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Phosphorus (P) and fluorine (F) are undesirable trace elements within coal. Societal tolerance levels for these elements are reducing, requiring methods to predict and mitigate their occurrence through selective mining. This study aims to classify the P-and/or F-bearing mineral phases as having formed early during the peat accumulation phase or late, post-coalification. This will be achieved by sampling a geological domain such as a seam split and undertaking spatial microanalysis. A seam split is where a single parent seam splits into two or more daughter seams.
Ten crushed raw and product coal samples were collected from the top, middle and bottom of coal seams from three drill holes (for a total of 30 samples) across a seam split as shown in Figure 1. A subset of these samples have been optically and chemically analysed via scanning electron microscopy with X-ray microanalysis, mineral liberation analysis and spectroscopic techniques.
Preliminary results in this study show that P and F contents strongly correlate and are elevated in the daughter seams. These contents decrease as the daughter seams coalesce into the parent seam. Within the split, the fusinite maceral of the inertinite group host most of the mineral matter within their available pore space (<10 to 30 microns). Fluorapatite, Ca5(PO4)3F, (FA) is the primary P-bearing and only F-bearing mineral phase observed. Although, additional P-bearing phases such as crandallite, CaAl3(PO4)(PO3OH)(OH)6, have also been observed. Optically, this in situ FA appears amorphous and is spatially undifferentiated. It primarily occurs within the cell lumens of inertinite macerals (FA-a) as both monomineralic aggregates and polymineralic aggregates with other inorganic phases such as kaolinite. However, additional fracture and/or cleat infilling phases (FA-b) have also been observed, although this does not appear to be stratigraphically or laterally constrained. Previous research by Ward et al (1996) suggests that cell-lumen infilling apatite formed during the early stages of the coal evolution. However, the distribution observed as part of this study could be palaeoenvironment, fault and/or hydrothermally controlled. For example, Figure 1 illustrates a possible conceptual model for contemporaneous emplacement during sediment loading and burial. However, pending further analysis, the FA observed could be interpreted as having formed from two separate fluid events, one early contemporaneous with subsidence and subsequent burial forming FA-a and one late – forming FA-b. By understanding the mechanisms controlling in situ P and F distributions within coal seams, coal operators could ensure P and F product specifications are met by selectively mining and refining blending schedules. CITATION: Davis, B, Rodrigues, S, Esterle, J and Golding, S, 2017. ‘It goes with the splits?’ Spatial variability of phosphorus and fluorine within coal, in Proceedings Tenth International Mining Geology Conference 2017, pp 451–452 (The Australasian Institute of Mining and Metallurgy: Melbourne).