Characterization of Silica Produced from Rice Husk Ash: Comparison of Purification and Processing Methods

Fernandes, Iara Janaína; Calheiro, Daiane; Sánchez, Felipe A. L.; Camacho, Alini Luísa Diehl; Rocha, Tatiana Louise Avila de Campos; Moraes, Carlos Alberto Mendes; Sousa, Vânia Caldas de

doi:10.1590/1980-5373-MR-2016-1043

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Articles • Mat. Res. 20 (Suppl 2) • 2017 • https://doi.org/10.1590/1980-5373-MR-2016-1043 linkcopy

Characterization of Silica Produced from Rice Husk Ash: Comparison of Purification and Processing Methods

Authorship SCIMAGO INSTITUTIONS RANKINGS

Abstract

As a byproduct of the combustion of rice husk to generate energy, rice husk ash (RHA) is formed by silica and carbon, apart from small amounts of other constituents. Several treatments can be used to increase the purity of the silica obtained, or even produce pure silica. The present study tested the efficiency of different techniques to obtain silica, characterizing and comparing the silicas obtained from RHA. A literature review was conducted, and then selected techniques were used to produce silica, which was characterized by XRF, XRD, particle sizing, specific weight, specific surface area, total carbon, and SEM. The literature review showed that most techniques include a pretreatment like acid or alkaline leaching followed by thermal treatment to increase the amount of silica produced by reduction of carbonaceous materials. The results showed that it is possible to produce silica from RHA using simple methods, and that these produced silica with purity above 98%. The treatments that afforded the best results were acid leaching followed by thermal treatment at 800ºC, and alkaline extraction at low temperature, with silica purity of 99.3% and 99.6%, respectively.

Keywords:
rice husk ash; silica; thermal treatment

Study	Objective	Summary of method used	Main results
Sankar et al.⁸	To produce nanosilicon powder from three kinds of rice husk.	• Combustion of rice husk in the open. • Acid leaching • Incineration at 700ºC under atmospheric conditions	Spherical, completely amorphous silica particles with large specific surface area and composed only by Si and O from all kinds of rice husk.
Bakar⁹	To investigate the ideal conditions to obtain high purity silica.	• Washing of rice husk with water • Acid leaching • Incineration at 500, 600, 700, 800, and 900 °C for 2 h under atmospheric conditions	All silicas had amorphous particles, and rice husk leached with HCl produced the highest content of silica (99.582%) at 600 ºC.
Ma et al.³	To develop a new, recycling-based technique to produce silica from RHA.	• Acid leaching of RHA • Extraction in reactor with NH₄F • Acid precipitation of silica	Spherical, completely amorphous silica particles measuring 50 to 60 nm in diameter and containing only Si and O, with yield of 94.6%.
Liou¹⁰	To investigate the effect of experimental conditions on the characteristics of the nanosilica obtained.	• Leaching of rice husk with water and then HCl • Incineration • Extraction with NaOH, forming silicate • Precipitation	Best results with pH 3, silicate 0.15 M, aging time of 12 h at 50ºC, with 99.48% purity silica.
Fernandes¹¹	To evaluate the leaching of RHA with NaOH to easily obtain highly reactive colloidal silica.	• Extraction of silica with NaOH 1M • Precipitation using H₂SO₄ drop by drop	The best Si:Na mass ratio was 4:1, and the highly reactive colloidal silica was obtained at low energy investment.
Yalçin and Sevinç¹²	To obtain high purity silica with large specific surface area and to evaluate the competitiveness of silica from rice husk.	• Washing of rice husk with water • Chemical treatments (acid leaching, alkaline leaching) • Incineration at 600ºC under different conditions (static oven, argon flow, oxygen flow, air flow)	Amorphous silica particles with maximum specific surface area of 321 m²/g 99.66% purity, depending on the treatment. The highest purity silica was obtained with prior acid treatment and incineration in oxygen atmosphere.
Kalapathy⁷	Investigate the efficacy of acid leaching of RHA before alkaline extraction and washing of sílica obtained with water.	• Acid leaching • Extraction of silica with NaOH • Precipitation with HCl • Washing with water	Initial acid leaching did not improve purity, the washing with water reduced Na and K levels, and silica yield was excellent when extraction was carried out with NaOH 1 N.
Conradt¹³	To obtain high purity silica with large specific surface area and to evaluate the competitiveness of silica from rice husk.	• Washing rice husk with water • Different chemical treatments (acid leaching, alkaline leaching, and enzymatic digestion) • Incineration at 600ºC (static oven, air flow, steam)	Except for leaching with NaOH, all other treatments produced amorphous silica with large specific surface area, and high purity silica was obtained with acid leaching with HCl.
Riveros⁴	To optimize the production of silica from rice husk.	• Washing rice husk with water • Acid leaching • Incineration • Leaching of RHA	Silica with approximate purity of 99.98% with three washing cycles, 6-h acid leaching, 6 leaching cycles with HCl 3% at 90ºC, and the main contaminant was Ca (100 ppm).

Method and References that treatments performed were based on	Treatment	Abbreviation
Thermal treatment of RHA at different temperatures with no pretreatment⁴, ⁸, ⁹, ¹² and ¹³	The samples were heated in a muffle oven in air at 700ºC and 800ºC for 1, 2, and 3 h with heating rate of 5 ºC/min and spontaneous cooling in the oven down to environment temperature.	TT 700 1h TT 700 2h TT 700 3h TT 800 1h TT 800 2h TT 800 3h
Acid leaching followed by thermal treatment⁴, ⁸, ⁹, ¹² and ¹³	Washing, contact time, and acid solution were selected based on a literature review. RHA samples were washed with HCl 1M (10 g sample/100 mL acid) for 1 h. RHA was filtered and washed with deionized water upon neutral pH. Samples were dried at 105ºC for 24 h and then treated in a muffle stove in air at 800ºC for 1 h at 5 ºC/min and cooling in the oven down to environment temperature.	LX TT 800 1h
Alkaline extraction at low temperature³, ⁷, ¹⁰ e ¹¹	Extraction based on the solubility of amorphous silica in alkaline pH and precipitation in lower pH. All parameters used were based on a literature review of the articles shown in Table 1. Briefly, 200 mL NaOH 1 M were added to 20 g RHA. The material was boiled for 1 h with constant stirring. After cooling, samples were filtered and the filtrate obtained was titrated with HCl 1 M upon neutral pH. After aging time of 48 h, water was added and the mixture was centrifuged and washed serially. The material was dried in a stove at 105ºC for 48 h.	EXT NaOH

Sample	Mean diameter (µm)	Specific weight (g/cm³)	Surface area (m²/g)
RHA FB	19.56 ± 0.49	2.22 ± 0.0028	11.35 ± 0.21
TT 700 1h	18.53 ± 0.04	2.23 ± 0.0030	2.83 ± 0.046
TT 700 2h	18.01 ± 0.10	2.23 ± 0.0017	2.78 ± 0.047
TT 700 3h	17.72 ± 0.01	2.24 ± 0.0017	2.74 ± 0.045
TT 800 1h	22.55 ± 0.23	2.23 ± 0.0011	1.70 ± 0.042
TT 800 2h	22.60 ± 0.30	2.21 ± 0.0026	1.70 ± 0.040
TT 800 3h	22.78 ± 0.11	2.22 ± 0.0036	1.46 ± 0.039
LX TT 800 1h	20.10 ± 0.59	2.20 ± 0.0034	2.97 ± 0.017
EXT NaOH	205.10 ± 4.60	2.13 ± 0.0162	290.03 ± 1.21

Sample/ Composition	RHA FB	TT 700 1h	TT 700 2h	TT 700 3h	TT 800 1h	TT 800 2h	TT 800 3h	LX TT 800 1h	EXT NaOH
SiO₂	96.73	98.71	98.79	98.68	98.87	98.94	98.99	99.25	99.61
Al₂O₃	0.09	0.09	0.07	0.07	0.04	ND	0.02	0.09	ND
P₂O₅	0.23	0.07	ND	0.06	0.04	ND	ND	ND	ND
SO₃	0.06	0.06	0.06	0.05	0.07	0.06	0.05	0.04	0.06
Cl	0.01	ND	ND	ND	ND	ND	ND	ND	0.08
K₂O	0.69	0.72	0.72	0.73	0.71	0.71	0.71	0.48	0.06
CaO	ND	ND	ND	ND	ND	ND	ND	ND	ND
MgO	ND	ND	ND	ND	ND	ND	ND	ND	ND
TiO₂	ND	ND	ND	ND	ND	ND	ND	ND	ND
MnO	0.01	0.02	0.02	0.02	0.02	0.02	0.02	0.04	0.10
Fe₂O₃	ND	ND	ND	ND	ND	ND	ND	ND	ND
ZnO	ND	0.01	0.01	0.01	0.01	0.01	0.01	ND	ND
TC	2.18	0.32	0.33	0.38	0.25	0.26	0.21	0.09	0.10

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