Silicon-Based Materials from Rice Husks and Their Applications

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REVIEWS Silicon-Based Materials from Rice Husks and Their Applications Luyi Sun* Department of Chemistry, The University of Alabama, Tuscaloosa, Alabama 35487-0336

Kecheng Gong Polymer Structure & Modification Lab, South China University of Technology, Guangzhou, 510640, P. R. China

Rice husk (RH) has now become a source for a number of silicon compounds, including silicon carbide, silica, silicon nitride, silicon tetrachloride, zeolite, and pure silicon. The applications of such materials derived from rice husks are very comprehensive. The methods of synthesizing these silicon-based materials from RHs and their applications are reviewed in this paper. Contents 1. Introduction 2. Structure and Components of RH 3. Silicon Carbide 3.1. Preparing Silicon Carbide from RHs 3.1.1. Pyrolysis Method 3.1.2. Plasma Method 3.2. Applications 3.2.1. Composites 3.2.2. Semiconductor Materials 3.2.3. Abrasive Materials 3.3. Summary 3.3.1. Energy 3.3.2. SiC Purity 3.3.3. Whisker Content 3.3.4. Improvements on Defects 3.3.5. Application of CVD 3.3.6. Preparing SiC using Microwave Radiation 3.3.7. Composites 4. Silica 4.1. Preparing Silica from RHs 4.1.1. Direct Combustion 4.1.2. Combustion after Pretreatment 4.1.3. Hydrothermal Method 4.1.4. Reaction with Sodium Carbonate 4.1.5. Reaction with Sodium Hydroxide 4.2. Applications 4.2.1. In Polymer Materials 4.2.2. In Cement 4.3. Summary

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4.3.1. Preparation of Ultrafine Silica Powders 4.3.2. Production of Silica Films 4.3.3. Dual-Phase Fillers 4.3.4. Energy 4.3.5. Applications in Alloys and Ceramics 5. Silicon Nitride 5.1. Preparing Silicon Nitride from RHs 5.2. Summary 6. Silicon Tetrachloride 6.1. Preparing Silicon Tetrachloride from RHs 6.2. Summary 7. Silicon 7.1. Preparing Silicon from RHs 7.2. Summary 8. Zeolite 8.1. Preparing Zeolite from RHs 8.2. Summary 9. Others 10. Conclusions 11. Literature Cited

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1. Introduction The production of rice, one of the major food crops in the world, generates one of the major wastes of the world, namely, rice husks (RHs). Efforts to utilize RHs have been handicapped by their tough, woody, abrasive * Author to whom correspondence should be addressed. Department of Chemistry, The University of Alabama, Tuscaloosa, AL 35487-0336. Tel.: 205-391-4502. Fax: 205-3489104. E-mail: [email protected].

10.1021/ie010284b CCC: $20.00 © 2001 American Chemical Society Published on Web 10/31/2001

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Table 1. Analysis of RHA1 component content

SiO2 86.9-97.3

K2 O 0.58-2.5

Na2O 0.0-1.75

CaO 0.2-1.5

nature; low nutritive properties; resistance to degradation; great bulk; and high ash content.1 Such efforts have resulted in minor usage, mostly in low-value applications in agricultural areas or as fuel. Little advantage is taken of the RHs and pollution is caused in such disposal processes. However, because of the high silicon content in RHs, the utilization of RHs has been significantly widened in the past few decades. At present, RHs are the raw materials for the production of a series of silicon-based materials, including silicon carbide, silica, silicon nitride, silicon tetrachloride, pure silicon, and zeolite.2-4 2. Structure and Components of RH Rice husks mainly contain lignin, cellulose, and hydrated silica. Reports have been published on the composition, properties, and intended uses of RHs since at least as early as 1871.1 Silicon enters the rice plant through its root in a soluble form, probably as a silicate or monosilicic acid, and then moves to the outer surface of the plant, where it becomes concentrated by evaporation and polymerization to form a cellulose silica membrane. There is quite general agreement that the silica is predominantly in inorganic linkages, but some of the silica is also bonded covalently to the organic compounds. This portion of the silica cannot be dissolved in alkali and can withstand very high temperatures.5 Characterizations by SEM, energy-dispersive X-ray analysis, AES, etc., suggest that the silica is mainly localized in the tough interlayer (epidermis) of the RH and that it also fills in the spaces between the epidermal cells.1,4,6,7 In general, rice husk ash (RHA) might well be considered slightly impure silica. The content of silica and all impurities in RHA vary depending on the variety, climate and geographic location.1 3. Silicon Carbide Silicon carbide is extremely hard and has a high thermal conductivity, high thermal-shock resistance, high hot strength, high melting point, a low coefficient of thermal expansion, good oxidation resistance, and good corrosion resistance to acid and base. It has been widely used in many industries. The process developed by Acheson of reacting sand and coke in a resistance furnace to produce silicon carbide is still the basic manufacturing process used today.8 The most critical manufacturing issues in recent years have been the cost and availability of coke, the cost of electricity, and environmental considerations.9 For the manufacture of high-performance ceramics by sintering or hot pressing, other methods of synthesis are sometimes used to prepare high-purity, very fine reactive powders. Such processes include plasma-arc synthesis, continuous feed through an induction furnace, batch reaction of silica and carbon in CO or inert gas, decomposition of polycarbosilanes, and chemical vapor deposition (CVD).8 Note that the variable costs for these fine reactive powders are an order of magnitude higher than those for materials produced with Acheson furnace. This has restricted wide-scale commercialization. The production of silicon carbide whiskers has problems similar to those

MgO 0.12-1.96

Fe2O3 trace-0.54

P2O5 0.2-2.85

Cl trace-0.42

involved in the production of ultrafine silicon carbide.10 Therefore, there is a need to identify new raw materials and new methods for manufacturing silicon carbide. 3.1. Preparing Silicon Carbide from RHs RHs contain 15-20 wt % silica and a number of organic constituents that will yield carbon when thermally decomposed. Therefore, RHA contains two necessary raw materials for the preparation of silicon carbide: SiO2 and C. With the very high surface area and intimate contact available for the carbon and silica in RHs, it is possible to form SiC at relatively low temperature6 (much lower than indicated by thermodynamic and kinetic calculations).11 In addition, the silica in the RHs can maintain the initial structure of the RHs. Both the low density and the space in the raw materials facilitate the production of silicon carbide.12 Therefore, RHs are the most economical and promising raw material for producing silicon carbide. Since the pioneering work of Lee and Cutler in 1975,13 many studies have been reported. The silicon carbide made from RHs is usually a mixture of silicon carbide powders and whiskers, but through control of the reaction conditions, silicon carbide whiskers can be obtained as the main product. Compared with traditional processing, this method is simpler and more economical,14 and it can dispose of RHs without pollution. 3.1.1. Pyrolysis Method Usually, the pyrolysis method of preparing SiC from RHs consists of two stages: first, the RHs are coked at 500-900 °C to obtain coked RHs; then, they are fired at higher temperatures (1500-1650 °C) in an inert or reducing atmosphere to form silicon carbide.15-17 However, the first stage is not necessary. Some research has even shown that the yield of SiC from raw RHs is higher than that from burned RHs.18,19 The produced SiC is usually cleaned with an acid treatment to remove small amounts of amorphous material and wool.20 Because the ratio of silica and carbon is less than the optimum ratio for silicon carbide formation, usually either silica is added or carbon is removed to regulate this ratio.21-23 According to whether pretreatment is performed and/ or a catalyst is used, pyrolysis methods can be divided into three categories. 3.1.1.1. Direct Pyrolysis. This is the most direct method, as neither pretreatment nor a catalyst is used. A series of experiments have shown that the results of this synthesis approach are acceptable.7,15,16,18-20,24-27 Patel et al. disclosed the reason a catalyst was not necessary: the existence of trace elements in RHs, some of which (Fe, Mg, etc.) can impart a catalytic effect.20,24 The control of temperature and pressure has always been found to be important.13 As reaction conditions change, the optimal temperature and pressure for forming SiC also change. In general, 1550-1600 °C is favorable. To increase the reaction rate and the production rate, researchers have regulated the reaction conditions and invented some new reaction methods. It was found that, instead of slow heating and holding for a long time at pyrolysis temperatures, rapid heating and holding for

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a short period at high temperatures increases the production rate in continuous operation, so the rapid heating method was adopted. In this method, raw RHs without any pretreatment are rapidly (300 ( 25 °C/min) heated to high temperature (1300-1600 °C). SiC forms from raw RHs heated to 1350 °C, whereas neither silica nor carbon crystallizes even at 1600 °C. In addition, some other studies have found that slow heating decreases the reactivity of silica and carbon in the RHs by increasing their degree of crystallization.25,28,29 In contrast with rapid heating, multistep pyrolysis, which involves the maintenance of different temperatures for different periods of time during the heating process, was found to result in an even higher production rate.18,19 The effect of pressure is also significant. It was found that both a high localized CO concentration and its continuous removal help to increase the total SiC yield.26 Therefore, inert gases such as argon and nitrogen have often been used to drive away CO, which results in the formation of good-quality SiC.15 Krishnarao et al.15,17 did systematic studies on the effects of pressure. By regulating the pressure from 101.3 to 165.5 kPa, they found that the increase of the pressure resulted in (i) an increase in the crystallization of silica and carbon in RHs, (ii) an increase in the temperature of SiC formation from 1100 to 1150 °C, and (iii) decreases in the formation of SiC whiskers and in the total SiC content. They also studied SiC formation through vacuum pyrolysis and found that, under such conditions, the formation of SiC from raw RHs is higher than that from burned RHs.26,30 3.1.1.2. Pyrolysis after Pretreatment. The most typical pretreatment is acid leaching, and HCl is most often used. Krishnarao et al.31 treated RHs by boiling them in 5 N HCl for 1 h. The washed and dried acidtreated raw rice husks (TRRHs) and untreated raw rice husks (RRHs) were directly pyrolyzed (without precoking) in argon atmosphere at different temperatures between 1050 and 1600 °C. Silica obtained from TRRHs has a lower level of impurities than that obtained from RRHs. Acid treatment was found to decrease the degree of crystallization of silica and carbon in RHs, but whisker formation was also decreased in TRRHs. Boric acid has also been used to pretreat RHs. Raman et al.32 mixed RHs with textile-grade polyacrylonitrile (PAN) fibers dissolved in dimethylformamide in a weight ratio of 10:1. Boric acid was added to the mixture with a silica/boric acid ratio of 10:1, and the mixture was stirred well. The dried mixture was heated in a furnace fitted with an alumina tube in argon atmosphere to 1450 °C for 4 h. The sample was also cooled in argon. It was found that the incorporation of boric acid into the mixture of RHs and PAN yielded SiC whiskers with a better aspect ratio. Enzymatic treatments, whose purpose is to remove excess carbon for silicon carbide formation in RHs, have also been used.21 Powdered RHs can be delignified with NaOH solution and treated with a mixture of cellulases (acucelase and Meicelase) to hydrolyze the cellulose of the husks. In RHs pretreated with 0.5% NaOH, silica was enriched to 29.5 wt %, and 288 mg of sugar was recovered from 1.0 g of RHs after a 5-day cellulase treatment. The C/SiO2 ratio reached 0.74, which is near the optimum ratio for silicon carbide formation. When this silica-enriched sample was heated at 1600 °C in an atmosphere of argon, R-SiC and β-SiC were formed,

and mature SiC whiskers were evident. Residual free carbon in the product was 1/11 of that for RHs that were heat-treated without enzyme treatment. Alkali pretreatment has also been reported.5 Strong alkalis such as NaOH can dissolve all of the SiO2 present in free form but cannot dissolve the SiO2 bonded to the organic molecules, which amounts to about 5%. In contrast, weak alkalis such as NH4OH cannot tackle the structure of RHs. In summary, the main purpose for pretreatment is to improve the purity of the SiC produced. 3.1.1.3. Pyrolysis with Catalyst. The incorporation of a catalyst greatly increases the reaction rate. Several types of catalysts, including Fe, Co, Ni, Pd, and Cr, have been tried.5 The effect of different catalysts on the formation of SiC from burned RHs was studied over a temperature range of 1200-1600 °C. It was shown that catalysts decreased the crystallization of carbon and silica in RHs and accelerated the formation of SiC.28,29 Narciso-Romero et al.33 compared the syntheses of SiC from RHs with and without the use of a catalyst (iron, cobalt, and nickel). They found that the introduction of a catalyst increased the reaction rate and that the yield was up to 3 times that for the uncatalyzed reaction. The general behaviors of the three catalysts were similar, although nickel was the most effective in terms of reaction rate and cobalt was best in producing larger crystal sizes. Other studies have shown that cobalt catalyst is not advantageous to the formation of SiC whiskers.28,29 The effect of the morphology of the catalyst was also evident. The reaction rate is enhanced with increasing surface area of the catalyst.13 Therefore, RHs have sometimes been impregnated in FeCl234 or FeSO435 solution to achieve a better catalysis effect. Catalysts can also decrease the reaction temperature. In the presence of CoCl2, the optimum temperature was g1400 °C, whereas when CoCl2 was not present, considerable quantities of SiC formed only above 1500 °C.36 New research has shown that alkali metal oxides used as catalyst-accelerators can further accelerate the formation of SiC. The reason is that the addition of alkali metal oxides can lower the melting point of silica, while the reaction of silica and carbon is the ratedetermining step.22 Another study showed that the addition of a mineralizer can further reduce the reaction temperature.34 When a catalyst is present, control of the temperature is still very important. For example, one study found that, when cobalt was present, rapid heating increased the formation of SiC whiskers,28 whereas slow heating (5 °C/min) stabilized the silica and carbon by increasing their degrees of crystallization.29 Another discovery that is worth mentioning is that, during vacuum pyrolysis, the addition of Si3N4 to RHA can increase the yield of SiC whiskers and reduce the amounts of SiC particles and excess carbon. At 1400 °C, mixtures with 50 and 60 wt % of Si3N4 were found to yield masses of SiC whiskers without any free carbon and with about 5 wt % of unreacted SiO2.30 Therefore, nitrogen can be considered to substitute the inert gas in the reaction to produce some Si3N4, which, in turn, can help in the production of silicon carbide whiskers. In summary, catalysts provide the three advantages of (i) increasing the reaction rate, (ii) increasing the yielding of silica, and (iii) decreasing the reaction temperature. In addition, catalysts can also determine the formation mechanism of SiC whiskers.37

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3.1.2. Plasma Method As mentioned above, the effect of CO from the reaction can be sufficiently significant to decrease the reaction rate; thus, CO needs to be constantly removed with argon. In addition, the reaction time can be reduced greatly by increasing the temperature. Both of the above processes can easily be attained in a thermal plasma reactor. Moreover, the very high temperatures (104 °C), steep temperature gradients (106 °C/min), and high quench rates (106 °C/s) associated with thermal plasmas can be a unique route for the preparation of ultrafine SiC from RHs. In contrast to other methods, it is very easy to realize continuous production and to control reaction parameters in plasma process.38-41 Posttreatment processes such as oxidation and acid treatment (with H2SO4, HCl, or HF) appear to be effective in removing the excess carbon, silica, and mineral oxides. Successive washings can remove chloride and fluoride ions.39,40 3.2. Applications 3.2.1. Composites After a lull during the 1960s and early 1970s, interest in SiC as a fiber reinforcement has experienced a revival in the past two decades. One of the important reasons for this lies in the successful preparation of relatively inexpensive high-grade SiC whiskers from RHs.42 Compared with many other ceramic materials, silicon carbide has a relatively high thermal conductivity and low coefficient of thermal expansion, giving it a relatively favorable thermal shock resistance.43,44 The strength and hardness of ceramics and alloys can be greatly enhanced by the addition of SiC particles into their matrixes. Generally, the reinforcing and toughening effects of SiC whiskers are better than those of SiC particles. The proper aspect ratio is 30-40, aswhiskers that are too long are hard to distribute. Therefore, a high content of whiskers is preferred in the production of SiC from RHs. Usually, thick whiskers are suitable for ceramic matrix composites, whereas thin whiskers are suitable for metal matrix composites.45 3.2.1.1. Ceramic Matrix Composites. Upon being reinforced reinforced by silicon carbide whiskers, a ceramic’s fracture toughness can be greatly enhanced, and its flexural strength and thermal conductivity can also be greatly improved.46 When the whiskers are being introduced into the ceramics, however, attention must be paid to avoid agglomeration, which will result in structural defects.47 Wang et al.44 have successfully prepared silicon carbide whiskers from RHs that have good reinforcing properties in ceramic matrix composites. 3.2.1.2. Metal Matrix Composites. As early as the beginning of the 1980s, the study of aluminum reinforced by silicon carbide from RHs was reported. The reinforced aluminum not only has a good combination of room-temperature specific strength and modulus and excellent thermal stability, but it also can be processed by normal metal working techniques. Such materials are increasingly considered for aerospace applications, where their high stiffness and strength-to-weight ratios are additional advantages.48-50 3.2.2. Semiconductor Materials Silicon carbide is an important semiconductor for applications in high-temperature electronics, ultraviolet sensors, and high-speed devices. These applications are

based on SiC’s wide energy band gap, high electron saturated drift velocity, high thermal conductivity, and other factors. Silicon carbide is also physically rugged and chemically inert, which is good for semiconductor devices operating in harsh environments. Its excellent properties were identified many years ago, but unfortunately, the presence of large densities of micropipe defects has prevented the routine production of highquality silicon carbide crystals.51,52 The silicon carbide from RHs has been reported to be used as semiconductor materials,53,54 but because of the high requirements on the purity, some special separation is needed in the manufacturing process. 3.2.3. Abrasive Materials When silicon carbide is used as an abrasive, the requirement on purity is not high. The purity of silicon carbide from RHs is high enough to match this requirement. The silicon carbide can be either directly used as abrasive material or hot-pressed with composites.55 3.3. Summary Since the first synthesis of SiC from RHs by Lee and Cutler in the 1970s,13 many improvements have been made, and this technology has been completely industrialized. However, further study on the following aspects is necessary. 3.3.1. Energy In the first step of the preparation of silicon carbide from RHs, making full use of the energy from the combustion of RHs is usually ignored. In fact, by regulating the proper reaction condition, combustible gas can be made as a byproduct while the quality of the produced silicon carbide is ensured.56 Such energy can be used in the other steps of manufacturing. 3.3.2. SiC Purity RHs contain some inorganic components other than silica, which become Fe2O3, K2O, etc., after pyrolysis. Sometimes, excess carbon is another source of impurities. Such impurities have little effect on low-grade silicon carbide, but for the high-grade silicon carbide, particular separation must be carried out.57,58 Now, more and more high-grade silicon carbide is needed in many fields, which urges the improvement of separation techniques and facilities. Some feasible and economic technologies for the separation and purification of silicon carbide whiskers from RHs have been reported by Xia et al.,59,60 but even further study to industrialize such technology is necessary. 3.3.3. Whisker Content Because the value of SiC whiskers is higher than that of SiC particles, improvements in the technology that increase the whisker content are important.59 3.3.4. Improvements on Defects Silicon carbide whiskers produced from RHs contain a high density of defects. These defects include (i) stacking faults and thin twins on close-packed planes lying normal to the whisker axis, (ii) cavities that are 1-20 nm in diameter and are usually confined to a

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region ∼100 nm wide at the whisker core, and (iii) partial dislocations lying in radial directions and apparently associated with the presence of cavities. Such defects greatly affect silicon carbide’s electric properties and thermal conductivity.7,61-63 Selecting the proper catalyst and regulating the reaction parameters can decrease such defects. 3.3.5. Application of CVD CVD has been widely used in the preparation of highgrade silicon carbide. At this point, silicon carbide powders,64,65 whiskers,66 fibers,67-69 and even films70 have been prepared by CVD, although few studies on the preparation of silicon carbide from RHs by CVD have been reported. Sharma et al. showed that the silicon carbide whiskers from RHs are very similar to those obtained by CVD with SiO and CO as the primary reactants.7 Recently, microcoiled silicon carbide fibers have been prepared by CVD using powder mixtures of SiO and coked RHs that were very quickly heated to the reaction temperature.71 Because SiO and CO are the intermediate products in the preparation of silicon carbide from RHs, using CVD to prepare high-grade silicon carbide from RHs should be possible. According to the reaction mechanism, SiO and CO can be produced from the redox of silica and carbon in RHs in the presence of a catalyst at high temperature. Then, in the second step, the produced SiO and CO can be used to synthesize silicon carbide whiskers (even film) by CVD. The technique might be complicated, but considering the product’s higher properties, such a method for producing silicon carbide should be economical. 3.3.6. Preparing SiC using Microwave Radiation Traditional heating usually is done by thermal radiation or thermal conduction, so the energy only affects the surface of the sample, which results in a low heating rate and long heating period. Considering that the preparation of silicon carbide from RHs is strict on temperature, some new heating methods have been developed, such as fluidized beds and microwaves. Especially microwave heating, which has been used to prepare silicon carbide powders, can achieve total and rapid heating and creates minimal pollution.72 3.3.7. Composites Compared with silicon-carbide-reinforced metal matrix composites and ceramic matrix composites, the research on silicon-carbide-reinforced resin matrix composites is limited. Considering the high price of the carbon fibers that are used in resin composites, cheap silicon carbide whiskers from RHs could be a suitable substitute for them. Another advantage of using silicon carbide whiskers as reinforcement lies in the low requirement on the purity of the silicon carbide whiskers. The main impurities in silicon carbide whiskers from RHs are carbon, silica, and silicon carbide particles, all of which have little negative effect on resin composites’ mechanical properties. Recently, Wang,73 et al. reported the synthesis of pentacoordinate silicon complexes from RHA. They are easily hydrolyzed to silica. Such complexes can also be reacted with other reactants to prepare thermally stable products. They can also be reacted in situ with SiO2 or Al2O3 to prepare ceramic matrix composites.

4. Silica Silica not only is an important starting material for semiconductors but also plays an important role in the plastics, rubber, and photoelectric material industries.74 There are two kinds of commercial silica: fumed silica and precipitated silica. The raw material of fumed silica is silicon halide. The purity and properties of fumed silica are excellent, but the amount of energy consumed and the cost are high. The silica produced through this method is mainly used as reinforcement in silicone rubber. The raw materials of precipitated silica are extensive and cheap, and the energy consumed is low. After modification with a silicon-coupling agent, the reinforcing properties of precipitated silica are close to those of carbon black, but its general properties are not as good as those of fumed silica.75,76 Because RHs naturally have high contents of silica and because the silica has a high reactivity, since the beginning of the 1980s, a new way to prepare highpurity silica from RHs has been intensively investigated. The silicas produced have different applications according to their qualities. 4.1. Preparing Silica from RHs 4.1.1. Direct Combustion RHs can be directly combusted to produce silica without any pretreatment.77-81 In the reaction process, the phase composition of silica in the ash and its surface area depend critically on the combustion temperature of RHs.77 Combustion instruments also strongly affect the quality of the silica produced. Properties of the ash obtained by combustion between 400 and 1500 °C have been investigated.78 The SiO2 in RHA formed by combustion below 800 °C was found to be amorphous. Particles of the ash having an average diameter of 20 µm were aggregates of small particles with a diameter of 2-5 µm. At combustion temperatures above 900 °C, the SiO2 in RHA consisted of cristobalite and a small amount of tridymite. The surfaces of the ash particles melted, and the particles bonded to each other. The particle size was 40-60 µm. Because the ash content is relatively high for combustion in a typical furnace, which will strongly affect the quality, Kapur77 designed a tube-in-basket (TiB) setup for combusting RHs. In a controlled manner, he found that the silica in RHs was noncrystalline up to 600 °C. The phase transformation in silica began between 600 and 800 °C, with the first appearance of the cristobalite phase in the 800 °C sample. The next transformation to tridymite started at about 1000 °C and became quite pronounced above 1200 °C. The small amount of quartz present in the samples burned at lower temperatures and disappeared completely by 1200 °C. The cristobalite and tridymite phases coexisted in the 1400 °C sample. The initial increase in the surface area from 60 m2/g in 350 °C burned ash to 80 m2/g at 600 °C was perhaps due to the burnoff of the residual carbon and the opening of new pores. Between 700 and 900 °C, a sharp drop occurred in the surface area from 40 to only 1 m2/g, which clearly points to the need for controlled combustion of RHs in order to produce reactive silica ash. Using this TiB burner, the silica ash produced consisted essentially of amorphous silica with a relatively high surface area of 65 m2/g. In addition to the

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temperature, the duration of heating is also critical to the quality of the silica produced .79 Luan et al.80 studied the combustion of RHs with and without coal in the presence of a pilot flame in a modified fluidized bed. The chemical and physical properties of both the top and the bottom products from the rice husk combustion were strongly influenced by two major factors: the feed rate and the feed composition. The addition of coal to RHs resulted in a much smoother temperature distribution in the reactor and the disappearance of agglomeration. The addition of coal also increased the bulk temperature and changed the composition, particle size distribution, and specific surface area of the ash.

of acid pretreatment. Microbial fermentation has also been applied to pretreat RHs at low temperatures to release silica in its natural, pure and highly reactive form. Such microbiological processes are relatively cheap, and if used in combination with acid leaching, it can result in the production from RHs of silica that is similar to a commercial xerogel. Its purity, specific surface area, and tendency to form fractural structures resembles the features of fumed silica.85,93 Bio-pretreatment results in a decrease in organic matter with a corresponding increase in silica content, greatly decreasing the carbon content in the silica produced . Its disadvantage lies in the long processing time.

4.1.2. Combustion after Pretreatment

In addition to silica, RHs contain organic compounds and metals. In high-temperature, high-pressure, and acidic media with strong oxidation activities, the organic compounds can be decomposed, and the trace metals can be turned into soluble ions; then, silica can be obtained. In this method, the reaction temperature is much lower than that used in the combustion method, and it is easy to retain the amorphicity of the silica in RHs. Some acids with strong oxidation activities such as H2SO4 and HNO3 are used; sometimes H2O2 is also used as the oxidative medium. According to Wu’s report,94 by controlling the ratio of RH, concentrated HNO3, and water at 1:5:5 (by weight) and allowing the reaction to proceed for 3 h at 160180 °C, high-purity and high-surface-area silica can be produced. Longer reaction times and higher contents of concentrated HNO3 do not affect the reaction results, but if too much water is added, which leads to a remarkable reduction in the content of concentrated HNO3, the result will be the incomplete oxidation of RHs. Another reported approach95 was similar to that of Wu, but included the further addition of H2O2. The weight ratio of H2O2 (30%) to RHA was 5:1, and the volume ratio of concentrated HNO3 to H2O2 was 1:10. Upon reaction in a sealed vessel at 150 °C for 3 h, highpurity (99.99%) silica can be produced.

Quite a few kinds of acids (HCl, H2SO4, HNO3, HF) have been reported to be used in pretreatment,79,82-90 but HCl is surely the most often used. Chakraverty et al.82,83 found that the leaching of RHs in 1 N HCl was effective in substantially removing most of the metallic impurities. Acid treatment of RHs prior to combustion does not affect the structural nature (amorphicity) of the silica produced . From the standpoint of the amorphicity of the silica produced , minimum time, energy requirements, and the cost of production, a furnace temperature of 500 °C and combustion time of 6 h are considered optimum for converting RHs into white amorphous silica. After acid leaching, the silica produced was completely white in color and had high purity.82 The report from Real et al.84 is more attractive. They found that the preliminary leaching of RHs with a solution of HCl before combustion at 600 °C could result in relatively pure silica (approximately 99.5%) with a high specific surface area (approximately 260 m2/g) that was maintained even after being heated at 800 °C. If the leaching with HCl was performed on the white ashes obtained from the combustion of RHs at 600 °C, an amorphous silica with the same purity was also obtained, but its specific surface area decreased to 1 m2/g. Because silica with a high specific surface area has a high reaction activity, acid leaching greatly improves the quality of the silica produced , which, in turn, widens its applications. Other acids, such as H2SO4, HNO3, or their mixture, have also been used in acid pretreatment.79,82,85-88 The general leaching effects of H2SO4, HNO3 and HCl are similar, but HCl leaching of RHs is superior to H2SO4 and HNO3 leaching in removing the metallic ingredients.82 In some cases, the chemical posttreatment of incinerated RHs has also been performed using HCl.85 Metal oxides strongly affect the production of silica. It has been found that some kinds of metal oxides, especially potassium oxide, contained in RHA cause the surface melting of SiO2 particles and accelerate the crystallization of amorphous SiO2 into cristobalite.78,79,84,86,91 This behavior is due to the strong interaction between the silica and the potassium contained in RHs, which leads to a dramatic decrease of the specific surface area if K+ cations are not removed prior to the annealing of the samples.78,84,92 Therefore, the main effect of acid leaching is to remove metal oxides, especially potassium oxides. Some alkalis, such as NaOH and NH4OH, have also been used to pretreat RHs.85,87,90 However, the effects of alkali pretreatment are not as obvious as the effects

4.1.3. Hydrothermal Method

4.1.4. Reaction with Sodium Carbonate RHs were first carbonated. The produced RHA was then reacted with Na2CO3 solution in the proper ratio for 3 h. After the product was washed, filtered, dried, and ground, silica with acceptable purity was obtained. A temperature that is too low will affect the purity of the silica produced , whereas a temperature that is too high will affect the yield and quality of the silica. When the concentration of Na2CO3 is higher than 15%, the yield will be above 90%. The proper reaction temperature and time are 600-650 °C and more than 3.5 h, respectively.96 The silica made from this method has good reinforcing properties in rubber. Another advantage of this method lies in the facts that all of the energy in the process is from the energy of the combustion of RHs, no extra fuel is needed, and the main reactant Na2CO3 can be recycled. Thus, this method not only is simple and economical but also generates little pollution.96,97 4.1.5. Reaction with Sodium Hydroxide In this method, RHA is first mixed with NaOH to produce sodium silicate. Then, the sodium silicate was reacted with NH4HCO3, (NH4)2SO4, or H2SO4 to produce

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SiO2.98-101 The key to this method is the reaction of RHA and NaOH. Generally, steam is used to keep the reaction at 150 °C, the reaction lasts 4-5 h, and the particle size of RHA should be about 100-mesh. The proper concentration of NaOH and mixing speed are also important.98 If the produced sodium silicate is titrated with 1 N HCl, silica gel can be precipitated when the pH falls below 10.102-104 4.2. Applications The main component of RHA is silica. However, the impurities contained in RHA have no obvious negative effect in many applications; in fact, they even have positive effect on some occasions. Therefore, purification usually is not necessary, and the silica produced can be used directly. 4.2.1. In Polymer Materials Using RHA as a filler in certain polymers results in composites with better physical, thermal, moisture resistance, and processing properties, as well as better economics.105 4.2.1.1. As Fillers in Rubbers. As early as the 1970s, researchers began to perform studies on RHA as a filler in rubbers. The combustion conditions are the key to success in such applications. The carbon-containing materials must be burned away, but the use of a temperature that is too high or a residence time that is too long will result in a fused crystallized mass with no useful properties at all.106 Burned RHA yields two grades of fillers, namely, white rice husk ash (WRHA) with a high silica content and black rice husk ash (BRHA) with a low silica content. WRHA can easily be ground to a fine powder. The ground product ranges from 0.1 to 2.0 µm in size (there are some agglomerates, too). It contains 10% carbon (occluded in the ash), as well as trace quantities of various metals, sometimes including iron, which are reported to have little or no effect on its cure or aging properties.107 This WRHA can also be used as a filler in natural rubber (NR), styrene butadiene rubber (SBR), nitrile butadiene rubber (NBR), butyl rubber (BR), ethylene-propylene-diene monomer (EPDM), etc.107-111 Especially in epoxidized natural rubber (ENR), the addition of WRHA can increase the tensile strength, tear strength, and hardiness of the rubber.112 As WRHA is predominantly silica, it responds effectively to silane-coupling agents in improving properties of the rubber compounds. WRHA that has been modified by silane-coupling agents not only can effectively improve ENR’s mechanical properties, such as tensile and tear strength, but also can improve ENR’s cure characteristics.107,108,112-114 The cure characteristics of WRHA-filled vulcanizates show a close resemblance to those of carbon black. This can be attributed to the similarities of filler-related parameters such as surface area, surface reactivity, particle size, and metal oxide content.113 Generally, WRHA is not as good as fumed silica and carbon black, especially in terms of tensile strength and tear strength,107,113 but it can replace or partially replace fumed silica and carbon black on some occasions.114 In addition, WRHA can sometimes be combined with other fillers, especially with reinforcing blacks. The properties obtained are linear functions of the amount of the particular filler present in the blend.108,115

BRHA has a lower silica content, typically about 54%, and a substantial carbon content of about 44%. It can also be used as a filler for rubber, but the effect is not as good as can be obtained with WRHA.113,116 4.2.1.2. As Fillers in Plastics. The application of RHA as a filler in plastics is relatively limited, mainly to polypropylene (PP). With an increase in the RHA loading, a PP composite’s flexural modulus and density increase, whereas its tensile strength, breaking elongation, and impact strength decrease. Yet, RHA still can replace some commercial fillers.117-119 When a silane-coupling agent is not applied, the melt shear stress and viscosity of PP increase with filler loading. However, if RHA is treated with a silanecoupling agent containing the peroxide bis(tert-butyl peroxy)diisopropyl benzene, both the melt shear stress and the viscosity of PP decrease. The degree of viscosity reduction increases with increasing filler, i.e., peroxide, concentration. A decrease in viscosity is attributed to the peroxide-induced degradation of the PP matrix.120,121 This silane-coupling agent can also improve the tensile strength.119 In addition to silane-coupling agents, titanate-coupling agents can improve a PP composite’s impact properties, and zirconate-coupling agents can bring about marginal improvements in the stiffness of PP a composite, but their general effects are not as good as those of silane-coupling agents.119 In addition to mechanical and processing properties, RHA also affects the thermal properties of the composites. The addition of BRHA raises the thermal degradation temperature while maintaining the oxidative stability; the thermal degradation temperature of WRHA composites was found to be independent of filler loading, but the oxidative stability deteriorated with increasing filler content.122 The addition of RHA fillers reduces the linear thermal expansion coefficient of the composites, which improves the dimensional stability of the composites against thermal effects.122 The stiffness of RHA composites can also be improved by increasing the filler loading, and fillers cause a significant modification to the damping properties of the matrix material. Overall, it can be inferred that the incorporation of RHA fillers does not result in any detrimental effects to the thermal behavior of the PP matrix and that BRHA composites seemed to have better thermal degradation and thermooxidative stabilities than WRHA composites.122 Compared to composites filled with commercial fumed silica, most of the RHA composites have better impact properties but low tensile and flexural strengths.119 RHA fillers can also act as weak nucleation agents and increase the degree of crystallinity of PP by a small margin.122 Studies on polyester filled with RHA have also been reported, but both the tensile and impact strengths of the resulting polyester composites decreased with increasing filler loading.123 In addition to being used in rubbers or plastics, RHA can also be used as a filler in rubber/plastic blends.124,125 Considering that transferring from RH to RHA results in a loss of about 80% of the weight of the RH and that the combustion process can cause pollution, RHs have also been directly used as filler in plastics, such as PP126 and polystyrene (PS).127 In such cases, the RHs can act as fibers to reinforce the plastics, and the silica in the

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RHs can also have some effect. With chemical modification of the RHs, the reinforcing effect is acceptable.

Considering the high value and wide application of nanoscale silica, when silica is prepared from RHs, nanoscale silica is desirable.

4.2.2. In Cement Because of high energy consumed in cement production, some countries short of energy often incorporate some other components into cement. RHA from the controlled combustion of RHs is highly pozzolanic.128-134 In fact, the incorporation of RHA into cement not only can decrease the cost but can also improve certain properties of the cement. The incorporation of RHA into cement paste does not increase its compressive strength,135-137 but if the RHA is filled in mortar138,139 or concrete,135,140,141 the compressive strength of both the mortar and concrete increase. The reason might lie in the reduced porosity, reduced Ca(OH)2 content, and reduced width of the interfacial zone between the paste and the aggregate when RHA is present.135 In addition to compressive strength, RHA can also increase concrete’s resistance to acid138,142,143 and sulfate,138 flexural strength,138 carbonation,143 penetration,143 etc. One of the main reasons for the improvement of concrete properties upon addition of RHA might be the formation of more C-S-H gel and less portlandite in concrete as a result of the reaction occurring between the RHA and the Ca2+, OH-, or Ca(OH)2 in the hydrating cement.143 RHA will affect the volume changes of the concrete, but the volume changes will be within the limit specified in the American standards.136 When expansive cement was partly replaced by RHA, its compressive strength increased markedly, and its chloride permeability decreased significantly,144 whereas its total expansion decreased slightly.144,145 Because RHA can react with lime and water, RHA has sometimes been mixed with lime first and then filled into cement.146-150 The properties of lime-RHA cement are acceptable. It can be used as the replacement of portland cement in some application. Sometimes, RHA has even been directly blended with lime and then used as a construction material.146 If kaolin clay is available, it can be added during the incineration of RHs to improve the binder quality. Blending portland cement with the incinerated kaolin can further increase the cost-effectiveness.139,151 4.3. Summary 4.3.1. Preparation of Ultrafine Silica Powders Because of their smaller-diameter particles, ultrafine silica powders have many technological applications, such as thixotropic agents, thermal insulators, composite filler, etc. They are widely used in the chemical and electrical fields.152 At present, ultrafine silica powders are mainly prepared by vapor-phase reaction or the solgel method. The high cost of preparation limits their wide application.153 Some recently developed methods using lasers, arcs, plasmas, etc., are hard to industrialize at present because of their highly demanding techniques and conditions.153 In fact, if the size distribution of the silica powder is not too wide, when the specific surface area of a silica powder is above 20 m2/g, the powder is considered nanoscale. Based on the research mentioned above, it is easy to prepare nanoscale silica from RHs under controlled conditions.

4.3.2. Production of Silica Films Almost all of the silica prepared from RHs is powder; few reports on silica films are available. Recently, because of the wide application of silica film in the field of photovoltaics, such films have attracted increasing attention. At present, the main methods for preparing silica films are sol-gel methods, sputtering processes, etc., with high technique demands and high costs.154 One of the methods for makinge silica film involves using a sol-gel technique to hydrolyze tetraethyl orthosilicate (TEOS), which is expensive. Recently, a novel pentacoordinate silicon complex73 having properties similar to those of TEOS has been synthesized from RHs. These relatively cheap pentacoordinate silicon complexes can be substituted for TEOS as the raw material to for preparing silica films. 4.3.3. Dual-Phase Fillers In 1995, one kind of new filler, carbon-silica dualphase (CSDP) filler, was developed by Cabot Corporation. This filler, as indicated by its name, consists of two phases: a carbon phase, with finely divided silica domains dispersed therein. In various polymers, the CSDP filler exhibits high filler-polymer and lower filler-filler interactions.155 Considering that RHA also contains both silica and carbon, it might also be able to be used directly as a CSDP filler. Worth mentioning are the intimate dispersion and contact of silica and carbon in RHA and the fact that the content of silica and carbon in RHA can easily be regulated by the combustion conditions. Although the silica content is high in RHA but low in CSDP fillers, considering the trend of the replacement of carbon by white fillers, RHA might be a kind of filler with huge potential in the future. 4.3.4. Energy All of the methods mentioned above do not consider the energy produced in the process. Lin et al. designed a gasification process that can produce amorphous silica and, at the same time, generate 10 kW of electric power by gasifying 28 kg/h of RHs.156 Such a measure, which can make full use of the thermal energy produced from combustion and ensure the good quality of the silica produced , must make the whole production process more economical. Thus, it is worth popularizing. 4.3.5. Applications in Alloys and Ceramics The silica from RHs is mainly used in polymer materials and cement, as there are few reports on applications in alloys. Compared with silicon carbide, its range of application is narrower. Some preliminary research on aluminum alloy-RHA composites157 and RHA-porcelain composites158,159 has been reported, but further studies are in process. 5. Silicon Nitride Silicon nitride is a new structural ceramic with high mechanical strength at room temperature and elevated temperatures, high thermal shock resistance, good wear

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resistance, good corrosion resistance, high fracture toughness, low density, low coefficient of thermal expansion, and high thermal conductivity. Thus, it has been under intensive study.160 Several of the most important methods for manufacturing silicon nitride are the direct nitridation of silicon powders, the carbothermal reduction of silica in a nitrogen-based atmosphere, and the reaction of chlorosilanes with a gas containing nitrogen or a nitrogen compound.161 Because high-purity fine silica and carbon are commercially available and inexpensive, the carbothermal reduction of silica is the main method for manufacturing silicon nitride powder.162,163 However, this method has a high requirement on raw materials, as silica with a high reactivity and a good distribution of the silica and carbon are important for the reaction. The better the distribution, the more helpful for nitridation.164 Therefore, RHs, which naturally contain both silica and carbon, came into the researchers’ minds. Very high surface area and intimate contact available for the carbon and silica mixture can be obtained by combusting RHs because RHs contain silica, lignin, cellulose, etc. More importantly, because of the uniform distribution of carbon and silica in the RHA, the reaction can occur more easily than by the conventional mechanical mixing technique.163,165-167 5.1. Preparing Silicon Nitride from RHs The production of silicon nitride from RHs was first reported in a U.S. patent, where the reaction temperature was within the range of 1100-1350 °C.168 Several other studies on the formation of silicon nitride from RHs have beem reported since then. Some reaction parameters have been identified as having an important role in the reaction system.165,169 In general, silicon nitride powders can be prepared from RHs at temperatures between 1260 and 1500 °C under a flow of nitrogen. The reaction temperature is relatively lower than that formed from the conventional SiO2/C mixture reaction, and the nitridation rate of the pyrolyzed RHs is distinctly faster than that for the conventional SiO2/C mixture process.165,170 Before nitridation, RHs have often been treated with acid solution to remove the metallic impurities and some organic elements (transfer to soluble ingredients) and then to obtain silicon nitride powders of high purity, 57,165,171 although in some reports, the RHs have also been used directly without any pretreatment.172 Typically, the carbon-to-silica ratio is such that an excess of carbon above the stoichiometric requirement for synthesizing silicon nitride is available for reaction.172 Silicon nitride is manufactured by the carbothermal reduction and nitridation of silica according to the overall reaction165

3SiO2(s)+6C(s)+2N2(g) f Si3N4(s)+6CO(g)

(1)

The generally accepted reaction mechanism is that an intermediate SiO is formed during the synthesis of silicon nitride and that the carbon reduction of SiO2 to SiO is the rate-determining step. Rahman and Riley173 prepared silicon nitride powder by nitriding pyrolyzed RHs under 95% nitrogen/5% hydrogen. They found that hydrogen addition is beneficial in accelerating the rate of nitride formation. However, when the reaction temperature is higher than 1450 °C, the content of byproduct will greatly increase. They also found that the silicon nitride particle mor-

phology is determined in part by the particle dimensions of the milled RHs. Silicon nitride crystals of hexagonal symmetry are obtained from starting powder of dimension ∼53 µm, whereas greater particle dimensions and lower packing densities yield silicon nitride with a whiskery morphology. The addition of preformed silicon nitride powder results in the formation of fine, equiaxed silicon nitride particles, or else undesirable whiskers comprise a substantial part of the product.172,173 In addition to nitrogen, purified and dried ammonia can also be used as a nitrogenation atmosphere.35,162 Henna et al. impregnated RHs in ferrous sulfate solution and then soaked them in an ammonia solution and coked them at 700 °C for 30 min in the absence of air. Silicon nitride was finally formed by firing the treated RHs at 1200-1500 °C in an ammonia atmosphere. The amount of silicon nitride increased with iron content to reach an optimum value, and beyond this value, an increase in the amount of silicon carbide formation was noted.170 Some metal oxides, such as Fe2O3, can catalyze the formation of silicon nitride.163,174 In addition to metal oxides, sodium fluoride has also been reported to have a good catalyst effect.163 Silicon nitride manufactured by this method is mainly R phase,11,166,175,176 but in the presence of V2O5 as a catalyst, β-Si3N4 is the predominant phase in the product.173,177 5.2. Summary A method for the production of silicon nitride from RHs has been known for more than 20 years, but it has not yet been industrialized. The main reason lies in the difficulty of obtaining high-purity Si3N4 as it is usually accompanied by SiC. This might be because of the presence of some alkali metals other than silica in RHs, which affect the purity and yield. What is more, the carbon-to-silica ratio is above the stoichiometric requirement for nitridation, so the extra carbon will result in SiC during reaction.57 Therefore, effective removal of the impurities before reaction is the key problem inhibiting industrialization. At present, the main method applied is acid leaching. Whereas the main effect for acid leaching is to remove the metal ions, it has little effect on organic compounds. The combination of acid leaching and a biochemical treatment that can effectively remove the organic component might solve this problem. On the other hand, recently, a SiC‚Si3N4 composite has attracted increasing attention. It has excellent physical and chemical properties and has been widely used in some industries.178-181 Therefore, one approach would be to produce some SiC on purpose and then directly prepare the SiC‚Si3N4 composite. The SiC and Si3N4 made in this way should have a good distribution and intimate contact, which should be helpful for improving the SiC‚Si3N4 composite’s properties. Because the ultrafine Si3N4 powders exhibit even better characteristics than normal Si3N4 powders, many methods have been developed for preparing them, but with higher costs.182-184 The silicon nitride powders from RHs usually are not as fine. Methods for improving the techniques for manufacturing smaller silicon nitride powders from RHs are worth further investigation. The main content of the Si3N4 made from above methods is particles; the whisker content is low. In addition, there are few reports on the preparation of Si3N4 fibers or films from RHs. Silicon nitride films are widely used in microelectronic device fabrication layers,

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gate dielectrics, and antireflective coatings.185 At present, silicon nitride films are mainly made from silane through some CVD methods, such as low-pressure CVD (LPCVD),186 plasma-enhanced CVD (PECVD),187 and laser-assisted PECVD (LAPECVD).187 The direct production of silicon nitride fibers or films from RHs seems impractical at present. However, RHs can be used as raw materials for producing the starting material for the production of silicon nitride film, silane,188,189 to reduce its high cost in other ways. Such studies are under way. 6. Silicon Tetrachloride Silicon tetrachloride is a prominent chemical material having a major impact in the semiconductor industry. It can be used as a silicon source material for the production of organosilicates, silicon esters, organosilicon halides, silicone polymer, etc. Recently, silicon tetrachloride has also been used for manufacturing solar-grade silicon, high-purity SiO2, SiC, and Si3N4.174,190 In the industrial process, silicon tetrachloride is usually produced as a byproduct with other metal chlorides or by the chlorination of SiC, ferrosilicon, or a SiO2/C mixture with Cl2.190 After pyrolysis at appropriate temperatures and pressures, the almost-pure C/SiO2 mixture can be obtained from RHs in a much more intimately dispersed form than could readily be accomplished by mechanical mixing. The mixture can be chlorinated to produce high-purity SiCl4, which avoids the separation procedure.190 6.1. Preparing Silicon Tetrachloride from RHs Chen et al.190 leached RHs in 3 N HCl in a glass round-bottomed flask, which was kept at about 100 °C within a thermostat for 1 h. After the leaching, the RHs were thoroughly washed with distilled water and then dried. The pyrolysis reaction was conducted in a tubular reactor under nitrogen atmosphere with heating at 900 °C for 1 h, and the final husk ash contained 51.18 wt % silica. The pyrolyzed RHs were chlorinated over the range of 700-1100 °C to produce silicon tetrachloride. The conversion of silica was found to be markedly dependent on the temperature of chlorination, and the reaction rate was distinctly faster than the rate for the pure SiO2/C mixture. The bulk of the chlorination of the pyrolyzed RHs occurred in about 7 min, and the subsequent rate of chlorination was very low. It can be inferred that the silica arising from pyrolyzed RHs is derived from the original RHs and that the degree of contact between SiO2 and C in the pyrolyzed RHs is much more intimate than would be obtained from mechanical mixing, giving a high reactivity to the chlorination reaction. In the process of the chlorination reaction, carbon plays the role of reducing agent, and the part of the silica that is dispersed intimately with the carbonaceous material has a higher reactivity. Hence, the reaction rate is very fast at the beginning of the reaction, and the reaction is almost complete after 7 min. To produce SiCl4 more efficiently from active SiO2 in RHs, pyrolyzed RHs can be chlorinated with both alkaline and alkaline earth additives such as potassium compounds in the temperature range of 600-1000 °C. These additives change to chlorides under the condition of chlorination. Potassium compounds accelerate the chlorination of the pyrolyzed RHs, but other alkaline

and alkaline earth compounds such as sodium, magnesium, and calcium inhibit the chlorination. The acceleration effect of potassium in the chlorination of SiO2 was explained by assuming that the diffusion of K+ ions in the SiO2 lattice causes the distortion of the SiO2 lattice and that the chlorinating species such as chlorinated carbon easily diffused into the SiO2 lattice. The inhibitory effect of the other elements was interpreted in terms of the absence of SiO2 lattice distortion, as the ionic radii of these elements are smaller than that of K+. It was also inferred that the melts of these chlorides covered the contact points of SiO2 and C.191,192 In addition, it was reported that the addition of sulfide could also increase the yield.193 6.2. Summary The attractiveness of the manufacture of SiCl4 from RHs lies in the low cost, the high reactivity of the silica content in RHs (low reaction temperature), and the low level of impurities that have to be removed from the product.194 The operation of the process is energetically self-sustaining. Combustion of the carbon residue after chlorination is more than adequate to provide both the heat needed for pyrolysis of the RHs and the heat required for the preheating of the chlorine. Because of the low bulk density of the RHs, the reactor will be larger than those of a conventional plant, but the lower temperature of operation and the absence of the need for electrical heating are expected to lead to comparable reactor costs.194 7. Silicon Low-cost solar-grade silicon is needed for the production of solar cells. At present, solar cells are manufactured using semiconductor-grade silicon. Because the purity level of solar-grade silicon is considerably less than that of semiconductor-grade silicon, the cost of manufacturing solar cells can be considerably reduced by investigating new and inexpensive source materials.195 Therefore, worldwide efforts are being directed at the development of a low-cost, high-volume, and commercially feasible process for the production of highpurity silicon to be used in solar cells. The preparation of pure silicon from RHs is under study. RHs are a relatively high-volume, low-cost byproduct commodity that contains the two basic components needed to produce silicon: silica and carbon. Impurity analyses indicate that RHs from various sources are compositionally similar and that they have low concentrations (10-20 ppmw) of aluminum and iron, the two major impurities in conventional raw materials used to prepare metallurgical silicon. The levels of the major impurities (Ca, K, Mg, and Mn) in RHs can be reduced by about a factor of 100 to around 20 ppmw by hot HCl leaching. The doping impurities boron and phosphorus, important in silicon intended for solar cells, are less affected by acid leaching. Their concentrations were found to be 1 and 40 ppmw, respectively, in leached RHs.196 Other impurities such as Mo, Ti, Ta, Ni, V, and Cr are either absent or present in very low concentrations and can also be easily removed through acid leaching. Hence, RH is a potential source for producing solar-grade silicon.197 7.1. Preparing Silicon from RHs Several approaches have been developed to produce silicon from RHs. Among the various methods of reducing the amorphous silica, reduction by metallic magne-

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sium has been found to be advantageous for the following reasons: (i) the reduction of silica can be carried out at a comparatively low temperature (around 600˚C), (ii) the supply of reasonably pure magnesium is assured, and (3) reaction products other than silicon can easily be removed by acid leaching.89 Usually, some pretreatments such as water leaching and acid leaching at high temperature are performed on the RHs before reaction. The pretreated RHs are first dried and burned to black ashes at relatively low temperature and then burned to white ashes at high temperature (about 600˚C). The prepared white ashes are leached with concentrated acid (usually HCl) at high temperature to remove any soluble impurities, and were then leached with pure water, dried, and ground to a fine powder. The prepared fine powder is reacted with magnesium at about 600 °C. Acid leaching is carried out once more at this point to remove any impurities in the product. Finally, the product is leached with pure water and dried to obtain the pure silicon. Ikram et al.195 acid leached and water leached the products of every step completely. The reduced sample was acid leached with HCl to remove MgO and other impurities that existed in the oxide form. Next, the sample was leached with HF and then with a mixture of HF and H2SO4 to remove, more or less completely, many impurities that were present in the form of silicates. The sample was then washed with distilled water and dried. The pure silicon (99.95%) was extracted from RHs at last. Sodium, magnesium, potassium, calcium, and iron were found to present in the range of 50-150 ppm; boron, aluminum manganese, and titanium were present in very low concentrations, each being less than 10 ppm. Phosphorus was not detected in the sample. Banerjee et al.89 performed some similar studies. They found that the silica in RHs can be reduced by magnesium at an even lower temperature (about 550 °C) than is employed in other conventional methods of reducing silica. This saves energy and, more importantly, minimizes the chances of contaminating the silicon by the reaction vessel and products. The reduction process is highly exothermic and is completed within a very short time (on the order of tens of seconds). The reaction seems to proceed via the interaction of vaporized magnesium with silica (gas-solid reaction), indicating that compaction and effective sealing of the total charge will increase the efficiency of the reaction. The contamination of silicon was probably mainly due to the laboratory-grade magnesium [which contained 0.05% (maximum) of acid-insoluble matter, 0.01% Cu, and 0.02% Fe) and the glassware used in the process. This suggests that the purification of magnesium and the use of Teflon will lead to better-quality silicon.89,198 Because the reaction is so rapid, MgO is sometimes added as a moderator in the mixture to dilute the matrix, thereby reducing the reaction rate. In addition to magnesium, calcium has also been used to reduce the silica in RHs.197,198 Calcium was selected because it is a stronger reducing agent and naturally more abundant than magnesium. When calcium is used in the reduction action, the reaction temperature is a little higher, at about 720 °C. Theoretically, Al and Ba can also be used as reducers in this reaction,197 but few such reports exist. In addition to magnesium and calcium, high-purity carbon can also be used to reduce silica to silicon. The required ratio of C to SiO2 for the production of solar-

grade silicon is 2:1. However, purified RHs have been pyrolyzed to produce a product containing a C/SiO2 ratio of about 4:1; therefore, additional silica was added to RHA or controlled combustion was carried out to adjust the C/SiO2 ratio to lower values.87,199 Hunt et al.196 leached RHs in hot HCl and then coked them in inert gas to further remove impurities. The coked RHs were extruded with sucrose as a binder to produce 5-mmdiameter pellets with an average bulk density of about 800 g/L. The pellets were pyrolyzed in an arc furnace to produce pure silicon. In addition to the above methods, pure silicon can also be prepared by hydrolyzing silicon tetrachloride, an intermediate product that can be made from RHs.200 7.2. Summary From the above methods, it is easy to see that the requirement on purity is very strict. Usually, the product requires acid leaching at high temperature in every step to improve the final product’s purity. Only in this way can the purity of the prepared silicon satisfy the requirements of solar cells. In addition to pure silicon powders, silicon films have also been prepared indirectly from RHs. Nandi et al. prepared hydrogenated amorphous silicon (a-Si:H) films by the chemical vapor deposition of silanes generated by the acid hydrolysis of magnesium silicide (Mg2Si) obtained from RHs.201 The growing interest in recent years in a-Si:H thin films is essentially due to the fact that it is an inexpensive material for the photovoltaic conversion of solar energy as well as for other interesting technological applications, such as thin film transistors, electrophotography, and optical recorders. The films are conventionally prepared by (i) glow discharge, (ii) photochemical vapor deposition, (iii) laser-induced CVD, and (iv) plasma CVD of silanes.201 The techniques mentioned above, however, require vacuum and expensive equipment for deposition. Compared with the above methods, this method is much more economic. The properties of the resulting a-Si:H films are somewhat inferior for immediate photovoltaic applications. By further optimizing the deposition parameters such as gas flow rate, substrate temperature, and geometry of the reactor, incorporating more hydrogen into the films, and minimizing the impurities contained in the films, better-quality films can be developed by this simple and inexpensive method. 8. Zeolite Zeolite, a typical microporous crystalline material, is an attractive material because of its solid acidity, ionexchange capability, adsorption/release capability, and molecular-level pores. Zeolite is usually synthesized under hydrothermal conditions from solutions of sodium aluminate, sodium silicate, or sodium hydroxide.202-204 Natural zeolite was discovered as early as 1756, but the synthesis of zeolite was not realized until 1948.205 By carrying out base treatment on the combusted RHs, high-activity silica that is suitable for zeolite synthesis can be obtained.206-209 As of now, mordenite,208,210 NaX zeolite,206,211 and ZSM zeolite207,212,213 have been successfully synthesized from RHs. 8.1. Preparing Zeolite from RHs Bajpai et al. synthesized mordenite using silica from RHA for the first time in 1981. The RHs were burned for 3 h at 1000 °C to obtain carbon-free ash. The ash

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thus obtained was finely ground and contained 88.86 wt % silica. The silica solution was prepared by treating RHA with 1-2 N NaOH maintained at 80 °C for a few hours with continuous shaking. Then, the solution was filtered through a linen cloth. The silica solution thus prepared was mixed with a solution containing sodium hydroxide and appropriate amounts of aluminum hydroxide. The hot solution of the starting mixture was kept in an autoclave for the hydrothermal reaction. The desired reaction temperature was attained within 3050 min. At the end of each run conducted for a specific period of time, the product was filtered and subsequently washed with hot distilled water until the pH of the filtrate reached 7. The solid product was then dried overnight at 120 °C and identified. The sequence of formation of the products with increasing temperature was in the following direction: amorphous to mordenite to analcime. It was shown that, when the temperature was lower than 135 °C, no mordenite was formed, whereas when the temperature was higher than 165 °C, analcime was formed. Therefore, the proper temperature is between 135 and 165 °C. The starting composition of the mixture is of paramount importance in governing the type of zeolite crystallized. Compared to the synthesis of mordenite using silica from a chemical source, for the synthesis of mordenite with silica from RHA, relatively less Na2O or a greater SiO2 content in the starting mixture is required. This probably can be attributed to the form of silica in the initial mixture. When silica from a chemical source was used, part of it was in the form of sodium silicate in solution, with the remaining part as silica gel. In the runs that involved silica from RHA, the silica was in the form of a silica solution. Because the crystallization of mordenite starts from the liquid phase of the starting sodium aluminosilicate gel, the solubilities of the materials in the final mixture to form the liquid phase that contributed to the mordenite crystallization appeared to be different in the two cases.208,214 Dalai et al.206 synthesized NaX zeolite using RHA as a source of silica for the first time in 1985. The synthesis of NaX zeolite is similar to that of mordenite. Rawtani et al.207 synthesized ZSM-5 zeolite from the sodium tetrapropylammonium (TPA) cation system for the first time using silica from RHA in 1989. In this study, RHA was the only source of the silica and alumina used in the synthesis. RHs were burned in a muffle furnace at 1000 °C for 10 h, and the resultant carbon-free ash was ground in a ball mill to a 200-mesh size. A stoichiometrically required amount of (TPA)OH was added to RHA, yielding a viscous TPA-silicatealuminate solution. The two solutions were mixed in a 150-mL stainless steel autoclave, and the lid of the autoclave was quickly closed to prevent TPA from absorbing carbon dioxide gas from the atmosphere. The reaction vessel was maintained at the desired temperature for a predetermined time. At the end of each run, the vessel was quenched immediately in cold water to stop the crystallization process. The solid products were filtered, washed, and dried overnight at 120 °C. When the mole percent of SiO2 varies from 70 to 80% and that for (TPA, Na)2O from 20 to 35%, pure ZSM-5 zeolite can be obtained as the final product. An increase in the hydroxide ion concentration can shorten the induction period, whereas higher alkalinity beyond a certain value will prolong the nucleation in the synthesis of ZSM-5 zeolite. The synthesis was carried out for temperatures

ranging from 125 to 200 °C and for synthesis durations of 6-120 h. Recently, ZSM-48 zeolite has also been successfully synthesized from RHs.212 It was prepared from a reaction mixture containing a source of silica (RHA), an organic compound (C6DN) having an amine functional group with pKa > 7, an alkali metal oxide (sodium oxide), and water. The mixture was maintained at 170 °C in a stainless steel autoclave until crystals of ZSM48 were formed. 8.2. Summary Zeolite membranes can be used as catalysts in reactors, as sensors for detection, and/or for liquid separations. These properties have attracted the interest of researchers for several years.215 As the silica film, zeolite membrane can also be made from tetraethyl orthosilicate (TEOS),216 so the novel pentacoordinate silicon complexes can also be used here.73 These relatively cheap pentacoordinate silicon complexes can substitute for TEOS as the raw material for preparing zeolite membrane. 9. Others In addition to the materials mentioned above, some other silicon-based materials made from RHs have also been reported, including cordierite,217,218 magnesium silicide,188,219,220 silanes,188,189 potassium silicate,2 sodium silicate,2,221,222 forsterite,223,224 Si-O-C fibers,225,226 sodium silicofluoride,227 sialon,228,229 and gehlenite.230 10. Conclusions Among the silicon-based materials mentioned above, only silicon carbide has a production method from RHs that has been industrialized. The industrializations of the other materials’ productions require further research. In addition, the products from RHs are mainly inorganic materials. In the past few years, Wang et al. have performed a series of studies on the organic applications of RH silica. One of their studies emphasized the synthesis of organosilicon complexes from treated RHs.73,231-233 Compared with carbothermal reduction, this method of synthesizing organosilicon is simple and economical and generates little pollution. Although there is general agreement that the silicon contained in RHs is in the phase of silica,1 because all of the silica derived from RHs has experienced an oxidation process (pyrolysis or oxidative media), no one can confirm that no organosilicon is contained in RHs. Some of the silica has also been found to be bonded covalently to the organic compounds.5 Considering that the formation of silicon-containing compounds in RHs is a biological process, the existence of organosilicon compounds in RHs is possible. If some of the silicon actually exists in the phase of organosilicon, then the direct derivation of organosilicon from RHs will be an easy and economical method of producing organosilicon. Moreover, with the development of bioengineering, direct “planting” of organosilicon in RHs might not be an impossible approach. In the past few years, the biochemistry characteristics of silicon have been a hot topic. On the basis of the developments in biomineralized nanostructured material chemistry, scientists are considering the synthesis

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and assembly of nanaostructured SiO2 using cell membranes and cell wall templates.234 The direct assembly of silicon-containing materials from RHs through biochemistry and nanochemistry might be a most potential future use of RHs. Acknowledgment The authors thank the four anonymous reviewers for helpful comments. L.Y.S. thanks Dr. Qiaoli Liang and Dr. Joseph S. Thrasher for helpful discussions. K.C.G. acknowledeges the financial support of the National Natural Science Foundation of China (No. 29974009). 11. Literature Cited (1) Houston, D. F. Rice: Chemistry and Technology; American Association of Cereal Chemists, Inc.: St Paul, MN, 1972 (2) Karera, A.; Nargis, S.; Patel, S.; Patel, M. Silicon-based materials from rice husk. J. Sci. Ind. Res. 1986, 45, 441. (3) Huang, Y. E. Comprehensive utilization of rice husk. Hebei Huagong (China) 1993, (4), 51. (4) Ding, M. Rice husk silicon and its applications. Inorg. Chem. Ind. 1992, 24 (6), 36. (5) Patel, M.; Karera, A. SiC whisker from rice husk: Microscopic study. Powder Metall. Int. 1991, 23 (1), 30. (6) Krishnarao, R. V.; Godkhindi, M. M. Distribution of silica in rice husks and its effect on the formation of silicon carbide. Ceram. Int. 1992, 18, 243. (7) Sharma, N. K.; Williams, W. S.; Zangvil, A. Formation and structure of silicon carbide whiskers from rice hulls. J. Am. Ceram. Soc. 1984, 67, 715. (8) Ault, N. N.; Crowe, J. T. Silicon carbide. Am. Ceram. Soc. Bull. 1989, 68, 1062. (9) He, E. G.; Wang, X. G.; Shang, W. Y.; Chen, W. The environment pollution in SiC industry and the prevention measure. Environ. Technol. 1999, 17 (3), 39. (10) Bray, D. J. Silicon carbide whiskers. Am. Ceram. Soc. Bull. 1993, 72 (6), 116. (11) Kumar, B.; Godkhindi, M. M. Studies on the formation of SiC, Si3N4 and Si2N2O during pyrolysis of rice husk. J. Mater. Sci. Lett. 1996, 15, 403. (12) Wang, Q. B.; Guo, M. X.; Xu, H. Study on synthesizing SiC whiskers from rice hulls and to be reinforcement for Si3N4 ceramic matrix composite materials. Adv. Ceram. 1997, 18 (4), 13. (13) Lee, J. G.; Cutler, I. B. Formation of silicon carbide from rice hulls. Am. Ceram. Soc. Bull. 1975, 54 (2), 195. (14) Sacher, I. Contributions to the use of rice husks and rice husk ash in ceramics. Sprechsaal 1988, 121, 1081. (15) Krishnarao, R. V.; Mahajan, Y. R.; Kumar, T. J. Conversion of raw rice husks to SiC by pyrolysis in nitrogen atmosphere. J. Eur. Ceram. Soc. 1998, 18, 147. (16) Krishnarao, R. V.; Godkhindi, M. M.; Chakraborty, M.; Mukunda, P. G. Formation of SiC whiskers from compacts of raw rice husks. J. Mater. Sci. 1994, 29, 2741. (17) Krishnarao, R. V.; Mahajan, Y. R. Formation of SiC whiskers from raw rice husks in argon atmosphere. Ceram. Int. 1996, 22, 353. (18) Krishnarao, R. V.; Godkhindi, M. M.; Chakraborty, M. Maximization of SiC whiskers yield during the pyrolysis of burnt rice husks. J. Mater. Sci. 1992, 27, 1227. (19) Krishnarao, R. V.; Godkhindi, M. M.; Mukunda, P. G.; Chakraborty, M. Direct pyrolysis of raw rice husks for maximization of silicon carbide whisker formation. J. Am. Ceram. Soc. 1991, 74, 2869. (20) Raju, C. B.; Verma, S. SiC whiskers from rice hulls: Formation, purification, and characterization. Br. Ceram. Trans. 1997, 96 (3), 112. (21) Mizuki, E.; Okumura, S.; Saito, H.; Murao, S. Formation of silicon carbide from rice husks using enzymatic methods for carbon control. Bioresour. Technol. 1993, 44 (1), 47. (22) Wang, Q. B.; Guo, M. X.; Han, M. F. A study on the ratedetermining step of growing SiC whisker. J. Mater. Eng. 1997, (1), 21. (23) Wang, Q. B.; Guo, M. X.; An, Z. Study on the growth of single β-SiC whisker and the role of catalyst. High Technol. Lett. 1996, 6 (10), 41.

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Received for review March 26, 2001 Revised manuscript received July 20, 2001 Accepted September 13, 2001 IE010284B