Literature Review on Iron Production Technologies and COREX Process Mathematical Models

1

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

2

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

1

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


2.5.2.2 Some Process Features .......................................................................................... 28

2.5.3 ENERGIRON Process ..................................................................................................... 28

2.5.3.1 Main Reactions Taking Place ................................................................................. 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

3

2.10.2 Melter-Gasifier Microscopic Analysis ......................................................................... 63

References ........................................................................................................................ 65

4

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.

5

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.

7

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

10

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

11

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]

13

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

14

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

15

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

16

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)

17

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.

18

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.

19

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)

20

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]

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.

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

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

24

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

25

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)


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

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.

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]


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

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

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


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.

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]

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

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]

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)


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

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.

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

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]

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

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)


Carburization:

3 Fe + 2 CO Fe3C + CO2 (5)

3 Fe + CO + H2 Fe3C + H2O (6)

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)


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

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]

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.

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.

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

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.

45

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

46

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

47

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

48

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]

49

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

50

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

51

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

52

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]

53

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]

54

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.

55

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.

56

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

Documents

Literature Review on Iron Production Technologies and COREX Process Mathematical Models