A mineralogical and geochemical study of three Brazilian coal cleaning rejects: Demonstration of electron beam applications

• Brazilian coals and coal-cleaning residues were studied.
• Sulfate minerals can be formed after pyrite in the residues.
• Sulfides cannot be completely removed from the clean coal.

The background and anthropogenic levels of hazardous elements in the surface soil of a coal mining area depend on the geological setting of the region and the underlying soil material, but may also be influenced by water-borne or aeolian transport of sediment from adjacent coal-related waste piles. Very few studies have focused on the chemical and mineralogical composition of Brazilian coal cleaning rejects (CCRs), which may represent significant sources of soil or water contamination. In this study, we have investigated the quantitative distribution of minerals and potentially hazardous elements in CCRs and a run-of-mine coal from the Brazilian states of Rio Grande do Sul and Santa Catarina. The major minerals, identified by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HR-TEM), and field-emission scanning electron microscopy/energy dispersive X-ray analysis techniques (FE-SEM/EDS) are kaolinite, quartz, mixed-layer illite–smectite, pyrite, jarosite, melanterite, gypsum, rutile, and calcite, while minor minerals include barite, hematite, siderite, sphalerite, and goethite. Galena, magnetite, zircon, and many other species may also occur as accessory/trace minerals. Pyrite and jarosite are relatively abundant in some cases, making up to around 4% or 5% of the mineral matter, with jarosite, melanterite, and gypsum probably formed by complex interaction of oxidation products from Fe-sulfides and clay or carbonate components, initiated by exposure and storage of the host material. Such atmospheric exposure promotes sulfide oxidation that releases substantial sulfate loads as well as Ca2 +, K+, Mg2 +, Cl−, and Al3 +. Metals with the most severe discharges were Zn, Cu, Mn, Co, Ni, and Cd. Most of the trace pollutants in the CCRs displayed a pH-dependent solubility, being immobile in near-neutral samples but mobile under the low-pH conditions associated with oxidized material. The results highlight the complex interactions among mineral matter components of the CCRs during storage, and the potential for release of potentially hazardous elements in association with longer-term exposure and storage.

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