1.   Introduction

Copper is a widely used metal in industry. Due to its excellent electrical conductivity, thermal conductivity, ductility, and plasticity, it is widely used in the chemical, medical, electronic, mechanical, national defense, energy, and construction fields [1–3]. Copper oxide and copper sulfide minerals are commonly used for the extraction of copper metal [4–5]. Because copper sulfide is consumed on a large scale, the available copper sulfide cannot meet the market demand. Therefore, the efficient utilization of copper oxide minerals is important for industrial production and the comprehensive utilization of resources [6–8].

Flotation is a common and economical mineralization method [9–11]. This method is divided into direct flotation and sulfidization flotation. Direct flotation can effectively enrich copper oxide minerals with simple ore properties. However, copper oxide minerals are generally of low grade, leading to poor flotation recovery by direct flotation, and the cost of efficient and selective reagents is high [12]. Therefore, sulfidization flotation is more effective in enriching this type of ore. During the sulfidization flotation process, S species interact with copper sites to form the corresponding copper sulfide species, which is beneficial for the adsorption of the collector, thereby improving the floatability of the mineral. During sulfidization flotation, the pH of the pulp is maintained between 8.5 and 9.5. Based on the relationship between the sulfur components and the pH of the Na₂S solution, the main component in the pulp is HS⁻ at this time. Various sulfidization products are generated on the surfaces of minerals [13–14]. The dosage of Na₂S affects the flotation of copper oxide. When the dosage is too low, the ions in the solution consume the sulfidization reagent, resulting in insufficient sulfidization and affecting the recovery of minerals. When the dosage of Na₂S is too high, the residual sulfur components consume the flotation reagents, thereby depressing mineral flotation [15].

Previous studies have found shortcomings in the application of sulfidization technology to copper oxide recovery, including the requirement of large amounts of sulfidizing agent, poor sulfidization efficiency, and unstable sulfidization layers, which result in the low flotation recovery of copper oxide. For a better flotation index of oxidized copper minerals, Zhang et al. [16] and Wang et al. [17] added Pb²⁺ and Cu²⁺ to the flotation process to activate the mineral surface, thereby promoting the sulfidization and flotation recovery. It has been shown that Pb²⁺ increases the number of interaction sites between mineral surfaces (such as malachite and chalcopyrite) and subsequent agents; Pb–S, Cu–S, and polysulfide species are generated, which enhance the stability of the sulfidization layer and promotes the interaction between samples and collectors, thereby improving the floatability of copper oxide minerals. Zhang et al. [18–19] observed that the concentration of Cu²⁺ in the pulp solution was too low, and a low number of Cu–S species and a high number of Pb–S species were found on the cerussite surface. However, when the concentration of Cu²⁺ was too high, the distribution of Pb²⁺ on the cerussite surface decreased, and the adsorption of Cu²⁺ covered the inherent Pb²⁺ on the sample surface, weakening the adsorption of the sulfur and xanthate components and resulting in a decrease in the flotation recovery. Dong et al. [20] observed that Cu²⁺ can also activate sulfide minerals, and arsenopyrite has good floatability under acidic conditions; however, the floatability of arsenopyrite decreases with increasing pH because it is easily oxidized to hydroxide components in an aqueous solution, which hinders the adsorption of xanthate. When Cu²⁺ is added, the concentration of SO${}_n^{2-}$ and Fe(III)–O components on the sample surface decreases, and Cu₂S components form on the surface, enabling arsenopyrite to obtain good floatability under alkaline conditions.

Previous studies found that Cu²⁺ and Pb²⁺ can activate malachite [17,21], which verifies that sulfur components can be adsorbed and promote the adsorption of collectors to improve their floatability. However, the mechanism of stepwise Cu²⁺ activation during the sulfidization flotation of malachite has not yet been reported. Therefore, it is necessary to study the activation mechanism of malachite flotation after stepwise activation with Cu²⁺. Zeta potential, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy and energy dispersive spectrometry (SEM–EDS), atomic force microscopy (AFM), and time-of-flight secondary ion mass spectroscopy (ToF–SIMS) experiments were used to explore the sulfidization products on the malachite surface. The effects of sulfidization and sulfidization product activity were determined during stepwise malachite activation with Cu²⁺. Fourier-transform infrared spectroscopy (FT-IR) characterization and contact angle measurement were used to study the action characteristics of the collector and the floatability of the malachite surface.

2.   Experimental

2.1   Mineral and reagents

Malachite samples were obtained from Yunnan Province, China. The handpicked samples were cleaned several times using ultrasonication, dried under vacuum, crushed manually, ground, and screened. The required particle sizes for the testing products were used in the flotation tests and other analytical experiments. The selected blocky samples were cut into regular cubes to meet detection requirements. As shown in the X-ray diffraction (XRD) test results (Fig. 1), the purity of the samples met the requirements for flotation studies and other detection methods. Analytically pure Na₂S·9H₂O, CuSO₄·5H₂O, terpineol oil, and sodium isoamyl xanthate (NaIX) were used as the flotation reagents. NaOH and HCl were used as the pH regulators. Deionized water was used in all the test processes.

Figure  1.  XRD pattern of malachite samples.

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2.2   Micro-flotation experiments

Flotation was conducted in a 40 mL XFG–II micro-flotation machine. A malachite sample (2 g) with particle sizes of 38–74 μm was used during the micro-flotation test. A CuSO₄ solution was used for pretreatment (for 5 min). Next, Na₂S, CuSO₄, NaIX, and terpineol oil were added successively, and the time required for reaction testing was determined after the addition of each reagent. After the reaction was complete, manual scraping (for 3 min) was performed to separate the concentrate from the tailings. All experiments were repeated three times, and the flotation recovery was calculated using the weighing method.

2.3   Zeta potential analysis

The zeta potentials of the malachite samples were determined using a Malvern Zetasizer. The malachite powders (0.05 g) with <5 μm particle sizes were used as test samples. The sample sequentially interacted with the flotation reagent for 5 min. During the experiment, pH was adjusted by adding HCl and NaOH solutions. After stirring, the solution was allowed to settle for 10 min, and the suspension was extracted to measure the mineral surface potential. The average of three zeta potential measurements is reported.

2.4   XPS investigation

The XPS investigation was performed using a PHI 5000 Versa Probe II instrument (ULVAC-PHI, Chigasaki, Japan). Two grams of the prepared malachite powder were used as test samples, and the flotation reagents were added in sequence. After the sample interacted with the flotation reagent for an appropriate time, the product was filtered and dried. Finally, the XPS spectra were analyzed using the MultiPak software.

2.5   ToF–SIMS characterization

The ToF–SIMS test instrument was produced by IONTOF (Germany). A block malachite was used as the experimental test sample. The required reagents were added to the reaction for 10 min and left to dry. During surface tests, the elemental properties of the samples were determined using a Bi${}_3^+$ (30 keV) primary ion source. During the profiling test, a Cs source (2 keV) was used as the profiling ion. The final experimental results were analyzed and presented using the IONTOF software.

2.6   SEM–EDS measurements

SEM–EDS measurements were performed using a MIRA LMS and Smart-EDX. The prepared block malachite samples were placed in a beaker, and different flotation reagents were added according to the test requirements. The test samples were subjected to a platinum spraying treatment, which increased the conductivity of the mineral surface and was removed during the semi-quantitative analysis. Finally, the samples were analyzed using Smart-EDX.

2.7   AFM measurements

The AFM characterization was performed using a dimension icon AFM, and selected 0.8 cm × 0.8 cm square peacock block malachite was used as the test sample. Before the experiment, the prepared malachite samples were cleaned and interacted with the reagent solution required for the test. Finally, conducted AFM detection after the sample was naturally dried.

2.8   FT-IR analysis

Fourier-transform infrared was characterized using a Nicolet iS50 infrared spectrometer. The malachite powder (below 38 μm) was used as a sample during analysis. First, ultrapure water (40 mL) was sequentially added and mixed with the flotation reagents. The dried product (approximately 1% by mass) was mixed with KBr (spectral purity) in an agate mortar and ground to <5 μm for subsequent spectral measurements. Finally, FT-IR spectra were recorded in the wavenumber range of 4000–400 cm⁻¹.

2.9   Contact angle measurements

Contact angle measurements were performed using a JY–82B contact angle analyzer. Before the experiment, 400, 1000, 2000, 3000, and 5000 mesh sandpapers were used to grind the malachite lump samples, and the final samples were cleaned via ultrasonication. Subsequently, the malachite sample was soaked in the required reagents according to the flotation test conditions and allowed to dry naturally for the contact angle test.

3.   Results and discussion

3.1   Effect of Cu²⁺ on the micro-flotation performance of malachite

The flotation recoveries of malachite using different reagent dosages are shown in Fig. 2. During sulfidization flotation, the pH of the pulp was adjusted to 9.3–9.5. In the single Cu²⁺ activation system (Cu²⁺ + Na₂S), an appropriate concentration of Cu²⁺ is beneficial to the sulfidization flotation of malachite. The flotation recovery reached its highest value of 39.00% when the concentration of Cu²⁺ is 3 × 10⁻⁴ mol·L⁻¹. However, the flotation of malachite was depressed with a continuous increase in the copper ion concentration. This indicates that adding low concentrations of Cu²⁺ before sulfidization can improve the flotation recovery due to an increase in the surface reactivity of malachite; however, high concentrations of Cu²⁺ would depress the flotation recovery, those results consistent with the results of Wang et al. [21]. In the stepwise Cu²⁺ activation system (Cu²⁺ + Na₂S + Cu²⁺), the flotation recovery of malachite increased to 84.18%, which was a 45.18% increase relative to the single activation system under the same conditions. This indicates that more active sites were generated on the mineral surface in the stepwise Cu²⁺ activation system, which promoted the reaction activity of the subsequent reagents. Therefore, it increases the floatability of malachite.

Figure  2.  Flotation recovery of malachite as a function of (a) Cu²⁺ and (b) Na₂S concentrations.

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The influence of the Na₂S concentration on malachite flotation in different systems was investigated. The concentration of Cu²⁺ is 3 × 10⁻⁴ mol·L⁻¹ in the first step activation, and the results are shown in Fig. 2. As shown in Fig. 2(b), the recovery of malachite initially increased and then decreased with increasing Na₂S concentration. This may be because the surface sulfidization of malachite is not sufficient when the amount of Na₂S is insufficient. The sulfidization effect of the mineral surface can be promoted when the amount of Na₂S is moderate, and the adsorption of the subsequent collector is increased. When the dosage of Na₂S is excessive, the residual S species in the pulp solution will be competitively adsorbed and xanthate species on the malachite surface, hindering the action of xanthate on the mineral surface and depressing the progress of flotation. Therefore, the concentration of Na₂S should be controlled to avoid excess residual ions in the pulp. In addition, the results also showed that the flotation recovery of malachite is highest after stepwise activation with Cu²⁺, with a maximum achievable 84.18%. The single Cu²⁺ activation is higher than that of direct sulfidization under the same conditions, indicating that compared with the system of direct sulfidization and single Cu²⁺ activation, stepwise activation with Cu²⁺ can increase the adsorption of the collector on the mineral surface and improve malachite floatability.

3.2   Zeta potentials of malachite treated with different reagents

Zeta potentials of malachite treated with different reagents are shown in Fig. 3. The surface potential of minerals shows a decreasing trend as the pH increases, which agrees with previous research [17]. After treatment with Cu²⁺, the malachite isoelectric point moved to the right, and the zeta potentials shifted to more positive values within the pH range, indicating that positively charged copper components were adsorbed to the mineral surface, thereby increasing the potential of the mineral surface and enhancing the mineral surface interaction with the subsequent negatively charged flotation reagents. The main components of Cu²⁺ in the solution at pH 5.9 are Cu²⁺ and Cu(OH)⁺ [18,22]. When the pH increased above 5.9, Cu(OH)_2(aq) became the main component of the solution. Therefore, at neutral pH, Cu²⁺, Cu(OH)⁺, and Cu(OH)_2(aq) were the main species. Compared with the surface during only Cu²⁺ activation, the potential of malachite treated with Na₂S shifted to a more negative direction; however, compared with the surface during direct sulfidization, the potential of malachite treated with Cu²⁺ and Na₂S shifted in a more positive direction, which indirectly indicates that the adsorption of negatively charged sulfur components on the malachite surface activated with Cu²⁺ was easier.

Figure  3.  Zeta potentials of malachite surfaces treated with different reagents: (a) malachite system and (b) copper ion activation system of malachite.

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As shown in Fig. 3(b), the zeta potentials moved in a positive direction in the stepwise Cu²⁺ activation system relative to the single Cu²⁺ activation. This demonstrates that the added Cu²⁺ was adsorbed on the sulfidized mineral surface, and the potential moved more positively, creating a favorable environment for the subsequent addition of flotation reagents. Compared with the zeta potentials of malachite untreated with NaIX, the zeta potentials were negatively shifted after the addition of NaIX, indicating that xanthate ions were adsorbed on the mineral. As shown in Fig. 3(b), after the addition of NaIX, the potential difference of the stepwise-activated surface was larger than that of the single-activated surface because Cu²⁺ increased the surface reactivity of the sample and promoted the sulfidization process. Therefore, the negatively charged xanthate ions are enhanced by the addition of Cu²⁺, resulting in a larger potential difference.

3.3   XPS analysis

The XPS of malachite was detected and shown in Fig. 4. Compared with the malachite sample untreated with Na₂S (Fig. 4(a) and (b)), the characteristic peak of S 2p appeared in the spectra after sulfidization of the malachite surface (Fig. 4(c) and (d)); the S 2p peak is more evident in the spectrum of malachite after stepwise Cu²⁺ activation (Fig. 4(d)). The atomic concentrations of specific elements on the malachite surface are listed in Table 1. Characteristic peaks of C 1s, O 1s, and Cu 2p were observed on the untreated malachite surface with atomic concentrations of 12.66%, 73.24%, and 14.10%, respectively. With the addition of Cu²⁺, the atomic concentrations of the C 1s, O 1s, and Cu 2p peaks were 12.85%，72.64% and 14.51%, respectively. This result shows that Cu²⁺ was adsorbed on the malachite surface, leading to a decrease in the atomic concentration of the O 1s peak and increase in the Cu 2p peak. For the stepwise activated surface, peaks corresponding to C, O, S, and Cu were observed at atomic concentrations of 12.74%, 69.88%, 2.61%, and 14.77%, respectively. These results show that after stepwise activation, the Cu concentration increased, and a higher concentration of S was observed for the stepwise activation system than for the single-activation system, indicating that Cu²⁺ acts on the sulfidized malachite surface. Stepwise Cu²⁺ activation promotes the sulfidization of the mineral surface, potentially because the Cu²⁺ and S components react and form Cu–S_n components, which change the chemical environment of the mineral surface, creating a favorable environment for subsequent reagent actions.

Figure  4.  XPS scan curves of malachite surfaces treated with (a) deionized water, (b) Cu²⁺, (c) Cu²⁺ + Na₂S, and (d) Cu²⁺ + Na₂S + Cu²⁺.

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Table  1.  Relative atomic concentrations of elements on malachite malachite under different experimental conditions %

Sample	C 1s	O 1s	S 2p	Cu 2p
Deionized water	12.66	73.24		14.10
Cu2+	12.85	72.64		14.51
Cu2++ Na2S	12.60	70.78	1.96	14.66
Cu2++ Na2S + Cu2+	12.74	69.88	2.61	14.77

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The O 1s spectrum of the untreated malachite surface could be resolved into two peaks at 531.46 and 532.32 eV, as shown in Fig. 5(a). Previous studies have shown that the position of the binding energy at 531.46 eV may be attributed to –Cu–O species, whereas the position of the binding energy at 532.32 eV may be attributed to –Cu–OH species [23–24]. After a single addition of Cu²⁺, the position of the binding energy of O 1s was shifted to 531.47 and 532.56 eV (Fig. 5(b)). Combined with the data listed in Table 2, the semi-quantitative proportion of total O in the –Cu–O species increased from 57.95at% to 63.38at%, and that of the –Cu–OH species decreased from 42.05at% to 36.62at%. The shift in the binding energy of the –OH species (0.24 eV) may be due to the interaction between the O species and the added Cu species, thereby forming more –Cu–O components. The O 1s binding energies of the single-activation system were 531.53 and 532.47 eV (Fig. 5(c)). The changes in the binding energies indicate that the O components reacted with the addition of S components. The position of binding energy was 531.57 and 532.44 eV when the malachite was stepwise activated (Fig. 5(d)). Compared with the O 1s XPS shown in Fig. 5(a)–(c), the proportion of –Cu–OH species was significantly reduced, and the reduction of hydrophilic species was beneficial for increasing the reaction activity of the mineral surface [25].

Table  2.  Binding energies and percentages in total O on malachite surfaces under different experimental conditions

Sample	Binding energy / eV			Percentage in total O / at%
	–Cu–O	–OH		–Cu–O	–OH
Deionized water	531.46	532.32		57.95	42.05
Cu2+	531.47	532.56		63.38	36.62
Cu2++ Na2S	531.53	532.47		77.47	22.53
Cu2++ Na2S + Cu2+	531.57	532.44		78.78	21.22

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Figure  5.  High-resolution O 1s XPS spectra of malachite surface treated with (a) deionized water, (b) Cu²⁺, (c) Cu²⁺ + Na₂S, and (d) Cu²⁺ + Na₂S + Cu²⁺.

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The characteristic XPS peaks of the S 2p spectrum are shown in Fig. 6. No obvious peak representing sulfur was observed in the S 2p characteristic peak of the unsulfidized malachite surface (Fig. 6(a) and (b)). In the S 2p spectra of the malachite surface after treatment with S species (Fig 6(c) and (d)), three pairs of S 2p peaks were fitted [26–27]. In the S 2p spectrum of the malachite surface after a single Cu²⁺ activation, the binding energies of S 2p are 162.07, 164.04, and 168.26 eV (Fig 6(c)). According to the location of binding energy, there may be the S²⁻, S${}_n^{2-}$, and SO${}_n^{2-}$ species, respectively. The S${}_n^{2-}$ species are slightly oxidized sulfide products that contribute to the increase of mineral surface activity. However, the SO${}_n^{2-}$ species are a product of excessive oxidation that is not conducive to the reactivity of mineral surfaces. When malachite was stepwise activated with Cu²⁺, the binding energies of S 2p shifted to 162.04, 164.29, and 168.18 eV. The proportion of SO${}_n^{2-}$ products on the surface was lower than that with single activation (Fig. 7). Combined with Table 1, the atomic concentrations of the S species increased with stepwise activation, indicating that highly active S species were adsorbed on the mineral surface and increased surface sulfidization, which improved the flotation behavior of malachite.

Figure  6.  High-resolution S 2p XPS spectra of malachite surfaces treated with (a) deionized water, (b) Cu²⁺, (c) Cu²⁺ + Na₂S, and (d) Cu²⁺ + Na₂S + Cu²⁺.

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Figure  7.  S components distribution of malachite surfaces treated with (a) Cu²⁺ + Na₂S and (b) Cu²⁺ + Na₂S + Cu²⁺.

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On the untreated malachite surface, the Cu 2p_3/2 and Cu 2p_1/2 peaks appeared at 935.12 eV (–Cu–O) and 954.92 eV (CuCO₃), respectively (Fig. 8(a)) [28–29], both of which are characteristic components of malachite (Cu₂(OH)₂CO₃). After Cu²⁺ treatment, the binding energies did not shift (Fig. 9(b)), demonstrating that solution-phase Cu²⁺ had negligible effect on the chemistry of the Cu atoms. However, as shown in Fig. 9, the atomic concentration of Cu increased slightly, indicating that the Cu components could be adsorbed onto the mineral surface. Double-peak characteristic peaks of copper were observed on the surface of the mineral (Cu 2p_3/2), as shown in Fig. 8(c) and (d). Owing to the sulfidization of the mineral surface, the intensities of the Cu 2p peaks corresponding to the copper sulfidization products increased. The binding energy of the peak at approximately 933 eV was attributed to the sulfidization products formed by the copper and sulfur species on the mineral surface. The binding energy of the bulk copper component (Cu(II)) on the malachite surface was approximately 935 eV [30]. The binding energy offset centered at 933.27 and 935.28 eV after stepwise Cu²⁺ activation. The Cu(I) and Cu(II) components accounted for 37.10at% and 62.90at% of the total Cu content, respectively. Compared with Fig. 8(c), the Cu(I) binding energy was significantly shifted, and the Cu(I) atomic concentration increased by 1.16%, accounting for 7.63% of the total Cu, which indicates that the yield and reactivity of the sulfidization products increased with stepwise Cu²⁺ activation.

Figure  8.  High-resolution Cu 2p XPS spectra of malachite surfaces treated with (a) deionized water, (b) Cu²⁺, (c) Cu²⁺ + Na₂S, and (d) Cu²⁺ + Na₂S + Cu²⁺.

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Figure  9.  Cu species distribution on malachite surfaces treated with (a) deionized water, (b) Cu²⁺, (c) Cu²⁺ + Na₂S, and (d) Cu²⁺ + Na₂S + Cu²⁺.

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3.4   ToF–SIMS analysis

The two-dimensional (2D) diagrams of the fragment peaks of S⁻, ${\text S}_2^-$, Cu⁺, and C₅H₁₁OCS${}_2^-$ under various reagent treatments are shown in Fig. 10. An S⁻ signal can be detected on the sulfidized malachite surface (Fig. 10(a)). Because no xanthate ions were added, only trace C₅H₁₁${\rm OCS}_2^-$ ion fragment signals could be detected. As shown in Fig. 10(b), the S⁻ and Cu⁺ signals were enhanced; this result indicates that the stepwise Cu²⁺ activation system can promote the sulfidization of the mineral surface because adsorbed Cu²⁺ forms Cu–S components, which provides active sites for subsequent NaIX adsorption. According to the above test results, the number of Cu–S components in the stepwise Cu²⁺ activation system increased. In the test condition with the collector (Fig. 10(c)), the S and C₅H₁₁${\rm OCS}_2^-$ signals were enhanced, and the Cu⁺ signal was weakened relative to the condition with the collector (Fig. 10(d)). Under the conditions of stepwise Cu²⁺ activation, the adsorption of C₅H₁₁OCS${}_2^-$ was enhanced, resulting in an increased S signal and increased adsorption of C₅H₁₁OCS${}_2^-$, thereby weakening the Cu⁺ signal. As indicated by zeta potential analysis, the adsorption of NaIX on the mineral surface was enhanced, which was also verified by ToF–SIMS.

Figure  10.  2D distribution images of secondary ions on malachite surfaces treated with (a) Cu²⁺ + Na₂S, (b) Cu²⁺ +Na₂S + Cu²⁺, (c) Cu²⁺ + Na₂S + NaIX, and (d) Cu²⁺ + Na₂S + Cu²⁺ + NaIX.

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The visual representation of S⁻, ${\text S}_2^-$, C₅H₁₁OC$\text{S}_2^-$, and ${\text{CO}}_3^-$ under various reagents treatment are shown in Fig. 11. The depth-profile curve corresponds to the distribution of negative ions, as shown in Fig. 12. For the single Cu²⁺ activation system, intersection of S⁻ and S${}_2^-$ and mineral body (CO${}_3^-$) curve at 38 and 58 s, respectively (Fig. 12(a)). For stepwise activation (Fig. 12(b)), the timing curve of the S⁻ signal intersects with the timing curve of ${\text {CO}}_3^-$ at 57 s. These results showed that the adsorption layer of S species on the malachite surface is thicker with stepwise activation. The three-dimensional (3D) distribution of S⁻ and ${\text S}_2^-$ (Fig. 11(a) and (b)) upper layer of CO${}_3^-$ distribution also verified this phenomenon.

Figure  11.  3D distribution images of negative ions on malachite surfaces treated with (a) Cu²⁺ + Na₂S, (b) Cu²⁺ +Na₂S + Cu²⁺, (c) Cu²⁺ + Na₂S + NaIX, and (d) Cu²⁺ + Na₂S + Cu²⁺ + NaIX.

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Figure  12.  ToF–SIMS negative-ion depth profile of malachite surfaces treated with (a) Cu²⁺ + Na₂S, (b) Cu²⁺ + Na₂S + Cu²⁺, (c) Cu²⁺ + Na₂S + NaIX, and (d) Cu²⁺ + Na₂S + Cu²⁺ + NaIX.

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The NaIX conditions were compared using the profile curve test results. According to Fig 12, the sputter time of S⁻ intersects with the timing curve of mineral body (${\mathrm{CO}}_3^-$) in Fig 12(d) is longer than Fig 12(c). Furthermore, in the complete depth profile, the sputter times of S⁻ intersects with the timing curve of ${\mathrm{CO}}_3^-$ were longer for stepwise activation than for single activation. Therefore, it can be determined that stepwise Cu²⁺ activation promoted the formation of sulfidization species and the adsorption of NaIX. The distribution of C5H11OCS₂⁻ on the malachite surface was denser under the conditions of stepwise activation (Fig. 11). This deeper distribution promotes the floatability of sulfidized malachite.

3.5   SEM–EDS and AFM measurements of malachite surface

SEM–EDS can be used to observe the microstructure of mineral surfaces and qualitatively and semi-quantitatively analyze the elemental composition, thereby providing a better understanding of the sulfidization characteristics of mineral surfaces and determining the changes in mineral surface morphology and material composition before and after flotation reagent action. AFM detection technology has usually been employed to detect mineral surface roughness and to explore the surface products of samples after the action of chemicals. Therefore, we used SEM–EDS and AFM to evaluate the surface of malachite under different sulfidization conditions, and the results are shown in Figs. 13 and 14.

Figure  13.  SEM–EDS of malachite surfaces treated with (a) Na₂S, and (b) Cu²⁺ + Na₂S.

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Figure  14.  AFM images of malachite surfaces treated with different reagents.

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As observed from the surface topography of malachite shown in Fig. 13(a), the generated sulfidization products were sparsely distributed and the particles were small, which indicated that the effect of direct sulfidization was poor and not conducive to flotation. The results of the atlas and element distribution maps indicate that the intrinsic elements C, O, and Cu of malachite have obvious signal strengths. The S signal was the sulfidization product generated after the malachite surface was sulfidized, and the content of the S element on the malachite surface was 4.59at%. According to the distribution map of elemental S, elemental S was distributed throughout the entire analysis interval, indicating that the generated granular products were sulfidization products (Fig. 13(b)). As shown in Fig. 14, compared with the AFM of the malachite surface, the surface morphology of the malachite treated with Na₂S is covered with a layer of columnar protruding structures. According to the AFM software, the value of R_q increased from 14.5 to 36.5 nm. Combined with the SEM–EDS and AFM results, the protruding columnar structure is a newly generated sulfidization product.

Compared with the direct sulfidization system, in a single Cu²⁺ + Na₂S system (Figs. 13(b) and 14(d)), more sulfidization products were generated, and the particles generated on the mineral surface became significantly larger and more varied. The distribution of S and Cu elements after treatment with Cu²⁺ is denser than the distribution of S and Cu on the mineral surface in the direct sulfidization system, with concentrations of 6.46at% and 74.30at%, respectively. In the activation system, the statistical results show that the highest R_q is 46.3 nm, which is significantly increased compared to the direct sulfidization system (R_q is 36.5 nm) and single Cu²⁺ system (R_q is 28.1 nm). This result further indicates that more Cu²⁺ species are adsorbed on the malachite surface after being modified by copper ions, which increases the number of active sites and thus enhances subsequent sulfidization.

3.6   FT-IR characterization of minerals surface

The FT-IR characterization results for the NaIX and malachite surfaces treated with different reagents are shown in Fig. 15. FT-IR spectra of NaIX showed the characteristic peak of –C=O, –C=S, and –O–C–S functional groups at wavenumbers of 1063, 1116, and 1151 cm⁻¹, respectively. Meanwhile, –C–H, –CH₂, and –CH₃ organic groups were observed at wavenumbers of 2873, 2930, and 2958 cm⁻¹, respectively. In addition, the –O–H group characteristic peaks are at 3385 and 1613 cm⁻¹ [31]. For untreated malachite, the wavenumbers near 3404 and 3314 cm⁻¹ are caused by –OH species, and the wavenumbers at 1493, 1390, and 1047 cm⁻¹ correspond vibrations of C${\text O}_3^-$ [32–33]. When xanthate species are added to the single copper activation system, the characterization peaks of xanthate species are detected on the mineral surface. The adsorption peaks at 2973 and 2919 cm⁻¹ were attributed to the stretching vibrations of –CH₂ and –CH₃, respectively, and the adsorption peak at 1196 cm⁻¹ is attributed to the bending vibration of –O–C–S. The intensity of the xanthate characterization peaks increased under stepwise activation, indicating that the adsorption capacity of the collector (NaIX) significantly increased.

Figure  15.  FT-IR spectra of NaIX and malachite surfaces treated with different reagents.

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3.7   Contact angle measurements of malachite surfaces

The contact angle is an important parameter used to determine the hydrophobicity of a mineral surface. Therefore, the contact angles of the malachite surfaces treated with different reagents were tested, and the results are shown in Fig. 16. The contact angle of the untreated malachite surface was 52.82°, indicating that malachite was hydrophilic [21,25]. After adding Na₂S and the collector, the hydrophobicity of the sample mineral surface increased to 73.25° (Fig. 16(b)), indicating that surface sulfidization increased the floatability of the sample. In the single Cu²⁺ activation system, the hydrophobicity of the sample increases (77.32°) (Fig. 16(c)). When malachite was stepwise activated with Cu²⁺, the hydrophobicity of the malachite surface reached a high value (87.59°) (Fig. 16(d)). This demonstrates that the addition of Cu²⁺ was beneficial to sample reactivity due to the enhanced sulfidization and further increased adsorption of the collector on the surface. Based on the above experimental results, after copper ion activation, the surface enhanced the sulfidization and adsorption of NaIX. This adsorption enhances hydrophobicity and increases flotation recovery.

Figure  16.  Contact angles of malachite surfaces treated with (a) deionized water, (b) Na₂S + NaIX, (c) Cu²⁺ + Na₂S + NaIX, and (d) Cu²⁺ + Na₂S + Cu²⁺ + NaIX.

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4.   Conclusions

In this study, micro-flotation experiments, zeta potential, XPS, ToF–SIMS, SEM–EDS, AFM, FT-IR, and contact angle analyses were used to study the sulfidization flotation mechanism of malachite treated with single and stepwise activation of Cu²⁺. This study provides new technological reserves for the effective enrichment of oxidized copper ores. The specific experimental conclusions are as follows.

(1) The floatability of malachite can be significantly enhanced with stepwise Cu²⁺ activation. The recovery of malachite in the stepwise activation system was 45.18% greater than that in the single activation system.

(2) The Cu²⁺ stepwise activation of malachite enhanced the sulfidization. More S components formed on the malachite surface and more sulfidization products with high activity were generated in the Cu²⁺ stepwise activation system. This provides favorable conditions for the subsequent adsorption of the collector.

(3) After stepwise Cu²⁺ activation, stronger chemical adsorption of the collector was observed. ToF–SIMS and FT-IR analyses indicated that a thick and dense xanthate layer was formed on the surface. The contact angle test indicated that the surface hydrophobicity significantly increased after Cu²⁺ stepwise activation.