Chemistry: electrical and wave energy – Processes and products – Electrostatic field or electrical discharge
Patent
1981-07-23
1984-01-24
Andrews, R. L.
Chemistry: electrical and wave energy
Processes and products
Electrostatic field or electrical discharge
204269, C25C 120, C25C 700
Patent
active
044275056
DESCRIPTION:
BRIEF SUMMARY
This invention relates to the recovery of metals from metal rich electrolytes.
The invention was developed primarily for use in association with automatic photographic film processors for the extraction of silver from the effluent solution from such processors and is described hereinafter primarily with reference to that application. However it will be appreciated that the invention is applicable generally to metal recovery units and not only to units for the recovery of silver.
Two types of recovery units are commonly used in associated with film processors, namely, the continuous, or flow through type, and the batch type. In one known form of the continuous type unit the metal rich solution is fed into the bottom of a plating-out cell and allowed to rise gently through that cell during which progress the metal is electrolytically plated onto electrodes utilizing a low current density. Another known continuous type unit employs agitation of the electrolyte which causes total mixing and may be operated at a somewhat higher current density. In both instances the depleted solution is discharged from the top of the cell.
As a general rule, the supply of solution from a film processor is not at a constant rate, indeed, depending upon the work load, it is customarily intermittent, the pH of the solution is apt to fluctuate from time to time affecting conductivity, the concentration of silver in the solution is also variable and of course the operation of the recovery unit itself may depart from preset values due, for example, to the deposit of sulphur on the anode thereby reducing the electrical conductivity of the cell and due to the development of faulty electrical contacts between the supply leads and the electrodes as a result of chemical splash and evaporation. Thus completely successful operation cannot be achieved simply by setting the amperage and the electrolyte flow rate in a flow-through type recovery unit to suit the conditions pertaining or anticipated at the start of operations.
That impossibility is recognised in the specification of Australian Pat. No. 444,212 which seeks, by providing an upstream holding tank and by switching the electricity supply to the electrodes on and off in concert with the operation of the metering pump producing flow of electrolyte through the cell, to correct or allow for the intermittent or variable nature of the rate of supply of feed electrolyte. However, this still allows the remaining causes for faulty operation to persist. Thus even with the apparatus described in that patent if, as is usually the case, the effective current has been set to a predetermined constant value, in some instances the current will be less than necessary to extract all, or nearly all, of the silver and the discharged solution may still contain significant quantities of silver.
On the other hand the amperage may be too high, in which event, after all, or nearly all, of the silver has been plated out, other elements, usually sulphur as silver sulphide, will be deposited, thereby undesirably contaminating the silver deposit. In more extreme cases of over-extraction, sulphur deposits will cover the anode causing a decrease in electrode efficiency, lowering the amperage and reducing the extraction rate proportionally.
Another known type of monitor which seeks to deliver a predetermined flow of electrolyte through the unit provides for a submersible pump feeding an elevated weir from which a flow control valve regulates the flow of electrolyte to the unit, but even though the electrolyte from the processor which contains free gelatin from the dissolved photographic emulsion, algae and other impurities may be filtered, some still persist and indeed grows within the holding tank where crystals of the salts may also form. These impurities interfere with the operation of the flow control valve even to the point whereby it becomes inoperative. This condition could go unnoticed until the next service were due.
The batch type unit has been developed in attempts to overcome most of the above-indicated
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