Oxidation of pyrite in low temperature acidic solutions: Rate laws and surface textures

Abstract

Rate laws have been determined for the aqueous oxidation of pyrite by ferric ion, dissolved oxygen and hydrogen peroxide at 30°C in dilute, acidic chloride solutions. Fresh, smooth pyrite grain surfaces were prepared by cleaning prior to experiments. Initial specific surface areas were measured by the multipoint BET technique. Surface textures before and after oxidation were examined by SEM. The initial rate method was used to derive rate laws. The specific initial rates of oxidation (moles pyrite cm −2 min −1) are given by the following rate laws (concentrations in molar units): r sp, Fe 3+ = −10 −9.74 M 0.5 Fe 3+ M −0.5 H+ ( pH 1–2) r sp , o 2 = −10 −6.77 M 0.5 O 2 ( pH 2–4) r sp , h 2 o 2 = −10 −1.43 M H 2 O 2 ( pH 2−4) An activation energy of 56.9 ± 7.5 kJ mole −1 was determined for the oxidation of pyrite by dissolved oxygen from 20–40°C. HPLC analyses indicated that only minor amounts of polythionates are detectable as products of oxidation by oxygen below pH 4; the major sulfur product is sulfate. Ferric ion and sulfate are the only detectable products of pyrite oxidation by hydrogen peroxide. Hydrogen peroxide is consumed by catalytic decomposition nearly as fast as it is by pyrite oxidation. SEM photomicrographs of cleaned pyrite surfaces indicate that prior to oxidation, substantial intergranular variations in surface texture exist. Reactive surface area is substantially different than total surface area. Oxidation is centered on reactive sites of high excess surface energy such as grain edges and corners, defects, solid and fluid inclusion pits, cleavages and fractures. These reactive sites are both inherited from mineral growth history and applied by grain preparation techniques. The geometry and variation of reactive sites suggests that the common assumption of a first-order, reproducible dependence of oxidation rates on surface area needs to be tested.