Leaching, Underground

a method of mining a useful mineral in which the mineral is selectively dissolved by chemical reagents in the ore body of the place of occurrence and then removed to the surface. Underground leaching is used for mining nonferrous metals and rare elements, and its use is feasible for mining phosphates and borates as well.

Underground leaching was used to mine nonferrous metals in 16th-century Spain. It was first practiced on a large industrial scale at the Cananea copper mine in Mexico in 1924 and at copper-pyrite deposits in the Urals from 1939 to 1942. Uranium ores have been mined by this method since 1957. Underground leaching is used in a number of countries, including the United States, the USSR, France, Japan, and the German Democratic Republic. In 1974 this method accounted for 20 percent of world copper production.

The selection of the solvent used in underground leaching depends on the composition of the ore and the nature of the chemical compound forming the useful component.

Underground leaching is a filtration process and is based on solid-liquid chemical reactions. In the underground leaching of permeable ore bodies, the deposit is mined by a series of wells arranged in series, polygons, or rings. The solvent is introduced into the wells, filters through the stratum, and leaches the useful components. The working solution is pumped out through other wells.

In the case of monolithic and impermeable ore bodies, the deposit is opened by underground mining excavations and the ore blocks are divided by boring and blasting. The solid mass is then irrigated in the upper level by a solvent, which flows downward and dissolves the useful mineral. The solutions are collected at the lower level and pumped to the surface for recovery.

A major drawback of underground leaching is the slow rate of reaction. To increase the rate of reaction, scientists are studying the way ore material is affected by electric and electromagnetic fields, prior heating, and firing. Nuclear reactions and microbiological methods are also being used for underground leaching.

Underground leaching permits the removal of deposits of poor ores and ore deposits that lie too deep for economical mining by conventional technology.