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Effect of solution chemistry on the flotation system of smithsonite and calcite
International Journal of Mineral Processing 119 (2013) 34–39
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International Journal of Mineral Processing journal homepage: www.elsevier.com/locate/ijminpro
Effect of solution chemistry on the flotation system of smithsonite and calcite Qing Shi, Guofan Zhang ⁎, Qiming Feng, Hong Deng Department of Minerals Engineering, School of Minerals processing and Bioengineering, Central South University, Changsha, 410083, China
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Article history: Received 11 September 2012 Received in revised form 16 December 2012 Accepted 29 December 2012 Available online 18 January 2013 Keywords: Smithsonite Calcite Flotation Solution chemistry
a b s t r a c t Both smithsonite and calcite are calcite-group minerals which are isomorphous with one another. They are similar in many physical and chemical properties, and may partially or fully replace one another, which make the separation of smithsonite from calcite inefficient and ineffective in flotation. The present study was undertaken to investigate the solubility properties of individual calcite and smithsonite minerals and their mixtures, and establish correlation between solution chemistry and flotation behavior. Solution chemical calculations show that the species distribution of smithsonite and calcite was strongly affected by solution pH value. With the increase of pH, the measured solubility of calcite is very close to the theoretical value, while that of smithsonite is less than the theoretical value, illustrating that surface precipitations of zinc hydroxide and hydrozincite are possible. In the mixed system, solution chemical calculations show that the conversion from smithsonite and calcite to hydrozincite and zinc hydroxide is spontaneous. The flotation behavior of smithsonite and calcite was studied with the supernatants, and XPS analysis was conducted to detect surface properties. Both simulation results and experimental data show that surface conversion from calcite to zinc species has a vital influence on the separation of smithsonite from calcite by flotation. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Smithsonite (ZnCO3) is one of the major sources of zinc oxide. As the primary source of zinc, sphalerite is steadily getting depleted. To meet the future zinc market demand, it is significant to process low grade oxidized zinc ores for a marketable product (Ejtemaei et al., 2011; Irannajad et al., 2009; Kashani and Rashchi, 2008). Froth flotation is the most widely used method to recover oxidized zinc minerals from ores (Hosseini and Forssberg, 2007; Kashani and Rashchi, 2008; Pereira and Peres, 2005). As the salt-type minerals, the dissolution characteristics of calcite and smithsonite play a major role in determining the nature of the interactions taking place in the bulk solution or on the mineral surfaces (Hu et al., 1995; Shi et al., 2012; Van Cappellen et al., 1993). The dissolved mineral species can undergo the reactions such as hydrolysis, complexation, adsorption, and surface and bulk precipitations, which could inhibit the selective interaction between reagents and minerals (Nunes et al., 2011; Sø et al., 2011; Vučinić et al., 2010). However, the definition of the generated surface species in the solution is still a challenge (Nunes et al., 2011). Solution chemical calculations play a major role in studying the behavior of flotation systems. By applying solution chemical calculations to a salt-type mineral and inorganic depressant flotation system, the critical depression pH and depressant concentration were predicted precisely (Yuehua et al., 2003). Surface species conversion between calcite and apatite was characterized based on thermodynamic data in the solution (Amankonah et al., 1985). However, ⁎ Corresponding author. Tel.: +86 731 88836817. E-mail address: [email protected] (G. Zhang). 0301-7516/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.minpro.2012.12.011
the solution chemistry of smithsonite and calcite has not been thoroughly studied and their interactions are not well understood. The shortage of fundamental studies addressing solution chemistry of smithsonite flotation system is one of the factors that hinder the development of selective flotation processes in the system under investigation. With the objective, we try to understand solution chemistry of individual calcite and smithsonite minerals and their mixtures better, and establish correlation between solution chemistry and flotation behavior. 2. Materials and methods 2.1. Materials and reagents Calcite and smithsonite used for all experiments were obtained from Yunnan and Hunan Province, China. The chemical analyses of smithsonite and calcite were shown in Table 1. The samples were ground and then sieved to collect the −150 μm fraction for the microflotation and XPS tests. Hydrochloric acid (HCl) and sodium hydroxide (NaOH) were used as pH regulators. Sodium oleate and sodium hexametaphosphate (SH) were used as collector and depressant, respectively. All the reagents used in this study were of analytical grade. Deionized water was used for all tests. 2.2. Experiments Single mineral flotation tests were carried out in a mechanical agitation flotation machine at a constant rate (Fig. 1). The mineral suspension
Q. Shi et al. / International Journal of Mineral Processing 119 (2013) 34–39
Κsp is the corresponding solubility product constant. The ion activity product (IAP) and saturation index (SI) of the mineral is given by:
Table 1 The chemical analysis of pure smithsonite and calcite. Component (%)
ZnO
Fe
SiO2
Al2O3
CaO
L.O.I
Amount in pure calcite Amount in pure smithsonite
\ 64.1
0.21 0.10
0.52 0.75
0.22 0.015
65.34 0.05
33.71 34.98
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 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. Mineral dissolution rate tests were carried out with the solid ratio 5% for the mineral solution, and the suspensions were agitated with 800 rpm at fixed time. When the concentration of ions was measured at different pH, the mineral-solution would be pre-conditioned for 10 min in order to obtain the dissolution equilibrium of minerals, and the sodium hydroxide or hydrochloric acid was added to get a desired pH value. After that, the solution was agitated for 10 min at 800 rpm and the pH was kept constant. All of the suspensions were centrifugally separated at 9000 rpm for 5 min by means of a centrifuge. The concentration of ions in the supernatant liquid was measured by ICP optical emission spectrometry. A Thermo Fisher Scientific K-Alpha 1063 XPS system was used for the X-ray photoelectron spectroscopy (XPS) measurements. The test chamber pressure was maintained below 10 −9 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. Theoretical background The formation of a particular mineral in a solution containing A n+ and B m− can be expressed by the equation nþ
Am Bn ðsÞ ¼ mA
m−
ðaqÞ þ nB
ðaqÞ
35
Κ sp :
h i nþ m m− n IAP ¼ A B ; SI ¼ log IAP− log K sp : According to the principles of solution chemistry, bulk precipitation of the mineral is anticipated when IAP>Ksp (SI >0), where [An+] and [Bm−] are, respectively, the actual activity of An+ and B m− in solution. The value above zero indicates the condition under which bulk precipitates are anticipated to be formed. Its value is negative when the mineral may be dissolved, and zero when the water and mineral are at chemical equilibrium. In this study, the ionic species distribution and saturation index calculations of a particular mineral were made with the Geochemical model PHREEQC (Charlton and Parkhurst, 2011; Parkhurst and Appelo, 1999), which uses ion-association and the extended Debye Hückel expressions to account for the non-ideality of aqueous solutions. All of pertinent reactions and thermodynamic data are listed in Table 2. 4. Results and discussion 4.1. Dissolution rate of smithsonite and calcite Mineral dissolution rate is defined as the quantity of lattice ions dissolved in solution at fixed time. It is proposed that the mineral dissolution rate correlates with the surface complexation reactions (Chen and Tao, 2004; Pokrovsky and Schott, 2002), so the dissolution rate can also be expressed by the solution pH as a function of stirring time. Fig. 2 shows the cation concentrations and solution pH as a function of stirring time after 2.0 g mineral added into 40 mL distilled water. Obviously, the degree of dissolution of calcite is greater. In an open system (Pco2 =10−3.5 atm), solution pH of carbonate minerals is affected by the protonation/deprotonation reactions of ions and CO2 in the atmosphere. The solution pH reaches to 9.6 immediately after the calcite is immersed in aqueous media, which illustrates that there is a large amount of CO32− in the solution and the proton reaction of CO32− occurs quickly (Table. 2 reaction 1), then the pH decreases slowly with reaction 2 taking place. It is consistent with the absorption of CO2 into carbonate solutions at room temperature, and reaction 2 is rate-controlling, while reactions 1 and 3 are everywhere at equilibrium (Savage et al., 1980). Calcite reached the dissolution equilibrium at pH 8.31 eventually. The smithsonite–water solution pH increases slowly with the increase of stirring time, which mainly due to the lower concentration of CO32− and lower dissolution rate of smithsonite, and smithsonite reached the dissolution equilibrium at pH 7.60. 4.2. Solubility of smithsonite and calcite
Fig. 1. Flotation cell for microflotation test.
There are large amounts of ionic species in the solution due to the dissolution and hydration of lattice ions, since smithsonite and calcite are sparingly soluble salt type minerals (Weast, 1973). A large amount of experimental evidences has shown that surface complex formation reactions correlate with the corresponding complexes in the solution (Pokrovsky et al., 2000; Stumm et al., 1983). Therefore, mineral–solution equilibria play a crucial role in determining the behavior of flotation systems, which influence the adsorption behavior of different species at the solid/liquid and liquid/air interfaces (Somasundaran et al., 1985). In practice, the conditioning time of flotation is limited, but mineral suspensions may require even several hours and/or weeks for full equilibrium (Amankonah et al., 1985; Rao et al., 1990; Somasundaran, 1968), which causes that the absorption of CO2 into carbonate solutions is hardly at equilibrium. Especially at the slight alkaline region, the increase of bicarbonate concentration in the solution makes the absorption of CO2 more difficult to occur (reaction 2). Therefore, we will focus on the properties of solution and mineral surface in the closed system.
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Table 2 Pertinent reactions and thermodynamic data for the solution chemistry calculation. Pertinent reactions and constant (1) (2) (3) (4) (5) (6) (7) (8) (9) (10)
CO32−+ H+ ⇔ HCO3− 10.33 CO2(g) + OH− ⇔ HCO3− 6.18 OH− + H+ ⇔ H2O 14.00 HCO3− + H+ ⇔ H2CO3 6.35 2+ − + Ca + OH ⇔ CaOH 1.40 Ca2+ + CO32− ⇔ CaCO3(s) 8.35 2+ − Ca + 2OH ⇔ Ca(OH)2(s) 5.22 1.11 Ca2+ + HCO3− ⇔ CaHCO3+ Ca2+ + CO32− ⇔ CaCO3(aq) 3.22 Zn(CO3)0.4(OH)1.2 ⇔ Zn2+ + 0.4CO32−+ 1.2OH− − 14.85
Pertinent reactions and constant (11) (12) (13) (14) (15) (16) (17) (18) (19)
Zn2+ + OH− ⇔ ZnOH+ Zn2+ + 2OH− ⇔ Zn(OH)2(aq) Zn2+ + 3OH− ⇔ Zn(OH)3− Zn2+ + 4OH− ⇔ Zn(OH)42− Zn2+ + HCO3− ⇔ ZnHCO3+ Zn2+ + CO32− ⇔ ZnCO3 Zn2+ + 2CO32− ⇔ Zn(CO3)22− Zn2+ + CO32− ⇔ ZnCO3(s) Zn(OH)2(s) + 2H+ ⇔ Zn2+ + 2H2O
5.00 11.10 13.60 14.80 2.10 5.30 9.63 10.00 11.50
Based on the previous consideration, the ionic species distribution of calcite as a function of pH in the closed systems is shown in Fig. 3(a). In accordance with these diagrams, the species Ca2+ are the main species in solution pHb 13. The concentrations of CaHCO3− and Ca 2+ decrease with increasing solution pH, while the concentration of CaOH+ increases with increasing solution pH. At pH >13, CaOH+ is the predominate species. Fig. 3(b) shows the ionic species distribution of smithsonite based on equilibria (1)–(4) and (10)–(19) as a function of pH in the closed systems, which shows the species Zn2+ predominate until pH 8.5. The specie Zn(OH)2(aq) is the predominant species between pH 8.5 and 11.5, while above pH 11.5 the species Zn(OH)3− and Zn(OH)42− predominate. According to the results of Fig. 3, there are various forms of lattice ions in the solution due to cationic hydrolysis. The solubility of mineral is defined as the total cation and cationic hydrolysis concentration in solution. Fig. 4 shows the solubility of calcite and smithsonite for various pH. It should be noted that there is significant difference between theoretical solubility of mineral in the open system (Pco2 = 10 −3.5 atm) and that in the closed system. This is mainly because CO2 in the atmosphere has an important influence on solubility of carbonate minerals. As can be seen from Fig. 4(a), the experimental measured solubility of calcite is very close to the theoretical value in the closed system at 7.5 b pH b 11.5, suggesting that the ionic species distribution is very close to that in the closed system under investigation. However, Fig. 4(b) shows that the experimental measured solubility of smithsonite is much less than the theoretical value for both the closed system and the open system (Pco2 = 10 −3.5 atm). The similar slop at 7.5 b pH b 8.5 between the measured solubility and the theoretical value is attributed to the slow dissolution of
smithsonite which did not reach the equilibrium in 10 min, while the different slop at 8.5 b pH b 11.5 is maybe due to the surface reaction or the adsorption of hydrolysis products. To estimate the possible surface reaction or bulk precipitation, Fig. 5 shows the saturation index of Zn species precipitates as a function of pH value based on the solution equilibrium with smithsonite. It is conceivable that carbonate ions and zinc ions dissolved from smithsonite reaction with hydroxyl or hydrogen ions in the solution to form Zn-precipitates. The extent of reaction depends on the concentration of total carbonate and zinc ions dissolved. Considering all of the hydrolysis
Fig. 2. Solution pH and cation concentrations of mineral–H2O–CO2(g) (Pco2 =10−3.5 atm) system as a function of stirring time.
Fig. 3. Influence of pH on the concentrations of ionic species in the closed solution system of calcite (a) and smithsonite (b) individually.
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Based on thermodynamic analysis of the smithsonite–solution equilibria involved in this system, it shows that the dissolution/precipitation behavior of smithsonite is very complicated in the alkaline range. The surface participates of zinc hydroxide and hydrozincite would play an important role in the surface property of smithsonite at 7 b pHb 12, as well as the soluble zinc hydroxide. 4.3. Surface conversion
Fig. 4. Calcite (a) and smithsonite (b) solubility as a function of pH.
During the processing of carbonates, surface conversion may occur as a result of the interactions of dissolved species with mineral surface in flotation systems. It is conceivable that suspensions of calcite and smithsonite previously equilibrated with calcite and smithsonite, respectively, and then the calcite solution is mixed with the smithsonite solution. Considering all hydrolysis and protonation reactions and using the thermodynamic data in Table 2, the calculated saturation index of bulk precipitates as function of pH is shown in Fig. 5. As seen from Fig. 5, the system is just about saturated with respect to smithsonite below pH 8.2 and undersaturated with respect to calcite below pH 9.8. The crossover point of the two curves defines a critical pH above which calcite is more stable than smithsonite. Therefore, surface conversion from calcite to smithsonite occurs below pH 8.2 possibly, while surface conversion from smithsonite to calcite is possible above pH 9.8 possibly. However, heterogeneous nuclear is also possible because its crystal structure is similar to that of the crystal being formed (Stumm et al., 1983). Precipitation of other zinc species is possible under these conditions. According to the data, the system is supersaturated with respect to hydrozincite and zinc hydroxide above pH 6.8 and pH 8.0 respectively. The values of saturation index show that bulk precipitation of hydrozincite and zinc hydroxide is more stable than smithsonite and calcite, suggesting that the conversion from smithsonite and calcite to hydrozincite and zinc hydroxide is spontaneous in the mixed system. Comparison between Figs. 5 and 6, which has very similar slop for hydrozincite and zinc hydroxide, illustrates that the similar surface reaction would occur on smithsonite surface between the single system and the mixed system. In other words, the dissolved species of calcite have little effects on the surface properties of smithsonite. 4.4. Micro-flotation tests
and protonation reactions and using the thermodynamic data in Table 2, the saturation index of hydrozincite as a function of pH was calculated. From this figure, bulk precipitates of hydrozincite and zinc hydroxide are anticipated above pH 6.4 and 7.5, respectively. At pH>10, the saturation index of zinc hydroxide is higher than that of hydrozincite, suggesting that zinc hydroxide is more stable than hydrozincite.
The results for the single-mineral flotation of calcite and smithsonite using 0.25 mM sodium oleate as the collector and calgon as the depressant in the absence and presence of supernatant are shown in Fig. 7. With increasing concentration of calgon, the recovery of calcite decreases but it has slight effects on smithsonite recovery in the deionized
Fig. 5. Saturation index of Zn precipitates as a function of pH value in the closed system.
Fig. 6. Saturation index of possible precipitates as a function of pH value in the mixture of calcite and smithsonite.
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Fig. 7. Flotation recovery of smithsonite and calcite as a function of calgon concentration.
water. The recovery of calcite decreases from about 90% to 20% at 10 mg/L SH concentration, while smithsonite recovery decreases from 92% to 82%. There is an obvious separation occurring in the single mineral flotation system. In order to illustrate the role of surface conversion, single-mineral flotation tests were conducted with the supernatants. In the supernatant of calcite, the recovery of smithsonite is similar with that in the deionized water, suggesting that the dissolved species of calcite have little effect on smithsonite flotation. However, the recovery of calcite in the supernatant of smithsonite is different from that in the deionized water, and calgon cannot depress the calcite effectively in the supernatant of smithsonite. The high floatability of calcite in the supernatant of smithsonite indicates surface reaction occurrence between the dissolved species of smithsonite and the surface of calcite, which inhibits the interaction between the reagents and the surface of calcite. There is no separation occurring in the mixed flotation system. These results correspond well with the solution chemistry calculations.
Fig. 8. XPS Ca2P and Zn2P peak fitting of calcite before (a) and after (b) conditioned in the supernatant of smithsonite.
4.5. Surface analysis To help interpret these simulation and flotation results, the XPS spectra were obtained from smithsonite and calcite single minerals in the presence and absence of supernatants, respectively. For the XPS survey scan of the calcite, the presence of carbon, oxygen and calcium peaks was confirmed from the spectra, in accordance with the molecular structure of calcite. No significant contamination was observed in the calcite. The high-resolution spectra of Ca 2p3/2 electrons were acquired for the calcite. As can be seen, the Ca 2P3/2 peak locates at 346.87 eV. The peaks were in good agreement with the literature data (García-Sánchez and Álvarez-Ayuso, 2002). When calcite was conditioned in the supernatant of smithsonite, zinc was detected on calcite surface by XPS survey scan. The high-resolution spectra of Zn 2p is shown in Fig. 8. It was deconvoluted into two peaks. The first peak at 1021.7 eV was due to hydrozincite, while the second peak at 1022.3 eV originated from zinc linked to oxygen in the zinc hydroxide (Dake et al., 1989; Moretti et al., 1989). It is worth noting that the binding energy shifts of Ca (−0.1 eV) are so small that could not be attributed to chemical shift. The high-resolution spectra of Ca2p3/2 and Zn2p3/2 were recorded on smithsonite before and after conditioned in the supernatant of calcite, and the results are shown in Fig. 9. As can be seen from Fig. 9, no Ca 2p3/2 peaks were observed on the high-resolution spectra of smithsonite before and after supernatant of calcite treatment, suggesting that Ca species can't precipitate on the smithsonite surface. Zn 2p3/2 peaks located at 1021.85 eV and 1021.98 eV respectively
before and after reacting with supernatant of calcite. There were no obvious binding energy shifts of Zn 2p3/2 electrons, indicating that the situation of Zn would keep constant before and after conditioned in the supernatant of calcite. The results are consistent with solution chemical calculations and flotation data. 5. Conclusions From the above results and discussion, the following conclusions can be drawn: (1) The dissolution rate of calcite was much faster than that of smithsonite in water, and the species distribution was strongly affected by solution pH value. The measured solubility of calcite is very close to the theoretical value in the closed system, while the measured solubility of smithsonite is much less than the theoretical value for both the closed system and the open system. Solution chemical calculations show that surface precipitates of zinc hydroxide and hydrozincite is possible. (2) In the mixed system, solution chemical calculations show that the conversion from smithsonite and calcite to hydrozincite and zinc hydroxide is spontaneous. The similar surface reaction would occur on smithsonite surface between the single system and the mixed system, suggesting that the dissolved species of calcite have little effect on the surface properties of smithsonite.
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References
Fig. 9. XPS Ca2P and Zn2P peak fitting of smithsonite before (a) and after (b) conditioned in the supernatant of calcite.
(3) Surface conversion from calcite to zinc species plays an important role on the separation of smithsonite from calcite. Surface reaction occurs between the dissolved species of smithsonite and calcite, and the precipitates hinder the interaction between the reagents and calcite. Surface analysis also shows hydrozincite and zinc hydroxide products on calcite surface, while no Ca 2p3/2 peaks were observed on the high-resolution spectra of smithsonite before and after supernatant of calcite treatment. Acknowledgments The authors acknowledge the National Natural Science Foundation of China (no. 51174229) and Hunan Provincial Innovation Foundation for Postgraduate (no. CX2011B121) for the financial support.
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Contents lists available at SciVerse ScienceDirect
International Journal of Mineral Processing journal homepage: www.elsevier.com/locate/ijminpro
Effect of solution chemistry on the flotation system of smithsonite and calcite Qing Shi, Guofan Zhang ⁎, Qiming Feng, Hong Deng Department of Minerals 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 11 September 2012 Received in revised form 16 December 2012 Accepted 29 December 2012 Available online 18 January 2013 Keywords: Smithsonite Calcite Flotation Solution chemistry
a b s t r a c t Both smithsonite and calcite are calcite-group minerals which are isomorphous with one another. They are similar in many physical and chemical properties, and may partially or fully replace one another, which make the separation of smithsonite from calcite inefficient and ineffective in flotation. The present study was undertaken to investigate the solubility properties of individual calcite and smithsonite minerals and their mixtures, and establish correlation between solution chemistry and flotation behavior. Solution chemical calculations show that the species distribution of smithsonite and calcite was strongly affected by solution pH value. With the increase of pH, the measured solubility of calcite is very close to the theoretical value, while that of smithsonite is less than the theoretical value, illustrating that surface precipitations of zinc hydroxide and hydrozincite are possible. In the mixed system, solution chemical calculations show that the conversion from smithsonite and calcite to hydrozincite and zinc hydroxide is spontaneous. The flotation behavior of smithsonite and calcite was studied with the supernatants, and XPS analysis was conducted to detect surface properties. Both simulation results and experimental data show that surface conversion from calcite to zinc species has a vital influence on the separation of smithsonite from calcite by flotation. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Smithsonite (ZnCO3) is one of the major sources of zinc oxide. As the primary source of zinc, sphalerite is steadily getting depleted. To meet the future zinc market demand, it is significant to process low grade oxidized zinc ores for a marketable product (Ejtemaei et al., 2011; Irannajad et al., 2009; Kashani and Rashchi, 2008). Froth flotation is the most widely used method to recover oxidized zinc minerals from ores (Hosseini and Forssberg, 2007; Kashani and Rashchi, 2008; Pereira and Peres, 2005). As the salt-type minerals, the dissolution characteristics of calcite and smithsonite play a major role in determining the nature of the interactions taking place in the bulk solution or on the mineral surfaces (Hu et al., 1995; Shi et al., 2012; Van Cappellen et al., 1993). The dissolved mineral species can undergo the reactions such as hydrolysis, complexation, adsorption, and surface and bulk precipitations, which could inhibit the selective interaction between reagents and minerals (Nunes et al., 2011; Sø et al., 2011; Vučinić et al., 2010). However, the definition of the generated surface species in the solution is still a challenge (Nunes et al., 2011). Solution chemical calculations play a major role in studying the behavior of flotation systems. By applying solution chemical calculations to a salt-type mineral and inorganic depressant flotation system, the critical depression pH and depressant concentration were predicted precisely (Yuehua et al., 2003). Surface species conversion between calcite and apatite was characterized based on thermodynamic data in the solution (Amankonah et al., 1985). However, ⁎ Corresponding author. Tel.: +86 731 88836817. E-mail address: [email protected] (G. Zhang). 0301-7516/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.minpro.2012.12.011
the solution chemistry of smithsonite and calcite has not been thoroughly studied and their interactions are not well understood. The shortage of fundamental studies addressing solution chemistry of smithsonite flotation system is one of the factors that hinder the development of selective flotation processes in the system under investigation. With the objective, we try to understand solution chemistry of individual calcite and smithsonite minerals and their mixtures better, and establish correlation between solution chemistry and flotation behavior. 2. Materials and methods 2.1. Materials and reagents Calcite and smithsonite used for all experiments were obtained from Yunnan and Hunan Province, China. The chemical analyses of smithsonite and calcite were shown in Table 1. The samples were ground and then sieved to collect the −150 μm fraction for the microflotation and XPS tests. Hydrochloric acid (HCl) and sodium hydroxide (NaOH) were used as pH regulators. Sodium oleate and sodium hexametaphosphate (SH) were used as collector and depressant, respectively. All the reagents used in this study were of analytical grade. Deionized water was used for all tests. 2.2. Experiments Single mineral flotation tests were carried out in a mechanical agitation flotation machine at a constant rate (Fig. 1). The mineral suspension
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Κsp is the corresponding solubility product constant. The ion activity product (IAP) and saturation index (SI) of the mineral is given by:
Table 1 The chemical analysis of pure smithsonite and calcite. Component (%)
ZnO
Fe
SiO2
Al2O3
CaO
L.O.I
Amount in pure calcite Amount in pure smithsonite
\ 64.1
0.21 0.10
0.52 0.75
0.22 0.015
65.34 0.05
33.71 34.98
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 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. Mineral dissolution rate tests were carried out with the solid ratio 5% for the mineral solution, and the suspensions were agitated with 800 rpm at fixed time. When the concentration of ions was measured at different pH, the mineral-solution would be pre-conditioned for 10 min in order to obtain the dissolution equilibrium of minerals, and the sodium hydroxide or hydrochloric acid was added to get a desired pH value. After that, the solution was agitated for 10 min at 800 rpm and the pH was kept constant. All of the suspensions were centrifugally separated at 9000 rpm for 5 min by means of a centrifuge. The concentration of ions in the supernatant liquid was measured by ICP optical emission spectrometry. A Thermo Fisher Scientific K-Alpha 1063 XPS system was used for the X-ray photoelectron spectroscopy (XPS) measurements. The test chamber pressure was maintained below 10 −9 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. Theoretical background The formation of a particular mineral in a solution containing A n+ and B m− can be expressed by the equation nþ
Am Bn ðsÞ ¼ mA
m−
ðaqÞ þ nB
ðaqÞ
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Κ sp :
h i nþ m m− n IAP ¼ A B ; SI ¼ log IAP− log K sp : According to the principles of solution chemistry, bulk precipitation of the mineral is anticipated when IAP>Ksp (SI >0), where [An+] and [Bm−] are, respectively, the actual activity of An+ and B m− in solution. The value above zero indicates the condition under which bulk precipitates are anticipated to be formed. Its value is negative when the mineral may be dissolved, and zero when the water and mineral are at chemical equilibrium. In this study, the ionic species distribution and saturation index calculations of a particular mineral were made with the Geochemical model PHREEQC (Charlton and Parkhurst, 2011; Parkhurst and Appelo, 1999), which uses ion-association and the extended Debye Hückel expressions to account for the non-ideality of aqueous solutions. All of pertinent reactions and thermodynamic data are listed in Table 2. 4. Results and discussion 4.1. Dissolution rate of smithsonite and calcite Mineral dissolution rate is defined as the quantity of lattice ions dissolved in solution at fixed time. It is proposed that the mineral dissolution rate correlates with the surface complexation reactions (Chen and Tao, 2004; Pokrovsky and Schott, 2002), so the dissolution rate can also be expressed by the solution pH as a function of stirring time. Fig. 2 shows the cation concentrations and solution pH as a function of stirring time after 2.0 g mineral added into 40 mL distilled water. Obviously, the degree of dissolution of calcite is greater. In an open system (Pco2 =10−3.5 atm), solution pH of carbonate minerals is affected by the protonation/deprotonation reactions of ions and CO2 in the atmosphere. The solution pH reaches to 9.6 immediately after the calcite is immersed in aqueous media, which illustrates that there is a large amount of CO32− in the solution and the proton reaction of CO32− occurs quickly (Table. 2 reaction 1), then the pH decreases slowly with reaction 2 taking place. It is consistent with the absorption of CO2 into carbonate solutions at room temperature, and reaction 2 is rate-controlling, while reactions 1 and 3 are everywhere at equilibrium (Savage et al., 1980). Calcite reached the dissolution equilibrium at pH 8.31 eventually. The smithsonite–water solution pH increases slowly with the increase of stirring time, which mainly due to the lower concentration of CO32− and lower dissolution rate of smithsonite, and smithsonite reached the dissolution equilibrium at pH 7.60. 4.2. Solubility of smithsonite and calcite
Fig. 1. Flotation cell for microflotation test.
There are large amounts of ionic species in the solution due to the dissolution and hydration of lattice ions, since smithsonite and calcite are sparingly soluble salt type minerals (Weast, 1973). A large amount of experimental evidences has shown that surface complex formation reactions correlate with the corresponding complexes in the solution (Pokrovsky et al., 2000; Stumm et al., 1983). Therefore, mineral–solution equilibria play a crucial role in determining the behavior of flotation systems, which influence the adsorption behavior of different species at the solid/liquid and liquid/air interfaces (Somasundaran et al., 1985). In practice, the conditioning time of flotation is limited, but mineral suspensions may require even several hours and/or weeks for full equilibrium (Amankonah et al., 1985; Rao et al., 1990; Somasundaran, 1968), which causes that the absorption of CO2 into carbonate solutions is hardly at equilibrium. Especially at the slight alkaline region, the increase of bicarbonate concentration in the solution makes the absorption of CO2 more difficult to occur (reaction 2). Therefore, we will focus on the properties of solution and mineral surface in the closed system.
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Table 2 Pertinent reactions and thermodynamic data for the solution chemistry calculation. Pertinent reactions and constant (1) (2) (3) (4) (5) (6) (7) (8) (9) (10)
CO32−+ H+ ⇔ HCO3− 10.33 CO2(g) + OH− ⇔ HCO3− 6.18 OH− + H+ ⇔ H2O 14.00 HCO3− + H+ ⇔ H2CO3 6.35 2+ − + Ca + OH ⇔ CaOH 1.40 Ca2+ + CO32− ⇔ CaCO3(s) 8.35 2+ − Ca + 2OH ⇔ Ca(OH)2(s) 5.22 1.11 Ca2+ + HCO3− ⇔ CaHCO3+ Ca2+ + CO32− ⇔ CaCO3(aq) 3.22 Zn(CO3)0.4(OH)1.2 ⇔ Zn2+ + 0.4CO32−+ 1.2OH− − 14.85
Pertinent reactions and constant (11) (12) (13) (14) (15) (16) (17) (18) (19)
Zn2+ + OH− ⇔ ZnOH+ Zn2+ + 2OH− ⇔ Zn(OH)2(aq) Zn2+ + 3OH− ⇔ Zn(OH)3− Zn2+ + 4OH− ⇔ Zn(OH)42− Zn2+ + HCO3− ⇔ ZnHCO3+ Zn2+ + CO32− ⇔ ZnCO3 Zn2+ + 2CO32− ⇔ Zn(CO3)22− Zn2+ + CO32− ⇔ ZnCO3(s) Zn(OH)2(s) + 2H+ ⇔ Zn2+ + 2H2O
5.00 11.10 13.60 14.80 2.10 5.30 9.63 10.00 11.50
Based on the previous consideration, the ionic species distribution of calcite as a function of pH in the closed systems is shown in Fig. 3(a). In accordance with these diagrams, the species Ca2+ are the main species in solution pHb 13. The concentrations of CaHCO3− and Ca 2+ decrease with increasing solution pH, while the concentration of CaOH+ increases with increasing solution pH. At pH >13, CaOH+ is the predominate species. Fig. 3(b) shows the ionic species distribution of smithsonite based on equilibria (1)–(4) and (10)–(19) as a function of pH in the closed systems, which shows the species Zn2+ predominate until pH 8.5. The specie Zn(OH)2(aq) is the predominant species between pH 8.5 and 11.5, while above pH 11.5 the species Zn(OH)3− and Zn(OH)42− predominate. According to the results of Fig. 3, there are various forms of lattice ions in the solution due to cationic hydrolysis. The solubility of mineral is defined as the total cation and cationic hydrolysis concentration in solution. Fig. 4 shows the solubility of calcite and smithsonite for various pH. It should be noted that there is significant difference between theoretical solubility of mineral in the open system (Pco2 = 10 −3.5 atm) and that in the closed system. This is mainly because CO2 in the atmosphere has an important influence on solubility of carbonate minerals. As can be seen from Fig. 4(a), the experimental measured solubility of calcite is very close to the theoretical value in the closed system at 7.5 b pH b 11.5, suggesting that the ionic species distribution is very close to that in the closed system under investigation. However, Fig. 4(b) shows that the experimental measured solubility of smithsonite is much less than the theoretical value for both the closed system and the open system (Pco2 = 10 −3.5 atm). The similar slop at 7.5 b pH b 8.5 between the measured solubility and the theoretical value is attributed to the slow dissolution of
smithsonite which did not reach the equilibrium in 10 min, while the different slop at 8.5 b pH b 11.5 is maybe due to the surface reaction or the adsorption of hydrolysis products. To estimate the possible surface reaction or bulk precipitation, Fig. 5 shows the saturation index of Zn species precipitates as a function of pH value based on the solution equilibrium with smithsonite. It is conceivable that carbonate ions and zinc ions dissolved from smithsonite reaction with hydroxyl or hydrogen ions in the solution to form Zn-precipitates. The extent of reaction depends on the concentration of total carbonate and zinc ions dissolved. Considering all of the hydrolysis
Fig. 2. Solution pH and cation concentrations of mineral–H2O–CO2(g) (Pco2 =10−3.5 atm) system as a function of stirring time.
Fig. 3. Influence of pH on the concentrations of ionic species in the closed solution system of calcite (a) and smithsonite (b) individually.
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Based on thermodynamic analysis of the smithsonite–solution equilibria involved in this system, it shows that the dissolution/precipitation behavior of smithsonite is very complicated in the alkaline range. The surface participates of zinc hydroxide and hydrozincite would play an important role in the surface property of smithsonite at 7 b pHb 12, as well as the soluble zinc hydroxide. 4.3. Surface conversion
Fig. 4. Calcite (a) and smithsonite (b) solubility as a function of pH.
During the processing of carbonates, surface conversion may occur as a result of the interactions of dissolved species with mineral surface in flotation systems. It is conceivable that suspensions of calcite and smithsonite previously equilibrated with calcite and smithsonite, respectively, and then the calcite solution is mixed with the smithsonite solution. Considering all hydrolysis and protonation reactions and using the thermodynamic data in Table 2, the calculated saturation index of bulk precipitates as function of pH is shown in Fig. 5. As seen from Fig. 5, the system is just about saturated with respect to smithsonite below pH 8.2 and undersaturated with respect to calcite below pH 9.8. The crossover point of the two curves defines a critical pH above which calcite is more stable than smithsonite. Therefore, surface conversion from calcite to smithsonite occurs below pH 8.2 possibly, while surface conversion from smithsonite to calcite is possible above pH 9.8 possibly. However, heterogeneous nuclear is also possible because its crystal structure is similar to that of the crystal being formed (Stumm et al., 1983). Precipitation of other zinc species is possible under these conditions. According to the data, the system is supersaturated with respect to hydrozincite and zinc hydroxide above pH 6.8 and pH 8.0 respectively. The values of saturation index show that bulk precipitation of hydrozincite and zinc hydroxide is more stable than smithsonite and calcite, suggesting that the conversion from smithsonite and calcite to hydrozincite and zinc hydroxide is spontaneous in the mixed system. Comparison between Figs. 5 and 6, which has very similar slop for hydrozincite and zinc hydroxide, illustrates that the similar surface reaction would occur on smithsonite surface between the single system and the mixed system. In other words, the dissolved species of calcite have little effects on the surface properties of smithsonite. 4.4. Micro-flotation tests
and protonation reactions and using the thermodynamic data in Table 2, the saturation index of hydrozincite as a function of pH was calculated. From this figure, bulk precipitates of hydrozincite and zinc hydroxide are anticipated above pH 6.4 and 7.5, respectively. At pH>10, the saturation index of zinc hydroxide is higher than that of hydrozincite, suggesting that zinc hydroxide is more stable than hydrozincite.
The results for the single-mineral flotation of calcite and smithsonite using 0.25 mM sodium oleate as the collector and calgon as the depressant in the absence and presence of supernatant are shown in Fig. 7. With increasing concentration of calgon, the recovery of calcite decreases but it has slight effects on smithsonite recovery in the deionized
Fig. 5. Saturation index of Zn precipitates as a function of pH value in the closed system.
Fig. 6. Saturation index of possible precipitates as a function of pH value in the mixture of calcite and smithsonite.
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Fig. 7. Flotation recovery of smithsonite and calcite as a function of calgon concentration.
water. The recovery of calcite decreases from about 90% to 20% at 10 mg/L SH concentration, while smithsonite recovery decreases from 92% to 82%. There is an obvious separation occurring in the single mineral flotation system. In order to illustrate the role of surface conversion, single-mineral flotation tests were conducted with the supernatants. In the supernatant of calcite, the recovery of smithsonite is similar with that in the deionized water, suggesting that the dissolved species of calcite have little effect on smithsonite flotation. However, the recovery of calcite in the supernatant of smithsonite is different from that in the deionized water, and calgon cannot depress the calcite effectively in the supernatant of smithsonite. The high floatability of calcite in the supernatant of smithsonite indicates surface reaction occurrence between the dissolved species of smithsonite and the surface of calcite, which inhibits the interaction between the reagents and the surface of calcite. There is no separation occurring in the mixed flotation system. These results correspond well with the solution chemistry calculations.
Fig. 8. XPS Ca2P and Zn2P peak fitting of calcite before (a) and after (b) conditioned in the supernatant of smithsonite.
4.5. Surface analysis To help interpret these simulation and flotation results, the XPS spectra were obtained from smithsonite and calcite single minerals in the presence and absence of supernatants, respectively. For the XPS survey scan of the calcite, the presence of carbon, oxygen and calcium peaks was confirmed from the spectra, in accordance with the molecular structure of calcite. No significant contamination was observed in the calcite. The high-resolution spectra of Ca 2p3/2 electrons were acquired for the calcite. As can be seen, the Ca 2P3/2 peak locates at 346.87 eV. The peaks were in good agreement with the literature data (García-Sánchez and Álvarez-Ayuso, 2002). When calcite was conditioned in the supernatant of smithsonite, zinc was detected on calcite surface by XPS survey scan. The high-resolution spectra of Zn 2p is shown in Fig. 8. It was deconvoluted into two peaks. The first peak at 1021.7 eV was due to hydrozincite, while the second peak at 1022.3 eV originated from zinc linked to oxygen in the zinc hydroxide (Dake et al., 1989; Moretti et al., 1989). It is worth noting that the binding energy shifts of Ca (−0.1 eV) are so small that could not be attributed to chemical shift. The high-resolution spectra of Ca2p3/2 and Zn2p3/2 were recorded on smithsonite before and after conditioned in the supernatant of calcite, and the results are shown in Fig. 9. As can be seen from Fig. 9, no Ca 2p3/2 peaks were observed on the high-resolution spectra of smithsonite before and after supernatant of calcite treatment, suggesting that Ca species can't precipitate on the smithsonite surface. Zn 2p3/2 peaks located at 1021.85 eV and 1021.98 eV respectively
before and after reacting with supernatant of calcite. There were no obvious binding energy shifts of Zn 2p3/2 electrons, indicating that the situation of Zn would keep constant before and after conditioned in the supernatant of calcite. The results are consistent with solution chemical calculations and flotation data. 5. Conclusions From the above results and discussion, the following conclusions can be drawn: (1) The dissolution rate of calcite was much faster than that of smithsonite in water, and the species distribution was strongly affected by solution pH value. The measured solubility of calcite is very close to the theoretical value in the closed system, while the measured solubility of smithsonite is much less than the theoretical value for both the closed system and the open system. Solution chemical calculations show that surface precipitates of zinc hydroxide and hydrozincite is possible. (2) In the mixed system, solution chemical calculations show that the conversion from smithsonite and calcite to hydrozincite and zinc hydroxide is spontaneous. The similar surface reaction would occur on smithsonite surface between the single system and the mixed system, suggesting that the dissolved species of calcite have little effect on the surface properties of smithsonite.
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References
Fig. 9. XPS Ca2P and Zn2P peak fitting of smithsonite before (a) and after (b) conditioned in the supernatant of calcite.
(3) Surface conversion from calcite to zinc species plays an important role on the separation of smithsonite from calcite. Surface reaction occurs between the dissolved species of smithsonite and calcite, and the precipitates hinder the interaction between the reagents and calcite. Surface analysis also shows hydrozincite and zinc hydroxide products on calcite surface, while no Ca 2p3/2 peaks were observed on the high-resolution spectra of smithsonite before and after supernatant of calcite treatment. Acknowledgments The authors acknowledge the National Natural Science Foundation of China (no. 51174229) and Hunan Provincial Innovation Foundation for Postgraduate (no. CX2011B121) for the financial support.
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