METHOD AND APPARATUS FOR PURIFYING A LIGHT METAL MELT, IN PARTICULAR ALUMINIUM

Abstract: Apparatus for purifying a melt which, besides one or more impurities to be volatilized, contains essentially a light metal, in particular aluminum. The apparatus comprises a vacuum processing vessel (4), a vacuum pump (P), one or more conduits for supplying the melt to the vacuum pro­ cessing vessel and means (12) for spraying the melt into the vacuum processing vessel. According to the invention, the vacuum processing vessel is provided with discharge means (7) for discharging impurities expelled from the melt. The dis­ charge means are connected to a means for separating the impurities in solid or liquid form, connected to the vacuum pump used for maintaining the subatmospheric pressure in the vacuum processing vessel. Connected to the vacuum pro­ cessing vessel are at least one supply conduit (9) and one return conduit (10) for the melt, the supply conduit being pro­ vided with a pump (11) for the melt to be purified and a spray nozzle (12) for atomizing the melt supplied by the pump into the vacuum processing vessel. During the purification treatment, a subatmospheric pressure is maintained in the apparatus by the vacuum pump and the temperature of the melt is maintained at 600°C-­ 900°C.

Description: Apparatus for, and method of, purifying a melt of a light metal, in particular aluminum. This invention relates to an apparatus for purifying a melt of a light metal, in particular aluminum, which melt may contain one or more volatilizable metallic impurities of relatively high vapour pressure, said apparatus comprising a vacuum processing vessel; means for creating a subatmospheric pressure in said vacuum processing vessel, supply conduit means extending from a bath of the melt to be purified into said vacuum processing vessel; pump means for pumping molten metal from the bath to the vacuum processing vessel; return conduit means; separating means for collecting impurities and discharge means for discharging impurities collected by the separating means. This invention further relates to a method of purifying a melt of a light metal, in particular aluminum using said apparatus which melt may contain one or more volatilizable metallic impurities wherein the light-metal melt is led through supply conduit means from a bath of the melt to be purified into a vacuum processing vessel which is partially filled with molten metal from said bath and wherein a subatmospheric pressure is maintained. An apparatus and a method of the above type are known from US-A-4456479, which discloses a process for refining an aluminum melt, in which metals with a lower vapour pressure than the parent metal are removed by directing a stream of liquid metal contained in a container or can into a chamber to be placed on the can. The chamber is provided with a riser tube and a downcomer tube which extend into the melt batch to be refined. The liquid metal of the melt to be refined is forced in the riser tube, by means of a carrier gas, upwards to the chamber, in which a partial vacuum is maintained. When the liquid metal enters the chamber through a goose-neck-shaped upper end of the riser tube bubbles of the carrier gas rapidly expand, thereby providing a stream of small drops of the liquid metal bombarding the surface of the molten metal bath in the chamber. A drawback of the known process is that all of the carrier gas used to force the liquid metal through the riser tube into the chamber has to be pumped off too. Thus a very large vacuum pump is needed to maintain the desired subatmospheric pressure in the chamber. Further, though the carrier gas bubbles and the subatmospheric pressure in the chamber do provide for a spray of small drops of the liquid metal a finer spray would be desirable in order to promote a fast and thorough removal of volatilizable impurities. Moreover, according to US-A-4456479 the separating means for collecting impurities have been designed in such a manner, that metal vapour condensing in the separating means must necessarily be kept liquid during the purification process. It is an object of the present invention to improve the known process and generally to provide a novel apparatus for purifying light-metal alloys, in particular aluminum alloys, with a broad field of application, ranging from a fast thorough removal of gases dissolved in the melt to the possibility of removing undesirable or less desirable metal components from the melt. For this purpose, according to the invention, there is provided an apparatus of the kind recited in the opening paragraph hereof, and which is characterized in that said pump means connected to said supply conduit means comprise a mechanical pump for forcing a continuous stream of liquid metal through said supply conduit means and in that a spraying nozzle device is provided at the end of the supply conduit means in the vacuum processing vessel for atomizing the melt supplied by the pump means, wherein the separating means include a condenser vessel connected by conduit means to the upper part of the vacuum processing vessel. A method of the kind described above is according to this invention characterized in that a continuous stream of molten metal is pumped from said bath into said vacuum processing vessel through a spraying nozzle device for atomizing the molten metal and volatilizing the impurities while continuously drawing off said volatilized impurities by means of a vacuum pump through a conduit connected to the vacuum processing vessel at a level above the molten metal in said vacuum processing vessel, wherein a condenser vessel is provided between said conduit and the vacuum pump and at least part of said impurities is collected in said condenser vessel. The separating means and the vacuum processing vessel in fact form a diffusion pump system of high capacity. By supplying the melt to be processed through the supply conduit to the vacuum processing vessel by a pump means, in accordance with this invention, whereby the melt passes a spraying means incorporated in the supply conduit, a vigorous atomization of the light-metal melt, for example, an aluminum melt, can be achieved. As a result, a large reaction area is obtained, which makes for good transfer of matter. Thus the apparatus according this invention makes it possible to expel from the melt consisting of, for example, aluminum contaminated with zinc or with zinc and magnesium, not only the undesirable gases dissolved in the melt, but also the zinc in the vaporous form, and to withdraw it from the vacuum processing vessel for it to be thereafter separated in an effective manner. The purified aluminum melt collecting in the bottom part of the vacuum processing vessel can be removed from the vacuum processing vessel through a return conduit. Advantageously, the supply conduit is connected, or the supply conduit and the return conduit are both connected, either directly or indirectly, to the bath of the melt to be purified, which bath may, for example, be maintained in a furnace. Thus it is possible for the melt bath being purified to be recycled one or more time, in which connection it is effective for the vacuum processing vessel to be disposed at about barometric height above the level of the bath of the melt to be purified and for the supply conduit and the return conduit to be respectively formed as a riser tube and a downcomer tube so that a liquid seal can be maintained in the downcomer tube, and the light-metal melt being processed can be automatically discharged from the vacuum processing vessel through the liquid seal. The apparatus according to the invention can be provided with means for the supply of oxygen gas or oxygen producing materials to the vacuum processing vessel and/or a point downstream of said vacuum processing vessel. When the light-metal melt to be purified is contaminated with, for example, zinc and/or magnesium, the separating means of the apparatus according to the invention may include a particles separating means which is connected through a connecting conduit to the vacuum processing vessel, said connecting conduit being provided with means for the supply of oxygen gas or oxygen producing materials. The zinc withdrawn from the vacuum processing vessel in the form of a vapour react(s) with the oxygen to form zinc and/or magnesium oxyde particles, which can be separated in the particles separating means, for example a dust filter. In another embodiment of the apparatus of the present invention, it is possible to separate vapour of a sublimable material, such as zinc vapour, withdrawn from the vacuum processing vessel, by precipitation as solid metallic zinc in a condenser. For periodic removal of the zinc metal, the condenser may be provided with a closable tap, and further with means for melting out the collected zinc metal. If the apparatus according to the invention takes the form in which it comprises a condenser and is combined with supply means for oxygen gas or oxygen producing materials, these means are destined preferrably for supply to the vacuum processing vessel. The condenser is, for that matter, also suitable for separating impurities in the liquid form therein. However, if desired, this condenser could also be used when the means for supplying oxygen gas or oxygen producing materials are connected to the connecting conduit downstream of the vacuum processing vessel. Advantageously the vacuum processing vessel and the supply and return conduit may be preheatable. For this purpose these are various possibilities, for example, inductive or electric heating or also by means of a burner. For better control of the process to be conducted in the apparatus according to the invention, a diaphragm may be provided in the connecting conduit between the processing vessel and the separating means. The main function of such a diaphragm is to control the diffusion pump system. Effectively the melt to be purified is maintained at a normal storage temperature in an aluminum furnace of 600°C-900°C, more in particular 710°C-740°C. As, in the apparatus according to the invention, the partial vacuum to be maintained in the vacuum processing vessel and the separating means, such as the condenser vessel, is not partly used for sucking in and spraying the melt being processed, the partial vacuum to be set may, if desired, be used as a means for controlling the purification process. Thus, for example, the vapour pressure of zinc in equilibrium with 0.1% zinc dissolved in aluminum is sufficiently high for the method according to the invention in which the zinc is separated from the aluminum melt to proceed at a high rate. On the other hand, the vapour pressure of magnesium that is in equilibrium with o.1% magnesium dissolved in aluminum is considerably lower, so that, at the relatively high pressure usable for the removal of zinc the separation of magnesium will lag behind considerably. If zinc has to be removed only, it is effective to maintain a pressure of 0.05-20 mbar (0.005-2 kPa), preferably 0.01-5 mbar (0.01-0.5 kPa) in the vacuum processing vessel, and when magnesium only is removed a pressure of 0.01-0.5 mbar (0.001-0.05 kPa), preferably 0.02-0.2 mbar (0.002-0.02 kPa). Surprisingly it has now been found that by supplying oxygen gas or an oxygen producing material to the vacuum processing vessel, a ready removal of the magnesium can also be realized. The pressure in the vacuum processing vessel is then effectively maintained at 0.05-20 mbar (0.005-2 kPa), preferably 0.1-5 mbar (0.01-0.5 kPa). The manner in which the oxygen is active in this connection is unknown. Possibly, the vapour phase in the vacuum processing vessel contains magnesium vapour which can be oxidized. The magnesium oxide formed partly falls back into the aluminum bath collecting in the bottom part of the vacuum processing vessel. When this processed aluminum melt is recycled to the furnace in which the aluminum melt is maintained, the magnesium oxide carried along will be scorified in the furnace and is removable. The other part of the magnesium oxide formed will be entrained as a solid by the zinc vapour from the vacuum processing vessel to the condenser and remain behind therein. During the periodic melting of the zinc, the magnesium oxide will float on the zinc as a slag, and may thus be withdrawn from the condenser and separated in that form. Due to the intense atomization of the melt in the vacuum processing vessel, the removal of magnesium from the aluminum melt to be purified can also be achieved by adding to the vacuum processing vessel chlorine and/or fluorine and/or a material producing chlorine and/or fluorine. It is true that the treatment of an aluminum alloy by means of a halogen or halogen compound is known from Netherlands patent application 7612653, but that publication is concerned with the expulsion of sodium from the aluminum alloy, while any magnesium that may be present should remain behind in the aluminum alloy. The invention will now be described with reference to the accompanying drawings, which show diagrammatic perspective views of two embodiments of the apparatus according to the invention by way of example. Fig.1 shows a first embodiment of the apparatus according to the present invention, fitted with a separate connecting conduit between a vacuum procesing vessel and a means for separating one or more impurities in solid or liquid form; Fig. 2 shows a different embodiment in which the vacuum processing vessel and the separating means for separating impurities in solid or liquid form are an integrated unit without a separate connecting conduit; and Fig.3 shows, in a larger scale and in cross-sectional view, the encircled detail III of Fig.2. In the embodiment of the apparatus according to the invention as shown, parts with like functions are designated by the same reference numerals. In the drawing, there is shown an aluminum melting furnace 1 to which open buffer vessels 2 and 3 are connected as a reservoir for the melt to be processed and as a supply container for melt that has been processed, respectively. Disposed at a barometric height above buffer vessel 2,3 is a vacuum processing vessel 4. This vessel 4 is connected through a conduit 6 to a condenser vessel 5. Condenser vessel 5 is in turn connected through conduit 15 and through dust separator 16 to a vacuum pump not shown(arrow P). Condensor vessel 5 is further provided at the bottom with a closable tap 7 which through line 8 is connected to a casting machine not shown. By means of a diaphragm or slide 17, the effective cross-sectional area of connector 6 (Fig.1) can be reduced. Oxygen or any other reactions or inert gas can be supplied through connections 18. Connected further to vacuum processing vessel 4 are a riser tube 9 and a downcomer tube 10, which extend into the open buffer vessels 2 and 3, respectively. Incorporated in riser tube 9 is a lifting pump 11 and a spray nozzle 12. The vacuum processing vessel 4 and condensor vessel 5 each have a door 13,14, respectively, giving access to the interior of vacuum processing vessel 4 and condenser vessel 5, respectively. In operation, a supply of aluminum melt to be purified is supplied from melting furnace 1 to such a level that riser tube 9 terminates below the bath surface. By means of a vacuum pump, the desired subatmospheric pressure is maintained through conduit 15 in vacuum processing vessel 4 and condenser vessel 5. The melt in the open buffer vessel 2 is pumped upwards in a continuous stream by pump 11 and atomized into the vacuum processing vessel 4 through spray nozzle 12. The processed aluminum melt collected continuously flows through the liquid seal formed in downcomer tube 10 under the influence of the partial vacuum in vessel 4 to the open buffer vessel 3 and thence back to the melting furnace 1. In condenser vessel 5, zinc vapour sucked off from the vacuum processing vessel 4 is collected and precipitated. Periodically, this zinc can be molten and removed through tap 7 and conduit 8 to the casting machine, where it can be cast into zinc ingots, for example. In the embodiment of the apparatus according to the invention as shown in Fig.2, the vacuum processing vessel 4 and the condenser vessel 5 merge into one another through an integrated connecting conduit 6. The melt supplied through conduit 9 is sprayed not through a spraying nozzle, but by inpinging the jet of aluminum melt upon a deflector plate 12. Owing to the lateral spread of the melt droplets, these can impinge one or more further times upon fixed plates 19 and are thus reactivated upon each impingement. Heating means are shown diagrammatically at 20 and 21. Naturally, the apparatus as described herein and shown in the accompanying drawing can be modified without departing from the scope of the invention. Although, by way of example, the removal of zinc and magnesium from an aluminum melt has been discussed in particular, the invention is not so limited. Thus, for example, sodium and cadmium, and also lithium, are volatilizable within the framework of the apparatus and method according to this invention, and to be removed from a light-metal melt in accordance with this invention, while the apparatus and method according to the invention are also applicable to a lead-zinc melt.

Claims: Apparatus for purifying a melt of a light metal, in particular aluminum, which melt may contain one or more volatilizable metallic impurities of relatively high vapour pressure, said apparatus comprising a vacuum processing vessel; means for creating a subatmospheric pressure in said vacuum processing vessel, supply conduit means extending from a bath of the melt to be purified into said vacuum processing vessel; pump means for pumping molten metal from the bath to the vacuum processing vessel; return conduit means; separating means for collecting impurities and discharge means for discharging impurities collected by the separating means, characterized in that said pump means (11) connected to said supply conduit means (9) comprise a mechanical pump for forcing a continuous stream of liquid metal through said supply conduit means; and in that a spraying nozzle device (12) is provided at the end of the supply conduit means in the vacuum processing vessel (4) for atomizing the melt supplied by the pump means, wherein the separating means include a condenser vessel (5) connected by conduit means (6) to the upper part of the vacuum processing vessel (4). Apparatus according to claim 1, characterized in that the means for creating a subatmospheric pressure in the vacuum processing vessel (4) includes a vacuum pump (P) connected to the condenser vessel (5). Apparatus according to claim 1 or 2, characterized by means (18) for supplying oxygen or oxygen producing material to the vacuum processing vessel (4) and/or a point downstream from the vacuum processing vessel. Apparatus according to any one of claims 1-3, characterized by means for supplying a halogen gas selected from the group consisting of chlorine and fluorine to the atomized materials generated in said vacuum processing vessel (4). Apparatus according to any one of claims 1-4, characterized in that the condenser vessel (5) has an outlet conduit (8) including a closable tap. Apparatus according to any one of claims 1-5, characterized in that the spraying nozzle device (12) for atomizing the melt supplied by the pump means includes at least one deflector device positioned in the path of atomized melt particles for further atomizing said particles. A method of purifying a melt of a light metal, in particular aluminum by using the apparatus of claim 1, which melt may contain one or more volatilizable metallic impurities wherein the light-metal melt is led through supply conduit means from a bath of the melt to be purified into a vacuum processing vessel which is partially filled with molten metal from said bath and wherein a subamospheric pressure is maintained, characterized in that a continuous stream of molten metal is pumped from said bath into said vacuum processing vessel through a spraying nozzle device for atomizing the molten metal and volatilizing the impurities while continuously drawing off said volatilized impurities by means of a vacuum pump through a conduit connected to the vacuum processing vessel at a level above the molten metal in said vacuum processing vessel, wherein a condenser vessel is provided between said conduit and the vacuum pump and at least part of said impurities is collected in said condenser vessel. A method according to claim 7, characterized in that the bath is maintained at a temperature of 710°-740°C. A method according to claim 7 or 8, characterized in that oxygen or oxygen producing material is supplied to the vacuum processing vessel and/or the conduit. A method according to claim 9, characterized in that oxygen or oxygen producing material is supplied to the condenser vessel. A method according to any one of claims 7-10, characterized in that in a first stage a relatively low vacuum is maintained in the vacuum processing vessel and that in a second stage, in which oxygen is added, a relatively high vacuum is maintained in the vacuum processing vessel. A method according to any one of claims 7-11, characterized in that a halogen or halogen producing material selected from the group consisting of chlorine and fluorine is introduced into said vacuum processing vessel.

Inventors: VAN WIJK, GIJSBERT WILLEM MEINDERT
Applicants: VAN WIJK, GIJSBERT WILLEM MEINDERT
Application year: 1987
Main classification: C22B21
IPC codes: C22B21, C22B9