Physiochemical Characterization of Ethiopian Mined Kaolin Clay through Beneficiation Process

Ayalew, Adane Adugna

doi:https://doi.org/10.1155/2023/9104807

Advances in Materials Science and Engineering

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Abstract Introduction Results and Discussion Conclusion Data Availability Conflicts of Interest Acknowledgments References Copyright Related Articles

Research Article | Open Access

Volume 2023 | Article ID 9104807 | https://doi.org/10.1155/2023/9104807

Physiochemical Characterization of Ethiopian Mined Kaolin Clay through Beneficiation Process

Adane Adugna Ayalew¹

Academic Editor: Ismail Demir

Received03 Aug 2022

Revised02 Feb 2023

Accepted01 Jul 2023

Published06 Jul 2023

Abstract

Kaolin mineral is a commercially solid powder with a comparatively low level of purity and is regularly used for a variety of applications, including filler, paints, ceramics, adsorbents, and paper. In Ethiopia, the kaolin clay mineral is significant for financial growth as the raw material used in the industry sector. However, slight consideration was given to the chemical, physical, mineralogical, and morphological properties of kaolin. In this study, the property of kaolin is investigated by using advanced instruments such as X-ray diffraction (XRD), X-ray fluorescence analysis (XRF), Fourier-transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), and differential thermogravimetric analysis (DTA). Based on the XRF test, the main component of kaolin clay contains SiO₂ (58.73%), Al₂O₃ (24.35%), K₂O (5.36%), and other impurities, including Fe₂O₃ (2.06%) and TiO₂ (0.13%). The FTIR spectra displayed the functional groups Si-O, Al-OH, Al-O, and Si-O-Al. The XRD diffractogram identified kaolin clay as the main mineral phase in the existence of quartz, halloysite, and chlorite.

1. Introduction

Kaolin is a white-brown powdery clay mineral. It has the main constituent of kaolinite of a hydrous aluminum silicate (Al₂O₃.2SiO₂.2H₂O) with a single tetrahedral silicate layer linked via oxygen molecules to a single alumina octahedral layer. The most common auxiliary minerals found with kaolin are maternal rocks such as mica, feldspar, ferruginous, quartz, and titaniferous [1, 2]. Some others are included, such as ilmenite, hematite, illite, bauxite, zircon, rutile, graphite, and montmorillonite, [3, 4]. Iron minerals are the most venomous impurities which inform a white kaolin color. The association of iron with kaolin can occur in three different forms such as (a) hematite, goethite, and pyrite, (b) substitution in the crystal network of minerals such as anatase, rutile, and mica, and (c) as surface absorption on montmorillonite and kaolin. Kaolin is used for various applications such as pottery, effluent waste treatment [5], composite filler manufacturing [6], and solid catalyst production such as zeolite [7–10].

Currently, Ethiopia has a vast kaolin source, which is found in different parts of the country. However, advanced and continuous research on natural kaolin minerals has not been widely investigated. Moreover, the presence of some impurities in kaolin makes it less commercially applicable [11]. To improve the quality of kaolin deposits to possibly meet some industrial requirements, the coloring impurities (mainly iron oxides and other small amounts of fluxing components) must be removed through efficient, economical, and environmentally friendly beneficiation methods.

Beneficiation of natural kaolin is an advanced process to improve kaolin’s whiteness and refine it from chemical and physical impurities such as metallic oxides and salts. Furthermore, beneficiation also removes dead mineral phases such as quartz, feldspar, pebbles, gris, muscovite, mica, titanium oxide, and iron oxide. Moreover, the beneficiation process can be used to enhance the quality of clay minerals, including particle size distribution, shape, chemical composition, brightness, and appearance intended for application. There are different beneficiation techniques widely applied such as chemical beneficiation using inorganic and organic acids such as citric acid [12, 13], sulfuric acid [14, 15], hydrochloric acid [16], oxalic acid [17, 18], sodium dithionite [19], and thiourea oxide. The biological beneficiation method is also another approach to purifying the raw kaolin by using bacteria [20–22] and fungi [23]. Physical beneficiation methods are preferentially used to improve the quality of raw kaolin due to the less economical, while biological beneficiation of kaolin is non-eco-friendly, energy-intensive, poorly maintained crystal structure, and very slow process for industrial mass production [24, 25]. Physical beneficiation methods are preferentially used to improve the quality of raw kaolin. The removal of gravel, sand, and other matter during the wet beneficiation of kaolin involves centrifugal separation [26], magnetic separation [27], chemical belching, filtering, flocculation [28], and magnetic flotation [29].

In Ethiopia, kaolin clay is significant for financial development as the raw material used for the industry sector. However, slight consideration was given to the chemical, physical, mineralogical, and morphological properties. The use of this mineral for different applications requires knowledge and study of the physiochemical properties. Although very limited studies on the synthesis and characterization of Ethiopian kaolin have been reported in the literature [30], the origin of Degen kaolin has not been studied. To fill this research gap, the present study was carried out to determine the characteristics of the kaolin clays and insight for various applications. The current study is intended to characterize the raw kaolin and beneficiated kaolin and to encourage the beneficiated kaolin for the manufacture of low-cost kaolin-based for various applications.

2. Experimental Methods

2.1. Materials and Chemicals

The studied natural kaolin clay was collected from the local area, Dejen Town, located in the state of the Amhara Region, Gojjam, Ethiopia. All laboratory-grade reagents such as sodium hydroxide (NaOH) and sulfuric acid (H₂SO₄) were used for the present study. Distilled water was also used in the present study to remove contamination from other sources.

2.2. Beneficiation of Raw Kaolin

The raw kaolin was beneficiated to advance the aluminum oxide (Al₂O₃) and silicon (SiO₂) oxide purity by removing some impurities (soluble salts, feldspar, quartz, gris, muscovite, mica, titanium oxide, and iron oxide) through a consecutive beneficiation process. The removal of impurities and dirt from raw kaolin was accomplished through physical separation, followed by the wet/soaking process according to the procedures reported by Mokwa et al. [31]. The powdered bulk sample was soaked in deionized water for 48 h. The slurry was plunged and screened through a 50 μm mesh sieve and then allowed to settle; the water was siphoned off and the samples were dried at 120°C for 3 h. After effective drying, it was crushed by using a jaw crusher and then sieved through a 0.15–0.3 mm sieve size. The powder of kaolin was calcined at 750°C for 2 h using Muffle Furnace (Nabertherm B180) for processing into metakaolin and then cooled for 1 h. The essence of this is to dehydroxylate the beneficiated kaolin to form an activated amorphous material called metakaolin. Then, the sample was stored for further characterization.

2.3. Characterization of Kaolin Clay

After the preparation of the activated samples, the chemical, mineralogical, and physical properties of the kaolin clay samples were characterized and evaluated by using X-ray diffraction (XRD), atomic absorption spectroscopic (AAS), Fourier-transform infrared spectroscopy (FTIR), thermal gravimetric analysis (TGA), and differential thermal gravimetric metric analysis (DTA).

2.3.1. X-Ray Diffraction (XRD) Analysis

XRD patterns of activated powdered samples were carried out using an equinox-1000 diffractometer Cu-Kα radiation with Cu anode (λ = 1.5406 Å) source was used to record the maximum diffraction lines and better identification of phases at the room. The quantitative and qualitative characterizations of the crystalline phases present were characterized by an X-ray diffractometer (MIN 3740) with a continuous scanning axis of 2θ with a scan range of 10–60° for fine particles.

2.3.2. Thermal Analysis

Thermogravimetric analysis (TGA) and differential scanning analysis (DSC) are used to analyze the degradation route of the sample and the thermal stability of the kaolin clay. The weight of the given sample is monitored continuously as a function of time and/or temperature. Thermal properties and mass loss of kaolin clay samples were analyzed by thermogravimetry (TGA Bj henven, Model HCT-1) with liquid nitrogen. The heating rate was set at 10°C/min from 25°C to 1000°C in the air stream (100 ml/min).

2.3.3. FTIR Analysis

Fourier-transform infrared spectroscopy (FTIR) spectral analysis of the kaolin clay was investigated using a JASCO-4100 spectrometer in the range of 400–4000 cm⁻¹. The samples were ground in a ceramic mortar and mixed with potassium bromide (KBr), and then, the pellets were formed by pressing them using a mechanical press.

2.3.4. XRF Analysis

The chemical compositions of the sample powders were analyzed using Horiba MESA-50 X-ray fluorescence analysis (XRF).

3. Results and Discussion

3.1. Chemical Composition Analysis

The chemical composition of raw kaolin is determined by using XRF, as presented in Table 1.

The characterization results showed that the composition of SiO₂ and Al₂O₃ varied between 58.73 and 60.25%, and 24.35 and 27.52%, respectively. The result data obtained are not much different from the result obtained from commercial kaolin based on the reported theory [32, 33], i.e., the composition of Al₂O₃ (21.85) and SiO₂ (59.03). The Al₂O₃/Fe₂O₃ and SiO₂/Al₂O₃ mass ratios were obtained in the range of 2.18–2.41 and 11.82–70.56, respectively, for both samples. The minimum value for the mass ratio of SiO₂/Al₂O₃ was obtained to be 2.41, confirming the presence of white kaolinite stages, while the maximum value for Al₂O₃/Fe₂O₃ is consistent with alumina rich with a definite white color. Having a higher mass ratio of Al₂O₃/Fe₂O₃ and its minor content of iron oxide and SiO₂/Al₂O₃, the beneficiated kaolin clay is an appropriate inexpensive raw material for various applications, specifically for water purification. Based on the closeness of the alumina and silicate composition found between raw kaolin and commercial kaolin, it was concluded that kaolin has alumina and silicate intensities comparable to commercial kaolin even though there are other impurities it holds such as Fe₂O₃ and TiO₂. Lastly, the beneficiating process can improve the properties of raw kaolin to get a high grade that is close to standard kaolin.

3.2. XRD Analysis

The XRD patterns of the kaolin samples allowed the determination of the qualitative and mineralogical phase composition of the materials. The physical alterations that looked at the raw kaolin and beneficiated clay minerals were investigated by using the X-ray diffraction method presented in Figure 1. The raw and beneficiated kaolin clay indicates well-defined diffraction at 2-theta values of 18.42, 24.16, 27.62, 51.6, and 58.52°, these major peaks are characteristically matching to the kaolinite [34]. Whereas, other peaks are equivalent to the 2-theta values of 29.57, 35.4, and 40.5° typically corresponding to quartz [35–37]. The obtained XRD showed that the beneficiated kaolin material is rich in kaolinite and quartz.

This is an endorsement that the level of quartz in the kaolin was not entirely removed during the beneficiation process. Thus, the existence of quartz in the beneficiated kaolin results in an increase in the content of silicon dioxide (SiO₂). This finding confirms the result of AAS, which is presented in Table 1.

3.3. TGA/DSC Analysis

The thermal analysis (TGA and DSC) of the raw and beneficiated kaolin samples was investigated. The heating rate was set at 10°C/min from 25°C to 1000°C in the air stream (100 ml/min). The TGA/DSC profile of the raw and beneficiated kaolin is presented in Figure 2. The TGA profiles of the two samples comprise two distinctive stages. In the first stage, the degree of mass loss of raw and beneficiated kaolin occurred in the range of 27°C–185°C, which is formed by an endothermic reaction. This noticeable mass loss can be attributed to the decomposition of poorly crystallized components such as fine-grained aluminum hydroxide, loss of void water, and loss of interlayer surface. In the second stage of the endothermic process, numerous mass losses can be observed in the range of 450°C–800°C for raw kaolin and 450–1000°C for beneficiated kaolin due to the loss of hydroxyl groups from the original kaolin structure [38, 39].

(a)

(b)

At this level, phase alteration from kaolin to metakaolin occurred. It can be seen in the DSC curve that an exothermic reaction occurred for all raw and beneficiated kaolin samples. The complete removal of the water molecules in the interlayer structure was observed in the beneficiated kaolin, which confirmed that there was no major endothermic peak. The results showed that both samples displayed an exothermic peak at 1005°C in the DSC array because of amorphous phase alteration. This alteration does not involve any mass loss.

3.4. SEM Observation

The surface morphology and porosity of raw and beneficiated kaolin materials were analyzed by scanning electron microscope (SEM). The SEM image of the raw kaolin sample is presented in Figure 3. As shown in Figure 3(a), the raw kaolin image has an irregular shape and a rough edge, and it is agglomerated and porous on the entire surface. The same phenomenon was reported in the physiochemical characteristics of kaolin clay by Yahaya et al., [40]. As the result shows, the pore holes are not observed continuously on the surface of raw kaolin. Moreover, the presence of large particles appeared to have been formed by several flaky particles stacked together to form a compact arrangement with a hexagonal shape, irregular bulk edges, and a flattened platelet structure of kaolinite. This indicates that it may be raw kaolin; a clear, layered, rectangular shape observed that indicates the natural kaolinite, without any treatment, is double-layered alumino-silicate clay [41]. As shown in Figure 3(b), due to the removal of impurities from raw kaolin, more distribution, fragmentation, and fewer aggregations occurred during beneficiation. This results in the formation of a more porous structure. In addition, the adsorptive and porosity natures on the surface are observed and form a complex structure for beneficiated kaolin. Many more holes are available in the beneficiated kaolin. It is also evident that the surface of beneficiated kaolin has homogenous and clear particles with a small flake shape. A related reflection was reported in the literature for kaolinite minerals [42–44]. Thus, the adsorptive nature of the surface helps for various applications such as adsorption and filler [44]. From the scanning morphological analysis in Figure 4(a), the average pore size of the raw kaolin is nearly 0.25 μm. The presence of rough and small pore sizes is due to the existence of impurities and soluble salts in the raw kaolin. While the morphology analysis revealed in Figure 4(b), the average pore size of the particle after benefaction is approximately 0.5 μm. The morphology of beneficiated kaolin is relatively well-defended and uniform.

(a)

(b)

(a)

(b)

3.5. FTIR Spectroscopy

The key functional groups, crystal structure, and other structural defects in the samples were identified using FTIR analyses. The FTIR spectra analyses of raw kaolin and beneficiated clay are presented in Figure 5. The peaks of both samples exhibit sharpness at their respective bands. Based on the absorption of FTIR spectra from raw kaolin and beneficiated sample, sharp peak uptakes of 1520 cm⁻¹ and 1630 cm⁻¹ are observed in the absorption of the –OH buckling vibration confined in the crystal framework. The peak found at 1630 cm⁻¹ is the bending mode of H₂O. The absorption bands looking at wave numbers 1015 cm⁻¹ and 1020 cm⁻¹ display the vibration of Si-O, which is a distinctive characteristic of kaolin clay minerals, whereas the peak of 3650 cm⁻¹ indicates the presence of –OH vibration that has bound to octahedral atoms on the surface of the layered silicate. Si-O vibration has a wave number range of 1010–1035 cm⁻¹, buckling vibration of -OH has a wave number range of 1635 cm⁻¹, and OH vibration has a wave number range of 3600 cm⁻¹. In the band region between 3600 cm⁻¹ and 3800 cm⁻¹, the most prominent absorption bands of kaolin appear, corresponding to the structural water molecule and Al-OH stretching vibrations. The absorption bands between 415 cm⁻¹ and 850 cm⁻¹ are consigned to Si-O, Al-O stretching, and OH deformation. The peak found at 720 cm⁻¹ is attributed to OH deformation [45] or Si-O [46]. In addition, the band occurs at 1000 cm⁻¹, also attributed to Si-O vibrations. The band at 500 cm⁻¹ is related to Si-O-Al stretching, and the band appearing at 462 cm⁻¹ and 418 cm⁻¹ is assigned to Si-O-Si bending vibrations [47]. The band found at 915 cm⁻¹ was corresponding to hydroxyl deformation of aluminum cation (Al³⁺). The bands found at 517 cm⁻¹ and 420 cm⁻¹ were attributed to the starching vibration of Al-O and Si-O and the symmetric vibration of Si-Si-O bonds. This result endorses the fact that kaolin contains a high content of silicon oxide compared to other constituents. Compared with raw kaolin FTIR spectra, the beneficiated kaolin observes some minor intensity peaks at 3750 cm⁻¹, which are attributed to the H-O-H stretching mode of water molecules [48]. Bands occurring at 3352 cm⁻¹ and 3420 cm⁻¹ in raw and beneficiated kaolin were allocated to the hydroxyl (OH) stretching water [49]. The results showed that the beneficiated kaolin does not enclose Al-O or –OH due to the complete loss of the physical hydroxyl group because of the calcination process of the sample at a higher temperature.

Generally, the FTIR spectra display that converting kaolin to metakaolin through thermal treatment is adequate. The formation of metakaolin can thus be used to appear in several bands in the FTIR spectrum in the range of 3550 cm⁻¹–3800 cm⁻¹. However, beneficiated kaolin has improved the rate of transmittance compared to raw kaolin.

4. Conclusion

This study investigated the physical and chemical properties of Ethiopian kaolin clay through characteristic techniques using XRD, FTIR, XRF, DSC/TGA, and SEM. Based on the XRF result, the main composition of Ethiopian kaolin was obtained as SiO₂ and Al₂O₃. The result of diffraction intensity using XRD can show that the minerals constituting of Ethiopian kaolin are quartz and kaolinite. The SEM result was obtained with a typical flake shape and pore sizes in the range of 0.2–1.4 µm. Raw kaolin has some impurities, such as iron oxide, soluble salts, feldspar, quartz, mica, and titanium oxide, which directly impact the chemical composition, optical, and mineralogical properties. However, the beneficiation process reduced the percentage of contaminants with the minor label. Wet physical beneficiation of kaolin was more effective in removing kaolin impurities. Benefaction results indicated significant removal of iron oxide and titanium oxide from raw kaolin. It was promising that the formation of a more porous structure and increasing kaolin whiteness occurred with this beneficiation method. Thus, this study could offer a new vision for the utilization of beneficiated Ethiopian kaolin for many industrial applications.

Data Availability

All the necessary information required for the replication of this work and/or conducting a secondary analysis is included in the article.

Conflicts of Interest

The author declares that he has no conflicts of interest.

Acknowledgments

The author would like to thank the Faculty of Chemical and Food Engineering, Bahir Dar Institute of Technology, for financial funding and for providing access to necessary materials for the achievement of the research.

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Copyright © 2023 Adane Adugna Ayalew. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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