CH627715A5 - Process for the production of aluminium chloride of high purity - Google Patents

Description

This invention relates to the production of aluminum chloride. More particularly, this invention relates to a method for controlling the particle size and purity of aluminum chloride.

In the production of aluminum chloride suitable for subsequent electrolytic reduction to metallic aluminum, by chlorination of materials containing aluminum compounds as well as other materials such as silicon, titanium and iron, the resulting chlorides must be separated to provide an aluminum chloride of sufficiently high purity for the satisfactory implementation of the subsequent electrolytic process. In US Patent No. 3,786,135 there is described and claimed a process for recovering high purity aluminum chloride from the gaseous effluent from the chlorination of aluminum compounds, which process includes a first cooling step initial of the hot gaseous effluent sufficient to selectively condense sodium aluminum chloride and other chlorides of high melting point, and separation of these initially condensed products and entrained particles, on the one hand, and l gaseous effluent, on the other hand, followed by further cooling of the gaseous effluent to a second predetermined lower temperature range to condense a high proportion of the remaining volatile constituents which are condensable above the temperature of aluminum chloride condensation. The final step claimed in this process relates to the direct desublimation of high purity aluminum chloride in a fluidized bed of aluminum chloride at a temperature range of about 30 to 100 ° C. It is the field of this third step that the improvements constituting the process of the present invention relate to.

In the aforementioned patent, there is described a fluidized bed containing fluidized particles of aluminum chloride in which the vapors pass at a speed not described. It is said that the vapors pass through the fluidized bed at a temperature of about 30-100 ° C to allow condensation of the vapors on the solid aluminum chloride particles. Filters above the fluidized bed prevent the loss of particles, particularly very fine particles, from the condenser. Means are provided for removing solid aluminum chloride from a location near the bottom of the condenser. As mentioned previously, it is stated that the operating temperature in the condenser is 30 to 100 ° C, preferably about 60 to 90 ° C, and more preferably in the narrower range of 50 to 70 ° C . The patent describes the effect of the condensation temperature on the particle size, noting that at lower temperatures in the specified range of 30 to 100 ° C, the average particle size of the condensed product is generally lower. The patent further indicates that, even in the range of 30 to 100 ° C, a certain amount of the gaseous aluminum chloride does not unplug. It therefore indicates the desirable side of using condensation temperatures at the lower end of the indicated range of 30 to 100 ° C.

Although the operation of the condensation process at the lower end of the interval as described in the aforementioned patent gives a satisfactory particle size as well as an advantageous yield, from an economic point of view, in aluminum chloride, it has been found that this can lead to undesirable condensation of by-products such as titanium tetrachloride. In addition, since the filing of the aforementioned patent in 1971, much has been learned about the operating conditions in the fluidized bed during condensation.

Although it seems that simply raising the condensing temperature would eliminate the contamination problem, it has been found that other operating parameters must also be controlled, in particular the input speed.

It is therefore an object of this invention to provide improvements in the operating parameters of the fluidized bed process for the condensation of aluminum chloride, in order to obtain for example aluminum chloride (A1C13) having a purity and a suitable particle size to a subsequent electrolytic reduction of metallic aluminum.

According to the invention, the gaseous aluminum chloride is passed through a fluidized bed of aluminum chloride particles at an entry speed of 18 m / s to 90 m / s.

Fig. 1 is a vertical section of a condensing apparatus for an embodiment of the method according to the invention.

Fig. 2 is a vertical section showing another embodiment of the invention in which two fluidized beds are used.

Let us first consider fig. 1; the aluminum chloride vapors which have been previously treated by an initial purification means, such as the first two purification stages described in the aforementioned US Pat. No. 3,786,135, enter the condensation chamber 18 via line 6 and entrance 30.

The inlet 30 for the gas containing the gaseous aluminum chloride is preferably provided with a means making it possible to maintain the temperature of the incoming gas at a high value, such as for example an auxiliary heating means and / or an isolation means. such as quartz, alumina, graphite, asbestos, etc., at its entry to minimize, if not prevent premature cooling and liquefaction and solidification of gaseous aluminum chloride passing through it, which would tend to seal by preventing or otherwise interfering with the desired operation of condensation or desublimation.

Due to the need to avoid premature condensation of gaseous aluminum chloride in places other than in the fluidized bed itself, considering the ambient conditions,

the entry 30, preferably, extends appreciably into the bed and stops far from all structural surfaces, including the

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walls of the chamber and the cooling means 26 placed in the chamber.

The gases are introduced into the condensation chamber 18 in order to condense or unblock on the fluidized particles constituting the fluidized bed 16. The fluidized bed 16 comprises particles of aluminum chloride having a particle size between 1 and 500 u. which are fluidized by a fluidizing gas which enters the chamber 18 via the pipe 8. The terms desublimation and desubliming as used herein denote the direct formation of solid aluminum chloride from the gas phase without any significant formation of an intermediate liquid phase, while the terms condensation and condense are used to denote the change from the gas phase to the liquid phase or to the solid phase.

Aluminum chloride vapors, preferably at a temperature of about 250 ° C, enter the bed at a minimum recommended speed of 18 m / s, but up to 90 m / s. Although one does not wish to be bound by any theory of operation, this entry speed allows an appropriate mixing of the hot vapors with the cold fluidized particles which, it is believed, provides a zone of condensation in the bed. near the nozzle.

It is believed that this area of apparent condensation counts in the discovery that the particle size can be at least partially controlled by changes in the input speed. It is believed that an increase in speed can cause the injection of aluminum chloride vapors at 250 ° C deeper into the bed, possibly creating an apparent lowering of the temperature of the condensation zone. These assumptions are based on the observed fact that increases in speed (without any change in bed temperature) result in a decrease in grain size.

The preferred temperature of 250 ° C is reached as the injection temperature of aluminum chloride by an equilibrium of several factors. Lower temperatures lead to the danger that the inlet 30 is clogged with solid aluminum chloride. Higher temperatures have the disadvantage that more heat must be removed from the fluidized bed. Higher temperatures also have a contrary effect on the removal, for example, of sodium aluminum chloride by precooling steps as described in US Patent No. 3,786,135 mentioned above. Nevertheless, in its broadest aspects, the present invention resides in the injection speed interval of 18 to 90 m / s, without limitation as to the injection temperature. If it were necessary to set a wide range for the injection temperature, it would go from a temperature just above the condensation temperature of aluminum chloride up to 350 ° C, and would preferably be from 150 to 300 ° C and better still from 220 to 300 ° C.

This control of the particle size by means of the control of the speed involves the control and the lowering of the particle size without otherwise lowering the overall temperature of the bed, which would cause the simultaneous condensation of a larger quantity of TiCl4 which would harm the purity of the A1C13 product.

Thus, the aluminum chloride vapors entering the bed condense on the particles and the remaining vapors of other impurities such as, for example, titanium chloride, etc., pass through the bed to its top and exit through line 38. Some of these gases are recycled to line 8 to be reused as fluidizing gases while the remaining gas passes through a purifier. The passage of solid aluminum chloride particles in line 38 is prevented by filters 36 which remove or trap all the solid particles.

The purity of the aluminum chloride obtained is maintained at 99.5% by weight or more, with a TiCl 4 content of less than 0.008% by weight, by operating the bed at a temperature of 60-80 ° C. The temperature is maintained in the fluidized bed 16 at 60-80 ° C using cooling coils 26 in which water is passed to a temperature low enough to maintain the bed at this temperature. Although this high temperature results in greater grain size, as stated in the US patent

No. 3786135, the use of the particular input speed of the invention gives a particle size interval which can be used in cells for subsequent electrolytic reduction. Higher bed temperatures (even above 80 ° C.) still give a usable particle size. It will also be noted therefore that the upper limit of the temperature range of the bed is not intended to maintain a correct particle size, but rather to minimize the losses of aluminum chloride which would occur at higher temperatures.

Preferably, the control of the particle size is also maintained in the preferred range of not more than 500 u., By periodically removing the particles of aluminum chloride through the outlet orifice 40 placed near the lower part of the fluidized bed. 16. Periodically, we mean removing 5 to 20% of the bed every hour. It is important for the preferred implementation of the invention that the removal of the particles is carried out near the lower part of the bed so that the larger particles (which are also difficult to fluidize) are removed. By the expression “near the lower part”, is meant a location situated at the lower part of the fluidized bed of the particles or in the lower 10% of the part of the bed to ensure the removal of large particles, as indicated above.

One can also use other means instead of or in addition to the drain at the bottom, to control the size of the particles.

For example, a gas such as an inert gas, that is to say CO2 or N2, can be periodically blown into the fluidized bed 16 at a speed of 90 m / s, for example by means of nozzles placed either in the lower part of the fluidized bed 16, that is to say near the lower part of the bed 16, as the position of the outlet orifice 40 in the drawing. These gas jets act as an attrition agent and cause abrasion between the large particles, which causes them to break into smaller particles, thus obtaining the same desired effect as when emptying with the lower part.

Or the particles can be removed from above or an intermediate portion of the bed and sieved to separate large particles and small particles. Small particles, that is to say less than 10 d and preferably less than 40 [*, will be returned to bed. The larger particles can then be used directly for feeding to a fusion cell or for other purposes, for example as a catalyst, etc., or they can be ground or crushed to the acceptable dimensions indicated above, then returned to the fluidized bed. Obviously, this process can also lead to periodic emptying at the bottom of the bed to remove large particles if the emptying at the top or at the intermediate part is not carried out often enough.

Another solution for controlling the particle size is to provide a mechanical means directly in the bed. This would include a blade in the bed, the control mechanism preferably being mounted outside the side walls of the fluidized bed condenser, the control shaft passing through the side wall (or the bottom wall) of the condenser.

To better illustrate the embodiment of FIG. 1, AlCl3 vapors are passed through a fluidized bed initially containing 50 g of aluminum chloride particles at an inlet speed of approximately 90 m / s while maintaining the bed temperature between 60 and 80 ° vs. Three 10 g samples are taken every hour. The particle size and purity are determined. The average particle size is approximately 300 days. The purity is greater than 99.5% by weight and the content of titanium tetrachloride is less than 0.008% by weight.

The embodiment of FIG. 1, in its preferred form, therefore constitutes a series of operating conditions which, in a single condenser or desublimation apparatus, seeks to control the purity of the product as well as the particle size while trying to limit the losses of chloride by operating the condensing or desublimation apparatus at a temperature of about 60-80 ° C and controlling both the entry speed of the chloride

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of aluminum gas and the overall production volume as well as taking steps to remove excessively large particles of aluminum chloride, preferably at the bottom or near the bottom of the condenser.

The embodiment of FIG. 2 provides another means for controlling the purity of aluminum chloride while maintaining the range of particle sizes and at the same time minimizing chloride losses.

According to the embodiment of FIG. 2, the aluminum chloride vapors are passed through a first condensing apparatus maintained at a temperature of 80-110 ° C with an inlet speed of 18 to 90 m / s. In a preferred embodiment, the aluminum chloride particles are removed near the bottom of the condenser. The remaining vapors are passed with an inlet speed of 18 to 90 m / s in a second condensing apparatus maintained at a temperature of approximately 20 to approximately 50 ° C. The resulting low temperature of the second condensing apparatus ensures the setting of practically all of the chloride entering the second device from the first condensing device.

Now consider fig. 2 in detail; in this figure, the structures similar to those of FIG. 1 received the same reference numbers; a first fluidized bed is shown in 2, comprising a container having a side wall 4 through which the aluminum chloride vapors penetrate through a pipe 6 which ends in a nozzle 30 which projects into a fluidized bed 2.

The chloride vapors condense in the fluidized bed 2 on the fluidized aluminum chloride particles having a particle size between 1 and 500 μm, which are fluidized by a fluidizing gas which enters the bed 2 through an inlet 8. As and as the aluminum chloride vapors condense or disublicate on the aluminum chloride particles, the particle size increases, the larger particles remaining near the bottom of the bed. The coarse particles are preferably removed periodically through the outlet port 40 which is placed near the lower part of the fluidized bed 2. By "near the lower part of the fluidized bed 2", is meant a position which is located at the lower part or within 10% of the lower part of the fluidized bed. By periodically, we mean a removal of 5 to 20% of the bed every hour. Or the larger particles can be removed by the other techniques already described above for the embodiment of FIG. 1.

According to the embodiment of FIG. 2, the fluidized particles of the bed 2 are maintained at a temperature of 80-110 ° C by cooling coils 26 which cool the fluidized bed to the desired temperature range. By keeping the bed at this temperature, impurities such as titanium chloride and chloride

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of silicon remain in the vapor state, which gives a purity of aluminum chloride greater than 99.5%. Note that the aluminum chloride vapors entering the fluidized bed 2 have an inlet temperature which can be as high as 150-250 ° C. As the aluminum chloride vapors condense on the particles in the fluidized bed 2, the remaining gases, including the fluidizing gas, rise to the top of the fluidized bed 2 where the solid particles are retained by filter bags 36, while the remaining gases and the volatile chlorides io leave the fluidized bed 2 via line 38.

Still according to this embodiment of FIG. 2, the hot gases leaving the fluidized bed 2 via line 38 are introduced into a second fluidized bed 52 of aluminum chloride particles, via a nozzle 80 which is similar to the nozzle 30 of the fluidized bed 2. In fact, the two fluidized beds can be identical to each other with regard to the fluidization mechanism, the outlet orifice, the filter bags and the cooling coils. However, according to this embodiment of the invention, the cooling coils 76 of the fluidized bed 52 maintain the temperature of the fluidized particles at about 20-50 ° C to ensure total trapping of all the chlorides contained. These chlorides can then be removed through the outlet orifice 90 which, as indicated above, is placed in a position similar to that of the outlet orifice 40 of the fluidized bed 2. The same removal speed can be used. particles only in the first fluidized bed, that is to say 5 to 20% / h. The remaining gases, including the fluidizing gas, then pass through the filters 86 to join the line 88 from where they are directed, via the line 94, to another purification, or else they are recycled via the line 92 to the lines 8 and / or 58 to be reused as fluidizing gases in the fluidized beds 2 and 52.

The following example serves to better illustrate the advantages of the embodiment of FIG. 2.

Aluminum chloride vapors are passed through a fluidized bed initially containing 50 kg of aluminum chloride particles at an inlet speed of about 90 m / s while maintaining the bed temperature at 80- 110 ° C. The uncondensed vapors pass through the fluidized bed then pass through a filter and an outlet orifice towards a second fluidized bed of aluminum chloride particles maintained at a temperature of 20-50 ° C 40 to cause condensation or desublimation remaining vapors of aluminum chloride. Periodic removal of 5-20% by weight of the aluminum chloride particles from the first bed provides particles which analysis shows to contain 0.004% by weight or less of titanium tetrachloride. Analysis of the 45 gases leaving the second fluidized bed shows a minimum quantity of aluminum chloride still remaining in the form of vapor, which indicates that the chloride losses have been reduced to a minimum value.

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Claims (6)

627,715 1. Process for the production of aluminum chloride by condensation of gaseous aluminum chloride in a fluidized bed of aluminum chloride particles, characterized in that the aluminum chloride is passed through the bed with a speed d entry from 18 m / s to 90 m / s. 2. Method according to claim 1, characterized in that the bed temperature is 60 to 80 ° C. 2 3. Method according to claim 1 or 2, characterized in that in addition to providing the entry speed between 18 and 90 m / s, measures are taken to control the size of the solid particles of aluminum chloride in the fluidized bed so that it is not greater than 500; j .. 4. Method according to claim 3, characterized in that the particles of condensed aluminum chloride are removed at a point close to the lower part of the fluidized bed. 5. Method according to claim 4, characterized in that removes every hour 5 to 20% by weight of the bed. 6. Method according to claim 1, characterized in that: a) the aluminum chloride vapors are introduced at an entry speed of 18 to 90 m / s into a first fluidized bed of aluminum chloride particles whose particle size is not greater than 500 ii, maintained at a temperature of 80-110 ° C; b) the size of the solid particles of aluminum chloride in the fluidized bed during the condensation is controlled, using measures other than the speed of entry, to prevent the development of particles greater than 500 a: c) passing the remaining non-condensed gases and vapors from the first fluidized bed into a second fluidized bed of aluminum chloride particles, with an inlet speed of 18 to 90 m / s, the aluminum chloride particles in the second fluidized bed having a size between 1 and 500 u. and being maintained at a temperature of 20 to 50 ° C, to recover the rest of the chloride from said gases and vapors.