Synthesis of pure silica from rice husk ash as an anti–caking agent for fertilizer industry In Vietnam - Nguyen Huu Hieu

Journal of Science and Technology 55 (xx) (2017) 193–201 SYNTHESIS OF PURE SILICA FROM RICE HUSK ASH AS AN ANTI–CAKING AGENT FOR FERTILIZER INDUSTRY IN VIETNAM Nguyen Huu Hieu1, 2, *, Tran Manh Hoang2, Pham Minh Hoang2, Tran Thi Thuy Tien2 1Faculty of Chemical Engineering, HCMUT–VNUHCM 268 Ly Thuong Kiet Street, Ward 14, District 10, Ho Chi Minh City, Vietnam 2Key Laboratory of Chemical Engineering and Petroleum Processing, HCMUT–VNUHCM 268 Ly Thuong Kiet Street, Ward 14, District 10, Ho Chi Minh City, Vietnam *Email: nhhieubk@hcmut.edu.vn Received: 30 December 2016; Accepted for publication: 6 March 2017 ABSTRACT Pure silica particles have been successfully synthesized from rice husk ash (RHA) by precipitation method. The characterization of silica was performed by X–ray diffraction, X–ray fluorescence, Fourier transform infrared spectroscopy, Brunauer–Emmett–Teller specific surface area, and transmission electron microscopy. The characterization results show that the obtained amorphous silica was 99.8 % purity with average particle size of 5–10 nm, and high specific surface area of 224 m2/g. Additionally, for the first time that RHA–silica was successfully applied as an anti–caking agent for a NPK–fertilizer. A suitable content of silica was evaluated through the level of anti–caking. The testing results show that with only 1.5 wt% of RHA–silica particles could replace kaolin as an anti–caking agent for the NPK–fertilizer. Keywords: silica, rice husk ash, anti–caking, precipitation, fertilizer. 1. INTRODUCTION Rice husk (RH) is considered as one of agricultural waste materials. According to Mekong Delta Development Research Institute (Vietnam), RH was over 4 million tons every year in Vietnam’s Mekong Delta. Nowadays, the RH was used as burning material for thermal processes in some factories. In 2020, RH will be taken advantage in the thermoelectric industry so that a large amount of rice husk ash (RHA) will be used. However, RHA is one of the most silica–rich raw materials containing over 60 % of silica, a small amount of metallic impurities, and ash of organic components [1]. Silica produced from RHA has microparticle size, exists in amorphous phase and is chemically inert. Because of these features, silica has many applications, such as adsorption Synthesis of pure silica from rice husk ash as an anti–caking agent for fertilizer industry 194 materials [2], catalyst [3], additives in rubber [4]. However, in Vietnam, there is not any research into ability of silica powder as an anti–caking agent in the fertilizer industry. In this work, silica particles were produced from RHA by precipitation method at pH 7. This is a simple and environmentally friendly method. The effects of synthesized conditions on the extracted yield and the purity of silica such as concentration of NaOH, reaction time, concentration of HCl, and volume percentage of ethanol were investigated. The characterization of the obtained silica was performed by X–ray diffraction (XRD), X–ray fluorescence (XRF), Fourier transform infrared spectroscopy (FTIR), Brunauer–Emmett–Teller (BET), transmission electron microscopy (TEM), and dynamic light scattering (DLS). The application of obtained silica as an anti–caking for NPK–fertilizer was studied and compared to kaolin which is a popular anti–caking agent in the fertilizer industry. 2. MATERIALS AND METHODS 2.1. Materials RHA was obtained from burning the husk at 200–300 °C, 45 minutes in Dong Thap province, Vietnam. The NPK–fertilizer was derived from Binh Dien Fertilizer Joint Stock Company, Long An Province, Vietnam. Hydrochloric acid (36.46 wt%) and sodium hydroxide (99 wt%) were purchased from Xilong Chemical, China. Ethanol (99.7 vol%) was purchased from ViNa Chemsol, Vietnam. All chemicals were analytical grade and used as received without further purification. 2.2. Investigation of the effect of synthesized conditions on the extraction and purity of silica 2.2.1. Effect of the concentration of NaOH on the extracted yield of silica Silica from RHA was synthesized using precipitation method [5]. In brief, 10 grams of RHA sample was stirred in 200 mL of NaOH solutions (0.2, 0.5, 0.8, 1.0, and 1.2 N, respectively) at 90 °C. After that, the filtrate was allowed to cool down to the room temperature and added 1 N HCl solution until pH 7. The formed silica gel was aged for 12 hours. The gel was washed with a mixture of ethanol and water, and then dried at 70 °C for 8 hours in a dry oven. The silica powder was obtained after milling the dried solid. 2.2.2. Effect of the reaction time on the extracted yield of silica The process was similar as shown in section 2.2.1 with the optimum obtained concentration of NaOH solution. The reaction time was changed into 30, 60, 90, 120, and 150 minutes, respectively. 2.2.3. Effect of the concentration of HCl on the purity of silica The process was similar as shown in section 2.2.2 with the optimum obtained reaction time. The concentration of HCl solution was changed into 1, 3, 4, and 5 N, respectively. 2.2.4. Effect of the volume percentage of ethanol on the purity of silica of 30 2.3 mi mi 2.4 D8 to of acq Pro the of Sp NO (M the Ma wa tim Un 2.5 (Fi Lo The proce HCl solution , 50, 80, and . Adding th Modified xed with 40 nutes in the . Character Crystal st –Discover i 80° at Instit the silica w uisition lif cessing, Vi silica were 400 to 4000 ecific surfac VA–3200e ANAR), Vi silica was terials Scie s measured e at Key La iversity Ho . Evaluation The abilit gure 1) and ng An Provi Nguyen ss was simil . The volum 100 %, resp e silica pow fertilizer wa 0 grams of rod mixer. ization ructure of s nstrument w ute of Appli ere determi etime of th et Nam Nati confirmed b cm–1 at Inst e area of th , Quantachr et Nam Nati studied by nce (IAMS– by DLS wi boratory of Chi Minh C of ability o y of anti–ca breaking e nce. Huu Hieu, T ar as shown e percentag ectively. der into the s obtained b commercia ilica was rec ith a CuKα r ed Materials ned by XRF e 60 at Key onal Univers y FTIR spe itute of Appl e silica was ome, Ameri onal Univer TEM with VAST), Ho th LA–950 Chemical En ity (VNUHC f anti–caki king of the m quipment (F Figure ran Manh H in section 2 e of ethanol NPK–fertil y dry–mix m l fertilizer a orded by X adiation (1. Science (IA using a S2 Laborator ity Ho Chi M ctrum with ied Material evaluated b ca at The sity Ho Chi JEM–1400 Chi Minh c Laser Partic gineering a M). ng odified fert igure 2) in 1. Compressi oang, Pham .2.3 with the in ethanol/w izer ethod. The t 0.5, 1.0, RD pattern 54060 Å) at MS–VAST Ranger Xp y of Chem inh City (V TENSOR–2 s Science (I y the multi– Center for Minh City ( Plus, Jeol, A ity. Average le Size, Hor nd Petroleum ilizer was st Binh Dien on equipment Minh Hoang optimum ob ater solution silica powde 1.5, and 2 w with a Bruk a scan veloc ), Ho Chi M lash, Bruke ical Enginee NUHCM). 7, Bruker, G AMS–VAST point BET a Molecular a VNUHCM) merica at particle siz iba, Japan i Processing udied by com Fertilizer Jo . , Tran Thi T tained conc was change r was dried, t% the sil er’s X–ray D ity of 2°/min inh city. Th r, Germany ring and P Functional g ermany in t ), Ho Chi M nalysis met nd Nanoarc . The morph Institute of e of silica i n 60 s of u , Viet Nam pression eq int Stock C huy Tien 195 entration d into 0, and then ica for 5 iffusion from 0° e oxides with an etroleum roups of he range inh city. hod with hitecture ology of Applied n the gel ltrasound National uipment ompany, Sy 19 on we ab 3.1 of con can sil 1 op nthesis of pu 6 The samp the sample re broken b ility of the sa . Effect of t Effect of silica increa centration o be explain icate solutio N, the amo timum conce F re silica from les were sim using the co y the breaki mple got. he NaOH co the NaOH c sed gradual f NaOH wa ed that when n (Na2SiO3) unt of Na2S ntration of N igure 3. Effe rice husk a ulated from mpression e ng equipme Figur 3. RESU ncentration oncentration ly when the s 1.2 N, the the NaOH increased. H iO3 solutio aOH is 1 N ct of the NaO sh as an an the real sto quipment fo nt. The low e 2. Breaking LTS AND and the re was shown concentrati yield of silic concentratio owever, w n extracted . H concentrat ti–caking ag ring conditio r at least 2 m er the break equipment. DISCUSSI action time in Figure 3 on of NaOH a did not inc n increased hen the conc from RHA ion on extract ent for fertili ns by creati onths. The ing force, th ON . It shows th increased. rease signif , the amoun entration of was maxim ed yield of th zer industry ng the same n the caking e better an at the extrac However, w icantly. The t of obtained NaOH was um. There e silica. pressure samples ti–caking ted yield hen the se results sodium reached fore, the inc wa 12 90 sol (te 3.2 the de HC at Effect of t reased grad s achieved w 0 minutes. These res °C for 120 ution was a mperature an . Effect of t Figure Figure 5 gel. It sho creased by h l. The size 5 N concent Nguyen he reaction ually when hen the rea Figure 4. ults can be minutes, th chieved. Th d time) of t he concentr 5. Effect of th shows effect ws that the alf as comp of silica dec ration of HC Huu Hieu, T time was sho the reaction ction time w Effect of the r explained th e maximum ese conditio he rice husk ation of HC e HCl concen of the conc size of the p ared to the reased gradu l, the size d ran Manh H wn in Figur time increa as 120 min eaction time at when stir amount of ns depend o [5]. l tration on av entration of recipitated precipitated ally when t id not chang oang, Pham e 4. It show sed, and the utes. Therefo on extracted y ring RHA i silica in RH n regional v erage particle HCl on the silica prepa silica prep he concentra e significan Minh Hoang s that the ex highest ext re, the optim ield of the sil n 1 N conce A extracted ariations an size of the si average par red at 3 N c ared at the 1 tion of HCl tly. The resu , Tran Thi T tracted yield racted yield um reactio ica. ntration of into sodium d burning c lica in the gel ticle size of oncentration N concent increased. H lts can be e huy Tien 197 of silica of silica n time is NaOH at silicate onditions . silica in of HCl ration of owever, xplained Sy 19 tha sol pre sil Na 3.3 of dis eth 3.4 3.4 nthesis of pu 8 t the precipi –silica prec cipitation w ica particles +and Cl– ion . Effect of t Figure 6 s ethanol wa persion of s anol was 50 Figure 6. Eff . Character .1. XRD pat re silica from tation silica ipitated mor as more slow became bigg s. he ratio eth hows that th s 0%, 100% ilica in etha % [7]. There ect of the volu ization of si tern F rice husk a was formed e quickly wh ly when co er and more anol:water e purity of t , and 50% nol/water s fore, the sili me percent o lica igure 7. The sh as an an from the sol en concent ncentration impure than he silica gra , respectiv olution was ca was wash f ethanol in th XRD pattern ti–caking ag –silica when ration of HC of HCl was l that of the dually increa ely. This re the highest ed better wi e ethanol/wa of the obtaine ent for fertili pH reached l was high. ow. The slo fast process sed when th sult can be when the v th 50 % etha ter solution on d silica. zer industry 7 [6]. There On the con w precipitati since the pre e volume pe explained olume perce nol solution purity of the fore, the trary, the on led to sence of rcentage that the ntage of . silica. Th res tha 3.4 ch 16 ab pe ob 3.4 XRD patt is strong bro earches [8, n 6 hours of .2. FTIR sp FTIR spe emical group 43 cm–1 wer sorbance pea aks between tained silica .3. TEM im Nguyen ern of silica ad peak su 9]. The resu burning tim ectrum ctrum in the s presenting e due to O ks at 1068 a 1068 and 4 was hydroph F ages F Huu Hieu, T in the Figur ggests chara lt can be exp e, the silica Figure 8 sh in silica are H groups of nd 467 cm– 67 cm–1 wer ilic. The res igure 8. The igure 9. The ran Manh H e 7 shows a cteristic of a lained that was the amo ows peaks a identified b silanol (Si– 1 were due t e attributed ult is simila FTIR spectra TEM images oang, Pham strong broa morphous p when RHA rphous phas t 3455, 1643 y the FTIR OH) and a o –Si–O–Si– to vibration r to the prev of the obtaine of the obtaine silica parti Minh Hoang d peak betwe hase and sim was burnt b e [10]. , 1068 and spectrum. T dsorbed wat and –Si–O– modes of th ious research d silica. d silica. cles , Tran Thi T en 22° and ilar to the elow 700 °C 467 cm–1. T he bands at er. The pred bonds of S e gel. There es [10–12]. huy Tien 199 23° (2θ). previous for less he major 3455 and ominant iO2. The fore, the Synthesis of pure silica from rice husk ash as an anti–caking agent for fertilizer industry 200 TEM images (Figure 9) show that size of silica particles approximated to 5–10 nm. However, the most of silica particles were aggregated together. This caused increasing particle size and porosity. Therefore, the obtained silica had porous structure, was appropriate for purpose of adsorbing moisture which was a primary caking agent for the fertilizer. 3.4.4. BET specific surface area The obtained silica had the specific surface area of 224 m2/g. This value is in medium level in comparison with the previous reports as shown in Table 1. Table 1. BET specific surface area of silica in the present work with other reports. BET specific surface area (m2/g) Reference 224 Present work 184 [11] 187 [11] 252 [12] 340 [9] 3.5. Effect of the content of silica powder in modified fertilizer Table 2. Effect of the anti–caking agent in the samples on the breaking force. Samples Anti–caking agent content, wt% Breaking forces, N 1 0 wt% of silica 95 2 1 wt% of kaolin 7 3 0.5 wt% of silica 9 4 1 wt% of silica 2 5 1.5 wt% of silica * 6 2 wt% of silica * (* The samples were not aggreated) Table 2 shows that the breaking force of the commercial fertilizer violently declined when adding 0.5 wt% the silica (from 95 N to 9 N). This result indicates that the anti–caking ability of commercial fertilizer was significantly improved. When the content of silica was 1.5 wt%, the fertilizer granules were not aggregated. In addition, at the same content of silica (1 wt%), the level of anti–caking when using the silica was better than using the kaolin. Therefore, the suitable anti–caking agent for the commercial fertilizer is the silica powder with the content of 1.5 wt%. Ability of anti–caking of silica was supposed that silica contained a lot of OH groups. These groups formed hydrogen bonds with moisture, simultaneous, the porous structure of silica kept the moisture from the fertilizer particles. 4. CONCLUSIONS In this study, the silica from RHA was successfully synthesized by precipitation method at pH 7. The effect of experimental factors on the purity and the extracted yield of silica were studied and suitable synthesized conditions were determined as 1 N of NaOH, 120 minutes of reaction time, 5 N of HCl and 50 % volume of ethanol in the washing solution. Nguyen Huu Hieu, Tran Manh Hoang, Pham Minh Hoang, Tran Thi Thuy Tien 201 The obtained amorphous silica was 82.4 % of extracted yield, 99.8 % purity with average particle size of 5–10 nm, and high specific surface area of 224 m2/g. The NPK coated silica fertilizer particles were fabricated by dry–mix method. The testing results show that at with 1.5 wt% of silica could be used as an anti–caking agent and replaced kaolin for the NPK–fertilizer. REFERENCES 1. Selvakumar K. 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