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Reduction of mill scale generated by steel processing
Article · January 2010
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Raaft Farahat, Mamdouh Eissa, Gamal Megahed, and Amin Baraka:
Reduction of mill scale generated by steel
processing
Mill scale is one of the by-products produced during steel processing and its specific production is considerably high
representing about 19-40 kg/t of hot rolled product, depending on the deformation technology used. The global production of steel during 2008 was 1125 million t and the corresponding mill scale produced is estimated to be 33
million t on average. On the other hand, mill scale is considered as a rich iron source (> 70 % Fe) with minimum impurities. Extensive research is being conducted for the recovery and utilization of the iron oxide that mill scale contains. Mill scale is used for magnetic storage, polishing, chemical manufacturing, pigment manufacturing, and biomedical application. The production of sponge iron from scale could be considered a highly profitable method of
beneficiation.
In this study, the reduction of composite pellets produced from mill scale using anthracite coal was investigated.
Laboratory scale trials have been conducted to study the effect of the amounts of reducing agent, reduction temperature and reduction time. The trial results show that it is possible to use mill scale as raw material in blast furnaces
as well as in direct reduction plants producing sponge iron characterised by 84 % Fetotal, 82 % metallic Fe and metallization degrees of more than 97 %.
Aims and scope
However, modern electric arc furnaces are equipped with
Until the last decade, the scale, slag, dust and sludge gen- oxygen lancing systems for melting and oxidation processes
erated by integrated steel plants was called waste, but now resulting in significant electric energy savings. Therefore,
this term has been replaced with by-product and sometimes huge amounts of mill scale are accumulated. Dumping of
product due to intensive re-utilization of these materials. The these mill scales in landfills would lead to continuously inmanagement of all these substances generated in steel plants creasing demand for more landfills and to the leaching of
has become an important issue due to ever-tightening envi- some percentages of heavy metals into soil and ground waronmental regulations.
ter, which would threaten environment.
Mill scale is one of these materials produced in the proThe normal practice of getting rid of mill scale is to sell it
cessing of steel during continuous casting, reheating and hot to cement plants. However, this portion of mill scale that
rolling operations. The scale formed during these operations used by Portland cement plants as a raw material in the manis removed by water sprays and then mill scale is accumulat- ufacturing of clinker is still rather small compared to the proed as a by-product in all
duced amount of mill
iron and steel companies,
scale.
either integrated iron and
Unfortunately, no techsteel companies or mini
nology has been implesteel mills and small mill
mented, en mass, to reshops. At EFS, fig. 1, for
cover and use such mateexample, sedimentation
rials. Furthermore, the
basins are available for
depletion of iron ores and
the collection of this
shortage of iron and steel
valuable by-product, fig.
scrap necessitate exten2. The specific producsive research work to
tion of mill scale is conreuse the secondary raw
siderably high representmaterials produced as
ing about 19 - 40 kg/t of
by-products in steel comhot rolled products
panies. On the other
[1...3], depending on the
hand, mill scale is condeformation technology
sidered the highest qualiused. The global producty iron oxide and a rich
tion of steel during 2008
iron source (> 70 % Fe)
Fig. 1: View of the Ezz Flat Steel (EFS) plant in Egypt
was 1125 million t and
with minimum impurithe corresponding mill
ties.
scale produced is estimated to be 33 million t on average. In
In a recent study [1], mill scale was used to prepare some
the past years, steelmakers used this mill scale as oxidizer in iron oxide pigments via specific precursors. It was shown
the conventional electric arc furnace steelmaking process. that it is possible to prepare magnetite (black), hematite
88
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(red), goethite (yellow) and maghemite (brown) pigments of Experimental
acceptable purity and with good morphological properties
(i.e. particle size, shape, color and surface area) from mill
Raw materials. The raw material used in this study was a
scales.
mixture of mill scale and anthracite coal milled to a size of
From the technical point of view, mill scales produced in less than 250 m as shown in table 1. Sieve analysis of mill
iron and steel companies can be
scale and coal used
reduced by carbonaceous matewas carried out by
rials. Reduction of iron oxides
using a vibratory
by carbonaceous materials is
sieve shaker. The
not only of industrial significhemical analysis
cance for the recycling of such
of mill scale was
huge amounts of substances,
carried out by usbut it is also of considerable
ing a wavelength
theoretical interest.
dispersive X-ray
The effect of process varifluorescence specables on the reduction of mill
trometer with an
scale by coal was studied at
Rh source and tube
four different temperatures
of 2.2 kW power.
from 1173 to 1323 K, particle
The XRF specsizes from 0.51 to 2.03 mm,
trometer was calicoal/mill scale ratios from 0.5
brated with certito 1.0 [4]. Within the range of
fied reference mavariables studied; an increase in Fig. 2: Mill scale collection from sedimentation basin
terial. The sample
reaction temperature and
was grinded by
coal/mill scale ratio results in
using a ball mill,
a high reduction rate, where- Table 1: Sieve analysis of the used mill scale and anthracite coal
dried at 110 °C
as an increase in the average
and fired at 1050
particle size decreases reduc°C to determine
tion rate.
the percentage
The reduction behaviour of
loss in ignition.
cold-bonded composite pelThe test results
lets, produced from the solid Table 2: Chemical analysis of the used mill scale
are shown in table
wastes of an integrated iron
2. The phase analyand steel company was investisis was carried out
gated in [5]. A rotary furnace
by using an X-ray
was utilized for this purpose
diffraction specand the effects of different reducing agents, the ratio of trometer of 2.2 kW max power. The phase analysis of used
Cfix/Fetotal, temperature and time on the reduction process mill scale is given in table 3. Fine coal was dried at 110 °C,
were studied. With increasing temperathen the volatile matter, ash content
ture, the degree of reduction was in- Table 3: Phase analysis of mill scale
and fixed carbon were determined
creased and, in comparison, equal degravimetrically, while sulfur was
grees of reduction were obtained with
determined instrumentally by a carshort reduction times.
bon and sulfur determinator. The
Gudenau et al. [6] studied the kinetic
instrument is equipped with a reand morphological assessment of
sistance furnace and infrared
self-reducing agglomerates precells. The chemical analysis
pared as pillow shape briquettes (45 Table 4: Chemical analysis of used coal
of the used coal is shown in
· 35 · 25 mm3) containing a mixture
table 4.
of hematite iron ore fines, coal
fines, fluxes and a binder agent.
Experimental work exeThe analysis of all gained data
cution program. Mill scale
showed, that the higher the temperand coal were dried to reature, the higher the metallization degree.
move the moisture, milled to sizes less than 250 m by using
The accumulation of huge amounts of the produced mill a ball mill. The fine mill scale was mixed with fine anthracite
scale in all iron and steel companies necessitates extensive coal, wetted by using about 10 % water, pelletized by using
studies on the recycling of such arisings. Therefore, in this manual pelletizer. The green pellets were dried at 140 - 150
study, the reduction of composite pellets produced from mill °C to remove the water. The dried pellets were placed in a
scale using anthracite coal was investigated to reveal the op- stainless steel crucible (of 50 mm height and 35 mm inner
timum reduction conditions through studying the most ef- diameter) and inserted into a laboratory muffle furnace for
fective parameters such the amount of reducing agent, re- reduction. The test was done in the temperature range from
duction temperature and reduction time.
800 to 1100 °C, reduction time varied between 10 and 80
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minutes and coal
amount ranged from
5 to 40 %. After having been discharged
from the muffle furnace, the crucible
with the raw material
was cooled very fast
to stop the reaction;
the temperature of
the material after reduction was measured and found to
have fallen below
400 °C within 20
seconds.
Process technology
Table 5: Test results of reduced mill scale with different percent of reducing agent for 60 min reduction time at 1100 °C
Table 6: Test results of reduced mill scale at different reduction temperatures at 30 % coal and 60 min reduction time
Evaluation of
the results
A lot of parameters
affect the reduction
of mill scale, such as,
the type of reducing
agent, the grain size
of mill scale and reducing agent, the
mixing, the properties of composite
pellets, the quantity
of the reducing
agent, temperature
and reduction time.
During this investigation, three parameters were studied: the
quantity of the reducing agent, temperature and reduction
time which have a
great effect on reduction rate.
Effect of reducing
agent. The amount of
reducing agent was
increased from 5 %
up to 40 %, starting at
low concentrations of
the reducing agent,
and increased up to
more than the stoichiometric ratio of
carbon to oxygen, at
constant temperature,
1100 °C, and constant
reduction time of 60
minutes. The percentage of metallic Fe
was measured as well
as total Fe, carbon,
90
Fig. 3: Behaviour of metallic iron produced against added coal at 1100
°C and 60 min reduction time
Fig. 4: Behaviour of carbon and oxygen in metallic iron produced
against added coal at 1100 °C and 60 min reduction time
Fig. 5: Behaviour of nitrogen in metallic iron produced against added
coal at 1100 °C and 60 min reduction time
oxygen, nitrogen and sulfur, as shown in table 5.
The metallic iron obtained was 8.33 % when 5
% coal had been used and
increased up to 81.89 %
for the use of 30 % coal.
The optimum amount of
reducing agent was found
to be 30 % coal (26 % C).
Increasing the reducing
agent of more than 30 %
results in a decrease in the
amount of metallic iron
produced, as shown in
fig. 3. The dilution effect
of excess carbon, i.e., carbon which has not been
consumed in reduction,
especially due to excess
ash content, is expected
to reduce the percentage
of metallic iron. Also, excess coal is expected to
hinder the reaction between iron oxide and carbon. The highest reduction corresponds to
Cfix/O2 = 1.04. The lower
the coal percentage used,
the higher the oxygen
content and the lower the
carbon content in the final product and vise versa, as shown in fig. 4. The
nitrogen content in reduced iron produced increased with increasing
coal content. It was 181
ppm when 10 % coal was
added and increased to
518 ppm for 40 % coal, as
shown in fig. 5. The excess coal has great effect
on nitrogen concentration
in produced iron due to
the high nitrogen content
in the coal employed. The
sulfur content was found
to increase with increasing coal percentage according to the stoichiometric calculation based
on the sulfur content in
coal.
Effect of temperature.
The effect of temperature
on the reduction rate was
studied for different temperatures from 800 to
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which, in turn, produces
1100 °C. The effect
CO2 through oxide reducof temperature was
tion. The reduction and
measured at constant
gasification reactions are
coal charge of 30 %
thus necessarily coupled.
(26 % C) and conTemperature should be
stant reduction time
higher than 1000 °C in orof 60 minutes. The
der to reach a high degree
highest percentage
of reduction. Since the carof metallic iron was
bon gasification reaction
obtained at 1100 °C
Fig.
6:
Behaviour
of
metallic
iron
produced
against
temperature
at
30
%
(Boudouard
reaction) is
as shown in table 6
coal
and
60
min
reduction
time
highly
endothermic,
a
and fig. 6. A higher
much
larger
amount
of
entemperature
was
ergy is required. Thus, the
studied, 1200 °C, but
rate of reduction is low at
due to the formation
lower temperatures and the
of very fine drops of
process does not reach
molten steel within
completion.
the reduced iron, the
products were not
analyzed accurately.
Effect of reduction
The carbon and oxytime. The effect of reducgen contents in the
tion time was studied from
reduced scale were Fig. 7: Behaviour of carbon and oxygen in metallic iron produced 10 minutes up to 80 minvery high at low against temperature at 30 % coal and 60 min reduction time
utes at constant temperatemperatures, and
ture 1100 °C and constant
both decreased with
coal amount of 30 % (26
increasing reduction
%C). The obtained test retemperature
as
sults are shown in table 7.
shown in fig. 7.
It was found that 50 to 60
It was also found
minutes is the optimum rethat the reduction
duction time as shown in
temperature has a
fig. 9. Carbon and oxygen
great effect on the niwere very high at low retrogen content in the
duction time meaning less
reduced iron proreduction, whereas their
duced. Nitrogen was Fig. 8: Behaviour of nitrogen in metallic iron produced against temper- amounts reduced to mini1129 ppm at 800 °C ature at 30 % coal and 60 min reduction time
Table 7: Test results of reand dropped to 318
duced
mill
scale
at
different
reduction
time
at
30 % coal and 1100 °C
ppm at 1100 °C, as shown in fig. 8.
Iron oxides are either reduced by carbon (direct
reduction) or by carbon monoxide (indirect reduction), formed by the gasification of carbon. The indirect reactions include:
3Fe2O3 + CO = 2Fe3O4 + CO2,
(1)
Fe3O4 + CO = 3FeO + CO2,
(2)
FeO + CO = Fe+CO2,
(3)
C + CO2 = 2CO.
(4)
The direct reactions include:
3Fe2O3 + C = 2Fe3O4 + CO,
(5)
Fe3O4 + C = 3FeO + CO,
(6)
FeO + C = Fe + CO.
(7)
The overall reaction involves a cyclic mechanism
in which CO2 is produced as a result of the reduction of iron oxides gasifying carbon to generate CO
STEEL GRIPS 8 (2010)
Fig. 9: Behaviour of metallic iron produced against reduction time at 30
% coal and 1100 °C reduction temperature
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mum as the time increased to 60 minutes, as shown in fig.
10. Nitrogen contents were reduced as the time of reduction
increased. Nitrogen has been observed to fall from 734 ppm
at 10 minutes to 257 ppm at 60 minutes, as shown in fig. 11.
Acknowledgement
The authors are so grateful to EZZ Flat Steel Company for
supporting this study.
Discussion of the results
The obtained results matched the results obtained by Camci et al. [5] as far as the possibility of
using mill scale in blast furnaces and direct reduction furnaces is concerned, as well as with respect
to the carbon required for reduction and the importance of reduction time. Also, the obtained results
agree with the results obtained by Gudenau et al.
[6], as the higher the temperature the higher the
metallization degree.
Comparison between the reduced mill scale obtained in this study and commercially produced
DRI is given in table 8. The metallic iron in commercial DRI is about 85 % and in reduced mill scale
is about 82 %. In DRI, metallization is 90 %, and in
reduced mill scale it is 97.6 %. In DRI, carbon is
1.6 % while reduced mill scale contains 6.6 %.
Fig. 10: Behaviour of carbon and oxygen in metallic iron produced
against reduction time at 30 % coal and 1100 °C reduction temperature
Conclusions
Based on this study, the following conclusions
can be drawn:
1. One of the methods for the beneficiation of mill
scale is its use for the production of composite pel- Fig. 11: Behaviour of nitrogen in metallic iron produced against reduclets which can be charged into blast furnaces and tion time at 30 % coal and 1100 °C reduction temperature
direct reduction plants producing sponge iron;
2. the usage of coal having high carbon contents
References
and low ash contents is recommended to reduce the percent
[1] M. A. Legodi and D. de Waal: Sience direct 74 (2007), p. 161/68.
of gangue material in the product. Coal containing 96 % [2] L.F. Rostik: EPA Region III Wast Minimization/Pollution Prevention
minimum fixed carbon is expected to give a better product;
Technic. Conf., February 4 - 7, 1996, Philadelphia, PA, USA, p. 465/73.
3. the high nitrogen content is not expected to create prob- [3] International Iron and Steel Institute, The management of steel industry
by-products and waste, Brussels Committee on Environmental affairs,
lems during melting. The gas content is expected to reach
1987.
few ppm at melting temperature. The effect of temperatures
[4] M. Rahman, R. Haque and M.M. Haque: Ironmak. Steelmak. 22
higher than 1100 °C on gases needs to be investigated;
(1995), No. 2, p.166/70.
4. to get the best results economically and environmentally, [5] L. Camci, S. Aydin and C. Arslan: Turk. Journ. Eng. Env. Sci. 26
it is recommended to work at the best parameters. In this
(2002), p. 37/44.
study, a temperature of 1100 °C, 26 % carbon and 50 – 60 [6] H.W. Gudenau, D. Senk, S. Wang, K. Martins and C. Steohany: ISIJ
Intern. 45 (2005) No. 4, p. 603/08.
minutes reduction time have been evaluated as such.
Raaft Farahat
Prof. Mamdouh Eissa
Assoc. Prof. Gamal Megahed
Prof. Amin Baraka
Chemist
Professor and Head of Steel
Technology Dept.
Plant Manager
Professor of
Physical Chemistry
Al-Ezz Flat Steel Company
(EFS)
Central Metallurgical R&D
Institute (CMRDI)
Al-Ezz Flat Steel
Company (EFS)
Faculty of Science
Cairo University
Egypt
Helwan, Egypt
Egypt
Cairo, Egypt
92
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