WO2020104343A1

WO2020104343A1 - Method and system for controlling suction of off-gases from electrolysis cells

Info

Publication number: WO2020104343A1
Application number: PCT/EP2019/081566
Authority: WO
Inventors: Eirik Manger; Are DYRØY; Morten Karlsen
Original assignee: Norsk Hydro Asa
Priority date: 2018-11-20
Filing date: 2019-11-18
Publication date: 2020-05-28
Also published as: AU2019382770A1; BR112021006307A2; ZA202102193B; NO20181482A1; EP3884083A1; CA3115415A1; EA202191402A1; NZ774481A

Abstract

The present invention relates to a method and a system for controlling the normal operational suction of off-gases from individual electrolysis cells for production of aluminium, where the cells can be of Hall-Héroult type, and further being provided with a hooding (CH) connected via a gas duct (GD) to a main gas duct (MGD) having means for generating a suction that transports the gas to a gas treatment centre (GTC), where the gas flow in said gas duct (GD) is controlled by a gas duct damper (GDD). One or more process variable/s such as pressure and/or temperature in the gas duct (GD) of each cell are measured and used as input signals to a controller (PLC) comprising a calculator where the controller calculates the actual mass flow in said gas duct (GD) based upon a pre-defined algorithm and produces an output set signal corresponding to a wanted flow rate, the signal is transmitted to an actuator (A) that regulates the position of the gas duct damper (GDD), and followingly the gas flow in the gas duct from each individual cell.

Description

Method and system for controlling suction of off-gases from electrolysis cells The present invention relates to a method and a system for controlling the normal operational suction of off-gases from electrolysis cells for production of aluminium, the cells can be of Hall-Heroult type, preferably with prebaked anodes.

The Hall-Heroult process, named after its inventors, is the most used method by which aluminium is produced industrially today. Liquid aluminium is produced by the electrolytic reduction of alumina ( Al₂0₃ ) dissolved in an electrolyte, referred to as bath, which mainly consists of cryolite ( Na₃AIF₆ ).

A sketch of a prior art alumina reduction cell is shown in Figure 1.

In an alumina reduction cell, hereafter referred to as the cell, several prebaked carbon anodes are dipped into the bath. The alumina is consumed electrochemically at the anode.

As can be seen from Equation (1 ), the carbon anode is consumed during the process (theoretically 333 kg C/t Al).

The lower part of the cell, the cathode, consists of a steel shell lined with refractory and thermal insulation. A pool of liquid aluminium is formed on top of the carbon bottom. The cathode, in the electrochemically sense, is the interface between the liquid aluminium and the bath, described by

AIF₃ + 3Na⁺ + 3e ® Al + 3NaF (2) and the total cell reaction becomes

Pure bath ( Na₃AIF₆ ) has a melting point of 101 1 °C. To lower the melting point, the liquidus temperature, aluminium fluoride (AIF₃) and calcium fluoride ( CaF₂ ), to mention the most important ones, are added to the bath. The bath composition in a cell may typically be 6-13 [wt%] AIF₃, 4-6 [wt%] CaF₂, and 2-4 [wt%] Al₂0₃. Lowering the liquidus temperature makes it possible to operate the cell at a lower bath temperature, but at the expense of reduced solubility of Al₂0₃ in the bath, demanding good Al₂0₃ control. It should be mentioned that if the concentration of Al₂0₃ gets too low (less than approx. 1 .8 wt%), the cell enters a state called anode effect. During anode effects, the cell voltage increases from the normal 4-4.5V up to 20-50V. Anode effect is a highly unwanted state, not only because it represents a waste of energy and a disturbance of the energy balance, but also because greenhouse gases ( CF₄ and C₂F₆ ) are produced at the anode. Very often the anode effect requires a manual intervention of an operator.

The bath temperature during normal cell operation is between 940 °C and 970 °C. The bath is not consumed during the electrolytic process, but some is lost, mainly due to vaporization. The vapour mainly consists of NaAIF4. In addition, some bath is lost by entrainment of small droplets, and water present in the alumina feed reacts to form HF.

In order to protect the environment, the gas is collected by a hooding and a gas suction system and further cleaned in a gas scrubbing system. More than 98% of the AIF₃ is recovered in the scrubbing system and recycled back to the cells. In addition, the content of sodium oxide (Na₂0) and calcium fluoride ( Ca₂F) in the fed Al₂0₃ neutralize AIF₃. The neutralized amount is also a function of the penetration of sodium into the cathode, and hence the cell age. As an example, one 170 kA cell emits about 60 equivalent kg AIF₃ pr. 24 hours and uses approximately 2500 kg Al₂0₃ pr. 24 hours. The amount of AIF₃ due to neutralization for one 170 kA cell is between 0 and 20 kg per 24 hours (dependent of cell age). However, since most of the AIF₃ is recycled, the real consumption of AIF₃ is very small compared to the consumption of Al₂0₃.

At the sidewalls of the cathode there is a frozen layer, called side ledge, which protects the carbon sidewall from erosion. The thickness of the side ledge is a function of the heat flow through the sides, which is a function of the difference in bath temperature and liquidus temperature.

The challenge is thereby to ensure stable cell operations resulting in a stable protective side ledge, while minimizing energy input and maximizing production. Given reasonable operational targets, it is an established operational practice that minimizing the process variations around target values results in good process operations in the sense of minimum pollution to the environment, maximum production and minimum expenditure. Used in the context of the alumina reduction cell the focus should be on achieving low anode effect frequency, good gas scrubbing efficiency and low deviation from target when it comes to alumina concentration, bath temperature and acidity. If the control of the alumina concentration is reasonably good, one has to focus on the bath temperature control and the AIF₃ control.

In controlling an electrolysis cell, there are, up till now, typically three main controlled variables: bath temperature, concentration of AIF₃ and concentration of Al₂0₃, and three control inputs: anode beam adjustments (controlling energy input), addition of AIF₃ and addition of A l₂0₃, and this is well documented in the prior art.

For instance, the applicant’s own W02009/067019 relates to a method for controlling the mass and energy balance of a cell by using a non-linear predictive model.

EP 2248605 A1 discloses an apparatus and a method for the removal of gasses from electrolysis cells by suction, the apparatus comprising a branch duct for each electrolysis cell, a main duct connecting the branch ducts to a gas treatment centre and a central suction fan providing for at least part of the suction, wherein one or more of the branch ducts are provided with supplementary suction means and wherein control means to control the supplementary suction means and pressure monitor means are provided, wherein the control means are adapted to control the supplementary suction means in dependence from changes in the monitored pressure with respect to a reference pressure.

The present invention relates to a method and a system for controlling the normal operational suction of off-gases from individual electrolysis cells (EC) in a plant for production of aluminium, the cells being of Hall-Heroult type, provided with a cell hooding (CH) connected via one gas duct (GD) to a main gas ducting (MGD) that transports the gas to a gas treatment centre (GTC) with suction means, where the gas flow in the gas duct (GD) can be controlled by a gas duct damper (GDD).

According to the invention one or more process variable/s such as pressure and temperature in the gas duct (GD) are measured continuously and used as input signals to a programmable logic controller (PLC) comprising a calculator where the controller calculates the actual mass flow in the gas duct (GD) based upon a pre-defined algorithm and produces an output set signal corresponding to a wanted flow rate, the signal is transmitted to an actuator (A) that regulates the position of the gas duct damper (GDD), and followingly the gas flow in the gas duct (GD) from individual cells.

The measured process variable/s as pressure and temperature in the gas duct (GD) may be compared with similar measured variables in ambient air outside the cell’s hooding to be able to establish one set of relative values with regard to the interior space of the gas duct and the ambient air.

To further optimize the energy control of Hall-Heroult cells for production of aluminium and also to be able to further contribute to the stabilization of the thermal balance of the cells, in particular in normal operation of the cells, the inventors have found that the suction of off gases from the hooding can be controlled in a new and inventive manner. Further benefits of the invention are that the off-gases from individual cells can be collected more efficient, in particular where the hooding is less gas-tight than designed, for instance due to wear, damages or the like.

The main idea is to in a first step tune the suction rate for each individual cell while the suction system in its normal operational modus. This corresponds to a situation where no maintenance work is done on the cells and thus all lids are closed.

Thus, the normal suction at each cell is balanced by individual needs of that specific cell and further adjusted in the following operations according to temperature and suction pressure inside superstructure.

This way of controlling the suction has shown to minimize the overall flow rate and savings can be achieved both in reduced effect from the fan but also less temperature loss from each individual cell.

The benefits of the present invention are in particular to control and optimise the amount of gas sucked off from electrolysis cell in a way that process variations in the cells can be reduced and emissions to the environment avoided/limited. By that, the electrolysis cells can be operated closer to operational targets and process limits, and it will be possible to achieve lower amount of emissions to the surroundings and lower energy consume per kg aluminium produced combined with more stable and efficient production process.

The control of the amount of gas sucked off involves a method and system for in-line measurements of pressure and temperature of the process gas, where these signals are used as input to a controller which produces an output signal to a damper controlled by an actuator that regulates the flow of said gas being sucked off individual cells.

The abovementioned advantages and further advantages can be obtained by the invention as defined in the attached claims.

The invention shall be described further by examples and figures, where:

Fig. 1 discloses a sketch of the main features of a prior art alumina reduction cell (Prebake) with its hooding,

Fig. 2 discloses schematically one hooding of a cell and its connection to a main gas duct of a suction system where pressure and temperature is measured in a gas duct,

Fig. 3 discloses similar details as shown in Fig. 2, where in addition pressure inside the hooding of the cell is measured,

Fig. 4 discloses plural cells monitored according to the principles of Fig. 2, preferably a group of cells connected to a common forced suction string, and similarly as above connected to a main gas duct,

Fig. 5 discloses two rows of cells monitored according the principles of Fig. 3, and further being connected to a GTC via a“H” formation.

In general, the invention as shown in Fig. 2 is based on utilising local measurements to balance and control the suction rates for each individual electrolysis cell (EC) in a pot line. By using clever and customised measurements/sensors, known relations between static pressure, temperature, and suctions rates as well as leakage break points, the emission/suction state and condition of each cell can be monitored. With automated dampers, commonly butterfly valves, controlled by a programmable logic controller (PLC), the rates can also be tuned and tailored so that cells with low hooding efficiency, i.e. high chances for leakage, receives more attention (i.e. increased gas suction flow rates) compared to cells with good hooding efficiency. In this manner, the total suction rate for the actual cell and also the plant might be reduced depending on specific needs and hooding handling.

In Fig. 2 the gas duct (GD) connecting the hooding of the cell (CH) and the main gas duct (MGD) and further transporting gas from the electrolysis cell, comprises a pressure transmitter (PT) and a temperature transmitter (TT), where both transmit signals to a programmable logic controller (PLC). The PLC controls the position of a gas duct damper (GDD) via an actuator (A) and thereby the flow rates through the gas duct in accordance to a predefined algorithm. In one embodiment, the controller can have an additional software enabling it to operate as a digital twin.

The static pressure and the temperature in the off-gas duct (GD) of the cells can be measured. These measurements can then be used to determine the net suction rates from the cells, either by known correlations or from model results. Calibration measurements might be needed to verify the function of the equipment.

The equipment collecting the measurements must be maintained regularly due to risk of fouling.

Monitoring of individual suction rates in combination with pressure, preferably static pressure, inside the cells’ superstructure as shown in Fig. 3 can even give warnings and act on low/bad hooding efficiency. By collecting pressure data from this area, measures or actions for limiting leakages of process gas to the surroundings can be taken, for instance by adjusting the damper (GDD) to allow for a higher suction rate.

The status of individual cells can be monitored on a screen as a process sheet having different colour codes (not shown),

An efficient Automated Cell Suction (APS) system according to the invention may also supplement or even substitute a forced suction system without any additional equipment, see Fig. 4, or it can act in parallel with such a system. A forced suction system is commonly known as a separate forced suction string (FSS) connected to a group of cells, typically 8- 10 and further having a suction channel with a booster fan blowing the extra suction into the main gas duct (MGD) of the plant’s gas extraction system. It can be arranged parallel to the ordinary gas suction channels between the cells and the main ducting and operated when there is a need for extra suction from at least one cell in the actual group of cells connected with the string. An example of a forced gas suction system is disclosed in Applicant’s own EP 1252373 A1 .

The forced suction system is operated for instance when anode change takes place and lids of a cell is removed and thus the hooding is punctured. The forced suction will be sufficiently strong to avoid substantial amounts of process gas entering out of the opening in the hooding during said operations.

In some arrangements it would be possible to eliminate the necessity of a separate forced suction system completely, by the implementation of the present invention. Such elimination should be verified by modelling and simulations before implementation.

Operating an APS system, the static pressure inside the cell’s hooding can be monitored as shown in Fig. 3 by one or more pressure transmitters (PT). This is one of the most challenging parts of the system, as the suction pressure here is quite low, of the order 5-8 Pa. In addition, scaling or fouling of pressure sensors can cause errors in the measurements over time. To reduce measurement errors, it is suggested to have at least two, preferably three to four pressure measurements (sensors / Pressure Transmitters) in the cell hooding. Trending averages can be used for control and calculating minimum and maximum values.

The pressure measuring points should preferably be placed on the same level as the gas skirt since this is where leakages will occur at too low suction rates. There is also a need to develop good control algorithms around the measurements, filtering out noise and disturbances. Additional flow measurements may be done by application of wing anemometers in the gas duct channels as these are believed to give a rather stable signal.

It is important to avoid deposits in the channels and on the dampers. To overcome this, it is suggested to have an automated procedure that completely opens and partially closes the dampers on a regular basis and running on one cell at a time. Adjusting the damper position on one cell will change this cells’ suction rate significantly, but the rest of the cells will not be affected. This also shows how a forced suction mode to some extent can be minimized and obtained by adjusting the damper.

In the final stage, see Fig. 5, the damper control of the dampers of all cells can be interlinked with fan control on the Gas Treatment Centre (GTC) to make up a full APS system. With such a connection, the fan power can be adjusted to account for e.g. sessional variations or overall changes in hooding efficiency. In this manner power can be saved and the gas treatment systems can potentially be used in a more optimal manner. Preferably, (see Fig. 3) the pressure (P) and temperature (T) at the gas duct (GD) connecting the hooding of the cell (CH) and the main gas duct (MGD) is monitored to keep the changes or variation under surveillance by use of a wireless pressure state or a pressure controller (pressure transmitter, PT) and a wireless temperature probe (temperature transmitter, TT) which is connected to an Ethernet system including one or more PLC’s (Programmable Logic Controllers), and further including a central PLC (CPLC) covering the entire pot line, see Fig. 4.

In certain situations, the APS system can be utilized to control and correct the cells’ heat balance. The energy evacuated with the off gas is directly proportional to the mass flow, and the control valve provides a measure for swiftly changing the heat flux out of the cells. The Idea is to be able to monitor the suction status from each individual cell, which is defined by the relationship between static pressure and temperature, to:

1 . Preserve the flow from each cell at a stable and constant value, or as commonly known as flow-balance of the suction system for the whole pot line (series of cells) or part of a pot line depending of the system size. The flow, expressed by pressure and temperature, should be as constant as possible to be able to maintain the emission level at a minimum. The flow from each cell should preferably be kept at a rate that captures substantially all process gases from the inside the hooding, even despite the hooding may be leaking or even punctured. In addition, during start-up conditions or during anode change, the suction rate must be adjusted accordingly by the PLC to achieve this.

2. Identify any decreasing pressure / increasing temperature at the relevant cell number or position. If such a case is occurring, it means that something is wrong with the suction system or cell control system and some actions have to be taken by the PLC to mitigate the changes.

3. Make a code for maintenance actions by use of different colours on actions level and type to be performed, which should be displayed/visualized for operators on one or more screens in the pot line control centre.

4. Set Start-up cells at the correct suction value/level via the PLC and monitor and set the control damper of the cell in the right position as the temperature of the cell rapidly decreases as it stabilizes after start-up to minimize the impact on the pot line emission and hence the environment. In practice this means, to use the tools of pressure and temperature which is displayed, to change the position of the damper as the pressure and temperature changes as the cell operation will be normalized. This can be done by the PLC.

5. Another aspect of the system, is to implement wireless surveillance of cell’s suction to automate the regulation and hence balancing of the suction system by use of motor controlled / pneumatic controlled regulation for the outlet duct dampers.

6. By applying the registered outlet pressure and temperature values as a state of heat losses to the heat balance control or surveillance system of the cells one will have a new tool to combine with existing system to control the energy input to the cell.

An appropriate APS system will:

• Reduce emissions

• Reduce operating costs by lowering the gas suction rates for individual cells

towards a minimum suction depending on precise measurements of the sub pressure inside the hooding compared with other information that can indicate the condition of the cell, for instance supported by models of the cell, such as a digital twin

• Reduce the total gas volume to be handled by the GTC

• Make the individual cells more autonomous

• Utilise the available gas suction rate better, for instance to create an enhanced suction on individual cells

• Allow sessional variations in gas suctions rates from GTC

• Provide possibilities for quantifying cell hooding efficiency

• Reduce thermal heat losses

• Reduce energy consume

• Give possibilities to control and quickly correct the energy flux with the off gas

The implementation of the system in a brown-field potline could be done as follows:

• Identify the cell in the suction string normally farthest away from the suction means in the GTC, as this cell will likely be most vulnerable with regard to a minimun, critical suction

Install in the cell the equipment needed for implementing the invention, such as pressure and temperature sensors to be able to monitor the temperature and pressure status related to this cell, and input these signals to a PLC. Arrange for a damper in the cell’s duct being controlled by an appropriate actuator via signals calculated by the PLC. · Tuning-in this individual cell according to a pre-defined level with regard to the measured pressure and temperature. Check that the suction generated by the GTC is sufficient, otherwise make appropriate adjustments.

Identify the next cell in the suction string next farthest away from the suction means in the GTC and repeat the above actions for this individual cell and check the GTC suction rate and make appropriate adjustments if necessary.

Repeat the above procedure for all cells in the pot line. · Take a final check of the suction from the GTC and if necessary adjust to an appropriate level.

In the alternative, the conversion to this system in a brown-field potline could be done during start-up of individual cells after re-lining. The conversion and installations can in this way be done in a situation where the cell is at room temperature.

The installation of the system in a green-field potline:

A green-field potline can be built with the necessary equipment installed in all cells, and the tuning-in of the suction for each individual cell may be done during start-up of the said individual cells.

Claims

1. A method for controlling the normal operational suction of off-gases from individual electrolysis cells (EC) in a plant for production of aluminium, the cells being of Hall-

Heroult type, provided with a hooding (CH) connected via one gas duct (GD) to a main gas ducting (MGD) that transports the gas to a gas treatment centre (GTC) with suction means, where the gas flow in the gas duct (GD) can be controlled by a gas duct damper (GDD),

characterised in that

one or more process variable/s among pressure and/or temperature in the gas duct (GD) are measured continuously and used as input signals to a controller (PLC) comprising a calculator where the controller calculates the actual mass flow in the gas duct (GD) based upon a pre-defined algorithm and produces an output set signal corresponding to a wanted flow rate, the signal is transmitted to an actuator

(A) that regulates the position of the gas duct damper (GDD), and followingly controlling the gas flow in the gas duct (GD) from individual cells.

2. A method in accordance with claim 1 ,

characterised in that

in addition, the pressure inside the hooding of the individual cell (CH) is measured by at least one pressure transmitter (PT) and an input signal is accordingly transmitted to the controller (PLC).

3. A method in accordance with claim 1 or 2,

characterised in that

the flow from each individual cell is optimised to a minimum where the onset of leakage is an individual property of each cell, varying with the hooding efficiency, and different suction rates are applied to each individual cell for keeping the cells sealed and avoid leakages to cell’s surroundings, while still keeping the suction rate as low as possible.

4. A method in accordance with claim 1 - 3,

characterised in that

the flow in the gas ducts (GD) of the individual cells in the plant are regulated towards an optimized minimum flow, where the gas flow generated by the suction means of the gas treatment centre (GTC) is adjusted correspondingly to optimize the amount of gas to be treated and the energy consume.

5. A method in accordance with claim 1 - 4,

characterised in that

the flow in the gas ducts (GD) of the individual cells can be controlled and changed to quickly adjust the heat balance of the cells.

6. A method in accordance with claim 1 - 5,

characterised in that

the flow in the gas ducts (GD) of the individual cells are controlled and verified with regard to a differential pressure based on measurements inside the cell’s hooding and the ambient air.

7. A method in accordance with claim 1 , that keeps the said automatic damper system free from deposits and scaling,

characterised in that

the damper opens and closes partially at regular intervals so that deposits on the damper blade is exposed to different air flows and cleaned.

8. System for controlling the normal operational suction of off-gases from individual electrolysis cells (EC) in a plant for production of aluminium, the cells being of Hall- Heroult type and further provided with a hooding (CH) connected via a gas duct (GD) to a main gas ducting (MGD) having means for generating a suction that transports the gas to a gas treatment centre (GTC), where the gas flow in the gas duct (GD) is controlled by a damper,

characterised in that

the system further comprises sensor/s for continuous measurement of one or more process variable/s among pressure (P) and/or temperature (T) in the gas duct (GD) where the continuous measured value/s are represented by input signals to a controller (PLC) comprising a calculator, the controller calculates the actual mass flow based upon a pre-defined algorithm and produces an output set signal corresponding to a wanted flow rate to an actuator (A) that regulates the position of the gas duct damper (GDD) and followingly the flow in the gas duct (GD).

9. System in accordance with claim 8,

characterised in that the system comprises in addition sensor/s (PT) for measurement of static pressure inside the cell hooding, where the measured value/s are represented by additional input signals to the controller (PLC) that produces an output set signal for controlling the flow in the gas duct (GD).

10. System in accordance with claims 8-9,

characterised in that

the flow in the gas ducts (GD) of the individual electrolysis cells (EC) in the plant are regulated towards an optimized minimum flow, where the gas flow generated by the suction means is adjusted correspondingly to optimize the flow.

11. System in accordance with claim 8,

characterised in that

the algorithm is based upon known relations between static pressure, temperature, and suctions rates of the gas.

12. System in accordance with claim 8,

characterised in that

the controller (PLC) is an integrated part of a local cell controller.

13. System in accordance with claim 8,

characterised in that

the controller (PLC) is an integrated part of a central controller (CPLC).

14. System in accordance with claim 8,

characterised in that

the signals are transmitted either wireless or by cable.

15. System in accordance with claim 8,

characterised in that

the controller (PLC) is able to control each individual electrolysis cell (EC) as an autonomous cell, maintaining the relative pressure inside the hooding (CH) and outside of it constant by controlling the gas duct damper (GDD) based upon appropriate measurements inside and/or outside the hooding (CH), and further that a minimum suction of the cell is always maintained by controlling the gas duct damper (GDD) and that changes in cell condition with regard to pressure and/or temperature is detected and compared with historical data stored in the controller (PLC), where appropriate actions regarding control of the gas duct damper (GDD) are taken.

16. System in accordance with claim 8,

characterised in that

the controller (PLC) comprises a digital twin model of the cell.