WO2013024041A1

WO2013024041A1 - Electrolytic process for the manufacture of fluorine and an apparatus therefor

Info

Publication number: WO2013024041A1
Application number: PCT/EP2012/065717
Authority: WO
Inventors: Holger Pernice; Reiner Fischer; Harald Krueger; Christoph Sommer; Michael OLPP; Antonio BERTANI; Alessandro FRANCHINO; Oliviero Diana; Philippe Morelle; Peter M. Predikant
Original assignee: Solvay SA
Current assignee: Solvay SA
Priority date: 2011-08-17
Filing date: 2012-08-10
Publication date: 2013-02-21
Anticipated expiration: 2014-02-17

Abstract

The present invention concerns an electrolytic process for the manufacture of fluorine comprising the electrolysis of an adduct of KF and HF wherein the electrolytic process is monitored by a thermographic camera, and an apparatus therefor. The method of monitoring the electrolytic process of fluorine production using conventional electrolytic cells, provides the possibility to automate the HF addition by reading the electrolyte level continuously. Additionally malfunctions like external fires (H₂ + O₂ recombination), internal H₂ + F₂ recombination due to broken partition walls, insufficient cooling, non optimal electrical connections can easily be detected, which leads to enhanced security of the process and less energy consumption.

Description

Electrolytic process for the manufacture of fluorine and an apparatus

therefor

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to European patent application

No. 11177834.6 filed on August 17, 2011, the whole content of this application being incorporated herein by reference for all purposes.

TECHNICAL FIELD OF THE INVENTION

The present invention concerns an electrolytic process for the manufacture of fluorine comprising the electrolysis of an adduct of KF and HF wherein the electrolytic process is monitored by a thermographic camera, and an apparatus therefor.

BACKGROUND OF THE INVENTION

In the manufacture of semiconductors, photovoltaic cells, thin film transistor (TFT) liquid crystal displays, and micro-electromechanical

systems (MEMS), fluorine (F₂) can be applied as etching gas, as chamber cleaning gas, often in concentrations from 1 to 50 % by volume in mixtures for example with nitrogen and/or argon.

Processes of this kind are for example described in WO 2007/116033 (which describes the use of fluorine and certain mixtures as an etchant and chamber cleaning agent), WO 2009/080615 (which describes the manufacture of MEMS), WO 2009/092453 (which describes the manufacture of solar cells), and in unpublished WO patent application PCT/EP2010/066109 which concerns the manufacture of TFTs. F₂ used for etching or chamber cleaning is often produced on site by electrolysis of HF in the presence of conducting salts, especially, as mentioned above, in the presence of KF which forms adducts with HF.

A molten HF adduct of KF 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 (I) by applying a voltage and passing electric current through the molten salt :

2 HF -> F₂ + H₂ (I) Practically, the voltage is often kept in a range of 8 to 11 Volt. Electrolysis may be performed in an apparatus comprising a vessel with a bottom, lid and walls containing the electrolyte and serving as cathode ;

alternatively, the vessel may comprise at least one cathode which extends into the molten electrolyte. The cell comprises at least one anode, preferably a multitude of anodes which extends or extend into the molten electrolyte. These anode(s) is/are often formed from carbon and may be cylindrical, flat, e.g. they may be present in square form, but principally, they may have any other desired shape. The apparatus also contains power supply lines connected with anode and cathode and lines to supply HF and to draw off produced F₂ and H₂. The invention is dealing with such kind of apparatus.

During electrolysis, HF is consumed (the KF only serves to provide conductivity to the electrolyte). The amount consumed may vary from electrolysis cell to electrolysis cell depending on the applied power, conductivity of the cell, temperature of the electrolyte, viscosity and the resulting current. As a consequence, the level of electrolyte decreases, and consumed HF must be replenished. HF may be supplied when the level reaches a certain lower limit, and the supply may be stopped when a certain upper level is reached.

US7351322 describes a method to control the level of electrolyte by level sensors which extend into the inner space of the vessel and which can detect the electrolyte level at five level stages.

Object of the present invention is to provide a simple technically feasible process to control not only the level of electrolyte in an electrolytic cell wherein fluorine (F₂) is produced electro lytically from solutions of KF in HF, but also to detect irregular process conditions, e.g. defective lids, too high resistance of power supply lines, defective partition walls in the vessel, cooling failure and other malfunctioning of the apparatus. Another object of the present invention is to provide a suitable apparatus for performing the electrolytic fluorine manufacture according to the present invention. These objects and other objects are achieved by the invention as outlined in the claims.

SUMMARY OF THE INVENTION

Accordingly, a process is provided for the electrolytic manufacture of fluorine from hydrogen fluoride containing a molten electrolyte containing HF and potassium fluoride dissolved therein as electrolyte salt in an apparatus comprising an electrolytic cell with a bottom, side walls, a lid, a power supply line for at least one anode, a power supply line for a cathode, a line to supply hydrogen fluoride, a partition wall, an anode room, a cathode room, a molten electrolyte comprising HF and dissolved KF, wherein electric current is passed through the anode and F₂ and H₂ are formed by electrolysis of the HF, wherein a thermographic camera identifies at least one process condition and provides a thermographic image.

BRIEF DESCRIPTION OF THE DRAWING

Figure 1 shows an apparatus wherein the process of the invention can be performed. The apparatus comprises an electrolytic cell (1) wherein the vessel formed by the walls (3) and the bottom (2) serves as cathode. The apparatus includes a thermographic camera (10) and a remote monitoring station (11). DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The thermographic image provides an actual visualization of the temperature of the apparatus, a region or regions of the apparatus or of parts of the apparatus. The thermographic image is useful to evaluate whether or if not the process is running properly. The actual thermographic image can be compared with a thermographic image or thermographic images taken when the process runs properly.

The term "process condition" denotes any process condition which has an impact on the thermographic image provided by the thermographic camera. The respective process condition may be any desired or undesired event or alteration of operating parts or of the progress of the electrolytic process having an impact such that the temperature visualization of the thermographic image indicates a temperature deviation to a lower temperature or to a higher temperature than a predetermined temperature range. The term "process condition" preferably denotes

· a change in the level of the electrolyte which exceeds a predetermined upper or lower level

• a temperature of the electrolyte which is higher than predetermined indicating a short cut in the cell possibly caused by a defective or broken anode or indicating defective partition walls isolating the space above the anode to prevent the dangerous heat releasing recombination reaction of F₂ and H₂ gas

• a too hot power supply line indicating a high resistance possibly caused by bad contacts between power supply line and anode or one or more anodes if several anodes are present, or power supply line and cathode or cathodes if several cathodes are present

· a hot spot above the lid indicating a broken lid and a respective reaction of H₂ with air or fluorine also leaving the reactor (i.e., a respective fire). The actual image of the thermographic camera can be evaluated by a human operator or by a monitoring station. Both the human operator and the monitoring station are preferably remote from the electrolytic cell. The actual image can be provided via cable or via electromagnetic waves.

A human operator may compare the actual image of the apparatus with an image or images taken under operable conditions. For example, the areas of the walls of the vessel not in contact with the electrolyte but in contact with the space above the electrolyte may have an orange color, and the areas of the walls of the vessel in contact with the electrolyte may be white (depending on the color allocated by the thermographic camera to specific temperatures). The operator can immediately realize if the level of the electrolyte represented by the border of white and orange is within the predetermined range. If the level is too low, the operator will initiate an HF supply until the desired electrolyte level is achieved.

If a bad electric connection between power supply line and the top of the anode provides a higher Ohmic resistance, or between the power supply line for the cathode or cathodes, the operator will realize a respective red or white color of the power supply line. If a hot spot can be realized above the lid, the lid may be broken or not perfectly closed.

If the temperature of the electrolyte is indicated to be too high, a faulty or maladjusted cooling system may be the cause. If there is a hot spot indicated, a broken anode may be causing an electric shortcut, or there may be a reaction between H₂ and F₂ due to broken partition walls around the anode or anodes, respectively.

Depending on the observations, the human operator may take his conclusions and stop the process for repair of anodes, lid, or partition walls, increase or decrease the cooling capacity, add HF, improve the electric contact between current conducting parts, or increase or decrease the electric

current/voltage.

If desired, the thermographic image may be automatically monitored by a monitoring station, preferably, a remote monitoring station.

In the monitoring station, the actual image provided by the thermographic camera is electronically evaluated by the automatic monitoring station, e.g. it is compared with an image or with several images providing a thermographic image or images representing acceptable or regular conditions of the electrolytic process for reference. This kind of automatic monitoring is described in US 2010/0044567 (now US patent 7,989,769) wherein the reference image is denoted as "mask".

In a preferred embodiment, the at least one process condition which is evaluated is selected from the group consisting of the electrolyte level, electrolyte temperature, current resistance in the power supply line or heat caused by a reaction between ¾ and oxygen or fluorine, and the monitoring station reacts by issuing an alarm, e.g. an acoustic or visual alarm, if the evaluation of the thermographic image indicates a deviation of the temperature which exceeds a predetermined threshold.

In another preferred embodiment, the at least one process condition is selected from the group consisting of the electrolyte level, the electrolyte temperature, current resistance in the power supply line for anode or anodes or cathode or cathodes or heat caused by a reaction between ¾ and oxygen or fluorine, and the monitoring station regulates the cooling capacity, it initiates the shut-off of the electrolytic process for the manufacture of fluorine if the evaluation of the thermographic image indicates a deviation of the temperature which exceeds a predetermined threshold, or even may initiate the introduction of extinguishing gases or liquids to extinguish any fire, e.g. a hydrogen fire.

According to a preferred embodiment, the monitoring station initiates the supply of HF into the cell, if necessary.

Depending on the result of the comparison, the monitoring station may even provide no reaction if the evaluation of the actual thermographic image shows that the process runs as desired ; the monitoring station may nevertheless protocol the result.

Thermographic cameras which are sensitive in a range which is included in the region of 3 to 15 μιη, are commercially available. Preferably, they comprise uncooled sensors. For example, the thermographic cameras sold by Testo AG, Germany, especially thermographic imagers sold as testo 880-1, testo 880-2 and testo 880-3, are very suitable. These thermographic cameras have a spectral range of 8 to 14 μιη. Other providers of thermal imaging systems suitable for use in the process of the invention are Fluke Corporation, Sierra Pacific

Innovations, from FLIR Systems Inc., and many others.

The advantage of the process of the invention is the simple way of monitoring and, if desired, of manually or automatically reacting when changes in the process conditions or faults are monitored. Another aspect of the present invention concerns an apparatus for the electrolytic manufacture of fluorine from HF comprising dissolved KF as electrolyte salt which apparatus comprising an electrolytic cell (1) with a bottom (2), side walls (3), a lid (4), a power supply line/anode (5), a power supply line/cathode (13), a line (6) to supply hydrogen fluoride, a partition wall (7), a cathode room (8), an anode room (9), a thermographic camera (10), optionally, a remote monitoring station (11), and, optionally, a line (12) to connect the thermographic camera (10) with the remote monitoring station (11). The molten electrolyte, when present in the apparatus, is indicated by reference sign (E)

Preferably, the apparatus comprises a remote monitoring station (11). The monitoring station (11) can be connected to the thermographic camera (10) via a cable (12) or via emitter and receiver for electromagnetic waves (e.g. emitting and receiving radio signals).

As mentioned above, there are several types of electrolytic cells. The vessel may serve as cathode ; this embodiment is shown in figure 1. In other alternatives, one or more cathodes are immersed into the liquid electrolyte.

The apparatus is very useful to perform the method of the invention in it. The method of the invention can, for example, it can be performed as follows in the apparatus :

KF and HF are filled into the electrolytic cell (1) such that the molar ratio of KF and HF are approximately in the range of 1 : 1.8 to 1 :2.3. For example, respective solid adducts of KF and HF can be applied. The electrolyte, indicated as (E) in figure 1, is heated until it is molten. The melting point is

approximately 72°C. It can even be heated to 100°C or even higher. Electric current is passed through line (5) and the anode connected thereto and through line (13) and the cathode connected thereto. Sometimes the walls (3) and the bottom (2) of the cell (1) serve as cathode. In another cell type neither the bottom nor even the walls but cathodes which surround each anode are used and thus are incorporated in the cell. F₂ forms and is collected in the anode room (9), and H₂ forms and is collected in the cathode room (8). To prevent recombination of F₂ and H₂, partition walls (7) are used. The thermographic camera (10) is switched on and provides actual thermographic images of the cell (1). The respective actual images can be compared with a thermographic image taken as reference image of the cell (1) earlier when the cell (1) ran properly. For the evaluation whether the process is running properly or not, a human operator should focus on the differences between the reference image and the actual thermographic image, especially concerning the temperature of the lines (5) and (13), the temperature of the electrolyte (E), the level of the electrolyte (E) as indicated by line separating the hot zone of the electrolyte (E) and the colder zones of the anode compartment (9) and the cathode compartment (8), and any hot spots in the proximity of the lid (4) which may indicate a hydrogen flame. If the current line (5) and/or current line (13) is or are too hot, a bad electric contact between the anode and the current line or the current line and the cathode, respectively, may be the cause. The flow of electric current should be stopped and the connection between line and anode or line and cathode should be fastened, cleaned, and/or non conducting layers/materials should be removed and/or optionally conducting material could be added prior rejoining. If the level of electrolyte (E) is undesirably low, HF is fed via line (6). If the electrolyte (E) is too hot, a broken anode, an insufficient cooling or a damaged partition wall (7) may be the cause. Other causes for improper operation of the process and the resulting alteration of the actual thermographic image may be identified by experienced operators.

In an alternative embodiment, a monitoring station performs the evaluation of the actual thermographic image or images in comparison with an image or images taken of the apparatus during a proper performance. The monitoring station may be remotely arranged from the apparatus. The actual images may be forwarded form the thermographic camera (10) to the remote monitoring station (11) by a cable (12) or otherwise, e.g. by radio signals. As described in US patent 7,989,769 the remote monitoring station receives actual

thermographic images from the thermographic camera. The monitoring station provides at least one mask corresponding to a thermographic image ; the mask provides temperature thresholds for different mask regions, e.g. for the electrolyte, the current lines or the space surrounding the lid. The monitoring station automatically compares temperatures of the actual thermographic image or actual thermographic images with the different mask regions to the temperature thresholds associated with the mask regions. An alarm may be generated, or other actions may be initiated, e.g. supplying HF to the electrolyte, shutting off the current, shutting off the electrolytic cell, or providing fire extinguishing agent. The monitoring station may also detect degradation associated with dirt on the lens of the camera. If desired, the apparatus can be integrated in a plant constructed according to the "skid" concept. Such a plant is especially suitable for the production of F₂ used in the manufacture of semiconductors, TFTs, micro-electromechanical devices and cells used in photovoltaic, and the plant preferably comprises skid mounted modules selected from the group consisting of

- a skid mounted module comprising at least one storage tank for HF, denoted as skid 1 ,

- a skid mounted module comprising at least one electrolytic cell according to the present invention to produce F₂, denoted as skid 2,

- a skid mounted module comprising purification means for purifying F₂, denoted as skid 3,

- a skid mounted module comprising means to deliver fluorine gas to the point of use, denoted as skid 4,

- a skid mounted module comprising cooling water circuits, denoted as skid 5, - a skid mounted module comprising means to treat waste gas, denoted as skid 6,

- a skid mounted module comprising means for the analysis of F₂, denoted as skid 7, and

- a skid mounted module comprising means to operate the electrolysis cells, denoted as skid 8.

Such a plant constructed according to the skid concept is described in unpublished US provisional patent applications N° 61/383204 and 61/383533, and published now as corresponding international patent application

WO 2012/034978, the whole content of which is incorporated herein for all purposes.

In such a plant according to the skid concept, the F₂ generating cell of the present invention is incorporated in skid 2.

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

The following examples are intended to explain the method of the invention in detail without intending to limit the scope of the invention.

Example 1 : Production of F₂ in an electrolytic cell

The electrolysis is performed in an electro lyzer apparatus wherein several electrolytic cells are assembled in a cell room. The cathode of each cell is presented by the cell vessel (also denoted as trough) which is made from stainless steel or nickel. The cell vessel is connected to the (-) pole of a rectifier or it is further linked onto the next anode bus bar in case of serial connection. Each cell contains a multitude of anodes, which are made from carbon. A single rectifier's (+) pole side (or a cathode in case of serial connection) is connected to a (+) bus bar mounted onto the electrolytic cell supplying different anodes in parallel.

The electrolytic cells are connected in series from (+) to (-), connecting the (+) pole of a main bus bar to the anode of the first cell and the (-) pole of a main bus bar to the cathode of the last cell ; in between, the respective cathode is connected with the respective anode by a short bus bar. In such a serial connection individual cells may be bypassed by a short circuit switch.

When a voltage of 10 to 11 V direct current is applied, the F₂ formed at the surface of the anodes is collected in the respective anode compartment.

A thermographic camera periodically makes images from the cell and sends the respective images via radio signals to a remote monitoring station. In the monitoring station, the actual images are compared with masks to identify deviations which indicate an irregular behavior or which indicate that the level of the electrolyte in the cell has reached a lower limit. If such a deviation of the values in the mask is identified, the monitoring station automatically sends an alarm, it initiates HF to be fed into the cell if the HF level is too low ; it stops the feeding of HF into the cell if the electrolyte reaches the desired upper level ; or in case of an irregularity, e.g. hot spots indicating bad electric connections causing respective higher electric resistance, or hot spots indicating a reaction

between H₂ and F₂ inside the cell or between H₂ and air outside the cell, the monitoring station initiates a stop of the HF feed into the cell, initiates a stop of the electric current, the total shut down of the F₂ producing cell or even the complete plant, and/or the feeding of extinguishing agent into the proximity of the cell. The monitoring station may also send a signal, e.g. to a control board, indicating which kind of irregularity caused the respective action.

Claims

C L A I M S

1. A method for the electrolytic manufacture of fluorine from hydrogen fluoride containing a molten electrolyte containing HF and potassium fluoride dissolved therein as electrolyte salt in an apparatus comprising an electrolytic cell with a bottom, side walls, a lid, a power supply line for the at least one anode, a power supply line for the cathode, a line to supply hydrogen fluoride, a partition wall, an anode room, a cathode room, a molten electrolyte comprising HF and dissolved KF, wherein electric current is passed through anode and cathode and F₂ and H₂ are formed by electrolysis of the HF, wherein a thermographic camera identifies at least one process condition and provides a thermographic image.

2. The method of claim 1 wherein the at least one process condition is selected from the group consisting of the electrolyte level, electrolyte

temperature, and current resistance in the power supply line.

3. The method of claim 1 or 2 wherein the at least one process condition is selected from the group consisting of heat caused by a reaction between H₂ and oxygen or fluorine.

4. The method of anyone of claims 1 to 3 wherein the thermographic image is monitored by a human operator.

5. The method of anyone of claims 1 to 3 wherein the thermographic image is automatically monitored by a monitoring station.

6. The method of claim 5 wherein the thermographic image is remotely monitored by a monitoring station.

7. The method of claims 5 or 6 wherein the monitoring station evaluates the thermographic image on a predetermined schedule to provide an automatic reaction if a predetermined temperature threshold is exceeded.

8. The method of claim 7 wherein the monitoring station provides at least one mask corresponding to a thermographic image from the thermographic camera wherein the mask provides temperature thresholds for different mask regions.

9. The method of anyone of claims 1 to 8 wherein the process condition is the electrolyte level.

10. The method of claim 9 wherein the monitoring station initiates the supply of HF to the electrolytic cell if the evaluation of the thermographic image indicates that the level of the electrolyte has dropped below a predetermined minimum level, or wherein the monitoring station stops the supply of HF to the electrolytic cell if the evaluation of the thermographic image indicates that the level of the electrolyte has exceeded a predetermined maximum level.

11. The method of anyone of claims 5 to 9 wherein the at least one process condition is selected from the group consisting of the electrolyte level, electrolyte temperature, current resistance in the power supply line or heat caused by a reaction between ¾ and oxygen or fluorine wherein the monitoring station issues an alarm if the evaluation of the thermographic image indicates a deviation of the temperature which exceeds a predetermined threshold.

12. The method of anyone of claims 5 to 9 wherein the at least one process condition is selected from the group consisting of the electrolyte level, the electrolyte temperature, current resistance in the power supply line or heat caused by a reaction between ¾ and oxygen or fluorine wherein the monitoring station initiates the shut-off of the electrolytic process for the manufacture of fluorine if the evaluation of the thermographic image indicates a deviation of the temperature which exceeds a predetermined threshold.

13. The method of anyone of claims 1 to 12 wherein the thermographic camera is sensitive in the region of 3 to 15 μιη and comprises uncooled sensors.

14. An apparatus for the electrolytic manufacture of fluorine from HF comprising dissolved KF as electrolyte salt which apparatus comprising an electrolytic cell (1) with a bottom (2), side walls (3), a lid (4), a power supply line/anode (5), a power supply line/cathode (13), a line (6) to supply hydrogen fluoride, a partition wall (7), a cathode room (8), an anode room (9), a thermographic camera (10), optionally, a remote monitoring station (11), and, optionally, a line (12) to connect the thermographic camera (10) with the remote monitoring station (11).

15. The apparatus of claim 14 further comprising a remote monitoring station (11).