RU2288974C2 - Method of automatic control of fluorine electrolyzer - Google Patents

Abstract

FIELD: automatic control of fluorine electrolyzer through maintenance of preset acidity of electrolyte melt.

SUBSTANCE: proposed electrolyzer is provided with level or acidity control sensor ensuring stabilization of optimal parameters of electrolyzer. Proposed method consists in maintenance of preset level of electrolyte by means of acidity control sensors or electrolyte level control sensors at retained preset acidity and temperature of electrolyte. Preset level of electrolyte is decreased or increased by 2-4% from height of gas-separating bell to minimum or maximum level, respectively; operation of electrolyzer is continued at new preset level; interval of change of levels ranges from 1.5 to 3.0 thousand of operating hours at each new level of electrolyte.

EFFECT: increased service life of gas-separating bells.

2 cl, 1 tbl

Description

The invention relates to a method for automatically controlling electrolysers with gas separation bells preventing the cathode and anode gases from mixing, and more particularly, to controlling an electrolyzer to produce fluorine.

Automatic control of a fluorine electrolyzer consists in maintaining a given acidity of the electrolyte melt, which ensures optimal performance (fluorine current and energy output, F ₂ and HF content in the anode gas) at given parameters of its operation (electrolyte temperature, current density on electrodes, force current). The required acidity is maintained by feeding acidic potassium fluoride melts with anhydrous hydrogen fluoride using sensors for monitoring the acidity or level of the melt subjected to electrolysis.

A known method for the automatic control of a fluorine electrolyzer (AS USSR No. 296626, 1989), which consists in the automatic search for the minimum possible values of acidity and temperature of the electrolyte while monitoring the voltage between the electrodes and its change in time, which reduces the entrainment of HF with electrolysis gases . There is also an automatic method for saturating a melt with hydrogen fluoride using an electrolyte level sensor in the electrolyzer body (N.P. Galkin, A.A. Mayorov and others. Chemistry and technology of uranium fluoride compounds. - M .: Gosatomizdat, 1961). The method fixes a change in the electrolyte level in the bath within ± 6.3 mm, which corresponds to a fluctuation in the concentration of hydrogen fluoride within ± 0.5%. This method is a prototype of the invention.

These methods stabilize the optimal operating mode of the operation of the electrolytic cells, provide minimal ablation of HF with exhaust gases, but have a significant drawback. They reduce the life of gas separation bells.

Fluorine is obtained by electrolysis of acidic potassium fluoride melts (KF · nHF). During the electrolysis of the melt, HF dissociates according to the scheme:

2HF = H ₂ F ⁺ + F ^- ,

H ₂ F ⁺ cations are discharged at the cathode:

2H ₂ F ⁺ + 2e = 2HF + H ₂ ,

anions (fluorine ions) are discharged at the anode:

2F ^- -2e = F ₂ .

Hydrogen is released at the cathode, and fluorine is released at the anode. A gas separation bell is necessary for separate removal of fluorine and hydrogen, and the electrolyte level at a given mark of the bell is a shutter that prevents their mixing. The gas separation bell is isolated from the cathode and anode. As a rule, the electrolyte is poured into the cell to a level corresponding to 1/2 the height of the bell. From the equations of the discharge of ions it is clear that during the electrolysis of an acidic potassium melt, the mass of potassium fluoride in the electrolyzer remains constant, anhydrous hydrogen fluoride decreases. In this case, the acidity of the melt decreases and at the same time the level of electrolyte in the cell decreases. Electrolyzer control means by means of sensors for monitoring the level or acidity of the melt maintain a predetermined electrolyte level by periodically or continuously supplying anhydrous hydrogen fluoride to the electrolyzer. This leads to the fact that the electrolyte level almost does not change or changes slightly. The bell material on the phase line is in the zone of maximum corrosion rate. It is impossible to increase the operating time of the bell to failure under these conditions.

An object of the invention is to increase the durability of the gas separation bells of the electrolyzer while maintaining the parameters of the electrolysis process, providing optimal performance. The problem is solved in that in a method for automatically controlling a fluorine electrolyzer, including maintaining a predetermined electrolyte level using acid control sensors or electrolyte level sensors while maintaining a predetermined acidity and electrolyte temperature, a predetermined electrolyte level is reduced or increased by 2-4% of the height of the gas separation bell to the minimum or maximum possible level, respectively, and continue the operation of the cell at a new predetermined level, with periodicity s change in levels of 1.5-3.0 thousand. hours of operation at each new level of electrolyte.

The basis of the proposed method is the following.

1. The main reason for the removal of the cell for repair is the electrochemical corrosion of the bell material at the interface between the liquid and gas phases (where the corrosion rate is maximum) to the through hole. According to the long-term industrial operation of STE-20 medium temperature electrolyzers, the service life of the steel bell on the electrolyzer under current load (hereinafter - the operating time of the bell) is in the range of 5600-14700 hours. Thus, the corrosion rate at the phase boundary (with a steel bell wall thickness of 6 mm) is 1 mm for 1000-1500 hours of operation at a current load of 9-15 kA.

2. A large interval of the durability of the bell is due to the different accuracy of maintaining the electrolyte level on individual electrolyzers. During the years of industrial development of medium-temperature electrolyzers, the manual method of level control using a calibrated rail was used. The accuracy of maintaining the level was low. Low accuracy leads to a "blurring" of the phase boundary, which seems to contribute to an increase in the durability of the bell, but at the same time it does not ensure the maintenance of the given acidity in the optimal range, which leads to a decrease in the durability of the bell. Modern sensors for monitoring the level of electrolyte type VM-100 ensure the maintenance of the level within ± 1 mm. High accuracy leads to the fact that the interface between the gas and liquid phases, where the corrosion rate is maximum, practically does not change. In this case, the durability of the bell is reduced even with the optimum acidity of the electrolyte, when the corrosion rate of the bell material is minimal. For this reason, despite the successes achieved in the automatic control of a fluorine electrolyzer, the operating time of the bell to failure has remained at the same level for decades. It is in the range of 6-14 thousand hours, and the average annual production of bells in fluorine production does not exceed 9 thousand hours.

3. Changing the set electrolyte level by 2-4% of the height of the bell every 1.5-3.0 thousand hours of operation of the bell helps to increase its service life by no less than 1.5-2 times. A change in the level of less than 2% of the height of the bell is impractical due to technological pressure drops of the anode and cathode gases. Changing the level of more than 4% of the height of the bell is impractical for economic reasons. The minimum operating time of the bell at a given level (1.5 thousand hours) is due to allowance for possible defects in the structure of the bell material, the maximum operating time (3.0 thousand hours) is selected taking into account the reliability of the remaining thickness of the bell wall (4 mm) during operation of the electrolyzer a constant electrolyte level.

4. The operating time of the bell increases due to the movement of the zone of maximum corrosion rate of its material.

Based on the foregoing, a new method for automatically controlling a fluorine electrolyzer is as follows. When commissioning the electrolyzer, the initial level of electrolyte is set, at which it is maintained for 1.5-3 thousand hours. Then the level is lowered (increased) by 2-4% of the bell height (5-10 mm) and continued operation at a new predetermined level and so on to the minimum (maximum) possible level. On a medium temperature fluorine electrolyzer STE-20 with a bell height of 300 mm, the permissible maximum electrolyte level from the upper edge of the bell is 80 mm, the minimum is 160 mm (with a standard permissible anode gas pressure of 15 mm Hg). Changing the level while maintaining constant acidity is carried out by draining the electrolyte from the electrolysis cell while lowering the level and supplying anhydrous hydrogen fluoride and potassium bifluoride to the electrolyte with increasing level.

Below are the results of industrial tests of the method on a medium temperature fluorine electrolyzer STE-20.

Control The total operating time of the bell, thousand hours / thousand kA · hour Prototype: With VM-100 level gauge (constant level for the entire period of operation) 6.9 / 69 New method (VM-100 level meter): E No. 1. Level H = 100 mm 1,5 / 15 level H = 95 mm in operation since 10/05/2004 E number 2. Level H = 140 mm 2.1 / 21 level H = 135 mm in operation since 11/18/2004 E number 3. Level H = 110 mm 1,5 / 15 level H = 115 mm 3.0 / 30 level H = 120 mm 4,5 / 45 level H = 125 mm in operation since 11/18/2004 Note: N is the electrolyte level from the upper edge of the bell.

The test schedule provides for a level change of 5 mm every 1.5-3 thousand hours of operation of these electrolyzers. It is supposed to carry out 4 level increases on electrolyzer No. 1 with subsequent descents (starting from the level of 100 mm), on electrolyzer No. 2 - to increase the level to the limit H = 80 mm; on electrolyzer No. 3 - lowering the level to the maximum possible N = 160 mm. An increase in the durability of the bell is expected to be no less than 1.5–2 times.

Claims (2)

1. A method for automatically controlling a fluorine electrolyzer by maintaining a given electrolyte level using acidity control sensors or electrolyte level sensors while maintaining a given acidity and electrolyte temperature, characterized in that the specified electrolyte level is lowered or increased by 2-4% of the height of the gas separation bell to a minimum or the maximum possible level, respectively, and continue to operate the electrolyzer at a new predetermined level, while the frequency of change of levels yaet 1,5-3,0 thousand. hours of operation at each new level of the electrolyte. 2. The method according to claim 1, characterized in that the operation of the electrolyzer begins at any value of the electrolyte level in a regulated interval.