Desulfurization of iron melts with fine particulate mixtures containing alkaline earth metal carbonates

Compositions for desulfurizing iron melts comprised of alkaline earth metal carbonates, such as calcium carbonates, and reductive metals, such as silicon alloys, and a process for desulfurizing iron melts therewith.

This invention relates to fine-grain desulfurizing mixtures for iron melts 
of alkali-earth carbonates as well as the compositions of same with 
reductive metals and a method for using the same. 
In order to be able to continue to produce high grade iron materials of 
constant quality, the desulfurization of pig iron and steel is acquiring 
increasing importance due to the declining quality of the ores and of the 
reducing agents such as coke and heavy fuel oil. 
Naturally occurring carbonates of the alkali-earth metals or carbonates 
thereof obtained by chemical reaction would be particularly suitable and 
inexpensive desulfurizing agents. As a result of the large quantities of 
gases which are produced upon the introduction of fine-particle carbonates 
into molten iron at temperatures of about 1,200.degree. to 1,700.degree. 
C., large amounts of iron would, however, be thrown out of the treatment 
vessels such as a ladle. Because of this disadvantage, namely the 
non-controllable development of gas, the low-cost, environmentally 
unobjectionable alkali-earth carbonates, which are available in sufficient 
quantity in nature, cannot be used directly for the desulfurizing of 
molten iron. 
This is all the more regrettable since, as it is known, the alkali earth 
oxides freshly formed in accordance with equations (1) and (2) have a very 
high reactivity due to their small crystal size. 
EQU CaCO.sub.3 .fwdarw. CaO + CO.sub.2 .DELTA.H.sub.1 = +42.8 kcal/Mol (1) 
EQU MgCO.sub.3 .fwdarw. MgO + CO.sub.2 .DELTA.H.sub.2 = +24.3 kcal/Mol (2) 
Other disadvantages in the use of alkali-earth carbonates as desulfurizing 
agents for molten iron result from the endothermic cleavage and carbon 
dioxide formation (see equations (1) and (2)), whereby the iron melt is 
considerably cooled. Furthermore, the reaction products produced such as 
calcium oxide, magnesium oxide, or calcium sulfide, cause a stiffening of 
the slags present at the surface of the melts, i.e., they increase the 
melting point of said slags and thus make removal from the treatment 
vessels more difficult. 
In all industrial methods used today for the deacidification of the 
alkali-earth carbonates, for instance the burning of lime, the dwell time 
of the resultant alkali-earth oxides at the acidification temperature is 
comparatively several orders of magnitude longer as against blowing into a 
pool of iron. Accordingly, industrially calcined lime, calcined dolomite, 
or calcined magnesite, due to recrystallization of the very small oxide 
crystallites (originally produced and which form larger crystals) is 
considerably slower in reaction then the alkali-earth oxides which are 
freshly formed by deacidification when blowing alkali-earth carbonates 
into molten iron and when, within one to five seconds, these rise through 
the melt and react with the sulfur dissolved in the melt. 
If the alkali-earth metal carbonates are replaced by the alkali-earth metal 
oxides produced industrially from them, the disadvantage of a 
non-controllable development of gas is, it is true, avoided, but the 
desulfurization effect when alkali-earth oxides are used is unsatisfactory 
in all the processes which have become known up to now, even when 
soft-burned alkali-earth oxides are used, since the calcium oxide crystals 
are too large and therefore too slow to react. Only a part of the 
alkali-earth oxides reacts with sulfur contained in the iron melt. 
It was therefore desired to develop new desulfurizing mixtures for iron 
melts which, starting from industrially readily accessible products, 
present in practically unlimited amount, and together with certain 
additives assure good desulfurization values with high velocity of 
reaction and scarcely increase the cost of the production of crude steel. 
This purpose has been achieved by means of fine particulate desulfurization 
mixtures based on alkali-earth carbonates, which are characterized by the 
fact that these contain further metals of reducing action (reductive 
metals) which bring about the superheating of the highly-active 
alkali-earth oxide, formed in situ in the iron melt, and suppress the 
development of gas. 
It has been found that in the presence of metals of reducing action, 
alkali-earth carbonates react exothermically with them at temperatures of 
about 1,100.degree.-1,300.degree. C. It was surprisingly found that a 
large quantity of carbon is formed and that scarcely any gas is liberated. 
A reaction mechanism in accordance with equations (1) to (6) (further 
shown herein) is thus assured. 
When blowing alkali-earth carbonates into iron melt, the dwell time of the 
alkali-earth oxide formed from the carbonates and of the carbon dioxide 
produced by thermal dissociation is only a few seconds even immersing a 
lance to a depth of 2 to 4 meters. The carbon dioxide which develops is 
removed by exothermal reaction by the addition, in accordance with the 
invention, of a metal of reductive action. The gas bubble produced 
collapses immediately and the alkali-earth oxide formed, which is strongly 
overheated due to the exothermal reaction of the carbon dioxide with the 
metal, avidly reacts with the sulfur present in the molten iron since no 
recrystallization or grain growth whatsoever take place during the short 
time of the ascent to the surface. The formation and collapse of the gas 
bubbles promote the agitating and mixing of the molten iron. 
As alkali-earth carbonates, which may be of a mineral nature or prepared 
synthetically, calcium carbonate, magnesium carbonate, dolomite as well as 
half-burned dolomite and diamide lime (calcium diamide carbonate) are in 
particular used. The latter is the residue obtained upon the production of 
cyanamide or dicyandiamide from calcium cyanamide and consists essentially 
of calcium carbonate and graphitic carbon. Upon contact with the molten 
iron, which has a temperature of at least about 1200.degree. C., these 
carbonates decompose into calcium oxide and carbon dioxide. The produced 
calcium oxide is a highly active desulfurizing agent at the time of its 
production. The cleavage and production of large quantities of carbon 
dioxide takes place with considerable consumption of heat (see equations 
(1) and (2)), as a result of which the temperature of the molten iron is 
reduced. 
This reduction in temperature is however more than compensated for by the 
addition of a metallic reducing agent. The exothermic reaction of the 
reducing agent -- which must be stronger under the given conditions than 
free carbon or its compounds with hydrogen -- with the carbon dioxide 
split-off from the carbonates exerts a favorable influence on the heat 
balance, which is affected by the endothermal decomposition of the 
alkali-earth carbonate (equations (3) and (4)). 
EQU CO.sub.2 + Si .fwdarw. SiO.sub.2 + C .DELTA.H = -123.7 kcal/Mol (3) 
EQU 1,5 CO.sub.2 + 2 Al .fwdarw. Al.sub.2 O.sub.3 + 1.5 C .DELTA.H = -238.9 
kcal/Mol (3) 
The reducing agent therefore has the purpose of so binding, by exothermal 
chemical reaction, the carbon dioxide which is liberated upon the thermal 
decomposition of the alkali-earth carbonate. Hence, only an intermediate 
development of gas takes place which, while it agitates the melt, no 
longer leads to splattering. Moreover, the overall heat balance remains 
positive and advantageous. Accordingly, the desulfurizing agent which is 
blown-in is superheated as compared with the melt. A certain residual 
development of gas is desirable for sufficient mixing of the melt. 
Reducing agents which satisfy these conditions are, for instance, silicon, 
aluminum, alloys of aluminum and silicon, manganese silicon and 
ferrosilicon having silicon contents of 15 to 98 percent silicon as well 
as mixtures of the aforementioned substances, powdered metal-containing 
wastes from metal working or crushing processes, aluminum filings, or 
dross, etc. 
The proportion of such reducing agents in the desulfurizing mixture can 
amount to up to 90 percent by weight. A content of 5 to 85 percent by 
weight is preferably used. 
Additionally, the reducing agent, when it is oxidized by the carbon 
monoxide and/or carbon dioxide, should, in addition, form alkali-earth 
oxide which then also exerts a desulfurizing action. 
By way of example, the following reaction equations may be given: 
EQU CO + Ca .fwdarw. CaO + C .DELTA.H = -125 kcal/Mol (5) 
EQU CO.sub.2 + 2 Mg .fwdarw. 2 MgO + C .DELTA.H = -193 kcal/Mol (6) 
As reducing agents which satisfy both conditions, mention may be made, by 
way of example, of calcium silicon, barium-calcium silicon, magnesium 
ferrosilicon, calcium, magnesium, strontium, barium, alloys of calcium, 
magnesium, strontium and barium, magnesium calcium silicon, ferrocalcium 
silicon and aluminum calcium silicon. Mixtures of these and other 
substances, particularly with iron, can also be used. In order to obtain a 
sufficient effect, up to 90 percent by weight, and preferably 10 to 25 
percent by weight, should be added in the desulfurizing mixture. 
Finally, the reducing agent, after its oxidation by carbon dioxide or 
carbon monoxide, should make a contribution to reducing the melting point 
of the slags from the desulfurization reaction without the walls of the 
treatment vessel being more strongly attacked by added fluxes such as 
fluorospar or colemanite. They must therefore be compounds which are 
present in the slag covering the melt even before the desulfurizing 
treatment, for instance, in the case of pig iron, compounds such as 
silicon dioxide, aluminum oxide, or silicates containing these and other 
oxides. The reducing agents which satisfy this condition are, for 
instance, calcium silicide, magnesium ferrosilicon, aluminum calcium 
silicide, as well as mixtures of said substances and others. Depending on 
the nature of the slag on the melt prior to the desulfurization treatment, 
the compostion of the reducing agent should be so selected that the slag 
does not become more viscous during the treatment. 
The amount of reducing agent of the aforementioned type in the 
desulfurizing mixture can amount to up to 90 percent by weight. A 
percentage of about 10 to 80 percent by weight is preferably added. 
All the metallic reducing agents mentioned here may be industrial products 
and contain the ordinary impurities resulting from their manufacture. No 
special requirements as to purity are made. In particular, iron may be 
present as impurity. 
The metallic reducing agents used in accordance with the method of the 
invention therefore fulfill entirely different purposes than the reducing 
agents contained in desulfurizing mixtures which have been previously 
described. The latter, as a result of the splitting-off or formation of 
hydrogen, carbon dioxide, carbon monoxide, or some other gas of reducing 
action are intended merely to create a protective atmosphere in order to 
protect the actual desulfurizing agent, such as for instance calcium 
carbide, from oxidation. In the present invention, the added metallic 
reducing agents, however, effect a binding, in exothermal reaction, of the 
carbon dioxide liberated upon the decomposition of the alkali-earth 
carbonates, and deliver additional active alkali-earth oxides in situ as 
well as reduce the melting point of the slags. The active, superheated 
calcium oxide reacts in known manner with the sulfur dissolved in the 
molten iron, for instance, in accordance with the following equation in 
case of pig iron: 
EQU CaO + S dissolved + C dissolved .fwdarw. CaS + CO (7) 
the calcium sulfide formed in accordance with equation (7) is taken up by 
the slag floating on the molten iron. 
By the use of different metals in the alkali-earth carbonate and in the 
metallic reducing agent, the melting points of the oxides or oxide 
mixtures resulting from the desulfurizing mixture can be controlled. In 
this way it is also possible to control the melting point and the 
consistency of the slags floating of the molten iron. In addition, the 
desulfurizing mixtures may also contain certain amounts of fluxes, such as 
inorganic fluorides or borates. 
In order to further increase the exothermal nature of the desulfurizing 
mixture introduced into the molten iron and thus the superheating of the 
alkali-earth oxide of desulfurizing action, proportions of known thermite 
mixtures, consisting of finely granulated iron oxide and powdered 
aluminum, may also be present. 
When using mixtures consisting essentially of calcium carbonate and 
ferrosilicon, it may prove advisable to add, furthermore, a few percent by 
weight of a calcium-magnesium or calcium-silicon alloy. 
The preparation of such desulfurization mixtures in accordance with the 
invention can be effected by simply mixing the individual components 
together in the corresponding ratio. However, it is preferred to grind the 
components, after intensive drying of the alkali-earth carbonate as lump 
starting products together with one or more reducing agents and bring 
these to granular sizes of less than 3 mm and preferably less than 0.3 mm. 
The particle size of the alkali-earth-carbonate can be coarser than that 
of the reducing agent, which would be present in very fine particle size. 
This finely granular mixture of alkali-earth carbonate and reductive metal 
can be conveyed pneumatically, such as by a gas feed stream, and by means 
of blast technology customarily used today, is introduced into molten iron 
present in the hearth of a blast furnace, in an open ladle, or in a 
transfer ladle, or in mixers. It is particularly advantageous for the 
reaction of the liberated carbon dioxide with the metal of reductive 
action that the desulfurizing mixture be introduced as deep as possible 
into the molten iron by means of the blast lance. The ferrostatic pressure 
or the additional pressure of the gas atmosphere has an advantageous 
effect in the sense of accelerating the reaction of the carbon dioxide 
with the metal. 
Should it be desirable instead of producing a mixture of alkali-earth 
carbonate and metal of reducing action, the individual components can be 
stored separately, measured out, and pneumatically conveyed separately, 
and then combined to form a mixture only shortly before the lance or 
within the lance. 
The following examples are intended further to explain the invention, 
without however limiting it to the mixtures or compositions indicated, 
their preparation or use.

EXAMPLE 1 
Desulfurization of pig iron with a mixture consisting of powdered magnesium 
and calcium carbonate. 
The desulfurizing mixture was prepared by simultaneous grinding of 
limestone, which had been precrushed to 0 to 5 mm, with powdered magnesium 
of a particle size of less than 1 mm in a tube mill using nitrogen as 
blanket gas. 
203 tons of pig iron were treated in a transfer ladle of a capacity of 230 
tons with the mixture of 40 percent by weight powdered magnesium and 60 
percent by weight ground limestone, the mixture being blown into the melt 
with an immersion lance (gas lance) at a depth of 1.85 m, with argon as 
conveyor or blast gas. 
The molten iron had a temperature of 1310.degree. C. The rate of conveyance 
was throttled until no substantial flame of burning magnesium could be 
noted any longer on the surface. This was obtained with an introduction 
rate of 17 kg per minute. 
The molten pig iron had an initial sulfur content of S.sub.I = 0.042 
percent. After a period of treatment of 11 minutes, 198 kg of 
desulfurizing agent had been blown-in. This corresponds to 0.98 kg per ton 
of pig iron. The sulfur content after the treatment was S.sub.E = 0.005 
percent. 
Thus, by calculation, there is a degree of conversion for calcium oxide 
formed from the calcium carbonate into calcium sulfide, for the magnesium 
oxide newly formed in the reaction into magnesium sulfide, and for the 
magnesium present (in excess) into magnesium sulfide, of a total of 46 
percent. 
EXAMPLE 2 
Desulfurization of a molten steel with a mixture of calcium carbonate, 
calcium silicide (calcium silicon) and aluminum. 
The molten steel which was at a temperature of 1620.degree. C. contained: 
0.07 percent by weight carbon 
0.13 percent by weight silicon 
0.35 percent by weight manganese 
as well as sulfur from 0.024 to 0.033 percent by weight, which was to be 
reduced by the desulfurization treatment on the average to 0.005 percent 
by weight. 
In order to avoid the oxidizing influence on the molten steel, a 
desulfurizing mixture having a deficit of calcium carbonate was selected. 
The small addition of aluminum present was intended to assure the desired 
aluminum content in the steel and, by partial reaction with calcium 
carbonate, to introduce aluminum oxide into the slag. 
In combination with a low-oxide, fluorspar-containing slag blanket, a 
desulfurizing mixture consisting of 
37 percent by weight calcium carbonate 
60 percent by weight calcium silicon 
3 percent by weight aluminum 
was introduced pneumatically into the molten steel contained in a 70-ton 
ladle. As conveyor or blast gas 6 to 10 liters of argon per kilogram of 
desulfurization mixture were used. The following table shows the results 
of the individual treatments: 
______________________________________ 
Treatment Consumption 
Number S.sub.I S.sub.E .DELTA.S 
kg/t 
______________________________________ 
1 0.031 0.005 0.026 2.4 
2 0.027 0.003 0.024 2.4 
3 0.030 0.002 0.028 2.7 
4 0.019 0.003 0.016 2.1 
5 0.013 0.002 0.011 2.0 
6 0.016 0.002 0.014 2.1 
7 0.033 0.008 0.025 2.2 
8 0.032 0.011 0.021 1.8 
______________________________________ 
The consumption of an average of 2.2 kg of desulfurizing mixture per ton of 
steel means a 50-65 percent better utilization of the desulfurizing agent 
than obtained with the previously customarily employed mixtures. 
EXAMPLE 3 
Desulfurization of pig iron with a mixture consisting of powdered calcium 
silicide and calcium carbonate. A mixture was prepared from 28.6 percent 
industrial calcium silicide (calcium silicon) and 71.4 percent calcium 
carbonate. The industrial calcium silicide contained 30.1 percent calcium 
and 60.3 percent silicon. The calcium carbonate was a precipitated 
product, prepared synthetically. The mixture was produced in a 
three-chamber tube mill. The fineness of grain of the mixture of the 
industrial calcium silicide and calcium carbonate leaving the mill was 98 
percent less than 0.1 mm. With this mixture, 196 tons of pig iron were 
treated in the transfer ladle. Air was used as blast gas. 
The charge was 12 liters of air (STP) per kilogram of mixture. 325 kg of 
the above-designated mixture were blown-in within 9.5 minutes. 
Before the treatment, the sulfur content was 0.047 percent and after the 
treatment it was 0.013 percent. Therefore 0.034 percent sulfur was 
removed, corresponding to a degree of desulfurization of 72 percent. The 
yield of the conversion of the desulfurizing agent into calcium sulfide, 
referred to the total content of calcium, was 59.8 percent. The 
consumption of desulfurizing agent was 0.5 kg/ton of pig iron per 0.01 
percent sulfur removed.