Open Cut » Geology
Increasing global restrictions on fluorine in product coal prompted this project, using data and samples from the Late Permian Bowen Basin coal measures as case studies. In 2015, few mines had tested for fluorine and modelling of its distribution for coal quality control through selective mining or beneficiation, required proxies while more data were gathered.
Fluorine is commonly associated with phosphorus as fluorapatite, Ca5(PO4)3F, in sedimentary rocks, suggesting phosphorus distributions, for which there is abundant data available, would suffice as a proxy for estimation. However, early company studies found this could also result in either an over or under-estimation of fluorine contents. Fluorine can be associated with other minerals, soils and groundwater. It was unclear how Fluorine varies within and between coal measures, and what is its most common mode of occurrence within the coal seams. Understanding the controls on the distribution of phosphorus and fluorine in Australian coals could assist in strategies for its management.
This project investigated the association of fluorine with phosphorus in public and company provided data, with coal grade, type and rank, with stratigraphic occurrence, and where able with geological features (e.g., faults, intrusions, seam splitting). It also investigated the association of fluorine and phosphorus with different minerals, and the geochemistry of the apatite found in coal, utilising work in another project.
In summary, all apatite found in this study was fluorapatite, regardless of its origins from volcanic ash falls or as a precipitate from hydrothermal fluids moving through the seams. The occurrence of detrital apatite, with a high fluorine>phosphorus signature is most common in high ash coals and those with abundant tuff partings. In these fluorine is also associated with clay minerals. This suggests that beneficiation may assist in fluorine reduction in Trend 1 coals (see report). The majority of the fluorapatite found was hosted in the pores of the inertinite group macerals. Fluorine increased as phosphorus increased in Trend 2 coals (see report) but showed no differentiation between raw and washed or clean coal composites, reducing beneficiation assistance. The inertinite host would suggest that coarser, duller coal might be beneficiated, but this has had varied success historically. Predicting the distribution of areas high in fluorapatite is still a problem, as the zones of saturation vary by seam, do not commonly show clear vectors to faults or intrusives, and crosscut seam lithotype profiles. Hence detailed sampling and analysis to understand individual sites is required to develop fit for purpose models for mitigation.