GB2109006A - Carbon elements for bipolar trickle towers and other electrochemical cells - Google Patents

Abstract

Carbon or graphite elements for use as packing material in bipolar trickle towers or as anodes in cells for recovering silver from exhausted photographic solutions are produced in the form of sheets or "waffle plates" by carbonising sheets (preferably perforated) of compacted and bonded fibrous cellulosic material (e.g. perforated hardboard, preferably laminated hardboard). Carbonisation may be affected in a single step, by heating in the absence of air, to a carbonising temperature which is high enough to impart the required electrical conductivity. Preferably, however, waffle plates or other elements of greater strength and less distortion are produced by first carbonising at a lower temperature, then impregnating with further carbonisable material (pitch, tar, resins, sugar and the like), and finally carbonising at a higher temperature.

Description

SPECIFICATION Bipolar trickle towers and other electrochemical cells, and carbon elements therefor This invention relates to electrically conductive carbon elements for bipolar trickle towers (referred to herein simply as trickle towers) and other electrochemical cells, and to an improved method of manufacturing such elements.

A trickle tower consists, as described, for example, in an article at pages 670-2 of "Chemistry and Industry" for 6th October 1979, of a tubular vertical tower or column packed with layers of rings, or other-shaped pieces of carbon or graphite (referred herein simply as carbon), these carbon layers being separated by interposed electrically insulating layers formed of nylon net or other suitable permeable insulating material.

The application of a voltage between electrode structures disposed at the top and bottom of the tower constitutes the carbon layers and interposed insulating layers into a series of reactor cells which may be employed to promote electrochemical reactions in a solution which is trickled down the tower as a thin film of liquid flowing down over the carbon surface and in surface contact also with any gaseous component of the reaction which may also be fed into the tower.

It has been suggested that the efficiency of such a trickle tower would be improved by replacing carbon rings, as the form in which the carbon is provided, by plate-like layers of carbon perforated by a spaced pattern of holes. It would not be practicable to manufacture such perforated platelike layers, which have been referred to as "waffle-plates", by extruding a rod of carbon with multiple holes extending through it in the direction of extrusion, and then slicing the rod transversely to yield the required apertured waffle plates.A more practical method would be to extrude a solid rod of carbon, cut it into slices and then multidrill the individual slices to give the required pattern of holes; and it may be that this will prove to be the only practical method of making "waffleplates" for large diameter trickle towers, where high compressive strength will be required property of the plates.

However, trickle towers used for many purposes, including the recovery of silver and other precious metals from solution, commonly have column diameters which do not exceed 250 or 300mm, or rectangular sections not in excess of 300mm square, and the stresses to which the waffle plates in such towers would be subjected would be correspondingly less.

It is one object of the present invention, as it relates to trickle towers, to provide an improved method of making apertured carbon plates ("waffle plates") at least for moderately sized trickle towers.

More generally it is an object of the invention to provide an improved method of making an electrically conductive carbon element for a trickle tower or other electrochemical cell.

According to the invention, there is provided a method of making an electrically conductive carbon electrochemical-cell element comprising the steps of producing a sheet of compacted and bonded fibrous cellulosic material and effecting carbonisation thereof in the absence of air by raising it to and maintaining it at carbonising temperature sufficient to impart the required electrical conductivity.

The invention also provides carbon elements made by this method, and trickle towers and other electrochemical cells furnished with such elements.

If shape and size are important, the sheet material may be shaped as required, with allowance for shrinkage, before carbonisation is effected. This shaping may, of course, include perforating the sheet with a distributed plurality of holes.

As is known, a body made of ceilulosic based materials can be carbonised by heating, out of contact with air, to produce a carbonised body which retains the original shape at least approximately, though it will be of reduced dimensions due to shrinkages which occur during the carbonisation.

A very suitable compacted and bonded fibrous cellulosic sheet material for the purpose of the invention is that known in Great Britain as hardboard. Sheets of hardboard, suitably of thickness between 3 and 1 Omms, may be perforated with a pattern of perforations and then carbonised by heating to a carbonising temperature in the absence of air. The individual perforations may suitably be round holes, or slots, or any shaped hole that can be drilled or punched through the board. For example, the individual perforations may suitably be rectangular slots about 5mm wide and 1 Omm long, and they may be aligned end-to-end in rows, with a spacing of about 1 Omm between adjacent ends of successive slots, these rows of slots being spaced suitably about 1 Omm apart from one another with the slots of one row opposite the spaces between the slots in the adjacent rows.

Such perforated sheets may be carbonised in a single step by heating to a temperature of, say, 1 2000C which is high enough to produce the required high electrical conductivity in the resulting carbonised body. This carbonisation process produces linear shrinkages which may be between 20% and 30% and which tend to produce distortions of the sheets, and although flatness is not essential in the final waffle plates of a trickle tower, it is desirable because although the carbonised sheets are strong in compression they are relatively weak in flexion and therefore not well adapted to be stacked upon one another if they show marked departures from flatness.

To reduce distortion during carbonisation, the sheets (if they are of hardboard) may be of the kind known as laminated hardboard, which consists of two or more relatively thin layers of laminae of hardboard bonded together by a suitable resin adhesive.

To improve both the flatness and the strength of the carbonised sheets, whether made from plain or laminated sheets, it is preferred to effect the carbonisation in three stages:- 1. Initial carbonisation by heating to a relatively low temperature, between about 4500 and 8000C for a period of say 6 hours in the absence of air; 2. After allowing the sheets to cool, impregnating them with carbonisable material such as pitch, tar, resins, sugar and the like; and 3. A further carbonisation by heating to a higher temperature of, say, between 1 2000C and 1 3500C, for a period of say 50 hours to complete the carbonisation of the impregnated sheets and to impart to them the high electrical conductivity which will be required of them as packing in a trickle tower or as a element of some other form of electrochemical cell.

Trickle tower waffle plates made in accordance with the invention as described above will, in general, be highly porous, and therefor highly suitable to receive catalysts which will promote the reactions to be effected in the trickle towers in which the waffle plates will be installed. A desired catalyst in solution may be added to the waffle plates after carbonisation is complete or, where the above described three stage carbonisation process is employed, after completion of the first stage if the catalyst is one which will not be adversely affected by the high temperature involved in the third stage. If the catalyst is to be added in solutuion after the first stage, and is compatible with, and soluble in the same medium as, the impregnating material to be added at the second, stage, both can be added to the same time.

It will be seen from the foregoing that the invention provides, inter alia, a method of making perforated sheets or plates of trickle tower packing material which comprises the steps of producing sheets of compacted and bonded fibrous cellolosic material, perforating the sheets with a distributed plurality of holes, and carbonising the perforated sheets by raising them to a carbonising temperature in the absence of air; and that according to a fruther aspect of the invention there is provided a bipolar trickle tower comprising a tubular column packed with layers of carbonised material separated by interposed electrically insulating layers and provided at its opposite end with electrode structures for application of a voltage therebetween and with reagent inlet and outlet means, wherein the said carbonised material is constituted by packing material made by the method outlined in the foregoing description.

The carbon elements made in accordance with the invention as described above can be used equally well in electrochemical cells other than trickle towers, for example as anodes in the -standard electrochemical cells used in the recovecy of silver from exhausted photographic developing and fixing solutions.

The anodes currently employed in such cells are composed of polycrystalline carbon made into solid sheets or rods. Solid sheets of carbonised hardboard made in accordance with the invention can replace existing anodes without modification of the equipment, but sheets of perforated carbonised hardboard can improve the performance of such cells by giving improved flow of the solution through and across the anode.

Claims (14)

Claims

1. A method of making an electrically conductive carbon electrochemical-cell element comprising the steps of producing a sheet of compacted and bonded fibrous cellulosic material and effecting carbonisation thereof in the absence of air by raising it to and maintaining it at a carbonising temperature sufficient to impart the required electrical conductivity.

2. A method as claimed in claim 1 and including the step of shaping the sheet material as required, with allowance for shrinkage, before effecting carbonisation thereof.

3. A method as claimed in claim 2, wherein the shaping of the sheet material includes the step of perforating the sheet with a distributed plurality of holes.

4. A method as claimed in any claims 1 to 3, wherein the sheet of compacted and bonded fibrous cellulosic material is a sheet of hardboard.

5. A method as claimed in any of claims 1 to 4, wherein carbonisation of the sheet is effected in a single step by heating in the absence of air to a temperature in the region of 1 2000C and maintaining such temperature until carbonisation is substantially complete, and the required electrical conductivity is obtained.

6. A method as claimed in any of claims 1 to 4 wherein an initial carbonisation of the sheets is effected by heating in the absence of air to a relatively low temperature, in the region between 4500 and 8000C, followed by a further heating in the absence of air to a higher temperature in the region between 12000 and 1 3500C until carbonisation is substantially complete and the required electrical conductivity is obtained.

7. A method as claimed in claim 6 and including, as an intermediate step after allowing the sheet to cool following the initial carbonisation of the sheet at the relatively low temperature, and before heating at a higher temperature, the step of impregnating the carbon.sed sheet with further carbonisable material.

8. A method as claimed in claim 7, wherein the carbonised sheet is impregnated with pitch, tar, a resin, sugar or the like, prior to the further heating at the higher temperature.

9. A method of making an electrically conductive carbon electrochemicalcell element by carbonisation of a sheet of compacted and bonded fibrous cellulosic material substantially as described herein.

1 0. An electrically conductive carbon electrochemical cell element made by the method of any claims 1 to 9.

11. A perforated electrically conductive carbon electrochemical-cell element made by the method claimed in claim 3 and any of the claim 4 to 8 as dependent thereon.

12. An electrochemical cell which includes at least one carbon element as claimed in claim 10 or claim 11.

13. An electrochemical cell in the form of a bipolar trickle tower containing layers of electrically conductive carbon elements separated by interposed electrically insluating layers wherein the carbon elements are elements as claimed in claim 10 or claim 11.

14. A bipolar trickle tower or other electrochemical cell substantially as described herein and containing carbon elements made by carbonisation of compacted and bonded fibrous cellulosic sheet material substantially as described herein.