WO2013092773A1

WO2013092773A1 - Liquid level control in an electrolytic cell for the generation of fluorine

Info

Publication number: WO2013092773A1
Application number: PCT/EP2012/076247
Authority: WO
Inventors: Holger Pernice; Peter M. Predikant; Oliviero Diana; Philippe Morelle; Christoph Sommer; Harald Krueger; Antonio BERTANI
Original assignee: Solvay Sa
Priority date: 2011-12-22
Filing date: 2012-12-19
Publication date: 2013-06-27
Also published as: CN104350181A; KR20140108292A

Abstract

The invention pertains to a method of measuring and/or controlling the electrolytic bath liquid level in an electrolysis cell for the generation of fluorine gas having an anode chamber and a cathode chamber separated from each other by a partition wall for generating fluorine gas by electrolyzing an electrolytic bath (electrolyte) comprising a hydrogen fluoride-containing mixed molten salt of an HF adduct of KF, wherein the method comprises the steps of (a) detecting the pressure in at least one of the anode chamber and the cathode chamber during generation of fluorine gas by pressure sensing means comprising a tube (liquid level sensing tube) reaching (submersing) into the electrolytic bath (electrolyte), and, preferably, (b) supplying through the tube of the pressure sensing means an effective amount of hydrogen fluoride to thereby remove and/or prevent plugging caused by potentially crystallized HF adduct of KF in the tube of the pressure sensing means at any time and/or during measuring and/or controlling the electrolytic bath (electrolyte) liquid level in the anode chamber and/or cathode chamber.

Description

LIQUID LEVEL CONTROL IN AN ELECTROLYTIC

CELL FOR THE GENERATION OF FLUORINE

The present inventions claims benefit of the priority of EP patent application N° 11195431.9 filed on December 22, 2011 the whole content of which is incorporated herein by reference for all purposes.

The invention concerns an improved method of liquid level control of an electrolyte in an electrolytic cell (electrolyzer) for the electrolytic generation of elemental fluorine from a molten salt of an HF adduct of KF; and preferably, to an improved method of liquid level control and simultaneously of preventing and/or removing plugs in a tube for controlling the liquid level of an electrolyte in an electrolytic cell (electrolyzer) for the electrolytic generation of elemental fluorine from a molten salt of an HF adduct of KF.

Electrolytically generated elemental fluorine is often intended for the supply (delivery) in a method for the manufacture of electronic devices.

Elemental fluorine (F₂) has no GWP (Global Warming Potential) and no impact on the ozone layer. Elemental fluorine is useful as fluorinating agent, e.g. for the manufacture of polymers which are f uorinated on the surface, for the manufacture of fluorinated solvents especially for Li ion batteries, as chamber cleaning agent and etchant for the manufacture of electronic devices, especially semiconductors, photovoltaic cells, micro-electromechanical

systems ("MEMS"), TFTs (thin film transistors for flat panel displays or liquid crystal displays), and the like.

As to the use as etchant for the manufacture of electronic devices, especially semiconductors, photovoltaic cells, MEMS and TFTs, several consecutive steps of deposition of layers and etching a part of them are necessary. Fluorine can be used for etching of layers constituted of very different constitution, for example, for etching silicon containing layers or other layers of compounds which form volatile reaction products, e.g. tungsten.

Etching can especially be performed photo-assisted, thermally or plasma- assisted.

As to the use for chamber cleaning, usually, during deposition processes performed in treatment chambers - often CVD chambers (chambers wherein layers are deposited on items via chemical vapor deposition, e.g. plasma- enhanced CVD, metal organic CVD or low pressure CVD) - undesired deposits form on the walls and on inner constructive parts of the chamber and must be regularly removed. This is achieved by treating the deposits thermally or plasma-enhanced with elemental fluorine as chamber cleaning agent.

Especially for the use of elemental fluorine as an etchant, but also when used as chamber cleaning agent, it is desirable that the elemental fluorine be very pure. The intrusion of water, carbon dioxide, nitrogen and oxygen is considered as undesired.

Elemental fluorine can be produced by various methods but is often produced electrolytically, as mentioned already above, from hydrogen fluoride (HF) as feed material for the electrolysis and source of the elemental fluorine. In the presence of an electrolyte salt, HF releases fluorine if a voltage of at least 2.9 V is applied. Practically, the voltage is often kept in a range of 8 to 11 Volt.

Typically, a molten HF adduct of KF, often having the

formula KF (1.8-2.3) HF, is the preferred electrolyte salt. HF is fed into the reactor containing the molten electrolyte salt, and F₂ is electrolytically formed from the HF according to the equation (1) by applying a voltage and passing electric current through the molten salt :

2HF -> H₂ + F₂ (1) Hydrogen fluoride is useful notably as feed material for chemical manufacturing processes such as manufacture by electrolysis of molecular fluorine (F₂), useful for example as chamber cleaning gas in the semiconductor industry, and the manufacture of other fluorinated chemicals such as fluorinated hydrocarbons.

After the fluorine is manufactured by the electrolytic manufacture (or any other method), it can be stored in pressurized cylinders and transported to the site of use. In plants with higher F₂ demand, it is preferred to produce the F₂ directly on site.

WO 2004/009873 discloses an apparatus and a method for generation of fluorine by the electrolysis of hydrogen fluoride. The fluorine is produced by electrolysis from HF in a fluorine generating cassette. The fluorine may be used in the manufacture of electronic devices, e.g. in the production of TFTs.

The apparatus comprises : a plurality of individual fluorine generating cassettes ; said individual fluorine generating cassettes being operably connected to a fluorine gas distribution system for the remote use and consumption of said fluorine gas ; said fluorine generating cassettes being individually isolatable from said gas distribution system and removable from the apparatus for remote maintenance. According to the reference WO 2004/009873 a supply of liquid hydrogen fluoride is held in a tank. A hydrogen fluoride vaporizer vaporizes liquid hydrogen fluoride from the tank and supplies it to the cassettes to maintain a constant concentration of electrolyte being composed of the molten HF adduct of KF as mentioned before.

During the electrolysis and the formation of elemental fluorine the hydrogen fluoride (HF) is consumed. The consumption of hydrogen

fluoride (HF) influences the amount of and consequently the level of the electrolyte, e.g. the molten HF adduct of KF, in the electrolytic cell.

Consequently, for the continued formation of fluorine, fresh HF must be fed into the electrolysis cell from time to time to keep the level of available HF as the source of fluorine in a certain range in which the electrolysis can be performed to yield fluorine with acceptable purity. Normally, e.g. in terms of the preferred electrolyte salt of a molten HF adduct of KF the range of HF is usually varying according to the formula KF-(1.8-2.3) HF. Also the height of the electrolyte (electrolyte level) must be kept in a certain range to make sure that the electrolysis is operated under appropriate conditions to produce elemental fluorine of the required quality. Therefore, it is important to monitor the electrolyte level in the electrolytic cell as an indicator of the HF content in the electrolytic cell and to regulate the amounts and intervals of feeding hydrogen fluoride as feed material into the electrolytic cell, e.g. to operate the electrolytic cell within the range of the required electrolyte level and with an electrolyte salt of a molten HF adduct of KF the range of HF is usually varying according to the formula KF (1.8-2.3) HF.

If the conditions in the electrolytic cell, including the liquid level of the electrolyte in the cell, are not adequately controlled and regulated to meet the before mentioned operating conditions in the electrolytic cell, this may cause problems with keeping the temperature and the composition of the electrolyte homogenous over room and time, and the consequence is that formation of impurities such as CF₄, OF₂, 0₂ may substantially increase leading to fluorine with low quality in terms of its purity. For many applications high purity fluorine or at least fluorine with minimal presence of impurities is required, in particular in on-site processes which should be as simple as possible, with low interference by personnel, continuous high purity fluorine production with minimal need of additional purification measures. Typically, in the state of the art an electrolytic cell for generating (highly) pure fluorine gas by electrolyzing an electrolytic bath comprising hydrogen fluoride in the form of a molten salt of an HF adduct of KF, will be equipped with liquid level sensing means capable of sensing the levels of the electrolytic bath (electrolyte) in the anode chamber and in the cathode chamber, respectively, For on-demand and on-site operation, automatic control of the electrolyte level in the electrolytic cell is indispensable to the safety in automatic operation. As regards the technology of controlling the fluctuation in electrolyte level, for instance, published patents EP0728228B1, EP0852267B1, EP0965661B1 and USP5688384 propose the so-called start/stop (on/off) control. However, when electrolysis is carried out using this technology, there arises a problem. Namely, the electrolysis is interrupted upon occurrence of a certain extent of fluctuation in liquid level, and the electrolysis cannot be restarted until the electrolyte level returns to the original level.

Published patent application EP1367149A1 discloses a fluorine gas generator for generating highly pure fluorine gas by electrolyzing an electrolytic bath comprising hydrogen fluoride in the form of a molten mixed salt which generator comprises an electrolytic cell divided, by a partition wall, into an anode chamber in which an anode is disposed and a cathode chamber in which a cathode is disposed, pressure maintenance means for maintaining the electrolytic cell inside at atmospheric pressure, and liquid level sensing means capable of sensing the levels of the electrolytic bath in the anode chamber and in the cathode chamber, respectively, at three or more level stages. According to this constitution, slight fluctuations in liquid level can be detected, and the anode chamber inside and cathode chamber inside can be maintained at atmospheric pressure by means of the pressure maintenance means. As a result, the level of the electrolytic bath as a whole is stabilized. Thus, the fluctuations in

electrolytic conditions during electrolysis can be reduced, and stable supply of fluorine gas becomes possible. Further, since the anode chamber inside and cathode chamber inside are maintained at atmospheric pressure, air or the like can be prevented from flowing into it from the outside, so that highly pure fluorine gas can be generated in a stable manner.

Furthermore, published patent application EP1457587A1 discloses a fluorine gas generator for generating fluorine gas by electrolyzing an electrolytic bath comprising a hydrogen fluoride-containing mixed molten salt which generator comprises an anode chamber and a cathode chamber separated from each other by a partition wall and is provided with electrolytic bath liquid level controlling means for controlling a height of electrolytic bath liquid level in at least one of the anode chamber and the cathode chamber during suspension of fluorine gas generation. The electrolytic bath liquid level controlling comprises pressure sensing means and pressure controlling means operated in association with the pressure sensing means. In a variant, the electrolytic bath liquid level controlling means, especially to determine the level in 5 stages, for controlling the liquid level in the anode chamber comprises a pressure sensing means, and a pressure controlling means operated in association with the pressure sensing means and controlling the pressure in the anode chamber to control the difference of the liquid level in the anode chamber and the cathode chamber by supplying a suitable current to the anode.

Object of the invention is to provide a method to measure and/or control the electrolyte level which allows to measure the level not in stages, but for any desired level.

The invention according to a first embodiment provides a method of measuring and/or controlling the electrolytic bath liquid level in an electrolysis cell for the generation of fluorine gas having at least one anode chamber and at least one cathode chamber separated from each other by a partition wall for generating fluorine gas by electrolyzing an electrolytic bath (electrolyte) comprising a hydrogen fluoride-containing mixed molten salt of an HF adduct of KF, wherein the method comprises the steps of (a) detecting the pressure in at least one of the anode chamber and the cathode chamber during generation of fluorine gas by pressure sensing means comprising a tube (liquid level sensing tube) reaching (submersing) into the electrolytic bath (electrolyte). Preferably, the liquid in the tube is displaced by means of an inert gas which displaces the liquid in the tube, and the pressure difference between the inert gas and the gas space of the at least one of the anode chamber and cathode chamber is determined.

A submerged tube is a preferred measure sensing means in the frame of the present invention.

It is preferred, in one alternative, to determine the electrolyte level of the anode chamber. In another alternative, it is preferred to determine the electrolyte level in the cathode chamber. Needless to say that construction materials used should be resistant to electrolyte and H₂ if the cathode level is analyzed (PTFE materials are for example suitable) and resistant to electrolyte and F₂ if the anode level is analyzed (for example, ceramic materials could be used).

A preferred way to determine the electrolyte level is to press an inert gas into the tube such that some inert gas leaves the tube. The pressure needed to displace the electrolyte in the tube is an indication of the height of the electrolyte level : the lower the level, the lower the pressure needed to displace the liquid in the tube, and accordingly, the lower the pressure difference to the gas space in the anode compartment and the cathode compartment. Depending on the respective tube, cell etc, a calibration between measured pressure difference and electrolyte level can be made. If necessary or desired, as indicated by the level determination, electrolyte can be supplied manually or automatically. Pressure differences might be determined, or the absolute pressure.

The electrolyte level may be determined in the cathode chamber or in the anode chamber. Often, the electrolyte level measurement will be performed in the anode chamber. N₂ is a preferred inert gas, especially for the measurement in t he cathode chamber, F₂ could also be used in the anode chamber; this preferred because hereby no contamination of the F₂ gas in the anode chamber can occur. If the liquid level is determined in the cathode chamber, N₂ or H₂ are preferred inert gases. Other inert gases could be used as well.

It is also possible to apply HF as inert gas for liquid level determination in both compartments but the result may not be as exact as if F₂ or N₂ were used.

It has to be noted that the method of the invention can be applied in electrolyzers having one or more anodes and one or more cathodes with respective chambers, and in apparatus having one or more anodes, and wherein the electrolyzer vessel is used as the cathode. The term "chamber" is intended to include the term "compartment". The method, as indicated above, allows the determination of any desired electrolyte level ; it is not limited to determine level stages. Still, this method of the invention still suffers a disadvantage which is described below.

It has been observed that when pressure sensing means are used to measure and control the level of the electrolyte, problems may arise due to the changing composition over time of the electrolyte salt of a molten HF adduct of KF, wherein the range of HF is usually varying according to the

formula KF (1.8-2.3) HF, as mentioned before. When the composition of the molten HF adduct of KF changes, e.g. when the content of HF in the salt according to the formula KF-(1.8-2.3) HF drops below the value 2, the melting point is getting relatively high and may cause crystallization of the adduct salt inside the submerged pressure measuring tube (the crystallized salt is also denoted as "frozen" salt) ; splashing electrolyte which then crystallizes also poses problems in the tube. As a consequence, the pressure measurement and the liquid level control of the electrolytic bath is impaired by such crystallization upon reducing of the diameter inside the tube and in particular when plugging of the tube occurs.

Therefore, a further objective of the present invention is to provide an improved method of measuring and/or controlling the electrolyte level by pressure sensing and controlling means in an electrolytic cell for generating fluorine, wherein the electrolyte salt is a molten HF adduct of KF. The improved method shall also allow for operating the electrolytic cell with reliable, and in particular continuous, liquid level measuring and/or controlling by pressure sensing and/or controlling means, in an electrolysis for generating (highly) pure fluorine gas stably and safely, e.g. in that the undesired crystallization of the HF-KF adduct salt inside the liquid level measuring and/or controlling tube can be minimized and plugging of the tube can be prevented.

The objective of the invention is achieved by the second embodiment of the invention which includes modifying the method of measuring and/or controlling the electrolyte level by pressure sensing and/or controlling means in an electrolytic cell for generating fluorine, wherein the electrolyte salt is a molten HF adduct of KF. The improved method of measuring and/or controlling the electrolyte level by pressure sensing and/or controlling means according to the second embodiment of the present invention shall be depicted by Figure 1 , and Fig. 2 depicts the first embodiment of the method of the invention.

The second embodiment of the invention as outlined in Fig. 1, i.e. the method including the improvement according to the present invention is a highly preferred embodiment, and, referring to the drawings, is further described in the following.

Brief description of the drawings

Figure 1 demonstrates the second embodiment, i.e. the improved method of measuring and/or controlling the electrolyte level by pressure sensing and/or controlling means in an electrolytic cell according to the present invention.

Figure 2 describes the first embodiment, i.e. the method of measuring and/or controlling the electrolyte level by pressure sensing and/or controlling means in an electrolytic cell by means of applying inert gas, e.g. nitrogen, for measuring a difference in pressure. The valve marked by an "X" is no longer necessary.

Detailed description of the invention

According to the first embodiment of the invention, (see Fig. 2) in an electrolyte level measurement from time to time nitrogen, or a suitable noble gas (e.g. helium or argon), or, as stated above, F₂ or H₂, respectively, or even HF, is applied to the electrolytic cell with a certain excess pressure by means of a tube reaching into the electrolyte. A difference in pressure, depending on the height of the electrolyte in the electrolytic cell, is detected as an indicator of the electrolyte level. In this constitution weekly up to daily blockage of the tube used for the pressure measuring may be caused by frozen HF adduct of KF if the HF content in the salt according to the formula KF-(1.8-2.3) HF drops below the value 2. Such frozen salt cannot be simply removed by blowing nitrogen gas through the tube. Therefore, from time to time the solidified electrolyte, e.g. the frozen HF adduct of KF must be removed by scratching what is quite

inconvenient.

The objective of the improved method of the invention (see Fig. 1) is achieved by a modified method of measuring and/or controlling the electrolyte level by pressure sensing and/or controlling means in an electrolytic cell for generating fluorine, wherein the electrolyte salt is a molten HF adduct of KF, the modified method according to the invention being characterized in that in addition to the nitrogen, or any other suitable inert gas, with a certain excess pressure also hydrogen fluoride (HF) is applied to the electrolytic cell by means of the same tube reaching into the electrolyte which is used for measuring and/or controlling the electrolyte level.

The word "inert gas" in the context of the invention is meant to designate a gas (which, if desired, may be a mixture of gases) that does not interfere, e.g. that does not chemically react, with the electrolyte and any product resulting from the electrolysis.

In particular, the objective of the invention is achieved by a method of measuring and/or controlling the electrolytic bath liquid level in an electrolysis cell for the generation of fluorine gas having at least one anode chamber and at least one cathode chamber separated from each other by a partition wall for generating fluorine gas by electrolyzing an electrolytic bath (electrolyte) comprising a hydrogen fluoride-containing mixed molten salt of an HF adduct of KF, wherein the method comprises the steps of (a) detecting the pressure in at least one of the anode chamber and the cathode chamber during generation of fluorine gas by pressure sensing means comprising a tube (liquid level sensing tube) reaching into the electrolytic bath (electrolyte), and (b) supplying through the tube of the pressure sensing means an effective amount of hydrogen fluoride to thereby remove and/or prevent plugging caused by potentially crystallized HF adduct of KF in the tube of the pressure sensing means at any time and/or during measuring and/or controlling the electrolytic bath (electrolyte) liquid level in the anode chamber and/or cathode chamber.

As mentioned above, other types of electrolyzer apply one or more anodes and use the vessel of the electrolyzer as cathode. Often, 20 anodes or more, up to 30 and even more, are contained in an electrolytic cell. The space around each anode is separated from the surrounding liquid e.g. by "skirts" to prevent formed F₂ to dissipate in the electrolyte or to arrive in the cathode compartment in gaseous form because the recombination of F₂ and H₂ in the cathode compartment to form HF causes a strong and dangerous chemical reaction. The gas space in such an anode compartment is formed from F₂ which is withdrawn and collected for further purification, storage or delivery. An especially suited apparatus of this type is described in international patent application

WO 2012/066054 (filing N° PCT/EP 2011/070286, filed November 16, 2011) the whole content of which is incorporated herein by reference.

The word "a", e.g. in an expression like "a step", is not intended to limit the expression to a single step. The term "comprising" includes the meaning "consisting of.

The word "tube" means a hollow line with an inner diameter that allows gaseous and/or liquid substances flowing through, and may be used in the art synonymously with the word "pipe" or "line".

In a preferred embodiment the invention concerns a method of measuring and/or controlling the electrolytic bath liquid level in an electrolysis cell for the generation of fluorine gas as defined here above, wherein the molten salt electrolyte is a molten HF adduct of KF, preferably a molten HF adduct of KF with a range of HF according to the formula KF-(1.8-2.3) HF.

The use of hydrogen fluoride (HF) according to the present invention for removing and/or preventing any blockage, which is caused by potentially crystalized HF adduct of KF in the tube of the pressure sensing means, is a simple and advantageous measure, because notably hydrogen fluoride (HF) is used anyway as feed material in the electrolysis for generating fluorine gas. It may therefore be drawn from the same storage containers which are used for the HF supply in the manufacturing of fluorine by electrolysis. By supplying an effective amount of hydrogen fluoride (HF) through the tube of the pressure sensing means any potentially frozen HF adduct of KF in the tube itself and/or in the neighborhood of the tube may be effectively dissolved or freezing of the HF adduct of KF may be effectively prevented by the local increase of HF. The words "effective" or "effectively" in this context of the invention have the meaning that the HF content in the salt according to the formula KF-(1.8-2.3) HF increased to a value of equal or greater than 2, in particular in the tube itself and/or in the neighborhood of the tube. Thereby, according to the invention, such frozen salt can be simply removed and/or prevented at any time and/or during the measuring and/or controlling the electrolytic bath (electrolyte) liquid level in the anode chamber and/or cathode chamber.

The hydrogen fluoride (HF) can be supplied to the liquid level sensing tube either as a liquid HF or as a gaseous HF. "Hydrogen fluoride" (HF) is understood to denote in particular anhydrous hydrogen fluoride. When included in a storage container, the hydrogen fluoride is generally liquid. Thus, in case of liquid HF supply the HF can be directly drawn in the required quantity from the storage container and be transferred to liquid level sensing tube, e.g. by pumping or simply by applying pressure to the container and pressing the HF into the liquid level sensing tube. In case of a gaseous HF supply, the storage container is additionally equipped with an evaporator, and the liquid HF then is evaporated from the storage container and transferred to the liquid level sensing tube.

Furthermore, the method according to the invention - steps (a) and (b) of the method - may be applied to either type of chamber of the electrolytic cell, e.g. it may be applied equally to the anode chamber (fluorine generating side) and to the cathode chamber (hydrogen generating side). Preferred is the anode chamber (fluorine generating side), and in this case the nitrogen or noble gas for the liquid level measurement may be optionally replaced by fluorine gas. If the invention is applied to the cathode chamber (hydrogen generating side), the in the in steps (a) and (b) of the method according to the invention, in this case the nitrogen or noble gas for the liquid level measurement may be optionally replaced by hydrogen gas.

Before describing the present invention in more detail, the invention shall be first put into context of an electrolytic cell for generating fluorine, wherein the electrolyte salt is a molten HF adduct of KF. Fluorine gas is produced in an electrolytic cell comprising an anode chamber and a cathode chamber. The electrolytic cell body is generally made of Ni, Monel, carbon steel or the like or other materials resistant toH₂, F₂, KF and HF. At the bottom of the electrolytic cell body, a bottom plate made of nickel or polytetrafluoroethylene or the like is disposed for preventing the hydrogen gas generated at the electrically conductive bottom from entering an anode compartment, being mixed with F₂ and entering into the reaction forming HF. The electrolytic cell body is filled with an electrolytic bath, namely a potassium fluoride-hydrogen fluoride system (herein usually referred to as "KF-HF adduct" or the like) in the form of a molten salt. The cell or bath is divided into at least one anode chamber or anode compartment and at least one cathode chamber or compartment by means of a skirt made of Monel or the like. Each cell preferably contains several anodes, typically 20 to 30, which may be, for example, made from nickel, carbon, sintered material, diamond-coated anodes or comparable materials, but usually are made from carbon. Upon applying a voltage between a carbon or nickel anode contained in the anode chamber and a nickel cathode contained in the cathode chamber, electrolysis occurs and fluorine gas is produced. The fluorine gas generated is discharged through a product line, and the hydrogen gas formed on the cathode side is discharged through a hydrogen gas discharge line.

Slight fluctuations in liquid level can be detected, and the anode chamber inside and cathode chamber inside can be maintained at atmospheric pressure or preferably slightly above atmospheric pressure by means of the pressure maintenance means. The pressure maintenance means in the fluorine gas generator usually comprises automatic valves operated (opened/closed) in association with pressure gauges connected to the anode chamber and cathode chamber, respectively, and automatic valves operated in association with the level sensing means disposed in the anode chamber and cathode chamber, respectively. This constitution makes it possible to control the electrolytic cell inside pressure in an easy and reliable manner. The operation of the automatic valves in association with the level sensing means and the supply of HF through the tube of the pressure sensing means an effective amount of hydrogen fluoride makes it possible to automatically control the level of the electrolytic bath.

Thus, a continuous and maintenance free electrolyte level measurement is easily possible in a reliable manner at any time and/or during measuring and/or controlling the electrolytic bath (electrolyte) liquid level in the anode chamber and/or cathode chamber. According to the invention, it is possible to detect the electrolytic bath liquid levels in the electrolytic cell at any time and/or during measuring and/or controlling the electrolytic bath (electrolyte) liquid level in the anode chamber and/or cathode chamber, and therefore, to detect even small changes in liquid level.

According to the embodiments of the invention, the method comprises detecting the electrolytic cell inside pressures by the pressure sensing means, e.g. the method comprises the step of (a) detecting the pressure in at least one of the anode chamber and the cathode chamber during generation of fluorine gas by pressure sensing means comprising a tube (liquid level sensing tube) reaching (submersing) into the electrolytic bath (electrolyte).

The determination can be performed at a pressure lower than ambient pressure, at ambient pressure and at a pressure which is higher than ambient pressure because the principle is the determination of a pressure difference.

Therefore, according to one embodiment of the invention, a noble gas, e.g. a noble gas such as helium gas (He gas), neon gas (Ne gas), argon gas (Ar gas) or krypton gas (Kr gas), or another inert gas, e.g. nitrogen gas (N₂ gas), or hydrogen gas (H₂) to the cathode chamber or cathode chambers, or fluorine gas (F₂ gas) to the anode chamber or chambers, is fed to the cathode chamber or/and anode chamber through automatic valves to provide a certain over pressure in the respective chambers. Other gases which do not react or interfere with the electrolyte or the produced gases might be applied, too. In a preferred embodiment of the invention the electrolysis is performed at a pressure slightly above atmospheric pressure, and then said noble gas or preferably nitrogen might be permanently applied to one of the respective chambers, e.g. the cathode chamber or anode chamber. Alternatively or additionally, the F₂ and H₂ produced during electrolysis may provide pressure. It has to be noted that said inert gases may cause undesired impurities especially in the F₂ produced.

This constitution may make it possible to more precisely control the liquid level fluctuations of the electrolyte due to the differential pressure-caused ascending or descending of the electrolytic bath and prevent the choking of filters or the like disposed in the downstream piping and lines due to splashing of the electrolytic bath, for instance.

This control makes it possible to ensure the operation in a safe and stable manner and to detect even small changes in liquid level. In the following, the present invention shall be described in more detail in the light of preferred embodiments and variants of measuring and/or controlling the liquid level of the electrolyte bath in the electrolytic cell for the generation of elemental fluorine.

The method of the invention comprising detecting the electrolytic cell inside pressures by the pressure sensing means, and then computing from the observed differential pressure the liquid level in the electrolytic cell, comprises the step of (a) detecting the pressure in at least one of the anode chamber and the cathode chamber during generation of fluorine gas by pressure sensing means comprising a tube (liquid level sensing tube) reaching into the electrolytic bath (electrolyte). The pressure may be detected in the anode chamber of the electrolytic cell, e.g. meaning the compartment wherein the fluorine is generated ("F₂-side"); but it is preferred to determine the pressure in the cathode compartment Thus, a preferred embodiment provides for a method of measuring and/or controlling the electrolytic bath liquid level in an electrolysis cell for the generation of fluorine, wherein the tube (liquid level sensing tube) reaching (submersing) into the electrolytic bath (electrolyte) is for pressure detection in the cathode chamber of the electrolytic cell ("H₂-side").

The invention is also directed to a method of measuring and/or controlling the electrolytic bath liquid level in an electrolysis cell for the generation of fluorine gas, wherein the electrolytic bath (electrolyte) liquid level is determined by measuring the differential pressure (delta-P) by means of a noble gas or nitrogen gas or any mixture thereof, preferably by means of a nitrogen (N₂) gas, which is pressed into the electrolytic cell by means of the tube (liquid level sensing tube) reaching (submersing) into the electrolytic bath (electrolyte). For measuring the differential pressure (delta-P) one of the above mentioned noble gases or nitrogen gas or any mixture thereof, preferably nitrogen (N₂) is pressed into the electrolytic cell by means of the tube (liquid level sensing tube) reaching into the electrolytic bath (electrolyte), under condition without pressure compensation ; under this condition the valve for pressure compensation between the tube and the gas zone above the liquid electrolyte in the electrolytic cell is closed (see Fig. 2) or is preferably even absent. Thereby, a pressure Pi is built up in the tube, and depending on the actual liquid level in the electrolyte and the pressure P₂ of the gas zone above the liquid electrolyte in the electrolytic cell, the differential pressure (delta-P, Δ-Ρ) is determined. By experience, the skilled person may then compute from the delta-P the liquid level of the electrolyte in the cell. Often, the computation is performed automatically by a Process Control System ("PCS"), or a Digital Control System ("DCS"). After the measurement, the pressure in the electrolytic cell is compensated and the cell returns to the previous condition before the measurement. However, by the pressure compensation the electrolyte may be heaved up into the tube and there is the risk that the molten electrolyte salt crystallizes within the tube and may subsequently lead to undesired plugging of the tube. It is possible to continuously supply N₂ pressure gas in a quantity that a slight amount of N₂ is permanently bubbling into the electrolyte, thereby trying to keep the tube free, but even with minimal amount of N₂ it cannot be prevented that little electrolyte sputters around and nevertheless causes crystallization and eventually plugging of the tube.

According to the invention it is foreseen to pass hydrogen fluoride together with nitrogen (HF + N₂) - which is preferred - or separately (only HF) through the tube. To prevent the backflow of electrolyte into the supply line when supply of HF is stopped, supply of N₂ can be continued. The HF tends to dissolve incrustations of the KF-HF adduct. Now, when the method according to the present invention is applied, e.g. if according to step (b) of the present invention an effective amount of hydrogen fluoride (HF) is supplied through the tube of the pressure sensing means hydrogen fluoride thereby it is possible to remove and/or prevent plugging caused by potentially crystalized HF adduct of KF in the tube of the pressure sensing means. The presence of N₂ or the supply of hydrogen fluoride (HF) has the further advantage that, in case liquid HF is used, the conventional pressure compensation as described here above is not obligatory. In case of liquid HF supply to the tube the pressure compensation can be effectuated by the added HF itself (see Fig. 1). Therefore, the pressure compensation valve shown in Fig. 2 may be eliminated, if simplification of the electrolytic cell and reduction of maintenance thereof is wished, for example in on-demand and on-site operation, where automatic and safe control of the electrolytic bath level in the electrolytic cell is indispensable, but manual maintenance should be minimized as much as possible by simplifying the electrolytic cell and the electrolytic process.

Although, it is possible to operate the invention with continuous supply of an effective amount of hydrogen fluoride in step (b) through the tube of the pressure sensing means, in a preferred embodiment the invention the method of measuring and/or controlling the electrolytic bath liquid level in an electrolysis cell for the generation of fluorine gas is operated in a manner, wherein the effective amount of hydrogen fluoride is supplied only periodically. In this embodiment of the invention, the hydrogen fluoride is preferably supplied together with nitrogen (HF + N₂); if the supply of N₂ continues after stopping the HF supply, the backflow of electrolyte into the supply line can be prevented. Alternatively, HF is supplied separately (i.e. only HF is supplied) through the tube (liquid level sensing tube) reaching (submersing) into the electrolytic bath (electrolyte). This, this embodiment of the method of measuring and/or controlling the electrolytic bath liquid level in an electrolysis cell for the generation of fluorine gas provides that the supply of N₂ continues when the supply of HF is interrupted.

The period (intervals) of supplying the effective amount of hydrogen fluoride may even be periods of from a few weeks to a weekly period

(e.g. seven days), a period of from several days (e.g. seven days) to daily period (e.g. 24 hours), a period of from several hours (e.g. 24 hours) to one hour (e.g. 60 minutes), or a period of from several minutes (e.g. 60 minutes) to one minute (e.g. 60 seconds), or even a period of from several seconds to a few seconds. The period or interval, respectively depends on the actual need of (b) supplying through the tube of the pressure sensing means an effective amount of hydrogen fluoride to thereby remove and/or prevent plugging caused by potentially crystalized HF adduct of KF in the tube of the pressure sensing means at any time and/or during measuring and/or controlling the electrolytic bath (electrolyte) liquid level in the anode chamber and/or cathode chamber, in the electrolytic cell under operation.

The word "few" in this context means that the duration of the respective period is greater than one, e.g. four, but usually not more than three or preferably two, if the period is measured in weeks ; or, if the period is measured in seconds, it means more than one, e.g. at least five, but usually at least ten or preferably at least 20, and more preferred about thirty (30) seconds.

As a matter of example, but without limitation, a period of one week or more preferably a daily period is recommendable, if the period is measured in weeks or days. If the period is measured in hours, shorter intervals of e.g. 1 to 5 hours, preferably 1 to 4 hours, more preferably 1 to 3 hours, and even more preferably 1 to 2 hours, are recommendable. If the period is measured in minutes, shorter intervals of e.g. 1 to 60 minutes, 1 to 50 minutes, 1

to 40 minutes, 1 to 30 minutes, 1 to 20 minutes, or 1 to 10 minutes are practically useful intervals ; preferably shorter intervals can be chosen, e.g. intervals of 1 to 5 minutes, preferably 1 to 4 minutes, more preferably 1 to 3 minutes, and even more preferably 1 to 2 minutes, and most preferably an interval of about one minute, are very recommendable. If the period is measured in seconds, still shorter intervals of e.g. 1 to 60 seconds, 1 to 50 seconds, preferably 20 to 50 seconds, 1 to 40 seconds, preferably 20 to 40 seconds, 1 to 30 seconds, preferably about 30 seconds, 1 to 20 seconds, or 1 to 10 seconds are practically useful and recommendable intervals ; even shorter intervals can be chosen, e.g. intervals of 1 to 5 seconds, 1 to 4 seconds, 1 to 3 seconds, 1 to 2 seconds, and an interval of about one second, but these very short intervals are less preferred.

In the more preferred embodiments of the invention the intervals are at least a weekly period, preferably a daily period, more preferably any period indicated above measured in minutes, and even more preferably any preferred period indicated above measured in seconds.

In the even more preferred embodiments of the invention the intervals are periods of 1 to 5 minutes, preferably 1 to 4 minutes, more preferably 1 to 3 minutes, and even more preferably 1 to 2 minutes, and most preferably an interval of about one minute.

In the even more preferred embodiments of the invention the intervals are periods 20 to 50 seconds, more preferably 20 to 40 seconds, and most preferably about 30 seconds.

According to the invention it is possible in step (b) to supply an effective amount of hydrogen fluoride through the tube of the pressure sensing means at any time and/or during measuring and/or controlling the electrolytic

bath (electrolyte) liquid level. Thus, it is possible to pass hydrogen fluoride together with nitrogen (HF + N₂) or separately (only HF) through the tube. In a preferred embodiment of the invention hydrogen fluoride (HF) and the nitrogen (N₂) for the pressure measurement are passed separately through the tube, e.g. meaning that the nitrogen (N₂) of step (a) and the hydrogen fluoride (HF) of step (b) are applied at different times or periods, e.g. such periods (intervals) as described here before. In this preferred embodiment the invention concerns in particular a method of measuring and/or controlling the electrolytic bath liquid level in an electrolysis cell for the generation of fluorine gas, wherein the steps of (a) detecting the pressure in at least one of the anode chamber and the cathode chamber and (b) supplying an effective amount of hydrogen fluoride are performed at different times, and preferably steps (a) and (b) are performed in an alternating mode. The alternating mode according to the invention is very advantageous, because it allows for more smooth conditions in the electrolysis and thus provides the opportunity of a practical realization of a continuous ("permanent") and maintenance free electrolyte level measurement. If short alternating periods are chosen, it has also a beneficial effect on the quality of the generated fluorine, because the level of impurities like 0₂, OF₂ and/or CF₄ is kept low due to the more stable and homogenous operation of the electrolysis. For example, the 0₂ and OF₂ concentrations oscillate only with a low amplitude and CF₄ level is kept below 20 ppmv at hour periods and can be even lower at minute periods.

The alternating periods of the steps (a) and (b) of the invention may be the same or differ from each other. For each of the steps (a) and (b) the

periods (intervals) indicated above may individually apply. Preferably, in the steps (a) and (b) of the method according to the present invention the

periods (intervals) indicated above are of approximately the same duration. Thus, in this preferred embodiment of the invention concerning the alternating mode both intervals of step (a) and (b) any period indicated above measured in minutes, and even more preferably any preferred period indicated above measured in seconds.

In the even more preferred embodiments of the invention with alternating mode of the intervals of step (a) and (b) the intervals are periods of 1

to 5 minutes, preferably 1 to 4 minutes, more preferably 1 to 3 minutes, and even more preferably 1 to 2 minutes, and most preferably an interval of

about one minute. In the even more preferred embodiments of the invention with alternating mode of the intervals of step (a) and (b) the intervals are periods 20 to 50 seconds, more preferably 20 to 40 seconds, and most preferably

about 30 seconds.

Thus, as an example but without limitation, in the method according to the invention of measuring and/or controlling the electrolytic bath liquid level in an electrolysis cell for the generation of fluorine gas having an anode chamber and a cathode chamber separated from each other by a partition wall for generating fluorine gas by electrolyzing an electrolytic bath (electrolyte) comprising a hydrogen fluoride-containing mixed molten salt of an HF adduct of KF, the step of (a) detecting the pressure in at least one of the anode chamber and the cathode chamber during generation of fluorine gas by pressure sensing means comprising a tube (liquid level sensing tube) reaching (submersing) into the electrolytic bath (electrolyte) is a period of 20 to 40 seconds, preferably a period of about 30 seconds, and the step of (b) supplying through the tube of the pressure sensing means an effective amount of hydrogen fluoride, to thereby remove and/or prevent plugging caused by potentially crystalized HF adduct of KF in the tube of the pressure sensing means, is a period of 20 to 40 seconds, preferably a period of about 30 seconds.

The before described embodiments of the invention are particularly useful if they are combined with another preferred embodiment of the invention which makes use of a pressure sensing means comprising a tube (liquid level sensing tube) reaching (submersing) into the electrolytic bath (electrolyte), wherein the tube is specified by a flow orifice with a specific inner diameter.

The inner diameter for a given cell depends on the inlet pressure with which the HF is supplied through the orifice, the pressure in the cell and the production capacity of the cell. At a high HF inlet pressure, the diameter of the orifice may be smaller than at a lower HF inlet pressure.

A plant to which HF is supplied in an amount of below 20 kg/h, and often is between 7 and 9 kg/h as described in detail below, relates to a preferred embodiment. Accordingly, in this preferred embodiment of the invention, the tube (liquid level sensing tube) reaching into the electrolytic bath (electrolyte) is characterized by a flow orifice in the tube of about 2.5 mm (inner) diameter. The term "about" in this context means that the value of 2.5 mm may slightly vary, e.g. of at maximum ± 0.1 to ± 0.25 mm, preferably by ± 0.1 to ± 0.2 mm. Thus, the flow orifice diameter may be characterized by as follows : 2.5 ± 0.25 mm, 2.5 ± 0.2 mm, or 2.5 ± 0.1 mm, and the like. It has to be noted that the orifice may vary depending on the amount of HF to be supplied per hour and the inlet pressure of the HF. For an hourly supply of more HF than below 20 kg/h, an orifice with larger diameter may be useful. For the supply of HF at a much lower rate than e.g. 6 kg/h, an orifice with a smaller diameter may be

advantageous. For an orifice of 2.5 mm, the cross sectional area is

approximately 5 mm². The cross sectional area should correlate to the amount of HF which is passed through it per hour. The relation between HF feed per hour and cross sectional area is assumed to be linear. It is assumed that if the amount of HF per hour is doubled, an orifice with a cross sectional area which also is twofold greater should be used. It is assumed that orifices with a diameter in the range of from 1 mm to 1 cm can be applied in supply lines. Nevertheless, orifices with a smaller or greater diameter could be used, too if necessary. In a further preferred embodiment of the invention the tube (liquid level sensing tube) reaching (submersing) into the electrolytic bath (electrolyte) in addition to its function according to step (a) of the method of the invention, it may serve at the same time as the (main) hydrogen fluoride feed supply line for the electrolysis to generate elemental fluorine. Then, according to this embodiment of the invention there is no need for a separate HF feed line and a separate liquid level sensing tube. This embodiment of the invention is particularly useful in combination with a liquid level sensing tube characterized by a flow orifice in the tube of about 2.5 mm (inner) diameter and the above described method according to the invention, wherein the step of (a) detecting the pressure in at least one of the anode chamber and the cathode chamber during generation of fluorine gas by pressure sensing means comprising a tube (liquid level sensing tube) reaching (submersing) into the electrolytic bath (electrolyte) is operated for a period of 20 to 40 seconds, preferably a period of

about 30 seconds, and the step of (b) supplying through the tube of the pressure sensing means an effective amount of hydrogen fluoride, to thereby remove and/or prevent plugging caused by potentially crystallized HF adduct of KF in the tube of the pressure sensing means, is operated for a period of 20

to 40 seconds, preferably a period of about 30 seconds. By this embodiment the invention provides a method for the manufacture of F₂ by electrolysis of HF contained in an electrolyte wherein F₂ and H₂ are formed and consumed HF is replenished with a supply of fresh HF wherein the supply of fresh HF is limited to an HF flow of at most 10 kg/h of HF per ton of electrolyte. Preferably, the HF flow is limited to at most 5 kg/h of HF per ton of electrolyte. The term "HF flow" is interchangeable with the term "HF feeding quantity per hour". The term "ton" in the frame of the present invention refers to metric tons.

It has to be noted that commonly used apparatuses for the electrolytic manufacture of F₂ (with H₂ produced as side product in essentially equimolar amounts) have a consumption of about 1 to 3 kg of HF per hour and per ton of electrolyte. Thus, an apparatus with a capacity of 2 tons of electrolyte must be supplied with 2 to 6 kg of fresh HF per hour to replenish consumed HF. Often, the consumption in such an apparatus having a capacity of about 2 tons of electrolyte is from 3 to 6 kg of HF per hour.

Thus, as already mentioned above, a continuous and maintenance free electrolyte level measurement is easily possible in a reliable manner at any time and/or during measuring and/or controlling the electrolytic bath (electrolyte) liquid level in the anode chamber and/or cathode chamber. In addition to the advantages in the liquid level control, and the removing and/or preventing the plugging of the tube (liquid level sensing tube) reaching into the electrolytic bath (electrolyte), the invention provides as further advantages that the impurity concentrations of 0₂ and OF₂ are low and oscillate with low amplitude. Also, by the invention automatic an HF feed system is realizable and an

approximate ("rough") HF feed measuring (differential pressure during HF feed) becomes possible, especially by adapting the liquid level sensing tube to the quantitative needs of supplying hydrogen fluoride as feed material in a given time, e.g. the required amount of hydrogen fluoride in kg/h, to an electrolytic cell (electrolyzer) for (electrolytically) generating elemental fluorine from a molten salt electrolyte in relation to the capacity of the electrolytic cell.

In the following, the present invention shall be described in more detail in the light of preferred embodiments and variants of measuring and/or controlling the liquid level of the electrolyte bath in the electrolytic cell for the generation of elemental fluorine, wherein the step (b) of supplying HF is performed in addition to the function in the liquid level measuring and/or controlling, also as the HF feed supply for the electrolytic generation of fluorine, e.g. meaning that the liquid level sensing tube reaching (submersing) into the electrolyte is also used as the single HF feed supply line.

According to this embodiment of the invention the HF consumed in the electrolysis is compensated by adding the required amount of fresh HF in a more smooth way during more beneficial intervals. The word "smooth" is e.g.

intended to mean less, potentially negative, interference with physicochemical process parameters or the electrolyte, thus preserving for more stable conditions in the electrolysis during the HF feeding, e.g. retaining the temperature and the composition of the electrolyte more homogenous over room and time, while at the same time not negatively impacting the production efficiency of fluorine. The word "smooth" also incurs that formation of impurities is minimized or even can be prevented. Therefore, this embodiment of the invention provides the overall advantages that, despite the addition of HF into the electrolytic cell, the temperature in can be kept more easily in a desired preset range and that the composition of the electrolyte stays more homogenous over room and time, e.g. throughout the electrolytic cell including the HF feeding or entry area. A further advantage is that formation of impurities can be minimized or prevented, thus yielding in fluorine which necessitates either only limited additional purification measures or yielding in fluorine with a purity that is ready for use in a process in which the fluorine shall be used.

Often, the HF supply lines are constructed for a much higher flow of HF for replenishment. For example, supply lines are constructed such that up to 80 kg/h of HF, and even more, could be introduced into the electrolytic apparatus. The amount of HF to be supplied per hour can simply be adapted by using appropriate lines. Lines with an inner diameter of 8 mm are sufficient to provide 80 kg/h of HF. But this is just mentioned as an example, without any intention of limitation. According to the invention, the maximal HF flow is considerably lower. Thus, the invention in this embodiment concerns a method for supplying hydrogen fluoride as feed material to an electrolytic cell

(electro lyzer) for (electro lytically) generating elemental fluorine from a molten salt electrolyte, which comprises supplying hydrogen fluoride (HF) via the liquid level sensing tube submersing into the electrolyte, which tube therefore also functions as the single HF feed supply line to the electrolytic cell, wherein the required amount of hydrogen fluoride feed for a defined fluorine production capacity is fed into the electrolytic cell with a, preferably gaseous, HF feeding quantity per hour (kg/h) over a defined feeding interval as described above, which HF feeding method therefore substantially differs from a full (maximum) HF feeding capacity of 80 kg/h, and even more, in short full HF feeding interval as applied in hitherto conventional methods.

Elemental fluorine in the context of this embodiment of the invention is produced by electrolysis using hydrogen fluoride (HF) as feed material for the electrolysis and source of the elemental fluorine. In the presence of an electrolyte salt, typically a molten HF adduct of KF preferably having the formula KF-(1.8-2.3) HF, HF releases fluorine if a voltage of at least 2.9 V is applied. Practically, the voltage is often kept in a range of 8 to 11 volt.

HF is advantageously supplied such that the level of electrolyte salt and HF in the respective cell does not exceed specific upper and lower levels.

Preferably, the electrolytic cell also includes sensors which determine the temperature in the cell, the level of liquid in the cell or cells, the pressures and pressure differences, the anode currents and voltages and gas temperatures. The cells are cooled with cooling water having a temperature of about 80 to 95°C.

In this embodiment the invention the required amount of hydrogen fluoride feed related to the fluorine production capacity of the electrolytic cell is in the range of a HF feeding quantity of 2-5 kg/h HF, preferably in the range of 3-4 kg/h HF in view of an apparatus having a capacity of 2 tons electrolyte.

Typically, in another embodiment the invention concerns a method for supplying hydrogen fluoride, wherein the HF feeding quantity per hour (kg/h) is below 20 kg/h, preferably below 15 kg/h, and more preferably below 10 kg/h, in view of an apparatus having a capacity of 2 tons electrolyte. In a variant of this embodiment of the invention the HF feeding quantity per hour (kg/h) is in the range of 1-20 kg/h, preferably in the range of 1-15 kg/h, and more preferably in the range of 1-10 kg/h, per 2 tons of electrolyte. In a preferred embodiment of the method of measuring and/or controlling the electrolytic bath liquid level in an electrolysis cell for the generation of fluorine gas, the method is applied in the manufacture of F₂ by electrolysis of HF contained in an electrolyte wherein F₂ and H₂ are formed and consumed HF is replenished with a supply of fresh HF wherein the supply of fresh HF is limited to an HF flow (HF feeding quantity) of at most 10 kg/h of HF per ton of electrolyte.

More specifically, the HF feeding quantity per hour (kg/h), depending on the pressure, is in the range of 2-10 kg/h, preferably in the range of 4-10 kg/h, and more preferably in the range of 6-10 kg/h, and even more preferably in the range of 7-9 kg/h, and most preferably the HF feeding quantity per hour is about 8 kg/h, per 2 tons of electrolyte. In this context the word "about" has the meaning that HF feeding quantity per hour can slightly vary around the value of 8 kg/h, e.g. being somewhat lower or somewhat higher than 8 kg/h. Thus the value may vary by about ± 0.5 kg/h, and consequently the HF feeding quantity then is preferably 8 ± 0.5 kg/h.

In this embodiment of the present the HF is fed, preferably as gas, into the electrolytic cell and the HF feeding quantity per hour (kg/h) is achieved by adjusting the diameter of the flow orifice in the liquid level sensing tube which simultaneously serves as the hydrogen fluoride supply line, preferably by means of a flow orifice of about 2.5 mm inner diameter. The term "flow orifice" means that the liquid level sensing tube which simultaneously serves as the hydrogen fluoride supply line, which itself in the section from the HF storage container to the location of the flow orifice and again thereafter in the section submersing into the electrolyte may have, and usually will have, a greater inner diameter than said 2.5 mm of the flow orifice. For the purpose of the invention it is fully sufficient that if the inner diameter of such an HF supply line with an inner diameter of greater than said 2.5 mm before and after the flow orifice, is reduced to a diameter of 2.5 mm at a location of the ending section of the HF supply line which is reaching into the electrolyte. This reduction of the inner diameter of the HF supply line may be achieved in the broadest sense by any suitable constructive means to reduce the inner diameter at a single point inside of a pipe, to result in an orifice with the required diameter.

The supply of hydrogen fluoride is preferably further characterized in that the HF feeding quantity (kg) and feeding interval (h) is regulated by an automatic valve. Typically, this automatic valve operates under preset conditions, optionally under modifying the HF feed parameters as appropriate from time to time during the process of manufacturing the fluorine, depending on the overall condition in the electrolytic cell as the electrolysis proceeds over time, e.g. depending on the temperature, the HF content, the level of the electrolyte, the current or any other relevant parameter in the electrolysis of the molten salt electrolyte, or the quality of the generated fluorine. Particularly, the HF feed quantity is, at least in addition to other means, also regulated by the above described method according to the invention of measuring and/or controlling the electrolytic bath liquid level. Preferably, the maximally possible HF flow per hour is greater than the total consumption of HF per hour. For example, the HF flow is 6 to 10 kg/h, and the HF consumed is 2 to 6 kg/h. In this case, the HF supply flow per hour may be greater than the actual

consumption, and thus, the supply of HF can be interrupted for a certain time range. During the time when no HF is supplied, it is advantageous to introduce an inert gas into the feed line for HF. In a preferred embodiment, the electrolyte level is determined during the time when no HF is supplied. The electrolyte level can, for example, be measured in a manner as described in the following paragraphs.

The liquid level sensing tube and HF supply line submerges into the molten electrolyte. Usually, the level of electrolyte in the submerged part of the supply line and the level of electrolyte surrounding the submerged line are essentially identical. When the HF supply is stopped and inert gas, as mentioned above, is pressed into the submerging liquid level sensing tube and HF supply line, essentially no electrolyte is in the submerging tube because the pressure of the inert gas prevents ingression of molten electrolyte. Depending on the level of the molten electrolyte, the pressure of the inert gas must be higher or lower to prevent ingression of the electrolyte into the submerged tube. It is possible to calibrate the pressure needed to prevent ingression of electrolyte, relative to the level of electrolyte in the cell. Depending on the determined pressure, the supply of HF can be regulated : if the level of electrolyte is comparably low, the HF flow may be regulated to a higher value, and/or the supply time may be extended ; and, if the level is comparably high, the HF flow may be regulated to a lower value, and/or the supply time may be shortened.

In a preferred embodiment, the ratio of HF flow in kg per hour and the HF consumption in kg per hour is between 1.2: 1 and 4 to 1. Preferably, it is between 1.5: 1 and 3: 1. Thus, it is possible to provide intervals wherein HF is supplied, and intervals wherein no HF is supplied, but the electrolyte level is determined as mentioned above by introducing an inert gas into the liquid level sensing tube and HF supply line. It is preferred to have a rather high frequency of HF supply and electrolyte level determination. For example, up to 30 periods of HF supply and correspondingly, up to 30 periods of electrolyte level determination can be foreseen per hour. In this case, depending on the ratio of HF flow and consumption, to give an example for a time ratio of 1.2: 1, HF supply would last for 33 seconds, followed by 27 seconds for making the level determination, followed by 33 seconds of HF supply, followed by 27 seconds of level determination and so on. This means that during the HF flow, an amount HF must be supplied which is approximately 1.9 times higher than would be supplied in a continuous HF supply mode without interruption. For a time ratio of 1 : 1 , e.g. if 30 seconds of HF supply would be interrupted for 30 seconds to determine the electrolyte level and so on, the amount of HF would be 2 times the amount supplied in a continuous mode without interruption. Of course, the expert can predetermine the duration of HF supply and level determination according to his will. The higher the frequency, the smoother the operation of F₂ production. The amount of HF consumed during both supply time and determination time is provided in the intervals of supply.

Of course, it is not necessary to determine the pressure in each of the intervals when no HF is supplied.

The pressure determination can be performed in the F₂ compartment of the cell or in the H₂ compartment. If it is performed in the F₂ compartment (which is preferred), N₂ or F₂ are preferred inert gases. If it is performed in the H₂ compartment (which is preferred), N₂ or H₂ are preferred inert gases.

It is even possible to use an automatic system regulating the amount of HF to be fed, the duration of HF feed and level determination depending on the data supplied by the level determination. In the context of the embodiments of the present invention the HF may supplied from any type of hydrogen fluoride storage containers. The hydrogen fluoride storage container can be a single hydrogen fluoride storage containers of varying sizes or it can be a hydrogen fluoride storage unit comprising a plurality of fixed or optionally transportable hydrogen fluoride storage containers which may be connected through a hydrogen fluoride supply line with the electrolyzer. An evaporator for evaporation of liquid HF may be provided with the hydrogen fluoride storage container or such an evaporator for evaporation of liquid HF is locally available at the plant where the HF is to be provided for the manufacture of fluorine, and can be connected to the hydrogen fluoride storage container or unit and to the hydrogen fluoride supply line. Other additional means, e.g. safety means like an HF sensor, an HF destruction system may be present at the location of the hydrogen fluoride storage container or unit, too. The HF storage containers used in the invention usually comprise an automatic HF level sensor. In particular the HF storage containers can be installed on weighing scales. In this embodiment, preferably, a process control system, in particular an automatic process control system is operable to close the remotely controlled valve of a first, empty HF container and to open the remotely controlled valve of another second HF-containing hydrogen fluoride storage container. This embodiment is particularly effective to avoid manual handling of HF valves and to ensure a continuous HF supply. Preferably, the valves are operable to close automatically in case of an anormal operation state, such as for example a process interruption in a process equipment connected to the HF supply line. Also, the valves are operable to close automatically in case of an HF leakage in the hydrogen fluoride supply unit according to the invention. Such HF leakage can for example be caused by a leakage of optional flange-connections inside the HF storage container. This avoids in particular the necessity to approach the hydrogen fluoride supply unit in this case.

Furthermore, the storage containers can be isolated from the HF supply line by double isolation valves having a closed isolation space. In that case, hydrogen fluoride supply unit according to the invention suitably further comprises at least one interspace vent valve in connection with one or more closed isolation space. The interspace vent valve is generally operable to remove optionally present hydrogen fluoride from the closed isolation space. Removal can be carried out, for example, by applying vacuum. Otherwise, the removal can be carried out, for example, by flushing the closed isolation space with an inert gas and/or a pressurized purging gas such as for example anhydrous air or, preferably, nitrogen. The removal can be carried continuously, but preferably, the removal is carried out discontinuously, in particular when an HF storage container is connected to and/or disconnected from the liquid level sensing and HF supply line. If appropriate, gases recovered from the closed isolation space are suitably vented to an HF destruction unit, for example a scrubber.

In an embodiment the present invention is characterized in that the elemental fluorine generated in the electrolytic cell is intended for the use in a method for the manufacture of electronic devices, preferably for the use as an etchant or use as a chamber cleaning agent in a method for the manufacture of electronic devices.

Preferably, such electronic devices are selected from the group consisting of semiconductors, photovoltaic cells, MEMS, and TFTs. In an embodiment of the invention the fluorine is used as chamber cleaning agent and etchant for the manufacture of electronic devices, especially semiconductors, photovoltaic cells, micro-electromechanical systems ("MEMS"), TFTs (thin film transistors for flat panel displays or liquid crystal displays), and the like. As to the use of fluorine as etchant for the manufacture of electronic devices, especially semiconductors, photovoltaic cells, MEMS and TFTs, several consecutive steps of deposition of layers and etching a part of them are necessary. Fluorine can be used for etching of layers constituted of very different constitution, for example, for etching silicon containing layers or other layers of compounds which form volatile reaction products, e.g. tungsten. Etching can be performed thermally or plasma- assisted. As to the use for chamber cleaning, usually, during deposition processes performed in treatment chambers - often CVD chambers (chambers wherein layers are deposited on items via chemical vapor deposition, e.g. plasma-enhanced CVD, metal organic CVD or low pressure CVD) - undesired deposits form on the walls and on inner constructive parts of the chamber and must be regularly removed. This is achieved by treating the deposits thermally or plasma-enhanced with elemental fluorine as chamber cleaning agent.

Especially for the use of elemental fluorine as an etchant, but also when used as chamber cleaning agent, it is desirable that the elemental fluorine be very pure. The intrusion of water, carbon dioxide, nitrogen and oxygen is considered as undesired. The elemental fluorine generated according to the present invention meets these quality requirements, as already explained in more detail above.

In yet another preferred embodiment the present invention is characterized in that the fluorine is generated "on site" or "over the fence" of a production plant, preferably for use in the method for the manufacture of electronic devices.

The fluorine can be manufactured, if desired, on site. This is a preferred embodiment of the invention. It can be produced in one or more satellite plants, e.g. in a fluorine generating cassette as described in WO 2004/009873. If desired, each cassette can be allocated to one or more process chambers wherein etching is performed ; or a plurality of fluorine generating cassettes is connected to a fluorine gas distribution system which is connected to the chambers. The inventive method for the low temperature purification can be integrated into the cassette. It can also be integrated into a plant according to the skid concept as described in PCT application WO 2012/034978 (filing N° PCT/EP2011/065773, filed September 12, 2011) the whole content of which is incorporated herein by reference.

After its manufacture and purification, the fluorine is delivered to the point of use. This is preferably performed under a pressure which is greater than ambient pressure.

In a preferred embodiment, the fluorine is pressurized by means of compressors, and no pressurizing gas is applied, unless elemental fluorine.

The step of storage, if foreseen, preferably denotes the storage of the elemental fluorine in suitable tanks, e.g. stainless steel bottles.

The fluorine is preferably generated on site of its point of use via electrolysis in an apparatus which is in fluid communication with the process chamber or process chambers. This means that the generated elemental fluorine is not filled into a tank or into pressurized bottles, which are then disconnected from the delivery line. If desired, the fluorine is stored in tanks or bottles only which remain connected to the delivery line. Often, the fluorine generator is located on the same plant as the tools wherein it is used, i.e. in a distance of less than 500 m from the manufacturing tools ; the generator often will be located near the tools, e.g. in a distance of 100 m or less from the tools. They can even be located in close proximity to the process chamber as the point of use, e.g. the distance can be 10 m or less.

The step of delivery preferably denotes passing the fluorine from the manufacturing apparatus to the point of use through pipes, especially through pipes which remain permanently connected to prevent intrusion of air into the fluorine, and to prevent fluorine to leak out.

In another aspect the invention also concerns an electrolytic cell for the generation of elemental fluorine by the electrolysis of a molten HF adduct of KF, wherein the electrolytic cell is equipped with a liquid level sensing tube which also serves as a hydrogen fluoride supply line connectable to a hydrogen fluoride supply unit, and wherein the liquid level sensing tube which also serves as a hydrogen fluoride supply line has a flow orifice with a diameter (this denotes an inner diameter) preferably in the range of from 1mm to 1cm, and especially preferably of about 2.5 mm (inner) diameter. The term "flow orifice" and the nature and means to achieve said inner diameter are already described above. Said description and explanations given above equally apply to this aspect of the invention which concerns an electrolytic cell for the generation of elemental fluorine by the electrolysis of a molten HF adduct of KF.

In a preferred embodiment of the present invention of an electrolytic cell for the generation of elemental fluorine by the electrolysis the fluorine is generated in a cassette, preferably in a cassette for producing fluorine "on site" or "over the fence" of production plants for electronic devices, preferably for electronic devices selected from the group consisting of semiconductors, photovoltaic cells, MEMS, and TFTs. The concept and nature and means to achieve "on site" or "over the fence" of production plants are already described above in the context of the method of supplying HF to an electrolysis cell for the manufacture of elemental fluorine. Said description and explanations given above equally apply to this aspect of the invention which concerns an electrolytic cell for the generation of elemental fluorine by the electrolysis of a molten HF adduct of KF.

To give an example of a very suitable cell of the present invention for the generation of elemental fluorine by the electrolysis the fluorine by the electrolysis of a molten HF adduct of KF, an electrolytic cell is provided for a daily (24 h) production capacity of 50 to 100 kg fluorine (F₂), preferably a daily (24 h) production capacity of 80 to 100 kg fluorine (F₂), and more preferably a daily (24 h) production capacity of about 80 to 90 kg fluorine (F₂). Other cells are also suitable, e.g. cells producing less F₂ or more F₂ per day. A preferred range of daily production is from 40 to 200 kg F₂ per day.

The method according to the first embodiment of the invention allows to determine any desired level of the electrolyte liquid with no limitation to stages ; the second embodiment additionally allows the determination for very long periods of time.

The following example is intended to explain the invention in detail without limiting it.

Example 1 : Liquid Level Detection with Supply of HF to Remove and/or

Prevent Plugging of the Liquid Level Measuring Tube, in an electrolytic cell for the manufacture of elemental fluorine

Conventionally, an electrolyte salt with a composition of about KF-2HF is filled into an electrolysis cell, heated to about 80 - 120°C and molten therein. A voltage of between 8 to 10 V is applied, and current is passed through the composition of electrolyte salt dissolved in the hydrogen fluoride ; the content of the cell is kept in a range of about 80 to 100°C. Elemental fluorine and elemental hydrogen form in the respective electrode compartments. The generated elemental fluorine is passed through a Monel metal frit to remove solids and pressurized by means of a compressor to about 10 Bar abs. and then passed through a trap cooled to -80°C ; in this trap, entrained HF condenses. The gaseous F₂ leaving the trap is and passed through a bed of NaF to remove any residual HF.

During the electrolysis, in certain periods, an amount of HF is introduced into the electrolytic cell through the liquid level sensing tube to remove and prevent further plugging of the tube during detection of the liquid level of the electrolytic bath. The HF supply is interrupted after a period of about 1 minute, and then the differential pressure in the electrolytic cell is determined

within 1 minute by pressing nitrogen into the liquid level sensing tube, located in the fluorine generating chamber of the electrolytic cell (anode) under condition without pressure compensation. Under this condition the valve for pressure compensation between the tube and the gas zone above the liquid electrolyte in the electrolytic cell is closed. A pressure Pi is built up in the tube, and depending on the actual liquid level in the electrolyte and the pressure P₂ of the gas zone above the liquid electrolyte in the electrolytic cell, a the differential pressure (delta-P) is determined. Then from the delta-P the liquid level of the electrolyte in the cell is computed. After the measurement, the pressure in the electrolytic cell is compensated and the cell returns to the previous condition before the measurement. The above liquid level detection may be repeated from time to time as appropriate as the electrolysis proceeds. Different periods may be applied for the above liquid level detection.

Example 2 : Liquid Level Detection with Supply of HF to Remove and/or Prevent Plugging of the Liquid Level Measuring Tube, in an electrolytic cell for the manufacture of elemental fluorine

Electrolysis is set up according to example 1. Further in accordance with example 1, during the electrolysis, a liquid level detection is performed, wherein a liquid level sensing tube with a flow orifice is used, wherein the inner diameter of the flow orifice is 2.5 mm and with a HF feeding quantity per hour of about 8 kg/h. The HF feeding quantity (kg) and feeding interval (h) is regulated by an automatic valve to provide a feed of 3 to 4 kg/h HF in view of the production capacity of the electrolytic cell (2 kg).

Example 3 : Alternating Liquid Level Detection and Feeding Gaseous HF into an electrolytic cell for the manufacture of elemental fluorine

An electrolyte salt with a composition of about KF-2HF is filled into an electrolysis cell, heated to about 80 - 120°C and molten therein. Gaseous HF is introduced into the electrolytic cell through an HF supply line wherein the inner diameter of the flow orifice is 2.5 mm and with a HF feeding quantity per hour of about 8 kg/h. The HF feeding quantity (kg) and feeding interval (h) is regulated by an automatic valve to provide a feed of 3 to 4 kg/h HF in view of the production capacity of the electrolytic cell (2 kg), depending on the alternating liquid level detection detailed further below. A voltage of between 8 to 10 V is applied, and current is passed through the composition of electrolyte salt dissolved in the hydrogen fluoride ; the content of the cell is kept in a range of about 80 to 100°C. Elemental fluorine and elemental hydrogen form in the respective electrode compartments. The generated elemental fluorine is passed through a Monel metal frit to remove solids and pressurized by means of a compressor to about 10 Bar abs. and then passed through a trap cooled to -80°C ; in this trap, entrained HF condenses. The gaseous F₂ leaving the trap is and passed through a bed of NaF to remove any residual HF.

During the intervals when HF supply is stopped, the electrolyte level is determined by pressing N₂ gas into the HF supply line. Depending on the determined electrolyte level, the supply of HF is reduced, increased or kept constant. In this example it is preferred to have a rather high frequency of HF supply and electrolyte level determination. The frequency is determined by the ratio of HF flow in kg per hour and of the HF consumption in kg per hour, which ratio in this example is a ratio of 1.8: 1. Therefore, up to 30 periods of HF supply and correspondingly, up to 30 periods of electrolyte level determination are performed per hour. In this example, with the time ratio of HF flow and consumption of 1.2: 1 , HF supply will last for 33 seconds, followed

by 27 seconds for making the level determination, followed by 33 seconds of HF supply, followed by 27 seconds of level determination, and so on.

In a variant of this example, for a ratio of 2:1, 30 seconds of HF supply would be interrupted for 30 seconds to determine the electrolyte level, and so on. Example4 : Alternating Liquid Level Detection and Feeding Gaseous HF into an electrolytic cell for the manufacture of elemental fluorine

An electrolyte salt with a composition of about KF-2HF is filled into an electrolysis cell, heated to about 80 - 120°C and molten therein. Gaseous HF is introduced into the electrolytic cell through an HF supply line wherein the inner diameter of the flow orifice is 2.5 mm and with a HF feeding quantity per hour of about 8 kg/h. The HF feeding quantity (kg) and feeding interval (h) is regulated by an automatic valve to provide a feed of 3 to 4 kg/h HF in view of the production capacity of the electrolytic cell (2 kg), depending on the alternating liquid level detection detailed further below. A voltage of between 8 to 10 V is applied, and current is passed through the composition of electrolyte salt dissolved in the hydrogen fluoride ; the content of the cell is kept in a range of about 80 to 100°C. Elemental fluorine and elemental hydrogen form in the respective electrode compartments. The generated raw elemental fluorine is passed through a Jet scrubber operated with liquid HF kept at about -75°C to -80°C to remove solids and a part of entrained HF. The partially purified F₂ is pressurized by means of a compressor to about 10 Bar abs. and then passed through a trap cooled to -80°C ; in this trap, entrained HF condenses. The gaseous F₂ leaving the trap is and passed through a bed of NaF to remove any residual HF.

During the intervals when HF supply is stopped, the electrolyte level is determined by pressing N₂ gas into the HF supply line. Depending on the determined electrolyte level, the supply of HF is reduced, increased or kept constant. In this example it is preferred to have a rather high frequency of HF supply and electrolyte level determination. The frequency is determined by the ratio of HF flow in kg per hour and of the HF consumption in kg per hour, which ratio in this example is a ratio of 1.8: 1. Therefore, up to 30 periods of HF supply and correspondingly, up to 30 periods of electrolyte level determination are performed per hour. In this example, with the time ratio of HF flow and consumption of 1.2:1, HF supply will last for 33 seconds, followed

Should the disclosure of any of the patents, patent applications, and publications that are incorporated herein by reference be in conflict with the present description to the extent that it might render a term unclear, the present description shall take precedence.

Claims

C L A I M S

1. A method of measuring and/or controlling the electrolytic bath liquid level in an electrolysis cell for the generation of fluorine gas having at least one anode chamber and at least one cathode chamber separated from each other by a partition wall for generating fluorine gas by electrolyzing an electrolytic bath (electrolyte) comprising a hydrogen fluoride- containing mixed molten salt of an HF adduct of KF, wherein the method comprises the steps of (a) detecting the pressure in at least one of the anode chamber and the cathode chamber during generation of fluorine gas by pressure sensing means comprising a tube (liquid level sensing tube) reaching (submersing) into the electrolytic bath (electrolyte).

2. The method of claim 1 of measuring and/or controlling the electrolytic bath liquid level in an electrolysis cell for the generation of fluorine gas having at least one anode chamber and at least one cathode chamber separated from each other by a partition wall for generating fluorine gas by electrolyzing an electrolytic bath (electrolyte) comprising a hydrogen fluoride-containing mixed molten salt of an HF adduct of KF, wherein the method comprises the steps of (a) detecting the pressure in at least one of the anode chamber and the cathode chamber during generation of fluorine gas by pressure sensing means comprising a tube (liquid level sensing tube) reaching (submersing) into the electrolytic bath (electrolyte), and (b) supplying through the tube of the pressure sensing means an effective amount of hydrogen fluoride to thereby remove and/or prevent plugging caused by potentially crystallized HF adduct of KF in the tube of the pressure sensing means at any time and/or during measuring and/or controlling the electrolytic bath (electrolyte) liquid level in the anode chamber and/or cathode chamber.

3. The method of claims 1 or 2 wherein an inert gas displaces the liquid in the tube, which method includes a step of determining the pressure difference between the inert gas and the gas space of the at least one of the anode chamber and cathode chamber.

4. The method of measuring and/or controlling the electrolytic bath liquid level in an electrolysis cell for the generation of fluorine gas according to claims 1 to 3, wherein the molten salt electrolyte is a molten HF adduct of KF, preferably a molten HF adduct of KF with a range of HF according to the formula KF (1.8-2.3) HF.

5. The method of measuring and/or controlling the electrolytic bath liquid level in an electrolysis cell for the generation of fluorine gas according to anyone of the claims 1 to 4, wherein the tube (liquid level sensing tube) reaching (submersing) into the electrolytic bath (electrolyte) is for pressure detection in the cathode chamber of the electrolytic cell ("H2-side").

6. The method of measuring and/or controlling the electrolytic bath liquid level in an electrolysis cell for the generation of fluorine gas according to anyone of the claims 1 to 4, wherein the tube (liquid level sensing tube) reaching (submersing) into the electrolytic bath (electrolyte) is for pressure detection in the anode chamber of the electrolytic cell ("F₂-side").

7. The method of measuring and/or controlling the electrolytic bath liquid level in an electrolysis cell for the generation of fluorine gas according to anyone of the claims 1 to 5, wherein the electrolytic bath (electrolyte) liquid level is determined by measuring the differential pressure (delta-P) by means of a noble gas or nitrogen gas or any mixture thereof, preferably by means of a nitrogen (N₂) gas, which is pressed into the electrolytic cell by means of the tube (liquid level sensing tube) reaching (submersing) into the electrolytic bath (electrolyte).

8. The method of measuring and/or controlling the electrolytic bath liquid level in an electrolysis cell for the generation of fluorine gas according to anyone of the claims 1 to 7, wherein the method is operated in a manner, wherein the effective amount of hydrogen fluoride is supplied only periodically, such that the hydrogen fluoride is supplied together with nitrogen (HF + N₂) or such that the hydrogen fluoride is supplied separately (only HF) through the tube (liquid level sensing tube) reaching (submersing) into the electrolytic bath (electrolyte).

9. The method of measuring and/or controlling the electrolytic bath liquid level in an electrolysis cell for the generation of fluorine gas according to claim 8, wherein the method is operated in a manner, wherein the effective amount of hydrogen fluoride is supplied only periodically, such that the hydrogen fluoride is supplied together with nitrogen (HF + N₂) through the tube (liquid level sensing tube) reaching (submersing) into the electrolytic bath (electrolyte).

10. The method of measuring and/or controlling the electrolytic bath liquid level in an electrolysis cell for the generation of fluorine gas according to claim 9 wherein the supply of N₂ continues when the supply of HF is interrupted.

11. The method of measuring and/or controlling the electrolytic bath liquid level in an electrolysis cell for the generation of fluorine gas according to anyone of claims 8, 9 or 10, wherein the effective amount of hydrogen fluoride is supplied periodically in periods of from several days (e.g. seven days) to daily period (e.g. 24 hours), a period of from several hours (e.g. 24 hours) to one hour (e.g. 60 minutes), or a period of from several minutes (e.g. 60 minutes) to one minute (e.g. 60 seconds), or even a period of from several seconds to a few seconds.

12. A method of measuring and/or controlling the electrolytic bath liquid level in an electrolysis cell for the generation of fluorine gas according to claim 11 , wherein an effective amount of liquid hydrogen fluoride is supplied in periods of 1 to 5 minutes, preferably 1 to 4 minutes, more preferably 1 to 3 minutes, and even more preferably 1 to 2 minutes, and most preferably an interval of about one minute.

13. The method of measuring and/or controlling the electrolytic bath liquid level in an electrolysis cell for the generation of fluorine gas according to claim 11 , wherein an effective amount of liquid hydrogen fluoride is supplied in periods 20 to 50 seconds, more preferably 20 to 40 seconds, and most preferably about 30 seconds.

14. The method of measuring and/or controlling the electrolytic bath liquid level in an electrolysis cell for the generation of fluorine gas according to anyone of the claims 1 to 13, wherein the steps of (a) detecting the pressure in at least one of the anode chamber and the cathode chamber and (b) supplying an effective amount of hydrogen fluoride are performed at different times, and preferably steps (a) and (b) are performed in an alternating mode.

15. The method of measuring and/or controlling the electrolytic bath liquid level in an electrolysis cell for the generation of fluorine gas according to claim 13 and/or 14, wherein the step of (a) detecting the pressure in at least one of the anode chamber and the cathode chamber during generation of fluorine gas by pressure sensing means comprising a tube (liquid level sensing tube) reaching (submersing) into the electrolytic bath (electrolyte) is a period of 20

to 40 seconds, preferably a period of about 30 seconds, and the step of (b) supplying through the tube of the pressure sensing means an effective amount of hydrogen fluoride, to thereby remove and/or prevent plugging caused by potentially crystallized HF adduct of KF in the tube of the pressure sensing means, is a period of 20 to 40 seconds, preferably a period of about 30 seconds.

16. The method of measuring and/or controlling the electrolytic bath liquid level in an electrolysis cell for the generation of fluorine gas according to anyone of the claims 1 to 15, wherein the tube (liquid level sensing tube) reaching (submersing) into the electrolytic bath (electrolyte) is characterized by a flow orifice, preferably by a flow orifice in the tube of about 2.5 mm (inner) diameter.

17. The method of measuring and/or controlling the electrolytic bath liquid level in an electrolysis cell for the generation of fluorine gas according to anyone of the claims 1 to 16, wherein the tube (liquid level sensing tube) reaching (submersing) into the electrolytic bath (electrolyte) in addition to its function according to step (a) serves at the same time as the (main) hydrogen fluoride feed supply line for the electrolysis to generate elemental fluorine.

18. The method of measuring and/or controlling the electrolytic bath liquid level in an electrolysis cell for the generation of fluorine gas according to anyone of the claims 13 to 17, wherein the method comprises a combination of a liquid level sensing tube with a flow orifice in the tube of about 2.5 mm (inner) diameter and the step of (a) detecting the pressure in at least one of the anode chamber and the cathode chamber during generation of fluorine gas by pressure sensing means comprising a tube (liquid level sensing tube) reaching

(submersing) into the electrolytic bath (electrolyte) is operated for a period of 20 to 40 seconds, preferably a period of about 30 seconds, and the step of (b) supplying through the tube of the pressure sensing means an effective amount of hydrogen fluoride, to thereby remove and/or prevent plugging caused by potentially crystalized HF adduct of KF in the tube of the pressure sensing means, is operated for a period of 20 to 40 seconds, preferably a period of about 30 seconds.

19. The method of measuring and/or controlling the electrolytic bath liquid level in an electrolysis cell for the generation of fluorine gas according to anyone of claims 1 to 18, wherein the method is applied in the manufacture of F₂ by electrolysis of HF contained in an electrolyte wherein F₂ and H₂ are formed and consumed HF is replenished with a supply of fresh HF wherein the supply of fresh HF is limited to an HF flow (HF feeding quantity) of at most 10 kg/h of HF per ton of electrolyte.

20. The method of measuring and/or controlling the electrolytic bath liquid level in an electrolysis cell for the generation of fluorine gas according to anyone of claims 1 to 19, wherein in the electrolysis the required amount of hydrogen fluoride feed related to the fluorine production capacity of the electrolytic cell is in the range of a HF feeding quantity of 2-5 kg/h HF, preferably in the range of 3-4 kg/h HF in view of an apparatus having a capacity of 2 tons electrolyte.

21. The method of measuring and/or controlling the electrolytic bath liquid level in an electrolysis cell for the generation of fluorine gas according to anyone of claims 1 to 20, wherein in the electrolysis the HF feeding quantity (kg) and feeding interval (h) is regulated by an automatic valve.

22. The method of measuring and/or controlling the electrolytic bath liquid level in an electrolysis cell for the generation of fluorine gas according to anyone of claims 1 to 21, wherein in the electrolysis the fluorine is generated "on site" or "over the fence" of a production plant, preferably for use in the method for the manufacture of electronic devices.

23. An electrolytic cell for the generation of elemental fluorine by the electrolysis of a molten HF adduct of KF, wherein the electrolytic cell is equipped with liquid level sensing tube for detecting the pressure in at least one of the anode chamber and the cathode chamber during generation of fluorine gas by pressure sensing means and which tube also serves as a hydrogen fluoride supply line connectable to a the hydrogen fluoride supply unit, and wherein the liquid level sensing tube and hydrogen fluoride supply line has a flow orifice of from about 1mm to 1cm (inner) diameter; preferably having a flow orifice of about 2.5 mm (inner) diameter.

24. The electrolytic cell for the generation of elemental fluorine by the electrolysis according to claim 21, wherein the fluorine is generated in a cassette, preferably in a cassette for producing fluorine "on site" or "over the fence" of production plants, preferably for electronic devices, preferably for electronic devices selected from the group consisting of semiconductors, photovoltaic cells, MEMS, and TFTs.

25. The electrolytic cell for the generation of elemental fluorine by the electrolysis according to claim 23 or 24, which provides for a daily (24 h) production capacity of 50 to 100 kg fluorine (F₂), preferably a daily (24 h) production capacity of 80 to 100 kg fluorine (F₂), and more preferably a daily (24 h) production capacity of about 80 to 90 kg fluorine (F₂).