The direct determination of the forms of sulphur in coal using microwave digestion and i.c.p-a.e.s analysis

A sequential digestion method is reported for the direct determination of sulphate, pyrite and organic sulphur concentrations in coals of either bituminous or sub-bituminous rank. A three stage extraction was developed, using acid digestion in a microwave oven. In the first stage, 5 M HCl is used to dissolve sulphate phases in the coal. Pyrite is then extracted from the stage 1 residue using 2 M HNO3,. The final stage, for the determination of organic sulphur, involves the use of concentrated HNO3, HCl, HF and boric acid for the complete decomposition of residue that remains following stage 2. The extract solutions from each stage are rapidly analysed for sulphur using i.c.p.-a.e.s. The sums of the three forms of sulphur have shown consistent agreement with certified total sulphur data for most of the coals studied. This correlation, in addition to the good precision achieved by this technique, suggests that the sequential digestion method is reliable. Furthermore, significant timesavings are achievable over some of the standard techniques demonstrating the suitability of the method for routine analyses. (C) 1999 Elsevier Science Ltd. All rights reserved.