Valorisation of waste galvanizing dross: Emphasis on recovery of zinc with zero effluent strategy.

Galvanizing dross-a waste product from steel industries but it can be a potential secondary resource for zinc through urban mining and recycling. In this concern, a novel and scalable recycling route with zero effluent strategy is developed for the recovery of zinc from galvanizing dross as high grade zinc salts along with value-added products through hydrometallurgical processing. In particular, as-such dross block was leached in 9% (v/v) sulphuric acid medium, wherein strong hydrogen gas effervescence results in alleviating the pulverization and stirring requirements; which are material and energy intensive. Leached zinc is purified and recovered as high purity ZnSO.(HO/7HO) and Zn(PO).4HO through controlled crystallization and phosphate precipitation respectively; which find application in fertilizers and anti-corrosive paints. Temperature difference method was opted for the crystallization of zinc sulfate salts, wherein 70 °C and 30 °C were found to be stability range of crystallization of ZnSO.HO and ZnSO.7HO respectively. ZnSO-HO phase diagram is developed using Factsage calculations to corroborate crystallization study. Moreover, exhaustive thermodynamic analysis of Zn-PO-HO system at 303 K on precipitation of zinc phosphate using di-ammonium hydrogen phosphate (80-240 g/L) is conducted and the results reveal that with increasing pH (3-6), intermediate hydrogen phosphate species (HPO, HPO and HPO) decompose to produce stable PO ions leading to zinc phosphate precipitation. Impurity like Iron and supernatant solution left after crystallization are recovered as hydrated iron-calcium sulfate mixture and ammonium sulfate salt respectively. This explored route is economical and easily adaptable with zero effluents, therefore, transcends serious challenges in terms of energy requirement, scale-up and effluent generation.