CZ226099A3 - Isolation process of tetrafluoromethane, hexafluoroethane from a gas - Google Patents

Abstract

The method consists in and / or removing inorganic fluorides from the offgas an electrolyzer for the production of aluminum containing inorganic fluorides and at least one of CF 4 and Cy 6, thereby obtaining purified degassing and contacting the cleaned degassing with the membrane at a temperature of 10 to 80 ° C, a pressure drop of 0.02 to 1.4 MPa; a flow rate between 10 -4 to 10 rpm / h 2 to obtain a retained current gas rich in at least one of CF4 and QFg and expired gas stream deprived of at least one of CF4 and C2F6.

Description

Technical field

The invention generally relates to a method for separating gases. In particular, the invention relates to a method for removing at least one of CF 3 and C ₂ F ₆ from a gas stream using a membrane. In particular, the invention also relates to a process for the production of aluminum, wherein at least one of CF ₄ and C ₂ F _{6 is} removed from the gas stream using a membrane.

BACKGROUND OF THE INVENTION

Normally, aluminum metal for commercial purposes is produced in two stages. The first step involves extracting alumina (Al ₂ O ₃ ) from bauxite using the Bayer process. The second step involves the reduction of alumina, which is dissolved in a mixture of molten cryolite (Na ₃ AlF ₆ ) and aluminum fluoride (AlF ₃ ) in an electrolyser at a temperature of about 950 to 960 ° C using the Hall-Heroult process. In a second step, the aluminum-containing ions are reduced electrochemically to form metallic aluminum on the surface of the metal cathode.

During normal electrolysis, carbon anodes are consumed as a result of the reaction of oxygen-containing ions with carbon anodes to form carbon dioxide and metallic aluminum according to the following reaction:

2A1 ₂ O ₃ + 3C -> 4A1 + 3CO ₂

However, under certain conditions, an anode effect occurs in the electrolyser, which is characterized by the following phenomena:

(1) the concentration of alumina in the electrolyte falls below 2% by weight;

2) when the concentration of oxygen-containing ions decreases, higher concentrations of fluoride ions predominate near the anode;

3) anode polarization voltage increases significantly,

(4) the critical current density at the anode is exceeded, at which the charge still only occurs at the anode for oxygen-containing anions; and

5) on the anode surface, where appropriate, elimination occurs F ₂ decomposition of cryolite.

During the anode effect, fluorine excreted on the anode

reacts with carbon anodes for creation CF ₄ and C ₂ F ₆ acc following reaction: 2Na ₃ AlF ₆ + 2C 2A1 + 2NaF + cf ₄ + c ₂ f ₆ . Said anode the effect is more detailed described in: Alton T. Tabereaux, Anode Effects, PFCs, Global Warming and the

Aluminum Industry, JOM, pp. 30-34 (November 1994).

For a normal cell, the daily emission of CF ₄ and C ₂ F _{g is} 0.25 kg. The aluminum production plant normally comprises 100 to 200 electrolysers, which in such a plant represents a daily emission of CF ₄ and C ₂ F _{b of} about 50 kg.

The emission of CF ₄ and C ₂ F _O from plants for the production of aluminum is normally discharged directly into the atmosphere. However, these gases, which are 10,000 times more greenhouse effect than carbon dioxide, have recently been classified as significantly contributing to the overall warming of our planet. After signing the United Nations Framework Convention on Climate Change and seeking to reduce the amount of gas emissions causing such global warming, it is desirable to achieve aluminum production facilities. · Minimize or eliminate completely the emissions of these emissions into the atmosphere.

It is an object of the present invention to accomplish this task within the aluminum industry.

The solution according to the invention will be apparent from the following description, the claims and the attached figures.

SUMMARY OF THE INVENTION

The invention relates to a process for the isolation of at least one of CF ₄ and C ₂ F ₆ from an off-gas from an aluminum production electrolyzer. The method comprises the following steps:

(a) removing inorganic fluorides from the off-gas containing inorganic fluorides and at least one of CF ₄ and C ₂ F _S to obtain a purified off-gas, and

b) contacting the cleaned offgase with the membrane under conditions to obtain a retained gas stream rich in at least one of CF ₄ and C ₂ F ₆ and a passed gas stream free of at least one of CF ₄ and C ₂ F ₆ .

The invention further relates to a method for producing aluminum. The method comprises the following steps:

(a) the electrolytic reduction of alumina dissolved in a mixture of molten cryolite and aluminum fluoride in an electrolyzer to produce aluminum;

b) withdrawing the offgas containing F ₂ , HF and at least one of CF ₄ and C ₂ F _b from the electrolyzer,

(c) contacting the off-gas with alumina under conditions in which aluminum fluoride and a gas stream containing at least one of CF ₄ and C ₂ F ₆ are formed;

d) recycling at least a portion of the aluminum fluoride from step c) to the electrolyzer; and

e) contacting a gas stream comprising at least one of CF ₄ and C ₂ F ₆ with a membrane under conditions to obtain a retained gas stream rich in at least one gas ·· ··· from CF ₄ and C ₂ F _b and a passing gas stream free of at least one of CF, and C..F ..

i. b

Brief description of the pictures

In the accompanying drawings, FIG. 1 shows a flow diagram of a method of insulating CF ₄ and C ₂ F ₆ according to the invention; FIG. 2 shows a graph showing the relative permeability of N ₂ ;

CF ₄ and C ₂ F _b through a specific membrane at different temperatures, and Figure 3 shows an embodiment of a membrane system useful in the present invention.

Most generally, the invention relates to a process for removing at least one of CF ₄ and C ₂ F _children from the gas stream. Preferably, the invention relates to a process for the common removal of CF ₄ and C ₂ F ₆ from a gas stream comprising these components.

Prior to said separation, the gas stream preferably comprises about 0.01 to about 20 vol% of at least one of CF ₄ and C ₂ F _ti . In addition to the content of at least one of CF ₄ and C ₂ F _6, the gas stream may also contain oxygen, carbon dioxide and nitrogen, as well as other gaseous components.

The process of the invention comprises the step of bringing a gas stream into contact with the membrane under conditions such that a retained gas stream rich in at least one of CF ₄ and C ₂ F ₆ and a passed gas stream free of at least one of CF ₄ and C ₂ F _a .

Preferably, the retained gas stream is devoid of oxygen, carbon dioxide and nitrogen, while the passed gas stream is rich in oxygen, carbon dioxide and nitrogen. If both fluorocarbon components CF ₄ and C ₂ F _b are present in the gas stream, contacting the membrane preferably results in a retention gas stream rich in both fluorocarbon components CF ₄ and C ₂ F _b and expired. a gas stream depleted of the two fluorocarbon components of CF ₄ and C ₂ F _O.

Within this specification and claims, the term rich means that the concentration of the respective component in the gas stream is higher than the concentration of the component in the gaseous stream before it is brought into contact with the membrane. Similarly, deprived means that the concentration of the respective component in the stream is lower than the concentration of that component in the gas stream prior to contact with the membrane.

The retained gas stream preferably comprises about 25 to about 100 vol% of at least one of CF ₄ and C ₂ F _o . The passed gas stream preferably contains about 0 to about 0.01 vol% of at least one of CF ₄ and C ₂ F ₆ .

According to a preferred embodiment of the invention, the purity of the passed and retained gas stream is improved by contacting the streams with additional membranes arranged in series. Such a method can be implemented as a multistage membrane separation system as shown in FIG. As can be seen from this figure, both the expired and the retained gas stream can be contacted with another membrane unit to increase the purity of these streams. The number of such contact steps man may vary depending on the desired purity of each of the two gas streams introduced. Using such a procedure, it is possible to achieve 100% isolation of at least one of CF ₄ and C ₂ F _S with a purity close to 100%.

Any membrane can be used in the process of the invention as long as it is able to selectively retain at least one of CF ₄ and C ₂ F ₆ and to pass the other gaseous components present in the gaseous stream. The membrane material should also be substantially inert to the gaseous components to be separated, which means that it should not react with these components.

The membranes useful in the present invention are preferably glassy membranes, such as polymeric membranes preferably made of polyimides, polyamides, polyamide-imides, polyesters, polycarbonates, polysulfones, polyether sulfones, polyether ketones, alkyl-substituted aromatic polyesters, mixtures of polyether sulfones, aromatic polyimides, aromatic polyamides, polyamide-imides, fluorinated aromatic polyimides, polyamides and polyamide-imides, glassy polymeric membranes, such as those described in USSN 08 / 247,125 (May 20, 1994), and membranes made of cellulose acetates, mixtures thereof, and mixtures thereof; Olymer β jakož iz polymer kop olymer kop kop kop kop kop kop kop kop kop kop olymer kop olymer kop olymer olymer olymer olymer olymer olymer olymer olymer olymer olymer olymer olymer kop olymer olymer olymer olymer kop kop olymer olymer olymer olymer alkyl, aryl), and the like.

Asymmetric membranes are prepared by precipitation of polymer solutions in non-solvents miscible with the solvents used. Such membranes are characterized by a dense separation layer supported by an anisotropic carrier, have a porosity to be detected and are prepared in a single step. Examples of such membranes and methods of making them are described in U.S. Patents 4,113,628, 4,378,324, 4,460,526, 4,474,662, 4,485,056, 4,512,893, 5,085,676 and 4,717,394. The last two of these patents disclose the preparation of asymmetric separation membranes from specific polyimides. Particularly preferred membranes are the polyimide asymmetric gas separation membranes described in said U.S. Patent 5,085,676.

In a pressurized separation process using a separation membrane to separate gases, one side of the separation membrane is contacted with a complex multicomponent gas mixture, with some of the gaseous components passing through the membrane faster than the other components. The separation membrane thus allows some of the gaseous components to pass through it, while for the other components of the gas mixture it constitutes a relative barrier. The relative velocity of gas passing through the membrane is a function of the composition of the material from which the membrane is made and its morphology. It has been established in the prior art that the intrinsic permeability of a polymer membrane is determined by a combination of gas diffusion through a membrane controlled by a partially filled and molecular free volume of material and by the solubility of gas in said material. Selectivity is expressed as the ratio of permeability of two gases separated by the membrane material. In this regard, it is highly desirable to provide defective dense separation layers ensuring high selectivity of gas separation.

Composite separation membranes typically have a dense separation layer on a preformed microporous substrate. The separation layer and the substrate usually have different compositions. Composite separation membranes having the structure ι A A * · ► A A · have been developed

A A ··

A A 4 A A 4

AAA ·· <

A <

AA AA

2,947,687, 2,970,106

2,953,502, 2,960,462,

No. 4,602,922, 4,132,824, consisting of an ultra-thin dense release layer supported by an anisotropic microporous support. Such composite membrane structures were obtained by laminating an ultra-thin dense release layer to a preformed anisotropic membrane support. Examples of such membranes and processes for their preparation are disclosed in U.S. Patents 4,644,669, 4,689,267, 4,741,829,

3,616,607, 4,714,481

4, 7.13,292, 4,086,310,

4,192,824, 4,155,793 and 4,156,597.

Alternatively, composite separation membranes intended for gas separation may be obtained by a multi-stage preparation process in which an anisotropic porous substrate is first formed, and then the substrate is contacted with the membrane-forming solution. Examples of such methods are described in U.S. Patents 4,826,599, 3,648,845 and 3,508,994.

U.S. Pat. No. 4,756,932 discloses a process for preparing composite hollow fiber membranes by co-extruding several layers of polymer solutions which subsequently precipitate in the non-solvent of said polymers as solvents.

It is miscible with used

In one embodiment of the method of the invention, the membrane may subsequently be treated or coated with a fluorinated or polymer layer to increase its resistance or co-perfluorinated to the harmful components present in the gas mixture from which CF and C, F ₆ are to be separated. this is at a low concentration of said harmful components or at temporary contact with such components.

The course of the variable, hollow fiber spinning membranes depends on many that may affect the morphology and properties of the hollow fibers. These variables include the composition of the polymer solution used to form the fibers, the composition of the fluid injected into the hollow fiber extrudate cavity during spinning, the spinneret temperature, the coagulation medium used to process the hollow fiber extrudate, the coagulation medium temperature, the coagulation rate of the polymer,

> · «

0 · ·· <

extrusion of the fiber, the speed of the winding fiber onto the winding drum, and the like.

The temperature of the gaseous mixture and / or membrane during the contact period may range from about -10 ° C to about 100 ° C. Preferably, the temperature is between about 10 and 80 ° C. More preferably, this temperature is from ambient temperature, i. from about 20 to about 25 ° C, to about 60 ° C.

In the process of the invention, it is preferred to achieve a pressure drop across the membrane of less than about 14 MPa. More preferably, said pressure drop should be between about 0.002 and about 1.4 MPa. Even more preferably said pressure drop should be about 0.14 to 0.42 MPa.

The desired pressure drop across the membrane can be achieved in one of two ways. In the first of these methods, the gas stream brought into contact with the membrane is compressed. Preferred compressors are sealed and oil-free compressors, such as those commercially available under the trade name Powerex by Powerex Harrison Company, Ohio. In the latter method, which is more advantageous, the pressure drop across the membrane is achieved by reducing the pressure on the side of the membrane leaving the gas flow through the membrane. A vacuum pump or any other suction device may be used to achieve such a pressure drop across the outlet side of the membrane.

The flow velocity of gas flow through the membrane can vary from about 0 to about 10 ⁵ Nm ³ / h per square meter of membrane available for separation. Preferably said flow rate is about asi ⁴ to 10 Nm ^! / hm ² . More preferably, this flow rate is about 0.1 to about 0.5 Nm ³ / wt ² .

In a preferred embodiment, the invention relates to a method of isolating at least one of CF ₄ and C ₂ F ₆ from an off-gas leaving an aluminum production electrolyzer. Such a gas comprises 1) gaseous components such as O ₂ , CO ₂ and N ₂ , 2) inorganic fluorides such as F ₂ , HF and NaAlF _4, and 3) at least one of CF 3 and C ₂ F ₆ . This off-gas φ φφφ φ φ *

it may also contain hydrocarbons and a large number of particulate materials.

The first step of the process involves removing said inorganic fluorides from the off-gas to obtain a purified off-gas. Said inorganic fluorides are preferably removed from the offgas using a caustic scrubber. The scrubber can be wet or dry. Dry scrubbers are unlike resin or soda-lime scrubbers, and some catalyst-containing dry scrubbers such as MnO ₂ may also be used. Examples of wet scrubbers that can be used in the present invention are described in the brochure entitled Selecting a CDO for your Particular Application from Delatch Corporation. If various harmful components are to be removed, it is preferable to use a dry scrubber or dry scrubbers in series with the wet scrubber or wet scrubbers.

Preferably, one or more filters are downstream of the scrubber or scrubbers to remove particulate materials from the offgas. Preferably, a filter having pores having a diameter of less than 20 microns, more preferably less than 10 microns, is used for this purpose.

The second step of the process comprises contacting the cleaned offgase with the membrane under conditions to provide a retained gas stream rich in at least one of CF ₄ and C ₂ F ₆ and a passed gas stream free of at least one of CF ₄ and C ₂ F ₅ . This separation membrane step can be carried out as described above.

In the event that a certain proportion of particulate material remains in the cleaned offgas before it is contacted with the membrane separation unit, it is envisaged in the present invention to use an additional filter or filters to remove this residual particulate material.

The invention further relates to a method for producing aluminum. The method comprises the following steps:

a) electrolytic reduction of alumina dissolved in the mixture

ί »9 • 9

99 9 9 9 999 · · .99 molten cryolite and aluminum fluoride in an electrolyzer to produce aluminum,

b) evacuating a flue gas comprising F ₂ , HF and at least one

CF ₄ and C ₂ F _ó from the electrolyzer,

(c) contacting the off-gas with alumina under conditions such that aluminum fluoride and a gas stream containing at least one of CF ₄ and C ₂ F _b are formed;

d) recycling at least a portion of the aluminum fluoride from step c) to the electrolyzer; and

e) contacting a gas stream containing at least one of CF ₄ and C ₂ F _b with a membrane under conditions to obtain a retained gas stream rich in at least one of CF ₄ and C ₂ F _b and an expired gas stream free of at least one from CF ₄ and C ₂ F ₆ .

The general features of this method are shown in FIG. Current

1. Alumina, cryolite and alumina are fed to an aluminum production electrolyzer 2 in which molten aluminum is produced by the electrolytic reduction of alumina. The operating conditions and equipment necessary for carrying out this step are well known to those skilled in the art. The molten aluminum is then removed from the electrolyzer 2 via line 3. The off-gas stream 4 is also removed from the cell 2. This offgas stream 4 contains O ₂ , CO ₂ , N ₂ , F ₂ , HF, NaAlF ₄ , CF ₄ , C ₂ F ₆ and a particulate fraction. The off-gas stream 4 is optionally passed through a filter 5 to remove said particulate fraction.

The filtered flue 6 is optionally removed from the filter 5 and fed to the dry scrubbing zone T. In this dry scrubbing zone, the filtered off-gas 6 is contacted with the alumina fed via line 8 under conditions such that inorganic fluorides such as F ₂ and HF are removed from the filtered off-gas. Such conditions of operation of the dry scrubber zone are well known to those skilled in the art. During the residence of the offgas in the dry scrubbing zone 1, the alumina reacts with the F ₂ and HF contained in the filtered offgas 6 to form aluminum fluoride according to the following reactions:

fe «fe fe • fe fefe fefe · fefe fefefe

A1 ₂ O ₃ + F ₂ -> A1F + ₂ 0 A1 ₂ O ₃ + HF -> A1F + H ₂ 0

The aluminum fluoride containing stream 9 is then withdrawn from the dry scrubbing zone and recycled to the stream 7. The dry scrubber zone 2 will also provide a purified gas stream 10 containing CF ₄ , C ₂ F ₆ , O ₂ , CO ₂ and N ₂ . The cleaned gas stream 10 is then fed to an additional filter 11, ^when it is necessary to remove any remaining particulate portion which is still present in the purified gas stream 10th The cleaned gas stream 10 is then fed to the separation membrane system 12, whose one possible embodiment is shown in Figure 3, in which the separation membrane system is obtained retentate gas stream 14 and passed through the gas flow 21 · Expired gas stream 13 comprises mostly of O ₂ , CO ₂ and N ₂ , while the retained gas stream 14 predominantly contains CF ₄ and C ₂ F ₆ . This retained gas stream 14 containing CF ₄ and C ₂ F _S can be further purified here or elsewhere to obtain a CF ₄ / C ₂ F ₆ stream that is suitable for use in the semiconductor industry.

Examples

In the following, the process according to the invention will be explained in more detail by means of specific examples thereof, which examples are for the purpose of illustration only and are not intended to limit the scope of the invention as defined by the claims.

Example 1

The gas stream containing CF ₄ , C ₂ F _fa and N ₂ is contacted with a polyimide asymmetric hollow fiber composite membrane at different temperatures to determine the relative permeability of CF ₄ , C ₂ F ₆ and N ₂ . The gas stream had a constant flow rate of 170cc. The feed gas pressure was kept constant at 0.3 MPa. The permeability of each component is ·

4 • 4

4 ·

4444 444

4 4

4 4 4 4

4 · · ·

4 4 · 4 44 «4 44

4 · 4

4 4 4

444 444

4 is shown graphically in FIG. As can be seen from FIG. 2, the selectivities CF ₄ / N ₂ and C ₂ F _Ó / N _{2 have} values of about 300.

Based on the above-mentioned selectivities, a computer simulation of a single-stage membrane separation unit was performed. Concentration and pressure of the feed gas stream, the passed gas stream and the retained gas stream, as well as the insulation yield

CF ₄ / C ₂ F ₆ are shown in Table 1 below.

Table 1

Introduced current Expired current Detained current Isolation yield CF ₄ / C ₂ F ₆ Flow 10 9.7 0.3 (Nm7h) [CF ₄ ] 0.1 1.2 x 10 ' 3.9 (% vol) [c ₂ fj 0.1 1.5 x 10 · 3.9 (% vol) Pressure 2 0.02 1.97

(MPa)

98.7%

Example 2

• 9. ·· · ·

9

9 9 ♦

9 9 9

9 · 99

99

9 9 9

9 9 9

999 999

The procedure of Example 1 is repeated using the feed gas concentration and pressure shown in Table 2 below, which also gives the simulated results obtained.

Table 2

Introduced current Expired current Detained current Isolation yield CF, / C ₂ F _s Flow 10 9.4 0, 6 (Nm ^{* 3} / hr) [CF ₄ ] 1.0 0.04 16.5 (% vol) C c ₂ f ₆ ] 5.0 0.16 83.1 (% vol) Pressure 3 0.02 2, 97

(MPa)

96.9%

Example 3

In a typical smelter, with 100 electrolysers, the flue gas flow is about

NM ³ / hr. The procedure described in Example 1 was repeated using the off-gas. The concentration and pressure of the flue gas supplied and the simulated results obtained for the flue gas are given in Table 3 below.

Table 3 • fl ·· »·· <

• fl • fl

Introduced current Expired current Detained current Yield, isolation cf ₄ / c ₂ f ₆ Flow 3 2.6 0.4 (Nm ³ / h) [CFJ 10.0 0.2 79.2 (% vol) [c ₂ fj 1.0 0.02 7.9 (% vol) Pressure 1 0.1 0, 97

(MPa)

98.1%

Example 4

In this example, the feed gas stream containing 1.14% CF ₄ and C ₂ F ₆ and the N ₂ residue is contacted with a polyimide asymmetric hollow fiber composite membrane at room temperature. The flow rates, concentrations, and pressures of the feed current, passed current, and retention current, together with the recovery yield of CF ₄ / C ₂ F _{6, are} shown in Table 4 below.

Table 4 ··············································

Introduced current Expired current Detained current Insulation yield CF ₄ / C ₂ F _ft Flow 313.8 310 3.8 (Nm7h) [CFJ - 0.45 <0.0 01 37.7 (% vol) [c ₂ fj 0, 69 0.003 57, 9 (% vol) Pressure θ ', 7 8 0.1 0.77

(MPa)

99,999%

While the process of the invention has been described in terms of preferred embodiments, it is to be understood that variations and modifications of the features described herein will be apparent to those skilled in the art.

Claims (20)

PATENT CLAIMS 32C0 - <Η ·· ·· A process for the isolation of at least one of CF ₄ and C ₂ F ₆ from an off-gas from an electrolyser for the production of aluminum, comprising: (a) removing inorganic fluorides from the off-gas containing inorganic fluorides and at least one of CF ₄ and C ₂ F ₆ to obtain a purified off-gas, and b) contacting the purified vent gas with a membrane at conditions which yield a retentate gas stream rich in at least one of CF ₄ AA expired gas stream stripped of at least one of CF ₄ and C ₂ F _a. The process according to claim 1, wherein the inorganic fluorides are F ₂ , HF and NaAlF ₄ . Method according to claim 1, characterized in that the step (a) is carried out using a caustic scrubber. 4. The method of claim 1 wherein said off-gas further comprises particulate materials and wherein said off-gas is contacted with a filter to effect removal of said particulate materials prior to step a). 5. The method of claim 1 wherein the cleaned off gas further comprises particulate materials and wherein the cleaned off gas is contacted with a filter to achieve removal of said particulate materials prior to the step. b). • ·· · # ·· ·· 9 · «9 9 9 9 ·· 9 9 · 999 · 9 · 9 · · · · · · · · · · · · · · · · · · · · · 9 9 ··· · 999 9 9 ·· ·· ** The method of claim 1, wherein the off-gas comprises CF ₄ and C ₂ F _h and wherein the retained gas stream is rich in CF ₂ and C ₂ F _r and said passed gas stream is free of CF ₄ and C ₂ F ₆ . The method of claim 1, wherein the off-gas further comprises O ₂ , CO ₂ and N ₂ . 8. The method according to claim 7, c h t en characterized in that the retentate gas stream is freed from 0 _2, C0 ₂ and N _2, and said expired gas stream rich in 0 _2, C0 ₂ and N _second The method of claim 1, wherein the off-gas comprises about 0.01 to about 20 vol% of at least one of CF ₄ and C ₂ F ₆ . The method of claim 9, wherein the retained gas stream comprises about 25 to about 100 vol% of at least one of CF ₄ and C ₂ F ₆ and the passed stream comprises about 0 to about 0.01 vol% of at least one from CF ₄ and C ₂ F ₆ . 11. The method according to claim 1 characterized in that said conditions include a temperature between about 10 and about 80 ° C, the pressure drop between about 0.02 and about 1.4 MPa and a flow rate between about 10 ^-4 and about 10 NmVh .m ² . 12. The method of claim 1 wherein the membrane is selected from a plurality of membranes made of polyimides, polyamides, polyamide-imides, polyesters, polycarbonates, polysulfones, polyethersulfone, polyetherketone, alkyl-substituted aromatic polyesters, and mixtures of polyethersulfone, aromatic polyimides, aromatic polyamides, polyamide-imides, fluorinated aromatic polyimides, polyamide and polyamide-imides. 9 9 9 9'9 • 9 13. A process for the production of aluminum, comprising: e (a) the electrolytic reduction of alumina dissolved in a mixture of molten cryolite and aluminum fluoride in an electrolyzer to produce aluminum; b) withdrawing the offgas containing F ₂ , HF and at least one of CF ₄ and C ₂ F _h from the electrolyzer, (c) contacting the off-gas with alumina under conditions such that aluminum fluoride and a gas stream containing at least one of CF ₄ and C ₂ F _b are formed; d) recycling at least a portion of the aluminum fluoride from step c) to the electrolyzer; and e) contacting a gas stream comprising at least one of CF ₄ and C ₂ F _tt with the membrane under conditions to obtain a retained gas stream rich in at least one of CF ₄ and C ₂ F _ft and a passed gas stream free of at least one from CF ₄ and C ₂ F ₆ . 14. The method of claim 13, wherein said gas stream comprises CF ₄ and C ₂ F _b and wherein the retained gas stream is rich in CF ₄ and C ₂ F _fa and the passed gas stream is devoid of CF ₄ and C ₂ F ₆ . The method of claim 13, wherein said gas stream further comprises The method of claim 15, wherein the retained gas stream is degassed and is rich in O ₂ , CO ₂ and N ₂ . The method of claim 13, wherein said gas stream comprises about one of CF ₄ and C ₂ F _b . ₂ , ₂ C and en n ₂ . ý t and m that z n and C e n Ý t. and m that WHAT 2 and N ₂ and expired current z n and C e n ý t and m that 0.01 to about: 20 May Vol. % at least · · · · · · · · · · · · · · · · · • * * * * * * * · * * · · · ··· tt ♦ · · · ·· 18. The method of claim 17, wherein t is 1 m, the retentate gas stream contains about 20 to about 100 vol.% Of at least one of CF ₄ and C ₂ F ₆ and passed stream contains about 0 to about 0.01 vol.% Of at least one of CF ₄ and C ₂ F _o . 19. The method according to claim 13, characterized i n that said conditions include a temperature between about 10 and about 80 ° C, the pressure drop between about 0.02 and about 1.4 MPa and a flow rate between about 10 "and about ⁴ 100 NNT / hm ² . 20. The method of claim 13, wherein the membrane is selected from a plurality of membranes made of polyimides, polyamides, polyamide-imides, polyesters, polycarbonates, polysulfones, polyethersulfone, polyetherketone, alkyl-substituted aromatic polyesters, and mixtures of polyethersulfone, aromatic polyimides, aromatic. polyamides, polyamide-imides, fluorinated aromatic polyimides, polyamide and polyamide-imides.