This study investigates strategies to enhance the silica content of ferronickel slag through an acid-hydrothermal treatment. The mechanism of silica enrichment is elucidated by examining the amorphization process, specifically the formation of amorphous silica phase resulting from the decomposition of magnesium silicate phase (clinoenstatite and forsterite). Our findings reveal that the hydrothermal process with 2 M H2SO4 at a temperature of 250 ℃ for 180 min can decompose the clinoenstatite phase structure that dominates the ferronickel slag so that the silica content increases to 70.90%, with the amount of amorphous phase doubled. The dissolved magnesium and iron reach 60.00% and 38.57%, respectively. The effect of hydrothermal process parameters on phase changes and amorphous levels is in line with the percentage of dissolved mass and changes in the chemical composition of the main elements of the ferronickel slag residue (SiO2, MgO, Fe2O3). This hydrothermal system is quite effective in facilitating waste valorization by extracting silica and other valuable elements from ferronickel slag. © The Minerals, Metals & Materials Society 2025.