Lecture 10 Biohydrometallurgy Of Copper General Principles, Mechanisms And Microorganisms

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1 Lecture 10 Biohydrometallurgy Of Copper General Principles, Mechanisms And Microorganisms Keywords: Bioleaching Of Copper, Leaching Reactions, Leaching Bacteria In lectures 10-12, bioleaching of copper ores and concentrates is critically analyzed with respect to use of different microorganisms, possible mechanisms, heap and bioreactor leaching [42-63]. Copper production is poised to reach to about 20mt by More than 20% of world copper is now produced through hydro and biometallurgy. To meet the ever - increasing demand, all types of ore resources including low grade ores, overburden and even wastes from mining operations need be economically processed. Biohydrometallurgy holds great promise in the economical and environment-friendly processing of such copper resources, besides treatment of copper concentrates. The most successful copper leach operations have been in the bioleaching of secondary copper sulfides and copper oxides (chalcocite, covellite, oxides and oxidized ores). However, chalcopyrite is the most abundant and the most refractory copper mineral which is not readily amenable to mesophilic biooxidation processes. However, efforts are being made to bioprocess chalcopyrite ores and concentrates using high temperature thermophilic organisms. For example, Straits resources tested chalcopyrite heap leaching with good results. Titan Resources, operated trial runs on mixed nickel sulfide-chalcopyrite heaps at Radio Hill. Mintek along with Iranian copper industries company are undertaking large scale pilot tests on heap bioleaching of chalcopyrite ores at the Sarcheshmeh mines in Iran. 1

2 Oxidative bioleaching of chalcopyrite involves role of ferric ions as an oxidant and formation of elemental sulfur. Reactions are controlled by redox potentials. Insoluble reaction products such as sulfur and jarosite compounds formed on chalcopyrite surfaces can affect dissolution rates. Sulfide ore dumps and heaps present a complex microbial habitat and many of the indigenous microorganisms participate in the bioleaching processes. In table 10.1, a few of the important mesophilic and thermophilic organisms isolated from sulfide deposits are listed, all of which could play a role in bioleaching of copper minerals. Acidithiobacillus ferrooxidans is the most widely studied organism with respect to copper bioleaching. Leptospirillum ferriphilum were found to outgrow At.ferrooxidans in many bioreactor operations. Predominant roles played by L.ferrooxidans and At.thiooxidans at high acidic levels in copper bioleach environments have been reported. Moderate and extreme thermophiles also are significant, especially at higher temperatures ( C). At.caldus is the dominant sulfur oxidizer in many bioreactor operations. Sulfolobus spp become significant at temperatures higher than 60 0 C. Microbial consortia participating in a bioleaching process (heaps or bioreactors) is indeed very complex. Major bioleaching reactions for chalcopyrite are given below: CuFeS 2 + 4Fe Fe ++ + Cu S 0 CuFeS 2 + 4H + = Fe ++ + Cu H 2 S 2Fe H O 2 = 2Fe H 2 O 2S + 3O 2 + 2H 2 O = 2H 2 SO 4 3Fe SO 4 + 6H 2 O + M + = MFe 3 (SO 4 ) 2 (OH) 6 + 6H + Where M = K +, Na + or NH + 4 2

3 Table 10.1: Some iron and sulfur oxidizing microorganisms (adapted from Watling H.R, Hydrometallurgy, 84 (2006), ) Acidianus brierleyi S - - ph Acidimicrobium ferrooxidans Mixotroph Fe ++ oxdn. and Fe +++ redn. Moderate thermophile ph 2 Acidiphilum spp Obligate heterotroph Mesophiles Acidithiobacillus ferrooxidans Acidithiobacillus thiooxidans Acidithiobacillus caldus Ferrimicrobium acidiphilium S, S - -, Fe ++ ph 2-4 S, S - - ph Mixotroph Moderate thermophile S, S - - ph Heterotroph Mesophile, ph 1-2 Fe ++, S - - Ferroplasma acidophilum Pyrite oxidation ph 1-2 Leptospirillum ferriphilum Fe ++ Mesophiles, some thermo tolerant Leptospirillum thermoferrooxidans Leptospirillum ferrooxidans Sulfobacillus acidophilus Sulfobacillus thermosulfidooxidans Sulfolobus metallicus Pyrite ph 1 2 Fe ++ Mesophile, ph 1 2 Fe(II) oxidation; Fe(III) reduction, S - - S ph Strict chemolithoautotroph, S, S - - Moderate thermophile. Hyperthermophile Sulfolobus acidocaldarius Heterotroph Hyperthermophile 3

4 Factors influencing bacterial mineral oxidation are given below: Physicochemical and microbiological parameters Mineral properties Bioleaching Temperature ph Redox potential Oxygen Carbon dioxide Mass transfer Nutrient availability Ferric ion concentration Pressure Surface tension Presence of inhibitors Microbial diversity, population and activity Metal tolerance and adaptation Mineral type, compositions, grain size and liberation Particle size, area Porosity Hydrophobicity Presence of secondary minerals Leaching mode (in situ, heap, dump or reactor leaching) Pulp density. Stirring rate (reactor) Heap geometry (heap leaching) 4

5 Important parameters influencing bioleaching processes are given in table Table 10.2: Some parameters influencing copper bioleaching processes Parameter Influence Reasons Particle size Pulp density Agitation Aeration Residence time Leaching rate increase with decrease in particle size High grinding costs for finer particles Bioleaching rates decrease with increase in pulp density. Bioleaching rates increase at higher impeller speeds upto critical leveldecrease if agitation rates increase further- High power consumption. Bioleaching rates increase with aeration rates Lower size of gas bubbles preferable. High power consumption with enhanced aeration rates and finer bubble sizes. High leaching rates with lower residence times, - low productivity. Higher surface area, better mass transfer and oxidation rate, Higher cell attachment. Mass transfer rates lower and abrasion effects high. Slurry density and viscosity high Uniform mixing and higher suspension of slurries, better mass transfer rates - Bacterial damage due to high shear and particle attrition. Better availability of O 2 and CO 2 High mass transfer. Slow kinetics, Higher bacterial wash-out. 5

6 Leaching methods for hydrometallurgical extraction of copper are illustrated in table Table 10.3: Leaching methods for hydrometallurgical extraction of copper Leach Nature of ore Grade Approximate Period Typical Cost method particle size operation In situ Oxide and years 4-5 million -- sulfides tonnes Dump Oxide / mm 2-15 years 4-5 million Low sulfides-run of mine and lean ores tonnes Heap Oxide and mm 3-6 months 3x10 5 tonnes Low secondary sulfides Percolation Oxide mm 5-10 days 5-15 vats High Agitation Oxide < 0.1mm 2-6 h tanks High concentrate Vat Roasted h A diagrammatic analysis illustrating the beneficial effect of high temperature leaching of chalcopyrite using thermophilic organisms (instead of acidophilic mesophiles) is given in fig

7 Fig. 10.1: Effect of temperature and thermophiles on copper dissolution For copper oxides and secondary sulfides, direct heap or dump leaching can be used and the efficiency of copper dissolution depends on the type of mineral and mineralogy. Copper oxides require only a few hours, where as secondary sulfides such as chalcocite and covellite take upto several months for acceptable copper recovery from heaps. On the otherhand, chalcopyrite would require years of leaching since its leaching rate is only about one fifth rate of chalcocite. Possible biooxidation reactions for various copper minerals are given in table

8 Table 10.4: Dissolution reactions of some copper minerals in heaps and dumps (Adapted from H.R.Watling, Hydrometallurgy, 84 (2006) ) Mineral Reactions Chrysocolla Tenorite Malachite Azurite Native copper Cuprite Chalcocite CuSiO 3. 2H 2 O + 2H + = Cu ++ + SiO 2. 3H 2 O CuO + H 2 SO 4 = CuSO 4 + H 2 O Cu 2 (CO 3 )(OH) 2 + 2H 2 SO 4 = 2CuSO 4 + CO 2 + 3H 2 O Cu 3 (CO 3 ) 2 (OH) 2 + 3H 2 SO 4 = 3CuSO 4 + 2CO 2 + 4H 2 O Cu + 1/2O 2 + H 2 SO 4 = CuSO 4 + H 2 O Cu 2 O + 1/2O 2 + 2H 2 SO 4 = 2CuSO 4 + 2H 2 O Cu 2 S + 1/2O 2 + H 2 SO 4 = CuS + CuSO 4 + H 2 O Cu 2 S + Fe 2 (SO 4 ) 3 = CuS + CuSO 4 + 2FeSO 4 Bornite Cu 5 FeS 4 + 2Fe 2 (SO 4 ) 3 = 2CuS + CuFeS 2 + 2CuSO 4 + 4FeSO 4 Covellite CuS + 2O 2 = CuSO 4 CuS + Fe 2 (SO 4 ) 3 = CuSO 4 + 2FeSO 4 + S o Enargite Cu 3 AsS l/2fe 2 (SO 4 ) 3 + 2H 2 O = 3CuSO 4 + 9FeSO 4 + 4S o + HAsO 2 + Chalcopyrite 11/2H 2 SO 4 CuFeS 2 + O 2 + 2H 2 SO 4 = CuSO 4 + FeSO 4 + 2S o + 2H 2 O 8