RU2368707C2 - Aluminium reference electrode - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: aluminium reference electrode contains molten aluminium and potential measuring device, which are placed into alumina diaphragm, protected by graphitic case, potential measuring device is implemented from solid aluminium. Diaphragm is manufactured from porous alundum with open porosity 0.5-3.0% (vol.).

EFFECT: improvement of electrode operation with simultaneous increasing of useful life.

2 cl, 2 dwg, 1 tbl

Description

The invention relates to the aluminum industry, in particular to the field of producing aluminum by electrolysis of cryolite-alumina melt.

A device is known (Burgman JW et al., Aluminum / Cryolite reference electrodes for use in cryolite-based melts. - J. Electrochemical Society, 1986, vol.l33 [3], p. 496-502), which consists of an alundum diaphragm ( case), in which aluminum, cryolite-alumina melt and a tungsten potential puller were placed. The interaction of tungsten with cryolite is ruled out using alunda and special high-temperature cement. Its main structural difference from the electrodes existing at that time is that the tungsten potential stripper is maximally isolated from cryolite, which leads to a rapid change in the electrochemical (thermodynamic) potential of the aluminum electrode, and as a result, to a reduction in the life of the electrode as a reference electrode. An electrode of this design is more stable than the electrodes existing at that time, but not stable enough for accurate and long-term measurements, since over time the alundum is impregnated with cryolite and interacts with the potential remover.

A significant drawback of this design is the presence of extraneous metal (tungsten potential puller) in the electrode. Its interaction with aluminum and cryolite-alumina melt leads to disruption of electrical contact in the electrode, as well as to a change in the chemical composition of the metal and melt in the electrode.

Common signs of the claimed and known device are the presence of an alundum diaphragm (case) and aluminum as a direct electrode material.

A device is also known (Aluminum reference electrode, US Patent 4764257, Int.C1.4 G01N 27/46), the structural difference of which compared with the above-described electrode is the presence of a boron nitride diaphragm, which practically does not interact with the cryolite-alumina melt. The fundamental difference in the operation of the electrode of this design is that the interaction of the tungsten stripper of the potential with the cryolite-alumina melt in it was completely excluded due to the compaction of the melt with barium fluoride. Aluminum in this design is located above the melt.

Significant disadvantages of the electrode of this design are the presence in it of a foreign metal (tungsten stripper potential) and foreign salt (barium fluoride), which leads to the emergence of a diffusion potential, thermal emf, and subsequently to a significant measurement error.

Common signs of the claimed and known device are the presence of a porous diaphragm (case) and aluminum as a direct electrode material.

The closest analogue (prototype) to the claimed invention according to the technical solution is the Piontelli electrode (R. Piontelli, B. Mazza, P. Pedeferri, Electrochimica Acta, 10, p. 1117-1126, (1965)).

The known aluminum reference electrode contains molten aluminum as an electrode material and a potential pickup, which are placed in an alundum diaphragm. The potential stripper is made in the form of a rod of tungsten (hereinafter also used Mo, Ta). To ensure electrical contact between aluminum and the molten bath (cell) in the alundum diaphragm, a hole was made (0.1-0.3 mm). In order to avoid the interaction of the alundum diaphragm with cryolite-alumina melts unsaturated with alumina (III), the structure was placed in a graphite cover.

The main disadvantage of this electrode is that the tungsten (molybdenum) potential stripper chemically interacts with both the melt and aluminum. Our chemical analyzes of the metal after using such an electrode showed that the solubility of W and Mo in aluminum reaches 10 at.%. In this case, the composition of the electrolyte located in the alundum diaphragm (case) changes, which, in turn, significantly changes the value of the electrochemical potential of this electrode during measurements. When measuring the potential difference between the studied electrode and the Piontelli electrode, the error in its value is introduced by the thermal emf between aluminum and tungsten (or the product of the interaction of tungsten and aluminum), the contact between which is carried out at the melt temperature.

Common features of the known and claimed electrodes are the use of molten aluminum as an electrode material and potential pickup, which are placed in an alundum diaphragm protected by a graphite cover.

An object of the invention is to increase the characteristics of the electrode while increasing its service life, that is, creating an electrode whose electrochemical potential would be stable, reversible and reproducible.

The problem is solved due to the fact that in the known aluminum reference electrode containing molten aluminum and a potential pickup, which are placed in an alundum diaphragm protected by a graphite cover, the potential pickup is made of solid aluminum. The diaphragm is made of porous alundum with an open porosity of 0.5-3.0 vol.%. An increase in porosity contributes to a decrease in the time of preliminary exposure of the electrode in the melt, which is very important if it is used for measurements at low temperatures (700-750 ° C), however, an increase in its value over 3 vol.% Will lead to a deterioration of the thermomechanical properties of the diaphragm.

Comparative analysis with the prototype allows us to conclude that the claimed technical solution differs from the known one in that the potential stripper is made of solid aluminum, and the diaphragm is made of porous alundum. There is no interaction between the constituent elements of the electrode, which allows you to create an electrode with a stable, reversible and reproducible electrochemical potential.

The proposed aluminum electrode (Figure 1) consists of a porous alundum diaphragm (1), in which there is molten aluminum (3) and a potential pickup (2) made of solid aluminum. The design is placed in a graphite cover (4).

An aluminum electrode is made as follows. The alundum diaphragm, protected by a graphite sheath, is placed in the melt. Aluminum is fused into the interior of the alundum diaphragm. Aluminum potential stripper is installed in liquid aluminum. The process of deposition of aluminum into the alundum diaphragm takes up to 30 minutes (at a melt temperature of 750 ° C), after which the electrode must be kept in the melt for 2-5 hours. During this time, the diaphragm immersed in the melt is completely impregnated and a stable electrical contact is established between the aluminum and the melt.

A direct effect on the duration of exposure of the electrode in the melt is exerted by the melt temperature, the thickness of the diaphragm, and the value of open porosity. The thickness of the alundum diaphragm is 1.5-4 mm. The porosity of the alundum diaphragm is 0.5-3.0 vol.%.

An increase in the porosity of the alundum diaphragm and a decrease in its thickness leads to a decrease in the time of preliminary exposure of the electrode in the melt and to a deterioration in the thermomechanical properties of the diaphragm.

The claimed electrode can be used for several thermal cycles, while it should be slowly heated (cooled) for 20-30 minutes, especially after long-term storage ("downtime") in a humid atmosphere.

Electrical contact between the melt and aluminum is carried out in the pores of the alundum diaphragm. There is no thermal emf between the aluminum electrode and the potential stripper, because the potential puller is also aluminum. The contact temperature of the aluminum stripper potential with the terminal of the measuring device is 20-50 ° C.

Figure 2 contains a diagram of an experimental cell for measuring the potential of the anode under conditions of electrolysis of a cryolite-alumina melt.

Based on several experiments in which the values of the potential of the claimed electrode and the potential of the Piontelli electrode were compared, it was revealed:

1) the initial potential difference between the Piontelli aluminum electrode and the claimed electrode is a positive value and is 20-70 mV. Moreover, the potential of the claimed electrode is more negative and closer to the potential of an ideal aluminum electrode;

2) the potential value of the aluminum Piontelli electrode varies relative to the potential value of the inventive electrode. In one of the experiments, it changed by 50 mV in 2.5 hours;

3) the potential of the Piontelli aluminum electrode changes faster (more) when the contact of the tungsten stripper of the potential with aluminum is constant.

The inventive aluminum reference electrode was investigated for the reversibility of its electrochemical potential, namely, the emf of the circuit between the carbon and the claimed aluminum electrode was measured. The following equilibria are possible in this chain:

The table shows the values of the measured emf, as well as the calculated thermodynamic values of the emf of these circuits. Analyzing the obtained values of the emf, we can conclude that the inventive aluminum electrode is reversible.

Table Temperature ° C 700 ± 4 750 ± 4 960 ± 5 * emf circuit 1, V 1,340 _{± 0,004} 1.311 _{± 0.003} 1,192 _{± 0,005} * emf circuit 2, V 1.399 _{± 0.004} 1.289 _{± 0.004} 1,075 _{± 0,003} Measured potential difference, V 1,328-1,338 1,297-1,303 1.07-1.20 Test duration, hours four 25 four * Values are obtained on the basis of reference thermodynamic data (Edited by V.P. Glushko. Thermodynamic constants of individual substances. Reference. Vol. 1-4. M .: Nauka, 1978-1982).

Thus, the data presented confirm that the electrochemical potential of the proposed electrode is stable, reversible and reproducible, which allows to increase the characteristics of the electrode while increasing the service life, that is, to solve the problem.

The inventive aluminum electrode can be used as a reference electrode for the following purposes:

1. Measurement of anodic and cathodic overvoltages under conditions of electrolysis (industrial electrolysis bath, laboratory cell).

2. Measurement of the stationary (corrosive) potential of the anode (industrial electrolysis bath, laboratory cell).

3. Electrochemical studies. Obtaining polarization, potentiometric, chronovoltammetric and other dependencies on various electrodes in molten fluorides at 700-1000 ° С.

FIG. 1. Aluminum reference electrode

1 - porous alundum diaphragm; 2 - aluminum potential pickup; 3 - molten aluminum; 4 - diaphragm made of porous graphite.

FIG. 2. Schematic of an experimental cell for measuring the anode potential under conditions of electrolysis of a cryolite-alumina melt

1 - porous alundum diaphragm; 2 - aluminum potential pickup; 3 - aluminum reference electrode; 4 - diaphragm made of porous graphite; 5 - alundum glass; 6 - melt; 7 - aluminum (cathode); 8, 10 - alundum tubes; 9 - current supply to the anode; 11 - current supply to the cathode (W, Mo, Ta); 12 - anode (carbon, MPA, etc.)

Claims (2)

1. Aluminum reference electrode, consisting of molten aluminum and a potential pickup, which are placed in an alundum diaphragm protected by porous graphite, characterized in that the potential pickup is made of aluminum, and the diaphragm is made of porous alundum. 2. The aluminum reference electrode according to claim 1, characterized in that alundum with an open porosity of 0.5-3.0 vol.% Is used.

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