Zinc-bearing sludge deposits resulting from metallurgical zinc production have been investigated. They were found to contain 11.0-13.0% of zinc, 1.4-1.5% of copper, 1.1-1.3% of arsenic, and ca 1% of lead, plus small amounts of cadmium and nickel (0.5 and 0.1%, respectively). The results of the leaching of these deposits with hydrochloric, sulfuric, and lactic acids, as well as with ammonia and NaOH, are presented. The composition of the leachates was dependent on the leaching reagent used. The effectiveness of leaching decreased in the following order of the reagents used: ammonia (10.5%), HNO₃ and NaOH (10.0% each), H₂SO₄ (9.5%), HCl (9.0%), and lactic acid (8.0%). Due to the poor selectivity of the strong mineral acids used, the most effective leaching reagents were concentrated ammonia, NaOH, and lactic acid. The recovery of zinc using electrolysis and solvent extraction also was evaluated. As much as 92.0-99.0% of zinc was deposited on the cathode for a sulfuric acid solution having a pH in the range 1-2. The extractants used in the extraction process were a 60% TBP solution in toluene for acidic solutions and 1-decyl-2-methylimidazole for weakly acidic and weakly alkaline solutions. From a solution having a pH of around 2.7, Zn(II) ions were most effectively extracted (99.0%), whereas from those of a pH>4, also Cu(II) and Cd(II) (98.5 and 96.0%, respectively) ions were co-extracted along with Zn(II). The Ni(II) ions were most effectively extracted at a pH around 5.5 (74.0%). From solutions left after leaching with NaOH, 1-decyl-2-methylimidazole extracted mostly Zn(II) and Cu(II), whereas from those left after leaching with ammonia, Cd(II) could be extracted in addition to Zn(II) and Cu(II). An optimum pH for zinc recovery was 7.5-8.0.
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