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Surface cleaning and oxidative effects of ultrasonication on the flotation of oxidized pyrite
Surface cleaning and oxidative effects of ultrasonication on the flotation of oxidized pyrite Qinbo Cao, Jinhua Cheng, Qicheng Feng, Shuming Wen, Bin Luo PII: DOI: Reference:
S0032-5910(17)30089-X doi:10.1016/j.powtec.2017.01.069 PTEC 12311
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Powder Technology
Received date: Revised date: Accepted date:
16 August 2016 5 December 2016 26 January 2017
Please cite this article as: Qinbo Cao, Jinhua Cheng, Qicheng Feng, Shuming Wen, Bin Luo, Surface cleaning and oxidative effects of ultrasonication on the flotation of oxidized pyrite, Powder Technology (2017), doi:10.1016/j.powtec.2017.01.069
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Surface cleaning and oxidative effects of ultrasonication on the flotation of oxidized pyrite a, b b Qinbo Cao *, Jinhua Cheng , Qicheng Fengb*, Shuming Wena, b, Bin Luob a State Key Laboratory of Complex Nonferrous Metal Resources Clean Utilization， Kunming 650093. b Faculty of Land Resources Engineering, Kunming University of Science and Technology, Kunming, 650093, Yunnan, PR China * To whom correspondence should be addressed. Tel：+86-0871-65187068 Qinbo Cao Email：[email protected]; Qicheng Feng Email: [email protected];
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Abstract: The effects of ultrasonication on the hydrophobicity of oxidized pyrite and the flotation of an oxidized pyrite ore were investigated in this work. Surface examinations show that ultrasonication (≥0.3 W/cm2) could result in surface cleaning and further oxidation of both slightly and heavily oxidized pyrite within 40 s. In the surface cleaning stage, the mechanical effects of ultrasonication removed the oxidation products from the pyrite surface. Further, in the surface oxidation stage, H2O2 and oxygen formed by ultrasonication immediately oxidized the pyrite, leading to a notable decrease in the contact angle. It was found that the influence of ultrasonication is governed by the time rather than the intensity of ultrasonication. Ultrasonication significantly improved the recovery of pyrite in the rougher and cleaner stages, when ultrasound was used only during flotation. The unfavorable effects of ultrasonication, such as oxidation, were not notable during flotation of the pyrite ore, due to the weak ultrasonic field in the flotation cells and the limited flotation time. Additionally, ultrasonication favored the formation of fine bubbles in the flotation cell, which is also benefit for pyrite flotation. Keywords: Ultrasound, Pyrite, Flotation, Surface cleaning, Oxidation.
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ACCEPTED MANUSCRIPT Surface cleaning and oxidative effects of ultrasonication on the flotation of oxidized pyrite
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1. Introduction Pyrite is a typical and abundant sulfide mineral[1]. It is well known that natural pyrite is hydrophobic in an anaerobic environment[2]. However, when pyrite is exposed to the atmosphere or aqueous conditions, oxidation occurring at the pyrite surface readily decreases hydrophobicity[3-5]. Such oxidation generates iron oxides/hydroxides and an underlying sulfur-rich surface[5-7]. It was recently revealed that the formation of H2O2 in the pulp accounts for the oxidation of pyrite[8]. H2O2 can be formed by pyrite in water without molecular oxygen[9], while with the molecular oxygen, ferrous iron and pyrite can also generate H2O2[10]. Nooshabadi et al. found a significant concentration of H2O2 in the pulp during wet-grinding of pyrite[8]. In particular, more H2O2 is generated when the pyrite is placed in water after dry-grinding [8]. H2O2 is also a strong oxidant for pyrite[11], and can significantly decrease the floatability of pyrite. After the oxidation with H2O2, a large number of iron oxide/hydroxide islands (1-2μm) are formed at the pyrite surface, which results in an extremely hydrophilic surface[12]. Ultrasonic pretreatment has been found to be a useful method to improve the floatability of oxidized pyrrhotite and other oxidized sulfide minerals[13]. The benefit of the ultrasonication is due to its surface cleaning effect. With ultrasonication, cavitation bubbles are generated in water during the fraction period of the ultrasound wave. When the cavitation bubble collapses near/at a solid surface, strong liquid jets are produced at an estimated speed of 100 m/s[14]. Such liquid jets can remove the surface coatings at the mineral surface[15-18]. In terms of pyrite flotation, the cleaning effect of ultrasonication can result in the removal of oxidation products at the pyrite surface, and thus improve the efficiency of pyrite flotation[19]. In contrast, ultrasound also shows a negative effect on the floatability of pyrite, which was found during desulfurization in coal flotation with ultrasonication[20]. This negative effect is caused by the chemical effects of ultrasonication. Ultrasonic irradiation in an aqueous solution can sonolyze the components in the solution. As a result, free radicals are produced by ultrasonication[21-22]. It is well-established that dissolved gas and water vapor in the cavitation bubble undergo thermal decomposition, resulting in the formation of OH and H radicals (reaction 1)[23]. Such radicals can be involved in secondary reactions (reaction 2-5)[20, 24]. H2O→•OH+H• (1) H•+H2O→H2+•OH (2) H• +O2 →•HO2 (3) •OH+•OH→H2O2 (4) •HO2 +•HO2→H2O2+O2↑ (5) The H2O2 and nascent oxygen can oxidize the pyrite surface, which decreases the floatability of pyrite. In fact, in previous research, only the surface cleaning effect of ultrasonication was found for the flotation of oxidized sulfide minerals[13]. The oxidative effect of ultrasonication was not observed for the oxidized pyrite. It is 2
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reasonable to predict that the surface cleaning and oxidative effects of ultrasonication on the oxidized pyrite are governed by the ultrasonication time as well as the sonic intensity. However, the influence of these ultrasonication parameters on the floatability of oxidized pyrite is still uncertain. On the other hand, Cliek et al. reported that ultrasonication can be successfully used during the flotation process[19]. This also benefits pyrite flotation because oxidation can happen during the flotation of pyrite. In previous works, single rougher flotation tests have been commonly employed to estimate the effects of ultrasonication on the flotation performance of sulfide minerals [13, 19]. However, for a run-of-mine pyrite ore, single rougher flotation usually cannot produce a satisfactory concentrate, and more stages are required. The influence of ultrasonication on different stages of pyrite flotation has not been fully revealed. Additionally, although ultrasonication shows notably beneficial effects on mineral flotation at the laboratory scale, its application in the flotation industry is still limited. One main problem is the attenuation of ultrasound in the slurry. The ultrasound loses energy during propagation in water, which is caused by multiple reasons, such as heat production and the formation of cavitation bubbles[25-26]. On the other hand, the solid particles in the pulp accelerate the attenuation of ultrasound[27]. The particle size and solid concentration have a significant impact on the attenuation of ultrasound[28-29]. Thus, a satisfactory ultrasonic field is essential for the scale-up of ultrasound-assisted flotation. However, the distribution of ultrasound in a flotation cell (laboratory scale) with slurry has not yet been fully investigated. Such information is valuable for the scale-up of flotation machines with ultrasound. The aim of the present work is to extend our knowledge regarding the effects of ultrasound on the flotation of oxidized pyrite. The dependence of the hydrophobicity of oxidized pyrite on ultrasonication conditions was examined through contact angle measurements. The oxidation states of pyrite samples with ultrasonication were further examined by EDX analysis. Furthermore, ultrasound was added during the flotation of a pyrite ore, and its effect on pyrite recovery at each flotation stage was determined. The distributions of sonic intensity and bubble size in flotation cells were also measured to further understand the effects of ultrasonication. 2 Experimental 2. 1 Reagents and samples Butyl xanthate and hydrogen peroxide (H2O2) were AR grade and purchased from Sinopharm Chemical Reagent Co., Ltd. Pine oil was provided by Hunan Minzhu Flotation Reagents Co., Ltd. These reagents were used as received. Pure pyrite pebbles were obtained from Yunnan Copper Industry Ltd. by Share Ltd., China. XRD analysis showed that the pyrite was triclinic, with space group P-1, and the purity was 99.9% (Fig. 1). The pyrite ore was taken from the Weixin area in Yunnan Province of China. The chemical compositional analysis of the pyrite ore is shown in Table 1. Further XRD results indicate that pyrite was the valuable mineral in this ore. Deionized (DI) water was used in the surface examinations, while batch flotation experiments were conducted with tap water. All of the experiments were performed at 23℃. 3
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2. 2 Contact angle measurements A GBX 3S tensiometer was employed to measure the contact angle of pyrite. Before the test, the pyrite pebble was polished with 2000-, 4000-, and 6000-grit Al2O3 sandpapers. The pyrite was further treated in two different ways: (1) by immersion in 1% H2O2 solution for a desired time; (2) further ultrasonication for different times and ultrasonic intensities, after oxidation in H2O2 solution. After the conditioning, the contact angles were measured with the captive-bubble method. The crystal was placed in a measurement cell filled with DI water. A 2-3 mm air bubble was generated under the crystal to attach to the surface. After the three-phase line was formed, the contact angles on both sides of the bubble were recorded. Each test was repeated three times. The average values were reported and the error of the measurements was less than ±2°. 2. 3 SEM-EDX analysis Surface analysis of pyrite samples that were oxidized with H2O2 and ultrasonicated was conducted with a FEI Quanta 200 SEM at 2000 × magnification. The accelerator voltage was 20 kV. The atomic concentrations of Fe, S, and O at the pyrite surface were measured. Images of pyrite after grinding were also obtained by the same instrument. 2. 4 Traditional flotation XFDMⅡflotation machines were used in the flotation tests. In each test, 500 g of pyrite ore was combined with 400 mL of tap water and 3 mL of H2O2 and ground in a ball mill for 3 min, to obtain a product that was 80 wt.% of solids passing through 200 mesh. After grinding, the slurry was filtered, and the ore was transferred into the 1.5 L flotation cell for rougher flotation, along with the pulp. Scavenger flotation was continuously conducted in the same 1.5 L flotation cell, and the cleaner flotation was further conducted in a 1 L flotation cell. The impeller speed of the flotation machines was 1500 rpm. The airflow rates were 4 L/min for the rougher/scavenger flotation stage and 3.5 L/min for the cleaner flotation stage. With a series of flotation tests, the optimal flotation conditions without ultrasonication were determined as shown in Table 2. 2. 5 Ultrasound-assisted flotation Ultrasonic transducers were equipped at the bottom of the flotation cells. The frequency of the ultrasonic transducer was 28 kHz. The energy of the transducer was 0-100 W. With this transducer, the highest ultrasonic intensity in water is 1.57 W/cm2. The flotation cell was installed in the XFDMⅡ flotation machine. Ultrasound was not used as a pre-conditioning method, which was used during the froth collection. Other grinding and mechanical conditions were identical to those for traditional flotation. The effect of ultrasound on each stage of pyrite flotation was investigated individually. 2. 6 Determination of ultrasonic intensity in the pulp A Megasonic Energy Meter (Hangzhou Success Ultrasonic Equipment Co. Ltd.) was employed to measure the ultrasonic energy at different spots in the vertical phase of the flotation cell, as shown in Fig. 2. The phases departed slightly from the center of the flotation cell, to avoid the stirring instrument. 4
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The pulp for the study was prepared under the conditions described for the traditional flotation. After the pulp was filled into the 1.5 L flotation cell, the ultrasonic intensity was directly measured without the addition of reagents and aeration. However, to measure the sonic intensity in the 1 L flotation cell, rougher flotation with ultrasound was conducted using another 500 g of pyrite ore to get a middling for cleaner flotation. After the middling was transferred into the 1 L flotation cell, the ultrasonic intensities at different spots were measured without aeration. All of the measurements were conducted while stirring. 2. 7 Bubble size measurement A high speed video system (Lighting RDTTM) was used to record the images of bubbles in a 6L of tempered glass cell with ultrasonic transducer. The pictures of bubbles in water with and without ultrasonication were both recorded, and were further analyzed with the software of Nano Measurer. In each case the size distribution of bubbles were obtained from the results of about 100 bubbles. The concentrations of pine oil and collector in water were same to that in rougher flotation. 3 Results and discussion 3. 1 Oxidation with H2O2 and cleaning effect of ultrasound In this work, natural pyrite pebble was oxidized in 1% H2O2 solution to generate a oxidative sample. Due to the strong oxidizing effect of H2O2, the pyrite samples at different oxidative levels can be readily obtained in a short time [30-31]. The hydrophobicity of pyrite decreased sharply with the increase in the treatment time in H2O2 solution (Fig. 3, curve a). The contact angle of natural pyrite was 57.1◦, while after 8 min of oxidation, the contact angle decreased to 18.5◦. The formation of ferric hydroxide/oxide at the pyrite surface accounted for this reduction in hydrophobicity[2]. The contact angles did not show an obvious reduction after 8 min of oxidation. It seems that the pyrite surface was completely coated by oxidation products after 8 min of oxidation. As reported by Jin et al., the contact angle decreased to 12◦ when pyrite was oxidized by H2O2 for 3 min[2]. Note that only 1% of H2O2 was used here, which is much lower than the concentration (30%) of H2O2 in their research. As a result, the rate of oxidation of pyrite here is relatively lower. When the oxidized samples were further treated ultrasonically (0.3 W/cm2) for 5 s, the contact angles were improved to some degree (Fig. 3, curve b). The improvement in hydrophobicity was significant for pyrite that was oxidized for less than 20 min. For instance, the contact angle was nearly 17◦ after 15 min of oxidation, and increased to 42◦ with 5 s of ultrasonication. For these oxidized samples, the increase in contact angle could be attributed to the cleaning effect of ultrasound, which partially removed the oxidation products at the pyrite surface. However, for pyrite oxidized for more than 20 min, ultrasonication showed a poor effect for raising the contact angle. The increase in contact angle after ultrasonication was less than 10◦. Newell et al. reported that ultrasonic conditioning failed to renew the floatability of heavily oxidized pyrrhotite [32]. They suggested that for the heavily oxidized sample, the oxidation process occurs not only at the mineral surface but also at the deeper layer beneath the surface. Further, the oxidation products are strongly 5
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attached at the pyrrhotite surface and are difficult to remove by ultrasound. In the present work, the pyrite is expected to be heavily oxidized after being treated with H2O2 for 20 min or more time. 5 s of ultrasonication is insufficient for eliminating the oxidation products and restoring the hydrophobicity of pyrite. 3. 2 Influence of ultrasonication on oxidized-pyrite surface The hydrophobicity of oxidized pyrite as a function of ultrasonication time and ultrasonic intensity was further studied. The pyrite samples at two oxidation levels, i.e., oxidized with H2O2 solution for 12 min and 20 min, were used in the tests. Such samples represent the lightly and heavily oxidized pyrite, respectively. Fig. 4 shows that for both oxidized samples, 0.1 W/cm2 of ultrasonication was insufficient to restore the hydrophobicity of the pyrite samples. On the other hand, ultrasound with intensity higher than 0.1 W/cm2 exhibited the opposite effect on the contact angles of oxidized pyrites, depending on the ultrasonication time. In terms of the slightly oxidized pyrite, the contact angle results with ultrasonic intensity above 0.1 W/cm2 may be discussed in two distinctive regions, as shown in Fig. 4A. In region 1 from 5 s to 20 s of ultrasonication, ultrasound improved the contact angle, and the maxima of contact angle were in the range of 55◦-58◦. These maximal values are close to the contact angle of natural pyrite, suggesting that the hydrophobicity of slightly oxidized pyrite is successfully restored by 20 s of ultrasonication. The positive effect of ultrasonication in this region could be due to its cleaning effect. In contrast, above 20 s of ultrasonication, i.e., in region 2, ultrasonication resulted in a sharp reduction in the contact angle. In particular, with 40 s of ultrasonication, the contact angle was below 31◦, which was even lower than that of the pyrite oxidized in H2O2 solution for 2 min. This phenomenon may be caused by the chemical effect of ultrasonication. After the original oxidation products are eliminated, cavitation bubbles are generated at the fresh pyrite surface. H2O2 and oxygen are also formed during this process, which are expected to further oxidize the pyrite. As a result, the hydrophobicity of pyrite is decreased. A similar trend was found for the heavily oxidized pyrite (Fig. 4B). 5 to 10 s of ultrasonication (above 0.1 W/cm2) could improve the contact angle to some degree. The highest contact angle was 39.5◦, which is much less than that of natural pyrite. As the ultrasonication time exceeded 10 s, the contact angle decreased again. The distribution of oxidation products at the pyrite surface is not inherently homogeneous [2]. For the heavily oxidized pyrite, some slightly oxidized areas are expected to be present at the surface. The oxidation products in these areas could be readily removed by 10 s of ultrasonication, which leads to an increase in the contact angle. However, in the heavily oxidized area, the oxidation products could form in the deeper layer of pyrite surface and are thus difficult to clean by ultrasound. Therefore, the beneficial effect of ultrasound on the hydrophobicity is limited for the heavily oxidized pyrite. In comparison, after more than 10 s of ultrasonication, and perhaps due to the oxidation effect of ultrasound, the pyrite surface became more hydrophilic than that after 10 s of ultrasonication. The above results essentially show that within 40 s of ultrasonication, both positive and negative effects of ultrasound were observed for the hydrophobicity of 6
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oxidized pyrite. Furthermore, the determining factor for the behavior of ultrasound was the ultrasonication time rather than the ultrasonic intensity. To further examine the positive/negative effects of ultrasound, SEM and EDX studies were performed to analyze the oxidation states of the oxidized pyrites. 3. 3 SEM and EDAX studies The oxygen concentration at the pyrite surface is crucial to evaluate the degree of oxidation of pyrite. EDX and XPS techniques are both capable of determining the oxygen content at a mineral surface. However, XPS analysis is difficult to perform on a mineral plate. Furthermore, air pollution also introduces oxygen during the sample preparation for the XPS experiments, which hinders the analysis. Given the disadvantages of XPS, we used the EDX technique to determine the elemental concentration at the surface of oxidized pyrite. Table 3 illustrates that 0.3 W/cm2 of ultrasound had a significant influence on the oxygen concentration at the pyrite surface that was oxidized by the H2O2 solution for 12 min. For this slightly oxidized pyrite, 20 s of ultrasonication significantly decreased the oxygen concentration at the mineral surface. The concentration ratio of S/Fe after 20 s of ultrasonication was 1.94. This suggests that the oxidation products are efficiently removed after 20 s of ultrasound irradiation, which is in line with the above contact angle results. On the contrary, with 40 s of ultrasonication, the oxygen concentration was twice that with 20 s of ultrasonication and was only 2% lower than that at the original oxidized surface. This indicates that 40 s of ultrasonic treatment improves the degree of oxidation of the pyrite surface compared to that under 20 s of ultrasonication. These results support the prediction that ultrasound can promote oxidation at a slightly oxidized pyrite surface with more than 20 s of ultrasonication. For the pyrite oxidized for 20 min, the oxygen concentration at the pyrite surface was 13.42% after oxidation, and further decreased to 8.92% after 10 s of ultrasonication. Again, the cleaning effect of ultrasound accounts for the reduction in oxygen content. The above contact angle tests show that 10 s of ultrasonication (0.3 W/cm2) could mostly improve the contact angle of this heavily oxidized pyrite. However, the oxygen concentration was still significant after 10 s of ultrasonication, suggesting the oxidation products at the surface are hard to remove completely. When the ultrasonication time was increased from 10 s to 50 s, the oxygen concentration also increased to some degree. This suggests that ultrasound could also exhibit oxidation effects at the surface of this heavily oxidized pyrite. Some authors have also suggested that over-irradiation with ultrasound may produce surface defects at the mineral surface, resulting in a decrease in hydrophobicity[15, 33]. Unfortunately, it is difficult to obtain a perfect pyrite surface without defects for the morphology study. Even when the pyrite was well-polished with sandpaper, the roughness at the surface was still significant, as shown in Fig. 5. The influence of ultrasound on the surface defects of pyrite was not investigated in this work, and should be studied in future. However, it is reasonable to expect that the oxidation effect of ultrasound is the main reason for the reduction in hydrophobicity, due to the increase in oxygen concentration at the mineral surface with over-ultrasonication. 7
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3. 4 Ultrasonic effects in different flotation stages Batch flotation tests were conducted to analyze the effect of ultrasonication on the pyrite flotation, where the ultrasound was used only during the flotation process. In our tests, it was found that the Fe grade did not respond to the ultrasonication. On the other hand, the Fe recovery was highly dependent on the ultrasonic energy. Therefore, only the recovery results are reported in Fig. 6. Although the natural pyrite surface is hydrophobic, the Fe recovery of this pyrite ore was only 6.3% by single rougher flotation with the addition of only 50 g/t of frother. It appeared that the pyrite had been oxidized to a certain extent after the wet-grinding and/or in the deposit. Using the collector and frother together, the Fe recovery was 78.47 % with the rougher flotation of traditional flotation (Fig. 6). With ultrasonication in the rougher stage, the recovery increased gradually with the increase in ultrasonic power, and reached a maximum (89.32%) at 70 W. As suggested, the oxidation products, such as ferric hydroxide, decrease the hydrophobicity of pyrite even in the presence of the xanthate collector[34]. It is reasonable to expect that the oxidation products at the pyrite surface are removed efficiently by the cleaning effect of ultrasound. Hence, the flotation performance of pyrite is improved. However, the recovery in the rougher stage was relatively lower at 90 W/100 W comparing to that at 70 W. This may be due to the following reasons. Firstly, as expected, the ultrasonic field at 90 W/100 W is extended to some degree in comparison with that at 70 W. The same pyrite particles might remain in the ultrasonic field of 90 W/ 100 W for more time. The surface of these pyrite particles could be oxidized to a certain degree. Secondly, the stronger ultrasonic intensity at 90 W/100 W could result in a more turbulent environment, which is not beneficial for the attachment of bubbles and particles[35]. Additionally, stronger ultrasonic power may introduce surface defects at the pyrite surface, resulting in a less hydrophobic pyrite surface than that at 70 W. In any event, it seems that the cleaning effect and unfavorable effects of ultrasound reach a balance at 70 W of ultrasonic power, for maximum improvement of the recovery in the rougher stage. It must be noted that the recovery at 100 W decreased by only 3.7 % compared to that at 70 W, which was still higher than that with traditional flotation. These results show that the unfavorable effects of ultrasound are not notable in the rougher stage. As for the cleaner stage, ultrasound showed more notable effects on increasing the pyrite recovery than that in the rougher stage. In this stage, the Fe recovery increased by 15.88% with 50 W of ultrasound, compared to that with traditional flotation. In fact, the amount of gauge minerals in the cleaner stage was much less than that in the rougher stage. Therefore, the ultrasonic irradiation could focus on pyrite, thus efficiently cleaning the pyrite surface. As the ultrasonic power exceeded 50 W, the recovery started to decrease. This reduction may be owing to the same reasons as those for the rougher stage, involving oxidation, over-turbulence, and surface-morphology damage. Additionally, the recovery in the scavenger stage was independent of the ultrasonic power, which could be attributed to the inefficient liberation of pyrite. It should be stressed that, in above flotation tests, ultrasonication was used in 8
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rougher and cleaner flotation stages respectively. As we tested, when ultrasonication was utilized successively in rougher stage (75 W of input power) and cleaner stage (50 W of input power), the Fe recovery reached 81.03% using 130 g/t of collector. However, with the traditional flotation (150 g/t of collector), the Fe recovery was only 61.09% after the cleaner flotation. Such results showed that ultrasonication can be used in the continuous flotation of this pyrite ore, and can reduce the demand for collector to some extent. 3. 5 Distribution of ultrasonic intensity As stated in the Introduction, the solid concentration has a significant impact on the transmission of ultrasound. To reveal the transmission of ultrasound in pulp, the ultrasonic intensities in the flotation cells with slurry were determined. In the rougher flotation stage, the solid concentration was 35.3% in the 1.5L of flotation cell. The ultrasonic intensity results in this flotation cell are showed in Fig. 7. The above contact angle results indicate that 0.3 W/cm2 of intensity was required for the ultrasound to exhibit the surface cleaning/oxidation effect for pyrite. With 75 W of ultrasonic power, 0.3 W/cm2 or higher intensity was measured within a height of 5 cm of the flotation cell (Fig. 7A). When the ultrasonic power was increased to 100 W, the ultrasonic field (≥0.3 W/cm2) was extended to some degree, but was still less than 7 cm of the height (Fig. 7B). After rougher flotation with another 500 g of ore using 75 W of ultrasonic power and 130 g/t of collector, the frother product was transferred into the 1 L flotation cell. The solid concentration (12.4%) in the cell was much less than that in the rougher flotation, while the ultrasonic field (≥0.3 W/cm2) was within 5 cm of height of flotation cell under 50 W or 100 W of ultrasonic power, Fig. 8. The solid particles in the pulp could adsorb and reflect the ultrasound waves[27]. In this regard, it was hard for ultrasound to propagate through the pulp, due to which the ultrasound was detected only in limited areas in the flotation cell. However, this ultrasound field was enough to exhibit benefits for the pyrite flotation, as demonstrated by the above flotation test. On the other hand, because the ultrasonic fields were weak under 100 W of ultrasonic power and the flotation time was limited, the unfavorable effects of ultrasound in the rougher/cleaner stage were not notable for the pyrite flotation. 3. 6 Effect on bubble size Bubble size has an important influence on mineral flotation, and fine bubble is commonly required[36]. Here we compared the bubble-size distributions in a flotation cell with and without ultrasonication. Since it is hard to obtain a clear picture of bubbles in slurry, the experiments were only performed in water without the pyrite ore. Fig. 9 shows that, without ultrasound, the mean size of bubbles was 2.55 mm, and 79.1% of bubbles were in the size range of 2.2 mm-4.6 mm. While, a smaller bubble-size range was obtained by 20 W of ultrasonic power, Fig. 10A. The mean size of bubbles decreased to 2.01 mm at this power. Furthermore, 72.2% of bubbles were less than 2.05 mm. It appeared that ultrasonication favored the formation of fine bubbles in the cell, which agrees well with the previous report[37]. 9
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It should be mentioned that ultrasound propagated well in water. With 20 W of input power, the ultrasonic intensity reached 0.21 W/cm2 at the top of cell (35 cm of height). While, at the bottom of the cell, the maxima ultrasonic intensity was 0. 82 W/cm2. Such ultrasonic intensity range is similar to that during rougher flotation at 75 W or cleaner flotation at 50 W. It is reasonable to expect that ultrasonication also promoted the formation of fine bubbles during the ultrasound-assisted flotation. Additionally, the mean size of bubbles at 100 W (Fig. 10B) was further decreased comparing with that at 20W, due to the stronger intensities at 100W, i.e., 1.55 W/cm2-0.75 W/cm2. However, as we known, it is still a challenge to produce such strong ultrasonic field in a slurry. 4 Conclusions A 0.3 W/cm2 or higher intensity of ultrasonication showed surface cleaning and oxidation effects for the oxidized pyrite samples. The effects of ultrasound were strictly dependent on the ultrasonication time. For the pyrite oxidized by H2O2 solution for 12 min, 20 s of ultrasonication efficiently eliminated the oxidation products, by which the hydrophobicity of the oxidized pyrite was restored. However, above 20 s of ultrasonication, H2O2 and nascent oxygen generated by the ultrasound were expected to further oxidize the pyrite surface. The cleaning effect of ultrasound was inefficient for pyrite oxidized by H2O2 solution for 20 min. Ten seconds of ultrasonication could partially improve the hydrophobicity of such heavily oxidized pyrite. More than 10 s of ultrasonication decreased the contact angle of the heavily oxidized pyrite, which may be due to the oxidative effect of ultrasound. In terms of the selected pyrite ore, the flotation recoveries in the rougher and cleaner stages were notably improved when ultrasound was used during the flotation. The adverse effects of ultrasound on pyrite recovery were not remarkable. Further study showed that the ultrasonic field (≥0.3 W/cm2) was within 5 cm of height in the flotation cells. For future scale-up, the flotation machine with ultrasound should have an ultrasonic field with the corresponding area and ultrasonic intensity, as revealed in this work. Besides the surface cleaning effect, another advantage of ultrasound is that ultrasonication could promote the formation of fine bubbles, which also could improve the flotation efficiency of pyrite. Acknowledgments The financial support from NSF grant No. 51304089 and the Ph.D. Programs Foundation of Ministry of Education of China 20135314120007 is gratefully acknowledged. References [1] X. Cazañas, P. Alfonso, J.C. Melgarejo, J.A. Proenza, A.E. Fallick, Source of ore-forming fluids in El Cobre VHMSdeposit (Cuba): evidence from fluid inclusions and sulfur isotopes, Journal of Geochemical Exploration, s 78–79 (2003) 85-90. [2] J. Jin, J.D. Miller, L.X. Dang, C.D. Wick, Effect of Surface Oxidation on Interfacial Water Structure at a Pyrite (100) Surface as Studied by Molecular Dynamics Simulation, International Journal of Mineral Processing, 139 (2015) 64–76. [3] K. Laajalehto, I. Kartio, E. Suoninen, XPS and SR-XPS techniques applied to sulphide mineral surfaces, International Journal of Mineral Processing, 51 (1997) 163-170.
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[11] H. Sun, M. Chen, L. Zou, R. Shu, R. Ruan, Study of the kinetics of pyrite oxidation under
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[12] J. Jin, J.D. Miller, L.X. Dang, C.D. Wick, Effect of surface oxidation on interfacial water structure at a pyrite (100) surface as studied by molecular dynamics simulation, International Journal of Mineral Processing, 139 (2015) 64-76.
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[13] A.J.H. Newell, D.J. Bradshaw, P.J. Harris, The effect of heavy oxidation upon flotation and potential remedies for Merensky type sulfides, Minerals Engineering, 19 (2006) 675-686. [14] K. Suslick, S. Doktycz, E. Flint, On the origin of sonoluminescence and sonochemistry, Ultrasonics, 28 (1990) 280-290.
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[15] N.E. Altun, J.-Y. Hwang, C. Hicyilmaz, Enhancement of flotation performance of oil shale cleaning by ultrasonic treatment, International Journal of Mineral Processing, 91 (2009) 1-13. [16] M. Celik, Effect of ultrasonic treatment on the floatability of coal and galena, Sep Sci Technol, 24 (1989) 1159-1166.
[17] A. Farmer, A. Collings, G. Jameson, Effect of ultrasound on surface cleaning of silica particles, International Journal of Mineral Processing, 60 (2000) 101-113. [18] A. Farmer, A. Collings, G. Jameson, The application of power ultrasound to the surface cleaning of silica and heavy mineral sands, Ultrasonics Sonochemistry, 7 (2000) 243-247. [19] E.C. Cilek, S. Ozgen, Effect of ultrasound on separation selectivity and efficiency of flotation, Minerals Engineering, 22 (2009) 1209-1217. [20] W.-z. Kang, H.-x. Xun, X.-h. Kong, M.-m. Li, Effects from changes in pulp nature after ultrasonic conditioning on high-sulfur coal flotation, Mining Science and Technology (China), 19 (2009) 498-507. [21] M. Anbar, I. Pecht, Sonolytic decomposition of organic solutes in dilute aqueous solutions. III. Oxidative deamination of ethylenediamine by OH radicals, The Journal of Physical Chemistry, 71 (1967) 1246-1249.
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OH-radical formation by ultrasound in aqueous solution – Part II: Terephthalate and Fricke dosimetry and the influence of various conditions on the sonolytic yield, Ultrasonics Sonochemistry, 5 (1998)
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[26] O. Louisnard, A simple model of ultrasound propagation in a cavitating liquid. Part I: Theory, nonlinear attenuation and traveling wave generation, Ultrasonics Sonochemistry, 19 (2012) 56-65. [27] G.-c. He, Y.-p. Mao, W. Ni, A new fractal modification of ultrasonic attenuation model for
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[28] K. Kubo, T. Norisuye, T.N. Tran, D. Shibata, H. Nakanishi, Q. Tran-Cong-Miyata, Sound velocity and attenuation coefficient of hard and hollow microparticle suspensions observed by ultrasound
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spectroscopy, Ultrasonics, 62 (2015) 186-194.
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[29] J.M. Furlan, V. Mundla, J. Kadambi, N. Hoyt, R. Visintainer, G. Addie, Development of A-scan ultrasound technique for measuring local particle concentration in slurry flows, Powder Technology, 215–216 (2012) 174-184.
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[30] A.M. Raichur, X.H. Wang, B.K. Parekh, Quantifying pyrite surface oxidation kinetics by contact angle measurements, Colloids & Surfaces A Physicochemical & Engineering Aspects, 167 (2000) 245–251.
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controlled redox potential, Hydrometallurgy, 155 (2015) 13–19. [32] A.J.H. Newell, D.J. Bradshaw, P.J. Harris, The effect of heavy oxidation upon flotation and potential remedies for Merensky type sulfides, Minerals Engineering, 19 (2006) 675-686. [33] D. Feng, C. Aldrich, Effect of Ultrasonication on the Flotation of Talc, Industrial & Engineering Chemistry Research, 43 (2004) 4422-4427. [34] M.B.M. Monte, F.F. Lins, J.F. Oliveira, Selective flotation of gold from pyrite under oxidizing conditions, International Journal of Mineral Processing, 51 (1997) 255-267. [35] G. Wang, J.B. Joshi, M. Sathe, G.J. Jameson, S. Zhou, G.M. Evans, Bubble Detachment from a Steel Ball in Turbulent Field: Application to Mineral Flotation Systems, Procedia Engineering, 102 (2015) 1046-1055. [36] W. Drenckhan, A. Saint-Jalmes, The science of foaming, Advances in Colloid & Interface Science, 222 (2015) 228-259. [37] S.G. Ozkan, H.Z. Kuyumcu, Investigation of mechanism of ultrasound on coal flotation, International Journal of Mineral Processing, 81 (2006) 201-203.
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6.57 9.87 1.35 0.21
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Element composition
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Table 2 Traditional flotation conditions for the pyrite ore Stage Reagent Dosage (g/t) Condition time (min) Butyl xanthate 150 2 Rougher flotation Pine oil 30 1 Butyl xanthate 75 2 Scavenger flotation Pine oil 20 1 Cleaner flotation -
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Table 3 Elemental concentrations and ratios at the pyrite plate surface (Sonic intensity was 0.3 W/cm2). Oxidation Atomic concentration Concentration time in (%) ratio Ultrasonication H2O2 time (s) solution Fe S O S/Fe O/Fe (min) 0 30.36 57.23 12.41 1.89 0.41 12 20 32.32 62.85 4.28 1.94 0.13 40 31.22 58.28 10.50 1.87 0.34 0 30.81 55.78 13.42 1.81 0.44 20 10 31.83 59.25 8.92 1.86 0.28 50 31.82 58.94 9.24 1.85 0.29
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Fig. 1 X-ray diffraction pattern of pyrite crystal.
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Fig. 2 Schematic diagram of the vertical phases in the 1.5 L (A) and 1 L (B) flotation cells for the ultrasonic intensity measurements (The vertical phase for the measurement is denoted by the dashed line.).
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Fig. 3 Contact angle results of pyrite (a, oxidized with H2O2; b, oxidized with H2O2 at different times and further ultrasonicated with an intensity of 0.3 W/cm2 for 5 s.).
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Fig. 4 Effect of ultrasound on the contact angles of pyrite samples oxidized by the H2O2 solution for 12 min (A) and 20 min (B).
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Fig. 5 SEM image of pyrite surface polished with 2000-grit, 4000-grit and 6000-grit sandpaper.
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Fig. 6 Influence of ultrasonic power on the Fe recovery at different stages.
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Fig. 7 Ultrasonic intensity (W/cm2) in the vertical phase of the 1.5 L flotation cell with slurry (A, 75 W of ultrasonic power; B, 100 W of ultrasonic power).
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Fig. 8 Ultrasonic intensity (W/cm2) in the vertical phase of the 1 L flotation cell with slurry (A, 50 W of ultrasonic power; B, 100 W of ultrasonic power).
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Fig. 9 Distribution of bubble size in water without ultrasound.
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Fig. 10 Distribution of bubble size in water with 20 W(A) and 100 W (B) of ultrasonic power.
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Ultrasonication showed opposite effects on the hydrophobicity of oxidized pyrite. The effects of ultrasound were determined by the ultrasonication time. 0.3 W/cm2 of intensity was required for the utilization of ultrasound. Negative effects of ultrasonication on the flotation of pyrite ore were not notable.
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S0032-5910(17)30089-X doi:10.1016/j.powtec.2017.01.069 PTEC 12311
To appear in:
Powder Technology
Received date: Revised date: Accepted date:
16 August 2016 5 December 2016 26 January 2017
Please cite this article as: Qinbo Cao, Jinhua Cheng, Qicheng Feng, Shuming Wen, Bin Luo, Surface cleaning and oxidative effects of ultrasonication on the flotation of oxidized pyrite, Powder Technology (2017), doi:10.1016/j.powtec.2017.01.069
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Surface cleaning and oxidative effects of ultrasonication on the flotation of oxidized pyrite a, b b Qinbo Cao *, Jinhua Cheng , Qicheng Fengb*, Shuming Wena, b, Bin Luob a State Key Laboratory of Complex Nonferrous Metal Resources Clean Utilization， Kunming 650093. b Faculty of Land Resources Engineering, Kunming University of Science and Technology, Kunming, 650093, Yunnan, PR China * To whom correspondence should be addressed. Tel：+86-0871-65187068 Qinbo Cao Email：[email protected]; Qicheng Feng Email: [email protected];
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Abstract: The effects of ultrasonication on the hydrophobicity of oxidized pyrite and the flotation of an oxidized pyrite ore were investigated in this work. Surface examinations show that ultrasonication (≥0.3 W/cm2) could result in surface cleaning and further oxidation of both slightly and heavily oxidized pyrite within 40 s. In the surface cleaning stage, the mechanical effects of ultrasonication removed the oxidation products from the pyrite surface. Further, in the surface oxidation stage, H2O2 and oxygen formed by ultrasonication immediately oxidized the pyrite, leading to a notable decrease in the contact angle. It was found that the influence of ultrasonication is governed by the time rather than the intensity of ultrasonication. Ultrasonication significantly improved the recovery of pyrite in the rougher and cleaner stages, when ultrasound was used only during flotation. The unfavorable effects of ultrasonication, such as oxidation, were not notable during flotation of the pyrite ore, due to the weak ultrasonic field in the flotation cells and the limited flotation time. Additionally, ultrasonication favored the formation of fine bubbles in the flotation cell, which is also benefit for pyrite flotation. Keywords: Ultrasound, Pyrite, Flotation, Surface cleaning, Oxidation.
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ACCEPTED MANUSCRIPT Surface cleaning and oxidative effects of ultrasonication on the flotation of oxidized pyrite
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1. Introduction Pyrite is a typical and abundant sulfide mineral[1]. It is well known that natural pyrite is hydrophobic in an anaerobic environment[2]. However, when pyrite is exposed to the atmosphere or aqueous conditions, oxidation occurring at the pyrite surface readily decreases hydrophobicity[3-5]. Such oxidation generates iron oxides/hydroxides and an underlying sulfur-rich surface[5-7]. It was recently revealed that the formation of H2O2 in the pulp accounts for the oxidation of pyrite[8]. H2O2 can be formed by pyrite in water without molecular oxygen[9], while with the molecular oxygen, ferrous iron and pyrite can also generate H2O2[10]. Nooshabadi et al. found a significant concentration of H2O2 in the pulp during wet-grinding of pyrite[8]. In particular, more H2O2 is generated when the pyrite is placed in water after dry-grinding [8]. H2O2 is also a strong oxidant for pyrite[11], and can significantly decrease the floatability of pyrite. After the oxidation with H2O2, a large number of iron oxide/hydroxide islands (1-2μm) are formed at the pyrite surface, which results in an extremely hydrophilic surface[12]. Ultrasonic pretreatment has been found to be a useful method to improve the floatability of oxidized pyrrhotite and other oxidized sulfide minerals[13]. The benefit of the ultrasonication is due to its surface cleaning effect. With ultrasonication, cavitation bubbles are generated in water during the fraction period of the ultrasound wave. When the cavitation bubble collapses near/at a solid surface, strong liquid jets are produced at an estimated speed of 100 m/s[14]. Such liquid jets can remove the surface coatings at the mineral surface[15-18]. In terms of pyrite flotation, the cleaning effect of ultrasonication can result in the removal of oxidation products at the pyrite surface, and thus improve the efficiency of pyrite flotation[19]. In contrast, ultrasound also shows a negative effect on the floatability of pyrite, which was found during desulfurization in coal flotation with ultrasonication[20]. This negative effect is caused by the chemical effects of ultrasonication. Ultrasonic irradiation in an aqueous solution can sonolyze the components in the solution. As a result, free radicals are produced by ultrasonication[21-22]. It is well-established that dissolved gas and water vapor in the cavitation bubble undergo thermal decomposition, resulting in the formation of OH and H radicals (reaction 1)[23]. Such radicals can be involved in secondary reactions (reaction 2-5)[20, 24]. H2O→•OH+H• (1) H•+H2O→H2+•OH (2) H• +O2 →•HO2 (3) •OH+•OH→H2O2 (4) •HO2 +•HO2→H2O2+O2↑ (5) The H2O2 and nascent oxygen can oxidize the pyrite surface, which decreases the floatability of pyrite. In fact, in previous research, only the surface cleaning effect of ultrasonication was found for the flotation of oxidized sulfide minerals[13]. The oxidative effect of ultrasonication was not observed for the oxidized pyrite. It is 2
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reasonable to predict that the surface cleaning and oxidative effects of ultrasonication on the oxidized pyrite are governed by the ultrasonication time as well as the sonic intensity. However, the influence of these ultrasonication parameters on the floatability of oxidized pyrite is still uncertain. On the other hand, Cliek et al. reported that ultrasonication can be successfully used during the flotation process[19]. This also benefits pyrite flotation because oxidation can happen during the flotation of pyrite. In previous works, single rougher flotation tests have been commonly employed to estimate the effects of ultrasonication on the flotation performance of sulfide minerals [13, 19]. However, for a run-of-mine pyrite ore, single rougher flotation usually cannot produce a satisfactory concentrate, and more stages are required. The influence of ultrasonication on different stages of pyrite flotation has not been fully revealed. Additionally, although ultrasonication shows notably beneficial effects on mineral flotation at the laboratory scale, its application in the flotation industry is still limited. One main problem is the attenuation of ultrasound in the slurry. The ultrasound loses energy during propagation in water, which is caused by multiple reasons, such as heat production and the formation of cavitation bubbles[25-26]. On the other hand, the solid particles in the pulp accelerate the attenuation of ultrasound[27]. The particle size and solid concentration have a significant impact on the attenuation of ultrasound[28-29]. Thus, a satisfactory ultrasonic field is essential for the scale-up of ultrasound-assisted flotation. However, the distribution of ultrasound in a flotation cell (laboratory scale) with slurry has not yet been fully investigated. Such information is valuable for the scale-up of flotation machines with ultrasound. The aim of the present work is to extend our knowledge regarding the effects of ultrasound on the flotation of oxidized pyrite. The dependence of the hydrophobicity of oxidized pyrite on ultrasonication conditions was examined through contact angle measurements. The oxidation states of pyrite samples with ultrasonication were further examined by EDX analysis. Furthermore, ultrasound was added during the flotation of a pyrite ore, and its effect on pyrite recovery at each flotation stage was determined. The distributions of sonic intensity and bubble size in flotation cells were also measured to further understand the effects of ultrasonication. 2 Experimental 2. 1 Reagents and samples Butyl xanthate and hydrogen peroxide (H2O2) were AR grade and purchased from Sinopharm Chemical Reagent Co., Ltd. Pine oil was provided by Hunan Minzhu Flotation Reagents Co., Ltd. These reagents were used as received. Pure pyrite pebbles were obtained from Yunnan Copper Industry Ltd. by Share Ltd., China. XRD analysis showed that the pyrite was triclinic, with space group P-1, and the purity was 99.9% (Fig. 1). The pyrite ore was taken from the Weixin area in Yunnan Province of China. The chemical compositional analysis of the pyrite ore is shown in Table 1. Further XRD results indicate that pyrite was the valuable mineral in this ore. Deionized (DI) water was used in the surface examinations, while batch flotation experiments were conducted with tap water. All of the experiments were performed at 23℃. 3
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2. 2 Contact angle measurements A GBX 3S tensiometer was employed to measure the contact angle of pyrite. Before the test, the pyrite pebble was polished with 2000-, 4000-, and 6000-grit Al2O3 sandpapers. The pyrite was further treated in two different ways: (1) by immersion in 1% H2O2 solution for a desired time; (2) further ultrasonication for different times and ultrasonic intensities, after oxidation in H2O2 solution. After the conditioning, the contact angles were measured with the captive-bubble method. The crystal was placed in a measurement cell filled with DI water. A 2-3 mm air bubble was generated under the crystal to attach to the surface. After the three-phase line was formed, the contact angles on both sides of the bubble were recorded. Each test was repeated three times. The average values were reported and the error of the measurements was less than ±2°. 2. 3 SEM-EDX analysis Surface analysis of pyrite samples that were oxidized with H2O2 and ultrasonicated was conducted with a FEI Quanta 200 SEM at 2000 × magnification. The accelerator voltage was 20 kV. The atomic concentrations of Fe, S, and O at the pyrite surface were measured. Images of pyrite after grinding were also obtained by the same instrument. 2. 4 Traditional flotation XFDMⅡflotation machines were used in the flotation tests. In each test, 500 g of pyrite ore was combined with 400 mL of tap water and 3 mL of H2O2 and ground in a ball mill for 3 min, to obtain a product that was 80 wt.% of solids passing through 200 mesh. After grinding, the slurry was filtered, and the ore was transferred into the 1.5 L flotation cell for rougher flotation, along with the pulp. Scavenger flotation was continuously conducted in the same 1.5 L flotation cell, and the cleaner flotation was further conducted in a 1 L flotation cell. The impeller speed of the flotation machines was 1500 rpm. The airflow rates were 4 L/min for the rougher/scavenger flotation stage and 3.5 L/min for the cleaner flotation stage. With a series of flotation tests, the optimal flotation conditions without ultrasonication were determined as shown in Table 2. 2. 5 Ultrasound-assisted flotation Ultrasonic transducers were equipped at the bottom of the flotation cells. The frequency of the ultrasonic transducer was 28 kHz. The energy of the transducer was 0-100 W. With this transducer, the highest ultrasonic intensity in water is 1.57 W/cm2. The flotation cell was installed in the XFDMⅡ flotation machine. Ultrasound was not used as a pre-conditioning method, which was used during the froth collection. Other grinding and mechanical conditions were identical to those for traditional flotation. The effect of ultrasound on each stage of pyrite flotation was investigated individually. 2. 6 Determination of ultrasonic intensity in the pulp A Megasonic Energy Meter (Hangzhou Success Ultrasonic Equipment Co. Ltd.) was employed to measure the ultrasonic energy at different spots in the vertical phase of the flotation cell, as shown in Fig. 2. The phases departed slightly from the center of the flotation cell, to avoid the stirring instrument. 4
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The pulp for the study was prepared under the conditions described for the traditional flotation. After the pulp was filled into the 1.5 L flotation cell, the ultrasonic intensity was directly measured without the addition of reagents and aeration. However, to measure the sonic intensity in the 1 L flotation cell, rougher flotation with ultrasound was conducted using another 500 g of pyrite ore to get a middling for cleaner flotation. After the middling was transferred into the 1 L flotation cell, the ultrasonic intensities at different spots were measured without aeration. All of the measurements were conducted while stirring. 2. 7 Bubble size measurement A high speed video system (Lighting RDTTM) was used to record the images of bubbles in a 6L of tempered glass cell with ultrasonic transducer. The pictures of bubbles in water with and without ultrasonication were both recorded, and were further analyzed with the software of Nano Measurer. In each case the size distribution of bubbles were obtained from the results of about 100 bubbles. The concentrations of pine oil and collector in water were same to that in rougher flotation. 3 Results and discussion 3. 1 Oxidation with H2O2 and cleaning effect of ultrasound In this work, natural pyrite pebble was oxidized in 1% H2O2 solution to generate a oxidative sample. Due to the strong oxidizing effect of H2O2, the pyrite samples at different oxidative levels can be readily obtained in a short time [30-31]. The hydrophobicity of pyrite decreased sharply with the increase in the treatment time in H2O2 solution (Fig. 3, curve a). The contact angle of natural pyrite was 57.1◦, while after 8 min of oxidation, the contact angle decreased to 18.5◦. The formation of ferric hydroxide/oxide at the pyrite surface accounted for this reduction in hydrophobicity[2]. The contact angles did not show an obvious reduction after 8 min of oxidation. It seems that the pyrite surface was completely coated by oxidation products after 8 min of oxidation. As reported by Jin et al., the contact angle decreased to 12◦ when pyrite was oxidized by H2O2 for 3 min[2]. Note that only 1% of H2O2 was used here, which is much lower than the concentration (30%) of H2O2 in their research. As a result, the rate of oxidation of pyrite here is relatively lower. When the oxidized samples were further treated ultrasonically (0.3 W/cm2) for 5 s, the contact angles were improved to some degree (Fig. 3, curve b). The improvement in hydrophobicity was significant for pyrite that was oxidized for less than 20 min. For instance, the contact angle was nearly 17◦ after 15 min of oxidation, and increased to 42◦ with 5 s of ultrasonication. For these oxidized samples, the increase in contact angle could be attributed to the cleaning effect of ultrasound, which partially removed the oxidation products at the pyrite surface. However, for pyrite oxidized for more than 20 min, ultrasonication showed a poor effect for raising the contact angle. The increase in contact angle after ultrasonication was less than 10◦. Newell et al. reported that ultrasonic conditioning failed to renew the floatability of heavily oxidized pyrrhotite [32]. They suggested that for the heavily oxidized sample, the oxidation process occurs not only at the mineral surface but also at the deeper layer beneath the surface. Further, the oxidation products are strongly 5
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attached at the pyrrhotite surface and are difficult to remove by ultrasound. In the present work, the pyrite is expected to be heavily oxidized after being treated with H2O2 for 20 min or more time. 5 s of ultrasonication is insufficient for eliminating the oxidation products and restoring the hydrophobicity of pyrite. 3. 2 Influence of ultrasonication on oxidized-pyrite surface The hydrophobicity of oxidized pyrite as a function of ultrasonication time and ultrasonic intensity was further studied. The pyrite samples at two oxidation levels, i.e., oxidized with H2O2 solution for 12 min and 20 min, were used in the tests. Such samples represent the lightly and heavily oxidized pyrite, respectively. Fig. 4 shows that for both oxidized samples, 0.1 W/cm2 of ultrasonication was insufficient to restore the hydrophobicity of the pyrite samples. On the other hand, ultrasound with intensity higher than 0.1 W/cm2 exhibited the opposite effect on the contact angles of oxidized pyrites, depending on the ultrasonication time. In terms of the slightly oxidized pyrite, the contact angle results with ultrasonic intensity above 0.1 W/cm2 may be discussed in two distinctive regions, as shown in Fig. 4A. In region 1 from 5 s to 20 s of ultrasonication, ultrasound improved the contact angle, and the maxima of contact angle were in the range of 55◦-58◦. These maximal values are close to the contact angle of natural pyrite, suggesting that the hydrophobicity of slightly oxidized pyrite is successfully restored by 20 s of ultrasonication. The positive effect of ultrasonication in this region could be due to its cleaning effect. In contrast, above 20 s of ultrasonication, i.e., in region 2, ultrasonication resulted in a sharp reduction in the contact angle. In particular, with 40 s of ultrasonication, the contact angle was below 31◦, which was even lower than that of the pyrite oxidized in H2O2 solution for 2 min. This phenomenon may be caused by the chemical effect of ultrasonication. After the original oxidation products are eliminated, cavitation bubbles are generated at the fresh pyrite surface. H2O2 and oxygen are also formed during this process, which are expected to further oxidize the pyrite. As a result, the hydrophobicity of pyrite is decreased. A similar trend was found for the heavily oxidized pyrite (Fig. 4B). 5 to 10 s of ultrasonication (above 0.1 W/cm2) could improve the contact angle to some degree. The highest contact angle was 39.5◦, which is much less than that of natural pyrite. As the ultrasonication time exceeded 10 s, the contact angle decreased again. The distribution of oxidation products at the pyrite surface is not inherently homogeneous [2]. For the heavily oxidized pyrite, some slightly oxidized areas are expected to be present at the surface. The oxidation products in these areas could be readily removed by 10 s of ultrasonication, which leads to an increase in the contact angle. However, in the heavily oxidized area, the oxidation products could form in the deeper layer of pyrite surface and are thus difficult to clean by ultrasound. Therefore, the beneficial effect of ultrasound on the hydrophobicity is limited for the heavily oxidized pyrite. In comparison, after more than 10 s of ultrasonication, and perhaps due to the oxidation effect of ultrasound, the pyrite surface became more hydrophilic than that after 10 s of ultrasonication. The above results essentially show that within 40 s of ultrasonication, both positive and negative effects of ultrasound were observed for the hydrophobicity of 6
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oxidized pyrite. Furthermore, the determining factor for the behavior of ultrasound was the ultrasonication time rather than the ultrasonic intensity. To further examine the positive/negative effects of ultrasound, SEM and EDX studies were performed to analyze the oxidation states of the oxidized pyrites. 3. 3 SEM and EDAX studies The oxygen concentration at the pyrite surface is crucial to evaluate the degree of oxidation of pyrite. EDX and XPS techniques are both capable of determining the oxygen content at a mineral surface. However, XPS analysis is difficult to perform on a mineral plate. Furthermore, air pollution also introduces oxygen during the sample preparation for the XPS experiments, which hinders the analysis. Given the disadvantages of XPS, we used the EDX technique to determine the elemental concentration at the surface of oxidized pyrite. Table 3 illustrates that 0.3 W/cm2 of ultrasound had a significant influence on the oxygen concentration at the pyrite surface that was oxidized by the H2O2 solution for 12 min. For this slightly oxidized pyrite, 20 s of ultrasonication significantly decreased the oxygen concentration at the mineral surface. The concentration ratio of S/Fe after 20 s of ultrasonication was 1.94. This suggests that the oxidation products are efficiently removed after 20 s of ultrasound irradiation, which is in line with the above contact angle results. On the contrary, with 40 s of ultrasonication, the oxygen concentration was twice that with 20 s of ultrasonication and was only 2% lower than that at the original oxidized surface. This indicates that 40 s of ultrasonic treatment improves the degree of oxidation of the pyrite surface compared to that under 20 s of ultrasonication. These results support the prediction that ultrasound can promote oxidation at a slightly oxidized pyrite surface with more than 20 s of ultrasonication. For the pyrite oxidized for 20 min, the oxygen concentration at the pyrite surface was 13.42% after oxidation, and further decreased to 8.92% after 10 s of ultrasonication. Again, the cleaning effect of ultrasound accounts for the reduction in oxygen content. The above contact angle tests show that 10 s of ultrasonication (0.3 W/cm2) could mostly improve the contact angle of this heavily oxidized pyrite. However, the oxygen concentration was still significant after 10 s of ultrasonication, suggesting the oxidation products at the surface are hard to remove completely. When the ultrasonication time was increased from 10 s to 50 s, the oxygen concentration also increased to some degree. This suggests that ultrasound could also exhibit oxidation effects at the surface of this heavily oxidized pyrite. Some authors have also suggested that over-irradiation with ultrasound may produce surface defects at the mineral surface, resulting in a decrease in hydrophobicity[15, 33]. Unfortunately, it is difficult to obtain a perfect pyrite surface without defects for the morphology study. Even when the pyrite was well-polished with sandpaper, the roughness at the surface was still significant, as shown in Fig. 5. The influence of ultrasound on the surface defects of pyrite was not investigated in this work, and should be studied in future. However, it is reasonable to expect that the oxidation effect of ultrasound is the main reason for the reduction in hydrophobicity, due to the increase in oxygen concentration at the mineral surface with over-ultrasonication. 7
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3. 4 Ultrasonic effects in different flotation stages Batch flotation tests were conducted to analyze the effect of ultrasonication on the pyrite flotation, where the ultrasound was used only during the flotation process. In our tests, it was found that the Fe grade did not respond to the ultrasonication. On the other hand, the Fe recovery was highly dependent on the ultrasonic energy. Therefore, only the recovery results are reported in Fig. 6. Although the natural pyrite surface is hydrophobic, the Fe recovery of this pyrite ore was only 6.3% by single rougher flotation with the addition of only 50 g/t of frother. It appeared that the pyrite had been oxidized to a certain extent after the wet-grinding and/or in the deposit. Using the collector and frother together, the Fe recovery was 78.47 % with the rougher flotation of traditional flotation (Fig. 6). With ultrasonication in the rougher stage, the recovery increased gradually with the increase in ultrasonic power, and reached a maximum (89.32%) at 70 W. As suggested, the oxidation products, such as ferric hydroxide, decrease the hydrophobicity of pyrite even in the presence of the xanthate collector[34]. It is reasonable to expect that the oxidation products at the pyrite surface are removed efficiently by the cleaning effect of ultrasound. Hence, the flotation performance of pyrite is improved. However, the recovery in the rougher stage was relatively lower at 90 W/100 W comparing to that at 70 W. This may be due to the following reasons. Firstly, as expected, the ultrasonic field at 90 W/100 W is extended to some degree in comparison with that at 70 W. The same pyrite particles might remain in the ultrasonic field of 90 W/ 100 W for more time. The surface of these pyrite particles could be oxidized to a certain degree. Secondly, the stronger ultrasonic intensity at 90 W/100 W could result in a more turbulent environment, which is not beneficial for the attachment of bubbles and particles[35]. Additionally, stronger ultrasonic power may introduce surface defects at the pyrite surface, resulting in a less hydrophobic pyrite surface than that at 70 W. In any event, it seems that the cleaning effect and unfavorable effects of ultrasound reach a balance at 70 W of ultrasonic power, for maximum improvement of the recovery in the rougher stage. It must be noted that the recovery at 100 W decreased by only 3.7 % compared to that at 70 W, which was still higher than that with traditional flotation. These results show that the unfavorable effects of ultrasound are not notable in the rougher stage. As for the cleaner stage, ultrasound showed more notable effects on increasing the pyrite recovery than that in the rougher stage. In this stage, the Fe recovery increased by 15.88% with 50 W of ultrasound, compared to that with traditional flotation. In fact, the amount of gauge minerals in the cleaner stage was much less than that in the rougher stage. Therefore, the ultrasonic irradiation could focus on pyrite, thus efficiently cleaning the pyrite surface. As the ultrasonic power exceeded 50 W, the recovery started to decrease. This reduction may be owing to the same reasons as those for the rougher stage, involving oxidation, over-turbulence, and surface-morphology damage. Additionally, the recovery in the scavenger stage was independent of the ultrasonic power, which could be attributed to the inefficient liberation of pyrite. It should be stressed that, in above flotation tests, ultrasonication was used in 8
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rougher and cleaner flotation stages respectively. As we tested, when ultrasonication was utilized successively in rougher stage (75 W of input power) and cleaner stage (50 W of input power), the Fe recovery reached 81.03% using 130 g/t of collector. However, with the traditional flotation (150 g/t of collector), the Fe recovery was only 61.09% after the cleaner flotation. Such results showed that ultrasonication can be used in the continuous flotation of this pyrite ore, and can reduce the demand for collector to some extent. 3. 5 Distribution of ultrasonic intensity As stated in the Introduction, the solid concentration has a significant impact on the transmission of ultrasound. To reveal the transmission of ultrasound in pulp, the ultrasonic intensities in the flotation cells with slurry were determined. In the rougher flotation stage, the solid concentration was 35.3% in the 1.5L of flotation cell. The ultrasonic intensity results in this flotation cell are showed in Fig. 7. The above contact angle results indicate that 0.3 W/cm2 of intensity was required for the ultrasound to exhibit the surface cleaning/oxidation effect for pyrite. With 75 W of ultrasonic power, 0.3 W/cm2 or higher intensity was measured within a height of 5 cm of the flotation cell (Fig. 7A). When the ultrasonic power was increased to 100 W, the ultrasonic field (≥0.3 W/cm2) was extended to some degree, but was still less than 7 cm of the height (Fig. 7B). After rougher flotation with another 500 g of ore using 75 W of ultrasonic power and 130 g/t of collector, the frother product was transferred into the 1 L flotation cell. The solid concentration (12.4%) in the cell was much less than that in the rougher flotation, while the ultrasonic field (≥0.3 W/cm2) was within 5 cm of height of flotation cell under 50 W or 100 W of ultrasonic power, Fig. 8. The solid particles in the pulp could adsorb and reflect the ultrasound waves[27]. In this regard, it was hard for ultrasound to propagate through the pulp, due to which the ultrasound was detected only in limited areas in the flotation cell. However, this ultrasound field was enough to exhibit benefits for the pyrite flotation, as demonstrated by the above flotation test. On the other hand, because the ultrasonic fields were weak under 100 W of ultrasonic power and the flotation time was limited, the unfavorable effects of ultrasound in the rougher/cleaner stage were not notable for the pyrite flotation. 3. 6 Effect on bubble size Bubble size has an important influence on mineral flotation, and fine bubble is commonly required[36]. Here we compared the bubble-size distributions in a flotation cell with and without ultrasonication. Since it is hard to obtain a clear picture of bubbles in slurry, the experiments were only performed in water without the pyrite ore. Fig. 9 shows that, without ultrasound, the mean size of bubbles was 2.55 mm, and 79.1% of bubbles were in the size range of 2.2 mm-4.6 mm. While, a smaller bubble-size range was obtained by 20 W of ultrasonic power, Fig. 10A. The mean size of bubbles decreased to 2.01 mm at this power. Furthermore, 72.2% of bubbles were less than 2.05 mm. It appeared that ultrasonication favored the formation of fine bubbles in the cell, which agrees well with the previous report[37]. 9
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It should be mentioned that ultrasound propagated well in water. With 20 W of input power, the ultrasonic intensity reached 0.21 W/cm2 at the top of cell (35 cm of height). While, at the bottom of the cell, the maxima ultrasonic intensity was 0. 82 W/cm2. Such ultrasonic intensity range is similar to that during rougher flotation at 75 W or cleaner flotation at 50 W. It is reasonable to expect that ultrasonication also promoted the formation of fine bubbles during the ultrasound-assisted flotation. Additionally, the mean size of bubbles at 100 W (Fig. 10B) was further decreased comparing with that at 20W, due to the stronger intensities at 100W, i.e., 1.55 W/cm2-0.75 W/cm2. However, as we known, it is still a challenge to produce such strong ultrasonic field in a slurry. 4 Conclusions A 0.3 W/cm2 or higher intensity of ultrasonication showed surface cleaning and oxidation effects for the oxidized pyrite samples. The effects of ultrasound were strictly dependent on the ultrasonication time. For the pyrite oxidized by H2O2 solution for 12 min, 20 s of ultrasonication efficiently eliminated the oxidation products, by which the hydrophobicity of the oxidized pyrite was restored. However, above 20 s of ultrasonication, H2O2 and nascent oxygen generated by the ultrasound were expected to further oxidize the pyrite surface. The cleaning effect of ultrasound was inefficient for pyrite oxidized by H2O2 solution for 20 min. Ten seconds of ultrasonication could partially improve the hydrophobicity of such heavily oxidized pyrite. More than 10 s of ultrasonication decreased the contact angle of the heavily oxidized pyrite, which may be due to the oxidative effect of ultrasound. In terms of the selected pyrite ore, the flotation recoveries in the rougher and cleaner stages were notably improved when ultrasound was used during the flotation. The adverse effects of ultrasound on pyrite recovery were not remarkable. Further study showed that the ultrasonic field (≥0.3 W/cm2) was within 5 cm of height in the flotation cells. For future scale-up, the flotation machine with ultrasound should have an ultrasonic field with the corresponding area and ultrasonic intensity, as revealed in this work. Besides the surface cleaning effect, another advantage of ultrasound is that ultrasonication could promote the formation of fine bubbles, which also could improve the flotation efficiency of pyrite. Acknowledgments The financial support from NSF grant No. 51304089 and the Ph.D. Programs Foundation of Ministry of Education of China 20135314120007 is gratefully acknowledged. References [1] X. Cazañas, P. Alfonso, J.C. Melgarejo, J.A. Proenza, A.E. Fallick, Source of ore-forming fluids in El Cobre VHMSdeposit (Cuba): evidence from fluid inclusions and sulfur isotopes, Journal of Geochemical Exploration, s 78–79 (2003) 85-90. [2] J. Jin, J.D. Miller, L.X. Dang, C.D. Wick, Effect of Surface Oxidation on Interfacial Water Structure at a Pyrite (100) Surface as Studied by Molecular Dynamics Simulation, International Journal of Mineral Processing, 139 (2015) 64–76. [3] K. Laajalehto, I. Kartio, E. Suoninen, XPS and SR-XPS techniques applied to sulphide mineral surfaces, International Journal of Mineral Processing, 51 (1997) 163-170.
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Table 2 Traditional flotation conditions for the pyrite ore Stage Reagent Dosage (g/t) Condition time (min) Butyl xanthate 150 2 Rougher flotation Pine oil 30 1 Butyl xanthate 75 2 Scavenger flotation Pine oil 20 1 Cleaner flotation -
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Table 3 Elemental concentrations and ratios at the pyrite plate surface (Sonic intensity was 0.3 W/cm2). Oxidation Atomic concentration Concentration time in (%) ratio Ultrasonication H2O2 time (s) solution Fe S O S/Fe O/Fe (min) 0 30.36 57.23 12.41 1.89 0.41 12 20 32.32 62.85 4.28 1.94 0.13 40 31.22 58.28 10.50 1.87 0.34 0 30.81 55.78 13.42 1.81 0.44 20 10 31.83 59.25 8.92 1.86 0.28 50 31.82 58.94 9.24 1.85 0.29
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Fig. 1 X-ray diffraction pattern of pyrite crystal.
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Fig. 2 Schematic diagram of the vertical phases in the 1.5 L (A) and 1 L (B) flotation cells for the ultrasonic intensity measurements (The vertical phase for the measurement is denoted by the dashed line.).
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Fig. 3 Contact angle results of pyrite (a, oxidized with H2O2; b, oxidized with H2O2 at different times and further ultrasonicated with an intensity of 0.3 W/cm2 for 5 s.).
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Fig. 4 Effect of ultrasound on the contact angles of pyrite samples oxidized by the H2O2 solution for 12 min (A) and 20 min (B).
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Fig. 5 SEM image of pyrite surface polished with 2000-grit, 4000-grit and 6000-grit sandpaper.
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Fig. 6 Influence of ultrasonic power on the Fe recovery at different stages.
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Fig. 7 Ultrasonic intensity (W/cm2) in the vertical phase of the 1.5 L flotation cell with slurry (A, 75 W of ultrasonic power; B, 100 W of ultrasonic power).
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Fig. 8 Ultrasonic intensity (W/cm2) in the vertical phase of the 1 L flotation cell with slurry (A, 50 W of ultrasonic power; B, 100 W of ultrasonic power).
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Fig. 9 Distribution of bubble size in water without ultrasound.
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Fig. 10 Distribution of bubble size in water with 20 W(A) and 100 W (B) of ultrasonic power.
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Ultrasonication showed opposite effects on the hydrophobicity of oxidized pyrite. The effects of ultrasound were determined by the ultrasonication time. 0.3 W/cm2 of intensity was required for the utilization of ultrasound. Negative effects of ultrasonication on the flotation of pyrite ore were not notable.
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