Production of acid water in a lead-zinc mine,Coeur d'Alene,Idaho

The Bunker Hill Mine in Idaho's Coeur d'Alene mining district produces approximately 10 m³/minute of acid water containing high concentrations of heavy metals. Field and laboratory studies indicate that much of the acid water is produced in a single ore body in the upper part of the mine. The ore of this body contains mainly sphalerite, galena, and pyrite in a siderite-quartz gangue. Ground water recharges this ore body through a near-vertical zone of high permeability, which is the result of mining by the caving technique. Ore samples from the caving area contained oxidized forms of iron and produced acid in a laboratory leaching test. Leaching experiments with several ore samples from the mine also indicated that the ratio of pyrite to calcite in the samples strongly controlled the resultant pH values. Oxidation of pyrite to sulfuric acid and compounds of iron is apparently responsible for the production of acid water in the mine. In contrast, dissolution of calcite in water results in a basic solution, with pH around 8.3, that can neutralize the acid produced by the oxidation process. Methods for prevention of acid mine drainage in this and other similar mines are noted.