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Corex Process - One of the dynamic routes for gel making with special reference to the success of JVSL

Dr S K Gupta Dr S S Gupta

ABSTRACT

Research and Development work is being carried out in different countries for last several decades to develop alternative routes to iron making. The reason for such development comes from the fact that the conventional route of Blast furnace iron making uses metallurgical coal, which is scarce. Also because of associated sinter plant and coke oven, the route has become highly capital intensive to make its environment friendly. Smelting reduction is an emerging technology for making hot iron using non-coking. Till today various smelting reduction processes like CORER, ROMELT, HISMELT, DIOS, AUSMELT etc. have been developed of which COREX is the first and so far only commercially established smelting reduction process, which is developed by Voest Alpine Industrianlagenbau (VAI), Austria. The stable and highly successfull operation of four COREX plants (POSCO, Korea, JVSL, India, SALDANHA, South Africa) confirms that COREX process is a proven and viable alternative to conventional blast furnace technology.

INTRODUCTION

In the world, the chief source of metallic iron today is through Blast furnace, as the technology is most established, energy efficient, and versatile both technologically and economically. However due to the metallurgical coal to produce BF grade coke, and setting up of coke ovens which pollutes the environment with its NOx and SOx emissions, and rigid quality BF has become highly capital intensive. The economic, environmental and cost pressures led to the development of Smelting-Reduction processes like COREX, ROMELT, HISMELT, DIOS, AUSMELT etc. COREX is the only Smelting-Reduction process so far commercialised and in India has been adopted by Jindal Vijayanagar Steel Limited.

COREX PROCESS

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.

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 850oC 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 250oC. 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 l, 2: 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 600-800oC 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 10OOoC to 11OOoC, 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 (850oC) 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 take over point and is termed as COREX export gas l, 2,4.

The efficiency of the CORER 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

Table 1 gives the comparison of JSVL, Posco and Saldanha Plant JVSL POSCO SALDANAH Corex-1 Corex-2 Total Production, tonnes 797969 826475 1624444 626931 707002 Annualised Melting rate, t/hr 91.09 94.35 92.72 71.57 80.71 Fuel Rate, Kg/thm 1004 1005 1005 1154 1122

Table 2 shows the progress of COREX performance in JVSL Year Production, mt Sp.Fuel consumption, kg/thm Hot metal Temp., C `Si', % `S', % 1999-2000 0.40 1163 1491 0.75 0.060 2000-2001 0.77 1071 1503 0.66 0.037 2001-2002 1.52 1082 1497 0.58 0.037 2002-2003 1.46 1041 1497 0.71 0.029 2004-2003 (Till Jan' 04) 1.36 1000 1487 0.76 0.027

RESULTS AND DISCUSSION

Samples were taken from each stage of production. One test piece was cast from each heat, it was normalized and tested for chemical analysis trough direct reading spectrometer, metallographic studies like micro structure, grain size and inclusion rating, hardness in BHN, mechanical tests like UTS, YS, %EI and bend-rebend tests were carried out. Test pieces were cut from each heat of 4 sets of rolled products and each set was tested for microstructure, hardness. UTS, YS and %EI. The results have been presented in tabular form in Table-2 for inclusion rating and Table-3 through Table-8 for mechanical tests.

Some of the unique features of JVSL COREX operation

Undersized iron ore (size 6-12mm) is being charged directly into the COREX melter-gasifier. It was realized that the surplus heat available at the top of the char bed (stable bed inside melter-gasifier) could be utilized for reduction of iron ore fines. Addition of iron bearing material 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.

Oxygen distribution inside melter-gasifier

During the startup of COREX Module-1, the practice was to feed about 15-18% of the total oxygen through the melter-gasifier dust burners and the balance trough the tuyeres. With this operational practice, there had been some problems, such as high sludge rate, sudden rise in pressure (pressure peaks), gas channeling through the char bed, rise in melter-gasifier dome temperature, unstable hot metal quality etc. With improved understanding of the process, the oxygen amount through the dust burner was increased gradually and oxygen through tuyeres was reduced correspondingly and positive results were observed. Presently, about 25 - 30% of the total oxygen is passed through dust burners and the balance through the tuyeres. In this process a major share of the metallurgical reactions, such as, residual reduction, calcination etc. are carried out in the upper part of the char bed which results in stable char bed condition, reduction in sludge rate and better hot metal quality.

Selection of non-coking coals

JVSL is importing coals from Australia, South Africa and China. All these coals have been initially tested in the laboratory and later found to be suitable for COREX operation. Over last two years of operation, JVSL COREX has used more than ten different type of coals either individually or as a blend of two or three types of coals. Wide ranges of volatile matter (20-34%) and coal ash (9-12%) have been tried in CORER. Additionally, other physico - chemical properties such as the strength and the reactivity of the coal char also varied widely. A high volatile coal is blended with low volatile coals so as to maintain the total volatile content around 30%. On similar, lines, it is preferred to blend high ash content, with low ash coal to maintain a moderate content in the coal blend. The mean particle size of the coal blend has significant influence on the process and is closely monitored besides other quality parameters.

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: Use of BOF Slag Mill scale Limestone and Dolomite fines

TARGET AHEAD

Recycling of various by products and plant wastes

At JVSL COREX, trials have been undertaken with indigenous coal. The trials had been taken place with unwashed coal from Singareni collieries. Though the percentage ash in indigenous coal was high due to low AI203 in ash of the selected coal, there was not much increase in slag rate with controlled proportion of indigenous coal in the coal blend. It was observed that about 10-15% of indigenous coal in the blend did not have any operational problems. The trials with other Indian coals will be conducted.

CONCLUSION

The performance of COREX process indicates brighter future smelting-reduction process. JSVL has carried out a number of developments within a short span of operation and is constantly striving for further improvements. The present pace of developments would definitely pave the way towards the lower cost steel production.

Steam injection

Steam injection which is practiced at POSCO, is proposed to be introduced at JVSL in near future. The advantages will be, increased raceway size and thereby improved distribution of gas flow and decrease in RAFT, which will help in reducing frequency of tuyere burning. Up to 100 gms of steam per ton of hot metal can be injected.

Use of waste plastics

Plastics having high calorific value (-10,000 kcal/kg) can reduce the fuel rate and can be directly fed with coal unlike in blast furnace where it is injected. This would be economical, as the expensive injection equipment will not be used. About 100 kg of plastics can be used per ton of hot metal.