Optimisation of the Sossego SAG mill

I-39 SAG 2006 DEPARTMENT OF MINING ENGINEERING UNIVERSITY OF BRITISH COLUMBIA Vancouver, B. C., Canada I-40 process 41,000 metric tons per day which is equivalent to 15 million metric tons per year from an open pit mine. The proved reserve is 255 Mt with an average grade of 1.0% Cu and 0.3 g/t Au. The ore is a granite with significant incidence of magnetite, with typical ball mill work indices of 17 to 20 kWh/t and very high indices of abrasiveness. Operation of Sossego mine and processing plant started in April 2004 targeting an average concentrate production of 540,000 metric tons per year with 30% Cu and 8 g/t Au. Industrial Circuit OPTIMISATION OF THE SOSSEGO SAG MILL Homero Delboni1, Marco A. Nankran Rosa2, Maurício G. Bergerman3, and Rinaldo P. Nardi4 1 Consultant – HDA Serviços Ltda., Lecturer – University of São Paulo, Brazil; 2Metallurgist, Companhia Vale do Rio Doce, Canaã dos Carajás, PA, Brazil; 3Metallurgist, Companhia Vale do Rio Doce, Canaã dos Carajás, PA, Brazil; 4Metallurgist Companhia Vale do Rio Doce, Belo Horizonte, MG, Brazil ABSTRACT Sossego was the first CVRD SAG mill operation to process copper-gold ore. It is located in Para state, south east of Amazon region in Brazil. The comminution circuit was designed to treat 41,000 metric tons per day comprising primary crushing, one 38 ft SAG mill operating in closed configuration with cone crushers, followed by two 22 ft ball mills. The circuit was designed on the basis of data obtained from a consistent pilot plant campaign. The ore variability was also assessed through simulation using data from a drilling core characterization program. After commissioning it became clear that a number of variables were contributing to the low throughputs obtained in the SAG mill. The role of SAG mill grate open area and aperture was evaluated in detail. This paper describes the main aspects of designing the circuit, as well as the work carried out at the industrial grinding circuit to increase its performance. INTRODUCTION Sossego is the first CVRD project in copper business. It is based on a copper-gold resource discovered in early 1997 comprising Sequeirinho and Sossego adjacent ore bodies. The mine is situated approximately 70 km south-west of Carajás area, near the town of Canaã dos Carajás, in the south of Para state, Brazil. The concentrator was designed to Sossego industrial plant comprises a typical high-tonnage primary crushing-SABC-flotation circuit based on large capacity equipment in each unit operation. A brief description of Sossego circuit follows. Primary crushing Run-of-Mine ore is delivered to a 6089 gyratory crusher by 240 ton reardump trucks. The primary crusher operates under nominal closed side setting of 5.5” which results in a P80 of 125-150 mm. Crushed ore is conveyed to a conical pile adjacent to the concentrator by a 4 km conveyor belt operating at a nominal rate of 2.300 t/h. Crushed ore stockpile The conical ore stockpile has a live capacity of 41 kt, equivalent to 24 h of plant operation. Three tandem apron feeders are located underneath the stockpile feeding a single conveyor belt that supplies the SAG mill. Grinding The grinding circuit consists of a single line configured as SABC with a nominal capacity of 1,841 t/h. A 38 foot diameter by 23 foot (EGL) SAG mill is driven by a 20 MW gearless motor. The SAG mill discharges onto two 12’ x 24’ horizontal vibrating screens whose oversize is conveyed to the recycle crushing building where two MP800 conical crushers reduces the pebbles to a nominal P80 of 12 mm. The crushed product is conducted to SAG mill feed closing the circuit. The screen undersize is pumped to two separated nests of 33” hydrocyclones. Each nest underflow is directed to one of the two 22’ x 32’ ball mill, equipped with 8 MW motor single pinion fixed speed drive. The targeted P80 of hydrocyclone overflow is 0.21 mm at 40% solids. Flotation and Regrinding The hydrocyclone overflow from each ball mill circuit feeds the rougher stage consisting of seven 160 m³ cells. Rougher tailings from both lines I-41 I-42 are sent by gravity to the tailing dam. Rougher concentrates are pumped to the regrind circuit consisting of two vertical mills (1,500 hp motor each) operating in a reverse mode with 15” hydrocyclones. The regrind circuit product, with a nominal P80 of 0.044 mm is pumped to the six cleaner flotation columns (4.27 x 10 m). The column tails feed the cleanerscavenger cells ie. One row of seven 70 m³ cells. The cleaner-scavenger concentrate is combined with the rougher concentrate, thus forming the circulating load of the flotation circuit. The cleaner-scavenger tails are pumped to the tailing dam, whereas the cleaner concentrate is pumped to a thickener. Thickening and filtering Final concentrate flows by gravity to a 20 m diameter thickener. The thickener overflow is recycled to the flotation feed and the underflow is pumped at 60% solids to the filtering plant tanks. From the filtering feed tank concentrate is pumped to two filters. The filtrate returns to the thickener and the cake with 9% moisture is stocked in a 5250 t capacity conical pile, located in a moisture controlled building. The concentrate is reclaimed by front end loaders to 35 t haul trucks which dump it in a storage building in the city of Parauapebas and then transported by a 800 km railway to the port of Itaqui, in Maranhao state. Figure 1 on the next page shows a summary of the Sossego flow sheet. Circuit Design A comprehensive pilot plant campaign was carried out at CIMM – Centro de Investigaciones Mineras y Metalurgicas in 2000 where various grinding circuit configurations and operating conditions were tested using both Sossego and Sequeirinho ore samples. The SABC configuration was selected and equipment designed to the circuit nominal capacity of 41,000 metric tons per day. It was clear from the pilot plant results that the performance of the primary grinding was strongly affected by the amount of critical size in the mill load. Accordingly, a combination of high ball charge and grate pebble port sizing resulted in a best overall performance of the circuit. The results indicated thus a competent ore as confirmed by DWT results which showed Axb of 32.2 and 49.5, respectively for Sequeirinho and Sossego ores. The results of grinding tests were the basis for equipment designing. Additionally, a study of ore variability was conducted on the basis of a drill core testing program. The variation of such characteristics on circuit performance was assessed by simulations carried out at JKSimMet simulator. Figure 1 : Sossego Flowsheet I-43 The final feasibility study was conducted by a consortium initially formed by Minerconsult and ECM, both Brazilian engineering companies, as well as with Bechtel. A review on the design criteria and methods were later conducted by Kvaerner. The main design specifications for the grinding circuit are given in Table 1. Table 1. Design specifications for Sossego Grinding Circuit SAG Mill Selected mill 38 foot diameter x 23 foot (EGL) Feed rate, t/h dry basis 1,841 Feed F80, mm 150 Motor power, MW 20 Ball charge, % 12% (operating); 15% (maximum) Speed Variable Average circulating load, % 30 Total charge, % 30 Speed, % critical Up to 82 Product, P80, mm 2.5 Specific power at motor, KWh/t 10.8 Ball mill F80, mm 2.5 Total installed power, MW 8x2 Ball charge, % 33 Ball size, inch 3 Final product P80, mm 0.21 Specific power at motor, KWh/t 8.7 I-44 Construction started in 2002 including a 80 km road between the city of Parauapebas and the mine, infrastructure for the city of Canaã dos Carajás, together with facilities to operate in a remote area. After a period of commissioning, Sossego plant started a ramp-up period in the first semester of 2004. The low throughputs obtained in the grinding circuit throughout the second semester of 2004 were attributed to a number of factors. The main ones are listed bellow: • • • Coarse size distribution of primary crusher, Limitations in SAG mill speed; Low rate of pebble extraction; In addition to the above issues variability in ore characteristics resulted in large variations of circulating loads at both primary and secondary grinding stages. Late in 2004 a comprehensive program was established in order to address each particular factor that affected the circuit performance. The analysis, actions and results are presented in the following sections. Size Distribution of Primary Crusher The highly abrasive characteristics of Sequeirinho ore was evident since the beginning of the primary crusher operation. The result of such a characteristics was that the nominal closed side setting (C.S.S.) of the primary crusher could be maintained for roughly 45 days after a new liner set was installed. After such a period the C.S.S. would increase steadily thus turning the SAG mill feed coarser. A survey program was established in order to daily assess the SAG mill feed size distribution. The obtained values would be compared with the targeted size distribution as showed in Table 2. Table 2. Targeted SAG Mill Feed Size Distribution Vertmill Product P80, mm 0.044 Specific regrinding energy, kWh/t 15.7 (Sequeirinho); 11.9 (Sossego) Grinding media Steel balls 1 Ore characteristics Bond Ball Mill WI, kWh/t JK DWT (A*b) Bond Abrasion Index 1. Grinding characterization study – CVRD/2000 16.0 33 0.48 I-45 As the feed size distribution has a great effect on SAG mill performance the liner changing frequency was then dictated by such a parameter. In addition, the blasting patterns were subsequently revised even though the drilling capacity of existing equipment was limited. Since early 2005 the powder factor has been increased, particularly in domains of high competent ores. New drilling equipment has also been incorporated to the mine thus giving more possibilities to adjust the blasting pattern. A SPLIT on-line size distribution analyzer was later installed at the SAG mill feed conveyor. All these actions resulted in a much more controlled size distribution of primary crusher discharge. Table 3 shows the results obtained during May 2006. Table 3. Current SAG Mill Feed Size Distribution I-46 Accordingly, the 32 grate discharge plates originally installed in the SAG mill operated from March to December 2004. Since this period a number of modifications were introduced in the grate design by increasing the open area and slot aperture in order to reduce the accumulation of pebble size material in the SAG mill charge. Table 3 shows in the next page the developments in discharge grate implemented at the Sossego SAG mill, briefly described as follows. First Generation Each plate of the first generation discharge grate included 16 slotted apertures of 2.5 inch. The plates were 3 inch thick made of Cr-Mo steel with a reported 325-375 HB hardness. Two complete sets of grates were installed respectively in the start-up and later in September 2004. During the period that the first generation grates were in operation, the average SAG mill throughput was 1,205 t/h. Second Generation Following a progressive change in the grate design, the first 10 plates of the second generation of discharge grates was installed in December 2004. The design of second generation included 3 inch slots as well as four 90 x 100 mm pebble ports. SAG Mill Speed Due to electrical problems related to the wrap-around motor, the SAG mill speed was limited to 10 r.p.m. throughout 2004. As the mill achieved its full speed ie. 10.3 r.p.m. the operators started to use high speed to cope with hard ore. However, the combination of high speed (up to 82% of critical) with high lifter bars resulted in a severe breakage of SAG mill liners. A different strategy was then implemented. Under such circumstances ie. Hard ore and high mill speed, the operators would run the mill aiming a total load of 32%, as opposed to an average of 27% for softer ore types. Pebble Extraction One of the main characteristics of the SAG mill operation in late 2004 was the severe accumulation of pebbles in its charge. Although no surveys were carried out on mill load it was evident the excessive amount of -3”+1” material in the mill chamber. Even though the circulating load averaged 20-25% it represented less than 300 t/h for a typical 1,100 t/h SAG mill new feed. An examination of the discharge grate design indicated an 8.6% open area with 2.5 inch slotted apertures. The encouraging results motivated the change of all 32 plates in February 2005. Such a sequence is shown in Table 3 ie. a combination of first and second generation followed by a full set of second generation grates. The open areas were respectively, 8.9% and 9.7% of the mill section. Accordingly, production rates increased to 1,247 and 1,416 t/h, the latter a 17% rise as compared with November 2004 figures. Third Generation The third generation of discharge grates introduced a further increase in the open area as well as combination of 3 and 3.5 inch slotted apertures, together with the four pebble ports designed for second generation plates. Plate thickness was changed to 4 inch. The strategy of progressive changing the plates was also adopted for the third generation. Accordingly, three combinations of grates were used, as listed in Table 3 (next page). The first combination installed in May 2005 included 22 second generation plates and remaining 10 plates of third generation. The resulted open area was 10.2% and the SAG mill throughput for the three month period averaged 1,559 t/h. I-47 I-48 All other two combinations of third generation grates were due to the accelerated plate wearing which forced the use of available plates at the local warehouse. In August 2005 a second configuration was installed. In this case first, second and third generation plates were included, respectively 18, 1 and 13 plates. The resulted open area was 9.7%. To prevent a reduction in SAG mill capacity due to a smaller opened area the SAG mill ball charge was increased to 12%. Consequently the average mill throughput was 1,572 t/h, leveling thus to the first grate combination. The third and final combination of third generation grates provided a 10.5% open area by using 9 plates of first generation together with 23 plates of third generation. The SAG mill production increased to an average of 1,639 t/h from September to December 2005. The pebble crusher operated at levels of 500 t/h, therefore a significant change if compared with 2004 figures. Fourth and Fifth Generations In December 2005 a full set of fourth generation grates was installed at Sossego SAG mill, followed by a fifth generation in February 2006. The difference between them was the manufacturer that provided the grates. During part of such a period the plant was operating with only one ball mill thus limiting the entire production. In February 2006 the SAG mill ball charge was further increased to 13.5%. The combination of higher ball charge, relatively high speeds and inadequate lifter profile resulted in severe liner damage and grate plate breakage. It was then decided to change the grate design to reinforce the “foot” of the plates as well as increasing the distance between contiguous apertures. Sixth and Seventh Generations The next two generations showed a progressive reduction in total grate open area as a function of new design. Plate thickness was increased to 6 inch. The mill speed was limited to 80% of critical value. Even though the grate was reinforced and speed limited, the most important strategy adopted to prevent both liner and grate breakage was to operate with a higher load level. Accordingly, the operators would run the mill targeting a 32% load, instead of a previously maximum of 27-28%. I-49 The implementation of such a strategy ceased damaging and breakage of liners and grates, as well as increased the mill throughput even under reduced open area and apertures. Figure 2 shows a graph that summarizes the relationship between grate open area and mill production, the latter calculated from Sossego’s monthly reported data base. 1800 SAG mill throughput flexibility to the operators by modulating the energy balance between the two grinding stages. After the implementation of new screening stage the SAG mill screen aperture will be addressed in detail. The current 12 mm mesh may be changed to adjust the amount of fines returning to the SAG mill. As the average grade of magnetite in both Sossego and Sequeirinho ores is 15%, the amount of magnetic pebbles separated by the magnets installed recycle crushers conveyor system is significant. Such material is mixed with broken and worn out steel balls. Tests will be carried out to evaluate the performance of optical sorters in separating the two materials. 1900 1700 1600 8% Ball Charge 1500 10% Ball Charge 13.5% Ball Charge ACKNOWLEDMENTS The authors wish to thank CVRD for permission to publish this paper. 1400 REFERENCS 1300 Delboni, H., 2004, Assessment and Optimization of the Sossego Grinding Circuit. Report (in Portuguese). 1200 1100 8.0 8.5 9.0 9.5 10.0 10.5 11.0 Grate open area (%) Figure 2. SAG mill throughput as a function of grate open area As shown in Figure 2 the grate open area has a strong effect on Sossego SAG mill throughput. The nominal capacity of the circuit should be achieved with a combination of 13.5% ball charge and 10.8% open area with 3 and 3.5 inch apertures. The mill is currently operating with 5.25 and 5.5 inch steel balls. Other issues addressed under the optimization program were: • • • I-50 Opening-up the pebble crushing circuit; Change the SAG screen aperture; Separation of magnetic pebbles and steel. An additional screen is currently being installed at Sossego plant in order to open-up the pebble crushing circuit. Accordingly the pebble crusher product will be conveyed to the new screen whose oversize will return to the pebble crusher, whereas the screen undersize will flow to the sump/pump that feeds the ball mill circuit. Such an option will give more