HK1206075B - Process and apparatus for vacuum distillation of high-purity magnesium - Google Patents

Description

Method and device for vacuum distillation of high-purity magnesium

The invention relates to a method for preparing high-purity magnesium by a reduced pressure distillation way and a device for implementing the method.

Magnesium has a lower boiling point than most other metals, which is why many processes for obtaining crude metals or recovering magnesium from scrap are carried out via vacuum distillation process steps, since this way magnesium can be obtained in one step substantially purified from less volatile metals. If these volatile substances are also to be removed for the production of high-purity magnesium, as is desired, for example, in the semiconductor industry, vacuum distillation plants with a plurality of condensation zones arranged in series are used, so that high-purity fractions having impurity concentrations only in the ppm range can also be obtained in fractions which are strongly contaminated with other volatile metals, such as zinc and cadmium. For example, EP 1335030 a1 describes a process in which the vapor rising from a crucible with a contaminated magnesium melt is passed through a plurality of deposition plates arranged in series and heated to progressively lower temperatures, and deposited on these plates in fractionated form.

The evaporation temperature of magnesium can be lowered below the melting point temperature with reduced pressure, and the metal has the characteristics that: even below the melting point, its vapor pressure is high enough to allow its use for the re-sublimation of commercially available high purity magnesium crystals. Therefore, according to the prior art for preparing high purity magnesium, most of the known vacuum distillation methods result in the deposition of solid magnesium crystals.

From a chemical point of view, these magnesium crystals, although they may be described as highly pure (due to their low content of impurity elements), nevertheless have a high surface/volume ratio and, when they are remelted to produce semifinished products or objects close to the final contour, the scale that was originally present on the surface of the magnesium crystals due to their high reactivity is distributed as a non-metallic inclusion in the melt and remains in the solidified material. Although these inclusions have only small concentration values, they still have an adverse effect on the corrosion properties, for example, of the high-purity magnesium.

According to one method described in EP 1335032 a1, a magnesium melt containing impurities is evaporated in an evaporation vessel made of highly pure graphite, wherein the vapor is subsequently condensed into a liquid melt in a condensation crucible likewise made of graphite. The two crucibles are surrounded by a graphite bell which prevents the magnesium vapour from coming into contact with the cold walls of the vacuum retort surrounding the bell and causing condensation there. There are two heating elements in the gap between the retort wall and the graphite bell jar to bring the evaporating crucible and the condensing crucible to the temperature required for the process while keeping the retort at a low temperature. In particular, the installation of the heating element in the low-pressure region and the protection of the evaporation and condensation regions by the graphite bell increases the construction costs, and also, because the graphite bell is not sealed and the interior space must be evacuated, magnesium vapors can also reach the heating element and the cold retort wall at these points to the outside.

In contrast to most processes of the prior art, the process according to the invention is to condense high-purity magnesium in the liquid state, wherein a high-purity magnesium melt free of non-metallic inclusions can be produced, which after solidification forms a compact mass which is suitable as a raw material for shaping processes in the semi-finished sense, without relatively large amounts of non-metallic inclusions having a negative influence on the mechanical and corrosion properties.

In particular, compared to the above-described method according to EP 1335032 a1, the present method has the advantage that the reduction pot can be heated from the outside, wherein the magnesium vapour can be brought into contact without problems with the inner wall of the reduction pot, since this inner wall is at such a high temperature that no solid magnesium crystals can deposit thereon. The reduction vessel wall can likewise consist of a material which dissolves in the magnesium melt to a small extent and thus contaminates the magnesium melt. However, provided that the reduction pot is composed of a material that does not release volatile impurities to the magnesium vapor.

Since the heating device can be arranged outside the reduction vessel and also allows contact with the hot inner walls of the reduction vessel, which slightly contaminate the magnesium melt, a particularly simple and cost-effective method can be produced compared to the prior art.

The method according to the invention will be explained below on the basis of three device examples.

Fig. 1 shows a principle drawing of the device according to the invention with the main inventive features according to the teaching of the independent claims.

FIG. 2 shows a cross-section of an exemplary apparatus for producing high purity magnesium.

FIG. 3 shows a cross-section of another exemplary apparatus for producing high purity magnesium.

The elements of the first device example are intentionally represented in figure 1 in a simple geometric form, aiming at emphasizing the fact that the core inventive concept is less based on the particular shape of these components, but on the function of these components with respect to the temperature distribution within the reduction tank. The starting material 2 in the form of a magnesium-containing metal melt is located together with the upper region 32 of the condensation vessel 3, preferably made of highly pure graphite, in the upper region 11 of the reduction vessel 1, for example made of stainless steel, with an arbitrary cross section, a cylindrical cross section being preferred in practice. The upper region 11 of the reduction vessel is brought to a temperature above the boiling point of magnesium by the heating elements 5 surrounding the reduction vessel (for example in the form of a resistance furnace) within the limits of the horizontal lines 8 and 81, and is then held at this temperature, so that the vapors rise from the boiling magnesium-containing metal melt 2 as indicated by the arrows 91 and fill the interior of the upper region 11 of the reduction vessel 1, wherein the vapors can also condense into a liquid state above the horizontal line 81 and then flow back again downward into the melt 2. Stainless steel is contaminated with alloying elements in contact with the magnesium melt because it reacts slightly with liquid magnesium, but these impurities in the vapor are not significant because these elements have a much lower vapor pressure than magnesium.

Contaminated melt dripping from the region above the horizontal is prevented from entering the opening 31 of the condensation vessel 3 by a cover plate 4, optionally also made of graphite, which returns impure magnesium to the melt 2. The cover plate 4 can be supported directly on the upper region 32 of the condensation vessel 3 or can also be connected to the inner wall of the reduction tank on the side or above. In any case, however, the fastening must be carried out such that the cover can be removed temporarily in order to remove the solidified high-purity magnesium 21 from the condensation vessel 3 without any problems.

For this example of the apparatus, the melt of the magnesium-containing raw material 2 is in direct contact with the outer surface of the condensation vessel 3 in the region 32 during the distillation. Since virtually all of the starting material has evaporated at the end of the distillation process, the condensation vessel is emptied again. The melt entering the gap between the region 31 of the condensation vessel and the region 12 of the reduction tank is of no consequence here, since the condensation vessel does not have to be removed from the reduction tank in order to remove the solidified magnesium. The vapor entering the upper region 32 of the graphite crucible as indicated by arrow 92 condenses below horizontal line 8 and collects in the lower region 33 of the graphite crucible as a high purity melt 21. To prevent boiling of this portion of high purity magnesium 21, a minimum pressure may be maintained within the reduction tank by an inert gas (e.g., argon) that causes the boiling point of magnesium to be higher than the temperature in region 33. The temperature profile of the lower part 33 of the condensation vessel 3 is determined by the independently adjustable heating elements 51.

The reduction tank 1 can be brought into communication with the prechamber 13 via a closure mechanism 6, for example in the form of a flat valve, when the ram 61 is moved in the direction of the arrow 93, wherein the closure mechanism 6 is pressed against a support 62, for example in the form of a ceramic or graphite felt seal, for example by the ram 61, which can be actuated from the outside and can be moved by means of the vacuum sleeve 63. The closure element 6 with the valve plate and the support 62 is located in the present exemplary embodiment above a horizontal line 81 together with the upper region 11 of the reduction vessel 1, i.e. at a temperature above the boiling point of magnesium, so that no solid magnesium deposits in the region of the closure element, as a result of which its function is ensured.

The antechamber 13 has, in the region of lower temperature above the horizontal line 82 representing the melting temperature isotherm, a cover 14, preferably designed as a removable vacuum flange, which has a plurality of connections in addition to the vacuum bushing 63. One of which leads via a line 71 and a valve 72 to a vacuum pump 73, the other via a line 74 to a vacuum gauge 75, and the third via a line 76, a valve 77, a pressure and/or flow regulating device 78 to an inert gas source 79, for example in the form of an argon cylinder.

After the reduction vessel 1 has been evacuated via the prechamber 13 and the temperature required for the distillation process has been reached, the closure mechanism 6 is opened only for a short time in order to set the pressure and to correct the pressure in order to avoid excessive condensation of magnesium vapor in the prechamber 13. The duration of the closing period depends on the pressure rise during the closing time. Ideally, the closure element 6 can be kept closed for a long time and opened only briefly for a longer time interval to check the pressure when a component that strongly releases gas, for example, consists of low-quality graphite, is sufficiently degassed, or when a component consisting of high-purity graphite is used. If the pressure rises above the upper setpoint value during this period, so that the evaporation process of the starting material 2 is impeded, the reduction tank 1 can be connected to the vacuum pump 73 for a certain time by opening the valve 72 until the pressure falls back to the setpoint value range. However, if the pressure drops below the target range, so that there is a risk of evaporation of the high-purity magnesium melt in the lower region 33 of the condensation vessel 3, the reduction tank 1 is connected via a flow and/or pressure regulating device 78 to an inert gas source 79 by opening the valve 77 until the pressure again rises to the target range. In both cases, the closure mechanism 6 is closed immediately after the nominal pressure range has been reached, in order to prevent excess magnesium vapor from entering the antechamber 31.

Once the distillation process according to the embodiment of the apparatus shown in fig. 1 is finished and the reduction tank 1 and its contents have cooled to room temperature, the reduction tank can be opened along the dashed horizontal line 83, for example by sawing, and the high purity magnesium 21 located in the lower region 33 of the condensation vessel 3 can then be removed from the crucible after the cover plate 4 has been removed, for example by turning the entire apparatus upside down. It is also possible to feed new starting materials 2 into the evaporation zone 111 of the reduction tank through the opening. The reduction tank 1 and the prechamber 13 must then be joined together again in a vacuum-tight manner, for example by welding or soldering.

The heating elements 5 and 51 used in the embodiment shown in fig. 1 relate to thermostatically controlled resistance heating elements which are located in the region of the reduction pot associated with the distillation process, i.e. for example in the upper region of the reduction pot where the melt of the starting materials contacts, for adjusting the temperature of the evaporation process, in the lower region 33 of the condensation vessel 3 and in the region immediately surrounding the closure mechanism 6 for checking whether the temperature in these regions is above the melting point of magnesium. Instead of the resistance heating elements 5 and 51, it is of course also possible to replace two induction coils, or a single induction coil wound according to a temperature profile, which heat the reduction tank and/or the material therein.

Fig. 2 shows an embodiment of the device according to the invention which is particularly suitable for operating properly under industrial conditions, wherein identical numerals indicate functionally identical parts as in fig. 1. The reduction vessel 1 is a centrally symmetrical tubular body welded with superalloys and has an upper region 11 with a larger diameter and a lower region 12 with a smaller diameter, wherein the lower region 12 extends in a tubular projection 15 into the upper region 11 of the reduction vessel. In this way, the melt of the starting material 2 is located in the annular crucible, the side walls of which are formed by the shell wall region 111 of the upper region 11 of the reduction vessel 1 and the tubular projection 15, so that the melt does not come into contact with the outer wall of the condensation vessel.

In this embodiment of the apparatus according to the invention, the tubular projection 15 and the lower region 12 of the reduction vessel have a continuously tapered inner wall inside, so that the condensation vessel 3 in the form of a crucible of highly pure graphite with a corresponding shell-face taper has no gaps into which the condensed magnesium melt can enter. Unlike the embodiment of the apparatus shown in fig. 1 in which the condensation vessel 3 has a cylindrical inner bore, since the solidified pure magnesium ingot shrinks much and is generally easy to remove, the condensation vessel 3 in this embodiment of the apparatus also has a tapered inner surface, thereby making it easier to remove the high purity magnesium ingot 21 after solidification.

The opening 31 of the condensation vessel 3 is covered by a V-shaped graphite cover plate 41, which graphite cover plate 41 has radial holes inclined upwards for the magnesium vapor to enter in the direction of the arrow 92. The cover plate 41 can also be designed more complex than shown in the figures, for example, side baffles can also be placed in front of the radial holes, so that the strongly boiling melt of the starting material 2 is not splashed directly into the condensation vessel 3 by centrifugation.

The closure mechanism is here configured in the form of a conical metal plug 64 which can be sealed against a conical seat 113 of the partition wall 112. This sealing process is supported by the liquid magnesium which prevents the passage of magnesium vapour condensing in the sealing gap between the metal plug 64 and the seat 113. The plunger 61 can be operated from the outside via the vacuum sleeve 63 and the sealing ring 631, with which the closure mechanism is closed in the direction of the arrow 92, either manually or automatically by means of control pulses. Instead of the sealing ring 631, the plunger 61 can also be introduced into the antechamber 13 in a sealed manner via the flange 14, for example by means of a metal bellows (metalbag). In the present exemplary embodiment, the flange 14 is sealed off from the cylindrical antechamber 13 by means of a sealing ring 141.

The lines 71, 74 and 76 lead, as in the example described above, to a vacuum pump 73, a vacuum gauge 75 and optionally an inert gas source, except that here the electrically controllable valves 72 and 77 are concerned, so that, in addition to the blocking mechanism 64 for connecting the reduction tank 1 to the prechamber 13, it is also possible to automatically reduce and increase the pressure in the control circuit to a certain nominal pressure range according to the concepts described in the preceding embodiments.

The apparatus differs from the preceding embodiment only in that the upper region 11 of the reduction tank 1 is heated by means of a heating element 5, for example in the form of a tubular resistance furnace, while its lower region 12 is surrounded by a heat-insulating layer 52 which is dimensioned such that the temperature of the condensed high-purity magnesium melt is maintained in the desired range between the melting and boiling point temperatures from the upper region by the heat conduction via the reduction tank shell wall 12 and the condensation vessel shell wall 32 and the heat reaching below by magnesium vapour condensation in combination with the cooling effect of the non-insulated region below the reduction tank shell wall 12. Furthermore, moving the reduction tank in the vertical direction relative to the heating furnace and the thermal insulation layer enables the temperature distribution in the reduction tank 1 to be changed within a certain range in a desired manner.

The temperature of the high-purity magnesium melt 21 can be controlled by means of a temperature measuring sensor 53, for example a K-or J-sheathed thermocouple which projects via the everted part of the shell wall 12 up to the root of the lower region 32 of the condensation vessel 3, and is adjusted appropriately, if necessary, so that it does not fall below the melting point of magnesium. The moment of evaporation of the last remaining starting material can be determined by means of a second temperature measuring sensor 54, for example a sheathed thermocouple, which likewise measures the temperature in the melt of the starting material 2 via the everted part of the shell wall of the region 111 of the reduction vessel. The temperature of the thermocouple suddenly rises at the point where the cooling effect of the evaporation process ceases. The third temperature measuring sensor 55 is located in the everted part of the lower region of the wall of antechamber 13 immediately above the locking mechanism 64 and allows checking whether the locking mechanism is above the melting point temperature of magnesium, since only this ensures that the locking mechanism functions properly. At the end of the furnace period, i.e. before the installation has cooled down, the closure mechanism 64 should be lifted from the seat surface 113 so that it does not weld to the seat surface.

In the first embodiment of the plant, which shows a heating plant variant with two separate heating elements, in the second embodiment the upper reduction tank zone is heated with one heating element 5, wherein the lower zone 12 surrounded by the insulation is indirectly heated jointly by the upper zone, wherein the heat generated by the condensation of magnesium vapour also contributes. According to the method of the present invention, it is also possible to use a device in which a plurality of furnaces are arranged in series in the vertical direction, so that the upper and lower parts of the reduction tank and also the regions of the locking mechanism 6 can be separately adjusted to their specific temperatures by using individual heating elements.

Fig. 3 shows a third embodiment of the device according to the invention, which is substantially similar to the device shown in fig. 2. However, in contrast to the latter arrangement, the condensation vessel 3 in the present exemplary embodiment still has a cylindrical outer surface, so that an annular gap is formed between this outer surface and the inner lateral surface of the lower region 12 of the reduction vessel 1. The crucible can be withdrawn downwardly from the reduction pot by removing the optionally water-cooled flange 15. In order to prevent magnesium vapour from entering the gap and condensing into a liquid state in its lower region or into a solid state below the crucible bottom 34, the flange 15 has an inlet pipe 761 with a flow regulating element 781, by means of which gas originating from an inert gas source can be fed into the space below the bottom 34 of the condensation vessel 3. The amount of inert gas introduced is calculated here such that its velocity in the gap outside the condensation vessel 3 is so high that it exceeds the diffusion velocity of the magnesium vapor which potentially migrates in the reverse direction. Although the pressure increase within the reduction tank is accelerated by the inert gas being introduced in this way, and the interval during which the closure mechanism 64 must be opened for controlling and regulating the pressure becomes comparatively short, this shortening is not significant, since only a small amount of inert gas is required to keep the gap free.

The regulating circuits and mechanisms required to maintain the temperature and pressure of the device are not shown in the drawings, since their mode of operation is not important for carrying out the process, regardless of the accuracy required to carry out the process.

Reference numerals

1 reduction pot, general

11 upper region of reduction tank

111 region of 11 in contact with the starting material

11211 and 13 in the column

113112 cone support 64

114 reduction of the everted part of the shell wall of the tank, as protection for the temperature measuring sensor 54

12 reducing pot narrowing lower zone

121 front chamber wall eversion as a protection for the temperature measuring sensor 53

13 antechamber

131 everted part of the reduction tank shell wall as protection for the temperature measuring sensor 55

14 vacuum flange

141 sealing ring

15 vacuum flange

151 sealing ring

2 Metal starting melt containing magnesium

21 high-purity magnesium melt

Condensation vessels, general

31 condensation crucible opening

32 condensation of the upper region of the crucible

33 condensation of the lower region of the crucible

4 cover plate

41 cover plate with side opening

Zone 5, 11 heating device

51 zone 12 heating device

Zone 52 insulation layer 12

53 condensation vessel bottom zone temperature measuring sensor

531 protective tube of temperature measuring sensor

54 evaporation zone temperature measuring sensor

55 latching mechanism 64 zone temperature measurement sensor

6 locking mechanism

61 push rod of locking mechanism

62 closure mechanism seal support

63 push rod vacuum sleeve

631 sealing ring

64 latching mechanism with tapered seat

71 connection to a vacuum pump

72 lockout mechanism to vacuum pump

73 vacuum pump

74 to the vacuum measuring device

75 vacuum measuring instrument

76 to the closure mechanism

761 connecting line to an inert gas flow regulating device

77 locking mechanism

78 inert gas flow regulating device

781 inert gas flow regulator

79 inert gas source

Lower horizontal limit of temperature range above boiling temperature of 8 deg.C

Upper limit of the level of the temperature range of 81 boiling temperature or higher

Upper horizontal limit of the temperature range of 82 or higher melting temperature

83 horizon of possible sawing

91 arrow, steam generation

Arrow 92, vapor enters the condenser vessel

93 arrow, direction of lifting of the closure mechanism 6

Claims (10)

1. A process for producing high purity magnesium by reduced pressure distillation, characterized in that,

-condensing high purity magnesium in the liquid state,

-the starting materials in the form of magnesium-containing melts are co-located with the upper region of the condensation vessel in the upper region of the reduction tank,

the reduction pot is made of a material that does not release volatile impurities to the magnesium vapour,

bringing the upper region of the reduction vessel to a temperature above the boiling point of magnesium within the limits of two horizontal lines and then keeping the temperature constant so that the vapors rise from the boiling magnesium-containing metal melt and fill the inner space of the upper region of the reduction vessel,

the vapor entering the upper region of the condensation vessel condenses below the lower level and integrates into a high-purity melt in the lower region of the condensation vessel; and is

The entry of contaminated melt dripping from the region above the upper level into the opening of the condensation vessel is prevented by a cover plate which allows impure magnesium to be returned into the melt.

2. The method according to claim 1, characterized in that the reduction tank is heated by heating means other than the reduction tank.

3. A method according to claim 1 or 2, characterized in that a minimum pressure is maintained inside the reduction tank, which pressure is such that the boiling point of magnesium is higher than the temperature in the lower region of the condensation vessel.

4. The method according to claim 1, characterized in that the reduction tank is evacuated via the antechamber and brought to the temperature required for the distillation process.

5. Method according to claim 4, characterized in that the pressure in the antechamber is checked.

6. Apparatus for carrying out the method according to any one of the preceding claims, comprising

A reduction tank and a condensation vessel protected by a cover plate,

-wherein the upper region of the condensation vessel is in the upper region of the reduction tank,

the upper region of the reduction tank is surrounded by a heating element adapted to heat the upper region of the reduction tank between two different levels to a temperature above the boiling point of magnesium,

and the condensation vessel is arranged in such a way that the lower region of the condensation vessel is located below the lower horizontal line.

7. The apparatus according to claim 6, characterized in that the reduction tank is composed of stainless steel.

8. A device according to claim 6 or 7, characterized in that the cover plate consists of graphite.

9. The apparatus according to claim 6, characterized in that the reduction tank communicates with a pre-chamber, wherein the pre-chamber is connected to a means for evacuating the reduction tank.

10. The apparatus according to claim 9, wherein the antechamber is disposed above the reduction tank.