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468

A comparative analysis of grinding

circuit design methodologies

Miguel Becerra

Anglo American Copper. Chi le

Peter Amelunxen

Amlnpro, Chile

Over the past decade the growth and development of mining projects have increased dramatically, as have the requirements to design and develop grinding circuits, mainly in the field of SAG and ball mill

circuits. There are several design methodologies available in the market or through the use of commercial software or expert

consultants. This article presents a comparative analysis of three

methods for grinding circuit design widely used in the market today,

Millpower 2000, JKSimMet and Aminpro, applied to a new mining sector of a current Anglo American mine. The parameters were

obtained from 100 drill core composites, each representing 30 metres of drill core, in a sector that represents the first five years of

production for a new concentrator. An important feature of this material is its high hardness for SAG grinding (average SPI of 140

minutes), and this perhaps provides an additional advantage to this

study as it represents an extreme condition. The analysis compares the resulting equipment sizes for each of the three methodologies,

considering a proposed tonnage of90 ktpd at 180 micron P80.

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Procemin2012

INTRODUCTION

9"' International Mineral Processing Conference 21 -t 23 November 2012 • Santiago. Ch1le

This study aims to provide a comparison between three commonly used grinding circuit equipment sizing methodologies. The three methods, MillPower 2000, ]K SimMet, and Aminpro, were applied to the same specific dataset of ore hardness determinations and assumed the same nominal mill throughput and grind targets. In all cases the database design is the same as the current operation at Las Bronces, specifically the Confluencia grinding circuit where the current operation has a SAG mill of 40 feet in diameter and 23.5 feet in length (EGL) associated with two ball mills, 26 feet in diameter and 41.5 feet in length (EGL). The installed power of these teams is 22000 kW and 16400 kW respectively, all operated by gearless systems. The circuit has a stage of crushing pebbles composed of two team MP-1000. The normal configuration of operation is of the type SABC-B.

METHODOLOGY

Los Bronces is a copper deposit covering two districts in the Andean high zone of the metropolitan regions of Lo Barnechea (municipality of Santiago) and Valparaiso (Los Andes municipality). It is located 65km northeast of Santiago in the upper basin of the Mapocho River and the sub basin, San Francisco, belongs to Anglo American. As part of the 2011 geometallurgical program development, between 200 and 300 samples from the South sector of the mine were selected for comminution and flotation testing, among others. These samples were obtained from 30m composites of diamond drill holes with geometallurgical objectives. Several comminution tests were conducted on each sample obtained; work index of balls, bars, abrasion, SPI and SMC test index. The development analysis was made using three methodologies:

• MillPower 2000 (Barratt, 1989)

• JKSimMet (Napier Munn et al., 1996)

• Aminpro (Aminpro, 2012)

The results were compared on the basis of the equipment sizing requirements and the estimated daily throughput calculations.

RESULTS AND DISCUSSION

The summary of the results appear in Table 1 and Figure 1 through Figure 3. According to these results, the analysed selection has a high average hardness considering the values obtained from the SPI and SMC. Figure 1, Figure 2 and Figure 3 show the cumulative frequency distribution for each of the parameters obtained and used in the analysis.

Procemin2012 9"' In ternational Mineral Processing Conference 21 -. 23 November 2012 • Santiago. Ch1le

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Figure 1 Cumulative frequency distribution: SPI & Ci

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Procemin2012 9"' International Mineral Processing Conference 21 -. 23 November 2012 • Santiago. Ch1le

Table 1 Statistical summC:Uy of hC:Udness database

SPI, min Ci BWi,kWh/t RWi,kWh/t DWi,kWhJm3 Minimun value 54.1 5.2 12.2 11.9 5.3 25th Percentile value 106.7 10.1 15.6 14.7 7.4 Median value 130.0 13.0 16.5 16.7 8.7 75th Percentile value 163.4 16.5 17.4 18.5 9.6 Maximun value 424.2 36.7 22.0 24.6 11.6 Average value 137.9 13.8 16.5 16.7 8.5 Count 308.0 308.0 297.0 112.0 91.0 Variance 2131.8 23.8 2.2 5.5 2.4 SD 46.2 4.9 1.5 2.3 1.6 Variation coefficient, % 33.5 35.4 9.0 14.0 18.4

MillPower 2000

Axb 22.8 27.1 30.0 35.7 50.9 32.3 91.0 43.9 6.6

20.5

DJB Consultants Inc. performed the MillPower 2000 circuit design calculations. The design was based on models configured using detailed information collected from the current grinding circuits. The following general objectives were considered for the design work:

• Identify the total energy requirements, considering the specific size reduction work required

• Determine the adequate transfer size required for the circuit

• Determine the size of the principle equipment

• General operating conditions were assumed

Based on the proposed daily mill throughput, DJB assumes that at least 75% of the samples are above the nominal mill throughput. Considering a mill runtime of 94% and a final P80 size of 180 micrometers, the proposed circuit consists of a SAG mill 40 feet in diameter and 25 feet in length (EGL), with an installed gearless motor of 30 MW and two 28 feet diameter by 44.5 feet in length ball mills equipped with 22 MW gearless motors. The circuit would operate in SABC-B configuration with two pebble crushers equipped with at least 750 kW motors. Table 2 shows the estimated processing rates for various points on the hardness distribution and various states of liner wear. The 90,000 TPD would be achieved 25% of the time in the case of half-worn liners. Lastly, Table 3 shows the results in terms of specific energy required for the SAG mill and ball mills.

Worn liners Hall-worn liners New liners

Table 2 Summary of treatment results

25th Percenti le, tid

92300 90800

89500

Median, tid

99100 97600

96200

75th Percentile, tid

108800 107200 105600


Table 3 Specific energy consumption estimations

Worn liners Half-worn liners New liners

JKSimMet

25th Percentile, t/d EsAC EBaJts 5.71 8.31 5.67 8.34 5.64 8.38

Median, t/d 751h Percentile, t/d EsAC Eaaus EsAC EsaJJs 6.26 9.12 6.72 9.80 6.22 9.16 6.68 9.84 6.19 9.20 6.64 9.89

The current analysis is based on the breakage and selection functions for the SAG mill and ball mills used in the JKSimMet model of the current Confluencia concentrator. The hardness parameters are in agreement with the Axb, ta, and work index results shown in Table 1. The run of mine size distribution was based on historic data collected from the Los Bronces operation. With these bases and recommendations made by JKTech (Baley et al, 2009), the evaluation of the required grinding circuit was performed. For this study, the same equipment was used as was recommended by DJB in order to evaluate the operational performance. Given the above, the proposed circuit consists of a SAG mill 40 feet in diameter and 25 feet in length (EGL), with an installed gearless motor of 24 MW and two 28 feet diameter by 44.5 feet in length ball mills equipped with 22 MW gearless motors. The circuit would operate in SABC-B configuration with two pebble crushers equipped with at least 750 kW motors. The power calculations do not include any additional design factors. The installed power was estimated based on the recommendation by Doll (Doll, 2012) considering the various power losses in the transmission system. The estimated SAG mill power includes a correction for the interstitial mill filling. In the case of the estimated ball mill power, the power was calculated using the Bond methodology. Table 4 shows the principle parameters obtained. The 25th percentile throughput rate corresponds to the 75th percentile Axb value. The ball miJl power shown is that for both mills.

Table 4 Summary of obtained results, JKSimMet

25th Percentile, t/d Median, t/d 75th Percentile, t/d

Hourly treatment, t/h 3990 4135 4460 Utilisation, % 94 94 94

Daily treatment, t/d 90014 93286 100618 SAG power, kW 22027 22059 22412

Ball Mill power, kW 40596 39318 40006 Pebbles power, kW 806 806 868 Pso, !liD 181 186 193 EsAC 5.72 5.53 5.22 Eaalls 10.17 9.51 8.97

Aminpro

Aminpro's method is based on SPI and Bond work index scale-up methods as described elsewhere (Amelunxen & Meadows, 2011). The approach is based on the use of industrial plant audit data and surveys to calibrate empirical energy-based models that are based on the SPI (in the case of the SAG mill) and Bond work index (for ball mills) hardness information. The calibration data for this study

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was taken from a series of 2003 plant surveys conducted at the Los Bronces concentrator (Minnovex, 2003).

SAG Mill O~ratlng Parameters Ball Mill Parameters Food Bond Feed Model Actual Feed Model Actual

Surwy Pebble Size SPI WI % Circ. Power Rate T80 SE SE Fsag Power Rate P80 SE SE CFnet No. CI\Jsher F80 Load (shell) (Dry) (shell) (shell) (shell) (Dry) (shell) (Shell)

(yes/no) (h) (min) (kWhit) (%) (kW) (Vhr) (um) (kWhlt) (kWh/t) (kW) (Vhr) (um) (kWh/t) (kWh/t)

1 SABC-B 1.8 81 16.3 13"/o 1121 1 2488 5200 5.79 4.51 0.78 16243 2488 213 8.8 6.5 0.74 2 SABC-AB 2.1 72 15.6 7% 10755 3173 3950 5.87 3.39 0.58 16505 3173 2.23 7.8 5.2 0.66

3 SABC-AB 1.2 67 16.2 3% 10716 2970 3300 5.93 3.61 0.61 16378 2970 229 7.8 5.5 0.71 4 SABC-AB 1.7 76 16.0 7% 11327 2922 2800 6.63 3.88 0.58 16496 2922 251 7.0 5.6 0.81 5 SABC·B 2.2 90 16.0 18% 13882 2656 5750 5.98 5.23 0.87 16374 2656 242 8.1 6.2 o.n 8 SABC·B 3.1 63 14.4 13% 13059 2344 4200 5.35 5.57 1.04 16182 2344 216 7.5 6.9 0.93 9 SABC·B 4.0 68 15.3 13"/o 12945 2553 5250 5.27 5.07 0.96 16268 2553 212 8.3 6.4 o.n 10 SABC·B 35 73 14.3 12% 12932 2511 3300 0. 22 5.15 0.83 16481 2511 202 7.5 6.6 0.88

11 SABC·B 2.8 74 15.5 16% 12847 2563 5300 5.50 5.01 0.91 16276 2563 231 8.0 6.4 0.80

Figure 4 Survey information for Los Bronces operation

The 2003 data show that the SPI calibration factor (Fsag) averages approximately 0.78 for the SABCB configuration. For the ball mill calibration, Amelunxen et al. (2011) proposed a relationship between the actual Bond correction factor and the P80 back-calculated from the Bond equation and the nominal CFnet for the grinding circuit. Comparing the 2003 Los Bronces data with the Candelaria and Bagdad data points published by Amelunxen et al. shows that while the Los Bronces mills operate at lower CFnet values than SAG mill circuits, this may be due to the relatively coarse grind that was observed during the time of the circuit surveys (Figure 4). When normalised for grind (right hand side of Figure 5), the data seem to indicate that the curve of CFnet versus P80 actually above those for other SABC-A and ABC-A grinding circuits, indicating lower ball mill efficiency for constant grind size. The difference may be due to fewer fines in the transfer stream at Los Bronces, suboptimal mill operation, cyclone configuration or ball diameter, or a combination of these factors.

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• Cerro Verde 0 los Bronces

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Figure 5 Specific energy consumption comparison and Cfuet estimation from Pso

1000

The circuits were sized to achieve 90,000 TPD nominal throughputs (i.e. the median) subject to a maximum allowable P80 of 185 microns for an ore represented by 75 individual hardness points (SPI and BWI for each) with equal weighting. An assumed runtime of 95% was used.


For the SABC-B circuit, the following equipment was selected:

• One 40' by 26 (inside shell by EGL) SAG mill equipped with a 27 MW gearless ring drive (effective power draw of 26 MW meter power)

• Two MP1250 pebble crushers in optional SABC-A or SABC-B configuration

• Two 26' by 44' (inside shell by EGL) ball mills equipped with 36 MW gearless drives, operating in reverse closed circuit with hydrocydones

For the SABC-B circuit, Table 4 summarises the Aminpro recomendations:

Table 4 Summary of results, Aminpro

25th Percentile, t/d Median, t/d 751h Percentile, t/d Hourly treatment, t/h 3602 4178 4539 Utilization, % 95 95 95 Daily treatment, t/d 82170 95251 103500 SAG power draw, kW 26338 26338 26338 Ball Mill power draw, kW 34448 34448 34448 Pebbles power draw, kW 1,585 1,585 1,585 Pso, om 174 183 180 EsAG 7.3 6.3 5.8 EBaUs 9.6 8.3 7.6

CONCLUSIONS

Figure 6 shows a comparison between the three approaches, and some key similarities and differences can be observed in the data. To wit:

• The SAG mill diameters and lengths are all very similar, but the installed power requirements differ quite substantially. The MillPower 2000 estimate provides for a 40' x 25' SAG mill with 30 MW of installed power (nameplate). The JKSimMet estimate recommends a mill of the same size but with 20% less installed power, at 24 MW. The Aminpro recommendation is a slightly longer SAG mill, at 40' x 26', but with 27 MW of installed power

• In spite of the 10% difference in the installed SAG mill power, the MillPower 2000 and Aminpro specific energy estimates are almost identical, at 6.2 kWh/t and 6.3 kWh/t, respectively. This indicates that the mill power differences may not be so much a result of differences in the mill sizing results, but in acceptable transfer size assumptions

• The Aminpro ball mills are 20% smaller than the MillPower 2000 or JKSimMet resul ts, at 36MW of total installed power (versus 44 MW). It appears that the difference is not due to differences in mill motor sizing; rather, the ball mill specific energy consumption is also about 20% lower and therefore the explanation rests in the energy model scale-up and/or calibration. Review of the 2003 dataset presented above indicates that Los Bronces mills, operating under the same configuration as the circuits being sized, were consuming only 6.1 kWh/t to achieve a P80 of 224 microns when treating ores with Bond work index values averaging 15.5 kWh/t. Using the median Bond work index of 16.5 kWh/t and the same CFnet values observed in 2003, the

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estimated ball mill specific energy required to achieve a P80 of 180 microns increases to 7.6 kWh/t. While closer to the Aminpro estimate than to the others, this value is still substantially less than all three of the scale-up estimates derived for this study

Parameter Units Mill Power 2000 JK SimMet Aminpro

Utilization % 94% 94% 95%

Median Th roughpue TPD 97600 93286 95251

Median Th roughpue tonnes/hr 4326 4135 4178 SAG Mil l no x diam x length 1x 40' x 25' 1x 40' x 25' 1x 40' x 26' SAG Mill Power(instal led) kW nameplate 30,000 24,000 27,000 SAG Specific Energy kWh/t (median) 6.22 5.53 6.3 Ball Mills no x diam x length 2 X 28' X 44.5' 2 X 28' X 44.5' 2x 26' x 44' Ball Mill Power (installed) kW nameplate 44,000 44,000 36,000 Ball Mill Specific Energy kWh/t (median) 9.2 9.5 8.3

1 assuming half-worn liners for Millpower 2000

Figure 6 Comparison between different methodologies analysed

ACKNO~EDGEMENTS

The authors wish to acknowledge Anglo American for the permission to publish this paper.

REFERENCES

Amelunxen, P., & Meadows, D. (2011). Not another HPGR trade-off s tudy! Minerals & Metallurgical Processing, 28(1), 1-7.

Amelunxen, P., Mular, M., Vanderbeek, J., Hill, L , & Herrera, E. (2011). The effects of ore variability on HPGR tradeoff economics. SAG 2011 . Vancouver.

Aminpro. (2012). Evaluation of Throughput Estimate for Los Bronces & Confluencia Plants.

Santiago.

Baley, C., Lane, G., Morrell, S. & Staples, P., (2009) 'What Can Go Wrong in Comminution Circuit

Design?', Tenth Mill Operators' Conference, Adelaide, Australia

Barrat, D. (1989) 'An Update on Testing, Scale-up and Sizing Equipment for Autogenous a.ns Semi

Autogenous grinding Circuits', Proceedings of the 1989 SAG Conference, Vancouver, Canada

Doll, A., (2012) 'Technical Memorandum: Mill Motor Power Definitions', Rev.S

Minnovex. (2003). Los Bronces SAG & Ballmill Benchmark Study. Toronto: Minnovex Technologies Inc.

Napier-Munn, T., Morrell, S., Morrison, R. & Kojovic, T., 'Mineral Comminution Circuits - Their

Operation and Optimization', Julius Krttschnitt Mineral Research Center, Australia

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