� .Hydrometallurgy 55 2000 119–136 www.elsevier.nlrlocaterhydromet � /Extraction of Ag I from aqueous thiocyanate solution with Primene JMT or TOA Sumiko Sanuki a,), Makoto Jyumonji a, Hiroshi Majima b a Department of Material Systems Engineering and Life Science, Faculty of Engineering, Toyama Uni˝ersity, 3190 Gofuku, Toyama 930-8555, Japan b Kyoto Uni˝ersity, Titan Kogyo K.K., 1978-25, Kogushi, Ube-city, Japan 755-8567 Received 3 November 1998; received in revised form 17 July 1999; accepted 13 September 1999 Abstract � .Using Primene JMT or TOA, the extraction of thiocyanic acid and of Ag I thiocyanato-com- plex from aqueous thiocyanate solution was studied. On extraction of thiocyanic acid using aqueous phase containing 1.0 M NH SCN and an organic phase containing 100–300 kg my34 � q y. � q y.Primene JMT or TOA, RNH PSCN and R NH PSCN were formed with apparent3 o 3 o y3 � .equilibrium constants, log K , of 8.7 and 6.5, respectively. The extraction of 1=10 M Ag Iex,a from aqueous ammonium thiocyanate solutions by the thiocyanate salts of Primene JMT or TOA at concentrations ranging from 100 to 300 kgPmy3, and in the presence of ammonium thiocyanate at concentrations of 0.5–3.0 M can be summarized as follows: K ex ,32y 2yq y q y2 RNH PSCN qAg SCN s RNH PAg SCN q2SCN , logK s1.6� . � .� . � .� .3 33 3 ex,3o 2 o K ex ,32y 2yq y q y2 R NH PSCN qAg SCN s R NH PAg SCN q2SCN , logK s2.5.� . � .� . � .� .3 33 3 ex,3o 2 o q 2000 Elsevier Science B.V. All rights reserved. � .Keywords: Primene JMT; TOA; Acqueous thiocyanate solution; Ag I ) Corresponding author. Tel.: q81-76-445-6817; fax: q81-76-445-6817. � .E-mail address:
[email protected] S. Sanuki . 0304-386Xr00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. � .PII: S0304-386X 99 00077-8 ( )S. Sanuki et al.rHydrometallurgy 55 2000 119–136120 1. Introduction The solvent extraction method is an effective energy saving process, since it is easy to separate the desired materials from others at room temperature and set up a flow as a closed circuit. To establish a recovering process for precious metals such as Au and Ag from aqueous thiocyanate solution using amine type extractants, and to determine the composition of extracted species as well as the equilibrium constants of extration � .reaction, the extraction of Ag I from aqueous thiocyanate solution with Primene JMT or TOA was studied. � . � .qSince the leached products of Au and Ag with thiourea TU are cations of Au TU 2 � .qand Ag TU , respectively, solvent extraction of these species using acidic extractants3 w xsuch as D2EHPA is known to be effective 1–4 . The solvent extraction method was applied to separate Au from cyanato-complexes w xusing Primene JMT by Caravaca and Alguacil 5 . In this study, the composition of extracted species and apparent equilibrium constant of extraction reaction were deter- mined. The extraction of Au cyanato-complexes using Primene JMT can proceed under acidic pH, generating HCN when free CNy ions are present in the aqueous phase, and therefore, the process is unsatisfactory. Thus, extraction of cyanato-complexes of Au or Ag using amine type extractants is not suitable for purification. However, aqueous ammonium thiocyanate solutions containing thiocyanato-complexes of Ag or Au are considerably stable in both acidic and alkaline solutions, and thus can be used effec- tively for purification purpose. The chemical properties of thiocyanate resemble those of halogen ions. AgSCN is an insoluble salt similar to silver halide, but it converts to soluble complexes with increases in thiocyanic acid concentration. By combining the solvent extraction process and precipitation stripping of insoluble salt, we expected to develop a useful purification technique to recover metals or metal salts. There have been no previous reports on the extraction of Au or Ag from aqueous ammonium thiocyanate solution containing thiocyanato-complexes. � .In the present study, the extraction equilibrium of Ag I from aqueous ammonium thiocyanate solution using amine type extractants was investigated to determine the composition of extracted species and the equilibrium constants of the extraction reac- tions. ( )2. Preliminary examination for the extraction of Ag I from aqueous ammonium thiocyanate using amine-type extractants The properties and apparent dissociation constants of amine hydrochloride of indus- w xtrially utilized amine type extractants are shown in Table 1 6 . Primene JMT, a primary amine, Amberlite LA-1 and LA-2 obtained from Rohn and Haas, both secondary amines, and Alamine 336 obtained from General Mills and � .Tri-octyl amine TOA supplied as commercial reagent, both tertiary amines, are widely used in industry. ( )S. Sanuki et al.rHydrometallurgy 55 2000 119–136 121 Table 1 w xProperties of amines and apparent dissociation constants of amine hydrochloride 6 Amine Formula Molecular Specific gravity Apparent dissociation weight at 298 K constants of amine � .hydrochloride pKa � .solution: benzene � .Primene JMT RNH RsC –C 350.8 0.849 6.902 18 24 � .Amberlite LA-1 R NH RsC –C 427.5 0.854 4.712 10 14 � .Amberlite LA-2 R NH RsC –C 365.4 0.826 5.512 10 14 � .Alamine 336 R N RsC –C 442.0 0.814 4.133 8 10 � .Tri-octyl amine R N RsC 353.7 0.808 4.103 8 As is shown in this Table, apparent dissociation constants of amine hydrochloride in benzene decrease in the order sequence primary, secondary, and tertiary amines. � .Therefore, ion-pair formation of these extractants to Ag I thiocyanato-complex and the extraction behavior to the organic phase will be affected by the extractants used. As a � .preliminary test, extraction of Ag I from ammonium thiocyanate solution was per- formed using these extractants. � .Fig. 1 shows the results of extraction of Ag I from aqueous ammonium thiocyanate solution using amine type extractants. In general, the extraction of metal ions by amine q � .proceeds to form ion pairs between metal complexes and R NH xO3 . Thus,x 4yx � .extraction is affected by pH. Log D where D is the distribution ratio is plotted against � .pH, as shown in Fig. 1, and percent extraction of Ag I from 1.0 M thiocyanate solution decreases with increase in pH. Above pH 6, percent extraction at same pH decreases in the order Primene JMT)Amberlite LA-1, TOA)Alamine 336. As shown in Table 1, � .Fig. 1. Extraction of Ag I from aqueous ammonium thiocyanate solution with various amines at 298 K. y2 w x y3Aqueous phase: C s1=10 Mq NH SCN s1 M; Organic phase: 100 kg m Extractantq50 kgAg �I.,aq 4 my3 n-decanol; Extractant: ‘ Primene JMT, ^ Amberlite LA-1, I Amberlite LA-2, e Alamine 336, v TOA. ( )S. Sanuki et al.rHydrometallurgy 55 2000 119–136122 this sequence coincides well with that of dissociation constant of hydrochloride of primary, secondary and tertiary amines. In other words, increases in the basicity of � .amines result in increasing formation of amine acid and Ag I thiocyanato-complex, and thus, the extraction is enhanced. The slopes observed in the plot of log D vs. pH increased in the order primary amine-secondary amine- tertiary amine, when the comparison was made using absolute values. � .�ny1.y � .Extraction of Ag SCN with primary amine, RNH , can be expressed byn 2 0 � .Eq. 1 by neglecting polymerization and solvation of the extractant in the extracted species. For extraction using secondary or tertiary amines, RNH was replaced with2 R NH or R N, and RNHq with R NHqor R Nq, respectively.2 3 3 2 2 3 � .ny1 yqny1 RNH q ny1 H qAg SCN� . � . � . � . n2 o Kex � .ny1 yqs RNH PAg SCN 1� . � .� .� /n3 ny1 o where n is the coordination number, and the suffix o denotes organic phase and �� q. � .�ny1.y. � .RNH PAg SCN is the extracted species of Ag I thiocyanato-complex.3 ny1 n o � . w xEq. 1 was derived from the equation reported by Caravaca and Alguacil 5 for the � .extraction of Au I from an aqueous solution of cyanide complex ions in the absence of free cyanide ions. � .In the present study, we performed the extraction of Ag I thiocyanato-complexes from aqueous ammonium thiocyanate solution containing a large amount of free ammonium thiocyanate. Under such conditions, the extraction of thiocyanic acid with amine, which is expressed by the following equations, must be taken into consideration. Generally, the extraction of metal complex ions with amine can proceed as follows. w � .xFirst, amine-acid salt is formed Eq. 2 and amine-acid and metal complex ion forms w � .x w xion-pair Eq. 3 6,7 . Kex,aq y q yRNH qH qSCN s RNH PSCN 2� . � .� .2 3o � .ny1 yq yny1 RNH PSCN qAg SCN� . � .� . n3 o Kex,n � .ny1 yq ys RNH P AgSCN q ny1 SCN 3� . � . � .� .� /n3 ny1 o � q y.where RNH PSCN is the extracted species of thiocyanic acid. When the extraction3 o � .of Ag I thiocyanato-complex using amine type extractant was performed with adjust- � . � .ment of pH, free thiocyanic acid was present so that reactions 1 and 2 occurred in parallel. The overall reaction in this case can be expressed as successive reactions of � . � .Eqs. 1 – 3 . Therefore, the extraction equilibrium of thiocyanic acid by amine, and the � .extraction of Ag I thiocyanato-complex ions by amine thiocyanate were studied to � .elucidate the extraction equilibrium of Ag I from ammonium thiocyanate with Primene JMT or TOA. � .Furthermore, we investigated the extraction of Ag I from ammonium thiocyanate solution by adjusting pH which can be expressed as successive reactions. ( )S. Sanuki et al.rHydrometallurgy 55 2000 119–136 123 3. Extraction of thiocyanic acid with Primene JMT or TOA In the extraction of metal ions using amine type extractants, ion-pairs formed between cationic protonated amine in the organic phase and anionic metal complexes are w xextracted 7 . Cationic protonated amine can be formed to extract acid to the organic phase. � .Prior to studying the extraction of Ag I from an ammonium thiocyanate solution, it is necessary to consider the formation of cationic protonated amines. Since amines are effective for extracting acid, they are often used industrially for the treatment of waste acid. 3.1. Equilibrium analysis for the extraction of thiocyanic acid with Primene JMT or TOA It is known that the ion pair of protonated amine and anion dissociates in an organic phase when a solvent with a high dielectric constant is used as the organic phase, while aggregation proceeds in a solvent with a low dielectric constant when the concentration w xof amine salt is high 8 . The extraction of thiocyanic acid by a kerosene solution of Primene JMT and TOA forms the third phase between aqueous and organic phases, and amine thiocyanate salt is contained is this third phase. By providing polarity to kerosene using n-decanol as a modifier to amine–kerosene solution, the intermediate phase disappears, and thus, amine thiocyanate salt dissolves into the kerosene phase. Since the polarity of the ion pair of amine thiocyanate is large, it cannot dissolve in a non-polar kerosene phase. Polymerization of the extracted ion pair species of amine extractant and amine thiocyanate salt is negligible in an organic solvent after promoting its polarity by adding n-decanol. Therefore, we analyzed the extraction equilibrium as a monomer in the present work. The extraction of thiocyanic acid from aqueous ammonium thiocyante solution can be � .expressed by Eq. 3 . The equilibrium constant K involves those of distributionex,a � .equilibrium and extraction equilibrium. K is shown by Eq. 4 , where the suffix aqex,a denotes aqueous phase. q yRNH PSCN� .3 oK s 4� .ex ,a q yw x w xRNH H SCN� . aq2 o Distribution ratio D is expressed as: q yRNH PSCN� .3 oDs 5� .yw xSCN aq � . � .Taking the logarithm of both sides of Eqs. 4 and 5 , we obtain: log Ds logK q log RNH ypH 6� . � .ex ,a 2 o ( )S. Sanuki et al.rHydrometallurgy 55 2000 119–136124 � .By denoting the concentration of Primene JMT in the organic phase by RNH , the2 o � . � . � .total initial concentration of RNH is expressed as RNH . Thus, Eq. 6 can be2 o 2 init converted to: q ylog Dy log RNH y RNH PSCN s logK ypH 7� . � .� .2 3 ex,ainit o � .Therefore, if the extraction of thiocyanic acid by amine proceeds according to Eq. 3 , � .the plots of the left hand side of Eq. 7 against pH, we can expect a straight line the slope of which is y1, and the value of log K can be determined from the intercept atex,a w� . � q y. xlog Dy log RNH y RNH PSCN s0.2 init 2 o 3.2. Experimental procedures 3.2.1. Preparation of test solutions Aqueous ammonium thiocyanate solutions, concentrations of which were 1.0 M, were prepared using deionized pure water with a specific resistivity of 5=104 Vm. Primene JMT was diluted without any further purification with kerosene which is commercially � 20 .available as a chemical pure reagent b.p. 175–3258C and d 0.79 to concentrations of4 100, 150, 200, and 300 kg my3. These solutions contained 50 kg my3 decanol as a modifier. 3.2.2. Experimental procedures Equal volumes of organic and aqueous solutions were taken into centrifugal separa- tion tubes and covered with a lid; then the pH was adjusted with 1.0 M H SO solution.2 4 Solution were shaken in a vertical motion type shaker for 0.5 ks, then in a thermostated shaker for 57.5 ks at 298 K for equilibration. After equilibriation, the organic phase was separated from the aqueous phase using a centrifugal separator. After measuring the pH of the aqueous phase by a pH meter calibrated using commercial buffer solutions, the SCNy ion concentration in the aqueous phase was determined by the Volhard method. The concentration of SCNy in the organic phase was determined by subtraction of that in the aqueous phase from the initial value. 3.3. Experimental results and discussion Thiocyanic acid was extracted from aqueous thiocyanate solution, the pH of which was adjusted with H SO , using Primene JMT or TOA at concentrations from 100 to2 4 300 kg my3. No extraction of H SO used for pH adjustment was detected.2 4 Figs. 2 and 3 show the experimental results, plotting log D values against pH. The distribution ratio of thiocyanic acid decreased with increasing pH regardless of the extractant concentration. At the same pH, log D increased with increases in extractant concentration. The extraction of thiocyanic acid using Primene JMT proceeded at higher pH than that using TOA. The distribution ratio using Primene JMT was larger than that using TOA at the same pH and same extractant concentration. At 100–300 kg my3 Primene JMT and TOA, the corresponding concentrations were 0.285–0.855 and 0.283–0.848 M, respectively, and these values were smaller than SCNy ion concentra- ( )S. Sanuki et al.rHydrometallurgy 55 2000 119–136 125 Fig. 2. Extraction of HSCN with Primene JMT at 298 K. Aqueous phase: 1 M NH SCN; Organic phase:4 Primene JMTq50 kg my3 n-decanol. � .tions in aqueous solution. If the reaction proceeds according to Eq. 4 , the maximum value of extraction should correspond to the concentration of extractant, and finally w xreach maximum loading 6 . Maximum loading was observed at pH lower than 7 or 5 with Primene JMT or TOA, respectively. At very low pH, however, extraction of � .thiocyanic acid over the stoichiometric amount shown in Eq. 4 occurred. The species extracted in this pH region were supposed to be solvated by excess thiocyanic acid �� q y . � q y ..RNH PSCN PmHSCN or R NH PSCN PmHSCN . The formation of such3 3 w xspecies has been reported by other researchers 9 , relating to the extraction of HCl and HNO .3 Fig. 3. Extraction of HSCN with TOA at 298 K. Aqueous phase: 1 M NH SCN; Organic phase: TOAq50 kg4 my3 n-decanol. ( )S. Sanuki et al.rHydrometallurgy 55 2000 119–136126 � .However, they did not deal with the extraction of Ag I at low pH. Exclusive of the experimental data obtained at very low pH, the composition of extracted species and the apparent equilibrium constant for the extraction reaction were determined in the region shown in the figures. Fig. 4 shows the relationship between the values corresponding to the left hand side � . w� . � q y. xof Eq. 8 , log Dy log RNH y RNH PSCN and pH. Although some2 init 3 o scattering of plots was detected, the slope of the linear portion of the line was estimated as unity both Primene JMT and TOA. Therefore, the extracted species can be considered � qas a monomer. From this finding, we assumed that the extracted species were RNH P3 y. � q y.SCN and R NH PSCN for Primene JMT and TOA, respectively.o 3 o w� . � q y. xFrom the intercept at log Dy log RNH y RNH PSCN s0, we estimated2 init 3 o the values of log K as 8.7 and 6.5 for Primene JMT and TOA, respectively. Theex,a values of log K determined using TOA were smaller than those using Primene JMT,ex,a showing a similar tendency to the dissociation constant of amine hydrochloride shown in Table 1. This may have been due to the basicity of the amine used. Therefore, the extraction of thiocyanic acid from 1.0 M aqueous thiocyanate solution using 100 to 300 kg my3 Primene JMT or TOA can be expressed as: Kex,aq y q yRNH qH qSCN s RNH PSCN , log K s8.7 8� . � .� .2 3 ex,ao o Kex,aq y q yR N qH qSCN s R NH PSCN , log K s6.5 9� . � .� .3 3 ex,ao The pH was adjusted for extraction of thiocyanic acid by Primene JMT or TOA using H SO . HCl has a stronger affinity to amine than that of H SO . The ionization2 4 2 4 constants of extractant by HCl, log K , were 6.9 with Primene JMT and 4.1 with TOAa � .Table 1 . In contrast, the extraction equilibrium constants using Primene JMT and TOA were 8.4 and 6.3, respectively, and these values were considerably larger than the ionization constant of HCl. As the extraction selectivity of HCl to amine is larger than �w x .Fig. 4. Relationship between log Dylog RNH – C and pH for the extraction of HSCN with2 init HSCN,o Primene JMT or TOA at 298 K. ( )S. Sanuki et al.rHydrometallurgy 55 2000 119–136 127 that of H SO , when H SO was used for pH adjustment, HSOy, SO2y ions, which2 4 2 4 4 4 might be present in the aqueous phase, had no significant effect in extraction. ( )4. Extraction of Ag I from aqueous ammonium thiocyanate solution using thio- cyanoto-complexes of Primene JMT or TOA � .Extraction of Ag I from an aqueous thiocyanate solution using Primene JMT or � . � .TOA may take the following two paths: 1 extraction of thiocyanic acid, and 2 � . w xextraction of Ag I thiocyanato-complexes by amine thiocyanic acid 7 . We demon- � q y. � q y.strated that RNH PSCN and R NH PSCN can be formed as extracted species3 o 3 o in the first path, respectively. Based on these findings, we extended the extraction of � .Ag I from ammonium thiocyanate solution using the thiocyanic acid salt of Primene JMT or TOA. ( )4.1. Prediction of extraction equilibria of Ag I from aqueous ammonium thiocyanate solutions using thiocyanic acid salts of Primene JMT and TOA The extracted species of thiocyanic acid with Primene JMT or TOA are expressed as � q y. � q y.RNH PSCN and R NH PSCN , respectively.3 o 3 o � . � .�ny1.yExtraction of Ag I thiocyanato-complex, Ag SCN , with thiocyanic acid saltn � .of Primene JMT can be written as shown in Eq. 3 . The equilibrium equation is expressed as: � . ny1ny1 yq yw xRNH PAg SCN SCN� .� .� /n3 ny1 oK s 10� .ex ,n ny1 � .ny1 yq yRNH PSCN Ag SCN� .� . n3 o � . w xOn the other hand, stability constants of Ag I thiocyanato-complex ion 10 is given by: � .ny1 yAg SCN� . n b s 11� .nn q yw x w xAg SCN w xThe values of b for ns2–4 were reported previously, as shown in Table 2 10 .n �� q. � . �ny1.y.Assuming that only RNH PAg SCN is extracted, the concentration of3 ny1 n o � .Ag I in the organic phase can be expressed as: � .ny1 yqC s RNH PAg SCN 12� . � .� .� /nAg ,o 3 ny1 o � .C , Ag I ion concentration in the aqueous phase can be expressed as shown in Eq.Ag, aq � . � .13 by considering the formation of various Ag I thiocyanato-complexes. y � .ny1 yqw xC s Ag q Ag SCN q Ag SCN q . . . q Ag SCN� . � . � .2 nAg ,aq n nq yw x w xs Ag 1q b SCN 13� .� n� 5 ns1 ( )S. Sanuki et al.rHydrometallurgy 55 2000 119–136128 Table 2 � . w xStability constants of thiocyanates complexes of Ag I at 298 K 10 Metal ion Ionic strength b b b2 3 4 8 9 9� .Ag I 0 corr. 1.94=10 3.43=10 5.69=10 � .Therefore, the distribution ratio, D, for Ag I ion is expressed by: � .ny1 yqRNH PAg SCN� .� .� /n3 ny1 oDsC rC s 14� .Ag ,o Ag ,aq n nq yw x w xAg 1q b SCN� n� 5 ns1 � � . � . � .Combining Eqs. 10 , 11 , and 14 , we obtain: ny1q y y y1w xDs RNH PSCN PK Pb SCN Pa 15� .3 ex,n no where: n nq yw x w xas1q Ag 1q b SCN 16� .� n� 5 ns1 � .Taking the logarithm of both sides of Eq. 15 , we obtain: q y yw xlog Ds ny1 log RNH PSCN q log SCN q logK b y log a� . � .3 ex,n no 17� . � .When the Ag I thiocyanato-complex ion concentration is negligibly small compared to � q y. yRNH PSCN and SCN ion concentrations in the aqueous phase, the concentration3 o change in SCNy ions due to the extraction can be neglected, then the plot of log w yxDq log a vs. log SCN appears to be linear with a slope of unity. From the slope of w yx w� q y. xthe log Dq log ay log SCN vs. log RNH PSCN , we can calculate ny13 o � .value, and thus the coordination number of Ag I thiocyanato-complex, and the compo- sition of extracted species can be determined. w yxFrom the intercept at log Dq log ay log SCN s0, we can determine the value of log K , the equilibrium constant.ex,n 4.2. Experimental procedures The aqueous phase was prepared by dissolving Ag O in ammonium thiocyanate to2 � . y3achieve Ag I and thiocyanate concentrations of 1=10 and 0.5–3.0 M, respectively, and diluting with deionized pure water with a specific resistivity of more than 5=104 Vm. The organic phase used was Primene JMT or TOA, at concentrations ranging from 100 to 300 kg my3, in which HSCN was preliminarily extracted from 1.0 M ammonium thiocyanate. As a modifier of the organic phase, n-decanol was added at 50 kg my3 and diluted with kerosene. Organic phases were prepared by adjusting the pH of the aqueous solution using H SO to extract stoichiometric amounts of thiocyanic acid. Extraction2 4 was done at pH 2 where we can expect the stoichiometric extraction. ( )S. Sanuki et al.rHydrometallurgy 55 2000 119–136 129 Equal volumes of organic and aqueous phases were added to a centrifugal precipita- tion tube with a lid, and shaken for 0.9 ks using a vertical motion type shaker. Then, the tube was shaken again in a thermostat maintained at 298 K for 57.6 ks. The test solution � .was separated by centrifugation, and the concentration of Ag I in the equilibriated aqueous solution was determined by atomic absorption photometry. 4.3. Experimental results and discussion ( )4.3.1. Extraction of Ag I from aqueous ammonium thiocyanate solution using thio- cyanate salts of Primene JMT and TOA y3 � .Solvent extraction experiments were performed for 1=10 M Ag I containing ammonium thiocyanate solution at concentrations ranging from 0.5 to 3.0 M using an organic solution containing the thiocyanate salt of Primene JMT or TOA ranging from 100 to 300 kgPmy3. Figs. 5 and 6 show the relationship between log D and concentration of ammonium thiocyanate. In extraction experiments, the increase in ammonium thiocyanate concentra- � .tion resulted in a decrease in the distribution ratio of Ag I . When the same concentra- � .tion of ammonium thiocyanate solution was used for extraction experiments, Ag I extraction increased with increases in extractant concentration. The distribution ratio of above 1 was obtained using the aqueous solution containing 0.5 or 1.0 M ammonium thiocyanate salt when Primene JMT or TOA was used, respectively. Using the data obtained in this work, the composition of extracted species and apparent equilibrium constants were calculated. Necessary stability constants for the calculation were taken from Table 2. For practical reasons, we did not add inert electrolyte to the aqueous phase to maintain a constant ionic strength. In other words, ionic strength of the aqueous phase used in the present experimental work was � .Fig. 5. Extraction of Ag I thiocyanate complexes by HSCN-loaded Primene JMT at 298 K. Aqueous phase: 5=10y3 M Ag OqNH SCN; Organic phase: HSCN-loaded Primene JMTq50 kg my3 n-decanol.2 4 ( )S. Sanuki et al.rHydrometallurgy 55 2000 119–136130 � . y3Fig. 6. Extraction of Ag I thiocyanate complexes by HSCN-loaded TOA at 298 K. Aqueous phase: 5=10 M Ag OqNH SCN; Organic phase: HSCN-loaded TOAq50 kg my3 n-decanol.2 4 impossible to maintain at a constant level, since the added electrolyte had the possibility of participating in extraction. However, the effect of ionic strength on the stability � .constants of Ag I thiocyanato-complexes is thought to be small compared to the large stability constants. Thus, we analyzed the results without consideration of activity. 4.3.2. Determination of the composition of extracted species and the apparent equilib- ( )rium constants for the extraction of Ag I from ammonium thiocyanate salts using thiocyanate salts of Primene JMT and TOA � . � .Based on Eq. 17 , the values of log Dq log a were plotted against log C ,SCN, aq � .as shown in Figs. 7 and 8. The value of log a was calculated from Eq. 16 using the data shown in Table 2. To determine the concentration of SCNy ions in the aqueous phase and that of amine thiocyanate in the organic phase, we neglected the changes due � .to the extraction reactions, since the Ag I concentration was far smaller than that of ammonium thiocyanate. As shown in the figure, a linear relationship with a slope of unity was observed when the extractant concentration kept at a constant level. Since the relationship between both parameters was obvious, we plotted the values of w yx w� q y. xlog Dq log ay log SCN against those of log RNH PSCN . The plots thus3 o w yxobtained are illustrated in Fig. 9. The values of log Dq log ay log SCN used for these figures were averaged values obtained using ammonium thiocyanate at 0.5, 1.0, � .2.0 or 3.0 M. As can be clearly seen from Eq. 17 , the slope of the straight line was � . � .ny1 , while the coordination number of Ag I thiocyanato-complexes concerning the extraction can be determined from this slope. As can be seen in Fig. 9, a linear relationship was observed, and from its slope, ny1s2, we obtained the coordination number, ns3. ( )S. Sanuki et al.rHydrometallurgy 55 2000 119–136 131 w x � .Fig. 7. Relationship between log Dqlog a and log NH SCN for the extraction of Ag I thiocyanato-com-4 plexes with HSCN-loaded Primene JMT. � .Therefore, the coordination number of Ag I thiocyanato-complex ion was found to � .2ybe three under the experimental conditions used, and we found that Ag SCN ions are3 � .involved in extraction of Ag I with thiocyanate salts of Primene JMT and TOA. Thus, the compositions of extracted species using thiocyanate salts of Primene JMT and TOA w� q. � .2yx w� q. � .2yxwere RNH PAg SCN and R NH PAg SCN , respectively.3 2 3 o 3 2 3 o w� q y. x w� qFrom the intercepts determined by log RNH PSCN s0 and log R NH P3 o 3 y. xSCN s0, we can calculate the apparent equilibrium constants, log K . Based ono ex,3 the results shown in Fig. 9, the values of log K b were 11.1 and 12.0 for Primeneex,3 3 JMT and TOA, respectively. The values of log K can be determined using the dataex,3 cited in Table 2, obtaining values of log K of 1.6 and 2.5 for Primene JMT and TOA,ex,3 w x � .Fig. 8. Relationship between log Dqlog a and log NH SCN for the extraction of Ag I thiocyanato-com-4 plexes with HSCN-loaded TOA. ( )S. Sanuki et al.rHydrometallurgy 55 2000 119–136132 w y x w� q y . w� q y .Fig. 9. Relationship between log Dqlog a ylog SCN and log RNH PSCN or log R NH PSCN3 o 3 o � .for the extraction of Ag I thiocyanato complexes with HSCN-loaded Primene JMT and TOA. � .respectively. Therefore, extraction of Ag I thiocyanato-complex using thiocyanate salts � . � .of Primene JMT and TOA can be expressed as Eqs. 18 and 19 , respectively. Kex,32y 2yq y q y2 RNH PSCN qAg SCN s RNH PAg SCN q2SCN ,� . � .� . � .� .3 33 3o 2 o logK s1.6 18� .ex ,3 Kex,32y 2yq y q y2 R NH PSCN qAg SCN s R NH PAg SCN q2SCN ,� . � .� . � .� .3 33 3o 2 o logK s2.5 19� .ex ,3 � .The Ag I extracted using thiocyanate salts of Primene JMT, a primary amine, and TOA, a tertiary amine, was in both cases the form of thiocyanato-complexes with the same coordination number, and no significant differences were detected due to differences in basicity of the amines used. The apparent equilibrium constant for extraction of thiocyanic acid using Primene JMT was stronger than that using TOA. It is noteworthy � .that the equilibrium constant for the extraction of Ag I thiocyanato-complex was larger using TOA than that using Primene JMT. ( )S. Sanuki et al.rHydrometallurgy 55 2000 119–136 133 5. Comparison of experimentally determined and calculated log D values for the ( )extraction of Ag I from aqueous NH SCN solution4 �� q. � .2y.Assuming that the extracted species were the same, i.e., RNH PAg SCN ,3 2 3 o � .the extraction of Ag I from aqueous ammonium thiocyanate solution using Primene � .JMT can be expressed by Eq. 20 . The equilibrium constant of extraction, K , isex � . � .shown by Eq. 21 , and the distribution ratio, D, is expressed as Eq. 22 . Kex2y 2yq q2 RNH q2H qAg SCN s RNH PAg SCN 20� . � . � . � .� .� .3 32 3o 2 o 2yqRNH PAg SCN� .� .� .33 2 oK s 21� .ex 2 2 2yqw xRNH H Ag SCN� . � . 32 o 2yqRNH PAg SCN� .� .� .33 2 oDsC rC s 22� .Ag ,o Ag ,aq q y1w xAg Pa � � . � . � . � .Combining Eqs. 21 , 22 and 11 at ns3, we obtain Eq. 23 : 2 2 3q y y1w x w xDs RNH H PK b P SCN Pa� .2 ex 3o yw xlog Ds2 log RNH y2pHq3 log SCN y log aq log K Pb 23� . � . � .2 ex 3o � � . � . � . w � � . � . � .xK values are derived from Eqs. 8 , 18 and 21 or Eqs. 9 , 19 and 21 and canex be expressed as: K sK 2 PK 24� .ex ex,a ex,3 � .Fig. 10. Comparison of calculated dashed line and observed values for the effect of Primene JMT � .concentration on the extraction of Ag I from aqueous ammonium thiocyanate solution by adjusting pH. ( )S. Sanuki et al.rHydrometallurgy 55 2000 119–136134 � .Fig. 11. Comparison of calculated dashed line and observed values for the effect of NH SCN concentration4 � .on the extraction of Ag I using Primene JMT by adjusting pH. Therefore, using experimentally determined values of K and K , the apparentex,a ex,3 equilibrium constants, K , were calculated.ex � .�ny1.yWhen the extraction of Ag SCN with Primene JMT or TOA was performedn � . � .by adjusting pH, free thiocyanic acid was present so that reactions Eqs. 8 or 9 and � .Fig. 12. Comparison of calculated dashed line and observed values for the effect of TOA concentration on � .the extraction of Ag I from aqueous ammonium thiocyanate solution by adjusting pH. ( )S. Sanuki et al.rHydrometallurgy 55 2000 119–136 135 � .Fig. 13. Comparison of calculated dashed line and observed values for the effect of NH SCN concentration4 � .on the extraction of Ag I using TOA by adjusting pH. � . � .20 occurred in parallel. The extraction of Ag I thiocyanato-complex with Primene � .JMT or TOA, as shown by Eq. 23 , can be calculated using K and K for eachex ex,a extraction. However, stability constants for the calculation were not directly determined for the particular solution system investigated in the present study. Therefore, calculated values differed considerably from those determined experimentally, as typically shown in Figs. 10–13. 6. Conclusions � .The extraction of Ag I from aqueous thiocyanate solution using Primene JMT or � .TOA was studied, and the main findings obtained were as follows: 1 In the extraction � .of Ag I from aqueous thiocyanate solution by Primene JMT or TOA, the extraction of � .thiocyanic acid and that of Ag I thiocyanato-complex proceeded in parallel with each � .other. 2 Extraction of thiocyanic acid was studied using an aqueous phase containing 1.0 M NH SCN and organic phase containing 100–300 kg my3 Primene JMT or TOA,4 and the following results were obtained. Kex,aq y q yPrimene JMT: RNH qH qSCN s RNH PSCN , log K s8.7� . � .2 3 ex,ao o Kex,aq y q yTOA: R N qH qSCN s R NH PSCN , log K s6.5� . � .3 3 ex,ao � . � . � .3 The extraction of Ag I from aqueous Ag I ammonium thiocyanate solutions by the thiocyanate salt of Primene JMT and TOA at Primene JMT and TOA concentrations ( )S. Sanuki et al.rHydrometallurgy 55 2000 119–136136 ranging from 100 to 300 kgPmy3, and in the presence of ammonium thiocyanate at concentrations ranging from 0.5 to 3.0 M can be determined as follows, respectively. Kex,32y 2yq y q y2 RNH PSCN qAg SCN s RNH PAg SCN q2SCN ,� . � .� . � .� .3 33 3o 2 o logK s1.6ex ,3 Kex,32y 2yq y q y2 R NH PSCN qAg SCN s R NH PAg SCN q2SCN ,� . � .� . � .� .3 33 3o 2 o logK s2.5ex ,3 References w x � .1 M. Tachibana, J. Shibata, M. Sano, S. Nishimura, Tech. Rep. Kansai Univ. 30 1988 61–69. w x � .2 T. Groenewald, B.J. Jones, Anal. Chem. 33 1971 1689–1691. w x � .3 M. Niinae, A. Oboso, Y. Takenaka, Y. Nakahiro, T. Yakamatsu, Nippon Kinzoku Gakkaishi 55 1991 867–873. w x � .4 S.G. Kim, H.Y. Lee, J.K. Oh, E.C. Lee, Hydrometallurgy 38 1995 7–13. w x � .5 C. Caravaca, F.J. Alguacil, Hydrometallurgy 31 1992 257–263. w x � . � .6 Kansaishibu Ed. , Novel Techniques for the Separation in Hydrometallurgy, MMIJ 1983 pp. 9–11 and � .pp. 96–97 in Japanese w x � .7 S.C. Dhara, Precious metals, mining extraction and processing, TMS-AIME 1984 199. w x � .8 W. Muller, R.M. Diamond, J. Phys. Chem. 70 1966 3469–3479.¨ w x � .9 M. Tanaka, in: Chemistry of Solvent Extraction, Kyoritsu-shuppan, Tokyo, 1977, p. 117, in Japanese . w x10 A.J. Bard, R. Parsons, J. Jordan, in: Standard potentials in aqueous solution, Marcel Dekker, New York, 1985, p. 834.