CA1311215C - Cell arrangement for electrometallurgical purposes, in particular aluminum electrolysis - Google Patents

Abstract

A B S T R A C T
A cell arrangement for electrometallurgical purposes, for example aluminum electrolysis. This arrangement provides a practical means for heat recovery as well as regulation and con-trol of the temperature conditions during cell operation. The cell arrangement comprises a cell box having an internal refract-ory lining in its bottom and walls, an anode, a heat exchanger comprising cooling chambers adapted to receive a circulating flow of a cooling medium which is regulated on the basis of signals from temperature sensor means connected to a system for tempera-ture control. The heat exchanger is directly incorporated in a closed circuit with an expansion engine, whereby the assembly of the heat exchanger, closed circuit and expansion engine is such that the cooling medium of the heat exchanger and the working medium of the expansion engine are the same fluid.

Description

~ 3~21 ~ 278~4 2 Heat Exchanging Arrangemen-t including Cells for electrometallurgical Purposes in particular Aluminum Electrolysis.

BACKGROUND OF THE INVENTION
During electrolysis in the electrometallurgical industry, for example in the aluminum melting industry, large amounts of power are lost in the form of heat from the cells employed in the processes. As far as the running and the efficiency of the actual process is concerned, it is also very important to take into account the cooling conditions.
Particularly, in recent time there has been a growing interest for energy economy and recovery, and thus there have been put Eorward various proposals for heat recovery in the above industry.
Examples of known proposals in this direction may be found in Published British Patent Applications No. 2,076,428 and No. 2,047,745. In the former British application there is des-cribed removal of heat by means of a number of cooling elements in the sidewall of the cell. The cooling is controllable, inter alia ~0 by means of valves for the flow of cooling medium in each element.
These cooling elements consist of pipes. The control takes place in response to heat sensors provided in the sidewall. The speci-~; fication, however, does not give any e~planation as to whether the purpose bf the arrangement is to recover energy. The arrangement proposed aims at controlling the temperature in the cell, and more particularly in the cell bath.
~ On the other hand British Patent Application ~o.
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~ 3 ~ 27884-2 2,047,745 describes recovery of energy with heat exchangers pro-vided above the bath and in the sidewalls respectively, possibly also in the bottom. The purpose of this is to produce steam or electricity at the same time as -the side coating (crust) shall be secured or maintained. The cell walls shall be well insulated.
~here is provided a cover above the bath so that the cell will be closed. A temperature sensor measures the electrolyte tempera-ture. Air is used as a cooling medium, which is highly hazardous in the environment concerned.
In the practical operation of control, cooling and heat recovery in the electrometallurgical industry it is of substantial significance to take into account the need for individual control of the temperature distribution at the side and bottom surfaces oE
cell boxes of the various types found within the electrometal-lurgic industry. Moreover, the high and diverse stresses to which cells and auxiliary equipment are subjected to within this indus-try, make it necessary that all equipment installed near the pro-cess either stand up to the stresses concerned or that the equip-ment at least cannot cause any significant damage if it should ~ail. This also applies to media used in the operation, such as coolants, for example.
In aluminum electrolysis for example cells are construc-ted with a cell box having an internal refractory lining in bottom and walls. ~he structure of the bottom and walls is to a sub-stantial degree aimed at withstanding the high temperatures and strong corrosive forces which occur by contact with the molten bath. Corresponding stresses act also on the bottom faces of the A

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anode. These contact surfaces or parts of the cell which essentially delimit the bath sideways, downwards and upwards, are decisively siynificant to the above heat and temperature conditions.
SUMMARY OF TH~ INVENTION
An object of the present invention is to satisfy khe requirements which are imposed on control systems and equipmen~ in the electrometallurgical industry as discussad above. On the one hand it is a question of making the operation of each cell more effective, and, on the other hand, to be able to utilize the heat output from the cell for recovering power.
Cell arrangement for electrometallurgical purposes electrolysis comprising a cell box having an internal refractory lining in the bottom and walls thereo~, said inkernal refractory lining providing contact surfaces against a cell bath; an anode partially immersed within said cell bath; and a heat exchanger associated with at least one of said contact surfaces, said heat exchanger comprising a plurality of cooling chambers, each of said cooling chambers having a base area covering a small proportion of the area of the contact surface and said plurality of cooling cha~bers together covering a substantial proportion of the area of the contact surface without any significant space between said cooling chambersr cooling chambers being adapted to rece1ve a throu~h-flow of a cooling medium which is controlled individually for each cooling chamber in response to a system for temperature control connected to temperature sensor devices within said cooling chambers; wherein said heat exchanger i5 directed 2 ~ ~

incorporated in a closed circuit with an expansion engine and said cooling medium in said heat exchanger is a working medium in said expansion engine.
Primarily it is oi interest to provide a heat exchanger in the sidewalls or the botto~ of the cell or in both the sidewall or the botkom of the cell box. However, in certain si~uation~ the heat exchanger may be located ln the anode, in particular whan . ~
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131~21~ 27884-2 contemplating new anode designs which may be developed. Advan-tageously the controllable heat exchanger can serve to secure a desired side coating or crust layer in the cell.
As will be described more closely below it is an advan-tage that the heat exchanger comprises cooling chambers each hav-ing a base area which covers only a small proportion of the area of ~he contact sur-face concerned, and which together cover a sub-stantial proportion of the area of the contact surface without any significant space between the cooling chambers, and that the cool-1~ ing chambers are adapted to have a through-flow of a cooling medium being controlled individually for each cooling chamber.
Concerning in particular the cell walls and the bottom respect ively, at the parts being covered by cooling chambers, the struc-ture can have a signiiicantly reduced total thickness and heat : transfer resistance compared to what would be required when the cooling chambers were not present.
With this sub-division of the cooling or heat recovery system by means of the comparatively small chambers, there are for~ed separate flow or recircu.lation circuits which by suitable control makes it possible to adapt the cooling and the heat output respectively, at the dif~erent portions of the cell with high accuracy according to the local temperature conditions therein, in particular in -the cell walls and bottom. Thereby it will be possible to obtain a cooling of the various portions of the cell ; so that there is obtained a desired temperature distribution in the cell itself and in particular in the cell walls, and also obtain an optimal heat recovery. In this way there is also ~3~121 ~ 27884-2 obtained an advantageous effect to the cell operation as such, since portions thereof having a tendency to for example undesire-ably increased temperatures, may be eliminated~ The cell design itself can thereby also be carried out simpler and cheaper than according to the manner of construction now being common, because the cooling and heat recovery system takes care of the heat developed in a more favourable way than what has been the case hitherto. ~ot the least the arrangement according to the inven-tion involves a possibility of operating with a significantly increased amperage and thereby an increased production, with the same cell design. This is due to the much more effective cooling effect which is obtained. This is to a substantial degree due to the direct circulation and control of the coolant and working medium, as will be described below. Since that part of the cell box which is between the cooling system and the process or melt bath, has a low heat capacity and a low thermal resistance, the cooling can be controlled quickly so that a cell row can be regulated in a short time for a lower or a higher current.
DETAILED DESCRIPTIO~ OF THE INVE~TIO~
The invention will be further described with reference to the accompanying drawings showing, by way of example, an embodiment of the invention. The particular illustrated embodi-ment is a cell for recovery of aluminum but it will be appreciated that the invention can be used in electrometallurgical cells other than cells for the recovery of aluminum.
Of the drawings:

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~ 3~ 2~ 27884-2 FI~. 1 shows a simplified cross-section of a part of the cell wall and bottom as well as the anode in an aluminum electrolytic cell included in an arrangement according to the invention, FIG. 2 is a simplified elevation view of a sidewall module or block which can be included in the arrangement of FIG.
1, and FIG. 3 shows highly schematically a recirculation circuit for a cooling medium included in a system for temperature control with the arrangement according to the inven-tion.
In accordance with common design practice the electro-lytic cell in FIG.l has an internal refractory lining which com-prises a bottom lining 1' and a wall lining 1. Suitably the lin-ing can consist of a material having good properties with respect to the ability to resist corrosive attacks from the electrolyte ~; and from molten aluminum, as well as reasonably good properties with respect to thermal and electrical conductivity. Nowadays it is common practice to use carbon based materials such as anthra-cite or graphite, but other materials can also be used for this Eunction. Possibly there may be applied a steel plate enclosure outside the lining, but this is not regarded as necessary in -~ connection with this invention, since the practical construction o~ such a cell intended for an arrangement according to this in-vention, can take place more effectively without such a continuous plate structure which is common in conventional aluminum electro-lytic cells.

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Above the cell bottom there is shown a layer of molten aluminum 4 and on top of this an electrolyte layer 3 consisting of molten alumina and cryolite. Moreover, there is shown a side coating 5 and a crust layer 5' consisting of solidified cryolite.
As known the side coating 5 has an important function in the cell operation, and it is very significan-t to effect control of the temperature conditions in the cell so that there is formed such a side coating 5 of suitable shape and -thickness. The side coating serves inter alia to protect the wall lining 1 against the strong corrosive effect which may be caused by the electrolytic bath 3.
In this connection the temperature gradient through the various layers from the melt bath 3, 4 out through the side coating 5 and the lining 1 is very important. The same also applies in part to ; the heat transfer conditions through the bottom structure of the cell.
The cell design according to FIG. 1 is specific in so far as the cell walls and bottom respectively, have a significant-ly reduced thickness of the lining and a low thermal resistance through the lining, compared to what has been used earlier in cell structures for electrometallurgical purposes, for example aluminum electrolysis. In this branch of inaustry there has been a very conservative attitude to the dimensioning of such cell boxes, perhaps in particular because of the expensive and potentially dangerous consequences which may occur when a cell box is molten through so that the molten coDtents may flow out. By providing a cooling system as described here it will be possible to reduce to a high degree the dimensions and the material requirement for ,~

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~constructing the.se cell boxes, since the necessary control and local cooling is ~ffected in a new and advantageous manner which is to be described in the following.
As will appear from FIG. 1 there is provided a heat exchanger system comprising side cooling chambers 6A, 6B and GC
engaging the wall lining 1 and bottom cooling chambers 6' beneath lining 1'. There are shown cooling chambers 51 in the anode 50 of the cell.
The cooling chambers 6A, 6B and 6C on the cell wall have a base area or surface of engagement covering a comparatively small proportion o~ the sidewal} of the cell. The base o~ the cooling chambers can advantageously have an approximate square æhape. The cooling chambers are located with an unsignificant spacing and are adapted to receive a through-flow of a cooling medium with individual control for each cooling chamber.
As seen from the interior of the cell the cooling chambers (heat exchanger elements) 6A, 6B, 6C lie behind the linlng 1 and further beh1nd the chambers there is mounted a heat distributing plate 16 which in the first place has a safe~y function. The plate 16 shall distribute the heat to adjacent chambers i~ one of the chambers should fail, possibly at connections thereto. Einally a highly insulating material can be pxovided behind the heat distributing plate 16.
FIGS. 1 and 2 illustrate somewhat more in detail the ~; cooling system for the cell wall. The cooling system (Fig. l) comprises supply plpes 7A, 7B, 7C having a com~on supply as indicated at 7. In FIG. 2 correspondiny supply pipes are denoted 2 ~ ~

7A', 7B' and 7C'. For each cooling chamber 6A, 6B, 6C (FIG. 1) there are inserted control valves 8A, 8B and 8C respectively, in the corresponding supply pipes. Moreover, for these chambers there is shown a common return conduit 9 with short pipe sections to each of the chambers, of which the pipe section 9A for chamber 6A has been indicated specifically.
As essen~ially parts of the sys~em for temperature control of the cell shown, there is illus~rated in FIG. 1 in a purely schema~ic and simplified manner, a control unit 40 which suitably can be a computer, and which delivers a setpoint through outputs indicated at 41, to a number of control devices 10 which in their turn actuate the above mentioned valves 8A, 8B and 8C.
In addition to a setpoint from the control unit 40 there is applied to the control devlces 10 one or more measurement value~
relating to the heat conditions in and in association with the cooling chamber 6A, 6B and 6C. Thus, in chamber 6C there is shown a te~perature measuring element 18 and besides a heat flux meter ;., 19, the measurement values from these elemen~s being lead each to a separate control device 10 as shown. Thereby the flow of cooling medium can be controlled individually for each cooling chamber. In accordance with conventional control methods the control uni~ or computer 40 can calculate the respe~tive setpoints on the basis of desired cell operation parameters and measurement values from dif~erent parts of the system or cell lnstallation.
In connection wi~h FIG. 1 there is only mentioned three cooling chambers 6A, 6B and 6C above (in FIG. 2 thxee chambers 6A', 6B'and 6C') but it is evident that a higher number of such :: 9 -~J~>~
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27~84-2 cooling chambers are provided along the whole length of an electrolytic cell in order ~o cover a subs~antial portion of the wall surface. Cooling chambers are mounted over all those parts of the wall surface which is of significance for the cooling and control during operation of the cell.
According to the invention an adva,ntageous embodiment consists therein that the cell wall is built up sectionally by moclular blocks, ~f which one block or module i5 shown in FIG~ ~.
Thiæ figure shows corresponding three cooling chambers 6A', 6B' and 6C', as in FIG. 1, with associated supply pipes 7A', 7B' and 7C' respectively. For simplicity the valves in these pipes are not included in FIG. 2. Possibly the valves can be located outside the modular hlock so that the structure thereof will be somewhat simplified. For each cooling chamber 6A', 6B' and 6C' there is indicated an associated square lining part lA, lB and lC
which can either be composed oX separate lining parts or may constitute a continuous element ~or the block. The cooling chambers are shown in FIG. 2 with a circular basic shape and have a central entry of the supply pipes 7A', 7B' and 7C'. The connection of a return conduit (not shown) from each of these chambers is indicated at 9A, 9B and 9C respectively. Like the supply pipe 7A', 7B' and 7C' the return conduit from each chamber can be extended vertically upwards for connection to the remaining circulation system at the upper edge of the cell wall, as lndicated in FIG. 1.
In order to obtain a favourable circulation and distribution of the cooling medium in each cooling chamber these ;, ~.

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can be provided with internal distribution walls, as shown specifically in the cooling ~ha~ber 6C' in FIG. 2. Thus, in relation to the ..

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~311 ~2~ ~ 27884-2 circ~lar shape of the cooling chambers shown therein, the distri-bution wall 29 in the chamber 6CIhas a spiral shape which leads the cooling medium in a spiral shaped flow path from the center out towards the connection to the return conduit at 9C adjacent the periphery of the chamber.
The measuring elements 18 and 19 are not shown in FIG.
2, but the location thereof will be in accordance with known principles for instrumer.tation. In addition to pure temperature measuremznt in the cooling medium, possibly in the wall lining, there can also be provided ~or measurement of heat flow in the chambers theat flux meters 19).
The modular block 20 as shown in FIG. 2 can be mass produced with all associated elements and plpe fittings ready for mounting and coupling in connection with the construction of a new cell or restoration o~ a cell which has been in operation and initially based on a system as described here - possibly also as a replacement Oe the lining in a cell which has been based on earlier technology.
An arrangement of cooling chamber~ on the cell wall~ has been described above. FIG. 1, however, also Rhows a heat exchang-er with cooling chambers 6' und2rneath the bottom lining 1' of the cell, with associated circulation pipes for a cooling medium. As th~ temper~ature and heat conditlons in thc bottom ar~ not ~o critical as they are along the cell walls, the cooling chambers 6' under the bottom do not have to be as small as explained in con-nection with the wall structure. Thus, the chambers 6' in the bottom can extend across a larger portion of the cell or possibly ,':
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over the whole length thereof. ~evertheless it may be an advan-tage to have a heat distributing plate 16' included.
For a more complete heat recovery and possibly a desired cooling effect, there is also in the anode S0 shown cooling cham-bers 51 provided with corresponding conduits, valves and control devices corresponding more or less to those discussed above in relation to the sidewall of the cell. Also in the anode there can ~e provided a heat distributing plate 56 behind the cooling cham-bers. The provision of such cooling chambers in the anode re-quires a modified design thereof in relation to what is conven-tional techniques. With such cooling of the anode in aluminum electrolytic cells great advantages can be obtained.
As a cooling medium, according to the present example, helium is used, as on the one hand it has favourable flow proper-ties and on the other hand it is a favourable medium for heat transportO Moreover, since helium is a one atom, inert gas it is not dangerous when employed in connection with electrolytic cells ; comprising high temperatures, electric current and other risk factors. The use of helium is particularly advantageous when the temperature control is also intended for heat recovery and not only for a pure cooling effect for purposes of the cell operation.
When the arrangement according to the invention is included in a system for heat recovery it is an important feature that the helium circulatlon takes place in a closed circuit for direct heat exchange to the high pressure side of a thermodynamic engine (expansion engine), for example a turbine, which utilizes heat recovered from the cell.

., ~' ~3~2~ 27884-2 Helium is a one atom gas having a high Cp/Cv ratio and a low viscosity. This makes helium well suited as a working medium - in a thermodynamic engine.
The principle for production of electric power by means of a closed gas circuit and a compressor, a high temperature heat exchanger, a gas turbine and a cooler is well known, and is desig-nated Joule's ideal gas cycle~ The theoretical maximum efficiency is lower than for Carnot's cycle, but it is not much lower. The equation for efficiency is given by:

N = l-(Pl / P2) (k-l) /k Pl = Minimum pressure P2 = Maximum pressure K = Cp/Cv ~;~ Cp = Specific heat at constant pressure Cv = Specific heat at constant volume~

For helium K is practically independent of temperature and pressure and equal to 1.67.
As shown by the equation, the efficiency increases with increasing pr~ssure ratio. The problem is that the temperature ; 20 in the gas increases strongly with an increasing degree of com-pression~ and this involves that less heat can be absorbed per .
cycle when the maximum temperature is given.
The principle of the heat r~covery is shown schemati-cally and simplified in FIG. 3~ FIG. 3 shows a heat exchanger 32 which comprises an arrangement of several cooling chambers as :
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~7884-2 described above. From this heat exchanger 32 helium circulates to the high pressure side 30A of a turbine which drives a generator 31, for example for producing electric power. Moreover, helium circulates to a second heat exchanger 33 at the low pressure side, with a possible subsequent control valve 34 and then to the low pressure side (the compressor part) 30B of the turbine. From there the helium flow goes back to the heat exchanger 32 on the electrolytic cell or cells. This direct heat exchange from the cell to the high pressure side of the turbine aggregate involves a strong simplification of the whole heat recovery system and has been made possible inter alia by employing helium as the cooling medium, which permits a lower maximum pressure in the circulation system.
The secondary heat exchanger 33 makes it possible to utilize still further portions of the waste heat, for example for water heating.
When the generator 31 shall supply electric alternating current at a substantially constant frequency, for example 50 Hz, the rotational velocity of the turbine 30A should be kept constant with a varying heat transfer to the high temperature heat exchang-er 3~. Such variations will occur during normal operation of aluminum electrolytic cells. The regulation thereof takes place ; through changes in the amount of circulating helium, i.e. by pressure changes in the closed circuit. Introduction of helium increases the pressure, whereas extrac-tion of helium from the circuit will lower the pressure therein. This is preferably done at point 39 in which there is a comparatively low pressure and low temperature,i.e., behind the low temperature heat exchanger 33.

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~3~ ~ 2~ 27884-2 Control of the pressure or amount of helium can be effected in various ways, but it is preferred to avoid a consumption or loss of helium in this connection. Thus, in FIG. 3 there is shown a pressure tank or accumulator 61 for helium and an associa-ted valve 63 which permits of a controlled supply of helium from the tank 61 to the circulation circuit at point 39.
Moreover, there is provided a compressor 62 which through another valve 64 serves to control the lowering o~ pressure in the circuit, by transferring (compress) helium to the tank 61. During such a pressure lowering operation valve 63 is obviously closed.
The regulation described here can take place under the control of a calculating unit 40' which suitably can be constitu-ted by or can be included as a part of the computer 40 in FIG. 1, whereby the relevant input signals for controlling the helium circulation will be obvious to an expert, the amperage at which the electrolytic cells are operated, being an important para-meter.
~; The regulation arrangement with the pressure accumulator tank 61 and compressor 62 and associated valves can be common to a number of or all cells in an electrolysis plant, or such arrange-ment can be provided for each cell.
Control for obtaining a substantially constant rotation-al veloclty as mentioned, is also advantageous with most interest-ing ty~es of expansion engine (turbine~ 30A and the associated compression engine (compressor) 30B~ These types of engine1 as a / rule have a relatively narrow range of rotational velocity with maximum efficiency.

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~3~2~.~ 27884-2 It is to be understood that the cell arrangement of the present invention can comprise a plurality of cells.

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Claims (10)

1. Cell arrangement for electrometallurgical purposes comprising a cell box having an internal refractory lining in the bottom and walls thereof, said internal refractory lining providing contact surfaces against a cell bath, an anode partially immersed within said cell bath; and a heat exchanger associated with at least one of said contact surfaces, said heat exchanger comprising a plurality of cooling chambers, each of said cooling chambers having a base area covering a small proportion of the area of the contact surface and said plurality of cooling chambers together covering a substantial proportion of the area of the contact surface without any significant space between said cooling chambers, said cooling chambers being adapted to receive a through-flow of a cooling medium which is controlled individually for each cooling chamber in response to a system for temperature control connected to temperature sensor devices within said cooling chambers; wherein said heat exchanger is directly incorporated in a closed circuit with an expansion engine and said cooling medium in said heat exchanger is a working medium in said expansion engine.

2. Cell arrangement for electrometallurgical purposes, comprising a cell box having an internal refractory lining in the bottom and walls thereof, said internal refractory lining providing contact surfaces against a cell bath; an anode partially immersed within said cell bath; a heat exchanger associated with at least one of said contact surfaces, said heat exchanger comprising a plurality of cooling chambers, each of said cooling chambers having a base area covering a small proportion of the area of the contact surface and said plurality of cooling chambers together covering a substantial proportion of the area of the contact surface without any significant space between said cooling chambers, said cooling chambers being adapted to receive a through-flow of a cooling medium which is controlled individually for each cooling chamber in response to a system for temperature control connected to temperature sensor devices within said cooling chambers; and a heat distributing plate of metal at the back of the cooling chambers and in good thermal contact therewith, said heat distributing plate being common to a plurality of cooling chambers.

3. Arrangement according to claim 1 or 2 wherein the cell wall is built up of modular blocks each having a height corresponding approximately to the height of the cell wall and a width corresponding to the width of a cooling chamber, and comprising internal lining parts, a number of cooling chambers with associated pipe fittings and a heat insulating layer outside the cooling chambers and around the pipe fittings.

4. Arrangement according to claim 1 or 2, wherein cooling chambers are provided in the anode of the cell, said cooling chambers being included in the system for temperature control.

5. Arrangement according to claim 1 or 2, wherein the system for temperature control comprises a control unit which on the basis of desired cell operation parameters and measurements delivers a setpoint for the regulation of valves in supply pipes for cooling medium to each cooling chamber.

6. Arrangement according to claim 1 or 2, wherein the cooling medium consists of helium.

7. Arrangement according to claim 1 wherein the expansion engine is adapted to drive a generator for producing electric alternating current at a substantially constant frequency, comprising means for regulating the pressure at a point in the closed circuit where there is a relatively low pressure and low temperature.

8. Arrangement according to claim 1, comprising a pressure tank which through a valve serves to increase the pressure in the closed circuit, a compressor which serves to lower the pressure in the closed circuit by transferring cooling medium therefrom to the pressure tank and another valve contributing to the control of the compressor, whereby the circulated amount of cooling medium is regulated by changing the pressure of the working medium.

9. Cell arrangement for electrometallurgical purposes, comprising a cell box having an internal refractory lining in the bottom and walls thereof, an anode, and a heat exchanger comprising cooling chambers adapted to receive a through-flow of a cooling medium being controlled in response to a system for temperature control connected to a temperature sensor device, said heat exchanger being directly incorporated in a closed circuit with an expansion engine, wherein the cooling medium in the heat exchanger is a working medium in the expansion engine.

10. Arrangement according to claim 1, 2 or 9 wherein the cell is a cell for the recovery of aluminum.