As mills move towards system closure and reduce water consumption and purge points, the levels of non-process elements increase, causing problems in the operation of the recovery cycle. Two such elements are aluminum (Al) and silicon (Si). These elements enter the mill mostly from contaminated wood, and from chemical make-up, and also from non-wood fibre sources. Al and Si form a range of aluminosilicate compounds such as sodalite, cancrinite, natrolite and hydrotalcite (in the presence of magnesium) in the alkaline pulping liquor, with their solubility being dependant on the hydroxide concentration, the ionic strength of the solution, temperature, the concentration of other species present and the initial quantities of Al and Si*.

Al and Si are most soluble in white liquor, and least soluble in black liquor. At constant temperature, their solubility increases as black liquor is concentrated. In mills where aluminosilicate deposits are a problem in black liquor evaporators, they are usually present as tenacious glassy scales which have to be removed by hydroblasting. Cancrinite limits the solubility of Al and Si in green liquor, and possibly in black liquor as well. To prevent aluminosilicate deposit formation, the input (i.e., from chemical make-up and wood) has to be low in these elements. The use of high purity chemical make-up may have to be considered when the input of Al and Si exceeds the solubility limit in black liquor. The normal purge points are the grits and the dregs.

* Wannenmacher, P.N., et al., Application of solubility data to predicting the accumulation of aluminum and silicon in alkaline pulp mills, pp. 303-308, 1996, TAPPI Minimum Effluent Mills Symposium.