OPTIMIZATION OF THE SAG MILL CIRCUIT AT

KINROSS PARACATU BRAZIL

Marcos Paulo Gomes¹, Luiz Tavarez Junior¹, Edis Siqueira Nunes Filho², Juliana Colacioppo²

Alex Jankovic² and Walter Valery²

¹ Kinross Paracatu, Brazil

² Metso Process Technology and Innovation

ABSTRACT

Kinross Paracatu and Metso Process Technology and Innovation (PTI) have reviewed and

optimized the operating strategies for the SAG mill circuit at Paracatu operation in Brazil. The

focus of this project was to reach design throughput and final grinding product size, conditions

which have not been achieved since commissioning in the beginning of 2009.

For the first time a full circuit survey was conducted followed by a complete mass balance and

model fit, and these have been utilized to investigate possible circuit changes and alternative

operating strategies. This project looked at the process variables, from ore characterization,

through SAG mill practices and finally ball mills and cyclones strategies and resulted in

significant improvement in overall comminution performance of the Kinross Paracatu operation.

Several options were investigated using the currently available equipment. Further simulations

were performed to evaluate a circuit expansion, with the inclusion of a third ball mill.

Keywords: Grinding, simulation, process optimizatio n, SAG milling.

INTRODUCTION

Kinross Paracatu is located in Brazil, 230 km south west of Brasilia in the west portion of the

State of Minas Gerais. The ore deposit is noted for its low grade of 0.44 g/t gold. The Paracatu

deposit is a metamorphic gold system with finely disseminated gold mineralization hosted within

an original bedded sedimentary host (phyllites). Gold grains typically average 50-150 microns in

size.

RPM has two processing plants, an old one constituted of five ball mill lines and a new SAB

circuit (a SAG mill followed by two Ball mill lines). The Run of Mine (ROM) ore sent o the SAG

mill plant is transported from the open pit by trucks and dumped to a grizzly. The oversize goes

to the primary crusher (an MMD sizer with 2x500HP motors), and the grizzly undersize is

combined with the MMD sizer product, conveyed and stockpiled before being fed to the grinding

circuit.

The Kinross Paracatu new grinding circuit consists of one 11.6 x 6.7m (38’ x 22’) SAG mill with

installed power of 20MW, followed by two 7.3 x 12.2m (24’ x 40’) ball mills with installed power

of 13MW each in closed circuit with cyclones. Two cyclone clusters have 18 units each with

660mm (26”) diameter, 152/165mm (6/6.5”) apex and 254mm (10”) vortex finder.

The SAG mill product discharges to a trommel with 17 mm slots apertures. The trommel

undersize discharges to a screen with 12.5 mm of aperture. The screen undersize is fed to the

ball mills and the oversize (pebbles) return to the SAG mill. There is no pebble crusher in the

circuit.

The new plant was designed to treat 5087t/h and achieve a final P80 (size at which 80% of the

mass of material will pass through) of 74 µm, however six month after the start up the average

throughput rate was around 3000t/h and P80 about 130µm. A full optimization project was

conducted to implement changes in the plant to achieve the design throughput and final

product.

ORE CHARACTERIZATION

Representative ore samples were collected in the SAG feed conveyor belt during the full survey

and were sent to Metso Laboratory Centre in Sorocaba, Brazil, for breakage and grinding

characterization tests. Tests conducted were Point Load (PLT), Drop Weight (DWT) and Bond

Ball Mill Work Index (BWi). Table 1 summarizes the PLT, DWT and BWi results for this sample.

Table 1 – Results of Point Load Index (I50), Drop Weight Test (A, b), Abrasion test (ta) and Bond

Ball Mill Work Index BWi

Sample Is50 (MPa) A* b A*b ta BWi (kWh/t)

SAG Feed 2,2 57,1 2,0 115,9 2,21 9,1

Additional ore characterisation data were provided by Kinross and they are summarized in

Table 2. The eleven ore samples tested represent the main ore types that will feed to the plant,

according to Kinross Paracatu mining plan, for approximately the next 2 years. Ore competency

varies from medium (A*b~60) to very incompetent (A*b~200). Ore hardness varies from medium

soft (Wi~13 kWh/t) to very soft (Wi~5.5 kWh/t). These ore characterization results were used as

JKSimMet inputs to analyze the effect of the ore variability in the circuit in terms of throughput

and final product.

Table 2 - Ore Characterisation Data Provided by Kinross Personnel

BWi UCS* Number A*b ta

(kWh/t) (MPa)

1 170.7 2.59 5.42 46.6

2 187.9 2.77 5.57 23.7

3 65.3 1.14 12.3 173.2

4 159.6 1.86 8.85 73.5

5 151.1 2.5 8.31 94.9

6 71.7 1.49 10.33 107.9

7 77.6 1.53 10.61 157.8

8 58 0.96 11.84 145

9 63.8 1.17 13.07 -

10 101.4 1.97 6.52 78.3

11 162 1.44 7.98 90.8

Average 115 1.77 9.16 99.2

Maximum 188 2.77 13.07 173.2

Minimum 58 0.96 5.42 23.7

*UCS calculated from PLT (IS50) results

The relationship between Axb and Bond Work Index for the Kinross Paracatu samples is also

shown in Figure 1, including the SAG mill feed sample collected during the survey. Clear trend

can be observed, although there is significant scatter that commonly occurs when correlating

Drop Weight and Bond Work Indices. More competent ore is also harder and vice versa.

y = 0.0001x2 - 0.0763x + 15.535R² = 0.6916

4

5

6

7

8

9

10

11

12

13

50 100 150 200

BW

i (kW

h/t)

A*b

Survey

Figure 1 – BWi vs. A*b for the data provided by Kinross

CIRCUIT SURVEY

One day before the survey, the SAG mill was ground out (i.e., run without feed to grind out the

ore and only retains balls inside the mill) and the ball charge was measured. The measured ball

charge was 18.3% by volume and it remained constant for the survey on the next day.

The standard PTI survey procedure was followed. The operating conditions were observed

during the survey for a period of two hours to ensure SAG mill circuit steady state conditions.

The campaign lasted one hour, during which five samples were taken in fourteen different circuit

points as shown in Figure 2. The samples were composited to reduce the error owing to stream

variance over the sampling period.

Figure 2 – Grinding Circuit Sampling Points

All slurry and dry samples collected around the SAG and ball mill circuits were dried and sieved

to determine percent solids and size distributions. After the survey the SAG mill was crash

stopped by cutting the feed, water and stopping the mill simultaneously. Thereafter, internal

dimensions necessary to calculate mill filling and pulp-lifter filling level were measured. The total

load level in the SAG mill was 22.7% by volume and the measurement of pulp level indicated

that it has sufficient capacity to transport higher pulp rates without excessive accumulation of

slurry in the mill and formation of a slurry pool. These measurements, together with the Pins

(plant operating) data and laboratory testwork results were used as inputs for the SAG and ball

mill JKSimMet models.

The circuit mill throughput during the survey was 4704 t/h, the feed size F80 was 89mm and the

final circuit product size P80 was140µm. This is compared to the design throughput of 5087 t/h

and final product size P80 of 74 µm. The SAG mill was reducing the feed to a trommel undersize

P80 of 3 mm and producing 253 t/h of pebbles. The two ball mills were performing quite similarly

producing a final grinding P80 size of approximately 140 µm.

MASS BALANCE AND MODELING

The survey data were mass balanced using JKSimMet to confirm the data quality and estimate

any stream flow rates that could not be measured.

The Kinross Paracatu schematic grinding circuit flowsheet showing the main mass balanced

results is shown in Figure 3, the “Solids” in the legend refers to tones per hour of dry solids.

Figure 3 – Circuit flowsheet and mass balance results obtained using JKSimMet

JKSimMet SAG, screen, ball mill and hydrocyclone models were calibrated based on survey

data collected in July 2009, mass balance results, plant operating data, ore characterisation

tests and historical data from the Metso Mine to Mill Process Integration and Optimisation

Project carried out at Kinross Paracatu in 2005. The models were then used to simulate

different options to improve grinding efficiency and reduce final product size.

OPTIMIZATION TROUGH SIMULATION

Simulations were conducted to assess if a finer final product may be achievable with the current

equipment at the same throughput as obtained in the survey, 4704 t/h. The main options

evaluated were: reduction of the vortex finder, reduction of the grinding media size, ball mill

charge increasing, introduction of a pre-classification stage before the current ball mill circuit, an

option to return portion of cyclone underflow back to SAG mill and reduce SAG screen aperture.

Further simulations were then performed to evaluate a circuit expansion, with the inclusion of a

third ball mill.

A summary of the simulations performed is presented as follow:

Simulation 1 – Reduced cyclones vortex finder

A smaller vortex finder of 240mm was simulated and compared to used 260mm size. The

results indicate that the final product P80 could be reduced from the base case (circuit survey)

of 140µm to 136µm with a corresponding increase in circulating load from 337% to 391%.

Potential benefits from this change were considered modest.

Simulation 2 – Reduced ball mill ball size

The 3 inch ball size are considered to big considering soft ore and the SAG mill product size.

The simulation with 2.5 inch balls indicated a reduction in P80 from 140µm to 133µm.

Simulations with 2 inch balls were also conducted showing a further reduction in P80 to 128µm;

industrial trial was proposed to confirm benefits from smaller ball size.

Simulation 3 – Increased ball mill ball charge

Another opportunity in the ball mills is to increase ball charge to fully utilize the installed power.

During the survey period, the ball charge was measured as 28% for both ball mills and a total

power draw of 20.5MW, representing 79% of the installed power of 26 MW. Although, there is

available motor power in both ball mills, the balls ejection from the mills is excessive when ball

charge is higher than 28%. Simulations were performed for 33% and 35% of balls and ball

retainers have been evaluated. The simulations indicated that P80 could be decreased from the

base case of 140 µm to 131µm and to 128µm for 33% and 35% of ball load respectively.

Simulation 4 - Introduction of the pre classificati on stage

A pre classification option was simulated including cyclones of the same dimensions as the

existing ball mill circuit cyclones (26”). The combined P80 of pre-classification and ball mill

cyclones was reduced to 132µm. This option was not recommended because, although the final

product was reduced, with the inclusion of a pre-classification stage, the water consumption

increased by 71% to 6729 m3/h, compared with 3940 m3/h during the survey. This could not be

implemented as the plant is already water limited.

Simulation 5 – Replacement 26 for 20 inches cyclone s diameter

The existing 26 inch cyclones were not considered appropriate for the design product size P80 of

74 microns. Smaller cyclones are more efficient in obtaining finer cut. As an illustration, the old

Kinross Paracatu plant operates with 20 inch cyclones and the final product target size is

achieved. Simulation results indicated that with 20 inch a P80 of 127µm could be achieved. The

water consumption increased by only 13% compared with the base case. The number of

cyclones required increases substantially (64 total) and new cyclone clusters would have to be

installed, however this change is recommended as substantially increases the circuit ability to

produce finer product.

Simulation 6 - Partial split of cyclone underflow b ack to SAG mill

RPM SAG mill pulp lifter was designed for high capacity and the calculations based on

measurement made during the survey confirmed that the SAG mill would have sufficient

transport capacity to receive part of the cyclones underflow. The ball mills were the bottleneck

of the circuit and therefore returning a part of the cyclone underflow to the SAG mill would

increase circuit capacity by higher utilisation of SAG mill power. Simulation with 10% of

underflow returning to the SAG mill indicated that P80 could be reduced from 140 to 132µm. By

increasing it to 20%, P80 could be further reduced to 128µm.

Simulation 7 – Reduced SAG discharge screen apertur e

Two simulations were conducted with 10mm and 8mm apertures. Simulation shows that by

reducing the apertures from 12.5mm to 10mm, screen oversize increased from 284 t/h to 530

t/h, while cyclone overflow P80 was effectively unchanged. With aperture of 8mm, the oversize

increased to 798 t/h, close to the pebble conveyor belt limit, and P80 reduced to 136µm.

Simulation 8 - Simulation of cumulative effect of c hanges

Simulations using the combination of changes described above were conducted. The simulation

using reduced vortex finder of 240mm, 2,5 inches balls, 33% ball mill ball charge, returning 20%

of the cyclone underflow back to SAG were considered feasible to implement in a short term

and indicated a final product size reduction to P80 of 115µm. The simulation with 35% ball

charge, 2½” balls, return of 20% of cyclone underflow to SAG and 20 inches cyclones

presented the best option, indicating a P80 reduction to 100µm. However this option is capital

intensive, as it requires replacement of cyclone batteries and an efficient ball retainer design.

Simulation 9 - Addition of the third ball mill

The simulation of the circuit options showed that the design throughput of 5087 t/h and P80 of 74

µm could not be achieved with the existent equipment. Simulations were therefore performed

with the addition of a third ball mill in parallel to existing two. In the first case, the same ball mill

charge as the base case was simulated and the P80 was reduced from 140 to 97µm. The

second simulation was performed utilizing almost full ball mill power capacity (36 of 39MW) and

the P80 achieved was 86µm.

For all simulations, P80 and throughput were plotted and are presented in Figure 4. For the

simulation without the third ball mill, the throughput was maintained constant and all the

changes aimed at reducing P80. It can be observed in the graph that the smallest P80 obtained

was 100µm. This was achieved by simulating cumulative effects of more than one change in the

grinding circuit. For the simulations with the addition of another ball mill the main objective was

still to reduce P80, but to also increase throughput to design.

Figure 4 – Throughput and P80 for all the simulations

EFFECT OF HARDNESS IN THROUGHPUT

The influence of the ore hardness variability on circuit throughput was investigated for the

following three scenarios:

1. Current grinding circuit with no modifications,

2. Optimised circuit (reduced vortex finder of 240mm, 2,5 inches balls, 33% of ball mills

ball charge, returning 20% of the cyclone underflow back to SAG),

3. Circuit with the addition of a third ball mill.

Table 3 shows the simulated throughput for the final product size P80=80µm. The Bond Work

Index values were chosen to represent the harder ore expected in next two years (from 8.5 to

13 kWh/t).

For the current grinding circuit, with an ore hardness of 8.5 kWh/t, the estimated throughput was

3800tph to achieve a P80 of 80µm, whereas with an ore hardness of 13 kWh/t, the throughput

would be reduced to 2400tph. For a work index of 9.2 kWh/t, the maximum throughput at P80 of

80µm would be 3550tph with the current grinding circuit, whilst a throughput of 4800tph is

achievable with the addition of a third ball mill, which is close to design values.

Table 3 – Throughput for 3 scenarios and 3 different ore hardness for the same P80 of 80µm

Scenarios Throughput (tph)

BWi = 8.5 BWi = 9.2 BWi = 13

1. Current Circuit 3800 3550 2400

2. Optimized Circuit 4200 3900 2700

3. Addition of Third Ball Mill 5100 4800 3400

Based on the simulations and operational analysis, it is concluded that the design throughput of

5087tph and P80 of 80µm can only be achieved with the addition of a third ball mill. With the

future ore hardness increases, it will become more difficult to achieve these values even with a

third ball mill.

GOLD RECOVERY IMPROVEMENT

The full survey was completed in June and the recommendations mentioned in this report

began to be implemented in September 2009. Figure 5 shows the average monthly data for

gold recovery and grind product size represented by percentage retained at 200# or 0.075mm

screen size (another indication of the final product size), during the year 2009. It can be seen

that in January and February, the start-up period, the gold recovery was very low due to the

coarse feed to the flotation circuit.

Approximately one year later, the gold recovery increased from 40 to more than 70%, and the

grind size reduced from 40% to 20% retained at 200#, matching the design size. It should be

noticed the SAG average throughput was around 3000tph, which is still significantly bellow

design of 5087tph and a third ball mill would have to be installed in order to achieve the design

targets.

Figure 5 – Gold Recovery and Grinding Product Size Average Monthly Data for 2009

CONCLUSIONS

Kinross Paracatu ore is is known to be quite soft, with low resistance to impact breakage.

However, in the next two years ore hardness will increase significantly as mine is going to

deeper levels. For the new SAG mill plant processing plant, the design throughput of 5087t/h

and final product of 74µm has not been achieved since commissioning. Considering that ore is

getting harder, it is unlikely that design production levels will be achieved without significant

changes in the grinding circuit.

A complete survey of the grinding circuit was conducted in June 2009, the circuit models were

developed and optimization opportunities were explored through simulation. The following were

found to be the key actions to focus on:

• Maintaining and possibly increasing the ball charge in the SAG mill to 20%;

• Operating the SAG mill in a optimal speed range (65-75% critical);

• Reducing the ball size in SAG mill from 5 to 4 inches balls;

• Returning up to 20% of the cyclone underflow back to the SAG mill;

• Reducing the cyclone vortex finder from 260 to 240mm;

• Reducing the ball size in ball mills from 3 to 2½ inches balls, followed by a further trial of

2 inches balls if successful;

• Increasing ball mill ball charge from 28% to 33%;

With majority of these measures put in place, significant improvement in gold recovery was

achieved owing to reduction in grinding circuit product size whilst the SAG average throughput

is maintained around 3000tph. This throughout is still significantly lower compared to the design

of 5087tph. In order to achieve throughput and a final product size close to the original design,

and considering increase in ore hardness in the future, installation of a third ball mill of same

size as the existing ones will be required.

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