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Thermodynamics of sodium aluminosilicate formation in aqueous alkaline solutions relevant to closed-cycle Kraft pulp mills Park, Hyeon

Abstract

Accumulation of Al and Si ions in the recovery cycle of a kraft pulp mill may cause sodium aluminosilicate scale formation. This glossy scale forms on process equipment and is very hard to remove. Thus, formation of the scale can create several operational problems in mills moving towards progressive system closure and should be prevented. The purpose of this study is to supply: (a) data on the precipitation conditions of sodium-aluminosilicates in green and white liquors of the recovery cycle; and (b) a model to predict such conditions. The data can either be used directly or to test process models for the design and optimization of progressive system closure strategies. The precipitation conditions of sodium aluminosilicates in synthetic green and white liquors at 368.15 K (95 °C) were determined. In the experiments, the effects of varying the Al/Si ratio and concentrations of OH⁻, C0₃²⁻, SO₄²⁻ , and HS⁻ were studied. The structure of the precipitates was identified by X-ray diffraction, thermogravimetry and chemical analysis. The precipitates were found to have the structure of hydroxysodalite dihydrate (Na₈(AlSi0₄)₆(OH)₂-2H₂0) except in a simulated green liquor system with low OH⁻ and high Cf concentrations where sodalite dihydrate (Nag(AlSi0₄)₆Cl₂-2H₂0) was formed. The precipitation conditions in mill green and white liquors at 368.15 K were also measured. The effects of varying the Al/Si ratio, NaOH, Na₂C0₃, and Na₂S concentrations were studied. The precipitates were found to have the structure of hydroxysodalite dihydrate. A thermodynamic model for sodium aluminosilicate formation in aqueous alkaline solutions was developed. Pitzer's method was adopted to calculate the activity of water and the activity coefficients of the other species in solution. The system under consideration contained the ions of Na+ , Al(OH) ₄⁻, Si0₃ ²⁻, OH⁻, C0₃²⁻, S0₄ ²⁻, Cl⁻, HS⁻ dissolved in water and in equilibrium with two possible solid phases (sodalite dihydrate : Na8(AlSi04)6Cl2-2H20 and hydroxysodalite dihydrate : (Na₈(AlSi0₄)₆(OH)₂-2H₂0) at 368.15 K. The equilibrium constants of sodalite dihydrate and hydroxysodalite dihydrate formation reactions were determined using the thermodynamic properties of the species involved. Property values that were not available in the literature were estimated by group contribution methods. The model calculates the molality of all species at equilibrium including the amount of solid precipitates. The calculations were compared with published data and were found to be in good agreement. Meanwhile, since the system contains the Si0₃²⁻ and Al(OH)₄ ⁻ ions, knowledge of the relevant Pitzer's model parameters is required. Osmotic coefficient and water activity data for Na₂Si0₃ and mixed Na₂Si0₃-NaOH aqueous solutions were obtained at 298.15 K by employing an isopiestic method. The binary Pitzer's parameters, β(0), β(1), and C[sup Φ], for Na₂Si0₃ and the mixing parameters, θ[sub 0H⁻Si0₃²⁻] and ¥ [sub Na⁺OH⁻SiO₃²] , were estimated using the osmotic coefficient data. In addition, osmotic coefficient data were obtained for the aqueous solutions of NaOH-NaCl-NaAl(OH)₄. The solutions were prepared by dissolving AICI36H2O in aqueous NaOH solutions. The osmotic coefficients of the solutions were measured by the isopiestic method at 298.15 K. The osmotic coefficient data were used to evaluate the unknown binary and mixing parameters of Pitzer's model for the aqueous NaOH-NaCl-NaAl(OH)₄ system. The experimental osmotic coefficient data were correlated well with Pitzer's model using the parameters obtained.

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