Thermodynamic study of K₂CrO₄-KAlO₂-KOH-H₂O and Na₂CrO₄-NaAlO₂-NaOH-H₂O systems

Phase diagrams of complex multi-component aqueous chromium salt system are the important theoretical bases for increasing the recovery ratio of chromium and reducing the poisonous waste in the chromate production process. Phase equilibrium of multi-component system has been calculated from the limited known data with thermodynamic models. The phase equilibriums of KOH-K₂CrO₄-H₂O, KOH-KAlO₂-H₂O, NaOH-Na₂CrO₄-H₂O and NaOH-NaAlO₂-H₂O systems were calculated with thermodynamic models. The solubilities of Cr and Al was measured in the systems at different temperatures. The results were compared with experimental data and they are consistent with each other. It shows that the lower concentration of KOH (or NaOH) is favorable for thecrystallization of KAlO₂ (or NaAlO₂) and higher concentration of KOH (or NaOH) is favorable for the crystallization of K₂CrO₄(Na₂CrO₄). Therefore, K₂CrO₄ (or Na₂CrO₄) can be separated from KAlO₂ (or NaAlO₂) with controlling the KOH concentration in solutions of KOH-K₂CrO₄-KAlO₂-H₂O or NaOH-Na₂CrO₄-NaAlO₂-H₂O systems.