Study On The Activities Of Na₂O-SiO₂-Al₂O₃-H₂O System

Silicon and aluminum are almost the richest content elements in the crust. Many earth chemical processes were related to their physico-chemical properties. Thermodynamic properties of the alkaline sodium aluminosilicate solution are important in various industrial sectors such as kraft pulping and the Bayer process for aluminum industry as well as in environmental geochemistry. The primary study content in this paper is described in the following:(1) The osmotic coefficients and water activities of the ternary system NaOH-Na2H2SiO4-H2O at303.15K have been determined by isopiestic method in the ionic strength ranging from0.5207to5.5347mol·kg-1and the modulus (mole ratio of SiO2and Na2O) ranging from0.1013to0.4547. The measured results have been represented by the Pitzer model. The Pitzer ion interaction parameters were evaluated from experimental data by using multiple regressions. One set of the results was as follows:β_Na2H2SiO4(0)=-0.10120,β_Na2H2SiO4(1)=1.80134, C_Na2H2SiO4φ=0.16770, θ_Na-,H2SiO42+=0.13478,Ψ_Na-,H2SiO42+,OH-=-0.08451. The osmotic coefficients are in reasonable agreements between the experimental data and that calculated from the Pitzer model. The mean activity coefficients of NaOH and Na2H2Si04in the NaOH-Na2H2SiO4-H2O system were obtained from the Pitzer model, and their variations with ionic strength and modulus were established by calculations.(2) Pitzer model was selected to predict and calculate the thermodynamic properties of the Na2O-Al2O3-SiO2-H2O system with the Pitzer parameters from the literatures and this paper. In order to test the accuracy of the calculated value, the osmotic coefficients of the quaternary system were determined by isopiestic method at303.15K. The relative deviation of osmotic coefficients and water activities between the experimental data and that calculated from the Pitzer model were less than0.04and0.004, respectively. The structure of Na2O-Al2O3-SiO2-H2O solution was investigated by infrared spectroscopy, which showed that the structure of the solution was stable during the experiment. The mean activity coefficients of NaOH, NaAl(OH)4and Na2H2SiO4in the quaternary system were obtained from the Pitzer model, and their variations with ionic strength were established by calculations.