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Modes of occurrence of Fe in kaolin from Yunnan China
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CERAMICS INTERNATIONAL
Ceramics International 40 (2014) 14579–14587 www.elsevier.com/locate/ceramint
Modes of occurrence of Fe in kaolin from Yunnan China Chunyu Zhoua,b, Yujun Lianga,b, Yansheng Gonga,b, Qi Zhoua,b, Yuting Chena,b, Xiumei Qiua,b, Hongquan Wanga,b, Wenjun Luoa,b, Chunjie Yana,b,n b
a Faculty of Material and Chemistry, China University of Geosciences, Wuhan 430074, China Engineering Research Center of Nano-Geomaterials of Education Ministry, China University of Geosciences, Lu Mo Road 388, Wuhan 430074, China
Received 23 April 2014; received in revised form 30 May 2014; accepted 9 June 2014 Available online 14 June 2014
Abstract Iron as the main contaminating element in kaolin gives an undesirable reddish color to this white clay, therefore analyzing the modes of occurrence of Fe in kaolin has important guidance for their removal. In this paper, the modes of occurrence of Fe were investigated on a very special kaolin from Yunnan China, which contains kaolinite and halloysite at the same time. Attempts were made to study the Fe species by correlating the XRD, microscopic investigations and XPS spectral information of the clay samples. Mineralogical characterization revealed that halloysite or kaolinite as the main mineral phases associated to illite, quartz and gibbsite as ancillary mineral in the Yunnan kaolin samples. Apart from the main mineral phases, goethite, magnetite and hematite are the three major iron impurities in this kaolin. Microscopic investigations (SEM/EDX and TEM/EDS) confirmed the both existence of pseudo-hexagonal kaolinite and tubular halloysite. The relationship between the mineral and iron impurities was also pointed out under microscope, the results showed that the iron impurity materials exist as free iron forms, and they tend to be present alone or adsorbed on the surface of kaolinite instead of halloysite. XPS characterization gives the specific content of the iron impurity, and the relative ratios of goethite, magnetite and hematite are 55.7%, 32.9% and 11.4%, respectively. This work is of great theoretical significance and guidance for removal of Fe from kaolin which has the kaolinite and halloysite minerals at the same time. & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: Kaolin; Iron occurrences; Removal; Mineralogical characterization
1. Introduction Kaolin is an important naturally occurring industrial clay. Due to whiteness, softness, easy dispersion, good plasticity and high adhesion properties, kaolin has been widely used in ceramics, pottery, paint, paper, plastic, cements, pharmaceuticals, cosmetics, and many other industries [1–5]. Since kaolin is a product of the decomposition of feldspars and micas, which presents in pegmatites micaceous schist, it is usually accompanied by other minerals such as quartz, sulfur, feldspars, micas, iron oxides and titanium oxides [6–8]. Among others, iron is the main n Corresponding author at: Engineering Research Center of NanoGeomaterials of Education Ministry, China University of Geosciences, Lu Mo Road 388, Wuhan 430074, China. Tel./fax: þ 86 27 67885098. E-mail address: [email protected] (C. Yan).
http://dx.doi.org/10.1016/j.ceramint.2014.06.042 0272-8842/& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
contaminating element in kaolin and gives an undesirable reddish color to this type of minerals [9,10]. Natural kaolin contains various levels of iron impurities, which greatly affect its quality (e.g., whiteness) [11–14] and thus decrease its commercial value (85–90% is required for filler clay and 90–92% for coating clay) [15–17]. Therefore, extensive research has been carried out on the nature of iron impurities in kaolin, which leads to the conclusion that iron may be present as a part of the kaolinite or ancillary mineral (mica or titania) structure (i.e., “structural iron” ) or as separate iron minerals such as oxides, hydroxides, oxy-hydroxides, sulfides and carbonates (ie., “free iron”) [18–21]. Iron (hydr) oxides (usually Fe3 þ forms) are commonly precipitated or adsorbed to the clay surfaces or admixed as a separate phase [22]. The free iron is present mainly as the following minerals: goethite, hematite, magnetite, pyrite and ilmenite [23].
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It is reported that the valance state of the transition metal ions and atomic position in the structure depends on the formation conditions of the mineral [24,25]. The amount of structural iron varies from kaolin to kaolin, depending on the origin of the deposit and the geologic processes [26]. The location of defects caused by the substitution of iron for aluminum has been widely studied, and electron paramagnetic resonance and Mossbauer spectroscopy have been used to detect them [27–29]. In these minerals, iron can form part of the kaolinite crystalline lattice [26,30] while whiteness is not affected for its low concent. In other secondary accompanying minerals in clays, such as goethite, magnetite and pyrite, the impurities can be removed by magnetic separation [31,32]. The presence of colloidal iron, usually as hydrated oxides, affects the white of clay and kaolin minerals notoriously. And when these minerals are calcined, its removal is more laborious [6]. Kaolinite and halloysite are two members of the kaolin family [33–35]. In natural state, kaolin is consisting principally of the kaolinite, which is seen to be hexagonal, platy crystals under the electron microscope [36]. The study is mainly focused on the Fe species in kaolinite, while the iron occurrence in halloysite is rarely investigated. The transformation from inerratic pseudo-hexagonal sheet morphology kaolinite [37] to hollow nanotubular structure halloysite [38] is known to occur due to the action of either weathering or hydrothermal processes [39]. However natural kaolin contained kaolinite and halloysite at the same time is very rare and the modes of occurrence of Fe in this kind of kaolin have not yet been reported until now. In the present investigation, the kaolin with kaolinite and halloysite existed at the same time, collected from Yunnan province, in south western of China, has been studied on its chemical, mineralogical and morphological properties. Attempts have been made to study the Fe species, namely the modes of occurrence of Fe therein. A comparative investigation is made between kaolinite and halloysite by electron microscopy analysis.
Fig. 1. Locational and geological map of study area.
water and screened through 45 μm American Standard Specification (A.S.S.) sieve. 2.3. Concentration of impurity minerals
2. Materials and methods 2.1. Geological setting The Yunnan kaolin sample has been collected from the village Bahu, Lincang district of Yunnan province in the south western part of China. It extends between longitude 1001030 and 1001040 E and latitude 231430 and 231450 N, which belongs to the granite weathered residual deposit (Fig. 1). The mine area is around 1320–1460 m in length, east–west width of 350–450 m, with an average thickness of 18.07 m, the thickness is up to 47 m, and the thinnest thickness is 3.5 m. 2.2. Beneficiation of clays Nearly 10 t kaolin samples were collected from the Bahu, Lincang mineral district, thoroughly blended, blunged with
The impurity minerals were concentrated by high gradient magnetic separation. Table 1 presents the results of the high gradient magnetic separation test. According to the firing whiteness, number 4 was chosen as the best processing conditions, for which the concentrate product has the highest whiteness (75.4%) and the tailing product has the lowest whiteness (34.5%). The best conditions for concentration of impurity minerals is as the following: feed concentration of 15%, slurry flow rate at 0.8 cm/s, and magnetic field intensity at 1.4 T. The chemical compositions of the raw kaolin sample, its concentrate and tailing products obtained at the best conditions are given in Table 2. As shown in Table 2, the contents of iron and titanium impurities in the raw kaolin sample are 1.60 wt% and 0.40 wt%, respectively. Obviously, iron is the main impurity in kaolin, which makes the kaolin sample brick red in color. The iron content in concentrate kaolin sample
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Table 1 High gradient magnetic separation test. Number
Feed concentration (%)
Slurry flow rate (cm/s)
Magnetic field intensity (T)
Product
Yield
Firing whiteness (%)
1
15
1.0
1.0
Concentrate Tailing Concentrate Tailing Concentrate Tailing Concentrate Tailing Concentrate Tailing Concentrate Tailing Concentrate Tailing
68.49 31.51 61.96 38.04 58.55 41.45 56.30 43.70 68.53 31.47 63.42 36.58 62.94 37.06
73.2 35.4 74.9 35.2 75.2 35.8 75.4 34.5 74.4 34.8 74.0 35.1 74.2 35.5
2
1.4
3
1.8
4
0.8
5
1.2
6
10
7
20
1.4
1.0
Table 2 Chemical compositions of kaolin samples (wt%). Sample
Al2O3
SiO2
TFe2O3
TiO2
K2 O
Na2O
CaO
MgO
P2O5
L.O.Ia
Raw Concentrate Tailing
36.51 37.09 34.89
46.42 46.06 44.12
1.60 1.13 4.16
0.40 0.24 1.35
0.95 0.91 1.11
0.31 0.32 0.22
0.05 0.05 0.09
0.34 0.31 0.49
0.03 0.13 0.29
13.34 13.74 13.20
a
Loss on ignition.
decreases from 1.60 wt% to 1.13 wt%, which indicates that the iron impurities can be removed in some extent by high gradient magnetic separator. The kaolin tailing sample which contents 4.16 wt% iron is subjected to detailed characterization to understand their properties and to explore the possible iron occurrences. 2.4. Characterization Whiteness measurements were taken with a commercial whiteness meter (WSB-3, Shanghai, China) for the pressed pellet samples. The bulk chemical composition in samples melted with lithium tetraborate was performed with an XRF PANalytical B.V. AXIOSmAX spectrometer. X-ray diffraction (XRD) measurements were carried out on a Germany Bruker D8-FOCUS powder X-ray diffractometer, using CuKα radiation (λ ¼ 0.154 nm) (40 kV, 30 mA) over the scanning range 2θ ¼ 5–701 with a step width of 21/min. The morphology of the sample surfaces and the energy dispersive X-ray spectroscopy (SEM/EDX) analyses were examined on a Hitachi SU8010 field emission scanning electron microscope, operated at 15 kV, in powder samples coated with Au. Transmission electron microscope (TEM) images were collected on a Philips CM-12 electron microscope with a field emission gun as the source of electrons operated at an acceleration voltage of 200 kV. Samples were prepared by
dispersing the particles in ethanol and placed on a Formvarcoated copper grid prior to loading in the electron beam. X-ray photoelectron spectroscopy (XPS) analysis was carried out by a VG Multilab 2000 spectrometer (Thermo Electron Corporation) with a monochromatic Mg Kα X-ray source (15 kV, 150 W). All XPS spectra were calibrated with the C1s peak at 284.6 eV. Survey spectra were collected with pass energy of 25 eV for Fe and 100 eV for all the elements.
3. Results and discussion 3.1. XRD analysis The diffraction patterns as shown in Fig. 2 reveal halloysite or kaolinite as the main mineral phase associated to illite, quartz and gibbsite as ancillary mineral in all the kaolin samples. The diffraction peak (001) at 12.11 in 2θ is corresponding to a basal spacing of 7.20 Å, the data is higher than that of kaolinite (7.1–7.2 Å) and lower than that of halloysite (7.3–7.5 Å), indicating that there may be the mixture of halloysite and kaolinite in this kaolin sample. Apart from the main mineral phases, there are three kinds of iron impurity mineral phases in the diffraction patterns, which are goethite (FeOOH) (PDF card no. 02-0273), magnetite (Fe3O4) (PDF card no. 02-1035) and hematite (Fe2O3) (PDF card no. 24-0072), respectively. They are all free iron according to the literature reported [23].
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Compared with raw and concentrate kaolin samples, the main mineral phase peaks of tailing kaolin sample is lower. What is more, since the content of TiO2 in tailing kaolin
sample increases to 1.35 wt% from 0.40 wt% in raw kaolin sample, the peaks of anatase appeared in tailing kaolin sample. 3.2. SEM/EDX analysis
Fig. 2. XRD patterns of Yunnan kaolin samples (a) raw, (b) concentrate, and (c) tailing.
SEM image of kaolinite in Yunnan tailing kaolin sample and the corresponding EDX spectra of three points are shown in Fig. 3. The SEM image at point A shows that the kaolinite in the sample has pseudo-hexagonal plates and stacks like book palisades, which directly prove that Yunnan kaolin is of high structural order [37]. The EDX spectrum of point A shows that there are only Si, Al, O three elements and the contents are 42.70%, 36.09%, 21.22%, respectively. The contents are similar to the theoretical chemical composition (SiO2, 46.54%; Al2O3, 39.50%; H2O, 13.96% [40,41]), indicating that the selected point A is pure kaolinite. As shown in SEM image in Fig. 3, some small lighter particles attach to the surface of kaolinite flakes intuitively. In order to analyze iron impurity material, two small lighter particles B and C are detected by EDX spectra analysis. The corresponding EDX spectra show that besides Si, Al, O elements, there is a certain amount of iron in the small lighter
Fig. 3. SEM image of kaolinite in Yunnan tailing kaolin sample and corresponding EDX spectra.
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Fig. 4. SEM image of halloysite in Yunnan tailing kaolin sample and corresponding EDX spectrum.
Fig. 5. SEM image of iron impurity mineral in Yunnan tailing kaolin sample and corresponding EDX spectrum.
particles B and C. The results demonstrate that the small particles of B and C contain iron impurities minerals and the iron content is 47.88% and 17.29%, respectively. The above data indicates that although both B and C are ferrous material, they are two different substances. The SEM image and the corresponding EDX spectrum of halloysite in Yunnan kaolin sample is shown in Fig. 4. Compared with kaolinite, apart from Si, Al, O elements, the K element also appears. Unlike kaolinite, no iron impurity material absorbs on the surface of halloysite. The above described results indicate that iron impurity material is more easily absorbed on the surface of kaolinite rather than halloysite. What is more, SEM/EDX investigations confirm the both existence of pseudo-hexagonal kaolinite and tubular halloysite. Besides the small iron particles which adsorbs on the surface of kaolinite, Fig. 5 also shows other iron impurity material particles independently. EDX analysis is done on the iron impurity material, the result shows that there is only a little of
Si and Al elements, the atomic ratio of Fe and O is close to 1:1. From the above results, it can be speculated that the iron impurity material may be goethite or a divalent iron compound during the transition process. TEM and XPS analyses are required to identify it. 3.3. TEM/EDS analysis Seen from the TEM image (shown in Fig. 6a), some black flocculent substance adsorbs on the surface of kaolin. In order to know the composition of the black material, SAED pattern and EDS spectrum are done on point 1#. Fe element appears in the EDS spectrum (Fig. 6c), indicating that the black flocculent substance is an iron impurity material. The SAED index analysis results of point 1# (listed in Table 3) shows that the data of indicator values agree fairly well with the standard values of goethite (FeO(OH)), indicating the presence of goethite phase. In the standard values and measured XRD spectrum, the diffraction intensity (I/I0 ¼ 20) of d ¼ 0.252 nm
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Fig. 6. TEM image of kaolinite in Yunnan tailing kaolin sample (a); SAED pattern of point 1# (b); EDS spectrum of point 1# (c); and EDS spectrum of point 2# (d).
Table 3 Selected area electron diffraction analysis data of point 1#. Indicator values
ED (d/nm)
0.418
0.252
0.245
0.171
Standard values
Goethite (d/nm) I/I0 Magnetite (d/nm) I/I0 Hematite (d/nm) I/I0
0.418 100
0.252 20 0.254 100 0.251 70
0.245 70 0.243 30
0.172 50 0.171 50 0.169 36
which corresponds to the crystal face of (121) is lower than that of d¼ 0.245 nm (I/I0 ¼ 70) which corresponds to the crystal face of (111). However, the SAED pattern of point 1#
as shown in Fig. 6b demonstrates that the intensity of diffraction ring of d¼ 0.252 nm is higher than the intensity of the diffraction ring d¼ 0.245 nm. Combined with XRD
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Fig. 7. TEM image of halloysite in Yunnan tailing kaolin sample (a); SAED pattern of point 3# (b); EDS spectrum of point 3# (c); and EDS spectrum of point 4# (d).
analysis, it can be drawn that the intensity of diffraction ring d ¼ 0.252 nm also corresponds to that of magnetite crystal face (311) and hematite crystal face (110). In addition, the intensity of diffraction ring d ¼ 0.245 nm diffraction ring comprises the third diffraction intensity of magnetite crystal plane (222). At the same time, the second diffraction intensity of magnetite crystal plane (422) and the third diffraction intensity of hematite crystal plane (116) both correspond to the diffraction intensity d¼ 0.171 nm. A conclusion can be drawn as the following: apart from goethite, magnetite and hematite also exist in this region.
For comparison, EDS spectrum analysis is also done on a lighter spot marked as point 2# (Fig. 6d), only Si and Al elements appear in the spectrum (Cu is from copper grid), indicating the existence of pure kaolinite. The above data prove that the iron impurity materials just absorb on the surface of kaolin. Besides kaolinite, halloysite with a mass of black needle material are also observed in TEM image shown in Fig. 7a. SAED pattern and EDS spectrum investigations are done on point 3# of the black needle material. The result is the same as point 1#, indicating the same kind of iron impurity material. By
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contrast, EDS spectrum is also done on the tube of halloysite at point 4#. Only Si and Al elements appear in the spectrum of Fig. 7d. The above results prove that the iron impurity material just exists as an independent material rather than in the structure or inside of halloysite. According to all TEM analysis, the following conclusion could be drawn: goethite, magnetite and hematite are the main iron minerals and the particle sizes are in hundreds of nanoscale. A part of them adsorb on the surface of kaolinite, some stand alone. The TEM results are consistent with SEM analysis.
3.4. XPS analysis XPS peak position and shape can provide information about the actual composition, chemical state of surfaces and molecular structure, furthermore the concentration or content of the elements could also be got from the XPS peak intensity [42]. To confirm the presence of iron in the tailing kaolin sample, XPS analysis is also performed. The black line in Fig. 8 shows the original XPS spectrum of Fe 2p. In order to analyze the data reasonably, peak mathematical curve fitting is done by Xps peak software using a least-squares fit of Gaussian–Lorentzian line shape. According to the literature reported, the original XPS spectrum of Fe 2p has been fitted to three fitting peaks separately in bands of Fe 2p3/2 and Fe 2p1/2. Contrasted with the binding energy listed in Table 4, the iron minerals in the surface of Yunnan kaolin are identified as goethite (FeOOH), magnetite (Fe3O4) and hematite (Fe2O3). Based on the peak areas in Xps peak software, the relative ratios of goethite, magnetite and hematite are calculated to be 55.7%, 32.9% and 11.4%, respectively. The results indicate that iron impurity materials in the Yunnan kaolin sample exist mainly in three forms, they are goethite, magnetite and hematite. Furthermore the most abundant is goethite, followed by hematite, and magnetite content is minimal. These results are consistent with the previous results.
Fig. 8. XPS spectrum of Fe 2p from the surface of Yunnan tailing kaolin sample.
Table 4 Binding energy of the corresponding material in XPS spectrum. Binding energy
Goethite (FeOOH) Magnetite (Fe3O4) Hematite (Fe2O3)
Fe (2p3/2)
Fe (2p1/2)
711.8 710.2 711.2
724.3 723.5 724.0
4. Conclusions XRD analysis revealed halloysite or kaolinite as the main mineral phases associated to illite and quartz as ancillary mineral in Yunnan kaolin sample. Apart from the main mineral phases, there are three iron impurity minerals in this kaolin. A comparative investigation was made between kaolinite and halloysite by SEM/EDX and TEM/EDS, the relationship between the mineral and iron impurities was pointed out, the results showed that the iron impurity materials exist as free iron forms, and they tend to be present alone or adsorbed on the surface of kaolinite instead of halloysite. The specific content of the iron impurity goethite, magnetite and hematite are calculated to be 55.7%, 32.9% and 11.4%, respectively by XPS analysis. This work has great theoretical guiding significance in iron removal aspect for kaolin which has the same component as described in this paper. Acknowledgments The financial supports from the Ministry of Land and Resources of the People's Republic of China and Public Service Project of the Chinese Ministry of Land and Resources (201311024) are greatly acknowledged. References [1] H.W. Ryu, K.S. Cho, Y.K. Chang, S.D. Kim, T. Mori, Refinement of low-grade clay by microbial removal of sulfur and iron compounds using Thiobacillus ferrooxidans, J. Ferment. Bioeng. 80 (1995) 46–52. [2] S. Chandrasekhar, S. Ramaswamy, Influence of mineral impurities on the properties of kaolin and its thermally treated products, Appl. Clay Sci. 21 (2002) 133–142. [3] S.K. Mandal, P.C. Banerjee, Iron leaching from China clay with oxalic acid: effect of different physico-chemical parameters, Int. J. Miner. Process. 74 (1–4) (2004) 263–270. [4] B. Kar, H. Sahoo, S.S. Rath, D.S. Rao, B. Das, Characterization and beneficiation studies for the removal of iron from a China clay from India, Clay Miner. 48 (5) (2013) 759–769. [5] J. Matusik, A. Gaweł, E. Bielańska, W. Osuch, K. Bahranowski, The effect of structural order on nanotubes derived from kaolin-group minerals, Clays Clay Miner. 57 (4) (2009) 452–464. [6] J. Gonzalez, M. Delcruiz, Bleaching of kaolins and clays by chlorination of iron and titanium, Appl. Clay Sci. 33 (3–4) (2006) 219–229. [7] E. Aghaie, M. Pazouki, M.R. Hosseini, M. Ranjbar, Kinetic modeling of the bioleaching process of iron removal from kaolin, Appl. Clay Sci. (65– 66) (2012) 43–47. [8] M.C. Cheshire, D.L. Bish, S.C. Brassell, Organic geochemical composition of the Georgia kaolins: insight s in to formation and diagenetic conditions, Clays Clay Miner. 60 (2012) 420–439.
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CERAMICS INTERNATIONAL
Ceramics International 40 (2014) 14579–14587 www.elsevier.com/locate/ceramint
Modes of occurrence of Fe in kaolin from Yunnan China Chunyu Zhoua,b, Yujun Lianga,b, Yansheng Gonga,b, Qi Zhoua,b, Yuting Chena,b, Xiumei Qiua,b, Hongquan Wanga,b, Wenjun Luoa,b, Chunjie Yana,b,n b
a Faculty of Material and Chemistry, China University of Geosciences, Wuhan 430074, China Engineering Research Center of Nano-Geomaterials of Education Ministry, China University of Geosciences, Lu Mo Road 388, Wuhan 430074, China
Received 23 April 2014; received in revised form 30 May 2014; accepted 9 June 2014 Available online 14 June 2014
Abstract Iron as the main contaminating element in kaolin gives an undesirable reddish color to this white clay, therefore analyzing the modes of occurrence of Fe in kaolin has important guidance for their removal. In this paper, the modes of occurrence of Fe were investigated on a very special kaolin from Yunnan China, which contains kaolinite and halloysite at the same time. Attempts were made to study the Fe species by correlating the XRD, microscopic investigations and XPS spectral information of the clay samples. Mineralogical characterization revealed that halloysite or kaolinite as the main mineral phases associated to illite, quartz and gibbsite as ancillary mineral in the Yunnan kaolin samples. Apart from the main mineral phases, goethite, magnetite and hematite are the three major iron impurities in this kaolin. Microscopic investigations (SEM/EDX and TEM/EDS) confirmed the both existence of pseudo-hexagonal kaolinite and tubular halloysite. The relationship between the mineral and iron impurities was also pointed out under microscope, the results showed that the iron impurity materials exist as free iron forms, and they tend to be present alone or adsorbed on the surface of kaolinite instead of halloysite. XPS characterization gives the specific content of the iron impurity, and the relative ratios of goethite, magnetite and hematite are 55.7%, 32.9% and 11.4%, respectively. This work is of great theoretical significance and guidance for removal of Fe from kaolin which has the kaolinite and halloysite minerals at the same time. & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: Kaolin; Iron occurrences; Removal; Mineralogical characterization
1. Introduction Kaolin is an important naturally occurring industrial clay. Due to whiteness, softness, easy dispersion, good plasticity and high adhesion properties, kaolin has been widely used in ceramics, pottery, paint, paper, plastic, cements, pharmaceuticals, cosmetics, and many other industries [1–5]. Since kaolin is a product of the decomposition of feldspars and micas, which presents in pegmatites micaceous schist, it is usually accompanied by other minerals such as quartz, sulfur, feldspars, micas, iron oxides and titanium oxides [6–8]. Among others, iron is the main n Corresponding author at: Engineering Research Center of NanoGeomaterials of Education Ministry, China University of Geosciences, Lu Mo Road 388, Wuhan 430074, China. Tel./fax: þ 86 27 67885098. E-mail address: [email protected] (C. Yan).
http://dx.doi.org/10.1016/j.ceramint.2014.06.042 0272-8842/& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
contaminating element in kaolin and gives an undesirable reddish color to this type of minerals [9,10]. Natural kaolin contains various levels of iron impurities, which greatly affect its quality (e.g., whiteness) [11–14] and thus decrease its commercial value (85–90% is required for filler clay and 90–92% for coating clay) [15–17]. Therefore, extensive research has been carried out on the nature of iron impurities in kaolin, which leads to the conclusion that iron may be present as a part of the kaolinite or ancillary mineral (mica or titania) structure (i.e., “structural iron” ) or as separate iron minerals such as oxides, hydroxides, oxy-hydroxides, sulfides and carbonates (ie., “free iron”) [18–21]. Iron (hydr) oxides (usually Fe3 þ forms) are commonly precipitated or adsorbed to the clay surfaces or admixed as a separate phase [22]. The free iron is present mainly as the following minerals: goethite, hematite, magnetite, pyrite and ilmenite [23].
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It is reported that the valance state of the transition metal ions and atomic position in the structure depends on the formation conditions of the mineral [24,25]. The amount of structural iron varies from kaolin to kaolin, depending on the origin of the deposit and the geologic processes [26]. The location of defects caused by the substitution of iron for aluminum has been widely studied, and electron paramagnetic resonance and Mossbauer spectroscopy have been used to detect them [27–29]. In these minerals, iron can form part of the kaolinite crystalline lattice [26,30] while whiteness is not affected for its low concent. In other secondary accompanying minerals in clays, such as goethite, magnetite and pyrite, the impurities can be removed by magnetic separation [31,32]. The presence of colloidal iron, usually as hydrated oxides, affects the white of clay and kaolin minerals notoriously. And when these minerals are calcined, its removal is more laborious [6]. Kaolinite and halloysite are two members of the kaolin family [33–35]. In natural state, kaolin is consisting principally of the kaolinite, which is seen to be hexagonal, platy crystals under the electron microscope [36]. The study is mainly focused on the Fe species in kaolinite, while the iron occurrence in halloysite is rarely investigated. The transformation from inerratic pseudo-hexagonal sheet morphology kaolinite [37] to hollow nanotubular structure halloysite [38] is known to occur due to the action of either weathering or hydrothermal processes [39]. However natural kaolin contained kaolinite and halloysite at the same time is very rare and the modes of occurrence of Fe in this kind of kaolin have not yet been reported until now. In the present investigation, the kaolin with kaolinite and halloysite existed at the same time, collected from Yunnan province, in south western of China, has been studied on its chemical, mineralogical and morphological properties. Attempts have been made to study the Fe species, namely the modes of occurrence of Fe therein. A comparative investigation is made between kaolinite and halloysite by electron microscopy analysis.
Fig. 1. Locational and geological map of study area.
water and screened through 45 μm American Standard Specification (A.S.S.) sieve. 2.3. Concentration of impurity minerals
2. Materials and methods 2.1. Geological setting The Yunnan kaolin sample has been collected from the village Bahu, Lincang district of Yunnan province in the south western part of China. It extends between longitude 1001030 and 1001040 E and latitude 231430 and 231450 N, which belongs to the granite weathered residual deposit (Fig. 1). The mine area is around 1320–1460 m in length, east–west width of 350–450 m, with an average thickness of 18.07 m, the thickness is up to 47 m, and the thinnest thickness is 3.5 m. 2.2. Beneficiation of clays Nearly 10 t kaolin samples were collected from the Bahu, Lincang mineral district, thoroughly blended, blunged with
The impurity minerals were concentrated by high gradient magnetic separation. Table 1 presents the results of the high gradient magnetic separation test. According to the firing whiteness, number 4 was chosen as the best processing conditions, for which the concentrate product has the highest whiteness (75.4%) and the tailing product has the lowest whiteness (34.5%). The best conditions for concentration of impurity minerals is as the following: feed concentration of 15%, slurry flow rate at 0.8 cm/s, and magnetic field intensity at 1.4 T. The chemical compositions of the raw kaolin sample, its concentrate and tailing products obtained at the best conditions are given in Table 2. As shown in Table 2, the contents of iron and titanium impurities in the raw kaolin sample are 1.60 wt% and 0.40 wt%, respectively. Obviously, iron is the main impurity in kaolin, which makes the kaolin sample brick red in color. The iron content in concentrate kaolin sample
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Table 1 High gradient magnetic separation test. Number
Feed concentration (%)
Slurry flow rate (cm/s)
Magnetic field intensity (T)
Product
Yield
Firing whiteness (%)
1
15
1.0
1.0
Concentrate Tailing Concentrate Tailing Concentrate Tailing Concentrate Tailing Concentrate Tailing Concentrate Tailing Concentrate Tailing
68.49 31.51 61.96 38.04 58.55 41.45 56.30 43.70 68.53 31.47 63.42 36.58 62.94 37.06
73.2 35.4 74.9 35.2 75.2 35.8 75.4 34.5 74.4 34.8 74.0 35.1 74.2 35.5
2
1.4
3
1.8
4
0.8
5
1.2
6
10
7
20
1.4
1.0
Table 2 Chemical compositions of kaolin samples (wt%). Sample
Al2O3
SiO2
TFe2O3
TiO2
K2 O
Na2O
CaO
MgO
P2O5
L.O.Ia
Raw Concentrate Tailing
36.51 37.09 34.89
46.42 46.06 44.12
1.60 1.13 4.16
0.40 0.24 1.35
0.95 0.91 1.11
0.31 0.32 0.22
0.05 0.05 0.09
0.34 0.31 0.49
0.03 0.13 0.29
13.34 13.74 13.20
a
Loss on ignition.
decreases from 1.60 wt% to 1.13 wt%, which indicates that the iron impurities can be removed in some extent by high gradient magnetic separator. The kaolin tailing sample which contents 4.16 wt% iron is subjected to detailed characterization to understand their properties and to explore the possible iron occurrences. 2.4. Characterization Whiteness measurements were taken with a commercial whiteness meter (WSB-3, Shanghai, China) for the pressed pellet samples. The bulk chemical composition in samples melted with lithium tetraborate was performed with an XRF PANalytical B.V. AXIOSmAX spectrometer. X-ray diffraction (XRD) measurements were carried out on a Germany Bruker D8-FOCUS powder X-ray diffractometer, using CuKα radiation (λ ¼ 0.154 nm) (40 kV, 30 mA) over the scanning range 2θ ¼ 5–701 with a step width of 21/min. The morphology of the sample surfaces and the energy dispersive X-ray spectroscopy (SEM/EDX) analyses were examined on a Hitachi SU8010 field emission scanning electron microscope, operated at 15 kV, in powder samples coated with Au. Transmission electron microscope (TEM) images were collected on a Philips CM-12 electron microscope with a field emission gun as the source of electrons operated at an acceleration voltage of 200 kV. Samples were prepared by
dispersing the particles in ethanol and placed on a Formvarcoated copper grid prior to loading in the electron beam. X-ray photoelectron spectroscopy (XPS) analysis was carried out by a VG Multilab 2000 spectrometer (Thermo Electron Corporation) with a monochromatic Mg Kα X-ray source (15 kV, 150 W). All XPS spectra were calibrated with the C1s peak at 284.6 eV. Survey spectra were collected with pass energy of 25 eV for Fe and 100 eV for all the elements.
3. Results and discussion 3.1. XRD analysis The diffraction patterns as shown in Fig. 2 reveal halloysite or kaolinite as the main mineral phase associated to illite, quartz and gibbsite as ancillary mineral in all the kaolin samples. The diffraction peak (001) at 12.11 in 2θ is corresponding to a basal spacing of 7.20 Å, the data is higher than that of kaolinite (7.1–7.2 Å) and lower than that of halloysite (7.3–7.5 Å), indicating that there may be the mixture of halloysite and kaolinite in this kaolin sample. Apart from the main mineral phases, there are three kinds of iron impurity mineral phases in the diffraction patterns, which are goethite (FeOOH) (PDF card no. 02-0273), magnetite (Fe3O4) (PDF card no. 02-1035) and hematite (Fe2O3) (PDF card no. 24-0072), respectively. They are all free iron according to the literature reported [23].
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Compared with raw and concentrate kaolin samples, the main mineral phase peaks of tailing kaolin sample is lower. What is more, since the content of TiO2 in tailing kaolin
sample increases to 1.35 wt% from 0.40 wt% in raw kaolin sample, the peaks of anatase appeared in tailing kaolin sample. 3.2. SEM/EDX analysis
Fig. 2. XRD patterns of Yunnan kaolin samples (a) raw, (b) concentrate, and (c) tailing.
SEM image of kaolinite in Yunnan tailing kaolin sample and the corresponding EDX spectra of three points are shown in Fig. 3. The SEM image at point A shows that the kaolinite in the sample has pseudo-hexagonal plates and stacks like book palisades, which directly prove that Yunnan kaolin is of high structural order [37]. The EDX spectrum of point A shows that there are only Si, Al, O three elements and the contents are 42.70%, 36.09%, 21.22%, respectively. The contents are similar to the theoretical chemical composition (SiO2, 46.54%; Al2O3, 39.50%; H2O, 13.96% [40,41]), indicating that the selected point A is pure kaolinite. As shown in SEM image in Fig. 3, some small lighter particles attach to the surface of kaolinite flakes intuitively. In order to analyze iron impurity material, two small lighter particles B and C are detected by EDX spectra analysis. The corresponding EDX spectra show that besides Si, Al, O elements, there is a certain amount of iron in the small lighter
Fig. 3. SEM image of kaolinite in Yunnan tailing kaolin sample and corresponding EDX spectra.
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Fig. 4. SEM image of halloysite in Yunnan tailing kaolin sample and corresponding EDX spectrum.
Fig. 5. SEM image of iron impurity mineral in Yunnan tailing kaolin sample and corresponding EDX spectrum.
particles B and C. The results demonstrate that the small particles of B and C contain iron impurities minerals and the iron content is 47.88% and 17.29%, respectively. The above data indicates that although both B and C are ferrous material, they are two different substances. The SEM image and the corresponding EDX spectrum of halloysite in Yunnan kaolin sample is shown in Fig. 4. Compared with kaolinite, apart from Si, Al, O elements, the K element also appears. Unlike kaolinite, no iron impurity material absorbs on the surface of halloysite. The above described results indicate that iron impurity material is more easily absorbed on the surface of kaolinite rather than halloysite. What is more, SEM/EDX investigations confirm the both existence of pseudo-hexagonal kaolinite and tubular halloysite. Besides the small iron particles which adsorbs on the surface of kaolinite, Fig. 5 also shows other iron impurity material particles independently. EDX analysis is done on the iron impurity material, the result shows that there is only a little of
Si and Al elements, the atomic ratio of Fe and O is close to 1:1. From the above results, it can be speculated that the iron impurity material may be goethite or a divalent iron compound during the transition process. TEM and XPS analyses are required to identify it. 3.3. TEM/EDS analysis Seen from the TEM image (shown in Fig. 6a), some black flocculent substance adsorbs on the surface of kaolin. In order to know the composition of the black material, SAED pattern and EDS spectrum are done on point 1#. Fe element appears in the EDS spectrum (Fig. 6c), indicating that the black flocculent substance is an iron impurity material. The SAED index analysis results of point 1# (listed in Table 3) shows that the data of indicator values agree fairly well with the standard values of goethite (FeO(OH)), indicating the presence of goethite phase. In the standard values and measured XRD spectrum, the diffraction intensity (I/I0 ¼ 20) of d ¼ 0.252 nm
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Fig. 6. TEM image of kaolinite in Yunnan tailing kaolin sample (a); SAED pattern of point 1# (b); EDS spectrum of point 1# (c); and EDS spectrum of point 2# (d).
Table 3 Selected area electron diffraction analysis data of point 1#. Indicator values
ED (d/nm)
0.418
0.252
0.245
0.171
Standard values
Goethite (d/nm) I/I0 Magnetite (d/nm) I/I0 Hematite (d/nm) I/I0
0.418 100
0.252 20 0.254 100 0.251 70
0.245 70 0.243 30
0.172 50 0.171 50 0.169 36
which corresponds to the crystal face of (121) is lower than that of d¼ 0.245 nm (I/I0 ¼ 70) which corresponds to the crystal face of (111). However, the SAED pattern of point 1#
as shown in Fig. 6b demonstrates that the intensity of diffraction ring of d¼ 0.252 nm is higher than the intensity of the diffraction ring d¼ 0.245 nm. Combined with XRD
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Fig. 7. TEM image of halloysite in Yunnan tailing kaolin sample (a); SAED pattern of point 3# (b); EDS spectrum of point 3# (c); and EDS spectrum of point 4# (d).
analysis, it can be drawn that the intensity of diffraction ring d ¼ 0.252 nm also corresponds to that of magnetite crystal face (311) and hematite crystal face (110). In addition, the intensity of diffraction ring d ¼ 0.245 nm diffraction ring comprises the third diffraction intensity of magnetite crystal plane (222). At the same time, the second diffraction intensity of magnetite crystal plane (422) and the third diffraction intensity of hematite crystal plane (116) both correspond to the diffraction intensity d¼ 0.171 nm. A conclusion can be drawn as the following: apart from goethite, magnetite and hematite also exist in this region.
For comparison, EDS spectrum analysis is also done on a lighter spot marked as point 2# (Fig. 6d), only Si and Al elements appear in the spectrum (Cu is from copper grid), indicating the existence of pure kaolinite. The above data prove that the iron impurity materials just absorb on the surface of kaolin. Besides kaolinite, halloysite with a mass of black needle material are also observed in TEM image shown in Fig. 7a. SAED pattern and EDS spectrum investigations are done on point 3# of the black needle material. The result is the same as point 1#, indicating the same kind of iron impurity material. By
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contrast, EDS spectrum is also done on the tube of halloysite at point 4#. Only Si and Al elements appear in the spectrum of Fig. 7d. The above results prove that the iron impurity material just exists as an independent material rather than in the structure or inside of halloysite. According to all TEM analysis, the following conclusion could be drawn: goethite, magnetite and hematite are the main iron minerals and the particle sizes are in hundreds of nanoscale. A part of them adsorb on the surface of kaolinite, some stand alone. The TEM results are consistent with SEM analysis.
3.4. XPS analysis XPS peak position and shape can provide information about the actual composition, chemical state of surfaces and molecular structure, furthermore the concentration or content of the elements could also be got from the XPS peak intensity [42]. To confirm the presence of iron in the tailing kaolin sample, XPS analysis is also performed. The black line in Fig. 8 shows the original XPS spectrum of Fe 2p. In order to analyze the data reasonably, peak mathematical curve fitting is done by Xps peak software using a least-squares fit of Gaussian–Lorentzian line shape. According to the literature reported, the original XPS spectrum of Fe 2p has been fitted to three fitting peaks separately in bands of Fe 2p3/2 and Fe 2p1/2. Contrasted with the binding energy listed in Table 4, the iron minerals in the surface of Yunnan kaolin are identified as goethite (FeOOH), magnetite (Fe3O4) and hematite (Fe2O3). Based on the peak areas in Xps peak software, the relative ratios of goethite, magnetite and hematite are calculated to be 55.7%, 32.9% and 11.4%, respectively. The results indicate that iron impurity materials in the Yunnan kaolin sample exist mainly in three forms, they are goethite, magnetite and hematite. Furthermore the most abundant is goethite, followed by hematite, and magnetite content is minimal. These results are consistent with the previous results.
Fig. 8. XPS spectrum of Fe 2p from the surface of Yunnan tailing kaolin sample.
Table 4 Binding energy of the corresponding material in XPS spectrum. Binding energy
Goethite (FeOOH) Magnetite (Fe3O4) Hematite (Fe2O3)
Fe (2p3/2)
Fe (2p1/2)
711.8 710.2 711.2
724.3 723.5 724.0
4. Conclusions XRD analysis revealed halloysite or kaolinite as the main mineral phases associated to illite and quartz as ancillary mineral in Yunnan kaolin sample. Apart from the main mineral phases, there are three iron impurity minerals in this kaolin. A comparative investigation was made between kaolinite and halloysite by SEM/EDX and TEM/EDS, the relationship between the mineral and iron impurities was pointed out, the results showed that the iron impurity materials exist as free iron forms, and they tend to be present alone or adsorbed on the surface of kaolinite instead of halloysite. The specific content of the iron impurity goethite, magnetite and hematite are calculated to be 55.7%, 32.9% and 11.4%, respectively by XPS analysis. This work has great theoretical guiding significance in iron removal aspect for kaolin which has the same component as described in this paper. Acknowledgments The financial supports from the Ministry of Land and Resources of the People's Republic of China and Public Service Project of the Chinese Ministry of Land and Resources (201311024) are greatly acknowledged. References [1] H.W. Ryu, K.S. Cho, Y.K. Chang, S.D. Kim, T. Mori, Refinement of low-grade clay by microbial removal of sulfur and iron compounds using Thiobacillus ferrooxidans, J. Ferment. Bioeng. 80 (1995) 46–52. [2] S. Chandrasekhar, S. Ramaswamy, Influence of mineral impurities on the properties of kaolin and its thermally treated products, Appl. Clay Sci. 21 (2002) 133–142. [3] S.K. Mandal, P.C. Banerjee, Iron leaching from China clay with oxalic acid: effect of different physico-chemical parameters, Int. J. Miner. Process. 74 (1–4) (2004) 263–270. [4] B. Kar, H. Sahoo, S.S. Rath, D.S. Rao, B. Das, Characterization and beneficiation studies for the removal of iron from a China clay from India, Clay Miner. 48 (5) (2013) 759–769. [5] J. Matusik, A. Gaweł, E. Bielańska, W. Osuch, K. Bahranowski, The effect of structural order on nanotubes derived from kaolin-group minerals, Clays Clay Miner. 57 (4) (2009) 452–464. [6] J. Gonzalez, M. Delcruiz, Bleaching of kaolins and clays by chlorination of iron and titanium, Appl. Clay Sci. 33 (3–4) (2006) 219–229. [7] E. Aghaie, M. Pazouki, M.R. Hosseini, M. Ranjbar, Kinetic modeling of the bioleaching process of iron removal from kaolin, Appl. Clay Sci. (65– 66) (2012) 43–47. [8] M.C. Cheshire, D.L. Bish, S.C. Brassell, Organic geochemical composition of the Georgia kaolins: insight s in to formation and diagenetic conditions, Clays Clay Miner. 60 (2012) 420–439.
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