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  • C04B35/00—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products
  • C04B35/515—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on non-oxide ceramics
  • C04B35/58—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on non-oxide ceramics based on borides, nitrides, i.e. nitrides, oxynitrides, carbonitrides or oxycarbonitrides or silicides
  • C04B35/5805—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on non-oxide ceramics based on borides, nitrides, i.e. nitrides, oxynitrides, carbonitrides or oxycarbonitrides or silicides based on borides
  • C04B35/58064—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on non-oxide ceramics based on borides, nitrides, i.e. nitrides, oxynitrides, carbonitrides or oxycarbonitrides or silicides based on borides based on refractory borides
  • C04B35/58071—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on non-oxide ceramics based on borides, nitrides, i.e. nitrides, oxynitrides, carbonitrides or oxycarbonitrides or silicides based on borides based on refractory borides based on titanium borides
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  • C04B35/00—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products
  • C04B35/515—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on non-oxide ceramics
  • C04B35/52—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on non-oxide ceramics based on carbon, e.g. graphite
• • C—CHEMISTRY; METALLURGY
  • C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
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  • C04B35/00—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products
  • C04B35/515—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on non-oxide ceramics
  • C04B35/52—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on non-oxide ceramics based on carbon, e.g. graphite
  • C04B35/528—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on non-oxide ceramics based on carbon, e.g. graphite obtained from carbonaceous particles with or without other non-organic components
  • C04B35/532—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on non-oxide ceramics based on carbon, e.g. graphite obtained from carbonaceous particles with or without other non-organic components containing a carbonisable binder
• • C—CHEMISTRY; METALLURGY
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  • C04B35/00—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products
  • C04B35/622—Forming processes; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products
  • C04B35/64—Burning or sintering processes
• • C—CHEMISTRY; METALLURGY
  • C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
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  • C04B35/00—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products
  • C04B35/622—Forming processes; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products
  • C04B35/64—Burning or sintering processes
  • C04B35/645—Pressure sintering
• • 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

• Aluminum metal has been produced for 90 years in the Hall cell by electrolysis of alumina in a molten cryolite salt electrolyte bath operating at temperatures in the range of 900o-1000oC.
• the reactivity of the molten cryolite, the need for excellent electrical conductivity, and cost considerations have limited the choice of materials for the electrodes and cell walls to the various allotropic forms of carbon.
• the Hall cell is a shallow vessel, with the floor forming the cathode, the side walls being a rammed coke-pitch mixture, and the anode being a block suspended in the bath at an anode-cathode separation of a few centimeters.
• the anode is typically formed from a pitch-calcined petroleum coke blend, prebaked to form a monolithic block of amorphous carbon.
• the cathode is typically formed from a pre-baked, pitch-calcined anthracite or coke blend, with cast-in-place iron over steel bar electrical conductors in grooves in the bottom side of the cathode.
• the ancde-cathode spacing is usually about 4-5 cm., and attempts to lower this distance result in an electrical discharge from the cathode to the anode through aluminum droplets suspended in the bath.
• the molten aluminum is present as a pad in the cell, but is not a quiescent pool due to the factors of preferential wetting of the carbon cathode surface by the cryolite melt in relation to the molten aluminum, causing the aluminum to form droplets, and the erratic movements of the molten aluzd-num from the strong electromagnetic forces generated by the high current density.
• the wetting of a solid surface in contact with two immiscible liquids is a function of the surface free energy of the three surfaces, in which the carbon cathode is a low energy surface and consequently is not readily wet by the liquid aluminum.
• the angle of a droplet of aluminum at the cryolite-aluminum-carbon junction is governed by the relationship where ⁇ 12 , ⁇ 13 , and ⁇ 23 are the surface free energies at the aluminum carbon, cryolite-carbon, and cryolite-aluminu ⁇ a boundaries, respectively.
• the cathode were a high energy surface, such as would occur if it were a ceramic instead of carbon, it would have a higher contact angle and better wettability with the liquid aluminum. This in turn would tend to smooth out the surface of the liquid aluminum pool and lessen the possibility of interelectrode discharge allowing the anode-cathode distance to be lowered and the thermodynamic efficiency of the cell improved, by decreasing the voltage drop through the bath.
• amorphous carbon is a low energy surface, but also is quite durable, lasting for several years duration as a cathode, and relatively inexpensive.
• a cathode or a cathode component such as a TiB 2 stud which has better wettability and would permit closer anode-cathode spacing could improve the thermodynamic efficiency and be very cost-effective.
• Titanium Diboride TiB 2 has been proposed for use as a cathodic element in Hall cells, giving an improved performance oyer the amorphous carbon and semi-graphite cathodes presently used. It had previously been known that Titanium Diboride (TiB 2 ) was useful as a cathode component in the electrolytic production of aluminum, when retrofitted in the Hall cell as a replacement for the carbon or semi-graphite form.
• the electrical efficiency of the cell was improved due to better conductivity, due mainly to a closer anodecathode spacing; and the corrosion resistance was improyed, probably due to increased hardness, chemical inertness and lower solubility as compared to the carbon and graphite fonts.
• TiB 2 -carbon composite which shows excellent performance as a cathode or cathode component in Hall aluminum cells.
• the method is markedly more economical, and also produces an unexpectedly improved cathode when its performance is compared to the traditional carbonaceous material.
• the method involyes the use of a titania (TiO 2 ) -graphite composite structure as a starting material. TiO 2 is dispersed in the mixture of coke particles and flour, then wetted and dispersed in a carbonizable liquid binder to form a plastic mass.
• the binder is preferably a coal tar pitch, however, petroleum pitches, phenolic resins, lignin sulfonate, and other carbonizable binders may also be used.
• the coke particles most useful are selected size ranges of calcined delayed petroleum coke, made by heating a heavy hydrocarbon fraction to about 500o-510oC and holding the material in a coker drum for about 18 hours, while taking the gas oils vaporizing off to a combination tower for separation and recycling. The solid coke re-sidue remaining is removed, xihen calcined at approximately 1200o-1300oC to form the calcined coke useful in Hall cell electrodes or electrode components, and for conversion to graphite.
• Regular coke is isotropic with a coefficient of thermal expansion (CTE) characteristic of from 10 to 30 x 10 -7 cm/cm/oC, over the range of 0o to 50oC, relatively uniform on all 3 geometric axes in physical properties, while an acicular or needle coke will generally be anisotropic having a CTE characteristic which is variant on the axes and less than 10 x 10 -7 cm/cm/oC on the principal axis.
• Coke flour may also be included, using a particle size range with about 50% passing a 79 mesh/cm (200 mesh per in.) screen.
• the filler carbon in the original formed article may also be obtained from other common sources, such as pitch coke, charcoal and metallurgical cokes from coal, with a mean particle diameter of about 3 mm being preferable, ana a high surface area/wt. ratio.
• the plastic mass is then molded or extruded to form the desired shape and baked on a cycle rising to 700o-1100oC oyer a period of 1 to 10 days to carbonize the binder, forming a solid C-TiO 2 composite.
• the baked carbon-TiO 2 composite shape produced is a structure containing TiO 2 and particulate carbon firmly bound in the matrix of carbonized pitch.
• the structure is highly porous due to the inherent porosity of the coke, incomplete packing, and the volatilization of about 30-40% of the initial weight of the pitch, and is specially formulated for high porosity.
• the baked composite shape is then impregnated in a pressure vessel under alternate cycles of vacuum and about 7 x 10 3 Pa (100 PSI) pressure with a boron compound alor.e or with a dispersion of B 2 O 3 and carbon black or other micronized carbon in H 2 O.
• a boron compound alor.e or with a dispersion of B 2 O 3 and carbon black or other micronized carbon in H 2 O Either B 2 O 3 or H 3 BO 3 may be used as B 2 O 3 is hydrolyzec to H 3 BO 3 in H 2 O by the reaction:
• the article may be impregnated with molten B 2 O 3 or boric acid or with a oarbcn black dispersion in a molten boron compound.
• a further modificaticn of the above procedure consists of mixing stoichiometric amounts of TiO 2 , carbon black and B 2 O 3 , heating the mixture to melt the B 2 O 3 , dispersing the TiO 2 and carbon in the molten B 2 O 3 , cooling, allowing the paste to harden to a solid, milling the solid to a powder, dispersing the powder in a binder, then using this dispersion as an impregnant.
• the boron compound ana carbon black may be dispersed in a molten pitch or other carbonizable binder such as a petroleum pitch with a 110o- 120°C softening point, and the resulting dispersion used as an impregnant.
• a molten pitch or other carbonizable binder such as a petroleum pitch with a 110o- 120°C softening point
• Each impregnating cycle will nomally require a bake to the 700o-1100oC range, carbonizing the binder.
• the process may also be used by mixing boron carbide (B 4 C) with coke particles and binder in the initial mix, baking, then impregnating the resulting baked piece with a TiO 2 -carbon black dispersion in a carbonizable binder.
• B 4 C boron carbide
• the unique aspect of the process provides a method whereby TiB 2 is formed during subsequent heat treatment to a temperature above 2000oC, while the carbon is being made graphitic.
• the carbon black or similar finely divided carbon acts as the reductant to minimize consumption of the article matrix during the reaction of TiO 2 and B 2 O 3 to form TiB 2 .
• the article After drying, the article is heated to the reaction temperature for the formation of TiB 2 , in the range of 1200o-1800oC.
• the reaction starts to take place at about 800oC but is quite slow below 1200oC and reaches a high reaction rate at about 1750oC.
• the heat treatment may be done in stages, with re-impregnation and reheating cycles to build up the desired concentration of TiB 2 . Due to the loss of the carbon black ana possibly a portion of the binder and coke as CO during the TiB 2 forming reaction, the article may develop excess porosity and consequently have low strength and be poor in other physical properties.
• a carbonizable binder preferably a petroleum pitch with a softening point in the 110o-123 range, although lignin sulfo ⁇ ate, phenolic resins and other pitches may be used, under about
• the article After impregnation, the article is again heated to the 600o to 1100oC range over a period of 2 to 10 days to carbonize the pitch, sealing the surface and strengthening the article.
• the last step in the process will generally include heating the article to 2000oC or higher, converting the carbon to the graphitic form.
• the temperature range preferred is about 2400o- 2500oC, although for particular processes any point in the 2000o- 3000o range may be used.
• a cathode shape is made by mixing coke particles with a mean diameter of 3 mm, coal tar pitch having a softening point of approximately 110o-120oC and TiO 2 in a high-shear heated mixer. The mix is heated to approximately 175 oand the coke and TiO 2 are well dispersed in the molten pitch.
• the cathodic element is molded at about 14 x 10 6
• Pa (2000 PSI) pressure then baked on a cycle rising to 720o in six days. After removal from the oven the shape is placed in an autoclave and impregnated with a dispersion of a rubber reinforcing grade of carbon black and B 2 O 3 in H 2 O, then removed and dried slowly to vaporize the H 2 O without loss of B 2 O 3 . The piece is next heated to 1500oC, at which temperature the TiO 2 and B 2 O 3 react, releasing CO. These gas-producing steps are carried out slowly in order to avoid fissuring due to too rapid gas evolution.
• the piece is then re-impregnated, using a petroleum pitch with a softening point of from 110o-120oC, baked to carbonize the pitch on a six day cycle, the temperature rising to 720oC, which fills the porosity left by theTiB 2 forming reaction, then graphitized by heating to 2400oC.
• a mixture is prepared haying the following composition: B 2 O 3 38 wt. %
• a conventional carbon body which has a high pore volume and is well suited for impregnation, is made from the following composition, by wt. :
• Example 4 The mix is blended, shaped and baked as in Example 1. The article is then impregnated under cycles cf vacuum and pressure above the melting point of B 2 O 3 with the mix prepared in Example 2, heated slowly to a TiB forming temperature above 1200oC, preferably 1750 , held at that temperature for one to four hours, cooled and impregnated with the same petroleum pitch under alternate cycles of vacuum and pressure as above, re-baked as above, and heat treated to a temperature of 2100o or higher.
• Example 4 Example 4
• a cathode shape is formed from pitch, coke, and TiO 2 and baked as in Example 1. It is then impregnated with a dispersion of carbon black in molten B 2 O 3 at 7 x 10 3 Pa (100 PSI). After impregnation, it is heated to 1500 for one hour to form TiB 2 , then cooled, impregnated with petroleum pitch under cycles of vacuum and 7 x 10 3 Pa at 250°C, re-baked for six days, the temperature reaching 720oC, then graphitized by heating to 2300°C.
• Example 5 A mixture is prepared having the following composition:
• a cathodic element for a Hall cell is formed by molding the dispersion trader about 1.4 x 10 7 Pa (2000 PSI) and baked on a cycle rising to about 800oC in six days. After cooling to ambient temperature, the element is impregnated with a dispersion of 30% TiO 2 by wt. (ceramic pigment grade) in petroleum pitch (S.P. 110o- 120°C) at 240°C under alternate cycles of vacuum and 6.9 x 10 5 Pa (100 PSI) pressure.
• Blends of the following dry ingredients are mixed in parts by wt. :
• Theoretical % TiB 2 in composite 1 2 8% 57% 79% 77% 1Assuming a 75-80% coke yield from the pitch during the bake cycle from ambient to 700o-1100oC. 2Assuming complete conversion of TiO 2 to TiB 2 .
• TheTiO 2 and coke are charged into a sigma type mixer heated to about 160°-175°C and thoroughly blended while being heated. When the dry blend has reached about 160oC, the pitch is added, melted, and the solid ingredients wetted by the molten pitch. After thorough mixing, the plastic mass is cooled and molded to the desired shape of the article.
• the article is baked on a slowly rising temperature cycle, reaching 720°C in a period of 6 days, and removed from the furnace and cooled. After re-heating to about 500°C, the article, at that temperature or higher, is impregnated with moltenB 2 O 3 , under 6.9 x 10 5 Pa pressure to a final pickup of sufficient boron-containing material to form the surface layer of TiB on further heat treatment.
• the TiO 2 -C composites of Example 6 are prepared and impregnated with molten H 3 BO 3 instead of B 2 O 3 , and further treated as in the Example.
• the TiO 2 -C composites of Example 6 are prepared and impregnated with a water solution of B 2 O 3 .
• B 2 O 3 is hydrated to H 3 BO 3 in water and thus the two are interchangeable.
• the article is impregnated under 1.7-6.9 x 10 5 Pa of pressure, dried at about 100oC, heat treated @ 1200o-2000oC and the process repeated to build up the desired amount of B compound in the structure of the article.
• Heat drives the reaction of TiO 2 and H 3 BO 3 , forming TiB by the overall reaction: TiO 2 + 2 H 3 BO 3 + 5 C ⁇ TiB 2 + 3 H 2 O + 5 CO.
• the article may be re-impregnated and re-baked to produce the TiB 2 -carbon composite, but if aTiB 2 -graphite composite is the desired end product, the article is further heated to 2200oC or higher, which temperature will convert the amorphous C to semigraphite or graphite.
• a re-impregnation under alternate cycles of vacuum and pressure step with pitch or a dispersion of TiO 2 or boron compound or with a mixture of both of the reactants (TiO 2 and a boron cog ⁇ pound) dispersed in a liquid carbonizable binder or impregnant may be used to seal this remaining porosity and densify the article.
• The. preferred impregnant is a petroleum pitch having a melting point in the 100o-120oC range used at about 165°-250°C.
• B 4 C (10 g) is dispersed with calcined delayed petroleum coke particles (90 g) having a mean diameter of 3 mm in a sigma mixer and heated to about 170oC, coal tar pitch (25 g) with a softening point of 110oC is added, and melted, and a plastic dispersion is formed.
• a cathodic element is molded under about 1.4 x 10 7 Pa (2000 PSI), baked on a cycle with the temperature rising to 800oC in six days. After baking, the element is cooled, then impregnated with a dispersion of TiO in petroleum pitch (30% by wt.) at 240°C with 6.9 x 10 5 Pa (100 PSI).
• the impregnation step is repeated with alternate vacuum and pressure cycles. After impregnation, the element is heated to 720oC over a six day period, then cooled. The impregnation-bake cycle is repeated several times to build up the requiredTiO 2 concentration firmly bound in the carbon matrix in the pore volume of the element. After baking, the element is further heated to 1750oC, which converts the reactants to TiB 2 . The reaction produces CO as shown, and to seal porosity resulting from the loss of C from the matrix, the element is impregnated with petroleum pitch and baked as above to seal the porosity and strengthen the structure.
• the element may be re-impregnated with the TiO 2 dispersion, baked, and re-heated as above. After heating to 1750oC, to form TiB 2 , the element is further heated to 2250oC to convert the carbon matrix to graphite.
• the final cathodic element has TiB 2 concentrated primarily on or near the surface.
• the process disclosed uses the reactions forming TiB 2 from TiO 2 , and B 4 C, B 2 O 3 , or other boron compounds to form a TiB 2 -graphite composite.
• the process may also be used to form other such composite structures from reactants forming refractory materials.
• the reactions are as follows:
• the process is in general the generalized reaction taking place at temperatures in the range of 800o-3000oC of: MO + B 2 O 3 + C ⁇ MB + CO (where M is a metal) or MO + B 4 C + C ⁇ MB + CO or MO + N + C ⁇ MN + CO (where N is a non-metal)
• EXAMPLE 10 The article of Example 6, after baking, is impregnated with a dispersion of B 4 C in petroleum pitch with a softening point of 110o- 120oC, at 240oC under several cycles of vacuum and pressure of 6.9 x 10 5 Pa (100 PSI). After impregnation, the article is re-baked as above, then further heated to 1750oC to drive the TiB 2 -forming reaction to completion, re-impregnated with petroleum pitch and re-baked, then heated to 2250oC to form the graphite-TiB 2 composite.
• the process is useful for the formation of a large number of composite structures containing the end product of a reaction occurring at high temperatures in the presence of carbon, whether it enters the reaction or not.
• the reaction may occur with a number of boron compounds including borax and borates, however B 2 O 3 and H 3 BO 3 are the most economical and available compounds.

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• 1982
  • 1982-07-22 EP EP19820902609 patent/EP0084059A4/fr not_active Withdrawn
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