Abstract
Natural monoclinic pyrrhotite particles (Fe 1− x S) were subjected to pressure leaching by oxygen in sulphuric acid solutions at temperatures ranging between 353 and 453 K (80–180°C). For temperatures below the melting point of sulphur (392 K), the rate of pyrrhotite oxidation shows a moderate dependence on temperature, while it is totally independent of sulphuric acid concentration. Nonetheless, in the absence of oxygen, as much as 30% of the mineral can be dissolved in 0.5 mol/l H 2SO 4. The conversion data were found to fit well to a shrinking-core model with mixed control by half-order surface reaction and oxidant diffusion though a product layer. Despite the high initial reactivity of pyrrhotite, complete oxidation of the mineral was never achieved at temperatures below 393 K, apparently due to an impervious sulphur product layer covering the particles. Complete pyrrhotite oxidation was achieved at temperatures above the melting point of sulphur and only with the use of lignin sulphonate as dispersant of molten sulphur. By analysing the conversion data with the shinking-core model, pyrrhotite oxidation in the high temperature range (403–453 K) was found to be surface-reaction controlled and of first order with respect to oxygen pressure.
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