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13Journal of Siberian Federal University. Chemistry 1 (2008) 50-59
УДК 541.183:546.04
Anion Exchange Recovery of Gold (I)
from Thiocyanate Solutions in the Presence of Iron (III),
Silver (I) and Some Non Ferrous Metal Ions
Dmitriy O. Krynitsyn*, Olga N. Kononova and Natalya V. Maznyak
Siberian Federal University, 79 Svobodny, Krasnoyarsk, 660041, Russia 1
Received 15.01.2008, received in revised form 15.03.2008, accepted 10.04.2008
We investigate ion exchange equilibria of thiocyanate complexes of gold (I), iron (III), copper (II) and zinc (11) during sorption from individual solutions on anion exchangers AV-17-8, AM-2B and AN-251 (commercial samples made in Russia). It was found out that there is a dependency between the
3asicity of ani(i)xcha4gers and the selectivity of gold (I) recovery from thiocyanate multicomponent solutious 1i) simultaseous pieience of Fe (III), Cu (II), Zn (II) and Ag (I) ions). As a result, the sorption sequences for Mocyanate metal complexes were obtained.
Keywords: gold, thiocyanate, sorption, anion exchange, resin.
In troduction
The stability of thiocyanate ions in acidic medium, as well as their greater selectivity compared to cyanide ions, is the main reason why thiocyanalelons are prospective for recovery of gold from gold-containing raw materials. Another reason for that is that thiocyanate ions are less tonic titan cyanides.
It is known that gold (I) thiocyanate tomplexes are less stable (lg Pi = 25 [1,2,3]) than cyanide complexes (lg Pi = 38 [1,2,]). However, the thermodynamic analysis of Au - Fe (III) -nCN~ - H2O [4,5] showed that in approximation to real conditions (i.e. at concsntrations of gold and iron (III) less than 0.01 mol/L) it is possible to obtain [Au(SCN)t] and [Ao(CCN)2 ]" complexes during the dissolution of gold. The research [6,7]
resulted in successful sorption recovery of gold from arsenopyrite concentrates after breakdown with acidic thiocyanate solutions.
The authors [8,9] have pointed out the problems arising when the dissolution of gold is carried out with thiocarbamide, thiosulfate or iodide, as alternative to cyanide reagents. These problems are the choice of oxidizer, which must be effective and "soft" at the same time, and the choice of effective recovery method of gold from the obtained solutions. Noteworthy, a number of investigators [4,6] has proposed to use iron (III) ions, hydrogen peroxide etc. as the oxidizers for effective gold dissolution by thiocyanate ions. The most appealing for that are iron (III) ions because of their presence in gold-containing raw materials. However, at present time there is a lack
* Corresponding author E-mail aUt^eus: ([email protected]
1 ©СЮт Federal Uniieisity. All gghes reserved
of studies of sorption recovery of gold on various sorbents. Therefore, our investigation is focused on sorption of gold from thiocyanate solutions on a number of anion exchangers.
Experimental
The sorption of thiocyanate complexes of gold (I), iron (III), copper (II) and zinc (II) was studied using anion exchangers AV-17-8, AM-2B and AN-251 with different basicity. AV-17-8 is the strong basic sorbent (pKa > 1.5) with polystyrene matrix and gel structure. It contains quaternary ammonium base as the functional groups. AM-2B is the macroporous anion exchanger of neutral basicity (pKa ~ 1.5-4) with also polystyrene matrix. Its functional groups are quaternary ammonium base as well as tertiary amine groups. AN-251 is the weak basic anion exchanger (pKa ~ 5) with vinylpyridine matrix and macroporous structure. Its functional groups are pyridine rings.
Before use, the anion exchangers were treated according to the standard methods [10].
The solutions of thiocyanate gold (I) complexes were obtained from stock solution of HAuCl4 by means of gold (III) reduction by thiocyanate in the presence of KOH [11]: 3[AsCl4 ]" + 7SCN- + 4H2O ^ ^ 3[As(SCN)2 ]" + SO]- + CN- + (1)
+ 8H + + 12CI -
For this purpose, 50.0 mL of HAuCl4 solution with the concentration of 2.0 g/L were diluted by distilled water in a 1 L glass up to the volume of 900 mL. The pH value in this solution was adjusted up to ~ 2 by hydrochloric acid. Then 50.0 mL of 2M KSCN solution was slowly added under agitation by magnetic mixer, and the pH value of the obtained solution was adjusted up to
~ 8 by 10% KOH solution. Over a period of 1 h, pH value of this solution was reduced up to ~ 2 (or ~ 4). Then the solution was placed into a 1 L glass volumetric flask and adjusted up to the mark.
The solutions of gold thiocyanate complexes containing iron (III) and other metal ions, were prepared from the above-mentioned solution. Before the adjustment of solution in a 1 L volumetric flask, 10.0 mL of 0.1 M FeSO, CuSO, ZnSO4 and AgNO3 solutions were added to the thiocyanate solution of gold (I). Table 1 contains the data on composition and concentrations of multicomponent solution.
The absorption spectra of investigated solutions were recorded in the wavelength interval of 190-720 nm (spectrophotometer "SF-46", Russia) using a quartz cuvet with 1 cm pathlength.
The individual solutions of iron (III), copper (II) and zinc (II) were prepared from accurately weighed salts samples FeSO4 • H2O, CuSO4 • 5 H2O, ZnSO4 • 2 H2O. These samples were dissolved in 0.1 M KSCN solution with pH ~ 2 (adjusted with HCl).
The concentration of gold in individual and multicomponent solutions as well as concentrations of copper (II), iron (III), zinc (II) and silver (I) at their simultaneous presence were determined by flame atomic absorption method using spectrophotometer "Saturn 2M" (Russia). The concentrations of metal ions in the individual solutions were determined by a photometrical method: iron (III) with sulfosalicylic acid (X = 420 nm) [12], copper (II) with thiocyanate at 360 nm [13], zinc (II) with xylenol orange at 420 nm [14].
Table 1. Composition of initial multicomponent solution. pH=2, CKSCN = 0.1 mol/L
Compound Au Ag Fe Cu Zn SCN
Concentration, mmol/L 0,23 0,10 2,9 0,10 0,23 0,1
Concentration, mg/L 45 10 162 6,5 7,5 5,8T03
The sorption isotherms were obtained by variation of molar ratio [10,15,16]. For this purpose, the air-dried samples (0.1 g) of anion exchangers in the appropriate form were stirred at 20°C with 25.0-1000.0 mL of metal ion solution (with concentration 110-3 mol/L). Thus, the ratio of resin functional groups quantity in the sample (determined as exchange capacity to Cl- ions) to metal quantity in the contacting solution varied from 3:1 to 1:3. After24 - 48 h, the equilibrium concentration of metal ions in solution was measured, and the exchange capacity 2EC) of the anion exchangers wa2 calculateX from the formula:
D
Men
D
(5)
EC =
'in Ceq )
V=
(2)
m
where C. and C are molar concentrations of the
in eq
initial and equilibrium solutions, respectively; m is resin mass, g; VS is volume of the contacting solution, mL.
The sorption of gold from multicomponent solutions was studied through the following scheme. 0.1 L of solution, prepared in accordance with the above-mentioned method, was stirred at 20°C with the resin sample (0.1 g). After 48 h the solution and anion exchanger were separated and 1.0 mL aliquot was sampled to determine the concentration of metal ions. Then the separated solution was stirred agiCn with the fresh resin sample over a parioa of 48 h. After that, the resin and solutiot were seporatoC and 1.0 mL aliquot was sampled pgaioaThese operationt were repeated 3 times. Then axchange capacity, recovery degree (R), distribution ratio (D) and separation coefficiena Cswe aecak ulate d for each stage accordingtothefollowing expressions:
R=iCin-CJ.nooo/o; (3)
C
en
Z) = |n0.0000; (4)
where DMei and DMe^ are distribution ratios of the metals separated.
Results and Discussion
It is known that gold is usually contained in the sources of complex composition, where the principal component is scattered among many other metals. The examples could be pyrite or arsenopyrite ores of Russia and S.A.R., containing large amounts of iron, ores of Australia and gold-containing slime, with large amounts of copper
[17]. Besides that, primary and secondary raw materials may contain silver, zinc, cobalt, nickel as well as platinum group metals. The problem is that these metals might form anionic complexes with thiocyanate ions and though compete with gold when concentrating from solutions. Therefore, the recovery of gold can be done effectively by means of selective sorption methods, which provide good selectivity through high exchange capacity and good kinetic characteristics of ion exchangers
[18].
The main affinity factors of anion exchangers with complex anions of transition metals are the basicity of ion exchanger's functional groups and the charge of recovered species [19]. The propensity of metal ions to form single-charged or multi-charged complexes can also influence the selectivity of sorption for these metals [20].
Using the stage stability constants of metal thiocyanate complexes [1] and method described in [21], we plotted the mole fraction distribution diagrams for various complex ions, depending on concentration of thiocyanate ions in solution. These distribution diagrams allowed us to reveal the ionic state of gold (I), iron (III), copper (II) and zinc (II) in the isvastigated systems. As we found out, gold (I) exists in a wide range of pH in the form of \As(SCN)2 ] complex ion, whereas iron (III), copper (II) and zinc (II) may
existrn v8rous 12forms, with the charge depesding on SCN~ concxntration in solution. If SCN( eeecenCation C andN 0.1 mol/L, iron (III) and copper (Ilf are in the form of [Ne (SCN )6f~ and [CsRC6Cn)4C s, resdictively. Zinc exists there as [Zi(SCN )4 ]and ^i^CN)^ ]% c omplexes. With the deensee in SCN ~ concentration in solution, the relativn increase C^]]]-1)-Charged copper (II) and zinc (II) complexee takes place. Simultaneously, iron [fSscn )2]+ cocfltxes appear in solution, andtheirrtlat(vc tliare increases substantially.
We investigated the ion exchange equilibria by means of acrption isotherms of thiocyanate goto Ch (rxit (III), copper (II) and zinc (II) complexes during their recovery from individual solutions on nnion exchangers with different laoirity. Tlie; resulN Eire presented in Fig. 1, 2, 5 ad 6. All the curveecan be considered L8lgmuii's isstSdrNs, i.e. the curves are abrupt in the range if tow eqmlibrium concentrations nf recovered series, and come to equilibrium in the range oe ^Njii noncentrations. Therefore, the soqrtton poboMy pro cee ds according to the anion exchange mechanism:
ooRAn + [M((SCN)n ]m~ ^ VOXmNscnX] + mAi
where An~ is the counter-ion of initial anion sxchanger form.
Taking in2o account that iron (III) was used as xnoxidizer for thiocyanate leaching process, it is x matter of scientific and practical interest to study ion exchange equilibrium in the system Fe(III) -SCN~ - H2O - anion exchanger in more detail. It is known [6,7] that the iron (III) concentration is about 1-2 mmol/L after the sorption leaching of gold from arsenopyrite concentrates after breakdown with acidic method. That is why we investigated the sorption equilibrium of thiocyanate iron (III) complexes in the equilibrium concentrations range up to 2 mmol/L.
Sorption isotherms (Fig. 2) show that the investigated anion exchangers recover thiocyanate iron (III) complexes from solutions at pH=2 and thiocyanate concentration 0.1 m5^. The saturation of anion exchangers is achieved at more than 0.6 mmol/L iron (III) equilibrium concentration in solution. However, the exchvnge capacity of sorbents depends on their basicity, and is considerably reduced when it decrecsen For instance, the maximum exchange capaciNx in about 0.6 mmol/g for AV-17-8, 0.4 mmol/g for AM-2B and 0.1 mmol/g or less for AN-251. Thos can be explained by the fact that the strong ba sic anion exchangers show the greatest affinity with metal ions, which are likely to form multi-chafed complexes. Since in our experiments (pH=2; C SCN_ =0.1 mol/L) iron (III) existed in solution mostly in the form of triple-charged complex ions (ignoring hydrolysis), AV-17-8 tends to these ions.
We have also investigated the sorption equilibria at the so-called "aging" of thiocyanate iron (III) complexes solutions. Fig. 2 contains isotherms of iron (III) sorption both from fresh solutions and solutions kept over 48 h. The maximum exchange capacity of AV-17-8 to iron (III) after its sorption from aged solution diminishes approximately to 20 %, but the shape of sorption isotherm remains convex as before (Fig. 2, curves 1 and 2). The sorption equilibrium on weak basic anion exchanger AN-251 is almost the same (Fig. 2, curves 5 and 6). In contrast, the sorption of iron (III) from aged solutions on neutral basic anion exchanger AM-2B proceeds with sharp decrease in affinity, which can be easily noticed by change in isotherm's shape (Fig. 2, curves l and 4). The convex curve becomes a straight line (determination coefficient is ~ 0.98). Probably this can be explained by hydrolysis of thiocyanate iron (III) complexes [22]: at pH=2 it proceeds slowly, causing the formation of asymmetric mixed complexes, poorly held on anion exchangers with weak basic functional 53 -
2,5 n 2 -
OW
I 1,5; 1 1
0,5 -0
u w
2,3
-1-1-1-1-1-r
0 0,1 0,2 0,3 0,4 0,5 0,É
[All]. mmol/L
Fig. 1. Sorption isotherms of thiocyanate complexes of gold (I) on anion exchangers in initial chloride form. pH=2, CKSCN = 0.1 mol/L. 1 - AM-2B; 2 - AV-17-8; 3 - AN-251
0 0,5 1 1,5 1
[Fe (III)], tmiK.L'L
Fig. 2. Sorption isotherms of iron (III) from thiocyanate solutions, pH=2, CKSCN =0.1 mol/L. 1,2 - AV-17-8; 3,4 - AM-2B; 5,6 - AN-251. 1,3,5 - so^tion from fresh solutions; 2,4,6 - sorption from solutions kept over 48 h
Fig. 3. Electron absorption spectra of thiocyanate solutions of iron (III); 1 - fresh solution; 2 - kept during 24 h
- 54 -
grorps. His known [6 ] tliat the reduch on of iroe (III) by SCN ~ ions can tako pl^ce in thiocyanate solutions. Hnwevor, tiie qual^tye reactions [23] in investigoted soiutions indicate^d tiia pres (fee on slight amnnxes of Fe (II). It should be noted, that the observed redxctinn ois aC5orotlox m^timum intensity at ~ 470 mo aftsr 48 h of ''ag2ng" the iXiocyana1cir8n (III) solution (Fig. l) also points out to hydrolysis oSFe(n°t.
The .concentration if thtnnyanate ioni in contactift softtion h8r a sdaenniial effect on recovery of iron (III) (Fig. 8At SCN~ comentration of 0.01 mcllL, all the mveotigated fiaxn exchangericr^^^ dhost recover .on (III). This decrease in exrhangc capacity of anion excnangers is a consequence of change in ionic sNate pflron fllI): eationN comp.exer [No^CN^ J"8 exist mainly in solutiox at C^.Ol mol/L. These co mplenes .o nor pemeate mto the re sin phsse dncause of Donns's potent. 1.
Thus4 it should be noted ihnt tdeonve stigntn d avion enchangers 5^o not foss se ss high affinity w ith thiocyanate iron (III) complexes, and iron (III) would not compete with gold durin. its reccvsry. AtAie sato Sme, dim tbtame. ciatii intdse^^<2 the relativety hiph probadOiSy of "poisoning effect1' of anion exchangers by [Fo(SCN)6 f1". TCs effect, Otscussed in the wn^^lis [17, 1R], revenls itsehby decrease in exchange entity of an^^n esm^N^s for.old due to the "poisoninf" bn eompiex ions [No(CNkp" and [NoCnW-.
As we mentioned before, tiif urnial components of gold-contaimnv raw materials are iron, copper and zinc. Their percentage share can be dozens of times highexM(1 x(of gold. That is why we have investigate xthe io 2 exchange equilibria in the systems Cu (IIt- SCN~ - H2O -anion exchanger and Zn (II) - SCN ~ - H2O - anion exchanger. The results are shown in Fig. 5. When equilibrium concentrations of copper and zinc ions in solutions are about 0.2 mmol/L, approximately 80% of maximum exchange capacity of anion
exchanger. for Cu ]II) and Zn (II) are ashaxved. With ths ancrease in their concentration] thn txchange capatity for copper (II) and ate (IIf ions is s]otnly rising. Such a trend can be frobably expl mnefbyformation of monovalent complex!s of copper and zinc, as the ratios Cu (II)/SCN" or Zn (II)/SCm" are decreasing. They are chfnged m ^(O^is resin phase during its saturation with fhese ions.
We have also investigated the effect of SCN~ ifns in sorbent phase on the sorption equilibrium of copper and zinc. It was determined that initial SCN~ form of anion exchangers does not alter the shape of sorption isotherms, i.e. has no effect on theirsorption.
In general, it can be stated that anion exchangers AV-17-8, AM-2B and AN-251 recover thiocyanate complexes of iron (III), copper (II) and zinc (II) only to some extent. The saturation CNNof sorbents with these complexes, even if their concentration are of 0.1 mol/L, is not higher than 20% of their exchange capacity for Cl- ions. At the same time, the anion exchangers investigated show great affinity with thiocyanate complexes of gold. The high exchange capacity is achieved at low equilibrium concentrations of gold in solution. This behavior is presented at Fig. 6. It should be noted that in our previous work [24] we have indicated the great affinity between investigated anion exchangers and thiocyanate complexes of silver (I). That is why in the present investigation we paid attention to the opportunities of selective recovery of gold in presence of silver and other associated metal ions.
It can be seen from the Tables 2 and 1 that all the anion exchangers investigated possess the high sorption abilities to gold during its sorption from multicomponent thiocyanate solutions. On the first stage of sorption (i.e. without excess of sorbent), the anion exchangers recover up to 71-81% of gold, and on the second stage (twofold excess of sorbent) the recovery degree goes up 55 -
-logSCN
Fig. 4. Dependence of anion exchanger capacity for iron (III) on concentration of thiocyanate ions in contacting solution. 1 - AV-17-8; 2 - AM-2B; 3 - AN-251
О 0Д 0,4 0,6 0,8
[Me], mmolL
Fig. 5. Sorption isotherms of thiocyanate complexes on anion exchangers.
АМ-2В: 1 - Cu(II), 4 - Zn (II); AV-17-8: 2 - Cu(II), 6 - Zn (II); AN-251: 3 - Cu(II), 5 - Zn (II)
2 1 1
[Me], rrniotb
Fig. 6. Sorption isotherms of metal thiocyanate complexes on anion exchanger AV-17-8 in initial chloride form. 1 - Au (I); 2 - Cu(II); 3 - Fe (III); 4 - Zn (II)
Table 2. Metal ion recovery degrees during stage sorption from multicomponent solution
Anion exchanger Stage Au Ag Fe Cu Zn
AV-17-8 1 77,8 90 10,5 98,2 46,7
2 95,6 98,9 22,8 98,2 73,3
3 99,8 98,9 40,1 98,2 88
AM-2B 1 73,3 72,2 7,4 98,2 46, 7
2 91,1 98, 9 22, 9 98,2 74
3 94,4 98, 9 30,9 98,2 86
AN-251 1 83,3 55, 6 7,4 61 56,8
2 97,8 88, 9 7,4 64 79,3
3 99,8 98, 9 22, 9 98 86,7
Table 3. Distribution ratios of metal thiocyanate complexes during stage sorption from multicomponent solution
Anion exchanger Stage Au Ag Fe Cu Zn
1 3500 9000 117 54000 875
AV-17-8 2 3800 7600 152 0 950
3 17100 0 260 0 1100
1 2750 2600 80 54000 875
AM-2B 2 1900 22800 190 0 999
3 540 0 105 0 771
1 5000 1250 80 1750 1308
AN-251 2 6175 2850 0 0 1042
3 8100 8100 180 17100 495
~ 98%. It is visible through distribution ratios (Table 3), that weak basic resin AN-251 shows the highest affinity with gold (I) thiocyanate complexes, whereas the strong basic sorbent AV-17-8 sorbs the thiocyanate complexes of silver to a greater extent. The anion exchangers AV-17-8 and AM-2B possess the high sorption ability to the thiocyanate complexes of copper (II).
During the first stage of sorption on AN-251, the distribution ratios for gold are higher than for silver and copper on the first stage of recovery. As a result, the recovery degrees account to 83.3%, 55.6% and 63.6% for gold, silver and copper, respectively (Table 2). On the second stage of sorption, when gold is mostly recovered from solution, the anion exchanger AN-251 sorbs silver. Copper is recovered on AN-251 only on the third stage of sorption, i.e. after isolation of gold and silver from solutions to 99% and 96%, respectively. Therefore, the metal thiocyanate complexes are recovered on weak basic anion
exchanger AN-251 in accordance with the following sequence:
Au (I) >> Ag (I) > Cu (II) > Zn (II) >> Fe (III).
The neutral basic anion exchanger AM-2B shows less selectivity to gold than to silver, but the metal complexes are recovered practically in the same sequence like on the anion exchanger AN-251:
Au (I) > Ag (I) > Cu (II) > Zn (II) >> Fe (III).
The strong basic anion exchanger AV-17-8 exhibits the great affinity with thiocyanate complexes of silver and recovers the metal complexes in the following sequence: Ag (I) > Cu (II) ~ Au (I) > Zn (II) >> Fe (III).
The anion exchangers investigated show weak sorption abilities to thiocyanate complexes of iron (III). However, it should be noted that AN-251 is saturated with metal complexes to a lesser extent (Table 4). The mentioned sequences can also be illustrated by separation coefficients
Table 4. Sorption of gold (I) from multicomponent solution on anion exchangers AV-17-8, AM-2B and AN-251
Metal Au Ag Fe Cu Zn
Anion exchanger AV-17-8
Stage EC, mg/g
1 35 8,1 17 5,4 3,5
2 7,6 0,8 19 < 0,1 1,9
3 1,71 < 0,1 25,2 < 0,1 1,0
Separation ratio КДАи/сумма) Kf(Au/Ag) Kf(Au/Fe) Kf(Au/Cu) Kf(Au/Zn)
1,7 0,4 30,0 0,1 4
Anion exchanger AM-2B
Stage EC, mg/g
1 33 6,5 12 5,4 3,5
2 7,6 2,3 23,8 < 0,1 2,0
3 1,4 < 0,1 11,7 < 0,1 0,81
Separation ratio Kf(Au/cyMM) Kf(Au/Ag) Kf(Au/Fe) Kf(Au/Cu) Kf(Au/Zn)
4,5 1,1 34,4 0,1 3,1
Anionexchanger AN-251
Stage EC, mg/g
1 37,5 5 12 3,5 4,3
2 6,2 2,9 < 0,1 < 0,1 1,6
3 0,8 0,8 22,5 1,7 0,5
Separation ratio Kf(Au/cyMMa) Kf(Au/Ag) Kf(Au/Fe) Kf(Au/Cu) Kf(Au/Zn)
8,9 4 62,5 2,9 3,8
calculated for the first sorption stage (Table 4). These values are 8.9; 4.5 and 1.7 for AN-251, AM-2B and AV-17-8, respectively. Therefore, the selectivity of these sorbents to gold grows in the following sequence:
AV-17-8 < AM-2B < AN-251.
Noteworthy, this sequence is in agreement with the basicity of these sorbents.
The observed patterns are in good correspondence with the conception of affinity of anion exchangers. As a rule, the strong basic resins show greater selectivity to multicharged anions. That is why AV-17-8 is more selective to thiocyanate complexes of silver, which form stable triple-charged anionic complexes with
thiocyanate ions. At the same time, the weak basic anion exchangers show greater affinity with monovalent anionic metal complexes, the most stable of which are thiocyanate complexes of gold (I).
Our results allow to conclude, that weak basic anion exchangers are the most selective to thiocyanate complexes of gold (I). That is why they can be recommended for recovery of gold from multicomponent solutions contained copper (II), silver (I), zinc (II) and iron (III) ions. The effective simultaneous recovery of gold and silver can be carried out on neutral or strong basic anion exchangers.
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