Bioleaching of Uranium and Gold

Bioleaching of Uranium

Uranium leaching or Microbial recovery of uranium from low grade ores proceeds is an indirect process. Thiobacillus ferrooxidans is involved in uranium extraction and it does not directly attack on ore but on the iron oxidants. 

     Low grade uranium ores are of two types- carbonate rich and pyrite containing. Insoluble tetravalent uranium oxide (UO2) is present in low grade ores. 

     Pyrite containing uranium deposits are leached by the action of Thiobacillus ferrooxidans. Ferric sulphate and sulphuric acid is obtained by the action of Thiobacillus ferrooxidans from the pyrite within uranium ore. 

 2FeS2+H2O+7 ½[O2]  Fe2[SO4]3+ H2SO4

 In the subsequent leaching process, H2SO4/FeSO4 solution oxidizes the insoluble tetravalent uranium in the ore to soluble hexavalent uranium sulphate UO2+Fe2(SO4)3  UO2SO4+2FeSO4 pH required for the reaction is 1.5-3.5. Temperature: around 350 C The dissolved Uranium is extracted from the leachate/leach liquor using organic solvents such as tributyl phosphate/ion exchange resins and subsequently precipitated out. Thiobacillus ferrooxidans does not directly interact with uranium minerals. The role of Thiobacillus ferrooxidans in uranium leaching is the best example of the indirect mechanism. Bacterial activity is limited to oxidation of pyrite and ferrous iron. 

  Bioliberation of Gold Gold is recovered from its ores by sodium cyanide leaching process which converts gold to a soluble cyanide complex. 4Au+8NaCN+O2+2H2O  4NaAu(CN)2+4NaOH Low grade sulfidic gold ores cannot be subjected to sodium cyanide extraction. Sulphidic gold ores are mainly two types: pyrite and arsenopyrite. Pretreatments like roasting or pressure oxidation is required to free gold from enclosing sulphides prior to cyanide leaching. Such pretreatments can be costly and are substituted by the action of iron and sulphur oxidising acidophilic bacteria. These bacteria can oxidise sulphidic ores which breaks the sulphide matrix and results in improved accessibility of gold for leaching with cyanide. Thus, bio-oxidation using Thiobacillus ferrooxidans is done in low grade sulphide ores followed by cyanide extraction which improves gold recovery from such low grade ores. This is a less costly, less polluting alternative to other oxidative pretreatments such as roasting and pressure oxidation. Recently, bio-oxidation of gold ores has been implemented as a commercial process, and is under study worldwide for further application to gold ores. Commercial exploitation has made use of heap leaching technology for refractory gold ores.

  Biomining Microorganisms • Need to survive the sulfuric acid produced in the process • Resist the high metal concentrations • Oxidize ferrous iron and sulfur compounds • Need to form a biofilm on the mineral surface Bioleaching process is brought about by bacteria like Thiobacillus ferrooxidans or Thiobacillus thioxidans. These are chemoautotrophic acidophiles and obtain energy from inorganic sources while growing in acidic medium. They can oxidise ferrous to ferric, sulphur and reduced sulphur compounds to sulphate/sulphuric acid, and cause leaching of metals from their oxide and sulphide ores. 

  Conclusion Bacterial leaching is a revolutionary technique used to extract various metals from their ores. Traditional methods of extraction such as roasting and smelting are very energy intensive and require high concentration of elements in ores. Bacterial leaching is possible with low concentrations and requires little energy inputs. The process is environment friendly even while giving extraction yields of over 90%. Bioleaching offers several advantages, such as: 1. The ability to economically process low grade sulfide ores 2. The ability to process ores that may not be feasible to be smelted for environmental reasons However, Bioleaching and mine wastes cause pollution – AMD or acid mine drainage, which is the acid rich effluent produced as a result which can result in pollution of environment.