Process for Refining Silver Bullion With Gold Separation

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    1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1us 20030154821Al(19) United States(12) Patent Application Publication

    Vanhoutte et al.(10) Pub. No.: US 2003/0154821 Al(43) Pub. Date: Aug. 21, 2003

    (54) PROCESS FOR REFINING SILVER BULLIONWITH GOLD SEPARATION The process comprises the steps of:

    Correspondence Address:ANTONELLI TERRY STOUT AND KRAUSSUITE 18001300 NORTH SEVENTEENTH STREETARLINGTON, VA 22209

    optionally removing Se as gaseous Se02 from themolten metallic phase by injecting air, which ispreferably O2 enriched, into the metallic phase at apreferred bath temperature of 1000-1100 c.;

    optionally slagging off the Pb by contacting the moltenmetallic phase with a silica and borax based flux ata preferred bath temperature of 1000-1150 c.;

    (76) Inventors: Dirk Vanhoutte, Langdorp (BE);SyboIt Brouwer, Berchem (BE)

    (21) Appl. No.: 10/182,121granulating the molten metallic phase in water, thereby

    forming Ag rich granules;

    PCT/EPOl/00613

    leaching the Ag rich granules with HN03 at a tempera-ture above 50 c.; preferably in an O2 enrichedatmosphere, followed by filtration, thereby separat-ing an Au bearing residue from an Ag rich liquor;

    (22) PCT Filed:(86) PCT No.:

    J a n . 17,2001

    (30) Foreign Application Priority DataJan. 28, 2000 (EP) 00200294.7

    heating the Ag rich liquor, thereby evaporating H20and forming an AgN03 bearing melt;Publication Classification maintaining the AgN03 bearing melt at a temperatureof 220-350 C. for at least 15 minutes, thereby

    forming a mixture of a purified AgN03 and of adenitration residue containing essentially all thePGM as oxides;

    (51) Int. CI? C22B l l /OO(52) U.S. CI. 75/634(57) ABSTRACTThe present invention concerns a process for refining silverbullion, i.e. raw silver containing generally more than 90%silver besides Se, Pb, Au, Cu and platinum group metals(PGM) as main impurities.

    separating the purified AgN03 from the denitrationresidue.By this process, the many drawbacks of the classical elec-trolytic refining of silver are avoided.

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    PROCESS FOR REFINING SILVER BULLIONWITH GOLD SEPARATION[0001] The present invention concerns a process for refin-ing silver bullion. Silver bullion is a raw silver alloyoriginating amongst others from lead-silver smelting, con-taining generally more than 90% silver besides Se, Pb, Au,Cu and platinum group metals (PGM) as main impurities.Silver bullion is normally available as Large castings.[0002] The present state of technology for refining silverbullion alloy is electrolytic refining: after casting anodes, thealloy is electrorefined in an AgN03-HN03 electrolyte. Thefollowing output streams are hereby produced:

    [0003] purifiedAg obtained as dendrites deposited ona stainless steel cathode sheet,

    [0004] anode slime, containing Au and PGM, whichis collected in fabric bags surrounding the anode;

    [0005] a bleed on the AgN03-HN03 electrolyte.[0006] The electrolyte bleed is necessary to prevent theaccumulation of impurities which anodically dissolve com-pletely as nitrates, such as Pb and Cu, or partially, such asPd.[0007] A first method for bleed treatment is the retrieval ofthe dissolved Ag by cementation with a less noble metal. Theimpurities such as Pb and Cu remain in the nitrate bearingbleed solution.[0008] A second method for bleed treatment is by deni-tration, also referred to as the black melt process in Ull-mann's Encyclopedia of Industrial Chemistry, 1993,vol.A24, p. 134. In this case, the water is evaporated fromthe electrolyte, and the anhydrous nitrate melt is heated to atleast 170 C. The nitrates of PGM decompose to insolubleoxides. Cu nitrate decomposes partially, to an extent deter-mined by the temperature of the melt. After reaction withwater, the oxides are separated from the AgN03 bearingsolution. However, if an excess of Pb and Cu has to beremoved from the electrolyte, at least part of the bleed mustbe treated according to the first method. Here again, the Aghas to be retrieved by cementation, producing an impure Pbbearing nitrate solution.[0009] Both methods thus necessitate a rather elaboratetreatment of the bleed whereby nitrate solutions are pro-duced. Their further treatment leads to the discharge ofnitrates.[0010] This disadvantage is further exacerbated whenhigher levels of Pb, Cu or Pd are present in the Ag bullion:a higher amount of impurities has indeed to be evacuated bybleeding a correspondingly larger quantity of electrolyte.[0011] Also, more than approximately 2% Pd in the rawsilver leads to problems as Pd then gets embedded in therefined Ag deposit.[0012] Both the electrolytic refining process and the anodeslime treatment are inherently slow. Consequently, metalshave a long residence time and the inventory of the refiningplant is high as are the ensuing financial costs.[0013] The present invention aims at resolving the abovementioned disadvantages. Moreover, the new process pro-duces Ag with a higher purity than the state of the artprocess.

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    [0014] Itshould be noted that lP-A-60224720 discloses aprocess for the recovery of Ag from Cu-electrolysis anodeslime. This process presupposes that Cu has been nearlycompletely removed from the slime and that a crude metallicAg is produced. This crude Ag is subjected to a melting stepwith injection of an O2 carrying gas, ensuring the oxidationand removal of impurities. The molten purified Ag is thengranulated and the granules are dissolved in HN03. Theobtained solution is cleaned up using a chelating resin,whereupon Ag is recovered from the solution by reductionwith hydrazine.[0015] The presupposed removal of Cu is however acomplex and lengthy process as it necessitates the chlori-nation (wet or dry) of the anode slime, followed by theconversion of the chlorides back to their metallic form.[0016] U.S. Pat. No. 5,000,928 describes a process for thepreparation of ultra-pure AgN03. As a first step, crude Ag isdissolved in HN03. It is disclosed that heating and aeratingpromotes the dissolution process. Further steps include theaddition of an alkaline agent to precipitate impurities and theuse of a selective reducing agent to precipitate Ag as ametallic powder. This powder is then again dissolved withHN03, whereupon ultra-pure AgN03 is crystallized from thesolution.[0017] The invention discloses a process for refining silverbullion. The process comprises the steps of

    [0018] leaching the silver bullion with HN03 at atemperature above 50 c., preferably in an O2enriched atmosphere, followed by filtration, therebyseparating an Au bearing residue from an Ag richliquor;

    [0019] heating the Ag rich liquor, thereby evaporat-ing H20 and forming an AgN03 bearing melt;

    [0020] maintaining the AgN03 bearing melt at atemperature of 300-350 C. for at least 15 minutes,thereby forming a mixture of a purified AgN03 andof a denitration residue containing essentially all theCu and PGM as oxides;

    [0021] separating the purified AgN03 from the deni-tration residue, either by[0022] filtrating the mixture at a temperature above220 c., thereby obtaining a purified molten

    AgN03; or by[0023] reacting the mixture with water, followed

    by filtration, thereby obtaining a purified AgN03solution.[0024] The leaching operation can be greatly acceleratedand thus rendered more economical if performed on gran-ules instead of on large castings. To this end the leachingstep can be preceded by the following steps:

    [0025] heating the silver bullion, thereby forming abath with a molten metallic phase;

    [0026] granulating the molten metallic phase inwater, thereby forming Ag rich granules;[0027] and the leaching step is performed on the Ag

    rich granules.

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    [0028] it is useful to take advantage of the above meltingoperation, which is needed for the granulation, to remove Seand Pb from the molten metallic phase. Se can be completelyremoved as gaseous Se02 from the molten metallic phase byinjecting air, which is preferably O2 enriched, into themetallic phase at a temperature of 1000-1250 c., or pref-erably of 1000-1100 C. Pb can be removed from the moltenmetallic phase by contacting this phase with an acidic fluxat a temperature of 1000-1300 c., or preferably of 1000-1150 c., whereby Pb is slagged off. In this way, thequantities of Se and Pb to be treated in the following stepsof the process are decreased substantially. Se and Pb can beremoved in either order.[0029] When purified dry AgN03 is obtained in the pro-cess as described above, it can be commercialized as such.Italso can be further processed to metallic Ag. To this end,the purified AgN03 is decomposed at a temperature above400 C. into elemental Ag and nitrogen oxides (NOx) whichare scrubbed with H20 in an oxidizing atmosphere, therebyforming HN03, which is optionally recycled to the leachingstep.[0030] When the purified AgN03 solution is obtained inthe process as described above, it can be commercialized assuch. It also can be further processed to metallic Ag.[0031] A first method is to hydrolyze the purified AgN03solution with NaOH or KOH, thereby forming an Ag20precipitate, which is separated by filtration and to decom-pose the Ag20 at a temperature above 300 C. into elementalAg and O2,[0032] A second method is to react the purified AgN03solution with HCI, thereby forming AgCI and HN03, whichare separated by filtration, the HN03 being optionallyrecycled to the leaching step. The AgCI is then transformedto elemental Ag and NaCI either by reacting the AgCI withan NaOH solution in presence of a reducing agent, or byheating the AgCl with a soda flux above 1000 C.[0033] The claimed process is well suited to refine rawsilver, especially when high levels of Pb, Au, Cu and Pd arepresent. No nitrates are discharged in the environment. Auand PGM are readily separated and recovered from the Agstream. The inventory of precious metals is thus much lowerthan with electrolytic refining.[0034] The mandatory and the preferred working condi-tions of the new process are explained hereunder.[0035] When Se is not removed before the molten metallicphase is granulated in water, it will concentrate in the Aubearing residue during the leaching of Ag with HN03. Thiswill considerably complicate the refining of Au from the Aubearing residue.[0036] During Se removal, the bath temperature shouldnot exceed 1250 C. The higher the temperature, the lowerthe rate of Se removal. A temperature of up to 1100 C. istherefore preferred. The bath should in any case remainabove 1000 c., i.e. well above the melting point of the rawsilver. Itis advisable to remove any slag from the surface ofthe metallic bath to enhance the volatilization rate of Se02.[0037] During Pb removal, the bath temperature shouldnot exceed 1300 C. With respect to refractory wear, atemperature of up to 1150 C. is preferred. The flux used forslagging off the Pb should be of the acidic type. An acidic

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    flux is a flux comprising phoshates, silicates or borates asessential components. A flux consisting of Si02 andNa2B407. 5aq in a weight ratio between 15:85 and 20:80 isrecommended.[0038] When leaching the Ag rich granules with HN03, aminimum temperature of 50 C. is needed. At a lowertemperature, there is a risk for the leaching reaction to slowdown or even to stop temporarily, and then to resumeviolently. Such unstable behavior is to be avoided forobvious safety reasons. Furthermore, it is recommended toraise the temperature gradually to at least 70 C. duringleaching to obtain a residue which is as rich as possible inAu.[0039] Preferably, the amount and concentration of HN03are chosen so as to obtain an AgN03 solution with approxi-mately 1000 gil of Ag and a free HN03 concentration of 1.5M. In this way, the volume of AgN03 solution to beprocessed is kept low, while crystallization of AgN03 isavoided as long as the solution is kept above 45 C.[0040] During leaching, it is preferred to maintain an O2enriched atmosphere above the reaction mixture. To this end,the vessel might be of the closed type or of the pressurizedtype. An oxidizing atmosphere is indeed helpful to recycleNOx' which are a byproduct of the leaching reaction, toHN03 in situ. The reactions are:

    [0041] 2NO+02~2N02 and[0042] 3N02+H20~2HN03+NO.

    [0043] A high O2 partial pressure is useful to enhance thereaction rates.[0044] The purpose of the next operation is to separate theCu, PGM, and the residual Pb from the Ag rich liquor. A heattreatment is applied whereby most impurities denitrate, i.e.their nitrate decomposes to oxide, forming an insolubleresidue. In this step, the temperature of the AgN03 bearingmelt should be kept between 220 and 350 C. A hold timeof at least 15 minutes is needed to ensure a sufficientdenitration level.[0045] However, when Cu has to be removed down to 10ppm or less as referenced to Ag, a higher minimal denitra-tion temperature of 300 C. has to be maintained to fullydecompose Cu nitrates.[0046] There are two methods to separate the mixture ofthe purified AgN03 and the denitration residue.[0047] The first method is the filtration of the mixture asa melt. The filtration has to be performed at a temperature ofat least 220 C. so as to stay well above the solidificationtemperature of AgN03. Purified molten AgN03 is obtained.[0048] The second method is to first react the mixture withwater and then to filtrate the obtained slurry. For similarreasons as for those explained above for the HN03 leaching,the amount of water with which the mixture is reacted isdetermined so as to obtain a purified AgN03 solution withpreferably about 1000 gil of Ag.[0049] Purified AgN03 or AgN03 solutions can be com-mercialized as such. However, if elemental Ag is needed,then the above process should be supplemented with aprocess for the transformation of purified AgN03 to Agmetal. To this end, any of the two methods describedhereunder can be applied.

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    [0050] In a first method, the purified AgN03 is injected ina furnace where it is decomposed at a temperature above400 C. The decomposition products are elementary Ag andNOx' A scrubber converts the NOx' to HN03 which can bereused in the leaching step. Any Ag compound present in theflue dust is also converted to AgN03 in the scrubber. Thedecomposition can be performed below or above the meltingpoint of Ag. The latter case lends itself more easily tocontinuous processing of AgN03.[0051] The above method will normally be applieddirectly on the purified molten AgN03; it is however alsoapplicable to the purified AgN03 solutions in which casewater is evaporated first.[0052] A second method starts from the purified AgN03solution. In a first step, an insoluble Ag compound such asAg20 is precipitated, e.g. by hydrolysis of the AgN03solution with KOH, or, preferably, with NaOH. In the lattercase, the byproduct of the hydrolysis is an NaN03 solution.This NaN03 solution is relatively pure and can be trans-formed into solid NaN03 by a known drying or crystalliza-tion process. For example, by using fluid bed drying, gran-ules of NaN03 are obtained which are similar in appearanceand in composition to commercially available prills.[0053] The Ag20 is optionally washed and is then ther-mally decomposed to Ag and O2 at a temperature of at least300 C. Sponge-like elemental Ag is obtained, which isre-melted and cast into ingots or granulated. If Pb has beenremoved to a large extent from the molten metallic silverbullion phase before leaching the Ag with HN03, then anytraces of Pb which are still present in the sponge-likeelemental Ag can be removed by natural volatilizationduring the re-melting and casting of Ag, or by absorption ofPb in a relatively small amount of silica-borax based flux.However, if Pb has not been removed from the moltenmetallic phase before the leaching step, then it can beremoved from the molten Ag by blowing air, or by absorp-tion in a relatively large amount of silica-borax based flux.[0054] An alternative to the precipitation of Ag20 is theprecipitation of a halide of Ag such as AgCl. This is done byreacting the purified AgN03 solution with HCI according tothe reaction:

    [0055] AgN03+HCI~AgCI+HN03'[0056] Thereafter, two different reaction schemes areavailable to obtain elemental Ag. According to a firstscheme, AgCI is transformed to metallic Ag by melting witha soda flux:

    [0057] 2AgCI+Na2C03~2Ag+2NaCI+C02+1/202'[0058] According to a second scheme, an aqueous AgCIsuspension is formed which is then reduced with H2 tometallic Ag under addition of NaOH. The reaction is:

    [0059] AgCI+NaOH+1/2H2~Ag+NaCI+H20.[0060] Both alternatives lead to the formation of NaCI,whether as an aqueous solution or as a solid, which normallycan be discharged without harm.[0061] Each essential step of the invention IS furtherillustrated with an example.

    EXAMPLE 1[0062] 250 kg of silver bullion is melted and brought at atemperature of 1050 C. 550 l/hr air enriched with 220 1/ hr

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    O2 is blown through the molten bath. Any slag which isformed during blowing is removed well before the slagcompletely covers the bath surface.[0063] After 90 minutes of blowing, 10 kg flux consistingof 83 wt % Na2B407. 5H20 and 17 wt % Si02 is added andcontacted for 20 minutes with the bath while maintaining theinjection of 5501/ hr air and 2201/ hr O2, After 20 minutes,the molten slag is drawn off. The addition of flux and theremoval of molten slag after 20 minutes is repeated 2 moretimes while maintaining the same gas injection. The moltenmetal is then granulated in water at 50 C.[0064] The composition of silver bullion and of theobtained Ag rich granules is reported in Table 1. Thisprocess proves to be capable to remove Pb and Se to below15 ppm and 4 ppm respectively. Because traces of Pb and Seremaining in the Ag are also partially removed in the nextoperations of the process, a refined Ag containing typically2 ppm Pb and

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    [0068] 270 kg of silver bullion is purified as described inExample 1.The obtained Ag rich granules are then leachedunder conditions as described in Example 2. The Ag richliquor, at a concentration of 1000 gil Ag, is first evaporatedand then further heated to a melt at 350 C. for 2 h. Partialdenitration of Pb and substantially complete denitration ofCu and PGM occurs at this temperature and the oxides ofthese metals form an insoluble denitration residue. There-after, the mixture of molten AgN03 and precipitated oxidesis transferred to a pressure leaf filter equipped with a layeredsintered stainless steel mesh with an absolute filter rating of5 ,um. The mixture is filtrated under a differential pressure of6 bars, at a temperature of 280 C.[0069] Table 3 shows the compositions. Itis clear that thepurified molten AgN03 is substantially free from PGM, Pband Cu. These impurities are concentrated in the denitrationresidue.

    TABLE 3Removal of PGM, Pb and Cu after denitration by filtration of

    Ag NO, in tge molten stateAg Pt Pd Pb Cu

    Ag rich matrix 1410 ppm 5175 ppm 32 ppm 3500 ppmgranulesAg rich liquor 1000 gil 1.3 gil 5.1 gil 0.03 gil 3.5 gilPurified molten matrix

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    3. Process according to claim 2, whereby Se is removedas gaseous Se02 from the molten metallic phase by injectingair, which is preferably O2 enriched, into the metallic phaseat a temperature of 1000-1250 c., and preferably of 1000-1100 C.

    4. Process according to claims 2 or 3, whereby the moltenmetallic phase is contacted with an acidic flux at a tempera-ture of 1000-1300 c., and preferably of 1000-1150 c., andPb is slagged off.

    5. Process according to claims 1 to 4, whereby the purifiedmolten AgN03 is further processed to elemental Ag, com-prising the steps of

    decomposing the purified molten AgN03 at a temperatureabove 400 C. into purified Ag and NOx;scrubbing the NOx' with H20 in an oxidizing atmosphere,thereby forming HN03, which is optionally recycled tothe leaching step.

    Aug. 21, 200356. Process according to claims 1 to 4, whereby the purified

    AgN03 solution is further processed to elemental Ag, com-prising the steps of eitherhydrolyzing the purified AgN03 solution with NaOH orKOH, thereby forming an Ag20 precipitate, which isseparated by filtration;decomposing the Ag20 at a temperature above 300 C.into elemental Ag and O2; orreacting the purified AgN03 solution with HCI, therebyforming AgCI and HN03, which are separated byfiltration, the HN03 being optionally recycled to theleaching step,transforming the AgCI to elementalAg and NaCI by either

    reacting the AgCI with an NaOH solution in presenceof a reducing agent, or by heating the AgCI with a sodaflux above 1000 C.

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