Quiescent flow leaching as a tool for assessing long-term weathering of ferronickel slag: insights from a 1.5-year experiment

Long-term leaching experiments provide estimates of the release of potentially toxic elements under controlled laboratory conditions. In this study, metallurgical slags originating from a ferronickel smelting operation were subjected to a leaching test under stagnant laboratory conditions at pH levels of 2.9 and 4.9 for 18 months. The results revealed significant variation in the leaching potential of the studied slags, with nickel and cobalt release ranging from less than 0.01% to as high as 90%. Of the samples analyzed, furnace slag produced by smelting predominantly Indonesian ores (IS-A slag) exhibited the greatest potential as a source of nickel, primarily due to the dissolution of nickel sulfides (mostly pentlandite) and the presence of finely dispersed, micron- to nanometer-scale ferronickel grains. The dissolution of the major silicate phases, particularly forsterite, was evidenced by the release of magnesium, reaching 1.9–2.1% in the same type of slag (IS-A). Slags from the same source (IS-B and IS-C), which were rich in magnesiochromite, contributed to chromium mobilization in the range of 0.3–0.6%. Furthermore, the dissolution of calcium silicate (mostly larnite) was confirmed by substantial calcium release (at least 13%) in 40-year-old slag formed after the magnetic separation (MS slag) of ores from the Republic of North Macedonia (Ržanovo Mine). This contrasts with slags lacking this phase. This study indicates that long-term stagnant-flow experimental conditions can illustrate the reactivity features of slags and demonstrate the growing influence of laboratory experiments in estimating the geochemical stability of individual slag phases. • Nickel and cobalt release varied widely, from <0.01% to 90%. • Nickel sulfides and fine ferronickel grains drove Ni mobilization. • Forsterite and larnite acted as Mg and Ca donors, with releases exceeding 15%.