Silica removal with sparingly soluble magnesium compounds. Part II

Silica scaling is one of the main bottlenecks in the reuse of papermaking effluents by reverse osmosis. The low hardness of deinking paper mill effluents makes necessary the addition of magnesium compounds to increase silica removal at high pH. Based on the results obtained in Part I, MgO was selected as the most efficient magnesium source. Its efficiency was tested at different dosages (150-10,000 mg/L), pH values (8.2-9.5) and temperatures (25-50 °C) and the optimization of the reaction time was also carried out. Silica removals over 95% were obtained at the 4 pHs and 3 temperatures with MgO dosages over 500 mg/L; however, MgO can only be applied if water temperature is higher than 35 °C, as the dissolution of MgO is limited. Moreover, the analysis of the solids obtained (SEM-EDX and FTIR) showed that the main mechanism for silica removal was co-precipitation of magnesium silicates (forsterite and antigorite) while adsorption was less significant.