US3257307A

US3257307A - Electrolytic cell for the production of aluminum

Info

Publication number: US3257307A
Application number: US201669A
Authority: US
Inventors: Jack L Henry; Robin D Holliday
Original assignee: Kaiser Aluminum and Chemical Corp
Current assignee: Kaiser Aluminum and Chemical Corp
Priority date: 1962-06-11
Filing date: 1962-06-11
Publication date: 1966-06-21
Anticipated expiration: 1983-06-21
Also published as: GB976031A; DE1184510B

Description

June 21, 1966 J. HENRY ETAL ELECTROLYTIC CELL FOR THE PRODUCTION OF ALUMINUM Filed June 11, 1962 5 Sheets-Sheet 1 CARBON FIG. El

CURRENT FIELD FLEX 3O ANODE 24 ELECTROLYTE 22 METAL LAYER I4 CELL LINING ANODES FIEl INSULATION 40 RHM BARS 4 FIG. Ed

ANODES gi jg CURRENT 46 FIELD RHM BARS F I E. E In FIG. El

INVENTOR. ROBIN D. HOLLIDAY JACK L. HENRY June 21, 1966 J. L. HENRY ETAL 3,257,307

ELECTROLYTIC CELL FOR THE PRODUCTION OF ALUMINUM Filed June 11, 1962 5 Sheets-Sheet 2 CELL WALL cAPPED RHM BARS PIE. 7

INVENTOR. ROBIN D. HOLLIDAY JACK L. HENRY June 21, 1966 J. L. HENRY ETAL 3,

ELECTROLYTIC CELL FOR THE PRODUCTION OF ALUMINUM Filed June 11, 1962 5 Sheets-Sheet 5 I04 ELECTROLYTE METAL PAD LINING INSULATION INVEN R. ROBIN D HOLLID JACK L. HENRY United States Patent 3,257,307 ELECTROLYTIC CELL FOR THE PRGDUCTION 0F ALUMINUM Jack L. Henry, Los Altos, and Robin D. Holliday, San

Jose, Calif., assignors to Kaiser Aluminum & Chemical Corporation, Oakland, alif., a corporation of Delaware Filed June 11, 1962, Ser. No. 201,669 8 Claims. (Cl. 204231) The present invention relates to a novel electrolytic cell arrangement. More particularly, the invention relates to an improved cell design utilizing refractory hard metal current-conducting elements of the type recently proposed for use in electrolytic cells for the production of aluminum. The invention contemplates cell designs wherein the refractory hard metal currentconducting elements are protected from corrosive action during operation and result in increased life of the current-conducting elements.

Recent innovations in the design of electrolytic cells for the production of aluminum incorporate the use of refractory hard metal current-conducting elements in the cathodic system in preference to previously used carbon material. Electrolytic cells utilizing refractory hard metal current-oonducting elements provide a number of advantages in operation which improve the efficiency and lower the cost of producing aluminum. The present invention provides for additional savings in the use of refractory hard metal current-conducting elements by prolonging the life during operation in the cell of the elements and reducing deterioration by corrosion. Refractory hard metal materials, as recognized in the art, include nonmetallic materials such as the carbides and borides of titanium, zirconium, tantalum and niobium, and mixtures thereof.

Among the electrolytic cell designs utilizing refractory hard metal elements are the so-called top entering designs wherein the refractory hard metal bars .are extended through the electrolyte into the metallic cathode layer in the cell from the top where they. are supported or suspended from overlying superstru-ctnres. One principal cause of deterioration of top entering refractory hard metal currentconducting elements during operation in an electrolytic cell is corrosion attack in the crust zone. The crust zone in the electrolytic cell is the top layer which overlies the electrolyte surface. The crust is gener-ally comprised of a mixture of alumina, aluminum fluoride, cryolite and other cell addition materials. The crust forms a hardened outer surface which performs certain useful functions, e.g., heat insulation. Top entering refractory hard metal current-conducting elements extend through the crust layer into the electrolyte. Deterioration of the elements by corrosion attack in the crust zone comes about by electrolytic conduction through the crust between the carbon anodes and the refractory hard metal cathodic current-conducting elements. The electrolytic conduction results in deposition of elemental sodium at the surface of the refractory hard metal element. Oxidation and hydrolysis of the sodium then results in a highly corrosive mixture of NaO'I-I, Na O, and other corrosive materials.

As an illustration of this corrosive attack on the refractory hard metal elements, the following experiments are described to show that sufficient current may be passed through solid flux (constituting the crust) to allow electrolysis and corrosion to occur.

Example I A small carbon-lined clay crucible filled with cryolite ,Temperature, C. Applied Voltage Current, amps.

An alkaline reaction and strong sodium flame are obtained from the cathode graphite after removal indicating the presence of sodium deposition at the cathode. The anode was neutral with very little sodium flame coloration.

Example II In another experiment a in. titanium boride bar was used as a cathode. The electrolyte was a mixture of cryolite with approximately 5% CaF and 5% A1 0 to more resemble the composition of the crust. The anode and cathode bars were rigidly gripped and held parallel. The temperature was controlled at 700 C. and the applied voltage maintained at 6.0 volts and with a current of 3.7 amps. After 70 hours it was found upon inspection that the graphite anode had been oxidized away. The flux was broken away from the cathode bar which was found to show the beginning of deterioration with necking about A in. below the electrolyte surface. Light sand blasting showed the attack to be most pronounced on the side facing the anode rod. In addition, the flux around the neck exhibited a strongly alkaline reaction whereas the area around the anode did not.

According to the invention there is provided a method of preventing corrosion attack by interposing current-conducting elements in the crust between the carbon anodes of the cell and the refractory hard metal current-conducting bars. These elements can be connected to the cathode bus bar system and cathodic deposition will be induced on the intervening elements rather than on the refractory hard metal bars. In corrison terminology the intervening elements can be described as sacrificial cathodes.

The invention is further described and explained by reference to the accompanying drawings wherein:

FIG. 1 is a schematic view partly in section depicting the general arrangement of the sacrificial cathode with respect to the refractory hard metal current-conducting element and the normal anode system of the electrolytic cell.

FIGS. 2a and 2b depict the anode-cathode arrangement, illustrating the current field and arrangements without and with the intervening sacrificial cathodes, respectively.

FIGS. 3, 4, and 5 describe some simple forms of sacrificial electrodes which may be employed according to the invention.

FIG. 6 is a top schematic view of another arrangement utilizing a protective sacrificial cathode barrier for the refractory hard metal cathodic elements.

FIG. 7 is a schematic side view of the arrangement in 'FIG. 6 illustrating one way the sacrificial cathode barrier may be connected to a cathodic system.

FIG. 8 is an illustration of another arrangement of sacrificial cathode and refractory hard metal cathode displacement wherein the refractory hard metal elements are centrally disposed with anodes and sacrificial cathode assemblies on both sides of the refractory hard metal cathodes.

not a highly critical factor.

FIG. 9 is a schematic side view of the plan illustrated in FIG. 8.

In FIG.'1 the relationship between the sacrificial cathode 30 and the refractory hard metal cathodic currentc-onducting element is illustrated partially in section Within an electrolytic cell 10. As can be seen the electrolytic cell It comprises a refractory side wall 12 which, together with bottom cell lining l4 and insulation 16, form the cell container for metal pad 22, molten electroly'te24, and crust layer 26. Crust layer 26 is the top layer and covers the electrolyte. sacrificial cathode 30 is electrically connected by electrical connection means 32 to the refractory hard metal cathode at the cap member 1.8 of the cathode element 20. The electrical connection means 32 may be suitably secured by fastening means 34- to the cap 18.

The action of the sacrificial cathode is seen in FIGS. 2a and 2b. As is illustrated in FIG. 2a, refractory hard metal current-conducting elements are disposed adjacent anodes 42 without any intervening sacrificial cathodes, a current field 44 is generated in the crust layer between the cathodes 40 and anodes 42. In FIG. 2b sacrificial cathodes 46 are interposed between refractory hard metal cathodic current-conducting elements 4t and anodes 42. The sacrificial cathodes only extend into and through the rust layer in the electrolytic cell. rent passing between the anode and cathode through the crust layer will be directed to the sacrificial cathode 46 and any sodium deposition or corrosion occurring will take place on the sacrificial cathode and not on the refractory hard metal cathodic current-conducting element.

FIGS. 3, 4 and 5 illustrate three embodiments of the sacrificial cathode. As is seen the sacrificial cathode may take many forms, e.g. a spike, fork or spade. In FIG. 3 the sacrificial cathode is in the form of a spike. The sacrificial cathode may either be mechanically bolted to a cap member or welded thereto. In FIG. 3 the sacrificial cathode spike 50 is depicted as being welded. FIG. 4 illustrates a fork arrangement of sacrificial cathodes wherein the sacrificial cathode member 54 comprises a plurality of spike members connected together and are shown attached to a bolted cap member. FIG. 5 illustrates a spade configuration of the sacrificial cathode 58 with a bolted cap member 60.

The sacrificial cathode elements shown in FIGS. 3, 4

I and 5 need not be of large size and, consequently,-when used in an arrangement as shown in FIG. 1 strength is In such cases, small rods of refractory hard metal material such as titanium boride may be used for the simple spikes.

Other possible cnostruction materials for sacrificial cathodes include carbon, silicon carbide, SnO or other conducting metals, oxides, or ceramics which possess suitable corrosion resistance combined with good electrical conductivity.

In FIG. 6 an embodiment is shown wherein the refratory hard metal cathodes are surrounded by a metallic frame which is connected to the cathode bus and acts as a sacrificial cathode for the refractory hard metal elements. In FIG. 6 electrolytic cell 70 is shown having cell wall 72 comprised of cell lining and insulation sections, and vertically disposed anodes 74. The anodes depicted are of the pre-baked type, however, other anode forms such as Soderberg anodes may be employed. As is seen, refractory hard metal cathodic current-conducting elements 76 are supported by means 78 extending from the side wall of the electrolytic cell 76 and extend toward the anodes which are centrally disposed. Surrounding the anodes is a metallic \frame 84 which, as has been described in FIG. 7, is electrically connected to the cathodic system. Both the refractory hard metal current-conducting elements and the metallic sacrificial cathode frame are electrically connected to terminals 82 and to the cathodic bus 80.

The manner in which the sacrificial cathode metallic Thus, curtory hard metal cathode through the crust layer.

frame 84 in FIG. 6 serves to protect the refractory hard metal current-conducting element from corrosion in the crust layer of :the cell is described schematically in FIG. 7. As can be seen the electrolytic cell 86 is shown with refractory brick contained within the furnace shell 87. Disposed internally within the cell are refractory hard metal current-conducting elements 88 which extend from the top through the crust layer and electrolyte into the moi-ten metal layer. Disposed between the refractory hard metal current-conducting element 88 and anode 96 is the sacrificial cathode frame 92 which, as shown, extends only through the crust layer and serves as cathodic protection for current passing from the anode to the refrac- The sacrificial cathode assembly may, as shown, be secured to a side wall. The sacrificial cathode assembly 90 is electrically connected to the cathodic bus system by electrical connection means 98. To avoid shorting, the sacrificial cathode assembly and the electrical connection can be insulated from the side wall by insulation 99. In the arrangement shown, as in the other sacrificial cathode arrangements within the purview of the invention, the sacrificial cathode without interfering with the current passage through the electrolyte and molten metal layer of the cell, prevents deposition on the cathode by current passing through the crust layer. The metallic sacrificial frame employed can be made of any suitable metallic material which would not dissolve significantly at the temperatures employed in electrolytic cell operation and which will maintain its physical integrity under cell operating conditions. In addition, the sacrificial cathode, may be electrically connected to an outside potential or other cathodic system instead of tothe same system to which the refractory hard metal cathode is electrically connected.

Although the selection of materials used as sacrificial cathodes is, as discussed above, very wide, the choice of possible construction materials must take into account the highly corrosive conditions in the crust at 800 C. One material which has been found to be particularly satisfactory ifO'I' large sacrificial cathode assemblies such as the fence in FIG. 6 is a high chromium cast iron which olfers a satisfactory compromise between service life and replacement costs for the sacrificial cathode fence shown in FIG. 6. Moreover, sucha fence would have the additional advantage of acting as a shock absorber to protect the refractory hard metal cathodic current-conducting elements from impact breakage during anode changes.

The most complete protection against corrosion attack resulting from electrolytic conduction through the crust between the carbon anodes and the refractory hard metal cathode elements is afforded by providing an electrically conducting screen in the crust that completely eliminates the possibility of direct conducting paths between the anodes and the refractory hard metal elements. One advantageous form of such protection is shown in FIGS. 6 and 7 described above, wherein a metallic frame of suitable electrically-conductive material is rigidly attached to the cell side Wall but electrically insulated therefrom. The sacrificial cathode frame extends into the crust region but does not penetrate significantly into the liquid melt zone. By connecting the sacrificial cathode assembly to a cathode bus, a complete electrolytic screen for the refractory hard metal elements is provided.

It is, of course, possible to modify this arrangement by using cathodic screen elements arranged in the crust region between the carbon anodes and the refractory hard metal elements which are electrically connected to the negative bus bar system but are not physically supported by the cell wall. Such electrodes might be made in several forms, all easily removable and replaceable. They should be arranged to reduce as far as possible the electrolytic current passage between the anodes and the refractory hard metal elements but need not form a physically continuous screen.

Still another embodiment of sacrificial cathodic protection for the refractory hard metal elements is described in FIGS. 8 and 9. As can be seen in FIG. 8, a cluster of refractory hard metal cathodes 100 are centrally disposed within the cell. On each side of the cluster of refractory hard metal cathodic elements are disposed support members 112 which extend across the cell and are supported by the side walls. Sacrificial cathode elements 108 are suspended from the hanging supports on both sides of the centrally disposed cluster of refractory hard metal elements. Conventional anodes arrangements may be employed on the remote side of the sacrificial cathode assembly, permitting the elements 108 of the sacrificial cathode assembly to protect the centrally disposed cluster of refractory hard metal elements from corrosion by current passage through the crust layer between the anode and the refractory hard metal cathodes.

The manner in which the anode, sacrificial cathodes, and refractory hard metal elements of FIG. 8 are disposed within the cell with relation to each other is further seen in FIG. 9, wherein the cluster of, refractory hard metal elements represented by the single element 100 is shown passing through the crust 102, electrolyte bath 104-, and the metal pad 106 in the cell. Disposed on either side of the refractory hard metal element are sacrificial cathode members 108'w hich, as is shown, extend through the crust layer 102 only. In this manner a current passage between anodes 110 and refractory hard metalelements through the crust layer 102 is interrupted by intervening sacrificial cathode members 108.

The arrangement shown in FIGS. 8 and 9 represents an alternative method of securing both mechanical and corrosion protection of top entry refractory hard metal cathode elements utilizing a fewer number of relatively large sacrificial cathode elements. These elements may be in the form of short, stubby, compact forms of refractory hard metal material. These sacrificial elements can be suspended from a rigid steel support welded or clamped to the shell of the furnace of the electrolytic cell. The short, -sacrificial elements will not be exposed to the molten aluminum and exposure to molten electrolyte will occur only occasionally.

The arrangement shown in FIGS. 8 and 9 illustrates a technique wherein a minimum of cell space and electrolytic cell area would be used by disposing the refractory hard metal cathode current-conducting elements centrally in the cluster arrangement illustrated. The refractory hard metal elements can be disposed in the cluster either at the ends of the cell or in a central compartment of the cell as shown. The anodes can hang on the opposite side of the cell with the sacrificial cathode members being interposed between the anodes and the refractory hard metal cathodes. This arrangement, as indicated above, will offer in addition to sacrificial cathode protection, significant protection from mechanical shock during breakage of the crust layer for anode changes.

It is apparent from the description that various changes and modifications may be made without departing from the invention. The scope of the invention should, therefore, be limited only by the appended claims, wherein What is claimed is:

1. In an electrolytic cell for the production of aluminum adapted to contain a molten metal cathode layer, an electrolyte layer above said metal layer, a crust layer above said electrolyte layer, and having an anodic system comprising at least one anode adapted to extend through said crust layer into said electrolyte layer and a cathodic system comprising at least one cathodic current conducting element adapted to extend through said crust and electrolyte layers into said metal layer, and wherein said element is comprised of refractory hard metal; the improvement comprising sacrificial cathode means comprising electrically conductive material interposed between said anode and refractory hard metal conductor element, said sacrificial cathode means adapted to extend into said crust layer, means to electrically connect said sacrificial cathode means to a cathodic system to enable current passage in the crust layer substantially between said anode and said sacrificial cathode means.

2. An improvement according to claim 1 wherein said sacrificial cathode means is electrically connected to the cathodic system of said electrolytic cell.

3. An improvement according to claim 1 wherein said sacrificial cathode means is electrically connected to a source of electrical potential separate from the cathodic system of the electrolytic cell.

4!. An improvement according to claim 1 wherein said sacrificial cathode means comprise refractory hard metal material.

5. In an electrolytic cell for the production of aluminum adapted to contain a molten metal cathode layer, an electrolyte layer above said metal layer, a crust layer above said electrolyte layer, and having an anodic system comprising at least one anode adapted to extend through said crust layer into said electrolyte layer and a cathodic system comprising at least one cathodic current conducting element adapted to extend through said crust and electrolyte layers into said metal layer, and wherein said element is comprised of refractory hard metal; the improvement comprising a sacrificial cathode barrier comprising electrically conductive material interposed between said anode and refractory hard metal conductor element, said sacrificial cathode barrier adapted to extend into said crust layer, means to electrically connect said sacrificial cathode barrier to a cathodic system to enable current passage in the crust layer substantially between said anode and said sacrificial cathode barrier.

6. An improvement according to claim 5 wherein said sacrificial cathode barrier comprises a steel frame disposed transversely between the anode and refractory hard metal cathodic current-conducting elements.

7. In an electrolytic cell for the production of aluminum adapted to'contain a molten metal cathode layer, an electrolyte layer above said metal layer, a crust layer above said electrolyte layer, and having a cathodic system comprising at least one centrally disposed refractory hard metal cathodic current-conducting element adapted to extend through said crust and electrolyte layers into said metal layer, and an anodic system comprising at least one anode disposed on either side-of said centrally disposed refractory hard metal cathodic element, the improvement comprising sacrificial cathode means comprising electrically conductive material interposed between said anodes and refractory hard metal conductor element, said sacrificial cathode means adapted to extend into said crust layer, means to electrically connect said sacrificial cathode means to a cathodic system to enable current passage in the crust layer substantially between said anodes and said sacrificial cathode means.

8. An improvement according to claim 7 wherein said sacrificial cathode means comprise refractory hard metal material.

References Cited by the Examiner UNITED STATES PATENTS 476,914 6/1892 Bernard 204-.-196 1,567,791 12/1925 Duhme 204231 3,028,324 4/ 1962 Ransley 204--67 JOHN H. MACK, Primary Examiner.

H. S. WILLIAMS, Assistant Examiner.

Claims

1. IN AN ELECTROLYTIC CELL FOR THE PRODUCTION OF ALUMINUM ADAPTED TO CONTAIN A MOLTEN METAL CATHODE LAYER, AN ELECTROLYTE LAYER ABOVE SAID METAL LAYER, A CRUST LAYER ABOVE SAID ELECTRLYTE LAYER, AND HAVING AN ANODIC SYSTEM COMPRISING AT LEASE ONE ANODE ADAPTED TO EXTEND THROUGH SAID CRUST LAYER INTO SAID ELECTRLYTE LAYER AND A CATHODIC SYSTEM COMPRISING AT LEAST ONE CATHODIC CURRENT CONDUCTING ELEMENT ADAPTED TO EXTEND THROUGH SAID CRUST AND ELECTROLYTE LAYERS INTO SAID METAL LAYER, AND WHEREIN SAID ELEMENT IS COMPRISED OF REFRACTORY HARD METAL; THE IMPROVEMENT COMPRISING SACRIFICIAL CATHODE MEANS COMPRISING ELECTRICALLY CONDUCTIVE MATERIAL INTERPOSED BE-TWEEN SAID ANODE AND REFRACTORY HARD METAL CONDUCTOR ELEMENT, SAID SACRIFICIAL CATHODE MEANS ADAPTED TO EXTEND INTO SAID CRUST LAYER, MEANS TO ELECTRICALLY CONNECT SAID SACRIFICIAL CATHODE MEANS TO A CATHODIC SYSTEM TO ENABLE CURRENT PASSAGE IN THE CRUST LAYER SUBSTANTIALLY BETWEEN SAID ANODE AND SAID SACRIFICIAL CATHODE MEANS.