A thorough investigation into the British Standard method for the determination of pyritic sulphur has shown that it is not suitable for all Australian coals, and that instead, the direct or non-simultaneous method should be adopted. Determinations of the iron which is not extractable with nitric acid, on both the original and demineralized coals, indicate that this iron is essentially non-pyritic; consequently size reduction of the sample to minus 300 B.S. mesh (as proposed in the revised British Standard method) is not necessary.
THE Standard methodr-3 for the determination of pyritic sulphur in coal is based on the assumption that pyritic iron is insoluble in hydrochloric acid but soluble in nitric acid, and that the iron soluble in hydrochloric acid is also soluble in nitric acid. It follows that the difference Fe(uNo3)-Fe(ucr) determined on separate portions of the sample should be equivalent to the pyritic iron and consequently to the pyritic sulphur. This procedure, however, involves two major problems: (1) the possibility that not all the pyrite in the sample is extracted by nitric acid under the conditions of the test;
(2) the presence in coal samples of iron-containing minerals soluble in hydrochloric acid but either insoluble or only partially soluble in nitric acid. A. H. Edwards et a14 showed that the penetration of nitric acid into some British coals was not sufficient to extract finely disseminated pyrite embedded in coal particles (with high-rank coals the penetration depths were less than 5 pm) and recommended crushing the coal to -300 B.S. mesh (53 pm) as suggested in the revised version of the Standard now in preparation instead of to -72 B.S. mesh (211 pm) as stipulated in the 1959 Standard, in order to allow for the extraction of at least part of this 'unavailable' pyrite. H. R. Brown et a15 confirmed Edwards's observation, but showed that the amount of pyrite involved was usually within the tolerance of the method.
The author has accumulated considerable evidence that the basic assumption of the method regarding the solubility of non-pyritic iron, is not always valid. With several coal samples it was found that the iron which was soluble in hydrochloric acid was either insoluble or only partially soluble in nitric acid, and consequently negative values were obtained for the pyritic iron content.
A. H. Edwards et UP also report instances of negative results with some British coals, and G. B. Belcher7 with some Australian coals.
The solution to the problem is the use of a direct (or non-simultaneous)