The effect of CaO and MgO addition and cooling rate on stability of slag obtained after jarosite and neutral leaching residue treatment in the Waelz process

Jarosite and neutral leaching residue (NLR) are the greatest environmental problems of hydrometallurgical zinc production due to their high quantity and hazardous content. Pyrometallurgical processes, such as the Waelz process, could be applied to recover valuable metals present in this waste. The paper investigates the possibility of forming environmentally stable Waelz slag after the Waelz process of jarosite and NLR. Waelz slag, obtained using CaO as a conventional alkaline additive in the Waelz process, showed high As and Sb release after the EN 12457-4 standard leaching test. Multiphase equilibrium composition calculations indicated that As and Sb were present as volatile and migratory species in the slag. In order to obtain Waelz slag with more stable matrices that would encapsulate and immobilize pollutants, MgO additions to CaO and different cooling conditions of the slag were further investigated. The efficiency of modification in additives composition and cooling conditions was assessed by EN 12457-4 leaching test and chemical and microstructural characterization (XRF and SEM-EDS analyses) of the Waelz slags. The results show that addition of MgO does not reduce the efficiency of the process, even more it increases leaching resistance of the slags. Concentrations of all investigated metals (As, Ba, Cu, Mo, Pb, Sb, Zn) after the leaching test were below defined limits for non-hazardous waste. Microstructural analyses revealed that MgO remained inert during Waelz process, and thus favoring the formation of amorphous stable structure, which was enhanced by increased cooling rate. All investigated Waelz slags with MgO additions are suitable for further use or safe disposal