WO1983000347A1

WO1983000347A1 - COMPOSITE OF TiB2-GRAPHITE

Info

Publication number: WO1983000347A1
Application number: PCT/US1982/001003
Authority: WO
Inventors: Lakes Carbon Corporation Great; Louis Arpad Joo'; Kenneth Wayne Tucker; Scott David Webb; Leslie Harrisville Juel; Frank Edward Mccown, (Jr.)
Original assignee: Individual
Current assignee: Individual
Priority date: 1981-07-27
Filing date: 1982-07-22
Publication date: 1983-02-03
Anticipated expiration: 1984-01-27
Also published as: EP0084059A4; BR8207804A; EP0084059A1; JPS58501173A

Abstract

Des articles composites en graphite et diborure de titane sont produits en mélangeant du TiO2, du coke de pétrole et un liant pour former une dispersion plastique. Des articles sont façonnés par moulage ou extrusion et sont cuits pour carboniser le liant et former un composite TiO2-carbone cuit. L'article est imprégné avec un composé de bore fondu ou dispersé, ou avec un composé de bore mélangé avec du noir de charbon, puis chauffé pour produire une réaction de formation de TiB2. L'article est encore chauffé jusqu'à une température de graphitisation pour former un composite de graphite-TiB2 utile comme composant cathodique dans une cellule de Hall de réduction d'aluminium.Composite articles of graphite and titanium diboride are produced by mixing TiO2, petroleum coke and a binder to form a plastic dispersion. Articles are shaped by molding or extrusion and are baked to carbonize the binder and form a baked TiO2-carbon composite. The article is impregnated with a molten or dispersed boron compound, or with a boron compound mixed with carbon black and then heated to produce a TiB2 formation reaction. The article is further heated to a graphitization temperature to form a graphite-TiB2 composite useful as a cathode component in an aluminum reduction Hall cell.

Description

COMPOSITE OF TiB₂-GRAPHITE

BACKGROUND OF THE INVENTION

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.

Typically 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. During operation of the Hall cell, only about 25% of the electricity consumed is used for the acnual reduction of alumina to aluminum, with approximately 40% of the current consumed by the voltage drop caused by the resistance of the bath. 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 α₁₂, α₁₃, and α₂₃ are the surface free energies at the aluminum carbon, cryolite-carbon, and cryolite-aluminuιa boundaries, respectively.

If 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. Typically, amorphous carbon is a low energy surface, but also is quite durable, lasting for several years duration as a cathode, and relatively inexpensive. However, a cathode or a cathode component such as a TiB₂ stud which has better wettability and would permit closer anode-cathode spacing could improve the thermodynamic efficiency and be very cost-effective.

Several workers in the field have developed refractory high free energy material cathodes. U.S. 2,915,442, Lewis, December 1, 1959, claims a process for production of aluminum using a cathode consisting of the bcrides, carbides, and nitrides of Ti, Zr, V, Ta, Nb, and Hf. U.S. 3,028,324, Ransley, April 3, 1962, claims a method of producing aluminum using a mixture of TiC and TiB₂ as the cathode. U.S. 3,151,053, Lewis, September 29, 1964, claims a Hall cell cathode conducting element consisting of one of the carbides and borides of Ti, Zr, Ta and Nb. U.S. 3,156,639, Kibby, November 10, 1964, claims a cathode for a Hall cell with a cap of refractory hard metal and dis closes TiB₂ as the material of construction. U.S. 3,314,876, Ransley, April 18, 1967, discloses the use of TiB₂ for use in Hall cell electrodes. The raw materials must be of high purity particularly in regard to oxygen content, Col. 1, line 73-Col. 2, line 29; Col. 4, lines 39-50, Col. 8, lines 1-24. U.S. 3,400,061, Lewis, September 3, 1968 discloses a cathode comprising a refractory hard metal and carbon, which may be formed in a one-step reaction during calcination. U.S. 4,071,420, Foster, January 31, 1978, discloses a cell for the electrolysis of a metal component in a molten electrolyte using a cathode with refractory hard metal TiB₂ tubular elements protruding into the electrolyte. S.N. 043,242, Kaplan et al. (Def. Pub.), filed Xay 29, 1979, discloses Hall cell bottoms of TiB₂. Canada 922,384, March 6, 1973, discloses in situ formation of TiB₂ during manufacture of arc furnace electrodes. Belgian 882,992, PPG Ind., October 27, 1980, discloses TiB₂ cathode plates.

Our co-pending applications, S.N. 186,181 and S.N. 186,182, filed September 11, 1980, disclose related subject matter.

SUMMARY OF THE INVENTION

Titanium Diboride, TiB₂ 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 (TiB₂) 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.

The principal deterrent to the use of TiB as a Hall cell cathode or cathode component has been the sensitivity to thermal shock and the great material cost, approximately $25/lb., as compared to the traditional carbonaceous compositions, which cost about $0.60/lb. Also, if the anode-cathode distance could be lowered, the % sayings in electricity would be as follows:

A-C distance % savings

3.8 cm. std.

1.9 cm. 20%

1.3 cm. 27%

1.0 cm. 30%

We have invented an improved process for producing a TiB₂-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 (TiO₂) -graphite composite structure as a starting material. TiO₂ 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-TiO₂ composite.

The baked carbon-TiO₂ composite shape produced is a structure containing TiO₂ 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 10³ Pa (100 PSI) pressure with a boron compound alor.e or with a dispersion of B₂O₃ and carbon black or other micronized carbon in H₂O. Either B₂O₃ or H₃BO₃ may be used as B₂O₃ is hydrolyzec to H₃BO₃ in H₂O by the reaction:

B₂O₃ + 3 H₂O - 2 H₃BO₃ After impregnation with the dispersion, the article is then dried slowly to 100ºC to minimize loss of the solid impregnants while vaporizing the water. Multiple impregnations, each followed by a drying cycle, may be necessary.

Alternately, the article may be impregnated with molten B₂O₃ 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 TiO₂, carbon black and B₂O₃, heating the mixture to melt the B₂O₃, dispersing the TiO₂ and carbon in the molten B₂O₃, 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. 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 (B₄C) with coke particles and binder in the initial mix, baking, then impregnating the resulting baked piece with a TiO₂-carbon black dispersion in a carbonizable binder.

The unique aspect of the process provides a method whereby TiB₂ 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 TiO₂ and B₂O₃ to form TiB₂.

TiO₂ + 5 C + B₂O → TiB₂ + 5 CO ↑

> 1200°C Initial mixing, shaping by molding or extrusion of TiO₂, coke, and binder pitch follow the standard practice of the carbon and graphite industry. The article is heat treated at 600º to 1100ºC to carbonize the binder, cooled and is then impregnated as described in a heated pressure vessel at temperatures from 100º-500ºC and pressures from 2-15 x 10³ Pa (50-200 PSI).

After drying, the article is heated to the reaction temperature for the formation of TiB₂, 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 TiB₂. Due to the loss of the carbon black ana possibly a portion of the binder and coke as CO during the TiB₂ forming reaction, the article may develop excess porosity and consequently have low strength and be poor in other physical properties. This can be remedied by additional impregnation with a carbonizable binder, preferably a petroleum pitch with a softening point in the 110º-123 range, although lignin sulfoπate, phenolic resins and other pitches may be used, under about

7 x 10³ Pa (100 PSI) at about 200°-250ºC in a heated pressure vessel.

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. Generally the temperature range preferred is about 2400º- 2500ºC, although for particular processes any point in the 2000º- 3000º range may be used.

Example 1

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 TiO₂ in a high-shear heated mixer. The mix is heated to approximately 175 ºand the coke and TiO₂ are well dispersed in the molten pitch. The cathodic element is molded at about 14 x 10⁶

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 B₂O₃ in H₂O, then removed and dried slowly to vaporize the H₂O without loss of B₂O₃. The piece is next heated to 1500ºC, at which temperature the TiO₂ and B₂O₃ 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 theTiB₂ forming reaction, then graphitized by heating to 2400ºC.

Example 2

A mixture is prepared haying the following composition: B₂O₃ 38 wt. %

Carbon black 29 wt. % TiO₂ 33 wt. %

100 wt. % The solids above are mixed in a sigma type mixer, then heated to the melting point of B₂O₃ or slightly higher, which may vary considerably with the purity. Pure B₂O₃ has a melting point of 450ºC but the commercially available grades usually melt at around 275° or slightly higher. The carbon and TiO₂ are thoroughly dispersed in the molten B₂O₃, then the mixture is dumped and allowed to cool and harden. The solid is ground to a fine particle size dispersion passing a 24 mesh/cm screen (60 mesh/in), then used to form an electrode mix of the following composition:

Above mix 100 parts by wt. Coal tar pitch, S.P. 110°-120°C 32 parts by wt.

This is mixed in a sigma type mixer heated to about 175ºC, dispersing the carbon black and reactive mixture in the melted pitch. After partial cooling, an article is molded, cooled, then baked to 720° over a six day period, carbonizing the binder. It is then heated to about 2000ºC, driving the TiB₂ forming reaction to completion while graphitizing the carbon residue. The body thus formed is a very porous semi-graphite-TiB₂ composite. The composite is impregnated with petroleum pitch having a 110º-120º softening point under alternate cycles of vacuum and pressure at about 240ºC, and re-baked on the above slow carbonizing cycle to 720º. The impregnation and baking steps are repeated, then the article is re-graphitized at 2300ºC, to form a strong graphite-TiB₂ composite with about 60% TiB₂ content by wt.

Example 3

A conventional carbon body, which has a high pore volume and is well suited for impregnation, is made from the following composition, by wt. :

Calcined coke particles, maximum particle size 60

12 mm, mean particle size 5 to 10 mm Coke flour, 50% passing 79 mesh/cm screen 40 Coal tar pitch, softening point 110°-120º 25

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 B₂O₃ 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

A cathode shape is formed from pitch, coke, and TiO₂ and baked as in Example 1. It is then impregnated with a dispersion of carbon black in molten B₂O₃ at 7 x 10³ Pa (100 PSI). After impregnation, it is heated to 1500 for one hour to form TiB₂, then cooled, impregnated with petroleum pitch under cycles of vacuum and 7 x 10³ 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:

% by wt.

B₄C 8

Coke particles (3 mm diam.) 36

Carbon black 36 Pitch (S.P. 110°). 20

100

The B₄C, coke, and carbon black are mixed in a heated sigma mixer at about 170ºC, the pitch added and the mixture wetted by the molten pitch to form a plastic dispersion. A cathodic element for a Hall cell is formed by molding the dispersion trader about 1.4 x 10⁷ 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% TiO₂ 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 10⁵ Pa (100 PSI) pressure. The impregnation and bake steps are repeated to fully impregnate the element. It is next heated slowly to about 1750ºC, at which temperature TiB₂ is formed and CO given off, and held at that temperature until the reaction is complete. To further strengthen the element and increase its density, it is re-impregnated with petroleum pitch, rebaked, then heated to about 2400ºC to convert the carbon matrix and particulate matter to a graphitic form. EXAMPLE 6

Blends of the following dry ingredients are mixed in parts by wt. :

A B C D TiO₂ 10 50 80 60

Regular Petroleum coke particles (calcined) (mean diam. 3 mm) 90 50 50 40 Coal tar pitch

(S.P. 110°-120°C) 26 28 38 20

Theoretical % TiB₂ 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 TiO₂ to TiB₂.

TheTiO₂ 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 moltenB₂O₃ , under 6.9 x 10⁵ Pa pressure to a final pickup of sufficient boron-containing material to form the surface layer of TiB on further heat treatment.

On further heating the reaction B₂O₃ + TiO₂ + 5 C → TiB₂ + 5 CO starts to take place at about 800°C, becomes quite apparent at about 1200°C, and reaches a high reaction rate around 1750ºC. Impregnation can be repeated with re-baking to build the desired quantity of TiB₂ in the composite. The article can be heated to 2200ºC or higher to graphitize the carbon, forming the final composite article of graphite-TiB₂, with the surface particularly rich in TiB₂. EXAMPLE 7

The TiO₂-C composites of Example 6 are prepared and impregnated with molten H₃BO₃ instead of B₂O₃, and further treated as in the Example.

EXAMPLE 8

The TiO₂-C composites of Example 6 are prepared and impregnated with a water solution of B₂O₃. B₂O₃ is hydrated to H₃BO₃ in water and thus the two are interchangeable. The article is impregnated under 1.7-6.9 x 10⁵ 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 TiO₂ and H₃BO₃, forming TiB by the overall reaction: TiO₂ + 2 H₃BO₃ + 5 C → TiB₂ + 3 H₂O + 5 CO. The article may be re-impregnated and re-baked to produce the TiB₂-carbon composite, but if aTiB₂ -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.

After heating to 1200ºC or higher, at which temperature TiB₂ begins to form, some porosity will be present at the surface due to the loss of CO or CO₂ formed by the overall reactions involved: TiO₂ + 2 H₃BO₃ + 5 C → TiB₂ + 3 H₂O + 5 CO 2 C + 2HBO₃ → B₂O₃ + H₂O + 2 CO TiO₂ + B₂O₃ + 5 C → TiB₂ + 5 CO 2 TiO₂ + Na₂B₄O₇ ^. 10 H₂O + 10 C → 2 TiB₂ + Na₂O + 10H₂O + 10 CO.

A re-impregnation under alternate cycles of vacuum and pressure step with pitch or a dispersion of TiO₂ or boron compound or with a mixture of both of the reactants (TiO₂ and a boron cogιpound) 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. After impregnation, the article is baked to

700º-1100ºC, and is re-heated to 2200°c or higher to graphitize the carbon residue, and form TiB₂. EXAMPLE 9

B₄C (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 10⁷ 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 10⁵ 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 requiredTiO₂ 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 TiB₂. 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. Alternately, the element may be re-impregnated with the TiO₂ dispersion, baked, and re-heated as above. After heating to 1750ºC, to form TiB₂, the element is further heated to 2250ºC to convert the carbon matrix to graphite. The final cathodic element has TiB₂ concentrated primarily on or near the surface.

The process disclosed uses the reactions forming TiB₂ from TiO₂, and B₄C, B₂O₃, or other boron compounds to form a TiB₂-graphite composite. The process may also be used to form other such composite structures from reactants forming refractory materials. In this instance the reactions are as follows:

TiO₂ +B₂O₃ + 5 C → TiB₂ + 5 CO. The reaction above probably proceeds through the formation of

B₄C as an intermediate

2 B₂O₃ + 7 C → B₄C + 6 CO

2 TiO₂ + B₄C + 3 C → 2 TiB₂ + 4 CO.

The process is in general the generalized reaction taking place at temperatures in the range of 800º-3000ºC of: MO + B₂O₃ + C → MB + CO (where M is a metal) or MO + B₄C + 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 B₄C in petroleum pitch with a softening point of 110º- 120ºC, at 240ºC under several cycles of vacuum and pressure of 6.9 x 10⁵ Pa (100 PSI). After impregnation, the article is re-baked as above, then further heated to 1750ºC to drive the TiB₂-forming reaction to completion, re-impregnated with petroleum pitch and re-baked, then heated to 2250ºC to form the graphite-TiB₂ composite.

As may be seen, from the above, 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.

We have found that the use of the approximate stoichiometric equivalents is preferable, e.g., TiO₂ (80 g) + B₂O₃ (70 g) + C (excess) → TiB₂ (70 g) + 5 CO ↑. The reaction Ti + 2 B → TiB₂ will also occur under these conditions, but is economically unattractive due to the high cost of the elemental reactants. The reaction with borax occurs but is unattractive due to the volume of volatiles produced.

The reaction may occur with a number of boron compounds including borax and borates, however B₂O₃ and H₃BO₃ are the most economical and available compounds.

Claims

ClaimsWe claim:

1. A process for manufacturing a carbon-TiB₂ composite article comprising blending coke, a first carbonizable binder and a first TiB₂ forming reactant to form a dispersion, shaping said article, baking said article to carbonize said binder, impregnating said article under alternate cycles of vacuum and pressure at least once with a second TiB₂-forming reactant in liquid form, and heating said article to aTiB₂ -forming temperature to form said carbon-TiB₂ composite article.

2. The process of Claims 1, 22, or 23 wherein the coke is a regular calcined petroleum coke having a mean particle diameter of approximately 3 mm.

3. The process of Claims 1, 22, or 23 wherein the binder is a molten coal tar pitch having a softening point from 100º to 120ºC, used at a temperature of approximately 160º to 175º.

4. The process of Claim 1 wherein the first TiB₂-forming reactant is TiO₂ and the second TiB₂-forming reactant comprises a boron compound selected from the group consisting of B₄C, B₂O₃ , H₃BO₃ and Na₂B₄0₇ ^.10H₂O.

5. The process of Claim 1 wherein the second TiB₂-forming reactant is a boron compound selected from the group consisting of

B₄C and B₂O₃, dispersed in a liquid selected from the group consisting of molten petroleum pitch and phenolic condensates.

6. The process of Claim 1 wherein the second TiB₂-forming reactant is molten B₂O₃, used at about 500ºC.

7. The process of Claim 1 wherein the second TiB₂-forming reactant is B₂O₃ , HBO₃, or Na₂B₄O₇ ^.10H₂O in water solution.

The process of Claims 1 or 15 wherein the article is baked on a temnppeerraature cycle rising to 700º to 1100°C over a period of 1 to 10 days.

9. The process of Claims 1 or 15 wherein the article is impregnated with the second TiB₂-forming reactant under alternate cycles of vacuum and a pressure of from 1.7 x 10⁵ to 6.9 x 10⁵ Pa (25 to 100 psi).

10. The process of Claims 1 or 15 wherein the article after impregnation is baked on a cycle with the temperature rising to 700º to 1100ºC over a period of 1 to 10 days, then further heated to a TiB₂- forming temperature of at least 1200ºC.

11. The process of Claims 1 or 15 wherein after heating to the TiB₂-forming temperature the article is further heated to a temperature of at least 2200°C.

12. The process of Claims 1 or 15 wherein the article after heating to the TiB₂-forming temperature is cooled, then re-impregnated at least once under alternate cycles of vacuum and pressure with the second TiB₂-forming reactant in liquid form, re-baked on a cycle rising to 700° to 1100ºC over a period of 1 to 10 days, and re-heated to the TiB₂-forming temperature of at least 1200ºC.

13. The process cf Claims 1, 15, 30, or 35 wherein the article after heating to the TiB₂-forming temperature is impregnated with petroleum pitch under alternate cycles of yacuum and pressure at about

240ºC and 6.9 x 10⁵ Pa, re-baked on a cycle rising to 700º to 1100ºC5 over a period of 1 to 10 days, ana heated to at least 2200ºC.

14. The process of Claims 1 or 15 wherein the article is re- impregnated with a dispersion of both the first and second TiB₂-forming reactants in petroleum pitch under alternate cycles of vacuum and pressure of 1.7 to 6.9 x 10⁵ Pa at about 240°C, baked on a cycle rising to 700º to 1100ºC over a period of 1 to 10 days, heated to at least 2200ºC to form a TiB₂-graphite composite.

15. The process of Claim 1 wherein the first TiB₂-forming reactant is a boron compound and the second TiB₂-forming reactant is TiO₂.

16. The process of Claims 1 or 15 wherein the article is shaped by molding at a pressure of about 1.4 x 10⁷ Pa.

17. The process of Claims 1 or 15 wherein the article is shaped by extrusion.

18. A process of manufacturing a cathodic element for a Hall aluminum cell comprising dispersing a firstTiB₂-forming reactant selected from the grout consisting of B₄C, H₃BO₃, and B₂O₃ and coke particles in molten coal tar pitch binder to form a plastic mass, shaping said mass to form said cathodic element, baking said element on a rising temperature cycle reaching 700º to 1100ºC over a period of 1 to 10 days, removing said element from said furnace, impregnating said element under a pressure of about 6.9 x 10⁵ Pa with TiO₂ dispersed in molten petroleum pitch, re-baking said element to 700º to 1100ºC over a period of 1 to 10 days, heating said element to 1750ºC to form a carbon- TiB₂ composite, re-inpregnating said element with said petroleum pitch at about 240°C under about 6.9 x 10⁵ Pa pressure, re-baking said element to 700° to 1100ºC over 1 to 10 days, and heating said element to 2250ºC in an argon atmosphere to form a graphite-TiB₂ composite cathodic element.

19. A process of manufacturing a cathodic TiB₂-graphite element for a Hall aluminum cell comprising dispersing TiO₂ and coke particles in molten coal tar pitch to form a plastic mass, forming said element by molding or extrusion, baking said element on a rising temperature cycle over a period of 1 to 10 days, the temperature reaching from

700º to 1100ºC, cooling said element, impregnating said element under alternate cycles of vacuum and a pressure of from 1.7 to 6.9 x 10⁵ Pa with a boron compound selected from the group consisting of molten B₂O₃, at approximately 500ºC, molten H₃BO₃ at approximately 500ºC, B₂O₃ in water solution, H₃BO₃ in water solution, and B₄C dispersed in molten petroleum pitch, drying said element at a temperature of approximately 100ºC if a water solution was used when impregnating said element, re-baking said element on a 1 to 10 day cycle, rising to 700º to 1100ºC, heating said element to approximately 1750ºC, cooling, and re-impregnating said element with said molten petroleum pitch, baking said element over a 1 to 10 day cycle, rising to 700º to 1100ºC, and heating to approximately 2250ºC to form said TiB₂-graphite element.

20. A product made by the process of Claims 1, 15, 18 or 19.

21. A method of operating an electrolytic aluminum reduction cell using as a cathode component the product made by the process of

Claims 1, 15, 18 or 19.

22. In a process for the production of a TiB₂-graphite composite article by an in situ reaction, of carbon, a boron compound and TiO₂, the improvement comprising the use. of carbon black as a source of carbon for said in situ reaction.

23. A process for the production of a composite TiB₂-graphitic carbon article comprising the steps of:

1) Mixing TiO₂, coke, and a liquid carbonizable binder to form a first plastic dispersion; 2) Shaping said dispersion to form an article;

3) Baking said article to carbonize said binder on a cycle from 1 to 10 days rising to a final temperature from 600º to 1100°C; 4) Dispersing carbon black in liquid B₂O₃ to form a second dispersion; 5) Impregnating said baked article with said second dispersion under a pressure from

2 to 15 x 10³ Pa at a temperature from

100º to 500ºC;

6) Heating the impregnated, baked article to at least 1200ºC to a Tι3 forming temperature and maintaining that temperature until the TiB₂ forming reaction is substantially completed;

7) Heating the article from 6) to at least

2100ºC to form saidTiB₂-graphitic carbon comoosite article.

24. The process of Claims 22, 23, 30, 32, or 35 wherein the carbon used is a rubber reinforcing grade of carbon black.

25. The process of Claims 23 or 35 wherein after the article is heated to the TiB₂ forming temperature, it is cooled, then impregnated under from 2 to 15 x 10³ Pa pressure at from 200º to 250ºC witn a petroleum pitch having a softening point from 110º to 120ºC, tnen naked on a cycle with the temperature rising to 600º to 1100ºC over 2 to 10 days before graphitization at 2300ºC .

26. The process of Claims 23 or 35 wherein the article is impregnated with the second dispersion and baked a plurality of times, dried, then heated to a TiB₂ forming temperature.

27. The process of Claim 23 wherein the second dispersion is a dispersion of carbon black in molten B₂O₃.

28. The process of Claim 23 wherein the second dispersion is a dispersion of carbon black and B₂O₃ in H₂O, the article then being heated to about 100ºC to vaporize the H₂O.

29. The process of Claim 26 wherein after heating to a boride forming temperature the article is re-impregnated with the second dispersion under alternate cycles of vacuum and pressure of from 2 to 15 x 10³ Pa at 200º to 250ºC, rebaked on a cycle of 2 to 10 days, the temperature rising to 700º to 1100ºC, then graphitized at a temperature of 2300°C.

30. A process for the production of a composite TiB₂-graphite article comprising the steps of:

1) Mixing B₂O₃, particulate carbon, and TiO₂ to form a mixture; 2) Heating said mixture to the melting point of said B₂O₃;

3) Dispersing said carbon and said TiO₂ in said B₂O₃ to form a plastic mixture;

4) Dumping said mixture, allowing it to harden to a solid;

5) Grinding said solid to particles passing a 24 mesh/cm (60 mesh/in) screen;

6) Mixing said particles with solid coal tar pitch, having a softening point of about 110° to 120°C;

7) Heating the mixture so formed to about 175ºC to melt said pitch;

8) Dispersing said particles in said molten pitch to form a dispersion; 9) Cooling said dispersion;

10) Molding an article from said dispersion;

11) Baking said article up to a temperature of about 700º to 1100ºC oyer a period of one ten days;

12) Heating said article to at least 2000ºC; and

13) Cooling said article.

31. The process of Claim 30 wherein 38 parts B₂O₃, 29 parts carbon black, and 33 parts TiO₂ by wt. are mixed in step 1.

32. The process of Claim 30 wherein 100 parts by wt. of the particles formed in step 5 are mixed with 32 parts by wt. of coal tar pitch.

33. The process of Claim 30 wherein the particulate carbon com prises a mixture of calcined petroleum coke particles and carbon black.

34. A process for the production of a composite TiB₂-graphite comprising forming a porous carbon article from relatively coarse coke particles having a maximum diameter of about 12 mm, coke flour, and coal tar pitch by melting said pitch and dispersing said particles and said flour in said pitch, shaping an article from the first dispersion thus formed, baking said article on a carbonizing cycle to carbonize said pitch, dispersing carbon black andTiO₂ in a liquid boron compound, to form a second dispersion, impregnating said article with said second dispersion, heating said article to a TiB₂ forming temperature above 1200ºC, cooling said article, impregnating said article with petroleum pitch, re-baking said article to 700º to 1100ºC over a period of one to ten days, and heating said article to at least 2100ºC to form aTiB₂ -graphitic carbon composite article.

35. A process for the production of a composite TiB₂-graphitic carbon article comprising the steps of:

1) Mixing a boron compound, coke, carbon, and a first liquid carbonizable binder to form a first plastic dispersion;

2) Shaping said dispersion to form said article;

3) Baking said article to carbonize said binder on a cycle of from 1 to 10 days rising to a final temperature from 600º to 1100°C;

4) Cooling said article to ambient temperature;

5) Dispersing TiO₂ in a second liquid car bonizable binder to form a second dispersion;

6) Impregnating said article with said second dispersion under alternate cycles of vacuum and a pressure of frcm 2 to 15 x 10³ Pa at a temperature from 100º to 500ºC;

7) Heating said article to at least 1200ºC to a TiB₂ forming temperature and maintaining that temperature until the TiB₂ forming reaction is substantially com pleted; and

8) Further heating said article to at least 2100ºC to form said TiB₂ -graphitic carbon composite.

36. The process or Claim 35 wherein the boron compound is B₄C.