CA1188522A - Recovery of silver from precious metal-containing intermediates - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
Silver is recovered from precious metals intermediate products such as anode sludges and the like by a process involving the steps of chlorination and then thiosulfate leaching the chlorination residue.

Description

2~

PC-213~
The invention is directed to a process for recovering silver from intermediate materials containing silvex along with other valuable metals includi~g metals of the platinum groupu It is known that many base metal ores such as those of copper, nickel, zinc, l.eadt etc~ also cGntain metals of the platinum group together with silver, gold, selenium, tellurium~ etc~ 5uch ores form important sources of the valuable metals which are k~own colloquially as I'precious metals"O
These metals occur in the various metal ore~ in small amounts and become concentrated during the working up o~ ~he b~se mfltals in the form of:~ariou~ inter~ediates including anode sludges, leach residues, cements, etc.
~ccordingly, t~le compositions of these precious metals-contain.ing materials vary widely depending upon the nature of the ore containing th~ sameO Despite ~he differences in composition these materials tend to lend themse~ves to a more or less common scheme of treakment and tend to contain ~he same ingredients to a great extent although the propor-tions of valuable metals therein can vary. Thus r me~als which may be present in the precious metals intermediate pro~ucts include all six members of the platinum group, gold, ~ilvery selenium, te].lurium, lead, arsenic, antimony~
tin, bismutht copper, nickel, zinc, iron and sulur. Once the material.5 are concentrated in the form Qf anode sludges, leach residues, etc. it then becomes important to recover the metal values as completely as possible and to produce metal concentrates oE respectable purltyO One known method r for example, can involve decopperizing of the preciou~ metal sz~

intermediate by leaching with sulfuric acid and then smelting in a Dor~ furnace in which silver may be recovered in the form of Dore metal. Such a procedure i5 expensive and can produce harmful emissions of ~elenium, arsenic, lead and other heavy metals.
It is known also to recover precious metal from intermediate products by dissolution in hydrochloric acid with chloxination followed by ammonia extraction o silver ~rom the resulting residues and recovery of platinum ~roup metals~ selenium, and tellurium from the leach liquor. It iS al50 known from the photographic art that thiosulfate solutions can be employed ~o dissolve silver from light unaffected por ions of film which is employed for photo-graphic purposes. Ammonium thiosulfate leaching of gold and silver from ammoniacal leach residues in ~he presence o ~upric ion and free ammonia is al~o known.
R~levant publications which can be cited in respect o~ the subject matter include _v ~ Metal 1963, ~ol. 36, No. 11, pages 85-86; and U.S. Patents No.
~o 3,658,510; No. 4~070,182; 4,269,622 and No. 4,2~9,~70.
~ t is desirable that a hydrometallurgical method be provided for working up precious metal intermediate materials which will provide for selec~ive and ef.ficien~
separation of silver therefrom.
S~MMARY OF THE INVENTION
Silver containing precious metal intermediate materials to be treated for the recovery of silver are first slurried with water and are leached with chlorine to dissolve essentially all of the precious metals and to ~o produce a leach residue containing the silver largely as silver chloride. Thereafter, the leach residue and the 5~;~

supernatant liquor containing dissolved chlorides are separated and the residue is contacted with an aqueous solu-tion of thio~ulfate to dissolve the silver. The thiosulfate solution is then worked up to recover silver therefromO
DETAILED DESCRIPTION OF_THE_INVENTION
Silver~containing precious metal intermediates to be treated in accordance with the invention are slurried with wat~r in the proportion of about 5 to about SO~ by : weight solidsO The resulting slurry is then chlorlnated for a time sufficient to convert the silver content to ~ilver chloride and to chlorinate most of the remaining metal values a~ chlorides. In the course of the chlorination a substantial quantity of hydrochloric acid can be formed by reactions between chlorine and elements such as sulfur, ~elenium, tellurium, arsenic, etc., or compou~ds thereof~
Chlorination is usually exothermic. Accordingly, cooling of the solution may sometimes be required. The addition rate of chlorine will be controlled ~o avoid possible overheating and/or excessive chlorine consumption.
A convenient temperature range for chlorination is about 60C to about 80DC.
After completion of the chlorine leach, the silver-containing residue is separat~d from the supernatant solution, which now contains the precious metals.
The silver-containing residue can then be leached, preferably at ambient temperature, with a thiosulfate solution e.g., sodium thiosulfate, to dissolve silver selec-tively with regard to impurities such as silica and ferrites which may be co~present in the silver-con~aining residue.
The thiosulfate leach may be conducted at a temperature of about 10 to about 80C. When lead i5 present ln the chlorine leach residue to an extent requiring selective removal of the silver, the pH of the thiosulfate leach solu-tion should be at least 7 and more preferably pH 9 or pH 10.
When the thiosulfate solution is sufficiently basic, the thiosulfate leach is highly selective or silver as compared to lead in the precipitate being di.ssolved. ~hiosulfate is used in approximately the proportions of two to four mols o thiosulfate ~or each mol o~ silver to be leached~ A further advantage of maintaining a thiosulfa~e ~olution basic has ~o do with the fact ~hat s~abili~y of the solutions is ~hereby increased. Small amounts of a sulfite, e.g. sodium sulfite added to the thiosulfake leach solution will also improve stability.
The thiosulfate leach solution containing silver from the residue may be readily treated to recover the silver in a variety of ways. For example, cementation with metals 5uch as iron, zinc or magnesium at amb.;ent temperature produces cements analyzing on ~he order of 90%
silver. Or~anic reducing agents, such as fructose, dextrose and lactose can be used to produc~ silver precipitates of high purity~ i.eL at least about 90% in silver, Electro-lytic recovery means may also be used.
As noted hereinbefore, the composition of the silver-containing precious metal intermedlates to be treated in accordance with the invention~ can vary widely depending upon the ore from which the intermediates are obtaine~. It may be convenient in connection with the working up/ ~or ~xample, of copper refinery sludges to recover silver, to subject the sludge to a decopperizing leach in sulfuric acid prior to treatment in accord2nce to the invention, Similarly, a lead removal step may be employed prior to 35~

treatment of ~he interme~iate in accordance w:ith the inven~
tion.

Some e~amples will now be givenO
EXAMPL~S

66.2 kg of a precious metals-containing feed, analyzing (%) 1~5 Pt~ 1.6 PdJ 0.40 Au, 0.16 Rh~ 0.09 Ru~ 7.6 Ag, 7~1 Pb~ 5.6 Se, 0.67 Tel 0~34 Cu was slurried in water at a solids density of ~450 9 solids per liter oE slurry and heated to 60C. Gaseous chlorine was sparged into the agitated slur~y ~or a ~otal of 4 h a~ a flowrate of 100Q/min. A~ter ~iltra~ion of ~he leached slurry about 4~.5 kg of leach res.idue was obtained, which was found to analyze (%) 0.035 Pt, 0.016 Pd, 0~005 Au, 10.1 Ag, 3O2 Pb. The ~ollowing metal extractions were obtained (~O 98 Pt, 99.3 Pd, 99.1 Au/ ~1 Ag.
The thiosulfate leach was conductecl as follows:
33.8Q of water was added to 15.5 kg of the above chlorine leach residue (39~ moisture). NaOH was added to the agitated slurry to bring the p~ to 10.0 at 22C. Then 3.75 kg oE ~2S23 was added (3.9 kg Na2S2O3/k~ Ag) and the slurry was agitated ~or 30 minutes at a pH of 10 (22C)~
The residue was filtered off and washed with one cake disp:lacement of water. Since the leach re~idue contained a large quantity oE filter-aid, which tends to trap large amounts o~ leach liquor~ the wet residue was su~jécted to a repulp leach using llQ o~ water and 0.4 kg of Na2S2O3~
Leaching was again conducted for 30 minutes at 22C ~pH 10).
~inally, the leach residue was separated f~om the solution by filtra~ion. The leach residue analyzed 0.11~ Ag and 5~2~

Pb. 99.1% of the silver was extracted w~-th only 0~8% of the : lead.
The thiosulfake leach liquor and the repulp leach li~uor were combined, resulting in a solution analyzing (g/l) 18.0 Ag and 0~07 Pba Sulfuric acid was added to the solution to reach a p~ of 4.0 at ~2S. Then iron powder w~9 added to cement the contained silver values. The pH was held at 4 by simul~aneous addition of sulfu~ic acid. When a redox potential of -400 mV(SCE~ was reached, the slurry was ~: 10 filtered. The silver cemen~ analyzed (%3 87.1 Ag, gO3 ~e, ~ 0.38 Pb, 1 r 4 S. More than 99.9% of the silver had been ; cemented and the barren solution analyzed 10 mg/l Ag.
!~
170 9 of a moist (39.4~ H20) precious metals containing ~eed analyzing (%) 1073 P~ 65 Pd, 0.33 ~u, 2.00 Ag, 3.43 Pb and 52.0% SiO2 was slurried with water and various amounts of hydrochloric acid, as shown in Table lo The resulting slurries ~30-35% solids) were agitated and heated to 80Co Gaseous chlorine was bubbled into the slurry at a flowrate of ~1 9/l slurry-min for a total of 4 h. The slurry~ was then f iltered and the leach residue and the leach liquor were analyzed.
As shown in Table 1, it was found ~hat the extrac-tion of the precious metals Pt, Pd and Au was essentlally una~fected by the init:ial acidity. ~Iowever, much less silver was extracted (only 002~) in the test where the feed was slurried with water. Thus, a be~ter separation of silver from Pt, Pd and Au was achieved.

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Exam~le 3 A chlorine leach residue analyzing~ in weight percent, 17.3 Ag and 9O9 Pb was subjected to a number of thiosulfate leach ~ests at p~ values between 2 and 12. The leach conditions were si.milar as .in Example 1 and are listed in Table 2, t~gether with the results of duplicate tests.
The extraction of silver w~s lowest (~97%) at a p~ of 2.0, and was generally g9~ or higher between p~ 4 and 12. The dissolution o lead was strongly influenced by the pH. At 10pH 6 and below~ more than 75% of the Pb was dissolved~
whereas at pH 8 less than 20~ Pb was extracted and at a pH
of 10 the dissolu~ion of Pb was only . TABLE 2 ; LEACHING OF CHLORTNE LEAC~I
ESIDUE WI.~ GorluM tYIOSU~-~T, eed Assa~ 17~3 Ag, 9.9 Pb Conditions: Leaching of 102 g C12 leach res.idue at 20~ solids in water, 3.85 g Na2S2O3/g Ag t 30 min. at 22C. ~epulp in 15 g/1 Ma2S~O3 solu~.ion (30 min at 22C) at ~he sam~ pH.

RESIDUE ASSAY~%~ L
Aq Pb Aq Pb 2.~ 0.gl 2.~0 96.2 33.2 2.0 0.~g 2.2~ 97.2 8~,9 4O0 0522 2~60 9~.0 83.7 4.0 ~.22 2561 99.1 ~2.9 6.0 0.13 3.40 99.5 77.6 6.0 0.20 3.44 9~.~ 77~1 8~0 0~16 10.8 g~.3 19.5 8.0 Q.09 10.8 99~5 ~7.9 1~.0 0.20 12.8 ~9.1 1.1 1~.0 0.25 13.4 98.9 0.8 1:;!.0 0~16 12~8 g9~3 lol 1250 0 r 30 13.1 98.7 ~.6 5~;~

Example 4 A thiosulfate leach liquo~ OQ) analyzing ~g/l) 18~4 Ag and 68 S203- at pH 10 was acidified to pH 4, and 3.3 g of Mg granules (70-80 mesh) was added over 2 h with simul-taneous addition o 151.5 ml of 50 g/l ~I2So4 solution ~o maintain pH 4. During this period the temperature rose from 22 to 31C and the redox potential of the solution decreased rom +70 m~ to ~250 m~ (Pt v~ SCE). After filtration and drying~ the solids ~20014 g~ analyzed (%)~ 90.9 Ag, 106 Mg and 3.15 S. The filtrate (1.085Q) contained 0~042 g/l AgO
Thus, 99.7% of the silver was recovered in the 501ids Example 5 A thiosulfate leach liquor (l.OQ) analyzing (g/l) 18.5 Ag, 0.11 Pb and 69 S~03= at pH 10 wa~ heated to 80~C
and 14.3 g of D-fruc~ose was added. The solution W~5 maintained at p~ 10 by addition of 72.9 ml of a lSO g/l NaO~I
solution. After 15 minutes the redox potential o~ the solu-tion had ~ecreased from O mV to -660 mV (Pt vs SCE) and the sil~er mirror~ originall.y plated on the sides of the beaker, disappeared leaving a flocculant precipitate (18023 g) which analyzed (%~: :97.5 ~g~ 0.55 Pb, and 0.66 S. The filtrate (1.044~) a~aly~ed <5 mg/l ~9. ~hus, 99~97% of the sil~er was recovered in the solids.
Example_6 ~ thiosulfate leach liquor tl.OQ) analyzing (g,~l) 17.9 Ag, 0.10 Pb, and 67 S203= at pH 10 was heated to 80~C
a~ld 14.3 g of ~-glucose was addedO The solution was maintained at pH 10 by addition of 265 ml of a 38 g/l NaOH
solution. After 30 minutes the redox potenkial of the solu-tion had decreased from +30 mV to -660 mV (Pt vs SCE) and the silver mirror, originally plated on the sides oE the ~ 5 ~ ~

beaker, disappeared leaving ~ 10cculant precipitate (17.45 9~ whlch analyzed (%) 97.9 Ag, 0.52 Pb, and 0.56 S. The filtrate ~1.26Q~ contained 40 mg/l ~g. Thus, 9~.7~ of the silver was .recovered in the solid 5 .
~xam~le 7 A thiosulfate leach liguor (l.OQ~ containing 17.9 g~l Ag and 67 g/l S~03- was adjusted to p~l 13 at 24C by adding 1~ 9 NaO~. Then 14.3 g of D-fructose was added.
After 2 hours the redox potential had decreased from +ÇO mV
to -225 mV (Pt vs SCE) and 8.85 g of precipitate was filtered off. After standing overnight, an additional 8.30 g of precipitate was recovered. The combined precipitates analyzed 99.6~ Ag while the solution contained only 70 mg/l Ag. Thus, 99.6~ of the silver was recovered in the solids.
xample 8 The compositions of ive other precious metal~
containing mater ials susceptible to treatment in accordance with the invention are shown in Table 3. Each of these mater.ials was chlorine leached under the conditiorls and for the times shown ;n Table 4. It will be seen from Table 4 that excellent extractions of platinum-group metals and gold were achieved with all the materials treated although the compositions thereof varied widely. On the other hand, extractions of silver were low. The compositions, in weight percent, of the chlorine leach residues are shown in Tahle 5. ~11 oE the chlorine leach residues were susceptib.le to thiosulfate leaching to di~solve silver~

~ 10 -352~

PM CONTAINING FEEDS TO CHLQRINE-WATER LEACHING
-- ASS~YS (%~ V

Feed ~lo. 1 2 3_ _ 4 5 S::u ().09 0.75 1042 1.8~ 38~6 N i 0 . 2 û 5 . 6 53 ~ 9 0 1 D 3 0 1 3 . 7 Fe - -- 0-74 0.13 4O45 S ~ -- ---- 19 . 4 S~ Oog6 8.40 -- 1600 10E~6 Te 0~14 0.60 -- ~.oo 0~14 Pb 3.7 5.90 11.8 2.75 3O08 Sb 0.05 0.03 -- 0.05 0.15 Sn (1 . 39 0 . 75 -- 0 O 3n o . 2~
Bi 0.04 0.15 -- 0.13 0.07 As 0.73 0.75 -- D.25 0.26 P~ 2.32 1.1~ 8.0 0.21 ~.04 Pd 12 1,81 8.0 0O52 2.22 ~u 0.47 0.36 2.70 0.25 0.5S
Rh 0.29 0.17 1.55 0.06 0.40 Ru 0.13 0.10 0.30 0.10 0~14 Ir 0.1 0.05 -- O~t)3 OolO
Ag 2,5 9,.S 34.3 13.1 7.32 SiO2 67 41.û

- 11 ~

:

8~2~

CHLORINE-WATER LE:ACHING OF PM FEEDS
Conditions: Pulp density 250-400 g/Q in water 1.0 2 .S g ~ 12/minQ slurry Feed 1 2 3 4 5 Leach Temperature 60 60 80 80 80 (C) I,each Time (h~ 6 6 5 6 5 gXTRACTIONS (~3 . . _. ., Cu 78 75 g9 96 ~99 ~ 9 2~i 7 15 99.2 89 99.
Fe ~ 21 99 S ~
Se 95 99 . 5 -- 99 . 7 80 Te 99 . 9 99 -- 99, 7 93, 3 Pb 60 98 89 93 99 2 0 .Sb 23 59 ~- 72 94 Sn 4 49 -~ 82 9~
Bi 72 94 -- 94 92 As 88 72 -- 88 99 Pt 99 . 2 99 . 2 99 . ~ 9g . 3 98 . 8 Pd 99.,9 99.4 99.6 99.8 g~.6 Au 99v9 98.2 98.9 99,û 99~4 Rh 99 . 7 97 96 98 97 Ru --- 98 97 g9 . 4 96 Ir 99 . 7 97 -- 96 88 Ag 0.6 1.7 ~ 3.3 S.iO2<0.2 0.0~ ---- _ __ ~8~

T~BLE 5 CHLORINE-WATER LEACH RESIDUE ASSA~S (~) . . . ~

Cu 0.02 0~37 a.o3s o.~ 0.05 Ni a.~g 6.35 0.065 0.~1o.n8 Fe -- -- -- 0.250018 S ~ 50.5 Se 0~07 0.07 -- 0.152.43 TeI OoOl OoOl ~~ 0~03~0~01 Pb 1.8 0.17 2090 0~53 . 0.08 Sb 0.06 0.04 -- 0~4 0O05 Sn 0,.51 0~46 ~ OolSO~OS
Bi 0.01 0.01 -- 0.020.04 As 0.11 0.28 -- 0.110.02 Pt 0.002 0O013 0.1~ 0~0050.14 Pd 00002 0~013 0~075 0~0040.051 Au<0~001 0.010 0~064 0~00700018 Rh<0.001 0.007 0.14 0.0040.057 Ru -- 0.004 0.020 0.0~20.033 Ir~0.001 0~003 -- 0~0030~0~7 Ag 3~43 12~0 74~0 30~6 33~8 SiO274.6 57.0 Although the present invention has been de~cribed in conjunction with preferred embodiments, it is to be understood that modi~ications and variations may be resorted to wikhout departing from the spirit and scope of the inven-tion, as those skilled in the art will readily understand.
Such modifications and variations are considered to be within the purview and scope of the invention and appended claimS.

Claims (8)

WE CLAIM:

1. A hydrometallurgical method for treating a precious-metal-containing intermediate material, to concen-trate and recover the silver content thereof which comprises slurrying said material in water, chlorinating the aqueous slurry to dissolve platinum group metals and gold and to produce a residue enriched with respect to silver, dissolving silver from said residue by contacting it with a thiosulfate solution and then recovering silver from the thiosulfate leach solution.

2. A process in accordance with claim 1 wherein said residue and said thiosulfate solution are contacted at a pH
of 2 or above.

3. A process in accordance with claim 1 wherein said residue also contains lead and wherein said thiosulfate leach is conducted at a pH in excess of 7 to selectively leach silver.

4. A process in accordance with claim 3 wherein said pH is at least 9.

5. A process in accordance with any of claims 1 r 2 or 3 wherein the temperature of the thiosulfate leach is between about 10° and about 80°C.

6. A process in accordance with any of claims 1, 2 or 3 wherein the thiosulfate leach is conducted at ambient temperature.

7. A process in accordance with any of claims 1, 2 or 3 wherein silver is recovered by cementation.

8. A process in accordance with any of claims 1, 2 or 3 wherein silver is recovered by reduction with an organic reductant.