GB2049452A

GB2049452A - Purification of aluminum chloride

Info

Publication number: GB2049452A
Application number: GB8013345A
Authority: GB
Original assignee: Aluminum Company of America
Current assignee: Howmet Aerospace Inc
Priority date: 1979-05-21
Filing date: 1980-04-23
Publication date: 1980-12-31
Also published as: NL8002806A; JPS55158121A; AU531330B2; FR2457256A1; FR2457256B1; JPS595525B2; GB2049452B; PL124027B1; OA06536A; AU5708480A; CA1121982A; PL224362A1; BR8003086A; GR67754B

Abstract

Aluminum chloride (AlCl3) is recovered from a mixture of gases containing aluminum chloride and other metal chlorides, including titanium tetrachloride (TiCl4), silicon tetrachloride (SiCl4) and ferric chloride (FeCl3). According to this recovery process, the gaseous chloride mixture is subjected to fractional distillation utilizing an array of distillation columns. A sufficient amount of titanium tetrachloride is maintained in any column which is operated to separate at least one metal chloride including silicon tetrachloride from a mixture of gases containing aluminum chloride and other metal chlorides and which is operated at temperatures which include the sublimation or melting points of aluminum chloride at the operating pressure of the column to prevent solidification of aluminum chloride in the column.

Description

SPECIFICATION Purification of aluminum chloride This invention relates to the production of aluminum chloride. More particularly, this invention relates to a process for recovering pure aluminum chloride from a mixture of metal chlorides.

In the production of aluminum chloride by the chlorination of a material such as kaolin clay, a gaseous mixture of metal chlorides will be obtained. This mixture, while predominating in aluminum chloride, will also contain the chlorides of silicon (SiCI4), titanium (TiC14) and iron (FeCI3), which metals are found as impurities in kaolin. The gaseous effluent from the chlorination reactor will also contain non-condensable gases such as nitrogen (N2), carbon monoxide (CO), carbon dioxide (CO2), hydrogen chloride (HCI), chlorine (Cl2), and very small amounts of other metal chlorides, as well as the aforementioned metal chlorides.

To recover the aluminum chloride from such a mixture of metal chloride gases, it is necessary to provide a separation process. Variation processes for the separation of one or more metal chlorides from a gaseous mixture containing various metal chlorides are known.

Several of these processes involve the selective condensation or fractional distillation of particular metal chlorides from the mixture. For example, Dewing's U.S. Patent No.3,436,211 describes a procedure for removing calcium and magnesium chlorides from aluminum chloride gas by selective condensation of the calcium and magnesium chlorides. In addition, U.S. Patent No.3,786,135 of King metal. describes a process by which sodium aluminum chloride is selectively condensed from the gaseous effluent from a chlorination reactor which has been utilized to chlorinate Bayer process alumina.

Arnold's U.S. Patent No. 2,387,228 discusses a separation process by which titanium tetrachloride and silicon tetrachloride may be separated from a mixture containing aluminum chloride and ferric chloride by heating the mixture above the boiling temperatures of the silicon tetrachloride (57.6"C. at atmospheric pressure) and titanium tetrachloride (136.4"C. at atmospheric pressure) to drive these chlorides off as gases, leaving a residue consisting essentially of aluminum and ferric chlorides. The Arnold Patent also discloses and claims a two-step fractional distillation process by which aluminum chloride may be separated from a mixture containing ferric chloride, silicon tetrachloride and titanium tetrachloride.The first step in this process reportedly removes gaseous silicon tetrachloride and titanium tetrachloride by fractional distillation at a pressure greater than the vapor pressure of the mixture at its melting temperature. The second step also involves fractional distillation under pressure, and reportedly separates gaseous aluminum chloride from liquid ferric chloride.

It has been our experience, however, that problems have arisen when a fractional distillation or selective condensation procedure has been utilized to separate aluminum chloride from a mixture of gases containing other metal chlorides. For example, the desublimation of aluminum chloride from a gas to a solid often interferes with a fractional distillation step by which silicon tetrachloride is separated from a molten mixture containing titanium tetrachloride, ferric chloride and aluminum chloride. This type of separation typically is carried out in a conventional distillation column having a reboiler, a condenser, a refluxing means and sufficient plates or packing to effect the desired separation.The temperature at the bottom of the column is maintained at a lever higher than the melting point of the mixture and the temperature at the top of the column is maintained at the boiling point of the silicon tetrachloride at the column's operating pressure.

Thus, silicon tetrachloride may be removed in a gaseous or liquid state at the top of the column. However, the range of temperatures through the column often includes or is sufficiently close to the sublimation temperature of aluminum chloride that appreciable amounts of aluminum chloride may desublime within the column from gas to solid, and thereby clog or plug the column, this interfering with its distillation function.

It is also known to use liquid chloride solvents to dissolve and thereby separate a particular metal chloride from a mixture of metal chloride gases. For example, Krchma's U.S. Patent No. 2,533,021 describes the separation of ferric chloride from a mixture of gases by dissolving the ferric chloride in a solvent consisting of a mixture of molten ferric chloride and sodium chloride. Lerner's U.S. Patent No.3,294,482 describes the separation of ferric chloride from a gaseous mixture of metal chlorides which contains the chlorides of iron, columbium, tungsten, molybdenum, and zirconium, by scrubbing the gaseous mixture with a molten mixture of ferric chloride and sodium chloride.

Finally, U.S. Patent No.3,938,969 of Sebenik metal. refers to a method of separating aluminum chloride from a mixture containing ferric chloride by dissolving the aluminum chloride in a titanium tetrachloride solvent. This Patent states that the solubility of aluminum chloride in titanium tetrachloride is relatively low, and that relatively large quantities of titanium tetrachloride are therefore required to dissolve a quantity of aluminum chloride. The Sebenik Patent also describes and claims a separation method by which aluminum chloride may be separated from a mixture of gaseous metal chlorides by employing a solvent which will preferentially dissolve aluminum chloride and which may dissolve ferric chloride, while failing to dissolve, or at most dissolving only sparingly other chlorides such as titanium tetrachloride and silicon tetrachloride.

Another use for the application of a liquid metal chloride to a gaseous mixture of chlorides is discussed by Dr. Robert Powell on page 91 of a 1968 publication of the Noyes Development Corporation, entitled "Titanium Dioxide and Titanium Tetrachloride". He describes the evaporation of liquid titanium tetrachloride in a stream of chlorination gases to cool said stream.

It has now been discovered that aluminum chloride can be separated and recovered from a mixture of gases containing aluminum chloride and other metal chlorides including titanium tetrachloride, a silicon tetrachloride and ferric chloride. The recovery may be carried out by subjecting the gaseous chloride mixture to a number of fractional distillation steps utilizing an array of distillation columns. According to the present invention, a sufficient amount of titanium tetrachloride is maintained in any column which is operated to separate at least one metal chloride including silicon tetrachloride from a mixture of gases containing aluminum chloride and other metal chlorides and which is operated at temperatures which include the sublimation or melting points of aluminum chloride at the operating pressure of the column to prevent solidification of aluminum chloride in the column.

According to a preferred embodiment of the invention, the gaseous chloride mixture is introduced into a first distillation column operated at temperatures sufficient to separate silicon tetrachloride by distillation from the mixture. The undistilled chlorides from the first column are then introduced into a second distillation column operated at temperatures sufficient to separate titanium tetrachloride by distillation from the remainder of the chlorides, and sufficient liquid titanium tetrachloride is added to the first column or upstream thereof to prevent precipitation of aluminum chloride and to dissolve any solid aluminum chloride condensing in the first column (and thereby clogging or plugging the column).The undistilled chlorides from the second column are then introduced into a third column to distill aluminum chloride from the remaining chlorides (which include ferric chloride), and purified aluminum chloride is recovered by condensing the aluminum chloride vapors distilled from the third column.

In accordance with the preferred embodiment of the process of the invention, at least a portion of the titanium tetrachloride recovered from the second distillation column may be recycled to the first distillation column to act as a solvent for the aluminum chloride therein.

In the accompanying drawings: Figure 1 is a schematic flow diagram generally illustrating the present invention as utilized in connection with the operation of an array of distillation columns in a clay chlorination process; Figure 2 is a schematic flow diagram generally illustrating an alternative array of distillation columns which may be utilized in place of the array of columns shown in Figure 1.

Referring particularly to the drawings, Figure 1 shows a clay chlorination reactor 10 in which suitable amounts of ore, such as kaolin clay, a carbon reductant and chlorine are reacted. The clay chlorination reactor produces a mixture of metallic chloride vapors and non-condensable gases. Because of the differences in the boiling points of the various metal chlorides, aluminum chloride product may be separated by fractional distillation of the vapors and gases through a series of distillation columns A, B and C.

The gaseous effluent from the chlorination reactor 10 typically includes the following metal chlorides: Metal Chloride Per Centby Weight AICI3 30-35 SiCI4 10-15 TiCI4 1-2 FeCI3 1-2 Other metal chlorides such as sodium chloride (NaCI) and calcium chloride (CaCI2) may be present in small amounts. However, it will become apparent from the following discussion that such heavier chlorides do not contaminate the fractionally distilled chlorides. Rather, the heavier metal chlorides, if present, remain in their liquid state and are removed from the distillation columns with the ferric chloride.

The gaseous effluent from the chlorination reactor 10 also includes the following non-condensable gases: nitrogen (N2) carbon monoxide (CO) carbon dioxide (CO2) hydrogen chloride (HCI) These gases constitute the significant balance of the percent by weight of the gaseous effluent. There may also be traces of additional non-condensable gases, such as phosgene (COCI2) and chlorine (Cl2).

In the process of producing aluminum chloride by fractional distillation of the metal chlorides in the chlorination reactor gaseous effluent, the gas stream is cooled. Cooling is typically accomplished by introducing a recycle stream 14 of metal chlorides from a downstream quench absorber 16 into the gaseous effluent line 12. Such recycle typically reduces the temperature of the gaseous effluent in line 12 from about 600 C. at the exit of the reactor to about 325"C. at the entrance of the quench absorber 16. It will be apparent to those skilled in the art that the cooled gas stream may also be passed through dust collectors, cyclones, or the like, (not shown) for cleaning purposes. Cooling of the gaseous stream to about 325"C. does not condense the gaseous chlorides of silicon, titanium, aluminum and iron.

The following table sets forth the melting and boiling temperatures for pertinent metal chlorides: "C. at one atmosphere pressure Metal Chloride Melting Point Boiling Point SiCI4 -70 57.6 TiCI4 -30 136.4 AICI3 * * FeCI3 282 315 NaCI 804 1413 CaCI2 772 > 1600 *AICI3 sublimes at a temperature of approximately 183"C. at a pressure of one atmosphere and melts at about 193"C. at a pressure of 2.3 atmospheres.

Prior to fractional distillation, the cooled gaseous effluent preferably enters a quench absorber 16 through line 12. The quench absorber 16 may be maintained with a top temperature of about 100"C. The gaseous effluent exits the quench absorber through line 18 and passes through at least one heat exchanger. In the preferred embodiment illustrated in Figure 1, the gaseous effluent from the quench absorber passes through two heat exchangers 20 and 22. The first heat exchanger 20 is maintained at a temperature of about 60"C.

which is sufficient to condense the majority of the metal chlorides in gas stream line 18. The second heat exchanger is maintained at a temperature of about -15 C. which is sufficient to condense essentially all of the remaining titanium tetrachloride and a portion of the silicon tetrachloride in the gas stream from the first heat exchanger 20. The condensate from the two heat exchangers 20 and 22 is fed back to the quench absorber 16 through return lines 24 and 26, respectively. It will be understood by those skilled in the art that the less volatile metal chlorides, ferric chloride in particular, remain condensed when they enter the quench absorber and do not pass through the heat exchangers. The condensed metal chlorides in the quench absorber 16 are preferably fed from the bottom thereof to a first distillation column A.The non-condensable gases and an appreciable quantity of silicon tetrachloride exit the second heat exchanger 22 through feed line 28. To begin fractional distillation, the condensed metal chloride mixture is fed from the quench absorber into the first distillation column a through feed line 30. By operating column A with a bottom temperature higher than the melting point of the mixture, such as about 200"C. at about 4.5 atmospheres pressure, and a top temperature which is approximately equal to the boiling point of silicon tetrachloride at the column's operating pressure, such as about 100 C. at about 4.5 atmospheres pressure, substantially all of the silicon tetrachloride is separated from the mixture, removed from column A through line 32, and condensed in condenser A'.A portion of this condensed silicon tetrachloride is returned to column A as reflux through line 34, and the balance is returned to reactor 10 through line 36. Alternatively, the column could be operated in conjunction with a partial condenser (not shown) to provide the required liquid silicon tetrachloride reflux stream. If such a condenser were employed, the uncondensed balance of the silicon etrachloride would return as a vapor to reactor 10 through line 36. It will be understood by those skilled in the art that the cooled gaseous effluent from the reactor 10 could be fed directly to first distillation column A without passing through intermediate quench absorber 16.If such were done, silicon tetrachloride would be driven from column A along with the non-condensable gases and would be separated from the gases downstream of condenser A', possibly in a second, lower-temperature condenser. By removing substantially all of the non-condensable gases in quench absorber 16, however, the condensate from condenser A' is substantially pure silicon tetrachloride. This silicon tetrachloride is recycled through line 36 to clay chlorination reactor 10 because the presence of additional silicon tetrachloride inhibits the further production of silicon tetrachloride in the clay chlorination reactor.

The temperature in first distillation column A is not high enough to vaporize the remaining metal chlorides; therefore, the liquid titanium tetrachloride, aluminum chloride and ferric chloride mixture is fed from the first column through feed line 38 to second distillation column B. By operating column B with a top temperature approximately equal to the boiling point of titanium tetrachloride at the operating pressure of the column, such as at a temperature between about 190" and 200"C. at about 4.5 atmospheres pressure and at a somewhat higher bottom temperature, such as at a temperature between about 230" and 235"C. at about 4.5 atmospheres pressure, substantially all of the titanium tetrachloride is driven out of the column through line 40 in the form of vapor to condenser B'.Liquid titanium tetrachloride exits condenser B' through line 42. A portion of this liquid is returned to column B through line 44 as reflux. A second portion may be recycled to first distillation column A through lines 48, 50, 80 and 82 or 84 and 86 to inhibit the accumulation of solid aluminum chloride therein. Alternatively, liquid titanium tetrachloride may be added upstream of column A, as at quench absorber 16 through lines 52 and 54, or to column A from outside source 90 through lines 88, 80, and 82 or 84 and 86. An additional portion of the titanium tetrachloride that passes out of condenser B' through lines 42 and 48 may be cooled and introduced via lines 50,52 and 76 into chamber 70, where it acts as a solvent for the gaseous silicon tetrachloride therein.This chamber and its use are more particularly described in the copending, commonly owned Patent Application entitled "Method of Removing a Low Boiling Point Metal Chloride from a Gaseous Stream". As described therein, the liquid solution of silicon tetrachloride and titanium tetrachloride is removed from chamber 70 through feed line 72 to quench absorber 16. The non-condensable gases which remain in the gas stream that is washed in chamber 70 are removed through line 74 for further treatment (not shown). Finally, the portion of the titanium tetrachloride from condenser B' that is not recycled to quench absorber 16, orto column A orto chamber70 may removed from the system, as for example, through lines 48 and 78.

The remaining liquid metal chlorides, aluminum chloride and ferric chloride, from column B pass through line 46 to column C where the aluminum chloride is separated as a gas. Aluminum chloride separation may be accomplished by operating the third distillation column with a top temperature of about 235"C. and a bottom temperature of about 350"C. at a pressure of about 4.5 atmospheres. It will be understood by those skilled in the art that if there are additional metallic chlorides in theSaseous stream from the chlorination reactor, such as sodium chloride or calcium chloride, these less volatile metallic chlorides will remain in the liquid state throughout the distillation process and will exit the third column through line 68 along with the ferric chloride.

Gaseous aluminum chloride passes from column C through line 56. A portion of this gas enters condenser C' through line 58, whereupon it is condensed and returned to column C as reflux through line 60. The gaseous aluminum chloride which is not passed into condenser C' flows through line 62 to desublimer 64.

The desublimer converts gaseous aluminum chloride to solid aluminum chloride product, which exits via line 66.

The present invention is particularly directed to a fractional distillation procedure in which a first distillation column is utilized to separate silicon tetrachloride from a gaseous chloride mixture containing titanium tetrachloride, ferric chloride and aluminum chloride. In accordance with the invention, as shown in Figure 1, additional amounts of titanium tetrachloride are added upstream of column A two quench absorber 16 to enter the column with the liquid mixture through line 30, orto column A either at the point offeed, located at 86, or at a higher point in the column, generally shown at 82.This additional titanium tetrachloride can be obtained from an independent source via lines 88 and 80 or may be fully or in part obtained, via lines 42,48, 50 and 80, from the titanium tetrachloride recovered from condenser B'.

The purpose of recycling titanium tetrachloride to distillation column A or upstream thereof is to inhibit the precipitation, desublimation or condensation of solid aluminum chloride on the plates or packing in that column. The titanium tetrachloride acts as a solvent for the aluminum chloride, and thus any aluminum chloride which has condensed or desublimed on the plates is dissolved thereby. For optimum effect, it is preferable to add the titanium tetrachloride both at 82 and at 86 and to quench absorber 16. The amount of titanium tetrachloride which is added for solvent purposes to column A or upstream thereof will vary with the amount of titanium tetrachloride already present in the effluent from reactor 10 and with the temperature and pressure conditions of the system.However, it is believed that, in accordance with the invention, an amount of at least 0.5 kilograms, and preferably an amount between 0.5 to 10 kilograms of titanium tetrachloride (depending on the amount oftitanium tetrachloride in the effluent from reactor 10) should be admitted to column A per kilogram of aluminum chloride introduced via line 30.

While the additional titanium tetrachloride fed to column A is shown as entering the column both at the feed entryway and a point above the entryway, it is within the scope of the invention to also add additional titanium tetrachloride at a point adjacent to the bottom of the column if this becomes necessary to aid in the transport of the mixed chlorides leaving the bottom to pass to column B. However, if a sufficient amount of titanium tetrachloride is added above this point or upstream of column A, it may not be necessary to add additional titanium chloride at the bottom of the first distillation column.

An alternative array of distillation columns in which the present invention may be utilized as illustrated in Figure 2. This array may be interchanged with the array of distillation columns shown in Figure 1. As shown in Figure 2, fractional distillation may be initiated by introducing the condensed metal chloride mixture from quench absorber 16 of Figure 1 into distillation column D through line 30. This column may be operated with a bottom temperature higher than the melting point of the mixture, such as about 205"C. at about 4.4 atmospheres pressure, and a top temperature which is approximately equal to the boiling point of titanium tetrachloride at the column's operating pressure, such as about 1 90"C. at about 4.1 atmospheres pressure.

By operating column D in this fashion, substantially all of the silicon tetrachloride and titanium tetrachloride is separated from the mixture, removed from column D through line 112, and condensed in condenser D'. A portion of the condensate is returned to column D as reflux through line 114, and the balance is passed to column E through line 116. Since the temperature in column D is not high enough to vaporize the metal chlorides other than titanium tetrachloride and silicon tetrachloride, the liquid remaining, consisting essentially of aluminum chloride and ferric chloride, is passed from column D to column F through line 118.

Column E is operated with a top temperature which is approximately equal to the boiling point of silicon tetrachloride at the operating pressure of the column, such as about 1 05"C. at about 4.1 atmospheres pressure. The bottom temperature of this column is below the boiling point of titanium tetrachloride at the operating pressure of the column, such as about 205"C. at about 4.4 atmospheres pressure. Operation of column E at these temperatures and pressures allows the major portion of the silicon tetrachloride to exit the column as a gas through line 120. This gas is condensed in condenser E', and a portion of the condensate is returned to column E as reflux through line 122. The remainder of the condensed silicon tetrachloride is returned to reactor 10 via line 124. Since the temperature in column E is not high enough to vaporize the titanium tetrachloride therein, it is removed from the column as a liquid through line 126. A porton of this liquid titanium tetrachloride may be recycled to column D through lines 128 and 130 or 132 to inhibit the accumulation of solid aluminum chloride therein. Alternatively, liquid titanium tetrachloride may be added upstream of column D, as at quench absorber 16 through lines 128, 134 and 135, orto column D from outside source 90 through lines 138, 128 and 130 or 132. An additional portion of the liquid titanium tetrachioride that passes out of column E through line 126 may be introduced via line 128, 134 and 137 into chamber 70, where it acts as a solvent for the gaseous silicon tetrachloride therein.Finally, the portion of the liquid titanium tetrachloride from column E that is not recycled to column D, or to quench absorber 16 orto chamber 70 may be removed from the system through lines 126 and 136.

The remaining liquid metal chlorides, aluminum chloride and ferric chloride, from column D pass through line 118 to column F where the aluminum chloride is separated as a gas. This separation may be accomplished by operating this column with a top temperature of about 235"C. and a bottom temperature of about 350"C. at a pressure of about 4.5 atmospheres. It will be understood by those skilled in the art that if there are additional metallic chlorides in the gaseous stream from reactor 10, such as sodium chloride or calcium chloride, these less volatile chlorides will remain in the liquid state throughout the distillation process and will exit column F through line 148 along with the ferric chloride.

Gaseous aluminum chloride exits column F through line 140. A portion of this gas enters condenser F' through line 142, whereupon it is condensed and returned to dolumn F as reflux through line 144. The gaseous aluminum chloride which is not passed into condenser F' flows through line 146 to desublimer 64.

Whereas the preferred embodiments of this invention have been described herein for purposes of illustration, it will be apparent to those skilled in the art that numerous variations thereof may be made without departing from the scope of the invention.

Claims

1. A process for the separation ofAICI3 from a mixture of gases containing AICI3 and other metal chlorides, including TiCI4, SiCI4 and FeCI3, by fractional distillation utilizing an array of distillation columns, which comprises maintaining a sufficient amount of TiCI4 in any column which is operated to separate at least one metal chloride including SiCI4 from a mixture of gases containing AIR13 and other metal chlorides and which is operated at temperatures which include the sublimation or melting points of AICI3 at the operating pressure of the column to prevent solidification of AICI3 in the column.

2. A process according to claim 1, wherein the mixture of gases is derived from the chlorination of a material containing aluminum oxide.

3. A process according to claim 1 or 2, comprising: (a) introducing the mixture into a first distillation column operated at temperatures sufficient to separate SiCI4 by distillation from the mixture; (b) introducing the undistilled chlorides from said first column into a second distillation column operated at temperatures sufficient to separate TiCI4 by distillation from the remainder of said chlorides; (c) adding sufficient liquid TiC14 to said first column to dissolve any solid AICI3 condensing in said first column; (d) introducing the undistilled chlorides from said second column into a third column to distill AICI3 from the remaining chlorides including FeCI3; and (e) recovering purified AIR13 by condensing the AICI3 vapors distilled from said third column.

4. A process according to claim 3, wherein said TiCI4 added to said first column is added with the mixture.

5. A process according to claim 3 or 4, wherein at least a portion of said TiCI4 added to said first column to dissolve AIR13 is recovered from said second column.

6. A process according to any one of claims 3 to 5, wherein said first column is operated at temperatures of about 200"C. at the bottom of the column and about 100"C. at the top of the column at 4.5 atmospheres.

7. A process according to any of claims 3 to 6, wherein said second column is operated at temperatures of about 230-235"C. at the bottom of the column and about 190-200"C. at the top of the column at 4.5 atmospheres.

8. A process according to any one of claims 3 to 7, wherein said third column is operated at about 4.5 atmospheres and at about 350"C. at the bottom of the column and about 235"C. at the top of the column.

9. A process according to any one of claims 3 to 8, wherein all of the TiCI4 recovered from said second column is recycled to said first column to dissolve solid AICI3 in said first column.

10. A process according to claim 9, wherein said recycled TiCI4 is added directly to said first column at the point of feed of the gases into said first column.

11. A process according to claim 10, wherein sufficient TiCI4 is added to said first column that from 0.5 to 10 kilograms of TiC14 per kilogram of AICI3 are present in the introduced mixture therein.

12. A process for the separation of AIC13 from a mixture of gases containing AICI3 and other metal chlorides, including TiC14, SiC14 and FeCI3 by fractional distillation utilizing an array of distillation columns, substantially as hereinbefore described with reference to the accompanying drawings.

13. Purified aluminum chloride, when prepared in accordance with the process of any one of the preceding claims.