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 ZnSO₄.(H₂O/7H₂O) and Zn₃(PO₄)₂.4H₂O 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 ZnSO₄.H₂O and ZnSO₄.7H₂O respectively. ZnSO₄-H₂O phase diagram is developed using Factsage calculations to corroborate crystallization study. Moreover, exhaustive thermodynamic analysis of Zn²⁺-PO₄^3--H₂O 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 (H₃PO₄, H₂PO₄^- and H₂PO₄^2-) decompose to produce stable PO₄^3- 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.