CONARC Process for Steelmaking
CONARC Process for Steelmaking
In an integrated steel plant, the primary steelmaking processes normally used are basic oxygen furnace (BOF) and electric arc furnace (EAF). These two processes also produce majority of the global steel. However, these process routes have their limitations in terms of raw material feed mix and total energy consumption. BOFs are designed to take up to 70 % to 90 % of hot metal (liquid iron) and 10 % to 30 % of solid charge like steel scrap, whereas EAFs are normally designed to take 100 % solid feed mix such as steel scrap, direct reduced iron (DRI), and pig iron as raw materials. So, to overcome these limitations, CONARC steelmaking process was developed which combines both BOF and EAF steelmaking processes.
CONARC steel making process are of two types. The first type is for the production of high-quality carbon steel. It has opened a new avenue in terms of process flexibility. The second type is for the production of stainless steel. The process for the production of stainless steel is described later in the article.
With the arrival of several steelmaking technologies and processes in the past century, it has become important for the industry to select the steelmaking processes which are efficient and optimized for maximum advantage in terms of finance and resource use. Steelmaking processes like the BOF and EAF have considerably influenced steel production over the last several decades and are being used extensively in the present-day era. However, these steelmaking processes have their limitations and short-comings in terms of energy consumption and process efficiency, and this has sparked a need for a more efficient steelmaking process which can also be used with a wide range of raw materials. One such steelmaking process is the CONARC steelmaking process, which has been used in several steel plants for the past few years because of its several advantages and broad applicability. The main reason for the development of the CONARC process is to get the benefits of both the BOF process and the EAF process under a single processing unit.
CONARC process for steelmaking was developed by Mannesmann Demag Huettentechnik (now it is part of SMS group). The objective for the development of this process was to utilize the benefits of both the conventional top blown converter steelmaking and EAF. The name of the process CONARC sums up the fusion of the two processes (CONverter ARCing). The technology of this process is based on the increased use of hot metal in the EAF and is aimed at optimizing energy recovery and maximizing productivity in such an operation. The process was developed for using any kind and mix of raw materials like hot metal, DRI, scrap, and pig iron for meeting the highest quality requirements for the production of all grades and qualities of steels covering a wide range from carbon steels to stainless steels. Depending upon the requirements of the finished products, CONARC process is followed by a ladle furnace or a vacuum degassing unit.
Hot metal is produced in blast furnace, Corex process, or Finex process. It contains around 92 % Fe (iron) and is at a temperature of 1,550 deg C. It is either transported to the steel melting shop either in the open top ladle or in torpedo ladle. Open top ladles are normally used when there exists a hot metal mixer in the steel melting shop.
The CONARC process also makes extensive use of DRI as one of its raw materials. The DRI contains low quantities of tramps or residual elements like copper (Cu), chromium (Cr), nickel (Ni), molybdenum (Mo), tin (Sn), arsenic (As), and zinc (Zn), and this implies that DRI can be blended suitably with scrap containing such impurities to reduce their presence in the final steel product. Also, the carbon content of the DRI can be controlled as per the requirements of a specific steelmaking practice. When DRI is to be melted under oxidizing conditions, the carbon content is kept as high as around 2 %. The metallic content of the iron in the DRI denotes the metallization of the DRI and is measured in percentage. Normally, around 92 % metallization is considered industrially viable, and this is achieved if gaseous hydrocarbons are used for its production.
There are certain requirements for some high-quality steel products and their consequences for the charged raw material and accordingly the technological steelmaking process is required to be to be selected. The requirements for maximum tolerable tramp elements (Cu + Cr + Ni + Mo) range from 0.13 % for deep-drawing steel to 0.35 % for heavy plates, and up to 0.8 % for seamless tubes and pipes (Fig 1). These tramp element requirements have a direct influence on the materials which can be charged in the steelmaking furnace. Tramp element levels of, e.g., 0,13 % or 0,35 % can only be realized by using large percentages of virgin materials such as DRI, HBI, pig iron, and hot metal. Scrap used as main charge material needs to be blended with a significant quantity of virgin materials. It is obvious that steelmaking shops based on scrap as charge material can hardly fulfil the stringent requirements imposed by most of the more sophisticated high-quality steel products. In this respect, CONARC process has flexibility with the percent of tramp elements as shown in Fig 1.
Fig 1 Quantity of hot metal in charge and tramp elements
The main principle of this process lies in the application of the steelmaking principles which are being used in both BOF and EAF processes. Also, this process aims to use hot metal extensively in the EAF part, to minimize energy wastage, and to maximize productivity. The BOF process is mainly used for the oxidation of the steel melt to remove impurities in hot metal (decarburization of the steel melt), while the EAF process is used for the refining and raising of the temperature of the steel melt.
CONARC process is the most flexible process for the production high-quality steel in a combination of the EAF process and BOF process. As seen in Fig 2, the CONARC process can operate with a variety of different starting materials. The twin-vessel process uses a top-blowing lance and electrically arcing electrodes which can alternatively be used in both identical vessels.
Fig 2 CONARC process of steelmaking
The basic set-up for the CONARC process consists of two similar furnace shells which are mainly lined with magnesia carbon refractories. It consists of two swivelling water-cooled top oxygen lance system and a swivelling electrode gantry which serve both the shells. Both the shells can be simultaneously operated with one shell on blowing stage and another shell on arcing stage, which increase the utilization and production of the furnace.
An average raw material feed mix used in CONARC steelmaking process consists hot metal (60 %) from iron making units like blast furnace, solid charge like DRI (38 %) and steel scrap (2 %). Fluxes like calcined lime and calcined dolomite are also added during the process for generating the slag. The hot metal contains carbon of around 4.5 %, manganese of around 0.7 %, sulphur of around 0.025 %, phosphorous of around 0.1 % and rest is iron. DRI contains Fe (iron) metallic of around 77 %, FeO (ferrous oxide) of around 9 %, carbon of around 1.7 %, sulphur of around 0.01 %, phosphorus of around 0.05 % and the rest is gangue containing SiO2 (silica), Al2O3 (alumina), MgO (magnesium oxide) and CaO (calcium oxide, or lime).
Principle of CONARC process
The basic concept of CONARC process is to carry out decarburization in one furnace shell and electric melting in another furnace. In Fig 3, the schematic principle of the process sequences can be seen. The process starts with the oxygen-blowing stage, which is followed by the electric arcing stage. The whole process operates in sequences inter-changeable in both furnace shells, starting with hot metal charging, then the oxygen lance is put into operation and start of the blowing. At the end of the blowing stage, solid charge materials like scrap and / or DRI are charged into the furnace shell, the electrodes are swung in the operating position and arcing starts. At the end of arcing, deslagging takes place. Finally, the liquid steel is tapped into the ladle.
Fig 3 Principle of CONARC process
Procedure and steps followed in the CONARC process
The procedure of steelmaking by the CONARC process consists several steps as described below.
The making of a heat in shells of the CONARC process starts with filling of hot metal into the charging ladle from a torpedo ladle or a hot metal mixer. The charging ladle then charge the hot metal into the liquid heel available in the shell. The process of charging of hot metal in the CONARC steelmaking is carried out either through the roof or through a launder. The charging method plays an important role in altering the energy consumption of the CONARC process and the subsequent processes. Because of the specific advantages and disadvantages of the charging methods, launder charging is practiced extensively in steel industry.
In case of charging through the roof, the significant advantage is that the temperature drop of the hot metal is minimized. However, the opening of the roof results into high quantities of radiation losses, hence, making roof charging unconventional in the steel industry. In case of charging using launder, the hot metal is poured at a specific rate into the vessel. The advantages of using launder charging are no heat losses by radiation, and no increase in the tap-to-tap time of the heat because of roof opening and closing.
After charging of the shell, oxygen lancing of the melt is done for carrying out the decarburization and for removing other detrimental impurities. As the decarburization is about to come to the end, solid materials e.g., DRI, and / or steel scrap are added into the furnace shell. At the end of the blowing stage, the electrodes are swung in the operating position and are lowered to start arcing for the melting process. The temperature of the liquid bath is increased to the needed level and then tapping out of the liquid melt is done in a heated steel teeming ladle.
In the procedure of steelmaking by the CONARC process, it can be seen that the CONARC steelmaking process is divided into two stages after the charging or the vessel with hot metal. These are oxygen blowing stage (the converter stage), and electric arcing stage (EAF stage), which is followed by tapping of steel into the ladle from the shell after achieving the required tapping parameters. The charging of the hot metal from the charging ladle into the shell is normally done through a refractory lined launder.
In the blowing stage, oxygen from the top oxygen lance perpendicular to the hot metal bath is blown at supersonic velocity into the liquid melt. At this stage, the chemical energy is produced because of the exothermic oxidation reactions. In this stage, the carbon, silicon, manganese, and phosphorous contents of the bath are oxidized and removed as slag, since these are the unwanted elements present in the steel melt. The chemical reactions associated with removing these impurities are exothermic, meaning that the reactions are accompanied by the generation of a large quantity of heat. This heat helps in the melting of cold materials such as scrap or DRI. This also results into the utilization of the generated energy because of the exothermic reactions taking place in the furnace shell, which in turn avoid overheating of the liquid steel bath in the furnace.
Fluxes are added during this stage for the generation of slag, which serves as a sink to remove the impurities generated from the oxidation reaction and also to protect the refractory lining. In this stage, silicon, manganese and phosphorus in the bath are converted to their respective oxides and combine with the fluxes and form slag. In this oxygen blowing stage, the carbon is oxidized to gaseous products like CO (carbon mono-oxide) and CO2 (carbon di-oxide) and removed through the flue gas, hence reducing the carbon content in the bath from initial percentage of around 4.5 % to 0.3 % – 0.5 %.
In the arcing stage which is the EAF stage, the electric energy is used to melt the solid charge and superheating the bath to the tapping temperature. During this stage, the remaining solid charge material like scrap or DRI is charged into the liquid bath for attaining the needed tapping weight of the heat.
The heat produced from the electric arcing through the 3 numbers graphite electrodes is used to increase the temperature of the bath and also melt the solid charge like scrap or / and DRI. Fluxes are also added in this stage for producing good foamy slag. During this stage, oxygen is blown through multiple CoJet injection lances for the oxidation of the bath and reduction of the carbon content. The CoJet injection lances are mounted on the furnace side wall. The carbon content normally is reduced to 0.02 % – 0.03 % as per the tapping steel chemical composition requirement. The end point percent carbon parameter is very important in CONARC steelmaking as it determines the productivity and quality of the steel produced.
While finishing of the heat, the temperature of the bath is increased to the required value, and after the required temperature is achieved, arcing is concluded, and deslagging is carried out. This is followed by tapping of steel into the steel teeming ladle.
Important tapping parameters of this primary steelmaking process are tapping temperature, tapping carbon, and phosphorus content in the steel. These are very critical for meeting the requirement of the downstream processes such as secondary steelmaking and continuous casting, and finally for meeting the requirements of the customers.
Decarburization in steelmaking is a very complex because of high temperature kinetics and thermo-dynamics, multi-phase and turbulent flow. End-point carbon of steel is very critical as it determines the productivity, and quality of the process. In the past, end point prediction has initially depended on the skill and experience of the operators. Presently several different ways of estimating the end point parameters in steel are available for ensuring the final product quality and process efficiency. Prediction of end point carbon in steelmaking process, has undergone through different stages like static prediction, dynamic prediction, and intelligent prediction. One of the most common methods is to do the mass balance and heat balance for estimating the final temperature and composition of the steel. This is a static method which mainly depends on the characteristics of the raw materials.
Major equipment for the CONARC process
The basic equipment of CONARC process consists of two identical furnace shells which are refractory lined, one slewable electrode structure with one set of electrodes serving both the furnace shells, one electric supply (transformer etc.) for both the shells, and one slewable water-cooled top oxygen lance system serving both the shells. Alternatively, two stationary top lances, one for each furnace shell for the blowing of oxygen can also be used. The furnace shells are equipped with multiple CoJet injection lances for the injection of oxygen as per the process requirements. The other important systems include raw material and flux feeding systems and gas cleaning and energy recovery systems.
The furnace shells are equipped with the bottom stirring devices integrated to the bottom of each of the furnace shell as well as burners and injectors systems in the shell for the injection of fuel and carbon as per the process requirements. Concerning the equipment used, there are automatic systems, so-called lance manipulators, and manually operated lances. With respect to the injected material one can distinguish between oxygen, carbon and dust injections.
The oxygen lancing system in CONARC furnace can be made more dynamic by using the jet-box system, which decreases energy consumption and contributes for improving the productivity of the CONARC process by reducing the tap-to-tap time, as the system increases the process dynamism significantly.
In the CONARC furnace, the furnace shell is constructed out of water-cooled panels and its height is increased. It facilitates the top lancing of oxygen in the furnace. However, the top blowing always results in emulsion formation during refining, which is deleterious as slopping is undesired during the top blowing of the oxygen.
It is important to note that the two CONARC shells can be used interchangeably, however, only one shell can operate either as a BOF shell or as an EAF shell at a particular instant, and hence, the second shell is required to operate in the other mode. Hence, decarburization of the melt is carried out in one shell, while electric melting of the raw materials is carried out in the other shell at a given instant.
The type of furnace which is used for melting plays an important role in affecting the energy consumption of the CONARC furnace. Both an AC (alternating current) EAF or a DC (direct current) EAF can be used for melting, and each of the two furnace types has its pros and cons. DC furnace is easier to operate mechanically, while AC furnace is easier to maintain electrically. Presently AC EAF is preferred for use though DC furnaces have also been used in the past.
Refractories for CONARC furnace shell
Magnesia-carbon (MgO-C) refractories are also used for the lining of the CONARC furnace shells. MgO-C refractories are used in the CONARC furnace shells because of their superior corrosion resistance and thermal spalling resistance. MgO-C refractory bricks of different thicknesses and quality are used in the different regions of the CONARC furnace shells based on the wear experienced by the refractory lining in each region. In the regions experiencing more wear, MgO-C refractories with a higher carbon content, higher density, and higher purity of raw materials are typically used. The refractory lining is hence installed in such a way so that the minimum quantity of refractory lining is left in all the places in the furnace after a specific time in service.
The MgO-C refractories used in the CONARC process experience severe wear as a result of the varying slag composition during the blowing phase. The failure of the MgO-C refractories leads to failure of the refractory linings of the CONARC furnace shells. If the refractory linings of the furnace shells are not replaced within time, it leads to enormous losses in production time, production equipment and sometimes in the product itself. Refractory manufactures take high care to optimize the mechanical, physical, and thermal properties of the refractory materials.
Failure of the refractories normally occurs as a result of either mechanical wear or chemical wear. Chemical corrosion occurs mainly because of the oxidation of carbon and the dissolution of magnesia grains by the slag. It is important to optimize operating conditions in a process in order to extend the lifetime of refractory materials.
Typical operating parameters
The performance parameters which are given here are from a CONCARC furnace of 170-ton capacity. The relationship between hot metal and DRI as charged material can be varied over a wide range. In production, the relationship of hot metal and DRI covers a range from 30 % to 70 % up to 75 % to 25 %. Fig 4 shows typical performance parameters of CONARC process.
Fig 4 Typical performance parameters of CONARC process
The graph in Fig 4 shows that (i) a power consumption of less than 200 kWh/t (kilo-watt-hour per ton) is achieved with 75 % 0f hot metal in the charge, (ii) the consumption of oxygen is dependent on the percentage of hot metal in the charge, and (iii) the tap-to-tap time is more or less constant over the full raw-material range.
The CONARC process for stainless steel
The successful results of the CONARC carbon steel process have given rise to the idea of introducing this innovative process for the production of stainless steel.
The conventional stainless steelmaking process starts either with scrap or with hot metal from a blast furnace and ferro-alloys. The hot metal has to be treated in a special process for decarburization, dephosphorization, and desiliconization (DDD unit) for satisfying the requirements of stainless-steel production. Depending on the finished product mix and its average carbon content, either the ‘duplex’ route or ‘triplex’ route is to be selected. The duplex route, represented by a one-step decarburization, is best suited for carbon levels higher than 4 ppm (parts per million). If ultra-low carbon levels (less than 2 ppm, super-ferrite) represent a greater part of the product mix, the triplex route with its two-step decarburization is the most economic process route.
The CONARC process for stainless steel which combines the EAF with the stainless-steel converter opens up a new economic window for stainless steel production. All types of raw materials can be charged, and even products with an extra-low carbon content (less than 2 ppm) can be produced using a VOD (vacuum oxygen decarburization) vacuum tank degasser. The process sequence is totally reverse to that of carbon steel production.
Fig 5 Schematic of CONARC stainless steel production process
The process sequence as shown in Fig 6 is totally reverse to that of carbon steel production. As in conventional stainless-steel production, the process starts with melting-down of the scrap and ferro-alloys in an EAF. The second stage, refining of the steel, takes place in the same furnace shell with oxygen blowing in combination with bottom stirring. The complete process is performed in only furnace shell which has outstanding economic advantages for the total production.
Fig 6 Process sequence for CONARC stainless steel production process
In comparison with the conventional ‘duplex’ or ‘triplex’ processes, the CONARC process for stainless steel yields a considerably higher productivity (up to 30 %) because of shorter tap-to-tap times for the entire process. Based on 150-ton melting units, capacities of more than 1.1 million tons per year can be achieved. Calculations prove that production cost savings of up to 50 USD per ton and a reduced specific plant investment of around 20 % in comparison with conventional stainless-steel routes can be expected.
Energy recovery
A two-stage boiler system is used for the waste heat recovery. In the first stage, the waste gas from the CONARC process is led through a boiler system consisting of a swivel type elbow, a post combustion chamber, and a hot gas line and cooled to 600 deg C. These components are designed as pressure parts for steam generation. In the second stage, the waste gas is cooled down to 200 deg C in a vertical pass boiler which is especially developed for this type of application.
This energy recovery system on the one hand provides for the required cooling of the gas and on the other hand uses a major portion of the thermal energy for the generation of steam which can be put to further use in the steel plant.
Advantages
The CONARC process is the technology which has been developed for the most economical way of making of steel because of its extremely high flexibility in terms of, charging of any kind and mix of raw materials, the use of different energy sources, and the production of all steel grades covering a wide range from carbon to stainless steel. The CONARC process is the best tailor-made solution for steelmaking which allows maximum flexibility without any equipment modifications when producing carbon or stainless steel.
The CONARC process has several advantages which include (i) high flexibility with respect to material input, scrap, liquid iron, and DRI which can be used in different mixing ratios as per the requirements of the steel quality and / or availability and / or unit prices of the materials, (ii) CONARC concept permits this process to cover the full range from pure EAF operation with 100 % scrap / 100 % DRI to pure converter operation, (iii) CONARC furnace is not a sheer electric melting unit but also act as a chemical reactor, hence, controlling the concentration of tramp elements in the steel because of the increased quantity of the use of virgin materials, (iv) CONARC process is flexible and can handle hot metal with changing contents of silicon, phosphorus and sulphur without any problems, (v) CONARC process also has advantages when treating hot metal with higher phosphorus content (up to 0.2 %) since, several aspects such as simple temperature control by adding DRI as coolant, addition of lime, as well as the withdrawal of phosphorus oxide rich slag by constant slag overflow through the slag door help to achieve an efficient phosphorus removal, (vi) tap to tap times of less than 40 minutes are easily achievable, (vii) large flexibility regarding energy resources, (viii) efficient energy recovery system reduces CO2 emission levels, (ix) effective energy recovery system of the process has a significant and sustainable contribution towards the energy efficiency of the steel plant, and (x) smooth network disturbance.
Comments on Post (1)
yogesh sajjanwar
Nice one Sir.
Regards