Optimization of silica removal with magnesium chloride in papermaking effluents: mechanistic and kinetic studies

The reuse of deinking paper mill effluent based on reverse osmosis is limited by silica scaling on the membranes. The removal of silica during softening processes is one of the most used approaches as it can treat large volumes of water at low cost, but when the water hardness is low, the addition of magnesium compounds is necessary. In the present investigation, MgCl₂·6H₂O was selected as magnesium source to analyze the effect of pH, dosage, temperature, and contact time on silica removal. Moreover, the silica removal mechanism was analyzed under different operational conditions. The results show that it is possible to obtain high silica removal rates (>70 %) at an intermediate dosage (750 mg/L of MgCl₂·6H₂O) either at high pH (12.0) and ambient temperature (20 °C) or lower pHs, i.e., pH = 10.5, but at higher temperatures 35–50 °C. The kinetic study demonstrates that contact times lower than 30 min are enough for silica removal with independence of the temperature. SEM-energy-dispersive X-ray spectroscopy (EDX) and Fourier transform infrared spectroscopy (FTIR) analysis of the solids obtained confirms that silica is removed through the formation of magnesium silicates. The EDX analysis showed that, independently of the operational conditions, the atomic Si/Mg ratio was around 0.7 which indicates that antigorite (Mg₃Si₂O₅(OH)₄) is the predominant specie formed.