J. Min. Metall. Sect. B-Metall. 49 (2) B (2013) 175 - 181 Journal of Mining and Metallurgy, Section B: Metallurgy DECARBURIZATION OF FERROCHROME AND HIGH ALLOY STEELS WITH OPTIMIZED GAS AND SLAG PHASES TOWARDS IMPROVED Cr RETENTION H. Wanga,*, M.M. Nzottab, L. Tengc, S. Seetharamanc a Department of Ferrous Metallurgy, University of Science and Technology Beijing, Beijing, China b Research & Development (R&D), Process Development (Metallurgy), Uddeholms AB, Hagfors, Sweden c Division of Materials Process Science, Royal Institute of Technology, Stockholm, Sweden (Received 13 August 2012; accepted 05 March 2013) Abstract Chromium is a high value metal and the retention of the same during the refining of high carbon ferrochrome as well as high alloy steel has significant economic and environmental impacts. The loss of chromium during the decarburization is generally minimized using argon-oxygen mixtures thereby reducing the oxygen partial pressure (PO2) of the oxidant gas. In the current study, experiments were carried out in an induction furnace and CO2 was introduced with the view to partly reduce PO2 and partly as an oxidizer. During these experiments, the decarburization of molten Cr-alloy was conducted using pure O2, pure CO2 or O2+CO2 mixtures. The results demonstrated that the Cr loss can be minimized under CO2 introduction. The kinetic analysis showed that the mass transfer is effective due to the production of 2CO gas molecules from one CO2 molecule during the reaction which will improve the stirring of the bath. Besides, CO2 reacts with carbon in melt is an endothermic reaction, introduction of CO2 could be a cooler during the refining process, hence the temperature could be controlled by controlling the diluting gas amount, in this case, the over heat of bath refractory could be prevented and the lifetime of refractory could be extended. Key words: CO2; Cr retention; Partial pressure; Decarburization; Temperature control. 1. Introduction also the high alloy steel making process. AOD as a traditional and effective way is being used widely in Traditionally, O2 is selected as an oxidizer for the world. Steam (H2O) is another profitable decarburization in ferrochrome refining process and alternative for Cr retention during AOD process also Electric Arc Furnace (EAF) process. However, as according to UHT’s (Uvån Hagfors Teknologi AB, reported by earlier researchers, it is realizable to Sweden) references and experience through its CLU decarburize with CO2 as well. The principle of technology [9, 10]. To the knowledge of the present decarburization of Fe-C melts using CO2 has been authors, no systematic investigation has been carried expounded by Mannion and Fruehan [1]. Their results out with respect to the simultaneous decarburization showed that nearly one-fifth of CO2 supplied was and Cr-loss in Fe-Cr-C melts with introduction of consumed during the decarburization reaction. Zughbi CO2. [2] reported higher decarburization rate and more In the present study, the Cr retention during M- vigorous reaction when O2 was used as the oxidizing LCFeCr making process and high alloy steel making gas instead of CO2. Sain and Belton [3] have studied process is investigated with introducing CO2 in the the decarburization of liquid iron by CO2 at great gas system. length. CO2 was also introduced into the argon- oxygen refining process by Heise et al.[4]. It is 2. Theoretical analysis reported that the carbon removal efficiency in argon- oxygen decarburization process (AOD) was improved The order of oxidation ability for O2, CO2 and O by using CO2 argon-oxygen mixtures. Some authors in Fe-C melts is shown in Table 1. The overall [5-8] also carried out the study on the surface chemical reaction of decarburization with CO2 can be decarburization of Fe-Cr-C melts. Besides this, there described by equation (1). In the present calculations, are some researchers who investigated as to how to ω[i] = 1% (Henrian) was chosen as the standard state reduce the Cr loss during ferrochrome refining and of C and O . * Corresponding author: [email protected] DOI:10.2298/JMMB120813010W MOLTEN12 - Conference Special Issue Ninth International Conference on Molten Slags, Fluxes and Salts C C C 176 H. Wang et al. / JMM 49 (2) B (2013) 175 - 181 Table 1. The oxidation ability for O2, CO2 and O . 0 00 0 Reactions No. rGHTS( G , J/mol (1873K) 0 G0 1 rGT138905 41. 63 C+ Og2 () CO(g) (1) 2 -216697 2 WeakO exothermic 0 G rGT140170 125. 6 C+Cr O2 (ggCO) 2 (g) (2) -97909 C WeakC endothermic G 0 rGT 21755 38. 74 C+O CO (g) (3) 2 -94082 C 3 Weak exothermic G O ( 2C 0 3 rG=-1170480+350.41T 2Cr Og223() CrOs () (4) 3C -514162 2 Strong exothermic) 0 0 r rGT321505 97. 47 23Cr CO2 () g Cr23 O ( s))()3CO g (5) -138944 Weak exothermic ( r . / p It is seen in Table 1 that CO2 is a weaker oxidizing Here, C Cr agent compared to O2. From a thermodynamic point log10 fecC [ C ]%% eC [ Cr ] …(9) of view, it is possible to use CO2 as an oxidizer for Cr C decarburization, realizing that Cr will also be oxidized log10 feCr Cr [] Cr% eCCr []% …(10) by CO during this process from reaction (5). One C Cr C 2 The interaction parametereC values %of e,e,e C C Cr and point to be noticed is that reactions (1), (3), (4) and (5) Cr eCr can be obtained from literatures [11-13]. are exothermic while reaction (2) is an endothermic e The value of the Gibbs energy change for reaction reaction. With mixing CO in the gas system, the G . 2 (6) thus is given by: temperature of the bath could be decreased and also rGTT6 748740 476.. 68 19 14 controlled in some extent. Except for the possibility to use CO as a 066 .[]. CC%% 00714 []lgr 3 2 decarburizer, the Cr retention during the []CCrpp%%23 lg[ ] lg(CO / ) decarburization process needs to be considered as …(11) well. It is also well known that, in order to prevent Cr The critical temperature Tc was calculated at in the molten metal phase from oxidation, at the same different situations and the result is presented in Table time decarburization, reaction (6) given below should 2. Cr can be kept in the metal bath only if the C C be prevented. This can be illustrated by introducing temperature of the melt exceeds Tc. This Cindicates that the parameter, referred to in the present work as the a lowerC value of Tc is better for the retention of Cr if ‘critical temperature Tc’ for Cr and C . Critical the bath temperature is constant. temperature is the temperature atC which r6G=0 . r 3 2Table 2. The critical temperature TC for different [C] and [Cr]. 3CO(g)+2Cr=Cr23 O (s)+3CC …(6) Cr C Composition of rGJmol6 (/ ) The standard states of and were chosen as ω 0 steel rG6 748740 No. PCO / Pa Tc /K [i] = 1% (Henrian). In the Ccase of Cr2O3 the slag, pure 476..68T ω [C]% ω [Cr]% substance standard state was0 taken. The standard state (/Jmol ) rG of 1 atm was for CO. of equation (6) is given 1 3.5 15 101325 -748740+486.31C T 1538 by equation (7). 2 2.5 15 101325 -748740+564.52T 1608 0 rGT6 748740 476.(/) 68 Jmol …(7) 3 1.5 15 101325 -748740+440.22 T 1701 C Assuming that the activity of (Cr O ) is 1 4 0.5 15 101325 -748740+400.19 T 1871 G l 2 3 C (precipitated as pure Cr O , without considering the C 2 3 5 3.5 18 101325 -748740+479.49T 1561 dissolutionr 6 in the slag phase), C 3 C C C aaC (CCr O ) 6 3.5 12 101325 -748740+494.39T 1514 0 23 rrC GGRT 66 ln C 23 7 3.5 9 101325 -748740+503.30T 1487 appCr(/) CO C C 33 0 fCC []% C rGRT6 ln Figure 1 illustrates the variation of Tc with respect 22 3 C …(8) fCrpCr [](% CCO /)p to Cr and C contents. From this figure, it can be seen l c clearly that Tc increases with the increasing of Cr where, a refers to thermodynamic activities and f content when C content is a constant. Further, Tc refers to Henrian activity coefficients; pCO is the θ decreases with the increasing of C content when Cr pressure of CO gas and p refers to total pressure. C CC C C C C C C H. Wang et al. / JMM 49 (2) B (2013) 175 - 181 177 content is stable. Thus, if the temperature of the melt compared to O2. It may be also noted that for every is kept constant, Cr-loss will increase with the CO2 bubble injection, two CO bubbles are formed; the progress of decarburization. Or in case the number of CO bubbles will be twice that temperature is kept constant, if the partial pressure of corresponding to O2 injection. This can facilitate good CO is decreased, the Cr will be kept in the melt and mixing between metal and slag phases leading the C will be removed. To realize decrease PCO, dilute subsequently towards a near-equilibrium situation, gas such as Ar could be used (the principle to AOD corresponding to equations (12) and (13). process) from the thermodynamics point of view. In the above considerations, the temperature of the Besides thermodynamic aspects, kinetic aspects metal bath was assumed to be constant. In reality, should also need to be considered. Ar introduction decarburization with O2 injection (reaction (1)) would could increase the stirring of the bath, hence increase be exothermic while, reaction (2) will be endothermic.