Leaching (minerals)
Leaching is a natural process that involves the movement and concentration of minerals through soil layers, which can also be harnessed for industrial mineral extraction. This process is particularly significant in the recovery of valuable metals like copper, uranium, and gold, especially when conventional methods are economically unfeasible. In cases where metals are poorly soluble or present in low concentrations, bioassisted leaching or bioleaching—utilizing specific microorganisms—becomes an effective strategy. For example, in copper extraction, bacteria such as Thiobacillus ferrooxidans oxidize low-grade ore, making the copper soluble and recoverable.
Leaching also plays a critical role in environmental systems, contributing to geochemical cycles necessary for nutrient availability in ecosystems. However, it can also lead to issues such as acid mine drainage, which poses environmental hazards by releasing sulfuric acid and heavy metals into waterways. Additionally, leaching can deplete essential nutrients like nitrogen and phosphorus from soil, potentially leading to ecological imbalances and health risks through contamination of drinking water. Understanding leaching is essential for both resource recovery and environmental management, reflecting its complex role in both human industry and natural ecosystems.
Leaching (minerals)
Leaching is the removal of insoluble minerals or metals found in various ores, generally by means of microbial solubilization. Leaching is significant as an artificial process for recovering certain minerals, as an environmental hazard, notably as a result of acid mine drainage, and as a natural geochemical process.
Background
Leaching is among the processes that concentrate or disperse minerals among layers of soil. Leaching is a natural phenomenon, but it has been adapted and applied to industrial processes for obtaining certain minerals. The recovery of important resource metals such as copper, uranium, and gold is of significant economic benefit. However, if the metal is insoluble or is present in low concentration, recovery through conventional chemical methods may be too costly to warrant the necessary investment. Bioassisted leaching, often referred to as microbial leaching or simply bioleaching, is often practiced under such circumstances. The principle behind such biotechnology is the ability of certain microorganisms to render the metal into a water-soluble form.
Bioleaching of Copper Ore
The production of copper ore is particularly illustrative of the leaching process. Low-grade ore containing relatively small concentrations of the metal is put into a leach dump, a large pile of ore intermixed with bacteria such as Thiobacillus ferrooxidans. Such bacteria are able to oxidize the copper ore rapidly under acidic conditions, rendering it water soluble. Pipes are used to distribute a dilute sulfuric acid solution over the surface of the dump. As the acid percolates through the pile, the copper is solubilized in the solution and is collected in an effluent at the bottom of the pile. Two forms of the copper are generally found in the crude ore: chalcocite, Cu2S, in which the copper is largely insoluble, and covellite, CuS, in which the copper is in a more soluble form. The primary function of the Thiobacillus lies in the ability of the bacteria to oxidize the copper in chalcocite to the more soluble form.
A variation of this method utilizes the ability of ferric iron, Fe+3, to oxidize copper ore. Reduced iron (Fe+2) in the form of pyrite (FeS2) is already present in most copper ore. In the presence of oxygen and sulfuric acid from the leaching process, the Thiobacillus will oxidize the ferrous iron to the ferric form. The ferric form oxidizes the copper ore, rendering it water soluble, but becomes reduced in the process. The process is maintained through continued reoxidation of the iron by the bacteria. Since the process requires oxygen, the size of the leach dump may prove inhibitory to the process. For this reason, large quantities of scrap iron containing ferric iron are generally added to the leach solution. In this manner, sufficient oxidizing power is maintained.
Generally speaking, those minerals that readily undergo oxidation can more easily be mined with the aid of microbial leaching. As illustrated in the foregoing examples, both iron and copper ores lend themselves readily to such a process. Other minerals, such as lead and molybdenum, are not as readily oxidized and are consequently less easily adapted to the process of microbial leaching.
Leaching of Gold
The extraction of gold from crude ore has historically involved a cyanide leaching process in which the gold is rendered soluble through mixing with a cyanide solution. However, the process is both expensive and environmentally unsound, owing to the highly toxic nature of the cyanide. In an alternative approach that uses bioleaching as a first stage, crushed gold ore is mixed with bacteria in a large holding tank. Oxidation by the bacteria produces a partially pure gold ore; the gold can then be more easily recovered by a smaller scale cyanide leaching. The process was first applied on a large scale in Nevada; a single plant there can produce 50,000 troy ounces (1.6 million grams) of gold each year.
Acid Mine Drainage
The spontaneous oxidation of pyrite in the air contributes to a major environmental problem associated with some mining operations: acid mine drainage. When pyrite is exposed to the air and water, large amounts of sulfuric acid are produced. Drainage of the acid can kill aquatic life and render water undrinkable. Some of the iron itself also leaches away into both groundwater and nearby streams.
Natural Leaching and Geochemical Cycling
The leaching of soluble minerals from soil contributes to geochemical cycling. Elements such as nitrogen, phosphorus, and calcium are all found in mineral form at some stages of the geochemical cycles that are constantly operating on the Earth. Many of these minerals are necessary for plant (and ultimately, human) growth. For example, proper concentrations of calcium and phosphorus are critical for cell maintenance. When decomposition of dead material occurs, these minerals enter into a soluble “pool” within the soil. Loss of these minerals through leaching occurs when soil water and runoff remove them from the pool. Both calcium and phosphorus end up in reservoirs such as those in deep-ocean sediments, where they may remain for extended periods of time.
Percolation of water downward through soil may also result in the leaching of soluble nitrogen ions. Both nitrites (NO2-) and nitrates (NO3-) are intermediates in the nitrogen cycle, converted into such forms usable by plants by the action of bacteria on ammonium compounds. Nitrate ions in particular are readily absorbed by the roots of plants. The leaching of nitrites and nitrates through movement of soil water may result in depletion of nitrogen.
In addition to the loss of nitrogen for plants, leaching can lead to significant environmental damage. Since both nitrite and nitrate ions are negatively charged, they are repelled by the negatively charged clay particles in soil, particularly lending themselves to leaching as water percolates through soil. High concentrations of nitrates in groundwater may contaminate drinking water, posing a threat to human health.
Bibliography
Atlas, Ronald M., and Richard Bartha. Microbial Ecology: Fundamentals and Applications. 4th ed. Menlo Park, Calif.: Benjamin/Cummings, 1998.
Burkin, A. R. “Chemistry of Leaching Processes.” In Chemical Hydrometallurgy: Theory and Principles. London: ICP, 2001.
Keller, Edward A. Environmental Geology. 8th ed. Upper Saddle River, N.J.: Prentice Hall, 2000.
Killham, Ken. Soil Ecology. New York: Cambridge University Press, 1994.
Madigan, Michael T., John M. Martinko, Paul V. Dunlap, and David P. Clark. Brock Biology of Microorganisms. San Francisco: Pearson/Benjamin Cummings, 2009.
Marsden, John, and C. Iain House. “Leaching.” In The Chemistry of Gold Extraction. 2d ed. Littleton, Colo.: Society for Mining, Metallurgy, and Exploration, 2006.
Robertson, G. P., and P. M. Groffman. “Nitrogen Transformations.” In Soil Microbiology, Ecology, and Biochemistry, edited by Eldor A. Paul. 3d ed. Boston: Academic Press, 2007.