Abstract

1. Introduction

Rice husk and rice husk ash (RHA) are interesting sources of considerable levels of high quality silica, which has several applications¹1 Ugheoke BI, Mamat O, Ari-Wahjoedi B. A direct comparison of processing methods of high purity rice husk silica. Asian Journal of Scientific Research. 2013;6(3):573-580.. The production of silica at industrial scale is based on mechanical, physical, chemical, and energy-intensive thermal operations at high temperatures using large amounts of acids, generating significant volumes of effluents²2 Haus R, Prinz S, Priess C. Assessment of High Purity Quartz Resources. In: Götze J, Möckel R, eds. Quartz: Deposits, Mineralogy and Analytics. Berlin, Heidelberg: Springer; 2012. p. 29-51.. In this sense, Ma et al.³3 Ma X, Zhou B, Gao W, Qu Y, Wang L, Wang Z, et al. A recyclable method for production of pure silica from rice hull ash. Powder Technology. 2012;217:497-501. highlight the fact that the conventional silica production method is based on the reaction between sodium carbonate and quartz at high temperatures. After sodium silicate is formed, silica is precipitated in a reaction with sulfuric acid. But this method also consumes high levels of energy and produces large amounts of liquid effluents and greenhouse gases.

Riveros and Garza⁴4 Riveros H, Garza C. Rice husks as a source of high purity silica. Journal of Crystal Growth. 1986;75(1):126-131. discussed the fact that, compared with other silica sources like sand, bentonite, and diatomaceous earth rice husk has very small amounts of contaminants that affect performance in applications requiring high purity, as in the manufacture of solar panels, for instance. Therefore, rice husk is an important source of silica and solar-grade silicon. Jung et al.⁵5 Jung DS, Ryou MH, Sung YJ, Park SB, Choi JW. Recycling rice husks for high-capacity lithium battery anodes. Proceedings of the National Academy of Sciences of the United States of America. 2013;110(30):12229-12234. also claims that rice husk has exclusive nanoporous silica layers, which have developed throughout years of natural evolution of the plant. This is why producing highly reactive silica from rice husk is a simple process with several advantages, compared with conventional production methods⁶6 Rafiee E, Shahebrahimi S, Feyzi M, Shaterzadeh M. Optimization of synthesis and characterization of nanosilica produced from rice husk (a common waste material). International Nano Letters. 2012;2:29..

High purity is required in several applications of silica. Silica is a basic raw material widely used in the semiconductors, ceramics, polymers, and materials industries. This silica is usually produced from quartz fused at high temperatures, which affords to obtain ultrapure polycrystalline silicon and silicon hydride. In addition to environmental and economic advantages, low-energy, simpler methods to obtain pure silica create opportunities for the development of new industrial applications of RHA⁷7 Kalapathy U, Proctor A, Shultz J. A simple method for production of pure silica from rice hull ash. Bioresource Technology. 2000;73(3):257-262..

Various methods are used to produce larger amounts or pure or ultrapure silica from rice husk or RHA. Table 1 shows a list of studies that looked into different treatments to reduce contamination, improve the properties of RHA, or even produce pure silica.

Table 1 also makes it clear that numerous approaches have been developed to produce high purity amorphous silica with large surface area using rice husk or RHA. Several studies³3 Ma X, Zhou B, Gao W, Qu Y, Wang L, Wang Z, et al. A recyclable method for production of pure silica from rice hull ash. Powder Technology. 2012;217:497-501.^,77 Kalapathy U, Proctor A, Shultz J. A simple method for production of pure silica from rice hull ash. Bioresource Technology. 2000;73(3):257-262.^,1010 Liou TH, Yang CC. Synthesis and surface characteristics of nanosilica produced from alkali-extracted rice husk ash. Materials Science and Engineering B. 2011;176(7):521-529.^,1111 Fernandes AA. Síntese de Zeólitas e Wolastonita a Partir da Cinza da Casca do Arroz. [Thesis]. São Paulo: Universidade de São Paulo; 2006. used alkaline extraction sol-gel method, which forms silicates, followed by silica precipitation by reaction with an acid. Kalapathy et al.⁷7 Kalapathy U, Proctor A, Shultz J. A simple method for production of pure silica from rice hull ash. Bioresource Technology. 2000;73(3):257-262. explained that it is possible to obtain amorphous silica from RHA by alkaline extraction at low temperatures. Also, at pH below 10, the solubility of amorphous silica is very low, but it rises suddenly when pH is increased above that. This solubility profile affords to extract silica from RHA by solubilization in alkaline medium and subsequent precipitation at lower pH values. Consuming comparatively lower amounts of energy, the alkaline extraction of silica by solubilization is not as costly nor as damaging to the environment as the quartz fusing method.

Other authors used thermal treatments at different temperatures and atmospheres with or without pretreatments⁴4 Riveros H, Garza C. Rice husks as a source of high purity silica. Journal of Crystal Growth. 1986;75(1):126-131.^,88 Sankar S, Sharma SK, Kaur N, Lee B, Kim DY, Lee S, et al. Biogenerated silica nanoparticles synthesized from sticky, red, and brown rice husk ashes by a chemical method. Ceramics International. 2016;42(4)4875-4885.^,99 Abu Bakar R, Yahya R, Gan SN. Production of High Purity Amorphous Silica From Rice Husk. Procedia Chemistry. 2016;19:189-195.^,1212 Yalçin N, Sevinç V. Studies on silica obtained from rice husk. Ceramics International. 2001;27(2):219-224.^,1313 Conradt R, Pimkhaokham P, Leela-Adisorn U. Nano-structured silica from rice husk. Journal of Non-Crystalline Solids. 1992;145:75-79.. Yalçin and Sevinç¹²12 Yalçin N, Sevinç V. Studies on silica obtained from rice husk. Ceramics International. 2001;27(2):219-224. demonstrated that incineration of rice husk at between 600ºC and 800ºC preceded by chemical and thermal pretreatment affords to obtain high purity silica. The authors used different chemical and thermal treatment approaches, but reported that acid leaching pretreatment followed by oxygen-rich incineration and combustion of rice husk produced the highest silica content. The atmosphere with the highest partial pressure of oxygen induced the chemical reduction of carbon, and was more efficient than the other atmosphere settings evaluated.

Despite the numerous approaches to produce and purify silica, RHA is virtually not used for this purpose. Most studies to date have addressed the production of silica directly from rice husk, not from its ash. Few investigations have looked into the potentialities of RHA as a source of silica, and the samples used generally were RHA produced under controlled laboratory conditions. The production of silica from RHA is very interesting from the industrial perspective, in view of fact that rice husk is normally used as direct source of energy. Yet another important aspect is that the purification method used today normally affects the features of the end product, changing morphology, structures, and reactivity¹⁴14 Kaviyarasu K, Manikandan E, Kennedy J, Jayachandran M, Maaza M. Rice Husks As A Sustainable Source Of High Quality Nanostructured Silica For High Performance Li-Ion Battery Requital By Sol-gel Method - A Review. Advanced Materials Letters. 2016;7(9)684-696..

This study characterized and compared the silicas produced from RHA using different extraction and purification methods. We analyzed the effect of each process on the characteristics of the silicas produced and discuss the advantages of each approach.

2. Materials and Methods

2.1 Materials

The RHA used in this study (called RHA FB) was supplied by a thermoelectric company established in the state of Rio Grande do Sul, Brazil. The company uses rice husk as biomass for the generation of energy by fluidized bed combustion. Briefly, rice husk is loaded into the reactor and instantly burned on a sand bed with ascending air flow at around 700ºC. After combustion, RHA is milled by the producer company.

2.2 Processing and purification of RHA

The different processing and purification techniques were compared for the amount and properties of silica produced based on the literature review of the studies listed in Table 1. The different processing and purification treatments performed are compiled in Table 2.

2.3 Characterization of RHA and silica

Particle sizing was carried out by wet laser diffraction in a particle analyzer (S3500, Microtac) using water as solvent containing one drop of surfactant. Flowrate was 25% and ultrasound time was approximately 30 s. All samples were considered to have a refraction index of 1.6.

Specific weight was determined using a helium pycnometer (AccuPyc II 1340, Micrometrics). After drying in a oven at 105ºC, approximately 5 g of each kind of RHA were placed in a 10-cm³ aluminum sample holder.

Specific surface area of samples was evaluated according to the nitrogen BET (Brunauer-Emmett-Teller) adsorption technique in a surface area analyzer (TriStar II Plus, Micrometrics). Samples were degassed under vacuum at 200ºC for 34 h.

Chemical composition was analyzed comparatively in an energy dispersive X-ray fluorescence (XRF) spectrometer (EDX 720 HS, Shimadzu). The results are expressed as percent by mass calculated over oxide content and normalized to 100% using carbon percent levels. The samples were previously ground and compared with a secondary RHA standard.

Total carbon (TC) was determined using previously dried samples according to the total combustion method at 1,350ºC and infrared detection in a carbon analyzer (SC-144DR, Leco).

The mineralogical composition of samples was analyzed by X-ray diffraction (XRD) in an X-ray diffractometer (X'PERT, Philips) with copper emission lines (CuKα → λ = 1.5418 Å). The experimental conditions were: 40 kV, 40 mA, 2θ scanning range from 5 to 75°, angular step of 0.05°. Peaks were identified using the software X1Pert High Score 2.0a.

The morphological characteristics of gold-plated samples were investigated by scanning electron microscopy (SEM) (EVO MA15, Zeiss) under high vacuum and detection of secondary electrons.

3. Results and Discussion

Table 3 shows particle sizing results (obtained by laser diffraction) and specific weight and specific surface area values. The specific weight of samples, including the control sample without treatment, RHA FB, varied little, between 2.13 g/cm³ and 2.24 g/cm³. Obtaining silica from RHA using different chemical and thermal treatments (500ºC - 800ºC), Yalçin and Sevinç¹²12 Yalçin N, Sevinç V. Studies on silica obtained from rice husk. Ceramics International. 2001;27(2):219-224. observed that the density of most samples increased with silica contents. In the present study, specific weight of samples did not vary with silica levels. However, this may have been due to the fact that silica levels likewise did not oscillate significantly between RHA samples obtained under the conditions assessed.

Nevertheless, the surface area and mean diameter of silica particles varied significantly using different settings. The positive correlation between surface area with size, shape, and porosity has been discussed in the liteature¹⁵15 Wypych G. Handbook of Fillers. 2nd ed. Toronto: ChemTec Publishing; 2000.. The results we obtained here show that specific surface area was influenced less by particle size than by porosity and shape, since the sample with the longest mean diameter was also the one with the largest surface area. At 290 m²/g, the sample obtained by sol-gel method at low temperature (EXT NaOH) had a significantly large surface area, compared to other samples. This may be associated with the sol-gel process used to produce EXT NaOH. Large surface area is desirable in many applications, such as in the reinforcement of polymer compounds. At any rate, it is necessary to consider the chances that, at approximately 205 µm, the diameter of the particles obtained by the EXT NaOH method reached that length because the material is an agglomerate, which may have influenced surface area due to the markedly irregular shape of particles.

The results of the present study also indicate that specific surface area decreases with the temperature of thermal treatment. Surface area is reduced with thermal treatments. The untreated sample had specific surface area of 11.35 m²/g. Yet, independently of time, specific surface area of silica particles obtained at 700ºC was around 2.8 m²/g, while for those obtained at 800ºC the values ranged between 1.4 m²/g and 1.7 m²/g. In previous studies by our research group¹⁶16 Fernandes IJ, Sánchez FAL, Jurado JR, Kieling AG, Rocha TLAC, Moraes CAM, et al. Physical, chemical and electric characterization of thermally treated rice husk ash and its potential application as ceramic raw material. Advanced Powder Technology. 2017;28(4):1228-1236., thermal treatments at even higher temperatures (1000ºC, 1200ºC, and 1400ºC) reduced specific surface area of RHA even more, reaching 0.44 m²/g. Such finding was also observed by Bie et al.¹⁷17 Bie RS, Song XF, Liu QQ, Ji XY, Chen P. Studies on effects of burning conditions and rice husk ash (RHA) blending amount on the mechanical behavior of cement. Cement and Concrete Composites. 2015;55:162-168. and Kapur¹⁸18 Kapur PC. Production of reactive bio-silica from the combustion of rice husk in a tube-in-basket (TiB) burner. Powder Technology. 1985;44(1):63-67., who showed that the surface area of RHA is influenced by (i) combustion temperature (the higher the temperature, the smaller the resulting specific surface area); (ii) the amount of potassium present (high potassium levels reduce specific surface area and increase mean diameter, since the element works as fusing agent); and (iii) the material structure (crystalline silica has smaller surface area, compared to amorphous silica). In the present study, the sample previously leached with acid solution (LX TT 800 1h) had slightly larger specific surface area, compared with the other samples treated at the same temperature. So, it may be said that prior acid leaching increased porosity, or that the larger surface area values recorded are associated with potassium levels.

We also observed that the mean diameter of particles increased with temperature. The treatments at 700ºC and 800ºC produced particles with mean diameters of approximately18 µm and 22.6 µm, respectively. This longer mean diameter may result from the mild pre-fusion of particle surfaces, possibly due to the presence of potassium. Mean diameter of particles of the sample pretreated with acid leaching was slightly shorter than that of the other samples treated at 800ºC, which may also be associated with the lower potassium content in this sample.

The sample EXT NaOH had mean diameter of 205.10 µm. In addition, the specific surface area of EXT NaOH particles was of another order of magnitude, not affording comparison with the other treatments performed in this study.

The results of the chemical characterization by XRF and TC levels are shown in Table 4.

All treatments produced RHA with high silica content and low TC levels. The thermal treatments used did not affect the chemical composition of samples consistently. Potassium was the main contaminant, varying between 0.48 and 0.70. Also, high treatment temperatures produced samples with lower TC. But the sample obtained by alkaline extraction at low temperature (EXT NaOH) had high silica level, with Cl as a contaminant. This is explained in view of the treatment adopted, which included silica precipitation with HCl. Additional washing with deionized water may help remove this residual contaminant. It is also possible to see that, at both 700ºC and 800ºC, incineration time did not substantially influence composition, meaning that the period of 1 h of thermal treatment is enough to produce desirable results.

The highest silica content was obtained for the RHA submitted to the EXT NaOH treatment (99.61%), followed by the one submitted to the LX TT 800 1h protocol (99.25%). The other thermal treatments at 700ºC and 800ºC produced similar contents of silica (between 98.7% and 98.9%). The two samples that had the highest silica levels were also those with the lowest TC content, EXT NaOH (0.10%) and LX TT 800 1h (0.09%).

Percent silica levels obtained in this study are considerably high, compared with data published in the literature (Table 1). The main reason may be the fact that the studies that obtained the highest silica contents, namely 99.98%⁴4 Riveros H, Garza C. Rice husks as a source of high purity silica. Journal of Crystal Growth. 1986;75(1):126-131., 99.66%¹²12 Yalçin N, Sevinç V. Studies on silica obtained from rice husk. Ceramics International. 2001;27(2):219-224., 99.58%⁹9 Abu Bakar R, Yahya R, Gan SN. Production of High Purity Amorphous Silica From Rice Husk. Procedia Chemistry. 2016;19:189-195., and 99.48%¹⁰10 Liou TH, Yang CC. Synthesis and surface characteristics of nanosilica produced from alkali-extracted rice husk ash. Materials Science and Engineering B. 2011;176(7):521-529. used rice husk as raw material, not RHA. In fact, considering the studies listed in Table 1, most⁴4 Riveros H, Garza C. Rice husks as a source of high purity silica. Journal of Crystal Growth. 1986;75(1):126-131.^,88 Sankar S, Sharma SK, Kaur N, Lee B, Kim DY, Lee S, et al. Biogenerated silica nanoparticles synthesized from sticky, red, and brown rice husk ashes by a chemical method. Ceramics International. 2016;42(4)4875-4885.^,99 Abu Bakar R, Yahya R, Gan SN. Production of High Purity Amorphous Silica From Rice Husk. Procedia Chemistry. 2016;19:189-195.^,1010 Liou TH, Yang CC. Synthesis and surface characteristics of nanosilica produced from alkali-extracted rice husk ash. Materials Science and Engineering B. 2011;176(7):521-529.^,1212 Yalçin N, Sevinç V. Studies on silica obtained from rice husk. Ceramics International. 2001;27(2):219-224.^,1313 Conradt R, Pimkhaokham P, Leela-Adisorn U. Nano-structured silica from rice husk. Journal of Non-Crystalline Solids. 1992;145:75-79. obtained silica from rice husk, while a smaller number³3 Ma X, Zhou B, Gao W, Qu Y, Wang L, Wang Z, et al. A recyclable method for production of pure silica from rice hull ash. Powder Technology. 2012;217:497-501.^,77 Kalapathy U, Proctor A, Shultz J. A simple method for production of pure silica from rice hull ash. Bioresource Technology. 2000;73(3):257-262.^,1111 Fernandes AA. Síntese de Zeólitas e Wolastonita a Partir da Cinza da Casca do Arroz. [Thesis]. São Paulo: Universidade de São Paulo; 2006. used RHA.

Ugheoke and Mamat¹⁹19 Ugheoke IB, Mamat O. A critical assessment and new research directions of rice husk silica processing methods and properties. Maejo International Journal of Science and Technology. 2012;6(3):430-448. explain that one of the reasons why it is more difficult to obtain silica with purity above 97% from rice husk by direct incineration is the presence of metallic contaminants, especially potassium and sodium oxides. These compounds affect the surface of silica particles, increasing their surface area and reactivity, and increase silica crystallization rate. Acid leaching followed by thermal treatment (LX TT 800 1h) induced the production of silica with the lowest carbon level (0.09%). Krishnarao et al.²⁰20 Krishnarao RV, Subrahmanyam J, Kumar TJ. Studies on the formation of black particles in rice husk silica ash. Journal of European Ceramic Society. 2001;21(1):99-104. explained that acid leaching prevents the formation of black particles, since it removes potassium, the main agent responsible for the presence of unburnt carbon in ash. The authors showed that potassium works as catalyst in the crystallization of silica and, when temperature rises above the dissociation temperature of K₂O (approximately 347ºC), the surface of ash particles begins to melt, blocking the transportation of oxygen and CO₂ and increasing the amount of unburnt carbon. Therefore, the low potassium level (0.48%) found in this sample may explain the smaller amount of carbon.

In this sense, a pretreatment strategy to remove metallic contaminants affords to obtain high purity silica with particles of larger specific surface area, compared with direct combustion¹⁹19 Ugheoke IB, Mamat O. A critical assessment and new research directions of rice husk silica processing methods and properties. Maejo International Journal of Science and Technology. 2012;6(3):430-448..

Figure 1 presents the diffractograms of the samples analyzed. Several studies have used different time and temperature conditions to produce amorphous or crystalline silica. However, most authors agree that amorphous silica is produced at temperatures below 800ºC. Several studies have shown that higher temperatures induce the formation of crystalline dots.¹⁸18 Kapur PC. Production of reactive bio-silica from the combustion of rice husk in a tube-in-basket (TiB) burner. Powder Technology. 1985;44(1):63-67.^,1919 Ugheoke IB, Mamat O. A critical assessment and new research directions of rice husk silica processing methods and properties. Maejo International Journal of Science and Technology. 2012;6(3):430-448.^,2121 Cordeiro LDNP. Análise da variação do índice de amorfismo da cinza de casca de arroz sobre a atividade pozolânica. [Dissertation]. Porto Alegre: Universidade Federal do Rio Grande do Sul; 2009.^,2222 Ferro WP, Silva LGAE, Wiebeck H. Uso da cinza da casca de arroz como carga em matrizes de poliamida 6 e poliamida 6.6. Polímeros. 2007;17(3):240-243.

The results of the present study confirm previous findings, since all treatments led to the generation of predominantly amorphous silica. Kapur¹⁸18 Kapur PC. Production of reactive bio-silica from the combustion of rice husk in a tube-in-basket (TiB) burner. Powder Technology. 1985;44(1):63-67. demonstrated that the cristobalite phase starts at 800°C. The results of the present study confirm the findings of the author, since, although they are predominantly amorphous, the samples treated at 800ºC presented the cristobalite phase as main peak. The silica obtained by alkaline extraction at low temperature (EXT NaOH) was completely amorphous. This is due to the fact that NaOH solubilizes only amorphous silica, which is subsequently precipitated. The sample pretreated with acid leaching (LX TT 800 1h) did not differ from the sample that was incinerated at the same temperature but without previous leaching (TT 800 1h).

In a study that tested various temperatures between 500ºC and 900ºC to incinerate rice husk, Bakar et al.⁹9 Abu Bakar R, Yahya R, Gan SN. Production of High Purity Amorphous Silica From Rice Husk. Procedia Chemistry. 2016;19:189-195. used raw samples and previously leached ones, and obtained amorphous silica. However, the authors highlighted that the sharpness of the peak increases with combustion temperature, mainly for non-leached samples, indicating that silica crystallization starts at around 900ºC and that the removal of alkaline species during acid leaching is an obstacle to the eutectic reaction of silica. The behavior predicted in Figure 1 is similar to the findings reported by Bakar et al.⁹9 Abu Bakar R, Yahya R, Gan SN. Production of High Purity Amorphous Silica From Rice Husk. Procedia Chemistry. 2016;19:189-195., since the sample heated to 800ºC for 3 h had a sharper peak, compared to the others.

Figure 2 shows the SEM microphotographs of silica obtained from RHA samples subjected to the different treatments tested. The silica particles produced from all samples were irregular and jagged. Can be observed agglomerates of particles below 10 µm. Although the different pretreatments used affected the chemical composition of the silicas obtained, it seems that the structure and the morphology of these particles was not altered.

4. Conclusion

The results of the present study show that it is possible to produce silica from RHA using simple processes. The various production methods tested afforded to obtain silica with purity above 98%, especially acid leaching followed by thermal treatment and sol-gel alkaline extraction at low temperature, when silica contents were between 99.3% and 99.6%, respectively. Alkaline extraction at low temperature also produced silica particles with large specific surface area (approximately 290 m²/g), compared with the other treatments evaluated, and proving to be the most advantageous technique.

The purity values observed, between 98.7% and 99.6%, are high, compared with data reported in the literature, especially considering the fact that most studies extracted silica directly from unburnt rice husk.

5. Acknowledgements

The authors acknowledge CAPES (Coordination of the Improvement of Higher Education Personnel), CNPq (National Council for Scientific and Technological Development), and FAPERGS (Foundation for Research Support of Rio Grande do Sul) for financial support.

6. References

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