Specialized metallurgical processes – compositions for use therei – Processes – Producing or treating free metal
Patent
1991-11-19
1993-05-18
Andrews, Melvyn J.
Specialized metallurgical processes, compositions for use therei
Processes
Producing or treating free metal
266218, C21C 534, C21C 548
Patent
active
052117430
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to the gas treatment of a melt, particularly a metal melt, in a converter, and particularly pertains to a method of intensifying and making more effective the contact between gas and melt, and of achieving effective admixture of the gas with the melt. The invention also relates to an arrangement for carrying out such treatment.
In the manufacture of steel, for instance pig iron, scrap or sponge iron is melted down in different furnaces, converters or like apparatus. The carbon content of the melt is reduced by refinement or decarburization of the melt, wherein oxygen gas is introduced into the melt in the case of certain processes. Gas or gas/powder mixtures can also be introduced into the melts for other reasons. In the majority of processes in which melts are treated with gas, a common desideratum is to achieve effective admixture of the gas with the melt, so that the greatest possible part of the melt comes into contact with the gas. The gas itself shall also be instrumental in achieving thorough admixture of the gas with the melt, to the greatest possible extent.
When manufacturing stainless steel in accordance with the AOD-process (Argon Oxygen Decarburization), the gas is introduced to the melt through a plurality of nozzles disposed in a converter. The nozzles are connected, via a vaporizer, to a storage container which contains gas in a liquid state. The storage container normally consists of a pressurized container having a working pressure of 16 or 25 bars. Because of pressure losses, the working pressure of 25 bars will give a maximum pressure of about 15 bars at the cold ends of the nozzles. In practice, this pressure is even lower, about 12 bars, because the process cannot be carried out with optimum values. The hydrostatic pressure in the converter in the locations of the nozzles can be about 4 bars, and hence the useful pressure-difference of the delivered gas flow is only about 8 bars
A pressure in the order of 8 bars will cause the gas delivered to the melt to bubble up towards the surface of said melt in the form of relatively large gas bubbles. Because of the low pressure, these bubbles will rise relatively close to the wall where the nozzle concerned is situated. Thus, the system operates with a two-phase system, a gas-phase in the form of discrete bubbles and a liquid phase in the form of separate, homogenous lumps of melt, these two phases remaining separate from one another.
The present invention is based on the realization that gas could be better introduced into the melt and admixed therewith if it were possible to deliver the gas at a much higher pressure, therewith providing a greater impulse and improved conditions for achieving a more effective process and of eliminating certain problems associated with present-day techniques and render the process more positive.
When using a higher pressure than is typical at present, at least twice as high and preferably 10-20 times as high as typical pressures, there will be produced a gas jet which penetrates deeply into the melt and which is therewith atomized so as to obtain a jet zone which includes an emulsion-like finely divided mixture of gas and melt which is surrounded solely by melt. The specific surface contact area in the jet zone is very high and the molten material bordering on the jet zone will also be entrained by the jet and admixed therewith, which further enhances turbulence in the melt and the admixture of gas therewith. The result is a highly effective mixing process with effective contact between gas and melt.
The use of relatively low pressures in present-day techniques can also result in a hammering effect on the nozzle and subsequently result in mechanical fatigue in and wear on the refractory lining.
This hammering effect is eliminated when a high pressure is used. The high pressure will also enable the diameters of the nozzles to be considerably increased. In turn, this means that if a fault should occur as a result of fatigue, or for some other reason, and cause a nozzle to be forced
REFERENCES:
patent: 2467413 (1949-04-01), Wildhack
patent: 4321088 (1982-03-01), Boehm
patent: 4382817 (1983-05-01), Vayssiere et al.
patent: 4627243 (1986-12-01), Schaub
patent: 5076837 (1991-12-01), Rathbone et al.
AGA Aktiebolag
Andrews Melvyn J.
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