The Activation Mechanism of Bi 3 + Ions to Rutile Flotation in a Strong Acidic Environment

Lead hydroxyl compounds are known as rutile flotation of the traditional activated component, but the optimum pH range for flotation is 2–3 using styryl phosphoric acid (SPA) as collector, without lead hydroxyl compounds in slurry solution. In this study, Bi3+ ions as a novel activator was investigated. The results revealed that the presence of Bi3+ ions increased the surface potential, due to the specific adsorption of hydroxyl compounds, which greatly increases the adsorption capacity of SPA on the rutile surface. Bi3+ ions increased the activation sites through the form of hydroxyl species adsorbing on the rutile surface and occupying the steric position of the original Ca2+ ions. The proton substitution reaction occurred between the hydroxyl species of Bi3+ ions (Bi(OH)n+(3−n)) and the hydroxylated rutile surface, producing the compounds of Ti-O-Bi2+. The micro-flotation tests results suggested that Bi3+ ions could improve the flotation recovery of rutile from 61% to 90%, and from 61% to 64% for Pb2+ ions.


Introduction
Rutile is the best raw material for high-end titanium pigment production and a high grade titanium to extract, which has important implications for the defense industry and high-end coating market [1,2].The majority of rutile ores belong to refractory ores, and its concentration is very difficult due to their fine grain sizes associated with the gangues, the complexity of the mineralogy, and the brittleness to easily over-ground [3].Although the concentration of rutile ores is obtained by a combination of gravity, magnetic and electrostatic separation techniques, flotation is one of the most efficient methods to solve the issue [4].
Sodium oleate, water-soluble petroleum sulfonate, styryl phosphoric acid (SPA), benzyl arsenic acid, alkyl imino bismethylene phosphoric acid and hydroximic acid are commonly used as collectors in rutile flotation [5][6][7].Liu and Peng [5] thought SPA was considered to be the most selective collector for rutile, but high consumption has limited its use.So, we investigated some activators to enhance the flotation effects of SPA.Many studies have reported that Pb(NO 3 ) 2 was the effective activator to the flotation of the minerals containing titanium.Li et al. [8] found that Pb(NO 3 ) 2 was an activator to effectively improve rutile floatability using salicyl hydroxamic acid as a collector, and the optimal pH range for the activating processing was 5.5-6.8.Fan and Rowson [9] investigated the effects of Pb(NO 3 ) 2 on the flotation separation of ilmenite using sodium oleate as a collector.They found that ilmenite could be activated by Pb(NO 3 ) 2 in the pH range of 4-8, but that it would be reduced when the pH range was 2-3.
Two mechanisms of metal ions adsorption in the activation of minerals have been proposed: (1) the formation of Me 1 -O-Me 2 + species (Me 1 : Zn, Mn, Fe, Sn, etc.; Me 2 : Pb 2+ and Cu 2+ ), which are the product of complex interactions between Me 2 (OH) + and the mineral surface [9]; (2) the formation of Me 2 S or Me 2 O by the exchange of Me 2 and Me 1 [10][11][12].However, these two mechanisms are just possible under certain conditions.The condition of mechanism (1) is the formation of hydroxy compounds of Me 2 ; and (2) is that the solubility product of Me 2 S or Me 2 O needs to be less than that of Me 1 S or Me 1 O.However, in the pH range of 1-3, the main specie of Pb(NO 3 ) 2 in the solution is free Pb 2+ ions and is adsorbed on the rutile surface through electrostatic forces [8], and the solubility product of PbO is much greater than TiO 2 .So, Pb 2+ ions have been unable to act as an activator for rutile flotation in this pH range.
It was reported that the Bi 3+ and Pb 2+ ions had lone pairs of electrons, which could be stereo chemically active, and exert a considerable effect on the structures of their complexes [13,14].Bi 3+ ions readily hydrolyze in aqueous solutions (pKa = 1.51) and have a high affinity to both oxygen and nitrogen ligands [13,15].These have two implications for us: (1) Bi 3+ ions have the same functional configuration with respect to the activation of Pb 2+ ions; (2) Bi 3+ ions have been hydrolyzed in the optimal flotation pH of 2-3.
We found that Bi 3+ ions could significantly improve the recovery rate of rutile flotation in pure minerals.In this study, the activation mechanism of Bi 3+ ions to rutile flotation and its subsequent flotation response to styryl phosphoric acid (SPA) were investigated as a novel activator to rutile flotation by micro-flotation tests, zeta potential measurements, adsorption capacity measurements and X-ray photoelectron spectroscopy (XPS) analyses.The activation mechanism of Bi 3+ ions to rutile flotation was discussed.The presented results are expected to be useful for the development and selection of high efficiency activators for rutile flotation.

Materials
Rutile samples used in this study were obtained from Zaoyang Mine, Hubei province, China.The samples were crushed and ground to particle size from 38 to 74 µm, and were then purified by sieving, several stages of tabling, and low and high intensity magnetic separation methods.The pure rutile samples were washed several times with distilled water and dried at room temperature.The result of particle size analysis was shown in Figure 1.The average particle size of rutile was 53.98 µm measured by a laser particle size analyzer (Mastersizer 2000, Malvern Instruments Ltd., Malvern, UK).Examination under an ultrasonic generator showed rutile grains had a clean surface, and were almost free from gangue minerals.All of the reagents are analytically pure grade in this study.
The crystal phase composition was determined by X-ray diffraction XRD (Germany Bruker-axes D8 Advance, Karlsruhe, Germany), whose detailed processes and methods were previously described by Han [16].The chemical composition of rutile was carried out by a wavelength dispersive X-ray Fluorescence (XRF, PANalytical B.V., Almelo, The Netherlands) spectrometer S4 Pioneer.The results of XRD and XRF are shown in Figure 2 and Table 1.The XRD results in Figure 2 demonstrate that the rutile samples were mainly composed of rutile, and the Ti element was only originated from rutile mineral.So, according to element content of XRF results in Table 1, the purity of rutile mineral in the sample was calculated to be 93.8%.The crystal phase composition was determined by X-ray diffraction XRD (Germany Bruker-axes D8 Advance, Karlsruhe, Germany), whose detailed processes and methods were previously described by Han [16].The chemical composition of rutile was carried out by a wavelength dispersive X-ray Fluorescence (XRF, PANalytical B.V., Almelo, The Netherlands) spectrometer S4 Pioneer.The results of XRD and XRF are shown in Figure 2 and Table 1.The XRD results in Figure 2 demonstrate that the rutile samples were mainly composed of rutile, and the Ti element was only originated from rutile mineral.So, according to element content of XRF results in Table 1, the purity of rutile mineral in the sample was calculated to be 93.8%.The crystal phase composition was determined by X-ray diffraction XRD (Germany Bruker-axes D8 Advance, Karlsruhe, Germany), whose detailed processes and methods were previously described by Han [16].The chemical composition of rutile was carried out by a wavelength dispersive X-ray Fluorescence (XRF, PANalytical B.V., Almelo, The Netherlands) spectrometer S4 Pioneer.The results of XRD and XRF are shown in Figure 2 and Table 1.The XRD results in Figure 2 demonstrate that the rutile samples were mainly composed of rutile, and the Ti element was only originated from rutile mineral.So, according to element content of XRF results in Table 1, the purity of rutile mineral in the sample was calculated to be 93.8%.

Micro-Flotation Tests
The purified mineral particles (2 g) were placed in a plexiglass cell (40 mL), which was then filled with distilled water and the pH value was adjusted.After keeping for 2 min, the metal ions and the collector were added, respectively.The suspension was agitated for 2 and 3 min, respectively.The pH was measured before the flotation, and the flotation was conducted for 4 min.The concentrates were weighed after filtration and drying, and the recovery rate was then calculated.

Adsorption Experiments
The treatment of rutile samples, the time and order of addition of the reagents were the same as micro-flotation tests, except that the flotation was not carried out.The suspension was agitated for 5 min.After each step of the extraction process the samples were centrifuged and filtered to separate the liquid and solid phases.After washing twice with distilled water, the filter liquor was moved to a 100-mL volumetric flask.The adsorption amount was calculated by following Equation (1): where, Q e is the mount of SPA adsorbed on the surface of rutile (mol/m 2 ), C 0 is the initial concentration of SPA (mol/L), V is the volume (L), S is the specific surface area of rutile (m 2 /g), and W is the mass of rutile (g).

Zeta Potential Measurements
The zeta potential of rutile surface was measured using a ZetaPALS instrument (Brookhaven, MS, USA).The samples were ground to less than 5 µm.The suspension was prepared by adding 50 mg of pure rutile to 5 mL of distilled, deionized water containing 10 −3 mol/L KCl as a supporting electrolyte.The resulting suspension was conditioned for 15 min during which suspension pH was measured.The pH was adjusted using either NaOH or H 2 SO 4 over the pH range of 2-12.

X-ray Photoelectron Spectroscopy
The chemical compositions of rutile surfaces were determined by XPS on a Thermo Scientific ESCALAB 250Xi (Thermo Fisher Scientific, Waltham, MA, USA) using Al Kα X-ray source operated at 200 W with 20 eV pass energy.The vacuum pressure was ranged from 10 −9 to 10 −8 Torr and the takeoff angle was 90 • .The data were collected and analyzed using the method under Thermo Scientific Avantage 4.52 software [17,18].

Micro-Flotation Tests
The flotation recoveries of rutile as a function of pH using SPA as collector are shown in Figure 3.In this study, Bi 3+ and Pb 2+ ions were added as the activators.Figure 3 shows that without the addition of Bi 3+ and Pb 2+ ions, the recovery of rutile is low (around 61%), and the optimal pH range of rutile flotation is 1-2.With the addition of Pb 2+ ions, the recovery of rutile slightly improves (around 65%), and the optimal pH range of rutile flotation is 1-2.7.With the addition of Bi 3+ ions, the recovery of rutile sharply increases (around 90%), and the optimal pH range of rutile flotation is 1-3.
Figure 4 shows ∆flotation recovery before and after the addition of activation ions as a function of pH.The ∆flotation recovery represents the activation ability of Pb 2+ and Bi 3+ ions on rutile flotation.It suggested that the activation ability of metal ions to rutile flotation improved with the different increase.The addition of Pb 2+ ions increased the rutile recovery when the pH was from 1 to 8. In strongly acidic conditions (pH < 2.5), the recovery of rutile was improved slightly (∆1 less than 5%), while in pH > 2.5, there was a great change (∆1 more than 15%).It indicated that Pb 2+ ions could not be used as the activator for rutile flotation in the pH range of 1-2.5, but could be used as the activator in the pH range of 2.5-7.0.Li et al. [8] used Pb 2+ ions as the activator for rutile flotation with the collector of salicyl hydroxamic acid (SHA) in the pH range of 4.5-6.5, which was similar to our results.Figure 4 shows Δflotation recovery before and after the addition of activation ions as a function of pH.The Δflotation recovery represents the activation ability of Pb 2+ and Bi 3+ ions on rutile flotation.It suggested that the activation ability of metal ions to rutile flotation improved with the different increase.The addition of Pb 2+ ions increased the rutile recovery when the pH was from 1 to 8. In strongly acidic conditions (pH < 2.5), the recovery of rutile was improved slightly (Δ1 less than 5%), while in pH > 2.5, there was a great change (Δ1 more than 15%).It indicated that Pb 2+ ions could not be used as the activator for rutile flotation in the pH range of 1-2.5, but could be used as the activator in the pH range of 2.5-7.0.Li et al. [8] used Pb 2+ ions as the activator for rutile flotation with the collector of salicyl hydroxamic acid (SHA) in the pH range of 4.5-6.5, which was similar to our results.The Δ flotation recovery as a function of pH: Δ1 is the difference before and after Pb 2+ ions addition; Δ2 is the difference before and after Bi 3+ ions addition; Δ3 is the difference between Pb 2+ and Bi 3+ ions.
The addition of Bi 2+ ions increased the rutile recovery when the pH was from 1 to 6.In strongly acidic conditions (pH < 4.8), the recovery of rutile improved sharply (Δ2 more than 15%), while pH > 6.0, there was slight change (Δ2 less than 5%).It suggested that Bi 2+ ions can be used as the activator  Figure 4 shows Δflotation recovery before and after the addition of activation ions as a function of pH.The Δflotation recovery represents the activation ability of Pb 2+ and Bi 3+ ions on rutile flotation.It suggested that the activation ability of metal ions to rutile flotation improved with the different increase.The addition of Pb 2+ ions increased the rutile recovery when the pH was from 1 to 8. In strongly acidic conditions (pH < 2.5), the recovery of rutile was improved slightly (Δ1 less than 5%), while in pH > 2.5, there was a great change (Δ1 more than 15%).It indicated that Pb 2+ ions could not be used as the activator for rutile flotation in the pH range of 1-2.5, but could be used as the activator in the pH range of 2.5-7.0.Li et al. [8] used Pb 2+ ions as the activator for rutile flotation with the collector of salicyl hydroxamic acid (SHA) in the pH range of 4.5-6.5, which was similar to our results.The addition of Bi 2+ ions increased the rutile recovery when the pH was from 1 to 6.In strongly acidic conditions (pH < 4.8), the recovery of rutile improved sharply (Δ2 more than 15%), while pH > 6.0, there was slight change (Δ2 less than 5%).It suggested that Bi 2+ ions can be used as the activator The addition of Bi 2+ ions increased the rutile recovery when the pH was from 1 to 6.In strongly acidic conditions (pH < 4.8), the recovery of rutile improved sharply (∆2 more than 15%), while pH > 6.0, there was slight change (∆2 less than 5%).It suggested that Bi 2+ ions can be used as the activator for rutile flotation in the pH range of 1-4.8, especially at pH 2.5, when ∆2 reaches the greatest value (about 40%), indicating that the activation ability was strongest at pH 2.5.
The ∆3 represents the difference of the activation ability of Pb 2+ and Bi 3+ ions on rutile flotation.Figure 4 shows that ∆3 was more than zero when 1.0 < pH < 4.0, and less than zero when 4.0 < pH < 8.0.It indicated that the activation ability of Bi 3+ ions was stronger than Pb 2+ ions when 1.0 < pH < 4.0, and weaker when Pb 2+ ions when 4.0 < pH < 8.0.The concentration of Ca 2+ ions in the pulp solution as a function of pH is shown in Figure 5. Figure 5 shows that the concentration of Ca 2+ ions in the pulp solution increased with the pH value declining, especially in the pH range of 1-2, when the concentration of Ca 2+ ions increased sharply.It indicated that the calcium impurity on the rutile surface was dissolved in the acidic conditions, especially in the pH range of 1-2, when it dissolved the most obviously.Combined with the curve of flotation recovery, without activation ions, the flotation recovery of the rutile decreased slightly with the pH value decreasing when the pH was 1-2.That was because the activating sites on rutile surface were dissolved to aqueous solution in the condition of strongly acidic conditions (pH < 2.0) [19,20].The flotation recovery of the rutile decreased slightly due to SPA adsorbed on the active sites of calcium, which made it more difficult.Table 2 shows that the radius of Ca 2+ ions was very close to the radius of Bi 3+ ions, less than that of Pb 2+ ions [21].It indicated that Bi 3+ ions can easily occupy the position of the original Ca 2+ ions on rutile surface, but that is was very difficult for Pb 2+ ions to occupy the vacancy left by Ca 2+ ions dissolution, due to the large radius and the steric hindrance.Since the Bi 3+ ions were +3 valences, it occupied the position of Ca 2+ ions, and it was still expressed as +1 valences, which was favorable for the adsorption of the anions of SPA on the substituted Bi 3+ ions sites.It explained the phenomenon that the flotation recovery and the adsorption capacity of SPA increased sharply in the presence of Bi 3+ ions in the pH range of 1-2.
for rutile flotation in the pH range of 1-4.8, especially at pH 2.5, when Δ2 reaches the greatest value (about 40%), indicating that the activation ability was strongest at pH 2.5.
The Δ3 represents the difference of the activation ability of Pb 2+ and Bi 3+ ions on rutile flotation.Figure 4 shows that Δ3 was more than zero when 1.0 < pH < 4.0, and less than zero when 4.0 < pH < 8.0.It indicated that the activation ability of Bi 3+ ions was stronger than Pb 2+ ions when 1.0 < pH < 4.0, and weaker when Pb 2+ ions when 4.0 < pH < 8.0.
The concentration of Ca 2+ ions in the pulp solution as a function of pH is shown in Figure 5. Figure 5 shows that the concentration of Ca 2+ ions in the pulp solution increased with the pH value declining, especially in the pH range of 1-2, when the concentration of Ca 2+ ions increased sharply.It indicated that the calcium impurity on the rutile surface was dissolved in the acidic conditions, especially in the pH range of 1-2, when it dissolved the most obviously.Combined with the curve of flotation recovery, without activation ions, the flotation recovery of the rutile decreased slightly with the pH value decreasing when the pH was 1-2.That was because the activating sites on rutile surface were dissolved to aqueous solution in the condition of strongly acidic conditions (pH < 2.0) [19,20].The flotation recovery of the rutile decreased slightly due to SPA adsorbed on the active sites of calcium, which made it more difficult.Table 2 shows that the radius of Ca 2+ ions was very close to the radius of Bi 3+ ions, less than that of Pb 2+ ions [21].It indicated that Bi 3+ ions can easily occupy the position of the original Ca 2+ ions on rutile surface, but that is was very difficult for Pb 2+ ions to occupy the vacancy left by Ca 2+ ions dissolution, due to the large radius and the steric hindrance.Since the Bi 3+ ions were +3 valences, it occupied the position of Ca 2+ ions, and it was still expressed as +1 valences, which was favorable for the adsorption of the anions of SPA on the substituted Bi 3+ ions sites.It explained the phenomenon that the flotation recovery and the adsorption capacity of SPA increased sharply in the presence of Bi 3+ ions in the pH range of 1-2.

Adsorption Amount of SPA
The adsorption amount of SPA on rutile surface in the presence and absence of Bi 3+ ions and Pb 2+ ions as a function of pH is presented in Figure 6. Figure 6 shows the optimal pH range of SPA adsorption was 1-2 without activation ions.With the addition of Pb 2+ ions, the adsorption amount of SPA on rutile surface changed slightly in the optimal pH range, but with the addition of Bi 3+ ions, the adsorption amount of SPA increased sharply in the optimal pH range, which was very consistent

Adsorption Amount of SPA
The adsorption amount of SPA on rutile surface in the presence and absence of Bi 3+ ions and Pb 2+ ions as a function of pH is presented in Figure 6. Figure 6 shows the optimal pH range of SPA adsorption was 1-2 without activation ions.With the addition of Pb 2+ ions, the adsorption amount of SPA on rutile surface changed slightly in the optimal pH range, but with the addition of Bi 3+ ions, the adsorption amount of SPA increased sharply in the optimal pH range, which was very consistent with the flotation recovery.It suggested that the adsorption amount of SPA was a key factor to determine the flotation recovery.With the increase of adsorption amount of the collector, the hydrophobicity of mineral surface increased, and led to the floatability of mineral improved [22][23][24].with the flotation recovery.It suggested that the adsorption amount of SPA was a key factor to determine the flotation recovery.With the increase of adsorption amount of the collector, the hydrophobicity of mineral surface increased, and led to the floatability of mineral improved [22][23][24].

Figure 6.
The adsorption amount of SPA on rutile surfaces before and after Bi 3+ ions modification as a function of pH.

Zeta Potentials
Adsorption of cations onto a mineral inevitably leads to a change in the surface charge.Figure 7 shows the zeta potentials of rutile as a function of pH in different conditions.In the absence of activators, the point of zero charge (PZC) of rutile occurred at pH 3.8 ± 0.1, which was almost in accord with the previous values reported by Parks [25].In the presence of 10 −4 mol/L Pb 2+ ions, the zeta potentials of rutile particles moved to more positive values and the PZC appeared at about pH 9.0 ± 0.1.In the presence of 10 −5 mol/L Bi 3+ ions, the PZC appeared at about pH 4.5 ± 0.1.Figure 8 shows that the zeta potentials of the rutile surface were affected slightly by Pb 2+ ions in the pH range of 2-4.At pH < 4.0, and all of the lead nitrate in the solution were present as free ions.The species distribution diagrams for Bi 3+ , Pb 2+ , Ti 4+ ions and SPA are shown in Figure 9 and help explain the results obtained from zeta potential experiments [19].On one hand, Pb 2+ ions adsorbed Figure 6.The adsorption amount of SPA on rutile surfaces before and after Bi 3+ ions modification as a function of pH.

Zeta Potentials
Adsorption of cations onto a mineral inevitably leads to a change in the surface charge.Figure 7 shows the zeta potentials of rutile as a function of pH in different conditions.In the absence of activators, the point of zero charge (PZC) of rutile occurred at pH 3.8 ± 0.1, which was almost in accord with the previous values reported by Parks [25].In the presence of 10 −4 mol/L Pb 2+ ions, the zeta potentials of rutile particles moved to more positive values and the PZC appeared at about pH 9.0 ± 0.1.In the presence of 10 −5 mol/L Bi 3+ ions, the PZC appeared at about pH 4.5 ± 0.1.with the flotation recovery.It suggested that the adsorption amount of SPA was a key factor to determine the flotation recovery.With the increase of adsorption amount of the collector, the hydrophobicity of mineral surface increased, and led to the floatability of mineral improved [22][23][24].
Figure 6.The adsorption amount of SPA on rutile surfaces before and after Bi 3+ ions modification as a function of pH.

Zeta Potentials
Adsorption of cations onto a mineral inevitably leads to a change in the surface charge.Figure 7 shows the zeta potentials of rutile as a function of pH in different conditions.In the absence of activators, the point of zero charge (PZC) of rutile occurred at pH 3.8 ± 0.1, which was almost in accord with the previous values reported by Parks [25].In the presence of 10 −4 mol/L Pb 2+ ions, the zeta potentials of rutile particles moved to more positive values and the PZC appeared at about pH 9.0 ± 0.1.In the presence of 10 −5 mol/L Bi 3+ ions, the PZC appeared at about pH 4.5 ± 0.1.Figure 8 shows that the zeta potentials of the rutile surface were affected slightly by Pb 2+ ions in the pH range of 2-4.At pH < 4.0, and all of the lead nitrate in the solution were present as free ions.The species distribution diagrams for Bi 3+ , Pb 2+ , Ti 4+ ions and SPA are shown in Figure 9 and help explain the results obtained from zeta potential experiments [19].On one hand, Pb 2+ ions adsorbed Figure 8 shows that the zeta potentials of the rutile surface were affected slightly by Pb 2+ ions in the pH range of 2-4.At pH < 4.0, and all of the lead nitrate in the solution were present as free ions.The species distribution diagrams for Bi 3+ , Pb 2+ , Ti 4+ ions and SPA are shown in Figure 9 and help explain the results obtained from zeta potential experiments [19].On one hand, Pb 2+ ions adsorbed on the rutile surface through electrostatic forces [8], resulting in the increase of zeta potential; on the other hand, because of the presence of salt solution, the thickness of the electric double layer of the rutile surface was compressed [26][27][28], resulting in the decrease of the zeta potential.In the pH range of 1-2.5, which was the optimal pH range for flotation, the rutile surface charge was positive.
The adsorption amount of Pb 2+ ions on rutile surface was very small [8], which suggested that Pb 2+ ions could not activate the rutile flotation using SPA as the collector.In the pH range of 5-9, because of the rutile surface with a positive charge, Pb 2+ ions could easily adsorb on the rutile surface.Figure 9d shows that lead nitrate in solution was primarily present as Pb(OH) + , primarily present as Pb 2+ and Pb(OH) + .While Pb 2+ decreased with the increase of pH, Pb(OH) + increased with increasing pH from 5 to 9. The increased adsorption of lead ions on the rutile surfaces, and more positively charged rutile surfaces from pH 5-9 (Figures 6 and 7), would therefore be attributed to the specific adsorption of Pb(OH) + on the rutile surfaces [8,19].
Figure 7 shows that the zeta potentials of the rutile surface dramatically increased in the pH range of 2-4 after the addition of Bi 3+ ions.In this pH range, bismuth nitrate in solution was primarily as a generation of hydroxyl compounds (Bi(OH) 2+ ) (Figure 9a).The sign of charge on the rutile surface was the same as that of Bi(OH) 2+ .It indicated that Bi 3+ ions on the rutile surface facilitated the large specific adsorption in the form of a generation of hydroxyl compounds (Bi(OH) 2+ ) In the optimum flotation range, the adsorption of metal cations on the mineral surface brought two advantages to the flotation process: (1) the zeta potentials of the mineral surface dramatically increased after the metal cations adsorption, which was better for the adsorption of anionic collectors on the mineral surface; (2) the metal cations adsorbed on the mineral surface in the form of the hydroxyl compounds, improved the activation sites for the collector adsorption on the mineral surface.
Figure 7 shows that the zeta potentials of the rutile surface obviously decreased after SPA addition, which indicated that SPA could adsorb on the rutile surface in the presence and absence of activation ions.However, the extent of decline was different.Figure 8 shows ∆zeta potential before and after SPA addition as a function of pH.The ∆zeta potential represented the adsorption capacity of SPA on the rutile surface under different conditions in solution.It suggested that the adsorption capacity of SPA on the rutile surface increased with different increases in activation ions.Figure 8 shows that ∆2 improved slightly more than ∆1, and ∆3 dramatically increased in comparison to ∆1 and ∆2 in the pH range of 2-3.It indicated that the effect of Pb 2+ ions on SPA adsorption was very small, and the adsorption capacity of SPA on the rutile surface dramatically increased in the presence of Bi 3+ ions.
The presence of Bi 3+ ions increased the surface potential, and facilitated the large specific adsorption in the form of the hydroxyl compounds, which greatly increased the adsorption capacity of SPA on the rutile surface, thus improving the hydrophobicity of the rutile surface, resulting in an increase in flotation recovery.
on the rutile surface through electrostatic forces [8], resulting in the increase of zeta potential; on the other hand, because of the presence of salt solution, the thickness of the electric double layer of the rutile surface was compressed [26][27][28], resulting in the decrease of the zeta potential.In the pH range of 1-2.5, which was the optimal pH range for flotation, the rutile surface charge was positive.The adsorption amount of Pb 2+ ions on rutile surface was very small [8], which suggested that Pb 2+ ions could not activate the rutile flotation using SPA as the collector.In the pH range of 5-9, because of the rutile surface with a positive charge, Pb 2+ ions could easily adsorb on the rutile surface.Figure 9d shows that lead nitrate in solution was primarily present as Pb(OH) + , primarily present as Pb 2+ and Pb(OH) + .While Pb 2+ decreased with the increase of pH, Pb(OH) + increased with increasing pH from 5 to 9. The increased adsorption of lead ions on the rutile surfaces, and more positively charged rutile surfaces from pH 5-9 (Figure 6 and 7), would therefore be attributed to the specific adsorption of Pb(OH) + on the rutile surfaces [8,19].
Figure 7 shows that the zeta potentials of the rutile surface dramatically increased in the pH range of 2-4 after the addition of Bi 3+ ions.In this pH range, bismuth nitrate in solution was primarily as a generation of hydroxyl compounds (Bi(OH) 2+ ) (Figure 9a).The sign of charge on the rutile surface was the same as that of Bi(OH) 2+ .It indicated that Bi 3+ ions on the rutile surface facilitated the large specific adsorption in the form of a generation of hydroxyl compounds (Bi(OH) 2+ ) In the optimum flotation range, the adsorption of metal cations on the mineral surface brought two advantages to the flotation process: (1) the zeta potentials of the mineral surface dramatically increased after the metal cations adsorption, which was better for the adsorption of anionic collectors on the mineral surface; (2) the metal cations adsorbed on the mineral surface in the form of the hydroxyl compounds, improved the activation sites for the collector adsorption on the mineral surface.
Figure 7 shows that the zeta potentials of the rutile surface obviously decreased after SPA addition, which indicated that SPA could adsorb on the rutile surface in the presence and absence of activation ions.However, the extent of decline was different.Figure 8 shows Δzeta potential before and after SPA addition as a function of pH.The Δzeta potential represented the adsorption capacity of SPA on the rutile surface under different conditions in solution.It suggested that the adsorption capacity of SPA on the rutile surface increased with different increases in activation ions.Figure 8 shows that Δ2 improved slightly more than Δ1, and Δ3 dramatically increased in comparison to Δ1 and Δ2 in the pH range of 2-3.It indicated that the effect of Pb 2+ ions on SPA adsorption was very small, and the adsorption capacity of SPA on the rutile surface dramatically increased in the presence of Bi 3+ ions.
The presence of Bi 3+ ions increased the surface potential, and facilitated the large specific adsorption in the form of the hydroxyl compounds, which greatly increased the adsorption capacity of SPA on the rutile surface, thus improving the hydrophobicity of the rutile surface, resulting in an increase in flotation recovery.Figure 8. ∆Zeta potential of rutile surface with a function of pH: ∆1 is the difference before and after SPA addition without activation ions; ∆2 is the difference before and after SPA addition with Pb 2+ ions; ∆3 is the difference before and after SPA addition with Bi 3+ ions.

XPS Analysis
The composition of the rutile surface after Bi 3+ ions adsorption was investigated by XPS. Figure 10 shows the survey scan and high-resolution XPS spectra of the rutile after Bi 3+ ions adsorption.An obvious peak at about 162 eV was showed in Figure 10a, which indicated that Bi 3+ ions could adsorb on the rutile surface in the form of a compound.The high-resolution XPS spectra of O, Ti and Bi elements were scanned in order to extract the information of the chemical states.The curve-fit results are shown in Figures 10bc, respectively, and their XPS peak parameters and chemical states are presented in Table 3.

XPS Analysis
The composition of the rutile surface after Bi 3+ ions adsorption was investigated by XPS. Figure 10 shows the survey scan and high-resolution XPS spectra of the rutile after Bi 3+ ions adsorption.An obvious peak at about 162 eV was showed in Figure 10a, which indicated that Bi 3+ ions could adsorb on the rutile surface in the form of a compound.The high-resolution XPS spectra of O, Ti and Bi elements were scanned in order to extract the information of the chemical states.The curve-fit results are shown in Figure 10b,c, respectively, and their XPS peak parameters and chemical states are presented in Table 3.

XPS Analysis
The composition of the rutile surface after Bi 3+ ions adsorption was investigated by XPS. Figure 10 shows the survey scan and high-resolution XPS spectra of the rutile after Bi 3+ ions adsorption.An obvious peak at about 162 eV was showed in Figure 10a, which indicated that Bi 3+ ions could adsorb on the rutile surface in the form of a compound.The high-resolution XPS spectra of O, Ti and Bi elements were scanned in order to extract the information of the chemical states.The curve-fit results are shown in Figures 10bc, respectively, and their XPS peak parameters and chemical states are presented in Table 3.The detailed scan spectrum of O 1s is presented in Figure 10b and composed of three components.As shown in Table 3, peak 3 was assigned to the O 2− state in rutile bulk (529.72 eV) [21,29], and peak 2 was assigned to the O 2− state in hydroxyl species on the rutile surface [30], including Ti-OH and Bi-OH (532.34 eV).It indicated that the hydroxylation process was present on rutile surface.Peak 1 was tentatively assigned to the O 2− state in Ti-O-Bi on the rutile surface (531.19) [21].
The detailed scan spectrum of Ti 2p is presented in Figure 10c and composed of two components.The characteristic of peaks 5 (464.24eV) and 4 (458.19eV) was obviously attributed to the XPS spectrum of Ti 2p in rutile bulk [8,31,32].Combined with the previous analysis, peak 4 (458.84eV) and peak 7 (461.19eV) were tentatively assigned to the Ti 4+ states in Ti-OH and Ti-O-Bi 2+ on the surface of rutile.
The detailed scan spectrum of Bi 4f7/2 is presented in Figure 10d and composed of four components.As shown in Table 3, Peaks 8, 9 and 10 were assigned to the Bi 4f7/2 of hydroxyl compounds of Bi 3+ ions, which agreed well with the results in the literature [33].This was very consistent with the previous results, which suggested that Bi 3+ ions adsorbed on the rutile surface in the form of hydroxides.Peak 11 had a binding energy of 160.10 eV, which was higher than that of the Bi 3+ states in hydroxyl compounds.It may be due to the formation of the complex of Ti-O-Bi 2+ on the rutile surface, because the complex of Ti-O-Bi 2+ was present, as Peak 2 proved.The detailed scan spectrum of O 1s is presented in Figure 10b and composed of three components.As shown in Table 3, peak 3 was assigned to the O 2− state in rutile bulk (529.72 eV) [21,29], and peak 2 was assigned to the O 2− state in hydroxyl species on the rutile surface [30], including Ti-OH and Bi-OH (532.34 eV).It indicated that the hydroxylation process was present on rutile surface.Peak 1 was tentatively assigned to the O 2− state in Ti-O-Bi on the rutile surface (531.19)[21].
The detailed scan spectrum of Ti 2p is presented in Figure 10c and composed of two components.The characteristic of peaks 5 (464.24eV) and 4 (458.19eV) was obviously attributed to the XPS spectrum of Ti 2p in rutile bulk [8,31,32].Combined with the previous analysis, peak 4 (458.84eV) and peak 7 (461.19eV) were tentatively assigned to the Ti 4+ states in Ti-OH and Ti-O-Bi 2+ on the surface of rutile.
The detailed scan spectrum of Bi 4f 7/2 is presented in Figure 10d and composed of four components.As shown in Table 3, Peaks 8, 9 and 10 were assigned to the Bi 4f 7/2 of hydroxyl compounds of Bi 3+ ions, which agreed well with the results in the literature [33].This was very consistent with the previous results, which suggested that Bi 3+ ions adsorbed on the rutile surface in the form of hydroxides.Peak 11 had a binding energy of 160.10 eV, which was higher than that of the Bi 3+ states in hydroxyl compounds.It may be due to the formation of the complex of Ti-O-Bi 2+ on the rutile surface, because the complex of Ti-O-Bi 2+ was present, as Peak 2 proved.
At pH 2.0, Bi 3+ ions in the form of the hydroxyl species (Bi(OH) n +(3−n) ) chemically interacted with the Ti-OH on rutile surface (after hydroxylation), forming the surface complex of Ti-O-Bi 2+ .

Figure 4 .
Figure 4.The Δ flotation recovery as a function of pH: Δ1 is the difference before and after Pb 2+ ions addition; Δ2 is the difference before and after Bi 3+ ions addition; Δ3 is the difference between Pb 2+ and Bi 3+ ions.

Figure 4 .
Figure 4.The Δ flotation recovery as a function of pH: Δ1 is the difference before and after Pb 2+ ions addition; Δ2 is the difference before and after Bi 3+ ions addition; Δ3 is the difference between Pb 2+ and Bi 3+ ions.

Figure 4 .
Figure 4.The ∆ flotation recovery as a function of pH: ∆1 is the difference before and after Pb 2+ ions addition; ∆2 is the difference before and after Bi 3+ ions addition; ∆3 is the difference between Pb 2+ and Bi 3+ ions.

Figure 5 .
Figure 5.The concentration of Ca 2+ ions as a function of pH.

Figure 5 .
Figure 5.The concentration of Ca 2+ ions as a function of pH.

Figure 7 .
Figure 7. Zeta potential of rutile surface with a function of pH.

Figure 7 .
Figure 7. Zeta potential of rutile surface with a function of pH.

Figure 7 .
Figure 7. Zeta potential of rutile surface with a function of pH.

Figure 8 .Figure 9 .
Figure 8. ΔZeta potential of rutile surface with a function of pH: Δ1 is the difference before and after SPA addition without activation ions; Δ2 is the difference before and after SPA addition with Pb 2+ ions; Δ3 is the difference before and after SPA addition with Bi 3+ ions.

Figure 8 .Figure 9 .
Figure 8. ΔZeta potential of rutile surface with a function of pH: Δ1 is the difference before and after SPA addition without activation ions; Δ2 is the difference before and after SPA addition with Pb 2+ ions; Δ3 is the difference before and after SPA addition with Bi 3+ ions.

Table 2 .
The radius of metal ions (All are 6 coordinated).

Table 2 .
The radius of metal ions (All are 6 coordinated).

Table 3
The O 1s, Ti 2p, and Bi 4f5/2 XPS parameters and chemical states for rutile after Bi 3+ ions adsorption.