Recovery of magnesium from ferronickel slag to prepare hydrated magnesium sulfate by hydrometallurgy method

Abstract This paper presents an economical and efficient technical method for Mg recovery from ferronickel slag to produce hydrated magnesium sulfate (MgSO4·nH2O), which can not only change waste into valuable material but also benefit environmental protection practices. Based on the characteristics of the ferronickel slag, the factors affecting the technical efficiency and reaction mechanisms involved in recovering Mg were studied. The results of the thermodynamic analysis revealed that the changes in the Gibbs free energy of the main leaching and crystallization reactions were negative within the investigated temperature range. The Mg leaching ratios using H2SO4 as a solvent in the single leaching and secondary leaching processes were 83.74% and 87.46%, respectively, under the following leaching conditions: a liquid–solid ratio of 20 L/kg, a leaching temperature of 180 °C, a leaching time of 120 min, a sulfuric acid concentration of 15 N, and a nitric acid dosage of 5%. The X-ray diffraction results revealed that both the ferronickel slag and its leaching residue are mainly composed of forsterite (Mg,Fe)2SiO4 and some amorphous silica. The kinetic analysis revealed that the leaching process follows an unreacted-shrinkage-nuclear-reaction model controlled by interfacial chemical reactions, and the apparent activation energy is 12.72 kJ/mol. The results of the Mg crystallization experiments showed that the Mg crystallization ratio could reach 84.25% under the following conditions: a seed coefficient of 0.5, a crystallization time of 10 h, an agitation rate of 50 rpm, and a final temperature of 25 °C. The results of the X-ray diffraction and energy dispersive X-ray spectroscopy revealed that the chemical composition of the final product is a mixture of MgSO4·nH2O (n = 3–5), and the crystal powders were combined with many micron-sized crystalline grains observed using scanning electron microscopy. The purity of the final product is about 99.2% based on the inductively coupled plasma mass spectrometry measurements. The method presented in this paper will make it possible to prepare magnesium salts using ferronickel slag as the raw material in the future.