Effects of seawater on the adsorption of xanthate onto galena and sphalerite

Seawater contains divalent calcium and magnesium cations. Under alkaline conditions, calcium and magnesium ions react with hydroxide ions to form insoluble hydroxyl complexes or hydroxide precipitates. The hydrophilic substances that may be adsorbed on the mineral surface during the flotation process hinder the adsorption of the collector, affecting mineral hydrophobicity, and thus reducing the floatability of the mineral. In this study, the effects of seawater on the adsorption of xanthate onto galena and sphalerite were investigated. The results show that under strong alkaline conditions, seawater has significant and slight adverse effects on sphalerite and galena, respectively. Flotation regulators such as ethylenediamine tetraacetic acid, sodium hexametaphosphate, and sodium silicate can eliminate the adverse effect on galena and sphalerite flotation to a certain extent. The mechanisms were revealed through microflotation experiments, contact angle measurements, bubble-particle attachment tests, zeta potential measurements, and XPS analysis.


Introduction
Pb and Zn are widely used in metallurgy, national defense, automotive, and electronics [1,2].Galena and sphalerite are the main sources of Pb and Zn extracted by these industries and often coexists in nature [3,4].Galena is naturally hydrophobic, making it suitable for flotation separation [5][6][7][8][9][10].The floatability of sphalerite, on the other hand, depends on its impurities, particularly the iron content.Generally, sphalerite is a sulfide mineral that usually does not exhibit flotation.The high floatability of sphalerite is attributed to the natural activation of heavy metal ions, especially copper ions, or mineral oxidation [11][12][13][14].
In the industrial practice of separating Pb and Zn through flotation, lime is commonly used as a pH regulator and inhibitor of sphalerite because the calcium in lime forms hydrophilic hydroxyl complexes that are adsorbed on the sphalerite surface under alkaline conditions, reducing its floatability.Seawater contains significant amounts of Mg 2+ and Ca 2+ [15][16][17][18][19], which can act as natural and inexhaustible inhibitors of sphalerite.Seawater flotation of Pb and Zn can conserve freshwater resources and reduce the amount of lime required.Therefore, it has attracted considerable research interest.
To analyze the effect of seawater on the flotation of galena and sphalerite, microflotation test was initially conducted using DI water and then repeated using seawater employing the same process and flotation reagent conditions.Three reagents, namely ethylenediamine tetraacetic acid (EDTA), sodium hexametaphosphate (SHMP) and sodium silicate (SS), were used to regulate Pb and Zn flotation in seawater.Among these reagents, EDTA was a chelating agent of calcium and magnesium ions [20,21], while SHMP [22][23][24] and SS [25,26] were the dispersants.By comparing the test results, the flotation mechanism was discussed through microflotation tests, contact angle measurements, bubble-particle attachment tests, XPS analysis, and zeta potential measurements.

Materials
The pure galena and sphalerite used in the experiments were obtained from Guilin, Guangxi and Taolin, Hunan, China.After crushing and screening, the ore sample was prepared into particle sizes of -74+38 μm and -38 μm.Samples with a particle size of -74+38 μm were individually packed into small vacuum bags, each weighing 20.0 g, for X-ray diffraction (XRD), XPS analysis, and microflotation tests.Furthermore, the -38 μm sample was further ground to -5 μm and used for zeta potential and FTIR measurements.An XRD analysis was performed to determine the chemical composition and purity of the sample.The results are presented in Figure 1 and Table 1.Artificially simulated seawater was prepared refer to the findings of Kester et al. [27].The concentrations of the major ions are listed in Table 2.  HCl and NaOH were used as pH-adjusting agents.EDTA, SHMP and SS were used as regulators.Pure samples (2.0 g) were added to a 30.0 ml flotation tank, and the pulp within the flotation tank was continuously stirred to ensure uniform dispersion of the samples in the solution.After one minute of stirring, the pH value of the pulp was adjusted, followed by the addition of the corresponding regulator at two-minute intervals.Next, PBX (stirring for 3 min) and MIBC (stirring for 1 min) were added, and the bubbles were manually scraped for 3 min.The concentrate and tailings were collected, dried, weighed, and sequentially tested to calculate the recovery.The test process is illustrated in Figure 2.

Contact angle measurements
Contact angle measurements are commonly used to determine the surface wettability of minerals.The most frequently-used sample preparation method for measuring contact angles of powder samples is the Powder Tablet Method [28][29][30].The contact angles of the galena and sphalerite were measured in DI water, seawater, and seawater containing EDTA, SHMP, and SS.For each measurement, adding 2.0 g pure mineral to a 30.0 ml solution.After treatment like those in the microflotation test, sample was dried in a vacuum oven at 40℃ for 24h and then pressed into 1-2 mm wafer under 10.0 MPa pressure for two minutes.A contact angle measuring instrument (JC2000A, Powereach, Shanghai, China) was used to measure the contact angle.Water with a particle diameter of approximately 2-3 mm was dropped onto the sample surface using a micro syringe.Each measurement was performed three times and the mean and standard deviation values were calculated [31,32].2.4.Bubble-particle attachment tests A self-constructed visual measurement system was used to measure the bubble-particle attachment.
The test system was primarily composed of a high-speed camera, vibration exciter, light source, lifting table, and computer.The samples were prepared in a square glass tank measuring 5 × 5 × 5 cm 3 .The preparation process and dosage were based on the flotation tests.After preparation, the samples were transferred to a lifting table, and the height was adjusted for subsequent tests.During the test, one end of the capillary tube was connected to the micro syringe, while the other end was placed below the liquid level of the square glass tank.Bubbles with a diameter of 2 mm were created using a micro syringe.The bubbles moved downward in each test, making contact with the mineral particle bed, and then returned to their initial position.The above was a complete round of testing.The device displacement was adjusted to ensure that each bubble was in contact with the sample particle bed, and different dwell times were set for each test.The process of mineral particles adhering to the bubbles was observed using a high-speed dynamic camera.Each experiment was repeated three times, and representative images were selected for analysis.
2.5.Zeta potential distribution measurements A Nano ZS90 nanometer particle potentiometer (Malvern Company) was used to measure the surface potential of the minerals before and after interaction with the reagents.The specific operation steps were as follows: For each test, a 30.0 mg sample with a particle size of -5 μm was placed in 30.0 ml KCl solution with a concentration of 1 × 10 -3 mol/L, and the container was placed on a magnetic stirrer for continuous agitation.The test process and dosing system were consistent with the flotation test.
After each test, the solution in the container was allowed to stand for 10 min, and the upper clarified solution was extracted for zeta potential analysis.Each group of samples was measured thrice, and the average value and standard deviation were calculated as the final test results.

X-ray photoelectron spectroscopy (XPS) analysis test
The processing method of XPS samples were referred to flotation test.After sufficient interaction with flotation agents according to the flotation process, the ore samples were filtered washed and vacuum dried for XPS testing.XPS analysis was performed using the Thermo KAlpha (ESCALAB 250xi) X-ray photoelectron spectrometer.The size of Al Kα microfocus monochromatic source was 5 μm, and the data acquisition power was 72 W. The vacuum degree in the analysis chamber should not exceed 2 × 10 -7 mbar.In the full-spectrum analysis, the pass energy and energy scan step size were 100 eV and 1 eV, respectively, and in the narrow-spectrum analysis, they were 50 eV and 0.1 eV.The acquired spectra were calibrated using standard C1s peaks (284.8eV), and XPS measurement data were analyzed using Thermo Vantage software.

Microflotation results
Figure 3 shows that in deionized water, the galena recovery remained stable above 90% within the pulp pH value range of 2-8, with the highest value of 94.65% (pH = 5.98).In addition, sphalerite recovery was low, with a maximum of 72.40% at pH 5.99, after which it began to decline.In seawater, galena recovery reached its highest value of 96.95% at a pH of 5.98, but sharply dropped when the pH exceeded 10.0.The sphalerite recovery was relatively low, with the maximum value of only 60.50% (pH = 5.97).It is evident that seawater has an inhibitory effect on both galena and sphalerite flotation, particularly on the latter.The separation of Pb and Zn is usually performed under alkaline conditions, which adversely affect the floatability of sulfide minerals in seawater.Therefore, this study investigated the changes in galena and sphalerite recovery after adding three regulators (EDTA, SHMP, and SS) to both DI water and seawater at a pH of 10.0, as shown in Figure 4.After adding three reagents, the recoveries of galena and sphalerite were increased to different extent.The variation range of galena is small while that of sphalerite is large.The results show that the three reagents are effective in eliminating the adverse effects of seawater on galena and sphalerite flotation.

Contact angle determination
The contact angle is an important parameter for measuring the wettability of a liquid on a mineral surface.It reflects the floatability of the minerals to a certain extent [30].Figure 5 shows the contact angles of galena and sphalerite in DI water, seawater, and seawater with EDTA, SHMP, and SS before and after interaction with the collector PBX.
Compared with DI water, the contact angles in seawater were smaller, indicating that hydrophilic substances were adsorbed onto the mineral surface.The contact angle of galena in seawater decreased to 66.53°.However, after the addition of EDTA, SHMP, and SS, the contact angles became 63.85°, 66.57°, and 69.85°, respectively, indicating that these three regulators had minute influence on the contact angle of galena in seawater.The contact angle of the sphalerite in seawater was reduced to 40.67°.However, after the addition of EDTA, SHMP, and SS, the contact angles changed to 61.02°, 65.41°, and 62.57°, respectively, indicating that the three regulators significantly increased the contact angle of sphalerite in seawater.The collector can increase mineral hydrophobicity, leading to an increase in the contact angle of the mineral.In seawater, the change in the galena contact angle was significantly greater than that of sphalerite after the addition of the collector PBX, indicating a higher adsorption amount of the collector PBX on the galena surface compared to that on sphalerite.

Bubble-particle attachment analysis
Contact angle measurements and bubble-particle attachment tests are commonly used to characterize mineral hydrophobicity [33,34].Among these tests, the bubble-particle attachment test is a kinetic measure of hydrophobicity, making it more suitable for describing flotation behavior [35][36][37][38].Figures 6 and 7 show the bubble-particle attachment under the three systems at dwell time 10, 20 and 30 ms.
Mineral particle adhesion on the bubble increased with a longer dwell time.In addition, at the same dwell time, galena particles have similar adhesion with bubbles in different systems.However, the adhesion of sphalerite particles to bubbles in seawater was significantly lower than that in DI water and was significantly improved when EDTA was added to seawater, indicating that seawater has a

Zeta potential distribution analysis
The adsorption of chemical substances on the mineral interface can affect their electrochemical properties, which is directly manifested as the alteration of zeta potential [39][40][41][42].These substances include hydroxyl complexes, ions, and other molecules.Figure 8 shows the zeta potentials of galena and sphalerite in different systems at a pH of 10.0.
In seawater, the zeta potentials of the galena and sphalerite shifted positively, indicating adsorption of positively charged substances onto the surface of the minerals.The zeta potential of galena increased from -17.67 mV to -11.27 mV, with an increase of 6.40 mV.Similarly, the zeta potential of sphalerite increased from -25.48 mV to -12.89 mV, with an increase of 12.59 mV, which was significant higher than that of galena, indicating higher adsorption amount of positively charged substances on the sphalerite surface compared to galena.However, after the addition of EDTA, SHMP and SS, the Zeta potentials of both minerals shifted negatively, indicating that the addition of the three regulators reduced the adsorption of positively charged substances on the mineral surface, but did not return to the value of DI water.When the PBX collector was added to the DI water system, the zeta potentials of both minerals decreased significantly, indicating that the anionic PBX collector was strongly adsorbed onto the mineral surface.In the seawater system, after the addition of the collector, the galena zeta potential decreased from -11.27 mV to -20.74 mV (reduction was 9.47 mV), and the sphalerite zeta potential decreased from -12.89 mV to -18.93 mV (reduction was 6.04 mV), which was lower than that of galena.Therefore, the adsorption capacity of PBX on the sphalerite surface was much lower than that of galena in seawater.This is consistent with the aforementioned conclusion regarding contact angle.

XPS analysis
Tables 3 and 4 show the surface element contents of galena and sphalerite in the different systems, respectively.Notably, the Mg content of galena in seawater was 0.43%, and the O content increased from 17.95% to 24.15%, indicating that small amounts of substances containing Mg and O were adsorbed on the galena surface.Furthermore, the Mg content increased to 0.89% after the addition of EDTA.Additionally, the increase in the N content indicated that the EDTA-complexes were adsorbed on the galena surface.However, after the addition of SHMP and SS, the Mg content decreased to 0.24% and 0.31%, respectively, indicating that the addition of SHMP and SS reduced the adsorption of Mg-containing substances.
- Similarly, the Mg content of the sphalerite in the seawater was 0.52%, and the O content increased from 10.62% to 14.77%, indicating that substances containing Mg and O were adsorbed on the sphalerite surface.In addition, the Mg content on the sphalerite surface was higher than that of galena, indicating that more hydroxyl complexes containing Mg were adsorbed on the sphalerite surface.Furthermore, the Mg content increased to 0.89% after the addition of EDTA.Similarly, the increase in the N content indicated that the EDTA-complexes were adsorbed on the sphalerite surface.After adding SHMP and SS, the Mg content decreased to 0.25% and 0.38%, respectively, indicating that the addition of SHMP and SS reduced the adsorption of Mg-containing substances.Notably, the decrease in the C and S contents on the surfaces of both sulfide minerals in seawater may be owing to the inhibition of the PBX adsorption on their surfaces.[43].However, several studies have suggested that the primary mechanism of xanthate-adsorbing sphalerite is the formation of double xanthate on its surface.At pH 10, the seawater mainly generated Mg(OH) + complexes, including a small amount of Ca(OH) + complexes and hydroxide precipitation.
The coatings of such hydrophilic substances on the mineral surface prevent adsorption of the collector, thereby reducing mineral recovery.The addition of EDTA can produce complex reactions with calcium and magnesium ions to reduce the ion content in seawater.The addition of SHMP and SS can produce precipitate reactions with calcium and magnesium ions.On the other hand, SHMP and SS can disperse particles, weaken the attraction between complexes and mineral particles, and reduce their adsorption onto mineral surfaces [44,45].These agents can reduce the adverse effects of seawater on the adsorption of xanthate on both galena and sphalerite surfaces.The contact Angle of sphalerite after adding PBX in seawater is much smaller than that in DI water, indicating that seawater greatly affects the adsorption of PBX on sphalerite surface.Bubble particle attachment analysis can more directly reflect the decrease degree of hydrophobicity of galena and sphalerite in seawater.Zeta potential distribution analysis showed that the potential drop of galena was more obvious when PBX was added in seawater.XPS analysis shows that there is more calcium and magnesium content on sphalerite surface in seawater.The above studies show that the adsorption of hydroxyl complexes on galena surface was less than that on sphalerite surface.Accordingly, the adverse effect of seawater on galena flotation was slighter than that of sphalerite.Galena and sphalerite are the most representative sulfide minerals of Pb and Zn, respectively.Flotation separation of galena and sphalerite is a common process in industry, which generally adopts preferential flotation, that is, flotation of galena first, then flotation of sphalerite.The application of seawater in the flotation separation of galena and sphalerite can reduce the consumption of fresh water resources.This study shows that seawater inhibits galena and sphalerite to different extents.On this basis, the effect of seawater on the flotation separation of galena and sphalerite is worthy of further research.

Conclusions
This study investigated the effect of seawater on galena and sphalerite flotation.
•Seawater has a significantly adverse effect on sphalerite flotation and a slightly adverse effect on galena flotation.
•The adverse effect is mainly due to the formation of hydroxyl complexes on the surface of minerals under alkaline conditions, which affects the adsorption of the collector PBX on the galena and sphalerite surfaces.
•The slight effect of seawater on galena flotation is due to the weak adsorption of hydrophilic substances on the surface of galena, and the characteristics of seawater to promote foaming make up for the loss of galena hydrophobicity.
•The adverse effects can be eliminated to some extent by the use of EDTA, SHMP, and SS, although their mechanisms differ.EDTA mainly reduces the content of magnesium and calcium ions in seawater flotation through chelation.Conversely, SHMP and SS mainly alter the forces of the hydroxyl complex and mineral particles, thereby reducing the adsorption of complexes on mineral particles.
the adhesion of sphalerite particles to bubbles.

Figure 8 .
Figure 8. Zeta potential of galena and sphalerite in different flotation systems when pH=10.0.
Figure 1.XRD pattern of pure galena and sphalerite.

Table 2 .
The concentrations of main ions in artificial simulated seawater (g/L).
2.2.Microflotation testsFlotation test was carried on a XFG model machine produced by Jilin Exploration Machinery Factory, China.The spindle speed set to 1992 RPM. Potassium butyl xanthate (PBX) was used as collector and methyl isobutyl carbinol (MIBC) was used as frother.

Table 3 .
Atomic concentration on the galena surface.

Table 4 .
Atomic concentration on the sphalerite surface.
-dissociates in water, it reacts with Pb atoms on the galena surface, producing lead xanthate through a process known as chemisorption