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Materials 2022, 15, 2946 13 of 20 The EDX results for the red sand (“as received” and after each heating test) are summarized in Table 5. The EDX results show that the red sand (as received) contained more elements than the white sand, indicating the presence of more impurities. Since the red sand had a minor component of ferrous oxide, it may have increased the chances for agglomeration at high temperatures. It was evident that the lesser the unwanted impurities in the silica, the lower the agglomeration would be. Like those for the white sand, the EDX results for the red sand showed a decrease in the O2 weight percentage with temperature. Table 5 also shows the presence of Fe in the red sand, whereas there was no Fe content in the white sand. The Fe disappeared after exposing the sample to cyclic heating at 1200 ◦C for 500 h. This meant that the change in color from red to white may have been due to evaporation of the Fe element in the sample, since Fe is what gave the red color to the sample of red sand. It is also possible that the color change upon heating could have resulted from a decrease in the density of oxygen and the corresponding change in the oxidation state of the metal ions in one or more of the red sand constituents that was not accompanied by a phase change. According to the EDX analysis shown in Table 5, the elements that disappeared in the aged sample were Fe, Ca, Mg, C, and K. On the other hand, only components of silica appeared in the sample after aging. A relevant qualitative observation was that the aged samples of white sand and red sand converted completely to pure silica. Table 5. Elemental composition of red sand. Element 800 ◦C 60.43 0.71 2.59 32.30 0.26 0.71 Red Sand (Weight %) Fresh 1000 ◦C 61.17 0.41 2.26 30.71 0.48 2.01 1200 ◦C 64.12 0.51 1.91 29.96 0.35 0.79 1200 ◦C—500 h 59.58 - 1.80 38.62 - - O 64.99 Mg 0.55 Al 2.14 Si 29.54 K 0.22 Ca 0.42 Ti - - 0.34 - - Fe 2.12 3.00 2.65 2.35 - Total 99.99 100.00 100.03 100.00 100.00 The XRD data for red sand “as received”, as well as for the samples that were exposed to heat treatment, are shown in Figure 7. The XRD results obtained for the samples that were not exposed to cyclic heating showed a clear matching in the peaks, with a small difference in the intensities. This peak intensity may be attributed to the temperature that was applied when heating. All the peaks of the red sand samples matched with the silica pattern, which proved that all the samples represented a strong existence of SiO2. A few peaks were also suppressed in the sample that was exposed to cyclic heating at 1200 ◦C for 500 h. This might have been because the sand initially had some impurities that were removed during the cyclic heating process. Like those of the white sand, the peaks of the samples “as received” and the samples subjected to a heat-treatment test for 6 h were assigned to α-SiO2 as the major phase and potassium aluminum silicate (KAl2O3·SiO2) as the minor phases; while the β-SiO2 was the major phase in the aged sample, and α-SiO2 keatite (SiO2) and aluminum silicate (Al2O3·SiO2) were the minor phases. It was difficult to observe the phases associated with iron oxide, either Fe3O4 or Fe2O3, or even Fe2TiO5, in samples that were not subjected to cyclic heating. This may have been due to the existence of these materials below the detection limit of the XRD.PDF Image | Low-Cost Particulates Used as Energy Storage and Heat-Transfer Medium
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