Abstract
Effect of calcium on the solubility of zinc oxide in the sodium hydroxide solution was studied in this paper. Results showed that ZnO equilibrium concentration was different at different temperatures when CaO or Ca(OH)2 was added to the saturated sodium zincate solution of Na2O-ZnO-H2O system. ZnO equilibrium concentrations had little change with Ca(OH)2 added at 100°C. But the concentrations became complicated at the temperature of 25°C, 50°C and 75°C, which decreased at Na2O ≤ 20% and then had no change at Na2O > 20% with Ca(OH)2 added at 25°C, 50°C and 75°C. The curves of the decreased equilibrium concentration formed arc shape when CaO or Ca(OH)2 added. Moreover, the curves of the concentration added CaO more crooked than that added Ca(OH)2. The dissociated Ca from Ca(OH)2 bonded to oxygen of ZnO22- and produced calcium zincate precipitation(Ca(Zn(OH)3)2•2H2O), which leads to zinc loss in the solution and decreases the balance concentration of zinc oxide. Therefore, in order to avoid the loss of zinc when CaO or Ca(OH)2 added to sodium zincate solution, Na2O concentration would be controlled above 20% or temperature be about 100°C.
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