Cathode blocks for aluminium electroysis and a method for producing same

Abstract

The present invention relates to a cathode block and a method for producing such cathode block (32), to be rodded with cathode bars (22, 22') and installed as a cathode assembly in a Hall-Heroult cell for electrolytic production of aluminium, where green pre-fabricated segments (31) of a composite material comprising hard material powders such as TiB2, HfB2, ZrB2, CrB2, WB2, or refectory metal carbide powder such as SiC, Cr3C2, B4C, TiC, or Al203, and coke and pitch, are formed in a small vibromould. These green composite segments (31) are then arranged selected areas in the bottom of a larger vibromould for making the cathode blocks, wherein the carbonaceous cathode paste is entered into the form, and the cathode paste the prefabricated segments are exposed to a forming process in the vibromould, fusing the pre-fabricated composite segments (31) with the cathode paste, thus forming a green cathode block with an integrated composite layer. The cathode block (32) is then baked and graphitized in a similar manner as for conventional cathode blocks.

Description

Cathode blocks for aluminium electroysis and a method for producing same The present invention provides a novel method for production of cathode blocks for aluminium electrolysis in Hall-Héroult cells, where the whole surface or parts of the surface of the cathode blocks is covered by a composite material. The invention also relates to cathodes produced by the method. Depending on the design, these cathode block can have many different purposes and improve the operational performance of the Hall-Héroult cells. The blocks can be rodded by cathode bars, Cu-rods or collector plates to form a cathode solution. Prior art Homogeneous composite cathode blocks for aluminium electrolysis based on C-TiB2 is normally produced in the same manner as conventional cathode blocks, but TiB2 powder is added to the dry coke aggregate and the pitch binder during preparation of the cathode paste. This C-TiB2 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-TiB2 composite material only in the top layer, in order to reduce the usage of expensive TiB2 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-TiB2 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-TiB2 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-TiB2 composite top layer is difficult to produce when using the C- TiB2 composite cathode paste in the forming process. WO0036187 discloses a multi layer cathode structure for aluminium electrolysis cells. Present invention 2 The present invention provides a novel method of producing cathode blocks with a C-TiB2 composite top layer, or a composite top layer comprising other metal boride powders such as HfB2, ZrB2, CrB2, WB2, or refectory metal carbide powder such as SiC, Cr3C2, B4C, TiC, or Al2O3 and carbon. The present method for producing a cathode block for a Hall-Héroult 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 TiB2 or other metal boride powder such as HfB2, ZrB2, CrB2, WB2, or refectory metal carbide powder such as SiC, Cr3C2, B4C, TiC, or Al2O3, 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-TiB2 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, 3 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. 4 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. 5 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 TiB2 powder or other metal boride powders such as HfB2, ZrB2, CrB2, WB2, or refectory metal carbide powder such as SiC, Cr3C2, B4C, TiC, or Al2O3, 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 TiB2 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 µm (d50), whereas the particles size distribution of the coke is typically 0.05 - 8 mm (d50). 6 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.

Claims

<pat:ClaimStatement>7 CLAIMS:< / pat:ClaimStatement> <pat:Claims com:id="claims"> <pat:Claim com:id="CLM-00001"> <pat:ClaimNumber>1< / pat:ClaimNumber> <pat:ClaimText>1. A method for producing a cathode block for a Hall-Héroult electrolysis cell for aluminium production, wherein pre-fabricated green composite segments with a certain shape and stiffness made of a composite cathode paste comprising a dry aggregate of a metal boride powder, a refractory metal carbide powder, or Al2O3 powder, together with a carbonaceous material, and a pitch binder, are formed or pre-fabricated in a compacting apparatus, which is a small vibromould, for production of said composite segments, and then positioned in a bottom of a large vibromould for production of conventional cathode blocks, and wherein a carbonaceous cathode paste is filled in the large vibromould and distributed evenly on top of the pre-fabricated green composite segments positioned in the bottom of the large vibromould, followed by vibromoulding to fuse the 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. < / pat:ClaimText> < / pat:Claim> <pat:Claim com:id="CLM-00002"> <pat:ClaimNumber>2< / pat:ClaimNumber> <pat:ClaimText>2. The method according to claim 1, wherein (i) the metal boride powder is TiB2, HfB2, ZrB2, CrB2, or WB2; (ii) the refractory metal carbide powder is SiC, Cr3C2, B4C, or TiC; and / or (iii) the carbonaceous material is coke particles. < / pat:ClaimText> < / pat:Claim> <pat:Claim com:id="CLM-00003"> <pat:ClaimNumber>3< / pat:ClaimNumber> <pat:ClaimText>3. The method according to claim 1 or 2, wherein the dry aggregate consists of 10-90 wt% of the metal boride powder, the refractory metal carbide powder, or the alumina powder, and 90-10 wt% of coke particles. < / pat:ClaimText> < / pat:Claim> <pat:Claim com:id="CLM-00004"> <pat:ClaimNumber>4< / pat:ClaimNumber> <pat:ClaimText>4. The method according to claim 1 or 2, wherein the dry aggregate consists of 20-40 wt% of the metal boride powder, the refractory metal carbide powder, or the alumina powder, and 80-60 wt% of coke particles. < / pat:ClaimText> < / pat:Claim> <pat:Claim com:id="CLM-00005"> <pat:ClaimNumber>5< / pat:ClaimNumber> <pat:ClaimText>5. The method according to any one of claims 1-4, wherein the particle size distribution (d50) of the metal boride powder, the refractory metal carbide powder, or the alumina powder is <semantics>0.1−100μm<annotation encoding="application / x-tex">0.1-100 \mu m< / annotation>< / semantics>, whereas the particles size distribution (d50) of the coke is <semantics>0.05−8μm<annotation encoding="application / x-tex">0.05-8 \mu m< / annotation>< / semantics>. < / pat:ClaimText> < / pat:Claim> <pat:Claim com:id="CLM-00006"> <pat:ClaimNumber>6< / pat:ClaimNumber> <pat:ClaimText>6. The method according to any one of claims 1-5, wherein the composite cathode paste is prepared by mixing the dry aggregate and 10-30 wt% of the pitch binder, relative to the dry aggregate, in a conventional mixer as used for preparing conventional cathode paste and in a similar manner as for preparation of conventional cathode paste. < / pat:ClaimText> < / pat:Claim> <pat:Claim com:id="CLM-00007"> <pat:ClaimNumber>7< / pat:ClaimNumber> <pat:ClaimText>7. The method according to any one of claims 1-6, wherein pre-fabricated green composite segments are formed by adding the composite cathode paste into a small vibromould for 8 production of said composite segments with two opposing form parts where one part has a spiky, zig-zag, conical, or pyramidal surface pattern, so that the pre-fabricated green composite segment will have the surface pattern on one side. < / pat:ClaimText> < / pat:Claim> <pat:Claim com:id="CLM-00008"> <pat:ClaimNumber>8< / pat:ClaimNumber> <pat:ClaimText>8. The method according to claim 7, wherein the pre-fabricated green composite segments are positioned on the bottom surface of the large vibromould for cathode block production with the spiky, zig-zag, conical, or pyramidal surface pattern pointing upwards, before addition of the carbonaceous cathode paste into the large vibromould. < / pat:ClaimText> < / pat:Claim> <pat:Claim com:id="CLM-00009"> <pat:ClaimNumber>9< / pat:ClaimNumber> <pat:ClaimText>9. The method according to any one of claims 1-8, wherein the pre-fabricated green composite segments are placed in both ends of the large vibromould, and extending 20- 100 cm towards the center of the vibromould, thus forming a green cathode block with a composite surface in the block ends. < / pat:ClaimText> < / pat:Claim> <pat:Claim com:id="CLM-00010"> <pat:ClaimNumber>10< / pat:ClaimNumber> <pat:ClaimText>10. The method according to any one of claims 1-8, wherein the pre-fabricated green composite segments are placed in the center of the bottom surface of the large vibromould for cathode block production and extending 20-50 cm toward each side of said vibromould, thus forming a green cathode block with a composite layer in the center of the cathode block surface. < / pat:ClaimText> < / pat:Claim> <pat:Claim com:id="CLM-00011"> <pat:ClaimNumber>11< / pat:ClaimNumber> <pat:ClaimText>11. The method according to any one of claims 1-8, wherein the pre-fabricated green composite segments are placed in specific areas in the bottom of the large vibromould for cathode block production extending from one end of the block to the other, forming parallel lines 5-20 cm wide along the vibromould, thus forming a green cathode block with a composite layer in parallel lines along the cathode block surface. < / pat:ClaimText> < / pat:Claim> <pat:Claim com:id="CLM-00012"> <pat:ClaimNumber>12< / pat:ClaimNumber> <pat:ClaimText>12. The method according to claim 7, wherein the pre-fabricated green composite segments are arranged onto a sloped surface in the bottom of the large vibromould for cathode block production with the spiky, zig-zag, conical, pyramidal surface pattern pointing upwards, before addition of the carbonaceous cathode paste into the vibromould, thus forming a green cathode block with a sloped composite surface. < / pat:ClaimText> < / pat:Claim> <pat:Claim com:id="CLM-00013"> <pat:ClaimNumber>13< / pat:ClaimNumber> <pat:ClaimText>13. The method according to any one of claims 1-12, wherein following vibromoulding, the green cathode block with the composite layer is baked at a corresponding temperature and duration, and in a corresponding manner as for conventional cathode blocks, before graphitization at a corresponding temperature and duration, and in a similar manner as for conventional cathode blocks. < / pat:ClaimText> < / pat:Claim> <pat:Claim com:id="CLM-00014"> <pat:ClaimNumber>14< / pat:ClaimNumber> <pat:ClaimText>14. A cathode block for a Hall-Héroult electrolysis cell for aluminium production, the block comprising an upper composite layer and a lower carbonaceous cathode material, 9 wherein the composite layer covers partly or wholly the upper part of the cathode block and is constituted by pre-fabricated green composite segments with a certain shape and stiffness made of a composite cathode paste comprising a dry aggregate of a metal boride powder, a refractory metal carbide powder, or Al2O3 powder, together with a carbonaceous material, and a pitch binder, wherein the cathode block is obtained by forming or pre-fabricating the green composite segments in a compacting apparatus, which is a small vibromould, for production of said composite segments, positioning the segments in a bottom of a large vibromould for production of conventional cathode blocks, and then fusing the segments to the carbonaceous cathode material during vibromoulding of the green cathode before baking and graphitization. < / pat:ClaimText> < / pat:Claim> <pat:Claim com:id="CLM-00015"> <pat:ClaimNumber>15< / pat:ClaimNumber> <pat:ClaimText>15. The cathode block according to claim 14, wherein the pre-fabricated green segments is made of a composite paste containing 10-90 wt% of the metal boride powder, the refractory metal carbide powder, or the Al2O3 powder, and the carbonaceous material, and the pitch binder. < / pat:ClaimText> < / pat:Claim> <pat:Claim com:id="CLM-00016"> <pat:ClaimNumber>16< / pat:ClaimNumber> <pat:ClaimText>16. The cathode block according to claim 14 or claim 15, wherein the pre-fabricated green segments have a spiky, zig-zag, conical, pyramidal surface pattern on the side fused with the carbonaceous cathode material. < / pat:ClaimText> < / pat:Claim> <pat:Claim com:id="CLM-00017"> <pat:ClaimNumber>17< / pat:ClaimNumber> <pat:ClaimText>17. The cathode block according to any one of claims 14-16, wherein (i) the metal boride powder is TiB2, HfB2, ZrB2, CrB2, or WB2; (ii) the refractory metal carbide powder is SiC, Cr3C2, B4C, or TiC; and / or (iii) the carbonaceous material is coke particles. < / pat:ClaimText> < / pat:Claim> < / pat:Claims>