US3729398A - Process and cell for the electrolytic recovery of aluminum - Google Patents

Abstract

PROCESS FOR THE ELECTROLYTIC RECOVERY OF ALUMINUM FROM A FUSED ALUMINA-CONTAINING BATH OF ALKALI ALUMINUM FLUORIDE IN WHICH THE FLUORIDE MELT IS KEPT IN DIRECT CONTACT WITH A MELT CONSISTING OF ELECTROLYTE HIGHLY CONCENTRATED IN ALUMINA AND SEPARATED FROM THE LIQUID CATHODIC ALUMINUM. THE ELECTROLYTIC CELL USED THEREFORE COMPRISES ONE OR MORE TROUGHS RESTING ON ITS BOTTOM AND PROVIDED FOR THE RECEPTION OF THE HIGHLY CONCENTRATED ELECTROLYTE.

Description

April 24, 1973 w. SCHMIDT-HATTING 3,729,398

PROCESS AND CELL FOR THE ELECTROLYTIC RECOVERY OF ALUMINUM Filed Feb. 9. 1971 UHIIHIIIHH HIHYI HHHN I HI] IN I Y IHIHH] 1'5 14 24 23 21 13 United States Patent PROCESS AND CELL FOR THE ELECTROLYTIC RECOVERY OF ALUMINUM Wolfgang Schmidt-Batting, Chippis, Switzerland, assignor to Swiss Aluminium Ltd., Chippis, Switzerland Filed Feb. 9, 1971, Ser. No. 113,960 Claims priority, application Switzerland, Feb. 17, 1970, 2,295/70 Int. Cl. C22d 3/02, 3/12 U.S. Cl. 204-67 3 Claims ABSTRACT OF THE DISCLOSURE In the electrolytic recovery of aluminum alumina is dissolved in a fluoride melt (molten cryolite) in a cell, which conventionally comprises a steel pot lined by an insulating lining of heat-resistant material which in turn is lined with carbon forming the bottom of the cell proper. Iron rods are embedded in the carbon bottom as the cathodic connections, and anodes of amorphous carbon dip into the fluoride melt (molten electrolyte) from above. Oxygen is given off at the anodes as a result of the electrolytic decomposition of the aluminum oxide and combines with the carbon of the anode to form carbon monoxide and carbon dioxide.

The fluoride melt itself is covered by a crust which forms by solidification of the electrolyte and a layer of aluminum oxide covers this crust. This aluminum oxide is introduced into the fluoride melt by periodic breaking of the crust. If the concentration of aluminum oxide, which normally lies between 2 and and advantageously between 5 and 7%, drops below about 2% in the electrolyte, the so-called anode effect occurs. The cell potential, which is normally from 3.7 to 4.5 volts, rises suddenly to a value between 20 and 50 volts. The crust on the electrolyte must be broken and new alumina introduced into the fluoride melt at this moment at the very latest.

The anode effect shows the operators that the concentration of alumina has dropped to a low value approximately between 1 and 2%.

The concentration of alumina in the electrolyte cannot be directly measured. It is indeed possible to take specimens of the melt and analyse these in a laboratory, but the time required is too long to allow corrections to the operation to be effected in good time.

When adding alumina to the fluoride melt, care is taken to supply approximately that amount which will bridge the time interval until the next breaking of the crust, and in this period to maintain the concentration of alumina at about 5% or more. The amount of alumina which is added to the melt during the servicing of the cell can not however be accurately controlled, but rather depends upon what happens when the crust is broken.

Thus it is possible that too little alumina enters the fluoride melt, with the result that the anode efiect occurs too frequently and the electrolyte is heated too much in consequence. The rise in temperature involved in this brings about a reduction in the current yield. The current yield is the ratio of the amount of metal actually produced to that which could theoretically be separated according to Faradays law. The difference in the two amounts of metals is essentially the result of re-oxidation of the aluminum dissolved in the melt by the anode gases.

If too much alumina is added to the cell, the limit of solubility of the alumina in the melt may be locally exceeded. Then some of the alumina sinks through the electrolyte without going into solution, enters the metal lying on the bottom of the cell, and passes through this to the bottom. There it forms a sludge which in the course of time can lead to encrustation of the bottom. Such encrustation in turn brings about increase in the electric resistance of the bottom of the cell and thus increase in the specific consumption of electrical energy (kw. h./ kg. Al). Moreover because of non-uniform distribution of the current in the bottom the bath may be disturbed by movement and arching of the metal with resultant reduction in the current yield.

As the effects of forming sludge and encrustation on the bottom are more harmful than those resulting from too frequent an anode effect, operators are often inclined to supply a little less alumina to the cell than is necessary and so cause rather more anode effects to occur. A consequence of this method of operation is that the current yield cannot be so high and the specific consumption of electrical energy so low as would be the case if the concentration of alumina in the melt were maintained constant. The current yield namel also drops when the concentration of alumina in the melt decreases.

Attempts have been made to maintain the alumina concentration constant and high by continuously introducing alumina into the fluoride melt by conveyor screws and other apparatus. The technical difliculties involved (cryolite is remarkably aggressive) as well as the absence of any possibility of continuously examining the concentration of alumina, have always led to such interruptions in the process that this method of operation has been abandoned. Today therefore in practice the alumina content in the fluoride melt alternates between about 2 and 10% with the risk of occasional supersaturation of the electrolyte as well as the formation of bottom sludge and encrustation on the one hand, or too high a frequency of the anode effect on the other hand.

My object in this invention is to eliminate the difliculties described.

According to the invention the electrolyte is kept during operation in direct contact with a melt which consists of electrolyte with high concentration of aluminum oxide and separated from the liquid cathodic aluminum. Because of the direct contact of the two melts and of the movement of the main melt in the cell, there is continuous interchange of melt highly concentrated or saturated in alumina with the electrolyte impoverished in alumina by the electrolysis. Preferably the highly concentrated melt rests on a lower layer of undissolved alumina so that as it loses alumina by interchange with the main melt it takes fresh alumina into solution from this lower layer.

In a conventional cell for the electrolysis of alumina, the thermal action and the escape of the anode gases normally bring about enough movement in the electrolyte to ensure the desired interchange of melts. If necessary, this movement of the main melt can be increased in any suitable way.

My invention includes apparatus for carrying out this process and comprising one or more troughs resting on the bottom of the cell for the reception of the highly concentrated or saturated melt, the upper edges of the trough or each trough being at such a height as to lie in operation in the layer of the electrolyte without reaching its surface and as the liquid cathodic aluminum in the main part of the cell cannot enter the trough. Such a trough should of course be made of a material resistant to the electrolyte.

The preferred form of cell is shown diagrammatically in the accompanying drawings, in which:

FIG. 1 is a longitudinal section;

FIG. 2 is a cross-section on the line A-A inFIG. l; and

FIG. 3 is a plan in which the anode supports and the current-conducting beam are omitted.

The cell shown comprises a steel pot 10 lined by heatresistant insulation 11, which in turn receives a carbon lining 12 in which iron cathode rods 13 are embedded. The layer of aluminum electrolytically separated during the operation is shown at 14, and on top of it there is the fluoride melt 15 containing alumina in solution. The melt is covered by a crust 16 and a layer of alumina 17 on top of it. Anodes 18 dip into the melt from above and are suspended by rods 19 from a beam 20 which carries the current.

A trough 21 is let into the carbon bottom of the cell, and its sides 22, which are made of carbon, project upwards into the melt 15, so that the aluminum 14 cannot enter the trough. This trough is filled with a melt 23 highly concentrated or saturated in alumina, which rests on a layer 24 of undissolved alumina. In operation alumina is supplied to the trough from above, and partly enters into solution and partly sinks to the bottom. The aluminum separated from the melt forms a single layer, that is to say the trough 21 does not divide the cell into two parts, and therefore the level of the liquid aluminum 14 is the same throughout the cell.

Because the highly concentrated or saturated melt 23 is in direct contact with the main melt 15 at the interface shown at 25, an interchange takes place at the interface 25, so that alumina enters the main melt from the trough. As the main melt 15 is in movement, a satisfactory content of dissolved alumina is maintained in it throughout the whole operation.

In the example of the cell construction shown, it is naturally impossible to prevent a small part of the anode current passing into the interior of the trough 21 and there producing :a small amount of aluminum, which must periodically be removed. Obviously the need for this removal can be prevented, for example, by coating the trough with a material which is a non-conductor of electricity.

There may of course be more than one trough in a large cell, and the shape of the trough can differ from that shown.

Although the trough 21 shown in unitary with the bottom of the cell it can be separately made, and indeed from some other material than carbon, and inset in the bottom of the cell.

The application of the process according to the invention is not restricted to cells of conventional construction, but rather the process can be employed in any aluminum electrolytic cell in which a melt of fluoride electrolyte containing alumina in solution is electrolysed.

By my invention it is possible to prevent the concentration of alumina from falling to values which lead to the anode effect. This affords, first, the advantage that the harmful heating of the melt produced by the anode effect is suppressed, and, second, that the average concentration of alumina remains higher. These two results increase the current yield by about 1 to in comparison with that obtained in cells of similar construction in which the 4 invention is not used. Thus, for example, the current yield may increase from 89% to over 92% and the specific consumption of electrical energy may be reduced by 0.5 kw. h./kg. Aland more.

In the electrolysis of aluminum value is often attached to the production of the anode effect at least occasionally, for example every 24 hours, because it gives an indication of the content of the dissolved alumina and it results in dissolving of the bottom sludge, which consists mainly of undissolved alumina and may lead to an encrustation of the cell bottom. The invention allows on the one hand the alumina content of the melt never to fall to so low a value that the anode effect can arise, and on the other hand to ensure that the melt outside the trough is never saturated in alumina, so that no sludge requiring the anode eflect to remove it is ever formed.

What I claim is:

1. In a process for the electrolytic recovery of aluminum from a main fluoride melt containing alumina in solution, wherein the separated liquid aluminum at the cathodic electrode forms a pool beneath the melt, and the main fluoride melt is kept in direct contact with a feeding melt in a walled trough which melt consists of electrolyte highly concentrated or saturated in aluminum oxide and separated from the liquid cathodic aluminum, the improvement comprising the interchange of said main fluoride melt and said feeding melt taking place above the wall of the trough.

2. An electrolytic cell for use in the recovery of aluminum from alumina dissolved in a main fluoride melt which is kept in direct contact with a feeding melt consisting of fluoride electrolyte highly concentrated or saturated in aluminum oxide and separated from the liquid cathodic aluminum,

comprising, in combination,

a receptacle for receiving the main fluoride melt, said receptacle having a carbon bottom portion forming the cathode electrode upon which a layer of liquid aluminum is collected,

a set of carbon anode blocks immersable into the main I References Cited UNITED STATES PATENTS 3,554,893 1/1971 De Varda 204245 X 3,400,061 9/ 1968 Lewis et al. 20467 3,502,553 3/1970 Gruber 20467 3,582,483 6/1971 Sem 20467 TA-HSUNG TUNG, Primary Examiner D. R. VALENTINE, Assistant Examiner US. Cl. X.R. 204243 R, 245

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