Acid mine drainage from sulphide ore deposits

Synopsis

Acid mine drainage is generally regarded as the principal environmental problem caused by the mining of sulphide ore deposits. Acid mine drainage results from the reaction of pyritic material with oxygen and water:

$$Fe{{S}_{2}} + 15/4{{O}_{2}} + 7/2{{H}_{2}}O = Fe{{(OH)}_{3}} + 2{{H}_{2}}S{{O}_{4}}$$

In general, oxidation proceeds by electrochemical mechanisms, with the cathodic reduction of oxygen being sufficiently strongly polarized that dissolution is slow. However, the solubility of Fe(III) increases with decreasing pH. This introduces an alternative, less strongly polarized cathodic reaction, which accelerates the oxidation of pyrite and release of acid. Oxidation may also be accelerated by naturally occurring bacteria, such as Thiobacillus ferrooxidans, which metabolize sulphide, sulphur and Fe(III) species. These bacteria are generally most active in acidic waters. Hence both the electrochemical and biological oxidation mechanisms are autocatalytic; once acid mine drainage develops, the problem tends to worsen and is extremely difficult to halt. Appreciable work has been done on acid mine drainage from coal mines. However sulphide ore deposits present an additional environmental threat; many sulphides are oxidized by acidic waters containing Fe(III), and release alarming amounts of heavy metals into affected drainage. Unfortunately, there is evidence that coal and ore pyrite exhibit different oxidation behaviour, thus existing preventive and mitigative technology addressing acid mine drainage from coal mines is not necessarily appropriate for sulphide ore deposits.

The West Shasta Copper-Zinc Mining District of northern California exemplifies the problems that can result from poor mining practices. This district contains a large number of abandoned and inactive mines in sulphide deposits. Large amounts of pyrite and other sulphides remain in the highly fractured ore-bodies, and in poorly situated tailings and waste piles. The local geology, the climate of mild, very wet winters and hot, dry summers, together with an extremely resilient strain of T. ferrooxidans all provide suitable conditions for rapid oxidation of pyrite, and release of large volumes of acid mine drainage. pH values as low as 0.6 have been measured in drainage from the Iron Mountain Mine, which is on the U.S. National Priority (Superfund) List. In addition to periodic fish kills, acid mine drainage from the district is responsible for much of the chronically high loading of heavy metals in the Sacramento River. Construction of a series of dams for irrigation of the Central Valley and hydroelectric power generation further exacerbated the problems, because this reduced the flow of clean water for dilution of the high volumes of contaminated runoff that follow heavy storms. The principal control and abatement strategies being used in the region are hydrologic controls and mine seals. The efficacy and cost of these methods are discussed critically, along with other techniques such as backfilling, water treatment, and proposed in-situ leaching. Although some of the practices that caused problems in the West Shasta Mining District, such as poor siting of waste management facilities, would not be allowed under current regulations, it is not certain that some of the problems might not arise from present and future development of sulphide ore deposits. The region provides useful insight into practices that should be avoided, and those that may be beneficial in ensuring that modern mines do not develop acid mine drainage problems.