Specialized metallurgical processes – compositions for use therei – Compositions – Solid treating composition for liquid metal or charge
Reexamination Certificate
2002-10-15
2004-09-21
Andrews, Melvyn (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Compositions
Solid treating composition for liquid metal or charge
C075S561000
Reexamination Certificate
active
06793708
ABSTRACT:
FIELD OF THE INVENTION
A slag composition comprised of from about 85 to about 99 weight percent of steelmaking slag and from about 1 to about 15 weight percent of a foaming additive. The foaming additive is comprised of from about 20 to about 80 weight percent of a source of elemental carbon, from about 5 to about 80 weight percent of calcium carbide, and from about 25 to about 75 weight percent of an oxide compound. The weight/weight ratio of the source of elemental carbon/calcium carbide is at least 2.
BACKGROUND OF THE INVENTION
The present invention is directed to providing an optimal composition for achieving desired properties in a steelmaking slag within an electric arc furnace, including the creation of appropriate basicity and chemistry for optimal foaming at each stage of the process, as well as a method and system for accomplishing the optimal composition.
In electric arc melting of steel the energy efficiency and overall cost is greatly improved by the creation of a basic foamy slag within the furnace. Foaming of the slag in order to engulf the electric arc is well known in the art, and is described, for example, in U.S. Pat. No. 4,447,265 and in U.S. Pat. No. 4,528,035. The procedures described involve the injection of carbonaceous material into the furnace slag to cause carbon monoxide gas evolution, and the escaping gas causes the slag to effervesce or foam. As a result of the effervescence or foaming, the slag is increased in its volume, engulfing the electric arcs and improving electrical and heat transfer efficiency.
In U.S. Pat. No. 4,473,397, the inventors discuss the injection of calcium carbide into a steel making convertor to decrease the amount of slag foaming. Contrary to that finding, U.S. Pat. No. 5,279,639 teaches that the introduction of calcium carbide into a slag with high oxygen potential causes slag foaming. Neither patent teaches the use of calcium carbide to create a foamy slag in an electric arc steel melting furnace.
In U.S. Pat. No. 5,279,639 the use of calcium carbide as the reductant in ladle slag treatment is discussed. The invention discloses that calcium carbide is also an effective slag foaming agent due to the release of carbon monoxide during reaction in the slag. This reaction product provides stirring and foaming of the slag with the benefits of mixing, enhanced reaction rates and arc stability, as described in the patent. It was noted that the release of gas was sustained over a period of time, and that foaming was sometimes considered excessive. The furnace slag could benefit from the sustained foaming observed during the use of calcium carbide. The present invention teaches a system and method to use calcium carbide and other materials to advantage in the creation of optimum foamy slag conditions in the electric arc melting furnace.
As described in the prior art, the use of injected oxygen aids in the production of carbon monoxide gas. Also described in the prior art, calcium oxide is sometimes injected along with the carbon and the oxygen in order to raise the slag basicity and stabilize the foam that is created. In addition, other cost benefits are derived such as reduced refractory lining wear, higher productivity, reduced graphite electrode consumption, and increased yield of iron units from the melted scrap. Foamy slag practice is arguably one of the greatest improvements to electric arc steel melting technology of the 20th century.
In the steel melting process, the chemical and physical properties of the slag can influence the economics of production and the quality of the final steel product. The slag is formed from several sources, possibly including added calcium and magnesium oxides, eroded refractory, residual slag from a previous steel batch, alloying elements that are present in the scrap charge that are oxidized during the steelmaking process, gangue materials that are present in the charge carbon or injected carbon materials, and entrained oxides that coat the scrap charge or are otherwise introduced into the process. Since the amount of these contributors to the slag may vary considerably from batch to batch, the chemistry of the slag on top of the steel may be quite variable, and hence the ability to cause and maintain a stable foamy slag may also be quite variable. As a result, the current art of slag foaming in the electric arc steel melting furnace is highly variable in its effectiveness.
The ideal chemistry for optimum slag foaming is different at different times in the process. Initially, while the iron oxide content of the slag is low, a lower basicity slag is required. Conversely, as the process progresses and iron oxide content of the slag increases, a higher basicity slag is better for a sustained foamy slag. Current technology, however, does not recognize or provide for this variance. Several characteristics are required in the furnace slag to optimize slag foaming at all stages of the process:
1. Slag with both fluid and solid components in appropriate ratio
2. Adequate viscosity to retain sufficient fraction of gas for a stable foam
3. Adequate sources of carbon and oxygen to sustain the foam
4. Control of FeO content of the slag by independent control of oxygen and carbon injection rates
5. Gas generation rate sufficient for foaming but not enough to cause foam collapse
6. Non-corrosive to refractory lining
7. Minimum slag weight that can accomplish required slag volume and chemistry
Again, the current state of the art does not consistently and reproducibly provide a foamy slag on each steel batch of steel that optimally meets the above criteria. The amount and composition of indigenous slag components are not accurately known, and the intentionally added components are typically designed around an average starting slag composition and an average desired final slag composition. This inaccuracy is particularly troublesome when attempting to create an optimum foamy slag in the furnace.
SUMMARY OF THE INVENTION
In accordance with this invention, there is provided a slag composition comprised of from about 85 to about 99 weight percent of steelmaking slag and from about 1 to about 15 weight percent of a foaming additive. The foaming additive is comprised of from about 20 to about 80 weight percent of a source of elemental carbon, from about 5 to about 80 weight percent of calcium carbide, and from about 25 to about 75 weight percent of an oxide compound. The weight/weight ratio of the source of elemental carbon/calcium carbide is at least 2.
In accordance with another aspect of the present invention, there is provided a method of forming foam in the slag produced during electric arc furnace steelmaking comprising the addition to said slag of an effective amount of a composition comprising from about 20 to about 80 wt % of a carbon source, from about 5 to 80% calcium carbide, the remainder consisting of a mixture selected from the group lime, dolomite, limestone, dolomitic limestone, complex low melting point oxide, byproduct slag, steelmaking slag, and baghouse dust.
In accordance with yet another aspect of the present invention, there is provided a method of creating and maintaining a foamy slag during the earlier stage of the electric arc furnace steelmaking cycle by concurrent addition of a carbon source, an oxygen source, and a complex oxide of melting point less than 2800° Fahrenheit (° F.).
In accordance with a further aspect of the present invention, there is provided a method of creating and maintaining a foamy slag during the later stage of the electric arc furnace steelmaking cycle by concurrent addition of a carbon source, an oxygen source, and a source of material that reacts endothermicly within the slag and concurrently increases slag basicity.
One aspect of the invention is based on the discovery that the monitoring and control of one or more of several key characteristics related to foamy slag formation leads to more consistent and optimal slag conditions. This discovery avoids problems that arise in conventional processes and results in improved control of chemical and physical prope
Jones Jeremy A. T.
Kemeny Frank L.
West William J.
Andrews Melvyn
Greenwald P.C. Howard J.
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