Graduate Thesis Or Dissertation
 

Solubility of aluminosilicates in kraft green and white liquors

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  • The increasing occurrence of aluminosilicate caused complications in krafi pulp mills can be attributed to the closing of purge points and the expanded use of contaminated feedstocks and recycled fiber sources. An active purge of aluminum and silicon from the chemical recovery cycle must be undertaken in order to maintain concentrations below the critical values at which black liquor evaporator scaling and other detrimental side effects occur. The 2-day solubility of aluminum and silicon in unclarified green and white liquors was investigated. All experimental runs were conducted at the mill sample temperature of 95°C. The mill liquors were spiked with various concentrations of aluminum and silicon. Beyond adding aluminum and silicon, in the form of A1C1₃6H₂0 and Na₂SiO₃⋅9H₂0, various amounts of CaO, TiO₂ and MgSO₄⋅7H₂0 were also added to test their effectiveness for removing aluminum and silicon from the green liquor. Secondary tests were conducted with 4MgCO₃⋅Mg(OH)₂⋅4H₂O and Mg powder to compare the results with those found when using MgSO₄⋅7H₂0. The effect of higher concentrations of potassium, added in the form of KC1 and K₂CO₃, was also explored in green liquor. White liquor experiments were conducted when the mill was using both fresh and reburned lime. Aluminum and silicon were added in the form of AlCl₃⋅6H₂O and Na₂SiO₃⋅9H₂O at various concentration levels. Only 1 to 1 ratios of magnesium to silicon were investigated for white liquor. Apparent 2-day solubility plots for aluminum and silicon, for both green and white liquors are presented. The addition of CaO, TiO2, 4MgCO₃⋅Mg(OH)₂⋅4H₂O, magnesium powder, KC1 and K₂CO₃ to the aluminum and silicon spiked green liquor had no noticeable effect on the solubilities of aluminum and silicon. The addition of MgSO₄⋅7H₂0 resulted in a noticeable decrease in both Al and Si solubility values in green liquor, while in white liquor addition of MgSO₄⋅7H₂0 appeared to only decrease the Al concentration. Equilibrium constants were calculated, by using Pitzer's method to calculate individual ion activity coefficients, for various aluminosilicate compounds found in this and other similar works. The results gave conservative log K values ranging between 34.2 and 37.2.
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