CA1181791A

CA1181791A - Apparatus for fusion electrolysis and electrode therefor

Info

Publication number: CA1181791A
Application number: CA000383628A
Authority: CA
Inventors: Konrad Koziol; Malcolm F. Pilbrow; Christine Zollner; Dieter H. Zollner
Original assignee: C Conradty Nuernberg GmbH and Co KG
Current assignee: C Conradty Nuernberg GmbH and Co KG
Priority date: 1980-10-27
Filing date: 1981-08-11
Publication date: 1985-01-29
Also published as: EP0050679B1; NO813601L; DE3071413D1; USRE32426E; DD201838A5; ATE17875T1; ES507055A0; NO156211C; EP0050679A1; ES8207595A1; NO156211B; JPS5776193A; HU188703B

Abstract

ABSTRACT OF THE DISCLOSURE
Electrode for fused melt electrolysis with a top portion (5) of metal, where appropriate with a cooling device (2,3) and the top portion (5) has an inner part (16) and an outer part (17) which are constructed so as to be detachable from each other and the inner part (16) extends substantially close to a connection or screw-mounting (1) and the top portion (5) or the inner part (16) or at least a partial zone thereof is protected by an insulating coating (4) of high temperature stability, and at least one bottom portion (6) of active material.
The electrode is suitable for the production of metals such as aluminium, magnesium, alkali metals or compounds thereof. It is characterised by reliable, energy saving operation and can be easily maintained and repaired.

Description

7~

~he invention relates to an electrode for fused melt electrol7sis, more particularly ~or the electro lytic production of metals such as aluminium~ magnesium, sodium, lithium or of compounds thereof r Carbon electrodes, made o~ hard carbon or ~raphite, are still mainly emplo~ed for the electrol~tio p:roduction o~ aluminium, magnesium, alkaline metals or compounds thereof on a commercial scale. Although the electrodes are intended mainly to carry current, they frequently ~0 also participate in the electrode reaction themselves.
~he actual electrodes consumption is therefore substan~
- tially higher than the theoretical rate Qf wear, due to the oxidation sensitivity of carbon electrodes under electrolysis conditions. ~he theoretical consumption rate in the fused melt electrolysis of aluminium is ~34 kg carbon/ton of aluminium~ but the actual carbon consumption amounts to approximately 450 kg of carbon/
ton o~ aluminium.
Similar problems arise for the electrodes used in the production o~ magnesium, sodium~ lithium and cerium metal mixes. ~ide reactions o~ an oxidizing kind on the electrode part which is immersed in the molten salt as well as losses due to atmospheric oxygen on the ,part which projects ~rom the melt result in irregular - 25 and premature wear of the electrodes. ~o this must be added the destructive action of the graphite deposits ~.

11~1 791 from electrode constituents on their products. Tests have already been undertaken, in which carbon electrodes are converted into a suitable electrode material by impregnation~ followed by thermochemical treatment and conversion into composite carbon-silicon carbide materials. However, in practice these tests have not led to any substantial improvement of fused melt electrolysis.
~he above-described disadvantages of carbon electrodes as well as rising costs of graphite and hard carbon have given rise to the development of form-stable electrodes.
It is hoped thereb~ not only to replace petro-car~on, a petrochemical raw material, the consumption of ~Jhich in the ~ederal German Republic for fused melt e:lectrolysis alone amounts to approximately 500 000 tans per annum, but also to achieve savings in energy consumption.
~o this end, a number of ceramic materials, for example in accordance with the British Patent Specifi-cation 1 152 124 (stabilized zirconium oxide), the US
Patent Specification 4 057 480 (substantially stannic oxide), the German Offenlegungsschrift 27 57 898 (sub-stantiall~ silicon carbide valve metal boride carbon), the South African Patent Application 77/19~1 (yttrium oxide with surface strata o~ electrocatalysts) or according to the German Offenlegungsschrift 24 46 314 (ceramic parent material with a coating of spinel compounds) have all been described.

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~ he disadvantage in the use of electrodes of ceramic materials in theix electrical conduc-tivity which is frequently only moderate to medium, even after the addition of conductivi-ty-increasing components. ~his is acceptable only for processes in which the electrode dimensions are small and the current path is therefore short. However, this applies primarily only to electro-lysis in aqueous media while electrodes for fused melt electrolysis, for example of aluminium, have substantial dimensions. ~or example, electrodes for the production of aluminium c~n have dimensions of up to 2250 x 950 x 750 mm ~hile typical graphite electrodes for the pro-duction of aluminill~ can have a size of 1700 x 200 x 100 mm or a cliameter of 400 x 2200 mm, depending on the t~pe of process. The production of such solid blocks of the above-mentioned ceramic materials is expensive ancL encounters substantial difficulties with respect to stability to alternating temperatures and electrical internal resistance. Recentl~, the efforts of current consuming industries ha~e been directed especially to a reduction of specific energy consumption and for this reason solid ceramic electrodes have not so far been accepted in practice.

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It is the object of the invention to provide a novel kind of electrode for fused melt electrolysis, in which the abo~e described disadvantages of th~3 prior art are ameliorated~ In particular, it is intended to provide a~ electrode capable of operatin~ rellab:Ly with an exceptionally low current/voltage lo~s and for which the ~pectrum of knowm and future acti~e materials can be used in the same ma~ner. ~he electrode should also be particularl~ easy to maintain and to repair.
~his kind o~ electrode is to be used preferably as anode.
The in~en-~ion provides an electrode for ~used melt electrolysis, comprising a metal top portion having an inner part and an outex part detachable from one ar.o~her, a b~ttom portion o~ active mat~3rial connected to said top ~ortion by screwmounting means, and an insulating coating of high temperature stability, wherein said inner part e~tend~ substantiall~ as far as said screwmounting means and wherein said insl1lating coating protects at least part of said inner part.
A further aspect of the invention provides an apparatus for ~usion electrolysis, particularly for the electrolytic production of metals such as aluminum, ~agnesium, sodium, lithium or compounds thereof having 25 an electrode which comprises a metallic top portion having an inner part with an upper and lower ~1~.1!.7 -4a-region and an outer par~ which outer part detachably receives at least the upper region of the inner lpart, a bottom portion o replaceable active material connected to the top portion by screwmounting means, and a:n i~sulating coating of high temperature stability, wherein said inner part extends ~ubstantially as far 2S ~aid screwmounting means and wherein said insulating coating is a detachably mounted moulding protecting at least the lower region oE said innex part.
~ i~uids, such as water, or gas, for example air can be used as coolants. ~he term "insulating" ,oating within the scope of the invention refers to a material which is inert and shielding with respect to the electrolysis media and can also be electrically insula-tlng.
~lectrodes comprising a cooled metal shank and a ~f , . ~,, graphi-te part screwmounted -thereon have already been proposed for use in the production o~ electric ~3teel in electric furnaces in which an arc extends from the electrode tip. ~he e~istence of -the arc and its possibility of travelling, the resultant extreme temperatures near to the arc as well as the atmosphere in the electric steel furnace and the kind of electrode process is so substantially different from fused melt electrolysis that the possibility of using such electro-des ~or performing fused melt electrolysis has not beenconsidered~ As regards the relevant prior art, reference should be made, for example, -to the ~ritish ~atent Speci-fication 1 223 162, the German Auslegesschri~-t 2~ 30 817 or the ~uropean Offenlegungsschrift 79302809.~. The ~5 electrodes mentioned in these documents are described by reference to the special requirements of the arc electrode and in terms of -the efforts made to meet the specific requirements of electro steel production.
According to one preferred embodiment of the electrode according to the invention the inner part and the outer part of the top portion are detachable from each other so that the inner part con-tains a gas - or liquid conducting chamber with a header duct and a return duct.
The outer part represents the terminal electrode and can be constructed of the same metal, for e~ample copper or metal alloys or other materials, like -the ~3~Y~

inner par-t.- Cooling ports or the like can be p:rovided in the ou-ter part. It is also possible -to provide the out;er part with retaining bores, for example for guiding ancl supporting insulating protec-tive s-tra-ta which are disposed below.
In one preferred embodiment of the electrode accor-ding to -the invention only a part region of the inner part is surrounded by the outer part so that -the metal shank in its entirety can be formed from a top region of larger diameter and a bottom region of smaller diameter.
~ he inner part of the elec-trode is extended into the nipple connection by means of which the top portion of metal and the bottom portion are interconnec1;ed. ~he gas or liquid cooling device, which may be required for the inner part ~ld extends axially therein is aclvan-tageously in-troduced into the screw nipple itse]f~ since this can be exposed to special heat stresses, depending on the material in use.
The co~nection between the inner and outer part can be obtained in different ways. A~s a rule, the connecting line extends parallel with the electrode axis. ~or example, the detachable connection can be obtained by screwthreading or by appropriate register fitting of the components. It is particularly preferred if the inner part is constructed as a register member of tapered or conical form and a par-t region of the outer and 7 :~:L~7~

inner part can be ~dditionally provided with scce~J-threading.
Connecting jaws can be attached to the outer par-G
by pockets or retaining means to which the current supply of the electrode is connected. Pocke-ts, into which graphite plates or segments are introduced to supply current, are a-ttached to the outer part in one preferred embodiment of the invention.
~he insulating coating of high temperature stability, representing a moulding in accordance with the invention~
can be an individual tube. Advantageously however the moulding can also be a series of tubular sections, seg-ments, half shells or the like which surround the bottom region of the top portion of the electrode and into the region of the screw nipple and where appropriate beyond the screw nipple. ~or most uses of the electrode or anode according to the inven-tion it is par-ticularly advantageous if a-t least the region of the moulding which can come into contact with the electrolyte and the resultant products, shields the metal shank and, where appropriate other metallic parts, more particularly the nipple, in gas--tight and liquid-tight manner. ~he material of the insulating moulding can also be, for - example, ceramic of high temperature stability but also, for example, graphite~ which is covered with an insulating coating. Such insulating ceramic or other materials of high temperature stability are known.

~ series of advan-tages, which will be described subsequently, is achieved by the use of a de-tachably surmountable moulding, more particularly in -the form of a series of tubular sec-tions, segments or hal:~
shells.
According to one preferrecl embodimen-t of the electrode according to -the in~ention the insulating moulding is disposed between a bottom part region o~
the top portion of metal and the bottom ac-tive portion so that the external edges of -the moulding extending in the direction of the electrode axis and the external edges of the outer region of -the top portion of me-tal are subs-tantially flush with each other.
~he electrode according to the invention is no-t subject to any restrictions regarding the abutment which suppoxts the insulating coa-ting or the moulding~
~his can also be made of a mating member of insulating metal, capable of withstanding high temperature stresses~
it can be the screw nipple itself and where appropriate can also be a component of the active part or a combi-nation thereof. Generally however the insulating moulding will not bear solely on the active part, to the extent to which this consists of a consumable material.J but will be supported at least partially by a non-consumable heat resistant material.
~ he position of the moulding can of course be controlled in suitable manner when the electrode is .

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produced. In one preferred embodimen-t of -the elec-trode accordin~ to the invention the insulating mouldlng can also be thrust on-to the abutment by pins, screw faste-ners etcO provided in bores in the top por-tion, for example by -the additional provision of sprlngs, even during operation o~ the electrode withou-t the need for re~oving the electrode from the electrolysis ~urnace.
Irrespective of -the provision of bores and screw fasteners or the like it can also be advantageous to ~0 mo~t the insulating moulding slidingly or loosely with respect to the metal shank so that in the event of failure o~ a part segment or breakage of an individual tube, ~or examp~e due to mechanical damage, the remaining part segments which are intact or the individual tube itsslf are able to slip forward, i.e. they are able to move ln the direction of longitudinal axis of the electrode.
~ here an extreme safe-ty design of the electrode is essential it is also possible -to cover the internally disposed metal shank~ which is protected by the insula--ting coating, with an additional highly stressable, conductive or insulating thin coating. ~his can be a cer~mic coating and can function as an additional heat shield or inert shield. ~he attack of electrolysis media can also be advantageously prevented by making the coa-ting very dense.
Depending on the use of the electrode it is possible ~l~a 791 to raount the insulating mouldi~g on retainers which are advantageously attached to the metal of the inner cooling unltO ~his will be considered primaril~ for uses of -the electrode where free movabili~y or advancing o~ intact (insulating or electrically conduc-tive) individual segments is not essential in the event of damage of one o~ the segments situated below.
Within the scope of the invention i-t is also possible for the insulating moulding -to surround not the entire region of the metal sha~k but an insulating, highly refractory injection compound, anchorad to retaining members, is used in place of the extending moulding in a zone where lower stresses can be expected~
Such insulating injection compounds are known and can be attached by means of retaining members, ~or example by means of soldering.
It is particularly advantageous îf the moulding is surmounted in gas-tight or liquid~tight manner at least in the region which can come into contact with the electrolyte and the resultant products.
~ he top and bottom portions can be interco~nected by means of a nipple which is cylindrical on the metal side and conical on the active side or vice versa. ~his part of the construction has proved itself, especially in tests. Metal, such as cast iron, nickel or a ternperature-resistant, corrosion-resistant metal alloy, can be considered as -the material of the nipple. Nipple .7~

connections of graphite itself are also considered as nipple connections, because of the high stabili-ty to alternating -temperatures.
According to a special embodiment of the invention the bottom portion can comprise several uni-ts ~rhich are retained by one or more nipple connections and the active unitis are arranged adjacently or one below the other. ~he inclusion of an "insert'l member bet~een the top portion and the bottom portion is advantageous, if the bottom consumable portion is connected -to the insert member by means of a nipple connection, fox example of graphite, because this nipple connection between the metal shank and the graphite insert member remains cooler and the consumable piece can be completely con-sumed without endangering the top portion. Ctherwise,to protect 1he nipple and the lower part of the top zone, a safety zone would have to be provided for -the consumable endpieceg and this safety zone would be lost.
It is also possible and in many cases desirable if the active part of the electrode is constructed of a plurality o~ tubes, rods and/or plates with a preferential orientation which corresponds to the current supply direction.

12 ~ .7~31 ~inally, iIl vi.ew of the temperature stresses impo,ed on the nipple it can be advantageous for this to be laterally slotted to compensate for thermal stresses~
According to one preferred embodiment of the electrode accordin~ to the invention the inner part and ~he bottom portion or active part thereof and/or its scre~ounting extend into a zone of high conducti-vity.. ~'his zone of high conductivity can, for e:~ample, represent a vessel which is filled under electro'lysis conditions with liquid metal of high conductivity.
t~his avoids energy losses i~ the region of the t~p part of the active rods, plates a:~d the li~e, for example if these consist of ceramic :material. ~he ceram:ic materials have an adequa-te conductivity only at elevated temperatureC so that in such a case it can be si~nificant to maintain the upper shank of the active (ceram.ic) rods at an elevated temperature. Metals with a suita'ble melting point, for example bismuth, can be menti~ned as suitable metals~
~he electrode according to the invention offers a series of advantages: specia.l mention should be made of the extremely low current or voltage losses o:n the path to the active part of the electrode~ ~y co:ntrast . to conventional solid blocks, whether or carbon, graphite or cer~mic material~ this enables a substantial energy ~L~ 91l saving to be achieved~ rthermore, side wear is minimized since it i- no longer the en-tire elect;rode but only the ac-tive par-t thereof which is exposed to the corrosive electrolysis medium and the result;ant reaction gases and vapours. ~inally, the electrode is versatile because its construc-tion permits the use of the entire spectrum o~ active materials which ccm fundamentally be employed in the ~ield of ~used melt electrolysis.
~urthermore7 during production the insulat-ing moulding can readily be introduced into a purpose adapted posi-tion. By the use of an insulating, externally disposed solid part it is possible to improve the mechanical stressabili-ty. By subdividing the insulating external zone into segmen-ts it is nol necessary, :in the event of defects or damage, to replace the entire electrode, since the damage can be economically and rapidly remedied by the introduction of a corresponding part member. ~he loose surmounting of the insulating moulding~ to the extent to which this is formed from segments, results in "automatic" follow up of the segments above in the event of mechanical or other damage of protective segmen-ts disposed below~ and this can be ensured where appropriate by attached springs. Accordingly, the elec-trode continues to be operational even after damage has already taken place, since the most endangered electrode region at t;he bo-ttom, neares-t to the operating zone of -the electrocle, is "automatically" protected by the sliding down of intac-t elements.
During manufacture the insulating moulding can also be introduced in a purpose-adapted position. ~he mechanical stressability can be improved by the use o~
an insulating, externally disposed solid part. ~y dividing -the insulating external zone into segments it will not be necessary to exchange the entire electrode in the event of breakdown or damage, since the damage can be economically and rapidly remedied by the intro-duction of the appropriate part member. ~uch loose mounting of the insulating moulding, to the extent to which this is formed from a plurali-ty of part members, leads to an "automatic" follow-up movement of -the above disposed segments in the event of mechanical or other destruction of defecti~e segments situated below, and this can be additionally ensured, where appropria-te, by attached springs. ~he electrode therefore continues to be operational, even when damage has already taken place, since the most endangered electrode region at the bottom, nearest to the working zone o~ the electrode, is "automatically" protected by the downwards sliding of elements which are intact.
Although the insulating moulding or the insulating coating 7 if this comprises a series of individual seg-ments or half shells, can have some cl0arance obtained by the kind of axial and internal support, the tongue and g,roo~e system will provide comple-te and comprehen sive protection for the sensitive metal region of the electrode. If the bo-ttom region of the "protective shield" of the electrode is nevertheless dam~ged., -the electrode can usually con-tl:rLue to operate, for as long as is necessary -to replace the consumable part, for example of graphite. W~Len the electrode is removed, the damaged individual segment e-tc~ can readily be replaced.
~ he invention subdivision of the metal shan~ also provides advantageous electrode properties. ~y virtue of the water conducted in the interior of the el.ectrode the latter remains intact even if the external part is damaged~ When the external region of the top portion is damaged it is therefore not necessary to shut; down the supply o~ coolant and to empty the electrode etc.
Owing to th.e simple detacha~ility of the outer portion the latter can be easily exchanged as a component if damaged whi~e the conventional construction wou].d call for complete repair of the metal shank or replacement thereof. ~he lateral current supply, for example by means of graphite contact jaws or segments, which are attached, for example in retaining pockets, makes it uD~Lecessary to remove the electrode in its entire-ty from the contact rail in the event of defects ln the region of the internal liquid supply means, sinc:e it 31.~9 11 i.s merely necessary for the internal part -to be ~etached.
By cons-tructing the top region as a portion of l~rger diameter and a portion of smaller diameter it is possible for an insulating protective layer of high tempe:rature resistance to be attached in a particularly compilct and convenient form and it may then not be necessary to additionally protect the outer part in lnsulating manner i~ said outer part is confined to the reg:ion of the current supply means.
~ome particularly preferred electrode const:ructions i.n accordance with the invention, intended especial1y or use as anoles~ are those in which the top portion of conduetive metal has an upper parl, of larger diameter and a lower part of s,naller dlameter~
~he part of smaller di~meter i.s then a-t least partial'-~covered by the insulating moulding. ~his arrangement is espeeially preferred within the scope of the ~nven-tion although the invention is neit'ner confined thereto nor is it restricted to the particularly ad~antageous embodiments in accordance with the illustrations below, Identieal components ha~e the same reference numeral in the illustrations in whieh:
~ig~ 1 is a longitudinal section through an electrode aecording to the invent:ion, ~ igs, 2 and 3 show a longitudinal section throuæh an electrode aceording to the invention, in whien the 1 7~

region ~otected by insulatiGn is not shown completely and the adjoining activ part is not shown;
Fig. 4 (found on the page containing Fig. 2) is a cross-section through the top portion oE me-tal or the part xegion thereof of larger diameter;
~iig. 5 is a longitudinal section through the ~iottom electrode portion with the intermediate member inr,erted;
~ig. 6 is a longitudinal section of an anode accor-ding to the invention, particularly intended for obtaining magnesium.
In the electrode, for example according to ~iig~ 1, -the cooling madium, such as water, air or irert gas, is introduced through the header duct 2 and retur]led through the return duct 3. The cooling system is disposed in the inner part 16 on which the outer part 17 is surmounted. Cooling medium also enters into a chamber within the screw nipple 1, for example fo~ned of cast iron. ~he top portion 5 of metal. for ex~nple Cu, comprises a top region of larger diameter and a lower region of smaller diameter extending as ~ar as the screw nipple 1 which forms the connection to the bottom portion 6 of acti~-e material, for example graphite.
~he insulating moulding 4 is supported by an abutment 7, for example of insulating ceramic of high thermal stability. In the top region the insulating mouldir.g 4 is defined by the top edge of the large diameter region of the metal shank. In the electrode illu3trated in ~iig. 1, the insulating moulding 4 is subdivided into /

segments which can slide in the direction of the electrode axis if a (bottom) segment should bre~ out.
~ igs. 1 to 3 show some of the preferred me~ms of connecting the inner part 16 and the outer part 17 as $ a register member 9 where appropriate with an adclitional partial screwthreading. Pins 9 or the like can be guided through bores 8 to retain the insulating coating 4 on an abutment 7 by means of the spri~g 10. q'he inSula~ng member can be additlonally secured by retaining means 14. ~he outer part reveals cooling ports 15 while connecting jaws 18, for example of graphite~ are shown on the outside~ ~he latter can be held in re-taining means or in pockets 19 which are attached to the outer edge o~ the metal æhank, as also shown in ~igs. 2 or 4.
~ig. 5 shows an insertion member 21, for example of graphite, which is connected to the top portion 5 by means of a nipple 1, slotted to compensate thermal stresses and advantageously consisting of copper. ~he insertion member 21 is then co~ected to the actual active member by means of an addi-tional nipple connection 22, which is advantageously formed from graphite. In this case, t,he active member is integrally constructed while in ~ig. 1 it is divided into individual tubes or rods 20.
Gas flushing ducts~ not sho-~n ln detail in t;he illustrations, can be provided batween the insulating stratum 4 and especially the inner part 16 of the top portion 5A Any damage to the insulatlng ceramic can be readily detected by gas flushing, for example by reference to a corresponding pressure drop. ~ certain cooling action is also possible. It is also within the scope of -the invention - also not sho~l in the illu-strations, - that the inner part 16 and/or the nipple connection or its external surfaces or the inser-tion member 21 can be provicled wi-th a coa-ting of high tempe rature stability~ ~he coating 12 of high -temperature stabili-ty associated with the top portion 5 can be electrically conductive or insulating depending on the dimensions of the insulating coating 4 of high ~empera-ture stabïli-ty situated thereabove. An insulating embodimen-t would thus provide a certain line of protec-tion which can come into action in the event of breakageof the externally dlsposed insulating coating L~,, If such an event is not expected, depending on operating conditions, the coating, for example of the inner part 16, can also consist of conductive material of high temperature stability, which will then perform the actlon of a "heat shield" or "insert shield" to protect the metal disposed -therebelow.
~ ig. 6 shows an anode especially intended for the production of aluminium. I-t comprises a plate 6, for exal~ple of graphite, with an annular, milled head part.
~his is connected by means of the appropriately con-str~lcted nipple 1, for example of pure nickel, to the ;1 3L~.~7~

-top portion 5. ~he interior of the nipple connec-tion contains a contact coating 12, for example of platinum.
~he insulating coating 4 is constructed in the form of a ceramic -tube, for example of high densi-ty silli-manite. It is supported by a refractory cement. ~heceramic tube extends beyond -the cell lid and shields the metal shank against corrosion. ~his solution to -the problem takes special account of the different coeEficients of expansion of graphite and metal.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. Apparatus for fusion electrolysis, particularly for the electrolytic production of metals such as aluminum, magnesium, sodium, lithium or compounds thereof having an electrode, said electrode comprising a metallic top portion having an inner part with an upper and lower region and an outer part which outer park detachably receives at least the upper region of said inner part, a bottom portion of replaceable active material connected to said top portion by screwmounting means, and an insulating coating of high temperature stability, wherein said inner part extends substantially as far as said screwmounting means and wherein said insulating coating is a detachably mounted moulding protecting at least the lower region of said inner part.

2. Apparatus as claimed in claim 1, wherein said inner part has a cooling device with a header duct and return duct.

3. Apparatus as claimed in claim 1, wherein said outer part is a terminal electrode.

4. Apparatus as claimed in claim 3, wherein said outer part is provided with cooling ports and retaining bores.

5. Apparatus as claimed in claim 1, wherein said insulating coating protects the lower region of said inner part in a fluid tight manner, at least in the region which can come into contact with the electrolyte and resultant products.

6. Apparatus as claimed in claim 1, wherein said screwmounting means is a screw nipple.

7, Apparatus as claimed in claim 1, wherein the detachable connection of said inner and outer parts is obtained by screw threading.

8. Apparatus as claimed in claim 1, wherein the detachable connection of said inner and outer parts is obtained by registered fitting.

9. Apparatus as claimed in claim 8, wherein said fitting is a taper fit and wherein additional screw threading means is provided in a region of said inner and outer parts.

10. Apparatus as claimed in claim 1, wherein graphite connecting jaws are mounted on said outer part by retaining means.

11. Apparatus as claimed in claim 1, wherein said moulding is tubular and surrounds the bottom region of said top portion substantially as far as said screwmounting means.

12. Apparatus as claimed in claim 11, wherein said tubular moulding is made of separable portions.

13. Apparatus as claimed in claim 11, wherein said moulding and the external edges of said top portion are substantially flush with one another.

14. Apparatus as claimed in claim 13, wherein said moulding is at least partly supported by said bottom portion.

15. Apparatus as claimed in claim 14, wherein said moulding is at least partly supported by said screw-mounting means.

16. Apparatus as claimed in claim 14, wherein a cut is provided in said top portion and an abutment is provided in the region of said screwmounting means, and wherein said moulding is supported between said cut and said abutment.

17. Apparatus as claimed in claim 16, wherein said moulding is resiliently retained on the abutment fastening means guided in bores of said top portion.

18. Apparatus as claimed in claim 1, wherein said top portion has a smaller diameter region which is cover-ed with a dense, high stressable, conducting coating of ceramic.

19. Apparatus as claimed in claim 13, wherein said insulating moulding consists of ceramic of high temperature stability covered with an insulating coating.

20. Apparatus as claimed in claim 1, wherein said insulating moulding consists of graphite tubing covered with an insulating coating.

21. Apparatus as claimed in claim 13, wherein said insulating moulding is mounted on retainers which are attached to the top portion.

22. Apparatus as claimed in claim 18, wherein in the top region of said smaller diameter region, the insulating moulding is partially replaced by an insulating highly refractory injection compound which is anchored to retaining members.

23. Apparatus as claimed in claim 21, wherein said insulating moulding is supported so that in the event of failure of a part segment or damage of the individual tube, the remaining intact partial segment or the individual tube itself can move in the direc-tion of the longitudinal electrode axis towards the stressing zone.

24. Apparatus as claimed in claim 1, wherein said screwmounting means comprises a screw nipple which is cylindrical on the side of one of said metal and consumable portions and conical on the side of the other of said portions.

25. Apparatus as claimed in claim 24, wherein said screw nipple comprises cast iron.

26. Apparatus as claimed in claim 25, wherein said screw nipple means comprises graphite.

27. Apparatus as claimed in claim 1, wherein said bottom portion comprises a plurality of adjacently arranged units which are retained by one or more nipple connections.

28. Apparatus as claimed in claim 1, wherein said bottom portion comprises a plurality of units arrang-ed one below the other, which are retained by one or more nipple connections.

29. Apparatus as claimed in claim 24, wherein the inner part of said top portion and said bottom por-tion are screwmounted to each other in addition to said screw nipple.

30. Apparatus as claimed in claim 27 or 28 wherein said screw nipple or nipples is or are slott-ed.

31. Apparatus as claimed in claim 1, wherein the inner part and the bottom portion or active part thereof and the screwmounting extend into a zone of high conductivity.

32. Apparatus as claimed in claim 31, wherein said zone of high conductivity is a vessel filled with highly conductive metal which is liquid under electrolysis conditions.

33. Apparatus as claimed in claim 1, wherein said insulating coating is in the form of a series of tubular sections, segments or half shells.