literature review on iron production technologies and corex process mathematical models
DESCRIPTION
In this document, the properties of different iron ores are discussed. The features, raw materials, modifications, and disadvantages of blast furnace are covered. Commercially applied direct reduction processes for iron production are explained. The industrially proven smelting reduction processes are described. Detailed description of the technical and commercial issues of COREX process is presented. The previous efforts in COREX macroscopic and microscopic analysis are summarized.TRANSCRIPT
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Literature Review on Iron Production Technologies and
COREX Process Mathematical Models
by
Ahmed Wafiq Abdel Mohsen Abolnasr M.Sc. of Chemical Engineering 2011 Cairo University
The Literature Review Chapter of the Thesis
(MICROSCOPIC MODELING OF THE FREE BOARD AND
FLUIDIZED BED INSIDE THE MELTER-GASIFIER OF
OPTIMIZED COREX IRONMAKING PROCESS)
in Partial Fulfillment of the Requirements for the Degree of
MASTER OF SCIENCE in
CHEMICAL ENGINEERING
Under the Supervision of
Prof. Dr. Tarek Mohamed Moustafa Dr. Ahmed Soliman Fawzy
Dr. Ahmed Fayez Nassar
Department of Chemical Engineering
Faculty of Engineering-Cairo University
FACULTY OF ENGINEERING, CAIRO UNIVERSITY
GIZA, EGYPT 2011
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LIST OF CONTENTS
IRON PRODUCTION LITERATURE REVIEW ......................................................... 4
2.1 Introduction ........................................................................................................................... 4
2.2 Principal Iron Bearing Materials ............................................................................................ 4
2.2.1 Different Iron Ores ......................................................................................................... 5
2.2.2 Impurities in Iron Ores .................................................................................................... 6
2.2.3 Classification of Iron Ores ............................................................................................... 6
2.2.4 Pelletisation .................................................................................................................... 7
2.2.5 Sintering .......................................................................................................................... 8
2.3 Blast Furnace ......................................................................................................................... 9
2.3.1 Blast Furnace Process Description.................................................................................. 9
2.3.2 Blast Furnace Gas Cycle ................................................................................................ 12
2.3.3 Production and Processing of Coke .............................................................................. 13
2.3.4 Required Coke Properties ............................................................................................. 14
2.3.5 Required Flux Properties .............................................................................................. 14
2.3.6 Required Air Properties ................................................................................................ 15
2.3.7 Blast Furnace Reactions ................................................................................................ 15
2.3.8 The Challenge of Coal Injection .................................................................................... 18
2.3.9 Environmental Analysis for the Blast Furnace Technology .......................................... 19
2.3.9.1 Sintering Plant Pollutants ...................................................................................... 19
2.3.9.2 Coking Plant Pollutants .......................................................................................... 19
2.3.9.3 Blast Furnace Pollutants ........................................................................................ 20
2.3.9.4 Greenhouse Gas Emissions for the Whole Industry .............................................. 20
2.3.10 Drawbacks of the Blast Furnace ................................................................................. 21
2.4 General Overview on Direct Reduction ............................................................................... 21
2.4.1 Metallization ................................................................................................................. 22
2.4.2 Use of DRI in Electric Arc Furnaces (EAF) ..................................................................... 22
2.4.3 DRI Oxidation and Briquetting ...................................................................................... 23
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2.4.4 Broad Classification of DR Processes ............................................................................ 24
2.4.5 Global DRI Production .................................................................................................. 24
2.5 Gas-Based DR Processes ...................................................................................................... 25
2.5.1 MIDREX Process ............................................................................................................ 25
2.5.1.1 Main Reactions Taking Place ................................................................................. 25
2.5.1.2 Process Description ............................................................................................... 25
2.5.2 HYL III Process ............................................................................................................... 27
2.5.2.1 Process Description ............................................................................................... 27
2.5.2.2 Some Process Features .......................................................................................... 28
2.5.3 ENERGIRON Process ..................................................................................................... 28
2.5.3.1 Main Reactions Taking Place ................................................................................. 29
2.5.3.2 Process Description ............................................................................................... 29
2.5.3.3 Some Process Features .......................................................................................... 30
2.6 Coal-Based DR Processes ..................................................................................................... 31
2.6.1 General Process Description of Rotary Kiln Technologies ............................................ 31
2.6.2 Encountered Reactions during coal-based DR Processes ............................................ 33
2.6.3 Comparison between Different Rotary Kiln DR Processes in Commercial Use ............ 34
2.6.4 Advantages of Rotary Kiln Processes ............................................................................ 34
2.6.5 Disadvantages of Rotary Kiln Processes ....................................................................... 35
2.7 Smelting Reduction ............................................................................................................. 35
2.7.1 Advantages of SR with respect to Blast Furnace .......................................................... 36
2.7.2 Advantages of SR with respect to DR ........................................................................... 36
2.7.3 Use of Hot Metal in Electric Arc Furnaces (EAF) ........................................................... 37
2.7.4 General Features of SR ................................................................................................. 37
2.7.5 Reaction Encountered in SR Processes ........................................................................ 38
2.7.5.1 Reactions Encountered in Pre-reduction Stage .................................................... 38
2.7.5.2 Reactions Encountered in Smelting Stage ............................................................. 39
2.7.6 COREX Process .............................................................................................................. 39
2.7.6.1 History of COREX ................................................................................................... 39
2.7.6.2 COREX Process Description ................................................................................... 40
2.7.6.3 Commercial Production ......................................................................................... 41
2.7.7 FINEX Process ............................................................................................................... 41
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2.8 COREX Process for Pig Iron Production ............................................................................... 42
2.8.1 Detailed Process Description ........................................................................................ 42
2.8.1.1 RS Process Description .......................................................................................... 43
2.8.1.2 MG Process Description ........................................................................................ 43
2.8.2 Dimensions of the Encountered Reactors .................................................................... 44
2.8.3 COREX Export Gas ......................................................................................................... 45
2.8.4 Raw Material Requirements for COREX Process .......................................................... 47
2.8.4.1 General Requirements .......................................................................................... 47
2.8.4.3 Use of Coke in COREX Process ............................................................................... 48
2.8.5 Factors Affecting the Efficiency of COREX Process ....................................................... 48
2.8.6 Environmental Analysis for COREX Process .................................................................. 49
2.8.7 Advantages of COREX Process ...................................................................................... 51
2.8.8 Disadvantages of COREX Process ................................................................................. 52
2.8.9 Case Study Jindal ....................................................................................................... 52
2.8.9.2 Using Iron Ore Fines .............................................................................................. 53
2.8.9.3 Recycling of various by products and plant wastes ............................................... 54
2.8.9.4 Improvement in Plant Operation .......................................................................... 54
2.8.9.5 Synergetic Combination of COREX and Blast Furnace ........................................... 54
2.8.10 Case Study SALDANHA ............................................................................................. 55
2.9 COREX Macroscopic Analysis ............................................................................................... 56
2.9.1 Reduction Shaft Macroscopic Analysis ......................................................................... 56
2.9.2 Melter-Gasifier Macroscopic Analysis .......................................................................... 57
2.9.2.1 Effect of Coal Size .................................................................................................. 57
2.9.2.2 Fuel Rate ................................................................................................................ 57
2.9.2.3 Factors Affecting Coke Addition ............................................................................ 58
2.9.2.4 Effect of amount of volatile matter in coal ........................................................... 59
2.9.2.5 Effect of Moisture .................................................................................................. 60
2.9.2.6 Minimization of Energy Consumption ................................................................... 61
2.9.2.7 Effect of %MgO and %Al2O3 on the slag ................................................................ 61
2.9.2.8 Modeling of COREX process for optimization of operational parameters ............ 62
2.10 COREX Microscopic Analysis .............................................................................................. 63
2.10.1 Reduction Shaft Microscopic Analysis ........................................................................ 63
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2.10.2 Melter-Gasifier Microscopic Analysis ......................................................................... 63
References ........................................................................................................................ 65
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IRON PRODUCTION LITERATURE REVIEW
In this chapter, the properties of different iron ores are discussed. The
features, raw materials, modifications, and disadvantages of blast furnace are
covered. Commercially applied direct reduction processes for iron production
are explained. The industrially proven smelting reduction processes are
described. Detailed description of the technical and commercial issues of
COREX process is presented. The previous efforts in COREX macroscopic
and microscopic analysis are summarized.
2.1 Introduction
In order to model and optimize certain new process for ironmaking, this
should be preceded by presenting the different global techniques used in the
field. Each of the three principal categories of ironmaking; blast furnace, direct
reduction, and smelting reduction have advantages and disadvantages.
Moreover, the direct reduction and smelting reduction comprise a lot of
different processes which are competing to reach the degree of commercial
application. Big steel complexes can successfully use the blast furnace
together with one of the new techniques to utilize the advantages of both.
2.2 Principal Iron Bearing Materials
As will be shown later, the nature of iron bearing materials is very
important parameter which can cause the success or failure of certain
technology. Nearly all technologies put constraints for the nature of iron
bearing materials to be used in the production of iron.
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2.2.1 Different Iron Ores
The ores of iron occur exclusively as its oxides. [6]
Hematite is the most
plentiful iron mineral mined, followed by Magnetite, Goethite limonite,
Siderite, Ilmenite, and Pyrite. [1]
The following table compares the different iron ores in terms of chemical
composition, percentage iron, color, crystal structure, and specific gravity.
1Table 2.1 Principal iron bearing materials[7]
Parameter Hematite Magnetite Goethite
(Limonite) Siderite Ilmenite Pyrite
Chemical
Name
Ferric
Oxide
Ferrous-
Ferric
Oxide
Hydrous Iron
Oxide
Iron
Carbonate
Iron-
Titanium
Oxide
Iron
Sulfide
Chemical
Formula Fe2O3 Fe3O4 HFeO2 FeCO3 FeTiO3 FeS2
Iron, wt% 69.94 72.36 62.85 48.2 36.8 46.55
Color Steel gray
to red
Dark gray
to black
Yellow or
brown to
nearly black
White to
greenish
gray to
black
Iron-
black
Pale
brass-
yellow
Crystal
Structure Hexagonal Cubic Orthorhombic Hexagonal Hexagonal Cubic
Specific
Gravity 5.24 5.18 3.3-4.3 3.83-3.88 4.72
4.95-
5.1
Limonite is famous with its extreme reducibility; thus, it is often employed
as a mixture with less reducible ores. On the other hand, Magnetite is
characterized by special magnetic behavior which causes easy separation of
the ore impurities. [6]
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2.2.2 Impurities in Iron Ores
Beside the iron oxide contained in any ore, there are also different
percentages of impurities (called gangue) which will affect the purity of the
produced iron especially if one of the direct reduction technologies (DR) is to
be used. Thus, DR technologies have big constraints on the purity of iron ore.
The following table shows the composition (in wt%) of 2 different iron ores;
one from a Swedish mine and the other from an Egyptian mine. It is apparent
that the iron ore from both mines greatly differ from each other. It is also
apparent that the iron percentage in the Egyptian mine is low, and thus it is not
a suitable raw material for the production of DRI.
2 Table 2.2 Iron ore composition from 2 different mines
Composition Swedish Ore[8]
Egyptian Ore[9]
Fe 66.74 58
Al2O3 0.25 2
BaO 0 1.5
CaO 0.47 0.7
K2O 0.035 0.3
MgO 1.45 0.02
MnO 0.085 3.5
Na2O 0.041 0.3
P2O5 0.021 0.2
S 0.0005 0.1
SiO2 2 8
TiO2 0.195 0.02
V2O5 0.21 0
2.2.3 Classification of Iron Ores
The most famous classification of iron ores is Bessemer versus non-
Bessemer. A Bessemer ore is one in which the phosphorous is low enough to
make the pig iron only contains 0.1 % phosphorous or less. [6]
Iron ores can
also be classified according to the particle form. So, it is frequent to hear that
the iron ore is classified into lump, fine, pellets, and sinter.
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2.2.4 Pelletisation
Pelletisation is a process of agglomeration of iron ore fines. It is one of the
industry trademarks. In this process, the particles smaller than 200 m of
which about 50% with 50 m size are converted into 12-15 mm pellets with
nodular shape as shown in figure 2.1. These iron ore fines are mainly
generated at mines. However, further grinding is also needed to reach the size
stated above. For efficient pelletizing process, the iron ore should be of very
high quality (low gangue). Thus, low grade ore fines should be first grinded
and cleaned. [10]
The ground ore is mixed with the proper amount of water and binder,
normally bentonite (Al2O3.4SiO2.H2O), and then rolled into small balls 915
mm in diameter in a balling drum or disk. These green (wet) pellets are dried,
then heated to 12001375oC to bond the small particles, and are finally cooled.
As shown in figure 2.2, the heating can be done on a traveling grate, in a shaft
furnace, or by a combination of a traveling grate and a rotary kiln (gratekiln).
The traveling grate and gratekiln are the most commonly used pelletizing
processes. [1]
1Figure 2.1 Iron ore pellets
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2Figure 2.2 Pellet-Firing system: a) Shaft furnace, b) Grate furnace,
c) Travelling grate furnace [10]
2.2.5 Sintering
Sintering is a principal section in integrated steel plants. Sintering consists
of igniting a mixture of wet iron-bearing limestone and coke fines on a
traveling grate to produce a clinker-like aggregate (sinter) suitable for use in
the blast furnace. The iron-bearing fines can include iron ore fines, flue dust,
or other steel mill wastes. As shown in figure 2.3, the traveling grate is shaped
like an endless loop of conveyor belt. The bed of material on the grate is first
ignited by passing under an ignition burner that is fired with natural gas and
air; then, as the grate moves slowly toward the discharge end, air is pulled
down through the bed. As the coke fines burn in the bed, the heat generated
sinters the particles. At the discharge end of the machine, the sinter is crushed
to remove extra-large lumps, then cooled, and is then finally screened. [1]
When the coke fines are combusted on the grate, partial fusion takes place for
the charge. On cooling, the different mineral phases crystallize and bond the
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Water
Mashing Stage
Fine Scrubbers
ESP
Thickener
Sludge
WaterTreatment Immobilisation
Depot
Slag
Recycling
Fe-Components
Sinter Machine
Process Air
Cleaned Water
DischargeWater
Sludge Tank
Floating Sludgeto BF
Water
Nat.GasReheating
EmissionMonitoring
Fan
Main Fan
Quench
MashingWater
SludgeWater
structure together to form a strong sinter. [11]
The sinter bed permeability
mainly controls the performance of sintering. It was found that water addition,
particle size distribution, ore porosity are very important factors in
determining the beds permeability. [12], [13]
3Figure 2.3 Flow sheet of iron ore sintering [14 ]
2.3 Blast Furnace
The blast furnace is the dominant unit operation for iron production till now.
Continuous improvements to the blast furnace have enabled it to compete with
the fast growing new direct reduction and smelting reduction technologies.
2.3.1 Blast Furnace Process Description
As shown in Figure 2.4, a hopper at the top discharges raw materials into
the furnace by using a pressurized gas seal system known as double bell. [1]
This system prevents gases and dust from escaping into the atmosphere. As
shown in figure 2.5, the charge consists of alternating loads of coke and a
mixture of iron bearing materials (iron ore, pellets, sinter) and flux (mainly
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limestone, and dolomite). [3]
These solids form a column that descends through
the furnace with a total residence time of about eight hours. [15]
Around the
circumference of the furnace near the bottom, water cooled nozzles (called
tuyeres) inject preheated air, often enriched with oxygen, into the furnace. In
most cases, gaseous, liquid, or powdered fuel are introduced together with the
preheated air. The heated air burns the injected fuel, and coke to produce the
heat required for the process, and to provide reducing gases that removes
oxygen from the ore. [16]
The reducing gases rapidly ascend through the
column and are expelled through a pair of stacks at the top in less than 20
seconds.[3]
The reduced iron melts, and settles in the bottom. The flux
combines with the impurities of the ore and coke to produce slag which also
melts and accumulates on the top of the molten iron. The gases exiting the
furnace are cleaned, and then are utilized in different ways.
4Figure 2.4 Double bell system used in blast furnaces
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5Figure 2.5 Blast furnace
The charging to blast furnaces is continuous, while the tapping of the hot
metal and slag is performed in batches. So, the process is considered semi-
continuous. The slags tapping hole is about 1.4 meters above the hot metals
tapping hole. From the operating experiences, the slag is tapped before the hot
metal. [17]
The hot metal is poured into refractory-lined railcars which are used
for transportation to the steelmaking section.
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2.3.2 Blast Furnace Gas Cycle
The off-gases leave the top of the furnace through uptake pipes, reverse
their direction in a down-comer, and enter the dust catcher. In the latter, dust is
separated from the gases. The dust is emptied into a rail car for transport to a
sinter plant for recycle or to a landfill.
After leaving the dust catcher, the off-gas is washed in a Venturi scrubber to
get rid of the remaining solid particles, and to condense the water vapor to
achieve higher gas calorific value. [18]
The cleaned gas can be utilized in steam
generation and power generation, firing steelmaking furnaces, firing of coke
ovens in the coke-processing nearby plant, and firing the blast furnace stoves.
The blast furnace stoves (also known as cawpers) are used for preheating
the air used in the blast furnace. There are usually three or four stoves lined
with refractory materials. Mainly air passes by one of the stoves, and the
others are being heated by the combustion of blast furnace gases. Thus, the
stoves alternate between absorbing heat generated by combustion of the blast
furnace off-gas, and releasing heat to the cold blast air as it passes down
through the stove. After leaving the stoves, the hot blast enters a large
refractory-lined bustle pipe which distributes the air on the tuyeres shown in
figure 2.6. [1]
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6Figure 2.6 Tuyeres around the blast furnace
2.3.3 Production and Processing of Coke
As shown in the blast furnace process description, coke is a main raw
material. Coke is produced from coking coals in a plant nearby the blast
furnace inside the integrated steel complex.
The process of producing coke involves heating coal in absence of oxygen
to about 2000oF in a distillation-like process.
[19] Many of the organic
substances inside the coal volatilize leaving the coke as the only solid product.
The volatilized gas is then subjected to sequentially lower temperature direct
contact condensing chambers, which capture tar (a mixture of many relatively
heavy organic compounds), oils, light oils, and finally low-molecular-weight
gases. Coke is pushed into a quenching car that transports it to quenching
towers. Here, the coke is sprayed with water to lower its temperature. [20]
Because of the direct contact of water with the coke oven gases and coke,
wastewater streams from the coking plant contain high concentrations of
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ammonia, phenol, sulfides, thiocyanates, and cyanides. Moreover, airborne
emissions also include SOx, NOx, ammonia, and particulate matters. [21]
2.3.4 Required Coke Properties
Coke has three primary functions in the blast furnace. First, the coke is a
reductant. It is gasified with the hot air to produce CO rich reducing gas that
converts the iron ore feed into iron. Second, the coke is a fuel. It provides
sufficient heat to melt the iron and the slag and promote the endothermic
reactions involved in the blast furnace. Third, the coke serves as a packed bed.
It provides a self-supporting porous bed that facilitates the contact between the
descending iron ore charge, and the ascending reducing gases. It also
facilitates the drainage of molten iron and slag phases. The composition of
coke is known by conducting proximate analysis where four parameters are
measured: Fixed carbon, volatiles, ash content, and moisture. Sulfur is the
most undesirable impurity in the industry. Despite being partially removed
during coking, still coke can contain about 1% sulfur. [6]
Size control is important in order to guarantee appropriate bed permeability,
so the minimum used size is about 15 mm. [17]
Through continuous research, and as an attempt to reduce the coke
consumption in the blast furnace, it is customary now to have solid, liquid, or
gaseous fuels, e.g. coal, fuel oil, or natural gas, added to the hot air blast at the
tuyeres to replace some of the coke. [1]
2.3.5 Required Flux Properties
If iron ores are reduced without flux, the impurities of the iron ore (mainly
silica and alumina) will react with iron oxides to form double silicates of iron
which is a heavy loss of iron. By the addition of a fluxing material (e.g.:
limestone), silica and alumina will have a great tendency to react with lime
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than iron. Flux also reacts with coke ash. Moreover, flux reacts with sulfur
(mainly from coke), and thereby reduces its concentration in the hot metal.
Fluxes are usually added in the form of either limestone or dolomite. The
main flux impurities are silica, alumina, sulfur, and phosphorous. The presence
of such impurities will reduce the percentage of lime and magnesia, and this
will require additional amount of flux to get rid of them. [6]
2.3.6 Required Air Properties
Air may also be considered a raw material. Over 1.5 ton of air is required to
produce 1 ton of hot metal. [1]
If the moisture content of the air is increased, the higher air temperatures
could be used satisfactorily. The added moisture promotes the endothermic
water gas reaction (C + H2O = CO + H2), and thus the temperature in the
combustion zones isnt extremely high, and consequently the furnace runs
more smoothly.
After further research, it was possible to inject auxiliary fuels as pulverized
coal, fuel oil, and natural gas with the air through the tuyeres in order to
decrease the coke consumption. Moreover, it is now possible to use oxygen
enriched air as high as 30%. Oxygen enrichment causes high production rates.
However, a good economic analysis is needed to determine the optimum
percentage of oxygen enrichment. [22]
So, recently the air used in the tuyeres is preheated to 900 - 1300o C, and is
mainly associated with moisture, oxygen, and auxiliary fuels.
2.3.7 Blast Furnace Reactions
Inside the blast furnace, many different unit operations occur
simultaneously including heat and mass transfer, reduction by gases, reduction
by solid carbon, high temperature gas generation, and finally smelting and
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liquid drainage.[23]
The countercurrent flow of gas and solids include heat
transfer from gases to solids, and oxygen transfer from solids to gases.[24]
From an overall heat and mass balance point of view, the blast furnace can
be divided into 4 zones:
a) Combustion zone
As the coke descends through the furnace, it is heated by the ascending
gases. When it reaches the raceway in front of the tuyeres, it reacts
immediately with the oxygen in the hot blast air according to equation 1. The
latter is actually the combination of coke combustion (equation 2) and coke
gasification (equation 3, also referred to as solution loss).
C + 0.5 O2 CO (1)
C + O2 CO2 (2)
C + CO2 2 CO (3)
C + H2O CO + H2 (4)
Coke gasification occurs just outside the raceway area where gaseous
oxygen is no longer available to completely combust the CO to CO2. The net
heat effect is exothermic; however, and as stated before, the endothermic water
gas reaction (equation 4) allows control of the temperature in front of the
tuyeres by controlling the moisture in the hot blast.
b) Melting (fusion) zone and final reduction of wustite:
H2 and CO from the previous reactions rise through the burden, contact
wustite (FeO) formed from previous reduction reactions in the upper part of
the furnace, and reduces it to iron.
CO + FeO CO2 + Fe (5)
H2 + FeO H2O + Fe (6)
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The iron absorbs carbon through contact with the coke, and melts owing to
its decreased melting point. Equations 3 and 4 combine with equations 5 and 6
in a cycle which effectively regenerates CO. Owing to the highly endothermic
nature of equation 3, the gases cool as they rise in the furnace.
c) Thermal reserve zone:
Once the gases have cooled to about 925oC, the thermodynamics for
equation 3 are no longer favorable. Because the predominant reaction is now
equation 5 which is slightly exothermic, and because the mildly endothermic
equation 6 occurs to a much lesser extent, the gases do not cool appreciably,
resulting in a thermal reserve zone. The net relative amounts of CO2 and H2O
produced by reduction are determined by equilibrium of the water gas reaction.
d) Reduction of hematite to wustite (upper shaft):
Only slight amounts of CO or H2 are required to reduce hematite to wustite.
CO + Fe2O3 CO2 + 2 FeO (7)
H2 + Fe2O3 H2O + 2 FeO (8)
Moreover, calcination and magnesium carbonate decomposition (from the
flux) takes place in this zone:
CaCO3 CaO + CO2 (9)
MgCO3 MgO + CO2 (10)
In this zone, the gas temperature falls off rapidly because of cooling by the
incoming materials, evaporation of moisture, and the net endothermic nature of
the above reactions.
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In addition to the principal reactions discussed, several others are also important,
including:
Fluxing of the sulfur into the slag,
S + CaO + C CaS + CO (11)
Reduction of other metallic oxides,
MnO + C Mn + CO (12)
SiO2 + C Si + 2 CO (13)
P2O5 + 5 C 2 P + 5 CO (14)
Equations 1013 result from contact between hot metal and slag, where the
produced manganese, silicon, and phosphorus are dissolved into the hot metal. [1]
2.3.8 The Challenge of Coal Injection
Despite that the new technologies have already started competing with the
blast furnace; experts believe that blast furnace will remain the principal
method for ironmaking. This is essentially because of the developments that
have taken place over the years in the technology, and engineering aspects. [25]
However, experts also see that the blast furnace would only be competitive
on the long run in case of increasing the coal usage, and decreasing the
dependence on coke. [5]
This trend has really started all over the world, and
coke rates of 300 kg/THM (Ton Hot Metal) with 190-200 kg/THM of coal
injection have already been achieved. [26]
The coal injection is adopted till the
maximum possible extent ensuring stable operation of the furnace.
However, still the coke is the dominant fuel in the blast furnace, and this is
one of its main drawbacks.
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2.3.9 Environmental Analysis for the Blast Furnace Technology
The blast furnace route for steel production is always criticized for its bad
environmental impacts, and there are increased pressures on it from the
environmental associations. The pollutants are mainly generated from the
sintering and coking plants.
2.3.9.1 Sintering Plant Pollutants
Emissions from the sintering process arise primarily from materials-
handling operations, which result in airborne dust, and from the combustion
reaction on the traveling grate. Combustion gases from the latter source
contain CO, CO2, SOx, NOx, volatile organic compounds (VOCs), dioxins and
furans, and oily mill scale. Iron sintering has been identified as a source of
polychlorinated dibenzoparadioxins (PCDD) and polychlorinated
dibenzofurans (PCDF). Combustion gases are most often cleaned in
electrostatic precipitators (ESPs), which significantly reduce dust emissions
but have minimal effect on the gaseous emissions.
There are no appreciable wastewater streams in the sintering plant . [27]
2.3.9.2 Coking Plant Pollutants
The coke oven is a major source of fugitive air emissions. Table 2.3 shows
the approximate amounts of emissions resulting for every ton of coke produced
if there is no vapor recovery system.
3Table 2.3Approximate amounts of emissions resulting for every ton of coke
produced in the absence of vapor recovery system [21]
Pollutant Amount produced in kg per ton of coke
Particulate matter (PM) 0.7 to 7.4
SOx 0.2 to 6.5
NOx 1.4
Ammonia 0.1
VOCs 3 (including 2 kg of benzene)
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Table 2.4 shows the approximate wastewater concentration resulting from
the coking plant. PAH stands for polycyclic aromatic hydrocarbons.
4Table 2.4Approximate wastewater concentration resulting from the coking
plant [21]
Pollutant Concentration in mg/lit
BOD5 1000
COD 1500 1600
TSS 200
Benzene 10
Phenol 150-2000
Ammonia 0.1-2
Cyanide 0.1-0.6
PAH 30
2.3.9.3 Blast Furnace Pollutants
As mentioned above, blast furnace gas is scrubbed before being used as a
fuel. The wastewater stream from the scrubbing process contains iron oxide
and carbon particulates. Moreover, it also contains ammonia and cyanide
which were absorbed from the gases. [1]
Another source of pollution occurs during tapping the blast furnace where
appreciable amount of dust emerges. Finally, when the blast furnace gas is
used as a fuel, SOx and NOx result from the combustion.
2.3.9.4 Greenhouse Gas Emissions for the Whole Industry
The blast furnace route followed by the basic oxygen furnace (BOF) used
for steelmaking results in emissions of about 1 ton of CO2 per ton of steel.
Thus, the industry is one of the largest contributors to greenhouse gas
emissions. Unfortunately, currently there is no economic substitute for the
reductant and energy requirements of the industry. So the only choice to
reduce these emissions is to improve the energy efficiency of the existing
plants and processes. [1]
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21
2.3.10 Drawbacks of the Blast Furnace
Despite being the predominant technology for iron production, and despite
being modified all the over the years, the blast furnace route for iron
production suffers from a lot of disadvantages that enabled the new
technologies to compete with it. The drawbacks of the blast furnace can be
summarized in the following points:
The capital cost of a conventional integrated iron and steelmaking complex
is very high, and very large new plants are required to assure profit. This
investment is difficult to be afforded by the private investors. On the other
hand, mini-mills can produce quality products at competitive cost at a much
smaller scale. Thus, this option is more attractive to the nowadays
investors. [2]
The inherent dependence of the process on the scarce coking coal. [5]
Any reduction in production rate results in a negative effect on the metal
quality, both in terms of chemistry and temperature. [4]
Since all the different unit operations takes place in a single reactor, there
are no means of ascertaining the efficiencies of individual process steps
taking place in the blast furnace. [5]
Extremely high negative environmental impacts. [2]
2.4 General Overview on Direct Reduction
Direct reduction (DR) includes many processes in which iron ore in the
form of lump or pellets is reduced in the solid state by either solid or gaseous
reducing agents. Reformed natural gas or non-coking coal is generally used as
the reductant. [5]
In the DR processes, the final product is solid. So, the gangue
won't be separated from the iron product as was the case in blast furnace.
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22
Consequently, and as DRI retains the chemical purity of the iron ore from
which it is produced, the iron ore should be very low in residual elements such
as copper, chrome, tin, nickel, and molybdenum.
Direct reduced iron (DRI) can be produced in pellet, lump, or briquette
form as DRI retains the shape and form of the iron oxide material fed to the
DR process. [1]
2.4.1 Metallization
Metallization is defined as the percent of total iron in the DRI which has
been converted to metallic iron. For example, DRI having a total iron content
of 90% and a metallic iron content of 77% has 85.5% metallization.
% =
100
DRI normally should at least have 85% metallization. Processes producing
solid
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23
2.4.3 DRI Oxidation and Briquetting
The removal of oxygen from the iron oxide during direct reduction leaves
voids. Thus, DRI (also called sponge iron) tends to have very small size of
grains, lower apparent density, greater porosity, and more specific surface area
than iron ore. [30]
Thus, DRI is subjected to oxidation during transportation. In general,
oxidation of DRI takes place in 2 forms: Reoxidation and Corrosion. [31]
Reoxidation occurs when the metallic iron in hot DRI reacts with oxygen in
the air to form either Fe3O4 or Fe2O3. On the other hand, corrosion occurs
when the metallic iron in DRI is wetted with fresh or saltwater and reacts with
oxygen from air to form rust, Fe(OH)3. Small amounts of hydrogen may be
generated when DRI reacts with water. [1]
This can form an explosive mixture
if it is stored in closed environment. [29]
Moreover, DRI saturated with water
can cause steam explosions if it is batch charged into an electric arc furnace.
Hot briquetting was found to be the best method for preventing reoxidation
of DRI. When DRI is hot briquetted, it is called HBI. HBI is produced by
molding hot (700oC) DRI into pillow-shaped briquettes using roll press. HBI is
almost twice as dense as non-briquetted DRI and it has substantially less
surface area, which makes it 100 times more resistant to reoxidation. [1]
Figure
2.7 shows the difference between the HBI and Pellet DRI.
7Figure 2.7 HBI and Pellet DRI
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2.4.4 Broad Classification of DR Processes
DR Processes are mainly classified according to the type of reductant used:
Reformed Natural gas, or non-coking coal. Table 2.5 shows a comparison
between gas-based and coal-based DR processes.
5Table 2.5 Comparison between gas-based and coal-based DR processes
Parameter Gas-Based Coal-Based
Reaction Kinetics Faster Slower
Temperatures Needed Lower Higher
Product's Purity Higher Lower
Energy Consumption Lower Higher
Raw Materials [30]
Natural Gas and Pellets Non-coking coal and Lump ore
Capital Cost [32]
Higher Lower
2.4.5 Global DRI Production
As stated before, the blast furnace share in ironmaking decreases with time.
Figure 2.8 shows the growth of DRI production globally from 1975 to 2007.
[30]. Midrex shown in the figure is the most dominant DRI production
technology, and it will be well-discussed in the following section.
8 Figure 2.8 Growth of global DRI production from 1975 to 2007
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2.5 Gas-Based DR Processes
In 2008, 75% of the DRI production was from the gas-based processes. [33]
In the gas based processes, the reduction of iron oxide is carried out by a
mixture of CO and H2 at a temperature of about 750-950C. [34]
The main
technologies for gas-based DR will be discussed below.
2.5.1 MIDREX Process
Surface Combustion Division of Midland-Ross Corporation developed the
Midrex Process. In the mid-1960s the Midrex division became a part of
famous German Korf Industries. The first commercial Midrex plant was
installed near Portland Oregon in USA and started production in 1969. By
1983, more than twenty Midrex modules were installed having a total capacity
of about 9 million metric ton per year. [34]
2.5.1.1 Main Reactions Taking Place
Reforming:
CH4 + H2O CO + 3 H2 (1)
CH4 + CO2 2 CO + 2 H2 (2)
Iron Ore Reduction:
H2 + Fe2O3 H2O + 2 FeO (3)
CO + Fe2O3 CO2 + 2 FeO (4)
H2 + FeO H2O + Fe (5)
CO + FeO CO2 + Fe (6)
2.5.1.2 Process Description
As shown in figure 2.9, reducing process gas enters the reducing furnace
through a bustle pipe and ports located at the bottom of the reduction zone.
The reducing gas flows counter currently to the descending solids. [34]
The
latter may be lump ore or pellets; however, pellets are preferred owing to their
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26
superior physicochemical characteristics. [35]
Iron oxide reduction takes place
according to the reduction reactions above.
The partially spent reducing top gas, containing about 70% carbon
monoxide plus hydrogen, flow from an outlet pipe located near the top of the
DRI furnace into the top gas scrubber where it is cooled and scrubbed to
remove the dust particles. The largest portion (about two third) of the top gas
is recompressed, enriched with natural gas, preheated to about 400oC and
piped into the reformer tubes. In the catalyst tubes, the gas mixture is purified
to form carbon monoxide and hydrogen according to the reforming reactions
(1 & 2) above. The hot reformed gas (over 900oC) which is about 95% carbon
monoxide plus hydrogen is then recycled to the DRI furnace.
The balance top gas (about one third) provides fuel for the burner in the
reformer. Hot flue gas from the reformer is used to reheat combustion air for
the reformer burners and also to preheat the process gas before reforming. This
has decreased the energy consumption to about 11.5 million kilojoules per
metric ton of DRI.
Cooling gases flow countercurrent to the burden in the cooling zone of shaft
furnace. The gas then leaves at the top of the cooling zone and flow through
the cooling gas scrubber. The cleaned and cooled gas is compressed, passed
through a demister, and is recycled to the cooling zone. [34]
When incorporating hot briquetting in the MIDREX process, the cooling gas
circuit is eliminated, and the hot DRI is continuously discharged from the shaft
furnace into a hopper and directly fed into a hot briquetting machine.
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27
9Figure 2.9 Flow sheet of MIDREX process for DRI production
2.5.2 HYL III Process
The HYL-III process was developed by Hylsa steel company in Mexico in
1980. It was a modification to the original batch HYL process. However, in the
HYL III process, a single shaft furnace with a moving bed is used in place of
the four original fixed bed reactors. The reactions taking place are the same
stated in MIDREX process. [34]
2.5.2.1 Process Description
The HYL III process is similar to the MIDREX process, however, it uses a
conventional steam reformer and pressurized shaft furnace. As shown in figure
2.10, fresh reducing gas is generated by reforming natural gas with steam. The
natural gas is preheated in the reformer's stack, desulfurized to less than 1 ppm
sulfur. It is then mixed with superheated steam, further preheated to 620oC in
the reformer's stack, and then reformed in alloy tubes filled with nickel-based
catalyst at a temperature of 830oC. The reformed gas is quenched to remove
water vapor, mixed with clean recycled top gas from the shaft furnace,
reheated to 925oC in an indirect fired heater, and injected into the shaft
furnace. For high (above 92%) metallization a CO2 removal unit is added in
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28
the top gas recycle line in order to upgrade the quality of the recycled top gas
and reducing gas. [1]
2.5.2.2 Some Process Features
The process can utilize high sulfur feed natural gas since it is equipped with
sulfur removal step.
Utilizing a CO2 removal circuit (typically PSA) in the circulating gas
system results in more positive control for the CO to H2 ratio in the
reducing gas. This allows controlling the % metallization of DRI
The higher gas pressure system reduces the tendency for bed fluidization,
and thus permits higher capacity. [16]
10Figure 2.10 Flow sheet of HYL III process for DRI production
2.5.3 ENERGIRON Process
For more than 50 years, HYL (now Tenova HYL) has developed
technologies designed to improve steelmaking competitiveness and
productivity for steel facilities. [36]
ENERGIRON is the innovative HYL direct
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29
reduction technology jointly developed by two premier companies Tenova and
Danieli. [37]
2.5.3.1 Main Reactions Taking Place
There are 3 sources for generating reducing gases in this scheme; self-
reforming in the furnace, feeding natural gas as make-up to the reducing gas
circuit, and injecting oxygen at the furnace's inlet.
Reforming and Oxidation:
CH4 + H2O CO + 3 H2 (1)
CO2 + H2 CO + H2O (2)
CH4 + 0.5 O2 CO + 2 H2 (3)
2 H2 + O2 2 H2O (4)
Iron Ore Reduction:
Reactions from 3 to 6 shown above in the MIDREX process
2.5.3.2 Process Description
As shown in figure 2.11, the natural gas stream is mixed with the reducing
gas recycle stream from the CO2 removal system. The reducing gas stream is
passed through the gas heater where it is heated up to 930oC. The reducing gas
temperature is further increased up to about 1020oC after the partial
combustion with oxygen before the furnace. The rest of the process is the same
as HYL III.
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30
Since all reducing gases are generated in the reduction section, utilizing the
catalytic effect of the metallic iron inside the furnace, an external reducing gas
reformer is not required. This is called zero-reformer process (ZR). The basic
scheme can also use the conventional steam-natural gas reforming and other
reducing agents such as hydrogen, gases from coal gasification, petroleum
coke and similar fossil fuels and coke-oven gas. [36]
11Figure 2.11 Flow sheet of ENERGIRON process for DRI production
2.5.3.3 Some Process Features
HYTEMP system allows the immediate transportation of hot DRI emerging
from the furnace to the EAF by the means of pneumatic transport system.
The ENERGIRON process can process high sulfur iron ores as the sulfur is
eliminated along with the CO2 in the CO2 absorption system, which is part
of the reduction circuit. [36]
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31
2.6 Coal-Based DR Processes
In 2008, 25% of the DRI production was from the coal-based processes. [33]
Despite not being a big percentage, the share of coal-based processes in DRI
production is gradually increasing. This may be attributed to the high global
reserves of coal which exceeds the natural gas as shown in figure 2.12.
12Figure 2.12 Global energy reserves
Moreover, experts are sure that on the long run coal will continue to be less
expensive and its price will be more stable than other forms of energy. [23]
In coal-based processes, rotary kilns are used as the reducing reactor. The
main differences in the individual processes are related to the control system
especially for temperature. [30]
2.6.1 General Process Description of Rotary Kiln Technologies
In all coal-based DR processes, lump ore or pellets (or both) together with
coarse fraction of non-coking coal are fed to the inlet end of the rotary kiln.
The size ranges of lump ore, pellets, and non-coking coal are respectively 4-20
mm, 9-20 mm, and 6-20 mm. This coal is referred to as co-current coal, and it
acts as a reducing agent, and as a major heat supplier to the kiln. A finer
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32
fraction of coal (-6 mm) is also injected from the discharge end of the kiln
using primary air as the carrier gas. This coal is called countercurrent coal, and
it helps in completing the reduction, and supplying heat.
A fluxing material like limestone or dolomite should also be added in order
to control the sulfur pick up by the reduced materials from the coal ash. The
flux is mainly in fine form (-4 mm), and is added with the countercurrent coal.
In these processes, optimizing the temperature of the bed charge is crucial.
At the inlet end, the temperature should be high enough so that the reduction
reactions proceed rapidly. On the other hand, the temperature should be low
enough to prevent the fusion of the coal ash. This is achieved by conserving a
balance between the solid-bed temperature, and the temperature in the
atmosphere above the bed (normally at least 100-150oC higher).
[30] This is
mainly achieved by burning combustibles released from the bed using
secondary air. The latter is blown by fans through burner tube space uniformly
along the length of the kiln. [34]
The product from the kiln is mainly a mixture of DRI and char. The
product's temperature is about 950-1000oC, and it is cooled in an indirectly
water-cooled rotary cooler to about 120oC. After that, the DRI is separated
from the coal char using magnetic separators, and finally screening is
performed. The separated char is mainly recycled as a feed material.
Waste gases leaving the kiln at the inlet end pass through a dust chamber
and a post-combustion chamber, before being cooled and cleaned in
electrostatic precipitators, scrubbers, or bag filters. Alternatively, the clean
kiln gases can be used in waste heat boilers to utilize the sensible heat in
producing steam. [5]
Figure 2.13 shows a schematic representation of DRI
production in rotary kilns. [30]
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33
13 Figure 2.13 A schematic representation of DRI production in rotary kilns
2.6.2 Encountered Reactions during coal-based DR Processes
The main reactions that take place within the rotary kiln are the frequent
reduction reactions.
CO + Fe2O3 CO2 + 2 FeO (1)
CO + FeO CO2 + Fe (2)
Reaction 2 takes place in the last 30% of the kiln's length.
The carbon monoxide results from combustion of coal in the presence of
controlled amounts of air
C + 0.5 O2 CO (3)
The produced carbon dioxide resulting from the reduction reacts quickly
with the carbon present in coal to produce carbon monoxide according to the
famous boudouard reaction
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34
C + CO2 2 CO (4)
This cycle continues to maintain the reducing conditions prevailing in the
kiln. Moreover, coal pyrolysis takes place inside the kiln, where volatiles tend
to evolve till about 600oC. However, it is to be noted that most of these
volatiles don't have real contribution to the actual process of reduction. Part of
these volatiles is being combusted by the secondary air injected to the kiln.
This combustion transfers heat to the charge directly by radiation, and also by
conduction from the kiln lining. [16]
2.6.3 Comparison between Different Rotary Kiln DR Processes in
Commercial Use
The coal-based DR processes are similar to great extent. The main
industrially applied processes are SL/RN, Codir, DRC, Jindal, and SIIL. The
main differences between them are in the tolerable size of raw materials, and
energy consumption. However, it worth noting that SL/RN process is the
mother of all the other coal-based DR processes, and it is the most widely
applied. [30]
2.6.4 Advantages of Rotary Kiln Processes
Rotary Kiln can effectively mix the solid charge as it undergoes
simultaneous heating and reduction. Intimate mixing of the charge helps in
diluting CO2 formed around the iron ore particles, and this helps the
reduction reactions to proceed.
Since large freeboard space is available above the solid charge in any kiln,
the gas phase can tolerate the presence of heavily dust-laden gases. In gas-
based processes, generation of dust can lead to channeling.
Rotary kilns are commercially proven, and there is a lot of operating
experiences with it especially in cement industry.
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35
The temperature of iron oxide reduction is much lower than that of blast
furnace (1000oC against 1300-1600
oC). As a result, less energy is required
for reduction. [30]
2.6.5 Disadvantages of Rotary Kiln Processes
The productivity is very low compared to shaft furnaces in gas-based DR
processes. In the latter, yield is up to 5 times more than the rotary kilns for
the same inner volume. Thus, for large capacity plants, multiple rotary kilns
are needed.
The reactor rotates at 0.4-0.5 rpm which makes it difficult to incorporate
process control and quality control systems. Moreover, the engineering of
such kilns is difficult.
The fact of cooling the product in order to perform magnetic separation is a
huge source of energy losses. Thus, these processes exhibit very low energy
efficiency.
Because of the repeated fall and rise of the charge during rotation, the
solids undergo size degradation. Thus, the coarser particles tend to float on
the top of the charge, and the fines tend to settle at the bottom, and thereby
increasing the tendency of adhering to refractory lining. The latter gives
rise to ring formation. Once rings are formed, uniform movement of the
charge becomes difficult, and shutdown of the kiln becomes a must. [30]
2.7 Smelting Reduction
As stated before, smelting reduction (SR) is the third route of ironmaking
after the blast furnace, and direct reduction. Smelting reduction produces pig
iron like the blast furnace; however, it has different features. The gradual
world shift from the integrated steel plants using the blast furnace basic
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36
oxygen furnace route to smaller mini-mills essentially based on EAF was the
main driving force for research and development in the field of SR. [35]
2.7.1 Advantages of SR with respect to Blast Furnace
SR processes use non-coking coal as fuel and reductant instead of the
scarce coal used in blast furnace.
SR processes are viable at lower production capacities, and this copes
with the gradual world shift from the integrated steel plants to smaller
mini-mills essentially based on EAF.
SR processes are more environmentally friendly compared to blast
furnace.
Some SR processes use un-agglomerated ore over 8 mm in size as the
ferrous feedstock. This wasn't possible in blast furnace. [38]
In SR, the same phenomena taking place in the blast furnace occurs;
however, they can take place separately in 2 or 3 units, and this
assures better process control. [39]
In SR, the dependence on pure oxygen instead of air prevents the
formation of cyanides. [5]
2.7.2 Advantages of SR with respect to DR
SR processes are characterized by operating at high temperatures so as to
produce molten iron. These high temperatures ensure faster rates of reaction,
and prevention of sticking problems associated with solid state reactions in DR
Processes. Having liquid phase in the reactor ensures increased transport rates
owing to convection, and remarkable increase in the conversion rate because
of the higher contact area resulting from the dispersed nature of phases.
Another advantage is the lower energy consumption when used in EAF (This
is discussed in the next section). [5]
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37
2.7.3 Use of Hot Metal in Electric Arc Furnaces (EAF)
In mini-mills, DRI or hot metal (HM) can be used as scrap substitutes in
EAF. As shown in figure 2.14, charging of HM as a scrap substitute leads to
great decrease in energy consumption, whereas, DRI causes slight increase. [5]
14Figure 2.14 Effect of different scrap substitutes on EAF energy
consumption
2.7.4 General Features of SR
Figure 2.15 shows a schematic diagram of SR processes. The phenomena
taking place in the blast furnace are mainly divided on 2 stages. Moreover, the
reactants aren't introduced together. Thermal non-coking coal is used as a fuel
and reductant instead of coke.
As stated before, liquid phase formation helps in having higher reaction
rates. The SR processes are characterized by reduction of molten FeO by CO.
It has been concluded that the controlling step for this reaction is mass
transfer. The overall reaction rate is proportional to the square root of the gas
flow rate. Therefore, for SR processes, one of the objectives is to increase the
amount of gas available for reduction. This will be mainly achieved by
increasing the amount of used coal. [35]
Post combustion mainly refers to the
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38
secondary oxygen or air introduced in the process for combusting volatiles for
heat production and volatiles cracking.
15Figure 2.15 Schematic representation of SR technology
2.7.5 Reaction Encountered in SR Processes
2.7.5.1 Reactions Encountered in Pre-reduction Stage
Iron Ore Reduction:
H2 + Fe2O3 H2O + 2 FeO (1)
CO + Fe2O3 CO2 + 2 FeO (2)
H2 + FeO H2O + Fe (3)
CO + FeO CO2 + Fe (4)
Carburization:
3 Fe + 2 CO Fe3C + CO2 (5)
3 Fe + CO + H2 Fe3C + H2O (6)
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39
2.7.5.2 Reactions Encountered in Smelting Stage
C + 0.5 O2 CO (1)
C + O2 CO2 (2)
C + CO2 2 CO (3)
C + H2O CO + H2 (4)
2 H2 + O2 2 H2O (5)
CO + H2O CO2 + H2 (6)
H2 + Fe2O3 H2O + 2 FeO (7)
CO + Fe2O3 CO2 + 2 FeO (8)
H2 + FeO H2O + Fe (9)
CO + FeO CO2 + Fe (10)
C in HM + FeO in slag Fe in HM + CO (11)
CO + FeO in slag Fe in HM + CO2 (12)
In addition, pyrolysis of coal, volatiles decomposition, and evaporation of
moisture takes place. [39]
2.7.6 COREX Process
Among the newly developed SR processes, COREX is the leader both in
terms of capacity and number of plants using this technology. [39]
COREX
produces a high quality HM using non-coking coal and pure oxygen in an
environmentally-friendly process.
2.7.6.1 History of COREX
The COREX process is a technology developed by Voest Alpine, Austria
(Now Siemens VAI) and Korf Engineering, Germany. The success achieved in
the early experiments led to the commissioning of a pilot plant at Kehl in
Germany in 1981 with a capacity of 60,000 tpa. The pilot plant was operated
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40
for six years, during which various grades of different iron ore forms as well
as different types of coal were tested.
Successful performance of the pilot plant encouraged the process developers
to set up a commercial unit. It was felt that a maximum scale up factor of 5
will be successful. Thus, a COREX unit was installed in 1988 in South Africa
in Iscor's Pretoria Works with a capacity of 300, 000 tpa. [39]
2.7.6.2 COREX Process Description
As shown in figure 2.16, the COREX process is based upon a reduction
shaft for iron ore reduction, and a melter-gasifier for coal gasification and iron
melting. In the reduction shaft, the iron oxide feed is in the form of lump ore
or pellets. As in blast furnace, the reduction gas (originated from the melter-
gasifier as will be shown below) moves counter-currently to the descending
burden. In the reduction shaft, about 75-95% metallization is achieved. The
off-gases are cleaned and then used as high caloric export gas. The solid
product from the reduction shaft is discharged via screw conveyors, and
transported via feed legs into the melter-gasifier. [16]
In the melter-gasifier, the pre-reduced iron is further heated and melted to
separate iron from slag. Hot metal is tapped at a temperature of approximately
1400-1500oC in a manner similar to the blast furnace. Inside the melter-
gasifier, the non-coking coal is introduced at room temperature, and it is dried
and devolatilized along the reactor. In the bottom, it is combusted using pure
oxygen in order to generate carbon monoxide essential for reduction. The
evolved coal volatiles are cracked in the top of the reactor, and thus huge
environmental problems are prevented. The reducing gases exit the melter-
gasifier at about 1000-1100oC. They are cooled to 800-900
oC, dedusted in a
hot dust cyclone, and conveyed back to the reduction shaft. [39]
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41
16 Figure 2.16 Flow Sheet of COREX process for HM Production
2.7.6.3 Commercial Production
Currently, 4 plants are utilizing COREX process for pig iron production.
These plants are Saldanha Steel Works in South Africa (0.8 mtpa), Jindal
South West Steel in India (2 * 0.8 mtpa), Posco in Korea (0.8 mtpa), and
Baosteel in China (1.2 mtpa).
2.7.7 FINEX Process
As shown in figure 2.17, FINEX is an SR process that uses ore fines in a
series of fluidized bed reactors for initial pre-reduction followed by a melter-
gasifier to produce pig iron. [35]
Since 1992, Siemens VAI and the Korean steel producer Posco have been
jointly developing the FINEX process. [40]
The objective of the process was
mainly utilizing the ore and coal fines generated during processing of the feed
required by the COREX unit in Posco. [35]
A commercial unit of 1.5 mtpa at
Pohang, Korea was commissioned in 2007 and is in operation since then. [41]
This is the only operating FINEX process till now.
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42
The generated FINEX export gas is a highly valuable product and can be
further used for DRI/HBI production, electric energy generation or heating
purposes. The hot metal and slag produced in the melter-gasifier is frequently
tapped from the hearth as in blast furnace or COREX process. [40]
17 Figure 2.17 Flow Sheet of FINEX process for HM Production
2.8 COREX Process for Pig Iron Production
As this thesis deals with optimization and modeling of COREX process, the
following section aims to zoom into the process from different view points.
2.8.1 Detailed Process Description
As mentioned previously, COREX consists of 2 reactors, the reduction shaft
and the melter-gasifier. The reduction shaft (RS) is placed above the melter-
gasifier (MG). The process operates at high pressure of about 3.5 bar which
ensures lowering the dimensions, and having high conversions for the different
reactions especially in the absence of nitrogen because of using pure oxygen.
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43
2.8.1.1 RS Process Description
Iron ore, pellets and additives (limestone and dolomite) are continuously
charged into RS via lock hopper system located on the top of the shaft. Some
amount of coke is also added to the shaft to avoid clustering of the burden
inside the shaft due to sticking of ore/pellets and to maintain adequate bed
permeability. The reduction gas is injected through the bustle located about 5
meters above the bottom of the shaft at about 850oC and over 3-bar pressure.
The gas moves in the counter current direction to the top of the shaft and exits
from the shaft at around 250oC. Percentage metallization ranges from 75% to
95%, and the solid product is termed as DRI. Subsequently, six screws
discharge the DRI from the RS into the MG.
2.8.1.2 MG Process Description
As shown in figure 2.18, the MG can be divided into 3 main reaction zones
namely: Gaseous free board zone (Dome), moving bed (middle part above
oxygen tuyeres) also called char bed, and fluidized bed (in the transition area
between the moving bed and the free board zone). The Hearth zone which is
the lower part below oxygen tuyeres can also be considered as the fourth zone.
The hot DRI at around 600-800oC along with partially calcined limestone
and dolomite are continuously fed into the MG through DRI down pipes. The
DRI down pipes are uniformly distributed along the circumference near the top
of the melter-gasifier so as to ensure uniform distribution of material over the
char bed. Additionally non-coking coal, iron ore fines, flux fines and some
coke are continuously charged by means of lock hopper system.
Oxygen plays a vital role in COREX process for generation of heat and
reduction gases. It is injected through the tuyeres, which gasifies the coal char
generating CO. The hot gases ascend upward through the char bed. [42]
It is to
be noted that the gas velocity in the lower part of the gasifier is adjusted to
maintain a stable fluidized bed before the secondary oxygen injection. [39]
The
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44
sensible heat of the gases is transferred to the char bed, which is utilized for
melting iron and slag and other metallurgical reactions. The hot metal and slag
are collected in the hearth and tapped in a manner similar to the blast furnace.
The dome temperature is maintained between 1000oC to 1100
oC, and this
assures cracking of all the volatile matter released from the coal. The gas
generated inside the MG contains fine dust particles, which are separated in
hot gas cyclones. The dust collected in the cyclones is recycled back to the
MG through the dust burners, where the dust is combusted with secondary
oxygen. There are four of these dust burners located around the circumference
of the melter-gasifier above the char bed. The gas from the MG is cooled to the
reduction gas temperature (850oC) through the addition of cooling gas. A
major part of this gas is subsequently fed to the RS. The excess gas is used to
control the plant pressure. This excess gas and the RS top gas are mixed and
termed as COREX export gas. [42]
18Figure 2.18 Zones in the Melter-Gasifier
2.8.2 Dimensions of the Encountered Reactors
COREX process can operate economically at small module sizes. Figure
2.19 shows the different modules currently available for the process. C-1000
stands for 1000 tpd, C-2000 stands for 2000 tpd, and similarly C-3000 stands
for 3000 tpd.
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19Figure 2.19 Different modules available for the reactors of COREX
process
The hearth diameter in the melter gasifier is the main dimension that
characterizes the reactor as shown in figure 2.19. As C-2000 is the module
used in most of the operating plants, it is important to know more about its
dimensions. As shown in figure 2.18, the free board is the dome shaped zone
inside the melter gasifier. Its diameter is about 14 meters, and its height is
about 8 meters. The whole melter gasifiers height is about 22 meters.
2.8.3 COREX Export Gas
Unless export gas is well-utilized, the process won't be cheaper than the blast
furnace route. [35]
Large volumes of export gas are generated from the process-
typically 1650 1700 Nm3/THM. The gas mainly consists of CO, H2, CO2, and
small amounts of CH4. The export gas can be utilized in heating purposes in a steel
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plant (in rolling mills), power generation (as shown in figure 2.20 a), the
production of oxygen used in MG (as shown in figure 2.20 a), synthesis gas in the
chemical industry, reductant to produce DRI in any gas-based DR processes (as
shown in figure 2.20 b), and finally can be used internally in the process as
reducing gas after CO2 removal. The latter results in appreciable reduction in the
consumption of coal and oxygen (as shown in figure 2.21). [39]
20Figure 2.20 Export gas from 3000 tpd COREX plant used in: a) combined
cycle power plant (th: thermal, el: electrical) and b) DRI production
21 Figure 2.21 In-Process utilization of COREX export gas
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2.8.4 Raw Material Requirements for COREX Process
To achieve reliable operation, there are specifications for the different raw
materials to be used in COREX process.
2.8.4.1 General Requirements
Table 2.6 summarizes the preferred and tolerable grain size, and some
chemical properties of the raw materials used in COREX process.
6Table 2.6 Raw materials requirements for COREX process
Specification Preferred Tolerable
Coal
a) % Volatile Matter
b) % Ash
c) % Sulfur
d) Grain size, mm
20 - 30 (Water free) 15 - 36 (Water free)
5 - 12 (Water free) 10 - 25 (Water free)
0.4 - 0.6 0.5 - 1.5
5 - 40 (50% should be +10)
Lump Ore
a) Fe%
b) Grain size, mm
62 - 65 55 (min.)
8 - 20 6 - 30
Pellets
a) Fe%
b) Grain size, mm
62 - 65 58 (min.)
8 - 16 6 - 30
Sinter
a) Fe%
b) Grain size, mm
50 - 55 45 - 50 (min.)
10 - 30 6 - 45
Limestone, dolomite
Grain size, mm
8 16
Limestone, dolomite fines
Grain size, mm
4 - 10
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2.8.4.2 COREX Insensitivity to Alkali Content
One of the most attractive features of COREX is its insensitivity to the
alkali content of the raw materials, so there is no buildup of alkalis as is the
case in the blast furnace. Since pure oxygen is used, no cyanides are formed at
the tuyere level, rather K2CO3 and Na2CO3 vapors are formed from the
reaction with CO2. These vapors are non toxic and are discharged via the
cooling gas scrubbers at the same input quantity. Moreover, if some alkali dust
emerges from the MG, the water used in the scrubbers dissolve them, and
thereby prevents accumulation. [5]
2.8.4.3 Use of Coke in COREX Process
Despite announcing in the beginning that COREX can operate totally
without coke, the actual practice has shown that coke quantities in the
operating plants are in the range of approximately 2-10 % of the coal charge.
Whereas POSCO plant in South Korea showed that it is possible to operate the
COREX plant at zero coke for long periods (coke consumption in total 1999
was in average 19 kg/t HM) by a careful operation and treatment of the raw
materials. However, SALDANHA plant of South Africa and JINDAL plant of
India could not reach that.
Another important aspect has to be taken into account regarding to the
required coke quality: In case coke is charged to the COREX plant, only low
quality coke is required. Consequently, coke can be seen as an "additive" for
the process as to the most extent thermal coal is directly used. Moreover, coke
will be of minor economic impact. [43]
2.8.5 Factors Affecting the Efficiency of COREX Process
In the reduction shaft, the metallization degree of the DRI and the calcination of
the additives are strongly dependent on the following parameters: [42]
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Amount and quality of the reduction gas flow
Temperature of the reduction gas
Reducibility of the iron bearing burden
Average particle size and the distribution of the solids charged
The efficiency of the whole process depends on the following parameters:
Size and chemical analysis of the raw materials especially the coal
% Metallization of the DRI in the RS
Optimum distribution of oxygen between the tuyeres and dust burners
Permeability of the char bed
2.8.6 Environmental Analysis for COREX Process
The elimination of coke-making operations and sintering has made COREX
process a very environmentally friendly process. The latter is one of the most
salient features for this process. Table 2.7 shows the differences between the
gaseous emissions and aqueous effluents between a modern blast furnace, and
an operating COREX unit. [5]
It is apparent that COREX is a very clean
technology with respect to the blast furnace.
7Table 2.7 Comparison between pollutants emerging from a blast furnace
and a COREX unit
Blast Furnace COREX Process
1) Aqueous Effluents in mg/THM
a) Ammonia
b) Phenol
c) Sulphide
d) Cyanide
590 50
80 0-1
60 7
20 1
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Table 2.8 compares the sulfur input and output balance in blast furnace and
COREX unit located in the same facility. [44]
It is apparent that despite that
COREX can sustain an input amount of sulfur twice as high as a blast furnace,
the sulfur content in the HM is similar to that of blast furnace. This is because
during the gasification of coal in the MG, sulfur is converted predominantly to
H2S. Moreover, small amount of SOx is formed by the combustion of H2S with
the oxygen in the dust burner region. The above H2S and SOx along with
reducing gases enter the RS where the following reactions take place:
CaO + H2S CaS + H2O
(Ca,Mg)O + H2S (Ca, Mg)S + H2O
4 CaO + 4 SO2 3 CaSO4 + CaS
Through these reactions, sulfur is captured in the calcined additives and
then is fed into the MG and finally dissolves into the molten-slag phase.[45]
8Table 2.8 Comparison between sulfur balance in a blast furnace and a
COREX unit
2) Gaseous Emissions in mg/THM
a) NOx
b) SOx
c) Dust
1900 21
1600 26
427 39
Sulfur input / output Blast Furnace COREX Process
1) Inputs in kg sulfur/THM
a) Ore
b) Fuel
Total
0.15 0.035
2.6 4.682
2.75 4.717
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2.8.7 Advantages of COREX Process
COREX process holds a lot of advantages, and this is why more than one
steel plant in the world uses this process in ironmaking. This is also why this
process is nearly the only competent to the blast furnace in the field of pig iron
production. The advantages can be summarized in the following points:
The dependence on thermal coal instead of coke allows conserving of the
scarce coking coal. [39]
The process is viable at lower production capacities, and this copes with the
gradual world shift from the integrated steel plants to smaller mini-mills
essentially based on EAF. [38]
The process is very environmentally friendly (as proved above).
The raw material requirement is not as stringent as in blast furnace, and
despite that the quality of the produced hot metal is not affected. [46]
Insensitivity to the alkali contents of the raw materials. [5]
Correction of hot metal and slag composition is easier and faster in COREX
than in blast furnace as additions can be made through the melter-gasifier.
The calorific value of COREX gas is 2.6 - .27 times higher than that of
blast furnace.
The start of the COREX furnace is easier after a shut down, and can reach
the rated capacity in one hour.
2) Outputs in kg sulfur/THM
a) Slag
b) HM
c) Sludge/ Dust
Total
2.42 4.051
0.18 0.19
0.15 0.476
2.75 4.717
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Specific melting capacity of COREX is about twice that of the blast
furnace. [46]
The cost advantage of COREX process with respect to the blast furnace
(after utilizing the export gas) varies from 10-20%. It is 10% at POSCO in
South Korea, and 19% at Jindal in India. [39]
COREX is suitable for two different steelmaking routes, the EAF route at
Saldanha Steel, South Africa, and the basic oxygen furnace (BOF) route at
Jindal, India. [4]
COREX is suitable for mini-mills and integrated steel plants. [5]
2.8.8 Disadvantages of COREX Process
Beside the various advantages of COREX, the process has also some
drawbacks which can be summarized in the following points:
The process can only have a maximum of 15.5% ore fines in the charge.
High volatile coals can't be used directly, and they must be blended with
low volatile ones.
The process won't be economically viable if the export gas isn't well
utilized. [41]
2.8.9 Case Study Jindal
Jindal Vijayanagar Steel Limited (JVSL) is a great example of COREX
process success. The company started its integrated steel operation in 1999,
based on COREX with a capacity of 0.8 mtpa. After success of the first Corex
unit, JVSL added the second module in 2001. After that, the company further
increased the production capacity to 3.8 mtpa by the commissioning of 2 blast
furnaces. Thus, the company utilizes the advantages of both COREX and the
blast furnace in a great synergetic way. [46]
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2.8.9.1 COREX Export Gas
The export gas from both COREX units is used in the generation of
electrical energy in two adjacent power plants each of 130 MW capacity, as
well as for the production of pellets in a pelletising plant of 3 mtpa. About
50% of these pellets is processed in COREX plants, while the rest is sold to
third parties. Using 70% pellets instead of 100 % lump ore increased the metal
output. [5]
During the decision making process of installing COREX modules in the
company, it was deduced that buying electricity from the state grid would have
meant paying Rs 4.32 per unit. On the other hand, generating power from a
COREX unit cost only Rs 2.60 per unit. This meant power costs reduced by
almost 40 per cent.
On the other hand, and during the decision making process of increasing the
production capacity, it was deduced that there will be surplus amount of export
gas if COREX is to be applied. Since no feasible application was found for this
surplus gas, the decision was to use blast furnace instead. The chief executive
officer of JVSL says that setting up a plant using COREX involves an
assumption that theres an assured buyer for the excess power produced. This
shows that the decision of returning back to blast furnace doesn't necessarily
mean that blast furnace is better than COREX. [47]
2.8.9.2 Using Iron Ore Fines
Undersized iron ore (size 6-12 mm) is being charged directly into the
COREX MG. It was realized that the surplus heat available in the free board
could be utilized for reduction of iron ore fines. Addition of fines via the coal
line increases the hot metal productivity, generates extra reduction gas for the
shaft and helps in controlling the process parameters more uniformly. On a
monthly average basis, maximum 15.5% of the total iron bearing material has
been substituted by iron ore fines addition. [42]
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2.8.9.3 Recycling of various by products and plant wastes
The drive towards reduction in hot metal price has prompted JVSL to adopt
innovative measures for recycling of various by products and plant wastes.
Some of these are the use of BOF Slag, mill scale, and Limestone and
Dolomite fines. [42]
2.8.9.4 Improvement in Plant Operation
Through continuous research, and by gaining more operating experiences,
the plant's performance was greatly enhanced as shown in Table 2.9. [42]
9Table 2.9 Progress of COREX performance in JVSL
Year Production,
mtpa
Fuel Consumption,
kg / THM
Hot Metal
Temperature, oC
% S in
HM
1999-2000 0.4 1163 1491 0.06
2000-2001 0.77 1071 1503 0.037
2001-2002 1.52 1082 1497 0.037
2002 - 2003 1.46 1041 1497 0.029
2003-2004 1.36 1000 1487 0.027
2.8.9.5 Synergetic Combination of COREX and Blast Furnace
JVSL has initiated a great engineering trend of having both COREX and
blast furnace operating in one integrated steel plant. The synergy of COREX
and blast furnace has helped JVSL to maximize the utilization of solid waste
and thereby reduced production cost of pig iron.
As shown in figure 2.22, the non-coking coal used in COREX is screened so
that the lump coal is fed to COREX, and the coal fines (- 6 mm) are fed to the
blast furnace as pulverized coal injection. Moreover, out of the total coke
produced, the lump coke is fed to the blast furnace, nut coke (6-25 mm) is fed
to COREX, and the coke breeze (-6 mm) is fed to the sinter plant.
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More than 70% of the plant wastes such as flux fines, mill scale, and BOF
slag are recycled into COREX either directly or indirectly through the pellet
and sinter plants. Moreover, the COREX export gas is used as a backup in the
blast furnace stoves, boilers, and in sinter and pellet plant. [48]
22 Figure 2.22 Synergy of COREX and Blast Furnace
2.8.10 Case Study SALDANHA
An Integrated Compact Mill, based on a COREX C-2000 (2000 tpd) unit in
combination with a COREX gas based direct reduction (DR) plant, was
started-up in 1999 at SALDANHA STEEL, South Africa.
Export gas from the COREX plant is used for the production of DRI in an
adjacent plant using a MIDREX shaft furnace and LINDE Vacuum Pressure
Swing Absorption plant (VPSA) for the removal of CO2. The DR plant is
operated with a mixture of about 65 % lump ore and 35 % pellets.
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The COREX plant is operated with mainly local iron ores comprising lump
ore (80 - 100%), pellets (0 - 20%) and local coal. All of the required additives
are also supplied locally. [43]
2.9 COREX Macroscopic Analysis
Because of having a multi-component multi-phase system, the macroscopic
analysis is very important to reach better understanding of the process, and
assess the effect of different process parameters.
2.9.1 Reduction Shaft Macroscopic Analysis
The most important parameter is the permeability of the burden inside the
shaft. Despite having better strength than lump, the particle size of pellets
decreases along the shaft because of attrition, and reaction. The cold crushing
strength (CCS) is the parameter which measures the pellets' strength. As this
parameter decreases, the pellets crumble and decrease the bed permeability,
and consequently pressure drop increases. The latter will cause channeling,
and low metallization. Statistical analysis has been used to study this
phenomenon, and the following curve results. [49]
23 Figure 2.23 Influence of CCS on the reduction shaft's pressure drop
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