GB2092178A

GB2092178A - Electrolytic silver recovery

Info

Publication number: GB2092178A
Application number: GB8102682A
Authority: GB
Original assignee: Pennellier D & Co Ltd
Current assignee: Pennellier D & Co Ltd
Priority date: 1981-01-29
Filing date: 1981-01-29
Publication date: 1982-08-11
Also published as: GB2092178B

Abstract

Silver is recovered from photographic waste solutions and the like by electro-deposition onto a rotating horizontal drum 1, the quality of the deposited silver being monitored by an optical sensor 9 which controls the electro-deposition current to maintain optimum deposition conditions. To avoid redissolution of deposited silver if power fails, the drum is not immersed in the solution but receives a film of solution. <IMAGE>

Description

SPECIFICATION Electrolytic silver recovery This invention relates to the electrolytic recovery of silver from silver-containing solutions, for example spent photographic fixer. In the electrolytic recovery of silver, it is not easy to ensure that optimum electrolysis conditipns are maintained for deposition of silver, and to ensure that electrolysis is stopped when the silver concentration has fallen to a value at which the deposited silver is of poor quality. Another disadvantage of known silver recovery equipment, which uses electrodes immersed in a bath of the silver-containing solution, is that if the power for electrolysis fails for any reason, the silver already deposited may redissolve in the solution.

According to one aspect of the present invention, electrolytic silver recovery is controlled by monitoring the optical properties of the deposited silver, in particular its colour or reflectivity.

Under correct silver plating conditions, the silver deposit is bright. If the plating current becomes too high, or if the concentration of silver in the solution fails as the silver is substantially completely plated out, the colour of the silver deposit becomes darker. The onset of this darkening can be detected by a suitable optical sensor, which can operate an alarm or can automatically control the electrolysis process to keep the electrolysis current below the value at which the deposit darkens, or to cut off the current, for example if reducing the current does not restore brightness to the deposit, thereby indicating that the silver has been almost completely plated out.

This means of control can ensure the deposition of silver of consistent high quality, whatever the silver concentration in the liquor, within wide limits, and can thereby ensure maximum total silver recovery within the minimum time, together with reliable termination of the silver plating operation.

The silver may be deposited onto immersed electrodes as in conventional silver recovery equipment. However, this makes it difficult to observe the colour of the deposited silver. It may be necesary to withdraw the plating electrodes from the liquor in order to monitor to the deposited silver. Preferably, the silver is deposited on to a moving surface of which at least a part is not immersed, and the silver deposit is monitored on a region of this surface which is free or substantially free of liquid. The surface may for example be a rotating drum or a belt. It may dip into a bath of the silver-containing solution, the immersed part of the surface receiving a plated deposit of silver, which deposit is then carried out of the bath of solution by the moving surface. The outwardly m-oving silver deposit can then be monitored without interference from the silver solution.

The use of a moving surface to carry the silver deposit has the further advantage that the deposited silver can be continuously removed for example by a scraper which separates it from the moving surface. The surface can move continuously or intermittently.

The use of a moving surface to transport deposited silver away from the region of deposition, with or without the optical monitoring mentioned above, constitutes a second aspect of this invention.

According to a third aspect of the invention, silver is deposited on to a moving surface which is in contact with a film of the silver-containing solution but is not immersed in a bath of the latter.

This arrangement has the advantage that, should the electro-plating current fail, the silver already deposited will not redissolve. Should the film of solution continue to be present, there may be local redissolution but most of the deposited silver will be safe from this. In general, however, the film of solution will be supplied by pumping and the pumping power will cease at the same time as the electrolysis current, so that there can be no redissolution of the deposited silver. The moving surface may move continuously or intermittently, and can be, for example, a rotating drum or a belt.

In a particularly advantageous arrangement, all of the above-mentioned aspects of the invention are combined. That is to say, silver is recovered electro-deposition on a moving surface which is in contact with a film of silver-containing solution but is not immersed therein, and the electrodeposition process is controlled by monitoring the optical properties, in particular the colour or reflectivity, of the silver deposited on the surface, at a position preferably clear of the film of solution. The deposited silver may then be continuously removed from the moving surface by a scraper.

By way of example only, the accompanying drawing illustrates the various aspects of the present invention.

The drawing shows a drum 1, of which at least the surface is electrically conductive and which rotates about a horizontal axis. The drum is mounted above a tank 2 which contains a bath 3 of photographic bleach fix. The surface of the bath is however preferably below the bottom of the drum.

Near the top of the drum is a device for applying to the descending surface of the drum a film of the bleach fix liquor. This is shown as a tray 4 with a free edge close to the surface of the drum. The liquor from the tank is pumped into the tray by a pump 5. At least part of this tray is electrically conductive, being made for example of a sheet of carbon, and is connected to one pole of a DC supply to act as an electro-plating anode.

The conductive surface of the drum is connected to the other pole of the DC supply to act as a cathode.

In operation, as the drum rotates, the silvercontaining liquor is pumped on to it so as to form a film on the upper part of the drum and thereafter a curtain 6 falling from the drum into the tank, leaving behind the silver which has been plated out onto the surface of the drum.

The power supplies to the pump on the one hand, and the drum and liquor tray on the other hand, are in common or are so inter-related that if the power supply for electro-deposition fails, the pump stops. Consequently the drum will then remain dry and the silver deposited on it will not be redissolved by contact with the liquor.

Instead of a tray, the film of liquor may be supplied to the drum in any other convenient way, for example by a perforated pipe parallel to the drum, or a series of spray nozzles.

The drum may be removed bodily for separation of the deposited silver. Preferably, however, the silver is continuously removed by a scraper blade 7, which may for example be arranged just before the liquor supply device.

The plating current is controlled by an optical monitoring system which monitors the colour of the deposited silver, preferably in the dry ascending region of the drum surface. Any suitable optical sensing system can be used. The drawing shows, very schematically, a light source 8 which directs a beam of light (not necessarily visible) on to the drum surface, and a photo-sensor 9 on to which the light beam is reflected from the silver surface. Suitable electronic circuitry detects the intensity of the reflected light and controls the silver plating current to maintain the current at the maximum value consistent with a bright silver deposit. If the silver becomes darker, the control circuitry initially reduces the current.If this does not restore brightness to the deposit, the control circuitry will determine that there is now insufficient silver in the liquor for satisfactory recovery, and will stop the plating process.

To allow for local variations in the silver deposit, several optical sensing devices may be provided, with an averaged output, or the sensor may be made to observe an appreciable area of the drum surface.

The drum surface may consist of stainless steel or any other suitable material.

In an alternative arrangement, the drum may be partly immersed in the liquor bath, and in this case the liquor supply device can be omitted. However in this arrangement, if there is a power failure, the deposited silver may be redissolved from the immersed part of the drum.

The drum can be replaced by an endless moving belt. This can provide a larger surface area of contact with the liquor film, in a given overall apparatus volume, because the liquor film can be in contact with the belt over substantially the whole of one side of the latter, whereas in the case of a drum only one quarter of the circumference of the drum can be in contact with the film of liquor.

Typically the rate of rotation of the drum may be 6 to 60 rpm, depending on whether the solution is fixer or bleach fix.

Claims

1. A method of electrolytic recovery of metal from a solution by electrolytic deposition of the metal, in which the deposition process is controlled in dependence on a monitored optical property of a deposited metal.

2. A method as claimed in claim 1 in which the metal is silver.

3. A method as claimed in claim 2 in which the monitored optical property is the colour or reflectivity of the deposited silver.

4. A method as claimed in claim 1, 2 or 3 in which the metal is deposited onto a moving surface by which the deposited metal is carried away from the solution.

5. A method as claimed in claim 4 in which the metal is deposited onto a drum which rotates about a substantially horizontal axis.

6. A method as claimed in claim 4, 5 or 6 in which the moving surface is partly immersed in a bath of the solution.

7. A method as claimed in claim 4, 5 or 6 in which the solution is delivered onto the moving surface.

8. Apparatus for the electrolytic recovery of metal from a solution, comprising electrodes disposed for contact with the solution in operation and for deposition of the metal on at least one electrode, at least one optical sensing device arranged to respond to an optical property of the deposited metal, and control means arranged to control the conditions of deposition of the metal in dependence on the monitored optical property.

9. A method of recovering a metal from a solution thereof in which the metal is electrodeposited onto a moving surface which is in contact with but is not immersed in the solution.

10. A method as claimed in claim 9 in which the said surface is the surface of a drum rotating about a substantially horizontal axis, the solution being supplied to an upper region of the drum surface.

1 A method of recovering silver from a silvercontaining solution, substantially as herein described with reference to the drawing.