Bioleaching is the use of bacterial microorganisms to extract precious metals, such as gold, from ore in which it is embedded. As an alternative to smelting or roasting, miners use bioleaching when there are lower concentrations of metal in ore and they need an efficient, environmentally responsible method to extract it. The bacteria feeds on nutrients in minerals, thereby separating the metal from the ore. Other metals that are commonly extracted via bioleaching include silver, zinc, copper, lead and uranium.
Uses Special Microorganisms
This process works because of how special microorganisms act on mineral deposits. These microorganisms are catalysts to speed up natural processes in the ore. The types of bacteria most often used in this process include Leptospirillum ferrooxidans, Thiobacillus ferrooxidans and certain species of Acidianus, Sulfolobus and Sulfobacillus.
Oxidation
The bacteria use a chemical reaction called oxidation to turn metal sulphide crystals into sulfates and pure metals. These constituent parts of ore are separated into valuable metal and leftover sulphur and other acidic chemicals. Eventually, enough material builds up in the waste solution to filter and concentrate it into recoverable metal.
Feasibility
For some types of metal, such as copper, bioleaching is not always economically feasible or fast enough, even with its low cost. In certain areas of the world or with other metals, however, this simple, effective and low-cost method is considered to be a smart choice. For example, developing countries often do not have the infrastructure or capital investment to begin smelting, but their land contains enough ore that its extraction can significantly improve their national economies. Many scientists believe that bioleaching might someday be used to mine metals such as zinc and nickel on the moon.
Low Impact on the Environment
As of the early 21st century, about 20% of the extracted copper in the world came from bioleaching. Mining companies must be careful of pollution that might result from solutions reaching a groundwater source. Overall, however, bioleaching produces a relatively small amount of air pollution and little damage to geological formations, because the bacteria occurs there naturally. An ideal metal deposit must allow a certain amount of water into the rock to carry the bacteria. It should be surrounded by rock that is impermeable to water, however, to make sure that no groundwater gets polluted with sulphur.