Removal of silicon from pulping whitewater using integrated treatment of chemical precipitation and evaporation

This article investigates and evaluates the technical applicability of integrated physico-chemical methods to treat thermo-mechanical pulping (TMP) whitewater collected from a local mill in Finland. Under optimized conditions, both chemical precipitation and evaporation were applied to remove silicon in the whitewater samples. The mechanism of silicon removal by the precipitant is also presented. Their treatment performances in removing silicon were statistically evaluated and compared to those of other reported studies. It is evident that both the treatments are applicable and effective to remove target pollutant from the samples. At the same SiO2 concentration of 200 mg/L, 0.10 g/L of dose, pH 10.5 and 303 K, the integrated chemical precipitation using Fennofix type FF40 and evaporation could remove 96% of silicon. This finding is slightly higher than those of similar precipitation using CaO and electro-coagulation in other studies, which could accomplish 93% and 95% of silicon removal respectively at initial SiO2 concentrations of 954 and 250 mg/L. Both chemical precipitation and evaporation could effectively treat a varying strength of TMP whitewater, as their treated effluents could meet the required silicon limit of less than 50 mg/L set by environmental legislation. Therefore, subsequent treatments are not required to complement the removal of silicon in the effluents prior to their discharge.