Selective recovery of nickel from stainless steel pickling sludge with NH3-(NH4)2CO3 leaching system

The recovery of valuable metals from stainless steel pickling sludge(SSPS) has great economic and environmental benefits. In this study, a new method is proposed for selective recovery of nickel from SSPS by NH3-(NH4)2CO3 ammonia leaching system. The Eh-pH diagram was used to analyze Ni, Fe, Cr leaching behavior during the ammonia leaching process. Nickel can be leached as the complex [Ni(NH3)n]2+, whereas Fe and Cr remain as precipitates in the leaching slag. The effects of NH3·H2O concentration, liquid–solid ratio, reaction temperature, and reaction time on the leaching efficiency of nickel in the ammonia leaching system were analyzed and optimized by single-factor study and response surface analysis, and the kinetics were analyzed. The optimal conditions for Ni leaching were found to be 28.28 min, 54.07 °C, a liquid–solid ratio of 23.7:1, and NH3·H2O concentration of 5.10 mol/L. Each factor had a greater effect on the rate of Ni leaching in the following order: liquid–solid ratio > NH3·H2O concentration > leaching time > leaching temperature. The ammonia leaching recovery system was controlled by chemical reaction and the activation energy was 58.17 KJ/mol. The results of scanning electron microscopy-energy dispersion spectrum (SEM-EDS), x-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) show that the leaching slag was in granular form with agglomerated particles and particle size of approximately 2.8 μm The major components of the leaching slag were Fe(OH)3, Fe2O3, Fe(OH)2, Cr(OH)3, and Cr2O3. Therefore, this study provides a new and effective way of using the resources of SSPS.