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Extraction studies of aurocyanide using Macronet adsorbents: physico-chemical characterization
REACTIVE & FUNCTIONAL POLYMERS
Reactive & Functional Polymers 60 (2004) 97–107
www.elsevier.com/locate/react
Extraction studies of aurocyanide using Macronet adsorbents: physico-chemical characterization J.L. Cortina a,*, R.M. Kautzmann b, R. Gliese c, C.H. Sampaio c a
c
Department of Chemical Engineering, Universitat Politecnica de Catalunya, Av. Diagonal 647, Barcelona E-08028, Spain b Department of Environmental Engineering, Universidad Luterana de Brasil, Canoas, Brazil Mineral Processing Laboratory (LAPROM), Department of Metallurgy, Universidad Federal Rio Grande do Sul, Porto Alegre, Brazil Received 14 November 2003; accepted 16 February 2004 Available online 7 June 2004
Abstract Ion exchange technology is situated during the last decade as an alternative to activated carbon in goldcyanide recovery process. The search for suitable resins to Au(CN) 2 recovery from alkaline cyanide solutions has prompted the synthesis of new resins incorporating new functionalities or modifying the polymer network properties that solve many of the existing problems. A new type of ion-exchange resins (Macronet Hypersol MN100 and MN300) incorporating mixtures of tertiary and quaternary groups, linked onto a styrene-divinylbenzene macroporous hyper-reticulated network are evaluated in this work. The total N content is 0.85 mmol/g for MN-100 and 1.14 mmol/g for MN-300 and the proton exchange capacity is 0.45 mmol/g for MN100 and 0.91 mmol/g for MN-300. Accurate titration curves were used to determine pKa values of the tertiary amine groups (pKHðaÞ ¼ 6:2 0:2 and 6.9 ± 0.2 for MN100 and MN300, respectively). The Hypersol Macronet resins (MN100 and MN300) extract Au(CN)2 via two different modes of metal extraction based on the tertiary amine groups of the resin and in the small portion of the quaternary ammonium groups present in the resins. The extraction isotherms of Au(CN) 2 show loadings of 16 mg Au/g and >40 mg Au/g for MN100 and MN300, respectively. Efficient stripping of Au(CN) 2 from the resin was achieved by using ethanol/water or acetone/water solutions of sodium hydroxide and sodium cyanide. Metal extraction from cyanide solutions, including Brazilian mine leach solution, showed considerable preference for gold and silver in comparison to base metals (copper, iron and nickel). Ó 2004 Elsevier B.V. All rights reserved. Keywords: Macronet resins; Ion-exchange resin; Goldcyanide extraction; Gold minerals; FTIR analysis
1. Introduction
*
Corresponding author. Tel.: +34-93-401-6570; fax: +34-93401-5814. E-mail address: [email protected] (J.L. Cortina).
The application of hydrometallurgy to the recovery of precious metals from dilute ore sources is increasing in use [1,2]. For example, dump, heap and vat leaching of selected ores are employed
1381-5148/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.reactfunctpolym.2004.02.015
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commercially in the gold mining industry, where greater use has been reached. In this case cyanidation leaching steps are combined with subsequent steps of gold cyanide adsorption with activated carbon, where a lack of selectivity is found due to the presence of mixtures of other metal cyanides [3,4]. Although, this processing is performed using activated carbon, in recent years, research efforts have attempted its substitution by Ion Exchange (IX). The main objective with the application of IX is solving existing problems associated in the activated carbon use by improving the selective recovery of goldcyanide; increasing loading and stripping efficiencies; and developing integrated processes of leaching and extraction as resinin-leach (RIL) and resin-in-pulp (RIP). Commercially available strong base anion exchange resins have been unable to compete with activated carbon due to poor selectivity, mechanical breakdown of the beads and the requirement for complex elution and regeneration processes [5,6]. However to overcome this problem weak base anion-exchange resins (WBA) could be use to extract Au(CN) 2 . WBA resins can be eluted by simply shifting the pH to higher values than the pKa and reverting the gold cyanide exchange reaction. New weak base anion-exchange resins, containing primary, secondary and tertiary amine functional groups, have been synthesized for the recovery of Au(CN) 2 from cyanide solutions [7– 10]. Typically, the selectivity of both weak and strong base resins resin appears to follow charge/ size considerations as well as hydration effects and polymer surface properties (e.g., hydrophobicity). Large size anions having a small charge density (Au(CN) 2 and Ag(CN)2 ) seem to be more compatible with the macroporous polystyrene resins [6]. The weakly hydrated aurocyanide anion tends to be adsorbed to a greater extent than the smaller size argentocyanide anion. In the last years new series of hypercrosslinked polymeric materials disclosed by Davankov and Rogozhin [11] have been developed by Purolite International Ltd. in collaboration with the Russian inventors [12,13]. Streat and Weetland [13] reported that a new family of polymers, Hypersol MacronetTM polymers are effective adsorbents for organic com-
pounds offering weaker binding energies in comparison with activated carbon leading to an easier reversion of the adsorption, i.e., better regeneration properties. Between them, Hypersol Macronets MN100 and MN300 resins functionalized with tertiary amine groups offer exchange capacity for anions as metal cyanides, high surface area and are mechanically strong suggesting a possible application on the hydrometallurgy of gold. This paper describes the evaluation of hyperreticulated macroporous ion exchange resins that aspires to fulfil the needs expressed. Resins have been characterised chemically and physically determining their acid–base properties, gold extraction capacity and dependence on pH. Its acid–base properties were characterised from titration data and were correlated with IR.
2. Experimental 2.1. Resins Hypersol MacronetTM sorbent resins (MN100 and MN300) were used as provided by Purolite Ltd. Resin properties are listed in Table 1. Resins in the wet form were conditioned by cyclic exhaustion with 1 M HCl and regenerations with 1 M NaOH and finally washed thoroughly with a large volume of distilled water and stored in water. Resins had 0.85 mmol N/g resin and 1.14 mmol N/g resin for MN-100 and MN-300, respectively (N was determined by elemental analysis). 2.2. Reagents and solutions NaCN solutions with pH maintained above 11 by using sodium hydroxide to avoid hydrogen cyanide formation, were used to dissolve metalcyanide complexes. Stock solutions were prepared by dissolving K2 Ni(CN)4 , KAu(CN)2 , K4 Fe(CN)6 , KAg(CN)2 and CuCN (Johnson Matthey and Aldrich, AR grade) in NaCN solution. Synthetic solutions were prepared by proper dilution with deionized water. The composition of these solutions are shown in Table 2.
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Table 1 Properties of Hypersol Macronet resins Properties
MN100
Polymer matrix structure Particle size (mm (US mesh)) Density (g/ml) Surface areaa (m2 /g) Pore volume (ml/g) Meso e macroporos d50 (Hg) (A) Volatile moistureb (%) Shrink/Swell factor, maxb (%) Functionalityc Capacity (mol/kg) Capacity of strong base groupsd (%) pH range (stability)
MN300 Crosslinked poly(styrene) 1.19–0.30 (16–50) 1.04 800–1000 1–1.1 850–950 57–62 5
WBA 0.6–0.8 10–20
WBA 1.2–1.4 5–15 0–14
a
BET – nitrogen. Ionic form – Cl . c Weak Base Anionic – (tertiary amine). d Capacity percentage attributed to strong base groups. b
Hydrochloric acid, sodium chloride and sodium hydroxide (Merck, AR) were used in the preparation of the different solutions for titration of the resin. Potassium bromide (Merck, for spectroscopy) was used for the preparation of the pressed disks for FTIR. 2.3. Mineral leaching solutions Real cyanide leach liquors were obtained by leaching of a gold mineral ore from Riacho dos Machados Mine (Brazil). The gold ore sample was crushed to 100% passing )100 mesh (<0.149 mm). A filtrated leaching solution (LIX) was treated with CaO to increase pH values around 9–10. The composition of LIX leach liquor is shown in Table 2. 2.4. Anion exchange capacity determination Macronet resins (MN) were converted to the chloride form by treating with 1 M HCl solutions Table 2 Composition of the metal cyanide solutions Metal
Mining leach solution (LIX, pH ¼ 10) (mg/l)
Synthetic solution (SYN, pH ¼ 10) (mg/l)
Au(I) Ag(I) Cu(I) Fe(II) Ni(II)
11.7 1.7 1.4 10.7 1.0
10.0 5.0 30.0 30.0 2.0
overnight. Finally were washed thoroughly with a large volume of distilled water and dried between filter sheets and stored in wet condition. The MN100 resin has a practical proton exchange capacity of 0.44 mmol/g (dry) as compared to the total N content of 0.85 mmol/g (dry) just 52% of the N groups are tertiary groups. For MN-300 0.94 mmol/g (dry) exchange capacity as compared to the total of 1.14 mmol/g (dry) by total N analysis represent 82% of the total tertiary groups. Practical exchange capacity of both resins was determined by two different procedures. In the first procedure, 0.2 g of resin in the chloride form was equilibrated with an excess of a standard NaOH solution overnight and later the excess of NaOH was titrated with a standard HCl solution. In the second procedure, titrations of 0.2 g of resins with a standard NaOH solution were performed. 2.5. Resin titration The acid–base properties of MN resins were investigated by titrating the free base form of the resin with standard hydrochloric acid solutions. The supporting electrolyte was a 0.2 M NaClO4 . A weighted amount (between 0.2 and 0.4 g) of the resin in the acid form suspended in a known volume of the aqueous solution. The acidity ranged between pH values of 2 and 10. The lower limit was chosen so that the junction potential could be
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kept constant. The titrations were followed potentiometrically at 25 °C with an Orion Research model 701A digital Ionalyzer, equipped with a Ross type Orion Research 810100 pH electrode with a salt bridge of the same composition of the external solution (1 M NaClO4 ), and with a porous glass junction to keep the resin outside the salt bridge. The cell was calibrated before each titration, during which the standard potential was determined by normal procedures [14]. 2.6. Gold-cyanide extraction procedures 2.6.1. Sorption experiments For measurement of equilibrium sorption, small-scale dynamic contacts between resin and metalcyanide complexes were used. Samples between 0.1 and 0.2 g of resins, were mixed mechanically in special glass stoppered tubes with an aqueous solution (20–250 ml) until equilibrium was achieved. The composition of the aqueous solutions varied depending on the nature of the experiment. After phase separation with a highspeed centrifuge, the equilibrium pH was measured using a Methrom AG 9100 combined electrode connected to a CRISON digital pH-meter. Metal content in both phases was determined by Atomic Absorption Spectrophotometry (Perkin– Elmer 2380 AAS with air-acetylene flame) or Inductively Coupled Plasma Spectrophotometry (ICP) (SpectroFlame, Kleve, Germany) depending on solution composition. The extent of sorption was calculated from the residual concentration of the metalcyanide complexes in the equilibrated solution. 2.6.2. Stripping experiments For measurement of equilibrium stripping, small-scale dynamic contacts between loaded resins and stripping solutions were carried out. Samples between 0.1 and 0.2 g of resins, were mixed mechanically in special glass stoppered tubes with an aqueous solution (20–250 ml) until equilibrium was achieved. The composition of the stripping solutions varied depending on the nature of the experiment. After phase separation the equilibrium metall content in both phases was determined as described previously.
2.7. FTIR spectroscopic studies FTIR spectra of MN100/MN300 resin samples equilibrated with aqueous solutions of different pHs (4, 6, 11 and 13) were recorded with a BOMEM MB120 Fourier Transform Infrared Spectrometer (4000–700 cm1 ) (32 interferograms were scanned at 2 cm2 resolution). Resins samples were dried at 60 °C overnight until constant pH.
3. Results and discussion 3.1. Acid–base characterisation. Acidity constant determination of MN100 and MN300 resins The necessity for protonating the amine groups of weak-base resins before interacting with metal cyanide anions promoted an investigation of acid–base properties of the ion exchanger. The basicity of a weak-base resin is determined by the pH value at which protonation of the functional group takes place. In turn this is influenced by factors such as the aqueousphase basicity of the amine; and inductive and steric effects. Such factors may be introduced by the attachment of the functional group to the matrix of the resin. Accurate titrations curves of the MN100 and MN300 resins were used for the determination of pKa value of the attached tertiary amine group. An example of the titration of the free base form of MN300 is shown in Fig. 1. The Modified Henderson–Hasselbach [15] equation was used as base for quantitative presentation of the potentiometric curves of the Macronet resins. By using this equation the acid– base base properties of ion exchangers are characterised by a constant pKHðaÞ expressed in the linear equation a ; ð1Þ pH ¼ pKHðaÞ þ n log 1a where a is the degree of dissociation of the functional groups of the ion exchanger and the pKHðaÞ and n are empirical constants [15,16]. If n ¼ 1 then Eq. (1) is a logarithmic form of the formal dissociation constant of the ion exchanger functional
J.L. Cortina et al. / Reactive & Functional Polymers 60 (2004) 97–107 10.0
pH
7 7
8.0
pH
101
6
6.0
6 4.0
5 2.0 0.00
-2 0.50
1.00
1.50
mmol HCl/g resin
Fig. 1. Potentiometric titration curve of the base (L) form of MN300 resin. Mass of resin 0.2 g and 1 M HCl solution were used for resin titration.
group. The physical meaning of constants pKHðaÞ is often named the dissociation constant index [15,16] with addition of some restrictive definitions (as half neutralisation degree, mean, etc.). Although a rigorous connection of this constant with physically-understandable properties has not been established, it is used as a measure of the acidity of ion exchangers, implying that it has much in common with the dissociation constant. Numerical analysis of the pH versus the degree of dissociation of the acid (a), were used to determine the mean pKHðaÞ value 6.2 ± 0.2 [1,2] for MN100 and 6.7 ± 0.2 (n ¼ 0; 99) for MN300 (means of four titrations) at 25 °C. An example of the pH ¼ f ðaÞ data of Fig. 1 is shown in Fig. 2. This pKHðaÞ values are lower than those of the corresponding linear amine groups of alkylamine ligands in water. Similar shifts between ligand and ligand-boundresin were previously observed by Green and Potgeiter [17] for imidazole and pyridine based resins and Warshawsky et al. [18] for piperezine based resins. Analysis of the IR spectra of the resins equilibrated to different pH values shown the absence of bands in the zone 2500–2200 cm1 which can be assigned to the N–H stretching vibrations, asymmetric and symmetric and the N–H deformation vibration of the protonated tertiary amine group for MN resins equilibrated at pH values above 8. Under these pH conditions both resins should be in its base form (P-NR2res ), according to the determined pKHðaÞ (6.2 and 6.7 for MN100 and
-1
0
1
log (a/1-a) Fig. 2. Variation of pH as a function of logða=1 aÞ (Modified Henderson–Hassenbach equation) for curve of Fig. 1. Linear analysis of this function gives a pKHðaÞ ¼ 6:9 0:2 and n ¼ 0; 99 for MN300.
MN300 respectively). Two strong bands at 2360 and 2330 cm1 were observed for MN resin samples equilibrated at pH below 6, where the resins are in its acid form (protonated tertiary amine: PNR2 Hþ Cl res ). 3.2. Physical characterisation Hypersol MacronetTM sorbent resins (MN100 and MN300) shown an appreciable portion of small micropores which create very high internal surface areas (800–1000 m2 /g) and an appreciable portion of macropores. Scanning electron microscopy examination of the polymers, presented in Fig. 3 for MN100, show a uniform globular structure. The globule diameter is 0.11–0.17 lm for MN100 and MN300 Macronets, thus creating large pores that will enable rapid diffusion of ionic species. A sample texture is apparent in the SEM at 15,000 magnification, possibly indicating the presence of large mesopores and macropores (around 0.01–0.02 lm) providing the high surface area while the micropores provide the former providing rapid access to the internal surfaces. 3.3. Goldcyanide extraction properties The equilibrium extraction of Au(CN) 2 from cyanide solution with both MN100 and MN300 resins were measured at different pH levels of a representative leaching solution containing 10 ppm
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J.L. Cortina et al. / Reactive & Functional Polymers 60 (2004) 97–107 100
Ex %
75 50 25 0 4
6
8
10
12
14
pH Fig. 4. Gold cyanide extraction percentage as a function of pH in the aqueous phase for MN100 resin. Phase ratio was 0.2 g resin/20 ml of solution, [Au(I)] ¼ 10 mg/l.
Fig. 3. Scanning electron micrographs of MN100 polymer bead at 15,000 magnification.
of gold. The pH adjustment in the range 4–13 was made with HCl and NaOH. From the results plotted in Fig. 4, a quasi S shaped function is observed with the extraction percentage, for MN100, falling rapidly from quantitative extraction at acidic pH values lower than 6 to low ex-
traction values for pH above 10. The extraction of 10 ppm is close to 85% at pH 8.0, but falls to 30% at pH 10. The resin loading capacity, in relation to the pH of the solution, was also determined (see Fig. 5, MN100), and as can be seen, the gold loading capacity varies from 0.08 mmol/g (16 mg Au/g) for MN100 resin to 0.2 mmol/g (40 mg Au/g) for MN300 resin. The basicity of a weak-base ion exchange resin has a profound effect on its capacity for goldcyanide at a particular pH value. The basicity of a weak base anion-exchange resin is determined by the pH value at which protonation of the functional group take place. This is influenced by factors as: (i) the aqueous-phase basicity of the amine and (ii) inductive effects, which may be introduced by the attachment of the functional group of the resin. Properties that are influenced by these factors could be the pKa , pH50 (Cl), pH50 (Au) (pH50
16 14
mgAu/g
12 10 8 6
MN100
4 2 0 0
100
200
300
400
Au ppm
Fig. 5. Gold cyanide loadings (mmol Au/g resin) as a function of goldcyanide concentration in the aqueous phase for MN100 resin. Phase ratio was 0.2 g resin/20 ml of solution.
J.L. Cortina et al. / Reactive & Functional Polymers 60 (2004) 97–107
are defined as the pH value where 50% of Au(CN) are extracted). The chloride 2 and Cl isotherms of both resins (MN100 and MN300) as a function of pH is plotted in Fig. 6 and loading values are summarised in Table 3. Both pH50 (Cl) and pH50 (Au), properties typically determined in applied studies, could be used as an estimation of the pKa value of the weak-base ion exchange resins. Table 4 shows the pH50 (Cl) and pH50 (Au) values of MN100 and MN300 resins and literature data for resins containing tertiary groups. Values of pH50 (Au) ranged for weak base anion exchange resins between 8 and 8.5, showed that efforts should be conducted to modified the acid–base properties of the functional groups up to 9–10. The extraction behaviour of MN resins may be attributed to the fact both resins, as determined earlier, contains two different types of functional
103
groups. Taking into account the pKHðaÞ values, the following reactions may be proposed to describe the gold cyanide extraction process: (1) at pH < pKHðaÞ the tertiary amine group will be protonated as (P-RNHRþ 2 ) and the mechanism of anion-exchange is based on the following step:
P-NR2 Hþ Cl res þ AuðCNÞ2
() P-NR2 Hþ AuðCNÞ2;res þ Cl
ð2Þ
(2) at pH > pKHðaÞ the anion-exchange reactions take place due to the existence of residual quaternary ammonium groups:
P-NRþ 3 Clres þ AuðCNÞ2 () P-NRþ 3 AuðCNÞ2;res þ Cl
ð3Þ
The infrared spectra of the loaded resin samples (MN100 and MN300) showed a sharp band at 2140 cm1 , extreme basic pH values 11 and 13
1
MN 300
0.8
mmol Cl-/g
MN100 0.6 0.4 0.2 0 2
3
4
5
6
7
8
9
pH
Fig. 6. Chloride loading (mmol Cl /g) as a function of pH in the aqueous phase for MN100 and MN300 resins. Phase ratio was 0.2 g resin/20 ml of solution.
Table 3 Loading properties for MN100 and MN300 resins Source of data
Capacity
MN100
MN300
PUROLITE
Total capacity (mmol/g) Capacity on strong base groups (%) Capacity on strong base groups (mmol/g) Capacity – Cl (mmol Cl /g) Capacity de N (mmol N/g) Loading Au (mmol/g) Loading Au (mg/g)
0.6–0.8 10–20 0.06–0.16 0.44 0.9–0.96 0.08 15
1.2–1.4 5–15 0.06–0.21 0.94 1.14–1.15 >0.2a >40a
Elemental (Cl) analysis Elemental (N) analysis Au(CN) 2 loading test a
Values from loading experiments where saturation was not achieved.
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Table 4 Loading, pH50 and pKa , for anion exchange resins Properties
MN100
MN300
PS-PIPa
NME2b
NET2c
Functionality Loading (mmol/g) pKa (pKHðaÞ ) pH50 (Cl ) pH50 (Au(CN) 2)
WB/SB 0.85 6.2 5.8 8.0
WB/SB 1.14 6.9 6.8 8.5
WB/SB 2.02 7.1 6.0 8.0
WB 5.92 8.1 6.6 8.2
WB 2.82 6.8 5.3 7.8
a
PS-PIP, piperazine based resin [18]. NM2, di-methyl amine based resin [9]. c NET2, di-ethyl amine based resin [9]. b
extraction percentage reach a constant value. This effect shows the cyanide competition of CN on Au(CN) 2 extraction. 3.4. Elution studies The loaded resin phases obtained in the extraction experiments were used to study the elution of Au(CN) 2 . Taking into account the extraction mechanisms involved, different types of elution solutions were used: (a) aqueous solutions, (b) ethanol/water and (c) acetone/water solutions of NaCN and NaOH. The elution data given in Table 5 point out that aqueous solutions of NaOH and NaCN gave partial recoveries (20–50%) while acetone and ethanol solutions of NaOH and NaCN achieved quantitative recoveries of Au(CN) 2 from MN 100 resin. For MN300 resin quantitative recoveries were achieved when the temperature was increased up to 60 °C. This behaviour may be understood taking into account the Au(CN) 2 ex100 90
Au %
while shown a sharp band of 2142 cm1 for samples at pH in the neutral and acidic range. The 2140 cm1 peak is assigned to the CN stretching vibration of Au(CN) 2 complex [19]. Comparing with potassium cyanide salt KAu(CN)2 [19] there is no appreciable change in the frequencies of the resins loaded at extreme basic pH values (10–13). This support the contention that the interaction between Au(CN) anion and the quaternary 2 amine groups is solely an ionic interaction and it is attributed to the presence of P-NRþ 3 Au(CN)2 as described by Eq. (3). This reaction has been previously described for strong anion exchange resins [20] and also for activated carbon [21]. The peak observed at 2142 cm1 exhibited by the resins loaded at acidic or neutral pH values (4–6) shown the ionic interaction between Au(CN) 2 anion and the tertiary amine through the formation of the complex P-NR2 Hþ Au(CN) 2;res as described by Eq. (2). Similar behaviour with changes in the frequencies to higher values have been described for weak anion exchange resins [22]. The effect of total cyanide concentration on goldcyanide extraction was evaluated in the concentration range expected from the processing of mineral ores. Cyanide concentration changes through the mineral leaching step from values of 1000 ppm (starting point) to 100–200 ppm (closing point). Fig. 7 shows the gold loading values for MN100 resin as a function of cyanide concentration in the aqueous phase. As can be seen, the extraction of Au(CN) 2 is reduced 10% by the presence of total cyanide content in concentrations higher than 250 ppm. For cyanide concentrations higher than 250 ppm and lower than 1000 ppm, simulating the final and initial leach solutions, the
80 70 60 25 ppmAu
50 40 0
200
400
600
[NaCN] mg/L Fig. 7. Goldcyanide extraction percentage (%E) for MN100 resin as a function of total cyanide concentration in the aqueous phase ([CN ]). Phase ratio was 0.2 g resin/20 ml of aqueous solution [Au(CN) 2 ] ¼ 25 mg/l.
J.L. Cortina et al. / Reactive & Functional Polymers 60 (2004) 97–107
3.5. Extraction efficiency and selectivity of MN resins for Au(CN) 2 from mineral cyanide solutions
100
Ex %
traction mechanisms involved (Eqs. (2) and (3)) and the nature of the eluents. In the case of aqueous solutions of NaOH and NaCN the deprotonation of the tertiary amine group occurs, causing release of the metal cyanide anions. In the case of a NaCN and NaOH aqueous/ethanol or acetone solutions quantitative elution of goldcyanide from the resin was achieved as can be seen in Table 5. Experience with Au(CN) 2 desorption from activated carbon [23] shows that two factors enhancing the rate of elution are the temperature and the effect of polar solvents, such as acetonitrile, acetone, methanol and ethanol. This second effect is attributed to an increase in the activity of the cyanide ion and a decrease in the activity of the aurocyanide ion in polar solvents as compared with water, and results in significant enhancement of the rate of elution. During elution operation cycles MN100 and MN300 resins were shown stable under operating conditions. Their chemical stability and mechanical strength is higher than that of most ion exchange resins. Because of their high porosity and surface area, osmotic forces are quickly dissipated hence resistance to osmotic shock is high.
75
Fe Ni
50
Cu Ag Au
25 0 0
100
200
300
t (min) Fig. 8. Extraction efficiency for the different metal cyanide complexes for MN100 resin in a synthetic solution (SYN). Phase ratio was 0.02 g resin/20 ml solution. For composition see Table 2.
using MN100 resin. As a general trend, the resin shows a fast extraction process (t1=2 20–30 min) and a selective extraction of gold and silver cyanide in the presence of other heavy metals. Besides the ability to extract Au(CN) 2 at the mineral leaching operating pH, the selectivity of the resin for the goldcyanide complex must be high. The selectivity factors (K SðAu=YÞ ) for this set of metal cyanides for MN100 resin when using a synthetic metal cyanide solution calculated were 5 (Au/Ag), 25 (Au/Cu) 10 (Au/Ni) and 33 (Au/Fe). K SðAuÞ was calculated by using the following equation: Ks ðAuÞ ¼
The extraction efficiency and selectivity of MN resins for Au(CN) 2 and other metal cyanide complexes from synthetic solutions (SYN) and gold ore mining leaching solutions (LIX) have been evaluated. Loading of metal cyanides ranging from 5 to 20 were obtained, similar to those achieved with activated carbon uses [22]. Fig. 8 shows the evolution of metal cyanide loadings as function of time for a resin in leach experiment
105
YAu XMe ; YMe XAu
ð4Þ
where Y and X represent the concentration of the metalcyanide in the resin and aqueous phase, respectively (in mg/g). The selectivity order for both resins may be arranged as follows: Au(CN) 2 > Ag(CN)2 > 2 2 3 Cu(CN)3 > Ni(CN)3 > Fe(CN)6 . This selectivity is in agreement with the fact that the hydrophilicity of the polymer matrix and the ionic density
Table 5 Stripping of Au(CN) 2 from MN100 and MN300 resins Stripant
MN100 (25 °C)
MN300 (25 °C)
MN300 (60 °C)
NaOH 1 M NaOH 1 M/ethanol 40% NaCN 5 g l1 NaCN 5 g l1 /ethanol 40% NaCN 5 g l1 /acetone 40%
10 65 20 100 99
10 55 15 90 85
20 70 35 99 100
106
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100 Fe
Ex (%)
75
Ni 50
Cu Ag
25
Au
0 0
100
200
300
400
t (min) Fig. 9. Extraction efficiency for the different metal cyanide complexes for MN100 resin in a mining leach solution (LIX). Phase ratio was 0.02 g resin/20 ml solution. For composition see Table 2.
(number of ionic groups per unit volume) play important roles in determining the selectivity characteristics of a resin. Ion–water interactions are stronger in the aqueous phase than in the resin phase, for the simple reason that more free water is available for solvation in the aqueous phase. Thus on this basis alone, the ions with the greatest ionic charge tend to have a lower extraction onto the resin phase. In the resin phase a low degree of hydrophylicity and a low ionic density increases the selectivity for gold and silver and favors those metals over the base metals, iron, copper and nickel. When evaluating the mineral leaching solutions gold, silver and copper are selectively extracted over iron and niquel as it is shown in Fig. 9. After more than 36 h no extraction of Ni and Fe onto the MN100 resin was observed.
4. Conclusions Hypersol Macronet Sorbent Resins MN100 and MN300 with a mixture of weak base functionality and a strong base functionality show promising results when applied to the extraction of goldcyanide from cyanide media in terms of pH behaviour, loadings and separation factors. This behaviour shown by Macronet resins appear to be determined by a combination of factors, among then the following could be pointed out: (a) the acid–base properties of the functional group; (b) the balanced
degree of hidrophobicity of the polymer matrix; (c) the high surface area of the resin. Gold cyanide extraction properties shows a typical S shape function congruent with an ion exchange mechanism of a weak-base resin, with a decrease in the extraction due to the deprotonation of the N of the tertiary group at pH values between 6 and 8. A distinctive behaviour was observed at higher pH values (pH range 10–13) where the extraction properties of the MN100 and MN300 resins show a constant extraction trend independently of the pH of the aqueous phase-contrary to the expected- due to the completed deprotonation of the weak base. Macronet resins with a weak base functionality (tertiary amine group) show not very high capacity when applied to the extraction of gold cyanide from cyanide media in terms of loading capacities achieving gold loadings of 16 mg/g MN100 and <40 mg Au/g MN300 at basic pH values close to those, normal conditions of resin in leach or resin in pulp operations. Those values are comparable with loading values achieved with activated carbon and weak-base anion exchange resins and slightly lower than those achieved with strong base resins [5]. Stripping of the gold cyanide complex from the MN resins was achieved efficiently by acetone or ethanol sodium hydroxide/or sodium cyanide solutions, while partial recovery was achieved when with aqueous sodium cyanide and hydroxide solutions.
Acknowledgements We wish to acknowledge MCYT Project QUI02-C02-002 (Ministerio de Educaci on y Ciencia de Espa~ na) and to Education Ministry of Spain and Brasil (CAPES) for their finantial support under the bilateral program for Science and Technology Cooperation project PH2000-53. We also thank Jim Dale and Isidor Almirall for Hypersol Macronet samples supply.
References [1] C.A. Fleming, The potential role of anion exchange resins in the gold industry, in: Mishra (Ed.), EDP Congress 1998,
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[2]
[3]
[4] [5] [6] [7] [8] [9] [10] [11]
[12]
The Minerals, Metals and Materials Society, 1998. p 95– 117. L. Bolinski, J. Shirley, Russian resin in pulp technology, current status and recent developments, Randol Gold Forum, Squaw Creek, California, April 1996, 419–424. B. Green, M.H. Kotze, J.P. Engelbrecht, Resin-in-pulp after gold, where next?, in: Mishra (Ed.), EDP Congress 1998, The Minerals, Metals and Materials Society, 1998. pp. 119–136. C. Gasparini, CIM Bull. 76 (1993) 141. J. Marsden, I. House, The Chemistry of Gold Extraction, Ellis Horwood, New York, USA, 1992. P.A. Riveros, Hydrometallurgy 33 (1993) 43–58. P.D. Fawell, C.F. Vernon, C. Klauber, H.G. Linge, React. Polym. 18 (1992) 47. C.F. Vernon, P.D. Fawell, C. Klauber, React. Polym. 18 (1992) 35. A.H. Schwellnus, B.A. Green, React. Polym. 12 (1990) 167. W.I. Harris, J.R. Stahlbush, W.C. Pike, R.R. Stevens, React. Polym. 17 (1992) 21. V.A. Davankov, S.V. Rogozhin, M.P. Tsyurupa, Patente URSS, 299165, 1969, Patente USA 3729465, 1969, Chem. Abstr. 75 (1971). PUROLITE Technical Bulletin (Macronet Resins). Available from.
107
[13] M. Streat, L.A. Weetland, React. Funct. Polym. 35 (1997) 99–109. [14] M. Pesavento, A. Profumo, R. Biesuz, React. Polym. 11 (1989) 37. [15] V. Soldatov, Ind. Eng. Chem. Res. 34 (1995) 2605. [16] V. Soldatov, in: J.A. Greig (Ed.), Ion Exchange in the Millenium, Imperial College Press, London, 2000, pp. 193– 207. [17] B.A. Green, A.H. Potgeiter, in: D. Naden, M. Streat (Eds.), Ion Exchange Technology, Ellis Horwoord, Chichester, 1984, p. 626. [18] A. Warshawsky, N. Kahana, V. Rampel, I. Rogachev, E. Meinhardt, J.L. Cortina, R.M. Kautzmann, C. Sampaio, Macromol. Mater. Eng. 283 (2000) 103. [19] K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds, Wiley, New York, 1978, p. 259. [20] L.H. Jones, R.A. Penneman, J. Chem. Phys. 22 (1954) 965. [21] M.D. Adams, Hydrometallurgy 31 (1992) 111. [22] C.F. Vernon, P.D. Fawell, C. Klauber, React. Polym. 18 (1992) 35. [23] F. Espiell, A. Roca, M. Cruells, C. Nu~ nez, Hydrometallurgy 19 (1988) 321.
Reactive & Functional Polymers 60 (2004) 97–107
www.elsevier.com/locate/react
Extraction studies of aurocyanide using Macronet adsorbents: physico-chemical characterization J.L. Cortina a,*, R.M. Kautzmann b, R. Gliese c, C.H. Sampaio c a
c
Department of Chemical Engineering, Universitat Politecnica de Catalunya, Av. Diagonal 647, Barcelona E-08028, Spain b Department of Environmental Engineering, Universidad Luterana de Brasil, Canoas, Brazil Mineral Processing Laboratory (LAPROM), Department of Metallurgy, Universidad Federal Rio Grande do Sul, Porto Alegre, Brazil Received 14 November 2003; accepted 16 February 2004 Available online 7 June 2004
Abstract Ion exchange technology is situated during the last decade as an alternative to activated carbon in goldcyanide recovery process. The search for suitable resins to Au(CN) 2 recovery from alkaline cyanide solutions has prompted the synthesis of new resins incorporating new functionalities or modifying the polymer network properties that solve many of the existing problems. A new type of ion-exchange resins (Macronet Hypersol MN100 and MN300) incorporating mixtures of tertiary and quaternary groups, linked onto a styrene-divinylbenzene macroporous hyper-reticulated network are evaluated in this work. The total N content is 0.85 mmol/g for MN-100 and 1.14 mmol/g for MN-300 and the proton exchange capacity is 0.45 mmol/g for MN100 and 0.91 mmol/g for MN-300. Accurate titration curves were used to determine pKa values of the tertiary amine groups (pKHðaÞ ¼ 6:2 0:2 and 6.9 ± 0.2 for MN100 and MN300, respectively). The Hypersol Macronet resins (MN100 and MN300) extract Au(CN)2 via two different modes of metal extraction based on the tertiary amine groups of the resin and in the small portion of the quaternary ammonium groups present in the resins. The extraction isotherms of Au(CN) 2 show loadings of 16 mg Au/g and >40 mg Au/g for MN100 and MN300, respectively. Efficient stripping of Au(CN) 2 from the resin was achieved by using ethanol/water or acetone/water solutions of sodium hydroxide and sodium cyanide. Metal extraction from cyanide solutions, including Brazilian mine leach solution, showed considerable preference for gold and silver in comparison to base metals (copper, iron and nickel). Ó 2004 Elsevier B.V. All rights reserved. Keywords: Macronet resins; Ion-exchange resin; Goldcyanide extraction; Gold minerals; FTIR analysis
1. Introduction
*
Corresponding author. Tel.: +34-93-401-6570; fax: +34-93401-5814. E-mail address: [email protected] (J.L. Cortina).
The application of hydrometallurgy to the recovery of precious metals from dilute ore sources is increasing in use [1,2]. For example, dump, heap and vat leaching of selected ores are employed
1381-5148/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.reactfunctpolym.2004.02.015
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J.L. Cortina et al. / Reactive & Functional Polymers 60 (2004) 97–107
commercially in the gold mining industry, where greater use has been reached. In this case cyanidation leaching steps are combined with subsequent steps of gold cyanide adsorption with activated carbon, where a lack of selectivity is found due to the presence of mixtures of other metal cyanides [3,4]. Although, this processing is performed using activated carbon, in recent years, research efforts have attempted its substitution by Ion Exchange (IX). The main objective with the application of IX is solving existing problems associated in the activated carbon use by improving the selective recovery of goldcyanide; increasing loading and stripping efficiencies; and developing integrated processes of leaching and extraction as resinin-leach (RIL) and resin-in-pulp (RIP). Commercially available strong base anion exchange resins have been unable to compete with activated carbon due to poor selectivity, mechanical breakdown of the beads and the requirement for complex elution and regeneration processes [5,6]. However to overcome this problem weak base anion-exchange resins (WBA) could be use to extract Au(CN) 2 . WBA resins can be eluted by simply shifting the pH to higher values than the pKa and reverting the gold cyanide exchange reaction. New weak base anion-exchange resins, containing primary, secondary and tertiary amine functional groups, have been synthesized for the recovery of Au(CN) 2 from cyanide solutions [7– 10]. Typically, the selectivity of both weak and strong base resins resin appears to follow charge/ size considerations as well as hydration effects and polymer surface properties (e.g., hydrophobicity). Large size anions having a small charge density (Au(CN) 2 and Ag(CN)2 ) seem to be more compatible with the macroporous polystyrene resins [6]. The weakly hydrated aurocyanide anion tends to be adsorbed to a greater extent than the smaller size argentocyanide anion. In the last years new series of hypercrosslinked polymeric materials disclosed by Davankov and Rogozhin [11] have been developed by Purolite International Ltd. in collaboration with the Russian inventors [12,13]. Streat and Weetland [13] reported that a new family of polymers, Hypersol MacronetTM polymers are effective adsorbents for organic com-
pounds offering weaker binding energies in comparison with activated carbon leading to an easier reversion of the adsorption, i.e., better regeneration properties. Between them, Hypersol Macronets MN100 and MN300 resins functionalized with tertiary amine groups offer exchange capacity for anions as metal cyanides, high surface area and are mechanically strong suggesting a possible application on the hydrometallurgy of gold. This paper describes the evaluation of hyperreticulated macroporous ion exchange resins that aspires to fulfil the needs expressed. Resins have been characterised chemically and physically determining their acid–base properties, gold extraction capacity and dependence on pH. Its acid–base properties were characterised from titration data and were correlated with IR.
2. Experimental 2.1. Resins Hypersol MacronetTM sorbent resins (MN100 and MN300) were used as provided by Purolite Ltd. Resin properties are listed in Table 1. Resins in the wet form were conditioned by cyclic exhaustion with 1 M HCl and regenerations with 1 M NaOH and finally washed thoroughly with a large volume of distilled water and stored in water. Resins had 0.85 mmol N/g resin and 1.14 mmol N/g resin for MN-100 and MN-300, respectively (N was determined by elemental analysis). 2.2. Reagents and solutions NaCN solutions with pH maintained above 11 by using sodium hydroxide to avoid hydrogen cyanide formation, were used to dissolve metalcyanide complexes. Stock solutions were prepared by dissolving K2 Ni(CN)4 , KAu(CN)2 , K4 Fe(CN)6 , KAg(CN)2 and CuCN (Johnson Matthey and Aldrich, AR grade) in NaCN solution. Synthetic solutions were prepared by proper dilution with deionized water. The composition of these solutions are shown in Table 2.
J.L. Cortina et al. / Reactive & Functional Polymers 60 (2004) 97–107
99
Table 1 Properties of Hypersol Macronet resins Properties
MN100
Polymer matrix structure Particle size (mm (US mesh)) Density (g/ml) Surface areaa (m2 /g) Pore volume (ml/g) Meso e macroporos d50 (Hg) (A) Volatile moistureb (%) Shrink/Swell factor, maxb (%) Functionalityc Capacity (mol/kg) Capacity of strong base groupsd (%) pH range (stability)
MN300 Crosslinked poly(styrene) 1.19–0.30 (16–50) 1.04 800–1000 1–1.1 850–950 57–62 5
WBA 0.6–0.8 10–20
WBA 1.2–1.4 5–15 0–14
a
BET – nitrogen. Ionic form – Cl . c Weak Base Anionic – (tertiary amine). d Capacity percentage attributed to strong base groups. b
Hydrochloric acid, sodium chloride and sodium hydroxide (Merck, AR) were used in the preparation of the different solutions for titration of the resin. Potassium bromide (Merck, for spectroscopy) was used for the preparation of the pressed disks for FTIR. 2.3. Mineral leaching solutions Real cyanide leach liquors were obtained by leaching of a gold mineral ore from Riacho dos Machados Mine (Brazil). The gold ore sample was crushed to 100% passing )100 mesh (<0.149 mm). A filtrated leaching solution (LIX) was treated with CaO to increase pH values around 9–10. The composition of LIX leach liquor is shown in Table 2. 2.4. Anion exchange capacity determination Macronet resins (MN) were converted to the chloride form by treating with 1 M HCl solutions Table 2 Composition of the metal cyanide solutions Metal
Mining leach solution (LIX, pH ¼ 10) (mg/l)
Synthetic solution (SYN, pH ¼ 10) (mg/l)
Au(I) Ag(I) Cu(I) Fe(II) Ni(II)
11.7 1.7 1.4 10.7 1.0
10.0 5.0 30.0 30.0 2.0
overnight. Finally were washed thoroughly with a large volume of distilled water and dried between filter sheets and stored in wet condition. The MN100 resin has a practical proton exchange capacity of 0.44 mmol/g (dry) as compared to the total N content of 0.85 mmol/g (dry) just 52% of the N groups are tertiary groups. For MN-300 0.94 mmol/g (dry) exchange capacity as compared to the total of 1.14 mmol/g (dry) by total N analysis represent 82% of the total tertiary groups. Practical exchange capacity of both resins was determined by two different procedures. In the first procedure, 0.2 g of resin in the chloride form was equilibrated with an excess of a standard NaOH solution overnight and later the excess of NaOH was titrated with a standard HCl solution. In the second procedure, titrations of 0.2 g of resins with a standard NaOH solution were performed. 2.5. Resin titration The acid–base properties of MN resins were investigated by titrating the free base form of the resin with standard hydrochloric acid solutions. The supporting electrolyte was a 0.2 M NaClO4 . A weighted amount (between 0.2 and 0.4 g) of the resin in the acid form suspended in a known volume of the aqueous solution. The acidity ranged between pH values of 2 and 10. The lower limit was chosen so that the junction potential could be
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J.L. Cortina et al. / Reactive & Functional Polymers 60 (2004) 97–107
kept constant. The titrations were followed potentiometrically at 25 °C with an Orion Research model 701A digital Ionalyzer, equipped with a Ross type Orion Research 810100 pH electrode with a salt bridge of the same composition of the external solution (1 M NaClO4 ), and with a porous glass junction to keep the resin outside the salt bridge. The cell was calibrated before each titration, during which the standard potential was determined by normal procedures [14]. 2.6. Gold-cyanide extraction procedures 2.6.1. Sorption experiments For measurement of equilibrium sorption, small-scale dynamic contacts between resin and metalcyanide complexes were used. Samples between 0.1 and 0.2 g of resins, were mixed mechanically in special glass stoppered tubes with an aqueous solution (20–250 ml) until equilibrium was achieved. The composition of the aqueous solutions varied depending on the nature of the experiment. After phase separation with a highspeed centrifuge, the equilibrium pH was measured using a Methrom AG 9100 combined electrode connected to a CRISON digital pH-meter. Metal content in both phases was determined by Atomic Absorption Spectrophotometry (Perkin– Elmer 2380 AAS with air-acetylene flame) or Inductively Coupled Plasma Spectrophotometry (ICP) (SpectroFlame, Kleve, Germany) depending on solution composition. The extent of sorption was calculated from the residual concentration of the metalcyanide complexes in the equilibrated solution. 2.6.2. Stripping experiments For measurement of equilibrium stripping, small-scale dynamic contacts between loaded resins and stripping solutions were carried out. Samples between 0.1 and 0.2 g of resins, were mixed mechanically in special glass stoppered tubes with an aqueous solution (20–250 ml) until equilibrium was achieved. The composition of the stripping solutions varied depending on the nature of the experiment. After phase separation the equilibrium metall content in both phases was determined as described previously.
2.7. FTIR spectroscopic studies FTIR spectra of MN100/MN300 resin samples equilibrated with aqueous solutions of different pHs (4, 6, 11 and 13) were recorded with a BOMEM MB120 Fourier Transform Infrared Spectrometer (4000–700 cm1 ) (32 interferograms were scanned at 2 cm2 resolution). Resins samples were dried at 60 °C overnight until constant pH.
3. Results and discussion 3.1. Acid–base characterisation. Acidity constant determination of MN100 and MN300 resins The necessity for protonating the amine groups of weak-base resins before interacting with metal cyanide anions promoted an investigation of acid–base properties of the ion exchanger. The basicity of a weak-base resin is determined by the pH value at which protonation of the functional group takes place. In turn this is influenced by factors such as the aqueousphase basicity of the amine; and inductive and steric effects. Such factors may be introduced by the attachment of the functional group to the matrix of the resin. Accurate titrations curves of the MN100 and MN300 resins were used for the determination of pKa value of the attached tertiary amine group. An example of the titration of the free base form of MN300 is shown in Fig. 1. The Modified Henderson–Hasselbach [15] equation was used as base for quantitative presentation of the potentiometric curves of the Macronet resins. By using this equation the acid– base base properties of ion exchangers are characterised by a constant pKHðaÞ expressed in the linear equation a ; ð1Þ pH ¼ pKHðaÞ þ n log 1a where a is the degree of dissociation of the functional groups of the ion exchanger and the pKHðaÞ and n are empirical constants [15,16]. If n ¼ 1 then Eq. (1) is a logarithmic form of the formal dissociation constant of the ion exchanger functional
J.L. Cortina et al. / Reactive & Functional Polymers 60 (2004) 97–107 10.0
pH
7 7
8.0
pH
101
6
6.0
6 4.0
5 2.0 0.00
-2 0.50
1.00
1.50
mmol HCl/g resin
Fig. 1. Potentiometric titration curve of the base (L) form of MN300 resin. Mass of resin 0.2 g and 1 M HCl solution were used for resin titration.
group. The physical meaning of constants pKHðaÞ is often named the dissociation constant index [15,16] with addition of some restrictive definitions (as half neutralisation degree, mean, etc.). Although a rigorous connection of this constant with physically-understandable properties has not been established, it is used as a measure of the acidity of ion exchangers, implying that it has much in common with the dissociation constant. Numerical analysis of the pH versus the degree of dissociation of the acid (a), were used to determine the mean pKHðaÞ value 6.2 ± 0.2 [1,2] for MN100 and 6.7 ± 0.2 (n ¼ 0; 99) for MN300 (means of four titrations) at 25 °C. An example of the pH ¼ f ðaÞ data of Fig. 1 is shown in Fig. 2. This pKHðaÞ values are lower than those of the corresponding linear amine groups of alkylamine ligands in water. Similar shifts between ligand and ligand-boundresin were previously observed by Green and Potgeiter [17] for imidazole and pyridine based resins and Warshawsky et al. [18] for piperezine based resins. Analysis of the IR spectra of the resins equilibrated to different pH values shown the absence of bands in the zone 2500–2200 cm1 which can be assigned to the N–H stretching vibrations, asymmetric and symmetric and the N–H deformation vibration of the protonated tertiary amine group for MN resins equilibrated at pH values above 8. Under these pH conditions both resins should be in its base form (P-NR2res ), according to the determined pKHðaÞ (6.2 and 6.7 for MN100 and
-1
0
1
log (a/1-a) Fig. 2. Variation of pH as a function of logða=1 aÞ (Modified Henderson–Hassenbach equation) for curve of Fig. 1. Linear analysis of this function gives a pKHðaÞ ¼ 6:9 0:2 and n ¼ 0; 99 for MN300.
MN300 respectively). Two strong bands at 2360 and 2330 cm1 were observed for MN resin samples equilibrated at pH below 6, where the resins are in its acid form (protonated tertiary amine: PNR2 Hþ Cl res ). 3.2. Physical characterisation Hypersol MacronetTM sorbent resins (MN100 and MN300) shown an appreciable portion of small micropores which create very high internal surface areas (800–1000 m2 /g) and an appreciable portion of macropores. Scanning electron microscopy examination of the polymers, presented in Fig. 3 for MN100, show a uniform globular structure. The globule diameter is 0.11–0.17 lm for MN100 and MN300 Macronets, thus creating large pores that will enable rapid diffusion of ionic species. A sample texture is apparent in the SEM at 15,000 magnification, possibly indicating the presence of large mesopores and macropores (around 0.01–0.02 lm) providing the high surface area while the micropores provide the former providing rapid access to the internal surfaces. 3.3. Goldcyanide extraction properties The equilibrium extraction of Au(CN) 2 from cyanide solution with both MN100 and MN300 resins were measured at different pH levels of a representative leaching solution containing 10 ppm
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J.L. Cortina et al. / Reactive & Functional Polymers 60 (2004) 97–107 100
Ex %
75 50 25 0 4
6
8
10
12
14
pH Fig. 4. Gold cyanide extraction percentage as a function of pH in the aqueous phase for MN100 resin. Phase ratio was 0.2 g resin/20 ml of solution, [Au(I)] ¼ 10 mg/l.
Fig. 3. Scanning electron micrographs of MN100 polymer bead at 15,000 magnification.
of gold. The pH adjustment in the range 4–13 was made with HCl and NaOH. From the results plotted in Fig. 4, a quasi S shaped function is observed with the extraction percentage, for MN100, falling rapidly from quantitative extraction at acidic pH values lower than 6 to low ex-
traction values for pH above 10. The extraction of 10 ppm is close to 85% at pH 8.0, but falls to 30% at pH 10. The resin loading capacity, in relation to the pH of the solution, was also determined (see Fig. 5, MN100), and as can be seen, the gold loading capacity varies from 0.08 mmol/g (16 mg Au/g) for MN100 resin to 0.2 mmol/g (40 mg Au/g) for MN300 resin. The basicity of a weak-base ion exchange resin has a profound effect on its capacity for goldcyanide at a particular pH value. The basicity of a weak base anion-exchange resin is determined by the pH value at which protonation of the functional group take place. This is influenced by factors as: (i) the aqueous-phase basicity of the amine and (ii) inductive effects, which may be introduced by the attachment of the functional group of the resin. Properties that are influenced by these factors could be the pKa , pH50 (Cl), pH50 (Au) (pH50
16 14
mgAu/g
12 10 8 6
MN100
4 2 0 0
100
200
300
400
Au ppm
Fig. 5. Gold cyanide loadings (mmol Au/g resin) as a function of goldcyanide concentration in the aqueous phase for MN100 resin. Phase ratio was 0.2 g resin/20 ml of solution.
J.L. Cortina et al. / Reactive & Functional Polymers 60 (2004) 97–107
are defined as the pH value where 50% of Au(CN) are extracted). The chloride 2 and Cl isotherms of both resins (MN100 and MN300) as a function of pH is plotted in Fig. 6 and loading values are summarised in Table 3. Both pH50 (Cl) and pH50 (Au), properties typically determined in applied studies, could be used as an estimation of the pKa value of the weak-base ion exchange resins. Table 4 shows the pH50 (Cl) and pH50 (Au) values of MN100 and MN300 resins and literature data for resins containing tertiary groups. Values of pH50 (Au) ranged for weak base anion exchange resins between 8 and 8.5, showed that efforts should be conducted to modified the acid–base properties of the functional groups up to 9–10. The extraction behaviour of MN resins may be attributed to the fact both resins, as determined earlier, contains two different types of functional
103
groups. Taking into account the pKHðaÞ values, the following reactions may be proposed to describe the gold cyanide extraction process: (1) at pH < pKHðaÞ the tertiary amine group will be protonated as (P-RNHRþ 2 ) and the mechanism of anion-exchange is based on the following step:
P-NR2 Hþ Cl res þ AuðCNÞ2
() P-NR2 Hþ AuðCNÞ2;res þ Cl
ð2Þ
(2) at pH > pKHðaÞ the anion-exchange reactions take place due to the existence of residual quaternary ammonium groups:
P-NRþ 3 Clres þ AuðCNÞ2 () P-NRþ 3 AuðCNÞ2;res þ Cl
ð3Þ
The infrared spectra of the loaded resin samples (MN100 and MN300) showed a sharp band at 2140 cm1 , extreme basic pH values 11 and 13
1
MN 300
0.8
mmol Cl-/g
MN100 0.6 0.4 0.2 0 2
3
4
5
6
7
8
9
pH
Fig. 6. Chloride loading (mmol Cl /g) as a function of pH in the aqueous phase for MN100 and MN300 resins. Phase ratio was 0.2 g resin/20 ml of solution.
Table 3 Loading properties for MN100 and MN300 resins Source of data
Capacity
MN100
MN300
PUROLITE
Total capacity (mmol/g) Capacity on strong base groups (%) Capacity on strong base groups (mmol/g) Capacity – Cl (mmol Cl /g) Capacity de N (mmol N/g) Loading Au (mmol/g) Loading Au (mg/g)
0.6–0.8 10–20 0.06–0.16 0.44 0.9–0.96 0.08 15
1.2–1.4 5–15 0.06–0.21 0.94 1.14–1.15 >0.2a >40a
Elemental (Cl) analysis Elemental (N) analysis Au(CN) 2 loading test a
Values from loading experiments where saturation was not achieved.
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J.L. Cortina et al. / Reactive & Functional Polymers 60 (2004) 97–107
Table 4 Loading, pH50 and pKa , for anion exchange resins Properties
MN100
MN300
PS-PIPa
NME2b
NET2c
Functionality Loading (mmol/g) pKa (pKHðaÞ ) pH50 (Cl ) pH50 (Au(CN) 2)
WB/SB 0.85 6.2 5.8 8.0
WB/SB 1.14 6.9 6.8 8.5
WB/SB 2.02 7.1 6.0 8.0
WB 5.92 8.1 6.6 8.2
WB 2.82 6.8 5.3 7.8
a
PS-PIP, piperazine based resin [18]. NM2, di-methyl amine based resin [9]. c NET2, di-ethyl amine based resin [9]. b
extraction percentage reach a constant value. This effect shows the cyanide competition of CN on Au(CN) 2 extraction. 3.4. Elution studies The loaded resin phases obtained in the extraction experiments were used to study the elution of Au(CN) 2 . Taking into account the extraction mechanisms involved, different types of elution solutions were used: (a) aqueous solutions, (b) ethanol/water and (c) acetone/water solutions of NaCN and NaOH. The elution data given in Table 5 point out that aqueous solutions of NaOH and NaCN gave partial recoveries (20–50%) while acetone and ethanol solutions of NaOH and NaCN achieved quantitative recoveries of Au(CN) 2 from MN 100 resin. For MN300 resin quantitative recoveries were achieved when the temperature was increased up to 60 °C. This behaviour may be understood taking into account the Au(CN) 2 ex100 90
Au %
while shown a sharp band of 2142 cm1 for samples at pH in the neutral and acidic range. The 2140 cm1 peak is assigned to the CN stretching vibration of Au(CN) 2 complex [19]. Comparing with potassium cyanide salt KAu(CN)2 [19] there is no appreciable change in the frequencies of the resins loaded at extreme basic pH values (10–13). This support the contention that the interaction between Au(CN) anion and the quaternary 2 amine groups is solely an ionic interaction and it is attributed to the presence of P-NRþ 3 Au(CN)2 as described by Eq. (3). This reaction has been previously described for strong anion exchange resins [20] and also for activated carbon [21]. The peak observed at 2142 cm1 exhibited by the resins loaded at acidic or neutral pH values (4–6) shown the ionic interaction between Au(CN) 2 anion and the tertiary amine through the formation of the complex P-NR2 Hþ Au(CN) 2;res as described by Eq. (2). Similar behaviour with changes in the frequencies to higher values have been described for weak anion exchange resins [22]. The effect of total cyanide concentration on goldcyanide extraction was evaluated in the concentration range expected from the processing of mineral ores. Cyanide concentration changes through the mineral leaching step from values of 1000 ppm (starting point) to 100–200 ppm (closing point). Fig. 7 shows the gold loading values for MN100 resin as a function of cyanide concentration in the aqueous phase. As can be seen, the extraction of Au(CN) 2 is reduced 10% by the presence of total cyanide content in concentrations higher than 250 ppm. For cyanide concentrations higher than 250 ppm and lower than 1000 ppm, simulating the final and initial leach solutions, the
80 70 60 25 ppmAu
50 40 0
200
400
600
[NaCN] mg/L Fig. 7. Goldcyanide extraction percentage (%E) for MN100 resin as a function of total cyanide concentration in the aqueous phase ([CN ]). Phase ratio was 0.2 g resin/20 ml of aqueous solution [Au(CN) 2 ] ¼ 25 mg/l.
J.L. Cortina et al. / Reactive & Functional Polymers 60 (2004) 97–107
3.5. Extraction efficiency and selectivity of MN resins for Au(CN) 2 from mineral cyanide solutions
100
Ex %
traction mechanisms involved (Eqs. (2) and (3)) and the nature of the eluents. In the case of aqueous solutions of NaOH and NaCN the deprotonation of the tertiary amine group occurs, causing release of the metal cyanide anions. In the case of a NaCN and NaOH aqueous/ethanol or acetone solutions quantitative elution of goldcyanide from the resin was achieved as can be seen in Table 5. Experience with Au(CN) 2 desorption from activated carbon [23] shows that two factors enhancing the rate of elution are the temperature and the effect of polar solvents, such as acetonitrile, acetone, methanol and ethanol. This second effect is attributed to an increase in the activity of the cyanide ion and a decrease in the activity of the aurocyanide ion in polar solvents as compared with water, and results in significant enhancement of the rate of elution. During elution operation cycles MN100 and MN300 resins were shown stable under operating conditions. Their chemical stability and mechanical strength is higher than that of most ion exchange resins. Because of their high porosity and surface area, osmotic forces are quickly dissipated hence resistance to osmotic shock is high.
75
Fe Ni
50
Cu Ag Au
25 0 0
100
200
300
t (min) Fig. 8. Extraction efficiency for the different metal cyanide complexes for MN100 resin in a synthetic solution (SYN). Phase ratio was 0.02 g resin/20 ml solution. For composition see Table 2.
using MN100 resin. As a general trend, the resin shows a fast extraction process (t1=2 20–30 min) and a selective extraction of gold and silver cyanide in the presence of other heavy metals. Besides the ability to extract Au(CN) 2 at the mineral leaching operating pH, the selectivity of the resin for the goldcyanide complex must be high. The selectivity factors (K SðAu=YÞ ) for this set of metal cyanides for MN100 resin when using a synthetic metal cyanide solution calculated were 5 (Au/Ag), 25 (Au/Cu) 10 (Au/Ni) and 33 (Au/Fe). K SðAuÞ was calculated by using the following equation: Ks ðAuÞ ¼
The extraction efficiency and selectivity of MN resins for Au(CN) 2 and other metal cyanide complexes from synthetic solutions (SYN) and gold ore mining leaching solutions (LIX) have been evaluated. Loading of metal cyanides ranging from 5 to 20 were obtained, similar to those achieved with activated carbon uses [22]. Fig. 8 shows the evolution of metal cyanide loadings as function of time for a resin in leach experiment
105
YAu XMe ; YMe XAu
ð4Þ
where Y and X represent the concentration of the metalcyanide in the resin and aqueous phase, respectively (in mg/g). The selectivity order for both resins may be arranged as follows: Au(CN) 2 > Ag(CN)2 > 2 2 3 Cu(CN)3 > Ni(CN)3 > Fe(CN)6 . This selectivity is in agreement with the fact that the hydrophilicity of the polymer matrix and the ionic density
Table 5 Stripping of Au(CN) 2 from MN100 and MN300 resins Stripant
MN100 (25 °C)
MN300 (25 °C)
MN300 (60 °C)
NaOH 1 M NaOH 1 M/ethanol 40% NaCN 5 g l1 NaCN 5 g l1 /ethanol 40% NaCN 5 g l1 /acetone 40%
10 65 20 100 99
10 55 15 90 85
20 70 35 99 100
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J.L. Cortina et al. / Reactive & Functional Polymers 60 (2004) 97–107
100 Fe
Ex (%)
75
Ni 50
Cu Ag
25
Au
0 0
100
200
300
400
t (min) Fig. 9. Extraction efficiency for the different metal cyanide complexes for MN100 resin in a mining leach solution (LIX). Phase ratio was 0.02 g resin/20 ml solution. For composition see Table 2.
(number of ionic groups per unit volume) play important roles in determining the selectivity characteristics of a resin. Ion–water interactions are stronger in the aqueous phase than in the resin phase, for the simple reason that more free water is available for solvation in the aqueous phase. Thus on this basis alone, the ions with the greatest ionic charge tend to have a lower extraction onto the resin phase. In the resin phase a low degree of hydrophylicity and a low ionic density increases the selectivity for gold and silver and favors those metals over the base metals, iron, copper and nickel. When evaluating the mineral leaching solutions gold, silver and copper are selectively extracted over iron and niquel as it is shown in Fig. 9. After more than 36 h no extraction of Ni and Fe onto the MN100 resin was observed.
4. Conclusions Hypersol Macronet Sorbent Resins MN100 and MN300 with a mixture of weak base functionality and a strong base functionality show promising results when applied to the extraction of goldcyanide from cyanide media in terms of pH behaviour, loadings and separation factors. This behaviour shown by Macronet resins appear to be determined by a combination of factors, among then the following could be pointed out: (a) the acid–base properties of the functional group; (b) the balanced
degree of hidrophobicity of the polymer matrix; (c) the high surface area of the resin. Gold cyanide extraction properties shows a typical S shape function congruent with an ion exchange mechanism of a weak-base resin, with a decrease in the extraction due to the deprotonation of the N of the tertiary group at pH values between 6 and 8. A distinctive behaviour was observed at higher pH values (pH range 10–13) where the extraction properties of the MN100 and MN300 resins show a constant extraction trend independently of the pH of the aqueous phase-contrary to the expected- due to the completed deprotonation of the weak base. Macronet resins with a weak base functionality (tertiary amine group) show not very high capacity when applied to the extraction of gold cyanide from cyanide media in terms of loading capacities achieving gold loadings of 16 mg/g MN100 and <40 mg Au/g MN300 at basic pH values close to those, normal conditions of resin in leach or resin in pulp operations. Those values are comparable with loading values achieved with activated carbon and weak-base anion exchange resins and slightly lower than those achieved with strong base resins [5]. Stripping of the gold cyanide complex from the MN resins was achieved efficiently by acetone or ethanol sodium hydroxide/or sodium cyanide solutions, while partial recovery was achieved when with aqueous sodium cyanide and hydroxide solutions.
Acknowledgements We wish to acknowledge MCYT Project QUI02-C02-002 (Ministerio de Educaci on y Ciencia de Espa~ na) and to Education Ministry of Spain and Brasil (CAPES) for their finantial support under the bilateral program for Science and Technology Cooperation project PH2000-53. We also thank Jim Dale and Isidor Almirall for Hypersol Macronet samples supply.
References [1] C.A. Fleming, The potential role of anion exchange resins in the gold industry, in: Mishra (Ed.), EDP Congress 1998,
J.L. Cortina et al. / Reactive & Functional Polymers 60 (2004) 97–107
[2]
[3]
[4] [5] [6] [7] [8] [9] [10] [11]
[12]
The Minerals, Metals and Materials Society, 1998. p 95– 117. L. Bolinski, J. Shirley, Russian resin in pulp technology, current status and recent developments, Randol Gold Forum, Squaw Creek, California, April 1996, 419–424. B. Green, M.H. Kotze, J.P. Engelbrecht, Resin-in-pulp after gold, where next?, in: Mishra (Ed.), EDP Congress 1998, The Minerals, Metals and Materials Society, 1998. pp. 119–136. C. Gasparini, CIM Bull. 76 (1993) 141. J. Marsden, I. House, The Chemistry of Gold Extraction, Ellis Horwood, New York, USA, 1992. P.A. Riveros, Hydrometallurgy 33 (1993) 43–58. P.D. Fawell, C.F. Vernon, C. Klauber, H.G. Linge, React. Polym. 18 (1992) 47. C.F. Vernon, P.D. Fawell, C. Klauber, React. Polym. 18 (1992) 35. A.H. Schwellnus, B.A. Green, React. Polym. 12 (1990) 167. W.I. Harris, J.R. Stahlbush, W.C. Pike, R.R. Stevens, React. Polym. 17 (1992) 21. V.A. Davankov, S.V. Rogozhin, M.P. Tsyurupa, Patente URSS, 299165, 1969, Patente USA 3729465, 1969, Chem. Abstr. 75 (1971). PUROLITE Technical Bulletin (Macronet Resins). Available from
107
[13] M. Streat, L.A. Weetland, React. Funct. Polym. 35 (1997) 99–109. [14] M. Pesavento, A. Profumo, R. Biesuz, React. Polym. 11 (1989) 37. [15] V. Soldatov, Ind. Eng. Chem. Res. 34 (1995) 2605. [16] V. Soldatov, in: J.A. Greig (Ed.), Ion Exchange in the Millenium, Imperial College Press, London, 2000, pp. 193– 207. [17] B.A. Green, A.H. Potgeiter, in: D. Naden, M. Streat (Eds.), Ion Exchange Technology, Ellis Horwoord, Chichester, 1984, p. 626. [18] A. Warshawsky, N. Kahana, V. Rampel, I. Rogachev, E. Meinhardt, J.L. Cortina, R.M. Kautzmann, C. Sampaio, Macromol. Mater. Eng. 283 (2000) 103. [19] K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds, Wiley, New York, 1978, p. 259. [20] L.H. Jones, R.A. Penneman, J. Chem. Phys. 22 (1954) 965. [21] M.D. Adams, Hydrometallurgy 31 (1992) 111. [22] C.F. Vernon, P.D. Fawell, C. Klauber, React. Polym. 18 (1992) 35. [23] F. Espiell, A. Roca, M. Cruells, C. Nu~ nez, Hydrometallurgy 19 (1988) 321.