RU2370573C2 - Method of alumina concentration estimate in cryolite-alumina melt and facility for implementation of this method - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: method consists in immersion of sensor into cryolite-alumina melt, in measuring current strength on sensor at forced linear change of voltage on sensor by means of controlled voltage source, in plotting volt-ampere curve on obtained values of current strength and voltage, in evaluation of current strength of peak on plotted volt-ampere curve corresponding to start of anode effect on anode of sensor and in evaluation of alumina concentration by gauge dependence. Simultaneously with measurement of current strength on the sensor, resistance between anode and cathode of the sensor is measured at superposition of alternate pulse voltage of 10-100 kHz frequency and variable component of current strength of corresponding frequency is registered; also ohm component is evaluated; volt-ampere curve is plotted less ohm component of voltage. The facility consists of the sensor with cathode and anode arranged coaxially to each other and insulated from each other with insulation out of pyrolytic boron nitride; also insulation of not less 1 mm thickness is applied on side surface of anode by method of chemical sedimentation out of gas phase producing insulating coating; further, the facility consists of an electronic control and registration unit with autonomous electric power source, of a controlled voltage source, of a recorder of current strength and voltage and of a controlled generator of alternate high frequency signals. Anode is placed inside cathode and is made out of graphite. Output of the unit is connected with anode of the sensor, while input is connected to the controlled source of voltage.

EFFECT: compensation of ohm component of voltage and elimination of outflow of cryolite-alumina melt between anode and pyrolytic boron nitride.

3 cl, 2 tbl, 6 dwg

Description

The invention relates to ferrous metallurgy, in particular to an electrolytic method for producing aluminum.

The concentration of alumina in the cryolite-alumina melt (electrolyte) of an aluminum electrolyzer is one of the key parameters that determine the technological process of electrolysis. A high concentration of alumina in the electrolyte leads to the formation of a precipitate at the bottom of the bath with negative consequences for the electrolysis technology, while a lack of alumina leads to the phenomenon of polarization of the anode, called the “anode effect”, which is observed during the operation of aluminum electrolyzers.

Alumina concentration in industrial aluminum electrolysis cells at the moment is determined only by analytical methods after sampling the electrolyte from the electrolyzer. This analysis is too slow and time-consuming and does not allow to obtain operational information on the concentration of alumina in an aluminum electrolyzer.

For the operational determination of the concentration of alumina in a cryolite-alumina melt, electrochemical methods and devices are known.

To determine the concentration of alumina in a cryolite-alumina melt, an electrochemical method is known (copyright certificate SU 1423627, publ. 10/20/1986), based on measuring the potential of the anode of the measuring sensor relative to the reference electrode when the anode is polarized with a direct current pulse with a density of 0.45 A / cm ² and measuring a polarization time of 200 mV.

Also known is the electrochemical method and device for determining the concentration of alumina in a cryolite-alumina melt (copyright certificate SU 1673645, publ. 09/21/1989), based on measuring the EMF values of a galvanic cell, including a reference electrode and a measuring electrode, which are related to the concentration of dissolved alumina by the Nernst equation .

To determine the concentration of alumina in the cryolite-alumina melt by the method of measuring the emf is also known a device comprising a reference electrode and a measuring electrode (US patent 4,639,304, publ. 01.27.1987).

The use of both a method of measuring anode overvoltage (copyright certificate SU 1423627, publ. 10/20/1986), and a method of measuring EMF (copyright certificate SU 1673645, publ. 09/21/1989; US patent 4,639,304, publ. 01/27/1987) to determine the concentration of alumina it is difficult in the cryolite-alumina melt of industrial aluminum electrolysis cells because of the significant influence on the anode overvoltage and EMF of gas evolution and intensive mixing of the electrolyte, as well as potential gradients in the electrolysis bath.

Known electrochemical method and device for determining the concentration of alumina in cryolite-alumina melt (US patent 4,935,107, publ. 06/19/1990), based on impedance measurements at low current and overvoltage between the indicator electrode, the reference electrode and the counter electrode. The use of the known electrochemical method (US patent 4,935,107, publ. 06/19/1990) to determine the concentration of alumina in the cryolite-alumina melt of industrial aluminum electrolysis cells will be associated with a large measurement error due to unsteady conditions in the industrial electrolysis bath due to intensive mixing of the molten electrolyte.

A device for determining the concentration of alumina in a cryolite-alumina melt (patent US 3,471,390, publ. 07.10.1969), including a test electrode probe, having a cathode graphite tip, an insulating insert of boron nitride and anode. The device (US patent 3,471,390, publ. 07.10.1969) serves as a miniature electrolyzer in which an increase in voltage causes a rise in current until an “anode effect” appears, leading to a decrease in current. This effect depends primarily on the concentration of alumina in the electrolyte. The electrode (US patent 3,471,390, publ. 07.10.1969) has a flat surface at the end of the rod, which is a graphite core surrounded by an insulating ring of boron nitride. In the source (patent US 3,471,390, publ. 07.10.1969), voltage changes are carried out in discrete steps for a given period of time, while the data is recorded using an arrow voltmeter and ammeter. However, the long duration of a separate reading leads to a significant consumption of electrodes, which leads to a decrease in the accuracy of measurements.

A probe device is known for electrochemical determination of alumina concentration in a cryolite-alumina melt (US patent 4,450,063, publ. 05.22.1984), different from the known probe from the source (US patent 3,471,390, publ. 07.10.1969) in that the working surfaces of the anode and the cathode of the probe, which are separated by an insulator, lie on a common surface. Further, the probe components — carbon anode, cathode, and boron nitride insulator — are tightly fitted to each other without the use of carbon-containing or refractory cement.

A disadvantage of the known device, a probe for electrochemical determination of alumina concentration in a cryolite-alumina melt (US patent 4,450,063, publ. 05.22.1984) is the insufficient isolation of the side of the probe anode from penetration of the melt at the point of mechanical contact with the pyrolytic boron nitride insulator, which will result to reduce the accuracy of measurements.

A known device and method for measuring the concentration of alumina in the molten electrolyte of the electrolyzer (US patent 6,010,611, publ. 04.01.2000). The method consists in the fact that, using a computer-controlled voltage source, fast cyclic voltage sweeps are applied to the anode of the electrode assembly, which cause a local anode effect, which is recorded when measuring the current strength. To measure the concentration of alumina, at least part of the following information is used: the voltage at which the local anode effect occurs; voltage at which the maximum current is observed; magnitude of maximum current (peak current); the total voltammogram area, and then the oxide ion content is calculated using a previously constructed calibration curve obtained at fixed known concentrations of alumina in the electrolyte.

The device proposed in the source (US patent 6,010,611, publ. 04.01.2000), is a sensor - electrode assembly containing an anode and a cathode located coaxially relative to each other and isolated from each other by insulation from pyrolytic boron nitride. Moreover, the anode is placed inside the cathode and made of graphite. The electronic control and registration unit contains an autonomous power supply, a controlled voltage source, a current and voltage recorder. In the working part of the sensor, intended for immersion in molten electrolyte, the anode is not protected by insulation. The anode is made of carbon and has an elongated cylindrical shape with a length to diameter ratio of more than 2.0.

When measuring the concentration of alumina in the electrolyte by initiating a local anode effect on the electrode assembly of a known device, the lateral surface of the carbon anode of an elongated cylindrical shape is oxidized due to its insecurity. In this case, the distance between the anode and cathode of the electrode assembly changes, which leads to an increase in measurement error. In addition, a too large distance between the anode and cathode surfaces is the reason for the large ohmic component of the voltage when current is passed through the electrode assembly and leads to a measurement error. Also, in the known method and device, registration and compensation of the ohmic component of the voltage in the electrolyte between the anode and cathode of the electrode assembly (I · R) are not provided, which also leads to an increase in measurement error.

By appointment and the presence of common essential features, the well-known decision was made as a prototype.

The objective of the invention is to increase the accuracy of determining the concentration of alumina in the cryolite-alumina melt of industrial aluminum electrolysis cells without sampling.

The technical result consists in compensating the ohmic component of the voltage and eliminating the penetration of cryolite-alumina melt between the side surface of the graphite anode and pyrolytic boron nitride.

The task provides a method and device for determining the concentration of alumina in cryolite-alumina melt.

The problem is achieved in that in a method for determining the concentration of alumina in a cryolite-alumina melt, including immersing the current sensor in a cryolite-alumina melt, measuring the current strength on the sensor with a forced linear voltage change on the sensor at a speed of + 10 ÷ + 50 V / s until the anode effect is achieved, and after the anode effect is achieved on the sensor - with a forced linear change in the voltage on the sensor at a speed of -50 ÷ -200 V / s until it returns to the original voltage on the sensor by controlling voltage source, building a current-voltage curve from the obtained current and voltage and determining the peak current on the obtained current-voltage curve corresponding to the beginning of the anode effect on the anode of the sensor, and determining the concentration of alumina from the calibration dependence, according to the claimed method, simultaneously with measuring the current at the sensor measure the resistance between the anode and cathode of the sensor when applying an alternating pulse voltage with a frequency of 10-100 kHz, with the registration of the variable state The current strength of the corresponding frequency and the determination of the ohmic component of the voltage, and the construction of the current-voltage curve is performed minus the ohmic component of the voltage.

The problem is achieved in that in a device for determining the concentration of alumina in a cryolite-alumina melt containing a sensor with an anode and a cathode located coaxially relative to each other and isolated from each other by insulation from pyrolytic boron nitride, in which the anode is located inside the cathode and is made of graphite, an electronic control and registration unit containing an autonomous power supply, a controlled voltage source, a current and voltage recorder, according to the claimed isob The insulation is made in the form of an insulating coating deposited by chemical vapor deposition from the gas phase onto the side surface of the anode, the electronic control and recording unit is equipped with a controlled generator of variable high-frequency signals, the output of which is connected to the sensor anode, and the input is connected to a controlled voltage source.

The device complements a private distinguishing feature, also aimed at solving the problem.

The thickness of the insulation coating may be at least 1 mm.

Distinctive features of the proposed solution from the prototype allow us to conclude that it meets the criterion of "novelty."

In the search process in patent and scientific and technical literature, no technical solutions were identified that are characterized by identical or equivalent features with the proposed solution, which allows us to conclude that the proposed solution meets the criterion of "inventive step".

The invention is illustrated graphic materials, where

figure 1 shows a typical experimental dependence of changes in current and resistance with a linear change in voltage at the sensor;

figure 2 shows a typical current-voltage curves with and without regard to the ohmic component of the voltage;

figure 3 shows the calibration dependence of the concentration of alumina on the peak current strength, which is obtained at fixed known concentrations of alumina in the cryolite-alumina melt;

figure 4 shows the current sensor of the device for determining the concentration of alumina in cryolite-alumina melt;

figure 5 shows a block diagram of a device for determining the concentration of alumina in cryolite-alumina melt;

figure 6 shows graphs of the experimental determination of the concentration of alumina in an industrial aluminum electrolyzer using the installation for measuring the concentration of alumina, the calculated theoretical dependence of the change in the concentration of alumina and the dependence of the change in the concentration of alumina according to chemical analysis.

With a forced linear change in the potential of the current sensor 1 at a speed of + 10 ÷ + 50 V / s until the anode effect is achieved, and after the anode effect is achieved on the current sensor 1, with a forced linear change in voltage at the current sensor 1 with a speed of -50 ÷ -200 V / s until returning to the initial voltage, simultaneously with measuring the current strength on the current sensor 1, measure the resistance between the anode 2 and the cathode 3 of the current sensor 1, which is determined by applying an alternating pulse voltage of 10-100 kHz, with the registration of the variable component of the current response of the corresponding frequency (figure 1).

The product of resistance and current is the ohmic component of the voltage, which must be determined and compensated. The ohmic component of the voltage is compensated automatically by subtracting it from the voltage values applied between the anode 2 and the cathode 3 of the sensor 1.

The dependence of the change in current strength on the voltage is presented in the form of a current-voltage curve, which is built minus the ohmic component of the voltage (figure 2).

The concentration of alumina in the cryolite-alumina melt is determined by measuring the peak current on the current-voltage curve corresponding to the beginning of the anode effect on the anode 2 of the sensor 1, obtained taking into account the ohmic component of the voltage (figure 2), and calculating the alumina content from the calibration current - concentration of alumina "(figure 3), which is obtained at fixed known concentrations of alumina in the cryolite-alumina melt.

To determine the concentration of alumina in the cryolite-alumina melt, a device is used that includes a sensor 1 and an electronic control and recording unit 5.

The sensor (figure 4) is intended for partial immersion in a cryolite-alumina melt and contains an anode 2 and a cathode 3, which form a single coaxial structure. Anode 2 and cathode 3 are isolated from each other by a layer of pyrolytic boron nitride 4. Anode 2 is an internal electrode and is made of graphite. Pyrolytic boron nitride 4 is deposited on the side surface of graphite anode 2 in a known manner (patent RU 2160224, publ. 10.12.2000) with the possibility of forming an insulating coating with a thickness of at least 1 mm. The insulating coating prevents the penetration of cryolite-alumina melt between the lateral surface of graphite anode 2 and pyrolytic boron nitride 4. The tight adhesion between the lateral surface of graphite anode 2 and pyrolytic boron nitride deposited on its surface completely eliminates the penetration of electrolyte, which makes it possible to clearly fix the working surface area of the anode 2 and increases the accuracy of measurements.

The electronic control and registration unit 5 is designed to measure the current strength on the sensor 1 during a forced linear change in voltage on the sensor 1 at a speed of + 10 ÷ + 50 V / sec until the anode effect is achieved, and after the anode effect is achieved on the sensor 1, upon forced linear change voltage on the sensor 1 with a speed of -20 ÷ -200 V / s until it returns to the original voltage on the sensor 1, measuring the resistance between the anode 2 and the cathode 3 of the sensor 1 when applying an alternating pulse voltage of 10-100 kHz, with the registration of the component of the current response of the corresponding frequency and determining the ohmic component of the voltage, constructing the current-voltage curve based on the obtained current values minus the ohmic component of the voltage (Fig. 2), determining the peak current on the obtained current-voltage curve corresponding to the beginning of the anode effect on the anode 2 of sensor 1, determine the concentration of alumina from the calibration dependence of the concentration of alumina on the peak current strength (figure 3).

The electronic control and registration unit 5 contains an autonomous power supply 6, a controlled voltage source 7, a controlled generator of variable high-frequency signals 8, a current and voltage recorder, including a shunt 9, an analog-to-digital and digital-to-analogue converter unit (ADC, DAC) 10 , control controller 11, radio data transmission unit (Bluetooth) 12.

The power supply from an autonomous source 6 is supplied to a controlled voltage source 7, which produces a variable and controlled signal in the range of 0-20 V, 0-20 A. Shunt 9 is used as a current to voltage converter. Block ADC, DAC 10 is a conventional analog-to-digital (ADC) and digital-to-analog (DAC) converters. The ADC periodically measures the current strength at the sensor, and the DAC issues commands to the controlled voltage source 7 to supply a controlled voltage to the sensor 1. The controlled alternating high-frequency signal generator 8 supplies an alternating high-frequency current signal (from 10 kHz to 100 kHz) to sensor 1. Registration and information processing is performed by the control controller 11. Data is transmitted to a mobile computer (notebook or laptop) 13 using a data transmission unit over the air (Bluetooth connection) 12.

To determine the concentration of alumina in the cryolite-alumina melt, a calibration dependence of the concentration of alumina on the peak current strength was constructed (Fig. 3). Using a device for determining the concentration of alumina in a cryolite-alumina melt in laboratory conditions, the peak current of the anode effect was recorded on sensor 1 at fixed known concentrations of alumina in the electrolyte.

The experimental determination of the concentration of alumina using a device for determining the concentration of alumina in a cryolite-alumina melt was carried out both in laboratory conditions and in the electrolyte of industrial aluminum electrolyzers.

In laboratory experiments, an electrolyte of the following composition was used:

NaF / AlF ₃ = 2.3;

CaF ₂ = 6.0% by weight .;

MgF ₂ = 0.2% of the mass.

The electrolyte of industrial aluminum electrolyzers had the following composition:

NaF / AlF ₃ = 2.2-2.4;

CaF ₂ = 5.5-6.5% of the mass .;

MgF ₂ = 0.2-0.3% of the mass.

The temperature of the electrolyte in laboratory experiments and in industrial electrolysis baths was in the range of 950-960 ° C.

During the experiments it was found that when measuring without taking into account the ohmic component of the voltage (I · R), there was an underestimation of the concentration of alumina in the electrolyte compared with measurements with compensation for the ohmic component of the voltage (I · R) (Fig. 2).

During the experimental determination of the concentration of alumina in an industrial aluminum electrolyzer using a device for determining the concentration of alumina in a cryolite-alumina melt, a chemical analysis of cryolite-alumina melt samples for alumina content was carried out in parallel, and the change in the concentration of alumina in the cryolite-alumina melt of an industrial electrolyzer was loaded using alumina loading (Fig.6, table 1).

Table 1 The concentration of Al ₂ O ₃ Discrepancy Installation data,% wt. Chemical data analysis,% wt. % wt. Al ₂ O ₃ % 2.12 2,31 0.19 8.2 1.71 1.81 0.1 5.5 2.12 2,32 0.2 8.6 1.88 1.91 0,03 1,6 1.96 2.02 0.06 3.0

The results are presented in table 1. The discrepancy between the values of the concentration of alumina, determined using a device for determining the concentration of alumina and the data of chemical analysis, is small and amount to 0.03-0.2% wt. Al ₂ O ₃ or 1.6-8.6% (Fig. 6, Table 1). Also observed is the correspondence of the obtained experimental data with the calculated data on the loading of alumina (Fig.6).

Table 2 presents comparative data on the error in determining the concentration of alumina in the cryolite-alumina melt by the proposed method and other electrochemical methods.

Table 2. Electrochemical express method for determining the concentration of alumina Error in determining the concentration of alumina, wt.% Al ₂ O ₃ Source Proposed method 0.2 EMF method 0.5 B. Mazza and P. Pedeferri "In Situ Determination of the Alumina Content in Aluminum Reduction Bath", G. Re. Metallurgica Ital., P. 582 (1976) Anode Surge Method 1,0 B.M. Mozhaev, S. S. Romanchenko, G. D. Kozmin, P. V. Polyakov, V. V. Burnakin, “Non-ferrous metals” 24 (4), pp. 49-51 (1983) Critical current method 0.5 U.S. Patent 3,471,390, publ. Oct 1969, U.S. Patent 4,450,063, publ. May 1984, A.T. Taberaux and N.E. Richards "An Improved Aluminum Concentration Meter", Light Metals TMS, p.p. 495-506 (1983) Method for measuring voltage with anode effect 0.5 R. G. Haverkamp, B. J. Welch and J. B. Metson "An Electrochemical Method of Measuring the Dissolution Rate of Alumina in Molten Cryolite", Bull. Of Electrochem. 8 (7), p.p.334-340 (1992) Chronopotentiometry 0.8 J. Thonstad "Chronopotentiometric Measurements on Graphite Anodes in Cryolite-Alumina Melts", Electrochimica Acta, 14, p. P. 127-134 (1969)

From the presented data it follows that the error in determining the concentration of alumina by the proposed method by the proposed device is less than other known methods of express determination of the concentration of alumina in a cryolite-alumina melt.

Claims (3)

1. A method for determining the concentration of alumina in a cryolite-alumina melt, including immersing the current sensor in a cryolite-alumina melt, measuring the current on the sensor with a forced linear voltage change on the sensor at a speed of + 10 ÷ + 50 V / s until the anode effect is achieved, and after reaching the anode effect on the sensor - with a forced linear change in the voltage on the sensor at a speed of -50 ÷ -200 V / s until it returns to the original voltage on the sensor using a controlled voltage source, building volts a black curve according to the obtained values of current and voltage, and determining the peak current on the obtained current-voltage curve corresponding to the beginning of the anode effect on the sensor anode, and determining the alumina concentration from the calibration curve, characterized in that, simultaneously with measuring the current strength on the sensor, the resistance between the anode and cathode of the sensor when applying an alternating pulse voltage with a frequency of 10-100 kHz with the registration of the variable component of the current strength of the corresponding frequency and determined the ohmic component of the voltage, and the construction of the current-voltage curve is performed minus the ohmic component of the voltage. 2. A device for determining the concentration of alumina in a cryolite-alumina melt, containing a sensor with an anode and a cathode located coaxially relative to each other and isolated from each other by insulation from pyrolytic boron nitride, the anode located inside the cathode and made of graphite, an electronic control unit and registration, containing an autonomous power source, a controlled voltage source, a current and voltage recorder, characterized in that the insulation is made in the form of an insulating coating I, deposited by chemical vapor deposition on the side surface of the anode, the electronic control and registration unit is equipped with a controlled generator of variable high-frequency signals, the output of which is connected to the sensor anode, and the input is connected to a controlled voltage source 3. The device according to claim 2, characterized in that the thickness of the insulation coating is at least 1 mm.

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