CH615700A5 - Method for operating a cell for the electrolytic decomposition of alumina into aluminium metal - Google Patents

Description

An object of the invention is to provide a new method for electrolytically reducing alumina to aluminum which overcomes many of the disadvantages of the conventional Hall-Heroult cell.

In the method according to the invention of the type mentioned, this is achieved by creating an atmosphere on the anode which contains water in amounts which are effective in order to prevent sputtering of the anode.

The invention is explained in more detail using an exemplary embodiment with reference to the accompanying drawings.

The drawing shows:

1 is a partial section through a cell of the Söderberg anode type for use in the present invention.

2 shows a sectional view through a cell with a pre-baked anode,

3 shows schematically and in section an arrangement to explain the principle of the present invention,

Fig. 4 shows part of a cryolite-AlF3 state diagram.

The term "A CONC" used in Fig. 4 is an abbreviation for A concentration and refers to the change in concentration at the indicated temperature index that is required to initiate crystallization.

5 schematically and partly in section a test measuring arrangement,

The operation of a Hall Heroult cell with bath weight ratios of NaF / AlF3 equal to or below 1.1: 1 resulted in a higher current efficiency due to lower bath operating temperatures. Higher C02 / C0 ratios mean lower consumption of carbon in the carbon-containing anodes. Experiments with baths with weight ratios of 1.1: 1 or less, however, have shown a problem of crust formation over the molten aluminum pad cathode during electrolysis. Analysis led to the discovery that this problem results from maintaining a 20-30 ° C difference between the electrolyte bath operating temperature and its liquidus temperature, i.e. A T = 20 to 30 ° C. This temperature difference has been measured at the hottest place in an industrial melting cell. Maintaining this 20 to 30 ° C difference was the practice when operating a conventional aluminum melt cell of the Hall-Heroult type.

Maintaining the 20-30 ° C difference in cell operation was the result of various considerations. For example, this temperature difference allowed the bath to form a protective solidified or frozen layer over and near the side linings of the cell, and it was known that for every 1 ° C increase in bath operating temperature above the bath liquidus temperature, the current efficiency decreased by approximately 0 , 22% occurred. Both of these factors indicated that a temperature difference above 30 ° is undesirable. Creating the lower limit of this temperature difference was the concept that the liquid space surrounded by the frozen bath on or near the side walls of the cell should not become too small for effective melting. With this well-founded basis for the temperature difference of 20 to 30 ° C, the problem of electrolyte crusting above the molten aluminum pad cathode during electrolysis in low bath conditions was not taken into account by experts in practice, namely to maintain this temperature difference there.

Analyzes showed that if the 20-30 ° C temperature difference is maintained during low bath operation, i.e. with a low weight ratio of NaF to A1F3, in the catholyte area of the electrolyte directly above the molten aluminum pad cathode, a concentration gradient influence occurs as a result of an annoying electrolyte crust over the molten aluminum pad cathode. It is believed that the gradient results from a depletion of acidic (AlF3-rich) components in the catholyte and an accompanying enrichment of basic (NaF) components of the catholyte.

m A difference of 20 to 30 ° between the operating temperature of the electrolyte bath and the liquidus temperature of the electrolyte bath is sufficient to prevent crusting in the catholyte area in high ratio baths that have been used up to now. However, it is insufficient to prevent incrustation at low ratios, for example 0.8. This is shown in Figure 4. Figure 4 is a state diagram for a dual fuel system, i.e. Cryolite and aluminum fluoride, and it should be noted that the situation will become somewhat different (e.g. lower 2 (i liquidus temperature) if other components such as A1203, CaF2, LiF etc. are added to the bath. However, Fig. 4 is sufficient To explain the concept, point A in Fig. 4 is a point 30 ° C above the liquidus temperature of an electrolyte bath with a ratio of 25 1.3 as it could be used in a Hall Heroult cell which Enrichment of sodium will move the actual catholyte composition in the direction of arrow Z, but changes in composition in the direction of arrow Z, as can be seen in Figure 4, will never occur in the area of the state diagram, where a solid phase could pass out. The situation is completely different for point B, which is 30 ° C above the liquidus temperature of a bath with a Ratio of 0.8. There, i5 cryolite (Na3AlF6) can precipitate as soon as an accumulation of sodium in the catholyte becomes large enough to move the active composition further to the left than the A concentration of 30 ° C., as shown in the figure. In this way, it becomes possible for a crust of 40 frozen electrolyte to form over the molten aluminum pad cathode when electrolysis proceeds in a low ratio electrolyte bath, with the 20 to 30 ° C difference between bath operating temperature and bath liquidus -Temperature is operated, as was previously the practice. 45 After considering the cause of the problem of crusting over the molten aluminum pad cathode in low ratio baths, this problem is eliminated by increasing the difference between the bath operating temperature and the bath liquidus temperature. This can be done by increasing the operating temperature or by using an additive such as LiF to lower the liquidus temperature. For example, it can be seen from Fig. 4 that crusting over the pad cathode at a bath ratio of 0.8 is prevented by performing the electrolysis 55 at a bath temperature that is at point C, which is 100 ° C above the liquidus temperature of a bath with a ratio of 0.8. At this higher operating temperature, a considerably larger concentration gradient can be tolerated in the catholyte without fear of the occurrence of 60 crusts, as is the case with the size of the A concentration 100 ° C. in the figure, compared to the size of the A concentration 30 ° C. the figure.

In order to determine the appropriate operating temperature for a given low ratio electrolyte, an estimate of an appropriate operating temperature is first made, for example, based on the nominal two-component composition of the electrolyte and Fig. 4. If a crust on the cathode interface -

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Bath occurs under the selected conditions, this can be noticed by the resistance which is opposed to a scanning or a lateral movement of a steel rod downwards at the interface in the electrolyte bath. Preferably the operating temperature is selected at which there is no noticeable crusting at the interface between the cathode and the electrolyte bath, taking into account that any rise above this minimum adequate temperature means loss of current efficiency.

While this method has been discussed for a constant bath ratio, it should be noted that a greater difference between the bath operating temperature and the bath liquid temperature can be achieved, for example, by adding more aluminum fluoride. Other substances, such as LiF, can also be used to lower the liquidus temperature while keeping the bath operating temperature constant.

The alumina used is generally added at a rate substantially the same as that at which it is consumed or converted to aluminum, i.e. at the rate of their electrochemical reduction. The term "substantially continuous" includes the addition of alumina continuously or in small, separate portions at frequent intervals.

The alumina, which is fed to a melting cell, must dissolve in the electrolyte at a rate that is at least equal to the rate of the electrochemical reduction, so that the dissolved Al203 portion of the electrolyte is not exhausted. If alumina is added to a cell faster than it can be dissolved, solids, which are referred to as "loops" or "dirt", accumulate on the crucible bottom with an accompanying negative influence on the operation of the cell. Factors affecting the formation of flakes or dirt include the maximum Al203 solubility in the electrolyte and the dissolution rate of the specially selected alumina. The method of adding and the amount of alumina added to the cell at any one time, along with the difference between the cell operating temperature and the liquidus temperature of the NaF-A1F3 electrolyte, are also important considerations with regard to the formation of clumps or dirt.

The solubility and dissolution rate of alumina in NaF-AlF3 electrolytes depends in part on the temperature and weight ratio of NaF / AlF3 (bath ratio) in the molten salt bath. The maximum solubility and dissolution rate are found in pure molten cryolite (bath ratio 1.5: 1) at elevated temperatures.

If the bath ratio is lowered by adding excess A1F3, the temperature at which a fully liquid, molten NaF-AlF3 salt system can be maintained will sharply lower the liquidus temperature. A decrease in the Al2Ö3 solubility and the dissolution rate accompanies a decrease in the bath ratio. While the use of low ratio molten salt mixtures as electrolytes in melt cells allows operation at lower temperatures, alumina addition is required with properties that improve their rate of dissolution in the electrolyte. The solubility of A1203 in a given bath at a certain temperature is independent of the physical form of the A1203 added to the electrolyte, but the rate of dissolution of the alumina in the bath is a function of properties of the added alumina.

The present invention makes use of the knowledge that alumina, which in comparison with “metal quality

Alumina », which is conventionally used for the production of aluminum metal by electrolytic reduction of A1203 in a cryolith-electrolyte, has a higher water content and a larger surface area and is charged directly in contact with molten electrolyte, shows a remarkably higher dissolution rate. It is believed that the higher water content, and particularly a higher chemically combined water content, acts to instantly disperse the charged alumina in the electrolyte by suddenly releasing steam when the charge comes into contact with the hot electrolytic bath. The well-dispersed particles then quickly dissolve in the bath.

The idea of introducing the considerable amounts of water into the alumina, which is used according to the invention for an electrolytic reduction bath, can raise the possibility of explosions. Thus it is mentioned in GB-PS 274 108 that in practice it has not been possible to hydrate or hydroxide alumina directly because of the more or less strong explosions caused by the material and because of the resultant ejection of glowing liquid to use. Methods have been proposed to overcome this problem, first by agglomerating the hydrate and only then placing it in a molten electrolyte bath, see for example DT-PS 472 006. The charging of alumina hydrate onto the crust over a molten electrolytic bath was also carried out proposed a Hall-Heroult cell, the addition to the bath taking place only after dehydration (see US Pat. No. 2,464,267). The practice of U.S. Patent 2,464,267 is in contrast to the present invention where a relatively high water content alumina is added directly to the molten electrolyte rather than being left on a crust over the molten electrolyte for a period of time. It has been recognized that substantially continuous addition of A1203, which contains significant amounts of combined water, does not cause explosions. The developed water only seems to quickly disperse the charged A1203 in the bath and thus promote the dissolution in the bath.

The thought of deliberately adding alumina to a cell that has a higher water content can also indicate a risk of a substantial increase in HF development. It was found that only 5% of the water on the aluminum bath was pyrohydrolysed to produce HF smoke.

The alumina to be used in the present invention can be added to individual cells or a series of cells in a row of crucibles. The cells can use either pre-baked anodes or in-situ baked anodes such as the Söderberg type.

Generally, calcining alumina hydrate, such as the alumina trihydrate made by the Bayer process, produces alumina for use in the invention. Generally, calcination temperatures in the range of about 300 to 600 ° C are suitable for this purpose. The device and method for heating alumina to the desired water content and surface area in ovens or so-called "relaxation heaters" (US Pat. No. 2,915,365, FR Pat. No. 1,108,011) are known.

Alumina with surface areas as high as 350 m2 / g can be obtained by heating a-alumina trihydrate (hydrargillite) in dry air at 400 ° C for 1 hour. Such materials can be quickly dissolved in electrolyte baths according to the invention.

For each alumina to be used as a batch to an aluminum smelting cell, it is possible to run a test as described in Example II or the like. is called to the

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to determine the desired minimum water content. The water content is preferably effective for dispersing the individual particles of the clay when the clay is added to the electrolyte bath. If the water content is too low, steam generation will not be sufficient to disperse the particles, and caking of the particles will result in a decrease in the effective dissolution rate of the alumina in the molten pool. The most preferred particle size for each alumina is 100 to 150 microns. The upper limit of the water content of the alumina is determined not by the need to prevent explosions, but by the amount of heat that must be removed from the bath to drive off water as steam.

This amount may not be sufficient to cause the bath to solidify around the solid alumina particles and thus contribute to the formation of slugs or dirt. In general, if this second criterion is taken into account, no dangerous explosions occur in accordance with the proposal according to the invention. In general, if the water content is effective to disperse the individual particles, it is also effective to prevent sputtering.

Certain guide lines are necessary in the covering of the upper part of the cell in the method according to the invention. The cover must

1.) prevent heat loss from the upper area of the bath to a sufficient degree so that the bath surface remains melted,

2.) prevent a significant amount of air from the diluting vapors from entering the cell and

3.) Eliminate combustion of the carbon-containing anode material in the air by drafts.

It is essential that the bath surface remains molten so that the alumina added contacts the molten bath. This causes the water in the alumina to disperse the particles for rapid dissolution in the bath and also to create water vapor that counteracts attack on the exposed carbon anode surfaces, as explained below. It can sometimes happen that an undissolved substance floats on the surface of the bath, but this undissolved substance does not have to be present in such quantities that the dispersion of the clay is interrupted and leads to the formation of lumps or dirt.

A basic concept, in addition to reducing the air entering the cell, is the undesirability of the resulting dilution of vapors such as gases emitted by the anode (e.g. hydrocarbons), such gases resulting from the electrolysis reaction (e.g. C02) and such gases which develop from the bath (e.g. gaseous fluorides). Of course, the cell cannot be completely closed, since openings have to be provided in order to withdraw the above-mentioned gases generated there from the cell.

Depending on the operating conditions, the consumption of carbon anodes in Hall Heroult process cells is in the range from A to 3A of one kg of carbon per kg of aluminum produced. The preferred conditions are such that they lead to the minimum stoichiometric consumption, 0.15 kg C / 0.45 kg / AI, which is prescribed by the reaction:

2A1203 + 3C-

3C02 + 4 AI

One of the problems with sealing the space above the electrolyte bath of a Hall-Heroult cell from air up to now has been that carbon sludge accumulates on the bath surface and even carbon dust is distributed in the electrolyte. This carbon sludge and

-dust results from decomposition of the carbon anodes. This phenomenon is called anode sputtering.

Carbon sludge causes problems with alumina addition. The carbon sludge has made it impossible to replenish used alumina during electrolysis. As the level of dissolved alumina in the bath decreases, the sludge formation accelerates. Carbon dust and sludge increase the bath viscosity and hinder diffusion of the oxygen-carrying ions to the anode, which limits the anode i (i current densities and affects the heat balance of the cell. Increasing the viscosity and density of the bath reduce the current efficiency and lead too bad metal coalescence The carbon in the slurry and in the dust is not available for reaction with oxygen at the anode i5, and thus the carbon consumption is increased by the dusting, because of the carbon sludge the bath movement caused by bubble formation at the anode is lowered, and the tendency of the electrolyte to solidify at the interface of the metal cushion bath is increased 2o.

Finally, enough carbon dust can be distributed in the bath in closed cells to allow electronic conductivity and complete loss of metal production. These conditions must be switched off for successful operation of a surrounding cell according to the invention.

Extending the life of the anodes will not only reduce carbon consumption, but in the case of pre-baked anodes, will decrease the amount of anode stumps that need to be recirculated to produce additional anodes, with problems affecting fluoride development during the Accompany baking by anodes, get smaller.

It has been found that the phenomenon of anode sputtering in closed cells can be prevented by creating a water-containing atmosphere at the anode within the cell. It is thus possible to create a protective jacket around the anode within the closed cell and to generate a water partial pressure within this protective jacket which is effective to prevent anode sputtering. The present invention makes use of the finding that when the above-described water-containing alumina is preferably added to the bath 45 at locations on the bath surface where gas evolves along the anode, the released water when the alumina contacts the bath surface , creates the water-containing atmosphere at the anode in a closed cell, which is necessary to prevent anode sputtering. If the alumina is added to the bath somewhere rather than along the anode, venting the cover over the cell should be provided and controlled so that the released gaseous water actually contacts the anode to prevent anode sputtering.

The prevention of anode sputtering means that the anode sputtering only has to be prevented to such an extent that no sputtering-related deterioration of the cell operation occurs.

Anode sputtering is believed to be caused by atmolite or sodium aluminum tetrafluoride, NaAlF4, in such vapor form. This substance attacks an anode and causes anode sputtering.

Thus, during the operation of the cell, a gaseous effluent, which contains gaseous fluorides, is developed from the electrolyte bath of dissolved alumina in molten cryolite (primarily cryolite or cryolite + additional fluorides such as excess A1F3, CaF2 and LiF), and among such gaseous ones Atmolite is present in fluoride.

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Attack by Atmolit on exposed carbon anode surfaces is particularly a problem when closed cells are used, for example cells which are sealed off at the top by a plate.

It is believed that the amount of water vapor to be created in an atmosphere around an anode surface depends to some extent on the amount of atmosphere to be neutralized. In general, it is desirable to create at least enough water vapor for Atmolit to react stoichiometrically with the water vapor according to the following equation:

NaAlF4 + | H20 9- NaF + | A1203 + 3HF

The water-containing atmosphere can be created by surrounding anode surfaces above the electrolytic bath with a protective jacket spaced from these surfaces and by introducing water vapor into the space between the protective jacket and the anode surfaces. Alternatively, the water vapor can be created by sealing off a space above the electrolyte bath and dissolving alumina in the electrolyte bath that contains enough water so that sufficient water vapor is evolved and rises into the enclosed space to provide the desired protection. Such alumina can be produced by heating alumina trihydrate to the desired water content by known methods, for example using rotary kilns. It is desirable to add the alumina to the bath near the anode or anodes.

The method according to the invention solves many of the problems that occur in conventional Hall-Heroult cell operation. For example, the current efficiency is improved and the operation with a low bath ratio, i.e. low sodium, helps eliminate bloating and shifting of the carbon liner. Also, the ability to seal the cell allows all of the vapor to be collected.

Then there is no loss of roof exhaust from the smelter. Since crucible gas is not diluted with air, the sprinkling treatment or the treatment according to US Pat. No. 3,503,184 is simplified, with only one hundredth of the gas volume to be treated which was previously treated. Lack of air brushes (e.g., U.S. Patent No. 3,708,414), low operating temperatures and low fluoride partial pressure reduce the amount of fluoride and other materials that need to be recovered. In conventional cells, for example, 12 kg of solids per ton of aluminum produced must be recovered; operation according to the invention at 800 ° C only requires a recovery of about 1.6 kg per ton, while operation at 900 ° C has resulted in only about 7.3 kg solids per ton.

Due to the fact that there is no crust on the bath, the alumina added to the bath can be carefully controlled and an optimal concentration can be maintained. The alumina with a higher water content enables even better control of the dissolved alumina concentration. A stable heat balance results in a minimal distance variation from anode to cathode. Cracking of the crust for changing the anode and for adding alumina is then eliminated.

For the first time, alumina with a higher water content enables closed Hall-Heroult cells to operate more satisfactorily. By minimizing carbon sludge formation in such closed cells, there is easy access for alumina to the molten bath surface.

Slightly increased RF radiation is more than a nuisance due to the complete trapping of crucible gases

Locking the upper part of the cell is enabled.

There is no distinguishable drop in current efficiency as a result of using higher water alumina in accordance with the present invention. 5 Carbon consumption can be lower in the invention due to the fact that there is practically no combustion of the anodes in air and because the operation is carried out at a higher ratio of C02 / C0.

The bath weight ratio NaF to A1F3 m is preferably less than 1.0. A ratio less than 0.9 can be used. It is preferred to keep the bath ratio at least greater than 0.5.

The concentration of A1203 dissolved in the bath should be above that at which an anode effect occurs, and it is selected to optimize the current efficiency of the cell. It is believed possible to have some solid particulate alumina in the bath, perhaps on a transitional basis. Soiling, i.e. sedimentation of excess amounts of solid alumina on the bottom of the cell; ii does not occur due to increased alumina solubility at the metal / bath interface caused by concentration gradients in the catholyte. In addition, because of the relatively small difference between the alumina concentration at which an anode effect begins and the alumina saturation concentration when operating with a low bath ratio according to the invention, it is preferred that alumina be added to the bath in a form which has a high dissolution rate, as described above.

Preferred embodiments of such alumina are described below.

While the bath can only consist of A1203, NaF and AIF3, it is possible to create in the bath at least one halide compound of the alkali and alkaline earth metals, apart from sodium, in an amount which is effective around the liquidus temperature of the bath lower the one that the bath would have if only A1203, NaF and A1F3 were available. Suitable alkali and alkaline earth metal halides are LiF, CaF2 and MgF2. In a preferred embodiment, the bath contains lithium fluoride in an amount of between 1 and 15% by weight.

The operating temperature of the bath is preferably kept at a temperature which is greater than 40 ° C. above the cryolite liquidus temperature of the bath. The cryolite-liquid temperature is the temperature at which cryolite 45 begins to crystallize when the bath is cooled. If the bath composition is such that cryolite is the first substance to crystallize on cooling, the intersection of the line of constant bath composition versus temperature with the top liquidus temperature. So where A1203 is the first substance to crystallize, a fairly good approximation of the cryolite liquidus temperature is the «euthectic» temperature, which is determined by the fact that the liquidus temperature for progressively decreasing A1203 content, corresponding increase in NaF + A1F3 and an SS constant bath ratio NaF / AlF3 is found, and the minimum liquidus temperature is selected on the basis of the resulting group of liquidus temperature values. The operating temperature must be effective to prevent bath crusting in the interface areas between the bath and the molten aluminum metal cathode. It is preferred that the operating temperature be below 935 ° C and baths have been successfully used at operating temperatures below 900 ° C, 850 ° C and 800 ° C. In some embodiments, the operating temperature is at least 65 degrees, sometimes at least 100 degrees above the liquidus temperature of the bath.

The electrolytic decomposition of A1203 according to the present invention can be carried out at an anode current density of 0.15 -

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3.1 amperes per cm2 are carried out, while current densities of 0.15-2.3 and 0.15-1.3 amperes per cm2 are the preferred current density ranges.

It is additionally preferred that carbon anodes used according to the invention are protected from a water-containing atmosphere. A suitable water-bearing atmosphere is created when the bath is sealed off from the air and the alumina is preferably supplied at locations on the bath surface where electrolysis gas develops along the anodes. The alumina is in the form of the alumina described here with a high dissolution rate and with a water content. The resulting water-bearing atmosphere prevents sputtering of the anode, a condition that cannot be tolerated in the present invention.

Up to 100% of the alumina to be added and at least 50% and preferably at least 90% by weight is high dissolution rate alumina containing enough water to create an atmosphere above the electrolyte bath which is effective to prevent sputtering of the anode . The alumina is essentially continuously fed directly to the molten electrolyte of the cell. The water content and the rate of dissolution are indicated by the total water and surface area of the alumina under other parameters. The expression “total water” is defined as follows: one exposes an alumina sample to a humidity of 100% for a few hours, sets the sample to 44% relative humidity at 25 ° C for 18 hours, then weighs exactly the sample, then heats it up 1100 ° C and weigh it again. The sample weight loss from the set condition at 44% relative humidity to the condition after heating to 1100 ° C, divided by the sample weight at 1100 ° C and multiplied by 100 is the percentage of total water.

The surface area is measured using the Brunauer-Enmett-Teller method (see Stephen Brunauer, P.H. Emnett, Edward Teller, J. of Am. Chem. Soc., V 60, pp. 309-19, 1938).

The use of alumina with a high water content according to the present invention is contrary to the general view, see page 34 of the publication "The Chemical Background of the Aluminum Industry" by Pear-son, published by The Royal Institute of Chemistry 1955, that alumina which used for the electrolytic production of aluminum should be moisture-free.

In addition, it is desirable that the alumina used in carrying out the process according to the invention is easy to handle and convert.

The properties according to the present invention, which increase the dissolution rate of alumina in molten NaF-AlF3 salt systems, also improve their handling and usability in plants such as according to US Pat. No. 3,503,184. Because the alumina used in the present invention has a higher water content , less energy compared to the energy used in the production of conventional “metal grade” alumina is required to produce it from hydrogenated alumina using the Bayer process.

The alumina supplied to the bath according to the invention can, if desired, be preheated as long as it maintains its above water content and surface area.

When alumina is added to improve the operation of an electrolytic cell to reduce alumina to aluminum, the preferred properties of the alumina used in the present invention include a particle size of 25 to 150 microns in addition to the total water content of 3-20% (optimum 55-145 below 100 mesh and above 270 mesh) and a spec. Surface area from 45 to 400 m2 / g.

Alumina with the above-mentioned properties is particularly suitable for producing aluminum by electrolytic decomposition of A1203 to aluminum metal in an electrolyte bath between an anode and a cathodic interface, which is formed between aluminum metal and an electrolyte bath, which essentially consists of A1203, NaF and A1F3 and has a weight ratio of NaF to A1F3 (bath ratio) up to 1.1: 1, preferably greater than 0.5: 1, while the bath is kept at an operating temperature which is 40 ° C. above the cryolite liquidus temperature of the bath and is effective to prevent bath crusting in interface areas between the bath and the aluminum metal. The cryolite liquidus temperature is the temperature at which cryolite initially crystallizes when the bath is cooled. If the bath ratio is equal to or less than 1.1: 1, the alumina preferably has a total water content of 8-20%, preferably 10-18%. The spec. Surface area may preferably be in the range of 135 to 180 m2 / g.

For conventional cells, for example at 950 ± 10 ° C, a bath ratio greater than 1.1: 1, generally between 1.15: 1 and 1.20: 1, and with an AT not greater than 30 ° C, are the preferred aluminum properties: a spec. Surface from 45 to 95 m2 / g and a total water content of 3 - 7%.

Maximum dissolution rate of alumina in a fluoride bath is achieved when heated, abrasion-resistant alumina with a large surface area, a total water content of 8-20% as particles with a diameter of 55-145 microns (-100 mesh + 270 mesh) directly from the unfrozen Surface of a moving bath at temperatures above its liquidus temperature can be added continuously or in small separate parts, ie in a time interval between separate additions equal to or less than 10 minutes. The term - "small separate parts" is underlined because of the importance of the AT at which the cell is operated. AT is the difference between the operating temperature and the liquidus temperature of a molten NaF-AlF3 salt mixture. The liquidus temperature can be reduced by adding other salts to the bath such as CaF2, Lif, MgFz, etc., but for simplicity a pure NaF-AlF3 system is considered. Conventional melting cells work with A T of 10 - 30 ° C. In conventional plants, a low A T is desirable because the current efficiency of the cell increases as the operating temperature decreases. Because of the improved control of conventional crucible series, the anode-to-cathode (ACD) distance was sometimes reduced to a nominal 2.54 cm distance when the cells were operating. Since the heat input to cells depends on the electrical current and resistance, the low ACD enables the AT to be reduced to, for example, 10 ° C. ± 5 ° C. While this low AT is advantageous from the standpoint of current and energy efficiency, it tends to do so To increase contamination problems in the cells even when an alumina is added with properties that maximize their dissolution rate. Automatic ore feeders can be used which, for example, only feed about 0.9 kg of A1203 to the bath per stage. This is a considerably slow rate of AI203 addition to the cell. However, if the A T1 of the bath is low, even this amount of clay can be so large that the heat removed from the bath to drive off water and bring the batch up to temperature to dissolve it can easily result in localized solidification of the electrolyte. When this occurs, alumina trapped in the solidified bath will sink to the bottom of the cell to create dirt instead of dissolving. Because of this, it is

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important to balance the size of the amount of AI203 that is fed to a crucible at a given time against the A T1 of the cell. A low AT and large alumina units will contaminate a crucible, especially if alumina is used which has a large surface area and contains a lot of water.

A suitable distribution of the particle size is advantageous in order to facilitate dissolving in a melting cell. Small alumina particles, for example, with a particle size of less than 44 microns (- 325 mesh) tend to atomize, agglomerate above the surface of the molten bath, and sink to the bottom of the cell, where they contribute to the pollution problems. Large particles with diameters greater than 150 microns (+ 100 mesh), for example, also contribute to contamination, especially if they are fed in large quantities to the crucible, which is operated with a low AT. The large particles form a layer of solidified electrolyte when touched with the molten bath, sinking to the bottom of the crucible instead of quickly dissolving. This is the same mechanism described above to explain soil build-up in cells that receive A1203 in amounts that are too large to match the low AT. The difference is that particle sizes larger than + 100 mesh lead to contamination, even if the AT value is close to 25 ° C. If the AT is low, attention must also be paid to the heat of vaporization of the water in the alumina.

The invention is further illustrated by the following examples.

Example I

The purpose of this example is to explain the basic principles relating to preventing anode sputtering using a partial pressure of gaseous water. 5, a graphite crucible 51 and an alumina crucible 52 are shown which have a hole 53 at their lower end. A molten aluminum metal pad 54 sits in the bottom of the alumina crucible and is in contact with the graphite crucible 53 so that it is in electrical contact with a cathode lead 55. An electrolyte bath 56, which contains 4% by weight of A1203 and NaF + A1F3 in a bath ratio of NaF / AlF3 = 0.8, rests on the cushion 54. The electrolyte bath has a temperature of 900 ° C. A carbon anode 57 (pre-baked, petroleum coke) is immersed in the electrolyte bath in order to create a current density of 0.75 amp. Per cm 2 at the anode. An alumina anode jacket 58 surrounds the anode 57 and is sealed at its upper part by a plug 59, which is provided with openings for the passage of the anode line 60 and the gas flow line 61. Suitable piping is provided to allow various amounts of argon gas to flow from the tank 62 through an impact bottle 63 which contains water 64 surrounded by an ice water bath 65. The carbon consumption was 0.33 to 0.38 kg per kg of aluminum produced using 100% electricity in 49 to 41 amp-hour experiments using water vapor shielding to prevent sputtering. With a water partial pressure in the argon of 4-22 torr, no carbon slurry or foam was found. When the impingement bottle 63 was bridged so that only argon moved down around the anode 57, a carbon scoop was formed on the bath and anode to cathode electronic shorting occurred through the carbon slurry.

Example II

This example and example III are intended to illustrate the increased dissolution rates that can be achieved with the alumina used to prevent sputtering in accordance with the present invention. The apparatus used in this example is shown in FIG. 3. A crucible surface 70, which was heated by electrical resistance heating 5, was used to bring a cryolite-based bath to a temperature of 740 ° C. in a graphite crucible 71, which rests on a refractory stone 72. The bath composition was 64% by weight cryolite and 36% by weight aluminum fluoride (A1F3). This corresponds to a bath weight ratio of NaF / AlF3 = 0.65. The bath amount was 500 g and 200 ml volume in the molten state. The bath contained 14 g or 2.8 wt% A1203 as an impurity. At 740 ° C this bath has melted and is free of crystals (liquidus temperature about 724.5 ° C). An amount of alumina of 1 g with a total water content of 17% and a spec. A surface area of 170 m2 / g was sprayed onto the exposed, non-encrusted surface of the molten bath. The bath was illuminated with a light source 73 and the time was measured at which no alumina particles 20 could be found in the bath through the viewing tube 74. This time was 2 minutes and 58 seconds, which corresponds to a dissolution rate in mg per ml bath / minute equal to 1.65. By comparison, a so-called "metal grade" clay with a spec. Surface of 40 m2 / g a dissolution rate of 0.14 mg per 2s ml bath / min. Under similar conditions.

Example III

Using the device according to FIG. 3 and an alumina with a total water content of 17% and a surface area of 170 m2 / g resulted in a dissolution rate of 16 mg with a bath weight ratio of NaF / AlF3 = 1.5 and a bath temperature of 980 ° C A1203 per ml bath per minute. The dissolution rate, which under the same conditions for an alumina with a total water content 35 of 20% and a spec. Surface area of 100 m2 / g measured was 8 mg of alumina per ml bath per minute. By comparison, a «metal grade» clay with a surface area of 30 m2 / g under the same conditions resulted in a dissolution rate of approximately 0.36 mg clay per ml 4 t) bath per minute.

Example IV

This example shows how alumina can be produced which is suitable for the present invention.

A Bayer process alumina hydrate was treated in a roasting oven to produce activated alumina suitable for use in the process of the invention. The furnace dimensions were 109.7 m long and 2.89 m inside diameter. The residence time of the material in the furnace was 1 -1 hour. The charged hydrate was countercurrent to the combustion gases that were introduced into the lower end of the furnace. A maximum temperature of 400 to 500 ° C was achieved 3.04 m to 4.57 m inwards from the lower end of the furnace. Natural gas was burned at 184060 liters (standard temperature and pressure) per hour to produce the combustion gases. The flow rate of this natural gas was selected by examining the product for the desired total water content. The volume ratio of air to gas was approximately 10: 1. An alumina with a total water content of 12.5% was produced. 88 to 95% by weight of the particles had a size larger than 325 mesh.

Examples V and VI Aluminum was produced in the cell according to FIG. 1. The maximum dimensions of the steel jacket 20 in the horizontal were 5.64 m by 3.56 m. Its maximum height was 1.14 m. The maximum dimensions of the molten aluminum

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tallpolsters 21 horizontal were 5.39 m by 2.85 m. The electrolyte bath had the same maximum dimensions as the metal cushion.

A mica mat 22 was placed between the steel jacket and the graphite block portion 20 to prevent current flow through the jacket 20. A mat thickness of 6 to 20 mils was used.

The molten aluminum metal pad 21 was supported on a carbonaceous cathode block liner 24 and a stamped carbonaceous liner 25. The carbonaceous liners were supported on an alumina fill 26 with a floor panel 27 disposed between the liner and the fill. A layer of bricks 28 was arranged between the graphite block 23 and the floor plate 27.

Figure 1 is a vertical sectional view through a cell and it is noted that, for example, similar graphite blocks 23 would repeat in other sectional views through the cell.

The anode 29 was a carbon anode of the Söderberg type. The charged composition to form this self-baking anode was 31% pitch with a softening point of 89-100 ° C (cube-in-air method) and 69% petroleum coke. The fraction of the coke was 30% coarse, 16% medium and 54% fine, the size distributions of the coarse, medium and fine coke being shown in Table 1.

Table I. Coke size distribution

Cumulative sieve% larger than sieve size

coarse medium fine

.371

31.1

3rd

50.6

4th

66.8

8th

91.7

11

14

97.9

48.9

28

98.8

75.5

48

99.1

93.5

2.4

100

99.4

98.0

10.4

200

99.7

99.0

39.7

crucible

100

100

100

"Gun-like" feeder, i.e. separate amounts of alumina are added at time intervals. In Examples V and VI, two feeders 41 were used and added alumina approximately every 5 minutes with the alumina levels adjusted to maintain the desired alumina concentration in the bath. It takes approximately 1 minute to expel the alumina, which was approximately 1500 g. The tube 42 is oriented so that alumina strikes the bath 31 where gas 44 rises along the anode. This ensures that the water developed from the charged alumina protects the anode against the generation of coal dust. This practice also promotes dissolution due to bath movement caused by gas evolution. By adding the 15 alumina in line with a row of tips (the tips 45a, b and c lie in a vertical plane parallel to the plane of Fig. 1, which also includes the tube 42) in the Soderberg anode (cracks usually occur in the Anode in line with the rows of tips), the dissolution rate is improved by the increased gas evolution at the cracks. The feeders 41 were operated with air as the fluidizing medium, emphasizing that this represents a small loss of air past the cover 32 to the bath.

25th

The particular alumina used in Examples V and VI had a total water content of 16.95%. This alumina was 98% + 325 mesh in size and its water content alone was sufficient to prevent sputtering, i.e. decomposition of the anode, so that carbon particles are built up in and on the bath.

The manufacturing data for Examples V and VI are given in Tables II to IV.

Table II. Production data designation

40

V

Example VI

The cathode current was supplied to the block lining 24 via steel collector fins such as the rod 30. The power supply is indicated by the plus and minus signs on the anode or the collector rod 30.

The space above the bath 31 was sealed from the ambient air by means of a closure 32, which is a cast iron pipe 33, a refractory Ceraform plate 34, which is a soft (for the purpose of achieving a good seal) fibrous electrical and heat-insulating tag, which is from the Johns Manville Co. is obtainable includes a steel jacket 35, a steel plate 36 and a refractory brick 37, for example 50% A1203 and 50% Si02. A part 38, which serves primarily for insulation, and a part 39, for example calcium aluminate-bonded tabular alumina, which was selected for its refractory properties, were provided within the jacket 35. The particular heat transfer situation was chosen to make the top surface 45 of the bath 31 substantially molten, i.e. free of any crust.

Alumina was charged from a container 40 through a fill valve and feed assembly 41 of the type described in U.S. Patent 3,681,229. Measured alumina quantities were fed to the exposed molten bath surface through an Inconel - 600 line 42. The distance between the bottom of the line 42 and the surface of the bath 31 was approximately 0.30 cm. The feeder 41 is a

Worked crucible days Total kg net aluminum (AI) kg net Al / crucible day 45 average% AI effectiveness of the electrical current kilowatt hour / kg AI anode effects / crucible day kg Söderberg paste kg net AI kg cryolite kg fluoride kg kg LiC03 distance from anode to cathode, mm

55 1st N.M. = not measured

32

15.827 449.6 99.74 92.6 16.6, 91, 56

812 2747 307 34.5

96

49.833 519.0 99.75

90.0 17.2

1.21 N.M. 1 1620

97.78 630

38.1

Table III. Electrical data designation

65 crucibles Average ampere kilowatt / crucible

Ohmic voltage drop in the bathroom

V

Example No. VI

5.13 5.17

66,874 72,207

343.1 373.3

1.70 1.68

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Table IV. Crucible bath data

Name Example No.

V VI

Wt% CaF2

3.11

3.17

% By weight A1203

4.09

4.00

% By weight A1F3

48.97

45.08

% By weight LiF

5.61

10,165

Wt% NaF

38.13

36.94

Wt% MgF2

.38

.28

Liquidus temperature ° C

882

906

Calculated weight ratio

.78

.82

NaF / AlF3

Calculated weight% cryolite

63.4

61.9

Calculated excess A1F3

April 23

20.5

Bath operating temperature ° C

898

922

“Eutectic” temperature0 C

799

814

Conductivity ohm "1 cm" 1

12.4

14.4

Bath depth, mm

209.8

193.6

Metal depth, mm

152.9

158.75

With respect to Table IV, the excess A1F3 indicates the amount of A1F3 above that under the cryolite, Formula 3NaF • A1F3. In each of Examples V and VI, AI203 is the first substance that crystallizes when it goes below the given liquidus temperature. In this case, the eutectic temperature calculates the liquidus temperature of the cryolite. The eutectic temperature is determined by finding the liquidus temperature for progressively decreasing A1203 content, correspondingly increasing NaF + A1F3 content and constant bath ratio NaF / AlF3, and selecting the minimum liquidus temperature based on the resulting group of liquidus temperature values. The AI 03 in solution is that at the special bath operating temperature. The conductivity data are similarly valid for the given operating temperature.

Gases evolved from the Söderberg anode (e.g., hydrocarbons), bath fluorides, and anode reaction gas (e.g., CO 2) were withdrawn from cover 32 through an opening (not shown) and passed through a burner to burn the hydrocarbons.

Since it is difficult to completely seal the bath from the air using cover 32, i. that there may be leaks in the cover 32, a pressure of 0.76 to 2.5 mm H2O from atmospheric pressure, measured negatively, is maintained between the cover 32 and the burner to prevent smoke loss from the cover 32. The burned gases are then fed to a wet cleaning system.

Example VII

Aluminum was produced in the cell shown in sectional view in FIG. 2. The cell had external dimensions of approximately 122 cm high, 226 cm long and 142 cm wide. Two pre-baked carbon anodes 10a and 10b were hung in an electrolyte bath 11 which rested on a cushion of molten aluminum 12. The molten bath and aluminum were laterally held by a refractory, non-conductive material 13. The refractory material 13 includes a side liner which is in contact with the molten bath and the molten aluminum and another externally arranged insulating material with internal structural parts made of steel, for example. Refractory alumina and silicon carbide bricks are the special 'side lining materials chosen in this example. The lining of the bottom of the cell was graphite blocks 14a to 14d, which were connected to the electrical system via steel rods 15a to 15d. Alumina became bath 11

through a suitable access (not shown) in the graphite layer 16. The special alumina used for loading had a surface area of 245 m2 / g and a total water content of 13%. The graphite layer 16 served to seal the bath from the air. The electrolyte bath 11 had a composition of 5% LiF and 4 to 5% A1203, the rest cryolite and A1F3 in such proportions that a weight ratio of NaF / AlF3 = 0.8. A1203 would be the first substance that would be excreted when bath 11 cooled. The liquidus temperature for Al203 excretion in the bath at 5% A1203 is 911.5 ° C; at 4% A1203 the liquidus temperature is 863.0 ° C. The bath operating temperature in Fig. 2 was 910 ± 10 ° C no crusting was observed in the interface between the molten aluminum cathode and the bath. The liquidus temperature, as determined by the eutectic temperature (as explained above), was at a bath ratio of 0.8 815 ° C.

Example VIII

5, a graphite crucible 51 is shown, which has a non-conductive, refractory lining 52 with a hole 53 at its lower end. A molten aluminum-metal pad 54 rests on the bottom of the alumina crucible and contacts the graphite crucible 51 so that it is in electrical contact with the cathode lead 55. On the pad 54 there is an electrolyte bath 56 with 4% by weight A1203 and NaF + A1F3 with a bath weight ratio of NaF / AlF3 = 0.8. The electrolyte bath has a temperature of 900 ° C. A carbon anode 57 (pre-baked, petroleum coke) is immersed in the electrolyte bath in order to create an electrical current density of 0.733 amperes per cm 2 at the anode. An alumina anode jacket 58 surrounds the anode 57, as shown, and is sealed at its upper end by means of a plug 59 which is provided with openings for the passage of an anode line 60 and a gas flow line 61. A suitable pipeline is provided so that different amounts of argon gas can flow out of the container 62 through the baffle bottle 63, which contains water 64, which is surrounded by an ice water bath 65. Thus, the argon gas supplied into the space between the anode 57 and the jacket 58 contained water vapor, which is taken up by the argon from the water in the bottle 63. In the operation of the cell to produce aluminum, the carbon consumption was 149.5 to 172.1 g per 453 g of aluminum produced with a current efficiency of 100% in 29 to 41 ampere-hours, using water vapor shielding to prevent anode sputtering has been. With 4 to 22 torr water partial pressure in the argon, no carbon foam or slurry was discovered. When the baffle bottle 63 was bridged so that only argon moved down around the anode 57, a carbon foam was formed on the surface of the bath 56 and an electronic wire entered through the carbon foam.

Example IX

In a cell for the electrolytic reduction of alumina to aluminum metal, the external dimensions were approximately 122 cm high, 226 cm long and 142 cm wide. Aluminum was produced by electrolysis of a molten bath, which contains 5% by weight lithium fluoride, 2 to 5% by weight A1203, the rest NaF and A1F3 at a weight ratio of NaF / AlF3 = 0.8 and a temperature between 850 and has 950 ° C. The cell is shown in longitudinal section in FIG. 2. Two pre-baked carbon anodes 10a and 10b were hung in the electrolyte bath 11, which rests on a cushion of molten aluminum 12. The molten bath and the aluminum were removed from the side

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40

45

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a refractory, non-conductive material 13 held. The refractory material 13 includes a side liner in contact with the molten bath and the molten aluminum and other exterior insulating material with internal structural members made of steel, for example. Refractory alumina and silicon carbide bricks were the special side lining materials chosen in this example. The lining of the bottom of the cell was graphite blocks 14a to 14d, which were connected to the electrical system via steel rods 15a to 15d. Alumina was supplied to the bath 11 through a suitable access (not shown) in a graphite cover 16. The graphite cover 16 was used to seal the bath from the air. The clays of Table V were used as the addition material.

Table V. Alumina properties

Alumina specific

No. Surface m2 / g

200-245 108-170 28

Total water

17-18 17-18 2-3

The dissolution rates of alumina 1 and 2 were sufficient to ensure cell operation without deposition. In contrast, the dissolution rate of the alumina 3 was noticeably slower with the prevailing agglomeration and persistent deposition.

Example X

Alumina with a total water content of 14%, a spec. A surface area of 200 m2 / g and a particle size of 100 microns (average diameter) was added essentially continuously as alumina addition at intervals of 1 to 2 minutes to a Hall Heroult cell, which was operated with a cryolite-based electrolyte, so that a Alumina content was maintained at approximately 5% by weight and the alumina was quickly dissolved when added to the bath. The current density of the cell was approximately 0.38 amperes / cm2. The NaF / AlF3 weight ratio of the bath was approximately 0.8, the liquidus temperature approximately 725 ° C and the bath temperature approximately 850 ° C. The rate of dissolution of the alumina exceeded the rate of alumina consumption by reduction to aluminum during continuous cell operation with a current efficiency greater than 90%, for 1 hour. The rate of alumina consumption was substantially equal to the rate of alumina addition so that the alumina in the bath was not exhausted. The aluminum produced was collected. The bath composition was approximately 63 wt% Na3AlF6.32 wt% A1F3 and 5 wt% A1203. The carbon consumption approached the theoretical value of 0.33 kg carbon per kg aluminum produced.

Example XI

This example explains the prior art. Various Hall-type electrolyte cells or so-called “crucibles” for the production of aluminum from A1203 were operated at approximately 980 ° C., bath conditions from 1.25: 1 to 1.45: 1 and AT from 15 to 20 ° C. . An increase in AT per 1 ° C reduced the current efficiency by 0.22%.

25th

Example XII

Test experiments were carried out, with a crucible being heated from the outside by means of a resistance furnace. Table VI shows the bath ratio for the cryolite-.1 »bath, percentages by weight A1203 and LiF in the bath and operating conditions during the production of aluminum from alumina, the minimum temperature being listed, at which no crusting was discernible over the metal cushion. The calculated liquidus temperature is also listed. 35 The presence of crusting was checked using a graphite probe, which is preferred to a steel probe, to prevent iron contamination. An AT of at least 100 ° C is present in each of the various attempts listed.

Table VI Data for Example XII bath ratio% A1203% LiF

0.7

0.7

0.7

0.565

0.565

5 5 5 5

10th

Current density amp./cm2

0.78 0.31 0.78 0.78 0.78

Minimum temperature for no crusting on the metal cushion (° C)

826 822 856 866

827

Calculated liquidus temperature (° C)

706 706 706 <700 <700

3 sheets of drawings

Claims (13)

615 700 PATENT CLAIMS 1. A method of operating a cell for the electrolytic decomposition of alumina to aluminum metal, the alumina being dissolved in a cryolite bath and the cell having a carbon anode, characterized in that an atmosphere is created at the anode which contains water in quantities which are effective to prevent sputtering of the anode. 2. The method according to claim 1, characterized in that a protective jacket is arranged around the anode, the atmosphere being created within the protective jacket. 3. The method according to claim 1, characterized in that the amount of water is at least sufficient to stoichiometric NaAlF4 according to the equation NaAlF4 + | H20 NaF + j Al203 + 3HF to react. 4. The method according to claim 1, characterized in that the upper part of the cell is closed. 5. The method according to claim 4, characterized in that alumina is used with a water content which is sufficient to be able to supply said amount of water. 6. The method according to claim 5, characterized in that an alumina is added substantially continuously to the molten bath surface, which has a total water content of 3-20 wt .-% and a specific surface area of at least 45 m2 / g, the alumina to makes up 100% of the A1203 added to the cell. 7. The method according to claim 5, characterized in that the alumina is added in portions at intervals of up to 10 minutes. 8. The method according to claim 5, characterized in that the alumina has a total water content of 8-20 wt .-%. 9. The method according to claim 5, characterized in that the alumina has a total water content of 10-18 wt .-%. 10. The method according to claim 5, characterized in that the water-containing alumina accounts for at least 50% by weight of the total amount of A1203 added to the cell. 11. The method according to claim 5, characterized in that the water-containing alumina makes up at least 90% of the A1203 added to the cell. 12. The method according to claim 5, characterized in that the alumina has a water content that is effective to disperse its individual particles after addition to the bath, but is insufficient to cause deposition. 13. The method according to claim 5, characterized in that the alumina has a specific surface area of 135 to 180 m2 / g. In conventional processes in the electrolytic aluminum production, exposed carbon is lost through combustion in the air. This limits the use of larger, pre-baked anodes in order to reduce laboratory costs because this advantage is eliminated by increased combustion in the air. Pre-baked carbon anodes must also be trimmed, i.e. molten cast iron is poured into a recess in the tip of the anode to form an electrical connection for receiving an anode rod to supply current to the cell. Trim is an expensive process. Used carbon anode dies are recirculated by breaking them and using the broken material as part of a mixture of carbon and binder that is melted and baked to form additional anodes. This re-in-5 recycle increases costs and continues to cause fluoride attack on the refractory components in the ovens used in the baking process. In some cases, self-baking carbon anodes (e.g. the so-called Soderberg electrodes) are used, but air burning is similarly a problem with such anodes. Furthermore, both pre-baked anodes and self-baking anodes are attacked by fluoride gas evolving from the electrolytic bath so that carbon dust is removed from the anodes, causing anode costs and other problems described below. A variety of factors help limit the life of a conventional Hall-Heroult cell. Sodium storage and the formation of sodium aluminum oxide cause swelling and cleavage of the cell lining with the consequent disturbance of the operating characteristics of the cell and shorten the service life, so that there is the need to periodically re-line the cell. Exhaust control is similarly a problem with the conventional Hall Heroult cell. An air flow that dilutes crucible gases up to 100 times is inherent in previous efforts to effectively extract the exhaust gas trap. The required deduction is expensive and requires considerable maintenance because individual sections are easily deformed. Fluoride, which is briefly fed m, must be added continuously in order to maintain the appropriate bath composition. With all precautionary measures, exhaust gas that is not caught by the exhaust vent escapes through roof openings. Process control is another problem area with the i5 conventional Hall Heroult cell. Adding alumina using crust insulation interferes with alumina control. Unnecessary anode effects reduce production. As a treatment for fluoride, A1F3 must be added in order to maintain the NaF / AlF3 ratio, which is necessary in order to achieve a high current efficiency. Ongoing additional anode-cathode spacing adjustment is required due to anode change and alumina feed, which requires the crust to break and disturb the heat balance. With respect to the cathodes of conventional Hall-Heroult cells, the carbon lining of the cell that forms part of the cathode assembly has a greater voltage drop than is desired, and as the cell ages and the bath penetrates the carbon lining, the voltage drop 50 increases the interface between the collector lamella and the lining continues. Magnetic influences result in decomposition of the carbon lining, reduce the aluminum production and shorten the life of the cell. The efficiency of the conventional Hall-55 Heroult cell method is also somewhat low. Conventional Hall-Heroult type aluminum melt cells use a molten aluminum metal pad as a cathode and, resting on the molten pad, in the ft (, essentially a cryolite (Na3AlF6) electrolyte bath to which aluminum fluoride is added to adjust the weight ratio of NaF to A1F3 (the Bath ratio) to a range greater than 1.1: 1 and up to 1.3: 1, thereby improving power utilization at operating temperatures of approximately f, 5 970 ° C. However, attempts have been made to work with progressively lower bath conditions , thwarted by the formation of a crust of frozen electrolyte over the molten aluminum pad cathode when the electro4 » 45 3rd 615 700 lysis progresses. This crust causes deposition of sodium, so that the current efficiency is deteriorated, the resistance at the cathode is drastically increased and the metal coalescence is reduced to the point at which a cell can no longer be operated.