The partitioning behaviour of boron from tourmaline during ashing of coal

Boron is an environmentally sensitive element that may be present in high concentrations in some coals. Three modes of occurrence for boron in coal are commonly recognised, namely, bound to the organic fraction, locked into clay minerals (mainly illite), and bound within the crystal lattice of tourmaline. The organically bound mode is generally considered to be the most likely. Following combustion in a pulverised fuel utility, boron is generally enriched in the fine fly ash waste, but in some cases, i): may also escape with the flue gas, suggesting variable partitioning behaviour. There is concern that boron may be leached from fly ash disposal impounds at concentrations toxic to higher land plants. A coal sample from the D Seam of the Strongman No. 2 Mine, West Coast, South Island, New Zealand has been used to test the hypothesis that boron present in tourmaline is less volatile in coal combustion, implying that mode of occurrence is a key control on the partitioning behaviour of this element. Six sample subsets were doped with increasing amounts of ground tourmaline. One subsample from each set was analysed by prompt gamma INAA to determine the concentration of boron in the coal. Two subsamples were ashed at 1000 degreesC. One ash sample from each set was analysed for boron, while the other ash sample was leached according to Australian standards. It was found that the relationship between boron in the doped coal and boron in the ash is approximately linear (with some losses noted during ashing), indicating boron present in tourmaline was substantially retained in the ash. Furthermore, no relationship was found between the boron content of the ash and boron leached from the ash samples by reagent water. The results suggest boron present in coal as tourmaline is retained in the ash and is unavailable to the environment following fly ash disposal. (C) 2002 Elsevier Science B.V All rights reserved.