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Production process for Sponge Iron

PROCESS DESCRIPTION

The process for the production of sponge iron consists of the reduction of iron ore with solid carbonaceous material such a coal, coke or lignite in a rotary kiln at high temperature and then cooled to room temperature in the rotary cooler with indirect water cooling system. The products are then screened and magnetically separated Sponge iron being magnetic gets attracted and gets separated from the non-magnetic char.

In the process for the production of sponge iron, the raw materials i.e. iron ore, feed coal and lime stone are feed to the rotary kiln through feed tube in a pre-determined ratio by electronic weighing equipment rotary kiln is of 42 Mts. in diameter and 3 Mts. long and is inclined at an angle of 1.432 Degrees approximately. It is internally lined with refractory of 200mm thick An AC variable speed motor at a stepless variable speed ranging from 0.2 to 1.0 rpm rotates it. Due to the inclination and the rotary motion of the kiln the material moves from the feed end of the klin to the discharge end in approximately 5.5 hrs. The fine coal is blown from the discharge end of the kiln to maintain the required temperature and the carbon concentration in the bed. The kiln has five shell air fans mounted on the top which blow air in the respective zones to maintain the required temperature profile. The material and the hot gasses move in the counter current direction as a result the iron ore gets pre-heated and gradually reduced by the time it reaches the discharge end.

The kiln can be divided process wise into two zones namely pre-heating zone and reduction zone .The pre-heating zone is normally 30% of the total length of the Kiln and the rest is taken as the reduction zone. The material gets heated to the reduction temperature in the pre-heating zone Up to 200 C, the iron ore, coal and lime stone gets dried and all the moisture is vapourised up to 800 C the iron ore gets roasted and any carbonates in it get calcinated and becomes active. The iron ore which is in the form of hematite, gets reduced to magnetite. After this the materials enter to the reduction zone where the magnetite is reduced to wusite and then to the metallic iron. The various stage wise reduction of iron ore is a shown below.

Fe2O3+ CO Fe3O4 +CO 2
Fe3O4+ CO FeO +CO2
FeO +CO    Fe +CO2

Thus the iron in the ore-gets reduced tot its metallic form. The sum total of the above reactions is endothermic. So to carry out these reactions to completion additional source of heat is required. This additional heat is obtained by burning the coal in the gas phase which transfer the heat to the bed material.

Coal contains sulpur in it During the decomposition of the coal the sulphure is released in the form of Iron sulphide During the reduction process of iron ore the sponge iron pick up sulphur by the following reaction

FeO +H2S FeS +H2O

The iron sulphide (FeS ) has deleterious effect in the steel making and is to be removed so lime stone is used to prevent the sulphur pick up by the sponge iron The reacting occurring is.

FeS + Ca+ CO Fe + CaS + CO2

All the above reactions are possible only in the presence of CO. The generation of the CO is most important reaction. Which is called the Boudard reaction. The Boudard reaction is as given below

C+ CO2 2CO

This reaction is highly endothermic which is also reversible. The conditions favorable for the forward reaction i.e. the generation of CO are.

1. The higher temperature favours the production of CO
2. The concentration of he reactants has to high so as that the forward reaction occurs
3. Low pressure favours the CO generation.

All the above reactions occur in the bed phase.
In the gas phase the following reactions occur

CH4+O2 CO2 +H2O
CO O2 CO2
C4+O2 CO2
All these reactions are exo-thermic. They supply the heat required for he reactions in the bed phase to occur. The oxygen required for the burning of these combustibles is supplied from the air tubes placed along the length of the klin by controlled combustion, the temperature in the various zones is maintained so that the reduction is proper and to sufficient degree.

The product quality is defined by the degree of metallisation. The degree of metallisation is defined a the ratio of the metallic iron to the total iron present.

                                              Fe (metallic)
Degree of Metallisation ………………… x 100
                                              Fe (total)

The reduction of iron ore is topo-chemical i.e the reduction proceeds from the surface to the core. The iron ore on partial reduction has all the different stages of the reduction.

The hot material after the reduction is complete is them transferred to the rotary cooler via the transfer chute The rotary cooler is 2.3 Mts in diameter and 22 Mts long made up of Mild Steel shell it is also inclined at 1.432 ‘ approximately and rotates at 1.4 It is driven by a AC motor the water is sprayed on the top of the shell. which cools the material inside the cooler indirectly. The heat from the material is extracted by the shell. In order to increase the surface area for the heat extraction fins are welded inside complete shell is covered by thin layer of water. The heat is transferred from the shell to the water by convection. By this the material gets cooled to 80 C and is discharge on the belt conveyor by the double pendulum valve. The double pendulum valve acts as the seal for the prevention of the atmospheric air into the kiln cooler system. The total kiln cooler system is kept under positive pressure about 3-05 mbar This prevent the atmospheric air from getting into the system. The kiln has to be always operated on positive pressure as any leakage into the system will cause the re-oxidation of the sponge iron there by causing the drop in the quality of the product.

The material after the discharge from the cooler is dropped on to the cooler discharge conveyor. A diversion chute is at the head end of this conveyor for diversion of the material in case of break down in the production separation. The material is then sent to the product separation system. In product separation system consisting of double deck screen, the material is screened to0.3mm and 3.20mm size fractions. The oversize i.e 420mm obtained is small quantity so it is taken on the floor or diverted to the sponge bin. The 0-3 mm size fraction is called the fines are fed to a drum type magnetic separator where the magnetic sponge iron fines and the non-magnetic dolochar separated and fed to the respective bins through the chutes and conveyor. The coarser fraction is similar separated by another magnetic separator and fed so respective bins. This magnetic fraction is called the sponge iron lumps and the non-magnetic as char, which is the unburned coal. This char can be recycled depending on the quality obtained after processing.

The gasses which flow in the counter current direction of the material go to the dust settling chamber where the heavier particles settle down. These particles are continuously removed by the wet scrapper system. The gasses then pass to the after burner chamber where the residual carbon or CO are burned by the excess air available The gasses are at high temperature and have lot of heat energy which can be utilised for the power generation through the waste heat recovery boiler . The hot gasses after the heat recovery boiler gets cooled to 200C The gasses are then scrubbed and let of to the atmosphere at 80 C through the Chimney. Alternatively the hot gasses are quenched and scrubbed to clean all the dust in it .And then they are let off to the atmosphere through the state.

Manufacturing Process

FURNACE :

Preparation :

First of all, crucible of the furnace is checked both electrically and mechanically. When the panel displays ok signal, then its coils are coated manually with high heat duty cement and dried by high power heater or 4 to 6, 100 watt bulbs. Generally it gets dried within 6 to 8 hours. Then one line of asbestos sheets is fitted all around the coil for protection. After fitting of asbestos sheets, an antenna made from stainless steel grade 304 is fixed to the bottom and connected to the crucible for earthing purpose.

Lining :

After the completion of above work, lining of the Crucible starts. Lining is done with 99% pure silica ramming mass mixed with boric acid as a binder. First of all, the ramming is done of the bottom up to a height of 6.5” to 7”. Duration of bottom ramming takes place for 4 to 5 hrs. After the completion of bottom ramming, former sheets are lowered for side lining. This takes another 4 hrs and thereafter tap hole and launder is made from silica ramming mass mixed with sodium silicate.

Charge and melting:

Next, the furnace is charged with good solid mild steel scrap. Thereafter, the panel is switched on and kept on zero kw. Thus, melting of the scrap starts through applied induction. Generally, any kind of induction furnace of any capacity completes the full melt within hours. The furnace gets charged from the top till the liquid metal reaches to the capacity of the furnace. During the melting process, the technicians remove waste and slag from the furnace in order to allow more fill up of scrap. Slag and waste generates from the non metallic rust dust paints from steel scraps. This slag is removed manually through special metallic spoons and dumped in a pot made from cast iron called ‘slag pot’.

Finished product :

When the casting of billets is over, Chemical and physical inspection is carried out and then the billets are lifted and finished product is ready for dispatch. This process is a continuous process.

Waste and slag :

Generation of slag, which is 10% to 25% of the metal melted, is removed from the liquid metal. Its end use is only for the filling and leveling. There is no other use of slag.