JPS5946317B2 - How to control an electrolytic cell - Google Patents

Description

[Detailed description of the invention] The present invention relates to a method for controlling an aluminum electrolytic cell, particularly.

The present invention relates to a method for maintaining stable operation of an electrolytic cell by maintaining a constant temperature. As is well known, aluminum
It is produced by electrolytically reducing alumina in an electrolytic bath mainly composed of cryolite.

Al dissolved in the electrolytic bath. Since alumina is consumed by the electrolytic reaction, alumina is supplied into the electrolytic bath continuously or at regular intervals. When the alumina concentration in the electrolytic bath decreases below a critical value, a phenomenon called the "anodic effect" occurs in which the electrolytic sliding voltage rapidly increases to 30 to 50V. Since normal electrolytic reactions are inhibited while the anode effect is occurring, alumina is promptly supplied to calm the anode effect. Anodic effect is an indicator of alumina supply, but it involves power loss, increased workload, and fluoride volatilization. Therefore, in order to keep the frequency of the anode effect within a certain control range, it is necessary to supply alumina continuously or at regular intervals, or to detect the occurrence of the anode effect in advance and supply alumina to prevent the occurrence of the anode effect. It is commonly done. Preferably, the electrolyzer is operated at a constant temperature.

If the electrolytic bath is overheated, the current efficiency will deteriorate. Furthermore, the layer of solidified electrolytic bath (self-lining) formed on the wall surface of the electrolytic cell in contact with the electrolytic bath is reduced, and the molten electrolytic bath comes into direct contact with the wall surface of the electrolytic cell, shortening the life of the electrolytic cell. On the other hand, if the electrolytic bath is overcooled, the self-lining becomes enlarged, which impedes the supply of alumina and the pumping out of the produced aluminum.

Furthermore, the solubility of alumina deteriorates, and the supplied alumina does not dissolve but settles and accumulates on the bottom of the electrolytic cell. the result,
The concentration of dissolved alumina in the electrolytic bath decreases, causing frequent anodic effects. In this way, if the temperature of the electrolytic cell is shifted either upward or downward, unfavorable results are brought about. The temperature of the electrolyzer during operation is determined by the amount of electricity supplied.

On the other hand, since the current value of each electrolytic cell in one series is the same and approximately constant, the temperature of each electrolytic cell is determined by the sliding voltage. This sliding voltage changes depending on the distance between the bottom surface of the anode and the molten aluminum surface in the electrolytic cell, that is, the distance between the electrodes, and the larger the distance between the electrodes, the higher the sliding voltage. Therefore, by measuring the temperature of the electrolytic bath, and if it falls below the standard value, the anode is raised to increase the distance between the electrodes, and in the opposite case, the anode is lowered to shorten the distance between the electrodes. , the temperature of the electrolytic bath can be kept constant. However, it is quite difficult to measure the temperature of the electrolytic cell, which is a prerequisite for this operation. As is well known, tens to hundreds of electrolytic cells are operated at the same time in an aluminum electrolytic plant, so installing temperature measuring devices in each electrolytic cell requires a great deal of expense, and maintenance and maintenance of the installed devices is costly. This is because the burden of inspection is also large. The present invention provides a method for maintaining the temperature of the electrolytic bath constant by adjusting the amount of power supplied based on other easily obtained indicators without directly measuring the temperature of the electrolytic bath.

According to the present invention, when the anodic effect occurs or is detected in advance, the amount of alumina supplied to the electrolytic cell from the previous occurrence or prior detection of the anodic effect to the current occurrence or prior detection of the anodic effect. The difference between the amount of alumina consumed in the same tank and the amount of alumina consumed between them is calculated, and an increase or decrease in the amount of alumina existing in the solid state in the aluminum electrolytic tank is detected based on this calculated value. By increasing or decreasing the amount of power supplied to the cell, the aluminum electrolytic cell can be maintained at a constant temperature and stable condition.

To explain the present invention in more detail, as mentioned above, in principle, alumina is continuously or intermittently supplied to an aluminum electrolytic cell in an amount slightly less than the amount corresponding to the alumina consumed by electrolysis, and the anode effect is increased. The interval between the two is kept within a certain control range.

On the other hand, the anodic effect occurs when the alumina concentration in the electrolytic bath drops below a critical value. Therefore, if the entire amount of alumina fed is immediately dissolved in the electrolytic bath, the interval of the anodic effect should be constant. However, in reality, it is nearly impossible to maintain a constant interval between anode effects. This is because the specific gravity of alumina is greater than that of the electrolytic bath and molten aluminum, so a part of the supplied alumina does not dissolve but settles and is deposited on the bottom of the electrolytic bath. Also, since the temperature of the electrolytic bath decreases when alumina is supplied, self-lining (which contains a large amount of alumina)
It also depends on how it grows. These phenomena are particularly noticeable when operating at low electrolytic bath temperatures to increase current efficiency. This is because lower temperatures reduce the solubility of alumina in the electrolytic bath. Therefore, even if the amount of alumina supplied matches the amount of alumina consumed, if the temperature of the electrolytic bath is low, the amount of solid alumina in the electrolytic bath will increase, causing the anode effect. On the other hand, if the temperature of the electrolytic bath is high, the alumina that exists in the solid state in the electrolytic bath will dissolve.
The anode effect that should occur no longer occurs. The present invention takes advantage of this phenomenon. In the method of the present invention, first, an increase or decrease in the amount of alumina present in a solid state in an electrolytic cell is detected. If the amount of alumina is increasing,
Since the temperature of the electrolytic bath is low, the anode is raised to increase the power supply. Conversely, if the amount of alumina is decreasing, the temperature of the electrolytic bath is high, so the anode is lowered to reduce the amount of power supplied. The increase or decrease in the amount of alumina present in the solid state in the electrolytic bath can be easily calculated by utilizing the fact that the anode effect occurs when the alumina concentration in the electrolytic bath reaches a certain value. In other words, if we calculate the amount of alumina supplied to the electrolytic cell from the time of occurrence or prediction of the previous anode effect to the occurrence or prediction of the current anode effect, and the amount of alumina consumed by electrolysis during that time, The difference is the increased amount of alumina present in the solid state. The amount of alumina supplied can be calculated by measuring with a measuring device provided in the alumina supply equipment.

Furthermore, the alumina consumption can be calculated using the following formula. COO
×102×10−3 Current efficiency (%) The amount of current flowing is determined by integrating the current values.

Under normal operating conditions, there is little change in the current value, so simply the average current value multiplied by the elapsed time may be used as the energization amount. Since the current efficiency usually does not change much, it is okay to use the average value. Therefore, the alumina consumption amount can be determined by multiplying the elapsed time by a constant. If it is found that the amount of alumina existing in the solid state in the electrolytic cell is increasing, the anode is raised to increase the distance between the electrodes and increase the amount of power supplied.

The amount of power supply should be increased is determined by the amount of heat required to heat the detected increased amount of alumina present in the solid state from the alumina supply temperature to the temperature of the electrolytic bath and dissolve it in the electrolytic bath. Defined based on. The minimum increase in the amount of power supplied is an amount corresponding to the amount of heat required to heat the increased amount of alumina from the supply temperature to the electrolytic bath temperature.

If the increase in power supply is less than this, it will not be possible to fully compensate for the amount of heat absorbed by the increase in solid state alumina. Therefore, the decreased temperature of the electrolytic bath was not fully recovered, and the anodic effect occurred due to poor dissolution of alumina due to the low electrolytic bath temperature, and the temperature of the electrolytic bath was further increased due to the temporary supply of alumina to eliminate the anode effect. There is a danger of falling into a vicious cycle of decline. The maximum increase in power supply is an amount corresponding to the amount of heat required to heat the increased amount of alumina to the temperature of the electrolytic bath and dissolve it in the electrolytic bath. If the power supply is increased beyond this, there is a risk of overheating the electrolytic bath. The increase in the amount of power supplied is determined as appropriate between the maximum value and the minimum value described above.

The amount of heat required to heat alumina (11<9) from room temperature to the electrolytic bath temperature is approximately 300 Kca1, and the amount of heat required to dissolve it in the electrolytic bath is approximately 500 Kca1.
It is desirable to increase or decrease the power supply quickly. That is, as soon as the anode effect occurs, the increase or decrease in the amount of alumina present in the solid state in the tank is calculated based on the difference between the amount of alumina supplied after the previous anode effect and the amount of alumina consumed during this time, and based on this, the amount of alumina is quickly calculated. It is most desirable to increase or decrease the amount of electricity supplied. In this case, the amount of increase in power supply is such that the supply is completed before the next anode effect occurs or is predicted to occur. That is, in the method of the present invention, in principle, the shortage in the amount of power supplied between the previous anode effect and the current anode effect is compensated for before the next anode effect. Accordingly, in the method of the present invention, once the anode effect has ended, power equal to or greater than the quotient of the increase in power supply divided by the average time between two consecutive anode effects is supplied in addition to the reference power amount. , a predetermined increased amount is supplied, and when the supply is finished, the amount of power supplied is returned to the reference value. Normally, the increased amount of power is supplied within 3 hours, preferably within 2 hours. The amount of power added to the reference amount of power may be constant over time, or may be increased initially and then gradually decreased.

Alternatively, the following simple method may be used, although the effect is slightly inferior to the above method. That is, this is a method of determining the increase or decrease in the amount of alumina present in a solid state in the electrolytic cell within a certain period, for example, every 24 hours, and changing the amount of power supplied based on this. For example, every 24 hours, based on the closest anodic effect and the previous anodic effect (if three or more anodic effects occur within a 24-hour period, the ) Calculate the amount of increase per unit time in the amount of alumina present in the solid state in the electrolytic cell, and add the amount of power corresponding to this increase per unit time to the following 24
It can be fed for hours.

According to this method, it is only necessary to adjust the amount of power supplied every 24 hours. The above is a case where the amount of alumina existing in the solid state in the electrolytic cell increases, but if the amount of alumina decreases, the amount of supplied power may be reduced in accordance with the above.

However, unlike the case where the amount of power supplied is increased, it is preferable that the amount of reduced power be constant over time. The amount of power supplied is increased or decreased by increasing or decreasing the distance between the poles.

The distance between the poles must be increased to the extent that the anode does not come out of the electrolytic bath. On the other hand, if the distance between the electrodes is reduced too much, the current efficiency will rapidly deteriorate. In this state, although the distance between the electrodes is small and the amount of power supplied is reduced, the heat absorbed by the reaction is further reduced and the temperature of the electrolytic bath increases. Therefore, the distance between the poles can be reduced only when there is a margin for the distance between the poles, and even in that case, it must be done with great care. Note that since calculation of the increase or decrease in the amount of alumina present in the solid state involves an error, it is preferable to adjust the inter-electrode distance only when the increase or decrease exceeds a certain value. One of the preferred operating methods according to the method of the present invention is to reduce the distance between poles and operate in a state where the amount of supplied power is slightly insufficient as a reference state.

In this way, the amount of alumina present in the solid state in the electrolytic cell always tends to increase. Therefore, after eliminating the anode effect, operation is performed with the inter-electrode distance expanded according to the increased amount of alumina detected, and once the predetermined amount of additional electricity has been supplied, the distance between the electrodes is increased based on the increased amount of alumina detected. Put it back and drive. In this way, since the distance between the poles is always adjusted in the direction of expansion, the distance between the poles does not decrease beyond the limit, and stable operation is possible. As explained in detail above, the present invention detects the temperature state of the electrolytic cell by increasing or decreasing the amount of solid state alumina in the cell, and increases or decreases the amount of power supplied based on the result, thereby stabilizing the electrolytic crab cell. This makes it possible to drive in the same condition.

Claims (1)

[Claims] 1. When the anodic effect occurs or is detected in advance, the amount of alumina supplied to the electrolytic tank from the previous occurrence or prior detection of the anodic effect to the current occurrence or prior detection of the anodic effect, and the amount of alumina supplied to the electrolytic tank during that period. The difference between the amount of alumina consumed in A method for controlling an aluminum electrolytic cell, characterized by increasing or decreasing the amount.