Effect of solution speciation on iron-sulphur-arsenic-chloride systems at 298 K

The speciation in solution of a number of metallurgically important iron-bearing systems are calculated from critically reviewed data. In acidic solution, Fe²⁺ and Fe³⁺ are the predominant iron species, although ferric ion hydrolyzes to Fe(OH)₂⁺ above pH 2. For sulphate and arsenate, HSO₄^- and H₃AsO₄ predominate up to pH to approximately 2 where deprotonation occurs and sulphate and dihydrogen arsenate ions become predominant. For arsenite, the major species is H₃AsO₃ for all acidic pH. In ferric systems where complexing anions are present, the situation is somewhat different, although all systems are substantially dependent upon the concentration of the anions. In the presence of arsenate and sulphate, ferric ion readily complexes to form charged anions, arsenate complexes being more favourable than sulphate in mixed solutions. Chloride plays only a minor role with complexation only occurring at concentrations close to unity. The importance of the solution speciation is discussed with reference to both leaching of iron minerals and in oxidative leaching using ferric ions.