Stability of Silicate Coatings on Pyrite Surfaces in a Low pH Environment

The suppression of pyrite oxidation due to the formation of stable coatings of Fe^3+-silica and/or Fe^3+-hydroxy-silica complexes over pyrite surfaces in near-neutral and neutral solution pHs is well documented. On the contrary, there is currently no convincing experimental evidence that proves the existence of such a coating under low pH environments. Aqueous continuous flow kinetic experiments were conducted using coating solutions containing Na2SiO3·5H2O and H2O2 at pH 6, 4, 3, and 2. Results showed that at pH 2, pyrite oxidation in the presence of silica is pseudofirst order and oxidation is not inhibited by silica. At pH 6, silica significantly reduced pyrite oxidation via a pseudofirst-order reaction that takes on several stages related to changes in the relative contributions of H2O2 and Fe^3+. Iron speciation data suggests a surface adsorption mechanism at pH 6 and an electrochemical mechanism at pH 2 for pyrite oxidation. Scanning electron microscopy and energy dispersive x-ray spectroscopy also support the kinetic model and showed the absence of any significant silica coating at pH 2.