Coal Preparation, 1990,Vol. 8, pp. 93-99
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SEMjEDX Studies on Association of
Mineral Matter in Coal
R. RAMESH and P. SOMASUNDARAN
Henry KrumbSchool of Mines, ColumbiaUniversity,New York,NY 10027.U.S.A.
(Recei.edOctober2. /988; infinolfomr January3/. /989)
Coal contains a wide variety of inorganic matter like quartz, the clay minerals,
carbonates,sulfidesand trace elements.The presenceof theseminerals is detrimental
to downstream processesinvolving combustion, liquefaction etc., and therefore it is
often necessaryto clean coal prior to utilization. Processesused for the recovery of
mineral matter vary from coal to coal, but is generally limited to the l~Jdily separable
phasesin discretepartings. Fine grained and disseminatedparticles an~minerals that
are intimately associatedwith or dispersedthroughout the coal matrix will not to a
large extent, be removed from the coal. A knowledge of the morphological relationship between the inorganic phasesand organic matter is helpful to devise suitable
schemesfor cleaning coals containing such particles.
Scanningelectron microscopy(SEM) coupled with energydispersiveX-ray analysis
(EDX) were used to determine the nature of the association betweenmineral matter
and bituminous coal from Bruceton mine, Pittsburgh, PA. The proximate and
ultimate analysisof the coal is given in Table I. The coal was crushedin a quaker mill
TABLEI
Analysisof Brucetonminecoal
Ultimate analysis as received
1.79
% Moisture
80.65
% Carbon
5.30
% Hydrogen
1.74
% Nitrogen
0 .10
% Chlorine
1.12
% Sulphur
3.29
% Ash
7.80
8/0Oxygen
(by diff.)
%
%
%
%
Proximate Analysis as recei~d
Moisture
1.79
Ash
3.29
Volatile Matter
36.29
Fixed Carbon
58.60
93
94
R. RAMESH and P. SOMASUNDARAN
FIGURE
and subsequentlyground in a ball mill using ceramic balls. Systematic analysis of
samplesfrom + 8, + 100, + 200, - 400 + 635 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. Representativephotomicrographs of inclusions
in coal and their elemental analysesare discussed.
Figure I showsa 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
ASSOCIATIONOF MINERAL MAlTER IN COAL
95
FIGURE 2
particle is a polydispersedclay associatedwith 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 + 100 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.
96
R. RAMESH and P. SOMASUNDARAN
FIGURE 3
The association of alumino silicates with a + 200 mesh sizecoal particle is shown
in Fig. 4. The inclusions are coated with coal dust (grey areas). The high degreeof
associationis evident from a comparison of the secondaryand back scatteredimages.
The micrograph in Fig. 5 is a backscattered image of pyrite (bright areas). An
elementalanalysisof the dark area under the pyrite particle revealedit to be composed
of aluminum silicates.Elemental sulphur in comparatively large quantities is noticed.
This micrograph is typical of the association betweenpyrite and clayey matter often
coated with coal dust probably due to grinding.
ASSOCIATION OF MINERAL MAlTER
IN COAL
91
FIGURE 4
Figure 6 shows a single liberated mineral particle in the - 400 + 635 mesh size.
Only in this size range large amounts of liberated mineral matter was observed.
Elemental analysis of this particle revealsit to be a complex substancecomposedof
iron oxide with magnesiumalumino silicate and smaller quantities of calcium sulfate,
calcium fluoride and possibly calcium carbonate. Manganeseis also present as an
oxide.
Detailed analyses also indicated that titanium was always found with feldspar,
phosphorous and chlorine with alumino silicates.
R. RAMESH and P. SOMASUNDARAN
FIGURE 5
CONCLUSIONS
The mineral matter in low ash coal is closely associatedwith the coal matrix. The
mineral matter at coarse sizes( + 200 mesh) is locked with each other and the coal
matrix. Only at - 400 + 635 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 particlesare coatedwith coal dust has important
ASSOCIATION OF MINERAL MATTER IN COAL
99
FIGURE 6
implications in cleaning processesbased on surface properties. Advanced cleaning
methods including addition of chemicalsduring grinding can lead to better liberation
and control of surface properties by preserving the differencesbetweenmineral and
coal surfaces.
Acknowledgements
This material is based upon the work supponeci by Department of Energy Grant No. DE-FG-2287-PC
79919.