7

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DesulfSteelm

7.1 INTRODUCTION

Except in free-cutting steels, sshortness in steels. Some decadthe maximum permissible sulfu0.02%. In special steel plates, tis a demand for ultra-low-sulfuHIC resistive steels, and alloyed

Sulfur comes into iron princby slag in a reducing environmeironmaking in a blast furnace. Vthe oxidizing environment. An eare produced through two-stage

In view of the consistent demto achieve it, external desulfurishop was developed. The proceessential feature of a modern in

Content below 0.01% mustcesses, such as the

MPE

proces

desulfurization is achieved to sokinetic energy of the tapping smolten steel and gas stirring (evacuum degassing) are also bei

However, only the injectionproducing ULS steel. ULS canstirring is required, so deep desnum in combination with calciuprocesses are capable of inclusi

Section 6.4.5 of this book hretard the nitrogen desorption rachieved in low-sulfur and low-is done before or during vacuum

Furnace slags contain oxide

the presence of a deoxidized stresult, some reversion of phospdeoxidizers, so it does not allocauses wear on the ladle lining.urization in Secondary aking

ulfur is considered to be a harmful impurity, since it causes hotes back, for common grades of steel cast through the ingot route,r content was 0.04%. In the continuous casting route, it should behe normal specification for sulfur is 0.005% these days, but therer (ULS) steel with as low as 10 ppm (0.001%), e.g., in line pipe, steel forgings.1ipally through coke ash. It is effectively removed from molten ironnt only. Hence, traditionally, sulfur control used to be done duringery little sulfur removal is possible in primary steelmaking due toxception is the electric arc furnace (EAF), where low-sulfur steels refining.

and for lower-sulfur steel and the incapability of the blast furnacezation of liquid iron in a ladle during transfer to the steelmakingss is capable of lowering sulfur content to 0.01% or so and is antegrated steel plant. be accomplished in secondary steelmaking. There are now pro-s of Mannesman and the EXOSLAG process of U.S. Steel,2 whereme extent during tapping by using synthetic slag and utilizing the

tream. Desulfurization by treatment with synthetic slag on top ofither in an ordinary ladle, in a ladle furnace or VAD, or during

ng practiced. of a powder such as calcium silicide into the melt is capable of be achieved only if the dissolved oxygen is also very low. Gasulfurization is associated with deep deoxidation. The use of alumi-m or rare earth (RE) metals achieves both. In addition, injection

on modification for further improvement of the properties of steel.as already stated that oxygen and sulfur dissolved in liquid steel

ate from steel in vacuum degassing. A low nitrogen level has beenoxygen steels. This is an additional benefit if deep desulfurization treatment.

s such as FeO, SiO2, P2O5, and MnO. These oxides are unstable ineel, especially when the slag and steel are intimately mixed. As ahorus into the steel occurs. This slag also partly consumes addedw proper utilization of them for steel deoxidation. The slag also

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Although these have been kbeen conducted on these effects

steel with slag during furnace taslags from primary steelmakinHowever, this is difficult to implsecondary steelmaking and othaims at the twin strategy, viz., (carryover slag by the addition

some extent) to render desirablcarryover, so this information n

7.2 THERMODYNAMIC

7.2.1 S

OLUTION

OF

S

ULFUR

At steelmaking temperatures, su

The dissolution of sulfur in mo

For the above reaction,

where

K

1

is the equilibrium conthe gas phase in atmosphere, a

1 wt.% standard state. Again,

lo

Equation (7.3) gives a little drespectively, at 1600C).

The interaction coefficients

on the activity coefficient of su

Appendix 2.3, where

h

S

=

f

S

W

further that the solubility of sul

7.2.2 R

EACTION

E

QUILIBRIA

Appendix 2.1 provides a compisulfides. Ca and Ba form CaS forms several sulfides

7

out of wforms an oxysulfide, Ce

2

O

2

S. Amay be noted, from thermodynnown for a long time, very little physicochemical investigation has. Turkdogan2 has considered some aspects of the reaction of liquidpping. This has already been discussed in Section 5.3. It is best if

g furnaces are not allowed into the secondary steelmaking ladle.ement. In addition, some slag is required for desulfurization duringer beneficial effects. Therefore, control of furnace carryover slaga) minimization of furnace carryover slag, and (b) modification ofof fluxes (principally CaO, but also Al, SiO2, Al2O3, and CaF2 toe properties to it. Section 5.3 discussed the minimization of slageed not be repeated here.

ASPECTS

IN LIQUID STEEL

lfur is a stable gas, with the most predominant molecule being S2.lten steel may be represented by the following equation:

1/2 S2 (g) = S (7.1)

(7.2)

stant for Reaction (7.1), denotes partial pressure of sulfur innd hS is the activity of dissolved sulfur in steel with reference to

g K1 = 0.964 (Ref. 5) (7.3)

iffering value from that based on Appendix 2.2 (335 and 348,

describing the influence of some common solutes (j) in liquid steellfur (fS) dissolved in liquid steel (i.e., ) at 1600C are given inS, WS being the weight percent of sulfur in steel. It may be noted

fur in molten steel is very high.

OF SULFUR

lation of the standard free energy of formation of some oxides andand BaS, respectively, upon reaction with sulfur, whereas ceriumhich CeS is the stablest one under steelmaking conditions. Ce alsoll of these compounds are solids at steelmaking temperatures. It

amic data on these compounds in any standard text, that all these

K 1hS[ ]

pS21 2---------

equilibrium

=

pS2

6535T

------------

esj

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elements form very stable sulfias well as desulfurizers and wo

Again, these compounds woof Ca-Si leads to the formation

Section 5.2. However, we do noverall reaction to be

For the limiting case of unit actequilibrium constant (

K

MS

) for R

The values of

K

MS

for different 7.1, reproduced from Turkdoga

weakest, with Ca and Ce lying Holappa

1

has reviewed the treaction. If the MO and MS adesulfurization reaction, viz.,

K MS

FIGURE 7.1 Oxygen/sulfur activdes as well as oxides. Therefore, they are both strong deoxidizersuld form both oxides and sulfides.uld not necessarily be present in a pure form. For example, addition of a CaO-SiO2-type deoxidation product as discussed earlier in

ot propose to get involved in these complexities and consider the

S + (MO) = O + (MS) (7.4)

ivities of MO and MS (i.e., assuming pure MO and pure MS), theeaction (7.4) is

(7.5)

systems can be calculated from the free energy of reaction. Figuren,8 shows the pattern. Ba is the strongest desulfurizer and Mg thein between.heoretical basis for sulfur removal in ladle treatment by slagmetalre not pure, then it is better to utilize the general ionic form of

[S] + (O2) = (S2) + [O] (7.6)

hO[ ]hS[ ]

----------

W O[ ]W S[ ]

-------------= = (at equilibrium)

ity ratio in liquid iron for some sulfide-oxide equilibria at 1873 K.9

K 6a

S2( ) hO[ ]hS[ ] ao2( )

------------------------=

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or,

If we replace with we

value of

K

6

(let it be ). The

where is known as the

mod

As discussed in Section 2.8,

sulfur, was originally defined b

where (

W

S

) is the weight percenof oxygen and sulfur as an

slag, and at a fixed temperatureof

C

S

, the better the desulfurizin

slag systems of interest in seco

Values of

C

S

for various slags a

C

S

is determined by equilibrpotential. However, it is the slamodified

C

S

(viz., ) as definThe relationship between

C

At 1600C, = 7.5

C

S

.

as

2

K6

K

CS

pO2

FIGURE 7.2 Sulfide capacities o

CS

lo

CS(7.7)

ight percent sulfur in slag (i.e., WS), then we may use a modifiedn,

(7.8)

ified sulfide capacity. the sulfide capacity of slag (CS), i.e., the ability of a slag to absorby Richardson9 as

(7.9)

t sulfur in the slag in equilibrium with a gas having partial pressuresd . Its usefulness stems from the fact that CS is a property of it is determined solely by slag composition. The higher the valueg ability of the slag. Figure 7.2 shows CS values for some typical

ndary steelmaking.9 The superiority of CaO-CaF2 slag is obvious.re available in Slag Atlas.10ating the slag with a gas mixture having known oxygen and sulfurgmetal equilibrium that is of interest. This requires the use of aed in Eq. (7.8). S and is

(7.10)

K6( ) ao2( )a

s2( ) hO[ ]hS[ ]

-----------------------=

6 ao2( )W S ]( ) hO[ ]

hS[ ]-------------------------- CS= =

CS W S( ) pO2/ pS2( )1 2

=

pS2

f some slags at 1873 K.9

CS

g CS log CS936T

--------- 1.375+=

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Another parameter of intere

(

L

S

), where

L

S

= (

W

S

)/[

W

S

]. Fr

equilibrium

,

h

O in liquid steel is typically daluminum. One may relate hO toappropriate to relate it to the foand aluminum content of metal fof more than 0.020% is general

7.2.3 TEMPERATURE AND CO

A simplified approach to this desulfurizer. For the reaction,

With the data on free energies i

K12 is the equilibrium constant f1600C (1873 K), Eq. (7.13) giv

Carlsson et al.11 have propo

FIGURE 7.3 Equilibrium sulfur pst is the equilibrium sulfur partition ratio between slag and metalom Eq. (7.8), if [hS] is taken as [WS], then, at slagmetal sulfur

(7.11)

etermined by the presence of a deoxidizer, especially dissolved the FeO content of slag as well. However, it has been found more

rmer. Figure 7.3 shows LS as a function of the CaO content of slagor CaO-Al2O3 slag.8 Therefore, for good desulfurization, Al contently recommended.1

MPOSITION DEPENDENCE OF CS

issue is to recognize that the CaO in slag is the predominant

CaO(s) + S = CaS(s) + O (7.12)

n Appendices 2.1 and 2.2,

(7.13)

or Reaction (7.12) and is same as KMS for a CaO-CaS reaction. Ates K12 as 0.013, whereas KMS from Figure 7.1 is approximately 0.03.sed an alternate correlation, viz.,

(7.14)

LSW S( )W S[ ]

------------

CShO[ ]

----------= =

artition ratio between liquid iron with dissolved Al and CaO-Al2O3 slags.8

logK 125140

T------------ 0.961+=

logK 125304

T------------ 1.191+=

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This yields a value of K12 at 160Toguri12 yields a value of 0.06.pure, but in solution in slag, the

Proceeding similarly as in the d

where m is a constant of propoCombining Eqs. (7.10), (7.1

logCS

Figures 7.4, 7.5, and 7.6 prand CaO-SiO2-Al2O3 systems aslag and only 0.01 in CaO-SiO2in CaO-Al2O3 than in CaO-SiOHence, CaO-Al2O3 slag is far suCS values of Figure 7.2.

Figure 7.6 gives values of temperature range of secondary of temperature at a fixed slag cdiagram such as Figure 7.2, it this can be made by invoking a

mK

FIGURE 7.4 Activity vs. compos0C of 0.028, matching Figure 7.1. The compilation of Zhang and Equation (7.14) is recommended for use. If CaO and CaS are notn

(7.15)

erivation of Eq. (7.8),

(7.16)

rtionality. 4), and (7.16),

= logm + log(aCaO) (7.17)

esent activity vs. composition relations in CaO-Al2O3, CaO-SiO2,t 1500 to 1600C13. At 50 wt.% CaO, aCaO = 0.33 in CaO-Al2O3 slag (approximately). To generalize, aCaO is 10 to 20 times larger2 in the composition ranges that are of interest in ladle refining.perior to CaO-SiO2 slag for desulfurization. This is reflected in the

CS in a ternary CaO-Al2O3-SiO2 system at 1600C. In the limitedsteelmaking, it seems good enough to assume aCaO to be independentomposition. Therefore, if CS is known at one temperature from acan be estimated at any other temperature. Further refinement on regular solution assumption for aCaO.

K 12aCaOS( ) hO[ ]aCaO( ) hS[ ]

-----------------------------=

12 aCaO( ) W S( )hO[ ]hS[ ]

---------- CS= =

4204T

------------ 0.184

ition diagram for CaO-Al2O3 system at 1773 to 1873 K.13

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For determining values of CTherefore, attempts are underwaet al.14 performed equilibrium mthey have proposed the followin

log CS = 3.44 (XCaO

FIGURE 7.5 Activity vs. compos

FIGURE 7.6 Sulfide capacities a1773 K.18S or , using diagrams as in Figure 7.2 is somewhat inconvenient.y to analytically represent CS as a function of slag composition. Tsaoeasurements. With the help of their own data, and those of others,g correlation by data fitting through statistical regression analysis.

+ 0.1 XMgO 0.8 (7.18)

ition diagram for CaO-SiO2 system at 1873 K.13

nd CaO-saturated liquidus (broken line) for CaF2-CaO-Al2O3 system at

CS

X Al2O3 XSiO2 )9894

T------------ 2.05+

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This may be useful for predictiCaF2-bearing slags. Gaye et al.on the basis of the optical basic

Combining with Eq. (7.10),

where B = 5.62 WCaO + 4.15

D = WCaO + 1.39 WMg

The conclusion drawn by the asitions leading to high LS are rarely on the ability to reach thsaturation. The authors also recbe subtracted in slag analysis.

7.2.4 TEMPERATURE AND CO

For good desulfurization, a largvalue of CS (i.e., ), but alsoit allows deeper deoxidation. Tfor deoxidation by aluminum m

From Eqs. (7.10) and (7.11

Again,

For which

From Appendix 5.1,

or, if it is assumed that hAl = W

CS

LSlog logC=

K Al

loghO13--- =on purposes within a factor of 2 to 3. Also, it is not applicable to15 employed the following correlation, arrived at by Duffy et al.16ity index (see Chapter 2, Section 2.8):

(7.19)

(7.20)

WMgO 1.15

O + 1.87

bove mentioned authors is that the domains of liquid slag compo-ther limited, and the efficiency of a sulfur removal treatment willese domains. The aimed compositions should be close to CaOommend that, in using Eq. (7.19), calcium present as CaF2 should

MPOSITION DEPENDENCE OF LS

e value of LS is required. This can be achieved not only by a large by a low value of hO. Here, aluminum is superior to silicon, sincehe thermodynamic relationship between LS with other parametersay be derived as described below.

),

(7.21)

(Al2O3) = 2[Al] + 3[O] (7.22)

(at equilibrium) (7.23)

(7.24)

Al,

(7.25)

logCSBD---- 2.82 13300

T---------------+=

logCSBD---- 1.445 12364

T---------------+=

W SiO2 1.46W Al2O3+

W SiO2 1.65W Al2O3+

S loghO logCS936T

--------- 1.375 loghO+=

hAl[ ]2 hO[ ]3aAl2O3( )

---------------------------=

logK Al64000

T--------------- 20.57+=

64000T

--------------- 20.57 2 logW Al log aAl2O3( )++

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Combining Eqs. (7.21) and

An important issue is variatto Eq. (7.26), we should also knExample 7.1Figure 7.2 presents values of predictions based on Eqs. (7.18CaO-Al2O3 systems.

SolutionValues of CS as read from Figu0.6 in all cases. = 0.4 for

In CaO-SiO2 at 0.6 mole fr

or,

In CaO-Al2O3 at 0.6 mole fract

or,

Hence, Figure 7.2 and Eq. (7.18)7.2.

Example 7.2At 1600C and for CaO-Al2O3

1. Calculate desulfurization2. Compare the above with3. Calculate the value of LS 4. Calculate the weight per

CaOSi

CaOAl

logLS logCS=

XSiO2

W CaO

W CaO (7.25),

(7.26)

ion of LS with temperature at a fixed slag composition. In additionow how CS varies with temperature.

sulfur capacity for some slags at 1600C. Compare these with) and (7.20) at a mole fraction of CaO of 0.6 for CaO-SiO2 and

re 7.2 are noted below. As for calculation from Eq. (7.18), XCaO = CaO-SiO2 slag, and = 0.4 for CaO-Al2O3 slag.

action CaO,

= 100 58.3 = 41.7%

ion CaO,

= 100 45 = 55%

give differing values. But Eq. (7.20) matches reasonably with Figure

slag with a mole fraction of CaO equal to 0.6,

efficiency of slag (i.e., [WO]/[WS] ratio). that of pure CaO.

if liquid steel contains 0.01 wt.% Al, and compare with Figure 7.3.cent sulfur in metal.

Values of Cs

Fig. 7.2 Eq. (7.18) Eq. (7.20)

O2 5.0 104 2.85 103 7.89 104

2O3 2.7 103 5.38 103 1.99 103

13--- log aAl2O3( )

23--- logW Al

20397T

--------------- 5.482+ +

XAl2O3

100 0.6 560.6 56 0.4 102+------------------------------------------------- 58.3%==

W SiO2

100 0.6 560.6 56 0.4 102+------------------------------------------------- 45%==

W Al2O3

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Assume slagmetal equilibrsolute elements.

Solution(a) From Eq. (7.11), and taking

Now, (WS) = 1, and CS = 2.7 1in these values,

(b) From Figure 7.1, for a CaO

Therefore, the slag with 1% sul

(c) The composition of slag co. Putting in other v

Figure 7.3 gives LS approximateFor comparison, LS is to be

before that, CS is to be estimateFrom Eq. (7.17),

log(CS)1923 lo

log(CS)1

Putting values into Eq. (7.26),

Assuming that the activity of AThe value of LS in Figure 7.3 is

(d) From (c),

Since (WS) = 1, [WS] =

aAl2O3 0.4=

log LS( )1923 2.5=

1 4ium and sulfur in slag as 1 wt.%. Ignore the interactions of other

hO = WO in metal phase,

(E1.1)

03 (Figure 7.2). From Eq. (7.10) at 1600C, = 7.5 CS. Putting

-CaS system at 1600C,

fur is as powerful a desulfurizer as pure CaO.

rresponds to 45 wt.% CaO and 55 wt.% Al2O3. From Figure 7.4,alues, i.e., CS = 2.7 103, WAl = 0.01, and T = 1873 K in Eq. (7.26),

log LS = 1.64, i.e., LS = 43.6

ly equal to 30 for 45 wt.% CaO and at 1600C. estimated at 1650C (1923 K) with the help of Eq. (7.26). Butd at 1650C using Eq. (7.17).

g(CS)1873 = 4208 = 0.0585 (E1.2)

923 = log(2.7 103) + 0.0585 = 2.510

l2O3 is independent of temperature, LS at 1923 K (1650C) = 28.8. approximately 20.

= 0.023% at equilibrium.

hO[ ]W S[ ]

------------

W O[ ]W S[ ]

-------------

CSW S( )

------------= =

CS

W O[ ]W S[ ]

------------- 2.03 10 2=

W O[ ]W S[ ]

------------- 2.5 10 2=

11923------------

11873------------

10 23---log0.01203971923--------------- 5.482

13--- 0.4log+ + 1.46=

LSW S( )W S[ ]

------------ 43.6= =

3.6

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7.2.5 SOME COMMENTS ON

As Example 7.1 shows, the corrExample 7.2, the following con

A slag that is not satura Pure CaO is also not an

Let us consider a slag of Casaturation, WCaO is 60 and = 0.1, LS is calculated as 220 Eq. (7.26), LS = 555. AccordingSiO2. A still larger value of LS cvalue of CS (Figure 7.2). This wreports show that such large va

Interest in CaF2-containing and Davies18 have reviewed the CaF2 dissolves oxides significanof oxides tend to be high in concentration, aCaO in a CaF2-Csystems. This results in higher capacities in CaF2-CaO-Al2O3 t

The importance of LS can bThe sulfur balance is

1000[WS

where MS1 = weight of slag in kAssuming the attainment o

Eq. (7.27) with Eq. (7.11),

degree of desulfuriz

Figure 7.7 presents some calculfor good desulfurization. Rewri

where Y = LS MS1/1000

7.3 DESULFURIZATION

7.3.1 INTRODUCTORY REMAR

As mentioned in Section 7.1, poto 20 ppm) only. Otherwise, desin an ordinary ladle, ladle furnacare as follows:

W Al2OLS

elation by Gaye et al.15 gave a better match with Figure 7.2. Fromclusions may be drawn:

ted with CaO is not an effective desulfurizer. effective desulfurizer.

O-Al2O3-SiO2-MgO with 5 wt.% SiO2 and 3 wt.% MgO. At lime is 32 (Figure 7.6). From Eqs. (7.20) and (7.26), and taking

at T = 1873 K and WAl = 0.01. If WAl = 0.04 wt.%, then, from to Gaye et al.15 even a value of 1000 is possible in CaO-Al2O3-an be obtained if some CaF2 is present in the slag due to a higheray, it is possible to obtain a value of LS larger than 1000. Literaturelues are indeed obtained in industrial practices.17slags originated from electroslag remelting processes. Richardson9thermodynamic properties of these slags, including sulfide capacity.tly. But it is a stable, neutral compound. Hence, activity coefficientscomparison to those in slags. For example, at comparable CaOaO slag is much higher than those in Al2O3-CaO and SiO2-CaOvalues of CS in CaF2-containing slags. Figure 7.6 presents sulfideernary at 1500C as determined by Kor and Richardson.19e demonstrated as follows.

]o + MS1(WS)o = 1000[WS] + MS1(WS) (7.27)

g/tonne steel and the subscript o indicates initial values.f slagmetal equilibrium and also that (WS)o = 0, and combining

ation (R) = (7.28)

ated curves based on Eq. (7.28). It shows the necessity of high LSting Eq. (7.28) we obtain,

(7.29)

WITH ONLY TOP SLAG

KS

wder injection is done to achieve an ultra-low sulfur level (S < 10ulfurization by treatment with synthetic slag on top of molten steele, or during vacuum treatment is quite all right. Principal additions

3aAl2O3

1 W S[ ]W S[ ]O---------------

LS MS1

1000 LS MS1+--------------------------------------

=

R Y1 Y+-------------=

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1. CaO. This is for the founsaturated.

2. Al. This is for reaction wjoins the slag phase. Thwell. Al also reacts with

3. CaF2,SiO2,Al2O3. Use th

Additions are made partly dladle. The tapping stream causeand desulfurization will occur. also exists, as for example, the Steel Corp. As a consequence oa molten top slag forms, consisthe beginning of the second stintroduced through porous botto

If the slagmetal sulfur equrization can be predicted by fprincipal question is whether suas literature reports are concerattained and sometimes not.

This topic shall be taken ureaction comes close to equilib

The equilibrium partitio There are no disturbing

lining into slag and a confrom the atmosphere.

As far as disturbing side reFigure 7.8 presents some data.2actual slag to that in CaO-satu

FIGURE 7.7 Desulfurization degrmation of a highly limy slag, either saturated with CaO or

ith dissolved oxygen in molten steel and to form Al2O3, whichis reaction is exothermic and raises the temperature of steel as the SiO2 of slag to some extent.is as required for slag formation.

uring tapping of the metal from the steelmaking furnace into thes violent stirring, and during this process some slagmetal reactionTo make it more effective, the practice of synthetic slag additionMPE process of Mannesman and the EXOSLAG process of U.S.

f these additions, the metal gets well deoxidized by aluminum, andting dominantly of CaO and Al2O3 with some CaF2,SiO2. This isage of the slagmetal desulfurization reaction. Stirring by argonm plugs is a must for speeding up mixing and mass transfer.

ilibrium is attained at the end of the process, the extent of desulfu-ollowing the procedure mentioned in Section 7.2.5. However, ach an equilibrium gets established in industrial processing. As farned, the slagmetal equilibrium for sulfur reaction is sometimes

p again later. For the time being, it will suffice to state that therium if

n coefficient (LS) is not too large. side reactions going on, such as dissolution of the refractorysequent change of slag composition, and/or absorption of oxygen

actions are concerned, a vacuum ladle degasser is least affected.0 The degree of CaO saturation simply means the ratio of WCaO inrated slag. The highest sulfur partition was obtained in the CaO-

ree for different sulfur distributions as a function of specific slag amount.

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saturated slag. Thermodynamic slags. The lower values in superssolid particles of CaO and hencon mixing and mass transfer in

In Figure 7.8, the maximum

parameter is about 25000 at a sapartition coefficient of 2920. Frvalue, perhaps indicating a closment of equilibrium also seemsspeed up the refining process. H

7.3.2 KINETICS OF DESULFUR

General Features

Chapter 4 briefly reviewed masin gas-stirred ladles. Several stubetween slag and metal. Chaptdegassing of steel, and their inflHence, there is no need to repearemarks.

The overall desulfurization

1. Transfer of sulfur dissol2. Transfer of O2 from the3. Chemical reaction at the

FIGURE 7.8 Sulfide capacity vs. by permission of Iron & Steel Soclimitations were held responsible for lower partition in unsaturatedaturated slags were attributed to kinetic factors. These slags contain

e tend to have higher viscosity with consequent retarding influence slag phase. value of the

turation degree of 1. At WAl = 0.04, this corresponds to an effectiveom discussions in Section 7.2.5, it is evident that it is quite a largee attainment of equilibrium. However, as stated earlier, nonattain- to be a common feature. Moreover, there is a worldwide effort toence, the kinetics of desulfurization are of considerable relevance.

IZATION REACTION WITH TOP SLAG

s transfer between two liquids as well as mixing and mass transferdies reported therein were in the context of the reaction of sulfur

er 6 presented the special features of flow, circulation in vacuumuence on mixing of liquid as well as gasmetal reaction kinetics.t this material, and discussions here are limited to a few additional

reaction consists of the following kinetic steps:

ved in liquid iron to slagmetal interface bulk of the slag to the slagmetal interface interface, i.e.,

[S] + (O2) = (S2) + [O] (7.6)

CaO-saturation degree of slag in ladle desulfurization of steel.20 Reprintediety, Warrendale, PA, U.S.A.

W S( )W S[ ]

------------

1W Al[ ]2 3

--------------------

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4. Transfer of S2 from the5. Transfer of [O] from the6. Mixing in slag phase7. Mixing in metal phase

Section 4.3 reviewed reactiperatures, especially at steelmathan interfacial chemical reactisothermal zone). Several laboraespecially in the ironmaking sitliquid iron, which is a product owith C, Si, Al etc. of liquid iron

The desulfurization reactionprocess with respect to concentrwith vacuum degassing and dec

where kS,emp is an empirical rate

This as such does not point and interfacial reaction can all bof solutes in slag are much larg(steps 2 and 4) are expected to

Of course, Eq. (7.30) woulcontrol rate. In that case, kS,emp would stand for sulfur, phase I fas compared to that of the metaland even emulsified due to gas to be fast. Transfer of oxygen ithat the reaction

would occur in the metal phaseincrease the mass transfer rate boundary layer mass transfer.21

Another way to consider thimechanism of slagmetal sulfuslag was always equal to the suthe data of one of their laboraequal to the sum of CO generasurprising. But what was puzzlinequilibrium during the course o

This was explained by invoreactions have been summarizreactions occurring simultaneouto that of all anodic reactions iat the interface, known as corro

interface into bulk slag interface into the bulk metal phase

on kinetics among phases. As stated there, reactions at high tem-king temperatures, are mostly controlled by mass transfer ratherion in a laboratory situation (i.e., melts well mixed and in antory investigations have been conducted on the reaction of sulfur,uation. It was established that the content of oxygen dissolved inf Reaction (7.6), has to be considerably lowered through its reaction if a good desulfurization is desired. has been found to behave approximately as a first-order reversibleation of sulfur in metal. In an analogy with Eq. (6.22) in connectionarburization, we may write

(7.30)

constant in s1

to any particular rate-controlling step, since mass transfer, mixing,e expressed as a first-order reversible process. Since concentrationser as compared to those in metal, mass transfer in the slag phasebe faster as compared to those in metal.d be valid even if the transfer of sulfur in metal and slag jointlywould be the parameter of Eq. (4.41), where solute ior liquid steel, and phase II for slag. Slag volume is much smaller. Moreover, as will be seen later, the top slag gets violently churnedstirring. Hence, mixing in the slag phase (step 6) is also expectedn the metal phase (step 5) is also likely to be fast, due to the fact

2[Al] + 3[O] = (Al2O3) (7.31)

close to the slagmetal interface in the metal itself. This wouldof [O] significantly due to the phenomenon of reaction-enhanced

s issue can be derived from the classic work of King et al.22 on ther transfer. They found that the rate of sulfur transfer from metal tom of the rates of oxidation of Fe, C, Si, etc. Figure 7.9 presents

tory experiments. The increase of sulfur in slag was numericallyted, and Fe + Si transferred to the slag. As such, the result is notg was the temporary overshoot of Fe and Si transfer to slag beyondf the reaction.king the electrochemical mechanism of slagmetal reaction. The

ed in Table 7.1. It shows that there are several electrochemicalsly. The sum of the rate of all cathodic reactions would be equal

n all cases. Moreover, there will be a common electrical potentialsion potential. In the initial stages, the cathodic reactions of sulfur

d W S[ ]dt--------------- kS ,emp= W S[ ]

W S( )LS

------------

A V( )km ,iI-II

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altered this potential to a valucouples. In later stages, these rsecondary steelmaking, thereforoxygen from slag to metal. Thethat of sulfur.

Investigators tried to ascertacontrolling. No clear-cut evidenthe situation.23,24 However, the trolled.25 Of course, as later inverather than phase boundary mamixing is not a problem in sma

If the backward reaction is as follows, if ks,emp is taken as i

TABLE 7.1Reactions Occurr

Type

Cathodic

Anodic

FIGURE 7.9 Increase of sulfur inexperiments of Ramachandran and

[

C[ ] +F[

1 2 S[2 3 A[e that led to a shifting of the equilibrium of Fe/Fe2+ and Si/Si4+eturned to equilibrium values. In the context of desulfurization ine, we may think of the transfer of Al from metal to slag rather than concentration of Al being larger, its transfer would be faster than

in whether the interfacial chemical reaction could be slow and ratece was available. Perhaps, in stagnant laboratory melts, this wasgeneral conclusion was that the reaction was mass transfer con-stigators concluded (Section 4.5), sometimes it may be bulk mixingss transfer that seems to be slow and rate controlling. Of course,ll laboratory melts.ignored in Eq. (7.30), then integration of Eq. (7.31) is simplified

ndependent of t:

ing during Sulfur Transfer from Liquid Iron to Slag

Initial Stages Later Stages

slag and equivalents of Fe, Si, and CO transferred or evolved in laboratory King.23

S] 2e+ S2( )= S[ ] 2e+ S2( )=Fe2 +( ) 2e_+ Fe[ ]=

1 2 Si4 +( ) 2e_+ 1 2( ) Si[ ]=2 3 Al3 +( ) 2e_+ 2 3 Al[ ]=

O2 ( ) CO 2e_+=e] Fe2 +( ) 2e_+=i] 1 2 Si4 +( ) 2e_+=l] 2 3 Al3 +( ) 2e_+=

C[ ] O2 ( )+ CO 2e_+=2 3 Al[ ] 2 3 Al3 +( ) 2e_+=

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Figure 7.10 is plotted accordingladle furnace. The slopes gave

The values of kS,emp increasebrief discussion on this issue, fwas noted there that, in generalinput per unit mass of the bath lreviewed mass transfer in ladleobtained by several investigatorn from 0.33 to 3.0.

Table 7.2 presents a summacompilation by Asai et al.27 It mmeasurements over a large rangshowed a much larger value inreported similar observation (sein a water-oil system. Figure 7ladle of pilot plant size.28

SlagMetal Emulsion and Rea

It has been established througemulsification of top liquid (slaleads to an increase of the interfissues are as follows:

1. Critical value of Q or m2. Change of the mass tran3. Enhancement of the inte

FIGURE 7.10 Rate of desulfuriza WAl2O3) = 2 4.26 Reprinted by p(7.32)

ly and is taken from an investigation by Ohma et al.26 in a 35 tonnevalues of kS,emp. with increasing volumetric gas flow rate (Q). Section 4.4.2 has arom mass transfer between two liquids in a gas-stirred vessel. It

, kmA Qn. Since Q is proportional to the rate of buoyancy energyiquid (m), kmA varies over a wide range. Asai et al.27 have refining processes. They have compiled the k vs. relationships in cold models and at high temperatures. These show a range of

ry of the behavior pattern of k vs. m. It is primarily based on theay be noted from Table 7.2 that, in cases where investigators madee of gas flow rate, log k vs. log m curves exhibited kinks, and n the high flow rate range. Several subsequent investigators alsoe Section 4.4.2). Figure 4.10 shows an example of such behavior

.11 presents a similar behavior for desulfurization in a gas-stirred

ction Rate

h water-model studies that this phenomenon is due to onset ofg, oil, etc.) into the bath liquid (steel, water, etc.). Emulsificationacial area and hence the kmA parameter. Technologically important

at which emulsification begins [QCr or m(Cr)]sfer coefficient (km) due to emulsificationrface area (A) as a result of emulsification

W S[ ]0W S[ ]

-------------- ln kS ,emp t=

tion of steel by slag refining and powder injection for (WCaO + WMgO/WSiO2ermission of Iron & Steel Society, Warrendale, PA, U.S.A.

mn

n

2001 CRC Press LLC

Some discussions, data, andabove. QCr depends on several vmetal. A more fundamental parhas presented an elaborate theoremulsification process. The ana

TABLE 7.2Correlation of Mass Transfe

System St

Slagsteel Ar gas

WaterHg N2 gas

Slagsteel Ar gas, mstirring

Slagsteel Ar gas

Slagsteel

Oilwater

Amalgamsaqueous sol.

nhexane N2 gas

Aqueous sol.

Amalgamsaqueous sol.Leadmolten salt

Slagsteel O2 gas

Liquid paraffinwater

Tetralineaqueous sol. Air

Q(1/min t) is assumedK: capacity coefficient of mass transf: mixing power density (W/t): fraction of slagt: tonne references have already been presented in Section 4.4.2 on item 1ariables. An important one is interfacial tension between slag and

ameter is the critical liquid velocity at the interface (uCr,i). Oeters29etical analysis for the conditions of drop formation that sets on thelysis leads to the correlation expressed in Eq. (7.33).

r in LiquidLiquid System27

irring Reaction Correlation Remarks

Desulfurization 2.5t converter

Reduction of quinone

echanical Dephosphorization

Desulfurization

= (amal) (aq.)

Dephosphorization

er = ak

K 0.25

60w/t