EP0044794B1 - Process and apparatus for accurately regulating the feeding rate and the alumina content of an igneous electrolysis, and use thereof in the production of aluminium - Google Patents

Abstract

A process and apparatus for controlling the rate of introduction and the content of alumina to a tank for the production of aluminium by the electrolysis of dissolved alumina in a cryolite-base bath, the upper part of which forms a solidified crust, and wherein the alumina content is maintained within a narrow range, of between 1% and 3.5%, wherein the alumina is introduced directly into the molten cryolite bath by way of at least one opening which is kept open in the solidified crust and the rate at which the alumina is introduced is modulated relative to variations in the internal resistance of the tank during predetermined periods of time, with alternation of the cycles of introducing alumina at a slower rate and at a faster rate than the rate corresponding to normal consumption within the tank.

Description

The present invention relates to a method and an apparatus for precisely regulating the rate of introduction and the alumina content of an igneous electrolysis cell, and their application to the production of aluminum according to the Hall-Héroult process. .

In recent years, the operation of aluminum production tanks has been progressively automated, both to improve the energy balance and regularity of operation, as well as to limit human intervention and improve the capture efficiency of fluorinated effluents.

One of the essential factors, to ensure the regularity of operation of an aluminum production tank by electrolysis of alumina dissolved in the molten cryolite, is the rate of introduction of the alumina into the bath. A defect in alumina causes the appearance of "the anodic effect, or" packaging which results in a sudden increase in the voltage across the terminals of the tank, which can go from 4 to 30 or 40 volts, and which affects on the whole series.

An excess of alumina creates a risk of soiling the bottom of the tank by alumina deposits which can transform into hard plates electrically insulating part of the cathode. This induces in the metal of the tanks the creation of very strong horizontal currents which, by interaction with the magnetic fields stir the sheet of metal and cause an instability of the bath-metal interface.

This defect is particularly annoying when it is sought to lower the operating temperature of the tank - which is very favorable to the Faraday yield - by adopting very "acid" baths (with a high content of AIF ₃ ) or containing various additives, such as than chlorides, lithium or magnesium salts. However, these baths have a substantially reduced capacity and speed for dissolving alumina, and their use implies that the alumina content is very precisely regulated, at relatively low concentrations and between two relatively close extreme limits.

Although it is possible to directly measure the alumina content of the baths by analysis of electrolyte samples, it has been chosen for many years to carry out an indirect evaluation of the alumina contents by following an electrical parameter reflecting the alumina concentration of said electrolyte.

e étant une image de la force contre-électromotrice de la cuve dont on admet généralement que la valeur est de 1,65 volt, U la tension aux bornes de la cuve et I l'intensité qui la traverse.This parameter is generally the variation of the internal resistance, or more exactly, of the internal pseudo-resistance which is equal to:

e being an image of the counter-electromotive force of the tank, the value of which is generally accepted to be 1.65 volts, U the voltage across the terminals of the tank and I the intensity passing through it.

By calibration, one can draw a variation curve of R as a function of the alumina content, and by measuring R at a frequency determined according to currently well known methods, one can know at any time the concentration symbolized by (Al ₂ O ₃ ].

Attempts have been made for many years to introduce alumina into the bath with a certain regularity so as to maintain its relatively stable concentration around a predetermined value.

The processes for automatic supply of alumina, more or less strictly controlled by its concentration in the bath, have been described previously. In particular, French patent FR-A-1 457 746 (US 3,616,316) in the name of REYNOLDS describes a regulation process in which the variation in internal resistance of the tank is used as a parameter reflecting the concentration of alumina, including l introduction into the bath is carried out by a distributor combined with a means of piercing in the frozen electrolysis crust: the process takes into account a variation in alumina concentration (fig. 4) which is not specified but extends largely on either side of the minimum internal resistance which is around 3.5% of A1 ₂ 0 ₃ , and alternately the supply of alumina can be adjusted to a low or high rate, the high rate being higher than the normal consumption of the tank, and the low rate being lower than this consumption.

French patent 1,506,463 to V.A.W. is based on the measurement of the time that elapses between stopping the supply of alumina and the appearance of the anode effect; American patent US-A-3400062 of ALCOA, implements a "pilot anode to obtain an early detection of the tendency to runaway and regulate the rate of introduction of alumina, which is distributed from a hopper fitted with a frozen electrolyte crust piercing device.

The means for supplying alumina is described, in a more complete manner, in US Pat. No. 3,681,229 assigned to REYNOLDS.

Finally, US-A-3371 026 assigned to REYNOLDS, describes a device making it possible to deliver doses of alumina of substantially constant weight, comprising means for keeping each loading orifice open.

More recently, regulatory methods based on the control of the alumina content have been described in particular in the Japanese patent application for SHOWA DENKO published on March 3, 1977 under No. 5,228,415 and in the patent US-A- 4,126,525 assigned to MITSUBISHI.

In the first of these patents, the alumina concentration is fixed in the range of 2 to 8%. We measures the variation AV, as a function of time t, of the voltage at the terminals of each tank, it is compared with a predetermined value and the rate of supply of alumina is modified to bring the OV / T back to the standard value. The disadvantage of this process is that its sensitivity varies with the alumina content, which is precisely minimal in the interval used, from 3 to 5% of AI ₂ 0 ₃ (table page 84).

In the second of these patents, the alumina content is also fixed in the range of 2 to 8% and, preferably, 4 to 6%. The tank is fed for a predetermined time t ₁ , with an amount of alumina greater than its theoretical consumption, until a predetermined alumina concentration is obtained (for example up to 7%), then the l 'feeding at a rate equal to the theoretical consumption for a predetermined time t ₂ , then the feeding is stopped until the appearance of the first symptoms of anode effect (“packaging”), and the feeding cycle is resumed at a rate higher than the theoretical consumption.

In this process, the alumina concentration varies, during the cycle, from 4.9 to 8% (example 1) or from 4.0 to 7% (example 2). These various processes lack precision and do not solve the problem posed, which is the regulation of the alumina content between narrow limits.

The object of the invention is a process for precise regulation of the rate of introduction and of the alumina content of a tank intended for the production of aluminum by electrolysis of alumina in a bath based on molten cryolite, the upper part of which forms a fixed crust, a process which maintains the alumina content in a narrow range chosen between 1 and 3.5% and which consists in introducing the alumina directly into the molten cryolite bath, in successive doses , of substantially constant weight and at variable time intervals, by at least one orifice kept open permanently in the frozen crust and to modulate the rate of introduction of alumina as a function of variations in the pseudo-internal resistance of the tank in predetermined time intervals, alternating sentences of undernourishment and supercharging of alumina with respect to the rate corresponding to the consumption of the tank.

Another object of the same invention is an apparatus for implementing the process for precise regulation of the alumina content, comprising means for delivering to each orifice successive doses of alumina of substantially constant weight, a means for measuring the the internal pseudo-resistance, a means of calculating the speed of variation of the internal resistance, means for varying the rate of introduction of the alumina doses as a function of variations in the internal resistance and means for varying the anode-cathode distance from the tank.

• La figure 1 représente la variation de la pseudo-résistance interne d'une cuve d'électrolyse en fonction de sa teneur en alumine, avec, en paramètre, la distance anode-cathode « DAM •.
• La figure 2 représente la variation de la pseudo-résistance interne d'une cuve d'électrolyse en fonction du temps et de la cadence d'introduction d'alumine selon l'invention.
• La figure 3 représente la variation de la pseudo-résistance interne d'une cuve d'électrolyse en fonction du temps et de la cadence d'introduction de l'alumine selon une variante de mise en oeuvre de l'invention.
• La figure 4 représente l'ensemble d'un doseur, de sa trémie d'alimentation et d'un dispositif destiné à maintenir ouvert, en permanence, l'orifice d'introduction de l'alumine.
• La figure 5 représente le doseur permettant de délivrer des doses d'alumine successives de poids sensiblement constant.
• La figure 1 montre que la pseudo-résistance interne d'une cuve passe par un minimum quelque peu flou aux environs de 3,5-4 % et augmente rapidement du côté des faibles teneurs en alumine et beaucoup plus lentement du côté des teneurs élevées. Pour avoir une bonne sensibilité, il y a donc intérêt à se placer du côté des faibles teneurs en alumine, sans toutefois descendre en dessous de 1 %, valeur autour de laquelle la pseudo-résistance interne augmente très rapidement quand la teneur en alumine diminue, ce qui correspond à l'effet d'anode ou « emballage _". Dans la suite, nous parlerons, pour simplifier, de résistance interne désignée par Ri pour désigner la pseudo-résistance interne.

Another object of the same invention is the application of the above process and apparatus to the production of aluminum by the Hall-Héroult process, either with a normal or slightly acidic electrolyte based on cryolite which may also contain , from 5 to 13% of AIF ₃ , and operating around 955 to 970 ° C, or with a very acidic electrolyte, which may contain from 13 to 20% of AIF ₃ and operating around 930 to 955 ° C , and may also contain Lithium, in the form of LiF, and operating at temperatures down to 910 ° C.

• FIG. 1 represents the variation of the pseudo-internal resistance of an electrolytic cell as a function of its alumina content, with, as a parameter, the anode-cathode distance "DAM •.
• FIG. 2 represents the variation of the internal pseudo-resistance of an electrolysis cell as a function of time and of the rate of introduction of alumina according to the invention.
• FIG. 3 represents the variation of the internal pseudo-resistance of an electrolysis cell as a function of time and of the rate of introduction of the alumina according to an alternative embodiment of the invention.
• Figure 4 shows the assembly of a metering device, its feed hopper and a device intended to keep open, permanently, the orifice for introducing alumina.
• FIG. 5 represents the dispenser making it possible to deliver successive doses of alumina of substantially constant weight.
• Figure 1 shows that the internal pseudo-resistance of a tank passes through a somewhat fuzzy minimum around 3.5-4% and increases rapidly on the side of low alumina contents and much more slowly on the side of high contents. To have a good sensitivity, it is therefore advantageous to place oneself on the side of low alumina contents, without however falling below 1%, a value around which the internal pseudo-resistance increases very rapidly when the alumina content decreases, which corresponds to the anode effect or "packaging _" . In the following, we will speak, for simplicity, of internal resistance designated by Ri to denote the pseudo-internal resistance.

The invention is based on the use of the part of the curve Ri = f [A1 ₂ 0 _3] comprised between alumina contents of approximately 1 to 3.5%, and on the possibility of evaluating, at any time - and correct - the alumina content of the cryolite bath and keep it between very narrow limits. As a result, in addition to a very high regularity of operation, the possibility of using electrolysis baths having a lower capacity for absorbing alumina but in return, leading to a substantially lowered operating temperature and a current efficiency called Faraday efficiency significantly increased.

The process which is the subject of the invention, which consists in modulating the feed rate according to variations in the internal resistance, comprises the following successive stages (identical stages, in the different variants, will be designated by the same letters).

A. We set a setpoint value Ro for the internal resistance Ri which is, for example, 13.9 µΩ for a modern 175,000 amp vessel with precooked anodes, and two upper and lower limit values between which the internal resistance will be allowed to vary, Ro + r and Ro - r, for example 13.9 ± 0.1 µΩ.

B. A regulation cycle is started when Ri is between 13.8 and 14.0 µΩ.

C. The tank is supplied at a rate known as slow (which will be denoted CL) 15 to 50% lower than the normal consumption corresponding to the electrolysis process, which will be denoted CN (over a long period of time, CN is approximately around 100 kg / h for a 175,000 amp tank). CL is deduced from CN by the equation CL = α · CN where a is an adjustable parameter. The tank will therefore gradually become depleted in alumina, the figurative point will go up in the direction of the arrow CL, FIG. 1, and Ri will grow (FIG. 2).

D. The successive values taken by the internal resistance are measured at equal time intervals t ₁ , t ₂ , t ₃ , etc., for example every 3 to 6 minutes. In practice, a large number of measurements are taken, the average of which is taken so as to eliminate the risk of outliers.

E. The slope p ₁ of the curve - in practice comparable to a straight line - of variation in internal resistance as a function of time during stage D is determined. If the slope p ₁ is less than a set value p ₁ , we gives an order of "tightening", that is to say of decreasing the anode-cathode distance or, more precisely, the anode-metal distance (DAM) by lowering the anode system by a predetermined value. When the internal resistance exceeds the upper limit value Ro + r (at t ₈ for example), the supply device is ordered to switch to fast rate (CR), 20 to 100% higher than normal consumption CN , for a predetermined time T which can be of the order of 1/2 hour to 1 hour. CR is deduced from CN by the equation CR = β · CN, where β is an adjustable parameter.

F. Due to the fast rate feed, the alumina content of the tank will gradually increase since it is supplied with more than the electrolysis consumes, the figurative point will go down in the direction of the arrow CR , Figure 1 and Ri will decrease. The successive values taken by the internal resistance are measured, at equal time intervals, tg and t ₁₆ , for example, every three to six minutes.

• a) on compare p₁ et p₂. Ils doivent être dans le rapport
  

G. At the end of time T, the supply is stopped at a rapid rate. Then, calculate the slope _P2 of the variation in internal resistance as a function of time during stage F and carry out the following operations:

• a) comparing p ₁ and p ₂ . They must be in the report
  

(p est en µΩ/mn et CL, par exemple en kg/mn)If this is not the case, we deduce that CN is badly centered and we recalculate a new value CN ₁ according to the equation:

(p is in µΩ / mn and CL, for example in kg / mn)

• b) si Ri est devenue inférieure à Ro - r ou si _P2 est supérieure à une valeur de consigne p₂ ⁰, on donne un ordre de desserrage, c'est-à-dire d'augmentation de la distance anode-cathode, d'une valeur prédéterminée ;
• c) on passe l'alimentation en cadence lente, éventuellement modifiée en fonction de la nouvelle valeur de CN₁ de la cadence normale, et on reprend ainsi un nouveau cycle, au stade C.

This calculation is normally carried out by the automaton which controls the tank and the CN registration is automatic, these operations being carried out by apparatus known to those skilled in the art and which are not part of the invention;

• b) if Ri has become less than Ro - r or if _P2 is greater than a reference value p ₂ ⁰ , an order of loosening is given, that is to say of increasing the anode-cathode distance, d 'a predetermined value;
• c) the feed is passed in slow cadence, possibly modified as a function of the new value of CN ₁ of the normal cadence, and a new cycle is thus resumed, at stage C.

In the process, the time T (fast rate feed) and the fast rate CR are adjusted so that the concentration of the electrolyte in alumina increases from 0.5 to 1% (in absolute value) and, preferably , from 0.5 to 0.6%. One thus moved on a reduced portion of the curve Ri = f [Al ₂ O ₃ ] which one can, therefore, and without appreciable error, to consider as linear in the interval.

This process therefore ensures a very high precision of the alumina content and, consequently, a very high regularity of operation of the tank.

• E₁ lorsque la résistance interne Ri a franchi la valeur limite haute Ro + r, on donne à la cuve un ordre de « serrage d'une valeur prédéterminée et on passe en cadence d'alimentation rapide CR pour un temps prédéterminé T.

It can be applied in two variants, of a simpler implementation; first variant: we carry out stages from A to D, then:

• E ₁ when the internal resistance Ri has crossed the high limit value Ro + r, the tank is given an order to "tighten a predetermined value and the fast feed rate CR is switched to a predetermined time T.

F due to the fast rate feed, the alumina content of the tank will gradually increase, since it is supplied with more than the electrolysis consumes, the figurative point will go down in the direction of the arrow CR , Figure 1 and Ri will decrease.

We measure the successive values taken by the internal resistance, at equal time intervals, t ₉ to t _l6 , for example every three to six minutes.

G ₁ when the time T has elapsed, we return to slow cadence. If at the end of time T, we have Ri <Ro - r, we give a release order proportional to the difference (Ro - r) - Ri, so as to reset the start of the cycle with Ri substantially equal to Ro - r .

In this variant, we no longer calculate the slopes _P1 and p ₂ , and we therefore no longer have the information "normal rate corrected CN ₁ •.

• E₂ lorsque la résistance interne Ri a franchi la valeur limite haute Ro + r, on donne à la cuve un ordre de « serrage d'une valeur prédéterminée. Si ce serrage amène la valeur suivante de Ri en dessous de Ro + r, on continue à alimenter en cadence lente jusqu'à ce que Ri repasse au-dessus de Ro + r. On donne alors un nouvel ordre de « serrage •. Si le premier ordre de serrage n'a pas permis à la valeur suivante Ri de repasser en dessous de Ro + r, on donne un deuxième, et éventuellement, d'autres ordres de serrage, mais on a fixé, à priori, et introduit dans l'automatisme, le nombre maximal N d'ordres successifs au-delà duquel on repasse en cadence d'alimentation rapide. Ce nombre N peut être 1, 2, 3, 4 ou 5. (Si N est égal à 0, on est ramené au cas précédent, stade E_i.) On passe alors en cadence rapide CR pendant un temps T prédéterminé.

A second variant consists in carrying out stages A and E as just described, and continuing as follows:

• E ₂ when the internal resistance Ri has crossed the upper limit value Ro + r, the tank is given an order to "tighten a predetermined value. If this tightening brings the next value of Ri below Ro + r, we continue to feed in slow cadence until Ri goes back above Ro + r. We then give a new order of "tightening". If the first tightening order did not allow the following value Ri to fall below Ro + r, we give a second, and possibly other tightening orders, but we fixed, a priori, and introduced in the automated system, the maximum number N of successive orders beyond which we return to rapid feed rate. This number N can be 1, 2, 3, 4 or 5. (If N is equal to 0, we are brought back to the previous case, stage E _i .) We then pass in rapid cadence CR for a predetermined time T.

F due to the fast rate feed, the alumina content of the tank will gradually increase, since it is supplied with more than the electrolysis consumes, the figurative point will go down in the direction of the arrow CR , Figure 1, and Ri will decrease.

G ₁ when the time T has elapsed, we return to slow rate CL. If at the end of time T, we have Ri <Ro - r, we give a release order proportional to the difference (Ro - r) - Ri, so as to reset the start of the cycle with Ri substantially equal to Ro - r .

The apparatus for implementing the invention comprises, first of all, a means for delivering, to each introduction orifice provided in the frozen electrolyte crust, successive doses of alumina of substantially constant weight, combined with an alumina storage means preferably located near the tank, which can be replenished periodically from a central storage.

Figures 4 and 5 show an alumina supply device according to the invention.

The alumina is stored in the hopper (1) placed in the tank superstructure. Its capacity can correspond, for example, to one or more days of walking, and it is itself replenished from a centralized storage, by all known means (pneumatic, fluidized transport, etc.).

The distributor (2) and the drilling tool (3) are placed inside the hopper and fixed on a plate (4) which forms the bottom. The distributor essentially comprises a metering device (5) and a distributor (6) which introduces the alumina into the orifice (7) formed and maintained in the frozen crust (8) on the surface of the electrolyte (9).

The metering device comprises a tubular body (10) in which slides a rod (11) actuated by the jack (12). This rod is provided with two conical plugs (13) (13 ') which cooperate with two conical bearings (14) (14') on which they can alternately come to bear in substantially sealed manner.

The tubular body (10) and the upper body (15) are joined coaxially by a plurality of ribs (16), leaving between them large spaces between which the alumina flows spontaneously by gravity when the shutter (13) is in the high position, so as to fill the tubular body whose capacity corresponds to a unit dose of alumina.

Under the action of the jack, the central rod (11) brings the shutter (13) in the low position, on the bearing surface (14) while the shutter (13 ') leaves its bearing surface (14') and thus allows the alumina dose to flow through the distribution chute (6) directly into the orifice (7).

The drilling tool (3) is also arranged in a tubular body (17) placed inside the hopper. It comprises a jack (18), the rod (19) of which is provided, at its end, with an easily interchangeable chuck (20), with a scraping means (21) which makes it possible to eliminate, during the ascent of the stick, the crusts of electrolyte which could have adhered to it.

The controls of the jacks (12) and (18), not shown, are transferred to the outside of the hopper in a known manner.

To prevent the chisel (20) from diving unnecessarily into the bath, it can be provided with a means for detecting the level of the electrolyte, such as an electrical contact, which gives the jack (18) the order to reassemble as soon as the crust has been broken and the end of the chisel has come into contact with the molten electrolyte.

The capacity of the dispenser is set according to the power of the tank and the number of feed points. A given tank may include one or more metering-distributing-picking assemblies, distributed for example between the two lines of anodes.

Of course, this type of dispenser is given only by way of example, and any other equivalent means for introducing alumina directly into the liquid electrolyte, through an open orifice, comes within the scope of the invention.

One can also provide, in the immediate vicinity of the orifice made and maintained in the crust, a means of capturing the gaseous effluents which emerge therefrom.

The measurement of internal pseudo-resistance can be carried out by various means known to man. art. The simplest consists of measuring the intensity I, the voltage U at the terminals of the tank and performing the operation:

The information collected and processed is ultimately used to ensure the timing of successive doses of alumina.

If, for example, the normal rate CN is 100 kg per hour, distributed between four orifices of introduction and each dose of alumina is 1 kg, CN corresponds to one dose every 110 seconds and CL = CN - 30% at a dose every 205 seconds.

These calculations and the triggering of orders to the metering distributor are carried out, in a known manner, by programmable logic controllers, equipped with microprocessors.

It is particularly advantageous to provide the device intended to keep the insertion opening open with a plugging detector for said opening, so that, pending manual or automatic unblocking, the metering distributors supplying the other open openings receive rate increase orders so that the total amount of alumina introduced into the tank remains constant.

• - soit de 5 à 13 % d'AIF₃, avec une température de fonctionnement comprise entre 955 et 970 °C ;
• - soit de 13 à 20 % d'AIF₃ (bains dits « très acides ») avec une température de fonctionnement de l'ordre de 930 à 955 °C, ces bains pouvant contenir, en outre, jusqu'à 1 % de lithium sous forme de fluorure de lithium avec, dans ce dernier cas, une température de fonctionnement pouvant descendre jusqu'à 910 °C.

The process and the equipment which have just been described apply to the series of tanks intended for the production of aluminum by electrolysis of alumina dissolved in baths based on molten cryolite and, in particular, in the case where the bath includes:

• - either from 5 to 13% of AIF ₃ , with an operating temperature between 955 and 970 ° C;
• - or from 13 to 20% of AIF ₃ (so-called “very acidic” baths) with an operating temperature of the order of 930 to 955 ° C., these baths can also contain up to 1% of lithium in the form of lithium fluoride with, in the latter case, an operating temperature which may drop to 910 ° C.

One can also consider other additives such as magnesium halides at a concentration of up to 2% magnesium or alkali or alkaline earth chlorides at a concentration of up to the equivalent of 3% CI.

These baths have a relatively low capacity for absorbing and dissolving alumina and are therefore well suited to the implementation of the process which is the subject of the invention, which ensures a regular supply of alumina. They have the advantage of ensuring a Faraday yield clearly superior to conventional baths operating at 960-970 ° C.

• On a fait fonctionner pendant plusieurs mois une série de cuves à anodes précuites, alimentées sous 180 000 ampères, en assurant la régulation de la teneur en alumine, selon l'invention, autour d'une valeur centrale de 2,9 % et des variantes extrêmes de 3,5 à 2,1 %. Le bain contenait 13 % d'AIF₃ et la température était voisine de 950 °C. On a obtenu un rendement Faraday moyen de 93,5 % (au lieu de 92 % moyens avec un bain à 8 % d'AIF₃ et 6 à 9 % AI_z0₃, à 960 °C).

Application example:

• A series of prebaked anode tanks, operated at 180,000 amperes, were operated for several months, regulating the alumina content, according to the invention, around a central value of 2.9% and variants extremes of 3.5 to 2.1%. The bath contained 13% AIF ₃ and the temperature was close to 950 ° C. An average Faraday yield of 93.5% was obtained (instead of an average of 92% with an 8% AIF ₃ bath and 6 to 9% AI _z 0 ₃ , at 960 ° C).

Then, the alumina content was lowered to a central value of 2.3% with extreme variations of 1.6 and 2.9%. The bath contained 14% AIF ₃ and 2% LiF, and the temperature was close to 935 ° C. An average Faraday yield of 95% was obtained.

It can, moreover, be taken for certain that the lowering of the temperature, permitted for the implementation of the invention, will appreciably increase the service life of the electrolysis cells.

Among the other advantages which the implementation of the invention provides, it is possible to indicate the elimination of accumulations of sludge on the bottom of the tanks, and the reduction of the average number of packaging, on each tank, to less than one per twenty. -four hours.

Claims (8)

1. A process for accurately controlling the rate of introduction and the content of alumina in a tank for the production of aluminium by the electrolysis of dissolved alumina in a cryolithe-base bath, the upper part of which forms a solidified crust, and wherein the alumina content is to be maintained in a narrow range of between 1 % and 3.5 %, wherein the alumina is introduced directly into the molten cryolithe bath by way of at least one opening which is kept open in the solidified crust, and the rate of introduction of the alumina is modulated in dependence on the variations in the internal resistance of the tank during predetermined periods of time, either by alternating cycles of equal duration of introducing alumina at a slower rate and at a faster rate than the rate corresponding to the consumption of the tank, or by introducing the alumina in successive amounts, of substantially constant weight, at variable periods of time, characterised in that the rate of introduction of the alumina in dependence on the variations in internal resistance is determined by the succession of the following operations which are performed on a repetitive cycle ;

A. a reference value Ro is fixed in respect of the resistance Ri and two upper and lower limit values Ro + r and Ro - r respectively, between which the internal resistance may vary, are fixed ;

B. a control cycle is begun at the moment at which Ri is between Ro - r and Ro + r;

C. the tank is fed at a slow rate CL which is from 15 to 50 % below its normal alumina consumption CN;

D. measurements are taken in respect of the successive values assumed by the internal resistance, which increases, at equal periods of time ;

E. the slope _P1 in respect of the variation in Ri in the course of stage D is determined ; p₁ is compared to a reference value _P1 ⁰ and, if p_i < _P1 ⁰ is found, an order to close up by a predetermined amount is given ; as soon as the internal resistance Ri exceeds Ro + r, the tank is fed at a rapid rate CR which is from 20 to 100 % higher than its normal consumption CN for a predetermined time T ;

F. measurements are taken in respect of the successive values assumed by the internal resistance, which decreases, at equal periods of time ;

G. at the end of the time T, the feed at the rapid rate CR is stopped, the slope _P2 in respect of the variation in internal resistance during stage F is calculated, and p₁ and _P2 are compared ; if

the rates CL and CR are not modified ; if

a fresh normal rate CN₁ is re-calculated in accordance with the formula

and the fresh value CN₁ is taken as a basis for calculation for the slow and fast rates of the following cycles, then Ri and Ro - r and _P2 and _P1 are compared ; if Ri < Ro - r or _P2 > p20, a predetermined reference value, an order for moving apart by a given distance is given ; and finally, the feed to a slow rate CL, which is possibly modified in dependence on the fresh value CN₁ of the normal rate, and a new cycle is begun at stage C.

2. A control process according to claim 1 characterised in that, at stage E, when the internal resistance Ri of the tank has exceeded the upper limit value Ro + r, the following operations are performed ;

E₁ when the internal resistance Ri has exceeded the upper limit value Ro + r, the tank is given an order to close up by a predetermined amount and the feed rate is changed to the rapid rate CR for a predetermined time, To ;

F measurements are taken in respect of the successive values assumed by the internal resistance, which falls, at equal periods of time ;

G₁ when the period of time T has elapsed, the feed is switched back to the slow rate ; if, at the end of the period of time T, Ri < Ro - r, an order to move apart in proportion to (Ro - r) - Ri is given.

3. A control process according to claim 1, characterised in that, at stage E, when the internal resistance of the tank has exceeded the upper limit value Ro + r, the following operations are performed :

E₂ a first order to close up by a predetermined amount is given and the internal resistance Ri is again measured ; if it is still higher than Ro + r, a second close-up order is given and so on until the internal resistance has again fallen below Ro + r ; when the number of successive close-up orders has exceeded a predetermined value N which is generally between 1 and 5, without the internal resistance having fallen to below Ro + r, the feed is switched to the rapid rate CR for a predetermined period of time T ;

F measurements are taken in respect of the successive values assumed by the internal resistance, which is falling, at equal periods of time ;

G₁ when the period of time T has elapsed, the feed is switched over again to the slow rate CL ; if, at the end of the period of time T, Ri < Ro - r, an order to move apart in proportion to the difference (Ro - r) - Ri is given and a fresh cycle is begun at stage C.

4. A control process according to any one of claims 1 to 3 wherein each opening for introducing alumina is kept open by means of a plunger which is displaced with a substantially vertical alternating movement and which is actuated in the period of time between the operations of introducing amounts of alumina, characterised in that the possible blockage of one of the introduction openings is detected and that any introduction of alumina at that point is stopped, and that the introduction of alumina at the other openings is proportionally increased until the blocked opening is unblocked.

5. A control process according to any one of claims 1 to 4 characterised in that at least one of the following additives is added to the bath of molten cryolite :

- 5 to 20 % aluminium fluoride,

- lithium salts in a concentration equal to or less than 1 % expressed in the form of Li,

- magnesium salts in a concentration equal to or less than 2 % expressed in the form of Mg, and

- alkali metal or alkaline-earth chloride in a concentration equal to or less than 3 % expressed in the form of Cl.

6. A control process according to any one of claims 1 to 5, characterised in that the temperature of the electrolyte is controlled to between 910 and 955 °C.

7. Apparatus for carrying out the process according to any one of claims 1 to 6 comprising a means for keeping each loading opening open, a means for supplying to each opening successive amounts of alumina, of substantially constant weight, a means for measuring the internal pseudo-resistance, a means for calculating the speed of the variation in internal resistance, means for varying the rate of introduction of the amounts of alumina in dependence on the variations in internal resistance, and means for varying the anode-cathode spacing of the tank, characterised in that it further comprises a means for detecting possible blockage of an introduction opening, a means for interrupting the feed at the blocked opening and a means for proportionally accelerating the feed rate at the other openings until the blocked opening is unblocked.

8. Use of the process and apparatus according to any one of claims 1 to 7 for the production of aluminium by electrolysis of dissolved alumina in a molten cryolite-based bath, wherein the alumina content is to be maintained in a narrow range of between 1 and 3.5 % with variations not exceeding ± 0.5 % with respect to the central value, the cryolite bath having added thereto from 5 to 20 % of AIF₃ and possibly up to 1 % of lithium in the form of LiF, magnesium halides in a concentration which may be up to 2 % of magnesium or alkali metal or alkaline-earth chlorides in a concentration which may be up to the equivalent of 3 % of Cl.

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