Metal Recovery Method and Apparatus

This invention relates to methods and apparatus for recovering metals from liquid solutions, and in particular to methods and apparatus relating to recovery of silver from fixer solutions by disposable cells, with no moving parts and having an extremely high efficiency.

Up to the present time, recovery of conductive metals from solution, and particularly recovery of silver from fixer solutions, has been a relatively expensive process, requiring substantial mechanism and moving parts, as well as close supervision or complex computer controls, to accomplish. This is because the recovery process is electrolytic in nature, and the reaction if not closely monitored can cause sulfiding, damage to the solution and loss of silver. Hence continuous agitation is required as well as close control of the current being supplied to the reaction.

For instance, X-Rite Company offers a number of silver recovery systems, all of which include some type of means for agitating the solution. Further, most of the systems offered by X-Rite have a cathode which is coiled, thus having a relatively small surface area.

Similarly, Roconex Corporation manufactures a number of lines of silver recovery systems and markets them under the "Rotex" trademark. All of these systems include some type of agitation, generally with a rotating cathode which must then be removed from the recovery unit and cleaned, and later reinstalled and reused.

Moreover, all of the systems referred to above are relatively expensive, and there is a need in the marketplace for systems which are less expensive and mechanically simpler, since mechanical simplicity brings with it a high degree of reliability.

This invention relates to improvements to the apparatus described above and to solutions to some of the problems raised thereby.

The invention relates to an apparatus for recovering a conductive metal from a liquid which contains that metal, and to a method for recovering that metal using that apparatus. The invention is particularly well suited for recovery of silver from photographic fixer solutions. The apparatus includes a generally closed non-metallic container defining a cavity, for containing the liquid. A first electrode is supported and affixed within the cavity. A second electrode is composed of a thin film applied to the inside surface of the container, the film including as its main constituent the same metal as that to be recovered from the solution. Finally, power supply means are electrically connected to the electrodes so as to result in the first electrode being an anode and the second electrode being a cathode, thus causing the metal from the solution to be deposited on the second, film electrode, lining the inside surface of the container with the metal to be recovered.

The invention further includes means for circulating metal laden liquid, that is, liquid containing the metal in solution, into the container and for removing demetalized solution from the container. This circulating means includes a liquid introduction tube for introducing the metal-containing liquid near the bottom of the container and a liquid drain tube at about the top of the fluid level for drawing off the demetalized solution in an overflow manner.

The invention requires no agitators or other moving parts, because the current density supplied by the power supply is low, on the order of about 1.5 milliamperes per square inch. The current density can be this low and still process a substantial volume of liquid because of the large surface area of the cathode, that is, the entire interior surface of the container. The recovery operation is further facilitated by the fact that the metal recovered thereby is not required to be removed from the cathode, because the container itself is disposable, constructed from materials such as plastic which can be added to the smelting furnace without contaminating the smelting operation.

The method of the invention includes providing a disposable, electrically insulating container for containing metal-containing solution and applying a thin film of the metal to the inside surface of the container. An electrode is affixed within the container so that it is immersed in the liquid and insulated from film on the inside surface of the container. The voltage potential of the electrode over that of the thin film is then raised to about 1.5 volts, thereby causing the metal in the solution to be electrolytically deposited on the film and to build up thereon. This voltage causes a current density between the electrode and the film of about 1.5 milliamperes per square inch.

Other objects and advantages of the invention will become apparent hereinafter.

The drawing figure is a sectional view, partially schematic, especially with respect to the electrical control circuit, of a metal recovery apparatus constructed according to one embodiment of the invention.

While the invention is applicable to removal of any type of conductive ionic metal in solution, it is particularly well suited to an application wherein silver is removed from photographic fixer solutions. The following description will refer to that silver removal application as exemplary, but it should not be considered as limiting the intended scope of the invention.

Referring now to the drawing figure, an apparatus 10 constructed according to one embodiment of the invention includes a container 12, containing an amount of fixer solution 14. The fixer solution 14 is a solution in which a ionic silver is dissolved. The entire inner surface 12a of the container is coated with a very thin film 12b of silver, the metal to be removed from the solution, such as by spray painting. The actual thickness of the film 12b as applied is not critical, as will be shown presently, as long as the interior surface of the container 12 is evenly coated sufficiently thick to conduct electricity. The thickness is commonly on the order of 1 mil. The material of the film may be generally any type of paint or other sprayable film containing silver, such as Acrylic 1, Part No. 73-00025, from Tecknit EMI Shielding Products, or E-Kote 3040 from ACME Conductive Coatings.

An electrode 16 is suspended near the center of the container, reaching substantially into the solution 14. This electrode 16 can be of any suitable and readily available material for such an electrode, such as carbon/graphite rod material. For ease of assembly and mounting of the electrode 16 to the container, the electrode can be mounted in the center of a cover 18, which is attached to the top of and closes the container 12. Preferably the cover 18 is formed of insulative material so as to insulate the electrode from the thin film 12b. The electrode 16 thus mounted reaches downward into the solution 14 for the majority of its length.

The apparatus 10 further includes a power supply 20 for providing energy for an electrolytic reaction to plate the ionic silver out of the fixer solution 14. This power supply 20 is preferably a 1.5 volt DC power supply, having a positive pole 22 and a negative pole 24. The electrode 16 is electrically connected to the positive pole 22 of the power supply 20, preferably by a positive connector 26, which fits into a receptacle 28 provided for that purpose at the top of electrode 16. Similarly, the thin film 12b coating the inside surface 12a of the container 12 is preferably electrically connected to the negative pole 24 by a negative connector 30 which fits into a receptacle 32 provided for that purpose. Hence the electrode 16, being connected to the positive pole of the power supply 20, is the anode for the electrolytic reaction, while the thin film 12b in effect acts as another electrode, becoming the cathode for the electrolytic reaction because of its connection to the negative pole of the power supply.

In the simplest mode of operation of the apparatus 10, then, an amount of fixer liquid 14 is placed in the container 12, and the cover 18 carrying the electrode rod 16 is placed on top of the container to close it. The electrode 16 and the film 12b are then connected to the power supply 20 and the electrolytic reaction begins. During the electrolytic reaction, silver from the solution 14 is deposited on the silver film 12b until all or a suitable amount of the silver is plated out of the solution. As the silver builds up on the film 12b, the cathode in effect increases in thickness, improving its performance. This is the reason that the original thickness of the film 12b is not critical, since it increases as the reaction progresses. Because of the relatively large surface area of the cathode film 12b with respect to the anode electrode 16, the current density will be extremely low, on the order of 1.5 milliamperes per square inch. Hence the apparatus 10 may run relatively unattended, since it is clear that the danger of sulfiding and/or damage to the solution, which problem is carefully and expensively controlled and guarded against in the prior art, is very remote at this low current density level. When a sufficient amount of silver has plated out, the power supply 20 may be disconnected and the electrode 16 and the liquid 14 removed.

The used container 12 may then be placed in its entirety, including the cover 18, in a silver smelting furnace (not shown) to refine the silver for reuse. This is a major advantage of the present invention. In most presently existing silver recovery devices, the deposited silver must be somehow removed from the cathode before smelting, whether by scraping or some other physical means or process. This can be a difficult, expensive and dangerous job. Moreover, some silver is inevitably lost in the process. In order that the container 12 may accompany its contained silver into the smelting furnace, it is required to be made entirely of some material that easily refines out of the molten silver in the smelting process, such as a common plastic pail. Of course a handle 34 may be provided for ease of handling the container 12, but if it is of a metal containing copper, such as stainless steel, or some other metal which does not easily refine out of silver, it must be removably attached to the container, so that it can be removed prior to placement of the container in the smelting furnace, and even reused, as is the electrode 16. Preferably, however, it is simply conventional iron or zinc-plated (galvanized) iron, for low cost.

While it is possible to use the apparatus 10 as described above in "batch mode", it is more efficient to use it in "continuous" mode, as will now be described. In continuous mode, silver-containing solution is constantly being circulated into the container 12, and de-silvered solution is constantly being removed. In the preferred embodiment as shown in the figure, the silver-containing solution is continuously introduced into the container 12 by means of a liquid introduction tube 36 which has its outlet 36a near the bottom of the container. To enhance flow, the liquid introduction tube 36 may also include a vent portion 36b which extends upwardly above the surface of the liquid 14. As silver is removed from the solution, the solution becomes lighter, rising to the top of the liquid 14 in the container 12 in a naturally occurring phenomenon referred to as "stratified transport". The solution at the top of the liquid, then, will be relatively more desilvered compared to that at the bottom of the liquid. This relatively de-silvered solution at the top is continuously removed from the container 12 by a liquid drain tube 38 the inlet 38a of which is located in the sidewall of the container at the level of the top of the liquid. By proper relative placement of the source of the silver-containing solution, above the container 12, and of the reservoir of relatively de-silvered solution, below the container 12, any necessity for pumps or other mechanically powered devices to move the solution is avoided, since the solution will move by siphoning. Further, since the relatively de-silvered solution may not be completely de-silvered from one pass through the apparatus 10, it may be desirable to connect a number of such apparatus together serially, so as to achieve the greatest possible silver extraction rate.

Both tubes 36 and 38 are preferred to be of plastic in order to be consistent with the objective, referred to above, that the entire container 12 may be placed in the smelting furnace when sufficient silver has been deposited.

Because of the extremely low current density allowed by the present invention, as mentioned above, the power supply 20 may be a generally conventional plug-in module type, as shown. As an additional aid in controlling and ensuring the integrity of the de-silvering reaction, the power supply 20 may also include certain additional features. In particular, in one embodiment of the present invention, the power supply 20 first includes a generally conventional transformer module 40, which plugs into a conventional 110 volt or 220 volt electrical outlet 42 and outputs 5 volts DC, up to one amp, via a positive lead 44 and a negative lead 44. A calculator-type transformer module with these characteristics is particularly well suited for this application. Connected between the two leads are a current limiting resistor R1 and a light emitting diode D1 connected in series. The purpose of the diode D1 is to indicate that the transformer module 40 is indeed receiving power from the outlet 42. Hence, whenever the outlet 42 is supplying power, the diode D1 is lit. Also connected to the positive lead 44 is a first lead of a voltage regulator IC1. The anode, or electrode 16, of the apparatus 10 is connected to a second lead of IC1. Finally, a third lead of IC1 is connected to the second lead by a resistor R2 and, via a potentiometer R3, to the negative lead 46 of the power supply transformer module 40. The purpose of the described arrangement of the voltage regulator IC1 and resistors R2 and R3 is to ensure that the current passing to the electrode 16 remains extremely low as described above. The potentiometer R3 allows adjustment of the circuit for tolerances of the components and for various sizes of containers 12. Preferably potentiometer R3 would be adjusted so as to provide 1.5 volts DC to the electrodes 12b and 16, resulting in the extremely low current density set forth above. Finally, flashing light emitting diode D2 is connected between the anode and cathode of the electrolysis circuit, that is, between the positive connector 26 and the negative connector 30. This diode D2 will flash if there is no current passing between the electrode 16 and the film 12b, thus acting as an indicator of the integrity of the electrolysis circuit.

While the apparatus hereinbefore set forth is effectively adapted to fulfill the aforesaid objects, it is to be understood that the invention is not intended to be limited to the specific preferred embodiment of metal recovery method and apparatus set forth above. Rather, it is to be taken as including all reasonable equivalents within the scope of the following claims.