Method of retrofitting of finite-gap electrolytic cells

Abstract

The present invention concerns a method of retrofitting of a membrane electrolysis cell, wherein a rigid cathode is shaped by plastic deformation of the regions in correspondence of cathodic supports; a pre-shaped conductive elastic element having compressed regions in correspondence of said cathodic supports is overlaid onto said rigid cathode; a flexible planar cathode provided with a catalytic coating is overlaid onto said conductive elastic element. The invention also concerns a correspondingly retrofitted electrolysis cell.

Claims

1. Method of retrofitting of an electrolysis cell comprising a cathodic compartment delimited by a cathodic back-wall and anodic compartment, separated by an ion-exchange membrane, said cathodic compartment containing a rigid planar cathode fixed to cathodic supports, said rigid planar cathode being spaced apart by 0.4 to 4 mm from said ion-exchange membrane, said anodic compartment containing an anode in contact with said ion-exchange membrane, the method comprising the simultaneous or sequential steps of: shaping said rigid cathode by plastic deformation of the regions in correspondence of said cathodic supports; overlaying onto said rigid cathode of a pre-shaped conductive elastic element having compressed regions in correspondence of said cathodic supports; overlaying a flexible planar cathode provided with a catalytic coating onto said conductive elastic element.

2. Method according to claim 1, wherein said anode in contact with said ion-exchange membrane is a louver-shaped anode, the method comprising the additional step of overlaying and fixing a planar anodic mesh provided with a catalytic coating onto said louver-shaped anode.

3. Method according to claim 2, wherein said planar anodic mesh is made of titanium and provided with an electrocatalytic film for chlorine evolution.

4. Method according to claim 1 wherein said planar rigid cathode is shaped by plastic deformation of the regions in correspondence of said cathodic supports in the range of 1 to 5 mm.

5. Method according to claim 1 wherein said pre-shaped conductive elastic element has compressed regions in correspondence of said cathodic supports having a thickness below 1 mm.

6. Method according to claim 1 wherein said planar rigid cathode consists of a punched sheet or expanded sheet or mesh made of nickel, provided with an electrocatalytic film for hydrogen evolution.

7. Method according to claim 1 wherein said cathodic supports consist of parallel ribs setting the distance between the rigid cathode and the cathodic back-wall.

8. Electrolysis cell comprising a cathodic compartment delimited by a cathodic back-wall and an anodic compartment separated by an ion-exchange membrane, said cathodic compartment containing a rigid planar cathode fixed to cathodic supports, said anodic compartment containing an anode in uniform contact with said ion-exchange membrane, wherein said electrolysis is retrofitted with said rigid planar cathode being curved into a rigid current distributor having regions comprised between said cathodic supports plastically deformed along the vertical axis by 1 to 5 mm, with a conductive elastic element having regions of thickness in the range of 0.1 to 1 mm in correspondence of said cathodic supports and with a flexible cathode consisting of a punched sheet or mesh of thickness ranging from 0.2 to 0.5 mm in uniform contact with said conductive elastic element on one side and with said ion-exchange membrane on the other side.

9. Electrolysis cell according to claim 8, wherein said anode is made of a louver-shaped base with a planar punched sheet or mesh of thickness ranging from 0.3 to 1 mm and provided with an electrocatalytic film fixed thereon.

10. Electrolyser consisting of a modular arrangement of a multiplicity of elementary cells according to claim 8.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) In FIG. 1 there is shown the assembly of a section of the cell comprised between two cathodic supports according to a mechanical design in accordance with the technology known as “finite-gap”.

(2) In FIG. 2 there is shown an assembly of a section of the cell comprised between two cathodic supports after a retrofitting according to the method of the invention.

(3) In FIG. 3 there is shown the assembly of a whole cell after a retrofitting according to the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

(4) FIG. 1 shows a front view of a section of the cell comprised between two cathodic supports 4 and two anodic supports 11 according to a mechanical design in accordance with the technology known as “finite-gap”, a rigid current distributor of planar geometry acting as cathode 1 facing an ion-exchange membrane 2 at a finite gap 10. Membrane 2 is in its turn overlaid and in contact with an anode having a louvered geometry 3.

(5) FIG. 2 shows a view of a detail of FIG. 3. More precisely, there is shown a front view of a section of the cell comprised between two cathodic supports 4 and two anodic supports 11 according to the invention. A current distributor 1 is obtained by curving of the cathode 1 of FIG. 1 in the regions 12 in correspondence of said cathodic supports 4. A pre-shaped conductive elastic element 5 is in contact with current distributor 1 on one side and flexible cathode 6 on the other, the latter being in intimate contact with ion-exchange membrane 2. Below ion-exchange membrane 2 there is depicted the anode comprised of a catalytically-coated planar mesh 7 welded on a portion of metal sheet of louvered geometry 3.

(6) FIG. 3 shows a front view of an electrolytic cell according to the invention wherein the two cathodic and anodic shells, respectively indicated with 8 and 9, cathodic current distributor 1, cathodic and anodic supports, respectively indicated with 4 and 11, the anode comprised of louver sheet 3 welded to planar catalysed anode mesh 7 and flexible cathode 6 are shown.

EXAMPLE 1

(7) An electrolytic cell was assembled according to the method of the invention with a result according to the scheme of FIG. 3. Starting from the components of a cell assembled in accordance to a “finite-gap” design, the following operations were carried out. The rigid cathode in form of 1 mm-thick sheet was bent in the regions between the contact surfaces with the cathodic supports in an area of about 2.5 mm. A conductive elastic element formed of interpenetrated coils of double nickel wires having a diameter of about 0.2 mm was also shaped by rolling so as to obtain compressed areas in correspondence of the areas of the rigid cathode in contact with the cathodic supports. A 0.3 mm-thick flexible cathodic mesh provided with a catalytic layer was then overlaid in intimate contact with the conductive elastic element. In the anodic compartment of the cell a 0.5 mm-thick planar titanium mesh coated with a catalytic layer of mixed oxides of platinum group metals was welded onto the pre-existing louver anode. The above elements were then assembled, obtaining a cell structure according to FIG. 3.

EXAMPLE 2

(8) The efficacy of cancelling the cathode-membrane gap by the retrofitting of a cathode of a cell originally having an internal geometry of “finite-gap” type and the installation of a new cathode coupled to a compressing elastic element as described in Example 1 was tested on a pilot electrolyser used for chlor-alkali membrane electrolysis. The electrolyser was equipped with eight single cells. The electrolyser was operated with 32% by weight caustic soda, sodium chloride brine at an outlet concentration of 210 g/l, at 90° C. and at a current density of 5 kA/m.sup.2. After a stabilisation period of about 1 week, the cells were characterised by an average voltage of 2.90 V, which remained essentially unchanged after 6 months of operation, when the electrolysis was discontinued and two single cells were extracted from the supports, opened and subjected to visual inspection of the components. The inspection did not emphasise any alteration worthy of note, and in particular the two membranes presented a surface essentially free of notches or other types of traces generated by abnormal compression of the cathode. As a term of comparison, the above described electrolyser showed energy savings of about 150 kWh per tonne of product caustic soda with respect to to an electrolyser equipped with the original cells prior to retrofitting, characterised by a membrane-cathode gap of 1.5 mm.

(9) The previous description shall not be intended as limiting the invention, which may be used according to different embodiments without departing from the scopes thereof, and whose extent is solely defined by the appended claims.

(10) Throughout the description and claims of the present application, the term “comprise” and variations thereof such as “comprising” and “comprises” are not intended to exclude the presence of other elements, components or additional process steps.

(11) The discussion of documents, acts, materials, devices, articles and the like is included in this specification solely for the purpose of providing a context for the present invention. It is not suggested or represented that any or all of these matters formed part of the prior art base or were common general knowledge in the field relevant to the present invention before the priority date of each claim of this application.