The extraction of gold from cyanide solutions by strong- and weak-base anion-exchange resins by C. A. Fleming* and G. Cromberget

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1 J. S. Atr. Inst. Min. Metal/., vo!. 84, no. 5. May pp The extration of gold from yanide solutions by strong- and weak-base anion-exhange resins by C. A. Fleming* and G. Cromberget SYNOPSIS This paper desribes the extration of gold, as the anioni auroyanide omplex, from solution by strong- and weakbase anion-exhange resins. The effets, on the rate of extration and the loading apaity for gold, of parameters suh as ph, temperature, ioni strength, agitation, and the presene of ompeting anions are reported. The elution and regeneration of strong- and weak-base resins will be examined in a seond paper. A third paper in this series will present the results of several small-sale pilot-plant investigations arried out with strong- and weakbase resins In yanide pulps from various South Afrian gold mines, and will ompare the metallurgial performane of resin-in-pulp with that of the arbon-in-pulp proess. SAMEVATTING Hierdie referaat beskryf die ekstraksie van goud, as die anioniese ourosianied-kompleks, uit 'n oplossing met sterken swakbasis-anioon-ruilharse. Die uitwerking van parameters soos die ph, temperatuur, ioonsterkte, roering en die aanwesigheid van mededingende anione op die ekstraksie-tempo en die laaivermoe vir goud word aangegee. Die eluering en regenerasie van sterk- en swakbasisharse sal in 'n tweede referaat ondersoek word; 'n derde referaat in die reeks sal weer die resultate aangee van verskeie kleinskaalse proefaanlegondersoeke wilt met sterk- en swak-basisharse uitgevoer is in sianiedpulp afkomstig van verskillende Suid-Afrikaanse goudmyne, en die metallurgiese werkverrigting van die hars-in-pulpproses met die van die koolstof-in-pulpproses vergelyk. Introdution Resin-in-pulp as an Alternative to Carbon-in-pulp Muh attention has been foused at the Counil for Mineral Tehnology (Mintek) on the arbon-in-pulp (CIP) proess for the extration of gold from yanide media. The suessful development of this proess led to a deision that the resin-in-pulp (RIP) proess should be examined as a possible alternative to CIP for gold reovery, beause resins have several harateristis that make them more attrative than ativated arbon. The following are examples. (a) Anion-exhange resins are superior to urrently available ativated arbons with respet to the rate and equilibrium loading of auroyanide. This means that either the resin inventory or the plant will be smaller in the RIP proess than in the CIP proess. (b) Resins an be eluted at room temperature, whereas the elution of arbon has to be arried out at high temperatures, preferably around 12 to 13 C, in a pressure vessel. () Ativated arbon requires periodi thermal reativation for the removal of adsorbed orgtmi materials-a step that is unneessary with resins. (This fator and (b) ould ontribute to redued apital and operating osts in an RIP appliation.) (d) Resins do not load alium arbonate to the same extent as ativated arbon, and the aid-washing step in the CIP proess will therefore probably be unneessary in an RIP appliation. * Assistant Diretor. t Senior Sientist. Both of the Mineral and Proess Chemistry Division, Counil for Mineral Tehnology (Mintek), Private Bag X315, Randburg, 2125 The South Afrian Institute of Mining and Metallurgy, SA ISSN X/33. + (e) Resins do not appear to be 'poisoned' by organi speies suh as flotation reagents, mahine oils and lubriants, or solvents, all of whih an severely inhibit the loading of gold onto ativated arbon. Similarly, speies suh as hematite and shales or lay-type minerals depress the loading of gold onto arbon but have little effet on resins. (f) A possible further advantage of RIP is that South Afria has developed a resin-manufaturing faility, whereas the long-term supply of ativated arbon is less assured. To be weighed against these advantages of resins over ativated arbon are the following disadvantages. (i) Anion-exhange resins are less seletive than ativated arbon for gold yanide over the base-metal yanides that are prevalent in yanide leah liquors. (ii) The partiles of'resin are smaller than those of arbon. Consequently, in RIP the pulp has to be sreened at about 35 mesh with 28-mesh interstage sreening, whereas in CIP the pulp is sreened at 28 mesh with 2-mesh interstage sreening. (iii) The physial strength of resins and their resistane to attrition and abrasion in the absorption tanks are largely unknown. (iv) Resins are less dense than ativated arbon and tend to aumulate at the surfae of the pulp unless it is agitated effiiently. In its projet on RIP, Mintek aims to weigh the advantages and disadvantages of RIP and CIP one against the other, and to establish whether there is a plae for RIP in the South Afrian gold-mining industry. Historial Bakground The use of resins to extrat gold from yanide solutions and pulps was first investigated in South Afria as early as 19611, although the D.S. Bureau of Mines onduted an even earlier investigation2. In the latter study, the use of weak-base resins to reover gold from JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY MA Y

2 yanide solution was examined, but the proess failed owing to the inability of weak-base resins to extrat anions effiiently at the high ph values of yanide leah liquors. The use of resins for the reovery of gold was also advaned by Burstall et al.3 and Burstall and Wells4 in the early 195s. In those studies, it was observed that gold in yanide solution ould readily be absorbed onto strongbase anion-exhange resins together with the omplex yanides of many other metals. By using a series of seletive eluting agents, they3.4 found it possible to remove the various metals suessively and to obtain a number of disrete eluates ontaining relatively high onentrations of the various metals. One of the eluates was an organi liquid that arried virtually all the silver and gold present in the original impure yanide pregnant solution that had been treated. Davison et al. I, using strong-base resins, also demonstrated the potentiality of the RIP proess, and expressed the view that it ould ompete with the onventional zin-preipitation proess provided that the pregnant solution was high in gold and relatively free of ompeting metal yanide omplexes. Their design philosophy was the extration of the auroyanide as seletively as possible in the absorption stages, followed by non-seletive elution of the resin. Pure gold was reovered from the eluate by eletrolysis. The most effiient reovery of gold was obtained when the elution and eletrolysis were arried out in a ontinuous and losed 'eletroelution' yle. Aidified thiourea and ammonium thioyanate were suessfully tested as non-seletive eluants. More reent progress in the development of RIP for gold extration has been made in the Soviet Union and Roumanias-8, and a 1978 report 9 indiates thatone ofthe largest gold mines in the Soviet Union is using an anionexhange resin to extrat auroyanide by an RIP proess. This is believed to have been the world's first appliation of RIP to the extration of gold, although 'RIP is now used widely for gold extration in the Soviet Union' 9. In their proess, non-seletive absorption is followed by seletive elution. Zin, nikel, and yanide are eluted first by dilute mineral aid. This is followed by the elution of opper with an ammoniaal solution of ammonium nitrate and, finally, gold and silver are desorbed and eletrowon ontinuously, with aidified thiourea as the eluant. The unattrative features of the proess developed by the Russians are the following: (i) its omplexity, sine three or four stages of elution are undesirable as regards apital and operating osts, (ii) the use of aid in the first stage of elution, whih onverts iron and obalt on the resin to omplex speies that desorb with diffiulty, and (iii) the signifiant perentage of thiourea (an expensive reagent) that is lost in eah yle. The Canadians reently looked into the possibility of using strong-base resins to.extrat gold from yanide solution 1. They onluded that the non-seletivity of gold absorption, oupled with the high ost of elution (ammonium thioyanate and aidified aetone were tested as eluants), rendered the proess unfavourable as ompared with zin preipitation. In their study, only larified solutions were investigated, and the eonomic benefits to be gained from the bypassing of filtration by an RIP appliation were not onsidered. Mintek first beame interested in resins for the extration of gold in 1975, when RIP was applied to the reovery of soluble gold at Durban Roodepoort Deep Gold Mine 11. Weak-br,se resin was tested in a solution whose ph value had been adjusted to 4, but the proess was abandoned beause of the massive formation of rystalline alium sulphate at that ph value. Strong-base resin was also tested, and it was demonstrated that gold ould be suessfully absorbed and eluted. An elution proedure (whih uses zin yanide as the eluant) proposed by Hazen 12 was tested beause of its ompatibility with the proesses used on most gold mines. That method uses a saturated solution of sodium zin yanide to elute gold from the resin, after whih the gold yanide passes through a bed of zin powder. The gold ements onto the zin, and the zin yanide that is generated is reyled to elution. It was reported 11 that the onentration of zin in a saturated solution of zin yanide was only 4 gjl and that, for this reason, vast volumes (32 resin bedvolumes) of eluate had to be used to elute the gold ompletely. Aordingly, the proess was assessed unfavourably11. In fat, in the present study it is shown that, at ambient temperature, sodium zin yanide solution an be prepared that ontains as muh zin as 3 gjl, whih has plaed the zin yanide elution proedure in a new perspetive. In the present study, whih is desribed in a series of three papers, the whole question of RIP for gold reovery is re-examined in the light of the suessful development of CIP, the advanes in the tehnology of ion-ex{jhange resins, and the hanged eonomi situation brought about by high gold-bullion pries. A onventional CIP plant omprising absorption, elution, and eletrowinning iruits should therefore be readily onvertible to RIP if it an be demonstrated that the RIP proess offers definite eonomi advantages. A further fator in the hanged situation sine the early RIP studies is the rapidly expanding resin market and the availability of a far wider range of strong- and weak-base resins suitable for use in an RIP appliation. The design philosophy in the present study is simpliity, and the omplex multi-elution proedure for strong-base resins proposed by earlier workerss-7 was therefore rejeted. Effiient reoveries of gold and silver an be ahieved by eletrolysis, even when this is preeded by non-seletive absorption and elution steps. Thus, the spent eletrolyte ontains the undesirable base-metal yanides that an be disarded periodially or removed in a bleed stream to regenerate the eluate. Extensive tests were arried out on a variety of strongand weak-base resins. These tests inluded laboratory experiments, whih are reported in this and the seond paper in the series, and small-sale pilot-plant investigations, whih are reported in the third. In the laboratory experiments, the effets on gold absorption of parameters suh as the ph value of the solution, temperature, agitation, and the presene of ompeting ions were determined. In addition, several reagents were tested 126 MAY 1984 JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY

3 under various onditions to establish optimum elution proedures for both types of resin. The Chemistry of Absorption and Elution The funtional group of a strong- base resin is a quaternary amine possessing a permanent positive harge. Gold yanide and other anions are therefore extrated by an ion-exhange reation, whih is shown here for auroyanide: I - NRa X- + Au(ON)"2"+- I -NRa Au(ON)2"+ X, , (1) where the symbol I - denotes the inert bakbone or matrix of the resin, whih is generally polystyrene. Other anions that our ommonly in yanide leah liquors are Ni( ONtt Co( ON) :;- Ou (ON).a,-Zn( ON);,-Fe (ON) :;-Fe( ON)s;. all of whih are extrated by a strong- base resin aording to an ion-exhange reation similar to that depited in equation (I). The omplex metal yanides on the resin an be eluted from the resin either by ion exhange, in whih the equilibrium depited in equation (I) an be fored to the left-hand side by an inrease in the onentration of X, or by onversion of the metal ions to non-anioni omplex speies. An example of the latter tehnique involves the thiourea ligand, whih, in the presene of strong mineral aids, forms a ationi omplex with gold, aording to equation (2): + I - NRa Au(ON) + 2(NH2)2 OS H2SO4 - I - NRa HSO [Au {(NH2)2 OS}2] HSO4 + 2HON..,. (2) As examples of the ion-exhange elution tehnique, anions suh as hloride, bisulphate, nitrate, thioyanate, or yanide an be used to reverse the ion-exhange equilibrium shown in equation (I). However, gold yanide is loaded far more strongly than these anions and, for the equilibrium to be shifted to the left-hand side of equation (1), the ativity of the eluant anion has generally to be inreased by the addition of a polar organi solvent suh as aetone or aetonitrile to the eluate. This forms the basis of the organi type of elution proedures that were tried in the pasta,4. Alternatively, an anion an be hosen that is absorbed onto a strong-base resin more strongly than auroyanide, in whih ase the equilibrium in equation (I) would lie naturally to the left-hand side. One suh anion, whih was tested and evaluated in this investigation, is the zin yanide anion Zn(ON)::- Beause this anion is absorbed so strongly onto a strong-base resin, the resin has to be regenerated before it is reyled to absorption. This an be arried out onveniently by treatment of the resin with a dilute mineral aid beause the zin yanide anion deomposes in aid solution. The elution of a strong- base resin with zin yanide followed by aid regeneration of the resin is depited in equations (3) and (4): I - NRa Au(CN); + Zn(ON) ( I - NRa)2 Zn(ON)4:+ 2 Au(ON)2'.... (3) and + 2- ( I - NRa)2 Zn(ON)4 + 3H2SO I - NRa HSO4 + ZnSO4 + 4HON.... (4) Weak-base resins ontain primary, seondary, or tertiary amine funtional groups (or a mixture of these), and the ion-exhange properties of weak-base resins are governed by the ph value of the solution in ontat with the resin, as an be seen from equation +- + (5): I - NR2 + HX - I - NR2HX. (5) In aid solution, the equilibrium lies to the right-hand side of equation (5), and the weak-base resin behaves like a strong- base resin, i.e. for the absorption of auroyanide, + - I - NR2 HX- + Au(ON); +- + I - NR2HAu(ON); + X (6) The great advantage of weak-base over strong-base resins is that they an be eluted by simple aid-base hydrolysis; that is, at high ph values, the equilibrium in equation (5) lies to the left-hand side, and the funtional groups of the resin, in the free-base form, are no longer able to hold anions. The elution of gold yanide from a weak-base resin by hydrolysis is illustrated (7): in equation I - NR2HAu(ON)2 + OH - I - NR2 + Au(ON);. (7) Weak-base resins an also be eluted in aid solution by an ion-exhange reation similar to that depited in equation (3), but in pratie the reation shown in equation (7) is very muh more effiient and is also simpler and heaper, and so would generally be preferred. For most ommerial weak-base resins, the pka, whih is defined as the ph value at whih 5 per ent of the funtional groups of the resin are protonated, is in the range 6 to 8. Therefore, in the ph range 1 to 11, whih is ommon for most gold yanide leah liquors, the equilibrium in equation (5) would be to the left-hand side, with the resin predominantly in the free-base form. In the past, this was the most serious limitation of the proess using a weak-base resin, and required that the ph value of the yanide solution in ontat with the weak-base resin should be adjusted to a point where the equilibrium in equation (5) lies substantially to the righthand side. In reent years, onsiderable suess has been ahieved in the development of new weak-base resins that absorb gold yanide effiiently between ph values of 8 and 1, and this was one of the major fators that promoted re-examination of the RIP proess based on the use of weak-base resins. Gold and silver an be reovered from all these eluates by eletrolysis with deposition of the metal (and the evolution of hydrogen) at a steel-wool athode, and the evolution of oxygen either at a stainless-steel anode (for alkaline eluates) or at a lead anode (for neutral or aidi eluates). Experimental Work Solutions and Materials In the experiments designed to show the effets of JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY MA Y

4 temperature, ph, ioni strength, agitation, and gold onentration on the rate of loading and the equilibrium loading of auroyanide on strong-base resins, a solution was prepared by the dissolution of potassium auroyanide (supplied by Johnson Matthey Limited) in water from a Millipore purifiation system. Adjustments to the ph value of the solution were made with ommerialgrade sulphuri or hydrohlori aids or sodium hydroxide, and adjustments to the ioni strength of the solution were made with sodium hloride. In the experiment designed to show the effet of ompeting ions on the equilibrium absorption of auroyanide onto strong- and weak-base resins, pregnant solutions from the plants at Grootvlei and Durban Roodepoort Deep Gold Mines were used. Finally, in the experiment designed to show the seletivity of strong- and weak-base resins for gold yanide over a variety of base-metal yanide omplexes as a funtion of ph, a stok solution was prepared that ontained eah of the following metal ions: gold, silver, opper, nikel, zin, obalt, and iron, and an exess of free yanide ions, at a molarity of approximately 1-4. Aliquot portions of this solution were adjusted to ph values in the range 2 to 11, and all the base metals and silver were dissolved as the analar-grade metal nitrates. Details of the various resins tested are given in Table 1. Unless stated otherwise, the samples of resin were onverted to the bisulphate form (by treatment with sulphuri aid) before use, and the temperature at whih the experiments were arried out was 22. Bath Equilibrium Tests Equilibrium isotherms were determined under onditions that resembled, as losely as possible, those that would be enountered in a ounterurrent RIP plant or in a ontinuous ion-exhange (OIX) olumn. In these appliations, the ratio of the flowrate of solution to that of the resin is generally between 5 and 1 to I. This ratio was maintained in the bath equilibrium tests designed to- show the effet of oompeting ions on the absorption of gold yanide. To 5 to 1 ml of resin, 5 litres of a plant pregnant solution was added and brought to equilibrium overnight on a roller. The solution was analysed (by atomi-absorption spetrophotometry) for the various metals at the start of the reation and again at equilibrium, the onentration of the metal ions on the resin being determined by differene. This gave one point on an isotherm for eah of the metals. The same sai.llple of resin was then added to 5 litres of fresh pregnant solution and brought to equilibrium one more, thus giving a seond point on the isotherm. This proedure was repeated six or seven times to produe an isotherm overing the full range of metal-ion onentrations in solution and on the resin that would be found in a typial ounterurrent appliation. In the determination of the influene of solution ph on the seletivity of strong- and weak-base resins for 8uroyanide, an exess of a stok solution ontaining equimolar onentrations of various metal yanide omplexes was pumped slowly in a single pass through a bed of resin in a olumn until equilibrium was attained. The solution and resin were then analysed so that the equilibrium loading ould be determined. This experiment was repeated at various ph values in the range 2 to 11. The strong-base ontent of weak-base resins was determined by treatment of the weak-base resins with an exess of sodium hydroxide. This treatment auses the strong-base groups to 18d hydroxyl ions while the weakbase groups revert to the free-base form. The hydroxyl ions are then eluted from the resin with sodium hloride and analysed by aid-base titration to give a measure of the strong-base apaity of the resin. The weak-base apaity of a resin was determined by protonation of the weak-base groups with a mineral aid, followed by titration of the resin with a standard sodium hydroxide solution. The Kinetis of Absorption The rate of loading and the equilibrium loading were measured under onditions simulating those in a ounterurrent extration proess, i.e. onditions in whih the onentration of gold yanide in solution remains approximately onstant as the loading of gold on the resin inreases and approahes its equilibrium value. Details of the experimental tehnique have been desribed elsewhere 13 for the loading of uranyl sulphate on a strongbase resin. Basially, the tehnique involves ontinuous monitoring of the onentration of gold in solution, gold being added to the reation vessel from an external soure at the same rate at whih it is being extrated from solution by the resin, thus maintaining a onstant onentration of gold in solution. The rate at whih gold is added from the external soure therefore yields a kineti DETAILS OF THE VARIOUS STRONG- AND WEAK-BASE RESINS USED IN THIS STUDY Resin Manufaturer Funtional group Matrix IRA4 Rohm & Haas Quaternary amine Polystyrene-DYE AIOlDU Diaprosim Quaternary amine Polystyrene A7 Diaprosim Seondary amine Phenol formaldehyde A3B Diaprosim Tertiary amine Epoxy polyamine A378 Diaprosim Tertiary amine Polystyrene-DYE IRA68 Rohm & Haas Tertiary amine Aryli amine IRA93 Rohm & Haas Tertiary amine Polystyrene-DYE A35 lona Polyamine* Epoxy amine A26 lona Polyamine* Aliphati amine MGl lonae Polyamine* Aryli-DVB * A mixture of primary, seondary, tertiary and, in some ases, small amounts of quaternary amine groups. DVB =- DivJnylbenzene 128 MA Y 1984 JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY I

5 profile of the reation, while the total amount of gold added from the external soure yields a measure of the equilibrium loading. The strong absorption of the auroyanide ion in the ultraviolet spetrum (239 nm) offers a onvenient method for monitoring of the gold in solu- ion at onentrations higher than 1 to 2 mgjl; for lower gold onentrations, a radioative gold isotope and isotope-ounting equipment were used. Two reation vessels were used: a olumn of 1 mm ross-setional diameter in whih the resin was fluidized by pumping of the gold yanide solution in an up-flow diretion through the bed, and a ylindrial, waterjaketed ell in whih the resin was agitated by an impeller driven by a Heidolph RZR 5 variable-speed motor. Absorption on Stron-base Resins The Effet of Gold Conentration The rate of extration of gold yanide over a range of onentrations of gold in solution was obtained from the following expression; t d[au]r [Au]R - [AU]R VEOE - - t VRt '.(5) where [Au]and [Au]; are the onentrations of gold on the resin at time t and respetively, V E is the volume of onentrated gold solution added from the external soure, OE is the onentration of gold in the external soure, and V R is the volume of resin in the reation vessel. For the first 2 to 3 minutes of reation, the rate was always onstant and independent of the onentration of gold on the resin. The results given in Tables Il to VI refer to this initial first-order reation. Results that show the rate of extration and the loading apaity as a funtion of the onentration of gold in solution are given in Fig. 1. These show that the rate of loading is first-order in the onentration of gold in solution over the range of onentrations studied; they Condltlona: 1,4 roiangth ::g1 M Partilealze 1realn <O,5>O,42mm 1,2 1, E!i,8!! o '6 ],6! :5,4 Theoretial maximum -_!!!.!.!,) also show that extremely high gold loading an be ahieved WftelrROompeting ions are present in solution. The Effet of ph The rate of loading and the equilibrium loading were examined at various ph values in the range 2 to 12. The solution ontained potassium auroyanide, and adjustments to the ph value were made with sodium hydroxide and hydrohlori aid. The ioni strength was kept onstant at,1 M by the addition of sodium hloride. The results are presented in Table Il and show that, over this range, the ph value has no effet on the rate or the equilibrium loading. THE EFFECT OF SOLUTION ph ON THE LOADING OF AUROCYANIDE ONTO A STRONG-BASE RESIN (ma4) Partile size of resin <,59 >,5 mm Gold in solution 1 mg/l Ioni strength,1 M n.d. ph value Initial loading rate Loading apaity (g/l)/min g/l = 2,1, ,, ,, ,,22 n.d. 6,, ,, ,1, ,, not determined The Effet of Ioni Strength The ioni strength of an auroyanide solution was varied from near to,5 M by the addition of sodium hloride, and the effet on the loading rate and the equilibrium loading was examined. The results (Table Ill) show that ioni strength has little effet on the rate of loading, but a fairly signifiant effet on the equilibrium loading. This is due to the inreased ompetition between Au(CN)2"and Cr-anions for ative sites on the resin. The Effet of Temperature The loading of gold onto a strong-base resin was examined at various temperatures in the range 3 to 8DC. No bakground eletrolyte was added to the neutral syntheti auroyanide solution. The results (Table IV) show that the rate of loading inreases with temperature III 'a. 1: THE EFFECT OF IONIC STRENGTH ON THE LOADING OF AUROCYANIDE 1 ONTO A STRONG-BASE RESIN (IRA1O1DU) Partile size of resin <,5 >,42 mm Gold in solution 1 mg/l ph value 6 Il,2 5 Ioni strength Initial loading rate Loading apaity M (g/l)/min g/l Gold In oolutlon, mgll 4 Fig. I-Rate of extration and loading apaity of auroyanide onto a strong-base resin (IRA4) as a funtion of gold in solution O,24 216,2,26 191,1,32 142,5,31 99 JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY MAY

6 (an Arrhenius plot reveals an ativation energy of 16,5 kjjmol), while the loading at equilibrium is slightly exothermi in nature. However, this latter effet is small ompared with the exothermi absorption of auroyanide onto ativated arbon, as shown in Table V. IV THE EFFECT OF SOLUTION TEMPERATURE ON THE LOADING OF AUROCYANIDE ONTO A STRONG-BASERESIN (IRA4) Partile size of resin <,59 >,42 mm Gold in solution 7,4 mg/l Ioni strength ph value 6 Temperature Initial] oading rate Loading apaity C (g/l)/min g/l 3, , , , , ,84 84 THE EFFECT OF SOLUTION TEMPERATURE ON THE LOADING OF AUROCYANIDE ONTO ACTIVATED CARBON (METSORB11) Partile size of arbon <,59 >,5 mm Gold in solution 3 mg/l ph value 1,4 Temperature Initial loading rate Loading apaity C (g/l)/min g/l 24, , , ,69 9 The Effe1 of Agitation The influene of mixing effiieny on the rate ofloading was examined in a fluidized-bed olumn and in a stirred ell. In the olumn, the solution was irulated at various flowrates, large-bead resin being used to maintain a stable fluidized bed over as wide a range of flowrates as possible (Table VI). The dependene of the loe-ding rate on the flowrate of the solution is indiative of a proess ontrolled by film diffusion. THE EFFECT OF SOLUTION FLOWRATE THROUGH A FLUIDIZED-BED COLUMN ON THE RATE OF EXTRACTION OF AUROCYANIDE BY A STRONG-BASE RESIN (IRA4) Partile size of resin < 1, >,85 mm Gold in solution 7,4 mg/l ph value 6 Ioni strength Inner diameter of olumn 1 mm V VI In the stirred ell, the stirring speed, ph value, ioni strength, and onentration of gold in solution were varied. These variations enabled the isolation and identifiation of areas in whih film-diffusion ontrol on the one hand, or intrapartile-diffusion ontrol on the other, are dominant. In this regard the following guidelines are relevant. (a) For proesses ontrolled by film diffusion, the rate inreases with improved mixing effiieny, whereas under ontrol by partile diffusion mixing has no effet on the rate. (b) Under onditions of film-diffusion ontrol there is a first-order dependene of the rate on the onentration of gold in solution; under partile-diffusion ontrol, this dependene ould be somewhat less than first-order. () The ph value and the ioni strength of the solution will have little effet on the rate under onditions of film-diffusion ontrol but ould have a marked effet under onditions of partile-diffusion ontrol. The effet of the onentration of gold in solution on the initial rate of loading at different stirring speeds is shown in Fig. 2. The straight-line log-log relationships of unit slope indiate that the rate has e- first-order dependene on the onentration of gold in solution between,8 and 1 mg/l, and that the rate inreases with inreasing stirring speed. These results point to film-diffusion ontrol under these onditions. The effet of stirring speed on the initial loading rate at different ph ve-iues and ioni strengths is shown in Fig. 3. Under eah of the three sets of onditions, the rate inreases with inreasing stirring speed, one again pointing to at least a partial ontribution from film-diffusion ontrol. The dependene of the rate on stirring speed dereases with dereasing ph and with inreasing ioni strength. (Partile-diffusion rates derease under these -1, 'i' o :;;.e! -2, #j. Partile size 1resin <1.OO>O,85mm ph value 1 solution B,O if'.- Ioni strength 1 solution -.p,. Stirring speed 5 r/mln. Boor/mln. 13OOr/mln ['] 2oor/mln 4 l/h Flowrate of solution m/s,4,14,6,21 1,4,49 2,5,87 4,9 1,74 5,7 6,2 2,2 2,19 Initial loading rate (g/l)/min,51,57,54,62,75,82,93-3, -1, 1, 2, log [Aul. Fig. 2-Effet of the onentration of gold in solution on the rate of loading of gold yanide onto a strong-base resin (IRA- 4) at different stirring speeds 13 MA Y 1984 JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY

7 1,1 1,,9 S 1ii ;;.,8 :;; ;:,7 :s,6,5 Gold in soiution Partile size 1 resin 5Omg/l <1,>O,85mm ph = 8: Ioni strangth = ph = 6: Ioni strength =,1M. ph = 2: Ioni strength =,1 M ;;,. ; I!! ID = g 8 iị. ::E ConditIons: Ratio 1 solution to resin ph value 5:1 11,5 Temperatura =2oC Stirring speed, rlmln Fig. 3-Effet of stirring speed on the initial rate of extration of auroyanide by a strong-base resin (IRA4) 2., 4 onditions.) At 5 r /min the rate is apparently ontrolled entirely by film diffusion (sine ioni strength and ph have little effet on the rate), whereas at 2 r/min it is probable that both film and partile diffusion ontribute to the loading mte. The broad onlusion to be drawn from these results is that film diffusion influenes the rate of gold extration to a greater or lesser extent under all the onditions likely to be enountered in RIP, Effiient mixing is therefore a most important riterion in the design of large-sale mixers for RIP absorption. The Effet of Competing Ions The equilibrium loeding of gold, silver, end several base metals in pregnent yanide solutions from two gold mines onto two ommerial strong-base resins is shown in Figs. 4 and 5, The trends in the two sets of isotherms are similar and indiate that opper, nikel, and zin are more strongly absorbed than gold, wherees obalt, silver, and iron are less strongly absorbed. The results in Figs. 4 and 5 are fairly representative of a general seletivity trend with minor variations that has been observed in the laboratory for a wide variety of plant pregnant solutions and different strong-base resins, The onlusions that an be drawn from this trend is that, for gold to be effiiently extrated from a plant pregnant solution, it is neessary for all the opper, nikel, and zin in solution to be extrated. This was generally onfirmed in the pilot-plant investigations 14. The stability of the gold yanide omplex, whih is greater than that of most base-metal yanide omplexes, offers the possibility that the gold loading an be improved by seletive preipitation of the base-metals from solution at low ph values. The order of stability15 of the yanide omplexes that are ommonly found in gold leah liquors is Co3+(log fi6 64) > Au+(log fi2 = 47) > Fe3+(logfi6= 44) > Fe2+(log fi6 = 35) > Ni2+(log fi4 = 3) > Cu+(log fi4 = 28) > > Zn2+(log fi4 = 19). At ph values between 6 and 3, the yanide omplexes of zin, nikel, and opper break Iron,1,2,3,4,5,6 Metal in solution, mgll Fig.4--Equilibrium loading of various metal yanide omplexes from a Grootvlei pregnant solution onto a strongbase resin (AIOIDU) 25 2 ii. 15. C I!! ID = iii ;; 1 :I Ratio 1 solution to resin 1:1 ph value 7, Temperature =2 C,2,4,8,8 1, 1,2 1,4 1,6 Metal in solution, Fig. 5-Equilibrium loading of various metal, yanide omplexes from a Durban Roodepoort Deep pregnant solution onto a strong-base resin (IRA4) mg/i JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY MA Y

8 1 Cobalt ;I. :f Q. 9 E " G 8 "' 7 " ' C 6 Fame,2 5 ;; a Copper.5 4 o ' '; 3 E I! 1ii ;; down to form insoluble neutral yanide speies in weakly aidi solutions, whih then re dissolve as the ationi metal salts in more strongly aidi solution. The results plotted in Fig. 6 show the effet of ph on the onentration of the anioni yanide omplex in solution. The effet of seletive base-metal preipitation on gold loading is shown in Fig. 7, where a syntheti solution ontaining a 1-4 M onentration of eah metal in exess yanide was adjusted to various ph values in the range 2 to 11 prior to equilibration with a strong- base resin. It an be seen that the loading of the gold improved signifiantly at ph values below 7. This is learly not an effet of ph per se (Table II), and an be due only to the fat that there is less ompetition from the base metals that have preipitated. The sharp derease in the zin loading between ph values of9 and 7 provides an explanation for j 4. the fat that zin loads very strongly from the Grootvlei pregnant solution at a ph value of 11 (Fig. 4), but only j marginally better than gold from the Durban Roode- if poort Deep pregnant solution at a ph value of 7 (Fig. 5). The loading of iron is anomalously high ompared with the loadings in Figs. 4 and 5. This is beause the simula- 3, tion of iron yanide loading is diffiult in the laboratory. i! It depends on the ratio of Fe(lI) to Fe{III) and also on!:i!': the formation of double metal yanide omplexes, whih our at lower ph values and Iow onentrations of free yanide. Nikel 8 ph Zin Total onentration of eah metal 1O-'M Temperature 2.C Fig.6-Effet of aidity on the stability of various metal yanide omplexes in aqueous solution 1 determination of the ion-exhange harateristis of a weak-base resin. Moreover, the effet of ph on the ion-exhange properties of a weak-base resin varies onsiderably from one resin to another. This aspet of the performane of weak-base resins was therefore examined in some detail. The effet of the ph value of the solution on the rate of loading and the loading apaity of auroyanide onto a weak-base resin was determined in the following experiment. Resin was added to a syntheti gold yanide solution in the stirred ell and, before the experiment started, the resin and solution were adjusted to the same ph value (in the range 2 to 1) to minimize drift in ph during the ourse of the experiment. Adjustments to the ph value were made with hydrohlori aid or ammonia, and the ioni strength was maintained at a onstant level by the addition of ammonium hloride. (Ammonia wp,s used rather than sodium hydroxide beause the sodium ion interferes in the detetion of gold by ultraviolet spetrosopy.) The effet of ph on the rate of loading and on the loading apaity onto two weak-base resins, A7 and MGl, was examined, and the results (Table VII) indiate that the rate of loading is more or less independent ofthe ph value ofthe solution at levels below about 8, and is similar to the rate ofioading onto a strong-base resin under the same onditions (Table II). In ontrast, the loading apaity is strongly dependent on the ph value of the solution, inreasing with inreasing aidity, as might be predited on the basis of equation (5). It is of interest that the maximum loading of gold ontoa7, whih ours at a ph value of6, is equal to about one-quarter of the loading onto a strong-base resin under 5, 2, Temperature 2.C I Absorption of Weak-base Resins The Effet of ph on Absorption In most respets, a weak-base resin behaves similarly to a strong-base resin with the same matrix. This is true of parameters suh as temperature, agitation, and ioni strength. However, the ph value of the solution, while playing little part in the mehanism of absorption on a strong-base resin, is the most import.ant parameter in the 1, ph Fig. 7-Effet of ph on the seletivity of a strong-base resin (AIOIDU) for metal yanide omplexes 132 MAY1984 JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY

9 ph THE EFFECT OF ph ON THE LOADING OF AUROCYANIDE ONTO Partile size of resin Gold in solution Ioni strength WEAK-BASE Initial loading rate Loading apaity (g/l)/min g/l A7 MGI A7 MGl 2,22,23 9,4 5,2 4,25,25 15,5 7,4 6,23,19 3, 8,6 8,22,22 17,7 11, 9,2,19 3,5 4, 1O <,2,11 2,,9 VII RESINS <,7>,42 mm 1 mg/l,1 M the same onditions. The derease in loading at low ph values is possibly due to ion-pairing of the weakly aidi auroyanide anion, although the reason why a similar effet is not seen with strong-base resins is not easily explained. In ontrast to strong-base resins, where the hemial performane differs only slightly from resin to resin, the apaity and the ph funtionality of weak-base resins differ widely from one resin to another. As an example of the wide variation in gold-loading apaity, isotherms for several weak-base resins are shown in Fig. 8. In this appliation, the pka is also a most important property of a weak-base resin sine, the higher the pka, the higher the ph value at whih gold extration will o.ur and, onsequently, the lower the onsumption of aid to redue the ph value of the gold yanide feed solution to an optimum level. The results in Table VII indiate that resins A7 and MGl have pka values in the ;;; C 6! A7 ph value 7 Ioni strength - Tempereture -2ODC Gold In solution, A375 A3O5 MG1 MG3 IRA68 Fig.8-Equilibrium loading of auroyanide onto various weak-base resins from a syntheti solution ontaining potassium auroyanide alone mall range 8 to 9 with maximum loading at ph values in the range 6 to 8. In fat, these results are fairly typial of a wide range of ommerial weak-base resins that were examined. These pka values are some 1 to 2 ph units higher than the values reported by the manufaturers, whih are generally determined by absorption titration with a strong aid suh as hydrohlori aid. This is probably due to the fat that, in addition to the protonation of the base, ion pairs are formed to whih the anion of the aid that has been titrated ontributes deisively. Thus, ion-pair formation between the large polymeri amine ation and the large, very stable, and highly polarizable anion Au(CN)2" will be far stronger than the ion pair formed between the amine and the hloride ionl6. The driving fore of ion-pair formation will therefore permit proton uptake from a more alkaline solution in the ase of Au(CN); than in the ase of C In reent work at Mintekl7, a resin was developed that has a pka value of more than 1; this ould be a signifiant advane in the development of a proess using a weak-base resin to extrat auroyanide. The Effet of Strong-base Funtional Groups Another property of a weak-base resin, whih an have a profound influene on its performane in this appliation, is the presene of strong-base funtional groups within the weak-base resin. These are quaternary amine groups, and are formed when two adjaent tertiary amine groups ross-link. Their ion-exhange properties are the same as those ofthe funtional groups in a strongbase resin, i.e. the absorption of anions on these groups is largely independent of the ph value of the solution. The impliation is that the auroyanide that is loaded onto the strong-base groups during absorption is not eluted by hydroxide ions in the elution yle. This gold is therefore returned to the next absorption yle, and this would result in poor extration effiieny. The weak-base and strong-base apaities of several ommerial weak-base resins are shown in Table VIII. In the determination of the effet that the strong-base apaity of a weak-base resin has on the ph funtionality of absorption and elution, the equilibrium loading of auroyanide on the resins was determined as a funtion of the ph value of the solution. The results are shown in Figs. 9 and 1. For the results shown in Fig. 9, the experimental on- I Weak- Proportion Resin base apaity of total eq/i % A7 1,91 99,4 A3B 1,99 85,3 A378 1,24 83,5 IRA68 1,57 97,6 IRA93 1,3 94,2 A35 1,91 88,8 A26 2,61 97, MGl 2,45 96,1 VIII CIAL WEAK-BASE RESINS THE STRONG- AND WEAK-BASE CAPACITIES OF SEVERAL COMMER- Strongbase apaity eq/i,12,343,245,38,8,24,8,98 Proportion of total %,6 14,7 16,5 2,4 5,8 11,2 3, 3,9 JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY MAY 1984 t33

10 ditions were arranged so as to yield less than 1 per ent utilization of the apaity of the resin, and it an be seen that gold is loaded very seletively onto the strong-base groups under these onditions; in fat, to suh an extent that resins IRA93 and A378 behave like strong-base resins, and the ph value has no effet on the loading of the gold. A7 is the only resin that would be fully eluted with sodium hydroxide (ph value greater than or equal 4 to 13), and IRA68 would also exhibit good elution harateristis. This is onsistent with the low strongbase ontents of these two resins (Table VIII). The results shown in Fig. 1 were obtained by the utilization of 5 to 1 per ent of the apaity of the resin. j 3 It an be seen that, at these higher loadings, the loading ;; r of gold onto eah of the resins showed a dependene on I! the ph value of the solution. The magnitude of the poten- l tial problem is most evident, however, in the results for: IRA93. Here the loading of about 3 g/t at high ph 8 2 values, whih is the amount of gold e.bsorbed irreversibly with respet to ph, is some 5 to 1 times greater than the loading one would normally ahieve in the treatment of pregnant solutions from gold plants. Moreover, the loading of 3 gjt represents only about 5 per ent utilization of the strong-base groups in this partiular resin. This weak-base resin would therefore behave more like a strong-base resin in most appliations, and the advntage of effiient, heap elution would be lost entirely r ";;; I! :: " " 4 CondItIons: Volume 1 solution Maaa 1 resin Initial onentration 1 gold In solution Ioni strength Temperature 1ml 1,Og 1mg/l,1 M lodc A ph IRA93, Fig. 9-Effet of ph on the equilibrium loading of auroyanide onto various weak-base resins (1, g of eah) from a solution, ontaining potassium auroyanide alone A MA Y 1984 JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY 5 1 Volume 1 solution Mass 1 resin Initial onentration 1 gold In solution Ioni strength Temperature 1ml,1 9 1mg/l,1 M 2DC ph Fig. IO-Effet of ph on the equilibrium loading of auroyanide onto weak-base resins (,1 g of eah) from a solution ontaining potassium auroyanide alone Experiments have shown that the ross-linking between adjaent tertiary or seondary amine groups, whih reates the strong-base groups in a weak-base resin, an be partly eliminated by fairly simple hemial means. Thus, heating of IRA93 in,1 M sodium hydroxide solution at 7 for 48 hours redued the strong-base ontent of the resin from 5,8 to 2, per ent. It is signifiant that the weak-base apaity of the resin was not affeted by this treatment, Le. there was no evidene of degradation ofthe tertiary amine groups. The Effet of Ioni Strength The ioni strength of e.n auroyanide solution was varied from to,5 M by the addition of sodium hloride, and the effet on the loading rate and the equilibrium lo8ding on A26 was examined. The results (Table IX) indiate that, as with strong-base resins, ioni strength has little effet on the rate but has a major influene on the loading apaity owing to the ompetition between hloride and auroyanide ions for funtional groups on the resin. The Effet of Agitation Auroyanide solution was pumped up-flow at various flowrates through a olumn ontaining a fluidized bed of the resin A 7, and the rate of gold extration was measured as a funtion of flowrate. The results (Table X) show that the initial loading rate varies with agitation, whih ir onsistent with a rate of loading ontrolled by film diffusion.

11 THE EFFECT OF IONIC STRENGTH ON THE LOADING OF AUROCYANIDE ONTO A WEAK-BASE RESIN (A26) Partiles size of resin <,5 > Gold in solution 1 mg/l ph value 4 Ioni strength Initial loading rate Loading <,Japaity M (g/l)/min g/l,22 14,5,28 91,1,2 57,5,21 1 IX,42 mm i 8 8 Ratio 1 solution ph alue to "'sin 5:1 7 Tampandu", 2oC As the gold loading approahes the equilibrium apaity of the resin, the ontribution from slow partile diffusion to the rate of extration inreases. So that a distintion ould be made between areas in whih filmdiffusion ontrol and partile-diffusion ontrol are dominant, the loading of gold onto MG1 was allowed to approah its equilibrium value in the stirred-ell reator, and the rate of loading was monitored antinuously. The results, whih are given in Fig. 11,indiate that the loading is ontrolled by film diffusion at gold loadings of less than 4 gjl (about 3 per ent of the loading apaity under the hosen onditions). At higher loading, the rate dereases owing to an inreasing ontribution from slow partile diffusion in the loading mehanism. E Flowrate of feed Resin-bed Initial loading solution expansion rate m/s % (g/l)/min,2,15,53 5,15 1,4 1,2 1,58 15,25 1,95 2,28 2,21 25,32 2,44 3,36 i t,1 ], -.,,h" '" lo"ptloo"'" "1< on," "'"-. THE EFFECT OF AGITATION ON THE RATE OF EXTRACTION OF GOLD CYANIDE BY THE WEAK-BASE RESIN A7 Partile size of resin <,5 > Gold in solution 1 mg/l ph value 6 Column diameter 1 mm G"d X,42 mm 1,'" Fig. I I-Effet of gold loading on the rate of extration of auroyanide by a weak-base resin (MGI),.! 4 a S ii :E 2,2,4,6,8 1, 1,2 1,4 1,8 Matal In solution, Fig. 12-Equilibrium loading of various metal yanide omplexes from Grootvlei pregnant solution onto a weak-base resin (A7) Interruption tests to provide evidene of this effet are a onvenient way by whih film diffusion and partile diffusion an be distinguished. During the loading yle, the absorption proess is interrupted for a period when the resin is separated from the feed solution. If the loading is under partile-diffusion ontrol, the period of interruption allows time for the higher onentration of gold in the peripheral regions of the resin to be distributed evenly throughout the resin matrix. The rate of loading immediately after the interruption should therefore be faster than it was before. If, on the other hand, the loading is under film-diffusion ontrol, the gold will always be evenly distributed throughout the resin matrix, and the period of interruption should have no effet on the rate of loading. Under the onditions of the experiment, the results in Fig. 11 are therefore onsistent with partile-diffusion ontrol at gold loadings greater than 4 gjl and with film-diffusion ontrol at loadings below 4 gjl. The Effet of Competing Ions In an examination of the loading of gold and various base metals onto the weak- base resin A 7, plant pregnant solutions from Durban Roodepoort Deep and Grootvlei Gold Mines were used, the isotherms being determined at a ph value of 7 (Figs. 12 and 13 respetively). The patterns of the isotherms are very similar to those obtained with strong-base resins, with zin, opper, and nikel generally being loaded more strongly than gold, obalt, iron, and silver, although the loading apaity of most of the metal omplexes is about one-half of that for strong- base resins under the same onditions. In a omparison of Figs. 4 and 12 with Figs. 5 and 13, it is interesting that obalt was loaded far better from mgn JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY MAY

12 8 8 ;. u; I!! :5 4 'ii i :I 2 Ratio of solution to resin 1:1 ph value 7 Temperature = 2.C,2,4,8,8 1, 1,2 Metal In solution, mg/l Fig. IJ-Equilibrium loading of various metal yanide orn. plexes from Durban Roodepoort Deep pregnant solution onto a weak-base resin (A7) the Grootvlei than from the Durban Roodepoort Deep pregnant solution. A possible reason for this ould be that the Grootvlei solution was inadvertently allowed to 'age' for several weeks before the loading experiments were arried out, whih would have inreased the onentration of the Co(CN):speies in solution at the expense of the Co(CN)s-speies18. It is also of interest that the seletivity of weak-base resins for gold and silver yanide over other ommon metal yanide omplexes is greatly enhaned at high ph values, as illustrated by Table XI. For these results, a. solution ontaining gold, silver, obalt, opper, nikel, zin, and iron (1-4 M eah), and an exess of yanide was pumped through a olumn of resin until equilibrium was attained. The experiment was repeated on four weak-base resins and one strong-base resin (AIIDU) at the two ph values of 11 and 6. The effet is best illustrated by the results for IRA93, whih show that the distribution oeffiients for gold and silver are very similar at both ph values, whereas the distribution oeffiients for obalt, opper, nikel, and iron are 1 to Nikel 2 times lower at a ph value of 11 than at a ph value of 6. These results form a ontrast with those for the strongbase resin, AIO1DU, whih shows no seletivity for gold or silver at either ph value. This effet might be due to a. redution in the harge (funtional-group) densityofthe weak-base resins at high ph values, whih would have Gold less effet on the single-harged gold and silver yanide anions than on the doubly and triply harged yanide omplexes of obalt, nikel, opper, iron, and zin. These latter ions require two or more positively harged funtional groups to be in proximity to one another to ahieve eletrial neutrality. At ph values greater than about 13, all the weak-base funtional groups will be in the freebase form, and the ion"exhange properties of the resin will be determined by the strong- base ontent of the resin. On the basis of this argument, it should be possible for 1,4 a resin with a low, optimum strong-base ontent that is evenly distributed throughout the resin matrix to ahieve exellent seletivity for gold and silver yanide. Russian workers19 have shown that good seletivity for gold yanide an be ahieved by a weak-base resin with a strongbase apaity of 16 per ent, whereas two other resins they examined (with strong-base apaities of 36 and 77 per ent) showed no seletivity whatsoever. The ph value.of their feed solution was 1,6. It therefore seems to be tehnially feasible for both good ion-exhange apaity and good seletivity to be ahieved for gold and silver in the ph range 1 to 11, whih is typial of most plant pregnant solutions. With these objetives in mind, the further development of weak-base resins should be aimed at the optimization of the pka of the weak-base groups and of their overall strong-base ontent. THE DISTRIBUTION OF AUROCYANIDE AND VARIOUS OTHER METAL CYANIDE COMPLEXES BETWEEN THE RESIN AND SOLUTION PHASES AS A FUNCTION OF THE ph VALUE OF THE SOLUTION DII and D6 = The onentrations of metal on the resin (gft) divided by the onentration in solution (mgfl) at equilibrium at ph values of Il and 6 respetively. The molarity of eah metal in solution at equilibrium was approximately IO-'M. XI Metal yanide A365 A368 A7 IRA93 AIOlDU DIl D6 DIl D6 DIl D6 DIl D6 DIl D6 Gold Silver Cobalt Copper Nikel 8 Il Iron Il Zin MA Y 1984 JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY

13 Aknowledgements This paper is published by permission of the Counil for Mineral Tehnology (Mintek). The exellent tehnial assistane of Mrs O. Wellington and Mrs M. Conradie is gratefully aknowledged. Referenes 1. DAVISON, J., READ, F.., NoAKES, F. D. L., and ARDEN, T.V. Ion exhange for gold reovery. Trans. Instn Min. 2. Metall., vo! p HuSSEY, S. J. Appliation of ion-exhange resins in the yanidation of a gold and silver ore. Washington, D.S. Bureau of Mines, R , BURSTALL, F. H., KEMBER, N. F., FORREST, P. J., and WELLS, R. A. Ion exhange proesses for the reovery of gold from yanide p solutions. Ind. Eng. Ohem., vo! BURSTALL,F. H., and WELLS, R. A. Reovery of gold from yanide solution by ion exhange. Ion Exhange and Its Appliations. London, Soiety of Chemial Industry, p DEMIDOV, V. I., KREINES, R. Z., IvANovA, M. 1., and KARTASHEvA, A. S. Pilot plant tests on absorptive extration of gold from waste yanide solutions. Soviet J. Nonferrous 6. Met., vo!. 8, no p. 53. FRIDMAN, 1. D., ZDOROVA,E. P., POCHKINA, L. E., BEK, R. Yu., MAsLu, A. 1., and IL'ICHEv, S. N. Pilot tests on extrating gold and silver from yanide pulp employing the ontinuous eletroelution method. Soviet J. Non-ferrous Met., vo!. 12, no p. 76; vo!. 12, no p FRIDMAN, 1. D., KLYNEVA, N. D., POCHKINA, L. E., and ZDORVA,E. P. Forms in whih yanide omplexes of metals are found in anionite AP-2 during sorption and regeneration proesses. Soviet J. Nonferrous Met., vo!. 13, no p.92. TATARu, S. A. Reovery of gold from yanide solution by ion exhange. Tenth International Mineral Oongress Jones, M. J. (ed). London, The Proessing Institution of Mining and Metallurgy, 1974, pp ANoN. Gold from Russia's Muruntau deposit. Goal, Gold Base Miner., vo! p. 75. STAMBOLIADES,K, MHARDY; J., and SALMAN, T. Ion exhange tehniques for the reovery of gold from yanide solutions. Can. Min. Metall. Bull., vo!. 71, no p.124. MuLLINDER, B. Counil for Mineral Tehnology, Randburg. Private ommuniation, HAzEN, W. C. Method of eluting absorbed omplex yanides of gold and silver. U.S. Pat FLEMING, C. A., and NICoL, M. J. A omparative study of kineti models for the extration of uranium by strong-base anion-exhange resins. J. S. Afr. Inst. Min. Metall., vo!. 8, no p. 89. FLEMING, C. A., and CROMBERGE,G. Pilot-plant tests on the extration of gold from yanide pulps by anion-exhange resins. J. S. Afr. Inst. Min. Metall. (To be published.) SMITH, R. M., and MARTELL, A. E. Critial stability onstants. Inorgani omplexes. New York, Plenum Press, vo!. 4. SCHMUCKLER,G. Counil for Mineral Tehnology, Randburg. Private ommuniation, Nov GREEN, B. R. Counil for Mineral Tehnology, Randburg. Private ommuniation, Jun FLEMING, C. A., and HANoK, R. D. The mehanism in the poisoning of anion-exhange resins by obalt yanide. J. S. Afr. Inst. Min. Metall., vol. 9, no. 11. p LASKORIN,B. N., SADOVNIKOVA,G. 1., PETROVA,L. N., and FEDOROV, V. D. Gold seletive ion exhangers. J. Appl. Ohem. USSR, vo!. 47, no p Asian mining The Asian Mining International Exhibition and Conferene that were to have been held in November 1984, as announed in our issue of August 1983, has been resheduled to take plae in Manila, Philippines, from 13th to 16th February, The Institution of Mining and Metallurgy, together with the Chamber of Mines of the Philippines, is organizing the Conferene, whih will be a forum for the disussion of mining projets and developments in Asia, and of opportunities for investment in mining within the region. The organizing ommittee will invite noted authors to present papers, and welomes additional submissions on tehnial and operational aspets of mining, treatment, and exploration, and broader topis suh as resoure investment and safety. Abstrats (2 to 3 words) of papers for onsideration and Conferene enquiries should be addressed to The Seretary, Institution of Mining and Metallurgy, 44 Portland Plae, London WIN 4BR, United Kingdom. Enquiries about the Exhibition should be made to Industrial Trade Fairs International Ltd, Radliffe House, Blenheim Court, Solihull, West Midlands B91 2BG, United Kingdom. Tel.: ; telex: JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY MA Y

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