US3031294A - Aluminum production method - Google Patents

Description

Patented Apr. 24, 1962 3,031,294 ALUMINUM PRUDUCTION METHOD Alan W. Searcy, Grinda, Calif., and David J. Meschi, Highland, Ind., assignors to the United States of America as represented by the United States Atomic Energy Commission No Drawing. Filed June 15, 1959, Ser. No. 820,565 1 Claim. (Cl. 75-68) The present invention refers to an aluminum produc tion method and, more particularly, to a method for producing aluminum by the reduction of aluminum varbide under controlled conditions.

The production of metallic aluminum has been accomplished, with varying degrees of success, by a variety of processes involving the reduction of an aluminum compound of higher valence to its lowest valence or metallic state. One of the less successful of these methods attempted by early researchers comprised the preparation of aluminum carbide by the carbon reduction of aluminum oxide and the subsequent distillation-reduction of the carbide to produce aluminum. Although it was possible to obtain metallic aluminum in this manner, the resulting product was of extremely poor quality being highly contaminated with carbon, aluminum carbide, aluminum oxide and other aluminum-carbon compounds; consequently, experiments along this line were largely abandoned. The present inventors, however, have succeeded in discovering those conditions under which aluminum carbide reduction may be performed to obtain an aluminum metal of much greater purity than was previously obtainable by this method.

In addition to advancing the state of the art by providing means for the successful reduction of aluminum carbide to aluminum metal, this invention also has considerable advantages from an economic standpoint in that this process requires considerably less energy than the electrolytic reduction of aluminum oxide which latter process is that employed by the majority of commercial aluminum metal manufacturers at present. In the electrolytic process 280 kilocalories are required to produce 2 mols of aluminum gas from aluminum oxide Whereas in the present invention only 96 kilocalories are required to produce the same amount of aluminum gas from aluminum carbide. This invention is also important in that it may be efficiently performed in geographical areas where large quantities of electrical energy are unduly expensive.

Accordingly, it is an object of this invention to provide an advantageous method for the production of aluminum metal by the thermal reduction of aluminum carbide.

It is another object of this invention to provide a method for reducing aluminum carbide whereby the resulting aluminum product has a purity greater than that obtained by previous aluminum carbide reduction methods.

It is still another object of this invention to provide a process for the production of metallic aluminum which requires a relatively small amount of energy in comparison with conventional aluminum production techniques.

It is a still further object of this invention to provide a method for the production of aluminum metal by the reduction of aluminum carbide under closely controlled conditions so as to minimize carbon contamination of the finished product.

Considering now the particulars of the invention, it has been known that metallic aluminum could be obtained by the thermal distillation of aluminum carbide according to the reaction:

This method as heretofore performed, however, suffered from the limitation that the aluminum metal so produced contained a relatively high proportion of undesirable contaminants thus rendering it unsuitable for further usage without extensive purification. Therefore, it has generally been considered impractical to employ this method in commercial operation. The present inventors, however, have investigated the method and successfully determined those factors which contributed to the poor quality of the aluminum. By the elimination, therefore, of these undesirable factors and by the selection ofcarefully determined and unique reaction conditions, the method of the present invention was developed.

Among the more objectionable features which were found to be inherent in the previously practiced method, and which are therefore eliminated from the method of the present invention, was the presence of moisture in the reaction system, the moisture being supplied either by a wet starting material or, more probably, by the atmosphere. The action of water on aluminum carbide causes hydrolysis of the latter compound with subsequent formation of aluminum hydroxide, methane and other low weight hydrocarbons. Due to their volatility the hydrocarbons escape easily and present no contamination problem. Aluminum hydroxide, however, will decompose on heating to release water, and thus effectively produce aluminum oxide. This oxide then reacts with the aluminum carbide starting material to give free aluminum and carbon monoxide.

On first consideration it would appear that this chain of reactions producing the desired end product, free aluminum, and a gaseous by-product carbon monoxide would be beneficial and in no way deleterious to a process designed for the production of high quality aluminum metal. Such is not the case, however, as studies have shown that this vaporized aluminum, as well as aluminum produced by the primary reaction of this method, is acted upon by the carbon monoxide at or before that surface of the reaction vessel at which the aluminum metal becomes condensed. Here the combination of these compounds produces both aluminum oxide and a variety of condensed carbon and aluminum-carbon products, the presence of which is considered undesirable in finished aluminum. It is one feature of the present invention therefore to prevent the formation of these contaminants by providing for the exclusion of moisture from the process. The manner in which the exclusion of moisture and other undesirable substances may be accomplished will be discussed in more detail hereinafter.

Considering further the distinguishing features of the present invention, investigations as to the inferiority of prior aluminum carbide reduction methods have also shown that it is undesirable to allow oxygen in the reaction system while the reaction mixture is hot, or at temperatures at which the aluminum carbide dissociation can be effected. The end result of oxygen contamination is the same as that due to moisture; that is the ultimate contaminants produced by each of these interfering sub stances are identical. In brief, the sequence of reactions initiated by oxygen are as follows: Aluminum oxide and carbon monoxide are formed by the interaction of oxygen and aluminum carbide. The aluminum oxide in turn reacts with aluminum carbide to produce free aluminum and carbon monoxide; as has been previously pointed out, this latter material has been shown to combine with free vaporized aluminum at or before the condensing surface whereupon there is again formed aluminum oxide as well as various carbon products.

It is an essential feature of the present invention therefore to perform the reduction of aluminum carbide under such conditions as to prevent oxygen from combining with the aluminum carbide dissociation products. The manner in which this is accomplished will be subsequently described.

Still another factor to be taken into consideration when thermally reducing aluminum carbide is the undesired effect on the final aluminum product due to the presence of nitrogen and the consequent interaction of this substance with aluminum carbide. It has been discovered that as nitrogen becomes available to vaporized aluminum carbide, a monovalent aluminum cyanide compound, AlCN, is formed. This compound per se is not found among the contaminants of the finished aluminum as it is not known to exist in the solid state; rather, on cooling, it decomposes to yield aluminum nitride and carbon. If during the cooling operation, however, aluminum cyanide comes into contact with gaseous aluminum, both the nitride and carbide of aluminum are likely to be formed. In either event the finished aluminum will be less pure, and consequently less commercially acceptable, in proportion to the amount of nitrogen which is permitted. Accordingly, it is another distinguishing feature of the present invention to provide for the reduction of aluminum carbide to aluminum metal in a nitrogen free atmosphere.

With respect now to the manner in which the afore mentioned features and conditions of the invention may be realized, both the aluminum carbide starting material and the gaseous atmosphere in which this material is to be reduced must be so chosen as to substantially exclude all traces of moisture, oxygen and nitrogen containing compounds.

Considering first aluminum carbide, care must be taken to select that material which contains little or none of the objectionable substances listed above. echnical grade aluminum carbide prepared by the reduction of aluminum oxide over an excess of carbon is quite satisfactory for this purpose as a high carbon atmosphere supports the reduction of virtually all aluminum nitride and oxide materials originally present in the aluminum oxide. Moisture, of course, will be driven out by the heat of this process and careful handling will insure the 'maintenance of the material in a dry state.

Control of the atmosphere in order to prevent moisture, oxygen and nitrogen from coming into contact with dissociating aluminum carbide, may be achieved by one of two methods. The first of these comprises conducting this process under a high vacuum whereby these substances are automatically removed by evacuation in proportion to the degree of vacuum achieved. Inasmuch as the partial pressure of aluminum is fairly independent of the total surrounding pressure, the vacuum pressure selected here is not critical to the thermal dissociation of aluminum carbide within the temperature range recommended below but is chosen for the express purpose of obtaining a non-contaminated atmosphere. For this reason it is desirable to employ as high a vacuum as is possible commensurate with efiicient and attainable operating conditions. A pressure of about atmospheres is suggested as satisfactory but should not be taken to be a limiting condition for this process.

An alternate method by which to prevent contamination of dissociating aluminum carbide comprises conducting this reduction process in the presence of a carefully dried, oxygen and nitrogen free inert gas such as helium or argon. In this manner, as with the above described vacuum technique, it is possible to selectively control the atmosphere with which the reactants are to come into contact and thus successfully eliminate those materials which have been found to adversely affect the quality of the finished aluminum. This atmospheric control is therefore an important aspect of the present invention.

In addition to the aforementioned precautions for the exclusion of certain substances from the reactants and surrounding atmosphere, it will be found that temperature is still another factor which is essential to performing the reduction of aluminum carbide in order to obtain a high purity aluminum product. The carryover of carbon contaminants into the distilled aluminum is increased at higher temperatures due to the formation of the unstable aluminum carbide, Al C This compound, which is known to exist in the gaseous state only, becomes entrained by the aluminum vapor produced by Al C thermal dissociation and on cooling, further reacts with this aluminum to give Al C again; this material in turn becomes physically mixed with the final aluminum metal product, thereby increasing the amount of contamination. Studies have shown that the formation of this unstable carbide is enhanced by increased temperature and, conversely, is minimized by lower heats. Therefore, the process of the present invention is best conducted in that temperature range which is sufiicient to provide energy for the dissociation of Al C but not so great as to cause excessive formation of Al C The range of about 1600 K. to about 2300 K. then has been found to satisfy the above requirement, and accordingly is prescribed as a necessary condition to practicing this invention.

In summary, the present invention may be described as an improved method for the production of aluminum metal by the dissociation of aluminum carbide under selected conditions which have been shown to effectively reduce the contamination of the finished metal, especially contamination by carbon compounds, which method comprises thermally reducing an aluminum carbide material substantially free of moisture, oxygen and nitrogen containing compounds in an atmosphere which is so controlled as to exclude these same objectionable substances and at a temperature within the range of about 1600 K. to about 2300" K.

The following example is offered as one illustration of the method and this invention in which the advantageous aspects thereof are demonstrated.

Example Into a covered graphite crucible having a central opening in the lid thereof, there was put a quantity of aluminum carbide which had been produced in such a manner as to contain substantially no aluminum oxide, aluminum nitride or other oxygen or nitrogen containing materials, and which also was carefully dried so as to insure the absence of moisture. This crucible was next placed in a dry vacuum chamber which was then closed and evacuated to an internal pressure of about 10 atmospheres and maintained at this pressure for the duration of the dissociation reaction. Radio-frequency electromagnetic induction was used as the heat source to bring the crucible contents to a temperature within the range of about 1600 to 2300 K. Under these conditions aluminum carbide became dissociated to form aluminum in the gaseous state and solid particles of carbon. As the dissociation proceeded the vaporized aluminum flowed from the crucible in a unidirectional stream and upon contacting a cooler portion of the reaction equipment condensed finally to yield solidified aluminum metal.

Heating of the crucible was continued until all aluminum carbide was converted to free aluminum and carbon. Examination of the aluminum recovered at the conclusion of this reaction process showed that under these carefully controlled conditions a minimum of contamination, particularly that due to carbon, was allowed. By spectroscopic procedures, the carbon content was shown to be considerably less than 1%, and by established procedures, removal of impurities could be easily accomplished.

What is claimed is:

A process for the production of aluminum metal comprising the production of aluminum carbide by reduction of a mixture consisting of aluminum oxide in the presence References Cited in the file of this patent of an excess of carbon, and the thermal reduction of said UNITED STATES PATENTS aluminum carb1de within a vacuum of about 10 atmospheres, said thermal reduction being carried out at a 2,776,884 Grunert 1957 temperature substantially Within the range 1600" K. to 5 FOREIGN PATENTS 2300 K, whereby aluminum is extracted in the gaseous state, said reaction being carried out in the absence of 482,150 Great Britain Mar. 24, 1938 nitrogen.

23,315 Great Britain 1909