Dissolution behavior of Fe, Co, and Ni from non-ferrous smelter slag in aqueous sulphur dioxide

Large amounts of non-ferrous slags containing Co, Ni, and Cu are produced and stockpiled by metallurgical industries. These industries also produce sulphur dioxide, an air pollutant. Using sulphur dioxide as a leaching reagent to leach the valuable metals from slag would reap both economical gains and environmental benefits. Extraction of these metals using aqueous sulphur dioxide (SO2(aq)) is being carried out in our group. I software, was applied to study the thermodynamics of slag–SO2 system. A private data bank was created and incorporated into the OLI software. Unknown thermodynamic data, mainly for metal-S(IV) complexes, were estimated and used for the simulation. The solubility of slag in SO2(aq) increased with increasing concentration of SO2(aq) and decreased with increase in temperature, ionic strength and pH. The main species in the system were the metal-S(IV) species. About 80% of dissolved Fe was present as FeSO3o, while FeHSO3+ and Fe2+ each accounted for about 10%. Co– and Ni–SO3 complexes contributed about 60% of the dissolved Co and Ni while the Co– and Ni–HSO3 complexes and free metal ions each contributed about 20%. As the total concentration of S(IV) increased in the system the species FeSO3o would become saturated and precipitate as FeSO3·3H2O. This suggested that iron could be removed from the system as sulphite and the Co and Ni in the solution could be enriched. Lower temperatures and ionic strengths enhanced the formation of this precipitate. The calculated solubility was compared with experimental results and a good agreement was obtained. mentally, when the pH of the leach solution was increased FeSO3·3H2O precipitate was obtained. As a result the ratio of Co to Fe in solution doubled while Ni to Fe ratio increased nearly four times. The increase of ratio was less than expected likely due to the coprecipitation of Co and Ni with Fe. The metal / Fe ratio in solution was sensitive to pH. High metal / Fe ratio is obtained within a pH range of 3–4; pH values above 5 result in overall removal of metals from the solution.