From metal retention to metal release in ferronickel slags

Abstract

Building on our previous study of both historical and fresh ferronickel slags produced from the smelting of lateritic Ni ores (Đorđević et al., 2024), we further investigated the retention and mobility of metals in electric furnace slags (EFS) using scanning and transmission electron microscopy (SEM and TEM), as well as Raman spectroscopy. Our analyses revealed the presence of crystalline Fe-rich forsterite, ortho- and clinopyroxenes (e.g., enstatite, pigeonite, augite), spinel-group phases (e.g., magnesiochromite, trevorite), an amorphous slag matrix as well as regions enriched in intermetallic Fe x Niy particles and Ni- and Fe- sulfides. On the microscale, Fe x Niy particles and sulfides are commonly intergrown. Additionally, Fe x Niy nanoparticles (<10 nm) were observed embedded in the glass matrix. Due to their high surface area, these nanoparticles may significantly influence slag reactivity, both in terms of potential applications and environmental metal release. To assess long-term metal mobility, we conducted an 18-month leaching experiment under quiescent-flow conditions using solutions adjusted to pH values of 2.9 and 4.8. Notably, older slags released up to 90 wt. % of their Ni content, primarily due to the dissolution of Fe-Ni- sulfides and fine-grained ferronickel alloys. Cobalt exhibited a similar trend. Magnesium mobilization (up to 2.1 wt. %) was associated with the breakdown of forsterite, while chromium release from magnesiochromite-bearing slags ranged from 0.3 wt. % to 0.6 wt. %. Leaching intensity was strongly pH-dependent: at pH 2.9, Ni concentrations in the leachate were up to 135 times higher than at pH 4.8 in some samples. Other potentially toxic elements (PTEs) showed similar behavior, with release rates 4 to 76 times greater under more acidic conditions (pH 2.9). Over time, concentrations of some metals in solution declined, suggesting the progressive formation of secondary phases capable of re-adsorbing or immobilizing dissolved metals, particularly under stagnant conditions.