The present invention concerns a method and a plant for cleaning and drying calcined electrodes made of carbon material, in particular nipple holes for anodes for use in electrolytic cells for the production of aluminium.

In modern electrolysis plants for the production of aluminium, anodes are used that consist of a prebaked or calcined carbon body attached to an anode hanger. The anode hanger usually consists of an upper part, often an anode bar made of aluminium, and a lower part that consists of a yoke with downward-projecting steel nipples.

The carbon body is usually attached to the anode hanger's steel part by means of a cast iron or contact paste connection. More precisely, the connection between the carbon body and the steel part is created by the nipples in the steel part first being arranged in such a way that they extend down into holes in the carbon body. The holes are then filled with molten cast iron or have contact paste pressed into them, thus ensuring a good electrical connection with adequate mechanical strength.

The holes may either be made when the carbon bodies are shaped, as so-called 'green carbons', prior to the calcination process, or they may be made after calcination, as described in NO patent 307795. When the anode carbons have been completed and calcined, it will usually take a while before they are assembled with an anode hanger in a so-called rodding plant. In many cases, anode carbons are made in one place and then transported to another place for assembly in a rodding plant, often with intermediate storage. Before the anodes are assembled with the hangers, it may be necessary to remove moisture, dirt and other contaminants from the nipple holes. Such anodes are often stored with the hole openings facing upwards so that any unwanted contaminants that enter the holes have a tendency to remain there. In addition, such holes, if they are exposed to free water, may take on considerable quantities of water. Any moisture and contaminants that may be in the holes before molten cast iron is added may result in reduced conductivity in the finished anode and, in a worst case scenario, lead to expansion of the liquid to form steam, with consequent ejection of molten cast iron during assembly. The prior art describes drying such holes using induction heating, with heating elements being inserted into the holes and emitting heat to them. The process is automated, with the positioning, insertion, dwell time and removal of the heating elements taking place according to a preprogrammed cycle.

The present invention comprises a solution that makes it possible to remove unwanted particulate material from the holes, at the same time as considerable quantities of unwanted liquid can be rapidly removed. The solution can be built with relatively low costs, while it is also efficient and inexpensive to use.

These and other advantages can be achieved with the invention as it is described in the attached claims. The present invention will be described in further detail in the following using examples and figures, where:

Fig. 1 shows a perspective view of a first embodiment of a plant in accordance with the present invention,

Fig. 2 shows a perspective view of a second embodiment of a plant in accordance with the present invention,

Fig. 3 shows the plant in Fig. 2 in a lateral view.

Figure 1 shows an anode 1 located on a transport device 2 comprising rollers 3, 4, 5. A stand 13 has means for cleaning and drying the nipple holes in the anode. In this embodiment, the means consist of nozzles 9, 10, 11 , 12 which are attached to a bracket 15 on the stand 13. The nozzles are designed to receive air via a pipe system 8, 7 which, at its upstream end, is connected to a fan 6. The air may either be heated before it reaches the fan or be heated downstream of the fan by means of a heating system.

Fig. 2 shows a second embodiment of the present invention, this time a more enclosed plant. An anode 21 is partially visible inside a tunnel-shaped enclosure 40. As in the previous example, the anode is located on a transport device 22. A fan 26 is also displayed. This is designed to blow air via a pipe system 27 to nozzles 32, 31 (only two are shown in Fig. 2). The tunnel-shaped enclosure 40 is equipped with an extraction hood 41 which converges into an extraction pipe 42 for the evacuation of air from the tunnel- shaped enclosure. Equivalent elements can be seen in Fig. 3, which also shows four nozzles 32, 31 , 30, 29. Four nozzles are used here as the anode has four nipple holes. A plant as shown above may be connected to a system for the supply of compressed air, for example 1-10 bar (not shown), to the nozzles 29, 30, 31 , 32, or alternatively via a separate pipe system for the supply of compressed air to separate compressed air nozzles arranged in the immediate proximity of the nozzles (not shown). The compressed air may be supplied at a pressure of 7-8 bar, for example, be dried and have a dew point of -30 ⁰ C to -40 ⁰ C. The air flow may have a speed of more than 20 m/s with a volume flow of less than 4 m ³ /minute.

The air flow may preferably have a speed of 50 m/s or higher from the nozzle opening and may be supplied in a first part of a cleaning cycle for the removal of unwanted particulate material and any free water. The enclosure of the anode ensures that these contaminants are not communicated to other areas of the plant. A collection device in the form of a pan with a drain (not shown) may be placed below the conveyor in connection with the tunnel- shaped enclosure for the collection of contaminants that have been removed. When the above cycle has been completed, for example in from 0.2 to 20 seconds, heated air may be supplied at a lower speed using the fan. The fan can supply 3,600 litres/minute, which is divided between four nozzles/heating elements. The heating elements can heat the room air at 20 ⁰ C to a temperature of approximately 590 ⁰ C by means of the 11 kW power of the heating elements. The heating elements are infinitely variable between 0 kW and 11 kW, thus producing an air temperature that varies accordingly. It will take approximately 1 minute to dry the nipple holes in an anode (a carbon). The temperature of the air during the drying process is preferably in the interval 450 to 500 ⁰ C. The air flow directed towards the anode's nipple holes in this stage may have a volume flow of less than 1 m ³ /minute at speeds lower than 5 m/s.

The drying unit undergoes the following cycle:

When an anode is detected on the conveyor belt into the drying unit, the fan starts and the heating elements are then switched on. When the anode is directly below the drying unit, the conveyor belt stops and normally remains stationary for around 60 seconds. During this time, the nipple holes are blown clean of loose material and any free water by means of the compressed air. Max. 7.5 bar for 5 seconds. Throughout the drying time of 50 seconds, 900 litres/min. hot air (max. 590 ⁰ C) is blown down into each nipple hole. If more carbons are detected on the conveyor ahead of the dryer, the dried carbon is conveyed out and a new carbon conveyed in for drying. If no more carbons are detected ahead of the drying unit, the heating elements are switched off and the fan continues for approximately 2 minutes before it is switched off. This is to extend the service life of the heating elements.

Automated control of processes such as those mentioned above, for example by means of PLC, etc., is within the expertise of an average expert and is, therefore, not discussed further here.

Please note that the embodiments of the plant shown are designed to handle anodes with four nipples. However, an expert will easily be able to adapt the plant to handle anodes with a different number of nipples.

In the examples described, one fan is shown supplying heated air to the anode's nipples via a joint manifold. It is also conceivable that one fan be installed combined with a heating unit (hot air blower) for each nipple hole in the anode, arranged in the immediate proximity of each nipple hole.

The heating unit may be fitted with overheating protection.

Although the word 'fan' is used in the description above, this comprises all types of pressure converter able to compress air such as air pumps, compressors, etc.

The air extracted will contain thermal energy that may be recovered with simple means. Alternatively, at least one sub-flow of the air may be recirculated in the plant, preferably after dehumidification.

Moreover, it is possible to install equipment to measure the relative humidity and temperature of the air extracted to make it possible to determine a state at which the drying can be concluded. Measurement signals in accordance with this may be fed to a control circuit that communicates with the rest of the control system for the plant and give this control system inputs. The information mentioned above may also be used to determine the energy supplied from the heating element and possibly also the speed of the air.