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The different effects of bentonite and kaolin on copper flotation
Applied Clay Science 114 (2015) 48–52
Contents lists available at ScienceDirect
Applied Clay Science journal homepage: www.elsevier.com/locate/clay
Note
The different effects of bentonite and kaolin on copper flotation Yanhong Wang a, Yongjun Peng a,⁎, Timothy Nicholson a, Rolf Andreas Lauten b a b
School of Chemical Engineering, The University of Queensland, Australia Pionera, Hjalmar Wesselsvei 10, 1701 Sarpsborg, Norway
a r t i c l e
i n f o
Article history: Received 11 March 2015 Received in revised form 7 May 2015 Accepted 10 May 2015 Available online xxxx Keywords: Clay minerals Copper flotation Slurry rheology Froth Entrainment
a b s t r a c t Clay minerals impose deleterious impacts on mineral processing. In this study, bentonite consisting primarily of montmorillonite and kaolin consisting primarily of kaolinite, were used to characterise the effect of clay minerals on copper flotation. It was found that bentonite and kaolin caused different issues in the process of flotation. Increasing the proportion of bentonite reduced the amount of froth on the top of slurry and decreased the copper recovery. On the other hand, increasing the kaolin content mainly decreased the copper grade with little effect on copper recovery and the bubbles on the top of froth became smaller with higher froth stability. The different roles of bentonite and kaolin in the flotation were associated with their unique properties. The findings in this study suggest that to mitigate the negative effect of clay minerals, different approaches might be applied according to specific problems caused by different clay minerals. © 2015 Elsevier B.V. All rights reserved.
1. Introduction In the mineral processing industry, an increasing volume of low grade ores containing different types and amounts of clay and other gangue minerals are currently being processed with the depletion of high quality ores. Different operations are characterised by the presence of different clay minerals. The Chilean porphyry copper ores operated at Disputada plants contain mainly illite and chlinochlore as clay minerals (Bulatovic et al., 1998); Kimberlite ores located in the vicinity of the South Africa–Zimbabwe border chiefly contain smectite and mica; and Norseman–Wiluna gold ores in Western Australia are composed of serpentine minerals (Burdukova et al., 2008). In coal mine, kaolinite and illite are the major clay minerals in high-clay-content coal mines at Central Queensland in Australia (Wang and Peng, 2014), while bentonite is the predominant clay in Xstrata Coal Mines at Central Queensland (Wang and Peng, 2013). However, processing the high clayey ores has raised new challenges as the clay minerals have deleterious effects on the flotation of valuable minerals, such as high slurry viscosity, no froth generated, and few valuable minerals loaded on the top of froth (Burdukova et al., 2008; Ndlovu et al., 2011). Currently, no reliable and effective ways are available to reduce these deleterious effects of clay minerals, especially in copper and gold flotation. Most investigators attribute the negative effect of clay minerals in flotation to slime coating (Edwards et al., 1980; Arnold and Aplan, 1986; Peng and Zhao, 2011), slurry viscosity (Merve Genc et al., 2012; Patra et al., 2012; Zhang and Peng, 2015), or entrainment (Wang and Peng, 2013; Liu and Peng, 2014). However, in a single study, they ⁎ Corresponding author. Tel.: +61 7 3365 7156; fax: +61 7 3365 4199. E-mail address: [email protected] (Y. Peng).
http://dx.doi.org/10.1016/j.clay.2015.05.008 0169-1317/© 2015 Elsevier B.V. All rights reserved.
usually compared the negative effects of different clay minerals from one aspect. The problem underlying these studies is that some clay minerals may not cause that specific problem at all. During the flotation process different clay minerals might mainly cause different corresponding problems, due to their unique properties. Mineral flotation is a complex process that takes place in both the slurry and froth, and the changes in slurry rheology may influence the subsequent bubble–particle collision and mobility of bubble–particle aggregates, which is further associated with the froth generated. Even if little variation occurs in slurry, it is entirely possible that the froth performance turns out to be completely different, because of the presence of different clay minerals. Clay minerals are phyllosilicate and non-phyllosilicate minerals comprising silica tetrahedral (T) sheets and octahedral (O) sheets joining together in certain proportions, based on which two structural units are classified for most clay minerals: 1:1 (T–O) and 2:1 (T–O–T) layer structures (Theng, 2012). The objective of this study is to evaluate the deleterious effects of different clay minerals on copper flotation and specify the underpinning mechanism. Swelling bentonite with a 2:1 structure and non-swelling kaolin Q38 with a 1:1 structure, were chosen in this study to represent two typical kinds of clays occurring in industry ore deposits. The influence of bentonite and kaolin on copper flotation was investigated by studying how they modified properties of both slurry and froth and caused different flotation behaviour. 2. Experimental 2.1. Material and reagents The copper–gold ore from Newcrest Telfer mine with 0.43 wt.% Cu, Telfer clean ore, was used as a reference system. The mineral composition
Y. Wang et al. / Applied Clay Science 114 (2015) 48–52
of this ore was analysed by quantitative X-ray Diffraction (XRD) using Bruker D4 Endeavor diffractometer with Co Kα radiation (1.7903 Å) generated using 35 kV and 40 mA. The scan rate and step size were 1 s/step and 0.02° 2θ, respectively. The diffraction patterns were acquired from 5 to 85° 2θ. Phase quantification was carried out using the DiffracPlus EVA software (Bruker) with the ICDD-PDF2 database (International Center for Diffraction Data, 2000). Bruker-AXS's TOPAS V4.2 software was then used to quantify each phase. Crystal structure information for all the minerals was obtained from the Bruker Structure Database. The analysis results in Fig. 1 indicated that the major valuable mineral is chalcopyrite, with pyrite, quartz, albite, muscovite, dolomite and amorphous as gangue minerals. Clay minerals may not coat chalcopyrite surface in flotation due to electrostatic repulsion between them (Peng and Zhao, 2011). A bentonite sample and a kaolin sample were purchased from Sibelco Group, Australia. The bentonite sample contains 63% montmorillonite, 25% albite and 12% quartz, while the kaolin sample contains 85% kaolinite Q38 with 4% quartz and 11% muscovite. The previous study has shown that the variation in the purities of bentonite and kaolin samples does not affect the experimental interpretations (Zhang and Peng, 2015). The presence of muscovite in the kaolin sample will not affect the flotation of Telfer clean ore when they are mixed since Telfer clean ore itself consists of 6.8 wt.% muscovite as shown in Fig. 1. Particle size distributions of these clay samples are similar with 70% of particles smaller than 10 μm. Industrial grade potassium isoamyl xanthate (PAX) obtained from Orica, Australia Pty Ltd, was used as the collector. Plant frother DSF004, an aliphatic alcohol based mixture from Newcrest Operation, was used in laboratory tests. The pH value of slurry in flotation was controlled by analytical grade hydrated lime (Chem-Supply, Australia). Brisbane tap water was used throughout the flotation process. 2.2. Mineral grinding and flotation Telfer clean ore was crushed to a size of −2.36 mm using jaw and roll crushers. Then 1 kg −2.36 mm ore sample was ground in a lab oratory rod mill with stainless steel rods at 66% solids to obtain an 80% passing of 106 μm (P80 = 106 μm). The ground slurry was then transferred to a 2.5 L J∙K. flotation cell. The artificial clayey ore was prepared as described in Zhang and Peng's (2015) study. Briefly, for each test a calculated amount of clean ore slurry was replaced by the same amount of a well-mixed bentonite or kaolin sample to keep the slurry density constant in the same mass fraction. The different particle sizes were found to not contribute to viscosity results. The slurry was firstly conditioned for 3 min with 30 g/t collector, and 15 g/t frother. The agitation speed during flotation was kept at 1000 rpm. The flotation froth was scraped every 10 s. Four flotation concentrates were collected after cumulative times of 1, 3, 6 and 10 min with an air flow rate of 3.0 L/min. After the first 3 min flotation, the second conditioning was followed with 15 g/t collector, and 8 g/t frother for another 2 min. During the flotation process, the pH was maintained constant at about 9.0.
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2.3. Rheology measurements Rheology measurements were conducted by an Ares rheometer (TA Instruments Ltd., U.S.). All tests were conducted with a 42 mm diameter cup and 28 mm diameter vaned rotor geometry. Each time 40 mL of slurry sample was poured into the cup and the rotor lowered until the gap between the rotor and the bottom of cup was 4 mm. Rheograms were generated in the shearing rate ranging between 1 and 300 s− 1 for 120 s. At least three duplications were performed for each sample. During measurements, the temperature was maintained at 25 °C with an accuracy of ±1 °C. 2.4. Froth image and stability measurement When flotation commenced, the froth was measured in-situ by using VisioFroth software. The camera was set up directly above the flotation cell and connected to a laptop device. The camera recorded the froth stability every 5 s automatically (Liu and Peng, 2014). The average froth stability values were calculated after each set of tests. At the same time, the froth image was recorded manually to observe the change of the amount of froth for each flotation test. The image capture area was 15.9 cm × 15.9 cm. 3. Results and discussion 3.1. Effect of clay minerals on copper flotation The effects of various amounts of bentonite and kaolin on copper flotation are shown in Fig. 2(a) and (b), respectively. Flotation tests were performed with Telfer clean ore as a laboratory baseline in the absence of clay samples. The presence of either bentonite or kaolin affected copper flotation. In Fig. 2(a) an increase in the amount of bentonite does not affect the copper grade but it reduces the copper recovery, in particular from the first concentrate. As the proportion of bentonite increased from 0 to 20 wt.%, the copper recovery in the first concentrate decreased from 76% to 25%. When the bentonite content increased to 25 wt.%, the flotation dynamic was completely hindered, with no froth generated on the top of pulp, which normally happens in industrial operations. In contrast to bentonite, increasing kaolin content in Telfer clean ore slightly lowered the flotation recovery, but reduced the copper grade substantially. In Fig. 2(b) with an increase in kaolin content in the ore from 0 to 25 wt.%, the copper grade decreased from 5% to 2.4% in the first concentrate. At the same time, the increase in kaolin content from 0 to 25 wt.% in the ore just slightly depressed the copper recovery of the first concentrate, from 80% to 70%. 3.2. Effect of clay minerals on slurry rheology The increase in clay content was also associated with the increase in the slurry viscosity, in particular for bentonite. The apparent viscosity of Telfer clean ore slurry as a function of the clay concentration is shown in
Fig. 1. X-ray diffraction of Telfer clean ore.
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Y. Wang et al. / Applied Clay Science 114 (2015) 48–52
inhibited the collision between bubbles and particles and the mobility of bubble–particle aggregates, which further reduced the amount of froth generated on the top of flotation cell, resulting in a lower copper recovery. This is in agreement with Zhang and Peng's (2015) study, where a relationship between copper recovery and the apparent viscosity was established. In this study, the relationship is more obvious for the first concentrate. The higher the apparent viscosity, the lower the copper recovery from the first concentrate. With regard to kaolin, the general trends indicated that, in contrast to bentonite, increasing the kaolin content had little influence on slurry viscosity and hence on copper recovery. However, it was found that an increase in kaolin proportion correlated with a decrease in copper grade (Fig. 2), which means that a portion of the kaolin was recovered in the concentrate. Since the rheological measurements revealed that there was a negligible change of slurry viscosity in the presence of kaolin, this indicates that most kaolinite particles are dispersed as the other solids in the ore. During the flotation process, fine kaolinite particles at a low concentration are easily suspended in the water and on the water film surrounding the bubbles, and hence they have more chances to transfer from the slurry phase to froth phase and then to be collected in the concentrate, resulting in a lower copper grade. At a higher kaolin concentration, although it has been found that both basal and edge faces of kaolinite are negatively charged in the solution at pH 9 (Gupta and Miller, 2010), the increasing particle concentration could reduce the mobility of particles and force kaolinite to adapt E–E association because of the strong repulsion between the facial platelets (Du et al., 2010). Further increasing the kaolinite concentration also increases the opportunity for interactions between different complex structures (Gupta et al., 2011). These network structures usually have low density, and move upwards into froth and consequently further reduce the copper grade. 3.3. Effect of clay minerals on froth performance Fig. 2. Cu grade as a function of Cu recovery in the flotation of Telfer clean ore in the absence and presence of clay minerals at different concentrations: (a) bentonite; (b) kaolin.
Fig. 3 at a shear rate of 100 s−1. Generally, the apparent viscosity of pulp increased exponentially with an increase in bentonite concentration, while there was merely a slight increase in the case of kaolin, which was consistent with the previous studies (Cruz et al., 2013; Zhang and Peng, 2015). For bentonite, both the aggregate structure and swelling property contributed to the dramatically elevated apparent viscosity. Bentonite with a 2:1 structure is more reactive than kaolin with a 1:1 structure. The larger interlayer distance of bentonite makes it easier for water molecules to penetrate between platelets, resulting in intercrystalline and osmotic swelling of bentonite, which facilitate strong interaction and formation of aggregate structures (Luckham and Rossi, 1999; Lagaly and Ziesmer, 2003). In flotation process, the high slurry viscosity
Fig. 3. Apparent viscosity as a function of clay mineral concentration in Telfer clean ore at a shear rate of 100 s−1.
During the flotation process, the presence of clay minerals also influenced the froth performance. Froth structure and stability play critical roles in determining mineral flotation recovery and selectivity (Cutting, 1989; Barbian et al., 2005). In Fig. 4(a0) to (a5), the volume of froth decreases when bentonite content in Telfer clean ore increased from 0 to 25 wt.%, which was in consistence with the flotation results in Fig. 2(a) that with an increase in bentonite content, the flotation recovery of the first concentrate decreased sharply. In the case of kaolin, upon increasing its content from 0 to 25 wt.%, the volume of froth seemed unchanged (Fig. 4(b0) to (b5)), which also agreed with the flotation results in Fig. 2 that the final copper recovery was unchanged. Meanwhile, the froth structure became completely different with an increase in clay concentration. Generally, with an increase in clay content, the number of large bubbles declined, resulting in the increasing number of medium bubbles in the case of bentonite. The bubble size decreased sharply in Fig. 4(b5) when the kaolin content increased to 25 wt.%, with only smaller bubbles appearing on the top of froth. These smaller bubbles might relate to the decrease of copper grade in Fig. 2(b), as it has been considered by some researchers that a close relationship exists between bubble size and flotation performance. A froth structure with small, spherical bubbles normally represents a low valuable mineral content in the froth (Moolman et al., 1995; Wang and Peng, 2013). The decrease of bubble size was also related to the increase of gas residence time which is an indicator of froth stability (Quinn et al., 2007). Therefore, froth stability in the flotation of Telfer clean ore in the presence of the two clay samples was also studied. In Fig. 5 the froth stability decreased with an increase in bentonite proportion but increased when kaolin proportion increased. The decrease of froth stability rendered it difficult to transport a considerable amount of mineral particles over the weir of flotation cell to the concentrate launder, which is also responsible for the decrease of copper recovery in the presence of bentonite. On the other hand, both higher froth stability and smaller
Y. Wang et al. / Applied Clay Science 114 (2015) 48–52
51
Fig. 5. Froth stability as a function of clay mineral concentration during the flotation process.
bubble size are typical characteristics of entrainment (Subrahmanyam and Forssberg, 1988; Wang and Peng, 2013; Liu and Peng, 2014; Wang et al., 2015). Therefore, the decrease of copper grade with the increase of kaolin proportion is also likely to be due to mechanical entrainment by which kaolinite minerals suspend in water due to their fine size, move upwards, enter the flotation froth and finally leave with the concentrate. 4. Conclusion The presence of bentonite and kaolin exacerbated the flotation of Telfer clean ore. Specifically, the copper recovery decreased with an increase in bentonite content in flotation because of the high slurry viscosity that reduced the frequency of bubble–particle collisions and the mobility of bubble–particle aggregates, resulting in a lower amount of froth. The copper grade was reduced with an increase in kaolin proportion. The addition of kaolin had little effect on slurry viscosity and the mobility of bubbles and particles was not affected. Due to the fine size of kaolin and the low-density aggregate structure, kaolin particles moved upwards to the froth layer reducing copper grade. The increase of kaolin content also caused severe entrainment characterised by smaller bubble size and higher froth stability. These observations are important in the industry, as different actions need to be taken in order to deal with clayey ores, according to the essence of the issue. For high bentonite content ores, the pulp viscosity should be reduced firstly to improve the flotation recovery, while for high kaolin content ores, increasing the concentrate grade is of vital importance in flotation, either by dealing with the fine size of particles or the low density of aggregates. Acknowledgements The authors gratefully acknowledge the financial support of this study from the Australian Research Council and Pionera (LP120200162) as well as the supply of practical ores from Newcrest Telfer operation. The first author also thanks the scholarship provided by the University of Queensland. References
Fig. 4. Froth image of the first concentrate in the flotation of Telfer clean ore in the absence and presence of clay minerals at different concentrations: (a0)–(a5), addition of different bentonite proportions; (b0)–(b5), addition of different kaolin proportions.
Arnold, B.J., Aplan, F.F., 1986. The effect of clay slimes on coal flotation. 1. The nature of the clay. Int. J. Miner. Process. 17 (3-4), 225–242. Barbian, N., Hadler, K., Ventura-Medina, E., Cilliers, J.J., 2005. The froth stability column: linking froth stability and flotation performance. Miner. Eng. 18 (3), 317–324. Bulatovic, S.M., Wyslouzil, D.M., Kant, C., 1998. Operating practices in the beneficiation of major porphyry copper/molybdenum plants from Chile: innovated technology and opportunities, a review. Miner. Eng. 11 (4), 313–331. Burdukova, E., Becker, M., Ndlovu, B., Mokgethi, B., Deglon, D., 2008. Relationship between slurry rheology and its mineralogical content. 24th Int. Minerals Processing Congress. China Scientific Book Service Co. Ltd., Beijing, China.
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Cruz, N., Peng, Y., Farrokhpay, S., Bradshaw, D., 2013. Interactions of clay minerals in copper–gold flotation: part 1—rheological properties of clay mineral suspensions in the presence of flotation reagents. Miner. Eng. 50–51, 30–37. Cutting, G., 1989. Effect of froth structure and mobility on plant performance. Miner. Process. Extr. Metall. Rev. 5 (1-4), 169–201. Du, J., Morris, G., Pushkarova, R.A., Smart, R.S.C., 2010. Effect of surface structure of kaolinite on aggregation, settling rate, and bed density. Langmuir 26 (16), 13227–13235. Edwards, C.R., Kipkie, W.B., Agar, G.E., 1980. The effect of slime coatings of the serpentine minerals, chrysotile and lizardite, on pentlandite flotation. Int. J. Miner. Process. 7 (1), 33–42. Gupta, V., Miller, J.D., 2010. Surface force measurements at the basal planes of ordered kaolinite particles. J. Colloid Interface Sci. 344 (2), 362–371. Gupta, V., Hampton, M.A., Stokes, J.R., Nguyen, A.V., Miller, J.D., 2011. Particle interactions in kaolinite suspensions and corresponding aggregate structures. J. Colloid Interface Sci. 359 (1), 95–103. Lagaly, G., Ziesmer, S., 2003. Colloid chemistry of clay minerals: the coagulation of montmorillonite dispersions. Adv. Colloid Interf. Sci. 100, 105–128. Liu, D., Peng, Y., 2014. Reducing the entrainment of clay minerals in flotation using tap and saline water. Powder Technol. 253, 216–222. Luckham, P.F., Rossi, S., 1999. The colloidal and rheological properties of bentonite suspensions. Adv. Colloid Interf. Sci. 82 (1-3), 43–92. Merve Genc, A., Kilickaplan, I., Laskowski, J., 2012. Effect of slurry rheology on flotation of nickel sulphide ore with fibrous gangue particles. Can. Metall. Q. 51 (4), 368–375. Moolman, D., Aldrich, C., Van Deventer, J., Bradshaw, D., 1995. The interpretation of flotation froth surfaces by using digital image analysis and neural networks. Chem. Eng. Sci. 50 (22), 3501–3513.
Ndlovu, B., Becker, M., Forbes, E., Deglon, D., Franzidis, J.-P., 2011. The influence of phyllosilicate mineralogy on the rheology of mineral slurries. Miner. Eng. 24 (12), 1314–1322. Patra, P., Bhambhani, T., Nagaraj, D.R., Somasundaran, P., 2012. Impact of slurry rheological behavior on selective separation of Ni minerals from fibrous serpentine ores. Colloids Surf. A 411, 24–26. Peng, Y., Zhao, S., 2011. The effect of surface oxidation of copper sulfide minerals on clay slime coating in flotation. Miner. Eng. 24 (15), 1687–1693. Quinn, J.J., Kracht, W., Gomez, C.O., Gagnon, C., Finch, J.A., 2007. Comparing the effect of salts and frother (MIBC) on gas dispersion and froth properties. Miner. Eng. 20 (14), 1296–1302. Subrahmanyam, T.V., Forssberg, E., 1988. Froth stability, particle entrainment and drainage in flotation — a review. Int. J. Miner. Process. 23 (1–2), 33–53. Theng, B.K.G., 2012. Chapter 1 — the clay minerals. In: Theng, B.K.G. (Ed.), Developments in Clay Science. vol. 4. Elsevier, pp. 3–45. Wang, B., Peng, Y., 2013. The behaviour of mineral matter in fine coal flotation using saline water. Fuel 109, 309–315. Wang, B., Peng, Y., 2014. The interaction of clay minerals and saline water in coarse coal flotation. Fuel 134, 326–332. Wang, L., Peng, Y., Runge, K., Bradshaw, D., 2015. A review of entrainment: mechanisms, contributing factors and modelling in flotation. Miner. Eng. 70, 77–91. Zhang, M., Peng, Y., 2015. Effect of clay minerals on slurry rheology and the flotation of copper and gold minerals. Miner. Eng. 70, 8–13.
Contents lists available at ScienceDirect
Applied Clay Science journal homepage: www.elsevier.com/locate/clay
Note
The different effects of bentonite and kaolin on copper flotation Yanhong Wang a, Yongjun Peng a,⁎, Timothy Nicholson a, Rolf Andreas Lauten b a b
School of Chemical Engineering, The University of Queensland, Australia Pionera, Hjalmar Wesselsvei 10, 1701 Sarpsborg, Norway
a r t i c l e
i n f o
Article history: Received 11 March 2015 Received in revised form 7 May 2015 Accepted 10 May 2015 Available online xxxx Keywords: Clay minerals Copper flotation Slurry rheology Froth Entrainment
a b s t r a c t Clay minerals impose deleterious impacts on mineral processing. In this study, bentonite consisting primarily of montmorillonite and kaolin consisting primarily of kaolinite, were used to characterise the effect of clay minerals on copper flotation. It was found that bentonite and kaolin caused different issues in the process of flotation. Increasing the proportion of bentonite reduced the amount of froth on the top of slurry and decreased the copper recovery. On the other hand, increasing the kaolin content mainly decreased the copper grade with little effect on copper recovery and the bubbles on the top of froth became smaller with higher froth stability. The different roles of bentonite and kaolin in the flotation were associated with their unique properties. The findings in this study suggest that to mitigate the negative effect of clay minerals, different approaches might be applied according to specific problems caused by different clay minerals. © 2015 Elsevier B.V. All rights reserved.
1. Introduction In the mineral processing industry, an increasing volume of low grade ores containing different types and amounts of clay and other gangue minerals are currently being processed with the depletion of high quality ores. Different operations are characterised by the presence of different clay minerals. The Chilean porphyry copper ores operated at Disputada plants contain mainly illite and chlinochlore as clay minerals (Bulatovic et al., 1998); Kimberlite ores located in the vicinity of the South Africa–Zimbabwe border chiefly contain smectite and mica; and Norseman–Wiluna gold ores in Western Australia are composed of serpentine minerals (Burdukova et al., 2008). In coal mine, kaolinite and illite are the major clay minerals in high-clay-content coal mines at Central Queensland in Australia (Wang and Peng, 2014), while bentonite is the predominant clay in Xstrata Coal Mines at Central Queensland (Wang and Peng, 2013). However, processing the high clayey ores has raised new challenges as the clay minerals have deleterious effects on the flotation of valuable minerals, such as high slurry viscosity, no froth generated, and few valuable minerals loaded on the top of froth (Burdukova et al., 2008; Ndlovu et al., 2011). Currently, no reliable and effective ways are available to reduce these deleterious effects of clay minerals, especially in copper and gold flotation. Most investigators attribute the negative effect of clay minerals in flotation to slime coating (Edwards et al., 1980; Arnold and Aplan, 1986; Peng and Zhao, 2011), slurry viscosity (Merve Genc et al., 2012; Patra et al., 2012; Zhang and Peng, 2015), or entrainment (Wang and Peng, 2013; Liu and Peng, 2014). However, in a single study, they ⁎ Corresponding author. Tel.: +61 7 3365 7156; fax: +61 7 3365 4199. E-mail address: [email protected] (Y. Peng).
http://dx.doi.org/10.1016/j.clay.2015.05.008 0169-1317/© 2015 Elsevier B.V. All rights reserved.
usually compared the negative effects of different clay minerals from one aspect. The problem underlying these studies is that some clay minerals may not cause that specific problem at all. During the flotation process different clay minerals might mainly cause different corresponding problems, due to their unique properties. Mineral flotation is a complex process that takes place in both the slurry and froth, and the changes in slurry rheology may influence the subsequent bubble–particle collision and mobility of bubble–particle aggregates, which is further associated with the froth generated. Even if little variation occurs in slurry, it is entirely possible that the froth performance turns out to be completely different, because of the presence of different clay minerals. Clay minerals are phyllosilicate and non-phyllosilicate minerals comprising silica tetrahedral (T) sheets and octahedral (O) sheets joining together in certain proportions, based on which two structural units are classified for most clay minerals: 1:1 (T–O) and 2:1 (T–O–T) layer structures (Theng, 2012). The objective of this study is to evaluate the deleterious effects of different clay minerals on copper flotation and specify the underpinning mechanism. Swelling bentonite with a 2:1 structure and non-swelling kaolin Q38 with a 1:1 structure, were chosen in this study to represent two typical kinds of clays occurring in industry ore deposits. The influence of bentonite and kaolin on copper flotation was investigated by studying how they modified properties of both slurry and froth and caused different flotation behaviour. 2. Experimental 2.1. Material and reagents The copper–gold ore from Newcrest Telfer mine with 0.43 wt.% Cu, Telfer clean ore, was used as a reference system. The mineral composition
Y. Wang et al. / Applied Clay Science 114 (2015) 48–52
of this ore was analysed by quantitative X-ray Diffraction (XRD) using Bruker D4 Endeavor diffractometer with Co Kα radiation (1.7903 Å) generated using 35 kV and 40 mA. The scan rate and step size were 1 s/step and 0.02° 2θ, respectively. The diffraction patterns were acquired from 5 to 85° 2θ. Phase quantification was carried out using the DiffracPlus EVA software (Bruker) with the ICDD-PDF2 database (International Center for Diffraction Data, 2000). Bruker-AXS's TOPAS V4.2 software was then used to quantify each phase. Crystal structure information for all the minerals was obtained from the Bruker Structure Database. The analysis results in Fig. 1 indicated that the major valuable mineral is chalcopyrite, with pyrite, quartz, albite, muscovite, dolomite and amorphous as gangue minerals. Clay minerals may not coat chalcopyrite surface in flotation due to electrostatic repulsion between them (Peng and Zhao, 2011). A bentonite sample and a kaolin sample were purchased from Sibelco Group, Australia. The bentonite sample contains 63% montmorillonite, 25% albite and 12% quartz, while the kaolin sample contains 85% kaolinite Q38 with 4% quartz and 11% muscovite. The previous study has shown that the variation in the purities of bentonite and kaolin samples does not affect the experimental interpretations (Zhang and Peng, 2015). The presence of muscovite in the kaolin sample will not affect the flotation of Telfer clean ore when they are mixed since Telfer clean ore itself consists of 6.8 wt.% muscovite as shown in Fig. 1. Particle size distributions of these clay samples are similar with 70% of particles smaller than 10 μm. Industrial grade potassium isoamyl xanthate (PAX) obtained from Orica, Australia Pty Ltd, was used as the collector. Plant frother DSF004, an aliphatic alcohol based mixture from Newcrest Operation, was used in laboratory tests. The pH value of slurry in flotation was controlled by analytical grade hydrated lime (Chem-Supply, Australia). Brisbane tap water was used throughout the flotation process. 2.2. Mineral grinding and flotation Telfer clean ore was crushed to a size of −2.36 mm using jaw and roll crushers. Then 1 kg −2.36 mm ore sample was ground in a lab oratory rod mill with stainless steel rods at 66% solids to obtain an 80% passing of 106 μm (P80 = 106 μm). The ground slurry was then transferred to a 2.5 L J∙K. flotation cell. The artificial clayey ore was prepared as described in Zhang and Peng's (2015) study. Briefly, for each test a calculated amount of clean ore slurry was replaced by the same amount of a well-mixed bentonite or kaolin sample to keep the slurry density constant in the same mass fraction. The different particle sizes were found to not contribute to viscosity results. The slurry was firstly conditioned for 3 min with 30 g/t collector, and 15 g/t frother. The agitation speed during flotation was kept at 1000 rpm. The flotation froth was scraped every 10 s. Four flotation concentrates were collected after cumulative times of 1, 3, 6 and 10 min with an air flow rate of 3.0 L/min. After the first 3 min flotation, the second conditioning was followed with 15 g/t collector, and 8 g/t frother for another 2 min. During the flotation process, the pH was maintained constant at about 9.0.
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2.3. Rheology measurements Rheology measurements were conducted by an Ares rheometer (TA Instruments Ltd., U.S.). All tests were conducted with a 42 mm diameter cup and 28 mm diameter vaned rotor geometry. Each time 40 mL of slurry sample was poured into the cup and the rotor lowered until the gap between the rotor and the bottom of cup was 4 mm. Rheograms were generated in the shearing rate ranging between 1 and 300 s− 1 for 120 s. At least three duplications were performed for each sample. During measurements, the temperature was maintained at 25 °C with an accuracy of ±1 °C. 2.4. Froth image and stability measurement When flotation commenced, the froth was measured in-situ by using VisioFroth software. The camera was set up directly above the flotation cell and connected to a laptop device. The camera recorded the froth stability every 5 s automatically (Liu and Peng, 2014). The average froth stability values were calculated after each set of tests. At the same time, the froth image was recorded manually to observe the change of the amount of froth for each flotation test. The image capture area was 15.9 cm × 15.9 cm. 3. Results and discussion 3.1. Effect of clay minerals on copper flotation The effects of various amounts of bentonite and kaolin on copper flotation are shown in Fig. 2(a) and (b), respectively. Flotation tests were performed with Telfer clean ore as a laboratory baseline in the absence of clay samples. The presence of either bentonite or kaolin affected copper flotation. In Fig. 2(a) an increase in the amount of bentonite does not affect the copper grade but it reduces the copper recovery, in particular from the first concentrate. As the proportion of bentonite increased from 0 to 20 wt.%, the copper recovery in the first concentrate decreased from 76% to 25%. When the bentonite content increased to 25 wt.%, the flotation dynamic was completely hindered, with no froth generated on the top of pulp, which normally happens in industrial operations. In contrast to bentonite, increasing kaolin content in Telfer clean ore slightly lowered the flotation recovery, but reduced the copper grade substantially. In Fig. 2(b) with an increase in kaolin content in the ore from 0 to 25 wt.%, the copper grade decreased from 5% to 2.4% in the first concentrate. At the same time, the increase in kaolin content from 0 to 25 wt.% in the ore just slightly depressed the copper recovery of the first concentrate, from 80% to 70%. 3.2. Effect of clay minerals on slurry rheology The increase in clay content was also associated with the increase in the slurry viscosity, in particular for bentonite. The apparent viscosity of Telfer clean ore slurry as a function of the clay concentration is shown in
Fig. 1. X-ray diffraction of Telfer clean ore.
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inhibited the collision between bubbles and particles and the mobility of bubble–particle aggregates, which further reduced the amount of froth generated on the top of flotation cell, resulting in a lower copper recovery. This is in agreement with Zhang and Peng's (2015) study, where a relationship between copper recovery and the apparent viscosity was established. In this study, the relationship is more obvious for the first concentrate. The higher the apparent viscosity, the lower the copper recovery from the first concentrate. With regard to kaolin, the general trends indicated that, in contrast to bentonite, increasing the kaolin content had little influence on slurry viscosity and hence on copper recovery. However, it was found that an increase in kaolin proportion correlated with a decrease in copper grade (Fig. 2), which means that a portion of the kaolin was recovered in the concentrate. Since the rheological measurements revealed that there was a negligible change of slurry viscosity in the presence of kaolin, this indicates that most kaolinite particles are dispersed as the other solids in the ore. During the flotation process, fine kaolinite particles at a low concentration are easily suspended in the water and on the water film surrounding the bubbles, and hence they have more chances to transfer from the slurry phase to froth phase and then to be collected in the concentrate, resulting in a lower copper grade. At a higher kaolin concentration, although it has been found that both basal and edge faces of kaolinite are negatively charged in the solution at pH 9 (Gupta and Miller, 2010), the increasing particle concentration could reduce the mobility of particles and force kaolinite to adapt E–E association because of the strong repulsion between the facial platelets (Du et al., 2010). Further increasing the kaolinite concentration also increases the opportunity for interactions between different complex structures (Gupta et al., 2011). These network structures usually have low density, and move upwards into froth and consequently further reduce the copper grade. 3.3. Effect of clay minerals on froth performance Fig. 2. Cu grade as a function of Cu recovery in the flotation of Telfer clean ore in the absence and presence of clay minerals at different concentrations: (a) bentonite; (b) kaolin.
Fig. 3 at a shear rate of 100 s−1. Generally, the apparent viscosity of pulp increased exponentially with an increase in bentonite concentration, while there was merely a slight increase in the case of kaolin, which was consistent with the previous studies (Cruz et al., 2013; Zhang and Peng, 2015). For bentonite, both the aggregate structure and swelling property contributed to the dramatically elevated apparent viscosity. Bentonite with a 2:1 structure is more reactive than kaolin with a 1:1 structure. The larger interlayer distance of bentonite makes it easier for water molecules to penetrate between platelets, resulting in intercrystalline and osmotic swelling of bentonite, which facilitate strong interaction and formation of aggregate structures (Luckham and Rossi, 1999; Lagaly and Ziesmer, 2003). In flotation process, the high slurry viscosity
Fig. 3. Apparent viscosity as a function of clay mineral concentration in Telfer clean ore at a shear rate of 100 s−1.
During the flotation process, the presence of clay minerals also influenced the froth performance. Froth structure and stability play critical roles in determining mineral flotation recovery and selectivity (Cutting, 1989; Barbian et al., 2005). In Fig. 4(a0) to (a5), the volume of froth decreases when bentonite content in Telfer clean ore increased from 0 to 25 wt.%, which was in consistence with the flotation results in Fig. 2(a) that with an increase in bentonite content, the flotation recovery of the first concentrate decreased sharply. In the case of kaolin, upon increasing its content from 0 to 25 wt.%, the volume of froth seemed unchanged (Fig. 4(b0) to (b5)), which also agreed with the flotation results in Fig. 2 that the final copper recovery was unchanged. Meanwhile, the froth structure became completely different with an increase in clay concentration. Generally, with an increase in clay content, the number of large bubbles declined, resulting in the increasing number of medium bubbles in the case of bentonite. The bubble size decreased sharply in Fig. 4(b5) when the kaolin content increased to 25 wt.%, with only smaller bubbles appearing on the top of froth. These smaller bubbles might relate to the decrease of copper grade in Fig. 2(b), as it has been considered by some researchers that a close relationship exists between bubble size and flotation performance. A froth structure with small, spherical bubbles normally represents a low valuable mineral content in the froth (Moolman et al., 1995; Wang and Peng, 2013). The decrease of bubble size was also related to the increase of gas residence time which is an indicator of froth stability (Quinn et al., 2007). Therefore, froth stability in the flotation of Telfer clean ore in the presence of the two clay samples was also studied. In Fig. 5 the froth stability decreased with an increase in bentonite proportion but increased when kaolin proportion increased. The decrease of froth stability rendered it difficult to transport a considerable amount of mineral particles over the weir of flotation cell to the concentrate launder, which is also responsible for the decrease of copper recovery in the presence of bentonite. On the other hand, both higher froth stability and smaller
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Fig. 5. Froth stability as a function of clay mineral concentration during the flotation process.
bubble size are typical characteristics of entrainment (Subrahmanyam and Forssberg, 1988; Wang and Peng, 2013; Liu and Peng, 2014; Wang et al., 2015). Therefore, the decrease of copper grade with the increase of kaolin proportion is also likely to be due to mechanical entrainment by which kaolinite minerals suspend in water due to their fine size, move upwards, enter the flotation froth and finally leave with the concentrate. 4. Conclusion The presence of bentonite and kaolin exacerbated the flotation of Telfer clean ore. Specifically, the copper recovery decreased with an increase in bentonite content in flotation because of the high slurry viscosity that reduced the frequency of bubble–particle collisions and the mobility of bubble–particle aggregates, resulting in a lower amount of froth. The copper grade was reduced with an increase in kaolin proportion. The addition of kaolin had little effect on slurry viscosity and the mobility of bubbles and particles was not affected. Due to the fine size of kaolin and the low-density aggregate structure, kaolin particles moved upwards to the froth layer reducing copper grade. The increase of kaolin content also caused severe entrainment characterised by smaller bubble size and higher froth stability. These observations are important in the industry, as different actions need to be taken in order to deal with clayey ores, according to the essence of the issue. For high bentonite content ores, the pulp viscosity should be reduced firstly to improve the flotation recovery, while for high kaolin content ores, increasing the concentrate grade is of vital importance in flotation, either by dealing with the fine size of particles or the low density of aggregates. Acknowledgements The authors gratefully acknowledge the financial support of this study from the Australian Research Council and Pionera (LP120200162) as well as the supply of practical ores from Newcrest Telfer operation. The first author also thanks the scholarship provided by the University of Queensland. References
Fig. 4. Froth image of the first concentrate in the flotation of Telfer clean ore in the absence and presence of clay minerals at different concentrations: (a0)–(a5), addition of different bentonite proportions; (b0)–(b5), addition of different kaolin proportions.
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