GB2135335A - Supports for carbon electrodes - Google Patents

Abstract

A carbon electrode has load-carrying means (5) secured thereto and a sprayed- on nickel coating (8) providing an electrical connection between the load-carrying means and the outer part of the electrode. The electrode may comprise an inner part (2) consisting of a dense carbon core, around which there is an outer part (3) formed by a layer of carbon having a higher degree of porosity and permeability than the core carbon. A support plate (4) of nickel has a threaded steel or nickel rod (5) welded to it and with its lower end (6) screwed into a tapped hole (7) in the core (2). The plate (4) is secured and electrically connected to the outer layer (3) by the sprayed-on nickel coating (8). The electrode can alternatively consist of a single part in block form with its bulk density sufficient to retain the screwed rod therein when the electrode is suspended thereby, whilst having the necessary permeability to achieve the desired electrode performance. The electrode is used in cells for the generation of fluorine by electrolysis of a fused electrolyte. <IMAGE>

Description

SPECIFICATION Supports for carbon electrodes This invention relates to carbon electrodes for use in electrolyticcellsforthegeneration offluorine by electrolysis of a fused electrolyte. The generation of fluorine using a fused electrolyte containing potas siurflfluoride and hydrogen fluoride is well-known.

Veryfew materials are suitable for use as anodes in electrolyticfluorine cells. Carbon and nickel have both been used, however, and carbon is preferred beeausè ithasthe higher current efficiency for fluorine generation. In 'high temperature cells' which operateatatemperaturn of approximately 250"C and use an electrolyte of composition KF.HF, the anodes can bagraphite butthese tend to degrade in the electrolyte in 'medium temperature cells' which operatatypically at 80-700"C and use an electrolyte of approximate composition KF.2HF.Consequently carbon anodes in medium temperature fluorine cells tend to have a lowgraphite content and normally comprise a material derived from coke particles mixed with atarorpitch binder, which is pressured into a block and carbonised by heating to 10000C or more. Such anodes may be of high or low permeabil ityora compositeofthetwo.

Low permeability carbon anodes have the dis advantagethattheytend to become non-wetted by electrolyte due to build up of carbon fluoride (CFx)n film and can become polarised. Bythisterm it is meant that the currentflowthrough the cell drops markedlyfora given appliedvoltageatwhich electrolysis earlier may have been proceeding satisfactorily. Application of high voltage (say SoV) for a short period can be used to overcome the polarisation. Alternatively, addition of metal fluorides such as those of lithium and aluminium to the electrolyte, or incorporation ofthese into the carbon can be used to promote anode wetting and hence discourage the occurrence of polarisation.The presence of dissolved nickel salts usuallytlErough slow corrosion of nickel orMonel cell componentscan also reducethe occurrencaofpolarisation. High permeability carbon anodes on the other hand are known to be relatively free from polarisation difficulties, particularly when operated ihe presence of dissolved nickel salts, at current densities per geometric area in excess of those possible with low permeability carbon. In addition,,the pores within the anode structure are used to transport the generated fluorine awayfrom the anode-electrolyte interface. Free bubbleformation canthereforn 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 oftypical high perm.eability carbons howeverarethattheyare mechanicallyweakand degradation can occur by chemical attack in use. Further, the material is difficult tosupport in the cell by means of clamps, bolts etc two the electrical connection hanger. One way in which an electrical connection has been made is by spraying molten nickel or copper over a nickel or copper hanger plate pressed onto the top ofthe carbon block.

Even so, after a relatively short period of use such a joint can fail due to cracking of the sprayed-on nickel at the edges ofthe plate. It is known from the published literature that other common metals would not be suitable because the joint would be adversely affected either by dissolution ofthe metal or by the formation of non-conducting surface films atthe anode operating potential. It is the purpose of the composite carbon anode and the means of support which are to be described to overcome problems associated with the use of high permeability carbon and yet to retain the advantages of having this material at the anode-electrolyte interface.

Acomposite 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 ofthe core, a 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 entirelytofacilitatethe generation of fluorine butto provide a reaction zoneforthe subsequentfluorination ofother 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 tightfriction fit between the parts is not sufficient and use of cements is not favoured due to their susceptibility to attack by the electrolyte. Improved conductivity is achieved by placing a layer offinely divided carbon between the parts.

According to the invention a composite carbon electrode has load-carrying means secured to its inner part, and a sprayed-on nickel coating providing an electrical connection between the load-carrying means and the outer part ofthe electrode.

The load-carrying means may comprise a nickel plate attached to the electrode by at least one screw-threaded member extending through the plate and into a hole drilled and tapped in the inner part so thatthe plate rests on the electrode, and at least one rod (which may be an extensionn ofthe screw threaded member) on the opposite side ofthe plate to the electrode and welded to the plate, the rod being adapted to provide means whereby the electrode may be suspended and for connection to an electrical supply, and the sprayed-on nickel coating covering the plate and the outer part of the electrode in the vicinity ofthe plate.

With an electrode supported in accordance with the invention good electrical connection to the high permeability outer carbon layer is ensured irrespective ofthe tightness ofthe friction fit between this and the core carbon. Thus the resistance or voltage drop problem between core and outer carbons is rendered unimportant and the requirement for any material to be placed between the machined carbon parts is removed. In an extension ofthe invention the composite carbon electrode may comprise inner and outer parts forming a bonded unit produced, for example, by applying to a dense core a coating of an appropriate precursor wh ich is then thermally treated to form a Iayerofcarbon ofthe desired high permeability. The need to machine separate inner and outer parts may thereby be avoided.In all cases most ofthe carbon weight ofthe electrode comprises the core and this is adequately carried by the threaded rod ofthe electrode support. This leaves the sprayed-on nickel largelyfree of mechanical stress and therefore able to maintain a reliable electrical connection between the support and outercarbon layer.

It has also been found that a single carbon material, carefully manufactured to have the required fine structure or bulkdensityfor inserting a screwthreaded member, and with sufficient permeability to achieve the desired electrode performance can also be suitable forsupporting in the described manner.

The electrode in this case is formed from a single b!jck of carbon withoutthe need for a dense carbon core.

The types of carbon suitable for forming the core and outerlayerwould have physical properties approximately as given in the Table below.

TABLE--PROPERTIES OF CARBONS SUITABLE FOR FORMING THE ELECTRODE Property Core Outer Layer Bulk density (g cm-3) 1.4-1.7 1.0-1.3 Open porosity (%) 20-25 30-45 Average pore size (C1) 10-20 30-100 Bulk permeability* 60.1 1.0-4.0 Tensile strength (kg cm-2) 100-250 40-80 *Measured astheflow rate of nitrogen in cm3 per second passing through 1 cm2 of specimen 2.5cm thick under a pressure of 5cm water gauge.

Asuitablethicknessforthe outer carbon layer is 5mm or less for a core thickness between 20 and 60mm. In terms of chemical composition, both carbons should have a low graphite content.

An electrode suitableforsupport in the previously described manner, butformed from a single block of carbon, should have a bulk densitytowards the higher end ofthe range shown forthe outer layer and a porosityatthe lower end ofthe range shown forthe outer layer.

The invention will now be described with reference to the accompanying drawings which are diagrammatic medial cross-sections and show in Figure 1 a composite electrode and support in accordance with the invention and in Figure 2 an electrolytic cell suitable for fluorine generation in which a composite electrode such as that shown in Figure 1 may be used.

Figure 1 shows a cylindrical electrode 1 of composite carbon structure comprising a dense carbon core 2 around which is an outer layer 3 of carbon having a higherdegree of porosityand permeabilitythanthe core carbon. 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 in a hole 7 drilled and tapped in the core 2.

The nickel plate overlaps the core 2 and thus rests on the outer layer 3 to which it is secured by a sprayed-on coating 8 of molten nickel to a depth of up to several millimetres over the plate 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 besubjectto dissolution at the anode operating potential. In order to ensure good adhesion ofthe sprayed-on nickel the surfaces ofthe nickel plate can first be roughened by mechanical abrasion. Whilst Figure 1 illustrates a cylindrical electrode, the principles of the invention may be readily applied to large rectangular etec- trodes.An electrode support in thiscase may comprises a rectanguFarplateto which may he welded one or more threaded rodsforsuspending the electrode in the cell: and one rod extending through the plate to be secured intotlie=!core carbons Alternatively, bolts extending through the plate from- thetop but notwelded to itmay be secured into holes drilledandtapped in the core carbon. Thesupport may be applied to a composite electrode orto ani electrode formed from a sing le grade of carbun.

An electrolyticfluorine cell suitablefor using the composite carbon electrode 1 as an anode is shown in Figure 2. In this Figure a steel cell body 10which acts as the cathodethrough a clamp connection at 11 to the electrical supply, is closed by a steel lid 12 from which it is electrically insulated by chloroprene rubberorfluorinated elastomer 13. The lid 12 supportsanickelorMonel gas separation skirt 14 extending below the electrolyte level 15. The gas separation skirt 14 prevents mixing of gases produced atthe anode and cathode byforming two separate compartments at the top ofthe 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 heatthrough the cell walls and a thermocouple to control the electrolytetemperature are necessary, butthese are not shown in the drawing. The cell is normally operated using an 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 enter the anode compartment Alternatively a layer or sheet of suitable polymersuchaspoly- (tetrafluoroethylene) can be used instead ofthesolid electrolyte layer to perform the same function The composite carbon anode 1 is supportedfromthe cell lid 12 and electrically insulatedtherefrom by means offluorinated elastomer 23 and other more rigid polymeric materials 24. The electrical supply is connected to the anode through an external clamp connection at 25.

The advantages of the invention are illustrated by the following examples.

Example 1 Acylinder of dense carbon,diameter40 mm and length 175 mm, with a hole drilled and tapped in the centre of one end to a depth of 30 mm, was placed inside a sleeve of carbon, having a permeability atthe upperendofthe rangefortheouterlayershown in the Table, closed at the lower end with a wall thickness overall of approximately 5mm. A nickel disc of diameter49mm through which had been welded a threaded steel rod was secured by means of the lower length of rod into the dense carbon, until the lower surface of the disc ways resting firmly on the surface of both core and sleeve carbons.The surfaces of the disc were earlier roughened by mechanical abrasion in orderto ensure good adhesion during the final stage of the composite anode preparation, in which molten nickel was sprayed over the disc and upper region (approx 30mm) of the sleeve carbon to a depth of 1 to 3mm. After cooling the composite anode was attached to the lid of an electrolytic cell of the type shown in Figure 2, although electrically insulated from it by means offluorinated elastomer and other more rigid polymeric materials, using an external nut around the protruding end ofthethreaded rod.

The cell was operated for 36 days without polarisation difficulties at current densities up to 100 mA cm -2 of the immersed anode area, with a cell voltage between anode and cathode up to 8.5V. Upon removal from the cell the anode was in good condition and there was no evidence of cracks in the sprayed-on nickel. Subsequently, a short period of operation in an electrolytic cell equipped with polycarbonate viewing ports in the cell wall and containing filtered electrolyte showed that, atcurrentdensi- ties up to 100 mA cm9, no bubbles offluorine appeared to break away from the carbon surface and thatfluorine transport away from the electrolyte interface therefore occurred through the pores of the anode.

Example 2 A composite anode having the same dimensions as in Example 1 and supported in a like manner, was fabricated using dense core material ofthe same composition, but with an outer carbon having a permeability at the lower end ofthe rangeforthe outer layer shown in the Table. Cell operation with this anode was carried out for 60 days at anode current densities upto 100 mA cm-2 with a cell voltage normally below 9.2V. A high current efficien- cy and absence offluorine escape to the cathode compartment were indicated by measurements of the anode off-gas flow rate.Measurements of cell voltage and the voltage between the anode and a palladiumihydrogen reference electrode, on occasions indicated a slight polarisation of the anode.

Upon removal from the cell the anode and support were in good condition. There was no evidence of dimensional change in the carbon anode and no visible cracks in the sprayed-on nickel. This Example indicates that a lower limit for the overall permeabil- ity of a composite anode suitableforfluorine generation has been reached.

Example 3 A single grade of carbon having a bulk density at the higher end ofthe range forthe outer layer shown in the Table (1.3g cm-3) and a porosity at the lower end of the range for the outer layer (30%), machined to a cylinder of diameter 43mm and length 180mm was attached to an appropriate diameter support using the mannerdescribed in Example 1.Withthis anode the cell was operated without polarisation difficulties for 49 days at anodecurrent densities up to 100 mA cm-2 and cell voltage Lip to 9.5V. A high current efficiency and absence of fPu o ri ne escape to the cathode compartment were indioated by measurements ofthe anode off-gasflow rate.Upon removal from the cell the anode and support were in good condition, showing no signs of swelling or cracking.

Example 4 (Comparison) Anodes of similar dimensions to the anodes of Examples 1-3 but fabricated enti rely from carbon of the outer layer type having a bulk density at the lower end ofthe range shown in the Table (1.1 g cm3) were attached to electrode supports simply by overspraying the nickel disc and upper region ofthe carbon with molten nickel, ie without the screw reinforcement. Operation with this type of anode was intially satisfactory.At current densities up to 100 mA cm -2 operation was free of polarisation difficulties and the cell voltage was notgreaterthan 8.5V under controlled conditions of electrolyte temperature and HFconcentration. Operation in the cell equipped with viewing ports showed no evidence offluorine bubbles at the anode surface. However, after several weeks operation a number of disadvantages became apparent. The sprayed-on nickel became cracked leading to weakening of the anode support. The carbon itself was sometimes subject to cracking, swelling and crumbling below the electrolyte immersion level.It was particularly susceptible to attack in fresh electrolyte containing high levels of impurities such as water or sulphate, and in electrolyte having a high HF concentration (43% by weight).

Example 5 (Comparison) Electrodes of diameter 43mm and length 180mm but fabricated entirely of dense carbon materials were attached to supports using the manner described in Example 1. Operation with this type of anode was made difficult by polarisation problems.

In one experiment, when this difficulty was overcome by controlling the impurity levels in the electrolyte, explosions insidethe cell could sometimes be heard even at current densities as low as 10 mA cm-2. This suggested that fluorine was escaping from the anode surfaces and because of the narrow anode-gas separation skirt gap (11 mm) thefluorinewas passing into the hydrogen compartment of the cell. Operation in the cell equipped with viewing ports confirmed the formation offree bubbles offluorine at the anode surface at current densities as low as 10 mA cm-2.

Upon removal from the cell each anode-supportjoint showed no evidence of cracks in the sprayed-on nickel.

Examples 1-3 demonstrate both a range of carbon electrode structures and a means of supporting such structures in an electrolytic cell, which are suitable for the generation offluorine. Comparison Examples4 and 5 demonstrate the disadvantages found with electrode structures based entirely on dense carbon (free fluorine bubbles and polarisation) and on high permeability carbon (chemical attack and failure of its attachment in service). Examples 1-3 show the improvmentswhich can be made by applying the present invention.

Claims (6)

1. A composite carbon electrode as hereinbetore defined and having load-carrying means secured to its inner part, and a sprayed-on nickel coating providing an electrical connection between the loadcarrying means and the outer part of the electrode.

2. An electrode according to claim 1, wherein the load-carrying means comprises a nickel plate attached to the electrode by at least one screwthreaded member extending through the plate and into a hole drilled and tapped in the inner part so that the plate rests on the electrode, and at least one rod, which may or may not be an extension of the screw-threaded member, on the opposite side ofthe plate to the electrode and welded to the plate, the rod being adapted to provide means whereby the elec trode can be suspended and for connection to an electrical supply, and the sprayed-on nickel coating covering the plate and the outer part ofthe electrode in the vicinity ofthe plate.

3. A composite carbon electrode according to either of claims 1 and 2, wherein the electrode comprises inner and outer parts in a bonded unit produced by applying to a dense core defining the inner part a coating of an appropriate precursor which is then thermally treated to form a layer of carbon ofthe desired high permabilityand forming the outer part.

4. Acarbon electrode comprising a single block part, a screw threaded member screwed into a hole drilled and tapped in the said block, a nickel plate resting on the block and having the screwthreaded member extending through it, and a sprayed-on nickel coating providing an electrical connection between the block and the plate, the block having sufficient fine structure or bulk density to retain the screw threaded membertherein when the block is suspended by said member whilst possessing sufficient permeability to achieve the desired electrode performance.

5. A composite carbon electrode, substantially as hereinbefore described with reference to the accompanying drawings.

6. Acarbon electrode, substantially as hereinbefore described with reference to Examples 1 to 3 of the foregoing specification.