COLLARS FOR THE PROTECTION OF ANODE HANGERS IN ALUMINIUM ELECTROLYSIS CELLS

Abstract: A protecting collar used to form a protecting cover of carbonaceous paste (6) over the cast iron connection (7) between a carbon anode (2) and stubs (4) of an anode hanger (1), and around the lower part of the stub (4). The collar (5,8,9) is placed around a stub and the carbonaceous paste (7) is then poured into the spacing between the collar (5,8,9) and the stub (4). The collar is made of cardboard, preferably brown paste board.

Description: The present invention relates to the securing of anodes, such as those used in the electrolytic production of aluminium, to anode hangers. More especially, it relates to the protecting collar used to form a mould around the stubs of the anode hanger which are connected to the anode by cast iron connections. A protecting cover of carbonaceous paste is formed by pouring the paste into the gap formed between the collar and the stub. Aluminium is mostly produced by electrolysis of aluminium oxide dissolved is a cryolite bath. The electrolysis cells enabling this consist of a steel shell with an inner carbon lining. Insulation is provided between the steel and the carbon and conducting rods are incorporated into the carbon lining at the bottom of the cell to provide a cathode. One or more carbon anodes are provided above the cathode and are partly submerged in the cryolite bath. The carbon anodes are manufactured by mixing petroleum coke and pitch, forming the mixture into blocks, complete with sockets for electrical connection, and subsequently baking them. Electrical connections are provided to the top and the bottom of the cells, and the cryolite is kept in its melted condition at a temperature close to the solidification temperature. The most common temperatures for these cells are between 930 and 980° C. The aluminium produced descends to the bottom of the cell and is deposited on the carbon lining to form a cathode of molten aluminium. Part of the aluminium produced is regularly removed from the cell by means of a suction pump and transferred to a casting ladle, and is in turn decanted to a melting pot. In an electrolysis cell of common size there are about 20-30 carbon anodes, and since these anodes are consumed gradually, they have to be changed after 20-30 days, depending on the size of the anodes. Thus, in each cell there is one anode exchanged every day. The carbon bodies are fixedly connected to anode hangers. The anode hangers serve two different purposes, namely to keep the carbon bodies at a predetermined distance from the cathode, and to conduct the electric current from an anode bar down through the carbon bodies. The anode hangers are releasebly fixed to an overhanging anode bar by means of a clamping device. As the carbon anodes are gradually consumed and as aluminium metal is removed from the cells, the anode bar, with the carbon anodes attached thereto, is lowered to keep a constant distance between the bottom side of the anodes and the cathode. The current required for the electrolysis cell lies between 100 000 and 300 000 amps. Electric current connections and bus bars are therefore made of industrial metals with good electric conductivity i.e. usually pure copper or aluminium. Since the lower part of the anode hangers is situated close to the cryolite bath which is of a high temperature, this part of the anode hanger is made of material which is resistant to the high temperature, i.e. usually steel. An anode hanger consists of aluminium or copper rods welded or bolted to steel stubs. The stubs are then positioned in the pre-formed sockets of the anodes and molten cast iron is poured around the stubs to produce a strong connection which has low electrical resistance A protecting ring of carbonaceous paste is formed around the lower part of the stubs and over the cast iron connection. This is done by firstly placing a protecting collar around the nipples and subsequently filling up the spacing between the collar and the stubs with the carbonaceous paste. The protecting ring of carbonaceous paste is finally hardened approximately 24 hours after the carbon anode has been inserted into the cell and prevents the electrolytic bath from coming into contact with the steel and the cast iron. Such contact would be liable to cause dissolution of the iron and steel which in turn may result in the stubs and cast iron becoming corroded such that the anode falls off the stubs. The protecting collars which have hitherto been employed have, due to the high temperature, been made of aluminium material (see e.g. DE-B-2547061). These aluminium collars have however been expensive to manufacture and to use. It is especially expensive to remove the aluminium material before the remains of the carbon anodes are processed for reuse. In accordance with the present invention an anode hanger for supporting an anode in an electrolytic cell for the production of aluminium by electrolysis, comprises a bar from which stubs protrude, the stubs being connected to the anode by means of cast iron connection, a protecting cover of carbonaceous paste being formed by pouring the paste within collars to form a cover over the cast iron and around the lower part of the stub, characterised in that each collar (5,8,9) is made of cardboard. Hereby large savings may be achieved by reduced manufacturing costs. Further, the problem related to the cleaning of the anode hangers is avoided since the card board collar burns away during the electrolysis process due to the high temperature, but after the carbonaceous paste has hardened. The collar is preferably made of brown past board and may be in the form of a strip with some connecting arrangements provided at either end of the strip. The invention will now be further described by way of examples and with reference to the accompaning drawings in which: Fig. 1 shows an anode hanger onto which is mounted a carbon anode, Fig. 2 shows an enlarged section of the connection between a stub and the carbon anode shown in Fig. 1, Fig. 3 shows an example of a card board collar according to the invention, and Fig. 4 shows another example of a card board collar according to the invention. Fig. 1 shows an anode hanger 1 supporting a so-called prebaked carbon anode 2 which is used in cells producing aluminium by electrolysis. The anode hanger 1 comprises an anode rod 3, of aluminium, and a cross member 15 of steel which is provided with three downwardly protruding steel stubs 4. The carbon anode 2 is fixedly connected to the stubs 4 by pouring cast iron 7 into the gap formed between the stub and the anode as shown in Fig. 2. Over the cast iron joint 7 and around the lower part of the stub 4 is provided a protecting ring made of carbonaceous paste 6. This, as previously mentioned, is made by at first placing a protecting collar 5 around the stub and thereafter filling up the empty space between the collar 5 and the stub 4. In Fig. 3 is shown an example of a protecting collar 8 of cardboard according to the invention. The invention is made of a cardboard stip which is provided with a locking arrangement at its ends. The locking arrangement as here shown consists of locking tongue 10 and a slit or cut 12. The collar is formed, as will be apparent from Fig. 3, by bending the collar so that the tongue may be led through the slit 12. Cuts 11 at the inner end of the tongue provide barbs which prevent the tongue from being pulled out of the slit. Fig. 4 shows another locking arrangement. Here the cardboard strip 9 is provided with transverse cuts 13,14 at its ends stretching from the side edge to the centre line in opposite directions. The collar is formed by bending the cardboard strip and letting the two slots (cuts) 12,14 grip into one another. The invention is not in any way limited to the above examples. Thus the collars may be made of cardboard strips where the ends are overlapping one another and are fixed by means of gluing, locking pin or the like.

Claims: 1. A protective collar for use with an anode hanger (1) for supporting an anode (2) in an electrolytic cell for the production of aluminium by electrolysis, the anode hanger (1) comprising a bar (15) from which stubs (4) protrude, the stubs (4) being connected to the anode (2) by means of cast iron connections (7), a protecting ring (6) of carbonaceous paste being formed by pouring the paste within collars (5,8,9) to form a cover over the cast iron (7) and around the lower part of the stub (4), characterised in that each collar (5,8,9) is made of cardboard. 2. A protecting collar as claimed in claim 1, characterised in that the cardboard is brown paste board. 3. A protecting collar according to claims 1 or 2, characterised in that the collar is made of a strip of cardboard and in that the ends of the card board strip are attached to one another by means of a connecting arrangement. 4. A protecting collar according to claim 3, characterised in that the connecting arrangement comprises a locking tongue (10) and a slot (12). 5. A protecting collar according to claim 3, characterised in that connecting arrangement comprises two cuts (13,14), one at each end of the strip. 6. A protecting collar according to claim 3, characterised in that the ends of the strip overlap and are glued to one another. 7. A protecting collar according to claim 3, characterised in that the ends of the strip overlap and are attached to one another by means of a locking pin.

Inventors: MEISINGSETH, LEIF
Applicants: NORSK HYDRO A/S
Application year: 1989
Main classification: C25C3
IPC codes: C25C3