Enhancing the separation of silica nanoparticles from backside grinding (BG) wastewater with synthesized magnetite

• Magnetite was applied to the silica removal in backside grinding wastewater.
• The best SiO2 removal was achieved to be about 97%.
• The optimum dosage of magnetite was determined as 16 mg/L.
• The pH should be maintained at less than 10 to prevent deterioration of SiO2 removal.
• Mixing and sedimentation in magnetic field were also important parameters.

Fine nano-sized magnetite particles were synthesized and then used to treat backside grinding (BG) wastewater for the removal of silica nanoparticles (SiO2). Batch experiments were conducted to examine the effects of the magnetite dosage, pH, stirring intensity (G) and duration (td), magnetic field strength and sedimentation time on the removal of SiO2. The optimum dosage of magnetite was determined as 16 mg/L, which had the best SiO2 removal of about 97%, under the conditions of initial pH 5.9, stirring intensity 900 S−1, stirring duration 30 min, sedimentation time 30 min and magnetic field strength 317 mT. The synthesis of nano-sized magnetite particles 10 nm in diameter led to a reduction in the magnetite dosage needed for the removal of SiO2. The pH was maintained at less than 10 to obtain the high SiO2 removal mentioned above. When the pH was over 10, the Fe detached from magnetite particles which subsequently led to a decrease in SiO2 removal. Magnetite aggregation was affected by both G and td. Increasing the values of G and td enhanced the removal of SiO2. When the multiple values of G and td, i.e., Gtd reached 12,000, a SiO2 removal of 95% was achieved. The relationship between the magnetic field strength and sedimentation time was also evaluated. An increase in the magnetic field strength improved the removal of SiO2. Specifically, the SiO2 removal was greatly dependent on the sedimentation time when the magnetic field strength was weak. The results of this study can be used in the design and operation of a magnetite aggregation process to treat BG wastewater for the removal of SiO2.