Abstract Fine nano-sized magnetite particles were synthesized and then used to treat backside grinding (BG) wastewater for the removal of silica nanoparticles (SiO 2 ). Batch experiments were conducted to examine the effects of the magnetite dosage, pH, stirring intensity (G) and duration (t d ), magnetic field strength and sedimentation time on the removal of SiO 2 . The optimum dosage of magnetite was determined as 16 mg/L, which had the best SiO 2 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 SiO 2 . The pH was maintained at less than 10 to obtain the high SiO 2 removal mentioned above. When the pH was over 10, the Fe detached from magnetite particles which subsequently led to a decrease in SiO 2 removal. Magnetite aggregation was affected by both G and t d . Increasing the values of G and t d enhanced the removal of SiO 2 . When the multiple values of G and t d , i.e., Gt d reached 12,000, a SiO 2 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 SiO 2 . Specifically, the SiO 2 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 SiO 2 .