Extraction of zinc sulfate from metallurgical waste to produce metal flux

Significant amounts of iron-containing steelmaking and blast furnace sludges with elevated zinc content accumulated on the territories of ferrous metallurgy enterprises. It is not possible to recycle these sludges into the blast furnace process for iron recovery, because zinc leads to the formation of scaffold buildups in the blast furnace throat zone; nor are they suitable for electric arc furnaces, as the iron in them is in an oxidized form. At the same time, zinc-containing sludges can become a valuable secondary product. Extracting zinc from these sludges is difficult, since zinc is present not only in oxide form but also, to a significant extent, in sulfate or sulfide forms. In this work, the possibilities of zinc extraction from sludges were evaluated using the FactSage software package. Direct carbothermic reduction of zinc sulfide requires temperatures above 2000 °C, which is technologically unattainable in most industrial units. An alternative, patent-protected approach was substantiated and thermodynamically evaluated: introducing calcium and magnesium carbonates (limestone, dolomite) into the charge before waelzing. Upon heating, limestone decomposes to form CaO, which undergoes an exchange reaction with ZnS, converting it into ZnO, after which zinc oxide is reduced by carbon at 1100 °C. Gibbs free energy calculations confirm the feasibility of zinc extraction via the proposed mechanism in the range of 1000 – 1200 °C. The effectiveness of the technology was experimentally demonstrated using sludges from the blast furnace and open-hearth shops of JSC EVRAZ NTMK with an initial zinc content of approximately 5 wt. % (in the form of Zn₃O(SO₄)₂). After roasting briquettes at 1200 °C, the residual zinc content decreased to a fraction of a percent, the degree of iron metallization reached 72 %, and zinc is not detected by phase analysis. Two marketable products were obtained – zinc sublimate and zinc-free metal flux, suitable for return to the metallurgical process.

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