Crystallisation kinetics of iron (oxyhydr)oxides from zinc leaching solution at 95-15C based on the Avrami equation

Here, the Avrami equation was employed to investigate the crystallisation process of iron (oxyhydr)oxides from the ZnSO4-FeSO4 solution. Hydrothermal conditions were as follows: initial Fe2+ concentration = 5-15 g L-1, Zn2+ concentration = 130 g L-1, temperature = 95-150 degrees C, oxygen partial pressure (P-O2) = 0.03-0.73 MPa, agitation speed = 300-600 rpm, seed addition = 5 g L-1. Results showed that increasing temperature, P-O2, and agitation speed can increase the crystallisation efficiency. The crystallisation activation energy of iron (oxyhydr)oxides is about 104.2 kJ mol(-1). During crystallisation, the goethite phase first transforms to hydro-haematite, then to haematite. In contrast, aggregates (precipitated at P-O2 = 0.73 MPa) with high-density acicular crystal have more holes that can adsorb the ZnSO4 solution than spherical crystal aggregates (precipitated at P-O2 = 0.03-0.33 MPa). Therefore, to produce high-grade crystallisation products while improving iron removal efficiency, the optimal conditions were controlled as follows: T = 150 degrees C, P-O2 = 0.3-0.4 MPa, agitation speed = 500 rpm, t = 180 min and adding seeds of 5 g L-1. The resultant (oxyhydr)oxides products containing > 55% Fe, < 1% Zn and < 3% S. Moreover, adding neutralisers can significantly increase the crystallisation efficiency and thus improve iron removal efficiency.