Magnesium recovery from ferronickel slag by reaction with sodium hydroxide

Ferronickel (FeNi) slag, as the main solid byproduct of the FeNi industry, has the potential to be utilized other than as roadbed material. To broaden the applications of FeNi slag (such as CO₂-capturing materials), there is a need for a treatment that breaks the magnesium silicate bonds and transforms the magnesium into MgO. As an initial study, this research treated FeNi slag by using sodium hydroxide followed by water-washing. The present authors evaluated several alkali treatment variables; such as temperature (523 K and 773 K), time (1–8 h), and mass ratio between FeNi slag and NaOH (1:0.75–1:4). The magnesium in FeNi slag was recovered as MgO. By using a FeNi slag:NaOH mass ratio of 1:2, a temperature of 773 K, and a time of 4 h, 97.34% of the magnesium was recovered, and 83.35% of the silicon was removed. During alkali treatment, the Mg-based compound in FeNi slag undergoes a transformation from MgSiO₃ to Mg₂SiO₄ to MgNa₂SiO₄ and, finally, to MgO. On the other hand, the Fe-based compound transforms olivine to Fe₂O₃ to spinel (Fe₂MgO₄ and Fe₃O₄) and, finally, to FeNaO₂. The alkali treatment process of FeNi slag using NaOH followed by water washing is also known to make the product morphology porous and has a large surface area (11.62 m²/g). In terms of economics, this process will be economical when considering the by-product (Na₄SiO₄) generated.