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

Abstract

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+-PO43--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 H2PO42−) decompose to produce stable PO43− 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.