An Atomic Force Microscopy Study of the Adsorption of Collectors on Chalcopyrite

Transcription

1 An Atomic Force Microscopy Study of the Adsorption of Collectors on Chalcopyrite Jinhong Zhang and Wei Zhang Department of Mining and Geological Engineering, The University of Arizona, Tucson, AZ, 85721, USA The adsorption of various collectors on chalcopyrite in aqueous solutions has been studied in situ by applying an atomic force microscopy (AFM). The obtained AFM images show that the morphology of mineral surface changes correspondingly with the change of solution chemistry, such as collectors type and dosage, suggesting that AFM is a powerful tool for the study of the adsorption of chemicals on mineral surface in froth flotation. By comparing the AFM images obtained at different conditions, one can also conclude the effectiveness of a collector and its optimum dosage during the flotation of chalcopyrite. Based on the findings, the application of different types of collectors in chalcopyrite flotation is discussed. Keywords: AFM; collector; flotation; chalcopyrite 1. Introduction Chalcopyrite (CuFeS 2 ), the most important copper mineral, is usually milled and further separated from the impurities, such as silicates and pyrite, by froth flotation, in which the copper-bearing minerals stick to aerated bubbles in the pulp and float to the surface, leaving the gangue at the bottom of the flotation cell where it is disposed of as mine tailings. In chalcopyrite flotation, collectors are usually added into the pulp (mixture of water and fine-grained minerals) to selectively adsorb onto the target mineral, i.e., chalcopyrite, and render its surface with high hydrophobicity, which is generally beneficial for a strong mineral-bubble attachment and thus a high flotation recovery. In industrial practice, various types of collectors such as xanthate, dithiolphosphate, and mercaptan (thiol), can be applied for the flotation of chalcopyrite. Generally, the choice of chemical scheme depends on the mineralogy, impurities and flowsheet. It is quite common that multiple collectors are used for a high metal recovery. For example, xanthate is often used as the primary collector with dithiophosphate collector acting as a secondary collector (promoter), because the latter is a better gold collector than xanthate with a higher selectivity against pyrite. The adsorption of collector on mineral surface is vital for a successful flotation process and it has been studied by applying various surface analysis techniques, such as IR (Infrared Spectroscopy), CV (Cyclic Voltammetry) and XPS (X-ray Photoelectron Spectroscopy) [1-6]. These studies have revealed a lot of information, such as the reaction, product and mechanism, of the adsorption of chemicals on mineral surface. It is also of great interest to directly get the image of adsorbed collectors on mineral surface changing with pulp chemistry, such as ph, redox potential, ionic strength and chemical s dosage. Polkin et al. (1955) [7] and Plaksin et al. (1956) [8] applied the autoradiography technique to obtain the images of xanthate radioactive isotopes absorbed on sulfide minerals. Kim et al. (1996) [9] and Smart et al. (2003) [10] applied scanning tunneling microscopy (STM) to study the change, i.e. oxidation, reaction and adsorption, of galena surface under flotation-related conditions. Zhang et al., (2010, 2012) [11-14] has applied the AFM imaging technique in the study of the adsorption of chemicals on mineral surfaces. By comparison, AFM has some advantages over other surface characterization techniques. First, by employing an AFM fluid cell, one can study the in-situ absorption of chemicals on solid in solution. On the other hand, some other analysis techniques, with the prerequisite of taking solid sample out of solution, may not collect the actual adsorption information because the adsorption usually changes when the solid sample is removed from solution. Second, a high vacuum is not required for an AFM measurement. Some chemicals such as dixanthogen, an important reaction product on sulfide surface to increase the mineral s hydrophobicity, are usually in a liquid form at room temperature. Thus, it is extremely hard to detect these chemicals without cooling the measuring system to a low temperature [6]. By now, a lot of research has been done on the flotation of chalcopyrite under different flotation conditions. However, the information of the adsorption patterns of collectors on chalcopyrite surface is barely available. In present investigation, an AFM has been applied to get the surface morphology of chalcopyrite in various aqueous solutions. By comparing the AFM images obtained under different conditions, one can study the adsorption of different collectors on chalcopyrite surface. The finding of present work will not only help clarify the adsorption mechanisms of different collectors on chalcopyrite, but also boost the development of novel collectors for a better chalcopyrite flotation practice in the future. 967

2 2. Experimental 2.1 Materials Research grade chalcopyrite (CuFeS 2 ) is obtained from Wards Natural Science Establishment Inc. Sample is finely polished, further cleaned by rinsing thoroughly with ethanol and water and a 1.2 cm 1.2 cm sample is used for the surface characterization experiments. The nanopure water used in present work has a conductivity of 18.2 MΩ-cm at 25 C and a surface tension of 72.5 mn/m at 22 C. The water is used without purposely removing the dissolved atmospheric CO 2. Thus, the ph of the water used is in the range of Potassium ethyl xanthate (KEX) (>98%) and potassium amyl xanthate (PAX) (>98%) are obtained from Alfa Aesar. Aero 350, 400, 404, 412 (Cytec Inc.) and Nalco 9740 (Nalco) are industrial flotation collectors which are being used in some copper concentrators. These chemicals are used without further purification. All the solutions are freshly prepared each time right before the experiments of surface characterization are carried out. They are also adjusted to ph 11 by the addition of NaOH. 2.2 AFM surface image measurement Surface images of a chalcopyrite sample soaked in various solutions are measured by using a Digital Instrument Nanoscope IIId AFM at room temperature (25+1 C). Silicon nitride NP-20 cantilevers are obtained from Veeco, CA. Triangular cantilevers with nominal spring constant of 0.12~0.58 N/m are used for surface image measurements. During the AFM imaging study, a 10 ml solution of a specific chemical at ph 11 is flushed through the AFM fluid cell and the AFM measurement is commenced after the chalcopyrite plate contacts the chemical s solution for a specific time. The AFM images reported in this communication include both height and deflection images obtained in the contact mode. They are processed by no image modification other than being flattened. All the images are obtained by scanning a mineral surface in a 10 µm 10 µm area unless specifically stated. 3. Results and Discussion 3.1 AFM surface images Figure 1 shows AFM images of a chalcopyrite surface obtained after the sample is soaked in M KEX for 10 minutes. Fig. 1A is the AFM height image with a 200 nm data scale. One can clearly see that many small patches appear on the chalcopyrite surface. During the experiments, it is noticed that these patches are relatively deformable under the scanning AFM probe; therefore, a minimal scan force has been applied in order not to disturb the absorbate. Fig. 1B is the AFM deflection image with a data scale of 50 nm, which clearly shows the shape and morphology of the patches. Fig. 1C is the 3-D image of Fig. 1A. The section analysis of Fig. 1A is shown as Fig. 1D, from which one can clearly see that the radius of one medium patch is about 469 nm and the height of the cap is about 31 nm. Thus, the angle formed by the radius and the height is only about 3.7. In addition, the survey of many patches of different size on the whole AFM image shows that the maximum angle is about 5, suggesting that the patches of the absorbate are in fact quite flat on the solid surface. Figure 2 shows AFM images of a chalcopyrite sample surface which has been soaked in M PAX solution for 10 minutes. Fig. 2A is the AFM height image with a 200 nm data scale. It shows that many patches absorbed on mineral surface with a quite high density within 10 minutes. Fig. 2B is the AFM deflection image with a data scale of 50 nm. Similar to the patches observed in KEX solutions, the absorbed patches are soft and deformable; therefore, a minimal scan force was also applied when the solid surface is scanned. Fig. 2C is the 3-D image of Fig. 2A. The section analysis of Fig. 2A is shown as Fig. 2D, from which one can see that the radius of a medium size patch is 332 nm and the height of the cap is 47 nm. The angle formed by the radius and the height is about 8, suggesting a quite flat patch. 968

3 Fig. 1 AFM images of a chalcopyrite surface soaked in M KEX for 10 minutes. A) the height image with a data scale of 200 nm; B) the deflection image with a data scale of 50 nm; C) the 3-D image and D) the section analysis. Fig. 2 AFM images of a chalcopyrite surface soaked in M PAX for 10 minutes. A) the height image with a data scale of 200 nm; B) the deflection image with a data scale of 50 nm; C) 3-D image and D) the section analysis. Figure 3 shows the AFM images of chalcopyrite surface soaked in 0.02 g/l Cytec 350 (PAX in an industrial product form) solutions for 5 minutes. Fig. 3A is the image of a bare chalcopyrite surface, which has been soaked in nanopure water in an AFM liquid cell for 10 minutes. In fact, no detectable change in surface morphology has been observed even after water was injected into the cell for one hour. From the image of a 10 µm 10 µm scan area, one can see that the solid surface is quite smooth in spite of the fact that there are some scratch lines on the sample surface due to surface polishing. Fig. 3B is the height image of a chalcopyrite surface soaked in 0.02 g/l Cytec 350 solutions for 5 minutes with a 50 nm data scale in a 10 µm 10 µm scan area. The image shows that that many small patches are uniformly distributed on the whole mineral surface. Fig. 3C and Fig. 3D are respectively the deflection image and the 3- D image of Fig. 3B. Fig. 3 AFM images of a chalcopyrite surface soaked in Aero 350 solution (0.02g/L) for 5 minutes. A) in water; B) the height image with a data scale of 100 nm; C) the deflection image with a data scale of 30 nm and D) 3-D image. Figure 4 shows the AFM images of a chalcopyrite surface soaked in 0.03 g/l Cytec 400 solutions for 30 minutes. Fig. 4A is the height image of the chalcopyrite surface obtained after a 30 minutes soaking time. Fig. 4B is the deflection image of Fig. 4A. Compare to the polished chalcopyrite image obtained in water as shown by Fig. 3A, Fig. 4A shows that the mineral surface is covered by a lot of adsorbate because of dramatic change in surface morphology and roughness. To confirm this finding, a much stronger scan force is applied when a small area (3 µm 3 µm) is scanned on the mineral surface. After that, the surface is scanned again in a 10 µm 10 µm area with a normal scan 969

4 force and the image is shown as Fig. 4C. By comparing it to Fig. 4A, one can clearly see that a 3 µm 3 µm window is shown in the center of Fig. 4C, which is due to the removal of the adsorbate under the strong applied scan force in the previous scan process in a small area. Fig. 4D is the deflection image of Fig. 4C, which helps show the change in surface morphology. Fig. 4 AFM images of a chalcopyrite soaked in Aero 400 solution (0.03g/L) for 30 minutes. A) the height image (in a 50 nm data scale); B) the deflection image (in a 30 nm data scale); C) the height image (in a 50 nm data scale) with a 3 µm 3 µm window being shown in the center and D) the deflection image of Fig. 4C with a data scale of 10 nm. Figure 5 shows the AFM images of a chalcopyrite surface soaked in 0.03 g/l Cytec 404 solutions for 30 minutes. Fig. 5A is the AFM height image with a 50 nm data scale. One can clearly see that a lot of adsorbate fully covers the mineral surface. Fig. 5B is the AFM deflection image with a data scale of 10 nm, which shows the shape and morphology of the adsorbate. Fig. 5C is the 3-D image of Fig. 5A. The section analysis of Fig. 5A is shown as Fig. 5D, from which one can clearly see that the height of the adsorbate is general less than 3 nm. Comparing it to those obtained with Fig. 1D, Fig. 2D and Fig. 3D, one can conclude that the morphology of the adsorbate is dramatically different from those of the patches and the height of the adsorbate is much smaller. Fig. 5 AFM images of a chalcopyrite soaked in 0.03g/L Aero 404 solution for 30 minutes. A) the height image in a 50 nm data scale; B) the deflection image of Fig. 5A with a 10 nm data scale; C) the 3-D image and D) the section analysis. Figure 6 shows the AFM images of a chalcopyrite surface soaked in 0.03g/L Cytec 412 solutions at ph 11 for 30 minutes. Fig. 6A is the 10 µm 10 µm height image with a data scale of 50 nm. The fact that a lot of adsorbate was observed on mineral surface suggests that Cytec 412 reacts with chalcopyrite at ph 11. Fig. 6B is the deflection image of Fig. 6A with a data scale of 10 nm showing the morphology of the adsorbate. Fig. 6C is the 10 µm 10 µm height image with a data scale of 50 nm after a 3 µm 1 µm 'window' is opened in the center by applying a large scan force 970

5 Microscopy: advances in scientific research and education (A. Méndez-Vilas, Ed.) following the same methodology as described in previous section. Fig. 6D is the deflection image of Fig. 6C with a data scale of 10 nm. Both images confirm the adsorption of Cytec 412 on chalcopyrite at ph 11. Fig. 6 AFM images of a chalcopyrite soaked in 0.03g/L Aero 412 solution at ph 11 for 30 minutes. A) the height image with a 50 nm data scale; B) the deflection image with a 10 nm data scale; C) the height image after a 'window' was opened in the center by applying a large scan force; and D) the deflection image of Fig. 6C. Figure 7 shows the AFM images of a chalcopyrite surface soaked in 0.02 g/l Nalco 9740 solutions for 5 minutes. Fig. 7A is the 10 µm 10 µm image of a bare chalcopyrite surface, which has been soaked in nanopure water in an AFM liquid cell for 10 minutes. In spite of some scratch lines due to the polishing process, the sample surface is smooth and this makes the polished mineral sample suitable for the study of the absorption of chemicals. Fig. 7B is the 10 µm 10 µm height image of a chalcopyrite surface soaked in 0.02 g/l Nalco 9740 solutions for 5 minutes with a 50 nm data scale. The image shows that there are also many small patches uniformly adsorbed on mineral surface. Fig. 7C and Fig. 7D are respectively the deflection image and the 3-D image of Fig. 7B. Fig. 7 AFM images of a chalcopyrite surface soaked in Nalco 9740 solution (0.02g/L) for 5 minutes. A) in water; B) the height image with a data scale of 50 nm; C) the deflection image with a data scale of 30 nm and D) the 3-D image. 3.2 Adsorption of xanthate on chalcopyrite surface Due to the importance of the adsorption of xanthate on sulfide minerals for froth flotation, many works have been carried out to clarify the adsorption mechanism. [15-16] In spite of the fact that there are still some debates, as reviewed by Leja (1982) [17], the deposition of metal xanthate with low solubility on sulfide mineral and the oxidization of xanthate into dixanthogen, are generally considered the two main mechanisms for the increase in hydrophobicity of sulfide minerals in flotation. 971

6 Figure 1, 2 and 3, the AFM images obtained with KEX, PAX and Aero 350 solutions show that some patch-like chemicals were absorbed on chalcopyrite surface. The change of surface morphology cannot be attributed to the oxidation of chalcopyrite in water, because in present investigation we found that the image of chalcopyrite surface remained the same even after the sample had been soaked in water for more than one hour. Therefore, the change in surface morphology must be due to the adsorption of xanthate. In the presence of oxygen in solution, xanthate can be oxidized into dixanthogen. Under ambient conditions, i.e. room temperature and normal pressure, dialkyl dixanthogen is usually in a liquid form and the melting point of diethyl dixanthogen, is 28. [18] or 32 [19]. The low melting point suggests that diethyl dixanthogen should be a soft solid at room temperature and it is usually an oily substance [18], when extracted at room temperature. In present study, xanthate solutions were not degassed and the AFM fluid cell was also open to air. In addition, the obtained AFM images show that the absorbate has a patch-like morphology and the edges of the patches are in general smooth and round, which fits well with the morphology of a hydrophobic oily substance in water. In addition, the AFM images also confirm the softness of the absorbate. The morphology of the absorbate on chalcopyrite surface observed in PAX solutions is similar as that obtained with KEX. As shown by Fig. 1, 2 and 3, the patches are flat with smooth and round edges. It fits well with the facts that oily diamyl dixanthogen is in general insoluble in water and the circular boundary is the direct result of the high interfacial tension between hydrophobic dixanthogen and water. It is also unlikely that these patches are insoluble metal xanthate, because in general each patch is uniformly flat with a smooth and round edge. In addition, the height of these patches is in tens of nanometers, which is too high for metal xanthate. The fact that no other evident changes in surface morphology except for the patches are detected after solid sample contacted xanthate solutions suggests that the adsorption of metal xanthate should be too sparse, compared to the round patches, for the absorbate to be detected under such a data scale as used in present work, without excluding the existence of metal xanthate on chalcopyrite surface. Comparing Fig. 1 to Fig. 2 and Fig. 3, one can conclude that the adsorption of PAX on chalcopyrite is much stronger than that of KEX, because the patches are detectable in only 5 minutes for the case of PAX. In addition, the patches observed with PAX solutions show a much lower profile in height, while with a much greater adsorption density and surface coverage. In flotation, as shown by the Cassie s Equation, a high adsorption density or surface coverage of hydrophobic collectors on mineral surface is beneficial for the increase of hydrophobicity and therefore floatability of minerals. The AFM images suggest that PAX is a more powerful collector than KEX for the flotation of chalcopyrite. In fact, PAX has been widely used in copper concentrators as a collector; while KEX is usually used for the research study purpose in academia. 3.3 Flotation collectors of chalcopyrite In industrial practice, as mentioned before, various types of collectors such as xanthate, dithiolphosphate, and mercaptan, can be applied for the flotation of chalcopyrite. In present investigation, KEX, PAX and Aero 350 are alkyl xanthate type collectors. Aero 400, 404 and 412 are the mixtures of dithiolphosphate and mecaptobenzothiazole. Nalco 9740 is mainly mercaptan. All these collectors have been applied as chalcopyrite collectors depending on the mineralogy, impurities and flowsheet. For all these collectors, xanthate is the most important collector for the flotation of sulfide minerals. It has been proposed that the deposition of metal xanthate with low solubility on sulfide mineral and the oxidization of xanthate into dixanthogen are generally the two main mechanisms for the increase in hydrophobicity of sulfide minerals in flotation. Dithiolphosphate is a very common promotes used in gold flotation in the combination of xanthate [20, 21] For example, Aero promoter 400 and 407 have been widely applied as effective promoters in gold flotation. Thiol collector (mercaptan) is by far the strongest collector for sulfide minerals because of the strong interaction of thiol with heavy metals. Mercaptobenzothiazole (MBT, Aero 404) is a specifically developed collector [22] for the flotation of gold and gold-carrying pyrite in acid circuits [23]. In general, MBT exists mainly in the non-ionized form in acid and alkaline solutions and both forms are more stable than the corresponding forms of xanthate. In present study, the AFM images show that all these collectors can successfully adsorb on chalcopyrite surface at ph 11. Therefore, the result confirms that all these chemicals can be used for the flotation of chalcopyrite in alkaline circuits. Of course, the difference in surface morphology after adsorption also suggests different adsorption mechanisms and efficiency of these collectors. For the case of xanthate, the patches are usually quite large in size and high in profile. This fits well with the oily property of the patches, which is usually bonded through the van der Waals force. On the other hand, as shown by Fig. 4 to Fig. 7, dithiolphosphate and mercaptan types of collectors adsorb on chalcopyrite in a much higher density and lower profile, which fits well with the properties of the metal salts of these chemicals. 4. Conclusions An AFM has been applied to study in situ the surface image of chalcopyrite in aqueous solutions. The AFM images show that all the studied chemicals adsorb on chalcopyrite after the mineral surface contacts solution at ph 11 after a specific time. The result confirms that all the chemicals can be used for the flotation of chalcopyrite in alkaline circuits. 972

7 However, the difference in surface morphology after the adsorption of various collectors also suggests that the adsorption mechanism and efficiency of different types of collectors are not of the same. The study of the morphology of the patches observed with xanthate type collectors suggests that the absorbate should be dixanthogen, which is an oily substance at room temperature. Metal xanthate could also be an adsorption production on chalcopyrite surface. However, the adsorption may too sparse for the absorbate to be detected. Results also show that the adsorption of PAX on chalcopyrite is much stronger than that of KEX, because the patches observed with PAX solutions show a much shorter adsorption time, a lower profile in height and a much greater adsorption density and surface coverage. The finding explains the fact that PAX instead of KEX has been widely used as a collector in copper concentrators in industrial practice. Acknowledgement J. Zhang is grateful to Freeport-McMoRan Copper & Gold, Inc. for sponsoring the Freeport McMoRan Copper and Gold Chair in Mineral Processing in the Department of Mining and Geological Engineering in the University of Arizona. The support from Resolution Copper Mining for the project is also greatly appreciated. Reference [1] Poling G.W. and Leja, J., (1963), J. Phys. Chem., 67 (10), pp [2] Allison, S. A., Goold, L. A., Nicol, M. J. and Granville, A. D., (1972), Metallurgical Trans. vol. 3, [3] Buckley, A. N., Woods, R., Appl. Surf. Sci. 17, 401 (1984) [4] Mielczarski, J.A., Yoon, R.H., J. Phys. Chem. 93, , [5] Mielczarski, J.A., Mielczarski, E., Cases, J.M., Int. J. Miner. Process. 52 (1998) [6] Kartio, I., Laajalehto, K., Suoninen, E., in Woods, R., Doyle, F.M., Richardson, P. E. (Eds.), Proc. 4th Int. Symp. Electrochemistry in Mineral and Metal Processing (Electrochem. Soc., Pennington 1996) p. 13 [7] Plaksin, I.N., Shafeyev, R.S., Zaiteseva, S.P., Proc. Acad. Sci. S.S.S.R., 108, No. 5 (1956) [8] Polkin, S.I., Kuzkin, S.F., Golov, V.M., Non-ferr. Metal, Moscow, No. 1 (1955) [9] Kim, B.S., Hayes, R.A., Prestidge, C.A., Ralston, J., Smart, R.St.C., Colloids and Surfaces A, 117 (1996) [10] Smart, R.St.C., Amarantidis, J., Skinner, W., Prestidge, C.A., Vanier, L. L., Grano, S.R., in Wandelt, K. and Thurgate, S. (ed), Solid Liquid Interfaces, Topics Appl. Phys. 85, 3 62 (2003). [11] Zhang, J. and Zhang, W., in Microscopy:, Formatex, ISBN-13: ,Vol. 3, Page , [12] Zhang, J. and Zhang, W., in 'Water in Mineral Processing' SME 2012a, ISBN: , p [13] Zhang, J. and Zhang, W., in 'Separation Technologies', SME, 2012b, ISBN: , p [14] Zhang, J. and Zhang, W., 'An AFM Study of the Adsorption of Collector on Arsenopyrite', XXVI IMPC, 2012c, India. [15] Gaudin, A.M. and Finkelstein, N.P. (1965), Nature, 207, [16] Fuerstenau, M.C., in: R.P. King (Editor), Principles of Flotation. SAIMM Monograph Series No. 3, pp [17] Leja, J., (1982), Surface chemistry of flotation, Plenum Press, New York. [18] Rao, S. R., (1971), Xanthate and related compounds, Dekker, New York. [19] Buckley, A. N., Hope, G. A, Woods, R., in Solid Liquid Interfaces, 85, (2003). [20] Nagaraj, D.R., 1997, Trans. Indian Inst. Metall. 50(5), [21] Allan, G.C., Woodcock, J.T., 2001, Miner. Eng. 14(9), [22] Finkelstein, N.P., Poling, J., 1977, Min. Sci. Eng. 9(4), [23] O Connor, C.T., Dunne, R.C., 1991, Miner. Eng. 4(7 11),