US3515661A - Electrolytic cells having detachable anodes secured to current distributors - Google Patents

Description

June 2, 1970 v. M. O. COULTER ET AL ELECTROLYTIC CELLS HAVING DETACHABLE ANODE SECURED TO CURRENT DISTRIBUTORS Filed Nov.

2 Sheets-Sheet 1 INVENTORS MICHAEL OLIVER COULT DAVID BELL ATTORNEYS June 2, 1970 Y M.O.COULTER ETAL' 3,515,661

ELECTROLYTIC CELLS HAVING DETACHABLE ANODES SECURED CURRENT DISTRIBUTORS Filed Nov. 2, 1966 2 Sheets-Sheet 2 5 1 F/G.4a [I "T w IN I 20 i Q J U v I i I I F i .a I 1; l I 5 R 3 I INVENTORS 1 5 4 r HAEL OLIVER COULTER C I W Vl D BELL BY WMXJv/M ATTORNEYS nited States Patent Ofifice 3,515,661 Patented June 2, 1970 3,515,661 ELECTROLYTIC CELLS HAVWG DETACH- ABLE ANODES SECURED T CURRENT DISTRIBUTORS Michael Oliver Coulter and David Bell, Holmes Chapel, near Crewe, England, assignors to Murgatroyds Salt and Chemical Company Limited, a British company Filed Nov. 2, 1966, Ser. No. 591,563 Claims priority, application Great Britain, Nov. 4, 1965, 46 ,728/ 65 Int. Cl. B011: 3/10 US. Cl. 204-263 11 Claims ABSTRACT OF THE DISCLOSURE An improved diaphragm cell for the electrolysis of aqueous solutions of alkali metal halides is provided having a cell casing a plurality of inverted U-shaped anodes, a plurality of elongate current distributors which are aluminum or copper bars covered with a titanium, niobium or tantalum sheath, and manifolding means. The plurality of anodes comprise electrode sections of a metal selected from the group consisting of titanium, niobium and tantalum which when made anodic within said diaphragm cell will form an electrical resistance of the same or greater order than that formed on titanium under identical conditions. Said electrode sections have at least one surface plated with a platinum group metal, each anode being secured in electrical contact by bolts to fins of titanium, niobium or tantalum welded to one of the said elongate current distributors. Each distributor traverses the cell and extends through at least one side wall thereof and has manifolding means exterior of the cell for con nection with the extensions of the other current distributors for attachment to a supply of direct current.

The invention relates to an electrolytic diaphragm cell for the electrolysis of aqueous solutions of alkali metal halides.

Electrolytic diaphragm cells for use in the production of gaseous chlorine and hydrogen together with caustic alkali by the electrolysis of aqueous solutions of alkali metal halides, in particular sodium chloride, are well known. Preferably such cells have employed a plurality of foraminous or perforated metal cathodes e.g. fabricated of steel wire mesh, covered with a liquid-permeable diaphragm or membrane interleaved within the cell with a plurality of anodes which in general are parallel, vertically disposed flat blades of graphite having their lower ends cast in a lead slab forming part of the base of the cell. In order to protect the lead from the corrosive attack of the chlorinated brine during the operational life of the cell a protective coating of a material such as concrete or bitumen is applied to the lead slab. In operation of such cells it has been found that the graphite anodes are attacked and wear progressively thinner during use. Such a thinning of the anodes leads to a progressive increase in cell voltage because the cross-section of graphite that is carrying the electrolysing current is reduced, and the current path through the electrolyte from anode to cathode is progressively increased.

Moreover, during electrolysis using graphite anodes the surface disintegration of the graphite leads to contamination of the electrolyte and the plugging of the cathode diaphragm or membrane by oily and carbonaceous material which impairs the eflicient operation of the diaphragm and shortens its life. Also carbon dioxide is formed by attack on the anodes and this contaminates the gaseous chlorine.

It is an object of the present invention to provide an electrolytic diaphragm cell which substantially avoids the above disadvantages.

Accordingly the present invention is a diaphragm cell for the electrolysis of aqueous solutions of alkali metal halides having cell casing, a plurality of anodes, an elongate current distributor and manifolding means wherein the plurality of anodes comprise electrode sections of a metal selected from the group consisting of titanium or similar metal having at least one surface plated with a platinum group metal each anode being secured in electrical contact with the elongate current distributor which traverses the cell and extends through at least one side wall thereof and has manifolding means exterior of the cell for connection with the extensions of other current distributors for attachment to a supply of direct current.

Throughout this specification the expression titanium or similar metal is defined as meaning titanium or titanium-base alloys or niobium or tantalum or alloys of titanium, niobium, or tantalum which when made anodic within a diaphragm cell of the type described herein forms an insulating film having an electrical resistance comparable to or greater than that formed on titanium under identical conditions.

By the term platinum group metal herein is meant any of those metals platinum, palladium, rhodium, ruthenium, osmium or iridium or alloys of any two or more such metals.

The anodes are suitably arranged in the cell each in a vertical plane parallel to one another and extending across the cell from side to side. The anodes are fabricated in titanium or similar metal in the form of sheet, wire mesh, perforated plate or expanded metal. The shape of the anodes may be any shape conveniently fabricated of the metal e.g. a flat sheet, mesh, plate etc., and may be suitably reinforced at its edges for example by folding or ribbing. In a preferred embodiment of the invention the anode takes the form of one or more pieces of metal sheet, mesh plate etc., bent into a U section. The straight arms of the U may be stiffened at the edges by folding. If desired the anode may alternatively or additionally be stiffened by fastening suitable spacing members between the arms of the U at suitable points. Alternatively, the anode may consist of a single flat piece of titanium or similar metal in the form of sheet, mesh perforated plate etc.

It is desirable to provide for rapid circulation of the anolyte, so that the anolyte which is contained in the narrow spaces between the vertical parts of the anode and cathode is replaced before the electrolyte dissolved in it has time to become depleted. If this is allowed to occur there will be a decrease in the current efficiency and an increase in the operating voltage of the cell. Rapid circulation can advantageously be achieved with the preferred anode design described by providing a number of perforations in the upper surface of the anode. Electrolyte will be carried upwards in the spaces between anodes and cathode through the gas lift action of the chlorine that is being generated, and will be able to flow through the perforations and downwards through the channel bounded by the inner faces of the arms of the U, and will be able to flow out into the lower portion of the cell below the cathode.

Anolyte circulation can be further promoted by forming the anode from two or more sections that are separated by a vertical gap.

The anodes may only be plated on those outer surfaces directly opposite to a cathode surface.

It is preferred to plate the anodes with platinum metal. A method of plating titanium or similar metal sheet, mesh, plate etc. with metals such as platinum to produce an adherent coating, is described and claimed in British specification 885,819.

The anodes are secured in electrical contact with the current distributors in any convenient manner e.g. welded or bolted. With an anode of U section the preferred construction takes the form of an anode having an inverted U section with both lower edges of the metal sheet, mesh, plate etc. welded to either side of the current distributor.

The elongate current distributor may be suitably a bar of copper or aluminium or other metal of high conductivity cast, soldered or brazed within a titanium or similar metal sheath. The interior of the titanium or similar metal sheath may first be electroplated with copper and then tinned in order to provide good electrical contact. Alternatively, the titanium or similar metal sheath may be formed by extruding a thin wall of titanium or similar metal over a copper or aluminium bar. If one end of the current distributor is mounted within the cell and is exposed to the electrolyte the end of the copper of aluminium core may be protected from corrosive attack by welding a plate or disc of titanium or similar metal to the exposed end of the titanium or similar metal sheath. With this form of construction the highly conducting metal such as copper necessary for conducting electric current to the anodes is completely protected from the corrosive electrolyte by the titanium or similar metal sheath, and the need to use bitumen, concrete or other sealing compositions that only have a limited life and are used in conventional cells is obviated.

In a preferred embodiment wherein the anodes are arranged in a vertical plane parallel to one another the current distributors are preferably placed in parallel horizontal alignment extending from side to side in the cell and above or below the anodes. It is also possible to stagger the horizontal alignment to present one row of distributors slightly below another. The current distributors preferably extend through only one side wall of the cell for manifolding for attachment to a supply of direct current, the free end of the distributor being held against the base or the opposite side wall by means of suitable sockets. Provision for sealing that part of the current distributor passing through the side wall of the cell may be made by use of any suitable sleeve bush or packed gland.

Manifolding of the extensions of the current distributors for connection to a supply of direct current may take any suitable form. For example the end of each distributor exterior of the cell may be held by a split clamp or it may be threaded and fastened to a busbar using a nut and back nut. In general all the anode elements in any one cell will be connected in parallel, and the assembly will be connected to the cathode of the adjoining cell by conventional means, so that a number of cells can be connected in series.

The cathode and general cell construction are well known and conventional in the art. The anode construction envisaged in the cell of the present invention does however allow for a somewhat lighter construction of the cell base, which hitherto because of the provision of heavy graphite anodes cast in a lead slab was of necessity fabricated of concrete or cast iron or like material. With a cell of the present invention the cell base may be fabricated of hard rubber lined mild steel, a composite construction consisting of rigid P.V.C. bonded to a resin/fibre glass system, polypropylene, polyvinyldichloride or chlorinated polyester resin (Hetron) which may be reinforced with a steel structure to provide adequate strength or other suitable materials known to those skilled in the art.

The construction of the particularly preferred embodiment for the anode assembly of the cell of the present invention is described in more detail with reference to the accompanying drawings in which FIG. 1 repersents schematically an anode according to 4 a preferred embodiment of the invention mounted on a current distributor.

FIG. 2 represents a perspective part sectional cut away view of the lower half of an electrolytic cell according to the invention showing the assembly of anodes and FIG. 3 shows a side elevation in section of a preferred method of sealing the extensions of the current distributors to the sides of the cell to prevent escape of electrolyte.

FIG. 4 represents schematically an anode mounted on a current distributor in an alternative manner.

FIG. 4a is top elevation of the construction according to FIG. 4.

FIG. 5 shows an alternative method of securing the anodes to the current distributor.

Referring to FIG. 1 the anode consists of tWo pieces of titanium or similar metal sheet 1 bent into an inverted U section with the lower edges 2 welded to the current distributor 3 which consists of a titanium of similar metal sheath 4 over a copper or aluminium bar 5. The outer facing surface of the U section of the anode is plated with a platinum metal (not shown).

Referring to FIG. 2 the lower part of the electrolytic, cell is shown generally as 6. A plurality of anodes 1 are assembled parallel to one another mounted on current distributors 3 arranged in parallel horizontal alignment below the anodes. Extensions 7 of one end of the current distributors pass through a side wall to be manifolded by connection to a busbar by means of threaded portions 8 for attachment to a supply of direct current (not shown). The other ends of the distributors are supported within the cell near the opposite side wall by titanium or similar metal brackets 9 fastened to a titanium or similar metal strip 10 extending the whole length of the side wall. This strip may be fastened to one or more side walls of the cell or supported by suitable brackets 11 from the base of the cell.

The extension of the current distributor passing through the side wall is sea ed to prevent escape of electrolyte through the side wall by the arrangement shown-in FIG.

3 in which. the seal is provided by a moulded sleeve 12 1 made from a suitable elastomer e.g. natural or synthetic rubber or Viton 1 (fiuoro-elastomer). The extension 7 of the current distributor is located within a branch 13 of the side wall of the cell by means of a flanged bush 14 fabricated of for example P.T.F.E. or polypropylene. The end of the extension 7 is threaded at 8 and carries the busbar 15 secured by a nut 16 and back nut 17.

Referring to FIG. 4 the anode consists of two pieces of titanium or simi ar metal sheet 1 bent into U sections with the free edges welded to a current distributor 3 i which consists of a titanium or similar metal sheath 4 over a copper or aluminium bar 5. The two U-shaped sections are disposed at C. to one another. The outer facing surfaces of the two U sections of the anode are plated with a platinum metal (not shown).

Referring to FIG. 5, this illustrates an a ternative method of securing the anodes to the current distributor. Thus the anode 1 is a U-section of metal sheet as before having holes drilled along the lower edge of each of the straight arms. Fins 18 of titanium or similar metal are welded parallel to one another either side of the titanium or similar metal sheath of the current distributor, the, fins being provided with holes corresponding to those in the lower edge of the anode U-section. The anode is then merely bolted to the fins by titanium or similar metal bolts 19 which pass through titanium or similar metal spacing elements so serving to prevent distortion of the anode structure on tightening the bolts. This method of construction enables the anodes to be removed from the current distributors for replating without need for removal of the current distributors themselvesQ An anode may similarly be constructed from a single 1 Registered trademark.

titanium plate bolted to a single fin that is welded to the current distributor.

As will be appreciated by one skilled in the art a cell constructed in accordance with the present invention finds numerous advantages over the cells employed hitherto in addition to avoiding the disadvantages of these previous cells enumerated above. Thus with an anode assembly of the type described removal of the assembly as a who e or of individual anodes with attached current distributor for repair or re-deposition of the platinum group metal coating as and when necessary is possible without the need for heavy manual labour in breaking the anodes out of a cast lead base with possible subsequent damage to the anode or to the cell structure. Also replacement of worn graphite anodes and the use of lead and special sealing materials for the cell base all of which increase the capital and operating cost of the cell is avoided. Also the life of diaphragms will be increased because they will not be contaminated by oily and carbonaceous material derived from graphite anodes. A further saving in diaphragm cost will be possible because when graphite anodes are used it is necessary to renew the diaphragm every time that the anodes have to be replaced. Since the platinum metal coating is expected to have several times the life of graphite anodes, the frequency of replacement for this reason will be reduced. A further advantage is that anodes constructed according to the invention will not be affected adversely by temperature changes resulting from variations in the current loading of the cells. The sealing compositions that have hitherto been used are liable to fail when substantial changes in current loading take place and particularly if it is necessary to switch the cells off altogether owing to breakdowns in electrical supplies or any other parts of the plant. When this occurs the lead used in the anode construction is liable to be attacked by the anolyte which results in a loss of lead from the cell and contamination of the caustic liquor produced by the cell.

When graphite anodes are used they have to have a substantial thickness when new, so that they will have an acceptable life before they require replacement. With the proposed design it is possible to make narrower anodes so that the cell can be made more compact, or a greater electrode area can be accommodated on the same floor area.

Since the weight of graphite and lead has been eliminated it may be possible to use a lighter floor construction.

For economic operation it is becoming necessary to design cells of increasing unit size. In order to minimise 'heat losses it is desirable that the outer dimensions of the cell should approximate to a cube so that with larger cells it is necessary to make the anode blades longer and the cathode deeper (i.e. to make the vertical extension of the cathode greater). With graphite anodes this resu ts in an increasing voltage penalty owing to the LR. drop through the graphite material. This can only be offset by using anodes that are thicker when new, but this will lead to a greater IJR. drop through the electrolyte when the anodes have worn thin, or to a greater consumption of graphite if the anodes are discarded while an appreciable thickness still remains. With the type of anode.

described the increased LR. drop through the anode plates can be offset by using thicker material, and the thickness of material can always be selected to achieve the optimum balance between capital and operating costs. The thickness of material can be optimised for every application according to the power cost and the cost of titanium metal.

We claim:

1. A diaphragm cell for the electrolysis of aqueous solutions of alkali metal halides having a cell casing a plurality of inverted U-shaped anodes, a plurality of elongate current distributors which are aluminum or copper bars covered with a titanium, niobium or tantalum sheath, and manifolding means, wherein the plurality of anodes comprise electrode sections of a metal selected from the group consisting of titanium, niobium and tantalum which when made anodic within said diaphragm cell will form an electrical resistance of the same or greater order than that formed on titanium under identical conditions, said electrode sections having at least one surface plated with a platinum group metal each anode being secured in electrical contact by bolts to fins of titanium, niobium or tantalum welded to one of the said elongate current distributors, each of which traverses the cell and extends through at least one side wall thereof and has manifolding means exterior of the cell for connection with the extensions of the other current distributors for attachment to a supply of direct current.

2. A diaphragm cell as claimed in claim 1 wherein the anodes are fabricated of the metal in the form selected from the group consisting of sheet, wire mesh, perforated plate or expanded metal.

3. A diaphragm cell as claimed in claim 2 wherein each anode takes a form comprising an inverted U- section of metal the ends of the straight arms of the section being secured one either side of the current dis tributor.

4. A diaphragm cell as claimed in claim 3 wherein each anode takes a form comprising at least two inverted U-sections separated by a vertical gap.

5. A diaphragm cell as claimed in claim 3 wherein each anode is fabricated from metal sheet and is provided with perforations in its upper surface.

6. A diaphragm cell as claimed in claim 2 wherein each anode takes a form comprising two U-sections of metal the ends of the straight arms of which are secured one either side of the current distributor and such that the sections are disposed at to one another.

7. A diaphragm cell as claimed in claim 2 wherein each anode takes a form comprising a fiat plate, one edge of which is secured to the current distributor.

8. A diaphragm cell as claimed in claim 1 wherein the anodes are plated with a platinum group metal only on those surfaces directly opposite to a cathode surface.

9. A diaphragm cell as claimed in claim 1 wherein the current distributor is a bar of copper, aluminum or other high conductivity metal contained within a titanium or similar metal sheath.

10. A diaphragm cell as claimed in claim 1 wherein the anodes are arranged in a vertical plane parallel to one another and secured to the current distributors placed in parallel horizontal alignment above or below the anodes.

11. A diaphragm cell as claimed in claim 10 wherein the horizontal alignment of the current distributors is staggered to present two rows of distributors.

References Cited UNITED STATES PATENTS 3,287,250 11/1966 Brown et al. 3,297,561 1/ 1967 Harrison et al. 3,313,721 4/1967 Teel. 3,342,717 9/1967 Leduc 204263 3,390,072 6/1968 Wiseman 204263 WINSTON A. DOUGLAS, Primary Examiner C. F. LE FEVOUR, Assistant Examiner US. Cl. X.R.

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