WO1998001226A1

WO1998001226A1 - Twin chamber pulp lifters for grate discharge mills

Info

Publication number: WO1998001226A1
Application number: PCT/AU1997/000418
Authority: WO
Inventors: Sanjeeva R. Latchireddi; Stephen Morrell
Original assignee: The University Of Queensland
Priority date: 1996-07-04
Filing date: 1997-07-02
Publication date: 1998-01-15
Also published as: AUPO083196A0

Abstract

A pulp lifter for a grate discharge mill used primarily in mining operations. The lifter comprises a plurality of chambers which radiate outwards. Each chamber comprises a transitional compartment (2) having apertures (19) into which the slurry enters and a collection compartment (21) to which the slurry passes. When the pulp lifter rotates, the slurry cannot flow back but must pass to the discharge.

Description

TWIN CHAMBER PULP LIFTERS FOR GRATE DISCHARGE MILLS

BACKGROUND OF THE INVENTION

This invention relates to grate discharge mills and is particularly concerned with the design of the pulp lifter which is an integral component thereof. Pulp lifters, also known as pan lifters, transport the slurry passing through the apertures in the associated grate of the grate discharge mill, into the discharge trunnion. In conjunction with the grate open area, the pulp lifter determines the maximum volumetric throughput of the mill. Typically, a pulp lifter comprises a plurality of radially orientated chambers which rotate with the mill in a vertical plane against a vertically disposed grate adjacent to the outlet of the mill. As the mill, and hence chambers, rotates, individual chambers pass through the slurry pool in the lower region of the mill. Slurry flows through the grate during this time and accumulates in the respective pulp lifter chambers. As the mill rotates further, the respective pulp lifters and their slurry contents are raised to an uppermost position at which the slurry begins to flow down the radial face of the pulp lifter towards the centre of the mill. The slurry is deflected at this point so that it flows through the trunnion and out of the mill. However, once the pulp lifter chamber leaves the slurry pool zone, slurry is no longer present against the face of the grate on the interior facing side of the mill and hence a portion of the slurry in the pulp lifter flows back into the mill by way of the grate holes through which it entered. This phenomenon is called "flowback" and it results in a higher pulp level in the mill which can have adverse effects on grinding efficiency.

The problem of "flowback" is further exacerbated with increase in the mill speed, and optimisation of the mill speed is therefore hindered.

OBJECT OF THE INVENTION It is therefore an object of the present invention to provide a pulp lifter which is designed to minimise the amount of flowback in a grate discharge mill.

SUMMARY OF THE INVENTION According to the present invention there is provided a pulp lifter for a grate discharge mill, said pulp lifter comprising a plurality of chambers radially arranged to rotate against the downstream side of a vertical grate, each of said chambers comprising a transitional compartment and a collection compartment, the transitional compartment being designed to enable slurry, passing through the grate from a mill slurry pool, to passage through it to the collection compartment for subsequent discharge but not back into the transitional compartment from the collection compartment, upon rotation of the pulp lifter.

The transitional compartment and the collection compartment preferably form two substantially contiguous segments of the pulp lifter. Such segments can be separately divided by a plate into identical sections. Preferably, the plates tilt slightly towards the grate side and are parallel with one another so that an area defined by one plate and one segment constitutes the transitional compartment adjacent to the grate, and an area defined by the other plate and the other segment constitutes the collection compartment, which is spaced from the face of the pulp lifter grate.

The arrangement is such that when slurry passes into the transitional compartment from the mill slurry pool, the collection compartment is at that instance below the level of the transitional compartment.

A plurality of apertures is suitably formed in the wall of the transitional compartment which faces the grate to enable the passage of slurry into the transitional compartment. The invention also extends to the pulp lifter chamber per se .

DESCRIPTION OF THE DRAWINGS Figure 1 is a cross-sectional view of a conventional prior art grate discharge mill;

Figure 2 is a peripheral cross-sectional view of a twin compartment of a pulp lifter according to one aspect of the present invention; Figure 3 is an orthogonal and partly exposed view of the twin compartment of Figure 2 ;

Figure 4 is a generalised peripheral cross- sectional view of a twin compartment of a pulp lifter according to a generalised aspect of the present invention;

Figure 5 is a generalised peripheral cross- sectional view of a twin compartment of a pulp lifter according to another aspect of the present invention;

Figure 6 is a generalised peripheral cross- sectional view of a twin compartment of a pulp lifter according to yet another aspect of the present invention;

Figure 7 is a generalised peripheral cross- section view of a twin compartment of a pulp lifter according to still another aspect of the present invention;

Figure 8 is an orthogonal view of the twin compartment of Figure 7;

Figures 9-11 are graphs showing the relation between the actual pulp lifter discharge flowrate for radial type pulp lifters (RPL) and feed flowrate;

Figures 12-14 are graphs showing the relation between the actual pulp lifter discharge flowrate for twin compartment pulp lifters (TCPL) and feed flowrate;

Figure 15 is a graph showing the relation between the holdup and discharge flowrates of RPL;

Figure 16 is a graph showing the relation between the holdup and discharge flowrates of TCPL; and

Figure 17 is a schematic plan view of a pulp lifter . DESCRIPTION OF PREFERRED EMBODIMENTS

Preferred embodiments of the invention will now be described with reference to the aforementioned drawings . In all of the drawings, like reference numerals refer to like parts.

Referring firstly to Figure 1, a conventional prior art grate discharge mill is illustrated. This comprises a rotating cylinder 10 having a slurry feed inlet 11 at one end and a discharge trunnion 12 at the other end. A grate 13 is vertically arranged at the discharge end and a pulp lifter 14 is located between the grate and the discharge trunnion 12.

Slurry 15 in the mill flows into the pulp lifter at position 16 and is raised by the pulp lifter to discharge into the trunnion at 17. As the slurry is raised, there is a tendency for a portion of it to flowback into the mill as indicated by the multiple arrows in region 18.

In Figures 2 and 3, a preferred twin compartment arrangement for a pulp lifter according to the present invention is illustrated. The arrangement comprises a transitional compartment 20 and a collection compartment 21. The transitional compartment 20 is in communication by way of apertures 19 formed in one side thereof, with the grate 22 of the mill. The illustration shows the passage of slurry (see dotted lines 23 in Figure 2 and solid lines 24 in Figure 3) from the grate 22 through the transitional compartment 20 into the collection compartment 21.

It will be quite clear from Figures 2 and 3 that the slurry which enters the transitional compartment 20 can only flow into the collection compartment 21 but cannot flow backwards from the collection compartment to the transitional compartment. Flowback is thereby avoided.

The actual dimensional specifications of the twin compartment pulp lifter depends on the mill dimensions. However, the dimension of the different segments, segment-a, segment-b, segment-c, bend 31 and bend 32 as is shown in Figure 4, are chosen to be in proportion to the grate hole size and pebble port 33. The position of the pebble port, if any, is as illustrated.

In order to evaluate the performance of twin compartment pulp lifters according to the present invention, tests were conducted in a laboratory mill with grate and pulp lifters made of clear perspex, using water. To determine the actual throughput of pulp lifters under different conditions, only one segment of the pulp lifters was attached to the grate and the flow through it was collected separately to determine the actual discharge flow rate of the pulp lifters. The flow rate of one pulp lifter multiplied by the total number of pulp lifters in an entire assembly gives the discharge flowrate of an entire assembly. The experimental conditions and the variables tested are given in Table 1 below:

TABLE 1

Variables kept constant Variables studied

Grate open area 10% Feed flow rate

Charge (lt/min) 5 levels

(including voids) 30% Mill speed

Total Number of pulp (% critical speed) 3 levels lifters 16 Pulp lifter size 3 levels

Essentially the width of the pulp lifter determines its capacity and hence the pulp lifter size is mentioned here as the percent ratio of the pulp lifter width to the mill length [Percent ratio = (Pulp lifter width/mill length) ^*100] . The sizes and percent ratios are given in Table 2 below:

TABLE 2

Pulp lifter size Percent ratio

Small 3.704

Medium 6.79

Large 9.877 Tests with Radial type Pulp lifters (RPL)

If the mill is operated as an end discharge mill without pulp lifters, then for a given level of water in the mill the discharge flowrate will be at its maximum and hence can be taken as the ideal condition.

The relation between the actual pulp lifter discharge flowrate for RPL and feed flowrate is shown in Figures 9-

11 together with the ideal condition. The relation shows a significant deviation from the ideal discharge flowrates which can be attributed mainly to the flowback phenomenon .

Tests with Twin chamber Pulp lifters (TCPL)

A prototype perspex model of the TCPL was made as illustrated in Figures 7 and 8. This was of equal volume to that of the RPL and tests were conducted under identical conditions in the laboratory mill.

The relation between the actual discharge rate of the TCPL and the feed flowrate is shown in Figures 12- 14. When compared with the RPL (Figures 9-11) the results are much closer to the ideal line. The point where these lines deviate from the ideal line indicates where the pulp flowrate through the grate is higher than the discharge capacity of the collection compartment of the TCPL. Hence part of the pulp remains heldup in the transition compartment, performs in a similar manner to the RPL. This clearly demonstrates that the flowback process can be considerably reduced by using TCPL over RPL. When the discharge flowrates of TCPL are compared with RPL, the advantages of TCPL over RPL can be easily observed. To illustrate this, a relation between the holdup and discharge flowrates of TCPL and RPL are plotted for one particular condition. It will be noted from Figure 15 that for the same pulp lifter size, at a given level of water in the mill, the discharge rate of the TCPL is much higher than that of the RPL. It is obvious from this that for the same level of water in the mill, the mill can be operated at higher throughput with the TCPL than with the RPL.

At a given feed flowrate, the mill hold-up

(level of water in the mill) is nearer to the ideal hold- up when the TCPL is used. Whereas when the RPL is used, the mill hold-up increases to a higher level due to flowback. This can inhibit the grinding efficiency by running the mill too wet - see Figure 16. Hence, compared to the RPL, using the TCPL is advantageous in maintaining the slurry levels nearer to the ideal conditions which results in better grinding efficiency.

Besides higher throughputs, the use of the TCPL is expected to give the following advantages over the

RPL. 1. Excessive pulp levels inside the mill can be reduced which enhances the grinding efficiency.

2. Reduction in the internal wear in the grate of the diaphragm assembly caused by flowback of pulp into the mill. 3. Changes in the grate open area will have less influence on the performance of the TCPL compared to the

RPL.

4. Minimise the recirculation of fines by reducing flowback . Figure 17 illustrates a schematic plan view of a pulp lifter according to the invention showing the radial arrangement of the chambers 40.

Whilst the above has been given by way of illustrative example of the invention, many modifications and variation may be made thereto by persons skilled in the art without departing from the broad scope and ambit of the invention as herein set forth.

Claims

CLAIMS :

1. A pulp lifter for a grate discharge mill, said pulp lifter comprising a plurality of chambers radially arranged to rotate against the downstream side of a vertical grate, each of said chambers comprising a transitional compartment and a collection compartment, the transitional compartment being designed to enable slurry, passing through the grate from a mill slurry pool, to passage through to the collection compartment for subsequent discharge but not back into the transitional compartment from the collection compartment, upon rotation of the pulp lifter.

2. A pulp lifter as claimed in claim 1, wherein the transitional compartment and the collection compartment form two substantially contiguous segments.

3. A pulp lifter as claimed in claim 2, wherein the segments are each divided by a pate into identical sections, said plates being tilted slightly towards the grate side, and being parallel to one another such that an area defined by one plate and one segment constitutes the transitional compartment adjacent to the grate, and an area defined by the other plate and the other segment constitutes the collection compartment spaced from the grate .

4. A pulp lifter as claimed in claim 1, wherein the relative positioning of the transitional compartment and the collection compartment is such that when slurry passes into the transitional compartment from the mill slurry pool, the collection compartment is below the transitional compartment.

5. A pulp lifter as claimed in claim 1, wherein the slurry passes into the transitional compartment by way of a plurality of apertures in the wall thereof which faces the grate.

6. A chamber for a pulp lifter comprising a transitional compartment and a collection compartment, the transitional compartment being designed to enable slurry, passing through it to passage to the collection compartment for subsequent discharge but no back onto the transitional compartment.

7. A chamber for a pulp lifter as claimed in claim

6, wherein the transitional compartment and the collection compartment form two substantially contiguous segments .

8. A chamber for a pulp lifter as claimed in claim

7, wherein the segments are each divided by a plate into identical sections, said plates being relatively tilted but parallel to one another.

9. A pulp lifter for a grate discharge mill substantially as herein described with reference to Figures 2-8 of the accompanying drawings.