Classifications

• • C—CHEMISTRY; METALLURGY
  • C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
  • C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
  • 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
• • C—CHEMISTRY; METALLURGY
  • C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
  • C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
  • 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
  • C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
  • 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
  • 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 a rammed coke-pitch mixture, and the anode a block suspended in the bath at an anode-carhode 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 anode-cathode spacing is usually ahout 4-5 cm., and attempts to lower this distance result in an electrical discharge from the cathode to the anode through aluminum droplets.
• 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 aluminum 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 sur faces, 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-aluminum boundaries, respective
• the cathode were a high energy surface, such as would occur if it were a ceramic instead of carbon, it would have a higher contac angle and better wettability with the liquid aluminum. This in turn would tend to smooth out the surface of the li ⁇ uid 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 TiB 2 stud as a component of the cathode which has better wettability and would permit closer anode-cathode spacing could improve the th ⁇ rmodynamic efficienc and be very cost-effective.
• Titanium Diboride, TiB 2 has been proposed for use as a cathode or cath ⁇ dic element or component. in Hall cells for the reduction of alumina, giving an improved performance over the amorphous carbon and semi-graphite cathodes presently used.
• Titanium Diboride (TiB 2 ) was useful as a cathode 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 anode-cathode spacing; and the corrosion resistance was improved, probably due to increased hardness, and lower solubility as compared to the carbon and graphite forms.
• TiB 2 precursors i.e., pigment grade titanium dioxide (TiO 2 ) and boron oxide (B 2 O 3 ) , or boron carbide (B 4 C) which are preferably added dry to the coke filler prior to addition of binder pitch.
• the normal method of production of monolithic carbonaceous pieces, either amorphous or graphitic carbon involves a dry blend of several different particle sizes of coke and/or anthracite fillers and coke flour (50%-200 mesh) (79 mesh/cm), followed by a dispersion of these solid particulates in melted pitch to form a plastic mass which is then molded or extruded, then baked on a gradually rising temperature cycle to approximately 700°-1100°C.
• the bake process drives off the low boiling molecular species present, then polymerizes and carbonizes the pitch residue to form a solid binder-coke composite. If the material is to be graphitized, it is further heated to a temperature between 2000oC and 3000oC in a graphitizing furnace.
• a non-acicular or regular petroleum coke or calcined anthracite may be used to avoid a mismatch of the Coefficient of Thermal Expansion (CTE) of the TiB 2 - coke mixture, or a needle coke may be used to form an anisotropic body.
• CTE Coefficient of Thermal Expansion
• the raw materials react in situ at temperatures above 1700oC to form a carbon-TiB 2 composite according to the following reactions: TiO 2 + B 2 O 3 + 5 C ⁇ - TiB 2 + 5 CO 2 TiO 2 + B 4 C + 3 C ⁇ - 2 TiB 2 + 4 CO.
• B C may be formed as an intermediate step in the above.
• TiB 2 -C composite in which the TiB 2 is of finer particle size and is better dispersed throughout the structure and is made at a much lower cost than by the addition of pure TiB 2 to the dry blend of coke particles and coke flour. It has been found easier to form TiB 2 in situ in graphite than to sinter TiB 2 powder into articles.
• the composite articles produced in this manner have greatly improved thermal shock resistance as compared to pure TiB 2 articles, and greatly improved resistance to intercalation and corrosion by the molten salt bath as compared to carbon articles.
• TiO 2 , B 2 O 3 or B 4 C may be used in place of TiO 2 , B 2 O 3 or B 4 C, such as elemental Ti and B, or other Ti or B compounds or minerals. We prefer these compounds for their ready availability and low price, however, others may be more suitable, based on special conditions or changes in supply and price.
• the impregnate the articles When manufacturing articles in this manner, it is preferred to impregnate the articles with a pitch and re-bake after the initial bake cycle. Alternately, the impregnation can be accomplished after heat treatment to 1700°-3000°C. Multiple impregnations may be advantageous. In this instance the reactions consume carbon from the coke and binder to form CO or CO 2 , which escape, leaving the article highly porous, it is advantageous to impregnate one or more times and re-bake the article before or after heating at the high temperature cycle to densify, strengthen and decrease porosity.
• the article is an electrode or component for a Hall cell, it may not be necessary to re-heat it to the 1700°-3000°C range, after the final impregnation, but rather to the 700o-1100oC range. If the article is to be used for an appli cation requiring heat resistance or other properties of graphite, it is necessary to reheat it to a high temperature of 2000o-3000oC to graphitize the coke remaining after this last impregnation.
• Preferred binders axe coal tar and petroleum pitches, although other binders such as phenolic, furan and other thermosetting resins, and organic and natural polymers may also be used.
• the principal requirements are an ability to wet the dry ingredients and have a carbon residue on baking to 700°-1100°C.
• a series of billets doped during mixing with TiB 2 precursors at 10 parts to 100 parts mix was molded and processed by heat treapments to 2400oC and 2700oC. After extensive analyses by X-ray diffraction (XRD) and X-ray fluorescence (XRF) , it was determined that a significant portion of TiB 2 , was formed from TiO 2 /B 2 O 3 and TiO 2 /B 4 C additives.
• XRD X-ray diffraction
• XRF X-ray fluorescence
• the mix used above was a mixture of acicular coke particles and coke flour, bonded with about 25 parts per hundred 110oC softening point coal tar pitch.
• Binders may be coal tar or petroleum pitches, with coal tar pitches preferred for their superior yield of carbon on coking.
• the articles are formed by molding or extrusion.
• Cathode blocks for Hall cells are molded or extruded, however, tubular or cylindrical inserts for cathodes are most economically produced as extrusions.
• Baking temperatures commonly reach from about 700o to 1100oC, with the practice normally followed in the examples below using a six day cycle, reaching a final temperature on a slowly rising curve typical of those normally followed in the electrode industry.
• the acicular needle cokes when heated to the graphitizapion temperatures of 2000o-3000oC, will form anisotropic graphite with coefficients of thermal expansion differing in at least two of the three geometric axes. Regular cokes will form isotropic graphite.
• graphitization of the carbon and reaction of the TiB 2 precursors can occur simultaneously during graphitization, forming an intimately dispersed, well bonded, homogenous composite.
• Example 1 The following compositions were produced as modifications of a standard carbon electrode mix.
• compositions above were made by premilling and blending the TiB 2 or the reactants with the coke particles and coke flour in a heated mixer, then the pitch was added, melted and the blend mixed while hot. A larger amount of pitch was added in C and D above to compensate for the increased surface area and binder demand of these blends.
• the pieces were molded using a pressure of 2000 psi (140.6 kg/cm 2 ) on a 3 3/4 in. (9.5 cm) diameter molding, baked to about 700oC, then trans ferred to a graphitizing furnace, and heated to 2400o or 2700oC.
• Example 2 The following compositions were made with higher concentrations of TiB 2 and precursors than in Example 1. The additives were incorporated at 100 pph level in the heated coke mix before the addition of binder. The formulations were mixed in a heated sigma mixer, molded at 2000 psi (140.6 kg/cm ) for 5 minutes at 113°-116oC, and baked to about 700oC, in a six day cycle, with results as follows:
• Example 3 Moldings were made using coke flour and TiB 2 at various percentages with results as follows, after mixing, molding and baking as in Example 1.
• Example 4 Pieces were formed by extrusion of mixtures made according to the procedure of Example 1, with the following compositions and results.
• Example 5 Moldings were made as in Example 1 with the following compositions:

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• 1980
  • 1980-09-11 US US06/186,181 patent/US4376029A/en not_active Expired - Lifetime
• 1981
  • 1981-09-11 JP JP56503168A patent/JPS57501377A/ja active Pending
  • 1981-09-11 BR BR8108789A patent/BR8108789A/pt unknown
  • 1981-09-11 AU AU76422/81A patent/AU544405B2/en not_active Expired - Fee Related
  • 1981-09-11 EP EP19810902691 patent/EP0059750A4/en not_active Withdrawn
  • 1981-09-11 WO PCT/US1981/001215 patent/WO1982001018A1/en not_active Ceased
• 1982
  • 1982-05-10 NO NO821537A patent/NO821537L/no unknown

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