AU2006201944A1 - Variable Split Sample Divider - Google Patents

Description

Variable Split Sample Divider 0 Background Art The invention described herein is intended primarily for use in the so-called preparation of nominally dry powdered or granulated materials prior to chemical or physical analysis or wherever it is necessary to extract a representative sub-sample of a material oof a specified mass from a larger lot or sample of powdered or granulated material. The device may also be used to extract a subsample of a flowing slurry (a suspension of osolids in water or other liquid).

In the chemical or physical analysis (referred to as analysis hereafter) of granular materials, it is usual to have a primary sample of the material than is larger in volume or mass than is necessary or convenient for the analysis procedures. For example, a 6 kg primary sample may have been taken, while the final analysis aliquot (that mass of material that is needed for the analysis) may be 20 g.

The generally accepted method of dividing the sample mass to a mass of 20 g, after the sample has been ground to a particle size small enough to reduce material heterogeneity effects to a desired level, is to use so-called 'rotary division' or a 'spinning riffle' to extract one or more subsamples from the original mass of material in a series of steps so as to arrive at one or more final subsamples of nominal mass 20 g and a residue of the original mass of material. The rotary division process involves the feeding of the granulated sample at a steady rate to a set of two or more containers mounted on a platform which rotates at a constant angular velocity. The containers are arranged on the platform in such a way as to ensure that the stream of granulated sample from the feeder, which is fixed in position adjacent to the rotating platform, will be collected, without significant loss, into the containers. The usual arrangement is such that the containers are of equal volume and size and the amount of material collected into each container is nominally identical. The term nominally identical, is intended to mean that the masses of granulated material collected into each container have masses and have chemical compositions that do not differ to a statistically significant extent, given the particulate nature of the material. The extents to which the nominally identical subsamples may vary in mass and composition are controlled by the particulate page 1 N heterogeneity of the material, which may be determined from the particle weights, the 0 O spectrum of particle compositions with respect to the analytes of interest and the nominal mass of the sample.

In the operation of a sample divider of any kind, material heterogeneity is overcome as far as possible by the extraction of a large number of increments from the original sample. In the case of the conventional rotary sample divider, each subsample will consist of N increments if the device makes N full rotations during the sample division Oprocess. The number of increments necessary to reduce the material heterogeneity between subsamples to a statistically insignificant level depends on the extent to which othe sample is well mixed before division. Accepted practice requires at least 0 increments to be collected in the formation of the subsamples, but more than increments is desirable.

The disadvantage of the usual design of rotary division apparatus is that the subsamples produced are all of the same mass. Devices are available for division into 4, 8, 10 12 and 16 subsamples. Some devices have been constructed that permit division into a variety of masses that are not equal. These devices have the objective of permitting the collection of subsamples from one or more of the containers so as to arrive at a subsample that is of a more convenient mass for further subdivision or for analysis.

However, the number of such unequal subsamples is limited and the fractions of the starting mass that the various subsamples represent is a fixed set.

In the case of the example mentioned above, namely division of a 6 kg sample down to g, the 20 g subsample is 0.003333 of the initial sample mass. If the only apparatus available is one that divides into 10 equal subsamples, the division cannot be made by rotary division alone, unless subsamples are repeatedly recombined and redivided.

It is known to have a rotary sample divider that is capable of pausing while collecting the material fed as sample or while directing the material flow to the residue. This device is not the subject of a patent but is manufactured by Rocklabs of New Zealand.

The Rocklabs device rotates in one sense only and uses a combination of the pausing of the rotation together with a manually adjustable sample collection aperture. In the Rocklabs machine, the sample is fed to a point off the axis of a metal cone opening upwards which has a manually adjustable aperture in the side of the cone. The cone is rotated by a motor and when the aperture is directly below the falling stream of material, the material falls through the aperture and is collected in a container page 2 N 65 designated as the subsample. When the aperture is not in the path of the falling stream, 0 O the material slides down the inside surface of the cone and is collect in a second container designated as the sample residue. It can be shown that this device cannot be reliably controlled to collect an arbitrary fraction of the original sample due to limitations in the timing of the pause which arises from the use of a synchronous alternating current motor and variable speed drive and the manual setting of the aperture in the cone.

OConventional cross-stream sample cutters and variable duty cycle Vezin samplers also effectively pause while not collecting sample. However, they differ from the invention (Ni odescribed herein in that it is not known to control such devices to pause while collecting C, 75 the full stream of material to be sampled and the cutters are commonly too narrow to permit the collection of the entire stream of falling material. Finally, it is known to have a subsample extraction device that has a central bent tube the first part of which is vertical and the second part of which is inclined at an angle to the vertical. Sample is fed at a nominally constant rate to the axis of the tube while the tube is caused to rotate about a vertical axis. A cone similar to that in the Rocklabs device is placed with its axis coincident with the axis of the rotating tube. Material that passes through an adjustable aperture in the cone is designated as subsample and material that slides down the interior surface to the cone to a collection vessel is designated as sample residue. It is not known to have such a device in which the central tube does not rotate in one direction only or in which the rotating tube starts and stops during subsample extraction.

The device has a limited range of mass fractions of the sample that the subsample may represent. This device is known as a rotating tube sample divider.

Disclosure Technical Problem The invention disclosed herein addresses the need to accurately extract a subsample from a sample of particulate material that represents an arbitrary but closely specified mass fraction of the sample. Given the current trend to have all activities in an analytical laboratory monitored, controlled and recorded by a Laboratory Information Management System (LIMS), there is a further need to have the subsample extraction device in communication with the LIMS. For example, the LIMS may be able to page 3 I provide the operator of the subsampling device with the desired mass fraction of the 0 O sample that is to be extracted as a subsample. Further, if the subsample extraction device can communicate electronically with the LIMS, the LIMS can provide the S 100 information on the desired mass fraction directly to the subsampling device without any tjf intervention by the operator.

To comply with the accepted procedures for the extraction of a subsample from a larger mass of material, the extraction should be made by collecting a large number of Oincrements using a mechanically correct sampling device from a falling stream of the S 105 material that has a substantially constant mass flow rate. The velocity of the sample ocutter as one or both of it edges pass through the stream must be constant and restricted to 0.6 meters per second. The rotary sample divider is a mechanically correct device and a sample cutter that mimics its operation is desirable. It is less desirable to use a rotating tube sample divider as the motion of the central tube may cause particles of 110 varying mass to follow different trajectories which may result in bias in favour of either the light or the heavy particles.

Technical Solution The variable split sample divider described herein has a moving sample cutter which passes back and forth through a falling stream of granular material which is fed at a 115 substantially constant rate from a hopper. A variety of means may be used to achieve this feeding function.

The sample cutter is a moveable chute which can direct a part or all of the flow of the granular material to a single subsample collection container. That part of the material flow that is not intercepted by the cutter is collected by fixed chute-work into a second 120 sample residue container.

The cutter motion may be rectilinear or along an arc. In what follows reference will be made to the movement along an arc, but a corresponding device based on reciprocating rectilinear motion can be constructed with similar methods of controlling the motion to achieve the same result.

125 A simplified general arrangement of the variable split sample divider is shown in Figure 1. The cutter moves back and forth along the arc, pivoting around the centre of rotation.

The design of the variable split sample divider recognises that the oscillating motion through the stream is technically equivalent to a simple continuous rotational motion, as page 4 I long as the speed of the cutter is substantially constant as one of the edges of the cutter 0 S 130 passes through the falling stream.

SThe cutter is wider than the falling stream which, ideally, but not of necessity, is confined to fall inside a chute immediately above the cutter. The surrounding chute work is such that the cutter can move to extreme positions remote from the falling stream such that none of the falling stream is intercepted by the cutter when the cutter is 135 in one of the remote positions.

The motion of the cutter is controlled by a motor operating under the control of a programmable logic controller (PLC) or other equivalent device. The motor may be of any suitable design as long as it can accurately and repeatably follow the required motion program. For ease of control and best dynamic response, a stepper motor is the 140 preferred actuator. However, it is possible to design a device based on the use of a servomotor or a linear motor.

One realisation of the required motion program for the cutter is diagrammed in Figure 2.

A positive velocity indicates a motion in one sense, say clockwise, and a negative velocity a rotation in the opposite sense.

145 Assuming that the plot of the motion commences with the cutter at rest in a position remote from the falling stream, the motor accelerates the cutter to a constant velocity, maintains a constant velocity and then decelerates to a stop. The acceleration and deceleration are such that the cutter achieves a constant speed, or angular velocity, before its edge nearest the falling stream moves through the stream and then comes to a 150 stop before the following edge enters the stream and after the leading edge has moved through the entire falling stream. It is a requirement to have the cutter stop in a position that is centred beneath the falling stream. After remaining at rest for a chosen time interval while collecting the whole of the falling stream, the cutter is accelerated to a constant speed and its trailing edge moves through the falling stream. It then 155 decelerates and comes to rest in a position remote from the falling stream in a position that is the mirror image of its starting position. It remains there for a chosen period of time and then repeats exactly the same pattern of motion, but in the opposite sense, coming to rest in the position from which the motion commenced. This motion defines one cycle of operation in which two increments are taken.

page 160 Define the arc length subtended between the edges of the cutter to be w and let the arc length from the start position of the leading edge of the cutter to a position in the centre of the falling stream be x, such that x is greater than or equal to w/2. Define the constant velocity of movement of the cutter to be v, and assume constant acceleration during the acceleration period so that the average velocity during acceleration is v/2.

165 Define the duration of the pause while collecting all of the stream to be t and the pause while collecting none of the stream to be t o Define the arc length over which acceleration and deceleration takes place to be xa. It can then be shown that the fraction of the stream collected into the subsample during the motion is w+ 2 xa +t i w+ 4xa 2x +t +to

V

+t i +t o 170 In the case in which 2x w, the fraction collected is w 2x a w+ 2 xa t.

V

2 (w +2xa) +t+to

V

from which it is clear that f 5 0.5 if the pause times are zero.

The total time for a half a cycle of motion is: Cycle duration T 2 w 2x a ti to

V

175 The time during which the stream is collected during the half cycle is: Collection time T s w 2x a t

V

The fraction of the sample collected may be made equal to any fraction by varying the times at which the device remains at rest, either in the stream or out of it. Figure 3 shows a plot of the fraction of the material collected as sample when the time at rest in 180 the stream is zero and the time at rest out of the stream is progressively increased. The unit of time on the abscissa is (w 2 Xa)/V. By setting the time at rest out of the stream to zero and increasing t a corresponding increase of the sample fraction collected as subsample is achieved.

page 6 The time scale on which the sample divider can be operated is limited by three factors: 185 1) the time required to accelerate the cutter to speed, 2) the desired maximum speed of the cutter through the stream and 3) the width of the falling stream. The usually accepted maximum speed of the cutter is 0.6 m/s. The width of the cutter should be 0.01 meter or three times the nominal top size of the particle size distribution; top size is usually taken to be the square sieve aperture which 95 per cent by mass of the sample 190 passes. However, these technically accepted values shall not limit the novelty and functionality of the device.

The time needed to accelerate/decelerate the cutter depends on the moment of inertia of the cutter and associated rotating drive parts about their axes of rotation and the available torque generated by the motor. The acceleration/deceleration time may also 195 be limited by timing restrictions imposed by the motor controller. Again, it is assumed that these practical aspects of design should not impact on the novelty of the design.

For given acceleration/deceleration times and a target fraction of sample of 0.5 and the condition that 2x w, the total time needed to divide the sample is dictated by the desired number of increments that are to be taken. For N increments, the time is then 200 td 2 N(w+ 2xa)

V

It should be noted that the fraction of the sample that is extracted is influenced by the number of increments that are cut from the falling stream. If the target fraction is to be accurately achieved, the number of increments extracted must be relatively high.

Extraction of 100 increments will permit a nominal accuracy of 1 part in 100 of the 205 desired fraction.

It can be demonstrated that a variable split sample divider of the nature disclosed herein need not adhere precisely to the motion diagram of Figure 2. Any correctly calculated motion diagram in which the cutter moves at a constant speed when one of its edges is passing through the limit of the falling stream will serve to correctly extract a desired 210 fraction of the starting material mass. For example, when the pause time in the stream is zero, the cutter need not come to a stop, but the omission of the stop must be included in the calculation of the fraction extracted in a correct manner. The critical factor is the accuracy of the control of the device.

page 7 It is also apparent that a device of this basic design can be used to extract a desired 215 fraction of a process stream on a wide range of physical scales.

A corresponding device which relies on the oscillatory motion of a chute which is caused to pause while directing a stream of granular solids or liquid or slurry to either a subsample collection vessel or to a residue collection vessel may also exploit the optimal design features of the device described above. Consider a fixed chute of 220 rectangular cross-section which communicates directly with a subsample collection vessel and has chutes on their side which both communicate directly with a residue collection vessel. Above these chutes is a flexible or otherwise movable conduit of a cross-section whose dimension in a vertical plane normal to the edges of the fixed chute leading to the subsample collection vessel is less that the distance between the edges of 225 the fixed chute leading to the subsample collection vessel measured in the aforementioned vertical plane. If the flexible conduit is set into motion such that it pauses in a controlled manner when the entire stream of material flowing through it is directed either to the subsample collection vessel or to the residue collection vessel, and it moves at a constant velocity while the stream of material discharging from it is falling 230 on the edges of the fixed chute which leads to the subsample collection vessel, this corresponding design is technically equivalent to the first design disclosed above. With appropriate attention to technical factors, this second or corresponding design may possibly achieve the same result as the first design, with a similar level of optimality.

Because the second design concept for implementation of a variable split sample divider 235 involves other than simple falling motion of the material to be subsampled, it may be subject to segregation effects that arise from the varying lateral acceleration of the stream, and in this respect it is deemed to be potentially inferior to the first design described.

This corresponding implementation of the design may also be effected using a movable 240 conduit that has a rotational motion, with a fixed chute for subsample collection that have edges that are parallel to radial lines from the centre of rotation of the moving conduit.

Further, it may be deemed desirable to control the motion and inertia and consequently the trajectory of the stream of material to be subsampled. To this end, the material to be 245 subsampled may be conveyed in an air stream or pumped in a liquid or slurry stream with the objective of controlling its trajectory through the apparatus.

page 8 IN Advantageous Effects 0 oThe novelty and utility of this design of variable sample divider is that it provides the most rapid possible extraction of subsamples while complying with all accepted S 250 technical rules of subsampling. The design is therefore of greater utility than alternatives. Construction of the sample divider has shown that the angle subtended by the cutter edges can be 18 degrees and the distance over which acceleration takes place can be reduced to zero (the cutter moves at a substantially constant velocity all the Otime). The amplitude of the motion measured from a central position can be 18 degrees.

0 e 255 Thus the total motion needed to take one increment is equivalent to 360 of rotation at o constant velocity (this choice of geometry makes 2x A conventional rotary ci sample divider operating at the same angular velocity must make 3600 of rotation to extract a single increment. The invention is therefore capable of extracting the subsample in a technically equivalent manner in one-tenth of the time.

260 By using the formulae above, it is possible to show that the time required to extract N increments with the new invention does not exceed the corresponding time needed by the conventional rotary sample divider until the fraction of the material wanted as a sample falls below 0.05 or exceeds 0.95.

The invention is novel in three respects: 1) continuous rotation of the device is not 265 necessary, 2) the fraction of the material collected as a subsample is effectively continuously and reliably variable as a result of the close control of the cutter motion achieved with the stepper motor or close servo motor control used to move the cutter, 3) the oscillatory rather than rotational motion of the cutter permits the sample division to be carried out in the shortest possible time, while using a fixed cutter velocity through 270 the falling stream.

It is the first feature above which permits minimisation of the time period required to divide the sample while ensuring that the sample division process involves the interception of many increments of material from the falling stream.

The variable split sample divider incorporates a programmable logic controller or other 275 similar device to direct its operation and to communicate with the user or a host computer. It may also include a user interface device such as a touch screen or visual display and keyboard.

page 9 IN Description of Drawings 0 280 Figure 1 provides plan and elevation views of a simplified assembly of the variable split sample divider. Figure 2 shows a schematic representation of one cycle of operation of the device. Figure 3 provides the relationship between the time required for the subsample extraction and the fraction of the sample that the subsample represents. The elements of the drawing are further described in relation to the preferred embodiment of 285 the device.

Preferred Form of the Invention 0 N The invention is exemplified by the following description. The item numbers refer to 0 O Figure 1. The dimensions given are specific to the preferred embodiment which may differ in scale from the drawing of Figure 1. The device may be constructed on a 290 variety of practical physical scales, without specific limitation to those described below.

The chute containing the falling stream of material subtends an angle of degrees of arc at the centre of rotation of the cutter. The cutter aperture subtends an angle of 18 degrees between the edges of the cutter and it travels through an angular displacement of 180 to either side of a central position beneath the falling stream of 295 material. The falling stream is located approximately 0.2 m from the axis of rotation of the cutter.

The cutter is fixed to a central shaft which is hollow, permitting collection of the subsample directly on the axis of rotation of the device, in the manner shown at The residue of the sample is collected in a second container The cutter is made of 300 light-weight composite carbon fibre epoxy composite) to reduce the moment of inertia of the cutter. The design of the cutter is such that lines coincident with the cutter edges that engage the falling stream of material will meet on the axis of rotation when projected in that direction. In the language of the technology of materials sampling, such a cutter is said to be of 'mechanically correct' design. The cutter is driven through 305 a pulley (not shown) on the central shaft which is in turn driven from a stepping motor (not shown) and a toothed belt (not shown). The device is equipped with inductive proximity switches (not shown) which permit positioning of the cutter head to within 1/1440 of a revolution or 0.250 and act as limit switches to prevent damage to the cutter.

The gearing of the drive pulleys is such that this tolerance corresponds to one step of the 310 stepper motor drive when it is configured to 800 steps per revolution. The speed of page stepping is one step every 2 ms. This speed may be varied according to the power of the motor. The design figures chosen here corresponds to an effective velocity of the cutter through the falling stream of 0.277 m/s which is approximately half the allowed speed according to accepted sampling practice and cutter design.

315 As a result of using a relatively powerful motor, the cutter head operates at constant velocity without the need for acceleration and deceleration periods. The duration of a pause of the motion can be made in increments of 2 ms.

Control of the device is accomplished using a programmable logic controller (PLC) (not shown) which directly supplies a direction signal and timing pulses to stepper motor 320 driving circuitry suited to the motor used. The PLC also monitors additional proximity switches (none shown) which sense the position of the feeder assembly, the safety cover over the moving cutter and the position of the subsample and sample residue containers.

The PLC controls the display of various messages to the operator of the device on a touchscreen (not shown). The operator's response to the messages are communicated to 325 the PLC from the touchscreen.

The operating parameters for a subsample extraction are transmitted to the divider via an RS232 serial link with a host computer or can be entered on the touchscreen. The host computer may determine the fraction of the sample to be collected or it may be determined by an operator.

330 The sample is fed to the divider from a variable rate feeder (not shown). The feeder is controlled by the PLC communicating with a variable speed drive on the feeder.

The basic operating parameters controlled by the divider controller are 1) feed rate of the sample, 2) length of time to pause when collecting the stream of material to subsample or to residue and 3) the total time expected to be required for the subsample 335 extraction.

An operating procedure may be described as follows. The operator or the LIMS resident in a host computer determines the mass of the material that is to be extracted as a subsample from the sample mass, fixing the target division ratio. Using a set of tables based on the formulae contained herein and the desired number of sample increments to 340 be taken, or using direct calculation, the feedrate setting for the sample feeder is then determined. The feedrate must be low enough to ensure that the desired number of increments can be taken from the falling stream. The pause time in the stream or page 11 I outside the falling stream is determined by the fraction of the sample to be retained.

0 o The feed rate and timing settings can be fixed in the PLC by communicating through the 345 host computer or through a manual data entry device connected to the PLC or by internal calculation in the PLC. Once these parameters are set, the PLC control program can be initiated and the sample division takes place. The PLC starts the motion of the sample cutter and the sample feeder. The motion is stopped either on an operator command or after a period which is guaranteed to cause the whole of the sample to pass CO 350 through the divider.

0Industrial Applicability oAnalytical laboratories are becoming more completely automated with the objective of reducing operator error and manpower requirements. If a laboratory is to completely automate the process of mechanically correct sample mass reduction from the point of 355 receipt of raw samples right down to the extraction of an analytical aliquot of material of a predetermined mass, it must in general extract a subsample of varying mass at some stage of the sample mass reduction process because the initial weights of the samples received will in general vary from one sample to the next. The invention described herein provides an optimal means of making such a sample mass reduction as it makes 360 the division in a mechanically correct manner, adhering to all technical rules for extraction of subsamples, and it carries out the subsample extraction in the shortest possible time for a given scale of the equipment.

The fact that the preferred embodiment of the variable split sample divider includes interface equipment to provide communication with a LIMS and provides a 365 continuously variable and accurate mass division ratio makes the device ideal for use within an automated sample preparation facility operated under computer control with robotic devices.

page 12

Claims (13)

1. A device for the accurate (unbiased and precise) extraction of a subsample that is a 370 user-defined fraction of the original mass of a sample of granular solid, liquid or slurry material comprising a cutter head of correct mechanical design (rectilinear or radial orientation of the cutter edges as the type of cutter motion requires) that is moved from one side to the other of a falling stream of granular material, in which the mass flow rate is constant, in a rectilinear or rotational oscillatory motion and is 375 accurately controlled to pause either while collecting the full stream of the flowing material or while not collecting the stream at all and which moves at a constant ovelocity when the cutter edges are passing through the flowing stream.

2. A device as in Claim 1 which is driven by a stepper motor and controlled by a programmable logic controller (PLC) or other functionally equivalent 380 programmable device.

3. A device as in Claim 1 which is driven by a stepper motor and controlled from the stepper motor controller itself.

4. A device as in Claim 1 which is driven by a device other than a stepper motor (for example, a servo motor) but achieves a similar accuracy of control of the motion of 385 the cutter head.

A device as in Claims 1 to 4, the cutter of which is constructed of light-weight composite material such as a carbon fibre reinforced material with the objective of reducing the moment of inertia of the moving part.

6. A device as in Claims 1 to 5 which is used to extract a representative subsample of a 390 flowing stream and communicates electronically with a host computer.

7. A device for the accurate (unbiased and precise) extraction of a subsample that is a user-defined fraction of the original mass of a sample of granular solid, liquid or slurry material comprising a fixed chute of correct design (rectilinear or radial orientation of the chute edges as the type of stream motion requires) communicating 395 with a subsample collection vessel, and a conduit carrying a flowing stream of granular material, in which the mass flow rate is constant, that is moved from one side of the fixed chute to the other in a rectilinear or rotational oscillatory motion and is accurately controlled to pause while either directing the full stream of the page 13 flowing material to the subsample collection vessel or while not directing the stream 400 to the subsample collection vessel and which causes the flowing stream of material to move at a constant speed normal to the chute edges while the flowing stream is in contact with the edges of the fixed chute.

8. A device as in Claim 7 which is driven by a stepper motor and controlled by a programmable logic controller (PLC) or other functionally equivalent 405 programmable device.

9. A device as in Claim 7 which is driven by a stepper motor and controlled from the stepper motor controller itself.

A device as in Claim 7 which is driven by a device other than a stepper motor (for example, a servo motor) but achieves a similar accuracy of control of the motion of 410 the flowing material stream.

11. A device as in Claims 7 to 10, the conduit of which is constructed of light-weight composite material such as a carbon fibre reinforced material.

12. A device as in Claims 7 to 10 which is used to extract a representative subsample of a flowing stream and communicates electronically with a host computer. 415

13. Devices as described in Claims 1 to 12 in which the material to be sampled is conveyed or pumped with the objective of controlling its trajectory page 14