Sulphide oxidation passivation technique

Abstract

It is known that once exposed to an oxidizing environment (water or oxygen) as series of reactions occur at the surface of sulfide minerals, as pyrite, and lead to acid drainage generation.The major steps of pyrite oxidation are: (1) Oxidation of sulfur in presence of atmospheric oxygen [1] FeS2 + 7/2O2 + H2O 􀃙 Fe2+ + 2SO42‐ + 2H+ (2) Oxidation of ferrous iron Fe(II) (production of ferric iron Fe(III)). At low pH Fe the reaction rates strongly increased by microbial activity (e.g.,Acidithiobacillus ferrooxidans). Bacteria use the reaction as an energy‐generating process, acting as a catalyze Fe2+ oxidation to Fe3+, with rates 5 or 6 times higher than in sterile conditions, increasing acid generation. The soluble Fe3+ formed under these conditions, can effectively scavenge electrons from S(‐1) in pyrite, generating more Fe2+ once again, and this process is recycled. [1,2] Fe2+ + 1/4O2 + H+ 􀃙 Fe3+ + 1/2H2O FeS2 + 14 Fe3+ + 8H2O 􀃙 15 Fe2+ + 2SO42‐+ 16H+ (3) Hydrolysis and precipitation of ferric complexes and minerals (ferryhidrite, schwertmannite, goethite or jarosite) when the acid mine water, rich in ferric iron , reaches the surface. Most of the acid is produced at this stage [1]. Fe3++ 3 H2O 􀃙 Fe(OH)3(s) + 3H+ The development of a feasible, low cost and long‐term inhibition of Fe2+ oxidation process at sulfide surface in solid waste accumulations could drive the mining industry towards a more sustainable future. Such technology should be focused on the understanding of the chemical and biological processes occurring on the surface of the sulphide minerals, as pyrite and pyrrhotite. Different strategies to this problem have been proposed in the literature in the last years. Among them, the technologies designed to generate a physical barrier between pyrite surface and the oxidants agents seem to be the most promising techniques. Iron‐phosphate or silicate coatings [3,4,5], complexation of ferric iron [6,7] and, more recently, the effect of lipids with two hydropobic tails on pyrite surface [8,9] prove to be effective in laboratory studies. All the techniques require the addition of specific chemical solutions to the waste rock as, phosphate or silicate rich solutions, H2O2, chemical complexing agent, lipids and organic solvents. However, none of these methodologies has been validated on field conditions mainly because of scale factor, which makes difficult to estimate the cost and environmental impact that these solutions may have. Boojum Research LTD has been actively trying to address this problem since 1992, developing simultaneously laboratory (waste rock drums) and field tests (Inco, Buchans, Stanrock, etc) where natural phosphate rock (NPR), alone or combined with other components (horse manure, straw, etc) was added to waste rock and tailings, respectively. This technology is proposed by this company has being suitable to groundwater and seepage treatment but, most of all, as a long‐term AMD passivation technique. Some parameters of this technology are not fully optimized but, field and lab demonstration tests have shown that a long‐term inhibition technique may be possible due to the chemical properties of NPR plus the initiation of a biofilm coating on pyritic surfaces. However, the passivation mechanisms still need to be clarified in order to develop a final commercial product. In this report we evaluate, for the first time, the surface of a complete set of rocks use by Boojum in two exposure periods of the Waste Rock Experiment (WRD) in order to gather more basic information for the passivation concept comprehension.