Behavior of metals in discard nickel smelter slag upon reacting with sulfuric acid

Characteristics of two nickel smelter slags (one fresh and one 30-years old) and their leaching behavior in sulfuric acid were investigated. X-ray diffractogram revealed that both slags were mainly iron silicate glass, with a small fraction of crystalline fayalite (Fe2SiO4). The fresh slag contained a very small fraction of maghemite (gamma-Fe2O3) in addition. In the 0.1 M sulfuric acid solution, over 35% of Ni and 45% of Co were extracted from the slag (L/S = 10) within 5 hours. Over 65% Ni and 75% Co extraction were reached after 20 hours. Higher acid concentrations accelerated the leaching process. No selective leaching of Ni and Co was observed in the acid at concentrations of 0.05 M or above. The fraction of Fe extracted was comparable to Co. The additives, including NH4+ and Cl-, showed little effect on the selectivity under the condition studied. Further decrease in the acid concentration, however, resulted in a preferential leaching of Ni and Co, which may explain the deficit of Ni and Co in the weathered old slag.