Strengthening Sulfidation Flotation of Hemimorphite via Pretreatment with Pb²⁺

Abstract

The conventional sulfidation-xanthate flotation process that consists of sulfidization with sodium sulfide, activation by heavy-metal ions, and collection with xanthate is not sufficiently efficient for treating hemimorphite, and the dosages of the sulfurizing reagent and metal ions are large. In this study, the sulfidation flotation (Pb²⁺ + Na₂S + Pb²⁺ + xanthate) of hemimorphite was strengthened by pretreating with Pb²⁺ before sulfidation. Microflotation test results indicated that the recovery of hemimorphite increased by 5–10% after pretreatment with Pb²⁺. The comprehensive results of adsorption experiments, scanning electron microscopy–energy-dispersive X-ray spectroscopy, atomic force microscopy, and X-ray photoelectron spectroscopy indicated that a large amount of Pb²⁺ was adsorbed on the hemimorphite surface and entered the lattice, forming Zn_(4−x)Pb_xSi₂O₇(OH)₂·H₂O. The newly formed component had an increased amount of surface sulfidation active sites and had the effect of induced crystallization, making the surface more effective for sulfidation. After the Pb²⁺ was added to the pulp, a large number of uniform and dense PbS species were formed on the hemimorphite surface, increasing the number of adsorption sites for xanthate and reducing the competitive adsorption of residual S²⁻ on the xanthate.

1. Introduction

Zn is an important nonferrous metal that is mainly used in galvanization and Zn alloys [1,2]. The most important source of Zn metal is the zinc sulfide mineral, followed by zinc oxide ores [3,4]. With the depletion of easy-enrichment zinc sulfide minerals, increasing attention has been paid to the utilization of zinc oxide ores in recent years [5,6]. Zinc oxide ores mainly include smithsonite (ZnCO₃), hemimorphite (Zn₄(H₂O)[Si₂O₇](OH)₂), and hydrozincite (Zn₅(CO₃)₂(OH)₆) [7]. Sulfidation–flotation is the primary method used to treat zinc oxide ores. Zinc oxide ores are first sulfurized with a sulfidizing agent, and then flotation is achieved by using cationic collectors (such as amines) or anion collectors (such as xanthate). The sulfidation process plays a dominant role in the sulfidation–flotation method, and the effect of sulfidation mainly depends on the mineral crystal structure, surface active components, etc [8,9].

Hemimorphite is a typical nesosilicate. In its crystal lattice, the Zn atom and the three O atoms belonging to the Si–O tetrahedrons and one hydroxyl group form a new tetrahedron [10]. During crushing and grinding, the bonding strength of Si–O bonds is higher than that of Zn–O bonds; thus, the Zn–O bonds are generally broken. When a Zn–O fracture occurs, SiO₄ and Zn bonds are often exposed to the surface of the mineral. However, due to the larger volume of SiO₄, the active component of Zn is often shielded by SiO₄ on the hemimorphite surface, and this surface exhibits a similar surface property to quartz under most conditions. Such structural properties lead to low-solubility minerals, low activity of surface Zn atoms, and low mobility of Zn²⁺; moreover, it is difficult for sulfurizing ions such as S²⁻ and HS^- to combine with Zn²⁺, resulting in an extremely low sulfidation rate of hemimorphite [11,12].

In actual production, sulfidation–amine flotation is more widely used than sulfidation–xanthate flotation. However, there is an unavoidable disadvantage in the processing of low-grade zinc oxide ore using amine flotation: it is sensitive to slime. In order to obtain a qualified zinc concentrate, it is necessary to desliming before the flotation, causing a large amount of zinc metal loss in the slime, rendering this technology unsuitable for zinc oxide ore flotation especially when the ore contains a lot of slime. By contrast, xanthate flotation is much more insensitive to slime [13,14,15], however, it is necessary to add metal ions such as Cu²⁺, Pb²⁺, Zn²⁺, and Ag⁺, to activate the sulfurized zinc oxide ore [16]. Researchers have studied the mechanism of sulfidation and the activation of metal ions in this system. Zhang et al. [17] found that sodium sulfide could react with the hemimorphite surface, generating ZnS. The ZnS reduced the polarity and hydrophilicity of the mineral surface. However, it also reduced the zeta potential of the mineral surface, resulting in the poor flotation results. Jia et al. [18] found that amorphous or poorly crystalline ZnS was formed on the surface of hemimorphite after hemimorphite was conditioned with Na₂S. The formation mechanism of the PbS species mainly comprised an ion-exchange process in which the added Pb²⁺ replaced the Zn²⁺ in ZnS, and the newly formed PbS species was cubic “synthetic galena” with a highly crystalline structure. This explains why it is necessary to add heavy-metal ions after sulfurization in sulfidation-xanthate flotation systems.

Although the flotation performance of hemimorphite can be significantly improved by adding metal ions after sulfidation, this method is still not sufficiently efficient for treating hemimorphite, and the introduction of a large number of heavy-metal ions increases the production cost and is harmful to the environment [19]. In this study, attempts were made to enhance the hemimorphite sulfidation and reduce the dosages of the sulfidizing agent and activator via pre-activation using Pb²⁺. Microflotation experiments were performed to compare the flotation performance effects of strengthening sulfidation (with Pb²⁺ pretreatment) and conventional sulfidation (without Pb²⁺ pretreatment). The strengthening mechanism of the hemimorphite sulfidation flotation was systematically investigated via adsorption experiments, scanning electron microscopy (SEM), atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS). Our research is of great significance because it enriches the sulfidation flotation theory of zinc oxide ores and improves the resource-utilization efficiency of zinc oxide ores.

2. Materials and Methods

2.1. Materials and Reagents

The high-purity hemimorphite used in all the tests was obtained from Lanping mine, China. First, a hammer was used to knock down high-purity lump ores to obtain samples for SEM, AFM, and XPS analysis. Selected samples were crushed, hand-sorted, ground, and sieved to obtain a product with a particle size of 45–105 μm for microflotation and adsorption experiments. The X-ray diffraction (XRD, D8 Advance, Bruker, Saarbrücken, Germeny) pattern of the powders is shown in Figure 1. Chemical analysis and the XRD results indicated that the purity of the samples was >95%, which satisfied the purity requirements for single-mineral flotation experiments.

Analytical-grade sodium sulfide (Na₂S·9H₂O) and lead nitrate (Pb(NO₃)₂) were used as the sulfidizing agent and activator, respectively. Sodium isoamyl xanthate (NaIX) and methyl isobutyl carbinol (MIBC) were employed as the collector and frother, respectively. Sodium hydroxide (NaOH) and hydrochloric acid (HCl) were employed as pH modifiers. All the reagents were purchased from Aladdin Chemical Reagent Co. Ltd., Shanghai, China. Ultrapure water (18 MΩ·cm) was used in all the experiments and was prepared by passing deionized water through a laboratory water-purification system (CASCADA І, Pall ForteBio Analytics (Shanghai) Co., Ltd., China).

2.2. Micro-Flotation Experiments

The flotation machine used in the flotation experiments was the XFGC II with a cell volume of 40 mL, and the impeller speed was maintained at 1992 rpm. In each test, a 2.00-g pure hemimorphite sample was placed in the flotation cell with 30 mL of ultrapure water. Then, specified amounts of Pb(NO₃)₂ (when Pb²⁺ pre-activation was needed), Na₂S·9H₂O, and Pb(NO₃)₂ were added in sequence, followed by conditioning for 5 min. Subsequently, the collector (NaIX, 8 × 10⁻⁵ mol/L) and frother (MIBC, 1 × 10⁻⁴ mol/L) were added sequentially with conditioning times of 2 and 1 min, respectively. Next, flotation tests were performed, with a total time of 3 min. In each experiment, with or without Pb²⁺ pre-activation, the molar ratio of Pb(NO₃)₂ to Na₂S was 1:1. The concentrate products obtained from the flotation were collected, filtered, weighed, and dried; then, the hemimorphite recovery was calculated.

2.3. Adsorption Experiments

Before the experiments, ion solutions with the desired concentrations were freshly prepared, and the Pb and S concentrations were determined accurately. Pb²⁺ adsorption measurement: 1.00 g of pure hemimorphite was placed in a vial containing 80 mL of a Pb²⁺ solution, followed by 5 min of magnetic stirring. The supernatant was then centrifuged, and the residual Pb²⁺ was measured accurately via an inductively coupled plasma optical emission spectrometer (iCAP 7000 Plus Series, Thermo Fisher Scientific, Waltham, MA, USA). S²⁻ adsorption measurement: A comparative experiment of S²⁻ adsorption was performed, in which the cases with and without the addition of Pb²⁺ before sulfidation at different concentrations of Na₂S·9H₂O were compared. Similarly, 1.00 g of pure hemimorphite was dispersed in 80 mL of the Pb(NO₃)₂ solution with magnetic stirring for 5 min. Then, Na₂S·9H₂O solutions with certain concentrations (based on the microflotation experiment conditions) were added, and the reaction was allowed to proceed for another 5 min. After centrifugation, the total residual amount of S in the supernatant solution was analyzed via inductively coupled plasma atomic emission spectroscopy (ICPS-1000ІІ, Shimadzu, Japan).

2.4. SEM Measurements

SEM (Quanta 250, Thermo Fisher Scientific, Waltham, MA, USA) was performed to observe the surface morphology of the hemimorphite samples, which were treated with conventional sulfidation or the strengthening-sulfidation process. The reagents were added in the same way as for the microflotation experiments. The concentrations of Na₂S·9H₂O and Pb(NO₃)₂ added in the conventional sulfuration process were both 3 × 10⁻³ mol/L, and the concentration of Pb²⁺ used for pretreatment was 1 × 10⁻⁴ mol/L. After adsorption, the physically adsorbed substance containing a black precipitation of PbS on the mineral surface was washed away by flowing water. Finally, the samples were dried under vacuum and used for analysis. Owing to their poor conductivity, it was necessary to sputter an Au film on the surface using a sputtering coater.

2.5. AFM Measurements

The sample surface for AFM measurements must be extremely smooth. The high-purity lump ores were cut into 10 × 10 × 5 mm³ pieces; then, the surface for morphological analysis was successively polished with #400, #600, #1200, #2500, and #5000 sandpaper sheets. The sample preparation methods for conventional sulfidation and strengthening sulfidation were identical to those for the SEM. Additionally, a blank control group that was conditioned with only deionized water for 5 min was used. The three samples were employed for analysis after vacuum drying. AFM measurements were performed using a 5500AFM instrument (Agilent, Santa Clara, CA, USA). The main operating parameters were as follows: the contact mode, a scanning range of 0–9 μm, and a scanning rate of 5 Hz.

2.6. XPS Measurements

The sample processing for XPS (ESCALAB250, Thermo Fisher Scientific, Waltham, MA, USA) was the same as that for the SEM measurements. Untreated samples and samples treated with Pb²⁺ (1 × 10⁻⁴ mol/L), Na₂S (3 × 10⁻³ mol/L), or Pb²⁺ (1 × 10⁻⁴ mol/L) + Na₂S (3 × 10⁻³ mol/L) were prepared for the XPS measurements. The main operating parameters were as follows: an Al/Mg double-anode target, a micro-focused monochromatic X-ray source, an energy resolution of 0.6 eV, and a spatial resolution of <3 µm. The spectral peaks were fitted using the Avantage software, and the C1s peak at 284.8 eV was obtained to calibrate all of the X-ray photoelectron spectra.

3. Results and Discussion

3.1. Microflotation

The effect of the Pb²⁺ concentration in the stage of pre-activation on the recovery of hemimorphite was investigated via microflotation experiments. The results are shown in Figure 2. They indicate that at the beginning, the recovery of hemimorphite slowly increased with the increasing Pb²⁺ concentration. The best recovery occurred when the Pb²⁺ concentration was 5 × 10⁻⁵ mol/L; however, the recovery decreased significantly for higher Pb²⁺ concentrations. The results reveal that the low Pb²⁺ concentration used for the pre-activation improved the flotation performance of the hemimorphite. This is attributed to an increase in the number of active sites on the hemimorphite surface that reacted with S²⁻ [20,21]. However, increasing the Pb²⁺ concentration could have reduced the concentration of S²⁻ in the flotation pulp, thus reducing the sulfidation efficiency.

Additional microflotation experiments were performed to compare the effects of hemimorphite sulfidation flotation with and without Pb²⁺ pretreatment at different concentrations of Na₂S·9H₂O. A comparison of the flotation results is shown in Figure 3. The recovery of the unpretreated hemimorphite increased with the Na₂S·9H₂O concentration in the range of 5 × 10⁻⁴ to 10 × 10⁻⁴ mol/L (the recovery increased from 43.25% to a maximum value of 77.20%). With the further increase in the concentration of Na₂S·9H₂O, the recovery declined, indicating that the flotation of hemimorphite was inhibited by excessive sodium sulfide [22]. The hemimorphite pretreated with Pb²⁺ exhibited the same tendency as the unpretreated samples, and the recovery reached a maximum value of 82.56% when the Na₂S·9H₂O concentration was only 9 × 10⁻⁴ mol/L. Notably, in the range of 5 × 10⁻⁴ to 10 × 10⁻⁴ mol/L, the floatability of the Pb²⁺-pretreated hemimorphite exhibited a significant improvement (5–10%) over that of the unpretreated samples. These results indicate that pretreatment with Pb²⁺ can not only improve the flotation of hemimorphite but also reduce the dosages of Na₂S·9H₂O and Pb(NO₃)₂.

3.2. Adsorption Experiments

The active sites on the surface of hemimorphite can significantly affect the sulfurizing effect and flotation performance. Ion-adsorption experiments were performed to investigate the adsorption capacity of the hemimorphite for Pb²⁺ and the adsorption capacity for S²⁻ with and without Pb²⁺ pretreatment. The absorptivity of 1.00 g of hemimorphite for Pb²⁺ at different concentrations of Pb(NO₃)₂ is shown in Figure 4. The results indicate that the hemimorphite had a strong adsorption capacity for Pb²⁺, and most of the Pb²⁺ was adsorbed to the surface of the hemimorphite samples in the range of concentrations tested. According to the results shown in Figure 3, the optimum concentration of Pb²⁺ for hemimorphite pretreatment in the microflotation experiments was 5 × 10⁻⁵ mol/L, and under this condition, the absorptivity reached 96.46%. It was assumed that the adsorption of Pb²⁺ on the hemimorphite surface was ideal monolayer adsorption and that all the adsorbed Pb²⁺ formed sulfidation active sites. The active-site density was calculated using the following formula:

$${\mathsf{\rho }}_{\mathrm{a}}=\frac{\mathrm{A}}{{\mathrm{S}}_{\mathrm{a}}}$$

(1)

where ρ_a represents the active-site density (mol/μm²), A represents the adsorbing capacity per unit mass (mol/g), and S_a represents the specific surface area (m²/g).

The specific surface area of the hemimorphite powder (particle size of 45–105 μm) was 11.233 m²/g, as measured via the Brunauer–Emmett–Teller method. The active-site density of Pb²⁺ on the hemimorphite surface was calculated as 3.435 × 10⁵ mol/μm² under the optimum flotation conditions.

According to the results of the microflotation experiments, the initial Na₂S·9H₂O concentration for the S²⁻ adsorption contrast tests were set as 5 × 10⁻⁴ to 13 × 10⁻⁴ mol/L, with steps of 1 × 10⁻⁴ mol/L, and the concentration for Pb²⁺ pretreatment was 5 × 10⁻⁵ mol/L. The comparison results for the S²⁻ adsorption are presented in

Table 1. Residual S concentration in the solution with and without Pb²⁺ pretreatment for different initial Na₂S·9H₂O concentrations.

Initial Na2S·9H2O Concentration (mol/L)	Residual S Concentration in Pulp Solution (mg/L)
	Without Pb2+ Pretreatment	With Pb2+ Pretreatment
5 × 10−4	2.464	1.280
6 × 10−4	3.331	1.772
7 × 10−4	4.652	2.731
8 × 10−4	6.117	4.023
9 × 10−4	7.701	4.929
10 × 10−4	8.928	6.925
11 × 10−4	11.273	9.226
12 × 10−4	14.015	11.723
13 × 10−4	16.732	13.628

. When the initial Na₂S·9H₂O concentration was low, the content of residual S was low; consequently, the S had a negligible influence on the subsequent flotation. This finding indicates that regardless of the pretreatment with Pb²⁺, the flotation behavior was limited by a deficiency of S²⁻ in the solution, which made the surface insufficient for hemimorphite sulfurization. With an increase in the Na₂S·9H₂O concentration, the hemimorphite could adsorb more S²⁻, which interacted on the surface, enhancing the degree of sulfurization and improving the flotation efficiency. When the sodium sulfide concentration was >10 × 10⁻⁴ mol/L, the adsorption capacity for S²⁻ on the mineral surface reached the “upper limit”, and additional sodium sulfide did not increase the S²⁻ adsorption. Moreover, the residual S²⁻ exhibited strong competitive adsorption with xanthate, which was detrimental to the collection performance [22]. This is the main reason why the flotation of hemimorphite was inhibited at high Na₂S·9H₂O concentrations.

As 0.177 × 10⁻⁵ mol/L (0.367 ppm) Pb²⁺ remained in the solution after pretreatment, it consumed an equivalent amount of S²⁻ (0.057 ppm). The comparison results indicate that regardless of the initial concentration of Na₂S·9H₂O in the solution, the residual S concentration in the solution after 5 min of Pb²⁺ pretreatment was lower than that without pretreatment. The recovery of hemimorphite subjected to the conventional sulfidation process was maximized when the concentration of Na₂S·9H₂O was 10 × 10⁻⁴ mol/L, at which 7.21 × 10⁻⁴ mol/L S²⁻ was adsorbed on the hemimorphite surface. The recovery of hemimorphite subjected to the strengthening-sulfidation process was maximized at a Na₂S·9H₂O concentration of only 9 × 10⁻⁴ mol/L, at which the adsorption capacity of the hemimorphite for S²⁻ was 7.46 × 10⁻⁴ mol/L. This indicates that the surfaces of the hemimorphite samples pretreated with Pb²⁺ absorbed more S²⁻ from the solution. A sufficient amount of S²⁻ on the surface of hemimorphite is beneficial for the subsequent activation of heavy-metal ions and adsorption of collectors. Additionally, the amount of residual S in the Pb²⁺-pretreated solution was smaller than that in the untreated solution; thus, the competitive adsorption of xanthate was relatively low [23,24,25]. These phenomena are consistent with microflotation.

3.3. SEM

The two-dimensional surface morphologies of the hemimorphite samples after sulfidation with and without Pb²⁺ pretreatment was observed via SEM. The secondary electron (SE) imaging results for hemimorphite treated with sodium sulfide alone are shown in Figure 5a. The surface was relatively uniform, with no obvious material generation and a large amount of natural fracture produced by crushing.

Due to its high atomic number (82), the Pb element exhibited a white color in the BE images. The phenomena of accumulation (Figure 5) and uniform distribution (Figure 6) were observed on the surfaces of the samples treated with conventional sulfidation and the strengthening-sulfidation process. In the hemimorphite samples treated with conventional sulfidation, several small bright masses were detected, the largest of which was a nearly 20-μm round bright region (Figure 5b). Energy-dispersive X-ray spectroscopy (EDS) confirmed that these masses were accumulated PbS species on the surface of the sample, and the Pb and S contents in the square marked area were 5.30% and 8.16%, respectively. The contents of O, Si, and Zn were lower than those of the samples treated with sodium sulfide alone. These PbS species were formed by the reaction of Pb ions and S²⁻ in the solution and were then adsorbed on the mineral surface. The deposits were not easily removed from the mineral surface, even when the surface was repeatedly washed with high-purity water. This is one of the reasons—in addition to the ion-exchange reaction between Pb ions and zinc sulfide forming lead sulfide—why Pb ions can activate sulfidized hemimorphite [18,26]. The surfaces of the samples pretreated with Pb²⁺ exhibited numerous irregular bright regions. They occupied an area that was at least five times larger than that occupied by the irregular bright regions on the hemimorphite treated with conventional sulfidation (Figure 5c). EDS results indicated that the Pb and S contents in the marked square area were 14.10% and 20.58%, respectively, which were more than twice those for the conventional sulfidation, and the contents of O, Si, and Zn decreased to 16.51%, 3.02%, and 12.19%, respectively. Clearly, the pre-adsorption of Pb²⁺ made the hemimorphite sample adsorb more PbS species and aggregate them into a group.

The SEM results (Figure 6) for the non-accumulated area of two sulfide samples revealed that regardless of whether the surfaces were pretreated with Pb²⁺, there were numerous white spots on the surfaces of both samples. The SEM–EDS results indicated that these white spots represent PbS species. The surface of the strengthening-sulfidation sample had more and denser white spots than that of the conventional-sulfidation sample. These spots represent PbS species grown on the hemimorphite; that is, Pb ions were first adsorbed, and then sulfide ions reacted with these adsorbed Pb ions. The newly formed PbS species were tightly bound to the solid surface and provided primary reagent adsorption sites for subsequent flotation. The increased PbS was attributed to the pre-adsorption of Pb ions on the hemimorphite surface, which induced crystallization [27,28,29]. When sodium sulfide was added to the flotation pulp, the surface of the hemimorphite that adsorbed the Pb ions reacted rapidly with the sulfide ions, forming PbS, and the hemimorphite surface had better effect of induced crystallization.

3.4. AFM

AFM was performed to observe the morphological changes in three dimensions for the hemimorphite surfaces treated with conventional sulfidation and strengthening sulfidation. The raw hemimorphite is shown in Figure 7a. As expected, the hemimorphite had a relatively smooth and uniform surface after manual polishing. The root-mean-square (RMS) surface roughness of the sample was 17.5 nm, and the scratches created by the sandpaper during polishing were observed. Figure 7b shows the surface morphology of the hemimorphite surface after conventional sulfidation treatment. Compared with that of the control group, it had rough, round protrusions. These protrusions may correspond to PbS species with a size of approximately 1 μm that were generated on the surface and irregularly distributed, similar to isolated islands. The RMS surface roughness of this sample was 47.8 nm. Figure 7c shows the surface morphology of the hemimorphite sample strengthened with sulfidation after Pb²⁺ pretreatment. Compared with the sample treated with conventional sulfidation, this sample had numerous round protrusions almost covering the entire scanning area on its surface. The RMS surface roughness of the sample was 71.0 nm. Comprehensive SEM and AFM results, it indicates that the pretreatment with Pb²⁺ increased the surface roughness of the hemimorphite, owing to the formation of lead sulfide on the mineral surface [30,31,32].

3.5. XPS

To investigate the strengthening mechanism of hemimorphite sulfidation, XPS analyses for S and Pb were performed. Figure 8 shows the high-resolution s²p X-ray photoelectron spectra, indicating the peaks for hemimorphite samples treated under different conditions. For the sample treated with Na₂S·9H₂O alone, the s²p³ peak (Figure 8a) with a binding energy of 161.34 eV and a doublet feature separated by 1.2 eV was consistent with ZnS species, indicating that the adsorbed S ions reacted with the surface of hemimorphite, forming ZnS [18,33,34,35]. In the presence of Na₂S·9H₂O + Pb²⁺ (Figure 8b), the s²p peaks were fitted reasonably well with two components around 161.39 and 160.58 eV, which correspond to the characteristic values of S in ZnS and PbS, respectively [30,36,37]. As expected, the ZnS content was reduced after the addition of Pb²⁺, indicating that some of the zinc sulfide was converted into PbS [26]. Figure 8c presents the s²p spectra of hemimorphite treated with the strengthening-sulfidation process. As before, the two peaks of ZnS and PbS were observed. However, the PbS content was significantly higher than that of the hemimorphite treated with conventional sulfidation, which is consistent with the SEM results.

Figure 9a presents the high-resolution Pb4f X-ray photoelectron spectra for hemimorphite treated with Pb²⁺ (1 × 10⁻⁴ mol/L) alone. The Pb4f peak was fitted into a doublet, with a peak area ratio of 4:3 and a binding-energy split of 4.8 eV. The Pb4f7 peak located at 139.13 eV corresponds to the newly generated component Zn_(4−x)Pb_xSi₂O₇(OH)₂·H₂O [18]. This indicates that the Pb²⁺ was adsorbed on the surface after the treatment and entered the hemimorphite lattice. In the range of pH values with good floatability, the following reactions may have occurred on the mineral surface [18,38].

$${\mathrm{Zn}}_4{\mathrm{Si}}_2{\mathrm{O}}_7{(\mathrm{OH})}_2\cdot {\mathrm{H}}_2\mathrm{O}+{x\mathrm{PbOH}}^{+}\to {\mathrm{Zn}}_{4-x}{\mathrm{Pb}}_x{\mathrm{Si}}_2{\mathrm{O}}_7{\left(\mathrm{OH}\right)}_2\cdot {\mathrm{H}}_2\mathrm{O}+{x\mathrm{ZnOH}}^{+}$$

(2)

Figure 9b shows the Pb4f peak of the sulfurized hemimorphite sample after pretreatment with Pb²⁺. The very weak Pb4f7 peak located at 139.15 eV corresponds to Zn_(4−x)Pb_xSi₂O₇(OH)₂·H₂O, whose amount was significantly smaller than that in the sample to which only Pb²⁺ was added. The stronger doublet with a Pb4f7 binding energy of 137.50 eV is attributed to PbS species [18,30,39,40]. The authors have reason to believe that the PbS species were formed by a reaction between Zn_(4−x)Pb_xSi₂O₇(OH)₂·H₂O and sodium sulfide [18]. Thus, Zn_(4−x)Pb_xSi₂O₇(OH)₂·H₂O can be considered as the sulfidation active site.

In summary, the sulfidation process of hemimorphite was strengthened by pretreating the hemimorphite with Pb²⁺, which improved the floatability. This was attributed to an increase in PbS species on the hemimorphite surface [22]. The increase was due to two factors: 1) the pre-adsorption of Pb ions increased the adsorption of S ions, which reacted with the subsequently added Pb ions to form more PbS species; and 2) the PbS formed on the hemimorphite surface had an induced crystallization effect on the subsequently formed PbS [22,41,42].

4. Conclusions

Microflotation experiments revealed that in the sulfidation-xanthate system, the sulfidation process of hemimorphite could be strengthened through pretreatment with a low concentration of Pb²⁺. The recovery of hemimorphite was improved by 5–10% compared with that of conventional sulfidation flotation; furthermore, smaller dosages of sodium sulfide and lead nitrate could be used. Ion-absorption experiments indicated that the hemimorphite powders had a large adsorption capacity for Pb²⁺, and the amount of sulfidation active sites on the surface could be increased. Thus, more S²⁻ was adsorbed from solutions, making the surface more effective for sulfuration. SEM–EDS and AFM revealed that Pb²⁺ pretreatment increased the amount of PbS species that could be generated and distributed on the strengthening-sulfidation surface of hemimorphite, making the surface rougher. XPS revealed that the pretreatment of hemimorphite with Pb²⁺ produced Zn_(4−x)Pb_xSi₂O₇(OH)₂·H₂O on the mineral surface, which provided sulfidation active sites.

The results of this study are helpful for the utilization of hemimorphite and provide theoretical support for the development of strengthening-sulfidation flotation of zinc oxide ores. In the next study, we will apply this technology to the flotation of actual ore and hope that this will have a positive effect on the recovery of zinc oxide ore.