Journal of Minerals & M aterials Characterization & Engineering, Vol. 8, No.7, pp 541-549, 2009 P rinted in the USA. All rights reserved
Metallurgical Use of Heat Altered Coal: A Case Study
Debjani Nag*, A.K. Singh and P.K. Banerjee
R&D Division, Tata Steel Limited, Jamshedpur-831001, India
* Corresponding Author:, Tel +916572148938
The effects of igneous intrusion on coal are observed in various parts of the world. It is found
that igneous intrusions h ave altered the quali ty and characteristics , especially th e coke ability of
the coals. It has been estimated that a large quantity of heat affected (jhama) coal is reserved in
the Jharia collieries of India. Nowadays, apart from difficulties of mining, its utilization is the
biggest issue. This paper presents a brief overview of heat altered coal and also highlights some
studies, undertaken in Tata Steel to make best possible metallurgical use of jhama especially in
steel industry.
Key Words: Jhama; igneous intrusion; thermally altered; natural coke.
Coal affected by igneous intrusion is a common phenomenon. Thermally affected coals are
known by various names such as Natural Coke, Geological Coke, Burnt Coke, Cinder and
Jhama. The kind of coal alteration depends on the temperature of the intrusion, the duration of
magmati c heat in g and the distance of coa l fro m dire ct con ta ct wi th th e i gneous rock. The thermal
influence depends on the thickness, nature and form of the intrusive body as well as on the
composition and rank of the intruded organic-rich rock [1]. In general, igneous intrusions cause
dramatic changes in vitrinite reflectance, mineralogy and geochemistry of the coal seams. In
most cases, these changes are gradual with distinct physical and optical properties [2-4].
Three zones in the intrusion affected coal seams can be demarcated as (i) normal coal (%Romax
0.8-1.2%) (ii) slightly thermally altered coal (%Romax 1.2-1.8%) and (iii) highly thermally
altered coal (%Romax 1.8-5.0%) [5]. Several authors have studied the different properties of
thermally altered coal. The impact of igneous intrusions on the organic component of coal has
been reported in the U.S.A [6-8]; U.K [9], Australia [10-14], Antarctica [15], China [16, 17],
India [18], Poland [19] and Turkey [20]. It has been found that volatile matter and total
hydrogen decrease towards the contact zone, while ash, total carbon and vitrinite reflectance all
542 Debjani Nag*, A.K. Singh and P.K. Banerjee Vol.8, No.7
steadily increase. The volatile matter and ultimate analysis characteristics of the highly affected
coals indicate that it is more like semi-coke, rather than a completely carbonized product.
Finkelman, et al. [21], Goodarzi and Cameron [22], Merritt [23], Querol, et al. [24, 25] and
Alexandra and Paul [26] studied the effect of igneous intrusions on the inorganic constituents of
coal. According to Finkelman et al [21], there are three ways by which the intrusion could
modify the inorganic element composition of a coal: (i) removal of elements by volatilization,
(ii) residual concentration of elements in refractory phases and (ii) addition or removal of
elements by fluids directly derived from the intrusion or from a hydrothermal system generated
at the intrusion-host boundary. According to Alexandra and Paul [26], majority of elements in
the altered coal have affinities to aluminosilicate minerals. Ankerite and siderite are products of
thermal alteration and only occur in the altered coal and dykes.
The microstructure of thermally affected coal has been investigated by several workers like
Singh, et al. [3], Kwiecinska and Petersen [1], Singh, et al.[2], Suchy, et al. [27], Bourrat, et al.
[28], Goodarzi and Murchison [29]. The optical changes of the coal induced by the thermal
metamorphism depend on the initial rank of the coal and its level of maturity at the time of
intrusion. In natural coke formed from bituminous coals, three groups of micro constituents may
be recognized: (i) matrix (groundmass) formed by total alteration of vitrinite and liptinite; (ii)
macerals of the inertinite group with preserved structures and textures visible in the unaltered
coal; (iii) new components, which are partly high-carbon material and partly mineral matter,
formed due to the intrusion. Entirely coked vitrinite, especially from bituminous coals, often
exhibits mosaic structure (fine, medium, coarse) showing variable degree of optical anisotropy.
The anisotropy and reflectance increase with the temperature of alteration. The occurrence of
mosaic structure is one of the primar y features which dist inguish natural coke fro m natural char.
All these findings discussed above are unique to coal samples because there are number of
factors that may cause differences in the chemical nature of thermally altered coal. In this paper
an attempt has been taken to give a brief description about the jhama coal of Jharia coalfields of
In India, a large amount of thermally metamorphosed coal has been found in the Damodar Valley
coalfields of Jharia, which is a part of Lower Gondwana coalfields. Here these coals are known
as jhama. In the Indian coal scenario Jharia coalfield occupi es a special status as this is th e only
storehouse of prime coking coal and has been meeting the coking coal needs of the country for
over a century. Coal mining in the coalfi eld was started in the last decade of the 19th century. It
is a sickle shaped coalfield occurring in the form of a basin truncated with a major boundary
fault on the southern flank. The arcuate coalfield, measuring about 40 km. in length and 12 km.
in width, occupies an area of nearly 450 sq. km.
Uncontrolled coal fires reportedly began in the Jharia coalfield in 1916, about 26 years after the
start of mining [30]. The origin of most coal- mine- fires in India is reported to be the result of
spontaneous combustion. Although the coals of the Jharia Coal Field are not as susceptible to
spontaneous combustion as some other coals in India [31], irresponsible mining practice prior to
Vol.8, No.7 Metallurgical Use of Heat Altered Coal 543
nationalization resulted in enhancing self heating of the coal. Large volumes of fragmented coal
in thick seams, crushed pillars, poor ventilation and surface subsidence fractures resulted in
conditions ideal for spontaneous combustion to occur. A study conducted by the Central Mine
Planning and Design Institute Limited (CMPDIL) of nearly one hundred fire sites in India
resulted in identifying spontaneous heating as the cause of about 67 percent of the fires. About
33 percent were attributable to some form of neglect, accident or design [32].
In Jharia, major part of the reserve of coking coal is altered due to igneous intrusion (Figure 1).
For example, the total insitu reserve of jhama at Jharia Division of Tata Steel is 154.10 million
tonne wherein the extractable reserve is only 8.64 million tonne. Due to metamorphism, the
pseudo carbonization takes place and coking coals are converted in to natural coke or jhama.
Problems related to mining of jhama were analyzed by Singh, et al [33]. They suggested that the
rating of a coaling machine should be increased for cutting of a coal seam with frequent igneous
intrusions. The nature and extent of influence of the igneous intrusions seem to be quite complex
and it has substantially altered the physico-chemical properties of the surrounding coal mass.
Here, the igneous intrusions causing regional in situ burning of a coal seam are normally mica-
peridotite and dolerite. After tearing the coal seam along the lower resistance path, the intruded
igneous materials appear in the form of a dyke and a sill in and around a coal seam. Thermo-
contact transformation, as a result of the igneous intrusion, resulted in a significant change in
physico-mechanical properties of a coal seam. A hypothetical schematic diagram showing the
effect of intrusions on coal seams has been presented in Figure 2.
Fig. 1. Heat affected coals in Jharia (Singh, et al., 2008).
544 Debjani Nag*, A.K. Singh and P.K. Banerjee Vol.8, No.7
Fig 2. Schematic presentation of magmatic intrusion affecting the coal seam in Jharia Coalfield,
Damodar Valley India (Singh, et al., 2008).
The presence of jhama causes the estimation of the quality and quantity of coal in any area or
seam to be problematic because of the coal’s heterogeneous alteration and compact nature. Due
to the various difficulties very few systematic characterisation was done earlier. Hence, proper
industrial utilisation of heat affected coal could n ot be made till now. Macroscop ical observation
revealed that jhama is usually dull, compact and hard. It has vacuoles (pores) which are empty or
infilled with mineral matter (Figure 3). The immediate contact between the igneous intrusive
body and the coal may be sharp and planar or it may be diffuse and irregular. The optical
changes of t he coal induced by the thermal metamorph ism depend on the initial rank of th e coal
and the level of maturity of the coal at the time of intrusion. In jhama three groups of micro
constituents may be recognized. They are matrix (groundmass) formed by total alteration of
vitrinite and liptinite, macerals of the inertinite group with preserved structures and textures
visible in the unaltered coal and new components, which are partly high-carbon material and
partly mineral matter, formed due to the intrusion [1]. Figure 4 presents some micrographs of
jhama coal taken from Jharia coal field.
Fig. 3. Field photograph of jhama coal.
Vol.8, No.7 Metallurgical Use of Heat Altered Coal 545
Fig 4. (A) Vitrinite maceral altering into mesophase sphere and mosaics; (B) Mesophase
spheres, mosaics and devolatilzation pores; (C) Natural coke groundmass with devolatilization
pores and cracks; (D) Same field showing mesophase spheres mosaics and fine flow structure
(E) Natural coke groundmass showing flow structure and vesicles; (F) Flow structures and
mosaics of different sizes with prominent anisotropy, fine flow structure and mineral matter
under crossed (Singh, et al., 2007).
Some preliminary studies have been made by Tata Steel Limited in the past. Some initiative has
taken for the washability study of jhama. This study has been done in CFRI, Dhanbad. Detailed
washability studies followed by laboratory flotation revealed that theoretical yield of 25.3% at
13% ash level are achievabl e when the raw jha ma is crushed to 75mm. Whil e, for the raw jhama
crushed to 10mm the theoretical yield improved to 44 2% at an ash level of 13.2%. In order to
use jhama as one of component in coal blend for coke making, it was characterised by some
preliminary tests like proximate analysis, ultimate analysis, fluidity, dilation, HGI, calorific
value, crucible swelling number and petrographic studies. Some of the properties of slightly
affected jhama are l isted in Table 1 (a, b, c). I t is characterized by high ash, low volatile matter
and high fixed carbon. Results indicate that jhama has significant amount of reactive material
546 Debjani Nag*, A.K. Singh and P.K. Banerjee Vol.8, No.7
(reflectance of desired level) (Table 1c) and it can also fulfill the r equire ment of reactives in co al
blend. It was found that the jhama collected from different seam have difference in their
properties. So the sample should be properly characterized before any application.
Table 1: Properties of jhama coal.
Proximate Analysis (%) Ultimate Analysis (%)
Ash Volatile
Matter Moisture Fixed
Carbon Carbon
16.83 11.70 1.93 69.54 89.90 3.28 0.44 2.25 4.13
Fluidity Dilatometry CSNHGICV
Nil 3 Nil 1 84.06414
Vitrinite Semi Vitrinite Exinite Inertinite Mineral Matter Average
Reflectance (R)
26.5 2.5 1.0 63.2 6.8 1.07
Jhama being partially carbonized is expected to lose all its coking properties and also have
difficult washability characteristics due to more uniform distribution of mineral matters in the
partially carbonized coal matrix. This makes its large-scale applications, such as, coke making or
PCI challenging to us. To achieve this objective, intensive and through research is needed to
investigate the detail characteristics (physical, microscopical and chemical) and beneficiation
study of jhama and finally to find out a guideline for industrial use.
Vol.8, No.7 Metallurgical Use of Heat Altered Coal 547
The authors gratefully acknowledge the Management of Tata Steel for granting permission to
publish the work.
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