GB1598306A

GB1598306A - Electrolytic method and apparatus

Info

Publication number: GB1598306A
Application number: GB5102276A
Authority: GB
Original assignee: Kodak Ltd
Current assignee: Kodak Ltd
Priority date: 1976-12-07
Filing date: 1976-12-07
Publication date: 1981-09-16

Description

(54) ELECTROLYTIC METHOD AND APPARATUS (71) We, KODAK LIMITED, a Company registered under the Law of England, of Kodak House, Station Road, Hemel Hempstead, Hertfordshire, do hereby declare this invention to be described in the following statement: The present invention relates to an electrolytic method and to apparatus for carrying out the method.

More particularly, the invention relates to an electrolytic method for recovering a metal from a solution containing metal ions and to an electrode assembly or an electrolytic cell for carrying out the method.

In general, cells for the recovery of metals comprise a container for the solution containing metal ions which forms the electrolyte. A pair (at least) of electrodes is located in the container in such a position as to be immersed in the electrolyte in use. One of the electrodes is arranged to be the cathode and the other is arranged to be the anode of the cell. The constitution of the electrodes depends on the nature of the electrolyte and the metal to be recovered. In use a D.C.

potential from a suitable source is applied across the electrodes and the metal ions are caused to migrate to the cathode where they are discharged and converted to metal atoms which generally adhere to the cathode. When sufficient metal has deposited on the cathode it is usually taken from the cell and the deposit removed therefrom, after which the cathode is replaced in the cell. Cells have been arranged to operate in a continuous manner in which case the electrolyte is passed through the cell from an inlet to an outlet at a predetermined rate.

The rate of passage of the electrolyte through the cell is such that a satisfactory proportion of the metal ions in the solution are discharged and converted to metal atoms at the cathode.

In accordance with the invention there is provided a method of recovering a metal from a solution containing metal ions which method comprises electrolyzing the solution with an anode and a permeable cathode, which cathode comprises carbon fibres as the electrically conducting material of which carbon provides the working surface and a permeable support for the carbon fibres formed of a combustible material, wherein substantially all the solution being electrolyzed is passed through the cathode in one direction as hereinafter defined.

In accordance with another aspect of the invention there is provided an electrode assembly comprising an anode, a permeable cathode comprising carbon fibres as the electrically conducting material of which carbon provides the working surface and a permeable support for the carbon fibres formed of a combustible material, and means to constrain an electrolyte to flow through the cathode in one direction as hereinafter defined.

The invention also provides an electrolytic cell comprising an electrolyte container having an inlet and an outlet which permit the flow of an electrolyte through the cell and, arranged in the cell, an anode and a permeable cathode, which cathode comprises carbon fibres as the electrically conducting material of which carbon provides the working surface and a permeable support for the carbon fibres formed of a combustible material, whereby when the cell is in use, substantially all the electrolyte flowing through the cell passes through the cathode in one direction as hereinafter defined.

The means to constrain an electrolyte to flow through the cathode in one direction may take a variety of forms, for example, it may be a pump which directs the flow of electrolyte through the cathode. Alternatively, it may be a housing to contain an electrolyte which is to be passed through the cathode under gravity.

Preferably, the electrolyte is pumped through the cathode.

The electrolyte being electrolyzed is passed through the cathode in one direction i.e.

at any given instant, electrolyte flow through the cathode is in one direction.

Over a period of time, the direction of flow may be reversed e.g. by reversing the flow of electrolyte or by reversing the cathode.

In this way, the electrolyte passing through the cathode comes into intimate contact with the effective surface of the cathode.

It is advantageous for the cathode to be uniformly permeable and, preferably, the cathode has a foraminate structure.

More preferably, the cathode comprises interlaced carbon fibres. U.K. Patent Specification No. 1,426,736 describes and claims a combustible electrode suitable for use in the present invention. Such an electrode comprises carbon fibres as the electrically conducting material and a solutionpermeable support therefor formed of a combustible material. A useful feature of this type of electrode is that it can be considered expendable and sent entire with its electrodeposit of metal for refining. The metal can be separated from the electrode by incinerating the electrode.

In a particular embodiment of the electrolytic cell, the cathode is mounted on a carrier adapted to be withdrawn from the cell. The carrier may be combustible so that for ease of metal recovery, the mounted cathode can simply be removed from the cell and incinerated. To facilitate withdrawal of the cathode from the cell the carrier may be provided with a handle.

The permeable cathode may have any convenient configuration e.g. flat, cylindrical or bagged. The anode may be disposed as is convenient for the particular configuration of the cathode.

In a particular embodiment of the electrolytic cell the cathode has a hollow cylindrical shape. Preferably, the anode also has a hollow cylindrical shape and is disposed co-axially with the cathode.

If the anode is outside the cathode it may be used as the container for the electrolyte.

It maay be advantageous for the anode to be a permeable electrode.

The electrolytic cell may comprise more than one cathode and/or anode. When more than one cathode is employed, theecathodes are preferably arranged so that, when the cell is in use, the electrolyte passes through the cathodes in turn. Alternatively, the cathodes may be arranged in parallel with respect to the flow of solution.

In order to maintain a sufficiently high rate of flow of electrolyte through the cathode it may be desirable for pumping means to be associated with the cell.

Clearly, as metal becomes deposited in the cathode it becomes less permeable and when the cathode becomes heavily deposited with metal there may be local areas where passage of the electrolytic is less free than in others. In order to avoid the cathode being subjected to excess pressure from the electrolyte, it may be desirable to provide the cell with means to permit the electrolyte to by-pass the cathode once a particular pressure has been exceeded.

It has been found that an electrode assembly and, more particularly, an electrolytic cell according to the invention is particularly useful for the recovery of silver from silver-containing photographic solutions such as fixing solutions.

For the efficient recovery of silver from fixing solutions it is necessary to use a high current density at the cathode (see G.I.P.

Levenson and C.J. Sharpe, J. Photogr. Sci., 23, 216-218, 1975). It is known in the art of the electrolytic de-silvering of fixer solutions at high current density that it is necessary to produce a high degree of agitation at the surface of the cathode in order to prevent a local depletion of silver ion. In the absence of silver ion, the voltage on the cathode falls enough to reduce the thiosulphate present to sulphide which diffuses into the body of the fixer and precipitates silver sulphide which can foul the system and impair subsequent plating on the cathode.

An electrode according to the invention enables the high agitation necessary to avoid sulphiding to be achieved in a particularly effective manner. Making the cathode expendable offers another advantage.

An electrolytic cell according to the invention can be applied to the fixing solution in current use in a photographic processing system, or to a reserve tank of used fixer or to a body of solution into which silverbearing fixer is supplied from a reserve tank at a metered rate so as to maintain a constant load of electrolytic recovery on the cell. The operative part of the cell, namely, cathode(s), anode(s) and, optionally, pumping means may be free-standing or it may be adapted for immersion into a tank of metalcontaining solution as an electrode assembly according to the invention.

The invention will now be described further by way of example, with reference to the accompanying drawings, in which: Figure 1 is a partially broken away, perspective view of an early experimental electrolytic cell which is convenient for illustrating the essential features of the invention.

Figure 2 is a cross-section of the cell shown in Figure 1 viewed from the end and taken half-way along the cell; and, Figure 3 is a schematic view of another electrolytic cell according to the invention.

The cell of Figures 1 and 2 comprises a rectangular box made from two shallow trays 15 and 15' clamped together on either side of a plane rectangular cathode with suitably interposed gaskets 18 and 18'. The two halves of the cell are clamped together by means of pairs of clamping bars 17 and The cathode comprises an open rectangular frame 11 which is clamped between the two halves of the cell. The opening defined by the frame 11 is covered by interlaced graphite fibres contained in netting 13 which divides the interior of the cell into two compartments. A wire 12 passes through a groove in the frame 11 and into electrical contact with the graphite fibres to serve as a cathode lead.

Stainless steel plates 14 and 14' serving as anodes are attached to the inner wall of each half of the cell facing the cathode by means of bolts 19.

Each half of the cell is provided with an inlet/outlet pipe shown as 16 and 16'. These permit an electrolyte to be passed in either direction through the cell and through the permeable cathode.

In operation, solution to be electrolyzed is passed through the cell from inlet to outlet thereby flowing through the cathode in one direction while electrolysis takes place.

The electrolytic cell shown in Figure 3 comprises a hollow cylindrical anode 11 made of a suitable grade of stainless steel.

The anode may form the wall of the cell or it may be mounted inside a cell wall of insulating material. The anode 11 has an open end in which a water-tight lid 12 is fitted. A lid is optional and would only be used if the cell was employed in an in-line mode or at a level below that of open parts of the circulation system. The lid 12 is provided with a terminal 13 on its outer face. A wire 23 connected to the terminal 13 passes through a hole in the lid 12 fitted with a water-tight gland and is connected at its other end to a permeable cathode 17 inside the cell. That part of the wire outside the cathode is insulated to prevent electrodeposition on itself. When a lid is not used, a terminal may be provided above the level of the electrolyte.

The cathode 17 is a hollow, cylindrical electrode made of carbon fibres sandwiched between layers of a netting material e.g.

Terylene netting ("Terylene" is a registered Trade Mark). The cathode is co-axially disposed within the anode and is mounted at each end on a non-conductive, substantially rigid carrier 14. The base of the carrier 14 is attached by screw thread to an inlet pipe 15 entering the sealed end of the cell 18.

Alternative means of attaching the carrier to the cell include bayonet attachment or cone attachment. Any form of attachment is suitable which holds the carrier firmly in place and provides easy release of the carrier from the cell. The inlet pipe 15 opens into a manifold chamber 19 forming the base of the carrier 14. The carrier 14 comprises a pillar 20 extending co-axially within and spaced apart from the cathode 17. The pillar 20 extends from the manifold chamber 19 to a point beyond the far end of the cathode 17 where it terminates in a handle 21. The wall of the pillar 20 tapers so that the gap between the pillar and the cathode is widest at the end nearest the inlet 15. A number of small holes 22 connect the space between the pillar and the cathode with the manifold chamber.

The sealed end of the cell 18 is also provided with an outlet pipe 16.

In operation, an electrolyte is passed through the cell during electrolysis via the inlet pipe 15 and the outlet pipe 16. On passing through the inlet pipe 15, the electrolyte enters the manifold chamber 19 and is distributed by holes 22 into the space between the carrier 20 and cathode 17. The space is relatively narrow so that the velocity of the eletrolyte moving on the inside of the cathode is increased, particularly when the cathode is heavily encrusted with deposited metal. The electrolyte passes through the cathode 17 towards the anode 11 and leaves the cell through the outlet pipe 16.

Relief holes may be provided in the end of the carrier 20 remote from the inlet so that when the cathode becomes blocked with deposited metal at least part of the electrolyte can pass through these holes.

The electrolyte may be pumped through the cell at a high rate e.g. 600 gallons per hour for a cell of 11 litres capacity.

When sufficient metal has been deposited at the cathode it may be detached from the cell by unscrewing the carrier and withdrawing from the cell using the carrier handle.

Thus a cathode can be removed and replaced with a new one very conveniently.

When the carrier is made of a combustible material, the metal can be recovered by simply incinerating the cathode/carrier combination.

The cell is especially useful for recovering silver from solution e.g. a spent fixer solution. Placing the cathode inside the anode has the advantage of reducing relatively the current density at the anode. As a result it is possible to use stainless steel as the anode and moreover, the rate of oxidation of bisulphite ion at the anode decreases with decreasing current density.

In an alternative construction of the cell described above, the carrier has a parallelsided pillar and the cathode is tapered so that the space between the pillar and cathode has the same configuration as the corresponding space in the above cell.

The invention will now be described by way of example.

Example 1 An electrolytic cell was constructed in the form of a rectangular Perspex resin box of internal dimensions 26 x 26 x 11 cms.

("Perspex" is a registered Trade Mark). For experimental convenience it comprised two shallow trays each measuring 26 x 26 x 5 cms. which could be clamped together on either side of a plane rectangular cathode with suitably interposed rubber gaskets. A stainless steel plate measuring 26 x 26 cms.

was bolted to the inside of both the halves of the cell so that the cathode was flanked on either side by an anode at a distance of about 5 cms.

The cathode was made by taking 75 cms.

of Grafil type AU ("Grafil" is a registered Trade Mark) which has about 7000 filaments of graphite, teasing it out to a loose tangle which was then sandwiched between two layers of nylon curtain netting. Before closing the sandwich, a wide loop of 0.7 mm. diameter silver wire was laid on the fibre and the end of the wire was brought out to serve as the connector. The sandwich was stretched on an open frame of PVC in which a groove had been cut to embed the connector wire to allow it to pass under the gasket. The PVC frame was made wider than the cathode area to provide a seating for the gasket on either side.

The cell was arranged with a header tank and a pump so that, in operation, the pump drew solution from the space on one side of the cathode and delivered it to the header tank from which it returned under gravity to the cell on the other side of the cathode.

The cell was run at a current of 5 amps (equivalent to a theoretical deposition rate of 20 g of silver per hour) while a conventional fixing solution based on ammonium thiosulphate was circulated. Silver bromide was added to the solution from time and the fixer components were replenished.

Silver was deposited on the carbon filaments of the cathode and was held there although the solution was streaming at about 1000 liters an hour through the cathode and no silver was shed onto the bottom of the cell.

On occasion, the silver concentration was allowed to fall as low as 0.2 g per litre of solution and no sign of sulphiding was observed.

Example 2 A cell of the type shown in Figure 3 was constructed with a cathode having one tenth of a square metre nominal area (i.e. not the actual surface area of the carbon fibre filaments) in a stainless steel cylinder of 220 mm diameter which also served as the anode. This cell could carry a current of 20 amps with the silver concentration in a fixing solution as low as 0.1 g/l without a sign of sulphiding. In practice, a current of about 10 amps was preferred to obtain a good metal deposit. In this experimental cell the cathode carrier was made integral with the lid of the cell to facilitate its rapid production and in effect, it worked like an inverted version of Figure 3.

WHAT WE CLAIM IS.: 1. A method of recovering a metal from a solution containing metal ions which method comprises electrolyzing the solution with an anode and a permeable cathode, which cathode comprises carbon fibres as the electrically conducting material of which carbon provides the working surface and a permeable support for the carbon fibres formed of a combustible material, wherein substantially all the solution being electrolyzed is passed through the cathode in one direction as hereinbefore defined.

2. A method as claimed in Claim 1 wherein the solution is pumped through the cathode.

3. A method as claimed in Claim 1 or Claim 2 wherein the carbon fibres are interlaced.

4. A method as claimed in any one of the preceding Claims wherein the cathode is uniformly permeable.

5. A method as claimed in any one of the preceding Claims wherein the solution to be electrolyzed contains silver ions.

6. A method as claimed in Claim 5 wherein the solution is a photographic fixing solution.

7. A method as claimed in Claim 1 substantially as hereinbefore described with reference to and as illustrated in Figures 1 and 2.

8. A method as claimed in Claim 1 substantially as hereinbefore described with reference to and as illustrated in Figure 3.

9. A method as claimed in Claim 1 substantially as hereinbefore described in Example 1.

10. A method as claimed in Claim 1 substantially as hereinbefore described in Example 2.

11. A method as claimed in any one of the preceding Claims wherein after electrolysis the cathode is incinerated to recover the metal.

12. A metal whenever recovered by a process as claimed in any one of the preceding Claims.

13. An electrode assembly comprising an anode, a permeable cathode comprising carbon fibres as the electrically conducting material of which carbon provides the working surface and a permeable support for the carbon fibres formed of a combustible material, and means to constrain an electrolyte to flow through the cathode in one direction as

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. way of example. Example 1 An electrolytic cell was constructed in the form of a rectangular Perspex resin box of internal dimensions 26 x 26 x 11 cms. ("Perspex" is a registered Trade Mark). For experimental convenience it comprised two shallow trays each measuring 26 x 26 x 5 cms. which could be clamped together on either side of a plane rectangular cathode with suitably interposed rubber gaskets. A stainless steel plate measuring 26 x 26 cms. was bolted to the inside of both the halves of the cell so that the cathode was flanked on either side by an anode at a distance of about 5 cms. The cathode was made by taking 75 cms. of Grafil type AU ("Grafil" is a registered Trade Mark) which has about 7000 filaments of graphite, teasing it out to a loose tangle which was then sandwiched between two layers of nylon curtain netting. Before closing the sandwich, a wide loop of 0.7 mm. diameter silver wire was laid on the fibre and the end of the wire was brought out to serve as the connector. The sandwich was stretched on an open frame of PVC in which a groove had been cut to embed the connector wire to allow it to pass under the gasket. The PVC frame was made wider than the cathode area to provide a seating for the gasket on either side. The cell was arranged with a header tank and a pump so that, in operation, the pump drew solution from the space on one side of the cathode and delivered it to the header tank from which it returned under gravity to the cell on the other side of the cathode. The cell was run at a current of 5 amps (equivalent to a theoretical deposition rate of 20 g of silver per hour) while a conventional fixing solution based on ammonium thiosulphate was circulated. Silver bromide was added to the solution from time and the fixer components were replenished. Silver was deposited on the carbon filaments of the cathode and was held there although the solution was streaming at about 1000 liters an hour through the cathode and no silver was shed onto the bottom of the cell. On occasion, the silver concentration was allowed to fall as low as 0.2 g per litre of solution and no sign of sulphiding was observed. Example 2 A cell of the type shown in Figure 3 was constructed with a cathode having one tenth of a square metre nominal area (i.e. not the actual surface area of the carbon fibre filaments) in a stainless steel cylinder of 220 mm diameter which also served as the anode. This cell could carry a current of 20 amps with the silver concentration in a fixing solution as low as 0.1 g/l without a sign of sulphiding. In practice, a current of about 10 amps was preferred to obtain a good metal deposit. In this experimental cell the cathode carrier was made integral with the lid of the cell to facilitate its rapid production and in effect, it worked like an inverted version of Figure 3. WHAT WE CLAIM IS.:

1. A method of recovering a metal from a solution containing metal ions which method comprises electrolyzing the solution with an anode and a permeable cathode, which cathode comprises carbon fibres as the electrically conducting material of which carbon provides the working surface and a permeable support for the carbon fibres formed of a combustible material, wherein substantially all the solution being electrolyzed is passed through the cathode in one direction as hereinbefore defined.

hereinbefore defined.

14. An electrode assembly as claimed in Claim 13 comprising pumping means to pump the electrolyte through the cathode.

15. An electrolytic cell comprising an electrolyte container having an inlet and an outlet which permit the flow of an electrolyte through the cell and, arranged in the cell, an anode and a permeable cathode, which cathode comprises carbon fibres as the electrically conducting material of which carbon provides the working surface and a permeable support for the carbon fibres formed of a combustible material, whereby when the cell is in use, substantially all the electrolyte flowing through the cell passes through the cathode in one direction as hereinbefore defined.

16. A cell as claimed in Claim 15 wherein the carbon fibres are interlaced.

17. A cell as claimed in Claim 15 or Claim 16 wherein the cathode is uniformly permeable.

18. A cell as claimed in any one of Claims 15 to 17 wherein the cathode is mounted on a carrier adapted to be withdrawn from the cell.

19. A cell as claimed in Claim 18 wherein the carrier is combustible.

20. A cell as claimed in Claim 18 or Claim 19 wherein the carrier comprises a handle for withdrawing the carrier from the cell.

21. A cell as claimed in any one of Claims 15 to 20 wherein the cathode has a hollow cylindrical shape.

22. A cell as claimed in Claim 21 wherein the anode has a hollow cylindrical shape and is disposed co-axially with the cathode.

23. A cell as claimed in any one of Claims 15 to 22 which comprises means permitting the electrolyte to by-pass the cathode once a particular pressure of electrolyte on the cathode has been exceeded.

24. A cell as claimed in any one of Claims 15 to 23 in association with pumping means to pump electrolyte through the cathode.

25. A cell as claimed in Claim 15 substantially as hereinbefore described with reference to and as illustrated in Figures 1 and 2.

26. A cell as claimed in Claim 15 substantially as hereinbefoe described with reference to and as illustrated in Figure 3.

27. A cell as claimed in Claim 15 substantially as hereinbefore described in Example 1.

28. A cell as claimed in Claim 15 substantially as hereinbefore described in Example 2.