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Digestion of silver in acidic ferric chloride and copper chloride solutions
Hydrometallurgy, 20 (1988) 219-233 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
219
Digestion of S i l v e r in Acidic Ferric Chloride and Copper Chloride Solutions BARBARA KO[.ODZIEJ Institute of Inorganic Chemistry and Metallurgy of Rare Elements, Technical University, 50370 Wroctaw (Poland) (Received June 18, 1987; accepted in revised form October 11, 1987)
ABSTRACT Kolodziej, B., 1988. Digestion of silver in acidic ferric chloride and copper chloride solutions. HydrometaUurgy, 20: 219-233. The digesting of silver has been carried out at different concentrations of leaching agents FeCl3 and CuCl2 in the range 30-90°C. On the basis of the kinetic curves obtained by experiment the initial digestion rates were calculated. It was found that at 0.01-0.1 M FeCl~ or CuCl2 the initial rates of digestion are about the same and a further increase in concentration of the leaching agent leads to a decrease in the initial rates of the digesting process. It should be stressed that the digestion of silver at a concentration of CuC12 of 1 M is much faster than in the case of 1 M FeC13. These differences are analysed by means of the potentiometric method and scanning analysis of the surface of silver after digestion.
INTRODUCTION T h e p o s s i b i l i t y o f a p p l i c a t i o n of h y d r o m e t a l l u r g i c a l p r o c e s s e s to r e c o v e r silv e r f r o m its w a s t e (scrap, c i n d e r s ) [1,2] a n d n a t u r a l m a t e r i a l s [3,4] s e e m s to be c o m p e t i t i v e to t h e t r a d i t i o n a l f u r n a c e m e t h o d s [5]. T h e a r g u m e n t s for t h e h y d r o m e t a l l u r g i c a l m e t h o d s are: e n v i r o n m e n t a l p r o tection, selective s e p a r a t i o n of silver f r o m t h e o t h e r c o m p o n e n t s of t h e t r e a t e d material and more economical small-scale operation. L e a c h i n g s o l u t i o n s of c y a n i d e s a n d of t h i o u r e a are t h e a g e n t s u s u a l l y a p p l i e d in t h e h y d r o m e t a l l u r g y o f silver [ 6, 7,8 ]. O u r p u r p o s e was to s t u d y t h e digesting of silver in s o l u t i o n s of ferric chloride a n d c o p p e r chloride. T h e r e a s o n s are o u t l i n e d as follows. Ferric chloride is still o n e o f t h e m o s t effective l e a c h i n g a g e n t s for sulphide m i n e r a l s a n d w a s t e m a t e r i a l s c o n t a i n i n g silver in t h e n a t u r a l or s u l p h i d e f o r m [9]. A p r e v i o u s p a p e r on t h e r e c o v e r y o f silver f r o m e l e c t r o n i c s c r a p m a t e r i a l s
220
by leaching with ferric chloride revealed the digesting of silver to be dependent upon the Fe ( I I I ) / F e (II) concentration ratio in the leaching solution. A decrease of the Fe ( I I I ) / F e ( I I ) concentration ratio as a result of leaching below a certain specific value (Fe ( I I I ) / F e ( I I ) < 1) results in a sudden increase in silver concentration in the leaching solution. The same effect was observed with copper ion concentration increase in the leaching solution [ 1 ]. The aim of the present work was to investigate the behaviour of silver in acid solutions of ferric chloride and copper chloride and dependence on leachant concentration and temperature, for its dissolution according to the following redox reactions: A g + nC1- = AgCI(~n - l ) - + e -, Fe+3+e-=Fe
n = 1,2,3,4
+2
Cu 2+ + n C 1 - + e - = CuCl(~n - l ) - ,
(1)
(2) n=2.3
2Cu 2+ + 4 C 1 - + 2 e - = Cu2C1]-
(3) (4)
EXPERIMENTAL
The digesting of silver in chloride solutions was examined. The sample of the metallic silver was a disc with an area of 1.77 cm e, rotating at a constant rate of 700 m i n - 1. The volume of the leaching solution was 250 mL and 2 mL samples were taken during leaching. The samples taken were analyzed for Ag(I), Cu(II) + C u ( I ) , and Fe(II) + F e ( I I I ) concentrations by the method of atomic absorption using a Perkin-Elmer 403 spectrophotometer. The potential of the silver in chloride solutions was measured vs. the saturated calomel electrode. Its changes were recorded on a "Sefram" type plotter. Measurements were performed in nitrogen saturated solutions. All solutions were prepared from analytical-grade reagents dissolved in distilled water. The chloride ion concentration in particular solutions was controlled using sodium chloride. Photographs of the silver surface after leaching were made by means of a Cambridge Instruments Stereoscan 180 electron-scanning microscope. LEACHING
The behaviour of silver in ferric chloride solution The effects of leaching time, ferric ion concentration and temperature on the silver digesting process in acid ferric chloride solutions was examined. Hydrochloric acid concentration and the total concentration of the chloride ions were held constant at 1 M and 4.2 M, respectively. Experiments were carried
221
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8
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TEMPERATURE 50°C
Z 0
~
150
0
50
Gt0
I 120 TIME
I 180
2&0
o rain
Fig. 1. Effect of ferric chloride concentration and of digestion time and temperature on digestion of silver; (1) 1 M FeCl~; (2) 0.5 M FeC13; (3) 0.1 M FeCl:~; (4) 0.01 M FeCl:,; (5) 0.001 M FeCl:,. The total chloride ion concentration is 4.2 M in all solutions.
600
e;, 2so .~
,'~
,~ 500
z 0
150
300
100 5
60
[ 120 TIME,
I 180
240
min
Fig. 2. Effect of ferric chloride concentration, of digestion time and of temperature on digestion of silver: (1) 1.01 M FeCI3; (2) 0.5 M FeOl3; (3) 0.] M FeC13; (4) 0.] M FeCI3; (5) 0.001 M FeC13. T h e total chloride ion c o n c e n t r a t i o n is 4.2 M in all solutions.
222 1s
3
g
~
0
.
5
O
-1
-2 -3 Io 9 [ Fe 3÷ } , n',ol .elm -3
Fig. 3. Dependenceof the initial silver digestion rate upon the logarithm of ferric ion concentration for different temperatures: (1) 30°C; (2) 50°C; (3) 70°C; (4) 90°C. out at ferric chloride concentrations from 10 -3 M to 1 M at 30, 50, 70 and 90°C. The kinetic curves are plotted in Figs. 1 and 2 and from these the initial rates of silver digestion were determined. The dependences of the initial silver digesting rates upon the logarithm of ferric ion concentration at various temperatures are shown in Fig. 3.
The behaviour of silver in copper chloride solution The effect of temperature and copper ion concentrations on the digestion of silver was examined for acid copper chloride solutions. Hydrochloric acid concentration and the total chloride ion concentration were held constant and equal to: 1M HC1 and 4.2 M C1- for 10 -3, 10 -2, 10 -1 M CuC12; 1 M HC1 and 3.7 M C l - for 0.92 M CuC12. The influence of temperature on silver digestion in solutions containing 0.92 M CuC12 at total chloride ion concentration 3.7 M C1- is shown in Fig. 4. Temperature increase is accompanied by silver digestion increase: after 3 h leaching at 90 ° C a rapid increase of silver concentration in the leaching solution was noted. The influence of copper ion concentration on silver digestion
223
700 '
Wl
E
500 3
II ,( 4.OO Z
o 300 ~J Z
200 i
,
100
I 180 240 TIME, rain
120
G0
I 300
Fig. 4. The effect of temperature and of digestion time on the digestion of silver in 0.92 M CuC12: (1) 50°C; (2) 70°C; (3) 90°C. The total chloride ion concentration is 3.7 M in all solutions.
300 i
fi
~ 250 200 Z
O U
150
m U
~I00
50
60
120 TIME,
180
24,0
rain
Fig. 5. The effect of CuCI2 concentration and of digestion time upon the digestion of silver at 50°C: (1) 0.92 M CuC12 total chloride ion concentration equal 3.7; (2) 0.1 M CuCl2; (3) 0.01 M CuC12; (4) 0.001 M CuCl2. The total chloride ion concentration is 4.2 M in all solutions.
224
"7
1'0I
0.5
-1 Iog(Cu 2+1,
-2 -3 mo|dm -3
Fig. 6. Dependenceof the initial rates of the digestionof silver on logarithmof copper ion concentration (at 50 °C), the total chlorideion concentrationis: ( O ) 4.2 M and ( • ) 3.7 M, respectively. at 50°C is presented in Fig. 5. The appropriate kinetic curves (Figs. 4 and 5) were used for determination of the initial rates. The influence of copper ion concentration and temperature on the initial rates are given in Fig. 6.
Comparison of the digestion of silver in ferric chloride and copper chloride solutions On the basis of relations shown in Figs. 3 and 6 it was proved that: (1). In the leaching solutions of 10 -3 M concentration at total chloride ion concentration about 4 M the digestion of silver in copper chloride solution is better t h a n t h a t in ferric chloride solution. The initial rates of digestion in copper chloride and ferric chloride at 50°C are 1.8 g A g / m 2 min and 1.2 g Ag/ m 2 min, respectively. (2). An increase in concentration of leaching solution from 10 -3 to 10 -2 M FeC13 or CuC12 results in a considerable increase of digestion of silver, and the initial rates of digestion in FeC13 and CuC12 solutions at 50 ° C are comparable (~ (C1-) about 4 M in all solutions). (3). The digestion rates in FeC13 and CuC12 solutions of higher concentrations (0.5 M, 1 M) decrease but copper chloride is more effective t h a n ferric chloride. (4). A temperature increase causes greater differences in the digestion of silver in concentrated solutions of FeC13 and CuC12. At 90°C the initial rate of the digesting of silver in 1 M FeC13 (~ (C1-) = 4 M) is 7.5 g A g / m 2 min , while in 0.92 M CuC12 (~ (C1-)--3.7) it is 16.9 g A g / m 2 min.
225
Differences in the digestion of silver in concentrated FeC13 and CuCl~ solutions are tentatively explained in terms of: strong effect of the chloride ions on solubility of the silver chloride, particularly at enhanced temperatures [ 10 ]; - ability of the chloride ions to form appropriate complex species with the iron and copper ions [11-13]; - possibility of the essential influence of oxidants such as FeC13 and CuC12 on the digestion process. With these possibilities in mind, - the free chloride ion concentration dependence on ferric chloride and copper chloride concentrations and the total chloride ion concentration in the system FeC13-HC1-H20 and CuC12-HC1-H20 was analyzed with the data available, - the behaviour of the silver electrode in copper chloride and ferric chloride solutions was examined, the change of the silver surface after leaching was investigated using scanning analysis. -
-
CONCENTRATION OF T H E FREE CHLORIDE IONS IN TH E FeCl~-HC1-H20 AND CuCI~-HCI-H~O SYSTEMS
With the assumption after Kimura [14], that the ferric and cupric ions in the presence of the chloride ions may form the following compounds: FeC1 +, FeCl~, FeC1 °, FeC14 and Cu 2+ , CuC1 +, CuC1 °, and on the basis of the equilibrium constant for the appropriate complexation reactions, the percent contribution of the free chloride ions dependence on FeCl~ or CuCl2 concentration and on the total chloride ion concentration in solution, was calculated. The results are presented in Tables 1 and 2. It should be noted that the results are approximate, because: - the data used for calculation are related only to normal temperature, TABLE1 Percent contribution of the free chloride ions and dependence on FeC13 concentration at total concentration of chloride ion 4M (25°C) [FeCl3 ] Contribution of the (M) free chloride ions
(%) 0.001 0.01 0.1 0.5 1.0
99.9 99.3 93.2 67.9 40.0
226 TABLE 2 Percent contribution of the free chloride ions and dependence on CuC12 concentration at total concentration of chloride ion 4 M (25 ° C) [CuCl2] Contribution of the free chloride ions
(%) 0.001 0.01 0.1 0.5 1.0
99.9 99.6 95.0 79.8 62.0
the concentrations of the appropriate ions, not their activities, are used; and were based on the available equilibrium constants values for complexation reactions, determined at the given ionic strengths of solutions, different from those of solutions used in leaching processes. The data obtained indicate the considerable differences in contribution of the free chloride ions in concentrated FeCl3 and CuC12 solutions. The free chloride ion concentration in 1 M FeC13 at total chloride ion concentration of 4 M is 20% lower than in the analogous CuC12 solution. The greater amount of free chloride ions in concentrated copper chloride solution affects the solubility of AgC1, the intermediate phase arising during the digestion of silver. Confirmation of the favourable influence of the copper chloride on the solubility increase of silver chloride in iron chloride in the temperature range 20-100 °C is found in the paper by Dinardo and Dutrizac [9]. -
-calculations
SILVER ELECTRODE POTENTIAL IN IRON CHLORIDE AND COPPER CHLORIDE SOLUTIONS
Studies on the behaviour of the silver electrode in iron and copper chlorides were performed to find out whether the enhanced effectiveness of the concentrated CuC12 compared with the concentrated FeC13 for digestion of silver results only from the smaller contribution of the free chloride ions in the FeC13 solution or depends upon the nature of the leaching agent alone. Measurements of the potential of a silver electrode were made in stationary conditions, in solutions of concentrations corresponding to concentrations of some selected solutions applied previously to the digestion of silver. The change of potential of the silver electrode vs. the saturated calomel electrode (SEK) was measured in the following solutions: 0.01 M FeC13, 0.01 M CuC12, 0.5 M FeC13, 1 M CuC12, ~ ( C 1 - ) = 4 M in all solutions, at 73°C. The appropriate relations o f EAg(vs.SEK) as a function of time are shown in Fig. 7. On these curves the sections characteristic of electrode processes are distinguished:
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Fig. 7. Variation of the silver electrode potential in digestion solutions at 73 ° C: (1) 0.01 FeCl:~; (2) 0.01 M CuC12; (3) 0.5 M CuC12; (4) 1 M CuCl2. The total chloride ion concentration is 4 M in all solutions.
AB - decrease of the silver electrode potential caused by formation of the "primary film" of the silver chloride, this effect could be called the "chloride shock of an electrode"; BC - stability of the silver electrode potential corresponding to the stability of the "primary film" of AgCl formed on the electrode surface; CD - increase of the silver electrode potential caused by dissolution of the AgC1 film and uncovering of the silver surface or by easy diffusion of the leaching agent through the AgC1 film; and DE - constant or intermediate stabilization of the electrode potential.
228 The intermediate stabilization of potential could be achieved by the "secondary" blocking of the electrode surface by the precipitating products of oxidation reaction (AgC1) and reduction reaction (CuC1), which could dissolve or appear in the form facilitating the access of solution to the electrode surface thus, EF - increase of the silver electrode potential caused by digestion of silver. In the solutions 0.01 M FeC13 and 0.01 M CuC12 the character of the relation EAg(vs.SEK)=f(time) is similar and the only differences are: - in FeCI~ solution the stability of the silver electrode potential for the "primary film" of AgC1 is observed in section BC; - in CuC12 solution the rapid drop of potential caused by the "chloride shock" of the electrode is followed by its equally rapid increase (section CD) related to the active digestion of the electrode: the electrode potential undergoes the intermediate stabilization (section DE) only in CuC12 solution; and - in FeC13 solution the stabilization potential of the "primary layer" of AgC1 (section BC) is followed by the slow systematic increase of the electrode potential. One of the reasons for differences in processes corresponding to sections AB, BC and CD could be the different structure of the silver chloride precipitating on the electrode surface. The probability is that the silver chloride formed in the copper chloride solution does not screen the electrode surface so well as in the FeC13 solution, and thus it is more accessible for the leaching agent. Thus, the effectiveness of the digestion of silver in 0.01 M CuC12 exceeding that in 0.01 M FeC13 should be noted only in the first minutes of leaching. In 0.5 M solution of FeCI~ the potential of the silver electrode exceeds that in solutions of 0.01 M FeC12 and 0.01 M CuC12. The sections of the curve EAg(vs.SEK)=f(time) corresponding to the "chloride shock" of the electrode, AB, to the stability of the primary layer of AgCl, BC, to the active digestion of the silver electrode, CD, and to the secondary stabilization of a potential, DE, can be distinguished. The constant potential value of the silver electrode suggests strong passivation of the electrode by the stable AgC1 layer, formed as the result of its secondary precipitation. In 1 M solution of CuC12 the potential of the silver electrode exceeds that in 0.5 FeC13 solutions (see Fig. 7). The immersion of the silver electrode in 1 M CuC12 solution was followed by the rapid increase of its potential section CD (see Fig. 7), and next, by its temporary stabilization (section DE). After the temporary stabilization, the electrode potential increased; this is indicative of digestion of the silver electrode. The colour of the surface of electrodes immersed in 1 M CuC12 solution of ( C1- ) -- 4 M at 73 ° C does not change, but the electrode surface becomes somewhat matt. This is different from electrodes immersed in the other solutions, where the surface changed colour to dark brown.
229 SCANNING ANALYSISOF THE SILVERSURFACEAFTER LEACHING The surface of the silver discs was analysed by scanning after leaching at 70 °C in the following solutions: 0.01 M FeC13, 0.01 M CuC12, 1 M FeC13 and 1 M CuC12, ~ (C1-) = 4 M in all solutions. The leaching was performed with and without stirring. The nature of the silver surface after leaching was found to depend on leaching (see Fig. 8.) After leaching under stirring in 0.01 M CuC12 solutions two layers could be observed on the silver surface, one amorphous and another covered with crystals (Fig. 8a). The silver surface after leaching under stirring in 0.01 M FeC13 is different (Fig. 8b). The layer on the silver surface is like a porous sponge. Similar differences in the structure of layers on the silver surface were observed after leaching under stationary conditions. The character of the surface of silver leached under the stationary conditions and under stirring indicates that the silver surface should be easily accessible for leaching. This was confirmed by electro-chemical studies and by the study of the leaching of silver in these solutions. The presence of the crystalline species on the surface of silver leached in 0.01 M CuC12 solution could be indicative of the influence of copper ions on the form of the secondary silver chloride formed or for the formation of mixed AgC1 and CuC1 crystals. The precipitate coating the surface of silver leached in 1 M FeCl~ or 1 M CuC12 is grained (under the same hydrodynamic conditions). In 1 M CuC12 solution the grains are finer than in 1 M FeCl3 (Fig. 8c). In 1 M FeC13 solution the precipitate grains are thicker and stick more closely to the silver surface (Fig. 8d). CONCLUSION Experiments on the digestion of silver in acid solutions of iron chloride and copper chloride at constant chloride ion concentration, calculations of the equilibrium concentration of the free chloride ions in the solutions, together with observation of the behaviour of the silver electrode in ferric chloride and cupric chloride allow us to state that: ( 1 ) There is a dependence of the initial rates of the digestion of silver on the leachant concentration. In ferric and cupric chloride solutions an increase in leachant concentration from 10 -3 to 10 .-2 caused a rapid increase in the silver digestion rate. In leaching solutions of concentration 10 -2 to 10-1 M rates are comparable. Further increase of concentration gives a considerable decrease of the initial rates of the digestion of silver.
t~
231
Fig. 8. Scanning electrode micrograph of the silver surface after digestion in the following solutions at 73°C (magnified×3000). (a) 0.1 M CuC12; (b) 0.01 M FeC13; (c) 1 M CuC12; (d) 1 M FeCl:~. The total chloride ion concentration is 4 M all solutions.
232
(2) Considerably higher digestion of silver is obtained in concentrated copper chloride solutions, compared to that in equally concentrated ferric chloride solution at comparable total chloride ion concentration. A temperature increase caused greater differences in the digestion of silver. (3) The contribution of the free chloride ions at total constant concentration in copper chloride solution always exceeds that in iron chloride (at 25 ° C ). (4) AgC1 is formed during the digestion of silver in all investigated chloride solutions. This was confirmed by means of the measurements of silver electrode potential and by scanning analysis of the surface of silver after leaching. The measurements of the silver electrode potential in 0.01 M CuC12 solution and in 0.01 M FeC13 solution show the possibility of greater effectiveness of silver digestion in CuC12 than in FeC13 solution (see section CD Fig. 7). The greater effectiveness of digestion of silver in CuC12 solution should be observed during the first minutes of leaching. An increase in digestion time shows that the rate depends on the solubility of AgC1 (see section EF and DF Fig. 7 (1,2)). In a dilute solution of CuC12 and FeCI~ at the same temperature and at comparable chloride ion concentration, the initial rates of digestion of silver were almost the same. (5) The differences in the passivation of silver in concentrated solutions of cupric and ferric chlorides, the higher free chloride ion concentration in CuC12 than in FeC13 solution, and the different form of silver chloride formed in these solutions, show why digestion of silver in concentrated cupric chloride is much more effective than in ferric chloride. Thus the initial rates of digestion in 0.92 M CuC12 and in 1 M FeC13 in the presence of total concentration of chloride ion of about 4 M are 10.1 g / m 2 min and 4.8 g / m 2 min at 70°C, 16.9 and 7.5 Ag/m e min at 90°C, respectively.
REFERENCES 1 2
3 4 5 6 7
Kotodziej, B. and Adamski, Z., A ferric chloride hydrometallurgical process for recovery of silver from electronic scrap materials, Hydrometallurgy, 12 (1984) 117-127. Nfifiez Alvarez, C. and Oliel, J.V., Study of Spanish pyrite cincders CI2-HCI leaching of fluidized-bed arsenical cinders, Trans Inst. Min. Metall. Sect. C; Mineral Process. Extr. Metall. 93 (December 1984) C 162-179. Reynolds, J.E. and Williams, A.R., Hydrometallurgical recovery of lead, Can. CA 1: 186-903. Prodak, J.J. and Vacker, A., Recovery of silver from leaching of ores, Fr. Domande, FR 2: (556) 370. Schack, C.H. and Clemmons, B.H., In: Butts A. (Ed), Silver: Economics, Metallurgy and Use, Van Nostrand, Princeton, NJ, 1967, pp. 57-77. Chamberlain, P.G. and Pojar, M.G., Gold and silver leaching practices in the United States, Bureau of Mines Information Circular, 1984. Schulze, R., Recovery of noble metals from ores, Ger. Often. DE 3 (347) 165.
233 8
Sumito Metal Mining Co. Ltd., Recovery of gold and silver in aqueous thiourea, Jpn. Kokai Tokkyo JP 60 {103) 138. 9 Dinardo, O. and Dutrizac, J.E., The solubility of silver chloride in ferric chloride leaching media, Hydrometallurgy 13 (1985) 345-363. 10 Seward, T.M., The stability of chloride complexes of silver in hydrothermal solutions up to 350°C, Geochim. Cosmochim. Acta, 40 (1976) 1329-1371. 11 Smith, R.M. and Martell, A.E., Critical stability constants, Vol. 4, Inorganic Complexes, Plenum Press, New York, NY, 1976 pp. 105-106. 12 Carlson, B. and Werrermark, G., Determination of the equilibrium constants and UV absorption spectra for Cu2+-C1 - complexes in 1 M HCI04, Inorg. Nucl. Chem. (1976) 15251527. 13 Sillen, L.G. and Martell, A.E., Stability Constants, Spec. Publ. No. 17, Chemical Soc., London, 1964. 14 Kimura, R.T., Haunschild, P.A. and Liddell, K.C., A mathematical model for calculation of equilibrium solution speciations for the FeCI.~-FeCI.~-CuC12-CuCI-HCINaC1-H20 system at 25°C, Metall. Trans. B, 15B (June 1984) 213.
219
Digestion of S i l v e r in Acidic Ferric Chloride and Copper Chloride Solutions BARBARA KO[.ODZIEJ Institute of Inorganic Chemistry and Metallurgy of Rare Elements, Technical University, 50370 Wroctaw (Poland) (Received June 18, 1987; accepted in revised form October 11, 1987)
ABSTRACT Kolodziej, B., 1988. Digestion of silver in acidic ferric chloride and copper chloride solutions. HydrometaUurgy, 20: 219-233. The digesting of silver has been carried out at different concentrations of leaching agents FeCl3 and CuCl2 in the range 30-90°C. On the basis of the kinetic curves obtained by experiment the initial digestion rates were calculated. It was found that at 0.01-0.1 M FeCl~ or CuCl2 the initial rates of digestion are about the same and a further increase in concentration of the leaching agent leads to a decrease in the initial rates of the digesting process. It should be stressed that the digestion of silver at a concentration of CuC12 of 1 M is much faster than in the case of 1 M FeC13. These differences are analysed by means of the potentiometric method and scanning analysis of the surface of silver after digestion.
INTRODUCTION T h e p o s s i b i l i t y o f a p p l i c a t i o n of h y d r o m e t a l l u r g i c a l p r o c e s s e s to r e c o v e r silv e r f r o m its w a s t e (scrap, c i n d e r s ) [1,2] a n d n a t u r a l m a t e r i a l s [3,4] s e e m s to be c o m p e t i t i v e to t h e t r a d i t i o n a l f u r n a c e m e t h o d s [5]. T h e a r g u m e n t s for t h e h y d r o m e t a l l u r g i c a l m e t h o d s are: e n v i r o n m e n t a l p r o tection, selective s e p a r a t i o n of silver f r o m t h e o t h e r c o m p o n e n t s of t h e t r e a t e d material and more economical small-scale operation. L e a c h i n g s o l u t i o n s of c y a n i d e s a n d of t h i o u r e a are t h e a g e n t s u s u a l l y a p p l i e d in t h e h y d r o m e t a l l u r g y o f silver [ 6, 7,8 ]. O u r p u r p o s e was to s t u d y t h e digesting of silver in s o l u t i o n s of ferric chloride a n d c o p p e r chloride. T h e r e a s o n s are o u t l i n e d as follows. Ferric chloride is still o n e o f t h e m o s t effective l e a c h i n g a g e n t s for sulphide m i n e r a l s a n d w a s t e m a t e r i a l s c o n t a i n i n g silver in t h e n a t u r a l or s u l p h i d e f o r m [9]. A p r e v i o u s p a p e r on t h e r e c o v e r y o f silver f r o m e l e c t r o n i c s c r a p m a t e r i a l s
220
by leaching with ferric chloride revealed the digesting of silver to be dependent upon the Fe ( I I I ) / F e (II) concentration ratio in the leaching solution. A decrease of the Fe ( I I I ) / F e ( I I ) concentration ratio as a result of leaching below a certain specific value (Fe ( I I I ) / F e ( I I ) < 1) results in a sudden increase in silver concentration in the leaching solution. The same effect was observed with copper ion concentration increase in the leaching solution [ 1 ]. The aim of the present work was to investigate the behaviour of silver in acid solutions of ferric chloride and copper chloride and dependence on leachant concentration and temperature, for its dissolution according to the following redox reactions: A g + nC1- = AgCI(~n - l ) - + e -, Fe+3+e-=Fe
n = 1,2,3,4
+2
Cu 2+ + n C 1 - + e - = CuCl(~n - l ) - ,
(1)
(2) n=2.3
2Cu 2+ + 4 C 1 - + 2 e - = Cu2C1]-
(3) (4)
EXPERIMENTAL
The digesting of silver in chloride solutions was examined. The sample of the metallic silver was a disc with an area of 1.77 cm e, rotating at a constant rate of 700 m i n - 1. The volume of the leaching solution was 250 mL and 2 mL samples were taken during leaching. The samples taken were analyzed for Ag(I), Cu(II) + C u ( I ) , and Fe(II) + F e ( I I I ) concentrations by the method of atomic absorption using a Perkin-Elmer 403 spectrophotometer. The potential of the silver in chloride solutions was measured vs. the saturated calomel electrode. Its changes were recorded on a "Sefram" type plotter. Measurements were performed in nitrogen saturated solutions. All solutions were prepared from analytical-grade reagents dissolved in distilled water. The chloride ion concentration in particular solutions was controlled using sodium chloride. Photographs of the silver surface after leaching were made by means of a Cambridge Instruments Stereoscan 180 electron-scanning microscope. LEACHING
The behaviour of silver in ferric chloride solution The effects of leaching time, ferric ion concentration and temperature on the silver digesting process in acid ferric chloride solutions was examined. Hydrochloric acid concentration and the total concentration of the chloride ions were held constant at 1 M and 4.2 M, respectively. Experiments were carried
221
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Fig. 1. Effect of ferric chloride concentration and of digestion time and temperature on digestion of silver; (1) 1 M FeCl~; (2) 0.5 M FeC13; (3) 0.1 M FeCl:~; (4) 0.01 M FeCl:,; (5) 0.001 M FeCl:,. The total chloride ion concentration is 4.2 M in all solutions.
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300
100 5
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[ 120 TIME,
I 180
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Fig. 2. Effect of ferric chloride concentration, of digestion time and of temperature on digestion of silver: (1) 1.01 M FeCI3; (2) 0.5 M FeOl3; (3) 0.] M FeC13; (4) 0.] M FeCI3; (5) 0.001 M FeC13. T h e total chloride ion c o n c e n t r a t i o n is 4.2 M in all solutions.
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.
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Fig. 3. Dependenceof the initial silver digestion rate upon the logarithm of ferric ion concentration for different temperatures: (1) 30°C; (2) 50°C; (3) 70°C; (4) 90°C. out at ferric chloride concentrations from 10 -3 M to 1 M at 30, 50, 70 and 90°C. The kinetic curves are plotted in Figs. 1 and 2 and from these the initial rates of silver digestion were determined. The dependences of the initial silver digesting rates upon the logarithm of ferric ion concentration at various temperatures are shown in Fig. 3.
The behaviour of silver in copper chloride solution The effect of temperature and copper ion concentrations on the digestion of silver was examined for acid copper chloride solutions. Hydrochloric acid concentration and the total chloride ion concentration were held constant and equal to: 1M HC1 and 4.2 M C1- for 10 -3, 10 -2, 10 -1 M CuC12; 1 M HC1 and 3.7 M C l - for 0.92 M CuC12. The influence of temperature on silver digestion in solutions containing 0.92 M CuC12 at total chloride ion concentration 3.7 M C1- is shown in Fig. 4. Temperature increase is accompanied by silver digestion increase: after 3 h leaching at 90 ° C a rapid increase of silver concentration in the leaching solution was noted. The influence of copper ion concentration on silver digestion
223
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o 300 ~J Z
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,
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I 180 240 TIME, rain
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I 300
Fig. 4. The effect of temperature and of digestion time on the digestion of silver in 0.92 M CuC12: (1) 50°C; (2) 70°C; (3) 90°C. The total chloride ion concentration is 3.7 M in all solutions.
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50
60
120 TIME,
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Fig. 5. The effect of CuCI2 concentration and of digestion time upon the digestion of silver at 50°C: (1) 0.92 M CuC12 total chloride ion concentration equal 3.7; (2) 0.1 M CuCl2; (3) 0.01 M CuC12; (4) 0.001 M CuCl2. The total chloride ion concentration is 4.2 M in all solutions.
224
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Fig. 6. Dependenceof the initial rates of the digestionof silver on logarithmof copper ion concentration (at 50 °C), the total chlorideion concentrationis: ( O ) 4.2 M and ( • ) 3.7 M, respectively. at 50°C is presented in Fig. 5. The appropriate kinetic curves (Figs. 4 and 5) were used for determination of the initial rates. The influence of copper ion concentration and temperature on the initial rates are given in Fig. 6.
Comparison of the digestion of silver in ferric chloride and copper chloride solutions On the basis of relations shown in Figs. 3 and 6 it was proved that: (1). In the leaching solutions of 10 -3 M concentration at total chloride ion concentration about 4 M the digestion of silver in copper chloride solution is better t h a n t h a t in ferric chloride solution. The initial rates of digestion in copper chloride and ferric chloride at 50°C are 1.8 g A g / m 2 min and 1.2 g Ag/ m 2 min, respectively. (2). An increase in concentration of leaching solution from 10 -3 to 10 -2 M FeC13 or CuC12 results in a considerable increase of digestion of silver, and the initial rates of digestion in FeC13 and CuC12 solutions at 50 ° C are comparable (~ (C1-) about 4 M in all solutions). (3). The digestion rates in FeC13 and CuC12 solutions of higher concentrations (0.5 M, 1 M) decrease but copper chloride is more effective t h a n ferric chloride. (4). A temperature increase causes greater differences in the digestion of silver in concentrated solutions of FeC13 and CuC12. At 90°C the initial rate of the digesting of silver in 1 M FeC13 (~ (C1-) = 4 M) is 7.5 g A g / m 2 min , while in 0.92 M CuC12 (~ (C1-)--3.7) it is 16.9 g A g / m 2 min.
225
Differences in the digestion of silver in concentrated FeC13 and CuCl~ solutions are tentatively explained in terms of: strong effect of the chloride ions on solubility of the silver chloride, particularly at enhanced temperatures [ 10 ]; - ability of the chloride ions to form appropriate complex species with the iron and copper ions [11-13]; - possibility of the essential influence of oxidants such as FeC13 and CuC12 on the digestion process. With these possibilities in mind, - the free chloride ion concentration dependence on ferric chloride and copper chloride concentrations and the total chloride ion concentration in the system FeC13-HC1-H20 and CuC12-HC1-H20 was analyzed with the data available, - the behaviour of the silver electrode in copper chloride and ferric chloride solutions was examined, the change of the silver surface after leaching was investigated using scanning analysis. -
-
CONCENTRATION OF T H E FREE CHLORIDE IONS IN TH E FeCl~-HC1-H20 AND CuCI~-HCI-H~O SYSTEMS
With the assumption after Kimura [14], that the ferric and cupric ions in the presence of the chloride ions may form the following compounds: FeC1 +, FeCl~, FeC1 °, FeC14 and Cu 2+ , CuC1 +, CuC1 °, and on the basis of the equilibrium constant for the appropriate complexation reactions, the percent contribution of the free chloride ions dependence on FeCl~ or CuCl2 concentration and on the total chloride ion concentration in solution, was calculated. The results are presented in Tables 1 and 2. It should be noted that the results are approximate, because: - the data used for calculation are related only to normal temperature, TABLE1 Percent contribution of the free chloride ions and dependence on FeC13 concentration at total concentration of chloride ion 4M (25°C) [FeCl3 ] Contribution of the (M) free chloride ions
(%) 0.001 0.01 0.1 0.5 1.0
99.9 99.3 93.2 67.9 40.0
226 TABLE 2 Percent contribution of the free chloride ions and dependence on CuC12 concentration at total concentration of chloride ion 4 M (25 ° C) [CuCl2] Contribution of the free chloride ions
(%) 0.001 0.01 0.1 0.5 1.0
99.9 99.6 95.0 79.8 62.0
the concentrations of the appropriate ions, not their activities, are used; and were based on the available equilibrium constants values for complexation reactions, determined at the given ionic strengths of solutions, different from those of solutions used in leaching processes. The data obtained indicate the considerable differences in contribution of the free chloride ions in concentrated FeCl3 and CuC12 solutions. The free chloride ion concentration in 1 M FeC13 at total chloride ion concentration of 4 M is 20% lower than in the analogous CuC12 solution. The greater amount of free chloride ions in concentrated copper chloride solution affects the solubility of AgC1, the intermediate phase arising during the digestion of silver. Confirmation of the favourable influence of the copper chloride on the solubility increase of silver chloride in iron chloride in the temperature range 20-100 °C is found in the paper by Dinardo and Dutrizac [9]. -
-calculations
SILVER ELECTRODE POTENTIAL IN IRON CHLORIDE AND COPPER CHLORIDE SOLUTIONS
Studies on the behaviour of the silver electrode in iron and copper chlorides were performed to find out whether the enhanced effectiveness of the concentrated CuC12 compared with the concentrated FeC13 for digestion of silver results only from the smaller contribution of the free chloride ions in the FeC13 solution or depends upon the nature of the leaching agent alone. Measurements of the potential of a silver electrode were made in stationary conditions, in solutions of concentrations corresponding to concentrations of some selected solutions applied previously to the digestion of silver. The change of potential of the silver electrode vs. the saturated calomel electrode (SEK) was measured in the following solutions: 0.01 M FeC13, 0.01 M CuC12, 0.5 M FeC13, 1 M CuC12, ~ ( C 1 - ) = 4 M in all solutions, at 73°C. The appropriate relations o f EAg(vs.SEK) as a function of time are shown in Fig. 7. On these curves the sections characteristic of electrode processes are distinguished:
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Fig. 7. Variation of the silver electrode potential in digestion solutions at 73 ° C: (1) 0.01 FeCl:~; (2) 0.01 M CuC12; (3) 0.5 M CuC12; (4) 1 M CuCl2. The total chloride ion concentration is 4 M in all solutions.
AB - decrease of the silver electrode potential caused by formation of the "primary film" of the silver chloride, this effect could be called the "chloride shock of an electrode"; BC - stability of the silver electrode potential corresponding to the stability of the "primary film" of AgCl formed on the electrode surface; CD - increase of the silver electrode potential caused by dissolution of the AgC1 film and uncovering of the silver surface or by easy diffusion of the leaching agent through the AgC1 film; and DE - constant or intermediate stabilization of the electrode potential.
228 The intermediate stabilization of potential could be achieved by the "secondary" blocking of the electrode surface by the precipitating products of oxidation reaction (AgC1) and reduction reaction (CuC1), which could dissolve or appear in the form facilitating the access of solution to the electrode surface thus, EF - increase of the silver electrode potential caused by digestion of silver. In the solutions 0.01 M FeC13 and 0.01 M CuC12 the character of the relation EAg(vs.SEK)=f(time) is similar and the only differences are: - in FeCI~ solution the stability of the silver electrode potential for the "primary film" of AgC1 is observed in section BC; - in CuC12 solution the rapid drop of potential caused by the "chloride shock" of the electrode is followed by its equally rapid increase (section CD) related to the active digestion of the electrode: the electrode potential undergoes the intermediate stabilization (section DE) only in CuC12 solution; and - in FeC13 solution the stabilization potential of the "primary layer" of AgC1 (section BC) is followed by the slow systematic increase of the electrode potential. One of the reasons for differences in processes corresponding to sections AB, BC and CD could be the different structure of the silver chloride precipitating on the electrode surface. The probability is that the silver chloride formed in the copper chloride solution does not screen the electrode surface so well as in the FeC13 solution, and thus it is more accessible for the leaching agent. Thus, the effectiveness of the digestion of silver in 0.01 M CuC12 exceeding that in 0.01 M FeC13 should be noted only in the first minutes of leaching. In 0.5 M solution of FeCI~ the potential of the silver electrode exceeds that in solutions of 0.01 M FeC12 and 0.01 M CuC12. The sections of the curve EAg(vs.SEK)=f(time) corresponding to the "chloride shock" of the electrode, AB, to the stability of the primary layer of AgCl, BC, to the active digestion of the silver electrode, CD, and to the secondary stabilization of a potential, DE, can be distinguished. The constant potential value of the silver electrode suggests strong passivation of the electrode by the stable AgC1 layer, formed as the result of its secondary precipitation. In 1 M solution of CuC12 the potential of the silver electrode exceeds that in 0.5 FeC13 solutions (see Fig. 7). The immersion of the silver electrode in 1 M CuC12 solution was followed by the rapid increase of its potential section CD (see Fig. 7), and next, by its temporary stabilization (section DE). After the temporary stabilization, the electrode potential increased; this is indicative of digestion of the silver electrode. The colour of the surface of electrodes immersed in 1 M CuC12 solution of ( C1- ) -- 4 M at 73 ° C does not change, but the electrode surface becomes somewhat matt. This is different from electrodes immersed in the other solutions, where the surface changed colour to dark brown.
229 SCANNING ANALYSISOF THE SILVERSURFACEAFTER LEACHING The surface of the silver discs was analysed by scanning after leaching at 70 °C in the following solutions: 0.01 M FeC13, 0.01 M CuC12, 1 M FeC13 and 1 M CuC12, ~ (C1-) = 4 M in all solutions. The leaching was performed with and without stirring. The nature of the silver surface after leaching was found to depend on leaching (see Fig. 8.) After leaching under stirring in 0.01 M CuC12 solutions two layers could be observed on the silver surface, one amorphous and another covered with crystals (Fig. 8a). The silver surface after leaching under stirring in 0.01 M FeC13 is different (Fig. 8b). The layer on the silver surface is like a porous sponge. Similar differences in the structure of layers on the silver surface were observed after leaching under stationary conditions. The character of the surface of silver leached under the stationary conditions and under stirring indicates that the silver surface should be easily accessible for leaching. This was confirmed by electro-chemical studies and by the study of the leaching of silver in these solutions. The presence of the crystalline species on the surface of silver leached in 0.01 M CuC12 solution could be indicative of the influence of copper ions on the form of the secondary silver chloride formed or for the formation of mixed AgC1 and CuC1 crystals. The precipitate coating the surface of silver leached in 1 M FeCl~ or 1 M CuC12 is grained (under the same hydrodynamic conditions). In 1 M CuC12 solution the grains are finer than in 1 M FeCl3 (Fig. 8c). In 1 M FeC13 solution the precipitate grains are thicker and stick more closely to the silver surface (Fig. 8d). CONCLUSION Experiments on the digestion of silver in acid solutions of iron chloride and copper chloride at constant chloride ion concentration, calculations of the equilibrium concentration of the free chloride ions in the solutions, together with observation of the behaviour of the silver electrode in ferric chloride and cupric chloride allow us to state that: ( 1 ) There is a dependence of the initial rates of the digestion of silver on the leachant concentration. In ferric and cupric chloride solutions an increase in leachant concentration from 10 -3 to 10 .-2 caused a rapid increase in the silver digestion rate. In leaching solutions of concentration 10 -2 to 10-1 M rates are comparable. Further increase of concentration gives a considerable decrease of the initial rates of the digestion of silver.
t~
231
Fig. 8. Scanning electrode micrograph of the silver surface after digestion in the following solutions at 73°C (magnified×3000). (a) 0.1 M CuC12; (b) 0.01 M FeC13; (c) 1 M CuC12; (d) 1 M FeCl:~. The total chloride ion concentration is 4 M all solutions.
232
(2) Considerably higher digestion of silver is obtained in concentrated copper chloride solutions, compared to that in equally concentrated ferric chloride solution at comparable total chloride ion concentration. A temperature increase caused greater differences in the digestion of silver. (3) The contribution of the free chloride ions at total constant concentration in copper chloride solution always exceeds that in iron chloride (at 25 ° C ). (4) AgC1 is formed during the digestion of silver in all investigated chloride solutions. This was confirmed by means of the measurements of silver electrode potential and by scanning analysis of the surface of silver after leaching. The measurements of the silver electrode potential in 0.01 M CuC12 solution and in 0.01 M FeC13 solution show the possibility of greater effectiveness of silver digestion in CuC12 than in FeC13 solution (see section CD Fig. 7). The greater effectiveness of digestion of silver in CuC12 solution should be observed during the first minutes of leaching. An increase in digestion time shows that the rate depends on the solubility of AgC1 (see section EF and DF Fig. 7 (1,2)). In a dilute solution of CuC12 and FeCI~ at the same temperature and at comparable chloride ion concentration, the initial rates of digestion of silver were almost the same. (5) The differences in the passivation of silver in concentrated solutions of cupric and ferric chlorides, the higher free chloride ion concentration in CuC12 than in FeC13 solution, and the different form of silver chloride formed in these solutions, show why digestion of silver in concentrated cupric chloride is much more effective than in ferric chloride. Thus the initial rates of digestion in 0.92 M CuC12 and in 1 M FeC13 in the presence of total concentration of chloride ion of about 4 M are 10.1 g / m 2 min and 4.8 g / m 2 min at 70°C, 16.9 and 7.5 Ag/m e min at 90°C, respectively.
REFERENCES 1 2
3 4 5 6 7
Kotodziej, B. and Adamski, Z., A ferric chloride hydrometallurgical process for recovery of silver from electronic scrap materials, Hydrometallurgy, 12 (1984) 117-127. Nfifiez Alvarez, C. and Oliel, J.V., Study of Spanish pyrite cincders CI2-HCI leaching of fluidized-bed arsenical cinders, Trans Inst. Min. Metall. Sect. C; Mineral Process. Extr. Metall. 93 (December 1984) C 162-179. Reynolds, J.E. and Williams, A.R., Hydrometallurgical recovery of lead, Can. CA 1: 186-903. Prodak, J.J. and Vacker, A., Recovery of silver from leaching of ores, Fr. Domande, FR 2: (556) 370. Schack, C.H. and Clemmons, B.H., In: Butts A. (Ed), Silver: Economics, Metallurgy and Use, Van Nostrand, Princeton, NJ, 1967, pp. 57-77. Chamberlain, P.G. and Pojar, M.G., Gold and silver leaching practices in the United States, Bureau of Mines Information Circular, 1984. Schulze, R., Recovery of noble metals from ores, Ger. Often. DE 3 (347) 165.
233 8
Sumito Metal Mining Co. Ltd., Recovery of gold and silver in aqueous thiourea, Jpn. Kokai Tokkyo JP 60 {103) 138. 9 Dinardo, O. and Dutrizac, J.E., The solubility of silver chloride in ferric chloride leaching media, Hydrometallurgy 13 (1985) 345-363. 10 Seward, T.M., The stability of chloride complexes of silver in hydrothermal solutions up to 350°C, Geochim. Cosmochim. Acta, 40 (1976) 1329-1371. 11 Smith, R.M. and Martell, A.E., Critical stability constants, Vol. 4, Inorganic Complexes, Plenum Press, New York, NY, 1976 pp. 105-106. 12 Carlson, B. and Werrermark, G., Determination of the equilibrium constants and UV absorption spectra for Cu2+-C1 - complexes in 1 M HCI04, Inorg. Nucl. Chem. (1976) 15251527. 13 Sillen, L.G. and Martell, A.E., Stability Constants, Spec. Publ. No. 17, Chemical Soc., London, 1964. 14 Kimura, R.T., Haunschild, P.A. and Liddell, K.C., A mathematical model for calculation of equilibrium solution speciations for the FeCI.~-FeCI.~-CuC12-CuCI-HCINaC1-H20 system at 25°C, Metall. Trans. B, 15B (June 1984) 213.