History of bioleaching

One of the earliest records of the practise of leaching is from the island of Cyprus. Galen, a naturalist and physician reported in AD 166 the operation of in situ leaching of copper. Surface water was allowed to percolate through the permeable rock, and was collected in amphorae. In the process of percolation through the rock, copper minerals dissolved so that the concentration of copper sulphate in solution was high. The solution was allowed to evaporate until copper sulphate crystallised. Pliny (23-79 AD) reported that a similar practise for the extraction of copper in the form of copper sulphate was widely practised in Spain.

Prior to invention of electrolysis, the only practical method for the recovery of copper from copper sulphate was by cementation, a process that derives its name from the Spanish word cementacion, meaning precipitation. It is thought that the cementation of copper was known in Pliny’s time, but no written record of its commercial application seems to have survived. The cementation of copper was also known to the Chinese, as documented by the Chinese king Lui-An (177-122 BC). However, the Chinese implemented the commercial production of copper from copper sulphate using a cementation process in the tenth century. The Chiangshan cementation plant started operation in 1096 with an annual production of 190 t per annum of copper. In the Middle Ages, the alchemist Paracelsus (AD 1493-1541) described the cementation of copper as an example of the transmutation of Mars (iron) into Venus (copper).

Heap leaching of copper sulphides on an industrial scale was carried out at the Rio Tinto mine in Spain at about 1752. The ore was crushed and laid on a gently sloping impervious pad. The layers of ore were alternated with beds of wood. Once the heap was constructed, the wood was ignited, resulting in the roasting of copper and iron sulphides. Water was then distributed over the top of the heap. As the water percolated through the heap, the copper and iron dissolved, forming copper and iron sulphates. In 1888, this method of extraction was prohibited by law, because of serious environmental damage caused by the clouds of sulphur dioxide formed. The process of heap leaching, without the roasting step, continued with success until the 1970’s at Rio Tinto. The reason for its success was unknown, but it was thought to be due to ‘some obscure quality either of the Rio Tinto ore or the Spanish climate’. Today, it is known that the microorganism Thiobacillus ferrooxidans played an important role in the success of the operation at Rio Tinto.

Several reports in the early part of last century associated soil microorganisms with weathering of sulphide minerals and of coals containing sulphides. Indeed, it was estimated that in 1940 the production of sulphuric acid due to the weathering of subbitumuous coals resulted in the discharge of several million tons of sulphuric acid into the Ohio River. This level of pollution was alarming, and universities and several US government institutions, such as the US Bureau of Mines, began to search for the causes of this sulphuric acid. These investigations found that the cause of the sulphuric acid was the oxidation of pyrite contained in subbitumuous coal, and that this oxidation occurred at rates in excess of those suggested by inorganic chemistry. In addition, the occurrence of sulphur oxidising bacteria was noted. A couple of years later, in 1950, a new species was identified and named Thiobacillus ferrooxidans. This organism is able to oxidise elemental sulphur and ferrous ions at much higher rates than can be achieved by inorganic chemistry. Indeed, it is this catalysis of the oxidation of ferrous ions that makes Thiobacillus ferrooxidans and other iron and sulphur oxidising microorganisms such important catalysts in the extraction of metals in bioleaching processes.

The isolation of sulphur-oxidising microorganisms from hot springs by Brierley and by Brock in the late 1960’s provided the opportunity of operating reactors at much higher temperatures. Woese proposed in the late 1970’s that these and similar microorganisms belonged to an entirely new kingdom of life, called the archae. Woese’s proposal that these thermophilic microorganisms are as different from bacteria as eucaryotes are from bacteria was verified by detailed genetic studies in 1996. Iron- and sulphur-oxidising archae are presently the microorganisms of choice in the development of tank leaching processes for the extraction of base metal sulphides.

Microorganisms currently used in commercial bioleaching operations (both stirred tank and bioassisted heap leaching) are ubiquitous in nature. Wherever a suitable ore is exposed to the surface and water is present, the microorganisms will be found occurring naturally. Microorganisms used in commercial operations are exactly the same as those found in nature, the only difference is that in some cases they have been selected for rapid growth on the ore or concentrate concerned. The growth of biomining microorganisms is inhibited or prevented in the presence of organic matter. The microorganisms also only function in specific temperature ranges, require iron or reduced sulphur as an energy source and grow optimally at pH < 2.5. These extreme conditions or requirements prevent the growth of the microorganisms on plants, insects or animals, including humans. They are therefore non-pathogenic and there have been no reports of illness due to these microbes.

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