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Introduction

Q1:- Describe the processes used to extract copper and gold from their ores using bacterial leaching. In each case, explain how the micro-organisms liberate the metal from its ores and describe how the metal is then extracted from the mixture. Bacterial Leaching of ores and other Materials Bacterial leaching is a process in which bacteria is used to extract heavy metals from low grade ores and tailings (left from an earlier traditional mining). In mining industry, bioleaching is a new technique used to extract minerals such copper and Gold from their low grade ores. The main agents used in bioleaching process are micro organisms such as Chemolithoautotrophs; they capture carbon for the synthesis of cellular components not from organic nutrients but from carbon dioxide (CO2) in the atmosphere. These bacteria are Thiobacillus Ferrooxidans and Thiobacillus Thiooxidans. Thiobacilli grow in acidic environment because they are acidophilic (acid loving). They drive energy by the oxidation of the Ferrous iron and Sulphur part of the mineral for their survival. They grow at PH value from 1 to 4 and strictly under aerobic conditions. Bacterial Leaching has been divided into two mechanisms:- * Direct Mechanism: - Direct mechanism is that in which bacteria is physically attached to the mineral surface. In this reaction, the electron passes into the bacterial cell which produces ATP (Adenotriphosphate), is the energy currency of the cell which reduces oxygen molecules to water. ...read more.

Middle

Thiooxidans S2O3-2(aq) + 2OH-(aq) 2S2O3-2(aq) not stable in alkaline solution S4O6-2(aq) +2e- S4O6-2(aq) + 2H+ (g) Thiooxidans H2S4O6 (400 words) Oxidation of Arsenic metal (III) to arsenic metal (V). Arsenic metal (V) is present in complex forms. As+3(aq) Fe+3 / FeS2 As+5 (aq) Thiobacillus bacteria The overall reaction for Arsenopyrite is:- 2FeAsS (s) + 7O2 (g) + 4H+ (g) + H2O (aq) 2Fe+3 (aq) +2HSO-4(aq) +2H3AsO4 (aq) Now, the Gold metal is left in the original solution which can be obtained by treating the solution with sodium cyanide (NaCN). Cynidation process increases the gold recovery from 10% to 100%. Extraction from Mixture:- Pure solid Gold metal can be extracted by Cynidation reaction in the presence of free oxygen. Au (aq) + 2NaCN (aq) + 1/4O2 + 1/2 O2 (g) Na [Au (CN) 2] (aq) + NaOH- (s) 2Na [Au (CN) 2] (aq) +Zn(s) 2NaCN (aq) + Zn (CN) 2 + Au (s) Extraction of copper Copper can be extracted from their low grade ores like chalcopyrite (CuFeS2), Pyrite (FeS2), Covellite and others. It is similar to the extraction process of gold. The process Stage 1:- The acid leaching solution (contains H2SO4, Thiobacillus thiooxidans and thioferrooxidans) is sprayed on mineral chalcopyrite (CuFeS2) which is then oxidised into copper (II) ions, ferrous ions (Fe+2), ferric ions (Fe+3), sulphate (II). ...read more.

Conclusion

Bioleaching is a slow process as it takes decades rather than years to extract metals but as traditional method is expensive but it is faster. As facts had revealed that bacterial leaching is the cause of acid mine drainage. Secondary extraction process for copper but as a primary extraction process for Gold:- (800 words) Bacterial leaching is considered as secondary process for copper for the fact that it is not able to perform on a large scale and copper is present in large amounts in their ores. It is also an extremely slow process and uneconomic. For gold however it proved to be cost effective as it recovered almost all the gold. Q3:-Summarise the development stages that a new mining process (such as bacterial leaching) must pass through before it can operate commercially? * It must be safe not damaging the environment. * Able to meet the demand of the trade market. * The process should be cost effective to produce the metal. * Funding should be provided by government and political powers to regulate a new process. * Tests should be carried out in labs to make sure the process works well. * The location used for bacterial leaching should be suitable to the favourable conditions of the process, * A large scale plant should be built for industrial use. (950 words) Summary:- Basically microbiological research focuses on the biochemistry, physiology and genetics of the involved micro-organisms and on the complex interrelationship in the community of the leaching species. ...read more.

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