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COAL BASED SPONGE IRON INDUSTRY
By Mr.Ashok Pandit, Mr. B.M.Sarangi, Mr. A.Kesava Babu & Mr. M.K. Sheshadri
Sponge Iron has now succeeded in becoming a preferred raw material in secondary steel making. The growth of the sponge iron industry is an indicator to this acceptability both in India and outside India. (Figure 1 – 4)
There has been continuous research and development both in blast furnaces and electric arc furnaces in terms of productivity, efficiencies, quality and environment. Recently there has been a growing environmental awareness which is receiving due attention with stricter government regulations and controls. The issue of green house gas emissions is of growing importance in the world mandating that green field primary metal production facilities incorporate technologies with a lower green house gas generation potential. However, brown field steel plants striving to improve their competitiveness are adopting new measures to improve efficiencies, quality and meet with the new environmental standards. (Figure - 5)
In electric arc furnace steel making some of these technologies involve the increased use of chemical energy in order to reduce electrical energy requirements and to use alternative metallics such as sponge iron.
In integrated steel plants, the main polluters are sinter plants and coke ovens where emissions of CO2 is very high. At present, there are no alternative coke free iron ore reduction technologies capable of fully replacing the modern high performance blast furnaces. But the charging of sponge iron into blast furnaces does reduce coke consumption and increases hot metal productivity.
Pandit, Ashok is the Managing Director of Tata Sponge Iron Limited and has more than 34 years’ experience in steel & sponge iron industry in India and abroad. Sarangi, B.M was working as EIC(O) in Tata Sponge Iron Limited till November, 2002 and has more than 30 years’ experience in steel & sponge iron industry. Babu, Kesav is working as Sr.Divisional Manager (Operation) in Tata Sponge Iron Limited. Limited and has more than 21 years’ experience in sponge iron technology & commissioning of sponge iron plants. Sheshadri, MK is working as Chief Marketing Manager in Tata Sponge Iron Limited and has more than 15 years’ of experience in steel & sponge iron industry.
Evolution of the sponge iron industry :
Though the industrial application of sponge iron started early in the 1950s, it could not fulfill industry expectations at that time in being used as a supplementary material for feed / alternative to blast furnaces. The main reasons were –
The gas based sponge iron plants could operate only where cheap (subsidised) natural gas was made available.
Sponge iron furnaces could not be developed into a single entity to produce liquid metal.
Increased scrap availability that was being traded world wide as a furnace raw material.
Blast furnace capacities increased from 0.7 million tones per annum in 1960s to over 4 million tones per annum. Today the largest gas based sponge iron module available is approx. 2.7 million tones per annum.
However, as discussed earlier, the environmental requirements and the capital cost required to set up new integrated steel plants have their own draw backs. In light of this, coal based sponge iron technology now indicates a new direction, particularly for Indian steel making conditions.
Development of coal based sponge iron processes with particular reference to India :
A coal based sponge iron plant was first built in 1980 at a place called Paloncha in Andhra Pradesh which had a capacity of just 0.03 million tones per annum. In a span of 20 years the industry has grown rapidly and the present estimated capacity is nearly 4.5 million tones. The industry has become well developed and is presently operating in eight different States of India. For a direct reduction in the inclined rotary kiln, ore and coal will normally pass through the inclined kiln in a counter current direction to the oxidising flue gases in the freeboard. After material heating, ore reduction and carbon gasification take place in close association with each other. The volatile constituents of the coal and carbon mononoxide from the bed material are burnt, over the entire length of the kiln with a controlled amount of air, thereby providing the necessary heat required for the metallisation process. The rotary kiln discharge is cooled in a rotary cooler connected to the kiln, screened and subjected to magnetic separation in order to remove the non magnetics from the sponge iron.
Numerous key developments that have taken place which has made this technology more successful. Some of the key innovations are discussed below :
Capital requirement : In the initial years nearly 80% of the equipment required was being imported. Presently, large scale indigenization has taken place and for a 1,20,000 tpa module 100% indigenised equipment is available and for a 1,50,000 tpa module only the kiln tyres and support rollers have to be imported. This has reduced the capital expenditure related costs considerably as can be seen by the estimates given below:
The sponge iron plants can also be set up quite rapidly and it now takes just 18 months to go on stream for a major plant. It also has a very low gestation period.
Raw materials : The main raw materials required are suitable iron ore, suitable coal, dolomite and power :
Iron ore : The iron ore used is hematite with an Fe content of 62-66% having low decerepitation characteristics. In the initial days the iron ore size was kept at 5-20 mm and was washed in a scrubber, but presently it has become a standard norm to use 5-18 mm ore as feed for a large kiln without scrubbing and/or washing. This has resulted in reducing the cost of iron ore fed to the kiln. The consumption of iron ore has also decreased from about 1600 KG per tonne of sponge iron to 1500 KG levels mainly due to a better understanding of the process, improvements of the equipment and increased levels of automation.
Coal : Non-coking coal is being used having certain important parameters considered necessary for the direct reduction of iron ore viz. reactivity, ash softening temperature, caking and swelling indices and sulphur content, etc. In India the availability of these coals is very low due to Government monopoly even though abundant resources of non-coking coal are available. Initially, only ‘B’ grade coals were being consumed whose availability has now become scarce. The industry has successfully adopted measures to utilize ‘C’ and ‘D’ grade coals through better process control, installing raw material heating systems, shale picking belts and coal washing plants. With these measures the *coal cost has been reduced by nearly 20-30% when compared with the usage of ‘B’ grade coal.
Dolomite : Dolomite is mainly used as a desulphurising agent to prevent the pick up of sulphur by the sponge iron from the sulphur released by the burning of coal inside the furnace. The initial specifications for dolomite were 1-4 mm, later it was found that 4-8 mm dolomite was far more suitable by which the consumption can be reduced by 50%. This was mainly due to the fact that lot of dolomite fines were being lost to waste gases and with 4-8 mm fraction this loss was minimized.
Power : The initial plants were high power consuming units due mainly to the wet waste gas cleaning, relay operated drives with low levels of automation. The power consumption levels used to be 110-130 units per tonne of sponge iron, with the advent of a dry gas cleaning system (electro-static precipitator), programmable logic operated drives and computers replacing the giant panels, the power consumption has been curtailed to 80-90 units per tonne of sponge iron.
Yield : The yield levels have increased considerably. Thanks mainly to the secondary steel sector and induction furnaces for using sponge iron fines. This fraction mainly –1mm initially used to be contaminated with fine non-magnetics dust particles and was presumed not fit for use, but with development of powerful magnetic separators even this fraction has become useable. In fact, in induction furnaces, sponge iron fines are occasionally preferred over lumps due to their higher metallisation. Simpler layouts have also helped in minimizing material handling losses.
Campaign days/capacity utilization: In coal based sponge iron plants, campaign days are defined as the continuous operation of the kiln between two shutdowns of the kiln. Kiln shutdowns require a complete removal of materials from the kiln and cooling of the kiln for maintenance and accretion breaking. After completion of the shutdown activities, the kiln is initially lighted up with oil, after which coal feeding is started only when desired temperatures have been reached. Then Iron ore feeding is begun. The period from ore feed stop to the restart of ore feed is normally taken as the shutdown period.
During the initial years, when understanding of the process and characteristics of the available raw materials was low, the operating campaign days used to be generally less than 100 days per campaign. Over the years, this has increased more than 175 days in well established plants. As the campaign days increase in a given financial year, it automatically increases the capacity utilization of the plant. The capacity utilization levels which used to be at 85-90% has now consistently crossed 100% in well established plants.
Power generation capabilities : Coal based sponge iron technology has gained higher economic viability by its ability to generate a considerable quantity of electricity through use of hot waste gases and kiln waste (char) materials.
In coal based sponge iron technology, the furnace (rotary kiln) fulfils various functions. It is used as a conveying, mixing and charring unit, as a heat exchanger and as a reactor for coal gasification and iron ore reduction. The advantage of these applications from single equipment in the rotary kiln, is partly offset since basically, the kiln is considered to be a poor heat exchanger. This is due to the reduced contact of gas and solids when compared with a shaft furnace resulting in high waste gas energy losses. In coal based sponge iron kilns, depending on the quality of reductant (coal) used, about 60% of the total heat input is utilised in the reduction process. About 40% of the heat input is discharged with the kiln waste gases and the kiln materials in the form of sensible or chemical heat. The hot waste gas and char produced thus contain considerable energy saving potentials. After deducting the internal power consumption, approximately 400-500 kwh of electric energy (depending on the reduction agent used) can be produced per ton of Fe by utilising the heat content of the kiln waste gases. This energy can be used for reducing the total external power requirement of about 900 kwh/t of billets for melting sponge iron in electric arc furnaces or in induction furnaces under Indian conditions.
The cost of captive power generation through kiln waste gases in India will be similar to hydel power generation costs. The cost difference between external energy and internal energy has a direct influence on the price of steel produced.
By burning coal fines, coal washery rejects and the non-magnetic kiln discharge (char) in a fluidized bed boiler, steam can be generated which can in turn be used for power generation. At present in India such boilers are in successful operation at M/s Jindal Steel & Power Limited (Chattisgarh), M/s Prakash Industries Limited (Chattisgarh) and M/s Bihar Sponge Iron Limited (Jharkhand) and M/s Sunflag Iron & Steel Company (Maharashtra). Few more coal based sponge iron plants are contemplating the setting up of similar boilers, including Tata Sponge Iron Limited (Orissa).
The power generation capacity for different modules are as shown below (through waste kiln gases only).
At an average electricity cost of Rs.3/Kwh the savings that can be achieved and the additional revenue generation if the net surplus power can be sold even at Rs.2.5 per Kwh would be :
The power generation capabilities are high and economical such that a company which was primarily producing sponge iron and steel, can now also become a major producer of power and add ‘POWER’ to its name.
Steel making through the sponge iron route (min integrated steel complex) : A one million tonne per annum sponge iron complex would consist of –
7 kilns of 1,50,000 tpy modules or 8 kilns of 1,20,000 tpy modules.
The above plants would generate surplus power* as below :
* = Calculated for 330 days operation at 90% efficiency 1 = To utilise coal fines & char (power generation will be higher if coal washery rejects are used)
Assuming 900 units are required to convert one tonne of sponge iron into steel billets, the following quantities of steel can be generated through the surplus power available :
Module (tpa) Steel production (tpa)
1,50,000 x 7 5,19,000
1,20,000 x 8 5,10,000
As per the above, nearly 60% of sponge iron produced can be consumed inhouse for steel making.
Value addition through steel making:
More than 100% value addition can be achieved in a steel marketing complex consisting of
• Facilities for both sponge iron and finished steel products (long / products) (Figure-6)
• Coal washing facilities for sponge iron making
• Increased power generation by utilizing coal washery rejects
Utilisation of sponge iron in iron making:
As usual, Tata Steel has again emerged a pioneer in utilizing sponge iron in blast furnaces. The charging of this alternative iron units into blast furnaces has two important advantages –
• Reduction in coke rates
• Increase in hot metal productivity
The mechanism by which sponge iron (DRI/pre reduced iron) improves blast furnace performance has been well researched and published both in USA and Japan. The thermodynamics of using sponge iron in the blast furnace burden has been published by M/s Agarwal & Pratt (the thermodynamic aspects of using partially reduced burdens. TMS-AIME iron making proceedings, vol 24, 1965; pages 33-36). For an iron oxide burden, most of the combined oxygen is removed by reduction by the rising stream of carbon monoxide formed by the combustion of carbon at the blast furnace tuyers region and this process is called indirect reduction ( Reactions 1, 2 & 3).
Fe2O3+CO = 2FeO + CO2 - (1)
Fe3O4+CO = 3FeO + CO2 - (2)
FeO+CO = Fe+CO2 - (3)
About one half of the iron in the blast furnace burden is metallised by indirect reduction. The remaining burden is reduced by a reaction that gasifies carbon endothermically. In a blast furnace operation this reaction is known as a "solution loss reaction" and it occurs normally at +9000C (+16500 F). The chemical reactions are :
FeO+CO = Fe+CO2 - 3
CO2+C = 2CO - 4
FeO+C = Fe+CO - 5
In the above instances, a combination of reaction 3&4 takes places or Reaction 5 proceeds and requires a certain heat input. When sponge iron is used in which most of the iron has already been metallised and is included as a significant part of the blast furnace charge, then the amount of endothermic reaction is reduced. Therefore, less thermal energy is required and consequently there is an equivalent coke rate reduction. The hot metal production from a blast furnace is proportional to the amount of blast volume per ton of hot metal. Since the blast volume per ton of hot metal is approximately proportional to the coke rate, the use of sponge iron in the burden thus increases the productivity of the furnace. (Figure 7 – 8)
Coke was and is often considered as the required evil for the blast furnace operation. Coking plants require very high investments and pose considerable environmental problems. Coke has also become an internationally traded product with 25 million tones having been traded as export in the year 2000. In the long run, a world wide demand of 290 million tones blast furnace coke and sinter plant coke breeze is anticipated for the steel industry. A suitable way to counter act these shortages would be to decrease the coke consumption rate further in the blast furnaces, which can be achieved by charging sponge iron which is now freely available in India. (Figure –9)
Summing up, for establishing a green field integrated steel plant, particularly under Indian conditions, the DR-EAF route may be adapted; the basis is in comparison with a blast furnace, a single rotary kiln is a very small unit both from the capital and the production point of view. By installing a series of kilns one has the simultaneous advantage of low capital investment since part of the coke making process is eliminated and the plant produces as much iron as a medium sized blast furnace would. Rotary kilns also provide a high degree of flexibility with regard to changes in the product price, availability of raw materials and fluctuations in steel demand since the DR/EAF route can be adjusted faster, easier and in smaller steps when compared to the BF/BOF route. Already a few major steel producers in the country have taken to this route of steel making to add steel capacities. For existing integrated steel plants based on the conventional BF – BOF route, building captive DR units would be an easier and viable option to increase their capacities without building additional and expensive coke making facilities which also depend on imported coking coals.
1. Experience with SL/RN Rotary Kilns at ISCOR by J L Ventor / A K Oosthseizes
2. M P T Metallurgical Plant & Technology International April’2002.
3. Use of DRI in Ironmaking by Edward J.Ostrowski, Retired, National Steel Corporation, Robert L.Stephenson, Retired (Deceased), United States Steel Corporation and updated by Davind A.Kercsmar, KNC Engineers, Pittsburgh, Pennsylvania, USA.
4. Operation of a Commercial Blast Furnace with Sponge Iron by Dr.Keiji Tsujihata, Isao Mitoma, Yajiro Fukagawa and Shin Hashimoto.
5. Energy utilisation in direct reduction using rotary kiln by Lothar
Formanek, Lurgi GmbH, Frankfut/Main; Heinz Eichberger, Korf Lurgi Stahl
Engineering GmbH, Frankfurt/Main; and Harry Serbent , Lurgi GmbH,