GB1595613A - Method of producing high purity aluminium chloride - Google Patents

Abstract

In a process for the production of aluminium chloride by condensation of gaseous aluminium chloride in a fluidised bed of aluminium chloride particles, the gaseous aluminium chloride is introduced into the bed at an inlet speed of 18 m/s to 90 m/s. The size of the particles of aluminium chloride in the bed is also maintained at less than 500 microns by periodically removing the larger particles which stay in the lower part of the fluidised bed. The aluminium chloride obtained has a purity enabling it to be used in an electrolytic reduction process for the production of metallic aluminium.

Description

(54) METHOD OF PRODUCING HIGH PURITY ALUMINUM CHLORIDE (71) We, ALUMINUM COMPANY OF AMERICA Corporation organized and existing under the laws of the State of Pennsylvania United States of America, of Alcoa Building, Pittsburgh, State of Pennsylvania, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to the production of aluminum chloride. More particularly, this invention relates to an improved process for the control of particle size and purity of aluminum chloride.

In the production of aluminum chloride suitable for subsequent electrolytic reduction to metallic aluminum by the chlorination of materials containing compounds of aluminum as well as other materials such as silicon, titanium, and iron, the resulting chlorides must be separated to provide a sufficiently high purity aluminum chloride for the subsequent electrolytic process to perform in a satisfactory manner.

In King et al U.S. Patent 3,786,135 there is disclosed and claimed a process for the recovery of high purity aluminum chloride from the gaseous effluent of chlorination of aluminum compounds which involves a first step of initially cooling the hot gaseous effluent sufficiently to selectively condense sodium aluminum chloride and other high melting point chloride values therefrom and separating such initially condensed values as well as entrained particles from the gaseous effluent followed by a further cooling of the gaseous effluent to a second and lower predetermined temperature range to condense a high proportion of the remaining volatile constituents that are condensible above the condensation temperature of aluminum chloride. The final step claimed in that process relates to the direct desublimation of high purity aluminum chloride values in a fluidized bed of aluminum chloride at a temperature range of from 3100"C. In a preferred embodiment of the present invention, the process of the present invention may be employed to carry out the final step of the above-mentioned process of U.S. Patent 3,786,135.

In the aforesaid patent there is illustrated a fluidized bed containing fluidized articles of aluminum chloride into which the vapors are passed at an undisclosed velocity. The vapors are said to pass through the fluidized bed at a temperature of 3(l000C. to provide 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. Provision is illustrated for removal of the solid aluminum chloride from a position adjacent the bottom of the condenser. As mentioned above, the operating temperature within the condenser is stated to be from 30--100"C., suitably within 60--90"C. and preferably within the narrower range of 50--70"C. The patentees go on to describe the effect on particle size of the condensation temperature noting that at lower temperatures within the specified range of 30--100"C. the average particle size of the condensed product is generally smaller. The patentees further note that even within the range of 3-100"C., a certain amount of the gaseous aluminum chloride values will not be desublime. They, therefore, indicate the desirability of using condensation temperatures at the lower end of the stated range of 30--100"C While operation of the condensation process at the lower end of the range as taught in the King et al patent does result in a satisfactory particle size as well as an economically attractive yield of aluminum chloride, it has been found that such operation can lead to undesirable condensation of the by-products such as titanium tetrachloride. Furthermore, since the filing of the aforementioned King et al patent in 1971, more has been learned as to the operating conditions within the fluidized bed during condensation.

While it would appear that simply raising the temperature of the condensation would eliminate the contamination problem, it has been discovered that other operating parameters, particularly entrance velocity must also be controlled.

It is therefore an object of this invention to provide improvements in the operating parameter of the fluidized bed process for condensation of aluminum chloride to yield, for instance, aluminum chloride (AICI3) having a purity and particle size suitable for subsequent electrolytic reduction to metallic aluminum.

In accordance with the invention, improvements for the process for the production of aluminum chloride in a fluidized bed comprises passing gaseous aluminum chloride into a fluidized bed of aluminum chloride particles at an entrance velocity of from 18 meters/sec. to 90 meters/sec.

Figure 1 is a vertical, cross section of a condensation apparatus operated in accordance with the invention.

Figure 2 is a vertical, cross section illustrating another embodiment of the invention wherein two fluidized beds are used.

Referring Just to Figure 1, aluminum chloride vapors which have been previously processed through initial purification means such as the first two stages of purification described in the aforesaid King et al 3,786,135 U.S. patent enter condensation chamber 18 via line 6 and inlet 30. The inlet 30 for the gaseous aluminum chloridecontaining gas is desirably provided with means to maintain the temperature of the incoming gas at an elevated value such as for example auxiliary heating means and/or insulation means such as quartz, alumina, graphite, asbestos, and the like at the entrance thereof to minimize, if not prevent, premature cooling and liquefaction and solidification of the gaseous aluminum chloride passing therethrough which would tend to clog the same to impede or otherwise deleteriously affect the desired condensation or desublimation operation.

Because of the need to avoid premature condensation of the gaseous aluminum chloride at locations other than in the fluidized bed itself considering the ambient conditions, the entrance of inlet 30 desirably projects appreciably into the bed and terminates remote from all structural surfaces therewith including the walls of the chamber and cooling means 26 located within the chamber.

The gases introduced into condensation chamber 18 to condense or desublime on the fluidized particles comprising fluidized bed 16. Fluidized bed 16 comprises aluminum chloride particles having a particle size range of from 1 micron to 500 microns which are fluidized by a fluidizing gas which enters chamber 18 through line 8.

"Desublimation" and "desublime" as utilized herein refer to the direct formation of solid aluminum chloride from the gaseous phase without any noticeable formation of an intermediate liquid phase while "condensation" and "condense" are intended to embrace change from the gaseous phase to either the liquid or solid phase.

In accordance with the invention, the aluminum chloride vapors, preferably at a temperature of about 250"C., enter the bed at a minimum velocity of 18 meters/sec and up to 90 meters/sec. While we do not wish to bound by any theory of operation, this entrance velocity provides for adequate mixture of the hot vapors with the cool fluidized particles which is thought to provide a condensation zone in the bed adjacent the nozzle.

This apparent condensation zone is thought to account for the discovery that the particle size can be at least partially controlled by changes in the entrance velocity. It is thought that an increase in velocity may inject the 250"C. aluminum chloride vapors deeper into the bed, thus perhaps creating an apparent lowering of the condensation zone temperature. These postulations are based on the observed fact that increases in velocity (without any change in the bed temperature) result in lowering of the particle size.

The preferred temperature of 250"C. as the aluminum chloride injection temperature is arrived at by balancing of several factors. Lower temperatures lead to danger that the inlet 30 will become plugged by solid aluminum chloride. Higher temperatures have the disadvantage that more heat has to be removed from the fluidized bed. Higher temperatures also run counter to the goal of removing, for instance, sodium aluminum chloride by precooling steps as described in the abovereferenced U.S. Patent 3,786,135.

Nevertheless, in its broader aspects, the present invention lies in the 18 meters/sec.

to 90 meters/sec. injection velocity range, without limitation as to injection temperature. If a broad range for the injection temperature were to be fixed, it would be from just above the aluminum chloride condensation temperature up to 350"C., preferably 150 to 3000C., more preferably 220 to 3000 C.

This control of particle size via velocity control thus results in control and lowering of the particle size without further lowering of the overall bed temperature which would otherwise cause greater amount of TiCI4 to also condense which would adversely affect the purity of the AlCI3 product.

Thus the aluminum chloride vapors entering the bed are condensed on the particles and the remaining vapors of other impurities such as, for example, titanium chloride, pass out to the top of the bed via line 38. Some of these gases are recirculated back to line 8 to be reused as fluidized gas while the remaining gas passes off to the scrubber. Passage of the solid aluminum chloride particles through line 38 is restrained by the filters 36 which remove or recapture all solid particles.

In accordance with the subsidiary aspect of the invention, purity of the aluminum chloride product is maintained at 99.5% by weight or higher, with a TiC14 content of less than 0.008% by weight, by operating the bed at a temperature of from 6e-80"C. The temperature in fluidized bed 16 is maintained at from 6--800C. via cooling coils 26 through which water is run at a temperature sufficiently low to maintain the bed at this temperature. While this elevated temperature does result in a larger particle size, as alluded to in King et al U.S. Patent 3,786,135, the use of the special entrance velocity of the invention results in a particle size range useable in subsequent electrolytic reduction cells. Higher bed temperatures (even above 80 C.) still result in a useful particle size. It should be further noted, therefore, that the upper limit of the temperature range of the bed is not to maintain correct particle size but rather to minimize aluminum chloride losses which would occur at higher temperatures.

In accordance with another aspect of the invention, particle size control to the preferred range of not greater than 500 microns is also maintained by periodic removal of aluminum chloride particles via exit port 40 located adjacent the bottom of fluidized bed 16. By periodic is meant removal of 5% to 20% of the bed every hour.

It is important to the preferred practice of the invention that the particle removal be carried out adjacent the bottom of the bed to insure that the largest particles (which also are difficult to fluidize) will be removed. By the term " . . adjacent the bottom . . . " is meant location either at the bottom of the fluidized bed of particles or in the lowest 10% of the bed height to insure large particle removal as discussed.

Other means can also be used either in place of, or a supplement to, bottom draining to control particle size.

For example, a gas such as an inert gas, i.e., CO2 or N2, may be periodically blown into fluidized bed 16 at a velocity of, for example 90 meters/sec. via nozzles located either in the bottom of fluidized bed 16 or adjacent the bottom of bed 16, such as the position of exit port 40 in the drawing.

These blasts of gas will act as an attriter to provide abrasion between the large particles causing them to break up into smaller particles, thus achieving the same desired effect as when bottom draining is used.

Alternatively, the particles may be removed via the top or an intermediate portion of the bed and screened to separate large and small particles. The small particles, i.e., under 100 microns and preferably under 40 microns would be returned to the bed. The larger particles would then either be used directly for feed to a smelting cell or for other purposes, i.e., catalyst, etc., or ground or crushed to the acceptable sizes stated above and then returned to the fluidized bed. Of course, this method may still entail periodic draining from the bottom of the bed to remove large particles if the top or intermediate draining is not done sufficiently often.

Another alternative for particle size control is the provision of a mechanical grinder directly in the bed. This would comprise a blade within the bed with the drive mechanism advantageously mounted exterior to the sidewalls of the fluidized bed condenser, with a drive shaft passing through the sidewall (or the bottom wall) of the condenser.

To further illustrate the embodiment of Figure 1, AIR13 vapors were passed through a fluidized bed initially containing 50 grams of aluminum chloride particles at an entrance velocity of about 90 meters/sec.

while maintaining the bed temperature at between 6--80"C. Three 10-gram samples were removed each hour. The particle size and purity were analyzed. The particle size averaged about 300 microns. The purity was over 99.5% by weight; and the titanium tetrachloride content was less than 0.008% by weight.

The embodiment of Figure 1 in its preferred form thus provides a set 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 attempting to mitigate the chloride losses by operating the condensing or desublimation apparatus at a temperature of 60--80"C. and controlling both the entrance velocity of the aluminum chloride gases and the overall throughput volume as well as by taking steps for removing oversize particles of aluminum chloride, preferably from the bottom or adjacent the bottom of the condensation apparatus.

The embodiment of Figure 2 provides an alternate means for controlling the purity of the aluminum chloride while maintaining the particle size range and at the same time minimizing the chloride losses.

In accordance with the embodiment of Figure 2, aluminum chloride vapors are passed through a first condensation apparatus maintained at a temperature of from 81 100C. at an entrance velocity of 18 meters/sec. to 90 meters/sec. In a preferred embodiment, the particles of aluminum chloride are removed adjacent the bottom of the condensation apparatus.

The remaining vapors are passed at an entrance velocity 18-90 meters/sec. to a second condensation apparatus maintained at a temperature of from 20"C. to 500C. The resultant low temperature of the second condensation apparatus insures capture of substantially all of the chloride values which enter the second apparatus from the first condensation apparatus.

Referring now to Figure 2 in detail where like structure to that in Figure 1 is given like numerals, a first fluidized bed is shown at 2 comprising a vessel having a sidewall 4 through which the aluminum chloride vapors enter via a conduit 6 which terminates in a nozzle 30 which protrudes into fluidized bed 2.

The chloride vapors condense within fluidized bed 2 on fluidized aluminum chloride particles having a particle size range of 1--500 microns which are fluidized by a fluidizing gas which enters fluidized bed 2 at an inlet 8. As the aluminum chloride vapors condense or desublime on the particles of aluminum chloride, the particles increase in size, with the larger particles remaining near the bottom of the bed. The larger particles preferably are periodically removed at exit port 40 which is located adjacent the bottom of fluidized bed 2. By adjacent the bottom of fluidized bed 2 is meant a position which is either at the bottom or within 10% of the bottom of the fluidized bed. By periodically is meant removal of 5 to 20% of the bed every hour. Alternatively, the larger particles may be eliminated by the other techniques already discussed above for the embodiment of Figure 1.

In accordance with this embodiment of Figure 2, the fluidized particles of bed 2 are maintained at a temperature of 8--110"C.

by cooling coils 26 which cool the fluidized bed down to the desired temperature range.

By maintaining the bed at this temperature, impurities such as titanium chloride and silicon chloride remain in the vapor state resulting in an aluminum chloride purity of greater than 99.5%. It should be noted that the aluminum chloride vapors entering the fluidized bed 2 have an inlet temperature which may be as high as 150250 C. As the aluminum chloride vapors condense on the particles in fluidized bed 2, the remaining gas, including the fluidizing gas, rises to the top of fluidized bed 2 wherein solid particles are restrained via filter bags 36 while the remaining gas and volatile chlorides leave fluidized bed 2 via line 38.

Further in accordance with this embodiment of Figure 2, the hot gases leaving fluidized bed 2 via line 38 are introduced into a second fluidized bed 52 of aluminum chloride particles via a nozzle 80 therein which is similar to nozzle 30 in fluidized bed 2. In fact, both fluidized beds may be identical to one another from the standpoint of the fluidizing mechanism, the exit port, the filter bags, and the cooling coils. However, in accordance with this embodiment of the invention, cooling coils 76 in fluidized bed 52 maintain the temperature of the fluidized particles at a temperature of 2-50"C. to insure complete capture of all chloride values therein. These chlorides may then be removed via exit port 90 which, as previously indicated, is positioned in a similar position to exit port 40 in fluidized bed 2. The same rate of removal of the particles can be used as in the first fluidized bed, i.e., 520% per hour.

Remaining gases, including the fluidizing gases, then pass through filters 86 into line 88 wherein they can be passed on via line 94 for further purification or recycled via line 92 back to lines 8 and/or lines 58 for reuse as fluidizing gas in fluidized beds 2 and 52.

The following example will serve to further illustrate the advantages of the embodiment of Figure 2.

Aluminum chloride vapors are passed through a fluidized bed initially containing 50 kilograms of aluminum chloride particles at an entrance velocity of about 90 meters/second while maintaining the bed temperature at 80--110"C. The uncondensed vapors pass through the fluidized bed and are then passed through a filter and exit port to a second fluidized bed of aluminum chloride particles maintained at a temperature of 2500C.

to cause condensation or desublimation of the remaining aluminum chloride vapors.

Periodic removal of from 520% by weight of the aluminum chloride particles from the first bed yields particles which can be analyzed to show 0.004% by weight or less titanium tetrachloride. Analysis of the off gases from the second fluidized bed show a minimum amount of aluminum chloride still remaining in vapor form indicating that the cloride losses have been reduced to a minimum.

WHAT WE CLAIM IS: 1. In a process for the production of aluminum chloride involving condensation of gaseous aluminum chloride in a fluidized bed of aluminum chloride particles the improvement which comprises passing the aluminum chloride into the bed at an entrance velocity of from 18 meters/second to 90 meters/second.

2. A process according to claim 1, wherein the temperature of the bed is 60 to 80"C.

3. A process according to claim 1 or 2, wherein measures in addition to the provision of the entrance velocity between 18 and 90 meters/second are taken to control the size of the solid particles of aluminum chloride within the fluidized bed to no larger than 500 microns.

4. A process according to claim 3, wherein particles of condensed aluminum chloride are removed from a point adjacent the bottom of the fluidized bed (as hereinbefore defined).

5. The process according to claim 4, wherein from 520% by weight of the bed is removed each hour.

6. The process according to claim 1, characterized by a) introducing the aluminum chloride vapors at an entrance velocity of 18 to 90 meters/second into a first fluidized bed having a particle size range to not greater than 500 microns of aluminum chloride particles maintained at a temperature of 8F110 C.; b) controlling, by measures in addition to entrance velocity, the size of the solid particles of aluminum chloride particles within the fluidized bed during condensation to prevent growth of the particles larger than 500 microns; c) passing the remaining uncondensed gases and vapors from said first fluidized bed at an entrance velocity of 18-90 meters/second into a second fluidized bed of aluminum chloride particles having a particle size range of 1--500 microns and maintained at a temperature of 20--50"C.

to recover the remainder of the chloride values from said gases and vapors.

7. A process for the production of aluminum chloride as claimed in claim 1 substantially as described.

8. Aluminum chloride whenever prepared by the process of claim 1 substantially as described.

Reference has been directed in pursuance of section 9, subsection (1) of the Patents Act 1949, to patent Nos 1407308 and 1402569

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (8)

**WARNING** start of CLMS field may overlap end of DESC **. gases from the second fluidized bed show a minimum amount of aluminum chloride still remaining in vapor form indicating that the cloride losses have been reduced to a minimum. WHAT WE CLAIM IS:

1. In a process for the production of aluminum chloride involving condensation of gaseous aluminum chloride in a fluidized bed of aluminum chloride particles the improvement which comprises passing the aluminum chloride into the bed at an entrance velocity of from 18 meters/second to 90 meters/second.

2. A process according to claim 1, wherein the temperature of the bed is 60 to 80"C.

3. A process according to claim 1 or 2, wherein measures in addition to the provision of the entrance velocity between 18 and 90 meters/second are taken to control the size of the solid particles of aluminum chloride within the fluidized bed to no larger than 500 microns.

4. A process according to claim 3, wherein particles of condensed aluminum chloride are removed from a point adjacent the bottom of the fluidized bed (as hereinbefore defined).

5. The process according to claim 4, wherein from 520% by weight of the bed is removed each hour.

6. The process according to claim 1, characterized by a) introducing the aluminum chloride vapors at an entrance velocity of 18 to 90 meters/second into a first fluidized bed having a particle size range to not greater than 500 microns of aluminum chloride particles maintained at a temperature of 8F110 C.; b) controlling, by measures in addition to entrance velocity, the size of the solid particles of aluminum chloride particles within the fluidized bed during condensation to prevent growth of the particles larger than 500 microns; c) passing the remaining uncondensed gases and vapors from said first fluidized bed at an entrance velocity of 18-90 meters/second into a second fluidized bed of aluminum chloride particles having a particle size range of 1--500 microns and maintained at a temperature of 20--50"C.

to recover the remainder of the chloride values from said gases and vapors.

7. A process for the production of aluminum chloride as claimed in claim 1 substantially as described.

8. Aluminum chloride whenever prepared by the process of claim 1 substantially as described.

Reference has been directed in pursuance of section 9, subsection (1) of the Patents Act 1949, to patent Nos 1407308 and 1402569