Metallurgical apparatus – Means for melting or vaporizing metal or treating liquefied... – By providing – treating or protecting environment – e.g. – vacuum
Reexamination Certificate
2000-03-15
2001-12-18
Kastler, Scott (Department: 1742)
Metallurgical apparatus
Means for melting or vaporizing metal or treating liquefied...
By providing, treating or protecting environment, e.g., vacuum
C266S200000
Reexamination Certificate
active
06331269
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a treatment vessel for liquid metal that is easily oxidable, said vessel comprising one or more devices to prevent gases that are harmful to the liquid metal or apparatuses in the vessel from entering the vessel, said vessel therefore being inert. Harmful gases generally contain oxygen and/or water vapor.
DESCRIPTION OF RELATED ART
Before casting oxidable liquid metals (particularly aluminum, magnesium or their alloys) into semi-finished products it is essential that they be treated so as to degas them (particularly of hydrogen) and/or to eliminate the alkalines or inclusions (particularly solid inclusions resulting from when the metals were oxidized).
The treatment in particular comprises blowing gas into the liquid metal (the gas, that is generally argon to which chlorine gas may or may not have been added, is blown using a dispersion rotor immersed in the liquid metal, the drive shaft and the turbine of the rotor being usually graphite) and/or simply filtering the metal through a bed of filtering material, for example a porous ceramic part or any other filtering medium.
Obtaining a low hydrogen content is crucial for certain metals and is always preferable for most other metals in order to avoid deterioration of mechanical properties. Mechanisms for regassing the liquid metal with the hydrogen that results from said metal reducing the water vapor contained in the surrounding atmosphere can restrict the degassing results.
Similarly, the absence of inclusions that results, in particular, from the liquid metal being oxidized by the oxygen in the air and/or water vapor is often essential in order to avoid defects in parts obtained subsequently, for example thin strips.
Contact between liquid metal and oxygen in the air and water vapor may also have the following consequences:
major melt loss with risk of the alloy composition becoming modified,
risk of plenty of solid dross forming thick layers that could contaminate the metal and be detrimental to the quality of the cast metal and that could erode the shaft of the rotor used to blow the gases, whereas the object is to eliminate the dross,
also the oxidable parts in said treatment vessel becoming prematurely worn by being oxidized, particularly said rotor shaft that is generally graphite.
It is therefore vitally important to avoid any contact between the liquid metal being treated and the surrounding temperature and/or the water vapor, for example during the periods when a constant temperature is maintained between two treatments, in order to improve the performance of the treatment vessel and the quality of the treated metal.
To achieve this aim European Patent EP 216393 and American Patent U.S. Pat. No. 3,870,511 describe maintaining or circulating a light gas flow with a low water or inert gas content, for example argon or similar gas, on the surface of the metal under the lid of the treatment vessel that comprises a leaktight seal. A device of this kind is, however, expensive in terms of the quantity of inert gas it consumes and is insufficiently effective. In particular only the surface of the bath is inert. Furthermore, the standard devices used to ensure a leaktight seal on the vessel are generally difficult and expensive to implement and use if they are to be efficient at preventing incoming air.
German patent DE 4307867 describes protecting liquid metal by creating a vacuum in the vessel. This solution is, however, expensive and difficult to implement and use and could even cause incoming air to enter the vessel.
Japanese patent application JP 62 240724 (Showa) describes a treatment vessel (
1
) and lid (
10
); leaktightness between the vessel and the lid is ensured by a projection (
13
) located on the periphery of lid (
10
) and embedded in a bath of sand (
9
) that is contained in a matching trough (
8
), said trough being located on the periphery of the upper section of vessel (
1
), and by a seal (
12
) capable of withstanding the heat between lid (
10
) and the top of the vessel (
11
). A leaktight device of this kind is not particularly simple to install or operate and the fact that it has two seals means that neither of the seals is sufficient on its own. Furthermore, there is a risk of the liquid metal becoming contaminated by the sand.
Also a pressurized gas box (
22
) is used to ensure leaktightness of the gas-blowing rotor (
4
) through the lid. This has the drawback of consuming gas and requiring the device to be monitored.
German patent application DE 2815011 describes a device for ensuring leaktightness in the vacuum of a melting vessel, the device comprising an inflatable tube (
16
) that operates in conjunction with an elastic leaktight strip (
17
) located on the periphery of said inflatable joint and that is pressed against the inner surface of trough (
21
) to ensure leaktightness of the vessel, the joint that ensures the compression being trapped in a receptacle closed by flange (
18
). Once again this composite leaktightness device is not particularly easy to produce or operate as it is designed to maintain a vacuum.
Another known method of protecting the surface of liquid metal uses a flux of melted salt. This protection is, however, imperfect due to the surface waves caused by the degassing treatment and the risk of inclusions being introduced into the liquid metal. Furthermore, the flux also has to be treated.
The applicant has, therefore, sought an inert vessel that prevents any damaging contact between the liquid metal and the external atmosphere, the vessel being just as compact as standard vessels and even more economical and easier to install and operate.
Also a number of types of vessel are known for treating oxidable liquid metals either continuously or semi-continuously.
These vessels comprise a vessel body that mainly consists of an outer steel shell the inner surfaces of which are lined with refractory brickwork that is relatively porous and that is as inert as possible relative to the metal being treated. The inner layout depends on the treatment(s) to which the metal is subjected.
The vessel brickwork described above can be divided into compartments using dividing walls also made of inert refractory brickwork.
The brickwork is at least partly made with mortar and refractory insulator. The shell comprises a plurality of holes to evacuate the water vapor and drying gases and to avoid the refractory brickwork from becoming distorted when they are dried and baked. The holes are also used to evacuate part of the water vapor that is taken up by the refractory lining and/or insulators or more generally by the lining coatings when the vessel is stopped or drained, another part of the vapor being able to pollute the metal treated in the vessel.
But when, on the other hand, the vessel is stopped the holes play a major part in the linings' recapturing humidity, humidity that will then be at least partly released into the treated metal and damage the quality, as described above.
The vessels also comprise a lid through which equipment passes that is necessary to the treatment method, said equipment often being removable, i.e. it can be removed from the vessel, and/or is movable. In particular, the equipment comprises means for injecting the treatment gas, usually dispersion turbines located at the end of a rotor shaft; the rotor as it is may pass through the lid or it may sometimes be covered with a fixed sheath, all the equipment being removable; the equipment can also comprise means for heating the liquid metal, for example an immersion or similar heater that can be removable and that is partly immersed in the liquid metal, or means for measuring the temperature (thermocouple).
Similarly to the lid, the vessel body comprises a metal outer surface that can be perforated, the inside of which is lined with refractory lining. There is generally a clearance between the lid and the surface of the liquid metal.
The vessels also comprise inlet and outlet means for the liquid metal that may be such that they avoid any contact between the met
Aluminium Pechiney
Dennison, Scheiner Schultz & Wakeman
Kastler Scott
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