NO317229B1 - point Mater - Google Patents

Description

This invention relates to a point feeder for dosing a material in powder form. More specifically, the invention relates to a point feeder for supplying powder materials to electrolysis cells used for the production of aluminum metal.

Background

Aluminum is most often produced by splitting aluminum oxide dissolved in a bath of molten cryolite using electric current, i.e. an electrolysis process. The smelter is located in a crucible/furnace box lined with coal, and where the lining serves as the cathode. The anode is also made of coal and is fed into the smelter from above. In principle, aluminum metal is produced by dissolving AI2O3 in the salt melt and splitting so that the Al<3+->ions can migrate to the cathode where they are supplied with electrons and thus reduced to elemental metal on contact with the cathode. At the same time, the 0<2>~ ions migrate to the anode where they react with the anode mass so that the carbon in the anode is oxidized to form CO2. This reaction emits electrons to the circuit. The anode becomes m.a.o. consumed and is a sacrificial anode, while the metal is formed at the cathode which is the lining of the furnace box.

Because of. the temperature in the process is the metal that forms liquid and it will settle at the bottom of the furnace box. The salt melt, i.e. the cryolite, has a lower density and settles on top of the metal bath. The anode must stick a little way into the molten salt for it to be a closed current circuit. Above the salt melt, a solid salt crust (also known as crust) will usually form which allows a thick heat-insulating layer of aluminum oxide to be placed on top of the melt.

This form of aluminum production is known as the Bayer-Hall-Héroult process, and as long as the process is supplied with electricity, aluminum oxide and anode mass and the produced metal is drained off, it can in principle go on forever in the same furnace box. Without going into detail, it is common practice to add some auxiliaries in addition to the raw material aluminum oxide. Most common is AIF3, an acidity regulator which also helps to lower the melting point of the salt melt (also commonly referred to as the bath) and to reduce the precipitation of sodium at the cathode. Both AIF3 and AI2O3 are added to the electrolysis bath in powder form. The anode is usually rod-shaped and is lowered into the smelt from above as it is consumed.

There are two types of anode in use today, both of which are carbon electrodes. One is known as a Søderberg anode and the other as a pre-baked anode. Without going into detail, it can be mentioned that the Søderberg anode is often designed as a long and thick carbon rod that covers almost the entire surface of the bath, but a pre-baked anode is much smaller. In return, therefore, a set of pre-baked anodes is often used, which are set up at a certain distance from each other in one or more rows to cover approximately the entire surface of the bath. For both types of anodes, it is common to enclose the furnace box with an anode mantle or jacket to capture exhaust gases from the smelting.

Known technique

Due to a number of factors such as control of the chemical properties and electrical conditions in the bath (the salt melt), it is important to have control over the quantities that are added and to achieve a homogeneous distribution of the additives in the bath as quickly as possible. It is very important to achieve the best possible control over the addition process in order to optimize yield and energy consumption in the process.

It has therefore become common to use a so-called point feeder for dosing powder materials to the electrolysis bath. A point feeder is a dosing device that delivers a specific dose of powder material to a specific area (point) on the surface of the bath in a short period of time. In order for the powder to go into solution as quickly as possible, a hole must be made in the crust so that the powder immediately comes into contact with the salt melt below. A lance, skewer or similar is usually used to pierce the crust just before the powder is dropped.

Several types of dosing devices are known which are used for dosing materials in powder form. NO 300602 shows a pneumatic dosing device for dosing aluminum oxide and aluminum fluoride into an electrolysis cell for the production of aluminium. The dosing device consists of a container which, in addition to an inlet and an outlet, includes fluidizing bodies. The fluidizing bodies are arranged so that air can be supplied to the material in the container periodically. Because the material in the container has fluidizable properties, the material will behave as a homogeneous fluid when air is supplied, and the dosing of the material through the outlet of the dosing device can thus be regulated. This dosing device is controlled by a PLC-controlled solenoid valve and can dispense doses of varying sizes.

Furthermore, reference should be made to publication NO 162774 where a further pneumatic dosing device is shown, where fluidization units are used to transfer powder material such as aluminum oxide and aluminum fluoride from a storage silo to a container. When the container is full, a further fluidization unit is activated which ensures that the material is fluidized and can flow out through an opening in the container, so that the container is emptied. This solution provides a relatively accurate dosage, but is a relatively complicated one and thus becomes a relatively expensive powder feeder which also requires a relatively high amount of maintenance.

GB 2 121 438 discloses a point feeder which is placed in the bottom of a silo. The feeder is filled by gravity. The feeder consists of a piece of pipe which is open at both ends, into which a rod with two conical discs is threaded which can close or upper and lower opening to the pipe. When the rod is moved to the lower position, the lower opening closes at the same time as the upper opening opens. Thus, oxide powder flows from the silo into the feeder. This solution builds too much to be suitable for and implemented on existing furnace systems, and it suffers from poor dosing control because both the upper and lower openings are open when the rod is moved between upper and lower positions.

A very important point in order to achieve the fastest possible homogenization of the additives for the bath is that these should be added in the areas of the bath where the flow in the bath is greatest so that the additives can be dissolved and distributed in the bath as quickly as possible. Because of. heat differences, electric fields, etc. sometimes strong currents occur in the liquid bath. However, the flow pattern in the bathroom is strongly dependent on conditions such as the physical design of the electrolysis furnace and the placement of anodes, so that the location of the point feeder(s) must be determined for each individual type of furnace and the operating parameters used.

A consequence of this need is that point feeders should be given a physical design that makes it easy to place them almost anywhere on the anode mantle of the electrolysis furnace. It should also be possible to place the point feeder(s) on the outside of the anode jacket. At the same time, the point feeder should be mechanically as simple as possible in order to reduce maintenance and installation costs to a minimum without compromising dosing accuracy and operational stability. None of the point feeders mentioned above satisfy these requirements.

The objective of the invention

It is therefore an objective of this invention to provide a point feeder which solves the problems mentioned above.

It is also a goal to provide a point feeder that is particularly well suited for dosing powdery materials into electrolysis baths in the production of aluminum metal.

Brief description of the invention

The objectives of the invention can be achieved by a point feeder as described in the following description with appended patent claims.

The point feeder according to the invention comprises a storage container which is designed with an inlet for filling a material in powder form in the storage container, an outlet for guiding the powder material out of the storage container and into a volumetric dosing container. By using at least one fluidizing element at the outlet of the storage container, it is ensured that the dosage container is completely filled so that each dosage is approximately the same size. At the bottom of the dosing container, there is an outlet that can be closed and opened as needed using a mechanical closing/opening device. The outlet of the dosing container is in communication with the opening in the melt's crust so that when the outlet is opened, the powder material is directed straight down into the feed hole. The feed hole is achieved using a conventional lance which is inserted through the crust and withdrawn just before the powder is released from the dosing container.

It is preferred that the storage container be equipped with one or more fluidizing elements which are activated during filling of the storage container and then at regular intervals to ensure a leveling of the amount of powder inside the container. To ensure that the powder in the storage container can be easily transported towards the outlet and further down into the dosing container, the bottom of the storage container is slightly sloping down towards the outlet. The top of the storage container preferably has one or more openings for refilling powder material. Furthermore, it is preferred that the dosing container has an outlet at the top so that air can escape during filling of the dosing container. This outlet can advantageously be in communication with the top of the storage container to take care of any powder material that comes with the air. It is preferred that the powder is led down into the smelter via a pipe from the outlet of the dosing container. In this way, great flexibility is achieved in choosing the location of the point feeder(s), in that the only thing that must physically be placed just above the bath and possibly go through the anode jacket is the supply pipe and the lance, and these can easily be dimensioned so that they have a small cross-section and thus requires little space. This makes it possible to place the point feeder over the most favorable flow areas in the smelter almost regardless of how the electrolysis furnace is designed or which anode solution is used.

The functionality of the point feeder can be described as follows: First, the outlet to the dosing container is closed. The fluidizing element is then activated at the outlet of the storage container so that the powder material is blown into and fills up the dosing container. As the air can escape through the roof/top of the dosing container, it is practically filled completely every time so that the dosing accuracy is very good. When the dosing container is filled, the lance is activated which pierces the crust. As soon as the lance is retracted to its upper starting position, the closing device on the outlet of the dosing container is activated so that the outlet is opened. The powder material inside the dosing container will then flow freely down the supply pipe, and then down into the opening in the crust and mixed into the melt. By placing a stop plate that goes a short distance into the storage container above its outlet, it is possible to avoid the flow of uncontrolled amounts of powder into the smelter because the stop plate and the properties of the powder mean that the powder flow always stops when a certain angle of descent of the powder inside is reached in the outlet of the storage container. As soon as the dosing container is empty, the outlet is closed and you are ready to repeat the process for a new dose.

The dosing containers according to the invention provide the same amount of powder each time. Any changes to the dosage quantities are achieved by using dosage units with different volumes. However, there is no limitation in the size, i.e. the volume of the dosage units used. If the process requires greater flexibility in the dosing quantities that a change in the dosing frequency can provide, this can be solved by using several dosing units with varying volumes or by designing the dosing unit so that you can quickly and efficiently change the dosing container during operation. Such solutions are obvious to a person skilled in the art, and thus belong to the idea of the invention.

With the point feeder according to the invention, a mechanically very simple solution for dosing powder material is achieved which is more accurate than the dosing devices known from NO 300602 and NO 162774. Thus, the point feeder is cheap to acquire and operate, and can also be easily mounted on existing furnaces that is in operation. By the fact that the point feeder's dosing device has such a high degree of accuracy in that the doses fed out from the point feeder are delivered with a constant size with a deviation of 10% or less, the following advantages are achieved with the point feeder according to the invention: better current yield, reduced anode effect, reduced emissions, reduced operating costs and lower maintenance costs.

The point feeder can be used for feeding aluminum oxide and aluminum fluoride both to a Søderberg electrolysis cell and to a pre-baked electrolysis cell. When using the point feeder in a pre-baked electrolysis cell, it is preferred to place the point feeders between the pre-baked anodes in each longitudinal row, as opposed to the more conventional solution of placing them in the middle of the electrolysis cell between the current rails. Alternatively, the powder material can be fed into the smelter either through the longitudinal slot between the pre-baked anodes or through a combination of longitudinal and transverse slots.

When using the point feeder in a Søderberg electrolysis cell, the powder material can either be fed into the smelter through or outside the anode mantle. This differs from the dosing device according to NO 300602 and NO 162774, where the powder material is fed outside the anode sheaths. When mounting in connection with a Søderberg electrolysis cell, it is preferred to mount the storage container with dosing container along the anode mantle of the furnace. It is entirely possible to associate more than one dosing container to each storage container.

Both when using the point feeder in a pre-baked electrolysis cell and in a Søderberg electrolysis cell, the number and location of the point feeders will depend on the flow pattern in the smelting for the individual electrolysis cell, but for furnaces with a rectangular cross-section, the flow will often form a figure-8-shaped pattern over the entire furnace cross section seen from above. In such cases, the point feeder(s) should be positioned so that the powder is fed down the "8" where the flow is most intense (see Figure 3).

For the Bayer-Hall-Héroult process, it is common today to add aluminum oxide and aluminum fluoride in separate point feeders. This is achieved by using at least one point feeder for AIF3 and at least one for AI2O3.

Although, for the sake of simplicity, the invention is described in connection with the dosing of aluminum oxide and aluminum fluoride in an electrolysis cell, it is obvious that a person skilled in the art will understand that the point feeder will also be suitable for dosing any powder for any imaginable process that is to be regularly fed with a certain amount of powder. Such processes may include, but are not limited to, processes within the metallurgical industry, food industry, pharmaceutical industry. In short, puiver feeding for every imaginable process chemical application and reactor design.

Detailed description of the invention

The point feeder's individual parts and constructive composition according to the invention shall be described in more detail below: The point feeder's dosing device is designed with a cavity and the dosing device is arranged adjacent to the container's outlet so that the material can be transferred from the container to the dosing device's cavity. The outlet opening of the storage container can advantageously be shared with the inlet opening of the dosing container, because the dosage amount is determined by the volume of the dosing container and the slope angle of the powder in the outlet of the storage container, see Figure 1.

The cavity of the point feeder is designed with a discharge opening equipped with an Ieder tube for transferring the powder material from the cavity of the dosing device to the melt of the electrolysis cell. Furthermore, the dosing device is equipped with a valve or closing device, which is arranged so that the dispensing opening of the dosing device can be closed and opened, and the material can respectively be kept inside or fed out through the dispensing opening. A vent pipe extends between the container and the dosing device for venting the cavity of the dosing device.

In a preferred embodiment of the point feeder, the closing device can consist of a cylinder. According to a further preferred embodiment of the point feeder, the cylinder is a pneumatic cylinder which is controlled by compressed air.

The cylinder comprises a cylinder rod which is arranged with a piston at one end, where the piston is arranged displaceably in a cylinder space. The other end of the cylinder rod is designed so that it fits in sealing contact with the container's discharge opening. This other end of the cylinder rod is designed with a part that has a diameter that is larger than the other diameter of the cylinder rod. This enlarged part of the cylinder rod can be achieved by using cylinders with fully fitted valve heads. Compressed air is fed into and out of the cylinder space and thereby moves the cylinder rod between a lower position where the other end of the cylinder rod is lowered below and opens the container's discharge opening so that the material flows out of the dosing container with the help of gravity, and an upper position where the other end of the cylinder rod is placed against and closes the discharge opening.

Within the framework of the patent claims, the closing device can be made up of devices other than a cylinder, where a minimum requirement for an alternative device is that the closing device is able to close and open the dispensing opening of the dosing device in a simple way.

An overall computer system can be used to control the point feeder and its individual components, such as closing and opening the discharge opening.

The point feeder's storage container can be filled by any commercial solution for filling powders into a tank.

The point feeder will subsequently be explained in detail with reference to the figures in which: Figure 1 shows a sketch of the storage container and the dosing unit of the point feeder according to the invention. Figure 2 shows a sketch showing three point feeders in Figure 1 arranged in connection with a Søderberg electrolysis cell. Figure 3 is a schematic diagram showing in a bird's eye view how four point feeders are thought to be arranged for a pre-baked electrolysis furnace where eight pre-baked anodes placed in two rows are used.

Figure 1 shows the storage container and the dosing unit of the point feeder in section from the side, where a container 9 is shown filled with powder material 14. The material is filled into the container 9 through the inlet 13. The container 9 is shown equipped with fluidizing elements 10,11, where the fluidizing elements are arranged compressed air supply shown by nozzles 12 and lines 1 and 2. Powder material will, when supplied with air from the fluidizing elements 10, 11, behave as a homogeneous fluid. When fluidizing elements 10 are activated, the amount of powder inside the container 9 will be leveled, and when fluidizing element 11 is activated, the powder mass is led out through the outlet 8a so that the dosing container's cavity 6a is filled up. The cavity 6a is further designed with a discharge opening 7a with a closing/opening device. The material can be fed from the cavity 6a through the discharge opening 7a down the guide tube 7 to the opening in the crust of the electrolysis cell's melt when the closing device is in the open position. A stop plate 8b is arranged over the opening 8a so that the powder inside the container 9 does not flow freely down into the cavity 6a when the closing device is in the open position. The amount of powder that is allowed to flow out of the dosing container is determined by the volume of the cavity 6a and the slope angle of the powder in the outlet 8a of the container 9.

The closing device in Figure 1 comprises a pneumatic cylinder comprising a cylinder rod 5 which is arranged with a piston 5a at one end. The piston 5a is displaceably arranged in a cylinder space 5b and the other end of the piston rod is designed with an enlarged part 5c which fits in sealing contact with the container's discharge opening 7a. Furthermore, a venting pipe 15 is arranged between the cavity 6a and the interior of the container 9. The function of the venting pipe 15 is to allow all air to escape from the cavity 6a during filling of the dosing container.

When the material is to be delivered from the dosing device to the smelter, the cylinder rod 5 is lowered to a lower position by supplying compressed air to the cylinder chamber 5b so that the piston 5a is guided downwards into the cylinder chamber 5b. Then powder inside the cavity 6a will flow out of the cavity 6a with the help of gravity and down into the guide pipe 7. After the powder flow has been stopped, the discharge opening 7a will be closed by raising the cylinder rod until the enlarged part 5c is brought into contact with the discharge opening 7a. The cycle can then be repeated by feeding powder material from the container 9 to the cavity 6a through the outlet 8a by activating the fluidizing element 11.

Figure 2 shows several point feeders attached along the anode mantle for a Søderberg electrolysis cell. It will be possible to use these top covers as walkways if necessary. Figure 3 shows an example of a preferred location of the point feeders in the case of a pre-baked oven where eight pre-baked anodes arranged in two rows are used. The Figure shows a frequently occurring flow pattern down in the forge, a flow in a figure of eight where the flow is most intense as indicated. In such a case, it is preferred to place the point feeder(s) at one of the points indicated by an x in the figure.

Example

A series of verification tests have been carried out on a dosing device according to the invention.

Three series of dosing of AIF3 were carried out where the degree of filling of the storage container was respectively 100%, 50% and 10%. Each dosing cycle consisted of the following steps: two second fluidization time (fluidization element 11), 5 second waiting, 10 second emptying time, and then 5 second waiting time before the cycle is repeated for new dosing. Each dose was collected and weighed. Each series consisted of ten doses. The density of the fluoride powder was 0.85 kg/dm, the feed nozzles of the fluidizing element 11 had a diameter of 0.8 mm and the feed pressure approx. 7.0 bar. The bevel plate 8b protruded 175 mm into the storage container 9. The storage container had a volume of 175 dm<3>.

The results were a dosage of 1.425±0.049 kg, 1.410±0.074 kg and 1.390±0.057 kg respectively for a degree of filling of 100%, 50% and 10%. This shows that the point feeder according to the invention gives the same dose within a wiggle room of 10%.

Claims (10)

1. Point feeder for dosing material in powder form, such as, for example, dosing oxide, fluoride in an electrolysis cell for the production of aluminum, where the point feeder comprises a storage container container (9) for the powder material and a dosing device (6) arranged in connection with the outlet of the storage container for dosing of a certain amount of powder material for the smelting, characterized in that the storage container (9) comprises a lid with at least one filling opening (13), side walls, a bottom surface, and at least one discharge opening (8a) comprising a stop plate (8b) to prevent powder from freely escaping through the opening, and where at least part of the bottom surface comprises one or more fluidizing elements 10 for leveling the powder level in the storage container (9) and at least one fluidizing element (11) for dosing the powder material, and where at least a part of the bottom surface slopes down towards the at least one discharge opening (8a) for guidance of material out of the storage container (9) and into the dosing unit (6), and - that the dosing device (6) comprises a cavity (6a) which will be filled with powder material when the at least one fluidizing element (11) is engaged, a discharge opening (7a) for transferring material from the dosing device's cavity (6a) to a guide tube (7), and a closing device (5) which is arranged so that the dosing device's discharge opening (7a) can be closed and is opened so that powder material can respectively be kept in the cavity (6a) or led out through the discharge opening and down the guide pipe (7) for delivery to the smelter. 2. Point feeder according to claim 1, characterized in that the closing device consists of a cylinder comprising a cylinder rod (5) which is arranged with a piston (5a) at one end, where the piston is arranged displaceably in a cylinder space (5b) and the other end of the piston rod (5c) is designed so that it fits in sealing contact with the container's discharge opening (7a), preferably in that this other end (5c) of the piston rod is designed with a part that has a diameter that is larger than the other diameter of the piston rod. 3. Point feeder according to one of claim 2, characterized in that the closing device (6) consists of a pneumatic cylinder which is controlled by compressed air being fed into/out of the cylinder space (5b). 4. Point feeder according to one of claims 1-3, characterized in that the outlet opening (8a) of the container is shared with the inlet opening of the dosing device. 5. Point feeder according to one of claims 1-4, characterized in that the storage container (9) is equipped with a stop plate (8b) above the container's outlet (8a). 6. Point feeder according to claim 4 or 5, characterized in that the dosing device (6) is a volumetric dosing container. 7. Point feeder according to one of claims 1-6 characterized in that a deaeration pipe (15) extends between the container and the dosing device. 8. Point feeder according to requirements 1-7, characterized in that the container (9) is given external dimensions so that it can be placed along the long sides of the gas jacket of an electrolysis cell, and where the height of the container (9) is adjusted so that the top surface of the container is flush with the top surface of the gas jacket. 9. Point feeder according to requirements 1-8, characterized in that the gas jacket is a gas jacket on a Søderberg-type electrolysis cell. 10. Use of point feeders according to claims 1-9 in an electrolytic cell where the anodes are prebaked.