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Stripping of silver from Primene JMT loaded with silver thiocyanate complexes
Hydrometallurgy 52 Ž1999. 123–135
Stripping of silver from Primene JMT loaded with silver thiocyanate complexes Sumiko Sanuki a
a,)
, Masumi Yata a , Hiroshi Majima
b
Department of Material Systems Engineering and Life Science, Faculty of Engineering, Toyama UniÕersity, 3190 Gofuku, Toyama 930-8555, Japan b Kyoto UniÕersity, Titan Kogyo, 1978-25, Kogushi, Ube 755-8567, Japan Received 12 August 1998; received in revised form 12 December 1998; accepted 13 December 1998
Abstract Stripping of silver from Primene JMT loaded with silver thiocyanate complexes was investigated, utilizing hydrolysis, anion exchange and reduction stripping reactions. The findings obtained are summarized as follows. Ž1. Precipitation stripping of AgSCN was almost impossible when water, aqueous NH 3 or NaOH was used as stripping solution. Ž2. Stripping or precipitation stripping reactions did not proceed quantitatively when HClO4 or NaClO4 , whose affinity for amine is strong, was used as stripping solution. Ž3. When NH 3 or NaOH solution containing NaBH 4 was used, reduction stripping proceeded producing aggregates of fine metallic Ag powder. Ž4. For efficient reduction stripping of Ag using a solution containing NaBH 4 , the addition of NH 3 or NaOH, and the use of NaBH 4 at more than twice the amount of AgŽI. in the organic phase, are required. Ž5. When AgŽI. extraction was done using Primene JMT that had been converted to the thiocyanate salt, the reduction efficiency deteriorated. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Primene JMT; Silver; Stripping
1. Introduction Precious metals have always been recycled because of their value. Recycling is done mostly using pyrometallurgical methods Žsmelting.. Cyanide and aqua regia are used mostly for analytical purposes and therefore, the environmental impact is small. Thiourea, thiosulphate, thiocyanate and especially bromine have been used occasionally to recycle )
Corresponding author. Telefax: q81-764-45-6817; E-mail: [email protected]
0304-386Xr99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 3 8 6 X Ž 9 9 . 0 0 0 0 2 - X
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S. Sanuki et al.r Hydrometallurgy 52 (1999) 123–135
precious metals, but the resulting concentrations are high and subsequent concentration by solvent extraction or ion exchange is generally not required. Generally speaking, however, recycling materials containing precious metals are usually contaminated with various metals. It is considered that hydrometallurgical methods become more important for recycling. Cyanidation using soluble cyanide and a dissolution method using aqua regia are commonly used for the recovery of Au or Ag w1x. However, these methods have serious flaws with regards to environmental and safety considerations. Thus, various alternative recovery procedures have been proposed, including the use of thiosulfate w2,3x, thiocyanate w4,5x, and acidic thiourea w6–8x. One of the authors ŽS.S.., has conducted a systematic study of the extraction of AgŽI. using amine-type extractants such as Primene JMT and TOA from an aqueous ammonium thiocyanate solution w9x. It was found that the extraction of thiocyanate concurred with the extraction of AgŽI.. Also, the composition and equilibrium constants of the species of thiocyanate extracted with Primene JMT and TOA, as well as the extraction of AgŽI. with thiocyanate salts of Primene JMT and TOA, were studied. In the present work, a method for the efficient stripping of silver from AgŽI. thiocyanate complex-loaded Primene JMT was studied to obtain information on the recovery of precious metals, and on the possibility of recovering insoluble salts of AgŽI. or metallic Ag directly from the organic phase.
2. Theoretical background for the stripping of silver from Ag(I)-loaded Primene JMT 2.1. RecoÕery of AgSCN by precipitation stripping method of Ag(I)-loaded Primene JMT Amine is the most common reagent among those extractants known as ion-pairing or basic type extractants, and extracts metallic ions based on the ion association principle w10x. Primene JMT ŽRNH 2 . used in the present study is a primary amine, and the chemistry of the extraction of AgŽI. thiocyanate complexes with RNH 2 has been studied extensively by Sanuki et al. w9x. Agq forms thiocyanate complexes, AgŽSCN.Žnny1.y Ž n s 1–4., in aqueous solution containing ammonium thiocyanate. The extraction reaction for this solution system can be expressed by Eq. Ž1.. 2y
2 Ž RNH 2 . o q 2Hqq Ag Ž SCN . 3 s
Ž RNHq3 . 2 Ag Ž SCN. 32y
o
Ž 1.
When reaction Ž1. proceeds, a side reaction, which is expressed by Eq. Ž2., can occur concurrently.
Ž RNH 2 . o q Hqq SCNys RNHq3 SCNy
o
Ž 2.
yx The concentrations of AgŽI. in the organic phase and wRNHq 3 SCN o are affected by the change in pH during the extraction of AgŽI.. There is no available information for free yx amine ŽRNH 2 . o , but it is known that the organic phase involves wRNHq 3 SCN o and 2y x w x wŽRNHq . Ž . 3 2 Ag SCN 3 o 9 . In general, metallic ions extracted by amine can be stripped
S. Sanuki et al.r Hydrometallurgy 52 (1999) 123–135
125
by deprotonation of protonated amine using neutral and alkaline aqueous solutions, since the metallic ion species are soluble in these solutions. The reaction for stripping of AgŽI. thiocyanate complex-loaded Primene JMT with alkaline solution is expressed by Eq. Ž3.. Ž . Ž RNHq3 . 2 Ag Ž SCN. 32q o q 2OHys 2 Ž RNH 2 . o q Ag Ž SCN. nny1 y
q Ž 3 y n . SCNyq 2H 2 O
Ž 3.
It is also presumed that the addition of anions, whose affinity for amine is stronger than that of AgŽSCN. 32y, causes an anion exchange reaction expressed by Eq. Ž4.. Ž ny1 .y q 2Xys 2 Ž RNH 3 X . o q Ag Ž SCN. n Ž RNHq3 . 2 Ag Ž SCN. 2y 3 o
q Ž 3 y n . SCNy
Ž 4.
Under the conditions of low SCNy concentration, AgSCN is an insoluble salt and thus there exists the possibility of recovering AgSCN by either of the reactions expressed by Eq. Ž3. or Eq. Ž4.. If the stripping of ŽHSCN. o occurs, the concentration of SCNy in that aqueous phase increases and thus AgŽI. is stripped as a soluble complex. Extraction of AgŽI. thiocyanate complexes by Primene JMT is difficult in the presence of NH 4 SCN at high concentration. Therefore, the dissociation reaction of AgŽI. thiocyanate complex-loaded Primene JMT may be regarded as a stripping method, which is expressed by Eq. Ž5.. Ž ny1 .y q Ž n y 3 . SCNys 2 Ž RNH 2 . o q Ag Ž SCN . n Ž RNHq3 . 2 Ag Ž SCN. 2y 3 o
q 2Hq
Ž 5.
2.2. Reduction stripping with NaBH4 NaBH 4 is known as a strong reducing agent and is utilized as a reductant for precious metal colloids and for electroless plating. This reagent is also useful for directly recovering ultrafine Pd powders from Pd-loaded amine solution, as reported by Sanuki et al. w11x. It is supposed that reduction to fine Ag powders using NaBH 4 as a stripping solution is possible, as expressed by Eq. Ž6.. q eys 2 Ž RNH 2 . o q Ag q 2Hqq 3SCN Ž RNHq3 . 2 Ag Ž SCN. 2y 3 o
Ž 6.
Eq. Ž6. can be more clearly explained by the following reactions. 2y
Reduction: Ag Ž SCN. 3
q eys Ag q 3SCNy
q y Oxidation: In acid solution BHy 4 q 3H 2 O s H 3 BO 3 q 7H q 8e y q y In alkali solution BHy 4 q 2H 2 O s H 2 BO 3 q 6H q 8e
Ž 7. Ž 8. Ž 9.
As is shown by Eqs. Ž8. and Ž9., the oxidation of NaBH 4 involves an eight-electron liberation reaction, so that one mole NaBH 4 theoretically may reduce eight moles AgŽI..
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However, the oxidation rate of NaBH 4 is so fast, that the evolution of hydrogen gas in acidic solution can proceed according to Eq. Ž10.. q BHy 4 q 3H 2 O q H s H 3 BO 3 q 4H 2
Ž 10 .
Therefore, an excess of NaBH 4 is required to reduce AgŽI. quantitatively.
3. Experimental 3.1. Reagents The organic phase used was Primene JMT solution, adjusted to a specific concentration by diluting with kerosene. AgŽI.-loaded Primene JMT solutions were prepared by extracting AgŽI. from the organic solution containing ammonium thiocyanate in two different ways. One solution was prepared by dissolving 5 = 10y3 M Ag 2 O into 1 M NH 4 SCN solution, and then AgŽI. was extracted at equilibrium pH around 7 or 2 using 100–300 kg my3 Primene JMT. The other organic phase was prepared by extracting AgŽI. from 0.5 and 1.0 M NH 4 SCN solution using thiocyanate salt of Primene JMT. This extractant was prepared by extracting HSCN using a definite amount of Primene JMT from 1 M NH 4 SCN, whose equilibrium pH was adjusted to 2 with H 2 SO4 . A modifier, n-decanol, was added to all organic phases to 50 kg my3 . The preparation of stripping solutions was done by dissolving reagent grade NaOH, NH 3 , HClO4 , NaClO4 and NH 4 SCN to obtain definite concentrations. Water used in the present study was deionized, with a specific resistivity of 5 = 10 4 V m. Stripping solutions used for the reduction stripping of AgŽI. were prepared by dissolving NaBH 4 ranging from 0.25 to 10 times the molar ratio of AgŽI. in an organic phase, into NaOH or NH 3 . 3.2. Experimental procedures Equal volumes of organic phase loaded AgŽI. and aqueous phase were taken into a centrifugal separation tube, then shaken for 0.9 ks using a vertical type shaker at ambient temperature. The tubes containing the solutions were kept at 298 K for equilibration, before being separated using a centrifugal separator operated at a speed of 50 rev sy1 for 0.9 ks. On reduction stripping, hydrogen gas evolved immediately after mixing, and so shaking was not started until after the gas evolution had slowed. 3.3. Analytical method The AgŽI. concentration in aqueous solution was determined by atomic absorption spectrophotometry. In contrast, AgŽI. in the organic phase was stripped twice using 4 M NH 4 SCN, then the combined solution was subjected to atomic absorption analysis. Precipitates formed by reduction stripping were filtered with a glass fibre filter. After the filtrate had been dried at 323 K, the sample was subjected to scanning electron microscopy and X-ray diffraction.
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4. Results and discussion 4.1. Stripping of Ag(I) with Õarious solutions containing acids or alkalis 4.1.1. Stripping of Ag(I) with aqueous alkali solutions As mentioned, stripping of AgŽI. was done using an alkaline solution containing NH 3 or NaOH, whose concentrations were 0.5 and 1.0 M, respectively. The results obtained are summarized in Table 1. Percentage stripping of AgŽI. using water or aqueous NH 3 solution was less than 1%, and no precipitation was detected. In contrast, the stripping with NaOH solution gave 15–33% stripping, exhibiting a slight formation of a white precipitate. However, it was almost impossible to produce quantitatively AgŽI. precipitates using the NaOH solutions tested in the present study. On the other hand, it is known that the extraction of HSCN due to the extraction of AgŽI. is decreased at higher pH w9x. In the present work, the effect of HSCN concentrations in the organic phase was not studied quantitatively. However, the percentage stripping for the organic phase obtained by extraction of AgŽI. at pH 1.6 is larger than that at pH 7.5 when the stripping was done with NaOH solution, as shown in Table 1. Therefore, it is considered that the stripping of AgŽI. is easier when the HSCN concentration in the organic phase is high and the pH of the aqueous phase is low. The AgŽI. stripped would be in the forms of complex ions, since the SCNy ion concentration in the stripping solution was high. 4.1.2. Stripping of Ag(I) by anion exchange Stripping of AgŽI. by anion exchange was examined. In the affinity for amine, the y y y w anions rank in decreasing order as follows: ClO4y) NOy 10x. In 3 ) Cl ) HSO4 ) F 2y y contrast, the affinity for amine of SCN and AgŽSCN. 3 has not been clearly indicated. To verify the effect of ClO4y ion on the stripping of AgŽI. from AgŽI.-loaded Primene JMT, stripping experiments were performed using aqueous solutions containing HClO4 or NaClO4 . The results obtained are summarized in Table 2.
Table 1 Stripping of AgŽI. from AgŽI.-loaded Primene JMT using various stripping solutions Stripping solution
AgŽI.-loaded Primene JMT Extracted at pH 7.5a
Water 0.5 M NH 3 1.0 M NH 3 0.5 M NaOH 1.0 M NaOH a
CAg , o s 7.8=10y3 M. CAg , o s8.7=10y3 M. c Slightly precipitated. b
Extracted at pH 1.6 b
Stripping Ž%.
pH
Stripping Ž%.
pH
0.27 0.87 0.53 17.4 c 15.4 c
6.1 6.9 7.5 13.2 13.4
0.24 0.24 0.32 33.0 c 33.0 c
3.0 6.5 7.0 13.0 13.4
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Table 2 Stripping of AgŽI. from AgŽI.-loaded Primene JMT using stripping solutions containing perchlorate ions Stripping solution
AgŽI.-loaded Primene JMT Extracted at pH 7.5a
1.0 M HClO4 2.0 M HClO4 1.0 M NaClO4 2.0 M NaClO4
Extracted at pH 1.6 b
Stripping Ž%.
pH
Stripping Ž%.
pH
2.6 4.5 c 2.8 7.0 c
0.54 0.15 7.2 6.4
3.1 9.6 2.9 9.5
0.42 0.19 2.6 2.7
a
CAg , o s 7.8=10y3 M. CAg , o s8.7=10y3 M. c Slightly precipitated. b
As is clear from this table, the percentage stripping was less than 9% under the conditions studied. This finding suggests that the affinity of ClO4y for amine is almost the same to that of AgŽSCN. 32y, and thus perchlorate solution is not suitable for precipitation stripping. 4.1.3. Stripping of Ag(I) with aqueous NH4 SCN solution Extraction of AgŽI. thiocyanate complexes using Primene JMT is reduced by the presence of NH 4 SCN at high concentration w9x. This finding indicates a possibility of stripping AgŽI. from AgŽI.-loaded Primene JMT by the exchange of anion between AgŽSCN. 32y and SCNy. However, in the presence of a large excess of SCNy ions, precipitation stripping may be impossible, since the stripped species are soluble AgŽI. thiocyanate complexes. Fig. 1 shows the results obtained by changing the NH 4 SCN concentration from 0.1 to 3.0 M. As is clear from the figure, the percentage stripping of
Fig. 1. Stripping of AgŽI. from AgŽI.-loaded Primene JMT using aqueous NH 4 SCN solutions at 298 K.
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AgŽI. increases with the increase in NH 4 SCN concentration, with 88% stripping at 3 M NH 4 SCN. The difference in pH value during the preparation of the organic phase, i.e., the difference in HSCN concentration in the organic phase, did not affect significantly the percentage stripping of AgŽI.. The quantitative stripping of AgŽI. using NH 4 SCN is possible even though no precipitation occurs, as such. If silver can be recovered from NH 4 SCN solution containing AgŽI. thiocyanate complexes by cementation, NH 4 SCN may prove useful as a stripping solution. Although carrying out stripping and reduction in one stage presents several practical problems, it would be interesting to know more about the processes. Also, cementation, without doubt, is the most promising alternative. These are interesting subjects for future study. From these findings, it is concluded that the precipitation stripping of AgSCN from AgŽI.-loaded Primene JMT under the experimental conditions employed in the present study is not feasible. 4.2. Reduction stripping from Ag(I)-loaded Primene JMT using NaBH4 4.2.1. Reduction stripping of Ag(I) thiocyanate complex ions from Primene JMT by NaBH4 The possibility of the reduction stripping of AgŽI. thiocyanate complex ions loaded onto Primene JMT was investigated. As mentioned before, NaBH 4 decomposes in acidic or neutral solution according to Eq. Ž10.. In this regard, Awadalla and Ritcey w12x reported that NaBH 4 can be used as a stabilized aqueous solution containing 12% by weight NaBH 4 and 40% NaOH. In the present work, neglecting such a stabilization, reduction stripping was performed by dissolving NaBH 4 into NH 3 or NaOH solution. Equal volumes of an organic solution containing AgŽI.-loaded Primene JMT and an aqueous NH 3 or NaOH solution containing NaBH 4 were mixed well to promote the reduction stripping of Ag. Upon mixing the organic and aqueous phases, a violent evolution of gas occurred, and dark precipitates formed. This gas is considered to be H 2 generated by the reaction of HSCN in the organic phase and NaBH 4 in the aqueous phase. Shaking of the solution system was started after the evolution of H 2 gas had slowed. When NaBH 4 concentration was low and reaction time was short, the precipitates formed were extremely fine and difficult to separate well from the organic phase. Under these conditions, the determination of AgŽI. concentration and filtration of the precipitates was not performed. Fig. 2 shows the percentage reduction and stripping determined at definite time intervals after the mixing was started in a thermostat at 298 K. Both the percentage reduction and stripping increased with the increase in retention time, reaching constant values after 57.6 ks. The difference between the percentage stripping and reduction corresponds to the AgŽI. concentration remaining in the aqueous phase, and this difference stayed almost constant with the elapse of retention time. Therefore, reduction stripping initially proceeds rapidly, then slows, and finally stops at around 57.6 ks. The precipitates recovered were subjected to X-ray diffractometry. Fig. 3 shows a typical diffraction pattern, which matches well that of metallic Ag. Thus, we conclude that the precipitates obtained are single phase metallic Ag.
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Fig. 2. Reduction stripping of Ag from AgŽI.-loaded Primene JMT using aqueous NaBH 4 solution at 298 K. CAg , o s 7.5=10y3 M Žextracted at pH 7.5.. wNaBH 4 xr CAg, o s 2. wNH 3 x s 0.5 M.
Fig. 4 shows the SEM photograph for the precipitates obtained by reduction stripping with NaBH 4 . From this picture, we confirmed that the stripping product is an aggregate of uneven fine-sized particulates. 4.2.2. Effect of NaOH or NH3 concentration on reduction stripping of Ag Since NaBH 4 is unstable in neutral or acidic solution, we added NaOH or NH 3 to the stripping solution. Fig. 5 depicts the effect of NaOH or NH 3 concentration in the stripping solution on the reduction stripping of Ag. In these experiments, the amount of NaBH 4 was kept constant at twice that of AgŽI. in the organic phase, while the alkali concentrations were changed from 0.1 to 1.0 M.
Fig. 3. X-ray diffraction pattern of the precipitates obtained by reduction stripping from AgŽI.-loaded Primene JMT.
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Fig. 4. SEM photograph of the precipitates obtained by reduction stripping from AgŽI.-loaded Primene JMT. Organic phase: 7.5=10y3 M AgŽI.-loaded 100 kg my3 Primene JMT. Aqueous phase: 0.5 M NH 3 solution containing 1.5=10y3 M NaBH 4 .
With the increase in NaOH or NH 3 concentration, both the percentage reduction and stripping increased. As is clear from this figure, a high percentage stripping is obtained above 0.3 M for NaOH and above 0.5 M for NH 3 . This may be due to the retardation of acid decomposition of NaBH 4 in high alkaline solution. It is noteworthy that a strong
Fig. 5. Effect of alkali concentration on reduction stripping of Ag from AgŽI.-loaded Primene JMT at 298 K. CAg , o s 7.9=10y3 M Žextracted at pH 7.5.. wNaBH 4 xr CAg, o s 2. Retention time: 86.4 ks.
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alkali, such as NaOH, is more effective than a weak alkali, such as NH 3 . To elucidate efficient stripping conditions, additional experiments were conducted using 0.5 M NH 3 solution containing NaBH 4 . The NH 3 was selected to be recycled for the leaching of Ag. 4.2.3. Effect of NaBH4 concentration on the reduction stripping of Ag When AgŽSCN. 32y is extracted with Primene JMT, HSCN is simultaneously extracted forming thiocyanate complexes. Thiocyanate complexes thus generated may participate in the acidic decomposition of NaBH 4 , affecting the reduction stripping reaction. Fig. 6 shows the results obtained. Two types of organic phase were used. One was prepared by extracting AgŽI. from Primene JMT pre-equilibrated with organic solution using thiocyanate, and the other by extracting AgŽI. at pH around 7. The concentrations of NaBH 4 were changed to give wNaBH4xrCAg, o ratios from 0.25 to 7.0. When the former organic phase was used, the concentration of HSCN loaded in the organic phase was higher. As is clear from this figure, the increase in NaBH 4 concentration results in an increase in both the percentage reduction and stripping, with values of more than 90% for wNaBH 4 xrCAg, o of 2.0 or more. In the range of wNaBH 4 xrCAg, o from 0.25 to 2.0, there is a difference in the percentage reduction and stripping due to the preparation of organic phase. The percentage reduction and stripping of Ag from an organic phase containing a large amount of HSCN, i.e., AgŽI. extracted using Primene JMT pre-treated with thiocyanate, were lower than that for the other organic phase. This is mainly caused by the contribution of HSCN in the organic phase to the acidic decomposition of NaBH 4 . Above a wNaBH 4 xrCAg, o ratio of 2.0, the effect of HSCN can be neglected. This phenomenon is attributed to the presence of a sufficient amount of NaBH 4 .
Fig. 6. Effect of wNaBH 4 xr CAg, o ratio on reduction stripping of Ag from AgŽI.-loaded Primene JMT at 298 K. Organic phase A: CAg , o s8.0=10y3 M Žextracted at pH 7.5.. Organic phase B: CAg, o s9.3=10y3 M Žextracted by HSCN-loaded Primene JMT.. wNH 3 x s 0.5 M. Retention time: 86.4 ks.
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4.2.4. Effect of concentration of extractant Fig. 7 shows the effect of extractant concentration on the reduction precipitation of AgŽI., for Primene JMT concentrations changing from 100 to 300 kg my3 . The upper figure shows the AgŽI. concentration in the organic phase prepared by two different methods. The concentration of AgŽI. in the organic phase increases with the increase in extractant concentration. It is also observed that the extraction of AgŽI. with the Primene JMT pre-treated to convert thiocyanate salt, is higher than that with the Primene JMT. Percentage reduction and stripping of AgŽI. from the organic phase obtained at pH 7 for extraction were around 75% and 80%, respectively, at an extractant concentration of 100 kg my3 . Above 150 kg my3 of extractant, both values decreased to about 30%. On the other hand, the percentage reduction and percentage stripping of Ag from the organic phase prepared by AgŽI. extraction using thiocyanate complexes of Primene JMT, decreased with the increase in extractant concentration. This is caused by the increase in HSCN concentration due to the increase in extractant concentration. The acid decomposition of NaBH 4 was enhanced, and the reduction stripping was retarded. 4.2.5. Effect of Ag(I) concentration in the organic phase Fig. 8 shows the effect of the concentration of AgŽI. loaded onto the organic phase. From this figure, it is obvious that the percentage reduction and stripping increase with the increase in AgŽI. concentration in the organic phase, exhibiting values of 99% at a AgŽI. concentration of 1.5 = 10y2 M or more. Below 1.5 = 10y2 M AgŽI. concentration, the AgŽI. concentration in the stripping solution increases with the decrease in AgŽI. concentration in the organic phase, suggesting difficulty of reduction.
Fig. 7. Effect of Primene JMT concentration on reduction stripping of Ag from AgŽI.-loaded Primene JMT at 298 K. Experimental conditions and symbols used in this figure are as shown in Fig. 6 unless otherwise stated.
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Fig. 8. Effect of AgŽI. concentration in organic phase on the reduction stripping of Ag using NaBH 4 from AgŽI.-loaded Primene JMT at 298 K. Primene JMT concentration: 100 kg my3 . Experimental conditions and symbols used in this figure are as shown in Fig. 6 unless otherwise stated.
No significant difference in stripping was detected for the two methods of preparing the AgŽI.-loaded organic phase under the present experimental conditions. This is mainly due to the large excess of NaBH 4 in the stripping solution used, as is understood from the value of wNaBH 4 xrCAg, o . 5. Conclusions Stripping of AgŽI. thiocyanate complex extracted by Primene JMT was investigated. The main results obtained are summarized as follows. Ž1. Precipitation stripping of AgSCN was almost impossible when water, aqueous NH 3 , or NaOH solution was used as stripping solution. Ž2. Stripping or precipitation stripping reactions did not proceed quantitatively when HClO4 or NaClO4 , whose affinity for amine is strong, was used as stripping solution. Ž3. When NH 3 or NaOH solution containing NaBH 4 was used, reduction stripping occurred producing aggregates of fine metallic silver powder. Ž4. For an efficient reduction stripping of Ag using a stripping solution containing NaBH 4 , the addition of NH 3 or NaOH, and the use of NaBH 4 at more than twice the amount of AgŽI. in the organic phase, are required. Ž5. When AgŽI. extraction was done using Primene JMT that had been converted to thiocyanate salt, the reduction efficiency deteriorated. References w1x A. Yazawa, M. Eguchi, Hydrometallurgy and Waste Water Treatment, Kyoritsu Suppan, Tokyo, 1975, p. 68 Žin Japanese.. w2x D. Zipperian, S. Raghavan, J.P. Wilson, Hydrometallurgy 19 Ž1987. 361–375.
S. Sanuki et al.r Hydrometallurgy 52 (1999) 123–135 w3x w4x w5x w6x w7x w8x w9x w10x
135
S. Sanuki, N. Minami, S. Sunada, K. Arai, Nippon Kinzoku Gakkaishi 53 Ž1989. 399–406. T. Yamashita, K. Nagatsuma, K. Hata, S. Goto, Shigen-to-Sozai 107 Ž1991. 562–568. T. Yamashita, K. Nagatsuma, K. Hata, S. Goto, Shigen-to-Sozai 109 Ž1993. 341–346. T. Groenewald, Hydrometallurgy 1 Ž1976. 277–290. C.K. Chen, T.N. Lung, C.C. Wan, Hydrometallurgy 5 Ž1980. 207–212. S. Goto, O. Ogawa, I. Asakura, S. Nakamura, Nippon Kogyo Kaishi 101 Ž1985. 75–79. S. Sanuki, M. Jyumonji, H. Majima, to be published. Novel Techniques for Separation in Hydrometallurgy, in: Kansaishibu ŽEd.., MMIJ, 1983, p. 9 Žin Japanese.. w11x S. Sanuki, A. Sugiyama, K. Takada, K. Arai, Nippon Kinzoku Gakkaishi 58 Ž1994. 1279–1287. w12x F.T. Awadalla, G.M. Ritcey, Randol Gold Forum, Squaw Valley, CA, 1990, pp. 295–306.
Stripping of silver from Primene JMT loaded with silver thiocyanate complexes Sumiko Sanuki a
a,)
, Masumi Yata a , Hiroshi Majima
b
Department of Material Systems Engineering and Life Science, Faculty of Engineering, Toyama UniÕersity, 3190 Gofuku, Toyama 930-8555, Japan b Kyoto UniÕersity, Titan Kogyo, 1978-25, Kogushi, Ube 755-8567, Japan Received 12 August 1998; received in revised form 12 December 1998; accepted 13 December 1998
Abstract Stripping of silver from Primene JMT loaded with silver thiocyanate complexes was investigated, utilizing hydrolysis, anion exchange and reduction stripping reactions. The findings obtained are summarized as follows. Ž1. Precipitation stripping of AgSCN was almost impossible when water, aqueous NH 3 or NaOH was used as stripping solution. Ž2. Stripping or precipitation stripping reactions did not proceed quantitatively when HClO4 or NaClO4 , whose affinity for amine is strong, was used as stripping solution. Ž3. When NH 3 or NaOH solution containing NaBH 4 was used, reduction stripping proceeded producing aggregates of fine metallic Ag powder. Ž4. For efficient reduction stripping of Ag using a solution containing NaBH 4 , the addition of NH 3 or NaOH, and the use of NaBH 4 at more than twice the amount of AgŽI. in the organic phase, are required. Ž5. When AgŽI. extraction was done using Primene JMT that had been converted to the thiocyanate salt, the reduction efficiency deteriorated. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Primene JMT; Silver; Stripping
1. Introduction Precious metals have always been recycled because of their value. Recycling is done mostly using pyrometallurgical methods Žsmelting.. Cyanide and aqua regia are used mostly for analytical purposes and therefore, the environmental impact is small. Thiourea, thiosulphate, thiocyanate and especially bromine have been used occasionally to recycle )
Corresponding author. Telefax: q81-764-45-6817; E-mail: [email protected]
0304-386Xr99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 3 8 6 X Ž 9 9 . 0 0 0 0 2 - X
124
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precious metals, but the resulting concentrations are high and subsequent concentration by solvent extraction or ion exchange is generally not required. Generally speaking, however, recycling materials containing precious metals are usually contaminated with various metals. It is considered that hydrometallurgical methods become more important for recycling. Cyanidation using soluble cyanide and a dissolution method using aqua regia are commonly used for the recovery of Au or Ag w1x. However, these methods have serious flaws with regards to environmental and safety considerations. Thus, various alternative recovery procedures have been proposed, including the use of thiosulfate w2,3x, thiocyanate w4,5x, and acidic thiourea w6–8x. One of the authors ŽS.S.., has conducted a systematic study of the extraction of AgŽI. using amine-type extractants such as Primene JMT and TOA from an aqueous ammonium thiocyanate solution w9x. It was found that the extraction of thiocyanate concurred with the extraction of AgŽI.. Also, the composition and equilibrium constants of the species of thiocyanate extracted with Primene JMT and TOA, as well as the extraction of AgŽI. with thiocyanate salts of Primene JMT and TOA, were studied. In the present work, a method for the efficient stripping of silver from AgŽI. thiocyanate complex-loaded Primene JMT was studied to obtain information on the recovery of precious metals, and on the possibility of recovering insoluble salts of AgŽI. or metallic Ag directly from the organic phase.
2. Theoretical background for the stripping of silver from Ag(I)-loaded Primene JMT 2.1. RecoÕery of AgSCN by precipitation stripping method of Ag(I)-loaded Primene JMT Amine is the most common reagent among those extractants known as ion-pairing or basic type extractants, and extracts metallic ions based on the ion association principle w10x. Primene JMT ŽRNH 2 . used in the present study is a primary amine, and the chemistry of the extraction of AgŽI. thiocyanate complexes with RNH 2 has been studied extensively by Sanuki et al. w9x. Agq forms thiocyanate complexes, AgŽSCN.Žnny1.y Ž n s 1–4., in aqueous solution containing ammonium thiocyanate. The extraction reaction for this solution system can be expressed by Eq. Ž1.. 2y
2 Ž RNH 2 . o q 2Hqq Ag Ž SCN . 3 s
Ž RNHq3 . 2 Ag Ž SCN. 32y
o
Ž 1.
When reaction Ž1. proceeds, a side reaction, which is expressed by Eq. Ž2., can occur concurrently.
Ž RNH 2 . o q Hqq SCNys RNHq3 SCNy
o
Ž 2.
yx The concentrations of AgŽI. in the organic phase and wRNHq 3 SCN o are affected by the change in pH during the extraction of AgŽI.. There is no available information for free yx amine ŽRNH 2 . o , but it is known that the organic phase involves wRNHq 3 SCN o and 2y x w x wŽRNHq . Ž . 3 2 Ag SCN 3 o 9 . In general, metallic ions extracted by amine can be stripped
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by deprotonation of protonated amine using neutral and alkaline aqueous solutions, since the metallic ion species are soluble in these solutions. The reaction for stripping of AgŽI. thiocyanate complex-loaded Primene JMT with alkaline solution is expressed by Eq. Ž3.. Ž . Ž RNHq3 . 2 Ag Ž SCN. 32q o q 2OHys 2 Ž RNH 2 . o q Ag Ž SCN. nny1 y
q Ž 3 y n . SCNyq 2H 2 O
Ž 3.
It is also presumed that the addition of anions, whose affinity for amine is stronger than that of AgŽSCN. 32y, causes an anion exchange reaction expressed by Eq. Ž4.. Ž ny1 .y q 2Xys 2 Ž RNH 3 X . o q Ag Ž SCN. n Ž RNHq3 . 2 Ag Ž SCN. 2y 3 o
q Ž 3 y n . SCNy
Ž 4.
Under the conditions of low SCNy concentration, AgSCN is an insoluble salt and thus there exists the possibility of recovering AgSCN by either of the reactions expressed by Eq. Ž3. or Eq. Ž4.. If the stripping of ŽHSCN. o occurs, the concentration of SCNy in that aqueous phase increases and thus AgŽI. is stripped as a soluble complex. Extraction of AgŽI. thiocyanate complexes by Primene JMT is difficult in the presence of NH 4 SCN at high concentration. Therefore, the dissociation reaction of AgŽI. thiocyanate complex-loaded Primene JMT may be regarded as a stripping method, which is expressed by Eq. Ž5.. Ž ny1 .y q Ž n y 3 . SCNys 2 Ž RNH 2 . o q Ag Ž SCN . n Ž RNHq3 . 2 Ag Ž SCN. 2y 3 o
q 2Hq
Ž 5.
2.2. Reduction stripping with NaBH4 NaBH 4 is known as a strong reducing agent and is utilized as a reductant for precious metal colloids and for electroless plating. This reagent is also useful for directly recovering ultrafine Pd powders from Pd-loaded amine solution, as reported by Sanuki et al. w11x. It is supposed that reduction to fine Ag powders using NaBH 4 as a stripping solution is possible, as expressed by Eq. Ž6.. q eys 2 Ž RNH 2 . o q Ag q 2Hqq 3SCN Ž RNHq3 . 2 Ag Ž SCN. 2y 3 o
Ž 6.
Eq. Ž6. can be more clearly explained by the following reactions. 2y
Reduction: Ag Ž SCN. 3
q eys Ag q 3SCNy
q y Oxidation: In acid solution BHy 4 q 3H 2 O s H 3 BO 3 q 7H q 8e y q y In alkali solution BHy 4 q 2H 2 O s H 2 BO 3 q 6H q 8e
Ž 7. Ž 8. Ž 9.
As is shown by Eqs. Ž8. and Ž9., the oxidation of NaBH 4 involves an eight-electron liberation reaction, so that one mole NaBH 4 theoretically may reduce eight moles AgŽI..
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However, the oxidation rate of NaBH 4 is so fast, that the evolution of hydrogen gas in acidic solution can proceed according to Eq. Ž10.. q BHy 4 q 3H 2 O q H s H 3 BO 3 q 4H 2
Ž 10 .
Therefore, an excess of NaBH 4 is required to reduce AgŽI. quantitatively.
3. Experimental 3.1. Reagents The organic phase used was Primene JMT solution, adjusted to a specific concentration by diluting with kerosene. AgŽI.-loaded Primene JMT solutions were prepared by extracting AgŽI. from the organic solution containing ammonium thiocyanate in two different ways. One solution was prepared by dissolving 5 = 10y3 M Ag 2 O into 1 M NH 4 SCN solution, and then AgŽI. was extracted at equilibrium pH around 7 or 2 using 100–300 kg my3 Primene JMT. The other organic phase was prepared by extracting AgŽI. from 0.5 and 1.0 M NH 4 SCN solution using thiocyanate salt of Primene JMT. This extractant was prepared by extracting HSCN using a definite amount of Primene JMT from 1 M NH 4 SCN, whose equilibrium pH was adjusted to 2 with H 2 SO4 . A modifier, n-decanol, was added to all organic phases to 50 kg my3 . The preparation of stripping solutions was done by dissolving reagent grade NaOH, NH 3 , HClO4 , NaClO4 and NH 4 SCN to obtain definite concentrations. Water used in the present study was deionized, with a specific resistivity of 5 = 10 4 V m. Stripping solutions used for the reduction stripping of AgŽI. were prepared by dissolving NaBH 4 ranging from 0.25 to 10 times the molar ratio of AgŽI. in an organic phase, into NaOH or NH 3 . 3.2. Experimental procedures Equal volumes of organic phase loaded AgŽI. and aqueous phase were taken into a centrifugal separation tube, then shaken for 0.9 ks using a vertical type shaker at ambient temperature. The tubes containing the solutions were kept at 298 K for equilibration, before being separated using a centrifugal separator operated at a speed of 50 rev sy1 for 0.9 ks. On reduction stripping, hydrogen gas evolved immediately after mixing, and so shaking was not started until after the gas evolution had slowed. 3.3. Analytical method The AgŽI. concentration in aqueous solution was determined by atomic absorption spectrophotometry. In contrast, AgŽI. in the organic phase was stripped twice using 4 M NH 4 SCN, then the combined solution was subjected to atomic absorption analysis. Precipitates formed by reduction stripping were filtered with a glass fibre filter. After the filtrate had been dried at 323 K, the sample was subjected to scanning electron microscopy and X-ray diffraction.
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4. Results and discussion 4.1. Stripping of Ag(I) with Õarious solutions containing acids or alkalis 4.1.1. Stripping of Ag(I) with aqueous alkali solutions As mentioned, stripping of AgŽI. was done using an alkaline solution containing NH 3 or NaOH, whose concentrations were 0.5 and 1.0 M, respectively. The results obtained are summarized in Table 1. Percentage stripping of AgŽI. using water or aqueous NH 3 solution was less than 1%, and no precipitation was detected. In contrast, the stripping with NaOH solution gave 15–33% stripping, exhibiting a slight formation of a white precipitate. However, it was almost impossible to produce quantitatively AgŽI. precipitates using the NaOH solutions tested in the present study. On the other hand, it is known that the extraction of HSCN due to the extraction of AgŽI. is decreased at higher pH w9x. In the present work, the effect of HSCN concentrations in the organic phase was not studied quantitatively. However, the percentage stripping for the organic phase obtained by extraction of AgŽI. at pH 1.6 is larger than that at pH 7.5 when the stripping was done with NaOH solution, as shown in Table 1. Therefore, it is considered that the stripping of AgŽI. is easier when the HSCN concentration in the organic phase is high and the pH of the aqueous phase is low. The AgŽI. stripped would be in the forms of complex ions, since the SCNy ion concentration in the stripping solution was high. 4.1.2. Stripping of Ag(I) by anion exchange Stripping of AgŽI. by anion exchange was examined. In the affinity for amine, the y y y w anions rank in decreasing order as follows: ClO4y) NOy 10x. In 3 ) Cl ) HSO4 ) F 2y y contrast, the affinity for amine of SCN and AgŽSCN. 3 has not been clearly indicated. To verify the effect of ClO4y ion on the stripping of AgŽI. from AgŽI.-loaded Primene JMT, stripping experiments were performed using aqueous solutions containing HClO4 or NaClO4 . The results obtained are summarized in Table 2.
Table 1 Stripping of AgŽI. from AgŽI.-loaded Primene JMT using various stripping solutions Stripping solution
AgŽI.-loaded Primene JMT Extracted at pH 7.5a
Water 0.5 M NH 3 1.0 M NH 3 0.5 M NaOH 1.0 M NaOH a
CAg , o s 7.8=10y3 M. CAg , o s8.7=10y3 M. c Slightly precipitated. b
Extracted at pH 1.6 b
Stripping Ž%.
pH
Stripping Ž%.
pH
0.27 0.87 0.53 17.4 c 15.4 c
6.1 6.9 7.5 13.2 13.4
0.24 0.24 0.32 33.0 c 33.0 c
3.0 6.5 7.0 13.0 13.4
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Table 2 Stripping of AgŽI. from AgŽI.-loaded Primene JMT using stripping solutions containing perchlorate ions Stripping solution
AgŽI.-loaded Primene JMT Extracted at pH 7.5a
1.0 M HClO4 2.0 M HClO4 1.0 M NaClO4 2.0 M NaClO4
Extracted at pH 1.6 b
Stripping Ž%.
pH
Stripping Ž%.
pH
2.6 4.5 c 2.8 7.0 c
0.54 0.15 7.2 6.4
3.1 9.6 2.9 9.5
0.42 0.19 2.6 2.7
a
CAg , o s 7.8=10y3 M. CAg , o s8.7=10y3 M. c Slightly precipitated. b
As is clear from this table, the percentage stripping was less than 9% under the conditions studied. This finding suggests that the affinity of ClO4y for amine is almost the same to that of AgŽSCN. 32y, and thus perchlorate solution is not suitable for precipitation stripping. 4.1.3. Stripping of Ag(I) with aqueous NH4 SCN solution Extraction of AgŽI. thiocyanate complexes using Primene JMT is reduced by the presence of NH 4 SCN at high concentration w9x. This finding indicates a possibility of stripping AgŽI. from AgŽI.-loaded Primene JMT by the exchange of anion between AgŽSCN. 32y and SCNy. However, in the presence of a large excess of SCNy ions, precipitation stripping may be impossible, since the stripped species are soluble AgŽI. thiocyanate complexes. Fig. 1 shows the results obtained by changing the NH 4 SCN concentration from 0.1 to 3.0 M. As is clear from the figure, the percentage stripping of
Fig. 1. Stripping of AgŽI. from AgŽI.-loaded Primene JMT using aqueous NH 4 SCN solutions at 298 K.
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AgŽI. increases with the increase in NH 4 SCN concentration, with 88% stripping at 3 M NH 4 SCN. The difference in pH value during the preparation of the organic phase, i.e., the difference in HSCN concentration in the organic phase, did not affect significantly the percentage stripping of AgŽI.. The quantitative stripping of AgŽI. using NH 4 SCN is possible even though no precipitation occurs, as such. If silver can be recovered from NH 4 SCN solution containing AgŽI. thiocyanate complexes by cementation, NH 4 SCN may prove useful as a stripping solution. Although carrying out stripping and reduction in one stage presents several practical problems, it would be interesting to know more about the processes. Also, cementation, without doubt, is the most promising alternative. These are interesting subjects for future study. From these findings, it is concluded that the precipitation stripping of AgSCN from AgŽI.-loaded Primene JMT under the experimental conditions employed in the present study is not feasible. 4.2. Reduction stripping from Ag(I)-loaded Primene JMT using NaBH4 4.2.1. Reduction stripping of Ag(I) thiocyanate complex ions from Primene JMT by NaBH4 The possibility of the reduction stripping of AgŽI. thiocyanate complex ions loaded onto Primene JMT was investigated. As mentioned before, NaBH 4 decomposes in acidic or neutral solution according to Eq. Ž10.. In this regard, Awadalla and Ritcey w12x reported that NaBH 4 can be used as a stabilized aqueous solution containing 12% by weight NaBH 4 and 40% NaOH. In the present work, neglecting such a stabilization, reduction stripping was performed by dissolving NaBH 4 into NH 3 or NaOH solution. Equal volumes of an organic solution containing AgŽI.-loaded Primene JMT and an aqueous NH 3 or NaOH solution containing NaBH 4 were mixed well to promote the reduction stripping of Ag. Upon mixing the organic and aqueous phases, a violent evolution of gas occurred, and dark precipitates formed. This gas is considered to be H 2 generated by the reaction of HSCN in the organic phase and NaBH 4 in the aqueous phase. Shaking of the solution system was started after the evolution of H 2 gas had slowed. When NaBH 4 concentration was low and reaction time was short, the precipitates formed were extremely fine and difficult to separate well from the organic phase. Under these conditions, the determination of AgŽI. concentration and filtration of the precipitates was not performed. Fig. 2 shows the percentage reduction and stripping determined at definite time intervals after the mixing was started in a thermostat at 298 K. Both the percentage reduction and stripping increased with the increase in retention time, reaching constant values after 57.6 ks. The difference between the percentage stripping and reduction corresponds to the AgŽI. concentration remaining in the aqueous phase, and this difference stayed almost constant with the elapse of retention time. Therefore, reduction stripping initially proceeds rapidly, then slows, and finally stops at around 57.6 ks. The precipitates recovered were subjected to X-ray diffractometry. Fig. 3 shows a typical diffraction pattern, which matches well that of metallic Ag. Thus, we conclude that the precipitates obtained are single phase metallic Ag.
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Fig. 2. Reduction stripping of Ag from AgŽI.-loaded Primene JMT using aqueous NaBH 4 solution at 298 K. CAg , o s 7.5=10y3 M Žextracted at pH 7.5.. wNaBH 4 xr CAg, o s 2. wNH 3 x s 0.5 M.
Fig. 4 shows the SEM photograph for the precipitates obtained by reduction stripping with NaBH 4 . From this picture, we confirmed that the stripping product is an aggregate of uneven fine-sized particulates. 4.2.2. Effect of NaOH or NH3 concentration on reduction stripping of Ag Since NaBH 4 is unstable in neutral or acidic solution, we added NaOH or NH 3 to the stripping solution. Fig. 5 depicts the effect of NaOH or NH 3 concentration in the stripping solution on the reduction stripping of Ag. In these experiments, the amount of NaBH 4 was kept constant at twice that of AgŽI. in the organic phase, while the alkali concentrations were changed from 0.1 to 1.0 M.
Fig. 3. X-ray diffraction pattern of the precipitates obtained by reduction stripping from AgŽI.-loaded Primene JMT.
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Fig. 4. SEM photograph of the precipitates obtained by reduction stripping from AgŽI.-loaded Primene JMT. Organic phase: 7.5=10y3 M AgŽI.-loaded 100 kg my3 Primene JMT. Aqueous phase: 0.5 M NH 3 solution containing 1.5=10y3 M NaBH 4 .
With the increase in NaOH or NH 3 concentration, both the percentage reduction and stripping increased. As is clear from this figure, a high percentage stripping is obtained above 0.3 M for NaOH and above 0.5 M for NH 3 . This may be due to the retardation of acid decomposition of NaBH 4 in high alkaline solution. It is noteworthy that a strong
Fig. 5. Effect of alkali concentration on reduction stripping of Ag from AgŽI.-loaded Primene JMT at 298 K. CAg , o s 7.9=10y3 M Žextracted at pH 7.5.. wNaBH 4 xr CAg, o s 2. Retention time: 86.4 ks.
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alkali, such as NaOH, is more effective than a weak alkali, such as NH 3 . To elucidate efficient stripping conditions, additional experiments were conducted using 0.5 M NH 3 solution containing NaBH 4 . The NH 3 was selected to be recycled for the leaching of Ag. 4.2.3. Effect of NaBH4 concentration on the reduction stripping of Ag When AgŽSCN. 32y is extracted with Primene JMT, HSCN is simultaneously extracted forming thiocyanate complexes. Thiocyanate complexes thus generated may participate in the acidic decomposition of NaBH 4 , affecting the reduction stripping reaction. Fig. 6 shows the results obtained. Two types of organic phase were used. One was prepared by extracting AgŽI. from Primene JMT pre-equilibrated with organic solution using thiocyanate, and the other by extracting AgŽI. at pH around 7. The concentrations of NaBH 4 were changed to give wNaBH4xrCAg, o ratios from 0.25 to 7.0. When the former organic phase was used, the concentration of HSCN loaded in the organic phase was higher. As is clear from this figure, the increase in NaBH 4 concentration results in an increase in both the percentage reduction and stripping, with values of more than 90% for wNaBH 4 xrCAg, o of 2.0 or more. In the range of wNaBH 4 xrCAg, o from 0.25 to 2.0, there is a difference in the percentage reduction and stripping due to the preparation of organic phase. The percentage reduction and stripping of Ag from an organic phase containing a large amount of HSCN, i.e., AgŽI. extracted using Primene JMT pre-treated with thiocyanate, were lower than that for the other organic phase. This is mainly caused by the contribution of HSCN in the organic phase to the acidic decomposition of NaBH 4 . Above a wNaBH 4 xrCAg, o ratio of 2.0, the effect of HSCN can be neglected. This phenomenon is attributed to the presence of a sufficient amount of NaBH 4 .
Fig. 6. Effect of wNaBH 4 xr CAg, o ratio on reduction stripping of Ag from AgŽI.-loaded Primene JMT at 298 K. Organic phase A: CAg , o s8.0=10y3 M Žextracted at pH 7.5.. Organic phase B: CAg, o s9.3=10y3 M Žextracted by HSCN-loaded Primene JMT.. wNH 3 x s 0.5 M. Retention time: 86.4 ks.
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4.2.4. Effect of concentration of extractant Fig. 7 shows the effect of extractant concentration on the reduction precipitation of AgŽI., for Primene JMT concentrations changing from 100 to 300 kg my3 . The upper figure shows the AgŽI. concentration in the organic phase prepared by two different methods. The concentration of AgŽI. in the organic phase increases with the increase in extractant concentration. It is also observed that the extraction of AgŽI. with the Primene JMT pre-treated to convert thiocyanate salt, is higher than that with the Primene JMT. Percentage reduction and stripping of AgŽI. from the organic phase obtained at pH 7 for extraction were around 75% and 80%, respectively, at an extractant concentration of 100 kg my3 . Above 150 kg my3 of extractant, both values decreased to about 30%. On the other hand, the percentage reduction and percentage stripping of Ag from the organic phase prepared by AgŽI. extraction using thiocyanate complexes of Primene JMT, decreased with the increase in extractant concentration. This is caused by the increase in HSCN concentration due to the increase in extractant concentration. The acid decomposition of NaBH 4 was enhanced, and the reduction stripping was retarded. 4.2.5. Effect of Ag(I) concentration in the organic phase Fig. 8 shows the effect of the concentration of AgŽI. loaded onto the organic phase. From this figure, it is obvious that the percentage reduction and stripping increase with the increase in AgŽI. concentration in the organic phase, exhibiting values of 99% at a AgŽI. concentration of 1.5 = 10y2 M or more. Below 1.5 = 10y2 M AgŽI. concentration, the AgŽI. concentration in the stripping solution increases with the decrease in AgŽI. concentration in the organic phase, suggesting difficulty of reduction.
Fig. 7. Effect of Primene JMT concentration on reduction stripping of Ag from AgŽI.-loaded Primene JMT at 298 K. Experimental conditions and symbols used in this figure are as shown in Fig. 6 unless otherwise stated.
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Fig. 8. Effect of AgŽI. concentration in organic phase on the reduction stripping of Ag using NaBH 4 from AgŽI.-loaded Primene JMT at 298 K. Primene JMT concentration: 100 kg my3 . Experimental conditions and symbols used in this figure are as shown in Fig. 6 unless otherwise stated.
No significant difference in stripping was detected for the two methods of preparing the AgŽI.-loaded organic phase under the present experimental conditions. This is mainly due to the large excess of NaBH 4 in the stripping solution used, as is understood from the value of wNaBH 4 xrCAg, o . 5. Conclusions Stripping of AgŽI. thiocyanate complex extracted by Primene JMT was investigated. The main results obtained are summarized as follows. Ž1. Precipitation stripping of AgSCN was almost impossible when water, aqueous NH 3 , or NaOH solution was used as stripping solution. Ž2. Stripping or precipitation stripping reactions did not proceed quantitatively when HClO4 or NaClO4 , whose affinity for amine is strong, was used as stripping solution. Ž3. When NH 3 or NaOH solution containing NaBH 4 was used, reduction stripping occurred producing aggregates of fine metallic silver powder. Ž4. For an efficient reduction stripping of Ag using a stripping solution containing NaBH 4 , the addition of NH 3 or NaOH, and the use of NaBH 4 at more than twice the amount of AgŽI. in the organic phase, are required. Ž5. When AgŽI. extraction was done using Primene JMT that had been converted to thiocyanate salt, the reduction efficiency deteriorated. References w1x A. Yazawa, M. Eguchi, Hydrometallurgy and Waste Water Treatment, Kyoritsu Suppan, Tokyo, 1975, p. 68 Žin Japanese.. w2x D. Zipperian, S. Raghavan, J.P. Wilson, Hydrometallurgy 19 Ž1987. 361–375.
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S. Sanuki, N. Minami, S. Sunada, K. Arai, Nippon Kinzoku Gakkaishi 53 Ž1989. 399–406. T. Yamashita, K. Nagatsuma, K. Hata, S. Goto, Shigen-to-Sozai 107 Ž1991. 562–568. T. Yamashita, K. Nagatsuma, K. Hata, S. Goto, Shigen-to-Sozai 109 Ž1993. 341–346. T. Groenewald, Hydrometallurgy 1 Ž1976. 277–290. C.K. Chen, T.N. Lung, C.C. Wan, Hydrometallurgy 5 Ž1980. 207–212. S. Goto, O. Ogawa, I. Asakura, S. Nakamura, Nippon Kogyo Kaishi 101 Ž1985. 75–79. S. Sanuki, M. Jyumonji, H. Majima, to be published. Novel Techniques for Separation in Hydrometallurgy, in: Kansaishibu ŽEd.., MMIJ, 1983, p. 9 Žin Japanese.. w11x S. Sanuki, A. Sugiyama, K. Takada, K. Arai, Nippon Kinzoku Gakkaishi 58 Ž1994. 1279–1287. w12x F.T. Awadalla, G.M. Ritcey, Randol Gold Forum, Squaw Valley, CA, 1990, pp. 295–306.