Journal of Minerals & Materials Characterization & Engineering, Vol. 6, No.2, pp 135-142, 2007 Printed in the USA. All rights reserved
Study on The Reliability of Coke Research Establishment
Micum 40 Formula to Predict Coke Micum 40 Strength
at The Ajaokuta Steel Plant, Nigeria
A.O. Adeleke,
A.B. Adahama,
R.S Makan,
S.A. Ibitoye
Dept of Materials Science and Engineering, Obafemi Awolowo University,
Ile-Ife, Nigeria.
National Metallurgical Development Centre (NMDC), P.M.B. 2116,
Jos, Nigeria.
Corresponding Author: E-mail:
G-values of 0.97, 0.93, 0.94 and 1.01 were determined for Polish Bellview blend 1 (BV1),
Polish Bellview blend 2 (BV2), Australian Agro-Allied blend (AA) and American Carbon
Energy blend (CE), respectively. The coking duration of 18 hours, 17 hours, 16 hours
and 22.5 hours, respectively were used to carbonize the coals each having volatiles
31.8%, 31.3%, 30.22%, and 21.90%, respectively. The Coke Research Establishment
(CRE) formula predicted M40 micum strength of 77.98%, 77.12%, 77.55% and 92.05%
for BV1, BV2, AA and CE blends, respectively. These predicted values were found to
deviate from the experimentally determined M40 indices of 77.80%, 70.80%, 78.20% and
64.16% determined for BV1, BV2, AA and CE respectively by 0.18 units, 6.32 units,
-0.65 units, and 27.89 units respectively. Thus, the best M40 index predicted was
determined for BV1 blend with 0.97 G-value and 18 hours coking time with only a small
allowable deviation of 0.18 units. The CRE formula has therefore been shown to be valid
to predict M40 index of coke produced from coal blends with G-value of about 0.97 and
carbonized at a moderate coking time of 18 hours. It has also been shown that the coking
conditions that produce the best M40 index also produced the best M10 index.
Keywords: coal, blends, coking, micum, indices
136 A.O. Adeleke, A.O.
A.B. Adahama,
R.S Makan, S.A. Ibitoye
Vol.6, No.2
The chemical composition of a coal depends on the proportions of the different chemical
components present in the parent plant debris, the nature and the extent of the changes
which these components have been subjected to since their deposition and the nature and
quantity of the inorganic matter present [1]. The rank of a coal is the degree of change of
chemical composition of the coal within the series of fossil fuel from the least mature
peat to the most mature anthracite. When a bituminous coal is subjected to high
temperatures, it undergoes some changes which among others include decomposition into
a number of complex compounds, the evolution of various gaseous and condensing
substances, conversion into plastic mass at specified temperatures as a result of melting
of its bitumen constituents and conversion of the formed plastic mass into non-plastic due
to further molecular decomposition of the organic mass [1,2].
This process of thermal decomposition of bituminous coal results in the formation of
coke, a solid residue with sufficient mechanical strength to withstand abrasive forces
when a column of smelted charge descends in a blast furnace. The coke mechanical
strength are indicated as micum 10 (M10) and micum 40 (M40) which are indices of
resistance to abrasion and fragmentation, respectively.
Dilatometers and plastometers are commonly used to study coals plastic properties. On
the basis of Ruhr dilatometric parameters, Simonis developed a mathematical formula to
determine the G-value coking capacity that indicates the cokeability of a coal[3,4].
The Simonis formula is :
(E) = Softening temperature (E)
(V )= Temperature of maximum dilatation(V)
(c) = Maximum contraction %(c)
(d) = maximum dilatation %(d)
Following extensive carbonization tests, the UK Coke Research Establishment (CRE)
developed an empirical formula to predict the micum 40 (M40) strength of coke[3]. The
formula is :
VxGM 88
(G) = G-value coking capacity of the coal charge
(V) = Volatile matter content (daf)
(T)= Time in hours of carbonization (to center temperature of
C) in an oven of average width of 450mm.
Vol.6, No.2
Study on The Reliability of Coke
The aim of this study is to compare the micum 40 indices obtained for coke produced
from coals imported for the Ajaokuta Steel Plant from Australia, Poland and United
States of America with the M40 indices obtained by the CRE formula. Since the CRE
formula was obtained following 170 carbonization tests in the same type of 250kg coke
oven [3] used for the imported coals, deductions from the comparison may provide useful
technical factors for successful industrial scale cokemaking operations.
Samples of Bellview 1, Bellview 2, Agro-Allied and Carbon Energy coals sourced from
Poland, Australia and United States of America, respectively.
Proximate Analysis
Proximate analysis of coal involves the determination of moisture, volatile matter, ash
and fixed carbon contents.
Moisture Content
The crucible was preheated at a temperature of 110
C for 1 hour. 1g of coal ground to
pass 250 microns was heated in the crucible at 110
C for 1 hour. The loss in weight
accounts for the moisture content.
Volatile Matter (VM)
The silica crucible was pre-heated in the muffle furnace for 7 minutes at 900
C and
cooled. 1g of sample ground to pass 250 microns was then placed in the crucible and
heated in the muffle furnace for 7 minutes at 900
C. The loss in weight accounts for the
volatile matter of the coal sample.
Ash Content
The silica crucible was pre-heated in a muffle furnace at 825
C for 1 hour. It was then
cooled and weighed. 1g of sample ground to pass 250 microns was then placed in the
crucible and heated in the muffle furnace at 825
C for 1 hour with the lid on. It was then
cooled in the desiccators and weighed. The incombustible residue constitutes the ash
Fixed Carbon
Fixed carbon of the coals were determined by calculation with the relation:
% Fixed carbon = 100 - % moisture - % Ash - % VM
138 A.O. Adeleke, A.O.
A.B. Adahama,
R.S Makan, S.A. Ibitoye
Vol.6, No.2
Ruhr Dilatometry
In Ruhr dilatometry, the variation in the length of a column of coal during heating is
measured. The coal sample ground to pass 250 micron sieve was compacted into a pencil
form. The pencil of coal was then placed in a metal tube and a piston rod was inserted
into the tube to rest on piston’s top. The other end of the piston rod was attached to a
rotating barrel to record the vertical movement of the piston. On heating, the column of
coal softened and contracted in length due to the plastic deformation under the action of
piston. When the coal softened, bubbles of gas were evolved causing the coal column to
swell up. The dilatation percent of the coal indicates its coking power.
Determination of Micum Strength
The coal samples were carbonized by normal charging in a 250-kg capacity coke oven.
Typical normal charging carbonization conditions used were:
Flue temperature – 1020
Bulk density – 830kg/m
Carbonization period – 15 hours
Charge temperature – 1250
The determination of micum indices involves coke stabilization, coke screening and
micum drum test. In coke stabilization, the coke yield was dropped from a height of 5m
into a metal receiver once. The stabilized coke was then screened through vibrating round
hole screens of sizes <10, 10-20, 20-40, 40-60, 60-80 and > 80mm. For micum drum test,
50 kg of the screened coke was subjected to 25rev/mm for 4min in a steel drum and
screened again. The micum 10 (M10) was determined as the percentage of the coke
residue below 10mm sieve size and micum 40 (M40) the percentage of the coke residue
above 40mm sieve size. The calculation of the micum indices for the Agro-Allied coal
blend is as follows:
Determination of screen distribution analysis (Table 3)
Determination of the percentages of coke retained on sieve sizes +40mm
(Table 4).
Calculation of the proportion of each sieve size range in the 50kg micum
drum sample (Table 5).
The results of the analyses are presented in Tables 1 to 5.
Vol.6, No.2
Study on The Reliability of Coke
Table 1: Proximate Analysis
S/No Parameters % BV1 BV2 AA CE
1. Moisture (ad) 1.5 0.8 0.87 1.47
2. Ash (db) 5.8 6.1 9.72 2.66
3. Volatile matter (daf) (V) 31.8 31.3 30.2 21.90
4. Fixed carbon (db) 61.0 61.8 63.01 75.28
Note: ad-as determined, daf –dried ash free
Table 2: Ruhr dilatometric parameters and coking parameters
S/No Parameters % BV1 BV2 AA CE
1. Softening temp.
c 406 404 395 408
2. Maximum contraction % 26 24 22 28
3. Maximum dilatation % 10 -7 -8 40
4. Maximum contraction temp.
440 439 424 437
5. Maximum dilatation temp
c 463 462 440 482
6. G-value (G) 0.97 0.93 0.94 1.01
7. Micum 10 (M10) 11.40 15.40 15.40 25.00
8. Micum to (M40) 77.80 70.80 78.20 64.16
9. Duration of carbonization (hrs) (T) 18 17 16 22.5
10. Calculated M40 77.98 77.12 77.55 92.05
11. Deviation 0.18 6.32 -0.65 27.89
12 Percentage deviation 0.23 8.93 0.83 43.47
Table 3: Screen distribution of Agro-Allied Coke
Sieve (mm) Weight (kg)
- 10 13.85
+10 – 20 2.45
+20 – 40 5.35
+40 – 60 20.17
+60 – 80 47.65
+80 64.82
Total 154.29
Table 4: Percentages of Coke retained +40mm sieve ranges
Sieve (mm) Weight (kg) Wt %
+40 – 60
+60 – 80
Total 132.64 100%
140 A.O. Adeleke, A.O.
A.B. Adahama,
R.S Makan, S.A. Ibitoye
Vol.6, No.2
Table 5: Proportions of +40mm sieve range in micum drum sample
Sieve (mm) Wt (%) Wt (kg)
+40 – 60
+60 – 80
The G-value determined for coal blends BV1, BV2, AA and CE are 0.97, 0.93, 0.94 and
1.01, respectively. The G-value of 0.97 and 1.01 for blends BV1 and CE, respectively,
fall within the range of 0.95 to 1.15 indicated by Simonis for most medium and strongly
coking coals [3]. The G-value of 0.97 was also determined for a German coal blend that
produced coke of micum 10 and micum 40 of 7% and 78.1%, respectively. The micum
40 of 77.80% determined for blend BV1 is observed to be only 0.4 units lower, while its
micum 10 of 11.40% is only 4.4units above that of coke from the German blend [5]. The
G-value thus gives a fairly reliable indication of coke strength. However, the poorer
micum 40 of 64.16% for blend CE when compared with 0.97 shows that micum 40
strength does not increase in direct proportion to G-value.
For micum 10, the best abrasion resistance was obtained for the BV1 coal blend and the
worst micum 10 value of 25% was obtained for the highest G-value of 1.01. These results
also indicate that the M10 index does not improves in direct proportion to the G-value. It
is also noted that the micum 10 indices for blends BV2 and AA with G-values 0.93 and
0.94 respectively, which falls below the Simonis range (0.95 to 1.15) produced coke with
better M10 indices than coal blend CE with the higher G-value. These results suggest that
the G-value coking capacity parameter gives a fairly accurate indication of coke strength
not only for coals within the Simonis range but also below.
For German cokemaking practice, a carbonization period of 18 to 24 hours is specified
for cokemaking in 250kg capacity coke ovens [6]. The carbonization periods used were
18 hours, 17 hours, 16 hours and 22.5 hours for blends BV1, BV2, AA and CE,
respectively. The blend AA with the lowest coking period of 16 hours gave the best M40
index of 78.20%, while the blend CE with the longest coking period of 22.5hours gave
the worst M40 index of 64.16%. The very high coking period for blend CE may account
for the low M40 index of its coke. It is also noted that the best M10 index of 11.40% was
obtained for the BV1 coke with 18 hours coking time, while the worst M10 index of 25%
was obtained for the CE coke carbonized for 22.5 hours. Both BV2 and AA coke
produced at 17 hours and 16 hours, respectively, have the same M10 index, while the
M40 index of the latter exceeds the former by 7.40 units. Coking durations which are as
low as 14 hours had been used for cokemaking in India [7]. These results suggest that
high coking period improves the M10 index, while low coking period promotes the M40
Vol.6, No.2
Study on The Reliability of Coke
index. Thus, moderate coking period of say 18 to 19 hours may be the most appropriate
to produce high-grade coke with good resistance to fragmentation and abrasion.
The volatile matter (daf) of 31.80%, 31.30%, 30.20% and 21.90% were determined for
blends BV1, BV2, AA and CE, respectively. Thus, blends BV1, BV2 and AA are high
volatile, while CE is low volatile [8]. The volatiles for the first three blends thus exceed
the 24.01% for a medium volatile Indian coking coal [7] but are lower than the volatiles
of 39.4% to 41.8% for coals carbonized to produce high grade coke in Japan [9]. Coals
with volatiles of 17-22%, which include the volatile of 21.70% for CE blend have also
been included in blends for cokemaking in France [7]. These results suggest that the
volatile contents of the four blends are such that they may produce metallurgical grade
From the analysis results, the best M40 index of 78.20% was determined for blend AA
with 30.22% volatile, while higher and lower volatiles gave poor M40 index. The best
M10 index of 11.40% was obtained for blend BV1 with higher volatile of 31.8%. Thus,
blends with moderately high volatiles may be the most appropriate to produce high-grade
coke. This deduction agrees with the specification that coals for cokemaking at Ajaokuta
have volatiles ranging from 27.7% to 30.30% [10].
The CRE formula shows that the M40 index increases with increasing G-value between
0.89 to 1.13, which includes the G-values for all the blends carbonized (i.e. 0.93 to 1.01).
The CRE formula is thus applicable to these coals. The fixed carbon content tends to
increase with decreasing value of volatile between 31.80 and 30.22%. The CRE M40
predicted is observed to generally decrease with carbonization time. Negative values of –
11, -13.9, -19.5, -30.9 units were calculated for the 18hrs, 17hrs, 16hrs and 22.5hrs
carbonization period in the terms containing T. The M40 index thus generally decreases
at too low coking period (e.g. 16hours) and too high coking period (e.g. 22.5hours). Thus,
moderate coking period of about 18 hours may be the most appropriate. The coking
period of 18 hours falls within the range of 13.9 to 19.5 hours specified by CRE to obtain
valid CRE prediction of M40 [3]. For a valid CRE formula prediction, the volatile matter
content range is 19% to 41%, which includes all the coal blends considered.
The deviation of calculated CRE M40 indices from the experimentally determined
indices are 0.18units, 6.32units, -0.65units and 27.89 units (that is, 0.23% 8.93% 0.83%,
and 43.47% in percentages), respectively. The deviation fall below the 3.2units
determined for blends carbonized at CRE except for BV2 and CE blends [3]. It is
observed that these unacceptable deviations occur at the lowest G-value of 0.93 and the
highest G-value of 1.01, while the least deviation occur at G-value of 0.97. These results
further confirm the BV1 blend as the best blend to produce the highest-grade coke. These
results also show that the coking conditions that give the best predicted M40 also
produced the best M10 index determined experimentally.
142 A.O. Adeleke, A.O.
A.B. Adahama,
R.S Makan, S.A. Ibitoye
Vol.6, No.2
The G-value, the volatile content and the coking periods of the blends carbonized agree
with the range of values of these parameters specified for coals to which the CRE M40
index may apply, except for CE blend which deviate in coking time. The best M40 index
was predicted for blend BV1 with 0.97 G-value and a moderate coking time of 18hours.
Furthermore, the least deviation of 0.18 units from the actual M40 index was determined
for BV1. The CRE M40 formula is thus applicable to predict the M40 index of coals to
produce coke at Ajaokuta using the bench scale parameters of G-value and volatile
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