US2355343A - Furnace for the electrothermal production of magnesium - Google Patents

Description

Aug. 1944- A. VON ZEERLEDER ET AL 2,355,343

FURNACE FOR THE ELECTROTHERMAL PRODUCTION OF MAGNESIUM Filed Sept. 17, 1942 &

\- Inventor: REA

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Patented Aug. 8, 1944 FURNACE FOR THE ELECTROTHERMAL PRODUCTION OF MAGNESIUM Alfred yon Zeerleder, Neuhausen-on-the-Rhelnfall, and Werner Sn, Sous-Geronde, Chippis, Valais, Switzerland, aulgnors to Societe Anonyme pour llndustrie de lAluminum, Chippls,

Switzerland Application September 17, 1942, Serial No. 458,750

- In Switzerland July 23, 1941 s'oiiimr. (oi. 13-8) cape from the mixture and condense on a less warm part of the furnace or in a special chainber outside the reaction chamber.

Furnaces for the electrothermal production of magnesium are already known in which an ironcontainer heated from the exterior holds the mixture to be reduced. Such furnaces are described for instance in the British Patents 526,669 and 412,417. Further furnaces are known which allow the continuous production of magnesium by electrothermal reduction and in which the reactive mixture is conveyed in a continuous stream past heating resistances at a definite distance therefrom. Such furnaces are described in the British Patents 439,535 and 450,064. They are however provided with very delicate parts which become easily injured and necessitate a careful attendance. Furthermore, a rotary furnace for the electrothermal production of mag-- nesium has become'known for instance through the British Patent 510,196, which furnace is also very sensitive.

The object of the present invention is a furnace for the electrothermal production of magnesium in which the heating of the charge is effected from the inside by means of an elongated electric heating element which is disposed in a space separated from the reaction chamber by a metallic protecting tube electrically united with the heating element at one of its ends, the electric connections to the poles being disposed in such a manner that the end of the tube united with the heating element is free; the tube serves therefore also as supply or return line for the current going to one end of the heating element or coming from it. Between the connecor rod of carbon or graphite and the protection tube surrounding the heating element is made suitably from wrought iron or steel, preferably from iron or steel protected against scaling or from scale-free material. A uniform heat distribution on the whole length of the heating element can be attained by a suitable choice of the electric resistan'cqior instance by using a graphite rod tapered at both ends.

One can also assemble tubes and rods of graphite to a heating element by means of threads in such a manner that the element has everywhere the desired resistance. If the furnace is placed vertically, the electric connections to the heating element and to the protecting tube are preferably disposed at the bottom so that the upper part of the iumace is kept free for collecting and condensing the magnesium vapours. It is most suitable to put the reaction mixture into a relatively thin-walled iron container which has substantially the form of a double-walled tube, whereupon the container is introduced into the furnace in such a manner that it surrounds the protecting tube containing the heating element. It is not absolutely necessary to take a container with closed walls; the walls can be provided with apertures or holes. The electric current flows at first through the heating element and then through the protecting tube or in the opposite way.

As the heating of the charge is effected from the inside, the utilisation of heat and therefore the current efiiciency are better than in furtions to the poles the current must therefore pass at first through the resistance and thereafter through the. protection tube or vice versa. It is suitable to dispose the furnace vertically. The heating element is preferably composed of a tube naces with crucibles or retorts heated from the outside. It may however be suitable to provide the furnace with an additional heating equipment which heats from the outside (Fig. 2); in this manner it is possible to reduce a thicker charge in the same time.

The heating element, especially if it is made from carbon or graphite, is sensitive to air; carbon for instance burns. However, according to a further feature of the invention, the heating element is surrounded by a metallic tube which separates it from the furnace atmosphere. It is therefore possible to keep the heating element always in an indifferent gas or even better in vacuo, so that it is protected when the furnace is opened for the extraction of the condensed magnesium.

Vacuum signifies here the reduced pressure designated normally in technics by this term. It is of course impossible to reach an absolute vacuum.

In the furnace according to the invention the heating element is protected not only against the air which penetrates into the reaction chamber when the furnace is opened, but also against gases especially water vapour and carbon dioxide, which can escape from the charge when it becomes heated up in the furnace. Even heating resistances made from graphite or carbon can have a very long working life as they can be maintained always in an indifierent gas or in vacuo.

As the one end of the protecting tube stands absolutely free, the corresponding end ofthe furnace can be opened for introducing the charge or for extracting the residues as well as the magnesium (if it becomes condensed in the furnace itself) without necessity of loosening any electric connection.

The difference between the thermal coefficient of expansion of the metallic protection tube and that of the heating resistance does not give any trouble if the connection to the heating resistance is made flexible or movable.

It is possible to unite (for instance by welding) the protecting tube to the outer wall of the reaction chamber in such a way that it forms a chamber closed at the bottom for the introduction of the charge container.

Two constructional examples of furnaces according to the invention are illustrated on the accompanying drawings.

Figure 1 shows a first example of a furnace according to the invention for working in vacuo. l is a heating resistance made from graphite in form of a tube. At the bottom the tube is fixed into a plain graphite cylinder 2 which is connected with a metallic electric conductor 3. The other end of the heating tube is also fixed into a lain graphite cylinder 4. The cylinder 4 is provided with a metallic cup-like part i which closes the heating chamber 8 on the top and is united with the outer tube 6. l are insulating sleeves made of asbestos board. The lower insulating sleeve does not separate the chamber 8 from the chamber 9 air-tightly; it is therefore possible to put the chamber 8 under vacuum by exhausting the air from the chamber 9. The flange l separates at the bottom the heating chamber 8 from the reaction chamber ll airtightly. The charge i2 is held in a container l3 which has substantially the form of a double walled tube which is made from relatively thin sheet iron. 14 is an insulating ring which retains the radiant heat coming from the charge l2. i6 is a thermal insulation. The whole space between the tube 6 and the furnace mantle l1 stands under the same pressure or vacuum. The metallic conductor 3 is connected to the bar l8 which penetrates into the chamber 9 hermetically and electrically insulated, so that the other connection l8 can be disposed on the mantle l7. l9 and 20 are pipe connections to vacuum pumps. I is a condensation hood for the magnesium vapours and 23 the cover which is removed when opening the furnace.

It is also possible to unite electrically the graphite cylinder 2 with the bottom plate 2| (this makes the conductor bar l8 unnecessary) and to dispose the electric insulation between the edge of the plate 2i and the flange 22 (Fig. 2).

When putting the furnace according to Fig. 1 into operation one opens the cover 23, introduces the container 13 which has become filled up with the charge l2, together with the condensation hood 15 into the heated chamber II, and closes the cover again. Immediately after having closed the cover one exhausts the air from the reaction chamber. Already before heating one may have exhausted the air from the chambers I and I. The magnesium vapours produced condense in the hood II in form of solid crystals. Alter the reaction (which of course is practically never absolutely completed) one suppresses the vacuum in the reaction chamber H (for instance by introducing nitrogen or carbon monoxide or chimney-gas), opens the cover 23, removes the container I3, introduces into the reaction chamber ll another similar container with a fresh charge, exhausts the gas introduced and carries out a new reduction.

In order to heat the charge more quickly, one may dispose between the insulation II and the container II a supplementary electric heating device, for instance a heating winding, as shown in Fig. 2.

The diflerence between the thermal coeiiicient of expansion of the metal forming the tube 0 and the coefiicient of the heating resistance I does not give any trouble as the heating resistance I can expand freely towards the end connected with the flexible conductor 3.

Figure 2 shows another example. The protection tube 6 is united to an outer tube 2|, preterably by welding, in such a manner that it forms a chamber closed at the bottom for the reception of the container ii. In this case the reaction chamber is separated from the furnace chamber. 2| is a supplementary electric heating device for heating from outside.

Figure 3 illustrates a heating resistance consisting of a graphite rod 26 tapering at both ends; and the heating resistance shown in Fig. 4 is built up of graphite tubes 21 and rods 28 screwed together.

We claim:

1. Furnace for the electrothermal production of magnesium by reduction of oxidic magnesium compounds with the aid of a reducing agent furnishing only non-gaseous oxidation products, comprising in combination, a reaction chamber,

. an elongated electric heating element adapted to heat the reaction mixture from the inside, a metallic protecting tube surrounding said heating element and separating it hermetically from the reaction chamber, said heating element and said protecting tube being united electrically at one end, and electric connections from the current supply conductors to the heating element and to the metallic tube disposed in such a manner that that end of the tube which is united with the heating element is free to permit access to the reaction chamber without unsealing the space between said tube and said element, and that the tube serves also to conduct the current from or to the heating element;

2. Furnace according to claim 1, characterized by the fact that the tube surrounding the heating element is made substantially from iron.

3. Furnace according to claim 1, characterized by the fact that the electric connections from the current supply conductors to the heating element and to the protecting tube are disposed at the bottom.

4. Furnace for the electrothermal production of magnesium by reduction of oxidic magnesium compounds with the aid of a reducing agent furnishing only non-gaseous oxidation products, comprising in combination, a reaction chamber, an elongated electric heating element adapted to heat the reaction mixture from the inside, a metallic protecting tube surrounding said heating element and separating it hermetically from the reaction chamber, said tube being united with the outer wall of the reaction chamber so as to form a chamber closed at the bottom for introduction of the container holding the charge, said heating element and said protecting tube being united electrically at one end, and electric connections from the current supply conductors to the heating element and to the metallic tube disposed in such a manner that that end of the tube which is united with the heating element is free to permit access to the reaction chamber without unsealing the space between said tube and said element, and that the tube serves also to conduct the current from or to the heating element.

5. A furnace for the electrothermal production of magnesium by reduction of oxide-containing -compounds thereof, comprising in combination, a reaction chamber, a heating element disposed to supply heat to said reaction chamber from the inside, a metallic protecting tube surrounding but spaced from said heating element and hermetically sealing it from the reaction chamber, said tube being electrically connected at one end to said heating element, electric connections to said heating element and to said tube at points remote from the point of connection of said tube to said element leaving said connection free and unobstructed and placing said tube in circuit with said heating element, and means permitting the opening of the reaction chamber for insertion or removal of a charge without breaking the hermetic seal between said protecting tube and said heating element.

6. A furnace for the electrothermal production of magnesium by reduction of oxide-containing compounds thereof, comprising in combination, a reaction chamber, a condensation hood covering said reaction chamber, a heating element disposed to supply heat to said reaction chamber from the inside, a metallic protecting tube surrounding but spaced from said heating element and hermetically sealing it from the reaction chamber, said tube being electrically connected at one end to said heating element, electric connections to said heating element and to said tube at points remote from the point of connection of said tube to said element leaving said connection free and unobstructed and placing said tube in circuit with said heating element, means permitting the opening of the reaction chamber for insertion or removal of a charge without breaking the hermetic seal between said protecting tube and said heating element, and means for hermetically enclosing the reaction chamber and the hood.

ALFRED von" ZEERLEDER. WERNER SYZ.

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