Use of Direct Reduced Iron in Electric Arc Furnace
Use of Direct Reduced Iron in Electric Arc Furnace
Electric arc furnace (EAF) operations have improved significantly over the past 30 – 40 years. Future significant improvements will need either new melting technologies or faster power input capabilities. In the meantime steel making in EAFs can benefit significantly from optimizing practices, increasing further the use of chemical energy and correctly using the direct reduced iron (DRI). Historically use of DRI in EAF was for the production of high quality low residuals steels with the anticipated expense of specific energy (kWh/ton), tap to tap time and loss of yield. Current educated use of DRI has developed practices which have demonstrated that DRI use can improve energy consumption, yields, productivity and above all operating costs.
Factors related to the Specification of DRI
During the production of DRI, oxygen is removed from iron ore reducing it to metallic iron and the more stable oxides such as silica (SiO2), alumina (Al2O3), lime (CaO) and magnesia (MgO) etc. As the oxygen is removed the concentration of these stable oxides increases when compared with their concentration in the feed material for the DRI. Hence the significant issues during steel making by the electric arc furnace (EAF) are as follows.
- Carbon content
- Non metallics coming with DRI (generally in case of DRI produced by rotary kiln)
The chemical composition of the DRI determines such important factors as yield, slag weight, energy consumption, carbon and raw material feeding rates and oxygen usage.
Analysis of DRI can vary based on the source and composition of iron ore used for making DRI, the process of making DRI and the process control practiced at the time of making DRI. DRI is available in the composition ranges as given below.
- Total Fe – 84.5 % to 94 %
- Fe Metallic – 78 % to 89 %
- Metallization – 83 % to 95 %
- Carbon – 0.02 % to 4 %
- Phosphorus – 0.005 % to 0.09 %
- Sulphur – 0.001 % to 0.03 %
- Silica (SiO2) – 1 % to 6 %
- Alumina (Al2O3) – 0.5 % to 4 %
- Lime (CaO) – 0.1 % to 2 %
- Magnesia (MgO) – 0.1 % to 1 %
- Residuals – Traces
- Non metallics – 0 % to 0.5 %
Metallization is normally expressed as a percentage which is Fe Metallic/Fe total x 100. Though metallization is an important parameter for a setel maker, yet it is very important to know the percentage of FeO which is present in DRI. With this one can know the amount of oxygen available in DRI.
Carbon is the another element in DRI important to steel maker. If carbon is maintained at a ratio of 0.75 to the available oxygen (in form of FeO) in the DRI then the carbon will form CO in a stoichiometric balance. Equivalent carbon content (ECC) is the percentage difference between carbon present in the DRI and the carbon needed to reduce the oxygen in the FeO. If the ECC is negative then addition of carbon to the bath is needed during steel making to reduce the FeO in the DRI. If the ECC is positive then it become necessary to blow oxygen in the bath to remove the excess carbon from the steel bath. Normally a positive ECC leads to a reduction in the consumption of electrical energy consumption and is always desired where bulk oxygen is available for blowing in the electric arc furnace. A negative ECC leads to bath decarburization
Gangue content is the other important parameter of the DRI which is important for the steel maker since it affects the slag composition. This content decides the addition of fluxes during steel making, amount of slag produced and the amount of energy going into the slag.
Besides the above steelmaker is to know that the gas based DRI is very reactive to free water and oxygen and is subject to a high degree of reoxidation. Self ignition can happen if there is a natural air draft available through the DRI storage pile. Fire can also result when dry pellets are placed on top of the wet DRI.
If the percentage of DRI in the EAF charge is more than 35 % of the total charge then it should be charged continuously.
In some of the EAFs DRI is used as a dilutant of the residual elements such as Cu, Ni, Sn, Mo and Cr so that critical steel gradeswith restricted chemical and physical specifications can be made. The boil caused by the continuously feeding DRI lowers the dissolved hydrogen and nitrogen levels in the liquid steel. In other EAFs DRI is used because of economical reasons since in the area either steel scrap is not available or is of very low uality and density which require multiple charging of the scrap.
Factors related to DRI feeding
Factors such as feeding and power input rates, flux consumption and tap tp tap time are greatly influenced by the DRI chemical composition. The final steel grade to be made is also important for deciding to decide the DRI feeding rate and the melting practice during the steel making by EAF.
While using DRI in EAF, a liquid heel ranging from 15 % to 30 % of the tapweight should be maintained. Precaution is necessary to prevent freezing of the liquid heel in case of big shut downs/breakdowns.
EAFs using 80 % to 100 % DRI needs to start the continuoyus feeding very quickly after initial arcing. DRI feed rate ranges from 5 Kg/min-MW to 35 Kg/min-MW. The DRI feed rate is increased in a series of steps. The maximum feed rate depends mainly on the quality of the DRI with respect to carbon, gangue and metallization. The feed rate is to be maintained in a such a way so that the liquid bath temperature is stabilized around 1570 deg C. This temperature of the bath when combined with the proper composition of the slag , oxygen and carbon produces a very good slag foaming in the EAF. Calculation of the proper DRI feed rate is done in terms of specific energy (kWh/charge ton). Calculating the specific energy for each par of a heat is a repititive process depending on the consistency of the DRI composition, additions, slag foaming and the furnace power parameters. The following example is given to understand the continuous feeding.
Operating parameters of EAF
Hot heel – 40 tons
Initial charge -45 tons
Total charge – 170 tons
Tap weight – 150 tons
Maximum power – 91 MW
DRI feed rate – 30 to 165 tons/ hr
The initial charge is melted first. DRI feeding is started at 200 kWh/ton as shownin Fig 1. The feeding and power input rates are quickly increased with the increase of specific energy. Near the end of the melting time the feed rate is decreased to produce an increase in the bath temperature. Fig 1 shows the relationship between feeding rate and the specific energy consumption while Fig 2 shows the power input at various stages during the melt.
Fig 1 Relationship between the feeding rate and the specific energy consumption
Fig 2 Power input at various stages during the melt
Power input is very much dependent on good slag foaming. Normally continuous charging of DRI is done into a flat bath. Due to this the foaming of the slag is necessary for insulation of the ac, protection of the furnace refractories and reduction of the slag density so that DRI can penetrate into the liquid melt. Power input practice is different from the EAFs with the scrap charging. The arc length and corresponding power input is kept low till the feed rate is increased with the progress of the heat and with the increase in the slag height due to foaming. During the heat, the arc lengths are adjusted for minimizing of hat time and energy consumption.
Feed rate of DRI needs careful observation and control. If it is fed too fast then a large solid mass of DRI with lime can be seen floating around the furnace. This is known as ‘Ferroberg’. Due to lower apparent density of DRI a small amount of this mass is immersed in the liquid steel and larger amount is exposed above the slag. Melting of Ferroberg is normally done by thermal radiation which is slow melting process when compared with heat transfer by conduction or convection. Hence feed rate is to immediately cut once a Ferroberg is seen.
Consistent chemistry of DRI is vital for controlling the fedding rate of DRI. If the chemistry of DRI is inconsistent then FeO level can increase or carbon content can decrease. In either case the feed rate is to be slowed which mean longer tap to tap time and higher consumption of specific enegy.
In case any shop does not have any facility for continuous charging of DRI and DRI is to be charged with the scrap bucket then DRI charge in the EAF get limited to 35 % of the weight of total iron bearing materials. This is because higher amount will cause sticking of the DRI with the furnace wall. In such case it is desirable that DRI is added to the furnace as the first furnace charge.
Factor related to slag formation
During steel making in EAF, it is essential that the slag produced during steel making is properly designed to neutralize the acid materials present in the gangue of the charge in order to protect the basic lining of the EAF, to remove P from the melt and to promote foaming. Basicity of lag ha direct effect on many parameters such as energy and refractory conumption, slag foaming,slag viscosity and slag weight etc. A highly viscous slag contains solid lumps of lime and does not react with the steel melt nor does it foam. Further knowledge of C and FeO content of the DRI is essential or deciding about the O2 and C injection during the heat. C and FeO content of the DRI will also decide whether the DRI is carburizing or decarburizing.