UDK 669.18:669.14.018.8
Reviewed scientific article/Pregledni znanstveni članek
ISSN 1580-2949 MTAEC9, 41(5)203(2007)
THE OXIDATION AND REDUCTION OF CHROMIUM DURING THE ELABORATION OF STAINLESS STEELS IN AN ELECTRIC ARC FURNACE
OKSIDACIJA IN REDUKCIJA KROMA IZ ŽLINDRE MED IZDELAVO NERJAVNIH JEKEL V ELEKTROOBLOČNI PEČI
Boštjan Arh, Franc Tehovnik
Institute of Metals and Technology, Lepi pot 11, 1000 Ljubljana, Slovenia bostjan.arh@imt.si
Prejem rokopisa — received: 2007-07-13; sprejem za objavo - accepted for publication: 2007-08-30
The oxidation of chromium during the elaboration of stainless steels occurs with oxygen in solution blown in the melt and with oxides in the slag. The loss of chromium during the steel bath processing increases the production costs and generates problems because of the high content of chromium oxide in the slag. A higher content of silicon in the furnace charge decreases the extent of the oxidation of chromium; however, the efficient reduction of chromium from the slag is very important for a minimal loss of chromium. In this survey, the theory of the oxidation of chromium, its reduction from the slag and the conditions for the formation of foaming slag are discussed.
Key words: electric arc furnace, chromium oxidation, foaming slag, slag reduction, stainless steel
Oksidacija kroma med izdelavo nerjavnih jekel poteka zaradi topnega kisika v talini, vpihanega kisika in zaradi oksidov v žlindri. Izguba kroma med izdelavo nerjavnega jekla v EOP nima za posledico samo višji materialni strošek, ampak tudi določene operativne posege zaradi prekomerne količine kromovega oksida v žlindri. Večja vsebnost silicija v založenem legiranem vložku zadrži oksidacijo kroma med njegovim taljenjem, vendar je le učinkovita redukcija žlindre po oksidaciji taline ključnega pomena za minimalno izgubo kroma. V prispevku bomo predstavili predvsem teoretične osnove oksidacije kroma, redukcijo kroma iz žlindre in pogoje za tvorbo peneče se žlindre pri izdelavi nerjavnih jekel v elektroobločni peči.
Ključne besede: elektroobločna peč, oksidacija kroma, peneča se žlindra, redukcija žlindre, nerjavna jekla
1	INTRODUCTION
The content of chromium in austenite stainless steels is generally in the range from 16 % to 24 %. More than 97 % of the chromium is lost during the melting of steels from scrap in an electric arc furnace (EAF). The oxidation of chromium occurs during the melting, and to an even greater extent it occurs during the blowing in of oxygen, aimed at decreasing the content of carbon in the bath. A smaller part of chromium is also oxidised while discharging the melt from the EAF. A high content of chromium increases the crusting of the slag, decreases its reactivity and impairs the formation of the foaming slag and the slag reduction during the process of steel elaboration. Stainless slags with a high content of chromium oxide cannot be recycled or used, and are for this reason also an ecological problem.
2	OXIDATION OF CHROMIUM IN THE MELT
The oxidation of chromium with oxygen in solution in the melt occurs in parallel with the oxidation of other elements, e.g., carbon, aluminium, silicon and manganese, and it depends on the temperature and the activity of these elements and the oxygen. The standard free energy of oxidation (AG0) for these elements is given by the following relations1:
3	[Cr] + 4[O] = (Cr3O4) AG° = -244.800 + 109.6 T (1)
[C] + [O] = CO(g)	AG° = -5.600 - 9.37 T (2)
[Sy + 2[O] = (SiO2)	AG° = -139.070 + 53.09 T (3)
2[Al] + 3[O] = (Al2O3)	AG° = -286.900 + 89.05 T (4)
[Mn + [O] = (MnO)	AG° = -57.530 + 24.73 T (5)
The equilibrium constant is deduced for every one of these reactions from the change in the standard free energy, and it is written for the oxidation of chromium as:
53.521 lg k =5±5±1 - 23.96
and
K,
34 a Cr ' a O
fcr • f4 • * c
(6)
(7)
with acr2O3, acr, a,O being the activity of (&2O3), [Cr], O ; fCr, fO being the activity coefficient for Cr and O, and wcr/%, W% being the mass fractions of Cr and O in the steel melt.
The equation giving the content of oxygen in solution in the steel bath of known composition (T304L) as a function of the temperature and the activity of chromium oxide is:
wj%
K i • fC • fO C
(8)
Materiali in tehnologije / Materials and technology 41 (2007) 5, 231-236
231
a
a
a
B. ARH, F. TEHOVNIK: THE OXIDATION AND REDUCTION OF CHROMIUM
Figure 1: Effect of chromium on the activity and the content of oxygen in the system Fe-Cr at 1873 K 2
Slika 1: Vpliv kroma na aktivnost in vsebnost kisika v sistemu Fe-Cr pri 1873 K 2
For a high content of chromium in the melt the activity of the oxygen and of the carbon is lower, as both interaction coefficients are negative. The effect of chromium on the content and the activity of the oxygen in the melt at 1600 °C is shown in Figure 12.
The dependences were calculated using an equation valid for a content of 9 % chromium and more in the steel bath (eq. 9)2,3. It is evident that for a high content of chromium the solubility of oxygen in the melt is greater (Figure 1). For this reason, and with an equal content of carbon, the content of oxygen is higher in the bath with chromium than in the bath without this element.
lg[wCr3/4 • fO • Wo)1 = -13380/T + 5,99
(9)
Chromium is a relatively strong deoxidiser. With a higher content of chromium, the oxidation products consist of iron-chromium spinels of the type FeOCr2O3. The transition from spinel to the saturation with &2O3 in the system Fe-O-Cr occurs when the critical content of chromium is exceeded4.
Fe(s) + / O2 (g) + Cr2O3(s) =
= FeOCr2O3(s) AG° = -307.600 + 66.82 T (10)
In Figure 2 the equilibrium between FeO.Cr2O3 and oxygen is shown for both a low and a high content of chromium in the bath. The critical content of chromium is shown for different temperatures when the equilibrium oxide phase changes from FeO-C^O3 to &2O3. The phase FeOCr2O3 in equilibrium with the Fe-Cr melt is changed to Cr2O3 when the actual content of chromium in the bath increases above the critical content. Pure &2O3 is the phase in equilibrium with the Fe-Cr bath for more than 7 % Cr in the bath.
The activity of &2O3 and of FeO-C^O3 in the FeOCr2O3 solid solution in equilibrium with the Fe-Cr melt decreases when decreasing the content of chromium below the w& critical.
In addition to the thermodynamic activity of different elements in the melt, the connection temperature-activity
Figure 2: Equilibrium between chromium and oxygen in the steel melt saturated with pure solid Cr2O3 and FeOCr2O3 4 Slika 2: Ravnotežje med kromom in kisikom v tekočem železu, nasičenem s čistim trdnim Cr2O3 in FeOCr2O3 4
of chromium oxide in the slag is of great importance, since this effect is significantly greater at lower temperatures. The activity of &2O3 depends on the alkalinity of the slag and on the content of CaO and SiO2. The dependence of the free enthalpy of oxidation of chromium, silicon and carbon on the temperature shows that, according to the affinity for oxygen in solution, silicon lies between carbon and chromium and, for this reason, it is oxidised first and its oxidation delays the oxidation of chromium.
The deoxidation of the Fe-Cr melt with silicon is shown in Figure 35 as the equilibrium of the contents of chromium and oxygen in the bath at 1600 °C in dependence of the silicon content. The initial content of oxygen in the Fe-Cr melt before the addition of silicon is shown by the upper dashed line, which represents the equilibrium Fe-Cr/Cr2O3 or Fe-Cr/FeOCr2O3 for the content of wsi/% = 0. The addition of silicon lowers the content of oxygen by a constant amount of chromium.
Figure 3: Equilibrium between the contents of chromium and oxygen in the steel bath at 1873 K for different contents of silicon5 Slika 3: Ravnotežje med vsebnostjo kroma in kisika v tekočem železu v odvisnosti od vsebnosti silicija pri 1873 K 5
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Figure 4: Thermodynamic equilibrium between carbon and chromium for different temperatures and a pressure of pco = 1 bar 2 Slika 4: Termodinamično ravnotežje med ogljikom in kromom v odvisnosti od temperature pri tlaku pco = 1 bar 2
The deoxidation power of silicon is lower with a higher content of chromium, e.g., by the addition of 1 % of silicon, the content of oxygen in the Fe + 5 % Cr bath is lowered from 350 |ig/g to approximately 73 |ig/g and for Fe + 18 % Cr it is lowered from 450 |ig/g to 110 |ig/g. For this reason, a low content of oxygen cannot be achieved with the addition of silicon alone. The oxidation-reduction equation for silicon and chromium
2Cr2O3 + 3 [Si] = 4 [Cr] + 3SiO2
(11)
The thermodynamics indicates that the content of silicon in the melt is proportional to the activity of SiO2 in the slag; thus, the content of silicon in the melt is higher with a higher content of SiO2 in the slag. The thermodynamics of the relations content of &2O3 in the slag, the content of chromium and silicon in the melt, the melt temperature and the activity of SiO2 in the slag were investigated by McCoy and Langerberg6. The results of these investigations show that the loss of chromium is higher with a low content of silicon in the melt and with a higher bath temperature6.
Furthermore, carbon, with an over critical content, delays the oxidation of chromium2. The equilibrium between chromium and carbon was, for the pressure of pco = 1 bar and the different temperatures in Figure 4, determined by applying the following relations and considering the coefficient of interaction of the first order:
Cr2O3 + 4C = 3Cr + 4 CO	(12)
lg[(Cr)3/4 • pCO/aC] = -11520/7 + 7.64 (13)
2.1 Stainless slags
The behaviour of chromium oxides (CrOx) in metallurgical slags is complex because of the existence of ions with several valences and of the high melting temperature of the slags containing chromium7. In slags the non-stoichiometric quantity of CrOx depends on the
temperature, on the slag's alkalinity and on the content of chromium oxides in equilibrium with the metallic chromium. For the mixture of Cr2O3 with the other components of the slag in contact with metallic chromium at high temperature, the equilibrium of the oxides is:
2CrO15 + Cr = 3CrO	(14)
Chromium is found in the slag as two- and three-valence ions. The activity of CrO and QO1.5 increases with the increase of the total content of CrOx and the basicity. In Figure 5 the activity of CrO and CrO1.5 in the system CaO-SiO2-CrOx at 1873 K is shown. At higher temperature, the activity is lower and the content of the bivalent chromium in the slag is increased. In contrast, with increased basicity the activity of chromium oxides is increased and the content of bivalent chromium in the slag is diminished (Figure 6). A greater activity of chromium oxides in the slag is found with greater alkalinity, a higher ratio of CaO/MgO and a lower temperature. The lower content of MgO in the slag increases the activity of the chromium oxides in the slag up to a limit; however, it does not affect the oxidation state and, as a result, the content of Cr2+ and Cr3+. Small additions of Al2O3 affect the activity of CrOx, while larger amounts have little effect. Slag with greater
Figure 5: Iso-activity diagrams for CrO1.5 and CrO in the CaO-SiO2-CrOx quasi-ternary system at 1873 K 7
Slika 5: Izoaktivnostni diagram molskih deležev (x/%) CrO1,5 in CrO v CaO-SiO2-CrOx v kvasiternarnem sistemu pri temperaturi 1873 K 7
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Figure 6: Effect of temperature on the activity of CrOi.5 in the system CaO-SiO2-CrOx and the effect of slag basicity on the activity of CrO1.5 in the system CaO-SiO2-AhO3-CrOx at 1873 K 7 Slika 6: Vpliv temperature na aktivnost CrO1;5 v sistemu CaO-SiO2-CrOx in vpliv bazičnosti žlindre na aktivnost CrO1;5 v sistemu CaO-SiO2-Al2O3-CrOx pri temperaturi 1873 K 7
system, CaO-MgO-SiO2, shown in Figure 7, is the saturation with CaO and MgO9, which is the necessary condition for a minimal content of Ö2O3 in the liquid slag and it is acceptable for the resistance of the refractory lining, also. The solubility of CrOx in these slags is small. During the blowing of oxygen in the bath, the elements are oxidised according to their affinity to oxygen and their activity in the melt. Al and Si have a greater affinity for oxygen; however, chromium is also oxidised due to its greater activity. If CrOx is absorbed in the molten slag, the equilibrium conditions change after the blowing of oxygen and the chromium is returned in the bath on the condition that there is a sufficient content of silicon. In the presence of chromium in the solid phase (MgCrO4, CaCrO4), the slag is rigid and the return of chromium in the bath is lower, also in the case of a higher content of silicon in the bath.
3 REDUCTION OF CHROMIUM FROM THE SLAG
Several procedures were developed for the reduction of chromium from the slag during the elaboration of the steel in the furnace and during the discharge in the ladle using silicon and aluminium. It is, however, very important that the loss of chromium is minimised during the oxidation melting and blowing of different reductants. In Figure 8 and 9 the equilibrium curves are shown for the oxidation of chromium with respect to the reactivity of silicon, carbon, aluminium and manganese
activity improves the reduction of chromium from the slag and increases the yield of chromium during the elaboration of stainless steels.
The stainless slag in the EAF consists mostly of CaO, MgO, Al2O3 and SiO28. The optimal composition in the
Figure 7: Isothermal section of the system CaO-MgO-SiO2 at 1600
°C 9
Slika 7: Izotermični prerez sistema CaO-MgO-SiO? pri 1600 °C 9
Figure 8: Effect of temperature on the ratio wCr/wSi and wCr/wC in the steel bath with 18 % Cr 10
Slika 8: Vpliv temperature na razmerji masnih deležev WCr/wSi in WCr/wC v talinah jekel z 18 % Cr 10
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Figure 9: Effect of temperature on the ratio wCr/wAl and wCr/wMn in the steel melt with 18 % Cr 10
Slika 9: Vpliv temperature na razmerji masnih deležev wCr/wAl in wCr/wMn v talinah jekel z 18 % Cr 10
at the temperature of the elaboration of the steel T304L10.
The thermodynamics of the relation wcr/wsi in the melt with 18 % Cr shows that with a temperature of approximately 1500 °C the oxidation of silicon starts at 0.2 % Si and at 1700 °C at 0.4 % Si. Thus, at 1500 °C a content of 0.2 % Si in the melt already arrests the oxidation of chromium, while at higher temperatures a greater content of silicon is required. The ratio of *Cr/*C for 18 % Cr in the melt shows that the reactivity of the chromium increases with the temperature. For this reason, at higher temperatures a lower equilibrium content of carbon impairs the oxidation of chromium. Figure 9 shows that the affinity of aluminium for oxygen is very high and that its activity decreases with increasing temperature. The activity of manganese is not affected by the temperature and in stainless steels with 2 % Mn the behaviour of manganese is different from that of chromium, carbon, aluminium and silicon.
Silicon impairs the oxidation of chromium at low temperatures, thus, during the melting of the charge, while carbon is efficient in this role and with sufficient concentration, at higher temperatures. The blowing in of the oxygen at high temperature (>1550 °C) increases the oxidation of carbon and strongly decreases the oxidation of chromium.
The slag basicity ((CaO+MgO)/(SiO2+Al2O3)) has a strong effect on the reduction of chromium oxides, and with a higher basicity the content of chromium oxide in the slag is lower. The activity of SiO2 and Al2O3 is also
Figure 10: Effect of slag basicity on the content of Cr2O3 in the slag11 Slika 10: Učinek bazičnosti žlindre na vsebnost Cr2O3 v žlindri11
lower, while the activity of CrO* is higher. All these levels of activity increase the rate of reduction of chromium oxide from the slag (Figure 10)11. According to this figure, the basicity should be approximately at the level of B = 1.8.
The efficient reduction of the furnace slag depends on the selection of a suitable reductant, dependent on the used procedure and the control of the furnace slag, which should ensure a high level of reduction of chromium oxide from the slag and should not contaminate the melt. The selection of the reductant depends on the furnace and the raffination practice.
However, of special importance is the decrease of the chromium losses during the oxidative melting with the blowing in of the different reductants. The first measure for the control of the content of &2O3 is the use of a charge and alloys (FeSi) with a high content of silicon, ensuring a content of 0.3 % Si after the melting of the charge. During the discharge, the reduction of chromium oxides is continued with silicon in solution.
For a decrease of the chromium oxidation, the injection of carbon is widely used because it is economically more suited than the reduction with Si and Al. These two elements are bound to form oxides, thereby decreasing the slag basicity, which requires the further addition of lime, resulting in the increase of the slag volume. With slag reduction as a result of the blowing in of carbon, carbon monoxide is formed, which is necessary for the formation of the foaming slag. However, for an efficient reduction of &2O3 with the blowing in of carbon, a high slag temperature is also necessary. For this reason, the blowing in of carbon is carried out with the parallel blowing in of oxygen. The reduction of chromium oxide with Si and C occurs by the reactions:
(Cr2O3) + 1,5 [Si] = 2 [Cr] + 1,5 (SiO2) (15) (G-2O3) + 3C(S) = 2 [Cr] + 3CO(g)	(16)
The reduction of chromium oxide is more efficient with conditions of:
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B. ARH, F. TEHOVNIK: THE OXIDATION AND REDUCTION OF CHROMIUM
•	a high activity of Cr2Ü3 with increased alkalinity,
•	a high activity of carbon, ac = 1 during the blowing in of carbon,
•	a low partial pressure,
•	a low activity of chromium in the metallic phase,
•	a high temperature.
Besides the used technology for the reduction with silicon and aluminium and the blowing in of carbon, the use of the blowing in of calcium carbide was reported also12. When this carbide reacts with oxides in the slag, the products of the reaction are chromium, lime and carbon monoxide. CaO has the function of a non-metallic addition for the formation of the slag, while carbon monoxide improves the slag foaming in comparison to the blowing in of carbon powder. The reduction of chromium oxide with calcium carbide (CaC2) proceeds according to the reaction:
Cr2O3 + CaC2 = 2Cr + 2CO(g) + CaO
(17)
The change of Gibbs free energy with temperature for the reactions of chromium oxide with silicon, carbon and calcium carbide are shown in Figure 11. It is clear that the reduction of chromium with carbon is more efficient at high temperature; therefore, in practice it is performed with the parallel blowing in of oxygen. The reaction between silicon and chromium oxide is not strongly temperature dependent and occurs, for this reason, also at lower temperatures. The suitability of calcium carbide as a reductant is shown by the reaction between this carbide and chromium oxide, which has a lower Gibbs energy in the temperature range of 1550 °C to 1700 °C.
It is characteristic for slags containing iron and chromium oxides, that on the boundary between the slag
Figure 12: Content of FeO and Cr2O3 with respect to time at 1600 °C and 1650 °C 13
Slika 12: Vsebnosti FeO in Cr2O3 v žlindri v odvisnosti od časa pri temperaturi 1600 °C in 1650 °C 13
and the carbon the reduction starts immediately and without any incubation period, while the reduction of Cr2O3 has an incubation period and is slower than the reduction of FeO13. The reduction of &2O3 begins only after the reduction of a considerable amount of FeO (Figure 12).
In Figure 13 the ratio ln(wa/wCro) with respect to time is shown for a slag with a different content of FeO with (%Cr) as the total content of chromium in the slag and with (Cro) as the reduced chromium in the slag at the time t13. The slope of the curves for the content of 5 % or 10 % of FeO shows that the reduced part of &2O3 is greater with a higher content of FeO in the slag. For a content of FeO = 0.5 % the incubation for the reduction becomes longer and the Cr2O3 reduction time is lengthened.
3.1 Foaming slag
The formation of foaming slag is a process dependent on a sufficient evolution of gases in a slag with a proper viscosity, which should not be too small, as a determined time of presence of gas bubbles in the slag is necessary
Figure 11: Temperature dependence of Gibbs free energy for the reduction of chromium with calcium carbide, silicon and carbon12 Slika 11: Temperaturna odvisnost Gibbsove proste energije za redukcijo kroma s kalcijevim karbidom, z ogljikom in s silicijem12
Figure 13: The kinetics of the reduction of chromium oxides for a different content of FeO in the slag and the slag alkalinity of CaO/SiO2= 1.15 13
Slika 13: Redukcija kromovega oksida v odvisnosti od vsebnosti FeO pri bazičnosti žlindre CaO/SiO? = 1,15 13
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to maintain sufficient foaming. If the viscosity is too great, the foaming cannot occur, or occurs with insufficient intensity.
The foaming characteristics are improved with the lowering of the surface tension (a) and density (p) and with the increase in the slag viscosity (n)14. The foaming stability and the foaming index (2) are determined from the average size of the bubbles (Db).
2 =115 n /( a • p)0 5	(18)
2 = nu/( a0,2 • p • Db0,9)	(19)
Db is the increase of volume due to the arising CO bubbles in the slag. Small bubbles are of special importance for the slag stability. The presence in the slag of a suspension of different solid phases has a stronger effect on the foaming than the slag surface tension or the viscosity. An optimised slag is not entirely liquid; however, it is saturated with CaO (Ca2SiO4) and MgO. The particles in suspension act as nuclei for gas bubbles and enable the formation of a large number of small bubbles.
For the effect of particles in suspension on the slag viscosity, the following relation was developed:
n e = n(1-1,350)-5/2	(20)
With: n e - the effective slag viscosity
n - the viscosity of the liquid gas without particles in suspension
6 - the volume share of the solid phase in the slag
In Figure 14 the dependence between the foaming index and the relative effective slag viscosity is shown15. If the relative effective slag viscosity is increased, the time of the presence of bubbles in the slag is lengthened, the foaming stability is increased and the foaming time is lengthened, also. As shown in Figure 15, the optimal quantity of solid particles is at the point G. Away from this point the slag becomes too crusty and the index of foaming is decreased.
The foaming slag has a positive effect in the EAF, since it decreases the electrodes radiation loss, decreases
	Optimum Over-Saturated Slag To°mlK;h second phase — particlcs or Cr>Oj ' ' ^precipitation
Liquidus Boundary	
Fully Liquid!	Increasing amounts of second phase particles
(0 = 0)	(©>0)
Effective Viscosity, t),
Figure 14: Foaming index versus the effective slag viscosity15 Slika 14: Indeks penjenja v odvisnosti od efektivne viskoznosti žlin-der15
Cromium Oxide, WCr2O3/%
Figure 15: Content of chromium oxide in the slag after discharge of the furnace in dependence of the slag foaming index16. Slika 15: Vsebnost kromovega oksida v žlindri po prebodu na peči v odvisnosti od indeksa penjenja žlindre16
the spraying of the slag on the lining and on the water panels, decreases the loss of electrodes and lowers the effect of variation of electrical tension with a longer electric arc16. It was found, also, that the foaming slag increases the return of chromium from the slag in the metallic bath (Figure 15).
By the elaboration of the carbon steels with FeO as major oxidation product (> 20 % of FeO in the slag), the propensity of the slag to foam is achieved simply with the blowing in of carbon in the slag. The base reaction for the formation of gas bubbles in the slag is:
FeO(S,l)+C(S)=Fe(l)+CO(g)	(21)
The foaming of stainless slag is different from that during the elaboration of carbon steels. The foaming capacity of stainless slags is decreased by a low content of iron oxide and a high content of chromium oxide in the slag. Solid particles of chromium oxide in the slag with a high melting point increase the slag viscosity and impair the foaming. With a low FeO content and with slow kinetics of reduction of chromium oxide, the addition of carbon, which increases the quantity of CO, does not fulfil the condition for an efficient foaming17.
The controlled formation of gas bubbles is very important for the formation of the foaming slag. During the elaboration of stainless steels, CO is produced, mainly with the carbon reduction of CrOx and FeOx. In stainless slags the Cr-O buffer appears at a much lower oxygen potential than the Fe-O buffer and, for this reason, chromium is oxidised before iron18. During the blowing in of oxygen in the melt CrOx is formed as an oxidation product in place of FeO and for this reaction it is characteristic that: • the solubility of CrOx in the slag is low and it depends upon the ratio CaO/SiO2 and the temperature,
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• the share of reduction of CrOx with carbon is small in comparison to the reduction of FeO. Consequently, the rate of formation of CO is small, since it forms mostly with the reduction of FeO. In presence of FeOx a greater quantity of CO is formed, and it can also be achieved with a greater degree of oxidation of the melt or the addition of FeO to the slag.
The second condition for the formation of the foaming slag is the proper slag fluidity. During the elaboration of stainless steels in the EAF the SiO2 and &2O3 are the main oxidation products. SiO2 is a flux component, while Cr2O3 is a component of the lining and it increases the rigidity of the slag. For this reason, the control of the Cr/Cr2O3 equilibrium in the melt is very important for achieving a foaming slag. The solubility of CrOx in these slags is small (< 5 % CrOx), and once the solubility is reached, the oxide CrOx is precipitated.
If a moderate amount of secondary particles is available in the slag, the viscosity and thus the foaming index increases. However, at higher chromium contents, large solid particles destroy the bubble network and become detrimental to the slag's foamability. The data from the investigation shows that the Cr2O3 content should be kept lower than 16 % for the slag composition, temperature and oxygen potential used in the research15.
The next parameter affecting the slag foaming is the temporal addition of additives during the elaboration of the steel in the furnace, since the composition of the slag is very important for the control of chromium and the achieving of the optimal degree of foaming. The formation of a fluid slag during the melting depends on the oxidation-reduction kinetics in the steel bath. An early fluid slag is achieved with the addition of wollastonite (CaSiO3). The addition of MgO in the system CaO-SiO2 increases the fluidity and decreases the
Figure 16: Areas of the tri-phase diagram with different activities of foaming/reduction17
Slika 16: Področja trifaznega diagrama s prikazom različnih lastnosti penjenja/redukcije17
melting point of the slags up to a content of 15 % MgO, above this level periclase or spinel are precipitated.
Considering the above analysis, it is understandable that achieving foaming in EAF slags rich in Cr2O3 is a difficult task, especially with regard to the formation of a sufficient quantity of gas bubbles. The investigation of the foaming and the reduction confirm that the area of stable foaming for EAF stainless slags is small19. This area and the area of the formation of gases are schematically shown in Figure 16.
Area I: A low basicity slag with a high viscosity gives a good foamability, according to Equation (18). Unfortunately, the gas generation is very small. The overall result of this is a poor foaming without any industrial interest.
Area II: This area shows the composition of the slag after a poorly controlled oxidation. The slag has a high viscosity and it has a large content of Cr2O3 solid particles. The great viscosity and the share of the solid phase in the slag decrease the kinetics of the formation of gases.
Area III: This part of the slag system is optimal. The basicity is high and the conditions for the formation of the CO gas are optimal. With particles of solid CaO and Cr2O3 the viscosity is increased and the index of foaming is increased, also.
The reduction of chromium oxide in the foaming occurs according to the following reactions18:
(CrO15) + % [Si] = [Cr] + % (SiO2) (22)
(CrO) + '/2 [Si] = [Cr] + 1/2 (SiO2) (23)
(CrO) + xC = xCO + [Cr]	(24)
4 CONCLUDING REMARKS
During the elaboration of stainless steels in an EAF the loss of chromium is reduced with a decrease of the oxidation during the melting of the charge, the control of the blowing in of the oxygen by partial oxidation of the melt, with the slag composition and with efficient slag reduction.
The share of the oxidation of chromium depends on the composition of the charge, which may contain chromium oxides. During the melting, chromium is oxidised with other oxides present and with oxygen in the air. The dependence of the oxidation enthalpy on temperature shows that silicon and aluminium prevent the oxidation of chromium and that the sequence of oxidation depends on the activity of the element.
The 0.3 % content of silicon and the addition of silicon decrease significantly the oxidation of chromium during the melting because silicon is oxidised first and, in this way, the content of chromium in the slag is reduced. The oxidation of silicon is efficient during the melting and the heating of the charge and during the discharge of steel and slag from the furnace. The decrease of the oxidation of chromium with the addition
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Materiali in tehnologije / Materials and technology 41 (2007) 5, 203-211
B. ARH, F. TEHOVNIK: THE OXIDATION AND REDUCTION OF CHROMIUM
of FeSi requires the addition of lime to maintain the right slag basicity (CaO/SiO2). These additions increase the volume of the slag.
During the refining period the reducing conditions must be secured by controlling the C/O tot injection ratio and the temperature of the blowing of oxygen. For this reason it is recommended that the blowing in of oxygen is carried out at a high temperature > 1600 °C, when the oxidation of carbon is strong and the oxidation of chromium is minimised.
After the charge was melted and the blowing in of oxygen stopped, a significant content of chromium oxides in the slag is obtained. The chromium is returned in the bath with the reduction of slag during the heating up and the raffination of the bath and during the discharge from the EAF. The selection of the reductant and the procedure of reduction are significant for achieving a maximal return of chromium and a minimal content of reductant in the melt.
Suitable reductants are ferrosilicon, aluminium, calcium carbide and carbon. However, silicon and carbon are used the most. A second possibility is the addition of ferrosilicon with parallel blowing in of carbon powder in the slag when CO and gas bubbles are formed. Both are necessary to obtain a foaming slag. However, the blowing in of carbon is efficient at a high bath temperature. The alternative is the injection of calcium carbide into the slag, when CO and CaO are obtained as products. The advantage of calcium carbide is that it is an efficient reductant for chromium oxide even at lower bath temperatures.
Good mixing between the steel and the slag during tapping provides excellent conditions for dissolving the silicon to reduce the &2O3 in the slag. The chemical composition and the basicity of the slags affect strongly the reduction of chromium oxides. For furnace slag, the lowering of chromium oxide depends also on the lowering of the activity of SiO2 and A^O3. For a higher slag basicity the activity of SiO2 and Al2O3 is lower and the reduction of chromium oxide greater. An basicity of 1.4 to 1.8 is the optimum range.
For the formation of the foaming slag an early presence of a sufficient volume of fluid slag is necessary. The final slag should be saturated with CaO and MgO (5-10 %) and have a viscosity suitable for the formation of the foaming process. The appropriate content of Si in the steel bath is necessary for the control of the ratio Cr/Cr2O3, since the solid chromium oxide increases the viscosity of the slag and impairs the foaming. Furthermore, the reactions should ensure the formation of small gas bubbles. The injection of carbon into the
slag with a high potential of oxygen in the bath and a high content of FeO generates the formation of CO bubbles. For the stable foaming of stainless slags with a low content of FeO it is necessary to maintain the generation of gas bubbles with the injection of carbon and iron oxide in the slag.
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