Graphitized cathode block for aluminum smelting

The present invention provides a graphitized cathode block for aluminum smelting obtained by forming a raw material containing at least one calcined coke selected from the group consisting of calcined petroleum coke and calcined pitch coke, and a binder; baking the formed material to give a non-graphitized block; and graphitizing the non-graphitized block by resistive heating; wherein both end portions of the block have a higher electrical resistivity than the center portion thereof. By employing the cathode block of the present invention, during electrolytically smelting aluminum, current may be uniformly flowed as the concentration of current at both end portions of the cathode block is lightened, and it becomes possible to maximize the advantages of graphitized cathode blocks.

The reference numerals indicated in the drawings represent a furnace 1 , transformer 2 , pusher 3 , terminal 4 , non-graphitized block 5 , thermal insulation packing 6 , current distributor 7 , clamping jig 8 , anode 9 , graphitized cathode block 10 and collector bar 11 . 
 DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention is now described in further detail with reference to the following examples. 
 EXAMPLE 1 Coal tar pitch (softening point 95° C.), 30 parts by weight was mixed with 100 parts by weight of calcined petroleum coke, kneaded at 150° C. for 60 minutes with a two-arm type kneader, cooled to 110° C., placed in a container, and molded into a size of width 700 mm×thickness 500 mm×length 3500 mm with a transverse extrusion press. Next, with a Ried-hammer type continuous furnace, a non-graphitized block was obtained by baking the obtained mold block in a reduction atmosphere in the condition of raising the temperature up to a maximum temperature of 1000° C. for about 20 days. Then, five non-graphitized blocks were serially aligned inside the direct energizing graphitization furnace, the contact portions of the adjacent blocks were secured by being uniformly covered with two graphite plates having a thickness of 5 cm, length of 70 cm and width of 66 cm and two graphite plates having a thickness of 5 cm, length of 70 cm and width of 47 cm, an electric current was applied to the blocks to graphitize the blocks at a temperature higher than 2000° C. The measurement results of the electrical resistivies of the center and end portion of the obtained graphitized block are shown in Table 1 below. Moreover, the life span of the electrolytic pot when using the obtained graphitized block in the electrolytic pot for aluminum smelting is also shown in Table 1 below. 
 EXAMPLES 2 to 6 A graphitized block was manufactured in a similar manner as with Example 1, other than that the thickness and length of the current distributor were as per Table 1 below. The electrical resistivities and the life span of the electrolytic pot when using the obtained graphitized block in the electrolytic pot for aluminum smelting are shown in Table 1 below. 
 EXAMPLES 7 to 12 Graphitized cathode blocks were manufactured in a similar manner as with Examples 1 to 6, respectively, other than that calcined pitch coke was used instead of calcined petroleum coke. The electrical resistivities and the life span of the electrolytic pot when using these obtained graphitized blocks in the electrolytic pot for aluminum smelting were both the same as with the case of using calcined petroleum coke as the raw material. 
 COMPARATIVE EXAMPLE 1 A graphitized block was manufactured in a similar manner as with Example 1, other than that a current distributor was not used. The electrical resistivities and the life span of the electrolytic pot when using the obtained graphitized block in the electrolytic pot for aluminum smelting are shown in Table 1 below. 
 COMPARATIVE EXAMPLE 2 The electrical resistivities and the life span of the electrolytic pot when using the baked block, which was obtained by molding the kneaded mixture of calcined anthracite containing graphite and pitch binder into a prescribed shape and baked it at 1000° C., in the electrolytic pot for aluminum smelting are shown in Table 1 below. 1 TABLE 1 Comparative Examples Examples 1 2 3 4 5 6 1 2 Thickness of 5 5 10 10 15 15 — — Current Distributor(cm) Length of 70 110 70 110 70 110 — — Current Distributor(cm) Electrical 13.1 13.2 15.8 16.2 17.9 18.0 9.9 45.3 Resistivity of Block End Portion (&mgr;&OHgr;m) Electrical 11.8 12.0 11.9 12.2 11.9 12.0 11.9 45.0 Resistivity of Block Center (&mgr;&OHgr;m) Life Span of 1898 1935 1971 2008 2059 2081 1643 1387 Electrolytic Pot (number of days) As a result of the above, it is clear that according to the present invention, a graphitized block having a higher electrical resistivities at the end portions of the block than the center portion thereof can be obtained and the life span of the electrolytic pot for aluminum smelting has been extended considerably.