US3431187A - Gold recovery - Google Patents

Description

March 4, 1969 L. E. LANCY GOLD RECOVERY I of 3 Sheet Filed Nov. 22, 1965 my 2 Na WL m5 m l a L fi/W Y B ll o Wm mmmm M? I nwmmn l|.|l ||l lilllll on -ll| in m f L. E. LANCY GOL-D RECOVERY March 4, 1969 Sheet Filed Nov. 22, 1965 INVENTOR. Les/1e .E Laney H/S ATTORNEYS United States Patent 3,431,187 GOLD RECOVERY Ellwood City, Pa., assignor to Laney Inc., Zelienople, Pa., a corporation of This invention relates to the recovery of gold carried over from a plating operation and particularly, to new and improved procedure and apparatus for removing gold from dragout or washing solutions.

The commercial plating of gold metal has many fields of use and is particularly important in the electronics field where lines are used for plating electrical contacts, printed circuits, etc. Each production line customarily has its own electrolytic gold plating solution. Due to the scarcity of gold and its cost, it is important to effect a maximum utilization of the metal in operations of this type. Gold from the solution, of course, is plated out on the surfaces which are undergoing the processing, but it has been found that a considerable amount is lost in the form of carry-over or dragout from the plating solution. That is, wet film droplets adhere to the surfaces of the work pieces when they are moved out of or removed from the gold plating solution.

There is a need for a recovery operation that will be practical for economically recovering gold dragout from various types of plating solutions which, in the new in stallations, involve electroplating solutions in the neutral or in the acid pH range. Also, some plating lines make use of cyanide solutions. The electroplating baths usually contain about 1 /2 to 8 g./l. of gold in solution.

There have been a number of methods employed in endeavoring to recover waste gold from such solutions and all to date have made use of means at the end of each individual plating line for this purpose. I have found that there is a need for a recovery system or procedure which will make possible the combining of the dragout solutions from a number of plating lines in an industrial plant and the recovery of gold in the plant in a practical, efiicient, and effective manner.

In endeavoring to recover gold, it has been the usual industrial practice to wash the workpiece with a stagnant water bath which is called a dragout recovery aqueous solution. It will be apparent that the gold concentration in the dragout solution will progressively increase and that unless the solution is exchanged for a new water solution, the gold concentration will tend to approach the same concentration as maintained in the gold plating solution, itself. Depending upon the Work load and in accordance with one method, the dragout solution from the end of each individual plating line is bottled once a month and shipped to the refinery for gold recovery.

Another approach to gold recovery has been to pass each dragout solution through an ion exchange cartridge, so that the ion exchange material will pick up the gold salts as the solution is circulated through the cartridge. This resin cartridge is then returned to the refinery operator who burns the resin to reclaim the gold value. These two recovery processes are cumbersome and costly and involve considerable manipulation. Although I have found that gold may be obtained from a plating solution by electrolysis when the concentration in the dragout solution approaches about 500 to 600' mg./l., the usual gold deposit is powdery and non-adherent. Thus, electrolysis for gold recovery has not been practiced by those skilled in the art because the electrolytically regained gold is lost as a powder in the electrolyte solution. I have investigated the possibility of filtering out this powder, but have found that with the quantities in question, it is uneconomical on the basis of the filter areas that have to be maintained and the need to burn the filter paper to recover the small quantity of gold dust.

I have also determined that gold recovery from a neutral or acid gold solution is even more hampered and that the efliciency of the electrolytic recovery approaches zero with a gold concentration in solution of about 200 mg/l. or less. Cathode efiiciency is considered to be the ratio between the actual weight of the deposit gained against the calculated theroretical efiiciency from the standpoint of an electrical unit employed for metal deposition. The efiiciency of the cathode used for electrolytic recovery becomes less and less as the concentration of the metal salts decreases. I have also found that acid gold solutions are particularly adverse to such a recovery operation, in that the gold ions are in a higher valency state (Auric III valent). This, of course, means that for the same weight of gold recovery, three times the amount of current is needed than when, for example, valency ions (Aurous Ivalent) are available.

I have analyzed a number of gold dragout solutions and have found that the usual concentration is around or about 1 g./l. and that it is quite usual to have a dragout of 2 to 4 g./hour. It is now also usual to maintain three, four or more parallel plating lines, particularly in the elec' tronics field.

In view of the above considerations, it has been an object of my inventionto develop new and improved recovery procedure and means for, in a practical economical manner, saving or recovering the gold values that are carried-over or dragged out from a gold plating solution or line;

Another object of my invention has: been to provide means that may beeffectively, etficiently and economically used for directly recovering gold values from the lines at a gold plating installation or plant and without the need for shipping the gOld containing dragout solution or other means to a refinery to recover the gold;

A further object of my invention has been to develop practical means for universal application to the recovery of gold values from a gold plating line and particularly, from a group or plurality of lines simultaneously, and irrespective of the type of plating solutions used in the lines;

These and other objects of by invention will appear to those skilled in the art from the illustrated embodiment and the detailed description thereof.

In the drawings:

FIGURE 1 is a typical plant layout illustrating a system constructed and employed in accordance with my invention for recovering gold values from a plurality or group of plating lines which lines may have ditferent types of plant solutions;

FIGURES 2 to 6 illustrate details of a construction of a suitable electrolytic recovery tank employing my invention;

FIGURE 2 is a side the tank construction;

FIGURE 3 is a horizontal section of the same scale taken through the tank with its electrodes removed to better illustrate compartment separators;

FIGURE 4 is a vertical section on the scale of FIG URES 1 and 2 showing electrodes in place within compartments;

FIGURE 5 is an enlarged side elevational detail in partial section, taken at an upper corner of the tank showing the mounting of electrode bars; and

FIGURE 6 is an end elevational detail in partial section on the scale of and taken along the line VI-VI of FIGURE 5.

In devising a solution to the problem heretofore involved in recovering gold values from dragout solutions,

perspective view in elevation of I have been able to develop a system, procedure or means which will assure that the gold ions are in their low valency state, that is effective for recovery of gold from lines using various types of plating solutions, that can be employed at a central area or point for recovery of gold from a number of plating lines and irrespective of the particular plating solutions used, and that will provide a practical and economical means of substantially fully recovering the gold content of dragout or washing solutions.

I have developed an alkaline cyanide electrolytic recovery solution which solves the problem and is maintained at a minimum pH that is above that at which poisonous cyanide gas will evolve. Such a solution provides the gold ions in their low valency state (Aurous I valent) and is non-toxic to operating personnel. For example, if an acid is added to a cyanide solution having a pH or less than 8, hydrocyanic gas is evolved which is highly toxic. In accordance with my invention, I fully avoid the production of such a gas, and fully effectively and efficiently deposit a gold film on cathodes, even where the dragout solution has a gold content of as low as about or 6 milligrams per liter, by maintaining a pH of at least about or higher, with an optimum of about 12. I have been able to maintain the pH, either by employing a continuous supply of an alkali metal hydroxide, such as caustic soda, to neutralize acid dragin, or through the use of a pH controller and the addition of the alkali to the solution as based upon the reading of such controller.

I formulate a gold electrolytic recovery solution by the addition of an alkali metal cyanide, such as sodium or potassium cyanide, and an alkali metal hydroxide, such as sodium or potassium hydroxide, to the dragout or washing water within which the work piece has been rinsed after leaving one or more plating solutions. The same gold alkaline cyanide recovery solution may be used, irrespective of the type or types of solutions used for providing the desired electrical contact, printed circuit or other type of workpiece gold plating operation. I recirculate the treated dragout solution for reuse, so that any minimal gold content (below a concentration of about 5 to 6 mg./liter) is not lost, but may be removed during the subsequent recirculating or return of the used dragout solution back to the electrolytic recovery tank or cell. In this way, I also have been able to continuously recover the gold which is entered through dragin from gold plating baths that are in use.

I have been able to maintain a gold recovery electrolyte which is effective with a gold concentration as low as about 5 or 6 mg./l. and, at the same time, recover all the gold that has entered the system in the form of an adherent uniform and peelable gold foil, operating at an available efiiciency of about 20% and an average current density of about 5 amperes per square foot. The electrolytic recovery system makes use of a plating cell constructed for good mixing, cathode agitation and recirculation of the recovery solution through the system.

I have found that a minimum of about .5 of an ounce/ gallon of alkaline cyanide (as added, for example, to the bath or tank at station D of FIGURE 1 of the drawings) is sufiicient for starting the recovery operation. As the operation proceeds, the CN level automatically increases, due to the formation of alkaline cyanide in the gold recovery bath (see, for example, the tank or bath 30 at station E of FIGURE 1 of the drawings). I prefer an initial addition of cyanide of about 2 ounces/gallon, and find that the upper limit is only controlled by economic considerations.

Since at about 100% current efficiency, 5.15 amperes per hour would be required to plate out about 38 grams of gold per hour at an available cathode efliciency, I have found that about amperes per hour are sufficient to reclaim 38 grams of gold per hour in a plant operating two gold solution lines. In this example, one line was an acid gold plating line containing approximately 7.5 g./l. gold and the other a cyanide plating line containing approximately 25 g./l. of gold in their plating solution. Assuming one gallon of dragout per hour from each line, this would amount to 28.4 g./ hr. from the acid gold solution and about 9.5 g./hr. from the cyanide gold solution. By maintaining an electrolytic recovery system employing 50 to 60 square feet of cathode area, I have been able to recover gold dragout at the same rate that it is introduced into the electrolyte and to maintain an electrolytic recovery solution having a gold content of about 5-20 mg./l. Any loss of gold from such a solution of dragout is of a negligible quantity from the standpoint of gold recovery, in that 2 gallons of electrolyte solution per hour would contain a total of about mg. of gold in comparison with a recovery of about 37.9 grams. The electrode material for the recovery cell or tank may be of any suitable cathode material, although I have found that a stainless steel sheet is highly effective and provides an easy peeled off gold film of several millimeters in thickness.

In FIGURE 1 I have shown a representative system employing the principles of my invention. In this figure, A, B, and C are individual gold plating lines along which work pieces 9, 9 and 9", respectively, are moved or advanced to produce a finished gold plated product or workpiece at the end of the line, such as indicated by 9a of the line C. By way of example, the line A may employ a cyanide type of gold plating electrolyte in its plating bath or tank 10, line B may employ a neutral type of electrolyte plating solution in its plating bath or tank 10, and line C may employ an acid electrolyte plating solution in its plating bath or tank 10". As indicated by the dotted lines and the arrows, the workpieces of each line enter and pass through the baths or solutions of the tanks 10, 10, and 10", at which time, they are provided with a suitable plated surface, and are then moved into aqueous solutions of dragout recovery tanks 11, 11, and 11 which wash-01f and collect the carried over residual gold from the workpieces for recovery purposes. Subsequently, the workpieces then leave the recovery tanks as finished workpieces, such as represented by 9a of the line C.

It will be noted that washing solution is introduced into each of the recovery tanks 11, 11, and 11" through upstream deli-very pipes 35, 35, and 35", and as controlled by regulating valves 34, 34' and 3 4". Washing solution containing the dragout gold of each of the tanks 11, 11', and 11 is taken oif from the tanks from overflow troughs 12, 12, and 12", through downstream delivery branch lines 13, 13, and 13" that are connected to a main downstream return line 14. The line 14 delivers the dragout solution from the tanks of each line to a sump tank 15 at a solution conditioning or treating zone D. The sump tank 15 serves to condition the solution in readiness for the electrolytic removal of its gold content. A strong neutralizing alkali, such as NaOH, is supplied to the solution of the tank 15 from a supply tank or vat 16, through line 17 and a regulator valve 18, and down pipe 17a. The operation of the valve 1 8 is controlled by an electric solenoid 19a that is electrically-connected through a pH controller 19 to an electrode system 20 (shown as having two electrode wires). In this manner, the requisite amount of alkali may be automaticallysupplied or fed to the solution in the right hand portion of the tank 15 to provide it with the requisite pH, as sensed by the electrode system 20 and controller 19.

The tank 15 is shown provided with a partition member 15a that separates the tank into two compartments or zones and provides an overflow edge for passing treated solution from the right hand compartment or zone into the left hand or downstream cmpartment or zone. It will be noted that a stirrer having an actuating motor 22 (such as an electric motor), a propeller shaft 23 and a propeller or mixing blade 24, is employed to agitate and quickly mix the solution in the right hand compartment.

I have shown an electrode system 21 extending into the solution of the left hand compartment. It has a short length wire or electrode 21a and a longer length wire or electrode 21b and is electrically-connected to the motor of a pump 26 (see the dotted line of FIGURE 1). When the liquid or solution level reaches the end of the wire 21a, the pump 26 is actuated to pump-out treated solution. 0n theother hand, when for any reason, the liquid level falls below the end of the wire 21b, the motor of the pump 26 is de-energized to stop the pumping action. Conventional control devices may be used in this connection, or the electric motor of the pump 26 may be manuallycontrolled on the basis of a visual determination of the availability of liquid in the left hand zone. In the arrangement shown, the system 21 and pump 26 may be omitted and gravity flow utilized where, for example, tank has an upper positioning with respect to the tank 30' at station E.

The gold containing solution, after being suitably conditioned in the tank 15 to provide an alkaline cyanide solution, is then gravity-flowed or drawn by means of downstream pump 26 from the bottom of the tank 15, through line 27, into a recovery tank or cell 30 at station E. It will be noted that the solution is properly pre-conditioned at station D within the tank 15 before it is introduced into the container or tank 30 at station E. After the gold has been removed from the solution in tank 30 by electroplating it out on cathode plates, it is then withdrawn through an outflow line 31 by a motor-operated pump 32 through upstream line 33 which returns the solution to each of the dragout recovery tanks 11, 11, and 11", through overhead header supply lines 35, 35', and 35".

In FIGURES 2 to 6, I have illustrated a gold recovering tank or container unit 30 suitable for carrying out my invention. The tank is shown of rectangular shape, having vertically-extending side walls 40, end walls 41, and a closing bottom wall 42 to define an open top container. The tank 30 is reinforced along its outer sides by a framework 44 made up of angle bars or pieces secured in place, asby welding. Upper portions of the reinforcing framework support a transverse or cross extending group of longitudinally spaced-apart channel members 45 on which cathode electrode bars 46 and anode electrode bars 47 are positioned.

It will be noted from FIGURE 2 that the electrode bars 46 and 47 extend longitudinally above the open mouth of the container 30 to rest upon the webs of the support channels 45 which are inverted and which, in turn, rest on the top edges of the tank and the reinforcing framework 44. As also shown in FIGURE 2, the bottom wall of the container is reinforced by transversely extending channels 43. A drain valve 49 in provided for the tank 30 for cleaning it out, as may be needed.

As shown in FIGURE 3, the tank is separated into zones, areas, or compartments by vertically-extending separator plates 48 and, as shown in FIGURE 4, a group of electrodes comprising a pair of anode plates a and a cathode plate 0 is suspended within each compartment defined by the separators 48. The electrode plates a and c are carried by hangers d which are suspended by their upper hook-like ends over the electrode bars 46 and 47 that are generally designated in FIGURES 5 and 6 as bars b. The lower ends of each hanger d are secured to the upper end of the respective electrode plates a and c. The anode plates a may be of a suitable material such as steel or stainless steel; and cathode plates c may be of a suitable material such as stainless steel.

In FIGURES 5 and 6, I have shown details of the mounting of the longitudinal-extending electrode bars b on the supporting channels 45. A U-shaped bolt or clamp 50 extends over the top edge of the edgewise-positioned electrode b, through a resin insulating piece or strip 51, a resin or insulating bushing 52, and thrOugh a resin or insulating washer 53 and a steel washer 54. It will be noted that the bushings 52 are positioned in holes or openings in the web of the channel 45 to space and insulate legs of the clamps 50 with respect thereto. Each clamp or bolt 50 is maintained in a tight holding position by nuts 50a which are secured on its threaded ends. It will thus be seen that the mounting of each clamp or bolt 50 is insulated from the metal of the container and the frame by the insulation of the parts 51, 52, and 53.

As will be noted, the system of FIGURE 1 may be continuously operated, its circulating aqueous solution may be maintained at a requisite pH for gold recovery, and the amount of gold recovered during each passage through the treatment zone E may be based on the most economical basis, since any gold remaining in the solution is recirculated and not lost. I customarily adjust the DC. current up or down, as applied to the electrodes at station E, to provide a power supply based on the gold content desired in the solution being returned or recirculated. The operation has been successful for recovery of gold from one or more plating lines that employ one or more types of plating solutions. In addition, as found to be desirable, the pumps 26 and 32, provide suitable agitation of the solution during the plating out of gold on the cathode plates 0 of the tank 30.

Although for the purpose of illustrating my invention, I have shown and described a representative system employing it and have shown and described a suitable electrode recovery tank, it will be apparent to those skilled in the art that various changes and modifications may be made without departing from the spirit and scope thereof, as indicated in the appended claims.

What I claim is:

1. An electroplating method of economically recovering gold in solution from a washing water waste solution employed to wash oif dragout from gold plated workpieces provided by a gold electroplating operation, including a waste solution having a low concentration of gold content, and irrespective of the type of solution used in the previous gold plating operation on the workpieces whether of a cyanide, acid or neutral type which comprises, collecting the waste solution from said washing operation, progressively converting the waste solution to an alkaline cyanide solution having a pH above that at which poisonous cyanide gas will be evolved by adding alkali metal cyanide and hydroxide as required, progressively moving the converted solution through an electroplating bath having anode and cathode plates, progressively plating out gold from the solution on the cathode plates in the form of metallic gold, and progressively returning said solution to said washing operation.

2. An electroplating method of recovering gold as defined in claim 1 wherein, the converted solution has a pH of about 12, and has a minimum content of about .5 oz./ gal. of cyanide in solution.

3. An electroplating method of recovering gold as defined in claim 1 wherein the converted solution has a minimum pH of about 10.

4. An electroplating method of recovering gold as defined in claim 3 wherein a minimum of about .5 02/ gal. of CN is maintained in the converted solution.

5. An electroplating method as defined in claim 3 wherein electric current supplied to the anode and cathode plates of the electroplating bath is increased with a lower concentration of dissolved gold content in the converted solution and is decreased for a greater concentration of dissolved gold content therein.

6. A method as defined in claim 3 wherein the plated out gold is removed from the cathode plates as a peelable gold foil.

7. A method as defined in claim 3 wherein washing water is provided in baths at the end of a group of electroplating lines along which gold plated workpieces are progressively passed which comprises, progressively washing off the gold plated workpieces in the baths at the end of the lines with water, progressively taking off the washing water waste solution from the baths of each of the plating lines and progressively converting it to an alkaline cyanide solution having the defined minimum pH, progressively moving the converted solution through the electroplating bath and progressively plating out gold from the solution on the cathode plates thereof, and progressively removing the solution from the electroplating bath and returning it to the baths of the lines of the group.

8. A method as defined in claim 3 wherein the solution is continuously agitated in the electroplating bath during the plating out of its solution gold content.

9. An electroplating method of recovering gold as defined in claim 3 wherein, a minimum of about .5 oz.. gal. of an alkali metal cyanide is maintained in the converted solution, and controlled amounts of an alkali metal hydroxide are added to the solution during its conversion to provide it with the defined minimum pH.

10. A method as defined in claim 9 wherein the alkali metal cyanide is of a class consisting of sodium and potassium cyanides and the alkali metal hydroxide is of a class consisting of sodium and potassium hydroxides.

11. A method as defined in claim 9 wherein, the alkali metal cyanide is initially added to the waste solution and the alkali hydroxide is progressively added to the waste solution in converting it for use in the electroplating bath, and the solution in the electroplating bath is progressively recycled for use as a dragout washing solution and for removing its solution gold content.

12. A method as defined in claim 9 wherein the gold of the waste solution is electroplated out on the cathode plates when its content is as low as about 5 mg./l.

13. An electroplating method of economically recovering gold in solution from a washing water waste solution employed to wash off dra gout from gold plated workpieces provided by a gold electroplating operation, including a waste solution having a low concentration of gold content, and irrespective of the type of solution used in the previous gold plating operation on the workpiece Whether of a cyanide, acid, or neutral type which comprises, collecting the waste solution, progressively converting the waste solution to an alkaline cyanide solution having pH above about 10, maintaining a minimum of about .5 oz./gal. of cyanide in said solution, progressively moving the converted solution through an electroplating bath having anode and cathode plates, and progressively plating out gold from the solution on the cathode plates in the form of metallic gold.

Metal Finishing, July 1950, pp. 48-49; May 1968, p. 59; pp. 60-65.

ROBERT K. MIHALEK, Primary Examiner.

R. L. ANDREWS, Assistant Examiner.

U.S. Cl. X.R.

Claims (1)

1. AN ELECTRPLATING METHOD OF ECONOMICALLY RECOVERING GOLD IN SOLUTION FROM A WASHING WATER WASTE SOLUTION EMPLOYED TO WASH OFF DRAGOUT FROM GOLD PLATED WORKPIECES PROVIDED BY A GOLD ELECTROPLATING OPERATION, INCLUDING A WASTE SOLUTION HAVING A LOW CONCENTRATION OF GOLD CONTENT, AND IRRESPECTIVE OF THE TYPE OF SOLUTION USED IN THE PREVIOUS GOLD PLATING OPERATION ON THE WORKPIECES WHETHER OF A CYANIDE, ACID OR NEUTRAL TYPE WHICH COMPRISES, COLLECTING THE WASTE SOLUTION FROM SAID WASHING OPERATION, PROGRESSIVELY CONVERTING THE WASTE SOLUTION TO AN ALKALINE CYANIDE SOLUTION HAVING A PH ABOVE THAT AT WHICH POISONOUS CYANIDE GAS WILL BE EVOLVED BY ADDING ALKALI METAL CYANIDE AND HYDROXIDE AS REQUIRED, PROGRESSIVELY MOVING THE CONVERTED SOLUTION THROUGH AN ELECTROPLATING BATH HAVING ANODE AND CATHODE PLATES. PROGRESSIVELY PLATING OUT GOLD FROM THE SOLUTION ON THE CATHOLE PLATES IN THE FORM OF METALLIC GOLD, AND PROGRESSIVELY RETURNING SAID SOLUTION TO SAID WASHING OPERATION.

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