Comparison of the HF-HCl and HF-BF 3 maceration techniques and the chemistry of resultant organic concentrates

Abstract

The common approach used to concentrate and isolate dispersed organic matter and to demineralize coal is by attack with HF. This reagent reacts with silicates to form SiF 4, a volatile gas, and leaves the organic matter largely unaltered. Hydrofluoric acid also reacts with other inorganics to form fluoride salts, the most important being CaF 2 which is very insoluble. Hydrochloric acid is usually used as a pretreatment to remove carbonate minerals and to inhibit fluoride salt formation in the final deashing step with HF. The approach taken in this work is to selectively dissolve neo-formed fluorides in a two step demineralizing process, by reaction with BF 3, which is a highly water soluble gas. The BF 3 is generated by the reaction of H 3BO 3 with HF, in water. It then reacts with neo-formed fluoride salts to form water soluble fluoroborates. Several coals and organic rich sediments were demineralized with both techniques. The total ash in the test samples varied from ∼1% in samples of Wyoming subbituminous coal and a North Dakota lignite to 33% in an Irati oil shale sample. For high ash samples, the HF-BF 3 technique resulted in a final deashed product which is lower in ash by an average of 19% over those from the HF-HCl technique. Elemental analysis, FTIR and microscopic examination did not indicate any relative difference in organic alteration. The HF-HCl macerated samples however, were found to have a much higher Cl and F content, suggesting a lesser degree of organic alteration for HF-BF 3 macerated samples.