GB2067598A

GB2067598A - Electrolytic recovery of silver

Info

Publication number: GB2067598A
Application number: GB8101414A
Authority: GB
Original assignee: Photographic Silver Recovery Ltd
Current assignee: Photographic Silver Recovery Ltd
Priority date: 1980-01-17
Filing date: 1981-01-16
Publication date: 1981-07-30

Abstract

A process and apparatus are provided for the recovery of silver from photographic solutions (11) by electrolytic plating in a tank (13), in which switching of the plating current and of a pump (22) for withdrawing desilvered solution from the plating tank (13) are controlled by level switches (19, 20) at two different levels in the tank above the cathode. The pump may be a metering pump and the upper and lower level switches may be respectively employed to switch the plating current and pump on and off together. Or the upper and lower level switches may be employed respectively to switch the plating current on and the pump off, the duration of the plating current and the switching on of the pump being determined by a timer. <IMAGE>

Description

SPECIFICATION Improvements in or relating to the recovery of silver This invention relates to the recovery of silver from the silver solutions that result in the processing of photographic and X-ray films and plates.

In our Patent No. 1285602 there is described a silver recovery system in which a metering pump withdraws solution to be treated from a storage or buffer tank and delivers it to a silver recovery unit in which the silver is deposited by electrolytic plating, the metering pump being switched on and off by level switches in the buffer tank. The present invention enables the buffer tank to be eliminated and facilitates the recycling of desilvered fix solution.

According to the present invention, there is provided a method of recovering silver by electrolytic plating from the silver solutions that result from the processing of photographic and X-ray films and plates, wherein an electrolytic plating cell, into which solution to be desilvered is delivered on a continuous or intermittent basis, is provided with liquidlevel-responsive switches situated at two different levels above the cathode and tne operations of those switches by the rising and falling of the solution level in the cell are employed to control switching of the plating current and of a pump that withdraws substantially desilvered solution from the cell.

In one embodiment, the plating bath is fitted with a level-sensing system which starts the metering pump to withdraw solution from the plating bath when the solution rises to a predetermined high level, and stops it when the solution falls to a predetermined lower level. As in the case of our prior patent, the plating current is switched on whenever the metering pump is switched on, and plating ceases when the pump stops. Thus a strict relationship between the plating current duration and the flow of solution through the unit is maintained.

The outlet from the metering pump is connected to a solution re-cycling system or, if the desilvered solution is not required, directly to a drain. An overflow from the plating bath higher than the aforesaid high level is fitted so that in the event of pump failure flooding is prevented. A high level warning device signals this condition.

Advantages of this system are: i) simplification od construction and manufacture by combining the functions of the storage or buffer tank arid the plating bath; ii) without the inclusion of another pump, the system is able to provide a pumped supply of desilvered solution for recycling at a pressure above atmospheric.

In an alternative arrangement, which also maintains the constant relationship between plating current duration and flow of solution through the unit, the plating current is switched on when the high solution level is reached and continues for a pre-selected time to remove a predetermined amount of silver.

This is achieved by the use of a cycle timing and switching device. The plating current is then switched off and a pump is started which reduces the level in the plating bath to the lower level. This pump need not be a metering pump. On the lower level being reached, the pump is stopped.

The cycle is repeated when the solution entering the plating tank from the processor restores the level to that at which the plating current is switched on.

The invention further provides apparatus for performing either of the above methods, comprising an electrolytic plating cell containing an anode or anodes and a rotary cathode, a solution feed pipe for delivering solution to be treated into the cell, two level switches in the cell at two different levels above the cathode, a solution outlet and associated withdrawal pump for withdrawing solution from the cell at a level below the top of the cathode, and circuitry whereby the switching on and off of the cell plating current and the pump are controlled by the operations of the level switches.

Arrangements in accordance with the invention will now be described in more detail by way of example with reference to the accompanying drawings, in which: Figure 1 is a diagrammatic representation of a silver recovery system associated with a photographic processor, Figs. 2 and 3 are diagrams of anode and outlet arrangements in the plating cell, and Figure 4 is a diagram of a cathode and inlet arrangement.

Referring firstly to Fig. 1, fixer solution to be treated flows from a processor 11 via a fixer input pipe 1 2 into the tank 1 3 of a silver recovery unit. Centrally within the lower part of the plating tank 1 3 is a rotary cathode 1 4 driven by a motor 1 5 through gearing 1 6 and a vertical shaft 17. The anodes 18 are located at the side of the tank. At higher and lower levels 'A' and 'B', both above the top of the cathode 14, are sensors 19, 20 for level switches. An outlet 21 is provided for desilvered fixer solution at a level below the top of the cathode.

When the fixer solution has risen to level 'A', the fixed level plating current, the cathode drive motor 1 5 and a constant flow metering pump 22 are switched on. The metering pump 22 then withdraws solution through the outlet 21 and delivers it to a replenisher tank 23, and it will run until the lower level 'B' is reached. Both the pump 22 and the plating current and then switched off.

Make-up solution for the processor 11 is provided by withdrawing desilvered fixer solution from the replenisher tank 23 by means of a replenisher pump 24.

The metering pump flow rate and the plat ing current amperage are chosen to remove all the silver in the input solution. For exam ple, assume the quantity of solution contained in the tank 1 3 between levels 'A' and 'B' is 1 litre and the initial silver content is 6 gm/litre.

If the plating current amperage is 1 5 it will plate out silver at a rate of 1 gm/min, so that 6 minutes is required to plate out all the silver in 1 litre of solution. The flow rate of the metering pump is therefore set at 1 66 ml/min .

In order to prevent. sofaras possible, silver bearing solution from being withdrawn from the plating tank via the outlet 21, the outlet is placed amongst or behind the anodes 18, e.g.

as in Fig. 2 or Fig. 3, while the inlet 12 is located at the opposite side of the tank away from the anodes. As the solution flows across the tank from inlet to outlet the silver cations will migrate to the intervening cathode and will not reach the outlet.

However, if a drum type cathode 25 is employed, as in Fig. 4, instead of the multiple plate type of Fig. 1, the solution inlet 1 2 should be directed to the interior of the drum rather than being located at the far side of the tank from the anodes.

The metering pump can be replaced with a non-metering, e.g. a centrifugal, pump if the system is operated in a different way. When the fixer solution reaches the level 'A' the silver recovery unit is switched on as before, but the pump is not. Silver is then desposited for a preselected time determined by a timer.

At the end of the preset time, the silver recovery unit is switched off and the pump 22 starts to return fixer to the replenisher tank 23 for reuse. When the fixer solution has dropped to the level 'B' the pump is switched off. The time of operation of the plating unit is selected so as to plate out all the silver from the quantity of solution subsequently pumped out by the pump, which, in the example given before, will be 6 minutes.

Claims

1. A method of recovering silver by electrolytic plating from the silver solutions that result from the processing of photographic and X-ray films and plates, wherein an electrolytic plating cell, into which solution to be desilvered is delivered on a continuous or intermittent basis, is provided with liquidlevel-responsive switches situated at two different levels above the cathode and the operations of those switches by the rising and falling of the solution level in the cell are employed to control switching of the plating current and of a pump that withdraws substantially desilvered solution from the cell.

2. A method according to claim 1, wherein the pump is a metering pump, and the pump and the plating current are switched on when the solution level reaches the upper level switch and off when the solution level falls to the lower level switch.

3. A method according to claim 1, wherein when the solution level reaches the upper level switch the plating current is switched on for a pre-selected period of time and the pump is inoperative, after which the plating current is switched off and the pump is switched on to reduce the solution level to the lower level switch whereupon the pump is switched off.

4. A method according to claim 2, wherein the metering pump flow rate and the plating current amperage are chosen to remove substantially all the silver from the treated solution.

5. A method according to claim 3, wherein the period of time for which the plating current is switched on is selected so as to plate out substantially all the silver from the quantity of solution subsequently pumped out by the pump.

6. Apparatus for performing the method according to claim 1, comprising an electrolytic plating cell containing an anode or anodes and a rotary cathode, a solution feed pipe for delivering solution to be treated into the cell, two level switches in the cell at two different levels above the cathode, a solution outlet and associated withdrawal pump for withdrawing solution from the cell at a level below the top of the cathode, and circuitry whereby the switching on and off of the cell plating current and the pump are controlled by the operations of the level switches.

7. Apparatus according to claim 6, further comprising a repieneisher tank into which the pump delivers.

8. Apparatus according to claim 7, further comprising a photographic processor from which silver solution flows to the feed pipe and into which fixer solution is pumped from the replenisher tank by a replenisher pump.

9. Apparatus according to claim 6 or claim 7 or claim 8, wherein the withdrawal pump is a metering pump.

10. Apparatus according to claim 6 or claim 7 or claim 8, wherein the circuitry includes a timer for causing the plating current to be switched on for a preset period of time by operation of the upper level switch and the pump to be started at the end of said preset period.

11. Apparatus according to any one of claims 6 to 10, wherein the anode or anodes are near the side wall of the cell and the outlet is placed behind or amongst the anode or anodes.

1 2. Apparatus according to claim 11, wherein the feed pipe delivers at a location at the opposite side of the cell away from the outlet.

1 3. Apparatus according to claim 11, wherein the cathode is of drum type and the feed pipe delivers to the interior of the drum.

14. Apparatus for recovering silver by electrolytic plating, substant.ally, as described with reference to Figs. 1 and 2 or 3, or as modified according to Fig. 4.