RU2540248C2 - Method of automatic monitoring of bath ratio - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: method of automatic monitoring of the bath ratio of the electrolyte of the aluminium electrolyser including its current and voltage measurement, calculation of actual values of the electrolyte resistance, and determination of the bath ratio of the electrolyte, comparison of the bath ratio with set value, and correction of the bath ratio of the electrolyte in case of deviation from the set value. This method determines the electrolyte resistivity during the anode frame movement with fixed duration, after equal time intervals in direction up and down, then resistivity is converted in movement coefficient, liquidus temperature is measured, and bath ratio of the electrolyte is determined depending on the movement coefficient and/or liquidus temperature. At that the movement coefficient is: U_res = TM × 6/V_AMG, where: U_res is resistivity of the electrolyte, movement coefficient [mV/mm]; V_AMG - speed of drive of movement gear of anode frame [mm/min]; TM - test movement [mV/s], determined as: TM = (ΔU_up + ΔU_down)/2/τ, where: ΔU_up is voltage difference during anode frame movement upward, mV; ΔU_down is voltage difference during anode frame movement downward, mV; τ is movement time, s. Electrolyte resistivity is determined during the anode frame movement within time interval from 0.5 s to 60 s after time intervals from 0.08 h to 24 h.

EFFECT: decreasing standard deviation of actual bath ratio from set value.

3 cl, 6 dwg

Description

The invention relates to ferrous metallurgy, in particular to the electrolytic production of aluminum, and in particular to the field of control of aluminum electrolysis.

Managing the composition of the electrolyte in the electrolytic production of aluminum is an important reserve in improving the technical and economic indicators of the aluminum electrolyzer and one of the primary tasks of managing the aluminum electrolysis technology as a whole.

A standard method for controlling the electrolyte composition is known when the cryolite ratio (the ratio of aluminum fluoride to sodium fluoride) (KO) is determined by crystal-optical or X-ray diffractometric methods in laboratory conditions after sampling. According to the factory instructions, electrolyte samples are taken once every 3 days by special samplers from the hole in the cryolite-alumina crust of the aluminum electrolyzer (Yanko E.A. Aluminum production. St. Petersburg State University, St. Petersburg: 2007, 376 pp.).

The disadvantage of this method is that the selection of electrolyte samples for analysis of its chemical composition is usually carried out once every three days, which is insufficient from the point of view of control efficiency, since the value of the cryolite ratio can vary significantly over several hours. In this regard, the electrolyzer for a long time works with the deviation of the parameters from the preset values, which entails a decrease in its performance indicators.

The well-known "Method of monitoring the technological parameters of the electrolyte of an aluminum electrolyzer" (patent RU 2471019, class C25C 3/20, publ. 12/27/2011). The method includes measuring the current strength and calculating the technological parameters, while measuring the current value I _e and level l of electrolyte _e is carried out using an automated punch isolated power system alumina as process parameters calculated electric resistance R _e of the electrolyte layer and bath ratio, measurement values of the DC prov DYT at the maximum amplitudes of the current, measuring the electrical resistance of the electrolyte layer - at a constant level electrolyte l _Oe and a constant voltage value equal to 24, the values of the electric electrolyte layer resistance is calibrated in accordance with the current value of the electrolyte bath ratio, calculated values cryolite ratio calibrated with values of standard electrolyte samples to obtain a calibration factor that takes into account the content of fluoride additives.

The disadvantage of this method is that it is not possible to maintain a constant electrolyte level on an existing aluminum electrolytic cell, since an aluminum electrolyzer is a self-regulating system, i.e. upon cooling, the electrolyte freezes on the cathode walls (the electrolyte volume decreases), the wells are tightened in the area of the punch of the automated alumina feeding system (there is a problem with the measurement) and vice versa. When heated, the volume of the electrolyte increases due to the melting of the electrolyte that has frozen on the cathode walls, the wells will bloom, and raw materials will stick to the punch, leading to additional insulation and measurement error. Also, the 24 V voltage for an aluminum electrolyzer is anomalous, in which, in addition to the energy consumption, there is a large emission of pollutants that destroy the ozone layer, which worsens the environmental situation.

A known method of regulating an electrolytic cell for producing aluminum by reduction from alumina dissolved in a bath of molten cryolite. The invention includes forming on the inner walls of the crust of the hardened cryolite melt, determining an indicator B, called a crust development index and sensitive to the development of the indicated crust of the hardened cryolite melt, and changing at least one of the electrolyzer control means, such as the pole distance (N), and / or at least one control operation, such as adding ALF ₃ , depending on the obtained value of the specified indicator. The indicator can be determined on the basis of electrical measurements on the electrolyzer and / or on the basis of measurements of the area of the liquid metal layer. The invention allows to effectively regulate the electrolyzer with currents up to 500 kA, with an electrolytic bath containing more than 11% AlF ₃ , and significantly reduce the number of measurements of AlF ₃ in the bath (patent RU 2280716, class C25C 3/20, publ. 27.07 . 2006).

The disadvantage of this method is that the measurement of the area of the liquid metal layer on the active electrolyzer can only be performed instrumentally (limited by the error of the measurement tool used) in several sections.

Closest to the proposed invention is a method for automatically controlling the cryolite ratio of the electrolyte, in which by moving the anode and registering changes in the resistance of the electrolyzer and the time of the movement of the anode, the value of the specific conductivity of the electrolyte is determined, then the cryolite ratio of the electrolyte is calculated, this value is compared with the set value and when the cryolite ratio from the set value, the composition of the electrolyte is adjusted (application No. 2000110682, class C25C 3/20. 04.2000, published on December 27, 2011).

The disadvantage of this method is that the more often the voltage adjustments take place on the electrolytic cell, the more often recounting takes place, and as you know, frequent adjustments occur on electrolyzers with a disturbed technological course, and the different value of the anode array’s travel time affects the calculated conductivity. When forecasting the cryolite ratio, it is taken into account that the alumina content in the electrolyte is constant, but in fact it is not possible to achieve a constant alumina content in the electrolyte in modern electrolyzers, the concentration varies from 1.8 to 4%.

The task of the invention is to increase the stabilization of the chemical composition of the electrolyte.

The technical result obtained by the implementation of the proposed technical solution is to reduce the deviation of the actual cryolite ratio from its target value.

The technical result is achieved due to the fact that in the method for automatically controlling the cryolite ratio of the electrolyte of an aluminum electrolysis cell, including moving the anode frame, measuring the current strength, voltage on the cell, calculating the current values of the specific resistance of the electrolyte and determining the cryolite ratio of the electrolyte, the specific resistance of the electrolyte is determined by moving the anode frames with a fixed duration at regular intervals in the up and down direction, depending on:

U _beats = TP × 6 / V _MPa ,

Where:

U _beats - specific resistance of the electrolyte [mV / mm],

V _MPa - the speed of the drive mechanism for moving the anode frame [mm / min],

TP - test movement [mV / s], defined as:

TP = (ΔU _up + ΔU _down ) / 2 / τ,

Where:

ΔU _up - voltage difference when moving the anode frame up, mV;

ΔU _down - voltage difference when moving the anode frame down, mV;

τ is the travel time, s,

measure the liquidus temperature and determine the cryolite ratio of the electrolyte depending on the specific resistance of the electrolyte and the liquidus temperature according to the equation:

KO _forecast = -3.47145-0.0051 × U _beats + 0.0063 × TL,

where: KO _forecast - the predicted value of the cryolite ratio,

U _beats - specific resistance of the electrolyte [mV / mm],

TL - liquidus temperature of the electrolyte [° C].

The electrolyte resistivity is determined when the anode frame is moved through time intervals from 0.08 h to 24 h, and the anode frame is moved for 0.5 s to 60 s.

The value of the time interval is set depending on the cycle of technological operations on the electrolyzer, the frequency of sampling the electrolyte to analyze the composition of the electrolyte and technological measurements, and the amount of displacement is set depending on the rated speed of the motor, the play of the jacks and the torque of the gearboxes installed on the electrolyzer.

The main difference of the proposed method is to measure the resistivity of the electrolyte on the cell by performing "test displacements" of the anode frame, in which the magnitude of the voltage change over the specified duration of movement is determined. Other differences from the proposed method according to the prototype are:

- measurement of liquidus temperature;

- fixed duration of movement of the anode frame;

- movement of the anode frame in both directions (up / down);

- measuring voltage changes at regular intervals.

The obtained method of calculating the cryolite ratio, in addition to the electrolyte resistivity, takes into account the actually measured liquidus temperature, which characterizes the current electrolyte composition (the content of alumina, calcium fluoride, aluminum fluoride, and other components).

The essence of the proposed invention is as follows.

The figure 1 shows an example of voltage regulation in the Elvis software, where one of the main control algorithms of a modern electrolyzer is a voltage regulation algorithm, the purpose of which is to maintain the actual voltage on the cell in accordance with the target voltage value.

To decide on the need for voltage regulation, if there are no regulation restrictions (being outside the dead zone), the necessary time is calculated for moving the anode frame. To calculate the time of movement, the parameter "regulation coefficient" is used - the number of millivolts per unit time.

T _REG = (U _GOAL _U _PR ) / K,

where: T _REG - voltage regulation time [s];

U _GOAL - target voltage [mV];

U _PR - the actual voltage [mV];

K is the regulation coefficient [mV / s].

Assessing the dynamics of the main technological parameters, using the principle: the “acid” the electrolyte, the higher its resistance, and vice versa, based on long-term observations, the relationship between the regulation coefficient and the cryolite ratio has been revealed (figures 2, 3). The main significant reason that does not allow to reveal the relationship between the coefficient of regulation and KO, is the lack of correlation dependence (figure 2) or positive correlation (figure 3).

The lack of correlation for individual electrolyzers is explained by the specifics of the algorithm for recalculating the regulation coefficient, where after regulation, assessing the difference between the calculated and the actual voltage change, the regulation coefficient is adjusted. Thus, the more often voltage adjustments take place on the electrolyzer, the more often the regulation coefficient is recalculated, and, as you know, frequent adjustments occur on electrolyzers with a disturbed technological process. Also, the magnitude of the regulation coefficient is affected by the different travel times of the anode frame.

Summarizing the above: the different frequency of regulation (unequal intervals of time) and the different duration of the movement of the anode frame only up or only down does not adequately reveal the dependence of the cryolite ratio on the regulation coefficient.

To assess the specific resistance of the electrolyte, it was proposed to perform “test displacements” of the anode frame on the electrolyzer, at which the magnitude of the voltage change over the specified duration of displacement is determined. In essence, “test displacements” determine the parameter as the “regulation coefficient”, however, the main differences are:

- fixed duration of movement of the anode;

- moving the anode frame in both directions;

- assessment at regular intervals;

- elimination of the influence of technological operations.

The figure 4 presents the dynamics of voltage changes when performing a "test move", where 1, 2, 3 is the interval of the voltage assessment; 4, 5 - the movement of the anode frame.

After successful adjustment of the anode frame “up - pause - down”, the “test movement” is converted into the specific resistance of the electrolyte, for ease of communication, the “displacement coefficient” is the number of millivolts per millimeter of movement of the drive mechanism for moving the anode frame [mV / mm].

U _beats = TP × 6 / V _MPA ,

where: U _beats - specific resistance of the electrolyte, the displacement coefficient [mV / mm];

TP - test movement [mV / s];

V _MPA is the speed of the drive mechanism for moving the anode frame [mm / min].

If we consider Ohm's law in a larger manner, the voltage drop of 1 cm of the MPR electrolyte is approximately

$$\Delta V=\rho *i=\frac{\mathrm{one}}{2O{m}^{-\mathrm{one}}*\mathrm{from}{m}^{-\mathrm{one}}}*0,85\frac{\mathrm{BUT}}{\mathrm{from}{m}^2}=42,5\frac{m\mathrm{AT}}{mm}$$

where: ΔV is the voltage (potential difference), V;

ρ is the resistivity, Ohm · m;

i is the current density, A / m ² .

Thus, the “test displacements” made it possible to more accurately than the regulation coefficient to evaluate the voltage drop in the electrolyte, and indirectly evaluate the resistance of the electrolyte during recalculation.

A further step in the development of the algorithm was a regression analysis to study the influence of the main technological parameters (independent variables) on the change in the “displacement coefficient” (dependent variable), predict the value of the dependent variable using independent ones, and determine the contribution of individual independent variables to the variation of the dependent one.

For this, the Elvis software implemented the calculation of the “displacement coefficient” relative to the performed “test displacements” of the anode frame (figure 5).

The figure 6 shows the correlation between the "displacement coefficient" and KO. The analysis showed that separately for the electrolytic cells, three main technological parameters (cryolite ratio, measured temperature of the electrolyte and liquidus) significantly affect the change in the "displacement coefficient" (figure 6, table 1). Between the analyzed parameters, there is a significant negative correlation with an average correlation coefficient of -0.71 to -0.85 (i.e., with an increase in one parameter, the other decreases).

Table 1 Electrolyte temperature and displacement coefficient Liquidus temperature and displacement coefficient Cryolite ratio and displacement coefficient Electrolyzer 010 r = -0.3346 r = -0.6478 r = -0.9349 Electrolyzer 011 r = -0.1640 r = -0.7163 r = -0.9801 Electrolyzer 012 r = -0.6286 r = -0.7346 r = -0.7769 Electrolyzer 013 r = -0.7378 r = -0.8122 r = -0.8409

In order to analyze the relationship between several independent variables (which of the independent variables contribute more to the prediction of CF), as well as construct an equation describing the dependence of the cryolite ratio on the displacement coefficient, electrolyte temperature, and liquidus, a multiple regression analysis was performed (Table 2).

It can be seen from the table that the t - criterion for the temperature of the electrolyte is quite low and the reliability level is> 0.05. This fact indicates a low effect of the temperature of the electrolyte on determining the value of the cryolite ratio.

The displacement coefficient and liquidus temperature have a significant impact on determining the value of the cryolite ratio (| t |> 2, p <0.05) ¹ ( ¹ Student criterion - the criterion is used to test the null hypothesis that the average values of the two sets are equal, | t |> 2 indicates that the significance level is high, p <0.05, error level <5%.) and this dependence is described by the following equation:

KO _forecast = -3.47145-0.0051 × U _beats + 0.0063 × TL,

where: KO _forecast - the predicted value of the cryolite ratio [unit],

U _beats - specific resistance of the electrolyte [mV / mm],

TL is the electrolyte liquidus temperature measured by the MITELIK device [° С].

The process control on electrolyzers with a capacity of 400 kA by the predicted cryolite ratio, determined by the specific resistance of the electrolyte by performing "test displacements" of the anode frame and determining the displacement coefficient, reduces the standard deviation of the actual cryolite ratio from the target value from 0.059 to 0.038.

To assess the adequacy of the resulting model in 2010-2013. comparative analyzes of the dynamics of the actual cryolite ratio and the predicted cryolite ratio by the displacement coefficient were carried out.

The results showed that the correlation between the predicted cryolite ratio in the displacement coefficient and the actual cryolite ratio is 0.89.

The application of the developed algorithm in the electrolysis technology control system, forecasting with high reliability provided an increase in the quality of the process by monitoring the thermal regime, stabilizing the electrolyte composition at a given optimal level.

Claims (3)

1. The method of automatic control of the cryolite ratio of the electrolyte of an aluminum electrolyzer, including the movement of the anode frame, measuring the current strength, voltage on the cell, calculating the current values of the specific resistance of the electrolyte and determining the cryolite ratio of the electrolyte, characterized in that the specific resistance of the electrolyte is determined when moving the anode frame with a fixed duration at regular intervals in the up and down direction according to:
U _beats = TP × 6 / V _MPA ,
Where:
U _beats - specific resistance of the electrolyte [mV / mm],
V _MPA is the speed of the drive mechanism for moving the anode frame [mm / min],
TP - test displacement [mV / s], determined by the equation:
TP = (ΔU _up + ΔU _down ) / 2 / τ,
Where:
ΔU _up - voltage difference when moving the anode frame up, mV,
ΔU _down - voltage difference when moving the anode frame down, mV,
τ is the travel time, s,
measure the liquidus temperature and determine the cryolite ratio of the electrolyte depending on the specific resistance of the electrolyte and the liquidus temperature according to the equation:
KO _forecast = - 3.47145 - 0.0051 × U _beats + 0.0063 × TL,
Where:
KO _forecast - the predicted value of the cryolite ratio,
TL - liquidus temperature of the electrolyte, [˚C].        2. The method according to claim 1, characterized in that the specific resistance of the electrolyte is determined when moving the anode frame at time intervals from 0.08 hours to 24 hours   3. The method according to claim 1, characterized in that the anode frame is moved for from 0.5 s to 60 s.

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