TITANIUM DIBORIDE-GRAPHITS COMPOSITES

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 operatinq 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.

Ty ^p ically 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 re-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 ahout 4-5 cm. , and attempts to lower this distance result in an electrical discharge from the cathode to the anode throuσh

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 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-aluminum-carbon junction is governed by the relationship cos θ = -12-***-*=I3- . ^α 23 where α , - and α _ are the surface free energies at the aluminum carbon, cryolite-carbon, and cryolite-aluminum boundaries, respective-

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 contac* angle and better wettability with the liquid aluminum. This in turn would tend to smooth out the surface of the liσuid 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 TiB stud as a com¬ ponent of the cathode which has better wettability and would permit closer anode-cathode spacing could improve the thεr odyna ic efficienc and be very cost-effective. ;

Several workers in the field have developed refractory high fre_ ' 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 Kf. U.S. 3,028,324, Ransley, April 3, 1962, claims a method of producing aluminum using a mixture of TiC and Ti3 as the caphode. U.S.

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, ibby, November 10, 1964, claims a cathode for a Hall cell with a cap of refractory hard metal and dis- closes TiB.. as the material of construction. U.S. 3,314,876, Ransley, 2

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 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 elec¬ trolysis of a metal component in a molten electrolyte using a cathode with refractory hard metal TiB_ tubular elements protruding into the electrolyte. The protruding elements enhance electrical conductivity and form a partial barrier to the mechanical agitation caused by magnetic effects.

SUMMARY OF THE INVENTION

Titanium Diboride, TiB has been proposed for use as a cathode or cathσdic 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.

It had previously been known that Titanium Diboride (Ti3_.) 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.

The principal deterrent to the use of Ti3. 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:

A-C distance % savings

3.8 cm. std.

1.9 cm. 2Q%- 1.3 cm. 27% 1.0 cm. 30%

We have invented an improved process for producing a Ti3 -carbon composite which shows excellent performance as a cathode or cathode component in Hall aluminum cells, and which is markedly more econom¬ ical. The method also produces an unexpectedly improved cathode when its performance is compared to the traditional carbonaceous material.

We have found that our method gives an unexpected advantage in that the articles produced in this manner are much more resistant to thermal shock than articles formed by prior art methods using TiB powder or reactants processed by previously known methods. In parti¬ cular, we have found that cathode components for Hall cells are mud- 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 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% Ti3 and a grade with 99.9% purity. The various grades are referred.to herein by their nominal purities as given above. The method involves the use of pre- ixed and ore-milled Ti3 precursors, i.e., pigment grade titanium dioxide (TiO_) and boron oxide ( ^B ₂ 0- i or boron carbide (.B..C) which are pre erably 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 ixpurs 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-TiB composite in situ. Here carbon includes graphipe as well as amorphous carbon.

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 2000 C and 3000 C 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 Ti3 - coke mixture, or a needle coke may be used to form an anisotropic body.

The raw materials react in situ at temperatures above 1700 C to form a carbon-TiB_ composite according to the following reactions: TiO + B O + 5 C →- TiB + 5 CO 2 TiO + B C + 3 C ^• * ^■ 2 Ti3 + 4 CO.

It may also be seen that B C may be formed as an intermediate step in the above.

2 B ₂ 0 ₃ + 7 C B„C + 6 CO

4

We have found that our method produces a TiB -C composite in which the TiB 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 to the dry blend of coke particles and coke flour. It has been found easier to form TiB in situ in graphite than to sinter TiB powder into articles. The composite articles produced in this manner have greatly im¬ proved thermal shock resistance as compared to pure Ti3_ 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 TiO„, 3_,0_, or B„C, such

2 2 3 4 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 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 advan ^¬ tageous. In this instance the reactions consume carbon from the coke and binder to form CO or CO., 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. 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 impregnation, but rather to the 700 -1100 C range. If the article is to be used for an appli- cation requiring heat resistance or other properties of graphite, it o o is necessary to reheat it to a high temperature of 2000 -3000 C to graphitize 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 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 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.

DESCRIPTION OF THE INVENTION

A series of billets doped during mixing with TiB precursors at 10 parts to 100 parts mix was molded and processed by heat treapments to 2400 C and 2700 C. After extensive analyses by X-ray diffraction (XRD) and X-ray fluorescence (XRF) , it was determined that a signifi¬ cant portion of TiB., was formed from Ti0_/B_o and Ti0„/S.C additives.

2 2 2 3 2 4 -

Positive identification of the Ti was made by XRD and distribution was observed by x-ray radiography. Further trials resulted in the production of moldings and

extrusions containing from 3.0-75% TiB after heat treatment in coke particle-flour-pitch, mixes.

The mix used above was a.mixture of acicular coke particles and o 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 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 700 to 1100 C, 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 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. In our process, graphitization of the carbon and reaction of the TiB precursors can occur simultaneously during graphitization, forming an intimately dispersed, well bonded, homogenous composite.

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Exa ple It The following compositions were produced as modifica¬ tions of a standard carbon electrode mix.

Composition

Coke particles (acicular) 1800 g 1800 g 1800 g 1800 g

Coke flour (acicular) 1200 g 1200 g 12Q0 g 1200 g

Coal tar pitch

(110 C softening point) 750 g 750 g 810 g 810 g

Lubricant 15 g 15 g 15 g 15 g

TiO. 160 g 223 g

140 g

^B 2°3

B ₄ C 77 g

TiB ₂ (99.5%) 300 g

Whole i piece AD*^, g/cc

Green 1.662 1.679 1.770 1.676

Baked 1.573 1.584 1.655 1.617

Heated at 2400°C 1.425 1.393 1.494 1.498

Heated at 2700°C 1.448 1.395 1.501 1.515

XRD Scan

2400°C C C,Ti3 * C,Ti3 * C,Ti3 *

2700°C C C,τiB ₂ * C,τi3 ₂ * C,Ti3 ₂ *

-" ^■ Apparent Density

*Weak, unidentified lines in X-ray diffraction.

The compositions above were made by premilling and blending the TiB 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") on a

3 3/4 in. (9.5 cm) diameter molding, baked to about 700 C, then t ferred to a graphitizing furnace, and heated to 2400 or 2700 C.

Results from X-ray diffraction and X-ray radiography indicat signi-ffiiccaanntt aammoouunntt ooff TTiiBB,. ffoorrmmaattiioonn jfrom the reactants in B and C above, at a calculated level of 7.38%.

Example 2; The following compositions were made with higher centrations of TiB and precursors than in Example 1. The additiv were incorporated at 100 pph level in the heated coke mix before t 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°-116 and baked to about 700 C, in a six day cycle, with results as foll

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Compositio , pbw H

Coke particles 60 60 60 60 Coke flour ² 40 40 40 40 Coal tar pitch 25 41.7 41.7 36.7 Lubricant 0.5 0.8 0.8 0. 8

^Ti °2 ^B 2°3 ³ 100 i0 ₂ /B ₄ C ^lf 100 iB ₂ (99.5%) 100

TiB , calculated % 46.8 32.2 42.2

Whole piece AD, 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. TiB (XRD) trace 3.4 34 28

Contaminants identified bv

XRD τio ₂ , -iC TiC

TiC

Condition after 2400°C OK weak, weak, OK porous porous

Av. dia . 3 mm acicular coke

^* - ^* 52% min. -200 mesh acicular

In stoichiometric ratio according to the equation i0 ₂ ÷ B ₂ 0 ₃ + 5 C **>- TiB ₂ + 5 CO.

In stoichiometric ratio according to the equation

2 TiO + B .C + C -* ^■ 2 TiB„ + 2 CO, . •*•> 4 2 2

C

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

Composition, obw K M N

10

15

20

25

^• 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/cm ² ) f _or 5 minutes at die temperatures of 115°-i20°c

OMPI

-12-

Example 4; Pieces were formed by extrusion of mixtures made according to the procedure of Example 1, with the following compo sitions and.results-

Composition, parts by weight

Isotropic coke flour 60.6 60. 6

TiB (90.9%) 39.4 39. 4

Coal tar pitch 32 32

Lubricant 1.5 1.5

TiB_, 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 o Die temperature, C 110 110

Extrusion pressure (psi) 500 500

(kg/cm ) 35 35

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Example 5; Moldings were made as in Example 1 with the followin compositions:

Composition, pbw ϋ

820

2 -* ^■

^•• Very high purity TiB , 99.9% + assay.

² ssuming 65% coke yield on coal tar pitch after baking to 700 -1100 C range.

Assuming reactions as in Example 2

Assuming the reaction:

2 TiO„ + Na„3 ,,0_- ^• 10 H_,0 + 10 C →- 2 TiB., + Na_0 + 10 H_,0 + 10 C 2 2 4 / 2 2 _.

-^U Γ_E ^