Minerals Engineering, Vol. 12, No. 8, pp. 893-904, 1999 Pergamon 0892-6875(99)00076-X © 1999 Elsevier Science Ltd All rights reserved 0892-6875/99/$ - see front matter BIOLEACHING OF COPPER SULPHIDE CONCENTRATE USING E X T R E M E THERMOPHILIC BACTERIA M. GERICKE and A. PINCHES Mintek, Private Bag X3015, Randburg, 2125, South Africa. E-maih mariekieg@mintek.co.za (Received 21 December 1998; accepted 5 March 1999) ABSTRACT An extreme thermophilic, iron-sulphur oxidising bacterial culture was isolated and adapted to tolerate high metal and solids concentrations at 70°C. Following isolation and adaptation, the culture was used in a continuous multi-stage pilot plant employing standard mechanically agitated and aerated tanks, for the bioleaching of a mixed secondary copper sulphide-chalcopyrite concentrate. The culture exhibited stable leach performance over the period of pilot plant operation and overall copper extractions of higher than 97% were maintained. It was shown, however, that solids concentration had a significant effect on the copper bioleach kinetics. Increased solids concentrations resulted in a drop in redox potential and copper extractions in the first stage reactor. Batch chemical ferric leaches, performed at controlled redox potentials, indicated that reduced residence times may be possible in bioleach processes, using thermophiles, if high redox potential levels can be maintained. © 1999 Elsevier Science Ltd. All rights reserved. Keywords Bioleaching; bacteria; bio0xidation INTRODUCTION Bioleaching of refractory gold-containing sulphide concentrates, typically containing pyrite and arsenopyrite, is now an established commercial technology. The further development of bioleaching, as a hydrometallurgical alternative to smelting for the processing of copper sulphide concentrates, is now being considered by research centers and copper producers around the world. The major challenge for copper sulphide bioleaching is the successful treatment of chalcopyrite (Lawrence and Poulin 1996), which is the major copper sulphide of commercial interest. Chalcopyrite typically exhibits slow leach kinetics and low copper extractions at low temperatures and ambient pressure in the acid ferric sulphate leach solutions characteristic of bioleach processes, due to passivation of the mineral surface (Mufioz et al. 1995). Secondary copper sulphides, such as chalcocite, digenite, bornite and covellite, generally leach far more easily. However, even with these copper sulphides, extended residence times are often required to achieve high copper extractions, using mesophilic Thiobacillus-Leptospirillum bacteria operating at temperatures in the range 35-45"C. One reason for this is that bioleaching of chalcocite and 893 894 M. Gerickeand A. Pinches bornite gives rise to intermediate mineral products, that exhibit significantly reduced leach rates at the low temperatures required for mesophilic bacteria (Neale 1998). The interest in thermophilic bacteria lies in the potential for improving the leach rates of sulphides such as pyrite and arsenopyrite, and in overcoming the problems encountered in bioleaching of refractory copper sulphides, such as chalcopyrite at lower temperatures. The potential to achieve this is clear from the extensive published data that has been obtained using shake flask and other laboratory-scale methods, but employing low solids concentrations and relatively mild agitation conditions (Brierley 1990). However, it has been claimed that the potential for using extreme thermophiles, such as Sulfolobus, at temperatures in the range 60-84°C may not be realised commercially unless their sensitivity to agitation at high solids concentrations can be overcome (Clark and Norris 1996). Solutions suggested by these authors include the selection or isolation of more robust bacteria or the development of improved reactor designs. The bioleaching of a pyrite-pyrrhotite-arsenopyrite gold-containing concentrate using Sulfolobus acidocaldarius strain BC65 at 65"C, has been demonstrated in a continuously operated pilot plant with mechanically agitated and aerated 1 m 3 leach tanks, employing turbine impellers (Sandstrrm and Petersson 1997). These bacteria exhibited superior leach kinetics compared to moderate thermophilic bacteria at 45°C and tolerated feed solids concentrations of 20%. Continuous pilot plant tests, treating chalcopyrite concentrate, using the Sulfolobus BC strain has also been reported (Le Roux and Wakerley 1988). A 101 air-stirred leach tank was used and copper extractions of 98% was achieved for a 14-day residence time, with Cu concentrations of up to 27g/1 being reached. Prior to the pilot plant trials the bacterial culture required a period of adaptation to increasing Cu concentrations. The work presented in this paper forms part of a major project at MINTEK to develop copper bioleaching technology capable of treating any copper concentrate. The paper describes the isolation of a Sulfolobus-like extreme thermophile culture, laboratory adaptation of the isolate, and the subsequent bioleaching of a mixed secondary copper sulphide-chalcopyrite concentrate in a continuous multi-stage pilot plant employing mechanically agitated and aerated leach tanks. The pilot plant facilities used are identical to those normally used for bioleaching using mesophilic bacterial cultures. Chemical ferric leaching tests on the concentrate are included to provide a comparison of copper leaching rates and mineral leaching behaviour. M A T E R I A L S AND M E T H O D S Isolation Samples were collected from a hot, sulphur-rich coal dump near Witbank, South Africa. For heterotrophic growth, yeast extract (0.1%), adjusted to pH 1.8 with H 2 S O 4 w a s used as enrichment medium. For autotrophic growth, 3g/1 ferrous iron, 5g/l sulphur or 0.2% pyrite in a standard culture medium comprising (g/l), (NH4)2SO 4 (0.5), MgSO 4 (0.24), KCI (0.05), and KzHPO4 (0.5) were used as substrates in separate enrichment cultures. Cultures were incubated in shake flasks at 70°C in temperature-controlled orbital shakers at 150 r/min and examined microscopically. Evaporation losses were compensated for by daily addition of distilled water. The enrichment cultures were subcultured in the same manner once substantial growth occurred. G r o w t h conditions on different substrates The ability of the isolates to adapt and grow on yeast extract, elemental sulphur, ferrous iron, pyrite and copper sulphides was evaluated in 500 ml Erlenmeyer flasks at 70°C in temperature-controlled shakers. The different substrates were added to 100 ml of the standard culture medium and the pH adjusted to 1.8 with H2SO 4. Growth was followed microscopically. Bioleaching of copper sulphide concentrate 895 Adaptation of the culture A mixed culture, prepared from the isolates obtained in the enrichment cultures, was adapted to increasing concentrations of a secondary copper sulphide and chalcopyrite (particle size, d~ = 101am) containing concentrate. The concentrate was washed with acetone prior to use. The adaptation was carried Out at 70°C in a 11 water-jacketed glass vessel, stirred at 500 r/min, using a magnetic stirrer. The reactor was fitted with a condenser to minimise evaporation. Air supplemented with 0.15% CO 2 was supplied through an air sparger. The pH was maintained at between 1.6 and 1.8 by addition of H2SO4. The adaptation involved addition of increasing amounts of concentrate to the medium up to 60g/1 over a two month period. The adaptation was continued in a 121 reactor with the same configuration as used in the pilot plant (see below) and the solids concentration was increased to 100g/1. Mineral characteristics The continuous study was performed on a copper sulphide concentrate, fine-milled in a stirred ball mill to a particle size of dgo = 10pm. Modal analysis of the sample (Table 1) showed that the major sulphides present were chalcopyrite (27%), chalcocite (12%), covellite (3%), bornite (2%) and pyrite (7%). The chemical composition of the concentrate is shown in Table 2. TABLE 1 Chemical analysis of the copper sulphide concentrate Element Cu Fe Si02 Sulphide Elemental sulphur Sulphate Mass (%) 26.7 15.4 18.9 19.6 <0.2 2.1 TABLE 2 Mineralogical analysis of the copper sulphide concentrate Mineral Phase Estimated Mass (%) Gangue Chalcopyrite (CuFeS2) Chalcocite (Cu2S) Covellite (CuS) Bornite (CusFeS4) Other Sulphides Pyrite (FeS2) TOTAL * Omitted from estimate 45 27 12 3 2 3 7 100 Estimated Portion of Total Cu Estimated Portion of Total Sulphide 42.1 43.2 9 5.7 38.1 19.5 8.1 4.1 100 30.2 100 (%) (%) Nutrients The culture was grown in a basal medium with the following composition: (NH4)SO 4 (lg/1), MgSO4 (0.24g/1), KCI (0.05 g/l) and K2HPO4 (0.5g/I). 896 M. Gericke and A. Pinches Mini pilot plant The testwork was carried out in continuously operated 121 polypropylene reactors at 70°C. The system consisted of a feed pulp tank and three reactors in series, with a container at the end for product collection. The first reactor was fed via a peristaltic pump from the feed tank and pulp transfer between the reactors was by means of gravity overflow. The reactors were fully baffled and agitation was by means of a dual impeller system, consisting of a 6blade flat-blade disc turbine (Rushton turbine) and a 6-blade 45 o pitch-blade turbine. The agitation speed was set at 300r/min. The air supply to the reactors was enriched with 0.15% CO 2 and supplied to the reactors by means of a sparger situated below the impeller. Air flow rates were controlled by a rotameter and air flow controller to each reactor. The reactors were fitted with condensers to reduce evaporation. The reactors were monitored daily for temperature, pH level, oxidation-reduction potential (ORP) and oxygen uptake rate. The ORP was measured with a platinum probe and Ag/AgCl.reference probe. The inlet and outlet gas streams from the reactors were analysed for oxygen content with a Hartmann-Braun gas analyser. The levels of copper and iron in solution were measured daily by atomic absorption spectrophotometry (Spectr AA-10). Residue solids were analysed for copper, iron and silicate using emission spectroscopy. Elemental sulphur, sulphide sulphur and sulphate were determined using wet-chemistry techniques. Ferric leach tests Ferric leach tests were done at 35°C and 70°C in 6.51 mechanically-agitated tanks. The initial total iron concentration was 30 g/1 and the Fe 3+ : Fe 2÷ ratio was adjusted to give ORP levels of 430 or 550 mV. The initial pH was adjusted to 0.5 with sulphuric acid to maintain strongly acidic conditions to keep iron in solution. A very high liquid : solids ratio was used to maintain the initial ORP and pH levels throughout the test. R E S U L T S AND D I S C U S S I O N Isolation and adaptation of the culture Samples were collected from a hot coal dump and by enrichment and serial transfer a number of isolates capable of growing at 70°C were isolated. The cultures were morphologically similar and consisted of round cells, sometimes irregularly shaped. Detailed taxonomical studies have not been done on the culture, but it was classified as Sulfolobus-like based on morphology. The ability of the isolates to grow on different substrates was evaluated. As expected, very good growth was obtained when the cultures were grown heterotrophically on yeast extract. The isolates were also able to grow autotrophically in the presence of 10g/1 elemental sulphur and high cell concentrations were recorded. The cultures could grow in 9K medium containing 10g/1 ferrous iron (pH 1.8), although the cell concentrations were lower than in sulphur medium. All the isolates grew very well in pyrite (d90 = ll5~tm) at low pulp densities (1%), but at a solids concentration of 5% no growth was observed. Sulfolobus species are known to be sensitive to high solids concentrations and the cultures were therefore gradually adapted to tolerate increasing concentrations of pyrite. The isolates adapted quickly and could eventually tolerate 5% pyrite in shake flasks. No growth occurred when the isolates were inoculated in 1% fine milled (d90 = 10~am), mixed secondary copper sulphide-chalcopyrite concentrate. It was not expected that shear-related damage of the cells could be a problem at such low solids concentrations, because good growth had been obtained on 5% pyrite at a coarser grind. From literature it is known that flotation reagents can have an inhibitory or toxic effect on the leaching ability of T. ferrooxidans (Loon and Madgwick 1995). It was therefore decided to acetone-wash the concentrate before adding it to the flasks. After overnight incubation in 0.5% acetone-washed Bioleaching of copper sulphide concentrate 897 concentrate, a high concentration of healthy looking cells was present. The solids concentration was then successfully increased to 5%. Growth occurred at these higher solids concentrations, but the cell numbers were much lower. As the different cultures showed very similar morphological and growth characteristics, a mixed culture consisting of the different isolates was prepared. This mixed culture was adapted to oxidise copper sulphides in a 11 water-jacketed vessel, while being gently agitated using a magnetic stirrer. The concentrate was acetone-washed prior to use and the solids concentration was increased gradually. The culture could tolerate 60g/1 concentrate and a copper concentration of approximately 15g/1 after a two month period. The adapted culture was used to inoculate a 121 reactor with the same configuration as used in the pilot plant studies. The culture was exposed to increasing concentrations of the copper sulphide mineral and eventually grew well in the presence of 100g/1 of the concentrate and approximately 25g/1 of copper in solution. Pilot p l a n t o p e r a t i o n The mini-plant was started by introducing bioleach pulp obtained from the 121 reactor, into the stage 1 reactor and initiating flow of feed pulp. The feed concentrate was not acetone-washed. The average feed solids concentration was 5% and the mean residence time, based on feed pulp flow to the reactor, was 3.2 days. The stage 2 reactor was started on day 34 by filling the reactor with nutrient medium (pH 1.8) and connecting the overflow from the stage 1 reactor. Stage 3 was started in a similar manner on day 60. During the 280 days of plant operation, adjustments were made to the feed solids concentration and the overall mean pulp residence time. These adjustments are shown in Figure 1. The corresponding time-course data for Cu extractions and ORP levels are shown in Figures 2 and 3. On day 178, oil from the agitator gearbox leaked into the stage 1 reactor and the plant had to be restarted. 14 ~0 o ~ .m e- ~ I ~° 12 o o°~© ~ 18 © =° ~Zi~ .................................................... i 7 o 10 o t= ,i= =,~° t~ m ~a o o o *° ' o D =- o o 8 g t~ 5~ E C 0 "0 .............. ~=~ ............................I o [] D O e. o 6 0 "~0 4 tr ii - Feed solids o Residence time I 2 0 I 0 Fig. 1 ; 50 r I 1 O0 I I 150 T i m e (days) i I 200 r 250 Adjustments to feed solids concentrations and residence times during pilot plant operation (arrow indicates plant re-start). The plant was operated without condensers during the first 39 days of the run and very high evaporation levels were recorded. The reactors were fitted with condensers and it was estimated from the differences between the feed pulp and the final overflow pulp flowrates that the decrease in pulp volumes due to evaporation was approximately 10% in each tank. Because of the small size of the reactors, it was estimated that a bigger error might have been introduced if make-up water was added to the reactors to compensate for the evaporation losses. 898 M. Gefickeand A. Pinches During the first 25 days of operation, the stage 1 reactor exhibited a degree of instability. The pulp ORP remained low, the pH rose to higher than 2.5 and the copper concentration increased to 28 g/1. Initially high cell concentrations were observed in the pulp, but around day 25 a proportion of these cells started to appear as "empty" cells, indicating that the cells were under stress. The assessment was that the apparent instability of the bacterial culture might be due to the need for an additional period of adaptation to the reactor conditions. The feed solids concentration was therefore reduced to 3% on day 26. Following this change, the copper concentration almost halved, the pulp ORP immediately increased to around 550 mV and more stable operation of the stage 1 reactor was observed. 60 i* stage1 °stage2 ,stage31 A ~A A 50 : % == AA 40 • ",t3,L', " ~' ° :o l /.'o o O .o/:'o° " ei,) n o 10 . o ° 0 0 I I 50 100 i 150 200 300 250 Time (days) Fig.2 Cu extraction data for the 3 reactor stages during pilot plant operation (arrow indicates plant restart). 700 600 0 500 03 <¢ ~ ~===r~Sa~'-~--=~'~= - I I* _ ° ~ " ~" = 0~ " V o©° * o o~ 4 0 0 ,,¢ > 300 n- 200 ~ = .%* ~ " f o O I* s t a g e 1 ° s t a g e 2 , stage31 100 0 50 100 150 Time 200 250 300 (days) F i g . 3 0 R P levels for the 3 reactor stages during pilot plant operation. On day 45 the feed solids concentration was increased to 4.3% and the overall residence time was increased to 5.2 days. No instability was observed following this change and the ORP stayed high. Bioleaching of copper sulphide concentrate 899 On day 67 the feed solids concentration was increased to 5.9%. At this point the pulp ORP in the stage 1 reactor showed a rapid decrease to around 400 mV. The pH increased to between 2.5 and 2.8. No instability was observed, however. From day 68 onwards, the pH in the first reactor was reduced to around 2, by manually adding H2SO 4. From day 80, HzSO 4 was added continuously to the stage 1 reactor at a rate equivalent to 140 kg H2SO 4 per ton of feed concentrate. This reduced the average pH in the first reactor to around 2. It can be assumed that pyrite oxidation was insignificant in the first reactor. The pyrite fraction of the concentrate was oxidised in the second and third reactors at the higher ORP values, producing acid and causing the pH to drop. On day 88, the residence time was increased to 7 days across the three reactor stages. Following this, the feed solids concentration was increased to 7.4% on day 107, 8.1% on day 145 and 9.4% on day 161. No instability was observed as a result of these changes, although the pulp ORP in the first stage reactor remained at around 400 mV and pH levels of around 2 were recorded. Following re-start on day 178, the plant was run at an overall residence time of 7.5 days and a feed solids concentration of 7.5%. After confirming that the leach performance was comparable to that previously obtained under the same operating conditions, the feed solids concentration was increased to 9.9% on day 233. Towards the end of the run solids accumulation occurred in the stage 3 reactor. This was associated with problems on the stage 3 reactor overflow rather than with insufficient mixing intensity in the reactor. The accumulation of solids, together with the effect of evaporation, explains the very high Cu concentrations observed in the stage 3 reactor from day 178 onwards (Figure 2). However, this had little influence on the overall Cu extraction, since more than 95% had been extracted over the stage 1 and 2 reactors (see below). The total Cu extraction was also routinely estimated based on analysis of the residue collected as the overflow from the final stage 3 reactor (Figure 4). The results indicate that Cu extractions of higher than 95% were maintained, except during the initial 25 day period when only the stage 1 reactor was operating and operational instability was being observed. 100 00 0 0 O0 0 00 0 0 0 0 0 0 0 0 0 0 0 O0 0 0 0 0000 0 0 0 GDO 0 o~ 90 v C o ,m ~ 80 X 0 70 > 60 0 50 0 i t r r 50 100 150 200 250 300 Time (days) Fig.4 Overall Cu extractions based on solid residue analysis. On a number of occasions, after the plant had been operating under reasonably steady conditions, larger pulp samples were collected from each reactor and the levels of Cu extraction and sulphide oxidation determined. The data are shown in Table 3. The pH level, ORP, Cu concentration, soluble Fe concentration and oxygen 900 M. GefickeandA. Pinches consumption data shown are the average values for the seven days prior to collection of the pulp samples. The data in Table 3 indicate that final Cu extractions higher than 98% and sulphide oxidation levels higher than 94% were consistently achieved for all the conditions employed. The data also show that more than 96% Cu extraction and 87% sulphide oxidation were consistently achieved in the stage 2 reactor. It is also important to note that in all the bioleach residues analysed, the amount of elemental sulphur measured was always insignificant (<0.2%). TABLE 3 Summary of steady state operating data during pilot plant operation Day 58 Overall residence time (days) Feed solids concentration (%) iPHdox potential (mV vs Ag/AgCI) Cu concentration (g/I) :e concentration (g/I) iOxygen consumption (kg/mS.d) Cu leached (%) Sulphide oxidlsed (%) Day 83 !OveraJJ residence time (days) Feed solids concentration (%) oH 2edox potential (mV vs Ag/AgCI) Cu concentration (g/I) :e concentration (g/I) Oxygen consumption (kg/mS.d) Cu leached (%) Sulphide oxidised (%) Day 101 Overall residence time (days) Feed solids concentration (%) pH Redox potential (mY vs Ag/AgCI) Cu concentration (g/I) Fe concentration (g/I) Oxygen consumption (kg/mS,d) Cu leached (%) Sulphide oxidised (%) Day 161 Overall residence time (days) Feed solids concentration (%) )H ~edox potential (mY vs Ag/AQCI) Cu concentration (g/I) Fe concentration (g/I) Oxygen consumption (kg/mS.d) Cu leached (%) Sulphide oxidised (%) Day 280 Overall residence time (days) Feed solids concentration (%) )H Redox potential (mV vs Ag/AQCI) Cu concentration (g/l) Fe concentration (g/I) Oxygen consumption (kg/m3 d) Cu leached (%) S,ulphide oxidised (%) 5 13 (over 2 stages) 43 Stage 1 Stage 2 Stage 3 222 2.09 551 607 12.7 146 1.0 1.2 69 11 94 ! 97 8 91.5 979 52 (over 3 stages) 59 Stage 1 Stage 2 STage3 196 154 1 53 412 590 639 1T3 181 195 1.9 54 5.7 63 75 20 83.8 97.0 980 603 94.5 972 7.01 (over 3 stages) 5.9 Stage 1 Stage 2 Stage 3 1.89 16 2 1.55 425 617 652 14.4 16.5 17.9 34 76 89.7 64.2 5.5 3.3 983 96.8 5.8 07 99 1 98.5 7.01 (over 3 stages) 8.1 Stage 1 Stage 2 Stage 3 1.92 1.65 1.59 424 570 617 21 5 27.2 29.8 6.1 9.2 9.6 9.6 5.2 11 78.1 97.6 992 52.9 95.3 98.5 728 (over 3 stages) 99 Stage 1 Stage 2 Stage 3 1.90 1.66 1 42 406 509 495 21.8 34.7 6t.) 65 107 18.8 69 80 10 526 962 98 2 237 87.4 94.4 Bioleaching of copper sulphide concentrate Data 901 interpretation The routine Cu extraction data plotted in Figure 4 show that it was possible to maintain > 97% Cu extraction for the mixed copper sulphide concentrate over the range of feed solids concentrations and residence times employed. However, the more detailed steady-state Cu extraction data in Table 3 show that changes in feed solids concentration and residence times had a significant effect on the copper bioleach kinetics. The effect is clearly shown by the Cu leaching data for the stage 1 reactor. The leaching performance of the stage 1 reactor will be most sensitive to the range of process variables that will effect growth and oxidative activities of the bacterial cells. The data for day 58 in Table 3 show that a relatively high ORP level (551 mV) and a Cu concentration of 12.7 g/1 were sustained in the stage 1 reactor at 4.3% feed solids and a 2.56 day residence time. Copper leached was 94.1% and sulphide oxidised was 91.5%. This result indicates that a major proportion of both the copper sulphides, including chalcopyrite, and the pyrite was oxidised in this reactor. On day 83 at an increased feed solids concentration of 5.9%, and a reduced residence time of 1.74 days in stage 1, the Cu extraction was lower (83.8%), which might be expected at the reduced residence time. The more notable effect was a major decrease in the ORP level to 412mV. The pH decreased from 2.22 to 1.96. For day 101 the feed solids concentration was maintained at 5.9%, but the stage 1 residence time was increased to 2.34 days, which is similar to the residence time on day 58. The ORP stayed low (425 mV), but the Cu extraction increased to 89.7%. The residence times on days 161 and 208 were the same as for day 101, but the feed solids concentrations were increased to 8.1 and 9.9%, respectively. The increase in feed solids concentration to 8.1% did not result in a significant change in the ORP level, but the Cu extraction decreased to 78.1%. The increase in feed solids to 9.9% resulted in both a significant drop in the ORP level to 406 mV and the Cu extraction to 52.6%. The stage 1 residence times for days 58, 101, 161 and 208 were similar, and therefore, the Cu extraction data for these days can be plotted (Figure 5) as the Cu leach rate vs. Cu feed rate (Cu concentration in feed pulp solids/residence time). This plot also serves to illustrate the drop in Cu extraction with increasing feed solids concentrations. 12 6 _ ~ (}day101 Qday 208 day 58 0 I I I I 2 4 6 8 Cu in feed (kg/m3.d) I 10 12 Fig.5 Relationship between the Cu leach rate and Cu feed rate for the stage 1 reactor. 902 M. Gefickeand A. Pinches The reduction in the Cu extraction rate evident from Figure 5 may be the result of an inhibitory effect on bacterial oxidation activity (Fe z+ and/or S 2- oxidation), resulting from the increasing Cu concentrations. While this may be a major factor, the high oxygen consumption rates supported in the stage 2 reactor at >30 g Cu/1 (Table 3) indicate that high bacterial oxidation rates are still supportable at these Cu levels. The effect of shear, related to the increasing solids concentration, on bacterial activity is another possible explanation for the drop in Cu extraction in the stage 1 reactor. The effect of shear on Sulfolobus using fine milled solids (cl~ = 10/am) has not been rigorously studied. Microscopic examination of the bacteria during the periods of high solids concentration provided no evidence that cell damage occurred. Evaluation of 02 and CO2 gas-liquid mass transfer efficiencies has not yet been carried out for bioleach processes using extreme thermophiles at high temperatures. The highest 02 consumption rate (9.6 kg/m3.d) was achieved in the stage 1 reactor for day 161 (Table 3). This suggests that O 2 mass transfer was probably not generally limiting. The 02 mass transfer efficiency is reported to be unaffected over the temperature range 35-70"C, whereas CO 2 mass transfer efficiency shows a decline with increasing temperature (Boogerd et al. 1990). The inlet air CO 2 content was set at 0.15%, but the possibility that the CO 2 supply rate was limiting bacterial growth cannot be excluded. A major decrease in Cu extraction rate occurred at a solids concentration of 9.9% (day 280) and this may be related to the very low ORP observed (406 mV). The highest Cu extraction observed at 4.3% feed solids, corresponds to the highest ORP observed (551 mV). At all other feed solids concentrations the ORP levels remained around 420 mV. Low ORP levels during Sulfolobus bioleaching of copper sulphide concentrates have been reported previously (Barr et al. 1992). The authors related this to the initial release of Fe 2+, which precedes Cu leaching during the bioleaching of copper sulphides. They were not able to provide a mechanism for this leach behaviour, but speculated that this mechanism is specific to thermophiles. However, the present results show that high ORP levels are possible at low feed solids concentrations, and it seems more likely that the appearance of low ORP is simply associated with limitation of bacterial growth kinetics. Similar initial low ORP levels have been observed for bioleaching of copper sulphide concentrates, using mesophilic bacteria (Neale 1998). The results of the ferric chemical leach tests are shown in Figure 6. At both 430 and 550 mV, there are two distinct leach regions--an initial period of rapid leaching and a slower secondary phase. The initial rapid leach rate is largely due to leaching of the secondary copper sulphides, while the slower secondary phase is mainly due to chalcopyrite leaching. The initial period of rapid leaching gives approximately 60% Cu extraction. This is consistent with the mineralogy and copper disposition data (secondary copper sulphides account for 57.9% of the Cu) in Table 2. It is clear that there is a significant enhancement of chalcopyrite leaching at higher ORP levels. At 550 mV the maximum (>98 %) Cu extraction is achieved after 24 hours and at 430 mV after 48 hours. In the pilot plant stage 1 reactor, the secondary copper sulphides would be expected to rapidly oxidise to completion, while any copper leached in excess of this amount, can be attributed to chalcopyrite leaching. Except for day 58, it can be assumed that pyrite will not have oxidised, since pyrite oxidation is not significant below 480 mV. The presence of pyrite may, however, have enhanced the rate of leaching of the copper sulphides, due to galvanic interaction with the pyrite (Ahonen and Tuovinen 1993) The stage 2 and 3 reactors were operating at much higher ORP levels, particularly at the lower feed solids concentrations, and the predominant reactions were expected to be oxidation of the pyrite and residual chalcopyrite. The magnitude of the oxygen uptake rates measured across the stage 2 and 3 reactors supports the view that pyrite oxidation made a significant contribution to oxygen consumption. Considering the day 101 data as an example, 89.7% of the Cu was leached in the stage 1 reactor and pyrite oxidation would have had to account for the major proportion of oxygen consumption in the stage 2 and 3 reactors. Bioleaching of copper sulphide concentrate 100 o o 903 ~ o o 80 o O C, o o O ¢J O. O. 0 60 40 [o550mV 20 0 o430mVl I I I I I 10 20 30 40 50 60 Time (hours) Fig.6 Cu extraction time-course data for chemical ferric leaches at 70 °C and 430 and 550 mV. CONCLUSIONS An extreme thermophilic, iron-sulphur oxidising bacterial culture was isolated from a burning coal dump. However, for the subsequent use of the culture in bioleach applications, it was necessary to carry out a series of adaptation steps. These included maintenance of bacterial oxidative activity at sufficient sulphide solids concentrations, tolerance to high metal ion concentrations such as copper and tolerance to flotation reagents associated with sulphide concentrates. Following isolation and adaptation, the bacterial culture was employed in a three-stage continuously operated pilot plant over a period of 280 days, treating a mixed secondary copper sulphide-chalcopyrite concentrate. Once established in the pilot plant, employing standard bioleach conditions, the extreme thermophile bacterial culture exhibited stable leaching performance over the period of pilot plant operation. The copper bioleach kinetics showed a significant reduction at feed solids concentrations approaching 10%. This effect was related to a drop in the ORP levels in the stage 1 reactor and was probably due to limitation of bacterial oxidative activity. The cause of this limitation is presently unknown. The stage 2 and 3 reactors exhibited much higher ORP levels and under all conditions >96% Cu extractions were achieved, even in the stage 2 reactors. Batch chemical ferric leach tests carried out on the concentrate under conditions of controlled ORP, showed that the Cu leach rate is significantly increased at an ORP level of 550 mV compared to an ORP of 430 mV. This indicates that significantly reduced residence times may be possible in bioleach processes, using the extreme thermophiles, if higher ORP levels can be maintained. Further research should now focus on assessing kinetic and other process variables which may be limiting the activity of the extreme thermophile bacterial culture, as the basis for maximising leaching rates and further minimising the residence times. ACKNOWLEDGEMENTS 904 M. Gerickeand A. Pinches This paper is published with the permission of MINTEK. 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