Seminar Report

Report Submitted by:

J Krishna Teja,

Bachelor in Engineering,

Metallurgy & Materials Engineering,

National Institute of Technology,

Durgapur.

Smelting reduction process

Corex Process in Iron Making

COREX PROCESS IN IRONMAKING

Report Submitted by: J Krishna Teja Bachelor in Engineering, Metallurgy & Materials Engineering, National Institute of Technology, Durgapur.

INTRODUCTION

For decades, there have been number of initiatives towards development of alternative smelting reduction route of iron making. The reason for such interest stems from the fact that the conventional blast furnace iron making depends on metallurgical coal, which is required for producing BF grade coke. Continued supply of metallurgical coal at a competitive price is becoming increasingly difficult with the depletion of coking coal reserves. Besides, the coke oven batteries are among the most environmentally hazardous reactors so far. COREX is the first and the only commercially established smelting-reduction process, as an alternative route to blast furnace, based on non-coking coal. This process, has been developed and is being marketed by VOEST-ALPINE INDUSTRIENLAGENBAU (VAI), Austria. The COREX process offers high smelting intensity and hence higher productivity, ability to use various types of non-coking coals, use of iron ore fines to an extent, low net operating cost, possible generation of power or other alternative use of export gas generated from the iron making unit, besides being eco-friendly. Jindal Vijaynagar Steel Limited (JVSL) is a Greenfield venture in Karnataka based on COREX iron making technology, with an investment of over Rs. 6,000 crore. It is supported by joint ventures, namely, Jindal Praxair Oxygen Company Ltd. (JPOCL), Jindal Thermal Power Company Ltd. (JTPCL) with co-generation concept and Vijaynagar Minerals Private Ltd. (VMPL). The world class performance of COREX plant so far has justified the vision and concept of such large investment in a cluster of industries co-existing. This conglomerate concept is the most adaptable in developing countries where there exists significant growing market for steel, power, cement, industrial gases, mining and mineral processing.

ESSENTIAL FEATURES

COREX consists of two reactors, the reduction shaft and the melter-gasifier. The reduction shaft is placed above the melter-gasifier and reduced iron bearing material descends by gravity. The volume of the reduction shaft and the melter-gasifier is about 600 m3 and 2200 m3 respectively.

Schematic diagram of a COREX plant

Reduction Shaft:

Iron ore, pellets and additives (limestone and dolomite) are continuously charged into the reduction shaft 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 850°C and over 3-bar pressure. The specific reduction gas flow is about 1200Nm3/ton of iron bearing burden charged to the shaft. The gas moves in the counter current direction to the top of the shaft

and exits from the shaft at around 250°C. About 5-6% 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 iron bearing material gets reduced to over 95% metallization in the shaft and is termed as DRI. Subsequently, six screws discharge the DRI from the reduction shaft into the melter-gasifier.

The metallization degree of the DRI and the calcination of the additives are strongly dependent on the following parameters:

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

Melter-Gasifier: The melter-gasifier can largely be divided into three reaction zones

Gaseous free board zone (upper part or dome)

Char bed (middle part above oxygen tuyeres)

Hearth zone (lower part below oxygen tuyeres)

Due to continuous gas flow through the char bed, there also exists a fluidized bed in the transition area between the char bed and the free board zone. The hot DRI at around 600-800 °C along with partially calcined limestone and dolomite are continuously fed into the melter-gasifier 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, quartzite and required quantity of coke are continuously charged by means of lock hopper system. The operating pressure, in the melter-gasifier is in excess of 3 bars.

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 generates CO. The hot gases ascend upward through the char bed. The 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. The efficiency of the furnace depends largely on the distribution of this gas in the char bed and utilization of the sensible heat of the gas. The dome temperature maintained between maintained between 1000°C to 1100°C, which assures cracking of all the volatile matter releases from the coal. The gas generated inside the melter-gasifier contains fine dust particles, which are separated in hot gas cyclones. The dust collected in the cyclones is recycled back to the melter-gasifier through the dust burners, where the dust is combusted with additional oxygen injected through the burners. There are four such dust burners located around the circumference of the melter-gasifier above the char bed. The gas from the melter-gasifier is cooled to the reduction gas temperature (850°C) through the addition of cooling gas. A major part of this gas is subsequently fed to the reduction shaft. The excess gas is used to control the plant pressure. This excess gas and the reduction shaft top gas are mixed prior to the takeover point and is termed as COREX export gas. The export gas has high net calorific value of

approximately 7,500 – 8000 kJ/m³ (STP). This gas is suitable for use for a wide range of applications like power generation etc.

Typical usage of materials: Coal use in the C-1000 plant is about 900 kg/thm. The oxygen requirement is 540 Nm3/thm. The production of export gas is around 11.5 GJ/thm. A small amount of electricity is required, 60 kWh/ton. On the basis of these data, the Specific energy consumption (SEC) of the C-1000 plant can be calculated to be 15.5– 17.5 GJ/thm. (All calculations of the Specific energy consumption (SEC) of smelting reduction processes are summarized in Table)

PROCESS CHEMISTRY

Corex is a two reactor but three-stage process. The blast furnace concept has been used, virtually splitting it into two at the cohesive zone interface. Accordingly a Corex plant has shaft unit, where iron ore pellets (with or without some closely sized lump ore) is reduced by gases emanating from the second unit to make hot sponge iron (first stage). This is mechanically transferred to the second unit or Melter-Gasifier where it is melted and carburized (second stage) by injection of both coal and oxygen. In the upper part of the Melter-Gasifier a fluidized bed of coal char is maintained (third stage), where any CO2 or H2O is converted to CO and H2. Since there is practically no CO2 or H2O in the gas leaving the Melter-Gasifier, we say that the degree of post combustion of Corex gas is zero, resulting in a gas rich in chemical energy.

Reactions in Reduction Shaft:

Following are the primary reactions taking place inside the reduction shaft:

Reduction of iron oxide by CO and H2 and transforming the iron oxides to metallic iron.

Fe2O3 → Fe3O4 → FeO→ Fe

Calcination of Limestone and Dolomite

CaCO3 → CaO+CO2 (endothermic)

CaCO3.MgCO3→CaO.MgO+2CO2 (endothermic)

Carbon deposition reaction and formation of Fe3C

2CO → CO2+C (exothermic)

3Fe+2CO → Fe3C+CO2 (exothermic)

Out of the above mentioned reactions, reduction of iron oxide by H2 and calcination reactions is endothermic. On the other hand, reduction of iron oxide by CO gas and carbon deposition reactions is exothermic in nature. The reduction gas is almost fully desulphurized in the shaft due to the presence of the burnt lime and dolomite according to the following reactions:

CaO + H2S → CaS + H2O

MgO + H2S → MgS + H2O

Low content of hydrogen sulphide of the top gas is important with respect to the further usage of the COREX gas.

Reactions in Melter-Gasifier:

Following are the reactions taking place in melter-gasifier:

Drying of Coal (100oC) Devolatilisation of coal (200oC to 95 oC) and liberation of methane and

higher hydrocarbons.

Decomposition of volatile matter Due to higher temperature prevailing in the melter-gasifier free board zone, the hydrocarbons are cracked into hydrogen and elementary carbon CnHm = nC + (m/2)H2

It is desirable that all higher hydrocarbons are cracked in the free board zone so as to assure generation of a good quality reduction gas. Maintaining dome temperature between 1000oC to 1100oC ensures the same.

Other reactions in the free board zone are:

CO2 + C = 2CO (Boudouard Reaction)

H2O + C = CO + H2 (Water Gas Reaction)

CO + H2O = CO2 + H2 (Shift Reaction).

Decomposition of the undecomposed limestone and dolomite. Residual Reduction of the iron oxide.

Direct reduction of FeO into the DRI takes place by carbon in the char bed. Combustion of coal char by oxygen. Burning of the coal char takes place near the tuyeres. The maximum temperature inside the melter-gasifier exists in front of the tuyeres. The following carbon gasification reactions takes place in the tuyeres area.

2C + O2 = 2CO 2

CO + O2 = 2CO2

C + CO2 = 2CO

Melting and formation of hot metal and slag respectively

A typical analysis of COREX hot metal and slag is give in table 2 & 3.

The typical analysis of various gases produced in COREX process is give in table 4.

The efficiency of the COREX process depends on the following parameters:

Size and chemical analysis of the raw especially the coal

Low C02 percentage in the reduction gas so as to ensure higher metallization of the DRI

Optimum distribution of oxygen between the tuyeres and dust burners

Permeability of the char bed

High system pressure

Higher melting rate operation

ADVANTAGES & LIMITATIONS:

Advantages of this process are following:

It substantially reduces specific investment costs compared with conventional blast furnace steelmaking process; lowers the production costs 15% to 25% compared with a blast furnace,

Outstanding overall environmental compatibility, due to reduction in CO2 generated per ton of iron production;

Use of COREX export gas for a wide range of applications, Use of a wide variety of iron ores and coals, Elimination of coking plants, Hot-metal quality suitable for all steel applications. Integrated electrical power generation possible. (viii) Combined production

of DRI possible. Highest operational flexibility with respect to production capacity,

production stops and raw material changes.

Limitations include:

It has the limitation in distributing the coal and DRI in the optimised manner in the melter-gasifier. This results in more peripheral flow of hot gases.

Absence of post combustion means that much chemical energy is lost in the exit gases and consequently the coal consumption in Corex is much higher than in any other iron making process.

The system is maintenance oriented, including cooling gas compressor for recycling part of COREX gas for cooling the hot gases from the melter-gasifier.

It is provided with very sophisticated gas cleaning facilities. There are three gas cleaning streams for cleaning the total gas generated in the process.

Hot DRI transfer and hot gas recycling are hazardous especially during their maintenance periods.

The melter-gasifier is subjected to high occurrence of pressure peaks on account of use of raw coal with poor char bed conditions. This results in jamming of dust recycling systems as well as gas cleaning systems thereby resulting in more downtime of the plant.

The process is sensitive to the quality of inputs particularly with respect to quality and granulometry, the input of fines, decrepitation, and

degradation behaviour of coal, iron ore and pellets at high temperature. Inferior quality of inputs result in frequent jamming of screws, DRI downpipes and DRS lines, affecting furnace availability.

WORLD SCENARIO:

Development of the COREX process dates back to the 1980s and was performed by Voest-Alpine (Germany/Austria). The first commercial COREX unit was constructed between 1985 and 1987 at ISCOR’s Pretoria works, after first testing the process in Germany. The plant had various problems (due to inexperience) after the start-up in December 1987. After reconstruction and de-bugging, the plant has been successfully in operation since 1989 and was given over to ISCOR(capacity 300,000- ktpa). Reconstruction of some parts and new operation conditions improved the performance greatly, leading to production of high quality iron, and high productivity and availability. The plant demonstrated to be economically attractive, with 30% lower production costs than the blast furnace on site, despite the low capacity of the COREX unit. The clean excess fuel gas is used on site in furnaces and coke ovens. The COREX process proved to be very flexible (with respect to the fuel rate, and additives addition), insensitive to high alkali content of the ore (and burden), and easy to operate.

The preliminary success of the first COREX plant lead to the decision to build a larger COREX (C-2000, 650,000 tonnes per year) in South Africa by Saldanha Steel, a subsidiary of ISCOR. Saldanha Steel is now a part of Mittal Steel South Africa which in turn is part of global steel company Arcelor-Mittal. In 1998, ISCOR decided to close the Corex-based Pretoria plant due to an “unprofitable economic situation”. The off-gases of the new COREX unit will be used to produce 800,000 tonnes per year of DRI, following a similar design to that at HANBO Steel, South Korea. This decision also seems to be based on the environmental performance of the COREX process, as the site is located near a nature preserve. This plant, which includes a thin slab caster, began operation in January 1999.

Since 1995 a COREX plant has been in operation in the Republic of Korea, with twice the capacity of the South African plant (C-2000). South Korea’s Pohang Iron and Steel Co. has a 600,000-700,000 ton per year unit COREX

plant. Several more orders have been placed for C-2000 plants in India, South Korea, and South Africa.

The Chinese steel producer Baosteel Pudong Iron and Steel Co. Ltd. (For Short Pudong steel) at Luojing, near Shanghai started up a Corex C-3000 plant in early November 2007, with a nominal production capacity of 1.5 million tons of hot metal per year. The project was completed under the management of Siemens Metals Technologies within a period of 29 months.

The latest generation of Corex plants, the C-3000, is ideally suited for integration into green- or brown-field steel works projects. It can replace the blast furnace, or can be used as a source of virgin iron for minimills. The economics of the Corex plant already provide an answer to future scrap and coke shortages, and the continually increasing demands placed on steel quality. Another alternative is the installation of a Corex C-3000 plant as a stand-alone merchant plant for the production of hot metal and/or pig iron.

In the United States, Geneva Steel has joined Air Products and Centerior Energy in a Department of Energy project to demonstrate the commercial viability of the Corex process in the United States.

INDIAN SCENARIO:

The COREX C-2000 Plant/Module 01 at Jindal Vijaynagar Steel Ltd., Tornagallu, Karnataka, India, Successfully started on August 8, 1999. Jindal Vijaynagar Steel Limited (JVSL) project is located in the midst of the rich iron ore belt of Bellary-

Hospet in Karnataka.The JVSL project is conceived as the one of the most modern, technologically efficient Greenfield plant with a capacity of 1.6 mt per annum.

As a highlight of this proven technology, more than 75% of the nominal capacity was reached shortly after start-up. The excellent hot metal quality fulfils the requirements of the LD (BOF) shop. The plant was installed by VOEST-ALPINE INDUSTRIENLAGENBAU (VAI), a company of the listed VA Technologic AG. This was the fourth plant which was put in operation for the production of hot metal based on VAI’s proprietary COREX technology. It is planned that the COREX export gas will be used for electrical power generation and for heating purposes within the steel complex. Hot metal from the COREX plant is processed to high quality steel in two 130 t LD (BOF) converters. The slag produced by the COREX plant is processed in the granulation plant and will be sold to cement industry.

The steel making facility (BOF-CCP shop) include two 120 T convertors and two single casters of latest design and state of art facility from Mannesmann Demag of Germany. The quality slab produced will be suitable for hot strip mill which has been engineered by Daniel United of USA, incorporating the latest technology of coil box and a 250 T walking beam furnace from Stein Heurty. A third generation mill is equipped with state of art electrical automation from Cegelac, USA and has all modern control features and equipment, to produce internationally competitive quality strips of 1.6mm to 12mm thickness up to a width 1250 mm.The JVSL project is visualised on co-generation concept and the unique feature of the project is its technology-cum-management model. Jindal Praxair Oxygen Company Limited, the joint venture company, supplies oxygen to the COREX plant. COREX off-gases will be used to produce power by another joint venture company, Jindal Tractebel Power Company Limited.

An overview of JVSL COREX Plant

Rated capacity: 1.6 million tpa

No. of units: 2 Module C-2000

Major units: Coal Blending Station

Coal Drying Plant

Stock House

COREX Tower

Cast House

Water recirculation

System Gas Cleaning and Distribution System

Slag Granulation Plant

Salient Technical features:

Use of non-coking coal. Availability of COREX gas of medium calorific value for use as fuel in the

steel plant as well as Power Plant. Environment friendly process.

REFERENCES:

“Smelting Reduction for Iron Making” by A. K. Jouhari, 2002 (Pages 44-58),

“Future technologies for energy-efficient Iron and steel making “by De beer, Worrell & Blok, 1998.

http://www.industry.siemens.com/metals-mining/en/ Ironmaking/corex.htm.

http://jpcindiansteel.nic.in/corex.asp Article by M. Gojic and S. Kozuh, Faculty of Metallurgy, University

of Zagreb, on “Development of Direct Reduction Processes and Smelting Reduction Processes for the Steel Production”.

Report by Lynn Price, Dian Phylipsen, Ernst Worrell on “Energy Use and Carbon Dioxide Emissions in the Steel Sector in Key Developing Countries”.