- Email: [email protected]
Study of the autoclave leaching of a tetrahedrite concentrate
Minerals Engineering, Vol. 6, No. I !, pp. 1117-1125, 1993 Printed in Great Britain
0892-6875193 $6.00+0.00 © 1993 Pergamon Press Ltd
STUDY OF THE AUTOCLAVE LEACHING OF A TETRAHEDRITE CONCENTRATE
M.J. NEIVA CORREIAQ, J.R. CARVALHOt and A.J. MONHEMIUS t § Dep. Eng. Quimiea, Institute Superior Tecnico, Av. Rovisco Pais 1096 Lisboa Codex, Portugal 1" Dep. of Mineral Resources Eng., Royal School of Mines, Imperial College, London SW7 2BP, UK
(Received 14 April 1993; accepted 17 May 1993)
ABSTRACT
Tetrahedrite is a sulphide of copper and antimony with the general chemical formulae Cu12 Sb¢91~. B is very difficult to fitut pure natural tetrahedrite because its original constituents are always partially substituted by other elements. Thus, arsenic can substitute for antimony leading to tenantite (Cul2As4SI~), while copper can be substituted by zinc, silver, mercury, iron, cadmium etc. This work describes the purification of an impure tetrahedrite/tenantite concentrate and presents the results obtained in some pressure leaching tests of the as-received or partially purified concentrate. The results show that, after two purification steps, it was possible to obtain a concentrate with 90 % tetrahedrite/tenantite and, under certain conditions, it was possible to leach 7096 of the copper wut 85% of the silver and zinc contained in the concentrate. Keywords Tetrahedrite, chloride, pressure leaching
INTRODUCTION Tetrahedrite and tenantite belong to a family of minerals usually called black coppers. The simplest chemical formulae for tetrahedrite and tenantite are CuI2Sb4SI3 and CuI2As4SI_3, respectively, which correspond to the following oxidation states: CUlo+ Cu22 + Sb43 + S132 "/CUlo+ Cu22 + As43 + S132 -. Natural tetrahedrites/tenantites always show a certain degree of substitution and it is known that Cu(I) can be substituted by silver, Cu(II) by zinc, iron, mercury or cadmium, and antimony or arsenic by bismuth [1]. Thus a possible formulae for a substituted tetrahedrite/tenantite is (CuAg)I0(CuZnHgCdFe)2(SbAsBi)4SI3. Tetrahedrites/tenantites are usually associated with copper ores or concentrates and they can be an important source of other valuable metals. However, their content of antimony, arsenic and mercury is usually very high and this makes the pyrometallurgical treatment of concentrates rich in these minerals undesirable due to pollution problems. Thus, hydrometallurgy seems to be a natural alternative to extract the valuable metals contained in these type of ores or concentrates. It is known [2,3] that tetrahedrite/tenantite minerals are very refractory to the most common leaching processes and, to cause significant dissolution, it is necessary to use high temperatures and pressures. Attempts have been made to dissolve tetrahedrite in cyanide solutions but the recovery of silver was extremely low [2]. With thiourea it was possible to extract 50% of the silver and 70% of the copper contained in a tetrahedrite concentrate [2]. The leaching of tetrahedrite/tenantite with ferric chloride was also studied [3], but the leaching rates and metal extractions were very low. 1117
1118
M.J. NEIVACORREIAet
al.
The pressure leaching of tetrahedrite concentrates has been investigated. Bahr et al [2] studied the leaching of a concentrate with 27 % copper and 2.7 % silver, contained mainly in tetrahedrite. They worked with an oxygen partial pressure of 39.5 atm and a temperature of 403 K and recovered 95 % of the copper and silver; however the very high pressure used is a significant disadvantage of this procedure. Alkaline leaching has been widely used to leach antimony ores and concentrates [4,5]. Under basic conditions, antimony is solubilised while the other metals, such as copper and silver, remain in the residue. Scheiner [4] used a mixture of sodium hydroxide and sulphur to remove antimony from a tetrahedrite concentrate, followed by pressure leaching of the residue with FeCI 2 at 373 K and an oxygen partial pressure of 3.7 atm. They were able to leach 98% of the copper, but to recover silver it was necessary to leach the autoclave residue with cyanide. The number of steps necessary to extract the various metals appears to be the major draw back of this process. Dayton et al [5] described the purification of a high silver, antimonial tetrahedrite with NaOH/NaHS to remove antimony and arsenic. After this operation, the solid residue could be treated by a pyre- or hydrometallurgical process to recover copper, silver, zinc etc. In this paper, the results of several pressure leaching tests of a tetrahedrite/tenantite concentrate in chloride medium are presented. This medium has been widely used in hydrometallurgy because of its high solubilising power due to the formation of chloride complexes. To favour the formation of these complexes, it is always necessary to have in solution a chloride donor and, in the leaching of sulphides, it is also necessary to use an oxidant agent. The most commonly used oxidising agents are Cu 2+ , Fe 3+, 02, and CI 2 and the usual sources of chloride ions are sodium chloride, calcium chloride or a mixture of both. A process to leach sulphide concentrates with Cu 2+ ,O 2 and NH4C1 as the solubilising agent was recently published in the literature [6,7]. It was claimed that NH4CI has several advantages, mainly because almost all metals form very stable chloride and ammonium complexes, which increase the solubilising power of the leaching solution. Furthermore, while the processes with Cu 2+ or Fe 3+ are carried out in acidic conditions, in leaching with NH4CI the pH is very close to neutral and this fact can have some influence on the mechanism of the leaching reactions and on metal extractions. Leaching reactions The leaching agents used in this work were cupric chloride, ferric chloride and ammonium chloride under an oxygen atmosphere. In order to write the chemical reactions involved in each leaching process it is necessary to define the change in the oxidation state of the species involved, namely the oxidation state of the sulphur obtained after the oxidation of sulphide. It seems that [8,9], if the leaching reaction is carried out in strong acidic medium most of the sulphur produced is in its elemental state, but, if the reaction is carried out under basic or neutral conditions, the proportion of elemental sulphur to sulphate will be lower. Lotens et al [10] found a relation of S:SO 4 from 3:1 to 2:2 depending on the type of oxidant used. Thus, the leaching reaction of a pure tetrahedrite with cupric chloride can be described by the equation: CuI2Sb4SI3 + 32 Cu 2+ = 44 Cu + + 4 Sb5+ + 13 S°
(1)
which assumes the oxidation of sulphide to elemental sulphur and the formation of Sb5+ instead of Sb3+ [3] (if Sb3+ is also formed lesser amounts of oxidant will be necessary). However, antimony undergoes hydrolysis and precipitates according to the reaction: Sb5+ + 4 H20 = H3SbO4 + 5 H +
(2)
Autoclave leaching of a tetrahedrite concentrate
1119
The oxygen used in the autoclave will regenerate the cupric ion, which acts as the oxygen carrier. Considering the oxidation of Cu + to Cu 2+, the general reaction of the oxygen leaching of tetrahedrite in the presence of copper can be described by the following equation:
(3)
Cul2Sb4Sl3 + 11 02 + 44 H + = 12 Cu 2+ + 4 Sb 5+ + 13 S + 22 H 2 0
The differences between the stoichiometries of reactions 1 and 3 and the stoichiometries of the leaching reactions of substituted tetrahedrites can be neglected. However, in this latter case, if it is intended to recover metals such as silver, lead or mercury, whose solubilities are very dependent on chloride concentration, it is necessary to add sodium eMoride to the solution to promote the formation of chloride complexes. The leaching reaction with ferric ion is similar to reaction 1 or 3 and can be described by:
(4)
CuI2Sb4SI3 + 44 F¢ 3+ = 12 Cu 2+ + 4 Sb 5+ + 13 S + 44 Fe 2+
The ammonium chloride leaching in the presence of copper is different because the reaction is carried out in nehtral conditions, the ammonium ion also taking part in the leaching reaction, and only about 75 % o f the sulphide sulphur produces elemental sulphur [7,10]. The overall leaching reaction under an oxygen atmosphere can be described by: CuI2Sb4S13 + 3 1 / 2 0 2 +38 NH4 + = 1 2 C u 2+ + 4 S b 5+ + 1 0 S + 3 SO42- + 3 8 N H 3 +19 H 2 0
(5)
The ammonium chloride added should provide an adequate concentration of chloride ions and should also supply the protons that are required in the leaching of the sulphides. Since almost all the metals of interest such as copper, silver, mercury and zinc form very stable ammonium complexes, the solubilising power of an ammonium chloride solution should be greater than a corresponding sodium chloride solution.
EXPERIMENTAL The experimental work was carried out with a tetrahedrite concentrate supplied by an Italian mining company ( Monte Avanza mine). This concentrate contained mainly tetrahedrite/tenantite with minor amounts of ehalcopyrite, pyrite and covelite dispersed in a carbonate and silicate matrix [11]. Its chemical composition and particle size distribution are presented in Tables 1 and 2, respectively. T A B L E 1 Chemical Analysis of the Tetrahedrite Concentrate Concentrate Element(%)
Mineral(%)
Cu- 16.5-17.0 Zn- 2.5-2.9 Ag- 0.073-0.076 TetrahedriteFe- 0.68-0.80 44.4-46.8 Sb- 9.8-10.0 As- 1.7-2.7 Hg- 1.3-1.5 S- 11.8 Ca-14.5-15.5 CaCO 3- 36.4-38.9 SIO2-14.3-19.2"
Acid Washed Concentrate Element(%) Cu- 25-27 Zn- 3.0-3.9 Ag- 0.110-0.125 Fe- 0.6-0.7 Sb- 14.5-16.5 As- 2.6-4.1 Hg- 2.0-2.4 S- 16.6
Mineral(%)
Tetrahedrite64.4-71.3
Cone. After Heavy Liquid Sep. Element(%)
Mineral(%)
Cu- 32.1-33.5 Zn- 4.4-5.3 Ag- 0.141-0.155 Fe- 0.7-0.8 Sb- 19.5-22 As- 3.4-4.0 Hg- 2.6-3.0 S- 23.0
Tetrahedrite85.8-91.8
SIO2-28.7-35.6"
Notes:
i) ii)
* - The SiO 2 content was calculated by the following formula: % SiO 2 = 100-(% Tth + % CaCO3) where Tth means tetrahedrite. Calcium content in the acid washed concentrate is less than 0.02%
~i0z-8.2-14.2"
M. J. NEIVA CORREIA et al.
1120
T A B L E 2 Particle size analysis of the as-received concentrate
Diameter (/Jm)
1
6
16
32
64
128
192
Cumulative curve (weight %)
4.7
22.2
34.5
42.1
53.7
81.6
100
The experimental work involved a study of the purification of the concentrate followed by some pressure leaching tests in order to identify the most important variables of the process. Purification tests The first step in the purification was the acid dissolution of the carbonate matrix. The type and concentration of acid were optimised in order to maximise the removal of the carbonates and minimise the extraction of copper and zinc ( [HCI]= 0.5 M). After the removal of the carbonates, it was necessary to separate the tetrahedrite from quartz. The process used was heavy liquid/separation which is based o n the difference of density between the two minerals. This process is very efficient if the granulometry of the solid is greater than 35 micron and if the amount of composite grains (formed by the two minerals that have to be separated) are not abundant. In this case, while tetrahedrite has a density of 4.5 - 5, that of quartz is only 2.6, which means that it should be possible to separate them if the separating medium has a density between these two values. The heavy liquid chosen was bromoform which has a density of 2.8; all the tests were carried out in a separating funnel. Prior to heavy liquid separation, the particles of less than 35 micron were eliminated by wet sieving. After each test, the denser phase was analysed for copper, zinc, silver, mercury, iron, antimony and arsenic by atomic absorption. Some of them were also analysed with a X ray difractometer and with an electronprobe microanalyser. Leaching tests The leaching tests were carried out in a titanium autoclave of 1 US gallon capacity supplied by Autoclave Engineers. In all tests the concentrate and the leaching solution were added to the reactor, the autoclave was assembled and the agitation and heating were started. When the desired temperature was reached, the introduction of oxygen was begun and this was considered the zero time. Samples were taken from time to time and were immediately filtered in a hot Buckner funnel in order to prevent the precipitation of any metal. At the end of each leaching test, the autoclave was depressurized and the temperature was allowed to decrease. The pulp was filtered and the cake was washed with a hot sodium chloride solution followed by hot distilled water. The solids used in the leaching tests were either the impure concentrate or the acid washed concentrate (without carbonates) and the experimental conditions used were: - pulp density - 10%; stirring speed - 1200 rpm; temperature - 378, 403,433 K; oxygen partial pressure - 3, 6 atm ; composition of the initial leaching solution: a) Cu 2+ - 2.7 g/l (as CuCI2), NaCI - 250 g/l, HCI - 0.5 M; b) Fe 3+ _ 2.4 g/l (as FeCI3), NaCI - 250 g/l, HCI - 0.5 M; c) Cu 2+ - 1.2 g/l (as CuCI2), NH 4 CI - 323 g/l. All the samples taken were analysed for copper, silver, zinc, iron, mercury, antimony and arsenic by atomic absorption, and the concentrations of Fe (II) and sulphate in the final leaching solutions were determined, respectively, by volumetric and gravimetric analysis. Elemental sulphur contained in the
Autoclave leaching of a tetrahedrite concentrate
1121
leaching residues was determined by extraction with benzene followed by evaporation of the solvent. The solid residues were analysed by X ray diffraction and optical microscopy.
RESULTS AND DISCUSSION The results of the purification tests are presented in Table 1. Thus, after the heavy liquid separation, it was possible to obtain a solid containing 90 % tetrahedrite. Microscopic observation of the heavy fractions showed that most of the grains were of pure tetrahedrite, or tetrahedrite with some inclusions of a transparent mineral (probably quartz), but there were also some grains of chalcopyrite and of another transparent mineral that was not separated with bromoform. This could indicate that this mineral was not quartz but another silicate with a density greater than 2 . 8 . Thus, to remove these species it would be necessary to use another heavy liquid with a density greater than 2.8, such as CH2I2 which has a density of 3.3, and it is also possible that the use of a narrower particle size distribution in the heavy liquid tests would also improve the results. Some of the results obtained in the leaching tests for copper and silver are presented in Figures 1 to 4. The leaching of the as-received concentrate (45 % of tetrahedrite) with cupric chloride (solution (a)) at 3 atm oxygen partial pressure (5 atm total pressure), and temperatures of 403 and 433 K, extracted 50 % of the silver (Figure 2) and zinc. However, the concentration of copper in solution (Figure I) and the extraction of iron and antimony decreased with time. The behaviour of Cu, Fe and Sb during the leaching was related with the change of the pH of the solution, which increased from an initial value lower than zero to a value greater than 4, due to the neutralisation reaction of the carbonate matrix of the impure concentrate. It is known that the precipitation of copper can occur during the leaching of sulphides in several forms, such as eovelite (CuS) [8], or as an oxyehloride (CuCI23Cu(OH)2) if the pH is greater than 1 [12]. In these tests the presence of the latter compound was identified from the XRD spectra of the leach residues. 3
[Cui (g/O
Cu Ext. (%)
100 I
[] Test 1
I Test 2
Test 3
¢ Test 4
2.5
I - 80
2
60 1.5
40
20
0.5
0 0
I
I
60
120
I
180 Time (rain.)
I
i
240
300
0
Fig. 1 Concentration/extraction of copper- experimental conditions: impure concentrate, PO2 --3 atm; Test 1- CuCI2, 433 K; Test 2- CuCI2, 403 K; Test 3- NH4CI, 378 K; Test 4- NH4CI, 403 K The extractions of iron and antimony were lower than 1% after 3 hours of reaction and they decreased with time. This precipitation may be due to a hydrolysis reaction that occur at pH greater than 1 , and/or
1122
M . J . NmvA CORREIA et al.
to the precipitation of a Fe/Sb compound. X ray diffraction analysis of the residues did not reveal any of the most common compounds, such as FeOOH, Fe203, FeSbO 4 or H3SbO4, but qualitative microprobe analysis of one of the residues showed that some of the tetrahedrite grains were, in fact, surrounded by a compound of Fe/Sb and Cu. The arsenic and mercury extractions in these tests were around 20 - 30% and the sulphate of the final leaching solutions was lower than 1 g/l. Ag Ext. (%) IO0 0 80
60 []
40
20
[]
Test 1
I
Test 2 Test 3
(> O ~
0
I
I
60
120
Test 4
I
I
180 Time (rain.)
I
240
300
Fig.2 Extraction of silver- experimental conditions: impure concentrate, PO2 = 3 atm; Test 1- CuCl 2, 433 K; Test 2- CuCI 2, 403 K; Test 3- NH4C1, 378 K; Test 4- NH4CI, 403 K Cu Ext.(=/,)
+
60
40
+
20
//J
÷
yj 0 0
o,,o
re,t7 I
I
60
120
I
180 Time (rain.)
I
240
I
300
Fig.3 Extraction of copper- experimental conditions: acid washed concentrate, 403 K ; Test 5- CuCl2, PO 2 =3 atm; Test 6- FeCI 3, PO 2 =3 atm; Test 7- FeC13, PO 2 = 6 atm
Autoclave leaching of a tetrahedrite concentrate
1123
Ag Ext. (%)
60 +
[]
I
[]
40
20
[]
Test 5
-I-
Test 6 Test ~r
0
I
0
60
I
120
I
180 Time (rain.)
I
240
I
300
Fig.4 Extraction of silver- experimental conditions: acid washed concentrate, 403 K ; Test 5- CuC12, PO 2 =3 atm; Test 6- FeCI3, PO 2 = 3 atm; Test 7- FeCI3, PO 2 = 6 atm The leaching experiments using ammonium chloride (solution (e)) were done with the as- received concentrate (45 % tetrahedrite) at 378 and 403 K with an oxygen partial pressure of 3 atm. The final pH of the solution was slightly greater than 6 and X ray diffraction analysis showed that the carbonate matrix did not react. For each test, 30 minutes or one hour from the end, the oxygen was purged from the autoclave and nitrogen was introduced. This was done in order to favour the reaction between Cu(II) and sulphide in the concentrate, with the consequent production of Cu(I), which is more soluble in ammonium chloride solutions. In this medium, the extraction of the metals is governed by the solubility of their ammoniacal complexes. At 403 K it was possible to extract 70% of the copper (Figure 1), 85 % of the silver (Figure 2), 80% of the mercury and 70 % of the zinc after 4 hours of reaction. Iron and antimony were not dissolved. As expected for a leaching reaction carried out in neutral conditions with Cu 2+ as the sulphide oxidant [10], the relation of elemental sulphur to sulphate was about 3 and the increase in temperature did not affect this value. The results obtained in these tests show that leaching with ammonium chloride is very efficient and it is possible that the results can be further improved. The leaching behaviour of the concentrate after acid washing was quite different. In this case, it was possible to extract, after 3 hours of leaching with cupric chloride at 403 K and 3 atm oxygen partial pressure, 40% o f the copper, silver, zinc, and mercury and 30% of the arsenic contained in the concentrate, while with ferric chloride copper,silver, zinc and mercury extraction increased to 60-70%, while the extraction of arsenie decreased to 15 % (Figures 3 and 4). The extraction of antimony was lower than 1%, whereas iron was almost completely extracted in the first hour but, subsequently began to precipitate, probably in one of the forms referred to above. In these experiments, the ratio S°: SO4 was higher than the one obtained in the leaching with the as-received concentrate, but the concentration of sulphate in solution was significant (SO4=4 g/l). The residue obtained in one of these tests was examined by optical microscopy and analysed with the electron microprobe. It was possible to see that there were several grains of unattacked tetrahedrite surrounded by elemental sulphur, an Fe/Sb compound, and by other unidentified compounds.
1124
M . J . NEIv^ COt~REIA et al.
From the results obtained, it is possible to conclude that the influence of increased temperature in the leaching tests with cupric chloride increased the initial rates of the leaching reactions but did not greatly affect the final metal extractions. On the contrary, in the leaching with ammonium chloride, the increase of 25 K in temperature increased the extraction of copper and silver by about 20%. The effect of increasing the oxygen partial pressure from 3 to 6 atm was studied in the ferric chloride leaching tests with the acid washed concentrate and the extractions have increased about 2% for copper and 13 % for silver after 5 hours of reaction (Figures 3 and 4). The increase in the oxygen partial pressure did not greatly affect the concentration of sulphate in solution, which means that oxygen did not favour the oxidation of sulphide sulphur to sulphate.
CONCLUSIONS The results obtained in the purification tests show that it is possible to prepare a purified concentrate containing 90 % tetrahedrite, by acid washing followed by heavy liquid separation with bromoform. The leaching results showed that ammonium chloride based leachants show great promise for the dissolution of tetrahedrite. With this reagent it was possible to extract 70 % of the copper and 85 % of the silver contained in the concentrate. As expected, mineralogical analysis of the leach residues showed the presence of elemental sulphur formed by oxidation of the sulphide matrix, but it was not possible to identify the compound in which iron and antimony had precipitated.
ACKNOWLEDGEMENTS The authors would like to acknowledge Professor G. Ferrara of Trieste University for supplying the samples of the tetrahedrite concentrate used in this work.
REFERENCES I.
2. 3.
.
.
6.
.
8.
9.
Pattrick, R.A.D. & Hall, A.J., Silver Substitution into Synthetic Zinc, Cadmium and Iron Tetrahedrites, Mineralogical Mag., 47, 441-5 (Dec. 1983). Bahr, A. & Priesemann, T., Recovery of Silver from Refractory Ores, XV/Int. Min. Prec. Cong., Stockholm, 1121-1134 (June 1988). Dutrizac, J.E. & Morrison, R.M., The Leaching of some Arsenide and Antimonide Minerals in Ferric Chloride Media in Hydrometallurgical Process Fundamentals, Bautista R.G. (Ed.), Plenum Press, 77-112 (1984). Scbeiner, B.J., Smyres, G.A., Haskett, P.R. & Lindstrom, R.E., Copper and Silver Recovery from a Sulfide Concentrate by Ferrous Chloride-Oxygen Leaching, U.S. Bur. Mines R.I. no. 8290, (1978). Dayton, S., Equity Silver on Line With Leach Plant; How Sb and As are Purged from a HighSilver Copper Concentrate before Smelting, Eng. Mining J., 78-83 (Jan. 1982). Limpo, J.L., Figueiredo, J.M., Amen, S. & Luis, A., The CENIM-LNETI Process: A New Process for the Hydrometallurgical of Complex Sulphides in Ammonium Chloride Solutions, Hydromet., 28, 149-161 (1992). Limpo, J.L., Luis, A. & Gomez, G., Reactions during the Oxygen Leaching of Metallic Sulphides in CENIM-LNETI Process, Hydromet., 28, 163-178 (1992). McDonald, G.W., Udovie, T.J., Dumesic, J.A. & Langer, S.H., Equilibria Associated with Cupric Chloride Leaching of Chalcopyrite Concentrate, H)dromet. 13, 125-135 (1984). Dutrizac, J.E., Elemental Sulphur Formation during the Ferric Chloride Leaching of Chalcopyrite, Hydromet., 23, 153-176 (1990).
Autoclave leaching of a tetrahedrite concentrate
10. 11.
12.
ME 6tl I--B
1125
Lotens, J.P. & Wesker, E., The Behaviour of Sulphur in the Oxidative Leaching of Sulphidic Minerals, Hydromet., 18, 39-54 (1987). Brusea, C., Ferrara, G. & Preti, U., Technologies Adapted for the Valorization of a Small MineThe Monte Avanza Polymetallie Sulphide (Tetrahedrite) Multibody Deposit. Part 2: Mineral Processing and Extractive Metallurgy, Proc. EC Seminar on Mineral Proc. and Ext. Met., Lisbon, (April 1991). Greig, J.A., Oxidative Chloride Leaching of Sulphide Concentrates, in Separation Processes in HydrometaUurgy, Davies G.A. (Ed.), Ellis Horwood, Chichester, 35-48 (1987).
0892-6875193 $6.00+0.00 © 1993 Pergamon Press Ltd
STUDY OF THE AUTOCLAVE LEACHING OF A TETRAHEDRITE CONCENTRATE
M.J. NEIVA CORREIAQ, J.R. CARVALHOt and A.J. MONHEMIUS t § Dep. Eng. Quimiea, Institute Superior Tecnico, Av. Rovisco Pais 1096 Lisboa Codex, Portugal 1" Dep. of Mineral Resources Eng., Royal School of Mines, Imperial College, London SW7 2BP, UK
(Received 14 April 1993; accepted 17 May 1993)
ABSTRACT
Tetrahedrite is a sulphide of copper and antimony with the general chemical formulae Cu12 Sb¢91~. B is very difficult to fitut pure natural tetrahedrite because its original constituents are always partially substituted by other elements. Thus, arsenic can substitute for antimony leading to tenantite (Cul2As4SI~), while copper can be substituted by zinc, silver, mercury, iron, cadmium etc. This work describes the purification of an impure tetrahedrite/tenantite concentrate and presents the results obtained in some pressure leaching tests of the as-received or partially purified concentrate. The results show that, after two purification steps, it was possible to obtain a concentrate with 90 % tetrahedrite/tenantite and, under certain conditions, it was possible to leach 7096 of the copper wut 85% of the silver and zinc contained in the concentrate. Keywords Tetrahedrite, chloride, pressure leaching
INTRODUCTION Tetrahedrite and tenantite belong to a family of minerals usually called black coppers. The simplest chemical formulae for tetrahedrite and tenantite are CuI2Sb4SI3 and CuI2As4SI_3, respectively, which correspond to the following oxidation states: CUlo+ Cu22 + Sb43 + S132 "/CUlo+ Cu22 + As43 + S132 -. Natural tetrahedrites/tenantites always show a certain degree of substitution and it is known that Cu(I) can be substituted by silver, Cu(II) by zinc, iron, mercury or cadmium, and antimony or arsenic by bismuth [1]. Thus a possible formulae for a substituted tetrahedrite/tenantite is (CuAg)I0(CuZnHgCdFe)2(SbAsBi)4SI3. Tetrahedrites/tenantites are usually associated with copper ores or concentrates and they can be an important source of other valuable metals. However, their content of antimony, arsenic and mercury is usually very high and this makes the pyrometallurgical treatment of concentrates rich in these minerals undesirable due to pollution problems. Thus, hydrometallurgy seems to be a natural alternative to extract the valuable metals contained in these type of ores or concentrates. It is known [2,3] that tetrahedrite/tenantite minerals are very refractory to the most common leaching processes and, to cause significant dissolution, it is necessary to use high temperatures and pressures. Attempts have been made to dissolve tetrahedrite in cyanide solutions but the recovery of silver was extremely low [2]. With thiourea it was possible to extract 50% of the silver and 70% of the copper contained in a tetrahedrite concentrate [2]. The leaching of tetrahedrite/tenantite with ferric chloride was also studied [3], but the leaching rates and metal extractions were very low. 1117
1118
M.J. NEIVACORREIAet
al.
The pressure leaching of tetrahedrite concentrates has been investigated. Bahr et al [2] studied the leaching of a concentrate with 27 % copper and 2.7 % silver, contained mainly in tetrahedrite. They worked with an oxygen partial pressure of 39.5 atm and a temperature of 403 K and recovered 95 % of the copper and silver; however the very high pressure used is a significant disadvantage of this procedure. Alkaline leaching has been widely used to leach antimony ores and concentrates [4,5]. Under basic conditions, antimony is solubilised while the other metals, such as copper and silver, remain in the residue. Scheiner [4] used a mixture of sodium hydroxide and sulphur to remove antimony from a tetrahedrite concentrate, followed by pressure leaching of the residue with FeCI 2 at 373 K and an oxygen partial pressure of 3.7 atm. They were able to leach 98% of the copper, but to recover silver it was necessary to leach the autoclave residue with cyanide. The number of steps necessary to extract the various metals appears to be the major draw back of this process. Dayton et al [5] described the purification of a high silver, antimonial tetrahedrite with NaOH/NaHS to remove antimony and arsenic. After this operation, the solid residue could be treated by a pyre- or hydrometallurgical process to recover copper, silver, zinc etc. In this paper, the results of several pressure leaching tests of a tetrahedrite/tenantite concentrate in chloride medium are presented. This medium has been widely used in hydrometallurgy because of its high solubilising power due to the formation of chloride complexes. To favour the formation of these complexes, it is always necessary to have in solution a chloride donor and, in the leaching of sulphides, it is also necessary to use an oxidant agent. The most commonly used oxidising agents are Cu 2+ , Fe 3+, 02, and CI 2 and the usual sources of chloride ions are sodium chloride, calcium chloride or a mixture of both. A process to leach sulphide concentrates with Cu 2+ ,O 2 and NH4C1 as the solubilising agent was recently published in the literature [6,7]. It was claimed that NH4CI has several advantages, mainly because almost all metals form very stable chloride and ammonium complexes, which increase the solubilising power of the leaching solution. Furthermore, while the processes with Cu 2+ or Fe 3+ are carried out in acidic conditions, in leaching with NH4CI the pH is very close to neutral and this fact can have some influence on the mechanism of the leaching reactions and on metal extractions. Leaching reactions The leaching agents used in this work were cupric chloride, ferric chloride and ammonium chloride under an oxygen atmosphere. In order to write the chemical reactions involved in each leaching process it is necessary to define the change in the oxidation state of the species involved, namely the oxidation state of the sulphur obtained after the oxidation of sulphide. It seems that [8,9], if the leaching reaction is carried out in strong acidic medium most of the sulphur produced is in its elemental state, but, if the reaction is carried out under basic or neutral conditions, the proportion of elemental sulphur to sulphate will be lower. Lotens et al [10] found a relation of S:SO 4 from 3:1 to 2:2 depending on the type of oxidant used. Thus, the leaching reaction of a pure tetrahedrite with cupric chloride can be described by the equation: CuI2Sb4SI3 + 32 Cu 2+ = 44 Cu + + 4 Sb5+ + 13 S°
(1)
which assumes the oxidation of sulphide to elemental sulphur and the formation of Sb5+ instead of Sb3+ [3] (if Sb3+ is also formed lesser amounts of oxidant will be necessary). However, antimony undergoes hydrolysis and precipitates according to the reaction: Sb5+ + 4 H20 = H3SbO4 + 5 H +
(2)
Autoclave leaching of a tetrahedrite concentrate
1119
The oxygen used in the autoclave will regenerate the cupric ion, which acts as the oxygen carrier. Considering the oxidation of Cu + to Cu 2+, the general reaction of the oxygen leaching of tetrahedrite in the presence of copper can be described by the following equation:
(3)
Cul2Sb4Sl3 + 11 02 + 44 H + = 12 Cu 2+ + 4 Sb 5+ + 13 S + 22 H 2 0
The differences between the stoichiometries of reactions 1 and 3 and the stoichiometries of the leaching reactions of substituted tetrahedrites can be neglected. However, in this latter case, if it is intended to recover metals such as silver, lead or mercury, whose solubilities are very dependent on chloride concentration, it is necessary to add sodium eMoride to the solution to promote the formation of chloride complexes. The leaching reaction with ferric ion is similar to reaction 1 or 3 and can be described by:
(4)
CuI2Sb4SI3 + 44 F¢ 3+ = 12 Cu 2+ + 4 Sb 5+ + 13 S + 44 Fe 2+
The ammonium chloride leaching in the presence of copper is different because the reaction is carried out in nehtral conditions, the ammonium ion also taking part in the leaching reaction, and only about 75 % o f the sulphide sulphur produces elemental sulphur [7,10]. The overall leaching reaction under an oxygen atmosphere can be described by: CuI2Sb4S13 + 3 1 / 2 0 2 +38 NH4 + = 1 2 C u 2+ + 4 S b 5+ + 1 0 S + 3 SO42- + 3 8 N H 3 +19 H 2 0
(5)
The ammonium chloride added should provide an adequate concentration of chloride ions and should also supply the protons that are required in the leaching of the sulphides. Since almost all the metals of interest such as copper, silver, mercury and zinc form very stable ammonium complexes, the solubilising power of an ammonium chloride solution should be greater than a corresponding sodium chloride solution.
EXPERIMENTAL The experimental work was carried out with a tetrahedrite concentrate supplied by an Italian mining company ( Monte Avanza mine). This concentrate contained mainly tetrahedrite/tenantite with minor amounts of ehalcopyrite, pyrite and covelite dispersed in a carbonate and silicate matrix [11]. Its chemical composition and particle size distribution are presented in Tables 1 and 2, respectively. T A B L E 1 Chemical Analysis of the Tetrahedrite Concentrate Concentrate Element(%)
Mineral(%)
Cu- 16.5-17.0 Zn- 2.5-2.9 Ag- 0.073-0.076 TetrahedriteFe- 0.68-0.80 44.4-46.8 Sb- 9.8-10.0 As- 1.7-2.7 Hg- 1.3-1.5 S- 11.8 Ca-14.5-15.5 CaCO 3- 36.4-38.9 SIO2-14.3-19.2"
Acid Washed Concentrate Element(%) Cu- 25-27 Zn- 3.0-3.9 Ag- 0.110-0.125 Fe- 0.6-0.7 Sb- 14.5-16.5 As- 2.6-4.1 Hg- 2.0-2.4 S- 16.6
Mineral(%)
Tetrahedrite64.4-71.3
Cone. After Heavy Liquid Sep. Element(%)
Mineral(%)
Cu- 32.1-33.5 Zn- 4.4-5.3 Ag- 0.141-0.155 Fe- 0.7-0.8 Sb- 19.5-22 As- 3.4-4.0 Hg- 2.6-3.0 S- 23.0
Tetrahedrite85.8-91.8
SIO2-28.7-35.6"
Notes:
i) ii)
* - The SiO 2 content was calculated by the following formula: % SiO 2 = 100-(% Tth + % CaCO3) where Tth means tetrahedrite. Calcium content in the acid washed concentrate is less than 0.02%
~i0z-8.2-14.2"
M. J. NEIVA CORREIA et al.
1120
T A B L E 2 Particle size analysis of the as-received concentrate
Diameter (/Jm)
1
6
16
32
64
128
192
Cumulative curve (weight %)
4.7
22.2
34.5
42.1
53.7
81.6
100
The experimental work involved a study of the purification of the concentrate followed by some pressure leaching tests in order to identify the most important variables of the process. Purification tests The first step in the purification was the acid dissolution of the carbonate matrix. The type and concentration of acid were optimised in order to maximise the removal of the carbonates and minimise the extraction of copper and zinc ( [HCI]= 0.5 M). After the removal of the carbonates, it was necessary to separate the tetrahedrite from quartz. The process used was heavy liquid/separation which is based o n the difference of density between the two minerals. This process is very efficient if the granulometry of the solid is greater than 35 micron and if the amount of composite grains (formed by the two minerals that have to be separated) are not abundant. In this case, while tetrahedrite has a density of 4.5 - 5, that of quartz is only 2.6, which means that it should be possible to separate them if the separating medium has a density between these two values. The heavy liquid chosen was bromoform which has a density of 2.8; all the tests were carried out in a separating funnel. Prior to heavy liquid separation, the particles of less than 35 micron were eliminated by wet sieving. After each test, the denser phase was analysed for copper, zinc, silver, mercury, iron, antimony and arsenic by atomic absorption. Some of them were also analysed with a X ray difractometer and with an electronprobe microanalyser. Leaching tests The leaching tests were carried out in a titanium autoclave of 1 US gallon capacity supplied by Autoclave Engineers. In all tests the concentrate and the leaching solution were added to the reactor, the autoclave was assembled and the agitation and heating were started. When the desired temperature was reached, the introduction of oxygen was begun and this was considered the zero time. Samples were taken from time to time and were immediately filtered in a hot Buckner funnel in order to prevent the precipitation of any metal. At the end of each leaching test, the autoclave was depressurized and the temperature was allowed to decrease. The pulp was filtered and the cake was washed with a hot sodium chloride solution followed by hot distilled water. The solids used in the leaching tests were either the impure concentrate or the acid washed concentrate (without carbonates) and the experimental conditions used were: - pulp density - 10%; stirring speed - 1200 rpm; temperature - 378, 403,433 K; oxygen partial pressure - 3, 6 atm ; composition of the initial leaching solution: a) Cu 2+ - 2.7 g/l (as CuCI2), NaCI - 250 g/l, HCI - 0.5 M; b) Fe 3+ _ 2.4 g/l (as FeCI3), NaCI - 250 g/l, HCI - 0.5 M; c) Cu 2+ - 1.2 g/l (as CuCI2), NH 4 CI - 323 g/l. All the samples taken were analysed for copper, silver, zinc, iron, mercury, antimony and arsenic by atomic absorption, and the concentrations of Fe (II) and sulphate in the final leaching solutions were determined, respectively, by volumetric and gravimetric analysis. Elemental sulphur contained in the
Autoclave leaching of a tetrahedrite concentrate
1121
leaching residues was determined by extraction with benzene followed by evaporation of the solvent. The solid residues were analysed by X ray diffraction and optical microscopy.
RESULTS AND DISCUSSION The results of the purification tests are presented in Table 1. Thus, after the heavy liquid separation, it was possible to obtain a solid containing 90 % tetrahedrite. Microscopic observation of the heavy fractions showed that most of the grains were of pure tetrahedrite, or tetrahedrite with some inclusions of a transparent mineral (probably quartz), but there were also some grains of chalcopyrite and of another transparent mineral that was not separated with bromoform. This could indicate that this mineral was not quartz but another silicate with a density greater than 2 . 8 . Thus, to remove these species it would be necessary to use another heavy liquid with a density greater than 2.8, such as CH2I2 which has a density of 3.3, and it is also possible that the use of a narrower particle size distribution in the heavy liquid tests would also improve the results. Some of the results obtained in the leaching tests for copper and silver are presented in Figures 1 to 4. The leaching of the as-received concentrate (45 % of tetrahedrite) with cupric chloride (solution (a)) at 3 atm oxygen partial pressure (5 atm total pressure), and temperatures of 403 and 433 K, extracted 50 % of the silver (Figure 2) and zinc. However, the concentration of copper in solution (Figure I) and the extraction of iron and antimony decreased with time. The behaviour of Cu, Fe and Sb during the leaching was related with the change of the pH of the solution, which increased from an initial value lower than zero to a value greater than 4, due to the neutralisation reaction of the carbonate matrix of the impure concentrate. It is known that the precipitation of copper can occur during the leaching of sulphides in several forms, such as eovelite (CuS) [8], or as an oxyehloride (CuCI23Cu(OH)2) if the pH is greater than 1 [12]. In these tests the presence of the latter compound was identified from the XRD spectra of the leach residues. 3
[Cui (g/O
Cu Ext. (%)
100 I
[] Test 1
I Test 2
Test 3
¢ Test 4
2.5
I - 80
2
60 1.5
40
20
0.5
0 0
I
I
60
120
I
180 Time (rain.)
I
i
240
300
0
Fig. 1 Concentration/extraction of copper- experimental conditions: impure concentrate, PO2 --3 atm; Test 1- CuCI2, 433 K; Test 2- CuCI2, 403 K; Test 3- NH4CI, 378 K; Test 4- NH4CI, 403 K The extractions of iron and antimony were lower than 1% after 3 hours of reaction and they decreased with time. This precipitation may be due to a hydrolysis reaction that occur at pH greater than 1 , and/or
1122
M . J . NmvA CORREIA et al.
to the precipitation of a Fe/Sb compound. X ray diffraction analysis of the residues did not reveal any of the most common compounds, such as FeOOH, Fe203, FeSbO 4 or H3SbO4, but qualitative microprobe analysis of one of the residues showed that some of the tetrahedrite grains were, in fact, surrounded by a compound of Fe/Sb and Cu. The arsenic and mercury extractions in these tests were around 20 - 30% and the sulphate of the final leaching solutions was lower than 1 g/l. Ag Ext. (%) IO0 0 80
60 []
40
20
[]
Test 1
I
Test 2 Test 3
(> O ~
0
I
I
60
120
Test 4
I
I
180 Time (rain.)
I
240
300
Fig.2 Extraction of silver- experimental conditions: impure concentrate, PO2 = 3 atm; Test 1- CuCl 2, 433 K; Test 2- CuCI 2, 403 K; Test 3- NH4C1, 378 K; Test 4- NH4CI, 403 K Cu Ext.(=/,)
+
60
40
+
20
//J
÷
yj 0 0
o,,o
re,t7 I
I
60
120
I
180 Time (rain.)
I
240
I
300
Fig.3 Extraction of copper- experimental conditions: acid washed concentrate, 403 K ; Test 5- CuCl2, PO 2 =3 atm; Test 6- FeCI 3, PO 2 =3 atm; Test 7- FeC13, PO 2 = 6 atm
Autoclave leaching of a tetrahedrite concentrate
1123
Ag Ext. (%)
60 +
[]
I
[]
40
20
[]
Test 5
-I-
Test 6 Test ~r
0
I
0
60
I
120
I
180 Time (rain.)
I
240
I
300
Fig.4 Extraction of silver- experimental conditions: acid washed concentrate, 403 K ; Test 5- CuC12, PO 2 =3 atm; Test 6- FeCI3, PO 2 = 3 atm; Test 7- FeCI3, PO 2 = 6 atm The leaching experiments using ammonium chloride (solution (e)) were done with the as- received concentrate (45 % tetrahedrite) at 378 and 403 K with an oxygen partial pressure of 3 atm. The final pH of the solution was slightly greater than 6 and X ray diffraction analysis showed that the carbonate matrix did not react. For each test, 30 minutes or one hour from the end, the oxygen was purged from the autoclave and nitrogen was introduced. This was done in order to favour the reaction between Cu(II) and sulphide in the concentrate, with the consequent production of Cu(I), which is more soluble in ammonium chloride solutions. In this medium, the extraction of the metals is governed by the solubility of their ammoniacal complexes. At 403 K it was possible to extract 70% of the copper (Figure 1), 85 % of the silver (Figure 2), 80% of the mercury and 70 % of the zinc after 4 hours of reaction. Iron and antimony were not dissolved. As expected for a leaching reaction carried out in neutral conditions with Cu 2+ as the sulphide oxidant [10], the relation of elemental sulphur to sulphate was about 3 and the increase in temperature did not affect this value. The results obtained in these tests show that leaching with ammonium chloride is very efficient and it is possible that the results can be further improved. The leaching behaviour of the concentrate after acid washing was quite different. In this case, it was possible to extract, after 3 hours of leaching with cupric chloride at 403 K and 3 atm oxygen partial pressure, 40% o f the copper, silver, zinc, and mercury and 30% of the arsenic contained in the concentrate, while with ferric chloride copper,silver, zinc and mercury extraction increased to 60-70%, while the extraction of arsenie decreased to 15 % (Figures 3 and 4). The extraction of antimony was lower than 1%, whereas iron was almost completely extracted in the first hour but, subsequently began to precipitate, probably in one of the forms referred to above. In these experiments, the ratio S°: SO4 was higher than the one obtained in the leaching with the as-received concentrate, but the concentration of sulphate in solution was significant (SO4=4 g/l). The residue obtained in one of these tests was examined by optical microscopy and analysed with the electron microprobe. It was possible to see that there were several grains of unattacked tetrahedrite surrounded by elemental sulphur, an Fe/Sb compound, and by other unidentified compounds.
1124
M . J . NEIv^ COt~REIA et al.
From the results obtained, it is possible to conclude that the influence of increased temperature in the leaching tests with cupric chloride increased the initial rates of the leaching reactions but did not greatly affect the final metal extractions. On the contrary, in the leaching with ammonium chloride, the increase of 25 K in temperature increased the extraction of copper and silver by about 20%. The effect of increasing the oxygen partial pressure from 3 to 6 atm was studied in the ferric chloride leaching tests with the acid washed concentrate and the extractions have increased about 2% for copper and 13 % for silver after 5 hours of reaction (Figures 3 and 4). The increase in the oxygen partial pressure did not greatly affect the concentration of sulphate in solution, which means that oxygen did not favour the oxidation of sulphide sulphur to sulphate.
CONCLUSIONS The results obtained in the purification tests show that it is possible to prepare a purified concentrate containing 90 % tetrahedrite, by acid washing followed by heavy liquid separation with bromoform. The leaching results showed that ammonium chloride based leachants show great promise for the dissolution of tetrahedrite. With this reagent it was possible to extract 70 % of the copper and 85 % of the silver contained in the concentrate. As expected, mineralogical analysis of the leach residues showed the presence of elemental sulphur formed by oxidation of the sulphide matrix, but it was not possible to identify the compound in which iron and antimony had precipitated.
ACKNOWLEDGEMENTS The authors would like to acknowledge Professor G. Ferrara of Trieste University for supplying the samples of the tetrahedrite concentrate used in this work.
REFERENCES I.
2. 3.
.
.
6.
.
8.
9.
Pattrick, R.A.D. & Hall, A.J., Silver Substitution into Synthetic Zinc, Cadmium and Iron Tetrahedrites, Mineralogical Mag., 47, 441-5 (Dec. 1983). Bahr, A. & Priesemann, T., Recovery of Silver from Refractory Ores, XV/Int. Min. Prec. Cong., Stockholm, 1121-1134 (June 1988). Dutrizac, J.E. & Morrison, R.M., The Leaching of some Arsenide and Antimonide Minerals in Ferric Chloride Media in Hydrometallurgical Process Fundamentals, Bautista R.G. (Ed.), Plenum Press, 77-112 (1984). Scbeiner, B.J., Smyres, G.A., Haskett, P.R. & Lindstrom, R.E., Copper and Silver Recovery from a Sulfide Concentrate by Ferrous Chloride-Oxygen Leaching, U.S. Bur. Mines R.I. no. 8290, (1978). Dayton, S., Equity Silver on Line With Leach Plant; How Sb and As are Purged from a HighSilver Copper Concentrate before Smelting, Eng. Mining J., 78-83 (Jan. 1982). Limpo, J.L., Figueiredo, J.M., Amen, S. & Luis, A., The CENIM-LNETI Process: A New Process for the Hydrometallurgical of Complex Sulphides in Ammonium Chloride Solutions, Hydromet., 28, 149-161 (1992). Limpo, J.L., Luis, A. & Gomez, G., Reactions during the Oxygen Leaching of Metallic Sulphides in CENIM-LNETI Process, Hydromet., 28, 163-178 (1992). McDonald, G.W., Udovie, T.J., Dumesic, J.A. & Langer, S.H., Equilibria Associated with Cupric Chloride Leaching of Chalcopyrite Concentrate, H)dromet. 13, 125-135 (1984). Dutrizac, J.E., Elemental Sulphur Formation during the Ferric Chloride Leaching of Chalcopyrite, Hydromet., 23, 153-176 (1990).
Autoclave leaching of a tetrahedrite concentrate
10. 11.
12.
ME 6tl I--B
1125
Lotens, J.P. & Wesker, E., The Behaviour of Sulphur in the Oxidative Leaching of Sulphidic Minerals, Hydromet., 18, 39-54 (1987). Brusea, C., Ferrara, G. & Preti, U., Technologies Adapted for the Valorization of a Small MineThe Monte Avanza Polymetallie Sulphide (Tetrahedrite) Multibody Deposit. Part 2: Mineral Processing and Extractive Metallurgy, Proc. EC Seminar on Mineral Proc. and Ext. Met., Lisbon, (April 1991). Greig, J.A., Oxidative Chloride Leaching of Sulphide Concentrates, in Separation Processes in HydrometaUurgy, Davies G.A. (Ed.), Ellis Horwood, Chichester, 35-48 (1987).