US3886055A - Electrolytic separation of metals - Google Patents

Abstract

A process for separating silver, gold or a silver/gold alloy from a composite metal body in which the silver, gold or alloy is adhered as an external layer over a ferritic or austenitic stainless steel substrate. Separation is effected by controlled potential electrolysis in which the metal composite serves as the anode and in which the electrolytic solution used contains between about 1% and about 10% by weight of an alkali metal cyanide and up to about 10% by weight of an alkali metal hydroxide. During electrolysis the anode voltage is controlled by reference to a standard electrode at a voltage at which the degree of dissolution of the stainless steel in the half cell comprising stainless steel and the solution is substantially less than the degree of dissolution of the silver, gold or silver/gold alloy in the half cell comprising silver, gold or silver/gold alloy and the solution thereby causing selective removal from the substrate of the silver, gold, or alloy thereof.

Description

United States Patent 1 Baboian et a1.

[451 May 27, 1975 I ELECTROLYTIC SEPARATION OF METALS [73] Assignee: Texas Instruments Incorporated,

Dallas, Tex.

[22] Filed: Dec. 12, 1973 [21] Appl. No.: 423,948

[52] US. Cl 204/146; 204/140 [51] Int. Cl. C23b 1/00; BOlk 3/00 [58] Field of Search 204/146, 140

[56] References Cited UNITED STATES PATENTS 2,735,810 2/1956 Gagliano 204/146 3,826,724 7/1974 Riggs, Jr. et a1. 204/146 OTHER PUBLICATIONS Metal Cleaning and Finishing, January 1937, pg. 31. Metal Finishing, November 1945, pp. 457 and 458. Corrosion, Vol. 16, No. 2, February 1960, pgs. 4754. Plating, October 1948, pgs. 1013 and 1044.

Primary ExaminerT. M Tufariello Attorney, Agent, or Firm-James P. McAndrews; John A. Haug; Edward J. Connors, Jr.

[57] ABSTRACT A process for separatingsilver, gold or a silver/gold alloy from a composite metal body in which the silver, gold or alloy is adhered as an external layer over a ferritic or austenitic stainless steel substrate. Separation is effected by controlled potential electrolysis in which the metal composite serves as the anode and in which the electrolytic solution used contains between about 1% and about 10% by weight of an alkali metal cyanide and up to about 10% by weight of an alkali metal hydroxide. During electrolysis the anode voltage is controlled by reference to a standard electrode at a voltage at which the degree of dissolution of the stainless steel in the half cell comprising stainless steel and the solution is substantially less than the degree of dissolution of the silver, gold or silver/gold alloy in the half cell comprising silver, gold or silver/gold alloy and the solution thereby causing selective removalfrom the substrate of the silver, gold, or alloy thereof.

10 Claims, 3 Drawing Figures I l I 29 fluff/WM: M575? fiiddkDE/Q L (0 E I I W;

M144 l I HMPL/F/ER 35) 1 I I I I 5 1 12 2 fla e aye I fern/N42 amt/r601.

I za/vmaa 47 VI/ nwz/z/m \a I I I :(JRR'A/T 45 Merl/4e I 5 I EKG/Q55 i porn/rm I (alt r402 I 557' Pd/NT I Ao/usr/wA/r I I I I I FOWiE ELECTROLYTIC SEPARATION OF METALS BACKGROUND OF THE INVENTION This invention relates to electrolytic separation of metals and more particularly to a controlled potential electrolytic method for selectively removing a gold, silver or gold/silver alloy external layer from a metal composite body having a substrate constituted by a ferritic or austenitic stainless steel.

Metal laminates or composites are widely used in a variety of different industrial and commercial applications. To produce such composites, a coating or cladding of one metal is applied to a substrate constituted by a different metal using various techniques. including vapor deposition, roll bonding, electroplating, etc. Both in the coating or cladding processes and in the fabrication of metal products from the metal composite stock, some amount of off-grade or scrap composite metal is inevitably produced. To conserve raw materials and minimize the cost of both the composite metal and products produced therefrom, it is highly desirable to recover the components of the scrap composite and reuse these components in further coating or cladding operations. Recovery of the scrap necessarily involves separation of the external layer of metal from the substrate metal. One technique which has been used commercially to accomplish this separation'is vaporization of one of the metals, normally that constituting the coating or cladding. vaporization techniques, however, require high temperatures and careful collection of vapors to avoid loss of metal vapor to the surroundings, a consideration which may be especially critical where the cladding metal is expensive, toxic or both. Vaporization recovery techniques have generally proved to be both costly and inefficient.

Chemical separation methods have also been used in the art to remove metal coatings or claddings from a substrate. Chemical separations are frequently unattractive, however, since it is difficult to devise solvent systems which will selectively dissolve one metal of a composite without at least partially dissolving the other. The consequent process complications, product purity problems, and cost of chemical separation are thus apparent.

A potential variant of chemical separation is separation by electrochemical processes in which the metal composite serves as an anode and one metal is preferentially removed by application of electrolytic current. However, electrolytic processes in general suffer from the same limitations as wet chemical procedures in that anodic dissolution is not normally selective, especially where the metals of the composite have oxidation potentials which are not substantially different. Moreover, where the metal to be removed is one normally considered to have substantially greater nobility than the substrate from which it is removed, electrochemical techniques might be anticipated to be especially unattractive.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a process for substantially quantitative recovery of gold, silver or a gold/silver alloy from a metal composite body in which the gold, silver or alloy is present as an external layer on a substrate constituted by a ferritic or austenitic stainless steel. It is also an object of the present invention to provide such a method in which removal is rapidly, efficiently and economically accomplished without significant attack, degradation or deterioration of the substrate. It is a further object of the present invention to provide such a method in which the gold, silver or alloy is recovered in pure form substantially free of the metals of the substrate. A particular object of the present invention is to provide such a method in which separation is effected by electrochemical means. Other objects and features will be in part apparent and in part pointed out hereinafter.

Briefly, the present invention is directed to a process for separating a first metal selected from the group consisting of silver and gold from a composite metal body in which said first metal is adhered as an external layer over a substrate comprising a second metal selected from the group consisting of austenitic and ferritic stainless steels. In this process, the metal composite body is immersed in a solution containing between about 1 and about 10% by weight of an alkali metal cyanide and up to about 10% by weight of an alkali metal hydroxide. Direct current is supplied to the body from the positive terminal of a direct current power source whose negative terminal is connected to a second electrode immersed in the electrolytic solution, thus establishing an electrolytic circuit in which the body is the anode and the second electrode is the cathode. The voltage at the anode is controlled by reference to a standard electrode at a voltage at which the degree of dissolution of the stainless steel in the half-cell comprising the second metal and the solution is substantially less than the degree of dissolution of the first metal in the half-cell comprising the first metal and the solution thereby causing selective removal of the first metal from the body.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a graph illustrating aspects of methods of this invention and showing the potentiostatic polarization curves for Type 430 stainless steel in various electrolytic solutions;

FIG. 2 is a similar graph showing the polarization curves for gold, silver and Type 430 stainless steel in a solution containing 5% sodium cyanide and 5% sodium hydroxide; and

FIG. 3 is a schematic diagram of apparatus suitable for practicing the instant invention.

DESCRIPTION OF PREFERRED EMBODIMENTS In accordance with the present invention, it has now been discovered that controlled potential electrolysis may be utilized to remove an external layer of silver, gold or a gold/silver alloy from a metal composite in which the substrate is a relatively less noble metal such as a ferritic or austenitic stainless steel. At certain anode voltages in a particular electrolytic solution, differential current densities at the gold/silver anode surface and the stainless steel anode surface favors the preferential dissolution of the former, and relative passivation of the latter. This phenomenon is illustrated by the polarization curves set forth in FIG. 2.

At an anode constituted by a particular metal immersed in a particular electrolytic solution, current density generally tends to increase as the absolute potential applied at that anode is made less negative. However, due to the passivity resulting from polarization at certain anode potentials, the current density curve for some metals, including austenitic and ferritic stainless steels, passes through minima at such potentials. At other voltages current density maxima may be observed. Minima on the polarization curve are fre quently attributable to the formation of passivating films at particular voltages. Diffusion limitations on .ansport of the reactants ant. zt tltzsts tit" tn. anodic reaction may also contribute to minimum current density.

As indicated in Fit]. 2, it has been found that at certain voltages stainless steels are more highly passivated than either gold or silver. Electrolysis in which the anode is controlled at such voltages affords preferential anodic dissolution of gold and/or silver and thus provides a uniquely advantageous method for the recovery of these metals from scrap composites in which they are present as coatings or claddings on stainless steel substrates.

in the process of the present invention, a composite metal body having an external layer of gold, silver or gold/silver alloy over a stainless steel substrate is immersed in an electrolytic solution containing between about 1 and about 10% by weight of an alkali metal cyanide and up to about l% by weight of an alkali metal hydroxide. A solution containing on the order of by weight sodium cyanide and 5% by weight sodium hydroxide is preferred.

The metal body is electrically connected to the positive terminal of a direct current power source whose negative terminal is connected to a cathode which is also immersed in the electrolytic solution. An electrolytic circuit is thus established in which the composite metal body is the anode. The cathode is preferably constructed of a material which affords a half-cell reaction resulting in the deposition thereon of the gold and/or silver dissolving at the anode. Conveniently, the cathode is constructed of metal of the same composition as that being recovered.

Electrolysis is effected by supply of direct current from the power source to the electrodes immersed in the solution. To provide selective removal of silver andor gold from the stainless steel substrate, the voltage at the anode is controlled at a voltage at which the degree of dissolution of the stainless steel in the half-cell comprising stainless steel and the electrolytic solution is substantially less than the degree of dissolution of the silver, gold or silver/gold alloy in the half-cell comprising silver/gold and the electrolytic solution. This voltage is selected from polarization curves of the type shown in FIGS. 1 and 2. A low degree of dissolution is indicated by a low current density at a particular voltage, while a high degree of dissolution results in a high current density with a consequently high rate of metal removal. The operating voltage selected should be one where the current density for the stainless steel substrate is at a minimum, while the current density for silyer/gold is reasonably high. For maximum selectivity, the ratio between the current densities at the voltage of choice should be as high as possible. It is particularly important that the current density ratio be high where the area of stainless steel substrate metal exposed to the electrolytic solution exceeds the exposed area of the gold/silver external layer. A difference between the current densities of two or more orders of magnitude will almost always provide clean separation. Where the exposed area of the external layer exceeds that of the substrate. lower ratios may also provide good separation.

As used in this disclosure, the term external layer of a metal composite simply means a layer which is exposed to the electrolytic solution for anodic dissolution. If the metal composite has two layers, the substrate to which the external layer is adhered will normally be exposed to the solution also, but the electrolyte and voltage are chosen to suppress its dissolution. The composite may also have the substrate sandwiched between two or more cladding layers. It will be understood that, in the present context, the terms external layer and -substrate carry no implication as to which metal was applied to which in the initial preparation of the composite.

To control the voltage during electrolytic separation operations, a standard electrode is immersed in the electrolyte solution in close proximity to the anode. The difference between the anode voltage and that of the standard electrode is constantly measured and, in response to this measurement, the voltage output of the power source is controlled to maintain the anode voltage at the predetermined voltage referred to the standard electrode. As a result, selective dissolution of the external layer of the metal composite is obtained.

As anodic dissolution of the external layer proceeds substantially to completion, the current density falls off to a low level and the separation is complete. Essentially quantitative removal of the external layer is thus achieved without significant attack on the substrate. Metal dissolved from the external layer at the anode deposits at the cathode in high purity. Where the cathode initially consists of this same metal, the cathodic product is simply washed and dried, and may then be suitable for use without further purification. Similarly, the denuded anode provides a relatively pure source of the stainless steel substrate metal. The process of the invention thus provides a simple, direct, rapid and economical method for separation and recovery of the constituent metals of the composite.

Apparatus for carrying out the process of the invention is illustrated in FIG. 3 and includes a potential measuring section and a potential control section. Shown at 1 is an anode constituted by a body of scrap metal composite. Through a lead wire 3, anode 1 is electrically connected to ground as indicated at 5 and to the positive terminal of a direct current power source (power amplifier) 7 which has a controllable variable output voltage. The negative terminal of power source 7 is electrically connected to cathode 11 through a conductor 9, a current meter 13 and a current recorder 15, the latter indicating the rate of electrolysis and power consumption of the electrolytic circuit, while a voltmeter 17 indicates the output voltage level of the power amplifier.

The voltage of anode l is sensed by a reference electrode l9 placed in proximity to the anode. Each of the threeelectrodes is immersed in an electrolytic solution 21 in a container 23. A signal lead 25 transmits the voltage of anode l to the input of a potentiometric control device 27 and to the positive input terminal ofa potential amplifier 29, while a signal lead 31 interconnects reference electrode 19 to one input terminal of a control amplifier 33 and to the negative input terminal of potential amplifier 29. Potential amplifier 29 thus provides a signal corresponding to an amplified difference in voltage between anode l and electrode 19, and this differential voltage is indicated by a potential meter 35 and recorded by a potential recorder 37.

Potentiometric control means 27 comprises a balance circuit including plus and minus temperature compensated zener diode regulated supplies 39 and 41 and a potential control set point potentiometer 43. The other input terminal of amplifier 33 is connected by a lead 45 to the rotor or arm of potentiometer 43. As long as the voltage of an anode l differs from the voltage of standard electrode 19 by an amount corresponding to the set point of potentiometric means 27, there will be no input error signal applied to the input of amplifier 33. However, upon the anode voltage straying from the set point the resultant difference between the voltages transmitted by leads 31 and 45 applies an error signal to control amplifier 33 which in turn transmits a control signal through line 47 to power amplifier 7 adjusting the total voltage output of the power source to bring the voltage of anode 1 back to the desired level. When the voltage of the anode becomes too positive relative to the standard, the output voltage of power source 7 is reduced to bring anode 1 back to the control level and, when the voltage of anode 1 becomes too EXAMPLE 3 Controlled potential electrolysis was carried out with an anode consisting of a one-inch square piece of type 430 stainless steel approximately 0.005 in. thick having a 0.000] in. thick cladding constituted by an alloy containing 75% gold and silver. The cathode was a 1- inch square piece of nickel. Electrolysis was conducted in 50 ml ofa 5% sodium cyanide/5% sodium hydroxide solution, using a potentiostat generally similar to that shown in FIG. 3 and described above. Such a potentiostat is available under the trade designation Duffers Model 600. The anode potential was controlled relative to a saturated calomel electrode also immersed in the electrolytic solution. Runs were made at several different voltages, and during each run the solution was rapidly stirred by means of a magnetic stirrer. At the end of each run, the anode was analyzed for silver and gold, while the cathode and the electolytic solution were analyzed for iron and chromium. The results of these runs are set forth in the following table.

CONTROLLED POTENTIAL ELECTROLYSIS OF 75% Au-25% Ag/430 SS SCRAP IN SODIUM CYANIDE SOLUTION Analysis of Electrolytic Controlled Stripping Time Solution Potential (Minutes) Analysis of Anode Analysis of Cathode Fe (ppm) Cr (ppm) of Scrap Au Ag Fe Cr .(V vs. SCE**) 0.50 240 0.0025 0.0005 None None I 8 1.0 0.30 60 0.0 0.0l4 0.00l 0.009 None L9 2.2 +0.20 30 0.013 0.00l 0.009 None 1.7 2.3 +0.30 20 0.0025 0.0002 0.001 None 3.5 1.4 +0.30 60* 0.0024 0.0002 0.0003 None 3.6 2.3 +0.30 120* 0.0025 0.0004 0.00l 0.0002 3.4 2.5 +0.40 l500 +0.70 SS Dissolves Only 20 Minutes Required "Saturated calomel electrode negative, the total output of power source 7 is increased to reestablish the control level.

The following examples illustrate the invention.

EXAMPLE 1 Polarization curves were obtained for 430 stainless steel in 1 N. sulfuric. 5% by weight sodium chloride so lution, and a solution containing 5% by weight sodium cyanide and 5% by weight sodium hydroxide respectively. To obtain the polarization curves, a 430 stainless steel anode having a surface area of 2 cm a large platinum cathode, and a saturated calomel sensing elec trode were immersed in 50 ml of the electrolytic solution at ambient temperature. Using a variable voltage source (such as that obtainable under the trade designation Beckman Electroscan), the anode potential was scanned at a rate of 8 v/hr. and current was measured as a function of voltage. The curves thus obtained are set forth in FIG. 1.

EXAMPLE 2 Using the technique described in Example 1, polarization curves were obtained at ambient temperature for silver and gold in a solution containing 5% sodium cyanide and 5% sodium hydroxide. The curves obtained are set forth in FIG. 2 which also includes a curve for 430 stainless steel essentially the same as that shown in FIG. 1.

As the above table indicates, optimum voltage for stripping a gold/25% silver alloy from a stainless steel substrate is on the order of +0.30 v relative to a saturated calomel electrode.

Where the anode used has a pure gold external layer on a 430 stainless steel substrate, pure gold may be selectively stripped at a voltage of approximately 0.5 v or approximately +0.3 v versus the saturated calomel electrode. If the anode has a pure silver external layer on a 430 stainless steel substrate, silver may be selectively stripped at potentials between about 0.6 v and +0.5 v versus the saturated calomel electrode. The optimum potential for silver removal is approximately 0.0

In view of the above, it will be seen that the several objects of the invention are achieved and other advantageous results attained.

As various changes could be made in the above methods without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

l. A process for separating a first metal selected from the group consisting of silver, gold and alloys of silver and gold from a composite metal body in which said first metal is adhered as an external layer over a substrate constituted by a second metal selected from the group consisting of ferritic and austenitic stainless steels. the process comprising the steps of:

immersing the body in a solution containing between about 1 and about 10% by weight of an alkali metal cyanide and up to about 1071 by weight of an alkali metal hydroxide; supplying direct current to said body from the positive terminal of a direct current power source whose negative terminal is connected to a second electrode immersed in said solution thereby establishing an electrolytic circuit in which said body is the anode and said second electrode is the cathode;

arranging a saturated calomel standard electrode ad jacent said composite metal body for continuously determining the voltage of said anode relative to said standard electrode; and

continuously regulating the voltage applied between said anode and cathode for maintaining the voltage of said anode relative to said standard electrode within the range from 0.6 to +0.5 volts for causing selective removal of said first metal from said body.

2. A process as set forth in claim 1 wherein said first metal is substantially pure gold and said voltage is controlled at approximately 0.5 volts relative to a saturated calomel standard reference electrode.

3. A process as set forth in claim 1 wherein said first metal is substantially pure gold and said anode voltage is maintained at a level of approximately +0.3 volts relative to a saturated calomel standard reference electrode.

4. A process as set forth in claim 1 wherein said first metal is substantially pure silver and said voltage is maintained at a level of between about -().6 and about +0.5 volts relative to a saturated calomel standard reference electrode.

5. A process as set forth in claim 1 wherein said first metal is substantially pure silver and wherein the voltage of said anode relative to said standard calomel electrode is maintained at a level of approximately 0.0 volts.

6. A process as set forth in claim I wherein said first metal is a gold/silver alloy and said voltage is maintained at a level of approximately +0.3 volts relative to said saturated calomel standard reference electrode.

7. A process as set forth in claim 1 wherein said electrolyte solution contains on the order of 5% by weight sodium cyanide and on the order of 5% by llveight sodium hydroxide.

8. A process as set forth in claim 1 wherein the volt- F age of said anode relative to said standard electrode is controlled by controlling the voltage outpui of said direct current power source automatically in response to the difference in voltage between said anoqil e and said standard electrode. 1

9. A process as set forth in claim 8 wherein said first metal is deposited at said cathode.

10. A process as set forth in claim 9 wherein the electrolytic solution is agitated during electrolysis.

Claims (10)

1. A PROCESS FOR SEPARATING A FIRST METAL SELECTED FROM THE GROUP CONSISTING OF SILVER, GOLD AND ALLOYS OF SILVER AND GOLD FROM A COMPOSITE METAL BODY IN WHICH SAID FIRST METAL IS ADHERED AS AN EXTERNAL LAYER OVER A SUBSTRATE CONSTITUTED BY A SECOND METAL SELECTED FROM THE GROUP CONSISTING OF FERRITIC AND AUSTENITIC STAINLESS STEELS, THE PROCESS COMPRISING THE STEPS OF: IMMERSING THE BODY IN A SOLUTION CONTAINING BETWEEN ABOUT 1 AND ABOUT 10% BY WEIGHT OF AN ALKALI METAL CYANIDE AND UP TO ABOUT 10% BY WEIGHT OF AN ALKALI METAL HYDROXIDE; SUPPLYING DIRECT CURRENT TO SAID BODY FROM THE POSITIVE TERMINAL OF A DIRECT CURRENT POWER SOURCE WHOSE NEGATIVE TERMINAL IS CONNECTED TO A SECOND ELECTRODE IMMERSED IN SAID SOLUTION THEREBY ESTABLISHING AN ELECTROLYTIC CIRCUIT IN WHICH SAID BODY IS THE ANODE AND SAID SECOND ELECTRODE IS THE CATHODE; ARRANGING A SATURATED CALOMEL STANDARD ELECTRODE ADJACENT SAID COMPOSITE METAL BODY FOR CONTINUOUSLY DETERMINING THE VOLTAGE OF SAID ANODE RELATIVE TO SAID STANDARD ELECTRODE; AND CONTINUOUSLY REGULATING THE VOLTAGE APPLIED BETWEEN SAID ANODE AND CATHODE FOR MAINTAINING THE VOLTAGE OF SAID ANODE RELATIVE TO SAID STANDARD ELECTRODE WIHTIN THE RANGE FROM -0.6 TO +0.5 VOLTS FOR CAUSING SELECTIVE REMOVAL OF SAID FIRST METAL FROM SAID BODY.

2. A process as set forth in claim 1 wherein said first metal is substantially pure gold and said voltage is controlled at approximately -0.5 volts relative to a saturated calomel standard reference electrode.

3. A process as set forth in claim 1 wherein said first metal is substantially pure gold and said anode voltage is maintained at a level of approximately +0.3 volts relative to a saturated calomel standard reference electrode.

4. A process as set forth in claim 1 wherein said first metal is substantially pure silver and said voltage is maintained at a level of between about -0.6 and about +0.5 volts relative to a saturated calomel standard reference electrode.

5. A process as set forth in claim 1 wherein said first metal is substantially pure silver and wherein the voltage of said anode relative to said standard calomel electrode is maintained at a level of approximately 0.0 volts.

6. A process as set forth in claim 1 wherein said first metal is a gold/silver alloy and said voltage is maintained at a level of approximately +0.3 volts relative to said saturated calomel standard reference electrode.

7. A process as set forth in claim 1 wherein said electrolyte solution contains on the order of 5% by weight sodium cyanide and on the order of 5% by weight sodium hydroxide.

8. A process as set forth in claim 1 wherein the voltage of said anode relative to said standard electrode is controlled by controlling the voltage output of said direct current power source automatically in response to the difference in voltage between said anode and said standard electrode.

9. A process as set forth in claim 8 wherein said first metal is deposited at said cathode.

10. A process as set forth in claim 9 wherein the electrolytic solution is agitated during electrolysis.

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