Preparing high purity white carbon black from rice husk

Abstract In this paper, rice husk (RH) was used as raw material to prepare white carbon black, and the key technological parameters of preparing white carbon black from RH were studied through single‐factor test, orthogonal experiment, and response surface analysis. Meanwhile, the characteristic of white carbon black was also analyzed. Through orthogonal experiment analysis, it was confirmed that the order of factors affecting the purity of white carbon black was calcination temperature > alkali treatment time > final pH > surfactant. Based on the response surface optimization analysis, the optimum parameters for preparation of white carbon black were as follows: calcination temperature 610°C, alkali treatment time of 2.3 hr, final pH of 10, CTMAB was used as the surfactant. Under this condition, the purity of silica prepared could be reached to 99.39%, and the particle size was uniform, spherical, and well dispersed, which satisfied the requirements of GB/T 34698‐2017 standard.

itation synthesis method in China because it is easy to control size, shape, and purity of the material (Fan, ; Xu et al., 2014;Yuvakkumar, Elango, Rajendran, & Kannan, 2014). Gao, Song, Yang, and Wu (2016) studied the preparation of white carbon black from water-quenching blast furnace slag based on acid precipitation. The results showed that the quality of SiO 2 in the product was increased from 84.43% to 91.32% after hydrochloric acid was used. Xu et al. (2014 prepared white carbon black from nano-husk ash by analkali dissolving-acid reaction method and investigated the effects of the pH and reaction time on the purity of the white carbon black. The results showed that the purity of white carbon black could reach to 98.48% when the NaOH solution with the RH ash was heated for 2 hr at 90°C and then finished its acid reaction at pH of 3 for 2 hr. Compared with acid precipitation method, alkali fusion method (involved dissolution of silica in NaOH solution followed by precipitation using acid solutions) was widely used. Shelke, Bhagade, and Mandavgane (2011) produced

| Preparation of white carbon black
The basic process was shown in Figure 1, and the process was mainly divided into four steps.
1. The pretreated RHs were firstly calcined at 500, 600, 700, and 800°C respectively to obtain RH ash. The RH ash was crushed and evenly mixed for further use.
2. A total of 2.0 g of carbonized rice hulls with 40 ml 10% NaOH solution were put into 100 ml three-flask and stirred by a magnetic stirring heater at 600 r/min under 120°C. The alkali treatment time was set as 1-4 hr to investigate the influence of alkali treatment time on the preparation process. After that, the mixture was washed and filtered. The residues were evaporated to 40 ml in a beaker at 80°C to obtain water glass solution.
3. To investigate the influence of surfactant on the content of SiO 2 and particle morphology of white carbon black, 0.4 g PEG-6000 (Polyethylene glycol 6000), SDS (sodium dodecyl sulfate), and CTMAB (cetyltrimethylammonium bromide) were added to water glass solution, respectively. Meanwhile, a blank control experiment was conducted. Then, the water glass solution was stirred on a magnetic stirring heater with a fixed reaction temperature 80°C at a speed of 600 r/min. 10% H 2 SO 4 solution was intermittent added to the water glass solution until the white precipitate was slowly appeared. To investigate the influence of final pH on the particle size and purity of white carbon black, the final pH of reaction was regulated at 7-10. 4. After 2 hr reaction, the heating and stirring were stopped and the solutions were placed for 1 hr aging. Then, the precipitate was washed until the filtrate was neutral. Finally, the washed precipitate was drying in the drying oven and the product of white carbon black was obtained.

| Analysis
The chemical composition of white carbon black was determined by X-ray fluorescence (XRF). The morphology and the size of the samples were examined by S-3000N scanning electron microscope (SEM). The spectra of the samples were recorded by using fourier transform infraredspectroscopy (FTIR, Nicolet 6700). Spectra were obtained from 4000 and 400 cm −1 . The characteristic of white carbon black was eval-

| Orthogonal experiment
In order to verify the accuracy of single-factor experimental results and to confirm the influence of various factors on the content of SiO 2 in white carbon black, the orthogonal experiment was designed. Four factors including calcination temperature (A), alkali treatment time (B), final pH (C), and surfactant (D) were selected. Three levels were selected for each factor based on the yield and the purity of white carbon black in the single-factor experiment. Orthogonal experiment was designed by using L 9 (3 4 ) orthogonal table, which could be seen in Table 1.  Table 2.

| The effect of calcination temperature, alkali treatment time, final pH, and surfactant on the content of SiO 2 in white carbon black
The effect of calcination temperature on the content of SiO 2 in white carbon black was shown in Figure 2a. The calcination temperature was an important factor in the process of preparation of white carbon black, which was related to the properties of the final product like content of SiO 2 . The impurity content could be significantly reduced by thermal decomposition process. The metals in the RH were probably carried out from the volatiles during thermal decomposition (Liou, 2004). With the increase of the calcination temperature, the content of SiO 2 was decreased. The highest content of SiO 2 in white carbon black was appeared at 500°C. When the calcination temperature was in the range of 500-700°C, there was not obvious difference on the purity of white carbon black, while the purity of white carbon black was decreased rapidly when the temperature was over 700°C. The RH was burned completely, the content of porous carbon in RH ash was very low, impurities could not be adsorbed, and the purity of white carbon black products was relatively low. Meanwhile, when the temperature was above 700°C, amorphous silica was also transformed into the undesirable crystalline forms, such as crystobalite or tridymite, which was not benefit for synthesis into other products (Wattanasiriwech, Wattanasiriwech, & Svasti, 2010). However, due to the incomplete calcination of RH at 500°C, there was a lot of carbon left. Considering the energy consumption, purity and yield of white carbon black, the calcining temperature of RH was chosen at 600°C, which was in the range of 500-700°C taken by many researchers (Ping & Hsieh, 2012;Xu et al., 2014;Yuvakkumar et al., 2014).
The effect of alkali treatment time on the content of SiO 2 in white carbon black was shown in Figure 2b. Water-soluble sodium silicate was obtained by reacting with sodium hydroxide according to the Equation (1). When the alkali treatment time was less than 2 hr, the purity of white carbon black products was displayed as a rising trend and reached the maximum value at 2 hr. When the alkali treatment time was longer than 2 hr, the purity of white carbon black decreased. Longer alkali treatment time resulted in high concentration of silicate and promoting the growth and reunion of silicate nuclear, which would catch up more metal impurity and SO 4 2− during the acid process. Meanwhile, higher concentration of the silicate caused violent particle collisions and fast precipitation polymerization, which resulted in higher content of impurities in silica products. However, when the alkali treatment time was greater than 3 hr, impurities such as SiO 2 and metals were  The effect of final pH on the content of SiO 2 in white carbon black was shown in Figure 2d. Silica was gradually precipitated to form a gel by sulfuric acid solution at pH 7 (Equation (2)).The purity of white carbon black was increased gradually with the rising of the final pH. Lower final pH needs more sulfuric acid, which caused higher concentration of H + and faster precipitation polymerization speed. Meanwhile, the impurities were easier to be trapped and packaged resulting in lower (1) SiO 2 + 2NaOH → Na 2 SiO 3 + H 2 O F I G U R E 2 The effect of calcination temperature (a), alkali treatment time (b), surfactant (c), and final pH (d) on the purity of white carbon black purity of white carbon black. While when the final pH was more than 11, the yield of white carbon black was decreased. Tzong-Horng Liou investigated effect of gelation pH and found that silica yields were increased with the rising of pH, with a maximum yield occurring at around pH 7. While when the pH was 11, these gels were almost dissolved in water (Liou & Yang, 2011). Considering the yield and purity of white carbon black, the final pH value was better to controlled at 10.

| Results of orthogonal experiment
The results of orthogonal experiment were shown in Table 3.
The range analysis showed that the orders of factors affecting the purity of white carbon black were A > B>C > D, that is, the calcination temperature > alkali treatment time > final pH > surfactants. Meanwhile, the results of variance analysis showed that calcination temperature, alkali treatment time, and final pH made significant influence on the purity of white carbon black, while the type of surfactant had not significant effect on white carbon black purity.
According to the results of orthogonal experimental, the best technological condition for preparing white carbon black from RHs was A1B1C3, that is, the calcination temperature was 600°C, the alkali treatment time was 2 hr, the final pH was 10, and the purity was the highest, which was consistent with those results of single-factor experiment.

| Results of response surface analysis
The interactions among calcination temperature, alkali treatment time and final pH were shown in Figure 4a-c. When C was fixed, the content of SiO 2 in white carbon black was decreased with the rising of A and was first increased and then decreased with the rising of B (Figure 4a).When B was fixed, the purity of white carbon black was decreased with the rising of A and was increased with the rising of C ( Figure 4b).When A was fixed, the purity of white carbon black was first increased and then decreased with B, but was increased with the rising of C (Figure 4c). This was consistent with the results of single-factor experiment.
The results of response surface analysis were displayed in Table 4 and Table 5. It could be seen that the model was significant (2) Na 2 SiO 3 + H 2 SO 4 → SiO 2 ⋅ H 2 O + Na 2 SO 4 F I G U R E 3 SEM micrographs of surfactants on the particle size and dispersion of white carbon black The process parameters of preparing white carbon black from RH were optimized and analyzed by Design-expert 10.0.4 software:

TA B L E 3 Code and level of factors
the calcination temperature was 612°C, the alkali treatment time was 2.29 hr, the final pH was 9.99, and the maximum purity of white carbon black could be reached to 99.94%.

| Characteristic of white carbon black
FTIR spectra of white carbon black were shown in Figure 5.   (Zhi & Guo, 2013). It indicated that the silica prepared in this experiment was amorphous hydrated silica with high purity and very low impurity content.  Figure 6, which was small and evenly distributed.
The silica content of all samples was higher than 90%, which satisfied the standards and market quality requirements.

ACK N OWLED G EM ENT
The

CO N FLI C T O F I NTE R E S T
The authors declare that we do not have any conflict of interest.

E TH I C A L R E V I E W
This study does not involve any human or animal testing.