Claims
- 1. A method of recovering metal-carbide scrap by treating the scrap with a low melting point metal, comprising the steps of: alloying metal-carbide scrap with a low melting point metal in an inner chamber arranged in a container in the presence of inert gas for bringing a metal-carbide matrix into solution at temperatures above the melting point of the alloy formed; directing metal vapor and inert gas from said inner chamber through a single opening onto condensation surfaces; circulating inert gas released from metal vapors through said inner chamber; carrying out the alloying step at pressures above substantially twice the partial pressure of the low melting point metal, and then vaporizing the low melting point metal at pressures below 1 mbar before condensing on said condensation surfaces; recirculating inert gas released from metal vapors from said condensation surfaces through a closed gas path formed by an annular gap between said container and said inner chamber, at least one capillary gap in said inner chamber, a vapor duct between said inner chamber and said condensation surfaces, and a return flow opening between said condensation surfaces and said container, whereby penetration of metal vapor in direction of inner surfaces and components of said container is prevented, said capillary gap comprising a gap left between crucibles stacked within said inner chamber to preclude a screen connection between contents of said inner chamber and inner faces of said container.
- 2. A method according to claim 1, and regulating temperature of the alloy by the pressure in the container.
- 3. A method according to claim 1, and using zinc as the low melting point metal, said alloying step being carried out at a pressure of between 1200 and 2000 mbars, and reducing the pressure in an isothermal step to below 1 mbar upon completion of formation of the alloy, and continuing treatment until the residue has a zinc content of below 100 ppm.
- 4. A method according to claim 3, and carrying out formatin of the alloy at approximately 850.degree. C., heating then the alloy to approximately 920.degree. C. and, carrying out the isothermal vaporization of zinc at said temperature of approximately 920.degree. C. until a zinc content of below 50 ppm is obtained.
Priority Claims (1)
| Number |
Date |
Country |
Kind |
| 3144284 |
Nov 1981 |
DEX |
|
Parent Case Info
This application is a continuation of Ser. No. 435,768 ; filed 3-14-88 now abandoned.
The invention concerns a method of recovering metal-carbide scrap by treating the scrap with a low melting point metal, which brings the metal-carbide matrix into solution, at temperatures above the melting point of the alloy formed, in a container in the presence of inert gas, in which method, first the alloying process is carried out at pressures above approximately twice the partial pressure of the low melting point metal and then the low melting point metal is vaporized at pressures below 1 mbar and is condensed on condensation surfaces.
Metal-carbide scrap occurs in considerable quantities, for example in connection with worn tools used in the machining of metals. A known example is constituted by what are called "turn-over plates". A problem that arises in this connection is that of recovering the metal-carbide scrap so that it can be used again in a suitably pure form as a starting material or as part of a mixture. The main constituent of the metal-carbide metal is cobalt.
A known method of the initially stated kind is based on the solubility of the metal-carbide matrix in a low melting point metal, such as zinc, for example. Depending upon the cobalt content of the metal carbide, such quantity of zinc is added to the scrap that an alloy having a solidus temperature of approximately 820.degree. C. is formed. Zinc is a metal having a very high vapour pressure, so that the alloying phase is carried out at an elevated protective-gas pressure, for example at a pressure of approximately 1500 mbars. The zinc penetrates the metal-carbide matrix by diffusion and breaks up the metal-carbide lattice. After the zinc has been driven off, all that remains in the equipment is a "cake", which is ground down in a comminuting process to form a fine powder. This power is retrieved for further use. In addition to zinc, cadmium may also be considered for use as the low melting point metal.
The known method exploits the partial pressure gradient in the zinc vapour between the heated alloying zone and the condensation surfaces, as well as the rate of diffusion of the zinc molecules between these zones. The concentration gradient is determined by the temperature gradient in the equipment required for carrying out the method, whereas the evaporation rate is determined by the rate of diffusion of the zinc molecules in the inert-gas atmosphere.
In the known method, the metal-carbide scrap is introduced, together with granulated zinc, into a crucible open at the top. To prevent the zinc from reacting in a harmful manner with the material of the crucible, the latter is made of graphite which is resistant to zinc. Two considerable disadvantages are, however, associated with this step:
In the first place, even a marked reduction in pressure in the container, following formation of the alloy, does not suffice to reduce the zinc content in the residue to values that are appreciably below 400 ppm. However, a high zinc content of this kind is too great to permit the recovered scrap to be used again, since such zinc content does not enable sufficient strength and length of service life to be obtained in the new metal-carbide tools. Consequently, it became necessary to apply further treatment to the embrittled residue involving additional complicated methods aimed at a zinc content of less than 400 ppm.
A further considerable disadvantage of the known procedure results from the screen connection between the contents of the crucible and the inner surfaces or components of the container. Consequently, it has not been possible to prevent the vaporized zinc from condensing, to some extent, on the components or inner faces of the container. Not only did these amounts of condensate constitute losses as regards the quantities of material deposited in the actual condensing vessel, but they also represented an undesirable contamination of the container and its components. Of very special importance is that zinc tends to react in an undesirable manner with the condensation surfaces and to form alloys which finally lead to the destruction of the components concerned.
In what is known as hot galvanizing, use is made of that inherent property of zinc that enables its contact surface to enter a normal serration. Whereas the very great adhesive strength of the zinc coating is extremely desirable in the end products produced in this way, the almost unbreakable connection between the zinc and the condensation surface is an undesirable subsidiary result when, for example, the condensed zinc has to be cleaned off the wall of the container at certain spaced areas. This is a practically insoluble problem.
An object of the present invention is, therefore, to provide a method of the initially described kind whereby the proportion of the low melting point metal left over in the residue can be reduced in a single operation to less than 100 ppm, and preferably less than 50 ppm, and wherein no metallic vapours are deposited on the inner faces or components of the container.
According to the invention and in the initially described method, this object is achieved in that the metal-carbide scrap and the low melting point metal are alloyed with each other in an inner chamber which is arranged in the container and from which the metal vapour and the inert gas are directed on to the condensation surfaces, and in that the inert gas, released from the metal vapours, is recycled through the inner chamber.
The inner chamber referred to is essential to the equipment for carrying out the method of the invention. It is to be understood as being a component of the container that affords to the metal vapours no free passage other than that leading to the condensation surfaces. The inner chamber is closed against the metal vapours in substantially all directions, and it has only one opening for passage of the metal vapours and through which these vapours pass directly on to the condensation surfaces. The inner chamber should, however, be sufficiently permeable by the inert gas present in the container to enable the gas to be cycled through the inner chamber. For this purpose the inner chamber may have extremely small openings or gaps, which preclude a screen connection between the contents of the inner chamber and the inner faces of the container or its components. At the same time, the flow paths for the inert gas in the walls of the inner chamber are so narrow that flow of metal vapour in the opposite direction is prevented.
By means of the arrangement in accordance with the invention, the vaporized metal molecules are moved in a preferential direction, i.e. towards the condensation surfaces. Thus, a motional mechanism is brought into action whereby the inert gas within the equipment is cycled between the inner chamber and the condensation surfaces.
This effect can be compared with the mechanism of the action of a diffusion pump. Since the inert gas escapes again from the condensation unit and enters the inner chamber through the above-described flow ducts, it is also cycled without the use of mechanical means such as circulating pumps, and simply as a result of the action of the stream of metal vapour. This stream of inert gas also prevents the flow of metal vapours in the opposite direction.
Since an appropriate configuration of the condensation surfaces easily renders it possible to condense the metal vapours to such an extent that the inert gas is completely freed from these vapours when it enters the container, the penetration of metal vapours in the direction of the inner surfaces and components of the container is prevented in this way. To some degree, the inert gas acts as a gas for flushing the space between the inner chamber and the wall of the container, and this leads to the equipment having an extremely lengthy service life.
Furthermore, the result of the above-described motional mechanism is that, in a single operation, the amount of low melting point metal in the residue ("cake") can be reduced to less than 100 ppm, and advantageously to less than 50 ppm.
Circulation of the inert gas interferes, in a positive sense, with the partial pressure gradient of the metal vapour corresponding to the temperature difference. A zone of low concentration of inert gas develops within the inner chamber, so that vaporization of metal can take place practically unimpeded. Outside the inner chamber, a greater density of inert gas and therefore increased protection of the wall of the container against attack by the metal vapour occur. The above-described motional mechanism is intensified when the total pressure in the condensation unit corresponds to the partial pressure of the metal vapour in the inner chamber.
Furthermore, in the known method and upon excessively rapid reduction in pressure, the associated excessive extraction of vaporizing heat results in the danger of a fall below the solidus line of the alloy formed and therefore of destruction of the inner chamber, i.e. the crucible, or of the vessel forming the inner chamber.
To prevent this and in accordance with a further feature of the invention, it is proposed that the temperature of the alloy be regulated through the pressure in the container. This preferably takes place by determining the temperature of the alloy directly or indirectly (for example by way of the temperature of the wall of the inner chamber) and thereby, at a given thermal capacity, regulating the suction capacity of the vacuum pumps in such a way that the temperature of the inner container is kept above a predetermined required temperature. Regulation of the suction capacity of the pumps, understood as relating to the container, can also be achieved by admitting foreign gas into the suction pipe by way of a regulating valve.
The temperature of the inner chamber therefore remains substantially constant, since small changes in temperature cause large changes in vapour pressure, whereas the vaporization rate is proportional to the quantity of heat applied. The danger of solidification of the alloy melts is to a large extent precluded in this way.
The invention also relates to equipment for performing the method, which equipment, in accordance with a further feature of the invention, is characterized in that the inner chamber consists of stackable crucibles each with an annular channel formed therein, which crucibles are placed one upon the other to leave capillary or diffusion gaps between them, and have central aligned vapour ducts, which are open only towards the condensation surfaces, and in that a return-flow duct for the inert gas is present below the inner chamber.
Finally, the invention also concerns a regulating means for performing the method, which means, in accordance with a further feature of the invention, is characterized by a temperature sensor associated with the inner chamber, and a pressure regulator, which is arranged downstream of the temperature sensor and regulates the vacuum in the container in such manner that the temperature of the inner chamber is kept above a predetermined required value.
Further advantageous forms of the subject-matter of the invention are set forth in the other subsidiary claims.
US Referenced Citations (2)
| Number |
Name |
Date |
Kind |
| 3767381 |
Bielefeldt |
Oct 1973 |
|
| 4407488 |
Wanetzky et al. |
Oct 1983 |
|
Continuations (1)
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Number |
Date |
Country |
| Parent |
435768 |
Mar 1988 |
|
Patent Grant
4818282
