Silica removal from newsprint mill effluents with aluminum salts

• Silica coagulation is necessary for the viability of RO treatments in paper mills.
• Silica removal is favored at high pH and dosages, the drawback is the conductivity increase.
• Polyamine modification of a polyaluminum salt is the most efficient coagulant.
• PANS-PA2 is able to reduce 97% silica concentration at pH 10.5 and 2500 ppm.
• Even without pH regulation, a 76% silica removal is obtained with PANS-PA2.

The main obstacle for the implementation of reverse osmosis (RO) in a treatment chain to reuse the effluent of a newsprint mill as fresh water is the high silica content of the water, which produces severe scaling on the membrane, thus, limiting its recovery. Coagulation is one of the preferred methods to reduce silica concentration. Five aluminum based coagulants have been tested at five dosages (500–2500 ppm) and three pHs (8.3, 9.5 and 10.5). All products showed their best efficiency at the highest dosage and pH, with the exception of alum, that was more efficient at intermediate dosages. A combination of a polyaluminum nitrate sulfate with a cationic quaternary polyamine (PANS-PA2), was the most efficient and versatile coagulant. It removed 97% of silica (5 ppm residual silica) at the optimal conditions (pH 10.5, 2500 ppm) and it was very efficient (76% silica removal) at pH 8.3, avoiding the need of any pH adjustment, and minimizing the conductivity and pH increase of the treated waters as well as obtaining some removal of the organic colloidal matter (≈25%).