About Durgapur Steel Plant-Introduction Set up in the late 50's with an initial annual capacity of one million tonnes of crude steel per year, the capacity of Durgapur Steel Plant (DSP) was later expanded to 1.6 million tonnes in the 70's. A massive modernization programme was undertaken in the plant in early 90's, which, while bringing numerous technological technological developments in the plant, enhanced the capacity of the plant to 2.088 million tonnes of hot metal, 1.8 million tonnes crude steel and 1.586 million tonnes saleable steel. The entire plant is covered under ISO 9001: 2000 quality management system. The modernized DSP now has state-of the-art technology for quality steel making. The modernized units have brought about improved productivity, substantial substantial improvement in energy conservation and better b etter quality products. DSPs Steel Making complex and the entire mills zone, comprising its Blooming & Billet Mill, Merchant Mill, Skelp Mill, Section Mill and Wheel & A xle Plant, are covered under ISO: 9002 quality assurance certification. With the successful commissioning commissioning of the modernized units, units, DSP is all set to produce 2.088 million tones of hot metal, 1.8 million tonnes of crude steel and 1.586 million tones of saleable steel annually. PRODUCT-MIX
TONNES/ANNUM
Merchant Products
2,80,000
Structural
2,07,000
Skelp
1,80,000
Wheels & Axles Semis Total Saleable steel
58,000 8,61,000 15,86,000
Iron ore, coal and limestone are the three basic raw materials for the steel industry. Durgapur Steel Plant draws its coal from the adjacent JhariaRanigunj coal belt. A good amount of prime coking coal, having fairly low ash content, is also imported. Bulk of iron ore lumps and fines come from the mines at Bolani in Orissa. Lime stone comes from a variety of sources: Birmitrapur Birmitrapur (Orissa), Jaisalmer (Rajasthan), ( Rajasthan), and Jukehi and Nandwara (Madhya Pradesh).
Raw Materials Handling To improve and ensure consistency in raw material quality, the facilities, which have been installed, are: y y y y y
Beneficiation/washing Beneficiation/washing facilities, both for lump ore and fines at Bolani Screening of lump iron ore inside the plant, Selective crushing of coal at Coal Handling Plant, Base blending facilities for Sinter Plant, Silo-cum- Blending bunkers
As part of the t he modernisation programme, new raw material handling storage and blending facilities with selective crushing of coal have been installed in order to ensure consistency in raw material quality. The beneficiation/washing beneficiation/washing facilities, both for lump ore and fines at Bolani, have a capacity to process 3.44 million tonnes (wet basis) per annum so as to be capable of catering to the entire requirement of the plant after modernization. Durgapur is the only steel plant in the country to have a coal washery at the plant site. Raw Materials Handling Complex Durgapur Steel Plant consumes about 7.4 million tonnes of different raw materials annually annually which comprises over 1.84 million tonnes of coal and 2.9 million tonnes of iron ore lump and fines. Besides the two major raw materials, the plant also requires limestone, dolomite, manganese ore, bauxite, silico manganese, ferro manganese, ferro silicon, etc. The coke ovens and coal chemicals zone is divided into four basic sections namely coal preparation plant, coal carbonisation plant, coke handling plant and coal chemicals. Presently, DSP is operating only three batteries. The Blast Furnace grade coke produced in Coke Ovens is directly used in Blast Furnaces while the undersized coke is used for sinter making. The volatile matters, which emanate during the process of coke making subsequently subsequently produce a variety of by-products like naphthalene oil, heavy creosote oil, light oil, crude tar partially distilled tar, Raja brand fertilizer, fertilizer, nitration grade benzene, nitration grade toluene, industrial grade toluene, light solvent naphtha etc. The coke oven gas is generally used in combination with the Blast Furnace gas and BOF gas as fuel and is carried through pipelines to the different areas of the plant. The adjoining Alloy Steels Plant under SAIL is also supplied with this fuel gas from DSP.
Raw Materials Handling To improve and ensure consistency in raw material quality, the facilities, which have been installed, are: y y y y y
Beneficiation/washing Beneficiation/washing facilities, both for lump ore and fines at Bolani Screening of lump iron ore inside the plant, Selective crushing of coal at Coal Handling Plant, Base blending facilities for Sinter Plant, Silo-cum- Blending bunkers
As part of the t he modernisation programme, new raw material handling storage and blending facilities with selective crushing of coal have been installed in order to ensure consistency in raw material quality. The beneficiation/washing beneficiation/washing facilities, both for lump ore and fines at Bolani, have a capacity to process 3.44 million tonnes (wet basis) per annum so as to be capable of catering to the entire requirement of the plant after modernization. Durgapur is the only steel plant in the country to have a coal washery at the plant site. Raw Materials Handling Complex Durgapur Steel Plant consumes about 7.4 million tonnes of different raw materials annually annually which comprises over 1.84 million tonnes of coal and 2.9 million tonnes of iron ore lump and fines. Besides the two major raw materials, the plant also requires limestone, dolomite, manganese ore, bauxite, silico manganese, ferro manganese, ferro silicon, etc. The coke ovens and coal chemicals zone is divided into four basic sections namely coal preparation plant, coal carbonisation plant, coke handling plant and coal chemicals. Presently, DSP is operating only three batteries. The Blast Furnace grade coke produced in Coke Ovens is directly used in Blast Furnaces while the undersized coke is used for sinter making. The volatile matters, which emanate during the process of coke making subsequently subsequently produce a variety of by-products like naphthalene oil, heavy creosote oil, light oil, crude tar partially distilled tar, Raja brand fertilizer, fertilizer, nitration grade benzene, nitration grade toluene, industrial grade toluene, light solvent naphtha etc. The coke oven gas is generally used in combination with the Blast Furnace gas and BOF gas as fuel and is carried through pipelines to the different areas of the plant. The adjoining Alloy Steels Plant under SAIL is also supplied with this fuel gas from DSP.
In order to enhance the productivity of blast furnaces, a high percentage of sinter charge is a prerequisite. Sinter is an agglomeration of iron ore fines, coke and limestone in the form of cakes. To ensure sinter burden in the blast furnaces at 75 per cent, a total of 3 million tonnes of sinter was envisaged for a production of about 2 million tonnes of hot metal. A technologically technologically modern and fuel efficient sintering machine having 198 sq metres sintering area has been added as part of the modernization scheme to produce 1.7 million tonnes of sinter. The balance requirement will be met from the revamped old sinter plant. Sinter mix, a mixture of fines of iron ore, limestone, coke, dolomite and flue dust, blended proportionally proportionally at the RMHC, is a prepared material which is self fluxing. In ignition strands it is burnt under controlled conditions to form a porous cake type substance called sinter, which used in blast furnaces enhances productivity productivity and reduces coke rate.
Steel Melting Shop Mixers - 2 x 1, 300 t Converters - 3 x 110 t (nominal heat size) Molten iron is further refined at the Steel Melting Shop (SMS) to produce steel, which is hard and malleable. At DSP, there are 3 converters (Basic Oxygen Furnace) of 110-130 tonnes each. The SMS also has a Vacuum Arc Degassing (VAD) unit for making special grades of steel. A major portion of the steel is r outed through the Continuous Casting Plant. Another major portion of the steel is taken to the teeming bay, where it is top poured into 8 tonne ingot moulds for making ingot steel. A portion of highly controlled controlled steel is cast at a t the Special Casting Bay into fluted ingots and special quality blooms. Fluted ingots are bottom poured and are used for making wheel steel for DSPs Wheel & Axle Plant. A portion of the liquid steel is also bottom poured to make axle ingots. Continuous Casting Plant The state of the art CCP has 2 Nos machines having 6 strands each. The other basic details are as follows: Design limits- 80-150 sq .mm, casting radius- 6 metres Casting time 85 minutes, Cut-off lengths- 6 / 9 / 12 metre No of ladle treatment stations-2 Mould level controller - Automatic (Radio-active Co-60)
The steel ladle from BOF is taken to the ladle treatment station. At the ladle treatment station, liquid steel is rinsed with nitrogen to homogenise its temperature and composition. After the rinsing, the ladle containing liquid steel is placed on the turret and brought over the tundish. The tundish acts as a buffer and enables the liquid steel to move homogeneously down through the six nozzles, provided at the bottom of the tundish into moulds. The automatic mould level controller controls the steel level in the mould. The subsequent primary and secondary cooling transforms the l iquid steel into billets of the required dimensions and is drawn out with the help of a withdrawal and straightener unit and cut into the required length by the shear provided in each strand. The continuous casting process is the result of a unique synchronization between Basic Oxygen Furnace and CCP. Once a ladle is emptied, another ladle is brought into casting position and the casting continues. The billets are gradually shifted to the cooling beds and then stacked orderly at the dispatch end for outside dispatch. The details about the cast number and quality of the billets are marked on the billet stack. The Merchant Mill of Durgapur Steel Plant utilises billets for rolling TMT bars and other merchant rounds, while a sizeable portion is sold in the d omestic and foreign markets.
Ingots weighing 8 tonnes each are heated in the soaking pits (numbering 20) for about 7 to 12 hours at around 1, 200 degrees centigrade and thereafter rolled in the 42 primary and the 32 secondary blooming mills. These are rolled further into different shapes and sizes in different finishing mills. Blooming mill Installed Mill capacity - 1.47 million tonnes/year Ingot weight - 8 tonnes 42" Mill: 42" x 102" reversible Blooming Mill Output bloom size (min) - 300 mm x 250 mm 32" Mill: 32" x 84" reversible Intermediate Mill Output bloom size (min) - 180 mm x 180 mm Billet Mill Installed Mill capacity - 0.957 tonnes / yr. Type - Continuous Morgan design Horizontal stands - 6, Vertical stands - 2 Product Range Billets
- 100 mm square to 125 mm square
Sleeper bars
- 352 mm x 12.5 mm
Skelp slabs
- 140 mm x 75 mm to 240 mm x 90 mm
The ingots after heating are rolled in the Blooming Mill to make blooms of the sizes mentioned in the table and then a part of the same are then further rolled in the Billet Mill for making rolled billets or slabs as per the above details. Section mill The Section Mill rolls out light and medium structural like joists, channels and angles. Mill capacity
- 0.2 million tonnes / year
Re-heating furnaces
- 2 x 40 t/hr
Roughing Mill
- 2 high reversible
Intermediate Mill
- 2 stands of 3 high non-reversible
Finishing Stand
- 2 high non-reversible
Product range: Joists
-
200 mm x 100 mm, 175 mm x 85 mm 150 mm x 75 mm, 116 mm x 100 mm
Channels
-
200 mm x 75 mm, 175 mm x 75 mm 150 mm x 75 mm, 125 mm x 65 mm
Angels
-
150 x 150 mm, 130 x 130 mm 110 x 110 mm, 100 x 100 mm
Fish plate bars for 52 kg rails
Merchant Mill The Merchant Mill produces plain round and Thermo-Mechanically Treated (TMT) bars in the range of 16mm - 28mm. The entire product range of TMT bars and rods at DSP is branded and has been able to create a niche market. Capacity
- 0.28 million tonnes / year
Type of mill
- continuous Morgan design
Horizontal stands
- 13, Repeaters - 4
Product range Plain rounds
- 12 - 32 mm dia
TMT bars
- 12 - 25 mm dia
Skelp Mill The Skelp Mill produces skelp in the range of 146 to 235 mm primarily for tubes and pipes making industry. Capacity
- 0.25 million tonnes / year
Type of Mill
- Continuous Loewy design
Horizontal stands
- 11
Vertical stands
- 6
Product range Strips & Skelps - 75-242 mm wide to 1.47-2.34 mm thick
Durgapur Steel Plant is the only major indigenous supplier of wheel sets, loco wheels, carriage and wagon wheels, and axles to the Indian Railways. As per demand of the Railways, the plant has developed loco wheels, which were imported earlier. The Wheel & Axle Plant is producing wheels manufactured as per the latest IRS specifications, i.e. R-19/93 for carriage and wagon wheels, R-34/99 for loco wheels and R-16/95 for axles. The wheel plant of the Wheel & Axle Plant is provided with six PLC c ontrolled band saws for accurate slicing of the 14 and 16 fluted ingots. A fully computerized 63/12 MN oil hydraulic press is there for forging and punching of the wheel blanks along with a fully computerized vertical wheel mill and other down stream facilities. All the wheels are 100 per cent rim-quenched, tempered and tested as per IRS specifications. Machining of these forged rolled and heat-treated wheel blanks are carried out in the 15 CNC machines. All the wheels are ultrasonically tested and inspected by RITES on behalf of the Indian Railways. A number of sophisticated and modern online testing facilities are there to conform to the stringent testing requirements of the Indian Railways. Wheel & Axle Plant Annual production of finished wheels
- 1,00,000 nos.
Production rate in rolling/forging
- 25 nos./hr
Production rate in machining
- 22 nos./hr
Durgapur Steel Plant has a number of captive engineering shops and supply of spare parts. The Central Engineering Maintenance Shop, Structural Shop, Fitting and Assembly Shops. The Foundry moulds and bottom plates for the steel melting shop. There are Repair Shops such as Electrical, Wagon and Loco repair.
for repairs has a Machine produces Ingot also Auxiliary
The Research & Control laboratories are entrusted with the responsibility of maintaining quality of products and also developing new products. It is well equipped for carrying out sophisticated chemical, metallurgical and other tests. An extensive computerization has been undertaken in DSP for personnel, commercial, process control, production and maintenance applications. The Production Planning and Control network is thoroughly used for tracking of customer orders, material, monitoring of quality parameters and ensuring availability of accurate, real time data to all agencies needing access to the data. In order to be fully competitive on the quality front, Durgapur Steel Plant has set out to acquire ISO 9000 c ertification for all its units. The Merchant Mill is the first to secure the p restigious ISO 9002 certificate. Subsequently, steel melting shop, basic oxygen furnace shop, continuous casting plant, and wheel and axle plant were also awarded the ISO 9002 certification and recently the Skelp Mill has been awarded the ISO 9002 certification. -----------------------------------------------------------------------------
Layout
of an integrated steel plant
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Report on Industrial Training at Durgapur Steel Plant, Durgapur From 28-6-2010 to 10-07-2010(Two Weeks)
Prepared by: Siddhartha Sinha Roll No- 075515 Department of Chemical Engineering Heritage Institute of Technology, Kolkata
Table of Contents
Topic 1. Introduction-About DSP 2. Basic of an integrated steel plant 3. Coke ovens 4. Basic oxygen furnace 5. Blast furnace 6. Oxygen plant
Pg.No.
The Coke Ovens
A coke oven battery Coking coals are the coals which when heated in the absence of air, first melt, go in the plastic state, swell and resolidify to produce a solid coherent mass called coke. When coking coal is heated in absence of air, a series of physical and chemical changes take place with the evolution of gases and vapours, and the solid residue left behind is called coke. Coke is used in Blast Furnace (BF) both as a reductant and as a source of thermal energy. Blast furnace operation demands the highest quality of raw materials, operation, and operators. C oke is the most important raw material fed into the blast furnace in terms of its effect on blast furnace operation and hot metal quality. Introduction of high quality coke to a blast furnace will result in lower coke rate, higher productivity and lower hot metal cost . Desired properties of coke1. 2. 3. 4. 5.
Should descend into the blast furnace smoothly without degradation. Lowest level of impurities Highest possible thermal energy Highest metal reducing capability Optimum permeability for flow of gaseous products upwards and molten products downwards.
6. Ability to withstand breakage at room temperature. 7. High coke strength after reaction with CO 2 (CSR) 8. Large mean size and narrow size variations, stable void fraction. The Durgapur Steel Plant obtains the coal partly from India and mostly from abroad due to dwindling reserves in India, also because of poor quality of Indian coal. Coal comes from Australia and Canada mainly. The domestic coal is obtained from coal mines in Sudamdih, Pathardih, Giridih, Jharia etc.
Coke productionThe entire coke making operation is comprised of the following steps: Before carbonization, the selected coals from specific mines are blended , pulverized , and oiled for proper bulk density control. The blended coal is charged into a number of slot type ovens wherein each oven shares a common heating flue with the adjacent oven. Coal is carbonized in a reducing atmosphere and the off-gas is collected and sent to the by-product plant where various by-products are recovered. Hence, this process is called by product coke making. The coal-to-coke transformation takes place as followsThe heat is transferred from the heated brick walls into the coal charge. From about 375°C to 475°C, the coal decomposes to form plastic layers near each wall. At about 475°C to 600°C, there is a marked evolution of tar, and aromatic hydrocarbon compounds, followed by resolidification of the plastic mass into semi-coke. At 600°C to 1100°C, the coke stabilization phase begins. This is characterized by contraction of coke mass, structural development of coke and final hydrogen evolution. During the plastic stage, the plastic layers move from each wall towards the center of the oven trapping the liberated gas and creating in gas pressure build up which is transferred to the heating wall. Once, the plastic layers have met at the center of the oven, the entire mass has been carbonized. The incandescent coke mass (see figure) is pushed from the oven and is wet or dry quenched prior to its shipment to blast furnace.
Incandescent coke waiting to be pushed Some modifications in coke making to improve yield and reduce energy consumption-
Partial Briquetting of Coal about 30% coal blend in the a binder (pitch/ pitch+tar) significantly improves as a
Charge (PBCC): The technology involves charging form of briquettes. Briquettes are prepared using upto 2 to 3.0% of charge. Coke quality result of increase in bulk density of charge.
Stamp Charging of Coal: The technology basically involves formation of a stable coal cake with finely crushed coal (88-90% - 3mm) by mechanically stamping outside the oven and pushing the cake thus formed inside the oven for carbonization. Coal moisture is maintained at 8-10% for the formation of cake. Due to stamping, bulk density of charge increases by 30-35% causing significant improvement CSR values of coke. Oven productivity increases by 10-12% & there is a possibility of using inferior coking coals to the extent of about 20%. Selective Crushing of Coals: In this technology, the aim is to improve homogeneity of reactive & inert components in coal by reducing the difference properties of coarse & fine size fractions. For petrographically heterogeneous coals like Indian coals, this technology is very helpful. Dry Coke Quenching: Dry quenching of coke is a major technology for the postcarbonization treatment which has come up in a big way. Here the red-hot coke is cooled by inert gases, instead of conventional water quenching. It not only effectively utilises the thermal energy of red-hot coke (80% of the sensible heat of coke can be recovered & made use of for production of steam) but also results in improvement of the coke quality. Factors affecting quality of cokeA good quality coke is generally made from carbonization of good quality coking coals. Coking coals are defined as those coals that on carbonization pass through softening, swelling, and resolidification to coke. One important consideration in selecting a coal blend is that it should not exert a high coke oven wall pressure and should contract sufficiently to allow the coke to
be pushed from the oven. The properties of coke and coke oven pushing performance are influenced by following coal quality and battery operating variables: rank of coal, petrographic, chemical and rheologic characteristics of coal, particle size, moisture content, bulk density, weathering of coal, coking temperature and coking rate, soaking time, quenching practice, and coke handling. Coke quality variability is low if all these factors are controlled. Coke producers use widely differing coals and employ many procedures to enhance the quality of the coke and to enhance the coke oven productivity and battery life. Bye-products of the coke oven.The
Blast Furnace grade coke produced in Coke Ovens is directly used in Blast Furnaces while the undersized coke is used for sinter making. The volatile matters, which emanate during the process of coke making subsequently produce a variety of by-products like naphthalene oil, heavy creosote oil, light oil, crude tar partially distilled tar, Raja brand fertiliser, nitration grade benzene, nitration grade toluene, industrial grade toluene, light solvent naphtha etc. The coke oven gas is generally used in combination with the Blast Furnace gas and BOF gas as fuel and is carried through pipelines to the different areas of the plant. The adjoining Alloy Steels Plant under SAIL is also supplied with this fuel gas from DSP.
No of batteries No. of ovens per battery
-3½ - 78
The coke ovens and coal chemicals zone is divided into four basic sections namely coal preparation plant, coal carbonisation plant, and coke handling plant and coal chemicals. Presently, DSP is operating only three batteries i.e. 273 ovens. Schematic diagram of a coke oven plant
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The Blast Furnace The purpose of a blast furnace is to chemically reduce and physically convert iron oxides into liquid iron called "hot metal". The blast furnace is a huge, steel stack lined with refractory brick, where iron ore, coke and limestone are dumped into the top, and preheated air is blown into the bottom. The raw materials require 6 to 8 hours to descend to the bottom of the furnace where they become the final product of liquid slag and liquid iron. These liquid products are drained from the furnace at regular intervals. The hot air that was blown into the bottom of the furnace ascends to the top in 6 to 8 seconds after going through numerous chemical reactions. Once a blast furnace is started it will continuously run for four to ten years with only short stops to perform planned maintenance.
Diagram showing process inside blast furnace
The Process inside Blast Furnace: Iron oxides can come to the blast furnace plant in the form of raw o re, pellets or sinter. The raw ore is removed from the earth and sized into pieces that range from 0.5 to 1.5 inches. This ore is either Hematite (Fe 2O3) or Magnetite (Fe 3O4) and the iron content ranges from 50% to 70%. This iron rich ore can be charged directly into a blast furnace without any further processing. Iron ore that contains a lower iron content must be processed or beneficiated to increase its iron content. Pellets are produced from this lower iron content ore. This ore is crushed and ground into a powder so the waste material called gangue can be removed. The remaining iron-rich powder is rolled into balls and fired in a furnace to produce strong, marble-sized pellets that contain 60% to 65% iron. Sinter is produced from fine raw ore, small coke, sand-sized limestone and numerous other steel plant waste materials that contain some iron. These fine materials are proportioned to obtain desired product chemistry then mixed together. This raw material mix is then placed on a sintering strand, which is similar to a steel conveyor belt, where it is ignited by gas fired furnace and fused by the heat from the coke fines into larger size pieces that are from 0.5 to 2.0 inches. The iron ore, pellets and sinter then become the liquid iron produced in the blast furnace with any of their remaining impurities going to the liquid slag. The coke is produced from a mixture of coals. The coal is crushed and ground into a powder and then charged into an oven. As the oven is heated the coal is cooked so most of the volatile matter such as oil and tar are removed. The cooked coal, called coke, is removed from the oven after 18 to 24 hours of reaction time. The coke is cooled and screened into pieces ranging from one inch to four inches. The coke contains 90 to 93% carbon, some ash and sulfur but compared to raw coal is very strong. The strong pieces of coke with a high energy value provide permeability, heat and gases which are required to reduce and melt the iron ore, pellets and sinter. The final raw material in the iron making process is limestone. The limestone is removed from the earth by blasting with explosives. It is then crushed and screened to a size that ranges from 0.5 inch to 1.5 inch to become blast furnace flux. This flux can be pure high calcium limestone, dolomitic limestone containing magnesia or a blend of the two types of limestone. Since the limestone is melted to become the slag which removes sulfur and other impurities, the blast furnace operator may blend the different stones to produce the desired slag chemistry and create optimum slag properties such as a low melting point and a high fluidity. All of the raw materials are stored in an ore field and transferred to the stockhouse before charging. Once these materials are charged into the furnace top, they go through numerous chemical and physical reactions while descending to the bottom of the furnace.
The iron ore, pellets and sinter are reduced which simply means the oxygen in the iron oxides is removed by a s eries of chemical reactions. These reactions occur as follows: 1) 3 Fe2O3 + CO = CO2 + 2 Fe3O4
Begins at 850° F
2) Fe3O4 + CO = CO 2 + 3 FeO
Begins at 1100° F
3) FeO + CO = CO 2 + Fe or FeO + C = CO + Fe
Begins at 1300° F
At the same time the iron oxides are going through these purifying reactions, they are also beginning to soften then melt and finally trickle as liquid iron through the coke to the bottom of the furnace. The coke descends to the bottom of the furnace to the level where the preheated air or hot blast enters the blast furnace. The coke is ignited by this hot blast and immediately reacts to generate heat as follows: C + O2 = CO2 + Heat Since the reaction takes place in the presence of excess carbon at a high temperature the carbon dioxide is reduced to carbon monoxide as follows: CO2+ C = 2CO The product of this reaction, carbon monoxide, is necessary to reduce the iron ore as seen in the previous iron oxide reactions. The limestone descends in the blast furnace and remains a solid while going through its first reaction as follows: CaCO3 = CaO + CO2 This reaction requires energy and starts at about 1600°F. The CaO formed from this reaction is used to remove sulfur from the iron which is necessary before the hot metal becomes steel. This sulfur removing reaction is: FeS + CaO + C = CaS + FeO + CO The CaS becomes part of the slag. The slag is also formed from any r emaining Silica (SiO2), Alumina (Al 2O3), Magnesia (MgO) or Calcia (CaO) that e ntered with the iron ore, pellets, sinter or coke. The liquid slag then trickles through the coke bed to the bottom of the furnace where it floats on top of the liquid iron since it is less dense. Another product of the ironmaking process, in addition to molten iron and slag, is hot dirty gases. These gases exit the top of the blast furnace and
proceed through gas cleaning equipment where particulate matter is r emoved from the gas and the gas is cooled. This gas has a considerable energy value so it is burned as a fuel in the "hot blast stoves" which are used to preheat the air entering the blast furnace to become "hot blast". Any of the gas not burned in the stoves is sent to t he boiler house and is used to generate steam which turns a turbo blower that generates the compressed air known as "cold blast" that comes to the stoves. In summary, the blast furnace is a counter-current realtor where solids descend and gases ascend. In this reactor there are numerous chemical and physical reactions that produce the desired final product which is hot metal. A typical hot metal chemistry: Iron
(Fe)
= 93.5 - 95.0%
Silicon (Si)
= 0.30 - 0.90%
Sulfur (S)
= 0.025 - 0.050%
Manganese (Mn)
= 0.55 - 0.75%
Phosphorus (P)
= 0.03 - 0.09%
Titanium (Ti)
= 0.02 - 0.06%
C arbon
(C )
= 4.1 - 4.4%
Diagram showing different equipments of a BF plant There is an ore storage yard that can also be an ore dock where boats and barges are unloaded. The raw materials stored in the ore yard are raw ore, several types of pellets, sinter, limestone or flux blend and possibly coke. These materials are transferred to the "stockhouse/hiline" (17) complex by ore bridges equipped with grab buckets or by conveyor belts. Materials can also be brought to the stockhouse/hiline in rail hoppers or transferred from ore bridges to self-propelled rail cars called "ore transfer cars". Each type of ore, pellet, sinter, coke and limestone is dumped into separate "storage bins" (18). The various raw materials are weighed according to a certain recipe designed to yield the desired hot metal and slag chemistry. This material weighing is done under the storage bins by a rail mounted scale car or computer controlled weigh hoppers that feed a conveyor belt. The weighed materials are then dumped into a "skip" car (19) which rides on rails up the "inclined skip bridge" to the "receiving hopper" (6) at the top of the furnace. The cables lifting the skip cars are powered from large winches located in the "hoist" house (20). Some modern blast furnace accomplish the same job with an automated conveyor stretching from the stockhouse to the furnace top. At the top of the furnace the materials are held until a "charge" usually consisting of some type of metallic (ore, pellets or sinter), coke and flux (limestone) have accumulated. The precise filling order is developed by the
blast furnace operators to carefully control gas flow and chemical reactions inside the furnace. The materials are charged into the blast furnace through two stages of conical "bells" (5) which seal in the gases and distribute the raw materials evenly around the circumference of the furnace "throat". Some modern furnaces do not have bells but instead have 2 or 3 airlock type hoppers that discharge raw materials onto a rotating chute which can change angles allowing more flexibility in precise material placement inside the furnace. Also at the top of the blast furnace are four "uptakes" (10) where the hot, dirty gas exits the furnace dome. The gas flows up to where two uptakes merge into an "offtake" (9). The two offtakes then merge into the "downcomer" (7). At the extreme top of the uptakes there are "bleeder valves" (8) which may release gas and protect the top of the furnace from sudden gas pressure surges. The gas descends in the downcomer to the "dustcatcher", where coarse particles settle out, accumulate and are dumped into a railroad car or truck for disposal. The gas then flows through a "Venturi Scrubber" (4) which removes the finer particles and finally into a "gas cooler" (2) where water sprays reduce the temperature of the hot but clean gas. Some modern furnaces are equipped with a combined scrubber and cooling unit. The cleaned and cooled gas is now ready for burning. The clean gas pipeline is directed to the hot blast "stove" (12). There are usually 3 or 4 cylindrical shaped stoves in a line adjacent to the blast furnace. The gas is burned in the bottom of a stove and the heat rises and transfers to refractory brick inside the stove. The products of combustion flow through passages in these bricks, out of the stove into a high "stack" (11) which is shared by all of the stoves. Large volumes of air, from 80,000 ft 3/min to 230,000 ft 3/min, are generated from a turbo blower and flow through the "cold blast main" (14) up to the stoves. This cold blast then enters the stove that has been previously heated and the heat stored in the refractory brick inside the stove is transferred to the "cold blast" to form "hot blast". The hot blast temperature can be from 1600°F to 2300°F depending on the stove design and condition. This heated air then exits the stove into the "hot blast main" (13) which runs up to the furnace. There is a "mixer line" (15) connecting the cold blast main to the hot blast main that is equipped with a valve used to control the b last temperature and keep it constant. The hot blast main enters into a doughnut shaped pipe that encircles the furnace, called the "bustle pipe" (31). From the bustle pipe, the hot blast is directed into the furnace through nozzles called "tuyeres" (30) (pronounced "tweers"). These tuyeres are equally spaced around the circumference of the furnace. There may be fourteen tuyeres on a small blast furnace and forty tuyeres on a large blast furnace. These tuyeres are made of copper and are water cooled since the temperature directly in front of them may be 3600°F to 4200°F. Oil, tar, natural gas, powdered coal and oxygen can also be injected into the furnace at tuyere level to combine with the coke to release additional energy which is necessary to increase productivity. The molten iron and slag drip past the tuyeres on the way to the furnace hearth which starts immediately below tuyere level.
Around the bottom half of the blast furnace the "casthouse" (1) encloses the bustle pipe, tuyeres and the equipment for "casting" the liquid iron and slag. The opening in the furnace hearth for casting or draining the furnace is called the "iron notch" (22). A large drill mounted on a pivoting base called the "taphole drill" (23) swings up to the iron notch and drills a hole through the refractory clay plug into the liquid iron. Another opening on the furnace called the "cinder notch" (21) is used to draw off slag or iron in emergency situations. Once the taphole is drilled open, liquid iron and slag flow down a deep trench called a "trough" (28). Set across and into the trough is a block of refractory, called a "skimmer", which has a small opening underneath it. The hot metal flows through this skimmer opening, over the "iron dam" and down the "iron runners" (27). Since the slag is less dense than iron, it floats on top of the iron, down the trough, hits the s kimmer and is diverted into the "slag runners" (24). The liquid slag flows into "slag pots" (25) or into slag pits (not shown) and the liquid iron flows into refractory lined "ladles" (26) known as torpedo cars or sub cars due to their shape. When the liquids in the furnace are drained down to taphole level, some of the blast from the tuyeres causes the taphole to spit. This signals the end of the cast, so the "mudgun" (29) is swung into the iron notch. The mudgun cylinder, which was previously filled with refractory clay, is actuated and the cylinder ram pushes clay into the iron notch stopping the flow of liquids. When the cast is complete, the iron ladles are taken to the steel shops for processing into steel and the slag is taken to the slag dump where it is processed into roadfill or railroad ballast. The casthouse is then cleaned and readied for the next cast which may occur in 45 minutes to 2 hours. Modern, larger blast furnaces may have as many as four tapholes and two casthouses. It is important to cast the furnace at the same rate that raw materials are charged and iron/slag produced so liquid levels can be maintained in the hearth and below the tuyeres. Liquid levels above the tuyeres can burn the copper casting and damage the furnace lining.
There are presently four Blast Furnaces working in DSP:
No 1
No 2
No 3 No 4 (being modernised)
Capacity (t/day)
1, 250
1, 820
1, 820
2, 340
Useful volume (cum)
1, 323
1, 400
1, 400
1, 800
Stoves
3
3
3
3
Productivity (t/cu m/day)
1.000
1.3
1.3
1.3
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The Basic Oxygen Furnace
Basic oxygen steel making is a method of primary steelmaking in which carbon-rich molten pig iron is made into steel. Blowing oxygen through molten pig iron lowers the carbon content of the alloy and changes it into low-carbon steel. The process is known as basic due to the pH of
the refractories - calcium oxide and magnesium oxide - that line the vessel to withstand the high temperature of molten metal. The scheme of a BO Furnace is as follows: Typical basic oxygen furnace has a vertical vessel lined with refractory lining. Only 8-12% of the furnace volume is filled with the treated molten metal. The bath depth is about 4-6.5 ft (1.2-1.9 m). The ratio between the height and diameter of the furnace is 1.2-1.5. The typical capacity of the Basic Oxygen Furnace is 250-400 t. The vessel consists of three parts: spherical bottom, cylindrical shell and upper cone. The vessel is attached to a supporting ring equipped with trunnions. The supporting ring provides stable position of the vessel during oxygen blowing. The converter is capable to rotate about its horizontal axis on trunnions driven by electric motors. This rotation (tilting) is necessary for charging raw materials, sampling the melt and pouring the steel out of the converter. The top blown basic oxygen furnace is equipped with the water cooled oxygen for blowing oxygen into the melt through 4-6 nozzles. Oxygen flow commonly reaches 6-8 m 3/min. The oxygen pressure is 1-1.5 MPa. Service life of oxygen lance is about 400 heats.
The basic oxygen steel-making process is as follows: 1. Molten pig iron (sometimes referred to as "hot metal") from a blast furnace is poured into a large refractory-lined container called a ladle; 2. The metal in the ladle is sent directly for basic oxygen steelmaking or to a pretreatment stage. Pretreatment of the blast furnace metal is used to reduce the refining load of sulfur, silicon, and phosphorus. In desulfurising pre treatment, a lance is lowered into the molten iron in the ladle and several hundred kilograms of powdered magnesium are added. Sulfur impurities are reduced to magnesium sulfide in a violent exothermic reaction. The sulfide is then raked off. Similar pretreatment is possible for desiliconisation and dephosphorisation using mill scale (iron oxide) and lime as reagents. The decision to pretreat depends on the quality of the blast furnace metal and the required final quality of the BOS steel. 3. Filling the furnace with the ingredients is called charging. The BOS process is autogenous: the required thermal energy is produced during the process. Maintaining the proper charge balance, the ratio of hot metal to scrap, is therefore very important. The BOS vessel is onefifth filled with steel scrap. Molten iron from the ladle is added a s required by the charge balance. 4. The vessel is then set upright and a water-cooled lance is lowered down into it. The lance blows 99% pure oxygen onto the steel and iron, igniting the carbon dissolved in the steel and burning it to form carbon monoxide and carbon dioxide, causing the temperature to rise to about 1700°C. This melts the scrap, lowers the carbon content of the molten iron and helps remove unwanted chemical elements. It is this use of oxygen instead of air that improves upon the Bessemer process, for the nitrogen (and other gases) in air do not react with the charge as oxygen does. High purity oxygen is blown into the furnace or BOS vessel through a vertically oriented water-cooled lance with velocities faster than Mach 1. 5. Fluxes (burnt lime or dolomite) are fed into the vessel to form slag, which absorbs impurities of the s teelmaking process. During blowing the metal in the vessel forms an emulsion with the slag, facilitating the refining process. Near the end of the blowing cycle, which takes about 20 minutes, the temperature is measured and samples are taken. The samples are tested and a computer analysis of the steel given within six minutes. A typical chemistry of the blown metal is 0.3-0.6% C, 0.05-0.1% Mn, 0.01-0.03% Si, 0.01-0.03% S and P. 6. The BOS vessel is tilted again and the steel is poured into a giant ladle. This process is called tapping the steel. The steel is further refined in the ladle furnace, by adding alloying materials to give the steel special properties required by the customer. Sometimes argon or
nitrogen gas is bubbled into the ladle to make sure the alloys mix correctly. The steel now contains 0.1-1% carbon. The more carbon in the steel, the harder it is, but it is also more brittle and less flexible. 7. After the steel is removed from the BOS vessel, the slag, filled with impurities, is poured off and cooled. Chemical changes in a BO furnaceThe basic oxygen furnace uses no additional fuel. The pig iron impurities (carbon, silicon, manganese and phosphorous) serve as fuel. Iron and its impurities oxidize evolving heat necessary for the process. Oxidation of the molten metal and the slag is complicated process proceeding in several stages and occurring simultaneously on the boundaries between different phases (gas-metal, gas-slag, slag-metal). Finally the reactions may be presented as follows: (square brackets [ ] - signify solution in steel, round brackets ( ) - in slag, curly brackets {} - in gas) 1/2{O2} = [O] [Fe] + 1/2{O 2} = (FeO) [Si] + {O 2} = (SiO2) [Mn] + 1/2{O 2} = (MnO) 2[P] + 5/2{O 2} = (P2O5) [C] + 1/2{O2} = {CO} {CO} + 1/2{O 2} = {CO2} Most oxides are absorbed by the slag. Gaseous products CO and CO 2 are transferred to the atmosphere and removed by the exhausting system. Basic Oxygen Process has limiting ability for desulfurization. The most popular method of desulfurization is removal of sulfur from molten steel to the basic reducing slag. However the slag formed in the Basic Oxygen Furnace is oxidizing (not reducing) therefore maximum value of distribution coefficient of sulfur in the process is about 10, which may be achieved in the slags containing high concentrations of CaO. Combined blowing processCombined blowing process consists of oxygen blowing from top and oxygen blowing from bottom or inert gas (Nitrogen or Argon) bottom stirring. The advantages over the above processes are - acceleration of blowing cycle by
25 % - higher yield - less slag - improved convertor lining life increased accuracy in achieving specific composition - reduced splashing. The steel produced in the basic oxygen furnace is sent to continuous casting or for ingot teeming.
The refractory lining and its maintenanceRefractory bricks for lining basic oxygen furnaces are made of either resin bonded magnesite or tar bonded mixtures of magnesite (Mg O) and burnt lime (C aO). The bonding material (resin, tar) is coked and turns into a carbon network binding the refractory grains, preventing wetting by the slag and protecting the lining the from chemical attack of the molten metal. The following measures allow prolonging the service life of the lining: y
y y
y
Control of the content of aggressive oxidizing oxide FeO in the slags at low level. Addition of MgO to the slags. Performing slag splashes - projecting residual magnesia saturated slag to the lining walls by Nitrogen blown through the lance. Repair the damaged zones of the lining by gunning refractory materials.
Properly maintained lining may serve 20000 heats. Capacity of a furnace is about 400 tons. A
production cycle (tap to tap) lasts for 40 mins.
*
The steel produced in the basic oxygen furnace is sent to continuous casting or for ingot teeming.
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The Oxygen Plant The process description of oxygen manufacture can be summarized in a step by step format below:
Step1:- COMPRESSION OF ATMOSPHERIC AIR Step2:- PURIFICATION OF AIR Step3:- COOLING OF AIR Step4:- SEPARATION OF LIQUID AIR INTO OXYGEN AND NITROGEN Step5:- COMPRESSION/WITHDRAWAL AND FILLING OF OXYGEN AND NITROGEN
The above steps in detail are explained below:
Step 1:The free dry-type opposed,
Air Compressor saturated air is sucked from atmosphere through a highly efficient suction filter into the first stage of the horizontally balanced lubricated reciprocating air compressor.
Step 2:- Purification of Air (Process Skid) This consists of purification of the air by removing moisture, oil traces and carbon-dioxide in the process air. Compressed air is chilled to 12°C in a chilling unit and evaporation cooler, compressed air passes through the coils of the chilling unit at a temperature of 12°C to a moisture separator, where the condensed moisture gets removed before entering into Molecular Sieve Battery. Before sending the air to MOLECULAR SIEVE BATTERY, air is passed through an OIL ABSORBER where air becomes oil free. Chilled air passes through the Molecular Sieve Battery consisting of Twin Tower packed with molecular sieves to remove moisture and carbon dioxide present in the air. Molecular Sieve Battery operates on Twin Tower System, when one tower is under production the other tower is regenerated by passing waste nitrogen gas at 200°C through a REACTIVATION HEATER. After interval of 8 to 10 hours, the tower under production gets exhausted and regenerated by
similar process before use and, thus the cycle continues. Any dust particle gets filtered in the DUST FILTER before air enters the AIR SEPARATION COLUMN All the equipments are mounted on process skid. Step 3:- Expander (Expansion Engine) The process air before liquefaction in the air separation unit needs to be cooled to temperatures sub-zero (cryogenic). The main portion of the air after the process skid enters the expansion engine through the heat exchanger no. I after pre-cooling. The temperature of the air drops to around -165degC by the Expander which is a very h ighly efficient advanced design with Teflon piston rings and completely hydraulic mechanism with leak proof ball valves. The rest of air at (-80) deg C from Heat Exchanger No. I enters into a highly efficient EXPANSION ENGINE, where the air further gets cooled down to (-150) deg C before entering into bottom column. The liquefied air from both these streams collected at the BOTTOM COLUMN is known as RICH LIQUID. Step 4:- Air Separation Unit After the process skid, the air enters the air separation unit (cold box) where the air converts into liquid air by deep cooling at cryogenic (low temperatures) and is separated into liquid oxygen and nitrogen. Chilled, Oil-free and moisture-free air enters into multi-pass HEAT EXCHANGER No. 1 where it gets cooled to (-80) deg C by cold gained from outgoing waste nitrogen and oxygen. A part of air, this enters a multipass HEAT EXCHANGER NO. II or LIQUEFIER made of special alloy tubes. This air cools to (-170) deg C before passing through an expansion valve. Due to Joule Thompson Effect, after the expansion valve, air gets further cooled down and gets liquefied before entering into Bottom Column. The RICH LIQUID in the BOTTOM COLUMN enters into feed tray of top column. Similarly the liquid nitrogen called POOR LIQUID enters into top column as a reflux & it takes away the latent heat of condensing oxygen and gets vaporized whereas the liquid oxygen flows down the trays of the TOP COLUMN into the Condenser. Liquid Oxygen from CONDENSER passes through a SUBCOOLER to a LIQUID OXYGEN PUMP. Step 5:- Cylinder Filling Station Liquid Oxygen & Nitrogen passes from the condenser to the cryogenic liquid oxygen pump for filling gas into cylinders. A. Capacity from 20 cubic Meter / hour t o 500cubic Meter/ hour B. Pressure (150 kg/cm2) upto 300 kg/cm2
Consisting of the following: - Cryogenic Pump - Internal Vaporizer - Cylinder Filling Manifold - Gas Lines Nitrogen filling from Gas Holder/Pipe Line by means of a separate nitrogen filling system from by product nitrogen coming from the cold box of the oxygen gas plant. A. Capacity from 20 cubic Meter/ hour to 500 cubic Meter/ hour B. Pressure upto 155/ 200/ 300 kg/cm2
Consisting of Following: - Gas Compressor - Gas Holder - Cylinder Filling Manifold - Gas Lines -----------------------------------------------------------------------------
