** Dept. of Materials Science and Engineering, Pohang University of Science and Technology

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SOLUBILITY OF NITROGEN IN CaO-Si- CaF SLAG SYSTEM Hyo Seok Song, Dong Sik Kim, Dong Joon Min* and Peter Chang Hee Rhee** Technical Research lab., Pohang Iron and Steel Co. (POSCO) *Dept.

1 SOLUBILITY OF NITROGEN IN CaO-Si- CaF SLAG SYSTEM Hyo Seok Song, Dong Sik Kim, Dong Joon Min* and Peter Chang Hee Rhee** Technical Research lab., Pohang Iron and Steel Co. (POSCO) *Dept.of Metallrgical Engineering, YONSEI Univ. ** Dept. of Materials Science and Engineering, Pohang University of Science and Technology ABSTRACT The solbility of nitrogen in CaO-SiOrCaF slag at high temperatre was measred to nderstand the dissolving mechanism of nitrogen by sing chemical eqilibrating techniqe. The effects of oxygen potential and temperatre on nitride capacity were shown good agreement with theoretical expectations. A minimm vale of the nitride capacity was fond at abot. slag basicity and the nitride capacity increased in both more acidic and basic regions. This may be explained by two mechanisms for nitrogen dissoltion; incorporated nitride ion and free nitride ion state. The effects ofbao and MgO to sbstitte CaO on nitride capacity showed similar behaviors. Both components were not effective for increasing nitride capacity of the slag. It was fond that nitride capacity and optical basicity had a close relationship even in the different basic oxide systems. dissoltion in the melts strongly depends on the activity of the basic component of the melt. On the other hand, it is also reported that the role of Si in the nitrogen dissoltion is significant. It is not qite clear whether nitrogen dissoltion is enhanced by the presence of basic or acidic components. In this stdy, the solbility of nitrogen in molten CaO Si-CaF and CaO(MgO, BaO)-SiOrCaF systems has been measred. The effects of oxygen partial pressre, temperatre and slag basicities on nitrogen dissoltion in slag are also discssed.. Experimental procedres The experiments carried ot by eqilibrating the slag in a graphite crcible with CO, N and Ar gas mixtres.. Figre 1 shows the schematics of the experimental apparats. The master slags were prepared by melting CaO, Si and CaF of reagent grade in a graphite crcible nder Ar atmosphere. The graphite crcible with 1g of slag was pt into the frnace after reaching the reaction temperatre. The SOOcc/min of mixed gas was blown onto the srface of the melts throgh the almina tbe which was placed 1 cm above the sample in order to ensre that oxygen potential was given by C-CO eqilibrim reaction 7. Partial pressres of oxygen and nitrogen were changed to investigate their effects on nitrogen solbility of slags. Gas otlet r 1. Introdction Nitrogen is one of the most important elements in steel de to its beneficial or detrimental effects on the mechanical properties of steel. In spite of many stdies and efforts tried in plant and laboratory, nitrogen has always been a very difficlt element to remove from liqid steel. For instance, different degassing processes that sally work well for hydrogen are not effective for nitrogen removal to the desired levels becase of the smaller diffsivity and its tendency to form nitride in steel 1. The nitrogen pick p from air dring tapping operation of the converter as well as secondary ladle refining provides another problem to control the nitrogen levef Recently there is a growing interest for nitrogen control by sing steelmaking slag. The solbility of nitrogen in varios slag systems has been stdied by several investigators to obtain reliable data which can be sed for nitrogen removal from molten steel as well as protection of steel from nitrogen absorption - 6. The recent work with CaO-CaF system indicated that nitrogen 1. Almina reaction tbe. Heating elements. Graphite crcible 4. Sample 5. Spporter 6. Mg trning 7. Mixing chamber 8. Mass flow controller 9. Silica gel Figre 1. Schematic diagram of experimental apparats In order to determine the time to reach eqilibrim, preliminary tests were carried ot. The reslts in figre show the changes of nitrogen contents in slag with reaction time at 16 C,.6atm of CO and.4atm of N partial pressre in CaO-SiOrCaF slag system. It can be seen that the system reaches eqilibrim after abot hors. After 4 hors of holding, the slag samples were taken ot of the frnace, qenched by argon and then prepared for chemical analysis. Nitride in slag was analyed by the "Kjeldahl method MOLTEN SLAGS, FLUXES AND SALTS 97 CONFERENCE - 58

2 1., * i.8,,,.5 E o.6,s! c c.4 QI Cl e "" Ncao i Ns1, = 1 Pco =.6 PN, = Time, hr Figre. Changes of nitrogen contents in Ca-Si -CaF system with time at 16 C _ ~ ~ -.1 Q, ~ Cl ~ ~ , log P, atm Figre. Nitrogen distribtion in slag as a fnction of oxygen partial pressre at l 6 C. Reslts and discssion. I Effects of oxygen partial pressre and temperatre Nitrogen dissoltion reaction between gas and slag, in general, may be expressed as follows N +-( ) = (N )+- (g) 4 (g) (I) P6, fn,. (%N ) K1 = 1 ~ () PJ, a~,- P6, (%NJ. ) I () PJ, It is expected that nitrogen distribtion ratio between gas and slag is dependent on oxygen partial pressre and its coefficient is -/4 from eqation () theoretically. Figre shows the relationship between nitrogen distribtion ratio and oxygen partial pressre and the slope of line agrees reasonably to the theoretical vale. The temperatre dependency of nitride capacity in slag is shown in figre 4. It increased with temperatre. The activation energy calclated from the slope of the data line for nitrogen dissoltion reaction is abot 6 kcal/mole. It can be inferred that this temperatre dependency is mainly de to the chemical reactions inclding absorption and desorption of nitrogen ions.. Effect ofcaf Spar is sed as a flxing component to indstrial slags in metallrgical processes. Nitrogen in slag which i, CJ-1.8 i,..q _ /T(K) x 1 4 Ncao/ Ns;o = Pee=.6 PN = Figre 4. Nitride capacity of Ca-Si -CaF system as a fnction of temperatre contained network former has shown to behave both as free nitride and as incorporated nitride 6. Figre 5 shows the changes of nitrogen contents with CaF concentration at the slag basicity, N cc o> I N cs;o,>,of. Nitrogen increased with CaF to reach a maximm vale at abot wt%caf and then decreased with frther CaF addition. In case of low CaF region, CaF and silicate reaction lead to the formation of florosilicates to increase the nmber of reaction site of silicate reslting in a higher nitrogen content. However, de to diltion of silicate with still higher CaF, the nitrogen content will decrease.. Effect of slag basicity As shown in figre 6, nitride capacity decreases with increasing CaO contents, to reach a minimm vale and then increases with frther increase in CaO mole fraction. It can be speclated from these reslts that dissoltion of nitrogen in a slag is characteried by two simltaneos reactions, one with a acidic component and the other with a basic component. Nitrogen may dissolve in slag as a MOLTEN SLAGS, FLUXES AND SALTS 97 CONFERENCE

3 1. #-.9 Ncao /Ns1 = oi lll ",!;.8 c " E.7 (,,).6 c Q) ).5 Q ~ _ (%CaF ) Figre 5. Effects of Cafii contents in slags on nitrogen solbility of Ca-Si -CaF system at 16 C J -1.6 ) ~ L] Xcao Figre 6. Effects of Xcao in slag on nitride capacity of CaO-Si -CaF system at 16 C incorporated nitride which is related to silica ions in slag and stable form in a acidic region becase nitrogen solbility is decreased with CaO contents increasing. In less basic slags, the nitrogen capacity decreases with CaO addition becase it redces the silica activity and the reaction site. The reaction of nitrogen dissoltion in this region can be expressed by eqation (4) N +- =N <g) 4 < ) p4 (wt%n) o, Ks ao CN. I.1. p N, a~,. f N. Where ff and N are nonbridging oxygen and nonbridging nitride incorporated with silica which is a network former in slag, respectively. Nitrogen dissoltion following eqation (1) may be referred to as "free nitride" and that following eqation (4) as "incorporated nitride". The experimental reslts for nitrogen dissoltion behavior in a (4) (5) acidic region in this stdy are in good agreement with other stdies 5 9 rn It is thoght that nitrogen may exist as a free nitride as in eqation ( 1) in case of frther increase of CaO concentration from this consideration. The free nitride is more stable than incorporated ion as the activity of oxygen ion is increased in basic region and it shows high free nitride capacity. These facts indicate that nitrogen dissoltion behavior may be related to the stability of nitride ions which is dependent very mch on the oxygen ions, slag basicity. The reaction mechanism can be changed according to the changes of stability for nitride ion with the basicity of slag Nitride capacities were plotted over NccaoJ I N(sio,J which is often sed to define basicity. As shown in Figre 7, the tendency of forming V-shape with minimm vale arond of basicity is more prononced. Nitride capacities of the system investigated in the present stdy are compared to those of several other slag systems in figre 8. Nitride capacities in this stdy are relatively high, 1.xlQ ll_ Difference in nitride capacities from those of Martine et al. 11 may have been cased by the temperatre difference of two stdies on same slag system. Other reslts 9 1 show steady decrease of nitride capacities with increasing basic oxide(mo) content in high basic region as shown in figre ~ ) ~ _ Ncao t Nsio Figre 7. Effects of slag basicity on nitride capacity of CaO-Si -CaF system at 16 C.4 Effects of other basic oxide on nitride capacities Effects of sbstitting other basic components (MgO and BaO) for CaO were investigated to examine the dependence of nitride capacity on basic oxides of the melt. One is weaker and the other is stronger basic component than CaO. The effects of sbstittion CaO by MgO on nitrogen dissoltion are shown for each slag basicity in figre 9. For slag basicities, 1 and, there are small increase in nitride capac1t1es with increasing the sbstittion ratio p to O. 5 of N <M&O> IN cc,op and then decrease with frther increasing of sbstittion ratio..5 MOLTEN SLAGS, FLUXES AND SALTS 97 CONFERENCE - 585

4 -11-1 ).Q This stdy Martine etal. CaO-Si -CaF,1soo c CaO-SiO~ Martine et al. CaO-CaF, 155 C Ito et al.-..._ Ca-Si,1~C -..._ Schwerdtfeger et al. ""9aO-Al,, 1 soo c Martine et al. The effects of BaO on nitride capacity was reported to increase the nitride capacity for basic slagslo_ The reslts in figre l O shows that nitride capacity decreased slightly p to.5 of Nc 8, l I Ncc,oJ and then increased for frther increase of BaO. Bt nitride capacity even in case of. of Ncs,oJ I Ncc,oi is almost the same as slag withot BaO. In case of small BaO addition, diltion effect of CaO by BaO and effect of capacity increase of oxygen ion spply by BaO in melt were nearly offset. Therefore this complex effect does not increase the activity of oxygen ions in melt. However, it is expected that BaO wold increase the activity of oxygen ion and nitride capacity in higher basicity slag only at very high BaO concentration....4 XMO.6.8 Figre 8. Nitride capacities of varios slag systems as a fnction of mole fraction of basic components ::::::::======::::::: U e Ncao/Nsio, = 1 NcaoiNsio, = O Ncao/Ns1, = : :&/ o_ e Figre 9. Effects of NMgO f Ncao ratio in slag on nitride capacity of CaO-SiO:!-MgO-CaF system at 16C , -1.5" N(CaO+BaO)/Ns1 = Pco =.6 PN, =.4 cj Q -1.7 ~ ,, Ntsaoi/N (CaO) Figre 1. Effects of Ntsaoi/N(cao) in slags on nitride capacity of CaO-BaO-Si-CaF system at 16 C The effects of sbstittion BaO for CaO on nitride capacity showed similar behaviors as MgO in this stdy. Those phenomena for sbstittion effects ofmgo and BaO cold be explained by complex effects of ionic strength and diltion between CaO and these basic oxides. The concept of optical basicity was sed as a measre of slag basicities 14. Figre 11 shows nitride capacities as a fnction of optical basicities. The nitride capacities of all slag systems However, there is more or less a steady decrease with increasing it in a higher basicity region of. The possible explanations for different behavior with slag basicities in the low basic region wold be de to presence of incorporated nitride as shown in figre 6. Nitride solbility increased as the activity of Si increased becase nitrogen was thoght to be bond with silicate network. In gi:;neral, in terms of oxygen ion spply capacity, MgO is less than that of CaO and activity of silicate network in melt wold increase with increasing MgO. For higher slag basicity region, nitride solbility decrease initially and then increased again as the sbstittion ratio of CaO by MgO increased. Slag is almost satrated with Cao at slag basicity of and l 6 C. Diltion of CaO by MgO in this region decreases the activity of oxygen ion and reslting in lower nitride capacity. r , -1.6 CJ, D cj -1.7.Q -1.8" ~,.-c.-=-,o,-.-c.-s,-,v-,ta-m~ in caoimnol-s1-caf B I,, I:> CeO(M )-51,.CeF,, B= CaOIMaOI-SI-CaF, 8:: -ii.q O CaO(Ba)-SJO:rCaF, B= Optical Basicity Figre 11. Relationship between optical slag basicity and nitride capacities of varios slag compositions at 16 C MOLTEN SLAGS, FLUXES AND SALTS 97 CONFERENCE

5 regardless of its constitents can be correlated with linear relationship. It shows an adverse basicity dependence at.65 as bondary. As seen in the figre, the effects ofmgo for basicities between 1 and and are different becase transition range which the reaction mechanism changed might be inclded for basicity. However, in the case of BaO addition, eqation (1) is valid for highly basic region. As discssed previosly, the diltion effect of CaO by BaO and effect of capacity increase of oxygen ion spply by BaO in melt were almost balanced, this complex effect does not increase the activity of oxygen ions in melt. 11. E. Martine and N. Sano, Met. Trans. 18, 199, p T. Shimoo, H. Kimra and M. Kawai, Jorn. Japan Inst. Met. 197, No.8, p K. Schwerdtfeger and H. G. Schbert, Met. Trans. 88, 1977, p J. A. Dffy and M.D. Ingram, J. Amer. Chem. Soc. 9, 1971, p Conclsion The solbility of nitrogen in molten CaO-SiOrCaF slag system in eqilibrim with carbon and gas mixtres of CO, N and Ar at 16 C was measred. The solbility of nitrogen increased with decreasing the oxygen potential and increasing the reaction temperatre. Nitrogen dissoltion was fond to be endothermic reaction and the activation energy was calclated to be 6kcal/mol. A minimm vale of the nitride capacity was fond at abot. slag basicity and the nitride capacity increased in both more acidic and basic regions. This may be explained by two mechanisms for nitrogen dissoltion; incorporated nitride ion and free nitride ion state. MgO increases the nitride capacity in case of slag basicity less than.. However, it decreases nitride capacity slightly with MgO contents at higher slag basicity. The effects of sbstitting BaO for CaO on nitride capacity showed similar behaviors as MgO. Those phenomena for sbstittion effect ofmgo and BaO cold be explained complex effects of ionic strength and diltion of CaO by other basic oxides. Nitride capacity has a clear relationship with optical basicity in spite of the different basic oxide system. References 1. R. J. Cholet and I. F. Masterson, Iron and steel making, 199 No., p.45.. E. Martine and N. Sano, Steel res. 58, 1987, p T. Shimoo, H. Kimra and M.Kawai, Jorn. Japan Inst. Met. 5, 1971, p K. Schwerdtfeger, W.Fix, H. G. Schbert,Ironmaking and steelmaking. 5, 1978, p F. Tomioka and H. Sito, Test-to-Hagane, 199, p E. Martine and N. Sano, Met. Trans. 18, 199, p E.T. Trkdogan, Physical chemistry of high temperatre technology, Academic Press, New York, JIS G18 : Method for determination of nitrogen in iron and Steel, K. Ito and R. J. Frehan, Met. Trans. 198, 1988, p D.J. Min and R. J. Frehan, Met. Trans. 1B, 199, p.15. MOLTEN SLAGS, FLUXES AND SALTS 97 CONFERENCE - 587

The Effect of Silica, Alumina, Calcia and Magnesia on the Activity Coefficient of Cobalt Oxide in Iron Silicate Slags

The Effect of Silica, Alumina, Calcia and Magnesia on the Activity Coefficient of Cobalt Oxide in Iron Silicate Slags highlight significant inconsistencies in the pblished data. The Effect of Silica, Almina, Calcia and Magnesia on the Activity Coefficient of Cobalt Oxide in Iron Silicate Slags Xliang Li* and Eric J. Grimsey**