Results of a preliminary study of the ammoniacal leaching of covellite (CuS) in an aqueous oxidizing system in the presence of a non-miscible organic sulfur solvent are described. Up to 60% of the sulfur can be recovered from the solvent as elemental sulfur while achieving high degrees of solubilization of the mineral.
Leaching tests, both with and without an added sulfur solvent were carried out in a two litre stainless steel autoclave which had been fitted with a magnetically driven stirrer. The autoclave was fitted with a modified turbine type agitator of the gas dispersion type, helical cooling coils and an external electrical heating jacket. Measured quantities of cupric sulfide, ammonium sulphate (when used) and deionized water were charged to the autoclave which was then heated, after flushing with oxygen, to somewhat above the desired operating temperature. The desired amount of aqueous ammonia and organic liquid were then added from a pressure cell into which they had previously been measured.
The effect on the reaction rates of an added immiscible organic phase to the leach solution was determined in duplicate tests with and without the organic. Results shows clearly that the presence of the organic phase did not significantly affect leaching rates at the conditions used.
An unusual feature of the leaching results which appears to be quite general for any conditions or any solvent was observed. The corresponding molar ratio of sulfur recovered in the solvent per mole of copper leached. This ratio remained constant over virtually the entire leaching time and indicates that the rate of sulfur extraction as elemental sulfur parallels very closely the rate of copper dissolution. Further, it suggests strongly that very little oxidation of the sulfur occurs once it is in solution in the organic phase. As mentioned, this nearly constant ratio of sulfur recovered per mole of copper leached is characteristic of all leaching tests.
A series of tests were carried out to investigate the effects of oxygen partial pressure. There is relatively little effect of oxygen variation except a modest one at the lowest pressures, on the fractional amount of sulfur recovered, although reaction rates varied widely over this range of oxygen pressures. Fractional sulfur recoveries increased only by about 20% over a more than twenty-five fold change in oxygen pressure. Hence, over modest ranges, it could be assumed that fractional recovery of sulfur is independent of the oxygen partial pressure. These results also offer good evidence that the leaching rate is not subject to rate limitations by oxygen mass transfer processes.
Ammonium sulfate additions were made in a series of tests in amounts varying from zero to approximately 2 moles/litre.
The preceding tests in which ammonium ion and aqueous ammonia concentrations were varied affect the hydroxyl ion concentration of the solution. It has been postulated that the rate of oxidation of elemental sulfur in ammoniacal solutions depends on the hydroxyl ion concentration according to the reaction,
2S° + 2OH- + O2 → S2O3= + H2O
The ability to recover elemental sulfur in an oxidizing ammoniacal system depends on two effects; first, a kinetic rate difference between the dissolution rate of copper from a mineral lattice and the surface oxidation of sulfur from the elemental form to soluble oxidized species, and secondly, the fact that elemental sulfur can be removed from the solid surface by a suitable solvent and protected from oxidation once it is dissolved.
The electrochemical model of leaching offers one description of some of the surface reactions proceeding. According to this model, the following steps are postulated,