The invention is described hereinafter with particular reference to the condensation of magnesium vapour. It is to be understood that this particular application is described by way of two non-limiting examples and that the principles of the invention can be applied to other volatile metals such as zinc, calcium, sodium and potassium. The metallic vapour may be mixed with an inert gas such as argon.

US patent No. 2,971, 833 entitled"Process of Manufacturing Magnesium" (J. Artru et al) describes what is known as a Magnetherm condenser used in the manufacture of magnesium. The Magnetherm process involves thermal production of magnesium vapour in a furnace and condensation of the vapour in a condenser. The vapour is conducted to a condensation zone in the condenser where it is condensed as partly liquid-and partly solid magnesium. The process is carried out under a vacuum of 4kPa to 10kPa and the condenser crucible is cooled from the outside either by water spray cooling or by immersion in a tank with circulating water. Since the furnace- condenser system is maintained under vacuum, and because the magnesium in the condenser is mostly solid, the process is essentially a batch process which is repeated every 12 to 24 hours. The vacuum has to be broken to remove the slag periodically from the furnace and to replace the full condenser crucible with an empty one.

SUMMARY OF INVENTION The invention provides a method of condensing metallic vapour which includes the steps of: (a) directing a stream of the vapour into a sealed condenser which includes a receiving crucible; (b) controlling the temperature inside the crucible so that the vapour condenses into, and is kept as, liquid metal ; and (c) tapping liquid metal from the crucible.

The metallic vapour may be vapour of a volatile metal such as zinc, calcium, magnesium, sodium and potassium.

In step (a) the stream may be directed at a controlled rate into the condenser.

The metallic vapour may be mixed with an inert gas such as argon.

In the case of magnesium the partial pressure of the vapour, entering the condenser, may be kept in the range of from 0,7 to 1,2 atm. The partial pressure of the inert gas may be maintained in the range of from 0 to 0,3 atm.

The pressure in the condenser may be maintained in the region of from 0,7 to 1,2 atm.

In step (b) the temperature inside the crucible may be controlled so that the vapour is condensed but so that the liquid metal is not allowed to solidify.

A substantial temperature gradient may exist inside the condenser. For example magnesium melts at a temperature of about 650°C and is vaporised at about 1100°C.

WO 03/048398 PCT/ZA02/00195 Clearly the vapour entering the condenser has a temperature above 1100°C while the liquid metal must be maintained at a temperature in excess of 650°C, but below 1100°C, which is the boiling point of magnesium.

The temperature inside the crucible may be controlled by circulating a heat transferring medium through a jacket around the condenser or by using any other heat exchange device. The heat transferring medium may be a liquid metal, eg. lead or tin, or a liquid salt. Initially the heat transferring medium may transfer heat from an external source to the crucible but thereafter, as the temperature of the crucible rises due to the ingress and condensation of the metallic vapour, heat may be extracted from the condenser and the heat transferring medium may be cooled.

The liquid metal may be tapped from the crucible in any appropriate way on a continuous or semi-continuous basis. For example the liquid metal may be tapped through an outlet or it may be siphoned from the crucible.

The invention also extends to apparatus for condensing metallic vapour which includes a sealed condenser which includes a receiving crucible, an inlet through which metallic vapour is directed into the condenser, a temperature control arrangement for controlling the temperature inside the crucible so that the metallic vapour condenses into, and is kept as, liquid metal, and an outlet through which liquid metal is drawn from the crucible.

The temperature control arrangement may comprise a heat exchange system which interacts with the liquid metal inside the crucible and a means for circulating a heat transferring medium through the heat exchange system.

The heat exchange system may include one or more jackets around the crucible, and may contain a tank positioned underneath the crucible.

The heat transferring medium may be a liquid metal or salt and preferably use is made of molten lead or molten tin.

The heat transferring medium may be used for heating the interior of the crucible, at least initially and, once the temperature of the crucible has reached a desired operating state, for removing heat from the crucible.

BRIEF DESCRIPTION OF THE DRAWINGS The invention is further described by way of examples with reference to the accompanying drawings in which: Figure 1 illustrates from the side and in cross section apparatus for condensing magnesium vapour according to a first form of the invention; and Figure 2 is similar to Figure 1 and illustrates apparatus according to a second form of the invention.

DESCRIPTION OF EMBODIMENT Figure 1 of the accompanying drawings illustrates from a side and in cross section apparatus 10 for condensing magnesium vapour according to a first form of the invention.

The apparatus includes a cylindrical receiver crucible 12, a condenser top section 14, an inlet 16 into the top section for introducing a mixture of magnesium vapour and an

WO 03/048398 PCT/ZA02/00195 inert gas such as argon, a condenser outlet 18, and thermally insulating refractory material 20 around the top section 14.

An outlet pipe 22 extends into the interior of the crucible and forms part of a secondary magnesium condenser or magnesium trap 24 which terminates in the outlet 18.

A concentric cooling jacket 26 extends around the condenser crucible 12. The jacket is connected to a temperature controlling circuit 28 which, in this example, is based on the use of liquid lead. The circuit 28 includes a lead pump 30, comprising an electric motor 32, a rotating shaft 34 and an impeller 36, and a lead sump 38 in which the impeller is immersed. A conduit 40 extends from a housing of the impeller to an upper region of the jacket and a return conduit 42 interconnects a lower part of the jacket to the sump.

A tap hole 44 extends from a lower region of the crucible through the jacket 26. An optional access port 46, which can be used for siphoning liquid magnesium from the crucible, extends through a side wall of the crucible at an upper region thereof.

The temperature of the cooling circuit 28 can be controlled by means of a gas burner 48 on top or underneath the lead sump 38. On the other hand heat can be extracted from the molten lead, in the sump, by cooling the sump by means of a water spray 50, by immersion of water-cooled steel pins, or by forced-draft air-cooling of the sump using any suitable technique.

The condenser is made airtight at its inlet 16 and outlet 18, at the ports for the tap hole and the siphon connection, and at the connection between the crucible 12 and

WO 03/048398 PCT/ZA02/00195 the top section 14, by means of water cooled 0-rings and high temperature gaskets.

These components are not shown for they are known in the art.

In use of the apparatus a mixture of magnesium vapour and inert gas, produced in a furnace, not shown, is supplied at a controlled rate to the condenser via the inlet 16.

A furnace which is able to generate magnesium, at atmospheric pressure, from magnesium-oxide containing feed materials is described for example in US patent No. 4,699, 653. The partial pressure of the magnesium, entering the condenser, is kept in the range of from 0,7 to 1,2 atm while the partial pressure, at the inlet of the condenser, of the inert gas, which normally is argon, is kept in the range of from 0 to 0. 2 atm. The pressure in the condenser is atmospheric or close to atmospheric and normally is in the region of between 0,7 and 1,2 atm.

The mixture of metallic vapour and inert gas, at the inlet 16, is above 1100°C. The -gas is forced downwardly as is indicated by means of a succession of arrows 52. In the lower region of the crucible the temperature is considerably reduced and the vapour is liquified. Liquid magnesium 54 is collected in the crucible 12.

The magnesium trap 24 is designed as a secondary condenser to recover at least part of the vapour which is not liquified. The magnesium trap may contain one or more deflection plates to limit magnesium bypassing the condenser and so enhance the recovery of magnesium. The deflection plates are designed such that sufficient cross sectional area is maintained to minimise blockages which can occur from time to time due to carry over of material which does not condense in the main condenser.

Most of the magnesium condensed in the secondary condenser runs back to the condenser crucible.

WO 03/048398 PCT/ZA02/00195 The temperature of the condensed metal inside the crucible can be regulated so that most of the metal is kept in the liquid state. The temperature of the circulating lead, and hence the temperature of the metal inside the condenser, are controlled by cooling or heating of the circulating liquid lead. The lead can be heated during the starting up phase and, for this purpose, use is made of the gas burner 48. Heating may also be employed to compensate for heat losses to the surroundings, during interruptions or during periods of low flow of metal vapour to the condenser, or when the apparatus is relatively small in which event the heat losses may be greater than the heat which is released by condensation.

The temperature at the condenser wall is kept in the region of from 500°C to 700°C by controlling the temperature of the lead which is pumped through the jacket 26. A spiral may be located between opposing walls of the condenser crucible and the jacket to reduce any possible large temperature differences in the jacket.

Once the system has reached steady state operating conditions cooling may be employed to removed the heat of condensation of the metal vapour entering the condenser. Cooling may be achieved by means of water which is directed from the sprays 50 onto the lead sump, by immersion of water-cooled steel pins, or by means of force-draft air-cooling of the sump.

The temperature of the condensed metal, inside the condenser crucible, is maintained at a level above the melting point of the metal, and the condenser is kept at, or near to, atmospheric pressure. It is therefore possible to tap the condensed metal 54 continuously or semi-continuously from the condenser through the tap hole 44, without interrupting metal vapour production and subsequent condensation.

Magnesium tapping is done such that the ingress of air during the operation is

WO 03/048398 PCT/ZA02/00195 minimized. The internal surface area of the condenser is designed to provide sufficient surface area for effective condensing even when the condenser is relatively full of liquid magnesium. The normal operating region is designed for reasonably steady state conditions to effect the required level of heat transfer from the inside of the condenser to the circulating coolant.

The siphon 46 is provided as an alternative to the tap hole 44, for drawing liquid metal from the crucible. Siphon techniques are known in the art and consequently are not further described herein.

Figure 2 of the accompanying drawings illustrates from a side and in cross section apparatus 60 for condensing magnesium vapour according to a second form of the invention.

The apparatus 60 is essentially the same as the apparatus 10 shown in Figure 1, except that the lead circuit and jacket are replaced by a lead tank, positioned underneath the condenser crucible.

The apparatus 60 includes a cylindrical receiver crucible 62, a condenser top section 64, an inlet 66 into the top section for introducing a mixture of magnesium vapour and an inert gas such as argon, a condenser outlet 68, and thermally insulating material 70 around the top section 64. An outlet pipe 72 extends into the interior of the crucible and forms part of a secondary magnesium condenser or magnesium trap 74 which terminates in the outlet 68.

A tank 76 is positioned underneath and surrounding the condenser crucible 62. A volume between opposing surfaces of the tank and the crucible 62 is filled with liquid lead 78.

A tap hole 80 extends from a lower region of the condenser crucible through a wall of the tank 76. The temperature of the lead 78 is controlled by means of gas burners 82 on top, on the sides, or underneath the lead tank 76. On the other hand, heat can be extracted from the molten lead 78 in the tank by cooling the tank by means of a water spray 84, by immersion of water-cooled steel pins, or by forced-draft air- cooling of the tank using any suitable technique.

The condenser is made airtight at its inlet 66 and its outlet 68, at the ports for the tap hole 80, and at the connection between the crucible 62 and the top section 64, by means of water-cooled 0-rings and high temperature gaskets.

Liquid magnesium 86 is collected in the lower region of the crucible 62.

As an alternative to molten lead it is possible to use molten tin in the temperature controlling circuit.

TEST 1 A first test was conducted using apparatus 10 of the kind shown in Figure 1.

A mixture of magnesium vapour and argon gas was supplied at a controlled rate of about 50kg/h magnesium vapour and 5 kg/h argon gas to the condenser, via the inlet 16, for a period of approximately 10 hours. The partial pressure of magnesium, entering the condenser, was accordingly kept at 0.8 atm, while the partial pressure of argon, at the inlet of the condenser, was kept at 0.05 atm, taking into account that the atmospheric pressure at the location of the test was approximately 0.85 atm. The lead was heated during the starting up phase by use of a propane burner (not

snown) underneath the lead sump 38. The temperature at the wall of the condenser crucible was kept in the region of 500°C to 650°C by controlling the temperature of the lead which was pumped through the jacket 26. Once the system had reached steady state conditions, the propane flow to the burner was reduced and adjusted to control the temperature. No cooling of the lead was required because of the relatively low flow of magnesium vapour to the condenser. The temperature inside the condenser crucible could be maintained between 650°C and 750°C, i. e. above 650°C, which is the melting point of magnesium. Liquid magnesium 54 was collected in the condenser crucible 12. After about 10 hours of operation, 265kg of liquid magnesium was tapped from the condenser via the tap hole 44.

TEST 2 A second test was conducted using apparatus 60 of the kind shown in Figure 2.

A mixture of magnesium vapour and argon gas was supplied at a controlled rate of about 50kg/h magnesium vapour and 5 kg/h argon gas to the condenser, via the inlet 66, for a period of approximately 10 hours. The volume between the crucible 62 and the lead tank 76 was filled with lead and was heated during the starting up phase by use of a propane burner 82 underneath the tank. The temperature of the wall of the condenser crucible, in the tank area, was kept between 500°C and 700°C by controlling the temperature of the lead. This was achieved by regulating the propane flow-rate to the burner. Once the system had reached steady state conditions, the propane flow to the burner was reduced and adjusted to control the temperature. No cooling of the lead was required because of the relatively low flow of magnesium vapour to the condenser. The temperature inside the crucible was kept between

650°C and 750°C, i. e. above 650°C, which is the melting point of magnesium. Liquid magnesium 86 was collected in the condenser crucible 76. After about 10 hours of operation, 320kg of liquid magnesium was tapped from the condenser, via the tap hole 80.

The facility provided by the present invention of being able to tap liquid magnesium continuously or semi-continuously from the crucible, without interrupting metal vapour production, should be contrasted with the situation which prevails in the conventional Magnetherm process wherein the magnesium vapour is condensed mainly as solid magnesium. In that process when the condenser crucible is filled with magnesium, the magnesium generating process is stopped, the vacuum is broken, the full crucible is removed, and an empty crucible is connected to the magnesium generating furnace. Downtimes of 15% to 20% of the total operating time are experienced in the Magnetherm process mainly due to the batch-wise operation of the condenser.