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Adsorption 2, 237-243 (1996)( 1996 Kluwer Academic Publishers. Manufactured in The Netherlands.

Gold Recovery from Cyanide Solutions with a New FibrousPolymer AdsorbentS. BELFER*

The Institutes or Applied Research, Ben-Gurion University of the Negev, Beer-Sheva, Israel

S. BINMANDepartmentof Chemistry, Ben-Gurion University of the Negev, Beer-Sheva, Israel

Received June 2, 1995; Revised February2, 1996; Accepted February12, 1996

Abstract. In a resin-in-pulp process for the recovery of gold cyanide from a very dilute solution it is desirablethat the resin should exhibit the ability to load gold at the natural pH of the leach liquor and be stripped by anaqueous alkali. The present work describes the results of gold cyanide adsorption by new ion-exchange hollowfibers prepared by the amination of sulfochlorinated polyethylene. The fibers, chopped into suitably sized pieces,showed very fast adsorption and desorption of gold from mixed cyanide solutions.Keywords: gold adsorption from dilute cyanide liquor, chelating hollow fibers, chemical modification of sul-fochlorinated polyethyleneIntroduction

Most of the total world production of gold-about 2000tons per annum-is obtained by the carbon-in-pulp(CIP) process, which comprises the following steps:grinding of the ore to a fine powder in a water slurry(pulp), cyanide leaching of the gold from the pulp, ad-sorption of the gold cyanide onto activated carbon, andfinally hot caustic elution and electrowinning to goldmetal. The activated carbon may be regenerated byheat treatment, but some of it is lost due to foulingwith organics and calcium. The considerable costs ofregenerating the activated carbon at temperatures above600C are sufficient reason to consider ion exchangeas an alternative process in gold recovery technology.There are two major reasons ion-exchange resinshave no t found extensive application in gold recov-ery: strong base resins require expensive eluantssuch as thiourea, thiocyanide, zinc cyanide and/or

*Contact address: The Institute for Chemistry &Chemical Technol-ogy, Ben-Gurion University of the Negev, P.O. Box 653, Beer-Sheva84105 Israel, Ph.: 972-7-461-934, Fax: 972-7-271612.

polar organic solvents, while weak base resins areineffective for loading gold at the normal leachingpH of 10-11. Therefore, a new sorbent for goldextraction would have to fulfil two main require-ments: loading at the high pH of the gold cyanideliquor and easy and complete stripping with aque-ous alkali. Recent reports of the synthesis of a se-ries of resins exhibiting the required properties haveopened up a new avenue of research. The resins, whichare medium base resins prepared from polyamine,1,3-diaminopropane, 2,4-diamino-2-methylpentane orchloromethylated styrene-divinylbenzene copolymer,have good selectivity for gold, high capacity at theleaching pH, and can be readily eluted with sodiumhydroxide solution (Harris et al., 1992). However, theyall suffer from the main drawback of granular ion-exchange resins, i.e., slow kinetic performance.

Our previous experience with gold cyanide extrac-tion showed that sorbents having high accessibility totheir active sites, e.g., silica modified by impregnationand subjected to y-radiation polymerization, exhibitextremely good kinetics (Belfer and Streat, 1986). Theeffectiveness of ion-exchange materials for the sorption


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238 Belfer and Binman

of gold can also be improved by using them in the formof ion-exchange fibers, which have improved diffusionproperties. Recently, several publications have out-lined the favorable performance of fibrous materials inhydrometallurgy and particularly in the gold industry(Soldatov, 1984; Tsumota et al., 1991; Belfer et al.,1992; Kotze and Cloete, 1992).

Our preliminary results with sulfochlorinatedpolyethylene (PESCI) fibers that were aminated withdiaminopropane and then reacted with alkylbromideshowed that the fibers have some strong base anionexchange functionality and are therefore able to ex-tract gold cyanide from alkaline solutions (Belfer andBinman, 1991). Th e combination of good kinetic prop-erties and good extraction ability at high pH of thesolution in contact with the hollow fibers makes thempotentially competitive with activated carbon. Accord-ing to published data (Granovskyi et al., 1982; Teleginaet al., 1982; Tsuchida et al., 1984), elution from acti-vated carbon takes several hours, while our fibers couldbe stripped within a few minutes. An additional advan-tage that makes that our hollow fibers suitable for thecommon CIP technology is the fact that they may be cutup into squares of suitable size. This will obviate one ofthe main disadvantages of resins-their small particlesize range, which calls for pulp-separating screens thatare somewhat smaller than those currently used in CIP(Muir, 1982). Moreover, the elasticity of our fibers, incomparison with carbon or beads, makes them moremechanically stable in processes involving strong at-traction forces.

This paper describes our results of extraction of goldcyanide from alkaline solution by means of hollowfibers.

ExperimentalFibersPESCI hollow fibers were aminated and quaternizedas follows. The PESCl obtained from the laboratoryof Dr. E. Korngold (Korngold and Vofsi, 1991) weresubjected to extraction with mixture of dichloroethane(DCIE) and hexane in a Soxhlet apparatus to removetraces of the soluble by-products of sulfochlorination.Single fibers (0.5-1 m in length) that had been allowedto swell in DC1E for 2-3 h at 45C were then placedin a solution of amine in solvent at room temperature(25C) overnight (24 h). Special care was taken toremove the water from both the diamine and the sol-vent (water analysis was performed with a Karl-Fisher

titrator). The fibers were then washed several timeswith 0.1 N HCl, 0.1 N NaOH and water. Swellingwas determined either by weighing the fiber first in thedry form and then in the wet form or by measuring thelength of the fiber, first dry and then wet. The aminatedhollow fibers in the deprotonated form were then quat-ernized with a solution of dimethylformamide (DMF)saturated with methyl bromide at 0C. Thereafter thefibers were separated, washed and analyzed. Their ca-pacity varied, depending on the time of reaction.

CapacityDeterminationTotal Ion-Exchange Capacity. Before measurementof the capacity, the fibers were quickly equilibratedalternately several times in 1.0 N HCl and 1.0 N NaOH.Th e dry samples were weighed (0.2 g), and each wasplaced in a 100-cm 3 volumetric flask, to which 50 cm 3of 0.1 N HC l was added. After shaking the suspensionfor 24 h 5 cm 3 of the solution was back titrated with0.05 N NaOH. The Cl- concentration in the HCl wasalso analyzed (Cl- capacity). The capacities of the ion-exchange fibers were calculated from the results of thetitration as H+ capacity (QH+) or Cl- (Qci-) capacity.

Capacity ofPartly Quaternized Fibers. The dry sam-ples in deprotonated form were weighed (0.2 g) andplaced in 100 cm 3 volumetric flasks, to which 50 cm 3of 10% NaCl were added. After shaking the suspen-sion for 24 h, the solution in contact with the fiberswas titrated with 0.05 N HCl with phenolphthalein asthe indicator. The value of the strong anion exchangecapacity of the fibers was calculated from the amountof added acid. Titration of the same solution was thencontinued with methyl orange as the indicator. Theamount of weak anion exchange groups was calculatedfrom the total amount of added acid.

Potentiometric TitrationTh e fibers (-1.0 g) in the deprotonated form wereplaced in a 250-cm 3 volumetric flask, to which wasadded 100 cm 3 of 0.1 N NaCl under an argon blanket.The suspension was shaken for 24 h, 1-2 cm 3 of 0.05 NHCl were added portion-wise, and shaking was contin-ued for a further 24 h. After each portion the acid wasadded to the solution, the pH of the solution in contactwith the of fibers was measured, and a potentiometrictitration curve was plotted.


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Gold Recovery from Cyanide Solutions 239

Metal ExtractionThe pieces of cut fibers, 2 to 3 cm or 0.5-1 cm inlength, were placed in a volumetric flask containing anaqueous cyanide solution (200 ppm CN-) of gold ions(up to 1.5 ppm) or gold ions in mixture with anotherions (pH 11). The suspension was then placed in ashaker, and 5-cm 3 samples of the solutions were takenevery 5-10 min. Finally, the fibers were filtered offand eluted with 5 M NaCNS or 0.1% NaCN in a 1%NaOH solution. The concentrations of the metal ions inthe aqueous solutions were determined by ICP-atomicspectroscopy (model OPTIMA 3000, Perkin Elmer).

Results and DiscussionGold Uptake by Aminated FibersThe PESCI fibers were aminated with four differentamines. The conditions of amination and the capacityof obtained fibers are summarized in Table 1. Theseresults are representative of numerous experimentalruns, in which parameters such as amine concentration,type of solvent, temperature and reaction time were

Table 1. Properties of aminated fibers prepared at 25C.CapacityConc. in Reaction

solvent time QH + QCi-Amine Solvent (%) (h) (meq/g) (meq/g)DAPa - 100 24 1.10 1.14DETAb Toluene 50 24 1.80 1.70TETAC DMF 20 24 1.25 0.70TETAd Toluene 50 24 1.07 0.75aDAPdiaminopropane, bDETA-diethylenetriamine,CTETA-triethylenetetramine, dDEPA-tetraethylenepentamine.

varied. The details of the preparation of aminatedPESCI is an issue of special interest and will be re-ported separately. The samples taken for this compar-ative study were chosen in terms of their high totalcapacity and good mechanical properties. A generalview of the fibers is presented in Fig. 1.

The kinetics of gold cyanide uptake by these fourtypes of fiber are presented in Fig. 2, in which goldconcentration in the solution (at pH 11) that was incontact with the fibers was plotted as a function of time.The experiment was planned to last for 1h because ourprevious results showed very fast kinetics (Belfer et al.,

Figure1. Aminated hollow fibers cu t into pieces. Scale is given in inches.


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0 20 40 60 80Time [min]

Figure 2. Decrease of gold concentration in solution as a func-tion of time for different fibers. DA P = diaminopropane, DETA =diethylenetriamine, TETA = triethylenextetramine TEPA = triethy-lene pentamine.

1992). From the Fig. 2 it can be seen that only the fiberaminated with diaminopropane showed gold extractionat this pH .

Th e other multifunctional diamines attached to thePESC1 fibers did not take up the gold cyanide underthese conditions. The ability of the ion exchangerprepared from a medium base diamine to extract goldcyanide at a high pH is in full agreement with previ-ously published data (Harris et al., 1992). In order toelucidate the ability of polyethylene-diaminopropanefibers to extract gold at high pH , potentiometric titra-tions were performed for all the fibers (Fig. 3). Theinitial pH value of 9.8 and the relatively rapid changeof pH in the vicinity of the equivalence point at pH13 characterized the base functional groups (-NH2)in the polyethylene-diaminopropane fibers. The otherthree potentiometric curves are characteristic of poly-functional exchangers with weaker base properties.The gradual change of pH at the neutralization pointis typical of multifunctional amines in which weakbase groups (-NH-) are present together with primaryamino groups, e.g., diethylenetriamine, triethylenete-tramine and triethylenepentamine.

It is likely that the polyethelene-DAP fibers havefunctional groups with stronger base properties. Inother words, polyethylene-diaminopropane behaves asa anion exchanger bearing amino groups that are activeat pH values higher than 7. There are some possible

HC I 0.05 N mllg

Figure3. Potentiometric titration of aminated polyethylene fibers.See legend to Fig. 2 for an explanation of the abbreviations.

explanations for this behavior. Firstly, polyethylene-DAP fibers show no differences in their Cl- and H+capacities. This means that no (or at least a very smallamount) of strong acid groups are present in the poly-mer matrix. (Note: strong acid groups may form an in-ternal salt with the amino groups, and their presence istherefore undesirable). Secondly, our aminated fibers,which represent a family of polysulfamido-amine poly-electrolytes, contain sulfamide groups --SO 2NH- thatmay act as weak cation exchangers at high pH val-ues (March, 1985). It appears that the presence of sul-famide groups is essential for both gold extraction andelution. When ion pair formation like Na + [Au(CN) 2] -occurs at high pH , the ion pair might be extracted by aweak acid group (sulfamide) and or a weak base group(primary amine), due to the close proximity of thesetwo types of groups in aminated PESC1 hollow fibers(Scheme 1, in which all possible reactions betweenthe fiber and the gold complex in solution are pre-sented). A mechanism has been suggested by Tye andGreen (1994) to explain the gold uptake by a phenol-formaldehyde-based weak-base resin.Scheme 1PE-SO 2NH(CH 2)3NH 2H X + Na[Au(CN)2](aq)anion exchange

PE-SO 2NH(CH 2) 3NH 2H OHanion exchangePE-SO 2NH(CH 2)3NH 2H -Au(CN) 2 + NaX(aq)ion pair exchangePE-SO 2NH(CH 2) 3NH 2 Na[Au(CN) 2] + HX(aq)PE-SO 2(Na)(CH 2) 3NH 2H [Au(CN 2)] + HX(aq)

0


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Table 2. Quatemization of polyethylene-diamino-propane hollow fibers.

Capacity (meq/g)Time Quaternization(h) Strong Weak Total (% )0 0 1.44 1.44 0

16 0.14 1.30 1.44 1024 0.23 1.21 1.44 1548 0.58 0.86 1.44 40

Gold Uptake by Alkylated Amino-FibersThe next series of experiments was aimed at enhanc-ing the basicity of the amine functionality by meansof alkylation. Table 2 summarizes the results of thealkylation of diaminopropane-containing fibers withmethyl bromide in DMF at different times. The per-centage of strong base amine groups increased withtime.

The effect of the amount ofstrong base groups (10%,15% or 40%) on the absorption gold cyanide from al-kaline solution was then evaluated. A remarkable dif-ference in uptake kinetics was clearly evident within aslittle as the first 4 min. The samples with 40% of thestrong-base groups adsorbed more than 50% of gold,the 10%-sample adsorbed less than 20% of the gold,and the unmodified fibers were able to extract only 5%of gold. The mechanism of gold cyanide uptake bypolyethylene diaminopropane is given in Scheme 2.

10 20 30Time [min]

Figure4. Decrease of gold concentration as a function of time fo rpolyethylene-diaminopropane fibers with different degrees of quat-ernization.

15

10

U

Scheme 2

SO 2NH(CH 2)3NH 2H X + Na[Au(CN)2](aq) --rSO2NH(CH 2) 3N(CH 3)3X

SO 2NH(CH 2)3NH 2Na[Au(CN)2]SO2NH(CH 2) 3N(CH 3) 3 Au(CN) 2 Time [min]

where - represents polyethylene.

Extraction of Gold rom a Mixed Cation SolutionThe ability our quaternized fibers to facilitate fast metalextraction from a solution containing gold or bothgold and copper cyanides at pH - 11 was tested (Figs. 4and 5) . The higher the degree of quaternization, thefaster the metal extraction. Fibers with 40% quat-ernization completely extracted the gold within 5min, while 10 min was needed for fibers with 10%

Figure 5. Decrease of Cu concentration as function of time forpolyethylene-diaminopropane fibers with different degrees of quat-ermnization.

quaternization. These results are in agreement withthe results for gold extraction from solutions contain-ing gold alone. The partially quaternized polyethylene-diaminopropane fibers were also able to extract copperions from aqueous cyanide solutions at high pH. Thedifference between the two cyanide anions was that thegold cyanide extraction was much faster.

The extraction of gold ions by 20% quaternizedpolyethylene-diaminopropane fibers (cut into -0.5 cm


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Table 3. Metals extraction by 20% quaternized polyethylenediaminopropane fibers to extract heavy metals.

Concentration in starting solution (ppm)Time(min) pH Au Cu Zn Co Ni0 11.2 1.4 11.4 3.4 3.5 3.05 11.0 0 0 0 0 0Ion capacity 0.07 0.57 0.17 0.17 0.15(mg/g)

100

8a o

-eam

60

40

20pieces) was also tested in presence of other heavy metalions at high pH . It was found that the extraction of goldwas not retarded by the presence of the whole range ofheavy metals that are usually found in the gold cyanideslurry (Table 3).

0 2 3 4

Number of elutions

Figure6. Degree of elution as function of number of elution stages.ElutionThe elution of gold ions by partially quaternizedpolyethylene diaminopropane fibers was tested withtwo different eluents, 5 N NaSCN or 0.1% NaCN in 1%NaOH aqueous solution. The elution with 5N NaSCNwas carried ou t on 20% quaternized fibers after ex-traction of gold was made in the presence of copperions. The concentration of CN- was 200 ppm; thevolume of feed solution was 30 ml; the volume of elu-ent was 15 ml. The results of extraction and elutionare presented in Table 4. The table shows the changesof metals concentration in the solution with the time.We can see that extraction of gold was accomplishedwithin 5 min (zero concentration), while 30 min wasrequired for copper extraction. The better diffusionperformance of gold cyanide was also revealed in theelution step. Mass balance calculation results, that the

Table 4. Extraction and elution* of Au and Cu by20% quaternized polyethylene-diaminopropane fibers.Extraction Elution

Time Au Cu Time Au Cu(min) (ppm) (ppm) (min) (ppm) (ppm)

0 1.5 12.2 10 4.0 13.65 0 8.1 15 4.0 14.0

10 0 4.615 0 1.730 0 0*With 5 N NaSCN.

gold was completely eluted within 15 min, while only57% of copper was eluted within this time.

Since a mixture of NaOH and NaCN appeared tobe the most suitable eluent for industrial applications(Muir, 1982; Gren and Potgieter, 1984), its efficacy asan eluent was also studied. In this series of experi-ments, a higher weight of fibers (10 g) was taken forloading of gold (1.2 ppm) at pH 11 from 200 cm 3 ofcyanide solution. Elution was performed with a mix-ture of 1g/lNaCN + 10 g/1 NaOH. From Fig. 6 it is evi-dent that complete elution of gold ions can be achievedwith this eluent in a three-step elution procedure. Acontact time of 10 min was sufficient for each step.

ConclusionsA remarkable sorption and desorption performanceof our chelating hollow fibers for a very dilute goldcyanide solution was observed, extraction and elutionbeing achieved within 10-15 min. The physical con-figuration and mechanical stability of the fibers makethem most attractive for the resin-in-pulp gold extrac-tion technology.

ReferencesBelfer, S. and S. Binman, Removal of Heavy Metals from Aqueous

Solutions by means of Hollow Fibers. The Institutes for AppliedResearch, Ben-Gurion University of the Negev, Beer-Sheva, Is-rael, Report No. BGU-ARI-43-92, 1991.

El m Eluentlg/1 NaCN+10 gl1 NaOH

V=15 ml; T=15 min

1


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Belfer, S. and M. Streat, "Gold Extraction with PolymericComposites Prepared by y-Radiation Polymerization Coupledwith Immobilization":' 5th Symp. on Ion Exchange, Hungary,1986.

Belfer, S., S. Binman, and E. Korngold, "Preparation of ChelatingHollow Fibers based on Chlorosulfonated Polyethylene and itsUses for Metal Extraction," Ion Exchange Advances, M.J. Slater(Ed.), p. 17, Elsevier Applied Science, London, 1992.

Granovskyi, A.I., L.S. Ivanova, and R.K. Storodjuk, "Desorption ofGold and Silver from Activated Carbon," Zh . Prikl. Khim., 12,2641 (1982) (in Russian).

Green, B.R. and A.H. Potgieter, "Unconventional Weak Base AnionExchange Resins, Useful for the Extraction of Metals, EspeciallyGold," Ion Exchange Technology, D. Naden and M. Streat (Eds.)p. 626, Ellis Hunwood, London, 1984.

Harris, W.I., I.R. Stanbush, and W.S. Pike, "Medium Base PolyamineIon Exchange Resins for the Extraction of Gold from CyanideSolutions," Ion Exchange Advances, M.J. Slater (Ed.) p. 374,Elsevier, London, 1992.

Korngold, E. and D. Vofsi, "Water Desalination by Ion ExchangeHollow Fibers," Proceedingof the 12th Int. Symp. on Desalinationand Water Reuse, Malta, 1991.

Kotze, M.H. and F.L.D. Cloete, "Ion Exchange Fibers for the Recov-ery of Gold Cyanide," Ion Exchange Advances, M.J. Slater (Ed.),p. 366, Elsevier Applied Science, L ondon, 1992.March, J., Advanced OrganicChemistry,p. 445, Wiley &Sons, NewYork, 1985.Muir, D.M., "Recovery of Gold from Cyanide Solution using Acti-vated Carbon," Symp. CIP Technology.fbr the Extraction of Gold,Melbourne, pp . 2-24, 1982.

Soldatov, V.S., "New Fibrous Ion Exchanges for Purification of Liq-uids and Gases," Stud. Envir. Sci., 23, 353 (1984).

Telegina, L.E., B. Ya Kofman, and K.B. Mazur, "Carbon-in PulpTechnology for Gold Extraction," Soviet J. Non-Ferrous Met.,23(2), 96 (1982).

Tsuchida, N., M. Ricane, an d D.M. Muir, "Studies on the Mecha-nism of Gold Adsorption on Carbon," Symp. on Carbon-in-PulpTechnologyfor the Extraction of Gold, Melbourne, 1984.

Tsumota, S., K. Saito, Sh. Fukusaki, T. Sugo, and J. Okamoto, "MetalCollection using Chelating Fiber Membrane," J.Memb. Sci., 58,221 (1991).

Tye, I. and B.R. Green, "Phenol-Formaldehyde Based Weak-BaseResins for the Recovery of Gold," Solvent Ext. IonEx., 12(4), 817(1994).

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