Thiourea leaching of silver from mechanically activated tetrahedrite

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hydrometallurgy Hydrometallurgy43 (1996) 367-377

ELSEVIER

Thiourea leaching of silver from mechanically activated tetrahedrite Peter B a l ~ a

a,*

Jana Ficeriovfi a Vladirnfr SepelLk Roland Kammel u

a

Institute of Geotechnics, Slovak Academy of Sciences, Watsonova 45, 043 53 Ko~ice, Slovak Republic b Technical University, Institute of Metallurgy, D-10623 Berlin, Germany

Received 13 October 1995; accepted 28 January 1996

Abstract

The thiourea leaching of silver from a tetrahedrite concentrate mechanically activated in a planetary mill or an attritor was studied. It was found that the two types of equipment gave rise to different rates of new surface formation and of crystal structure disordering. The rate of thiourea leaching of silver from tetrahedrite (Cu,Ag)lo(Zn,Fe)E(Sb,As)4S 13 is a structure-sensitive quantity, while the dependence of the rate constant of leaching on the empirical coefficient S g / ( l - R) (S A = specific surface, R = disordering of tetrahedrite structure) exhibits a linear character with equal slope for both types of mills. The results are also of prognostic character because they enable us to propose suitable equipment for intensive grinding depending on the demand for fineness or reactivity of the solid substances.

1. I n t r o d u c t i o n

at~ • considerable natural resource of silver. Gasparrini mentions 200 ~ re'her in major, minor and variable amounts [1]. However, of these, IO-!:2 ~ are of practical importance. These are, in order of leachability: m'htut,~ ~ halides and silver sulphides [2]. Of ~ m'lver-eontaining sulphides, tetrahedrite (general formula {IEM,~g~]0(Zn,Fe)2(Sb,As)4St3) exhibits very poor extractability of silver. Experiments

* Corresponding author. 0304-386X/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. PH S0304-386X(96)0001 5- 1

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in the U.S.A. by the Bureau of Mines have shown that only 25% of silver can be extracted by cyanide leaching from tetrahedrite without any pretreatment [3]. Similarly, Stofko indicates recovery of up to 10% of silver for leaching tetrahedrite with thiourea [4]. When tetrahedrite was pretreated in different ways, silver leaching improved considerably. Frenay applied biological pretreatment and cyanided the residue to achieve 85% leaching of the silver [5]. Anderson [6] reground the leached tetrahedrite and used sodium nitrite/sulphuric acid pressure leaching for silver recovery. This method of pretreatment made it possible to achieve recoveries of 92.5% Ag. Besides biological and chemical pretreatment, increased extraction of precious metals from refractory concentrates can also be achieved by ultrafine grinding. Suitable mills are being intensively tested and marketed in South Africa and Australia for processing refractory ores containing precious metals [7,8]. The extraction of silver can also be improved by changing the method of leaching. For example, thiourea, CS(NH2) 2, represents an attractive leaching alternative, stabilizing silver ions as a stable complex [9,10]: Ag + + 3CS(NH2) 2 ~ Ag[CS(NH2)2] ;

(1)

Pesic and Seal [11] have stated that the dissolution of silver in thiourea also requires ferric ion as the oxidizing agent in the solution. The reported advantages of acidic thiourea solution in the presence of Fe 3+ over classical cyanide leaching are: low toxicity, faster dissolution rate and higher selectivity [12]. In solid-state reactions stimulated by mechanical treatment the problem of decisive factors determining their course is very important. It was Senna [13] who analysed the problem: which of the structural parameters or the surface area (i.e., surface area determined by the air permeability method, the area of the external surface of the boundary area between the crystallites) is predominant for the reaction. Later, TkA~ovA and Balfi~ introduced into the kinetics of mechanically activated solids the empirical coefficient SA/X (S A = surface area; X = crystallinity) as a measure of the structural sensitivity [14]. Subsequently, its usefulness was verified in reactivity investigations of other leaching reactions [15]. The aim of this study was to demonstrate the usefulness of mechanical activation as a method of physical pretreatment to enhance the thiourea leaching of silver from a refractory tetrahedrite concentrate.

2. Experimental 2.1. M a t e r i a l s

The investigations were carried out with a tetrahedrite flotation concentrate (M~iriaR o ~ a v a deposit, Slovakia) with the following chemical composition: 21.0% Cu, 10.8% Sb, 28.3% S, 21.5% Fe, 2.15% Zn, 1.06% As, 1.05% Hg and 0.21% Ag. X-ray diffraction analysis showed the following proportions of dominant phases: 41% tetrahedrite, 12% chalcopyrite and 25% pyrite.

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2.2. Mechanical activation The mechanical activation of 'as received' concentrate was performed in two mills: (1) Planetary mill Pulverisette 4 (Fritsch, Germany) under the following conditions: tungsten carbide grinding of 350 cm 3 volume; weight of sample 20 g; tungsten carbide balls (25 pieces of 10 mm diameter plus 5 pieces of 25 mm diameter) as grinding means; grinding medium air; relative acceleration (against gravitational acceleration) of the mill b / g = 10.3; grinding times 2, 5, 10, 15, 20, 30, 45, 60 and 90 min. (2) Stirring ball mill (attritor) Molinex P E / 0 7 5 (Netzsch, Germany) under the following conditions: volume of grinding chamber 500 ml; weight of sample 20 g; iron balls (2000 g of 2 mm diameter) as grinding means; grinding medium water; grinding times 10, 20, 40, 80 and 160 min.

2.3. Physico-chemical characteristic 2.3.1. Surface area The specific surface area, SA, was determined by the low temperature nitrogen adsorption method in a Gemini 2360 sorption apparatus (Micromeritics, USA). 2.3.2. X-ray diffraction analysis X-ray diffraction is a very convenient method of analysing the structural changes which take place in minerals during grinding [16]. The increase in structural disorder in

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t M [rain] Fig. l. The value of AR ~0vs. grindingtime, tM, for tetrahedriteactivatedin: 1 = planetarymill; 2 = attritor.

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P. Bald~ et al. / Hydrometallurgy 43 (1996) 367-377

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P. Bal6~ et al. / Hydrometallurgy 43 (1996) 367-377

371

the solid is responsible for the decrease in the integral intensity of X-rays diffracted by the (hkl) plane of the analysed phase. The quantitative examination of disordering of the samples by mechanical activation was accomplished using a diffractometer DRON 2.0 with goniometer GUR-5 (Techsnabexport, Russia) under the conditions: F e K a radiation, 24 kV, 10 mA and rate of goniometer 10 min- 1. The disordering of the sample structure, R, was defined by: R=

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(2)

where I * and I ° are integral intensities (proportional to diffraction peak area) for activated and 'as received' samples and Ch, T and P are chalcopyrite (hkl = 100, d = 0.303 nm), tetrahedrite (hkl = 100, d = 0.300 nm) and pyrite (hkl = 111, d = 0.313 nm), respectively.

2.3.3. Leaching The leaching was investigated in a 500 ml glass reactor into which 400 ml of leaching solution (4 g CS(NH2) 2 + 2 g F e 2 ( S O 4 ) 3 • 9H20 + H 2 S O 4 ( 1 % ) ; pH = 1) and

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P. Bal6~ et al./ Hydrometallurgy 43 (1996) 367-377

372

1 g of concentrate were added. The leaching was performed at 293 K at a stirring rate of 8.33 s -1 . Aliquots (10 ml) of the solution were withdrawn at appropriate time intervals for determination of the contents of dissolved silver by atomic absorption spectroscopy. The leaching kinetics were best fitted to the kinetic equation [17]: e ~ = k I + k t + k 2 t2

(3)

where a = silver recovery (~AC divided by 100); k~, k and k 2 = empirical coefficients proportional to rate constants; and t = leaching time. As Dutrizac [17] states, Eq. (3)

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t L [rain ] Fig. 5. Silver recovery, EAg , VS. leaching time, t L , for tetrahedrite concentrate mechanically activated in planetary mill. Time of activation: 1 = 2 min, 2 = 5 min, 3 = l0 min, 4 = 30 rain, 5 = 15 rain, 6 = 20 mill, 7 = 60 min, 8 = 90 min, 9 = 45 min.

P. BaM~et al. / Hydrometallurgy 43 (1996) 367-377

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enables evaluation of rate constants not only for initial leaching rates (at t = 0) but also for middle 'linear' sections of the leaching curves.

3. Results and discussion

3.1. Physico-chemical properties of mechanically activated samples In Figs. 1 - 3 the values of ARl0 (percentage of fines under 10 p~m), S A (specific surface area) and R (structure disorder) of tetrahedrite concentrate samples are plotted

Fig. 6. SEM photographs of tetrahedrite sample. (a) As-received sample. (b) Sample leached in thiourea solution.

P. Bald~ et al./ Hydrometallurgy 43 (1996) 367-377

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against the time of mechanical activation by dry grinding in a planetary mill (1) and by wet grinding in an attritor (2). The grinding in water favours the formation of finer particles (Fig. 1), owing to which the overall surface is greater (Fig. 2). Moreover, in the course of grinding in aqueous medium in an attritor the specific surface increases with the grinding time, while in the course of dry grinding in a planetary mill the surface values attain a maximum at t M = 15 min and at higher grinding times a stagnation of these values is observed. Stagnation of surface area values of tetrahedrite, chalcopyrite and pyrite by dry grinding was observed in our previous papers [18,19], and since the tetrahedrite concentrate was composed of these minerals, is was to be expected that the same mechanism would apply. The degree of dispersal and the magnitude of the newly formed surface, however, have little influence on structure disorder of the mechanically activated samples in our case. It follows from Fig. 3 that the process of activation in a planetary mill brings about a much more intensive violation of tetrahedrite concentrate structure. For all samples the values of structure disorder is at least two times greater for planetary grinding than they are for grinding in an attritor.

3.2. Thiourea leaching of silver from mechanically activated samples The samples activated in an attritor or in a planetary mill were subjected to thiourea leaching. The results of leaching are summarized in Figs. 4 and 5. Under the conditions used of mechanical activation and leaching, the maximum recovery was achieved from the samples activated in a planetary mill. In this case recovery of 48% Ag was obtained

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P. BaM~ et al. / Hydrometallurgy 43 (1996) 367-377

for a sample ground for 45 min and leached for 120 min. The recoveries from the samples activated in an attritor were lower: under the same leaching conditions recoveries of less than 30% Ag were obtained. The recoveries obtained for the 'as received' sample (without mechanical pretreatment) were less than 10% Ag [4,20,21]. These results indicate that the disordering of the structure of tetrahedrite is a decisive process from the viewpoint of silver extraction from this mineral. The results are illustrated by SEM photographs in Fig. 6. Mechanical activation and subsequent thiourea leaching result in comminution and the heterogeneity of the particle morphology.

3.3. Surface/bulk properties and thiourea leaching of mechanically activated samples: quantitative relationship The relations presented in Figs. 1-5 suggest: 1. a different rate of new surface formation and of structure disordering in the course of mechanical activation in attritor and planetary mill; 2. different recoveries of thiourea leaching of silver for samples activated in the mills. The relationship between total silver recovery and leaching rate are presented in Fig. 7. It follows that, by grinding in an attritor, higher rates of recovery can be obtained. By I

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P. Bal~£ et aL / Hydrometallurgy 43 (1996) 367-377

dry grinding in a planetary mill the leaching process is slow down by agglomeration effects and, in consequence, the rate constant is limited (Note: the start of agglomeration can be seen from the stagnation o f the specific surface, S A, for t M > 15 min in Fig. 2). Fig. 8 represents the quantitative relationship between thiourea leaching and s u r f a c e / b u l k properties o f the mechanically activated samples investigated. The rate constant calculated by Eq. (3) has been correlated with the empirical coefficient S A / ( 1 -- R), which represents the s u r f a c e / b u l k disordering ratio for the values o f S A and R given in Figs. 2 and 3 (the value R was calculated by Eq. (2) and divided by 100). The plot in Fig. 8 shows that the extraction of silver from tetrahedrite is a structure-sensitive reaction. Simple proportionality expresses the equal influence of surface increase and volume disordering o f the samples by mechanical activation on the thiourea leaching o f silver. An equal rate of leaching can be attained by mechanical activation either in an attritor (i.e. in a mill producing larger surface and smaller disordering in bulk), or in planetary mill (where the disordering in bulk is great and the formation o f new surface is minor). This observation is also o f prognostic character because it enables us to propose suitable grinding equipment according to the demand for fineness or reactivity o f the solid substances.

Acknowledgements This work was supported by the Slovak Grant Agency for Science (grants 2 / 1 3 6 8 / 9 4 and 2 / 1 3 6 9 / 9 4 ) .

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[11] Pesic, B. and Seal, T., Dissolution of silver with thiourea: The rotating disc study. In: M.C. Iha and S.D. Hill (Editors), Precious Metals '89 (1988), pp. 307-339. [12] Lewis, A., Thiourea: A potential alternative for A u / A g leaching. Eng. Min. J., February (1982): 59-63. [13] Senna, M., Determination of effective surface area for the chemical reaction of fine particulate materials. Part. Part. Syst. Charact., 6 (1989): 163-167. [14] "I'k~i~ovfi, K. and Bal~, P., Structural and temperature sensitivity of leaching of chalcopyrite with iron (III) sulphate. Hydrometallurgy, 21 (1988): 103-112. [15] Bal~., P. and Ebert, I., Oxidative leaching of mechanically activated sphalerite. Hydrometallurgy, 27 (1991): 141-150. [16] Tkfi~ovfi, K., Mechanical Activation of Minerals. Elsevier, Amsterdam (1989). [17] Dutrizac, J.E., The kinetics of dissolution of chalcopyrite in ferric ion media. Metall. Trans. B, 9B (1978): 431-439. [18] Ba152, P. and Brian~in, J., Reactivity of mechanically activated pyrite. Solid State Ionics, 63-65 (1993): 296-300. [19] Balfi,~, P. and Brian~in, J., New surface formation and agglomeration in mechanically activated sulphides. Fizykochem. Probl. Mineral., 28 (1994): 91-97. [20] B a l ~ , P., Kammel, R., Ku~nierovfi, M. and Achimovi~ovfi, M., Mechano-chemical treatment of tetrahedrite as a new non-polluting method of metals recovery. In: Hydrometallurgy '94. Chapman and Hall, London (1994), pp. 209-218. [21] BaltiC, P., Kammel, R. and Achimovi(:ovfi, M., Selektive Hydrometallurgische Gewinnung yon Sb, Hg und Ad aus mechano-I 2 chemisch behandelten Tetrahedrit Konzentraten. Metall, 48 (1994): 217-220.