1. The preparation of silica white from fly ash and its characterization
Zhitong YAO1, 2, Jinhui LI1, Meisheng XIA2, Ying YE2
1Department of Environmental Science and Engineering, Tsinghua University, Beijing, China
2Department of Ocean Science and Engineering, Zhejiang University, Hangzhou, China
Abstract: The silica white was prepared using fly ash as raw material. It comprised alkaline fusion of fly ash followed by extraction with hydrochloric acid. The acid concentrations, reaction time, S/L ratio and stirring rate on silica extraction efficiency were investigated. The prepared silica white was also characterized by XRD and N2 adsorption-desorption. The results showed that, the extracting rate displayed an increasing trend with the increase of acid concentrations, reaction time and stirring rates. The specific surface area and average pore size of the prepared silica white were 364.8m2/g and 6.02nm, respectively.
Keywords: fly ash; silica white; extraction efficiency; hydrochloric acid
Introduction
Silica white (SiO2·nH2O) is a highly-refined chemical product, with nH2O in the form of surface hydroxyl groups. Its chemical characteristic is stable and possesses excellent properties. Therefore, it has been widely applied in rubberized foot wear, paint, dyes, printing ink, and plastic products. Regarding to the preparation, it is usually prepared by chemical vapor deposition method using silicon tetrachloride as raw material and acid precipitation method using water glass solution and acid as precursors.
Experimental
1. Material and methods
The fly ash was derived from a thermal power plant in Zhejiang province, China. After pretreatment by froth floatation removing the unburned carbon and floating beads, its chemical components were determined. It consists of following oxides (in wt.%): SiO2 50.5, Al2O3 35.9, Fe2O3 5.3, CaO 4.6, TiO2 1.5, K2O 1.3 and others 0.9.
2. Preparation procedure
(1) Activation
The ash was mixed with sodium carbonate anhydrous in the weight ratio 1:0.5 and milled for 2 h. Then, the mixture was calcined in a muffle furnace at 950◦C for 1.5h. Finally, the fused product was cooled to room temperature and crushed into powder.
(2) Silica extraction
In the extracting process, the fused product was added into a three-neck flask fitted with an impeller stirrer. The hydrochloric acid with different concentrations (4.5, 5.0, 5.5, 6.0, 6.5 and 7.0M) were transferred into flask at different S/L (g/ml) ratios (1:4, 1:4.5, 1:5, 1:5.5, 1:6 and 1:7).
The phase analysis was carried out by Rigaku D-Max/IIB diffractometer with Cu Kα radiation operated at 40kV and 30mA. Data collection was carried out in the 2θ range 10~80°. The N2 adsorption measurement was carried out to evaluate the specific surface area and pore volumes. N2 adsorption-desorption isotherms were recorded using a Micromeritics model ASAP 2020 adsorption analyzer.
3. Results and discussion 
1. The effect of HCl concentrations
In order to testing the effect of HCl concentrations (4.0, 5.0, 5.5, 6.0, 6.5 and 7.0M) on silica extraction efficiency, a series of experiments were undertaken. From fig. 1, it can be seen that the extracting rate increased with an increase of hydrochloric acid concentration. As the acid concentration increased from 4.5M to 6.0M, the extraction rate increased significantly from 37% to 76%. However, the extracting rate increased slowly when the acid concentration is greater than 6.0 M. Moreover, it decreased when the acid concentration reached 7.0M.
Characterization and testing
Figure 1. The effect of HCl concentrations on silica extracting rate
(Reaction conditions: S/L ratio: 1/6, reaction time: 90min, stirring rate: 300r/min)
2. The effect of S/L ratio
Fig.2 displayed the effect of S/L ratio on silica extracting rate. On the premise of acid concentration keeping constant, larger S/L ratio can reduce the consumption of acid. However, larger S/L meant the reduction of acid amount in reaction system and this would incur incomplete reaction. Besides, it can result in the increase of slurry viscidity, which went against the reaction.
Figure 2. The effect of S/L ratio on silica extracting rate
(Reaction conditions: HCl concentration: 6.5M, reaction time: 90min, stirring rate: 300r/min) 
3. The effect of stirring rates
Fig.3 illustrated the effect of stirring rates on silica extraction efficiency. It can be seen that, the extracting rate increased at higher stirring rates. This may be attributed that, higher stirring rates can create turbulence, speed the diffuse, enhance mass transfer and then promoted the reaction. However, when the stirring rate is higher than 300r/min, the extracting rate increased slowly. .
Figure 3. The effect of stirring rates on silica extracting rate
(Reaction conditions: HCl concentration: 6.5M, S/L ratio: 1/6, reaction time: 90min)
4. The effect of reaction time 
The effect of reaction time on silica extraction rate was also investigated and the results were showed in Fig.4. It was clear that the extracting rate increased with the extending of time. It tended to increase slowly when the reaction time reached 90min.
Figure 4. The effect of reaction time on silica extracting rate
(Reaction conditions: HCl concentration: 6.5M, S/L ratio: 1/6, stirring rate: 300r/min)
5. XRD analysis
The XRD pattern of silica white obtained was displayed in Fig.5. It was well corresponding to the values in JCPDS for silica white. Besides, there existed a broad hump in the range (2θ) of 15~35°, which indicated that the silica white was amorphous.
Figure 5. The XRD pattern of silica white
6. N2 adsorption-desorption and pore size distribution
Figure 6. N2 adsorption-desorption isotherms of silica white and corresponding pore diameter distribution pattern (inset)
Fig.6 illustrated the N2 adsorption-desorption plots for silica white. The pattern corresponded to the type IV of the BDDT classification[11]. There was a steep increase in the amount of nitrogen adsorbed when the relative pressure (P/P0) was higher than 0.6.
Conclusion
From the experimental results the following conclusions can be drawn:
1) The optimal preparation conditions, such as acid concentration, reaction time and stirring rate were determined as 6.5M, 90min and 300r/min, respectively. The silica extracting rate displayed an increasing trend with the increase of acid concentrations, reaction time and stirring rate. The extraction efficiency decreased with the increase of S/L ratio and the optimal S/L ratio was found to be 1:6.
2) The prepared silica white was amorphous. Its specific surface area and average pore size of were 364.8m2/g and 6.02nm, respectively.
Acknowledgement
Author: Zhitong YAO Department of Environmental Science and Engineering, Tsinghua University.