Corex Process for Production of Iron
During the late twentieth century, several new initiatives have been taken for the development of the smelting reduction technology which can become alternative route for the production of liquid iron (hot metal) since the conventional blast furnace (BF) ironmaking depends on metallurgical coal, which is required for producing BF coke needed for the production of hot metal in the blast furnace. Metallurgical coal is not only costly but is associated with environmental issues during its conversion to BF coke in the coke oven batteries. Smelting reduction process is that process which is based on smelting reduction technology and hence in this process the production of hot metal is carried out without the use of metallurgical coke. Corex process is one of these initiatives. It is the first and the only commercially established smelting-reduction process based on non-coking coal which is available as an alternative route to the blast furnace for the production of hot metal.
Corex process was developed by the Austrian technology supplier VOEST in the late 1970s, and its feasibility was confirmed during the 1980s. The first pilot plant was installed in Kehl, Germany, in 1981. Commercialization, however, was reached together with the South African steelmaker ISCOR where the C-1000 (C – 0.5 M) module was commissioned in November 1989 at its Pretoria works. This first generation reactor which is called melter-gasifier had a hearth diameter of 5.5 m and a hot metal production rate ranging from 40 tons per hour to 60 tons per hour. The plant rated capacity was 300,000 metric tons per year. The general applicability of this first generation process was limited and a lot of technical problems had to be solved. Nevertheless, it helped to overcome the critical demonstration stage for this smelting reduction technology.
After the success of the smelting reduction technology was achieved through operation of C-1000 module, C-2000 (C-0.8 M) module was introduced by the process developer. This module has a hearth diameter of 7.3 m and a hot metal production rate ranging from 80 tons per hour to 100 tons per hour. The plant rated capacity is ranging from 600,000 tons per year to 800,000 tons per year. The first plant to be commissioned with this module was in 1995 at Pohang works of POSCO. The commissioning was in 1995. Since then one C- 2000 module has been commissioned in at Arcelor Mittal South Africa – Saldanha Works (December 1998), two modules at JSW Bellary plant in India (August 1999, and April 2000), and two modules at ESSAR plant at Hazira, India.
After the commercial success of C-2000 module, Corex process developer started offering also higher capacity modules namely (i) C-1.0 M module with a hearth diameter of 7.8 m and a hot metal production rate ranging from 100 tons per hour to 125 tons per hour and with a rated capacity is ranging from 800,000 tons per year to 1 million tons per year, (ii) C-1.3 M module with a hearth diameter of 9.4 m and a hot metal production rate ranging from 125 tons per hour to 160 tons per hour and with a rated capacity is ranging from 1.0 million tons per year to 1.3 million tons per year, (iii) C-3000 (C-1.5 M) module with a hearth diameter of 9.6 m and a hot metal production rate ranging from 160 tons per hour to 180 tons per hour and with a rated capacity is ranging from 1.3 million tons per year to 1.5 million tons per year, and (iv) C- 2.0 M module with a hearth diameter of 11.5 m and a hot metal production rate ranging from 210 tons per hour to 240 tons per hour and with a rated capacity is ranging from 1.7 million tons per year to 2.0 million tons per year. Of these higher capacity modules two numbers C-3000 (C-1.5 M) modules have been commissioned by Baosteel group China at their Luojing works in Shanghai one in November 2007 and the second in March 2011.
Today Corex technology is an acknowledged process for producing liquid hot metal in a quality which is identical to hot metal produced in a blast furnace. It is an industrially and commercially proven direct smelting reduction process which allows for cost-efficient and environmentally compatible production of hot metal directly from iron ore and non-coking coal. The process is the only alternative to the conventional blast furnace route consisting of sinter plant, coke oven and blast furnace.
Corex process distinguishes itself from the blast furnace route by (i) direct use of non-coking coal as reducing agent and energy source, (ii) Iron ore can be directly and feasibly charged to the process in form of lump ore, and pellets, and (iii) use of pure oxygen instead of nitrogen rich hot blast. Direct use of non-coking coal is possible since the coal is charred inside the melter gasifier. The high dome temperature exceeds 1,000 deg C which results into complete cracking of the hydrocarbons released by the non-coking coal and avoids the formation of tar. The typical ore burden for Corex is 30 % lump ore and 70 % pellets. Operational results have shown that stable operation is even possible with a lump ore fraction upto 80 %. Use of high purity oxygen in the Corex process generates of nitrogen free top gas. Due to its high calorific value, this gas can be recycled for reduction work or used for heat or energy generation.
The attractiveness of the Corex process is due to (i) hot metal quality suitable for steelmaking, (ii) low investment and operational costs due to the elimination of coke ovens and by product plant, (iii) low process related emission rates, (iv) use of a wide variety of iron ores and especially non-coking coals, and (v) generation of a highly valuable export gas which can be used for various purposes such as electric power generation, DRI production, or natural gas substitution.
The process description
The most innovative feature of the Corex process is the separation of the iron reduction and smelting operations into two separate reactors, namely reduction shaft and melter-gasifier. In the two-stage operation of the process DRI produced from a shaft furnace is charged into a melter-gasifier for smelting. In the melter-gasifier non coking coal is gasified by injecting oxygen and pre-reduced iron ore/ pellets are melted. The outgoing gas is used for reduction of iron ore/pellets in the upstream shaft furnace. The schematic process flow sheet is shown in Fig 1 and the schematic view of main Corex plant sections is shown in Fig 2.
Fig 1 Schematic flow sheet of Corex process
Fig 2 Schematic view of main Corex plant sections
Iron ore (lump ore, pellets, or a mixture thereof) is charged into the reduction shaft, where it is reduced to direct reduced iron (DRI) by the reduction gas in counter flow. The materials descend in the reduction shaft by gravity. The reduction gas around 800 deg C to 850 deg C and with more than 3 kg/sq cm pressure moves in the counter current direction to the top of the shaft and exits from the shaft at around 250 deg C to 300 deg C. The iron-bearing material is directly reduced to above 90 % metallization in the shaft, and is termed as DRI (direct reduced iron). The metallization degree of the DRI and the calcination of the additives are strongly dependent on four parameters namely (i) amount and quality of the reduction gas, (ii) temperature of the reduction gas, (iii) reducibility of the iron bearing burden, and (iv) average particle size and the distribution of the solids charged. Subsequently, the hot DRI (around 800 deg C) and partially calcined limestone and dolomite are discharged into the melter-gasifier from the reduction shaft via speed controlled discharge screw conveyors.
The melter-gasifier can largely be divided into three reaction zones namely (i) gaseous free board zone (upper part or dome), (ii) char bed zone (middle part above oxygen tuyeres), and (iii) 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 melter-gasifier operates at a pressure of 3 kg/sq cm to 5 kg/sq cm and comprises an upper fluidized bed area at around 1500 deg C and a lower melting and liquid collection area at around 1550 deg C. Non-coking coal, limestone, and quartzite are charged by means of a lock hopper system into the freeboard above the fluidized bed area where they are heated rapidly to 1000 deg C to1200 deg C. 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 volatile matter is driven off and shattered fixed carbon particles fall into the gasification zone where oxygen is injected through blast furnace type tuyeres to burn the carbon to carbon monoxide (CO). Injected oxygen gasifies the coal char and generates CO. The sensible heat of the hot gases is transferred to the char bed, which is utilized for melting iron and slag and other metallurgical reactions. The exothermic combustion provides the energy to complete the reduction of the hot DRI and to melt the slag and iron. Besides final reduction and melting, all other metallurgical reactions also take place in the melter-gasifier. Hot metal and slag are tapped as in conventional blast furnace practice.
Reducing gas for the shaft furnace is produced by partial combustion of coal with oxygen in the fluidized bed of the melter-gasifier. The gas at the temperature range of 1,050 deg C to 1,100 deg C from the melter-gasifier is cooled to the reduction gas temperature (800 deg C to 850 deg C) by the addition of cooling gas. The gas is cleaned in a hot cyclone to recycle entrained fines. A portion of the clean gas is then introduced into the shaft furnace as reducing gas containing more than 94 % CO plus H2 (hydrogen). The remaining gas is mixed with the cleaned off gas from the shaft furnace and the mixture is used as export fuel gas. The export fuel gas is also being known as Corex gas.
The calorific value of Corex gas is around 2000 kcal/N cum. The major components of Corex gas by volume are around carbon monoxide – 44 %, carbon dioxide – 30 %, hydrogen – 21 %, moisture – 1 %, nitrogen – 2 %, methane – 1.5 %, and dust content – less than 5 mg/N cum. The pressure of Corex gas supplied to various consumers is normally around 700 mm water column.
The heat and mass balance calculation for an ironmaking process plays an important role during the operation of Corex process. .
A recycling system consisting of a compressor station and CO2 removal makes it possible to utilize more export gas for metallurgical work. This raises export gas utilization to a higher level and improves the overall economics of the process besides making it more environmentally friendly. Due to this increased gas utilization which is based on gas recycling, gas production in the melter gasifier can be significantly lowered, which is directly reflected in lower fuel and oxygen consumption. With this slag production is also reduced by around more than 20 %.
The chemistry of the process
In a Corex process, the blast furnace concept is used but the blast furnace is virtually split into two halves at the cohesive zone interface (Fig 3). The process has three stages. The first stage of the process takes place in the reduction shaft, where iron ore burden is reduced by gases emanating from the melter-gasifier and is converted to hot DRI. Hot DRI is mechanically transferred to the melter-gasifier where the second and third stage of the process takes place. The second stage of the process consists of melting and carburizing of hot DRI by the coal and oxygen which is added in the meter-gasifier. In the process third stage which takes place in the upper part of the melter-gasifier, a fluidized bed of coal char is maintained. Here CO2 (carbon di oxide) and moisture (H2O) is converted to CO and H2. Since there is practically no CO2 or H2O in the gas leaving the melter-gasifier, the degree of post combustion of Corex gas is zero, resulting in a gas rich in chemical energy.
Fig 3 Comparison of concepts of blast furnace route and Corex route
The reactions taking place in the reduction shaft are as follows.
Reduction of iron burden by CO and H2 and its metallization in several stages. Fe2O3-> Fe3O4 -> FeO-> Fe.
Calcination of limestone and dolomite. CaCO3 = CaO + CO2 and CaCO3.MgCO3 = CaO.MgO + 2CO2.
Carbon deposition reaction and formation of iron carbide (Fe3C). 2CO = CO2 + C and 3Fe + 2CO = Fe3C+ CO2.
Out of the above mentioned reactions, reactions for the reduction of iron oxide by hydrogen and calcination are endothermic, while the reactions for the reduction of iron oxide by CO gas and carbon deposition are 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 reactions CaO + H2S = CaS + H2O and 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.
The reactions taking place in the melter-gasifier are as follows.
Drying of coal which takes place at 100 deg C.
Devolatilization of coal which takes place at the temperature range of 200 deg C to 950 deg C and liberation of methane (CH4) and higher hydrocarbons (CnHm).
Decomposition of volatile matter takes place due to the higher temperature prevailing in the melter-gasifier free board zone. In this area, the hydrocarbons are cracked into hydrogen and elementary carbon (CnHm = n C + (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. This is achieved by maintaining dome temperature in the range of 1050 deg C to 1100 deg C. Other reactions which take place in the free board zone are (i) CO2 + C = 2CO (Boudouard reaction), (ii) H2O + C = CO + H2 (water gas reaction), and (iii) CO + H2O = CO2 + H2 (shift reaction).
Calcination of the uncalcined limestone and dolomite.
Reduction of the residual iron oxide in the sponge iron.
Direct reduction of FeO in the DRI takes place by carbon in the char bed.
Combustion of coal char by oxygen takes place near the tuyeres since the maximum temperature inside the melter-gasifier exists in front of the tuyeres. The carbon gasification reactions which take place in the tuyeres area are (i) 2C + O2 = 2CO, (ii) 2CO + O2 = 2CO2, and (iii) C + CO2 = 2CO.
Melting of iron and slag and separation of hot metal and liquid slag.
Product characteristics and specific consumptions
Typical analysis of hot metal from the Corex process consists of carbon – 4.5 %, silicon – 0.5 %, manganese – 0.08 %, sulphur – 0.03 %, and phosphorus – 0.1 %. The hot metal temperature is around 1470 deg C to 1500 deg C.
Typical analysis of slag from Corex process consists of CaO – 35 %, MgO – 13 %, SiO2 – 31 %, Al2O3- 15 %, FeO – 0.4 %, TiO2 – 0.5 %, and S – 1.2 %. The temperature of liquid slag is around 1520 deg C to 1580 deg C.
Typical analysis of Corex process top gas consists of around CO – 42 %, H2 -19 %, CO2 – 31 % and CH4 – 1.9 %. Typical analysis of the reduction gas is around CO – 62 %, H2 – 23 %, CO2 – 9 %, and methane – 2 %. Typical analysis of export gas is around CO – 44 %, H2 – 21 %, CO2 – 30 %, moisture – 1 %, N2 – 2 %, CH4 – 1.5 %, and dust content – less than 5 mg/N cum.
Typical specific consumption figures in per ton of hot metal are around 940 kg for dry non-coking coal, 265 kg for additives and 520 cum for oxygen. The corresponding typical specific consumption figures with the recycling of export gas in per ton of hot metal are around 770 kg for dry non-coking coal, 185 kg for additives and 455 cum for oxygen. Typical generation figures in per ton of hot metal are around 340 kg for liquid slag and around 16, 500 thousands cum of export gas with a calorific value of around 1910 Kcal/N cum. The corresponding typical generation figures with the recycling of export gas in per ton of hot metal are around 265 kg for liquid slag, and around 14,100 thousands cum of export gas with a calorific value of around 1790 Kcal/N cum.
In the Corex process around 45 % of the total energy input is used for ironmaking and the rest goes to export fuel gas. The hot metal produced has carbon and silicon contents similar to blast furnace hot metal. However, nearly all of the sulphur in the non-coking coal enters the slag and hot metal. In this regards, organic sulphur in the coal gasifies and is absorbed by the DRI and returned to the melter-gasifier as iron sulphide.
Environment aspects of the process
Corex process captures most of the pollutants in an inert state in the slag and the released hydrocarbons are destroyed in the dome of the melter-gasifier. Further due to the in-situ coking of the coal in the melter-gasifier, a large portion of sulphur is captured in the slag, dramatically decreasing emissions of gaseous SO2 or H2S. Also, since pure oxygen is used instead of the hot air blast, Corex process significantly reduces nitrogen emissions in the form of NOx and provides the advantages with respect to dust emissions. The emission levels with the Corex process are much lower than the BF route of production which consists of blast furnace, sinter plant and the coke ovens.
Corex process emits at least 15 % less CO2 as compared to BF route for hot metal production. The air emissions are also lower than the conventional BF units. CO2 emission is around 1420 kg/tHM as compared to around 1900 kg/tHM for BF. SO2 emission is around 55 Kg/tHM as compared to around 1400 kg/tHM in BF route of production. Particulates and NOx emissions in the Corex process are around 10 % of the BF route of production. Phenols discharge is around 0.04 g/tHM, while in BF route of production it is 100 g/ tHM. Sulphides discharge is around 0.01 g/tHM while for BF route of production it is 180 g/ tHM. Ammonia discharge for Corex process is 60 g/tHM while for BF route of production it is 900 g/tHM. Cyanide discharges in the waste-water are completely eliminated in the Corex route.
Advantages and limitations of Corex process
Advantages of the Corex process include (i) reduction in the specific investment cost compared with conventional blast furnace route of production, (ii) lower production cost, (iii) better environmental performance because of lower emissions and discharges, (iv) higher calorific value of export gas makes it suitable for use in a wide range of applications, (v) flexible with regards to the raw material uses since a wide variety of iron ores and coals can be used, (vi) good operational flexibility with respect to production capacity, production stops and raw material changes.
Limitations of the Corex process include (i) optimized distribution of coal and DRI is needed in the melter-gasifier to avoid peripheral flow of hot gases, (ii) absence of post combustion results into the loss of the chemical energy in the export gases resulting into high consumption of coal, (iii) many of the equipments such as cooling gas compressor are maintenance oriented, (iv) transfer of hot DRI and recycling of the hot gas are hazardous especially during their maintenance periods, (vi) melter-gasifier is subjected to high occurrence of pressure peaks on account of use of raw coal with poor char bed conditions resulting into jamming of dust recycling systems as well as gas cleaning systems, (vii) sensitiveness of the process inputs quality parameters such as granulometry, percentage of fines in the inputs, decrepitation, and degradation behaviour of coal, iron ore and pellets at high temperature.