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Effect of the lattice ions on the calcite flotation in presence of Zn(II)
Minerals Engineering 40 (2013) 24–29
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Effect of the lattice ions on the calcite flotation in presence of Zn(II) Qing Shi, Guofan Zhang ⇑, Qiming Feng, Leming Ou, Yiping Lu Department of Mineral Engineering, School of Minerals Processing and Bioengineering, Central South University, Changsha 410083, China
a r t i c l e
i n f o
Article history: Received 2 May 2012 Accepted 18 September 2012 Available online 8 November 2012 Keywords: Calcite Flotation Zinc(II) Lattice ions
a b s t r a c t The effect of Zn2+ on calcite flotation and the effect of lattice ions on calcite flotation in presence of Zn2+ were investigated through microflotation tests, zeta potential measurements, and surface analysis. The results show that Zn species inhibit the interaction between the reagents and calcite through the formation of surface precipitation coating on the calcite surface, when the concentration of Zn2+ is higher than 105 mol/L. The effect of Zn species on calcite flotation can be decreased with the increase of Ca2+, instead 2+ and CO2 of CO2 3 . In presence of Ca 3 , the change of floatability suggests that Zn species can be desorbed with an optimum ratio of Ca2þ =CO2 3 . The zeta potential measurements and surface analysis show the existence of a mixture of hydrozincite and zinc hydroxide products on mineral surface, and zinc surface coating can be desorbed due to the strong chemical adsorption of Ca2+ and the formation of hydrozincite in the bulk solution. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Calcite is the common and important component of sediments and sedimentary rocks (Morse, 1990). Also it is one of the most extensive carbonate gangue in the flotation of smithsonite, apatite, scheelite and fluorite (Arnold et al., 1978; Hosseini and Forssberg, 2009; Takamori and Tsunekawa, 1983). Separation of valuable minerals and calcite is extremely complex due to the interaction of minerals with several dissolved metals, leading to sorption processes such as precipitation/coprecipitation, adsorption, and ionic substitution. In the case of smithsonite, surface conversion inhibits the selective separation of smithsonite from calcite as a result of reactions of the dissolved species with mineral surface, considerably altering interfacial characteristics of minerals (Hu et al., 1995). Though reagents such as sodium silicate (type N), starch, sodium hexa methaphosphate, and methylcarboxylate can be used as calcite depressants in smithsonite flotation, the process is not selective enough in practice due to the similar surface properties and surface conversion (Ejtemaei et al., 2011; Irannajad et al., 2009a; Kashani and Rashchi, 2008; Marabini et al., 2007). However, few literatures focused on the effect of Zn2+ on calcite flotation and how to eliminate the effect of Zn2+ through changing the surface components of calcite. The behavior and mechanism of Zn2+ adsorption on calcite have been noted by numerous investigators. When total aqueous Zn concentration is less than 105.25 mol/L, Zinc adsorption occurs via exchanging with Ca2+ in the surface-adsorbed layer on calcite ⇑ Corresponding author. Tel.: +86 731 88836817. E-mail address: [email protected] (G. Zhang). 0892-6875/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.mineng.2012.09.016
(Zachara et al., 1988). A thermodynamic study suggests that adsorbed Zn2+ on calcite remains hydrated, unlike its behavior on magnesite and dolomite (Jurinak and Bauer, 1956). Sorption/ desorption experimental results suggest that the sorption is totally reversible at low alkaline range (Tertre et al., 2010; Zachara et al., 1989), which results from the hydrated metal ions distributed on the calcite surface or within a surface layer undergo exchange equilibria with the aqueous phase (Zachara et al., 1991). Cheng et al. (1998) reported that Zn2+ substituted Ca2+ in the surface layer of calcite with structural relaxation of the coordinating CO3 ligands, in response to the size of Zn2+ which is smaller than that of Ca2+. SEXAFS results indicated that the ZnAO radial distance was very similar to the ZnAO bond length in pure ZnCO3, which was also found by Reeder et al. (1999) for Zn incorporated into calcite during crystal growth (Cheng et al., 1998; Reeder et al., 1999). Elzinga showed the results of in situ extended X-ray absorption fine structure (EXAFS) spectroscopy studies of Zn2+ complexes formed at the calcite surface. Zn(II) was found to coordinate at Ca sites on the calcite surface, forming mononuclear inner-sphere adsorption complexes (Elzinga and Reeder, 2002). When Zn2+ concentration approached and exceeded the solubility of known zinc carbonate solids, the sorption and precipitation of Zn was investigated by Zachara et al. (1989). Surface-enhance precipitation was not observed and CaCO3 did not nucleate Zn solids, when the aqueous ion activity product (IAP) was below the equilibrium IAP of the least soluble, kinetically viable Zn phase. The CaCO3(s) surface was not requisite for Zn precipitation. When CaCO3(s) was presented as the precipitation formed a surface coating or discrete Zn particles bound to the surface, XPS, XRD, EDS of both Zn-treated CaCO3(s) and isolated Zn particles implied that the precipitation was hydrozincite or its hydrated form (Zachara et al., 1989).
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The objective of this study is to investigate the effect of Zn2+ on calcite flotation and the effect of calcite lattice ions on calcite flotation in presence of Zn2+. In the present study, the effects of various concentrations of Zn2+ and the lattice ions on pure calcite at various pH values were assessed using flotation, and surface analysis was used to interpret the results of flotation. 2. Materials and methods 2.1. Samples and reagents The calcite used for all experiments was obtained from Changsha, Hunan Province, China. Mineralogical and X-ray powder diffraction data confirmed that the calcite sample was of high 99% purity. The samples were ground and then sieved to collect the 100 lm fraction for the microflotation and XPS tests. Hydrochloric acid (HCl), sodium hydroxide (NaOH), and sodium carbonate (Na2CO3) were used as pH regulators. Sodium oleate and sodium hexametaphosphate (SH) were used as collector and depressant, respectively. Calcium chloride and zinc sulfate were also prepared. All the reagents used in this study were of analytical grade. Deionized water was used for all tests. 2.2. Experiments 2.2.1. Flotation tests Single mineral flotation tests were carried out in a mechanic agitation flotation machine at a constant rate. The mineral suspension was prepared by adding 2.0 g of minerals to 40 mL of solutions. The pH of the mineral suspension was adjusted to a desired value by adding NaOH, Na2CO3 or HCl stock solutions. The prepared SH solution was added at a desired concentration, and the slurry was conditioned for 5 min. Then samples were conditioned with collector for 3 min and flotation was carried out for a total of 5 min. Both floated and unfloated particles were collected, filtered and dried. The flotation recovery was calculated based on solid weight distributions between these two products. 2.2.2. Zeta potential measurements Zeta potential measurements on calcite were carried out using a plus zeta potential meter (Brookhaven, Zetaplus, USA). Potassium
Fig. 2. Flotation recovery of calcite as a function of Zn2+ concentrations in presence of 10 mg/L SH and 2.5 104 mol/L NaOL at pH 10.0.
chloride was used to maintain the ionic strength at 103 mol/L. Samples of 0.030 g in the size-5 lm were added into 50 mL of the electrolyte solution or reagent solution with desired pH using sodium carbonate (Na2CO3). The suspensions were then magnetically stirred for 10 min, and the final pH was measured and recorded. The zeta potential of samples was then measured in the absence/presence of Zn2+. 2.2.3. Surface analysis A Thermo Fisher Scientific K-Alpha 1063 XPS system was used for the X-ray Photoelectron Spectroscopy measurements (XPS). The test chamber pressure was maintained below 109 mbar during spectral acquisition. Binding energy scale was corrected based on C 1s peak from contaminations (around 284.8 eV) as the internal binding energy standard. The samples were conditioned with the same conditioning regime as the flotation tests without the addition of SH and collector, then filtrated and rinsed thoroughly with water in order to remove any weakly adsorbed particles. The prepared samples were dried in a vacuum oven at room temperature for 24 h. 3. Results and discussion
Fig. 1. Flotation recovery of calcite as a function of pH: (1a) in presence of 0.5 104 mol/L NaOL; (1b) in presence of 0.5 104 mol/L NaOL and 1.0 104 mol/L Zn2+; (2a) in presence of 2.5 104 mol/L NaOL; (2b) in presence of 2.5 104 mol/L NaOL and 1.0 104 mol/L Zn2+; (3a) in presence of 2.5 104 mol/L NaOL and 10 mg/L SH; (3b) in presence of 2.5 104 mol/L NaOL, 1.0 104 mol/L Zn2+, and 10 mg/L SH.
The effect of Zn2+ on flotation recovery of calcite as a function of pH without and in presence of sodium hexametaphosphate (SH) is shown in Figs. 1 and 2. Water molecules at calcite surfaces can form tightly packed monolayer structure, suggesting that the calcite surface is easily hydrated by water molecules due to its high surface polarity (Fa et al., 2006). Oleic acid and sodium oleate have been the most extensively used collectors for calcite (Changgen and Yongxin, 1983; Irannajad et al., 2009b; Mishra, 1982; Young and Miller, 2000). In the pH range from 7.0 to 11.5, the floatability of calcite with 0.5 104 mol/L sodium oleate is higher than 75% (Fig. 1curve1a). The higher recovery is attributed to chemisorption of oleate on calcite at low concentrations (Antti and Forssberg, 1989; Young and Miller, 2000). When the concentration of sodium oleate increases to 2.5 104 mol/L, the recovery of calcite is higher than 90% in the pH range from 7 to 11.5 (Fig. 1curve2a). With the addition of 1.0 104 mol/L Zn2+, the floatability of calcite decreases from 80.2% at pH 7.5% to 36% at pH 11 in presence of 0.5 104 mol/L sodium oleate (Fig. 1curve1b), while the recovery of calcite remains higher than 90% in presence of 2.5 104 mol/L sodium oleate (Fig. 1curve2b). The results suggest that calcite is depressed by the Zn species and this depression effect depends on pH at the alkaline range. Meanwhile, the higher floatability of
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calcite in presence of 2.5 104 mol/L sodium oleate and 1.0 104 mol/L Zn2+ indicates that the zinc species can also react with the collector. Sodium hexametaphosphate (SH), also known as Calgon, has been widely used as the depressant of calcite (Changgen and Yongxin, 1983; Wu and Zhu, 2006). The floatability of calcite decreases drastically after addition of as low as 10 mg/L SH (Fig. 1curve3a), suggesting that SH is sufficient for the depression of calcite in flotation with sodium oleate. However, with the 1.0 104 mol/L Zn2+ (Fig. 1, curve 3b), the floatability of calcite is higher than that without Zn2+ (Fig. 1, curve 3a), indicating that the Zn species activate the flotation of calcite in presence of SH. The double roles of Zn2+ on the flotation of calcite illustrate that some interactions exist between Zn species and calcite, which interfere the interaction between calcite and SH, as well as the collector. It is clear from Fig. 2 that the Zn2+ has little effect on the depression of calcite when the concentration of Zn2+ is lower than 1.0 105 mol/L at pH 10.0. As suggested by several authors, the adsorption of Zn2+ onto calcite in low concentration occurs essentially by means of exchanging with Ca2+ in a surface-adsorbed layer (Cheng et al., 1998; Tertre et al., 2010; Zachara et al., 1991; Zachara et al., 1988). When the concentration of total aqueous Zn is higher than 105.0 mol/L, the interaction between Zn2+ and anions (CO2 3 and/or OH on the surface or solution) leads to the formation of insoluble surface precipitates on the surface of the mineral (Zachara et al., 1989). Therefore, it suggests that the formation of insoluble surface precipitates is the main reason for the effect of Zn2+ on calcite flotation. Hydrozincite is reported to be the most stable phase under atmospheric CO2(g) and exhibits rapid formation and dissolution kinetics, while smithsonite is reported to be formed slowly under low temperature conditions (Preis and Gamsjäger, 2001; Stumm and Morgan, 1970; Zachara et al., 1989). Saturation index (SI), defined as: SI = log(IAP/Ksp), is used to present the degree of saturation. PHREEQC was used for calculating the saturation index of zinc speciation based on the equilibrium of calcite in the close system (Parkhurst and Appelo, 1999). Fig. 3 shows the saturation index of Zn-precipitations as a function of pH with 1.0 104 mol/L Zn2+, which indicates the solution are supersaturated with respect to hydrozincite and zinc hydroxide at pH > 8.0. Therefore, hydrozincite and zinc hydroxide are possible species on the calcite surface, which agrees with earlier observations (Garcı´a-Sánchez and Álvarez-Ayuso, 2002; Zachara et al., 1989). According to the results, Zinc
surface precipitates on calcite, possible hydrozincite and/or zinc hydroxide, is the primary reason for the change of calcite surface properties. The goal of the following experiments is to change the flotation recovery of calcite through removing the surface coating of zinc species from the mineral surface. Sodium carbonate, known as pH regulator used in minerals processing, and calcium chloride 2+ can provide a large amount of CO2 3 and Ca , which are the lattice ions of calcite. These lattice ions can be strong competitors to trace elements on the mineral surface and their exchange reactions with carbonate minerals may be occurred. Fig. 4 shows the floatability of calcite for various pH values using the Na2CO3 as the pH regulator without and in presence of Zn2+. As seen from Fig. 4, the floatability of calcite increases slightly and SH is also the effective depressant of calcite with the Na2CO3. In presence of Zn2+, the flotation recovery of calcite reduces from 82% at pH 8.5 to 55% at pH 9.7 with low collector concentration, while the flotation recovery of calcite increases from 54% at pH 8.5 to 94% at pH 10.0 with the SH and high collector concentration. The influence of Zn species on the floatability with Na2CO3 is similar to that using NaOH as pH regulator, which illustrates that surface precipitates of zinc can also be formed and interfere the interaction between reagents and calcite. In order to investigate whether Zn species can be desorbed from the mineral surface by Ca2+ or not, the influence of various Ca2+ concentrations on flotation recovery of calcite is presented on Fig. 5. To avoid the influence of superfluous Ca2+ on the flotation of calcite, the aqueous solution was removed by filtration, and new solution with the same pH was added to the CaCO3(s). Then flotation was performed after the addition of SH and sodium oleate. The results show that the flotation recovery of calcite decreases with the increase of Ca2+ concentration in the pH range from 7.8 to 11.0. Based on these observations, it is proposed that the Zn species on the mineral surface can be partly substituted by the Ca2+ in the solution, but the interaction between Ca2+ and the surface is so weak that high Ca2+ concentration is demanded. With a high concentration of Ca2+ and CO2 3 , calcium carbonate precipitation could be spontaneous onto the calcite surface (Ogino et al., 1987), which may have a stronger competition with the surface coating of zinc species than Ca2+. To investigate the effect of both Ca2+ and CO2 3 , Fig. 6 shows the floatability of calcite for various Ca2+ concentrations using the Na2CO3 as pH regulator. It can be seen from Fig. 6 that the floatability decreases with increasing
Fig. 3. The saturation index [log(IAP/Ksp)] of zinc species as a function of pH: 1. hydrozincite; 2. zinc hydroxide; 3. smithsonite.
Fig. 4. Flotation recovery of calcite for various pH using the Na2CO3 as pH regulator: (1a) in presence of 0.5 104 mol/L NaOL; (1b) in presence of 0.5 104 mol/L NaOL and 1.0 104 mol/L Zn2+; (2a) in presence of 2.5 104 mol/L NaOL and 10 mg/L SH; (2b) in presence of 2.5 104 mol/L NaOL, 1.0 104 mol/L Zn2+, and 10 mg/L SH.
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Fig. 5. Flotation recovery of calcite for various Ca2+ concentrations in presence of 2.5 104 mol/L NaOL and 10 mg/L SH: (1) pH = 7.8; (2) pH = 9.0; (3) pH = 10.0; (4) pH = 11.0.
Fig. 7. Zeta potential of calcite particles as function of Ca2+ concentrations at pH 10.0: (1) calcite with Ca2+; (2) calcite with Ca2+ and 1 104 mol/L Zn2+.
Ca2+ concentration. The optimum pH value for calcite depression is pH 10.0 (flotation recovery from 90% to 15% with 1.0 103 mol/L Ca2+), indicating that there is an optimum proportion of Ca2þ =CO2 3 for desorption of zinc species. To help interpret these flotation results, Fig. 7 shows zeta potential of calcite particles before and after the treatments with zinc ions and calcium ions at pH 10.0 using Na2CO3 as pH regulator. It is proved that the potential determining ions for calcite are Ca2+ and + CO2 3 , instead of H and OH . The role of pH is only to control carbonate ion speciation (Thompson and Pownall, 1989). The neutral sites on the calcium carbonate surface may also be represented as „CaOH0 and „CO3H0 (Van Cappellen et al., 1993). At pH 10.0, the concentration of CO2 in the solution is over that of Ca2+ in the 3 solution using Na2CO3 as pH regulator. To keep the pH constant, calcium carbonate will precipitate from an excess of CO2 over 3 Ca2+ after the addition of Ca2+, which causes that surface „CO 3 site increases faster than that of surface „Ca+ site (Fig. 7, curve 1). Therefore, zeta potential of calcite decreases with an increase of Ca2+ concentration in this condition. This result is in accordance with Chibowski et al. (2003).
In presence of 1 104 mol/L Zn2+, Zn species would mainly precipitate on surface „CO 3 site. Zeta potential of calcite in presence of Zn2+ is close to that without Zn2+ at pH 10.0, suggesting zeta potential of Zn precipitates is similar to that of calcite (Fig. 7, curve 2 and curve l). The solution is supersaturated with respect to calcite after the addition of Ca2+ and CO2 3 , and calcium carbonate precipitation could be spontaneous onto the calcite surface (Ogino et al., 1987), illustrating that Ca2+ and CO2 are ad3 sorbed on surface carbonate site and surface calcium site respectively. The increase in zeta potential of calcite from 19 mv to 3 mv at 1 103 mol/L Ca2+ (Fig. 7curve2) indicates that the chemical adsorption of Ca2+ on the „CO 3 site is strong, so that Zn species is possible desorbed from „CO 3 site. The results are correlated well with calcite flotation, that Ca2+ can partly eliminate the effect of Zn species instead of CO2 3 . Meanwhile, the large amount of CO2 favors the formation of hydrozincite in the bulk 3 solution (Freij et al., 2005), which facilitates Zn species desorbed from calcite surface. Therefore, the optimum flotation result can be obtained with both Ca2+ and CO2 3 . To further confirm surface property of calcite, XPS analysis of calcite samples was conducted at pH 10.0 using Na2CO3 as the pH regulator (Fig. 8a). The proportions of elements on the surface of calcite without and in presence of Zn2+ and Ca2+, as measured by XPS, are shown in Table 1. According to the obtained results, XPS is not able to detect surface Zn on natural calcite sample. In presence of Zn2+, the XPS atomic percent of Zn confirms zinc surface precipitates reporting on calcite surface. The same quantity of Ca between the natural calcite sample and that with Zn2+ implies that zinc precipitates mainly link to carbonate site instead of exchanging with calcium on the mineral surface, which confirms the earlier assumption. In presence of Zn2+, the slight increase of Ca atomic percent after the addition of Ca2+ illustrates that calcium adsorb on calcite surface through chemical reaction, which is consistent with the results of zeta potential. The atomic percent of zinc is down from 3.69% to 0.49% with the addition of Ca2+, indicating that zinc surface precipitates can be substituted. Analysis of the binding energy (BE) and kinetic energy (KE) of the zinc XPS signals gives information about the state of the zinc on the mineral surface (Fig. 8b). The Zn2p3/2 peak and LMM peak locate at 1022.0 eV and 988.4 eV respectively, which states that BE and KE are close to hydrozincite and zinc oxide (Hosking et al., 2007; Moretti et al., 1989). The Zn2p3/2 peak is fitted with two components, the Zn2p3/2(1) located at 1022.3 eV corresponded to zinc linked to oxygen in zinc hydroxide, and the Zn2p3/2(2)
Fig. 6. Flotation recovery of calcite for various Ca2+ concentrations using the Na2CO3 as pH regulator in presence of 2.5 104 mol/L NaOL and 10 mg/L SH: (1) pH = 9.0; (2) pH = 10.0; (3) pH = 11.0.
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Fig. 8. XPS spectra of calcite (a) and XPS narrow spectra of Zn2p (b) at pH 10.0 using Na2CO3 as pH regulator: (1) natural calcite sample; (2) calcite sample with Zn2+; (3) calcite sample with Ca2+ and Zn2+.
Table 1 Concentrations (at.%) of the elements measured by XPS on the surface of calcite. Sample
Natural calcite Calcite with Zn2+ Calcite with Zn2+ and Ca2+
Elements analysis C1s
Ca2P
O1s
Zn2p
42.93 33.58 38.12
15.32 15.35 16.75
41.75 47.38 44.64
– 3.69 0.49
middle is attributed to carbon contamination originating from hydrocarbon molecules presented in the ambient air. The data show the presence of a mixture of hydrozincite and zinc hydroxide products on the mineral surface, and the zinc surface coating can be desorbed through the formation of calcium carbonate precipitation on the surface. 4. Conclusions
shifted by 0.6 eV towards low binding energies in comparison with the first component, attributed to hydrozincite, situated at 1021.7 eV (Boulton et al., 2003; Zhang et al., 2009). The position of the Ca and Zn XPS peak remain unchanged with increasing Zn and Ca surface concentrations. The Ca2p3/2 peak is located at around 346.8 eV (not shown), attributed to CaCO3. In the spectrum of C1s, two individual peaks can be seen. The one having a binding energy of near 290.0 eV is attributed to carbon in CaCO3 (Stipp and Hochella, 1991), and the major peak in the
From the above results and discussion, the following conclusions can be drawn: (1) In the alkaline range, Zn ions can interfere the interactions between the reagents and calcite through the formation of insoluble surface precipitates, when the concentration of Zn2+ is higher than 1.0 105 mol/L. The solubility of zinc minerals suggests that the possible surface precipitates are hydrozincite and/or zinc hydroxide.
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(2) The CO2 3 cannot eliminate the influence of zinc ions on calcite flotation, while the high concentration of Ca2+ declines the effect of zinc ions. With the addition of both Ca2+ and CO2 3 , the influence of zinc on the depression of calcite can be removed (flotation recovery from 90% to 15% with 1.0 103 mol/L Ca2+). (3) Zeta potential measurements and surface analysis show the presence of a mixture of hydrozincite and zinc hydroxide products on calcite surface, which mainly link to carbonate site instead of exchanging with calcium on the mineral surface. The zinc surface coating can be desorbed from calcite surface due to the strong chemical adsorption of Ca2+, and the large amount of CO2 3 favor the formation of hydrozincite in the bulk solution. So that the interaction between the reagents and calcium carbonate can reappear.
Acknowledgments The authors acknowledge the National Natural Science Foundation of China (Nos. 51174229) and Hunan Provincial Innovation Foundation for Postgraduate (Nos. CX2011B121) for the financial support. References Antti, B.M., Forssberg, E., 1989. Pulp chemistry in calcite flotation. Modelling of oleate adsorption using theoretical equilibrium calculations. Minerals Engineering 2 (1), 93–109. Arnold, R., Brownbill, E.E., Ihle, S.W., 1978. Hallimond tube flotation of scheelite and calcite with amines. International Journal of Mineral Processing 5 (Compendex), 143–152. Boulton, A., Fornasiero, D., Ralston, J., 2003. Characterisation of sphalerite and pyrite flotation samples by XPS and ToF–SIMS. International Journal of Mineral Processing 70 (1–4), 205–219. Changgen, L., Yongxin, L., 1983. Selective flotation of scheelite from calcium minerals with sodium oleate as a collector and phosphates as modifiers. II. The mechanism of the interaction between phosphate modifiers and minerals. International Journal of Mineral Processing 10 (3), 219–235. Cheng, L., Sturchio, N.C., Woicik, J.C., Kemner, K.M., Lyman, P.F., Bedzyk, M.J., 1998. High-resolution structural study of zinc ion incorporation at the calcite cleavage surface. Surface Science 415 (1–2), L976–L982. Chibowski, E., Hotysz, L., Szczes´, A., 2003. Time dependent changes in zeta potential of freshly precipitated calcium carbonate. Colloids and Surfaces A: Physicochemical and Engineering Aspects 222 (1–3), 41–54. Ejtemaei, M., Irannajad, M., Gharabaghi, M., 2011. Influence of important factors on flotation of zinc oxide mineral using cationic, anionic and mixed (cationic/ anionic) collectors. Minerals Engineering 24 (13), 1402–1408. Elzinga, E.J., Reeder, R.J., 2002. X-ray absorption spectroscopy study of Cu2+ and Zn2+ adsorption complexes at the calcite surface. Implications for site-specific metal incorporation preferences during calcite crystal growth. Geochimica et Cosmochimica Acta 66 (22), 3943–3954. Fa, K., Nguyen, A.V., Miller, J.D., 2006. Interaction of calcium dioleate collector colloids with calcite and fluorite surfaces as revealed by AFM force measurements and molecular dynamics simulation. International Journal of Mineral Processing 81 (3), 166–177. Freij, S.J., Godelitsas, A., Putnis, A., 2005. Crystal growth and dissolution processes at the calcite-water interface in the presence of zinc ions. Journal of Crystal Growth 273, 535–545 (copyright 2009, The Institution of Engineering and Technology). Garcı´a-Sánchez, A., Álvarez-Ayuso, E., 2002. Sorption of Zn, Cd and Cr on calcite. Application to purification of industrial wastewaters. Minerals Engineering 15 (7), 539–547. Hosking, N.C., Ström, M.A., Shipway, P.H., Rudd, C.D., 2007. Corrosion resistance of zinc–magnesium coated steel. Corrosion Science 49 (9), 3669–3695.
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Hosseini, S.H., Forssberg, E., 2009. Smithsonite flotation using mixed anionic/ cationic collector. Transactions of the Institutions of Mining and Metallurgy. Section C: Mineral Processing and Extractive Metallurgy 118 (Compendex), 186–190. Hu, Y., Xu, J., Luo, C., Yuan, C., 1995. Solution chemistry studies on dodecylamine flotation of smithsonite/calcite. Journal of Central South University of Technology 26 (5), 589–594. Irannajad, M., Ejtemaei, M., Gharabaghi, M., 2009a. The effect of reagents on selective flotation of smithsonite-calcite-quartz. Minerals Engineering 22 (Compendex), 766–771. Irannajad, M., Ejtemaei, M., Gharabaghi, M., 2009b. The effect of reagents on selective flotation of smithsonite–calcite–quartz. Minerals Engineering 22 (9–10), 766–771. Morse, J.W., 1990. Geochemistry of sedimentary carbonate. Elsevier Amsterdam, Oxford – New York – Tokyo. Jurinak, J.J., Bauer, N., 1956. Thermodynamics of zinc adsorption on calcite, dolomite and magnesite-type minerals. Soil Science Society of America Journal 20 (4), 466–471. Kashani, A.H.N., Rashchi, F., 2008. Separation of oxidized zinc minerals from tailings: influence of flotation reagents. Minerals Engineering 21 (12–14), 967– 972. Marabini, A.M., Ciriachi, M., Plescia, P., Barbaro, M., 2007. Chelating reagents for flotation. Minerals Engineering 20 (10), 1014–1025. Mishra, S.K., 1982. Electrokinetic properties and flotation behaviour of apatite and calcite in the presence of sodium oleate and sodium metasilicate. International Journal of Mineral Processing 9 (1), 59–73. Moretti, G., Fierro, G., Lo Jacono, M., Porta, P., 1989. Characterization of CuOAZnO catalysts by X-ray photoelectron spectroscopy: precursors, calcined and reduced samples. Surface and Interface Analysis 14 (6–7), 325–336. Ogino, T., Suzuki, T., Sawada, K., 1987. The formation and transformation mechanism of calcium carbonate in water. Geochimica et Cosmochimica Acta 51 (10), 2757–2767. Parkhurst, D.L., Appelo, C.A.J., 1999. User’s Guide to PHREEQC, Version 2. U.S. Geological Survey Water Resource, Version 2, 99–4259. Preis, W., Gamsjäger, H., 2001. Solid+solute) phase equilibria in aqueous solution. XIII. Thermodynamic properties of hydrozincite and predominance diagrams for (Zn2++H2O+CO2. The Journal of Chemical Thermodynamics 33 (7), 803–819. Reeder, R.J., Northrup, P.A., Lamble, G.M., 1999. XAFS study of the coordination and local relaxation around Co{sup 2+}, Zn{sup 2+}, Pb{sup 2+}, and Ba{sup 2+} trace elements in calcite. American Mineralogist 84 (7–8), 1049–1060. Stipp, S.L., Hochella Jr, M.F., 1991. Structure and bonding environments at the calcite surface as observed with X-ray photoelectron spectroscopy (XPS) and low energy electron diffraction (LEED). Geochimica et Cosmochimica Acta 55 (6), 1723–1736. Stumm, W., Morgan, J.J., 1970. Aquatic Chemistry. Wiley InterScience, New York. Takamori, T., Tsunekawa, M., 1983. Separation of calcite from fluorite ore by the adsorption-washing-flotation method. In Preprints – XIV International Mineral Processing Congress: Worldwide Industrial Application of Mineral Processing Technology. CIM, Toronto, Ont, Can, pp. V. 1. 1–V. 1. 12. Tertre, E., Beaucaire, C., Juery, A., Ly, J., 2010. Methodology to obtain exchange properties of the calcite surface–application to major and trace elements: Ca(II), and Zn(II). Journal of Colloid and Interface Science 347 (1), 120–126. Thompson, D.W., Pownall, P.G., 1989. Surface electrical properties of calcite. Journal of Colloid and Interface Science 131 (1), 74–82. Van Cappellen, P., Charlet, L., Stumm, W., Wersin, P., 1993. A surface complexation model of the carbonate mineral-aqueous solution interface. Geochimica et Cosmochimica Acta 57 (15), 3505–3518. Wu, X.Q., Zhu, J.G., 2006. Selective flotation of cassiterite with benzohydroxamic acid. Minerals Engineering 19 (14), 1410–1417. Young, C.A., Miller, J.D., 2000. Effect of temperature on oleate adsorption at a calcite surface. an FT-NIR/IRS study and review. International Journal of Mineral Processing 58 (1–4), 331–350. Zachara, J.M., Cowan, C.E., Resch, C.T., 1991. Sorption of divalent metals on calcite. Geochimica et Cosmochimica Acta 55 (6), 1549–1562. Zachara, J.M., Kittrick, J.A., Dake, L.S., Harsh, J.B., 1989. Solubility and surface spectroscopy of zinc precipitates on calcite. Geochimica et Cosmochimica Acta 53 (1), 9–19. Zachara, J.M., Kittrick, J.A., Harsh, J.B., 1988. The mechanism of Zn2+ adsorption on calcite. Geochimica et Cosmochimica Acta 52 (9), 2281–2291. Zhang, B., Zhou, H.-B., Han, E.-H., Ke, W., 2009. Effects of a small addition of Mn on the corrosion behaviour of Zn in a mixed solution. Electrochimica Acta 54 (26), 6598–6608.
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Effect of the lattice ions on the calcite flotation in presence of Zn(II) Qing Shi, Guofan Zhang ⇑, Qiming Feng, Leming Ou, Yiping Lu Department of Mineral Engineering, School of Minerals Processing and Bioengineering, Central South University, Changsha 410083, China
a r t i c l e
i n f o
Article history: Received 2 May 2012 Accepted 18 September 2012 Available online 8 November 2012 Keywords: Calcite Flotation Zinc(II) Lattice ions
a b s t r a c t The effect of Zn2+ on calcite flotation and the effect of lattice ions on calcite flotation in presence of Zn2+ were investigated through microflotation tests, zeta potential measurements, and surface analysis. The results show that Zn species inhibit the interaction between the reagents and calcite through the formation of surface precipitation coating on the calcite surface, when the concentration of Zn2+ is higher than 105 mol/L. The effect of Zn species on calcite flotation can be decreased with the increase of Ca2+, instead 2+ and CO2 of CO2 3 . In presence of Ca 3 , the change of floatability suggests that Zn species can be desorbed with an optimum ratio of Ca2þ =CO2 3 . The zeta potential measurements and surface analysis show the existence of a mixture of hydrozincite and zinc hydroxide products on mineral surface, and zinc surface coating can be desorbed due to the strong chemical adsorption of Ca2+ and the formation of hydrozincite in the bulk solution. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Calcite is the common and important component of sediments and sedimentary rocks (Morse, 1990). Also it is one of the most extensive carbonate gangue in the flotation of smithsonite, apatite, scheelite and fluorite (Arnold et al., 1978; Hosseini and Forssberg, 2009; Takamori and Tsunekawa, 1983). Separation of valuable minerals and calcite is extremely complex due to the interaction of minerals with several dissolved metals, leading to sorption processes such as precipitation/coprecipitation, adsorption, and ionic substitution. In the case of smithsonite, surface conversion inhibits the selective separation of smithsonite from calcite as a result of reactions of the dissolved species with mineral surface, considerably altering interfacial characteristics of minerals (Hu et al., 1995). Though reagents such as sodium silicate (type N), starch, sodium hexa methaphosphate, and methylcarboxylate can be used as calcite depressants in smithsonite flotation, the process is not selective enough in practice due to the similar surface properties and surface conversion (Ejtemaei et al., 2011; Irannajad et al., 2009a; Kashani and Rashchi, 2008; Marabini et al., 2007). However, few literatures focused on the effect of Zn2+ on calcite flotation and how to eliminate the effect of Zn2+ through changing the surface components of calcite. The behavior and mechanism of Zn2+ adsorption on calcite have been noted by numerous investigators. When total aqueous Zn concentration is less than 105.25 mol/L, Zinc adsorption occurs via exchanging with Ca2+ in the surface-adsorbed layer on calcite ⇑ Corresponding author. Tel.: +86 731 88836817. E-mail address: [email protected] (G. Zhang). 0892-6875/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.mineng.2012.09.016
(Zachara et al., 1988). A thermodynamic study suggests that adsorbed Zn2+ on calcite remains hydrated, unlike its behavior on magnesite and dolomite (Jurinak and Bauer, 1956). Sorption/ desorption experimental results suggest that the sorption is totally reversible at low alkaline range (Tertre et al., 2010; Zachara et al., 1989), which results from the hydrated metal ions distributed on the calcite surface or within a surface layer undergo exchange equilibria with the aqueous phase (Zachara et al., 1991). Cheng et al. (1998) reported that Zn2+ substituted Ca2+ in the surface layer of calcite with structural relaxation of the coordinating CO3 ligands, in response to the size of Zn2+ which is smaller than that of Ca2+. SEXAFS results indicated that the ZnAO radial distance was very similar to the ZnAO bond length in pure ZnCO3, which was also found by Reeder et al. (1999) for Zn incorporated into calcite during crystal growth (Cheng et al., 1998; Reeder et al., 1999). Elzinga showed the results of in situ extended X-ray absorption fine structure (EXAFS) spectroscopy studies of Zn2+ complexes formed at the calcite surface. Zn(II) was found to coordinate at Ca sites on the calcite surface, forming mononuclear inner-sphere adsorption complexes (Elzinga and Reeder, 2002). When Zn2+ concentration approached and exceeded the solubility of known zinc carbonate solids, the sorption and precipitation of Zn was investigated by Zachara et al. (1989). Surface-enhance precipitation was not observed and CaCO3 did not nucleate Zn solids, when the aqueous ion activity product (IAP) was below the equilibrium IAP of the least soluble, kinetically viable Zn phase. The CaCO3(s) surface was not requisite for Zn precipitation. When CaCO3(s) was presented as the precipitation formed a surface coating or discrete Zn particles bound to the surface, XPS, XRD, EDS of both Zn-treated CaCO3(s) and isolated Zn particles implied that the precipitation was hydrozincite or its hydrated form (Zachara et al., 1989).
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The objective of this study is to investigate the effect of Zn2+ on calcite flotation and the effect of calcite lattice ions on calcite flotation in presence of Zn2+. In the present study, the effects of various concentrations of Zn2+ and the lattice ions on pure calcite at various pH values were assessed using flotation, and surface analysis was used to interpret the results of flotation. 2. Materials and methods 2.1. Samples and reagents The calcite used for all experiments was obtained from Changsha, Hunan Province, China. Mineralogical and X-ray powder diffraction data confirmed that the calcite sample was of high 99% purity. The samples were ground and then sieved to collect the 100 lm fraction for the microflotation and XPS tests. Hydrochloric acid (HCl), sodium hydroxide (NaOH), and sodium carbonate (Na2CO3) were used as pH regulators. Sodium oleate and sodium hexametaphosphate (SH) were used as collector and depressant, respectively. Calcium chloride and zinc sulfate were also prepared. All the reagents used in this study were of analytical grade. Deionized water was used for all tests. 2.2. Experiments 2.2.1. Flotation tests Single mineral flotation tests were carried out in a mechanic agitation flotation machine at a constant rate. The mineral suspension was prepared by adding 2.0 g of minerals to 40 mL of solutions. The pH of the mineral suspension was adjusted to a desired value by adding NaOH, Na2CO3 or HCl stock solutions. The prepared SH solution was added at a desired concentration, and the slurry was conditioned for 5 min. Then samples were conditioned with collector for 3 min and flotation was carried out for a total of 5 min. Both floated and unfloated particles were collected, filtered and dried. The flotation recovery was calculated based on solid weight distributions between these two products. 2.2.2. Zeta potential measurements Zeta potential measurements on calcite were carried out using a plus zeta potential meter (Brookhaven, Zetaplus, USA). Potassium
Fig. 2. Flotation recovery of calcite as a function of Zn2+ concentrations in presence of 10 mg/L SH and 2.5 104 mol/L NaOL at pH 10.0.
chloride was used to maintain the ionic strength at 103 mol/L. Samples of 0.030 g in the size-5 lm were added into 50 mL of the electrolyte solution or reagent solution with desired pH using sodium carbonate (Na2CO3). The suspensions were then magnetically stirred for 10 min, and the final pH was measured and recorded. The zeta potential of samples was then measured in the absence/presence of Zn2+. 2.2.3. Surface analysis A Thermo Fisher Scientific K-Alpha 1063 XPS system was used for the X-ray Photoelectron Spectroscopy measurements (XPS). The test chamber pressure was maintained below 109 mbar during spectral acquisition. Binding energy scale was corrected based on C 1s peak from contaminations (around 284.8 eV) as the internal binding energy standard. The samples were conditioned with the same conditioning regime as the flotation tests without the addition of SH and collector, then filtrated and rinsed thoroughly with water in order to remove any weakly adsorbed particles. The prepared samples were dried in a vacuum oven at room temperature for 24 h. 3. Results and discussion
Fig. 1. Flotation recovery of calcite as a function of pH: (1a) in presence of 0.5 104 mol/L NaOL; (1b) in presence of 0.5 104 mol/L NaOL and 1.0 104 mol/L Zn2+; (2a) in presence of 2.5 104 mol/L NaOL; (2b) in presence of 2.5 104 mol/L NaOL and 1.0 104 mol/L Zn2+; (3a) in presence of 2.5 104 mol/L NaOL and 10 mg/L SH; (3b) in presence of 2.5 104 mol/L NaOL, 1.0 104 mol/L Zn2+, and 10 mg/L SH.
The effect of Zn2+ on flotation recovery of calcite as a function of pH without and in presence of sodium hexametaphosphate (SH) is shown in Figs. 1 and 2. Water molecules at calcite surfaces can form tightly packed monolayer structure, suggesting that the calcite surface is easily hydrated by water molecules due to its high surface polarity (Fa et al., 2006). Oleic acid and sodium oleate have been the most extensively used collectors for calcite (Changgen and Yongxin, 1983; Irannajad et al., 2009b; Mishra, 1982; Young and Miller, 2000). In the pH range from 7.0 to 11.5, the floatability of calcite with 0.5 104 mol/L sodium oleate is higher than 75% (Fig. 1curve1a). The higher recovery is attributed to chemisorption of oleate on calcite at low concentrations (Antti and Forssberg, 1989; Young and Miller, 2000). When the concentration of sodium oleate increases to 2.5 104 mol/L, the recovery of calcite is higher than 90% in the pH range from 7 to 11.5 (Fig. 1curve2a). With the addition of 1.0 104 mol/L Zn2+, the floatability of calcite decreases from 80.2% at pH 7.5% to 36% at pH 11 in presence of 0.5 104 mol/L sodium oleate (Fig. 1curve1b), while the recovery of calcite remains higher than 90% in presence of 2.5 104 mol/L sodium oleate (Fig. 1curve2b). The results suggest that calcite is depressed by the Zn species and this depression effect depends on pH at the alkaline range. Meanwhile, the higher floatability of
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calcite in presence of 2.5 104 mol/L sodium oleate and 1.0 104 mol/L Zn2+ indicates that the zinc species can also react with the collector. Sodium hexametaphosphate (SH), also known as Calgon, has been widely used as the depressant of calcite (Changgen and Yongxin, 1983; Wu and Zhu, 2006). The floatability of calcite decreases drastically after addition of as low as 10 mg/L SH (Fig. 1curve3a), suggesting that SH is sufficient for the depression of calcite in flotation with sodium oleate. However, with the 1.0 104 mol/L Zn2+ (Fig. 1, curve 3b), the floatability of calcite is higher than that without Zn2+ (Fig. 1, curve 3a), indicating that the Zn species activate the flotation of calcite in presence of SH. The double roles of Zn2+ on the flotation of calcite illustrate that some interactions exist between Zn species and calcite, which interfere the interaction between calcite and SH, as well as the collector. It is clear from Fig. 2 that the Zn2+ has little effect on the depression of calcite when the concentration of Zn2+ is lower than 1.0 105 mol/L at pH 10.0. As suggested by several authors, the adsorption of Zn2+ onto calcite in low concentration occurs essentially by means of exchanging with Ca2+ in a surface-adsorbed layer (Cheng et al., 1998; Tertre et al., 2010; Zachara et al., 1991; Zachara et al., 1988). When the concentration of total aqueous Zn is higher than 105.0 mol/L, the interaction between Zn2+ and anions (CO2 3 and/or OH on the surface or solution) leads to the formation of insoluble surface precipitates on the surface of the mineral (Zachara et al., 1989). Therefore, it suggests that the formation of insoluble surface precipitates is the main reason for the effect of Zn2+ on calcite flotation. Hydrozincite is reported to be the most stable phase under atmospheric CO2(g) and exhibits rapid formation and dissolution kinetics, while smithsonite is reported to be formed slowly under low temperature conditions (Preis and Gamsjäger, 2001; Stumm and Morgan, 1970; Zachara et al., 1989). Saturation index (SI), defined as: SI = log(IAP/Ksp), is used to present the degree of saturation. PHREEQC was used for calculating the saturation index of zinc speciation based on the equilibrium of calcite in the close system (Parkhurst and Appelo, 1999). Fig. 3 shows the saturation index of Zn-precipitations as a function of pH with 1.0 104 mol/L Zn2+, which indicates the solution are supersaturated with respect to hydrozincite and zinc hydroxide at pH > 8.0. Therefore, hydrozincite and zinc hydroxide are possible species on the calcite surface, which agrees with earlier observations (Garcı´a-Sánchez and Álvarez-Ayuso, 2002; Zachara et al., 1989). According to the results, Zinc
surface precipitates on calcite, possible hydrozincite and/or zinc hydroxide, is the primary reason for the change of calcite surface properties. The goal of the following experiments is to change the flotation recovery of calcite through removing the surface coating of zinc species from the mineral surface. Sodium carbonate, known as pH regulator used in minerals processing, and calcium chloride 2+ can provide a large amount of CO2 3 and Ca , which are the lattice ions of calcite. These lattice ions can be strong competitors to trace elements on the mineral surface and their exchange reactions with carbonate minerals may be occurred. Fig. 4 shows the floatability of calcite for various pH values using the Na2CO3 as the pH regulator without and in presence of Zn2+. As seen from Fig. 4, the floatability of calcite increases slightly and SH is also the effective depressant of calcite with the Na2CO3. In presence of Zn2+, the flotation recovery of calcite reduces from 82% at pH 8.5 to 55% at pH 9.7 with low collector concentration, while the flotation recovery of calcite increases from 54% at pH 8.5 to 94% at pH 10.0 with the SH and high collector concentration. The influence of Zn species on the floatability with Na2CO3 is similar to that using NaOH as pH regulator, which illustrates that surface precipitates of zinc can also be formed and interfere the interaction between reagents and calcite. In order to investigate whether Zn species can be desorbed from the mineral surface by Ca2+ or not, the influence of various Ca2+ concentrations on flotation recovery of calcite is presented on Fig. 5. To avoid the influence of superfluous Ca2+ on the flotation of calcite, the aqueous solution was removed by filtration, and new solution with the same pH was added to the CaCO3(s). Then flotation was performed after the addition of SH and sodium oleate. The results show that the flotation recovery of calcite decreases with the increase of Ca2+ concentration in the pH range from 7.8 to 11.0. Based on these observations, it is proposed that the Zn species on the mineral surface can be partly substituted by the Ca2+ in the solution, but the interaction between Ca2+ and the surface is so weak that high Ca2+ concentration is demanded. With a high concentration of Ca2+ and CO2 3 , calcium carbonate precipitation could be spontaneous onto the calcite surface (Ogino et al., 1987), which may have a stronger competition with the surface coating of zinc species than Ca2+. To investigate the effect of both Ca2+ and CO2 3 , Fig. 6 shows the floatability of calcite for various Ca2+ concentrations using the Na2CO3 as pH regulator. It can be seen from Fig. 6 that the floatability decreases with increasing
Fig. 3. The saturation index [log(IAP/Ksp)] of zinc species as a function of pH: 1. hydrozincite; 2. zinc hydroxide; 3. smithsonite.
Fig. 4. Flotation recovery of calcite for various pH using the Na2CO3 as pH regulator: (1a) in presence of 0.5 104 mol/L NaOL; (1b) in presence of 0.5 104 mol/L NaOL and 1.0 104 mol/L Zn2+; (2a) in presence of 2.5 104 mol/L NaOL and 10 mg/L SH; (2b) in presence of 2.5 104 mol/L NaOL, 1.0 104 mol/L Zn2+, and 10 mg/L SH.
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Fig. 5. Flotation recovery of calcite for various Ca2+ concentrations in presence of 2.5 104 mol/L NaOL and 10 mg/L SH: (1) pH = 7.8; (2) pH = 9.0; (3) pH = 10.0; (4) pH = 11.0.
Fig. 7. Zeta potential of calcite particles as function of Ca2+ concentrations at pH 10.0: (1) calcite with Ca2+; (2) calcite with Ca2+ and 1 104 mol/L Zn2+.
Ca2+ concentration. The optimum pH value for calcite depression is pH 10.0 (flotation recovery from 90% to 15% with 1.0 103 mol/L Ca2+), indicating that there is an optimum proportion of Ca2þ =CO2 3 for desorption of zinc species. To help interpret these flotation results, Fig. 7 shows zeta potential of calcite particles before and after the treatments with zinc ions and calcium ions at pH 10.0 using Na2CO3 as pH regulator. It is proved that the potential determining ions for calcite are Ca2+ and + CO2 3 , instead of H and OH . The role of pH is only to control carbonate ion speciation (Thompson and Pownall, 1989). The neutral sites on the calcium carbonate surface may also be represented as „CaOH0 and „CO3H0 (Van Cappellen et al., 1993). At pH 10.0, the concentration of CO2 in the solution is over that of Ca2+ in the 3 solution using Na2CO3 as pH regulator. To keep the pH constant, calcium carbonate will precipitate from an excess of CO2 over 3 Ca2+ after the addition of Ca2+, which causes that surface „CO 3 site increases faster than that of surface „Ca+ site (Fig. 7, curve 1). Therefore, zeta potential of calcite decreases with an increase of Ca2+ concentration in this condition. This result is in accordance with Chibowski et al. (2003).
In presence of 1 104 mol/L Zn2+, Zn species would mainly precipitate on surface „CO 3 site. Zeta potential of calcite in presence of Zn2+ is close to that without Zn2+ at pH 10.0, suggesting zeta potential of Zn precipitates is similar to that of calcite (Fig. 7, curve 2 and curve l). The solution is supersaturated with respect to calcite after the addition of Ca2+ and CO2 3 , and calcium carbonate precipitation could be spontaneous onto the calcite surface (Ogino et al., 1987), illustrating that Ca2+ and CO2 are ad3 sorbed on surface carbonate site and surface calcium site respectively. The increase in zeta potential of calcite from 19 mv to 3 mv at 1 103 mol/L Ca2+ (Fig. 7curve2) indicates that the chemical adsorption of Ca2+ on the „CO 3 site is strong, so that Zn species is possible desorbed from „CO 3 site. The results are correlated well with calcite flotation, that Ca2+ can partly eliminate the effect of Zn species instead of CO2 3 . Meanwhile, the large amount of CO2 favors the formation of hydrozincite in the bulk 3 solution (Freij et al., 2005), which facilitates Zn species desorbed from calcite surface. Therefore, the optimum flotation result can be obtained with both Ca2+ and CO2 3 . To further confirm surface property of calcite, XPS analysis of calcite samples was conducted at pH 10.0 using Na2CO3 as the pH regulator (Fig. 8a). The proportions of elements on the surface of calcite without and in presence of Zn2+ and Ca2+, as measured by XPS, are shown in Table 1. According to the obtained results, XPS is not able to detect surface Zn on natural calcite sample. In presence of Zn2+, the XPS atomic percent of Zn confirms zinc surface precipitates reporting on calcite surface. The same quantity of Ca between the natural calcite sample and that with Zn2+ implies that zinc precipitates mainly link to carbonate site instead of exchanging with calcium on the mineral surface, which confirms the earlier assumption. In presence of Zn2+, the slight increase of Ca atomic percent after the addition of Ca2+ illustrates that calcium adsorb on calcite surface through chemical reaction, which is consistent with the results of zeta potential. The atomic percent of zinc is down from 3.69% to 0.49% with the addition of Ca2+, indicating that zinc surface precipitates can be substituted. Analysis of the binding energy (BE) and kinetic energy (KE) of the zinc XPS signals gives information about the state of the zinc on the mineral surface (Fig. 8b). The Zn2p3/2 peak and LMM peak locate at 1022.0 eV and 988.4 eV respectively, which states that BE and KE are close to hydrozincite and zinc oxide (Hosking et al., 2007; Moretti et al., 1989). The Zn2p3/2 peak is fitted with two components, the Zn2p3/2(1) located at 1022.3 eV corresponded to zinc linked to oxygen in zinc hydroxide, and the Zn2p3/2(2)
Fig. 6. Flotation recovery of calcite for various Ca2+ concentrations using the Na2CO3 as pH regulator in presence of 2.5 104 mol/L NaOL and 10 mg/L SH: (1) pH = 9.0; (2) pH = 10.0; (3) pH = 11.0.
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Fig. 8. XPS spectra of calcite (a) and XPS narrow spectra of Zn2p (b) at pH 10.0 using Na2CO3 as pH regulator: (1) natural calcite sample; (2) calcite sample with Zn2+; (3) calcite sample with Ca2+ and Zn2+.
Table 1 Concentrations (at.%) of the elements measured by XPS on the surface of calcite. Sample
Natural calcite Calcite with Zn2+ Calcite with Zn2+ and Ca2+
Elements analysis C1s
Ca2P
O1s
Zn2p
42.93 33.58 38.12
15.32 15.35 16.75
41.75 47.38 44.64
– 3.69 0.49
middle is attributed to carbon contamination originating from hydrocarbon molecules presented in the ambient air. The data show the presence of a mixture of hydrozincite and zinc hydroxide products on the mineral surface, and the zinc surface coating can be desorbed through the formation of calcium carbonate precipitation on the surface. 4. Conclusions
shifted by 0.6 eV towards low binding energies in comparison with the first component, attributed to hydrozincite, situated at 1021.7 eV (Boulton et al., 2003; Zhang et al., 2009). The position of the Ca and Zn XPS peak remain unchanged with increasing Zn and Ca surface concentrations. The Ca2p3/2 peak is located at around 346.8 eV (not shown), attributed to CaCO3. In the spectrum of C1s, two individual peaks can be seen. The one having a binding energy of near 290.0 eV is attributed to carbon in CaCO3 (Stipp and Hochella, 1991), and the major peak in the
From the above results and discussion, the following conclusions can be drawn: (1) In the alkaline range, Zn ions can interfere the interactions between the reagents and calcite through the formation of insoluble surface precipitates, when the concentration of Zn2+ is higher than 1.0 105 mol/L. The solubility of zinc minerals suggests that the possible surface precipitates are hydrozincite and/or zinc hydroxide.
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(2) The CO2 3 cannot eliminate the influence of zinc ions on calcite flotation, while the high concentration of Ca2+ declines the effect of zinc ions. With the addition of both Ca2+ and CO2 3 , the influence of zinc on the depression of calcite can be removed (flotation recovery from 90% to 15% with 1.0 103 mol/L Ca2+). (3) Zeta potential measurements and surface analysis show the presence of a mixture of hydrozincite and zinc hydroxide products on calcite surface, which mainly link to carbonate site instead of exchanging with calcium on the mineral surface. The zinc surface coating can be desorbed from calcite surface due to the strong chemical adsorption of Ca2+, and the large amount of CO2 3 favor the formation of hydrozincite in the bulk solution. So that the interaction between the reagents and calcium carbonate can reappear.
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