The most common steel making technology is the Bf-Bof Route. Coke is used in Blast Furnace (BF) both as a reductant and as a source of thermal energy. It involves reduction of ore to liquid metal in the blast furnace and and refining in convertor to form steel. The various stages of the steel plant is described below.
COKE MAKING - COAL CARBONISATION : 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.
Conventional cokemaking is done in a coke oven battery of ovens sandwiched between heating walls. They are carbonised at a temperature around 1000o-1100o C upto a certain degree of devolatization to produce metallurgical coke of desired mechanical and thermo-chemical properties. A schematic diagram of Coke Oven Battery is given in Fig.below.
COKE MAKING - COAL CARBONISATION
Schematic Diagram of Coke Oven Battery
During carbonisation, coking coals undergo transformation into plastic state at around 350o-400o C swell and then resolidify at around 500o-550o C to give semi-coke and then coke. In coke ovens, after coal is charged inside the oven, plastic layers are formed adjacent to the heating walls, and with the progress of time, the plastic layers move towards the centre of ovenfrom either side and ultimately meet each other at the centre. During cokemaking, two opposite reactions take place, viz. condensation and pyrolysis.
The quality and quantity of plastic layer is of extreme importance and it determines the inherent strength of coke matrix. For producing coke of good quality, coals should have certain degree of maturity (rank 1.1-1.3), good rheological properties (about 200-1000 ddpm by Gieseler Plastometer), wide range of fluidity and low inerts.
The various modification in the coke making which have improved coke yield and reduced sp. Energy consumption are :
Partial Briquetting of Coal Charge (PBCC): The technology involves charging about 30% coal blend in the form of briquettes. Briquettes are prepared using a binder (pitch/ pitch+tar) upto 2. to 3.0% of charge. Coke quality significantly improves as a 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 carbonisation. 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 in micum indices and 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 post-carbonisation 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 (M10 index can be improved by 1 point).
Sintering is a technology for agglomeration of iron ore fines into useful Blast Furnace burden material. This technology was developed for the treatment of the waste fines in the early 20th cenmtury. Since then sinter has become the widely accepted and preferred Blast Furnace burden material. Presently more than 70% of hot metal in the world is produced through the sinter. In India, approximately 50% of hot metal is produced using sinter feed in Blast Furnaces.
The major advantages of using sinter in BFs are :
- Use of iron ore fines, coke breeze, metallurgical wastes, lime, dolomite for hot metal production
- Better reducibility and other high temperature properties
- Increased BF productivity
- Improved quality of hot metal
- Reduction in coke rate in blast furnaces
SINTERING PROCESS A Sinter Plant typically comprise the following sub-units as shown below.
The raw materials used are as follows - Iron ore fines (-10 mm), coke breeze (-3 mm), Lime stone & dolomite fines (-3mm) and other metallurgical wastes. The proportioned raw materials are mixed and moistened in a mixing drum. The mix is loaded on sinter machine through a feeder onto a moving grate (pallet) and then the mix is rolled through segregation plate so that the coarse materials settle at the bottom and fines onto the top.
The top surface of the mix is ignited through stationary burners at 1200oC. As the pallet moves forward, the air is sucked through wind box situated under the grate. A high temperature combustion zone is created in the charge -bed due to combustion of solid fuel of the mix and regeneration of heat of incandescent sinter and outgoing gases. Due to forward movement of pallet , the sintering process travels vertically down. The different zones created on a sinter-bed are shown in the adjoining figure.
Sinter is produced as a combined result of locally limited melting , grain boundary diffusion and recrystallisation of iron oxides.
On the completion of sintering process, finished sinter cake is crushed and cooled. The cooled sinter is screened and + 6 mm fraction is despatched to blast furnace and -6 mm is recirculated as return sinter.
The Blast furnace iron making process basically consists of the conversion of iron oxide to iron in liquid form . This requires reductant for reduction of iron oxide and heat for the above reduction reaction to take place and for melting the products of smelting. The primary source to fulfill both these requirements is carbon (in the form of coke), which shares major portion of cost of hot metal production.
The blast furnace is a vertical counter-current heat exchanger as well as a chemical reactor in which burden material charged from the top descend downward and the gasses generated at the tuyere level ascend upward.
The inside profile of the furnace from top to bottom is termed furnace throat, shaft, belly , bosh and hearth. The throat is the top part of the furnace and includes the installation necessary for charging coke and burden materials and drawing off the top gasses . The top gas containing the flue dust is routed from the furnace top to the gas purifiers and then to the consumption zones. The profile of the furnace widens in the shaft that follows. The widening of the furnace chamber from top to bottom is necessary to avoid hanging and scaffolding of the burden in the blast furnace when they expand during heating . The height of the shaft is about 3/5 of the total height of the furnace. The shaft is followed by the belly , in the bosh below this , the profile again narrows, as this is the part of the furnace where the stock column starts to melt and volume of the furnace can be reduced. The hearth is the lower cylindrical part of the furnace where the fluid slag and the hot metal accumulate. Arranged in the upper part of the hearth are water-cooled tuyeres made of copper. The hot air for combustion is injected through these into the blast furnace. Hot metal is tapped through the tap hole, which is opened by power driven drills into a train of ladles kept below the runner of the cast house. Slag comes along with the metal and is skimmed off with the help of skimmer plate towards slag runner and is collected in slag thimbles. The tap hole is tightly sealed with a mud gun after tapping process is complete.
Raw material ( ore, sinter , coke ) are screened before being charged into the blast furnace through conveyors or skip. Air for combustion in the blast furnace is blown from turbo blowers which are preheated in hot blast stoves to temperatures around 1300oC , which is then blown through tuyers into theblast furnace. Each blast furnace is equipped with two or more stoves which operate alternatively. Preheating of air helps in reducing fuel consumption in the furnace.
From top to bottom of the furnace the following process occurs :
-Drying , preheating, ejection of hydrate water
In the top third of the shaft, gas delivers its heat so that the charging materials are preheated and dried . When a temperature of 400o to 500oC is reached , the water which was fixed with the burden is ejected. Indirect reduction by carbon monoxide occurs below 1000oC . At temperatures above 1000 oC, iron oxide not yet reduced into iron is directly reduced. After melting, the reduction process is completed as hot metal flows through layers of coke.
Hot metal produced in the blast furnace is sent to Basic oxygen Furnace for steel making or to Pig casting machines for pig iron casting in ladles.
Pre-treatment of Hot metal
Hot metal from blast furnaces is treated to remove undesired elements like sulphur , silicon or phosphorous before being transformed to steel. Desulphurising agents are applied to reduce sulphur content of the metal.
BASIC OXYGEN FURNACE
The basic oxygen process is the most common process for producing steel. The basic oxygen furnace ( LD convertor) is a pear shaped vessel lined inside with refractory bricks . The vessel lining consists of tar bonded dolomite /magnesia carbon bricks or other refractories. The vessel can be rotated 360 o on its axis . Oxygen is blown into the vessel with the help of water cooled lance.
The 'heat' begins with the addition of scrap into the slightly tilted convertor, hot metal is then added after straightening the convertor, Oxygen is blown into the bath through the lance .The necessary fluxes are added during blowing .Flux addition is done automatically and precisely through bunkers situated above the convereters. A sample is taken after blowing for 16-18 minutes and temperature is measured using a thermocouple.The steel is tapped by tilting the convertor to the tapping side and alloying elements are added via chutes while metal is being tapped The convertor is tilted to the charging side in order to remove the floating slag .
During blowing operation, oxygen oxidises iron into iron oxide and carbon into carbon monoxide.The iron oxide immediately transfers the oxygen to the tramp elements.The center of the reaction has temperatures of around 2000o-2500oC .The development of carbon monoxide during refining process promotes agitation within the molten bath. The reaction of the tramp elements with the oxygen and the iron oxide developed in the center of reaction leads to formation of reactive slag. As blowing continues, there is a continuous decrease of carbon,phosphorous, manganese and silicon within the melt.Phosphorous is removed by inducing early slag formation by adding powder lime with oxygen. The refining process is completed when the desired carbon content is attained.
Various other blowing process in practice are : Oxygen bottom blowing process : In this process, pure oxygen is blown into the bath from below through a cooled nozzle. It results in a lower tap to tap time and greater output due to more intensive mixture
Combined blowing process
Combined 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 achieveing specifice composition - reduced splashing The steel produced in the basic oxygen furnace is sent to continuous casting or for ingot teeming.
Continuous casting technique accounts for more than 60% of total liquid steel in the world. The main advantages of steel processing through CC route are higher yield, lower energy consumption, elimination of primary mills.
THE PRODUCTION STAGES
During continuous casting, the liquid steel passes from the pouring ladle, with the exclusion of air, via a tundish with an adjustable discharge device into the short, water-cooled copper mould. The shape of the mould defines the shape of the steel. Before casting, the bottom of the mould is sealed with a so-called dummy bar. As soon as the bath reaches its intended steel level, the mould starts to oscillate vertically in order to prevent the strand adhering to its walls. The red-hot strand, solidified at the surface zones, is drawnfrom the mould, first with the aid of a dummy bar, and later by driving rolls. Because of its liquid core, the strand must be carefully sprayed and cooled down with water. Rolls on all sides must also support it until it has completely solidified. This prevents the still thin rim zone from disintegrating.
THE PRODUCTION STAGES
Once it has completely solidified, the strand can be divided by mobile cutting torches or shears. Intensive cooling leads to a homogeneous solidification microstructure with favourable technological properties. High casting speeds are achievable nowadays; depending on dimensions and the number of strands that are simultaneously cast, speeds of about 0.6 to 3.5 m/min are possible for slabs.
The primary features of continuous casting now-a-days are
- Sequence casting and composite casting with the aid of turrets, which take up two ladles, and swivelling devices for the wear-exposed tundishes. This allows dissimilar grades of steel to be cast directly after each other.
- Some kind of protection for the pouring stream between the ladle and tundish as well as between the tundish and the mould, with the aid of inert gases for improving the cleanness (preventing reoxidation).
- Oscillating moulds and fully automated addition of casting fluxes for improving the strand surface, as well as adjustable moulds for changing strand dimensions during casting.
- Mould level control.
- Constant casting temperature.
- Precise strand guidance and consistent strand cross-section by means of matching roll design of high precision and/or split rolls.
- Intensive secondary colling of the strand with the aid of uniform and metered spraying.
- Electromagnetic stirring in the mould to improve surface quality (no surface shrinkage cavities or inclusions close to the surface).
- Electromagnetic stirring of the partly solidified strand to obtain a globular - non-directional - solidification microstructure with no segregation zones in the centre of the strand.
- Strand backup and matching straightening and driving rolls for forming the straight and straightened strands in order to prevent surface tension and cracks, as well as internal cracks.