PYRRHOTITE LEACHING IN ACID MIXTURES OF HCL AND H2SO4

Pyrrhotite (Fe7S8) was leached in air-equilibrated pH 3.0 HCl H2SO4 ac id mixtures with Cl-:SO42- ratios of 1:0, 3:1, 1:1, 1:3 and 0:1, AES d epth profiles and XPS analyses of reacted surfaces were consistent wit h several compositional layers including a surface coating of Fe(III)- oxyhydroxide, an underlying zone of sulfur enrichment which decreased in sulfur content with depth, and finally unaltered pyrrhotite. Leachi ng experiments showed that iron concentrations increased linearly in s olution as a function of the square-root of time. This relationship wa s indicative of a diffusion-limited reaction. Sulfate concentrations i ncreased rapidly to 1.0 ppm within the first few minutes of reaction, then remained unchanged over the duration of the experiment. These res ults demonstrate that sulfate release was a rapid one-time event in th e earliest stages of pyrrhotite dissolution. Pyrrhotite leaching in ac id solutions proceeded via the diffusion of iron to the mineral surfac e. With the removal of iron from the pyrrhotite structure, polysulfide replaced monosulfide as the dominant sulfur species. The Fe(III)-oxyh ydroxide was determined to be the product of reaction between oxygen a nd iron species at the surface, Pyrrhotite surfaces reacted in solutio ns containing the greater sulfate concentrations were found to have th e thickest Fe(III)-oxyhydroxide layers. In contrast, surfaces reacted with solutions containing appreciable chloride developed sulfur-rich n ear surfaces with an overlying thin veneer of Fe(III)-oxyhydroxide. Re sults of the study suggest that chloride inhibited the formation of su rface Fe(III)-oxyhydroxides and promoted the development of sulfur-ric h sublayers. Cl-:SO42- ratios in solution did not appear to have any s ignificant effect on leach rates of iron.