Chemical speciation in concentrated alkaline aluminate solutions in sodium, potassium and caesium media. Interpretation of the unusual variations of the observed hydroxide activity

Abstract

A detailed electrochemical investigation using H₂/Pt electrode potentiometry as well as Raman and NMR spectroscopy was carried out to develop a comprehensive chemical explanation for the unusual patterns of hydroxide concentrations observed in strongly alkaline, highly concentrated aluminate solutions (Bayer-liquors). For this, aluminate solutions with various alkaline metal background cations were investigated. The effect of the temperature on the observed patterns was also studied, and for comparison with solutions of similar concentrations, the chemical speciation of borate solutions was also studied. The formation of the NaOH⁰ ion-pair has been proven with the formation constant (defined in terms of activities) β₀ = 0.78 ± 0.08. The formation of analogous KOH⁰ or CsOH⁰ ion-pairs under the experimental conditions applied is negligible. Assuming the formation of the NaAl(OH)₄⁰ ion-pair is not necessary for modeling the experimental findings, as its formation causes only secondary effects on the potentiometric patterns. It has also been shown that all experimental data can be interpreted quantitatively if the formation of the doubly charged dimeric aluminate species is included in the calculation of the changes in the mean activity coefficients. The formation constant of the aluminate dimer could not be estimated purely from the H₂/Pt potentiometric data but a lower limit for its formation constant (defined in terms of activities) has been derived. These conclusions are in full congruency with those derived from the Raman spectra of solutions with similar concentrations, so the two independent experimental methods lead to the same set of chemical species in highly concentrated alkaline aluminate solutions.