Improvements in the selective cementation of Cd and Ni/Co from zinc industrial electrolyte

The selective cementation of cadmium, cobalt, and nickel from synthetic and an industrial zinc sulfate (150–160 g.L−1) electrolyte was investigated. The desired cadmium removal (from 640 to 740 mg.L−1 to <1 mg.L−1) was achieved at ambient temperature, with staged addition of zinc dust, using a Zinc/Metals molar ratio in excess of 300%, which is about 50% lower than typical ratios used in industries. An enriched Cd cement (29% wt. vs. app. 7% wt. in industrial samples) with low nickel (0.05% wt.) content was produced in one purification stage. The zinc content in the Cd cement was 20% lower relative to that in the industrial cement. In a second stage, fast and high cobalt and nickel removals were obtained after the pre-activation of zinc dust with copper and antimony. The presence of 150–160 g.L−1 Zn did not affect cobalt removal but made it difficult for nickel cementation. The industrial specification for the purified solution that feed the zinc electrowinning process was achieved for cobalt (< 0.2 mg.L−1) and nickel (< 0.5 mg.L−1), after 60 min of reaction time. The nature, morphology, and chemical composition of the cementation products were evaluated by X-ray diffraction analyses and electron microscopy techniques. Finally, an alternative two-stage purification flowsheet was proposed to produce a rich cadmium cement with low nickel content (0.05% wt.), and a cobalt‑nickel cement. The process allows for a reduction in the consumption of zinc dust and energy, as well as the reaction time.