Specialized metallurgical processes – compositions for use therei – Processes – Producing or treating free metal
Reexamination Certificate
2000-05-24
2003-03-04
King, Roy (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Processes
Producing or treating free metal
Reexamination Certificate
active
06527826
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method for operating furnace equipment for magnesium alloys in which a bath surface is covered with a protective furnace gas containing a sulfur compound.
THE PRIOR ART
In installations where liquid magnesium is present it is necessary to take special precautions to reliably prevent the access of atmospheric oxygen because magnesium is extremely combustible in the liquid state. SF
6
is used as such a protective furnace gas, for example, which reacts with the magnesium of the melt and forms a protective layer which protects the melt from the access of atmospheric oxygen. Occasionally, mixtures of SF
6
and nitrogen are used for this purpose. Since SF
6
concerns a greenhouse gas, its use is problematic for reasons of environmental protection. Moreover, the costs for this gas are relatively high.
It has already been considered as an alternative to use SO
2
as a protective furnace gas. SO
2
is available at relatively low cost and, like SF
6
, forms a protective layer on a magnesium melt. The application of SO
2
has been prevented up until now because it concerns an extremely unpleasant smelling gas which in medium to high concentrations is detrimental to health and promotes corrosion. During the use of SO
2
in the conventional manner it is not possible to prevent the escape of gas from the furnaces or the like which leads to an impermissible burden on the ambient environment. In particular, maximum workplace concentrations will be exceeded by far.
It is further known to use gas mixtures of SO
2
and dried air as a protective furnace gas for magnesium furnaces. It has not yet been managed, on the one hand, to control the toxic and corrosive effect of SO
2
and, on the other hand, to realise a secure operation of the furnace.
It is the object of the present invention to provide a method which, on the one hand, allows a secure treatment of magnesium melts and, on the other hand, causes the lowest possible burden on the environment.
SUMMARY OF THE INVENTION
This object is achieved in accordance with the invention in such a way that a mixture of SO
2
and an inert gas is used as a protective furnace gas, with the volume share of SO
2
being set precisely to a value which is in a range of between 0.3% and 1.5%, preferably between 0.5% and 1.0%. It was surprisingly noticed that in the case of SO
2
, there is a small range in which there is a sufficient protective effect on the one hand and a burden on the environment can be substantially prevented on the other hand. The relevant aspect of the present invention is the setting of a precise value of the SO
2
concentration in the protective furnace gas. Since very low SO
2
concentrations are used, precise process control and the avoidance of fluctuations in the gas composition is very important to prevent fires.
Nitrogen is used particularly preferably as the inert gas. Nitrogen is available at low cost and is not critical to the environment. In a particularly preferable embodiment of the method in accordance with the invention, it is provided that the mixture of SO
2
and the inert gas is performed in an air-conditioned room whose temperature is kept above 22° C., and preferably in a range of between 25° C. and 35° C. It has been noticed that a satisfactory mixture of SO
2
and nitrogen is only possible from a certain minimum temperature at a performance pressure >2 bars. Moreover, temperature-induced fluctuations in volume and pressure of the involved gases can be securely prevented by the mixture in an air-conditioned room. A temperature of approx. 30° C. is particularly preferable.
Moreover, the present invention relates to an apparatus for producing a protective furnace gas for furnace equipment for magnesium alloys with a storage tank for a sulfur compound, a storage tank for an inert gas and a mixing device. The apparatus in accordance with the invention is characterized in that the mixing device is arranged to provide a precise flow rate control of SO
2
and nitrogen and is arranged in an air-conditioned room. Safety gas cells are provided in an air-conditioned room which receive the SO
2
cylinders. Nitrogen is taken from a conventional tank. The mixture of SO
2
and nitrogen is performed through electronic mass flow meters which are arranged in the air-conditioned room. The gas mixture thus produced is conveyed to the respective furnace equipment via pipelines and adjusted to consumption by way of local control systems. The furnace equipment can concern smelting furnaces, holding furnaces, dosing furnaces and pig casting belts for magnesium alloys.
SO
2
sensors are provided in the safety gas cells which already respond to low SO
2
concentrations. Once an SO
2
cylinder has been emptied, the same is scavenged with nitrogen together with the pipelines in order to exclude any health hazards during the exchange of the cylinders. In this manner it can be prevented reliably that any SO
2
escapes into the environment.
As a result of the low SO
2
concentration of the protective furnace gas, any pollution of the environment and any annoyance caused by bad smell, particularly in the furnace area, can be securely prevented. Secure operations can be ensured by the precisely set mixture ratio.
REFERENCES:
patent: 1898969 (1933-02-01), Barstow
patent: 5087485 (1992-02-01), Cho
patent: 2018407 (1971-02-01), None
patent: 4203193 (1993-08-01), None
patent: 9614439 (1996-05-01), None
J.F. Heffron et al., “Practical Aspects of Using Sulphur Hexafloride in Fluxless Melting of Magnesium and Its Alloys” in North American Die Casting Assn. Conference: 17thInt. Die Casting Congress . . . , Bd. 94, No. 2, Oct. 18-21, 1993, pp. 322-329.
J. W. Fruehling et al., “Protective Atmospheres for Melting Magnesium Alloys” inAFS Transactions, vol. 56, No. 2, Aug. 1969, pp. 159-164.
H. Gjestland et al., “Use of SF6in the Magnesium Industry: An Environmental Challenge” inInstitute of Materials Conference: Proceedings of the Third International Magnesium Conference, Apr. 10-12, 1996, pp. 33-41.
Y. P. Aleksandrova et al., “Interaction of Magnesium with Gases” inMet. Sci. Heat Treat., vol. 19, No. 3-4, Mar.-Apr. 1997, pp. 218-221.
Dykema Gossett PLLC
King Roy
McGuthry-Banks Tima
TCG Unitech Aktiengesellschaft
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