Oxidation of Synthetic and Natural Samples of Enargite and

The oxidation of synthetic and natural samples of tennantite and enargite is compared by measuring the dissolution and the zeta potential of these min...
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Langmuir 1999, 15, 4524-4529

Oxidation of Synthetic and Natural Samples of Enargite and Tennantite: 1. Dissolution and Zeta Potential Study D. Fullston, D. Fornasiero,* and J. Ralston Ian Wark Research Institute, University of South Australia, The Levels Campus, Mawson Lakes SA 5095, Australia Received October 28, 1998. In Final Form: March 9, 1999 The oxidation of synthetic and natural samples of tennantite and enargite is compared by measuring the dissolution and the zeta potential of these minerals. The changes in zeta potential with pH and oxidizing conditions are consistent with the presence of a copper hydroxide layer covering a metal-deficient sulfurrich surface and with the extent of this copper hydroxide coverage increasing with oxidation conditions. Analysis of the zeta potential data reveals that during the acid titration of the minerals, dissolution of the surface copper hydroxide layer occurs at neutral pH values, whereas precipitation of copper hydroxide on the mineral surface is observed during the base titration. Hysteresis between the zeta potential acid and base titration curves is only observed in oxidizing conditions and is attributed to the dissolution of copper from the mineral lattice at acidic pH values. Arsenic does not appear to contribute to the mineral oxidation; in particular, its dissolution is much less than that of copper. This study has shown that in alkaline conditions the natural samples of tennantite and enargite oxidize more than the synthetic samples and tennantite oxidizes more extensively than enargite.

Introduction The metal impurities that are commonly associated with sulfide ore bodies, such as arsenic, antimony, bismuth, and mercury, are often enriched along with the valuable minerals in the flotation concentrate. The presence of these elements in the processed metals has a detrimental effect on their physical properties and also causes serious toxicological and environmental problems during smelting.1,2 High financial penalties are imposed by the smelters to treat minerals containing these elements; hence, the term “penalty” is often used to describe them.3 Although postflotation techniques such as hydrometallurgy or pyrometallurgy may be used to remove these penalty elements, it would be economically and environmentally beneficial to separate them from the valuable minerals during flotation. Their separation is difficult as they have generally a flotation behavior similar to that of the minerals with which they are associated. This is the case in the separation of iron sulfide minerals such as pyrite and arsenopyrite or copper sulfide minerals such as chalcocite, chalcopyrite, enargite, tennantite, and tetrahedrite. Apart from arsenopyrite, the amount of literature dealing with the separation of these penalty element minerals is limited. One of the separation methods relies on the selective oxidation of these minerals based on differences in their electrochemical properties.4 Oxidation can promote the adsorption of collectors such as xanthate at low oxidation potential values or prevent their adsorption by creating a physical barrier for their diffusion to the mineral surface at high oxidation potential values. The present work is a study of the oxidation of the copper arsenic sulfide minerals enargite (Cu3AsS4) and tennantite (Cu12As4S13) in view of their possible separation by flotation from nonarsenic copper sulfide minerals. Only a few * Corresponding author. Fax: [email protected].

+61-8-8302-3683. E-mail:

(1) Ozberg, E.; Guthrie, R. I. L. Mater. Sci. Technol. 1985, 1, 12. (2) Dutr’e, V.; Vandecasteele, C. J. Hazard. Mater. 1995, 40, 55. (3) Morizot, G.; Ollivier, P. Miner. Eng. 1993, 6, 841. (4) Byrne, M.; Grano, S.; Ralston, J.; Franco, A. Miner. Eng. 1995, 8, 1571.

studies have been devoted to the oxidation of enargite and tennantite. Recent investigations include cyclic voltammetry and impedance spectroscopy studies of enargite5,6 and an X-ray photoelectron spectroscopy (XPS) study of tennantite.7 With the absence of information on mineral purity in the first case and the presence of a large amount of impurities in the sample in the last case, it is difficult to ascertain if, in these studies, the results of mineral oxidation are truly characteristic of enargite and tennantite. In the present study, the samples of enargite and tennantite are characterized, and the effects of impurities in the natural samples on their surface oxidation are evaluated by comparing the oxidation of synthetic and natural samples. Their surface oxidation is monitored by measuring the dissolution and the zeta potential of these minerals as a function of oxidizing conditions. Experimental Section All chemicals were of analytical grade quality. High purity gas (nitrogen or oxygen from CIG Ltd) was scrubbed by bubbling it through a silica dispersion prior to introduction into the reaction vessel. High purity water, produced by reverse osmosis, two stages of ion exchange, and two stages of activated carbon prior to final filtration, was used in all experimental work. This water was “pretreated” at pH 11.0 with KNO3 (0.01 mol dm-3) as electrolyte and by bubbling the required gas into it. The natural samples of enargite and tennantite were purchased from Continental Minerals (Tucson, AZ). Their chemical analysis revealed that the tennantite sample contains 45.7% (weight %) Cu, 13.4% As and 23.6% S with impurities of Fe (4.5%), Pb (0.03%), and Zn (0.04%) and the enargite sample contains 57.2% Cu, 11.7% As, and 28.3% S with impurities of Fe (3.0%), Pb (0.02%) and Zn (0.2%). Optical microscopy examination of the polished surface of these minerals revealed the presence of a small amount of bornite and chalcocite in these samples. The presence of iron on the mineral surface was also confirmed by XPS. (5) Pauporte, Th.; Schuhmann, D. Colloids Surf., A 1996, 11, 1. (6) Cordova, R.; Gomes, H.; Real, S. G.; Schrebler, R.; Vilche, J. R. J. Electrochem. Soc. 1997, 144, 2628. (7) Mielczarski, J. A.; Cases, J. M.; Alnot, M.; Ehrhardt, J. J. Langmuir 1996, 10, 2519.

10.1021/la981526o CCC: $18.00 © 1999 American Chemical Society Published on Web 05/13/1999

Synthetic and Natural Samples of Enargite and Tennantite Synthetic enargite and tennantite were produced from stoichiometric amounts of the elemental components of copper (Aldrich, 99.98% pure), arsenic (Aldrich, 99.9998% pure), and sulfur (Aldrich, 99.99% pure) in a silica tube.8 The silica tube was sealed under a vacuum of 10-3-10-4 Torr and heated to 450-550 °C for 6-26 days. For enargite, because of incomplete mixing of the reactants, the content of the silica tube was ground and reacted a second time. Only one phase was present on the polished surface of the synthetic samples. Just prior to use, 0.25 g of mineral was ground in a small amount of pretreated water with a ceramic mortar and pestle inside a glovebox flushed with nitrogen gas. The mineral dispersion was then transferred to a water-jacketed, closed reaction vessel and conditioned in 0.5 dm3 of a 0.01 mol dm-3 KNO3 solution at pH 11.0 in the presence of the required gas. The reaction vessel was equipped with a tightly fitting lid containing sealable ports for gas entry and exit, electrodes, and a temperature probe. The temperature inside the reaction vessel was maintained at 21.0 ( 0.2 °C. The mineral dispersion was stirred continuously throughout the experiment. The pH was kept at a fixed value of interest by adding small quantities of a concentrated solution of nitric acid or potassium hydroxide. Minerals were conditioned for 20 min in the presence of nitrogen gas bubbling into the solution, for 60 min with oxygen gas, or for 60 min with oxygen gas and hydrogen peroxide (addition of 0.4 cm3 of a 30% w/v hydrogen peroxide solution). Conditioning with hydrogen peroxide was only required if the change of zeta potential between nitrogen and oxygen gas conditioning was too small. A short conditioning time with nitrogen gas and a long conditioning time with oxygen gas should provide conditions of slow and fast oxidation for the minerals, respectively.9,10 At the end of the mineral conditioning period, 10 cm3 of the mineral dispersion was transferred into a vertically mounted cell inside a Rank Brothers Microelectrophoresis Mark II apparatus (the temperature inside the cell was also maintained at 21 °C). At least 10 mobility measurements at each of the two stationary planes were performed (successively reversing the platinum electrode polarization), and the average mobility was converted to zeta potential using the Smoluchowski equation. The particle mobility was measured from pH 11.0 to 5.0 and back to pH 11.0 at half pH intervals. The mineral dispersion in the reaction vessel was equilibrated for 15 min at each new pH value before particle mobility measurement. For the mineral dissolution study, the conditioning of 2.5 g of mineral in 0.5 dm3 of solution at pH 11.0 and the rate of pH change from pH 11.0 to 5.0 were the same as that used in the zeta potential study. Aliquots of the pulp solution were taken when pH 5.0 was reached and at various time intervals thereafter, filtered through a 0.22 µm filter, and acidified before being analyzed for copper and arsenic with an inductively coupled plasma emission spectrophotometer (Spectroflame M Spectro).

Results Mineral Synthesis. The crystal structure of the synthetic and natural mineral samples was measured by X-ray powder diffraction (XRD). The more intense XRD peaks are reported in Tables 1 and 2 together with reference XRD peaks of the two arsenic minerals. The matching of the various XRD peaks (position and relative intensity) of the natural and synthetic samples used in this study to that of the reference XRD spectra is very high. However, for the natural enargite sample, additional peaks of relatively low intensities are observed at 2.38, 1.96, and 1.92 Å (Table 1). These peaks are certainly associated with the impurities of chalcocite and bornite found in the natural sample by optical microscopy examination. Indeed, the most intense reference XRD (8) Vaughan, D. J.; Craig, J. R. In Mineral Chemistry of Metal Sulfides; Harland, W. B., Agrell, S. O., Cook, A. H., Hughes, N. F., Eds.; Cambridge University Press: New York, 1978; p 264. (9) Fornasiero, D.; Li, F.; Ralston, J. J. Colloid Interface Sci. 1994, 164, 345. (10) Fornasiero, D.; Li, F.; Ralston, J.; Smart, R. St. J. Colloid Interface Sci. 1994, 164, 333.

Langmuir, Vol. 15, No. 13, 1999 4525 Table 1. Reference and Experimental XRD Peaks for the Enargite Samples Cu3AsS4a d (Å)

Cu3AsS4a

Int

d (Å)

Int

3.21 3.07 2.84

35 + 75 3.21 100 95 3.07 41 25 + 80 2.85 83

2.22

20 + 35 2.22

1.85 55 + 35 1.73 100 1.60 20 1.59 45 + 35 1.55 19 1.42 10 a

1.86 1.73 1.60 1.59 1.55 1.42

Cu3AsS4b

natural

synthetic

d (Å) Int d (Å) Int d (Å) Int

3.21 100 3.20 100 3.19 100 3.08 90 3.06 42 3.06 58 2.85 100 2.83 86 2.83 96 2.38 24 26 2.23 50 2.21 42 2.21 46 1.96 31 1.93 36 72 + 62 1.86 70 1.85 98 1.85 95 48 1.73 80 1.72 79 1.72 92 11 1.60 42 1.60 34 33 + 33 1.59 50 1.58 66 1.58 66 11 1.55 44 1.55 37 6+6 1.42 37 1.42 28

Reference 11. b Reference 12.

Table 2. Reference and Experimental XRD Peaks for the Tennantite Samples (Cu,X)12As4S13 X ) 4% Fe (iron type)a d (Å)

Int

2.94 2.55 2.40 1.99 1.86

100 30 20 20 20

1.80 1.65

80 20

1.54

50

a

(Cu,X)12As4S13 X ) 14% Hg (mercurian type)a d (Å)

Int

2.97

100

2.57 2.42 2.02 1.88 1.82

40 20 20 30 90

1.67 1.55

30 70

natural d (Å)

Int

2.93 2.54 2.40 2.00 1.86

100 25 19 19 29

1.80 1.65

62 21

1.54

39

synthetic d (Å)

Int

2.97 2.92 2.53 2.39 1.99 1.85 1.82 1.80 1.65 1.56 1.53

45 100 28 20 20 30 35 77 23 27 47

Reference 11.

peaks for chalcocite11 are found at 2.40 (Int ) 100) and 1.97 (Int ) 76) and for bornite at 1.94 (Int ) 100). Because of their relatively lower intensities in the natural enargite XRD spectrum, the other XRD peaks of chalcocite and bornite could not be observed. Gaudin and Dickie13 reported two types of tennantite that could be differentiated by their different rates of iridescent filming and crystal shapes: one tennantite is isotropic and films slowly in HCl-CrO3 and H2SO4-CrO3 solutions, whereas the other tennantite forms elongated, rosette-shaped crystals and films considerably faster. Furthermore, two distinct types of tennantite are presented in the Mineral Powder Diffraction File Data Book:11 a copper iron arsenic sulfide, (Cu, Fe)12As4S13, and a copper mercury arsenic sulfide, (Cu, Hg)12As4S13 (Table 2). The various XRD peaks of the synthetic tennantite in Table 2 indicate that the two types of tennantite reported in the literature, the iron type and the mercurian type, are present in this sample, although no iron or mercury was present in the reactants. It was found that for a short period of heating (6 days at 550 °C) in a slightly sulfurdeficient mixture and without a homogenization step, the mercurian type of tennantite prevails, but in a slightly sulfur-rich mixture for short periods of heating (6 days at 550 °C) or after long homogenization periods (26 days at 450 °C) the iron type of tennantite is formed. Table 2 shows that only one type of tennantite is present in the natural (11) Mineral Powder Diffraction File, Data Book; International Centre for Diffraction Data: Swarthmore, 1986. (12) Picot, P.; Johan, Z. In Atlas of Ore Minerals; BRGM-Elsevier: Amsterdam, 1982. (13) Gaudin, A. M.; Dicke, G. Econ. Geol. 1939, 34, 49.

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Fullston et al.

Figure 1. Zeta potential versus pH curves of natural enargite conditioned at pH 11.0 for 20 min in nitrogen (circle) and for 60 min in oxygen (triangle). Filled and empty symbols refer to a pH change from high to low pH values and from low to high pH values, respectively (the arrows show the direction of pH change).

Figure 3. Zeta potential versus pH curves of natural tennantite conditioned at pH 11.0 for 20 min in nitrogen (circle) and for 60 min in oxygen (triangle). Filled and empty symbols refer to a pH change from high to low pH values and from low to high pH values, respectively (the arrows show the direction of pH change).

Figure 2. Zeta potential versus pH curves of synthetic enargite conditioned at pH 11.0 for 20 min in nitrogen (circle), for 60 min in oxygen (triangle), and for 60 min with H2O2 (square). Filled and empty symbols refer to a pH change from high to low pH values and from low to high pH values, respectively (the arrows show the direction of pH change).

Figure 4. Zeta potential versus pH curves of synthetic tennantite conditioned at pH 11.0 for 20 min in nitrogen (circle), for 60 min in oxygen (triangle), and for 60 min with H2O2 (square). Filled and empty symbols refer to a pH change from high to low pH values and from low to high pH values, respectively (the arrows show the direction of pH change).

sample (iron type). This may be due either to the 4.5% iron impurity it contains or to the conditions under which the mineral was formed. Therefore, for a better comparison between the oxidation of the natural and synthetic samples, only the iron type synthetic tennantite was used in the zeta potential and dissolution studies. Zeta Potential Study. The zeta potential versus pH curves of the natural and synthetic samples of enargite and tennantite conditioned at pH 11.0 are presented in Figures 1-4. Conditioning the minerals for 20 min in the presence of nitrogen gas bubbling into the solution produces negative zeta potential versus pH curves. The zeta potential value increases monotonically from approximately -60 mV at pH 11.0 to -40 mV at pH 5.0. Within experimental error, no difference is observed between the

zeta potential acid (high to low pH change) and base (low to high pH change) titration curves. The zeta potential versus pH curves of the minerals conditioned for 60 min with oxygen gas are less negative and even become positive. The effect of oxygen conditioning on the zeta potential of the minerals (acid titration curves) is larger for tennantite than for enargite and larger for the natural minerals than for the synthetic ones. A large hysteresis in the zeta potential values is observed between the acid and base titration curves for only the natural samples. This hysteresis is maximum in the pH region between 7 and 9, with the base titration curve having the less negative or the more positive zeta potential values. As the differences in the zeta potential curves for the synthetic samples conditioned with nitrogen and oxygen gases are very small, hydrogen peroxide (60 min condi-

Synthetic and Natural Samples of Enargite and Tennantite

Langmuir, Vol. 15, No. 13, 1999 4527 Table 3. Arsenic Concentration (10-6 mol m-2) in Solution at pH 5.0 natural nitrogen oxygen

Figure 5. Copper dissolved from natural enargite and tennantite at pH 5.0 in nitrogen (empty circles) and oxygen (filled circles).

synthetic

tennantite

enargite

tennantite

enargite

3.9 1.6

1.5 0.5

1.4 1.9

3.4 3.8

concentration is negligible in the case of nitrogen conditioning but increases with oxygen conditioning to 3.6 × 10-6 mol m-2 for synthetic enargite, 6.2 × 10-6 mol m-2 for synthetic tennantite, 2.7 × 10-5 mol m-2 for natural enargite, and 4.1 × 10-5 mol m-2 for natural tennantite. The concentration of arsenic in solution at pH 5.0 is much less than the concentration of copper and remains almost constant with time. In more acidic conditions and in the presence of ferric ions, similar dissolution rates of copper and arsenic were observed for enargite14 and tennantite.15 The amount of arsenic dissolved from the natural minerals is higher for nitrogen conditioning than for oxygen conditioning, but the opposite is true for the synthetic minerals, as shown in Table 3. The larger amount of copper hydroxide formed on the natural mineral surface may act as a barrier for the diffusion of arsenic away from the surface, and this may explain the opposite trend in arsenic dissolution between synthetic and natural samples. Discussion

Figure 6. Copper dissolved from synthetic enargite and tennantite at pH 5.0 in nitrogen (empty circles) and oxygen (filled circles).

tioning) is added to produce a more oxidizing environment. Under these conditions, the zeta potentials of the synthetic samples are further reduced and even become positive. Hysteresis is now observed between the acid and base titration curves. Dissolution Study. The minerals were conditioned at pH 11.0 for the same amount of time and with the same gases as in the zeta potential study. The rate of pH change from pH 11 to 5 was also the same. The amount of copper and arsenic in solution was only monitored as a function of time when the pH value of 5.0 was reached. The amount of copper dissolved from the natural and synthetic minerals is shown in Figures 5 and 6, respectively. More copper is dissolved at pH 5.0 from the minerals conditioned in oxygen than in nitrogen, from the natural minerals than from the synthetic ones, and from tennantite than from enargite. The concentration of copper measured at the time of 0 min corresponds to the amount copper dissolved from the minerals during the acid titration (a parallel study has shown than copper could only be detected in solution at pH values lower than 7). This copper

The oxidation of enargite has been previously studied by cyclic voltammetry and impedance spectroscopy.5,6 Although no comment on the purity of the enargite sample used in the study was made, it was shown that only copper is involved in the first step of oxidation. The results of cyclic voltammetry and impedance spectroscopy were consistent with the incongruent dissolution of copper forming a copper-depleted passivating film on the enargite surface. The XPS study of tennantite conditioned at pH 10 with air indicated that only a small amount of surface oxidation product is present on the tennantite surface.7 The trends in the zeta potential of enargite and tennantite with pH and oxidizing conditions observed in this study are similar to those observed for copper sulfide minerals such as chalcocite.16,17 Although some chalcocite is present in the natural samples, the similar zeta potential behavior of the synthetic and natural samples indicates that these zeta potentials are representative of copper arsenic sulfide minerals. The zeta potential versus pH curves of sulfide minerals characteristic of weakly or nonoxidized surfaces and of fully oxidized surfaces are shown schematically in Figure 7 by curves 1 and 2, respectively. The isoelectric point, IEP, is characteristic of the electrical properties of the minerals and is defined as the pH value where the zeta potential value is zero. For pH values higher and lower than the IEP, the zeta potential is negative and positive, respectively. The zeta potential value reflects the charge on the mineral surface. This charge results from the protonation or deprotonation of surface metal sulfide groups (14) Dutrizac, J. E.; MacDonald, R. J. C. Can. Metall. Q. 1972, 11, 469. (15) Dutrizac, J. E.; Morrison, R. M. NATO Conf. Ser. 1984, 6, 77. (16) Healy, T. W.; Moignard, M. S. In Flotation, A. M. Gaudin Memorial; Fuerstenau, M. C., Ed.; AIME: New York, 1976; Vol. 1, p 275. (17) Liu, J. C.; Huang, C. P. Langmuir 1992, 8, 1851.

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Figure 7. Schematic representation of the zeta potential versus pH curves of sulfide minerals with low (curve 1), high (curve 2). and intermediate (curve 3) levels of surface oxidation.

-MSH + H+ S -MSH2+ -MSH S -MS- + H+ (1) or surface metal hydroxide groups

-MOH + H+ S -MOH2+ -MOH S -MO- + H+ (2) The IEP of nonoxidized sulfide minerals has been obtained (by extrapolation) at acidic pH values between 1 and 2,9,16,18 while the IEP of metal hydroxides has a wider pH range but occurs generally at higher pH values. For example, IEP values of 6.5 for ferric oxide/hydroxide, 9.5 for copper oxide/hydroxide, and 12 for magnesium oxide/hydroxide19 are found. In the initial stage of oxidation, as the metal leaves the mineral lattice, the surface becomes metal-deficient and therefore sulfur-rich with reaction 1 controlling the charging of the sulfide groups.9,10,20 The corresponding zeta potential becomes more negative with increasing pH values (curve 1 in Figure 7). As oxidation proceeds, the surface sulfide groups oxidize to polysulfide, Sn2-, or elemental sulfur.16,20 It is therefore not surprising to observe similar zeta potentials for elemental sulfur and for sulfide minerals conditioned in nonoxidizing environments,9,16,20 as is the case in this study for all of the copper arsenic minerals conditioned with nitrogen. Elemental sulfur has also been detected on the surfaces of tennantite and enargite in leaching experiments in acidified ferric solutions.14,15 In the presence of oxygen and at higher pH values, metal hydrolysis occurs on the mineral surface and in solution, followed by precipitation of metal hydroxide on the sulfurrich surface. Reaction 2 controls the charging of the metal hydroxide groups. The zeta potential versus pH curve of a fully oxidized sulfide mineral is similar to that of the metal hydroxide covering its surface (curve 2 in Figure 7). For a partially oxidized surface, the zeta potential versus pH curve lies between that of elemental sulfur and that of the corresponding metal hydroxide, as in curve 3 of Figure 7.9,16,20 In this study, the mineral dissolution results and the change in zeta potentials (acid titration) between the minerals conditioned in nitrogen and oxygen indicate that at alkaline pH values more copper hydroxide is present on tennantite than on enargite and more on the natural samples than on synthetic samples. (18) Fornasiero, D.; Eijt, V.; Ralston, J. Colloids Surf. 1992, 62, 63. (19) Parks, G. A. Chem. Rev. 1965, 65, 177. (20) Fairthorne, G.; Fornasiero, D.; Ralston, J. Int. J. Miner. Process. 1997, 49, 31.

Fullston et al.

Large differences in zeta potentials have been observed when sulfide minerals such as chalcocite, galena, or sphalerite are conditioned at acidic or alkaline pH values.9,16 For example, conditioning galena at alkaline pH values produces a zeta potential versus pH curve similar to curve 2 (Figure 7) with an IEP value of 9, which is consistent with a layer of lead hydroxide covering the galena surface. In contrast, when galena is conditioned at acidic pH values, lead dissolves from the mineral lattice, leaving behind a sulfur-rich surface with a zeta potential versus pH curve very similar to that of elemental sulfur. As the pH is increased, the lead ions hydrolyze in solution and eventually precipitate as lead hydroxide on the galena surface at a pH value corresponding to the precipitation edge of lead hydroxide. For higher pH values, the zeta potential of galena reflects the presence of lead hydroxide covering the mineral surface. The mechanism of metal hydroxide interaction with mineral surfaces is well accepted, and the corresponding zeta potential behavior, illustrated with curve 3 in Figure 7, is well documented. This behavior also occurs when metal ions (e.g., Pb(II), Cu(II), Fe(II), Ni(II), Co(II), Ca(II), etc.) are introduced into the system and precipitate as metal hydroxides on silica or sulfide minerals, such as sphalerite or pyrite.21-25 The processes of metal hydroxide precipitation onto and dissolution from mineral surfaces are not necessarily pH reversible and depend on several thermodynamic and kinetic factors. In the conditions used in this study, the most important factor is mainly related to the kinetics of pH change during the acid and base titration of the mineral. In particular, the differences in zeta potentials between curves 2 and 3 in Figure 7 observed at acidic pH values may be explained by different thicknesses of the metal hydroxide layer that require different times to dissolve. The strength of interaction (chemical, electrostatic, hydrogen bonding) of the metal hydroxides with the surface groups may also control the metal hydroxide surface covering or dissolution and therefore affects the shape of the zeta potential versus pH curves. In this study, all the zeta potential curves converge at the acidic end of these curves to approximately the same zeta potential value, irrespective of the conditioning of the minerals (N2, O2 and H2O2) and of the direction of pH change (acid or base titration). This behavior is indicative of the dissolution of the copper hydroxide from the mineral surface, exposing the sulfur-rich surface at acidic pH values. In fact, the abrupt change in zeta potential values during the acid titration of natural tennantite conditioned in oxygen (Figure 3) indicates that copper hydroxide starts to dissolve at pH values lower than 8. If the same copper hydroxides which have dissolved during the mineral acid titration reprecipitate during the base titration, we should expect similar values of the zeta potential maximum. This is not the case for the minerals conditioned in oxygen, with their zeta potentials during the base titration being less negative or more positive than those during the acid titration. This extra amount of surface copper hydroxide observed during the base titration certainly results from the dissolution of copper ions from the mineral lattice at acidic pH values (as observed in the dissolution results in Figures 5 and 6), (21) James, R. O.; Healy, T. W. J. Colloid Interface Sci. 1972, 40, 42. (22) Wang, X.; Forssberg, E.; Bolin, N. J. Miner. Process. Extr. Metall. Rev. 1989, 4, 167. (23) Healy, T. W. In Principles of Mineral Flotation; Jones, M. H., Woodcock, J. T., Eds.; The Wark Symposium, AIMM: Parkville, 1984; p 43. (24) Mackenzie, J. M. W.; O’Brien, R. T. Trans. Am. Inst. Min., Metall. Pet. Eng. 1969, 244, 168. (25) Pugh, R. J.; Tjus, K. J. Colloid Interface Sci. 1987, 117, 231.

Synthetic and Natural Samples of Enargite and Tennantite

which precipitate as copper hydroxide during the mineral base titration at pH values larger than 6 (precipitation edge for copper hydroxide26). The extent of zeta potential hysteresis between the acid and base titrations therefore reflects the level of mineral dissolution at acidic pH values; no or little copper dissolution occurs for the minerals conditioned in nitrogen, and therefore no hysteresis is observed in their zeta potentials. Similar trends are observed for the zeta potentials of the synthetic samples in Figures 2 and 4. However, their dissolution and the change in zeta potential values with increasing oxidation conditions are much less than in the case of the natural samples, which in turn implies that the synthetic samples oxidize more slowly than the natural samples. A similar conclusion was obtained in a comparative XPS and scanning tunneling microscopic study of the oxidation of natural and synthetic galena samples.27 The presence of impurities in the natural samples may be at the origin of the more extensive oxidation of the natural samples than of the synthetic samples. We note in the zeta potential versus pH curves (acid titration) of the synthetic samples conditioned with hydrogen peroxide that the change in zeta potential values corresponding to the dissolution of copper hydroxide occurs at a more acidic pH value (pH ) 6) than for the natural samples. These zeta potential results indicate that the surface of the synthetic samples are covered with less oxidation products (copper hydroxide) than the surface of the natural samples but a lower pH is necessary to dissolve them. It is possible that these oxidation products are more resistant to acidic conditions because they are more strongly attached or/ and their layer is thicker on the synthetic samples than on the natural samples. The former explanation is more likely as XPS results of the same samples28 indicate that the thickness of the copper hydroxide layer is more or less the same on the surfaces of the synthetic and natural samples. A difference in the nature of the surface sites where these copper hydroxides are formed is possibly at the origin of the different affinities of these copper hydroxides for synthetic and natural samples. A recent study has shown that oxidation of a synthetic galena surface occurs preferentially on the edges and dislocations rather than on the faces of the galena surface, as is the case for a natural galena sample.27 The following equations illustrate the reactions that may occur on the enargite (and tennantite) surface, as discussed during the interpretation of the zeta potential data. oxidation at pH 9.0:

Cu3AsS4 + x/2O2 + x H2O f Cu3-xAsS4‚‚‚(Cu(OH)2)x dissolution of copper hydroxide during the acid titration: (26) Brookins, D. G. In Eh-pH Diagrams for Geochemistry; SpringerVerlag: Berlin, 1988; p 61. (27) Kim, B. S.; Hayes, R. A.; Prestidge, C. A.; Ralston, J.; Smart, R. St. C. Appl. Surf. Sci. 1994, 78, 385. (28) Fulston, D.; Fornasiero, D.; Ralston, J. Langmuir 1999, 15, 4530.

Langmuir, Vol. 15, No. 13, 1999 4529

Cu3-xAsS4‚‚‚(Cu(OH)2)x + 2xH+ f Cu3-xAsS4 + xCu2++ 2xH2O dissolution of copper from the mineral lattice at low pH values:

Cu3-xAsS4 + 2yH+ + y/2O2 f Cu3-x-yAsS4 + yCu2+ + yH2O precipitation of copper hydroxide during the base titration:

Cu3-x-yAsS4 + (x + y)Cu2+ + 2(x + y)H2O f Cu3-x-yAsS4‚‚‚(Cu(OH)2)(x+y) + 2(x + y)H+ Complementary XPS studies of these minerals will be performed to confirm the results of this zeta potential analysis and to provide more direct evidence for the very slow oxidation of arsenic on the mineral surface.28 Conclusions Samples of tennantite and enargite have been synthesized. XRD analysis has revealed that two types of tennantite an iron type and a mercurian type, can be formed. Their proportion depends on the conditions used in the synthesis but may also reflect the presence of impurities, as is the case for the natural samples. The changes in zeta potential with pH and oxidizing conditions are consistent with the presence of a copper hydroxide layer covering a metal-deficient sulfur-rich surface and with the extent of this copper hydroxide coverage increasing with oxidation conditions. Analysis of the zeta potential data reveals that during the acid titration of the minerals dissolution of the surface copper hydroxide layer occurs at pH values less than 8, whereas during the base titration, precipitation of copper hydroxide on the mineral surface is observed at pH values higher than 6. Hysteresis between the zeta potential acid and base titration curves is only observed in oxidizing conditions and is attributed to the dissolution of copper from the mineral lattice at acidic pH values. This dissolution is larger for the natural samples than for the synthetic samples, hence the larger hysteresis observed in the zeta potentials of the natural samples. Arsenic does not appear to contribute to the mineral oxidation. In particular, its dissolution is much less than that of copper. This study has shown that in alkaline conditions the natural samples of tennantite and enargite oxidize more than the synthetic samples and tennantite oxidizes more extensively than enargite. Acknowledgment. The financial support for this work from the Australian Research Council and Rio Tinto Ltd. is gratefully acknowledged. LA981526O