CRITERIA FOR SELECTION OF RAW FEED COALS TO COKING COAL WASHERY FOR PRODUCING CLEAN COAL OF DESIRABLE PROPERTIES

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1 CRITERIA FOR SELECTION OF RAW FEED COALS TO COKING COAL WASHERY FOR PRODUCING CLEAN COAL OF DESIRABLE PROPERTIES T. K. Das, D. G. Naolakhe & B. N. Prasad RDCIS, SAIL, Ranchi Abstract Coking coal is a natural resource and is always contaminated with mineral matters. Generally, the ash (oxides of mineral matters like silica, alumina, etc.) content of raw coking coals is higher than the desired limit for its use in BF coke making. Through beneficiation of raw coals, ash content can be brought down to the desired level and quality can be improved. Most of the Indian coking coal washeries are now fed from a number of sources and detailed characterization of raw feed coals are required to be carried out for linking of any mine to any washery. In general, mineral liberation and washability characteristics are determined for raw coals for linking of any mine to a washery. However, studies carried out at RDCIS, SAIL clearly demonstrated that the above studies are not adequate for achieving the desired coking properties of clean coal, produced from any washery. It has been established that petrographic study of coal, in addition to the above is also required to arrive at the desired coking properties. Key Words: Coking Coal, Mineral Liberation, Float & Sink, Petrography, Reactive Maceral Introduction The properties of coking coal vary significantly with its composition. In general, raw coking coals are beneficiated to achieve the desired composition of clean coal. There are different technologies available for beneficiation it may be just manual separation of shale materials from coal materials or wet beneficiation using jig, dense media bath, dense media cyclone, flotation unit, spiral separator etc. Earlier pit-head washeries were installed to beneficiate the raw coal of specific mine. But in India at present each washery is fed from a number of mines. In such cases, there are some specific properties of raw feed coals which need to be examined for linking to any washery for producing clean coal of desired quality

2 This paper describes in brief the different properties to be determined for selection of raw feed coal sources for any coking coal washery. Mineral Liberation Characteristics Coal is a natural resource and always associated with mineral matters varying in quantity depending upon geological location and environment during its formation. When runs-of-mine coal is crushed to smaller sizes, preferential distribution minerals takes place. Generally coal-rich particles are softer and reported in smaller particles. However the mineral-rich particles are harder and reported in bigger particles. Thus, size reduction is essential to liberate minerals. Mineral liberation characteristics determine the top size to which coal is to be crushed to obtain optimum liberation. Indian coals are generally containing high ash, low vitrinites and are very difficult to wash. Minerals are generally highly disseminated and needs finer crushing for liberation of minerals. Salient results of the studies on mineral liberation characteristics carried out at RDCIS, SAIL, Ranchi is presented in Figs.-I and II Fig.-I: Finely disseminated mineral matters in Indian coals - 2 -

3 Values, % Fines Generation Gross Yield at 19% Ash NGM at19% Ash <75 mm <20 mm <10 mm Crushing Level Fig.-II: Improvement in mineral liberation with finer crushing of raw coal As shown in Fig.-II, when a coal is crushed to a top size of 75 mm, the yield of clean coal is 54% at 19.0% ash content, the same improved to 60% when crushed to a top size of 20 mm and to 65% for a top size of 10 mm. The improvement in the clean coal yield is due to the increase in the liberation of minerals with finer crushing. The other improvement is the reduction in the content of near gravity materials (NGM) from 54% at top size of 75 mm to 44% at top size of 20 mm and to 37% at a top size of 10 mm. However, when top size is reduced through finer crushing, generation of fine fraction (<0.5 mm) is increased. The increase is not high when top size is reduced from 75 mm to 20 mm but it is very high when further reduced to 10 mm. The handling of fines in washery is expensive and difficult. Thus, the optimum crushing is with top size of about 20 mm. Beneficiation Characteristics Most beneficiation processes are based on gravimetric separation. Thus, Float & Sink tests for different size fractions are necessary. Following information is available from this test Yield of clean coal at a specified ash content Yields of middling and reject and their ash contents Washability characteristics content of near gravity materials (NGM) at the cut density, difficulty level in washing, etc

4 This information is essential to know the suitability of the washery equipment in washing such coals in any existing washery or in designing new washeries. Typical results of Float & Sink tests carried out at RDCIS, SAIL, Ranchi on a coal are presented in Fig.-III. Cumulative Yield, % Float 90 Characteristics 80 Sink Ash, % 100 Cu. Sink, % Fig.-III: Washability curves Petrographic Characterization Mineral liberation characterization and Float & Sink test are very basic and essential for all raw coal for their beneficiation. But these are not adequate in ensuring desired coking properties of clean coal. It has been observed that by beneficiation, increase in the content of reactive macerals takes place in addition to the reduction in ash content. But the level of increase in the content of reactive macerals is not same in all coals. The coking properties are highly dependent on the content of reactive macerals and ash. This can be seen from the results of investigation carried out at RDCIS, SAIL, Ranchi presented in Tables-1 &

5 Table-1: Effect of beneficiation on petrographic composition for coals of higher vitrinite content (>40%) in clean coal Parameters Kuju IX Seam Tandsi Tapin North Raw Clean Raw Clean Raw Clean Proximate Analysis Analytical Moisture, % Volatile Matter, % Ash, % Petrogrpaphic Analysis Maceral Composition Vitrinite, % Reactive fusinite, % Liptinite, % Inertinite, % Mineral Matter, % MMR, % Caking/ Coking Properties CSN ½ 3½ 3½ LTGK Coke Type F G G G 3 G 1 G 2 Maximum Fluidity, ddpm Table-2: Effect of beneficiation on petrographic composition for coals lower vitrinite content (<40%) in clean coal Parameters Karo Parej-II, Top Seam Jarandih Raw Clean Raw Clean Raw Clean Proximate Analysis Analytical Moisture, % Volatile Matter, % Ash, % Petrogrpaphic Analysis Maceral Composition Vitrinite, % Reactive fusinite, % Liptinite, % Inertinite, % Mineral Matter MMR, %

6 Caking/ Coking Properties CSN LTGK Coke Type D F C F - F Maximum Fluidity, ddpm It can be seen from Table-1 that when raw coal from Kuju IX Seam is beneficiated, the ash content is reduced from 30.8% to 17.8%, the content of vitrinite, which is the main reactive maceral is also increased significantly. Whole Liptinite is reactive maceral. Vitrinite and reactive fusinite of certain range of rank are reactive macerals. The effect could be seen from improvement in coking properties represented by CSN, LTGK coke type and maximum fluidity. Similar results were observed in Tnndsi and Tapin North coal. In clean coal from all these coals, the vitrinite content is more than 40%. The level of improvement varied from coal to coal. From Table-2, it can be observed that in Karo, Parej-II and Jarandih, the reduction in ash content to desired level by beneficiation did not improve the vitrinite contents to a significant level. As a result, the coking properties of the clean coal samples produced from these coals were poor. Thus, while washing these coals, it is essential to blend some coal which provides much higher vitrinite content and as a result clean coal contains desired vitrinite content to maintain coking properties. Perographic analysis also provided rank (MMR or ROR), which are very important property to classify coals in prime, medium or blendable coals. This can be roughly done with the help of volatile matter content and with the help of petrographic analysis it can be assessed precisely. Logistics The distance between the mine and the washery, transportation facility of both raw and clean coals may also be considered during linking. Conclusions When any coking coal washery is fed from a number of raw coal sources, traditional properties, namely mineral liberation and beneficiation characteristics are not adequate to ensure desired coking properties of clean coal. Coking properties are determined by not only ash content but also petrographic properties. In case of a coal having low vitrinite, a compensating coal is to be - 6 -

7 blended at a proportion such that the desired vitrinite level in clean coal can be achieved