Solution Chemistry of Sodium Silicate and Implications for Pyrite

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Solution Chemistry of Sodium Silicate and Implications for Pyrite Flotation Bo Feng, Yiping Lu,* Qiming Feng, and Hao Li School of Mineral Processing and Bioengineering, Central South University, Changsha 410083, China ABSTRACT: The solution chemistry of sodium silicate and implications for pyrite flotation have been studied. In addition to flotation and sedimentation tests, electrophoresis, inductively coupled plasma (ICP) tests, and X-ray photoelectron spectroscopy (XPS) have been used. The flotation and sedimentation results show that lizardite causes problems in the flotation of pyrite by adhering to the pyrite particles. Addition of the sodium silicate could make the mixed sample of pyrite and lizardite more disperse in the alkaline condition and significantly reduce the adverse effect of lizardite on the flotation of pyrite. ICP tests and XPS analysises show that sodium silicate can adsorb onto the lizardite surface and change the surface characteristic of lizardite. Sodium silicate mainly exists in the form of SiO(OH)3− in the pH range that sodium silicate can restore pyrite flotation recovery. The adsorption of SiO(OH)3− ions at the lizardite/solution interface overcompensates the positive charge on the lizardite particle and its ζ potential is rendered negative. The total interaction energy between lizardite and pyrite is changed from attractive energy to repulsive energy in the presence of sodium silicate, according to the calculation of the Derjaguin−Landau−Verwey−Overbeek theory.

1. INTRODUCTION Lizardite is a common gangue mineral encountered in complex sulfide ores.1,2 In nickel sulfide ore processing, lizardite may report to flotation concentrate via composite particles or through attachment to the valuable minerals as “slime coatings”.3−5 As a kind of magnesium silicate (MgO) gangue mineral, large quantities of lizardite in flotation concentrates can cause problems during smelting, often resulting in the imposition of smelter penalties for mineral processing companies.6 In addition, these hydrophilic lizardite minerals may interfere with the flotation of valuable sulfide minerals, such as pentlandite.3,7 A coating of hydrophilic slime particles will decrease the hydrophobicity of the sulfide particle and may also reduce collector adsorption.8 Either of these flotation mechanisms will cause the sulfide particle to have impaired floatability. For example, a hydrophobic sulfide particle coated with hydrophilic slime particles may become increasingly hydrophilic. Consequently the once hydrophobic particle may take longer to attach to a rising bubble or may even not float at all.4,9 In order for improving the flotation of the nickel sulfide ore, sodium hexametaphosphate, carboxymethyl cellulose (CMC), and other agents are used to disperse slime particles of MgO type minerals from sulfide surfaces.10,11 Sodium silicate is widely used as a depressant or dispersant in the flotation of nonsulfide mineral,12 which is also called water glass and has a composition expressed by mNa2O·nSiO2. In aqueous solution, sodium silicates have three major species: Si(OH)4 (uncharged silica gel) at pH < 9.4, SiO(OH)3− at pH > 9.4, and SiO2(OH)2− at pH > 12.6. The negatively charged species are useful for silica depression and dispersion.13 The focus of the present work is the study and characterization of the surface chemistry of lizardite with respect to lizardite−pyrite interactions. The aim of this study is to be able to modify the lizardite surface chemistry to reduce or eliminate undesirable interactions between lizardite and pyrite and understand the mechanisms involved. © 2012 American Chemical Society

2. THEORETICAL BACKGROUND Heterocoagulation is usually described by the Derjaguin− Landau−Verwey−Overbeek (DLVO) theory.14,15 The colloidal forces considered include the electrostatic double-layer force and the van der Waals force. 2.1. Electrostatic Double-Layer Interaction. The model used to describe the electrostatic double-layer interaction energy is based on the Poisson−Boltzmann equation, which describes the electrostatic potential in an ionic solution as a function of position relative to the particle surface and has been found to be accurate down to separations of a few nanometers.16 The interaction energy at constant surface potentials is often used and can be described by the following equation: VE =

πε0εrR1R 2 (ψ 2 + ψ2 2) · (R1 + R 2) 1 ⎧ 2ψ ψ ⎡ 1 + exp( −κH ) ⎤ ⎨ 2 1 2 2 ln⎢ ⎥ ⎩ (ψ1 + ψ2 ) ⎣ 1 − exp( −κH ) ⎦ ⎪ ⎪

⎫ + ln[1 − exp( −2κH )]⎬ ⎭

⎪ ⎪

(1)

where the radius of lizardite particle R1 is 3.085 μm and the radius of pyrite particle R2 is 46.9 μm; κ−1 is the thickness of the electric double-layer, κ = 0.180 nm−1; ε0 and εr represent the vacuum dielectric constant and the relative dielectric constant of the continuous phase, with a given value of 6.95 × 10−10 C2/ (J·m); H represents the distance between particles;10 and ψ1 Received: Revised: Accepted: Published: 12089

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were collected and kept separately for various studies. The −37 μm size fraction was used for the electrokinetic studies, and the −150 + 75 μm fraction was used for the flotation and sedimentation tests. The average diameter of the flotation sample was 93.8 μm Potassium nitrate was used to maintain the ionic strength, and HCl (hydrochloric acid) and NaOH (sodium hydroxide) were used as pH regulators. All the reagents used in this study were of analytical grade. Deionized double distilled water was used for all tests. 3.2. Experiments. 3.2.1. Flotation Tests. Mineral flotation was carried out in duplicate in a mechanical agitation flotation machine (Figure 2) with the impeller set at 1000 rpm. The

and ψ2 are the surface potential (when contact time between the particles is short, the assumption of constant surface charge is appropriate).17 2.2. Van der Waals Interaction. The van der Waals interaction energy is A R1R 2 VW = − 6H R1 + R 2 (2) The Hamaker constant for pyrite/water/lizardite is not available in literature. For pyrite, Sharma lists a value of the Hamaker constant acting through vacuum as A11 = 1.2 × 10−19 J. 18 As lizardite is a magnesium silicate mineral, an approximation using the Hamaker constant for mica in vacuum could be used A22 = 9.7 × 10−20 J.1 The Hamaker constant (A123) for two different materials (1 and 3) interacting through media (3) is A132 = ( A11 −

A33 )( A 22 −

A33 )

(3)

−20

When medium 3 is water (A33 = 3.7 × 10 J), a value of the Hamaker constant of 1.83 × 10−20 J was calculated for the pyrite/water/lizardite system on the basis of eq 3 and is used in the current study.

3. MATERIALS AND METHODS 3.1. Samples and Reagents. The lizardite was obtained from Donghai, Jiangsu Province, China. Mineralogical and Xray powder diffraction data confirmed that the lizardite sample was of high purity with trace amounts of chlorite and amphibole (Figure 1). The chemical analysis of the sample is

Figure 2. Schematic of flotation machine.

mineral suspension was prepared by adding 2.0 g of pyrite and 0.1 g of lizardite to 40 mL of solution. The pH of the mineral suspension was adjusted to a desired value by adding KOH or HCl stock solutions. It was at the beginning of the conditioning stage that sodium silicate was added and conditioned for 5 min (if required). The prepared PAX (Potassium amyl xanthate) solution was added at a desired concentration and conditioned for 5 min. The frother MIBC (methyl isobutyl carbinol) was then added to the slurry, and flotation was carried out for a total of 4 min. The floated and unfloated particles were collected, filtered, and dried. The flotation recovery was calculated based on solid weight distributions between the two products. 3.2.2. Sedimentation Tests. Coagulation and dispersion between lizardite and pyrite were studied using the sedimentation tests. For the sedimentation tests, 0.1 g of sample powder was taken and made up to 100 mL after the addition of desired amounts of 0.001 M KNO3. The suspensions were then agitated for half an hour using a magnetic stirrer at 25 °C and transferred to 100 mL graduated flasks. The solution was then settled at a fixed time (3 min), and the supernatant liquor of fixed height (25 mL) was pipetted out and measured by scattering turbidimeter. The dispersion of the supernatant liquor was characterized by its turbidity. The higher the turbidity value is, the better dispersed the sample is. 3.2.3. ζ Potential Measurements. The ζ potential of particles can be determined through measuring their electrophoretic mobility. The electrophoretic mobility is measured by

Figure 1. XRD of lizardite.

Table 1. Bulk Chemical Analyses of Lizardite and Pyrite chemical composition lizardite pyrite

MgO

SiO2

Al2O3

CaO

TFe

S

32.92

37.11

0.80

0.21

5.17 44.96

52.98

shown quantitatively in Table 1. The sample was dry ground and screened. The −10 μm size range was used in flotation and sedimentation tests. The particle size distribution determined using a Malvern Instruments mastersizer was 100% −10 μm, with a D50 of 3.94 μm, a D90 of 9.62 μm, and an average diameter of 6.17μm. Pyrite was obtained from Yunfu, Guangdong Province, China. The chemical analysis of the sample is also shown in Table 1. The samples were dry ground and screened through 150, 75, and 37 μm series sieves. All the different size fractions 12090

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timing the motion of several particles over a known distance as they migrate, through electrostatic attraction, toward an oppositely charged electrode. The magnitude of the charge is reflected in the speed of the particle, and the sign of the charge is indicated by the direction of migration. The electrophoretic mobility of various samples was measured in a ζ potentiometer (Coulter Delsa440 SX). A dilute mineral suspension was prepared by adding 0.03 g mineral to 100 mL of 0.001 M potassium nitrate solution, thermostatically controlled at 20 °C. No attempt was made to exclude oxygen from the system. The suspension was stirred for 30 min to allow the system to equilibrate at the required pH prior to the removal of a subsample for electrophoretic mobility measurement. The pH of the suspension was adjusted with small additions of dilute HCl or KOH. The electrophoretic mobility is converted to ζ potential by the equipment. 3.2.4. Inductively Coupled Plasma (ICP) Tests. The concentrations of Mg and Si species were determined in the presence and absence of the sodium silicate by the ICP method. The ICP tests were carried out at constant ionic strength (0.001 M KNO3). Lizardite powder was taken and made up to 40 mL in 100 mL Erlenmeyer flasks. When needed, sodium silicate was added at the beginning of the conditioning period. The suspensions were agitated using a magnetic stirrer for 1 h. The sample was then centrifuged, and the concentration of species left in solution was analyzed using the ICP. Five different runs were accomplished for each experiment, and the average was taken as the amount of species present in the liquid. 3.2.5. X-ray Photoelectron Spectroscopy (XPS). The surface chemical composition of serpentine samples was determined by XPS. This analysis was conducted with a Perkin-Elmer Physical Electronics Division (PHI) 5100 spectrometer using Mg Kα Xray source operated at 300 W with a pass energy of 35.75 eV. The vacuum pressure during the analysis ranged from 10−8 to 10−9 Torr. The energy scale was calibrated using the Fermi edge and the 3d5/2 line for silver (Eb = 932.67 eV). All samples were analyzed at a takeoff angle of 45°. The mineral surfaces were examined in survey mode over the binding energy range from 0 to 1200 eV in order to identify all the species.

Figure 3. Recovery of pyrite as a function of pH (PAX = 1 × 10−4 M; MIBC = 1 × 10−4 M).

Figure 4. Recovery of pyrite as a function of sodium silicate concentration (PAX = 1 × 10−4 M; MIBC = 1 × 10−4 M, pH = 9).

4. RESULTS AND DISCUSSION 4.1. Effect of Sodium Silicate on the Flotation Performance of Pyrite. The effect of pH on the floatability of pyrite in the absence and presence of sodium silicate is shown in Figure 3. It is evident from the picture that the flotation recovery of pyrite is steadily decreased with increasing pH from 3 to about 11 in the absence of sodium silicate when fine lizardite particles were added prior to collector addition, which is in agreement with earlier observation.1,3 In the pH range of 3−6.2, the use of sodium silicate lowers the flotation recovery of pyrite, while in the pH range of 6.2−11, pyrite recovery could be somewhat restored. The effect of sodium silicate concentration on the flotation performance of pyrite that has been depressed by lizardite is shown in Figure 4. The result shows that the addition of sodium silicate could significantly weaken the depression effect of lizardite on the flotation performance of pyrite. A maximum increase in pyrite recovery was obtained with 100 mg/L sodium silicate, while higher sodium silicate concentrations produced little gain in recovery. The attachment of lizardite slimes to the valuable minerals as “slime coatings” is the main reason that lizardite slimes interfere

with pyrite flotation. The effect of sodium silicate on the aggregation and dispersion behavior of pyrite and lizardite is studied by sedimentation tests. Turbidity technique, due to its noninvasive, noncontact properties, is well suited to studies of colloidal suspensions. The turbidity value of suspensions reflects the particle numbers of colloidal suspensions. A decrease of turbidity value indicates a decrease in particle number, which is the result of particle aggregation. The turbidity of mixed ores as a function of pH in the absence and presence of sodium silicate is shown in Figure 5. It can be seen from Figure 5 that the sodium silicate could effectively disperse mixed ores in the pH range of 6.2−11, as the turbidity of mixed ores in the presence of sodium silicate is higher than the value in the absence of sodium silicate. Figure 6 shows the turbidity of mixed ores as a function of sodium silicate concentration at pH 9. The results illustrate that sodium silicate has a significant dispersion effect on mixed ores; the turbidity of mixed ores markedly increased up to a sodium silicate concentration of 150 mg/L, with further sodium silicate addition producing little gain in turbidity. 4.2. Effect of Sodium Silicate on the Surface Characteristic of Lizardite. It has been determined that the 12091

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The atomic concentrations of the different elements present on the surface of the lizardite were determined based on the intensity of the C(1s), O(1s), Mg(1s), Fe(2p), and Si(2p) signals produced. The results of this surface analysis are summarized in Table 3. Analysis of the chemical species present Table 3. Relative Atomic Concentrations of the Surface of Lizardite Samples Using XPS elements (%) sample

C(1s)

O(1s)

Si(2p)

Mg(2p)

Fe(2p)

lizardite lizardite + sodium silicate

13.4 8.2

53.1 60.7

9.1 11.8

23.8 18.8

0.6 0.5

on the surface of each sample by XPS indicated that the surface of the sample contained slightly more silicon and oxygen in the presence of sodium silicate. This increased concentration of silicon and oxygen on the surfaces of lizardite is the result of sodium silicate adsorption. Sodium silicate yields a number of silicate species in solution as a function of pH. The φ−pH diagram of Na2SiO3 solution is shown in Figure 7. It can be seen that Si(OH)4 is predominant

Figure 5. Turbidity of lizardite and pyrite as a function of pH in the presence of sodium silicate.

Figure 6. Turbidity of lizardite and pyrite as a function of sodium silicate concentration at pH 9. Figure 7. Distribution coefficients of various species of sodium silicate as a function of pH.

electrokinetic behavior of lizardite aqueous suspensions is mainly a function of the Mg/Si atomic ratio on surface. The removal of Mg cations from the lizardite surface will result in a shift of the zero charge points to a lower pH.19 The ICP test results in Table 2 show that the concentration of Mg cations

when pH is less than 9.4 whereas SiO(OH)3− is predominate when pH is greater than 9.4 and SiO2(OH)2− is predominate when pH is above 12.6. It can be seen from a comparison of Figures 2 and 6 that the pH range in which sodium silicate restores pyrite flotation recovery corresponds to the range where SiO(OH)3− begins to form. It can therefore be concluded that the presence of teh SiO(OH)3− species is the main reason why sodium silicate can restore the flotation recovery of pyrite that has been depressed by lizardite. The adsorption of sodium silicate on lizardite surface may change the surface characteristic of lizardite. As a direct surface chemistry investigation of the different minerals, individual electro-kinetic studies were undertaken of pyrite and lizardite particles as a function of pH in 0.001 M KNO3. It can be seen from Figure 8 that lizardite has an IEP of ca. pH 11.8. The ζ potential of lizardite is positive in the pH value range of 2−11.8. The surface of pyrite is negatively charged in the pH value range of 2−12. At pH value 9, where flotation of nickel sulfide ores is routinely performed, the surface potential of lizardite and pyrite are opposite, and the positively charged fine lizardite

Table 2. ICP Test Results of Lizardite in the Presence of Sodium Silicate sodium silicate concn (mg/L)

Mg (mg/L)

adsorbed sodium silicate concn (mg/L)

0 50 100 200

1.27 0.89 1.57 2.01

0 22.25 48.73 108.55

dissolved from the lizardite surface does not change in the presence of sodium silicate. However, the results show that the amount of sodium silicate that adsorbed on the surface of lizardite increased with the increasing of sodium silicate concentration. To confirm the adsorption of sodium silicate on the lizardite surface, the surface of lizardite sample was analyzed using XPS. 12092

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particles is no longer observed, and a repulsive force exists between the pyrite and lizardite particles.

5. CONCLUSIONS From the results of this investigation, the following conclusions can be drawn: (1) Sodium silicate can effectively disperse the lizardite and pyrite, reducing the coverage of lizardite on pyrite and improving the flotation performance of the pyrite in the pH range of 6.2−11. (2) The solution chemistry of sodium silicate illustrates that Si(OH)4 is predominant when pH is less than 9.4 whereas SiO(OH)3− is predominate when pH is greater than 9.4 and SiO2(OH)2− is predominate when pH is above 12.6. (3) The IEP of lizardite is ca. pH 11.8 and shows an acidic shift in the presence of sodium silicate. The ICP tests and XPS analysis show that the adsorption of negatively charged SiO(OH)3− on lizardite surface is responsible for the acidic shift of IEP. (4) DLVO theory shows that a repulsive force exists between the pyrite and lizardite particles in the presence of sodium silicate, complementing the result that lizardite does not interfere with pyrite flotation in the presence of sodium silicate.

Figure 8. ζ potential of lizardite and pyrite particles as a function of pH.

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particles will attach to the negatively charged pyrite particle surface through electrostatic attraction. The adsorption of negatively charged SiO(OH)3− on lizardite surface changes the surface characteristics of lizardite and shifts the IEP of lizardite from pH 11.8 to pH 5. At pH 9, both pyrite and lizardite are negatively charged, and the electrostatic interaction between pyrite and lizardite will be negligible. Particle interaction energies in aqueous solution are commonly described through application of DLVO theory, which allows quantitative prediction of the interaction energy. The ζ potential values for the lizardite and pyrite particle surface can be determined from the electro-kinetic results in Figure 8. The ζ potential value of pyrite at pH 9 is −34 mV, and these values of the lizardite in the absence and presence of sodium silicate are 25.58 mV and −21.2 mV, respectively. The total interaction energy E was calculated by replacing the relative data into eqs 1 and 2, and the result is shown in Figure 9. The total interaction energy between lizardite and pyrite is negative, as shown in curve 1 of Figure 9. They attract each other, and the aggregation of each other occurs easily. In the presence of sodium silicate, attraction between the two mineral

AUTHOR INFORMATION

Corresponding Author

* Tel./Fax: +86-731-88836817. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the National Natural Science Foundation of China (No. 51174229) REFERENCES

(1) Bremmell, K. E.; Fornasiero, D.; Ralston, J. Pentlandite-lizardite interactions and implications for their separation by flotation. Colloids Surf., A 2005, 252, 207. (2) Wiese, J.; Harris, P.; Bradshaw, D. The response of sulphide and gangue minerals in selected Merensky ores to increased depressant dosages. Miner. Eng. 2007, 20, 986. (3) Edwards, G. R.; Kipkie, W. B.; Agar, G. E. The effect of slime coatings of the serpentine minerals, chrysotile and lizardite on pentlandite flotation. Int. J. Miner. Process. 1980, 7, 33. (4) Wellham, E. J.; Elber, L.; Yan, D. S. The role of carboxy methyl cellulose in the flotation of a nickel sulphide transition ore. Miner. Eng. 1992, 5, 381. (5) Pietrobon, M. C.; Grano, S. R.; Sobieraj, S.; Ralston, J. Recovery mechanisims for pentlandite and MgO-bearing gangue minerals in nickel ores from Western Australia. Miner. Eng. 1997, 10, 775. (6) Beattie, D. A.; Huynh, L.; Kaggwa, G. B.; Ralston, J. Influence of adsorbed polysaccharides and polyacrylamides on talc flotation. Int. J. Miner. Process. 2006, 78, 238. (7) Li, Z. H. The effect of gangue minerals containing magnesium on pentlandite flotation. Kuangye Xueyuan Xuebao. 1993, 24, 36. (8) Learmont, M. E.; Iwasaki, I. Effect of grinding media on galena flotation. Miner. Metall. Process. 1984, 1, 136. (9) Trahar, W. J. A rational interpretation of the role of particle size in flotation. Int. J. Miner. Process. 1981, 8, 289. (10) Feng, B.; Lu, Y. P.; Feng, Q. M.; Zhang, M. Y.; Gu, Y. L. Talcserpentine interactions and implications for talc depression. Miner. Eng. 2012, 32, 68. (11) Kirjavainen, V.; Heiskanen, K. Some factors that affect beneficiation of sulphide nickel-copper ores. Miner. Eng. 2007, 20, 629. (12) Rao, D. S.; Vijayakumar, T. V.; Angadi, S.; Prabhakar, S.; Raju, G. B. Effects of modulus and dosage of sodium silicate on limestone flotation. Int. J. Sci. Technol. 2010, 4, 397.

Figure 9. Interaction energy between a pyrite and lizardite particle. 12093

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(13) Rao, D. S.; VijayaKumar, T. V.; Rao, S. S.; Prabhakar, S.; Raju, G. B. Effectiveness of sodium silicate as gangue depressants in iron ore slimes flotation. Int. J. Miner., Metall. Mater. 2011, 18, 515. (14) Adamczyk, Z.; Weroński, P. Application of the DLVO theory for particle deposition problems. Adv. Colloid Interface Sci. 1999, 83, 137. (15) Missnan, T.; Adell, A. On the applicability of DLVO theory to the prediction of clay colloids stability. J. Colloid Interface Sci. 2000, 230, 150. (16) Mitchell, T. K.; Nguyen, A. V.; Evans, G. M. Heterocoagulation of chalcopyrite and pyrite minerals in flotation separation. Adv. Colloid Interface Sci. 2005, 114, 227. (17) Nguyen, A. V.; Evans, G. M.; Jameson, G. J. Approximate calculations of electrical double-layer interaction between spheres. In Encyclopedia of surface and colloid science; Hubbard, A. T., Ed.; Marcel Dekker: New York, 2002. (18) Sharma, P. K.; Rao, K. H. Adhesion of Paenibacillus polymyxa on chalcopyrite and pyrite: surface thermodynamics and extended DLVO theory. Colloids Surf., B 2003, 29, 21. (19) Tartaj, P.; Cerpa, A.; Garcia-Gonzalez, M. T.; Serna, C. J. Surface instability of lizardite in aqueous suspensions. J. Colloid Interface Sci. 2000, 231, 176.

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