Sphalerite dissolution kinetics at low hydrothermal conditions

A macroscopic (mixed-flow reactor) and microscopic (hydrothermal atomic force microscopy, HAFM) approach was used to quantify sphalerite (ZnS) dissolution rates and to investigate the stoichiometry of dissolution and leached layer formation. HAFM observations of reacted (110) surfaces at room temperature revealed surface roughening which was likely generated by pit formation, local surface swelling or re-deposition of sulphur compounds, as also evidenced by a decreased hardness of the roughened surface. At 125 °C, appearance of hillocks with three-sided facets was observed on the (110) surface, most likely of the forms {111} or {1¯1¯1¯}. On the facets, terraces were separated by steps. At these steps, preferential release of material took place during dissolution and caused the steps to retreat.Macroscopic dissolution rates measured in mixed-flow reactors at pH 2 and 125, 150 and 200 °C are in reasonable agreement with literature data and yielded an apparent activation energy of Zn-release of 96 ± 4 kJ/mol. Elevated dissolution rates were observed during the initial stage of the experiment. The time necessary to attain steady-state ranged from 110 to 250 h. Measured Zn/S concentration ratios in the reactor effluent were at average 1.1 ± 0.1. These ratios do not indicate extensive leached layer formation and agree with HAFM observations that revealed a morphology transition from {110} to {111} or {1¯1¯1¯}. The addition of FeCl3 to the solution yielded a strong rate increase with an apparent reaction order with respect to Fe3+(aq) ranging from 0.41 to 0.48.

Research highlights
► Sphalerite dissolution was studied by mixed-flow reactor and HAFM experiments.
► At 125 °C, {110} surfaces likely transformed into {111} facets with visible step retreat.
► The morphological transition correlated with an initial dissolution rate maximum.
► Zn/S release rate ratios did not support extensive leached layer formation.
► A high apparent activation energy pointed to surface controlled dissolution.