DIRECT REDUCTION OF FERROUS OXIDES TO FORM AN IRON ...

International Iron & Steel Symposium, 02-04 April 2012, Karabük, Türkiye
DIRECT REDUCTION OF FERROUS OXIDES T O FORM A N IRON-
RICH A LTERNATIVE CHARGE MATERIA L A ND ITS
CHA RA CTERIZATION
Abstract
H. İbrahim Ünal a, * , Enes Turgut b , Ş. Hakan Atapek c , Attila Alkan d
a, * Kocaeli University, Kocaeli, Turkey, E-mail: iunal@kocaeli.edu.tr
b Kocaeli University, Kocaeli, Turkey, E-mail: turgut.enes@gmail.com
c Kocaeli University, Kocaeli, Turkey, E-mail: hatapek@kocaeli.edu.tr
d Kocaeli University, Kocaeli, Turkey, E-mail: aalkanstem@yahoo.com
In this study, production of sponge iron by direct reduction of oxides and the effect of reductant on metalization
were investigated. In the first stage of the study, scale formed during hot rolling of slabs was reduced in a rotating
furnace using solid and gas reductants. Coal was used as solid reductant and hydrogen released from the
combustion reaction of LNG was used as the gas one. The sponge iron produced by direct reduction was melted
and solidified. In the second stage, Hematite ore formed as pellets were reduced using solid carbon in a furnace
heated up to 1100 ºC for 60 and 120 minutes. Reduction degree of process was evaluated as a function of time
and the ratio of Cfix / Fetotal. In the third stage, final products were examined using light and scanning electron
microscopes and microanalysis was carried out by energy dispersive x-ray spectrometer attached to the electron
microscope. It is concluded that (i) direct reduction using both solid and gas reductants caused higher
metallization compared to using only solid reductant, (ii) as the reduction time and ratio of Cfix / Fetotal increased
%-reduction of ore increased.
Key words : Reduction, mill scale, metalization, characterization.
1. Introduction
One of the wastes generated in steel plants is the mill scale which represents about 2% of the steel produced. It is
formed during the continuous casting and rolling mill processes when steel is submitted to thermal gradients in
oxidant atmospheres, which promotes the growth of iron oxides layer at the surface of steel. Generally, the
steelmaking by-products such as dust and mill scale are recycled by the metallurgical processes such as the blast
furnace and/or the direct reduction reactors that use coal as reducing agent to produce pre-reduced pellets
intended for the remelt in electric steel plant. In the integrated plants, the mill scale is habitually used as a raw
material at the sintering plants. This waste is recycled as briquettes used in BOF steelmaking as well as an
addition to the iron ore pellets designated for blast furnace process. Recycling part of these by-products is already
supported by the powder metallurgy where the economic recovery is more favorable. It is possible to use the
scale for pure iron powder production. The reduced iron powder is the most widely used material in powder
metallurgy industry. The direct reduction process has commonly been used by many companies to obtain metallic
iron powder by the reaction of iron oxide (magnetite, hematite ore or mill scale) and reducing gases (CO/H2) at
high temperatures (> 1000°C) [1-3].
Sponge iron is the metallic form of iron produced from reduction of iron oxide below the fusion temperature of iron
ore by utilizing hydrocarbon gases or carbonaceous fuels as coal. The reduced product having high degree of
metallisation exhibits a honeycomb structure due to which it is named as sponge iron. As the iron ore is in direct
contact with the reducing agent throughout the reduction process, it is often termed as direct reduced iron (DRI).
Sponge iron is produced primarily both by using non-coking coal and natural gas as reductant and therefore
classified as coal based and gas based process, respectively. High purity sponge iron powder is currently
manufactured from high purity iron ore following the basic steps of magnetic separation and milling, primary
reduction process, hydrogen annealing, and final milling and classification. Due to a promising availability of coal,
the coal based sponge iron plants share the major amount of its production [4-8]. In this study, direct reduction of
mill scale in a in a rotating furnace using gas and solid reductants was carried out and then final product was
analysized to determine the degree of metallization. On the other hand, the effects of reduction time and ratio of
Cfix / Fetotal on %-reduction was discussed for hematite ore formed as pellets.
2. Experimental study
2.1. Materials
The charge materials, used in production of sponge iron in a rotating furnace, are mill scale, coal and slag
formers. Mill scale is a steelmaking by-product from the rolling mill in the steel hot rolling process. Mill scale
contains both iron in elemental form and three types of iron oxides: wustite (FeO), hematite (α-Fe2O3) and
421

422
Ünal, H. İ., Turgut, E., Atapek, Ş. H.and Alkan, A.
magnetite (Fe3O4). The iron content is normally around 70 %, with traces of non-ferrous metals and alkaline
compounds. Mill scale is formed by flaky particles of a size of generally less than 5.0 mm. The size distribution
depends on the stage in the process where the mill scale is generated [9]. In this study, the particle size of the mill
scale is between 400-500 μm. Table 1 shows the contents of raw materials for 1 ton. The slag formers having a
particle size of ~ 200 μm include bentonite, dolomite, limestone and are added to charge as 20 kg. Table 2 shows
the contents (%) of slag formers used in the study. The content of coal used in mill scale processing is 67.25 %
and its particle size is ~ 150 μm.
Table 1. The contents of raw materials (%).
Raw materials %
Mill scale 76.39
Coal 22.06
Slag formers 1.550
Table 2. The contents of slag formers used in the study (%).
Slag formers %
Bentonite 10
Dolomite 40
Limestone 50
Hematite ores in pellet form was supplied from ERDEMİR A.Ş. and its reduction by solid carbon having a purity of
99 % was investigated as a function of of reduction time and the ratio of Cfix / Fetotal. Cfix / Fetotal ratio can be
described as the amount of carbon necessary to reduce completely the iron oxide that exists in the system [10].
Table 3 shows x-ray fluorescence (XRF) analysis of both mill scale and ore materials.
Table 3. XRF anaylsis of mill scale and ore material used in the experimental study.
Materials Fe Mn Si Al Ca Cr K O
Mill scale 69.10 0.66 0.11 - 0.21 0.10 - 29.82
Hematite
ore
62.40 - 1.20 0.30 0.50 - 0.10 35.50
2.2. Reduction procedure
Industrially iron is produced from iron ores, principally hematite (Fe2O3), magnetite (Fe3O4) by a carbothermic
reaction that is reduction with carbon, in a blast furnace at temperatures about 800-1600 °C. In the blast furnace,
iron ore, carbon in the form of coke, and a flux such as limestone are fed into the top of the furnace, while blast of
heated air is forced into the furnace at the bottom. Reduction of iron oxides occurs either by carbon or by carbon
monoxide, formed by the gasification of carbon. The reduction process carried out by the carbon is called as
direct reduction and the reaction can be defined in equation 1.
FexOy + yC = xFe + yCO (equation 1)
On the other hand, the reduction process conducted with CO is called as indirect reduction and its reaction is
given in equation 2 and 3. The reduction of ferrous materials can be indirectly achieved by hydrogen and
equations 4-6 indicate the stages of reactions. The reduction of iron ores by hydrogen is a gas-solid reaction
which occurs in two or three stages. For temperatures higher than 570°C, hematite (Fe2O3) is first transformed
into magnetite (Fe3O4), then into wustite (Fe1-yO), and finally into metallic iron whereas at temperatures below
570°C, magnetite is directly transformed into iron since wustite is not thermodynamically stable.
FexOy + yCO = xFe + yCO2 (equation 2)
yCO2 + yC = 2yCO (Boudouard Reaction) (equation 3)
3Fe2O3 + H2 → 2Fe3O4 + H2O (equation 4)
Fe3O4 + H2 → 3FeO + H2O (equation 5)
FeO + H2 → Fe + H2O (equation 6)

423
Ünal, H. İ., Turgut, E., Atapek, Ş. H.and Alkan, A.
The iron also can be produced from its ore by the direct reduction of iron ore by a reducing agent which is coal
based or may be a gaseous reducing agent, which is called direct reduced iron (DRI) or sponge iron. Direct
reduced iron (DRI) is a good substitute of scrap for making steel in electric arc furnace, basic oxygen furnace etc.
and there has been a rapid worldwide growth in its production. DRI is a solid state product of direct reduction
processes which is produced either in the form of lump or pellet. Availability of huge amounts of non-coking coal,
scarcity of coking coal deposits and industrial significance of DRI led to many efforts for the development of many
direct reduction processes [6, 11].
In this study, two different techniques were applied for the reduction of ferrous oxide materials. In the first stage,
the reduction of mill scale was carried out in a rotating furnace using solid and gas reductants to produce sponge
iron. The coal is used as solid reductant and its features have been given in Section 2.1. The source of hydrogen
used as gas reductant is liquefied natural gas (LNG). The mixture of gas formed after the decomposition by
combustion of LNG has 95 % H2. The flow of reduction in the rotating furnace is summarized in Table 4. The
reduction in the rotating furnace were carried out in two media : (i) solid coal and (ii) solid coal and hydrogen gas.
After reduction, produced sponge irons are smelted and the decomposition of metal-slag was done. In the second
stage, the reduction of hematite ore formed as pellets was carried out using solid carbon in a furnace. The
furnace is heated up to 1100 ºC step by step with 10 °C/min. The pellets having different Cfix / Fetotal were
reduced at 1100 ºC for 60 and 120 minutes. Table 5 shows the reduction conditions for ore as pellet form. In order
to reduce all iron oxides, carbon ratio was selected as 1.5 and 2 times of the theoretical amount. The amount of
binder in pellet was neglected.
Table 4. The flow of reduction in the rotating furnace used.
Process
Charging of materials (mill scale + coal + slag formers) to furnace
1 st reduction (entrance to furnace) : ~ 850 – 900 °C
2 nd reduction (middle of furnace) : ~ 950 – 1000 °C
3 rd reduction (close to nozzle) : ~ 1200 °C
Table 5. The reduction conditions for pellet material.
Carbon ratio Cfix / Fetotal Temperature, °C Time, min.
2.3. Calculation of ore reduction
1.5 0.48
60
1100
2 0.64 120
The stoichiometric amount of carbon was determined using the reaction given in equation 7. All reduced products
were cooled from selected reaction temperature to room temperature in the furnace and scaled to determine %reduction
(R). This value was calculated using equation 8.
Fe2O3 + 3C = 2Fe + 3CO (equation 7)
% R = (removed oxygen/total oxygen) x 100 (equation 8)
2.4. Microscopic examinations
Solidified sponge iron and its slag were prepared by metallographic methods. Grinding was carried out with 320,
600 and 1000 mesh size SiC abrasives, respectively and then ground surfaces were polished with 3 μm diamond
solution. Etching was done with nital (% 3 HNO3) to characterize the microstructure. Zeiss Axiotech 100 light
microscope and Jeol JSM 6060 and JSM 840A scanning electron microscopes (SEM) were used for
metallographic examinations. Energy dispersive x-ray spectrometer (EDS) was used for elemental analysis of the
phases observed in the electron microscope.

3. Results and Discussions
3.1. Microstructural characterization of reduced mill scale
424
Ünal, H. İ., Turgut, E., Atapek, Ş. H.and Alkan, A.
In iron-steel making industry, it is desired that sponge iron as an alternative charge material must have high
degree of iron. The effect of reductant in the reduction conditions is very important in obtaining the final product.
Figure 1a and b show SEM micrographs of solidified sponge iron reduced by only solid carbon (coal). The solid
metal form includes coal particle in dark contrast within the matrix and it indicates that the reduction of ferrous
oxides by coal is unsufficient. An EDS analysis is given in Figure 1c and it shows the degree of metallization in
the selected image area. The amount of carbon is very high and the amount of iron is lower than that of traditional
sponge iron (55-65 Fe%). Iron ore can also be reduced by hydrogen. Wagner et al. reported that (i) most of the
reaction features are very similar to that of the reduction by carbon monoxide and many mechanisms are
common to both of them, (i) the reduction with hydrogen is endothermic, whereas it is exothermic with carbon
monoxide and conversely, thermodynamics are more favorable with hydrogen than with carbon monoxide above
800°C, (iii) with hydrogen, the hot gas fed has to bring enough calories to heat and maintain the solid at a
temperature sufficiently high for the reaction to occur, (iv) kinetics are also faster with hydrogen [11]. Figure 2a
and b show SEM micrographs of sponge iron reduced by solid + gas reductants. Hydrogen accompanies coal for
the reduction mechanism and causes a high degree of metallization. EDS analysis given in Figure 2c indicates
that the content of iron is higher and the amount of carbon is lower than the sponge iron reduced by coal only.
3.2. Effect of reduction time and ratio of Cfix / Fetotal
Reaction kinetics in iron ore reduction deal with the rate at which iron oxides are converted to metallic iron by the
removal of oxygen. The rate of a chemical reaction increase with increase in temperature. For this reason the
reaction kinetics are not generally a matter of great importance in the blast furnace because of the high
temperatures at which the furnace is operated. On the other hand, in DR processes where the iron is reduced in
the solid state, the maximum temperature is below the melting temperature and the reaction rates are slower. For
direct reduction of iron ore, the mechanisms are complex because the oxide must go through a series of step
wise changes before the conversion is complete. The slowest step in the process determines the overall reaction
rate and is referred to as the rate controlling step [12]. Aguilar et al. reported the reduction process for a given
spherical pellet material and they illustrated a schema explaining the reduction from outside to core. The
reduction of hematite to metallic iron is carried out by the following reactions (equation 9, 10 and 11) [13].
3Fe2O3 + CO = 2Fe3O4 + CO2 (equation 9)
Fe3O4 + CO = 3FeO + CO2 (equation 10)
FeO + CO = Fe + CO2 (equation 11)
Figure 3 shows the the effects of reduction time and ratio of Cfix / Fetotal on %-reduction. As it is seen clearly, as
reduction time increases %-reduction increases. The reduction of the iron oxides takes place in a series of
sequential steps. The overall rate will be determined by the slowest process in the series. Baliarsingh et al.
summarized the possible consecutive steps which are (i) transport of gaseous reductant from the bulk gas phase
to the particle surface through a boundary gas film, (ii) molecular diffusion of the gaseous reductant through the
product layer to the reaction interface, (iii) adsorption of the gaseous reductant at the interface, (iv) reaction at the
interface, (v) desorption of the gaseous products from the interface, (vi) mass transport of iron and oxygen ions
and transformations in the solid phase, formation and growth of reaction products e.g magnetite, wustite, iron, (vii)
molecular diffusion of gaseous products through the product layer to the particle surface, (viii) transport of the
gaseous products from the particle surface through the boundary gas film to the bulk gas phase [12]. All these
mechanisms require adequate reaction time and this explains why the metallization, in other words, %-reduction
increases as a function of time. On the other hand, carbon and its solid/gas compounds are the driving force for
all mechanisms. An increase in the ratio of Cfix / Fetotal directly affects the metallization in the positive direction and
as a result a linear relationship between the ratio of Cfix / Fetotal and %-reduction is obtained. Figure 3 shows that
as the ratio of Cfix / Fetotal increases %-reduction directly increases.

(a)
(c)
425
Ünal, H. İ., Turgut, E., Atapek, Ş. H.and Alkan, A.
Figure 1. (a) and (b) SEM micrographs of solidified sponge iron after reduction by solid reductant (coal), (c)
EDS analysis of matrix illustrated in (b).
(a)
(c)
Figure 2. (a) and (b) SEM micrographs of solidified sponge iron after reduction by solid + gas reductants
(coal+H2), (c) EDS analysis of matrix illustrated in (b).
(b)
(b)

4. Conclusions
426
Ünal, H. İ., Turgut, E., Atapek, Ş. H.and Alkan, A.
Figure 3. %-reduction as a function of the reduction time and the ratio of Cfix / Fetotal.
In this study, reduction of ferrous oxide materials like mill scale and hematite as pellet was studied. Two distinct
tenhniques were used for the reduction. In the first stage of the study, a mixture consisting of mill scale, coal and
slag formers was tried to be reduced in a rotating furnace. The reduction was carried out using solid reductant as
coal and also hydrogen which was obtained from the combustion LNG. After reducing, the metallization was
clarified by phase analysis. In the second stage, hematite ore as pellet was reduced in a furnace. Its %-reduction
was evaluated as a function of the reduction time and the ratio of Cfix / Fetotal. The results can be given as follows;
(i) Reduction of mill scale could be achieved using solid plus gas reductants. EDS analysis displayed that higher
metallization (higher iron and lower oxygen contents) was obtained in the system having adequate coal plus
hydrogen concentration.
(ii) Reduction of hematite ore as pellet depended on the reduction time and ratio of Cfix / Fetotal. As the reduction
time in addition to the ratio of Cfix / Fetotal increased, %-reduction indicating the metallization increased.
5. References
[1] Bagatini M. C., Zymla V., Osorio E and Vilela A. C. F., Characterization and reduction behavior of mill scale,
ISIJ International, 51(7), 1072-1079, 2011.
[2] Benchiheub O., Mechachti S., Serrai S. and Khalifa M. G., Elaboration of iron powfer from mill scale, Journal
of Materials Environmental Science, 1(4), 267-276, 2010.
[3] Mazurov E. F., Gnuchev S. M., Skripchuk S., Markin A. A. and Lyalin E. S., Sponge iron as a charge material,
Metallurgist, 8(11), 602-604, 1964.
[4] Selan M., Lehrhofer J., Friedrich K., Kordesch K. and Simader G., Sponge iron : economic, ecological,
technical and process-specific aspects, Journal of Power Sources, 61(1-2), 247-253, 1996.
[5] Leshchenko I. P., Tereshchenko V. T., Martynov O. V., Trakhimovich V. I. and Borzenkov D. V., The use of
sponge iron in steel smelting, Metallurgist, 17(7), 491-494, 1973.
[6] Turgut E., Doğrudan redüksiyon ile sünger demir üretimi, Yüksek Lisans Tezi, Kocaeli Üniversitesi, Fen
Bilimleri Enstitüsü, Metalurji ve Malzeme Mühendisliği Bölümü Ana Bilim Dalı, Kocaeli, 2010.
[7] German R. M., Powder metallurgy science, II. Edition., MPIF, Princeton, New Jersey, 1994.
[8] Prasad, A. K., Prasad R. K. And Khanam S., An investigation for generation of energy conservation
measures fro sponge iron plants using process integration principles, IJRRAS, 6(1), 77-88, 2011.
[9] Martín M. I., López F. A., Rabanal M. E. and Torralba J. M., Obtainment of sponge iron by reduction of a
steelmaking by-product, I. Spanish National Conference on Advances in Materials Recycling and Eco-
Energy, Proceedings, 107-110, Madrid-Spain, 12-13 November 2009.
[10] Çamcı L., Aydın S. and Arslan C., Recution of Iron oxides in solid wastes generated by steelworks, Turkish
Journal of Engineering & Enviromental Sciences, 26, 37-44, 2002.
[11] Wagner D., Devisme O., Patisson F. and Ablitzer D., A laboratory study of the reduction of iron oxides by
hydrogen, Sohn International Symposium, Proceedings, Vol. 2, 111-120, San Diego-USA, 27-31 August,
2006.
[12]Baliarsingh S. K and Mishra B., Kinetics of iron ore reduction by coal and charcoal, Thesis, National Institute
of Technology, Department of Metallurgical and Materials Engineering, India, 2008.
[13]Aguilar J. A. and Gomez I., Microwaves applied to carbothermic reduction of iron ore pellets, International
Microwave Power Institute, 32(2), 67-73, 1997.