US2991063A

US2991063A - Apparatus for the continuous vacuum treatment of metals

Info

Publication number: US2991063A
Application number: US799233A
Authority: US
Inventors: Scheuer Ernst; Howells Thomas John
Original assignee: International Alloys Ltd
Current assignee: International Alloys Ltd
Priority date: 1958-03-28
Filing date: 1959-03-13
Publication date: 1961-07-04
Anticipated expiration: 1978-07-04

Description

July 4, 1961 E. SCHEUER ET AL 2,991,063

APPARATUS FOR THE CONTINUOUS VACUUM TREATMENT OF METALS Filed March 13, 1959 5 Sheets-Sheet 1 July 4, 1961 E. SCHEUER ET AL 2,991,063

APPARATUS FOR THE CONTINUOUS VACUUM TREATMENT OF METALS Filed March 13, 1959 3 Sheets-Sheet 2 APPARATUS FOR THE CONTINUOUS VACUUM TREATMENT OF METALS Filed March 13, 1959 3 Sheets-Sheet 3 outside pressure at the temperatures in question.

. 2 991 063 APPARATUS FOR 'IIlE C ONTIN'UOUS VACUUM TREATMENT OF METALS Ernst Scheuer, Stone, and Thomas John Howells, Aylesbury, England, ass'gnors to International Alloys Limited, Aylesbury, England- Filed Mar. 13, 1959, Ser. No. 799,233 Claims priority, application Great Britain Mar. 28, 1958 i 5 Claims. (Cl. 26634) 2,991,063 Patented July 4., 1961 viding for electrical heating inside the space between said shell at said tube.

feature of the invention resides in bringing about the liquid-tight joint by shaping the outer shell as a cup surrounding the suitably shaped bottom end of the inner 7 tube, said cup having a; tubular expendable extension This invention relates to apparatus for the continuous In this specification reference will be made to aluminium alloys as the liquid metal to be treated, but we wish it to be understood that the invention extends to the treatment of other metals such as lead, copper and antimony for example. Similarly reference will be made to graphite as avmost suitable refractory material of con- 'struction, but we do not limit ourselves to this material as other refractories such as silica carbide, chromium .oxide and so forth may be used.

It is known to degas or free from the more volatile constituents molten metals and alloys in a continuous way by providing two containers, one for the untreated and the other for the treated metal, connected by an inverted U-tube the two legs of which dip into the molten metal in the containers and by applying a vacuumat the top of the U-tube, at which top a small chamber is usually provided. If the level of metal in thecontainer for the untreated metal is maintained higher than the level of metal in the container for the treated metal, and

the height of the legs of the U-tube is not greater than corresponds to atmospheric pressure expressed .in' the height of metal, a continuous flow of metal through the evacuated top chamber from the first container into the second container will occur. The two tubes leading to and from the chamber at the top of the U-tube are hereinafter called the barometric legs of the apparatus.

Whilev the principle of this method is consideredv tech nically sound and economic, its introduction into practice has proved diflicult, especially in the case of metals of high melting point that form alloys with metallic ma- :terials' of construction, e.g. aluminium, where it has been impossible to find a material for. thenbarometric legs which is impermeable to. gas, is not dissolved by the liquid aluminium, and is strong enough to withstand the Materials such as graphite or silicon-carbide are inert to aluminium at the temperatures in question andare strong istruction of. apparatus that will. overcome the disadvantage inherent in the permeability of materials of the (kind just referred to. V g L To the attainment of this object the invention provides for the continuous vacuum treatment of. liquid metals a barometric leg consisting of an inner tube madeof material which is permeable to gas and impermeable and resistant to said liquid metal at the temperature of operation, said inner tube being surrounded by a vacuum-tight shell, the bottom end of the shell being shaped foi' being dipped -into liquid metal and being connected to the bottom of said inner tube by aliquid-tight joint, and provision being made to'maintain the metal in the liquid state inside said tube at the desired temperature by proconductor of which dips into the sump of liquid metal.

i In practice the temperature withinthe cup is controlled so as to freeze part of the metal thereby providing a solid liquid-tight joint between the porous tube and the shell.

,Another featureofthe invention resides in having the vacuum-tight shell large enough to afford access through a manhole therein provided'for the maintenance'or re.- placement ofthe heater without dismantling the inner tube of the-barometric'leg. i t

Various forms of electric heaters may be employed, for instance resistance or induction heaters.

By way of example one embodiment of the invention is illustrated on the accompanying. drawing to which reference is now made.

FIG. 1 is a diagrammatic plan of this embodiment.

FIG. 2 is a longitudinal section on a larger scale in the planes AA of FIG. 1. Y

FIG. 3 shows on a largerscale and in longitudinal section details of one of the bottom seals.

FIGS. 4-and 5 show respectively in longitudinal section and plan details of the supports of the barometric legs.

FIG. 6 is a longitudinal section on a larger scale of one of the barometric legs.

The whole vacuum part of the apparatus is enclosed in a welded mild steel shell 1, the temperature of which is kept so low as to hold the mechanical properties. of the steel at a safe level as regards. danger of creep. The shell is large enough to allow access to the barometric legs by means of a manhole for maintenance purposes. This is important as it is desirable to leave certain parts of the inner structure undisturbed while others may need occasional replacement and/or service.

Reference 2 is a sump containing liquid metal to be treated; for instance it may be the crucible of a bale-out furnace. The bottom end 3 of a feed tube 4 of the apparatus dips into this sump, the top end of this tube leading through anoverflow5 to an evaporator tray 6. Another overflow 7 leads theliquid metal from the evaporator -tray into a tube 8 of the discharge leg, the bottom end of which tube dips into a liquid metal sump 9. This discharge sump 9 is similar to the feed sump 2. but is arranged at alower level.

The two sumps are arranged in a manner so as to allow of raising and lowering them with respect to the tubes dipping into them. This enables adjustment of the height of the liquid metal column in each of the legs to suit the outside barometric pressure and exposure of the bottom ends of the tubes for repair or replacement.

The principle of operation of the follows: I

Both sumps 2 and 9 are filled with liquid metal. The system is then evacuated by a vacuum pump (not shown) connectedat 11 to the steel shell in a condenser 10. The atmospheric pressure will then push up liquid metal in the two barometric legs until the hydrostatic pressure of the metal columns counterbalances it. The metal level of the feed sump 2 is then raised by adding liquid metal to this sump until metal starts overflowing at 5 into the evaporator tray 6. The liquid metal in the sump 9 is held by an outflow L2 or any other suitable arrangement at such a level that the metal level in the discharge leg 8 remains well below the overflow 7. Liquid metal is now added either in a controlled stream or intermittently in measured apparatus is as responding to the inflowing metal overflows at 5 into the liquid metal, the time of exposure in the evaporator tray is controlled. The treated metal flows from the evaporator tray through the discharge tube or leg 8 and the discharge sump 9 from which latter it is removed continuously or at intervals. The vapours of volatile substances removed from the metal in the evaporatortray flow upwards into the condenser 10 which is equipped with suitable means of temperature control, e.g. insulation and cooling arrangements (not shown) and in which the less volatile part of the vapour condenses (for ex ample Zn and Mg) whilst gaseous substances such as sulphur dioxide, carbon monoxide and hydrogen are removed through the vacuum pump.

Difficulties in the continuous feeding in and out of liquid metals of higher melting points arise from the necessity of avoiding in the path of the liquid metal the use of metallic materials and of the normal ceramic material such as glass and porcelain used in the construction of vacuum-tight containers. The only material which is at the same time chemically resistant, refractory, and suitable for machining to accurate shape and sufliciently resistant against mechanical stress and thermal shock, is graphite; silicon carbide may also be useable but has to be moulded to final shape as it cannot be machined. Unfortunately these materials are porous, so that they cannot be made to form the wall of a vacuum container.

Considering now as an example the case of liquid aluminium and graphite, the main constructional problem of the feed and discharge tubes lies in effecting between the steel shell 1 and each of the graphite tubes 4 or 8 carrying the metal a seal that will prevent liquid metal from penetrating into the space. between the shell and the graphite tube. This seal has to allow for the difference in the thermal expansion and in the mechanical properties of graphite and steel. We have found the following arrangement effective:

A short and thick walled steel tube 3, FIG. 3, of about 1%" internal diameter and /2" wall thickness is welded or tightly joined in some other way into a conical steel cup 14 flanged at 16 to the bottom of the steel shell 1 of the apparatus. Other metals of sufliciently high softening temperatures and resistance to liquid aluminium may also be used in place of steel. This tube 3, suitably coated with a refractory wash, dips into the liquid metal in the sump 2 or 9 as the case may be. In the conical cup 14 is arranged a second conical cup 15 of the same metal and held by the same main flange 16 but independent of the cup 14. This cup 15 holds the internal graphite (17 in FIG. 3) so that the cup 14 can be removed and replaced without interfering with the refractory parts. Inside the cup 15 is seated a graphite cup 17, the outside shape of which fits the inside shape of the cup 15 with a gap of less than A". The cup 17 contains a central boss, and the wall thickness of the bottom round the boss and of the sides is dimensioned in such a way as to provide as much resistance to radial heat flow as can be achieved without endangering the mechanical resistance of the cup. The graphite cup 17 is held down against the upward thrust exerted by the liquid metal entering under atmospheric pressure by a steel ring 18 fitted between the main flange 16 and the flange of the cup 15. Into the central boss the graphite tube 4 or 8 of say 3" diameter and A "wall thickness is screw-threaded and tightened against penetration of liquid metal. This graphite tube is extended by means of sealed screw-threaded or other suitable connections (FIG. 6) to the height necessary to balance the atmospheric pressure by the liquid metal head. It ends at the top in the aforesaid overflow 5 or 7, FIG. 2.

The operation of the described seal is as follows: When the liquid metal enters the bottom end of the tube 4 or 8 it penetrates into. the gap between the steel cup 15 and the graphite cup. 17. The temperature distribution in. this gap is controlled in such a way as to keep the centralpartof the metal above its melting point, whilst in 4 the zone indicated at 20 (FIG. 3) near the top edge of the cups, the temperature drops below the freezing point of the metal. As soon as the liquid metal enters this part of the gap it will freeze and seal the gap. There is no rigid connection between the graphite part and the steel shell. Should the gap open either by distortion of the steel shell or differences in thermal expansion between the steel shell and the graphite, the leak will automatically be sealed by liquid metal entering into and freezing in the expanded gap.

The proper operation of the whole arrangement requires keeping the temperature of various parts of the apparatus at different and well defined levels. For this purpose we have found the following heating and cooling arrangements suitable: Electric resistance heaters 21 are arranged inside the vacuum-tight shell around the graphite tubes 4, 8 from the bottom of the cups 17 to the overflows 5 and 7. In addition, conventional heating, for example gas or electric (not shown in the drawing), is provided around the outside of the cups 14 where they join the steel tubes 3 and for the upper parts of the steel tubes 3 themselves. Furthermore, cooling by air blast (not shown in the drawing) is provided outside the zone 20 of the steel cup 14 to assist in maintaining the required lower temperature in this zone.

The metal temperature in the barometric legs is preferably kept at not les than 100 C. above the melting point of the alloy to be treated, in order to prevent undue wear of the steel tubes 3 dipping into the sumps 2, 3 and of the graphite tubes 4, 8 and the electric heating resistors 21. Should a higher temperature be required for the actual evaporation process, it can be produced by heating the metal on its way to and through the evaporation section namely in the overflow 5A, FIGS. 1 and 2. If desirable a cooling section 7A can be arranged between the evaporator tray and the discharge tube 8 in order to prevent unnecessary wear of the discharge leg.

Each graphite tube 4 or 8 together with its bottom cup is a rather sensitive assembly and should be disturbed as little as possible. It has therefore been found necessary to make arrangements for the protection of this vital part.

In order to prevent mechanical stress being set up by thermal expansion between the outer shell and each graphite tube the graphite tube is supported only on the carbon base, namely the central boss of the cup 17, FIG. 3; it is steadied against bending stresses by suitable guide rings 25, FIG. 6, supported on brackets 24, FIGS. 4, 5 and 6, which are attached to the outer shell 1.

On the same brackets 24 are supported the electric resistance heaters 21, FIGS. 3 and 5, and insulation blocks or radiation shields 22. As the resistance heaters may sometimes require servicing or replacing both they and the surrounding insulation rings or radiation shields are split longitudinally to enable them to be removed from the brackets without the graphite tubes being disturbed. For the same reason the outer shell is made large enough to contain both barometric legs and to permit access to the space around them by manhole.

We claim:

1. For use in the continuous vacuum treatment of a liquid metal, an inverted U-tube apparatus each barometric leg comprising an inner tube of a material permeable to gas and impermeable and resistant to said metal at the temperature of operation, a sump for said liquid metal below said tube, a vacuum-tight shell surrounding and spaced from said tube and having its lower end dipping into said sump, means for enabling a liquid-tight seal of said metal in the frozen state to be formed above the level of the liquid metal in said sump between the lower end of said tube and the inner wall of said vacuum-tight shell, and electrical heating means inside the space between said shell and said tube to maintain the metal inside said tube in the liquid state at the desired temperature.

2. For use in the continuous vacuum treatment of a 5 liquid metal, an inverted U-tube apparatus each barometric leg comprising an inner tube of a material permeable to gas and impermeable and resistant to said metal at the temperature of operation, a sump for said liquid metal below said tube, a vacuum-tight shell surrounding and spaced from said tube, means for enabling a liquid-tight seal of said metal in the frozen state to be formed above the level of the liquid metal in said sump between the lower end of said tube and the inner wall of said vacuumtight shell, said enabling means including a cup-shaped portion of said shell surrounding the bottom of said inner tube and presenting a tubular expendable extension dip ping into said sump, and electrical heating means inside the space between said shell and said tube to maintain the metal inside said tube in the liquid state at the desired temperature.

3. For use in the continuous vacuum treatment of a liquid metal, an inverted U-tube apparatus each barometric leg comprising an inner tube of a material premeable to gas and impermeable and resistant to said metal at the temperature of operation, a sump for said liquid metal below said tube, a vacuum-tight shell surrounding and spaced from said tube, means for enabling a liquid-tight seal of said metal in the frozen state to be formed above the level of the liquid metal in said sump between the lower end of said tube and the inner wall of said vacuumtight shell, said enabling means including a cup-shaped portion of said shell surrounding the bottom of said inner tube and presenting a tubular expendable extension dipping into said sump and an inner cup of said permeable material, said cup-shaped portion and cup receiving therebetween a flash of said metal sealing between said tube and shell, and electrical heating means inside the space between said shell and said tube to maintain the metal inside said tube in the liquid state at the desired temperature while freezing part of the metal inside said cup to afford a solid liquid-tight joint between said tube and said shell.

4. For use in the continuous vacuum treatment of a liquid metal, an inverted U-tube apparatus each baromet ric leg comprising an inner tube of a material permeable to gas and impermeable and resistant to said metal at the temperature of operation, a sump for said liquid metal below said tube, a vacuum-tight shell surrounding and spaced from said tube and having its lower end dipping into said sump, means enabling a liquid-tight seal of said metal in the frozen state to be formed above the level of the liquid metal in said sump between the lower end of said tube and the inner wall of said vacuum-tight shell, an electrical heater inside the space between said shell and said tube to maintain the said metal in the liquid state inside the said tube at the desired temperature, and removable closure means giving access to the interior of said shell for the maintenance of said heater without dismantling said tube.

5. For use in the continuous vacuum treatment of a liquid metal, an inverted U-tube apparatus each barometric leg comprising an inner tube of a material permeable to gas and impremeable and resistant :to said metal at the temperature of operation, a sump for said liquid metal below said tube, a vacuum-tight shell surrounding and spaced from said tube and having its lower end dipping into said sump, means for enabling a liquid-tight seal of said metal in the frozen state to be formed above the level of the liquid metal in said sump between the lower end of said tube and the inner wall of said vacuum tight shell, electric heating elements inside the space between said shell and said tube, elements of insulating material screening said heating means, and means supporting said elements for replacement without dismantling said tube, said elements being subdivided longitudinally.

References Cited in the file of this patent UNITED STATES PATENTS Dumas June 13,1933