CA2643442A1 - Preheating of electrolytic cell - Google Patents

Abstract

A process for preheating an electrolytic cell, which involves disposing at least one sheet of compressed particles of exfoliated graphite across at least a portion of the surface of cathode blocks in the cell.

Description

DESCRIPTION
PREHEATING OF ELECTROLYTIC CELL
TECHNICAL FIELD
[0001] The present invention relates to a material and process useful for starting (i.e., preheating) an electrolysis cell, especially an aluminum electrolysis cell. More particularly, the present invention relates to the use of compressed particles of exfoliated graphite for resistively heating an aluminum electrolysis cell.
BACKGROUND ART
[0002] In the electrolytic production of some metals, an electrolyzing current is passed from the anode to the cathode of an electrolytic cell through an electrolyte comprising a molten compound of the metal, which compound can also be dissolved in a molten solvent. One of the more common metals produced by such a process is aluminum.

[0003] The electrolytic cell in common use today for the preparation of aluminum is of the classic Hall-Heroult design. This cell typically utilizes one or more carbon anodes and cathodes formed of a carbon or graphite material, which also function as the bottom or floor of the cell. The anodes extend into the cell from above and make contact with the electrolyte.
Metallic current collector bars are embedded in the cathodes and are connected electrically to the cathodic side of the source of current. The electrolyte, called hereunder bath, used typically consists primarily of molten cryolite in which is dissolved alumina and which contains other material such as fluorspar. Molten aluminum resulting from the reduction of alumina accumulates at the bottom of the electrolytic cell as a molten pool over the bottom cathode and serves as a molten metal cathode.

[0004] Production of metals in such a cell can be interrupted by a loss of power or other operational problem. In addition, erosion and other effects can require cell shutdown for removal and replacement of the anodes and cathode blocks. Of course, new cells being readied for being placed into operation are shutdown. Since the cell temperature usually cannot be maintained during shutdown, the metal and electrolyte often cannot remain molten. As the cell cools, molten metal and electrolyte that is not removed will solidify to a hard mass that is not easily removed when the cell is to be restarted.

[0005] Regardless of the reason for cell shutdown, starting (or restarting of a Hall-Heroult cell) requires bringing the cell from ambient to operating temperature, which is on the order of about 950 C to 1000 C or higher. Generally, this entails lowering the anode of the cell onto the cathode, e.g., the cell bottom, which may be covered with a layer of granular material, coke or graphite (FR 2 844 811), called hereafter "resistor", and then turning on the electrolyzing current. The heat generated by the electrical contact resistance between the anode and cathode and the resistance of the granular material is sufficient to bring the cell to operating temperature. When the cell reaches a sufficiently high temperature, the anodes are raised, molten bath and molten aluminum are successively added to the cell, the anode is lowered into the molten bath to the desired distance from the cathode and electrolysis begins.

[0006] Generally, the preheating process takes about 48 hours to bring the cell from an ambient temperature to an operating temperature of 900 C
or higher. The necessary resistance for heating the electrolytic cell is the summation of the resistance of the contacts between, on one end, the anode and the resistor and, on the other end, the cathode and the resistor and the intrinsic resistance of the resistor. The intrinsic resistance for each resistor is determined by the following equation:
Rresistor = h ~
S
where Rresistor is the intrinsic resistance of the resistor, p is the specific resistivity of the resistor, h is the thickness of the layer, and S is the surface area of the resistor. As known to one skilled in the art, the specific resistivity of granular coke is approximately four times the specific resistivity of crushed synthetic graphite. Thus, a smaller area of crushed graphite is needed when compared to the cell floor coverage by coke to achieve a similar overall resistance resulting in the temperature increase of the electrolytic cell.

[0007] Additionally, the exact thickness of the layer of granular coke or crushed synthetic graphite on the cell floor is dependent upon the selected resistor. The approximate thickness of a layer of coke is about one centimeter while the layer is about 2 to 3 centimeter thick if crushed synthetic graphite is used as the resistor.

[0008] The use of granular coke, described in "Cathodes in Aluminium Electrolysis, M. Sorlie and H.A. Oye, Aluminium Verlag, 1994 pages 77-83) requires the use of shunts because its resistivity is too high to ensure a sufficiently low heating rate of about 20 C/hour. This heating rate is required to safely bring, without too high thermal gradients, all the elements of the cell to working conditions. The use of shunts necessitates manpower to remove them periodically during the preheating period of approximately 2 to 3 days. It may be also a safety issue as high current intensity is crossing the shunts. In order to avoid the use of shunts, a more conductive material, granular synthetic graphite, has been proposed by Jouaffre, Basquin and Vanvoren (FR 2 844 811). Granular graphite is placed on a fraction of the cathode surface under the anodes and does not require the use of shunts because of the graphite's lower specific resistivity. Otherwise stated, granular graphite handles the heating in a safer manner and does not cause localized thermal gradients.

[0009] However, as the resistivity of graphite decreases with temperature, the heating rate will decrease as the temperature of the joint increases. Thus, lengthy preheating periods may be necessary. No adjustment of current is possible as no shunt is available and the full load of the potline is used to preheat the cell.

[0010] During its temperature raise, the cell is bending and therefore the contact surface and the contact pressure of the anode on the granular material are modified. Periodical monitoring of each anode current uptake is recommended to avoid high concentration of current resulting in "hot spots"
and inhomogeneous cell temperatures. Specifically, the anodes must be checked at about every three hours to insure proper current distribution during the preheating. If either high or low current uptakes or "hot spots" or inhomogeneous cell temperatures are detected, then the anode position has to be modified.

[0011] Another inconvenient of the granular material is the change of thickness during the preheating resulting from vibration and high pressure applied by the anode.

[0012] After preheating step is finished, bath is added in the cell, causing the granular resistor to float to the surface of the bath. It is not recommended to leave the granular material in the bath as it will deteriorate the thermal insulation properties of the bath and hence the power consumption of the cell. It is then necessary to skim the grains manually from the hot bath. This task creates safety issues caused by the close proximity of a high electrical current, high temperature and hot liquids.

[0013] Furthermore, another issue with the use of granular materials in preheating electrolytic cells is that with granular materials of few millimeters grain size it is difficult to ensure a uniform and constant bed thickness of the 1 to 4 cm all across the cell.

[0014] Different approaches for more efficiently preheating an electrolytic cell have been suggested. For instance, Steiger and Holden, in U.S. Patent Nos. 4,181,583 and 4,181,584, claim a system utilizing resistance heater bars (denoted 20 and 20' in the patents). The patents make reference to the use of, inter alia, "Grafoil graphite sheets" to ensure good electrical contact between the anodes and resistance heater bars (see, U.S. Patent No.
4,181,583 at col. 10, lines 23-29 and U.S. Patent No. 4,181,584 at col. 9, lines 21-27). In this case the graphite sheets are used to minimize the contact resistance between the anode and the resistor.

[0015] In U.S. Patent No. 4,175,022, Vadla and Wilder teach the use of a graphite sheet to protect the insulation at the bottom of an electrolytic cell from corrosive attack. The Vadla and Wilder graphite sheet is disposed below the cathodic bottom of the cell and above the insulting layer protecting the cell casing. Due to this location current does not cross the graphite sheet during the preheating process and also during the electrolytic reaction.

[0016] Graphites are made up of layer planes of hexagonal arrays or networks of carbon atoms. These layer planes of hexagonally arranged carbon atoms are substantially flat and are oriented or ordered so as to be substantially parallel and equidistant to one another. The substantially flat, parallel equidistant sheets or layers of carbon atoms, usually referred to as graphene layers or basal planes, are linked or bonded together and groups thereof are arranged in crystallites. Highly ordered graphites consist of crystallites being highly aligned or oriented with respect to each other and having well ordered carbon layers. In other words, highly ordered graphites have a high degree of preferred crystallite orientation. It should be noted that graphites possess anisotropic structures and thus exhibit or possess many properties that are highly directional e.g. thermal and electrical conductivity and fluid diffusion.

[0017] Briefly, graphites may be characterized as laminated structures of carbon, that is, structures consisting of superposed layers or laminae of carbon atoms joined together by weak van der Waals forces. In considering the graphite structure, two axes or directions are usually noted, to wit, the "c" axis or direction and the "a" axes or directions. For simplicity, the "c"
axis or direction may be considered as the direction perpendicular to the carbon layers. The "a" axes or directions may be considered as the directions parallel to the carbon layers or the directions perpendicular to the "c"
direction. The graphites suitable for manufacturing flexible graphite sheets possess a very high degree of orientation.

[0018] As noted above, the bonding forces holding the parallel layers of carbon atoms together are only weak van der Waals forces. Natural graphites can be treated so that the spacing between the superposed carbon layers or laminae can be appreciably opened up so as to provide a marked expansion in the direction perpendicular to the layers, that is, in the "c"
direction, and thus form an expanded or intumesced graphite structure in which the laminar character of the carbon layers is substantially retained.

[0019] Graphite flake which has been greatly expanded and more particularly expanded so as to have a final thickness or "c" direction dimension which is as much as about 80 or more times the original "c"
direction dimension can be formed without the use of a binder into cohesive or integrated sheets of expanded graphite, e.g. webs, papers, strips, tapes, foils, mats or the like (typically referred to as "flexible graphite"). The formation of graphite particles which have been expanded to have a final thickness or "c" dimension which is as much as about 80 times or more the original "c" direction dimension into integrated flexible sheets by compression, without the use of any binding material, is believed to be possible due to the mechanical interlocking, or cohesion, which is achieved between the voluminously expanded graphite particles.

[0020] In addition to flexibility, the graphite material, as noted above, has also been found to possess a high degree of anisotropy with respect to thermal and electrical conductivity and fluid diffusion, comparable to the natural graphite starting material due to orientation of the expanded graphite particles and graphite layers substantially parallel to the opposed faces of the sheet resulting from very high compression, e.g. roll pressing.
Sheet material thus produced has excellent flexibility, good strength and a very high degree of orientation.

[0021] Briefly, the process of producing flexible, binderless anisotropic graphite sheet material, e.g. web, paper, strip, tape, foil, mat, or the like, comprises compressing or compacting under a predetermined load and in the absence of a binder, expanded graphite particles which have a "c" direction dimension which is as much as about 80 or more times that of the original particles so as to form a substantially flat, flexible, integrated graphite sheet.
The expanded graphite particles that generally are worm-like or vermiform in appearance, once compressed, will maintain the compression set and alignment with the opposed major surfaces of the sheet. Controlling the degree of compression can vary the density and thickness of the sheet material. The number of sheets can also be varied. The density of the sheet material can be within the range of from about 0.04 g/cm3 to about 2.3 g/cm3.
The flexible graphite sheet material exhibits an appreciable degree of anisotropy due to the alignment of graphite particles parallel to the major opposed, parallel surfaces of the sheet, with the degree of anisotropy increasing upon roll pressing of the sheet material to increase orientation.
In roll pressed anisotropic sheet material, the thickness, i.e. the direction perpendicular to the opposed, parallel sheet surfaces comprises the "c"
direction and the directions ranging along the length and width, i.e. along or parallel to the opposed, major surfaces comprises the "a" directions and the thermal, electrical and fluid diffusion properties of the sheet are very different, by orders of magnitude, for the "c" and "a" directions.

[0022] The binderless graphite sheet material could possess an anisotropic ratio between about 2 and about 250 (with respect to thermal anisotropy) or between about 200 and about 5000 (with respect to electrical anisotropy), the anisotropic ratio defined as the ratio of in-plane conductivity to through-plane conductivity. Furthermore this material's specific resistivity exhibits a much lesser dependence upon both temperature and pressure, reducing the necessary anode adjustments during preheating of the electrolytic cell.

[0023] Additionally the use of graphite sheet material in the preheating of an electrolytic cell may preclude skimming of carbonaceous material on the surface of the bath. The graphite sheet may be stuck to the anode and subsequently follow the anode at the end of the preheating when the anode is moved up. Because the binderless graphite sheet material is a carbon based material it will be consumed by the electrolysis process as the anode does. No skimming will be necessary.
DISCLOSURE OF THE INVENTION

[0024] The present invention provides a method for preheating an electrolytic cell for starting or restarting the operation of the cell. The method includes disposing at least one sheet of compressed particles of exfoliated graphite across the surface of one or more cathode blocks in the cell. This material may cover totally or partially the cathodic surface. One or more of the anodes is brought into contact with at least one sheet of compressed particles of exfoliated graphite. A current is then passed from the anode(s) into and through the sheet(s) of compressed particles of exfoliated graphite to resistively heat the graphite sheet(s) and thereby preheat the electrolytic cell.

[0025] The graphite sheet useful in the present invention should exhibit sufficient resistivity such that, when current is passed therethrough, the sheet heats up to the extent needed to preheat the electrolytic cell. More specifically, the resistivity of the graphite sheet should be sufficient to raise the temperature of the electrolytic cell to at least about 900 C. Because sheets of compressed particles of exfoliated graphite are anisotropic in nature, with respect to electrical conductivity, the electrical conductivity of the sheets is substantially higher across the length and width of the sheet (the "c"
direction) than through the thickness of the sheet (the "a" direction). Because of this, the graphite sheet will help to rebalance any current density distortion applied to certain areas of the sheet by spreading the current across the length and width of the sheet preferentially. In this way, the temperature generated by the graphite sheet is more uniform across the surface of the cell that would be observed when a more isotropic material is used. Additionally, as graphite sheets also present high thermal conductivity in the "c" direction, they will disperse the heat generated more uniformly across the surface of the cell than would be observed if a more isotropic material was used. These characteristics of graphite sheets reduce or suppress the formation of "hot spots" which can deteriorate the anode and the cathode.

[0026] In addition, because the resistivity of a sheet of compressed particles of exfoliated graphite can be controlled, the temperature to which the electrolytic cell is raised can also be controlled. For instance, if a layer of coke is used to preheat the cell, as is conventional in the art, that layer has a certain resistivity; thus, the temperature increase rate, which is applied to the cell is limited by that resistivity. As known in the art, the high resistivity of coke also requires the use of shunts to monitor the amount of current injected in the cell and therefore the temperature increase rate. Contrariwise, when a sheet of compressed particles of exfoliated graphite is used, the temperature to which the electrolytic cell is raised can be controlled by controlling the electrical conductivity (the inverse of resistivity) and does not require the use of shunts to monitor current.

[0027] One of the ways this directional alignment of the graphene layers can be achieved is by the application of pressure to the flexible graphite sheet, either by calendering the sheet (i.e., through the application of shear force) or by die pressing or reciprocal platen pressing (i.e., through the application of compaction), with calendering more effective at producing directional alignment. For instance, by calendering the sheet to a density of 1.1 g/cc, as opposed to 1.7 g/cc, or higher.

[0028] Alternatively, if a laminate is formed, the directional alignment of the graphene layers which make up the laminate in gross is increased, such as by the application of pressure, resulting in a density greater than the starting density of the component flexible graphite sheets that make up the laminate. The pressure can be applied by conventional means, such as by die pressing or calendering. Pressures of at least about 60 MPa are preferred, with pressures of at least about 550 MPa, and more preferably at least about 700 MPa, needed to achieve densities as high as 2.3 g/cc.

[0029] Additionally, the resulting aligned laminate also exhibits increased strength, as compared to a non-"aligned" laminate.

[0030] Generally, for use in preheating an electrolytic cell to temperatures of about 900 C or higher, a sheet of compressed particles of exfoliated graphite should have a thickness of about 0.1mm to about 25mm and a density of about 2.3 g/cm3 to about 0.04g/cm3. The electrical resistivity of the sheet should be at least about 500 Ohm-m.

[0031] Therefore, it is an object of the present invention to provide an efficient system for preheating an electrolytic cell, especially a cell for the electrolysis of aluminum.

[0032] Another object of the present invention is to provide a system for the preheating of an electrolytic cell by resistance heating.

[0033] Still another object of the present invention is to provide a material, which can be easily and uniformly placed in an electrolysis cell to facilitate preheating and to reduce manpower. Also, the use of a template necessary to pour the granular material on the cathode surface is avoided.

[0034] Yet another object of the present invention is to provide a material which can be easily and uniformly placed in an electrolysis cell to facilitate preheating, and which has the desired electrical characteristics.

[0035] Another object of the present invention is to provide a material suitable for preheating an electrolysis cell by resistance heating, and yet which is more efficient, safer and cost effective than conventional materials and methods for accomplishing the same.

[0036] Another object of the present invention is to provide a material suitable for preheating an electrolysis cell by resistance heating, and yet which can be stuck (or glued) to the anode and therefore avoiding the skimming of granular material after the addition of bath.

[0037] Another object of the present invention is to provide a material suitable for preheating an electrolysis cell by resistance heating, and yet the resistivity of which is less dependant upon pressure compared to granular material. This will enable to reduce the number of anode position adjustments during preheating, resulting from the bending of the cell submitted to temperature gradient.

[0038] These objects and others, which will be apparent to the skilled artisan upon reading the following description, can be achieved by providing a process for preheating an electrolytic cell, which includes disposing at least one sheet of compressed particles of exfoliated graphite, totally or partially, across the surface of one or more cathode blocks in the cell. The process then contemplates bringing one or more of the anodes of the electrolytic cell into contact with at least one sheet of compressed particles of exfoliated graphite (optionally by gluing the at least one sheet of compressed particles of exfoliated graphite to the anode), and passing a current from the one or more anodes into and through the at least one sheet of compressed particles of exfoliated graphite to resistively heat the graphite sheet and thereby preheat the electrolytic cell.

[0039] Preferably, the electrolytic cell is preheated to temperatures of about 900 C or higher. The at least one sheet of compressed particles of exfoliated graphite used in the inventive process advantageously has a thickness of about 0.075 mm to about 25 mm and a density of about 0.5 g/cc to about 1.7 g/cc.. The electrical resistivity of the sheet should be at least about 500 Ohm-m.

[0040] It is to be understood that both the foregoing general description and the following detailed description present embodiments of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed.
The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate various embodiments of the invention, and together with the description serve to explain the principles and operations of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Fig. 1 is a side cross-sectional view of an aluminum electrolysis cell in operation.

[0042] Fig. 2 is a top plan view of the aluminum electrolysis cell of Fig.
1.

[0043] Fig. 3 is a side cross-sectional view of an aluminum electrolysis cell having at least one sheet of compressed particles of exfoliated graphite disposed across the surface of one or more cathode blocks in the cell.

[0044] Fig. 4 is a top plan view of the aluminum electrolysis cell of Fig. 3.

[0045] Fig. 5 is a top plan view of an aluminum electrolysis cell having a plurality of sheets of compressed particles of exfoliated graphite disposed across the surface of one or more cathode blocks in the cell, such that the sheets of compressed particles of exfoliated graphite are disposed so as to correspond to the approximate footprint of the anodes.
BEST MODE FOR CARRYING OUT THE INVENTION

[0046] As noted, the inventive method uses at least one sheet of compressed particles of exfoliated graphite, commonly known as flexible graphite. Graphite is a crystalline form of carbon comprising atoms covalently bonded in flat-layered planes with weaker bonds between the planes. By treating particles of graphite, such as natural graphite flake, with an intercalant of, e.g. a solution of sulfuric and nitric acid, the crystal structure of the graphite reacts to form a compound of graphite and the intercalant. The treated particles of graphite are hereafter referred to as "particles of intercalated graphite." Upon exposure to high temperature, the intercalant within the graphite decomposes and volatilizes, causing the particles of intercalated graphite to expand in dimension as much as about 80 or more times its original volume in an accordion-like fashion in the "c"
direction, i.e. in the direction perpendicular to the crystalline planes of the graphite. The exfoliated graphite particles are vermiform in appearance, and are therefore commonly referred to as worms. The worms may be compressed together into flexible sheets that, unlike the original graphite flakes, can be formed and cut into various shapes.

[0047] Graphite starting materials suitable for use in the present invention include highly graphitic carbonaceous materials capable of intercalating organic and inorganic acids as well as halogens and then expanding when exposed to heat. These highly graphitic carbonaceous materials most preferably have a degree of graphitization of about 1Ø As used in this disclosure, the term "degree of graphitization" refers to the value g according to the formula:
g - 3.45 - d(002) 0.095 where d(002) is the spacing between the graphitic layers of the carbons in the crystal structure measured in Angstrom units. The spacing d between graphite layers is measured by standard X-ray diffraction techniques. The positions of diffraction peaks corresponding to the (002), (004) and (006) Miller Indices are measured, and standard least-squares techniques are employed to derive spacing which minimizes the total error for all of these peaks. Examples of highly graphitic carbonaceous materials include natural graphites from various sources, as well as other carbonaceous materials such as graphite prepared by chemical vapor deposition, high temperature pyrolysis of polymers, or crystallization from molten metal solutions and the like. Natural graphite is most preferred.

[0048] The graphite starting materials used in the present invention may contain non-graphite components so long as the crystal structure of the starting materials maintains the required degree of graphitization and they are capable of exfoliation. Generally, any carbon-containing material, the crystal structure of which possesses the required degree of graphitization and which can be exfoliated, is suitable for use with the present invention.
Such graphite preferably has a purity of at least about eighty weight percent.
More preferably, the graphite employed for the present invention will have a purity of at least about 94%. In the most preferred embodiment, the graphite employed will have a purity of at least about 98%.

[0049] A common method for manufacturing graphite sheet is described by Shane et al. in U.S. Patent No. 3,404,061, the disclosure of which is incorporated herein by reference. In the typical practice of the Shane et al. method, natural graphite flakes are intercalated by dispersing the flakes in a solution containing e.g., a mixture of nitric and sulfuric acid, advantageously at a level of about 20 to about 300 parts by weight of intercalant solution per 100 parts by weight of graphite flakes (pph). The intercalation solution contains oxidizing and other intercalating agents known in the art. Examples include those containing oxidizing agents and oxidizing mixtures, such as solutions containing nitric acid, potassium chlorate, chromic acid, potassium permanganate, potassium chromate, potassium dichromate, perchloric acid, and the like, or mixtures, such as for example, concentrated nitric acid and chlorate, chromic acid and phosphoric acid, sulfuric acid and nitric acid, or mixtures of a strong organic acid, e.g.
trifluoroacetic acid, and a strong oxidizing agent soluble in the organic acid.
Alternatively, an electric potential can be used to bring about oxidation of the graphite. Chemical species that can be introduced into the graphite crystal using electrolytic oxidation include sulfuric acid as well as other acids.

[0050] In a preferred embodiment, the intercalating agent is a solution of a mixture of sulfuric acid, or sulfuric acid and phosphoric acid, and an oxidizing agent, i.e. nitric acid, perchloric acid, chromic acid, potassium permanganate, hydrogen peroxide, iodic or periodic acids, or the like.
Although less preferred, the intercalation solution may contain metal halides such as ferric chloride, and ferric chloride mixed with sulfuric acid, or a halide, such as bromine as a solution of bromine and sulfuric acid or bromine in an organic solvent.

[0051] The quantity of intercalation solution may range from about 20 to about 350 pph and more typically about 40 to about 160 pph. After the flakes are intercalated, any excess solution is drained from the flakes and the flakes are water-washed. Alternatively, the quantity of the intercalation solution may be limited to between about 10 and about 40 pph, which permits the washing step to be eliminated as taught and described in U.S.
Patent No. 4,895,713, the disclosure of which is also herein incorporated by reference.

[0052] The particles of graphite flake treated with intercalation solution can optionally be contacted, e.g. by blending, with a reducing organic agent selected from alcohols, sugars, aldehydes and esters which are reactive with the surface film of oxidizing intercalating solution at temperatures in the range of 25 C and 125 C. Suitable specific organic agents include hexadecanol, octadecanol, 1-octanol, 2-octanol, decylalcohol, 1, 10 decanediol, decylaldehyde, 1-propanol, 1,3 propanediol, ethyleneglycol, polypropylene glycol, dextrose, fructose, lactose, sucrose, potato starch, ethylene glycol monostearate, diethylene glycol dibenzoate, propylene glycol monostearate, glycerol monostearate, dimethyl oxylate, diethyl oxylate, methyl formate, ethyl formate, ascorbic acid and lignin-derived compounds, such as sodium lignosulfate. The amount of organic reducing agent is suitably from about 0.5 to 4% by weight of the particles of graphite flake.

[0053] The use of an expansion aid applied prior to, during or immediately after intercalation can also provide improvements. Among these improvements can be reduced exfoliation temperature and increased expanded volume (also referred to as "worm volume"). An expansion aid in this context will advantageously be an organic material sufficiently soluble in the intercalation solution to achieve an improvement in expansion. More narrowly, organic materials of this type that contain carbon, hydrogen and oxygen, preferably exclusively, may be employed. Carboxylic acids have been found especially effective. A suitable carboxylic acid useful as the expansion aid can be selected from aromatic, aliphatic or cycloaliphatic, straight chain or branched chain, saturated and unsaturated monocarboxylic acids, dicarboxylic acids and polycarboxylic acids which have at least 1 carbon atom, and preferably up to about 15 carbon atoms, which is soluble in the intercalation solution in amounts effective to provide a measurable improvement of one or more aspects of exfoliation. Suitable organic solvents can be employed to improve solubility of an organic expansion aid in the intercalation solution.

[0054] Representative examples of saturated aliphatic carboxylic acids are acids such as those of the formula H(CH2)nCOOH wherein n is a number of from 0 to about 5, including formic, acetic, propionic, butyric, pentanoic, hexanoic, and the like. In place of the carboxylic acids, the anhydrides or reactive carboxylic acid derivatives such as alkyl esters can also be employed.
Representative of alkyl esters are methyl formate and ethyl formate.
Sulfuric acid, nitric acid and other known aqueous intercalants have the ability to decompose formic acid ultimately to water and carbon dioxide.
Because of this, formic acid and other sensitive expansion aids are advantageously contacted with the graphite flake prior to immersion of the flake in aqueous intercalant. Representative of dicarboxylic acids are aliphatic dicarboxylic acids having 2-12 carbon atoms, in particular oxalic acid, fumaric acid, malonic acid, maleic acid, succinic acid, glutaric acid, adipic acid, 1,5-pentanedicarboxylic acid, 1,6-hexanedicarboxylic acid, 1,10-decanedicarboxylic acid, cyclohexane-1,4-dicarboxylic acid and aromatic dicarboxylic acids such as phthalic acid or terephthalic acid. Representative of alkyl esters are dimethyl oxylate and diethyl oxylate. Representative of cycloaliphatic acids is cyclohexane carboxylic acid and of aromatic carboxylic acids are benzoic acid, naphthoic acid, anthranilic acid, p-aminobenzoic acid, salicylic acid, o-, m- and p-tolyl acids, methoxy and ethoxybenzoic acids, acetoacetamidobenzoic acids and, acetamidobenzoic acids, phenylacetic acid and naphthoic acids. Representative of hydroxy aromatic acids are hydroxybenzoic acid, 3-hydroxy-l-naphthoic acid, 3-hydroxy-2-naphthoic acid, 4-hydroxy-2-naphthoic acid, 5-hydroxy-l-naphthoic acid, 5-hydroxy-2-naphthoic acid, 6-hydroxy-2-naphthoic acid and 7-hydroxy-2-naphthoic acid.
Prominent among the polycarboxylic acids is citric acid.

[0055] The intercalation solution will be aqueous and will preferably contain an amount of expansion aid of from about 1 to 10%, the amount being effective to enhance exfoliation. In the embodiment wherein the expansion aid is contacted with the graphite flake prior to or after immersing in the aqueous intercalation solution, the expansion aid can be admixed with the graphite by suitable means, such as a V-blender, typically in an amount of from about 0.2% to about 10% by weight of the graphite flake.

[0056] After intercalating the graphite flake, and following the blending of the intercalant coated intercalated graphite flake with the organic reducing agent, the blend is exposed to temperatures in the range of 25 to 125 C to promote reaction of the reducing agent and intercalant coating. The heating period is up to about 20 hours, with shorter heating periods, e.g., at least about 10 minutes, for higher temperatures in the above-noted range. Times of one half hour or less, e.g., on the order of 10 to 25 minutes, can be employed at the higher temperatures.

[0057] The above described methods for intercalating and exfoliating graphite flake may beneficially be augmented by a pretreatment of the graphite flake at graphitization temperatures, i.e. temperatures in the range of about 3000 C and above and by the inclusion in the intercalant of a lubricious additive, as described in International Patent Application No.
PCT/US02/39749.

[0058] The pretreatment, or annealing, of the graphite flake results in a significantly increased expansion (i.e., increase in expansion volume of up to 300% or greater), when the flake is subsequently subjected to intercalation and exfoliation. Indeed, desirably, the increase in expansion is at least about 50%, as compared to similar processing without the annealing step. The temperatures employed for the annealing step should not be significantly below 3000 C, because temperatures even 100 C lower result in substantially reduced expansion.

[0059] The annealing of the present invention is performed for a period of time sufficient to result in a flake having an enhanced degree of expansion upon intercalation and subsequent exfoliation. Typically the time required will be 1 hour or more, preferably 1 to 3 hours and will most advantageously proceed in an inert environment. For maximum beneficial results, the annealed graphite flake will also be subjected to other processes known in the art to enhance the degree expansion - namely intercalation in the presence of an organic reducing agent, an intercalation aid such as an organic acid, and a surfactant wash following intercalation. Moreover, for maximum beneficial results, the intercalation step may be repeated.

[0060] The annealing step of the instant invention may be performed in an induction furnace or other such apparatus as is known and appreciated in the art of graphitization; for the temperatures here employed, which are in the range of 3000 C, are at the high end of the range encountered in graphitization processes.

[0061] Because it has been observed that the worms produced using graphite subjected to pre-intercalation annealing can sometimes "clump"
together, which can negatively impact area weight uniformity, an additive that assists in the formation of "free flowing" worms is highly desirable. The addition of a lubricious additive to the intercalation solution facilitates the more uniform distribution of the worms across the bed of a compression apparatus (such as the bed of a calender station conventionally used for compressing (or "calendering") graphite worms into flexible graphite sheet.
The resulting sheet therefore has higher area weight uniformity and greater tensile strength. The lubricious additive is preferably a long chain hydrocarbon, more preferably a hydrocarbon having at least about 10 carbons. Other organic compounds having long chain hydrocarbon groups, even if other functional groups are present, can also be employed.

[0062] More preferably, the lubricious additive is an oil, with a mineral oil being most preferred, especially considering the fact that mineral oils are less prone to rancidity and odors, which can be an important consideration for long term storage. It will be noted that certain of the expansion aids detailed above also meet the definition of a lubricious additive. When these materials are used as the expansion aid, it may not be necessary to include a separate lubricious additive in the intercalant.

[0063] The lubricious additive is present in the intercalant in an amount of at least about 1.4 pph, more preferably at least about 1.8 pph.
Although the upper limit of the inclusion of lubricous additive is not as critical as the lower limit, there does not appear to be any significant additional advantage to including the lubricious additive at a level of greater than about 4 pph.

[0064] The thus treated particles of graphite are sometimes referred to as "particles of intercalated graphite." Upon exposure to high temperature, e.g. temperatures of at least about 160 C and especially about 700 C to 1200 C and higher, the particles of intercalated graphite expand as much as about 80 to 1000 or more times their original volume in an accordion-like fashion in the c-direction, i.e. in the direction perpendicular to the crystalline planes of the constituent graphite particles. The expanded, i.e. exfoliated, graphite particles are vermiform in appearance, and are therefore commonly referred to as worms. The worms may be compressed together into flexible articles that, unlike the original graphite flakes, can be formed and cut into various shapes and provided with small transverse openings by deforming mechanical impact as hereinafter described.

[0065] It is possible, but not preferred, to treat the flexible graphite material with resin since the absorbed resin, after curing, can enhance the moisture resistance and handling strength, i.e. stiffness, of the flexible graphite article as well as "fixing" the morphology of the article. This may not be desired, since stiffness may be an undesirable trait in this application.

[0066] If desired, however, suitable resin content is preferably at least about 5% by weight, more preferably about 10 to 35% by weight, and suitably up to about 60% by weight. Resins found especially useful in the practice of the present invention include acrylic-, epoxy- and phenolic-based resin systems, fluoro-based polymers, or mixtures thereof. Suitable epoxy resin systems include those based on diglycidyl ether of bisphenol A (DGEBA) and other multifunctional resin systems; phenolic resins that can be employed include resole and novolac phenolics. Optionally, the flexible graphite may be impregnated with fibers and/or salts in addition to the resin or in place of the resin. Additionally, reactive or non-reactive additives may be employed with the resin system to modify properties (such as tack, material flow, hydrophobicity, etc.). Of particular interest are additives, which increase the electrical resistivity of the graphite sheet. These resistivity-increasing additives include metal, polymer, synthetic graphite, alloy and combinations thereof. These additives may added as dispersed components or as layers or as embedded sheet in the graphite sheet.

[0067] Alternatively, the flexible graphite sheets of the present invention may utilize particles of reground flexible graphite sheets rather than freshly expanded worms, as discussed in International Patent Application No. PCT/US02/16730. The sheets may be newly formed sheet material, recycled sheet material, scrap sheet material, or any other suitable source.

[0068] Also the processes of the present invention may use a blend of virgin materials and recycled materials.

[0069] Flexible graphite materials prepared according to the foregoing description can also be generally referred to as compressed particles of exfoliated graphite. Since the materials may be resin-impregnated, the resin in the sheets needs to be cured before the sheets are used in their intended application.

[0070] According to the invention, flexible graphite materials prepared as described above are compressed to the desired thickness and shape, commonly a thickness of about 0.075 mm to 25 mm, at which time the impregnated flexible mats have a density of about 0.5 g/cc to about 1.7 g/cc.

[0071] One type of apparatus for continuously forming compressed flexible graphite materials, which may or may not be resin-impregnated, is shown in International Publication No. WO 00/64808, the disclosure of which is incorporated herein by reference.

[0072] If it is desired to employ flexible graphite sheets having adapted resistivity (that is, a total resistance, including the resistance of the graphite material itself, plus it's surface-to-surface contact resistance) greater than that of graphite sheets prepared as described above, resin-impregnated sheets may be required and further alignment of the graphene layers of those graphite sheets is also necessary. To do so, following the compression step (such as by calendering), the impregnated materials are cut to suitable-sized pieces and placed in a press, where they are cured at an elevated temperature. The temperature should be sufficient to ensure that the lamellar structure is densified at the curing pressure, while the thermal properties of the structure are not adversely impacted. Generally, this will require a temperature of at least about 90 C, and generally up to about 200 C. Most preferably, cure is at a temperature of from about 150 C to 200 C. The pressure employed for curing will be somewhat a function of the temperature utilized, but will be sufficient to ensure that the lamellar structure is densified without adversely impacting the electrical properties of the structure. Generally, for convenience of manufacture, the minimum required pressure to densify the structure to the required degree will be utilized. Such a pressure will generally be at least about 7MPa, (equivalent to about 1000 pounds per square inch), and need not be more than about 35 MPa (equivalent to about 5000 psi), and more commonly from about 7 to about 21 MPa (1000 to 3000 psi). The curing time may vary depending on the resin system and the temperature and pressure employed, but generally will range from about 0.5 hours to 2 hours. After curing is complete, the composites are seen to have a density of at least about 1.8 g/cm3 and commonly from about 1.8 g/cm3 to 2.0 g/cm3.

[0073] Although the formation of sheets through calendering or molding is the most common method of formation of the flexible graphite materials useful in the practice of the present invention, other forming methods can also be employed. For instance, the exfoliated graphite particles can be compression molded into a net shape or near net shape.
Thus, if the end application requires an article assuming a certain shape or profile, that shape or profile can be molded into the flexible graphite article, before or after resin impregnation. Cure would then take place in a mold assuming the same shape; indeed, in the preferred embodiment, compression and curing will take place in the same mold. Machining to the final shape can then be effected.

[0074] Likewise, it is also feasible that expansion of the particles of intercalated graphite can take place in situ in the compression mold, rather than by passing the graphite particles through a flame, followed by compression, resin impregnation and cure.

[0075] As discussed, the inventive method comprises the use of the thus-produced flexible graphite sheets for heating an electrolytic cell for the electrolytic production of metal from a molten compound of the metal, e.g., a salt or oxide, or a compound of the metal dissolved in a molten solvent. One commercial electrolytic cell to which the present invention is applicable is the Hall-Heroult cell for the manufacture of aluminum by electrolysis of alumina. Other metals produced by electrolysis in a fused electrolyte bath include magnesium, sodium, lithium, beryllium, boron, cerium, columbium (niobium), molybdenum, zirconium, tantalum, titanium, thorium and uranium.

[0076] Referring now to Figs. 1 and 2, a portion of an electrolytic cell for the production of aluminum is shown. The electrolytic cell comprises an outer shell 11, adjacent to which is an insulating lining 13 of a material such as alumina, bauxite, clay, magnesite, or aluminum silicate. Optionally, a refractory wall 15 is located adjacent the insulating lining 13. Adjacent insulating lining 13 and at the bottom of the cell is floor 17, which can be formed of a carbon material. When floor 17 is used to carry the current to current collector bars 6, it must be electrically conductive; when leads are used for carrying the current, floor 17 can be made of a non-conductive material.

[0077] Most preferably, floor 17 is formed of cathode blocks, formed of carbon, a semi-graphitic material or graphite, such as described in U.S.
Patent No. 6,723,212, to Paulus and Dreyfus, the disclosure of which is incorporated herein by reference.

[0078] Floor 17 and walls 15 of cell 10 generally define a chamber 18 having a lower zone adapted to receive a pool of molten aluminum 20 and an upper zone adapted to contain a body or charge of molten electrolyte or flux 30. Disposed at least partially within chamber 18 and partially immersed in electrolyte layer 30 is a plurality of anodes 40, which are usually of carbon, suspended from hangers 42, which can be of aluminum, iron, or copper. The position of the anodes can be adjusted vertically, i.e., raised or lowered, by conventional means. The hangers are connected to a bus bar (not shown) to connect the anode to the positive pole of the source of supply of electrolyzing current (not shown). As anodes 40 are consumed during the operation of cell 10, they are fed downwards. Anodes 40 are commonly arranged in a double row extending the length of the cell, as illustrated in Figs. 1 and 2.

[0079] Embedded in floor 17 are current collector bars 6. Current collector bars 6 serve to complete the electrical circuit by connection to a cathode bus system (not shown). Other means for withdrawing current from the cell can be employed also. However, in the arrangement described and during electrolysis, current passes serially from the anode bus system through anodes 40, electrolyte 30, molten aluminum pad 20 and floor 17 to collector bars 6 and thence to the cathode bus system which is connected to the cathode (negative) pole of the source of supply of electrolyzing current.

[0080] During operation of electrolytic cell 10, anodes 40 are immersed in molten electrolyte 30 and are spaced from pad of molten aluminum 20. In the event of a power outage or other operational difficulty that requires electrolysis to be discontinued, cell 10 will cool and the electrolyte 30 and elemental aluminum left in cell 10 will solidify to a hard mass, which is not easily removed.

[0081] Referring now to Figs. 3-5, in the practice of the present invention, after the frozen electrolyte 30 surrounding anodes 40 is loosened and anodes 40 are raised away from it, a material 50 capable of resistance heating is disposed in cell 10 over floor 17. As noted above, material 50 comprises one or more sheets of compressed particles of exfoliated graphite.

[0082] Generally, for use in preheating electrolytic cell 10 to temperatures of about 900 C or higher as is conventionally desired, the at least one sheet of compressed particles of exfoliated graphite which forms material 50 should have a thickness of about 0.075 mm to about 25 mm and a density of about 0.5 g/cc to about 1.7 g/cc. The electrical resistivity of the sheet should be at least about 500 Ohm-m.

[0083] In one embodiment, material 50 covers a major portion of floor 17, most preferably substantially the entirety of floor 17, to ensure uniform heat spreading across floor 17, as shown in Figs. 3 and 4. Alternatively, it is possible to simply apply the heat generated in material 50 in the area of floor 17 corresponding to the "footprint" of anodes 40, that is, the area of floor that would be contacted by anodes 40 if anodes 40 were extended down into contact with floor 17, as shown in Fig. 5. In addition, coverage of material can be in an area smaller than the "footprint" of anodes 40 (not shown). The coverage of material 50 on floor 17 can be adjusted according to the electrical resistance of material 50.

[0084] To reheat electrolytic cell 10, anodes 40 are lowered until they contact conducting material 50 directly beneath them. When current is permitted to flow through anodes 40, it passes through material 50 and thence through floor 17 to the cathodic current collection system.

[0085] Material 50 is heated by its internal resistance to the flow of current, and it in turn conducts and radiates heat to the interior of cell 10, i.e., to the solid electrolyte 30, metal pad 20, and the internal elements of cell 10, thereby providing the heat to melt solid electrolyte 30 and metal pad 20 and bring cell 10 to its operational temperature. This transfer of heat, principally by conduction and radiation, from material 50 to the interior of the electrolytic cell results in a gentle heating of the internal elements of the cell to the desired temperature.

[0086] By interior of cell 10 is meant those elements of cell 10 which are at the operational temperature of cell 10 during typical cell operation, such as walls 15, floor 17 and other internal parts of cell 10. Naturally, the exterior of cell 10 and layers of material between the exterior and interior of cell 10 will become heated (by conduction and convection) from the heat present in the interior of cell 10. However, it is necessary only to heat the internal elements to the desired temperature so that electrolysis can begin.
Thereafter, the heat generated during electrolysis will maintain cell 10 at its operating temperature.

[0087] The above description is intended to enable the person skilled in the art to practice the invention. It is not intended to detail all of the possible variations and modifications, which will become apparent to the skilled worker upon reading the description. It is intended, however, that all such modifications and variations be included within the scope of the invention, which is defined by the following claims. The claims are intended to cover the indicated elements and steps in any arrangement or sequence, which is effective to meet the objectives intended for the invention, unless the context specifically indicates the contrary.

Claims (10)

1. A process for preheating an electrolytic cell, comprising disposing at least one sheet of compressed particles of exfoliated graphite across at least a portion of the surface of the cathode blocks in the cell.

2. The process of claim 1, further comprising bringing the anodes of the electrolytic cell into contact with at least one sheet of compressed particles of exfoliated graphite.

3. The process of claim 2, wherein a current is then passed from the anodes into and through the at least one sheet of compressed particles of exfoliated graphite to resistively heat the graphite sheet and thereby preheat the electrolytic cell.

4. The process of claim 3, wherein the electrolytic cell is preheated to temperatures of about 900°C or higher.

5. The process of claim 4, wherein the at least one sheet of compressed particles of exfoliated graphite has a thickness of about 0.075 mm to about 25 mm.

6. The process of claim 5, wherein the at least one sheet of compressed particles of exfoliated graphite has a density of about 0.5 g/cc to about 1.7 g/cc.

7. The process of claim 6, wherein at least one sheet of compressed particles of exfoliated graphite has an electrical resistivity of at least about 500µOhm-m.

8. The process of claim 2, wherein the at least one sheet of compressed particles of exfoliated graphite is glued to the bottom of the anode.

9. The process of claim 1, further comprising imbedding the exfoliated graphite particles with a resistivity additive.

10. The process of claim 9 wherein the resistivity additive is selected from the group consisting of metal, polymer, synthetic graphite, alloy and combinations thereof.