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| Jelínek P., Buchtele J., Miškovský F., Straka P. |
| INFLUENCE OF GRAIN SIZE AND MACERAL COMPOSITION OF COAL ON YIELD OF LUSTROUS CARBON IN THERMAL DEGRADATION PROCESS |
Abstract
Work is aimed to the thermal degradation of coal pyrolysis products during a forming of casts from graphitizing alloys (research project of Grant Agency of Czech Rep [1]. In the thermal degradation process, coal and additives act as the sources of pyrolytic carbon. Amounts of coal and additives are 3-4% in the moulding mixtures. Yields ofsolid carbon products obtained from two-stage pyrolysis in laboratory scale are as follows : 64-78% coke, 9.3-16.2% amorphous carbon and 0.18-0.93% lustrous carbon. Even if the yield of lustrous carbon is very small, its influence on surface quality of cast is decisive. The lustrous carbon formed on the inactive part of mould enables to create a smooth surface of the cast with high quality. A role of coke/semicoke is also important, because it compensates a thermal stress in the mould caused by dilatation of sand grain owing to ? ? conversion of SiO2. Finally, the role of amorphous carbonis, together with coke/semicoke, to clog interstices between grains in the mould preventing an ingress of metal. In our contribution, an influence of grain size and maceral composition of coal on the lustrous carbon creation is investigated. For experiments, bituminous coal from Ostrava-Karviná Coal District (OKD) was taken (86.5%C, 0.54% moisture, 4.99% ash, 31.1% volatiles). Coal in the question was choosen from a set of 12 coals of OKD with respect to coalification, maceral and chemical composition, technological properties and porous and optical texture of solid products of pyrolysis [2,3]. On this basis we find coal used as typical in the frame of OKD. The most important criterion we considered the yield of the lustrous carbon in the standard test according to Bindernagel [4]. A hypothesis was accepted that the high yield of lustrous carbon (8.5%) is caused mainly by aliphatic aromatic structure of liptinite (17%) in our coal. In literature, different grain size of coal in bentonite moulding mixtures are suggested. With regard to surface quality of casts from graphitizing alloys, a grain size under 0.03 mm but also 0.1-1 mm are recomanded in contradiction. In our case, coal sample was crushed under 3 mm and fractions 0-0.2, 0.2-0.5, 0.5-1 and 1-3 mm were prepared. The yields of lustrous carbon, coke and volatiles by Bindernagel’s test were determined. The lowest yield of lustrous carbon was obtained from the finest fraction 0-0.2 mm. It corresponds with a low yield of tar/volatiles from this fraction caused by the insufficient development of porous structure [5]. The main reason is that the mass transfer from solid to liquid phase is worse during pyrolysis of pulver particles under 0.2 mm owing to their lower swelling and plasticity. An influence of maceral composition of coal was by mentioned standart test proved. For testing, separated fractions of other coal sample of OKCD enriched by vitrinite or inertinite were used. As with fractions enriched with vitrinite (76% vitrinite, 28.7% volatiles) the yield of lustrous carbon 8.0% was achieved, with inertinite fraction (85% inertinite, 22.4% volatiles) the yield was 4.8%. These results agree with our hypothesis. An aimed selection of coals and additives for uniform bentonite moulding mixture with regards to chemical composition, technological properties and thermal degradation process parameters enables to optimize the mixtures composition. Thus, the high surface quality of castings from graphitizing alloys can be achieved. The castings are traditionaly important comodity of our export.
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| No 1 (1998), p. 71-78 |
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