Abstract:The thermodynamic properties of the most important NaOH−NaAl(OH)₄−H₂O system in Bayer process for alumina production were investigated. A theoretical model for calculating the equilibrium constant of gibbsite dissolved in sodium hydroxide solution was proposed. New Pitzer model parameters and mixing parameters for the system NaOH−NaAl(OH)₄−H₂O were yielded and tested in the temperature range of 298.15−373.15 K. The results show that the proposed model for calculating the equilibrium constant of gibbsite dissolution is applicable and accurate. The obtained Pitzer model parameters of β⁽⁰⁾(NaAl(OH)₄), β⁽¹⁾(NaAl(OH)₄), C^Φ(NaAl(OH)₄) for NaAl(OH)₄, the binary mixing parameter of θ(OH⁻Al(OH)₄⁻) for Al(OH)₄⁻ with OH⁻, and the ternary mixing parameter of ψ(Na⁺OH⁻Al(OH)₄⁻) for Al(OH)₄⁻ with OH⁻ and Na⁺ are temperature-dependent. The prediction of the equilibrium solubility of gibbsite dissolved in sodium hydroxide solution was feasible in the temperature range of 298.15−373.15 K.

Key words: Pitzer model; sodium aluminate solution; gibbsite; equilibrium constant; equilibrium solubility