| Summary: | 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. === Applied Science, Faculty of === Chemical and Biological Engineering, Department of === Graduate
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