Journal of Minerals & Materials Characterization & Engineering, Vol. 7, No.4, pp 301-306, 2008 Printed in the USA. All rights reserved
Assessment of Some Nigerian Coals for Metallurgical Application
I. O. Akpabio
, M. M. Chagga
and A. Jauro
Chemistry Programme, Abubakar Tafawa Balewa University, P. M. B. 0248, Bauchi,
Bauchi State, Nigeria.
Scientific Services Department, National Metallurgical Development Centre (NMDC),
P.M. B 2116, Jos, Plateau State, Nigeria.
* Correspondence Author: E-mail:
In the quest for a search of a suitable coal deposit of metallurgical importance, the qualities of
three Nigerian coal samples, Lafia-Obi, Doho and Lamja, were assessed for possible utilization
in metallurgical coke production. Several parameters that determine coking quality, such as
moisture content, ash, volatile matter, free swelling index, sulphur content, Gray-King coke test,
dilatation and plasticity (fluidity), were investigated. Lafia-Obi has the lowest moisture and
volatile matter and the highest fixed carbon content. Its ash and sulphur contents are greater
than that required by a coking but washing prior to use is expected to improve it. The volatile
matter places Lafia-Obi in the medium-volatile bituminous rank, while Lamja and Doho are of
medium-volatile bituminous rank. The rheological properties show that Lafia-Obi is the only
caking coal. This put together single out Lafia-Obi as a potential candidate for coke making.
Doho and Lamja coals may be useful for steam raising, liquefaction or chemicals manufacture.
Keywords: proximate analysis, coking coal, plastic properties, coke
In order to curtail the importation of metallurgical spares and consumables, the Ajaokuta Steel
Company Ltd was built to transform Nigeria Technologically. This dream is yet to be realized
due to unavailability of prime coking coal locally. In spite of the reported occurrence of several
coal deposits in the country, report on their coking potentials is still lacking [1]. Coal is a vital
resource that can transform a country technologically and also substitute imported sources of
302 I. O. Akpabio, M. M. Chagga and A. Jauro Vol.7, No.4
energy supplies. If the indigenous coal deposits can be explored and exploited, Nigerian
economy can be diversified, leading to a smooth industrial and technological transition from the
present petroleum based economy [2]. Coking coal is a coal that softens, fuse and re-solidify
when heated to form the carbon-rich material called coke. Metallurgical coke is one of the major
raw materials needed in the production of iron by the blast furnace method. It acts as a reducing
agent, reducing the iron ore to its elemental form.
This paper reports the results of proximate analysis and rheological properties of three Nigerian
coals, Lafia-Obi (Nasarawa State), Doho (Gombe State) and Lamja (Adamawa State) and uses
the results in assessing the coking potentials of the coals.
2.1 Materials
The coal samples were collected directly from the coal fields at Lafia-Obi (Nasarawa state),
Doho (Gombe state) and Lamja (Adamawa state). Samples collected were kept in an airtight
polyethylene bags.
2.2 Methods
The coal samples were air dried, pulverized and sieved to pass 200µm sieve size [3]. The
following parameters were determined.
(a) Inherent Moisture: 1g of the powdered sample was placed in a pre-weighed silica
crucible and subjected to a temperature of 105 ºC to 110 ºC
for 1 hour in the absence of air,
until a constant weight was attained [3].
(b) Ash Content: 1g of pulverized sample was weighed in to a platinum crucible and
subjected to a temperature of 825 ºC
in a muffle furnace for about 4 hrs until a constant
weight was attained [3].
(c) Volatile Matter: 1g of powdered coal sample was weighed and covered in a 10ml
platinum crucible. The sample was subjected to a temperature of 925 ºC
in a muffle furnace
for 7 minutes [4].
(d) Gray-King Coke Test: 20g of powdered sample was weighed into a retort tube, pressed to
a layer of about 15cm long and then plugged with a cotton wool. A U-tube placed in a beaker
filled with ice blocks was placed in a tubular furnace set between 325 ºC 600
ºC at a heating
rate of 5 ºC/min. The coke button obtained was compared with the standard shape profiles
(e) Free Swelling Index: 1g of the pulverized coal sample was weighed into a translucent
crucible. It was covered and heated over a Bunsen burner for about 7 minutes. The button
obtained was compared with the standard numbered profiles from 1-9 in ½ Units [4].
Vol.7, No.4
Assessment of Some Nigerian Coals
(f) Total Sulphur (Eschka method): 1g of pulverized sample was mixed with 3g Eschka
mixture. The mixture was heated to a temperature of 800 ºC for 1 hour. The mixture was
digested in water. The sulphur dissolved in water was precipitated as barium sulfate by the
addition of barium chloride. The precipitate was filtered; reduced to ashes and the final
weight determined gravimetrically [3].
(g) Plasticity (Fluidity): 5g of powdered sample was weighed into a metal crucible equipped
with a motorized stirrer. The sample was stirred as it was heated at a rate of 1ºC/minute to a
final temperature of 300
ºC. Stirring force required as phase changes was recorded by the rate
of stirrer’s rotation [3].
(h) Dilatation: 5g of the pulverized coal sample was pressed into a pencil and heated in a
tubular finance with a piston weight on top. As the coal softened the sample contracts due to
the piston weight. Also, as soften mass increased, the volatile components trapped in the
pencil caused a dilatation (i.e. increase in volume of soften mass) [6].
Table 1 shows the results of proximate analysis, Gray king assay, free swelling index, dilation
and fluidity obtained from coal samples. Lafia-Obi has the lowest moisture content of 2.91%,
followed by Lamja with a value of 3.08%. The highest moisture content of 3.90% was recorded
in Doho sample. High moisture content would result in a decrease plant capacity and an increase
in operating costs. This is by affecting the calorific value and the concentration of other
constituents [7]. The moisture content required by a good coking coal is 1.5% [3]. Therefore
values recorded in the coal samples are generally higher than that stipulated for a good coking
Table 1: Proximate Analysis, Sulphur Content and Rheological Properties of Some
Nigerian Coals.
Parameters Samples Identity
1. Proximate Analysis Lafia-obi Doho Lamja
Moisture (wt %)
Ash (% db)
Volatile matter (% db)
Fixed carbon (%)
Total sulphur (%)
2. Gray king Assay G3 A B
3. Free Swelling Index 6 0 0
4. Dilation % 23 -5 -5
5. Fluidity (ddpm) 21 0 0
db = dry-basis, ddpm = dials division per minute
304 I. O. Akpabio, M. M. Chagga and A. Jauro Vol.7, No.4
The lowest ash content of 11.87% was observed in Lamja sample, followed by Lafia-Obi with a
value of 20.66%. Doho sample recorded the highest ash content of 29.91%. Lower ash content is
an essential requirement for coke making coals, because some of the ash would end up in the
coke on carbonization, and in the blast furnace, the ash influences slag volume and composition.
An ash content of less than 10% is recommended for a good coking coal [8]. Industrial
experience indicates that a 1wt. % increase of ash in the coke reduces metal production by 2 or 3
wt. % [9].
Lafia-Obi sample possess the lowest volatile matter of 27.29%, followed by Lamja with a value
of 40.01% and then Doho with the highest value of 43.44%. The volatile matter content of
medium volatile bituminous coal is around 22%. This suggests that the Lafia-Obi coal is of
medium volatile bituminous rank. Doho and Lamja coals may be of high volatile non-caking or
weakly caking class of coals which are known to have volatile matter content of 36% and above.
Volatile matter apart from its use in coal ranking is one of the most important parameters used in
determining their suitable applications [10]. Volatile matter does not form part of the coal; it is
usually evolved as tar during carbonization.
The fixed carbon content of the coal samples shown on Table 1 revealed that Lafia-Obi sample
has the highest carbon content of 46.23% followed by Lamja with a value of 44.23%. The least
carbon content of 21.98% is found in Doho sample. The fixed carbon content of the coal is the
carbon found in the material after volatile materials are driven off. It is used as an estimate of the
amount of coke that will be yielded from a sample of coal [11]. This suggests that Lafia-Obi
sample with the highest value has more carbon for coke formation, followed by Lamja and then
Doho coal has the lowest sulphur content of 0.77%, followed by Lamja with a value of 0.81%.
The highest sulphur content of 2.91% is observed in Lafia-Obi sample. In the carbonization
process sulphur is undesirable because most of it would be retained in the coke. This sulphur has
to be removed from iron and steel-making by either modifying the blast-furnace burden with a
consequence reduction in iron output or by external desulphurizing technique which are both
money and time-consuming. An acceptable level for sulphur in coking coal blends for furnace
coke production should not exceed 1.5-1.60% [12]. Therefore the sulphur contents of both Doho
and Lamja are within the required range of a good coking coal while that of Lafia-Obi is beyond.
Coals are purified by washing prior to carbonization. Therefore a decrease in sulphur and ash
contents to the coking coal requirement is expected of Lafia-Obi, when purified. Walker, et al.,
reported a significant reduction in sulphur and ash yield between washed and unwashed coals
The Gray-king test and free swelling index (FSI) shown on Table 1, gave a comparative G3
profile for Lafia-Obi coal, indicating that the coal has a good coking potential. A good
Vol.7, No.4
Assessment of Some Nigerian Coals
metallurgical coal usually possesses a Gray-king profile between G1 to G9. Doho and Lamja
coals have Gray-king type A and B profiles respectively which suggest that they are of poor
coking qualities. This observation is supported by the Free Swelling Index which gave zero
values for both Doho and Lamja, shows that they are non-agglomerating and hence non-caking.
The FSI depends on both rank and coal type. If a single coal is to be used for coke manufacture,
an intermediate value (4-6) for this parameter is probably desirable [11, 14]. Therefore, Free
Swelling Index of six (6) recorded for Lafia-obi sample show that it has a good agglomerating
character that is expected of a good coking coal. The dilatation and fluidity for Lafia-Obi sample
are 23% and 21 dial division per minute (ddpm) which is good for a coking coal. Zero values
recorded for both Doho and Lamja, also support their non-caking behavior.
The studies conducted revealed that Lafia-Obi coal has the lowest moisture, volatile matter and
highest fixed carbon content, followed by the Lamja and then Doho. The volatile matter content,
places Lafia-Obi in the medium-volatile bituminous rank, while Lamja and Doho are classed as
high-volatile bituminous. Even though the ash and sulphur content of Lafia-Obi is higher than
that of Lamja, Doho and that required by a coking coal, purification prior to use is expected to
improve its quality. Gray-king assay and free swelling index show that only Lafia-Obi is caking.
The results combined together, single out Lafia-Obi as the only potential candidate for
metallurgical coke making.
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Assessment of Some Nigerian Coals