GB2135334A - Composite carbon electrode - Google Patents

Abstract

In a composite carbon electrode comprising an inner part (2) and an outer part (3), the outer part having a greater porosity than the inner part, the inner and outer parts are bonded together. The outer part (3) can be formed in position on the inner part (2) and thereby or thereafter be bonded thereto.The two parts can be formed simultaneously or separately. The outer part can be at least one coating of a precursor applied to a core comprising a precursor for the inner part and the parts be carbonised either in one stage or successively. Where more than one layer is employed for the outer part, these can be of increasing porosity. Carbon fibre precursor can be employed for the outer part. The electrode is used for the generation of fluorine by electrolysis of a fused electrolyte. <IMAGE>

Description

SPECIFICATION Composite carbon electrodes This invention relates to carbon electrodes for use in electrolytecellsforthegeneration of fluorine by electrolysis of a fused electrolyte. The generation of fluorine using a fused electrolyte containing potassium fluorine and hydrogen fluoride iswell-known.

Veryfew materials are suitable for use as anodes in electrolyte fluorine cells. Carbon and nickel have both been used, however, and carbon is preferred because it has the higher current efficiencyforfluorine generation. In 'high temperature cel Is' which operate at a temperatureof approximately250 C and use an electrolyte of composition KF.HF the anodes can be graphite butthesetendto degrade in the electrolyte in medium temperature cells' which operate typically at 80-1 00 C and use an electrolyte of approximate composition KF.2HF.Consequently carbon anodes in medium temperature fluorine cells tend to have a low graphite content and normally comprise a material derived from coke particles mixed with a tar or pitch binder, which is pressed into a block and carbonised by heating to 1 0000C or more. Such anodes may be of high or low permeability or a composite ofthe two.

Low permeability carbon anodes have the disadvantage that they tend to become non-wetted by electrolyte due to build up ofcarbonfluoride (CFx)n film and can become polarised. Bythisterm itis meant thatthe currentflowthrough the cell drops markedly for a given applied voltage atwhich electrolysis earlier may have been proceeding satisfactorily. Application of high voltage (say 50V) for a short period can be used to overcome the polarisation. Alternatively, addition of metal fluorides such as those oflithium and aluminium to the electrolyte, or incoporation of these into the carbon can be used promote anode wetting and hence discourage the occurrence of polarisation.

The presence of dissolved nickel salts usually through slow corrosion of nickel or Monel cell components can also reduce the occurence of polarisation. High permeability carbon anodes on the other hand are known to be relatively free from polarisation difficulties, particularly when operated in the presence of dissolved nickel salts, at current densities per geometric area in excess of those possible with low per meability carbon. In addition, the pores within the anode structure are used to transport the generated fluorine away from the anode-electrolyte interface.

Free bubbleformation can therefore be controlled, and operation using a narrow gap between the anode and gas separation skirt is possible. This is a factor in minimising the cell working voltage. The disadvantages of typical high permeability carbons however are that they are mechanically weak and degradation can occur by chemical attack in use. Further, the material is difficult to support in the cell by means of clamps, bolts etcto the electrical connection hanger. It is the purpose of the composite carbon anode which is to be described to overcome problems associated with the use of high permeability carbon and yet to retainthe advantages of having this material atthe anode-electrolyte interface.

A composite carbon electrode as the term is used in this specification is an electrode in which an inner part comprises carbon forming a dense core and an outer part comprises carbon forming on the core, but not necessarily on the top and bottom surfaces of the core, at least one layer having a higher degree of porosity and permeability than the carbon forming the core.

Such composite carbon electrodes are already known for use in electrochemical fluorination processes in which fluorine is produced forfluorination of material in the more porous outer part ofthe electrode. In these processes the composite electrode is not designed entirely to facilitate the generation of fluorine butto provide a reaction zone for the subsequentfluorina- tion of other material introduced at the lower end of the electrode. The problem of ensuring a good electrical connection between the two carbon parts of the electrode has however been recognised. It has been reported that a tig htfriction fit between the parts is notsufficientand useofcements is notfavoured due to their susceptibility to attack by the electrolyte.

Improved conductivity is achieved by placing a layer of finely divided carbon between the parts.

The present invention consists in a complete carbon electrode in which the inner and outer parts forum a bonded unit.

The invention also consists in the production of a composite carbon electrode by forming the outer part in position on the inner part and thereby or thereafter bonding the partstogether.

The inner part may be formed simultaneously with the outer part, for example, during carbonisation of a core which is the precursorofthe inner part, at least one coating of an appropriate precursorforthe outer part being applied to the core prior to carbonisation of the core and even prior to pressing into its final shape.

Alternatively, such coating or coatings may be applied aftercarbonisation of the core.

An electrode may be built up of a succession of layers of carbon of gradually increasing porosity and permeability, by carbonisation of coatings individual lyortogether,thecarbonisation procedure being chosen to limitthe tendency of the thermal treatment to produce strains sufficient to disruptthe composite.

The gradually increasing porosity and permeability may be achieved from coke-binder mixtures differing in particle size distribution ofthefiller coke.

Typically, for use in an electrolytic cells for the generation of fluorine by electrolysis of a fused electrolyte the outer part hasa thickness of 5 mm or -less and the inner part has a thickness of at least 20mm. These dimensions are considered generally desirable if the full benefit of the use of a composite carbon electrode is to be obtained but with an electrode in accordance with the invention the problem of electrical resistance at the junction of the carbon layers is not as great as in the case of separately machined parts. The minimum thickness of the outer layer is not limited by its effect on the voltage drop along the working electrode, because reliable conduction can be maintained from the electrode support to the outer iayerthrough the carbon of the inner part. The carbons forming each part should have a low graphite content and physical properties as given in the Table below are considered most suitable.

TABLE-- PROPERTIES OF CARBONS SUITABLE FOR FORMING THE COMPOSITE ELECTRODE Property Core Outer Layer Bulkdensity(gcm-3) 1.4-1.7 1.0-1.3 Open porosity (%) 20 - 25 30 - 45 Average pore size ( 10 - 20 30 - 100 Bulkpermeability* S 0.1 1.0-4.0 Tensile strength (kg cm-2) 100 -- 250 40 - 80 * Measured asthe flow rate of nitrogen in cm3 second passing through 1cm2 of specimen 2.5 thick under a pressure of 5cm water gauge.

Afurthertype of carbon suitable forforming the outer part ofthe composite electrode is carbon fibre.

It is not necessary for the outer part to coverthe top and bottom surfaces of the core or inner part. Thus load-carrying means may be secured to the core and a sprayed-on nickel coating supply an electrical cc nnection between the load-carrying means and the other part ofthe electrode. It may also be desirable for the outer part to cover only the lower section of the core length, up to a level which is just below the intended immersion level forthe electrode in the electrolyte. This leaves the high permeability outer layer available fortransport of product gas from the site of generation to a level inside the desired cell compartment.In this case, load-carrying means may be secured to the core and a sprayed-on nickel coating maybe used to improve the electrical connection between the load-carrying means and the core.

Production of electrodes in accordance with the invention may be based on procedures known in the carbon manufacturing industry. A controlled size distribution of coke filler particles derived from coal tar or petroleum is thoroughly mixed attemperatures from ambientto about 220"C with a pitch binder derived from coal tar, petroleum pitch or synthetic polymer resins, eg phenol formaldehyde and furfuryl alcohol polymers. The core and subsequent layers are formed in turn by pressing or extrusion, with some cooling from the mixing temperature permissi bleduringtheforming. Removal of volatiles and carbonisation or baking take place after each forming stage orofthe inner and outer parts together.With the material contained in a furnace under inert atmosphere or sealed from the outside atmosphere, the procedure is to raise the temperature slowly over a period extending to several days upto 1000 1 300 C. This is followed by slow cooling, again over periods up to several days in order to avoid crack formation.

Pore size distribution and density of each carbonised part are conveniently controlled by changing the particle size distribution of filler coke, or by subsequentvacuum impregnations with pitch or synthetic resins and further re-carbonisation.

A commercially available dense carbon or one made according to the above procedure can be used forthe inner part of a composite electrode in which the outer part comprises carbon fibre based material, processes for manufacturing permeable carbonfibre based material being known in the art.

The invention will now be described with reference to the accompanying drawings which are diagrammatic medial cross-sections and show in Figures 1 and 2 alternative constructions of composite electrodes including supports in accordance with the invention, and in Figure 3 an electrolytic cell suitableforfluorine generation in which a composite electrode such as that shown in either Figure 1 or Figure 2 may be used. Like reference numerals indicated like parts in the drawings.

Figures 1 and 2 show cylindrical electrodes 1 of composite carbon structure in which the dense inner core 2 and outer high permeability part3 form a bonded unit. In the Figure 1 construction, the electrode 1 is provided with support comprising a machined flat nickel plate 4to which is welded a threaded steel or nickel rod 5 secured at its lower end 6 by being screwed into a hole 7 drilled and tapped in the core 2. The nickel plate rests on the core, and may as shown in this particular Figure, also overlap the core, to rest on the outer part3 to which it is joined by a sprayed-on coating 8 of molten nickel to a depth of up to several millimetres overthe plates and upper region ofthe electrode length.This length should be such that the nickel-covered region does not extend to the intended immersion level ofthe electrode in the cell electrolyte, otherwise the nickel would be subjectto dissolution atthe anode operating potential. In orderto ensure good adhesion ofthe sprayed-on nickel the surfaces of the nickel plate can first be roughened by mechanical abrasion.

In the Figure 2 construction the outer carbon part 3 covers only the lower section ofthe core 2, up to a level which is just below the intended immersion level of the electrode in the electrolyte. The nickel plate 4 covers only the core 2to which it is joined by a sprayed-on coating 8 of molten nickel as described for Figure 1. With an electrode ofthis type, electrical conduction from the supportto the outer part is through the core. Whilst Figures 1 and 2 illustrate cylindrical electrodes the principles ofthe invention may be readily applied to large rectangular electrodes.

An electrolytic fluorine cell suitable for using the composite carbon electrode 1 as an anode is shown in Figure 3. In this Figure a steel cell body 10 which acts as the cathode through a clamp connection at 11 to the electrical supply, is closed by a steel lid 12 from which it is electrically insulated by chloroprene rubber orfluorinated elastomer 13. The lid 12 supports a nickel or Monel gas separation skirt 14 extending below the electrolyte level 15. The gas separation skirt 14 prevents mixing of gases pro duced atthe anode and cathode by forming two separate compartments atthetop of the cell for which pipes, 16, 17,18, 19 are provided to remove off-gas and to effect purging with nitrogen.The cell is also provided with a dip pipe 20 through which hydrofluoric acid used in the electrolysis is replenished.

Means to supply heat through the cell walls and a thermocouple to control the electrolyte temperature are necessary, but these are not shown in the drawing. The cell in normally operated using in electrolyte of composition KF.2HF (41 % by weight HF) at a temperature in the range 80 - 1 000C. Awater cooling jacket 21 maintains a layer of frozen electrolyte 22 on the cell base. This prevents the base acting as a cathode and producing hydrogen which could then enterthe anode compartment. Alternatively a layer or sheet of suitable polymer such as poly(tetrafluoroethylene) can be used instead ofthe solid electrolyte layer to perform the same function.The composite carbon anode 1 is supported from the cell lid 12 and electrically insulated therefrom by means offluorinated elastomer23 and other more rigid polymeric materials 24. The electrical supply is connected to the anode through an external clamp connection at 25. In use ofthe cell evolved fluorine mightattackthe anode 1 above the immersion level.

This may be suppressed to some extent by diluting thefluorine in the anode gas compartmentwith a nitrogen flow.

The advantages of the invention are illustrated by the following: Three cylinders of length 180 mm and diameters 33,38 and 41 mm were machined from a commercially available dense carbon, and a hole in which to secure a supportwas drilled and tapped to a depth of 30 mm in the centre of one end of each cylinder.

These cylinders were then used as cores in the assembly of bonded composite electrodes of outside diameter43 mm. The outer high permeability parts of thickness 5,2.5 and 1 mm comprised carbonised carbon fibre. The composite electrodes were attached to supports as follows. A nickel disc of diameter 35 mm through which had been welded a threaded steel rod wassecured by means ofthe lower length of rod into the dense carbon, until the lower surface of the disc was resting firmly on the core surface, or on the surface of both core and outer carbon. The surfaces of the disk were earlier roughened by mechanical abrasion in order to ensure a good adhesion during thefinal assembly stage, in which molten nickel was sprayed overthe disc and upper region (approx. 30 mm) ofthe outer carbon to a depth of 1 to 2 mm.

The performance of each electrode as a fluorine cell anode was studied in an electrolytic cell ofthetype shown in Figure 3. Immersion periods were 14,7 and 9 days respectivelyforthe 5,2.5 and 1 mm fibre layer anodes. In all cases a high current efficiency and absenceofflourine escape to the cathode compartmentwere indicated by measurements ofthe cell off-gas pressures and anode off-gas flow rate. At the end of the experimentsthe anodes were in good condition and without dimensional changes below the electrolyte immersion level. Above this level the fibre layers were pitted.

Overall, this shows that a high porosity, high permeability outer part only 1 mm thick can be used in a bonded composite electrode to transport fluorine awayfrom the site of generation thereby reducing the incidence of polarisation whilst retaining high current efficienty. The cell voltage is not adversely affected by using such a thin layer which indicates that satisfactory electrical conduction is achieved from the supportto the outer part via the core.

In comparison,considerthetesting of anodes of similar length and outside diameterto the anode constructions referred to earlier, but in this instance fabricated entirely from carbon fibre material. Each anode was attached to a support simply by overspraying the nickel disc and upper region ofthe carbon with molten nickel, ie without the additional screw connection used in the earlier constructions.

Immersion periods forthese anodes were up to 30 days. The cell was operated largelywithoutpolarisa- tion difficulties. In one experiment using a cell equipped with polycarbonate viewing ports and electrolyte containing an above normal HF concentration, it was possible to demonstrate that the anode remained free offluorine bubbles at all applied current densities. Chemical attack did not occur at the immersed sections ofthe anodes in electrolytes containing upto 44.6% by weight of HF and in fresh electrolyte containing high levels of impurities such as water and sulphate the anodes were susceptible to attack byfluorine above the immersion level, as shown by the pitting ofsome specimens. In the case of the specimen subjected to the highest current densitiesthe attack eventually led to the immersed section becoming detached from the remainder of the upper section.

Overall, this comparison shows that a high porosity carbon fibre anode can be used to generate fluorine largelywithout polarisation problems and that the carbon fibre has high resistance to chemical attack by the electrolyte. The major disadvantage is fluorine attack above the electrolyte level.

Claims (13)

1. A composite carbon electrode as hereinbefore defined, wherein the inner and outer parts form a bonded unit.

2. A process of forming an electrode according to claim 1, comprising forming the outer part in position on the inner part and thereby or thereafter bonding the parts together.

3. A process according to claim 2, wherein the inner part is formed simultaneously with the outer part.

4. A process according to claim 3, wherein at least one coating of an appropriate precursorforthe outer part is applied to the core prior to carbonisation of such core which is the precursor ofthe inner part.

5. A process according to claim 4, wherein the said at least one coating is applied prior to pressing to final shape.

6. A process according to claim 2, wherein the or each coating of an appropriate precursorforthe outer part is or are applied after carbonisation of the core.

7. A process according to any of claims 2-6, including building up a succession of layers ofcarbon of gradually increasing porosity and permeability by carbonisation of coatings individually ortogether.

8. A process according to claim 2, wherein the outer part comprises carbon fibre.

9. A process according to any of claims 2-8, including incorporating a load carrying means by securing ittothe inner part, and spraying on a nickel coating between the load-carrying means and the outerto constitute an electrical connection.

10. Aprocessofforming a composite carbon electrode, substantially as hereinbefore described.

11. A composite carbon electrode when made by a process according to anyofclaims2-10.

12. A composite carbon electrode substantially as hereinbefore described with reference to Figures 1 and 2 ofthe drawings.

13. An electrolyte cellforthe generation of fluorine and having a composite carbon electrode as claimed in claim 11 orclaim 12.