Petrographical and mineralogical analysis of coal after biological leaching

Abstract

For coal utilization the sulphur content is a decisive parameter for the quality of the coal. In many countries clean coal technologies have to be applied on the basis of regulations concerning fuel quality and emission standards for dust, CO, SO2 and NOx in the flue gas. It becomes quite obvious that it is always preferable to keep the sulphur levels in coal at a minimum. Bacterial oxidation of the sulphur present in coal could well be thought of as an effective alternative.Desulphurization by bacteria Thiobacillus ferrooxidans was applied to coal sample from Sokolov mine. Bacteria growing in batch culture on ferrous iron at initial pH 1.6 were harvested at the later growth phase. The ferric iron precipitates were separated from the cells by centrifugation and the rest medium by membrane filtration. Cell pellet captured on the filter was washed by hydrochloric acid and distilled water and finally suspended in fresh Waksmann & Joffe medium. The medium was previously acidified by 5M H2SO4 to pH 2. Initial concentration of sulphates in medium was 1.8 g L-1. Desulphurization of coal was studied in a stirred batch reactor at 10% w/v pulp density in diluted H2SO4 at pH = 2.The effect of bacterial leaching on mineral and organic matter of coal was followed by optical microscopy. Petrographic evaluation of the coal matter samples consisted of the determination of reflectance of gelified huminite macerals (R0), determination of maceral group contents of huminite, liptinite, inertinite and determination of the mineral content with emphasis on the various forms of iron sulphides. The sample examined were brown coal with higher liptinite and pyrite contents. The huminite reflectance of 0,33 % is corresponding to the lignitic metatype. Huminite concentration is 65,7 % and is the most abundant maceral group. The maceral ulminite and densinite contributes to the high huminite content. The concentration of attrinite, who be pass to liptodetrinite, textinite, gelinite and corpohuminite contents does not exceed 5 %. The maceral composition of liptinite was determined by a fluorescence measurement. The remaining liptinite macerals, i.e. liptodetrinite and bituminite, resinite, cutinite and fluorinite are accessories. The content of inertinite (fusinite, sklerotinite and inertodetrinite) is low (4,5 %).Coal contains sulphur in inorganic form (mainly pyrite and marcasite) and sulphur in organic form. Biological lixiviation has caused relatively significant change in FeS2, in some cases full or part elimination of dispersed framboids at mineralised detrite as well as at huminite macerals resulting in empty holes and hudge FeS2 grains were slightly disintegrated at the surface or dike - pyrite was relased irregularly after lixiviation