US3068092A

US3068092A - Process for the recovery of aluminum from aluminum-aluminum carbide mixtures

Info

Publication number: US3068092A
Application number: US68617A
Authority: US
Inventors: Menegoz Charles Daniel
Original assignee: Pechiney SA
Current assignee: Pechiney SA
Priority date: 1959-11-18
Filing date: 1960-11-14
Publication date: 1962-12-11
Anticipated expiration: 1979-12-11
Also published as: NL258035A; CH400577A; ES262528A1; LU39423A1; FR1461309A; GB964792A; DE1155252B; BE597195A

Description

United States Patent C) PROCESS FOR THE RECOVERY OF ALUMINUM The present invention relates to the recovery of aluminum from mixtures comprising aluminum and aluminum carbide, and it concerns more particularly a process for treating such mixtures with a flux.

Previous work has made available a process wherein aluminum is recovered by reducing aluminum oxide with carbon at high temperatures. In this process, there is ob tained as intermediary products, mixtures consisting primarily of aluminum and aluminum carbide. This prior process consists more particularly of heating aluminum oxide and carbon in the form of mixtures or agglomerates to temperatures superior to about 2300 C. The following reactions result:

This process is described more particularly in Serial No. 10,581, filed February 24, 1960, now U.S. Patent No. 2,974,032, and Serial No. 798,365, filed March 10, 1959, now abandoned.

According to the aboveprocess, aluminum may beextracted from said aluminum-aluminum carbide mixtures by means of various procedures, for example, by means of a flux consisting of molten metallic halides. The quantities of flux employed are quite large and, by a weight comparison, are at least equal to and usually greater than the quantity of the treated mixtures.

. In order to obtain high extraction rates for aluminum in the course of this procedure, it was stated that the aluminum-aluminum carbide mixtures containing up to about 3% aluminum oxide should be cooled sufiiciently slowly to allow aluminum carbide contained therein to crystallize out in the form of large crystals. The metal-- lic aluminum was imprisoned or interspersed within these crystals. When cooling at a rate of about 200 C. per hour between 2400-1800" C., crystals of aluminum carbide with sizes of the order of -20 mm. could be obtained.

The foregoing procedure, however, presents very important drawbacks from an industrial and economic point of view. Lt proved difficult at the solidifying temperatures of the mixtures to cool them sufiiciently slowly in order to obtain the desired large crystals. This was due to the high temperatures of the system, and can be illustrated by considering the melting points of various mixtures, which are as follows:

About 2450 C. for an aluminum content of 50% by weight About 2250 C. for an aluminum content of 60% by weight About 2150 C. for an aluminum content of 70% by weight About 2000" C. for an aluminum con-tent of 80% by weight the aluminum contained in the mixtures.

required high power consumption in order to bring them to, and maintain them at, the treatment temperatures. Moreover, as the slurries which are formed contain some flux, the aluminum carbidein the mixtures and a large quantity of non-separated metallic aluminum became difficult to isolate from the flux. There is a tendency for solidified masses of the slurr'ies to contain a fairly largequantity of the flux.

Finally, the rather close densities of pure aluminum,-

flux and the slurries tend to hinder the cooling of liquid aluminum into a distinct mass, and also tend to slow down and limit the decantation and separation of the three components.

sults in the elimination completely, orto a substantial degree, of the various drawbacks mentioned. The process enables a quick and substantially complete recovery of the free aluminum contained in the mixtures of alumi- This can be achieved ir-- respective of the aluminum oxide contentof the mixtures and independent of the manner in which the alumi-. num-aluminum carbide mixtures were obtained. Likewise, the process of this invention presents economic ad-l num and aluminum carbide.

vantages in that limited quantities of flux are employed.

object of this invention is the recovery of aluminum from mixtures of aluminum and aluminum carbide obtained from the carbothermal reduction of aluminum oxide. 1

A further object of this invention is the recovery of aluminum in a manner which will permit the recovery of substantially all the elemental aluminum values contained in the aluminum-aluminum carbide mixtures.

An additional object of this invention is to provide a process for the recovery of aluminum in a highly efficient and economical manner.

. A further object of this invention is to provide a process for extracting the aluminum contained in mixtures comprising aluminum and aluminum carbide wherein said mixture is added to a flux, the amount by weight of the added mixtures being greater than the amount of the flux employed.

These and other objects of the invention will be more readily apparent when considering the following description and claims.

It has been found that contrary to prior processes in which mixtures consisting substantially of aluminum and aluminum carbide are treated with amounts of flux greater than the amount of the mixture, the quantity of mixtures comprising aluminum and aluminum carbide treated by flux according to this invention may be heavier than the quantity of flux. It has been surprisingly dis covered that the aluminum may be more efficiently recovered when smaller amounts of flux are employed. In

; process is such that the quantity of flux used in comparison with that of the mixture is preferably of the same order or magnitude as the quantity of aluminum carbide contained in the mixtures. Thus, for example, in the case of'treating a mixture containing about 47% aluminum Furthermore; the large quantities of flux employed in these processes It then becomes necessary to con-j siderably lengthen the duration of the decantation step, 3

which will accordingly increase the power consumption. The instant invention consists of a process which re-I carbide by weight, an amount of flux, between 50-55% of the weight of the total mixture, may be employed.

The flux compositions employed in accordance with this invention preferably include the chlorides, bromides, or fluorides of the alkaline and alkaline earth metals. For example, sodium, potassium, and lithium fluoride, and chlorides of sodium and potassium may 'be employed alone or in combination. As a further feature, additions of aluminum fluoride may be employed whereby the surface tension of'the flux relative to that of the treated mixture is reduced. On the other hand, the introduction of calcium chloride is to be avoided because it tends to increase the surface tension, thus impeding the intermixing of the mixture and flux.

According to a preferred embodiment, mixtures containing aluminum and aluminum carbide at a temperature not exceeding about 1200" C. are introduced progressively into the molten flux. The flux is carefully stirred in order to enable rapid and intimate contact with said mixture. Formation of an emulsion of metallic aluminum is to be avoided since such an emulsion would tend to separate out together with the slurry which will be formed.

The temperature of the flux, according to a preferred embodiment, is greater than the melting point of the flux by an amount of at least 100 C. It is preferred not to heat the flux to a temperature too close to that of the boiling point because the increase in vapor pressure tends to impede the progress of the separation. Use may be made of a fiux'maintained at a temperature between about 900-1000 C., preferably about 950 C.

It has been established that on lowering the temperature of the flux in the course of the present process to a value about 50 above the solidification temperature of the flux used, separation of liquid aluminum from the nearly congealed slurry is effected much more readily. This cooling may be effected either progressively in the course of the treatment or'only at the end of the treatment, when practically all the mixture has been added to the flux.

In the application of progressive cooling, the following technique may be employed. Portions of the mass may be subjected to cooling from the bottom to the top and then from the top to the bottom a variable number of times. That is, a heat exchanger will traverse the mass vertically, extracting heat in a progressive fashion. In any case, the last traverse will always be from the bottom to the top. A procedure of this type has been found to facilitate the separation of the sizes formed. In some instances, cooling may be effected only by a regularly ascending procedure from the bottom to the top of the mass.

As a further alternative, cooling may be effected in a non-uniform or discontinuous manner. For example, the mass being treated may be submitted to successive coolings, followed-by partial reheatings.

Another feature of this invention which may be utilized in combination with or independently of the cooling feature, consists in adding to the flux or slurry a small quantity of a substance in powdered form which does not melt at the process temperatures and is inert towards the flux components and treated mixtures. These addition agents include aluminum oxide, carbon, and/or aluminum carbide. A setting or congealing of the mass and the substantially complete separation of liquid aluminum is more readilyobtained by the use of these additives. The quantity of these additives or combinations thereof corresponds to a few percent, for example, between 2-15% of the weight of the flux, and preferably about 5% of the weight of the flux. The congealment is, of course, obtained without these additives when the final quantities of aluminum carbide containing mixtures have been added.

It has been determined that the mixtures comprising aluminum carbide and aluminum are preferably obtained in the form of grains of sizes nearly equal to or smaller than 20 mm. in their largest dimension. These particles may be obtained by crushing or grinding the raw, solidified products coming from the carbothermal reduction of aluminum oxide. The crushing or grinding may be carried out on cold or heated material. The material is heated in many cases since the masses coming from the carbothermal reduction are of considerable size and very hard when cold. To take advantage of the malleability of the metallic aluminum during the comminuting, the masses are heated to temperatures above or below the melting point of aluminum. If above the melting point, the particles obtained comprise particles of aluminum and aluminum carbide, and also particles of free aluminum which has exuded due to the pressures of the crushing operation. Where the temperatures are below the melting point of aluminum, only the aluminum-aluminum carbide particles are obtained. The comminuting is preferably carried out in an anhydrous or inert atmosphere, and jaw or cylindrical crushers are cited as an example of the apparatus that may be employed. The working surfaces of the apparatus may be sprinkled with powdered, fusible salts which will protect the metallic surfaces from corro sion by the liquid aluminum.

In some instances, it may be desirable to submit the mixtures coming from the carbothermal reduction to a coarse crushing, followed by a second substantially identical fine crushing operation. This second crushing is preferably carried out on the cold material. The action of a ball or rolling mill upon this material induces a shearing action in the grains. The mixtures thus obtained are in the form of flakes having a thickness of the order of 1-3 mm., with the largest dimensions about 10-20 mm. It is preferable not to convert the mixtures into too fine a powder. The size of the grains should not be less than about 3 mm. in their smallest dimension. If in too small a 'form, the aluminum carbide 'will tend to be hydrolyzed in contact with a not strictly anhydrous atmosphere. In order to avoid any such possibility, it is contemplated that powder obtained in too fine a form, or in fact all the powders which are to be used, may be agglomerated into pellets. This powder generally exhibits the property of being self-compacting. However, non-aqueous binding agents, such as heavy hydrocarbons and pitch, may be employed. It is also contemplated that the flux may be agglomerated in the desired proportions with the powdered mixture and the process carried out from this point.

A special feature of this invention consists in a process wherein the aluminum-aluminum carbide mixtures are pretreated with a molten fiux. This flux is substantially of the type previously described, but it is employed in an amount by weight greater than the amount of the mixture. Amounts of 1.l2.5 times the amount of the mixture, and particularly 1.3 times the amount of the mixture, will be employed. In the course of this pretreatment, the mixtures are added rapidly to the molten flux which has been heated to between about 850-l000 C. The mixtures may be heated up to about 1200 C. or may be added cold, the latter alternative being resorted to in order to'avoid overheating and vaporization of the flux. The pretreatment is carried out in a rotary furnace with an approximately horizontal axis, and in which the feeding axis is sloping against the rotation axis. The pretreatment will consist of kneading for a few minutes to several hours, the duration of-the kneading varying inversely with the temperature of the treated mixture, and varying directly with the quantity of preheated flux and flux temperature. After the kneading, decantation for a few minutes results in a bottom layer consisting substantially of molten aluminum, a top layer of flux, and an intermediate layer comprising fiux, aluminum carbide and metallic aluminum. This latter layer 'is in the form of an emulsion or slurry and can be used as the flux in the aluminum extraction process of this invention. alternative, the flux and slurry may be used again in another'pretreating operation before a process in accordance with this invention is resorted to. It can be seen As an that the pretreatment accomplishes, among other things,

a partial recovery of the aluminum and supplies starting Example 1 An electric furnace was employed comprising a ver tical crucible of 40 cm. internal diameter and 100 cm. heighth. The furnace was equipped with a stirrer made out of a graphite vertical bar of about cm. diameter and it was induction heated.

Into this furnace there was introduced 52.5 kg. of sodium chloride which was melted and heated to about 950 C. Into the thus obtained flux there was progressively introduced Within 1 hour, 100 kg. of a cold mixture comprising about 50% free aluminum by weight and more than 47% aluminum carbide by weight. This mixture was composed of flakes of about 1 mm. thickness, the other dimensions thereof being in the order of 820 mm. The mixture was obtained by cold rolling the product of a carbothermal reduction of aluminum oxide with a cylinder crusher of 1 mm. opening. The temperature was maintained at about 950 C. during the addition of mixture to the flux.

When the addition was completed, the flux had been converted to a slurry due to the introduction of aluminum carbide from the mixture, and had become very viscous. A mass of liquid aluminum h-ad separated out.

The stirring was continued for about minutes While progressively lowering the temperature down to about 850 C. The slurry had then become practically solid and had congealed against the walls and bottom of the crucible while the liquid aluminum had gathered in the core. The aluminum was then poured out by tilting the furnace.

By this procedure, 48 kg. of pure aluminum was recovered which corresponded to a yield of 96% by weight compared to the free aluminum in the treated mixture.

Example 2 A rotating furnace with a horizontal axis was provided containing 250 kg. of flux substantially consisting of a eutectic mixture of sodium chloride and potassium chloride, melted and heated to a temperature of the order of 900 C. Into this furnace there was progressively introduced within about 2 hours, 450 kg. of a mixture identical to that of Example 1. The mixture was similarly composed of flakes of about 3 mm. thickness, the other dimensions being of the order of 8-20 mm. The rotating speed of the furnace was about 6 revolutions per minute.

After about minutes kneading of the comparatively viscous mass, the latter was poured off into a refractory ladle heated to about 900 C.

While agitating the contents of the ladle energetically, there was rapidly added 15 kg. of aluminum oxide in the form of a powder. After a few minutes, a congealed, almost solid slurry was obtained. The distinct phase of liquid aluminum was then .poured out by simply tilting the ladle. 214 kg. of pure aluminum was recovered which corresponded to an efficiency of 95%.

Example 3 A pretreatment was carried out in a rotating furnace with a horizontal axis containing 2800 kg. of a flux consisting substantially of molten sodium chloride. The flux was heated to about 950 C. and there was added thereto 2100 kg. of a mixture comprising 65% aluminum by weight. This mixture was introduced at a temperature of about 700-l000 C. by 7 additions of 300 kg. lasting 5-10 minutes each. The mixtures had been obtained by hot crushing and grinding with a jaw crusher and a cylinder crusher provided with a 3 mm. opening. This equipment was placed above the furnace in order that the latter was able to receive the hot particles issuing from the apparatus practically without any delay.

Kneading was maintained for about 20 minutes while the rotating speed of the furnace was about 3 revolu tions per minute. This was followed by decantation for about 15 minutes, and the following values were obtained:

The pretreatment thus permitted the extraction of about 55% by weight of the aluminum contained in the initial mixture.

After having poured out the thus separated aluminum,

there was added in a virtually identical manner, 2100 kg;

of the same hot mixture. This mass was maintained at about 950 C. Subsequent to this addition, stirring was carried on for about 20 minutes, followed by a decantation step of about 15 minutes. obtained from this procedure:

Flux (sodium chloride) 300 kg.(about).

The following values were Slurry 5270 kg. (about). Aluminum (ingot cast) 680 kg. (about).

the slurry containingi I Sodium chloride 2500 kg. (about). Free aluminumunma 1300'kg. (about); Residue (consisting substantially of aluminum carbide) 1470 kg. (about).

This second treatment thus enabled the extraction of about 50% by weight of the aluminum contained in the latertreated mixture.

After having poured out the thus separated aluminum, the remaining flux and slurry were transferred into a furnace of the same type as the one used in Example 2, and were maintained at a temperature of about 950 C. The rotating speedof the furnace was 6 revolutions per minute. 3450 kg. of the same mixture were then progressively introduced into this flux and slurry. This mixture'was introduced practically cold and in the form of flakes of about 3 mm. thickness. After the addition and after the resulting viscous mass had been kneaded for about l hour, the latter was poured off into refractory ladles which had been preheated to about 900 C. While agitating the mass energetically, there was rapidly added about kg. of aluminum oxide in the form of a powder of a size up .to about 0.15 mm. After a few minutes a congealed, Epractically solid slurry was obtained. The liquid aluminum was then poured out by tilting the ladle. 3190 kg. ofpure aluminum was recovered which, in addition to thel430 kg. previously recovered in the two pretreatment steps, corresponded to an efliciency of 93% by weight of the free aluminum contained in the treated mixtures. H It will be understood'that changes may be made in the foregoing process in a manner to provide the characteristics of this invention without departing from the spirit thereof, especially as defined in the following claims.

I claim:

1. A process for recovering aluminum from a mixture comprising aluminum and aluminum carbide, comprising the steps of preparing a molten flux capable of forming a slurry of said flux and aluminum carbide in said mixture and wherein the flux is at least one of the compounds selected from the group consisting of alkali metal halides and alkaline earth metal halides, progressively adding to said flux an amount by weight of a mixture of aluminum and aluminum carbide such that the aluminum carbide content of said mixture is approximately equal to the amount by weight of said flux but where in the amount of aluminum plus aluminum carbide is greater than the amount by weight of the flux, and agitating the combination of said mixture and said flux whereby substantially all the aluminum is separated out.

2. The process of claim 1 in which the aluminum carbide comprises about 47% by weight of said mixture and the flux comprises 50-55% by weight of said mixture.

3. The process according to claim 1 in which said mixture consists of particles of up to about 20 mm. obtained by comminuting the solid material product of a carbothermal reduction of aluminum oxide.

4. The process as claimed in claim 1 wherein the flux additionally contains a small amount of aluminum fluoride.

5. A process for recovering aluminum from a mixture comprising aluminum and aluminum carbide, comprising the steps of preparing a molten flux capable of forming a slurry of said flux and aluminum carbide in said mixture and wherein the flux is at least one of the compounds selected from the group consisting of alkali metal halides and alkaline earth metal halides, maintaining said flux at a temperature at least 100' C. above its melting point, progressively adding to said flux an amount by weight of a mixture of aluminum and aluminum carbide such that the aluminum carbide content of said mixture is approximately equal to the amount by weight of said flux but wherein the amount of aluminum plus aluminum carbide is greater than the amount by weight of the flux, and agitating the combination of said mixture and said flux whereby substantially all the aluminum is separated out.

6. The process according to claim 5 wherein the flux comprises at least one member selected from the group consisting of the chlorides, bromides and fluorides of sodium, potassium and lithium.

7. The process according to claim 6 wherein said flux is maintained at a temperature between about 900-1000 C.

8. A process for recovering aluminum from a mixture comprising aluminum and aluminum carbide, comprising the steps of preparing a molten flux capable of forming a slurry of said flux and aluminum carbide in said mixture and wherein the flux is at least one of the compounds selected from the group consisting of alkali metal halides and alkaline earth metal halides, maintaining said fiux at a temperature at least 100 C. above its melting point, progressively adding to said flux an amount by weight of a mixture of aluminum and aluminum carbide such that the aluminum carbide content of said mixture is approximately equal to the amount by weight of said flux but wherein the amount of aluminum plus aluminum carbide is greater than the amount by weight of the flux, agitating the combination of said mixture and said flux, and lowering the temperature of said combination to a point about 50 C. above the melting point of said flux whereby substantially all the aluminum is separated out.

9. The process according to claim 8 wherein the temperature is progressively lowered during the addition of said mixture.

10. The process according to claim 8 wherein the temperature is progressively lowered subsequent to the addition of said mixture.

11. The process according to claim 8 wherein the temperature is alternately raised and lowered a plurality of times prior to the final cooling step.

12. The process according to claim 8, including the step of adding to the combination in the amount of 215% by weight of said flux at least one material selected from the group consisting of aluminum oxide, carbon and aluminum carbide.

13. The process as claimed in claim 8 in which the flux additionally contains a small amount of aluminum fluoride.

14. The process of recovering aluminum from a mixture comprising aluminum and aluminum carbide, comprising the steps of treating an amount of said mixture with a molten flux maintained at a temperature between 850-1000 C., and wherein the flux is at least one of the compounds selected from the group consisting of alkali metal halides and alkaline earth metal halides, the amount of said flux being 1.1-2.5 times greater by weight than the amount of said mixture, whereby there is obtained liquid aluminum, flux, and a slurry composed of aluminum, aluminum carbide and flux, separating out said slurry, progressively adding to said slurry an amount by Weight of a mixture of aluminum and aluminum carbide such that the aluminum carbide content of said mixture is approximately equal to the amount by weight of said slurry but wherein the amount of aluminum and aluminum carbide is greater than the amount of flux, and agitating the combination of said mixture and said slurry whereby substantially all the aluminum is separated out.

15. The process according to claim 14 wherein the fiux comprises at least one member selected from the group consisting of the chlorides, bromides and fluorides of sodium, potassium and lithium.

16. The process according to claim 14 wherein said mixtures are in the form of particles having dimensions of up to about 20 mm. obtained by comminuting the solid material product of a carbothermal reduction of aluminum oxide.

17. The process as claimed in claim 14 in which the flux contains small amounts of aluminum fluoride.

18. A process for the separation of aluminum from a mixture of aluminum and aluminum carbide comprising the steps of admixing the aluminum-aluminum carbide mixture with a flux which is capable of forming a slurry with aluminum carbide and wherein the flux is at least one of the compounds selected from the group consisting of alkali metal halides and alkaline earth metal halides and in which the flux is employed in an amount by weight at least as great as the amount of aluminum carbide but less than the total of aluminum and aluminum carbide, agitating the combination of flux and aluminum-aluminum carbide whereby aluminum is freed from the aluminum carbide while the aluminum carbide forms into a slurry with the flux.

19. The process as claimed in claim 18 in which the fiux contains a small amount of aluminum fluoride.

References Cited in the file of this patent UNITED STATES PATENTS

Claims

1. A PROCESS FOR RECOVERING ALUMINUM FROM A MIXTURE COMPRISING ALUMINUM AND ALUMINUM CARBRIDE, COMPRISING THE STEPS OF PREPARING A MOLTEN FLUX CAPABLE OF FORMING A SLURRY OF SAID FLUX AND ALUMINUM CARBIDE IN SAID MIXTURE AND WHEREIN THE FLUX IS AT LEAST ONE OF THE COMPOUNDS SELECTED FROM THE GROUP CONSISTING OF ALKALI METAL HALIDES AND ALKALINE EARTH METAL HALIDES, PROGRESSIVELY ADDING TO SAID FLUX AN AMOUNT BY WEIGHT OF A MIXTURE OF ALUMINUM AND ALUMINUM CARBIDE SUCH THAT THE ALUMINUM CARBIDE CONTENT OF SAID MIXTURE IS APPROXIMATELY EQUAL TO THE AMOUNT BY WEIGHT OF SAID FLUX BUT WHERE IN THE AMOUNT OF ALUMINUM PLUS ALUMINUM CARBIDE IS GREATER THAN THE AMOUNT BY WEIGHT OF THE FLUX, AND AGITATING THE COMBINATION OF SAID MIXTURE AND SAID FLUX WHEREBY SUBSTANTIALLY ALL THE ALUMINUM IS SEPARATED OUT.