This study presents a novel approach for the alkaline hydrothermal process that enables selective dissolution of magnesium silicate phases (MgSiO3 and Mg2SiO4) in ferronickel slag (TFN) to recover high purity nanosilica (SiO2) and brucite (Mg(OH)2). The findings reveal that, although the addition of water theoretically lowers the Gibbs free energy of the reaction, controlling and restricting the amount of water content plays a critical role in promoting the reaction between NaOH and TFN particles, thereby significantly improving silica extraction. The maximum silica recovery of 84.50% was obtained at 250 °C for 5 h with an NaOH/TFN mass ratio of 2 and an H2O/NaOH mass ratio of 0.75. Under these conditions, silicate phases decomposed rapidly into sodium silicate and brucite within a short reaction time. Acid precipitation of the resulting sodium silicate filtrate yielded nanosilica particles sized 37–152 nm, with a high chemical purity of 96.93 wt.% SiO2. The brucite byproduct offers potential for further purification or use for a variety of functional applications. © The Minerals, Metals & Materials Society 2026.