WO2006088375A1

WO2006088375A1 - Method and means for control of heat balance

Info

Publication number: WO2006088375A1
Application number: PCT/NO2006/000063
Authority: WO
Inventors: Bjørn Petter MOXNES; ASBJøRN SOLHEIM
Original assignee: Norsk Hydro Asa
Priority date: 2005-02-16
Filing date: 2006-02-15
Publication date: 2006-08-24
Also published as: CA2598067C; CA2598067A1; AU2006214865A1; AU2006214865B2; IS8670A; NO20050844D0

Abstract

A method and means for superheat control in an electrolysis cell for production of aluminium. The cell comprises a superstructure with one or more suspended anodes. Such anode being suspended by an anode hanger comprising a yoke 2 with one or more stubs integrated in one anode block 3. The yoke (2) is cooled by a cooling medium, such as gas, applied thereto. Heated medium is extracted from the cell and energy being recovered in a heat exchanger.

Description

Method and means for control of heat balance

The present invention relates to a method and means for control of the heat input in an electrolysis cell for production of aluminium. In particular, the invention relates to control of the heat loss in the current leads of the electrodes in the cell and more specific it relates to the cooling of the anode yokes connected to the anode hangers.

Conventionally, anode suspenders consist of a rod or a stem attached at its upper end to the anode beam in the superstructure of the cell, its other end is connected to an anode yoke that comprises one or more studs or stubs that are integrated with the anode carbon block. The anode rod can be made out of aluminium while the yoke is conventionally made out of steel material and one conventional way of integrating the stubs in holes in the carbon block is by means of cast iron.

The anode yoke on an anode hanger plays an important role in the Hall Heroult prebaked cell. The yoke can be defined as the constructional part between the anode stubs connected to the anode carbon and the anode stem connected to the anode beam. Due to the different tasks it has to full fill, the construction and chose of materials is a balance between different properties.

• The yoke needs to be strong enough to avoid deformation of the yoke due to the high temperatures involved.

• The yoke needs to have a certain surface to perform sufficient heat transportation from the stubs • The yoke needs to have a reasonable good electrical conductivity to avoid a too high voltage drop.

Approximately 50 % of the heat loss from the Hall-Heroult prebaked cell is through the top of the cell, and then again approximately 50 % of this heat loss is from the stubs and yokes.

The normal way of raising the amperage in the prebaked cells is done by increasing the anode dimension or/and reducing the interpolar distance (ACD). This will keep both the gross and the net heat input constant and the specific energy consumption will be reduced.

A recently new way of increasing the amperage has been to increase the number of stubs or stub dimension and thereby increase the heat loss from the cell. In this way the ACD is kept constant even if the amperage is raised. This approach focus on keeping net heat input constant, while the gross heat input is increasing. Since the gross heat input will increase with this way of increasing the amperage the specific energy consumption will increase.

In addition to an increase in the specific energy consumption, an increase of the heat loss from the stubs/yoke will also result in an increased temperature in the raw gas. Increased temperature in the raw gas will lead to a higher temperature on the raw gas entering the dry scrubbers and thereby increase the maintenance cost of the filter bags in the dry scrubber. At the same time the efficiency of the ventilation fans will be reduced due to a reduced density of the air sucked through the system. An increase in the raw gas temperature will lead to an increase in the gas pressure in the cell and thereby an increase possibility for puncturing the cell resulting in an increased emission of HF gas and dust to the working environment.

An increased temperature of the raw gas will also increase the heat loss to the surroundings, and thereby increase the heat stress on the operators working in vicinity of the cell

A technical way to reduce the raw gas temperature is to increase the suction rate from the cell, assumingly from 5000 to 7000-8000 Nm3/h. This way of solving the problem will be expensive due to the need of scaling up the equipment related to the dry scrubber system and also the energy consumption of the fans will increase.

Another way of solving the problem with an increase in the raw gas temperature avoiding to increase the suction rate is to cool down the raw gas by spraying water mist into the raw gas channel, as disclosed in WO 2004064984. One disadvantage related to this way of cooling the raw gas could be that the corrosion in the raw gas ducts will increase and the moisture content of the alumina may increase resulting in a higher HF outlet to the surroundings.

The main purpose of cooling the anode yoke as described in accordance with the present invention, is to be able to raise the amperage on the cell while maintaining the side and end ledge (frozen bath) in the bath phase without reducing the ACD, without increasing the dimension of the stub and yoke and thereby without increasing the temperature of the raw gas. Removing heat from the yoke with an active cooling will also increase the efficiency of stub and yoke as a heat sink for heat leaving the interpolar distance where most of the heat is generated. The reason for this is because the specific electrical and thermal conductivity of steel will increase and thereby leading to an increased heat loss through the stub and yoke and also because less internal heat will be generated in the material (steel). Calculation on a heat balance model with active cooling of the yokes has shown possibility for a 10 % increase in the amperage maintaining the interpolardistance and keeping the side ledge constant.

These and further improvements and advantages can be achieved with the present invention as defined in the accompanying claims.

The invention shall be further described by examples and figures where;

Fig. 1 discloses part of an anode, an anode stem and a yoke with means for cooling, seen from one side,

Fig. 2 discloses a yoke with a heat exchanger, seen from above,

Fig. 3 is a diagram showing the temperature on anode with use of active cooling of yoke,

Fig. 4 is a diagram showing heat loss from cell with use of active cooling of yoke,

Fig. 5 is a diagram showing relative resistance with use of active cooling of yoke,

Fig. 6 discloses one embodiment of a yoke with a heat exchanger, where the cooling gas is supplied/extracted in a central area of the yoke, seen from above,

Fig. 7a-d discloses one alternative embodiment of a yoke with a heat exchanger, where the cooling gas is supplied/extracted in a central area of the yoke, seen in different views.

As disclosed in Fig. 1 and Fig. 2 the anode comprises of a stem 1 , a yoke 2 and a carbon block 3. Means for supplying a cooling medium such as gas and extraction of the heated gas comprises flexible hoses 9 provided with quick coupling terminals 7, 8. Attached to the yoke is shown a heat exchanger 4 comprising ducts for supply/extraction of gas. The heat exchanger is provided with quick couplings 5,6 that communicates with the quick couplings 7, 8.

In one embodiment, the yoke or yokes in the superstructure of the cell can be cooled by pressurised gas, in particular air. The pressurised air can be provided by a pump, a compressor, a fan or the similar. The cooling of the yoke can then be done by arranging an air hose with a double pair of air tubes connected to each individual anode yoke. A cold air tube and a warm air tube can be applied. From the "cold air tube" cold pressurized air (7- 8 bar, 25° C) is blown through a channel in/on the yoke. The gradually warmed up air (warm air) is lead through the channel in/on the yoke before it ends up in the second air tube, the "warm air tube". The warmed up air from all the yokes ends into a collector channel on the top of the cell. After leaving each individual cell the warmed up air is collected from numerous cells before it ends up in a heat exchanger for possible energy extraction.

Fig. 3 is a diagram showing the temperature on anode by use of active cooling of yoke.

Fig. 4 is a diagram showing heat loss from cell with use of active cooling of yoke

The values in diagrams can be calculated by the following equations:

The air hose with the double tube is connected to the yoke with quick couplings on air hose and on the yoke. The coupling on the yoke has a valve that opens when the pressurized air is coupled on the valve. This is to prevent lumps of bath to get blocked in the channel in/on the yoke. Before one old anode is removed, the coupling is loosened with a handgrip and due to a spring system (the same technical solution as filling on a petrol station) the air hose is pulled back in upper position. After a new anode has been put into position, the air hose is again connected to the yoke by pulling the air hose down from its upper resting position. Due to reduced current pick up on new set anodes the coupling will be done 24- 48 hours after the new anode is installed in the cell.

The material used in the lower part of the "warm air hose" has to sustain at least 300 ⁰C. A steel coated flexible tube is an optional material.

The design of the yoke with a cooling channel could be done in at least two different ways. In one embodiment an air channel is made through the yoke. The disadvantage with this solution relates to expensive production of the yoke and possibility for oxidation and closing up of the channel inside the yoke.

One other embodiment that is a cheaper solution with less maintenance cost is to arrange a channel/channels on the outside of the yoke. The channel/channels is fixed to the yoke by welding or other appropriate means. Normally, this channel/channels will be made of steel, but if it could be fixed or arranged properly to the yoke also aluminium or another material could be used as material for the channel/channels. Another option is to make a track on the yoke so the aluminium channel/channels could slide into to the right position on the yoke. But of cause this have to be made air tight not to get air leakages.

Fig. 6 discloses one embodiment of a yoke 20 with a heat exchanger where the cooling gas is supplied/extracted in a central area of the yoke. Inside the yoke there are arranged a channel or ducting structure. In this embodiment, cold gas such as air is divided into substantially two equal flows at the inlet side 21 of the exchanger. The gas flows to both lateral ends of the yoke 20, were it is directed towards one centrally arranged outlet 22.

Figure 7a-d discloses one alternative embodiment of a yoke 30 with a stem 35 and studs 36 to be integrated in one carbon block. The yoke 30 is provided with a heat exchanger. In this embodiment, the supply of gas have inlet means 31 is arranged at a central part of the yoke, whereby the gas flows to one lateral extension of the yoke. In the end region, the flow is directed towards the opposite end, where it is directed towards one centrally arranged outlet 32. In Fig. 7a the yoke is seen from one side (similar to that of Fig. 1 ), and in Fig. 7b the yoke is seen from one lateral extension where the stem 35, the yoke 30 with inlet means 31 and one stud 36 is disclosed.

In Fig. 7c, the yoke is seen from above (similar to the view in Fig. 6 and Fig. 2), and the arrows indicate the direction of flow inside the yoke 30. In addition, one inlet 31 and outlet 32 means are disclosed. Fig. 7d represents a cross section view of the cut A-A in Fig c. This cross section discloses the yoke 30 and gas flow channels 33 and 34.

It should be understood that the flow pattern of the cooling medium as shown in the embodiments of Figs. 6 and 7, can in principle be achieved both by the arrangement of a heat exchanger fixed at the outside of the yoke or by arrangement of internal ducts in the yoke.

Other operational, environment and investment improvements by active cooling of the anode yokes besides potential for amperage increase are claimed below.

• Reduced temperature in the raw gas will lead to a lower pressure in the cell resulting in less Nm3 air needed to be sucked from the cell (reduced energy consumption on fans) to keep a certain under pressure in the cell.

• Less Nm3 sucked from the cell means less dimensions (reduced investment) on the dry scrubber system. Lower temperature on the raw gas means less maintenance (reduced maintenance cost) on the filter bags in the dry scrubber.

• With less heat given away from the stub and yoke to the cell, less heat will be lead through the hoods and into the working zone, in other words it will be less heat stress on the operators. This is especially important in the summer time or in parts of the world with a high temperature in the pot room.

• The hot air lead away from yokes will have a temperature of estimated 250 - 350

⁰C. With such a high temperature on the warm air it is possible to produce warm water/vapour and to extract energy with a heat exchanger and steam turbine. The air from the yokes will be clean and to use of a simple heat exchanger design with a high efficiency will be much easier than for instance using a heat exchanger on polluted raw gas.

• By turning on and off the cooling of the yokes it will be possible to regulate the net heat input into the cell. This can be used when the power in the pot line is reduced for a shorter or longer time by removing less heat from the yoke. In this way the number of cells that has to be shut down due to lack of enough power will be reduced. This is not possible to do, if a solution with increased stubs/ yoke dimension is chosen as a mean to increase the amperage. • The proposed technical solution can also be used by regulating the effect input to the cell under normal operation instead of moving the anode up and down (power pulsing). If the cell needs more heat, less warm air is removed from all the yokes on the cell, and if the cell needs less heat more air could be removed from the yokes than normal. In this way the need for upwards and downwards movements of the anode to increase or reduce the heat input to the cell will be less and therefore it will be possible to keep a more constant interpolar distance (ACD). By keeping the ACD more constant the fluctuation in the bath level will be reduced, and also the process control will be improved since movements of the anode normally will disturb the resistance signal to the regulator deciding the alumina addition.

• By cooling the yoke with air the need for long anode stubs (typical 30 cm) will be reduced, and thereby it will it be possible to reduce the specific energy consumption due to lower voltage drop in the stubs. A reduction of 10 cm should not be a problem. This will also increase the heat loss from the stubs.

• Reduced length of the stubs will allow for higher anodes without increasing the height of the superstructure. (Reduced investment cost)

• A colder anode yoke will reduce the maintenance cost of the bimetallic plate in the hanger due to lower temperature in the bimetallic plate, and also reduce the cow boy effect due to less thermal expansion of the yoke, and thereby less expansion force working on the stubs.

• If the temperature on the stubs is reduced, the possibility of anode cracking due to a higher thermal expansion on the stubs than on the anode will be reduced.

• A lower temperature on the yoke will also make it more easy to use other materials in the yoke than steel, by instance copper with a higher thermal conductivity and higher electrical conductivity than steel. Even an aluminium yoke could be considered. Calculation of reduced resistance with reduced temperature on the yoke and stub by the following equations:

Figure 5 is a diagram showing relative resistance with use of active cooling of yoke, based upon the above standing calculations.

In the examples above, pressurised air or gas is mentioned as an appropriate cooling medium. However, it should be understood that any other appropriate cooling medium can be applied in accordance with the present invention.

Further, the examples relates in its major part to cooling of the yoke. It should be understood that the way of integrating the heat exchanger can at least partly include other parts of the anode hanger, such as the suspender and the studs.

Claims

1. A method for superheat control in an electrolysis cell for production of aluminium, the cell comprising a superstructure with one or more suspended anode(-s), the anode being suspended by an anode hanger comprising a yoke (2) with one or more stubs integrated in the anode block (3), characterised in that the yoke (2) being cooled by a cooling medium applied thereto.

2. A method in accordance with claim 1 , characterised in that the cooling medium is applied to the inside or outside of the yoke (2).

3. A method in accordance with claim 1 , characterised in that the cooling medium is applied to the yoke (2) via channels arranged in a heat exchanger (4) attached at the outside or inside thereof.

4. A method in accordance with claim 1 , characterised in that heated medium is extracted from the yoke through an extraction system comprising flexible hoses (9).

5. A method in accordance with claim 1 , characterised in that the cooling medium is supplied to the yoke (2) through a ducting system that is provided with a quick coupling (7) and attached to the yoke (2) as a new anode is replaced.

6. Means for control of superheat in an electrolysis cell for production of aluminium, the cell comprising a superstructure with one or more suspended anode(-s), the anode being suspended by an anode hanger comprising a yoke (2) with one or more stubs integrated in the anode block (3), characterised in that the yoke comprises channels for ducting a cooling medium.

7. Means in accordance with claim 6, characterised in that the channels are arranged inside or outside the yoke.

8. Means in accordance with claim 6, characterised in that the channels are arranged in a heat exchanger (4) attached to the yoke (2).

9. Means in accordance with claim 6, characterised in that the cooling medium is supplied via one or more flexible hoses (9) and quick couplings (7) arranged in the superstructure of the cell.

10. Means in accordance with claim 6, characterised in that heated medium is extracted from the yoke through a medium extraction system comprising one or more flexible hoses (9) and quick couplings (8).

11. Means in accordance with claim 10, characterised in that the medium extraction system extracts heated medium from one or more cells, and comprises further a heat exchanger unit for extracting energy from the heated medium.