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
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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.
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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.
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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.
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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 )
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.
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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
D i s c h a r g e r a t e ( l / m )
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
D i s c h a r g e r a t e ( l / m )
RPL
Ideal
Charge volume - 30%
Open area -10%
0
100
200
300
400
500
0 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 - 15%
Open area - 3.6%
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 - 15%
Open area - 7%
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 - 15%
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 - 45%
Open area - 3.6%
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 - 45%
Open area - 7%
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 - 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.
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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
D i s c h a r g e r a t e ( l / m )
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
D i s c h a r g e r a t e ( l / m )
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
D i s c h a r g e r a t e ( l / m )
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
D i s c h a r g e r a t e ( l / m
)
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
D i s c h a r g e r a t e ( l / m )
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
D i s c h a r g e r a t e ( l / m )
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
D i s c h a r g e r a t e ( l / m
)
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
D i s c h a r g e r a t e ( l / m )
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
D i s c h a r g e r a t e ( l / m )
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.
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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.
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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).
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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.
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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).
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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
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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.
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187S. Latchireddi, S. Morrell / Int. J. Miner. Process. 79 (2006) 174 – 187