Petrographic analysis of hard coal in grain class <1mm

There are presented results of petrographic analysis of the finest class of hard coal (<1mm). Examined Medium-Rank coals – orthobituminous C – came from four seams of western part of Upper Silesian Coal Basin. The coal samples have been obtained from froth flotation in processing plant. Microscopic analysis comprised defining of vitrinite reflectance and determining of percentage of maceral groups and group of microlithotypes. Petrographic characteristic of examined coals enable to count them to high-vitrinite coals with vitrinite content V = 22-88%vol., low liptinite content L = 2-3%vol. and medium inertinite content I = 3-11%vol. (where the most often semifusinite and inertodetrinite occurred). The highest content of mineral matter (72%vol.), carbominerites and rock have been stated obviously in tailings. Concerning grain size (<1 mm) detailed determining of the art of mineral matter using microscope in transferring light was impossible – there were distinguished only quartz and feldspars. There was mostly approved occurring of isolated macerals. Too fine granulation (<1mm) did not allow to conduct combine analysis, because grain size classified counting mainly as mentioned `isolated` macerals or `on the border of the grain`. The amount of sections of the net did not allow to distinguish microlithotypes, among which there was only possibility to observe vitrite as `observed` microlitothype.


Introduction
Coal in grain class <1mm is one of the most numerous grain classes occurring in the coal mines. This class, named very often dust, originating due to the mining works, transport of hard coal and in processing, occurs both underground (in workings), as on the surface (e. g. in processing plants) of the coal mine [1,2].
Concerning specific character, it is a raw material difficult for management and causing lots of problems for the mines [3]. In the last time in the connection with intensive re-structuring of hard coal mining branch, appeared new conceptions of ecologic management of the finest grain class of these rocks, e.g. different projects connected with production of briquettes. Production of these fuels requires just in the beginning and even before planning stage detailed recognition of the raw material quality, which would be used as a fuel.
Investigations of coal quality comprise very wide spectrum of analysis, depending mostly on its rank (generally power coal/coking coal) and concerning usually technical, chemical and elementary analysis, the analysis of coking parameters and they end usually in more or less advanced analysis of petrographic composition [4].
If in the case of all from the beginning mentioned types of laboratory tests the subject dust class is required (because it diminishes the stage of preparing samples for lab tests allowing often to omit process of crushing of coal to the grain size <1mm), so in the petrographic composition analysis it makes a big difficulty, because mentioned microscopic tests are based on samples in the form of briquettes, consisted of grains with size of 1mm. Smaller grain sizes did not fulfil recommended standards and influence the representativeness and reliability of obtained results. These results thus are required by potential producers of fuels, who often do not realise that there are special requirements.
The content of the paper and presented results have purpose to bring forward the problem signed already earlier, basing on the results of conducted research [5][6][7][8].     The coals from study area belong generally to Medium-Rank coalsorthobituminous C.

Characteristic of analyzed hard coals
The tests concerned hard coals coming from four coal seams: 401, 403 and 405 from Załęże Beds and 415 from Ruda Beds, which mean values of quality parameters are showed below: (table 1).

Sampling and samples
Samples of coal of class <1mm have been collected in processing plant, where is applied froth flotation to the enrichment of the finest part of minerals. This is the most suitable place for collecting of representative samples to subject studies and its choice is connected with market conditions, where mentioned fuels are the most often produced from flotation concentrate or they contain this concentrate in different or even large amount. There were taken 12 samples of coal blend comprising output from examined seams in ratio: 24% seam 401, 40% seam 403, 15% seam 405 and 21% seam 415. Moment of sampling was precisely defined for the time when the mine exploited chosen seams with the same productivity and output. Process of sampling lasted four weeks. The samples were taken during first shift in the middle of the week (Wednesday), when mining and preparation processes are the most stable i.e. the output in time is constant. In each week there were taken three samples including: feed directed to flotation (N7, N13, N20, N27) and two output products in the form of flotation concentrate (F7, F13, F20, F27) and flotation tailings (O7, O13, O20, O27): (figure 4). Each taken sample is a collective of single samples taken every half an hour during the whole shift.

Laboratory (microscopic) analysis
Microscopic analysis covered determining of random reflectance of vitrinite and defining of percentage of maceral groups and percentage of microlithotypes groups. Maceral analysis had in view defining of percentage of macerals in examined sample of coal. The differences in optical features of macerals, observed under the microscope, prove the differences in chemical composition and differentiation of their technological properties. On the other hand the aim of microlithotype analysis was fixing of percentage of microlithotypes, carbominerites and rock in collected samples [10,11,12].
These tests have been done on briquettes, which have been made from coal coming both from the feed directed to froth flotation cells, and output products, meaning flotation concentrate and flotation tailings.
Measurements of vitrinite reflectance R were done using microscope Axioskop of company Zeiss in polarized reflected light applying immerse liquid with coefficient of light reflection n0=1.5180. Microscope have been equipped in double ocular with magnification 10× and immerse lens with magnification 50×. The measurements were done using software "Photan" of company Zeiss, which enabled making of reflectograms.
Analysis of petrographic composition comprised quantity analysis of particular maceral and microlithotype groups, i.e. participation of particular macerals, microlithotypes and mineral matter content. In this purpose there was applied so called combined analysis, which rule is based on using 20 point net for signing microlithotypes, where one and the same point signs in the point maceral. It gives then a measurement of maceral in given microlithotype. This analysis was conducted using microscope of company Leitz in polarized reflected light applying immerse liquid with coefficient of light reflection n0=1.5180. The tests were done using machine counting particular maceral groups of company Leitz.

Reflectance
Coals making examined coal blend, both in seams, and after flotation showed similar values of parameters.

Petrographic composition
As a result of conducted microscopic analysis of petrographic composition it may be noticed that values of particular parameters are changing in the way showed in the tables below: (table 3 and table 4).     figure 6 and figure 7). The highest vitrinite content was observed in feed V=88%vol., in concentrate it counts V=85%vol., and in tailings there is minimum V=22%vol.  Liptinite content in the coals of examined seams counts L=3-10%vol.: (table 1). Conducted tests showed low content of this group counting L=2-3%vol.: (table 3, figure 8 and figure 9). The lowest liptinite content was determined in feed L=2%vol. and concentrate L=2%vol. The highest liptinite content L=3%vol. was determined in tailings.  Inertinite content in the coals of analyzed seams varies between I=19-32%vol.: (table 1). Analysis of percentage of this group in petrographic composition showed content of inertinite I=3-11%vol.: (table 3, figure 10 and figure 11). In the feed this content results I=6%vol., concentrate contains the largest amount I=11%vol. and in tailings this content is the lowest I=3%vol. Between macerals of this group the most often occurred inertodetrinite and semifusinite. There were seen traces of macrinite, micrinite and fusinite: (  Figure 10. Inertodetrinite (sample F7). Figure 11. Semifusinite (sample O20).
Mineral matter content, in the coals from seams creating coal blend: (table 1), have been determined in the borders 1-13%vol. Conducted examinations of petrographic composition showed share of MM in the limits 2-72%vol.: (table 3, figure 12 and figure 13). Share of MM in feed counts 4%vol. and in the concentrate its content is the lowest: 2%vol. For tailings it is on the high level and counts 72%vol. Precise determining of the art of MM was not possible because of too small grains (<1 mm). In its composition there could be distinguished pyrite, quartz and feldspars. Pyrite occurred in the form of concretions (in framboid form) making larger grains, and in the form of single, small grains and spatters occurring within the limits of macerals and between MM. Pyrite made also accretions with macerals, and filled the fissures of cracks.

Summary and conclusions
Conducted petrographic characteristic of examined coals allow to count them to the high-vitrinite coals with vitrinite content V=22-88%vol., low liptinite content L=2-3%vol. and medium inertinite content I=3-11%vol. Dominant macerals group is vitrinite. Macerals of liptinite group constitute the lowest share. Amongst macerals of inertinite group occurred the most often semifusinite and inertodetrinite. Usually during microscopic examinations there was determined occurring of isolated macerals. The analysis of petrographic and microlitothype composition allowed to define the highest amount of mineral matter, carbominerites and rock in tailings, what obviously prove the role of preparation process.
Precise determination of the art of mineral matter under microscope in transmitted light was impossible because of grain size (<1 mm). There were distinguished only quartz and feldspars.