Pretreatment However, impurities made of quartz and very

Pretreatment of the raw kaolin was carried out by a sedimentation method. The difference in the settling times of the kaolin and the associated impurities forms the basis of a simple mechanism to separate the impurities from the raw kaolin. Sodium hexametaphosphate was used as the soluble dispersing agent. The dispersing agent ionizes to produce cations that are attracted to the negative charges on the clay particles. Each kaolinite plate or stack has a similar charge and thus they repel each other. The settling process during sedimentation allowed the coarse grit particles to settle to the bottom of the beaker. Minerals such as mica, particles of which are flake shaped, do not settle as rapidly as the quartz and heavy minerals. Once the settling product was removed and the supernatant was taken, kaolinite was separated from the major impurities. After the pretreatment process, fine kaolin powder with a high kaolin concentration was produced. However, impurities made of quartz and very small quantities of clay mineral (Johnson et al., 2014) were present in the refined kaolin. Table 1 shows the X-ray fluorescence (XRF) results for the chemical composition of both the raw and refined kaolin. In the XRF analysis, the chemical compounds present in the kaolin and their concentrations were obtained, with the major elements present expressed in the form of their oxides. Figure 1 demonstrates the XRD patterns of the refined kaolin samples.
The kaolin clay undergoes metakaolinization, called dehydroxylation, transforming it into an amorphous, but highly reactive and metastable, phase that a very suitable source for zeolite synthesis (Kovo et al., 2009). According to Chandrasekhar (1996), kaolin was found to have maximum reactivity in terms of zeolite formation when it is calcined at 900 °C for 1 h. Phase change in the sample after the metakaolinization process was verified by using XRD. Formation of metakaolin due to the dehydroxylation reaction was confirmed in the XRD pattern, where the refined kaolin in Figure 2 shows the disappearance of kaolinite peaks. The metakaolinization of kaolin by heating the refined kaolin for 2 h at 900 ºC reassembled all the kaolinite peaks, except for the peaks due to the admixed impurities. Thermal treatment of kaolin causes the kaolinite crystal lattice to collapse and form a highly disordered and amorphous structure. The only crystalline phase in the metakaolin was quartz, because quartz is not affected by the thermal treatment during calcination. The activation of kaolin produces structural changes to this mineral, promoting its reactivity for synthesizing zeolitic materials (Kovo et al., 2009).


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