CA1163762A - Titanium diboride-graphite composites - Google Patents

Abstract

A B S T R A C T
A cathode component for a Hall aluminum cell is economically produced from a mixture of a carbon source, preferably calcined petroleum coke, and optionally calcined acicular needle petroleum coke, calcined anthracite coal;
a binder such as pitch including the various petroleum and coal tar pitches; titanium dioxide, TiO2; and boric acid, B2O3 or boron carbide, B4C; forming said mixture into shapes and heating to a TiB2-forming temperature.

Description

~ 1 6376~

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 900-1000C. The reactiv-ity of the molten cryolite, the need for excellent electrical conduc-tivity, and cost considerations have limited the choice c,f materialsfor the electrodes and cell walls to the various allotropic forms of carbon.
Typically the Hall cell is a shallow vessel, with the f~oor forming the cathode, the side walls a rammed coke-pitch mixture, and 0 the anode 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 elec-tricity consumed is used for the actual 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 anode-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 ~., ;

g ~ ~3762 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 electro-magnetic forces generated by the high current density.
The wetting of a solid surface in contact ~ith two immiscible liquids is a function of the surface free energy of the three sur-10 faces, in which the carbon cathode is a low ener~y surface and con-sequently is not readily ~et '3y the liquid aluminum. The angle of a drople~ of aluminum at the cryolite-alumin~-carbon junction is governed by the relationship cos ~ = -12 - 13-~23 where 12~ al3, and ~23 are the surface free energies at the aluminum carbon, cryolite-carDon, and cryolite-aluminum 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 20 angle and better wettability with the liquid aluminum. This in turn would tend to smooth out the surface of Ithe liquid alùminum 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. ~owever, a cathode or a TiB2 stud as a com-ponent of the cathode which has better wettability and would permit closer anode-cathode spacing could improve the thermodynamic efficiency 30 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 borides, carbides, and nitrides of Ti, Zr, V, Ta, Nb, and Hf.
35 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.

~ J 6376~

3,151,054, 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, ~ovember 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. 3,31~,876, Ransley,April 18, 1967, discloses the use of TiB2 for use in Hall cell elec~
trodes. ~he raw materials must be of high purity paxticularly in regard to oxygen content, Col. 1, line 73-Col. 2, line ?~; Col. 4, lines 39-50, Col. 8, lines 1-24. U.S. 3,400,061, Iewi~, SeDtelr~er 3, 10 1968 discloses a cathode comprising a rerractory harci netal and carbon, whicn may be formed in a one-s~ep reaction during calcination. U.S.
4,071,420, Foste~, January 31, 1978, discloses a ceil for the elec-trolysis of a metal component in a molten electrolyte using a cathode with refractory hard metal TiB2 tubular elements protruding into the 15 electrolyte. The protruding elements enhance electrical conductivity and form a partial barrier to the mechanical agitation caused by magnetic effects.

Titanium Diboride, TiB2 has been proposed for use as a cathode or cathodic element or component in Hall cells for the reducticn of 20 alumina, giving an improved performance over the amorphous carbon and semi-graphite cathodes presently used.
It had previously been known that Titanium Diboride (TiB2) was useful as a cathode in the electrolytic produc.ion of aluminum, when retrofitted in the Hall cell as a replacement for the carbon or semi-25 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 reslstance was improved, probably due to increased hardness, and lower solubility as compared to the carbon and qraphite forms.
The principal deterrent to the use of Ti~2 as a Hall cell cathode has been the great cost, approximately $25/lb. as compared to the traditional carbonaceous compositions, which cost about $0.60/lb., and its sensitivity to thermal shock. If the anode-cathode distance could be lowered, the % savings in electricity would be as follows:

~ ~ 6~7fi~

A-C distance ~ savinqs _ _ 3.~ 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, and which is mar;~ed1v inore econom-ical. The method also produces an unexpectedly im~roved cathode 10 when its performance is compared to the traditional carbonaceous material.
We have found that our method gives an uneYpected advanta~e in that the articles produced in this manner are much more resistant to thennal shock than articles formed by prior art methods using TiB2 15 powder or reactants processed by previously known methods. In parti-cular, we have found that cathode components for Hall cells are much more resistant to the severe thermal shock imposed on them at the temperature of operation in molten cryolite.
We have also found another unexpected advantage in that we do not 20 need to use the highly purified raw materials specified in the pre-viously known methods. We have also used a commercially pure grade specified to assay at least 98% and typically 99.5~ TiB2 and a grade with 99.9% purity. The various grades are referred to herein by their nominal puric'es as given above.
The method involves the use of pre-mixed and pre-milled TiB2 precursors, i.e., pigment grade titanium dioxide (TiO2) and boron oxide (B203), or boron carbide (B4C) which are preferably added dry to the coke filler prior to addition of binder pitch. These reactants are then intimately mixed and well dispersed in the coke-pitch mixture 30 and firmly bonded into place during the bake cycle. We have found that the reaction proceeds well at or above 1700 C, forming the bonded carbon-TiB2 composite in situ. Here carbon includes graphite as well as amorphous carbon.
The normal method of production of monolithic carbonaceous pieces, 35 either amorphous or graphitic carbon, involves a dry blend of several different particle sizes of coke and/or anthracite fillers and coke 1 ~ 63762 flour t50~-200 mesh) (?9 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 700-1100C. The ba~e 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 2000C and 3000C in a graphiti~ing furnace. ~ n~n-~ciculdr or regular petroleum coke or calcined anthracite may be used to avoid 10 a mismatch of the Coefficient of Thermal Expansion (~T~) of the TiB~-coke mixture, or a needle coke may be used to form an ani-.otropic body.
The raw materials react in situ at temperatures above 1700 C to form a carbon-TiB2 composite according to the following reactions:
TiO2 + B2O3 + 5 C T 2

2 TiO2 + B4C + 3 C -~ 2 TiB2 + 4 CO.
It may also be seen that B4C may be formed as an intermediate step in the above.
2 B2O3 + 7 C ~ B4C + 6 CO
We have found that our method produces a TiB2-C composite in 20 which the TiB2 is of finer particle size and is better dispersed throughout the structure and is made at a much lower cost t~Lan by the addition of pure TiB2 to the dry blend of coke particles and coke flour. It has been found easier to form TiB2 in situ in graphite than to sinter TiB2 powder into articles.
The composite articles produced in this manner have greatly im-proved thermal shock resistance as compared to pure TiB2 articles, and greatly improved resistance to intercalation and corrosion by the molten salt bath as compared to carbon articles.
Other reactants may be used in place of TiO2, B2O3 or B4C, such 30 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.
When manufacturing articles in this manner, it is preferred to 35 impregnate the articles with a pitch and re-bake after the initial bake cycle. Alternately, the impregnation can be accomplished after i 1 6~76~

heat treatment to 1700-3000 C. Multiple impregnations ~ay be advan-tageous. In this instance the reactions consume carbon from the coke and binder to form CO or CO2, which escape, leaving the article highly porous, it is advantageous to impregnate one or more ti~es and re-bake S the article before or after heating at the high temperature cycle to densify, strengthen and decrease porosity. If the article is an elec-trode or component for a Hall cell, it may not be necessary to re-heat it to the 1700 -3000 C range, after the final impresnation, but rather to the 700 -1100 C -ange. If the article is to be used for an appli-10 cation requiring heat resistance or other properties of graphite, itis necessary to reheat it ';o a nigh temperature of 2000 -3000 C to graDhiti7e the coke remaining after this last impregnation.
Another unexpected advantage is found in that articles made in this manner may be molded or extruded, in contrast to the previously lS known methods of cold pressing and sintering. Extrusion particularly is preferred where large quantities are to be made. Molding and ex-trusion methods are preferable to cold pressing and sintering as more economical in practice, more adaptable for production of various shapes and not requiring as complex equipment.
Other useful sources of carbon include solvent refined coal cokes, metallurgical coke, and charcoals.
Preferred binders are 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 25 requirements are an ability to wet the dry ingredients and have a carbon residue on baking to 700 -1100C.

A series of billets doped during mixing with TiB2 precursors at 10 parts to 100 parts mix was molded and processed by heat treatments to 2400 C and 2700 C. After extensive analyses by X-ray diffraction 30 (XRD) and X-ray fluorescence (XRF2, it was determined that a signifi-cant portion of TiB2 was formed from TiO2/B2O3 and TiO2/B4C additives-Positive identification of the TiB2 was made by XRD and distribution was observed by X-ray radiography.
Further trials resulted in the production of moldings and 1 J B~76~

extrusions containing from 3.0-75~ TiB2 after heat treatment in coke particle-flour-pitch mixes.
The mix used above was a mixture of acicular coke particles and coke flour, bonded with about 25 parts per hundred 110 C softening point coal tar pitch.
Various useful forms of carbon include the acicular needle type and regular types of petroleum coke, calcined anthracite, metallurgi-cal coke and other selected mineral and vegetable carbons. Binders may be coal tar or pet ole~m pitches, with coal tar pitches preferred 10 for their superior yield of carbon on coking.
The articles are formed bv 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 700 to 1100 C, 15 with the practice normally followed in the examples below using a six day cycle, reaching a final temperature on a slowly rising curve typ-ical of those normally followed in the electrode industry.
The acicular needle cokes, when heated to the graphitization temperatures of 2000 -3000 C, will form anisGtropic graphite with 20 coefficients of thermal expansion differing in at least two of the three geometric axes. Regular cokes will form isotropic graphite.
In our process, graphitization of the carbon and reaction of the TiB2 precursors can occur simultaneously during graphitization, forming an intimately dispersed, well bonded, homogenous composite.

~8--Example 1: T~e following cQmpositions were produced a~ modifica-tions of a ~tandard car~on electrode mix.

Composition A s C D
Coke particle~
tacicular) 1800 g 1800 g 1800 g 1800 g Coke flour (acicular) 1200 g 1200 g 1200 g 1200 g Coal tar pitch (110C ~oftening point~ 750 g 750 g 810 g 810 g Lubricant 15 g lS g 15 g lS g TiO2 160 g 223 g B203 140 g 4 77 g TiB2 (99 5~ 300 g Whole piece ADl, g/cc Green 1.662 1.679 1.770 1.676 Baked 1.573 1.584 1.655 1.617 Heated at 2400 C1.425 1~393 1.494 1.498 Heated at 2700 C1.448 1.395 1.501 1.516 XRD Scan 2400 C C C,TiB2* C,TiB2*C,TiB2*
2700 C C C,Ti82* C,TiB2*C,TiB2*

lApparent Density *Weak, unidentified line~ in X-ray diffraction.

The compo~itions above were made by premilling and hlending the TiB2 or the reactants with the coke particles and coke flour in a heated mixer, then the pitch waY added, melted and the blend mixed while hot.
A larger amount of pitch was added in C and D above to compensate for 30 the increa~ed surface area and binder demand of these blend~. The piece~ were molded u~ing a pressure of 2000 p5i tl40.6 kg/cm ) on a `~ 1 63~62

3 3/4 in. ~9.S cm) diameter ~olding, baked to about 700 C, then trans-ferred to a graphitizing furnace, and heated to 2400 or 2700C.
Rèsults from X-ray diffraction and X-ray radiography indicate a significant amount of TiB2 formation from the reactants in B and C
above, at a calculated level of 7.38%.

Example 2: The following compositions were made with higher con-centrations of TiB2 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 10 mixer, molded at 2000 psi (140.6 kg/cm ) for 5 minutes at 113 -116 C, and baked to about 700 C, in a six day cycle, with results as follows:

Composition, pbw E F G H
Coke particlesl 60 60 60 6Q
Coke flour2 40 40 40 40 Coal tar pitch 25 41.7 41.7 36.7 S Lubricant 0.5 0.8 0.8 0.8 TiO2/B203 100 Tio2/B4C4 100 TiB2 (99 5%) 100 TiB2, calculated % 46.8 32.2 42.2 Whole piece ~D, g/cc Green 1.682 1.943 2.118 2.134 Baked 1.531 1.593 2.075 2.097 Heated-2400 C 1.450 1.104 1.605 1.974 Approx. TiB2 (XRD) % trace 3.4 34 28 Contaminants identified by XRD TiC TiC TiC

Condition after 2400 C OK weak, weak,OK
porous porous lAv. diam. 3 mm acicular coke 252~ min. -200 mesh acicular 3In stoichiometric ratio according to the equation Ti2 + B203 + S C T 2 4In stoichiometric ratio according to the equation 2 TiO + B C + C ' 2 TiB ~ 2 CO

1 ~ 63762 -11.~

Example 3: Moldings were made using coke flour and TiB2 at various percentages with results as follows, after mixing, molding and baking as in Example 1.

Composition, pbw I _ _ J K L ~ N
.

S Coke flour, isotropic 80.1 61.4 37.8 Coke flour, acicular 79.9 61.4 37.8 TiB2 (90 9%) 19.9 38.6 62.2 20.1 38.6 62.2 Coal tar pitc~ 38 32 27 38 32 27 Lubricant 1 1 1 1 1 Calc. TiB2 15 30 50 lS 30 50 Whole piece AD, g/cc Green 1.818/ 1.988/ 2.307/ 1.866/ 2.024/
1.817 1.989 2.294 1.857 2.005 2.322 Baked 1.733/ 1.931/ 2.237/ 1.693/ 1.900/
1.742 1.918 2.213 1.702 1.863 2.242 Impregnated, wt. ~
pickupl 7.4 3.6 0.6 7.8 1.4 1.6 Rebaked AD, g/cc 1.81 1.981/ 2.258 1.763/ 1.927/ 2.261 1.979 1.793 1.908 Heated to 2400 C, AD 1.84 2.263 2.217 TiB2 by XRD,~ 7 24 19 Impregnated with petroleum pitch with a softening point of 115 -120 C and rebaked to about 700 C.

Two moldings were made for most of the above formulations, molded at 30 2000 psi (140.6 kg/cm2) for 5 minutes at die temperatures of 115_l2ooc Example 4: Pieces were formed by extrusion of mixtures made according to the procedure of Example 1, with the following compo-s:itions and results.

Composition, parts by weight 0 P
Isotropic coke flour 60.6 60.6 TiB2 (90'9%) 39.4 39.4 Coal tar pitch 32 32 Lubricant 1.5 1.5 TiB2, calculated %29.5 29.5 Whole piece AD, g/cc Green 1.962 1.973 Baked 1.891 1.902 Extrusion conditions Mud pot C 115-120 C 115-120 C
Die temperature, C110 110 Extrusion pressure (psi) 500 500 (kg/cm ) 35 35 ~ ~ 63762 Example 5: Moldings were made as in Example 1 with the following compositions:

Composition, pbw Q R S T U
Coke flour, isotropic 15 52.5 71.3 50 TiB2 - 99.9%1 100 85 TiO2 35.3 21.3 15

4 12.2 7.4 Borax 35 Piteh, 110 C 21 24 32 38 25 Green whole pieee AD, g/cc 3.050 2.750 2.0401.850 1.820 Caleulated TiB2 % 88%2 74%231%2,319%2,341%2 4 lVery high purity TiB2, 99.9~ + assay.
2Assuming 65~ eoke yield on eoal ':ar pitch after baking to 700 -1100 C range.
3Assuming reaetions as in Example 2 4Assuming the reaetion:
2 TiO2 + Na2B4O7 10 H2O + 10 C ~ 2 TiB2 + Na2O + 10 H2O + 10 CO

Claims (3)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A process for the production of TiB2-carbon composite comprised of the steps of mixing and dispersing particulate carbonaceous matter and TiB2-forming reactants in a carbon-forming binder to form a plastic mixture, -forming a shaped article from said mixture, baking said article at 700° to 1100°C., impregnating said baked article with a carbon-forming binder and rebaking said article to 700° to 1100°C.

2. A process for the production of a TiB2-car-bon composite article comprised of the steps of mixing and dispersing particulate carbonaceous matter and TiB2-forming reactants in a carbon-forming binder to form a plastic mixture, forming a shaped article from said mixture, baking said article to 700° to 1100°C., heating said article to a TiB2-forming temperature from 2000° to 3000°C. wherein the composite article is impregnated with a carbon-forming binder and is re-baked to 700° to 1100°C.

3. A process for the production of a TiB2-carbon composite article comprised of the steps of mixing and dispersing particulate carbonaceous matter and TiB2-forming reactants in a carbon-forming binder to form a plastic mixture, forming a shaped article from said mix-ture, baking said article to 700° to 1100°C., further heating said article to a TiB2-forming temperature of 2000°C to 3000°C. wherein the composite article is then impregnated with a carbon-forming binder, re-baked to 700° to 1100°C. and re-heated to 2000° to 3000°C.