application of the shrinking core model to the kinetics of extraction of goldi, silveri and nickelii...

Ž .Hydrometallurgy 56 2000 323–336www.elsevier.nlrlocaterhydromet

Application of the shrinking core model to thež / ž /kinetics of extraction of gold I , silver I and

ž /nickel II cyanide complexes by novel anionexchange resins

Greg W. Dicinoski a,1, Lawrence R. Gahan b, Peter J. Lawson c,),John A. Rideout c

a School of Chemistry, UniÕersity of Tasmania, Hobart, Tasmania 7001, Australiab Department of Chemistry, UniÕersity of Queensland, St. Lucia, Queensland 4072, Australia

c School of Chemical and Biomedical Sciences, Central Queensland UniÕersity, Rockhampton,Queensland 4702, Australia

Received 27 May 1999; received in revised form 15 February 2000; accepted 21 February 2000

Abstract

Ž . Ž .This paper describes an investigation of the kinetics of loading of gold I , silver I , andŽ .nickel II cyanide complexes onto two novel anion exchange resins with high selectivity for the

Ž . Ž .linear dicyanoaurate I and dicyanoargentate I complexes. The kinetic data fit well to a shrinkingcore theoretical model, and indicate that in all three complexes, the loading is controlled by therate of diffusion of the ions penetrating the reacted layer. Diffusion coefficients were determined

Ž . Ž . Žand those of Au I and Ag I cyano complexes are similar to one another but much higher 30 to. Ž .60 times those of the Ni II cyano complex on these two resins. q 2000 Elsevier Science B.V. All

rights reserved.

Keywords: Aurocyanide; Argentocyanide; Gold and silver selective resins; Resin-in-pulp; Shrinking coremodel; Kinetics of loading of precious metals onto anion exchange resins

) Corresponding author.Ž . Ž .E-mail addresses: [email protected] G.W. Dicinoski , [email protected] P.J. Lawson .

1 Also corresponding author.

0304-386Xr00r$ - see front matter q2000 Elsevier Science B.V. All rights reserved.Ž .PII: S0304-386X 00 00082-7

( )G.W. Dicinoski et al.rHydrometallurgy 56 2000 323–336324

1. Introduction

Cyanidation of gold and silver ores has remained the dominant hydrometallurgicalw xtechnology over the last century for economic recovery of these precious metals 1 .

Very few significant technological improvements occurred in the industry in the first 70years of the 20th century. The release of the price of gold however in 1968 from thefixed US$35roz gold standard resulted in a rapid tenfold increase in its price and theimproved profitability of gold processing catalysed significant investment in researchand development over the last 30 years. This has transformed precious metal recoveryinto a ‘‘high tech’’ hydrometallurgical process capable of economically extractingincreasingly complex ore types and lower grades of ores.

Zinc cementation of precious metals from cyanide leachates via the Merrill–CroweŽ . Ž .process has largely been replaced in the last 25 years by carbon-in-pulp CIP or

Ž .carbon-in-leach CIL concentration processes coupled with electrolytic recovery of thew xprecious metals 2 .

Despite this success, carbon’s lack of selectivity has triggered comparisons of CIPŽ . w xwith resins-in-pulp RIP over the last 15 years 3–8 . Although commercial anion

w xexchange resins have been used at the Golden Jubilee Mine in South Africa 8 , and inw xthe former Soviet Union 6 , economic viability was doubtful. However, the develop-

w xment of new gold specific resins at Mintek in South Africa 9–14 and elsewherew x w x15–19 has rejuvenated interest in the RIP vs. CIP debate. Johns and Marsh 20 haverecently presented convincing evidence that gold-specific resins are an economicallyviable alternative.

Over 20 sterically hindered gold and silver cyanide selective quaternary ammoniumfunctionalized anion exchange resins have been synthesised and characterised at the

Ž . w xCentral Queensland University CQU 21,22 . The kinetics of the extraction of theŽ . Ž . Ž .Au I , Ag I and Ni II cyanide complexes by two of the most promising of these resins

Ž . w xhas been evaluated in this paper, and a Shrinking Core Model SCM 23–25 applied tothe interpretation of this data. The structures and abbreviated names of the two resins ofinterest are illustrated in Fig. 1. Both resins are derivatives of polystyrenerdivinyl

Ž .benzene polymers. NOTREN was synthesised by capping a tris 2-aminoethyl amineŽ .TREN resin derivative. TEA-BE was synthesised by forming a tribenzyl ether of a

Ž . Ž . w xtris 2-hydroxyethyl amine TEA resin derivative 26 . These bulky mono-cationic sitesŽ . Ž .selectively adsorb linear gold I and silver I cyanide complexes but reject the tetrahe-

Ž . Ž .Fig. 1. a NOTREN resin, b TEA-BE resin.

( )G.W. Dicinoski et al.rHydrometallurgy 56 2000 323–336 325

Ž . Ž . Ž . Ž . Ž .dral and octahedral complexes of copper I , zinc II , iron II and III and cobalt III andŽ .partially load the square planar nickel II cyanide complex.

2. Experimental

All acids and complexes used were of A.R. quality. The atomic absorption spectro-scopic analyses were performed with either a Varian AA1475 or a Varian SpectrAA-300

Žspectrometer. Details of syntheses of the two novel resins used in this study and. w xnumerous other CQU resins will be reported elsewhere 26 . These two resins were

derivatives of polystyrene approximately 3% crosslinked with divinyl benzene andŽ . Ž . Ž .chloromethylated to 4 mmol Clrg dry 20–60 mesh Polysciences Merrifield resin .

Confirmation of the syntheses was gained by FT-IR difference spectroscopy, CPrMAS13C NMR, anion exchange capacity tests and microanalyses.

2.1. Determination of the diffusion coefficient for each resin

w xA shallow bed micro scale procedure similar to that of Riveros and Cooper 7 , whichavoids the formation of a concentration gradient across the resin bed, was followed here.The resins of interest were screened to achieve uniform bead sizes, NOTREN to a size

Ž . Ž .of 30–40 mesh av. 0.51 mm and TEA-BE to a size of 25–30 mesh av. 0.66 mm .Ž . Ž .NOTREN had a degree of substitution D of S of 1.2 mmolrg dry and TEA-BE a D

Ž .of S of 0.92 mmolrg dry . In each case, 0.025 g of dry resin in the chloride form wasdegassed in water and wet-loaded into a 120-mm long by 10 mm I.D. glass column andretained by a sintered glass frit.

Solutions were passed through the resin beds at 200 mLrh, as follows:

w Ž . x Ž . ŽAu: 1000 mgrL Au as K Au CN 5 mmolrL Au , 130 mgrL KCN 2.02. Ž .mmolrL , 100 mgrL NaOH 2.5 mmolrL at pH G11 and 258C.

w Ž . x Ž .Ag: 814 mgrL Ag as K Ag CN 7.5 mmolrL Ag qKCN and NaOH as above.2w Ž . x Ž .Ni: 300 mgrL Ni as Na Ni CN PH O 5 mmolrL Ni qNaCN and NaOH same2 4 2

molarity as above.

Flow times of 5, 10, 20, etc., to 60 min were run on each column for each metalcyanide solution. The resins from each time period were filtered off, washed copiouslywith water and methanol and air-dried, then dried in a vacuum oven overnight at 508C.Each resin was completely digested in acid for quantitative metal ion analysis, a typicalprocedure for which follows: 0.10 g of loaded resin was charred in 10 mL of conc.sulphuric acid with heating in a fume cupboard. The temperature was then loweredbelow 1008C and 10 mL of conc. nitric acid added and the mixture reheated untildigestion was complete. It was again cooled below 1008C and 10 mL of conc.hydrochloric acid added and the solution further heated to solubilise the gold. Thesolution was then cooled and diluted appropriately in a volumetric flask and the solutionanalysed for metal ions by atomic absorption spectroscopy.


2.2. Determination of mass transfer coefficients for each resin and each metal cyanidecomplex

The method used to determine the mass transfer coefficient is similar to that used inSection 2.1, but metal cyanide complex concentrations were one-tenth of those usedpreviously and KCN and NaOH concentrations were unaltered:

In each case, 0.50 mmolrL of M nq was used as metallocyanides, plus 130 mgrLŽ . ŽKCN to give 2 mmolrL free cyanide and 100 mgrL NaOH to give 2.5 mmolrL

.base at pH G11, at 258C.

3. Shrinking core mathematical model

Ion exchange processes can be separated into two broad classes. The first contains thefast, reversible electrostatic exchange processes involving simple cations or anions,which occur uniformly between the ions in solution and the opposite ions of the poroussolid phase. The second category includes those ion exchange processes, which areaccompanied by fast chemical reactions, such as the abstraction of the metal cyanocomplexes, where the interacting ions progress in shells into the resin matrix. A sharpmoving boundary between the reacted outer shell and the shrinking unreacted inner corecan be observed during the ion exchange process. The likelihood of such a process

w xoccurring in spherical resin beads was proposed by Helfferich 27 and was laterinvestigated in various processes. This section considers the hypothesis that the anionexchange of the gold, silver and nickel cyano complexes are accompanied by fastchemical reactions which are controlled by the diffusion of these complexes through thereacted layer of the resin beads. This will be achieved by employing the ‘‘Shrinking

w xCore Mathematical Model’’ which was developed by Smuckler et al. 23,24 .A summary of this model as developed by these workers and modified by Riveros

w xand Cooper 7 follows.Consider a spherical anion exchange resin bead of radius R in contact with an

Žexternal solution containing an anion A, with a concentration of C which for this trialA.remained essentially constant by experimental design . As the ion exchange reaction

takes place, a layer of adsorbed complex is formed on the surface of the unreacted bead.This layer is porous, so the reaction can continue by the diffusion of A ions through thislayer and which subsequently takes part in a rapid reaction at the interface of the reactedlayer and unreacted core. This process is illustrated in Fig. 2, where the development ofthe concentration gradient from C to zero is traced.A

w xIt was also assumed by Bischoff 28 that for a pseudo-steady state system, the rate ofshrinkage of the unreacted core, d r rd t, was small when compared to the velocity ofc

diffusion of ion A through the product layer. The concentration gradient of reactantw xmolecules at any shell radius, r, in the reacted layer was given by 23,24 :

d N dCA 2y s4p r D 1Ž .d t d r


ŽFig. 2. Representation of the shell progressive mechanism in a spherical ion exchange bead Reprinted fromw x .Fig. 1 of Ref. 24 , with permission from Elsevier Science .

where D is the effective diffusion coefficient, in cm2rs, for A through the porousreacted layer. The initial boundary conditions for the above equation are:

at rsR cm , CsC mmolrgŽ . Ž .A

while

at rsr , Cs0c

because the reactant is consumed as rapidly as it diffuses to the surface of the unreactedŽ .core. Thus, integration of Eq. 1 yields:

d N CA As4p Rr D 2Ž .cd t Ryrc

As the reaction progresses, the decrease of the radius of the unreacted core accompany-ing the consumption of the liquid reactant develops as given by:

d N d rA c2sr 4p r 3Ž .B cd t d t

where r is the capacity of the resin in mmolrg.BŽ . Ž .Equating Eqs. 2 and 3 and separating the similar variables yields:

r 1 1 tc 2yr y r d r sDC d t 4Ž .H HB c c Až /r RR 0c


which upon integration will give the relationship between t and the decrease in theradius of the unreacted core as shown by:

2 2 3r R r rB c cts 1y3 q2 5Ž .ž / ž /6DC R RA

The fractional conversion, a , of the ion exchange polymer, is the ratio between themeasured and maximum capacity of the resin. This is also referred to as the fractionalapproach to equilibrium loading of the resin and has values between zero and one. a is

Ž .determined experimentally and can be related to the radii, R, in Eq. 5 as given by:

3rc1yas 6Ž .ž /R

Ž . Ž .Substituting Eq. 6 into Eq. 5 produces the rate expression in terms of the fractionalapproach to equilibrium for the resin loading:

2r RB 2r3ts 1y3 1ya q2 1ya 7Ž . Ž . Ž .6DCA

Ž .Eq. 7 can be further modified to accommodate an ion exchange process where thestoichiometric number is not equal to one, by inserting, n, the number of sites on theresin occupied by the absorbing anion, A. The above equation can be rearranged to a

Ž .form amenable to graphical representation, Eq. 8 . Plotting the fractional conversionŽ 2 .term against time allows the determination of the diffusion coefficient D cm rs for

the anion, A, through a resin matrix. Thus, this form of the equation modelling thew xshrinking core process is given as 7,25 :

6nDCA2r31q2 1ya y3 1ya s t 8Ž . Ž . Ž .2ž /r rB

Ž .Eq. 8 is valid for systems in which fast chemical reactions are occurring, there isnegligible resistance to diffusion from the Nernst layer, the external solution has anapproximately constant composition and the anion exchange resin beads are of constantsize and have spherical geometry. The shrinking core model also predicts that when lowconcentrations of metal complexes are used, a regime of film diffusion control applies.In this case, the fractional approach to equilibrium, a , exhibits a linear relationship withtime, since the Nernst diffusion layer and the external solution concentration areconstant. This is shown by:

3nK Cm Aas t 9Ž .

r rB

Ž .where K is the mass transfer coefficient cmrs for each complex between themw xexternal solution and the resin beads 7 . Like the previous equations, this one is valid

only if the concentration of the metal complex anion in the external solution is constantand the resins beads are spherical and of constant size.


The shrinking core model and variations thereof have been widely used in theliterature to describe the kinetics of reactions accompanying fluid–solid interactions,

w xsome examples of which involving resins or polymers are given in Refs. 29–34 .

4. Results and discussion

From earlier work using two- and three-element systems on NOTREN and TEA-BEresins, it was noticed that the gold complex loaded faster than the silver which in turnloaded much faster than the nickel complex. We propose that the small sizes of the goldand silver cyano complexes allow faster diffusion than is possible for the larger nickelcyano complex.

With the aim of investigating these points further, the extraction of the three metalcyano complexes from single-element solutions was studied using the shallow bed

w xtechnique on a micro scale 7 . In this procedure, an aqueous solution is passed througha thin layer of resin beads at a high flow rate. This method avoids the formation of aconcentration gradient through the resin bed. The loading of each of the metal cyanocomplexes on the NOTREN resin is represented in Figs. 3 and 4. The first displays theloading in millimoles of metal adsorbed per gram of resin against time while the second

Ž .portrays the fractional approach to equilibrium loading a with increase in time. Thegraphs for the TEA-BE resin are not illustrated here, but were very similar to those forthe NOTREN data.

The fractional approach to equilibrium of the gold and silver cyano complexes ismore rapid than for the nickel cyano complex despite the higher ionic charge on the

Ž . Ž . Ž .Fig. 3. Rate of loading of Au I , Ag I and Ni II cyano complexes at high metal concentration on theNOTREN resin.


Ž . Ž . Ž .Fig. 4. Fractional loading rate of Au I , Ag I and Ni II cyano complexes at high metal concentration onNOTREN resin.

latter. The nickel cyano complex has a reaction stoichiometry of two, while the preciousmetal cyano complexes have values of one. The parabolic kinetics for the loading of themetal cyano complexes on the resin is consistent with those evidenced in other trialsŽ .temperature, ionic strength, etc. . They strongly suggest a particle diffusion-controlledextraction, since the concentration of the metal cyano complexes in the external solutionremained constant throughout the trial.

w Ž . Ž .Ž2r3.xFig. 5 shows that a plot of 1q2 1ya y3 1ya vs. time is a straight line.A linear regression analysis of the data for each resin, along with calculation of thediffusion coefficient for each metal complex, is presented in Table 1.

The magnitude of the D values for the gold and silver cyano complexes are similarin the NOTREN resin, but the silver complex appears slightly more mobile in theTEA-BE resin matrix than in the gold complex. This could be explained by the slightlysmaller size of the silver complex. These D values for the gold complex are approxi-mately 60 and 30 times that of the nickel complex, on the NOTREN and TEA-BE,respectively. The D values for the silver complex are some 60 and 50 times those forthe nickel complex on the NOTREN and TEA-BE, respectively. Values for D have beendetermined for the loading of the gold, silver and zinc cyano complexes by Riveros and

w x Ž y5 y5 y6 2Cooper 7 on DOWEX MSA-1 1.08=10 , 0.903=10 , and 3.00=10 cm rs,.respectively . From these values, it is observed that the diffusivity of the T zincd

complex is about one-third of that for the gold or silver complexes. The values for thegold and silver complex diffusivities within the DOWEX MSA-1 resin matrix are abouthalf or less those for the NOTREN and TEA-BE resin matrices, presumably due to thedifferences in the individual adsorbent matrix composition. The crosslinking of the CQU

Ž .resins is fairly low ;3% whereas that of the DOWEX MSA-1 macroreticular resin isŽ . w Ž . x2yconsiderably higher ;12% . The low D values for Ni CN on the CQU resins4


Ž . Ž . Ž .Fig. 5. Application of the SCM for determination of the diffusion coefficients for the Au I , Ag I and Ni IIcyano complexes on the NOTREN resin.

Žcan be explained by the fact that it is square planar and has an n value of two two sites.required for nickel adsorption and these resins have quite low D of S, and highly

Ž .sterically hindered cationic sites see Fig. 1 . The gel type CQU resins are highlyŽ . Ž .selective for the linear dicyanoaurate I and dicyanoargentate I complexes which are

presumably able to access a higher proportion of cationic sites than the more bulkyŽ . w Ž . x2ytetracyanonickelate II complex. Although the Zn CN is tetrahedral and has an n4

value of two, and is considerably more sterically hindered than the gold and silvercomplexes, the lower selectivity of the DOWEX MSA-1 is probably caused by the moreuniform pores in the macroreticular resin, the greater accessibility of these cations, andthe lower steric hindrance of the trimethyl substituents on these quaternary ammoniumsites. The selectivities of the NOTREN and TEA-BE for the precious metal cyano

w xcomplexes are in agreement with the hypothesis of Green et al. 11 , that suchselectivities increase as alkyl substituents on the ammonium group are enlarged. If thesehypotheses are correct, then the value determined for D is both a measure of theselectivity of the resin and the diffusivity of the complex within the resin matrix. The

Table 1Determination of the diffusion coefficients of the gold, silver and nickel cyano complexes on the NOTRENand TEA-BE resins

2 y1 2Ž . Ž .Resin Metal Intercept r Slope h n D cm rsy5NOTREN Gold y0.025 1.00 0.689 1 1.97=10y5Silver y0.041 0.99 1.090 1 2.07=10y7Nickel 0.001 0.98 0.024 2 3.42=10y5TEA-BE Gold y0.046 0.99 0.909 1 2.21=10y5Silver y0.020 0.99 1.016 1 3.73=10y7Nickel y0.001 1.00 0.039 2 7.23=10


proximity of the intercept on the y-axis to zero in Fig. 5 indicates that the contributionŽfrom film diffusion was negligible. For this to be so, the value of the mass transfer or

.film diffusion coefficient from the Nernst layer must be considerably larger than thediffusion coefficient.

Ž .Mass transfer coefficients K were determined to test this assumption. To do this,m

the concentrations of the metal ions were one-tenth of those in the previous test, whilethe free cyanide and base concentrations remained the same. The results for this series oftests are displayed in Figs. 6 and 7 for the NOTREN resin. Again, only the NOTRENdata is displayed due to the similarity between the two sets of results. Table 2summarises data for both resins.

Ž .The relationship between the resin loading mmolrg and contact time is linear asshown in Fig. 6. The loading rates of the gold, silver and nickel cyano complexes on theNOTREN resin were 0.4357 mmolrgrh, 0.2838 mmolrgrh, and 0.04174 mmolrgrh,respectively and were 0.4190 mmolrgrh, 0.3757 mmolrgrh, and 0.05584 mmolrgrh,respectively for the TEA-BE resin.

The linearity suggests a regime of film diffusion control since the Nernst diffusionlayer and the external solution concentration are constant. The shrinking core model alsopredicts that in the case of film diffusion control, the fractional approach to equilibrium

Ž .a bears a linear relationship with respect to time, as shown earlier by Eq. 9 . This isillustrated in Fig. 7.

Ž Ž ..It is seen from the intercept on the y-axis of Fig. 7, theoretically zero from Eq. 9Ž 2 .and the linear correlation r , that a good fit to the model was obtained with each resin

for the three metal cyano complexes. Thus, the loading is also influenced by diffusioncontrol. The K values for the three metal complexes on each resin indicate that goldm

Ž . Ž . Ž .Fig. 6. Rate of loading of Au I , Ag I and Ni II cyano complexes at low metal concentration on theNOTREN resin.


Ž . Ž . Ž .Fig. 7. Fractional loading rate of Au I , Ag I and Ni II cyano complexes at low metal concentration fordetermination of mass transfer coefficients on NOTREN resin.

and silver complexes migrate to the resin bead surface about 10 times as fast as thenickel complex. Also, the gold complex moves faster than the silver complex despite itslarger size. This can be explained by the more polarisable nature of the gold complexgiving it a slightly higher effective negative charge than the silver complex. This resultis the opposite of that obtained for the diffusion coefficient and supports the loadingtrends of these two complexes observed previously, with the initial loading rate of thegold complex being faster than that of the silver complex, while the final equilibriumloading of the two complexes are comparable. The results for these two CQU resins are

w xcomparable with those obtained by Riveros and Cooper 7 for the loading of the gold,Ž y3silver and zinc cyano complexes on the DOWEX MSA-1 resin, 4.42=10 , 6.11=

y3 y3 .10 , and 3.53=10 cmrs, respectively . Finally, the magnitude of K for eachmŽcomplex on the two resins is much larger than the value for D by a factor of

.approximately 100 for the gold and silver complexes and 1000 for the nickel complexes .

Table 2Determination of the mass transfer coefficients for the gold, silver and nickel cyano complexes loading on theNOTREN and TEA-BE resins

2 y1Ž . Ž .Resin Metal Intercept r Slope h n K cmrsm

y3NOTREN Gold 0.0009 1.00 0.363 1 4.09=10y3Silver 0.0089 1.00 0.284 1 2.67=10y4Nickel 0.0040 0.99 0.070 2 3.92=10y3TEA-BE Gold 0.0136 0.99 0.454 1 5.08=10y3Silver 0.0262 1.00 0.407 1 4.09=10y4Nickel 0.0087 0.99 0.121 2 6.77=10


This confirms that the loading is diffusion-controlled and is less influenced by the masstransfer of the anion to the Nernst layer.

After studying the selectivity of several commercial weak and strong base anionw Ž . xy w xexchange resins for Au CN over base metal cyano complexes, Riveros 32 has2

suggested that the resin matrix has a significant bearing on such selectivity. HeŽ .concluded that low hydrophilicity and a low ionic density isolated cationic sites

w Ž . xyincrease the affinity of a resin for Au CN , whereas high hydrophilicity and a high2w Ž . x3yionic density increase resin affinity for the multivalent hydrated ions such as Cu CN 4

w Ž . x4yand Fe CN . Such relationships have also been discussed by Fleming and Cromberge6w x w x3 and Green et al. 10 . In this work, the resin matrices were held constant and althoughthe polystyrenerdivinyl benzene matrix is quite hydrophobic, the quarternary ammo-nium derivatives are somewhat polar and hydrophilic and these resins would therefore

Ž .have intermediate hydrophilicity. However, their low D of S or ionic density and theirŽ .sterically hindered cationic sites render them highly selective for the linear gold I and

Ž .silver I cyano complexes.w xHiskey and Ette 25 showed that the weak base resin, PAZ-4, designed especially to

be selective for the gold and silver cyano complexes, also displayed behaviour consistentwith the SCM. Their adsorption model relies on the exchange of all the chloridecounter-ions on the resin by cyanide ions. This is assumed to also occur in the present

Ž .study as the cyanide anion is present in a reasonable concentration ;2.0 mmolrL , hasa slightly larger ionic radius, and is more polarisable than the chloride ion. It is thenassumed that the gold cyano complex is transported into the resin matrix by a bridgingand billiard-ball mechanism. This mechanism is illustrated in Fig. 8 and functions by the

w xFig. 8. Hiskey and Ette’s 25 model depicting the mechanism of aurocyanide loading onto an anion exchangeresin.


Scheme 1. Reaction for the abstraction of the aurocyanide complex by an anion exchange resin according tow xthe shrinking core model and the model of Hiskey and Ette 25 .

gold atom bridging between sites by co-ordinating with the adsorbed cyanide groupalready in place.

The gold complex advances into the resin when an aurocyanide complex ion collideswith a bridged gold atom already bound to the resin. Thus, the gold advances into the

Ž .resin matrix by the collision and formation of R–NC–Au–CN bridges. This conclu-sion is consistent with the observed experimental data above, with the known lability of

Ž . w xdicyanoaurate I 35 and the shrinking core model. According to this mechanism, theloading of the gold cyano complex onto the strong base resin proceeds via the reaction

Ž .presented in Scheme 1. This mechanism will also apply to the dicyanoargentate Icomplex which is similarly labile.

5. Conclusion

The initial extraction rates of these cyano complexes on the two CQU resins iscontrolled by liquid film diffusion. Gold and silver are extracted at considerably higherrates than nickel because of higher diffusivities in the liquid film.

As the reacted layer increased in thickness, the extraction is controlled by particlediffusion in the resin matrix. The differences in resin diffusivities of these complexeswere confirmed by fitting single element kinetic data to a shrinking core model. The

Ž . Ž .diffusion coefficients of Au I and Ag I cyano complexes are significantly higher thanŽ .those of Ni II on these two resins. The selectivity of these resins for the precious metal

cyano complexes is thought to be predominantly caused by the sterically hindered natureof the quaternary ammonium cationic sites.

Acknowledgements

The authors are grateful for a Central Queensland University research grant, and oneŽ .of us G.W.D. , for an Australian postgraduate award. They wish to gratefully acknowl-

edge the generosity of Prof. J. Brent Hiskey of the University of Arizona for allowingŽ w x.use of Fig. 8 Fig. 13 of Ref. 25 , and for supplying a copy of this. They also are

Ž w x.grateful to Elsevier Science for permission to use Fig. 2 Fig. 1 of Ref. 24 .


References

w x Ž .1 C.A. Fleming, Hydrometallurgy 30 1992 127–162.w x Ž .2 P.R. Bailey, Application of activated carbon to gold recovery, in: G.G. Stanley Ed. , The Extractive

Metallurgy of Gold in South Africa, S. Afr. Inst. Min. Metall. Monograph, Ser. M vol. 71987, pp.379–614.

w x Ž .3 C.A. Fleming, G. Cromberge, J. S. Afr. Inst. Min. Metall. 84 1984 125–137.w x Ž .4 C.A. Fleming, G. Cromberge, J. S. Afr. Inst. Min. Metall. 84 1984 269–280.w x5 J.W. Hosking, in: Proc. Aust. Inst. Min. Metall., Perth and Kalgoorlie Branches Regional Conf. on Gold

Mining, Metallurgy and Geology,1984, pp. 127–139.w x Ž .6 E.S. Borovskii, E.M. Rakhmanko, A.R. Tsyganou, Russ. J. Inorg. Chem. 30 1985 1157–1159.w x Ž .7 P.A. Riveros, W.C. Cooper, Solvent Extr. Ion Exch. 6 1988 479–503.w x Ž . Ž8 C.A. Fleming, D. Seymore, in: H. von Michaelis Ed. , Proc. Randol Gold Forum ’90 Squaw Valley,

.CA , Randol Intl., Golden, CO, 1990, pp. 237–242.w x Ž .9 B.R. Green, A.H. Schwellnus, M.H. Kotze, in: C.E. Fivaz, R.P. King Eds. , Proc. Gold 100, Interna-

tional Conf. on Gold, S.A.I.M.M., Johannesburg, 1986, pp. 321–333, vol. 2.w x Ž .10 B.R. Green, K.G. Ashurst, T.E. Chantson, in: R.B. Bhappu, R.J. Harden Eds. , Gold Forum on

Ž .Technology and Practices, World Gold ’89, Reno, NV ,Proc. 1st Joint Int. Meet. SME and AusIMM,Soc. Min. Metall. Exploration, Colorado, 1989, pp. 341–353, Chap. 40.

w x .11 B.R. Green, I. Tyc, A.H. Schwellnus, Australian Patent No. Au-A-468931r89 , 1990.w x Ž . Ž .12 M.W. Johns, B.R. Green, in: H. von Michaelis Ed. , Proceedings Randol Gold Forum ’91 Cairns ,

Randol Intl., Golden, CO, 1991, pp. 363–368.w x Ž .13 M.H. Kotze, B.R. Green, G. Steinbach, in: B. Hiskey, G. Warren Eds. , Hydrometallurgy — Fundamen-

tals, Technology and Innovations, A.I.M.E, 1993, pp. 395–406, Chap. 25.w x Ž .14 P.J. Conradie, M.W. Johns, R.J. Fowles, Hydrometallurgy 37 1995 349–366.w x Ž .15 M. Asker, R.Y. Wan, J. Miller, D.R. Quillen, S.D. Alexandratos, Metall. Trans. B 18B 1987 625–633.w x16 L.L. Wilson, P.L. Mattison, M.J. Virnig, U.K. Patent Appl. No. G.B. 2186563, Aug. 1987.w x Ž .17 W.I. Harris, J.R. Stahlbush, W.C. Pike, R.R. Stevens, React. Polym. 17 1992 21–27.w x Ž .18 J.H. Hodgkin, R. Eibl, React. Polym. 9 1988 285–291.w x19 G.A. Kordosky, M.H. Kotze, J.M.W. Mackenzie, M.J. Virnig, Proc. XVII Int. Min Proc. Congress

Ž . Ž .Sydney 1993 1195–1203.w x Ž . Ž .20 M.H. Johns, D. Marsh, in: H. von Michaelis Ed. , Proc. Randol Gold Forum ’93 Beaver Creek, CO ,

Randol Intl., Golden, CO, 1993, pp. 293–299.w x21 P.J. Lawson, G.W. Dicinoski, J.A. Rideout, Prov. Australian Patent P.M. 2274, 1993.w x Ž .22 P.J. Lawson, G.W. Dicinoski, J.A. Rideout, in: H. von Michaelis Ed. , Proc. Randol Gold Forum ’93

Ž .Beaver Creek, CO , Randol Intl., Golden, CO, 1993, pp. 301–308.w x Ž .23 G. Smuckler, M. Nativ, S. Goldstein, in: M. Streat Ed. , Theory and Practice of Ion Exchange, an

International Conference held at Churchill College, University of Cambridge, Society of ChemicalIndustry, London, UK, 1976.

w x Ž .24 M. Nativ, S. Goldstein, G. Smuckler, J. Inorg. Nucl. Chem. 37 1975 1951–1956.w x Ž .25 J.B. Hiskey, A.O. Ette, in: D.R. Gaskell, J.P. Hager, J.E. Hoffmann Eds. , Proc. Reinhardt Schuhmann

Ž .Intl. Symposium of TMS-AIME, Colorado Springs , TMS-AIME, Warrendale, PA, 1986, pp. 295–310.w x26 G.W. Dicinoski, L.R. Gahan, P.J. Lawson, J.A. Rideout, in press.w x Ž .27 F. Helfferich, J. Phys. Chem. 69 1965 1178–1187.w x Ž .28 K.B. Bischoff, Chem. Eng. Sci. 18 1963 711–713.w x Ž .29 C.-J. Kim, Drug Dev. Ind. Pharm. 22 1996 307–311.w x30 W.A.C. Somers, A.J.J. Smolders, W.A. Beverloo, H.J. Rozie, J. Visser, F.M. Rombouts, K. van’t Riet,

Ž .Bioseparation 4 1994 285–298.w x Ž .31 Z. Lewandowski, F. Roe, Biotechnol. Bioeng. 43 1994 186–187.w x Ž .32 V.M. Bandari, V.A. Juvekar, S.R. Patwardhan, Sep. Sci. Technol. 27 1992 1043–1064.w x Ž .33 S.H. Gehrke, G. Agrawal, M.C. Yang, Polyelectrolyte gels, ACS Symp. Ser. 480 1992 211–237.w x Ž .34 P.A. Riveros, Hydrometallurgy 33 1993 43–58.w x35 A.G. Sharpe, The Chemistry of Cyano Complexes of the Transition Metals, Academic Press, New York,

1976, p. 17.

application of the shrinking core model to the kinetics of extraction of goldi, silveri and nickelii...

Documents