EP2102378B1

EP2102378B1 - Method and device for removal of elements resolved in a metal melt

Info

Publication number: EP2102378B1
Application number: EP07851993.1A
Authority: EP
Inventors: Karl Venaas
Original assignee: ALU INNOVATION AS
Current assignee: ALU INNOVATION AS
Priority date: 2006-12-13
Filing date: 2007-12-12
Publication date: 2013-06-19
Anticipated expiration: 2027-12-12
Also published as: ES2427162T3; US20100206128A1; NO329797B1; EP2102378A1; CA2671529A1; NO20065893L; WO2008079015A1; EP2102378A4; CA2671529C

Description

The present invention relates to a method as stated in the introductory part of claim 1, for removal of elements from a metal melt in a container, which melt contains a base metal and elements of undesirable constituents, in which the metal melt is heated to a level maintaining the content melted, with an electrode immersed into the container. Moreover, the invention comprises a device for removal of such elements, as stated in the introductory part of claim 1.

Background

On treating metals, the need for cleansing of a metal for another metal arises. This relates particularly to remelting of metals with impurities of one or more other metals.
An example is at remelting of aluminium containing magnesium, Mg. To provide sufficient purity, the magnesium remnants in most cases should be removedd. Currently, this is done by

1 Adding Cl2(g), which is reacting with Mg according to the equation: Mg + Cl2 = MgCl2
2 By adding active salts reducing the Mg content.

Common to both methods is the development of MgCl2, which is a salt depositing on the melt surface together with Al2O3 slag, which is removed periodically. Such slag is transferred to melting works recovering aluminium, while the rest containing oxides, chlorides etc. normally is stored on land or is dumped at sea. This is undesirable for environmental reasons.
EP 1 225 236 discloses a continuous vacuum refining method of molten metal wherein impurities in the molten metal are eliminated by evaporation by stirring the molten metal in a molten metal stirring chamber. The elements to be removed, such as magnesium, are vaporized and subsequently condensed in a separate condenser vessel.
US 4 842 643 relates to an apparatus for purifying a melt which, besides one or more impurities to be volatilized, contains essentially a light metal such as aluminium. The apparatus comprises a vacuum processing vessel and means for vaporizing and subsequently condensing the various elements in separate condenser vessel.
WO 2004/076699 reveals a vacuum container for heating metals in which electrodes are used. The device comprises a container subjected to a vacuum. A rotating electrode is arranged in the container, producing a electric arc for supply of heat to the melt.
US 5 385 601 relates to a furnace for melting aluminium dross heated with a plasma-arch to promote vaporization of volatile components. The volatile gases having left the furnace are passed through one or more condensers.

Object

The main object of the invention is to provide a method and a device for removal of undesirable metals from metal melts, reducing said problems. It is particularly an object to provide a method and a device for remelting aluminium containing magnesium, in which Mg is removed during the melting treatment. It is an object to provide a device being advantageously built, operated and maintained.
The invention should be integrated in a device according to Norwegian patent specification 318848 , with the general scheme of operation: In a hollow rotating body being immersed in a melt, an electrode conducting gas is arranged. Between the electrode and the melt a flame arch is burning, from which the heat energy is transferred directly to the melt.

The Invention

In the following, the method according top the invention is defined by claim 1 and the device is correspondingly defined by claim 2.
The invention may for example be used for removing of magnesium from aluminium melts.
The magnesium vapour is mixed with further gas which is added in the rotor. The gas mixture is led through a vacuum exit to a condenser for depositing of liquid magnesium. The exhaust gas from the condenser is fed through a vacuum pump into the atmosphere. The emission is without CO2 gas. If the aluminium melt contains other alkalis they will evaporate correspondingly.

Example

The invention is described in more detail in the following and with reference to the figures, wherein

Figure 1 shows a connection between vapour pressure and temperature for some metals which could be in the melt, while
Figure 2 show a schematic side view of a plant comprising a device according to the invention, provided for removal of undesirable metals from an aluminium melt.

The example is based on a device vaporizing magnesium directly from an aluminium melt without oxidizing Al and Mg. The device is provided for bringing heat energy to the melt. In Figure 1 a diagram is showing the connection between temperature and vapour pressure for aluminium, magnesium and zinc in the range 700 - 1200 °C. The diagram shows that the vapour of aluminium in this range is negligent to the vapour pressure for the other metals.
Figure 2 shows schematically a plant with a device according to the invention. It comprises a cylindrical or rectangular upright container 1 with an outlet in the container 1 in the form of an outlet conduit 2. The melt treated flows through a tube 3 at the lower end of the outlet conduit 2. The melt to be treated is transferred via a chute to a reservoir 24 and is sucked into the lower end container 1 through a tube 4 and is lifted by a sub pressure into the container provided by a vacuum pump 29.
The vacuum pump 29 is connected to a series of three condensers 27 being connected to the container 1 by a tube 5. In the container 1, a rotor 6 is provided, powered by a motor 7 over a drive belt 8 or similar, to a pulley 9 attached to a tubular shaft connected to the rotor 6. The motor is attached to a bracket 11. The bracket 11 can be attached to the container 1 or to a separate rack. The sealing between the rotor shaft 10 and the container 1 is
provided by a sealant 12. The bracket 11 has a bearing 16 journaling the rotor shaft 10. Centrally in the shaft an electrode 13 is arranged, the upper end of which is connected to a cable with a cable shoe 14. The electrical current, which can be DC or AC, is transferred to the device through the cable shoe 14 and through an electrode connection 28 on the container 1.
The metal melt of the example is heated to a temperature between 800. and 1200 °C, preferably ca. 1000 °C by the electrode 13 and 26.
The electrode 13 has a central bore for supplying gas from a connection 15 at the outer end of the electrode 13. The gas supplied through the electrode 13 is preferably argon or nitrogen or a mixture thereof. Other gases may also be utilized. The gas over an upper melt level 21 will comprise of a mixture of gas supplied to the rotor and gas emitted from the melt. The gases emitted from the melt are hydrogen and the vapour of magnesium, zinc, sodium etc. The gases overthe melt level 21 are extracted through the tube 5 to the condensers 27, in which magnesium, zinc and sodium etc. are condensed to melts. The residual gas is fed through the vacuum pump 29 to the environment, possibly through a purification device.
Between the rotor shaft 10 and the electrode 13 a ring 16 is arranged for acting as a sealant and an electrical insulation. The ring 16 has a passage for gas and particles through a tube 17 connected to a powder supply 18. The reservoir 24 has a dividing wall 20 and a gatevalve 19 (the operation of which is not illustrated), to allow continuous handling of the melt, but the reservoir may also be a melting pot filled with melt. At the lower end of the rotor 6, multiple holes 25 are arranged between the circumference and the melt level 23. The level 23 of the melt in the rotor is governed by the gas pressure of the rotor 6.
The plant is started with the gate valve 19 closed and the reservoir 24 is filled to a certain level. When the vacuum is provided by the vacuum pump 29 through the condensers 27 and the tube 5, gas is provided simultaneously through the electrode 13 and/or the ring 16 to the rotor 6. The melt is brought to the level 21. The rotor 5 is operated and the voltage from a rectifier or transformer is applied. The current is led through the cable shoe 14 to the electrode and through the contact 28, alternatively through the rotor shaft 10 via a collector ring (not shown). A flame arch 22 is created between the electrode 13 and the top level of the melt 23. The rotation of the rotor 6 will pump the heated melt in the rotor through the holes 25 and mix it into the melt of the container. Simultaneously gas bubbles are created at the outlet of the holes 25. The gas bubbles, containing gases emitted from the melt, hydrogen and vapours of magnesium, zinc, sodium, etc. The gas bubbles are rising in the melt, through the upper level 21 and are released through the tube 5 and to the condensers 27, wherein magnesium, zinc and sodium etc. are condensed to melts which can be drained through the valve connection 30. The holes 25 of the rotor 6 can be circular or polygonal, and be arranged at the lover end of the rotor, being notches therein. When the melt has reached a desirable temperature, the valve 19 is opened and the metal may exit from the container 1 for further processing.
The invention is not restricted to the example of Figure 2. Thus, the plant can comprise two, three or more containers in a series. Each container may have multiple rotors with a flame arch.

Claims

Method for removal of elements from a metal melt in a container, which melt contains a base metal and elements of undesirable constituents, in which the metal melt is heated to a level maintaining the content melted, with an electrode immersed into the container, characterized in that the metal melt is heated to a level causing vaporization of an element to be removed, but under the level providing gas pressure for the base metal, and that the container with the metal melt is brought under a reduced pressure in at least one compartment in which the temperature is maintained under the vaporization level of the element to be removed, and in that the gas is transported through a cascade arrangement with declining temperature in succeeding steps of the cascade.
Device for the removal of elements from metal melts, with a container (1) for the metal melt, which comprises a flame arch (22) for providing heat, and with an exit tube (5) over the level of the metal melt, for vapour from the interior of the container (1), characterized in that the exit tube (5) exits into a compartment (27) which is kept at a temperature under the vaporization temperature, for condensing the vaporized metal, and in that multiple compartments (27) with declining temperature are arranged in a series, for condensing different metal vapours, the metal with the highest vaporization point is condensed in the first compartment (27) and the metal with the lowest vaporization point is condensed in the last chamber.