*3.1. AFM Image of Minerlal Surface in Various Xanthate Solutions*

Figure 1 shows the surface images of a bare bornite surface obtained in air. Figure 1A is the 5 µm × 5 µm height image with a data scale of 20 nm, which shows that the solid surface was quite smooth despite some scratch lines on the sample surface due to surface polishing. A smooth bare mineral surface is beneficial for the identification and analysis of the adsorbate when the surface contacts the solutions of various collectors. Figure 1B is the 3D image of Figure 1A. Figure 1C is the section analysis of Figure 1A, which confirms that the polished bornite surface was quite smooth. Figure 1D is the deflection image of Figure 1A with a 10 nm data scale.

**3. Results**

Figure 1A with a 10 nm data scale.

*3.1. AFM Image of Minerlal Surface in Various Xanthate Solutions*

Figure 1 shows the surface images of a bare bornite surface obtained in air. Figure 1A is the 5 µm × 5 µm height image with a data scale of 20 nm, which shows that the solid surface was quite smooth despite some scratch lines on the sample surface due to surface polishing. A smooth bare mineral surface is beneficial for the identification and analysis of the adsorbate when the surface contacts the solutions of various collectors. Figure 1B is the 3D image of Figure 1A. Figure 1C is the section analysis of Figure 1A, which confirms that the polished bornite surface was quite smooth. Figure 1D is the deflection image of

**Figure 1.** AFM images of a bornite surface in air. (**A**) The 5 µm × 5 µm height image with a data scale of 20 nm, (**B**) the 3D 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 bornite surface in air. (**A**) The 5 µm × 5 µm height image with a data scale of 20 nm, (**B**) the 3D 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 shows the surface images of a bare bornite surface which contacted nanopure water in an AFM liquid cell for 10 min. Figure 2A is the 10 µm × 10 µm height image, Figure 2 shows the surface images of a bare bornite surface which contacted nanopure water in an AFM liquid cell for 10 min. Figure 2A is the 10 µm × 10 µm height image, which shows that the solid surface was still largely smooth, with little adsorbate on the sample surface detected by the AFM probe. Figure 2B is the 3D image of Figure 2A. Figure 2C is the section analysis of Figure 2A. Figure 2D is the deflection image of Figure 2A with a 10 nm data scale.

a 10 nm data scale.

**Figure 2.** AFM images of a bornite surface soaked in water for 10 min. (**A**) The 5 µm × 5 µm height image with a data scale of 20 nm, (**B**) the 3D 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 bornite surface soaked in water for 10 min. (**A**) The 5 µm × 5 µm height image with a data scale of 20 nm, (**B**) the 3D 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 bornite surface which contacted the 5 × 10−<sup>5</sup> M KEX solution at pH 6 for 10 min. Compared to Figure2A, a significant amount of adsorbate can be observed on the bornite when the mineral surface contacted the xanthate solution for 10 min. Figure 3B is the 3D image of Figure 3A. Figure 3C is the section analysis of Figure 3A, which clearly shows that the surface roughness increased due to the adsorption. Figure 3D is the deflection image of Figure 3A with a 10 nm data scale. Figure 3A is the height image of a bornite surface which contacted the 5 <sup>×</sup> <sup>10</sup>−<sup>5</sup> <sup>M</sup> KEX solution at pH 6 for 10 min. Compared to Figure 2A, a significant amount of adsorbate can be observed on the bornite when the mineral surface contacted the xanthate solution for 10 min. Figure 3B is the 3D image of Figure 3A. Figure 3C is the section analysis of Figure 3A, which clearly shows that the surface roughness increased due to the adsorption. Figure 3D is the deflection image of Figure 3A with a 10 nm data scale.

which shows that the solid surface was still largely smooth, with little adsorbate on the sample surface detected by the AFM probe. Figure 2B is the 3D image of Figure 2A. Figure 2C is the section analysis of Figure 2A. Figure 2D is the deflection image of Figure 2A with

**Figure 3.** AFM images of a bornite surface soaked in the 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 3D 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 bornite surface soaked in the 5 <sup>×</sup> <sup>10</sup>−<sup>5</sup> M KEX solution at pH 6 for 10 min. (**A**) The 5 <sup>µ</sup><sup>m</sup> <sup>×</sup> <sup>5</sup> <sup>µ</sup><sup>m</sup> height image with a data scale of 20 nm, (**B**) the 3D 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 4A is the height image of a bornite surface, which was in contact with the 1 × 10−<sup>4</sup> M KEX solution at pH 6 for 10 min. Compared to Figure 2A, a significant amount of adsorbate can be observed on the bornite when the mineral surface contacted the 1 × 10−<sup>4</sup> M KEX solution for 10 min. In addition, compared to Figure 3A, the mineral surface became rougher, suggesting that more precipitates were formed at the solid/liquid interface when the KEX's concentration increased from 5 × 10−<sup>5</sup> M to 1 × 10−<sup>4</sup> M. Figure 4B is the 3D image of Figure 4A. Figure 4C is the section analysis of Figure 4A, which confirms that the surface roughness increased due to the adsorption of the collector. Figure 4D is the deflection image of Figure 4A with a 10 nm data scale. Figure 4A is the height image of a bornite surface, which was in contact with the <sup>1</sup> <sup>×</sup> <sup>10</sup>−<sup>4</sup> <sup>M</sup> KEX solution at pH 6 for 10 min. Compared to Figure 2A, a significant amount of adsorbate can be observed on the bornite when the mineral surface contacted the <sup>1</sup> <sup>×</sup> <sup>10</sup>−<sup>4</sup> <sup>M</sup> KEX solution for 10 min. In addition, compared to Figure 3A, the mineral surface became rougher, suggesting that more precipitates were formed at the solid/liquid interface when the KEX's concentration increased from 5 <sup>×</sup> <sup>10</sup>−<sup>5</sup> M to 1 <sup>×</sup> <sup>10</sup>−<sup>4</sup> M. Figure 4B is the 3D image of Figure 4A. Figure 4C is the section analysis of Figure 4A, which confirms that the surface roughness increased due to the adsorption of the collector. Figure 4D is the deflection image of Figure 4A with a 10 nm data scale.

**Figure 4.** AFM images of a bornite surface soaked in the 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 3D 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 bornite surface soaked in the 1 <sup>×</sup> <sup>10</sup>−<sup>4</sup> M KEX solution at pH 6 for 10 min. (**A**) The 5 <sup>µ</sup><sup>m</sup> <sup>×</sup> <sup>5</sup> <sup>µ</sup><sup>m</sup> height image with a data scale of 20 nm, (**B**) the 3D 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 bornite surface soaked in the 1 × 10−<sup>4</sup> M KEX solution at pH 6 for 20 min and further rinsed with 10 mL ethanol and 10 mL water consecutively. The images were finally obtained when the mineral sample contacted the water. Compared to Figure 4A, it can be observed that the precipitates, as observed from Figure 4A, still existed on the bornite surface, and were not dissolved or rinsed off the mineral surface. Figure 5B is the 3D 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 bornite surface soaked in the 1 <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 10 mL water consecutively. The images were finally obtained when the mineral sample contacted the water. Compared to Figure 4A, it can be observed that the precipitates, as observed from Figure 4A, still existed on the bornite surface, and were not dissolved or rinsed off the mineral surface. Figure 5B is the 3D 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 bornite surface soaked in the 1 × 10<sup>−</sup><sup>4</sup> M KEX solution at pH 6 for 20 min and further rinsed with ethanol and water. (**A**) The 5 µm × 5 µm height image with a data scale of 20 nm, (**B**) the 3D 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 bornite surface soaked in the 1 <sup>×</sup> <sup>10</sup>−<sup>4</sup> M KEX solution at pH 6 for 20 min and further rinsed with ethanol and water. (**A**) The 5 µm × 5 µm height image with a data scale of 20 nm, (**B**) the 3D 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 bornite surface soaked in the 5 × 10−<sup>4</sup> M KEX solution at pH 6 for 10 min. By comparing Figure 6A, the height image, to Figure 2A, a significant amount of bornite can be observed after the mineral surface contacted the xanthate solution. In addition, comparing Figure 6A to Figures 3A and 4A, when the KEX's concentration increased, more precipitates formed at the solid/liquid interface, and the mineral surface became much rougher. Figure 6B is the 3D image of Figure 6A. Figure 6C, the section analysis of Figure 6A, confirms that the surface roughness increased greatly when the bornite surface contacted a high concentration of KEX. Figure 6D is the deflection image of Figure 6A with a 10 nm data scale. Figure <sup>6</sup> shows the AFM images of a bornite surface soaked in the 5 <sup>×</sup> <sup>10</sup>−<sup>4</sup> M KEX solution at pH 6 for 10 min. By comparing Figure 6A, the height image, to Figure 2A, a significant amount of bornite can be observed after the mineral surface contacted the xanthate solution. In addition, comparing Figure 6A to Figures 3A and 4A, when the KEX's concentration increased, more precipitates formed at the solid/liquid interface, and the mineral surface became much rougher. Figure 6B is the 3D image of Figure 6A. Figure 6C, the section analysis of Figure 6A, confirms that the surface roughness increased greatly when the bornite surface contacted a high concentration of KEX. Figure 6D is the deflection image of Figure 6A with a 10 nm data scale.

**Figure 6.** AFM images of a bornite surface soaked in the 5 × 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 3D 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 bornite surface soaked in the 5 <sup>×</sup> <sup>10</sup>−<sup>4</sup> M KEX solution at pH 6 for 10 min. (**A**) The 5 <sup>µ</sup><sup>m</sup> <sup>×</sup> <sup>5</sup> <sup>µ</sup><sup>m</sup> height image with a data scale of 20 nm, (**B**) the 3D 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 KEX on bornite in the solution, after Figure 6 was obtained, a 3 µm × 3 µm area was scanned once, applying a much larger scan force to intentionally remove the adsorbate. Further, the same position was scanned again in a 5 µm × 5 µm area, applying a normal scan force. The result is shown in Figure 7. From Figure 7A, the height image, a 3 µm × 3 µm 'window' can be observed in the center of the image due to the removal of some adsorbate from mineral surface under the previously applied large scan force. That is, the 'window' in the center with a low profile is the bornite surface covered by the adsorbate, which was partially removed by the applied large scan force. The surrounding area with a high profile is the mineral surface covered by the adsorbate, which was not disturbed by the large scan force. By comparing Figure 7B, the 3D image of Figure 7A, to Figure 6B, a pit on the mineral surface can be easily observed, with adsorbate covering the surrounding area. Figure 7C, the section analysis of Figure 7A, To verify the adsorption of the KEX on bornite in the solution, after Figure 6 was obtained, a 3 µm × 3 µm area was scanned once, applying a much larger scan force to intentionally remove the adsorbate. Further, the same position was scanned again in a 5 µm × 5 µm area, applying a normal scan force. The result is shown in Figure 7. From Figure 7A, the height image, a 3 µm × 3 µm 'window' can be observed in the center of the image due to the removal of some adsorbate from mineral surface under the previously applied large scan force. That is, the 'window' in the center with a low profile is the bornite surface covered by the adsorbate, which was partially removed by the applied large scan force. The surrounding area with a high profile is the mineral surface covered by the adsorbate, which was not disturbed by the large scan force. By comparing Figure 7B, the 3D image of Figure 7A, to Figure 6B, a pit on the mineral surface can be easily observed, with adsorbate covering the surrounding area. Figure 7C, the section analysis of Figure 7A, shows the height difference between the

shows the height difference between the 'window' (as shown by the green markers) and

'window' (as shown by the green markers) and the surrounding area being covered with adsorbate (as shown by the red markers). Figure 7D is the deflection image of Figure 7A with a 10 nm data scale. the surrounding area being covered with adsorbate (as shown by the red markers). Figure 7D is the deflection image of Figure 7A with a 10 nm data scale.

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**Figure 7.** AFM images of a bornite surface soaked in the 5 × 10-4 M KEX solution at pH 6 for 20 min. (**A**) The 5 µm × 5 µm height image with a data scale of 20 nm, (**B**) the 3D image, (**C**) the section analysis and (**D**) the deflection image with a data scale of 10 nm. **Figure 7.** AFM images of a bornite surface soaked in the 5 <sup>×</sup> <sup>10</sup>−<sup>4</sup> M KEX solution at pH 6 for 20 min. (**A**) The 5 <sup>µ</sup><sup>m</sup> <sup>×</sup> <sup>5</sup> <sup>µ</sup><sup>m</sup> height image with a data scale of 20 nm, (**B**) the 3D image, (**C**) the section analysis and (**D**) the deflection image with a data scale of 10 nm.

Figure 8 shows the AFM images of a bornite surface soaked in the 1 × 10−<sup>5</sup> M PAX solution at pH 6 for 10 min. By comparing Figure 8A, the height image, to Figure 2A, some precipitates can be observed on the bornite when the mineral surface contacted the PAX solution. Figure 8B is the 3D image of Figure 8A. Figure 8C, the section analysis of Figure 8A, shows that the surface roughness increased due to the adsorption of PAX. Figure 8D is the deflection image of Figure 8A with a 10 nm data scale. Figure <sup>8</sup> shows the AFM images of a bornite surface soaked in the 1 <sup>×</sup> <sup>10</sup>−<sup>5</sup> M PAX solution at pH 6 for 10 min. By comparing Figure 8A, the height image, to Figure 2A, some precipitates can be observed on the bornite when the mineral surface contacted the PAX solution. Figure 8B is the 3D image of Figure 8A. Figure 8C, the section analysis of Figure 8A, shows that the surface roughness increased due to the adsorption of PAX. Figure 8D is the deflection image of Figure 8A with a 10 nm data scale.

**Figure 8.** AFM images of a bornite surface soaked in the 1 × 10<sup>−</sup><sup>5</sup> M PAX solution at pH 6 for 10 min. (**A**) The 5 µm × 5 µm height image with a data scale of 20 nm, (**B**) the 3D 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 bornite surface soaked in the 1 <sup>×</sup> <sup>10</sup>−<sup>5</sup> M PAX solution at pH 6 for 10 min. (**A**) The 5 <sup>µ</sup><sup>m</sup> <sup>×</sup> <sup>5</sup> <sup>µ</sup><sup>m</sup> height image with a data scale of 20 nm, (**B**) the 3D 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 9 shows the AFM images of a bornite surface soaked in 5 × 10−<sup>5</sup> M PAX solution at pH 6 for 10 min. By comparing Figure 9A, the height image, to Figure 2A, a significant amount of adsorbate can be observed on the bornite when the mineral surface contacted the 5 × 10−<sup>5</sup> M PAX solution for 10 min. By comparing Figure 9A to Figure 8A, it can be seen that, when the PAX concentration increased from 1 × 10−<sup>5</sup> M to 5 × 10−<sup>5</sup> M, the mineral surface became much rougher, with more precipitates forming at the solid/liquid interface. Figure 9B is the 3D image of Figure 9A. Figure 9C is the section analysis of Figure 9A, which confirms that the surface roughness increased with the increasing PAX concentration. Figure 9D is the deflection image of Figure 9A with a 10 nm data scale. Figure <sup>9</sup> shows the AFM images of a bornite surface soaked in 5 <sup>×</sup> <sup>10</sup>−<sup>5</sup> M PAX solution at pH 6 for 10 min. By comparing Figure 9A, the height image, to Figure 2A, a significant amount of adsorbate can be observed on the bornite when the mineral surface contacted the 5 <sup>×</sup> <sup>10</sup>−<sup>5</sup> M PAX solution for 10 min. By comparing Figure 9A to Figure 8A, it can be seen that, when the PAX concentration increased 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 became much rougher, with more precipitates forming at the solid/liquid interface. Figure 9B is the 3D image of Figure 9A. Figure 9C is the section analysis of Figure 9A, which confirms that the surface roughness increased with the increasing PAX concentration. Figure 9D is the deflection image of Figure 9A with a 10 nm data scale.

**Figure 9.** AFM images of a bornite surface soaked in the 5 × 10<sup>−</sup><sup>5</sup> M PAX solution at pH 6 for 10 min. (**A**) The 5 µm × 5 µm height image with a data scale of 20 nm, (**B**) the 3D 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 9.** AFM images of a bornite surface soaked in the 5 <sup>×</sup> <sup>10</sup>−<sup>5</sup> M PAX solution at pH 6 for 10 min. (**A**) The 5 <sup>µ</sup><sup>m</sup> <sup>×</sup> <sup>5</sup> <sup>µ</sup><sup>m</sup> height image with a data scale of 20 nm, (**B**) the 3D 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 10 shows the AFM images of a bornite surface soaked in the 1 × 10−<sup>4</sup> M PAX solution at pH 6 for 10 min. By comparing Figure 10A, the height image, to Figure 2A, a significant amount of adsorbate can be observed on the bornite when the mineral surface contacted the 1 × 10−<sup>4</sup> M PAX solution for 10 min. By comparing Figure 10A to Figure 8A and Figure 9A, when the PAX concentration increases, the mineral surface became much rougher, with more precipitates forming at the solid/liquid interface. Figure 10B is the 3D image of Figure 10A. Figure 10C, the section analysis of Figure 10A, shows that the surface roughness increased with the increasing PAX concentration. Figure 10D is the deflection image of Figure 10A with a 10 nm data scale. Figure <sup>10</sup> shows the AFM images of a bornite surface soaked in the 1 <sup>×</sup> <sup>10</sup>−<sup>4</sup> M PAX solution at pH 6 for 10 min. By comparing Figure 10A, the height image, to Figure 2A, a significant amount of adsorbate can be observed on the bornite when the mineral surface contacted the 1 <sup>×</sup> <sup>10</sup>−<sup>4</sup> M PAX solution for 10 min. By comparing Figure 10A to Figures 8A and 9A, when the PAX concentration increases, the mineral surface became much rougher, with more precipitates forming at the solid/liquid interface. Figure 10B is the 3D image of Figure 10A. Figure 10C, the section analysis of Figure 10A, shows that the surface roughness increased with the increasing PAX concentration. Figure 10D is the deflection image of Figure 10A with a 10 nm data scale.

**Figure 10.** AFM images of a bornite surface soaked in the 1 × 10<sup>−</sup><sup>4</sup> M PAX solution at pH 6 for 10 min. (**A**) The 5 µm × 5 µm height image with a data scale of 20 nm, (**B**) the 3D 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 10.** AFM images of a bornite surface soaked in the 1 <sup>×</sup> <sup>10</sup>−<sup>4</sup> M PAX solution at pH 6 for 10 min. (**A**) The <sup>5</sup> <sup>µ</sup><sup>m</sup> <sup>×</sup> <sup>5</sup> <sup>µ</sup><sup>m</sup> height image with a data scale of 20 nm, (**B**) the 3D 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 bornite in the solution, after Figure 10 was obtained, a 2 µm × 2 µm area was scanned once, applying a much larger scan force to intentionally remove the adsorbate. Further, the same position was scanned again in a 5 µm × 5 µm area, applying a normal scan force. The result is shown as Figure 11. As shown in Figure 11A, the height image, a 2 µm × 2 µm 'window' is shown in the center of the image due to the partial removal of the adsorbate from the mineral surface under the previously applied large scan force. That is, the 'window' in the center with a low profile is the bornite surface covered by the adsorbate, which was partially removed by the applied large scan force. The surrounding area with a high profile is the mineral surface covered by the adsorbate, which was not disturbed by the large scan force. This finding is as that shown in Figure 7. By comparing Figure 11B, the 3D image of Figure 11A, to Figure 10B, a pit on mineral surface can be easily observed, with adsorbate covering the surrounding To verify the adsorption of the PAX on bornite in the solution, after Figure 10 was obtained, a 2 µm × 2 µm area was scanned once, applying a much larger scan force to intentionally remove the adsorbate. Further, the same position was scanned again in a 5 µm × 5 µm area, applying a normal scan force. The result is shown as Figure 11. As shown in Figure 11A, the height image, a 2 µm × 2 µm 'window' is shown in the center of the image due to the partial removal of the adsorbate from the mineral surface under the previously applied large scan force. That is, the 'window' in the center with a low profile is the bornite surface covered by the adsorbate, which was partially removed by the applied large scan force. The surrounding area with a high profile is the mineral surface covered by the adsorbate, which was not disturbed by the large scan force. This finding is as that shown in Figure 7. By comparing Figure 11B, the 3D image of Figure 11A, to Figure 10B, a pit on mineral surface can be easily observed, with adsorbate covering the surrounding

area. Figure 11C is the section analysis of Figure 11A, which shows the height difference

area. Figure 11C is the section analysis of Figure 11A, which shows the height difference between the 'window' and the surrounding area covered with adsorbate (indicated by the red markers). Figure 11D is the deflection image of Figure 11A with a 10 nm data scale. The adsorbate strongly combined with the mineral surface strongly, and a quite large scan force had to be applied during the experiment. Therefore, the obtained 'window' was slightly deformed, as shown in Figure 11A. between the 'window' and the surrounding area covered with adsorbate (indicated by the red markers). Figure 11D is the deflection image of Figure 11A with a 10 nm data scale. The adsorbate strongly combined with the mineral surface strongly, and a quite large scan force had to be applied during the experiment. Therefore, the obtained 'window' was slightly deformed, as shown in Figure 11A.

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**Figure 11.** AFM images of a bornite surface soaked in 1 × 10<sup>−</sup><sup>4</sup> M PAX solution at pH 6 for 20 min. (**A**) The 5 µm × 5 µm height image with a data scale of 20 nm, (**B**) the 3D image, (**C**) the section analysis and (**D**) the deflection image with a data scale of 10 nm. The 2 µm × 2 µm blank 'window' in the center of the image occurred due to the removal of the adsorbate from the mineral surface under the intentionally applied large scan force. **Figure 11.** AFM images of a bornite surface soaked in 1 <sup>×</sup> <sup>10</sup>−<sup>4</sup> M PAX solution at pH 6 for 20 min. (**A**) The 5 <sup>µ</sup><sup>m</sup> <sup>×</sup> <sup>5</sup> <sup>µ</sup><sup>m</sup> height image with a data scale of 20 nm, (**B**) the 3D image, (**C**) the section analysis and (**D**) the deflection image with a data scale of 10 nm. The 2 µm × 2 µm blank 'window' in the center of the image occurred due to the removal of the adsorbate from the mineral surface under the intentionally applied large scan force.

Figure 12 shows the AFM images of a bornite surface soaked in the 5 × 10 <sup>−</sup><sup>4</sup> M KEX solution at pH 10. Figure 12A–C are the height images with a 20 nm data scale obtained after the mineral surface contacted the KEX solution, respectively, for 5, 10 and 20 min. Similar to Figure 6, in all the images, a significant amount of adsorbate can be observed on the bornite. Figure 12D is the 1 µm × 1 µm (large magnification) height image collected Figure <sup>12</sup> shows the AFM images of a bornite surface soaked in the 5 <sup>×</sup> <sup>10</sup>−<sup>4</sup> M KEX solution at pH 10. Figure 12A–C are the height images with a 20 nm data scale obtained after the mineral surface contacted the KEX solution, respectively, for 5, 10 and 20 min. Similar to Figure 6, in all the images, a significant amount of adsorbate can be observed on

right after Figure 12C was obtained.

the bornite. Figure 12D is the 1 µm × 1 µm (large magnification) height image collected right after Figure 12C was obtained. *Minerals* **2021**, *11*, x FOR PEER REVIEW 15 of 24

**Figure 12.** AFM height images (5 µm × 5 µm) with a data scale of 20 nm of a bornite surface soaked in the 5 × 10<sup>−</sup><sup>4</sup> M KEX solution at pH 10 (**A**) for 5 min, (**B**) for 10 min, (**C**) for 20 min and (**D**) for 20 min (1 µm × 1 µm). **Figure 12.** AFM height images (5 <sup>µ</sup><sup>m</sup> <sup>×</sup> <sup>5</sup> <sup>µ</sup>m) with a data scale of 20 nm of a bornite surface soaked in the 5 <sup>×</sup> <sup>10</sup>−<sup>4</sup> M KEX solution at pH 10 (**A**) for 5 min, (**B**) for 10 min, (**C**) for 20 min and (**D**) for 20 min (1 µm × 1 µm).

Figure 13 shows the AFM images of a bornite surface soaked in the 1 × 10−<sup>4</sup> M PAX solution at pH 10. Figure 13A–C are the height images with a 20 nm data scale obtained after the mineral surface contacted the PAX solution, respectively, for 5, 10 and 20 min. Similar to Figure 10, in all the images, a significant amount of adsorbate can be observed on the bornite. Figure 13D is the 1 µm × 1 µm (large magnification) height image collected right after Figure 13C was obtained. Figure <sup>13</sup> shows the AFM images of a bornite surface soaked in the 1 <sup>×</sup> <sup>10</sup>−<sup>4</sup> M PAX solution at pH 10. Figure 13A–C are the height images with a 20 nm data scale obtained after the mineral surface contacted the PAX solution, respectively, for 5, 10 and 20 min. Similar to Figure 10, in all the images, a significant amount of adsorbate can be observed on the bornite. Figure 13D is the 1 µm × 1 µm (large magnification) height image collected right after Figure 13C was obtained.

**Figure 13.** AFM height images (5 µm × 5 µm) with a data scale of 20 nm of a bornite surface soaked in the 1 × 10<sup>−</sup><sup>4</sup> M PAX solution at pH 10 (**A**) for 5 min, (**B**) for 10 min, (**C**) for 20 min and (**D**) for 20 min (1 µm × 1 µm). **Figure 13.** AFM height images (5 <sup>µ</sup><sup>m</sup> <sup>×</sup> <sup>5</sup> <sup>µ</sup>m) with a data scale of 20 nm of a bornite surface soaked in the 1 <sup>×</sup> <sup>10</sup>−<sup>4</sup> M PAX solution at pH 10 (**A**) for 5 min, (**B**) for 10 min, (**C**) for 20 min and (**D**) for 20 min (1 µm × 1 µm).

To clearly show the difference of the adsorbate in the morphologies of bornite and chalcopyrite, the adsorption of xanthate on chalcopyrite was also studied, and the AFM image is shown in Figure 14. Figure 14A,B were obtained with 5 × 10−<sup>4</sup> M KEX. Figure 14C,D were obtained with 5 × 10−<sup>5</sup> M PAX. The AFM images clearly show that there was patch-like adsorbate on the chalcopyrite surface, which was flat with smooth and round edges. This morphology fits well with the fact that oily dialkyl dixanthogen is generally insoluble in water, and the circular boundary is the direct result of the high interfacial tension between the hydrophobic dixanthogen and water [17,18]. In addition, this adsorbate had a completely different morphology as the one shown in Figures 3–13. To clearly show the difference of the adsorbate in the morphologies of bornite and chalcopyrite, the adsorption of xanthate on chalcopyrite was also studied, and the AFM image is shown in Figure 14. Figure 14A,B were obtained with 5 <sup>×</sup> <sup>10</sup>−<sup>4</sup> M KEX. Figure 14C,D were obtained with 5 <sup>×</sup> <sup>10</sup>−<sup>5</sup> M PAX. The AFM images clearly show that there was patch-like adsorbate on the chalcopyrite surface, which was flat with smooth and round edges. This morphology fits well with the fact that oily dialkyl dixanthogen is generally insoluble in water, and the circular boundary is the direct result of the high interfacial tension between the hydrophobic dixanthogen and water [17,18]. In addition, this adsorbate had a completely different morphology as the one shown in Figures 3–13.

**Figure 14.** AFM images of a chalcopyrite surface in the xanthate solution at pH 6 for 30 min. (**A**) The 10 µm × 10 µm deflection image with a data scale of 200 nm at 5 × 10<sup>−</sup><sup>4</sup> M KEX, (**B**) the 3D image of (**A**), (**C**) the 10 µm × 10 µm deflection image with a data scale of 200 nm at 5 × 10<sup>−</sup><sup>5</sup> M PAX and (**D**) the 3D image of (**C**). **Figure 14.** AFM images of a chalcopyrite surface in the xanthate solution at pH 6 for 30 min. (**A**) The 10 µm × 10 µm deflection image with a data scale of 200 nm at 5 <sup>×</sup> <sup>10</sup>−<sup>4</sup> M KEX, (**B**) the 3D image of (**A**), (**C**) the 10 <sup>µ</sup><sup>m</sup> <sup>×</sup> <sup>10</sup> <sup>µ</sup>m deflection image with a data scale of 200 nm at 5 <sup>×</sup> <sup>10</sup>−<sup>5</sup> M PAX and (**D**) the 3D image of (**C**).

#### *3.2. AFM Surface Force Measurement 3.2. AFM Surface Force Measurement*

The interaction force (F) between an AFM probe and a polished bornite plate is measured when the plate contacts various PAX solutions at pH 6 for 10 min. Using an AFM force measurement, one can obtain both the approach force curve and the retract force curve, which are shown as Figures 15 and 16, respectively. The interaction force (F) between an AFM probe and a polished bornite plate is measured when the plate contacts various PAX solutions at pH 6 for 10 min. Using an AFM force measurement, one can obtain both the approach force curve and the retract force curve, which are shown as Figures 15 and 16, respectively.

*Minerals* **2021**, *11*, x FOR PEER REVIEW 18 of 24

**Figure 15.** The approach force (F) measured between an AFM probe and a bornite plate soaked in solutions at pH 6 as a function of the separation (H) between the probe and the plate by applying an AFM force measurement. (Δ) in water; (×) in the 1 × 10<sup>−</sup><sup>5</sup> M PAX solution, (o) in the 5 × 10<sup>−</sup><sup>5</sup> M PAX solution and (□) in the 1 × 10<sup>−</sup><sup>4</sup> M PAX solution. The inlet (◊) shows the approach force curve obtained with CuFeS<sup>2</sup> in the 5 × 10<sup>−</sup><sup>5</sup> M PAX solution. **Figure 15.** The approach force (F) measured between an AFM probe and a bornite plate soaked in solutions at pH 6 as a function of the separation (H) between the probe and the plate by applying an AFM force measurement. (∆) in water; (×) in the 1 <sup>×</sup> <sup>10</sup>−<sup>5</sup> M PAX solution, (o) in the 5 <sup>×</sup> <sup>10</sup>−<sup>5</sup> <sup>M</sup> PAX solution and () in the 1 <sup>×</sup> <sup>10</sup>−<sup>4</sup> M PAX solution. The inlet (♦) shows the approach force curve obtained with CuFeS<sup>2</sup> in the 5 <sup>×</sup> <sup>10</sup>−<sup>5</sup> M PAX solution. **Figure 15.** The approach force (F) measured between an AFM probe and a bornite plate soaked in solutions at pH 6 as a function of the separation (H) between the probe and the plate by applying an AFM force measurement. (Δ) in water; (×) in the 1 × 10<sup>−</sup><sup>5</sup> M PAX solution, (o) in the 5 × 10<sup>−</sup><sup>5</sup> M PAX solution and (□) in the 1 × 10<sup>−</sup><sup>4</sup> M PAX solution. The inlet (◊) shows the approach force curve obtained with CuFeS<sup>2</sup> in the 5 × 10<sup>−</sup><sup>5</sup> M PAX solution.

**Figure 16.** The detach force (F) measured between an AFM probe and a bornite plate soaked in solutions at pH 6 as a function of the separation (H) between the probe and the plate by applying an AFM force measurement. (Δ) in water, (×) in the 1 × 10<sup>−</sup><sup>5</sup> M PAX solution, (o) in the 5 × 10<sup>−</sup><sup>5</sup> M PAX solution and (□) in the 1 × 10<sup>−</sup><sup>4</sup> M PAX solution. The inlet (◊) shows the detach force curve obtained with CuFeS<sup>2</sup> in the 5 × 10<sup>−</sup><sup>5</sup> M PAX solution. **Figure 16.** The detach force (F) measured between an AFM probe and a bornite plate soaked in solutions at pH 6 as a function of the separation (H) between the probe and the plate by applying an AFM force measurement. (Δ) in water, (×) in the 1 × 10<sup>−</sup><sup>5</sup> M PAX solution, (o) in the 5 × 10<sup>−</sup><sup>5</sup> M PAX solution and (□) in the 1 × 10<sup>−</sup><sup>4</sup> M PAX solution. The inlet (◊) shows the detach force curve obtained with CuFeS<sup>2</sup> in the 5 × 10<sup>−</sup><sup>5</sup> M PAX solution. **Figure 16.** The detach force (F) measured between an AFM probe and a bornite plate soaked in solutions at pH 6 as a function of the separation (H) between the probe and the plate by applying an AFM force measurement. (∆) in water, (×) in the 1 <sup>×</sup> <sup>10</sup>−<sup>5</sup> M PAX solution, (o) in the 5 <sup>×</sup> <sup>10</sup>−<sup>5</sup> <sup>M</sup> PAX solution and () in the 1 <sup>×</sup> <sup>10</sup>−<sup>4</sup> M PAX solution. The inlet (♦) shows the detach force curve obtained with CuFeS<sup>2</sup> in the 5 <sup>×</sup> <sup>10</sup>−<sup>5</sup> M PAX solution.

Figure 15 shows that the approach force curves measured in water and various PAX solutions were similar to each other. The "jump-in" occurred where the separation was less than 5 nm, which was within the range of the van der Waals force. Figure 16 shows that the detach force measured between an AFM probe and bornite surface in water was about 3 nN. The value increased slightly to 4 nN when in the 1 <sup>×</sup> <sup>10</sup>−<sup>5</sup> M PAX solution. Further increasing the concentration of the PAX solution did not significantly change the detach force. The fact that the "jump-off" point was sharp and that the "jump-off" point occurred where the separation was close to 0 nm suggest that the adsorbate is physically rigid in nature. The inlet shows the detach force curve obtained with CuFeS<sup>2</sup> in <sup>5</sup> <sup>×</sup> <sup>10</sup>−<sup>5</sup> <sup>M</sup> PAX solution, and the detach force was about 5.2 nN. The fact that the "jump-off" point was not sharp and that the "jump-off" point occurred at above 50 nm confirms that the oily dixanthogen is deformable. Figure 15 shows that the approach force curves measured in water and various PAX solutions were similar to each other. The "jump-in" occurred where the separation was less than 5 nm, which was within the range of the van der Waals force. Figure 16 shows that the detach force measured between an AFM probe and bornite surface in water was about 3 nN. The value increased slightly to 4 nN when in the 1 × 10−<sup>5</sup> M PAX solution. Further increasing the concentration of the PAX solution did not significantly change the detach force. The fact that the "jump-off" point was sharp and that the "jump-off" point occurred where the separation was close to 0 nm suggest that the adsorbate is physically rigid in nature. The inlet shows the detach force curve obtained with CuFeS<sup>2</sup> in 5 × 10−<sup>5</sup> M PAX solution, and the detach force was about 5.2 nN. The fact that the "jump-off" point was not sharp and that the "jump-off" point occurred at above 50 nm confirms that the oily dixanthogen is deformable.

#### *3.3. AFT-FTIR Results 3.3. AFT-FTIR Results*

Figure 17 shows the ATR-FTIR spectra of the adsorbate on bornite after the mineral surface 5 <sup>×</sup> <sup>10</sup>−<sup>4</sup> M KEX solution at pH 6 for 1 h. On the spectra, the main peaks shown at 1195 cm−<sup>1</sup> and 1126 cm−<sup>1</sup> were due to the bonds of C-O-C, and the peaks at 1049 cm−<sup>1</sup> and 1032 cm−<sup>1</sup> were due to the bonds of S-C-S. The results are in line with the FTIR spectra of cuprous xanthate as reported by Poling [23] and Leppinen et al. [24]. That is, the obtained FTIR spectra, as shown in Figure 17, confirms that the adsorbate on bornite in xanthate solutions is essentially CuX, with no dixanthogen detected. Figure 17 shows the ATR-FTIR spectra of the adsorbate on bornite after the mineral surface 5 × 10−<sup>4</sup> M KEX solution at pH 6 for 1 h. On the spectra, the main peaks shown at 1195 cm−<sup>1</sup> and 1126 cm−<sup>1</sup> were due to the bonds of C-O-C, and the peaks at 1049 cm−<sup>1</sup> and 1032 cm−<sup>1</sup> were due to the bonds of S-C-S. The results are in line with the FTIR spectra of cuprous xanthate as reported by Poling [23] and Leppinen et al. [24]. That is, the obtained FTIR spectra, as shown in Figure 17, confirms that the adsorbate on bornite in xanthate solutions is essentially CuX, with no dixanthogen detected.

**Figure 17.** ATR-FTIR spectra of a bornite surface after it contacted the 5 × 10<sup>−</sup><sup>4</sup> M KEX solution at pH 6 for 1 h. **Figure 17.** ATR-FTIR spectra of a bornite surface after it contacted the 5 <sup>×</sup> <sup>10</sup>−<sup>4</sup> M KEX solution at pH 6 for 1 h.

## **4. Discussion**

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

As shown in Figures 3–13, a significant amount of adsorbate can be observed on the bornite surface when it contacted xanthate solutions for a specific time. The roughness analysis of the AFM images of a bornite surface in xanthate solutions is summarized and listed as Table 1. In general, during the same timeframe, i.e., 10 min, surface roughness increased when the concentration of xanthate increased. For example, at pH 6, when the concentration of KEX increased from 5 <sup>×</sup> <sup>10</sup>−<sup>5</sup> M to 1 <sup>×</sup> <sup>10</sup>−<sup>4</sup> M, the roughness Ra value increased from 1.919 nm to 2.372 nm, and the value increased to 2.412 nm when the concentration was further increased to 5 <sup>×</sup> <sup>10</sup>−<sup>4</sup> M. A similar trend was also observed for the case of PAX, although the change in values was not as significant as the one as obtained with KEX. The change of the morphology of the bornite surface cannot be attributed to the reaction of the mineral surface with water, because the AFM images obtained with the addition of xanthate solutions are completely different from those shown in Figure 2, which were captured within same timeframe. Therefore, the adsorbate shown in Figures 3–13 must be due to the adsorption of xanthate at the bornite/liquid interface.

**Table 1.** Roughness analysis of the AFM images of a bornite 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.

> The adsorption of metal xanthate with a low solubility and the oxidation of xanthate into dixanthogen on a sulfide mineral surface in an aqueous solution, as summarized by Leja [5] and Woods [6], are generally considered the main mechanisms for the increase in hydrophobicity of sulfide minerals in flotation. Previous AFM studies with chalcopyrite and pyrite [17–19] have shown that 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 addition, under ambient conditions, i.e., room temperature and normal pressure, dialkyl dixanthogen is usually in a liquid form with a low melting point. [25] During an AFM scanning process, a minimal force must be applied because of the softness of dixanthogen [17–19]. In this investigation, as shown in Figures 3–13, the adsorbate had no smooth and round edges, and the morphology was completely different compared to that observed for dixanthogen, which is shown in Figure 14. In addition, the adsorbate on the bornite surface was not 'soft,' and its morphology was not disturbed, even under

an elevated scan force. For example, as shown in Figures 7–11, a large scan force must be applied to remove the adsorbate and open a 'window' in the center of the AFM image. In addition, Figures 15 and 16 show that the adsorbate on the bornite in the xanthate solution was rigid, with sharp "jump-in" and "jump-off" points on the force curves. Therefore, we ruled out the possibility that observed adsorbate on the bornite was dixanthogen.

This finding, as obtained from the AFM imaging analysis results, is in line with what has been previously reported. For example, Allison et al. [3] reported that xanthate adsorbed on bornite in the form of metal xanthate with low solubility, i.e., CuX. It has been suggested that the final adsorption products on sulfides are highly associated with the semiconductor type of sulfide minerals. That is, dixanthogen is usually formed on n-type minerals, while metal xanthate is observed on p-type minerals. Bornite is classified as a p-type mineral [26], which favors the formation of metal xanthate.

According to Buckley et al. [9], the adsorption of xanthate on bornite is an electrochemical process depending on the potential. When the potential is above −0.35 v, bornite is oxidized in water and yields an iron–free copper sulfide, the reaction of which is shown as follows:

#### *Cu***5***FeS***4***+***3***H***2***O=Cu***5***S***4***+Fe(OH)***3***+***3***H <sup>+</sup>+***3***e* (1)

Comparing Figures 1 and 2, the bornite surface that contacted the water was rougher than the surface that contacted air. In the present investigation, the solution potential was not controlled, and the potential value of DI water was higher than −0.35 V. These results suggest that the bornite surface will undertake oxidation reaction to some extent when it contacts water following the reaction, as shown by Equation (1).

Zachwieja et al. [10] also proposed that the adsorption of xanthate on bornite is due to the following simplified anodic reaction simplified, introducing insoluble cuprous xanthate on the bornite surface:

$$\text{Cu}\_5\text{S}\_4 + nX^- = n\text{CuX} + \text{Cu}\_{5-n}\text{S}\_4 + n e^- \tag{2}$$

That is, xanthate adsorbs on bornite mainly in the form of insoluble cuprous xanthate at a low solution potential. The production of dixanthogen on bornite occurs only when the potential is above the rest potential of X/X<sup>2</sup> couple. In the present study, the solution potential was not controlled, and the value was generally below −0.1 V. In addition, as mentioned before, no noticeable dixanthogen was observed from the obtained AFM images. Therefore, the adsorbate, as shown in Figures 3–11, is mainly cuprous alkyl xanthate with a low solubility.

In addition, the ATR-FTIR results of the adsorption of KEX on bornite show that the main peaks on the obtained spectra were at 1195 cm−<sup>1</sup> (C–O–C), 1126 cm−<sup>1</sup> (C–O–C), 1049 cm−<sup>1</sup> (S–C–S) and 1032 cm−<sup>1</sup> (S–C–S) (Figure 16). The results are almost identical to those that have been reported for the FTIR spectra of CuX by Poling [23] and Leppinen et al. [24]. In addition, the fact that the characteristic peaks of ethyl xanthate dixanthogen (X2), namely those at 1020 cm−<sup>1</sup> and 1260 cm−<sup>1</sup> , were not observed on the spectra as obtained rules out the existence of X<sup>2</sup> on the bornite surface. Therefore, the irregular adsorbate, as shown in Figures 3–11, is basically insoluble cuprous xanthate (CuX). In this sense, the adsorption behavior of xanthate on the bornite is very similar to the adsorption behavior applicable for the case of chalcocite/xanthate systems [20].

#### *4.2. Effect of the Hydrocarbon Chain of Xanthate*

In froth flotation, the rank and concentration of a collector are two important parameters in determining the collectivity and selectivity of the collector. In general, a xanthate collector with a high rank has a high collectivity and, therefore, a low selectivity, and vice versa. In the present work, the effect of the collector's rank on the adsorption of xanthate on bornite was studied using both KEX and PAX as collectors. Figures 3–11 show that, when the collector's concentration is constant, PAX adsorbs on the bornite surface with a significantly higher surface coverage and a more uniform layer structure. This can be attributed to the fact that the formation of cuprous amyl xanthate occurs at a lower surfactant

concentration than that for cuprous ethyl xanthate because of the lower-solubility product of cuprous amyl xanthate. For example, it has been reported that the solubility product of cuprous amyl xanthate and cuprous ethyl xanthate is 8.0 <sup>×</sup> <sup>10</sup>−<sup>22</sup> and 5.2 <sup>×</sup> <sup>10</sup>−20, respectively. [27,28] 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 is due to the longer hydrocarbon chain of the former and, therefore, an increased lateral hydrophobic attraction between hydrocarbon chains. Therefore, the higher-rank xanthate, i.e., PAX, is more powerful than the lower rank-xanthate, i.e., KEX, for adsorption on the bornite surface. Therefore, the higher-rank xanthate provides a highly improved flotation collectivity.

Allison et al. [3] observed that "no product of reaction with the methyl and ethyl homologues could be detected, although both reacted very extensively with the surface." They further explained this by stating that "the reaction products are not detected because the lower homologues of cuprous xanthate are extremely insoluble in CS<sup>2</sup> and most other solvents and consequently are not extracted from the surface." As shown in Figures 3–7 obtained with the present work, the KEX did adsorb on the bornite intensively, with the mineral surface being fully covered. The binding of the adsorbate, i.e., cuprous ethyl xanthate, and bornite is very strong, and the adsorbate was not removed even after applying a large scan force. In addition, Figure 5 shows that the adsorbate was not extracted from the bornite surface by rinsing with ethanol alcohol.
