Physiochemical Characterization of Ethiopian Mined Kaolin Clay through Beneficiation Process

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Introduction
Kaolin is a white-brown powdery clay mineral. It has the main constituent of kaolinite of a hydrous aluminum silicate (Al 2 O 3 .2SiO 2 .2H 2 O) with a single tetrahedral silicate layer linked via oxygen molecules to a single alumina octahedral layer. Te 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. Te association of iron with kaolin can occur in three diferent 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, efuent waste treatment [5], composite fller manufacturing [6], and solid catalyst production such as zeolite [7][8][9][10].
Currently, Ethiopia has a vast kaolin source, which is found in diferent 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 fuxing components) must be removed through efcient, economical, and environmentally friendly benefciation methods.
Benefciation of natural kaolin is an advanced process to improve kaolin's whiteness and refne it from chemical and physical impurities such as metallic oxides and salts. Furthermore, benefciation also removes dead mineral phases such as quartz, feldspar, pebbles, gris, muscovite, mica, titanium oxide, and iron oxide. Moreover, the benefciation process can be used to enhance the quality of clay minerals, including particle size distribution, shape, chemical composition, brightness, and appearance intended for application. Tere are diferent benefciation techniques widely applied such as chemical benefciation 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. Te biological benefciation method is also another approach to purifying the raw kaolin by using bacteria [20][21][22] and fungi [23]. Physical benefciation methods are preferentially used to improve the quality of raw kaolin due to the less economical, while biological benefciation of kaolin is non-eco-friendly, energyintensive, poorly maintained crystal structure, and very slow process for industrial mass production [24,25]. Physical benefciation methods are preferentially used to improve the quality of raw kaolin. Te removal of gravel, sand, and other matter during the wet benefciation of kaolin involves centrifugal separation [26], magnetic separation [27], chemical belching, fltering, focculation [28], and magnetic fotation [29].
In Ethiopia, kaolin clay is signifcant for fnancial development as the raw material used for the industry sector. However, slight consideration was given to the chemical, physical, mineralogical, and morphological properties. Te use of this mineral for diferent 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 fll this research gap, the present study was carried out to determine the characteristics of the kaolin clays and insight for various applications. Te current study is intended to characterize the raw kaolin and benefciated kaolin and to encourage the benefciated kaolin for the manufacture of low-cost kaolin-based for various applications.

Materials and Chemicals.
Te 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 (H 2 SO 4 ) were used for the present study. Distilled water was also used in the present study to remove contamination from other sources.

Benefciation of Raw Kaolin.
Te raw kaolin was benefciated to advance the aluminum oxide (Al 2 O 3 ) and silicon (SiO 2 ) oxide purity by removing some impurities (soluble salts, feldspar, quartz, gris, muscovite, mica, titanium oxide, and iron oxide) through a consecutive benefciation process. Te 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]. Te powdered bulk sample was soaked in deionized water for 48 h. Te slurry was plunged and screened through a 50 μm mesh sieve and then allowed to settle; the water was siphoned of and the samples were dried at 120°C for 3 h. After efective drying, it was crushed by using a jaw crusher and then sieved through a 0.15-0.3 mm sieve size. Te powder of kaolin was calcined at 750°C for 2 h using Mufe Furnace (Nabertherm B180) for processing into metakaolin and then cooled for 1 h. Te essence of this is to dehydroxylate the benefciated kaolin to form an activated amorphous material called metakaolin. Ten, the sample was stored for further characterization.

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 difraction (XRD), atomic absorption spectroscopic (AAS), Fourier-transform infrared spectroscopy (FTIR), thermal gravimetric analysis (TGA), and diferential thermal gravimetric metric analysis (DTA).

X-Ray Difraction (XRD)
Analysis. XRD patterns of activated powdered samples were carried out using an equinox-1000 difractometer Cu-Kα radiation with Cu anode (λ �1.5406Å) source was used to record the maximum difraction lines and better identifcation of phases at the room. Te quantitative and qualitative characterizations of the crystalline phases present were characterized by an X-ray difractometer (MIN 3740) with a continuous scanning axis of 2θ with a scan range of 10-60°for fne particles.

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

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 . Te 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.

XRF Analysis.
Te chemical compositions of the sample powders were analyzed using Horiba MESA-50 Xray fuorescence analysis (XRF).

Chemical Composition Analysis.
Te chemical composition of raw kaolin is determined by using XRF, as presented in Table 1.
Te characterization results showed that the composition of SiO 2 and Al 2 O 3 varied between 58.73 and 60.25%, and 24.35 and 27.52%, respectively. Te result data obtained are not much diferent from the result obtained from commercial kaolin based on the reported theory [32,33] 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 Fe 2 O 3 and TiO 2 . Lastly, the benefciating process can improve the properties of raw kaolin to get a high grade that is close to standard kaolin.

XRD Analysis.
Te XRD patterns of the kaolin samples allowed the determination of the qualitative and mineralogical phase composition of the materials. Te physical alterations that looked at the raw kaolin and benefciated clay minerals were investigated by using the X-ray difraction method presented in Figure 1. Te raw and benefciated kaolin clay indicates well-defned difraction 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][36][37]. Te obtained XRD showed that the benefciated kaolin material is rich in kaolinite and quartz.
Tis is an endorsement that the level of quartz in the kaolin was not entirely removed during the benefciation process. Tus, the existence of quartz in the benefciated kaolin results in an increase in the content of silicon dioxide (SiO 2 ). Tis fnding confrms the result of AAS, which is presented in Table 1.

TGA/DSC Analysis.
Te thermal analysis (TGA and DSC) of the raw and benefciated kaolin samples was investigated. Te heating rate was set at 10°C/min from 25°C to 1000°C in the air stream (100 ml/min). Te TGA/DSC profle of the raw and benefciated kaolin is presented in Figure 2. Te TGA profles of the two samples comprise two distinctive stages. In the frst stage, the degree of mass loss of raw and benefciated kaolin occurred in the range of 27°C-185°C, which is formed by an endothermic reaction. Tis noticeable mass loss can be attributed to the decomposition of poorly crystallized components such as fne-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 benefciated kaolin due to the loss of hydroxyl groups from the original kaolin structure [38,39].
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 benefciated kaolin samples. Te complete removal of the water molecules in the interlayer structure was observed in the benefciated kaolin, which confrmed that there was no major endothermic peak. Te results showed that both samples displayed an exothermic peak at 1005°C in the DSC array because of amorphous phase alteration. Tis alteration does not involve any mass loss.

SEM Observation.
Te surface morphology and porosity of raw and benefciated kaolin materials were analyzed by scanning electron microscope (SEM). Te 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. Te 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 faky particles stacked together to form a compact arrangement with a hexagonal shape, irregular bulk edges, and a fattened platelet structure of  kaolinite. Tis 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 benefciation. Tis 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 benefciated kaolin. Many more holes are available in the benefciated kaolin. It is also evident that the surface of benefciated kaolin has homogenous and clear particles with a small fake shape. A related refection was reported in the literature for kaolinite minerals [42][43][44]. Tus, the adsorptive nature of the surface helps for various applications such as adsorption and fller [44]. From the scanning morphological analysis in Figure 4(a), the average pore size of the raw kaolin is nearly 0.25 μm. Te 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. Te morphology of benefciated kaolin is relatively well-defended and uniform.

FTIR Spectroscopy.
Te key functional groups, crystal structure, and other structural defects in the samples were identifed using FTIR analyses. Te FTIR spectra analyses of raw kaolin and benefciated clay are presented in Figure 5. Te peaks of both samples exhibit sharpness at their respective bands. Based on the absorption of FTIR spectra from raw kaolin and benefciated sample, sharp peak uptakes of 1520 cm −1 and 1630 cm −1 are observed in the absorption of the -OH buckling vibration confned in the crystal framework. Te peak found at 1630 cm −1 is the bending Te 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. Te 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]. Te band found at 915 cm −1 was corresponding to hydroxyl deformation of aluminum cation (Al 3+ ). Te 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. Tis result endorses the fact that kaolin contains a high content of silicon oxide compared to other constituents. Compared with raw kaolin FTIR spectra, the benefciated 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 benefciated kaolin were allocated to the hydroxyl (OH) stretching water [49]. Te results

Advances in Materials Science and Engineering
showed that the benefciated 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. Te 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, benefciated kaolin has improved the rate of transmittance compared to raw kaolin.

Conclusion
Tis 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 SiO 2 and Al 2 O 3 . Te result of difraction intensity using XRD can show that the minerals constituting of Ethiopian kaolin are quartz and kaolinite. Te SEM result was obtained with a typical fake 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 benefciation process reduced the percentage of contaminants with the minor label. Wet physical benefciation of kaolin was more efective in removing kaolin impurities. Benefaction results indicated signifcant 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 benefciation method. Tus, this study could ofer a new vision for the utilization of benefciated 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
Te author declares that he has no conficts of interest.