Sodium aluminosilicate crystal formation in alkaline solutions relevant to closed cycle kraft pulp mills

Metal ions, such as those of Al, Si, and Ca known as non‐process elements (NPEs), are a major concern in the development of technology for achieving closed cycle operation in Kraft pulp mills. For example, the build‐up of NPEs such as Al and Si ions causes sodium aluminosilicate complex formation in the recovery cycle of Kraft pulp mills. These complexes precipitate on process equipment and are very hard to remove. The purpose of this study is to supply basic data on the precipitation conditions for such complexes. These data can be used to develop process models for design and optimization of closed mills. The precipitation conditions of sodium aluminosilicate complexes in synthetic green and white liquors were determined. In the experiments, the effects of varying the A1/Si ratio and concentrations of OH‐, Na 2 S, CO 3 2 ‐, and SO 4 2 ‐ were studied. The structure of the precipitates was identified by X‐ray diffraction, thermo‐gravimetry and chemical analysis. The precipitates were found to have the structure of hydroxysodalite and/or sodalite.