Removal of Silica from Bleach Plant Filtates, 1996 Minimum Effluent Mills Symposium Proceedings

Olli Välttilä, Susanna Jaarmo, Risto Järvinen - The Finnish Pulp and Paper Research Institute

Erkki Kiiskilä - A.Ahlström Corporation

The aim of the study was to remove silicon direct from bleach plant filtrates. Laboratory experiments were carried out using calcium, aluminium and magnesium compounds as precipitating agents for silicate solutions. Optimum chemical charge, pH and time were determined for pure silicate solutions and synthetic bleach filtrate. The results were confirmed using a mill sample of TCF bleach plant filtrate at process temperature. The results proved that it is possible to remove silicon from bleach plant filtrates to low residual concentration. Treatment at pH 10.5-l1 in the presence of magnesium resulted in good removal of silicon and other non-process elements (except calcium) and produced a settleable precipitate. Silica was also coprecipitated effectively in the presence of aluminium and calcium, but in the first case the precipitate produced did not settle and in the second case the treated filtrate had a high residual calcium concentration.