Deoxidation of Liquid Steel with Molten Slag by Using Electrochemical Method

Transcription

1 SJ nternational, Vol. 5 (0), No., pp Deoxidation of Liquid Steel with Molten Slag y Uing Electrochemical Method Guo-Hua ZHANG,,) * Kuo-Chih CHU,) and Fu-Shen L,) ) State Key Laoratory of Advanced Metallurgy, Univerity of Science and Technology Beijing, Beijing China. ) School of Metallurgical and Ecological Engineering, Univerity of Science and Technology Beijing, Beijing China. (Received on June 8, 0; accepted on Augut 8, 0) A new unpolluted deoxidation method for liquid teel wa given in thi paper. Four deoxidation experiment were done, under the condition of applied voltage U0, V, V and 6 V etween the liquid teel and Ca(5 wt%) Al 3(55 wt%) molten lag. The reult howed that the deoxidation rate increaed a increaing of applied voltage. n addition, aed on the oxygen diffuion ehavior under the gradient of electrochemical potential, the kinetic formulae for deoxidation of teel with molten lag electrochemitry method are derived. The formulae can well decrie the change of diolved oxygen content in the teel. From the formulae it can e concluded that deoxidation rate could e enhanced y trengthening the tirring of teel, increaing the applied voltage, and electing lag ytem with a high aicity and a low vicoity, etc. KEY WRDS: unpolluted deoxidation; reolved oxygen; zirconia olid electrolyte; molten lag.. ntroduction Deoxidation of liquid teel i one of the mot important tak in the teelmaking proce, which could directly affect the performance of the teel product. The mot widely ued method i the precipitation deoxidation y adding alloy element into the liquid teel, uch a Al, Fe Si, Fe Mn and Ca Si etc., which can comined with the diolved oxygen to generate tale oxide. Thi method can decreae the diolved oxygen to a very low level quickly. However, the deoxidation product, epecially thee with very mall ize, are very hard to e removed from the teel, which contitute the main ource of incluion. Conuently, the demand for a new unpolluted deoxidation method ecome more and more urgent. By the way of directionally leading the tranfer of oxygen y uing the olid electrolyte, electrochemical method could uccefully remove the oxygen from metal melt without the pollution of incluion. Lot of invetigation were carried out on the deoxidation of liquid teel and copper y uing Zr 7) olid electrolyte. n thi proce, the olid electrolyte act a the tranmiion route of oxygen ion and imultaneouly eparate the melt with the deoxidation product, therey it can accomplih the unpolluted deoxidation of liquid teel. However, thi method ha the diadvantage of i) high price of olid electrolyte; ii) poile crack of olid electrolyte; iii) inconvenient operation, which make it hard to e applied on a large cale. Recently, a new unpolluted deoxidation method wa developed, which replaced the olid electrolyte y molten lag and realized the deoxidation y exerting electric field * Correponding author: ghzhang_ut@63.com D: etween lag and metal melt. 8) Deoxidation of liquid copper wa achieved y Lu et al. y uing Na 3 AlF 6 Al 3 melt. 8) n the preent tudy, thi method will e ued to liquid teel. Furthermore, the doxidation kinetic will alo e developed.. Experiment.. Experimental rocedure 0 g lag and 00 g electrolytic iron powder were ued in the preent tudy. The compoition of electrolytic iron i hown in Tale. The compoition of lag wa Ca Al 3 (55 wt%), which wa pre-melted for h at 580 C efore ue. Baed on the calculated reult y Factage 6, the aturation concentration of alumina at 580 C i around 60 wt%. Since alumina crucile wa ued a the reaction container, Al 3 content of the lag wa et to 55 wt%. The concentration of oxygen in the teel wa adjuted y adding Fe 3 of a high purity. The ytematic diagram of experiment apparatu i hown in Fig.. The ued alumina crucile ha an inner diameter of 5 mm and a height of 00 mm. The anode material wa graphite, while Mo Zr cermet for the cathode material. Cermet i compoed of metal phae and ceramic phae, and when it i ued a electrode, Mo phae act a the path of charge tranfer, while Zr could protect Mo from continually diolving into the liquid teel. Cermet i already widely ued in the high temperature field, for it excellent corroion reitance, high temperature rei- Tale. Compoition of electrolytic iron (wt%). C S Mn Si SJ

2 SJ nternational, Vol. 5 (0), No. tance and thermal hock reitance, for intance, protective pipe of thermocouple when meauring the temperature of liquid teel. During the experimental proce, the ottom of the graphite electrode hould e immered into the lag and higher than the lag-teel interface o a to avoid the open circuit and hort-circuit. The powder mixture of electrolytic iron and Fe 3 were put into the alumina crucile, which wa placed in the tue of high temperature furnace with the heating element of MoSi. Then the furnace wa heated to 580 C and held for 30 mm under the protection of argon ga. After adding the lag for 0 min, the graphite and cermet electrode were put into the lag and liquid teel, repectively. Then the voltage wa applied and the deoxidation kinetic wa monitored y meauring the concentration of diolved oxygen at different time y the oxygen proe... Meaurement of Diolved xygen Content The oxygen proe with the reference electrode of Cr/ Cr 3 and olid electrolyte material of Zr tailized y Mg wa ued to meaure the concentration of diolved oxygen. The chematic diagram of oxygen proe i alo hown in Fig.. t hould e pointed out that a inerting oxygen proe through lag layer when it i the time to meaure the concentration of oxygen, a thin heet iron wa ued to protect the Zr tue from lag. Becaue of the exitence of oxygen potential difference etween liquid teel and reference electrode, a concentration cell could e formed. The tandard forming Gi free energy change of Cr 3 wa given a follow: 9) 3 Cr + Cr 3... () Δ Θ, 3 G T ln... () Θ The tandard Gi free energy change for the proce of diolving of oxygen ga into liquid teel wa: 0) []... (3) a, [] G T ln ( / )... () Δ Θ Θ Conidering the electron conduct and electron hole conduct of Zr olid electrolyte, the Nernt uation will e E + e + ' h' ln + ln... (5) F + e + ' h' where R i the ga contant, 8.3 J/(mol K); T i aolute temperature, K; F i Faraday contant, C/mol; e' i the characteritic oxygen preure of electron, a; h' i the characteritic oxygen preure of electron hole, a. n the cae of liquid teel, characteritic oxygen preure of electron hole could e neglected. Conuently, Eq. (5) could e implified a E F ln (6) The Mg content of the olid electrolyte material ued in the preent tudy wa.0.8 wt%, and the correponding characteritic oxygen preure of electron i: ). T... (7) e ' 0 Baed on the aove formulae, the activity of oxygen in the liquid teel could e calculated a EF ( + e' ) exp e' a... (8) Θ 05. Δ Θ, G ( ) exp Becaue of the low purity level in the electrolytic iron, f [] could e approximately aumed, therey, the activity of oxygen could e conidered a it concentration e' e' 3. Reult The preent tudy invetigated the deoxidation of liquid Fig.. Schematic diagram of experimental apparatu. : alumina protective tue; : molydenum rod; 3: graphite electrode; : molten lag; 5: liquid teel; 6: Zr tue; 7: Cr/Cr 3 powder; 8: alumina crucile; 9: cermet electrode. Fig.. The relation etween oxygen concentration and time in teel at different applied voltage. 0 SJ 768

3 SJ nternational, Vol. 5 (0), No. teel under the voltage of 0 V, V, V and 6 V. Experiment of V, V and 6 V were done two time. The change of diolved oxygen concentration with time at different applied voltage were hown in Fig., from which it can e een that the deoxidation rate decreae with the decreae of oxygen content, ut increae a increaing the applied voltage. When the voltage wa 6 V, the diolved oxygen can e decreaed to e a very low level in 5 min. The utilization of graphite anode provided a very low oxygen potential, which i eneficial for the deoxidation of liquid teel. From Fig., it can e een that deoxidation occur even when the voltage i 0 V, ecaue in thi cae the oxygen potential i different etween the urface of graphite and liquid teel. xygen could e removed from the liquid teel to react with graphite through lag under the drive of chemical potential difference ecaue of the aence of applied voltage.. Deoxidation Kinetic.. Kinetic Mechanim Analye Under the external electric field, the deoxidation proce could e aumed to e compoed of the following tep, and the correponding chematic diagram i hown in Fig. 3. ) The diolved oxygen in the liquid teel diffue to the lag-teel interface through the concentration oundary layer of oxygen in liquid teel.... (9) ) xygen atom in the lag-teel interface ecome oxygen ion after otaining the electron. + e ( )... (0) 3) xygen ion diffue to the urface of graphite anode through the molten lag, and react with caron, ( ) + C C + e... () ) The charge accumulated in the two electrode can e removed through the external circuit... Diffuion Flux under the Motivation of Electrochemical otential Becaue of the exitence of electric field, oxygen ion diffuion i driven y the electrochemical potential difference. Conuently, the diffuion flux hould e decried y the generalized Fick firt law μ ZFU J M M... () ( μc + ) x x where μ i the electrochemical potential; μ C i the chemical potential; U i the applied voltage; M i the diffuion flux when the gradient of electrochemical potential i unit, which i imilar with the cae of electrical conductivity (current denity under unit electric field trength)... (3) where j, σ and E are current denity, electrical conductivity and electric field trength. t i known to u that the electrical conductivity i compoition dependent. Conuently, it could e reaonaly peculated that M hould alo e related to compoition. Next, the relation etween M and compoition will e developed. When the external electric field i aent, the Fick firt law could e decried a J... () where D, C and x are diffuion coefficient, concentration and ditance, repectively. n the cae of U0, Eq. () will ecome μ C C J M M μ x C x... (5) Θ ( μc + ln a) C a C M M C x ln C x When the activity i ual to concentration, Eq. (5) ecome ln a C M C J M C x C x By comining Eq. () and (6), j σ E D C x M CD... (6)... (7).3. Derivation of Deoxidation Kinetic Equation The oxygen diffuion in the liquid teel and lag may e the rate controlling tep during the deoxidation proce. t Fig. 3. Schematic diagram of oxygen tranfer SJ

4 SJ nternational, Vol. 5 (0), No. i aumed that the concentration of oxygen atom and oxygen ion C in the lag-teel interface are the ame; the concentration of oxygen ion in the lag-graphite electrode ' interface C i ual to the oxygen concentration C in uilirium with caron,... (8)... (9) Ma tranfer of oxygen ion in the molten lag i mainly driven y two effect: the firt one i the diffuion flux j under concentration gradient; the other one i flux j under electric field, j... (0) ( j M ZFU ) DFE x C DFE C Z (here )... () where L i the ditance from the ottom of graphite electrode to the lag-teel interface, m; D i the diffuion coefficient of oxygen ion in molten lag, m /. Conuently, the total tranfer flux of oxygen can e expreed a... () The diffuion flux j ' of oxygen from liquid teel to lagteel interface i... (3) DFU f k, Eq. () could e otained conidering the relation j j ' According to the ma conervation law,... ()... (5) where m i the weight of liquid teel, kg; ρ i the denity of liquid teel, kg/m 3 ; C i the concentration of oxygen, mol/m 3 ; A i the area of lag-teel interface, m. The tranformation formula etween the concentration C and ppm i given a follow, ρ C... (6) M 0 3 where M i the atomic weight of oxygen element, 6 g/mol. According to Eq. (5) and (6), r D C x C ' C C C... (7)... (8) 0 SJ 770 C ' k Δ C k ( C C ) k ( C C ) DZFU j j + j k ( C C ) + C m dc ρ dt ' j k ( C C ) 3 k3( k + k) C k k C j 3 3 Aj A k k k C k k C 3( + ) 3 3 md A k k k k k C 3( + ) 3 ρ dt 3 d Aρk3( k + k) A k k dt mk ( k k) [ ] ρ 3 + mk ( k k) [ ] 3 Aρ k Aigning 3( k + k) Aρk C, k3, C mk ( + k + k) mk ( + k + k) d dt... (9) Baed on the initial condition of [] [] 0 when t 0, Eq. (9) could e integrated to e C k f C3, C k + k 3 C + C 3... (30)... (3) Thu, the deoxidation kinetic uation under the gradient of electrochemical potential i otained a Eq. (3), with the adjuted parameter of C, C 3, []. When uing graphite a the anode material, [] i very low and could e conidered approximately to e zero. n thi cae, Eq. (3) will ecome 0 Ct e... (3).. Comparion etween Meaured and Calculated Deoxidation Kinetic Curve When utituting the correponding expreion into Eq. (3), the change of oxygen content [] with time t under the condition of different initial oxygen content [] 0 and different applied voltage U could e expreed a follow, t DF... (33) m k U k e Equation (33) i ued to fit the experimental reult y a commercial mathematic oftware topt, and the comparion etween meaured and calculated deoxidation kinetic curve are given in Fig., with the correlation coefficient DF r> The optimized value of parameter k, and k 3 are m/, m/( V) and 0 m/, repectively. From Fig., it can e een that the preent model could well decrie the deoxidation kinetic of liquid teel. 3 Fig.. The comparion etween the model etimated reult and experimental value at different applied voltage. 0 C C e + C C Ct Ct ( C ) e + C DF A k k + U ρ 3

5 SJ nternational, Vol. 5 (0), No. 5. Dicuion () Baed on Eq. (9), it can e een that a low uilirium oxygen content [] i eneficial for the remove of oxygen in liquid teel. Furthermore, according to the relation etween C, C 3 and parameter k, k and k 3, the deoxidation rate could e enhanced y the following method: i) increaing k. arameter k i ued to decrie the diffuion coefficient of oxygen ion in molten lag which i compoition dependent. Generally, the higher the aicity i, the larger the diffuion rate will e. There are three kind of oxygen ion in the molten lag: ridging oxygen, non-ridging oxygen and free oxygen. ) Both the ridging oxygen and non-ridging oxygen are onded with the network forming cation uch a Si ion, o their moile ailitie are weak under electric field. While the free oxygen ion i only onded with cation from aic oxide, therey, a larger diffuion rate could e anticipated. ii) increaing k. Becaue DFU k, k could e increaed y uing lag with a high aicity which poee a large diffuion coefficient of oxygen ion; increaing the applied voltage U; controlling the thickne of lag. iii) increaing k 3. k 3 i ued to decrie the diffuion of oxygen in liquid teel. Therefore, in the practical indutrial production, k 3 could e enhanced y increaing the tirring intenity of liquid teel y lowing Ar ga. () n the preent tudy, the deoxidation kinetic wa propoed aed on the aumption that the whole deoxidation proce could e conidered a the diffuion of oxygen from liquid teel to the interface of graphite electrode. However, during the deoxidation proce with external electric field, the electrolyi may e happened in the molten lag. From the viewpoint of thermodynamic uilirium, the lag hould contain iron oxide conidering the exitence of oxygen in liquid teel. Therey, the true compoition of the lag will e Ca Al 3 Fe x. f the electrolyi reaction occur, oxygen ion alo ha to diffuion to the urface of graphite anode to generate C, while the cation uch a Fe + and Fe 3+ diffuion to the lag-teel interface to otain the electron and diolved into liquid teel. So, even if the occurrence of electrolyi reaction, the deoxidation of teel could e approximately conidered a the diffuion proce of oxygen from teel to graphite electrode. t hould e pointed out that when the concentration of oxygen i very low, the content of iron oxide in lag hould alo e very low from the viewpoint of thermodynamic uilirium. n thi cae, the compoition of lag will e Ca Al 3, and the electrolyi reaction of Al 3 will e occurred ecaue of the exitence of applied electric field, which could change the concentration of aluminum in liquid teel. n further, much work will e done aout thi point. 6. Concluion The preent tudy invetigated the deoxidation of liquid teel y uing the electrochemical method. t wa found that the application of external electric field etween lag and teel could enhance the deoxidation rate of liquid teel. Furthermore, the deoxidation rate increae greatly a increaing the applied voltage. According to the tranfer regularity of oxygen under the gradient of electrochemical potential, the deoxidation kinetic formulae were developed, and the model calculated reult accord well with the meaured value. Thi deoxidation method overcome the hortage of the traditional deoxidation method, and doen t introduce incluion to liquid teel. t i anticipated that thi method may e applicale to the practical dexodiation proce, even if lot of fundamental work are till needed to invetigate. Acknowledgement Thank are given to the financial upport from National Natural Science Foundation of China (53008). REFERENCES ) W. A. Ficher and D. Janke: Scr. Metall., 6 (97), 93. ) K. E. erg, L. M. Friedman, W. M. Boortein and R. A. Rapp: Metall. Mater. Tran. B, B (973), 75. 3) Z. Haham, U. al and K. C. Chou: J. Electrochem. Soc., (995), 69. ) S. Yuan, U. al and K. C. Chou: J. Electrochem. Soc., (99), 67. 5) S. Yuan, U. al and K. C. Chou: J. Am. Ceram. Soc., 79 (996), 6. 6). Soral, U. al, H. R. Laron and B. Schroeder: Metall. Mater. Tran. B, 30B (999), ) X. J. Hu, L. Xiao, F. S. Li, X. G. Lu, L. F. Li and G. Z. Zhou: Acta Metall. Sin., 35 (999), 36. 8) X. G. Lu, X. W. Liang, M. S. Sun, W. Z. Ding and G. Z. Zhou: Acta Metall. Sin., (005), 3. 9). Brain and. Knacke: Thermochemical ropertie of norganic Sutance, Springer-Verlag, Berlin, (973). 0) G. K. Singworth and J. F. Elliot: Met. Sci., 8 (97), 98. ) S. L. An: hd Thei, Univerity of Science and Technology Beijing, Beijing, (99). ) G. H. Zhang, K. C. Chou and K. Mill: SJ nt., 5 (0), SJ