First insights into mineralogy and weathering of the slags produced by smelting lateritic Ni ore at Vozarci, North Macedonia

The laterite Ni ore smelting operations at Vozarci, North Macedonia, which produces ferronickel since 1982, have produced large amounts of smelting wastes dumped at the smelter. In May 2022 we have sampled three different types of slags occurring in these slag dumps. Slag 1 was produced using predominantly ore imported from Indonesia, slag 2 was formed using the ore from the nearby Ržanovo mine and slag 3 was produced using predominantly ore imported from the Ivory Coast. X-ray fluorescence (XRF) spectroscopy analyses showed that slags 1 and 3 contain between 43–47 wt.% SiO₂ and slag 2 35 wt.% SiO₂. Relative to slags 1 and 3, slag 2 is enriched in iron (40 wt.% Fe₂O₃). Furthermore, slag 2 is characterized by a large Cr content (ca. 17400 ppm of Cr₂O₃). A combination of powder X-ray diffraction, Raman spectroscopy and SEM-EDS analyses (of polished aliquots) showed that the slags consist of silicate glass, synthetic equivalents of orthopyroxenes (enstatite), clinopyroxenes (clinopyroxene, diopside, pigeonite, augite), olivines (forsterite, fayalite) and subordinate spinel-group phases (chromite, magnesiochromite, magnetite, trevorite), sulphides (pyrrhotite and FeS-NiS phases) and intermetallic compounds (metallic Fe with up to 10 at.% Ni and remnants of ferronickel). All three slags contain significant amounts of the following potentially toxic elements: Co (20-87 ppm), Cr (9600-17400 ppm), Ni (500-730 ppm) and Zn (150-380 ppm). The phase assemblages and textures in the Vozarci slags are similar to those in other pyrometallurgical slags produced during reworking of silicate and sulphide ores. Although the slags 1 and 2 have been exposed to atmospheric conditions for about 40 years, those occurring in the dumps are not much affected by weathering. At present, we have identified the following alteration products: (i) abundant iron oxy-hydroxides (including lepidocrocite) (slag 1), (ii) minor nesquehonite, MgCO₃·3H₂O (slag 2) and (iii) syngenite, K₂Ca(SO₄)₂·H₂O (slag 1). Since slag 3 is newly deposited slag, it is not weathered at all.

Financial support of the Federal Ministry of Education, Science and Research (BMBWF) and Austria´s Agency for Education and Internationalisation (OeAD), (WTZ Grant MK 06/2022) is gratefully acknowledged.