Characterization and beneficiation of Ethiopian kaolin for use in fabrication of ceramic membrane

The leaching experiments were carried out in a 300 ml round bottom flask placed in a magnetic stirrer hot plate with temperature control. The hydrous kaolin material was mixed with the oxalic acid solution at different concentrations (0.1 M, 0.5 M, 1.0 M, 1.5 M, and 2.0 M) at a 1:10 solid to solution ratio and maintained at different targeted temperature (80 °C, 90 °C, 100 °C, and 120 °C) for 2 h under continuous moderate stirring (300 rpm). A 10 ml slurry sample was taken every 20 minutes till the reaction time of 120 min. it was filtered using a nylon syringe filter with a porosity of 0.2 μm then centrifuged immediately at 1000 rpm for 10 min. Since the resulting solution is a higher concentration, a 1:100 dilution was used for the subsequent analysis. The clear diluted solution was analysed for iron content by Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES, Optima 2100 DV, Perkin Elmer Instruments, Shelton, CT, USA). A wavelength of 238.204 nm was used for iron detection but an alternative wavelength was also used for controlling possible interferences from other elements (239.562 nm) [56]. All experiments were performed in triplicate, all data are expressed as the mean ± standard deviation.

The residue product was then filtered, repeatedly washed with distilled water to remove any unspent oxalic acid, dried at 103 °C for 24 h. Finally, the acid-treated hydrous kaolin was activated through thermal treatment in a muffled furnace at 700 °C for 3 h, resulting in the formation of calcined kaolin (see figure 1). Its transformation to metakaolin, an amorphous Al-Si phase from which aluminum is easily leached by acid leachants (sulfuric, hydrochloric and, oxalic acid) [57, 58]. According to the previous studies [13, 32, 57], physical beneficiation and calcination were widely applied first and then followed by leaching with acid. In this study, the physical beneficiation and leaching with oxalic acid were applied first and then followed by calcination. In order to remove a high level of iron oxides from kaolin clay, chemical leaching of the clay prior to high-temperature calcination was applied. Moreover, leaching with oxalic acid before calcination is important to prevent the extraction of aluminum from kaolin through the oxalic acid leaching process. Another study proves that the structural ferric ions and ferric oxides from kaolin were difficult to remove using acid as a leaching reagent; therefore, iron removal by acids should be conducted prior to calcination [13].

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The loss on ignition was determined using the expression equation (1).

$$\begin{eqnarray}&&{\rm{LOI}}\,=\,\displaystyle \frac{{M}_{2}\,-\,{M}_{1}}{{M}_{{\rm{2}}}-\,{M}_{3}}\,\end{eqnarray} \tag{ 1 }$$

Where M₁ is the weight of empty crucible; M₂ is the weight of crucible and sample before calcination; M₃ is the weight of crucible and sample after calcination.

The flat green ceramic bodies prepared previously were dried in an oven at 110 °C for 24 h and fired to a final temperature of 500 °C, 600 °C, 700 °C, 800 °C, 900 °C, 1000 °C, and 1100 °C for 1h. The shrinkage percentage that occurred during the firing process was determined using the dimensions (volume) of the flat green ceramic specimen before and after the firing process. The total linear shrinkage percentage of the flat green kaolin-based ceramic specimen was determined by using equation (3).

$$\begin{eqnarray}&&{\rm{Total}}\,{\rm{linear}}\,\mathrm{shrinkage}\,=\left(\displaystyle \frac{{V}_{0}-{V}_{f}}{{V}_{o}}\right)\times {\rm{100}}\end{eqnarray} \tag{ 3 }$$

where V_o is the volume of the specimen before firing and V_f is the volume specimen after firing.

Water absorption and porosity of the flat ceramic specimens were determined by the boiling water method. The flat ceramic specimens prepared in the linear shrinkage part and sintered to different final temperatures ranging from 500 °C to 1100 °C were immersed in boiling water for 2 h and cooled to room temperature. After boiling the specimens were immersed in water for 24 h and then the surface was wiped with tissue paper. The percentage of water absorption was evaluated using the expression equation (4).

$$\begin{eqnarray}&&{\rm{Water}}\,{\rm{absorption}}\,=\left(\displaystyle \frac{{M}_{s}\,-\,{M}_{d}}{{M}_{d}}\right)\times {\rm{100}}\end{eqnarray} \tag{ 4 }$$

where M_s is the mass of the saturated specimen, M_d is the mass of the dry specimen.

The porosity of the flat ceramic body was evaluated using the expression equation (5).

$$\begin{eqnarray}&&{\rm{Porosity}}\,=\left(\displaystyle \frac{\displaystyle \frac{{M}_{s\,}-\,{M}_{d}}{{\rho }_{w}}}{{V}_{f}}\right)\times {\rm{100}}\end{eqnarray} \tag{ 5 }$$

where M_d is the mass of the dry specimen (g), M_s is the mass of the saturated specimen (g), ρ_w is the density of water (1 g cm⁻³ at 4 °C), and V_f is the volume of the fired specimen (cm³).

Contact angle measurement has been performed on ceramic specimens to evaluate their surface hydrophilic characteristics. The wettability properties of flat ceramic specimens fired to different final temperatures ranging from 500 °C to 1100 °C were measured by the sessile drop method (Kruss DSA 10Mk2, Germany) using distilled water at room temperature. All contact angle readings were taken 10 min after a 0.5 ml water droplet was placed on the membrane surface.

Chemical resistance tests were carried out in an acidic or basic environment. A flat ceramic specimen fired at different temperature (500 °C, 600 °C, 700 °C, 800 °C, 900 °C, 1000 °C, and 1100 °C) was immersed in acid solution (hydrochloric acid solutions at pH = 1.0 and alkali solution (sodium hydroxide solutions at pH = 13.0) at ambient temperature for 24 h. The flat ceramic specimens were removed from the solutions (acid and alkali), rinsed with distilled water, and dried in an oven at 110 °C for 24 h. The weight of the flat ceramic specimen was measured before and after the test and the chemical resistance of the specimen was calculated by the following equation (6).

$$\begin{eqnarray}&&{\rm{Chemical}}\,{\rm{resistance}}\,=\left(\displaystyle \frac{{M}_{0\,}-\,{M}_{{\rm{pH}}}}{{M}_{0}}\right)\times {\rm{100}}\end{eqnarray} \tag{ 6 }$$

where M_o is the mass of the specimen before pH attack and, M_pH is its mass after removing the specimen from the acidic (HCl, pH = 1.0) or basic (NaOH, pH = 13.0) solutions.

The density of the beneficiated kaolin is 2.64 gcm⁻³ as compared to 2.5 gcm⁻³ for the raw kaolin (see table 1). This weight increase may be due to an increase in Al₂O₃ content from 24.9 to 29.4% during the beneficiation process. The pH values of the raw, beneficiated & acid leached, beneficiated, acid leached and calcined, and commercial kaolin were 5.85, 5.05, 5.15, and 5.41, respectively. This showed that the surface of the raw, beneficiated & acid leached, beneficiated, acid leached and calcined, and commercial kaolin body is slightly acidic. Similar results are reported in the literature [33] for raw and beneficiated kaolin.

The particle size distribution of raw, beneficiated & acid leached, beneficiated, acid leached & calcined, and commercial kaolin is mainly in the range of 3.52–45 μm (D₅₀ = 10.8 μm), 3.37–30.5 μm (D₅₀ = 10.6 μm), 3.7–42.3 μm (D₅₀ = 11.4 μm), and 0.48–0.99 μm (D₅₀ = 0.8 μm), respectively. The slight decrease of particle size of the beneficiated sample compared to the raw kaolin size is due to the dissolution of the iron oxides and oxyhydroxides. There is a substantial increase in particle size of the beneficiated, leached, and calcined sample compared to the raw, beneficiated, and leached kaolin as a result of calcination. According to these results, the particle sizes of beneficiated and acid leached, and beneficiated, acid leached and calcined samples are adequate for uses of the ceramic products because of containing fine particles.

Figure 2 shows the color changes of the kaolin before and after beneficiation; in the raw kaolin sample, a beige coloration is evident, the beneficiated and acid leached kaolin is essentially white, while imparts color to calcined kaolin (low brightness due to iron oxide impurity) and commercial kaolin (high brightness). The color of beneficiated and acid-leached kaolin is very close to the commercial kaolin. As additional data, figure 3 shows the images of the kaolin clay sample before and after beneficiation with a particle size of less than 45 μm observed at an optical microscope (SMZ25). The observation with reflected light microscopy indicates that: the raw kaolin clay appears in different colors (show by the arrows): slightly white with a brownish red, yellow, and black color impurity due to iron oxide, titanoferrous, and pyrite (iron sulphide)/ilmenite, respectively, having the lowest whiteness index value of 72.0%; the beneficiated and acid leached kaolin is essentially white, having the highest whiteness index value of 81.4%. A similar observation was obtained by Scorzelli et al [59]. After the beneficiated, acid leached and calcined process, a yellow color pigment was appeared in the sample (see figure 3(c)). This is due to the presence of iron-stained titania. Thus, the microscopic observation indicates that oxalic acid leaching has not completely removed the titanium dioxide (TiO₂) content. It was observed that 84.83% of iron impurities were removed from raw kaolin with an increase of whiteness index from 72 to 81.4%. This reveals that the beneficiated kaolin reaches an adequate whiteness index for the ceramics industry. According to the literature, the ceramic industry demands an ISO whiteness index above 80.0% [19].

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The surface area of separate kaolin samples was measured by the Brunauer-Emmett-Teller (BET) multipoint method. The measured specific surface area of kaolin ranges from 11.6 to 30.7 m² g⁻¹ for the various treatment. The lowest specific surface area (11.6 m² g⁻¹) was found for the raw kaolin clays while the highest specific surface area (30.7 m² g⁻¹) was obtained for the beneficiated and acid leached kaolin (see table 1). Similar results are reported in the literature [15, 60, 61] for natural kaolinite clays. As can be seen in table 1, the surface area is increased with oxalic acid treatment. It increases in the order of BALK > BALCK > CK > RK. The increase in the surface area from raw kaolin clay to the beneficiated and acid-treated kaolin clay is due to the removal of the impurities within the pore kaolin mineral through the beneficiation process. However, the surface area of the beneficiated and acid leached kaolin clay treated at high temperature (700 °C) for 3 h is slightly decreasing to a value of 19.4 m² g⁻¹, whereas during calcination the pore volume and pore size increased due to losing their structure [61, 62].

The structural changes that appeared on the raw kaolin and beneficiated clay material were studied using the x-ray diffraction technique. The raw, beneficiated, and acid leached, beneficiated, acid leached and calcinated and commercial kaolin clay shows well-defined diffractions at 2 theta value of 12.18–13.8°, 23.55–24.9°, and 62.5°, these peaks are typically corresponding to kaolinite [32, 42]. On the other hand, the peaks corresponding to the 2 Theta values 27.5° and 41.6°, which are typical characteristics of quartz [65–67]. The obtained x-ray diffraction patterns indicated that the beneficiated, acid-leached, and calcinated material is rich in kaolinite and quartz. This is a confirmation that the quartz material was not completely removed through beneficiation, acid leaching, and calcination [67]. The presence of quartz in the mineralogical composition of the beneficiated, acid-leached, and calcinated kaolin sample results in increasing the silicon dioxide (SiO₂) content [66]. This result supports the x-ray fluorescence result presented in table 2. A small peak corresponding to the 2 Theta values of 68.9° for beneficiated, acid leached, and calcined kaolin figure 4(c), which is typically characteristic of titanium oxide (TiO2). This is a confirmation that the titanium oxide is released after calcination [32]. This result supports the optical microscope image of beneficiated, acid-leached, and calcined kaolin presented in figure 3(c).

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The thermal analyses (DSC/TGA) of the powdered raw, beneficiated and acid leached, beneficiated, acid leached and calcined, and commercial kaolin materials were performed using DSC/TGA, Model Q600 apparatus operating under a constant dry airflow at a heating rate of 10 °C min⁻¹ between room temperature and 1100 °C. Commercial kaolin was used as reference material for all experiments. The DSC/TGA profiles of the raw, beneficiated, and acid leached, beneficiated, acid leached and calcined, and commercial kaolin are shown in figures 5(a)–(d). The total weight loss of mass was 6.9 wt% for raw, 9.14 wt% for beneficiated and acid leached, 0.55 wt% for beneficiated, acid leached and calcined, and 9.14 wt% for commercial kaolin. TGA profile of the studied samples consists of two distinct stages. In the first stage of the endothermic reaction, the mass loss of raw, beneficiated and acid leached, beneficiated, acid leached and calcined, and commercial kaolin in the temperature range 30 °C–140 °C was 0.3 wt%, 1.25 wt%, 0.099 wt%, and 0.16 wt%, respectively which is attributed to the loss of surface, adsorbed/interlayer, and void water [64, 68, 69]. From 140 °C to 360 °C, a net weight gain of 0.062 wt% and 0.34 wt% was recorded for beneficiated, acid leached, and calcined kaolin, and commercial kaolin, respectively, which is attributed to oxygen chemisorption occurring prior to the onset of combustion (figures 5(b), (d)).

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In the second stage of the endothermic reaction, several mass losses were observed in the temperature range of 400 °C–800 °C due to the loss of hydroxyl groups (dehydroxylation of the kaolinite) from the kaolin structure [44, 64, 68–71]. In this stage, phase transformation chemical reaction occurred kaolin (Al₂Si₂O₅(OH)₄) to metakaolin (Al₂Si₂O₇) and showed a significant mass loss of up to 6.09 wt% of the raw, 7.2 wt% of the beneficiated and acid leached, and 0.38 wt% of beneficiated, acid leached and calcined, and 8.9 wt% of the commercial kaolin. This major mass loss and intense endothermicity are because of the combustion of organic matter and carbon. The conversion of kaolin to metakaolin is further supported by the results from the XRD analysis by the disappearance of kaolinite peaks.

It can be determined in DSC curves that exothermic peaks appeared for all the kaolin powder samples. The exothermic peak occurs at a temperature between 50 °C to 60 °C for all indicated the removal of the free water molecules present in the kaolin mineral. The endothermic peak experienced at 140 °C for beneficiated and acid leached kaolin was corresponding to hygroscopic water loss (figure 5(b)). The endothermic peak experienced at 504°C for raw, beneficiated and acid leached, and commercial kaolin was corresponding to organic matter decomposition and the transformation of kaolin to metakaolin caused by the dehydroxylation of a coordinated and structural water molecule [61, 71]. The absence of any major endothermic peak at 504 °C for beneficiated, acid leached, and calcinated kaolin confirmed that almost complete removal of structural water could be achieved in beneficiated, acid leached, and calcinated kaolin sample (figure 5(c)). All samples exhibit an exothermic peak at 1003 °C–1013 °C in the DSC pattern due to an amorphous transformation and formation of spinel and/ or mullite in the minerals (figures 5(a)–(d)). This transformation doesn't engage in any mass loss. Similar results were obtained in their studies with kaolin [61, 68, 69, 71].

FTIR was used to identify the crystal structures, functional groups existing in the samples, and other structural defects. FTIR spectra of the raw, beneficiated and acid leached, beneficiated, acid leached and calcined, and commercial kaolin are illustrated in figures 6(a)–(d). The peaks of the raw, beneficiated, and acid leached, and commercial kaolin clearly shows a sharpness at those bands but the beneficiated, acid leached and calcined kaolin exhibit an insignificant peak. Bands at 763 and 690 were related to the perpendicular vibrations of Si-OH [72]. A band at 910 cm⁻¹ was attributed to OH deformation linked to iron cation (Fe³⁺) and aluminum cation (Al³⁺) [60].

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Bands at 1114 cm⁻¹ and 1018 cm⁻¹ were attributed to the stretching vibration of Si-O and/or Al-O and the symmetric stretching vibrations of Si-O-Si bonds. This confirms that the studied kaolin contained a high proportion of silicon oxide. The spectra of the beneficiated and acid leached kaolin (figure 6(b)) show the new small intensity peaks at 1634 cm⁻¹, comparing to the spectrum of the raw, beneficiated, acid leached and calcined, and commercial kaolin which could be described as H-O-H bending modes of water [73]. Bands at 3620 cm⁻¹ and 3693 cm⁻¹ in raw, beneficiated, and acid leached, and commercial kaolin was assigned to the OH stretching of water [74]. Beneficiated, acid leached and calcined do not contain OH⁻ and Al-O because of the loss of the structural hydroxyl group due to the calcination process (calcination leads to the dehydroxylation at temperatures in the range of 400 °C–800 °C), while Si-O bands appeared during the calcination process (figure 6(c)). The FTIR result shows that thermal treatment was adequate to convert kaolin to metakaolin. The formation of metakaolin can be inferred by the disappearance of multiple bands in the FTIR spectrum of kaolin in the wavenumber range of 3600–3700 cm⁻1. Beneficiated, acid-leached and calcined kaolin has enhanced percent transmittance than the raw and commercial kaolin.

The scanning electron micrographs of the different kaolin samples are presented in figures 7(a)–(d). The SEM image (figure 7(a)) reveals the presence of large particles that appeared to have been formed by several flaky particles stacked together to form a compact arrangement in hexagonal shapes, irregular bulk edges, and surfaces, and flattened platelets structure of kaolinite [3, 18, 21, 32, 34, 75]. In comparison to the commercial kaolin clay (figure 7(d)), the beneficiated and acid leached product (figure 7(b)) was similar morphological features. After oxalic acid leaching and calcination figure 7(c), the platelet structure/larger particles of the kaolin were broken down, disaggregated, and produced a more porous structure which caused an increase in surface area (table 1). Moreover, due to the removal of the impurities from raw kaolin, more distribution, fragmentation, and fewer aggregations occurred during beneficiation. This results in the formation of a more porous structure. it is also evident that the surface of beneficiated, acid-leached, and calcinated kaolin has homogenous and clear particles with a small flake shape. A similar observation was reported in the literature [18, 32, 34, 61, 75, 76] for kaolinite minerals.

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The effect of oxalic acid concentrations (0.1 M to 2.0 M) at different leaching times on iron removal from the hydrous kaolin is shown in figure 8. The dissolution curves appear to be parabolic as oxalic acid concentration increases from 0.5 M to 2.0 M. It can also be observed that iron dissolution was very slow at an oxalic acid concentration of 0.1 M. The maximum removal of iron at different concentrations of oxalic acid, i.e., 0.1 M, 0.5 M, 1 M, 1.5 M, and 2 M at 120 °C for 120 min was 27.68 ± 0.74, 77.65 ± 0.5, 73.49 ± 0.2, 83.02 ± 0.1 and 84.83 ± 0.24%, respectively. As summarized in figure 8, the minimum and maximum iron ore dissolution are 19.97 ± 0.4% and 84.83 ± 0.24% at a contact time of 120 min, leaching temperature of 120 °C for an oxalic acid concentration of 0.1 M and 2.0 M, respectively. Thus, 2.0 M oxalic acid was to be the maximum concentration for leaching of iron from hydrous kaolin. This suggests that the oxides, hydroxides, and hydrated oxides of total iron impurities associated with kaolin clay are easily solubilized in oxalic acid. According to the results of the present study, it can be concluded that the removal of iron increased by increasing the reaction time and oxalic acid concentration, indicating that both parameters play a major role in the dissolution of iron. Several studies also observed a similar trend during the removal of iron from kaolin clay [11], and other clay materials [23, 77, 78].

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From figure 9, it can be observed that the removal of iron increased with leaching temperature and reaction time, indicating that both parameters play a major role in the extraction of iron. As can be seen in the figure, the maximum removal efficiency of iron is found to be 46.47 ± 0.31%, 62.91 ± 0.1%, 74.69 ± 0.8%, and 84.83 ± 0.24% at a temperature of 80, 90, 100, and 120 °C, respectively. This shows a marginal increase in the amount of the hydrous kaolin reacted with leaching time within the studied temperature ranges (80 °C–120 °C). It is also indicated that iron dissolution using oxalic acid could be thermally activated to improve its removal efficiency. Therefore, a higher leaching temperature and longer time enhance iron dissolution due to the enhancement in diffusion rate or chemical reaction. Several studies also observed a similar trend during the removal of iron from kaolin clay [11, 43], and other clay materials [23, 77, 78].

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