A review of element partitioning in coal preparation

With the current focus on critical elements and their potential recovery from coal and coal-related sources, such as coal refuse, coal-fired power plant ash, overburden, underclays, and partings, coal preparation technologies are being evaluated for their role in recovering or rejecting these elements. It is also important to review data from past studies on trace element removal from coal, as many of those elements considered hazardous air pollutant precursors in the 1990s are considered critical elements today. These include Be, Cr, Mn, Co, F (as the mineral fluorspar), Ni, As, and Sb. More recent work has focused on the rare earth elements (REEs) in the lanthanide group, Y, and Sc. Following a brief overview of coal cleaning processes, methods of predicting the partitioning of these elements in coal preparation plant processes are reviewed, and those coal preparation technologies that have been applied in laboratory, pilot, and commercial-scale testing for recovery or rejection of these elements are discussed. Opportunities for future work in element partitioning are then considered including element association with coal macerals and comprehensive evaluations of the use of hydrometallurgical principles to recover elements from organic matter. It will be concluded that the mode of occurrence of these elements plays a major role in the recovery or rejection of these elements in conventional coal preparation processes as modes of occurrence change from coal seam to coal seam and within a coal seam. Sample identification must include location and depth, and collection must be broadened to analyze a coal seam throughout a basin to begin to develop predictions based on depositional environments. This is important for the coal measure itself but also for any associated strata and any coal-related waste product—coarse refuse, tailings, fly ash, bottom ash, or, potentially, acid mine drainage. Development of a comprehensive picture of the occurrence of elements in coal and their disposition in waste products will be critical in establishing the most valuable targets for critical element recovery, both for REEs and other critical minerals.