Froth flotation is probably the best-established method of cleaning minus 0,5 mm fines and is in extensive use abroad. In South Africa, however, froth flotation was confined until recently to the upgrading of coking-coal fines. Although these fines are classified, according to South African standards, as being amenable to froth flotation, the process is far from straightforward. Studies directed towards a search for more selective reagents included various frothers4, like Aerofroth 65, Aerofroth 73, Aerofroth 77, pentanol, tri-ethoxy-butane (TEB), crude tar acids, and eucalyptus oil, in conjunction with paraffin as collector. Eucalyptus oil appeared a very promising reagent, even when used on its own. It was also found effective for certain weakly coking coals5, 6. Other collectors5,6, such as kerosene, dieselene, and turpentine, were investigated, but the outcome was not entirely satisfactory. It is possible that more careful studies would reveal more effective reagents, and this is being pursued.
In contrast, the flotation of non-coking coals proved disappointing in the extreme. Their poor floatability is attributed to the small amount of ‘bright’ coal they contain.
As a means of cleaning fine coal, water-only cyclones appear attractive for two reasons: they offer likely capital-cost advantages, and they can easily be integrated in existing dense-medium washing plants. However, it has been found7,8 that the accuracy of separation is not as a rule sufficiently high to enable this type of separator to rank as an alternative to any of the established dense-medium processes for small coal. A degree of cleaning is undoubtedly achieved, but the ecart probable is about 0,25.
Tests have indicated that the water-only cyclone operates both as a density separator and as a classifier. This results in the cutpoints of various size fractions of the feed being distributed over a wide range of relative densities, and the overall cutpoint is noticeably higher if a considerable amount of fine material is present. The separating effect in the minus 150 f1-mfraction is almost negligible, and that on the coarser fractions is merely an ash reduction; for example, the ash content can be reduced from 17,6 to 15,4 percent at a yield of 75,2 per cent, while the theoretical yield according to the washability characteristics is as high as 94,0 per cent at the same ash content.
Tests on the compound water cyclone9 or tricone produced promising results. For example, a raw coal smaller than 0,5 mm with an ash content of 18,2 per cent was washed to give a product with an ash content of 12,8 per cent. The ash content of the plus 150 f1-msize fraction was found to be 9,4 per cent, and that of the minus 150 f1-m fraction was still as high as 16,7 per cent. Appropriate desliming can be applied, but that leaves the fraction between 150 and 75f1-m almost untreated. It is therefore considered unlikely that the hydrocyclone will find wide application in the beneficiation of minus 0,5 mm coal.
Concentrating tables are widely used in the U.S.A. but they are not yet in commercial use in South Africa. Researchlo, however, has indicated that the low capacity of these units can be a major drawback: a quarter-size table cannot handle much more than 0,5 tlh if coal of 7 per cent ash is to be produced from coal smaller than 1 mm. Ecart probable values were about 0,08 with serious ‘tails’ increasing the ash content, and the organic efficiencies were as low as 72 per cent.
Various combinations of flotation, water-only cyclones, and tables were investigated5,6, but a product of about 7 per cent ash could be obtained only when the table was used as the second-stage unit. However, this configuration has the disadvantage of low capacity.