METHOD FOR REMOVING CARBON ANODES IN ALUMINIUM ELECTROLYSIS CELLS

Abstract: A method of removing anodes in cells producing aluminium by electrolysis according to the Hall-Heroult process. Each cell comprises a cathode containing a fused salt bath of aluminium oxide dissolved in cryolite, and above the cathode is provided one or more carbon anodes (22) which are partly submerged in the bath and are partly covered with and surrounded by a crust. Immediately before a used anode is to be removed a through-going cut is made in the crust all the way around the anode and as close to the anode as possible. A device for performing the method is in the form of a crust cutter (1) comprising a cutting head (20) which is rotatably mounted around a vertical axis on the outer end of a telescopic arm (2) or the like, the inner end of which is rotatably mounted in a housing or frame construction (8) which can be raised or lowered.

Description: The present invention relates to the exchange of anodes in cells producing aluminium by electrolysis according to the Hall-Heroult process. Industrial production of aluminium is accomplished by electrolysis of aluminium oxide dissolved in melted cryolite in which is added small amounts of additives, mainly aluminium fluoride and calcium fluoride, at temperatures from 950°C to 970°C. With the so-called Hall-Heroult process, named after the inventors, aluminium is produced in a liquid state and deposited on the cathode such that the molten aluminium becomes the actual cathode. Carbon anodes, made of petrol coke and pitch, are partly submerged in the electrolyte which is usually called the melt bath. On the bottom side of the prebaked anodes, aluminum oxide is decomposed under the creation of oxygen which promptly reacts with the carbon of the anode to carbon dioxide. Usually there are about 20 prebaked anodes in an electrolysis cell, and since the anodes are gradually consumed, each anode has to be exchanged after 20 to 24 days. Each cell therefore has one anode exchanged every day. The melt bath is covered with a crust composed of solidified cryolite melt and an aluminium oxide layer. Since the anodes are partly submerged in the melt, the crust partly covers and fixedly holds the anodes. When the anodes are completely used, they may as well be fully covered with crust. Thus, when the anodes are exchanged they have to be "released" from the crust before being removed. This is for the most part done by breaking up the crust around the anodes. In some cases the anodes are just pulled out, without initially releasing them from the crust. In both cases , however, parts of the crust, and also part of the anodes, will fall down to the bottom of the cells. These crust parts or pieces will, if they are not removed, cause problems and disturbances during the electrolysis process. Removing the crust pieces increases the work of the operational personnel which again results in higher operational costs. There is also a certain probability that not all of the pieces will be removed, and this again will cause operational disturbances. In accordance with the present invention a method of removing anodes from cell used in the production of aluminium by electrolysis wherein each cell comprises a cathode immersed in a bath of liquid electrolyte and one or more anodes positioned above the cathode, at least partly submerged in the bath is characterised in that immediately before a used anode is to be removed, the crust which is produced during electrolysis and which surrounds the anodes is cut through around the used anode as close to the anode as possible. This method overcomes the problem of pieces of the crust falling to the bottom of the cell and hence removes the time consuming and expensive operation of retrieving the fallen pieces. A suitable apparatus for performing this method consits of a crust cutter which comprises a cutting head rotatably mounted around a vertical axis on the outer end of a telescopic arm, the inner end of which is rotatably mounted in a housing or frame construction which can be raised or lowered. The cutting head may be made of a steel plate and is preferably provided with a cutting edge which may be two sided, comprising a forwardly disposed cutting edge and a rearwardly disposed cutting edge. The crust cutter may be mounted on a vehicle, or a crane or the like. The invention will now be further described by way of example and with reference to the accompanying drawings in which:- Figure 1 shows an arrangement or device being used for performing the method and which is in the form of a crust cutter, Figure 2 shows a horizontal section of the crust cutter along the line A-A in Figure 1, Figure 3 shows an alternative cutter share, and Figure 4 shows a crust cutter mounted on a vehicle which is placed along side an electrolysis cell. As previously mentioned, the anodes have to be exchanged when they have reached their lowermost position. The method according to the present invention is characterised in that there is made a through-going cut in the crust around and as close as possible to the anodes before they are pulled out of the cell. In Figures 1 and 2 is shown a device for performing the method and which is in the form of crust cutter 1. As can be seen from the drawings, the crust cutter comprises a cutting head 20 which is turnably mounted on the outer end of telescopic arm 2 by means of cylinder/piston arrangement 9, 10. The telescopic arm comprises an inner part 3, an intermediate part 4 and an outer part 5. The intermediate and outer parts can be moved in their longitudinal direction by means of a piston/cylinder arrangment 6, 7 respectively. Further, the telescopic arm can be rotated relative to its longitudinal axis in a housing or frame construction 8, and the housing itself can be raised or lowered by means of parallel arms, 11, 12 respectively, and a cylinder/piston arrangement 13. Thus, the crust cutter may be raised or lowered and can be mounted on a vehicle 14 as shown in Figure 4, or it can be mounted on a crane or the like (not shown). The cutter share 1 consists of a steel plate 18 which is provided with a partly circular cutting edge 15 and a declining cutting edge 16. Alternatively the cutter can be designed as shown in Figure 3, where the cutting edge 17 is semi circular. The steel plate 18 is connected to a pivot 19, and the plate 18 is turnable through an angle of at least 90° in both side directions. The turning motion of the cutter can, as previously mentioned be obtained by means of a cylinder/pistol arrangement 9, 10. Other arrangements for turning of the cutter may also be used, such as a motor/toothed wheel transmition. Figure 4 shows a crust cutter according to the invention mounted on a vehicle 14. The vehicle is placed alongside an electrolysis cell. Only a part of the cell is shown, and for practical reasons and to be able to see the cutter in operation, the anode bar, the current connections, the anode super structure etc. is not indicated in the drawing. The cutting of the crust along the sides of the anodes is accomplished by firstly forcing the cutting head 20 (the edge 15, 16, 17) through the crust, and thereafter extending or retracting the telescopic arm. At the short ends of the anodes the crust is, however, cut in the following way: the telescopic arm is extended to a position where the cutting head 20 is at the outer or inner end of the anode being exchanged. Thereafter the cutting head is turned through an angle of 90° relative to the longitudinal axis of the telescopic device. This position is shown in Figure 4. The cutting is now accomplished by turning the telescopic arm to the right, or by lowering the telescopic arm so that the cutting edge is forced through the crust. Since the length of the cutting head is shorter than the length of the ends of the anodes, a corresponding cutting operation has to be performed from the other side of the anode. After having cut the crust all the way around the anode, the anode can be extracted and exchanged with a new one. By means of the here described method it has been possible to avoid the crust pieces falling into the bottom of the cell. This implies that the subsequent cleaning of the cell bottom, operational disturbances and problems caused by the remaining crust pieces on the cell bottom, are avoided.

Claims: A method of removing anodes from cells used in the production of aluminium by electrolysis in which each cell comprises a cathode immersed in a bath of liquid electrolyte and one or more anodes are positioned above the cathode, and at least partly submerged in the bath, characterised in that immediately before a used anode is to be removed, the crust which is produced during electrolysis and which surrounds the anodes is cut through around the used anode as close to the anode as possible Apparatus for performing the method according to Claim 1 characterised in that a crust cutter (1) comprises a cutting head (20) rotatably mounted around a vertical axis on the outer end of a telescopic arm (2), the inner end of which is rotatably mounted in a housing or frame construction which can be raised or lowered. Apparatus according to Claim 2, characterised in that the cutting head is made of a steel plate which is provided with a cutting edge. Apparatus according to Claim 3, characterised in that the cutting edge is two-sided, thus comprising a forwardly disposed partly circular cutting edge and a rearwardly disposed, declining edge. Apparatus according to Claim 4, characterised in that the cutting edge is semi circular. Apparatus according to any of Claims 2 to 5, characterised in that the cutting device is mounted on a vehicle and can be raised or lowered by means of parallel bars and a cylinder/piston arrangement. Apparatus according to any of Claims 2 to 5, characterised in that the crust cutter is mounted on and/or is moveable by means of a crane or the like.

Inventors: DRONNESUND, KJARTAN, HEGGO, STALE
Applicants: NORSK HYDRO A/S
Application year: 1989
Main classification: C25C3
IPC codes: C25C3, B25D17