8. Spent Electrolyte Regeneration and Recovery of Associated Valuable Metals from Lean Leaching Solution

The impurities in the electrowinning solution can be accumulated during the cycle of alkaline solutions. The integrated process of regeneration and purification of spent electrolyte by causticization is developed. The addition of solid NaOH can be used to separate Na₂CO₃ in the spent electrolyte. The Na₂CO₃ is causticized using CaO to regenerate NaOH. During this separation and causticization process, the impurity ions in the spent electrolyte are removed simultaneously. The causticized alkaline solution is recycled to the next leaching step. Moreover, Mo, W, Nb, Ta, etc., will be also dissolved in alkaline solution under the leaching conditions of zinc and lead hazardous wastes. The Mo and W can be recovered with ion flotation and adsorbing colloid flotation process using primary amine dodecylamine (RNH₂) as collector and ferric ions or magnesium chloride as co-precipitants or depressant (modifier). Adsorbing colloid flotation (ACF) and ion flotation (IF) using ferric ions as the co-precipitant or sorbent and sodium dodecyl sulfate as the collector can be used for the effective removal of arsenic and molybdenum from aqueous solution. The synergistic extraction of phosphorus, arsenic, and silica from tungstate and molybdate solutions with primary amines and TBP can be realized. Mixed solutions of 12-molybdophosphoric acid and its reduced molybdenum blue solutions are found to be an effective scrubbing process for the combined removal of H₂S, SO₂, and NO_x from gas streams. When the molar ratio of NO₂/(SO₂ + H₂S) in the waste gases is just equal to 1/2, H₂S, SO₂, and NO_x will be removed quantitatively, while the regeneration of scrubbing solution is not needed. H₂S and SO₂ react with molybdophosphoric acid solution to form elemental sulfur and H₂SO₄, respectively, while molybdophosphoric acid is reduced into blue species, molybdenum blue solution. The reactions of H₂S are rather quickly, while those of SO₂ are much more slowly. The resulting molybdenum blue solution is used further for the removal of NO_x, in which molybdenum blue is rapidly oxidized back into molybdophosphoric acid and NO_x is reduced into N₂. Porous white tungstic acid (white tungstic acid), whose composition is firstly revealed in this work, is used to produce new tungstoniobates and tungstotantalates and a series of heteropoly compounds. Moreover, a quick determination method of tungsten concentration is proposed according to the quantitative relationship between the precipitate of white tungstic acid and pH value of a certain range.