The solubility of aluminum sulfate in sulfuric aqueous solution was determined and analyzed thermodynamically. Aluminum sulfate dissolved in sulfuric acid aqueous solution not spontaneously, because the dissolution enthalpy is a positive value. The solubility of aluminum sulfate had a temperature dependency and hardly dissolved with higher concentration of sulfuric acid. On the other hand, aluminum sulfate solubility did not depend on the existence of oxalic acid, which was dissolved to saturation concentration as an additive. Moreover, the solubility curves of aluminum sulfate and oxalic acid were different in the range between 1.5 M and 6 M sulfuric acid aqueous solution at 25°C. Therefore, using the difference of solubility characteristics, aluminum sulfate could be recovered selectively from waste acid containing oxalic acid. An aluminum sulfate crystal generated from the model waste acid, which was 3.5 M sulfuric acid aqueous solution containing 20 g·L-1 oxalic acid, was determined as one of aluminum sulfate hydrate by powder-XRD and its purity was 94%.
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