FLUORINE-GENERATING ELECTROLYTIC CELLS

Abstract: A fluorine-generating electrolytic cell has the base 12 thereof insulated electrically by a layer 10 of plastics material such as PTFE which is held against the cell base by means of structure extending between cooling coils 24 of the cell and the layer 10.

Description: This invention relates to fluorine-generating electrolytic cells. The production of fluorine by the electrolysis of a fused electrolyte containing potassium fluoride and hydrogen fluoride is well known. During electrolysis heat is generated and the electrolyte must be cooled. The cooling of the electrolyte has been achieved by the provision of cooling tubes immersed in the electrolyte. In one form of cell used for the large scale production of fluorine the electrolyte is cooled by cooling coils. A cell of this kind is hereinafter referred to as being a fluorine-generating electrolytic cell of the kind specified. In such a cell, the cooling coils may also act as cathodes. The cooling coils can be mild steel. Hitherto, cells of the kind specified have been operated with a water-cooled base so that a layer of "frozen" electrolyte is formed on the base so as to electrically insulate the base and hence prevent the generation of hydrogen at the cell base, this generation being undesirable since the hydrogen could otherwise migrate to the anode compartments of the cell where it could interact with fluorine with potentially serious consequences. Patent Specification GB-A-2135334 discloses an alternative approach in which, instead of insulating the cell by means of a solid layer of electrolyte, a polymeric material such as polytetrafluoroethylene is applied to the cell base. The polymeric layer need only be of the order of 2 mm thick (in contrast with a solid electrolyte layer typically of the order of 50 mm thick) with the advantage that the anodes can be made longer. The present invention addresses the problem of securing the insulating layer to the cell base without adversely affecting the integrity of the cell base. According to the present invention there is provided a fluorine-generating electrolytic cell having cooling coils for cooling the electrolyte and in which the base of the cell is insulated electrically by a layer of material characterised in that the layer is held against the cell base by means acting between the cooling coils and an upwardly presented face of the layer. By using the cooling coils in holding the insulating layer in place, the need to make changes to the cell vessel is avoided and installation and removal of the layer is relatively easy and inexpensive. Also, by using an insulating layer, all of the electrolyte is molten with the advantage that a smaller temperature gradient through the depth of the electrolyte results, which leads to improvements in mass transfer. The invention will now be described by way of example only with reference to the accompanying drawings in which: Figure 1 is a fragmentary vertical section through a fluorine-generating electrolytic cell showing one form of the invention; Figure 2 is a plan view of the clamping frame of the clamping assembly shown in figure 1; Figure 3 is a side view of the clamping frame; Figure 4 is a diagrammatic plan view showing part only of a cell in which the cooling coils/ cathodes are provided with protective barriers or guards (the anodes being omitted for clarity); Figure 5 is a plan view of one sheet of the insulating base layer; and Figure 6 is a front view of a protective barrier or guard. Typically, a fluorine-generating electrolytic cell comprises a mild steel tank jacketted on the sides so that steam can be applied to the jackets to maintain the KF.2HF electrolyte in molten condition when the cell is not in production. In conventional fluorine cells, the base also is jacketted so that the electrolyte in this region can be frozen by the application of coolant but this is unneccessary in the present invention. A series of water-cooled coils connected to inlet and exit headers divide the tank transversely and function as water-cooled cathodes. The cell lid has a series of openings into which anode assemblies fit so that each anode assembly is interposed between a pair of coils. Each anode assembly consists of a flat plate of mild steel to the underside of which is attached a rectangular Monel gas separating skirt inside which is located a pair of anode blocks. The anodes are insulated from the skirt assembly and the cell top by means of neoprene or fluoro-elastomer gaskets depending on the duty. To ensure no mixing of the gaseous products, each skirt protrudes a short distance into the electrolyte to divide the cell into a series of fluorine compartments and one hydrogen compartment. Fluorine from the anode assemblies is collected in a common manifold while the hydrogen leaves at an offtake located at one end of the cell. Provision is made in the cell lid for a liquid hydrogen fluoride feed pipe, electrolyte sample dip pipe, electrolyte thermocouple wells, and a nitrogen purge to both the hydrogen side and each individual anode compartment. Electrical contact to each anode is provided by a mild steel/nickel hanger secured to the block by means of a nickel oversprayed coating. The hanger, which has a nickel base plate, has two mild steel vertical threaded studs which protrude through the top of the gas separating skirt. Electrical connections from the pairs of anodes are made to a positive busbar running the length of the cell. The negative busbar is connected to the cell body which is thus at the same potential as the cathodes. Referring now to Figures 1-3, the frozen layer of electrolyte is replaced by a thin layer 10 (typically 2 mm) of a plastics material such as a fluorinated polymer, eg polytetrafluoroethylene or polyvinyldifluoride, or polypropylene. The layer may comprise a number of separate sections or sheets disposed side-by-side. The layer 10 having an upwardly presented face 9 is held against the base 12 of the cell by a frame 14 (which is shown in greater detail in Figures 2 and 3). The frame 14 is of generally rectangular configuration and has a number of cross-members 16 extending between its sides. A number of studs or struts 18 extend upwardly from the frame cross members. At its upper end each stud 18 is threaded and locates a clamping plate 20 which can be adjusted towards and away from the cell base by means of a nut 22. The clamping plates 20 are designed to bridge the space between a pair of cooling coils/cathodes 24 and the frame is so located that the studs 18 extend generally medially of adjacent pairs of coils 24. In use, the nuts 22 are adjusted to engage the plates 20 against the coils thereby forcing the frame downwardly to hold the layer 10 firmly against the cell base and thereby prevent seepage of electrolyte beneath the layer 10. Figures 4, 5 and 6 illustrate an alternative arrangement in which the cathode cooling coils 24 are provided with protective guards 26 which are secured to the coils by welds 28. In Figure 4, reference 30 depicts the side walls of the cell tank and reference 32 depicts the side walls steam jackets. Each guard 26 comprises a box-section structure having main walls 34 of expanded metal which allow electrolyte flow therethrough but prevent large pieces of debris from impinging against the coils. The guard has a channel-section sides 36 and at the bottom edge of the structure there is a channel-section bridging piece 38 which spans the space between the main walls 34. Each guard 26 engages the insulating layer 10 via the ends of the sides 36 and the bridging piece 38. As shown in Figure 4, the layer 10 is made up of a number of side-by-side sections 10a, b, c...., the abutting edges being depicted by reference 40. The arrangement is such that the joints 40 extend approximately medially of each pair of coils 24 so that the sections 10a, b, c.... are held down against the cell base at their adjoining edges. It will be understood that the spaces between each pair of cooling coils will be occupied by anode assemblies (not shown). Figure 5 illustrates one of the sections 10a, b, c.... It is formed with a number of apertures 42 through which the vertical inlet and the outlet pipe sections 44 (see Figure 1) of the cooling coils extend as a close fit. A slit 46 extends from each aperture 42 to the adjacent short edge of the section 10a, b, c.... to allow the section 10a, b, c.... to be assembled to the pipe sections 44.

Claims: 1. A fluorine-generating electrolytic cell having cooling coils (24) for cooling the electrolyte and in which the base (12) of the cell is insulated electrically by a layer of material, characterised in that the cooling coils (24) are displaced from the base (12), and the layer (10) is held against the cell base (12) by means (14-22; 34, 36) for bridging the space between the cooling coils (24) and an upwardly presented face (9) of the layer (10). 2. A cell as claimed in Claim 1 characterised in that the layer (10) comprises a number of sheets (10a, 10b, 10c) of plastics material arranged side-by-side. 3. A cell as claimed in Claim 1 or Claim 2 characterised in that said bridging means (14-22) comprsises a clamping member or members (14) pressed against the layer (10) by struts (18) extending between the cooling coils (24) and the clamping member or members (14). 4. A cell as claimed in Claim 3, characterised in that the clamping member comprises a frame (14) to which the struts (18) are secured. 5. A cell as claimed in Claim 3 or Claim 4, characterised in that each strut (18) engages a pair of cooling coils (24) through the agency of a bridging piece (20) which is adjustable lengthwise of the strut (18). 6. A cell as claimed in Claim 1 or Claim 2, characterised in that the cooling coils (24) are provided with protective barriers (26) attached to the coils (24), and the bridging means comprises portions (36, 38) of the protective barriers (26) which portions (36, 38) seat against the layer (10a, 10b, 10c) to hold the same against the cell base (12). 7. A cell as claimed in any one of Claims 1 to 6, characterised in that the layer (10) comprises a fluorinated polymer. 8. A cell as claimed in any one of Claims 1 to 6, characterised in that the layer (10) comprises polypropylene.

Inventors: LEEMING, ROLAND
Applicants: BRITISH NUCLEAR FUELS PLC
Application year: 1988
Main classification: C25B9
IPC codes: C25B1, C25B9