Innovations in refining process technique Combined blowing vacuum converter with CO2

Transcription

1 Lutz Rose Jan Reichel Thomas Germershausen SMS Siemag AG Düsseldorf / Germany INTRODUCTION Innovations in refining process technique Combined blowing vacuum converter with CO2 The vacuum converter has been developed on the basis of the AOD process and the VOD process. The new converter process eliminates disadvantages of the ladle metallurgy refining process under vacuum, Fig.1. The principle of this technology is the combination of advantages of the AOD process with those of the VOD process. For this purpose, the oxygen blowing capacity is increased, the reaction volume adapted to slopping minimizing and the mixing energy optimized to ensure shorter treatment times. The vacuum converter is supposed to be more cost-effective than the AOD process, to achieve better metallurgical results and to increase the production. The total investment for the vacuum converter is lower than the complete investment for the AOD and VOD plant configuration plus auxiliary equipment. The new process can be used for the production of stainless steel as: - austenitic steel grades - ferritic steel grades - superferritic steel grades - duplex steel grades - martensitic steel grades - special-purpose steels as well as for the refining of ferroalloys such as - Fe Cr HC - Fe Mn HC The metallurgical process is divided into two process steps. The main decarburization takes place at atmospheric conditions, while the final decarburization is performed under vacuum. Moreover, the new process provides the possibility of using carbon dioxide as inert gas. Oxygen, or an oxygen/co2 mixture, or an inert gas is blown in through the bottom tuyères. Especially in the case of premetal with high initial carbon and silicon contents, this process distinguishes advantages which will be presented in detail in the next chapters. VOD (Vacuum Oxygen Decarburization) process In the years occurred the development of the VOD process Patents: TEW Witten, Dr. Hans Georg Bauer Dr. Josef Otto, Finkl & Sons Chicago, Chuck Finkl Attainable carbon contents: 0.005% [C] Page 1

2 With the development of the VOD process in the process line with an electric arc furnace as melting unit, the production of stainless steel became economically efficient for the first time. By utilizing the vacuum technology, it was possible to reduce the partial pressure of CO in liquid metal to less than 1 mbar. The chromium loss in the final decarburization phase was reduced drastically. Fig. 1: VOD plant VODC (Vacuum Oxygen Decarburization Converter) process The VODC process, Fig.2 is a further development of the VOD process. The liquid metal is treated in a converter vessel. Just as in the VOD plant, the converter vessel is closed with a vacuum cover. This process also utilizes a top lance and a bottom stirring system. Thanks the larger reaction space in the converter vessel, it is possible to operate with an increased specific oxygen blowing capacity. As a rule, the value lies at approx. 0.7 Nm3/(t xmin). These measures led to significantly reduced decarburization time. Apart from that, the same metallurgical results are achieved as in the VOD process. Fig. 2: VODC converter plant Page 2

3 AOD (Argon Oxygen Decarburization) process Developed in 1967 and patented by William A. Krivsky dated June 27, Attainable carbon contents: 0.015% The AOD process (Argon, Oxygen, Decarburization) is the first production process for stainless steel grades in which the treatment time in a converter under atmospheric conditions corresponds to the casting time of a slab caster for many steel grades. On this way an essential prerequisite for the economically efficient production of stainless, acidand heat-resistant steel grades could be fulfilled, Fig. 3. Fig. 3: Typical process lines for stainless steelmaking and refining of ferroalloys The vacuum converter process The vacuum converter process combines the advantages of the AOD process with those of the VOD process. For all stainless, acid- and heat-resistant steel grades, the treatment time including the vacuum treatment is shorter than in the conventional AOD process. Furthermore, the process provides the option of refining FeCr HC or FeMn HC. By using CO2 as refining gas, the consumption of argon is reduced drastically, leading to significant savings on production costs and investment costs, Fig. 4. Fig. 4: Reference comparison AOD vs. vacuum converter as operating costs for the final decarburization in the converter Page 3

4 The partial pressure reduction of CO is effected by injecting relatively small amounts of CO2 / inert gas via side tuyères and by means of a vacuum in the converter vessel. With this process technology, lower partial pressures and a higher mixing energy can be attained than in the AOD process, Fig.5. Fig. 5: CO partial pressure as a function of carbon removal efficiency The reaction equilibrium between chromium and carbon is much more favorable compared to the AOD and the VOD process, Fig. 6 and 7. Fig. 6: Chromium - Carbon equilibrium AOD Converter - vacuum conv. Fig. 7: Chromium - Carbon equilibrium VOD - vacuum converter On this way, the metallurgical prerequisites for the decarburization reaction are more favorable as well. The decarburization speed is higher and the final carbon contents which can be attained are lower than conventional. Higher decarburization speed reduces the total treatment time increasing the production, Fig. 8. Fig. 8: Reference comparison in operating time AOD vs. vacuum converter The same applies for the nitrogen content at the end of the converter process, Fig. 9. Page 4

5 Fig. 9: Chromium-nitrogen equilibrium for ferritic stainless steel grades Thanks the optimized conditions during the decarburization reaction, the chromium loss is reduced significantly. In the production of stainless steel grades, a chromium yield of >98% can be attained. As a result, less reducing agents and slag formers are required. VACUUM CONVERTER PROCESS OPERATION AND EQUIPMENT Vacuum converter process. Fundamentals The technology serves for the production of stainless steel / ferroalloys for different refining grades. High initial carbon and silicon contents classify all refining processes as strong exothermic processes requiring intensive cooling. However, regardless of the main element in the metal, the decarburization step of this process has the same characteristics as the carbon or stainless steel refining processes. The treatment is principally divided into two steps, the main decarburization and the final decarburization, Fig. 10. Main decarburization Final decarburization 0.15%/min -dc/dt 0.06%/min C 1.6% 0.7% Vessel pressure O2-Blowing rate 0.015% Fig. 10 Decarburization course of vacuum converter process Both steps are separated by the so-called critical carbon content. All recorded decarburization processes confirm these decarburization characteristics. Page 5

6 In the main decarburization step, the time span depends on the blowing parameters, and in the final period it depends on the thermodynamic potential of the carbon oxidation. In the main period, the oxygen blowing rates determine the decarburization speed, eq. (1). After the critical carbon concentration the decarburization speed is controlled by the thermodynamic potential of carbon oxidation, expressed by the difference between the current and the equilibrium carbon concentration, eq. (2). dc dt 100 W Q O 2C (1) where: - oxygen flow rate reacting with carbon Q 2 W O C - metal weight So far the critical carbon concentration is obtained, the speed of the carbon transportation to the reaction places determines the decarburization rate. The decarburization rate may be then approximated by dc dt where: - mass transfer coefficient k C A - reaction surface V - volume of metal C, C*- current and equilibrium carbon concentration in the metal k C A V C C * (2) From a practical point of view, it is difficult to predict accurately either kc or the interfacial reaction area A. However, estimation can be made regarding the magnitude of these quantities based on the turbulence theory, since both the interfacial reaction area and the mass transfer coefficient are related to the rate of turbulence energy dissipation. For this reason, the two quantities can be combined to a single parameter 1, the overall oxygen exchange coefficient: 1 k C A V (3) The dimension of τ is the time second; that is why it is convenient to define it as a reaction time constant of the decarburization. Thus, the rate of decarburization is expressed by dc dt 1 C C * (4) Unfitted oxygen blowing to the thermodynamical conditions after passing the critical point leads to high metal oxidation with an extreme temperature increasing. To avoid such effects the process control requires a blowing pattern adaptation. This adaptation changes the oxygen supply to the decarburization process and consequently results in minimum oxidation of metallic components indirectly preventing an excessive temperature increase. It can be said that optimal process control during the final step determines the economic effects of the overall process. Finding the optimal conditions requires an analysis of the process kinetics and the principles of the decarburization process at a given circumstances. The decarburization mechanism is determined by the following reactions: Page 6

7 C O CO n (5) Me m O Me O (6) n m Me O mc nme mco From eq. (7) the process equilibrium can be expressed as and at the carbon activity as n m (7) a p K( T ) (8) a n m Me CO m Me O a n m C Thus the carbon equilibrium can be expressed as where * a C fc C (9) n O m CO C * * V Me, X, T, a Me p (10) Me - main component of the melt V* - complex function depending on metal chemistry, temperature, slag oxide activity X - co-existing elements such as Si, P, S, Mn, Cr etc. T - melt temperature a - oxide activity of the main element Me n O m pco - carbon monoxide partial pressure The carbon monoxide partial pressure is defined as p CO N CO p (11) N N CO IG whereby dc N CO k 1 (12) dt NCO - number of removed CO moles from the metal and and N IG k2 Q IG (13) N IG - number of inert gas moles at an operating flow rate Q IG. In combination of the eq. (4) and eq. (10) the decarburization control by vacuum converter can be expressed as Page 7

8 dc dt dc k1 1 dt C dc k1 k2q dt IG p (14) It can be seen that in the final decarburization there are two parameters forcing the decarburization speed, i.e. the outside pressure p and the inert gas flow rate QIG. In extreme cases, if the outside pressure equals the atmospheric pressure, the only one possibility to control the decarburization is the inert gas flow rate (typical AOD process). In other cases, if the inert gas flow rate is reduced to the stirring rate, the decarburization can be controlled via the vacuum pressure, respectively. Combining both parameters one obtain extreme low carbon equilibriums and in consequence low carbon contents. Besides this effect, in cases of standard decarburization the inert gas consumption can be significantly reduced when the pressure level in the vacuum system is decreased. The control takes two-dimensional form at a constant temperature providing an optimal solution at maximal decarburization speed. Using carbon dioxide as inert gas the control of the decarburization is reduced to the outside pressure only. Fig.11 presents the carbon equilibrium of a ferrochromium treatment dependent on the outside pressure and temperature. At CO2 as inert gas and at 1700 C, the carbon equilibrium is established at 1% if the operated vacuum obtains a level of 200 mbar. Fig.11 Carbon equilibrium of a FeCr MC alloy Corresponding to the Boudouard reaction, the operation of CO2 enables additional decarburization and desiliconization of the melt as below CO O CO 2 (15) or + C CO = 2CO (16) CO O CO 2 (17) Si 2 (18) 2 SiO Both reactions are strongly endothermic and support the main oxidation reactions with additional oxygen being free from the decomposition of CO2. Page 8

9 Vacuum converter equipment and operation Fig. 11 shows the general design of the vacuum converter. The connection between the converter vessel and the movable cover is established by an insulated water cooled flange below the converter mouth. The converter is equipped with movable top lance and a movable water-cooled stack bottom part. Fig. 11 General geometry of the vacuum converter vessel This movable device for traversing is also equipped with a vacuum cover, Fig.12. Fig. 12: Vacuum converter during vacuum operation The process gas supply oxygen, carbon dioxide, argon and nitrogen is connected to the bottom tuyères via a trunnion ring. The number of tuyères varies depending on the converter size. Temperature control is ensured by a continuous material addition system with a charging rate that is contrary to the increasing temperature gradient of the process. Page 9

10 Samples are taken and temperature measurements performed at the process start; after the desiliconization, the main and the final decarburization as well as the tapping and complete the treatment. In the first step the process is started with the main decarburization using a top lance and bottom tuyères under atmospheric conditions. In the second step the vacuum cover is used for the final decarburization. Oxygen blowing during the vacuum step is performed by bottom only. Both steps, the main decarburization and the final decarburization, are completed by a conventional slag reduction process. Via vacuum pump system pressures can be controlled across the entire range between 1 and 1000 mbar. STAINLESS STEEL PRODUCTION The new vacuum converter process developed by SMS Siemag for the production of stainless steel combines the advantages of the AOD process with an operation under vacuum conditions, leading to deepened reducing carbon, hydrogen and nitrogen in the steel, Fig. 13. O 2 injection Moveable lance Vacuum cover AOD Argon Oxygen Decarburization Converter vessel VOD Vacuum Oxygen Bottom blowing system O 2 /CO 2 Inert gas injection Vacuum converter Figure 13: Innovated vacuum converter with CO2 injection for refining special steels and ferroalloys The conversion of existing AOD converters to vacuum converters will be possible in most cases. The installation of a transfer car for a combined VOD and converter hood as well as a vacuum pump system will be required. For an AOD with a capacity of 100 tons, the investment for the additional equipment will be approximately 6 million. The possibility to reach lower final carbon, hydrogen and nitrogen contents in the steel compared to the AOD process gives producers the opportunity to improve the process reliability and expand their product portfolio in the field of special low carbon and nitrogen grades. When operating with reduced pressure, the expensive process gas mix of oxygen and argon used in the AOD process can be partly replaced by the cheaper mix of oxygen and CO2. The use of argon can be limited due to favorable thermodynamic process conditions for decarburization and nitrogen removal under vacuum conditions. The decarburization speed in the final process stages and the oxygen efficiency are higher compared to the conventional AOD process. The vacuum converter technology provides potential for treatment time reducing by 3.5 minutes per heat due to the higher decarburization speed. The productivity of steelmaking plants can be increased up to 5%. The implementation of the newly Page 10

11 developed vacuum converter technology has the potential to reduce the specific argon consumption by approx. 10 Nm3/t for low carbon grades. Additional, the process allows to save oxygen and silicon for reduction, which in total reduce operating costs at approximately 10 /t. The vacuum converter process is highly efficient process, which offer great potential for the reduction raw materials and media consumptions and enables not only cost reduction for the steel producer but is also an environmental technology. The overall environmental effect results from lower energy consumption and greenhouse gas emissions. One example for a high energy-demanding process is the production of oxygen and argon within an air separation plant. Considering a yearly production of 1,000,000 tons and a specific energy consumption of 5 kwh/nm3 in the air separation plant, it will have an ecological benefit in reducing of energy consumption by 50 kwh/t or 50 MWh per year in total. Ferroalloy refining Industrial-scale production of ferroalloys such as FeCr and FeMn usually takes place in submerged arc furnaces. The vacuum converter with CO2 as inert gas is an ideal device for refining ferroalloys with high carbon and silicon contents. In recent times, excess capacities for these products have been leading on the international market to the price decline, Fig. 14: Sales price of FeCr HC and FeCr LC Fe Cr HC Price 0,975 $/lb Cr 1,656 /kg Fe Cr LC Price 1,93 $/lb Cr 3,27 /kg Delta Price 1.62 Delta Price 1,621 /t FeCr Conversion Cost /t FeCr Capital Cost /t FeCr Transport Cost 65 /t FeCr Profit /t FeCr Typical Production 65,000t / Year FeCr Profit 62,465,000 / Figure 14: Profit estimation ferrochrome refining * Without taxes Calculation on Market Basis On the customer side the steel industry places ever-increasing demands on the ferroalloy production. The market asks for alloys with low and ultra-low contents of carbon and Page 11

12 phosphor and other accompanying elements for the production of new steel grades with high chromium or manganese contents. This applies for both the production of carbon steel as well as for stainless grades and special steels. Here the typical steel grades with high Cr and Mn contents such as TWIP / TRIP / P550 / duplex and superferritic steel grades. These requirements are the main reason for developing of the vacuum converter process by means of which the production of such ferroalloys is possible. Thanks to the new vacuum converter using CO2, these requirements can be now implemented both in terms of economy and ecology. By means of this technology, the decarburization process occurs with maximum yields of chromium and manganese. While large amounts of argon and/or steam are used for conventional converter processes, in the new process the so-called greenhouse gas CO2 is employed. Two effects are achieved here: 1. The injected carbon dioxide is decomposed to carbon monoxide and oxygen by means of a strong endothermic reaction; it means the gas acts as coolant. While conventional refining processes require up to 30% of cost-intensive coolants, the new process depending on the converter size works completely without coolant, thus economizing the consumption figures and the yield of the refined material. 2. Furthermore, high temperatures in the converter vessel are avoided since the optimum conditions for the decarburization reaction are set via the vacuum. Compared to conventional processes, the wear of the converter vessel refractory lining is significantly reduced thanks to the lower temperature level and shorter treatment times. Moreover, the generated carbon monoxide acts as a reducing agent and improves the kinetics in the reaction vessel. From one cubic meter of injected CO2, two cubic meters of high-density carbon monoxide are produced. Compared to other processes, injecting this medium generates a higher momentum, which results in a higher mixing energy. The slagging of chromium or manganese is reduced to a minimum. That is why the new process also helps significantly to reduce the consumption of expensive reducing agents and slag formers. In the conventional production lines, slags with their properties can be disposed on special landfills only. With the new process, the content of critical elements can be reduced by 80%. As mentioned before the vacuum converter technology enables production of ferroalloys in the ranges of Medium Carbon/Low Carbon/Ultra Low Carbon. It extends product spectrum and enables to enhance better prices for a high-quality product with unique selling points. Vacuum converter with CO 2 industrial test On the basis of laboratory test results from the AGH University of Science and Technology / in Cracow, Poland, SMS Siemag AG decided to run industrial tests on ferrochrome high carbon refining in a modified 10t vacuum converter at Metso Minerals Oy in order to prove the novel refining technology and its industrial functionality and viability. The tests were carried out in the foundry in Tampere, Finland, Fig.15. The vacuum converter and the EAF, both with a capacity of original 15 t and designed for the production of austenitic and ferritic steel grades, have been modified by SMS Siemag and fitted to required test conditions, Fig.16. The EAF dolomite refractory was replaced by chromite-magnesite due to the high melting temperature of the FeCr HC resulting from the high liquidus temperature. High temperature losses during the melt transfer and charging into the converter increase the necessity of a higher EAF tapping temperature. The original blowing system of the vacuum converter, consisting of a top lance only, has been extended with two additional bottom tuyères. The gas control was implemented in Page 12

13 the form of a valve station and a computer control system developed especially for this configuration. Figure 15: FeCr HC refining process line by vacuum converter The metallurgical test results confirmed the theoretical process calculations and the benefits of CO2 application. Fig. 16: Impressions of the test installations at the Metso Minerals Oy facilities. The test results performed the technology concept of the FeCr HC refining by oxygen and CO2. The initial chemistry: C% Si% Cr% P% S% was transformed into FeCr MC with: and with a chromium yield of 95.6%. Page 13

14 SUMMARY The vacuum converter technology combines the advantages of the AOD converter -and the VOD technologies. In the vacuum converter process, the final decarburization of stainless steel grades and ferroalloys takes place under vacuum. The dilution of process gases known from the AOD process is largely omitted. Argon, which is relatively expensive, is partly or fully replaced with CO2. In the chemical industry, many processes generate carbon dioxide as a waste product, what explain the low price of the gas in comparison to argon. Furthermore, this gas has an excellent cooling effect, so that for the production of ferroalloys less of the cost-intensive coolants are required. For stainless, acid- and heat-resistant steel grades, the production costs for the final decarburization are reduced by approx. 7%. Depending on the initial and the target carbon contents, significantly higher values can be attained for ferroalloys. The vacuum converter technology is suitable for production of duplex steel grades with high nitrogen contents as well. In this case the nitrogen increasing process is carried out by means of much higher nitrogen blowing rates by bottom tuyères, approx. 1 Nm3/(t min) in comparison with typical VOD stirring blowing rates, approx Nm3/(t min). The product portfolio for high-chromium steel grades with minimum carbon and nitrogen contents can be expanded. Ferroalloys such as FeCr and FeMn can be refined to carbon contents lower than 0.1%. Due to the low conversion costs, this process is able to generate high added value for high-quality ferroalloys in the Low Carbon and Ultra Low Carbon ranges. The productive output of a plant using the vacuum converter technology is increased because the final decarburization requires less time due to the high oxygen blowing capacity and high decarburization speeds. The total investment costs are approx. 40% lower than for a plant configuration with an AOD converter and VOD plant. An industrial test under production conditions was successfully completed. The economic benefits can be summarized as: - Significant reduction of TCO approx. 7% - Savings on investment costs for equipment approx. 9% - Savings on building space costs for construction part approx. 7% - Expansion of product portfolio - Reduced energy loss approx. 12% - Savings on operating personal approx. 40% - Savings on production costs (Refractory, argon, reducing agents) approx. 5% Page 14