NO146390B - PROCEDURE FOR THE PREPARATION OF ALUMINUM CHLORIDE BY GAS-CONDENSATION CONDENSATION - Google Patents

Description

This invention relates to a method for producing aluminum chloride by condensing gaseous aluminum chloride in one or two fluidized beds of aluminum chloride particles. Reference is made to claim 1 f, where the method according to the invention is stated, and to claims 2-4, which state preferred embodiments.

In the production of aluminum chloride which is suitable for electrolytic reduction to metallic aluminium, by chlorination of materials containing compounds of aluminum as well as other materials, such as silicon, titanium and iron, the resulting chlorides must be separated so that an aluminum chloride with a sufficiently high purity for the electrolytic process to proceed satisfactorily. In the U.S. patent no.

3 786 135 describes a method for extracting high-purity aluminum chloride from the gaseous product from chlorine

ring of aluminum compounds, comprising a first step in which the hot gaseous product is first cooled sufficiently

equal to the selective condensation of sodium aluminum chloride and other high-melting chlorides, with the separation of these initially condensed materials as well as trapped particles from the gaseous product, followed by further cooling of the gaseous product to another and lower, predetermined temperature range

prevail, whereby a large proportion of the remaining volatile components that are condensable above the aluminum chloride's condensation temperature are condensed. The last step in said process concerns the direct desublimation of high-purity aluminum chloride in a fluidized bed of aluminum chloride in a temperature range of approx. 3 0-100°C. The further development that comprises the method according to the invention applies to this third step.

In the above-mentioned patent, a fluidized bed containing fluidized particles of aluminum chloride is shown, into which the vapors are fed at an unspecified speed. The vapors are indicated to pass through the vortex with a temperature of approx. 30-100°C, whereby the vapors are condensed on the solid aluminum chloride particles. The filter above the fluidized bed prevents the loss of particles, especially very fine particles, from the condenser. Means are shown to remove the solid aluminum chloride from a location near the bottom of the condenser. As mentioned above, the operating temperature in the condenser is stated to be 30-100°C, advantageously 60-90°C and preferably within the narrower range of 50-70°C. In the patent document, the effect of the condensation temperature on the particle size is further described, and it is stated that at low temperatures within the specified range 30-100°C, particles of condensed product with a generally smaller average size are obtained. Furthermore, it is stated in the patent that even in the range 30-100°C there will be a certain amount of gaseous aluminum chloride that does not desublimate, and that it is therefore desirable to use condensation temperatures in the lower part of the stated range of 30 -100°C.

While carrying out the condensation process at relatively low temperatures within the above range as suggested in the U.S. patent 3,786,135 results in a satisfactory particle size as well as an economically attractive yield of aluminum chloride, it has been found that such an embodiment can lead to unwanted condensation of the by-products, such as titanium tetrachloride. Since the filing of the application corresponding to the above-mentioned patent in 1971, further experiments have been carried out with regard to the operating conditions in the fluidized bed during condensation.

While one would assume that raising the condensing temperature would eliminate the contamination problem, it has been discovered that other operating parameters, particularly the inlet velocity, must also be regulated.

The present invention thus aims to provide improvements in the operating parameter for the fluidized bed process for the condensation of aluminum chloride, by which, for example, aluminum chloride (AlCl^) is obtained with a purity and particle size that is suitable for the later electrolytic reduction to metallic aluminum.

The present invention thus relates to a method for producing aluminum chloride in one or two swirling layers of aluminum chloride particles, characterized in that the aluminum chloride is fed into the swirling layer with an inlet velocity of 18-90 m/sec. Fig. 1 is a vertical section of a condensing apparatus for use in carrying out the invention. Fig. 2 is a vertical section illustrating another embodiment of the invention where two fluidized beds are used.

In fig. 1 comes aluminum chloride vapors which have already undergone a first cleaning treatment, such as e.g. the first two cleaning steps described in the above-mentioned patent 3 786 135, into a condensation chamber 18 via a line 6 and an inlet 30. The inlet 30 for the aluminum chloride-containing gas is appropriately provided with means to maintain the temperature of the incoming gas at an elevated value, such as auxiliary heating devices and/or insulating agents such as quartz, alumina, graphite, asbestos etc., at the inlet to minimize, if not prevent, premature cooling and condensation and solidification of the flowing-through gaseous aluminum chloride, as this easily leads to clogging, whereby the desired condensation or desublimation is prevented or adversely affected in some other way.

Due to the necessity to avoid premature condensation of the gaseous aluminum chloride in places other than in the fluidized bed itself in consideration of the ambient conditions, the mouth of the inlet 30 expediently protrudes a good distance into the fluidized bed and ends at a safe distance from all apparatus surfaces, including the walls of the chamber and the cooling device 26

in the chamber.

The gases are introduced into the condensation chamber 18 for condensation

or desublimation on the fluidized particles that make up

the fluidized bed 16. The fluidized bed 16 consists of aluminum chloride particles with sizes e.g. in the range 1-500 µm,

which particles are fluidized by means of a gas introduced into the chamber 18 through a line 8. "Desublimation" and "de-sublime" are used herein in connection with the direct formation of solid aluminum chloride from the gaseous phase without any significant formation of an intermediate liquid phase,

while "condensation" and "condense" are intended to include change from the gaseous phase to either liquid or solid phase.

According to the invention, the aluminum chloride vapours, preferably at a temperature of approx. 250°C into the fluid bed with a recommended minimum speed of 18 m/sec. and up to 9 0* m/sec. Without the invention being understood to be limited by any theory, it is mentioned that this inlet velocity provides adequate mixing of the hot vapors with the cold fluidized particles, which means

to provide a condensation zone in the fluidized bed near the nozzle.

The aforementioned condensation zone is believed to explain the discovery that the particle size can be at least partially regulated by changes in the inlet velocity. It is believed that an increase in the speed can cause the 250°C hot aluminum chloride vapors to be led deeper into the vortex layer, whereby a reduction in the temperature in the condensation zone appears to be caused. These postulates are based on the observed fact that an increase in velocity (without any change in the temperature of the vortex layer) results in a decrease in particle size.

The preferred temperature of 250°C as the aluminum chloride supply temperature has been arrived at by balancing several factors. Lower temperatures entail a risk of the inlet 30 being blocked by solid aluminum chloride. Higher temperatures have the disadvantage that more heat must be removed from the fluidized bed. Higher temperatures are also contrary to the goal of removing, for example, sodium aluminum chloride in the precooling step, as described in the above-mentioned U.S. Pat. patent 3 786 135. In its broader aspects, the present invention nevertheless lies in the supply velocity range of 18-90 m/sec., without limitation with regard to the supply temperature. If a wide range for the feed temperature were to be determined, it would have to be from immediately above the condensation temperature of the aluminum chloride and up to 350°C, preferably 150-300°C and most preferably 220-300°C.

This regulation of the particle size by means of speed regulation thus results in regulation and reduction of the particle size without further reduction of the temperature of the entire fluidized bed, which would also mean that an increased amount of TiCl^ would also condense, which would have an adverse effect on the purity of AlCl the ^ product.

The aluminum chloride vapors that enter the layer are thus condensed on the particles, and the other vapors from pollution

gives as e.g. titanium tetrachloride or the like, passes out from the top of the layer via line 38. Some of these gases are recycled to line 8 to be used again as fluidizing gas,

while the rest of the gases are led to the washer. Passage of the solid aluminum chloride particles through line 38 is essentially prevented by the filters 36, which remove or retain all solid particles.

According to a preferred feature of the invention, it is used, when there are few impurities present in the gaseous aluminum chloride,

a fluid bed with a temperature of 60-80°C. An aluminum chloride product purity of 99.5% by weight or higher can then be achieved, with a TiCl 2 content of less than 0.008% by weight. The temperature in the fluidized bed 16 is kept between 60 and 80°C by means of cooling pipes 26 through which water with a sufficiently low temperature to keep the fluidized bed at said temperature is passed. While this temperature results in a larger particle size, as suggested in the U.S. patent 3 786 135, the use of the special inlet velocity according to the invention results in a particle size range suitable for the later use in electrolysis cells. Higher fluidized bed temperatures (even above 80°C) still result in a suitable particle size. However, it should also be noted that the upper limit for the temperature range of the fluidized bed is not for maintaining the correct particle size, but rather for minimizing aluminum chloride loss which would be effective at higher temperatures.

Another preferred embodiment of the invention is that particles of condensed aluminum chloride are taken out, in amounts of 5-20% by weight of the fluidized bed per hour, from a point near the bottom of the fluidized bed, to control the size of the solid aluminum chloride particles in the fluidized bed to no more than 500 µm. The removal of the 5-20% of the vortex layer per hour is carried out appropriately periodically. By the expression "near the bottom" is meant either at the bottom of the vortex layer of particles or in the lowest 10% of the layer height, whereby the removal of large particles is ensured as mentioned.

Other means can also be used either instead of or as a supplement to drains or outlets near the bottom to regulate the particle size.

For example, a gas, such as an inert gas, i.e. CC>2 or N2, can be periodically blown into the fluidized bed 16 at a speed of, for example, 90 m/sec. via nozzles arranged either at the bottom of

the vortex layer 16 or near the bottom of the vortex layer 16, for example arranged as the outlet opening 40 in the drawing. These gas blow-ins will act as a scrubbing device that provides abrasion between the large particles, whereby they are broken up into smaller particles, so that the same desired effect is achieved as when an outlet or drain at the bottom is used.

Alternatively, the particles can be removed from the top of the fluidized bed or from an intermediate part of the fluidized bed and sieved to separate large and small particles. The small particles, i.e. particles below 100 μm and preferably below 40 μm would then be returned to the fluidized bed. The larger particles would then either be used directly as starting material for melt bath cells or for other purposes, i.e. catalyst purposes etc, or ground or crushed to the acceptable sizes specified above and then returned to the fluid bed. This method can, of course, still entail periodic extraction from the bottom of the fluidized bed for the removal of large particles if extraction from the top or the intermediate part of the fluidized bed is not carried out often enough.

Another alternative for regulating the particle size is the use of a mechanical crushing or grinding device directly in the fluidized bed. This could consist of a wing or plate arranged in the fluidized bed with a power transmission mechanism suitably mounted on the outside of the side walls of the fluidized bed condenser with a drive shaft laid through the side wall (or bottom)

of the capacitor.

To further illustrate the embodiment of fig. 1, AlCl₂ vapor was passed through a fluidized bed initially containing 50 g of aluminum chloride particles, with an inlet velocity of approx. 90 m/sec., while the fluid bed temperature was kept between 60 and 80°C. Three samples of 10 g were taken per hour. The particle size and purity were investigated. The particle size was found to be approx. 3 00^um on average. The purity was above 99.5% by weight, and the content of titanium tetrachloride was less than 0.008% by weight.

In its preferred form, the embodiment of FIG. 1 thus a set of operating conditions which, in a single condenser or desublimation apparatus, seek to regulate the purity of the product as well as the particle size, while at the same time striving to reduce the chloride losses by using a temperature of approx. 60-80°C in the condensation or desublimation apparatus and regulation of both the inlet speed of the aluminum chloride gases and the total flow volume as well as by taking precautions for removing excess aluminum chloride, preferably from the bottom or somewhere near the bottom of the condensation apparatus.

The embodiment in fig. 2 provides an alternative means of controlling the purity of the aluminum chloride, while maintaining the particle size range while minimizing chloride losses.

In fig. 2, corresponding parts are given the same reference number as on

fig. 1. A first fluidized bed is shown at 2 comprising a container with a side wall 4, through which the aluminum chloride vapors are supplied via a line 6 which ends in a nozzle 30, which projects into a fluidized bed 2.

Fig. 2 illustrates the preferred embodiment according to claim 4, which is particularly useful when relatively large amounts of contaminants are present in the gaseous starting material. Aluminum chloride vapor is supplied with an inlet velocity of 18-90 m/sec. to a first fluidized bed 2 of aluminum chloride particles with particle sizes up to 500 µm, usually in the range 1-500 µm, the particles being fluidized by a fluidizing gas which is supplied to the fluidized bed 2 through an inlet 8. The fluidized bed is kept within the temperature range 80-110°C . The size of the solid particles of aluminum chloride in the fluidized bed 2 during the condensation is regulated by removing particles of condensed aluminum chloride, in amounts of 5-20% by weight of the fluidized bed per hour, from a point near the bottom of the fluidized bed (outlet opening 40), so that particle growth beyond sizes of 500 µm is prevented. The remaining uncondensed gases from the fluidized bed 2 are fed with an inlet velocity of 18-90 m/sec. into another fluidized bed 52 of aluminum chloride particles having a particle size within the range 1-500 µm, and which is kept at a temperature of 20-50°C, whereby the rest of the chlorides are recovered from the gases.

Alternatively, the relatively large particles can be removed on them

other ways already described above in connection with fig. 1.

According to this embodiment of fig. 2, the fluidized particles in the fluidized bed 2 are kept at a temperature of 80-110°C by means of cooling pipe 26, which cools the fluidized bed down to the desired temperature range. By keeping the fluidized bed at this temperature, impurities such as titanium tetrachloride and silicon tetrachloride will remain in the vaporous state, resulting in an aluminum chloride purity of over 99.5%. It should be noted that the aluminum chloride vapors supplied to fluidized bed 2 have an inlet temperature which can be as high as 150-250°C. While the aluminum chloride vapors are condensed on the particles in the fluidized bed 2, the remaining gas, including the fluidizing gas, rises to the top of the fluidized bed 2, where solid particles are retained by filter bags 36, while the remaining gas and volatile chlorides leave the fluidized bed 2 through line 38.

The hot gases which in the embodiment of fig. 2 leaves the vortex layer 2 via line 38, is fed into another vortex layer 52

of aluminum chloride particles via a nozzle 80, which is of the same type as the nozzle 30 in the fluidized bed 2. The two mentioned fluidized beds can be completely similar with regard to the fluidization mechanism, the outlet opening, the filter bags and the cooling pipes. However, according to this embodiment of the invention, the cooling pipes 76

in the fluidized bed 52 the temperature of the fluidized particles is 20-50°C, whereby a complete capture or retention of all chloride material is ensured. The chlorides can then be removed via the outlet opening 90, which, as described above, is arranged in a similar position to the outlet opening 40 in the fluidized bed 2. The same removal rate can be used for the particles as in the first fluidized bed, i.e. 5-20% per

hour. Remaining gases, including the fluidizing gases, then pass through the filter 86 to line 88, in which they can be carried on via line 94 for further purification or recycled via line 92 back to line 8 and/or line 58 to be used again as fluidizing gas in vortex layers 2 and 52.

The following example will further illustrate the advantages of the embodiment of fig. 2.

Aluminum chloride vapor is passed through a fluidized bed which initially contains 50 kg of aluminum chloride particles, with an inlet velocity of approx. 90 m/sec., the temperature of the vortex layer being kept at 80-110°C. The non-condensed vapors flow through the fluidized bed and then through a filter and an outlet opening to another fluidized bed of aluminum chloride particles, which is maintained at a temperature of 20-50°C, thereby causing condensation or desublimation of the remaining aluminum chloride vapors. Periodic removal of 5-20 weight percent of the aluminum chloride particles from the first fluidized bed provides particles that can be analyzed and shown to contain 0.004 weight percent titanium tetrachloride or less. been reduced to a minimum.

Claims (4)

1. Process for producing aluminum chloride by condensing gaseous aluminum chloride in one or two fluidized beds of aluminum chloride particles, characterized in that the aluminum chloride is fed into the fluidized bed at an inlet velocity of 18-90 m/sec. 2. Method according to claim 1, where there is little contamination present, characterized in that a fluidized bed with a temperature of 60-80°C is used. 3. Method according to claim 1 or 2, characterized in that particles of condensed aluminum chloride are taken out, in quantities of 5-20% by weight of the fluidized bed per hour, from a point near the bottom of the fluidized bed, to control the size of the solid aluminum chloride particles in the fluidized bed to no more than 500 µm. 4. Method according to claim 1, where large amounts of contaminants, for example TiCl^, are present, characterized in that a) aluminum chloride vapor is supplied with an inlet velocity of 18-90 m/sec. to a first fluidized bed of aluminum chloride particles with particle sizes up to SOO^um, which fluidized bed is kept at a temperature of 80-110°C, b) the size of the solid particles of aluminum chloride in the fluidized bed during condensation is regulated by removing particles of condensed aluminum chloride, in amounts of 5-20% by weight of the vortex layer per hour, from a point near the bottom of the fluidized bed, so that particle growth is prevented beyond sizes of 500^ura, c) the remaining uncondensed gases from said fluidized bed are fed with an inlet velocity of 18-90 m/sec. into another fluidized bed of aluminum chloride particles having a particle size in the range of 1-500 µm, and which is kept at a temperature of 20-50°C, whereby the rest of the chlorides are recovered from the gases.