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The diatomite powder purchased from Linjiang Imerys Diatomite Company Co., Ltd. China was used to prepare porous diatomite ceramic and the preparation process was as follows: First, diatomite powder was mixed with PEG (polyethylene glycol 4,000; 18.6 wt.%) for 10 minutes using a stirrer; second, the diatomite slurry was distributed on a plate then oven dried at 60°C for 1.5 hours; third, the dried diatomite was gridded into powder, and 3.0 g dried power was injected into a square mold (35 × 35 mm) then pressed at different pressure of 2, 5 and 8 MPa; finally, the prepared sample was heated in air to 600°C at a heating rate of 10°C/min for 2 hours to remove the PEG binder, then the binder-free sample was subsequently sintered at a temperature range of 900 to 1,400°C with a heating rate of 30°C/min for 1 hour.

Prepared porous diatomite ceramic filter (PDCF) was modified with H₃PO₄ (85.0%) and NaOH (98.0%) through the following steps: First, two different catalyst solutions were prepared by mixing 40 wt.% catalyst (NaOH or H₃PO₄) with 20 wt.% distilled water and 40 wt.% binder. Second, the prepared PDCF sample was immersed into catalyst solutions for 2 minutes then naturally dried for 24 hours. The modified samples were defined as H₃PO₄/PDCF (for NH₃ adsorption)and NaOH/PDCF (for H₂S adsorption), respectively. Prior to adsorption experiment, the dried sample was oven dried again at an optimized temperature of 90°C for 20 minutes.¹²⁾

An enclosed chamber (0.1 m³) was designed as experimental apparatus as shown in Fig. 1(a). 0.5 litter of H₂S (H₂S 4.02% and N2 balance) gas was injected into the chamber to set up the initial concentration of 200 ppm for the adsorption experiment by NaOH/PDCF sample. When it comes to NH₃, a certain amount of NH₃ (28.0~30.0%, 48.8 μL) liquid was put on the hot pad and heated them for 15 minutes to have an initial concentration of 200 ppm for the adsorption experiment by H₃PO₄/PDCF sample. Then, the electric fan was operated at a flow velocity of 4.8 m/s. The concentration of toxic gases was measured at 30 min, 60 min and 120 min using a syringe type detector with a gas tube as shown in Fig. 1(b).

Fig. 1 
Schematic diagram of (a) the experimental apparatus, and (b) syringe type detector

XRF (X-Ray Fluorescence Spectrometer, SHIMADZU (Japan), XRF-1700) was used to identify the chemical composition of diatomite. XRD ((X-Ray Diffractometer, Rigaku (Japan), Ultima IV) was used to analyze the crystal strucure of raw diatomite. FE-SEM (Field Emission Scanning Electron Microscope, TESCAN (Czech), MIRA 3 LMH In-Beam Detector) was employed to observe the surface morphology of raw diatomite and PDCF samples. Thermo Gravimetric Analyzer (Perkin Elmer (U.S.A), TGA 7, Pyris 1) was employed to perform the thermal gravimetric analysis of raw diatomite. EDS (Energy Dispersive X-ray, TESCAN(Czech), VEGA II LSU) was used to determin the key element of catalyst and adsorbed air pollutants. The compression strength of sintered samples was determined by using universal testing machine at a rate of 0.5 mm/min. The density of sintered samples was determined by calculating the dimensional volume and mass of samples. All experiments and analyses were performed at least twice and the averages were reported in the results section.

Table 1 shows the chemical composition of raw diatomite. As expected, diatomite mainly comprises of silica (92.43%) as well as other metal oxides. According to Martinoyic et al.,¹³⁾ if the silica (SiO₂) content is greater than 85%, the diatomite is considered to be a high-purity diatomite, suitable for development of a commercially available celite filter. Fig. 2(a) shows the FE-SEM image of diatomite and it is clearly that the diatomite consists mainly of plates and broken particles, and the plate exhibits an inherently highly porous structure. The information regarding density and textural properties of the diatomite powder is provided in Table 2. TGA curve in Fig. 2(b) shows that no significant change in the weight loss was found, which indicated that diatomite does not contain volatile impurities. In addition, The XRD pattern of diatomite in Fig. 2(c) shows that the diatomite is mainly composed of cristobalite. High-temperature calcination treatment is able to transform the original amorphous silica of diatomite into cristobalite without significant shape change. Due to its excellent chemical and thermal stability, cristobalite is superior to amorphous silica in many applications.¹⁴⁾

Fig. 2 
(a) the FE-SEM image of as-received diatomite, (b) the TGA curve of diatomite, and (c) X-ray diffraction pattern of diatomite

As can be seen from Fig. 3(a), as the temperature increased, the density of PDCF increased, but the density suddenly increased greatly since 1,200°C. This was due to the low temperature (<1,200°C) can not cause the sintering, thereby it still retained inherently highly porous structure of diatomite as shown in Fig. 4(I)(a), and (b). When the temperature increased to 1,100°C, the diatomite particle began to partially melt, and the liquid phase accumulated (formation of cristobalite) in this region as shown in Fig. 4(I)(c). Furthermore, with the increase of temperature, the liquid phase continued to expand until it completely covered the surface of diatomite ceramic at 1,400°C as shown in Fig. 4(I)(d), (e) and (f). This indicated that the sintering of diatomite occurred at around 1,200°C, and the sharp condensation of diatomite particle during sintering caused a significant increase in density. Roccket et al.¹⁵⁾ reported that the formation of cristobalite was occurred at about 1,500°C. But, in this study, the sintering temperature was found to be around 1,100~1,200°C, which is less than 1,500°C. This could be due to impurities (such as Na₂O, K₂O, Al₂O₃, etc.) promoted low temperature eutectic reactions and the formation of liquid phase. A similar phenomenon was also found in other studies,^16,17) in which it was reported that the eutectic reaction of diatomite sintering occurred at around 1,150°C (close to 1,100°C in this study). This was further confirmed by the EDS analysis (Fig. 4(II)), in which the presence of peaks of Na and Al at 1,100°C demonstrated that the metal oxides did facilitate the low temperature eutectic reactions. On the other hand, a result similar to density behaviour was obtained for the variation of compression strength as a function of temperature as shown in Fig. 3(b). That is, with the increase of temperature, the compression strength increased. In addition, it can be seen that the modeling pressure had a light effect on the density, ie. the greater the modeling pressure, the greater the density as shown in Fig. 3(a). no significant changes in compression strength as a function of modeling pressure was found as shown in Fig. 3(b).

Fig. 3 
Density and compressive strength of diatomite ceramic filter as a function of sintering temperature, (a) Density and (b) Compression strength.

Fig. 4 
(I) of FE-SEM images of the fractured surfaces of the diatomite ceramic filter at different sintering temperatures of: (a) 900°C, (b) 1,000°C, (c) 1,100°C, (d) 1,200°C, (e) 1,300°C and (f) 1,400°C, and (II) of sintered diatomite ceramic filter at a molding pressure of 2 MPa and sintering temperature of 1,100°C.

Fig. 5 shows the adsorption results of H₂S and NH₃ by diatomite ceramic filter. It can be seen that as the sintering temperature increased, the adsorption rate decreased, and only NaOH/PDCF at 1,000°C and 1,100°C can completely remove H₂S within 120 min as shown in Fig. 5(a). Similar results were also found for adsorption of NH₃ by H₃PO₄/PDCF as shown in Fig. 5(b). This was due to the lower the sintering temperature, the lower the density, meaning the more porosity was retained. This was consistent with the effect of temperature on the density above mentioned. However, the lower the sintering termperature, the lower the compression strength. Akhtar et al.¹⁸⁾ reported that if the compressive strength of diatomite ceramic is 18 MPa, that is sufficient for water and air filtration. Therefore, considering the adsorption performance and compression strength, the diatomite ceramic filter sintered at 1,100°C was regarded as optimized sample due to its high compressive strength (around 20 MPa see Fig. 3(b)) and good adsorption performance.

Fig. 5 
Adsorption results of (a) H₂S by NaOH/ PDCR, and (b) NH₃ by H₃PO₄/NaOH

In order to investigate the effect of NaOH and H₃PO₄ on the adsorption performance of H₂S and NH₃, SEM-EDS analysis was performed to observe the surface of PDCF modified with NaOH and H₃PO₄, and identify the adsorbed air pollutant molecules. Based on the above analysis, PDCF at 1,100°C was selected as a sample for SEM-EDS analysis as shown in Fig. 6(a), and the bimodal porous structure of inter-particle pores and intra-particle pores was obviously observed, which is beneficial for the adsorption. After modification with NaOH and H₃PO₄, the peaks of Na and P were determined, indicating the presence of NaOH and H₃PO₄ on the DPCF as shown in Fig. 6(b) and (d). Acid or base leaching led to the dissolution and erosion of diatomite particle, producing a large number of reactive sites and functional groups and making the surface very rough and irregular, increasing surface area as shown in Fig. 6(b) and (d). Similar foundlings were found in other study, ⁹⁾ which reported that chemical activation can alter surface properties, such as increasing surface roughness and irregularities, and creating more new pores, increasing their pore properties and surface area. After adsorption of H₂S and NH₃, a peak of "S" (around 2 wt.%) was identified as shown in Fig. 6(c), showing the presence of adsorbed H₂S on NaOH/PDFC. Correspondingly, the bimodal porous structure was covered obviously by air pollutant molecules on the SEM image as shown Fig. 6(c). Similar phenomenon was also found in the SEM image of Fig. 6(e), which indicated that the NH₃ molecules were adsorbed and attached on the surface of H₃PO₄/PDCF.

Fig. 6 
SEM-EDS of (a) PDCF at 1,100°C, (b) NaOH/PDCF, (c) H₂S adsorbed NaOH/PDCF, (d) H₃PO₄/PDCF and (e) NH₃ adsorbed H₃PO₄/PDCF