Microwave assisted alkaline roasting-water leaching for the valorisation of goethite sludge from zinc refining process

Microwave (MW) alkaline roasting followed by water leaching was studied to selectively extract valuable (Zn, Pb) and hazardous (As) elements for the decontamination of the goethite sludge. During alkaline roasting stable ZnFe2O4 was decomposed to ZnO. The influence of alkali source (KOH, NaOH) and the heating source (conventional vs. microwave) were systematically investigated. Generally, ZnFe2O4 decomposed better upon increasing roasting temperatures. For the water leaching step, different liquid-to-solid ratios (L:S = 10, 3, 2) and addition of alkaline agents (KOH, NaOH) were tested to enhance Zn extraction. NaOH addition to the leaching solution (60 degrees C, 30 min, L:S = 10) gave the best extractions for Zn and Pb for both MW KOH roasted (84.3 +/- 0.4% Zn and 38 +/- 3% Pb) and MW NaOH roasted (83 +/- 1% Zn and 36.3 +/- 0.3% Pb extraction) materials (400 degrees C, 60 min). Alternatively, decreasing the L:S of water leaching to 2 gave a similar Zn extraction (83 +/- 1%) and reduced alkali consumption. By increasing the set MW roasting temperature to 450 degrees C, Zn leachability was 87 +/- 3% (NaOH roasting, water leaching L:S = 2, 60 degrees C, 30 min). Additionally, As removal in the studied system was 98-100%. Interestingly, roasting with KOH increased the Mn solubility as (per)manganate and decreased Zn leachability by consuming OH- for disproportionation reactions of manganate. The leachability of matrix elements Ca and Fe was negligible.