Abstract
Zinc electrolyte crystallization sediment (ZECS) generated in the zinc hydrometallugy industry contains valuable metals of strontium (Sr), manganese (Mn), and zinc (Zn), which has economical value but poses potential detrimental to the environment. However, the extraction and recovery of Sr is extremely difficult due to the thermodynamically stable Ca and Sr sulfate cocrystallization structure in ZECS. The destruction of refractory sulfate cocrystallization structure is a prerequisite for Sr extraction, and the carbothermal reduction roasting method is demonstrated to be efficient for the phase reconstruction. In this study, a stepwise Sr, Mn, and Zn recovery process from ZECS via carbothermal reduction roasting-acid leaching-selective precipitation-multistage extraction was proposed. In the reduction roasting procedure, all the sulfates including CaSO4, SrSO4, and Ca0.7Sr0.3SO4 are reduced to soluble sulfides. Meanwhile, most MnO2 is converted to a low-valence Mn product (MnO) that is soluble in acid, and the reduced Zn(g) is volatilized which can be collected from the flue dust. In the acid leaching procedure, the leaching efficiencies of Sr, Ca, and Mn are as high as 97.34 %, 97.44 %, and 95.49 % respectively, while 99.50 % Si in the form of silica is enriched in the leaching residue. After that, high-purity Sr solution was obtained after selective precipitation for Mn removal and D2EHPA multistage extraction for Ca removal. 97.41 % Mn in the leaching solution is precipitated by adjusting the pH value to 9.5 with NaOH solution. After four-stage extraction, 79.60 % Sr and only 0.35 % Ca are retained in the aqueous phase. The purified solution contains 2.88 g/L Sr, 0.052 g/L Ca, and 0.014 g/L Mn, which can be used to prepare industrial SrCO3 product via the carbonation operation. Thermodynamic behavior and reduction roasting reconstruction mechanism of Ca and Sr sulfate cocrystallization structure in ZECS were discussed in this work. The distributions of main metal elements (Sr, Ca, Mn, and Zn) in ZECS during the carbothermal reduction roasting-acid leaching-selective precipitation-multistage extraction process were also systematically studied.
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