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CHAPTER 17

PHYSICAL CHEMISTRY OF SLAG-METAL REACTIONS

B ASIC open-hearth slags have no obviously unique features when compared with slags from other metallurgical opera-

tions. Open-hearth slags form and exist a t temperatures ranging from 2500 to 3100 F (1370 to 1700 C ) . In the course of an open-hearth heat, the liquid slag changes slowly and continuously in chemical analysis-the changes reflecting changes in temperature, changes in metal analysis, solution of charged materials, solution of feed additions, and solution of refractories. Through- out the process, the slag performs specific primary functions that are responsible for a large measure of the technical and economic importance of basic open-hearth steelmaking. I t is intended in this chapter to present the chemical reactions by which the primary functions of basic open-hearth slags are carried out, to describe the physical and chemical principles governing the extent to which these reactions proceed and can be regulated, and finally to indicate the limitations imposed on other features of the process by the fulfillment of the primary functions.

Basic open-hearth slags perform three primary functions. First and foremost, the slag provides a means of removing phosphorus from the liquid metal. The ability to remove phosphorus is large!y responsible for the widespread use of the basic open- hcarth process. Second, the slag serves as a controlling factor in transferring the oxygen required for the other oxidizing reactions. Third, the slag provides a means of eliminating some sulfur from the open-hearth bath. While the extent of sulfur removal is not comparable to that of phosphorus, the ability to remove some sulfur contributes much to the value of the process.

The effectiveness with which the slag fulfills its functions varies with temperature and composition. Since operating temperatures are fixed largely by the specified nature of the product and the equipment, .the speed and extent to which the functions of the slag are carried out depend largely on its composition. The necessity for regulating slag composition to accomplish the re-

725

726 Chapter 17-SLAG-METAL REACTIONS

quired dephosphorization and desulfurization with greatest econ- omy establishes, in large measure, the distribution of other elements (such as silicon, manganese, and chromium). In normal open-hearth practice, variations in operating conditions and 'charged materials affect the extent to which the slag-metal reactions can proceed. As a result, the analyses of finishing liquid steel, as determined by slag composition, are not exactly re-. producible from heat to heat but vary in a definite but usually relatively small range.

Formation of Open-hearth Slags. The common components of basic open-hearth slags are listed in Table 17-1. This table also shows the chief sources of the various components and indicates the range of percentages found in refining slags. The percentages listed in Table 17-1 merely indicate the proportion of thos? components determined chemically. It should not be con- strued that the compounds and elements listed here necessarily exist as such in either liquid or solid slag. Typical analyses of slags taken a t key stages of open-hearth heats are given in Chap-

Component ~ Chief sources

Range of analysis in finishin

open-hearth syags, per cent

FeO . . . . . . . . . . . . . . . . Fe20s . . . . . . . . . . . . . . .

A120,. . . . . . . . . . . . . . . CaO . . . . . . . . . . . . . . . . MgO . . . . . . . . . . . . . . . MnO . . . . . . . . . . . . . . .

CaF2 . . . . . . . . Fe (metallic)

Ore and oxidation of iron Ore, oxidation of iron, and oxidation

of FeO Oxidation of silicon in pig iron and

scrap, refractories, and ore Scrap, refractories, ore, fluxes Fluxes, refractories Refractories, fluxes Oxidation of manganese in pig iron

and scrap Oxidation of phosphorus in pig iron

and scrap . Pig iron, scrap, and fuel Oxidation of chromiuln in scrap and

plg lron Fluxes Mechanically included

- - -

" T h e P20; content varies considerably wi th type of open-hearth practice (see Chapter 8). On certain duplex and high-metal practices the P,OJ in the finishing elags can exceed 10 per cent.

FORMATION OF OPEN-HEARTH SLAGS 727

ters 2, 7, and 8. To indicate general trends in the evolution of open-hearth slag, the following qualitative description of a heat made with scrap and hot-metal practice is given.

The earliest slags in the open-hearth furnace are formed during the first stages of melting scrap. During the meltdown period, as well as during the remainder of the heat, the oxygen pressure in the furnace gases is of the order of atm while that in the liquid iron pools of melted scrap is around atm (see Chapter 22). Accordingly, the iron oxidizes rapidly during melting, an immiscible oxide film being visible as the liquid runs off the scrap piles. The oxide collects over the pools of molten metal ; and a t the time of the hot-metal addition, a thin but very definite high iron oxide slag layer is present over all pools of metal. In addition to i ts high iron oxide content this early slag contains small amounts of CaO, MgO, SOz , and MnO. These oxides presumably originate from furnace refractories and from alloying elements or foreign material included in the charge.

As a result of the hot-metal addition, considerable silicon, manganese, phosphorus, and carbon react with the iron oxide formed on melting. If no ore has been charged, there is a n immediate sharp reduction in slag iron oxide content. In heats to which ore is charged, the iron oxide consumed in reactions with hot metal is steadily replaced by charged ore rising into the slag. Thus, on ore heats there is only a gradual reduction in slag iron oxide following hot-metal addition. I t is common practice to flush off a large part of the early slag on ore heats. As the rest of the scrap melts and the lime boil occurs, the lime content of the slag increases. By the time all the scrap has melted, the only marked difference in the analysis of slags from ore heats and non-ore

. heats is usually the lower manganese content of the former. This distinguishing feature persists throughout the refining period.

During the period following melt, the changes in slag analysis merely reflect the gradual solution of lime that has risen during the lime boil and additions made to accelerate the carbon drop. In low-carbon heats there is usually a definite rise in iron oxide content corresponding to the higher oxygen content of the liquid metal. In all heats, the most important slag composition i s that existing just before deoxidation or tap (a t the end of the refining period). The percentages of the elements phosphorus, sulfur, manganese, and chromium that remain in the liquid steel depend


largely on equilibrium relations between the metal and the liquid slag existing a t the end of the refining period. These equilibria are reviewed in the last part of this chapter.

In any evaluation of slags based on analyses, consideration must be given to the physical condition of the slag before sam-

Fig 17-1. Partial composition phase diagram for the ternary system CaO-FeO-SiOz (Bowen, Schairer and Posniak,' Korber and Oelsen.')

Heavy dotted lines show schematically the compositions of open-hearth type slags. Temperature contours outline liquidus surface. Circles represent compounds.

pling. Obviously, chemical analysis of slags containing undissolved solid particles will give a false indication of the composition and chemical behavior of the liquid phase. Thus in evaluating the basicity or oxidizing power of either operating or experimental slags precautions should be taken to allow for any solid lime, lime silicate, iron oxide, or magnesia in the sample to be analyzed.

Application of Phase Diagrams to Open-hearth Slags. For a preliminary consideration of the physical and chemical behavior

PHASE DIAGRAMS AND OPEN-HEARTH SLAGS 729

of basic open-hearth slags, recourse is made to existing phase diagrams. As indicated in Table 17-1, basic open-hearth slags represent an extremely complex system that includes significant percentages of a t least nine different components. Obviously, i t would be impossible to represent such a system in any simple type of phase diagram. As a first approach, however, it is in- formative to analyze several selected diagrams; and while the information to be gathered is only qualitative in nature, i t is quite helpful in explaining many aspects of the behavior of operating slags.

In conventional operations, the only component added specifically to impart basic properties to the slag is lime. The dominant acid constituent is silica. Because of the high oxygen gradient between the furnace atmosphere and the liquid metal, the slag system necessarily includes ionsiderable iron oxide. Thus, i t is only logical to begin the study of open-hearth slags by referring to availab!e information regarding the ternary system CaO-FeO- SO2. This diagram, which is shown in Fig 17-1, includes information regarding liquidus temperatures1 and solubility limits a t 1600 C (2912 F ) .2 This temperature was selected because i t represents open-hearth operating conditions. These data are amplified by the binary section representing the join Ca,SiOl- Fe,Si04 shown in Fig 17-2 and by the phase diagram of the two- component system CaO-Si02 shown in Fig 18-6 (p. 756).

To obtain some concept of the phases present in open-hearth slags, their solubility limits, and their melting temperatures, i t would be desirable to indicate the areas of the phase diagrams in which operating slags exist. Because of the complexity of open- hearth slag systems, this cannot be done with any degree of ac- curacy. As a rough approximation, however, it is possible to consider that, a s f a r a s their effect on melting point and solubility relations is concerned, the components CaO and MgO behave alike. Similarly SiO, and Al20:{ can be grouped together, as can FeO arid BlnO. Using such combinations, a crude picture can be developed to depict the nature of phase changes encountered in the development of open-hearth slags. The general nature of compositioil changes encountered in the development of slags from ore-charge heats and from heats with no ore charge are indicated by the two heavy broken lines in Fig 17-1. It should be recognized that when slags contain large percentages of flux-


ing oxides such as Fe203 and A1203 (also CaF2) the actual liquidus temperatures can be considerably lower than the values shown in Fig 17-1 for the simple ternary system.

\ Since meltdown slags are largely composed of FeO, such slags \ exist near the wustite area of Fig 17-1. The FeO activity in such

slags approaches that of the pure iron oxide. As a result, meltdown slags are capable of transferring considerable oxygen as 0 to the molten steel below the slag. The metal existing below these preliminary slags must therefore be highly oxidized and low in carbon, manganese, and silicon.

The early slags that form immediately following hot-metal addition are relatively acid as a result of the rapid oxidation of silicon. When ore is charged, this acid slag must exist for a time in contact with relatively pure solid iron oxide. Although such iron orride dissolves rather rapidly'in the slag, there is generally a period early in ore-charge heats when the slags must border on the wustite area, and the FeO activity in that slag is necessarily very high. In this period, then, reactions favored by high iron oxide activity proceed rapidly. In such heats, silicon, manganese, and even phosphorus are removed rapidly until they approach an equilibrium condition with the early slag. With the depletion of the charged ore the slag composition moves away from the wustite area and the FeO activity is reduced. If such an acid slag were allowed to remain within the furnace, the oxidized phosphorus would tend to revert back to the metal. One purpose of the slag runoff is to prevent such phosphorus re- versions.

The early acid slags on heats in which no ore has been charged exist in a low-melting zone of low iron oxide activity and therefore are fairly fluid but not very reactive with respect to oxygen transfer to the liquid metal. As a result, the oxidation of manganese in the early period of such heats is relatively slow and there is essentially no phosphorus elimination.

While the curves for the two different types of slags originate in different sections of the diagram, the general nature of the changes in liquidus temperatures during the early stages in their development is similar. Early acid slags with either practice exist in low-melting-point areas. Such slags are generally quite fluid. As small increments of lime dissolve into such slags, there is no great change in the liquidus temperatures and as a result

PHASE DIAGRAMS AND OPEN-HEARTH SLAGS 73 1

such slags remain fairly fluid. With solution of the large quantities of lime that become available during the lime boil, both slag-composition lines cross through a zone in which the melting point rises sharply. This rise i n liquidus temperatures is associated with the appearance of the high-melting silicate CazSiO,, in equilibrium with the liquid slag. Because of the resultant high melting point, such ,slags include considerable solid silicate

Fig 17-2. The 2Ca0-Si01-2Fe0-Si0; %action of the Fe0-COO-Si02 diagram. (Bowen, Schairer and Posniak.')

and generally become extremely heavy and viscous. As indicated in Fig 17-1, the actual melting point of the high-lime slags would be above ordinary open-hearth operating temperatures were i t not for the presence of fluxing oxides. The sharp curve toward higher FeO that is present a t the end of each line simply reflects a characteristic increase in slag iron oxide a t low bath carbon. Such an increase in iron oxide is necessary to dissolve the large proportion of lime present in many finishing slags.. This section of the curve, of course, would not apply t o heats tapped a t high bath-carbon contents.

732 Chapter I7-SLAG-METAL REACTIONS

The predominant change occurring during the development of open-hearth slags is the solution of lime. The primary source of this lime is limestone rising from the furnace bottom during the lime boil. The binary system CaO-SiOs, given in Fig 18-6 (p. 756), shows that the series of solid compounds, CaO, Ca,SiO, (3CaO *Si02) and CaPSi04 (2CaO Si02) must be present within an individual lump and that the layer next to the liquid phase a t operating temperatures must be Ca2Si04 (2CaO-SiO,). There- fore, the lime-solution process is not one of lime dissolving directly into early acid slags but rather one involving solution of the orthosilicate.

By increasing slag temperatures i t should be possible, a t least in theory, to develop a situation where i t is no longer necessary that solid compounds form in the process of lime solution. Under such conditions lime will dissolve directly into the liquid slag. I n the presence of fluxing oxides, the temperature a t which lime solution occurs without the formation of Ca2Si0, (2CaO SiOz) is markedly reduced. When ore is added to slags containing large amounts of solid lime, the percentage of fluxing Fe203 is increased temporarily and a period of rapid direct solution of lime (or of Ca3Si0,) is often encountered. Similar direct solution occurs when lime is added to the highly oxidized slags that exist a t relatively high temperatures over low-carbon heats.

Figs 17-1,17-2, and 18-6 demonstrate that with increasing lime solution and a t operating temperatures a situation is encountered where slags can exist only in equilibrium with solid Ca2Si04. Without fluxing additions, once the solubility limit of the orthosilicate is reached further lime solution can be accomplished only by markedly increasing slag temperatures. Since such slags exist in equilibrium with the compound Ca,SiO,, the lime activity of that slag must be only that of the orthosilicate. This lime activity is presumably considerably less than i t would be were the slag in equilibrium with pure lime. In this respect i t is interesting to note from Fig 18-6 that if a sufficiently high temperature could be reached (in excess of the melting point of CanSiO,), the liquid slag phase could exist in equilibrium with pure lime; under such conditions a much higher lime activity would be expected.

When slags become too thick as a result of solid lime being present, i t is common practice to add a fluxing agent capable of

PHASE DIAGRAMS AND OPEN-HEARTH SLAGS 733

forming low-melting combinations with lime. The commonest of such fluxes is fluorspar (CaF,). Unlike silica, the compound CaF, forms no intermediate compound (such as Ca2Si04) with lime. Thus the presence of local high spar concentrations around lumps results in direct and effective lime solution without any reduction in lime activity. While it is theoretically possible to add sufficient fluorspar to dissolve comp1etel.y all solid lime-bear- ing phases in the slag, this is not the practice used. Actually, fluorspar is used to reduce rapidly the proportion of solid lime lumps in the slag and to increase the amount of lime available in the slag. Such slags, however, are generally still saturated with Ca,SiO,.

Insofar as open-hearth reactions are concerned, it is desirable to dissolve a maximum of lime into the slag in order to accomplish most effective phosphorus and sulfur removal. In both reactions, especially that for sulfur removal, most effective elimination begins only after the solid Ca,Si04 phase appears. Thus, the problems of dephosphorization and desulfurization often in- volve dissolving more lime into a slag that is already saturated with lime (or Ca,Si04). At a given temperature, i t is obvious, that the addition of more lime to such a slag would have no bene- ficial effect. Thus, if the temperature is not raised, the proportion of fluxing components must be increased before more lime can dissolve. The line in Fig 17-1 representing the lime solubility limits a t 1600 C (2912 F ) illustrates this situation. This line indicates that to permit further solution of lime in a saturated slag some increase in the ratio FeO/SiO, must be effected. (The use of spar, which would increase the ratio CaF2/Si02, would exert a similar effect.) For example, a slag with a composition corresponding to the intersection of the two heavy dotted lines in Fig 17-1 will contain solid silicate particles a t 1600 C (2912 F ) . Ry increasing the FeO/SiO, ratio of that slag, its composition could be moved to a position beyond the end of the dotted lines. At this composition, the slag would be completely liquid and capable of dissolving more lime.

In early refining slags, operating temperatures and FeO content are relatively low. As shown in Fig 17-1, i t should be quite difficult to dissolve large percentages of lime into slags under such conditions. With the characteristic bui!d-up of FeO (as well as Fe2O:<) and the increased temperature of low-carbon

.734 Chapter 17-SLAG-METAL REACTIONS

heats, the actual amount of lime that can be dissolved into such slags increases considerably.

Higher operating temperatures move the lime-solubility curves closer to the lime corner and can be used to promote the formation of slags with high lime activity. Thus, the attainment of the high-lime slags needed to carry out the primary furnace reactions is favored by high operating temperatures, high slag FeO, or high fluorspar content.

PROPERTIES OF LIQUID SLAGS The properties of liquid slag a re reflected by i ts appearance in

the furnace as well as the rate a t which certain primary processes will proceed. While i t would be desirable to accumulate basic information regarding such properties a s viscosity, electrical conductivity, thermal conductivity, density, and vapor pressure of liquid slags, the high temperatures involved often pre- clude the application of standard measuring techniques. ' The limited measurerhents that have been made indicate how such properties are governed by the temperature and composition of

I the slags. These features and the s!ag characteristics they reflect will be reviewed.

Viscosity. Slag viscosity has often been related to reaction rates, diffusion rates, slag composition, and temperature. Gen- erally, viscosity is measured by timing flow through a capillary tube, by determining the damping of the oscillations of a pendulum through a liquid or by measuring the retarding effect of the liquid on an immersed rotating cylinder. Obviously, the use of such viscosity-measuring techniques on open-hearth slags would be greatly hampered by the existing high temperatures. Accordingly, true viscosity data on operating slags a re very limited.

If the force of viscosity (the force resisting flow) is R, it may be determined through the equation :

where q is called the viscosity coefficient, A is the cross-sectional area being considered, and du/dz is the velocity gradient per- pendicular to A. It is evident from this equation that the greater the resistance to flow, the greater will be the viscosity coefficient.

PROPERTIES O F L IQUID SLAGS . 735

Some values of the viscosity coefficient for slags that are given in Table 14-5, and in Fig 17-3, show that the value of 7 in slag varies through the wide range from about 0.01 to 10 poises.

As temperature increases, the viscosity of a liquid decreases. This relation has been expressed by an equation of the form

where A and B are constants whose values depend on the particular liquid being considered and T is the temperature. An il-

'

lustration of the effect of temperature on the viscosity of open- hearth slag is shown in Fig 17-3.

Fig 17-3. Relation .between viscosity of basic open-hearth slags and temperature. (Endell, Heidtkamp and Hax.=)

In practice, widespread application has been made of the empirical viscosity measurement carried out with the Herty viscosi- meter. As pointed out in Chapter 6, p. 204, this measurement is influenced by temperature and a number of incidental variables


and gives, a t best, only a qualitative indication of the fundamental property, viscosity. Measurements of this type have demonstrated that, a t a given temperature, the viscosity of basic open-hearth slag increases with increasing lime content and decreases with increased percentages of fluxes such as Fe203, MnO, CaF,, and A1203 or increased amounts of acid oxides such as Si02 and P205. It is likely that these effects can be related to constitution of the liquid but the exact nature of such relations has not been thoroughly investigated. It is hoped that continued improvement in refractories and high-temperature techniques will eventually provide a satisfactory method for making true viscosity measurements.

In addition to the effects of composition and temperature, the presence of undissolved particles in the slag exerts considerable influence on its viscosity. The solid particles of lime and calcium orthosilicate that frequently are present in operating slags increase the viscosity appreciably. In fact, during the period when lime is being dissolved most rapidly, slags are often so viscous that they cannot be poured readily from the sampling spoon.

In Chapters 15 and 22 i t is pointed out that the rate a t which diffusion processes proceed in the open hearth depends largely on the viscosity coefficient of the medium being considered. Thus, the rate of a reaction involving diffusion through slag, such as the transfer of oxygen from the atmosphere to the bath, depends considerably on slag viscosity. However, only the rate of such reactions is affected by viscosity, not the ultimate equilibrium condition. Actually, turbulence is of greater importance than diffusion as a rate-determining factor. Again, however, turbulence is affected directly by viscosity.

Vapor Pressure. In Chapter 14 it was explained tha t when a liquid is placed in a closed vessel a part of the liquid will vaporize and the vapor will occupy the space not taken up by the liquid. The pressure exerted by the vapor when the liquid and its vapor exi:3t in equilibrium is called the vapor pressure of the liquid.

Raoult's law states that in an ideal solution the vapor pressure ,of any component is proportional to its mole fraction. It was pointed out in Chapter 14 that vapor pressure is also a measure of the activity of any substance. Thus, for ideal solutions a direct relation exists between vapor pressure, activity, and mole

PROPERTIES OF LIQUID SLAGS 737

fraction. Since open-hearth slags are solutions, these considerations can be applied to them. The vapor pressure of slags can be studied with reference to a given component and fundamental information can thus be gained regarding slag constitution, but unfortunately, vapor-pressure measurements applicable to open- hearth processes have not been made.

Electrical and Thermal Conductivity. Electrical conductivity measurements have been made on slags of simple binary and ternary oxide systems. When combined with other measurements, electrical conductivity determinations not only provide data regarding the ionization of slag but also indicate the nature of the ions present and the degree of ionization.

Electrical conductivity, or the flow of an electric current in a conductor, is either metallic or ionic. In the first type the current consists of a flow of electrons while the second type consists of flow of electrically charged particles (ions). A third type, in which the flow consists of both electrons and ions, is called mixed conductivity. In general, metallic conductors have high conductivity while that of ionic conductors is relatively iow; the conductivity of metallic conductors decreases and that of ionic conductors increases as the temperature is raised.

Wejnarth4 made conductivity measurements on slags of the simple system Ca0-Fe0-Si02 that indicated to him that slags are mixed conductors. This indication is based on the fact that his conductivity values, while low compared with those of metals, are very high when compared with known strong electrolytes in aqueous solutions. Wejnarth found that the conductivity of such slag systems increased with increased iron or manganese oxide and decreased with increasing silica content. While data from electrical conductivity measurements are as yet quite meager, all available information (5,6v7vS) is in agreement with the thesis that metallurgical slags are ionized to a considerable degree, which accounts for their conductivity.

Ionic conductance is a function of slag viscosityQnd therefore can be related to temperature by an equation similar to that already given for viscosity. In this equation i t is generally preferred to use the resistance of the slag rather than its conductivity. The equation then reads as follows:

A l o g R = - + B

T

738 Chapter 17-SLAG-METAL. REACTIONS

where R is the resistance in ohms and A and B are constants depending on the nature of the slag. The relation between viscosity and conductivity can be explained by the fact that large ions move relatively slowly in any potential field, and thus cannot carry as much current as can smaller, more numerous, and highly mobile ions that characterize fluid slags.

Another property related to electrical conductivity is thermal conductivity. For open-hearth slag, thermal conductivity obviously is associated with heat transfer and reaction rates. In practice, however, the transfer of heat and oxygen to the bath is largely determined by the physical condition of the slag and bath action or turbulence rather than any feature of its composition. Thus, slags that are foamy and fairly viscous have low heat conductivity due to the presence of considerable entrapped gas. Since open-hearth slags, a t best, a re relatively poor conductors of heat, it is fortunate that the process does not depend on simple conduction for the necessary high refining temperatures, but instead is able to obtain adequate heat transfer through con- vection stimulated by the continuous boiling action of the bath.

Miscellaneous Properties. Data are available on miscellaneous properties of solid slags such as specific heat,1° refractive indices of solid constitutents,ll density,12 and magnetic susceptibility.13 Such properties may find some application a s control measures, but give little promise of being useful for determining the fundamental nature of liquid slags.

BASICITY AND OXIDIZING POWER

The two terms used most commonly in describing behavior of slags are basicity and oxidizing power. These terms a r e used in both mill and laboratory studies with the same general meaning.

The common slag-forming oxides may be classified in the following general way with regard to their acid or basic behavior:

Bases Mild Bases Mild Acids Acids CaO MnO A1203 Si02 MgO FeO Fez01 PzOa

The term basicity refers to some measure of the oxides that are available to carry out the slag functions of dephosphorization

BASICITY AND OXIDIZING POWER 739

and desulfurization. The measure usually is some function of the relative amounts of basic and acid oxides. In general, the basic oxides of this classification combine with acid oxides to give the structures and configurations that occur in solid and liquid slags. For example, strong bases, such as CaO, combine with strong acids, such as SiO,, to form relatively stable compounds, such as CaZSiO,. Often, less active bases may combine with acid oxides to form silicates, such as Fe2Si04, or possibly some combination between basic oxides and mild acids occurs to form compounds, such as Ca2Fe205. In the latter case it is likely that the tendency for the compound to dissociate a t high temperatures is relatively great. These formulas must be interpreted as indicating only the compositions of the phases present in the solidified crystalline slag. They do not mean that individual molecules with this limited number of atoms or ionic groups can be isolated or that the known properties of the crystals can be interpreted in terms of such small molecules. The same reasoning applies to any extension of these concepts to the liquid phase. However, in a limited number of cases some ideas of the size of effective molecular aggregates can be derived from studies of slag-metal equilibria. I t seems that Ca,Si208 groups may predominate in basic slags while Ca3Si30, groups may predominate in acid slag. l4, l5

Various expressions for the difference between the amounts (or concentrations) of basic and acid oxides have been used to describe available base. In certain cases some ratio of the amounts (or concentrations) of the two types of oxides is used as a measure of basicity. In the latter part of this chapter some examples of basicity measurements are given in conjunction with descriptions of specific slag-metal reactions.

The preceding discussion of the term basicity simply indicates the behavior of the dominant basic open-hearth slag-forming oxides and offers no general definition of acids or bases. For an acceptable definition, recourse is made to modern concepts of acid and basic molecules as originally developed by G. N. Lewisl6 and applied specifically to oxide and slag systems by Sun and Silverman17 and by Chipman and Chang.14 According to the Lewis theory, the fundamental distinction between acid and basic molecules appears in the nature of the valence shell of the atom and the manner in which electrons from this shell behave in the


presence of other molecules, atoms, ions, o r radicals. On tnls basis, a basic molecule (atom, ion, or radical) is one that can donate an electron pair from its valence shell to the valence shell of another molecule (atom, ion, or radical). By sharing an electron pair in this way a coordinate bond is formed between the participating particles. By the same definition, then, an acid molecule (atom, ion, or radical) is one that can accept an electron pair into its valence shell. The reaction between acids and bases in which covalent or coordinate bonds are formed through shared electrons is referred to as neutralization. In such neutralization reactions, the bases, the acids, and the neutralization products may be neutral molecules or ions. The product of a neutralization reaction may itself be capable of either donating or accepting an electron pair and thus in turn behave either as a base or acid.

The classification of an oxide as basic or acid is not determined by its chemical formula but by its tendency to donate or accept electrons. The exact classification depends on the specific reaction being considered. In any given-group of oxides, such as exists in basic open-hearth slags, the behavior is reasonably predictable. The previous classification of oxides is entirely consistent with this modern concept of bases and acids.

Oxidizing power of a slag refers to the amount of oxygen'that a slag is capable of transferring to the liquid steel bath. As will be shown, a reasonably consistent concept of oxidizing power can be developed when i t is considered that the oxygen available for transfer to the bath exists as iron oxide. On this basis, the oxidizing power of a slag refers to iron oxide activity in that slag. At equilibrium, the iron oxide (or oxygen) activity in the slag is equal to the oxygen activity in the liquid metal.

For operating conditions i t is difficult to define oxidizing power in terms of bath oxidation, since the effect of carbon content on the oxygen content of liquid steel nearly erases any relation with slag composition. It has been shown that other features of the open-hearth process are as important in deterniining the bath oxygen content as the slag composition.

Frequently, the term oxidizing power is used in referring to the iron oxide content of the slag. Defined in this way, i t has proved useful in describing the distribution of the elements phosphorus, manganese, and chromium. The distribution of each of

SLAG CONSTITUTIC~N 74 1

these elements between the slag and the metal is affected directly by the iron oxide in the slag.

SLAG CONSTITUTION

The constitution of slags after solidification has been well established for practically all types of open-hearth practice. Some methods that have contributed to the understanding of the nature of solid slags a r e petrographic examination, X-ray analysis, and phase-diagram studies. Each method has its own particular advantages and limitations. All have the shortcoming that their application has been limited to the investigation of the solid phases.

I t is important to realize that observations made a t temperatures where slags a re solid do not necessarily reveal the constitution that exists in the liquid slag in contact with a steel bath. In the absence of more direct measurements, recourse has been made to relations observed in solid slags as a first approximation to actual conditions existing in liquid slags. Available information substantiates the statement that the mineralogic nature of solid slags has no direct application to the liquid state beyond providing a general basis for further assumption.

Liquid Slag Constitution. I t has been shown in Chapter 14 that classical methods of physical chemistry may be used to evaluate the equilibrium constant for a given reaction and to determine the influence of temperature and composition on this constant. To make use of these methods, however, i t is necessary to be sure that the reaction is correctly written and to make use of one of two alternative approaches for expressing the concentration of the reactants.

The first approach involves activities of the reactants. This method will be illustrated in the part of this chapter that describes the distribution of oxygen between slag and molten iron. It will become apparent that general use of this method would require a complete set of activity data for all chemical constituents in the slag and metal, and that such data are not available a t the present time. When proper activity data are available, such a treatment is useful but still leaves a great deal to be desired. It must be regarded as a formalized procedure for producing the rabbit without disclosing anything about what


goes on inside the hat. In more direct language, when activities are known, they can be used to evaluate the final state of equilibrium that will be attained by the system, but they do not help much in understanding the mechanism by which this equilibrium is reached or the factors that govern the rate a t which this mechanism operates. For example, the work of Fetters and Chip- man1* and of Taylor and ChipmanlQhows that the activity of oxygen in a slag containing 30 per cent FeO is the same as that of a slag containing 80 per cent FeO, with only a slight change in the ratio (%CaO)/(%SiOP), but i t does not tell us any of the reasons for this situation. Since equilibrium may be regarded as the condition reached when the net rate of a reaction is zero, i t is obvious that no one in the steel industry, whether his immediate responsibility is as an operator or as a research man, can be entirely satisfied with the "activity" approach to the slag- constitution problem.

The other approach is to describe in the best possible way all the chemical species present in a system and use some concentration term such as mole fraction instead of activity in the evaluation of the equilibrium constant. Here again there is a dearth of information, but useful approximations have been made. This concept of slag constitution also provides an approach to the study of the rates and mechanisms of these reactions.

There are a number of methods that contribute to knowledge of the constitution of steelmaking slags. Viscosity, vapor pressure, and electrical conductivity have all been described as properties of liquid slag which may be measured as functions of composition and temperature and which may be interpreted in terms of the molecular make-up and types of interatomic forces present in the slag.

The study of two-phase (slag-metal) equilibria has probably yielded more information than any other method. The metallurgist is interested in slags because of their reaction with liquid metal, and a great many data are available on the distribution of various elements between the slag and metal phases encountered in commercial processes. Examples are given in later sections of this chapter dealing with the oxygen, phosphorus, and silicon reactions. The more general possibilities of the method as a research tool should not be overlooked. Any slag-metal, slag-gas, slag-matte, or two-slag system can be used, and in some cases

SLAG CONSTITUTION 743

Fig 17-4. Thin section from early open-hearth slag. 500X. Dark phase [Fe, M n J a 0 4 ; light phase ( C a , M g , Mn, Fe)zSiO,.

Fig 17-5. Thin section from intermediate open-hearth slag. 500X. Dark phase CazFe,OE; light phase CazSiOI + phosphate.


the problem can be simplified by using a n unconventional system. An example is the distribution of aluminum between iron and silver studied by Chipman20 to estimate the activity of aluminum in iron. The theory of this method has been explained in Chapter 14;

Constitution of Solidified Slags. Because of possible relation to liquid slag constitution, i t is of interest to review briefly some of the more obvious mineralogic features of solidified open-hearth slag.

Solidified open-hearth slags (including those that are formed before addition of hot metal) are remarkedly similar in general mineralogic appearance despite variations in open-hearth charging practice. Early slags generally consist of only two phases, a complex CaO, MgO, MnO, FeO orthosilicate that may be described by the formula (R'R)2 SiO,, and a spinel-type oxide phase, (R'R)30,. As more lime enters the slag, the composition and optical properties of the silicate phase and the oxide phase gradually change. The general mineralogic appearance, however, does not change greatly until sufficient lime is present to form dicalcium silicate. Intermediate slags a s well as some finishing slags from high-carbon heats are composed predominantly of the dicalcium silicate and an oxide phase. With further lime solution that occurs in the finishing stages of heats,,compounds such as Ca2FePOj, Ca,Si05, and CaFe20, often appear in the solid slag samples. Figs 17-4, 17-5, 17-6, and 17-7 are photomicrographs

Composition Type of Slag in which Compound is Found

CaO.. . . . . . . . . . . . . . . . Finishing slags; raw slags MgO. . . . . . . . . . . . . . . . Intermediate and finish~ng slag (R'R)30ra. . . . . . . . . . . . Early slags

. . . . CaFe20r. . . . . . . . . . . . . . Finishing slags Ca2Fe205. . . . . . . . . . . . Intermediate and finishing slags . .

(R'R)2Si04n. . . . . . . . . . Early slags . .

CazSi04. . . . . . . . . . . . . . Intermediate and finishing slags CarSiO5.. . . . . . . . . . . . . Finishing slags Ca3M '(Si04)2. . . . . Intermediate slags . ~ a , ~ 2 8 i 2 0 , ~ . . . . . . . : : : High-phosphorus (southern) slags Ca5P2Si012.. . . . . . . . . . High-phosphorus (southern) slags and basic bessenler slags Ca4P209. . . . . . . . . . . . . Basic bessemer slags

The cornpo~u~ld Ca,P,O* has been reported to exist in solution with the silicate in all of these slags.

" R' R indicates isomorphous solution of Ca, Mg, Mn, and Fe.


Fig 17-6. Thin section from finishing open-hearth slag. 500X. Dark phase CaFeiO, and CazFe,Oe; light phase predominantly Ca.SiOl.

Fig 17-7. Thin section from finishing open-hearth slag. 500X. Dark phase CaFe,04; light phase CaaSiO,.

Chapter 17-SLAG-METAL REACTIONS

illustrating the appearance of thin sections from typical slags taken a t four different stages of open-hearth heats.

Other compounds appear in solidified basic open-hearth slags but their presence usually reflects some peculiarities in charging or operating practice. A list of compounds that reportedly have been identified in solidified basic slags is given in Table 17-2.

Identification of the phases in solidified slags represents the first approach to the problem of determining the constitution of liquid slag. In interpreting such information, i t should be recognized that the phases comprising the constitution of the solid sample may be vastly different from those existing in liquid slags in the furnace. Compounds may be present that cannot exist in molten slag. Crystals may appear that are stable only in some intermediate temperature range. Recognition of such limitations is essential to prevent false conclusions concerning the constitution of liquid slag.

General Concepts of Slag Constitution. It is now in order to develop a general concept of slag constitution that will permit the correlation of observed slag properties with known slag behavior by proper application of the principles of physical chemistry.

1. Association o f Oxides. The simplest useful concept was originally developed and used extensively by Herty21 and S ~ h e n c k ~ ~ in 1934 and has since been modified by many other investigators, most recently by Winkler and C h i ~ m a n ~ ~ in their study of the phosphorus reaction. This example will be considered in detail later in this chapter, but the general features of the concept will be outlined here.

In terms of the commonly reported chemical analysis, an open- hearth slag contains the various oxides, CaO, MgO, SiO,, A1203, P205, MnO, Fe203, and others. In order to have real value, a concept of slag constitution must be related to such an analysis, for these are the only quantities that usually are determined directly. Experience has shown that slags do not act as though these oxides formed ideal solutions. For example, if a wide range of slag compositions is considered, the amount of oxygen dissolved in iron is not directly proportional to the percentage of FeO in the slag. It is necessary therefore to assume that these oxides combine as chemical compounds so as to render certain


components inactive. Such compounds might be Ca,Si04 or Ca4P209, and the oxides that are not required for such postulated compounds a re regarded as "free" or available for reaction. This procedure has proved overly simple, and a compromise has been more satisfactory. In this compromise, all constituents are considered as combining in some compound, which in turn is par- tially dissociated. On the basis of observed data and simplifying assumptions, SchenckZ2 developed a series of dissociation constants that enabled him to describe qualitatively the behavior of the entire range of steelmaking slags (acid, basic, oxidizing, and reducing). The example of the phosphorus reaction will illustrate the value of this type of treatment.

Although such work has been valuable and useful, it does not appear promising as a means of developing the basic science much beyond its present status. The principal objection is that it assumes ideal behavior of slag components that exist in large concentrations, and this is not in accord with general observations in other liquid systems that have been studied more extensively and whose behavior is better understood.

2. The Silicate Network and Ionic Dissociation. In order to understand slags, i t is necessary to have some concept of the chemical nature of the interatomic forces acting in slags and the manner in which these forces are influenced by factors such as temperature and composition that enter into the control of metallurgical operations.

In seeking answers to questions regarding the nature of interatomic forces in slags, the metallurgist can learn much from ceramists, geologists, geophysicists, physicists, and physical chemists, who share with him a fundamental interest in the properties of glasses, molten magmas, and slags. The results of their investigations can be explained by considering the simplest possible case, pure liquid Si02. This case is well understood and i t can be extended to include the liquid silicates that are the basis of most metallurgical slags.

It has been observed from X-ray diffraction patternsZ4 of SiO, that no sharp transition occurs in passing from crystalline materials of increasingly small particle size to amorphous or glassy material and liquids. However, distinct characteristic differences appear, which can be interpreted in terms of the spatial arrangement of atoms in these various physical states. The


structure of crystalline silica (cristobalite) is such that every silicon atom is surrounded by four oxygen atoms, so that the silicon may be regarded as a t the center of a tetrahedron and the oxygen atoms a t its corners. Each oxygen is shared by two silicons, and the structure should be regarded as extending in- definitely in all directions. The various ways in which this sharing of oxygens may occur are illustrated schematically in

0 1 2 3 4 5 6 7 8 9 l O l i l ~ ~ ~ ~ ~ ~ f ~ ~ ~ ~ ~ ~ ~ t l t j

Scale in Angstrom units

Fig 17-8.' Crystalline silicates. (Bragg.")

Fig 17-8. Crystals are characterized by the fact tha t they possess this perfectly regular ar ray of atoms in all directions. It is also true that in all silicates the spacing is determined by the close packing of the larger oxygen ions, 2.64 d diameter, the smaller silicon ions, 0.78 A diameter, fitting into the interstices. The observed diffraction patterns of fused silica can be interpreted best by considering it to possess the same features of silicate tetrahedra with shared oxygen ions a t the corners, but the linkage is of a more random nature so that the tetrahedra do not assume a geometrical relation to each other.

For convenience in discussion, this structure can be repre-


sented schematically in two dimensions as shown in Fig 17-9a. Here, just as in the three-dimensional model, each silicon is coordinated with four different oxygens (coordination number is 4) and each oxygen is shared by two silicons. Now consider

Fig 17-9. Schematic diagram of silicate networks: ( a ) crystalline silica; ( b ) silicate linkage broken by lime; [ c ] aluminate substituted for silicate tetrahedra.

addition of another constituent such as CaO to this structure, a s shown in Fig 17-9b. Two silicon-oxygen bonds have been broken and replaced by calcium-oxygen bonds, but the over-all electrical neutrality of the slag has been maintained. This process can continue with further addition of CaO until the limiting composition of CaaSi04 is reached. At this time, all silicons are still coordinated with four oxygens in the fundamental silicate tetrahedra, but the tetrahedra no longer share all corners with each other. The extent to which the valence electrons of the oxygen-calcium bond are attracted to these silicate tetrahedra (polarization) determines the ionization of the slag. In simple

750 Chapter I 7-SLAG-METAL. REACTIONS

terms, it may be said that, since Ca2+ has a diameter of 2.06 A compared with 0.78 A for Si4+, the closer force fie!d of silicon will predominate and the slag will be highly ionized. ,

Some other simple cases should be considered to illustrate further the principles and flexibility of this method of slag interpretation. Aluminum is much like silicon in that i t has a very small ion and frequently is observed to be coordinated with four oxygens-that is, aluminum has a coordination number of four. In natural minerals these alumina tetrahedra may substitute for silica tetrahedra in the mineral structure, with the provision that the difference in positive charge between A13+ and Si4+ is also compensated by a smaller number of cations in the lattice. It would, therefore, seem reasonable to expect that when small amounts of A1203 are present in a slag i t will first enter the struc-

Fig 17-10. FeO activity in liquid slags containing COO and FoO. (Fetters and Chip- man.''] .

. .

ture in this way by simply substituting for silica tetrahedra in the complex, as indicated schematically in Fig 17-96. Pronounced changes in properties from. such a .substitution of alumi.num. for silicon would not be expected. However, i t is o.bserved that,when

'OXYGEN DISTRIBUTION. 75 1

larger amounts of aluminum are present in minerals, the aluminum may also have a coordination number of 6 , in which case it does not substitute for silicon in the tetrahedra. I t may, therefore, be expected that as the A1,03 concentration of slags is increased, a point will be reached where the aluminum will cease to substitute for silicon in the tetrahedra and a discontinuity in properties will result. Discontinuities have been observed in measurements of electrochemical potential, viscosity, and desul- furizing power.6

The basis for extending the picture further has now been developed. The sulfide ion is only slightly larger than the oxide ion, and these two ions are known to have much in common with regard to chemical properties. Sulfur in slag probably substi- tutes for oxygen in the silicate network, and the structure is stabilized by the presence of the more positive calcium. Pros- phorus is another small ion that has chemical properties similar to silicon and may, therefore, substitute for silicon in the centers of tetrahedra. This is consistent with the mineralogical observations of Barrett and M ~ C a u g h e y ~ ~ and the knowledge that some phosphorus may be removed by relatively acid slags.27 Thus, the silicate-structure picture is more consistent with the known metallurgical facts than the simpler associated oxide theories that require a molecule of the type Ca,P20,* to stabilize phosphorus in the slag.

SLAG-METAL REACTIONS

Oxygen Distribution between Slag and, carbon-free Iron Bath. In open-hearth operations an equilibrium distribution of oxygen between the slag and the liquid steel bath is seldom, if ever, attained. This situation results from the constant reaction of the bath oxygen with carbon. Studies of oxygen distribution between slags and carbon-free baths are of value, however, since they provide information regarding the amount of oxygen that any slag is capable of transferring a t a given temperature. More important, studies of oxygen distribution provide an insight into liquid slag constitution. In consideration of slag-metal reactions, the distribution of oxygen is therefore of primary importance.

For iron oxide slags and for those composed of iron oxide, manganese oxide, and silica (as is essentially true of acid slags), the


distribution of oxygen a t 1600 C (2912 F ) .has been shown by Korber28, 2D to approximate :

In order to discuss oxygen distribution in more basic slags, i t is necessary further to clarify the term iron oxide activity. The iron oxide or oxygen activity for any slag can be determined by obtaining the ratio between the analyzed oxygen content of the metal in equilibrium with that slag and the saturation oxygen of the metal a t the temperature under consideration.

The constancy of the value L in acid slags can be explained only on the basis that FeO exists in an uncombined state and, therefore, does not depend on the amount of silica present. Solid- state studies indicate that iron oxide and silica combine readily. I t must, therefore, be concluded that any compound of FeO and S i02 that exists a t lower temperatures is largely dissociated a t 1600 C (2912 F ) . The mole fraction of any of the oxides such as FeO in such an acid slag can be determined by obtaining the ratio of the moles of that oxide to the total moles in the slag. The necessary data can be obtained by chemical analysis for the constituent oxides.

Using as a reference state the system in which oxygen- saturated metal is in equilibrium with a pure FeO slag and as- signing a value of unit activity to the FeO in that system, the following equation can be written :

N F ~ O = aFeO = 00

The investigations of K o r b e r 2 h n d of Taylor and Chipman'" demonstrate that in acid slags the mole fraction of FeO in the slag is approximately equal to its activity. In other words, Raoult's law can be directly applied to such acid slags since they behave as nearly ideal solutions. In Chapter 16, the saturation value of oxygen in iron was shown to be given by the equation :

Thus, for example, the saturation oxygen a t 1649 C (3000 F ) can be calculated to be 0.28 per cent. If the metal in equilibrium with a particular slag a t tha t temperature contains 0.12 per cent oxygen, its FeO activity is 0.12/0.28, or 0.43. If the slag behaves'as an ideal solution, this activity will be equal to the mole fraction of FeO in the slag.

OXYGEN DISTRIBUTION 753

How closely basic slags approach ideal behavior has not been established with certainty. Available data indicate that, within the limits of experimental error, considerations based on the assumption that slags behave ideally offer a logical explanation for many observed features of slag-metal reactions. The bulk of the information regarding the constitution of basic slags has been provided through the excellent researches of Chipman with Fetters, Taylor, and Winkler.ls. 191 23

The distribution of oxygen between carbon-free iron and slags approaching the simple system Ca0-Fe0-Fe203 was studied by Fetters and Chipman.ls In order to compare the activity of FeO with the mole fraction of FeO, it is necessary to develop a satisfactory concept of slag constitution. A first approach to the problem of slag constitution in the simple system Ca0-Fe0-Fe203 may be made by converting the Fe203 to FeO on the basis of oxygen content a s shown in Eq 6-4 (p. 195), (this type of conver- sion has frequently been used in investigations of open-hearth data) . To simplify the calculations further, the magnesia present in the experimental slags is added to the lime. On this basis, the relation between oxygen activity and mole fraction of FeO is compared with the line predicted by Raoult's law in Fig 17-10. The resultant points show considerable spread. In addition, the best line through the points deviates perceptibly from the line describing ideal behavior, the deviation being greatest in slags of highest lime content.

In the explanation for such deviations lies the basis for our current knowledge of liquid slag constitution. I t might be rea- soned that slags are not ideal, and the departure from Raoult's law may be compensated through the use of an activity coefficient. On the other hand, apparent deviations from Raoult's law frequently result from lack of knowledge of correct constitution, and the resultant inability to determine the correct values of mole fractions. For the simple slag system Ca0-Fe0-Fe203, recourse is made to petrographic observations where the existence of calcium ferrites (as well a s orthosilicates) has been noted. Recalculating the mole fraction of FeO on the basis that Fe,03 combines with CaO to form the compound CaFe,O, (and making an adjustment for the presence of small amounts of silica, on the assumption that Ca,SiO, is formed) results in a closer approach to Raoult's law as well as a decrease in the spread between the


points. These results are plotted in Fig 17-11. The example in Table 17-3 is given to illustrate .the nature of the calculations in-

Fig 17-1 1 . FeO activity in liquid slags containing C O O , FeO, and CaFelO,. (Fetters and Chipman.")

Slag Composition

Moles .

MgO

6 .65 0.166

FeO -

. . . . . . . . . . . . Weight. per cent.. 27.65 49.03 e per 1 g a s g . . I 0 4 1 4 , 1 0 6 8 2

Mole Fraction

Slag Constitution

FelOa --

15.01 0.094

0.094 0 . 0 7 0.024 0 . 0 2 0.519 0 .39 0.652 0 .52

OXYGEN DISTRIBUTION 755

volved in the determination of the mole fractions of the various assumed slag constituents.

An alternative assumption regarding slag constitution involves the ionization of the various constituents. Since studies of crystal s t ructure have led to the view that most salts a s well as metallic oxides and silicates a r e an assemblage of ions, i t appears appropriate to consider slag-forming compounds a s being highly ionized in the solid state. However, very few data a r e available regarding the extent to which these compounds a re electrolytically dissociated (ionized) when in the molten state.

Samarin, Temkin, and Shvarzmanw assumed comp!ete ionization of slag-forming compounds and, on the basis tha t the ions behave a s ideal solutes, they explained certain quantitative relations regarding both sulfur distribution and the solubility of magnesia. The dominant dissociation reactions used by Samarin et al. were :

C:10 = C:1++ + 0- Ca2S101 = ?Ca++ +:,F1014- CnFc204 = C:c++ + FrrOd- TeO = FP++ + 0- go = MK++ + 0-

While this concept was used with some success in developing expressions fo r sulfur distribution and the MgO solubility product in selected basic slags, i t was unsatisfactory fo r the case of oxygen distribution.

Chipman and Chang,14 however, have successfully applied the ionization concept to the same slags a s a re considered in F i g 17-11. In order t o develop this concept, consideration was first given to the ionization of pure FeO to form F e + + and 0=. An activity value of unity was assigned to both FeO and the product ions. On this basis,

n ~ e O = m e + + X no-

This expression eliminates the need for specifying the degree of ionization. Using the expression fo r the activity of FeO, E q 17-3 may be wri t ten to describe the distribution of oxygen between molten iron and a pure iron oxide slag phase:

I'rO(1) = Fe'? + 0- = Fe(1) + O -

From the assumption tha t the activities of Fe++ and 0- 'are


equal to unity, it is evident that K is equal to the saturation oxygen content, and the expression can then be written :

-6320 log K = -

T + 2.734 [17-3al

In order to deal with iron oxide slags containing dissolved lime or magnesia, it is necessary to know something about the nature and activity of the ions present. Based on available information, Chipman and Chang assumed that the'most probable ion species in such slags would be Ca+ +, Mg+ +, Fe+ +, 0=, Si044-, and Fe2OS4-. When the expression

aFeo = are++ X a0-

is used to describe ironoxide activity in slags containing lime, magnesia and silica, it is necessary to know the proper activity (a ) and activity coefficient (r) to substitute in the equation. As an approach to this problem, Chipman and Chang assumed that the activities of t h e ions ~ a + +, Mg+ +, and Fe+ + were equal to their respective ion fractions Nc,++, NMg++, and N F , + + . A value for a~,,, was obtained using the relation shown in Eq 17-3. On this basis, Eq 17-3 could be written:

-- [% O1 - N F ~ + + . 7 No- K

To illustrate the calculation, the previous slag sample is used. The metal in equilibrium with the slag a t 1649 C (3000 F ) contained 0.120 per cent O=. At that temperature, K = 0.278. Sub- stituting the following data

Moles positive ions Moles negative ions

a (CaO + MgO + FeO - 2Si02 - 2FezOe)

in the preceding equation gives : 0.120 0.682 - - -- - 1.107

0.278 1.343 X r - 1.225

Thus it can be determined for this particular case that r = 0.94. The results of similar calculations for slags of increasing lime

plus magnesia content are plotted in Fig 17-12. This figure demonstrates that the values for the activity coefficient of the oxygen ion in these slags were close to unity. Thus, it may be concluded that the relation obtained by plotting the iron oxide

OXYGEN DISTRIBUTION 75 7

activity as determined by the oxygen content of the iron against the FeO activity measured as the product of ion fractions Nn++ x No= would approach the line predicted by Raoult's law.*

The ionic consideration was presented to illustrate that alternative hypotheses can be used to explain observed results in slag-metal reactions. Since the application of ionic theory to more complex slag systems has not yet been developed, no further consideration is given to the ionic theory of slags.

Fig 17-12. Activity coefficient of oxide ion in slags containing the oxides of iron, calcium, and magnesium. [Chipman and Chang.")

Basic open-hearth slags a re considerably more complex than the simple system used in the preceding illustrations. An approach to such slags was made by Fetters and ChipmanIs and Taylor and Chipmanl9 in their studies of the lime-silica-iron oxide system. Again the experimental slags contained the incidental compounds Fe203 and MgO.

The distribution of iron oxide between synthetic basic slags

' In this same paper, Chang and Chipman demonstrate that such ideal behavior cannot be generally assunled for all types of slag-forming ions. Calculations based on analysis of the same samples used in F i g 17-12 show a 100-fold change in activity coefficient for the Fe?O.'- ion.


,Mi70 CaOf MgO, per cent

Fig 17-13. Distribution of oxygen between synthetic slag and iron a t 1600 C (2912 F). (Fetters and Chipman."J

CaO+ 90 80 70 60 50 40 . , 30 20 10 ,FeO M-0 CaOtMgO, per cent .

Fig 17-14. Oxygen content of iron in equilibrium with' synthetic slog a t I 6 0 0 ' C (2912 F ) . (Taylor and Chipman.")

. . ' OXYGEN DISTRIBUTION

Mgo Mole fraction of CaOtMgO

Fig 17-15. Activity of iron oxide in basic slags. (Taylor and Chipm~n.'*] . .

and carbon-free metal is shown in Figs 17-13, 17-14, and 17-15. The distribution in these three figures is indicated in the ternary diagrams for the system Ca0-Fe0-Si02 by three different values. In Fig 17-13 the ratio of the percentage of oxygen in metal to the percentage of FeO in the slag is plotted. In Fig 17-14 the oxygen content of the iron is plotted on the ternary diagram; in Fig 17-15 the FeO activity is shown. These figures summarize results obtained in two independent researches and must be considered a s excellent checks in view of the possible experimental error.

I t appeared from studies of the simple CaO-FeO-Fe20, system previously discussed that molten slags contain such compounds as CaO and CaFe20,. The symmetry of the curves in Figs 17-13, 17-14, and 17-15 about the orthosilicate composition supports the assumption that this compound is of primary importance in establishing the constitution of liquid slags based on the CaO- SiO, system. Assuming the compounds CaO, FeO, CaFe20, and Ca2SiOa to be present in those slags of the Ca0-Fe0-Si0, system that are more basic than the orthosilicate composition, i t becomes possible to plot activity of FeO against mole fraction for this more complex system. The results of such calculations, made


FeO in slaq, mole fraction

Fig 17-16. FeO activity in liquid slags containing CatSiO,, FeO, and CaFerOd. (Taylor and Chipman."]

FeO in slaq.mole fraction Fig 17-17. FeO activity in liquid slags containing (CazSiO,)r, FeO and CaFezO,. (Taylor and Chipman?')

OXYGEN DISTRIBUTION 76 1

from the data of Chipman with Fetters and Taylor, are plotted in Fig 17-16.

The best line through the points of Fig 17-16 deviates considerably from the straight line predicted by Raoult's law. Again it is desirable to seek some explanation for the departure from ideal behavior.

Taylor and Chipman found a satisfactory explanation for the deviation from ideal behavior shown in Fig 17-16 in the assumption that calcium orthosilicate forms a double molecule according to the equation

2 Ca2Si04 = Ca4Si208

(Association reactions of this type occur quite commonly among compounds in which the binding is not ionic.) Recalculation of the data of Fig 17-16 on the basis that the double molecule forms eliminates the deviation from ideal behavior. The agreement with Raoult's law, as shown in Fig 17-17, is well within the mar- gin of experimental error.

FeO in slag,mole fraction

Fig 17-18. FeO activity in liquid slags containinq ( C a ~ s i O l ) ? , FeO, , C a O , CaFee01, CazAI2O5, and CarP:O,. (Winkler and Chiprnan.23)


The same principles can be extended to include more complex slags containing fluorspar, alumina, etc., with quite satisfactory results, as shown in Fig 17-18. The data for this figure were obtained from the study on phosphorus distribution by Winkler and C h i ~ m a n , * ~ and the assumptions required will be discussed in detail in the following section.

Despite the satisfactory agreement with Raoult's law, this particular concept of molecular association in liquid basic slags must be considered tentative and liable to revision. The results obtained fit assumed formulas and equations well enough to be useful in describing specific basic open-hearth reactions. However, further elaboration of the diss~ciation concept is required to account for known differences in the stability of lime, magnesia, and manganese silicates and to describe more accurately the behavior of the less stable ferrite and aluminate compounds. It is likely that modifications and refinements in the proposed slag constitution or even an entirely different concept will ultimately be developed that may explain the reactions even more satis- factorily.

Phosphorus Distribution. The qualitative effects of operating variables on phosphorus elimination a re readily apparent to open-hearth operators. It has long been recognized that phosphorus elimination is favored by high basicity, high slag oxidation, and low temperature. It is also common knowledge that each of these variables exerts a strong effect on the rate of the reaction.

The quantitative aspects of the phosphorus reaction, however, are fairly complex. As a result, there has been little agreement between the reiults obtained by various i n v e ~ t i g a t o r s . ~ l - ~ ~ The lack of agreement is somewhat surprising since practically all investigators have been in accord with respect to the fundamental nature of the phosphorus-elimination reaction. The dis- crepancies among the various studies can be attributed, in large measure, to the lack of a complete understanding of liquid slag constitution. Specifically, a large part of the difficulty has arisen from the absence of a satisfactory method of calculating mole fraction of the active constituents in liquid slag and the lack of a universally acceptable concept of available lime.'

PHOSPHORUS -DISTRIBUTION - . 763

The phosphorus-elimination reaction may be written: 4Ca0 (zn slag) + 2 P_ + 5Fe0 ( z t ~ slag) = Ca4P209 (zn slog) + 5& (17-4)

The equilibrium constant for this reaction is : (acs4p200) ( o ~ e ) ~

K' = - [17-4a) (UP)' ( a ~ e o ) ~ (acso)'

It has been proposed by several investigators that phosphorus may exist in liquid steel as Fe,P. Since a single atom of phosphorus is included in the compound Fe3P, such a refinement would make no difference in the quantitative considerations that follow.

Inasmuch a s mole fraction of iron consistently approaches unity, a value of one may be substituted for the term a ~ , in Eq 17-4a. Moreover, since the phosphorus concentration of the bath is always very small, i t is reasonable to assume that tHe activity of phosphorus ap is proportional to the weight percentage of that element.

The iron oxide activity aseo, was defined earlier by the equation '. ' I% 01 , . . aFeo = -

I% O..tl For any temperature, then, aseo = const. [% 01. Accordingly, Eq 17-4a may now be written:

I n order to calculate the equilibrium constant expressed in Eq 17-4b, i t is necessary to determine the activities of the compound Ca,P200 and the available lime, CaO'. A reasonable first approach to this problem could be made if the correct mole-fraction values of these two liquid slag constituents were known. Using the concept of molecular association and assuming the mole fractions of the slag constituents to be equiva!ent to their respective activities, Eq 17-4b can be rewritten:

It is apparent from Eq 17-4c that the dephosphorization reaction may also be written:

4 CaO' ( in slag) + 2 P + 5 Q = Ca4P2Og (in slag), Winkler and C h i ~ m a n ~ ~ approached the problem of determin-

ing the proper mole-fraction values by utilizing the best available molecular constitution data as determined from oxygen-


distribution studies. Through a n ingenious series of assumptions regarding liquid slag behavior, they obtained mole-fraction values for both Ca,P,O, and CaO'. From earlier work, they accepted the formation of the compounds Ca,Si20,, Ca,P20,, FeO, CaFe204. Their own data indicated that best results were obtained when the compounds Ca2A1206 and CaaFpP,09 were assumed to form in slags containing substantial percentages of A1203 and CaF,, respectively.

To obtain a single measure of slag basicity, Winkler and Chip- man combined the magnesia and manganese oxide with the lime. This assumption apparently fits their data quite well for slags containing less than 12 per cent MnO. They were not successful in developing a single expression for basicity that could be used throughout the entire range of manganese oxide analyses. (In the rest of this section, the term CaO refers to the sum of the basic oxides rather than to the lime content itself.)

In addition to the preceding assumptions, Winkler and Chip- man proposed that available base included both the excess over that combined in the previously listed compounds and the lime resulting from dissociation of the predominant double orthosilicate molecule, Ca4Si208. An assumption of this type was necessary to explain the fact that considerable removal of phosphorus occurs in certain slags where the lime content is f a r below that required to combine with silica. The proposed dissociation reaction was written

Ca4Si208 = Ca2S~20, + 2 (CaO') 117-51

for which the dissociation constant may be expressed as

Winkler and Chipman used a method of successive approximations to determine from their data that thk value of KD was '0.01 and that the constant was independent of temperature. It should be recognized that, although this particular dissociation concept is not entirely satisfactory, a t present it provides a useful approach to the quantitative interpretation of phosphorus distribution. Further refinements or entirely new concepts may be provided by future studies.

The determination of the dissociation constant K , made i t possible for Winkler and Chipman to calculate K p for a wide range

PHOSPHORUS DlSTRlBUTlON 765

of experimental slag analyses and operating temperatures. A sample calculation from their data is given in Table 17-4.

TABLE SLAG SLAG TEST NO. 22, HEAT E-36e Temperature, 1595 C (2903 F)

I I

I Slag analysis anal yeis I Bath

Total moles basic oxides. . . . . . . . . 0.370 + 0.263 + 0.035 = 0.668 Moles of base in excess of that re-

quired to combine as CaFe20c and C a P z 0 9 . . . . . . . . . . . . . . . . . (C) = 0.668 - 0.031 - (4 X 0.012) = 0.589

Total moles of silicate per 100 grams of slag, [Sz] = 0.171 a Winkler and C h i ~ m a n . ~ ~

Chemical analysis, . . . . . . . . . . . per cent

Moles per 100 grams slag. . . . . . . . . . . . . . .

Adopting the symbols (C2S2) for moles of CazSizO6 and (C4S2) for Ca4Siz08, the following expression may be written for the moles of free lime per 100 grams of slag :

(CaOr) = (C) - 4 ( C ~ Z ) - ~ ( C & Z ) [17-61 Moreover,

(S2) = (Csz) f ( C Z ~ Z ) (17-71

If Ng is the total number of moles per 100 grams of slag, Ns = CaO' + (82) + FeO + CaFe20r + Ca4P20g (1 7-81

KD given in Eq 17-5a may now be expressed

K D = (C2Sz) (Ca01)2

[17-91 (CIS:) (Ns)'

By substituting for C4S2 its equivalent, Sz - C2Sz (Eq 17-7), and the value KD = 0.01, i t i s possible to obtain the following

CaO -

20.76

0.370

result : (CzSz)

( C Z ~ Z ) ~ + [ (0 - 4(sz) 1 (czsz)' + - 4(sz)' + 0.01 N:I 7 - 0.0025 N s 2 ( ~ z ) = 0 [IT-101

For the particular slag and metal samples being considered, this equation becomes :

MgO -

10.51

0.263

SiOl -

20.50

0.342

(0.0090 + 0.01 Ns2) (CZSZ)~ - 0:035 (CZSZ)~ - -- - CZS? - 0.0043 ~ 8 2 = 0

4 117-111

The only unknowns in the preceding equation are CzS2 and N8.

MnO -

2.51

0.035

P20b -

1.67

0.012

b e 0 'Fe.0. P - -

38.86

0.540

0 -

0.0300.160

-

4.98

0.031


A solution to the equation may be obtained if a satisfactory estimate of Ns can be made. Such an estimate may be made simply by adding a reasonable value for CaO' to the known terms involved in the total moles calculation (see Eq 17-8). The assumed value can be corrected if calculations indicate considerable error in the estimate.

For this slag, Ns = CaO' + 0.171 + 0.540 + 0.031 + 0.012 N s = CaO' + 0.754

Since previous work indicates a value of CaO' approaching 0.100, a tentative solution of Eq 17-11 may be made by substituting N s = 0.85. Eq 17-11 then becomes:

(Cd2)' - 0.095 (CUSZ)~ + 0.0040 (CUSz) - 0.00032 = 0 from which it may be determined that

CzSz = 0.089 C4S2 = 0.082 (see Eq 17-7) CaO' = 0.053 (see Eq 17-6) Ns = 0.837 (see Eq 17-8)

Eq 17-11 can now be recalculated using this new value of N8. The recalculated results, however, would not change the preceding values significantly. Using the preceding values,

0.083 the mole fraction o f CaO' or (Nc.01) = - = 0.099

0.837 0.012

the mole fraction of Ca4PzOo or (Nc.4Pzo,) = - = 0.014 0.837

Kp = 0.014

= 1.56 X 10, (0.030)2 (0.160)6 (0.099)'

log Kp = 9,192 The values of K p for all the samples of ~ i n k l e r and Chipman

are plotted against the reciprocal of the absolute temperature in Fig 17-19. Despite definite deviations on both sides of the straight 'line,' the relationship is surprisingly good considering the complex nature of the calculations.

The equation of the line best fitting the points of Fig 17-19 is

The equation demonstrates quantitatively the marked sensitivity of the phosphorus-elimination reaction to temperature.

In many studies of phosphorus distribdtion, data regarding oxygen content of the steel are not available. Accordingly, it is desirable to transform Eqs 17-4 and 17-12 so that they may be

' PHOSPHORUS DISTRIBUTION , 767

applied to studies where FeO activity is determined through consideration of slag analyses.

I t was shown earlier that FeO activity is equal to mole fraction

Fig 17-19. Effect of temperature on phosphorus equilibrium constant

Ca4P200 K P = (Winkler -and Chip~nan.~~)

1% pi2 [% oi6 -(cao1)4

of FeO when the slag constitution is determined by the method used in the derivation of Kp (see Fig 17-18). Thus,

[% 01 = (NFCO) [% Ossil [17-141

If the dephosphorization constant using mole fraction of FeO is defined as

(Nc~,P,o,) K'p = [17-151

[% PI2 ( N F , ~ ) ~ (Nc.o')~ i t follows that

K'P = K P [% Osai16 [17-161 and

log K'P = log K P + 510g [% Omail 117-171

The value [%OBnt] can be determined through the equation -6320

log [% OartI = + 2.734 (1 7-21


Substituting the expressions for log [%O] (Eq 17-2) and log KP (Eq 17-12) into Eq 17-17 leads to an expression

Numerous other investigators have attempted to derive expressions for the phosphorus reaction that would allow calculation of constants equivalent to Kp. As pointed out previously, these attempts have been hampered by the lack of understanding of liquid slag constitution. Some investigations 3 4 7 4 1 have proved moderately successful in determining a constant for slags in a narrow composition range. For limited ranges, i t is not always necessary to have a complete understanding of slag constitution in order to calculate a suitable constant. The application of such a constant to a wider range of slag compositions immediately exposes the shortcomings of the results.

One other investigation of the phosphorus reaction deserves mention in that, while i t was very complete, the results were not entirely in accord with those already presented. Balajiva, Quar- rell, and VajraguptaZ7 have studied phosphorus distribution between low-carbon iron and numerous high-phosphorus slags. The slag composition range covered was not a s great a s that included in the study by Winkler and Chipman and entirely different melting techniques were used in the two investigations. The largest part of-the work of Balajiva e t al. was done a t 1585 C (2885 F ) by melting for a specified time in a small specially constructed arc furnace. The investigation by Winkler and Chip- man covered the temperature range from 1530 C (2786 F ) to 1738 C (3160 F ) , and the melting time was determined by the time actually required to reach eauilibrium.*

To avoid confusion, the curves presented by Balajiva et al. are not reproduced. Instead, i t is interesting to survey the results obtained when their data are replotted using the concepts of slag constitution developed by Winkler and Chipman. The distribution of log KP for 88 pairs of slag and metal samp!es obtained a t 1585 C (2885 F ) is given in Fig 17-20. The values are distribu- ted rather symmetrically about a mean log Kp value of 6.40.

* By using radioactive phosphorus, the time required to reach equilibrium was determined rapidly and simply by measuring the radioactivity of successive bath samples.

PHOSPHORUS DISTRIBUTION 769 ,

This mean value .is almost identical to the log Kt , of 6.45 that can be calculated from Eq 17-15a for 1585 C (2885).

log If;

Fig 17-20. Comparison of K'P values at 1585 C. (Winkler and Chipman" and Balaiiva, Quarrell, and Vairagupta.")

The spread of the points in Fig 17-20 is three to four times a s )I a

great as that reported by Winkler and Chipman. Accordingly, <I. t

the two investigations cannot be considered to verify each other ?.,

despite the remarkable agreement between calculated mean values of log K',. In the latter investigation, the greatest depart- tures from the mean value were obtained on the high-basicity, low-FeO heats. As a result of their study, Balajiva et al. concluded that for each slag basicity (as measured by lime-silica ratio) there exists an optimum FeO value for phosphorus elimination; and when this value is exceeded the dephosphorization ratio (%P205) /[%PI decreased.

Data were also obtained by Balajiva and QuarrellZ7 a t temperatures of 1535 C (2795 F ) and 1635 C (2975 F ) . The data available a t these temperatures were not adequate for deriving frequency curves of the type given in Fig 17-20. However, the general features of these results were similar to those obtained a t 1585 C (2885 F ) .


Practical Aspects of Phosphorus Equilibrium Data. Obviously, Eqs 17-4 and 17-12, in which [OJoO] is used in determining the phosphorus-distribution constant, cannot be applied directly to conditions existing in the basic open-hearth furnace, since the presence of substantial percentages of carbon precludes any reasonable approach to actual equilibrium oxygen distribution. It is interesting, however, to examine available open-hearth data with regard to the applicability of Eqs 17-15 and 17-15a, in which FeO activity is used in the phosphorus-distribution equation.

: Some 30 pairs of bath and slag samples were provided* for

*The data fo r these calculations were made available by the Inland Steel Company.

..PHOSPHORUS EQUILIBRIUM 77 1

which complete analytical data were available as well as care- fully checked temperature readings. (The temperature readings were obtained with immersion platinum thermocouples, and these values were checked with readings made with a Leeds and Northrup "blowing tube" radiation pyrometer.) Each pair of samples was obtained just before final additions were made to selected open-hearth heats. Lines representing Eqs 17-12 and 17-15a are shown in F ig 17-21. The points plotted in Fig 17-21 were obtained by calculating the values of Ktp (Eq 17-15) from the open-hearth data. These points approach the calculated equilibrium curve fairly closely despite the fact that the metal samples contained substantial and varying percentages of carbon. It appears that Eqs 17-15 and 17-15a can be used to describe phosphorus distribution in commercial basic open-hearth operations.

Two corollaries to the preceding conclusion deserve mention. First, the phosphorus reaction in the basic open hearth reaches equilibrium during the working of a heat. Second, slag FeO is a reasonable measure of oxidation in the slag-metal system involved in the phosphorus reaction.

Fig 17-22. Schematic curves showing approximate effect of slag lime-silica rotio and slag FeO on residual phosphorus for conventional open-hearth practice. Caleula+ed from Winkler and Chipmon?'

Since the preceding equations apparently are applicable to open-hearth operations, it is desirable to transform their curves


into terms more convenient for use by operators. In conventional stationary open-hearth practice, finishing slags usually contain 1.5 to 3.0 per cent P,O,. Dephosphorization a t 1600 C (2912 F ) for slags in this range is shown as a series of residual phosphorus curves in Fig 17-22. The basicity index used in Fig 17-22 is the simple ratio (% CaO) / (% Si02) while the iron oxide is expressed as % FeO. The dotted lines drawn in Fig 17-22 are, a t best, approximations that really should be shown as bands including substantial areas on both sides of the residual phosphorus curve. The curves presented in this semiquantitative manner bring out very clearly the general features of the phosphorus reaction. It is evident from Fig 17-22 that both high iron oxide and high slag basicity are necessary for optimum phosphorus elimination. In the usual range of slags in stationary open-hearth practice, increasing the lime-silica ratio is effective in dephosphorization up to about 2.75 to 3.00; beyond this value only increased FeO is effective. For a given lime-silica ratio over 2.00, increased FeO content over 25 per cent is not as effective in eliminating phosphorus as corresponding changes in the lower iron oxide ranges.

Increasing temperature displaces the family of curves in F ig 17-22 to the right. Consequently, higher temperatures result in higher residual phosphorus for a given basicity. This is equivalent to stating that dephosphorization is favored by low temperatures.

Manganese Distribution. The reaction tha t usually is written to represent the distribution of manganese between slag and iron has already been considered quantitatively in Chapter 16 :

Mn + FeO (in slag) = Fe + MnO (in slag) - - [16-261

This reaction will be considered further here as i t relates to slag constitution and slag behavior, for the various methods of evaluating the constant for the reaction throw some light on these problems. One of the most useful forms of the constant i s

where the slag terms are written as mole fractions and the manganese as weight per cent. Since manganese has nearly t h e same molecular weight a s iron, this is equivalent to writing a molar

MANGANESE DISTRIBUTION 773

concentration as the activity term for manganese, and this is a reasonable approximation for the small amounts that usually are present in the open-hearth bath.

This reaction has been studied extensively by Kijrber and O e l ~ e n ~ ~ for nearly pure FeO-MnO slags and for such slags saturated with Si02. Their work shows that these systems behave almost ideally and that up to 3.6 per cent manganese in iron the composition can be calculated from K b I l l for silica-free slags. It also shows that the oxygen in the iron under these slags is determined primarily by the FeO in the slag and that the MnO- Si02 complex in the slag is much stronger than the FeO-SiO, complex ; i.e., MnO behaves a s a stronger base than FeO.

It is necessary to consider more complicated slags if we are to learn more about MnO in the basic open hearth. The study by Darken and L a r ~ e n ~ ~ is selected for this purpose because i t is based on operating data and illustrates a general method of approach to such problems. Their results are in good agreement with many other studies a s to the general influence of temperature and slag composition on the manganese reaction. Solving the Eq 16-26 for [% Mn] gives:

and this was used with Korber's value of KM, to calculate, the [% Mn] for a number of open-hearth heats. In order to make such calculations and interpret them over a reasonable range of compositions, i t is necessary to make some assumptions regarding slag constitution, and it is generally considered that the assumptions that are in best agreement with observed results are not only the most useful but also the most nearly correct. The following assumptions were made :

1. All the manganese in the slag is present as "free" or uncombined MnO; i.e., the total MnO reported in the slag analysis should be used in calculating the (MnO) term.

2. All the iron in the slag is present as free FeO or Fe203. 3. As a result of the reaction

Fe203 (slag) +?Fe = 3 FeO (slag)

each mole of Fe203 reported by analysis should be calculated as three moles of FeO.

774 Chapter I 7-S LAG-METAL REACTIONS

4. The influence of other oxides can be characterized by a basicity ratio :

(C.10). - 4 (Pzo5) L =

(SlOz) This implies that all the phosphorus in the slag is combined as CaaP20g and that other oxides such a s A1203 and MgO are without influence in the range of composition considered.

The calculations made by this scheme were related to the observed values of [% Mn] by the ratio

M = I% Mnlob. [% Mnl,,~,

for 150 heats for which good samples and temperature measurements were available. This ratio M was plotted against the

MOL RATIO QO;::os' IN SLAG

Fig 17-23. Comparison of ratio M of observed to calculated residual manganese in open-hearth heats as related to lime-silica ratio L. (Darken and Lar~en .~ ' )

basicity ratio L a s shown in Fig 17-23. When L L 2.4, M - 1, indicating that for these values of L the assumptions.made regarding slag constitution are useful and perhaps valid. For more acid slags, L < 2.4, the calculated values of [YO Mn] are too great, and i t appears that some of the MnO should be regarded as combined in a stable silicate complex. This is in accord with Korber's interpretation and the observed differences between basic and acid open-hearth practice. I n the latter case, a

SULFUR DISTRIBUTION 775

relatively high MnO can be carried in the slag with a low residual manganese in the steel. It also conforms with other experience (see oxygen-distribution section) regarding the critical nature of the basicity ratio of about 2.0, corresponding to the composition CazSiO,, below which typical basic slag behavior is not found. Since the calculations were made from operating data, the results also imply that the open-hearth bath approaches this equilibrium closely in normal practice.

It was shown in Chapter 16 that the influence of temperature on this equilibrium is represented by the equation

which is also in agreement with operating experience of higher manganese residuals a t higher temperatures. Eq 16-26a is based on the recent work of Chipman, Gero, and Winkler,38 which provides the most accurate evaluation of KMn but does not change the concepts on slag constitution that have been discussed here as originally set forth by Korber and Oelsen.

Other assumptions have been made regarding the mode of combination of MnO in basic slags with equally valid results, but most observers are in qualitative agreement that MnO should be regarded a s a basic oxide with a slightly more basic character than FeO; the latter difference becomes more significant as the slag basicity decreases (see following section).

Sillfur Distribution. In spite of i ts importance, the reactions and slag properties governing the distribution of sulfur are probably the least understood of any steelmaking reaction. The work of Grant and C h i ~ m a n ~ ~ presents the most recent analysis of the subject. They approached the problem by determining the assumptions of slag constitution that would give the most satisfactory results for simple slag systems and extended this to include a wide variety of more complex conditions. Their system of slag constitution was designed to give a measure of the excess base or acid in a slag system and was based on the following conditions and requirements for a neutral slag, all stated on a molar basis :

1. FeO and CaF, are neutral. 2. CaO, MnO, MgO are bases of equal value in neutralizing

acid constituents according to the ratios: 2 base : 1 SiOz 4 base : 1 P205 2 base : 1 A1203 1 base : 1 Fe20s

The Fe203 was treated as an acid only when sufficient base was available for the other three acid constituents ; with a deficiency of base the Fe203 was treated as neutral.

When the sulfur-distribution ratio (S) /[S] was plotted against the excess acid or base calculated in this way, the data established a curve that was regarded as standard (see F ig 17-24). It was then shown that large variations in MnO, CaFZ,

HEATS E -29 THRU E - 4 4

. 10 -,05 0 + O 5 1.10 5 ..20 2 -30 1.35 r.40 EXCESS BASE OR ACID - I N MOLES PCR IQO G. OF SLAG

Fig 17-24. Effect of available base on desulfurization ratio in the temperature range 1580 to 1650 C .(2876 to 3002 F ) . (Gran t and Ch ip~nan .~ ' ) This plot shows all points used in establishing the standard line.

AI2O3, FeO, and temperature caused no significant variations from this standard curve. These results indicate that the slag- constitution scheme adopted provides a means of measuring the excess base as related to the sulfur reaction for the types of basic, oxidizing slags that are encountered in the open hearth. De- sulfurization of iron is favored by excess base, determined in this way, and is relatively independent of the amounts of neutral constituents in the slag or of the temperature. This neutral behavior of FeO appears to eliminate the most generally accepted reaction

CnO ( i l l slng) + FeS = CaS (in slng) + FeO (in slag) -

SULFUR DISTRIBUTION 777

as the reaction controlling the sulfur distribution, and Grant and Chipman propose the reactions :

FeS = FeS (in slag) -

MnS = MnS (in slag) -

as more compatible with the facts. The influence of slag basicity is then to change these distributions and not to convert the sulfur in the slag to a slightly dissociated CaS. It is unfortunate that when the calculations are applied to operating data, the scatter is considerably greater than for the laboratory heats.3Q

Another approach is the treatment of Darken and L a r ~ e n , ~ ~ which follows from their study of the manganese equilibrium and is based on a similar concept of slag constitution. It results in an equation that assigns significant roles to the slag FeO and the manganese in the bath. The complete development of the equation is beyond the limitations of this book, but its qualitative meaning may be outlined:

(Qt" - = N , (A + B' = + C [Mn],,) [Slw (FeO)

(S), = weight per cent sulfur in slag [S], = weight per cent sulfur in metal

N , = a measure of the total mo!es in the slag A, B', C = constants thzt can be evaluated from experimentaldata.

(CaO), = a measure of the free lime in the slag depcndent upon the slag constitution used to describe the manganese reaction

(FeO) = the total equivalent FeO as used to describe the manganese react.ion [Mn], = weight per cent manganese in metal

This equation assigns specific effects to the (FeO) of the slag and the manganese of the metal, which are in accord with many observations from practice. This equation apparently fits operating data better than does the expression developed from the laboratory study of Grant and C h i ~ m a n . ~ ~

The sulfur distribution found in open-hearth operations, without reference to slag constitution, is found in the work of Fettem and Chi~man.~O This includes a statistical analysis of data from several different shops with the general conclusion that desulfurization is more effective as the slag basicity increases and also when the bath is high in carbon. In another paper,ls the same authors reported sulfur distribution for a large number of laboratory experiments as summarized in Fig 17-25. These data

778' Chapter 17-SLAG-METAL REACTIONS

again show the general dependence of the sulfur 'reaction on basicity, but give no new information on slag constitutio'n.

These treatments were developed by evaluation of large masses of data, rather than by physical-chemical treatment. The di-

Fig 17-25. Distribution of sulfur between synthetic slag and metal a t temperatures ranging from 1550 to 1650 C (2822 to 3002 F] . [Fetters and Chipman."']

vergent ideas they present indicate that the sulfur reaction is not yet fully understood. It is hoped that a more complete development of the si!icate network and ionic principles will contribute a solution t o this problem but no successful treatment has been made.

Silicon Distribution. Despite the fact tha t silica is present in all basic open-hearth slags, only a trace of silicon is ever present in the steel bath during the refining period. The explanation for this situation lies in the fact that the analyzed silica is combined either in molecules such as Ca,Si,Os or as complex ions such as

The absence of silicon in the bath simply means that the activity of silica, as,,,, is very small. Either the concept of molecular association or that of ionization adequately explains the behavior of silicon in the basic open-hearth system. In acid- furnace operation and in some aspects of deoxidation, considerations of silicon distribution do have some practical significance.

REFERENCES

The general reaction for oxidation of silicon may be written : Si + 2 0 = SiOn (solid); AF" = -129,440 + 48.44 2' - -

asio, [{ = -

[Sil [012

This means that liquid metal containing 0.015 per cent oxygen (which is in equilibrium with 0.40 per cent carbon and which is also in contact with pure silica, aslo2 = 1) would also contain 0.15 per cent silicon. As the activity of silica in slag approaches zero, i t is obvious that the,residual silicon in the active bath must also approach zero.

The application of the preceding equation to deoxidation has already been discussed in Chapter 16.

SUMMARY

An attempt has been made in this chapter to indicate the present state of knowledge regarding the constitution of basic slags. Two general concepts of slag constitution have been developed and both have met with some success. A molecular association hypothesis has been applied by various investigators to explain observations and data regarding the distribution of oxygen, phosphorus, sulfur, manganese, and silicon. The ionization concept has not been applied as generally but has been used successfully in connection with limited data on oxygen, sulfur, and manganese distribution. There is no reason to believe that broader application of either theory will not be made in future studies.

The approach used in applying these concepts to explain specific sets of data is more or less empirical. No completely satisfactory concept of slag constitutionr capable of universal application to all types of slag is as yet available; the status of knowledge regarding slag constitution must be considered tentative. A great deal of fundamental work is being done now in this field and considerable clarification of existing uncertainties may be expected.

REFERENCES

1. BOWEN, N. L., J. F. SCHAIRER, and E. POSNJAK: The System, CaO 17eO- SiO ... Am. ,I. Sei., 1933, ser. 5, v. 26, pp. 193-284.

2. KORBER, F., and W. OELSEN: Die Schlackenkunde als Grundlage der Metallurgie der Eisenerzeugung (The Science of Slags as a Basis of

Chapter I7-SLAG-METAL REACTIONS

the Metallurgy of Ferrous Production Metallurgy). S t a h l u. E i sen , 1940, v. 60, pp. 921-930.

3. ENDELL, V. K., G. HEIDTKAMP, and L. H ~ x : ~ b e r den Fliissigkeitsgrad von Kalksilikaten, Kalkferriten und Basischen Siemens-Martin- Schlacken bis 1625 (On the Viscosity of Lime Silicates; Lime Fcrr i tes and Basic Open-hearth Slags up to 1625 C) , Arch. Eisenhii t tenw., 1936, v. 10, pp. 85-90.

4. WEJNARTH, A.: The Current-Conducting Properties of Slags in Electric Furnaces. Tq-ans. Electrochem. Soc., 1934, v. 65, pp. 329-343.

5. MARTIX, A. E., and G. DERGE: The Electrical Conductivity of Molten Blast-furnace Slags. Tran,s. A I M E , 1913, v. 154, pp. 104-115.

6. CHANG, L. C., and G. DERGE: An Electrochemical Study of the Proper- ties of Molten Slags of the Systems CaO-SiO, and Ca0-A1,O1-Si0,. Trans . .4IME, 1947, v. 172, pp. 90-120.

7. BOCKRIS, J . O'M., J . A. KITCHENER, S. IGNATOWICZ, and J . W. TOMLIN- SON: The Electrical Conductivity of Silicate Melts: Systems Contain- ing Ca, Mn, and Al. Discussions of t he Faraday Socie ty , 1948, v. IV, pp. 265-281. Also F. D. RICHARDSON: The Constitution and Thermo- dynamics of Liquid Slags. Discussions of t he F n r a d a y Socie ty , 1948, V. IV, pp. 244-257. .

8. HERASYMENKO, P., and G. E. SPEIGHT: Ionic Theory of Slag Metal Equilibria: P a r t I-Derivation of the Fundamental Relationships. J. I ron S tee l Ins t . (London) , 1950, v. 166, pp. 169-183.

9. SCHELLINGER: A. K., and R. P. OLSEN: The Relationship between Elec- trical Conductivity and Composition of Molten Lead Silicate Slags. Trans . AIIME, 1949, v. 185, pp. 984-986.

10. NAESER, G.: ~ b e r den Wiirmeinhalt von Schlacken (On the Hea t Con- tent of Slags). Mit t . Ka i se r -Wi lhe lm Ins t . Eisenforsch. , 1930, v. 12, pp. 7-12.

. 11. TENENBAUM, M., and T. L. JOSEPH: The Use of the Reflecting Micro- scope in the Examination of Open-hearth Slag. Blas t Furnace S tee l Plant , 1941, v. ,29, pp. 403-407, 522-523, 551-552. .

12. GOFF, I.: Estimating Open-Hearth Slag Composition. Blas t Furnace Steel Plnnt , 1934, v. 22, pp. 640-641, 656, 6913-694. ,

13. ROGERS, B. A,, and K. 0. STAMM: The Magnetic Properties of a Series of Basic Open-Hearth Slag Samples. T r a n s . Am. Soc. Metals, 1937, v. 25, pp. 420-434.

14. CHIPMAN, J., and L. C. CHANG: The Ionic Nature of Metallurgical Slags. Simple Oxide Systems. Tl ,ans . A I M E , 1949, v. 185, pp. 191-197.

16. DERGE, G.: The Distribution of Oxygen between Molten Iron and Iron Oxide-Silica Slags. Yearbook A m . Iron. S tee l Inst . , 1949, pp. 368-395.

REFERENCES 78 1

16. LEWIS, G. N.: Acids and Bases. .J. Franklin Znst., 1338, v. 226, pp. 293- 313.

17. SUN, G. K., and A. SILVERMAN: Lewis Acid-Base Theory Applied t c Glass. J. Am. Cernm. Soc., 1 9 4 5 , ~ . 28, pp. 8-11.,

18. FETTERS, K. L., and J. CHIPMAN: Equilibria of Liquid Iron and Slags of the System Ca0-Mg0-Fe0-Si0,. Trans. AZME, 1941, v. 145, pp. 95-112.

19. TAYLOR, C. R., and J. CHIPMAN: Equilibria of Liquid Iron and Simple Basic and Acid Slags in a Rotating Induction Furnace. Trans. AZME, 1943, v. 154, pp. 228-247.

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22. SCHENCK, H.: Physikalische Chemie d m Eisenhuttenprozesse (Physical Chemistry of Steelmaking Processes). Springer, Berlin, v. 1, 1932, IX+306 pp.; v. 11, 1934, VIII+274 pp.

23. WINKLER, T. B., and J. CHIPMAN: An Equilibrium Study of the Distri- bution of Phosphorus between Liquid Iron and Basic Slags. Trans. AZME, 1946, v. 167, pp. 111-133.

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26. BARRETT, R. L., and W. G. MCCAUGHEY: The Role of Basic Slags in the Elimination of Phosphorus from Steel. Trans. AZME, 1944, v. 158, pp. 87-97.

27. BALAJIVA, K., and P. VAJRAGUPTA: The Effect of Temperature on the Phosphorus Reaction in the Basic Steelmaking Process. J . Zron Steel Znst. (London), 1947, v. 155, pp. 563-567. Also K. BALAJIVA, A. G. QUARRELL, and P. VAJRAGUPTA: A Laboratory Investigation of the Phosphorus Reaction in the Basic Steelmaking Process. J. Zron Steel Znst. (London), 1946, v. 163, pp. 115-148.

28. KORBER, F., and W. OELSEN: ~ b e r die Beziehungen Zwischen Mangan- haltigem Eisen und Schlacken, die fas t Nur aus Manganoxydul und Eisenoxydul Bestehen (On the Relations between Manganiferous Iron and Slags Consisting almost entirely of MnO and FeO). Mitt. Kaiser-Wilhelm Znst. Eisenforsch., 1932, v. 14, pp. 181-204. Also F. KORBER: Untersuchungen iiber das Verhalten des Mangans bei der Stahlerzeugung (Investigations of the Behavior of Manganese in Steelmaking). Stahl u . Eisen, 1932, v. 62, pp. 133-144.


29. KORBER, F.: Forschungsarbeiten zur Physikalischen Chemie Der Metall- schlacken-Reaktionen (Investigations in the Physical Chemistry of Metal-Slag Reactions). 2. .Elektrochem., 1937, v. 43,,pp. 450-460.

30. SAMARIN, A., M. TEMKIN, and L:.SHVARZMAN: Distribution of Sulphur between Metal and Slag from the Viewpoint of the Ionic Nature of slag. ,Aeta Physicochim., U.S.S.R.., 1945, v. 20, pp. 421-440.

31. SCHACKMANN, H., and W. KRINGS: ~ b e r Gleichgewichte zwischen Metallen und Schlacken . im Schmelzfluss. IV. Das Gleichgewicht 5 F e 0 + 2P =-- P:O.<+ 5Fe (On Equilibria between Metals and ,Slags i n the Melt. IV. The ~ ~ u i l i b r i u m 6FeO + 2 P = PzOj -I- 5Fe) . 2. anorg. u. allgem. Chsm., 1933,,v. 213, pp. 161-179.

32. HERTY, C. H., JR.: Chemical Equilibrium of Manganese, Carbon and Phosphorus in Basic Open-hearth Process. Trans. AIME, 1926, v. 73, pp. 1107-1134.

33. DIEPSCHLAG, E., and H. S C H ~ R M A N N 1 Untersuchung ."her die Entphos- phorung von Eisen durch Oxydation (Investigation of . the Dephos- phorization of Iron by Oxidation). Angew. Chem., 1933, v. 46, pp. 61-62. . ..

34. MCCANCE, A.: he Application of Physical 'Chemistry to Steelmaking. . J. Iron Steel Inst. (London), Physical Chemistry of Steelmaking

Symposium, 1938, pp. 331-371.

36. BISCHOF, W., and E. MAURER: Die Verteilung des Phosphors zwischen Eisen und kalkhaltigen Eisenphosphatschlacken (Distribution of Phosphorus between Iron and Iron-phosphate Slags Containing Lime). Arch. Eisenhiittenw., 1932, v. 6, pp. 415-421. .

36. OELSEN, W., and H. MAETZ: Das Verhalten des Flusspates und der ~ a l z i u m ~ h o s p h a t e gegeniiber dem Eisenoxydul im Schmelzfluss und seine metallurgische Bedeutung (The Behavior of ~ l u o r s ~ a r and of Calcium Phosphate Toward Ferrous Oxide in the ~ e i t and I t s Metal- lurgical Significance). Mitt. Kaiser-Wilhelm Inst. Eisenforsch., 1941, V. 23, pp. 195-246. '' . .

37. DARKEN, L. S., and B. M. LARSEN: Distribution of Manganese and of Sulphur between Slag and Metal in the Open-hearth Furnace. Trans.

, . AIME, 1942, V. 160, pp. 87-112. .. ,

38. CHIPMAN, J., J. B. GERO, and T. B. WINKLER: The Manganese Equi- librium. T.rans. AIME, 1950, v. 188, pp. 341-346.

39. GRANT, N. J., and J. CHIPMAN: Sulphur Equilibria between Liquid Iron and Slags. Trane. .4IME, 1946, v. 167, pp. 134-154.

40. FETTERS, K. L;, and J. CHIPMAN : slag-metal' Relationshi'ps in the Basic _ Open-hearth Furnace. Trans. AIME, 1940, v. ,140, pp. 170-203.

41. TDNENBAUM, M.: . Phosphorus Distribution in the Basic Open-hearth Furnace. Trans. AIME, 1944, v. 27, pp. 152-159.

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