Precipitation and characterization of zinc borates from hydrometallurgical processing of zinc ash

Zinc ash has been used as a zinc source for precipitation of zinc borates through leaching-purification-precipitation route. Zinc ash containing about 71% Zn, 0.6% Fe, 28% Cl−, 0.2% Pb, and traces other elements such as Cu, Cd, As etc. in ppm level were leached in sulfuric acid. The leach solutions generated were purified by pH adjustment, followed by precipitation of borates using borax (Na2B4O7·10H2O). It has been observed that, the phase compositions of precipitated zinc borates as well as their morphologies are quite sensitive to B/Zn molar ratio, solution pH, precipitation temperature and ageing period. Precipitation of particular zinc borate phase of interest (3ZnO·3B2O3·3.5H2O) was found to be facilitated at higher precipitation temperature (≥95 °C) and in the pH range 5.5-6.0 with B/Zn molar ratio >4.0.