NO172451B - CELL FOR ELECTROLYTIC REDUCTION OF ALUMINUM OXYDE, COATING MATERIAL AND CATHOD FOR SUCH CELL AND METHOD OF PRODUCING CATODE SURFACE MATERIAL - Google Patents

Description

The present invention relates to a cell for electrolytic reduction of aluminum oxide, coating material comprising refractory, hard material and carbon, aluminum wettable cathode for an aluminum reduction cell as well as method for producing cathode surface material for said cell.

Production of aluminum is conventionally carried out by the electrolytic reduction process according to Hall-Heroult, whereby aluminum oxide is dissolved in molten cryolite and electrolysed at temperatures from 900 to 1000°C. This process is carried out in a reduction cell which typically comprises a steel shell provided with an insulating lining of suitable refractory material, which in turn is provided with a lining of carbon which comes into contact with the molten constituents. One or more anodes, typically made of carbon, are connected to the positive terminal of a direct current source, and suspended in the cell. One or more conductor rods connected to the negative pole of a source of direct direct current are embedded in the carbon cathode substrate comprising the floor of the cell, thus causing the cathode substrate to become cathodic when current is applied. If the cathode substrate comprises a carbon liner, it is typically constructed from a number of burnt cathode blocks which have been rammed together with a mixture typically consisting of anthracite, coke and coal tar pitch.

In this conventional design of the Hall-Heroult cell, the molten aluminum pool or pool formed during the electrolysis itself acts as part of the cathode system. The duration of the carbon liner or cathode material may average 3-8 years, but may be shorter under adverse conditions. Degradation of the carbon lining material is due to erosion and ingress of electrolyte and liquid aluminum as well as deposits of metallic sodium, which cause swelling and deformation of the carbon blocks and stamping mixture.

Difficulties in cell operation have included surface effects on the carbon cathode beneath the aluminum build-up, such as accumulation of undissolved material (sludge or dirt) forming insulating areas on the cell bottom. Penetration of cryolite through the carbon body causes displacement of the cathode blocks. Aluminum penetration into the iron cathode rods results in excess iron content in the aluminum metal, or in more severe cases, a draining. Another serious disadvantage of the carbon cathode is that it is not wetted by aluminum, necessitating the maintenance of a substantial pond height or accumulation height of metal to ensure effective contact of molten aluminum across the cathode surface. A problem with maintaining such an aluminum pond is that electromagnetic forces create movements and standing waves in the molten aluminum. To avoid short-circuiting between the metal and the anode, the anode-to-cathode distance (ACD) must be kept at a safe 4-6 cm in most designs. For any given cell plant, there is a minimum ACD below which there is a severe loss of current efficiency due to short-circuiting of the metal (aluminum) build-up to the anode, resulting from instability of the metal build-up combined with increased back-reaction under conditions of strong agitation. The electrical resistance of the inter-electrode distance traveled by the current through the electrolyte causes a voltage drop in the range of 1.4-2.7 volts, which represents 30-60# of the voltage drop in a cell, and is the largest single voltage drop in a given cell.

To reduce ACD and associated voltage drop, extensive research using refractory hard materials (RHM), such as TiB£, as cathode materials has been conducted since the 1950s. T1B2 is only very poorly soluble in aluminium, is highly conductive and is wetted by aluminium. This wettability property allows an aluminum film to be electrodeposited directly onto an RHM cathode surface and avoids the need for an aluminum build-up. Because a titanium diboride and similar refractory hard materials are wetted by aluminum, resist the corrosive environment of a reduction cell, and are excellent electrical conductors, several cell designs using refractory hard materials have been proposed in an effort to save energy, in part by reducing the anode-to-cathode distance.

The use of current-conducting elements of titanium diboride in electrolytic cells for the production or refining of aluminum is described in the following exemplary US patents: 4,915,442, 3,028,324, 3,215,615, 3,314,876, 3,330,756, 3,156,639, 3,274. 094 and 3,400,061. Despite the rather extensive efforts that have been made in the past, as these and other patents indicate, and the potential advantages of the use of titanium diboride as a conductive element, such compositions do not appear to have been commercially adapted on any significant scale in the aluminum industry. Lack of acceptance of TiB2~ or RHM current conducting elements in the prior art is due to their lack of stability when used in electrolytic reduction cells. It has been reported that such current-conducting elements fail after relatively short periods of use. Such a failure has been associated with penetration into the self-bonded RHM structure of the electrolyte, and/or aluminium, thereby causing critical weakening with consequent crack formation and failure. It is well known that liquid phases penetrating the grain boundaries of solids can have undesirable effects. For example, RHM tiles in which oxygen impurities tend to segregate along the grain boundaries are susceptible to rapid attack by aluminum metal and/or cryolite baths. Previously known techniques for combating TiB2~ chip disintegration in aluminum cells have been to use highly refined TiB2~ powder for the production of the chip, containing less than 50 ppm oxygen at 3 or 4 times the price of commercially pure TiB2 powder containing approx. 3000 ppm oxygen. Manufacturing also significantly increases the price of such tiles. However, it is not known that any cells using TiB2 ~ tiles have been operated successfully for extended periods of time without loss of adhesion of the tiles to the cathode, or disintegration of the tiles. Other suggested reasons for the failure of RHM tiles and coatings have been the solubility of the composition in molten aluminum or molten flux, or the lack of mechanical strength and resistance to thermal shock. Furthermore, various types of TiB2 coating materials, applied to carbon substrates, have failed due to differential thermal expansion between the titanium diboride material and the carbon cathode block. To the best of our knowledge, no prior known RHM-containing materials have been successfully in operation as a commercially used cathode substrate due to thermal expansion mismatch, bonding problems, etc.

TJS-PS 3,400,061 to Lewis et al., assigned to Kaiser Aluminum, teaches e.g. a cell construction with a drained and wetted cathode, where the cathode surface of refractory hard material consists of a mixture of refractory hard material, at least 5% carbon and usually 10-20 wt-# of pitch binder, fired at 900°C or higher. According to the patent, such a composite cathode has a higher degree of dimensional stability than previously available. The composite cathode coating material according to this patent can be stamped into place in the cell bottom. However, this technique has not been widely used because of its susceptibility to attack by the electrolyte bath, as taught in a later Kaiser Aluminum US-PS 4,093,514 to Payne.

Said US-PS 4,093,524 to Payne defines an improved method for bonding titanium diboride and other refractory hard materials to a conductive substrate such as graphite or silicon carbide. The cathode surface is made from titanium diboride tiles with a thickness of 0.3-2.5 cm. However, the large differences in thermal expansion coefficients between such tiles of refractory, hard material and carbon preclude the formation of a bond that will be effective both at room temperature and at the operating temperatures of the cell. The bond is consequently formed in situ at the interface between the chip of refractory, hard material and the carbon by a reaction between aluminum and carbon to form aluminum carbide close to the cell's operating temperature. However, since the bond does not form until high temperatures are reached, the tiles are easily displaced during the start-up operation. The bond is accelerated by passing electric current over the surface, resulting in a very thin aluminum carbide bond. Attack of aluminum and/or electrolyte on the bond results. However, if the tiles are mounted too far apart, and if the plates are mounted too close together, they bulge at operating temperature which results in rapid breakdown of the cell lining and in disruption of cell operations. Consequently, this idea has not been widely used.

In US-PS 3,661,736, Holliday defines an inexpensive and dimensionally stable composite cathode for a drained and wetted cell, comprising particles or lumps of arc-melted "RHM alloy" embedded in an electrically conductive matrix. The base material consists of carbon or graphite, and a powdered filler, such as aluminum carbide, titanium carbide or titanium nitride. When operating such a cell, however, electrolyte and/or aluminum attack grain boundaries in the arc-melted alloy of refractory, hard material, as well as the large areas of carbon or graphite matrix, at a rate of about 1 cm per second. years, which leads to early destruction of the cathodic surface.

US-PS 4,308,114 to Das et al., describes a cathode surface refractory, hard material in a graphite matrix. In this case, the refractory, hard material is compounded with a pitch binder and subjected to graphitization at or above 2350°C. Such cathodes are subject to early failure due to rapid ablation and possible encrustation and erosion of the graphite matrix.

In addition to the above-mentioned patents, a number of other references concern the use of titanium diboride in chip form. Titanium diboride tiles of high purity and density have been tested, but they generally exhibit poor resistance to thermal shock and are difficult to bond to carbon substrates used in conventional cells. Mechanisms for bond dissolution are believed to involve high stresses developed by the thermal expansion mismatch between the titanium diboride and the carbon, as well as aluminum intrusion along the interface between the tiles and the adhesive holding the tiles in place, due to wetting of the bottom surface of the tile with aluminium. In addition to bond dissolution, disintegration of even higher tiles can occur due to aluminum penetration into grain boundaries. These problems, together with the high price of titanium diboride tiles, have discouraged widespread commercial use of titanium diboride in conventional electrolytic cells, and limited their use in new cell designs. It is the purpose of the present invention to overcome the disadvantages of the previous attempts to use refractory, hard materials as a surface coating for carbon cathode blocks, and for monolithic cathode surfaces.

The present invention relates to aluminum cells where the improvement includes the use of a coating material of carbon and refractory, hard material, containing refractory, hard material, a thermosetting binder system, carbonaceous filler and additive as well as modifying agents, which is applied to a carbon cathode and hardened in situ to a hard, tough surface comprising RHM in a carbonized binder primer. The carbonaceous additive used in said material applied to the cathodes in the cells according to the present invention may include carbon fibers which act as crack stoppers and reinforcements. Sufficient RHM is incorporated into the coating material to ensure continuous aluminum wetting of the surface. Coating thus ensures an economical and efficient application of the advantages of RHM in cathodes, while the need for more expensive RHM tiles is eliminated.

According to the present invention, it has been found that cathode structures can be coated with refractory hard material (RHM) combined with specified thermosetting binders and other materials to improve the conventional carbon lining in an aluminum reduction cell. Such coated cathodes combine the advantages of conventional carbon liners, such as structural integrity and low cost, with desired properties obtained by using the refractory, hard materials. Such improvements include wettability of molten aluminum, low solubility in the environment of molten aluminum cryolite, good electrical conductivity and reduced adhesion of dirt. In addition, the present invention can be used in connection with existing reduction cells without the costs and time of a complete cell reconstruction, or the high costs of manufacturing RHM tiles or RHM alloy tiles as proposed in the prior art. The coating according to the present invention can also be used in cell constructions that use slanted cathodes.

According to the present invention, a cell for the electrolytic reduction of aluminum oxide to aluminum is provided, which has a cathode comprising a cathode substrate with an aluminum wettable surface layer bonded thereto, the layer comprising refractory, hard material and carbon, characterized in that said layer comprises refractory, hard material In a non-graphitizable carbon matrix having an ablation rate during operation of the cell substantially equal to the combined rate of wear and dissolution of the refractory hard material, wherein said surface layer is obtained by curing and carbonizing a coating material comprising: 10-90 wt. -# refractory, hard material; 0.5-40 wt-56 thermosetting resin, wherein the thermosetting resin comprises at least one resin different from a phenolic resin, a furan resin precursor or a mixture thereof; 2-40 wt-See mixing fluid; 1-60 wt & particulate carbonaceous filler having a particle size less than 100 mesh (0.147 mm); and up to 70 weight-# carbonaceous additive in the form of particulate material having a particle size greater than 100 mesh (0.147 mm) and/or carbon fibers, wherein the thermosetting resin forms a resinous binder system having a char yield greater than 25# and wherein the hardened and carbonized coating shows a percentage expansion between 800 and 1000°C which differs by less than 0.2 from the percentage expansion shown by said cathode substrate.

Furthermore, according to the invention, a coating material comprising refractory, hard material and carbon is provided, characterized in that it is composed of 10-90% by weight of refractory, hard material; a thermosetting binder system comprising 0.5-40 wt-# of non-graphitizing thermosetting resin, wherein the thermosetting resin comprises at least one resin different from phenolic resins, furan resin precursors or mixtures thereof; 2-40 wt-# mixing fluid; 1-60 wt.-5é particulate carbonaceous filler having a particle size less than 100 mesh (0.147 mm); and up to 70 wt-# of carbonaceous additive in the form of particulate matter having a particle size greater than 100 mesh (0.147 mm) and/or carbon fibers; wherein the binder system, the carbonaceous filler, and the carbonaceous additive exhibit an ablation rate after curing and carbonization that is substantially equal to the combined rate of wear and dissolution of the refractory hard material in the environment of an aluminum reduction cell during operation thereof, wherein the thermosetting resin forms a resinous binder system which has a char yield greater than 2556 and where the hardened and carbonized material exhibits a percentage expansion between 800 and 1000°C that differs by less than 0.2 from the percentage expansion of the cathode substrate in the aluminum lumen reduction cell.

The invention also provides an aluminum wettable cathode for an aluminum reduction cell comprising a cathode substrate having a surface layer containing refractory, hard material and carbon, characterized in that the surface layer is a hardened and carbonized composition of the type defined above.

Finally, the present invention provides a method for producing a refractory, hard material and carbon-aluminum wettable cathode surface for an aluminum reduction cell, and this method is characterized by the fact that a coating material of the type defined above is applied to a cathode substrate, the coating material is hardened to form a relatively hard, adherent surface layer, and this layer is carbonized to form an adherent aluminum wettable surface layer comprising refractory hard material in a non-graphitizable carbon matrix, said surface layer having a percentage expansion between 800 and 1000°C which differs from the percentage expansion of the cathode substrate by less than 0.2, where the thermosetting resin forms a resinous binder system having a char yield greater than 2556.

When understanding the idea of the present invention, it is important that certain differences and definitions are observed. The following definitions shall therefore apply in connection with the present invention.

The term "coating material" used in the present invention includes refractory, hard material, carbonaceous additive, carbonaceous filler and binder system. As used herein, the term "coating composition" or "coating material" shall include the combination of all these materials. The term "coating", on the other hand, may include less, depending on the state of drying, curing, or carbonization, since e.g. mixing liquid can evaporate and/or polymerize during curing and carbonization.

The term "hard refractories" is usually defined as the borides, carbides, silicides and nitrides of the transition metals in the fourth to sixth groups of the periodic table, often referred to as refractory hard materials, and alloys thereof.

The term "resinous binder" shall be used to denote a polymerisable and/or crosslinkable, thermosetting carbonaceous substance.

The term "mixing liquid" used in the present invention acts in a number of different ways in the coating material according to the invention, depending on the specific composition. It may be present to allow easy and uniform mixing of the solid components of the coating material and to provide a light, spreadable mass. Certain mixing fluids, such as furfural, may also allow an increase in the amount of carbonaceous filler that can be incorporated into the coating material. The mixing liquid can also promote internal bonding and bonding between the coating and the carbon substrate when it is a solvent containing the resinous binder, and/or constitutes the resinous binder or part of the resinous binder. This is because a dissolved resin or liquid resin can more easily penetrate and impregnate permeable components in the coating as well as the carbon substrate. The mixing liquid also allows the resin to enter the spaces between the particles in the coating material by capillary action through a wicking effect. The mixing fluid may act solely as a solvent for the resinous binder (already present in the solids of the binder system), such as methyl ethyl ketone, (which could dissolve a novolak if present in the solids), and be evaporated during hardening and carbonization operations. If, on the other hand, the mixing liquid is present only as an inert carrier liquid, then it can also be evaporated during curing and carbonization. Otherwise, the mixing liquid can function as a combined solvent and resin-forming agent, such as furfuryl alcohol and furfural, some of which is volatilized during heating, while the rest is incorporated into the resinous binder. In another case, the mixing liquid may be the resinous binder per se, such as where the resinous binder is a liquid, such as furfural (usually in combination with phenol), furfuryl alcohol, or low polymers thereof, or a resol. The mixing fluid may also comprise the resinous binder In the case of a solid resin, such as a novolak, dissolved in a solvent (whose solvent portion may be volatilized during heating), or a resin of high viscosity, such as a partially polymerized resol diluted with a solvent. The mixing liquid may also contain gas release agents, modifiers and curing agents.

The term "binder system" shall be used to indicate resinous binder, mixing fluid, and, if desired, gas release agents, modifiers and curing agents.

The term "gas release agent" shall mean agents present which form liquid phases which seep through the coating and then evaporate, to create small channels in the coating, so that volatile substances can be released.

The term "modifying agents" shall mean materials added to the resinous binder to modify e.g. curing, electrical properties or physical properties such as flexural strength or impact strength prior to carbonization of the coating.

The term "curing agents" shall mean agents necessary to either copolymerize with the resin or to activate the resin to a state in which the resin can polymerize or copolymerize. Cross-linking or activating agents fall into this category, as do catalysts required for most polymerization and cross-linking reactions.

The term "carbonaceous filler" shall be interpreted to mean those carbonaceous materials present, either as a component of a known carbon cement or as part of a proprietary or common carbon system, with a C:H ratio greater than 2:1, which is less than 100 mesh (0.147 mm) in size. While a carbonaceous filler may have reactive groups present and need not be fully carbonized, such materials do not polymerize with themselves, as is the case with the resinous admixture material. Furthermore, the carbonaceous filler is substantially insoluble in commonly used solvents, such as methyl ethyl ketone or quinoline, while the resinous binder (in its incompletely cured state) is usually soluble therein.

The term "carbonaceous additives" shall mean those carbonaceous materials present either as a component of a known carbon cement or as part of a proprietary or common carbon system, with a C:H ratio greater than 2:1, which includes particulate carbon aggregate with a particle size range from 4 mesh (4.699 mm) to 100 mesh (0.147 mm) and/or carbon fibers.

The term "carbon system" shall include binder system plus carbonaceous additive and carbonaceous filler; or coating material minus RHM.

The term "carbon cement" shall mean a commercially available carbonaceous cement or adhesive, usually comprising a resinous binder, mixing fluid, carbonaceous filler and curing agents, the solid and liquid parts of which may be packaged separately to increase shelf life, or as a premixed cement. Gas release agents and/or modifiers may be present in such a system, or may be added thereto for use in the present invention. Carbonaceous additives are usually added to such systems for use in the present invention if they are not present in the commercially available formulation.

Pitch may be present as part of the resinous binder, as a modifying material, but requires the presence of a suitable curing agent, such as hexamethylenetetramine. Such a curing agent may already be present as a component of the resinous binder, or may be added thereto to facilitate cross-linking between the resinous binder and the pitch, or bond formation between the pitch and the carbonaceous filler, or self-bonding between the polynuclear aromatics which comprises the majority of the brook. Although pitch is known to constitute a graphite precursor, graphitization is not realized in the present invention. The graphite precursor is thus dispersed in the resinous binder, which is an amorphous carbon precursor. Pitch can seep through the coating to provide gas release channels and, in the presence of suitable curing agents, can be cross-linked with the resinous binder and/or the carbonaceous filler.

The coating material is composed in such a way that a number of critical purposes are achieved. Firstly, the coating material can yield to allow for shrinkage and expansion differences, so that the coating adheres to the cathode substrate over temperatures up to approx. 800°C, at which temperature a slowly heated coating material has been completely hardened and carbonized to a rigid mass. It then has a thermal expansion coefficient which is very similar to that of the substrate on which it is applied. Furthermore, the carbon content and type of resinous binder in the coating material is such that a high total char formation occurs during carbonization. This minimizes the formation of large, closed cavities and excessive gas development. The carbon matrix in the carbonized coating also has an in-use ablation rate equal to or very little above the combined rate of wear and dissolution of the refractory, hard material in an aluminum cell environment, thereby ensuring uniform wear of the coating surface, and continuous exposure of the refractory, hard the material at the surface. "Ablation" is defined here to include the loss and consequent dilution of a material through a combination of mechanical and chemical mechanisms. Purely graphitic structures fail in this respect due to significantly faster rates of loss of the anisotonic structure to the graphitic groundmass, caused e.g. of sodium deposits, and the weak bonds between atomic layers in graphitic material. In order to achieve these critical objectives, it has been discovered that it is necessary to provide a carbon matrix that has very specific properties, so that the carbon matrix of the present invention provides a high degree of strength in 3 dimensions, as opposed to having the planar weakness that is present in graphite.

First, in order to achieve an adherent coating over the entire temperature range to which the coated cathode block can be exposed, it is essential that the coating material exhibits a reasonable degree of dimensional flow over the temperature range from ambient to about 800°C, in which range the coating releases volatile substances and undergoes hardening and carbonization into a solid, rigid mass. In this temperature range, the coating material may undergo "thinning" or compression, a vertical thickness change, to allow for volumetric contraction or shrinkage of the coating vis a vis the positive linear expansion of the block to which it is applied. It is noted that the resinous binder per se undergoes significant expansion and contraction over this temperature range and that the presence of carbon fiber is very effective in strengthening the coating during this curing and carbonization stage by minimizing harmful cracking and allowing fine cracking, which allowing stress relief and helping to accommodate expansion differences between the coating and the substrate. However, once the coating material has been carbonized to a rigid, hard substance, it is essential that the coefficient of thermal expansion of the coating be substantially equal to the thermal expansion coefficient of the cathode substrate. Thus, the value for the percentage expansion of the carbonized coating must be within approx. ±0.2 of the value for the percentage expansion of the cathode substrate, and preferably within approx. ±0.1 over the temperature range from 800 to 1000<*>C, e.g. Thus, if the cathode substrate to be coated exhibits an expansion of +0.1556 over the temperature range of 800 to 1000°C or higher, the coating should have an expansion (over the same range) of -0.05 to +0.35$6 and preferably from +0.05 to +0.2556.

It is desirable that the amount of shrinkage that the cured binder system undergoes during carbonization is as small as possible. This can be achieved by selecting a carbonaceous resin which, when used in accordance with the present invention, will provide a coating material which, when subjected to carbonisation, exhibits a shrinkage of the coating on the substrate which

is less than that which would induce coating failure, cathode block failure or separation of the coating from the cathode substrate. Fine, vertical cracking in the carbonized coating is an acceptable stress-release mechanism. The presence of carbonaceous additive and/or filler is useful.

Both shrinkage and expansion of the cured binder system and the coating material can be measured in the following way. A sample of the material to be tested is prepared and spread in a Teflon®-coated mold with dimensions 5 cm x 1.27 cm x 0.64 cm deep. The material is hardened and allowed to cool before removal from the mold. The piece is then divided into four test pieces, 2.54 cm x 0.64 cm x 0.64 cm, which are then dried to constant weight at 250°C in an alumina crucible. A test sample is measured using a micrometer, then heated from room temperature to 1000°C in a dilatometer continuously flushed with argon. Expansion/contraction is recorded continuously as a function of temperature on a curve recording device. Two expansion-shrinkage values are calculated: one based on the initial sample length and final length at 1000°C; the other based on the original sample length and final length after return to room temperature (this is termed "total contraction"). Samples are also measured with micrometers after cooling as a check of the curve recording apparatus. It is desired that the full cycle or total contraction is preferably less than approx. 1.056.

In addition to the above considerations, it has been found critical to use a binder system which, when exposed to carbonisation, has a charring yield of over approx. 2556. The charring yield is defined here as the mass of stable carbonaceous residue that is formed by thermal decomposition of a unit mass of the binder system. Thermogravimetric analyzes of various binder systems have demonstrated that the amount of char yield is a function of the aromaticity of the resin structure. In general, carbon rings that are bonded at two or more places will usually remain as charred material. Rigid polymers are the most stable, as they lose hydrogen and give a very high carbon charring yield.

The char yield of a binder system, as used here, is determined by curing a proposed carbon system, i.e. binder system plus carbonaceous filler (for a 24 hour period to thereby achieve polymerization and/or cross-linking, followed by heating at 250°C in a sufficient time to achieve constant weight to thereby eliminate volatiles, polymerization products and/or unreacted liquid. The sample is then sintered at 1000°C in a non-oxidizing atmosphere and the weight of the remaining charred mass is determined. Likewise, the charring weight is determined of carbonaceous filler present in the carbon system and subtracted from the char weight of the carbon system to determine the char weight of the binder system From the weight of the carbon system at 250°C and the known weight of carbonaceous filler at 250°C, one can calculate the weight of the binder system at 250 ° C. The charring yield of the binder system is then calculated, as a percentage proportion, from the charring weight of the binder system and the weight of the binder system at 150°C. It has been observed that binder systems exhibiting a char yield of over approx. 2556, gives acceptable coatings on curing and carbonization, while a binder system exhibiting 856 charring yields gave an unacceptable carbon matrix on carbonization. Charring yields above 5056 are preferred.

In order to achieve a long-lasting coating material in the environment of an aluminum cell, it is necessary that the ablation rate of the hardened and carbonized carbon system is close to that of the refractory hard material in such an environment. As the refractory hard material is removed from the coating, its carbon matrix is removed at a similar or only slightly faster rate, thereby exposing additional refractory hard material to the cellular environment. In this manner, the coated cathode surface remains substantially constant, with respect to refractory hard material content, thereby improving cell operation as measured by uniformity of performance. In previous attempts to provide cathodes coated with refractory hard material, ablation and/or inter-granular attack has resulted in rapid surface degradation due to depletion of either the refractory hard material or the carbon matrix at a rate greater than the other, resulting in periods when there are limited areas that have either a surface composition that is rich in refractory hard material with sufficient bonding ability, or a carbon rich surface with insufficient refractory hard material. The present invention overcomes these shortcomings by providing a coating in which the refractory, hard material and the carbon matrix dissolve or are otherwise depleted at approximately equal rates.

It is important to clarify or distinguish between carbonization and graphitization applicable when heating carbonaceous bodies in the context of the present invention. "Carbonisation" is normally carried out by heating a carbonaceous body, either in unitary or particulate form, for the purpose of removing volatile substances, and progressively increasing the ratio of carbon to hydrogen, and to progressively eliminate hydrogen from the body. In the carbonization process, the temperature is gradually increased to allow for the slow development of volatile substances, such as decomposition products, thereby avoiding blistering, and to allow volumetric shrinkage (which will occur at some point in the operation), and progress gradually to avoid the formation of large cracks. While curing is considered to take place at temperatures up to approx. 250°C, carbonization temperatures normally range from 150 to 1000°C, although higher temperatures up to 1600 or higher can also be used. While carbonation can be continued to approx. 1000"C or higher, the carbonization of the carbonaceous materials present is substantially complete at about 800"C, and the resinous binder has been carbonized to bond the filler materials and RHM into a durable structure. The initial curing and initial steps in the carbonization operation are normally carried out in a conventional radiant or convection type furnace, heated with gas or oil, the heat being supplied to the carbon by indirect heat transfer or direct flame contact. At one point or another, e.g. above 250°C, the carbon becomes sufficiently electrically conductive to allow resistance heating, if desired.

It should be noted that the coating material is preferably in the form of a workable paste which is smoothed or leveled to a desired thickness and surface smoothness. This coating hardens by polymerization and/or cross-linking, and loses volatile substances when slowly heated to approx. 150°C, whereby the coating has reached the thermosetting stage and formed a relatively solid mass. Carbonization, at temperatures above 250°C, then converts this coating to a rigid matrix, consisting essentially of non-graphitized carbon with RHM, carbonaceous additive and carbonaceous filler therein.

A difference must be drawn between this material and previously known materials which only use pitch binders, in the absence of cross-linking or polymerisation agents. Pitch does not crosslink or polymerize by itself, but actually passes through a liquid "plastic" state or zone between 50 and 500°C in which temperature range significant swelling occurs followed by a period when the carbonaceous mass solidifies (and shrinks together) into a hard, solid body of coke. The coating material according to the present invention, on the other hand, begins polymerization and/or cross-linking at temperatures which can be as low as approx. 20° C and hardens to a relatively hard resin state at temperatures below approx. 150°C, depending on curing time and coating thickness, followed by processive hardening through carbonisation.

The entire heating cycle during carbonization is somewhat time-consuming. Carbonization typically results in the loss of volatiles and the removal of volatile reaction products from thermal decomposition. However, there is no significant change in the crystallographic structure of the carbonaceous additive or filler, and the carbonized resin continues to behave amorphously, although it may bind together a substantial amount of graphite material or material containing graphitic crystals.

Graphitization is easily distinguishable from carbonization, as described, in that it requires significantly higher temperatures and longer periods of time, and produces drastic and easily observable changes in the atomic structure. When graphitizing, the temperatures used vary from just over 2000°C up to 3000°C, with more typical temperatures varying from 2400°C or 2500<*>C to 3000°C, as these temperatures are usually associated with the higher quality grades of graphite This heating occurs over a fairly long period of time, typically around 2 weeks. The heating is carried out in a non-oxidizing atmosphere, typically by passing electric current directly through the carbon to thereby heat the carbon internally and directly with the help of its own electric resistance, as opposed to the more conventional furnace and heating method used in carbonization. Graphitization drastically changes and rearranges an amorphous or partially graphitic internal structure by developing a graphite-crystal atomic arrangement. A graphite structure possesses the well-known , densely packed, layered and specially oriented graphitic structural arrangement. Graphitization is generally only feasible with well-known te graphite precursor substances, such as pitch.

To illustrate some of the differences in internal structure when comparing graphite with non-graphitic or amorphous carbon, the d<g>o2~ °g Lc dimensions are useful. The "Lc" dimension relates to the crystal or crystallite size in the "c" direction, the direction normal to the ground plane, and the "dgg2" dimension is the interlayer spacing. These dimensions are normally determined by X-ray diffraction techniques. R. E. Franklin defines amorphous carbon with an interlayer spacing (døQ2) of 3.44Å and crystallinity of 3.35Å (Acta Crystallographica, vol. 3, p. 107

(1950) ; Proceedings of the Royal Society of London, vol. A209, p. 196 (1951); Acta Crystallographica, vol. 4, p. 253

(1951) ). During the graphitization process, the amorphous structure of the graphite precursor carbon material is changed to the crystalline structure of graphite, which is shown by an increase in the Lc dimension and a decrease in the dgg2 dimension. In amorphous carbon, the Lc dimension normally varies from 10 to 100 Angstrom units (Å), while most graphite material typically shows an Lc dimension of over approx. 350 or 400 Å, typically from over 400 Å to 1000 Å. There is another significant graphitic structure in which Lc is between 100 Å or more, up to 350 or 400 Å, and this is sometimes referred to as "semi-graphitic", as it has the same general atomic arrangement and

—configuration in its structure like the graphite just described, but differs somewhat from the more common X-ray diffraction pattern of graphite due to a small

difference 1 orientation of successive atomic planes. Both graphite structures have a doo2_dimension smaller than approx. 3.4 Å. In general, graphitization at temperatures from 2000°C up to 2400°C tends to give the "semi-graphitic" structure, while temperatures above 2400°C tend to give the "normal" graphitic structure.

An acceptable practice for the manufacture of carbonaceous coatings for aluminum cell cathodes. according to the present invention, is to use particulate graphite as a filler material which is added to the binder and other components. The mixture is then spread, hardened and carbonized. While this carbonized carbonaceous material may contain some graphite, it is not bound by the graphite, but instead contains both graphite particles from the filler and amorphous carbon derived from the binder and/or components of the carbonaceous filler. When carrying out the present invention, it is important that the carbonized cathode coating consists of a non-graphitizing binder to thereby ensure the correct combination of electrical and thermal conductivity, expansion coefficient, ablation rate and stability properties in the surface of carbon-refractory, hard metal.

While the borides, carbides, silicides, and nitrides of elements in groups IV through VI of the periodic table generally all have high melting points and hardness, good electronic and thermal conductivity, are wetted by molten aluminum, and are resistant to aluminum and aluminum-cryolite melts, TiB2 is the preferred RHM due to its relatively low cost and high resistance to oxy-fluoride melts and molten metal. Particle sizes for refractory, hard material can conveniently vary from sub-micron size to approx. 10 mesh, and preferably from sub-micron size to less than 100 mesh (0.147 mm) and most preferably less than 325 mesh (0.043 mm).

It is generally believed that grain boundaries between (TIB2 or other refractory, hard material), crystals are sensitized, ie made more susceptible to attack by the segregation of oxide impurities. Migration of oxygen dissolved in single crystals of T1B2 to crystal boundaries is also believed to be possible by diffusion processes at the temperatures at which aluminum cells are normally operated, i.e. around 1000<*>C. Thus, oxygen impurities, whether or not initially segregated at the surface of TiB2~ crystals, can migrate to the intercrystalline boundaries and render them susceptible to attack. Attack on the entire area of intercrystalline boundaries results in the loss of TiB2~ crystals.

It has now been shown that single crystals of T1B2, bi-crystals of TIB2, open clusters of crystals of T1B2, and/or crushed and ground crystals of TIB2, in intimate contact with the carbon matrix, do not crack or disintegrate when exposed to bath- and molten aluminum in aluminum cells over a longer period of time. All of this is to be included in the term "single crystals" as used here. It should also be understood that while the discussion focuses on TIB2, other RHM materials are also included. The common feature for all these particles is that they generally have more exposed, free crystal surfaces or transcrystalline fractures than internal grain boundaries between 2 or more crystals. The binder system is thus able to adhere to almost every TiB2~ crystal by binding to crystal facets or broken surfaces. The open clusters of smaller TiB2~ crystals can be penetrated by the carbonaceous binder system, so that even if the grain boundaries between TiB2~ crystals should be attacked by the bath, the TiB2~ crystals will still be held in the coating by the binder. This has been observed in broken pieces of the carbonized coating. TiB2 tends to break up in a conchoidal (or glass-like) manner, so that fragments have surfaces that cut across crystal planes, and the binder system forms strong bonds to such surfaces.

Both carbon and TiB2 components in surfaces produced according to the present invention are shown to slowly dissolve or erode so that new TiB2 crystals are continuously exposed to the molten aluminium. Both TIB2 and carbon dissolve through different chemical mechanisms in the aluminum metal in the cell, up to saturation concentrations. However, they can be present beyond the saturation limit as undissolved particles of the elements, or in combination containing the elements. The rate at which both TiB2 and carbon are lost from the cathode surface is dependent on such factors as temperatures, movement of the metal, bath and dirt, and overall bath chemistry. It is assumed that some of the TiB2 particles are carried away into the aluminum before they are completely dissolved. The TiB2 concentration in the aluminum metal is typically found to be only slightly above the solubility limit for the temperature at which the cell is operated. The rate of carbon loss is a critical factor in RHM-containing wetted cathode cells as the carbon is used to bind the RHM particles in the cathode coating. An excessively high rate of carbon loss results in undercutting of TiB2~ particles in the matrix, and a subsequent increase in TiB2~ loss from the cathode surface. Too low a carbon loss would result in areas of local depletion of TiB2 particle concentration at the cathode surface and consequently a loss of aluminum wetting. The TiB2 material preferred for use in the present invention is typically specified as less than 325 mesh (0.043 mm). If the TiB2 material is produced by carbothermic reduction of titanium and boron oxides and carbides, individual particles will usually fit the assumed category of single crystals. This also applies to TiB2 produced by plasma methods described in US-PS 4,282,195 to Hoekje in PPG Industries. Production of TiB2 by an arc melting process usually results in a polycrystal comprising relatively large individual crystals. Thus, when crushed to less than 20 mesh (0.833 mm), this would normally produce particles each having only short sections of internal grain boundary. If less than 325 mesh (0.043 mm) TiB2~ powder is produced by crushing arc-melted TiB2~ lumps, then each of the resulting particles is smaller than the original individual crystals so that each particle consists of a fragment of a larger crystal (or e.g. eg two or three crystals joined at non intercrystalline boundaries). Thus, very few crystals would be completely isolated from the binder system, and even if their intercrystalline boundaries were completely dissolved by the bath, very few crystals of TIB2 would break away.

TiB2 produced by carbothermic reduction of powders is typically composed of material that easily crushes to less than 325 mesh (0.043 mm) without breaking many of the individual crystals. When this material is viewed in a scanning electron microscope, it is seen that it is composed of very small single crystals, open clusters of TIB2~ single crystals and broken pieces of TiB2 single crystals.

Other RHM materials can be successfully used instead of TiB2 in the coatings described here when appropriate changes to the coating material are made to account for the differences in wettability, surface area, particle size, porosity and solubility of the RHM material. Sufficient RHM is incorporated into the coating material to ensure aluminum wetting, while thermal expansion mismatch effects are minimized and a refractory hard material dissolution rate is achieved that is less than the rate of loss of the carbon matrix in the coating. While discussion of the invention will focus on the use of TiB 2 as the preferred RHM material, it is understood that any suitable RHM material, such as ZrB 2 or alloys of refractory hard materials, may be used. Sufficient RHM is provided in the coating material to ensure wettability. In general, the RHM material may comprise from 10 to 90 wt-56 of the coating material and preferably from 20 to 7056. It has been found that wettability can be achieved at concentrations below 1056, but better results are obtained at ranges from 2056 and upwards, being from 35 to 6056 is the most preferred area.

The resinous binders used in the present invention may comprise any material which satisfies the previously mentioned criteria. Typical resins which may be used include phenolic, furan, polyphenylene, heterocyclic resins, epoxy, silicone, alkyl and polyimide resins. Examples of phenolic resins that can be used include phenol formaldehyde, phenolacetaldehyde, phenol furfural, m-cresol formaldehyde and resorcinol formaldehyde resins Epoxy resins that can be used include the diglycidyl ether of bisphenol A, the diglycidyl ether of tetrachlorobisphenol A, the diglycidyl ether of resorcinol and the like, and especially the novolaks. Preferred epoxy coating materials include glycidyl ethers, such as the glycidyl ethers of phenols, and especially those prepared by reacting a dihydric phenol with epichlorohydrin, e.g. the diglycidyl ether of bisphenol A, and epoxy novolac. The silicone polymers which can be used include methylsiloxane polymers and mixed methylphenylsiloxane polymers, e.g. polymers of dimethylsiloxane, polymers of phenylmethylsiloxane, copolymers of phenylmethylsiloxane and dimethylsiloxane, and copolymers of diphenylsiloxane and dimethylsiloxane. Examples of heterocyclic resins are polybenzimidazoles, polysiloxalines and pyrones. Any of the well-known specific alkyds, especially those modified with phenol formaldehyde, and polyimide resins may be used. The phenolic resins and furans are the preferred class of resins, especially in view of relatively low prices.

Furan resins are very advantageously used as the resinous binder in the present invention. Acids and bases are usually used as catalysts for furan resin polymerization, but may not be necessary as furan can copolymerize with other resins in the absence of catalysts. Suitable acid catalysts include inorganic or organic acids, such as hydrochloric acid, sulfuric acid, nitric acid, orthophosphoric acid, benzenesulfonic acid, toluenesulfonic acid, naphthalenesulfonic acid, maleic acid, oxalic acid, malonic acid, phthalic acid, lactic acid and citric acid. This family of catalysts also includes organic anhydrides, such as maleic anhydride and phthalic anhydride. Examples of further satisfactory conventional acid catalysts include mineral acid salts of urea, thiourea, substituted urea compounds, such as methylurea and phenylthiourea; mineral salts of ethanolamines, such as mono-, di- and triethanolamine; and mineral acid salts of amines such as methylamine, trimethylamine, aniline, benzylamine, morpholine, etc. Preferred acid catalysts have a Ka value of at least 10"^.

Suitable basic catalysts include alkali hydroxides, such as lithium hydroxide, sodium hydroxide, potassium hydroxide and the like; alkaline earth hydroxides, such as magnesium hydroxide, calcium hydroxide, etc. Other satisfactory base catalysts include e.g. amine catalysts, such as primary amines such as ethylamine, propylamine, etc.; secondary amines, such as diisopropylamine, triethylamine, etc. Examples of other satisfactory base catalysts are mixtures of alkali hydroxides and alkaline earth hydroxides, such as mixtures of sodium hydroxide and calcium hydroxide. A mixture of alkali hydroxides and alkaline earth hydroxides provides a thermosetting binder that gives unexpectedly high carbon yields. Appropriate amounts of catalyst or curing agents are included in the resinous binder, depending on the specific binder selected. Determination of the appropriate curing agent and the appropriate amount thereof will be realized by one skilled in the art.

By the term "acid-cured furan resins" are meant resins such as homopolymers of furfuryl alcohol, homopolymers of furfuryl alcohol cross-linked with furfural, copolymers of furfuryl alcohol and formaldehyde, copolymers of furfuryl alcohol and phenol, copolymers of furfural and phenol, or monomeric materials containing the furan ring somewhere in the structure, and which can be hardened to a final hardened and hardened mass by adding an acid catalyst thereto.

Resin conversion of the compositions in question is dependent on hydrogen ion concentration and is accelerated by heating, as is well known. Because the reaction caused by the addition of the acid catalyst to the resin is exothermic, accuracy must be exercised in the choice and amount of catalyst used, otherwise the resin conversion may proceed too quickly and the resulting mass may become unusable. For this reason, inorganic acids such as hydrochloric acid, sulfuric acid, phosphoric acid and chromic acid are usually not used. On the other hand, inorganic acid salts such as zinc chloride, sodium bisulfate and mercuric chloride are often used, which is also the case for latent acid catalysts, such as maleic anhydride and phthalic anhydride, which only upon heating provide the necessary acid catalyst.

In general, ordinarily organic compounds are preferred for achieving the desired properties in acid-cured furan resin products. In addition to the aromatic sulfonic acids, aromatic sulfonochlorides, such as benzenesulfonochloride, para-acetylbenzenesulfonochloride; the aliphatic amino salts of the aromatic sulfonic acids, including the ammonium salts, such as ammonium paratoluenesulfonate, dimethylaminobenzenesulfonate, diethylaminotoluenesulfonate, ammonium benzenesulfonate and

—disulfonate, ammonium phenolsulfonate, ammonium naphthalenesulfonate and —disulfonate, ammonium anthracene sulfonate and —disulfonate, ammonium sulfanylate; the aromatic amino salts of aromatic sulfonic acids, such as the aniline salt of benzenesulfonic acid, the aniline salt of paratoluenesulfonic acid and the pyridine salt of phenolsulfonic acid; the organic salts of strong inorganic acids, such as glyoxal sulfate; the metallic salts of chlorosulfonic acid, such as sodium chlorosulfonate and potassium chlorosulfonate; the aliphatic and aromatic salts of strong

inorganic acids, such as triethanolamine chloride, aniline hydrochloride, ammonium sulfamate, pyridine sulfate, pyridine bisulfate, and aniline sulfate; the amino salts of sulfanilic acids, such as aniline sulfanilate and pyridine sulfonate; the ferric salts of sulfonic acids, such as ferric trichlorobenzene; the acid anhydrides, such as phosphoric anhydride and maleic anhydride; the ammonium salts of alkanesulfonic acids, such as ammonium methanesulfonate; the ferric salts of sulfonic acids, such as ferricbenzene sulfonate and ferrictoluene sulfonate; and the ammonium salts of organically substituted inorganic acids, such as ammonium methyl phosphate, have been used as catalysts for furan resins.

Alkaline catalyzed furan binders conveniently include furfural plus a phenol, furfural plus a ketone, and furfuryl alcohol plus an aldehyde and an amine. The furan binder can optionally be mixed with pitch to form a binder material. This binder material can be mixed with a carbon aggregate or filler. Pitch materials, such as coal tar pitch, may be present as a modifying component of the binder system, and high-boiling condensed aromatic systems from such are often found as impurities in filler materials derived from pitch. Furan binders as intended herein suitable for use in the present invention include the following (with suitable catalysts): a. Furfural plus a phenol; particular example: furfural and

phenol;

b. furfural plus a ketone; examples: furfural and acetone

or furfural and cyclohexanone;

c. furfuryl alcohol plus an aldehyde and an amine; special

Example; furfuryl alcohol and formaldehyde and urea.

An example of a furfural binder which is suitable for use in the present invention and which is relatively liquid at room temperature and has a suitable charring yield, is a mixture consisting essentially of between 50 g 75 wt-£ coal tar pitch with a softening point above 100°C, and between 50 and 25 wt-56 monomer polymerizable thermosetting dispersants consisting of furfural and a material selected from the group consisting of phenol, cyclohexanone and compounds of the formula:

where R is a hydrocarbon group with between 2 and 4 carbon atoms, inclusive, and a catalyst. The hydrocarbon group in the above formula may be saturated or unsaturated, and straight or branched. Suitable amounts of RHM and carbonaceous fillers can then be added to this binder to obtain an effective coating material. The time and temperature required to solubilize the coal tar pitch in the dispersants will vary with the softening point of the coal tar pitch, the total amounts of the dispersants, and the relative amounts of the different dispersants. Excessive heating should be avoided to prevent premature polymerization of the binder. While the time and temperature parameters necessary to solubilize the coal tar pitch in the dispersants cannot be generally stated, they can be readily determined by one skilled in the art. In this type of coating material, pitch does not act as a modifier for the binder, but rather as a functional component that has a desired high charring yield.

In general, molar ratios of furfural to phenol between 0.5 and 2 moles of furfural to 1 mole of phenol are preferred when phenol is the second component of the dispersant component; and molar ratios between 1 and 2 moles of furfural to 1 mole of cyclohexanone, methyl aliphatic ketone of the above formula, or mixtures thereof, are preferred when cyclohexanone, methyl aliphatic ketone or mixtures thereof are the second constituent of the dispersant component.

A currently commercially available alkaline-catalysable furan binder useful in the present invention is that obtainable from the Quaker Oats Company under the commercial or trade name "QX-362". It is now assumed that the main ingredients in this binder are furfural and cyclohexanone, although some furfuryl alcohol may also be present.

A group of furfuryl resins which are considered suitable in the present invention are furfuryl alcohol copolymers prepared by reacting maleic acid or maleic anhydride with a polyhydroxy compound, such as methylene glycol. This forms an ethylenically unsaturated polycarboxylic acid ester prepolymer. The ester prepolymer is then copolymerized with furfuryl alcohol to produce the furfuryl alcohol copolymer. The described furfuryl alcohol copolymers are advantageous due to certain properties such as relatively low volatility, ease of storage, release of minimum water during curing, resistance to excessive shrinkage and a relatively short curing reaction. Such copolymers may also be quite suitable for use in accordance with the present invention due to other properties, namely their ability to remain highly flexible after curing.

In addition to those listed as components of the commercially available carbon cements, such as UCAR® C-14, as discussed below, a number of different novolak resins can be used as the basic resinous binder in the present invention. The term novolak refers to a condensation product of a phenolic compound with an aldehyde, the condensation being carried out in the presence of an acid catalyst and generally with a molar excess of phenolic compound to form a novolak resin in which there are practically no methylol groups, such as present in resols, and in which the molecules of the phenolic compounds are bound together by means of a methylene group. The phenolic compound may be phenol or phenol in which one or more hydrogens have been replaced by any of various substituents attached to the benzene ring, some examples being the cresols, phenylphenols, 3,5-dialkylphenols, chlorophenols, resorcinol, hydroquinone, xylenols, etc. The phenolic compound may instead be naphthyl or hydroxyphenanthrene or another hydroxyl derivative of a compound having a fused ring system. It should be noted that the novolak resins are not thermosetting per se. Novolak resins are cured in the presence of curing agents, such as formaldehyde with a base catalyst, hexamethylenetetramine, paraformaldehyde with a base catalyst, ethylene diamine formaldehyde, etc.

For the purposes of the present invention, any fusible novolak which can be polymerized further with a suitable aldehyde can be used. Put another way, the novolak molecules should have two or more accessible sites for further polymerization and/or cross-linking. Apart from this limitation, any novolak can be used, including modified novolaks, i.e. those in which a non-phenolic compound is also included in the molecule, such as diphenyl oxide or bisphenol-A modified phenol-formaldehyde novolak. Mixtures of novolaks can be used or novolaks containing more than one type of phenolic compounds can be used.

Novolaks generally have a number average molecular weight in the range of 500 to 1200, although in exceptional cases the molecular weight may be as low as 300 or as high as 2000 or more. Unmodified phenol formaldehyde novolaks usually have a number average weight in the range from 500 to 900, most of the commercially available materials falling within this range.

Novolaks with a molecular weight of 500 to 1200 are preferably used in the present invention. When a novolak with a very low molecular weight is used, the temperature at which such novolaks soften and become sticky is usually relatively low. A mixture of pitch and novolak can be formed by any suitable technique, such as dry mixing or melting the pitch and the novolak by heating together to form a homogeneous mixture. Different pitch materials can be used for this purpose as mentioned earlier. In addition to the novolak-phenolic resins, the use of resols is also within the scope of the present invention.

Resol resins are most often produced by condensation of phenols and aldehydes under alkaline conditions. Resols differ from novolaks in that polynuclear methylol-substituted phenols are formed as intermediates in resols, and resols are produced by reacting phenolic substances with excess aldehyde in the presence of an alkaline catalyst. A resol produced by condensation of phenol with formaldehyde most likely proceeds through intermediate products that have the following structural type:

In a typical synthesis, resoles are prepared by heating 1 mol of phenol with 1.5 mol of formaldehyde under alkaline conditions.

The resol resins are produced by condensation of phenols with formaldehyde, or more generally, by reaction of a phenolic compound having 2 or 3 reactive aromatic ring-hydrogen positions, with an aldehyde or an aldehyde-releasing or inducing compound which can undergo phenolaldehyde condensation. Illustrative of phenols are phenol, cresol, xylenol, alkylphenols, such as ethylphenol, butylphenol, nonylphenol, dodecylphenol, isopropylmethoxyphenol, chlorophenol, resorcinol, hydroquinone, naphthol, 2,2-bis(p-hydroxyphenyl)propane and the like, and mixtures of such phenols. Large aliphatic groups substituted on the benzene ring can lead away from the present invention because these are lost during heating and can thus reduce charring yield and increase the emission of volatile substances. Illustrative of aldehydes are formaldehyde, paraformaldehyde, acetaldehyde, acrolein, crotonaldehyde, furfural, etc. Illustrative of aldehyde-inducing agents is hexamethylenetetramine. Ketones, such as acetone, can also be condensed with the phenolic compounds to form phenolic resins.

The condensation of a phenol and an aldehyde is carried out in the presence of alkaline reagents such as sodium carbonate, sodium acetate, sodium hydroxide, ammonium hydroxide and the like. When the condensation reaction is complete, if desired, the water and other volatile materials can be removed by distillation and the catalyst neutralized.

The resol resins denoted thermosetting resins. That is under the application of heat, these resins progressively polymerize until they are finally insoluble, non-fusible and fully cured. For the purposes of the present invention, the curable phenolic resins are considered to be those which have not progressed so far in their polymerization that they have become non-fusible.

Furfuryl alcohol can be used as the mixing liquid in the phenolic carbonaceous binder, and is believed to react with the phenolic resin as it cures, serving as a resin modifier. The use of furfuryl alcohol is preferred as it has been found that bonds having the high strength that can be obtained using this mixing liquid cannot be produced when other mixing liquids are used instead of furfuryl alcohol. When, for example, furfuraldehyde is used instead of furfuryl alcohol in otherwise identical compositions, bonds are formed which only have about half the strength of the bond formed when furfuryl alcohol is used.

Since the desired net end effect is to achieve a surface layer composed essentially of RHM and carbon, the binder system should be easily decomposable, in high yield, to a carbon residue. Such components as resinous binder should comprise from 1 to 4056 of the coating material either as part of the carbon cement or as a regular carbon system. The resin per se can amount to 5056 or more, calculated on the weight of the coating material. Although higher resin concentrations are possible, no particular advantage is gained and longer curing and carbonization cycles may be required. The carbon system should comprise from 10 to 9056 of the coating material, preferably from 30 to 8056, and most preferably from 40 to 6556 of the coating material applied to the substrate.

One can use suitable mixtures of carbon and phenolic resin or other thermosetting resinous binders of suitable particle sizes, or alternatively commercial compositions. The mixing fluid component of the coating material can vary from 2 to 40 wt-56 for reasonable evaporation and curing rates, with from 5 to 2556 being preferred for achieving workable consistency. It is desirable that the coating material is workable and can be spread easily, as with a trowel. Insufficient liquid will make the mixture dry and non-spreadable, while too much liquid results in difficulties in curing and firing.

Various modifying agents may be present to modify the nature of the resinous binder during mixing, curing and carbonization of the coating material. These can typically make up from 0 to 10% by weight of the coating material. Suitable modifying agents for e.g. phenol formaldehyde resins, include rosin, aniline, copolymers, resin "alloys", etc. Rosin-modified phenolics have an important combination of solubility, viscosity and drying properties. A well-known method of producing phenolic resins for use in surface coatings involves heating and mixing phenol-formaldehyde condensates with rosin. The condensate used is preferably one produced from alkylphenols by alkaline condensation.

Aniline-modified phenolic substances are produced by treating a phenol-formaldehyde condensate intermediate with aniline or aniline derivatives, by co-condensation, or by mixing a phenol-formaldehyde condensate with an aniline-formaldehyde condensate. Aniline-modified phenolic substances have particularly good electrical properties. Phenolic resins can be copolymerized with such materials as chlorinated phenols, nitromethane and organosilicon compounds, e.g. siloxane.

Some of the materials used for the treatment of phenol formaldehyde intermediate condensates are epichlorohydrin, ether resin, ethylene oxide polymers, hydrogen peroxide, ketones, methylaniline HCl, polyvalent salts of hexanoic acid, stannous chloride and terpene phenol products.

In addition, mixtures of various resinous materials with phenolic resins can be prepared. Examples of the more common resin "alloys" are phenolic resins and epoxy resins, ketone-aldehyde condensates, melamine and urea resins, natural and synthetic rubber, polyvinyl chloride and polyvinyl acetate resins. Through such modifications, it is entirely possible to significantly modify the binder system according to the present application. Replacement of e.g. phenol with meta-resole will give a resin which is soluble in alcohol, and with a fast drying time. This can be beneficial under certain conditions and curing time conditions. Replacing phenol with meta-alkyl phenol gives a resin that is more rubbery and flexible, but has less tensile strength. Replacement of phenol with substituted phenols, e.g. p-tertiary butylphenol gives resins that are oil-soluble, while replacing phenol with naphthalene- and anthracene-derived phenols does not change the phenolic properties to a greater extent, but can provide greater compatibility with pitch, in binder systems that use pitch. Replacement of formaldehyde with higher aldehydes, such as acetaldehyde, results in the resin becoming oil soluble.

The addition of glycerol to phenol formaldehyde resols acts as a plasticizer in the binder systems used. Anthracene oil, on the other hand, can be added to furfuryl alcohol as a stabilizer prior to the addition of acid catalyst. Stabilization helps control the polymerization rate in the presence of acid catalysts.

Certain special blends are worth noting. E.g. Phenol formaldehyde modified with phenol furfuryl resin has a flat plasticity curve and does not go through a rubbery stage as is the case for phenol formaldehyde. Thus, mixtures of phenol formaldehyde with phenol furfuryl are suitable. They provide excellent impact resistance, chemical resistance, rapid curing at high temperatures and a high capacity for filler. Mixtures of phenol formaldehyde with certain thermoplastic substances give resins which are suitable for bonding to metal, such as may be used with metallic cathode substrates. Blends with butadiene/acrylonitrile copolymer rubber provide improved impact resistance, while blends with resorcinol-formaldehyde resins provide faster reaction rates and lower curing temperatures. However, these resins are somewhat expensive for use in the present invention.

Pitch is often present in the coating material as a modifier or binder, in concentrations up to 50 or even 7556 when present as a functional component of the binder system. When present in concentrations up to 10 wt-56 of the coating material.

In addition to the RHM and the binder system, which itself may include a filler material, it is desirable to provide additional particulate carbon. Some particulate carbon, either amorphous or graphitic, is often present in commercially available cements as mentioned above. Furthermore, particulate carbon can be added, either as a fine powder or as a coarse aggregate, or mixtures thereof, in the form of amorphous carbon or graphitic carbon.

It is highly desirable to have a carbonaceous filler material present, either as a component of an exclusive carbon system or present in a commercial cement, or as an addition to a commercial cement. Such carbonaceous fillers are less than 100 mesh (0.147 mm) and preferably less than 325 mesh (0.043 mm) and may include fine carbonaceous meal, graphite meal, crushed coke, crushed graphite, carbon black, and the like. The presence of such fine flour provides improved packing density for the granulometry used, which, through a soaking action, absorbs resin-forming liquids to form a dense, strongly bound carbon matrix during carbonization.

Carbonaceous filler such as fine flour should comprise from 1 to 6056 of the coating material, with from 10 to 4056 being preferred.

The carbonaceous additive or aggregate material, if present, may be from less than 4 mesh (4.699 mm) to over 100 mesh (0.147 mm), and is preferably from less than 8 mesh (2.362 mm) to over 20 mesh (0.833 etc.). Such a coarse aggregate obviously allows micro-cracking, helps the release of volatile substances, reduces shrinkage and contributes to a high carbon yield. Carbonaceous additive, such as aggregate and/or fiber, should comprise from 0 to 7056 of the coating material, with from 5 to 1556 being preferred.

As mentioned earlier, it is preferred that carbon fiber is added to the coating material which is a crack stopper. When such fiber is used, certain variations have been found in composition areas. When carbon fibers are used, they can preferably be produced from pitch precursors, organic fiber precursors, such as polyacrylonitrile or rayon. Bek fibers are considerably cheaper and therefore preferred.

Fiber weight may vary from 0 to 10 weight-56 of the coating material, preferably from 0.05 to 1.0%, and more preferably from 0.05 to 0.556. Concentrations above approx. 10& will, however, be relatively expensive, but little clearly added benefit. Carbon fibers with lengths varying from 0.16 to 1.27 cm are preferred. Short fibers allow easier mixing and application, and can be used in higher concentrations. Bonded fibres, consisting of parallel fiber threads bound together by means of a material which is soluble in the mixing liquid, are particularly preferred, since they mix very easily with the binder system. Fiber orientation can vary and the fibers can be mixed as an integral part of the coating material to facilitate the application procedure.

It is also possible to modify the carbonaceous filler used. In this regard, a variety of modifiers can be added to the filler. E.g. silica can be added to the carbonaceous filler to give the binder system non-sintering properties. In general, however, inorganic fillers make the resinous binder more difficult to process according to the present invention, and reduce the charring yield.

Gas releasing agents are suitably included in the coating material to avoid blisters and/or excessively large cracks. Suitable gas release agents include flammable oils, soaps and waxes.

A flammable oil can be used as a gas releasing agent in the carbonaceous binder. To avoid volatilization of the oil while the phenolic resin is curing, it is desirable that the oil has a boiling point that is higher than the resin's curing temperature. For this reason, oils with a boiling point above 150°C, preferably above 200°C, are most useful, with oils having a boiling point above 250°C being particularly preferred. While petroleum-based oils, such as paraffin oils, aromatic oils and asphalt oils are preferred, other oils, such as animal and vegetable oils, can also be used. Among the petroleum-based oils, the paraffin oils are preferred. Illustrative of animal and vegetable oils that can be used are palm kernel oil, olive oil, peanut oil, beef tallow oil, cottonseed oil, corn oil, soya oil, etc. Amounts of oil up to 3 weight-56 of the coating material, preferably from 0.4 to 2.0 weight-56, are suitable.

Soaps can also be used as gas release agents. While the soap used may be any of the metal-one quaternary ammonium salts of the fatty acids, binders made with either the neutral or acid quaternary ammonium soaps are more resistant to oxidation than cements made with the more common metal soaps, so that the use of non-metallic soaps are preferred. Such non-metallic soap is produced by reacting a fatty acid with triethanolamine. The fatty acids used, such as fatty acids used for the production of metal soaps, usually contain from 10 to 24 carbon atoms, and can be either saturated or unsaturated. Among the saturated fatty acids that can be used are capric acid, lauric acid, myristic acid, palmitic acid, stearic acid, arachinic acid, behenic acid, tetracosanoic acid and others. Typical unsaturated fatty acids include palmitoleic acid, oleic acid, linoleic acid, arachidonic acid, cetoleic acid, eucinic acid, selacoleic acid and the like. Amounts of soaps from 0 to 15 wt-56 or higher of the coating material, preferably from 0.5 to 5.0 wt-56, are suitable.

Various waxes can also be used as gas release agents. Suitable waxes include various grades of petroleum wax including such common paraffin waxes as refined paraffin mud, sweated paraffin, paraffin flakes, paraffin block and microcrystalline wax.

A preferred binder system is that commercially designated as UCAR® C-14, marketed by Union Carbide. This composition is believed to comprise a mixture of an oil, a soap, finely divided carbonaceous particles, furfuryl alcohol, a phenolic resin of the novolak type, and a curing agent for the phenolic resin. The mixture of the oil, the finely divided carbonaceous particles, the phenolic resin and the phenolic resin hardener can be prepared by mixing the carbonaceous particles, the phenolic resin and the phenolic resin hardener in any conventional manner, e.g. in a thumb container, spraying the oil into the resulting mixture and further mixing the material until the oil has been incorporated therein and a substantially homogeneous mixture is formed. The mixture of soap and furfuryl alcohol can be prepared by heating the soap to a temperature of approx. 100°C to liquefy this, and then dissolve the molten soap in the furfuryl alcohol. Upon cooling, the soap remains dissolved in the furfuryl alcohol as a stable solution which can be stored until it is ready to be mixed with the mixture of oil, finely divided carbonaceous particles, phenolic resin and phenolic resin hardener. The two mixtures, one liquid and the other substantially solid, can be easily mixed at room temperature, either manually or mechanically.

Many phenolic resins of the novolak type can be used in UCAK® C-34 cement. Such resins are produced by condensation of phenols, such as phenol itself, m-cresol, p-cresol, o-cresol, 3,5-xylenol, 3,4-xylenol, 2,5-xylenol, p-ethylphenol, p — tert-butylphenol, p-amylphenol, p-tert-octylphenol, p-phenylphenol, 2,3,5-trimethylphenol, resorcinol, etc., with aldehydes, such as formaldehyde, furfuraldehyde, acetaldehyde, etc. In practice, an unsubstituted phenol formaldehyde resin can be used based on price considerations. Curing the novolak resin to the thermoset state can be effected by any curing agent conventionally used to cure such resins. Such curing agents are conventional materials, such as paraformaldehyde, furfural or hexamethylenetetramine, with suitable catalysts when necessary, which on application of heat develop aldehydes which react with the resin and cause it to be cross-linked. The novolak resin is suitably used in the UCAR® C-34 cement in an amount from 0.5 weight-* of the coating material to 15 weight-*, most preferably from 2.5 to 8 weight-*. The curing agent for the resin is used in an amount sufficient to cure such a resin to the thermoset state, i.e. in an amount which will provide at least sufficient formaldehyde to react with and cross-link the resin, and provide sufficient alkaline catalyst for the reaction.

Many forms of finely divided carbon or graphite can be used as components in said UCAR® C-34 carbonaceous cement. Suitable carbon-containing materials include graphite meal, petroleum coke meal, stream coke meal, calcined lamp soot meal, thermal carbon black (produced by passing natural gas over hot, refractory substances), etc. Amounts of the carbonaceous flour from 1 weight-* of the coating material to 60 weight-*, preferably from 10 to 40 weight-*, are suitable. The carbonaceous flour is most preferably a mixture of graphite and thermoatomic carbon black, the graphite flour being present in an amount of from 2 to 50 weight-* and the thermoatomic carbon black being present in an amount of from 0.5 to 25 weight.

Appropriately, furfuryl alcohol is used in UCAR® C-34 cement, and can be present in an amount from 2 weight-* of the coating material to 40 weight-*, most preferably from 4 to 20 weight-

Additional suitable carbon cements and commercially available, such as UCAR® C-38 (Union Carbide), a material very similar to UCAR® C-34 but containing an oxidation inhibitor; Stebbins AR-25-HT, a furan resin composition comprising furfuryl alcohol, partially polymerized furfuryl alcohol in forms such as difurfuryl ether, and a latent catalyst which may be phthalic anhydride or a derivative thereof; Stebbins Ar-20C, comprising a partially polymerized furan resin in forms such as difurfuryl ether, together with furfuraldehyde, and a latent catalyst which may be phthalic anhydride or a derivative thereof; and Atlas CARBO-KOREZ, a phenolic resin composition comprising a phenolic novolak or resol resin in a solvent as a mixing liquid, cured by combination with a phenolic novolak in the solids. The solvent is likely to be an aliphatic alcohol, such as butyl alcohol. Other suitable carbon cements include Atlas CARBO-ALKOR, comprising furfuryl alcohol monomer and partially polymerized forms such as difurfuryl ether and a latent catalyst which may be phthalic anhydride or a derivative thereof; DYLON GC, comprising furufuryl alcohol as a solvent and monomer which cures together with a phenol formaldehyde and hexamethylenetetramine curing agent; and Aremco GRAPHI-BOND® 551-R, comprising furfuryl alcohol and a latent catalyst.

The coating material can be applied to the cathode of an aluminum cell as a single layer or multiple layers. A coating system consisting of several layers can provide a stronger bond due to greater penetration into the pore structure of the carbon cathode by a first bonding layer that does not include T1B2, and allows easier and faster curing of the coating. Furthermore, the use of multiple layers can also reduce the size and number of shrinkage cracks in the TiB2-containing top layer. In a further preferred embodiment, the coating material can typically comprise up to 10 weight-* of carbon fiber which inhibits cracking, strengthens the coating and reduces any tendency for peeling of the coating, especially at any point of contact with the bath. The carbon cement applied to the cathode substrate as a bonding layer can contain up to 40* additional carbonaceous filler and additive, which helps prevent cracking of the substrate due to tensile forces occurring during curing and carbonization of the coating, by modifying the strength of the bond between the substrate and the binding layer, and the properties in the binding layer.

The coated cathodes described here can be successfully used in conventional aluminum cells or in cells in which the anode and cathode are inclined relative to the horizontal plane. Such aluminum cells have been proposed in earlier references, such as Lewis et al., US-PS 3,400,061; Holliday, US-PS 3,661,736; and Payne US-PS 4,093,524. Such configurations are claimed to have the advantage of easy drainage of the aluminium, so that there is only a thin film of aluminum on an aluminium-moistened cathode surface, thus allowing much narrower anode-to-cathode distances. However, previous attempts to achieve commercially acceptable coated inclined cathodes, or RHM tile surfaces for cathodes, have not been successful.

In addition, the idea described here is also applicable to aluminum reduction cells where non-consumable anodes are used. Such anodes may consist of porous electronically conductive material, such as platinum black, separated from the molten electrolyte by a ceramic oxygen ion conductive layer which is impermeable to and resistant to the electrolyte. The oxygen ions are led through the oxide layer and discharged at the anode, forming oxygen gas. Silk anodes are described in Marcinek, US-PS 3,562,135.

In US-PS 3,578,580 to Schmidt-Hatting and Huwyler, anodes are described based on similar principles, but including either molten silver or an auxiliary electrolyte such as molten PbO, and a solid electrical conductor or platinum. Anodes based on SnO 2 are described in Klein, US-PS 3,718,550; and Adler, US-PS 3,930,967, 3,960,678 and 3,974,046. In addition, De Nora et al. described, in US-PS 4,089,669, the use of sintered electrodes with a matrix of Y2O3 and other compounds with electronic conductivity and with electrocatalytic material on the surface. More complex oxide anodes for Hall-Heroult aluminum electrolysis, based on oxides with spinel, perovskite, delafossite, pyrochlore, scheelite and rutll structure with electronic conductivity, and mixtures thereof, are described in patents to Yamada et al., US-PS 4,039,401 , GB-PS 1,508,876, Japan Kokai 77,140,411 (1977) and Japan Kokai 77,153,816 (1977).

The advantages of inert anodes are less ACD and more compact cell construction, which has less heat loss. While inert anodes have so far shown a disadvantage in terms of initial electrical energy requirements compared to consumable carbon anodes, inert anodes will have less irregular surfaces and significantly longer lifetimes, thereby avoiding the necessity to change anodes frequently with accompanying disruption of cell operating conditions.

Such anodes can be used in conjunction with cathodes that have aluminum wettable surfaces, as described herein, with advantages such as narrower ACD openings, and extended lifetime. A further advantage may be expected from the use of such anodes in combination with inclined or drained cathodes having RHM carbon coated surfaces as taught herein.

It is also thought that the cell specified in the present invention may comprise a significantly thinner carbon cathode block than those currently used in conventional aluminum cells. Considering the potential for extended coating life that is available, the necessity for thick cathode blocks is reduced, resulting in a reduction of temperature in the conductive medium used, and substantial cost reduction and power savings. Furthermore, it is included that cells according to the present invention can utilize non-carbon-containing cathode substrates protected by the aluminum wettable REM surface layer described here.

The improved operation of full scale (105K amp) VSS aluminum reduction cells constructed with coated cathodes of the present invention continues to demonstrate the productivity improvements resulting from the invention. While the examples set forth below illustrate the preferred specifications for the improved coating material and method, it is possible to achieve a viable coating by many similar methods. The examples were prepared by coating the cathode blocks prior to their assembly on the collector rods and the cell stamping process. In commercial practice, it is possible that an i-cell coating would be used to coat all or part of the bottom and sidewall rafts in a stamped cathode. The entire cathode coating could then be hardened and carbonized in a single operation using e.g. a shielded heating device located above the coated stamped cathode. This could result in savings in time and coating costs, and provide a completely monolithic cathode without stamp joints, as well as a reduction in emissions during firing and improvement of the firing process to achieve an improved burnt stamp material in the slots and side walls.

The coating material can be applied to each cathode block, cured and carbonized before being placed in position. Alternatively, the cathode can be assembled and stamped, the coating applied and then cured. The carbonization process would take place in the cell in this case. Curing can be achieved in stages whereby the coating substrate is gradually raised to the desired curing temperature whereby a relatively hard surface is achieved, followed by carbonisation at temperatures up to 1100° C. In the initial stages of such curing, volatile components, such as volatile substances in the mixing liquid and the reaction product removed, while at higher temperatures, e.g. 250-1100°C carbonaceous materials, such as cross-linked phenolic resin, decompose, leaving a non-graphitizing, carbonaceous matrix containing RHM. This carbonization step can be carried out directly after the initial curing by heating the coated carbon substrates to the desired temperature, or after said cathode substrate is placed in the electrolytic cell. Alternatively, the carbon cathode block can be placed in position in the cell after coating and initial curing, followed by "baking in" of the coated cathode at cell startup and operation. Alternative sequences for coating, curing, carbonisation, assembly, assembly and stamping can of course be used.

The coated area can vary from the entire inner surface of the cathode cavity to less than 10* of the cathode surface under the anode or anodes. The area to be coated varies from 50 to 100* of the cathode surface directly below the anode or anodes, the preferred area varying from 70 to 100* of said area. However, it may be desirable to leave some of the area uncoated to allow degassing of the cathode stamp material during e.g. cell heating and

— start-up.

The RHM coating need not be continuous over the entire cathode surface. In the case of TiB2 tiles, small spaces between adjacent tiles (1-5 mm) will be bridged by the molten metal. Likewise, TiB2~ particles in a carbon surface at a suitable concentration will give a pseudo-continuous aluminum wetting film by bridging between adjacent TiB2~ particles. In the case of the coating material according to the invention, 20 wt-* T1B2 in the surface will give a pseudo-aluminium-wetted surface. A preferred total content of T1B2 in the surface layer of 35-60 wt-* will allow mixing of inhomogeneities and a longer coating life. Modification of the RHM particle properties and/or changing the coating formulation and/or the RHM distribution in the coating may enable the use of smaller amounts of RHM. Cracks in the coating should be less than 5 mm in width, preferably less than 1 mm wide.

Two different types of TiB2 powder have been used in the test cells. No difference could be detected in the mixing and coating methods when the pure (+99.5) T1B2 was replaced with less pure (98*) material. A carbothermic process was used to produce the higher purity powder while an arc melting process was used to produce the cheaper, less pure powder.

While no minimum or maximum coating thickness has yet been defined, thicker coatings provide longer coating life. However, the tendency is greater for cracking and higher coating costs with thicker coatings. The preferred coating thickness is from 1 cm to 1.6 cm to minimize the tendency for blistering or cracking of the coating during curing and carbonization. The maximum coating thickness should be 1 in accordance with the expected cell lifetime, i.e. there is no need for a coating thickness to last for 10 years if the cell lifetime is expected to be only 7 years.

It is also included that the coating material can be applied as a single layer or as several layers, which layers can be cured individually between applications. According to this feature, it is possible to produce a substantial coating thickness (eg 5 cm or more) by successively applying thin layers of coating material and curing such layers individually. For greatest bond strength, it may be desirable to treat the surface of each hardened layer by scraping or steel brushing, e.g. prior to applying the next layer. It is also possible using this technique to achieve a graded RHM content in the coating by changing the RHM concentration in successive layers of the coating material.

The general procedure used to coat individual cathode blocks, assemble the cathodes and connect cells is summarized below. 1. Steel brushing and vacuum cleaning of the top surface of the cathode blocks.

2. Attach the shape around the top of each block.

3. Mix dry coating components.

4. Preheat coating materials and blocks.

5. Mix the coating material.

6. Apply a thin coating and work it into the block surface. 7. Complete coating application and use a rod to level the coating with the top of the mold. 8. Wait 15-30 minutes before using a metal trowel to partially smooth the coating surface. 9. Insert a thermocouple into the side of the coating and place coated blocks in a curing oven.

10. Curing.

11. Remove coating forms from the blocks.

12. Place the hardened blocks in a metal box, cover with coke and heat for partial carbonization.

13. Cool the blocks before removing them from the cooking layer.

14. Cast the blocks on collector rods.

15. Stamp the collector rod assemblies into the cathode shell and stamp the side walls. 16. Brush away excess tamping material from the coated block surface. 17. Follow conventional methods of cathode installation, burning and connecting.

The uniformity, workability and bonding properties of the coating are strongly influenced by temperature. The preferred premix temperatures for the mixing liquid and solids of the coating material are 40-45°C and 30-35°C, respectively. A premix temperature range of 20-45°C has been used for the mixing liquid and solid components without any serious problems. The temperature of the cathode block or the surface to be coated should be between 20 and 65°C, preferably between 40 and 50°C.

A low premix temperature resulted in non-uniform mixing and the need to add more solvent to make the coating workable. Both parts produced small blisters in the coating. A high premix temperature causes a partially premature curing of the coating, which resulted in poor machinability and poor bonding to the carbon substrate.

A low block temperature made it difficult to spread the coating evenly and achieve the necessary wetting of the carbon substrate that is needed for good bonding. A high block temperature partly caused premature hardening and too strong shrinkage in the coating. Too much shrinkage increased the tendency for cracking and delamination from the carbon substrate.

As indicated, there are certain ranges of acceptable temperatures for preheat treatment of the coating material and cathode blocks. It is also noted that the coating thickness can vary from about 0.6 to 1.6 cm, or higher. Preferred areas are indicated in Table I.

The tendency for blistering in the coating was influenced by the degree of and technique used for finishing the top surface of the applied wet coating. A finished smooth surface achieved by either dry or wet processing of the coating surface showed a greater tendency for blistering than an uneven surface applied with a trowel. A smooth surface promoted rapid formation of a film that sealed the surface and inhibited the release of blistering gases developed during curing. In contrast, imperfections in the partially smoothed surface promote the release of gas during the curing cycle. The surface of 8 blocks coated with the formulations indicated in example 6 was finished and made smooth by both wet and dry processing, and blisters were observed in the coatings of these 8 blocks after curing. Blister-free coatings on 52 subsequent blocks using the same formulation were achieved by only partially smoothing the surface of the wet coating.

The surface texture for different finish treatments of coatings produced from the coating material in example 8 has also been characterized. The technique in this case involved taking a 2 cm x 2 cm x 2 cm sample of coated block and placing it in an epoxy resin filled with white powder to achieve a white background against the black coating. The assembled sample was then sectioned and polished to reveal the detailed profile of the coating surface. This was photographed at 12 times magnification and the contour of the coating surface was recorded on ordinary paper. "Typical" 5 mm sections of this profile were then analyzed expressed as a maximum peak-to-trough height and averaged. Values for these measurements are given in table II. It was noted that coated cathodes given a smooth finish in fully dry and fully wet conditions showed blistering, while those given a semi-smooth finish resulted in an acceptable coating. One can therefore imagine that a surface structure with average peak-to-valley heights of less than 0.65 mm should be avoided.

The high porosity of graphite particles (UCAR® BB-6) or other equivalent porous aggregate, promoted the escape of solvent and other gases during the curing and carbonization cycle, compared to that of regular coke material, such as UCAR® 6-03 coke. Addition of a porous aggregate to the coating reduced the tendency for blistering and the extent of coating shrinkage during curing. Too much shrinkage weakens the bond strength between coating and substrate, and promotes crack formation.

Example 1: Carbonization yield.

Accurately known weights of mixing fluid and solid components for various carbon systems were mixed in a Teflon®-coated mold with dimensions 5.1 cm x 1.27 cm x 0.63 cm deep. When the system included a separate curing agent, the weight of curing agent was included with the weight of the mixing liquid. Each carbon system was run through the normal cure cycle, then weighed and removed from the mold. The cured piece was then heated to constant weight in an alumina crucible at 250°C in air, and the weight was corrected for any loss that might have occurred when the piece was removed from the mold (eg, breakage, dusting, or adhesion to the mold). The extra polished weight was thus the "real" weight that the sample would have achieved at 250°C in the absence of physical treatment. The piece was then sintered at 1000°C over a period of 24 hours in an alumina crucible under an argon atmosphere. After cooling in the oven, the sample was weighed and the weight was again corrected to correspond to the original weight of composition (instead of the weight used in the sintering phase). The adjusted char weight thus represents the total char weight resulting from the original sample.

An accurately determined weight of the original solids was extracted with boiling methyl ethyl ketone (MEK) in a soxhlet extractor for 4 hours, or longer if the extract continued to come out colored after that time. The weight of extracted material was determined by evaporating the rich solvent to dryness (at least 24 hours) in a tall beaker kept in a vacuum oven at ambient temperature. The increase in weight above that of the empty beaker was taken as the weight of extracted material. This was cross-checked with the weight of insoluble solids present in the soxhlet extraction vessel (also evaporated to dryness and constant weight under vacuum).

The weight of MEK-insolubles in the original carbon system is then calculated from the known weight of solids originally present and the percentage of these solids that comprise the MEK-insolubles. These solids were assumed to undergo no weight loss up to 250°C. The weight of "resin" (in this case defined as anything other than MEK insolubles in the carbon system cured at 250°C) in the specimen at 250°C is thus given by the "actual" specimen weight at 250°C minus the weight of MEK insoluble substances originally present.*

The char weight due to the MEK insolubles was determined by sintering the extraction residue from the soxhlet vessel at 1000°C under an argon atmosphere in an alumina crucible. The resulting char weight was again adjusted to correspond to the actual weight by subtracting the MEK insolubles found in the sample to begin with. Thus, the char weight from the resin was the total char weight minus the char weight from MEK insolubles. And "char yield" was the char weight from the resin as a percentage of the total resin weight in the "slug" material at 250'C.

Results from these measurements are as follows:

Example 2: Expansion values.

A sample of a coating material was prepared and spread into a Teflon®-coated mold with dimensions 5.1 cm x 1.25 cm x 0.63 cm deep. This was run through the normal curing cycle, allowed to cool, and the sample was removed from the mold. The sample was divided into 4 test pieces with dimensions 2.5 cm x 0.6 cm x 0.6 cm, which were dried to constant weight in an alumina crucible at 250°C (usually 16-24 hours).

A test piece was measured using a micrometer, then heated from room temperature to 1000°C in a dilatometer continuously flushed with argon. The expansion/contraction was recorded continuously as a function of temperature on an XY recorder. Two shrinkage values were calculated; one based on the initial sample length along with the length at 1000"C; the other based on the initial length along with the final length once the sample had cooled back to room temperature. Final lengths of the samples were also measured with a micrometer after cooling as a double control of the registration system The samples tested were as indicated in Table IV.

While the overall expansion of the coating materials seems to show relatively poor reproducibility, despite very careful sample preparation and uniform temperature cycling, it is noted that after carbonization, expansion of the coating material between 800 and 1000°C shows good reproducibility and is very similar to block expansion. It is further noted that the UCAR® C-34 cement itself shows an unacceptable expansion above this temperature range.

It is assumed that up to the point of full carbonation (approx. 800'), the coating has sufficient flow to allow for any mismatch in the C.T.E. between coating and cathode substrate. Beyond this point, the coating becomes stiff and brittle, so that if a difference in C.T.E. exists, this can only be adapted by developing internal stresses or by opening larger cracks.

Example 3

A coating material (CM-24A) was prepared by combining and mixing the following components (percentages are by weight): 36* TiB2, less than 325 mesh (0.043 mm); 34.2* Union Carbide UCAR® C-34 carbon cement solid; 19.7* UCAR® C-34 mixing fluid; 10.1* Union Carbide Calcined Petroleum Coke Particles, Grade 6-03.

The resulting coating material was applied to a 7.5 cm x 15 cm cathode block substrate. Sufficient material to achieve a layer about 0.16 cm deep was applied and worked into the block surface. Additional material to obtain a layer about 1.59 cm thick was added, smoothed and leveled.

The coating was cured by heating at 25°C/hour to 100°C, holding for 5 hours, heating at 25°C/hour to 140°C, holding for 24 hours, and air cooling to room temperature.

After curing, several small cracks could be seen, but the coating-to-substrate bond was intact.

Example 4

A coating material (CM-37) was prepared by combining and mixing the following components: 36*T1B2, less than 325 mesh (0.043 mm); 29.4* UCAR® C-34 cement solids; 32.4* UCAR® C-34 Mixing Fluids; 2.2* Great Lakes Carbon FORTAFIL® unbonded fiber with lengths 0.63 cm.

The resulting coating material was applied to a 7.5 cm x 15 cm cathode block substrate and cured in a manner similar to that described in Example 3.

After curing, the coating showed no cracks as was the case in the coating with the carbon aggregate used in example 3. The carbon fibers seem to act as crack stoppers in the cathode coating and should therefore result in a longer cathode lifetime.

Example 5

A 7.5 c x 15 cm piece of cathode block was used as a substrate for a coating material (CM-38A) consisting of 37.5* TIB2, 30.6* UCAR® C-34 cement solid, 29.6* UCAR® C- 34 mixing fluid and 2.3* F0RTAFIL® unglued carbon fiber. This material was applied in a manner similar to that in Example 3 to a final thickness of 0.95 cm.

The coated substrate was then cured according to the following cycle: The coated block was heated to 100°C at a rate of 25°C per minute. hour and kept at this temperature for 1 3 hours. Heating was continued at 25°C per hour to 140°C, at which temperature the block was held for 16 hours. The coated block was then removed and allowed to air cool to room temperature. No surface blisters, cracks or mix defects were visible upon inspection.

The hardened block was then carbonized by heating to 1000°C over a 24 hour period in an argon atmosphere to avoid oxidation. The block was then allowed to cool to below 200°C under an argon atmosphere, removed and cooled to room temperature. After cooling, examination showed no defects. Integrity and substrate binding remained unaffected by carbonization.

Example 6

The mixing liquid content was reduced from approx. 20-32 weight-* to approx. 15-19 wt-* to step up the coating process for coating small test blocks (7.5 cm x 15 cm) to that of full scale cathode blocks (50.2 cm x 52.1 cm). Blisters were formed during the curing cycle when formulations with higher solvent content were used to coat the larger cathode blocks. As the surface area to perimeter ratio of the block to be coated increased, there was an increasing tendency for the coating to blister, ie the solvent could not escape during the curing cycle. Blistering also tended to increase with coating thickness, i.e. 0.32 cm thick coatings had little tendency to blister, while 1.27 cm thick coatings had a strong tendency to blister. Blister-free coatings were obtained on cathode blocks using the following formulation (CM-66): 36 wt* TiB2 powder, less than 325 mesh (0.043 mm),

36.35 wt-* UCAR® C-34 cement solids.

19.3 wt-* UCAR® C-34 mixing fluid,

0.35 wt-* Great Lakes Carbon FORTAFIL® 3, unbonded

carbon fibers, 0.63 cm length,

8.0 wt-* UCAR® BB-6 graphite particles.

A commercial VSS aluminum reduction cell was operated experimentally for 310 days using a cathode coated with a 0.63 cm thick coating of the above formulation. The cell energy efficiency was 0.14 kWh per pounds of aluminum produced lower than the average for plant 1 in the last nine months of operation.

After 310 days of operation, the test cell (A-43) was shut down and the molten metal and bath drained, leaving less than 5 cm of metal at the bottom of the cathode cavity. Examination of a 4.5 cm diameter core sample from the cold test cathode showed that the coating loss was consistent with the expected coating life as determined from metal analysis (ie approximately 1/3 of the original coating thickness had been lost). In isolated areas of the cathode there were signs that the entire coating had been lost.

Core samples from test cell A-46 in Table IV, which was also shut down, confirmed that the coating loss was in accordance with the planned coating life determined from metal analysis. There was no evidence of isolated areas of coating failure or fouling on the cathode surface in this cell.

Example 7

The tendency for blistering was reduced by reducing the fiber content of the coating, because reducing the fiber content reduced the amount of solvent required to make the coating workable. However, there was a tendency for the formation of fine vertical cracks in the coating. These fine vertical cracks represented a means by which the solvent could escape during the curing cycle and thus blistering was reduced. Blister-free but finely cracked coatings were obtained on cathode blocks using the following formulation (CM-78B): 46.0 wt-* TiBg powder less than 325 mesh (0.043 mm), 28.5 wt-* UCAR® C-34 cement solids,

15.5 wt-* UCAR® C-34 mixing fluid,

10.0 wt-* UCAR® BB-6 graphite particles.

Many problems resulted during the initial stage of this coating experiment due to an attempt to form a very smooth coating surface. After applying the coating material to the cathode substrate, it was moistened with excess mixing liquid and completely smoothed.

Such smooth coatings developed strong blistering during curing. The additional mixing liquid apparently formed a "skin" on the surface during curing, inhibiting the escape of evolved gases. This resulted in 1 blistering and imperfect coating. When the surfaces of subsequent coatings were given a light surface finish without excess liquid, no blistering occurred and acceptable hardened surfaces resulted. It was also noted that ambient temperatures had an effect on the mixing and spreadability of the coating material, with colder temperatures resulting in a stiffer, more difficult coating application.

As long as vertical cracks remain small, they do not inhibit the performance of the coating. The TiB2~ content was increased in this formulation to increase coating life, based on an assumed constant TiB2 dissolution rate. A commercial VSS aluminum reduction cell was operated experimentally for more than 225 days using a cathode coated with the above formulation at a thickness of 0.95 cm. The cell energy efficiency was 0.19 kWh per pounds of aluminum produced lower than the average for the plant in the last 6 months of operation.

Example 8

The formulation in Example 7 was modified to reduce the number of vertical cracks in the coating, but a low solvent content was maintained throughout to prevent the formation of blisters. Preferred formulations (CM-82 and CM-82A), whose cured resinous binder has a char yield of 94*. as determined by charring yield analysis of this coating, follows:

CM-82

46.0 wt-* TiB2~ powder, Carborundum less than 325 mesh

(0.043mm),

28.0 wt-* UCAR® C-34 cement solids,

15.8 weight-* UCAR® C-34 mixing fluid,

0.2 wt-* Great Lakes Carbon FORTAFIL® 3, carbon fibers bonded with UCAR® C-34 cement, 0.32 cm length, 10.9 wt-* UCAR® BB-6 graphite particles.

CM-82A

46.0 wt-* TiB2 powder, Metallurg less than 325 mesh

(0.043mm),

28.0 wt-* UCAR® C-34 cement solids,

15.8 wt-* UCAR® C-34 mixing fluid,

0.2 wt-* Great Lakes Carbon FORTAFIL® 3, carbon fibers bonded with UCAR® C-34 cement, 0.32 cm length,

10.0 wt-* UCAR® BB-6 graphite particles.

Cathode blocks for two commercial size VSS aluminum reduction test cells were successfully coated to a thickness of 0.95 cm with the above formulations and operated for 125 days. The number of vertical cracks in the coatings was reduced to practically zero. Cell energy effect in the weeks was 0.08 and 0.05 kWh per pounds of aluminum produced lower than that which was lower than the average for the plant, respectively, in the last two months of operation.

In preparing the coated cathode blocks in this example, the blocks were preheated to 40-50°C prior to coating, due to the cooler ambient temperature, to allow good coating material workability without the need to add additional mixing fluid. It was also noted that preheating the dry and liquid components of the coating material prior to mixing improved the mixing and application properties of the coating material. Furthermore, at lower mixing liquid concentrations less blistering occurred.

Further experiments were carried out using different coating materials and preheating the cathode blocks. It was found that preheating the block to 70-80°C resulted in a very smooth and light coating application, but caused premature curing, resulting in severe shrinkage and bond weakening. It was found that preheating temperatures of 40-50°C resulted in the best combination of coating material workability and elimination of shrinkage effects.

The previous coating formulations and additional formulations produced and used for commercial VSS aluminum cell cathode blocks are summarized in the following table V, where energy efficiency is expressed as kWh per pounds of aluminum produced, and planned coating life is based on dissolution of T1B2 in the aluminum product. At an energy cost of 22/1000 per kWh, savings from the order of 10 dollars per ton. In addition to the commercially available carbon cement UCAR® C-34 used in the previous examples, a large number of other thermosetting carbon cements have been used for the production of coating materials. In most cases, good coatings have been produced. In most cases where poor quality coatings have resulted after curing and carbonization, it is believed that good coatings can be produced by making special modifications to the coating formulation.

Example 9

A coating material (CM-89) was prepared by combining and mixing the following components: 36.5* TiB2, 34.6* UCAR® C-37 carbon cement solid, 19.2* UCAR® C-37 mixing fluid and 9.7* UCAR ® BB-6 graphite aggregate. This material was essentially identical to the composition of UCAR® C-34 carbon cement, with the addition of acid-washed carbonaceous filler in the form of graphite flakes for expansion. The coating material was relatively workable and fluid, and could easily be spread with a small tool on a Sumitomo SK cathode block, 7.5 cm x 15 cm x 5 cm, to a thickness of 0.95 cm.

This coated block was cured by raising the temperature from room temperature to 100°C for a 1-hour period. Stay for 4 hours, heating to 135°C for 1 hour, stay at 135°C for 4 hours, heating to 150°C for 1 hour, stay at 150°C for 12 hours, heating to 165°C for 1 hour, and stay for 3 hours at 165°C, followed by cooling in air. The cured coating showed bond weakening due to shrinkage at the outer edges of the substrate surface with less internal porosity, very small lateral cracks, and no surface defects.

The coated substrate was then carbonized by heating to 1000°C in an argon atmosphere over a 24-hour period and allowed to cool to room temperature. The coating was substantially destroyed by carbonization, as practically no coating remained on the substrate surface. This example is illustrative of the importance of the choice of the carbonaceous additive used in the coating material. In this particular case, the coating material included an incompatible additive in the carbon cement resin, specifically a graphite flake additive suitable for causing expansion.

Example 10

A coating material (CM-90) was prepared from a mixture of 35* T1B2" less than 325 mesh (0.043 mm), 33.4* UCAR® C-38 carbon cement solid, 22.2* UCAR® C-38 mixing fluid and 9, 4* UCAR® BB-6 graphite particles. The UCAR® C-38 cement is believed to be essentially identical to the UCAR® C-34 carbon cement with the addition of an oxidation inhibitor. The produced coating material showed good workability, fluidity and was easy to spread on a Sumitomo SK block as indicated in the previous example. This coated block was then cured as in Example 9. The cured coating material illustrated very large blisters under the surface, good bonding, good density and no surface defects.

After carbonization as indicated in Example 9, the carbonized coating showed weakening of bond due to shrinkage of the coating material without any surface defects appearing in the carbonized coating. It is believed that this coating material could well be used in the present invention by providing a binding layer of carbon cement between the coating material and the cathode substrate.

Example 11

A coating material (CM-87) consisting of 33.6* T1B2, 37.5* Stebbins AR-20-C carbon cement solid and 98.9* Stebbins AR-20-QC mixing fluid was prepared. The Stebbins AR-20-QC mixing fluid is believed to comprise a partially polymerized furan resin such as a furfuryl alcohol low resin polymer. Furfural and a latent catalyst, such as phthalic anhydride, are also believed to be present. The cement solids in this carbon cement are believed to comprise a carbon powder with a catalyst, and an anilln modifier.

This coating material was applied to a cathode substrate, as stated previously, and cured at 30°C for 24 hours, heated to 110°C for 3 hours, and held for 24 hours at 110°C. The coating material, which showed poor workability, and was stiff and tacky prior to application to the substrate, showed very little lateral cracking after curing. The cured coating was hard without any blisters or large cracks, had good density and a very strong bond to the substrate. After carbonization as indicated in Example 9, it showed carbonized coatings a slight shrinkage crack, a slight weakening of the bond due to subsequent shrinkage, good density, but had large areas of peeling.

Example 12

A similar coating material (CM-88) was prepared using 38.7* Stebbins AR-25-HT carbon cement solid, 28.3* Stebbins AR-25-HT mixing fluid and 33* titanium diboride. This cement is believed to comprise a modified furan resin, of furfuryl alcohol and a low resin polymer of furfuryl alcohol and a latent catalyst. The coating material was stiff and sticky, and showed poor workability. The coating substrate was cured as indicated in Example 11, resulting in a coating substrate exhibiting the same properties as those of the coating in Example 11. After carbonization, the coating showed more shrinkage than that in Example 11, causing bond weakening. However, no peeling or cracks were detected in the surface layer.

Example 13

A polyester resin with carbon filler, marketed as Atlas CARBO-VITROPLAST carbon cement, was mixed with 35* T1B2 to form a coating material (CM-92). The coating material showed good workability, good fluidity, was not sticky, and could be easily spread on a cathode substrate as indicated in Example 9. The coated substrate was cured by heating to 100°C for 1 hour, holding at 100°C for 3 hours, heating to 165°C for 1 hour, held at 165°C for 10 hours, heating to 200°C for 1 hour, held for Vt hour, and air cooling. The cured coating showed excellent bond to the substrate, excellent density, and no surface defects. The coated substrate was then subjected to carbonization, as indicated in Example 9. The carbonized coated substrate showed complete bond weakening, the coating being complete but having extensive fine porosity. The coating showed no surface defects, but was very bright and pulverized easily. It is noted that the resinous binders used in the production of this coating material showed a char yield of only 8*, illustrating the critical importance of this parameter.

Example 14

Atlas CARBO-ALKOR, a carbon cement based on furan resin with carbon filler, was prepared in accordance with the manufacturer's instructions and mixed with sufficient titanium diboride to comprise 35* of the coating material. This coating material (CM-93) showed good workability. After curing as indicated in Example 13, the coating showed good bonding through the substrate, extensive surface peeling, and reasonable density. After carbonization, the coating showed slight bond weakening, more extensive peeling, and large cracks in the surface due to shrinkage. When the coating material was reformulated using 3.5* soap as the gas release agent instead of an equal amount of mixing liquid, an acceptable coating was obtained.

Example 15

A coating material (CM-94) was prepared as indicated in Example 13 using Atlas CARBO-KOREZ carbon cement, believed to be a phenolic resin based cement with carbon filler, and with a butyl alcohol solvent. The coating material showed very poor processability, it was very stiff and difficult to spread on the carbon substrate. After curing as indicated in Example 13, the coating material showed good bonding, good density and no surface defects. However, after carbonization, bond weakening due to shrinkage was observed. It is believed that the use of a furfuryl alcohol solvent instead of the butyl alcohol mixing fluid in the Atlas CARBO-KOREZ resin would result in improved impregnation of the cathode substrate, and therefore better bonding.

Example 16

A coating material was prepared using 35* TiB2 and 65* premixed DYLON carbon cement grade GC, the DYLON carbon cement is believed to comprise less than 10* pulverulent coal tar pitch, less than 20* phenolic resin and less than 35* furfuryl alcohol. The coating material showed very good workability, but after curing as indicated in example 9, extensive severe cracking in the coating and bond weakening was observed. Significant lateral cracking and high porosity were noted. This coating was not considered worthy of carbonization in view of its poor condition after curing. This coating material was later reformulated with the addition of 3.5* soap as a gas release agent, and produced an excellent coating after curing and carbonization. This illustrates that the choice of additional materials for the coating material is crucial, and the failure of a coating material due to a lack of porosity through which volatile substances can escape.

Example 17

A coating material was prepared using 45* T1B2 and 55* premixed Aremco GRAPHI-BOND® 551-R carbon cement. This carbon cement is believed to comprise 60* graphite and 40* furfuryl alcohol, and is specially formulated for use in reducing atmospheres. The coating material was smooth and relatively easy to apply to the cathode substrate. The coating substrate was then cured as indicated in Example 9, and this resulted in good bonding, reasonable density with vertical cracking near the edges and extensive fine horizontal cracking. After carbonization, bond weakening occurred near the edges due to shrinkage, but the coating showed reasonable density, with some large vertical cracks and some fine horizontal cracks. It is believed that the addition of gas release agent and carbonaceous additive would overcome such problems.

Example 18

A coating material as set forth in Example 17 was prepared using Aremco GRAPH-BOND® Grade 551-R, which is believed to comprise 32* graphite and 68* aluminum sodium silicate. The coating material was very liquid and allowed itself to be poured instead of spread. The coated substrate was cured as in Example 9, but a large blister developed in the coating after 4 hours at 100°C. A good bond was formed, although the coating was very porous. After carbonization, the coating was found to be visibly unchanged, but the had become more brittle and crumbly, and a slight weakening of the bond was evident near the edges.This example illustrates a coating material unsuitable for the present invention.

Example 19

An additional coating material was prepared using 45* T1B2 and premixed Great Lakes Carbon Grade P-514 carbon cement believed to be based on graphite particles and a binder. The coating material was easy to process and was easily smoothed onto a cathode substrate. After curing, as indicated in Example 9, strong porosity and scaling occurred after 4 hours at 100°C. A good bond was formed, with good density in areas between holes. The coating was shiny with a glassy appearance in the pores and under the peeling areas. After carbonization, the coating substrate was found to be unchanged except that large vertical cracks had developed and a slight bond weakening had occurred along the edges. When the coating material was reformulated using 3.5* soap as a gas release agent instead of an equal amount of mixing liquid, an acceptable coating was obtained.

Example 20

A difficult-to-process (infusible) polyphenylene polymer is produced by polymerization of 1,3-cyclohexadiene with Ziegler-Natta catalysts followed by halogenation and dehydrohalogenation. This gives a para-phenylene polymer with a molecular weight of approx. 4000. This resin alone is unsuitable for use in the present invention due to its relatively low molecular weight and relative insolubility.

A fusible polyphenylene resin (soluble, e.g. in trichlorobenzene) is prepared by cationic oxidative polymerization of 1,3,5-diphenylbenzene. The reaction product contains difficult-to-process polyphenylene by-product which is removed by fractional distillation. The distilled fusible polyphenyl has a molecular weight between 1000 and 1500.

A coating material is prepared using these two resins as follows. The two resins are ground separately to less than 325 mesh (0.043 mm) and then thoroughly mixed for 15 minutes in a high-speed mixer, at a ratio of 1.6*

(calculated on the weight of the coating material) of difficult-to-process polyphenylene to 3.3* fusible polyphenylene. 16.3* graphite flour (UCAR® GP 38 P) and 8.2* REGAL® 400 pellets (carbon black from Cabot Corporation) are added to this mixture.

A mixing liquid is prepared containing 1.6* triethanolamine (TEA) oleate and 11.4* chloroform. The TEA oleate is first heated to a liquid state by heating to 100°C and then poured into the chloroform.

To the dry components (including polyphenylene resin powders, graphite powders and carbon black) are now added 47.3* Carborundum less than 325 mesh (0.043 mm) T1B2, 9.8* Union Carbide UCAR® BB-6 graphite aggregate and 0.5* Great Lakes Carbon FORTAFIL® 0.32 cm glued fiber. The components are dry mixed and the mixing liquid is added until a uniform dispersion of the components is observed.

The coating material is spread on a cathode block in the usual manner to form a 0.95 cm thick layer. This block is then exposed to air for 1 hour to allow some of the solvent to evaporate. Curing is achieved by slowly heating the coated block to 70°C to volatilize most of the mixing liquid, followed by slowly heating to 80°C and holding at this temperature for 2 hours. Again the temperature is slowly raised to 200°C and held for 8 hours The block is then cooled to room temperature.

A partial carbonization is then carried out by heating for 9 hours at 135°C, followed by 24 hours of slow temperature increase to 300°C and holding at 300°C for 6 hours. A satisfactory coating is obtained.

Example 21

A coating material (CM-91) consisting of 44.9* T1B2 (less than 325 mesh (0.043 mm) Carborundum), 28* UCAR® C-34 carbon cement solid, 14.7* UCAR® C-34 mixing fluid, 12* UCAR® BB-6 graphite and 0.4* Great Lakes Carbon FORTAFIL® 3 0.32 cm bonded fiber were prepared.

This material was difficult to mix due to its high fiber content. TIB2 dispersion was poor and the material required re-mixing halfway through the coating operation.

This material was coated onto a full size cathode block, preheated overnight to 50°C. The material was difficult to spread until it was heated by the hot block.

The coating was flattened to a thickness of 1.27 cm, semi-smoothed and cured in the same manner as indicated in Example 9. No defects were noted after cooling.

The coating was then carbonized, which produced a satisfactorily bonded coating with an acceptable surface.

Claims (28)

1. Cell for the electrolytic reduction of alumina to aluminium, which has a cathode comprising a cathode substrate with an aluminum wettable surface layer bonded thereto, the layer comprising refractory, hard material and carbon, characterized in that said layer comprises refractory, hard material in a non-graphitizable carbon matrix which has an ablation rate during operation of the cell that is substantially equal to the combined rate of wear and dissolution of the refractory hard material, wherein said surface layer is obtained by curing and carbonizing a coating material comprising: 10-90 weight-* refractory hard material; 0.5-40 wt-* thermosetting resin, wherein the thermosetting resin comprises at least one resin different from a phenolic resin, a furan resin precursor or a mixture thereof; 2-40 weight-* mixing liquid; 1-60 wt* particulate carbonaceous filler having a particle size less than 100 mesh (0.147 mm); and up to 70 weight* of carbonaceous additive in the form of particulate material having a particle size greater than 100 mesh (0.147 mm) and/or carbon fibers, wherein the thermosetting resin forms a resinous binder system having a char yield greater than 25* and wherein the hardened and carbonized coating shows a percentage expansion between 800 and 1000°C which differs by less than 0.2 from the percentage expansion shown by said cathode substrate. 2. Cell according to claim 1, characterized in that the refractory hard material comprises titanium diboride. 3. Cell according to claim 2, characterized in that the thermosetting resin includes one or more phenol resins, furan resin precursors or mixtures thereof. 4. Cell according to any one of claims 1-3, characterized in that the thermosetting resin forms a resinous binder system showing a charring yield of over 50*. 5. Coating material comprising refractory, hard material and carbon, characterized in that it is composed of 10-90 weight-* refractory, hard material; a thermosetting binder system comprising 0.5-40 wt* non-graphitizing thermosetting resin, wherein the thermosetting resin comprises at least one resin different from phenolic resins, furan resin precursors or mixtures thereof; 2-40 weight-* mixing liquid; 1-60 wt-* particulate carbonaceous filler having a particle size less than 100 mesh (0.147 mm); and up to 70 weight* carbonaceous additive in the form of particulate material having a particle size greater than 100 mesh (0.147 mm) and/or carbon fibers; wherein the binder system, the carbonaceous filler, and the carbonaceous additive exhibit an ablation rate after curing and carbonization that is substantially equal to the combined rate of wear and dissolution of the refractory hard material in the environment of an aluminum reduction cell during operation thereof, wherein the thermosetting resin forms a resinous binder system which has a charring yield of more than 25* and where the hardened and carbonized material exhibits a percentage expansion between 800 and 1000°C that differs by less than 0.2 from the percentage expansion of the cathode substrate in the aluminum reduction cell. 6. Coating material according to claim 5, characterized in that the refractory, hard material is titanium diboride. 7. Coating material according to claim 5 or 6, characterized in that the binder system comprises a resinous binder selected from polyphenylene, heterocyclic, epoxy, silicone, alkyd and polyimide resins, and mixtures thereof, and in that the refractory, hard material is selected from single crystal , bicrystal and open clusters of single crystal titanium diboride. 8. Coating material according to any one of claims 5-7, characterized in that the thermosetting resin includes one or more phenolic resins, furan resin precursors or mixtures thereof. 9. Coating material according to any one of claims 5-8, characterized in that the carbonaceous filler has a C:H ratio greater than 2:1. 10. Coating material according to any one of claims 5-9, characterized in that the carbonaceous additive is selected from carbon aggregates having a particle size of from 4 mesh (4.699 mm) to 100 mesh (0.147 mm) carbon fiber, and mixtures thereof, and comprises 5 -15* of the coating material. 11. Coating material according to any one of claims 5-10, characterized in that it shows a total contraction of less than 1.0* when cycling from 20°C to 100°C to 20°C. 12. Aluminum wettable cathode for an aluminum reduction cell comprising a cathode substrate having a surface layer containing refractory, hard material and carbon, characterized in that the surface layer is a hardened and carbonized composition including 10-90 weight-* refractory, hard material; thermosetting resinous binder comprising 0.5-40 wt* non-graphitizing thermosetting resin, wherein the thermosetting resin comprises at least one resin other than a phenolic resin, a furan resin precursor or a mixture thereof; 2-40 weight-* mixing liquid; 1-60 wt-* particulate carbonaceous filler having a particle size less than 100 mesh (0.147 mm); and up to 70 weight* carbonaceous additive in the form of particulate material having a particle size greater than 100 mesh (0.147 mm) and/or carbon fibers, wherein the thermosetting resin forms a resinous binder system having a char yield greater than 25* and wherein the surface layer has a percentage expansion between 800 and 1000<*>C which differs from the percentage expansion of the cathode substrate by less than 0.2*. 13. Aluminum wettable cathode according to claim 12, characterized in that the thermosetting resin includes one or more phenol resins, furan resin precursors or mixtures thereof. 14. Aluminum wettable cathode according to claim 12 or 13, characterized in that the refractory, hard material is titanium diboride. 15. Aluminum wettable cathode according to any one of claims 12-14, characterized in that the cathode surfaces are inclined relative to the horizontal plane. 16. Aluminum wettable cathode according to any one of claims 12-15, characterized in that the carbon in the surface layer has an ablation rate which is substantially equal to the rate of wear and dissolution of the refractory, hard material in the operating environment of an aluminum reduction cell. 17. Process for producing a refractory, hard material and carbon-aluminum wettable cathode surface for an aluminum reduction cell, characterized in that a coating material is applied to a cathode substrate comprising: 10-90 weight-* refractory, hard material, 0.5-40 weight- * non-graphitizing thermosetting resin, wherein the thermosetting resin comprises at least one resin different from a phenol resin, a furan resin precursor or a mixture thereof; 2-40 weight-* mixing liquid; 1-60 wt* particulate carbonaceous filler having a particle size less than 100 mesh (0.147 mm); and up to 70 wt* of carbonaceous additive in the form of particulate matter with a particle size greater than 100 mesh (0.147 mm) and/or carbon fibers; the coating material is hardened to form a relatively hard, adherent surface layer, and this layer is carbonized to form an adherent aluminum wettable surface layer comprising refractory hard material in a non-graphitizable carbon matrix, said surface layer having a percentage expansion between 800 and 1000°C which separates differs from the percentage expansion of the cathode substrate by less than 0.2, where the thermosetting resin forms a resinous binder system having a char yield of over 25*. 18. Method according to claim 17, characterized in that titanium diboride is used as refractory, hard material. 19. Method according to claim 17 or 18, characterized in that the thermosetting resin is selected from polyphenylene, heterocyclic, epoxy, silicone, alkyd and polyimide resins, and mixtures thereof. 20. Method according to any one of claims 17-19, characterized in that a thermosetting resin is used which includes one or more phenol resins, furan resin precursors, or mixtures thereof. 21. Method according to any one of claims 17-20, characterized in that a carbonaceous mixture is used in the carbonized surface layer which has an ablation rate substantially equal to the combined rate of wear and dissolution of the refractory, hard material in the environment of the aluminum reduction cell during operation hence. 22. Method according to any one of claims 17-21, characterized in that the coating mixture is applied to carbon cathode blocks outside the cell, and is cured before placement in the cell. 23. Method according to claim 22, characterized in that the coating material is carbonized before placement in the cell. 24. Method according to claim 22, characterized in that the coating material is carbonized in the cell. 25. Method according to any one of claims 17-21, characterized in that the coating material is applied to carbon cathode blocks outside the cell, and then hardened and carbonized in the cell. 26. Method according to any one of claims 17-21, characterized in that the coating material is applied to the cathode substrate in the cell, and then hardened and carbonized. 27. Method according to any one of claims 17-26, characterized in that the coating material is applied in several layers. 28. Method according to claim 27, characterized in that the content of refractory, hard material changes in successive layers.