Method and Composition for the Desulfurization of Molten Metals

The present invention relates to compositions for the desulfurization of molten metals, especially of molten pig iron. The compositions contain calcium carbide and/or calcium cyanamide with an additive increasing the desulfurization effect.

Sulfur contained in pig iron impairs especially the mechanical properties of ferrous materials and therefore appreciable amounts of it in such materials are undesirable. Since the selection and procurement of starting materials for the manufacture of low-sulfur ferrous products is becoming increasingly difficult, the molten irons in general must be subjected to a desulfurizing treatment. However, in the production of steel from pig iron the removal of the sulfur from the molten iron is difficult and uneconomical; it is more advantageous to initially reduce the sulfur in the pig iron to sufficiently low levels.

Known methods of desulfurizing molten iron outside of the melting unit make use of desulfurizing agents consisting of two or more solids in fine powdered form. These are fluidized by means of a carrier current of gas--air, nitrogen, argon, natural gas and other neutral gases or gases having a reducing action may be used--and blown into the molten iron. The reaction between the solid desulfurizing agent and the sulfur bound to the iron takes place on the surface of the desulfurizing agent.

Also known are desulfurization processes in which calcium cyanamide or calcium carbide are blown into the molten iron together with fine powdered carbon materials such as soft coal, anthracite, brown coal, coke, petroleum coke and other products containing carbon, which provide a reducing atmosphere conducive to desulfurization.

A definite advance has been achieved by desulfurization with combinations of calcium cyanamide or calcium carbide and diamide lime (W. German Pat. Nos. 1,583,268 and 1,758,250). Such agents not only create within the molten metal the desired reducing atmosphere in which the desulfurizing agent produces its effect without delay, but also, by the simultaneous yielding of gas from the diamide lime, they promote the uniform distribution of the desulfurizing agent into all parts of the melt and accelerate the precipitation of the desulfurization products.

In spite of these good results, there has been a need in metallurgical plant practice to improve desulfurizing agents based on calcium cyanamide and/or calcium carbide with regard to the degree of desulfurization which they achieve and with regard to their accuracy and reliability in achieving low sulfur content levels.

The present invention provides a desulfurization composition capable of achieving these objectives.

Essentially, the invention comprises a desulfurizing composition based on calcium carbide and/or calcium cyanamide and containing an agent which yields hydrogen and/or water at the temperature of the molten metal being treated with the solid desulfurizing compositions.

Suitable agents are, for example:

a. For yielding H.sub.2 : calcium hydride and the hydrides of other alkaline earth and alkali metals, organic polymers containing hydrogen, e.g., polyolefins such as polyethylene and polypropylene, polyamides, polystyrene, and polyacrylonitrile, either individually or in mixtures, as well as urea, guanidines, biguanidines, dicyandiamide, dicyandiamidine and melamine.

b. For yielding H.sub.2 O: calcium hydroxide (hydrate of lime, Ca(OH).sub.2), alkaline earth borates containing water of crystallization, such as colemanite and pandermite, aluminum hydroxides, perlite, kaolin, clays and other such minerals, carbohydrates such as sugar and starch, solid organic oxygen compounds such as phthalic acid and glycolic acid, organic polymers containing hydrogen and oxygen such as polyvinyl alcohol and polyvinyl acetate, and polyalcohols such as sorbitol.

The organic polymers may be prepared by many different polymerization processes and in many different degress of polymerization. The nitrogen simultaneously yielded by nitrogen-containing additives during the treatment does not impair the desulfurization effect.

Hydrate of lime is preferred as the H.sub.2 O yielding agent, since it is available at low cost virtually anywhere in the world without high transportation cost.

The decomposition of the powdered agent of the invention forms a desirable reducing atmosphere even before the actual desulfurizing agent begins its action. The agents decompose spontaneously at the temperatures of the molten metal (from about 1200 to 1450.degree. C. in the case of iron) with the formation of water or hydrogen, nitrogen in some cases, and in some cases very finely divided carbon. The carbon, in the active form in which it is thus produced, exercises an advantageous action partially by binding the small amounts of oxygen dissolved in the iron, but mainly by forming carbon monoxide with the oxygen content of the desulfurizing agent or reacting with the oxygen in the carrier gas, or by forming carbon dioxide from carbonate components. The gases that are produced intensify the turbulence in the melt, increase the movement of the bath and assure the reducing status.

It has been found desirable for the amount of the agents to range from 0.3 to 60% by weight, the amount of hydrogen gas yielding substances being best between 0.3 and 20%, the amount of water yielding substances between 1 and 60%, preferably 5 and 40%; in the case of carbohydrates 1 to 30% will suffice.

According to a special embodiment of the invention, the desulfurizing composition of the invention will additionally contain deoxidizers such as aluminum or calcium silicon in amounts of up to about 10 % by weight or carbon in amounts of up to about 20 % by weight. In this manner, the desired reducing atmosphere is favored. The basic desulfurizing agent, calcium carbide or calcium cyanamide, is present in an amount of at least 30%, preferably at least 45%, by weight.

Especially advantageous mixtures have the following composition as shown in the indicated tables below showing performance data.

All percentages given refer to the weight, unless otherwise specified.

The desulfurizing agents of the invention are prepared by mixing the components, whereupon moisture adhering to the agent reacts with the basic desulfurizing agent with the formation of acetylene (in the case of CaC.sub.2) or Ca(OH).sub.2, so as to assure that the agent can contain only bound H.sub.2 O.

The desulfurizing agents of the invention provide additional effects when they are used, so that the amount of desulfurizing agent used is less than it has been in the case of the agents known hitherto, or the degree of desulfurization is greater for the same amount. Final sulfur contents are attained of 0.02 % S.sub.E to 0.01 % S.sub.E for a starting sulfur content of 0.04 to 0.15 % S.sub.A, with the accuracy desired in modern-day practice.

With the mixtures of the invention equally good results are achieved in the desulfurization of molten pig iron and ferrous alloys such as ferrochromium and ferronickel, and also in nonferrous molten metals such as nickel, copper and the like.

The invention will be explained with the aid of the following examples. Examples 1-6 contain comparisons with desulfurizing agents of the prior art, and Examples 7-24 show the effect of the desulfurizing agent of the invention.

The rest of the agents named are also usable in the same manner. Which agent is actually used will vary locally according to economic criteria.

The results given in the following table are averages obtained from up to 6 desulfurization tests where experiments on a pilot plant scale (HT) are involved. Where the results are based on factory tests (B) the desulfurization was performed in torpedo ladles containing approximately 200 metric tons of pig iron, based on an average of more than 20 treatments.

In all experiments, the powdered desulfurization agents were blown into a pig iron melt through refractory-jacketed blowing lances using air as the carrier-gas.

The .alpha.-value given in the table is a characteristic which expresses the consumption of desulfurization agent in kilograms per metric ton of pig iron and a decrease of 0.01 % in the sulfur content of the pig iron.

It will be understood that the foregoing specification and examples are illustrative but not limitative of the present invention inasmuch as other embodiments within the spirit and scope of the invention will suggest themselves to those skilled in the art.