Silica removal from newsprint mill effluents with aluminum salts

Abstract

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 sulphate 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%).

Efficiency of chitosans for the treatment of papermaking process water by dissolved air flotation

“…Efficiency differences among the different treatments cannot be clearly assessed as these differences are very close to the experimental error of the silica measurement method. Although coagulation has been used in the literature to remove silica from papermaking waters, significant removals can be achieved only at high pHs and dosages [34][35][36] which is not the present case.…”

Silica removal with sparingly soluble magnesium compounds. Part I

“…Membrane treatments, such as ultrafiltration (UF) and reverse osmosis (RO) [2,3], allow to produce the water quality required to reuse the effluent. However, effluents from deinking paper mills are characterized by high silica content, ranging from 50 to 250 mg/L as SiO2 [2,4,5]. This makes the removal of silica a key factor for the reuse of the effluent to work on the RO membrane at recoveries higher than 20% [3] without scaling problems.…”

“…Coagulation with aluminum-based salts is very effective at ambient temperature, but high dosages (2500-5000 mg/L) are required to achieve high removal rates and the use of hybrid coagulants which increases considerably the treatment cost [4]. On the other hand, when silica removal is carried out during softening, it is necessary to ensure that enough hardness is present in the water.…”

“…When the concentration of soluble silica exceeds its solubility, it precipitates on the membrane surface alone or with other products. In the case of colloidal silica, fouling occurs due to the accumulation of the colloids formed in the bulk solution, and accumulates on the membrane surface [16,17].Although there are many silica removal techniques proposed in the literature [18], it is usually carried out during softening processes or by coagulation at high pH [2,4,5,19,20].Coagulation with aluminum-based salts is very effective at ambient temperature, but high dosages (2500-5000 mg/L) are required to achieve high removal rates and the use of hybrid coagulants which increases considerably the treatment cost [4]. On the other hand, when silica removal is carried out during softening, it is necessary to ensure that enough hardness is present in the water.…”

“…Although there are many silica removal techniques proposed in the literature [18], it is usually carried out during softening processes or by coagulation at high pH [2,4,5,19,20].…”