TECHNICAL ASPECTS OF MANUFACTURE OF SPONGE IRON
We have produced 55 million tons plus of steel in our country in the year 2008.This production of steel has come thro both conventional blast furnace route , electric arc/induction furnace route and thro new iron making technology COREX. Process. But for these newer developments, we would be requiring around 70 million tons of coking coal to produce 55 million tons of steel. We neither have resources nor the infrastructure to sustain this level of coking coal production.
Realizing these deficiencies in our country and in view of large reserves of high grade hematite or and vast reserves of non coking coal , Govt. Of India concentrated on developing alternate technologies to produce iron with non coking coal since 1970. Both NML and TATA STEEL have done basic groundwork in the sponge iron technology development and put up their pilot plant in 1972-1975. Subsequently UNIDO came forward in bringing SL/RN technology of sponge iron making to our country with LURGI GERMANY in1979. A demonstration sponge iron plant of 30000tpa capacity was set up with liberal assistance from UNIDO, Govt.of India and LURGI. This humble beginning has grown tremendously to 22 million tons of installed capacity today.
All the major sponge iron processes like SL/RN, ACCAR, CODIR,TDR,HYL and MIDREX have been adopted successfully with our raw materials and local conditions
We are all very proud as on today to say loudly the following achievements of our country during the last two decades:
We are the largest producer of sponge iron in the world since 2003 and maintaining the no 1 position till date
We have adopted all technologies of sponge iron making and optimized the performance with our own raw materials.
We are the largest producer of sponge iron using non coking coal as a reductant and and are having the largest coal based plant in the world.
We shall discuss in detail all technical aspects of coal base sponge iron making with particular reference to control of important process parameters to achieve quality sponge iron in this august gathering.
2.0 DESCRIPTION OF PROCESS:
Flow chart of SL/RN process has been shown in fig. 1.Sized Iron Ore and Non Coking Coal along with Dolomite are fed continuously to the charge end of Rotary Kiln inclined at 2.5%. Coal is also injected through an Injection Pipe at the discharge end of Kiln. Due to the inclination and rotation of Kiln the charge material moves along the length of Kiln where it is discharged continuously after processing. To supply heat for the process air is blown into the Kiln through central burner and air pipes mounted on the Kiln shell. As the charge moves along the kiln, it is heated by the gases which flow in opposite direction.
The first section, approximately half of kiln is called preheating zone where iron ore, coal and dolomite are dried and heated to reaction temperatures using the heat released from the combustion of volatile matter and carbon in the coal . The second half of kiln is called reduction zone where major amount of Oxygen contained in the iron ore is removed leaving metallic iron (Fe). De-sulphurisation in the Rotary Kiln is effected by calcined limestone/dolomite. The chemical reactions taking place at various zones in the rotary kiln are as given below:
C + 02 > CO2
CaCO3 > CaO + CO2
MgCO3 > MgO + CO2
C + CO2 > 2CO
3Fe2O3 + 3CO > 2Fe3O4 + 3CO2
Fe3O4 + CO > 3FeO + 3CO2
FeO + CO > Fe + CO2
FeS + Cao> FeO + CaS
The reduced iron ore -SPONGE IRON with unburnt char from rotary kiln gets discharged into a Rotary Cooler. The heat will be dissipated indirectly by water spraying into the outside of Cooler Shell. The material gets cooled to around 120 deg C.
The waste gases leaving rotary kiln pass through a dust settling chamber to an after burning chamber where the combustibles like carbon, soot particles are burnt out. The gases that are coming out of after burning chamber are cooled in an evaporation cooler to around 250 deg C and then led to ESP.
The product coming out of cooler is screened and magnetically separated as SPONGE LUMPS and SPONGE FINES.
RAW MATERIAL REQUIREMENTS:
All the Sponge Iron processes are very sensitive to the physical an chemical characteristics of raw materials. Extensive testing is required for evaluating the suitability of these raw materials for the manufacture of Sponge Iron.
Iron Ore:
Lump iron ore in the size range of 5-20mm can be used in all coal based rotary kiln processes. In gas based processes 70% iron ore pellets and 30% lump iron ore of 10-30mm are being used. In general high grade iron ore of 65% minimum Fe (total) with less decrepitating characteristics is preferred for use in rotary kiln.
Reductant:
Non coking coal is used as reductant in the rotary kiln process. As low ash ( 20 – 22% ) high. Fixed carbon content ( 45 – 50% ), medium volatile has been found to be most suitable for these processes. Other important characteristics which can not be over looked are the reactivity and ash softening temperature of the coal under reducing conditions, preferably 1250 deg C and above.
CONTROL OF PROCESS PARAMETERS:
In the operation of rotary kiln for direct reduction the important process parameters which require close monitoring and control are as follows:
-Feed rates of raw materials
-Temperature profile
-Air profile
-kiln revolution
-Gas pressure in the kiln
- Cooler discharge temperature
- Coal Injection
- Mean Particle Size
-Retention Time
Feed Rate of Raw materials:
After the initial heating of the kiln with oil and coal fines for a period of 12 to 18 hrs, the feeding of raw material commences with the feeding of Feed Coal. When the inlet temperature is stabilized at 800 deg C ,Iron ore feed is introduced at 60% of the design feed rate(4t/hr for 100TPD kiln) and then increased to 80% progressively with maintaining the inlet temperature more than 750 deg C. This method is to ensure stabilization of the process in the shortest period. With stabilized process conditions and consistent product quality , the feed rate of iron ore can be increased by 0.2t/hr at 8 hours interval to the design capacity in a period of two/three days. The aim shall be to achieve maximum thru put with stabilized process condition and minimum back flow. The feed rate of coal is a function feed rate of iron ore with fixed carbon/ iron ratio 0.45 to 0.50.The proportion of coal to be fed from the feed end and discharge end requires close attention (60%/40%), so that carbon starvation at region is prevented inside the kiln. The feed rate of limestone is based on the sulphur content of the coal and is generally at the rate of 3-6% of the iron ore feed rate.
The typical feed rates for 100TPD kiln
IRON ORE BELLARY 5-18MM 7.0TPH
FEED COAL SOUTH AFRICA 10-20MM 1.1TPH
FEED COAL SOUTH AFRICA 4-10MM 1.1TPH
INJ.COAL SOUTH AFRICA 0-4MM 1.3TPH
INJ.COAL SOUTH Africa 4-10mm 0.4TPH
Temperature Profile:
The temperature profile along the length of the kiln are adjusted so as to heat the iron ore to the reduction temperature with in short time and maintain this bed temperature to carry out the reduction to achieve the desired Fe metallic/degree of metallization. The temperature profile is unique for different iron ores depending up on the reducibility characteristics. In general the temperature profile of the material bed is from 750deg C to 900 deg C in the preheating zone (25% of the kiln lenth) and 950 deg C to 1050 deg C in the reduction Zone. The gas/bed temperature in the reduction zone shall be limited to 150-200 deg C lower than the softening temperature of coal ash under reducing conditions to minimize the accretion formation.
The typical temperature profile for 100tpd kiln;
Temp T1 T2 T3 T4 T5 T6 T7
REC 750 860 960 1020 1020 1030 1030
GAS 930 1030 1080 1080 1090 1100
BED 850 980 1020 1030 1040 1050
Air Profile:
The rotary kiln is equipped with 7 to 9 shell air fans and the air input is controlled by varying minutely the position of the damper . The total quantity of air injected varies from 1800 - 2500 Nm3/hour/ton of iron ore. The distribution of this air over the length of the kiln is adjusted carefully for each ore depending up on the reducibility and the decrepitating behaviour. The air profile is optimized to have very mild oxidizing conditions in the reduction zone for effective heat transfer and almost neutral atmosphere in the preheating zone for minimizing the heat losses in the waste gases. Minor adjustments in the air profile are done with regard to variation in quality level of the reluctant coal , keeping in view the temperature profile to be maintained. The increment/decrement of air at one time in one shell air fan shall be not more than 200-300 Nm3/hour.
The typical air profile based on anemometer readings for 100tpd kiln is as given below:
V1 V2 V3 V4 V5 V6 V7 CB LOBE
NM3/HR 3000 3500 3500 2500 2000 2500 2500 3200 1600
Gas Pressure in the kiln:
The gas pressure in the kiln is so regulated to eliminate any air ingress which shall lead to excess air conditions in the kiln. The gas pressure inside the kiln is controlled so as to have a slight positive pressure of the order of 0.3 to 0.5 millibar(3 to 5mmWC) at the kiln outlet. This is ensured by continuous monitoring of the damper of the I.D.fan/speed of the I.D.fan. High gas pressure of more than 1.5mbar at the kiln outlet shall affect the quality of the product.
Kiln Revolution:
The kiln revolution is an important process parameter with respect uniform Fe metallic of all thr grain sizes. It is regulated so as to have optimum retention time of the materials inside the kiln for carrying out the reduction and at the same time ensuring minimum back flow of the feed materials. Lower kiln rpm gives good preheating temperatures and higher kiln rpm gives longer campaign life. The optimum rpm for 100tpd kiln is 0.47-0.52, for 350tpd kilns 0.45 - 0.48, for 500tpd kilns 0.40 -0.44.
Temperature of the cooler discharge product:
The temperature of the cooler discharge product is maintained well below 120deg C. This will ensure sponge lumps temperature of 80degC and below to the product silo/bin. This temperature is significant in preventing thr reoxidation of the product during storage. In case of cooler discharge temperature of more than 150 deg C product shall be diverted separately and cooled in covered shed in open air by spreading layers of not more than 250mm thick.
Coal Injection:
In general a mix of fine and coarese coal 40% to 50% of the total coal requirement for the process is fed from the discharge end thro a coal injector. Coal is injected in such a way that fresh coal falls at various zones of the kiln so as to maintain the temperature profile. A good coal injection system consisting of separate weigh feeders for fine coal /coarse coals , a twin lobe blower(air cooled/water cooled) , rotary airlock feeder and properly aligned coal injection pipe to ensure coal throw up to start of reduction zone. This system ensures consistency in the product quality.
Mean Particle Size:
In general sized iron ore of 5-18mm is used in the process of manufacture of sponge iron on all types of rotary kiln with +16mm not exceeding 15-20%.The mean particle size of iron ore shall be restricted to 11mm in order to achieve uniform Fe metallic of all grain size fraction of the product. The mean particle size ratio of iron ore to feed coal shall be in the range of 1.2 to 1.5 depending up on the characteristics of coal.
Retention Time:
The time traveled by the materials in the kiln is taken as a reference for the assessment og the kiln condition.In case of a fresh kiln the retention time for a 100TPD kiln at 0.45rpm is 6-7 hrs,10-11hrs for a 350TPD kiln at 0.47rpm,.12-13hrs for 500tpd kiln at 0.43rpm.With the formation of coating the retention time increases progressively. The retention time can be easily determined by feeding 30mm size hexagonal nuts in feed conveyor and collecting the same at cooler discharge conveyor. Any abnormal increase in retention time clearly indicates severe accretion formation hindering the smooth flow of materials.
QUALITY OF THE PRODUCT:
The important constituents determining the quality of sponge iron are the Fe total, Fe metallic , sulphur, phosphorous, carbon and gangue content.
Fe total:
The Fe total content in sponge iron purely depends on Fe con tent in iron ore and the degree of reduction achieved in the process. Fe total content of 90%min is required for steel making requirement.
Fe metallic:
Fe metallic content in sponge iron depends on the effective control of process parameters
Like temperature profile, retention time , mean particle size of iron ore/coal. Fe metallic content of 78-82% is desirable for steel making. Fe metallic content of grain size fractions of 4-12mm,+12-16mm, +16mm should also be determined once in a week for control of of consistency in product quality.
Sulphur content:
The sulphur content in sponge iron is controlled by adding 1-6mm of limestone/dolomite along with feed materials which picks up sulphur from the coal preventing the sulphur pick up in sponge iron. the sulphur content in sponge iron can be controlled in the range 0.012 to 0,025%
Phosphorous content:
The phosphorous present in the iron ore is retained as P2O5 in sponge iron with out any change. The phosphorous content in sponge iron is generally varies from 0,06% to 0.09%
Gangue content:
As the removal of oxygen in iror ore takes place in solid state, the gangue content present in iron ore(mainly silica and alumina) remains in sponge iron with out any change. In general the gangue content in sponge iron varies from 4% to 8% .
Carbon content:
The carbon content in sponge iron produced by coal based rotay kiln processes are generally low and is in the range og 0.08% to 0.20%.The carbon content in gas based processes can be controllrd to a level of 1.5% to 2.0% based on requirement for steel making.
Size Distribution:
The size distribution of sponge iron lumps varies with decrepitating behavior of iron ores. As the Fe metallic content in sponge iron varies for each size fraction control of feed size of iron ore is very important for achieving uniform Fe metallic content. The typical size distribution and Fe metallic for each fraction is as given below.
+16MM +12MM +10MM +5MM -4MM
Fe metal 79.36 82.28 83.35 81.54 85.01
Fe total 90.00 91.31 92.55 91.76 95.55
Percent 4.21 14.74 27.37 33.68 20.00
In general -1mm fraction in sponge fines of 0-4mm shall be restricted to less than 30% and +12mm in sponge lumps of 4-18mm shall be restricted to 20-30%. for effective use in induction furnaces .
Material Balance and Energy Balance:
The typical material balance and energy balance for 100tpd kiln is as given below.
RECOMMENDATIONS :
Based on our experience an ideal coal based sponge iron plant shall have the following facilities.
As the manufacture of sponge iron is very sensitive to the physical and chemical characteristics of raw materials, a well designed raw material preparation plant with crushing/screening facility for iron ore and coal to control the size of input materials to the kiln is a prime requiste.
In order to have effective control and monitoring of process parameters, Direct reduction plant comprising rotary kiln, rotary cooler, waste gas cleaning system, product separation system, in plant dedusting system, PLC system, Measuring and control systems shall be equipped with standard equipments.
All records of the process parameters shall be maintained properly to serve as a reference for analysis and for future requirements.
COMPARISON BETWEEN GAS AND COAL BASED PROCESS
Gas Based Coal based
a) Iron Ore Requirements
Chemical:
Fe(T) % 67.0 min. 65-67
SiO2 + Al2 O2 % 3.0 max. 4.0 max.
S % 0.008 max. 0.03 max.
P % 0.03 max. 0.06 max.
Physical:
Size +10-35 mm +5-20 mm.
Tumbler index ( +6.3 mm) 90 % min. 85 % min.
b) Specific Consumption
Iron Ore / ton of DRI 1450 KGS 1500 KGS
Reductant / ton of DRI 2.5 Gcal 5.5 Gcal
Power KWH / ton of DRI 110 80
c) Analysis of Product
Fe(T) % 90-94 90-93
Fe(M) % 83-89 80-84
Metallisation % 92-95 88-92
C % 1.0 - 2.5 0.08-0.2
P % 0.05-0.09 0.04-0.06
Gangue % 3.0-5.0 3.0-5.0
Bulk density % 1.6-1.9 1.8-2.0
LIST OF INDIAN STANDARDS ON SPONGE IRON:
IS 10812: 1992 Classification of sponge iron/direct reduced iron(DRI) for steel
making.
IS 10852: 1984 Guidelines for storage and transportation of sponge iron/direct reduced iron(DRI)
IS 11092: 1991 Iron ore pellets for direct reduction process.
IS 11093: 1991 Iron ore lumps for direct reduction process.
IS 11284: 1985 Rotary tube test for iron bearing materials for the manufacture of
sponge iron/direct reduced iron(DRI).
IS 1285 : 1985 Various tests on raw materials to be used for coal based rotary kiln
direct reduction.
IS 11795: 1986 Guidelines for ash softening studies on solid reductants for direct
reduction.
IS 11860: 1986 Guidelines for non-coking coals for solid reductant base direct
reduction in rotary kilns.
IS 12052: 1987 Method for determination of re-oxidation rate of sponge iron/direct
reduced iron(DRI) in contact with humid air and in direct contact
with water.
IS 12263: 1987 Method for determination of rigidity of sponge iron/DRI during
transportation and handling.
IS 12381: 1987 Method for determination of coal(char) reactivity for direct reduction process.
IS 12668: 1989 Melting characteristics of sponge iron/direct reduced iron(DRI)
IS 12929: 1990 Feedstock (iron oxides and natural gas) for gas based direct
reduction-guidelines.
IS 13839: 1993 Specification for sponge iron/direct reduced (DRI) fines/briquettes
for steel making.
IS 13885: 1993 Non-magnetic/char in sponge iron – method of determination.
IS 13905: 1993 Hot briquette sponge iron(HBI) for steel making.
