US2199391A - Process of gold refining - Google Patents

Description

Patented May 7, 1940- In the gold refining process; in the a'rt', as the Wohlwill process, and describedjin vari-jf ou's Wohlwill patents, particularly the Patents? Nos. 25,8 3, 625,864fa'nd 961,924, aneleetrolytic process is described for refining gold of the order silver forming moreor less insoluble chlorides. The dissolved 'gold 'from'theanode'is then elec trolytically deposited in very' pureiqrm on the cathode; Theimpuritiesieplace the gold ions of the gold chlorideelectrolyte which" are then fde'posited on the cathodei As the gold-chloride electrolyte, I-IAuClrisgradnaHy'exhausted by this action, itbecomes' necessary in order to maintain the process, that this expensive electrolyte be added to mepann; -Woh1will'has also proposed 7 the" use-of 'a-"sm'all amount 'of- NaLCI' which he found assists the electrolytic aidtibna's wll as;

ti'on'tothe whole, in other worda -that tiv'ely fine '=gold haviii g alrelatlvelylow prcent' eurrem qerisifiy andhiererarean rate refimng is liniited ime mmnatio to 'inatiomcf siichc My invention relates to novelfapparatiis for. Q. and methods of electrolytic parting and refining of metals, and more particularly; relates'to novel apparatusfor and methods or electrolytic part;

sibleinthe Q'Will patents.

H e ,"A further objectfof my inyention' is tofprovide of .850fine inan electrolytic bathof HAuCh, or I AuCls plus "HCl, with al cathode starting sheet" of pure gold Whenexternal heat applied toa 7 temperature of 65 70 Cfand ajcurrent density of approximately 100 amperes per square foot, the gold together with" the impurities on the anodeis dissolved in the-electrolytefl'lead andy.

, with a concentrated hydrochloric acid electrobe operated at considerably higher speeds om jusrm for *renmng relats 1am .jamng the: seaso ,n nt in ie mme 'w hii rel ivel re cc ce t hydro'chl'orifacidf content "than has heretofore benconsidered practicaljnecessary,or'even posprior art as exemplifiedby theWOhI- c qcor elwan J icfi7 i[ n vi i n to sprovide jnovel pparatus for; and methods of electrolytically parting and refining metals;

ovel apparatus for and methodsoj electrolytically parting'and'refininggoldi' j H Still another object of my invention is to provide ja novelfelctrolytic process for parting gold lyte.

Still a further object of my invention is to provide a novel electrolytic process which -len'ds ,ltself to repea ted operation for producing gold o hi h u y-M.

, Another obj ect ofiny'in tio i a provide a novel electrolytic process which isadapted for both parting relatively impure goldand for re- Still another object of my inventionfkisto provide a novel electrolytic process which is adapted 'for're'fining gold of a-highsilverFcontent.

v Still afurtherobject of niyinvention'isto provide a nov'el niethod of an apparatus for electrolyticallyeissmvin gold into' solution:

"f'A further "object of my'invention is to provide i16vel apparatus "for' and methods of electrolytmany amng and refining -"go1d*which' require only minor investments in the electrolyt'e'jwhich v than has heretofore been possible; andfWhich requires --no external heating of the electrolyte or mechanical scraping-oi the anode.

"which ra f et? l qfiitefi l fi ess employing concentr'ated hydrochlor the eleetrolyt 'th gold'b nodes from eighteen carat (.417 to .750 fine) grades, and the scrap bullion offered for refining, generally .400 to .600 fine may be refined with my process.

Platinum and palladium are invariably present in this scrap bullion in amounts varying from a trace to as high as two percent whichindicates the admixture of dental scrap. Lead also is generally present in this nondescript alloy. Dental gold usually has a high gold content and may contain up to twenty-five percent platinum and palladium. The dental scrap bullion averages about 18 carat. In recent years, alloys containing to 50% palladium have come into use with correspondingly lower percentages of gold and platinum.

High grade scrap bullion which may be refined by my process is generally a mixture of jewelry and dental scrap, a typical composition being:

Percent Gold 60 Silver 15 Copper 17 Zinc 4 Nickel 1 Balance consisting of tin, lead, iron, platinum, palladium, other metals, silicon and sulphur.

Gold 4.0 Silver 1.5 Copper 70.0 Nickel 41) 17.0 'Iln 2,0

Balance lead, iron, platinum, palladium, I other metals, silicon and sulphur.

The base bullion anodes may be cast in an open mold with a small head or lug at the top and a one-eighth inch diameter copper or silver A wire inserted in the lug before the metal solidifies. The conducting wire may be welded to the .lug with a torch flame, if desired. The upper end of the anode is cast thicker'than the lower to compensate for localized current densi-- ties.

The cathodes may consist of strips of sterlin silver. fine gold or tantalum, or preferably sterling silver plated or faced with the fine gold or a metal of the platinum group. Sterling silver tubing may serve as cathodes and cooling water may :be

passed through them to cool the bath. They maywith a small brass bolt. 1 .Well known means for agitating the electrolyte are provided.

The electrolyte is concentrated (commercial grade) hydrochloric acid. In this case, if anodes .600 fine with 15% silver and 25% copper are used, a current density of about 300 amperes per square foot of anode surface may be employed. At 180 amperes per square foot, an anodeefiiciency of about 63% is realized with anodes of this composition. By adding even a slight amount of sodium chloride to the electrolyte with the same anodes, the current density may be increased to 630 amperes per square foot or more without generating chlorine at the anode. Using a current density of 180 amperes, the anode efliciency increased to 75% as compared to the chiciency without sodium chloride of 63%. I have noticed similar effects in using potassium, calcium and bariumchlorides.

'No external heat is applied, the bath being self-,- heating. Some cooling is required to prevent boiling, but I prefer to provide sufficient cooling to maintain the bath temperature below C.

The electrochemical eii'ects visible during electrolysis-are as follows: Let us assume that we start with an anode of a composition within the usual range-say, about 60% gold, 15% silver, and 15 to 25% copper. At the beginning of electrolysis, with fresh concentrated hydrochloric acid, the anode goes into solution in the electrolyte and if the bath is not agitated, the dark green solution sinks to the bottom of the cell so that little or no deposit is formed. Silver chloride is formed at the anode, most of it dropping off immediately or adhering loosely, and a small amount being dissolved. During this initial period, an assay of the colored liquid would show the presence of gold. There is also strong gassing at the electrodes, oxygen at the anode, and hydrogen at the cathode. This gassing is reduced in amount, and with it the potential across the cell, if salt is added, or if superimposed alternating current is used. As electrolysis continues, the heavy colored solution atthe bottom increases in amount and also diffuses to some extent towards the top, especially with rising temperature. As this solution containing the anode' metal .rlses and comes into contact with the cathode, a

deposit of gold rapidly forms on the cathode. In addition, a reaction is momentarily noticeable which is always seen during the entire operation of the Wohlwill process, namely, a precipitation of finely divided gold throughout the bath. I assume that this precipitation is the well known decomposition of aurous chloride or chloraurous acid into metallic gold and auric chloride or chlorauric acid. This reaction, however, is only momentary and is not again visible during the continuance of electrolysis. If the bath is stirred or agitated from the beginning,.thls reaction apparently does not occur. It appears reasonable to assume from the above that aurous ions are not formed at the anode, and that theyare formed at the cathode momentarily, when the solution coming into contact with the cathode,

contains an excess of auric ions beyond the amount which can be immediately reduced to metallic gold. Almost immediately after deposi-'.-

tion begins, the bath becomes depleted of gold and continues in this condition thereafter. Both cupric and cup'rous chloride are found in the solutains only gold and silver, but if any large amount of platinum group metals is dissolved from the anode, small amounts of these may be present in the cathode deposit.

'If the gold content of the anode is low and the copper content correspondingly high, the cathode deposit may consist largely of copper mixed with in the solution undeposited on the cathode.

The free acid in the bath is gradually consumed in the formation of chlorides of silver, copper, platinum, etc. The anode eificiency and the production rate drop ofl correspondingly. -In one instance, usingdirect current alone, at an anode current density of 175 amperes per square foot, the averageanode loss in weight per ampere hours was 5.15 ounces during the first eight hours of operation. The average for the second eight hours period was 3.50 ounces per 100ampere hours, the average for the sixteen hours being 4.3 ounces. Thus, at the end of sixteen hours, the hath no longer gives good production. The bath acts somewhat differently when using superimposed alternating current on the direct current, initial anode consumption as high as 7.5

ounces per 100 ampere hours being obtained at the same current density.

To improve the average anode eiliciency over a longer period of time, the bath maybe continuously changed or be removed at intervals such as four or eight hours and also the copper and silver and platinum group metals may be regularly electrodeposited atlow current density with insoluble anodes. The acid of the bath is thus renewed to some extent permitting the further use of the bath until impurities accumulate in suflicient amount to interfere with the gold cathode deposit. As the action proceeds, the

-electrolyte contains a relatively high amount'of silver chloride in solution which does not precipitate on cooling but precipitates on dilution of the bath with water. Cuprous chloride and iron silicate similarly precipitate.

The electrolytic action is accompanied by strong" polarization and evolution of oxygen at the anode and hydrogen at the cathode. The polarization and gassing is reduced by addition of a soluble chloride and by use of superimposed alternating current or rectified alternating current.

The voltage under'these conditions must always up thickly on the cathode and soon drops ofi by its own weight to the bottom of the tank, where it mixes with silver chloride and anode particles which have also dropped down. The tank slimes are removed at convenient intervals, filtered, washed with hot water and refined by any suitable method, to obtain fine gold. An assay of the metallic part of the slimes (after removing the admixed AgCl by leaching with a solvent) shows gold .920 to .986 fine, if anodes of the usual range of composition are used. The balance is mainly metallic silver, only traces of other metals being found as arule.

If the amount of gold in the anode is low, other metals such as copper or platinum may be found in greater or less amount in the, cathode deposit,

but do not hinder the subsequent refining. Un-

der certain conditions, .as' when much platinum is present in the anode, chlorine may be generated at the anode but the anode continues to pass into solution, nevertheless. 1

My process thus permits the rapid solution of base bullion anodes at high current :density,

without regard to the silver content and with practically no interference or retardation from the formation of silver chloride. It also provides for the depositionof fairly pure gold at the cathode in a form that is easily removed and rapidly refined. The success of'my method with high silver anodes at high current density, I assume to be at least in part due to the fact that silver chloride partly dissolves under the conditions of myprocess. A assume it is this extra solubility which prevents the silver forming a passive coating on the anode. The high current density I find it possible to employ is in part due to the fact that AgCl does not interfere .with the dissolution of the anode. However, the principal factor permitting the use of high current. density is that in my electrolyte, which contains the maximum possible amount of free HCl and other chlorine compounds such as soluble chlorides, gold does not become passive when polarized at the anode at tremendously high current densities. The same is true of palladium and to a lesser degree of platinum.

The solubility of AgCl in hydrochloric acid decreases rapidly on dilution of the acid, but increases rapidly with rising-temperature. The solubility of AgCl in acid diluted to 63.5% by volume (20% anhydrous HCl) when heated to 107 C. (boiling point 108.5" C.) approaches its solubility inboiling concentrated acid. The presence of soluble chlorides increases the solubility of AgCl in hydrochloric acid and this effect also increases rapidly with increasing temperature. Although the acid strength diminishes in the course of electrolysis the relatively high solubility of silver chloride is maintained by the increasing concentration of the soluble chlorides of metals dissolved from the anode.

- The comparatively large solubility of AgCl in the electrolyte is shown qualitatively by the following: if the bath be slightly diluted with water, a copious precipitate of cuprous chloride and silver chloride is formed which redissolves on boiling. If a large amount of water be added, the

AgCl precipitate does not redissolve on boiling, to

any visible amount.

This solubility of AgCl leads-t0 one slight difficulty. 'That is, the deposition of metallic silver at the cathode to the extent of 1.5% to 8% of the total cathode deposit, being apparently alloyed with the gold. This silver content somewhat-retards, but does not seriously interfere with the refining of the cathode deposit, when refined by the aqua regia or the Wohlwill methods. This-silver content can be controlled to a large degree by the use of high cathode current density, by cooling, and by using superimposed alternating current or rectified current, or with an auxiliary electrolysis to remove the silver from the electrolyte before the latter comes into contact with the cathode. With superimposed alternating current and a cathode density about 350 amperes per square foot, I havebeen able to reduce the cathode silver to 1.4%. In the same test, cathode density of 180 amperes per square foot gave a silver content of 3.72% with superimposed alternating current and with direct ourrent'alone, 6.22%.

Lead in the anode acts similarly to silver, but lead chloride is more soluble than silver chloride in a'hot bath. For this reason, sulphuric acid is added to the Wohlwill bath, to form lead sulphate, which is practically insoluble. Lead sulphate forms a passive coating similar to silver chloride, but as the amount of lead is usually small, it does not interfere. I have found that in the Wohlwill bath containing more than the usual amount of gold and of hydrochloric acid, the addition of sulphuric acid was not effective if the anode contained much lead, the bath being filled with lead chloride crystals when cold. In my present process although lead is always present in the anode,.I have at no time encountered trouble from this source. The lead is apparently completely dissolved and does not crystallize when cold, and does not usually contaminate the cathode deposit.

The remarkable increase in rate of decomposition of the anode, accompanied by increased anode efficiency, on the addition of sodium chloride, was pointed out hereinabove. Even a mi,- nute amount of salt, for example, a pinch of salt in a two gallon cell produces this effect. This effect is still more remarkable, considering the very slight solubility of salt in concentrated hydrochloric acid, about 0.1%.

There would seem to be a relation between the effect of the minute amount of salt and the minute gold content of the bath.

If a sample of the bath be taken shortly after electrolysis has started, with ordinary anodes,

containing 50-60% gold with the bath agitated,

an ordinary qualitative test for gold in solution will be negative. This condition is theoretically impossible, since gold is continuously dissolving from the anode, and a more delicate test would no double reveal traces in solution, if a sample be taken near the anode. (With anodes of higher gold content, and a low base metal content, there is, of course, a large proportion of dissolved gold remaining undeposited.) This indicates that copper and other base metals dissolving, displace gold' from the bath with extraordinary rapidity. We can consider, for practical purposes, that the gold is deposited as fast as it dissolves, and that traces of salt are sufficient to combine with the traces of gold chloride present, forming a double chloride, which has a lower decomposition-voltage than chlorauric acid, and the maximum current density limit of passivity of gold is raised. There may also be an effect of the added chlorine ions on the base metals present.

Fine gold itself exhibits passivitythat is, above a certain current density gold ceases to ,dissolve and chlorine is set free. This is ascribed by Allmand and Puri (Trans. Faraday Soc. 1925) and others to the formation of an oxide film. Wohlwill found that sodium chloride or HCl added to his chlorauric acid bath formed a double chloride and enabled him to increase the current density. Superimposed alternating current has been found by Allmand and'Puri to raise this limiting current density for fine gold anodes in dilute hydrochloric acid, and they also In the Wohlwill process, the limiting factor is the passivating coating of silver chloride. 'Superimposed alternating current loosens the silver chloride and incidentally raises the gold passivity limit of current density. Hydrochloric acid or sodium chloride has the same effect with 'respect to gold. The gold effect here is of minor import-. ance, as the silver effect limits the process.

My observations have led me to think that the silver content has little or no limiting effect in my process. I have succeeded in decomposing even fine silver rapidly. I believe the only limiting effect present is anode passivity of gold; probably also of platinum, iridium and osmium when present. The presence of salt, forming double chloride, raises the limited current density of gold, and use of superimposed alternating cur- 93% gold and 7% silver derived from the cathode.

slimes as follows: I washed the cathode deposit with ammonia water to remove silver chloride,

melted the residue into an anode, and treated the anode by the process in the same manner as for parting as hereinbefore set forth, but the bath was not stirred. Most of the gold dissolved and remained inthe bath, while some of it deposited on the cathode in the ,usual way. When most of the anode had been used up, I added a little nitric acid to the bath and the cathode deposit immediately dissolved, indicating that little or no metallic silver had deposited. After diluting the solution and filtering off the silver chloride, the gold was precipitated and proved to be of high purity.

In another case, I treated anodes of a somewhat unusual composition, approximately:

This worked very satisfactorily inthe usual manner. There was no appreciable amount of gold in the bath at the end of the operation, and there was very little platinum metal in the oathode deposit.

As stated hereinabove, fine silver can be decomposedv rapidly. Pure platinum as anode is attacked less rapidly (with direct current) but nevertheless does dissolve, with liberation of chlorine.

If refining the cathode slimes obtained by the above parting process is to be done by the Wohlwill process, the greater part of the silver chloride is preferably removed before melting the anodes. This may be done chemically in for example one of three waysi.

Another method is to separate the silver chloride in the course of melting, under oxidizing condi- 75 1 If theuslinies'rare tocbe refined by acid trea ment; the silver. chloride need not :be; removed The washed if slimes i are -.:treated', 'iwith -?h0t: .8q l.8:: regia until all-,the .gold;is dissolv .The; solu tionise'vaporated almost-1 tozdryness. ,with. addi tions of hydrochloric i;acid;.until all, the .nitricliacld has been drivenromwThe; crystallizedrresidue o syrupy liquid ls dis'solvedwinz5 hydrochloricxacid allowed: to stand until. a all the; dissolved. silver. chloride precipitates' and settles, andzsthe. ;:soluJ-' sulphate or sodium bisulphite, isziaddedrto 1th filtrate andithe gold precipitatedsii e'I'haipre cipitate is washed with'hot watergdriedgndemelt ed, and poured into;= bars about @9996 fine-3111f ahigher-.purityiberdesire theergoldc preci ita at once'zredissolvedt and:reprecipitatedni 1.1:;

I propose:- as a?furtherrpracticalrmethodioir1153: fining the cathode deposit, or slimes, obtained.

in accordance witlifiiii yisprocess described abgve to melt it into anodes for a second treatment by my process, by the method described on the preceding page. This involves removing the admixed AgCl by one of the methods described on thepreceding page, before melting. Part of the gold is deposited and part remains in the electrolyte. When sufiicient gold has deposited, the deposit with admixed AgCl and the electroLvte may be removed together, and the deposited gold brought into solution by the addition of nitric acid. For this method, the silver content of the cathode deposit may be kept low by high cathode current density and cooling, and by superimposed alternating current or rectified current, in which case, very little nitric acid is required. The solution is diluted and filtered free of silver chloride, and the gold is precipitated in the filtrate in pure form. Another application of my process for refining the anodes made from the cathode slimes of the parting process involves dissolving the gold of the anode electrolytically and entirely preventing the formation of a deposit, by the familiar use of porous cups surrounding the cathode. The anode solution is diluted and filtered, separating the silver chloride, and fine gold precipitated chemically from the filtrate.

Superimposed alternating current upondirect current or rectified alternating current may-be applied in the refining process as well as in parting, and the use of concentrated hydrochloric acid and the addition of soluble chloride is desirable but not essential in refining high gold con-- tent anodes.

To refine the spent electrolyte and washings, the spent baths are accumulated in a final precipitating tank. The silver, the platinum metals and traces of gold are precipitated together by hanging strips of copper or zinc in'the tank.

' in this specification andin the claims as concentrated hydrochloric acidis concentrated hy- .drochloric acidot substantially 1 commercial zs a as heiaat gsiaaacg do eat mass; .st a lshas irgnaetiesatl amr asan anode.

3. The method of parting a gold bullion anode of any purity containing silver in a bath initially .ec a. oces'sew comprising commercialconcentrated HCl free of gold salts which comprises passing current of a density of the order of fromlOO to 900 amperes per square foot between the cathode and anode to part the gold electrolytically from the anode and without the formation of passive AgCl on the anode.

4. The method of gold parting a gold bullion anode of any purity which comprises electrolytically dissolving the gold' in an electrolytic bath initially comprising commercial concentrated HCl free of gold salts and superimposing a current to prevent polarization. 4

5. The method of gold parting and refining a gold bullion anode of any purity which comprises electrolytically dissolving the gold in a gold bullion anode of any purity which comprises,

electrolytically dissolving the gold in a bath of commercial concentrated HCl and electrolytically refining the gold in a concentrated HCl bath an depositing the gold,

7. The method of gold partingand refining a gold bullion anode of any purity which comprises electrolytically dissolving the gold in an electrolytic bath initially comprising commercial concentrated 1101 free of gold salts and refining the gold chemically.

8. In an electrolytic system having an anode of any purity of goldbullion, an initial bath of commercially concentrated HCl free of gold salts and a cathode, the method of gold parting which comprises passing a current through the anode, electrolyte and cathode, removing the gold from the anode and depositing it on the cathode in the form of an impure loose spongy mass and refining the cathode deposit by dissolving the gold in aqua regia and chemically precipitating the gold from the aqua regia solution.

9. The method of gold parting which comprises passing an electric current from an anode of gold bullion of relatively low to high fineness through a bath of commercial concentrated hydrochloric acid, depositing metallic gold from the solution on the cathode in the form of a 78 mercial concentrated hydrochloric acid, depositing metallic gold from the solution on the cathode in the form of a loosespongy mass, and permitting the cathode deposit to drop to the bottom of the solution.

11. The method of gold parting which com prises passing an electric current from an anode of impure gold through a bath of commercial concentrated hydrochloric acid containing a small amount of a chloride of a metal selected irom the group of alkali and alkali earth metals, depositing metallic gold from the solution on-the cathode in the form of a loose spongy mass, and permitting the cathode deposit to drop to the bottom of the solution.

12. The method of gold parting which comprises passing an electric current of a density of the order of from 150 to 900 amperes per square foot from an anode of impure gold containing silver through a bath of commercial concentrated hydrochloric acid, depositing metallic gold from the solution on the cathode in the form of a loose spongy mass,- and permitting the oathode deposit to drop to the bottom of the solution.

13. The method of gold parting which comprises passing an electric current from a gold anode containing silver to a cathode through a bath of commercial concentrated hydrochloric acid at current densities of the order of from 150 to 900 amperes per square foot, forming silver chloride on the anode in the-form of a loose spongy mass, depositing gold on the cathode in the form of a loose, spongy mass, and maintainingthe concentratation of hydrochloric acid sufficientNto, maintain the electrolytic action at a predeterinined rate.

14. Themethod of gold parting which com SAMUEL J. BLAUT.