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Biooxidation of arsenopyrite concentrate using BIOX® process: Industrial experience in Tamboraque, Peru
Hydrometallurgy 83 (2006) 90 – 96 www.elsevier.com/locate/hydromet
Biooxidation of arsenopyrite concentrate using BIOX® process: Industrial experience in Tamboraque, Peru Martha E. Ly Arrascue a,1 , Jan van Niekerk b,⁎ b
a Golder Associates Peru S.A., Peru Gold Fields Ltd., 24 St. Andrews Road, Parktown, 2193, Johannesburg, South Africa
Available online 18 May 2006
Abstract The Tamboraque Plant is located in the mining district of Viso-Aruri, approximately 90 km east of Lima, Peru, at an altitude of 3000 m above sea level. In 1980 the previous owners of the mine and processing plant, Minera Lizandro Proaño S.A. (MLPSA), initiated an investigation into the available technologies for the recovery of refractory gold from old tailings. The Coricancha mine is a polymetallic ore deposit and lead and zinc concentrates can be produced by flotation. The zinc flotation tailings can be treated further by differential flotation to produce an arsenopyrite concentrate containing the bulk of the gold. The “TAMBORAQUE” project involved the expansion of the ore treatment capacity of the plant from 200 to 600 ton/day. The expansion also involved the installation of a separate flotation section for the zinc tailings and a BIOX® and CIL plant for the treatment of the arsenopyrite concentrate. The first phase of the project involved the installation of a flotation and BIOX® plant for the treatment of an existing zinc flotation tailings dump accumulated over the years. The second phase involved the expansion of the capacity of the existing plant to treat fresh ore from the mine. Initial batch tests indicated that the arsenopyrite concentrate could be treated successfully using biooxidation. A 50 kg/day biooxidation pilot plant was commissioned in Lima, using a native bacterial culture isolated from acid mine drainage from the Coricancha mine. MLPSA operated the pilot plant successfully for 20 months. At this stage MLPSA contacted Gencor (now Gold Fields Limited) for assistance in the optimization of the process. The continuous testwork indicated that 90% gold recovery can be achieved after only 80% sulfide oxidation. A licence agreement was concluded between MLPSA and Gencor in 1995 for the use of the BIOX® technology at the Tamboraque plant. As part of the agreement, Gencor carried out the process design of the biooxidation plant. Continuous plant operation commenced at the end of August 1998 after successful commissioning of the plant. The feed rate through the plant was, however, below the design due to concentrate shortages. The plant reached the design capacity during 2002, achieving 85% gold recovery on a bulk pyrite and arsenopyrite concentrate produced from fresh ore from the mine. The mining and metallurgical activities were, however, shut down in October 2002 due to financial and mining related problems. © 2006 Elsevier B.V. All rights reserved.
1. Introduction ⁎ Corresponding author. E-mail addresses: [email protected], [email protected] (M.E.L. Arrascue), [email protected] (J. van Niekerk). 1 Wiese Sudameris Leasing until 2003. 0304-386X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.hydromet.2006.03.050
The Coricancha mine is located in the Mining District Viso Aruri, at an altitude of 4000 m above sea level, and the Tamboraque plant, at an altitude of 3000 m above sea level. The plant is located approximately 90 km east of the city of Lima on the Central highway, a few
M.E.L. Arrascue, J. van Niekerk / Hydrometallurgy 83 (2006) 90–96
kilometers from the town of San Mateo. The mine has been almost constantly in production since the Spanish Colonial Stage. Ownership of the mine was first held by Negociación Minera Lizandro Proaño until 2000, when ownership was transferred to Wiese Sudameris Leasing. 1.1. Mineralogy The Coricancha mine consists of a filoneano field (low sulfurization type) with quartz veins and sulfide polymetallics of Au, Ag, Cu, Pb and Zn that filled the main system fractures (Constancia and Wellington) as well as others of tensional type (Rocío, Colquipallana, etc.), that cross volcanic formations of the Rímac Group. The gold is mainly found as finely disseminated particles within the crystal structure of the arsenopyrite, causing it to be refractory. A small fraction of the gold is associated with pyrite, but is rarely associated with silver, galena and sphalerite [1].
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was, however, causing environmental problems due to the arsenic. The first flotation cells were installed in 1930 to separate the arsenopyrite from lead and silver. MPLSA, a small lead and zinc concentrates producer, initiated an investigation to evaluate alternative process options for gold recovery from its flotation tailings. The gold bearing tailings was subjected to a differential flotation process to obtain a pyrite concentrate and an arsenopyrite concentrate. Both of the concentrates were highly refractory, achieving less than 10% gold recovery after direct cyanidation. The arsenopyrite concentrate, due to its higher gold content, was selected for further testwork and subjected to roasting, pressure leaching and bacterial leaching processes. Based on the laboratory results, biooxidation was selected as the preferred process route due to the technical and economic advantages the process offered. 1.3. Process description
1.2. History Mining activities in the Viso Aruri district started in the middle IX century, initially developing the zones rich in silver. The tenacity and impulse of Don Lizandro Proaño resulted in the startup of the Tamboraque smelter in 1906. Operation of the smelter
The ore from the Coricancha mine was previously treated at a rate of 200 ton/day to produce lead and zinc concentrates. The zinc flotation tailings, still containing 3.14 g/ton gold and 33.9 g/ton silver, was deposited on Tailings dams 1 and 2 for further processing at a later stage.
Crushing
Ore Mine
Milling Lead Concentrate
Lead Flotation TAILINGS DAM
Zinc Concentrate Zinc Flotation Pyrite concentrate
Pyrite Flotation
General flotation tailings
Arsenopyrite Flotation
BIOX® Process
overflow
Neutralisation
Neutralisation Residue
underflow
Gold in activated carbon
′ Cianuracion (CIL)
Cyanide Destruction Caro′s Acid
Cyanidation tailings
Fig. 1. Flow sheet of Tamboraque plant—Phase 2.
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The objective of the Tamboraque Project was to increase the milling capacity of the plant to 600 ton/day and at the same time to construct facilities to recover gold and silver from the arsenopyrite. The project was scheduled to be executed in two phases. During phase 1 an estimated 19,000 oz gold and 28,000 oz silver was to be recovered from the 260,000 tons of gold bearing tailings previously deposited on Tailings dams 1 and 2. This phase included the construction of additional flotation capacity as well as the BIOX® and conventional CIL plants for gold recovery [2]. The second phase of the project was the expansion of the existing plant to treat 600 ton/day of fresh ore from the mine to produce the lead and zinc concentrates. Pyrite and arsenopyrite concentrates would be produced from the zinc flotation tailings. The planned gold production for phase 2 was 23,500 oz per year. The Tamboraque flow sheet for phase 2 of the project is shown in Fig. 1. 2. Background The following section will give a brief description of the steps followed up to the installation and operation of the Tamboraque BIOX® plant. 2.1. Bacteria isolation from the acid rock drainage of the Coricancha mine The Coricancha mine is an underground mine with several levels for ore extraction. The presence of acidic water (acid mine drainage) can be observed in all areas of the mine. It was determined that the 710 extraction level of the Constancia vein has the most acidic pH values (2.0) and the highest iron (9.5 g/l) and arsenic (55 mg/l) solution concentrations. It was decided to isolate mesophilic native bacteria present in this acidic water. This mixed culture, with a population density of 2 × 108 cells/ml, was adapted to arsenopyrite concentrate through the progressive increase of the pulp solids concentration from 1% to 5%. Regular sub-culturing was carried out by transferring 10% of the biooxidized pulp to a new culture media with increased solid concentration. The adapted bacterial culture was used in further laboratory tests. 2.2. Laboratory tests Several laboratory tests were carried out in a 20 l stainless steel tank using the mesophilic bacteria adapted to the arsenopyrite concentrate. The tank was supplied with constant agitation, aeration and
temperature control. Excellent bacterial development and growth was observed during the tests, obtaining populations of 109 cells/ml when a pulp density of 20% solids was reached. The high metal concentrations in solution did not inhibit the bacteria, rather the bacterial cultivation showed a good adjustment to the arsenopyrite concentrate. The testwork also confirmed that using acidic water from 710 level in the mine did not affect the process negatively. On the contrary it proved to be very beneficial as it eliminated the sulfuric acid consumption, thereby realizing an operating cost saving. The testwork indicated that a sulfide sulfur oxidation of 85% was sufficient to achieve a gold recovery of 92% from the biooxidation product. 2.3. Pilot tests A continuous biooxidation pilot plant, capable of treating up to 50 kg/day concentrate was donated by the German organization GTZ and the pilot plant was installed at the TECSUP Institute in Lima. The pilot plant consisted of five 300 l stainless steel reactors. Each reactor was equipped with agitation and an air sparger. The first four tanks were also equipped with stainless steel cooling coils. The pilot plant was operated with the first two reactors (primary reactors) in parallel and the three secondary reactors in series, giving a total of 4 stages. The pH was controlled manually by adding either sulfuric acid or lime to the reactors. The pulp temperature in each reactor was also controlled manually by opening and closing valves in the heating/cooling circuits. The process proved to be acid consuming when using water from the Rímac river for feed dilution. The acid consumption was, however, drastically reduced by using acid mine drainage. The concentrate is expected to be acid consuming due to the high arsenopyrite concentration. MLPSA achieved stable continuous biooxidation treatment of the arsenopyrite concentrate. A weight loss of 30% to 35% was measured at a plant retention time of 10 days and a feed solids concentration of 15% by mass. Gold recoveries in excess of 90% were recorded during this period. In June 1994 Minera Lizandro Proaño S.A. contacted Gencor Process Research (now Gold Fields Limited) for assistance with the optimization of the continuous pilot plant testwork to ensure that sufficient information can be obtained for the design of the industrial scale plant. The pilot plant testwork continued, achieving gold recoveries in excess of 90% at 83% sulfide sulfur oxidation. The plant retention time was also reduced to
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6 days while maintaining the feed solids concentration at 15% [3,4]. 3. BIOX® process at Tamboraque 3.1. Design of the BIOX® plant Gencor supplied MLPSA with a detailed process design package for the Tamboraque BIOX® plant. The plant had a design capacity of 60 ton/day concentrate with grades of 22–30% As; 30–34% Fe; 21.8–31.1 g/ton Au; 49.8–62.2 g/ton Ag and 24–30% S2−. The design was based on data collected from the pilot plant test. The pyrite concentrate, with grades of 1.8–2% As, 44–46% Fe, 4.6–5.5 g/ton Au, 133 g/ton Ag and 7–9% S2− would go the tailings facility along with the flotation tailings, cyanidation tailings and neutralization residues. The typical process parameters were [5]: 1. pH 2. DO concentration 3. Feed solids concentration 4. BIOX® retention time 5. Pulp temperature
1.2–1.8 2 ppm minimum 20% by mass 4–5 days 40–45 °C
The main criteria for the plant design are summarized in Table 1. Table 1 Main criteria for the design of the Tamboraque BIOX® plant Atmospheric pressure Dry maximum temperature Wet maximum temperature Plant capacity Plant availability Existing ore analysis Sulfur as sulfide Arsenic Mineralogic analysis Pyrite Arsenopyrite % Solids in the feeding pulp Total retention time Number of primary reactors in parallel Number of secondary reactors in series Temperature of the biooxidation pulp Weight loss Total air needed Heat generated in the reaction Number of CCD stages Washing water ratio in CCD (washing water: solid flow) Water needed for CCD Neutralization stages Retention time per neutralization stage
70.1 kPa 20 °C 16 °C 60 ton/day 95% 30.0% 26.0% 35.3% 56.5% 20% 5 days 3 3 40 °C 30% 15,400 N m3/h 13,000 kW 3 8.2 9.0 m2/ton/h 6 1h
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3.2. Toxicity tests During the construction of the plant, several toxicity tests were performed on material to be used in the industrial process, for example the different types of rubber, fertilizers to be used as nutrients, lime and limestone from different suppliers to control the pulp pH, cement used in liners. These tests were performed in glass vessels with 9K nutrient medium. The tests were inoculated with the same mixed bacterial culture to be used in the industrial plant and the tests were placed in an orbital agitator at 36 °C. All tests were performed in triplicate and few tests showed any deviation from the performance of the control tests. The rate of ferrous to ferric conversion and bacterial growth were used to evaluate the performance of the 9K shake flask tests. Batch biooxidation tests were also performed on concentrates produced using different flotation reagents. The tests indicated that the mercaptobenzotiazole reagent inhibits bacterial activity and the reagent was thus not used in the industrial plant. 3.3. Inoculum build-up for the industrial BIOX® plant The bacterial inoculum isolated from the Coricancha mine was adapted to the arsenopyrite concentrate over a 4-year period. The volume of active bacterial inoculum was increased using a series of progressively larger bioreactors. The volumes of the reactors used were 10 l, 100 l, 1 m3 and 10 m3 and each stage of the inoculum build-up required approximately 7 to 10 days. For each stage sufficient concentrate was milled to 85% − 45 μm. The nutrient medium for the 10 l and 100 l reactors were made up using 9K nutrient salts, while fertilizers (ammonium sulfate, potassium sulfate and diamonic phosphate) were used for the 1 m3 and 10 m3 reactors. Acid mine drainage was used for dilution in all the stages of the process. This increased the ferric concentration in the tests and also reduced the initial acid consumption for the tests. The main control parameters, measured on a daily basis, were temperature (36–40 °C), dissolved oxygen (2–5 mg/l) and pH (1.3–1.6). The ferrous and ferric iron concentrations, bacterial population, arsenic concentration in solution and redox potential were also measured on a daily basis to evaluate the bacterial activity in the tests. Sulfide sulfur oxidations of 96%, 94% and 90% were achieved corresponding to a gold recovery of 92%. The first BIOX® reactor was subsequently inoculated using the bacterial inoculum built up using a native bacterial
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culture grown on arsenopyrite in the acid mine drainage medium. 4. Description of the Tamboraque BIOX® plant Differential flotation of the zinc flotation tailings produced two concentrates. The first product, a pyrite concentrate was not expected to be treated in the BIOX® plant due to the low gold content. The arsenopyrite concentrate was first remilled to 90% − 200 mesh before being pumped to the concentrate thickener for dewatering. A concentrate stock tank was installed ahead of the BIOX® reactors to ensure that a constant feed rate can be maintained to the BIOX® reactors. Variable speed pumps were used to pump the concentrate from the stock tank to the BIOX® splitter. The density of the slurry was controlled to 20% solids by injecting mine water into the suction end of the BIOX® feed pumps. Both the concentrate feed rate and the slurry density could be controlled either manually or automatically by the PLC. The nutrient solution was also pumped to the BIOX® feed splitter. The feed splitter split the feed equally between the three primary BIOX® reactors operating in parallel. Overflow from the primary reactors flowed to the first secondary reactor through launders. The three secondary reactors were operated in series. The BIOX® reactors had an operating volume of 262 m3 each and were constructed from stainless steel. Each reactor was equipped with a mild steel, rubber lined agitator and a stainless steel sparge ring. Each reactor was also equipped with stainless steel cooling coil baffles. The slurry temperature in each reactor was automatically controlled at 40 °C by passing cold water from the Rímac River through the cooling coils. The oxidation reactions are strongly exothermic and constant cooling of the reactors was required even when the ambient temperature decreased to below 6 °C. The oxidation of sulfide minerals also requires large quantities of oxygen and low-pressure air was injected into the reactors through the sparge rings to meet this requirement. The Lightning A315 impellers were used for the agitation and air dispersion. The A315 is a high solidity, axial flow down pumping hydrofoil impeller and is very efficient in applications where high gas rates are used. The pH was measured and controlled manually to between 1.2 and 1.6 by the addition of either limestone or sulfuric acid. During biooxidation of the arsenopyrite concentrate, iron, arsenic and sulfur are solubilized. These elements were washed from the BIOX® product through the countercurrent decantation stages in a series of three
thickeners. High rate thickeners, with a cationic flocculant, were selected for this application. The washed biooxidation product solids were pumped to the Carbonin-Leach (CIL) section for gold recovery using cyanide. The acidic overflow liquor from the first thickener was pumped to the neutralization section. The neutralization section consisted of six agitated and aerated stainless steel reactors. Air was injected into all the reactors to maintain oxidative conditions in the reactors. A two-stage neutralization process was used. In the first stage (tank 2) the pH of the slurry was increased to 5 by the addition of limestone. The pH was then further increased to 7 by the addition of lime in the last three tanks. The neutralized effluent was combined with the flotation tailings and pumped to the tailings treatment section for disposal in the tailings dam [6,7]. 5. First industrial operation period: 1998–2000 Commissioning of the industrial BIOX® plant commenced with the inoculation of the first primary reactor using 10 m3 of active bacterial slurry from the inoculum build-up phase. Semi-continuous feeding of the reactor commenced once the bacteria in the reactor was active. The overflow from the reactor was used to fill the remaining two primary reactors. Continuous feeding commenced once all the primary reactors were active and the overflow from the primary reactors were used to fill the secondary reactors. Acid mine drainage (mine water) was used at all times as dilution water. The plant did not reach the design capacity during the first phase of operation due to concentrate shortages. 5.1. Phase 1: treatment of tailings dams 1 and 2 material The inoculation and filling of the BIOX® reactors took considerably longer than initially scheduled. The delays in the program were mainly caused by prolonged power interruptions experienced during the commissioning period (up to 70 h/month) causing interruptions in the air supply to the reactors. Concentrate shortages, caused by difficulties experienced in the repulping of the tailings material, also caused delays. The treatment of the tailings dams were finally abandoned due to the inability of maintaining a constant supply of ore and all focus was shifted to phase 2 of the project. 5.2. Phase 2: treatment of ore from the Coricancha mine Phase 1 was initially scheduled to last for 2 years, but due to problems experienced during commissioning of
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the tailings dam reclamation system, the second phase had to be started earlier. Phase 2 was therefore started during July 1999. The full plant, including the lead and zinc flotation sections were commissioned and fresh concentrate from Coricancha mine were treated to produce the lead and zinc concentrates. The tailings were then floated to produce a bulk sulfide concentrate. One of the problems experienced during this period was the control of the thiocyanate concentration in the feed to the BIOX® reactors. Cyanide was added to the lead flotation step to depress the pyrite and arsenopyrite. The cyanide reacted with the concentrate to form thiocyanate. Thiocyanate is toxic to the bacteria even at very low concentrations and the concentrate had to be washed to reduce the thiocyanate concentration to less than 1 mg/l before the concentrate could be fed to the BIOX® reactors. Limestone was also added to the concentrate as a source of carbon (in to form of carbon dioxide) for the bacteria. This led to an increase in the activity of the bacteria and thus to the rate of oxidation in the primary reactors. Sulfide oxidation values in excess of 90% were recorded during this period with corresponding gold recoveries also in excess of 90%. A weight loss of 30– 35% was measured across the BIOX® reactors. The neutralization section also performed satisfactory using limestone in the first three neutralization tanks and lime in the second three reactors. The mine was, however, experiencing significant problems with grade control and dilution of the ore with waste and this caused the feed grade to the plant to be too low. 6. Second industrial operation period: 2002 During 2000, ownership of the mine and Tamboraque plant was transferred to Wiese Sudameris Leasing and the operation of the plant was stopped. The plant was kept under care and maintenance and an active bacterial culture was maintained in the laboratory bioreactors and the 10 m3 reactor. This proved to be a good decision as the filling of the BIOX® reactors were accomplished in only 20 days after the decision was taken to recommence operations early in 2002. It also took only a further 2 months for the plant to reach the design capacity once all the reactors were full. Power interruptions were still a major problem, causing the dissolved oxygen in the reactors to drop too low and thus resulting in fluctuations in the activity of the bacteria. This problem was solved, in part, by the installation of two smaller blowers. These blowers could
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Fig. 2. Bioreactor at Tamboraque BIOX® plant.
be started and operated from the standby power and was able to maintain at least 1 mg/l dissolved oxygen in the BIOX® reactors. This reduced the recovery period considerably and continuous feeding could be restarted within a day after the power failure. The design capacity could also be reached within a week from start-up. The BIOX® reactors were fed with a bulk flotation concentrate (pyrite and arsenopyrite) with an average gold grade of 0.7 oz/ton (∼ 21.8 g/ton). The gold recovery from the biooxidation product was in excess of 85% with a retention time of between 4 and 6 days in the BIOX® reactors. The mine was, however, still experiencing dilution and grade control problems. The control parameters in the BIOX® reactors were the same as during the previous period and the washing of the concentrate was continued to ensure that the thiocyanate concentration remained below 1 mg/l. The washing of the concentrate also reduced the foaming on the BIOX® reactors due to the removal of flotation reagents from the concentrate. Due to the use of mine water for dilution, it was still not necessary to add sulfuric acid to the reactors to maintain the pH. The addition of limestone slurry into the primary reactors as a source of carbon for the bacteria was also continued. All the reagent addition rates were optimized during this period and in each case a saving of up to 20% could be realized. This included BIOX® nutrients and limestone in the neutralization section. Due to environmental reasons, an additional neutralization tank was installed to treat excess acid mine drainage from other levels of the mine (Fig. 2). Acknowledgements The authors would like to thank Mr. Pieter van Aswegen, Senior BIOX® Manager from Gold Fields Limited for permission to present this paper and to Mr.
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Hennie Marais who suddenly passed away in 2000 for his constant support. Also to Mr. Cesar Loayza for the metallurgical information and support. References [1] Loayza, C., Glave, W., Vargas, M., y Yupanqui, R., Gold recovery from L. Proaño flotation tailings: evaluation of alternatives. Presentado en I International Metallurgical Congress, Lima, Peru. [2] Dew, D.W. 1995. Comparison of performance for continuous biooxidation of refractory gold ore flotation concentrates. In Biohydrometallurgical Processing Vol. 1: Proceedings of the International Biohydrometallurgy Symposium IBS-95, held at Viña del Mar, Chile ed. Universidad de Chile, pp. 239–251, ISBN: 956-19-0209-5. [3] Loayza, C., Glave, W., Ly, M.E., Gold recovery from flotation tailings by biooxidation process. . Presentado en XXIII Convención de Ingenieros de Minas del Perú, September 22–26, Arequipa, Perú.
[4] Loayza, C., Ly, M.E., Yupanqui, R., Roman, G., 1999. Laboratory Biooxidation tests of arsenopyrite concentrate for the Tamboraque industrial plant. In Biohydrometallurgy and the Environment toward the mining of the 21st century. Part A.: Proceedings of the International Biohydrometallurgy Symposium IB´99, held in San Lorenzo de El Escorial, Madrid Spain. ed Elsevier. pp. 405–410, ISBN: 0-444-50193-2 (A and B). [5] van Aswegen, P.C., Marais, H.J., Advances in the application of the BIOX® process for refractory gold ores. . In: Kawatra, S.K., Natarajan, K.A. (Eds.), Mineral Biotechnology Microbial Aspects of Mineral Beneficiation, Metal Extraction, and Environmental Control. Society for Mining, Metallurgy, and Exploration, Inc.. ISBN: 0-87335-201-7, 121–134. [6] Rawlings, D.E., Heavy metal mining using microbes. . Annu. Rev. Microbiol., 56 (2002), 65–91. [7] Olson, G., Brierley, J.A., Brierley, C.L., Bioleaching review part B: progress in bioleaching: applications of microbial processes by the minerals industries. . Appl. Microbiol. Biotechnol., 63 (2003), 249–257.
Biooxidation of arsenopyrite concentrate using BIOX® process: Industrial experience in Tamboraque, Peru Martha E. Ly Arrascue a,1 , Jan van Niekerk b,⁎ b
a Golder Associates Peru S.A., Peru Gold Fields Ltd., 24 St. Andrews Road, Parktown, 2193, Johannesburg, South Africa
Available online 18 May 2006
Abstract The Tamboraque Plant is located in the mining district of Viso-Aruri, approximately 90 km east of Lima, Peru, at an altitude of 3000 m above sea level. In 1980 the previous owners of the mine and processing plant, Minera Lizandro Proaño S.A. (MLPSA), initiated an investigation into the available technologies for the recovery of refractory gold from old tailings. The Coricancha mine is a polymetallic ore deposit and lead and zinc concentrates can be produced by flotation. The zinc flotation tailings can be treated further by differential flotation to produce an arsenopyrite concentrate containing the bulk of the gold. The “TAMBORAQUE” project involved the expansion of the ore treatment capacity of the plant from 200 to 600 ton/day. The expansion also involved the installation of a separate flotation section for the zinc tailings and a BIOX® and CIL plant for the treatment of the arsenopyrite concentrate. The first phase of the project involved the installation of a flotation and BIOX® plant for the treatment of an existing zinc flotation tailings dump accumulated over the years. The second phase involved the expansion of the capacity of the existing plant to treat fresh ore from the mine. Initial batch tests indicated that the arsenopyrite concentrate could be treated successfully using biooxidation. A 50 kg/day biooxidation pilot plant was commissioned in Lima, using a native bacterial culture isolated from acid mine drainage from the Coricancha mine. MLPSA operated the pilot plant successfully for 20 months. At this stage MLPSA contacted Gencor (now Gold Fields Limited) for assistance in the optimization of the process. The continuous testwork indicated that 90% gold recovery can be achieved after only 80% sulfide oxidation. A licence agreement was concluded between MLPSA and Gencor in 1995 for the use of the BIOX® technology at the Tamboraque plant. As part of the agreement, Gencor carried out the process design of the biooxidation plant. Continuous plant operation commenced at the end of August 1998 after successful commissioning of the plant. The feed rate through the plant was, however, below the design due to concentrate shortages. The plant reached the design capacity during 2002, achieving 85% gold recovery on a bulk pyrite and arsenopyrite concentrate produced from fresh ore from the mine. The mining and metallurgical activities were, however, shut down in October 2002 due to financial and mining related problems. © 2006 Elsevier B.V. All rights reserved.
1. Introduction ⁎ Corresponding author. E-mail addresses: [email protected], [email protected] (M.E.L. Arrascue), [email protected] (J. van Niekerk). 1 Wiese Sudameris Leasing until 2003. 0304-386X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.hydromet.2006.03.050
The Coricancha mine is located in the Mining District Viso Aruri, at an altitude of 4000 m above sea level, and the Tamboraque plant, at an altitude of 3000 m above sea level. The plant is located approximately 90 km east of the city of Lima on the Central highway, a few
M.E.L. Arrascue, J. van Niekerk / Hydrometallurgy 83 (2006) 90–96
kilometers from the town of San Mateo. The mine has been almost constantly in production since the Spanish Colonial Stage. Ownership of the mine was first held by Negociación Minera Lizandro Proaño until 2000, when ownership was transferred to Wiese Sudameris Leasing. 1.1. Mineralogy The Coricancha mine consists of a filoneano field (low sulfurization type) with quartz veins and sulfide polymetallics of Au, Ag, Cu, Pb and Zn that filled the main system fractures (Constancia and Wellington) as well as others of tensional type (Rocío, Colquipallana, etc.), that cross volcanic formations of the Rímac Group. The gold is mainly found as finely disseminated particles within the crystal structure of the arsenopyrite, causing it to be refractory. A small fraction of the gold is associated with pyrite, but is rarely associated with silver, galena and sphalerite [1].
91
was, however, causing environmental problems due to the arsenic. The first flotation cells were installed in 1930 to separate the arsenopyrite from lead and silver. MPLSA, a small lead and zinc concentrates producer, initiated an investigation to evaluate alternative process options for gold recovery from its flotation tailings. The gold bearing tailings was subjected to a differential flotation process to obtain a pyrite concentrate and an arsenopyrite concentrate. Both of the concentrates were highly refractory, achieving less than 10% gold recovery after direct cyanidation. The arsenopyrite concentrate, due to its higher gold content, was selected for further testwork and subjected to roasting, pressure leaching and bacterial leaching processes. Based on the laboratory results, biooxidation was selected as the preferred process route due to the technical and economic advantages the process offered. 1.3. Process description
1.2. History Mining activities in the Viso Aruri district started in the middle IX century, initially developing the zones rich in silver. The tenacity and impulse of Don Lizandro Proaño resulted in the startup of the Tamboraque smelter in 1906. Operation of the smelter
The ore from the Coricancha mine was previously treated at a rate of 200 ton/day to produce lead and zinc concentrates. The zinc flotation tailings, still containing 3.14 g/ton gold and 33.9 g/ton silver, was deposited on Tailings dams 1 and 2 for further processing at a later stage.
Crushing
Ore Mine
Milling Lead Concentrate
Lead Flotation TAILINGS DAM
Zinc Concentrate Zinc Flotation Pyrite concentrate
Pyrite Flotation
General flotation tailings
Arsenopyrite Flotation
BIOX® Process
overflow
Neutralisation
Neutralisation Residue
underflow
Gold in activated carbon
′ Cianuracion (CIL)
Cyanide Destruction Caro′s Acid
Cyanidation tailings
Fig. 1. Flow sheet of Tamboraque plant—Phase 2.
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The objective of the Tamboraque Project was to increase the milling capacity of the plant to 600 ton/day and at the same time to construct facilities to recover gold and silver from the arsenopyrite. The project was scheduled to be executed in two phases. During phase 1 an estimated 19,000 oz gold and 28,000 oz silver was to be recovered from the 260,000 tons of gold bearing tailings previously deposited on Tailings dams 1 and 2. This phase included the construction of additional flotation capacity as well as the BIOX® and conventional CIL plants for gold recovery [2]. The second phase of the project was the expansion of the existing plant to treat 600 ton/day of fresh ore from the mine to produce the lead and zinc concentrates. Pyrite and arsenopyrite concentrates would be produced from the zinc flotation tailings. The planned gold production for phase 2 was 23,500 oz per year. The Tamboraque flow sheet for phase 2 of the project is shown in Fig. 1. 2. Background The following section will give a brief description of the steps followed up to the installation and operation of the Tamboraque BIOX® plant. 2.1. Bacteria isolation from the acid rock drainage of the Coricancha mine The Coricancha mine is an underground mine with several levels for ore extraction. The presence of acidic water (acid mine drainage) can be observed in all areas of the mine. It was determined that the 710 extraction level of the Constancia vein has the most acidic pH values (2.0) and the highest iron (9.5 g/l) and arsenic (55 mg/l) solution concentrations. It was decided to isolate mesophilic native bacteria present in this acidic water. This mixed culture, with a population density of 2 × 108 cells/ml, was adapted to arsenopyrite concentrate through the progressive increase of the pulp solids concentration from 1% to 5%. Regular sub-culturing was carried out by transferring 10% of the biooxidized pulp to a new culture media with increased solid concentration. The adapted bacterial culture was used in further laboratory tests. 2.2. Laboratory tests Several laboratory tests were carried out in a 20 l stainless steel tank using the mesophilic bacteria adapted to the arsenopyrite concentrate. The tank was supplied with constant agitation, aeration and
temperature control. Excellent bacterial development and growth was observed during the tests, obtaining populations of 109 cells/ml when a pulp density of 20% solids was reached. The high metal concentrations in solution did not inhibit the bacteria, rather the bacterial cultivation showed a good adjustment to the arsenopyrite concentrate. The testwork also confirmed that using acidic water from 710 level in the mine did not affect the process negatively. On the contrary it proved to be very beneficial as it eliminated the sulfuric acid consumption, thereby realizing an operating cost saving. The testwork indicated that a sulfide sulfur oxidation of 85% was sufficient to achieve a gold recovery of 92% from the biooxidation product. 2.3. Pilot tests A continuous biooxidation pilot plant, capable of treating up to 50 kg/day concentrate was donated by the German organization GTZ and the pilot plant was installed at the TECSUP Institute in Lima. The pilot plant consisted of five 300 l stainless steel reactors. Each reactor was equipped with agitation and an air sparger. The first four tanks were also equipped with stainless steel cooling coils. The pilot plant was operated with the first two reactors (primary reactors) in parallel and the three secondary reactors in series, giving a total of 4 stages. The pH was controlled manually by adding either sulfuric acid or lime to the reactors. The pulp temperature in each reactor was also controlled manually by opening and closing valves in the heating/cooling circuits. The process proved to be acid consuming when using water from the Rímac river for feed dilution. The acid consumption was, however, drastically reduced by using acid mine drainage. The concentrate is expected to be acid consuming due to the high arsenopyrite concentration. MLPSA achieved stable continuous biooxidation treatment of the arsenopyrite concentrate. A weight loss of 30% to 35% was measured at a plant retention time of 10 days and a feed solids concentration of 15% by mass. Gold recoveries in excess of 90% were recorded during this period. In June 1994 Minera Lizandro Proaño S.A. contacted Gencor Process Research (now Gold Fields Limited) for assistance with the optimization of the continuous pilot plant testwork to ensure that sufficient information can be obtained for the design of the industrial scale plant. The pilot plant testwork continued, achieving gold recoveries in excess of 90% at 83% sulfide sulfur oxidation. The plant retention time was also reduced to
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6 days while maintaining the feed solids concentration at 15% [3,4]. 3. BIOX® process at Tamboraque 3.1. Design of the BIOX® plant Gencor supplied MLPSA with a detailed process design package for the Tamboraque BIOX® plant. The plant had a design capacity of 60 ton/day concentrate with grades of 22–30% As; 30–34% Fe; 21.8–31.1 g/ton Au; 49.8–62.2 g/ton Ag and 24–30% S2−. The design was based on data collected from the pilot plant test. The pyrite concentrate, with grades of 1.8–2% As, 44–46% Fe, 4.6–5.5 g/ton Au, 133 g/ton Ag and 7–9% S2− would go the tailings facility along with the flotation tailings, cyanidation tailings and neutralization residues. The typical process parameters were [5]: 1. pH 2. DO concentration 3. Feed solids concentration 4. BIOX® retention time 5. Pulp temperature
1.2–1.8 2 ppm minimum 20% by mass 4–5 days 40–45 °C
The main criteria for the plant design are summarized in Table 1. Table 1 Main criteria for the design of the Tamboraque BIOX® plant Atmospheric pressure Dry maximum temperature Wet maximum temperature Plant capacity Plant availability Existing ore analysis Sulfur as sulfide Arsenic Mineralogic analysis Pyrite Arsenopyrite % Solids in the feeding pulp Total retention time Number of primary reactors in parallel Number of secondary reactors in series Temperature of the biooxidation pulp Weight loss Total air needed Heat generated in the reaction Number of CCD stages Washing water ratio in CCD (washing water: solid flow) Water needed for CCD Neutralization stages Retention time per neutralization stage
70.1 kPa 20 °C 16 °C 60 ton/day 95% 30.0% 26.0% 35.3% 56.5% 20% 5 days 3 3 40 °C 30% 15,400 N m3/h 13,000 kW 3 8.2 9.0 m2/ton/h 6 1h
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3.2. Toxicity tests During the construction of the plant, several toxicity tests were performed on material to be used in the industrial process, for example the different types of rubber, fertilizers to be used as nutrients, lime and limestone from different suppliers to control the pulp pH, cement used in liners. These tests were performed in glass vessels with 9K nutrient medium. The tests were inoculated with the same mixed bacterial culture to be used in the industrial plant and the tests were placed in an orbital agitator at 36 °C. All tests were performed in triplicate and few tests showed any deviation from the performance of the control tests. The rate of ferrous to ferric conversion and bacterial growth were used to evaluate the performance of the 9K shake flask tests. Batch biooxidation tests were also performed on concentrates produced using different flotation reagents. The tests indicated that the mercaptobenzotiazole reagent inhibits bacterial activity and the reagent was thus not used in the industrial plant. 3.3. Inoculum build-up for the industrial BIOX® plant The bacterial inoculum isolated from the Coricancha mine was adapted to the arsenopyrite concentrate over a 4-year period. The volume of active bacterial inoculum was increased using a series of progressively larger bioreactors. The volumes of the reactors used were 10 l, 100 l, 1 m3 and 10 m3 and each stage of the inoculum build-up required approximately 7 to 10 days. For each stage sufficient concentrate was milled to 85% − 45 μm. The nutrient medium for the 10 l and 100 l reactors were made up using 9K nutrient salts, while fertilizers (ammonium sulfate, potassium sulfate and diamonic phosphate) were used for the 1 m3 and 10 m3 reactors. Acid mine drainage was used for dilution in all the stages of the process. This increased the ferric concentration in the tests and also reduced the initial acid consumption for the tests. The main control parameters, measured on a daily basis, were temperature (36–40 °C), dissolved oxygen (2–5 mg/l) and pH (1.3–1.6). The ferrous and ferric iron concentrations, bacterial population, arsenic concentration in solution and redox potential were also measured on a daily basis to evaluate the bacterial activity in the tests. Sulfide sulfur oxidations of 96%, 94% and 90% were achieved corresponding to a gold recovery of 92%. The first BIOX® reactor was subsequently inoculated using the bacterial inoculum built up using a native bacterial
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culture grown on arsenopyrite in the acid mine drainage medium. 4. Description of the Tamboraque BIOX® plant Differential flotation of the zinc flotation tailings produced two concentrates. The first product, a pyrite concentrate was not expected to be treated in the BIOX® plant due to the low gold content. The arsenopyrite concentrate was first remilled to 90% − 200 mesh before being pumped to the concentrate thickener for dewatering. A concentrate stock tank was installed ahead of the BIOX® reactors to ensure that a constant feed rate can be maintained to the BIOX® reactors. Variable speed pumps were used to pump the concentrate from the stock tank to the BIOX® splitter. The density of the slurry was controlled to 20% solids by injecting mine water into the suction end of the BIOX® feed pumps. Both the concentrate feed rate and the slurry density could be controlled either manually or automatically by the PLC. The nutrient solution was also pumped to the BIOX® feed splitter. The feed splitter split the feed equally between the three primary BIOX® reactors operating in parallel. Overflow from the primary reactors flowed to the first secondary reactor through launders. The three secondary reactors were operated in series. The BIOX® reactors had an operating volume of 262 m3 each and were constructed from stainless steel. Each reactor was equipped with a mild steel, rubber lined agitator and a stainless steel sparge ring. Each reactor was also equipped with stainless steel cooling coil baffles. The slurry temperature in each reactor was automatically controlled at 40 °C by passing cold water from the Rímac River through the cooling coils. The oxidation reactions are strongly exothermic and constant cooling of the reactors was required even when the ambient temperature decreased to below 6 °C. The oxidation of sulfide minerals also requires large quantities of oxygen and low-pressure air was injected into the reactors through the sparge rings to meet this requirement. The Lightning A315 impellers were used for the agitation and air dispersion. The A315 is a high solidity, axial flow down pumping hydrofoil impeller and is very efficient in applications where high gas rates are used. The pH was measured and controlled manually to between 1.2 and 1.6 by the addition of either limestone or sulfuric acid. During biooxidation of the arsenopyrite concentrate, iron, arsenic and sulfur are solubilized. These elements were washed from the BIOX® product through the countercurrent decantation stages in a series of three
thickeners. High rate thickeners, with a cationic flocculant, were selected for this application. The washed biooxidation product solids were pumped to the Carbonin-Leach (CIL) section for gold recovery using cyanide. The acidic overflow liquor from the first thickener was pumped to the neutralization section. The neutralization section consisted of six agitated and aerated stainless steel reactors. Air was injected into all the reactors to maintain oxidative conditions in the reactors. A two-stage neutralization process was used. In the first stage (tank 2) the pH of the slurry was increased to 5 by the addition of limestone. The pH was then further increased to 7 by the addition of lime in the last three tanks. The neutralized effluent was combined with the flotation tailings and pumped to the tailings treatment section for disposal in the tailings dam [6,7]. 5. First industrial operation period: 1998–2000 Commissioning of the industrial BIOX® plant commenced with the inoculation of the first primary reactor using 10 m3 of active bacterial slurry from the inoculum build-up phase. Semi-continuous feeding of the reactor commenced once the bacteria in the reactor was active. The overflow from the reactor was used to fill the remaining two primary reactors. Continuous feeding commenced once all the primary reactors were active and the overflow from the primary reactors were used to fill the secondary reactors. Acid mine drainage (mine water) was used at all times as dilution water. The plant did not reach the design capacity during the first phase of operation due to concentrate shortages. 5.1. Phase 1: treatment of tailings dams 1 and 2 material The inoculation and filling of the BIOX® reactors took considerably longer than initially scheduled. The delays in the program were mainly caused by prolonged power interruptions experienced during the commissioning period (up to 70 h/month) causing interruptions in the air supply to the reactors. Concentrate shortages, caused by difficulties experienced in the repulping of the tailings material, also caused delays. The treatment of the tailings dams were finally abandoned due to the inability of maintaining a constant supply of ore and all focus was shifted to phase 2 of the project. 5.2. Phase 2: treatment of ore from the Coricancha mine Phase 1 was initially scheduled to last for 2 years, but due to problems experienced during commissioning of
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the tailings dam reclamation system, the second phase had to be started earlier. Phase 2 was therefore started during July 1999. The full plant, including the lead and zinc flotation sections were commissioned and fresh concentrate from Coricancha mine were treated to produce the lead and zinc concentrates. The tailings were then floated to produce a bulk sulfide concentrate. One of the problems experienced during this period was the control of the thiocyanate concentration in the feed to the BIOX® reactors. Cyanide was added to the lead flotation step to depress the pyrite and arsenopyrite. The cyanide reacted with the concentrate to form thiocyanate. Thiocyanate is toxic to the bacteria even at very low concentrations and the concentrate had to be washed to reduce the thiocyanate concentration to less than 1 mg/l before the concentrate could be fed to the BIOX® reactors. Limestone was also added to the concentrate as a source of carbon (in to form of carbon dioxide) for the bacteria. This led to an increase in the activity of the bacteria and thus to the rate of oxidation in the primary reactors. Sulfide oxidation values in excess of 90% were recorded during this period with corresponding gold recoveries also in excess of 90%. A weight loss of 30– 35% was measured across the BIOX® reactors. The neutralization section also performed satisfactory using limestone in the first three neutralization tanks and lime in the second three reactors. The mine was, however, experiencing significant problems with grade control and dilution of the ore with waste and this caused the feed grade to the plant to be too low. 6. Second industrial operation period: 2002 During 2000, ownership of the mine and Tamboraque plant was transferred to Wiese Sudameris Leasing and the operation of the plant was stopped. The plant was kept under care and maintenance and an active bacterial culture was maintained in the laboratory bioreactors and the 10 m3 reactor. This proved to be a good decision as the filling of the BIOX® reactors were accomplished in only 20 days after the decision was taken to recommence operations early in 2002. It also took only a further 2 months for the plant to reach the design capacity once all the reactors were full. Power interruptions were still a major problem, causing the dissolved oxygen in the reactors to drop too low and thus resulting in fluctuations in the activity of the bacteria. This problem was solved, in part, by the installation of two smaller blowers. These blowers could
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Fig. 2. Bioreactor at Tamboraque BIOX® plant.
be started and operated from the standby power and was able to maintain at least 1 mg/l dissolved oxygen in the BIOX® reactors. This reduced the recovery period considerably and continuous feeding could be restarted within a day after the power failure. The design capacity could also be reached within a week from start-up. The BIOX® reactors were fed with a bulk flotation concentrate (pyrite and arsenopyrite) with an average gold grade of 0.7 oz/ton (∼ 21.8 g/ton). The gold recovery from the biooxidation product was in excess of 85% with a retention time of between 4 and 6 days in the BIOX® reactors. The mine was, however, still experiencing dilution and grade control problems. The control parameters in the BIOX® reactors were the same as during the previous period and the washing of the concentrate was continued to ensure that the thiocyanate concentration remained below 1 mg/l. The washing of the concentrate also reduced the foaming on the BIOX® reactors due to the removal of flotation reagents from the concentrate. Due to the use of mine water for dilution, it was still not necessary to add sulfuric acid to the reactors to maintain the pH. The addition of limestone slurry into the primary reactors as a source of carbon for the bacteria was also continued. All the reagent addition rates were optimized during this period and in each case a saving of up to 20% could be realized. This included BIOX® nutrients and limestone in the neutralization section. Due to environmental reasons, an additional neutralization tank was installed to treat excess acid mine drainage from other levels of the mine (Fig. 2). Acknowledgements The authors would like to thank Mr. Pieter van Aswegen, Senior BIOX® Manager from Gold Fields Limited for permission to present this paper and to Mr.
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Hennie Marais who suddenly passed away in 2000 for his constant support. Also to Mr. Cesar Loayza for the metallurgical information and support. References [1] Loayza, C., Glave, W., Vargas, M., y Yupanqui, R., Gold recovery from L. Proaño flotation tailings: evaluation of alternatives. Presentado en I International Metallurgical Congress, Lima, Peru. [2] Dew, D.W. 1995. Comparison of performance for continuous biooxidation of refractory gold ore flotation concentrates. In Biohydrometallurgical Processing Vol. 1: Proceedings of the International Biohydrometallurgy Symposium IBS-95, held at Viña del Mar, Chile ed. Universidad de Chile, pp. 239–251, ISBN: 956-19-0209-5. [3] Loayza, C., Glave, W., Ly, M.E., Gold recovery from flotation tailings by biooxidation process. . Presentado en XXIII Convención de Ingenieros de Minas del Perú, September 22–26, Arequipa, Perú.
[4] Loayza, C., Ly, M.E., Yupanqui, R., Roman, G., 1999. Laboratory Biooxidation tests of arsenopyrite concentrate for the Tamboraque industrial plant. In Biohydrometallurgy and the Environment toward the mining of the 21st century. Part A.: Proceedings of the International Biohydrometallurgy Symposium IB´99, held in San Lorenzo de El Escorial, Madrid Spain. ed Elsevier. pp. 405–410, ISBN: 0-444-50193-2 (A and B). [5] van Aswegen, P.C., Marais, H.J., Advances in the application of the BIOX® process for refractory gold ores. . In: Kawatra, S.K., Natarajan, K.A. (Eds.), Mineral Biotechnology Microbial Aspects of Mineral Beneficiation, Metal Extraction, and Environmental Control. Society for Mining, Metallurgy, and Exploration, Inc.. ISBN: 0-87335-201-7, 121–134. [6] Rawlings, D.E., Heavy metal mining using microbes. . Annu. Rev. Microbiol., 56 (2002), 65–91. [7] Olson, G., Brierley, J.A., Brierley, C.L., Bioleaching review part B: progress in bioleaching: applications of microbial processes by the minerals industries. . Appl. Microbiol. Biotechnol., 63 (2003), 249–257.