Journal of the Chinese Chemical Society, 2010, 57, 411-416
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Study on the Thermochemical and Kinetic Characteristics of Alkali Treated Rice Husk I. G. Markovska,a B. Bogdanov,a N. M. Nedelchev,b,* K. M. Gurova,c M. H. Zagorchevac and L. A. Lyubcheva a
b
Dept. of Silicate Technology, Prof. Assen Zlatarov University, Burgas 8010, Bulgaria Dept. Comp. & Informat. Technology, Prof. Assen Zlatarov University, Burgas 8010, Bulgaria c Central Research Laboratory, Prof. Assen Zlatarov University, Burgas 8010, Bulgaria
The effective utilization of waste rice husk requires detailed studies on the kinetics and mechanism of their thermal and chemical treatment. The present work is a comparative study on the thermo-chemical and kinetic characteristics of thermal degradation of raw and treated with NaOH rice husk. The optimal conditions for extraction of SiO2 from rice husk in the form of Na2SiO3 were determined using thermal analysis (TA), scanning electron microscope (SEM) and elemental analysis. The experiments were carried out under conditions of non-isothermal heating in air and nitrogen atmosphere. Using different kinetic models, the values of the effective kinetic parameters, characterizing the destruction process were determined. Keywords: Rice husk; Alkaline treatment; Kinetics; Thermal analysis; SEM.
INTRODUCTION Rice husk (RH) is widely spread agricultural waste product in the world. They can not be fully utilized as food for livestock, fertilizer or fuel, so the methods of their utilization are under extensive investigation. Rice husk are known to contain about 20% chemically active SiO2 which can be used for production of adsorbents.1,2 They are cheap starting material for a number of silicon containing compounds,3-6 as well as active carbon with high specific area.7 The development of different technologies for the utilization of the lignin-cellulose materials by biological or thermal decomposition is impossible without in-depth knowledge on the processes of their thermal destruction.8-10 The methods of dynamic thermal analysis allow observing the processes under the influence of different agents in wide temperature interval. There are mathematical procedures based on the data from thermogravimetric analysis (TGA) which can be used to determine a number of thermochemical and kinetic characteristics of the substance destruction processes11-13 including RH, as it has been shown in our earlier papers.14,15 The present manuscript is a continuation of our effort but using alkali treated rice husk. Thorough investigations on the changes taking place in RH when treated with bases were not found in the avail-
able literature.16 The different by chemical nature components were found to change in some way, part of them decompose and basically cellulose remains in the RH. It could be expected that the estimation of the changes in the thermal and structural characteristics of the raw RH would allow determining the change in the ratio between the organic and inorganic components of RH which would show the ways for their more effective utilization. The aim of the present paper is to study the processes of thermal and thermo-oxidative destruction of raw and treated with NaOH rice husk with regard to the maximal extraction of SiO2. THEORY AND EXPERIMENTAL Materials The objects of the study were raw RH and RH obtained by chemical treatment with NaOH. The husk contained 74.5% organic matter (cellulose, hemicellulose and lignin), moisture and inorganic matter consisting of 20% SiO 2 and 5.5% mixture of the following oxides: CaO, Fe2O3, MgO, Al2O3, Na2O, K2O, MnO2, as well as traces of Cu and Pb.17 For the experiments, the raw RH was subjected to hot alkali treatment (at 100 °C) with 2N, 4N, 6N and 9N NaOH
* Corresponding author. Tel: +359-56-858258; Fax: +359-568-80249; E-mail:
[email protected]
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J. Chin. Chem. Soc., Vol. 57, No. 3A, 2010
in two series of samples treated for 1 and 3 hours, respectively. Further, the samples were denoted according to base normality and treatment time, as follows: samples of the first series - RH-2/1; RH-4/1; RH-6/1; RH-9/1 and second series: RH-2/3; RH-4/3; RH-6/3 and RH-9/3. For the thermal analysis, the samples were cut to pieces about 1 mm long and 0.5 mm wide. Methods The thermal studies (TG and DTG) were carried out on an apparatus for complex dynamic thermal analysis – derivatograph (MOM, Hungary) under the following conditions: temperature interval 20-1000 °C, heating rate 10 deg min-1, sample weight 100 ± 1 mg, static air medium and nitrogen medium with flow rate of 17 l h-1. The electron microscope photographs were taken using scanning electron microscope Tesla BS 340 (Czech Republic) in a regime of secondary electrons at acceleration of 20 kV. The carbon and hydrogen content were determined by automatic gas analyzer Carlo Erba 1104 (Italy). The silica content in the solid residue was determined after treatment with hydrofluoric acid by the method of rapid determination of silicon dioxide in quartz materials.18 Alkaline treatment 100 g of rice husk was mixed with a liter of sodium hydroxide solution (2, 4, 6 and 9n). Then, the mixture was heated to boiling for 1 or 3 h. The rice husk were separated by filtration and rinsed with distilled water until neutral pH was achieved (five washes, on the average). After rinsing, the RH was dried in an oven for 24 h at 100 °C. Theoretical approach and calculation procedures The kinetics of thermal degradation reactions is described by various equations taking into account the special features of their mechanisms. To determine the effective kinetic parameters of destruction, the data from the TG and DTG curves in the temperature range 180-400 °C considered to be the range of basic destruction were used and the calculations were performed using software developed on the basis of the integral method of Coats and Redfern.19,20 The check for the most probable mechanisms of the rate determining stages of the thermochemical reactions was carried out by 33 kinetic models used for studies on the nonisothermal kinetics of decomposition of solid state.11,12 These methods involve the process of nuclei growth due to motion on the interfacing plane, transport (diffusion) phenomena and chemical reactions. The basic equation of the non-isothermal kinetics in
Markovska et al.
its general form can be written as follows: da/dt = k(T)f (a),
(1)
where: f (a) – function, the form of which depends on the reaction mechanism; k(T) – temperature dependence of the rate constant usually described by the Arrhenius equation: k = Aexp(–E/RT),
(2)
where A – pre-exponential or frequency factor; E – activation energy; R – universal gas constant (8.314 J mol–1 C–1). Under constant heating rate: dT/dt = q = const.
(3)
After substitution in eq. (1) and some transformations, the following equation was obtained: a
ò da /
T
f (a ) =
0
A æ E ö expç ÷dT . q ò0 è RT ø
(4)
After substitution and taking logarithm, eq. (4) becomes: g (a ) =
ART 2 qE
æ 2RT ö æ E ö . ÷ ÷ expç ç 1E ø è RT ø è
(5)
Dividing both sides by T 2 and taking logarithm, eq. (5) turns into: ln
g (a ) T
2
= ln
AR æ 2RT ö E . ÷ç 1qE è E ø RT
(6)
Since 2RT/E
