Bioleaching and Bio-Oxidation

Bioleaching and Bio-Oxidation

	Bioleaching and Bio-Oxidation As destructive and as unstoppable as ARD is the catalytic activity of the mesophilic and thermophilic chemoautotrophs that has been harnessed for cost-effective, efficient and environmentally acceptable commercial processing technologies called bioleaching and mineral bio-oxidation. Bioleaching is the bacterial oxidation of sulphide minerals, whereby metals of value (copper, uranium and zinc) are released into solution.
Mineral bio-oxidation is a biological process in which iron sulphide minerals such as pyrite and arsenopyrite are degraded by bacteria and precious metals (gold and silver) are liberated for recovery by conventional metallurgical techniques. Since 1950, bacteria have been used to bioleach sulphidic, mineral-bearing waste rock from surface copper mines. The waste rock is piled near mining operation, a dilute sulphuric acid solution is applied to the top surfaces of the waste piles by using drip irrigation, and the solution is percolated through the pile.
The moist, acid environment along with air entering from the tops and sides of the waste rock pile provides a conducive environment for naturally occurring Thiobacillus and Leptospirillum species to develop. Bacterial numbers reach 106 to 107 per ml of each solution. The bacteria oxidise copper sulphide minerals releasing soluble copper and ferric iron which is carried from the waste pile by the percolating acid solution. The copper is recovered by solvent extraction, and high grade cathode copper is produced by electrowinning. Some 20% of the world's copper is estimated to be produced by bioleaching. Even higher grade copper sulphide ores are bioleached in a process called bacterial thin layer leaching.
The copper sulphide ore is crushed to less than 6.3 mm and placed on an impermeable pad to a height of 3-6 m. A dilute sulphuric acid solution is applied along with a mixed culture of mesophilic iron and sulphide oxidising bacteria, and bacterial catalysis commences. After 7-9 months, bioleaching is complete, with about 80% of the copper extracted from the ore. A heap leaching method has been developed to process precious metal. ores in which elemental gold and silver are encased in sulphide minerals. Precious metal ores, amenable to mineral bio-oxidation, are called refractory sulphidic precious metal ores.
	As with bacterial leaching of copper sulphide ores, the refractory sulphidic precious metal ores are crushed but usually to a larger size and stacked on impermeable pads to heights of approximately 6-12 m. Bacteria can be added to the crushed ore as it is stacked onto lined pads. With drip irrigation of the heap using dilute sulphuric acid containing low concentrations of ammonium and phosphate ions, the bacteria in close proximity to the sulphide minerals rapidly oxidise the pyrite and arsenopyrite in which the gold and silver are embedded.

Within several months, depending on the ore characteristics, the bacteria and ferric iron have oxidised the sulphide minerals, exposing the elemental gold and silver. The ore is then washed with water to remove acid, soluble heavy metals and iron.

The ore is neutralised and treated with a dilute sodium cyanide solution or other reagents (thiourea or thiosulphate) that solubilise the precious metals. Bio-oxidation of refractory sulphidic precious metal ores is called pretreatment because the ore is subjected to an additional treatment process before undergoing conventional metallurgical extraction with gold solubilising reagents.

Applications of Microbial Interactions Part 2
Causes of Corrosion
Oxygen Influencing Corrosion
Anaerobic Microbial Corrosion
Pitting Corrosion
Protection from Corrosion
Microbially Induced Concrete Corrosion
Bio-Mining
Microbiology and Chemistry of Ard
Bioleaching and Bio-Oxidation
Bio-Accumulation
Xenobiotics
Bio-Geochemical Cycles
Carbon Cycle
Sources of Carbon Cycle
Scources of Organic Carbon
Mineralisation of Carbon
Decomposition and Carbon Dioxide Evolution
Carbon Assimilation
Nitrogen Cycle
Steps in Nitrogen Cycle

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