WO1993014248A1

WO1993014248A1 - Trickle alumina feeder

Info

Publication number: WO1993014248A1
Application number: PCT/AU1993/000010
Authority: WO
Inventors: Barry John Welch; David John Stretch
Original assignee: Comalco Aluminium Limited
Priority date: 1992-01-10
Filing date: 1993-01-08
Publication date: 1993-07-22
Also published as: IS3963A; AU3338293A

Abstract

A feeding apparatus allowing alumina to be trickle fed to an aluminium reduction cell comprises a container (10) for holding alumina. The container includes a discharge means having a first flow passage and a second flow passage. In use, continuous gravitational discharge of alumina through the first flow passage occurs when alumina is present in the container. When a higher flow rate of alumina to the aluminium reduction cell is required, extra alumina is fed to the container and this causes alumina to also flow through the second flow passage. In one embodiment, the first flow passage comprises an outlet (22) in the container and the second flow passage comprises a further outlet connected to a pipe (26) extending inwardly into the container such that alumina will flow throuh the second flow passage only when the level of alumina in the container is above the upper level (28) of the pipe (26). In an alternative embodiment, flow restricting outlets are placed in the outlet chute or conduit. The apparatus is suitable for retro-fitting to point feeders to convert them to continous feeders.

Description

TITLE: TRICKLE ALUMINA FEEDER Background of the Invention

This invention relates to an apparatus and a method enabling the substantially continuous feeding of alumina to an electrolytic cell for the production of aluminium by the Hall-Heroult process. In this process, aluminium is produced by electrolytic reduction of alumina (A1₂0₃) dissolved in a bath of electrolyte contained in the cell which electrolyte is based on molten cryolite. The metal is formed at the molten aluminium cathode and oxygen is discharged at the carbon anode but reacts with and consumes it to form carbon oxides.

An important factor in the efficient production o_f aluminium by this process is the method whereby the alumina is introduced into the bath. A method using traditional technology involves breaking the frozen crust alumina matrix which covers the surface of the molten electrolyte in the cell in normal operation to form an opening and then dumping a volu etrically measured amount of alumina into the opening. This dumping is done either at periodic intervals or on receipt of a demand feed signal. By way of example, one approach is to break all the crust along half the centre channel width of the cell and dump the volumetric equivalent of approximately 65 kg of alumina into the cell every 30-100 minutes or on a demand feed basis. This causes a major thermal disturbance as the alumina concentration of the electrolyte increases an_d subsequently decreases as it is electrolytically removed.

Recent experiments with the composition of the electrolyte and the operation of electrolytic cells have indicated that the crust added during the breaking action contributes to operating problems as does the mass of frozen electrolyte alumina matrix that is generated _by the thermal shock of a dumping. Consequently, smaller alumina additions have become favoured to reduce the sludging of the alumina and thus increase the efficiency of the system. This has led to the development of point feeding systems where a smaller hole is broken through the crust and the mass of alumina added during each feeding action varies between 0.5 and 3 kg, but the feeding frequency is increased to every 1 to 5 minutes depending on the feed strategy used. Because of the more frequent feeding of smaller quantities of alumina, more than one hole is formed in the crust by pneumatically or mechanically driven pickles (crust breakers) . Examples of point or s'pot feeding systems are given in United States Patents 3,400,062, 3,689,229 and 4,437,964.

While the point feeding systems have been a big improvement on the larger dump feeding systems, the discontinuous addition of discrete quantities of alumina to the cell still causes inefficient alumina dissolution, a limited amount of sludge formation, and fluctuations in the concentration of alumina in the electrolyte. Modern understanding indicates the desirability of preventing any sludge build up and minimising concentration fluctuations, especially where it is desired to operate the cell with an electrolyte near its freezing point or near its alumina saturation point. It was with this in mind that the apparatus and method of the present invention were devised. In particular, it is an object of the present invention to provide an apparatus and a method allowing for substantially continuous trickle feeding of alumina to an electrolytic cell.

A problem that can arise in an electrolytic cell used for the production of aluminium is the "anode effect". This occurs when the alumina concentration in the electrolyte becomes so low that the electrolyte itself starts breaking down resulting in a gas ilm forming about the anodes which considerably increases the electrical resistance. The voltage at which the electrolytic cell operates usually ranges between about 4 to 5 volts. When the anode effect occurs the voltage rises rapidly to about 100 volts for example, meaning a large increase in the power consumption and in the cost of running the electrolytic cell. To control this effect when a point or spot feeder supplies alumina to the cell, its frequency of periodic feeding is increased to produce an additional surge of alumina fed to the cell until the anode effect is suppressed. Another solution sometimes used is to move the anodes closer to the cathode and then to return the anodes to their usual positions when the alumina concentration in the cell has been restored to the desired level or within the desired range of concentrations. Therefore, it is also an object of the preferred forms of the present invention to provide means for suppressing the anode effect if it should occur in the electrolytic cell.

Summary of the Invention

In one aspect the present invention broadly consists in an apparatus allowing alumina to be trickle fed into a cell for the production of aluminium by electrolysis of alumina dissolved in molten electrolyte, comprising: a container for receiving alumina located or locatable over the cell; discharge means which allows discharge of alumina from the container, said discharge means including a first flow passage which allows continuous or substantially continuous gravitational discharge of alumina from the container, said discharge means further including a second flow passage which is arranged to allow continuous gravitational discharge of alumina from the container when the alumina in the container is present in an amount that exceeds a predetermined level, with alumina being discharged through the first flow passage at the same time; and means for conveying alumina discharged from the container to a desired location within the cell.

Preferably the container is arranged to receive alumina from a point alumina feeder ^" which periodically discharges discrete quantities of alumina, the frequency of the discharge being capable of adjustment between a lower value where the quantity of alumina delivered to the container is insufficient to cause discharge from the second flow passage but causes a continuous or a substantially continuous trickle discharge from the first flow passage, and a higher value where the quantity of alumina delivered to the container is sufficient to cause discharge from both the first flow passage and the second flow passage.

In one embodiment, the discharge means may comprise an outlet from the container, the outlet being connected to conduit means for conveying alumina discharged from the container to a desired location within the cell, said outlet and/or conduit means including flow restricting means.

Preferably, the flow restricting means includes at least one insert positioned within the conduit means to partially close the conduit means, said insert having a first opening to define the first flow passage and a second opening to define the seπond flow passage.

The first opening may comprise an orifice located in a lower part of the insert, with the second opening being located above the first opening. The second opening may be larger than the first opening.

In an alternative embodiment, the flow restricting means may comprise a partial closure positioned in the conduit, said partial closure having a slot in a lower part thereof to define the first flow passage, said slot enlarging at its upper end into a larger opening, said larger opening defining the second flow passage.

In yet another embodiment the flow restricting means may comprise a wall positioned in the outlet conduit to act as a weir to partially block the conduit, said wall including a gap to allow alumina to flow therethrough, said gap defining the first flow passage, and the second flow passage being defined between the upper edge of the wall and the upper part of the conduit.

The size of the gap may be adjustable to alter the size of the first flow passage.

In a further embodiment, the discharge means comprises an outlet from the container, a discharge chute operatively associated with said outlet, said discharge chute including weir means comprising a wall positioned across said chute, a gap positioned in said wall or positioned between said wall and a wall of said chute, said gap defining the first flow passage, wherein in use alumina flows through the outlet into the discharge chute and backs up against the weir means whilst a portion of the alumina flows through said gap, and when alumina is present in the container above the predetermined level, alumina overflows the weir means and allows continuous gravitational discharge of alumina through the gap and over the weir means, said second flow passage being formed by the space above the weir means.

Again, the size and height of the gap may be adjustable.

In an especially preferred embodiment of the present invention the discharge means comprises a first outlet from the container which allows continuous or substantially continuous gravitational discharge of alumina from the container when the alumina in the container is above the first outlet and a second outlet from the container which is arranged to allow continuous gravitational discharge of alumina from the container when the alumina in the container is present in an amount that exceeds a predetermined level, with alumina being discharged through the first outlet at the same time.

In one embodiment, the second outlet is, in use, located above the first outlet and allows continuous gravitational discharge of alumina from the container when the alumina in the container is above the second outlet.

In another embodiment, the second outlet includes an outlet pipe extending from the second outlet inwardly into the container, said outlet pipe having an open end located within said container wherein continuous gravitational discharge of alumina from the second outlet occurs when alumina in the container is above the open end of the outlet pipe. Preferably, the means for conveying alumina discharged through the outlets to a desired location within the cell comprises conduit means.

Preferably, the conduit means comprises a single feed pipe sloping away from the container and having an upper end connected to the container.

Preferably, the feed pipe is connected . to the container at or adjacent its lowermost point.

Preferably, the feed pipe is detachable from the container.

Preferably, the feed pipe has a closure at or towards its upper end with the first and second outlets passing through the closure which otherwise blocks the pipe.

Preferably, the first outlet comprises a hole through a lower portion of the closure.

Preferably, the second outlet comprises a hole through an upper portion of the closure.

Preferably, the size of the second outlet hole is greater than that of the first outlet hole. Preferably, the second outlet also comprises an outlet pipe, an end of which is connected to the second outlet hole in the closure and the other open end of which projects upwardly into the container.

Preferably the closure comprises a disc. In a second aspect, the present invention broadly consists in a method of feeding alumina into a cell for the production of aluminium by electrolysis of alumina dissolved in molten electrolyte, and of maintaining the concentration of alumina in the electrolyte substantially at a predetermined level or substantially within a predetermined range of concentrations during the production process, the method comprising the steps of: supplying alumina to a point alumina feeder which periodically discharges discrete quantities of alumina and which has an adjustable frequency of discharge; receiving the periodic discharges of alumina in a trickle feeder which converts the periodic discharges into a continuous or a substantially continuous flow of alumina; delivering the continuous or substantially continuous flow of alumina to the cell; obtaining a measure of the concentration of the alumina dissolved in the molten electrolyte in the cell; comparing the measured concentration obtained with said predetermined level or range; and controlling the frequency of discharge of the point alumina feeder so that its rate of discharge of alumina to the trickle feeder and hence the rate of discharge from the trickle feeder to the cell maintains the alumina concentration in the electrolyte substantially at the predetermined level or range.

Preferably, the method also enables the anode effect to be countered if it should occur in the cell, the method then comprising increasing the frequency of discharge of the point alumina feeder to a level such that an overflow occurs in the trickle feeder which produces a surge in the quantity of alumina delivered to the tank until the anode effect is suppressed.

Preferably, the method uses as the trickle feeder the apparatus as defined above.

Preferably, measures of the concentration of alumina in the electrolyte are obtained substantially continuously and are substantially continuously compared with the predetermined value or range. Preferably, the concentration of alumina in the electrolyte is determined from measurements of the electrical resistance of the electrolyte but other techniques can be used.

At least preferred embodiments of the invention provide:

1. An apparatus and a method whereby the rate of addition of alumina to an electrolytic cell can be substantially controlled to any set value between prescribed limits. 2. An apparatus and a method to direct the alumina to a hole maintained open in the crust in a manner that reduces the chance of closure of that hole. 3. An apparatus that reduces the amount of crust broken into the electrolytic cell as a consequence of the continuous or substantially continuous alumina feeding method employed. 4. An apparatus and a method whereby the anode effect can be substantially controlled and that allows rapid response to the onset of the anode effect. 5. An apparatus that gives efficient utilisation of the one or more alumina storage hoppers above the cell. It is a feature of the invention that it can be applied to conventional storage hoppers used in modern centre-worked or point fed cells. The apparatus is suitable for retrofitting to current cells to convert them from point-fed cells to substantially continuously fed cells.

This invention may also be said broadly to consist in the parts, elements and features referred to or indicated in the specification of the application, individually or collectively, and any or all combinations of any two or more of said parts, elements or features, and where specific integers are mentioned herein which have known equivalents in the art to which this invention relates, such known equivalents are deemed to be incorporated herein as if individually set forth. The invention consists in the foregoing and also envisages constructions of which the following give examples.

Brief Definition of the Drawings

FIGURE 1 shows a front end view of a preferred trickle feeder;

FIGURE 2 shows a section on A-A of FIGURE 1; FIGURE 3 shows an upper end view of the preferred closure disc having the first and second outlets .for discharge of alumina from the trickle feeder; and

FIGURE 4 shows, on a smaller scale, a view similar to that of FIGURE 2 illustrating the operation of the trickle feeder.

FIGURES 5a and 5b show alternative embodiments of flow restricting inserts suitable for use in the present invention; FIGURES 6, 6A, 7 and 7A show schematic views of an alternative trickle feeder according to the present invention utilizing inserts similar to those illustrated in Figure 5; FIGURE 8 shows an alternative construction to the apparatus of FIGURE 6;

FIGURE 9 is an end view of a weir means for use in the present invention; and FIGURE 10 shows an embodiment of a trickle feeder incorporating a weir in the outlet from the container.

Description of Preferred Embodiments

Referring to the drawings, the preferred trickle feeder of the present invention has a container in the form of a bin 10 to receive alumina from a source of alumina. It is envisaged that the usual source of alumina will be a point feeder (also known as a spot feeder) which, for example, may be of the type disclosed in United States

Patent No. 4,437,964. The point feeder will usually be installed in the bottom of an alumina hopper. Delivery of alumina directly from a point feeder to the electrolytic cell (not shown) means the alumina is fed to the cell in discrete quantities at periodic intervals. Usually the point feeder allows the frequency of its discharge to be adjusted according to the feed demands of the cell. As previously indicated, the point of this is to try and maintain the concentration of alumina dissolved in the electrolyte in the cell at about a predetermined value or substantially within a predetermined range of concentrations. This object can be more readily achieved, especially where a low concentration of alumina in the electrolyte is desired, by having the alumina fed to the cell continuously or at least substantially continuously. A purpose of the trickle feeder of the present invention is to convert the periodic discharges received from the point feeder to a continuous or substantially continuous flow of alumina for delivery to the cell.

The bin 10 may have an open top 12 or an opening in the top to receive alumina from the point feeder. Alternatively, the alumina from the point feeder can be delivered to the tank through its back 14. The bottom 16 of the bin preferably slopes so that the alumina in the bin tends to flow to a lowermost point in the bin.

The apparatus includes a feed pipe 18 for conveying alumina from the bin to a desired location within the cell. The feed pipe is arranged to slope away from the bin at an angle equal to or greater than the angle of flow of the alumina and has its upper end connected to the bin at or adjacent its lowermost point. It is preferred that the connection of the upper end of the feed pipe to the bin be releasable so that the feed pipe is detachable from the bin.

Within the feed pipe, at or towards its upper end, is a closure in the form of a disc 20. This closure has two outlets in it for the passage of alumina therethrough. The first outlet comprises a hole 22 through a lower portion of the disc 20. The second outlet also comprises a hole 24 through the disc, preferably in an upper portion of the disc, and an outlet pipe 26 which has one end connected to the second outlet hole 24 and the other end projecting upwardly into the bin as shown in FIGURES 2 and 4. The outlet pipe is open at its upper end 28.

As long as the level of the alumina in the bin in the vicinity of the top end 28 of the outlet pipe 26 is below that top end then the first outlet hole 22 provides the only outlet for discharge of alumina from the bin. This is the situation shown in FIGURE 4 where the alumina 30 in the bin has a level in the vicinity of the top end 28 of the outlet pipe 26 which is close to but below the level of that top end. However, whenever the level of alumina in the bin in the vicinity of that top end goes over the top end then alumina discharges through the outlet pipe 26 and second outlet hole 24 as well as from the first outlet hole 22, the two streams of alumina then being delivered together to the cell through the common feed pipe 18.

The feed pipe 18 directs the alumina which passes through the bottom aperture 14 to the hole maintained through the crust which covers the surface of the molten electrolyte in the electrolytic cell in normal operation. The electrolytic cell and the crust breaking means are not shown in the drawings and are not further described as they may be any suitable types known to those skilled in the art. It is sufficient to say that the crust breaking means, having formed the hole of the crust is operated to maintain that hole so that alumina can be continuously or substantially continuously fed to the molten electrolyte beneath the crust. It is highly desirable that the hole be maintained open whenever continuous or substantially continuous feeding of alumina to the electrolytic cell is taking place.

The dimensions chosen for the parts of the trickle feeder and the size and frequency of the quantities of alumina delivered to the bin from the point feeder are chosen so that in normal use alumina is discharged from the bin 10 through only the first outlet hole 22 in a continuous or substantially continuous flow. Thus, the periodic dumpings of alumina into the bin from the point feeder are smoothed out into a continuous or a substantially continuous flow for delivery to the cell which assists in maintaining the desired concentration of alumina in the electrolyte in the cell. Ideally, the quantity of alumina fed to the bin in a single dumping from the point feeder equals the quantity of alumina discharged through the first outlet hole 22 in the period between two dumpings and the flow through the first outlet hole 22 is substantially continuous, and preferably substantially constant, throughout that period.

If during operation of the tank the anode effect should be experienced then the point feeder is made to operate faster so that the frequency at which alumina is delivered from it is increased sufficiently to cause the alumina in the bin to overflow into the outlet pipe 26. This causes a sudden surge in the alumina fed through the feed pipe 18 to the cell which is maintained until the anode effect is suppressed. By way of example, if the first outlet hole 22 is sized (for example, having a diameter of 13.5 mm) to discharge alumina at the rate of 1 kilogram per minute and the point feeder delivers 2.5 kilograms of alumina to the bin in each dumping, then the point feeder must deliver a dumping of alumina every 2.5 minutes to prevent the bin from emptying or filling to cause overflow into the outlet pipe 26. Of course, delivery of alumina from the point feeder at a lower frequency of operation will still allow continuous flow of alumina through the first outlet hole 22 though not for the full period between dumpings of alumina from the point feeder. This is what is meant in the specification when reference is made to "substantially continuous flow".

Again, by way of example, the second outlet hole 24 and outlet pipe 26 can be sized (for example, having a diameter of 25 mm) so as to discharge alumina at the maximum rate of 5.8 kilograms per minute. When operating at this level then the total discharge of alumina through the feed pipe is 6.8 kilograms per minute which means that the rate of discharge from the point feeder can be about 2.72 dumps per minute, that meaning a period of about 22 seconds between dumps.

The operation of the point feeder is preferably controlled by sensing means known in the art which senses the concentration of alumina in the electrolyte and operates the point feeder to maintain the concentration substantially at the predetermined level or substantially within the predetermined range of concentrations during the period of production of aluminium by electrolysis in the cell. Because the electrical resistivity of the electrolyte varies according to the concentration of alumina therein, measurements of the electrical resistance of the electrolyte can. be used to generate signals to electrical control means controlling operation of the point feeder. This measurement can be performed continuously and continuously compared with the predetermined value or range so that while the point feeder and trickle feeder together will normally provide continuous or substantially continuous feeding of alumina to the cell during its operation to replace the alumina being consumed by the process, the rate of replacement can vary according to any variation in the rate of consumption. However, it will be appreciated that the first outlet hole 22 can cope with discharge of alumina from a zero flow rate up to a certain maximum flow rate only, for example 2.5 kilograms per minute in the example given above. After that, a greater rate of delivery from the point feeder will lead to an overflow through the outlet pipe 26. Normally such a greater rate of delivery from the point feeder would occur only when the anode effect was experienced in the cell. It is of course possible to determine the onset and duration of the anode effect from a measurement of the voltage at which the cell is operating or of the power consumption of the cell.

The above describes a preferred form of the present invention but it will be realised by those skilled in the art to which the invention relates that various modifications can be made without departing from the scope of the invention as has been broadly defined. The components can be made to any suitable sizes and shapes and be made of any suitable materials of construction. It is not necessary to have the bin discharge from its lowermost point though that is preferred so as to enable the bin to be substantially emptied of alumina through the first outlet. The first and second outlets need not be located within a feed pipe 18 but could be located directly in a wall or sloping floor of the bin. It is possible to have the alumina discharged through each outlet separately conveyed to the cell though it is convenient to use a single feed pipe 18 for this purpose. The feed pipe 18 need not be as shown in Figure 2 but may be of any suitable design that will direct the flowing alumina powder to the opening in the crust. The use of an outlet pipe 26 means that the open end 28 is spaced some distance above the first outlet hole 22. This allows for some fluctuation in the level of alumina in the bin during normal operation of the tank. If the second outlet hole 24 can be spaced sufficiently above the first outlet hole 22 to provide for this fluctuation then it is not necessary to use an outlet pipe 26.

FIGURES 5 to 9 show alternative embodiments of the present invention. FIGURES 5a and 5b are schematic diagrams of flow restricting inserts that are suitable for placement in the outlet and/or the conduit associated with the alumina container. Referring to FIGURE 5a the flow restricting insert 50 comprises solid portion 52 shaped to fit into the conduit. The insert includes slot 54 having walls 56 and 58. At the upper end of slot 54 the slot enlarges outwardly to define a larger opening 60. Opening 60 has walls 62, 64, 66 and 68. Slot 54 defines the first flow passage whilst opening 60 defines the second flow passage.

Referring now to FIGURE 5b, an alternative pipe insert 70 comprises solid section 72 shaped to fit into the conduit. The insert 70 includes orifice 74, which defines the first flow passage and opening 76 which defines the second flow passage.

The use of pipe inserts 50 and 70 is illustrated in FIGURES 6 and 7. As shown in FIGURE 6, container 80 has an outlet 82. A conduit 84 is attached to the outlet 82.

Conduit 84 includes pipe insert 86. The pipe insert 86 may comprise insert 50 as shown in FIGURE 5a or insert 70 as shown in FIGURE 5b. In either case, insert 86 includes a first flow passage located at a lower part thereof. When alumina is added to container 80, the alumina exits the container through outlet 82 and flows down the conduit until it strikes flow restricting insert 86. Alumina 88 is thereby caused to back up against flow restricting insert 86, as is shown schematically in FIGURE 6. During normal operation of the container, the level of alumina in the container is such that the alumina backs up against flow restricting insert 86 to a level such that the alumina will flow only through the first flow passage (which may be slot 54 of FIGURE 5a or orifice 74 of FIGURE 5b). However, when a higher flow rate of alumina to the cell is required, such as is the case when the cell is experiencing an anode effect, the rate of addition of alumina to the container can be increased which causes the alumina to back up to a much higher level against flow restricting 88. This is schematically shown in FIGURE 7. As the alumina is now backed up against a much larger proportion of the flow restricting insert 86, the alumina will flow through both the first flow passage and the second flow passage of the flow restricting insert. This then allows a much greater flow rate of alumina to be fed to the cell. It will be appreciated that the invention as shown in FIGURES 6 and 7 may be subjected to many variations. For example, the flow restricting insert 86 may be positioned in the outlet of container 80. This is schematically shown in FIGURE 8, where the dashed lines across the outlet indicate flow restricting insert 86. Furthermore, the flow restricting insert may be placed in any suitable orientation in the outlet and/or conduit. It is also possible to use two or more flow restricting inserts positioned in the outlet and/or conduit to control the flow rate of alumina from the container. This is illustrated in Figure 6A which shows two flow restricting inserts 86 and 86a. As shown in Figure 7A, three flow restricting inserts 86, 86a and 86b may be used. The use of two or more inserts allows for better control of the flow rate.

It will also be appreciated that the actual shape of the flow restricting insert may vary quite a deal, the only requirement being that the insert includes a first flow passage and a second flow passage.

In some instances, the flow restricting means of the present invention may comprise a weir means. The weir means may be positioned in the outlet of the container or in a conduit or a shoot associated with the outlet. Referring to FIGURE 9, the weir means 90 comprises walls 91,92 extending upwardly from the bottom 93 of the weir. The weir includes a gap 94 which allows alumina to flow therethrough. Gap 94 defines the first flow passage for the alumina. It will be appreciated that in use of the weir means, alumina is caused to back up against walls 91 and 92 and the alumina flows through gap 94. In situations where a high flow rate of alumina to the cell is required, the rate of addition to the container is increased and this causes the alumina backing up against the weir 90 to overflow walls 91 and 92. In this case, it will be realised that the second flow passage is defined by the space above the top edges of walls 91 and 92.

In an especially preferred embodiment of the invention that uses weir means, gap 94 is adjustable in size. This allows for further control over the flow rate of alumina in normal flow situations.

The weir means 90 may be placed in either the outlet from the container or in a conduit or shoot that is associated with the outlet. FIGURE 10 shows the situation where the weir means 90 is placed in the outlet of the container. FIGURE 10 shows container 100 having an outlet 101. Weir means 90, represented in FIGURE 10 by the dashed lines, is positioned across outlet 101. Alumina 102 backs up against the weir 90 and flows through gap 94. If the rate of addition of alumina to container 100 is increased, the level of the alumina 102 will extend above walls 91 and 92 of the weir and will flow over the top of the walls 91 and 92. It will be appreciated that alumina will flow through gap 94 at the same time.

Weir means 90 may be subject to many variations over that shown in FIGURE 9. For example, the angle of the bottom of the weir may alter over a wide range of angles. Also, the height of walls 91,92 may be varied to suit the requirements of each application. Gap 94 need not be located in the middle of the weir, and indeed in some instances gap 94 may be formed by spacing either of walls 91 or 92 from the edge of the outlet, conduit or shoot.

Use of point feeders has led to an improvement in the control that can be obtained during the production of aluminium. Use of a trickle feeder according to the present invention allows a further improvement in the control that can be achieved.

Claims

CLAIMS :

1. An apparatus allowing alumina to be trickle fed into a cell for the production of aluminium by electrolysis of alumina dissolved in molten electrolyte, comprising: a container for receiving alumina located or locatable over the cell; discharge means which allows discharge of alumina from the container, said discharge means including a first flow passage which allows continuous or substantially continuous gravitational discharge of alumina from the container, said discharge means further including a second flow passage which is arranged to allow continuous gravitational discharge of alumina from the container when the alumina in the container is present in an amount that exceeds a predetermined level, with alumina being discharged through the first flow passage at the same time; and means for conveying alumina discharged from the container to a desired location within the cell.

2. Apparatus as claimed in claim 1 wherein the container is arranged to receive alumina from a point alumina feeder which periodically discharges discrete quantities of alumina, the frequency of the discharge being capable of adjustment between a lower value where the quantity of alumina delivered to the container is insufficient to cause discharge from the second flow passage but causes a continuous or a substantially continuous trickle discharge from the first flow passage,

-and a higher value where the quantity of alumina delivered to the container is sufficient to cause discharge from both the first flow passage and the second flow passage.

3. Apparatus as claimed in claim 1 or claim 2 wherein the discharge means comprises an outlet from the container, the outlet being connected to conduit means for conveying alumina discharged from the container to a desired location within the cell, said outlet and/or conduit means including flow restricting means.

4. Apparatus as claimed in claim 3 wherein said flow restricting means includes at least one insert positioned within the conduit means to partially close the conduit means, said insert having a first opening to define the first flow passage and a second opening to define the second flow passage.

5. Apparatus as claimed in claim 4 wherein the first opening comprises an orifice located in a lower part of the insert and the second opening is located above the orifice, said second opening being larger than said first opening.

6. Apparatus as claimed in claim 4 wherein the first opening comprises a slot in a lower part of the insert, said slot enlarging at an upper part thereof into a larger opening, said larger opening defining the second flow passage.

7. Apparatus as claimed in claim 3 wherein the flow restricting means comprises a wall positioned in the outlet conduit to act as a weir to partially block the conduit, said wall including a gap to allow alumina to flow therethrough, said gap defining the first flow passage, and the second flow passage being defined between the upper edge of the wall and the upper part of the conduit.

8. Apparatus as claimed in claim 2 wherein the discharge means comprises an outlet from the container, a discharge chute operatively associated with said outlet, said discharge chute including weir means comprising a wall positioned across said chute, a gap positioned in said wall or positioned between said wall and a wall of said chute, said gap defining the first flow passage, wherein in use alumina flows through the outlet into the discharge chute and backs up against the weir means whilst a portion of the alumina flows through said gap, and when alumina is present in the container above the predetermined level, alumina overflows the weir means and allows continuous gravitational discharge of alumina through the gap and over the weir means, said second flow passage being formed by the space above the weir means.

9. Apparatus as claimed in claims 7 or 8 wherein the size of the gap is adjustable.

10. Apparatus as claimed in claims 1 or 2 wherein the discharge means comprises a first outlet from the container which allows continuous or substantially continuous gravitational discharge of alumina from the container when the alumina in the container is above the first outlet and a second outlet from the container which is arranged to allow continuous gravitational discharge of alumina from the container when the alumina in the container is present in an amount that exceeds a predetermined level, with alumina being discharged through the first outlet at the same time.

11. Apparatus as claimed in claim 10 wherein the second outlet is, in use, located above the first outlet and allows continuous gravitational discharge of alumina from the container when the alumina in the container is above the second outlet.

12. Apparatus as claimed in claim 10 wherein the second outlet includes an outlet pipe extending from the second outlet inwardly into the container, said outlet pipe having an open end located within the container wherein gravitational discharge of alumina from the second outlet occurs when alumina in the container is above the open end of the outlet pipe.

13. Apparatus as claimed in claim 10 wherein the means for conveying alumina discharged through the outlets to a desired location within the cell comprises conduit means.

14. Apparatus as claimed in claim 13 wherein the conduit means comprises a feed pipe sloping away from the container and having an upper end connected to the container.

15. Apparatus as claimed in claim 13 wherein the feed pipe is connected to the container at or adjacent its lowermost point.

16. Apparatus as claimed in claim 14 wherein the feed pipe has a closure at or towards its upper end with the first and second outlets passing through the closure which otherwise blocks the pipe.

17. Apparatus as claimed in claim 16 wherein the first outlet comprises a hole through a lower portion of the closure and the second outlet comprises a hole through an upper portion of the closure.

18. Apparatus as claimed in any one of claims 10 to 17 wherein the size of the second outlet is greater than that of the first outlet.

19. A method of feeding alumina into a cell for the production of aluminium by electrolysis of alumina dissolved in molten electrolyte, and of maintaining the concentration of alumina in the electrolyte substantially at a predetermined level or substantially within a predetermined range of concentrations during the production process, the' method comprising the steps of: supplying alumina to a point alumina feeder which periodically discharges discrete quantities of alumina and which has an adjustable frequency of discharge; receiving the periodic discharges of alumina in a trickle feeder which converts the periodic discharges into a continuous or a substantially continuous flow of alumina; delivering the continuous or substantially continuous flow of alumina to the cell; obtaining a measure of the concentration of the alumina dissolved in the molten electrolyte in the cell; comparing the measured concentration obtained with said predetermined level or range; and controlling the frequency of discharge of the point alumina feeder so that its rate of discharge of alumina to the trickle feeder and hence the rate of discharge from the trickle feeder to the cell maintains the alumina concentration in the electrolyte substantially at the predetermined level or range.

20. A method as claimed in claim 19 wherein measures of the concentration of alumina in the electrolyte are obtained substantially continuously and are substantially continuously compared with the predetermined value or range.

21. A method as claimed in claim 19 or claim 20 wherein the concentration of alumina in the electrolyte is determined from measurements of the electrical resistance of the electrolyte.

22. A method as claimed in any one of claims 19 to 21 wherein the method further comprises detecting the onset of anode affect in the cell, increasing the frequency of discharge of the point alumina feeder to a level such that a substantial increase in the flowrate of alumina from the trickle feeder occurs which produces a surge dLn the quantity of alumina delivered to the tank until the anode effect is suppressed.

23. A method as claimed in any one of claims 19 to 22 wherein the trickle feeder comprises the apparatus as claimed in any one of claims 1 to 18.