US2735810A

US2735810A - Cathode

Info

Publication number: US2735810A
Authority: US
Publication date: 1956-02-21
Anticipated expiration: 1973-02-21

Description

Feb. 21, 1956 F. P. GAGL!ANO RECOVERY OF GOLD FROM SCRAP CONTAINING IT CATHODE Filed Feb. 10, 1954 ANODE CONDUCTOR CATHODE "'1 GOLD SCRAP ELECTROLYTE CONTAlNING FERROCYANDE- INVENTOR fiankfl Gaylz'ano United States Patent RECOVERY OF GOLD FROM SCRAP CONTAINING 1T Francis P. Gagliano, Jackson Heights, N. Y., assignor to Secon Metals Corporation, White Plains, N. Y., a corporation of New York Application February 10, 1954, Serial No. 409,475

8 Claims. (21. 204-410 This invention relates to the recovery of gold from gold plated or clad material, such, for example, as the scrap produced in the manufacture of grids and other parts of electronic tubes and other sources of gold plated or clad material.

Substantial amounts of scrap in the form of gold plated or clad wire, ribbon, sheets, etc., which scrap contains gold in amount of from 0.1% to 6%, copper or silver or both in amounts of from less than 1% to as high as 90% of copper and from less than 1% to as high as 10% of silver, is produced in the manufacture of electronic tubes. Such scrap may contain other metals including one or more of the followingnickel, iron, tungsten, molybdenum and cadmium.

All percentages and parts herein are on a weight basis.

Attempts heretofore made to recover the gold content of such scrap by electrolysis employing a cyanide as the electrolyte have usually resulted in the copper or silver plating out preferentially to the gold and otherwiseinterfering with the recovery of the gold to an extent rendering such process uneconomical and inefiicient, particularly in that the recovery process is very slow. It has been found that, as the concentration of the copper or silver builds up in the electrolytic bath, the eificiency of the bath decreases until it reaches a point where it can no longer be used practically to efiect the recovery of the gold.

This invention is predicated on the surprising and unexpected discovery that the gold content of gold plated or clad material containing copper or silver or both can readily and efiiciently be recovered electrolytically by employing a bath containingfrom about 3% to about 16% alkali metal or ammonium ferrocyanide, from about 0.25% to about 8% alkali metal or ammonium cyanide, and maintaining the bath at a temperature of from 20 to 80 C., and at a pH of from about 7.5 to about 12. The ratio of the ferrocyanide to the cyanide in the bath should be within the range of from 2:1 to 60:1.

Preferably, the bath contains from about 2% to 5% alkali metal or ammonium ferrocyanide, from 0.5% to 2% alkali metal or ammonium cyanide, a ratio of ferrocyanide to cyanide of 521. The bath, in use, is maintained at a temperature offrom about 40 to 50 C. and a pH of from 8 to 10. The ferrocyanide, cyanide and salts or alkali hydroxides added for purposes of maintaining the desired pH, preferably have the 'same cation. Thus, when employing potassium ferrocyanide as a constituent of the electrolytic bath, potassium cyanide is used along with potassium carbonate, potassium phosphate and/ or potassium hydroxide, as will be explained more fully hereinafter. Y

Particularly preferred is the use of a bath containing 4% potassium ferrocyanide, 0.8% potassium cyanide, 0.5 potassium carbonate at a temperature of about 45 C. and a pH of about 9. This pH is maintained by the addition of monopotassium phosphate (KH2PO4) and potassium hydroxide as needed.

As the ferrocyanideemployed in the bath, potassium or sodium ferrocyanide is preferred, although other alkali 2,735,810 Patented Feb. 21, 1956 or francium ferrocyanide may be used. Ammonium ferrocyanide may also be employed.

As the cyanide, sodium or potassium cyanide is preferred, although the other above-mentioned alkali metal or ammonium cyanides may be used. The cyanide is added to'the bath because it forms complexes with metals, such as iron, copper, nickel, silver, chromium, etc., which complexes are soluble and remain in solution. In the absence of the cyanide, insoluble compounds, such as Fe(CN) 2, a brown precipitate, Fe2[Fe(CN) 1s, Fe(OH)s, Fe(OH)z, Ni(CN)2, CuzFe(CN)s and Cu4Fe(CN)s result which contaminate the bath and reduce its efficiency. As the electrolysis proceeds employing a bath having the composition above noted the free CN concentration decreases due to the formation of complexes of the type above noted. Accordingly, from time to time, as needed, additional cyanide is added to replenish the cyanide thus used up and maintain its cencentration within the range above noted.

As above indicated, preferably, the cation of the ferrocyanide and the cyanide, as well as of any other salts or bases incorporated in the bath, are the same.

In order to maintain the pH within the range above noted, alkali metal or ammonium carbonates, bicarbonates, hydroxides or phosphates, or mixtures of these salts 'or bases may be added to the bath as neded. Desidably, a mixture of alkali metal phosphate and carbonate is used. The alkali metal carbonate serves to maintain the pH above 7.5, and the phosphate appears to have a buffer action to maintain the pH at the desired value substantially constant over relatively long periods of time.

A bath having the composition above noted has the advantage of minimizing the efiluent disposal problem. Spent baths employed in this invention can readily be treated, for example, by chlorination, to condition them to provide an eflluent which can be disposed of in the usual sewage or other industrial drain systems.

Moreover, a ferrocyanide-cyanide bath containing at least 2 parts of ferrocyanide per part of cyanide is much less toxic than the cyanide baths heretofore employed. This is the case, because the ferrocyanides are much more stable than the cyanides. Furthermore, in view of the relatively small amounts of cyanide contained in the baths employed in practicing this invention, there is ma terially less tendency for evolution of hydrogen cyanide to take place'than there is from the cyanide baths heretofore used.

The cathode may be any desired electrical conductor, preferably metallic, on which the gold will deposit. Thus, gold plates or bars may be used as the cathode.

, Alternatively, stainless steel may be used from which the metal ferrocyanides, suchas lithium, rubidium, caesium,

with contact rods on which the charge of scrap metal is supported so that the electrical current is supplied efficiently to the scrap metal. -It will be understood the anode simply serves as an electrical conductor for supplying current to the scrap metal and that in effect the scrap metal is the anode, once electrolysis has commenced.

The voltage and current density used will, of course, depend on the particular equipment employed, the size of the bath, the size of the cathodes, anodes, etc. In the recovery of gold from scrap employing a relatively small bath for treating charges of from about to about 500 grams, from 2 to 6 volts and a current density of from 1 to 6 amperes per square foot have been used with satisfactory results. However, as above noted, the voltage and current density may be varied within wide limits and will depend on the equipment used.

Metal which may be treated in accordance with this invention contains from 0.1% to 6% gold, an appreciable amount of either silver or copper or both, the copper content not exceeding 90% and the silver content not exceeding 10%, from to 90% nickel, from 0% to 80% iron, from 0% to 94% tungsten, from 0% to 94% molybdenum, and from 0% to 20% chromium. It will be understood the process of this invention is applicable not only to the recovery of gold from such scrap, but also from any gold clad or plated material containing other metals, particularly copper or silver or both in amounts of from 1% to 90% copper, if copper is present, and from 1% to silver, if silver is present.

The charge is left in the bath as long as it is necessary to remove or strip the gold. This usually will be evident from a change in color of the charge. The charge should not be left in the bath for an excess of time beyond that necessary to strip substantially all of the gold, because this will result in unnecessary contamination of the bath.

In the accompanying drawing is shown a vertical section through one form of an electrolytic bath which may be employed in practicing this invention. It will be understood this showing is for purposes of illustration only and other types of equipment may be used.

In the single figure of the drawing, 10 indicates a container provided with an acid resistant lining 11, desirably of polyethylene, rubber, or other plastic. Cathodes 12, which in the embodiment shown, are of stainless steel, are immersed in the bath 13. Centrally disposed within the container 10 is a perforated basket 14, desirably of acid resistant plastic material, such as polyethylene. Basket 14 receives the charge of scrap 15 to be treated. Anode conductor 16 of any electrically conducting material, e. g., copper, steel, etc., is provided with contact rods 17 which extend through the pile of scrap 15 providing satisfactory electrical contact therewith.

The following examples are given for purposes of illustrating the invention; it will be understood the invention is not limited to these examples. In all of these examples the material treated was industrial scrap produced in the production of grids for electronic tubes. It contained approximately 0.75% gold, 10% copper, 70% iron and nickel and the rest molybdenum and tungsten. All examples were carried out in equipment of the type shown in the drawing employing a stainless steel cathode. Upon the stripping of the gold from each charge or during each run, the charge was removed and a fresh charge introduced.

EXAMPLE I The bath used in this example was made by mixing 600 grams of potassium ferrocyanide K4Fe(CN)s3H2O, 10 grams of potassium cyanide and 1 gallon of water. The resultant solution was used as the electrolytic bath in recovering the gold content of 6 successive charges of the scrap. The data on these 6 charges (referred to as Run 1, 2, etc. in the table) is given in Table 1 which follows:

Analysis of the cathode plate showed it contained 40% gold. The gold removed from the cathode could readily be recovered in substantially pure form, for example, by subjecting it to an acid refining to remove the other metal chiefly copper.

It is noted that in a comparative run using the same scrap but in which the bath consisted of a solution of potassium cyanide, the bath was soon fouled and it was impossible to recover the gold content of even one charge of about 200 grams.

EXAMPLE II The composition of the bath was 500 grams of potassium ferrocyanide, grams potassium cyanide to 1 gallon of water. Potassium hydroxide was added after the second run to bring the pH of the bath up to 12. Additional potassium hydroxide was added at the beginning of Run 4 to maintain the pH of the bath at 12. Data on these four runs isgiven in Table 2 which follows:

Table 2 l'ilaime in inutes iit Taken to Tom era- Run char 6 Recover Voltage Amperage pH of N0. Substan- Used Used Bath B g tially a I Grams A of the Gold The cathode plate was heavy and showed a 60% gold content corresponding approximately to an 80% gold recovery from the gold content of the scrap.

EXAMPLE III The composition of the bath used in this example was 200 grams potassium ferrocyanide, 75 grams potassium cyanide, 40 grams potassium carbonate and 1 gallon of water. At the conclusion of Run 3 a small amount of monopotassium phosphate was added, also small amounts of potassium hydroxides were added to the bath to maintain the pH at the values indicated. Data on these runs are given in Table 3 which follows:

Table 3 Time in Weight ,ilgg fif Run ef 8 Recover Voltage Amperage pH of TEE???" No. st gs en- Used Used Bath Bath 00 y Grams An of the Gold Analysis of the cathode plate showed approximately 100% gold recovery from the gold content of the scrap.

The expression gold plated meta is used in the claims in a broad sense and is intended to include gold clad materials as well as plated.

Since certain changes may be made in carrying out the above process without departing from the scope of the invention, it is intended that all matter contained in the above description shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. An electrolytic process of recovering gold from gold plated metals, which comprisessubjecting the gold plated metal to electrolysis employing an electrolytic bath containing a ferrocyanide from the group consisting of alkali metal and ammonium ferrocyanide and a cyanide from the group consisting of alkali metal and ammonium cyanides, the ratio of ferrocyanide to cyanide in the bath being within the range of from 2:1 to 5:1 by weight.

2. The process as defined in claim 1, in which the bath contains from 3% to 16% by weight of ferrocyanide and from 0.25 to 8% by weight of cyanide.

3. A process of recovering gold from gold plated metals, which comprises subjecting the gold plated metals to electrolysis employing an electrolytic bath containing from 3% to 16% by weight of a ferrocyanide from the group consisting of the alkali metal and ammonium ferrocyanides, from 0.25 to 8% by weight of a cyanide from the group consisting of by weight, the alkali metal and ammonium cyanides, the ratio of ferrocyanide to cyanide being within the range of from 2:1 to 5:1, the bath during the electrolytic decomposition being maintained at a pH of from 7.5 to 12 and at a temperature of from to 80 C.

4. A process of recovering gold from gold plated metals, which comprises subjecting the gold plated metals to electrolysis employing an electrolytic bath containing from 2% to 5% by Weight of a ferrocyanide from the group consisting of the alkali metal and ammonium ferrocyanides, from 0.5% to 2% by weight of a cyanide from the group consisting of the alkali metal and ammonium cyanides, the ratio of ferrocyanide to cyanide being about 5:1 by weight, the bath during the electrolytic decomposition being maintained at a pH of from 8 to 10 and at a temperature of from to C.

5. A process of recovering gold from gold plated metals, which comprises subjecting the gold plated metals to electrolysis employing an electrolytic bath containing about 4% by weight of an alkali metal ferrocyanide and about 0.8% by weight of an alkali metal cyanide, the bath during the electrolytic decomposition being maintained at a pH of about 9 and at a temperature of about 45 C.

6. A process of recovering gold from gold scrap containing approximately from 0.1% to 6% by weight of gold and containing a metal from the group consisting of copper and silver, which comprises subjecting the scrap to electrolysis in an aqueous bath containing from 3% to 16% by weight of ferrocyanide from the group consisting of alkali metal and ammonium ferrocyanides, and from 0.25% to 8% by Weight of a cyanide from the group consisting of alkali metal and ammonium cyanides at a pH of from 7.5 to 12 and a temperature of from 20 to C., the ratio of ferrocyanide to cyanide being within the range of from 2:1 to 5:1 by Weight.

7. The process as defined in claim 6, in which the bath contains from 2% to 4% by Weight of potassium ferrocyanide, from 0.5% to 2% by Weight of potassium cyanide, potassium carbonate and potassium phosphate in amount to maintain the bath at a pH of about 9 and the temperature of the bath is maintained at approximately 45 C.

8. An electrolytic process of recovering gold from gold-containing metals, which comprises introducing the gold-containing metals into an electrolytic bath containing a ferrocyanide from the group consisting of alkali metal and ammonium ferrocyanides and a cyanide from the group consisting of alkali metal and ammonium cyanides, the gold-containing metal thus introduced being made the anode, said bath also containing a cathode, the ratio of ferrocyanide to cyanide in the bath being within the range of from 2:1 to 5:1 by weight, and passing current through said bath to eiiect deposition of the gold on said cathode.

References Cited in the file of this patent Kern: Transactions of the Electrochemical Society, vol. 24 (1913), pp. 241 to 270. Pages 260 to 265 and 268 pertinent.

Kushner: Products Finishing, January 1942, pages 46 to 56.

Kushner: Products Finishing, December 1941, pages 26 and 28.

Claims

1. AN ELECTROLYTIC PROCESS OF RECOVERING GOLD FROM GOLD PLATED METALS, WHICH COMPRISES SUBJECTING THE GOLD PLATED METAL TO ELECTROLYSIS EMPLOYING AN ELECTROLYTIC BATH CONTAINING A FERROCYANIDE FROM THE GROUP CONSISTING OF ALKALI METAL AND AMMONIUM FERROCYANIDE AND A CYANIDE FROM THE GROUP CONSISTING OF ALKALI METAL AND AMMONIUM