Solubility of pyrite (FeS2) ground for different periods was investigated in acid solutions in air. In the initial dissolution, up to 50 h, the dissolution rate, r, was high, due to oxidized species such as FeSO4 formed by grinding. After 50 h, the surface of pyrite was exposed and the dissolution rate decreased. The formation and dissolution of oxidized species were confirmed by X-ray photoelectron spectroscopy, XPS. From the dissolution rates of Fe and S species, rFe and rS, for 50-200 h, it was found that pyrite dissolves nonstoichiometrically ( rS rFe < 2) and that grinding promotes the dissolution of sulfur species leading to the stoichiometric dissolution ( rS rFe = 2). The increase in rS can be explained by the displacement of S atoms in the crystal due to the grinding. The effective Debye-Waller parameter, Beff, depends on the displacement of constituent atoms in the crystal from their equilibrium positions and is measured from the diffraction intensities of X-rays. The parameter for S atoms in pyrite, Beff(S), increased by one or two orders of magnitude with increasing grinding time, tG, while that for the pyrite, Beff(FeS2), did not change markedly. It was established that rS is closely related to Beff(S). The phenomenon may be regarded as a type of mechanical activation.
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