GB1593204A - Electrolytic cell for producing aluminium - Google Patents

Description

(54) AN ELECTROLYTIC CELL FOR PRODUCING ALUMINIUM (71) We, NIPPON LIGHT METAL COM- PANY LIMITED, a Japanese corporation, of 3-5, 7-chome, Ginza, Chuo-ku, Tokyo, Japan, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement The present invention relates to an electrolytic cell for producing alumium.

Aluminium has been made by holding and electrolyzing a molten halide salt electrolytic bath containing molten alumium chloride as, for example, an electrolytic bath of AlCl3-NaCl-LiCl system or AlCl3-MgCl2 -NaCl system, at a temperature above the melting point of aluminium. This has various advantages in that it can be operated at an electrolyzing temperature near the temperature of 700or which is about 300"C lower than in the Hall Heroult process and that, as the anode reaction product by the electrolysis is a chlorine gas, no reaction with graphite used as an electrode material will take place so that the electrode is not eroded.

Although recognised as saving energy and resources, this method has not yet been applied industrially.

The electrolytic cell considered most feasible to date is an electrolytic cell having horizontal bi-polar electrodes manufactured recently by ALCOA, U.S.A. and described in U.S. patent No. 3,822,195.

In the ALCOA electrolytic cell many horizontal rectangular graphite electrode plates are arranged between the two electrodes of an electrolytic cell filled with a molten halide salt containing aluminium chloride with a proper clearance from the inner wall of the cell, and such that the aluminium chloride in the bath between the respective laminated electrodes is electrolyzed by passing an electric current between the two electrodes so as to generate chlorine gas between the anodes of the respective electrodes and molten alumiun grains on the cathode surfaces.

The chlorine gas produced at the anodes rises through the passage defined by the air gap or clearance between the electrodes and the inner wall of the cell on one side of the rectangular electrodes, and cause circulation of the electrolytic bath. On the other hand, the molten alumium grains produced at the cathodes move on the cathode surfaces due to the above mentioned circulation, reach the gas rising passage, flow downwardly in countercurrent with the circulation flow by virtue of their own weight and accumulate in the bottom of the cell.

There are however, defects in the ALCOA electrolytic cell. For example, the chlorine gas and molten alumium move countercurrently through the same passage and the chlorine gas produced at the anodes accumulates on one side of the rectangle, the gas content of the electrolytic bath between the electrodes on the discharging side will be so large and the chances of the aluminium and chlorine gas contacting each other will be so many that the aluminium will be rechlorinated and the current efficiency will be reduced.

One object of this invention is to avoid the defects of conventional electrolytic cells and an efficient electrolysis of aluminium chloride can be made.

According to this invention, an electrolytic cell for producing aluminium comprises, in an upper part, a raw material aluminium chloride feeding port and a chlorine gas discharging port, in a lower part, a molten metal reservoir and a middle part, wherein, a molten salt electrolytic bath containing aluminium chloride is electrolyzed to produce molten aluminium in the metal reservoir, at least three vertically spaced electrodes including a cathode, an anode and at least one bi-polar electrode between the cathode and anode and which are downwardly inclined toward the centre of the cell, a first communicating means in the centre of the cell and a second communicating means formed between the outer edge side surface of the electrodes and the inner wall of the cell.

Embodiments of the invention will now be described by way of example with refer ence to the accompanying drawings of which: Fig. 1 is a vertically sectional view of an embodiment of an electrolytic cell; Fig. 2 is a developed diagrammatic front view of flange parts and sleeves in the respective electrodes in the embodiment in Fig. 1; Fig. 3 is a vertically sectional view of another embodiment of the electrolytic cell according to the present invention; Fig. 4 is a plan view of the embodiment in Fig. 3.

In the embodiment in Figs. 1 and 2, numeral 1 indicates a cylindrical sealed type electrolytic cell formed of an outer plate 2 made of iron, an insulating glasswool layer 3, a refractory aluminium material 4 and a refractory nitride material 5. Numeral 6 indicates a sealing lid part provided in the top part of the electrolytic cell 1 and formed the same as the electrolytic cell 1. A raw material feeding port 7 for introducing a raw material aluminium chloride vapor into a bath is provided in the centre part of the lid part 6. A plurality of gas discharging ports 8 for discharging a chlorine gas generated by an electrolysis out of the cell are provided in the peripheral side part of the lid part 6.

A reservoir of molten aluminum obtained by the electrolysis is formed in the bottom part of the electrolytic cell 1 and bricks made of graphite are used for the inner wall 10 of this part. Numeral 11 indicates an outlet port for collecting molten aluminum accumulated in the molten aluminum reservoir 9. A temperature regulating mechanism 12 is provided on the periphery of the outlet port 11 so that, by properly controlling the temperature of the inner wall of the outlet port 11, the thickness of the soidified metal layer 13 deposited on the inner wall can be adjusted and the metal deliverying velocity can be thereby adjusted.

14, 15, 15 and 16 are funnel-shaped electrodes made of graphite, having respectively centre holes 17 and peripheral clearances and downwardly inclining toward the centre holes 17.

The respective electrodes 14, 15, 15 and 16 are arranged laminately at a proper distance between them on the vertical center axis of the electrolytic cell 1 and the uppermost electrode 14 and lowermost electrode 16 are fitted respectively with current passing terminals 18 and 19 so as to respectively form an anode and cathode. A sleeve 17a made of such a refractory material as of alumina or a nitride chemically resistant to the bath components is fitted in each center hole 17 as a sealing materia!.The electrodes 15 located intermediately between the electrodes 14 and 16 act as bi-polar electrodes having cathode and anode functions on upper and lower surfaces respectively, 20, 201, 2011, 20111 and 201111 are sleeves made of refractory material such as alumina or a nitride chemically resistant to the bath components for coating and protecting the outside surfaces of the electrodes 14, 15, 15 and 16 and holding the respective electrodes 14, 15, 15 and 16 at a fixed distance between them. The respective electrodes 14, 15, 15 and 16 are supported by the above mentioned sleeves 20, 201, 2011, 20111 and 201111 on flange parts 21, 211, 2111 and 21111 provided respectively on the peripheries of the lower ends.Further, the above mentioned respective sleeves and flanges are provided respectively with incisions 23, 231, 2311 and 23111 and 24, 241, 2411, 24111 communicating at proper intervals. As seen in the developed view shown in Fig. 2, the incisions 24, 241, 2411 and 24111 formed respectively in the above mentioned flange parts 21, 211 221l and 21111 preferably have the openings made gradually larger towards the upper part of the cell in which the flow of gas is greater. 25 is a hood made of such refractory material as of alumina or a nitride chemically resistant to the path components, connecting the centre hole 17 of the upper most funnel-shaped electrode 14 with the above mentioned raw material feeing port 7 and having on the lower peripheral surface a plurality of passages 26 through which the electrolytic bath flows in. By the way, the hood 25 may be made by cylindrically stacking firebricks.

The operation of the electrolytic cell formed as in this embodiment shall be described in the following.

When the cell 1 is first filled to the bath level 27 with a halide electrolytic bath containing aluminum chloride and an electric current is passed betweeen both electrodes 14 and 16, the respective funnel-shaped electrodes 15 between the electrodes 14 and 16 will become bi-polar electrodes and their upper surfaces and lower surfaces will function respectively as cathodes and anodes.

Therefore, by the electrolysis of aluminum chloride in the electrolytic bath present in the spaces between the respective electrodes 14, 15, 15 and 16, a chlorine gas will be produced on the anode surfaces and molten aluminum will be deposited in the form of grains on the cathode surfaces. Now, as the respective electrodes 14, 15 15 and 16 are funnel shaped, the molten aluminum grains produced on the cathode surfaces will move centripetally toward the center holes 17 along the sloped upper surfaces of the funnels and will fall into the center holes 17 to be accumulated in the molten metal reservoir 9. On the other hand, on the anodes, the produced chlorine gas will diffusely rise in the peripheral direction along the sloped lower surfaces of the funnels, will rise through the peripheral clearances (gas rising passages) through the incision 23, 231, 231l and 23111 of the sleeves and the incisions 24, 241, 241l and 241ll of the flanges and will be discharged out of the cell through the gas discharging ports 8 provided in the peripheral part of th lid part 6 in the cell top part.

In such case, the electrolytic bath contained in the above mentioned gas rising passages will produce a rising current due to the buoyant effect by the rising force of the chlorine gas. On the contrarty, a falling current will be produced in the center hole 17 of the electrode. Thus, as shown by the arrows in Fig. 1, the electrolytic bath will form a circulating current which will pass through between the respective electrodes 14, 15, 15 and 16 from the center hole 17, will reach the peripheral part of the cell, will rise through the peripheral clearances (gas rising passages), will be separated from the chlorine gas in the uppermost part and then will return to the center hole 17 again from the passages 26 of the hood 25.

In the case of laminating a plurality of funnel-shaped electrodes 14, 15, 15 and 16 at a fixed distance between them, the electrodes having flanges which are also supporters are used in the drawing but, for example, the electrodes may be held by setting separators of stays by a plurality of fine alumina pipes. It is needless to say that, in such case, a proper gas rising passsage will have to be provided between the electrodes and the inner wall of the cell. Also, the electrodes may be held only by cylindrical sleeves which may be made to communicate with the peripheral clearances (gas rising passages) by providing incisions.

In this embodiment, the raw material feedport 7 and the center hole 17 of the electrode are connected with each other through the hood 25 but the raw material aluminum chloride vapor can be blown directly into the center hole of the electrode from the raw material feeding port without using the hood.

The embodiment in Figs. 3 and 4 shall be described in the following. In this embodiment, the same reference numerals are attached to the same respective parts as in Figs. 1 and 2.

The greatest difference between this embodiment and the above described embodiment is that pairs of right and left inclined electrode plate groups are provided instead of the funnel-shaped electrodes made of graphite and used in the above described embodiment. That is to say, 14a, 15a-15a, 16a and 14'a, 15'a-15'a, 16'a are pairs of right and left inclined electrode plate groups provided as respectively opposed to each other on the right and left. 14a and 14'a are anodes. 16a and 16'a are cathodes.

1Sa, 1 5a-l 5'a, 1 5'a-(respectively four pairs in the drawing) are bi-polar electrodes formed respectively between the anodes and cathodes 14a, 16a and 14'a, 16'a. The respective upper surfaces function as cathodes and the respective lower surfaces function as anodes.

The respective electrode plates are kept at a fixed distance between them respectively by spacers 28, 28,-28', 28'-, the anodes 14a and 14'a and cathodes 16a and 16'a are held in an outer casing respectively by conductive rods 18, 18' and 19, 19' and the conductive rods 19 and 19' are positively supported by refractory materials 30. These right and left electrode plate groups are provided as opposed to each other with a fixed clearance between them. A falling passage 1 7a for the electrolytic bath and molten metal is formed between them.Further, rising passages 29 and 29' for the electrolytic bath and chlorine gas are formed between the rspective electrode plate groups 14a, 15a,-15a. 16a and 14'a, 15'a,-15'a, 16'a and the inner wall (refractory material layer 5) of the electrolytic cell. By the way, it is preferable that the above mentioned rising passages 29 and 29' are made wider with the approach to the upper part of the cell in which the flow of the gas is greater.

Now, the operation of the electrolytic of this embodiment is substantially the same as of the above described embodiment.

When the cell is first filled to the bath level 27 with a halide electrolytic bath containing aluminum chloride and an electric current is passed between both electrodes 14a, 16a and 14'a, 16'a of the right and left electrode plate groups, the respective electrode plates 15a and 15'a present as held between both electrodes 14a and 16a' and between both electrodes 14'a and 16a' will operate as bi-polar electrodes in which their upper surfaces will function as cathodes and their lower surfaces will function as anodes.The aluminum chloride in the electrolytic bath present in the spaces between the respective electrodes 14a, 15a,-15a, 16a and between the respective electrodes 14a', 15a',-15a', 16, will be electrolyzed, a chlorine gas will be produced on the anode surfaces and molten aluminum will be deposited in the form of grains on the cathode surfaces.However, as the respective electrode plates 14a, 15a,--15a, 16a and 14a', 15a',- 15a', 16a' slope inwardly, the molten aluminum grains produced on the cathode surfaces will fall due to their own weight inward of the cell along the sloped upper surfaces of the electrode plates, will further fall into the passage 1 7a formed in the clearance between both electrode plate groups and will be accumulated in the molten metal reservoir 9.

On the other hand, the chlorine gas produced on the anode surfaces will rise outward of the cell along the sloped lower surfaces of the respective electrode plates, will rise through the rising passages 29 and 29' formed of the clearances between the outer ends of the electrode plates and the inner wall of the cell and will be discharged out of the cell through the gas discharging ports 8 provided in the lid part in the top part of the cell.

In such case, the electrolytic bath contained in the above mentioned rising passages 29 and 29' will be subjected to the buoyancy effect by the rise of the chlorine gas and will produce a rising current. On the other hand, a falling current will be produced on the contrary in the falling passage 1 7a formed between the right and left electrode plate groups and, therefore, as indicated by the arrows in Fig. 3, the electrolytic bath in the cell will form a circulating current which will go outward in the cell through between the respective electrode plates 14a, 15a,-- 15a, 16, 14'a, 15'a,--15'a 16'a, will rise through the gas rising passages 29 and 29', will reach the upper part of the cell and will return again to the falling passage 17a between the respective electrode plate groups.

As explained in the above, according to the present invention, as the electrode part arranged in the intermediate part of the electrolytic cell is inwardly inclined, the chlorine gas produced on the anode surfaces of the electrodes will quickly rise along the sloped lower surfaces of the electrode plates and, on the other hand, the molten aluminum grains deposited on the cathode surfaces of the electrodes will quickly lower due to their own weight along the sloped upper surfaces of the electrode plates and therefore the chances of the aluminum being re-chlorinated will be remarkably low, Further as the chlorine gas rising passage and the molten metal falling passage are respectively independent of each other, the chances of the aluminum and chlorine gas contacting each other in these parts will be also low and therefore it will be possible to substantially perfectly prevent the loss of Al by re-oxidation.

From the viewpoint of the structure and the current efficiency in the case of the electrolysis, it is proper that the angle of inclination of the inclined electrode plates set in the embodiment of the present invention is 10 to 50 degrees.

The present invention is not limited to the above mentioned embodiment. For example, in the case of obtaining an electrolytic cell of a large capacity, the above mentioned pair of electrode plate groups may be made one set and a plurality of such sets may be arranged in parallel.

WHAT WE CLAIM IS: 1. An electrolytic cell for producing aluminium having, in an upper part, a raw material aluminium chloride feeding port and a chlorine gas discharging port, in a lower part, a molten metal reservoir and a middle part, wherein, a molten salt electrolytic bath containing aluminium chloride is electrolyzed to repoduce molten aluminium in the metal reservoir, at least three vertically spaced electrodes including a cathode, an anode and at least one bi-polar electrode between the cathode and anode and which are downwardly inclined toward the centre of the cell, a first communicating means in the centre of the cell and a second communicating means formed between the outer edge side surface of the electrodes and the inner wall of the cell.

2. A cell according to Claim 1 wherein each electrode means is a funnel-shaped electrode having a central hole.

3. A cell according to Claim 1 wherein each electrode means comprises a pair of right and left inclined electrodes, a passage formed with a fixed clearance on the respective inside surfaces of the pair of electrodes.

4. A cell according to Claim 2 wherein a hood interconnects the raw material feeding port and the uppermost funnelshaped electrode, passages for the discharge of gaseous reaction products of the electrolytic bath being formed in the hood.

5. A cell according to Claim 2 wherein the second communicating means consistss of incisions of many flanges provided at the lower ends of the outer peripheries of said funnel-shaped electrodes and incisions of sleeves fitted to the inner wall of the electrolytic cell.

6. A cell according to Claim 5 wherein the insions of the flanges and sleeves are made gradually larger in the openings with the approach to the upper part of the cell.

7. A cell according to Claim 3, wherein the second communicating means is a passage defined between the outer edge side surfaces of the electrode plates and the inner wall of the electrolytic cell.

8. A cell according to Claim 7 wherein the passage is gradually wider with the approach to the upper part of the cell.

9. A cell according to Claim 3 wherein the angle of inclination of the inclined electrodes is from 10 to 50 degrees.

10. A cell according to Claim 2 wherein the funnel-shaped electrodes are kept with a fixed spacing between them by the many flanges provided at the lower ends of the outer peripheries of the electrodes and the sleeves fitted to the inner wall of the electrolytic cell supporting the flanges.

11. A cell according to Claim 3 wherein the electrode plates are located by spacers.

12. An electrolytic cell having in the centre of a lid thereof a raw material feeding port, in a peripheral portion of the lid, gas discharging ports and, in the lower part

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (14)

**WARNING** start of CLMS field may overlap end of DESC **. will rise through the rising passages 29 and 29' formed of the clearances between the outer ends of the electrode plates and the inner wall of the cell and will be discharged out of the cell through the gas discharging ports 8 provided in the lid part in the top part of the cell. In such case, the electrolytic bath contained in the above mentioned rising passages 29 and 29' will be subjected to the buoyancy effect by the rise of the chlorine gas and will produce a rising current. On the other hand, a falling current will be produced on the contrary in the falling passage 1 7a formed between the right and left electrode plate groups and, therefore, as indicated by the arrows in Fig. 3, the electrolytic bath in the cell will form a circulating current which will go outward in the cell through between the respective electrode plates 14a, 15a,-- 15a, 16, 14'a, 15'a,--15'a 16'a, will rise through the gas rising passages 29 and 29', will reach the upper part of the cell and will return again to the falling passage 17a between the respective electrode plate groups. As explained in the above, according to the present invention, as the electrode part arranged in the intermediate part of the electrolytic cell is inwardly inclined, the chlorine gas produced on the anode surfaces of the electrodes will quickly rise along the sloped lower surfaces of the electrode plates and, on the other hand, the molten aluminum grains deposited on the cathode surfaces of the electrodes will quickly lower due to their own weight along the sloped upper surfaces of the electrode plates and therefore the chances of the aluminum being re-chlorinated will be remarkably low, Further as the chlorine gas rising passage and the molten metal falling passage are respectively independent of each other, the chances of the aluminum and chlorine gas contacting each other in these parts will be also low and therefore it will be possible to substantially perfectly prevent the loss of Al by re-oxidation. From the viewpoint of the structure and the current efficiency in the case of the electrolysis, it is proper that the angle of inclination of the inclined electrode plates set in the embodiment of the present invention is 10 to 50 degrees. The present invention is not limited to the above mentioned embodiment. For example, in the case of obtaining an electrolytic cell of a large capacity, the above mentioned pair of electrode plate groups may be made one set and a plurality of such sets may be arranged in parallel. WHAT WE CLAIM IS:

1. An electrolytic cell for producing aluminium having, in an upper part, a raw material aluminium chloride feeding port and a chlorine gas discharging port, in a lower part, a molten metal reservoir and a middle part, wherein, a molten salt electrolytic bath containing aluminium chloride is electrolyzed to repoduce molten aluminium in the metal reservoir, at least three vertically spaced electrodes including a cathode, an anode and at least one bi-polar electrode between the cathode and anode and which are downwardly inclined toward the centre of the cell, a first communicating means in the centre of the cell and a second communicating means formed between the outer edge side surface of the electrodes and the inner wall of the cell.

2. A cell according to Claim 1 wherein each electrode means is a funnel-shaped electrode having a central hole.

3. A cell according to Claim 1 wherein each electrode means comprises a pair of right and left inclined electrodes, a passage formed with a fixed clearance on the respective inside surfaces of the pair of electrodes.

4. A cell according to Claim 2 wherein a hood interconnects the raw material feeding port and the uppermost funnelshaped electrode, passages for the discharge of gaseous reaction products of the electrolytic bath being formed in the hood.

5. A cell according to Claim 2 wherein the second communicating means consistss of incisions of many flanges provided at the lower ends of the outer peripheries of said funnel-shaped electrodes and incisions of sleeves fitted to the inner wall of the electrolytic cell.

6. A cell according to Claim 5 wherein the insions of the flanges and sleeves are made gradually larger in the openings with the approach to the upper part of the cell.

7. A cell according to Claim 3, wherein the second communicating means is a passage defined between the outer edge side surfaces of the electrode plates and the inner wall of the electrolytic cell.

8. A cell according to Claim 7 wherein the passage is gradually wider with the approach to the upper part of the cell.

9. A cell according to Claim 3 wherein the angle of inclination of the inclined electrodes is from 10 to 50 degrees.

10. A cell according to Claim 2 wherein the funnel-shaped electrodes are kept with a fixed spacing between them by the many flanges provided at the lower ends of the outer peripheries of the electrodes and the sleeves fitted to the inner wall of the electrolytic cell supporting the flanges.

11. A cell according to Claim 3 wherein the electrode plates are located by spacers.

12. An electrolytic cell having in the centre of a lid thereof a raw material feeding port, in a peripheral portion of the lid, gas discharging ports and, in the lower part

of the cell, a molten metal reservoir and a metal outlet port, the cell further comprising a plurality of funnel-shaped electrodes having central holes and arranged in spaced relationship so as to form at least one bipolar electrode between a cathode and an anode, and a gas rising passage between the outer peripheral side surface of each of the electrodes and the inner wall of the cell, the said feeding port being connected to the central hole of the uppermost funnel-shaped electrode through a hood and the hood having passages for the discharge of gaseous reaction products of the electrolytic bath.

13. A electrolytic cell having in an upper part, a raw material aluminium chloride feeding port and a chlorine gas discharging port, a lower port with a molten metal reservoir and a middle part wherein a molten salt electrolytic bath containing aluminium chloride is electrolyzed to produce molten aluminium in the metal reservoir at the bottom part, pairs of vertically spaced, in a laminer manner along a vertical axis of the cell, right and left inclined electrode plate groups opposed to and spaced from each other and being downwardly inclined toward the centre of the cell at an angle of from 10 to 15 degrees, the electrode plates in the respective electrode plate groups being arranged in spaced relationship to define at least one intermediate bi-polar electrode between the cathode and the anode.

14. An electrolytic cell for producing aluminium constructed and arranged substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.