Prior art

Homogeneous composite cathode blocks for aluminium electrolysis based on C-TiB ₂ is normally produced in the same manner as conventional cathode blocks, but TiB ₂ powder is added to the dry coke aggregate and the pitch binder during preparation of the cathode paste. This C-TiB ₂ composite block is then vibromoulded or extruded to form a green cathode block, before baking and subsequent graphitization. Alternatively, a two-layered cathode block design can be prepared with C-TiB ₂ composite material only in the top layer, in order to reduce the usage of expensive TiB ₂ powder. In this case one thick base layer of conventional cathode paste is evenly distributed in the bottom of a vibromould. Then a top layer of C-TiB ₂ composite cathode paste is applied evenly on top of the base layer, before vibromoulding to form the green block followed by baking and graphitization. However, the accuracy during application C-TiB ₂ paste for the top layer is limited, and it may be difficult to achieve an even and thin top layer. Moreover, more complex cathode block designs with C-TiB ₂ composite top layer is difficult to produce when using the C- TiB ₂ composite cathode paste in the forming process.

W00036187 discloses a multi layer cathode structure for aluminium electrolysis cells.

Present invention The present invention provides a novel method of producing cathode blocks with a C-TiB ₂ composite top layer, or a composite top layer comprising other metal boride powders such as HfB ₂ , ZrB ₂ , CrB ₂ , WB ₂ , or refectory metal carbide powder such as SiC, Cr ₃ C ₂ , B C, TiC, or Al ₂ 0 ₃ and carbon. The present method for producing a cathode block for a Hall-Heroult electrolysis cell for aluminium production can be performed by that one or more pre-fabricated green composite segment(s) are positioned in a bottom of a large vibromould for production of conventional cathode blocks, and where a carbonaceous cathode paste is dumped in the vibromould and distributed evenly on top of the pre-fabricated green composite segments positioned in the bottom of the vibromould, followed by vibromoulding to fuse the said composite segments with the carbonaceous cathode paste, thus forming a green cathode block with an integrated composite layer on the cathode block surface or parts of the cathode block surface.

Said pre-fabricated green composite segment(s) can be made of a green composite material or segments comprising a dry aggregate of TiB ₂ or other metal boride powder such as HfB ₂ , ZrB ₂ , CrB ₂ , WB ₂ , or refectory metal carbide powder such as SiC, Cr ₃ C ₂ , B ₄ C, TiC, or Al ₂ 0 ₃ , together with carbonaceous materials such as coke particles and pitch binder, and formed or pre-fabricated in a small vibromould.

The novel and inventive production method will simplify the production, increase the precision during production, facilitate more complex designs of the cathode block and the C-TiB ₂ top layer, and possibly reduce the production cost.

In the following, the present invention shall be further explained by examples and figures where:

Fig. 1 discloses production of a pre-fabricated green composite segment in a small vibromould,

Fig. 2 discloses details of a pre-fabricated green composite segment, seen from one side, Fig. 3 discloses a conventional vibromould seen from one side, for forming the green cathode block based on conventional cathode paste and pre-fabricated green composite segments,

Fig. 4 discloses the same cathode block as in Fig.3 after vibromoulding, now turned up-side down relative to the position in the mold and with a complete composite cathode surface layer,

Fig. 5 discloses the same block as in Fig.4 as seen from the top, Fig. 6 discloses the same block as in Fig. 4 as seen from the block end, and with two recesses for cathode bars,

Fig. 7 discloses a vibromould from one side with pre-fabricated green composite segments placed in specific areas, and where conventional cathode paste is distributed on top of the composite segments,

Fig. 8 discloses the same cathode block as in Fig.7 after vibromoulding, and turned up-side down relative to the position in the mold, and with a composite layer on parts of the cathode surface

Fig. 9 discloses the same cathode block as in Fig. 8 as seen from the top,

Fig. 10 discloses a vibromould from the side with a sloped bottom covered by pre-fabricated green composite segments, and where conventional cathode paste is distributed on top of the composite segments.

Fig.11 discloses the same cathode block as in Fig.10 turned up-side down relative to the position in the mold, and with a complete composite layer on the sloped cathode surface

Fig. 12 discloses the same block as in Fig.11 as seen from the top, Fig. 13 discloses in a top view of one other cathode block design showing two areas with a composite layer extending from one end of the block to the other, produced in the same manner by placing pre-fabricated green composite segments in certain positions in the bottom of the vibromould,

Fig. 14 discloses the same block as in Fig.13 as seen from the block end, Fig. 15 discloses a side view of a cathode block with a composite layer arranged in a transversal direction, produced in the same manner by placing pre-fabricated green composite segments in certain positions in the bottom of the vibromold,

Fig. 16 shows a top view of the cathode block shown in Fig. 15. Detailed description:

Fig. 1 , discloses production of a green composite composite segment 11 in a small vibromould 10, where a composite cathode paste is poured into a mould part 10” and a vibrating weight 10’ compacts the mass to form a composite segment with a certain shape and stiffness called a pre fabricated green composite segment. Fig. 2 discloses details of a pre-fabricated green composite segment 20.

Fig. 3 discloses a vibromould 30 seen from the side where the pre-fabricated green composite segments 31 covers the whole bottom of the vibromould and where conventional cathode paste 32 is distributed evenly on top of these segments. This way, the green cathode block is formed upside down.

Fig. 4 discloses the same cathode block 32 as in Fig.3 after vibromoulding. Now with a complete composite layer 31 on top of the cathode paste 32.

Fig. 5 discloses the same cathode block as in Fig.4 as seen from the top. From this view only the composite layer 31 can be seen. Fig. 6 discloses the same cathode block 32 as in Fig. 4 as seen from the block end, and provided with two recesses 21 , 21 ’ for cathode bars 22, 22’, and the composite layer 31 .

Fig. 7 discloses the vibromould 30 as seen from the side where pre-fabricated green composite segments 31 ’, 31 ” are carefully placed only in some specific areas in the ends of the bottom of the vibromould and where conventional cathode paste 32 is distributed evenly on top of these. This way the cathode block is produced upside down.

Fig. 8 discloses the same cathode block 32 as in Fig.7 after vibromoulding. Now turned around with the composite layers on top towards the ends of the cathode block.

Fig. 9 discloses the same cathode block as in Fig. 8 as seen from the top. From this view the two areas with composite layer 31 ', 31 ” can be seen in the block ends, separated by an area based on conventional cathode paste 32.

Fig. 10 discloses a vibromould 40 as seen from the side where the bottom of the vibromould have a sloped surface and where this sloped bottom of the vibromould is completely covered with pre fabricated green composite segments 41 and where conventional cathode paste 42 is distributed evenly on top of these segments 41 . Fig.11 discloses the same cathode block 42 as in Fig.10 after vibromoulding. Now with a sloped composite layer 41 on top.

Fig. 12 discloses the same block as in Fig.10 as seen from the top. From this view only the composite layer 41 can be seen. Fig. 13 discloses an example of another cathode block design made possible by using pre fabricated green composite segments. Here the cathode block is seen from above, showing two areas with the composite material extending from one end of the block to the other end, prepared by careful positioning of the pre-fabricated green composite segments 51 , 51 ’. The conventional cathode paste is shown at 52, 52’, 52”, 52”’.

Fig. 14 discloses the same block as in Fig.13 as seen from the left block end, and with two recesses 21 , 21 ’ for cathode bars 22, 22’.

Fig. 15 discloses a side view of a cathode block with one area with a composite layer on the cathode surface arranged in a transversal direction in a cathode block 62, prepared by careful positioning of the pre-fabricated green composite segments 61 .

Fig. 16 shows a top view of the cathode block 62 shown in Fig. 15 with the composite segment 61.

The method for making the cathode can be based upon some main steps: i) A special composite cathode paste is prepared for making the composite segments by adding a dry aggregate of TiB ₂ powder or other metal boride powders such as HfB ₂ , ZrB ₂ , CrB ₂ , WB ₂ , or refectory metal carbide powder such as SiC, Cr ₃ C ₂ , B C, TiC, or AI ₂ C> ₃ , and coke in addition to a pitch binder in a similar mixer as used for preparation of conventional cathode paste and by using a similar mixing procedure as used for preparation of conventional cathode paste. Alternatively, instead of conventional coke, the composite cathode paste can be prepared using coke with inclusions of the said borides, carbides, or alumina, in addition to the pitch binder and TiB ₂ powder. ii) The dry aggregate consists of 10-90 wt% of the said boride, carbide or alumina powder, and 10-90 wt% of coke particles, but more typically the aggregate consists of 20-40 wt% of the said boride, carbide or alumina powder, and 60-80 wt% of coke particles. 10-20 wt% of pitch binder, relative to the mass of the dry aggregate, is added in the mixing prosess. As for production of conventional cathode paste, the temperature during the mixing process depends on the softening point of the binder pitch. Typically, the temperature during mixing is 120-200 °C. iii) The said borides, carbides or alumina powder in the dry aggregate has typically a particle size of 0.1 - 100 pm (d50), whereas the particles size distribution of the coke is typically 0.05 - 8 mm (d50). iv) After mixing, green composite segments are formed by placing the composite cathode paste in a small pre-heated vibromould. The bottom of the mold may have a spiky, zig zag, conical, pyramide or similar type of pattern to increase the surface area of one of the surfaces of the green composite segment. The segments formed in this process are called ‘pre-fabricated green composite segments’, since these segments can be produced beforehand and stored until they will be used for cathode block production at a later stage. v) Before cathode block production, the pre-fabricated green composite segments are preheated and then placed in the bottom of the vibromould used for preparation of green cathode blocks, with the spiky pattern upwards. vi) The conventional cathode paste is filled into the vibromould and distributed evenly on top of the pre-fabricated green composite segments, before vibromoulding of the cathode block is started. During vibromoulding the pre-fabricated green composite segments are fused with the cathode paste, and after vibromoulding these segments are an integrated part of the green cathode block. This way the green cathode block is produced upside down, with an integrated composite top layer resting on the bottom of the vibromould. vii) After vibromoulding the green cathode block is baked and then graphitized in a similar manner as for conventional cathode blocks. viii) Machining and rodding of the cathode blocks can be done in any manner known in the art, for instance by cathode bars, Cu rods or by collector plates.

The composite layer can be arranged so that it covers partly or wholly the upper part of the cathode block.