2.1.3 DESULFURIZIATION IN SECONDARY STEELMAKINGExcept in free-cutting steels, sulfur is considered to be a harmful impurity, since it causes hot shortness in steels. Some decades back, for common grades of steel cast through the ingot route, the maximum permissible sulfur content was 0.04%. In the continuous casting route, it should be 0.02%. In special steel plates, the normal specication for sulfur is 0.005% these days, but there is a demand for ultra-low-sulfur (ULS) steel with as low as 10 ppm (0.001%), e.g., in line pipe, HIC resistive steels, and alloyed steel forgings.Sulfur comes into iron principally through coke ash. It is effectively removed from molten iron by slag in a reducing environment only. Hence, traditionally, sulfur control used to be done during ironmaking in a blast furnace. Very little sulfur removal is possible in primary steelmaking due to the oxidizing environment. An exception is the electric arc furnace (EAF), where low-sulfur steels are produced through two-stage rening.

In view of the consistent demand for lower-sulfur steel and the incapability of the blast furnace to achieve it, external desulfurization of liquid iron in a ladle during transfer to the steelmaking shop was developed. The process is capable of lowering sulfur content to 0.01% or so and is an essential feature of a modern integrated steel plant.

Content below 0.01% must be accomplished in secondary steelmaking. There are now pro-cesses, such as the MPE process of Mannesman and the EXOSLAG process of U.S. Steel, where desulfurization is achieved to some extent during tapping by using synthetic slag and utilizing the kinetic energy of the tapping stream. Desulfurization by treatment with synthetic slag on top of molten steel and gas stirring (either in an ordinary ladle, in a ladle furnace or VAD, or during vacuum degassing) are also being practiced.

However, only the injection of a powder such as calcium silicide into the melt is capable of producing ULS steel. ULS can be achieved only if the dissolved oxygen is also very low. Gas stirring is required, so deep desulfurization is associated with deep deoxidation. The use of alumi-num in combination with calcium or rare earth (RE) metals achieves both. In addition, injection processes are capable of inclusion modication for further improvement of the properties of steel. A low nitrogen level has been achieved in low-sulfur and low-oxygen steels. This is an additional benet if deep desulfurization is done before or during vacuum treatment. Furnace slags contain oxides such as FeO, SiO2, P2O5, and MnO. These oxides are unstable in the presence of a deoxidized steel, especially when the slag and steel are intimately mixed. As a result, some reversion of phosphorus into the steel occurs. This slag also partly consumes added deoxidizers, so it does not allow proper utilization of them for steel deoxidation. The slag also causes wear on the ladle lining. (Although these have been known for a long time, very little physicochemical investigation has been conducted on these effects. Turkdogan has considered some aspects of the reaction of liquid steel with slag during furnace tapping. It is best if slags from primary steelmaking furnaces are not allowed into the secondary steelmaking ladle. However, this is difcult to implement. In addition, some slag is required for desulfurization during secondary steelmaking and other benecial effects. Therefore, control of furnace carryover slag aims at the twin strategy, viz., (a) minimization of furnace carryover slag, and (b) modication of carryover slag by the addition of uxes (principally CaO, but also Al, SiO2, Al2O3, and CaF2 to some extent) to render desirable properties to it. 2.2.3 THERMODYNAMIC OF DESULFURIZIATION IN SECONDARY STEELMAKINGA. SOLUTION OF SULFUR IN LIQUID STEELAt steelmaking temperatures, sulfur is a stable gas, with the most predominant molecule being S2. The dissolution of sulfur in molten steel may be represented by the following equation:1/2 S2 (g) = S(1)For the above reaction,

K1= ---------

(2)where K1 is the equilibrium constant for Reaction (1), pS denotes partial pressure of sulfur in the gas phase in atmosphere, and hS is the activity of dissolved sulfur in steel with reference to 1 wt.% standard state.Again,log K1 = ----------- 0.964(3)The interaction coefcients describing the inuence of some common solutes (j) in liquid steel on the activity coefcient of sulfur (fS) dissolved in liquid steel (i.e., es ) at 1600C. where hS = fS WS, WS being the weight percent of sulfur in steel. It may be noted further that the solubility of sulfur in molten steel is very high.

B. REACTION EQUILIBRIA OF SULFURCa and Ba form CaS and BaS, respectively, upon reaction with sulfur, whereas cerium forms several suldes7 out of which CeS is the stablest one under steelmaking conditions. Ce also forms an oxysulde, Ce2O2S. All of these compounds are solids at steelmaking temperatures. It may be noted, from thermodynamic data on these compounds in any standard text, that all these elements form very stable suldes as well as oxides. Therefore, they are both strong deoxidizers as well as desulfurizers and would form both oxides and suldes.

Again, these compounds would not necessarily be present in a pure form. For example, addition of Ca-Si leads to the formation of a CaO-SiO2-type deoxidation product . However, we do not propose to get involved in these complexities and consider the overall reaction to be

S + (MO) = O + (MS)(4)

For the limiting case of unit activities of MO and MS (i.e., assuming pure MO and pure MS), the equilibrium constant (KMS) for Reaction (4) is

KMS = ---------- = ------------- (at equilibrium)(5)The values of KMS for different systems can be calculated from the free energy of reaction. Figure 1, reproduced from Turkdogan, shows the pattern. Ba is the strongest desulfurizer and Mg the weakest, with Ca and Ce lying in between.

Holappa has reviewed the theoretical basis for sulfur removal in ladle treatment by slagmetal reaction. If the MO and MS are not pure, then it is better to utilize the general ionic form of desulfurization reaction, viz.,[S] + (O2) = (S2) + [O](6)

K = --------------------------- (7)FIGURE 1 Oxygen/sulfur activity ratio in liquid iron for some sulde-oxide equilibria at 1873 K.

(K 6 ) (a o 2 - ) = -----------------------(7)

If we replace a 2 with weight percent sulfur in slag (i.e., WS), then we may use a modied value of K6 (let it be K 6 ). Then,

K 6(a 2) = -------------------------- = C S(8) where C S is known as the modied sulde capacity.

the sulde capacity of slag (CS), i.e., the ability of a slag to absorb sulfur, was originally dened by Richardson as

CS = (WS)(pO2/pS2)1/2(9)where (WS) is the weight percent sulfur in the slag in equilibrium with a gas having partial pressures of oxygen and sulfur as pO and pS . Its usefulness stems from the fact that CS is a property of slag, and at a xed temperature it is determined solely by slag composition. The higher the value of CS, the better the desulfurizing ability of the slag. Figure 2 shows CS values for some typical slag systems of interest in secondary steelmaking.9 The superiority of CaO-CaF2 slag is obvious. Values of CS for various slags are available in Slag Atlas.10CS is determined by equilibrating the slag with a gas mixture having known oxygen and sulfur potential. However, it is the slagmetal equilibrium that is of interest. This requires the use of a modied CS (viz., C S ) as dened in Eq. (8).

The relationship between CS and C S is

log CS = log C S + -------- 1.375(10)

At 1600C, C S = 5 CS.

FIGURE 2 Sulde capacities of some slags at 1873 K.9(Another parameter of interest is the equilibrium sulfur partition ratio between slag and metal (LS), where LS = (WS)/[WS]. From Eq. (8), if [hS] is taken as [WS], then, at slagmetal sulfur equilibrium,

LS = ---------- = ----------(11)hO in liquid steel is typically determined by the presence of a deoxidizer, especially dissolved aluminum. One may relate hO to the FeO content of slag as well. However, it has been found more appropriate to relate it to the former. Figure 3 shows LS as a function of the CaO content of slag and aluminum content of metal for CaO-Al2O3 slag. Therefore, for good desulfurization,Al content of more than 0.020% is generally recommended.h

pS2

S

2

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[hs] [WO2- ]

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[hs]

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[Ws] [ho]