INDUSTRIAL AND ENGINEERING CHEMISTRY
1648
D
=
length of periphery of heat transfer
= surface areaof heat transfer/length of catalyst zone, meters
function heat received by catalyst zone per unit volume per unit time through its periphery, kcal. per hour per cu. meter of catalyst kl k’, R“, . . . ., K , K’,K”, , . . = constants in rate equation, which are functions of temperature only = length of catalyst zone, meteis 1 nL = flow rate of 8,. moles per hour P = total pressure in reactor, atmospheres p , = partial pressure of A,, atmospheres = heat of reaction, kcal. per mole of A I produced = total sectional area of catalyst zone, square meters T = reaction temperature, O C. 0’ = over-all heat transfer coefficient, kcal./sq. meter hour C. u = linear velocity of fluid, meters per hour W = packed volume of catalyst, cubic meters Y = space time yield, moles of A I per hour per cubic meter of catalyst zL = mole fraction of A, = mole ratio of A , to A2 in feed = n10/n20 = stoichiometric ratio of A , to A1 in reaction, having posiY$ tive and negative sign accordingly as A1 belongs to the initial or final system-Le., V I = -1, P Z < 0, and v 1 = 0 if = A , is an inert component = space velocity, moles of feed per hour per cubic meter of u catalyst = reaction rate, moles of 41 per hour per cubic +, \E, rp, meter of catalyst = fractional conversion relating to component A2 =
f H
Vol. 45, No. 8
Subscript 0 designates entrance conditions Subscript e designates exit conditions
= a =
.
. .
2
O
n20
- %e
literature Cited (1) Benton, A. F., a n d Elgin, J. C., J . Am. Chena. Soc., 49, 2426 (1927). (2) Biill, R., J . Chem. Phys., 1 9 , 1047 (1951). ( 3 ) D o d d , R. H., a n d Watson, K . &I.,TTans. A m . Inst. Chem. Engrs., 4 2 , 2 6 3 (1946). H a y . R.G.. Coull. J..a n d E m m e t t . P. H.. I w . ESG.CHEX..41. 2809 (1949). Kal’kova, K. T., a n d T e m k i n , M., Zhur. Fiz. Khim.,23, 695 (1949). K i p e r m a n , S. L., a n d Granovskaya, V. Sh., I b X , 2 5 , 5 5 7 (1951). K o d a m a , S., F u k u i , K . , a n d T a n a k a , H., J . Chem. Soc. Japan (Ind.Ghem. Sec.), 54, 303 (1951). K u m m e r , 5. T., a n d E m m e t t , P. H., IND. EXG.CHEU.,35, 677 (1943). Laupichler, F . G., Ibid., 30, 578 (1938). P e r r y , J. H., “Chemical Engineer’s H a n d b o o k , ” 3rd ed., p. 329, New Y o r k , hlcGraw-Hill Book Co. (1950). Pyzhev, ST., a n d T e m k i n , M., Acta Physicochim. (U.R.&’,,S.), 1 3 , 3 2 7 (1940). Tschernitz, J., Bernstein, S., Beckman, R. B . , a n d Hougen, 0. A , , T?ans. Am. Inst. CLem. Engm., 42, 883 (1946). Uyehara, 0. A., a n d W a t s o n , K. M., IND.ENG.CHEM., 35, 541 (1943). Wilhelm, R. H., Chem. Eng. Progr., 45, 208 (1948). W i n t e r , E., 2.phys. Chem., B13, 401 (1931). Wynkoop, R., a n d Wilhelm, R. H., Chem. Eny. Proyr., 4 6 , 3 0 0 (1950). RECEIVED for review M a y 26, 1952.
n2o
a a a
BCCEPTED
February 12, 1953.
Ion Exchange Process for Recovery of Gold from Cyanide Solution F. H. BURSTALL, P. J. FORREST, N. F. KEMBER,
AND
R. A. WELLS
Chemical Research Laborafory, Teddington, Middlesex, England
T
HE precipitation of gold from cyanide solutions with the aid
of metallic zinc is a well-established procedure of the gold mining industry. It combines the advantages of cheapness and simplicity for the treatment of a large number of ores. There are cases, hon-ever, where the usual procedure of cyanidation, followed by filtration and precipitation of gold in the filtrate with zinc, can be carried out only w-ith difficulty. On grinding, some ores give a large proportion of slimes, which make filtration both difficult and expensive. Others contain elements soluble in cyanide solution t o form complexes, which inhibit the precipitation of gold. Adsorption of the gold on activated carbon has been suggested many times in the past as a means of overcoming these difficulties. More recently, a number of workers ( 2 , 3 ) have demonstrated the possibility of adding activated charcoal directly t o leach pulps and of recovering the charcoal, containing adsorbed gold, by floating or screening. The process ha3 been made cyclic by eluting the adsorbed gold, together with any silver that may be present, from the charcoal n-ith hot sodium cyanide solution (8). The charcoal may then be used for a further adsorption stage. .4 number of gold mills are operating plants based upon this cyclic process. The possibility of using an anion exchange resin for the recovery of gold was first demonstrated using a chloride solution of gold ( 7 ) . Later, Hussey ( 4 ) investigated the possibility of using
the Reakly basic anion exchange resin, Amberlite 1R-4B, for the adsorption of gold from cyanide solutions. By using a countercurrent adsorption and regeneration system an adsorption of 95.4% of the gold and 79.0% of the silver present in a pregnant solution of a leach pulp, prepared from ore from the Buckthorn mining district, was obtained. Loss of resin by attrition mas shown t o be less than 1% after 33 days agitation with a pulp of minus 100-mesh ore containing 31% solids. I n view of the high p H (>10.0) of a normal pregnant solution it is perhaps surprising that the gold capacity of the resin was sufficiently high to make it possible to operate the process, but it must be emphasized that good recovery of gold was achieved only by the use of a very high resin t o pulp ratio. This paper describes an investigation into the adsorption of gold from cyanide solutions on the strongly basic resin, Amberlite IRA-400. The work has been extended to a study of the behavior of other metals, including silver, copper, iron, cobalt, nickel, and zinc and anions other than cyanide, such as sulfate, thiosulfate, and thiocyanate, likely t o occur in commercial cyanide solutions. Experiments were carried out on clear solutions and not on unfiltered pulps, but in view of the experiences of earlier workers, it appears likely that the process suggested could be readily applied, after suitable modification of practical technique] to pregnant pulp solutiona. All the heavy metal complex cyanides examined, including
August
INDUSTRIAL AND ENGINEERING CHEMISTRY
1953
gold, were strongly adsorbed by the resin. Using pure solutions of aurocyanide up to 0.66 gram of gold were adsorbed per gram of dry resin. Adsorption of mixtures of complexes showed that the monovalent aurocyanide ion could be displaced by multivalent complex cyanides. The presence of sulfate had no effect on the adsorption of aurocyanide but the affinity of the resin for gold was decreased to some extent by the presence of thiosulfate or thiocyanate. PREGNANT LIQUOR
4
RESIN (AMBERLITE IRA-400) ADSORBS HEAVY METAL COMPLEX CYANIDES
0.2N HCI
I
I
.)
ELUTED
1
2N NaCN ELUTED
4
I COPPER ,IRON
I
IACETONE-Sn HCI
ELUTED
0 % m ,
COBALT
2 N KCNS
SlLVL R QOLD
ELUTED
t l i
COBALT
.
the resin. In later experiments a further adsorption stage was carried out immediately after eluting with the aqueous reagents and this cycle was repeated until a sufficient quantity of gold had been adsorbed by the resin to justify use of the organic solvent. In this way a considerable saving in the solvent and resin required for a given volume of pregnant solution was achieved. Some 99.5% of the gold and 100% of the silver were ultimately eluted from the resin with the acetone solvent. A similar multistage experiment was finally carried out on a pregnant solution obtained by the cyanidation of a low grade gold ore. The recovery of gold was particularly satisfactory in view of the fact that while the pregnant solution contained only 1 p.p.m. of gold its copper content was a hundred times greater. On elution with an acetone-hydrochloric acid mixture, 95% of the gold was recovered; the rest was retained on the resin. The proposed scheme ( 1 ) for treating a pregnant liquor is simply illustrated by the accompanying flow sheet. Each step outlined would not be necessary in each cycle, and with some ores it would be possible to leave out some steps completely. Experiments to Determine Capacity of Resin for Complex Cyanides
d
SILVER ,OOLD,COBALT
1649
Process for Treating Gold-Bearing Solution from Cyanida tion
I n agreement with other workers, elution of the adsorbed gold from the resin was found to be difficult and the aqueous eluting agents tried gave only partial elution of the gold. Previous chromatographic work (6),in which organic solvents had been used successfully for the removal of gold, led to the suggestion that it might be possible to elute gold from the resin with the aid of mixtures of organic solvents and mineral acids. It was found that a large variety of organic solvents would elute gold both rapidly and efficiently. During this investigation, elution with acetone containing hydrochloric acid was studied in some detail. Fortunately, complex cyanides, other than gold, proved less difficult t o elute and many of them could be removed with normal aqueous eluting agents. Moreover, suitable choice of eluting agent enabled selective elution of the metallic complexes to be carried out. Advantage has been taken of these differences in elution properties to obtain gold concentrates, and a procedure has been developed by which the removal of metallic cyanides, other than gold, is effected by elution with aqueous solvents which remove no gold. Gold is subsequently removed from the resin with an organic solbent and recovered as a high grade concentrate. The resin is then in a suitable form for undergoing a further adsorption cycle. In experiments using synthetic pregnant solutions saturated resin samples were eluted first with 0.2N hydrochloric acid, which removed nickel and zinc, and secondly with IN sodium cyanide, which removed copper and iron. Gold was then recovered in better than 98% yield together with 80% of the silver, by elution with acetone containing 5% hydrochloric acid (density 1.18) and 5% water. The remaining gold and silver was still adsorbed by
Single Components. The capacity of Amberlite IRA-400 for a number of complex cyanides normally present in pregnant liquor was determined. ilbatch of the commercial grade resin in the chloride form was dried in vacuo over calcium chloride for 24 hours and the product sieved. The portion ietained on British Standard Sieve 80 and passing BSS 20 was used in subsequent investigations. The results of all experiments were related t o the dry weight of the resin; the moisture content was determined by drying a sample a t 110' C. for 6 hours. The conversion of the resin to the hydroxide form, in order to measure its capacity for the univalent OH '-t C1 exchange, proved t o be unusually difficult. Even after the passage of 1500 ml. of 2N sodium hydroxide through the equivalent of 1 gram dry weight of resin, in the chloride form, some traces of chloride were still being eluted. The conversion of the resin from the chloride to nitrate form was much inore readily carried out and this method of standardization was used throughout the investigation. The equivalent of 1 gram dry weight of the resin was eluted with 100 ml. of 1N sodium nitrate. The released chloride was determined and gave a capacity of 3 . i O millimoles of nitrate per gram of dry resin. The capacity of the resin for complex metallic cyanides was determined by a series of breakthrough experiments. A solution containing2 milliequivalents(meq.)per liter of the complex cyanide in 0.15% sodium cyanide was passed through a 1-gram column (in all experiments, when 1 gram resin was used column dimensions were 3 em. X 1 cm. diameter) of the resin a t a rate of 200 ml. per hour. This was continued until the concentration of metal in the effluent was equal to that in the influent. I n most cases equilibrium was reached after the passage of 2 to 3 liters of solution. The resin was then destroyed by ashing and the metal determined.
Table I.
Capacity o f Amberlite IRA-400 for Single Complex Cyanides
Metal
2 cu
Fe (Ferrocyanide) co Ni Zn
Metal Capacity Adsor~Fd, G. Dry Resin Mg. Millimoles 340.8 3.16 659.1 3.33 82.1 1.29 48.8 0.87 76.3 1.29 106.9 1.82 81.6 1 25
-
Ratio of Cawcity for C1NosExchange t o Capacity for Metal 1.17 1.11 2.87 4.25 2.87 2.02 2.96
1650
INDUSTRIAL AND ENGINEERING CHEMISTRY
summation of the quaqtities adsorbed from each 2 liters of influent. I t Mas shomm later that it was possible to predict approximately the point of breakthrough of gold of any known mixture of complex cyanides on the basis of the figure of 3.78 meq. of metals on the resin a t breakthrough. Effect of Other Anions. The effect of the presence of a number of different anions, normally found in cyanide liquors, on the adsorption of gold was examined. The presence of sulfate had little or no effect on the adsorption of gold by the resin. Three 1-gram columns of resin were prepared and through each was passed one of the following solutions:
.____,
0
5 10 15 20 2-LITER F R A C T I O N S O F EFFLUENT COLLECTED
Figure 1 . 100
25
I. Two liters of 0.15% w/v sodium cyanide containing 4 millimoles sodium aurocyanide. 11. The same solution 0.01N (o.07170) with respect to idfate. 111. The same solution 0.1N (0.71%) with respect to sodium sulfate,
30
Adsorption of Mixed Gold, Copper, and Iron Cyanides
r
I
100 200 T H R O U G H P U T , ML.
Figure 2.
300
400
Elution of Aurocyanide
Vol. 45, No. 8
Each sample of resin wac: then ashed and the gold determined For columns I and I1 a capacity of 3.32 meq. Au(CN)*- per gram dry resin and for column I11 3.29 meq. ,4u(CN)z- per gram was obtained. The total sulfate was measured in both influent and effluent and none was found to have been adsorbed. The presence of thiosulfate reduced the adsorption of gold slightly. Thiocyanate also showed a more marked effect, but in neither case was the reduction serious. Three columns were prepared, each containing 1 gram of resin and through each was passed one of the following solutions:
I. Two liters of 0.15% w/v sodium cyanide solution containing 3.3 millimoles aurocyanide. 11. The same solution 0.01S (0.081%) with remeet to sodium thiocyanate. 111. The same solution 0.01s (0.079’%) x i t h respect to sodium thiosulfate.
Eluting solutions
1. 2. 3.
1 N NH4NOa in 0.2N NH4OH 1N NH4N03 1N N02SOa
The last column in Table I indicates the effective valency of the complex-i.e., silver and gold, univalent; nickel, bivalent; copper, cobalt, and zinc, trivalent; and iron, quadrivalent. The apparent trivalency of the copper and zinc complexes is particularly noteworthy in view of the commonly accepted bivalency of both in cyanide solution. Mixtures. By means of a breakthrough experiment, information was gained as t o the relative affinity of the resin for a number of different complex cyanides. A solution of a mixture of ferro-, cupro-, and aurocyanides in O.15y0sodium cyanide solution was passed through a 1-gram column of resin at a rate of 300 ml. per hour. Each fraction of 2 liters of effluent was evaporated t o low bulk after the addition of excess acid and analyzed for each of the three metals. This was continued until the concentrations of each metal in the efluent was equal t o its concentration in the influent. The resin was then ashed and analyzed. The results of analysis of each fraction of effluent, plotted in Figure 1, show t h a t after the passage of 12 liters the adsorbed gold was partially displaced from the resin by the other complexes, and later cuprocyanide was partially displaced by ferrocyanide. At equilibrium all three complex cyanides were present on the resin and the aspparent total capacity was greater than that for the C1- -c NO3- exchange. This wasprobably due to the formation of complexes between the cyanides; thus copper and iron may have been adsorbed together as CuaFe(CS)s- or Cu2Fe(CN)a--. Later work also showed the possibility of higher complex formation between aurocyanide, argentocyanide, and other cyanides. The figures in the last column of Table I1 are estimated by
Table 11.
Metal Fe
CU .4u
Total
Adsorption of Mixture of Complex Cyanides
Coiiiplex Fe(CS)a---Cu(CN)d--.4u(CN)?-
In 2 liters of Influent AIg 3Ieq.
At Equiiibiiuin Mg Meq
1.53 0,109 36.8 0.217 29.0 4.60 1 9 . 8 0.100 .0 _ _ - _ _ _ _9 8_ 25.93 0.426 163.8
2.63
At Gold
Breakthrough
(Estd.) Meq. 13.6 0.97
Ng.
1.37
40.8
4.60
227.5
0 _1 7 3 . 1 _ 0_. 5 _
1.93 0.88
3.78
The resin samples were then wet oxidized and gold and sulfate determinations carried out in each case. On column I, 3.08 meq. of gold mere adsorbed; on column 11, 2.42 meq. of gold and 0.79 meq. of thiocyanate; and on column 111, 2.86 meq. of gold and 0.14 meq. of thiosulfate. These results must be treated with some reserve since they give only some indication of loadings of gold which may be achieved. Measurements of the loading of columns a t the point of gold breakthrough and a t equilibrium, in the presence of these other anions, remain to he carried out. Complex Cyanides Are Eluted with Aqueous Eluting Agents
I n many cases the elution of adsorbed complex cyanides has proved difficult. Most of the more normally used eluting agents have proved only partially efl’ective, but in some cases selective elution has been obtained. Aurocyanide. The eluting agents, 211’ sodium chloride, 5N ammonium chloride, 211’, 4N, 5 N , and 11N hydrochloric acid, IN sulfuric acid, 2N sodium hydroxide, 4N ammonium hydroxide, 1N sodium carbonate, and 1N sodium cyanide proved ineffective in removing 10 mg. of gold adsorbed on 2 grams of resin. Under
INDUSTRIAL AND ENGINEERING CHEMISTRY
August 1953
Table 111. Extraction of Gold, Silver, and Cobalt Cyanides with 2N Potassium Thiocyanate Silver Fraction 2 N KCNS Aeetone-5% HC1-6% water Resin ash
%
88.5 Nil Nil
100.0
Nil Nil
Cobalt
Gold
Mg.
%
Mg.
180.2 9 2 . 4 14.3 7.3 0.5 0.3
Mg.
%
18.5 100.0
Nil Nil
Nil Nil
100 80
s;
60
I-
3 Y
40
PO 0
PO0 THROUGHPUT,
0
300
I00
400
or without the addition of ammonia. Elution curves are shown in Figure 2 and compared with t h a t for 1N sodium sulfate. The nitrate is more efficient than the sulfate in removing the gold complex, contrary t o expectation from the normal order of affinities, but the elution still leaves much t o be desired. Later work showed that 2 N potassium thiocyanate was a very effective eluting agent for aurocyanide (see discussion of cobalticyanide), and further investigation of this reagent is required. Cuprocyanide. A number of columns were prepared with 30 mg. of copper as cuprocyanide adsorbed onto 2 grams of resin. Elutions were carried out with 2 N and 4N hydrochloric acid (Figure 3) and 2 N sodium chloride (Figure 4), and the latter was shown t o be the most effective, eluting 80% in the first 100 ml. Subsequent experiments on mixtures showed that cuprocyanide could be efficiently removed with sodium cyanide solution. Ferricyanide. The removal of ferricyanide from the resin was not studied in great detail, since in practice gold cyanide liquors rarely contain iron as ferricyanide. Elution curves carried out in the same way as for cuprocyanide are shown in Figure 3 for 2N and 4N hydrochloric acid and i n Figure 4 for 2N sodium chloride. Of these reagents, 2 N sodium chloride was the most effective.
ML.
100
Figure 3. Elution of Ferro-, Ferri, Cupro-, and Zincocyanides with Hydrochloric Acid Cu(CN)4---wlth 4 N HCI 2. Cu(CN)d--- with 2N HCI 3. Fe(CN16 - - - - with 4 N HCI 4. Fe(CN16 - - -- with 2N HCI 5. Fe(CN)e--- with 4 N HCI 6. Fe(CN)e- - - with 2 N HCI 7. Zn(CN)s - - - with 2N HCI
1651
80
1.
8 '
60
d P 40
PO
the same conditions, traces of gold were eluted with equal volume mixtures of 4N hydrochloric acid with 4N nitric acid. Using more heavily loaded samples of resin, partial extraction of gold with 4N hydrochloric acid was observed. Thus, 18.2% of 100 mg. of gold adsorbed on 2 grams of resin was eluted with 400 ml. of 4N hydrochloric acid and 39.6% of the gold was eluted under the same conditions, when the resin was loaded to capacity with gold. During elution with hydrochloric acid, breakdown of the aurocyanide occurred, leaving a fine yellow precipitate on the resin which it was impossible to remove. Considerably better elution of gold was obtained using 1N ammonium nitrate, with
0 0
Elution of Ferro-, Ferri-, and Cuprocyanides 1. 2. 3.
4. 5. 6.
7.
Eluting solutions 1 N NH4NOa-0.2N NH4OH lNNaN03 2 N NaCl 2 N NaCl 2 N NaCl 1N (NHhS04 2NNaOH
1
PO0
ELUTING AGENT,
Figure 5.
300
400
ML.
Elution of Zincocyanide with Hydrochloric Acid
Dilute
Ferrocyanide. Columns of resin (2 grams), each containing 30 mg. of iron as ferrocyanide, were eluted with various solutions. The results, shown graphically in Figure 3, indicate the poor elution obtained with 2N and 4 N hydrochloric acid. I n Figure 4 elutions with 2N sodium chloride, 1N ammonium nitrate 0.2N ammonium hydroxide, 1N sodium nitrate, 1N ammonium sulfate,and2N sodiumhydroxide are compared. With both nitrate solutions a highly efficient elution was obtained. With 2N sodium chloride a successful but less speedy extraction was observed, and with the ammonium sulfate and sodium hydroxide solutions extraction was poor. I n later experiments, sodium cyanide solution was shown to be extremely efficient in removing ferrocyanide. Zincocyanide. I n preliminary experiments on the elution of zincocyanide, using 2N or 4N hydrochloric acid as eluting agent, very little zinc was found in the eluate but a large quantity was found in the subsequent water wash. This phenomenon was further investigated using a number of 1-gram columns, on each of which was adsorbed 20 mg. of zinc as zincocyanide. Elutions were carried out using dilute hydrochloric acid solutions of decreasing strength. The results (Figure 5 ) showed that the rate of elution of zinc increased rapidly with decrease in acidity, reached a maximum a t 0.1N hydrochloric acid, and then decreased. Nickelocyanide. The elution studies carried out on zincocyanide using hydrochloric acid solutions of varying strength were repeated on columns of resin bearing adsorbed nickelocyanide.
+
Figure 4.
1
100
INDUSTRIAL AND ENGINEERING CHEMISTRY
1652
Vol, 45, No. 8
100 I
The results (Figure 6 ) indicated that nickel is desorbed in a normal manner-i.e., more rapidly with increasing acidity. Cobalticyanide. Cobalticyanide proved t o be one of the most difficult of complex cyanides t o remove from the resin. A number of columns (1 gram), each containing 2 i mg. of cobalt as cobalticyanide, n-ere treated with the various eluting agents examined for other complex cyanides. Kone was effective in eluting more than 10% of the cobalt, with the exception of IK sodium cyanide I\ hich removed 24%. A hitherto untried eluting agent, potassium thiocyanate, proved most effective in removing cobalticyanide. A 100-ml. fraction of 21L- potassium thiocyanate eluted 99.6% of the cobalt complex. The efficiency of potassium thiocyanate as an eluting agent was demonstrated by adsorbing 1 meq. of each of the complex cyanides of gold, silver, and cobalt on a 1-gram column of resin. The column was eluted with 100 nil. of 2 N potassium thiocyanate, followed by 100 ml. of acetone-5% hydrochloric a ~ i d - 5 7 water ~ mixture. (Reasons for t h e use of an organic solvent are dealt with in detail later.) The thiocyanate solution extracted 100% of the silver and cobalt, together with 92.4% of the gold (Table 111). The remainder of the gold was still extractable with the organic solvent mixture.
90
-
80
-
io0 SOLVENT, ML.
Figure 7.
4.
100
PO0
300
5. 6. 7. 8.
400
300
Extraction of 10 Mg. Gold with Organic Solvents 1. 2. 3.
0
PO0
Solvents Methanol-5% HCl-5% water Absolute olcohol-5~o HCI Ether-5% HNOa Methanol-1 0% HCI-570 water Methanol-1 0% HCI Acefone-5% HCI Absolute alcohol-lO% HCI Ethyl acetote-1 0% HN03-5% water
ELUTING AGENT, ML.
Figure 6.
Elution of Nickelocyanide with Dilute Hydrochloric Acid
Argentocyanide. Complete removal of adsorbed argentocyanide proved almost as difficult as the complete removal of aurocyanide. As indicated in Table 111, potassium thiocyanate proved particularly effective. Less effective, but complete elution, mas also achieved with a mixture of 1X ammonium nitrate and 0.2X ammonium hydroxide. Traces of silver Tere eluted with sodium cyanide solution, but the behavior of adsorbed silver depended largely upon its relative concentration, the nature of other cyanide complexes present, and the previous treatment of the resin. Organic Solvents Containing Mineral Acids Used for Elution of Complex Cyanides
The suggestion that organic solvents containing mineral acid might be useful eluting agents for aurocyanide was based upon the assumption that the presence of mineral acid would bring about a t least partial ionization of the gold-resin complex. T h e complex gold ion would then be free to form, in the presence of mineral acid, a nonionized covalent complex with the solvent, which would show little tendency to be readsorbed. A number of organic solvent-mineral acid mixtures was tested in the following manner. After the adsorption stage the resin was washed with water and the level of the supernatant liquor reduced t o a minimum. The organic solvent was then allowed t o flow through the column, usually a t a rate of 50 ml. per hour per square em. After the addition of water the solvent was distilled from each fraction
of eluate, leaving a small volume of dilute acid solution containing the extracted matter. In early experiments on the elution of metals other than gold some erroneous results were obtained, usually accompanied by the formation of highly colored compounds on the surface of the resin beads during elution. Once formed, these compounds proved difficult t o remove. Their formation was found t o be due to starting with resin in the chloride form and eluting with solvent containing nitric acid. It was thought probable that 100
se
@3
80
60
W 4
9
s
40
PO I
I
I
1 P 1OO.ML. FRACTIONS
0
3
Figure 8. Elution of Aurocyanide with Acetone-Hydrochloric Acid Mixture 1. 2.
3.
Solvents Acetone-5yo HCi Acetone-5% HCl-lO% water Acetone-5Y0 HCl-5% water
I
4
August 1953
s
INDUSTRIAL AND ENGINEERING CHEMISTRY
traces of chloride ion left on the resin after the adsorption stage combined with the nitric acid, forming local oxidizing conditions. The effect was most marked in the case of ferrocyanide, where insoluble compounds such as ferric ferrocyanide could be formed. No such compound formed when the resin was in the nitrate form before eluting with a solvent containing nitric acid, This effect was noticed when only one complex was present on the resin and is not to be confused with intercomplex formation between several metals. Aurocyanide. The quantitative removal of gold from a cellulose adsorbent using ethyl acetate containing 10% nitric acid (density 1.42) and 5% water has already been recorded (6). The same solvent was used t o elute 10 mg. of gold a s aurocyanide adsorbed on a 2-gram column of resin. Virtually complete removal of the gold was obtained (Figure 7 ) . A number of other solvents containing added nitric or hydrochloric acids was examined, and it became immediately apparent t h a t a wide variety of solvents could be used for the elution of gold (Figure 7 ) . Acetone was chosen for a more detailed study. A number of columns of resin ( I gram each) was brought t o capacity with aurocyanide and eluted with acetone containing varying amounts of water and hydrochloric acid. The rate of elution of the com-
Table IV. Au Recovered from Effluent (Adsorption Stage), G. 0.0246 0.0297 0.0345 0.0425 0.0474 0.0524
Cycle 1 2 3 4 5 6
1653
Recycling Experiment A' G. 0.6300 0.6249 0.6201 0.6121 0.6070 0.6022
Mep. 3.20 3.18 3.15 3.11 3.08 3.06
Au Eluted by Acetone, G. 0.6309 0.6257 0.6181 0.6134 0.6084 0.6030
100 80
'd
60
3
40
z
PO 0
I00
*d
0
Figure 1 0.
80
300
Elutions with Acetone-5% Acid-5% W a t e r
400
Hydrochloric
60
3 w
n 40
-I
s
100 PO0 THROUGHPUT, ML.
PO 0 0
Figure
9.
1 9. 100-ML. FRACTIONS
3
4
Elution of Aurocyanide with Acetone-Nitric Acid Mixtures I. 2. 3.
Six cycles were comp1ete.d and the gold estimated in each of the solutions from every cycle (Table IV). After six cycles the take-up of gold per cycle had fallen by 5%. At the completion of the sixth cycle the resin was ashed and a little over 20% of the fall in capacity was shown to be due to gold remaining on the resin. The remaining loss in capacity,*equivalent t o a decrease of from 3.20 t o 3.09 meq. per gram of resin, was partially due t o the accumulation of traces of other metals, present as impuritiese.g., ferrocyanide, which showed as a faint blue-green color at the top of the column.
Solvents Acetone-Z% HNO~-Z% water Acetone-370 H N 0 r 5 % water Acetone-5% " 0 8 4 % water
plex increased rapidly as the acidity was increased from 1 to 5%. On the addition of water to acetone containing 5% volume/ volume hydrochloric acid (density 1.18),the rate of extraction of the gold increased to an optimum value when the water concentration was 5%. With this solvent, acetone-5% hydrochloric acid (density 1.18)-5% water, 99.9% of the gold was extracted in 400 ml. of solvent, 96.2% being recovered in the first 100 ml. (Figure 8). I n a second series of experiments using nitric acid in place of hydrochloric acid a rate of extraction almost identical with that of acetone-5% hydrochloric acid (density 1.18)-570 water (Figure 9) was obtained. I n order t o consider the commercial application of this type of elution, an experiment was carried out to determine the stability of the resin t o repeated treatment with the solvent. A series of cycles were carried out, each consisting of an adsorption and elution stage using a column containing 1 gram of resin in the nitrate form. Each cycle consisted of four stages: 1. The resin was brought to near capacity by passing through 2 liters of solution containing 0.6546 gram of gold a s aurocyanide in 0.15y0sodium cyanide at a rate of 100 ml. per hour. 2. The column was washed with 100 ml. of water which was combined with the effluent from step 1. 3. The column was eluted with 250 ml. of acetone-5yo nitric acid (density 1.42)-5% water at a rate of 50 ml. per hour. 4. The column was washed with 100 ml. of water which was combined with the eluate from step 3.
100
80
*d
60
3
40
C
PO 0
0
Figure 1 1.
100 PO0 THROUGHPUT, ML.
Elutions with Acetone-5% Water
300
400
Nitric A c i d 4 7 0
Other Complex Cyanides. The elution of a number of cyanides with acetone containing 5% hydrochloric acid and 5y0water and acetone containing 5% nitric acid and 5Oj, water was studied. Results obtained f%rsilver, copper, iron, and zinc are compared with those for gold in Figures 10 and 11. I n each experiment the extraction was carried out on a 1-gram sample of resin saturated with respect to the particular complex under examination. The poor extraction of all metals other than gold with acetone containing nitric acid appeared to be a possible basis for a separation procedure. The extraction of silver was particularly poor, less than 1 % of the argentocyanide being extracted with 400 ml. of solvent. On extraction of the argentocyanide with the ace-
INDUSTRIAL AND ENGINEERING CHEMISTRY
1654
tone-hydrochloric acid mixture, some breakdown occurred and silver chloride was deposited in the column. A roughly quantitative experiment was carried out on the elution of cobalticyanide with the acetone-hydrochloric acid mixture. Results a-ere similar t o those obtained for silver. 100
Table V.
Column C
Vol. 45, No. 8
Elution of Gold-Silver Mixtures
Solvent 4cetone-5%HSOs6 % water hcetone-b% HC1-5%
Gold Eluted, 2nd 100 ml. mi. Total 36.4 1 . 5 37.9 6 8 . 7 4 . 7 73.4 95.1 3 . 8 98.9 1st 100
Silver Eluted, % 1st 100
nil.
.. 22:7
2nd
100 ml.
.
Total ..
30:3
53.'0
,
water 80 60
a
E
3 w
40
I
20
0 0
100
200
300
400
THROUGHPUT, ML. Figure 12.
Elutions with Ethyl Alcohol-1 0% Hydrochloric Acid
Two further elutions were carried out on adsorbed cuprocj-anide and ferrocyanide using ethyl alcohol containing 10% v/v hydrochloric acid (density 1.18). The results (Figure 12) indicate the possibility of using this solvent for the separation of adsorbed complex cyanides. Complex Cyanides Removed by Aqueous and Organic Solvent Elution in Full-Cycle Experiments
From the studies made of the elution of the separately adsorbed cyanide complexes it appeared t h a t it might be possible, by suitable choice of eluting agents, t o obtain a satisfactory concentration of the gold. This might be achieved by extracting the gold first, leaving the other metals on the resin, t o be extracted a t a later stage, or by the reverse procedure. I n view of the promising results obtained with organic solvents containing nitric acid the first alternative was investigated. Preliminary Experiments. Experiments on single components indicated the possibility of separating gold from iron, copper, and silver by using a n acetone-nitric acid-water mixture. A mixture of complex cyanides in dilute sodium cyanide solution was passed through a column containing 5 grams of resin in the nitrate form until gold was just detected in the effluent, The molar proportions of the metals in the solution were silver, 0.7; gold, 1.0; copper, 7.0; and iron, 3.0. The column was eluted with acetone-5% nitric acid-5% water mixture. During the early stages of the elution, highly colored compounds formed in bands down the column, turning the top reddish-brown, the center yelloffish-green, and the bottom gray. It is believed t h a t the reddish-brown compound was cupric ferrocyanide, formed by the oxidation products of the cuprocyanide reacting with the ferrocyanide, Later evidence indicated that the gray compound was due to similar interaction between the gold and silver complexes, or possibly to the formation of silver ferrocyanide. .4nalysis of the solvent eluate showed that only 4% of the gold was extracted. A similar experiment was carried out, omitting silver for the sake of simplicity and eluting with acetone-5% hydrochloric acid-5% water solvent. Again the formation of highly colored compounds was observed: all but the bottom of the column turned deep blue-green. Analysis of the eluate showed that 85% of the gold had been extracted, together with most of the copper and some iron. A number of aqueous eluting agents was then passed through the column in a n attempt to remove the remaining complexes. The most effective of these was 1N ammonium nitrate-0.2iV
ammonium hydroxide-0.211' sodium cyanide, the cyanide being included in the mixture in an attempt t o reform the soluble complex cyanides. This mixture largely cleaned up the colored compounds from the column but did not elute any more gold, since the remainder of the gold was later recovered from the resin ash. From the results of these early experiments it was apparent that use of an organic solvent eluting agent was to be avoided for the first stage of an elution cycle. The second alternative, that of first eluting metals other than gold and silver with aqueous eluting agents and subsequently rerovering the gold, was therefore investigated. I n view of the promise of the ammonium nitrate-ammonium hydroxide-sodium cyanide mixture used in the earlier experiment, its components were tested to determine the most useful eluting agent. All combinations were tested and it was found that 1N sodium cyanide eluted ferrocyanide very rapidly, cuprocyanide less rapidly, and that no gold or silver was detected in the eluate. At a later stage in the program the strength of the cyanide solution was increased to 2N in order t o increase the rate of elution of the copper, without elution of gold or silver. Following these experiments, a full-cycle elution vias started by adsorbing onto a column of resin a mixture of the complex cyanides of silver, gold, copper, and iron. The column was then eluted with IN sodium cyanide to remove the iron and copper; a solvent elution was then carried out with acetone-5% nitric acid-5% water mixture. The first stage of the elution was successful, but the solvent elution removed only 650/, of the gold present on the resin. It appeared possible, therefore, that in the presence of the acetone-nitric acid solvent some interaction occurred between the gold and silver complexes similar to that previously observed between copper and iron. To examine this possibility, two columns were prepared, each containing 1 gram of resin in the nitrate form. The following solutions were then passed through the columns: Column A. 0,275 gram gold and 0.126 gram silver a s complex cyanides Column B. 0.275 gram gold and 0.032 gram silver as complex cyanides Table VI. Column A B (overloaded)
Effect of Overloading on Elution Adsorbed on Column, hlg. Au .4g 28.3 103.2 22 0 163.8
Au Eluted with IiVNaCN Mg.
5%
0.8 60.8
39.6
0.8
S g Eluted with 1 N NaCN Lk. % 2.6 9.1 12 6 57.3
Both columns m-ere eluted with acetone-5yo nitric acid-5 yo water solvent, and two fractions of 100 ml. each were collected. The results in Table V indicate that an increase in the weight of silver present in the column decreases the amount of gold extracted. KO silver was detected in either eluate. During the elution the bottom of each column darkened, probably because of the formation of a gold-silver complex cyanide precipitate. A third column similar to column A was prepared and eluted with acetone-5 % hydrochloric a ~ i d - 5 7water ~ mixture. The results, also shoivn in Table V, indicate that the use of this solvent is completely effective and that the extraction of gold is not affected
Table VII. Fraction (100 ml.)
Eluting -4gent 1'V NaCX
Three-Stage Elution Ag ____
1 2
Total 1NNHdNOa-0.2N NHaOH
Nil
Nil
Nil
Nil
199.4 27.0 47.3 17.6 30.5 8.2 207.6 44.6 77.8 10.0 Nil 5.7 Trace _ _ _ _ - Nil Nil 5.7 10.0 Nil Nil Nil
96.6 4.0 100.6 Nil Nil Nil Nil
1 2
..
Table VIII.
h-aCN
Four-Stage Elution
FracA' Ag. tion hlg. % 1 , , ,, ... ... 2 ,. ,. .,. ,.. Water , , ., .,, ... Total , . , , , , , ... ... 1 .. .. . .. . ... 2 .. .. .. . .. Total ,. .. . 1 69.5 84.0 3 . 5 39.2 2 12.1 14.6 3.7 40.8 Water ,, ,, Trace Trace Total 8 1 . 6 9 8 . 6 7 . 2 80.0
. .
Acetone
h-HjNOah &OH Column residue
1
.. .
2
- - -
Total Column residue
Eluting Agent HC1
%
Mg.
11 2
_ -11 6 4
Total Acetone-5% HC1-5% water
cu
Au
%
Mg.
6.4
%
cu ME. % 5.7 1.5 6.2 23.3 -1 3_. 9 _ 3 . 7 4 2 . 9 11.4 123.332.9 45.6 168.945.1
..
..
...
...
0.5
0.1
1.1
1.3
1.8
20.0
0.9
0.2
Mg. 57.1 206.4 61.0 4.0
Fe
%
M
n
582
3 4
850
9 8 4 6
0 1 0 1
2 5 2 6
-
38
Y%
4 8
0.2
5.0
0.2 Nil 0.2 Nil
Nil Nil Nil Nil
Nil Nil Nil Nil
.s N il Nil 1 .1 1 4.0 6.6 0.2 5.0 0 1 Nil 0.1
_ .
Nil
__
This solution was passed through a column containing 2 grams of resin in the chloride form. The column was then washed with 200 ml. of water. No traces of gold were detected in the effluent. Elution was carried out in three stages:
I. 400 m l . 1 N s o d i u m cyanide 11. 200ml. acetone-5%, hydrochloric acid-5% water 111. 200 ml. 1K ammonium nitrate4.2-V ammonium hydroxide
The column was washed with 100 ml. of water between each % Mg. % stage. Each wash was com56.3 1 1 7 . 0 6 5 . 9 bined with the last fraction of 41.5 17.9 10.1 the preceding stage. After the 8 _2 3_ . 0 _1 _ 3.0 _1 ._ 9 9 . 6 157.9 8 9 . 0 e x p e r i m e n t , the resin was 0.1 4.4 2.5 ashed and the residue analyzed 3.4 (Table VII). 7.0 0.8 0.2 10.5 5.9 The three eluting agents 27.9 0.3 0.1 0.8 0.5 .. 0.4 0.1 3.6 2.0 may be used successfully t o .. 0.2 3.8 2.1 separate a mixture of the 27.9 0.9 0.3 8.2 4.6 cyanide complexes of silver, 9 . 3
