Composition and method for the desulfurization of molten iron

Finely granular desulfurizing agents for iron melts, consisting of at least one alkaline earth metal carbonate and at least one reducing metal carbide and optionally a reducing metal or an alloy thereof, are outstandingly effective in desulfurizing action and, based on the high degree of utilization of said agent, increase the amount of slag formed only to a negligible degree.

The invention relates to a finely granular desulfurizing agent suitable for 
molten pig iron and steel, which is based on carbonate and carbide, and to 
a method for the desulfurization of iron melts. 
The desulfurization of pig iron and steel is gaining importance on account 
of the deteriorating quality of the ores and the increasing use of 
high-sulfur coke and heavy furnace oil. The high-quality iron and steel 
needed today can be produced only by desulfurization in the blast furnace 
or between the blast furnace and the steel mill, or by desulfurization 
after the steelmaking process. 
Alkaline earth oxides, such as lime, and alkaline earth carbonates, such as 
limestone or dolomite, have long been known as desulfurization agents for 
molten iron. 
A great number of molten iron desulfurizing agents consist of alkaline 
earth oxides to which other substances are added. For example, processes 
have become known in which finely ground lime is blown with natural gas 
into pig iron, the natural gas being cracked endothermically to carbon and 
hydrogen. In another process, lime dust is mixed with powdered magnesium 
and this mixture is blown through a lance into the molten pig iron. The 
desulfurization is caused by the magnesium vaporizing with the absorption 
of heat. Mixtures of lime or calcium carbonate and soda are frequently 
recommended for the desulfurization of pig iron. However, such 
compositions, although very inexpensive, are rarely used nowadays, for 
environmental reasons, and on account of the corrosive effect they have on 
the lining of ladles as well as the temperature drop they cause in the 
melt. 
The thermal dissociation of the alkaline earth carbonates, which occur when 
they are injected into the molten iron, releases large amounts of gas 
which violently agitates the bath and can cause molten metal to be 
splashed out. On account of this disadvantage of the uncontrollable 
formation of gas, the inexpensive and environmentally safe alkaline earth 
carbonates have not been used for the injection method of desulfurizing 
molten iron. The thermal dissociation of the alkaline earth carbonates is 
an endothermal reaction which cools the molten iron, as shown by the 
following equations: 
EQU CaCO.sub.3 .fwdarw.CaO+CO.sub.2 H=+42.8 kcal/mole 
EQU MgCO.sub.3 .fwdarw.MgO+CO.sub.2 H=+24.3 kcal/mole. 
If it is desired to make better use of these reactions, the formation of 
gas--that is, the splitting off of carbon dioxide in the thermal 
dissociation of the alkaline earth carbonates--must be suppressed. 
The alkaline earth oxides produced from alkaline earth carbonates by 
present-day calcination methods are slow to react on account of the long 
time they spend in the kiln. Even in the case of low calcination, which is 
performed at the lowest possible temperature, the alkaline earth oxide is 
exposed to the calcination temperature for at least 20 minutes. In the 
calcination process, the finely crystalline active alkaline earth oxide, 
which is primarily formed from the alkaline earth carbonate by thermal 
dissociation, rapidly recrystallizes under the kiln conditions to a coarse 
oxide which is relatively inactive with respect to the sulfur dissolved in 
molten iron. 
Technical calcium carbide, with and without additives, has already been 
proposed as a desulfurizing agent. In recent years there has been an 
interest especially in molten iron desulfurizing agents based on calcium 
carbide, to which precipitated or finely ground calcium carbonate, diamide 
lime (a mixture of finely crystalline calcium carbonate and carbon), water 
or hydrogen yielding compound, such as borates, alkaline earth hydroxide, 
hydrocarbons such as polyethylene, polypropylene, polyesters, tar, heavy 
oil and other substances, have been added. These additives intensify the 
movement and circulation of iron melts by yielding gas when the finely 
granular desulfurizing agent is added, thereby producing a good contact 
between the actual desulfurizing agent, that is, the calcium carbide, and 
the molten iron. At the same time, the yielding of gas and the oxidation 
of the carbon to carbon monoxide, or the yielding of hydrogen, was 
supposed to create a reducing atmosphere whereby the oxidation of the 
calcium carbide acting as a desulfurizing agent by the carbon dioxide 
forming from the carbonates would be prevented. The carbon dioxide was 
supposed to be reduced to carbon monoxide by the carbon and/or hydrocarbon 
of the desulfurizing agent. 
These more recent desulfurizing agents, however, still have disadvantages. 
Particularly the relatively low yield of the desulfurization reaction 
taking place in accordance with Equation 1--with respect to the calcium 
carbide put in--called for more careful investigation of the processes 
taking place during the reaction. 
EQU CaC.sub.2 +S.fwdarw.CaS+2C (1) 
the low yield was all the more surprising inasmuch as, towards the end of 
the treatment, calcium carbide could no longer be detected in the slag. 
The invention, therefore, is addressed to the problem of finding a more 
effective desulfurization mixture which, based on easily available 
starting products, and with a high rate of reaction, would assure a very 
high degree of utilization of the desulfurizing agent put in, thereby 
increasing the amount of slag to only a negligible degree. 
This problem has been solved by a finely granular desulfurizing agent for 
iron melts based on carbonate and carbide, which is characterized in that 
it contains no additional carbon, and consists of at least one alkaline 
earth carbonate and at least one reducing metal carbide, as well as, in 
some cases, a reducing metal or an alloy thereof. 
It has been found that alkaline earth carbonates react exothermically with 
metal carbide beginning at approximately 1100.degree. C. It is surprising 
that in this reaction a large amount of carbon forms and hardly any 
formation of gas occurs. An explanation is offered by a reaction in 
accordance with Equation 4. This assumption gains further support from the 
fact that calcium carbide reacts very exothermically at the said 
temperatures also in accordance with Equation 3, with the release of 
carbon and the formation of calcium oxide. 
When alkaline earth carbonates are blown into iron melts, the time of stay 
of the alkaline earth oxide formed from the carbonates and of the carbon 
dioxide formed by thermal dissociation is only a few seconds, even when 
the lance is immersed to a depth of two to four meters. The carbon dioxide 
that develops is captured in an exothermic reaction by the addition, in 
accordance with the invention, of a reducing carbide; the gas bubble that 
develops immediately collapses, and the superheated oxide that forms 
produces the desulfurization together with the oxide formed from the 
alkaline earth carbonate. The formation and the collapse of the gas 
bubbles promotes the movement and mixing of the iron melt. 
The very small, highly active oxide crystallites formed from the alkaline 
earth carbonate in situ react, in the time of their ascent, with the 
sulfur dissolved in the molten iron, with a considerably higher degree of 
transformation than normal technically burnt lime, since in this short 
period no recrystallization and grain growth takes place whatever. 
Alkaline earth carbonates, therefore, react exothermically with carbidic 
reducing agents when blown into iron melts of temperatures between about 
1200.degree. and 1700.degree. C., with the formation of highly active 
alkaline earth oxide. In the case of calcium carbonate and calcium 
carbide, the reaction equations are as follows: 
EQU CaCO.sub.3 .fwdarw.CaO+CO.sub.2 H.sub.2 =+43 kcal/mole (2) 
EQU CO.sub.2 +2CaC.sub.2 .fwdarw.2CaO+5C H.sub.3 =-181 kcal/mole (3) 
EQU CaCO.sub.3 +2CaC.sub.2 .fwdarw.3CaO+5C H4+-138 kcal/mole (4) 
According to the gross reaction of Equation 4, the agent composed of 
calcium carbonate and calcium carbide or the calcium oxide formed 
therefrom, which is blown into the iron melt, superheats to several 
hundreds of degrees Celsius above the iron temperature. The same applies 
accordingly to the other alkaline earth metals and lithium. 
The highly active alkaline earth oxide formed in situ has an outstanding 
desulfurizing action in accordance with Equations 5 and 5a. 
EQU CaO+[S]+C.fwdarw.CaS+CO (5) 
EQU mgO+[S]+C.fwdarw.MgS+CO (5a) 
The formation of gas as a result of the splitting of carbon dioxide out of 
the alkaline earth carbonate will then take place in an only intermediate 
manner. The carbon dioxide is reduced to carbon by the reducing carbide, 
such as calcium carbide for example, active alkaline earth oxide, 
superheated on account of the great exothermy of this reaction, forming 
additionally, which has an outstanding desulfurizing action. The carbon 
monoxide formed in accordance with Equation 5 is exothermically reduced to 
carbon, reactive calcium oxide again forming from calcium carbide for 
example (Equation 6). 
EQU CO+CaC.sub.2 .fwdarw.CaO+3C .DELTA.H=-106 kcal/mole (6) 
In the agent of the invention, therefore, the alkaline earth carbonate acts 
as the actual desulfurizing agent after the corresponding oxide has been 
formed, while the carbide, by reacting with the carbonate, suppresses the 
undesirable gassing and is transformed into the oxide which also has a 
desulfurizing action. 
Suitable alkaline earth carbonates are all naturally occurring carbonates, 
especially calcium carbonate and dolomite; also suitable are half-burned 
dolomite, magnesite, strontium carbonate and barium carbonate, and the 
alkaline earth carbonates which are produced in technical reactions, nical 
reactions, as by-products for example, such as the calcium carbonate 
formed in the washing of carbon dioxide. Lithium carbonate is likewise 
suitable. 
In the case of a mixture of magnesium carbonate and calcium carbide, the 
processes which take place when it is blown into an iron melt are 
described by the following equations: 
EQU MgCO.sub.3 .fwdarw.MgO+CO.sub.2 .DELTA.H=+24.3 kcal/mole (7) 
EQU CO.sub.2 +2CaC.sub.2 .fwdarw.2CaO+5C .DELTA.H=-181 kcal/mole (8) 
EQU MgCO.sub.3 +2CaC.sub.2 .fwdarw.2CaO+MgO+5C .DELTA.H=-157 kcal/mole (9) 
The desulfurizing agent composed of magnesium carbonate and calcium carbide 
in accordance with Equation 9 is even more exothermic than the agent 
composed of calcium carbonate and calcium carbide in accordance with 
Equation 4. The finely divided, highly active, superheated magnesium oxide 
that forms according to Equation 9 is a substance which has an outstanding 
desulfurizing action. 
Calcium carbide, barium carbide, aluminum carbide, magnesium carbide, 
lithium carbide, boron carbide, titanium carbide and other carbides are 
suitable reducing carbides. Mixtures of the reducing carbides can also be 
used. 
The surface area of the active alkaline earth oxide formed in situ is 
substantially larger than that of the alkaline earth carbonate injected 
into the molten iron, and also larger than that of the reducing carbide. 
The high activity of the alkaline earth oxide is explained by this large 
surface that is available for the desulfurizing reaction. The particle 
size of the alkaline earth oxides approaches that of pyrogenically 
produced dusts. The surface area of such dusts exceeds by several orders 
of magnitude the surface enlargement that can be achieved by the grinding 
of solids. 
The composition of the desulfurizing agent of the invention can vary within 
wide limits. The alkaline earth carbonate content will amount preferably 
to from 85 to 5% by weight, and the reducing carbide content to 15 to 95% 
by weight. For use in steel melts, the percentages of the carbonate will 
be in the lower range. Agents are preferred for this purpose which have a 
content of from 3 to 50% by weight, and especially preferred are those 
containing from 10 to 40% alkaline earth carbonate by weight. To control 
the physical properties of the slags and to enhance the reducing activity 
of the desulfurizing agent, the latter can additionally contain reducing 
metals, such as magnesium, aluminum, calcium or other alloys such as 
calcium silicon, for example, in amounts up to 10% by weight. 
The desulfurizing agents of the invention are incorporated pneumatically 
into the pig iron or steel melts in a known manner, e.g., by means of a 
submersible lance. In the brief period of the ascent of the desulfurizing 
agent, the exothermic formation of the highly active alkaline earth oxide 
takes place, with reduction of the intermediately released carbon dioxide 
to carbon. The intermediate gas formation is important for the 
distribution of the desulfurizing agent in the melt and for the stirring 
of the melt. 
The components of the agent of the invention can be added separately or in 
mixture. If separately, the components can be conveyed pneumatically 
separately and combined either just ahead of or within the lance to form 
the mixture. 
Through the use of different metals in the carbide and of lithium carbonate 
or of different metals in the alkaline earth carbonate, the melting points 
of the resulting oxides or oxide mixtures can be controlled, thereby also 
permitting control of the melting point or of the sintering action of the 
slags floating on the molten iron. The desulfurizing agents on the basis 
of alkaline earth carbonate and carbide yield, when the reaction is 
complete, the following oxides and oxide mixtures, for example: 
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Oxide or 
Equation No. 
Desulfurizing Mixture 
Oxide Mixture 
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(4) CaCO.sub.3 + 2CaC.sub.2 
3CaO 
(9) MgCO.sub.3 + 2CaC.sub.2 
MgO + 2CaO 
(10) 1/2CaMg(CO.sub.3).sub.2 + 2CaC.sub.2 
1/2MgO + 21/2 CaO 
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The components of the desulfurizing agent must be thoroughly ground up. The 
grain size of the alkaline earth carbonate can be larger than that of the 
carbide. The carbide must be as fine as possible so as to offer the 
greatest possible surface area to the intermediately forming carbon 
dioxide for the reaction. This reaction is ultimately a "burning" of the 
carbide in the momentarily present carbon dioxide and/or carbon monoxide. 
The exothermic desulfurizing agent of the invention has the advantage that 
it can be injected easily, by means of the blowing techniques commonly in 
use today, into the iron melts contained in the hearth of the blast 
furnace, in open ladles or torpedo ladles or mixers. In this case, the 
alkaline earth carbonate and the carbide react with one another either 
through intermediately formed carbon dioxide or, if both substances are 
very finely ground and have large surface areas, directly. 
It is advantageous for the reaction of the carbon dioxide with the carbide 
if the desulfurization mixture is injected through the blowing lance as 
deeply as possible into the iron melt. Immersion depths of about 1 to 3 
meters correspond to excess pressures of about 0.72 to 2.16 bars. 
An additional overpressure of the gas atmosphere on the molten iron has an 
advantageous effect accordingly. The higher concentration of the carbon 
dioxide in the gas phase has an accelerating effect on the reaction with 
the carbide and reduces the risk of the separation of gas and solid matter 
.

EXAMPLE 1 
Desulfurization of a molten steel with a mixture of calcium carbonate, 
calcium carbide and a small content of aluminum. 
A series of melts of what are called carbon steels of the following 
analysis: 
0.31 wt.-% carbon 
0.31 wt.-% silicon 
0.55 wt.-% manganese, 
and a starting sulfur content S.sub.A of 0.017 to 0.031 wt.-%, at a 
temperature of 1600.degree. C., are to be reduced by the desulfurizing 
treatment to an average final sulfur content S.sub.E of 0.004 wt.-%. 
To limit the oxidizing effect on the steel bath, the desulfurizing mixture 
contains a slight deficiency of calcium carbonate in relation to the 
calcium carbide. The small aluminum content is intended to assure the 
required aluminum content in the steel and to produce a refining action 
through the formation of CaO.Al.sub.2 O.sub.3. 
In conjunction with a slag cover that was low in oxides and contained 
fluorspar, a desulfurization mixture consisting of 
32 wt.-% of calcium carbonate 
65 wt.-% of calcium carbide and 
3 wt.-% of aluminum 
was pneumatically injected into the molten steel contained in a 70 metric 
ton ladle. Six to ten liters of argon per kilogram of desulfurizing 
mixture were used as the carrier gas. 
The following table shows the result of the individual treatments. 
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Treatment Consump- 
No. S.sub.A S.sub.E .increment.S 
tion kg/t .alpha. 
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1 0.031 0.004 0.027 2.2 0.74 
2 0.034 0.004 0.030 2.3 0.76 
3 0.034 0.002 0.032 3.0 0.93 
4 0.017 0.003 0.014 1.7 1.20 
5 0.024 0.003 0.021 2.0 0.95 
6 0.026 0.005 0.021 1.8 0.86 
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The consumption of an average of 2.1 kg of desulfurizing mixture per metric 
ton of steel is substantially lower when compared with the desulfurizing 
mixtures known heretofore. 
The alpha value is characteristic of the specific consumption of 
desulfurizing mixture in kg per metric ton of iron for each 0.01% of the 
decrease in the sulfur content. 
EXAMPLE 2 
Desulfurization of pig iron with a mixture consisting of dolomite, calcium 
carbide and aluminum. 
160 metric tons of pig iron at a temperature of 1330.degree. to 
1336.degree. C. were treated in an open ladle with a mixture consisting of 
60 wt.-% dolomite, 35 wt.-% calcium carbide and 5 wt.-% aluminum. For each 
kilogram of desulfurizing agent, 6 to 10 liters of nitrogen were used for 
the injection of the mixture into the molten pig iron. 
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Desulf. Mix. 
Treatment consumption 
No. S.sub.A S.sub.E S in kg/t 
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1 0.047 0.014 0.033 3.1 0.93 
2 0.054 0.015 0.039 3.3 0.85 
3 0.039 0.007 0.032 3.0 0.93 
4 0.062 0.016 0.046 3.8 0.83 
5 0.046 0.006 0.040 3.4 0.85 
6 0.051 0.004 0.047 4.4 0.93 
7 0.044 0.004 0.040 3.5 0.88 
8 0.071 0.013 0.058 4.9 0.84 
9 0.027 0.002 0.025 3.0 1.20 
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The expenditure of an average of 3.6 kg of desulfurizing mixture is 30 to 
40% lower than in the case of the desulfurizing agents used heretofore. 
EXAMPLE 3 
Desulfurization of a steel melt with a mixture of calcium carbonate, 
calcium carbide and aluminum. 
A series of steel melts, commonly called soft steels, of the following 
analysis: 
0.03 wt.-% carbon 
0.20 wt-% silicon 
0.30 wt.-% manganese, and 
a starting sulfur content S.sub.A of 0.012 to 0.026 wt.-%, are to be 
reduced to an average final sulfur content S.sub.E of 0.03 wt.-% by the 
desulfurization treatment. 
The steel bath was covered with a low-oxide slag containing flux. The 
desulfurizing agent had the following composition: 
35 wt.-% calcium carbonate 
59 wt.-$ calcium carbide 
6 wt.-% aluminum, 
and it was blown by an argon stream of 6 to 10 liters per minute into an 
open ladle containing 90 metric tons of steel. 
The following table gives the results of the individual treatments: 
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Treatment Consump- 
No. S.sub.A S.sub.E S tion kg/t 
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1 0.021 0.004 0.017 1.7 1.0 
2 0.026 0.003 0.023 1.9 0.83 
3 0.013 0.003 0.010 1.0 1.0 
4 0.017 0.005 0.012 1.2 1.0 
5 0.019 0.002 0.017 2.1 1.2 
6 0.025 0.003 0.022 2.0 0.9 
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The consumption averaging 1.6 kg of desulfurizing mixture per metric ton of 
steel is approximately 25 to 35 percent lower in comparison to previously 
known desulfurization mixtures.