I-133

SLURRY POOLING AND TRANSPORT ISSUES IN SAG MILLS

Malcolm Powell1 and Walter Valery2

1Head Comminution Group, Mineral Processing Research Unit, University of Cape Town, Rondebosch, 7700, South Africa, mpowell@chemeng.uct.ac.za. 2General Manager, Metso Minerals Process Technology Australia & Asia-Pacific, 24 Lavarack Ave, Eagle Farm, Brisbane, QLD 4009, Australia. Walter.valery@metso.com

ABSTRACT

Slurry pooling, excessive accumulation of slurry in the mill, and the associated loss of throughput and grind, are well recognised on low-aspect, single-stage SAG mills. However, it is becoming apparent that a wide range of the high-aspect mills also suffer from pooling issues. This is associated with the high throughput of mills treating less competent ores, excessive pebble porting, and large (>36ft) mills being closed with cyclones. Data are presented from a number of mills operating in different applications to support this contention. The issue raised is that mill throughput is controlled by discharge capacity, and a better understanding of this is required to enable higher mill throughputs.

INTRODUCTION

It is generally thought that only low aspect mills suffer from slurry pooling, and even for those mills most operators are unaware the existence of pooling in their mills. It is the contention of the authors that milling equipment and mill liner suppliers have an inadequate knowledge of the issues surrounding transport issues in general in mills, and are not adequately responding to the case histories that are clearly indicating that many mills are suffering from discharge issues. It is the aim of this paper to highlight, through some well documented case studies and

I-134 some empirical correlations, the transport limitations and limiting factors that are coming to light in high throughput mills.

DEFINITION OF TERMS AND THEIR SIGNIFICANCE

First, it is appropriate to clearly define some of the terminology that is used in this field of application, and to clarify its usage and significance.

Slurry pooling The development of a pool along the length of a mill, arising from slurry in excess of what can be held within the grinding charge. The issues surrounding slurry pooling were highlighted by Morrell and co-workers, and the consequences are elucidated in a number of publications, Latchireddi and Morrell (1997), Morrell et al. (1996, 2000).

Slurry pool

Poor impact

Reduced attrition

Cataracting material

Toe

Shoulder

Figure 1 Slurry pooling

Figure 1 illustrates the notion of slurry pooling. In this end view of a laboratory mill, the shaded area shows the zone occupied by slurry. This has overrun the charge volume and formed a slurry pool in the toe region. This is the impacting region, so results in impact grinding being considerably reduced through the falling cataracting material splashing into a pool instead of crashing onto the solid toe region of the charge. In the bulk region of the charge that is ascending to the shoulder there is considerable attrition through shearing of layers. This action is

DEPARTMENT OF MINING ENGINEERING UNIVERSITY OF BRITISH COLUMBIA

Vancouver, B. C., Canada

SAG 2 0 0 6

I-135 responsible for the production of fine material. The pooling causes a dilution of the slurry leading to a lowered viscosity, and the well know associated drop in milling efficiency. Additionally, the pool running the length of the mill washes suspended particles, (particles up to 500µm are easily suspended in a flowing slurry) straight out through the mill. The pool resides on the opposite side to the bulk of the charge and consequently produces a counter-torque that results in a reduction of mill power. Techniques of detecting slurry pooling are presented in Powell et al. (2001). In essence the slurry level should be just below the charge level after a crash stop. The sum effect of these pooling effects is reduced throughput, coarsening of grind, and a drop in power.

Crash stop The mill feed, all mill inlet water, and the mill are stopped simultaneously. This is often referred to in surveying procedures and in the literature, but the authors have found that few operators actually appreciate the meaning of a true crash stop. What is not appreciated is the term ‘simultaneously’, often this is loosely interpreted to mean “around about the same time, preferably within a minute or so”. The issue is that once the feed is stopped a mill can pump out a significant portion of the resident slurry in only a few rotations. Generally crash stopping entails tripping the mill, closing off inlet water, and stopping the pumps to cyclones closed with the mill. A check should be made that belt wash water is on the auto-valve, or it should be switched off manually at the time of the stop. The auto switching off of the water valves should be checked beforehand, to ensure that they do switch off properly and to allow for shut-off time. It is sometimes necessary to activate the valves first, and stop the mill as they are almost closed. As the cyclone underflow has to be stopped, the sump pumps have to be stopped, a particularly unpopular move with operators. However, knocking open the pump drain valve as it stops allows the slurry to drain to spillage, and prevent blockages from occurring. Feed to screens can be maintained, as only a small mass of screen oversize reports to the inlet of the mill, and can be ignored in mill filling measurements. A crash stop is conducted to measure mill filling and slurry level, and the mill internal dimensions. The mill filling should be measured in at least 3 points along the mill, preferably by the vertical height to the roof of the mill (a laser range meter is ideal for this). The slurry level need only be measured at one point, as the slurry is horizontal.

I-136

Aspect ratio Aspect ratio is the ratio of mill diameter to mill length. High aspect mills are classic of the Americas, where often the diameter is twice the length, giving an aspect ration of 2. These are ideal for high throughputs and a coarse product to feed to a secondary ball mill for further size reduction. Medium aspect mills are common in Australia, with aspect ratios between 1.2 and 1.5. Low aspect mills are common in South Africa and Scandinavia, The length can be up to twice the diameter, to give an aspect ratio of 0.5. These mills ensure a long residence time, which yields a finer grind size. They are often operated as single stage mills that produce the final product. To achieve this they are closed with a classifier, usually a fine 1mm screen, or cyclones. When in closed circuit the mill often has to handle a significantly higher slurry discharge rate. Specific issues relating to operation of low aspect mills and the slurry pooling issues are available in Powell et al. (2001) and Mainza et al. (2006).

Discharge grate The discharge grate is designed to retain oversize material and balls in the mill, and allow product to discharge. Often pebble ports are used to allow the discharge of larger rocks, but they also allow intermediate size material to discharge. The open area of new grates can be calculated from drawings. It is not uncommon for the quoted open area figures to be markedly different to the final grate design, so beware of the figures quoted by liner suppliers. For worn grates a sample of slot widths should be taken to check the worn width. Two mm of wear on each edge for 15 and 20mm slot widths, results in a 25% to 20% increase in open area. The relative radial position is a number used to assess how close to the periphery of the mill the open area is positioned, equation 1. In effect it is the average radial position of the open area divided by the mill radius. This can be calculated for one panel, or any subset of panels that are representative of the whole grate.

[ ]slotall slots

mill

slot area x r1Rel. Radial Posn. =

total open areax

R

∑ Equation 1

Rmill = mill radius to liner plate Rslot = radial dist to centre of each slot

I-137 Morrell and Stephenson (1996) found this factor to impose a strong influence on pulp discharge capacity, so it is important to include it in any grate assessment.

Pulp lifter The pulp lifters act as a pump to lift the pulp up from the pool to overflow through the discharge trunnion They are radial vanes between the grate and the end of the mill extending out from the discharge cone to the periphery of the mill. Most pulp lifters are straight radial arms. A more efficient form of pulp lifter is a spiral or curved. To measure the depth of the lifters it is easy to insert a tape measure through a slot and push it against the rear wall. Measure the total depth to the rear wall, then measure the grate thickness and get the pulp lifter depth by difference. This should be done in a couple of radial positions to check that the chamber has a constant width. The number of pulp lifters must be counted and noted whether they extend the full radial length or, as is often the case, every second row is only half length. This layout is to prevent flow constriction at the centre of the discharge, caused by the convergence of the thick pulp lifter bars.

SLURRY POOLING ISSUE

A number of regions in the mill contribute to the hold-up and discharge of the slurry. This is presented in some detail by Condori and Powell (2006). A description of the key practical aspects is given here.

Mill charge Strangely this aspect of the slurry hold up is generally overlooked. The physical composition of the charge has a significant influence on the resistance to flow that the charge contents present. There is a huge difference in flow resistance of the slurry from AG milling to ball milling, and from a coarse to a fine mill charge. Thus in shifting from open circuit to closed with a fine screen, the slurry hold-up in a mill increases considerably, even though there is typically less than 10% recirculating load when closing with a screen. The driving force here is the finer mill charge driven by the longer residence time and recycle of sandy (1-10mm) material. This effect can also be observed when the feed to a mill changes in size distribution. This can lead to susceptible mills drifting in and out of slurry pooling. This effect was observed at the Los Bronces site in Chile, on a 34ft mill, Powell et al. (2006). During a mill circuit survey the power to the no 2 SAG mill was found to have dropped by 1.5MW, despite load creeping up and �ederate increasing at the onset of the power drop, and

I-138 then being maintained, Figure 2. Upon inspection of the mill after a crash stop it was found to be dramatically slurry pooling.

6000

6500

7000

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10:04:48 10:19:12 10:33:36 10:48:00 11:02:24 11:16:48

time

pow

er, k

W

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200

250

300

350

400

450

pebb

le d

ischa

rge,

tph

SAG 2 PowerLoadx100SAG 2 pebble dischargefeedrate/10

Figure 2 Los Bronces onset of pooling

On 6m diameter single stage AG mill closed with a 1mm screen, it was found that the mill was usually in a slurry pooling condition. However, when a coarser feed was received the mill would suddenly experience a surge in power, increasing power draw by about 400kW in less than 2 minutes, despite feedrate being held constant and mill load not changing. This could be reproduced by switching the coarse feeders on and off, and correlated directly to slurry pooling.

Grate The grate can discharge a massive flowrate, well in excess of that required by the mill. In fact an open ended mill, which means that there is only the grate retaining the mill charge and the slurry can pour out the end of the mill, discharges at an excessive rate. This was found by Mokken et al. (1975) who tackled the slurry pooling issues in South African Gold mine mills, by converting to open ended mills. They found that they had to block off a considerable portion of the grate open area to retain adequate slurry in the mill for efficient grinding. This clearly indicates that the constraint to slurry discharge does not lie with the grates.

Pulp lifters The work of Mokken et al. (1975) forms an early record of the inefficiency of the pulp lifters, for it was through their removal that the slurry discharge rate increased beyond the required limit in closed

I-139 circuit production mills with a recirculating load in excess of 200%. This was investigated in some detail on the pilot scale by Latchireddi and Morrell (2003), who demonstrated that with a grate only discharge, the maximum discharge rate was more than double that for a standard radial pulp lifter arrangement for the same slurry hold-up in the mill. What these studies highlighted is the inefficiency of the pulp lifter system. When one considers that the pulp lifters are effectively a centrifugal pump running in the reverse direction to that required by a pump, one can begin to appreciate the issue. A pump draws liquid in at the centre and flings it to the periphery, whereas the pulp lifters move slurry from the periphery to the centre. The slurry flows through the grate and into the pulp chamber. The studies of Latchireddi and Morrell (2003) demonstrated that the majority of discharge takes place at the base of the charge. From there it is lifted up but at the same time accelerated outwards by the rotary motion of the pulp lifters. This is key, as it results in a net lowered radial acceleration towards the discharge end of the pulp lifter. This increases the residence time on the lifter, the slurry flows far slower down the lifter than if it was poured onto a static sloping channel. Net acceleration = 2Acc sin(θ) rg ϖ= − For: θ= instantaneous angle of the lifter from the horizontal g = gravitational acceleration ω = angular velocity, in radians/s r = radius along the pulp lifter Thus as the mill rotates faster, although the swept area of the pulp lifters increases, which will increase pumping capacity, the flowrate off them decreases. The net effect is that the pumping capacity of the pulp lifters passes through a peak as mill speed is increased, and then decreases. This peak appears to be at over 85% of critical speed, so does not seem important to most mills. However, the pumping capacity is strongly levelling off from about 80% of critical speed, so will compromise the discharge capacity of higher speed mills.

Flowback What Latchireddi and Morrell (2003) did clearly identify is that a significant portion of the slurry does not reach the end of the pulp lifter in each revolution of the mill. Once the pulp lifter has passed the profile of the charge the very grate holes through which the pulp flowed out of the mill are now available for the slurry to flow-back through into the mill as it flows rather slowly down the pulp lifter. This effect is exacerbated by: • large holes – minimal flow resistance,

I-140 • holes positioned flush with the leading face of the pulp lifter – the

full depth of slurry flowing down the channel, it is exposed to the holes,

• holes positioned towards the centre line of the mill – the slurry has more contact time with these and is deeper towards the centre of the discharge.

Additionally, the holes towards the centre line of the mill are not exposed to charge on the inside of the mill, so have zero contribution to discharge capacity, they only contribute to flowback.

Ultimate limit There is no realistically achievable limit to the slurry discharge capacity. It has been noted that there appears to be a step increase in discharge capacity as the mill enters slurry pooling. This may correlate to the low flow resistance within the pool, that flows like a river above the toe of the charge. There is then a dramatic increase in discharge capacity as the level reaches the discharge trunnion and the mill switches to overflow discharge. From this point on the discharge rate is not controlled or limited by the pulp lifter arrangement, the slurry simply bypasses that. This has been observed on some single stage fine grinding applications where the mill is operating with a high charge filling. The symptom of this is that after crash stopping the mill, the slurry continues to flow out of the discharge trunnion. This is absolute concrete evidence that it is dramatically slurry pooling and is operating as an overflow mill. One such instance was reported by Powell et al. (2001), for a mill treating UG2 platinum ore and operating with a charge filling of over 40%. What should be noted from this discussion is that being told that a mill is managing to discharge the full slurry requirements, does not imply that it is operating without a slurry pool.

PEBBLE DISCHARGE

A throughput limiting factor in the large open circuit SAG mills is the rate at which pebbles can be discharged. Massive increases in feedrate can be achieved by discharging pebbles at as high a rate as possible, and rates exceeding 50% of the RoM feedrate can be achieved. To obtain these rates the discharge grate slots are enlarged to all be pebble ports. Generally the total open area does not exceed 10%. It is proposed that the resulting massive slots in the grates allow maximum flowback of the slurry and thus compromise slurry discharge

I-141 capacity, which then becomes the new rate limiting factor to the mill operation. Additionally, Installing ports increases the discharge requirements of the mill, as not only the extra pebbles are discharged, but also intermediate size material, that falls between the �rammel aperture and the desired pebble size – about 15 to 30mm, is discharged and has to be recycled to the mill.

SOME SITE EXAMPLES

Los Bronces The outcomes of survey work conducted at the Los Bronces mine of AngloChile have been reported, Condori et al. (2006). In this work a marked difference was noted between the slurry discharge of the two SAG mills, as illustrated in Figure 3.

Figure 3 Mill discharge and load for the Los Bronces mills

The 28ft SAG mill 1 has good discharge, with the slurry flowing out in the circled area on the photograph, and the charge being dry on the surface. Upon removing the hood that covers the discharge, the 34ft number 2 SAG mill was noted to have a violent splashing discharge. The slurry only begins to discharge after the vertical, to the left of the dotted line. This is observed on many mills, and graphically demonstrates how the centrifugal force slows down the flowrate of the slurry, to such an extent that it only reaches the centre as the pulp lifter reaches the vertical. The

I-142 slurry sprays out under pressure, indicating that the trunnion discharge is not coping with the slurry flowrate and it is producing slurry carry-over in the pulp lifter chamber. From these observations it was immediately concluded that the mill will slurry pool. Sure enough, upon entering the mill after a crash stop, a massive slurry pool was found in the mill.

Figure 4 Los Bronces SAG mill 2 discharge grate

As illustrated in Figure 4, the mill has classic massive pebble porting, typical of the operations aiming for maximum pebble discharge rate. The open area is successful at achieving this, but at an associated cost that is generally not appreciated. The mill has 14% open area, and undoubtedly a massive slurry flowback problem. In order to evaluate this issue, the superficial discharge flow velocity (SDV) was calculated. This is the volumetric flowrate per cross-section discharge open area, in m3/h per m2, giving units of m/h. As can be seen in Table 1, the value for SAG mill 1 is 25% higher than for SAG mill 2. The lower SDV in SAG 2 is due to its larger open area, 14% versus 10% for SAG 1. Interestingly dropping the open area to the same as that of SAG mill 1, results in the same calculated SDV as SAG mill 1. The SAG 1 mill was used as base case in order to fit parameters of the slurry hold-up model (Latchireddi and Morrell, 2003), after that the model was applied to the SAG 2 mill under the current operating conditions. From the simulation it was calculated that a deeper 550mm pulp lifter is required, as opposed to the current depth of 465 mm. By reducing the open area, increasing the pulp lifter depth and increasing the trunnion discharge area, the slurry discharge restriction in the SAG 2 mill should be reduced and the mill throughput can be increased. This is backed by the simulations in which the SAG 1 model was used for SAG 2, and a 10% increase in throughput was achieved. This indicates that this is a crucial area of circuit optimisation that is well worthwhile pursuing.

SAG 2SAG 1 Unutilised space

I-143 Clearly the impact of this on the pebble discharge rate has to be assessed. The procedure would be to remove slots from the inner radial edge of the grates to achieve the required open area. As can be seen in the detail on the right of Figure 4, there is space at the periphery of the grate. Placing a long slot in this area will replace the open area of two of the inner slots, and no flowback will result from this outermost slot. To achieve this, the filler ring behind the shell liner has to be reduced and the grate lengthened to slide a bit behind the lifter bar, so that the outer structure of the grate can be maintained. Chamfering of the end of the lifter bar on the end shell liner will allow the grate to be removed without removing liners.

Open vs. closed circuit In a set of work conducted at Morila Gold mine, an AngloGold Ashanti/Randgold Resources joint venture in Mali, the SAG mill was operated in closed and open circuit configurations during a period of low throughput requirements. The data is presented in Table 1, courtesy of Aubrey Mainza, of the MPRU of University of Cape Town. The open circuit has double the �ederate of the closed circuit configuration, but almost the same slurry discharge rate, due to the cyclone recycle stream, giving almost the same SDV. This seems to indicate that the absolute flowrate limit of the mill may be controlling the mill throughput.

Figure 5 Sticky, almost pooling charge in a closed-circuit high

aspect mill A further illustration of the effect of closing a circuit on the discharge capacity is provided by Figure 5. This is for a high aspect mill operating in closed circuit with a 1mm screen, and a recirculating load of less than 10%. As explained earlier, the effect of closing the circuit is to develop a considerably finer charge, and this results in an increased hold-up of the slurry in the charge. Thus it is the charge that becomes the rate controlling factor in the maximum possible slurry discharge rate. Clearly, converting a mill to a closed circuit operation immediately makes it

I-144 susceptible to slurry pooling issues, and this is not just a function of the extra flowrate from the recycle stream. The warning is that in closing a circuit, careful consideration must be taken of the mill discharge capacity, as it may be inadequate and consequently limit the grinding potential of the mill.

Influence of ball charge It is not well appreciated how much of an influence the fractional ball filling in a SAG mill has on the discharge capacity. The extremes are given by AG milling and operating in RoM ball mill mode, where the fraction of balls can exceed 0.8 of the total charge. The effect on slurry hold-up is illustrated in Figure 6. The mill in the left images was converted from AG milling to RoM ball milling, and the effect on the slurry hold-up is dramatic, with the pooling and stickiness apparent in the AG mode being replaced by a dry charge surface. This is despite the mill receiving considerably more feed in the RoM ball mill mode. The right images are two different mills, with the lower AG mill showing considerable pooling.

Figure 6 UG2 mills operating as a primary RoM ball mill (top) and

AG mill (bottom). The left images are from the same mill.

I-145

Discharge grate modification In many instances the discharge grate is progressively modified in order to steadily increase the pebble discharge rate, to meet ever-increasing production demands. From data collected during these exercises the relationship shown in Figure 7 was developed. This shows the relative pebble production at different % pebble port open area and ball charges.

0

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0 20 40 60 80 100% pebble ports open area

% s

cats

rec

ircu

latio

n

AG/SAG, HardAG, Moderate Soft

0%

4% 6%

8%

8%

8%

Figure 7 Relationship between pebble scats recirculation and open

area, for different ball charges 28ft AG mill treating a soft ore The AG mill discharge grate design in a moderately soft Pb/Zn/Ag ore plant, has been modified throughout the life of the circuit in order to increase throughput. Initially, a grate with 17mm slots was used and at one stage a grate with all 65mm ports was tested. It can be observed from Figure 7 that the pebble production increased from 20% for the grate without pebble ports to over 100% for the grate with all pebble ports. Although the grate open area was also increased from 5.6% with no pebble ports to 7.6% with all pebble ports, the fraction of pebble ports area dictates the amount of pebbles from the mill. An optimal grate design with pebble ports at 65% of the open area was found to allow high mill throughput (over 400tph) and to limit pebble production to the pebble conveyor capacity. As shown in Table 1, this mill operates at high rock load, around 40%, at high slurry filling but rarely experience slurry pooling. This is due to relatively moderate slurry recirculation around 150 – 250% sufficient discharge grate open area and pulp lifter capacity.

I-146 36ft AG mill treating a hard ore The 36ft AG mill discharge grate design in a hard ore plant, has been modified during the commissioning in order to reduce the pebble discharge to design level. The first discharge grate consisted of equal number of panels with 40mm and 70mm pebble ports. During the first days of commissioning, the mill was operated in fully autogenous mode without pebble crushing and pebble recirculation was in excess of 100%. A dramatic reduction in pebble production and increase in feed rate occurred with addition of 4% balls (Figure 7). The ball charge was then increased to 8% and the pebble production dropped below 50% of feed rate. The pebble production (t/h) was above the capacity of the discharge screen and pebble return conveyor and therefore, grates with pebble ports were gradually replaced by grates with 20mm slots until design values for pebble production and mill feed rate were achieved. The final design had 6 panels with 40mm ports, 6 panels with 70mm ports and 24 panels with 20mm slots. The total open area has decreased from 9.8% with all pebble ports to 7.4%. Plant operating experience and inspections of mill internals suggest that slurry pooling is not an issue. This mill has the highest SDV of the current database, suggesting an efficient discharge design.

CAPACITY RELATIONSHIPS

A summary of a selection of mills, with detailed information on operating conditions and mill charge filling, is presented in Table 1. The analysis presented below is in no way meant to be a modelling exercise, but rather a demonstration of some of the key drivers in discharge capacity. However, the extension of this high quality data base will be used to test existing relationships and develop new ones in the near future. Two key areas are assessed from the data; slurry and pebble discharge capacity. Both are expressed as the superficial discharge velocity (SDV), which is the volumetric flowrate per unit open area. For convenience the units used are based on the figures familiar to mill operators, of flowrate in m3/h and grate open area in m2, yielding an SDV value in m/h. The usefulness of the SDV is that for a possible open area and engineering design parameters, the likely maximum flowrate out of the mill can be calculated. Trend lines are used to give a first view of the likely key parameters, and simple linear regression analysis is used to weed out the non-dependent variables and highlight the important factors that influence the discharge rate.

I-147

Slurry discharge capacity Figure 8 shows some trends for the slurry SDV. There is a surprisingly poor correlation with pulp lifter length, as an absolute number or as a percentage of the mill length. There is a trend with the superficial discharge velocity off the lip of the pulp lifter, but the data is scattered. There is a good correlation with the fitted JKSAG mill discharge coefficient, which is encouraging. It is also noted that a couple of the sites lie well outside the main band of trends.

Slurry discharge

0

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50 70 90 110 130 150 170 190

superficial velocity, m/h

Disc

harg

e co

effic

ient

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pul

p di

sch

vel,

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m

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JKSAG disch coeffsup vel pulp lift, m/hpulp lifter, mmpulp lifter, %mill Lx50

Los Bronces 2 pooling

Amandel AG

Figure 8 Correlations with superficial discharge velocity

Te st of slurry supe rficial discharge rate fi t

50

70

90

110

130

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170

190

50 70 90 110 130 150 170 190

expe rimental

pred

icte

d

predicted super vel

out liers

Sv = 164 - 59.5*frac balls+714 pulpL/mill vol-267*rel radial posn

Adjusted R2 = 0.842Std Error = 12.6

Figure 9 Correlation of simple fit to discharge data

I-148 When conducting a linear regression, the Pulp lifter was converted to length per mill volume, as the milling capacity of a mill is directly related to its volume. The parameter of the fraction of balls in the charge was investigated, as highlighter earlier in this paper. The relative radial position of the grate open area was also assessed. A number of other parameters were assessed, and found to have little or no correlation with the SDV. Surprisingly the slurry level in the mill, and mill filling had no correlation. The relationship that was derived is presented in equation 2, and the correlation of predicted and measured data presented in Figure 9. The fraction of balls came out as a strong correlation, and it is interesting to note that this factor is not used in any current slurry discharge models. SDV = 164 - 59.5*frac balls + 714 pulpL/mill vol - 267*rel radial posn Equation 2 The reason for the two outliers (which were excluded from the analysis) is not clear, but both have unexpectedly high SDV’s. Equation 2 has great potential for quick engineering calculations, as for a given pulp discharge requirement, and possible open area (usually 7 – 9% of the mill end area), and fraction of balls, a likely pulp lifter depth requirement can be calculated.

Pebble discharge capacity The trends shown in Figure 10, show scattered trending with open area, but this figure suffers from being interdependent as the SDV includes the open area. There is no correlation with pebble port size, but a strong relationship to the recirculating load of pebbles. The two outlier sets are for mills for which the majority of pebbles are recycled with little or no crushing, hence they build up an unusually high pebble recycle load. In the linear regression the obvious factors to try were found to show no correlation to the SDV. Only the % of recirculating pebbles and the F80 were found to have any correlation with the pebble SDV. The relationship is given by equation 3. SDV pebble (m/h) = 560 – 2.8*F80 (mm) + 27* % recirc. Pebbles Equation 3 The goodness of fit is rather good and is shown in Figure 11. The F80 is an indicator of the fraction of coarse material in the feed that can form pebbles. It would be preferable to use the number of rocks in and above the pebble size class, but rather more detailed feed size information is required. It is intended to test this more rigorously. The correlation with the recirculation of pebbles was extremely strong, and using this accommodated all the data, even the outliers that were noted in the trending exercise of Figure 10 (page after next).

I-149

Table 1 Site data used in the analysis

I-150 Equation 3 can be used to calculate the required grate open area for a design pebble discharge rate, for a given feed size and an expected recycle rate.

pebble discharge

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2.00

4.00

6.00

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10.00

12.00

14.00

400 600 800 1000 1200 1400 1600 1800

superfical pebble flow rate

0

10

20

30

40

50

60

70

80

open area, m2 open area, % pebble width % recirc pebbles

Only partial pebble recrush

Figure 10 Correlations with pebble SDV

Test of pebble superficial discharge rate fit

0200400600800

10001200140016001800

0 200 400 600 800 1000 1200 1400 1600 1800

experimental

pred

icte

d

Adjusted R2 = 0.85Std Error = 140

SV pebble = 560-2.8*F80+27*% recirc pebbles

Figure 11 Correlation of simple fit to pebble SDV

site Los

Bro

nces

1

Los

Bro

nces

2

Nav

acha

b

Nav

acha

b

Mor

ila O

C

Mor

ila C

C

Gol

d or

e, S

AG

36

ft m

ill

Lead

/Zin

c/S

ilver

28

ft A

G m

ill

UG

2, A

G

Diam shell, ft 28 34 16 16 26 26 36 28 20Length, shell, ft 14 17 32 32 20 20 18 15 24Diam inside, m 8.26 10.12 4.72 4.72 7.80 7.80 10.81 8.37 5.92Length inside, m 4.189 4.722 9.49 9.49 5.47 5.47 4.95 4.03 7.1Aspect ratio 2.0 2.1 0.5 0.5 1.4 1.4 2.2 2.1 0.8cone angle, O 12 12 0 0 0 0 15 17 22.5rpm 11 10 17.3 17.3 11.35 11.35 8.8 11 13.2% crit. 76.9 74.4 88.9 88.9 74.9 74.9 68.4 75.2 75.9total, % 23.7 17.5 40.6 40.0 33.7 44.0 27.0 41.0 27.9balls, % 12.7 13.9 9.7 7.76 6.7 6.7 8 0 0balls frac charge 0.54 0.79 0.24 0.19 0.20 0.15 0.30 0.00 0.00slurry, % 21.0 23.4 39.1 42.1 33.2 40.0 21.0 19.0 30.0slurry fraction 0.89 1.33 0.96 1.05 0.99 0.91 0.78 0.46 1.08Power, kW 3917 7855 3034 2899 4876 3242 9500 4900 2580grate width 53 70 18 18 33 33 20 17 20pebble width 60 78 76 76 77 77 70 65 20open area, m2 5.44 11.17 2.45 2.45 5.20 5.20 6.90 3.80 1.45open area, % 10.17 13.9 14.0 14.0 10.9 10.9 7.5 6.9 5.3outer slot to liner, mm 250 219 240 240 243 243 350 290 170frac pebble ports 1.00 1.00 0.54 0.54 0.77 0.77 1.00 1.00 0.30rel radial posn 0.802 0.847 0.71 0.71 0.800 0.800 0.79 0.81 0.640trunnion diam, m 2.000 2.200 1.195 1.195 2.170 2.170 2.2 2 1.450trommel aper, mm 13 18 18 18 21 21 8 10 12JKSAG disch coeff 10000 6500 3000 3000 2000 2000 9800 7460 3500depth 0.375 0.464 0.300 0.300 0.270 0.270 0.430 0.300 0.200number 30 32 8 8 13 13 18 14 20shape radial radial curved curved radial radial radial radial radial% mill L 9.0 9.8 3.2 3.2 4.9 4.9 8.7 7.4 2.8RoM tph 919 1505 137 153 411 188 550 410 377F80, mm 57 57 142.0 142.0 115 145 105 120 65% -1mm 1.18 1.18 5.7 5.7 10 12 7.0 11 41A*b 38.0 38.0 84.1 81.1 37.9 37.9 35.0 88 145.0ta 0.62 0.62 0.51 0.48 0.39 0.39 0.24 0.34 1.25density 2.59 2.59 2.84 2.84 2.75 2.75 2.80 3.40 3.40solids, tph 919 1505 425 406.0 482 640 1781 1190 401% solids 65.4 67 73.1 77.1 66 79 72.3 82.6 79.6water m3/h 486 741 156 121 248 170 682 250 103total flow, m3/h 841 1322 306 264 424 403 1318 600 221pebbles, tph 81 335 25.6 33.2 71.2 49 150 205 6.6slurry %solid (no peb) 63.3 61.2 71.9 75.6 62.3 77.6 70.5 79.8 79.3% -Xm (approx 1mm) 60.6 58.7 86.46 83.94 60.0 87.6 80 59 92.8total m/hr 154 118 125 107 81 77 191 158 152solids, m/hr 65 52 61 58 34 45 92 92 81pebble m/hr 574 1158 682 885 647 445 776 1587 446sup vel pulp lift, m/h 524 549 330 285 279 266 523 391 537total 0 0 211 166 17 240 224 190 6.4slurry 0 0 192 144 0 214 205 140 4.6pebbles 8.8 22.3 18.7 21.8 17.3 26.1 27.0 50.0 1.8R

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CONCLUSIONS

In closing a SAG mill circuit, careful consideration must be taken of the mill discharge capacity, as it may be inadequate and consequently limit the grinding potential of the mill. The examples presented here show that mills can successfully operate with up to 250% circulating load, so long as adequate pulp discharge capacity is available and the grate is appropriately designed. It is generally reported that the recirculating load should be kept below 300% otherwise the mill goes off the grind, and the recirculating load then shoots up, coinciding with slurry pooling. The finer the mill product, the finer the mill charge and the more susceptible the mill is to pooling due to the charge flow resistance increasing. Higher ball loads decrease the slurry hold-up function of the mill and allow a higher slurry discharge rate. Pebble porting a mill dramatically increases its discharge requirements, and it has been shown that a large porting open area has an adverse effect on slurry discharge efficiency. It is therefore strongly recommended that the mill discharge capacity is assessed, and possibly improved through careful grate design, and possibly even modification of the discharge chamber, before an entire circuit expansion is implemented that can be doomed to never achieving the expected outcomes. The slurry superficial discharge flow velocities obtained from the data set indicate a normal range of 100 to 150m/h, but that figures of up to 200 are attainable. The pebble SDV is in the range 500 to 1200 m/h, but again a very high value of 1600m/h was obtained. In design these guideline limits should not be exceeded. The SDV relationships given by equations 2 and 3, allow a better indicator of the likely discharge capacity for given milling conditions and discharge key design parameters, and should be useful in checking supplier equipment recommendations. It is proposed to continue this work to develop more robust relationships that can link into the existing published modelling relationships.

ACKNOWLEDEGEMENTS

To the hard work of our co-workers, including Aubrey Mainza, André van der Westhuizen and Percy Condori, and of Ian Smit, of AngloGold Ashanti who was involved in a number of the surveys that contributed to this excellent data base. We would also like to thank the numerous mines who have supported the research work and allowed us to stop mills. Some of the data was collected as part of AMIRA P9 projects, and this research has been supported by the South African government Thrip funding.

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REFERENCES

Condori, P. and Powell, M.S. (2006). A Mechanistic model of SAG mill slurry discharge. Proceedings International autogenous and semiautogenous grinding technology 2006, Sep. 24-27, Ed. Mular et al, Published CIM. Latchireddi, S. R. and Morrell, S., 1997. “A laboratory study of the performance characteristics of mill pulp lifters”. Minerals Engineering, Vol. 10, no. 11, pp. 1233-1244. Latchireddi, S.R. and Morrell, S., 2003, “Slurry flow in mills: grate-pulp lifter discharge systems (Part 2)”. Mineral Engineering, 16, pp. 635-642. Morrell, S., Stephenson, I., 1996, “Slurry discharge capacity of autogenous and semi-autogenous mills and the effect of grate design”, International Journal of Mineral Processing, Vol. 46, pp 53 - 72. Mokken, A., Blendulf, G., Young, G., 1975, “A study of the arrangements for pulp discharge on pebble mills and their influence on mill performance”, J. S.A. Inst. Min. Metal., May., pp. 257-280. Morrell, S. and Kojovic, T., 1996. “The influence of slurry transport on the power draw of autogenous and semi-autogenous mills”. Proceedings of International conference on Autogenous and Semiautogenous grinding Technology, Vancouver, Canada, pp. 373-389. Morrell, S. and Latchireddi, S., 2000. “The Operation and Interaction of Grates and Pulp Lifters in Autogenous and Semi-Autogenous Mills”. In Proceedings of Seventh Mill Operators Conference. AusIMM, Kalgoorlie, Australia,pp 13-20. Powell, M.S., Morrell, S. and Latchireddi, S., 2001. “Developments in the understanding of South African style SAG mills”. Minerals Engineering. Vol. 14 No. 10, pp. 1143-1153. Powell, M.S, Condori, P, Smit, I, and Valery, W. (2006). The value of rigorous surveys – the Los Bronces experience. Proceedings International autogenous and semiautogenous grinding technology 2006, Sep. 24-27, Ed. Mular et al, Published CIM. Mainza, A.N., Powell, M.S., and Morrison, R.D. (2006). A review of SAG circuits closed with hydrocyclones. Proceedings International autogenous and semiautogenous grinding technology 2006, Sep. 24-27, Ed. Mular et al, Published CIM.