TITLE:
Methodology to improve strength after reaction (CSR) of coke for
stamp charge battery.
FIELD OF INVENTION:
The invention relates to a process for improving coke strength of coke
for stamp charge battery.
BACKGROUND OF THE INVENTION:
CSR, or coke strength after reaction with CO2, has become the more
important means of evaluating the quality of coking coal and of
controlling blast furnace performance. It is now a principal criterion by
which coals are selected to make blast furnace coke. With the
increase size of up-coming blast furnaces and enhanced PCI
(pulverized coal injection) rate it is necessary to improve the coke
strength after reaction of the coke. Presently Tata Steel is using
around 20% prime coking coal in blend. Also prime coal reserve is
limited. If by some means coke strength after reaction can be
increased then use of prime coking coal can be minimized.
This will be beneficial from the cost point of view. In Tata Steel coke
strength after reaction has been done following the NSC method. In
which 200 g coke of 19-21 mm size is heated in a reaction tube (78
mm diameter X 210 mm length) at 1100°C for two hours during which
CO2 is passed a 5 l/min. The percentage loss in weight of coke during
the above reaction is reported as the coke reactivity test (CRI). This
reacted coke is further tested by rotating in a I drum (127 mm
diameter X725 mm length) for 30 min at a speed of 20 rpm. The coke
is then screened on a 10 mm sieve and the % of + 10 mm fraction is
reported as the coke strength after reaction (CSR).
Present study introduces a new concept of "differential screening". In
this method coal first crushed in such a manner that 90% of it should
pass through 3.2 mm size sieve. This coal then screened in several
fraction. It was observed that different size fraction of coal has
different properties in terms of ash, volatile matter, crucible swelling
number, fluidity, reactives/inerts amount etc. By enhancing one of the
fractions in blend it is possible to improve the coke strength after
reaction (CSR) of coke.
OBJECTS OF THE INVENTION:
An object of this invention is to propose a process for improving coke
strength of coke for stamp charge battery.
Another object of this invention is to propose a process based on
differential screening to increase the coke strength (coke strength
after reaction) of coke.
Further object of this invention is to propose a process of using a
specific fraction of coal in the blend to achieve higher coke strength.
BRIEF DESCRIPTION OF THE INVENTION:
According to this invention there is provided a process for improving
coke strength of coke comprising:
crushing the prime coking coal (1) and weakly coking coal (2)
subjecting the crushed coals to the step of screening;
seperating out the different fractions:
(+3.2,-3.2+1.6, -1.6+0.8, -0.8+0.5, -0.5+0.2 and -0.2)
A, B, C, D, E & F respectively.
subjecting the different fraction to the step of characterization based
on its coking properties, blending the different fractions to produce
coke with the best coke strength.
DETAILED DESCRIPTION OF THE INVENTION:
Concept of differential screening: Conventionally coal used in
carbonization tests is crushed in such a manner that 90% of it should
passes through 3.2 mm screen. In present study a new method of
screening has been designed. In which coal after crushing in
conventional manner passed through different screen (+3.2, -3.2+1.6,
-1.6+0.8, -0.8+0.5,-0.5+0.2). Then complete characterization of
different fractions for coking was done. It was found that some
specific fraction of a coal has richer coking property. This method
suggests isolation of the richer fraction of coal by screening and then
adding it to the blend. Experiments have been done with a good
coking and a weakly coking coal.
In conventional coal blending coal is crushed in such a manner so
that 90% of it should pass through 3.2 mm screen. In proposed
invention coal is first crushed in similar fashion. Then it screen to
separate out the different fraction (+3.2, -3.2+1.6, -1.6+0.8, -0.8+0.5,-
0.5+0.2). All the fractions then characterize for its coking properties
separately. Then the best fraction is blended.
The above mentioned process, named as differential screening is not
used anywhere in blend design for coke making. Another advantage
of this process is that, by applying selective screening we can take
out a specific fraction from a non-coking or thermal coal. This method
had been tested with thermal coal. It was found that certain fraction of
thermal coal has better coking properties than its parent coal. So by
applying the selective crushing it is possible to separate out one
specific fraction of thermal or non-coking coal and utilize in
metallurgical coke making.
This investigation has done with two individual coals one is prime
coking coal (1) and another is weakly coking coal (2). In normal
stamp charging the coal/blend is to crushed in such a manner that
90% of it should be -3.2 mm. In this study -3.2 mm fraction of coal
has been screened in order to separate the different fractions +3.2,-
3.2+1.6, -1.6+0.8, -0.8+0.5, -0.5+0.2 and -0.2. These fractions have
been designated as A, B, C, D, E and F respectively.
Characterization: These fractions for the two coals have been
characterized in terms of ash, volatile matter, crucible swelling
number (CSN), fluidity and petrography.
Ash determination: Ash is determined by following ASTM standard D
3174-11. 1 gm of 250 mm size sample is taken to a weighed capsule.
Then the sample is placed in a cold muffle furnace and heated
gradually at such a rate that the temperature reached 450°C to 500°C
by 1 hr. At the end of the 2 hr it will reach 950°C. After cooling the
weight of the sample then measured and ash is calculated by weight
difference.
VM determination: Ash is determined by following ASTM standard D
3175-11. In this test 1 gm of 250 mm size sample is taken in a
covered platinum crucible and heated in a furnace of 950°C for 7 min.
The VM is calculated by weight difference.
Crucible swelling number: Crucible swelling number test has been
done by following ASTM D720-91 (2010). In which 1 gm of sample (-
0.212 mm size) is taken in a translucent squat shaped silica crucible
and the sample is leveled by tapping the crucible 12 times. The
crucible is covered with a lid and heated under standard conditions,
either by a special type of gas burner or muffle furnace. After the test
the shape of coke button is compared with a standard chart and
accordingly, the crucible swelling number (0 to 9) is assigned to the
coal sample.
Fluidity: This test is done following ASTM. D2639-08 This test is
performed in a constant torque Gieseler plastometer. 5 gm sample of
-0.425 mm particle size is packed around a stirrer in a cylindrical steel
crucible (21.5mm ID x 35 mm length). The crucible along with the
stirrer and the sample is immersed in a solder bath which is heated at
a constant rate of 30C/min. A constant torque of 40 g inch is applied
on the stirrer which remains stationary at the beginning and starts
rotating as the coal becomes plastic and again stops, when the coal
resolidifies into a coke mass. The speed of stirrer is noted at 1 min
intervals as dial divisions per minute (ddpm). The data are reported in
terms of maximum fluidity.
Petrography of coal: This is done by following ASTM standard D
2797M-11 a, D2798-11 a and D 2799-1. For microscopic studies 5 kg
coke sample is crushed to minus 3 mm size and by using coning
and quartering method 100 gm sample is prepared and mounted on
carnauba wax. The mounted sample is polished using silicon carbide
coated grinding papers of 320, 400, 600 grids respectively, followed
by buffing on blazer cloth using alumina polishing compounds (1.0
and 0.3 mm). The polished specimen is dried using an air dryer prior
to microscopic studies. The microscopic studies were done by Leica
DMS4000 microscope along with QWin software.
Carbonization tests: A number of carbonization tests were conducted
in the 7-kg test oven, under stamp charging conditions using a
standard procedure established at R&D, Tata Steel. The series of
carbonization tests were carried to study the influence of specific size
fractions of coal under stamp charge conditions. Water was added to
the coal blend to obtain the desired value of moisture content. The
coal cake was made inside a cardboard box keeping the bulk density
1150 kg/m3. Base tests were done with 100% of the two individual
coal (prime coking and weakly coking). Then certain amount of the
blend is replaced by the specific fractions and again carbonization
tests were done. Before charging the coal cake into the oven, it was
ensured that the empty oven temperature is 900±5°C. After 5 h of
carbonisation time, the hot coke was pushed out and quenched with
water. The coke samples were tested for coke strength after reaction
(CSR) and CRI (coke reactivity indices).
Results
Table 1(a and b) shows the properties of different fractions of two
individual coal. Table 2 shows the carbonization tests results. It is
found that replacing individual coal by specific fractions (-0.8+0.5
mm) leads to higher coke strength after reaction (CSR) and lower
coke reactivity indices (CRI). In case of both prime and weakly coking
coal the increment in coke strength after reaction (CSR) is
approximately 3 units which is quite significant. This may be due to
the improved surface to volume ration of the fractions.
WE CLAIM:
1. A process for improving coke strength of coke comprising:
crushing the prime coking coal (1) and weakly coking coal (2)
subjecting the crushed coals to the step of screening;
separating out the different fractions:
(+3.2,-3.2+1.6, -1.6+0.8, -0.8+0.5, -0.5+0.2 and -0.2)
A, B, C, D, E & F respectively.
subjecting the different fraction to the step of characterization
based on its coking properties, blending the different fractions
to produce coke with the best coke strength.
2. The process as claimed in claim 1 wherein the coal in crushed
in such a manner that 90% of the coal should be of -3.2 mm.
3. The process as claimed in claim 1 wherein the most preferred
fraction is (-0.8 + 0.5m) ie D which where added to 90% coal
(1) or coal (2) gives higher coke strength after reaction (CSR)
and lower coke reactivity indices (CRI).
4. The process as claimed in claim 1 wherein the addition of
fraction D (-0.8 + 0.5 mm) to coal enhances coke strength after
reaction (CSR) approximately by 3 units.

ABSTRACT

A process for improving coke strength of coke comprising: crushing
the prime coking coal (1) and weakly coking coal (2) subjecting the
crushed coals to the step of screening; seperating art the different
fractions: (+3.2,-3.2+1.6, -1.6+0.8, -0.8+0.5, -0.5+0.2 and -0.2) A, B, C, D, E &
F respectively, subjecting the different fraction to the step of characterization based on its coking properties, blending the different
fractions to produce coke with the best coke strength.