WO2011026190A1

WO2011026190A1 - A method and apparatus for testing a shearing device

Info

Publication number: WO2011026190A1
Application number: PCT/AU2010/001139
Authority: WO
Inventors: Richard Triglavcanin; Ian Arbuthnot; Chad Loan
Original assignee: Outotec Oyj
Priority date: 2009-09-03
Filing date: 2010-09-03
Publication date: 2011-03-10
Also published as: AU2010291875B2; AU2010291875A1; CL2012000567A1; CA2772088A1

Abstract

A method (100) of testing a shearing device (42) for a separation device (40), comprises: submerging (104) a portion (103) of the shearing device in a test tank (101) to apply shear; calculating (105) a first speed of the shearing device for a first time period at a first predetermined distance; calculating (107) a second speed for the shearing device portion at a second predetermined distance that corresponds to the first speed at the first predetermined distance; calculating (109) the time difference between the first time period and a second time period for moving the shearing device portion at the second speed; and moving (110) the shearing device portion at the second speed for the second time period and stopping (111) movement of the shearing device portion for the time difference, to simulate the shear applied by the shearing device at the first predetermined distance over the first time period.

Description

"A METHOD AND APPARATUS FOR TESTING A SHEARING DEVICE"

FIELD OF THE INVENTION

The present invention relates to separation devices for suspensions and pulps and in particular to a method for controlling the application of shear to pulp in a separation device. It has been developed primarily for use in thickeners and will be described hereinafter with reference to this application. However, it will be appreciated that the invention is not limited to this particular field of use.

BACKGROUND OF THE INVENTION The following discussion of the prior art is intended to present the invention in an appropriate technical context and allow its significance to be properly appreciated. Unless clearly indicated to the contrary, however, reference to any prior art in this specification should not be construed as an admission that such art is widely known or forms part of common general knowledge in the field. Separation devices, such as thickeners, clarifiers and concentrators, are typically used for separating solids from suspensions (typically containing solids suspended in a liquid) and are often found in the mining, mineral processing, food processing, sugar refining, water treatment, sewage treatment, and other such industries. These devices typically comprise a tank in which solids are deposited from a suspension or solution and settle toward the bottom as pulp or sludge to be drawn off from below and recovered. A dilute liquor of lower relative density is thereby displaced toward the top of the tank, for removal via an overflow launder. The suspension to be thickened is initially fed through a feed pipe, conduit or line into a feedwell disposed within the main tank. A rake assembly is conventionally mounted for rotation about a central drive shaft and typically has at least two rake arms having scraper blades to move the settled material inwardly for collection through an underflow outlet.

In its application to mineral processing, separation and extraction, a finely ground ore is suspended as pulp in a suitable liquid medium such as water at a consistency which permits flow, and settlement in quiescent conditions. The pulp is settled from the suspension by a combination of gravity with or without chemical and/or mechanical processes. Initially, coagulant and/or flocculant can be added into the suspension to improve the settling process. The suspension is then carefully mixed into the separation device, such as a thickener, to facilitate the clumping together of solid particles, eventually forming larger denser "aggregates" of pulp particles that are settled out of suspension. Typically, several zones or layers of material having different overall densities gradually form within the tank, as illustrated in Figure 1. At the bottom of the tank 1 , the pulp forms a relatively dense zone 2 of compacted pulp or solids 3 that are frequently in the form of networked aggregates (i.e. the pulp aggregates are in continuous contact with one another). This zone 2 is typically called a "pulp bed" or networked layer of pulp. Above the pulp bed 2, a hindered zone 4 tends to contain solids 5 that have not yet fully settled or "compacted". That is, the solids or aggregates 5 are not yet in continuous contact with one another (un-networked). Above the hindered zone 4 is a "free settling" zone 6, where solids or aggregates 7 are partially suspended in the liquid and descending toward the bottom of the tank 1. It will be appreciated that the hindered zone 4 is not always a distinct zone between the networked layer 2 and the free settling zone 6. Instead, the hindered zone 4 may form a transition or an interface between the networked layer 2 and the free settling zone 6 that blends between the two zones. Above the free settling zone 6 is a clarified zone 8 of dilute liquor, where little solids are present and the dilute liquor is removed from the tank 1 by way of an overflow launder (not shown). Figure 1 also illustrates the feedwell 9 and underflow outlet 10 for removing the compacted pulp 3 from the tank 1.

It has hitherto been conventionally thought that to ensure that an appropriate underflow density is maintained within the pulp bed 2, it and the hindered zone 4 should be undisturbed to permit settling of the dense aggregates of solid particles into their desirable compacted arrangement. As a consequence, most developments in separation device technology concern the improvement of the settling process, either in the feedwell or the free settling zone 6, rather than any processes which may disturb the compacted arrangement of the solids particles in the pulp bed 3 or the partially compacted solids in the hindered zone 4. It has also been found that as the pulp bed 2 increases in depth, it becomes increasingly difficult for released liquid to permeate through the pulp bed 2 and migrate upwardly into the clarified zone 8. One solution has been to provide dewatering pickets mounted to the rake arms to aid removal of such liquid, thereby increasing the underflow density and thus the efficiency of the separation process. These pickets are typically arranged at equally spaced intervals to produce dewatering channels in the pulp bed equally spaced across the diameter of the tank, and are designed to minimise disturbance of the pulp bed.

The inventors discovered unexpectedly and surprisingly that the application of a disturbance, preferably in the form of shear, to pulp can result in improved efficiency in the separation process, especially the settling process in a thickener. It is believed that by causing a disturbance substantially uniformly across a disturbance zone in an upper region of the networked layer of pulp, the networked pulp is disrupted, by breaking up, disturbing, re-arranging, re-orienting or "shaking" the continuous contact between the pulp, or subjecting it to a force. This disruption of the networked pulp enables the release of trapped liquid upwardly towards the clarified zone of dilute liquor and increases the density of the pulp below the disturbance zone relative to pulp density above the disturbance zone. At the same time, the disturbance creates turbulence that inhibits or prevents the formation of donuts in the pulp bed. Moreover, the inventors have determined that if the networked layer is disturbed too much, fractionation of the networked pulp into smaller pieces occurs, resulting in the smaller pieces settling more slowly. Too little disturbance fails to disrupt the networked pulp sufficiently to release enough liquid to improve settling efficiency. Thus, the inventors have determined that by controlling the amount of the disturbance to an optimum level, as distinct from a minimum level, the improved separation efficiency can be maintained continuously over the work cycles of the separation device.

Throughout the description and claims, the terms "disrupt", "disrupting", "disruption" and its variants are taken to mean breaking up, disturbing, re-arranging, re-orienting or "shaking" particles or a substance, as well as applying a force to the particles or substance. In the context of the present invention, these terms are taken to mean breaking up, disturbing, re-arranging, re -orienting, applying a force to, or "shaking" the organised structure of the networked pulp, including but not limited to the continuous contact between the networked pulp. As a result of this unexpected and surprising discovery, the inventors developed a method and a separation device for controlling a disturbance, preferably in the form of shear, to pulp in the separation device, which for convenience will be described throughout the description as the "disturbance control invention". In the disturbance control invention, a suspension comprising pulp is fed into a tank of a separation device at a flux, the pulp being allowed to separate from the suspension and a disturbance causing device, preferably a shearing device, is submerged for shearing pulp at least partially within a region of the tank. In the case of a shearing device that created the disturbance through mechanical agitation, one or more shearing parameters for the shearing device were controlled with respect to the flux and/or one or more operational parameters to controllably apply an optimal shear to pulp passing through the region. These shearing parameters were selected from the group consisting essentially of the speed of the shearing device, the shape of the shearing device and the depth of the shearing region. The operational parameters were selected from the group consisting essentially of the pulp composition, the pulp particle size, the pulp flow velocity in the tank, the pulp yield stress, the pulp viscosity, the underflow specific gravity, the underflow weight per weight percentage and the rate at which flocculant is added to the suspension. Throughout the description and claims, the term "flux" means the rate of flow of solids suspended in a fluid (generally a liquid) suspension and is measured in tonnes per square metre hour (t/m²h). In the context of minerals separation, the flux is used to refer to the flow of suspended pulp solids in the slurry. Although the solids concentration or pulp density of the suspension may change as the pulp moves through the tank, the flow of solids may be regarded as independent of the pulp density and so is expressed as a flux.

It will be appreciated that this method is not directly dependent upon a specific configuration of the disturbance causing device or its preferred form of applying a shear through a shearing device. Rather, the mechanism by which shear is applied to cause the disturbance can take a number of forms. For example, one shearing mechanism is to use liquid or gas jets to inject a liquid or gas towards, into or through the disturbance zone 16 to apply shear substantially uniformly across the disturbance zone. Similarly, a fluidiser can direct fluid flow towards, into or through the disturbance zone 16 to apply shear substantially uniformly across the disturbance zone. Other shearing mechanisms include subjecting the disturbance zone 16 to mechanical vibration using a suitable vibratory apparatus or ultrasonic impulses to apply shear substantially uniformly across the disturbance zone. While these shearing mechanisms are suitable for implementing the disturbance control invention, the inventors have determined that a preferred shearing mechanism is mechanical agitation. Where the mechanical agitation is performed using a shearing device, it is expected that most of these shearing devices would have at least one arm carrying a plurality of shearing elements. However, the shearing device may vary in its overall shape and dimensions, the number of shearing arms and shearing elements on each respective shearing arm, and the arrangement of the shearing arms and shearing elements. As a consequence, it is necessary to test the design of any proposed shearing device to determine whether it will produce the required optimal shear in accordance with this method.

It has been found in practice that for the purposes of testing shearing devices, it is preferred to limit the variables involved to accurately measure the shear produced by a particular shearing device. Accordingly, the variables for the shearing and operational parameters are limited to the shearing device speed and the flux of the suspension. Thus, this type of testing can determine whether a particular configuration of the shearing device results in an optimal amount of shear, and if so, the optimal shearing device speed and frequency for that particular shearing device configuration.

In most conventional testing regimes, a scaled down version of a separation device, for example a thickener, is typically used as a continuous process testing apparatus. A correspondingly scaled down version of the shearing device is used as a test shearing device and is placed inside the test apparatus. The test shearing device is operated over various speeds and for a predetermined operational cycle, to simulate conditions in the full scale thickener. The underflow density of the settled pulp is then measured to determine the effectiveness of the shearing device. Thus, it is important to simulate the correct amount of shear, as the effectiveness of the shearing device is determined indirectly by measuring the underflow density of the settled pulp in the test apparatus after being sheared by the test shearing device. However, where tests are conducted in a conventional testing apparatus using a scaled down version of the shearing device, these tests do not provide accurate results. The test apparatus does not fully and/or accurately simulate the operation of the shearing device in the thickener because the amount of shear that is applied to the pulp in the tank is a function of the velocity of the shearing device (the radial distance of the shearing device from its axis of rotation), the profile of the shearing device and the frequency of the shear events applied by the shearing device. Thus, it is important to simulate the correct amount of shear and the frequency (number) of shear events, as the effectiveness of the shearing device is determined indirectly by measuring the underflow density of the settled pulp in the test apparatus after being sheared by the test shearing device. Consequently, it is difficult to reproduce in a testing apparatus the correct amount of shear and the frequency of shear events at each radial distance for the same predetermined time period or cycle. In other words, results for shear obtained at one radial distance cannot be extrapolated to obtain accurate results for a greater radial distance over the same cycle, due to the differing amounts of shear and frequency of shear events that would be applied at each radial distance over the common cycle.

As a result, the test results must be manipulated statistically to take into account the effect of these scale differences between the test apparatus and test shearing device and the full scale thickener and shearing device upon the shear. However, in the case of the shear profile and the frequency or number of shear events, it is difficult to adjust the test data to accurately take these factors into account. Consequently, this introduces errors into the test results that make it difficult to gauge the effectiveness of the shearing device. It is an object of the invention to overcome or ameliorate one or more of the deficiencies of the prior art, or at least to provide a useful alternative.

SUMMARY OF THE INVENTION

According to one aspect, the invention provides a method of testing a shearing device for a separation device, wherein the separation device comprises a tank for receiving a feed material, wherein feed material settles in the tank and the pulp forms into aggregates, the pulp aggregates settling and forming a first networked layer of pulp towards the bottom of the tank, and the shearing device is moveable to apply shear substantially uniformly across a first disturbance zone in an upper region of the first networked layer, so as to disrupt the networked pulp in the first disturbance zone within a predetermined period of time, the method comprising the steps of:

providing a test tank for a feed material to settle and pulp to form into aggregates, the pulp aggregates settling and forming a second networked layer of pulp towards the bottom of the test tank;

submerging a portion of the shearing device at least partially within the second networked layer in the test tank to apply shear in a second disturbance zone in an upper region of the second networked layer; calculating a first speed of the shearing device for a first time period in which the shearing device is expected to apply shear in the first disturbance zone at a first predetermined distance;

positioning the shearing device portion at a second predetermined distance in the test tank;

calculating a second speed for the shearing device portion at the second predetermined distance that corresponds to the first speed at the first predetermined distance;

calculating a second time period for moving the shearing device portion at the second speed that corresponds to the first time period at the first predetermined distance;

calculating a time difference between the first time period and the second time period; and

moving the shearing device portion in the second disturbance zone at the second speed for the second time period and stopping movement of the shearing device portion for the time difference, so as to simulate the application of shear in the first disturbance zone by the shearing device at the first predetermined distance in the separation device over the first time period.

Unless the context clearly requires otherwise, throughout the description and the claims, the words "comprise", "comprising", and the like are to be construed in an inclusive sense as opposed to an exclusive or exhaustive sense; that is to say, in the sense of "including, but not limited to".

Preferably, the method comprises repeating the moving step after the stopping step. More preferably, the method comprises successively repeating the moving and stopping steps. Preferably, the moving step comprises rotating the shearing device portion.

Preferably, the first and second speeds are rotational speeds of the shearing device and the shearing device portion, respectively. More preferably, the method comprises calculating the rotational speed of the shearing device portion so that its linear speed at the second predetermined distance is substantially equal to the linear speed of the shearing device at the first predetermined distance. In one preferred form, the linear speeds of the shearing device portion and the shearing device are average linear speeds. In this case, the average linear speed is the average of linear speeds across the respective widths of the shearing device portion or shearing device.

Preferably, the method further comprises adjusting the second speed by a scaling factor. More preferably, the scaling factor is calculated according to the relationship:

where ε is the scaling factor;

Q is the circumference of a circle travelled by the shearing device at the first predetermined distance in metres; and

Qis the circumference of the test tank in metres. Alternatively, the scaling factor is calculated according to the relationship:

where ε is the scaling factor;

i is the first predetermined distance in metres; and

Ix is the second predetermined distance in metres. In one particularly preferred form, the adjusting step comprises applying the scaling factor to the rotational speed of the shearing device to obtain the rotational speed of the shearing device portion. In this case, the rotational speed for the shearing device portion is calculated according to the relationship: where co_t is the rotational speed of the shearing device portion in rpm;

ε is the scaling factor; and

cot is the rotational speed of the shearing device in rpm.

Preferably, the first time period is the time for one revolution of the shearing device in the tank of the separation device. Preferably, the second time period is calculated by applying a scaling factor to the first time period. More preferably, the second time period is calculated according to the relationship: where tct is the first time period in seconds;

ε is the scaling factor; and

t_C£ is the second time period in seconds. More preferably, the scaling factor is calculated according to the relationship:

where ε is the scaling factor;

Qis the circumference of the test tank in metres.

Alternatively, the scaling factor is calculated according to the relationship:

where ε is the scaling factor;

i is the first predetermined distance in metres; and

it is the second predetermined distance in metres.

In a further alternative, the scaling factor is calculated according to the relationship: where ε is the scaling factor;

on is the rotational speed of the shearing device portion in rpm; and cot is the rotational speed of the shearing device in rpm.

Preferably, where the shearing device has N shearing arms, the method further comprises adjusting the second time period according to the relationship:

where tct is the second time period in seconds;

N is the number of shearing arms of the shearing device;

ε is the scaling factor;

G)_t is the rotational speed of the shearing device portion in rpm;

(¾ is the rotational speed of the shearing device in rpm; and t_C£ is the first time period in seconds.

Preferably, the first predetermined distance is proportional to the second predetermined distance.

Preferably, the method comprises the step of measuring the separation of pulp from the fluid in the test tank after completion of the first time period to determine whether the shearing device would apply the expected optimal shear at the first predetermined distance. More preferably, the measuring step is performed after a predetermined number of successive repetitions of the moving step and the stopping step.

Preferably, the first predetermined distance is a radial distance from a centre of the separation device. Preferably, the second predetermined distance is a radial distance from a centre of the test tank. More preferably, the second predetermined distance is a radial distance from a centre of the test tank to a selected radial point on the shearing device portion. In one preferred form, the second predetermined distance is a radial distance from a centre of the test tank to an outer edge of the shearing device portion.

Preferably, the method comprising adjusting one or more of the first speed, the first time period and the time difference in response to one or more shearing parameters. Preferably, the shearing parameters are selected from the group consisting essentially of the shape of the shearing device and the depth of the shearing region.

Preferably, the method comprises the step of adjusting one or more of the first speed, the first time period and the time difference in response to changes in the flux. Preferably, the method comprises the step of adjusting one or more of the first speed, the first time period and the time difference in response to changes in one or more of the operational parameters.

Preferably, the operational parameters are selected from the group consisting essentially of the pulp composition, the pulp particle size, the pulp flow velocity in the tank, the pulp yield stress, the pulp viscosity, the underflow specific gravity, the underflow weight per weight percentage and the rate at which flocculant is added to the suspension.

Preferably, the method comprises the step of reversibly rotating the shearing device. Preferably, the moving step further comprises periodically reversing the rotation of the shearing device. Preferably, the method comprises the step of moving the shearing device portion to apply a substantially uniform number of shear events to the networked pulp in the second disturbance zone within the second time period.

Preferably, the method comprises the step of moving the shearing device portion to apply a substantially uniform cumulative shear to the networked pulp in the second disturbance zone within the second time period. Throughout the description and the claims, the term "cumulative shear" means the sum of the shear that is applied to a typical pulp aggregate or particle passing through a defined region. In this context, the cumulative shear is the total sum of shear that a typical pulp aggregate or particle experiences between its entry into and exit from the region, which is determined by the sum of "shear" events that have occurred and the magnitude of those shear events; that is, the number of times the typical pulp aggregate or particle has been "hit" (a shear force has been applied to it). These shear events not only include direct "hits" of the pulp aggregate or particle by the shearing device but also disturbances or "shaking" of the pulp aggregate or particle caught in the wake of the passage of the shearing device, which the inventors call a "zone of turbulence". These zones of turbulence are sufficient to apply a shear force to the pulp aggregate or particle, albeit less than the amount of shear directly applied by the shearing device.

Alternatively, the moving step further comprises rotating the shearing device portion about an axis of rotation that is parallel, eccentric or offset with respect to a central axis of the test tank. Preferably, the method comprises the step of moving the axis of rotation relative to the central axis. More preferably, the axis of rotation rotates, revolves or orbits at least partially around the central axis. In one preferred form, the axis of rotation at least partially traverses a regular path around the central axis. Alternatively, the axis of rotation at least partially traverses an irregular path around the central axis. In some embodiments, the axis of rotation moves in a circular path. In other embodiments, however, the axis of rotation moves in a non-circular path, which may be geometrically regular or irregular.

Preferably, the shearing device portion has a plurality of shearing elements. Preferably, the method comprises the step of defining a zone of turbulence for each shearing element to disturb, re-arrange or break-up the pulp aggregates and/or to release liquid.

Preferably, the method further comprises the steps of spacing apart the shearing elements along at least one arm of the shearing device portion to define respective intervals therebetween and applying a substantially uniform average shear to the networked pulp in at least two intervals along a line parallel to or coincident with the at least one arm. More preferably, the average shear in all the intervals between the shearing elements along the line is substantially uniform or the same. Throughout the description and the claims, the term "average shear" means the average of shear applied to pulp between any two predetermined reference points. In this context, the two reference points typically (but not necessarily) coincide with adjacent shearing elements disposed on the at least one arm of the shearing device. It will be appreciated that the line may be non-linear in whole or part. For example, the line may include a portion that is arcuate, angled or offset with respect to a straight portion of the line. In one preferred form, the line is a radial line.

In one preferred form, the at least one arm extends outwardly. Preferably, the at least one arm extends radially outwardly. More preferably, the at least one arm extends radially outward substantially to an outer perimeter of the region.

Preferably, the method comprises the step of applying substantially uniform average shear along the length of the arms.

Preferably, the method comprises the step of disposing one or more shearing elements on the at least one arm. Preferably, the method comprises the step of arranging the shearing elements to apply shear along the at least one arm.

Alternatively, the method comprises the step of disposing one or more shearing elements along the axis of rotation to extend radially outwardly. Preferably, the method comprises the step of removably mounting the one or more shearing elements to a drive shaft of the shearing device portion. According to another aspect, the invention provides an apparatus for testing a shearing device, wherein the shearing device is for a separation device comprising a tank for receiving a feed material, wherein feed material settles in the tank and the pulp forms into aggregates, the pulp aggregates settling and forming a first networked layer of pulp towards the bottom of the tank, and the shearing device is moveable to apply shear substantially uniformly across a first disturbance zone in an upper region of the first networked layer, so as to disrupt the networked pulp in the first disturbance zone within a predetermined period of time, the apparatus comprising:

a test tank for receiving a feed material to settle and pulp to form into aggregates, the pulp aggregates settling and forming a second networked layer of pulp towards the bottom of the test tank, and a portion of the shearing device submerged at least partially within the second networked layer in the test tank to apply shear in a second disturbance zone in an upper region of the second networked layer;

wherein the shearing device portion is positioned at a second predetermined distance in the test tank;

the shearing device portion is moveable in the second disturbance zone at a second speed for a second time period, said second speed corresponding to a first speed of the shearing device for a first time period in which the shearing device is expected to apply shear in the first disturbance zone at a first predetermined distance, and said second time period corresponds to the first time period at the first predetermined distance; and

the shearing device portion is able to be stopped for a time difference between the first time period and the second time period, so as to simulate the application of shear in the first disturbance zone by the shearing device at the first predetermined distance in the separation device over the first time period. Preferably, the shearing device portion rotates in the test tank. Preferably, the first and second speeds are rotational speeds of the shearing device and the shearing device portion, respectively.

Preferably, the shearing device portion applies a substantially uniform number of shear events to the networked pulp in the second disturbance zone within the second time period.

Preferably, the shearing device portion applies a substantially uniform cumulative shear to the networked pulp in the second disturbance zone within the second time period.

Preferably, the shearing device portion rotates about an axis of rotation that is parallel, eccentric or offset with respect to a central axis of the test tank. Preferably, the axis of rotation moves relative to the central axis. More preferably, the axis of rotation rotates, revolves or orbits at least partially around the central axis. In one preferred form, the axis of rotation at least partially traverses a regular path around the central axis. Alternatively, the axis of rotation at least partially traverses an irregular path around the central axis. In some embodiments, the axis of rotation moves in a circular path. In other embodiments, however, the axis of rotation moves in a non-circular path, which may be geometrically regular or irregular. Preferably, the shearing device portion has a plurality of shearing elements. Preferably, the shearing elements each define a zone of turbulence to disturb, re-arrange or break-up the pulp aggregates and/or to release liquid.

Preferably, the shearing elements are spaced apart along at least one arm of the shearing device portion to define respective intervals therebetween and apply a substantially uniform average shear to the networked pulp in at least two intervals along a line parallel to or coincident with the at least one arm. More preferably, the average shear in all the intervals between the shearing elements along the line is substantially uniform or the same. In one preferred form, the shearing device portion applies substantially uniform average shear along the length of the at least one arm.

Preferably, the shearing device portion has at least one arm that extends outwardly. In one preferred form, the at least one arm extends radially outwardly. More preferably, the at least one arm extends radially outward substantially to an outer perimeter of the region.

Preferably, one or more shearing elements are disposed on the at least one arm. Preferably, one or more shearing elements are arranged to apply shear along the at least one arm. Alternatively, one or more shearing elements are disposed along the axis of rotation to extend radially outwardly. Preferably, the one or more shearing elements are removably mounted to a drive shaft of the shearing device portion.

The test tank can be configured to be any suitable shape corresponding the tank of the separation device. Preferably, the test tank is configured to be at least one of the following shapes: a substantially cylindrical in shape, rectangular in shape, a trough that can be open or closed, a partially arcuate in shape and an arcuate shape that corresponds to a circumferential arc of the separation device.

Preferably, the separation device is a thickener. BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which:

Figure 1 is a schematic cross-sectional view of the typical zones of material within a separation device; Figure 2A is a schematic cross-sectional view illustrating the settling process in the separation device of Figure 1;

Figure 2B is a schematic diagram illustrating the inventive concept of causing a disturbance substantially uniformly across in an upper region of the networked layer; Figure 3 is a schematic diagram view of one example of the disturbance control invention;

Figure 4 is a schematic diagram of another example of the disturbance control method of Figure 3;

Figure 5 is a cross-sectional view of a separation device for according to the disturbance control invention;

Figures 6A and 6B are respective schematic partial cross-sectional and plan views of a shearing device for a separation device according to one embodiment of the disturbance control invention;

Figures 7A and 7B are respective schematic partial cross-sectional and plan views of a dewatering picket assembly in the prior art;

Figure 8A is a cross-sectional view of a separation device according to a further embodiment of the disturbance control invention;

Figure 8B is a cross-sectional view of a separation device according to yet another embodiment of the disturbance control invention; Figure 8C is a cross-sectional view of a separation device according to a further embodiment of the disturbance control invention;

Figure 9 is a schematic diagram of one embodiment of the method of the invention;

Figures 10A to IOC are schematic diagrams further illustrating the operation of the method of Figure 9; Figure 11 is a cross-sectional view of test apparatus for implementing the method of

Figure 5;

Figure 12 is a plan view of the test apparatus of Figure 11; Figure 13 is a schematic diagram superimposing the test apparatus of Figures 11 and 12 on the separation device of Figure 5 to illustrate the method of Figure 9;

Figure 14A and 14B are plan views of a cylindrical test tank and a thickener illustrating the linear velocities of the shearing device portion and the shearing device, respectively;

Figure 15 is a plan view of a test apparatus according to another embodiment of the invention; and

Figure 16 is a plan view of a test apparatus according to a further embodiment of the invention. PREFERRED EMBODIMENTS OF THE INVENTION

To fully understand and appreciate the current invention and place it in an appropriate context, the general technical field and the inventors' disturbance control invention will be discussed below.

A preferred application of the invention is in the fields of mineral processing, separation and extraction, whereby finely ground ore is suspended as pulp in a suitable liquid medium, such as water, at a consistency which permits flow, and settlement in quiescent conditions. The pulp, which includes both solid particles and liquid, is settled from the suspension by a combination of gravity with or without chemical and/or mechanical processes. The pulp gradually clumps together to form aggregates of pulp as it descends from the feedwell towards the bottom of the tank. This process is typically enhanced by the addition of flocculating agents, also known as flocculants, which bind the settling solid or pulp particles together. These denser pulp aggregates settle more rapidly than the individual particles by virtue of their overall size and density relative to the surrounding liquid, gradually forming a networked layer or pulp bed 2, where the pulp is in a compacted arrangement and continuous contact with each other, as best shown in Figure 1.

The settling of pulp as it passes through the zones in a thickening tank 1 is described in more detail with reference to Figure 2A, where corresponding features have been given the same reference numerals. Within the feedwell 9, flocculant 11 is added and adsorbs onto discrete pulp particles 12, as best shown in Figure 2A(a). The flocculant 1 1 and pulp particles 12 grow and loosely bind together into porous pulp aggregates 13 within the feedwell 9 and/or as the pulp particles 12 flow out of the feedwell 9 into the free settling zone 6, as best shown in Figure 2A(b). Due to their porous nature, liquid 14 is trapped within individual pulp aggregates. As the pulp aggregates 13 further descend in the tank 1 through the free settling zone 6 and into the hindered zone 4, they become crowded and impede settling of each other, as best shown in Figure 2A(c). Gradually, the pulp aggregates 14 consolidate and compact together into an organised networked layer 2 of pulp, also called a pulp bed, as best shown in Figure 2A(d). Nevertheless, despite this compacted arrangement of the networked pulp layer 2, it has been found that areas occur within the networked pulp layer where liquid remains trapped within and between the aggregates in the networked layer of pulp. As it is difficult for this trapped liquid to escape the pulp bed into the clarified zone of dilute liquor, the underflow density of the pulp is diminished.

Thus, the inventors have unexpectedly and surprisingly found that the optimal disturbance for achieving this improved separation efficiency continuously over the work cycles of the separation device is obtained by causing the disturbance substantially uniformly across a disturbance zone in an upper region of the networked layer, as best shown in Figure 2B where corresponding features have been given the same reference numerals. As shown in Figures 2B(a) to 2B(d), flocculant is added into the feedwell 9 to adsorb onto discrete pulp particles 12 to promote the formation of aggregates 13 that descend and form a networked layer of pulp. Unlike the conventional settling process illustrated in Figure 2A, where the pulp aggregates 13 are left alone during formation of the networked layer 2, a disturbance 15 is caused substantially uniformly across within a disturbance zone 16 in an upper region 17 of the networked layer 2, as best shown in Figure 2B(e). As a consequence, a proportion of the networked pulp 3 (being the networked pulp that passes through the disturbance zone 16) is disrupted to release liquid 14 trapped within the networked pulp, thus increasing the relative density of the pulp 18 below the disturbance zone 16, as best shown in Figure 2B(f).

The inventors have found that a preferred and convenient way of causing the disturbance is to apply shear substantially uniformly across the disturbance zone, although other forms of disturbance may be used, for example creating turbulence across the disturbance zone. The disturbance, preferably the application of shear, substantially uniformly across the disturbance zone 16 results in an increased probability of the networked pulp receiving a disturbance that disrupts its generally organised structure. The disturbance may also disrupt the continuous contact between the networked pulp. The disruption can take the form of shaking or disturbing the networked pulp. Alternatively, or cumulatively, the disruption can take the form of re-arranging, re-orienting or breaking up the networked pulp. In both cases, the disruption has the effect of releasing liquid 14 trapped in the networked pulp, either between pulp aggregates or within a pulp aggregate. Thus, a substantial proportion of this trapped liquid 14 is released upwardly out of the networked pulp bed 2. It is believed that the application of shear to the networked pulp "shakes", re-arranges or breaks up its structure and/or continuous contact between the networked pulp so that the pulp below the disturbance zone becomes more dense, which results in an enhancement of their settling rate and/or their packing density. Moreover, the disturbance is not so excessive as to cause fractionation of the networked pulp into smaller pieces, which settle more slowly. The relatively denser pulp tends to reform into a networked pulp layer below the disturbance zone, due to its own weight applying compaction forces to the pulp. As a result, the invention provides the appropriate amount of disturbance to increase the settling rate and/or underflow density of the pulp in the networked layer or pulp bed 2, thus leading to increased efficiency and performance of the separation device. The inventors have discovered that the disturbance, preferably by way of shear, induces a stepwise increase in the density of the pulp below the disturbance zone. In the context of the application of the invention to a thickening process, the inventors have found that by controlling the disturbance, preferably shear, to an optimal amount using at least one or more of three primary options that will be discussed in more detail below, this stepwise increase in density of the pulp below the disturbance zone is at least a 5% increase compared to the density of pulp above the disturbance zone. In one preferred form, there is at least a 10% increase compared to the density of pulp above the disturbance zone. In other preferred forms, the density of the pulp below the disturbance zone is at least 25%, preferably at least 35% and more preferably at least 50%, greater than the density of pulp above the disturbance zone.

It will be appreciated that during operation of the separation device, the depth of the networked pulp layer 2 will gradually increase. Alternatively, the separation device may have a relatively low networked pulp layer 2 for operational requirements. Consequently, the disturbance zone 16 may initially occupy a larger proportion of the networked pulp layer 2, and in such cases the disturbance zone may be within an upper 75% of the networked layer of pulp. Where a typical depth of the networked pulp layer is present in the tank, the disturbance zone is within an upper half of the networked layer of pulp. However, the method of the invention may still be implemented where the disturbance zone 16 is within an upper 30% of the networked layer of pulp, an upper 10%> of the networked layer of pulp, or even at or adjacent the top of the networked layer of pulp.

The inventors have also surprisingly discovered that where shear is applied by way of mechanical agitation, this improved settling effect is best achieved by carefully controlling the shear applied to the pulp bed so as to keep the shear at an optimum level, as distinct from a minimum level. If too much shear is applied, fractionation of the aggregates into smaller pieces occurs, resulting in the smaller pieces settling more slowly. Too little shear fails to disrupt the networked pulp sufficiently to release enough liquid to improve settling efficiency. This resulted in the inventors developing the disturbance control invention and its particular application to a disturbance created by mechanical agitation using a shearing device. Where a shearing device is used in the disturbance control invention, the optimal shear for achieving this improved separation efficiency continuously over the work cycles of the separation device is obtained by controlling or adjusting one or more shearing parameters with respect to the flux (throughput) of the incoming suspension into the separation device, one or more operational parameters or a combination of both. These shearing parameters comprise the shearing device speed (linear or rotational), the depth of the disturbance zone and the (three-dimensional) shearing device shape. This unexpected and surprising discovery meant that the amount of shear applied to the pulp could be optimally controlled in accordance with operational requirements, especially variations in the supply of the suspension, whilst maintaining the improved separation efficiency in the separation device. In the case of a thickener, the application of the disturbance control invention results in improvements in the recovered underflow density of the settled pulp relative to the amount of flux or throughput of the liquid slurry that is processed by the thickener.

It will be appreciated by those skilled in the art that the concept of causing a disturbance, for example by applying shear, in a disturbance zone in the networked layer 2, which may include at least a portion of the hindered zone 6, is contrary to conventional thought and has not been contemplated as such in the prior art. In the prior art, it was preferred not to disturb the pulp bed 2 or the hindered zone 4, since most of the aggregates are compacted or almost compacted (in the case of the hindered zone 4), and improvements were focussed on improving the efficiency of the settling process, either in the feedwell 9 or in the free settling zone 6 in the tank 1. This was reflected in the design of thickeners specifically to minimise motion within the pulp bed 2. For example, equally spaced predominantly vertically extending pickets were mounted on the rake arms to create vertical dewatering channels to release liquid. However, the configuration and spacing of the pickets were designed to ensure that the pickets moved gently through the pulp bed to minimise any turbulence created by the pickets or their associated dewatering channels. A further advantage of the disturbance control invention is that disturbing the networked pulp layer 2 in the disturbance zone 16, for example by the application of shear, tends to inhibit the formation of donuts in the networked layer.

One example of the disturbance control invention is schematically illustrated in Figure 3. The method 20 for controlling shear applied to pulp within a separation device comprises the steps of feeding a suspension comprising pulp into a tank of the separation device at a flux (step 21), allowing the pulp to separate out of the suspension (step 22), submerging a shearing device for shearing pulp at least partially within a region of the tank (step 23), and controlling one or more shearing parameters to controllably apply an optimal shear to pulp passing through the "shear" region (step 24). This involves controlling the shearing device speed (step 25), the depth of the disturbance zone (step 26), the three- dimensional shape of the shearing device (step 27), or any combination of these shearing parameters, with respect to the flux and/or one or more operational parameters.

In particular, it has been discovered that optimal shear is obtained by controlling one or more of the shearing parameters in accordance with the following equation:

where So is the optimal shear;

ίι(λ) is the shear factor function;

f (y) is the shearing device speed function;

f₃(h) is the disturbance zone height or depth function; f₄(f) is the flux function; and

f₅(p) is the operational parameter function.

As the shear factor λ is a variable representing the average shear applied to the aggregates by the shearing device, it is therefore a function of the three-dimensional shape and the shearing device speed. Consequently, the shearing device speed is both a shearing parameter and a factor influencing the shear factor λ.

Therefore, altering the three-dimensional shape of the shearing device will influence the optimal shear applied to pulp passing through the disturbance zone through the shear factor λ. The shearing device may alter its three-dimensional shape by having movable shearing elements that can adjust their angle of incidence with respect to the direction of movement of the shearing device. Alternatively, the shearing device may have shearing elements that can be added or removed during operation to change its three-dimensional shape. However, in most practical commercial applications the shearing device shape has a predefined shape for simplicity, as providing a shearing device with an adjustable shape adds complexity to the design. It is therefore contemplated that the shearing device speed and the disturbance zone depth will be commonly selected to comply with the above relationship expressed in equation (1) and obtain optimal shear.

The operational parameter function f₅(p) represents the one or more variable operational parameters of the separation device. These operational parameters typically comprise the pulp composition, the pulp particle size, the pulp flow velocity in the tank, the pulp yield stress, the pulp viscosity, the underfiow specific gravity, the underfiow weight per weight percentage and the rate at which fiocculant is added to the suspension. As the operational parameter function f₅(p) can represent several variables, it is usually adjusted where one or more operational parameters are constant, or are assumed to be constant. For example, where the pulp viscosity or the underfiow specific gravity is known and does not vary significantly over the operation of the separation device, the operational parameter function f₅(p) is then adjusted to f*₅(p) multiplied by a constant representing the known pulp viscosity or the underfiow specific gravity value.

In one embodiment of the disturbance control invention, the selected shearing parameter(s) are kept proportional to the flux. That is, equation (1) becomes:

where So is the optimal shear;

λ is the shear factor;

y is the speed of the shearing device;

h is the height or depth of the disturbance zone;

f is the flux; and

f₅(p) is the operational parameter function.

Where all the operational parameters remain or are assumed to be constant, equation (2) becomes:

where S₀ is the optimal shear;

λ is the shear factor;

y is the speed of the shearing device;

h is the height or depth of the disturbance zone;

f is the flux; and

k_p is a constant representing the operational parameters.

In the method 20, the shearing device speed is initially set to move at the required speed, and both the shearing device shape and the disturbance zone depth are predetermined, in relation to the flux to ensure that an optimal shear is applied to the pulp. The disturbance zone depth is controlled simply and directly by controlling the submersion of the shearing device, as this will control the extent to which the shearing device will apply shear to the pulp. Alternatively, the disturbance zone depth is controlled by controlling the level of the suspension in the tank, and thus the extent in which the shearing device is submerged. As discussed above, the shearing device shape is usually predefined, although it is possible to have shearing devices with movable or adjustable shearing elements that change the shearing device shape.

As the disturbance zone depth and the shearing device speed are controlled through a suitable control unit or system of the separation device, it is relatively straightforward for these two shearing parameters to be set at the same time to ensure that an optimal shear is applied to the pulp during operation of the separation device.

It will be appreciated from equations (1) to (3) that the optimal shear is a function of the shearing parameters, operational parameters and the flux. Accordingly, the selected shearing parameter(s) can be controlled with respect to one or more of the operational parameters of the separation device instead of the flux. The selected shearing parameter(s) can also be controlled with respect to both the flux and the operational parameters.

A particular advantageous example of the disturbance control invention is illustrated in Figure 4, where corresponding features have been given the same reference numerals. The method 30 is applied to the settling process in a thickener, where at step 21 a slurry containing a mixture of liquid and pulp aggregates or particles is fed into a thickening tank at an initial flux. At step 22, the pulp is then allowed to settle out of the slurry. At step 23, a shearing device is submerged within a region of the tank. It will be appreciated that the shearing device may be submerged within the region prior to the settling step 22. The method 30 further comprises monitoring at step 31 the flux of the suspension during the feeding step 21, using a suitable sensor (not shown) in communication with a control unit or system (not shown) of the thickener. In response to the monitoring step, the control unit controls one or more of the shearing parameters relative to the flux, to apply an optimal shear to the pulp at step 24, being the shearing device speed (step 32), the depth of the disturbance zone (step 33), the three-dimensional shape of the shearing device (step 34), or any combination of these shearing parameters, to meet the above relationship expressed in equations (1), (2) or (3). At step 35, one or more of the operational parameters are monitored with corresponding sensors in communication with the control unit or system. In response to any change in an operational parameter, the control unit or system adjusts one or more of the shearing parameters at step 36 to maintain the relationship of equations (1) or (2). In addition, any changes in the flux detected at step 31 may prompt adjustment of one or more of the shearing parameters at step 36. This involves adjusting the shearing device speed (step 32), the depth of the disturbance zone (step 33), the three-dimensional shape of the shearing device (step 34), or any combination of these shearing parameters, to maintain the relationship to the changed flux and/or operational parameter(s) in equations (1) or (2). Thus, in this embodiment the shearing parameter(s) are adjusted in response to the flux of the incoming suspension and/or operational parameters so that optimal shear is applied to the pulp at all times.

The adjusted shearing parameter need not be the same parameter initially selected at step 24. For example, the shearing device speed may be controlled initially at step 32, but subsequently, the disturbance zone depth is adjusted at step 33 to maintain its relationship to the flux, thus maintaining the optimal shear. It will also be appreciated that the method may be implemented by using any one of the shearing parameters while keeping the remaining shearing parameters constant. For example, the method can be limited to control and/or adjustment of the three-dimensional geometry of the shearing device, while the shearing device speed and the disturbance zone depth are preset. In this example, the shearing device shape can be controlled and/or adjusted by adding or removing shearing arms or elements. Alternatively, it is contemplated that one or more shearing elements are movable to adjust their angle of incidence to the direction of rotation of the shearing device.

In addition, the flux monitoring step 31 or the operational parameter monitoring step 35 may be omitted where it is desired to only control or adjust the shearing parameters with respect to only the operational parameter(s) or the flux. However, those skilled in the art will recognise that the most accurate control of the shearing parameters is obtained by monitoring both the flux and the selected operational parameter(s). In addition, only one or some of the operational parameters can be selected for monitoring at step 35 as desired, or where the other operational parameters are constant or are assumed to be constant.

Referring to Figure 5, a separation device in accordance with one embodiment of the disturbance control invention is illustrated, where corresponding features have been given the same reference numerals. The separation device is in the form of a thickener 40 and comprises a tank 1, an inlet 41 for feeding the suspension at a flux into the tank via a centrally located feedwell 9, and a disturbance causing device in the form of a shearing device 42 causing a disturbance substantially uniformly across a disturbance zone 16 in an upper region 17 of the networked layer 2, so as to disrupt the networked pulp 3 in the disturbance zone 16 within a predetermined period of time, thereby releasing entrained liquid 14 from the networked pulp in the disturbance zone 16 and increasing the relative density of the pulp 18 below the disturbance zone. One or more shearing parameters are controlled with respect to the flux and/or one or more operational parameters, to controllably apply an optimal shear to the pulp passing through the disturbance zone 16. In this embodiment, the thickener 40 is configured as a bridge-type thickener, having a supporting bridge 44 located diametrically across and above the tank 1 and a circumferential overflow launder 45. A central drive assembly 46 operates a central drive shaft 47 to rotate a rake assembly 48 and the shearing device 42 about a central axis 49 of the tank 1. The rake assembly 48 comprises rake arms 50 having scraper blades 51 extending downwardly towards the bottom 52 of the tank 1 to move settled and compacted pulp towards an underflow outlet 53. The entire tank 1 is supported by columns 54.

The shearing device 42 comprises two outwardly extending radial arms 81 , with a plurality of shearing elements in the form of angled linear rods or pickets 82 mounted to each radial arm. The pickets 82 are inclined at an angle of approximately 45° with respect to a vertical plane and are spaced at uneven intervals 83 to each other, with the pickets progressively decreasing in number from the axis of rotation 49 to respective outer edges 84 of the radial arms 81. This progressive increase in the intervals 83 is in proportion to the distance of their associated pickets 82 from the axis of rotation 49. As a result, the inner pickets 82a are densely located relative to each other towards the rotational axis 49, compared to the outer pickets 82b near the outer edges 84.

In operation, a suspension of pulp in the form of a slurry is fed into the feedwell 9 through the inlet 41. The slurry may be fed tangentially into the feedwell 9 to improve the residence time for mixing and reaction with reagents, such as flocculants, that help create the aggregates or "fiocs" of higher density pulp solids. Tangential entry also assists in dissipating the kinetic energy of the slurry in the feedwell 9, thus promoting settling within the tank 1. The suspension then flows downwardly under gravity out of a restricted outlet 64 into the tank 1 , where it settles to form the various zones of material, including the pulp bed 2, hindered zone 4, free settling zone 6 and clarified zone 8. The relatively dense pulp bed 2 displaces the upper clarified zone 8 of relatively dilute liquor towards the top of the tank 1. The thickened pulp is drawn off through the underflow outlet 53, while the dilute liquor is progressively drawn off through an overflow launder 45.

As the depth of the pulp bed 2 increases to encompass the disturbance zone 16 as part of its upper half, the shearing device 42 rotates around the tank 1 , causing the pickets 82 to apply shear substantially uniformly across the disturbance zone 16 to the pulp aggregates or particles descending from the feedwell outlet 64 into the disturbance zone 16. As discussed above, the disruption of the networked pulp in the disturbance zone 16 results in the release of trapped liquid or liquor and increases the relative density of the pulp below the disturbance zone 16. The denser pulp 18 below the disturbance zone 16 tends to reform a substantially higher density relative to the pulp above the disturbance zone, and thus settle quickly without excessive fractionation and detrimentally affecting the settling process. The shear is applied either as direct "hits" from the pickets 55, 61 and 63 or as disturbances in the zones of turbulence associated with the wake of the passage of the pickets 55 through the disturbance zone 16.

As previously described in relation to Figures 3 and 4, the shearing device speed, the depth of the disturbance zone 16, the three-dimensional shape of the shearing device 42, or any combination of these shearing parameters are controlled to maintain the relationship of equations (1), (2) or (3) with respect to the flux and/or one or more operational parameters. Furthermore, these shearing parameters, individually or in combination, are adjusted in response to changes in the flux and/or one or more operational parameters to ensure optimal shear is continuously applied to the pulp. This results in shear being applied in a sufficient amount to the pulp aggregates or particles before they exit the region, to release trapped liquid and disturb, "shake", re-arrange or break-up aggregates into aggregates that settle quickly without excessively applying shear that would fractionate the aggregates and detrimentally affect the settling process. The optimal shear is applied either as direct "hits" from the pickets 82 or as disturbances in the zones of turbulence associated with the wake of the passage of the pickets 82 through the disturbance zone 16.

The inventors have found that in the invention the shearing parameters of the shearing device speed, disturbance zone depth and shearing device shape tend to dominate the relationship with the flux over the other possible operational parameters (for example, the pulp composition, the pulp particle size, the pulp flow velocity in the tank, the pulp yield stress, the pulp viscosity, the underflow specific gravity, the underflow weight per weight percentage and the rate at which f occulant (if any) is added to the suspension) to achieve an optimal shear profile. However, it is conceivable to apply the disturbance control invention so that the shearing parameters are controlled in relation to the one or more of the operational parameters instead of the flux. In practice, the shearing parameters are controlled in relation to the flux and the operational parameters, so that these operational parameters are used to further adjust the shearing parameters of shearing device speed, disturbance zone depth and shearing device shape, to further enhance the application of optimal shear to the pulp.

In addition, the specific configuration of the shearing device does not directly affect the optimal shear profile that is obtained from the shearing device speed to flux ratio, provided that the shearing device is configured to apply shear substantially uniformly across the disturbance zone to the networked pulp. It will be appreciated that the disturbance control invention can thus be implemented to any shearing device employed in a separation device, and so is not limited to a particular shearing device configuration. The inventors have, however, determined that there are several preferred configurations for the shearing device as they are generally more efficient in achieving an optimum shear profile, which are described below.

Thus, the inventors have discovered that the optimal amount of shear that results in improved and optimal thickener performance can be achieved primarily where the shearing device configuration results in at least one of, or a combination of, the following:

(1) a substantially uniform cumulative shear being applied to the networked pulp in the disturbance zone within a predetermined period of time;

(2) a substantially uniform average shear is applied to the networked pulp in at least two intervals between shearing elements spaced apart along at least one arm of the shearing device, along a line parallel to or coincident with the at least one arm; and

(3) a substantially uniform number of shear events is applied to the networked pulp in the disturbance zone within a predetermined period of time.

The separation device 40 of Figure 5 has a shearing device 42 that implements the concept of substantially uniform cumulative shear. The concept of substantially uniform cumulative shear is discussed in more detail below with reference to Figures 5A and 6A, where corresponding features have been given the same reference numerals.

If a pulp aggregate or particle is settling at a distance i from the centre at a rate v m s^-1, and the depth of the disturbance zone is d m, then the time taken by the particle to move through the disturbance zone is represented by

Θ = d/v seconds ... (4) Assuming that the disturbance is caused by the application of shear by a shearing device 69 having, for example, four rotating radial arms 70 (carrying angled pickets 72) mounted on a centre shaft 47 travelling at a rotational speed of ω revolutions per second, the number of "passes" in time Θ is represented by:

η = 4.θω ... (5)

This number of passes can also be regarded as the number of shear "events" experienced by each pulp aggregate 13 or particle 12 as the shearing pickets 72 move past. In this context, the shear applied by any individual picket not only includes a direct "hit" of the pulp aggregate by the picket 72 but the disturbance or "shaking" of the pulp aggregate caught in the wake of the passage of the picket, which the inventors call a "zone of turbulence". These zones of turbulence are sufficient to apply a shear force to the aggregate or pulp particle, albeit less than the amount of shear directly applied by the pickets 72.

Comparing the shearing picket configurations of Figures 6A and 7A, the probability of a pulp aggregate 13 or particle 12 being subjected to varying shear rates during the n shearing events is greater for the configuration of Figure 6A than the prior art configuration of Figure 7A, assuming that the number of shear events is significantly greater than 1. Hence, the total shear applied to a layer of settling pulp aggregates or particles becomes more uniform as n increases and the angle of the pickets φ is increased. However, the inventors believe that increasing φ several degrees beyond 45° is not beneficial because of fluid flow considerations, and substantially uniform cumulative shear is optimally obtained by inclining the shearing elements, such as the pickets 72, at 45° to the vertical.

With reference to the embodiment of Figure 5, in operation the shearing device 42 is rotated about the central axis 49 by the central drive shaft 47 to apply a substantially uniform cumulative shear to the pulp passing through disturbance zone 16 of the pulp bed 2 in accordance with principles described above. That is, shearing device 42 makes several passes through the disturbance zone 16 and the pickets 82 are angled so that the pulp aggregates or particles are subjected to several varying shear events, either by way of a direct "hit" or being caught in a zone of turbulence, as indicated by equations (4) and (5). Thus, the cumulative shear applied to pulp exiting the region 43 is substantially uniform or the same. Additionally, the inventors have discovered that where the shearing device comprises a plurality of shearing elements spaced apart along at least one arm to define intervals therebetween, an optimal amount of shear is obtained by providing a substantially uniform average shear in at least two intervals along a line parallel to or coincident with the at least one arm, and more preferably all the intervals between the shearing elements along the line.

In most cases, the shearing device will employ two or more outwardly extending radial arms and thus the substantially uniform average shear applied in the intervals between the shearing elements will be along a radial line in alignment with the radial arms. In other words, the line along which the substantially uniform average shear is applied in the intervals generally corresponds to the profile of the shearing device when viewed in plan. However, it will be appreciated that where the shearing device is partially or fully non-linear in cross-section, the line will correspondingly be partially or fully non-linear in conformity with that cross-section of the shearing device. For example, the shearing device may have arms that are sinuous, partially curved or even zigzag-like in shape, in which case the substantially uniform average shear would be applied along a sinuous, partially curved or zigzag-like line, respectively.

This concept of applying a substantially uniform average shear is discussed in more detail below with reference to Figures 5, 6B and 7B. The shearing device 80 of Figure has pickets 82 that are spaced at uneven intervals 83 to each other, with the pickets progressively decreasing in number from an axis of rotation 49 to respective outer edges 84 of the radial arms 81. This uneven spacing of the pickets 82 along the radial arms 81 results in the average shear in the intervals 83 between each pair of pickets 82 being substantially the same or uniform along a radial line defined by the radial arms 81. In particular, the inventors have determined that the shear applied to pulp aggregates or particles is generally a function of the linear speed or velocity of the pickets (or other shearing elements) and the distance between the picket and the pulp aggregate or particle. Since the linear velocity of the picket is also a function of the rotational speed of the drive shaft and the distance of the picket from the axis of rotation, the inventors have determined that as the distance from the axis of rotation increases, the linear velocity of the picket increases proportionately. This relationship between the shear and the distance between pickets is described in more detail below with reference to Figures 5B and 6B.

ut is the linear velocity of the picket in m-s ¹,

ξ is the distance between the picket and the pulp aggregate or particle in metres, and

k is a constant, which is a function of material properties of the pulp.

Also,

where ω is the rotational speed of the shaft in s^" ; and

I is the distance from the centre in metres.

Equation (7) can be simplified, by using revolutions per minute (rpm) as the unit for the rotational speed ω and 2π being a constant b, to

where ui is now expressed in m/min instead of m/s.

Referring to Figures 6A and 6B, equations (6), (7) and (8) indicate that as the distance

from the axis of rotation 49 increases, the linear velocity of the picket 82, ¾, increases proportionally as

is a product of 2πω and i_\,

For a set of particles (or aggregates 13) between any two pickets 82, in order to ensure that the average shear is substantially the same or uniform along the line parallel or coincident with the radial arms (ie. along the length of the radial arm 81), the spacing (ξ) between the pickets and the aggregates needs to increase proportionally to the linear velocity. That is, the distance or gap

between the pickets 82 is in proportion to their distance

from the axis of rotation 49 along the radial arm 81. Hence, the requirement for a substantially constant or uniform average shear can be met by increasing the distance or gap between the pickets in proportion to their distance along the radius. By way of contrast, this substantially constant or uniform average shear cannot be achieved by means of a set of evenly spaced pickets or rods fixed to a radial arm, since the linear speed of any such rod is proportional to its distance from the centre, as illustrated in Figures 7A and 7B. The configuration of the shearing device 42 results in a substantially uniform average shear being applied in the intervals 83 between the pickets 42 along a radial line defined by the radial arms 81. The outer pickets 82b provide a higher shear force than the inner pickets 82a due to the outer pickets 82b having a greater linear velocity, as indicated by equations (6), (7) and (8). However, due to the denser distribution of the inner pickets 82a compared to the outer pickets 82b, aggregates closer towards the axis of rotation 49 of the shearing device 80 have a more uniform shear profile over a smaller range of shear (in the amount of shear) than that applied to aggregates further from the axis of rotation 49. The shear profiles in the intervals 83 toward the outer edges 84 of the radial arms 81 are relatively less uniform and extend over a larger range or amplitude of shear than the shear profiles closer towards the axis of rotation 49. However, due to the differential spacing, the average shear applied to the pulp aggregates in the intervals 83 defined between the pickets 82 will be substantially uniform across the radial arms 81.

Thus, both the cumulative shear from the total number of shear events and the average shear between the pickets 82 are each substantially uniform (although not generally the same value) due to the arrangement of the angled pickets 82 on the radial arms 81. This causes the disruption of pulp aggregates to release trapped liquid, improving the overall density of the pulp bed 2, and to create denser aggregates that settle quickly in the pulp bed 2, thus improving the separation efficiency. Furthermore, the inventors have unexpectedly discovered that the application of a substantially uniform number of shear events across the disturbance zone 16 will also achieve an optimal shear profile that disrupts the networked pulp, thereby releasing trapped liquid 14 and increasing the density of the pulp 18 below the disturbance zone. The inventors have discovered that so long as the number of shear events received by the pulp passing through the disturbance zone 16 is substantially uniform over a predetermined period of time (for example, the period it takes for an x number of revolutions), then shear is being applied substantially uniformly across the disturbance zone, as indicated by equation (4). Thus, the necessary disruption to the networked pulp is obtained, along with the associated release of trapped liquid 14 and increase in the density of the pulp 18 below the disturbance zone 16. It follows that a uniform number of shear events does not require substantially uniform cumulative shear or substantially uniform average shear to be applied at the same time, since the number of shear events is significant and not the amount of each shear event.

Accordingly, Figures 8A, 8B and 8C illustrate shearing devices that achieve a uniform number of shear events without applying substantially uniform cumulative shear or substantially uniform average shear.

In Figure 8A, where corresponding features have been given the same reference numerals, the shearing device 85 has two outwardly extending radial arms 81, with a plurality of shearing elements in the form of angled linear rods or pickets 86 mounted to each radial arm. The pickets 86 are inclined at an angle of approximately 45° with respect to a vertical plane and are spaced at even intervals 87 to each other from an axis of rotation 49 to respective outer edges 84 of the radial arms 81. The shearing device 85 makes several passes through the disturbance zone 16 and the pickets 86 are angled so that the networked pulp aggregates 13 or particles 12 receive the same number of shear events between entry and exit of the pulp into and out of the disturbance zone 16. However, the even spacing of the pickets 86 means that the average shear in the intervals 87 between each pair of pickets 86 is not the same or uniform along a radial line defined by the radial arms 81. In addition, as the pickets 86 are not arranged to compensate for the progressive increase in linear velocity of the pickets 86 towards the outer edges 84 of the radial arms 81. Thus the amount of shear and therefore the cumulative amount of shear is not the same or uniform.

Similarly, in Figure 8B, where corresponding features have been given the same reference numerals, the shearing device 88 has two outwardly extending radial shearing arms 89 that apply shear across their respective lengths, and thus substantially uniformly across the disturbance zone 16. As there are no shearing elements other than the radial arms 89 that occupy the depth of the disturbance zone 16, there are no intervals for average shear nor any way to compensate for the progressive increase in linear velocity of the shearing arms 89 towards their respective outer edges 84.

In Figure 8C, where corresponding features have been given the same reference numerals, the shearing device 90 has two outwardly extending radial arms 81, with a plurality of shearing elements in the form of substantially vertical linear rods or pickets 91 mounted to and equispaced along each radial arm. In this embodiment, the pickets 91 are grouped closely together in a tight concentration to increase the area of the disturbance zone 16 to approximately 50% of the cross-sectional area of the upper region 17, and hence 50% of the networked pulp in the upper region, that receives a shear event during a pass of the shearing device 90. The shearing device 90 makes several passes through the disturbance zone 16 and the concentration of pickets 91 ensures that 50% of the networked pulp aggregates 13 or particles 12 receive the same number of shear events between entry and exit of the pulp into and out of the disturbance zone 16. As the pickets 91 are equispaced along the radial arms 81 , there is no uniform average shear between each pair of pickets 91 along a radial line defined by the radial arms 81. In addition, the pickets 91 are not arranged to compensate for the progressive increase in linear velocity of the pickets 91 towards the outer edges 84 of the radial arms 81, and hence, the amount of shear. Consequently, the cumulative amount of shear is not the same or uniform. In one variation, another set of radial arms 81 are provided with pickets 91 offset to the pickets 91 on the first set of radial arms 81 to apply shear in the intervals and thus increasing the disturbance zone 16 to encompass the entire upper region 17 (100%), and thus apply shear to the entire (100%)) networked pulp passing through the upper region.

The inventors have unexpectedly and surprisingly discovered that it is particularly advantageous for the shearing device 42 to be located in the upper half of the pulp bed 2, as the liquid is readily able to escape the pulp bed 2 into the clarified zone 8 of dilute liquor. By way of contrast, applying shear in only the bottom half of the pulp bed 2 will release liquid upwardly, however, the undisturbed upper layer of the pulp bed tends to produce a blanketing effect that hinders or even prevents further upward migration of the liquid into the clarified zone 8. Thus, the improved efficiency attained by the shearing device 42 is not as effectively achievable in the bottom half of the pulp bed 2, as it is in the upper portion, particularly in the upper half. In addition, the shear applied in the disturbance zone 16 is not constrained by the need to minimise the rotation speed of the shearing device 42, as it has been unexpectedly and surprisingly found that a greater amount of shear produced by the increased rotational speed does not adversely affect the compaction of the pulp solids in the pulp bed 2. The inventors also contemplate that this advantageous effect can be extended to include a portion of the hindered zone 4 above the pulp bed 2, especially a lower portion of the hindered zone. This is particularly the case where the hindered zone 4 is relatively shallow compared to the networked pulp layer 2 or the free settling zone 6 or forms an interface between the networked pulp layer 2 and the free settling zone 6. It has also been determined that an optimal shear can be obtained by either providing a substantially uniform cumulative shear, a substantially uniform average shear between the shearing elements or a substantially uniform number of shear events independently of each other, or a combination of any two or all three. In particular, it has been discovered that an advantageous implementation of the disturbance causing step 27 is to apply shear substantially uniformly across the disturbance zone 16. The mechanism by which shear is applied in the disturbance causing step 27 can take a number of forms. For example, one shearing mechanism is to use liquid or gas jets to inject a liquid or gas towards, into or through the disturbance zone 16 to apply shear substantially uniformly across the disturbance zone. Similarly, a fluidiser can direct fluid flow towards, into or through the disturbance zone 16 to apply shear substantially uniformly across the disturbance zone. Other shearing mechanisms include subjecting the disturbance zone 16 to mechanical vibration using a suitable vibratory apparatus or ultrasonic impulses to apply shear substantially uniformly across the disturbance zone. While these shearing mechanisms are suitable for implementing the disturbance causing (shearing) step 27 in the method of the disturbance control invention, the inventors have determined that a preferred shearing mechanism is mechanical agitation, advantageously by way of a shearing device that moves through the disturbance zone 16 to apply shear substantially uniformly across the disturbance zone. In one preferred form, the shearing device is rotated in the tank in accordance with the disturbance causing (shearing) step 27.

While the above discussion of the disturbance control invention describes three particularly preferred configurations for the shearing device, other shearing device configurations can be designed to obtain an optimal shear profile. Design of these alternative shearing device configurations involves testing the effectiveness of a specific shearing device configuration in a test apparatus, as well as determining the optimal shearing device speed and/or flux of the suspension. While this may seem straightforward, the inventors have found in practice that conventional testing regimes have been generally unsatisfactory in obtaining accurate results for the performance of a specific shearing device configuration. In particular, the inventors have found that the shear applied by a test shearing device in a test tank does not correlate to the shear that is applied by a shearing device in a thickener. This is due to the fact that shear is dependent on the linear velocity u of the shearing device and linear velocity u is in turn dependent on the rotational speed ω and the distance I from the centre of the thickening tank, as outlined in equations (6) and (7) above. This means that to properly simulate the shear applied at a designated distance greater than the radius of the test tank, it is necessary to have the test shearing device move at the same linear velocity as the linear velocity of the shearing device at the designated distance. This would involve increasing the rotational speed ω to compensate for the reduced distance i in the test tank. This causes a further problem in that increasing the rotational speed ω of the test shearing device results in a decrease in the period in which the test shearing device performs a cycle in the test tank. This decreased time period does not equate to the time period in which the full scale shearing device performs a cycle in the thickener. While the test results could be manipulated statistically to obtain approximate results, this necessarily introduces errors into the data and thus reduces the accuracy of the test results. Therefore, the inventors have developed a method for accurately testing a shearing device by simulating the expected conditions for optimal shear in a full scale thickener for a portion of the shearing device in a test apparatus. Referring to Figure 9, one embodiment a method of testing a shearing device for a separation device according to the present invention is schematically illustrated. In the method 100, a test tank 101 is provided for containing the fluid comprising the pulp (for example, a feed slurry) at step 102. A portion 103 of the shearing device 42 is submerged at least partially within the fluid held in the test tank 101 at step 104. As best shown by a comparison of Figure 5 with Figure 9, the shearing device portion 103 is a full scale "slice" or portion of the shearing device 42 and is not a scaled down version of the shearing device. Using a full scale portion 103 of the shearing device 42 is preferred in order to obtain the most accurate test results, as it provides the same shear profile as the shearing device 42.

A first speed of the shearing device 42 is calculated at step 105 for a first time period in which the shearing device is expected to apply an optimal shear to the pulp in the disturbance zone 16 at a first predetermined distance. In this embodiment, the first speed is the rotational speed of the shearing device 42, as this determines the linear speed or velocity of the shearing device 42 at the first predetermined distance. The first time period is the time in which the shearing device travels the first predetermined distance at its linear velocity. The linear speed is frequently determined by controlling the rotation speed ω of the shearing device 42. However, it will be appreciated that in other embodiments, the first speed is the linear speed or velocity, rather than the rotational speed. The shearing device portion 103 is positioned at a second predetermined distance in the test tank 101 at step 106. For the purposes of the calculation, the second predetermined distance is taken to be the distance from the centre of rotation 49 to the outer edge of the shearing device portion 103. A second speed for the shearing device portion 103 at the second predetermined distance is calculated at step 107 that corresponds to the first speed at the first predetermined distance. In this embodiment, the second speed is the rotational speed of the shearing device portion, and is calculated so that the corresponding linear velocity of the shearing device portion 103 is equal to the linear velocity of the shearing device 42 at the first predetermined distance. This corresponding relationship is based on equations (7) and (8). As shear is proportional to the linear velocity u, if the shearing device portion 103 and the shearing device 42 have the same linear velocity, then they will apply the same shear rate. This enables the same amount of shear that is applied at the first predetermined distance to be applied at the second predetermined distance, since the shearing device portion 103 has the same profile as the shearing device 42. While equation (7) appears to suggest a simple proportional relationship, the inventors have discovered that the calculation of the rotational speed of the shearing device portion 103 has to be adjusted for two primary factors. Firstly, the distance travelled by the shearing device portion 103 in one revolution of the test tank 101 at the linear velocity u_t is equal to the circumference C_t of the test tank, as best shown in Figure 10A. However, the distance travelled by the shearing device 42 at the linear velocity ¾ at the radial distance i will be less than, the same as or greater than the circumference C_t of the test tank 101. The shearing device 42 would travel along a circumferential arc of a circle Ct that is equal to C_t at the radial distance i. Therefore, the inventors have determined that the rotational speed for the shearing device portion 103 needs to be adjusted by a scaling factor ε equal to the ratio of the circumference of the circle Q travelled by the shearing device 42 at the linear velocity ¾ to the circumference C_t of the test tank 101 so that their respective linear velocities are equal. That is,

where ε is the scaling factor;

Q is the circumference of the circle travelled by the shearing device 42 at the linear velocity ¾ in metres; and

C_tis the circumference of the test tank in metres. The relationship in equation (9) is derived from the fact that the circumference Q forms part of a circle having a circumference Q = 2πί, as best shown in Figure 10B. Thus, from equations (7) and (8),

Similarly, the linear velocity of the shearing device portion 103 in the test tank 101 is also expressed as

As discussed above, for the magnitude of shear to be the same at radial distance it in the test tank 101 and at the radial distance i in the thickener, u_t = ¾. This means that

In addition, as

Thus, for u_t = ¾, equation (12) can be rewritten as

Thus, the scaling factor ε can be calculated from either the ratio of the respective circumferences of the test tank and thickener, the ratio of the radial distances of the shearing device portion 103 and the shearing device 42 from their respective axes of rotation 49 or the ratio of the rotational speeds of the shearing device portion 103 and the shearing device 42. In addition, these ratios remain the same whenever the linear velocities of the shearing device portion 103 and the shearing device 42 are the same.

From equation (13), it can be seen that this adjustment for the rotational speed of the shearing device portion can be calculated by directly applying the scaling factor ε to the rotational speed cot of the shearing device 42. Thus, as part of step 107, the rotational speed for the shearing device portion 103 is calculated as:

where co_t is the rotational speed of the shearing device portion 103 in the test tank

101 in rpm;

ε is the scaling factor from equation (9); and

co_t is the rotational speed of the shearing device 42 in rpm. The second adjustment that is required relates to the fact that the rotational speed co_t of the shearing device portion 103 tends to be greater than the rotational speed cot of the shearing device, where the second predetermined distance is less than the first predetermined distance. This means that the shearing device portion 103 is providing a higher frequency or number of shear events at the second predetermined distance over the first time period, resulting in excess shear being applied. The magnitude of each shear event (μ) is the same for the shearing device 42 and the shearing device portion 103 because the latter is a full scale "slice" of the former. However, the frequency of shear events for the shearing device 42 and the shearing device portion 103 are not the same and are proportional to the scaling factor ε. Accordingly, a second time period for moving the shearing device portion 103 at the second speed is calculated at step 108 that corresponds to the first time period at the first predetermined distance. That is, in step 108 the second time period for the shearing device portion 103 moving at the second rotational speed co_t is calculated as a proportion of the first time period in the following manner set out below in more detail.

The circumference of the test tank 101, typically being smaller than the thickener tank, means that C_t is a proportion of Q. Thus, there are Q/C_t sections of C_t in Q, as best shown in Figure IOC. As u_t = ut, the time for one shear event to occur in a circumferential arc equal to C_t is tc_t and the time for one shear event to occur in the thickener having the circumference Q is tct-

As the distance travelled by the shearing device 42 is equal to the circumferential arc Ct and linear velocity is equal to distance travelled over time, then

and so

From equation (11), u_t = 2nro_t-f_t and therefore

Similarly,

tcx = l/corN ... (16) where N is the number of arms of the shearing device 42.

This N term in equation (16) takes into account where the shearing device 42 has more than one shearing arm 81.

Dividing equation (15) by equation (16),

and so,

or

Where N = 1 (ie. one shearing arm), then equation (18) simplifies to tct = tce/ε.

The second time period that is calculated at step 108 is the same as the time tc_t, because it is the time period in which the shearing device 42 would apply shear in a circumferential arc equal to the circumference C_t of the test tank 101 at the radial distance i. The inventors call this second time period tct the shear or "on" time period, t_on, as it is the time in which the shearing device portion 103 is moved in the test tank 101.

The time difference between the first time period and the second time period is then calculated at step 109. The inventors call this time difference the "off time period, toff, since it is the time in which movement of the shearing device portion 103 is stopped. That is,

toff = time for one cycle or revolution in the test tank - t_on

The shearing device portion 103 is then moved at the calculated second rotational speed for the second time period at step 110. Movement of the shearing device portion 103 is then stopped for the time difference f at step 111. The moving step 110 and the stopping step 111 simulate the application of shear by the shearing device 42 at the first predetermined distance in the separation device over the first time period.

At step 112, the method determines whether to repeat steps 110 and 111. If so, then the method returns to steps 110 and 111 and this may be repeated successively to acquire statistically meaningful test results for the shearing device portion 103 and to allow for aggregate movement through the disturbance zone 16. Once the required number of cycles of steps 110 and 111 has been completed (which can include a single cycle if desired), then the underflow of settled pulp is withdraw from the test tank 101 and measured at step 113 to determine the effectiveness of the shearing device portion 103 in improving settling efficiency at the second predetermined distance. Due to the simulation of shear in a full scale thickener, these results can be directly applied for the shearing device at the first predetermined distance.

The method 100 can then be repeated for another first predetermined distance, essentially by returning to step 105 to enable the recalculation of another linear speed for the shearing device 42 at a different predetermined distance for the same first time period or another time period, at the same distance but with a different rotational speed, or any combination of these parameters. In this way, the method 100 can successively test the shearing device 42 at various predetermined distances and/or rotational speeds in the separation device using a single test tank 101.

However, the inventors have determined that it is not necessary to repeat the tests of the shearing device portion 103 at different radial distances because the amount of shear applied per pulp particle is substantially the same for both the test tank 101 and the full scale thickener 40. Calculations performed by the inventors have established that the amount of shear applied in the test tank 101, as moves from the inner end of the shearing device portion 103 to its outer extremity is equivalent to the amount of shear applied by the shearing device 42 in the thickener 40 from its inner end of the shearing device arm 81 to its outer end 84. This relationship holds where the linear or "tip" speeds of the respective shearing arms are essentially the same and the shearing device profiles are the same (that is, the size of the pickets, distance between pickets and overall geometrical shape of the shearing device). Consequently, a series of tests simulating the shear applied by the shearing device portion 103, and by extension the shearing device 42, along various radial distances is unnecessary due to the shearing device portion 103 being a full scale "slice" of the shearing device 42, and thus having the same shearing device profile. Rather, the inventors contemplate that testing the shearing device portion 103 at the same tip speed or "outer circumferential" (linear) speed as the outer end 84 of the shearing arm 81 of the shearing device 42 would obtain the most accurate results. Even so, it is still necessary to perform the method 100 and in particular the rotational speed calculation step 107, the second time period calculation step 108 and the time difference calculation step 109 for the test tank 101.

Thus, the method 100 simulates the operational speeds and time periods in which the shearing device 42 applies shear in a separation device, for example, a thickener. By moving the shearing device portion 103 at the second rotational speed so that the linear velocity of its outer edge is the same as the linear velocity of the shearing device at a predetermined distance, and adjusting the second time period to take into account the differences in the circumferential arc lengths travelled by the shearing device portion 103 and the shearing device 42, the method 100 reproduces the shear that would be applied at the first predetermined distance in the test tank 101. Furthermore, the intermittent motion of the shearing device portion caused by the stopping step 111 ensures that the shearing device portion 103 does not apply more shear than would occur at the first predetermined distance, thus ensuring that the correct amount of shear is applied to the pulp at the second predetermined distance and that the measuring step 113 obtains the correct value for the underflow density for the settled pulp. In other words, stopping movement of the shearing device portion 103 for the time difference simulates the time that passes between successive applications of shear to any one section of pulp by the shearing device 42 at the first predetermined distance.

By way of contrast, where scaled down versions of a shearing device and a thickener are used as a test apparatus under a conventional test regime, it is clear from equations (6) and (7) that the results obtained for the conventional test apparatus cannot be readily extrapolated or adjusted to fit a practical in situ shearing device in a full scale thickener. This is because the amount of shear that is applied in the conventional test apparatus is not the same as the amount of shear that is applied at radial distances greater than the maximum radius of the test apparatus. The amount of shear cannot be adjusted, since what is measured is the effect of the shear upon the underflow density of the settled pulp, so it is difficult to determine how to adjust the measured underflow density values for the variance in shear. Moreover, the shearing device in the test apparatus is typically a scaled down version of the actual shearing device and therefore does not apply the same amount of shear as a full scale portion of the shearing device at the radial distance of the thickener tank.

As equation (7) can enable one to determine the linear velocity of the shearing device at a specific radial distance i from its axis of rotation 49, this enables the method 100 to calculate the rotational speed of the shearing device portion 103 in the test apparatus to reproduce the same expected amount of shear at the specific radial distance i. However, simply adjusting the rotational speed of the shearing device portion 103 as described above is insufficient to replicate the amount of shear at the first predetermined distance. As discussed above, the method 100 reduces the time period for moving the shearing device portion 103 to take into account its increased rotational speed and the reduced circumference C_t of the test tank 101, using equations (9), (13), (14), (18) and (19). Thus, by moving the shearing device portion 103 at a different rotational speed to the rotational speed of the shearing device 42 at the first predetermined (radial) distance, but over a reduced time period, the shearing device portion 103 is able to travel at the same linear speed as the shearing device 42 over the first time period, thus reproducing the same amount of shear that is applied at the first predetermined distance in the test apparatus.

It will be appreciated that in the method 100, the second time period reflects the time between successive applications of shear to pulp at any particular point on the circumference of the tank of the separation device, described by the circle having the specified radial distance. That is, for any section of fluid or pulp in the circumference, there is an interval between successive applications of shear by the shearing device 42 as it travels around the circle. Therefore, to reproduce the correct number of shear events for a section of pulp at any particular part of the circumference of the circle that has a radial distance i in the test tank 101, movement of the shearing device portion 103 must be intermittently paused between successive movements where more than one cycle is performed.

Furthermore, steps 105 and 106 can performed in reverse order (ie. step 106 before step 105) or at the same time. In practice and for convenience, it is preferred to fix the second predetermined distance of the shearing device portion 103 at the radius of the test tank 101.

Referring now to Figures 11 and 12, a test apparatus 120 in accordance with one embodiment of the invention is illustrated, where corresponding features have been given the same reference numerals. The test apparatus 120 comprises a one metre diameter tank 121, an inlet 41 for feeding a suspension comprising pulp at a flux into the tank via a centrally located feedwell 9 and a shearing device portion 103 for shearing pulp within the tank 121. As described earlier, the shearing device portion 103 is a full scale portion of the shearing device 42 of Figure 4 that is installed in a full size thickener. Accordingly, the shearing device portion 103 provides a more accurate simulation of the shear that would be applied by the shearing device 42 than a scaled down version of the shearing device.

A central drive assembly 46 operates a central drive shaft 47 to rotate a rake assembly 48, comprising rake arms 50 having scraper blades 51 extending downwardly towards the bottom 52 of the tank to move settled and compacted pulp towards an underflow outlet 53. The test tank 121 is supported by columns 54. In addition, a feed pipe 122 connects the feed inlet 41 to a supply of suspension used in the test apparatus 120. A launder 45 is provided to collect diluted liquid from the top of the test tank 121. EXAMPLES

Referring now to Figure 13, operation of the test apparatus 120 and implementation of the method 100 according to the embodiment of the invention will now be described in relation to several examples, where corresponding features have been given the same reference numerals. These examples were conducted as separate tests, in which the shearing device portion 103 was used to simulate different segments of the shearing device 42. Figure 13 schematically illustrates the test apparatus 120 having a diameter of 1 m superimposed upon a thickener that has a tank 1 with a diameter of 24 m and hence a radius of 12 m.

In the examples, the shearing device 42 has six separate shearing arms 81 that extend radially outwardly from the centre of the thickener tank 1. Each shearing arm 81 has a plurality of shearing elements in the form of pickets 82. The shearing arms 81 are equispaced from each other at approximately 60°. The shearing device portion 103 is a full scale slice of one of the shearing arms 81.

For the sake of simplicity and ease of reference, the rotational speeds of the single shearing device 42 and the single shearing device portion 103 will be expressed in revolutions per minute (rpm). In the following examples, the rotational speed of the single arm of the shearing device 42 in the thickener will be taken to be 0.167 rpm, which corresponds to a typical speed in a full scale thickener. At this rotational speed, the shearing device 42 completes a revolution of the tank 1 in a first time period or cycle of 5.988 ~ 6 minutes. This means that the fluid at any radial distance I would receive six applications of shear for every six minute cycle. As noted above, the test tank 121 has a diameter†_t = 1 m and hence a radius or radial distance

and a circumference of

In addition, the outer position of the shearing device portion 103 is fixed at the radial distance t_t= ½ m in the test tank 121. Example 1

In accordance with the method 100 and step 105, a radial distance i _\ = 12 m for the shearing device 42 in the full scale thickener tank 1 is selected for testing at a rotational speed of 1/6 or 0.167 rpm. Thus, the linear velocity of the shearing device 42 in the thickener at i \ = 12 m would be ui = b-corf i = 2π x 0.167 rpm x 12 m = 24π x 0.167 m/min ~ 12.59 m/min.

The rotational speed for the shearing device portion 103 at i _t = ½ m to apply the same shear as the shearing device 42 at i \ = 12 m is then calculated in accordance with step 107. As discussed above, this is achieved by equalising the linear velocities of the outer edge of the shearing device portion 103 and the shearing device 42 at i _\ = 12m; that is, setting u_t = ui. To achieve this, the rotational speed co_t is adjusted by the scaling factor ε relating to the ratio of the circumference C_t of the test tank 121 to a circumferential arc Ci traversed by the shearing device 42 at the linear velocity ui = 24π x 0.167 m/min ~ 12.59 m/min. At a radial distance i \ = 12 m, the shearing device 42 would transcribe a circumference of a circle Ci =24π ~ 75.40 m. Therefore, the scaling factor ε = Q/C, = 24π/π ~ 75.40/3.14 = 24 and the rotational speed for the shearing device portion 103 is:

= 24 x 0.167 = 4 rpm

If the shearing device portion 103 were allowed to rotate at 4 rpm in the same one minute cycle as the shearing device 42, then it would perform 4 revolutions in a one minute cycle. By way of contrast, the shearing device 42 would only perform 0.167 of a revolution in a one minute cycle, assuming a six-arm shearing device would apply one shear event in this time. This would result in the shearing device portion 103 applying at least four times the number of shear events to the pulp than the shearing device 42 because it is rotating faster in the test tank 121 compared to the shearing device 42 in the thickener. Consequently, to ensure that the pulp receives the same frequency or number of applications of shear (or shear events) over the same one minute cycle, at step 108 a "shear" or "on" time period is calculated for the shearing device portion 103 to apply shear at i_t = ½ m in an equivalent frequency of shear events as the shearing device 42 applies at l = 12 m for 1 minute. This is achieved by reducing the shear or on time period in proportion to the increase in the rotational speed so that the same frequency or number of shear events is performed by the shearing device portion 103 as the shearing device 42. Also, the shear time period needs to be adjusted by the number of arms (N) on the shearing device 42, because the shearing device portion 103 is only a slice of one of these arms. Since co_t is expressed in revolutions/min, then the shear time period t_on = N'COt/co_t = 6 x 0.167/4 min = 15 s. In order to properly simulate the time between successive applications of shear by the shearing device 42, at step 108, the time difference between the shear time period ton and the cycle ti for the shearing device 42 is calculated to obtain a pause or "off time period in which the shearing device portion 103 is stopped and thus prevent the further application of shear to the fluid in the cycle ti . For £i = 12 m, the off period for the shearing device portion 103 is thus calculated as _f = ti - t_on = 1 min - 15 s = 45 s.

Thus, in accordance with step 110, the shearing device portion 103 at t_t = ½ m is operated at co_t = 4 rpm for t_on = 15 s to reproduce the same linear velocity ui = 12.59 m/min as the shearing device 42 at i_\ = 12 m rotating at o¾ = 0.167 rpm for ti = 1 minute. The shearing device portion 103 is then paused for t_0ff = 45 s in accordance with step 111. At step 112, a decision is made whether to repeat the rotation step 110 and the stopping step 111. Where more than one rotation is required for test purposes (and usually is to obtain more statistically meaningful data and to allow for pulp aggregate movement through the disturbance zone), rotation of the shearing device portion 103 at step 110 is resumed, followed by the required stopping step 111. When the desired number of "on'V'off cycles has been completed, then the underflow of settled pulp is removed from the test tank 121 and its density is measured at step 113 to determine the effectiveness of the shearing device portion 103, and hence the shearing device 42, in applying an optimal shear to the pulp.

Example 2

In this example, the radial distance £₂ = 9 m is selected for the shearing device 42 in the full scale thickener tank 1 for testing at step 105 of the method 100. Thus, the linear velocity of the shearing device 42 is u₂ = b-(or£₂= 2π x 0.167 rpm x 9 m = 18π x 0.167 m/min ~ 9.44 m/min. The rotational speed for the shearing device portion 103 at 1₁ = ½ m to apply the same shear and frequency of shear events as the shearing device 42 at i ₂ = 9 m is then calculated in accordance with step 107 by equalising the linear velocity u_t of the shearing device portion 103 with the linear velocity u₂. At a radial distance i ₂ = 9 m, the shearing device 42 would transcribe a circumference of a circle C₂ = 18π ~ 56.55 m. Therefore, the scaling factor ε₂ = C₂/C_t = 18π Ιπ ~ 56.55/3.14 = 18 and the rotational speed for the shearing device portion 103 is:

co_t = ε₂·ω«

= 18x 0.167 = 3 rpm. As discussed in example 1 above, this means that if the shearing device portion 103 were allowed to rotate at 3 rpm for the same one minute cycle as the shearing device 42, then it would perform 3 revolutions compared to the shearing device 42 only performing 0.167 of a revolution in the one minute cycle. This would result in the shearing device portion 103 applying three times the number of shear events to the pulp than the shearing device 42 because it is rotating faster in the test tank 121 compared to shearing device 42 in the thickener. Consequently, to ensure that the pulp receives the same frequency or number of applications of shear (or shear events) over the same one minute cycle, at step 108 the "shear" or "on" time period is calculated so that the shearing device portion 103 applies shear at i _t = ½ m in an equivalent amount of time as the shearing device 42 applies at i ₂ = 9 m for 1 minute. Again, this is achieved by reducing the shear or on time period in proportion to the increase in the rotational speed so that the same frequency of shear events is performed by the shearing device portion 103 as the shearing device 42. Also, the shear time is adjusted for the increased number of arms on the shearing device 42 compared to the shearing device portion 103. Thus, the shear time period t_on = N^'COt/co_t = 6 x 0.167/3 = 0.334 min = 20 s.

Once again, to properly simulate the time between successive applications of shear by the shearing device 42, at step 108, the time difference between the calculated shear time period t_on and the cycle t₂ for the shearing device 42 is calculated to obtain the pause or "off time period in which the shearing device portion 103 is stopped and thus prevent the further application of shear to the fluid in the cycle t₂. For i ₂ = 9 m, the shearing device portion 103 should be stopped for a period t_0ff = t₂ - t_on = 1 min - 20 s = 40 s. Once again, steps 110 and 111 may be repeated where more than one rotation is required for test purposes. When the desired number of "on'V'off ' cycles have been completed, then the underflow of settled pulp is removed from the test tank 121 and its density is measured at step 113 to determine the effectiveness of the shearing device portion 103, and hence the shearing device 42, in applying an optimal shear to the pulp. Thus, in accordance with step 110, the shearing device portion 103 at i_t = ½ m should be operated at co_t = 3 rpm for t_on = 20 s to reproduce the same linear velocity ui = 9.44 m/min as the shearing device 42 at i ₂ = 9 m rotating at o¾ = 0.167 rpm for t₂ = 1 minute. The shearing device portion 103 is then paused for toff = 40 s in accordance with step 111. Again, at step 112, the method 100 determines whether more than one rotation is required for test purposes. If so, then rotation of the shearing device portion 103 at step 110 is resumed, followed by step 111. When the desired number of "on'V'of ' cycles has been completed, then the underflow of settled pulp is removed from the test tank 121 and its density is measured at step 113 to determine the effectiveness of the shearing device portion 103, and hence the shearing device 42, in applying an optimal shear to the pulp at the rotational speed ω₂.

Example 3

In this example, the radial distance i 3 = 6 m is selected for the shearing device 42 in the full scale thickener tank 1 for testing at step 105 of the method 100. Thus, the linear velocity of the shearing device 42 is u₃ = b-co_r-f₃= 2n x 0.167 rpm x 6 m = 12π x 0.167 m/min ~ 6.29 m/min.

The rotational speed for the shearing device portion 103 at i _t = ½ m to apply the same shear force as the shearing device 42 at i ₃ = 6 m is then calculated in accordance with step 107 by equalising the linear velocity u_t of the shearing device portion 103 with the linear velocity u₃ = 6.29 m/min. At a radial distance i ₃ = 6 m, the shearing device 42 would transcribe a circumference of a circle C₃ = 12π m ~ 37.70 m. Therefore, the scaling factor ε₃ = C₃/C_t = 12π/π = 12 and the rotational speed for the shearing device portion 103 is co_t = ε₃·ω£ = 12 x 0.167 = 2 rpm.

Again, if the shearing device portion 103 were allowed to rotate at 2 rpm in a one minute cycle, then it would perform 2 revolutions, whereas the shearing device 42 would only perform 0.167 of a revolution. As the shearing device 42 has six radial arms 81 applying shear to the pulp, it applies one shear event per minute. This would result in the shearing device portion 103 applying twice the number of shear events to the pulp in the fluid than the shearing device 42 because it is rotating faster in the test tank 121, but has one sixth of the number of shearing arms, compared to the shearing device 42 in the thickener. Consequently, to ensure that the pulp in the fluid receives the same frequency or number of applications of shear (or shear events) over the same cycle for each shear event, at step 108 the "shear" or "on" time period is calculated so that the shearing device portion 103 applies shear at i_t = ½ m in an equivalent amount of time for each radial shearing arm 81 of the shearing device 42 to apply shear at l₃ = 6 m. Again, this is achieved by reducing the shear or on time period in proportion to the increase in the rotational speed so that the same frequency or number of shear events is performed by the shearing device portion 103 as the shearing device 42. Also, the shear time is adjusted for the increased number of arms on the shearing device 42 compared to the shearing device portion 103. Thus, the shear time period t_on = N^'COt/co_t = 6 x 0.167/2 = 0.5 min = 30 s. In order to properly simulate the time between successive applications of shear by the shearing device 42, at step 108, the time difference between the shear time period to_n and the cycle t₃ for the shearing device 42 is calculated to obtain a pause or "off time period in which the shearing device portion 103 is stopped and thus prevent the further application of shear to the pulp in the fluid in the cycle t₃. For (3 = 6 m, the shearing device portion 103 should be stopped for a period t_off = t₃ - t_on = 1 min - 30 s = 30 s.

Thus, in accordance with step 110, the shearing device portion 103 at t_t = ½ m is operated at ω₃ = 2 rpm for t_on = 30 s to reproduce the same linear velocity u₃ = 12π ^x 0.167 m/min ~ 6.29 m/min as the shearing device 42 at £₃ = 6 m rotating at o¾ = 0.167 rpm for t₃ = 1 minute. The shearing device portion 103 is then paused for toff = 30 s to take into account the six shearing arms 81 of the shearing device 42, in accordance with step 111. Again, steps 110 and 111 may be repeated at step 112 where more than one rotation is required for test purposes. When the desired number of "on'V'off cycles have been completed, then the underflow of settled pulp is removed from the test tank 121 and its density is measured as per step 113 to determine the effectiveness of the shearing device portion 103, and hence the shearing device 42, in applying an optimal shear to the fluid at the rotational speed ω₃. Example 4

In yet another example, the radial distance la, = 3 m is selected for the shearing device 42 having six radial shearing arms 81 in the full scale thickener tank 1 for testing at step 105 of the method 100. Thus, the linear velocity of the shearing device 42 is

The rotational speed for the shearing device portion 103 at i _t = ½ m to apply the same shear as the shearing device 42 at i ₄ = 3 m is then calculated in accordance with step 107 by equalising the linear velocity uo of the shearing device portion 103 with the linear velocity u₄ = π m/min - 3.14 m/min. At a radial distance i ₄ = 3 m, the shearing device 42 would transcribe a circumference of a circle C₄ = 6π m ~ 18.85 m. Therefore, the scaling factor

and the rotational speed for the shearing device portion 103 is co_t = ε₄·ω£ = 6 x 0.167 = 1 rpm. As in example 3 above, the shearing device 42 has six radial shearing arms 81 and so will provide one shearing event per minute. This means that the frequency of shear events at the rotational speed co_t for the shearing device portion 103 is the same as the frequency of shear events at the rotational speed cot of the shearing device 42, despite the differences in radial distance.

The shearing device portion 103 thus applies shear to the pulp in the fluid the same number of times as the shearing device 42 having six radial arms. Therefore, at step 108 the time shear or "on" period is calculated so that the shearing device portion 103 applies shear at i _t = ½ m in an equivalent frequency of shear events as the shearing device 42 applies at l₄ = 3 m for 1 minute. Thus, the shear or on time period t_on = N^'COt/co_t = 6 x 0.167/1 min= 60 s. Thus, there is no time difference between the two periods as t_0ff = t₄ - t_on = 1 - 1 niin = 0, meaning that the shearing device portion 103 may be rotated in the test tank 121 continuously without stopping and still reproduce the same number of shear events and the same amount of shear as would be applied by the shearing device 42 having six radial shearing arms 81 at the radial distance £₄ = 3 m for a 1 minute cycle.

Consequently, with the data generated from the operation of the test apparatus 120 in this method 100, various configurations of the shearing device can be designed so as test whether an expected optimal shear is applied by each configuration of the shearing device at the calculated shearing device speed for selected radial distances. The intermittent rotation of the shearing device portion 103 in the test apparatus 120 provides a more accurate set of data for designing the shearing device in a full scale thickener, as opposed to extrapolating results for a scaled down version of the shearing device operated continuously in the test apparatus 120.

Further tests were conducted using examples 3 and 4. That is, a portion of a full scale six arm shearing device was rotated in a 1 m diameter tank to simulate the performance of the six arm shearing device in a 24 m diameter thickener. A summary of the results obtained with these further tests are set out in Table 1 below.

Table 1: Test Results for Examples

Experiments have been conducted in accordance with Table 1. The various distances along the "Full Scale" shearing device of 12 m length (i.e. suitable for a 24 m diameter thickener tank) operating at a rotational speed of 0.167 rpm and having 6 radial shearing arms, are simulated by operating the shearing device portion at the speeds and on:off cycle times shown.

The results of these tests are tabulated in Table 2, together with an array of data simulating other operating conditions.

It can be seen from these experimental tests that optimal underflow densities are obtained when varying the operating cycle in accordance with the on/off shear time periods described in Examples 1 to 4. From the results in Table 2, it can then be determined what shear magnitude and frequency is optimal at different radii within the thickener, thus allowing for optimal design of the shearing mechanism across the entire radius of the thickener. Thus, the experimental tests demonstrate that the method 100 enables an accurate measurement of the effectiveness of the shearing device configuration. As a consequence of these experimental tests, the inventors conclude that employing the method 100 of Figure 5 will result in accurate testing for shearing devices in terms of their effectiveness in applying an optimal shear to the pulp that is consistently maintained and provide the same level of improvement in the overall performance of a thickener employing the shearing devices.

In the preferred embodiments of the invention, the test apparatus 120 employs a cylindrical tank 101. This results in the linear velocity u_t of the shearing device portion 103 approaching zero close to the rotational axis 49, as best shown in Figure 14A. Where the shearing device 42 is being tested at a radial distance i that is substantially displaced from the rotational axis 49 of the thickener tank 1, for example at the outer edge of the tank between radial points ri and r₂, the linear velocity u_ri closest to the rotational axis 49 is unlikely to approach zero. Therefore, even though the shearing device portion 103 has the same length W_r as the shearing device 42 between radial points ri and r₂, there is a disparity between the range of linear velocities ui, u₂, u₃.... u_n of the shearing device portion 103 and the linear velocities u_ri to u_r2 of the shearing device 42. This disparity increases as the radial distance i increases from the rotational axis 49. In the previously described embodiments, the inventors have used the respective outermost edges of the shearing device 42 and the shearing device portion 103 for the first and second predetermined distances, in order to minimise or reduce any error in the test results caused by this disparity in linear velocities.

If more precise results are required, then the convergence of the linear velocities u_t in a cylindrical test tank 101 towards zero is taken into account by using an average linear velocity u_t(av) for the shearing device portion 103 that is the same as the average linear velocity u_r(av) of the shearing device 42 over the same length W_r as the shearing device portion 103. That is, the average linear velocity u_t(av) would be the average of the linear velocities ui, u₂, u₃ ...u_n taken over length W_r of the shearing device portion 103, as best shown in Figure 14 A. Likewise, the average linear velocity u_t(av) of the shearing device 42 would be the average of the linear velocities u_ri, ... u_r2 between the radial points ri and r₂, as best shown in Figure 14B. The method 100 is equally applicable to either case, whether the linear velocities are taken as the linear velocities of the respective outermost edges of the shearing device 42 and the shearing device portion 103, or the average linear velocities edges of the shearing device 42 and the shearing device portion 103 over their respective lengths W_r.

Another embodiment of the invention which takes into account this disparity in linear velocities is illustrated in Figure 15, where corresponding features have been given the same reference numerals. In this embodiment, the test apparatus 230 has an arcuate trough or tank 231 with the shearing device portion 103 extending across its width W_r. The shearing device portion 103 is attached to a moveable arm 232, which is mounted to a central drive shaft 233 for rotational movement about an axis 49. The use of an arcuate trough or tank 231 ensures that the linear velocity ui does not approach zero close to the rotational axis 49. Instead, the innermost edge of the tank 231 is radially displaced from the rotational axis 49 of the shearing device portion 103. In addition, the average linear velocity is not used, since the linear velocities ui, u₂, u₃, ... u_n of the shearing device portion 103 along the width W_r, would correspond to the same linear velocities ui, ... u_n of the shearing device 42 along the same width W_r on the shearing arm 81. Thus, there is a more accurate simulation of the shear at the radial distance i. This is because the linear velocity ui closest to the rotational axis 49 is less likely to approach zero, since it is spaced from the rotational axis 49 by a distance of i_x = l_n - i_\, as best shown in Figure 15. A further embodiment of the invention is illustrated in Figure 16, where corresponding features have been given the same reference numerals. In this embodiment, the test apparatus 240 has a rectangular trough or tank 241 having a length d, with the shearing device portion 103 extending across its width W_r. The shearing device portion 103 is attached to a moveable arm 242, which is mounted to a carriage 243 for linear movement along a track 244 parallel to the tank 241.

In this embodiment, there is no need to calculate a rotational speed for either the shearing device 42 or the shearing device portion 103. Instead, the linear velocities ¾, u_t of the shearing device 42 and the shearing device portion 103 are used to test the shearing device portion 103 in the straight tank 241. This results in the method 100 being simplified in that the calculating step 107 is now reduced to simply equating the linear velocity ¾ of the shearing device 42 to the linear velocity u_t of the shearing device portion 103. In addition, as C_t is effectively the length d of the tank 241, the second or "on" time period is calculated in step 108 according to equations (18) and (19) as

The "off time period is then calculated in accordance with step 109 and the shearing device portion 103 is moved for t_on along the tank 241 and stopped for toff. Ideally, the shearing device portion 103 would travel the length of the trough 241 during the on time period t_on and would simply wait during the "off time period t_off to move in the reverse direction back to its original starting point at the next on time period t_on.

It will be appreciated that while shearing device portion 103 has been described as extending across the respective radii of the tanks 101, 231 and 241, in further embodiments, the shearing device portion 103 occupies a fraction of this distance to enable lateral adjustment of its position in the test tank 101, 231 and 241. As discussed above, the average velocity of the width of the shearing device portion 103 would be calculated at step 107 to match the corresponding average linear velocity of the shearing device 42 over the same width and range of radial distances.

In other embodiments, the method 100 employs a variety of shearing device configurations other than the configuration illustrated in the Figures 4, 6 and 7. For example, the shearing device portion 103 may have several arms to simulate a multiple arm shearing device, as discussed in Examples 3 and 4. The pickets arranged on the shearing device may also vary in both orientation and configuration. For example, in the preferred embodiment, the shearing device 42 has been described and illustrated with pickets angled with respect to a vertical plane that is at right angles to the radial arm. However, it will be appreciated that the pickets can be angled with respect to other vertical planes, such as a vertical plane parallel to or coplanar with the radial arms so that the pickets have an angle of incidence with respect to the direction of rotation of the shearing device. In other embodiments, the pickets may be only angled with respect to the vertical plane parallel to or coplanar with the radial arms.

Whilst the preferred embodiments of the invention have been described as employing shearing elements in the form of linear pickets or rods, it would be appreciated by one skilled in the art that other configurations for the shearing elements can be used, such as V- shaped angled rods, half or semi-circular tubes or other shearing elements having different polygonal cross-sections. In particular, the pickets themselves can be altered in shape to produce the desired shear profile. For example, a non-linear picket can be used, such as a spiral or helical shape. It will be appreciated by one skilled in the art that by simulating the same linear velocity and shear profile of the shearing device the invention ensures that the correct amount of shear for a predetermined cycle can be reproduced in a test tank, enabling accurate results to be obtained directly, without having to manipulate the test data to take into account the differences in scale between the test apparatus and the full scale thickener. Thus, in the invention, equalisation of the linear velocities of the shearing device portion and the full scale shearing device is achieved by adjusting the rotational speed and the shear time period to ensure that the simulation accurately reflects the amount of shear that is applied at any radial distance in the full scale thickener. By obtaining accurate test results for shearing device configurations and varying rotational speeds, the design of shearing devices is improved and more efficient, thus reducing development costs involved with designing shearing device configurations that achieve an optimal shear in a separation device, for example, a thickener. In all these respects, the invention represents a practical and commercially significant improvement over the prior art.

Although the invention has been described with reference to specific examples, it will be appreciated by those skilled in the art that the invention may be embodied in many other forms.

Claims

CLAIMS:- 1. A method of testing a shearing device for a separation device, wherein the separation device comprises a tank for receiving a feed material, wherein feed material settles in the tank and the pulp forms into aggregates, the pulp aggregates settling towards the bottom of the tank and forming a first networked layer of pulp, and the shearing device is moveable to apply shear substantially uniformly across a first disturbance zone in an upper region of the first networked layer, so as to disrupt the networked pulp in the first disturbance zone within a predetermined period of time, the method comprising the steps of:

providing a test tank for a feed material to settle and pulp to form into aggregates, the pulp aggregates settling towards the bottom of the test tank and forming a second networked layer of pulp;

submerging a portion of the shearing device at least partially within the second networked layer in the test tank to apply shear in a second disturbance zone in an upper region of the second networked layer;

calculating a first speed of the shearing device for a first time period in which the shearing device is expected to apply a shear in the first disturbance zone at a first predetermined distance;

calculating a time difference between the first time period and the second time period;

2. The method of claim 1, further comprising repeating the moving step after the stopping step.

3. The method of claim 1 or 2, further comprising successively repeating the moving and stopping steps.

4. The method of any one of claims 1 to 3, wherein the moving step comprises rotating the shearing device portion.

5. The method of claim 4, wherein the first and second speeds are linear speeds of the shearing device and the shearing device portion, respectively.

6. The method of claim 4, wherein the first and second speeds are rotational speeds of the shearing device and the shearing device portion, respectively.

7. The method of claim 6, further comprising calculating the rotational speed of the shearing device portion so that its linear speed at the second predetermined distance is substantially equal to the linear speed of the shearing device at the first predetermined distance.

8. The method of claim 7, wherein the linear speeds of the shearing device portion and the shearing device are average linear speeds.

9. The method of claim 8, wherein the average linear speed is the average of linear speeds across the respective widths of the shearing device portion or shearing device.

10. The method of any one of claims 1 to 9, further comprising the step of adjusting the second speed by a scaling factor.

11. The method of claim 10, wherein the scaling factor is calculated according to the relationship:

where ε is the scaling factor;

Q is the circumference of a circle travelled by the shearing device at the first predetermined distance in metres;

Qis the circumference of the test tank in metres;

i is the first predetermined distance in metres; and it is the second predetermined distance in metres.

12. The method of claim 10, wherein the adjusting step comprises applying the scaling factor to the rotational speed of the shearing device to obtain the rotational speed of the shearing device portion.

13. The method of claim 12, wherein the rotational speed for the shearing device portion is calculated according to the relationship:

where co_t is the rotational speed of the shearing device portion in rpm;

ε is the scaling factor; and

cot is the rotational speed of the shearing device in rpm.

14. The method of any one of claims 1 to 13, wherein the first time period is the time for one revolution of the shearing device in the tank of the separation device.

15. The method of any one of claims 1 to 14, wherein the second time period is calculated by applying a scaling factor to the first time period.

16. The method of claim 15, wherein the second time period is calculated according to the relationship:

where tee is the first time period in seconds;

ε is the scaling factor; and

tct is the second time period in seconds.

17. The method of claim 15 or 16, wherein the scaling factor is calculated according to the relationship:

where ε is the scaling factor;

C_tis the circumference of the test tank in metres;

18. The method of claim 15 or 16, wherein the scaling factor is calculated according to the relationship:

where ε is the scaling factor;

cot is the rotational speed of the shearing device portion in rpm; and cot is the rotational speed of the shearing device in rpm.

19. The method of any one of claims 1 to 18, wherein the first predetermined distance is proportional to the second predetermined distance.

20. The method of any one of claims 1 to 19, further comprising the step of measuring the separation of pulp from the fluid in the test tank after completion of the first time period to determine whether the shearing device would apply the expected optimal shear at the first predetermined distance.

21. The method of claim 20, wherein the measuring step is performed after a predetermined number of successive repetitions of the moving step and the stopping step.

22. The method of any one of claims 1 to 21, wherein the first predetermined distance is a radial distance from a centre of the separation device.

23. The method of any one of claims 1 to 22, wherein the second predetermined distance is a radial distance from a centre of the test tank.

24. The method of any one of claims 1 to 22, wherein the second predetermined distance is a radial distance from a centre of the test tank to a selected radial point on the shearing device portion.

25. The method of claim 24, wherein the second predetermined distance is a radial distance from a centre of the test tank to an outer edge of the shearing device portion.

26. The method of any one of claims 1 to 25, wherein the shearing device has N shearing arms and the method further comprises adjusting the second time period according to the relationship:

where t_Ct is the second time period in seconds;

N is the number of shearing arms of the shearing device;

ε is the scaling factor; co_t is the rotational speed of the shearing device portion in rpm;

cot is the rotational speed of the shearing device in rpm; and tee is the first time period in seconds.

27. The method of any one of claims 1 to 26, further comprising adjusting one or more of the first speed, the first time period and the time difference in response to at least one of one or more shearing parameters, changes in the flux and changes in one or more of the operational parameters.

28. The method of claim 27, wherein the shearing parameters are selected from the group consisting essentially of the shape of the shearing device and the depth of the shearing region.

29. The method of claim 27, wherein the operational parameters are selected from the group consisting essentially of the pulp composition, the pulp particle size, the pulp flow velocity in the tank, the pulp yield stress, the pulp viscosity, the underflow specific gravity, the underflow weight per weight percentage and the rate at which flocculant is added to the suspension.

The method of any one of claims 1 to 29, wherein the shearing device portion is equivalent to a full scale portion of the shearing device.

The method of any one of claims 1 to 30, further comprising the step of reversibly rotating the shearing device or periodically reversing the rotation of the shearing device.

32. The method of any one of claims 1 to 31, further comprising the step of moving the shearing device portion to apply a substantially uniform number of shear events to the networked pulp in the second disturbance zone within the second time period.

33. The method of any one of claims 1 to 32, further comprising the step of moving the shearing device portion to apply a substantially uniform cumulative shear to the networked pulp in the second disturbance zone within the second time period.

34. The method of any one of claims 1 to 33, wherein the shearing device portion has a plurality of shearing elements and further comprising the steps of spacing apart the shearing elements along at least one arm of the shearing device portion to define respective intervals therebetween and applying a substantially uniform average shear to the networked pulp in at least two intervals along a line parallel to or coincident with the at least one arm.

35. The method of claim 34, wherein the average shear in all the intervals between the shearing elements along the line is substantially uniform or the same.

36. The method of claim 34 or 35, further comprising the step of applying substantially uniform average shear along the length of the at least one arm.

37. The method of any one of claims 1 to 36, wherein the shearing device portion has at least one arm that extends outwardly or radially outwardly.

38. The method of claim 37, further comprising the step of disposing one or more shearing elements on the at least one arm.

39. The method of claim 38, further comprising the step of arranging the shearing elements to apply shear along the at least one arm.

40. The method of any one of claims 1 to 39, wherein the shearing device portion has a plurality of shearing elements.

41. The method of claim 42, further comprising the step of defining a zone of turbulence for each shearing element to disturb, re-arrange or break-up the pulp aggregates and/or to release liquid.

42. The method of claim 40 or 41, further comprising the step of disposing one or more shearing elements along the axis of rotation to extend radially outwardly.

43. The method of any one of claims 40 to 42, further comprising the step of removably mounting the one or more shearing elements to a drive shaft of the shearing device.

44. The method of any one of claims 1 to 43, further comprising the step of rotating the shearing device portion about an axis of rotation that is parallel, eccentric or offset with respect to a central axis of the test tank.

45. The method of claim 44, further comprising the step of moving the axis of rotation relative to the central axis.

46. The method of claim 45, wherein the axis of rotation rotates, revolves or orbits at least partially around the central axis.

47. The method of claim 45, wherein the axis of rotation at least partially traverses a regular path around the central axis.

48. The method of claim 45, wherein the axis of rotation moves in a circular path.

49. The method of claim 45, wherein the axis of rotation at least partially traverses an irregular path around the central axis.

50. The method of claim 45, wherein the axis of rotation moves in a non-circular path, which may be geometrically regular or irregular.

51. The method of any one of claims 1 to 50, wherein the test tank is at least one of a substantially cylindrical shape, substantially rectangular shape, partially arcuate shape, an open trough in shape, a closed trough shape, and an arcuate shape that corresponds to a circumferential arc of the separation device.

52. An apparatus for testing a shearing device, wherein the shearing device is for a separation device comprising a tank for receiving a feed material, wherein feed material settles in the tank and the pulp forms into aggregates, the pulp aggregates settling and forming a first networked layer of pulp towards the bottom of the tank, and the shearing device is moveable to apply shear substantially uniformly across a first disturbance zone in an upper region of the first networked layer, so as to disrupt the networked pulp in the first disturbance zone within a predetermined period of time, the apparatus comprising:

a test tank for receiving a feed material to settle and pulp to form into aggregates, the pulp aggregates settling and forming a second networked layer of pulp towards the bottom of the test tank, and

a portion of the shearing device submerged at least partially within the second networked layer in the test tank to apply shear in a second disturbance zone in an upper region of the second networked layer;

wherein the shearing device portion is positioned at a second predetermined distance in the test tank; the shearing device portion is moveable in the second disturbance zone at a second speed for a second time period, said second speed corresponding to a first speed of the shearing device for a first time period in which the shearing device is expected to apply shear in the first disturbance zone at a first predetermined distance, and said second time period corresponds to the first time period at the first predetermined distance; and

the shearing device portion is able to be stopped for a time difference between the first time period and the second time period, so as to simulate the application of shear in the first disturbance zone by the shearing device at the first predetermined distance in the separation device over the first time period.

53. The apparatus of claim 52, wherein the shearing device portion rotates in the test tank.

54. The apparatus of claim 56 or 53, wherein the first and second speeds are rotational speeds of the shearing device and the shearing device portion, respectively.

55. The apparatus of any one of claims 52 to 54, wherein the shearing device portion applies a substantially uniform number of shear events to the networked pulp in the second disturbance zone within the second time period.

56. The apparatus of any one of claims 52 to 55, wherein the shearing device portion applies a substantially uniform cumulative shear to the networked pulp in the second disturbance zone within the second time period.

57. The apparatus of any one of claims 52 to 56, the shearing device portion has a plurality of shearing elements and the shearing elements are spaced apart along at least one arm of the shearing device portion to define respective intervals therebetween and apply a substantially uniform average shear to the networked pulp in at least two intervals along a line parallel to or coincident with the at least one arm.

58. The apparatus of claim 57, wherein the average shear in all the intervals between the shearing elements along the line is substantially uniform or the same.

59. The apparatus of claim 57 or 58, wherein the shearing device portion applies substantially uniform average shear along the length of the at least one arm.

60. The apparatus of any one of claims 52 to 59, wherein the shearing device portion has at least one arm that extends outwardly or radially outwardly.

61. The apparatus of claim 60, wherein one or more shearing elements are disposed on the at least one arm.

62. The apparatus of any one of claims 52 to 63, wherein the shearing device portion has a plurality of shearing elements.

63. The apparatus of claim 62, wherein one or more shearing elements are disposed along the axis of rotation to extend radially outwardly.

64. The apparatus of claim 62 or 63, wherein the one or more shearing elements are removably mounted to a drive shaft of the shearing device portion.

65. The apparatus of any one of claims 52 to 64, wherein the test tank is at least one of a substantially cylindrical shape, substantially rectangular shape, partially arcuate shape, an open trough in shape, a closed trough shape, and an arcuate shape that corresponds to a circumferential arc of the separation device.