The removal of pyritic sulfur from coal was studied, using three techniques: bacterial leaching, froth flotation with bacteria as a pyrite depressant, and column flotation. The bacterial leaching experiments studied the benefit of improved aeration on the leaching rate of pyrite from coal with Thiobacillus ferrooxidans. Using air-agitated reactors rather than conventional shake-flasks, the leaching rate could be approximately doubled. However, the process still required over a week to remove half of the pyrite from the test coals. The study then shifted to the use of T. ferrooxidans and other microorganisms to depress the pyrite during froth flotation. These experiments showed two things: first, that all of the tested microorganisms were able to depress the flotation of pyrite which had become hydrophobic; and second, that pyrite was not normally hydrophobic under the conditions where coal flotation is carried out (pH = 5-9, with neutral-oil collectors). The liberated pyrite which reaches the coal froth was shown to be carried there by entrained water, and not by true flotation. As a result, pyrite depressants will be of little use in reducing froth sulfur contents, and much greater benefit will come from reducing entrainment.; Based on these results, a novel type of flotation column was designed and built which uses horizontal baffles to improve froth scrubbing efficiency and product quality in coal flotation. These baffles also reduce the necessary column height, do not greatly reduce the column capacity, and make the performance of the column more consistent. This column was first tested on a laboratory scale, and then on a pilot scale in an operating coal-cleaning plant (Empire Coal Co., Gnadenhutten, Ohio), with excellent results.
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