CATHODE PROTECTION AND CONTROL OF AMD

Acid mine (or rock) drainage – AMD or ARD – forms when metal sulfides are exposed to water and oxygen. For pyrite, the acid producing reaction is usually simplified to: 2FeS2 + 7.5O2 + 7H2O = 2Fe(OH)3 + 8H+ + 4SO42- (Stumm and Morgan 1996)(1). A pH of 3 or less is generated and the commonest approach to amelioration is to add lime, a treatment of the symptoms rather than the disease. Alternatively, the problem can be attacked pre-emptively, by preventing the formation of AMD at source, using a technique similar to that employed in the prevention of corrosion by oxidation in metal structures (cathode protection). Corrosion is prevented by converting the structure into a cathode by coupling it with a sacrificial anode. The anode acts as a source of electrons, which move to the cathode through whatever aqueous phase permeates the local environment (groundwater, sea-water or existing AMD for example). The cathode is thereby prevented from oxidizing. Now consider the oxidation of pyrite in terms of the following electron-producing half-reaction: FeS2 + 11H2O = Fe(OH)3 + 19H+ + 2SO42- + 15e- (compare Carlson et al., 1992)(2). In nature O2 is the common electron acceptor. Creating an electrochemical cell with pyrite as cathode provides a mechanism for pumping electrons into the right hand side of the half-reaction, stabilizing pyrite and preventing the formation of AMD. This simple set-up was first tested at the Sherman Mine in Northern Ontario as part of the PhD thesis of Gene Shelp. There, a sulphide ore body was wired to a scrap-iron anode and using the ambient AMD of a mine pit as battery fluid, a galvanic cell was maintained for 9 months. A parallel laboratory experiment was performed to demonstrate a rise in pH from 3 to 5.7. Ideally, to minimize AMD,a site would be wired before commencement of mining. However, post-operational treatment of AMD is also feasible and a particulate mixture of sulphide and a more electropositive metal may be employed as a mini-galvanic filter to treat existing AMD (Chesworth & Shelp, 2001)(3). (1) "Aquatic Chemistry" Wiley-Interscience: NY. (2) Applied & Environmental Microbiology 58: 1046-9. (3) ARCSACC Proceedings 01: 240-244.