WO2014003571A1 - Electrode and a method for making same - Google Patents

Description

Electrode and a method for making same The present invention relates to a carbon based electrode and a method of making same. In particular the invention relates to cathodes for electrolytic production of aluminium. In an embodiment it can be applied to anodes as well.

Commonly, for the fixation of steel collector bars in cathode blocks, there are preformed slots in the blocks that allow the bars to be entered into them. The space or void between the wall of the slots and the bars can be filled with melted cast-iron and/or a conductive paste can be applied.

In a similar way, pre-baked carbon anodes are fixed to steel studs forming part of an anode hanger. The anode has pre-formed bores which allow the steel studs to be entered into them. The fixation between the stud and the anode is commonly performed by pouring melted cast-iron in the annular space between each individual stud and the corresponding bore in the anode. The use of melted cast-iron require ovens that can melt the cast-iron and a corresponding distribution and pouring system. Further, before pouring the cast-iron into the recess in the cathode, the electrode and collector bars must be pre-heated due to the large differences in temperatures between the melted metal and the electrode (room temperature). For the anode, the studs are in some technologies preheated.

A pre-baked carbon anode is normally worn out after approx. 30 days in the cell due to the consumption of the carbon material it consists of. It then must be replaced. The worn out anode (butts) is transported to a facility where the studs of the anode hanger is cleaned by removing rest material of the anode together with cast iron residues. Commonly, this is performed by the use of mechanical frapping tools.

The operations for assembly of a new anode and removing the butts of a worn out anode are time consuming and expensive. Similar will apply for the more long lasting cathode blocks, where the collector bars and the carbon material has to be separated before disposal or recycling. Aluminium producers have been looking for more simple and energy efficient connection methods of the current leaders to the calcined carbon body. The importance of efficient solutions to this challenge will be even more important as the electrolysis cells becomes more energy intensive.

In the race toward low specific energy consumption for aluminium production, one well known and potent tool is the reduction of the cathodic voltage drop. Indeed, reducing cathodic voltage drop reduces ohmic energy loss in the cathode, allowing operators to either increase potline amperage and/or reduce pot voltage that ultimately results in a reduction of the specific energy consumption per ton of produced aluminium.

Many means have been used to achieve cathodic voltage drop reduction, and one that is commonly known is the use of copper inserts to improve the conductivity of the steel collector bars.

Examples are given in WO04031452 which discloses collector bars of steel having a copper core, US5976333A and WO016301 which both discloses various designs of a copper rod inserted in a steel tube embedded in a slot in a cathode block. It has been demonstrated in tests that copper inserts in the collector bars can reduce the cathodic voltage drop by about 60mV with regard to conventional steel collector bars.

Another benefit of using copper as a high conducting element in cathodes is the more uniform cathodic current density achieved with such designs. For graphitized cathodes especially, a more uniform current density decreases the maximum erosion rate, thereby increasing cathode life.

However, each mV saved with solutions involving insertion of highly conductive elements is expensive, because in addition to the expensive copper rods used, assembly (collector bar drilling and copper bar insertion) nearly triples the cost of copper alone.

During the inventors strive to find designs with lower costs, yet keeping the benefits of applying a highly conductive material, a number of alternative designs were evaluated. One alternative that has the potential of being both cheaper and more easy to implement than that described in the prior art was to apply a cast metal with good electrical conductivity in the rodding of electrodes. Applying for instance cast copper for collector bar rodding has shown to deliver significant benefits in a model study. In accordance with the present invention there is applied an electric conductive cast material between the electrical current lead and the calcinated carbonaceous material in the electrode, that represent considerable advantages. The cast material is sealed off by means of a ramming paste, that preferably is electric conductive such as a carbonaceous ramming paste.

The use of the present cast material may facilitate the re-use of the material, as the high value Cu part is located at the exterior of the steel part.

Further, the electrical resistance in an electrode where the invention is applied is observed to be improved with reference to the commonly used cast iron.

This and further advantages can be obtained by the invention as defined in the accompanying claims 1-13.

The present invention will in the following be further described by figures and examples where; Fig. 1 discloses a cross-section view through a cathode with a collector bar that discloses various casting height,

Fig. 2 discloses in part one collector bar removed from a cathode block after casting, Fig. 3 discloses a diagram where electrical current versus days in operation has been measured for a cathode block with Cu rodding and adjacent standard blocks,

Fig. 4 discloses a sketch where an electrode is tilted down to the left when cast copper is poured to control the distribution of cast material in the void,

Fig. 5 discloses a sketch where an electrode is tilted down to the right when cast copper is poured to control the distribution of cast material in the void.

En an experimental set up, a casting trial took place after the similar procedure of normal cast iron casting, see Fig. 1 where a carbon electrode 1 is provided with slots or recesses 6,7 having electrical current leads 2,3 therein. In each recess there is a void 8, 9 that is partly filled with cast Cu 4, 5. An induction furnace was emptied and loaded with 200 kg of Cu. The amount of Cu was too little to get an efficient melting, and preferable more Cu should be added. There were much sparking in the initial phase of the melt down. The charge was also added 2 dl of graphite fines (< 0.2 mm) to prevent oxidation. However, the air flow and the fineness the grains burned away the graphite in less than a minute. Coarser grained graphite or coke was suggested as improvement.

The charge with Cu was heated to 1280°C before it was transferred to the casting ladle. The casting ladle was newly lined and preheated with a gas burner. The casting started immediately, but the flowability of Cu seemed to be rather low, and not better than cast iron. The superheat was close to 200 degrees in the induction furnace. Probably the temperature drop upon transferring to the casting ladle was large, but this was not measured.

One example of a final casting height of the Cu in the slots is shown in Fig. 1 for the purpose of disclosure. It shows two different levels: one slot 7 with 10 cm Cu and one slot 6 with ~4 cm see Fig. 1.

Inspection of the casted area after removing the bars from the block revealed a very good coverage of the casted Cu, see Fig. 2 where the Cu cast part is indicated at 4 and the bar at 2. More than 95% of the contact area was filled in a continuous Cu block. The only voids observed were in connection with graphite floaters that were inserted to seal off the casting volume. A piece of the casted Cu was cut of for chemical analysis and the Cu did not show any significant pick-up of impurities. A small amount of Fe and Si pick-up was expected since the induction furnace was used for cast iron and not cleaned appropriately.

No wing, longitudinal or transversal cracks were observed in the carbon block after the casting. The collector bars were pulled out of the blocks by a forklift, and were qualitatively stuck to the same degree as collector bars with cast iron.

The copper used for rodding can be any type, as long as it is relatively pure and is easy to transport with existing equipment at the rodding shop. Copper scrap, copper shots, and at worst copper bar would be fit as raw copper for rodding. Copper contaminated with volatile products (oil or solvents) should be avoided. No alloyed copper (brass or bronze) is adequate for this application, because of higher resistivity of these alloys.

After the casting, the void that was not filled with cast material was rammed with ramming paste. The sealing with ramming paste was done to prevent leakage of molten Cu in the unlikely event of temperatures well above 1 100°C in the cell. The ramming paste will be baked during start-up of the cell.

In another example, a full cathode block (4 collector bars) was cast with Cu according to the procedure above. Approximate half of the slot height was filled with Cu and the rest of the slot was filled with carbonaceous ramming paste. One normal block with cast iron rodding was then replaced with the block with Cu-rodding in an otherwise normal electrolysis cell. After start-up the current distribution was measured, see Fig 3. The block with Cu-rodding has higher pull of current, as expected due to the lower resistivity of Cu. Furthermore, the there is no development or drift in the current distribution for the block with Cu in the period investigated.

The resistivity of the individual collector bars in the cell was measured after 53 days in operation, and the results are given in Table 1 below. The collector bars with Cu rodding had in average 13% lower resistance than standard collector bars.

Table 1. Measurements of collector bar resistivity after 53 days in operation.

In Fig. 3 there is disclosed a diagram where electrical current versus days in operation has been measured for a cathode block with Cu rodding and standard cathode blocks adjacent said Cu rodded block. The cathode block with Cu rodding was positioned in the middle of the cell, and the two adjacent reference standard blocks were positioned on each side of the cathode block with Cu rodding. The current in each collector bar was measured with an amp meter and summarized for each cathode block.

It should be understood that in the void between the recess in the carbon body and the current lead there could be filled-in cast Cu only. In an alternative, it could be filled-in solid Cu or another material with good electrical conductivity together with cast Cu. The solid material may be particulate or rod shaped. Another advantage with the method described here, is the flexibility in the amount of Cu used by only varying the fill height during casting. A high amount of Cu might be desirable in times of low Cu-price and/or high electricity costs. Furthermore, the casting/rodding can be done in a tilted configuration. This makes it possible to further optimise a reduction in CVD, as shown in Fig. 4 with more Cu 21 towards the end of the cathode block 1. The bas is indicated at 20. Alternatively one could improve current distribution as shown in Fig 5. by having more Cu 23 in the centre of the cathode block 1 , by just tilting the assembly during casting with Cu. The bar is indicated at 22.

Claims

Claims

1. An electrode for use in an electrolysis process for production of aluminium, the electrode comprises a body of calcinated carbonaceous material connected with an electrical current lead, where said current lead is embedded in a recess in said carbonaceous body, the recess being wider than the lead where the resulting void being filled at least partly with an electric conductive material

characterised in that

the electric conductive material comprises cast copper that is sealed off by a ramming paste.

2. An electrode in accordance with claim 1 ,

characterised in that

the electric conductive material comprises a solid material in the shape of particles or rods.

3. An electrode in accordance with claim 2,

characterised in that

the solid material is copper.

4. An electrode in accordance with claim 1 ,

characterised in that

the electrical current lead is made out of steel.

5. An electrode in accordance with claim 1,

characterised in that

the electrical current lead is made out of copper.

6. An electrode in accordance with claim 1 ,

characterised in that

the electrode is a cathode.

7 An electrode in accordance with claim 1 ,

characterised in that

the electrode is an anode.

8. An electrode in accordance with claim 1,

characterised in that

the ramming paste is an electric conductive material, for instance a carbonaceous ramming paste.

9. Method for producing an electrode for use in an electrolysis process for production of aluminium, the electrode comprises a body of calcinated carbonaceous material connected with an electrical current lead, where said current lead is embedded in a recess in said carbonaceous body, the recess being wider than the lead and leaving a void wherein it is filled in an electric conductive material that at least partly fills the void,

characterised in that

the electric conductive material comprises cast copper wherein, after solidification of the cast copper, the copper is sealed off with a ramming paste to prevent any leakage of melted copper.

10. Method in accordance with claim 9,

characterised in that

before filling the cast copper there is filled a solid electrical conducting material in the void.

11. Method in accordance with claim 9,

characterised in that

the cast copper partly melts the solid material when being filled in the void.

12. Method in accordance with claim 9,

characterised in that

the electrode is tilted when pouring the cast copper in the void.

13. Method in accordance to claim 9,

characterised in that

the void to be filled by cast copper is sealed off lengthwise by sealing elements.