7.3. Results and Discussion
7.3.1 Ceramics synthesis and their properties
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167 7.2.4. TCLP test
The TCLP tests were conducted in accordance with EPA Method 1311 [24] to evaluate the leachability of selenate from the synthesized ceramics. The ceramic samples which were calcined at various temperatures were ground (˂ 1 mm). According to the TCLP, the ceramic samples were extracted with a leachate-to-solid mass ratio of 20:1 at pH 4.93 containing 0.10 M acetic acid and 0.064 M NaOH. After 18 h of leaching, the extracted solution was separated from the solids by filtration and supplied for determination of selenate by an inductively coupled plasma optical emission spectrometer (ICP-OES) (Perkin Elmer, Optima 8300, US). The chemical stability of the selenate-containing ceramics (800 °C) was examined separately using water with pH adjusted to 2.00 ± 0.01, 4.00 ± 0.01, 6.00 ± 0.01, 8.00 ± 0.01, 10.00 ± 0.01, and 12.00 ± 0.01 with 0.1 M H2SO4 or NaOH. The experimental procedures were performed in the exactly same manner as the procedures described above.
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wt % of SiO2, and the other 2 wt% was K2O, CaO, Fe2O3, P2O5, ZnO and other compounds.
Table 7.1 Chemical composition (wt%)and specific density, d (g/cm3) of granular blast furnace slag (GBFS) and silica fume (SF) determined by XRF
Sample MgO Al2O3 SiO2 P2O5 SO3 K2O CaO MnO Fe2O3 ZnO
GBFS 6.09 14.70 33.90 0.02 2.42 0.33 42.60 0.31 0.39 –
SF – 0.56 98.70 0.39 0.06 0.02 0.04 – 0.19 0.06
The XRD patterns of the GBFS and SF are shown in Fig. 7.1, and both raw materials appeared to be amorphous. However, some impurity of calcium zirconium oxide (CaZrO3, PDF# 01-078-3350) was found in SF. The XRD pattern for the synthesized selenate-ettringite was identified (Ca6Al2(SeO4)3(OH)12·26H2O, PDF# 00-042-0224), and no other phases were observed. In addition, the SEM images of GBFS and SF were considerably different, as shown in Fig. 7.2. No distinguishable morphology was observed in GBFS. By contrast, SF showed approximately 0.5-2 µm regular spherical particles.
To determine whether selenate could be immobilized in the synthesized ceramics after calcination, the thermal stabilities of pure sulfate ettringite and pure selenate ettringite were compared. Fig. 7.3 (a) and (b) depicted the TG curves of pure sulfate ettringite and pure selenate ettringite. In previous reports, the mass loss of pure ettringite was attributed to water loss when the temperature was lower than 800 °C [4, 25]. For pure ettringite, most of the mass loss occurred in the range of 90 to 108 °C. Approximately 33% of the mass loss corresponded to a loss of freely associated H2O.
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5 10 15 20 25 30 35 40 45 50 55 60 65 70
(111) 60.02o
(112) 50.25o
(101) 30.20o CaZrO 3
(110) 15.56o (114) 22.75o CaZrO3
GBFS slag Silica fume
Intensity/a.u.
Diffraction angle, 2 [Cu K/degree CaZrO 3
Pure selenate ettringite
(100) 8.98o
Fig. 7.1 XRD patterns of raw materials used for the production of ceramics. Numbers indicate the miller index and 2θ of the main diffraction peaks of the confirmed phases.
Fig. 7.2 SEM images of (a) silica fume, and (c) GBFS. The squares in (a) and (c) are expanded in (b) and (d).
(b)
5 µm 2 µm
5 µm 2 µm
(d) (a)
(c)
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0 200 400 600 800 1000 1200
50 60 70 80 90 100
(a) TG
DTG (ug/s)
Weight loss (%)
Temperature (oC) DTG
-10 -8 -6 -4 -2 0
0 200 400 600 800 1000 1200
20 30 40 50 60 70 80 90 100
DTG
DTG (ug/s)
Weight loss (%)
Temperature (oC)
-8 -6 -4 -2 0
(b)
TG
1040 o C 940 o C
Fig. 7.3 Thermogravimetric analysis of (a) pure ettringite and (b) pure selenate ettringite.
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Fig. 7.4 Photographs of calcined products at various temperatures.
Approximately 46% of the mass loss occurred between 108 to 800 °C; this should have resulted from the removed H2O molecules associated with the minerals [25] (Fig. 7.3(a)).
However, pure selenate ettringite exhibited characteristics similar to those of pure sulfate ettringite until the temperature reached 940 °C, as shown in Fig. 3(b). As the temperature increased, selenate began to evaporate, which resulted in an approximate 30% mass loss.
Thus, the calcination temperatures should be maintained at temperatures lower than 940 °C to stabilize selenate in ceramics.
The mixtures were calcined at various temperatures to optimize the ideal synthetic conditions of ceramics. Photographs of the tablets of calcined products at various temperatures are shown in Fig. 7.4. The color of the tablets changed from white to light yellow up to 600 °C. As the calcination temperature increased, the color of the tablets became dark brown at higher than 800 °C. This color change is related to crystallization of colorless phases in the black colored glass matrix, which then causes light scattering at the grain
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172 boundaries [13].
The XRD patterns of the calcined samples are shown in Fig. 7.5. Crystallization was observed at calcination temperatures higher than 800 °C. In addition, gehlenite (Ca2Al2SiO7, PDF# 01-076-7534) was observed as the major phase with pseudo-wollastonite (CaSiO3, PDF# 01-074-0874) and nepheline (NaAlSiO4, PDF# 01-083-2279) as an impurity (Fig.
7.5(a)). Rietveld refinement was performed to quantify the crystal phases in synthesized glass ceramic (800 oC) and the glass phase was quantified by JADE 6.5 peak fitting. After Rietveld refinement calculation, the weight fraction of gehlenite, pseudo-wollastonite, nepheline, and silicon dioxide is 51.8%, 20.4%, 20.5%, and 7.3%, respectively (Fig. 5(b)). Based on the result of JADE 6.5 peak fitting, the crystallinity of synthesized ceramic (800 oC) is 48.36%, indicating the amorphous glass phase is 51.64%.
Fig. 6 illustrates the SEM images of the calcined mixture at 200 °C and 800 °C. The calcined product at 200 °C exhibited some amorphous and regular spherical particles (Fig. 6 (a)). This result could be in good agreement with the morphology of the raw materials described in the above. Since the mixture primarily consists of GBFS and SF, the morphology of the GBFS and SF is not clearly changed after calcination at 200 °C. By contrast, when the mixture was calcined at 800 °C, the morphology of the mixture was significantly changed to be compacted with a smooth surface.
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173 5 10 15 20 25 30 35 40 45 50 55 60 65 70
(202) 29.07o
(002) 22.12o(200) 20.88o
(060) 45.90o
• (-204)
31.75o
(-202) 27.51o (201) 29.11o (312) 52.10o
(211) 31.38o
uncalcined 200 oC 400 oC 600 oC 800 oC 900 oC
º º
ººº
ºº
ººº º
º
º º º º
º •
Psedowollastonite (CaSiO
3) º Gehlenite (Ca2Al2SiO7)
Intensity/a.u.
Diffraction angle, 2[Cu K /degree
• (a)
Nepheline (NaAlSiO4)
5 10 15 20 25 30 35 40 45 50 55 60 65 70 800 oC
Intensity/a.u.
Diffraction angle, 2[Cu K /degree
Observed pattern Calculated pattern Background Residue
SiO2 Wt. frac. 7.3%
NaAlSiO
4 Wt. frac. 20.5%
CaSiO3 Wt. frac. 20.4%
Ca2Al
2SiO
7 Wt. frac. 51.8%
Rwp 5.93%
(b)
Fig. 7.5 (a) XRD patterns of calcined products at various temperatures. Numbers indicate the miller index and 2θ of the main diffraction peaks of the confirmed phases. (b) Rietveld refinement of calcined products at 800 °C.
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Fig. 7.6 SEM images of mixtures calcined at (a) 200 °C and (c) 800 °C. The squares in (a) and (c) are expanded in (b) and (d).
0 100 200 300 400 500 600 700 800 900 1000 80
82 84 86 88 90 92 94 96 98 100
DTA (uV)
Weight loss (%)
Temperature (oC)
-8 -4 0 4 8 12 16 20 24 28 -1.1%
708o C
exothermic
570o C
endothermic
DTA -2.6%
endothermic 94oC TG
Fig. 7.7 Thermogravimetric analysis of the GBFS and SF mixture without selenate ettringite.
5 µm 2 µm
5 µm 2 µm
(a) (b)
(d)
(c)
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To elaborate further on the synthetic processes of ceramics, the TG and DTA curves of the pure GBFS and SF are shown in Fig. 7.7. It is worth noting that the weight loss of ettringite under high temperature could attribute to water loss and this would affect the TG-DTA curves [4, 25]. It is better to understand the ceramics formation process without the effect of ettringite dehydration. Thus, we chose the raw material without loading selenate-substituted ettringite to interpret ceramics formation process. The TG curves exhibited a 3.7 % mass loss when the temperature was increased to 400 °C. Endothermic peaks were observed that resulted from water loss (2.6 %) and evaporation (1.1 %) of thermally unstable components (P4O10) from 94 to 450 °C. As mentioned in the previous reports, SF and GBFS both have a high specific surface area [11] and contain some water-absorbing components, such as CaO and MgO (Table 7.1), which easily absorb water from the air. Since the temperature was increased, desorption of the adsorbed water in SF and GBFS occurred in the temperature range of room temperature to 250 °C, resulting in a 2.6 % mass loss. The remaining 1.1 % mass loss in the material could be attributed to the decomposition of the low thermal-stability compounds (e.g., P4O10) found in GBFS and SF (Table. 7.1). One endothermic peak that appeared at approximately 570 °C resulted from the melting of the mixture. This result is consistent with the color change of the mixed pellet at 600 °C (Fig. 7.4). The exothermic peak observed from approximately 708 °C corresponded to the crystallization of the mixture. This result was coincident with the XRD patterns (Fig. 7.5) in which gehlenite and
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pseudo-wollastonite were identified after calcination at 800 °C, indicating glass ceramics was formed under this temperature.
The total and leached concentrations of selenate in the calcined products were plotted against the calcination temperature, as shown in Fig. 7.8. The digestion experiment revealed the total concentrations of selenate in the mixture did not decrease, although the mixture was calcined at 900 °C. This result is consistent with the TG analysis (Fig. 7.3), which indicated that selenate in ettringite had not been evaporated during heat treatment at temperatures lower than 940 °C. However, mixtures heat-treated at various temperatures exhibited different leaching characteristics. As shown in Fig. 7.8, approximately 300 mg/L of selenate was leached from the mixture when the calcination temperature was lower than 600 °C.
0 200 400 600 800 1000
0 50 100 150 200 250 300 350 400 450 500
Total and leached concentrations of Se (mg/kg)
Temperature (oC) Total
Leached
Fig. 7.8 Total and leached amounts of Se from heated ceramics depending on the calcination temperatures.
Based on the XRD (Fig. 7.5) and TG results (Fig. 7.7), it was concluded that the water
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molecules in the material desorbed from the amorphous raw material (GBFS, SF) during calcination. By increasing the temperature to 600 °C, GBFS and SF began to melt, as described in above. In addition, ettringite was transformed to meta-ettringite (dehydrated ettringite) at high temperatures [26]. It can be assumed that the melted GBFS and SF partially covered the surface of the meta-selenate-ettringite, which prohibited leaching of selenate from the mixture that was calcined at 600 °C. In the ceramics, it was found that the concentration of selenate that leached from the ceramics was approximately 150 mg/L.
Moreover, when the calcination temperature was increased to 800 °C, approximately 0.1 mg/L selenate was leached out from the ceramics material. As shown in Fig. 7.5, the mixture began to crystallize at 800 °C and the formation of gehlenite and pseudo-wollastonite stabilized selenate uptaking absorbent in the glass ceramics, resulting in higher stability.