Distribution of selected trace elements in density fractionated Waterberg coals from South Africa

The Waterberg Coalfield, located in the Limpopo Province (north-western area) of South Africa, contains a significant proportion of the South African coal resource. This Permian coalfield typically contains high vitrinite, high ash coals in the Kungurian Grootegeluk Formation, and high inertinite, low ash coals in the Artinskian Vryheid Formation. Four run-of-mine coals and density fractioned samples were analysed to determine their trace element content. The concentrations of most trace elements in the run-of-mine coals exceed the global averages and certain global ranges, and generally exceed values reported for other South African coalfields. Specifically, Hg concentrations are high to very high in all the coals (up to 2.43 ppm in a sample from the Vryheid Formation), Cd and Se concentrations are comparable to or lower than global averages, and As is very low in the Vryheid Formation (1.57 ppm). A sample from the Grootegeluk Formation is enriched in siderophile elements, and a sample from the Vryheid Formation is depleted in chalcophile elements. Studies of the density fractionated samples indicate that Cd exhibits an organic affinity in the Grootegeluk Formation samples, and possibly a pyritic (or sulphide) affinity in the Vryheid Formation sample. Selenium has a greater affinity for the middle, clay-rich density fractions in all samples. Arsenic reports to the mineral-rich sink fractions, specifically the fractions enriched in pyrite. Mercury reports to both the float and sink fractions, indicating an organic and inorganic affinity in these coals, although there is more enrichment in the sink fractions in the Grootegeluk Formation samples, and a definite organic correlation in the Vryheid Formation sample.

► Selected trace elements were determined in South African Waterberg coals.
► Trace elements were generally high compared to global and South African averages.
► Se and Cd were the exception, being lower.
► Hg is highly enriched in some samples, with organic and inorganic affinities.
► As has a positive correlation with pyritic sulphur.