an efficient pulp lifter for ag-sag mills.pdf

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Slurry flow in mills with TCPL — An efficient pulp lifter

for ag/sag mills

Sanjeeva Latchireddi ⁎, Stephen Morrell

JKMRC, University of Queensland, Isles Rd., Indooroopilly 4068, Australia

Received 14 December 2005; accepted 23 February 2006

Available online 24 April 2006

Abstract

The difficulties associated with slurry transportation in autogenous (ag) and semi-autogenous (sag) grinding mills have become

more apparent in recent years with the increasing trend to build larger diameter mills for grinding high tonnages. This is particularly

noticeable when ag/sag mills are run in closed circuit with classifiers such as fine screens/cyclones.

Extensive test work carried out on slurry removal mechanism in grate discharge mills (ag/sag) has shown that the conventional

pulp lifters (radial and curved) have inherent drawbacks. They allow short-circuiting of the slurry from pulp lifters into the grinding

chamber leading to slurry pool formation. Slurry pool absorbs part of the impact thus inhibiting the grinding process.

Twin Chamber Pulp Lifter (TCPL) — an efficient design of pulp lifter developed by the authors overcomes the inherent

drawbacks of the conventional pulp lifters. Extensive testing in both laboratory and pilot scale mills has shown that the TCPL

completely blocks the flow-back process, thus allowing the mill to operate close to their design flow capacity. The TCPL

performance is also found to be independent of variations in charge volume and grate design, whereas they significantly affect the

performance of conventional pulp lifters (radial and curved).

© 2006 Elsevier B.V. All rights reserved.

Keywords: sag milling; comminution; grinding; autogenous; grates; pulp lifters

1. Introduction

Pulp lifters, also known as pan lifters, are an impor-

tant component of grate discharge mills (GDM). TheGDM include autogenous (ag), semiautogenous (sag)

and grate discharge ball mills. The purpose of the pulp

lifters is simply to transport the slurry passing through

the grate holes into the discharge trunnion. The

performance analyses of conventional design of pulp

lifter have shown that a large amount of slurry flows

back from pulp lifter into the mill. The degree of slow-

back depends on the size and design of the pulp lifters.The ideal slurry flow in a typical grate discharge mill is

schematically shown in Fig. 1.

The geometry of conventional pulp lifters is such that

the slurry, once passed through the grate into pulp lifter

will always be in contact with the grate until it is com-

pletely discharged, which makes the ‘flow-back ’ pro-

cess inevitable (Latchireddi, 2002). Though the impact

of flow-back may be of lower magnitude in open circuit

grinding, it can make a noticeable impact when the mills

are operated in closed circuit, especially with cyclones

Int. J. Miner. Process. 79 (2006) 174–187

www.elsevier.com/locate/ijminpro

⁎ Corresponding author. Current address: Outokumpu Technology

Inc., 10771 E Easter Ave., Centennial, CO 80112, USA.

E-mail address: [email protected]

(S. Latchireddi).

0301-7516/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.minpro.2006.02.005

mailto:[email protected]

http://dx.doi.org/10.1016/j.minpro.2006.02.005

http://dx.doi.org/10.1016/j.minpro.2006.02.005

mailto:[email protected]



and fine screens, where very large amounts of slurry

pass through the mills.

Based on the analysis and understanding of the me-

chanism of slurry removal system in ag/sag mills ope-

rating with conventional pulp lifters, Latchireddi and

Morrell (1997a, in press-a,b) have summarized the slurry

transportation process in grate discharge mills as shown

in Fig. 2.Although the carry-over of slurry in pulp lifter occurs

only at higher mill speeds, eventually it flows through

the grate back into the mill by the time it starts a new

cycle.

It is essential to stop the flow-back process to im-

prove the performance of pulp lifters, and any reduction

in flow-back fraction would directly result in higher

flow capacity. There are two possible ways to achieve

this aim. The first of them is to increase the width/depth

of the pulp lifter to such an extent that the slurry inside

the pulp lifter does not get exposed to the grate holes.

However, this option would increase the cost of the mill

considerably besides introducing high frictional resis-

tance to the slurry flow. The other option is to change the

design of pulp lifter, and one such design development

is the Twin Chamber Pulp Lifter — TCPL (Latchireddi,

2002; Latchireddi and Morrell, 1997b).

This paper describes the development of the TCPL

and its performance in comparison to the conventionaldesigns based on the test work carried out in laboratory

and pilot mills. Also briefly presented are the results of

the first industrial installation at Wagerup Refinery of

Alcoa World Alumina.

2. Development of a twin chamber pulp lifter

The only way to stop ‘flow-back ’ is to ensure that

once the slurry entered the pulp lifter, is not exposed to

the grate holes or slots. The importance of this aspect is

illustrated by considering two contiguous segments of the radial pulp lifter (RPL) as shown in Fig. 3. The two

segments look like two rectangular boxes sitting one

upon each other. It is implicit from Fig. 3 that the slurry

present inside the pulp lifter will always be in contact

with grate holes.

The two contiguous segments were modified by

feathering the radial face and as shown in Fig. 4. This

arrangement was done to facilitate the slurry to flow

away from the grate holes.

It is apparent from Fig. 4 that the slurry first enters

the section exposed to the grate, the Transition chamber

(TC) and then flows into the lower section, the

Flow into thepul

Flow outtrunnion

Flow into thepulp lifter

Flow outtrunnion

Flow out oftrunnion

Flow into thepul

Flow outtrunnion

Flow into thepulp lifter

Flow outtrunnion

Flow out oftrunnion

Grate

Fig. 1. Ideal slurry flow in a typical grate discharge mill.

Water

Ore

Mill

Shell Grat

Pulp

Lifter

Carry -over

Flow-back

Water

Mill

Shell Grat

Pulp

Lifter

Carry -over

Flow-back

Mill

Shell Grate

Pulp

Lifter Discharge

Carry -over

Flow-back

Fig. 2. Different stages of material transportation in a grate dischargemill.

175S. Latchireddi, S. Morrell / Int. J. Miner. Process. 79 (2006) 174 – 187



Collection chamber (CC). The collection chamber is not

exposed to the grate holes at all. This mechanism

ensures the pulp unable to flow or drain backwards into

the mill. Hence the flow-back process is prevented up to

the capacity of the collection chamber. Since the new

design consists of two chambers for different purposes,

it was named the “Twin Chamber Pulp Lifter ” (TCPL).

The TCPL can be precisely designed to handle the

designed flow capacity of the mill whose dimensions

depends on the operating conditions such as mill speedand number of pulp lifter segments. It is important to

note that the cross-sectional area of the slot through

which slurry flows from transition chamber into collec-

tion chamber should be at least equal to the total area of

grate holes in that section to allow free flow of slurry.

3. Experimental

3.1. Laboratory mill

Prototype models of the TCPL (Fig. 4) were

fabricated using a 2 mm thick clear acrylic, equal in

volume to that of the three different sizes of radial pulplifters (RPL). For a given mill diameter, the width of the

pulp lifter determines its capacity. In the present

investigation, the pulp lifter size (PLS) was represented

Radial face

Grate

Peripheral view

Fig. 3. Two contiguous radial pulp lifters as seen from the mill discharge end.

Fig. 4. The schematic of TCPL arrangement.

176 S. Latchireddi, S. Morrell / Int. J. Miner. Process. 79 (2006) 174 – 187



as a fraction of mill diameter (PLS = Depth of pulp

lifter/ Mill diameter). The details of the pulp lifter sizes

that were used in the test work are given in Table 1. The

volume of pulp lifters (TCPL/RPL) was kept same for

the purpose of comparison.

Tests were conducted after fixing a single segment of

the pulp lifter to the grate whose discharge was collected

independently via the central trunnion arrangement as

shown in the schematic of laboratory mill (0.3 m dia-meter×0.15 m length) in Fig. 5.

For each test, first a timed sample of pulp lifter dis-

charge was taken to estimate its discharge rate and then

the instantaneous hold-up was measured. To obtain the

discharge capacity of the entire pulp lifter assembly at the

measured hold-up, the discharge rate of the single pulp

lifter segment was multiplied by the total number (six-

teen) of pulp lifters.

3.2. Pilot scale mill

The complete assembly of the 1 m diameter by 0.5 mlength pilot mill is shown in Fig. 6, where the conven-

tional radial pulp lifter was shown fixed to the mill, and

the pilot size TCPL kept standing at the bottom. The size

of the pulp lifter was kept same (PLS=0.335) for all thedesigns for the purpose of their comparison.

Table 1

The normalized size of pulp lifters used in test work (TCPL/RPL)

Pulp lifter size PLS

Small 0.018

Medium 0.0335

Large 0.0495

Pump

Discharge

funnel

Pulp lifter segment

Pulp lifterdischarge

Transparent

grate

Grinding

media

Flowmeter

SamplerTwister

Collection

chute

Fig. 5. The schematic diagram of the laboratory mill (0.3×0.15 m).

Fig. 6. The complete section of pilot scale TCPL.




Tests were conducted with TCPL, RPL and CPL,

independently at the same operating conditions where

charge volume, mill speed, grate open area were varied

over a range of feed flowrates.

In each test the mill was set to rotate at the required

speed, after fixing the desired grate and pulp lifter to thedischarge end of the mill and filling with a known

volume of charge/grinding media. Then the pump was

switched on and the flow was allowed to pass into the

mill by opening the knife-gate valve whose flowrate was

estimated manually by taking timed samples. Upon al-

lowing the mill to reach steady state condition, the mill

feed and rotation were simultaneously stopped, and the

water that surged out of grate was diverted by the sam-

pling arrangement into a separate container. The leftover

fluid inside the mill was drained out through a separate

drain valve fixed on the mill shell. The volume of thefluid collected was reported as the instantaneous hold-

up inside the mill at the given flowrate.

The detailed discussions on the results obtained with

the conventional designs of pulp lifters have been pub-

lished elsewhere (Latchireddi and Morrell, in press-a,b)

by the authors. The performance analysis of TCPL in

comparison to the conventional pulp lifters is discussed

in the following sections.

4. Results and discussion

4.1. Influence of pulp lifter size

To understand how the increasing size of the pup

lifter influences the performance of TCPL in transport-

ing the slurry passing through the grate, a set of results

obtained at a particular condition (30% charge volume,

70% critical speed, 7.05 open area) are plotted as shown

in Fig. 7(A). For the purpose of comparison, the perfor-

mance of radial pulp lifters (RPL) under the same con-

ditions is shown in Fig. 7(B).

The important observations that can be made from

Fig. 7A compared to Fig. 7B are:

• the TCPL performance matches the ideal (grate-only)

system over a much greater range of discharge rates.

• the increasing pulp lifter size increases the discharge

rate, and hence the range over which it matches ideal

discharge rates.

• the deviation point of the discharge lines from the

ideal line indicates that the volume of fluid flowing

into the pulp lifter through the grate exceeds the

capacity of the collection chamber — hence part of

the fluid remains held-up in the transition chamber,

which performs in a similar manner to that of a RPL.

• the TCPL allows the mill to operate at its maximum

flow capacity as obtained by grate-only discharge

system.

The above observations and the typical design of

TCPL amply demonstrate that the flow-back process

can be eliminated, up to the capacity of the collection

chamber.

4.2. Influence of the variables on performance of TCPL

Besides overcoming the major problem of flow-

back, which is unavoidable in case of conventional

pulp lifter designs, the unique design of TCPL offers

many other advantages. The most important one is that

its performance does not get affected due to variations

in:

♦ grate open area, and

♦ volume of grinding media (charge) inside the mill.

0

5

10

15

20

25A

B

0.05 0.1 0 .15 0 .2

Fractional Hold-up

D i s c h a r g e r a t e ( l / m )


0

5

10

15

20

25

0.05 0.1 0.15 0 .2

Fractional Hold-up

Small

MediumGrate-only

Small

Medium

Grate-only

Large

Large

Fig. 7. A: Performance of different sizes of TCPL in laboratory mill.

B: Performance of different sizes of RPL in laboratory mill.




In ag/sag mills the volume of the grinding media

(balls and coarse ore) tends to change with the type of

ore which also has strong interaction with grate open

area and influences the performance of pulp lifter.

Hence, the effects of both these variables are shown

together and discussed. To illustrate the above points thevariation in mill hold-up–discharge rate relation with

change in grate open area and charge volumes are shown

in Figs. 8 and 9 respectively for RPL and TCPL.

It has been a usual practice to increase the grate open

area to obtain an increased discharge rate. This would

be successful with grate-only discharge mechanism

(Latchireddi and Morrell, in press-a). However, it was

found from the test work with grate-pulp lifter

discharge systems that the performance of RPL intransporting the slurry flowing out of the discharge

grate, deteriorates with increasing open area and is

particularly high in magnitude when the mill is running

Charge volume - 30%

Open area - 3.6%

0

100

200

300

400

500

0 0.1 0.2 0.3 0.4

Fractional hold-up


RPL

Ideal

Charge volume - 30%

Open area - 7%

0

100

200

300

400

500

0 0.1 0.2 0.3 0.4

Fractional hold-up


RPL

Ideal

Charge volume - 30%

Open area -10%

0

100

200

300

400

500

0 0.2 0.3 0.4

Fractional hold-up


RPL

Ideal

Charge volume - 15%

Open area - 3.6%

0

100

200

300

400

500

0 0.1 0.2 0.3 0.4

Fractional Hold-up


RPL

Ideal

Charge volume - 15%

Open area - 7%

0

100

200

300

400

500

0 0.1 0.2 0.3 0.4

Fractional hold-up


RPL

Ideal

Charge volume - 15%

Open area - 10%

0

100

200

300

400

500

0 0.1 0.2 0.3 0.4

Fractional hold-up


RPL

Ideal

Charge volume - 45%

Open area - 3.6%

0

100

200

300

400

500

0 0.1 0.2 0.3 0.4

Fractional hold-up


RPL

Ideal

Charge volume - 45%

Open area - 7%

0

100

200

300

400

500

0 0.1 0.2 0.3 0.4

Fractional hold-up


RPL

Ideal

Charge volume - 45%

Open area - 10%

0

100

200

300

400

500

0 0.1 0.2 0.3 0.4

Fractional hold-up

D i s c h a r g e r a t e ( l / m

)

RPL

Ideal

Charge volume

G r a t e o p e n a r e a

0.1

Fig. 8. Performance of RPL with variations in charge volume and open.




with lower charge volume. This trend can be seen in

Fig. 8 in terms of the difference between the grate-only

(ideal) and grate-pulp lifter lines. Though this trend

remains the same, the magnitude of inefficiency

gradually reduces with increasing charge volume. This

is simply because the amount of flow-back is propor-

tional to the number of grate holes that are exposed to the

fluid inside the pulp lifter, which reduces with increasing

charge volume.

The observation of decreasing discharge rates with

increasing grate open area is in accordance to the state-

ment made by Rowland and Kjos (1975), that if the pulp

lifters do not have enough capacity, the typical approach

of increasing the grate area does not improve the

situation but makes it worse by allowing the slurry to

flow back into the mill, causing it to run too wet. Morrell

and Kojovic (1996) have mentioned that the presence of

excessive slurry pool inside the mill reduces the

grinding efficiency.

The results presented in Fig. 8 amply illustrate that

the performance of conventional radial pulp lifters, in

transporting the slurry, is highly influenced by the

Charge volume - 30%

Open area - 3.6%

0

100

200

300

400

500

0 0.1 0.2 0.3 0.4

Fractional Hold-up


Ideal

TCPL

Charge volume - 30%

Open area - 7%

0

100

200

300

400

500

0 0.1 0.2 0.3 0.4

Fractional Hold-up


Ideal

TCPL

Charge volume 30%

Open area - 10%

0

100

200

300

400

500

0 0.1 0.2 0.3 0.4

Fractional Hold-up


Ideal

TCPL

Charge volume - 15%

Open area - 3.6%

0

100

200

300

400

500

0 0.1 0.2 0.3 0.4

Fractional Hold-up


)

Ideal

TCPL

Charge volume - 15%

Open area - 7%

0

100

200

300

400

500

0 0.1 0.2 0.3 0.4

Fractional Hold-up


Ideal

TCPL

Charge volume 15%

Open area - 10%

0

100

200

300

400

500

0 0.1 0.2 0.3 0.4

Fractional Hold-up


Ideal

TCPL

Charge volume - 45%

Open area - 3.6%

0

100

200

300

400

500

0 0.1 0.2 0.3 0.4

Fractional Hold-up


)

Ideal

TCPL

Charge volume

Charge volume - 45%

Open area - 7%

0

100

200

300

400

500

0 0.1 0.2 0.3 0.4

Fractional Hold-up


Ideal

TCPL

Charge volume 45%

Open area - 10%

0

100

200

300

400

500

0 0.1 0.2 0.3 0.4

Fractional Hold-up


Ideal

TCPL

G r a t e o p e n a

r e a

Fig. 9. Performance of TCPL with variations in charge volume and open area.




variations in grate open area and charge volume inside

the mill.

Contrary to the observations made from the RPL

results shown in Fig. 8, it may be seen from Fig. 9 that

the performance of the TCPL is not adversely affected

by changes in grate open area and charge volume. This

observation is clearly seen up to the discharge capacity

of the collection chamber. This is because once the

slurry flows into the collection chamber it is not exposed

to the grate holes. However, the influence of the chargevolume and open area, which is similar to that in the

RPL, can be seen at discharge rates exceeding the ca-

pacity of the collection chamber.

Comparing Fig. 8 with that of Fig. 9, it is quite

evizdent that up to the capacity of the collection

chamber, the performance of TCPL is not adversely

influenced by changes in grate open area and charge

volume inside the mill.

4.3. Performance comparison of TCPL with conven-

tional pulp lifter designs

To illustrate the superiority of the TCPL over con-

ventional pulp lifter designs, the relationship between

the hold-up and discharge rates of the mill operating

with the same size of TCPL, RPL and CPL are plotted

for a particular condition as shown in Fig. 10.

It may be noted from Fig. 10 that for the same pulp

lifter size,

➣ at a given slurry hold-up in the mill, the discharge

rate with the TCPL is significantly higher than

that of the RPL/CPL, which makes it obvious that

for the same level of hold-up in the mill, the mill

can be operated at a higher throughput with the

TCPL compared to that with the RPL or CPL.

➣ at a given flow-rate, the slurry hold-up inside the

mill can be kept close to the grate-only (ideal)

hold-up with TCPL in use. Whereas with either RPL or CPL in use, the mill hold-up increases

significantly to a higher level due to flow-back,

leading to the formation of a slurry pool, which

has adverse effects on the grinding efficiency.

It is apparent from Fig. 10 that using TCPL is

advantageous in maintaining slurry levels closer to the

ideal conditions (grate-only) without slurry pooling — the

condition that is required for the best grinding performance.

5. Full-scale industrial installation of TCPL

The clear ability of the TCPL to achieve a higher

discharge rate for a given hold-up prompted Alcoa World

Alumina Australia to install this design in one of their

severely flow restricted SAG mills at its Wagerup Refinery

in Western Australia. This is the world's first full-scale

industrial trial of the TCPL whose installation and com-

missioning was carried out during August/September

1999 and was subsequently installed in all 9 mills (Denis

et al., 2001) of both Wagerup and Pinjarra refineries.

Based on the simulations, preliminary design details

of the TCPL concept with critical dimensions were pro-vided to Alcoa. Prototype models were used to convey

the design concept and understand the issues of flow-

back and pitfalls of the current radial pulp lifters to

designers and other related plant personnel involved. To

ensure better understanding of the design for retrofitting

and installation, ALCOA had fabricated a scale model of

grate-TCPL assembly.

0

100

200

300

400

500

0 0.1 0.2 0.3 0.4

Fractional hold-up

D i c h a r g e r a

t e ( l / m i n )

TCPL

Radial

Curved

Grate-only

Fig. 10. Comparison of different pulp lifter designs (pilot mill data at

15% charge, 7.05% grate open area and 70% critical speed, pulp lifter

size — 6.7%).

Fig. 11. The schematic of #4 mill circuit together with sampling points.




To assess the effect of the TCPL compared with the

existing RPL, complete grinding surveys around the sag

mill circuit were conducted both before and after the

installation. The important observations made are

discussed in this section.

5.1. Description of milling circuit and data acquisition

The schematic of the mill #4 circuit is shown in

Fig. 11. Fresh ore was fed via a conveyor from a 2000 t

capacity mill feed bin, which was kept full to minimise

size segregation during the mill surveys and trials.

Slurry discharged from the mill passes through the

trommel, where oversize is returned to the mill via a

central pipe with assistance of a liquor jet. The undersize

of trommel flows into a sump from where a variablespeed pump delivers it to the DSM screens via rotary

distributor. The DSM oversize and the spent liquor

combines together and enters the mill along with the

fresh feed.

0

100

200

300

400

500

22:06 22:06 22:06 22:06 22:06

Time (Hours)

Feed Rate (TPH)

0

100

200

300

400

500

A

B

17: 30 20: 30 23: 30 2:30 5:30 8:30 11: 30 14: 30

Time (Hours)

Feed Rate (TPH)

Power (x10, KWH)

Mill Spill Point

Fig. 12. A: Mill fed rate (TPH) over a period of 5 days (pre-installation). B: Spill points where slurry overflows (pre-installation).




To assess the true effect of the TCPL, a complete mill

survey around the sag mill circuit was conducted both

before and after the installation of the TCPL. The

sampling points from where the representative samples

were collected at intervals of every 15 min over a period

of 45 min are also shown in Fig. 11. All the surveys werefollowed by a crash stop of the mill to measure the

steady state charge and slurry volumes.

5.2. Pre-installation performance

To understand the normal performance before instal-

lation of the TCPL, 6 min average process data were

obtained over a period of 5 days from the #4 sag mill

at Wagerup and is depicted in Fig. 12A. A feed rate of

400 tph was seen occasionally with an average value of

390 tph. A closer look at the data over few hours, spillage

of the mill through the feed trunnion can be observed,

which trips the feed till the mill settles down. Two of these

spikes in mill feed rate response can be seen in Fig. 12B.

The spilling of the sag mill over the feed trunnion

indicates that the volume of slurry inside the mill (hold-

up) has increased so much that the mill starts to operate

similar to an overflow mill. This situation arises due tothe poor performance of its discharge assembly,

consisting of a very small size of pulp lifters.

A significant amount of slurry was observed to be

spilled over the feed trunnion soon after the crash stop of

the mill and the slurry level up to the lip of the feed

trunnion can be seen in Fig. 13 where a 700 mm deep

slurry pool was measured above the charge level (grind-

ing media+ coarse ore).

5.3. Simulation of pre- and post-installation conditions

The models developed by Latchireddi (2002) were

used to predict the hold-up–discharge rate curve for the

ideal or the grate-only discharge system as well as with

pulp lifters. The simulated results thus obtained are

graphically shown in Fig. 14.

The large difference between the discharge rates

through the grate and the pulp lifters at any hold-up

clearly shows the inefficient performance of the existing

pulp lifters. It can be observed from Fig. 14 that at the

current mill discharge rate of 440 m3/h, the slurry hold-

up inside the mill is significantly higher than the ideal

hold-up. This results in a huge slurry pool, as observedduring the crash stop of the mill (Fig. 13A), causing the

mill to run too wet which leads to inefficient grinding

(Rowland and Kjos, 1975; Austin et al., 1984). The wear

pattern caused by the flow-back on inner side of the

discharge grate can be seen in Fig. 13B.

Fig. 13. A: Slurry pool inside the mill (pre-installation). B: Wear on

grate due to flow-back (pre-installation).

0

100

200

300

400

500

600

0 0.05 0.1 0.15 0.2

Fractional slurry hold-up

D i s c h a r g e r a t e ( m 3 / h )

Grate-only

Proposed TCPL

Existing RPL

Current operation

Fig. 14. Performance of pulp lifters based on simulated results.




However, with the TCPL in operation, the slurry

hold-up was predicted to come very close to the grate-

only value, suggesting better grinding conditions inside

the mill without any slurry pool.

6. Post-installation performance

To understand the impact of the TCPL over the pre-

installation condition, a complete mill survey was con-

ducted under the same operating conditions by setting

the mill to run at 390 tph of feed rate. To isolate the effect

of change in feed ore characteristics, it was made sure

that the bauxite ore came from the same stockpile.

The very first observation made during this survey

was a significant increase in mill noise with individual

impacts being easily identifiable. The noise observed

during pre-installation survey was very quiet with fewdiscernible impacts due to the presence of the slurry

pool. Upon crash stopping the mill after the survey, no

slurry was either overflowed or found on top of the

solids inside the mill (Fig. 15), which confirms the

efficient transportation of slurry by the TCPL. Further,

the load inside the mill was found to have reduced

significantly.

6.1. Assessment of impact

The operational differences, process data obtained

from the control room and the measured data on slurryand load volumes inside the mill for both pre- and post-

installation surveys are given in Table 2.

To assess the true influence of the pulp lifter design,

initially it was planned to reinstall the old grates to

isolate all the possible factors that could affect the slurry

transportation. However, due to the inability to refit

worn components into the mill, the original dischargegrates could not be refitted. Although the position of the

holes remained the same, the total grate open area was

reduced from 10.5% (14.7% total, 4.2% pegged) to

7.9% (8.2% total, 0.3% pegged).

Considering the shortened mill length (from 3.66 to

3.48 m), smaller grate hole size (from 26.8 to 18.5 mm)

and reduced grate open area (from 10.47% to 8.16%),

the load volume is expected to be more than the pre-

installation condition for the same feed rate. Howev-

er, a significant reduction in the load volume (from

40% to 15.41%) was observed. The possible reasons

for this are explained by analyzing the grinding pro-cess during both pre- and post-installation conditions.

This is discussed and schematically described in the

following.

There are a number of breakage mechanisms that

have been reported to cause size reduction in ag/sag

mills (Digre, 1969; Stanley, 1974). In a broad sense, all

the different mechanisms can be divided into two

principal groups based on the type of product they

generate, as shown below:

♦ Coarser product — impact breakage♦ Finer product — chipping, abrasion and attrition.

A large amount of size reduction occurs by impact of

coarse media and grinding balls falling from shoulder

position onto a bed of media in the toe region of the

charge. The grinding capacity of ag/sag mills largely

depends on impact breakage whose efficiency depends

on how well impact energy is imparted to the target

rocks at the toe.

In the pre-installation operation with RPL, the impact

energy of the falling media particles from the shoulder

position gets dissipated into the dense slurry pool

Table 2

The process and operating data from the pre- and post-installation

surveys

Parameter Unit Pre Post

Throughput tph 390 390

Gross power kW 2400 2430Power usage kW h/t 6.15 6.23

Total load % vol 40 15.41

Ball load % vol 12.4 12.4

Rock and slurry % vol 32 8

Mill diameter m 7.73 7.73

Mill length (EGL) m 3.66 3.48

Mill speed % critical 70 70

Grate open area % 10.5 8.2

Average hole diameter mm 26.8 18

Fig. 15. Slurry level inside the mill after crash stop (post-installation).




present near the toe region (Fig. 16a), instead of being

used to cause breakage of particles. This inefficient

usage of grinding energy reduces the grinding capacity.

However, with the TCPL installed, the slurry pool

completely disappeared due to stoppage of the flow-

back process. In absence of excessive slurry pool, theimpact energy of the falling grinding ball/particles will

be efficiently utilised in the breakage of particles

(Fig. 16 b). Thus the increased impact breakage of

particles reduces the coarse ore in the load resulting in

reduced volume of total charge inside the mill.

Further, breakage of fine particles due to attrition is

also expected to increase as the probability of particles

getting caught in the shearing layers of balls and rocks

of the tumbling charge increases due to the presence of

slurry within the interstices of the grinding media.

The improved breakage of coarse and f ine particles due to removal of slurry pool, created by

efficient slurry transportation with TCPL can be seen

in size analysis data of different streams as given in

Table 3.

6.2. Mill operation with TCPL

The significantly lower operating load volume in the

mill with TCPL has provided opportunity for increase

in throughput. To optimise the grinding capacity, the

mill feed rate was increased at increments and the mill

operation at 450 tph was found to be achievablewithout overloading the mill in either power or slurry

pooling. The mill load of 27% was estimated at

450 tph without excessive slurry on surface of the

charge.

Denis et al. (2001) have reported an average of

470 tph throughput over a period of one year with peak

operation at as high as 520 tph for a 1 week duration

with load cells installed which allowed better control of

the mill. The operating data of the mill over 24 h of

continuous operation at an average of 510 TPH is

graphically shown in Fig. 17.

It can be observed from Fig. 17 that the mill was

running consistently as long as uniform feed was

provided, as indicated by the bin level. It is knownthat there will be a segregation of coarse particles along

the periphery when a stream of crushed ore falls into a

bin or a stockpile. Consistent maintenance of its level is

essential to provide a uniform feed to the system. If the

level goes down significantly, the segregated coarse

particles start dominating the bin's discharge, which

enters the mill. The same thing has occurred when the

bin level dropped from 92% to 50% approx (around

9:24 AM — Fig. 17) resulting in significantly coarse

feed to the mill which leads to an overloading situation

as the coarse particles need more residence time to

break to the size of grate aperture. Due to increase inthe load, the mill draws more power and once it reaches

the set point (in this case 2900 kW) the control system

reduces the feed rate to bring the system back to

normal.

The average power draw at 510 tph feed rate was

observed to be 2814 kW which gives the specific energy

PoorImpact

Poor

Attrition

SlurryPool

Pre Installation

Impact

Attrition

Post Installation

a b

Fig. 16. Slurry profile a) Pre-installation and b) post-installation of TCPL.

Table 3

DSM feed and product sizes before and after TCPL installation

Parameter 80% passing size (mm)

Pre Post

DSM feed 1.968 1.328

DSM oversize 4.165 3.459DSM undersize 0.249 0.246




of 5.52 kWh/t. Comparing this value with that of the

pre-TCPL installation (6.15 kWh/t), there is a signif-icant saving of energy, which amounts to 7.7 MW h

per day.

7. Summary and conclusions

Proper design of the pulp lifter discharge system is

essential for successful mill operation. A new design of

pulp lifter called the Twin Chamber Pulp Lifter (TCPL)

overcomes the slurry transportation problems associated

with conventional pulp lifters (Radial and Curved) in

grate discharge mills. The experimental results obtained

from both laboratory and pilot scale mills have amplydemonstrated several advantages of the TCPL over

conventional designs.

• TCPL eliminates the flow-back process, which is

unavoidable with radial and curved pulp lifters — the

conventional designs.

• TCPL allows the mill to operate as close as possible to

its maximum flow capacity at any operating condition

compared to conventional pulp lifter designs.

• With TCPL the dependency of the pulp lifter's

performance on the grate design and the volume of grinding media inside the mill can be eliminated.

This leaves the grate design as the major controlling

factor for mill capacity, which is relatively easier and

less capital intensive.

• TCPL can be precisely designed to handle the re-

quired flow capacity during the design stage.

The World's first industrial installation of TCPL in

26 ft diameter sag mill at Wagerup Refinery of Alcoa

world alumina has proved the advantages of TCPL at

industrial scale. With TCPL, the mill throughput has

increased from 390 (with RPL) to 470 tph on average by

ensuring the best grinding environment inside the mill

without slurry pool. Peak operation at as high as 510 tphfor 1-week duration were also achieved depending on

the type of bauxite ore. Mill power consumption on

kW h/t basis has dropped by approximately 15–20%

with increase in mill throughout. This trial has also

demonstrated the ease of design and retrofitting of

TCPL in existing mills to improve their operation.

Acknowledgements

The fellowship provided by AusAID and the finan-

cial support of the sponsors of the AMIRA P9L project

at JKMRC are gratefully acknowledged. The authors arealso grateful to ANI Mineral Processing for providing

the pilot sag mill and to Wagerup Refinery, Alcoa World

Alumina, Western Australia for conducting the world's

first industrial trials.

References

Austin, L.G., Klimpel, R.R., Luckie, P.T., 1984. Process Engineering

of Size Reduction: Ball Milling. SME-AIME, New York.

Denis, N., Morrell, S., Chapman, B., Latchireddi, S., 2001. The

development and installation of the Twin Chamber Pulp Lifter at

Alcoa. Proc. SAG'01, Vancouver, Canada.

Digre, M., 1969. Autogenous grinding in relation to abrasion con-

ditions and mineralogical factors. Proc. Autogenous Grinding

Seminar, Trondheim, p. A1.

Latchireddi, S.R., 2002. Modeling the performance of grates and pulp

lifters in autogenous and semiautogenous mills. PhD Thesis,

University of Queensland, Australia.

Latchireddi, S.R., Morrell, S., 1997a. A laboratory study of the per-

formance characteristics of mill pulp lifters. Minerals Engineering

10 (11), 1233–1244.

Latchireddi, S.R., Morrell, S., 1997b. A new design of pulp lifter for

grate discharge mills. Sixth Mill Operators Conference, Madang,

PNG, pp. 57–61 (October).

Latchireddi, S. R. Morrell, S., in press-a. Slurry flow in Mills:Grate_only discharge mechanism (Part-1). Minerals Engineering.

0

100

200

300

400

500

600

6:24 AM3:24 AM12:24 AM9:24 PM6:24 PM3:24 PM12:24 PM9:24 AM

Bin Level (%)

Feed (TPH)

Power (x10 Kw)

z

Fig. 17. Mill performance at 510 tph of feed rate.




Latchireddi, S. R. Morrell, S., in press-b. Slurry flow in Mills:

Grate_Pulp lifter discharge mechanism (Part-2). Minerals

Engineering.

Morrell, S., Kojovic, T., 1996. The influence of slurry transport on the

power draw of autogenous and semi-autogenous mills. Proc.

SAG'96, Vancouver, Canada, vol. 1, pp. 378–389.

Rowland, C.A, Kjos, J.M., 1975. Autogenous and semiautogenous mill

selection and design. Australian Mining, pp. 21–35 (September).

Stanley, G.G., 1974. The autogenous mill — a mathematical model

derived from pilot and industrial scale experiment. PhD Thesis,

University of Queensland, Australia.


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