**1. Introduction**

In copper sulfide extractive metallurgy, the most efficient and widely applied separation technique is froth flotation, for which the adsorption of the collector on the mineral surface is vital to achieving satisfactory flotation efficiency and metal recovery. By now, some works have been carried out to clarify the adsorption mechanism of collectors on copper sulfides. [1–6]. For the flotation of chalcocite, xanthate and dialkyl dithiophosphate have been applied as collectors in flotation practice.

For the case of the adsorption of xanthate on chalcocite surface in an aqueous solution, the work has attracted the interest of many researchers. For example, Gaudin and Schuhmann [1] studied the solubility of potassium ethyl xanthate and its reaction products in organic solvents and firstly proposed there was initially chemisorbed xanthate (X−) on chalcocite surface followed by insoluble cuprous xanthate (CuX). Allison et al. [3] studied the reaction products of various sulfide minerals with xanthate solutions and reported that the measured rest potential of chalcocite in 6.25 <sup>×</sup> <sup>10</sup>−<sup>4</sup> M KEX solution at pH 7 was +60 mV and the reaction product of PAX on chalcocite was cuprous xanthate. Mielczarski and Suoninen [7,8] applied X-ray photoelectron spectroscopy (XPS) and studied the adsorption of potassium ethyl xanthate on cuprous sulfide. It was reported that there was a relatively rapid formation of a well-oriented monolayer of xanthate ions followed by the slow growth of disordered cuprous xanthate molecules on top of this layer. Richardson et al. [9] carried

**Citation:** Zhang, J.; Zhang, W. AFM Image Analysis of the Adsorption of Xanthate and Dialkyl Dithiophosphate on Chalcocite. *Minerals* **2022**, *12*, 1018. https:// doi.org/10.3390/min12081018

Academic Editors: Jan Zawala and Jean-François Blais

Received: 30 June 2022 Accepted: 11 August 2022 Published: 13 August 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

out an electrochemical study of chalcocite flotation by using ethyl xanthate as a collector. It was proposed that cuprous xanthate and possibly cupric xanthate were adsorbed hydrophobic species on chalcocite surfaces due to the exchange and charge transfer oxidation reactions. Leppinen et al. [10] applied an in situ Fourier-transform infrared spectroscopy (FTIR) study of ethyl xanthate adsorption on sulfide minerals under conditions of controlled potential and they concluded that, for chalcocite, there were three different potential regions of xanthate adsorption, including chemisorption, copper ethyl xanthate formation on the surface and multilayer formation.

Some studies have been carried out to study the adsorption of dialkyl dithiophosphate on the chalcocite surface and therefore its impact on chalcocite flotation. Goold and Finkelstein [11] reported that only Cu(DTP)<sup>2</sup> was present on copper sulfide. Solozhenkin and Koptsia [12] found cuprous diethyl dithiophosphate (CuDTP) and cupric diethyl dithiophosphate (Cu(DTP)2). Chander and Fuerstenau [13] studied the effect of potassium diethyl dithiophosphate (DTP) on the electrochemical properties of copper sulfide in aqueous solutions. It was reported that multilayers of reaction products, i.e., CuDTP and Cu(DTP)<sup>2</sup> were formed when copper sulfide was immersed in reagent solutions of dithiophosphate.

Zhang et al. [14] studied the hydrophobic flocculation of marmatite fines in aqueous suspensions by the addition of butyl xanthate (KBX) and ammonium dibutyl dithiophosphate (ADD) using laser particle size analysis, microscopy analysis, electrophoretic light scattering, contact angle measurement and infrared spectroscopy. It was claimed that the chemisorption of butyl xanthate ions or dibutyl dithiophosphate ions on marmatite resulted in hydrophobic flocculation, and therefore a greatly improved flotation response of marmatite because of the formation of flocs.

Recently, Dhar et al. [15] investigated the interaction of dithiophosphate and a mixture of xanthate and dithiophosphate collector on copper ore sample using zeta potential, quantitative adsorption, FTIR studies and Hallimond tube flotation. It was reported that using this mixture of collectors can improve both the grade and recovery of copper flotation concentrate.

All the studies reviewed above have revealed a lot of information, such as the reaction, product and mechanism, of the adsorption of xanthate and dithiophosphate collectors on chalcocite surface. It is also of great interest to directly obtain the image of collectors on mineral surfaces changing with pulp chemistry, such as pH and chemical dosage. For example, the AFM imaging technique has been widely used in the surface characterization of various materials, and recently the technique has been successfully applied to study in situ the adsorption of chemicals on the surface of minerals [16–22]. The novel analysis method has greatly expanded the understanding of the impact of solution chemistry on the collectors' adsorption on the mineral surface and the flotation mechanism as well.

In the present investigation, an AFM image analysis technique has been applied to obtain the surface morphology of chalcocite in various collectors' solutions, i.e., KEX, PAX and dialkyl dithiophosphate, at different pHs. By comparing the AFM images obtained under different conditions, such as the collector's type, dosage, and contact time, one can study the impact of water chemistry on the adsorption of collectors on chalcocite. The image analysis results will help answer some of the questions, for example, what is the morphology of the adsorbate on the chalcocite surface? What is the impact of the collector's dosage, the contact time and solutions' pH on the adsorption of the collector on the chalcocite surface? What is the binding strength between the adsorbate and chalcocite? What is the adsorption morphology on chalcocite by using a mixed collectors scheme of xanthate and dialkyl dithiophosphate? All this information will help clarify the reaction and adsorption mechanisms of xanthate and dialkyl dithiophosphate on chalcocite changing with solutions' chemistry, and therefore its impact on chalcocite flotation in industry practice.

#### **2. Materials and Methods**

#### *2.1. Materials*

Research-grade chalcocite (Cu2S) was obtained from Wards Natural Science Establishment Inc. (Rochester, NY, USA). Mineral samples were finely polished by consecutively using #800, #1200 and #2400 sandpaper, and then a diamond-polishing paste of 10, 5, 2.5 and 1 microns. (MTI Inc., Richmond, CA, USA) Mineral samples were further cleaned by rinsing thoroughly with ethanol and water. A 1.2 cm × 1.2 cm sample was used for the AFM surface image analysis. The DI (deionized) water used in the present work had a conductivity of 18.2 MΩ·cm−<sup>1</sup> at 22 ◦C and surface tension of 72.8 mN/m at 22 ◦C. Potassium amyl xanthate (PAX, >98%), potassium ethyl xanthate (KEX, >98%) and NaOH (>99%) were obtained from Alfa Aesar and used without further purification. Cytec Aerofloat 238 (sodium di-butyl dithiophosphate) was obtained from Cytec (Tempe, AZ, USA). Chemical solutions were freshly prepared at various concentrations and pH levels as needed each time right before an experiment was carried out.

#### *2.2. AFM Surface Image Analysis*

AFM surface image measurements were carried out with a Digital Instrument Nanoscope IIID (Veeco, San Jose, CA, USA) AFM using the contact mode at room temperature (22 ± 1 ◦C). SNL cantilevers were obtained from Veeco, San Jose, CA, USA. Triangular Si3N<sup>4</sup> cantilevers with a nominal spring constant of 0.12~0.58 N/m were used for both AFM imaging and force measurements.

To study the mineral surface in water, surface image measurements were carried out after 5 mL of DI water was gently injected into an AFM fluid cell. Extreme care was taken to avoid the entrapment of air in the cell. After surface images were collected in water, a 10 mL solution of a specific chemical's concentration was flushed through the liquid cell, and the cell was left undisturbed for the adsorption of chemicals on the mineral surface. AFM image analysis measurement was commenced after the exposure of the mineral plate to the chemical solution for a specific time. The AFM images as reported in this study, which were processed by no image modification other than being flattened, include both height and deflection images obtained in the contact mode. The same silicon nitride probe was applied to obtain the AFM image of the mineral plate in the solutions at different conditions.

#### **3. Results**

#### *3.1. AFM Image of Mineral Surface in Xanthate Solutions*

Figure 1 shows the surface images of a bare chalcocite surface that has been covered by nanopure water in an AFM liquid cell for 10 min. Figure 1A is the 10 µm × 10 µm height image, from which one can see that the solid surface is still largely smooth with very few adsorbates on the sample surface detected by the AFM probe. Figure 1B is the 3-D image of Figure 1A. Figure 1C is the section analysis of Figure 1A. Figure 1D is the deflection image of Figure 1A with a 10 nm data scale.

**Figure 1.** AFM images of a chalcocite surface in water for 10 min. (**A**) The 10 μm × 10 μm height image with a data scale of 20 nm; (**B**) the 3‐D image; (**C**) the section analysis (the red arrows indicate the top and the bottom of an average asperity); and (**D**) the deflection image with a data scale of 10 nm. **Figure 1.** AFM images of a chalcocite surface in water for 10 min. (**A**) The 10 µm × 10 µm height image with a data scale of 20 nm; (**B**) the 3-D image; (**C**) the section analysis (the red arrows indicate the top and the bottom of an average asperity); and (**D**) the deflection image with a data scale of 10 nm.

Figure 2A is the height image of a chalcocite surface in 5 × 10−<sup>5</sup> M KEX solution at pH 6 for 10 min. Compared to Figure 1A, a lot of adsorbate shows up on chalcocite when the mineral surface contacts the xanthate solution for 10 min. Figure 2B is the 3‐D image of Figure 2A. Figure 2C is the section analysis of Figure 2A and the surface roughness clearly increases due to the adsorption. Figure 2D is the deflection image of Figure 2A with a 10 Figure 2A is the height image of a chalcocite surface in 5 <sup>×</sup> <sup>10</sup>−<sup>5</sup> M KEX solution at pH 6 for 10 min. Compared to Figure 1A, a lot of adsorbate shows up on chalcocite when the mineral surface contacts the xanthate solution for 10 min. Figure 2B is the 3-D image of Figure 2A. Figure 2C is the section analysis of Figure 2A and the surface roughness clearly increases due to the adsorption. Figure 2D is the deflection image of Figure 2A with a 10 nm data scale.

nm data scale.

**Figure 2.** AFM images of a chalcocite surface in 5 × 10<sup>−</sup><sup>5</sup> M KEX solution at pH 6 for 10 min. (**A**) The 5 μm × 5 μm height image with a data scale of 20 nm; (**B**) the 3‐D image; (**C**) the section analysis (the red arrows indicate the top and the bottom of an average asperity); and (**D**) the deflection image with a data scale of 10 nm. **Figure 2.** AFM images of a chalcocite surface in 5 <sup>×</sup> <sup>10</sup>−<sup>5</sup> M KEX solution at pH 6 for 10 min. (**A**) The 5 µm × 5 µm height image with a data scale of 20 nm; (**B**) the 3-D image; (**C**) the section analysis (the red arrows indicate the top and the bottom of an average asperity); and (**D**) the deflection image with a data scale of 10 nm.

Figure 3A is the height image of a chalcocite surface in 1 × 10−<sup>4</sup> M KEX solution at pH 6 for 10 min. Compared to Figure 1A, a lot of adsorbate exists on chalcocite when the mineral surface contacts 1 × 10−<sup>4</sup> M KEX solution for 10 min. In addition, by comparing it to Figure 2A, one can find that the mineral surface becomes rougher, suggesting more precipitates are forming at the solid/liquid interface, when the KEX's concentration in‐ creases from 5 × 10−<sup>5</sup> M to 1 × 10−<sup>4</sup> M. Figure 3B is the 3‐D image of Figure 3A. Figure 3C is the section analysis of Figure 3A and it confirms that the surface roughness increases due to the adsorption of the collector. Figure 3D is the deflection image of Figure 3A with a 10 Figure 3A is the height image of a chalcocite surface in 1 <sup>×</sup> <sup>10</sup>−<sup>4</sup> M KEX solution at pH 6 for 10 min. Compared to Figure 1A, a lot of adsorbate exists on chalcocite when the mineral surface contacts 1 <sup>×</sup> <sup>10</sup>−<sup>4</sup> M KEX solution for 10 min. In addition, by comparing it to Figure 2A, one can find that the mineral surface becomes rougher, suggesting more precipitates are forming at the solid/liquid interface, when the KEX's concentration increases from 5 <sup>×</sup> <sup>10</sup>−<sup>5</sup> M to 1 <sup>×</sup> <sup>10</sup>−<sup>4</sup> M. Figure 3B is the 3-D image of Figure 3A. Figure 3C is the section analysis of Figure 3A and it confirms that the surface roughness increases due to the adsorption of the collector. Figure 3D is the deflection image of Figure 3A with a 10 nm data scale.

nm data scale.

**Figure 3.** AFM images of a chalcocite surface in 1 × 10<sup>−</sup><sup>4</sup> M KEX solution at pH 6 for 10 min. (**A**) The 5 μm × 5 μm height image with a data scale of 20 nm; (**B**) the 3‐D image; (**C**) the section analysis (the red arrows indicate the top and the bottom of an average asperity); and (**D**) the deflection image with a data scale of 10 nm. **Figure 3.** AFM images of a chalcocite surface in 1 <sup>×</sup> <sup>10</sup>−<sup>4</sup> M KEX solution at pH 6 for 10 min. (**A**) The 5 µm × 5 µm height image with a data scale of 20 nm; (**B**) the 3-D image; (**C**) the section analysis (the red arrows indicate the top and the bottom of an average asperity); and (**D**) the deflection image with a data scale of 10 nm.

Figure 4 shows the AFM images of a chalcocite surface in 5 × 10−<sup>4</sup> M KEX solution at pH 6 for 10 min. By comparing Figure 4A, the height image, to Figure 1A, one can observe a lot of adsorbate showing up on chalcocite after the mineral surface contacts the xanthate solution. In addition, by comparing it to Figures 2A and 3A, one can find that, when the KEX's concentration increases, more precipitates are forming at the solid/liquid interface and the mineral surface becomes much rougher. Figure 4B is the 3‐D image of Figure 4A. Figure 4C, the section analysis of Figure 4A, confirms that the surface roughness increases greatly when the chalcocite surface contacts a high concentration of KEX. Figure 4D is the deflection image of Figure 4A with a 10 nm data scale. Figure <sup>4</sup> shows the AFM images of a chalcocite surface in 5 <sup>×</sup> <sup>10</sup>−<sup>4</sup> M KEX solution at pH 6 for 10 min. By comparing Figure 4A, the height image, to Figure 1A, one can observe a lot of adsorbate showing up on chalcocite after the mineral surface contacts the xanthate solution. In addition, by comparing it to Figures 2A and 3A, one can find that, when the KEX's concentration increases, more precipitates are forming at the solid/liquid interface and the mineral surface becomes much rougher. Figure 4B is the 3-D image of Figure 4A. Figure 4C, the section analysis of Figure 4A, confirms that the surface roughness increases greatly when the chalcocite surface contacts a high concentration of KEX. Figure 4D is the deflection image of Figure 4A with a 10 nm data scale.

**Figure 4.** AFM images of a chalcocite surface in 5 × 10<sup>−</sup><sup>4</sup> M KEX solution at pH 6 for 10 min. (**A**) The 10 μm × 10 μm height image with a data scale of 20 nm; (**B**) the 3‐D image; (**C**) the section analysis (the red arrows indicate the top and the bottom of an average asperity); and (**D**) the deflection image with a data scale of 10 nm. **Figure 4.** AFM images of a chalcocite surface in 5 <sup>×</sup> <sup>10</sup>−<sup>4</sup> M KEX solution at pH 6 for 10 min. (**A**) The 10 µm × 10 µm height image with a data scale of 20 nm; (**B**) the 3-D image; (**C**) the section analysis (the red arrows indicate the top and the bottom of an average asperity); and (**D**) the deflection image with a data scale of 10 nm.

Figure 5A is the height image of a chalcocite surface in 5 × 10−<sup>4</sup> M KEX solution at pH 6 for 20 min and further rinsed with 10 mL ethanol and 5 mL water consecutively. The images are finally obtained when the mineral sample is placed in water. Compared to Figure 4A, one can see that the precipitates as observed in Figure 4A still exist on the chalcocite surface without being dissolved and rinsed off from the mineral surface. Figure 5B is the 3‐D image of Figure 5A. Figure 5C is the section analysis of Figure 5A. Figure 5D is the deflection image of Figure 5A with a 10 nm data scale. Figure 5A is the height image of a chalcocite surface in 5 <sup>×</sup> <sup>10</sup>−<sup>4</sup> M KEX solution at pH 6 for 20 min and further rinsed with 10 mL ethanol and 5 mL water consecutively. The images are finally obtained when the mineral sample is placed in water. Compared to Figure 4A, one can see that the precipitates as observed in Figure 4A still exist on the chalcocite surface without being dissolved and rinsed off from the mineral surface. Figure 5B is the 3-D image of Figure 5A. Figure 5C is the section analysis of Figure 5A. Figure 5D is the deflection image of Figure 5A with a 10 nm data scale.

**Figure 5.** AFM images of a chalcocite surface in 5 × 10<sup>−</sup><sup>4</sup> M KEX solution at pH 6 for 30 min and further rinsed with ethanol and water. (**A**) The 10 μm × 10 μm height image with a data scale of 20 nm; (**B**) the 3‐D image; (**C**) the section analysis (the red arrows indicate the top and the bottom of an average asperity); and (**D**) the deflection image with a data scale of 10 nm. **Figure 5.** AFM images of a chalcocite surface in 5 <sup>×</sup> <sup>10</sup>−<sup>4</sup> M KEX solution at pH 6 for 30 min and further rinsed with ethanol and water. (**A**) The 10 µm × 10 µm height image with a data scale of 20 nm; (**B**) the 3-D image; (**C**) the section analysis (the red arrows indicate the top and the bottom of an average asperity); and (**D**) the deflection image with a data scale of 10 nm.

Figure 6 shows the AFM images of a chalcocite surface in 1 × 10−<sup>5</sup> M PAX solution at pH 6 for 10 min. By comparing Figure 6A, the height image, to Figure 1A, one can find that there is a little adsorbate showing up on chalcocite when the mineral surface contacts PAX solution at pH 6 for 10 min. Figure 6B is the 3‐D image of Figure 6A. Figure 6C, the section analysis of Figure 6A, shows that the surface roughness increases a little bit due to the adsorption of PAX. Figure 6D is the deflection image of Figure 6A with a 10 nm data Figure <sup>6</sup> shows the AFM images of a chalcocite surface in 1 <sup>×</sup> <sup>10</sup>−<sup>5</sup> M PAX solution at pH 6 for 10 min. By comparing Figure 6A, the height image, to Figure 1A, one can find that there is a little adsorbate showing up on chalcocite when the mineral surface contacts PAX solution at pH 6 for 10 min. Figure 6B is the 3-D image of Figure 6A. Figure 6C, the section analysis of Figure 6A, shows that the surface roughness increases a little bit due to the adsorption of PAX. Figure 6D is the deflection image of Figure 6A with a 10 nm data scale.

scale.

**Figure 6.** AFM images of a chalcocite surface in 1 × 10<sup>−</sup><sup>5</sup> M PAX solution at pH 6 for 10 min. (**A**) The 10 μm × 10 μm height image with a data scale of 20 nm; (**B**) the 3‐D image; (**C**) the section analysis (the red arrows indicate the top and the bottom of an average asperity); and (**D**) the deflection image with a data scale of 10 nm. **Figure 6.** AFM images of a chalcocite surface in 1 <sup>×</sup> <sup>10</sup>−<sup>5</sup> M PAX solution at pH 6 for 10 min. (**A**) The 10 µm × 10 µm height image with a data scale of 20 nm; (**B**) the 3-D image; (**C**) the section analysis (the red arrows indicate the top and the bottom of an average asperity); and (**D**) the deflection image with a data scale of 10 nm.

Figure 7 shows the AFM images of a chalcocite surface in 5 × 10−<sup>5</sup> M PAX solution at pH 6 for 10 min. By comparing Figure 7A, the height image, to Figure 1A, one can observe that there is a lot of adsorbate showing up on chalcocite when the mineral surface contacts 5 × 10−<sup>5</sup> M PAX solution for 10 min. By comparing Figure 7A to Figure 6A, one can also find that when the PAX's concentration increases from 1 × 10−<sup>5</sup> M to 5 × 10−<sup>5</sup> M, the mineral surface becomes much rougher with more precipitates forming at the solid/liquid inter‐ face. Figure 7B is the 3‐D image of Figure 7A. Figure 7C is the section analysis of Figure 7A and it confirms that the surface roughness increases with increasing the PAX's concen‐ tration. Figure 7D is the deflection image of Figure 7A with a 10 nm data scale. Figure <sup>7</sup> shows the AFM images of a chalcocite surface in 5 <sup>×</sup> <sup>10</sup>−<sup>5</sup> M PAX solution at pH 6 for 10 min. By comparing Figure 7A, the height image, to Figure 1A, one can observe that there is a lot of adsorbate showing up on chalcocite when the mineral surface contacts <sup>5</sup> <sup>×</sup> <sup>10</sup>−<sup>5</sup> <sup>M</sup> PAX solution for 10 min. By comparing Figure 7A to Figure 6A, one can also find that when the PAX's concentration increases from 1 <sup>×</sup> <sup>10</sup>−<sup>5</sup> M to 5 <sup>×</sup> <sup>10</sup>−<sup>5</sup> M, the mineral surface becomes much rougher with more precipitates forming at the solid/liquid interface. Figure 7B is the 3-D image of Figure 7A. Figure 7C is the section analysis of Figure 7A and it confirms that the surface roughness increases with increasing the PAX's concentration. Figure 7D is the deflection image of Figure 7A with a 10 nm data scale.

**Figure 7.** AFM images of a chalcocite surface in 5 × 10<sup>−</sup><sup>5</sup> M PAX solution at pH 6 for 10 min. (**A**) The 10 μm × 10 μm height image with a data scale of 20 nm; (**B**) the 3‐D image; (**C**) the section analysis (the red arrows indicate the top and the bottom of an average asperity); and (**D**) the deflection image with a data scale of 10 nm. **Figure 7.** AFM images of a chalcocite surface in 5 <sup>×</sup> <sup>10</sup>−<sup>5</sup> M PAX solution at pH 6 for 10 min. (**A**) The 10 µm × 10 µm height image with a data scale of 20 nm; (**B**) the 3-D image; (**C**) the section analysis (the red arrows indicate the top and the bottom of an average asperity); and (**D**) the deflection image with a data scale of 10 nm.

Figure 8 shows the AFM images of a chalcocite surface in 1 × 10−<sup>4</sup> M PAX solution at pH 6 for 10 min. By comparing Figure 8A, the height image, to Figure 1A, one can observe that there is a lot of adsorbate showing up on chalcocite when the mineral surface contacts 1 × 10−<sup>4</sup> M PAX solution for 10 min. By comparing Figure 8A to Figures 6A and 7A, one can observe that when the PAX's concentration increases, the mineral surface becomes much rougher with more precipitates forming at the solid/liquid interface. Figure 8B is the 3‐D image of Figure 8A. Figure 8C, the section analysis of Figure 8A, shows that the surface roughness increases with increasing the PAX's concentration. Figure 8D is the de‐ flection image of Figure 8A with a 10 nm data scale. Figure <sup>8</sup> shows the AFM images of a chalcocite surface in 1 <sup>×</sup> <sup>10</sup>−<sup>4</sup> M PAX solution at pH 6 for 10 min. By comparing Figure 8A, the height image, to Figure 1A, one can observe that there is a lot of adsorbate showing up on chalcocite when the mineral surface contacts <sup>1</sup> <sup>×</sup> <sup>10</sup>−<sup>4</sup> M PAX solution for 10 min. By comparing Figure 8A to Figures 6A and 7A, one can observe that when the PAX's concentration increases, the mineral surface becomes much rougher with more precipitates forming at the solid/liquid interface. Figure 8B is the 3-D image of Figure 8A. Figure 8C, the section analysis of Figure 8A, shows that the surface roughness increases with increasing the PAX's concentration. Figure 8D is the deflection image of Figure 8A with a 10 nm data scale.

**Figure 8.** AFM images of a chalcocite surface in 1 × 10<sup>−</sup><sup>4</sup> M PAX solution at pH 6 for 10 min. (**A**) The 10 μm × 10 μm height image with a data scale of 20 nm; (**B**) the 3‐D image; (**C**) the section analysis (the red arrows indicate the top and the bottom of an average asperity); and (**D**) the deflection image with a data scale of 10 nm. **Figure 8.** AFM images of a chalcocite surface in 1 <sup>×</sup> <sup>10</sup>−<sup>4</sup> M PAX solution at pH 6 for 10 min. (**A**) The 10 µm × 10 µm height image with a data scale of 20 nm; (**B**) the 3-D image; (**C**) the section analysis (the red arrows indicate the top and the bottom of an average asperity); and (**D**) the deflection image with a data scale of 10 nm.

To verify the adsorption of the PAX on chalcocite in solution, after Figure 8 is ob‐ tained, a 10 μm × 5 μm area is scanned for one time with a much larger scan force being applied to intentionally remove the adsorbate. Further, the same position is scanned again in a 13 μm × 13 μm area with a normal scan force being applied and the result is shown in Figure 9. One can see from Figure 9A, the height image, that a 10 μm × 5 μm blank 'win‐ dow' is shown in the center of the image due to the removal of the adsorbate from the mineral surface under the previously applied large scan force. That is, the 'window' in the center is the bare chalcocite surface and the surrounding area is the mineral surface still To verify the adsorption of the PAX on chalcocite in solution, after Figure 8 is obtained, a 10µm × 5µm area is scanned for one time with a much larger scan force being applied to intentionally remove the adsorbate. Further, the same position is scanned again in a 13 µm × 13 µm area with a normal scan force being applied and the result is shown in Figure 9. One can see from Figure 9A, the height image, that a 10 µm × 5 µm blank 'window' is shown in the center of the image due to the removal of the adsorbate from the mineral surface under the previously applied large scan force. That is, the 'window' in the center is the bare chalcocite surface and the surrounding area is the mineral surface still covered by the adsorbate without being disturbed by the applied large scan force. By

covered by the adsorbate without being disturbed by the applied large scan force. By com‐ paring Figure 9B, the 3‐D image of Figure 9A, to Figure 8B, one can easily observe a pit

'window' (as shown by the green markers) and the surrounding area being covered with

comparing Figure 9B, the 3-D image of Figure 9A, to Figure 8B, one can easily observe a pit existing on the mineral surface with adsorbate covering the surrounding area. Figure 9C is the section analysis of Figure 9A and it shows the height difference between the blank 'window' (as shown by the green markers) and the surrounding area being covered with adsorbate (as shown by the red markers). Figure 9D is the deflection image of Figure 9A with a 10 nm data scale. *Minerals* **2022**, *12*, x FOR PEER REVIEW 12 of 34 adsorbate (as shown by the red markers). Figure 9D is the deflection image of Figure 9A with a 10 nm data scale.

**Figure 9.** AFM images of a chalcocite surface in 1 × 10<sup>−</sup><sup>4</sup> M PAX solution at pH 6 for 20 min. (**A**) The 13 μm × 13 μm height image with a data scale of 20 nm; (**B**) the 3‐D image; (**C**) the section analysis (the arrows indicate the top and the bottom of average asperities of different zones); and (**D**) the deflection image with a data scale of 10 nm. **Figure 9.** AFM images of a chalcocite surface in 1 <sup>×</sup> <sup>10</sup>−<sup>4</sup> M PAX solution at pH 6 for 20 min. (**A**) The 13 µm × 13 µm height image with a data scale of 20 nm; (**B**) the 3-D image; (**C**) the section analysis (the arrows indicate the top and the bottom of average asperities of different zones); and (**D**) the deflection image with a data scale of 10 nm.

Figure 10 shows the AFM images of a chalcocite surface in 5 × 10−<sup>5</sup> M KEX solution at pH 10 for 10 min. By comparing Figure 10A, the height image to Figure 1A, one can find a lot of adsorbate showing up on chalcocite when the mineral surface contacts the KEX Figure <sup>10</sup> shows the AFM images of a chalcocite surface in 5 <sup>×</sup> <sup>10</sup>−<sup>5</sup> M KEX solution at pH 10 for 10 min. By comparing Figure 10A, the height image to Figure 1A, one can find a lot of adsorbate showing up on chalcocite when the mineral surface contacts the

solution. Figure 10B is the 3‐D image of Figure 10A. Figure 10C is the section analysis of

KEX solution. Figure 10B is the 3-D image of Figure 10A. Figure 10C is the section analysis of Figure 10A and the surface roughness clearly increases due to the adsorption of KEX. Figure 10D is the deflection image of Figure 10A with a 10 nm data scale. *Minerals* **2022**, *12*, x FOR PEER REVIEW 13 of 34

**Figure 10.** AFM images of a chalcocite surface in 5 × 10<sup>−</sup><sup>5</sup> M KEX solution at pH 10 for 10 min. (**A**) The 10 μm × 10 μm height image with a data scale of 20 nm; (**B**) the 3‐D image; (**C**) the section analysis (the red arrows indicate the top and the bottom of an average asperity); and (**D**) the deflec‐ tion image with a data scale of 10 nm. **Figure 10.** AFM images of a chalcocite surface in 5 <sup>×</sup> <sup>10</sup>−<sup>5</sup> M KEX solution at pH 10 for 10 min. (**A**) The 10 µm × 10 µm height image with a data scale of 20 nm; (**B**) the 3-D image; (**C**) the section analysis (the red arrows indicate the top and the bottom of an average asperity); and (**D**) the deflection image with a data scale of 10 nm.

Figure 11 shows the AFM images of a chalcocite surface in 1 × 10−<sup>4</sup> M KEX solution at pH 10 for 10 min. Figure 11A is the height image, from which one can see a lot of adsorbate showing up on chalcocite. In addition, by comparing it to Figure 10A, one can find that the mineral surface becomes much rougher, suggesting more precipitates are forming at the solid/liquid interface, when the KEX's concentration increases from 5 × 10−<sup>5</sup> M to 1 × 10−<sup>4</sup> M at pH 10. Figure 11B is the 3‐D image of Figure 11A. Figure 11C, the section analysis Figure <sup>11</sup> shows the AFM images of a chalcocite surface in 1 <sup>×</sup> <sup>10</sup>−<sup>4</sup> M KEX solution at pH 10 for 10 min. Figure 11A is the height image, from which one can see a lot of adsorbate showing up on chalcocite. In addition, by comparing it to Figure 10A, one can find that the mineral surface becomes much rougher, suggesting more precipitates are forming at the solid/liquid interface, when the KEX's concentration increases from 5 <sup>×</sup> <sup>10</sup>−<sup>5</sup> M to <sup>1</sup> <sup>×</sup> <sup>10</sup>−<sup>4</sup> <sup>M</sup> at pH 10. Figure 11B is the 3-D image of Figure 11A. Figure 11C, the section

of Figure 11A, confirms that the surface roughness increases with increasing the concen‐

analysis of Figure 11A, confirms that the surface roughness increases with increasing the concentration of KEX. Figure 11D is the deflection image of Figure 11A with a 10 nm data scale. *Minerals* **2022**, *12*, x FOR PEER REVIEW 14 of 34

**Figure 11.** AFM images of a chalcocite surface in 1 × 10<sup>−</sup><sup>4</sup> M KEX solution at pH 10 for 10 min. (**A**) The 10 μm × 10 μm height image with a data scale of 20 nm; (**B**) the 3‐D image; (**C**) the section analysis (the red arrows indicate the top and the bottom of an average asperity); and (**D**) the deflec‐ tion image with a data scale of 10 nm. **Figure 11.** AFM images of a chalcocite surface in 1 <sup>×</sup> <sup>10</sup>−<sup>4</sup> M KEX solution at pH 10 for 10 min. (**A**) The 10 µm × 10 µm height image with a data scale of 20 nm; (**B**) the 3-D image; (**C**) the section analysis (the red arrows indicate the top and the bottom of an average asperity); and (**D**) the deflection image with a data scale of 10 nm.

Figure 12 shows the AFM images of a chalcocite surface in 5 × 10−<sup>4</sup> M KEX solution at pH 10 for 10 min. By comparing Figure 12A, the height image, to Figure 1A, one can notice a lot of adsorbate showing up on chalcocite after the mineral surface contacts the xanthate solution. In addition, by comparing it to Figures 10A and 11A, one can find that, when the KEX's concentration increases, more precipitates are forming at the solid/liquid interface and the mineral surface becomes much rougher. Figure 12B is the 3‐D image of Figure Figure <sup>12</sup> shows the AFM images of a chalcocite surface in 5 <sup>×</sup> <sup>10</sup>−<sup>4</sup> M KEX solution at pH 10 for 10 min. By comparing Figure 12A, the height image, to Figure 1A, one can notice a lot of adsorbate showing up on chalcocite after the mineral surface contacts the xanthate solution. In addition, by comparing it to Figures 10A and 11A, one can find that, when the KEX's concentration increases, more precipitates are forming at the solid/liquid interface and the mineral surface becomes much rougher. Figure 12B is the 3-D image

12A. Figure 12C, the section analysis of Figure 12A, confirms that the surface roughness

of Figure 12A. Figure 12C, the section analysis of Figure 12A, confirms that the surface roughness increases greatly when the chalcocite surface contacts a high concentration of KEX. Figure 12D is the deflection image of Figure 12A with a 10 nm data scale. *Minerals* **2022**, *12*, x FOR PEER REVIEW 15 of 34

**Figure 12.** AFM images of a chalcocite surface in 5 × 10<sup>−</sup><sup>4</sup> M KEX solution at pH 10 for 10 min. (**A**) The 10 μm × 10 μm height image with a data scale of 20 nm; (**B**) the 3‐D image; (**C**) the section analysis (the red arrows indicate the top and the bottom of an average asperity); and (**D**) the deflec‐ tion image with a data scale of 10 nm. **Figure 12.** AFM images of a chalcocite surface in 5 <sup>×</sup> <sup>10</sup>−<sup>4</sup> M KEX solution at pH 10 for 10 min. (**A**) The 10 µm × 10 µm height image with a data scale of 20 nm; (**B**) the 3-D image; (**C**) the section analysis (the red arrows indicate the top and the bottom of an average asperity); and (**D**) the deflection image with a data scale of 10 nm.

Similar to as obtained in Figure 9, to verify the adsorption of PAX on chalcocite at pH 10, after Figure 12 is obtained, a 5 μm × 5 μm area is scanned with a much larger scan force being applied to intentionally remove the adsorbate. Further, the same position is scanned again in a 10 μm × 10 μm area with a normal scan force being applied and the result is shown in Figure 13. One can observe from Figure 13A, the height image, that a 5 μm × 5 μm 'window' is shown in the center of the image due to the removal of the adsorbate from Similar to as obtained in Figure 9, to verify the adsorption of PAX on chalcocite at pH 10, after Figure 12 is obtained, a 5 µm × 5 µm area is scanned with a much larger scan force being applied to intentionally remove the adsorbate. Further, the same position is scanned again in a 10 µm × 10 µm area with a normal scan force being applied and the result is shown in Figure 13. One can observe from Figure 13A, the height image, that a 5 µm × 5 µm 'window' is shown in the center of the image due to the removal of the

the mineral surface under the previously applied large scan force. That is, the 'window'

still covered by the adsorbate without being disturbed by the applied large scan force. By comparing Figure 13B, the 3‐D image of Figure 13A, to Figure 12B, one can easily observe a pit existing on the mineral surface with adsorbate covering the surrounding area. Figure 13C is the section analysis of Figure 13A and it shows the height difference between the 'window' (as shown by the green markers) and the surrounding area being covered with

adsorbate from the mineral surface under the previously applied large scan force. That is, the 'window' in the center is the bare chalcocite surface and the surrounding area is the mineral surface still covered by the adsorbate without being disturbed by the applied large scan force. By comparing Figure 13B, the 3-D image of Figure 13A, to Figure 12B, one can easily observe a pit existing on the mineral surface with adsorbate covering the surrounding area. Figure 13C is the section analysis of Figure 13A and it shows the height difference between the 'window' (as shown by the green markers) and the surrounding area being covered with adsorbate (as shown by the red markers). Figure 13D is the deflection image of Figure 13A with a 10 nm data scale. *Minerals* **2022**, *12*, x FOR PEER REVIEW 16 of 34 adsorbate (as shown by the red markers). Figure 13D is the deflection image of Figure 13A with a 10 nm data scale.

**Figure 13.** AFM images of a chalcocite surface in 5 × 10<sup>−</sup><sup>4</sup> M KEX solution at pH 10 for 20 min. (**A**) The 10 μm × 10 μm height image with a data scale of 20 nm; (**B**) the 3‐D image; (**C**) the section analysis (the red arrows indicate the top and the bottom of an average asperity); and (**D**) the deflec‐ tion image with a data scale of 10 nm. The 5 μm × 5 μm 'window' in the center of the image is due to the removal of the adsorbate from the mineral surface under the intentionally applied large scan force. **Figure 13.** AFM images of a chalcocite surface in 5 <sup>×</sup> <sup>10</sup>−<sup>4</sup> M KEX solution at pH 10 for 20 min. (**A**) The 10 µm × 10 µm height image with a data scale of 20 nm; (**B**) the 3-D image; (**C**) the section analysis (the red arrows indicate the top and the bottom of an average asperity); and (**D**) the deflection image with a data scale of 10 nm. The 5 µm × 5 µm 'window' in the center of the image is due to the removal of the adsorbate from the mineral surface under the intentionally applied large scan force.

pH 10 for 30 min and further rinsed with 10 mL ethanol and 5 mL water consecutively. The images are finally obtained when the mineral sample is placed in water. By comparing it to Figure 12A, one can observe that the precipitates as observed in Figure 12A still exist on the chalcocite surface without being dissolved and rinsed off from the mineral surface. Figure 14B is the 3‐D image of Figure 14A. Figure 14C is the section analysis of Figure

14A. Figure 14D is the deflection image of Figure 14A with a 10 nm data scale.

Figure 14A is the height image of a chalcocite surface in 5 <sup>×</sup> <sup>10</sup>−<sup>4</sup> M KEX solution at pH 10 for 30 min and further rinsed with 10 mL ethanol and 5 mL water consecutively. The images are finally obtained when the mineral sample is placed in water. By comparing it to Figure 12A, one can observe that the precipitates as observed in Figure 12A still exist on the chalcocite surface without being dissolved and rinsed off from the mineral surface. Figure 14B is the 3-D image of Figure 14A. Figure 14C is the section analysis of Figure 14A. Figure 14D is the deflection image of Figure 14A with a 10 nm data scale. *Minerals* **2022**, *12*, x FOR PEER REVIEW 17 of 34

**Figure 14.** AFM images of a chalcocite surface in 5 × 10<sup>−</sup><sup>4</sup> M KEX solution at pH 10 for 30 min, and further rinsed with 10 mL ethanol and 5 mL water consecutively. (**A**) The 10 μm × 10 μm height image with a data scale of 20 nm; (**B**) the 3‐D image; (**C**) the section analysis (the red arrows indicate the top and the bottom of an average asperity); and (**D**) the deflection image with a data scale of 10 nm. **Figure 14.** AFM images of a chalcocite surface in 5 <sup>×</sup> <sup>10</sup>−<sup>4</sup> M KEX solution at pH 10 for 30 min, and further rinsed with 10 mL ethanol and 5 mL water consecutively. (**A**) The 10 µm × 10 µm height image with a data scale of 20 nm; (**B**) the 3-D image; (**C**) the section analysis (the red arrows indicate the top and the bottom of an average asperity); and (**D**) the deflection image with a data scale of 10 nm.

Figure 15 shows the AFM images of a chalcocite surface in 1 × 10−<sup>5</sup> M PAX solution at pH 10 for 10 min. From Figure 15A, the height image, one can find that there is a little

ysis of Figure 15A, shows that the surface roughness increases due to the adsorption of

PAX. Figure 15D is the deflection image of Figure 15A with a 10 nm data scale.

Figure <sup>15</sup> shows the AFM images of a chalcocite surface in 1 <sup>×</sup> <sup>10</sup>−<sup>5</sup> M PAX solution at pH 10 for 10 min. From Figure 15A, the height image, one can find that there is a little adsorbate showing up on chalcocite when the mineral surface contacts PAX solution at pH 10 for 10 min. Figure 15B is the 3-D image of Figure 15A. Figure 15C, the section analysis of Figure 15A, shows that the surface roughness increases due to the adsorption of PAX. Figure 15D is the deflection image of Figure 15A with a 10 nm data scale. *Minerals* **2022**, *12*, x FOR PEER REVIEW 18 of 34

**Figure 15.** AFM images of a chalcocite surface in 1 × 10<sup>−</sup><sup>5</sup> M PAX solution at pH 10 for 10 min. (**A**) The 10 μm × 10 μm height image with a data scale of 20 nm; (**B**) the 3‐D image; (**C**) the section analysis (the red arrows indicate the top and the bottom of an average asperity); and (**D**) the deflec‐ tion image with a data scale of 10 nm. **Figure 15.** AFM images of a chalcocite surface in 1 <sup>×</sup> <sup>10</sup>−<sup>5</sup> M PAX solution at pH 10 for 10 min. (**A**) The 10 µm × 10 µm height image with a data scale of 20 nm; (**B**) the 3-D image; (**C**) the section analysis (the red arrows indicate the top and the bottom of an average asperity); and (**D**) the deflection image with a data scale of 10 nm.

Figure 16 shows the AFM images of a chalcocite surface in 5 × 10−<sup>5</sup> M PAX solution at pH 10 for 10 min. Figure 16A is the height image, from which one can see a lot of ad‐ sorbate showing up on chalcocite. In addition, by comparing it to Figure 15A, one can find that the mineral surface becomes much rougher, suggesting more precipitates are forming

ysis of Figure 16A, confirms that the surface roughness increases with increasing the con‐ centration of PAX. Figure 16D is the deflection image of Figure 16A with a 10 nm data

scale.

Figure <sup>16</sup> shows the AFM images of a chalcocite surface in 5 <sup>×</sup> <sup>10</sup>−<sup>5</sup> M PAX solution at pH 10 for 10 min. Figure 16A is the height image, from which one can see a lot of adsorbate showing up on chalcocite. In addition, by comparing it to Figure 15A, one can find that the mineral surface becomes much rougher, suggesting more precipitates are forming at the solid/liquid interface, when the PAX's concentration increases from <sup>1</sup> <sup>×</sup> <sup>10</sup>−<sup>5</sup> M to 5 <sup>×</sup> <sup>10</sup>−<sup>5</sup> <sup>M</sup> at pH 10. Figure 16B is the 3-D image of Figure 16A. Figure 16C, the section analysis of Figure 16A, confirms that the surface roughness increases with increasing the concentration of PAX. Figure 16D is the deflection image of Figure 16A with a 10 nm data scale. *Minerals* **2022**, *12*, x FOR PEER REVIEW 19 of 34

**Figure 16.** AFM images of a chalcocite surface in 5 × 10<sup>−</sup><sup>5</sup> M PAX solution at pH 10 for 10 min. (**A**) The 10 μm × 10 μm height image with a data scale of 20 nm; (**B**) the 3‐D image; (**C**) the section analysis (the red arrows indicate the top and the bottom of an average asperity); and (**D**) the deflec‐ tion image with a data scale of 10 nm. **Figure 16.** AFM images of a chalcocite surface in 5 <sup>×</sup> <sup>10</sup>−<sup>5</sup> M PAX solution at pH 10 for 10 min. (**A**) The 10 µm × 10 µm height image with a data scale of 20 nm; (**B**) the 3-D image; (**C**) the section analysis (the red arrows indicate the top and the bottom of an average asperity); and (**D**) the deflection image with a data scale of 10 nm.

Figure 17 shows the AFM images of a chalcocite surface in 1 × 10<sup>−</sup><sup>4</sup> M PAX solution at pH 10 for 10 min. By comparing Figure 17A, the height image, to Figure 1A, one can notice

PAX's concentration increases, more precipitates are forming at the solid/liquid interface and the mineral surface becomes much rougher. Figure 17B is the 3‐D image of Figure 17A. Figure 17C, the section analysis of Figure 17A, confirms that the surface roughness increases greatly when the chalcocite surface contacts a high concentration of PAX. Figure

17D is the deflection image of Figure 17A with a 10 nm data scale.

Figure <sup>17</sup> shows the AFM images of a chalcocite surface in 1 <sup>×</sup> <sup>10</sup>−<sup>4</sup> M PAX solution at pH 10 for 10 min. By comparing Figure 17A, the height image, to Figure 1A, one can notice a lot of adsorbate showing up on chalcocite after the mineral surface contacts the xanthate solution. In addition, by comparing it to Figures 15A and 16A, one can find that, when the PAX's concentration increases, more precipitates are forming at the solid/liquid interface and the mineral surface becomes much rougher. Figure 17B is the 3-D image of Figure 17A. Figure 17C, the section analysis of Figure 17A, confirms that the surface roughness increases greatly when the chalcocite surface contacts a high concentration of PAX. Figure 17D is the deflection image of Figure 17A with a 10 nm data scale. *Minerals* **2022**, *12*, x FOR PEER REVIEW 20 of 34

**Figure 17.** AFM images of a chalcocite surface in 1 × 10<sup>−</sup><sup>4</sup> M PAX solution at pH 10 for 10 min. (**A**) The 10 μm × 10 μm height image with a data scale of 20 nm; (**B**) the 3‐D image; (**C**) the section analysis (the red arrows indicate the top and the bottom of an average asperity); and (**D**) the deflec‐ tion image with a data scale of 10 nm. **Figure 17.** AFM images of a chalcocite surface in 1 <sup>×</sup> <sup>10</sup>−<sup>4</sup> M PAX solution at pH 10 for 10 min. (**A**) The 10 µm × 10 µm height image with a data scale of 20 nm; (**B**) the 3-D image; (**C**) the section analysis (the red arrows indicate the top and the bottom of an average asperity); and (**D**) the deflection image with a data scale of 10 nm.

is a lot of adsorbate showing up on chalcocite when the mineral surface contacts the Aer‐ ofloat 238 solution at pH 6 for 10 min. The figure also shows that the morphology of the adsorbate, which shows round and smooth edges, differs greatly from those obtained with xanthate, for which the adsorbate is generally precipitates with irregular sharp edges. Figure 18B is the 3‐D image of Figure 18A. Figure 18C is the section analysis of Figure 18A, and Figure 18D is the deflection image of Figure 18A with a 10 nm data scale.

*3.2. AFM Image of Mineral Surface in Cytec Aerofloat 238 Solutions*

#### *3.2. AFM Image of Mineral Surface in Cytec Aerofloat 238 Solutions*

Figure 18 shows the AFM images of a chalcocite surface in 200 ppm Aerofloat 238 solution at pH 6 for 10 min. From Figure 18A, the height image, one can observe that there is a lot of adsorbate showing up on chalcocite when the mineral surface contacts the Aerofloat 238 solution at pH 6 for 10 min. The figure also shows that the morphology of the adsorbate, which shows round and smooth edges, differs greatly from those obtained with xanthate, for which the adsorbate is generally precipitates with irregular sharp edges. Figure 18B is the 3-D image of Figure 18A. Figure 18C is the section analysis of Figure 18A, and Figure 18D is the deflection image of Figure 18A with a 10 nm data scale. *Minerals* **2022**, *12*, x FOR PEER REVIEW 21 of 34

**Figure 18.** AFM images of a chalcocite surface in 200 ppm Aerofloat 238 solution at pH 6 for 10 min. (**A**) The 10 μm × 10 μm height image with a data scale of 20 nm; (**B**) the 3‐D image; (**C**) the section analysis (the red arrows indicate the top and the bottom of an average asperity); and (**D**) the deflec‐ tion image with a data scale of 10 nm. **Figure 18.** AFM images of a chalcocite surface in 200 ppm Aerofloat 238 solution at pH 6 for 10 min. (**A**) The 10 µm × 10 µm height image with a data scale of 20 nm; (**B**) the 3-D image; (**C**) the section analysis (the red arrows indicate the top and the bottom of an average asperity); and (**D**) the deflection image with a data scale of 10 nm.

Figure 19 shows the AFM images of a chalcocite surface in 200 ppm Aerofloat 238

sorption time increases from 10 min to 20 min. Again, the figure also shows that the ad‐ sorbate has round and smooth edges and it is greatly different from those obtained with xanthate, the adsorbate of which is generally precipitates with irregular sharp edges. Fig‐ ure 19B is the 3‐D image of Figure 19A. Figure 19C is the section analysis of Figure 19A, showing the surface roughness increases with adsorption time increases. Figure 19D is the

deflection image of Figure 19A with a 10 nm data scale.

Figure 19 shows the AFM images of a chalcocite surface in 200 ppm Aerofloat 238 solution at pH 6 for 20 min. By comparing Figure 19A, the height image, to Figure 18A, one can see that the patches adsorbed on chalcocite become larger in size when the adsorption time increases from 10 min to 20 min. Again, the figure also shows that the adsorbate has round and smooth edges and it is greatly different from those obtained with xanthate, the adsorbate of which is generally precipitates with irregular sharp edges. Figure 19B is the 3-D image of Figure 19A. Figure 19C is the section analysis of Figure 19A, showing the surface roughness increases with adsorption time increases. Figure 19D is the deflection image of Figure 19A with a 10 nm data scale. *Minerals* **2022**, *12*, x FOR PEER REVIEW 22 of 34

**Figure 19.** AFM images of a chalcocite surface in 200 ppm Aerofloat 238 solution at pH 6 for 20 min. (**A**) The 10 μm × 10 μm height image with a data scale of 20 nm; (**B**) the 3‐D image; (**C**) the section analysis (the red arrows indicate the top and the bottom of an average asperity); and (**D**) the deflec‐ tion image with a data scale of 10 nm. **Figure 19.** AFM images of a chalcocite surface in 200 ppm Aerofloat 238 solution at pH 6 for 20 min. (**A**) The 10 µm × 10 µm height image with a data scale of 20 nm; (**B**) the 3-D image; (**C**) the section analysis (the red arrows indicate the top and the bottom of an average asperity); and (**D**) the deflection image with a data scale of 10 nm.

Similar to as obtained in Figure 13, to verify the adsorption of Aerofloat 238 on chal‐ cocite, after Figure 19 is obtained, a 10 μm × 5 μm area is scanned with a much larger scan

scanned again in a 10 μm × 10 μm area with a normal scan force being applied and the result is shown in Figure 20. One can find from Figure 20A, the height image, that a 10 μm × 5 μm 'window' is shown in the upper image due to the removal of the adsorbate from the mineral surface under the previously applied large scan force. That is, the upper 'win‐ dow' is the bare chalcocite surface and the lower section is the mineral surface still covered by the adsorbate without being disturbed by the applied large scan force. Figure 20B, Fig‐ ure 20C, and Figure 20D, are, respectively, the 3‐D image, the section analysis, and the

deflection image of Figure 20A with a 10 nm data scale.

Similar to as obtained in Figure 13, to verify the adsorption of Aerofloat 238 on chalcocite, after Figure 19 is obtained, a 10 µm × 5 µm area is scanned with a much larger scan force being applied to intentionally remove the adsorbate. Further, the same position is scanned again in a 10 µm × 10 µm area with a normal scan force being applied and the result is shown in Figure 20. One can find from Figure 20A, the height image, that a 10 µm × 5 µm 'window' is shown in the upper image due to the removal of the adsorbate from the mineral surface under the previously applied large scan force. That is, the upper 'window' is the bare chalcocite surface and the lower section is the mineral surface still covered by the adsorbate without being disturbed by the applied large scan force. Figure 20B, Figure 20C, and Figure 20D, are, respectively, the 3-D image, the section analysis, and the deflection image of Figure 20A with a 10 nm data scale. *Minerals* **2022**, *12*, x FOR PEER REVIEW 23 of 34

**Figure 20.** AFM images of a chalcocite surface in 200 ppm Aerofloat 238 solution at pH 6 for 20 min. (**A**) The 10 μm × 10 μm height image with a data scale of 20 nm; (**B**) the 3‐D image; (**C**) the section analysis (the red arrows indicate the top and the bottom of an average asperity in a zone of adsorbate and the green arrows indicate those in a zone with adsorbate being removed); and (**D**) the deflection image with a data scale of 10 nm. The 10 μm × 5 μm 'window' in the upper part is due to the removal of the adsorbate from the mineral under the intentionally applied large scan force. deflection image of Figure 21A with <sup>a</sup> <sup>10</sup> nm data scale. **Figure 20.** AFM images of a chalcocite surface in 200 ppm Aerofloat 238 solution at pH 6 for 20 min. (**A**) The 10 µm × 10 µm height image with a data scale of 20 nm; (**B**) the 3-D image; (**C**) the section analysis (the red arrows indicate the top and the bottom of an average asperity in a zone of adsorbate and the green arrows indicate those in a zone with adsorbate being removed); and (**D**) the deflection image with a data scale of 10 nm. The 10 µm × 5 µm 'window' in the upper part is due to the removal of the adsorbate from the mineral under the intentionally applied large scan force.

smooth edges, which is greatly different from those obtained with xanthate. Figure 21B, Figure 21C, and Figure 21D, are, respectively, the 3‐D image, the section analysis, and the

Figure 21 shows the AFM images of a chalcocite surface in 200 ppm Aerofloat 238 solution at pH 10 for 10 min. From Figure 21A, the height image, one can find that there

Figure 21 shows the AFM images of a chalcocite surface in 200 ppm Aerofloat 238 solution at pH 10 for 10 min. From Figure 21A, the height image, one can find that there are many patches being adsorbed on chalcocite when the mineral contacts the Aerofloat 238 solution at pH 10. Again, the figure also shows that the adsorbate has round and smooth edges, which is greatly different from those obtained with xanthate. Figure 21B, Figure 21C, and Figure 21D, are, respectively, the 3-D image, the section analysis, and the deflection image of Figure 21A with a 10 nm data scale. *Minerals* **2022**, *12*, x FOR PEER REVIEW 24 of 34

**Figure 21.** AFM images of a chalcocite surface in 200 ppm Aerofloat 238 solution at pH 10 for 10 min. (**A**) The 10 μm × 10 μm height image with a data scale of 20 nm; (**B**) the 3‐D image; (**C**) the section analysis (the red arrows indicate the top and the bottom of an average asperity); and (**D**) the deflection image with a data scale of 10 nm. **Figure 21.** AFM images of a chalcocite surface in 200 ppm Aerofloat 238 solution at pH 10 for 10 min. (**A**) The 10 µm × 10 µm height image with a data scale of 20 nm; (**B**) the 3-D image; (**C**) the section analysis (the red arrows indicate the top and the bottom of an average asperity); and (**D**) the deflection image with a data scale of 10 nm.

Figure 22 shows the AFM images of a chalcocite surface in 200 ppm Aerofloat 238 solution at pH 10 for 20 min. By comparing Figure 22A, the height image, to Figure 21A, one can find that when adsorption time increases, there are more patches being adsorbed on chalcocite when the mineral contacts the Aerofloat 238 solution at pH 10. Figure 22B,

deflection image of Figure 22A with a 10 nm data scale.

Figure 22 shows the AFM images of a chalcocite surface in 200 ppm Aerofloat 238 solution at pH 10 for 20 min. By comparing Figure 22A, the height image, to Figure 21A, one can find that when adsorption time increases, there are more patches being adsorbed on chalcocite when the mineral contacts the Aerofloat 238 solution at pH 10. Figures 22B, 22C and 22D are, respectively, the 3-D image, the section analysis, and the deflection image of Figure 22A with a 10 nm data scale. *Minerals* **2022**, *12*, x FOR PEER REVIEW 25 of 34

**Figure 22.** AFM images of a chalcocite surface in 200 ppm Aerofloat 238 solution at pH 10 for 20 min. (**A**) The 10 μm × 10 μm height image with a data scale of 20 nm; (**B**) the 3‐D image; (**C**) the section analysis (the red arrows indicate the top and the bottom of an average asperity); and (**D**) the deflection image with a data scale of 10 nm. **Figure 22.** AFM images of a chalcocite surface in 200 ppm Aerofloat 238 solution at pH 10 for 20 min. (**A**) The 10 µm × 10 µm height image with a data scale of 20 nm; (**B**) the 3-D image; (**C**) the section analysis (the red arrows indicate the top and the bottom of an average asperity); and (**D**) the deflection image with a data scale of 10 nm.

Figure 23A is the height image of a chalcocite surface in 200 ppm Aerofloat 238 solu‐ tion at pH 10 for 20 min, and further rinsed with 10 mL ethanol and 5 mL water consecu‐ tively. The images are finally obtained when the mineral sample is in water. By comparing it to Figure 22A, one can tell that the adsorbate as observed in Figure 23A does not exist

respectively, the 3‐D image, the section analysis, and the deflection image of Figure 23A

with a 10 nm data scale.

Figure 23A is the height image of a chalcocite surface in 200 ppm Aerofloat 238 solution at pH 10 for 20 min, and further rinsed with 10 mL ethanol and 5 mL water consecutively. The images are finally obtained when the mineral sample is in water. By comparing it to Figure 22A, one can tell that the adsorbate as observed in Figure 23A does not exist on the chalcocite surface anymore because it has been dissolved and rinsed off from the mineral surface by flushing with ethanol. Figure 23B, Figure 23C, and Figure 23D, are, respectively, the 3-D image, the section analysis, and the deflection image of Figure 23A with a 10 nm data scale. *Minerals* **2022**, *12*, x FOR PEER REVIEW 26 of 34

Figure 24A is the height image of a chalcocite surface in the mixture of 1 × 10−<sup>4</sup> M PAX and 300 ppm Aerofloat 238 solution (1:1 vol. ratio) at pH 6. One can clearly see that there

one is the morphology of patches and it is similar to those as obtained with the addition of Aerofloat 238; as indicated by the "Blue" arrow, the one is the morphology of precipi‐ tates, and it is similar to those as obtained with the addition of xanthate. Figure 24B, Figure 24C, and Figure 24D, are, respectively, the 3‐D image, the section analysis, and the deflec‐

are two different types of adsorption morphology, i.e., as indicated by the "Red" arrow, 26

tion image of Figure 24A with a 10 nm data scale.

Figure 24A is the height image of a chalcocite surface in the mixture of 1 <sup>×</sup> <sup>10</sup>−<sup>4</sup> <sup>M</sup> PAX and 300 ppm Aerofloat 238 solution (1:1 vol. ratio) at pH 6. One can clearly see that there are two different types of adsorption morphology, i.e., as indicated by the "Red" arrow, one is the morphology of patches and it is similar to those as obtained with the addition of Aerofloat 238; as indicated by the "Blue" arrow, the one is the morphology of precipitates, and it is similar to those as obtained with the addition of xanthate. Figure 24B, Figure 24C, and Figure 24D, are, respectively, the 3-D image, the section analysis, and the deflection image of Figure 24A with a 10 nm data scale. *Minerals* **2022**, *12*, x FOR PEER REVIEW 27 of 34

**Figure 24.** AFM images of a chalcocite surface in the mixture of 1 × 10<sup>−</sup><sup>4</sup> M PAX and 300 ppm Aero‐ float 238 solution (1:1 volume ratio) at pH 6 for 20 min. (**A**) The 10 μm × 10 μm height image with a data scale of 20 nm; (**B**) the 3‐D image; (**C**) the section analysis (the red arrows indicate the top and the bottom of an average asperity); and (**D**) the deflection image with a data scale of 10 nm. The "Red" arrow indicates the patches due to the adsorption of Aerofloat 238 and the "Blue" arrow indicates the precipitates due to the adsorption of PAX. **Figure 24.** AFM images of a chalcocite surface in the mixture of 1 <sup>×</sup> <sup>10</sup>−<sup>4</sup> M PAX and 300 ppm Aerofloat 238 solution (1:1 volume ratio) at pH 6 for 20 min. (**A**) The 10 µm × 10 µm height image with a data scale of 20 nm; (**B**) the 3-D image; (**C**) the section analysis (the red arrows indicate the top and the bottom of an average asperity); and (**D**) the deflection image with a data scale of 10 nm. The "Red" arrow indicates the patches due to the adsorption of Aerofloat 238 and the "Blue" arrow indicates the precipitates due to the adsorption of PAX.

changes the surface morphology of chalcocite accordingly to changing solution chemistry such as collector's concentration, solution pH and adsorption time. The roughness analy‐ sis of the AFM images of a chalcocite surface in collector solutions is summarized and

*4.1. Adsorption of Xanthate on Chalcocite Surface*

**4. Discussion**

listed in Table 1 as follows.

## **4. Discussion**

#### *4.1. Adsorption of Xanthate on Chalcocite Surface*

Figures 2–17 show that a significant amount of adsorbate can be observed on the chalcocite surface when it contacts xanthate solutions for a specific time. The adsorbate changes the surface morphology of chalcocite accordingly to changing solution chemistry such as collector's concentration, solution pH and adsorption time. The roughness analysis of the AFM images of a chalcocite surface in collector solutions is summarized and listed in Table 1 as follows.


**Table 1.** Roughness analysis of the AFM images of a chalcocite surface in xanthate solutions.

Note: \* Ra(Sa): arithmetic average of the absolute values of the surface height deviations. \*\* Rms(Sq): root mean square average of height deviations taken from the mean image data plane.

Table 1 shows that, during the same timeframe, i.e., 10 min, surface roughness in general increases with increasing the concentration of xanthate. For example, as shown by Figures 2–4, at pH 6, when the concentration of KEX increases from 5 <sup>×</sup> <sup>10</sup>−<sup>5</sup> M to <sup>1</sup> <sup>×</sup> <sup>10</sup>−<sup>4</sup> M, the roughness Ra value increases from 1.110 nm to 1.752 nm, and the value increases to 5.242 nm when the concentration is further increased to 5 <sup>×</sup> <sup>10</sup>−<sup>4</sup> M. A similar trend is also observed for the case of PAX, although the change in values is not as significant as the one obtained with KEX. At pH 10, the same conclusion is also applicable. For example, as shown by Figures 10–12, when the concentration of KEX increases from 5 <sup>×</sup> <sup>10</sup>−<sup>5</sup> M to <sup>1</sup> <sup>×</sup> <sup>10</sup>−<sup>4</sup> M, the roughness Ra value increases from 1.349 nm to 3.097 nm, and the value increases to 7.400 nm when the concentration is further increased to 5 <sup>×</sup> <sup>10</sup>−<sup>4</sup> M. A similar trend is also observed for the case of PAX as well.

In the above section, as shown by Figures 2–17, when chalcocite contacts xanthate solutions for a specific time, a lot of adsorbates will show up on the mineral surface and the adsorption changes the surface morphology. Clearly this change in surface morphology of chalcocite cannot be attributed to the reaction of the mineral with water because the AFM images obtained with the addition of various xanthate solutions are totally different from Figure 1, which is captured during the same time frame. Therefore, the adsorbate shown in Figures 2–17 must be due to the adsorption of xanthate at the mineral/liquid interface. The same conclusion can also be drawn for the cases of Aerofloat 238, as shown by Figures 18–24. That is, both these collectors can effectively adsorb on the chalcocite surface in a short time frame as applied for the AFM imaging analysis.

According to Leja [5] and Woods [6], generally, the main mechanisms for the increase in hydrophobicity of sulfide minerals in flotation by the addition of collectors include (1) the adsorption of metal xanthate with low solubility and (2) the oxidation of xanthate into dixanthogen on a sulfide mineral surface in an aqueous solution. Previous AFM studies with chalcopyrite and pyrite [16–18] have shown that, because under ambient conditions, i.e., room temperature and normal pressure, dialkyl dixanthogen is usually in a liquid form with a low melting point [23], the adsorbed dixanthogen on sulfides in an aqueous solution demonstrates patches with smooth and round edges, which fits well with the fact that oily dixanthogen is generally insoluble in water and that the circular boundary is the direct result of the high interfacial tension between hydrophobic dixanthogen and water. In the present investigation, as shown by Figures 2–17, the adsorbate shows no evidence of smooth and round edges, and the surface morphology of chalcocite after adsorption is similar to what has been obtained with the bornite/xanthate system instead [22]. In addition, Figures 8 and 13 clearly show that the adsorbate is not soft at all, because even when a large scan force is applied to remove the adsorbate and open a 'window' in the center of the AFM image, there is still some residual precipitates left on the mineral surface. Further, as shown in Figure 14, the precipitate persists on the chalcocite surface after the mineral surface is rinsed with copious ethanol alcohol, suggesting that the adsorbate is insoluble in ethanol, which is clearly not an essential property of nonpolar oily dixanthogen. As such, it is reasonable to rule out the possibility that the observed adsorbate on the chalcocite in xanthate solutions is dixanthogen.

It has been proposed that the semiconductor type of sulfide minerals determines the final adsorption products on sulfides. For example, on n-type minerals, dixanthogen is usually formed, while on p-type minerals metal xanthate is observed. Chalcocite is "known as a fairly good but variable conductor", and it is classified as "a consistently p-type mineral" [24], which favors the formation of metal xanthate. As reviewed in the previous section, the adsorption of xanthate on the chalcocite surface in an aqueous solution is mainly due to the initially chemisorbed xanthate (*X* −) on the chalcocite surface followed by insoluble cuprous xanthate (CuX) [1,3,7–10]. Clearly, the findings obtained from the current AFM imaging analysis results are in line with what has been previously reported.

In general, the 'chemisorption' of xanthate on chalcocite occurs via: [10]

$$
\lambda^- = \lambda\_{\text{ads}} + \mathbf{e}^- \tag{1}
$$

The adsorption of xanthate on chalcocite is due to an anodic reaction simplified as follows introducing insoluble cuprous xanthate on the chalcocite surface [10,25]:

$$\mathbf{C}\mathbf{u}\_2\mathbf{S} + \mathbf{n}\mathbf{X}^- = \mathbf{n}\mathbf{C}\mathbf{u}\mathbf{X} + \mathbf{C}\mathbf{u}\_{2-\mathbf{n}}\mathbf{S} + \mathbf{n}\mathbf{e}^- \tag{2}$$

The section analysis of the obtained AFM images also suggests that the irregular adsorbate cannot be attributed to the chemisorbed xanthate, of which the maximum adsorption peaks at a monolayer surface coverage [1,10] because the height of the adsorbate is generally above 5 nm and the value is dramatically larger than a monolayer length of ethyl xanthate and amyl xanthate as used in the present study. In the present study, solution potential is not intentionally controlled, and it suggests that the chalcocite surface will undertake an oxidation reaction to some extent when it contacts water.

However, the claim that the observed irregular adsorbate is cuprous xanthate does not rule out the existence of chemisorbed xanthate on the chalcocite surface. In the present study, chemisorbed xanthate may co-exist with cuprous xanthate on the chalcocite surface; however, it is technically too difficult to observe the substance from the obtained AFM images. The difficulty of detecting chemisorbed xanthate using an AFM lies in two facts. Firstly, the hydrocarbon chain length of KEX and PAX as used in the present study is very short, i.e., less than 1 nm, and this makes it difficult to identify such a small change in surface morphology of a very soft substance, i.e., xanthate, with high confidence. Secondly, the chalcocite mineral sample as studied in the present work is prepared by polishing the

mineral surface, and the polishing process results in some degree of surface roughness. As shown in Figure 1, there are some scratch lines existing on the polished chalcocite surface and the preexisting surface roughness makes it unrealistic to study the surface morphology change in angstrom. Therefore, it is possible that xanthate chemisorbs on chalcocite surface in a very low profile without being successfully detected by an AFM.

In order to infer the binding strength of the adsorbate and chalcocite mineral surface in quality, as shown in Figure 9, a 10 µm × 5 µm area is scanned for one time by applying a much larger scan force to intentionally remove the adsorbate, and therefore a 10 µm × 5 µm 'window' is shown in the image due to the partial removal of the adsorbate from the mineral surface under the intentionally applied large scan force. Similarly, Figure 13 is obtained in the same methodology. Both Figures 9 and 13 show that there is still some residual adsorbate adsorbing in the "window" area although a large force is applied. In addition, Figure 14 shows that rinsing with copious ethanol does not remove the adsorbate. This conclusion coincides well with those reported by Allison et al. [3], quote, "no product of reaction with the methyl and ethyl homologues could be detected, although both reacted very extensively with the surface". As such, xanthate adsorbs on chalcocite intensively with the mineral surface being covered quickly by adsorbate. The binding of the adsorbate, i.e., cuprous ethyl xanthate, and chalcocite is very strong, and the adsorbate cannot be completely removed by applying a large scan force. Rinsing with ethanol alcohol cannot extract the adsorbate from the chalcocite surface.

#### *4.2. Effect of the Rank of Xanthate*

In froth flotation, the rank of a collector is an important parameter determining the collectivity and the selectivity of the collector. In general, a xanthate collector with a low rank has a low collectivity and therefore a high selectivity and vice versa. In the present work, the effect of the collector's rank on the adsorption of xanthate on chalcocite has been studied by using both KEX and PAX as collectors. By comparing Figures 2–5 to Figures 6–8 and Figures 10–14 to Figures 15–17, one can see that when the collector's concentration is the same, PAX adsorbs on the chalcocite surface in much higher surface coverage and a more uniform layer structure. The formation of cuprous amyl xanthate occurs at a lower surfactant concentration than that of cuprous ethyl xanthate, and it is due to a lower solubility product for the former. For example, it was reported that the solubility product of cuprous amyl xanthate is 8.0 <sup>×</sup> <sup>10</sup>−<sup>22</sup> and that for cuprous ethyl xanthate is 5.2 <sup>×</sup> <sup>10</sup>−<sup>20</sup> [26,27]. Allison et al. [3] also reported that the percentage of reacted xanthate increased by one fold with the carbon number of xanthate increasing from 2 to 5 at the same surfactant concentration. Therefore, the fact that cuprous amyl xanthate forms a more uniform layer than cuprous ethyl xanthate does is due to the longer hydrocarbon chain of the former and therefore an increased lateral hydrophobic attraction between hydrocarbon chains. Therefore, a high-rank xanthate, i.e., PAX, is more powerful than a low-rank xanthate, i.e., KEX, for the adsorption on chalcocite surface; and therefore, a highly enhanced flotation collectivity. The same conclusion is also applicable to the case of the xanthate/bornite system [22].

#### *4.3. Effect of the Concentration of Xanthate*

Equation (2) shows that increasing the concentration of xanthate, i.e., the reactant, facilitates the reaction to proceed rightward, resulting in more reaction product, i.e., CuX, on the mineral surface. The AFM images obtained in the present work clearly show such a concentration effect of xanthate. For example, by comparing Figures 2, 3 and 5, one can easily observe that the surface coverage and the height of the adsorbate (surface roughness) increase, suggesting that at pH 6 the amount of adsorbate increases greatly when the concentration of KEX increases from 5 <sup>×</sup> <sup>10</sup>−<sup>5</sup> M to 5 <sup>×</sup> <sup>10</sup>−<sup>4</sup> M. The same conclusion can also be drawn for the case of the PAX/chalcocite system. For example, by comparing Figures 7–9, one can observe that when the concentration of PAX increases from <sup>1</sup> <sup>×</sup> <sup>10</sup>−<sup>5</sup> <sup>M</sup> to 1 <sup>×</sup> <sup>10</sup>−<sup>4</sup> M, the amount of adsorbate increases with the same contact time frame. The

same concentration effect of xanthate (KEX and PAX) is also observed at pH 10. All these results show that during a wide pH range in flotation practice, increasing the concentration of xanthate is beneficial for a high surface coverage of cuprous xanthate, and therefore a high flotation recovery.

#### *4.4. Effect of Adsorption Time of Xanthate*

By comparing Figure 4 to Figure 5, Figure 8 to Figure 9, and Figure 12 to Figure 13, one can see that when the adsorption time increases from 10 min to 20 min in xanthate solution, the surface coverage and the height of the adsorbate increase as well as. The same trend is applicable for both pH 6 and pH 10. It is then concluded from these AFM images that the adsorption of xanthate on chalcocite increases with the adsorption time. The finding is in line with a common industrial practice of copper ore beneficiation by adding collectors in a mill, the objective of which includes increasing the adsorption time and promoting the adsorption of the collector on the freshly exposed mineral surface.

#### *4.5. Adsorption of Dialkyl Dithiophosphate on Chalcocite Surface*

The AFM images of a chalcocite surface in Aerofloat 238 solution at pH 6 for different contact times are shown in Figures 18–20, from which one can see that dialkyl dithiophosphate adsorbs intensively on chalcocite at pH 6. In addition, increasing contact time from 10 min to 20 min increases the amount of adsorbate on the chalcocite surface by increasing both surface coverage and surface roughness as shown in Table 1. Figures 21–23 are the AFM images of a chalcocite surface in Aerofloat 238 solution at pH 10 for different contact times. The figures show that at pH 10 dialkyl dithiophosphate can adsorb intensively on chalcocite as well, suggesting a wide pulp pH range for the flotation of chalcocite using Aerofloat 238 as a collector.

Figure 18 shows that there is a lot of adsorbate existing on chalcocite when the mineral surface contacts the Aerofloat 238 solution and the morphology of the adsorbate, which shows round and smooth edges, differs greatly from those as obtained with xanthate, for which the adsorbate is generally precipitates with irregular sharp edges. In addition, Figure 19 shows that when the mineral surface is scanned with a much larger scan force being applied to intentionally remove the adsorbate, a 10 µm × 5 µm 'window' is shown in the upper image due to the removal of the adsorbate from the mineral surface under the applied large scan force. As the 'window' area is close to the bare chalcocite surface, it suggests that the binding strength between the adsorbate and chalcocite is weak, and the finding is different from the one obtained in Figure 9, which shows a strong binding strength between the precipitate of cuprous xanthate and chalcocite.

Additionally, as shown in Figure 23, rinsing the chalcocite surface with 10 mL ethanol after it contacts the Aerofloat 238 solution for 20 min removes the adsorbate almost completely from chalcocite, and it suggests that the adsorbate is generally an oily substance of high solubility in ethanol, but not in water.

As to the adsorbate on chalcocite, there have been some studies with different conclusions [11–13]. For example, Goold and Finkelstein [11] reported that only Cu(DTP)<sup>2</sup> was present on copper sulfide. Solozhenkin and Koptsia [12] reported that the products were both CuDTP and Cu(DTP)2. Chander and Fuerstenau [13] proposed that the oxidation of DTP− at copper sulfide was a two-step process consisting of the discharge of the DTP− ion to form a free DTP· radical (DTP− = DTP· + e−) followed by the subsequent reaction of DTP· . They also claimed that the most probable reaction involving DTP· at copper sulfide were:

$$\text{Cu}\_2\text{S} + (2-\text{y})\text{DTP}^\cdot + \text{yDTP}^- = 2\text{CuDTP} + \text{S} + \text{ye}^- \tag{3}$$

$$\text{Cu}\_2\text{S} + (4-\text{m})\text{DTP}^\cdot + \text{mDTP}^- = 2\text{Cu}(\text{DTP})\_2 + \text{S} + \text{me}^-\tag{4}$$

Equations (3) and (4) show that CuDTP and Cu(DTP)<sup>2</sup> are formed when copper sulfide is immersed in reagent solutions of dithiophosphate.

Zinc dialkyl dithiophosphate (Zn(DTP)2) has been widely applied as anti-wear additives in lubricants. It is characterized as a nonpolar oily liquid at room temperature, which is insoluble in water. As cupric dialkyl dithiophosphate (Cu(DTP)2) has a similar molecular structure as that of zinc dialkyl dithiophosphate, Cu(DTP)<sup>2</sup> is an oily liquid substance as well at room temperature. Figures 18–23 clearly show that the adsorbate is generally an oily substance of high solubility in ethanol, and the finding confirms well with the claim that Cu(DTP)<sup>2</sup> is the adsorbate formed when copper sulfide is immersed in reagent solutions of dithiophosphate. As such, Cu(DTP)<sup>2</sup> is the main adsorbate on the chalcocite surface in dialkyl dithiophosphate solutions. Of course, CuDTP can also co-adsorb together with Cu(DTP)<sup>2</sup> on chalcocite, but its amount is too sparse to be observed from AFM images when compared to Cu(DTP)2.
