Gradient Separation and Recovery of Pb, Se, Cu, and Hg from Acid Sludge by a Sustainable Hydrometallurgical Process

Acid sludge, a by-product of the Cu smelting process rich in Pb, Se, Cu, Hg, and other valuable metals, is a highly recyclable smelting material. Due to its high selenium content and various phase structures that form inter-chemical and inclusion structures with associated minerals such as copper and mercury, selective separation and recovery of Pb, Cu, Se, Hg and other components are limited. To address this problem, a cascade separation process of “H₂SO₄ + NaClO₃ oxidized coordinated leaching—HCl and Na₂SO₃ selective reduction of selenium—H₂C₂O₄ reduction of copper precipitation—NaH₂PO₂ reduction of mercury precipitation” was used for the efficient recovery of these metals from acidic sludge. The results showed that excellent outcomes have been obtained under optimal process parameters at each stage. In the oxidation leaching stage, Pb remains in the slag, Se, Cu, and Hg are leached into the solution, and the leaching rate is above 99%. Under appropriate concentrations of hydrochloric acid, Se was selectively separated in a complexation reaction with Na₂SO₃. The precipitation rate for Se was almost 100%, with Se product purity reaching up to 99.4%. After that, the precipitation rate of Cu in the oxalic acid precipitation is more than 99%, and the precipitation rate of Hg in the sodium hypophosphite reduction process is more than 99%. In addition, 99.09% of total lead, 97.64% of total selenium, 98.97% of total copper and 98.08% of total mercury in the acid sludge entered their separation products. During the process, the acid sludge's metals are effectively separated without introducing difficult impurity ions.

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