Use of Direct Reduced Iron in Electric Arc Furnace
Use of Direct Reduced Iron in Electric Arc Furnace
Steelmaking by the electric arc furnace (EAF) has very good flexibility with respect to the selection of charge materials. The traditional charge material for the EAF process has been 100 percent cold scrap but as the issues regarding scrap such as its availability and quality, market price fluctuations and restrictions imposed by scrap in making some steel grades due to residual elements and nitrogen level etc. have increased, EAF operators intensified the search for alternative iron containing charge materials.
Direct reduced iron (DRI) like pig iron and hot metal is one of the alternative iron charge material which has been used in varying percentage in the EAF steelmaking process. DRI usage can have distinct effects on the melting process. Energy consumption, productivity and yield are affected by DRI chemistry, percentage of DRI used in the scrap mix and operating practices. DRI is utilized as a scrap replacement in the EAF steelmaking process. Fig 1 shows a typical melting profile with DRI in the charge mix in a 150 ton EAF.
Fig 1 Typical melting profile with DRI in the charge mix in a 150 ton EAF
The normal advantages associated with the use of DRI in the EAF steelmaking process are (i) constant size and dimension, (ii) known composition, (iii) almost absence of tramp elements, (iv) purity, or the absence of non-metallic substances leading to better productivity and energy consumption, (v) easy use in the EAF, since DRI can be charged into an EAF along with scrap, using buckets or by the continuous feeding, (vi) better availability as compared to the low-residual scrap, (vii) availability of the associated carbon content which yield energy during the steelmaking process, (vii) easy foamy slag generation, (vii) direct charging in case of the use of hot DRI with hot DRI reducing the energy consumption by as much as 16 % to 20 % by making use of the energy value of the DRI at temperatures higher than 600 deg C, (viii) easy handling and storage with the possibility of feeding without opening the furnace roof by continuous feeding system resulting into stable steel bath surface which reduces the risk of electrode breakages, (ix) possibility of blending of DRI with scrap which means that cheaper scrap grades can be used, and (x) more environmentally friendly since it avoids problems of hazardous contaminants such as lead (Pb) or cadmium (Cd) in EAF dusts, and reduces the possible formation of dioxins and furans.
Historically use of DRI in EAF was limited to the production of high quality low residuals steels with the anticipated higher expenses due to the specific energy (kWh/ton), tap to tap time, flux consumption, increased FeO in the slag, loss of yield, and higher refractory and electrode wear. This has been the case since the DRI has been used without identifying and understanding the inherent and unique properties and modifying practices accordingly.
EAF operations have improved considerably since late 1970s – early 1080s. Steelmaking in EAFs has been benefitted significantly from optimizing practices, increasing further the use of chemical energy, and with the use of the DRI in a proper way. The practices developed regarding the use of DRI have demonstrated that DRI use can improve energy consumption, yields, productivity, and above all the operating costs. Further DRI with higher carbon content allows the EAF steelmaking to reap substantial financial and technical benefits. The use of high carbon, hot DRI can results into improved operating capabilities, coupled with the cost effective productivity.
The main parameters affecting the operation of EAF are composition of raw materials (% gangue / chemistry, metallization, % carbon, % phosphorus, and energy content), operating practices (power profiles, foamy slag, and melting practices), and furnace design (heel, oxygen use and tools, AC/DC etc.). Non metallics coming with DRI (usually in case of DRI produced by rotary kiln) also has adverse effect on EAF steelmaking. Without attention to these factors, use of DRI in the EAF steelmaking can have adverse impact on the operating parameters of the EAF.
DRI is the product which is produced by the direct reduction of iron ore or other iron bearing materials in the solid state by using non-coking coal or natural gas. Processes which produce DRI by reduction of iron ore below the melting point of the iron are normally known as the direct reduction processes. The reducing agents are carbon monoxide (CO) and hydrogen (H2), coming from reformed natural gas, syngas, or coal. Iron ore is used mostly in pellet and/or lumpy form. Oxygen (O2) is removed from the iron ore by chemical reactions based on H2 and CO for the production of highly metalized DRI.
In the direct reduction process, the solid metallic iron (Fe) is obtained directly from solid iron ore without subjecting the ore or the metal to fusion. Direct reduction can be defined as reduction in the solid state at O2 potentials which allow reduction of iron oxides, but not of other oxides (Al2O3, MnO, and SiO2 etc.), to the corresponding elements. Since reduction is in the solid state, there is very little chance of these elements dissolving (at low thermodynamic activity) in the reduced iron, so the oxides which are more stable than iron remain essentially unreduced. DRI has a porous structure.
DRI is produced in many forms. These are lump, pellets, hot briquetted iron (HBI), fines, and cold briquetted iron (CBI). HBI and CBI are densified forms of DRI. When DRI is discharged from the furnace in cold condition (temperature around 50 deg C) then the product is known as cold DRI (CDRI) and when DRI is discharged in hot condition (temperature around 650 deg C) for use in hot condition in steelmaking then the product is known as hot DRI (HDRI).
Iron content in the DRI is in two forms. One is in metallic form which is known as metallic iron, Fe (M), and the second form of iron which is present in residual iron oxides, Fe (O). The total iron, Fe (T), in DRI is the sum of these two iron components. Metallic iron is the aggregate quantity of iron, either free or combined with carbon (as cementite) present in DRI. The chemical and physical properties of DRI produced by the different processes normally vary.
The quality of the DRI is important since it impacts parameters of steelmaking such as yield, flux consumption, slag quantity, energy consumption, and carbon, oxygen and raw material feeding rates. The quality of DRI is dependent upon (i) quality of input materials mainly iron ore/pellets and fuel/reductant which determines the gangue materials/impurities in the DRI, (ii) production process which determines the carbon content in the DRI, and (iii) process parameters which determines the metallization of the DRI.
A low degree of metallization means more FeO has to be reduced in the EAF. On the other hand, a high metallization degree results in less CO generation and lower bath agitation in the EAF, which in turn reduces the heat transfer efficiency and accordingly increases the furnaces energy requirements. The best results are obtained when metallization of DRI lies between 94 % and 96 %.
There are two reasons for to use DRI as a part of charge mix in the EAF steelmaking. These are (i) residual control, and (ii) non-availability of premium scrap. The non availability of the premium scrap creates a pressure on the EAF operators to turn to lower-grade scrap sources, such as obsolete scrap, as a source of iron units. The problem with obsolete scrap is its quality. Further, the growth in FAF steelmaking has led to an unavoidable quicker turn-around of scrap and, as a result, to the increased contamination of scrap by other elements. Residuals such as chromium(Cr), nickel (Ni), molybdenum (Mo), copper (Cu), and tin (Sn), ranging from 0.15 % to 0.75 % depending on the type of scrap, have adverse effects on some mechanical properties of the steel. Hence, the use of scrap-only-based steelmaking in EAFs for the production of quality steels, as well as low-carbon steel products, is generally avoided.
Another problem associated with the scrap-only-based EAF steelmaking is nitrogen. The nitrogen content of EAF steels is higher than that of basic oxygen furnace steels. As a result, steels produced in the scrap-only-based EAF steelmaking have normally poor aging characteristics which make them practically unsuitable, for example, for deep-drawing applications.
For the production of high quality special steel grades from scrap with varying quality and chemical composition, the compliance with high purity levels is sometimes only achieved with the dilution of unwanted tramp elements such as Pb, Cu, Cr, Ni, Mo, and Sn with highly pure substituting materials such as DRI. The tramp elements level decreases linearly when DRI percent in EAF charge mix increases. The thumb rule equation for calculating total tramp elements in liquid steel at the time of tapping is ‘percent (Cr + Ni + Cu +Sn) = 0.3225 − 0.001174 x percent DRI.
Also with the increase of the DRI percent in the charge mix, nitrogen levels show a similar reduction with proper slag foaming. The nitrogen reduction enables EAFs to produce many special steels. Further, DRI is also used for economic high quality steel production with very low phosphorus and hydrogen content.
DRI has high bulk density, which is greater than that of the most types of the steel scrap. Its density is higher than that of the slag in the furnace, which facilitates its melting at the slag / metal interface. The remaining FeO in the DRI reacts with the carbon in the liquid metal bath to improve the foaming slag which shields the refractory from the electric arc. However, DRI feeding rate is an important parameter of the EAF process which is to be controlled. The optimum feeding rate depends on the DRI chemical composition, the bath temperature, and the stirring energy provided by the oxygen-carbon injectors and the bottom stirring plugs. DRI feeding rates in most of the DRI charged furnaces are in the range of 27 kg/min MW to 35 kg/min MW.
The carbon content of the DRI is important with respect to its use in the EAF steelmaking. Carbon in DRI can be present in elemental form or it can be present as a combination of both the elemental as well as in the bonded form. The bonded form of carbon in DRI is iron carbide (Fe3C) which is a stable compound of iron and carbon. Normally around two-thirds of the carbon contained in DRI is present as iron carbide, and the balance is in the form of elemental carbon. When the carbon is in the bonded form then there is no loss of the not-bonded C due to its combustion in the EAF atmosphere. Tab 1 gives typical specification of gas based DRI.
|Tab 1 Typical specification of gas based DRI|
|Basis- Fe in iron ores / pellet – 65.5 % to 68 %|
|6||Phosphorus as P2O5||%||0.005-0.09|
|Note: Residuals are unreduced oxides such as silica, manganese oxide, alumina, lime, and magnesia|
It is normally seen that in the gas based DRI, the carbon content of the DRI is generally more than the stoichiometric requirements needed to reduce the FeO content remaining in the DRI product. The excess carbon has significant impact on the FeO content of the slag and on the slag foaming which is needed for an efficient EAF steelmaking process. In case of negative excess carbon, the necessary addition of anthracite coal for FeO reduction is beneficial late in the EAF steelmaking process. However, not all the FeO is reduced into Fe since a portion of the FeO does always exist in the furnace slag. This means that the practical amount of excess carbon of DRI which is available for combustion in the EAF steel bath is more than the excess carbon calculated for DRI reduction. This term is called combustible carbon and defined by the equation ‘Combustible carbon = carbon in DRI – stoichiometric carbon x (FeO in DRI – FeO in slag).
The combustible carbon reacts with the oxygen injected to the EAF steel bath to release heat in the steel bath and also contribute CO gas for the slag foaming. With increasing combustible carbon in the EAF, the nitrogen content of the tapped steel also decreases. Excess carbon from the DRI decreases the input of anthracite coal which is a major source for the dissolved nitrogen in the EAF bath (0.1 % N2) besides infiltrated air. A second benefit obtained from the carbon in DRI is through the energetic benefits of the iron carbide. Fe3C yields energy through the exothermic reactions obtained during its dissociation in the steel bath (- 0.4 kWh / kg C), in contrast to the endothermic dissolution of carbon particles in the steel bath (0.62 kWh / kg C).
Metallic yield and quantity of slag
Metallic yield of the liquid steel is influenced during EAF steelmaking with the addition of DRI in the EAF charge. It is seen that the metallic yield is decreased when DRI percent in the EAF charge mix is increased. This is mainly because of the increase in the slag volume.
The slag volume increases as the DRI percent in the EAF charge mix increases. The thumb rule equation for the slag quantity is ‘Slag quantity (kg/ton of liquid steel) = 127 + 2.43 x percent DRI’. As per this thumb rule equation, a 10 % increase of DRI in the EAF charge mix leads to an increase in slag weight by 24.3 kg. The slag weight depends mainly on the content and composition of the gangue in the DRI and the basicity of the slag.
Slag chemistry and volume affect yield in the EAF. With DRI in the EAF charge, the operator is to be careful with the slag so that good foaming takes place with the minimum volume of the slag at the required basicity. Because of the endothermic reduction reaction of FeO by carbon, (FeO + C = Fe + CO), and the higher slag volume encountered due to the DRI usage, the electrical power consumption of the EAF normally increases with increasing percentage of DRI in the metallic charge. Normally, the electrical power consumption increases more or less linearly with the increase in the percent of DRI in the EAF.
Electric power consumption
Electric power consumption (kWh per ton of liquid steel) during EAF steelmaking increases when there is an increase in the DRI percent in the EAF charge mix. As per the thumb rule, with every increase of 10 % in DRI percent leads to an increase in electric power consumption by 14.5 kWh/ton of liquid steel under certain conditions.
Many factors tend to increase the electric energy consumption when using DRI in the EAF steelmaking. With good slag foaming, an EAF melting 100 % cold scrap and without other energy inputs typically consumes energy in the range of around 400 kWh/ton to 435 kWh/ ton of liquid steel. For comparison purposes, an EAF having a charge mix consisting of 98.2 % DRI with very good slag foaming has achieved an average energy consumption level of 635 kWh/ton of liquid steel.
DRI metallization affects energy consumption. The lower metallization level of DRI means higher FeO level. Chemical reduction of FeO is an endothermic reaction. Reduction of one ton of FeO to Fe needs around 800 kWh at steelmaking temperatures. Increasing levels of SiO2 in the DRI increase the electric power requirements. SiO2 needs the addition of lime to maintain the basicity ratio. Melting one ton of slag needs around 530 kWh of energy. Increasing amounts of SiO2 need increasing amounts of CaO to maintain the basicity ratio. Both the SiO2 in the DRI and calcined lime consume energy during the melting process. Fig 2 shows the relationship between the gangue content of DRI and the energy consumption per ton of liquid steel. Furthermore, there are a number of additional factors which affect the steelmaking process. These include yield, lime requirements, and the oxygen and carbon injection needs.
Fig 2 Effect of gangue content on energy requirement per ton of liquid steel
Phosphorous and sulphur contents can have a negative effect on energy consumption due to lime requirements. The CaO in the lime absorbs phosphorous from the bath. If the EAF is being operated at a constant FeO percentage then the only way to remove more phosphorous is by adding more lime. Increase of the amount of lime results in increased energy consumption and Fe yield loss. Efficient sulphur removal needs the use of a reducing slag. EAFs typically operate with a basic oxidizing slag. While it is possible to remove some sulphur from the EAF by increasing the amount of lime addition, this results in increased energy consumption and is not very effective.
Carburized DRI has a positive effect on the energy consumption if high volume oxygen is available to inject into the bath. Energy consumption can be reduced by 2 kWh/N cum to 4 kWh/N cum of oxygen, if it is injected with the correct amount of carbon and a good foaming slag is produced.
Charging hot DRI saves on energy but oxidation is an issue. Transporting hot DRI directly from the DRI module is to be done under a sealed nitrogen or process gas atmosphere before charging to the EAF.
Effect of DRI addition in EAF charge on power on time
The power on time is increased when DRI percent in the EAF charge mix is increased. The thumb rule equation for calculating the power on time is ‘EAF power on time = 46.36 +0.1320 x DRI percent. The substitution of steel scrap with DRI increases the time needed for melting the EAF charge (power-on time). This is attributed to the lower melting rate of DRI caused by the FeO which needs to be reduced. Moreover, having an acidic slag caused by the SiO2 and Al2O3 containing gangue materials in the DRI. It is also obvious that the specific consumption of lime and dolomite increases to manage the appropriate slag basicity near (CaO / SiO2) equal to 2. Due to the increasing slag quantity due to the increasing the DRI in metallic charge, again longer melting time is needed to bring the slag into solution and accordingly there is higher electrical power consumption, and this is also the reason for increasing lime, total flux consumption and then slag quantity.
Effect of metallization on yield and other parameters
Liquid steel yield from DRI is a function of the metallization rate, total gangue content and carbon injection and addition practices. A typical charge of DRI can contain 93 % total iron with 86 % metallic iron for a metallization of 92 %. If 100 % reduction of the FeO is possible then the DRI charge gives a liquid steel yield of 93 %. In practice, this result is not achievable in the EAF. If higher yields are desired, then the DRI need to have a higher metallization. Further, as the metallization goes down, it has negative influence on all the parameters. Fig 3 shows typical relationship of metallization with yield and power consumption experienced in an EAF.
Fig 3 Typical relationship of metallization with yield and power consumption
Effect of DRI percent in the charge mix on lime and total flux
DRI normally contains silica as the main gangue constituent together with low levels of other impurities such as sulphur and phosphorus. According to the concentrations of these components in the DRI and the proportion of DRI in the metallic charge, varying quantities of lime is to be added into the EAF in order to slag the silica and remove the sulphur and phosphorus to the allowable levels of these elements for the grade of the steel to be produced.
The total lime consumption increases with the increase in the DRI percent in the EAF charge mix. As per thumb rule, there is an increase in lime consumption by 2.6 kg/ton of liquid steel with every 10 % increase of DRI in the charge mix. The thumb rule equation for calculating total flux consumption (in kg per ton of liquid steel) is ‘Flux consumption (kg /t) = 45.31 + 0.2416 x DRI percent.
With the increased use of DRI in EAF steelmaking, charging methods have changed. With a captive DRI plant, the percent of DRI in the EAF charge is normally high such plants. In such plants, direct supply to the EAF is done through a continuous charging system to a fifth hole in the roof of the EAF. Invariably there are storage bins or large warehouses to accept DRI supply when the EAF shop is down for maintenance or delays or when stockpiling is required for DRI plant shutdowns. Fig 4 shows different charging practices being followed with DRI usage in EAF steelmaking.
Fig 4 Different charging practices being followed with DRI usage in EAF steelmaking
Continuous feeding is usually employed above 25 % to facilitate matching feeding rate to power and chemical energy input and to prevent ‘iceberg’ or ‘ferroberg’ formation. Prevention of ferrobergs is determined in part by the feed rate and the power input available.
Bucket charging is used in the EAFs which are normally using less than 25 % to 30 % DRI in the charge-mix, though it is preferable to continuously charge DRI. DRI in the bucket aids charge densification as well as lowering residuals in liquid steel. Bucket charging avoids the cost of a continuous charging system. DRI is normally charged on top of heavy scrap or bundles (Fig 4) to maximize densification of the charge, and the rest of the bucket is loaded according to the site-specific practice. DRI is normally divided between buckets in multiple-bucket charges, with more in the last bucket to improve the melt refining where the lower % C and increased O2 content can compensate for lack of O2 input capacity and minimize blow down at the end of the heat. If a single bucket charge practice is used, DRI is input in multiple layers. DRI is charged higher up (third and penultimate layers) in the bucket to prevent DRI falling through the bottom of the bucket.
The roof through a fifth hole is preferred for DRI and is definitely it is more efficient when using more than 30 % DRI. Continuous charging facilitates coordination of the feed rate with the power input and flux feeding to ensure slag control (foaming height, and viscosity etc.) and prevent ferrobergs, which occurs when cold DRI is charged too quickly.
Continuously charging hot (600 dg C) DRI can reduce energy required by as much as 16 % to 20 %. Continuous feeding considerably reduces the EAF energy requirement since it affords closed-door operation. This negates heat and time losses from roof swing(s) and charging, also the potential nitrogen pick-up arising from air ingress occurring when the roof is open. Hot charging DRI reduces the power requirement by 20 kWh/100 deg C to 30 kWh/100 deg C. There have been many different hot charging methods. One method is hot charging by transporting the DRI from the DRI plant to the EAF in insulated trucks. Another method uses a pneumatic system conveyor with gravity feed through the fifth hole. Several plants make use of conveyors or direct gravity feed. A benchmarking study based on published data from 150 EAFs shows that some of the EAFs charging HDRI are matching the energy consumption of scrap-based EAFs.