Mineralogy, elemental composition and modes of occurrence of elements in Canadian feed-coals

Feed-coals used in some Canadian power plants were examined for their mineralogical and elemental composition. The mode of occurrence of elements (organic/minerals) in these samples were determined using sequential leaching by H₂O, NH₄OAc and HCl.Canadian feed-coals examined in this study are subbituminous to bituminous rank (calorific values: 19.62-29.88 MJ/kg). The sulphur content of these coals is 0.32-3.55%.Quartz and aluminosilicates (clay minerals and feldspar) dominate the mineralogy of these samples. The accessory minerals consists of pyrite, sphalerite, barite, calcite, anhydrite, chromite, zircon, biotite, and monazite, which occur as both primary and secondary cell filling types. An interesting mineral found in one of the subbituminous feed-coals is the gorceixite that is normally associated with degraded volcanic ash.The sulphur content of the feed-coals is an indication of their geological setting with feed-coal formed in a fresh water setting containing the least amount of S and those associated with evaporites having the highest S content.The concentrations of Cd, Cr, Cu, Hg, Ni, Se, V, and Zn in these Canadian feed-coals are low compared to world coals. Mercury content of these feed-coals ranges from 0.04 to 0.16 ppm. Mercury is mostly associated with the pyretic portion of coal and has direct correlation to As and S content in these coals. Arsenic in low sulphur coals is mostly associated with coal macerals and in higher sulphur coals with pyrite. About 47% of As in high sulphur coal is removed after leaching by HCl.Water soluble elements consist of Co, Ni and Mn, which are associated only with bituminous feed-coals. The removal of these elements by water is possibly due to an increased acidity of leaching solution and presence of soluble Cl and S.Beryllium and vanadium in low S coals are mostly removed after leaching by HCl, indicating a possible association with clay minerals. Limited Be and V is removed from lower clay mineral content coals; however, in high S content coals, more Be and V are removed.The speciation of As, Cr and Ni indicates that As is mostly present in the less-toxic form As⁺⁵ and Cr is present entirely as Cr³⁺, an essential human trace nutrient, found in both subbituminous and bituminous ranked coals. Nickel is present mostly as Ni²⁺ in oxygen coordination in these milled coals and carcinogenic compounds of nickel are not present in these feed-coals.