Langmuir 2006, 22, 1671-1679
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Sulfidation of Lead-Loaded Zeolite Microparticles and Flotation by Amylxanthate Alain Walcarius,*,† Bouchaı¨b Marouf,‡ Ali M. Lamdaouar,†,‡,§,| Khadija Chlihi,| and Jacques Bessiere† Laboratoire de Chimie Physique et Microbiologie pour l’EnVironnement, Unite´ Mixte de Recherche UMR 7564, CNRS - UniVersite´ H. Poincare´ Nancy I, 405 rue de VandoeuVre, F-54600 Villers-les-Nancy, France, Ecole Nationale de l’Industrie Mine´ rale, B.P. 753, Rabat, Morocco, Laboratoire d’Electrochimie et de Chimie Analytique, UniVersite´ Mohamed V, Faculte´ des Sciences, De´ partement de Chimie, AVenue Ibn Battouta, Rabat, Morocco, and Laboratoire de Chimie Analytique et Electrochimie Applique´ es, UniVersite´ Ibn Tofaı¨l, B.P. 131 Ke´ nitra, Morocco ReceiVed August 17, 2005. In Final Form: December 4, 2005 The possibility of recovering lead-loaded zeolite Y microparticles (PbY) by flotation after sulfidation was investigated using amyl xanthate (AMX) as the collector. The sulfidation process (by aqueous Na2S) was first studied as a function of the medium composition (Na2S concentration, pH), and the solid phases were characterized by various physicochemical techniques (X-ray photoelectron spectroscopy, high-frequency impedance measurements, and electrochemistry). Progressively increasing the sulfidation level resulted in the concomitant transformation of Pb(II) species ion-exchanged in the zeolite into PbS clusters that were mostly located at the external boundaries of the zeolite particles while remaining attached to the aluminosilicate (PbS-Y). Similar to what occurred for galena particles, the zeolite-supported PbS clusters were likely to adsorb the AMX collector, the remaining (nonsulfided) ion-exchanged Pb(II) species being transformed into a Pb(AMX)2 precipitate when using AMX in large excess. Owing to such AMX adsorption on PbS-Y, the zeolite particles were found to flocculate and to float in the presence of air bubbles. If rather high AMX concentrations (>5 × 10-3 M) were necessary to ensure the flotation of nonsulfided PbY particles (Walcarius, A.; Lamdaouar, A. M.; El Kacemi, K.; Marouf, B.; Bessiere, J. Langmuir 2001, 17, 2258), significantly lower concentrations (down to 1 × 10-4 M) gave rise to high flotation yields (ca. 95%) upon PbY sulfidation. It is noteworthy that the sulfidation level should be maintained at a value high enough (>10%) to produce the minimal PbS amount ensuring flotation but not too high (90%) were observed over a wide pH range (from 5 to 12), contrary to what happened for non-sulfided PbY particles that were only recovered in high yields at pH values higher than 11 because a Pb(OH)2 overlayer on the zeolite particles was necessary to enable the collection process by amyl xanthate.22 Without Na2S treatment, no detectable flotation was observed in an AMX concentration as high as 5 × 10-4 M (curve b in Figure 8). This points out the interest in such a preconditioning event from an environmental point of view for the purpose of remediation of lead-contaminated effluents using fine zeolite powders, which can then be recovered quite easily by flotation using small amounts of collector. By comparison, AMX concentrations in the 10-5 M range were usually large enough to ensure complete flotation of galena particles 50-100 µm in diameter,26 whereas much more concentrated collector solutions (mM range) were necessary to flocculate and recover galena fine particles (µm size).58 The flotation of sulfide-treated cerussite (PbCO3 with overcoated PbS clusters) set in only after adding AMX at rather high concentrations, in the 5 × 10-4 to 10-3 M range as a function of the sulfide conditioning time, because enough xanthate species are required to precipitate all of the aqueous lead species originating from the cerussite suspension.26 Similar values have been reported for the flotation of malachite with xanthates by sulfidation in aqueous medium.59 In the present case, it is not necessary to consume all of the Pb2+ (and Pb4(OH)44+) cations from the zeolite because the chemisorption of a rather small amount of amyl xanthate on the PbS clusters is enough to increase the hydrophobicity of the particles and enable their flotation. The flotation yields reached here are of the same order of magnitude as those reported for the flotation of metal-loaded zeolites using nonselective surfactant collectors.12,13,17,18 The recovery of 1 mmol of Pb(II) contained in 1 g of zeolite requires only 0.1 mmol of sulfide and 0.01 mmol of collector. (58) Song, S.; Lopez-Valdivieso, A.; Reyes-Bahena, J. L.; Bermejo-Perez, H. I.; Trass, O. J. Colloid Interface Sci. 2000, 227, 272. (59) Bustamante, H.; Castro, S. Trans. Inst. Mining Metal., Sect. C 1975, 84, 167.
Walcarius et al.
Figure 9. Influence of the Na2S concentration on the flotation yield of lead-doped zeolite particles (10 g L-1) by 7 × 10-5 M potassium amyl xanthate after treatment with 1 mM Na2S (9, pH 6.6; 0, pH 11).
However, flotation was inhibited when using an excess of sulfide in solution (over the lead content in the zeolite), as illustrated by curve c in Figure 8 depicting zeolite recoveries lower than 10% in the presence of 3 × 10-2 M Na2S. The influence of the Na2S concentration on the flotation yield was more extensively studied, and typical results are shown in Figure 9 for two pH values (6.6 and 11). Clearly, it is shown that a Na2S concentration as low as 1 mM, corresponding to 10% of the lead content in the zeolite for the solid-to-solution ratio applied in the experiments, is sufficient to ensure high flotation yields. Such high recoveries were maintained up to Na2S concentrations of 5-7 mM depending on the operating pH and then decreased dramatically in more concentrated sulfide media. As mentioned previously, sulfide concentrations in solution that are too high have a deleterious effect on the interaction between AMX and PbS (as well as many other sulfide minerals60) because they contribute to depress galena by lowering the potential of the suspension below the value of lead xanthate formation.25,61 This deleterious effect was slightly more pronounced at pH 11 than in a neutral medium. 3.2.3. Depression Tests: Comparison with Galena Flotation. The above results have already pointed out several similarities between the sulfided lead-doped zeolite particles and the galena mineral with respect to their interaction with amyl xanthate and subsequent flotation. To argue further in this direction, some additional depression tests have been performed to evaluate their effects on hindering the flotation processes (Figure 10). Basically, galena depression with respect to its flotation by AMX occurred when the potential of the medium was too low to enable the formation of hydrophobic species (AMXads, Pb(AMX)2, or (AMX)2) on the mineral surface or, alternatively, by using reactants likely to make the PbS surface more hydrophilic. The commonly used galena depressors are sulfide26,62 or sulfite63-65 species as well as chromate and dichromate ions.65-68 (60) Woods, R.; Richardson, P. E. In AdVances in Mineral Processing; Somasundran, P., Ed.; Society of Mining Engineers: Littleton, CO, 1986; pp 154-170. (61) Guy, P. J.; Trahar, W. J. In Flotation of Sulphide Minerals; Forssberg, K. S. E., Ed.; Elsevier: Amsterdam, 1985; pp 91-109. (62) Dutra, A. J. B.; Espinola, A.; Sampaio, J. A. J. Braz. Chem. Soc. 1997, 8, 193. (63) Grano, S. R.; Prestidge, C. A.; Ralston, J. Int. J. Miner. Process. 1997, 52, 1. (64) Grano, S. R.; Prestidge, C. A.; Ralston, J. Int. J. Miner. Process. 1997, 52, 161. (65) Shimoiizaka, J.; Usui, S.; Matsuoka, I.; Sasaki, H. In Flotation; Fuerstenau, M. C., Ed.; AIME: New York, 1976; Vol. 1, pp 393-413. (66) Okada, S.; Majima, H. Can. Metall. Q. 1971, 10, 189.
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depression tests confirm the similar behavior of sulfided PbY zeolite and galena with respect to their flotation when using amyl xanthate as the collector.
4. Conclusions
Figure 10. Influence of the depressor concentration on the flotation yield of lead-doped zeolite particles (10 g L-1) by 7 × 10-5 M potassium amyl xanthate after treatment with 4 mM Na2S (pH 7): (a) K2Cr2O7 and (b) Na2SO3.
The role played by sulfide species in preventing the interactions between AMX and sulfided PbY zeolite particles has been discussed in section 3.2.1, and the consequence of this effect on hindering their effective flotation has been illustrated in Figure 9. A similar decrease in the flotation yields of PbY has been observed when adding sulfite ions to the medium (Figure 10, curve b), but this was less pronounced than when using sulfide as the depressor because of the weaker reducing properties of sulfite in comparison to those of sulfide. This led to flotation yields of the same order of magnitude as those reported for galena recovery by ethyl xanthate in the presence of added sulfite species.63 Finally, dichromate species were found to have a dramatic influence on inhibiting the flotation of sulfided PbY zeolite particles (Figure 10, curve b), similar to what occurred for galena by preventing AMX collector chemisorption because of the formation of lead chromate on the PbS surface.65 These (67) Cecile, J. L.; Bloise, R.; Barbery, G. In Complex Sulphide Ores; Jones, M. J., Ed.; Institute of Mineral Metallurgy: London, 1980; pp 159-70. (68) Paalsson, B. I. Int. J. Miner. Process. 1991, 33, 207.
Zeolite particles ion-exchanged with Pb(II) ions can be readily and quantitatively sulfided in aqueous Na2S. The resulting PbS clusters remain strongly attached to the aluminosilicate framework and have the tendency to enrich the external boundaries of the zeolite particles. They display dielectric and electrochemical properties similar to those of the galena mineral. The partially sulfided PbY particles are likely to interact with amyl xanthate (AMX) according to two different pathways: the first one involves the precipitation of Pb(AMX)2 by the reaction of nonsulfided Pb(II) species with AMX, and the second one implies the chemisorption of AMX onto the surface of the PbS clusters. This last process is at the origin of the possibility of separating the solid microparticles from the liquid phase by dispersed-air flotation. High yields (>90-95%) were obtained using AMX collector concentrations 100 times lower than those required to recover nonsulfided PbY particles by a similar procedure. This was possible over a wide pH range extending from 5 to 12 whereas the recovery of non-sulfided PbY was restricted to pH values above 10-11. A minimal sulfidation level of 10% was necessary to ensure zeolite recoveries higher than 90%. The mechanism responsible for the flotation of lead-loaded zeolite microparticles after sulfidation is probably similar to that implied in galena flotation, as suggested by depression tests performed under reducing conditions. Acknowledgment. This work was supported by a cooperation grant between France and Morocco (A.P.A.P.E., no. 1115), for which Professor K. El Kacemi is gratefully acknowledged. We also thank J. Lambert (LCPME) for recording the XPS spectra. Supporting Information Available: Speed of PbY sulfidation for various starting Pb/S ratios. Dielectric monitoring of PbY sulfidation at pH 11. This material is available free of charge via the Internet at http://pubs.acs.org. LA0522404
