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Journal of Minerals & Materials Characterization & Engineering, Vol. 7, No.4, pp 347-353, 2008
jmmce.org Printed in the USA. All rights reserved
Effects of Stamped Charging on the Strength of Coke from the Weakly
Caking Australian Agro-Allied Coal Blend Mixed with Coke Breeze
, A.O Adeleke
, A.O. Olulana
, S.A. Ibitoye
Dept. of Materials Science & Engineering,
Obafemi Awolowo University, Ile-Ife. Nigeria.
National Metallurgical Development Centre (NMDC),
P.M.B 2116, Jos. Nigeria.
The weakly caking Australian Agro-Allied coal sample mixed with 7% un-reactive coke breeze
was subjected to stamped charging prior to carbonization for 22 hours in a 250kg pilot plant to
improve the resulting coke strength. The coal was carbonized with a bulk density of 1,000kg/m
and heating flue temperature of 1,340
C. The screen distribution analysis and the micum tests
conducted on the coke gave
of 15.90% and 73.20%, respectively. These results
show that the Agro-Allied coke produced is inferior to the coke from a normal top charged
indices of 15.40% and 78.20%, respectively and do not satisfy the
requirements for the blast furnace ironmaking process. These results thus suggest that the
stamped charging coking improvement method designed for high caking, high volatile coals may
not be able to improve the strength of coke resulting from a weakly caking, medium volatile coal
mixed with coke breeze.
Keywords: coal, caking, stamped, carbonization, coke, micum
Coking coal to produce metallurgical grade coke comes from the bituminous grade coals, which
constitutes about 52% of the world’s resources of coals. However, only about 5% of the world’s
supply of coals is prime coking and suitable for straight carbonization [1, 2]. Coke is an essential
input in blast furnace iron production. Consequently, the volume of coke produced and used
348 A.B. Adahama, A.O Adeleke, A.O. Olulana and S.A. Ibitoye Vol.7, No.4
depends on the blast furnace activity. For instance, as at 1973 when the blast furnace iron output
was 96.2 million in the United States, blast furnace coke consumption was 60.7 million tons .
When coal is heated, it softens when heat breaks the coal structure producing liquids and gases.
The evolved low molecular weights components of the coal may escape as gases, while some
molecules may condense as a complex mixture of liquids called tars and the large molecular
weight species may re-combine and solidify as coke .
A coking coal is required to have low ash content because a large amount of inorganic material
can dilute the plastic stage, adversely affecting coke formation. The coal should also contain low
sulphur to reduce the amount of this heteroatom that may get into iron and reduce its mechanical
strength [4, 5]. Bench and pilot scale studies are normally conducted on coal blends prior to
industrial scale cokemaking in a battery of ovens. Several coking techniques such as preheating,
stamped charging and partial briquetting have been developed to improve the micum strength of
coke from coal blends .
Coke breeze is the fine screening that result from the crushing of coke. Coke breeze typically
passes a 0.5 inch or 0.25 inch screen opening. It has been estimated that a coke plant typically
produces 4.3 tons of coke breeze for 134 tons of furnace coke . The addition of coke breeze
to coal blends allows the use of high volatile coal (and poor caking coals) as up to 70% of blend.
Coal blends containing coke breeze produces coke with higher lumpiness and improved
resistance to fissuring but with a slightly reduced abrasion resistance .
The aim of this research is to determine the micum strength obtainable in carbonizing the
imported weakly caking Australian Agro-Allied coal (mixed with coke breeze) and assess the
usability or otherwise of the resultant coke when the blast furnace at the Nigerian Ajaokuta steel
plant begin operation.
2. MATERIALS AND METHODS
Drums of Agro-Allied coal blend imported from Australia and coke breeze by-product at the
Coke Oven pilot plant, National Metallurgical Development Centre (NMDC), Jos, Nigeria.
The coal sample was carbonized in a 250 kg coke oven plant and the coke produced was
subjected to screen distribution analysis and micum drum tests to determine the micum strength.
Pilot scale coal carbonization
The as-received coal was subjected to further crushing to obtain a sieve analysis such that
>3mm size fraction is >70% and < 0.5mm is < 35%. The bulk density test was performed to
obtain a bulk density in the range 690 to 760 kg/m
. The coal was then mixed with 7%
weight percent coke breeze and subjected to stamping in the steel stamping box to increase
Vol.7, No.4 Effects of Stamped Charging on the Strength of Coke 349
the bulk density to 1000 kg/m
and the cake formed was pushed into the coke oven for
During carbonization, the temperatures of the six heating flues were maintained at 1,340
by careful adjustment of air/liquefied petroleum gas ratio, at a heating rate of 1.1
charge temperature rose progressively from 60
C to about 1250
C within a carbonization
period of 22 hours. The coke produced was pushed into a quenching facility where cooling
was done for about 3 hours by water circulating in the system but not in direct contact with
The coke produced was then stabilized by dropping the coke from a hopper placed at a height
of 5.6m. Afterwards, the stabilized coke were screened through round hole sieves 0 – 10, 10
– 20, 20 – 40, 40- 60, 60 – 80 and + 80mm in a vibrating screening machine. For micum test,
50kg sample obtained from selected screen fractions specified proportions was charged into a
micum drum where it was subjected to rotation at a rate of 25 revolutions per minute for 4
minutes. The coke product of this test was screened again on the vibrating screen machine to
determine the M
indices that indicate coke mechanical strength.
3. RESULTS AND DISCUSSION
The carbonization conditions used are presented in Table 1, while the results of screen
distribution analysis and micum drum tests are shown in Tables 2 and 3, respectively.
Table 1. Carbonization conditions for the stamped Agro-Allied coal.
S/No Coking parameters Agro-Allied coal
1. Weight of charge (kg) 248.95
2. Bulk density (kg/m
3. Flue temperature (
4. Carbonization time (hours) 22
350 A.B. Adahama, A.O Adeleke, A.O. Olulana and S.A. Ibitoye Vol.7, No.4
Table 2. Screen Distribution Analysis of Agro-Allied coke.
S/N Sieve Sizes (mm) Wt (kg) %
1 -10 9.80 5.49
2. +10-20 3.75 2.10
3. +20-40 4.70 2.63
4. +40-60 7.70 4.32
5. +60-80 17.30 9.70
6 +80 135.15 75.76
Total weight retained 178.40
Table 3. Micum indices of Agro-Allied coke.
5. Micum 10 (M10) % 15.90%
6. Micum 40 (M40) % 73.20%
3.2 Discussion of Results
The volatile matter (daf) of 30.2% determined for the Agro-Allied coal blend is less than the
33% required to necessitate stamped charging and coke breeze addition prior to carbonization in
the German Zentralkokerei Saar plant [8,9]. Thee free swelling index of 2.5 for the Agro-Allied
coal is also lower than the value of 6.5 for the German blend. The 7% coke breeze added to the
Agro-Allied coal is higher than 5% for the German blend [8, 9]. These results indicate that both
the volatile content and the caking value of the Agro-Allied coal do not necessitate its stamped
charging carbonization with coke dust additions. The more expensive stamped charging
carbonization with coke breeze additions are applied to high caking and high volatile coals to
reduce the possibility of crack formation during carbonization and improve coke strength .
The bulk density of 1,000kg/m
used for the stamped charged Agro-Allied coke is higher than
the 800 to 820kg/m
employed at UK Coke Research Establishment (CRE) for normal top
charging of coals. The latter bulk density for normal top charging has been reported to offer a
greater margin of safety as regards internal pressure generation than opting for the lower mean
density of 720 to 750 kg/m
. The bulk density of coal charge has been found to have great
effects on internal pressure generation in the coke oven. It has been suggested that coking
pressure is related to the fifth power of bulk density . The stamped density for the Agro-
Allied coal is lower than the 1,200kg/m
for stamped charge carbonization in the German
Zentralkokerei Saar plant  but similar to 800 to 1,000kg/m
used for preheating/stamped
charging in China . It is also within the range of 800 to 1,160 kg/ m
for stamped charge coal
in India. The high bulk density allow the coal particles to come closer together thus facilitating
effective bonding of inert particles by the plastic mass .
These results strongly suggest that
the bulk density used at NMDC for the successful carbonization of Agro-Allied stamped coal is
Vol.7, No.4 Effects of Stamped Charging on the Strength of Coke 351
appropriate and the absence of excessive pressure generation during the carbonization showed
that the bulk density selected was appropriate.
The first phase of lateral shrinkage of coke during coal carbonization has been shown to depend
on the charge bulk density and coal composition, while the second phase is controlled by coking
time and coking rate. The second phase of contraction was reported to be almost the same for all
the coals carbonized, while the total internal contraction was found to decrease with increasing
charge bulk density . The bulk density of 800kg/m
produced lateral contraction of about
13.5mm, while a charge density of 680kg/m
gave about 18mm . The bulk density of
for the carbonization of Agro-Allied coal exceeds the range of 790 to 850kg/m
carbonization at Krupp Mannesmam coke oven battery 
The size consists requirement of
mixture of coal particles of >3.0mm and <0.5mm used for the carbonization of the Agro-Allied
coal at NMDC is similar (though of different proportions) to >3.15mm, <2.0mm and <0.5mm
mixture used at the German Krupp Mannessmann Coke oven for carbonization of coals .
These results show that the charge density employed for the stamped Agro-Allied coal may
produce a coke with a lateral contraction lower than 13.5mm and with no adverse effect on the
coke oven refractory linings.
The heating flue temperature of 1,340
C used at the NMDC coke oven for the carbonization of
the stamped Agro-Allied cake is higher than1,200
C used at Wuhan, China but the same as
C at the German Zentralkokerei Saar plant [5,8]. These results indicate that the
carbonization of the stamped cake at NMDC was conducted at a flue temperature that falls
within the range for standard practice in the steel industries.
abrasion resistance of 15.9% determined for Agro-Allied coke with coke breeze
addition is far higher (and thus the coke has a poorer abrasion resistance) than 6.2% for the
German coke from the stamped charge cake of 33% volatile matter plus coke breeze. The M
73.2% determined for the Agro-Allied coke is lower (thus, lower resistance to fissuring) than
76.9% for the German coke . The micum 10 for Agro-Allied coke is also lower than 5.3 to
5.5% determined for some Indian stamped coal blends coke . The micum strength of the
coke is also lower than the specification of 9% (maximum) M
and 78.8% (minimum) M
coke for the blast furnace ironmaking at the Ajaokuta steel plant, Nigeria . The micum 10
strength parameter determined for the Agro-Allied coke is also inferior to the 9% (maximum)
and 5% to 8% specified for Phione steel plant and European steel mills, respectively [16,17]. The
micum strength determined for the stamped Agro-Allied coke is also inferior to the 15.40% and
78.20% for the coke from the Agro-Allied coal normally top charged under slightly different
conditions which include a lower coking time of 16 hours . These results show that although
stamped charging has been proved to be effective in improving coke strength of high volatile
coals with high free swelling indices by eliminating the incidence of crack formation, it is not
appropriate for a medium volatile, low caking coal such as Agro-Allied with coke dust addition.
The coking time of 22 hours used for the carbonization of the Agro-Allied coals far exceeds the
range of 18 to 20 hours used for carbonization at the Taranto steel works coke oven battery
The coking period of 22 hours is also 2.5 hours higher than the 19.5 hours used for carbonizing
stamped coal in the German Zentralkokerei Saar plant , but fall within the range of 18 to 24
hours for typical German practice . The coking periods used for the pilot scale carbonization
352 A.B. Adahama, A.O Adeleke, A.O. Olulana and S.A. Ibitoye Vol.7, No.4
of Agro-Allied coals thus fall within the range for standard coking practice in recognized steel
industries and the result of micum drum test in the coke produced are thus likely to be reliable
indices of Agro-Allied coke strength.
The weakly caking Agro-Allied coal mixed with 7% coke breeze was subjected to stamped
charging prior to carbonization for 22 hours according to standard practice to improve its coke
strength. The results obtained from proximate analysis and crucible swelling tests showed that
the coal is a medium volatile, low caking coal for which the application of the more expensive
stamped charging with coke breeze addition is in-appropriate. The micum strength 10 and 40 of
15.90% and 73.20% obtained for the resultant coke is inferior to the 15.40% and 78.20%
determined for the normally charged Agro-Allied blend carbonized for 16 hours. These results
showed that the application of the stamping coking improvement method may not lead to
improved coke strength for a medium volatile, low caking coal mixed with the un-reactive coke
breeze, particularly when carbonized at more than 16 hours.
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