Description

COMPOSITE OF TiB -GRAPHITE

BACKGROUND OF THE INVENTION

Aluminum metal has been produced for 90 years in the Hall cell by electrolysis of alum na in a molten cryolite salt electrolyte bath operating at temperatures in the range of 900 -1000 C. The reactiv¬ ity of the molten cryolite, the need for excellent electrical conduc¬ tivity, 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 petroleu ^***** . 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-i -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 elec¬ tricity consumed is used for the acnual reduction of alumina to alu¬ minum, 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 en. , and attempts to lower this distance result i _n electrical discharge from the cathode ^* εo the anode through

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alu inum droplets suspended in the baih.

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 alu -num from the strong electro¬ magnetic 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 con¬ sequently is not readily wet by the liquid aluminum. The angle of a droplet of aluminum at the cryolite-aluaim_n-carbon junction is governed by the relationship cos S ^** = -12- ^:: - ^≥ *i3- where < ^" *-,-.» ^a -, -- r -a d α are the surface free energies at the aluminum carbon, cryolite-carbon, and cryoli ^* -:e-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 -co smooth out the surface of the liquid aluminum pool and lessen the possibility of in ^* cεrelec ^* roάe discharge allowing the anode-cathode distance to be lowered ar-d the thermodynamic efficiency of the cell improved, by decreasing ^* che 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 ca-chode or a cathode component such as a TiB stud which has better v≤-ctabiiity and would permit closer anode-cathode spacing could improve ^* he -hermodynamic 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 = ^■• " ^• -'-.-"- ^* using a cathode consisting of the bcrides, carbides, and nitrides of Ti, Zr, V, Ta, Nb, and Hf. U.S. 3 _Λ 028,324, Ransley, April 3, 1962, claims a method of producing aluminum using a mixture of iC and Ti-3„ as the cathode. U.S.

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3,151,053, Lewis, September 29, 1964, claims a Hall cell cathode con¬ ducting 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- 5 closes TiB as the material of construction. U.S. 3,314,876, Ransley, April 18, 1967, discloses the use of TiB for use in Hall cell elec¬ trodes. The raw materials must be of high purity particularly in re¬ gard 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 10 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 TiB tubular elements protruding into the electrolyte. S.N. 15 043,242, Kaplan et al. (Def. Pub.} _. , filed ay 29, 1979, discloses Hall cell bottoms of TiB . Canada 922,384, March 6, 1973, discloses in situ formation of TiB during manufacture of arc furnace electrodes. Belgian 882,992, PPG nd. , October 27, 1980, discloses TiB cathode plates.

Our co-pending applications, S.N. 186,181 and S.N. 186,182, filed 20 September 11, 1980, disclose related subject matter.

SUMMARY OF THE INVENTION

Titanium Diboride, TiB--, 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-

25 it had previously been known that Titanium Diboride (TiB ) was useful as a cathode component in the electrolytic production of alu¬ minum, 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-

30 cathode 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

35 shock and the great material cost, approximately $25/lb. , as

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compared to the traditional carbonaceous compositions, which cost about $Q.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 TiB -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 titani (TiO )-graphite composite structure as a starting material. TiO 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_ _ c o about 500 -510 C 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 120Q -130Q C 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 (CTΞ)_ characteristic of from 1Q to 30 x 10 cm/cm/ C over the range of 0 to 50 C, relatively uniform on all 3 geometric axes in physical properties, while an acicular or needle coke will generally be anisotropic having a CTΞ characteristic which is variant on the axes and less than 10 x 10 cm/cm/ C 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 ob¬ tained 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 700 -1100 C oyer a period of 1 to 10 days to carbonize the binder, forming a solid C-TiO composite.

The baked carbon-TiO composite shape produced is a structure con¬ taining T 0 ₂ and particulate carbon firmly bound in the matrix of car¬ bonized 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

3 under alternate cycles of vacuum and about 7 x 10 Pa (100 PSI) pressure with a boron compound alor.e or with a dispersion of B O and carbon black or other micronized carbon in H O. Either B„0 or H BO may be used as B O is hydrolyzec to H BO- in Ξ 0 by the reaction: After impregnation with the dispersion, the article is then dried slowly to 100 C to minimize loss of the solid impregnants while vapor- izing the water. -Multiple impregnations, each followed by a drying cycle, may be necessary.

Alternately, the article may be impregnated with molten B O 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 , carbon black and B_0_, heating the mix¬ ture to melt the B-Q-, _/ dispersing the Ti0 and carbon in the molten B 0 , 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 i-mpregr.εr-t. The boron compound ana carbon black may be dispersed in a molten pitch or other carbonizable binder such as a petroleum pitch with a 110 - 120°C softening point, and the resulting dispersion used as an impregnant. Each impregnating cycle will nomally require a bake to the 700 -1100 C

range, carbonizing the binder.

The process may also be used by mixing boron carbide (B.C) with coke particles and binder in the initial mix, baking, then impregnating the resulting baked piece with a TiO -carbon black dispersion in a carbonizable binder.

The unique aspect of the process provides a method whereby TiB is formed during subsequent heat treatment to a temperature above 2000 C, 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 and B O to form TiB . i0 ₂ + 5 C + B ₂ 0 →- TiB + 5 CO i

>_ 1200°C Initial mixing, shaping by molding or extrusion of TiO , coke, and binder pitch follow the standard practice of the carbon and graphite industry. The article is heat treated at 600 to 1100 C to carbonize the binder, cooled and is then impregnated as described in a heated o o pressure vessel at temperatures from 100 -500 C and pressures from 2-15 x 10 ³ Pa (50-200 PSI) .

After drying, the article is heated to the reaction temperature for the formation of TiB , in the range of 1200 -1800 C. The reaction starts to take place at about 800 C but is quite slow below 1200 C and reaches a high reaction rate at about 1750 C. The heat treatment may be done in stages, with, re-impregnation and reheating cycles to build up the desired concentration of Ti-- . Due to the loss of the carbon black ana possibly a portion of the binder and coke as CO during the TiB 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 addi¬ tional impregnation with a carbonizable binder, preferably a etroleum o pitch with a softening point in the 110 -123 range, although lignin sulfoπate, phenolic resins and other pitches may be used, under about

7 x 10 Pa (100 PSI) at about 200°-250 C in a heated pressure vessel. o o

After impregnation, the article is again heated to the 600 to 1100 C 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

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O article to 2000 C or higher, converting the carbon to the graphitic form. Generally the temperature range preferred is about 2400 - 2500 C, although for particular processes any point in the 2000 - 3000 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 approxi¬ mately 110 -120 C and TiO„ in a high-shear heated mixer. The mix is heated to approximately 175 and the coke and TiO_ are well dispersed in the molten pitch. The cathodic element is molded at about 14 x 10

Pa (2000 PSI) pressure, then baked on a cycle rising to 720 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 0 in H O, then removed and dried slowly to vaporize the H O without loss of B 0_ . The piece is next heated to 1500 C, at which temperature the TiO and B O, 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-impregna ed, using a petroleum pitch with a o o softening point of from 110 -12Q C, baked to carbonize the pitch on a o six day cycle, the temperature rising to 720 C, which fills the porosity left by the TiB fo-ting reaction, then graphitized by heating to 2400 C.

Example 2

A mixture is prepared haying the following composition: B ₂ 0 ₃ 38 wt. %

Carbon black 29 wt. % TiO 33 wt. %

100 wt. % The solids above are mixed in a sigma type mixer, then heated to the melting point of 3 0 or slightly higher, which may vary considerably with the purity. Pure B O has a melting point of 450 C but the com¬ mercially available grades usually melt at around 275° or slightly higher. The carbon and TiO are thoroughly dispersed in the molten B O , 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 sig a type mixer heated to about 175 C, dis¬ persing 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 0 about 2000 C, driving the TiB forming reaction to completion while graphitizing the carbon residue. The body thus formed is a very porous semi-graphite-TiB composite. The composite is impregnated with petro¬ leum pitch having a 110 -120 softening point under alternate cycles of vacuum and pressure at about 240 C, and re-baked on the above slow 5 carbonizing cycle to 720 . The impregnation and baking steps are re¬ peated, then the article is re-graphitized at 2300 C, to form a strong graphite-Ti3 composite with about 60% TiB content by wt.

Example 3

A conventional carbon body, which has a high pore volume and is 0 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 5 Coal tar pitch, softening point 11Q°-120 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 B O with the mix prepared in Example 2, heated slowly to a TiB forming temperature above 12CQ C, preferably 1750 , held at that Q temperature for one to four hours, cooled and impregnated with the same petroleum pitch under alternate cycles of vacuum and pressure as above, o re-baked as above, and heat treated to a temperature of 2100 or higher.

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Example 4

A cathode shape is formed from pitch, coke, and TiO„ and baked as in Example 1. It is then impregnated with a dispersion of carbon black in molten B O at 7 x 10 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 Pa at 250°C, re-baked for six days, the temperature reaching 720 C, then graphitized by heating to 2300°C.

Example 5 A mixture is prepared having the following composition:

% by wt.

B ₄ C 8

Coke particles (3 mm diam.) 36

Carbon black 36 Pitch (S.P. 110°). 20

100

The B C, coke, and carbon black are mixed in a heate-d sig a mixer at about 170 C, 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 10 Pa (2000 PSI) and baked on a cycle rising to about 800 C in six days. After cooling to ambient temperature, the element is impregnated with a dispersion of 30% TiO by wt. (ceramic pigment gra e) in petroleum pitch (S.P. 110 - 120°C) at 240°C under alternate cycles of vacuum and 6.9 x 10 Pa (100 PSI) pressure. The impregnation and bake steps are repeated to fully impregnate the element. It is next heated slowly to about 1750 C, at which temperature TiB 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 2400 C 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 TiO 10 50 80 60

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

(S.P. 110°-120°C) 26 28 38 20

Theoretical % TiB_ in composite ^{1 2} 8% 57% 79% 77%

¹ Ass ^* uming a 75-80% coke yield from the pitch during the bake cycle from ambient to 700 -1100 C. Assuming complete conversion of TiO to TiB .

The TiO and coke are charged into a sig a type mixer heated to about 160°-175°C and thoroughly blended while being heated. When the dry blend has reached about 160 C, 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, reach ^¬ ing 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 molten ^ ₂ °3 ^{r under 6} - ⁹ - -^ ^{10 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 B O ÷ i0 ₂ + 5 C - ^* - ^* - TiB ₂ + 5 CO starts to take place at about S00°C, becomes quite apparent at about 1200°C, and reaches a high reaction rate around 1750 C. Impregnation can be repeated with re-baking to build the desired quantity of Ti3 ₂ in the composite. The article can be heated to 2200 c or higher to graph- itize the carbon, forming the final composite article of graphite-TiB ₂ , with the surface particularly rich in iB ₂ .

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EXAMPLE 7

The TiO -C composites of Example 6 are prepared and impregnated with molten H BO instead of B O , and further treated as in the Example.

EXAMPLE 8

The TiO -C composites of Example 6 are prepared and impregnated with a water solution of B O . B O i- ³ hydrated to H,BO in water and thus the two are interchangeable. The article is impregnated under 1.7-6.9 x 10 Pa of pressure, dried at about 100 C, heat treated @ 1200 -2000 C 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_ and H BO , forming TiB by the overall reaction: TiO 4* 2 H BO 4* 5 C -> TiB + 3 H O + 5 CO. The article may be re-impregnated and re-baked to produce the TiB -carbon composite, but if a TiB -graphite composite is the desired end product, the article is further heated to 2200 C or higher, which temperature will convert the amorphous C to semigraphite or graphite.

After heating to 1200 C or higher, at which temperature TiB begins to form, some porosity will be present at the surface due to ^" the loss of CO or CO formed by the overall reactions involved: TiO ÷ 2 H BO ÷ 5 C →- TiB ₂ + 3 H ₂ 0 + 5 CO 2 C + 2HB0 →- B 0 + H O + 2 CO TiO ÷ B O ÷ 5 C - TiB t 5 CO 2 TiQ + Na B^Q • 10 H--.0 ÷ 10 C - ^* • ^*** 2 T E ^* ₂ + N ₂ 0. + 10 H,,0 + 10 CO.

A re-impregnation under alternate, cycles of vacuum and pressure step with pitch or a dispersion of TiO or boron compound or with a mixture of both of the reactants (TiO 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 100 -120 C range used at about 165°-250°C. After impregnation, the article is baked to

700 -1100 c, and is re-heated to 2200°c or higher to graphitize the carbon residue, and form TiB .

EXAMPLE 9

B C (10 g) is dispersed with calcined delayed petroleum coke particles (90 g)_ having a mean diameter of 3 mm in a sig a mixer and heated to about 170 C, coal tar pitch (25 g) with a softening point of 110 C is added, and melted, and a plastic dispersion is formed. A

7 cathodic element is molded under about 1.4 x 10 Pa (2000 PSI), baked on a cycle with the temperature rising to 800 C 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 Pa (100 PSI) . The impregnation step is repeated with alternate vacuum and pressure cycles. After impregnation, the element is heated to 720 C over a six day period, then cooled. The impregnation-bake cycle is repeated several times to build up the required TiO concentration firmly bound in the carbon matrix in the pore volume of the element. After baking, the element is further heated to 1750 C, which converts the reactants to TiB . 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 TiO dispersion, baked, and re-heated as above. After heating to 1750 C, to form TiB , the element is further heated to 2250 C to convert the carbon matrix to graphite. The final cathodic element has TiB concentrated primarily on or near the surface.

The process disclosed uses the reactions forming TiB from TiO , and 3 C, B O , or other boron compounds to form a TiB -graphite composite. The process may also be used to form other such composite structures from reactants forming refractory materials. In this in¬ stance the reactions are as follows:

TiO„ ÷ 3_0., + 5 C →- TiB, + 5 CO. 2 -. 3 2 The reaction above probably proceeds through the formation of

B,C as an intermediate 4

2 B O ÷ 7 C - ^* • ^* - B.C + 6 CO

2 TiO_ ÷ 3 C + 3 C →- 2 TiB + 4 CO. 2 4 2

The process is in general the generalized reaction taking place at temperatures in the range of 800 -3000 C of:

MO + B O + C ^• * MB + CO C here M is a metal) or MO + B + C ^*** MB + CO 4 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 C in petroleum pitch with, a softening point of 110 - 120 C, at 240 C -under several cycles of vacuum and pressure of 6.9 x 10 Pa (100 PSI). After impregnation, the article is re-baked as above, then further heated to 1750 C to drive the TiB -forming reaction to completion, re-impregnated with petroleum pitch and re-baked, then heated to 2250 C to form the graphite-TiB 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. ,

TiO (80 g) + B O ( _. 70 g)_ + C (excess). →- iB ₂ (70 g). + 5 CO +. The reaction Ti + 2 B →- TiB 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 B , and LBO are the most economical and available compounds.

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