Slag heaps over years may evolve into complexly weathered zones, which are a challenging material for analyses as they contain phases from numerous sources and at different stages of weathering. However, the weathered zones are important parts of slag heaps, because they contain both primary and secondary phases enriched in metal(oid)s that may become soluble under specific conditions. The weathering reactions related to metal release or precipitation may be recorded in a heavy mineral fraction as the fraction contains predominately minerals with elevated toxic elements concentrations. Therefore, an automated SEM analysis on a polished section of included heavy mineral particles was applied in this paper for a rapid recognition of phases in a complex setting and their classification into detrital, primary and secondary phases. The approach was applied to a slag heap in Świętochłowice (Upper Silesia, Poland) and it consisted of analyzing magnetic and non-magnetic heavy mineral fractions from three distinct horizons noted A, B and C. Materials had been previously interpreted as being sourced from the heap itself (lowermost horizon C) and from artificially added materials used later for superficial site remediation (upper horizons A and B). Instead, automated SEM analysis demonstrated that horizon C is derived from the slag heap weathering, horizon B is derived predominately from the artificially added materials, whereas horizon A is a mixture of the B and C horizons. Additionally, when slag particles in horizons A and C are compared, the lowermost horizon C contains more slag-derived secondary phases, whereas horizon A contains more primary slag phases. Therefore, horizon A remains the most prone to releasing toxic elements because, considering its position as the uppermost horizon, it can be submitted to climatic solicitation (fast water circulation).
We have gone through the comments on our review paper entitled: “Copper metallurgical slags–current knowledge and fate: a review” published recently in the Critical Reviews in Environmental Science...
Three kinds of slags occur in the waste dump of pyrometallurgical slags produced during reworking of lateritic ores of Ni in Szklary, Lower Silesia, southwestern Poland. Slags 1 and 2 contain 34–44 wt.% SiO2, and slag 2 is enriched in Ca and impoverished in Fe relative to slag 1; slag 3 is characterized by extreme Ca content ( ca. 50 wt.% CaO). The slags consist of silicate glass, synthetic equivalents of clinopyroxenes (diopside, hedenbergite), melilite, olivines (forsterite, fayalite) and subordinate spinel (chromite), sulfides (pyrrhotite, pentlandite, heazlewoodite, digenite) and intermetallic compounds (awaruite, bronze, metallic Fe and Cu). One type of slag contains significant amounts of the potentially toxic elements (PTE) Co (>380 ppm), Cr (>6400 ppm), Ni (>4000 ppm) and Zn (>352 ppm). The phase assemblages and textures in the Szklary slags are similar to those in other pyrometallurgical slags produced during reworking of silicate and sulfide ores, but they contain fewer sulfides. Although the slags have been exposed to atmospheric conditions for 30–80 years, those occurring in the dump are not affected by weathering, and small vitreous fragments of slag occurring in nearby agricultural fields have only thin (<100 μ m) crusts due to weathering. Nanometric inclusions of sulfides and metallic alloys are embedded in the silicate glass of vitreous slags. Some PTE are concentrated in these silicates: diopside is enriched in Cr (up to 2.3 wt.% Cr2O3), forsterite in Ni (up to 1.7 wt.% NiO), and melilite in Zn (up to 0.7 wt.% ZnO), but their reactivity was found to be limited in the alkaline soils surrounding Szklary.
The aim of this study was to evaluate the feasibility of (bio)hydrometallurgical methods for metal extraction from historical copper slags. Two types of slags (amorphous slag—AS, and crystalline slag—CS) were subjected to 24 to 48 h of leaching with: (i) Sulfuric acid at 0.1, 0.5, and 1 M concentrations at 1%, 5%, and 10% pulp densities (PDs); and (ii) normality equivalent (2 N) acids (sulfuric, hydrochloric, nitric, citric, and oxalic) at pulp densities ranging from 1% to 2%. Bioleaching experiments were performed within 21 days with Acidithiobacillus thiooxidans accompanied by an abiotic control (sterile growth medium). The results demonstrated that the most efficient treatment for amorphous and crystalline slag was bioleaching at 1% PD over 21 days, which led to extraction of Cu at rates of 98.7% and 52.1% for AS and CS, respectively. Among the chemical agents, hydrochloric acid was the most efficient and enabled 30.5% of Cu to be extracted from CS (1% PD, 48 h) and 98.8% of Cu to be extracted from AS (1% PD, 24 h). Slag residues after leaching were characterized by strong alteration features demonstrated by the complete dissolution of fayalite in the case of CS and the transformation of AS to amorphous silica and secondary gypsum. Based on this study, we conclude that amorphous slag is a more suitable candidate for potential metal recovery because of its generally high susceptibility to leaching and due to the generation of residue significantly depleted in metals as the end product. The inventory of economically relevant metals showed that 1 ton of historical copper slag contains metals valued at $47 and $135 for crystalline and amorphous slag, respectively, suggesting that secondary processing of such materials can potentially be both economically and environmentally viable.
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