Desulfurization agent for ferrous melts and method of using the same

Improved desulfurization of ferrous melts is obtained by injecting into the melt in finely divided form a mixture by weight 48% to 95% calcium carbide, 2% to 40% of carbon-containing calcium carbonate and from about 1% to 12% of high volatile, bituminous coal dust.

This invention relates to an improvement in desulfurization agents for 
ferrous melts, e.g., pig iron, cast iron and steel melts, and to an 
improved ferrous metal desulfurization process. 
It is known from U.S. Pat. No. 3,598,573 that ferrous melts can be 
effectively desulfurized by treatment with a desulfurization agent 
comprising calcium carbide and a precipitated carbon-containing calcium 
carbonate known in industry as diamide lime formed as a by-product in an 
industrial process for manufacturing dicyandiamide. In accordance with 
this commercial process an aqueous calcium cyanamide suspension is treated 
with carbon dioxide to precipitate calcium carbonate in very finely 
divided form and intimately admixed with finely divided and 
co-precipitated carbon. The precipitate contains about 70% to 85% by 
weight of calcium carbonate, about 8% to 12% of carbon and minor 
proportions of impurities such as ferric oxide, aluminum oxide and silicon 
dioxide. As disclosed in U.S. Pat. No. 3,598,573, this diamide lime can be 
mixed to the extent of 2% to 40% with calcium carbide to form a mixture 
that is a good desulfurizing agent for ferrous metals. 
The present invention is based on applicant's discovery that the 
desulfurizing agent of U.S. Pat. No. 3,598,573 can be substantially 
improved by admixing therewith finely divided bituminous coal, preferably 
a relatively high volatility bituminous coal. We were surprised to find 
that by incorporating a suitable amount of finely divided bituminous coal 
in the previously known mixture of calcium carbide and diamide lime, a 
desulfurizing agent is obtained which can be used in a smaller amount to 
produce a given desulfurization per ton of ore than the mixture of calcium 
cyanamide and diamide lime previously proposed as a desulfurizing agent. 
While we do not wish to be bound by any particular theory as to why this 
improved result is obtained with the composition of the invention, it is 
our present understanding that this improved efficiency is due at least in 
part to the development of reducing gases upon heating of the bituminous 
coal powder, which reducing gases protect the carbides from burning in the 
oxidizing gases developed from the calcium carbonate and in this way 
result in an increased degree of utilization of the calcium carbide. A 
further advantage of the present desulfurizaton agent resides in the fact 
that to the extent that the bituminous coal is not consumed in its passage 
through the iron melt, it leaves a residue that is only carbon, which is 
not absorbed by the iron melt since the latter is already saturated in 
respect to carbon. The remaining carbon therefore retains the pulverulent 
form in which it was injected into the melt; it does not slag but burns 
harmlessly on the surface of the melt without developing irritating gases 
or gases that contaminate the environment. These results are attained 
without adverse effect on the previously described synergistic interaction 
of diamide lime and calcium carbide. 
In general the present desulfurizing agent consists essentially of a 
mixture of calcium carbide, calcium carbonate containing finely divided 
carbon dispersed therein and from about 1% to 12% by weight of the mixture 
of finely divided bituminous coal, preferably a bituminous coal containing 
a relatively high proportion of volatile matter. The proportions of 
calcium carbide and diamide lime used are desirably similar to those 
disclosed in U.S. Pat. No. 3,598,573, i.e., a calcium carbide/diamide lime 
weight ratio of about 59:1 to 1.5:1. The preferred proportions of the 
several components of the desulfurizing agent are by weight 48% to 95% of 
calcium carbide, from about 2% to 40% of the carbon-containing calcium 
carbonate (diamide lime) and from about 1% to 12% of the coal dust. The 
resulting composition may have a total carbon content of about 5% to 12% 
by weight. 
The present composition may be used in the same way as the previous calcium 
carbide/diamide lime desulfurizing agents. Because of its finely divided 
form, the desulfurizing agent may be added by blowing it into the melt or 
stirring it into the melt in a shaking ladle or dispersing it in the melt 
with a mechanical agitator. Basically introduction of the desulfurizing 
agent into the melt may be carried out in any manner known to be suitable 
for the addition of other pulverulent material to the melt either in the 
ladle or in the furnace. 
The absolute amount of the desulfurizing agent to be added in accordance 
with the invention depends upon the initial sulfur content of the iron and 
the final sulfur content desired. With iron of the usual sulfur content, 
e.g., about 0.04% to 0.06%, an addition of sulfurizing agent of the order 
of about 0.5 to 2.5 kg per ton of ferrous metal per 0.01% desired 
reduction in sulfur will usually be required. On the average, the amount 
required is likely to be of the order of magnitude of 0.9 to 1 kg per ton 
of iron for removal of 0.01% of sulfur. 
The desulfurizing agent of the invention is not only highly selective, but 
is also economical to employ. It constitutes an improvement over the 
calcium cyanamide/diamide lime desulfurizing agent of U.S. Pat. No. 
3,598,573 in that a lesser amount of desulfurizing agent is required to 
produce a given desulfurization effect. Moreover, the amount of slag is 
even further reduced and thus the iron loss is also further reduced. 
In order to point ot more fully the nature of the present invention, the 
following Example is given of a typical embodiment thereof.

EXAMPLE 
60% of technical grade calcium carbide, 35% of dried diamide lime (a 
mixture of precipitated calcium carbonate with 10% of finely divided 
carbon dispersed therein) and 5% of dried bituminous coal powder were 
ground together in a tube mill to form a powder. The bituminous coal used 
had a 38% by weight volatile content. 
This mixture was used to desulfurize pig iron employing the injecton 
technique. For this purpose an injection lance was introduced into the 
opening of a submarine ladle which contained about 185 tons of pig iron at 
a temperature of 1340.degree. C. The desulfurizing agent was added to the 
melt at an average feed rate of 28 kg per minute and for injection of the 
powdered desulfurizing agent dry air was used at the rate of 11 liters 
(standard temperature and pressure) per kg of mixture fed. The sulfur 
content of the melt was decreased by this treatment from an average 
initial value of 0.048% to an average final value of 0.009%. To effect 
this reduction in sulfur content 3.6kg of desulfurizing agent was used per 
ton of pig iron. 
For purposes of comparison a desulfurizing agent as disclosed in U.S. Pat. 
No. 3,598,573 comprising 60% of calcium carbide and 40% of diamide lime 
was used to desulfurize a similar bath. About 5.0 kg of desulfurizing 
agent per ton of pig iron was required in order to obtain the same degree 
of desulfurization as that obtained with the composition of the present 
invention. 
It is of course to be understood that the foregoing description is intended 
to be illustrative only and that numerous changes can be made in the 
ingredients, proportions and conditions described without departing from 
the spirit of the invention as defined in the appended claims.