Use of Hot Metal for Steelmaking in Electrical Arc Furnace
Use of Hot Metal for Steelmaking in Electrical 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 % 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. alternative charge materials have been used in varying percentage successfully by EAF operators. The alternative charge materials are direct reduced iron (DRI), hot briquetted iron (HBI), pig iron, or hot metal.
The use of hot metal is more popular in those areas where there is shortage of scrap and /or electric power or electric power has high cost. The popular source of hot metal is blast furnace hence hot metal can be used in those EAFs which are in close proximity of the blast furnace, otherwise the EAF operator has to use pig iron. Pig iron needs extra energy for its melting. A large range of variation in the proportions of hot metal and scrap are possible in EAF steelmaking. With 100 % scrap operation at one end of the scale; the FAF can also be operated with a charge of only 20 % scrap and 80 % hot metal. There are at present many EAFs which have been designed for using to 80 % of hot metal in the charge.
Combining a charge of hot metal and scrap to EAF helps in improving the operating performance of the process. Hot metal has dissolved carbon and silicon which are important sources of heat available with their oxidation. The heat of oxidation of these elements along with sensible heat available in hot metal helps in substantial reduction in power consumption during steelmaking in the EAF. Further, hot metal is free of foreign non-metallic materials which have been removed as slag during iron making process. However, the EAF operators are required to take care of strong reaction taking place in the furnace due to the carbon in the hot metal. The hot metal can be charged in a controlled manner to take care of carbon content in liquid metal bath in the furnace.
The major benefits associated with hot metal charging in EAF include enhanced productivity, improved slag foaming, and increased carbon content in the charge. The high purity, low gangue content of hot metal allows for the production of those steel products which need low residual content. Further, the hot metal has known and consistent chemistry certified by analysis and this offsets the wide chemistry fluctuations common with the use of the obsolete scrap.
The chemical energy contained in the hot metal is delivered efficiently by contained carbon, which promotes faster melting and increased productivity. Hot metal is extremely beneficial for increasing EAF productivity and achieving short tap-to-tap times provided the furnace design does not restrict the rate of decarburization. Hot metal also allows for considerable flexibility in the selection of scrap. Due to the low residual content of the hot metal, it is possible to utilize low-grade scrap when higher proportion of hot metal is used in the EAF.
Hot metal provides benefits similar to pig iron, with the added benefit that the material is already at a temperature of around 1,300 deg C or above. Thus, the major portion of the energy requirement in the EAF (required for melting the Fe) is already provided. One ton of hot metal at 1,430 deg C supplies around 250 kWh in the form of sensible heat, based only on the Fe content. For the use of pig iron in the EAF, typical energy savings are typically in the range of 3.1 kWh/percent pig iron to 3.6 kWh/percent pig iron. Using hot metal increases the savings to 4.8 kWh/percent hot metal. Using large quantities of hot metal can decrease the power consumption to the level of 200 kWh per ton of hot metal and below and thus can be very beneficial for locations which have a weak electrical grid.
Influence of HM on key parameters of EAF process
In recent times, the main emphasis in EAF steelmaking has been related to achieving maximum energy efficiency. Further the feed charge materials are influencing the design of the EAF and its operation practice. The influence of HM as a charge material on various key parameters of an EAF process of steel making is detailed below.
Residual elements – Residual elements also known as tramp elements cannot be removed from the steel during processing. Hence, the amount of these elements in the product is a direct function of the amount of these elements charged to the steelmaking process through charge materials. High levels of residual elements can affect casting and rolling operations besides affecting the product quality. By adding HM, which is a clean iron unit, in the charge mix the level of residual elements in the liquid steel can be reduced to acceptable levels through the process of dilution.
Nitrogen level in steel – Nitrogen is generally considered as undesirable impurity which causes embrittlement in steels and affects strain aging. Nitrogen in the liquid steel is present in the form of solution. During the solidification of steel in continuous casting, nitrogen is the main reason for the formation of blow holes. Hot metal because of its lower nitrogen levels provides a dilution effect and also results into generation of carbon mono oxide (CO) gas within the steel bath through C boil which further leads to lowering of the nitrogen content in the steel.
Removal of hydrogen – CO gas evolved during the decarburization process helps in the removal of hydrogen. It has been demonstrated that decarburization at a rate of 1 % per hour can lower hydrogen levels in the steel from 8 ppm to 2 ppm in 10 minutes.
Influence on productivity and other operating parameters – With the use of HM in the charge mix there is larger oxygen consumption which helps in improvement in productivity of EAF, in shorter tap to tap time, improved Fe yield and reduction in specific power consumption. With the use of HM in the charge mix, the saving is in the range of 4 kWh per percent hot metal to 8 kWh per percent hot metal in the charge. Fig 1 shows relationship of productivity and specific power consumption with percentage of hot metal in the EAF charge.
Fig 1 Relationship between hot metal in charge with power consumption and productivity
Role of carbon – During steel making in EAF, carbon is needed in order to react with the oxygen and iron oxide to help in slag foaming. Some carbon is also needed for meeting of the product requirement. Carbon is also charged in the EAF to react with in the bath with injected oxygen to produce CO gas as well as to give sufficient chemical energy input for saving consumption of electrical energy. The generation of CO gas within the bath helps in achieving low concentration of dissolved gases in steel. The CO gas bubbles also helps in slag foaming (create an emulsion) which helps to contain the electric arc and improve energy transfer to the steel bath instead of to the furnace shell and transfer to the bath by radiation energy. The transfer efficiencies under various slag conditions are given in Tab 1.
|Tab1 Type of arc and transfer efficiency of electrical energy|
|Sl. no.||Type of arc||% efficiency of electrical energy|
|2||Partly surrounded by foaming slag||65|
|3||Totally surrounded by foaming slag||93|
|4||Partly resistance heating||97|
|5||Total resistance heating||100|
It can be seen that totally immersing the electric arc in the foaming slag has huge positive effect on the energy transfer. Thus heat losses through radiations can be reduced. The total amount of carbon addition needed in operation of EAF depends on several factors namely (i) carbon content of feed materials (ii) planned level of oxygen consumption (iii) desired tap carbon level (iv) economics of Fe yield as compared to the carbon cost and (v) capacity of exhaust gas system. Carbon is normally added in the EAF in the form of coal or coke which has got an ash content which is an unwanted material in steelmaking. Hot metal has carbon in the range of 3.8 % to 4.5 %. Hence the use of hot metal saves a lot of cost when compared with the addition of carbon to the bath from external sources.
Due to erratic charge carbon recovery in the EAF, many EAF operators have turned to high-carbon feed materials such as pig iron or hot metal etc. as a way to reduce the variations in steelmaking operations. When large quantities of pig iron or hot metal are used, it is not necessary to add charge carbon at all. Every 1 % of hot metal in the charge supplies 0.435 kg per ton of charge carbon (assumes 4 % C in hot metal, and 92 % scrap yield). Thus, 20 % hot metal in the charge supplies the equivalent of nearly 9 kg per ton of charge carbon. The recovery of carbon contained in metallic feed materials is very high (typically 90 % to 100 %). When improved carbon recovery is taken into account, this amount of hot metal can replace 10 kg per ton of charge carbon to 60 kg per ton of charge carbon.
Design features – The key issue of the EAF is its decarburization capacity. High carbon content in the charge requires additional time for decarburization. EAF cannot utilize oxygen injection rates which are typical for BOF steelmaking practice. Hot metal share exceeding 40 % has been considered as a maximum limit above of which the EAF productivity is reduced due to insufficient oxygen injection capacity. However, presently EAFs are available which are designed to use upto 80 % of hot metal.
The oxygen injection limits in the normal designed EAFs are usually related to problems with extensive splashing phenomena, backfire, electrode consumption increase and erosion of refractory lining as well as reduced life of roof panels and refractory delta centre piece. Typical decarburization rates in normal designed EAFs ranges from 0.06 % to 0.1 %. Too high carbon level in the feed charge materials in such EAFs can increase tap to tap time due to this limitation. Typical relationship between decarburization rate and hot metal charge in the EAF charge materials is shown in Fig 2.
Fig 2 Hot metal charge in electric arc furnace
Comparison with carbon content of DRI and/or HBI – Further in case of DRI/HBI (EAF feed material) carbon is also needed to reduce iron oxide present in the DRI and /or HBI. This increases the requirement of charge carbon. In case of hot metal such requirement does not exist since hot metal is having 100 % metallization.
Silicon and manganese content of hot metal – Hot metal contains substantial quantity of silicon and manganese. These elements when they get oxidized provide chemical energy which further reduces consumption of electrical energy. Also these oxides being acidic in nature increases consumption of flux (CaO and MgO) to keep the required level of slag basicity in the furnace bath.
Hot metal charging
The charging of hot metal to the EAF sounds like a simple proposition though it is in fact quite complex. Charging of hot metal in an EAF needs extra care since contact with highly oxidized furnace slag or cold scrap can result in violent reactions. Loss of control during hot metal charging can result in overflowing of slag and metal from the EAF. Similar violent reactions normally happen when large carbon concentration gradients develop in the liquid bath during superheating phase. Loss of control during hot metal charging ends up with overflow of slag and steel from the furnace. In extreme cases, damages of electrode arms are also observed during violent eruptions in the furnace.
Charging of hot metal can be done in two locations namely through roof or slag door. Some plants charge hot metal to the EAF by swinging the roof and pouring it into the furnace. This causes very rapid mixing of the hot metal with the highly oxidized slag in the EAF and sometimes due to this explosions do occur. Hence, for this mode of operation, it is recommended that a slag deoxidizer is added prior to hot metal addition. Typical deoxidizers are silicon fines, aluminum fines and calcium carbide.
The points which are important during hot metal charging in the EAF are (i) hot metal charging is to be carried out with power on to avoid productivity losses, (ii) hot metal ladle tilting control is to be precise enough to ensure stable pouring rates, (iii) hot metal runner is to be as short as possible to avoid hot metal freezing, and (iv) the runner is to be preheated between the pouring operations.
In order to utilize the advantage of hot metal temperature, its charging into the furnace is to be done with a closed roof. The alternative method of charging the hot metal to the EAF is to pour it down a launder which is inserted into the side of the EAF. This method requires more time for charging of the hot metal but results in a much safer operation.
Normally, the logistic and layout limitations problems do not leave too much freedom to select the place where hot metal ladles can be delivered to the EAF shop, i.e. on the charging or tapping side of the furnace. The EAF design itself imposes additional limitations. Position of the transformer, off-gas exhaust, etc. seriously limit the available space, where hot metal runner can be inserted into the furnace and the actual runner positioning is a compromise among various considerations. The typical solutions are given in Fig2. The runner inserted through slag door is to be movable (by means of a dedicated hot metal charging car). In other positions, the runner can be either fixed on the furnace shell or on charging car.
The most serious disadvantage of slag door charging is pouring of hot metal against the flow of slag. In some cases this can result in poor phosphorus removal from the bath. Besides, pouring can be started only after the area behind the slag door is free from slag. Side-wall position of the runner is problematic in case of hot metal overflow. At that place, it is difficult to collect spilled metal. Furthermore, any overflow creates a risk for all piping installed in the neighbouring area. The runner located on the EBT balcony seems to be the most advantageous. Because of the limited scrap presence in that area, charging of hot metal can be started very early. In case of overflow, hot metal can be collected in the tapping pit below the furnace.
Most modern operations inject oxygen at several locations instead of using the single slag door lance. Under this condition, it has been established that hot metal charge of 30 % to 40 % is more suitable for EAFs. Hot metal charging upto 50 % has been successfully used in some of the EAFs. However, hot metal charging of more than 50 % results in operational problems as excessive heat is generated through oxidation of elements, such as carbon, manganese and silicon, which can lead to overheating of the furnaces.
With the new advancements in EAF and process technology, high-speed carbon removal from hot metal can be achieved in the EAF without losing yield or overloading the off gas system. As EAF technology has evolved, the economic utilization of hot metal in the EAF has increased from a maximum of 50 % to as high as 80 %. The full utilization of the hot metal energy content can be now achieved.