Classifications

• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25C—PROCESSES FOR THE ELECTROLYTIC PRODUCTION, RECOVERY OR REFINING OF METALS; APPARATUS THEREFOR
  • C25C3/00—Electrolytic production, recovery or refining of metals by electrolysis of melts
  • C25C3/06—Electrolytic production, recovery or refining of metals by electrolysis of melts of aluminium
  • C25C3/08—Cell construction, e.g. bottoms, walls, cathodes

Definitions

• the invention relates to graphitized cathode blocks, a process for their production and their use in particular for the electrolytic production of aluminum.
• electrolysis cells which comprise a base composed of a multiplicity of blocks, which acts as a cathode.
• the electrolyte is a melt, essentially a solution of aluminum oxide in cryolite.
• the working temperature is around 1000 ° C, for example.
• the electrolytically deposited molten aluminum collects on the bottom of the cell under a layer of the electrolyte.
• Around the cells is a metallic housing (preferably steel) with a lining made of high temperature resistant material.
• the material of the cathode blocks is preferably carbon because of the required chemical and thermal resistance, which can be partially or completely graphitized by thermal treatment.
• mixtures of pitches, cokes, anthracite and / or graphite in selected particle sizes or particle size distributions for the solids are mixed, shaped and fired and optionally (partially) graphitized.
• the firing (carbonization) usually takes place at temperatures of approx. 1200 ° C, the graphitization usually at temperatures of over 2400 ° C.
• While graphitized cathodes are preferred because of their higher electrical conductivity, they show greater wear during operation, corresponding to an average annual decrease in their thickness of up to 80 mm. This wear is not evenly distributed over the length of the cathode blocks (corresponding to the width of the cell), but changes the surface of the cathode blocks into a W-shaped profile. Due to the uneven removal, the service life of the cathode blocks is limited by the places with the greatest removal.
• One way to equalize the removal over the length of the cathode block and thus to extend the service life is to design the cathode blocks so that their electrical resistance varies over the length such that the current density (and thus the Wear) is uniform over its length or at least exhibits the smallest possible deviation over the length from its mean value.
• a solution is described in DE 20 61 263, in which composite cathodes are formed either from several carbon blocks with different electrical conductivity, which are arranged in such a way that a uniform or approximately uniform current distribution results, or from carbon blocks, the electrical resistances of which are in the direction of the cathodic Derivatives increase continuously.
• the number of carbon blocks and their electrical resistance depend on the cell size and type, they must be recalculated for each case.
• Cathode blocks made of a large number of individual carbon blocks require a great deal of effort in the construction; the joints must also be properly sealed to prevent the liquid aluminum from flowing out at the joints.
• WO 00/46426 describes a graphite cathode which consists of a single block which has an electrical conductivity which is variable over its length, the conductivity at the ends of the block being lower than in the middle. This uneven distribution of electrical conductivity is achieved, while during the graphitization the end zones are brought to a temperature of 2200 to 2500 ° C, while the other zone is exposed to a temperature of 2700 to 3000 ° C.
• This different heat treatment can be achieved according to this teaching in two ways: first, the heat dissipation in the graphitization furnace can be limited differently, or heat sinks can be introduced in the vicinity of the end zones, which increase the heat loss.
• the density of the heat-insulating bed is changed so that the heat loss becomes uneven over the length of the cathodes and the desired temperatures are thus set.
• the heat loss in the vicinity of the ends can be increased by different designs of the heat-insulating bed, or heat-dissipating bodies made of graphite, which cause a greater heat outflow to the furnace wall, are preferably introduced for this purpose.
• the difference in the heat treatment can be done by locally changing the current density, with the result of different heat development.
• the current density can be changed by different resistances of the conductive bed between two cathodes in an Acheson furnace (cross-graphing); no solution of this type is specified for a longitudinal graphitization process.
• cathode blocks are graphitized using the longitudinal graphitization method, an electrical transition with an increased resistance compared to the resistance inside the individual cathode blocks or the connecting element results at the joints between the individual cathode blocks with one another or with electrically conductive connecting elements arranged between them.
• This increased resistance leads to increased heat development and thus to a higher temperature, that is to say an acceleration of the graphitization reaction. Therefore, the electrical resistance in longitudinal graphitization at the ends of the cathode blocks is usually lower than that in the center of the cathode blocks. This distribution of the resistance or the electrical conductivity over the length of the cathode block is precisely the opposite of the desired course.
• cathode blocks with the desired course can be produced in a simple manner by placing the cathode blocks described above in the middle cut apart and reassembled in the opposite direction. This results in a profile of the electrical resistance in the form of an N. rounded on the legs.
• the present invention therefore relates to graphitized cathode blocks for the production of aluminum by electrolytic reduction of aluminum oxide in a bath of molten cryolite, characterized in that the cathode blocks are composed of at least two parts and have a V-shaped profile of their electrical resistance over their length, wherein the resistance in the center of the cathode blocks has a discontinuity and increases steadily towards the ends, such that the resistance at the ends of the parts is at least 1.05: 1 to that in the center.
• the cathode blocks are preferably composed of at least two parts, the electrical resistance of which increases continuously over their length, such that the resistance at the ends of the parts is at least 1.15: 1 in relation to that in the middle. This ratio is particularly preferably 1.3: 1.
• FIG. 2 shows a side view of a cathode block which has been separated and reversed in the middle, a connecting layer of ramming mass being introduced in the middle,
• Fig. 3 is a side view of a cathode block which has been separated in the middle and reversed '' ', with an adhesive joint connecting the two parts in the middle
• Fig. 4 is a side view of a cathode block separated in the middle and assembled in reverse, the two parts being merely flush with one another.
• FIG. 1 shows the course of the specific electrical resistance p shown in the interior of the side view of a cathode block 4, calculated as (R & /), where R is the electrical resistance of a cuboid test specimen, a its cross-sectional area, and 6 its length, along the length of the cathode block.
• R is the electrical resistance of a cuboid test specimen, a its cross-sectional area, and 6 its length, along the length of the cathode block.
• the ends of the block are marked with A.
• the cathode block is cut apart along the line BB, the end faces at A being designated as 4-1, and the separating surface along the line BB in the side section is called 4-2.
• the separated cathode block is now assembled as shown in FIGS. 2 to 4, that the ends A and the end faces 4-1 are in the middle of the assembled cathode block.
• Fig. 2 shows an embodiment, wherein between the two ends A now located in the middle with the end surfaces 4-line position of the ramming mass 5, which is otherwise also used to seal the contact surfaces between the individual cathode blocks on the bottom of the tub of the electrolytic cell becomes.
• Ramming compounds based on anthracite and graphite with a density of approx. 1700 kg / m 3 such as BST 17/1 from SGL Carbon AG, are suitable for this.
• the areas 4-2 that were previously inside have now become outside areas.
• the course of the specific electrical resistance p is now such that the lowest value lies in the center of the cathode block, and the specific electrical resistance now rises symmetrically towards the center towards the ends.
• the course of the electrical conductivity is then reversed, namely descending from a peak in the middle of the cathode block to the ends.
• FIG. 3 A further preferred embodiment is shown in which case the two half-blocks are each joined together with ends A by a layer of adhesive 6 with the required temperature resistance.
• FIG. 4 shows an embodiment in which an adhesive or intermediate layer has been dispensed with and the two half blocks have only been joined together with their ends A.
• the required surface pressure is applied due to the thermal expansion of the half-blocks, which after flush installation are pressed together during heating in the electrolysis cells. It has been shown that the pressing force is large enough to ensure a secure and tight connection of the two half blocks if the end faces were sufficiently flat before the division.
• the graphitized cathode blocks according to the invention show in the production of aluminum by electrolytic reduction of aluminum oxide in a bath of molten cryolite a more uniform wear over the length of the cathode compared to the conventional ones with homogeneous distribution of the electrical conductivity and therefore a significantly increased service life.

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Electrochemistry (AREA)
• Materials Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Electrolytic Production Of Metals (AREA)

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• 2001
  • 2001-12-28 DE DE2001164008 patent/DE10164008C1/de not_active Expired - Fee Related
• 2002
  • 2002-12-18 AR ARP020104963 patent/AR037912A1/es unknown
  • 2002-12-19 BR BR0215323-8A patent/BR0215323A/pt not_active IP Right Cessation
  • 2002-12-19 EP EP02796687A patent/EP1481115B1/de not_active Expired - Lifetime
  • 2002-12-19 PL PL369969A patent/PL201672B1/pl not_active IP Right Cessation
  • 2002-12-19 WO PCT/EP2002/014548 patent/WO2003056068A2/de active IP Right Grant
  • 2002-12-19 DE DE50205232T patent/DE50205232D1/de not_active Expired - Fee Related
  • 2002-12-19 CA CA002470753A patent/CA2470753A1/en not_active Abandoned
  • 2002-12-19 AU AU2002361174A patent/AU2002361174A1/en not_active Abandoned

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