SEMjEDX Studies on Association of Mineral Matter in Coal

Transcription

1 Coal Preparation, 1990, Vol. 8, pp Reprints available directly from the publisher Photocopying permitted by license 1990 Gordon and Breach Science Publishers S.A. Printed in the United Kingdom SEMjEDX Studies on Association of Mineral Matter in Coal R. RAMESH and P. SOMASUNDARAN Henry Krumb School of Mines, Columbia University, New York, NY U.S.A. (Recei.ed October 2. /988; infinolfomr January 3/. /989) Coal contains a wide variety of inorganic matter like quartz, the clay minerals, carbonates, sulfides and trace elements. The presence of these minerals is detrimental to downstream processes involving combustion, liquefaction etc., and therefore it is often necessary to clean coal prior to utilization. Processes used for the recovery of mineral matter vary from coal to coal, but is generally limited to the l~jdily separable phases in discrete partings. Fine grained and disseminated particles an~minerals that are intimately associated with or dispersed throughout the coal matrix will not to a large extent, be removed from the coal. A knowledge of the morphological relationship between the inorganic phases and organic matter is helpful to devise suitable schemes for cleaning coals containing such particles. Scanning electron microscopy (SEM) coupled with energy dispersive X-ray analysis (EDX) were used to determine the nature of the association between mineral matter and bituminous coal from Bruceton mine, Pittsburgh, PA. The proximate and ultimate analysis of the coal is given in Table I. The coal was crushed in a quaker mill TABLE I Analysis of Bruceton mine coal Ultimate analysis as received % Moisture 1.79 % Carbon % Hydrogen 5.30 % Nitrogen 1.74 % Chlorine 0.10 % Sulphur 1.12 % Ash /0 Oxygen 7.80 (by diff.) Proximate Analysis as recei~d % Moisture 1.79 % Ash 3.29 % Volatile Matter % Fixed Carbon

2 94 R. RAMESH and P. SOMASUNDARAN FIGURE and subsequently ground in a ball mill using ceramic balls. Systematic analysis of samples from + 8, + 100, + 200, mesh fractions were conducted using a SEM Cambridge 250 Mark 2 coupled with a Kevex 8000 EDX analy7.er. Samples for SEM were prepared from grain mounts fixed to aluminum stud sample holders and carbon coated prior to analysis. Representative photomicrographs of inclusions in coal and their elemental analyses are discussed. Figure I shows a micrograph of an inclusion in a + 8 mesh particle. The elemental analysis of this particle indicates that it contains Na, Ca, Si, P, AI, Fe, S, Ct. The

3 ASSOCIATION OF MINERAL MAlTER IN COAL 95 FIGURE 2 particle is a polydispersed clay associated with oxidation products of pyrite in the form of sulfates. Figure 2 shows the secondary and back scattered images of a section in a pyrite encrustation on a mesh coal particle. The micrograph illustrates the high topography, porous nature of the inclusion, and disposition of the pyrite particles on the coal surface. Trace amounts of Mg are noted. Fig. 3 is a micrograph showing the intimate association of an altered form of feldspar probably cerisite with the coal. Trace amounts of Ti are noted.

4 96 R. RAMESH and P. SOMASUNDARAN FIGURE 3 The association of alumino silicates with a mesh size coal particle is shown in Fig. 4. The inclusions are coated with coal dust (grey areas). The high degree of association is evident from a comparison of the secondary and back scattered images. The micrograph in Fig. 5 is a back scattered image of pyrite (bright areas). An elemental analysis of the dark area under the pyrite particle revealed it to be composed of aluminum silicates. Elemental sulphur in comparatively large quantities is noticed. This micrograph is typical of the association between pyrite and clayey matter often coated with coal dust probably due to grinding.

5 ASSOCIATION OF MINERAL MAlTER IN COAL 91 FIGURE 4 Figure 6 shows a single liberated mineral particle in the mesh size. Only in this size range large amounts of liberated mineral matter was observed. Elemental analysis of this particle reveals it to be a complex substance composed of iron oxide with magnesium alumino silicate and smaller quantities of calcium sulfate, calcium fluoride and possibly calcium carbonate. Manganese is also present as an oxide. Detailed analyses also indicated that titanium was always found with feldspar, phosphorous and chlorine with alumino silicates.

6 R. RAMESH and P. SOMASUNDARAN FIGURE 5 CONCLUSIONS The mineral matter in low ash coal is closely associated with the coal matrix. The mineral matter at coarse sizes ( mesh) is locked with each other and the coal matrix. Only at mesh individual mineral particles are seen. Single inorganic crystals of complex composition are noticed. The high topography of the coal surface, and the fact that mineral particles are coated with coal dust has important

7 ASSOCIATION OF MINERAL MATTER IN COAL 99 FIGURE 6 implications in cleaning processes based on surface properties. Advanced cleaning methods including addition of chemicals during grinding can lead to better liberation and control of surface properties by preserving the differences between mineral and coal surfaces. Acknowledgements This material is based upon the work supponeci by Department of Energy Grant No. DE-FG-2287-PC