Abstract: Studies on the solublisation of copper from chalcopyrite by direct (contact)and indirect leaching using Thiobacillus ferrooxidan were carried out. Thiobacillus ferrooxidan produced up to 2380 ppm acidic Fe+3 solution of pH 1.5 in 480 h. The direct leaching method was slow for the solublisation of copper from the ore and only 28.52% of copper leached in 70 days. While on the other hand 87.57% of copper was brought in to solution in 192 h by indirect leaching technique utilizing biologically oxidized acidic Fe+3 solution at 30 C. The increase in temperature of the indirect leaching process, shorten the metal recovery time. At 60°C maximum leaching (88.30%) was obtained in 8 hrs only.
Introduction
Microbial leaching processes are getting importance in the recovery of metals from sulphide ores, because of low energy requirement, minimum pollution and maximum yield. Copper, zinc, gold etc., can be recovered from their sulphide ores by microbial leaching. Three groups of microorganisms are used for leaching process: autotrophic bacteria (Troma and Bosecker, 1982), heterotrophic bacteria (Keil and Schwartz,1980; Munier-Lamy and Berthelin, 1987) and fungi (Keil and Schwartz, 1980; Siegel, 1973). Chemoautotrophic iron and sulphur oxidizing bacteria actively participate in leaching of sulphide minerals. Among iron oxidizing microorganisms belonging to the genus Thiobacillus, cultures of T. ferrooxidan and T. thiooxidan are of great importance in biometallurgy and related processes. A lot of information has been reported on the vital role of T. ferrooxidan and T. thiooxidan in leaching of copper and other metals from low grade ores (Brierley, 1978, Rahman and Gul, 1999; Rahman and Gul, 2001; Kelly et al., 1977), wastes (Lundergren and Malouf, 1983), desulfurisation of petroleum (Devis, 1967) and coal (Dugan and Apel, 1978, Chandra et al., 1980; Kwang and Yen, 1990; Mannivannan et al., 1994). T. ferrooxidan is the most widely used organism in microbial leaching. Metals can be recovered from their sulphide ores by `direct (contact) leaching’ and `indirect leaching’ by acidic ferric (Fe+3) iron that is generated by bacterial oxidation from ferrous (Fe+2) iron. The studies presented here deals with the comparison of direct and indirect leaching of copper from the chalcopyrite ore of saindak by using Thiobacillus ferrooxidan. The effects of temperature on copper leaching in 2-stage process were also carried out.
Materials and Methods
Substrate: The chalcopyrite ore supplied by Saindak Metals Limited was used for these studies. This ore contained chalcopyrite (CuFeS), copper oxide (CuO), Pyrite (Fe2S) and silica (SiO2) as major constituents. The ore used in these studies contained 0.45% copper including 0.038% copper as copper oxide. The ore was grinded and # -100, +150 was used through out the studies.
Organism: The strain Thiobacillus ferrooxidan of Rahman and Gul (1999) was used through out the studies. The strain was maintained in a medium containing (g l-1) ammonium sulphate 3.0, potassium chloride 0.1, dipotassium hydrogen phosphate 0.5, magnesium sulphate 0.5 calcium nitrate 0.01 and ferrous sulphate 4.0. The pH of the medium was adjusted at 2.5 with 10 N H2SO4. The strain was sub cultured after every 20 days.
Inoculum preparation: The inoculum was prepared by growing Thiobacillus ferrooxidan in conical flasks having sterilized 9K medium for 15 days at 35±1oC. The bacterial cells were harvested by centrifugation and resuspended in 0.005 M H2SO4. The final concentration of inoculum was ~5X 105 bacteria ml-1.
Preparation of acidic ferric sulphate solution: The acidic ferric sulphate solution was prepared by using mineral salt solution (g l-1) ammonium sulphate 3.0, Potassium chloride 0.1, Magnesium sulphate 0.5 and Fe++ iron 3000 pm as ferrous sulphate. The pH of the solution was adjusted at 2.5 by 10 N H2SO4. 2.5 L mineral salt solution was inoculated by 10% v/v inoculum and incubated at 35±1oC. The aeration of the medium was started after 24 h and continued up to 432 h of incubation. After than the medium was kept standing for further 48 h. The medium containing acidic ferric sulphate was centrifuged and supernatant used as copper leachant.
Microbial leaching technique: Direct leaching of copper was carried out in 250 ml conical flasks containing 10 g ore and 100 mL medium containing (g l-1) ammonium sulphate 3.0, potassium chloride 0.1, dipotassium hydrogen phosphate 0.5, magnesium sulphate 0.5 and calcium nitrate 0.01. The pH of the medium was adjusted at 2.5 with 10 N sulphuric acid. The medium and ore were sterilized at 121oC for 15 minutes and inoculated. The flasks were incubated at 35oC on rotatory shaker at an operating speed of 150 rpm.
The indirect leaching was carried out in 250 mL conical flasks containing 40 g ore and 100 mL biologically oxidized acidic ferric sulphate solution in presence of sodium azide 10 mM, to inhibit microbial growth. The flasks were incubated at 30oC on rotatory shaker, adjusted at a speed of 150 rpm. After regular intervals samples were withdrawn and analyzed for copper and iron contents.
Studies on the effects of temperature on leaching of copper were carried out at 30, 40, 50 and 60oC.
Analytical methods: The analyses of the leached solution was done after filtration through 2 μm-pore size membrane to remove bacterial mass and other insoluble materials.
The copper contents in the leached solution and ore residue were determined by atomic absorption spectrophotometer AAS 2380, Perkin Elmer, USA, using standard conditions.
The Fe2+, Fe3+ and total iron contents were analyzed by o-phenanthroline method (Herrera et al., 1989).
Results and Discussion
Direct leaching of copper: Fig. 1 show the results of leaching of copper from chalcopyrite ore in presence of Thiobacillus ferrooxidan. The dissolution of iron, its oxidation and leaching of copper were started after 24 of incubation. The initial dissolution of copper was a chemical reaction in which copper oxide present in the ore was solubilized with the action of sulfuric acid in the medium. The dissolution of iron in Fe2+ form was quite high than the other two processes. The higher leached concentration of Fe2+ in the medium may be due to the presence of iron in forms other than pyrite (Fe2S) i.e., pyrorhotite Fe1-xS, that dissolved with the action of sulfuric acid. The concentration of ferrous iron in the medium increased up to 40 days and than decreased gradually.
| Fig. 1: | Direct ‘intact’ leaching of copper from chalcopyrite by Thiobacillus ferrooxidan |
| Fig. 2: | Course of Fe2+ Oxidation by Thiobacillus ferrooxidan |
| Fig. 3: | Leaching of copper by biologically oxidized ferrous sulfate solution from chalcopyrite |
The decrease in Fe2+ contents could be attributed to increased rate of oxidation by establishment of Thiobacillus ferrooxidan in the medium. The rate of oxidation Fe2+ almost doubled after 40 days of incubation i.e., from 0.089 mg to 0.187 mg per day.
| Fig. 4: | Effects of temperature on copper leached by biologically oxidized ferrous sulfate solution from chalcopyrite |
Niemela et al. (1994) have also reported the initial solubilization of Fe2+ iron from Black Schist ore and subsequent oxidation to Fe3+ iron. They described the dissolution of iron as chemical reaction with some enhancement by bacteria whereas the oxidation of Fe2+ to Fe3+ iron was bacterial mediated with negligible concentration from chemical reaction.
The pattern of leaching of copper and oxidation of Fe2+ iron were more or less the same except the rate of copper leaching was little high. The results show two phases of copper leaching and iron oxidation. The 1st phase lasted up to 30 days and 2nd from 31st to 70th day. The 1st phase was slow in respect of copper leaching and Fe2+ iron oxidation, which show the establishment of Thiobacillus ferrooxidan in the medium. In the next i.e., 2nd phase, the rate of both the processes i.e., leaching of copper and Fe2+ oxidation, increased and reached to as high as 2.33 and 0.192 mg per day in last 40 days, respectively. The rates of Fe2+ oxidation and leaching of copper were statistically the same during the 50th to 60th day of the experiment i.e., Fe2+ oxidation 0.263 mg and copper leaching 0.270 mg per day. This phase of bacterial Fe++ oxidation show that Thiobacillus ferrooxidan had established itself and fully functional. At the end of experiment the medium contained 12.835 mg copper and 16.539 mg iron (9.211 mg Fe3+ and 7.328 mg Fe2+). Our results are much better than reported by Choi and Hopkin (1987) who achieved 30% dissolution of copper contained in the ore in over 21 months by direct leaching whereas in present studies 28.52% leaching of copper was obtained in only 70 days. An increase in the pH of the medium to 3.2 was also observed in the initial phases of the experiment, this may be because of the chemical reaction of H2SO4 on the ore which solubilised the copper from copper oxide and iron from the ore. However, the pH of the medium later on decreased to 2.1 because of the production of H2SO4 by Thiobacillus ferrooxidan.
Oxidation of Fe++ iron: Data regarding oxidation of iron by T. ferrooxidan (Fig. 2) show three distinct phases of iron oxidation. The first lag phase of bacterial growth lasts up to 96 h where the rate of oxidation was quite slow, the 2nd log phase between 96 to 432 h where the rate of oxidation was as high as 6.4 mg h-1. The oxidation of Fe2+ was statistically non significant in 3rd and last stationary phase from 432 to 480 h of incubation. The slow rate of oxidation in 3rd phase may be due to the toxicity of high concentration of Fe3+ ions to Thiobacillus, which retard the metabolism. Pakinkar and Agate (1987) have also reported the intolerance of metal ion in Thiobacillus sp. The changes in pH were more or less inversely proportional to the growth of bacteria and oxidation of Fe2+ iron. After 480 h the oxidized Fe3+ contents of the medium reached to 2379 mg with pH 1.5.
Indirect leaching of copper: Acidic ferric sulfate is an important oxidizing agent and lixiviant for sulphide minerals of copper. The oxidized acidic iron solution plays an important role in dump and situ leaching. 40 gm ore was subjected to leaching with 100 mL biologically oxidized acidic ferric sulfate solution. The results regarding leaching of copper, utilization of Fe3+ and changes in pH of the leachant are depicted in fig. 3. The rate of indirect copper leaching was quite high as compared to direct leaching. In this process Fe3+ iron attacked the copper compounds of the ore. Fe3+ iron oxidized monovalent copper ion (Cu1+) to create a divalent copper ion (Cu2+), which then combined with SO4-2 ion with the contribution of sulfuric acid to form copper sulfate. By this process maximum of 87.57% of copper present in the ore was recovered in 192 h. The utilization of Fe3+ iron was more or less directly proportional to the leached copper. The pH of the ferric sulfate solution was changed from 1.5 to 4.6 during the leaching of copper from the ore.
Effects of temperature on indirect leaching of copper: Temperature, play an important role in chemical reactions. The rate of copper leaching was increased with the increase in temperature. At 30oC maximum leaching of copper was achieved in 192 h (Fig. 3), while at 40 and 50oC in 48 and 20 h, respectively (Fig. 4). The fastest rate of leaching was observed at 60oC, where the maximum leaching was observed after 8 h. After than, although there was an increase in copper contents of leaching solution but it was not significant. The average rate of copper dissolution in initial 2 h was 5.4 mgh-1 at 30oC, 22.62 mgh-1 at 40oC, 53.65 mgh-1 at 50oC and 69.11 mgh-1 at 60oC. It was also observed that the leaching process slow down after 86% leaching of copper. This probably may be due to the accumulation of Fe2+ iron around the chalcopyrite particles and leaving small sites of attack for the attack of Fe3+ iron.
From the results of the studies presented here, it can be safely concluded:
| I) | That indirect leaching method is efficient than direct (contact) leaching. |
| ii) | That indirect leaching permits the advantage of separate enhancement of each the two processes involved in the bio-recovery of copper. |
| iii) | That in indirect leaching chemical attack of Fe+3 to release copper from the ore can be performed at higher temperatures for enhanced recovery rate, in order to reduce the processing time. |