Capítulo 2: La población precolombina
2. La revolución agrícola
Rice husk is a waste by-product of the rice milling industry. It constitutes about 20% the weight of a harvested rice paddy and the mineral ash content of the rice husks ranges between 15-30%
of which 87-97% is amorphous silica depending on the combustion technique and conditions employed [4], [6],[21], [22]
The rice plant has high amorphous silica content because it naturally absorbs from the soil and transports silicon in the form of silicic acid to its outer surfaces. The silicic acid on reaching the outer surfaces of the rice plant becomes concentrated due to evaporation and is subsequently polymerized into silica cellulose membrane[23]. Because of this natural selectivity, the rice plant limits the uptake of heavy metallic elements that are found in large concentration in other cheap sources of silica such as quartz, bentonite and diatomaceous earth. A typical proximate analysis of rice husk is shown in Table 3.1
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An estimated one hundred and twenty million (120 × 106) metric tonnes of rice husk are generated annually worldwide [24]. For this huge amount of waste, the economic importance of utilizing rice husk has attracted several research interests in recent years.
3.2 Silicon from Rice Husk - A Review of the Literature
The main industrial uses of rice husk ash (RHA) are as a pozzolan in the cement and concrete industry and as an insulator in the steel industry. The synthesis of silicon from RHA has to date remained as a laboratory curiosity.
Singh and Dindhaw [23] reported obtaining silicon of 6N (99.9999%) purity by reducing white rice husk ash with magnesium at temperature of 800 oC followed by several successive acid (mixtures of HF, H2SO4 and HCl) leaching treatments. The reduction was also investigated at temperatures of 850 and 900 oC. The silica in their rice husk fired beyond 800 oC was observed to have attained some degree of crystalinity. They also suggested the possibility of obtaining silicon of similar purity by directly smelting the purified amorphous silica with carbonaceous reductants in electric furnace followed by leaching with acids and repeating the smelting and leaching for about nine times. The authors however did not disclose the method used to analyze their silicon to the 6N purity.
Table 3. 1 Typical proximate analysis of rice husks[21]
Moisture 6%
Ash 16.92%
Volatiles 51.98%
Fixed Carbon 25.10%
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Amick et al. [25] also patented a process for producing rice husk silica with adjusted silica to carbon ratio for direct reduction into high purity silicon with no addition of carbonaceous reductants. The method as described by Amick et al. comprise of leaching rice husk in semiconductor grade hydrochloric acid followed by pyrolysis of the leached husk at 900 oC in an atmosphere of 1% anhydrous HCl/Ar gas stream for a period of about one hour. The pyrolyzed rice husk which has a carbon -to -silica ratio of 4:1 is further processed in a conventional fluid bed combuster with Ar/CO2 atmosphere at a temperature of 950 oC to obtain stoichiometric carbon -to- silica ratio of 2:1. Reduction of the ash so produced at a temperature of 1900 oC reportedly yielded silicon with total impurity less than 75 ppm. The boron and phosphorus content were reported to be less than 10 ppm each.
Subsequently, Hunt et al. [26] investigated the possibility of producing high purity silicon from rice husk by purifying the rice husk silica according to the method of Amick et al, followed by pelletizing and reduction in a modified electric arc furnace. The pelletizing was carried using carbon black as a reductant and sucrose as a binder. The authors concluded that purified RHA could be a potential silica source for solar grade silicon production.
Bose et al. [22] subjected powdered silicon obtained by magnesium reduction of rice husk ash at a temperature of 600-650 oC to melting and directional solidification and found that boron was the active impurity in the polycrystalline silicon ingot obtained. They determined the minority carrier life time of their polycrystalline silicon material to be of the order of 1-5µs and concluded that to be promising for photovoltaic applications. A minimum carrier lifetime requirement for efficient solar cells fabricated from multicrystalline silicon wafers is however estimated as 25 µs [2].
The magnesium reduction of rice husk ash has also been investigated by Banerjee et al. [27] and by Ikram and Akther [10]. Banerjee et al reduced acid leached rice husk ash by intimately mixing the ash with magnesium powder and firing the powdered mixture in a sealed graphite crucible in a muffle furnace. The reaction product was successively leached in mineral acids (HCl, H2SO4, and HF) in a Teflon beaker. A spectrochemical analysis of the final silicon product showed a high boron content of 20-200ppm as well as high magnesium (50-1000ppm) and
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aluminum (10-200 ppm). They attributed the contamination of the silicon to the use of laboratory grade magnesium and also from glassware. In comparison with the silica produced by Singh and Dhindaw, Barnerjee et al. reported the silica had attained some degree of crystalinity when produced from roasting of husk at temperatures between 500-600 oC.
Following a similar approach but with 4N purity magnesium, Ikram and Akhter reported silicon of 99.95% purity with Boron content of approximately 2ppm.The process steps followed by Ikram and Akhter [10] , comprise boiling rice husk in 1:10 HCl and distilled water for 15 minutes, burning of the acid treated husk in air to obtain black ash, firing of the black ash in a muffle furnace to obtain white ash, leaching of the white RHA in dilute HCl , reduction of the leached white RHA with magnesium of 4N purity followed by sequential leaching with HCl, HF and a mixture of HF and H2SO4.
Contrary to the report by Banerjee et al and in agreement with Singh and Dhindaw, these authors reported that no crystalinity was observed in the RHA produced at 620 oC. Only the RHA fired to 900 oC had attained significant crystallinity with reflections or sharp peaks of different phases of SiO2 in their XRD pattern. They concluded that the silicon can be upgraded to solar grade silicon by conventional refining methods. The XRD patterns of the magnesium reduced white RHA product by Ikram and Akhter is shown in Figure 3.1.
Calciothermic reduction of purified rice husk ash was reported by Mishra et al.[28] They mixed a stoichiometric composition of granular calcium and purified rice husk silica and subsequently fired the powdered mixture in a sealed sillimanite crucible in a muffle furnace at temperature of about 720 oC. The reduction product was milled to fine powder and successively leached with concentrated nitric acid (HNO3) and hydrofluoric (HF) acid to obtain silicon of 99.9% purity with a boron content of 10 ppm. They suggested that the use of MgO coated crucibles and high purity reagent can lead to producing solar grade silicon by this method.
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Figure 3. 1 XRD pattern of Magnesium reduced RHA at 620 oC [10]
3.3 Silicon from Rice Husks - Summary
It can be seen from a review of the relevant literature that the main process steps involved in the efforts to synthesize silicon from rice husks are combustion of the husk, reduction of the ash with suitable reductant and purification of the reduction products with appropriate acidic reagents.
The sequence of process steps and process parameters (temperature, time, type of reductants, type and concentrations of leaching reagent) have to be optimized in order for this approach to solar grade silicon to be both technically and economically feasible.
3.4 Thermodynamics of Metallothermic Reactions for RHA (SiO
2)
The choice of reducing reagent for silica influences the thermodynamics and kinetics of the reactions. It was shown from the Ellingham diagram in Section 2.3 that C, Mg, Ca, Al and Ti are common and readily available reductants for SiO2. The carbon reduction of SiO2 is only feasible at temperatures in excess of 1900 oC and this represents the commercially established
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carbothermic process already described in Section 2.5. The metallic elements Mg, Ca, Al and Ti however can reduce SiO2 at comparatively lower temperature and forms mixtures of condensed phase products. The relevant overall reactions and their corresponding free energy change and adiabatic temperature rise per mole of silicon is shown in Table 3.2
Table 3. 2 Silica-Metal Reaction Thermodynamic Data*
SiO2 + 2Ca = Si + 2CaO
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CHAPTER FOUR: Experimental Work
This chapter presents the experimental approach followed throughout this research work to investigate the feasibility of synthesizing silicon of high purity from rice husk ash. The scheme of experimental work is presented in Figure 4.1.
Figure 4.1 Scheme of experimental work
●FactSage™ Study
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4.1 Materials and Reagents
Rice husk and rice husk ash used in this study were of Indian origin and were provided by Process Research Ortech Incorporated (PRO). The method and apparatus used by PRO in processing the rice husk into rice husk silica or rice husk ash (RHA) will be described briefly in the section under Sample Preparation. Table 3.1 provides a summary of the samples, reagents and reducing agents used in this work.
Table 4. 1 Summary of materials and reagents
Material Description Source
Rice Husk Ash
Processed in Torbed® Reactor at 800 oC
Process Research Ortech (PRO)
Magnesium Turnings 99 wt% pure Fisher Scientific
Magnesium Granules
98 wt% pure (metal basis),
-40 + 230 µm Sigma Aldrich
Hydrochloric Acid 36.5-38 wt% Standard stock Caledon, TMG Nitric Acid 70 wt% Standard stock Caledon TMG Hydrofluoric acid 48 wt% Standard stock Caledon TMG Polyvinyl Alcohol
(PVA) 98-99 wt% Alfa Aesar
Argon > 6.3 grade Lindy BIP
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4.2 Sample Preparation – Rice Husk Ash 4.2.1 Combustion of Rice Husks
The expanded Torbed reactor (ETR) is the apparatus used by Process Research Ortech Inc. to process the rice husks into rice husk ash (RHA). The ETR is essentially an upright cylindrical reactor with stainless steel interior walls. The base of the reactor is equipped with inlets through which a high velocity stream of air-gas mixture (process gas) is blown into the reactor to supply heat for reaction. A schematic of the ETR is shown in Figure 4.2.
The reactor, after pre-heating to a wind–box temperature of about 500 oC was fed with dry rice husks from the top through a feed chute. The jet stream of air–gas mixture blown into the reactor holds the diffuse feed material (rice husk) in a cyclonic motion. The feed materials falling to the bottom of the reactor are met by the process gas stream and are forced radially outwards to the walls of the reactor by centrifugal forces incipient to the design of the reactor. The falling diffuse materials reaching the base of the reactor are re-entrained in the process gas stream for continuous solid- gas contact [29].
Products attaining sufficient combustion (800 - 830 oC for this test) exit into an adjoining cyclone where products separate into the base of the cyclone and exhaust gases through a bag house – scrubber system. The reactor is equipped with real time feedback control systems, which enable on-line close control and monitoring of the combustion process.
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Figure 4.2 Schematic of expanded Torbed reactor [29]
4.2.2 Moisture Content Determination
The moisture content of the rice husk ash was determined by the Drying Oven method. Using a Mettler HK 160 analytical balance of sensitivity 0.0001 g, a quantity of the rice husk ash was weighed into a cleaned and dried ceramic boat of predetermined tare weight. The boat and content was weighed and then placed in a Precision thelco mechanical convection oven and dried to constant weight at 105 oC over 4 to 5 hours. The ceramic boat and content was re-weighed and the moisture content determined. The moisture content was found to be below 0.5 wt% hence all weighting of RHA were considered to be on dry basis without moisture content corrections.
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4.3 Characterization of RHA Sample 4.3.1 Chemical Composition Analysis
The bulk of the as received rice husk ash (RHA) was blended and riffled using a Jones riffles to ensure homogenous and representative samples were selected for both chemical composition and other analysis. Analyses requested were multi-acid digestion Inductively Coupled Plasma (ICP-MS/OES), Whole Rock Assay, and Leco total carbon content. The ANALEST laboratory of University of Toronto Chemistry Department was used for in-house chemical composition analysis. External laboratory which was involved in the chemical compositional analysis of samples from this research is International Plasma Laboratory (IPL)-Vancouver.
4.3.2 Particle Size Analysis
Laser diffraction particle size analysis method was used to characterize the particle size of the as received RHA. The Malvern Master Sizer S instrument was used. The measurable particle size range of the instrument is 0.05-3000 µm and it is equipped with a small volume sample dispersion unit. A lens range of 300RF, a beam length of 2.4mm, and a presentation of 30AD with polydisperse analysis was used for this measurement. About 0.5 g of the rice husk ash was dispersed in de-ionized water in the sample dispersion unit of the instrument, vigorously mixed for about two (2) minutes at speed of 2100 rpm, and sonicated for 45 seconds. The ultrasonic waves were used to break or minimize any particle agglomerates that may be present in the suspension. Measurements were taken and the diffraction data and graphs recorded by the instrument software program.
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4.3.3 Surface Area Measurement
The Coulter SA 3100 Analyzer together with the BET (Brauner Emmet Teller) calculation model was used to determine the surface area of the as-received RHA.
For sample preparation, about 2 g of the rice husk ash was weighed into the glass tube sample holder of the instrument and the free space volume (space in tube not occupied by sample) was measured using helium gas. The sample tube and contents were then outgassed with the system’s in-built vacuum pump. Analysis for the surface area began by introducing known doses of nitrogen (adsorbate gas) into the previously evacuated tube containing the sample. The glass tube was maintained at a constant temperature by immersion into a Dewar of nitrogen. When the pressure in the tube was equilibrated following each dose of the adsorbate gas, the pressure reading was recorded and then used to compute the volume of gas adsorbed on to the surface and pores of the sample.
The saturation pressure was measured for every sample tube pressure data point. The isotherm data obtained were used to plot isotherm curves and a BET plot from which the surface area was calculated. The relation between the volume of adsorbed gas and the relative pressure in the sample tube is given by the BET equation in the form;
Ps
C = Constant related to enthalpy of adsorption.
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The BET model assumes a cross-sectional area of the adsorbate molecule (Am) to be 0.162 nm 2 for the nitrogen gas, which enables the specific BET surface area (SBET) in m2/g to be calculated from the expression: [30]
SBET = VmNAAm
Mv 4.2
NA and Mv are respectively the Avogadro’s number and gram molar volume of an ideal gas.
4.3.4 XRD Analysis
Philips Diffractometer (model PW 3710) with X’PERT graphics software package was used to analyze the structures of the rice husk ash sample. An aluminum-glass composite sample holder with a rectangular slot measuring 2 cm × 1 cm × 0.2 cm was filled with the powdered sample using the front loading method. The samples were analyzed using a CuKα radiation (λ = 1.54056Å) with a nickel filter. Bragg’s angle (2theta) range of 10 -50o and a scan speed of 0.72 degree per minute with a step-size of 0.015° was used. The Philips diffractometer was operated at 40 kV and 40 mA. The structural pattern was recorded and analyzed with X’PERT HighScore™ software.
4.3.5 Scanning Electron Microscopy (SEM) Studies
The Hitachi S2500 scanning electron microscope was used to characterize the particle morphologies of the samples. Sample specimens with diameter of 12 mm each were gold coated in a gold sputter coater for 90 seconds at 15 mA current output. The gold coating was necessary to ensure a conducting surface was obtained for electron bombardment and characterization. The scanning electron microscope was operated at 20 kV and a working distance of 15 mm. Selected areas of interest were focused and micrographs were taken. For samples requiring SEM and EDX analysis, the Hitachi S570 scanning electron microscope was used.
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4.4. Purification Treatment of RHA 4.4.1 Leaching of RHA
Acid leaching of the rice husk ash was carried out to remove soluble elemental impurities and hence increase the purity of the silica content. Leaching was carried out at 10 wt% solids in 10 wt% HCl. The HCl solution was prepared from a standard HCl stock of mean concentration 37 wt% and density 1.19 g/mL.
The leaching reactor was 1-2L teflon beaker with teflon lid. Holes were drilled in the lid to accommodate a separator funnel for introduction of preheated acid to the pre-heated solid to be leached; as well a thermometer and a three blade teflon coated steel impeller. The port for the separator funnel doubled as a sampling port. The experimental set-up for leaching is shown in Figure 4.3.
Leaching time was varied between 1and 4 hrs and the temperature was varied between 60 and 90
oC with continuous stirring at 250 rpm. Vacuum assisted filtration was used to separate the residue and leach liquor. The residue was thoroughly washed with cold de-ionized water. The residue obtained was oven dried to constant weight at 105 oC. Samples of the dry residue were digested and analyzed by ICP- OES/MS to determine impurity reduction under the various leaching conditions.
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Figure 4.3 Leaching experiment set-up
4.4.2 Roasting of Acid Leached Ash
Acid leached rice husk ash was heat treated to reduce the amount of carbon content before reduction and hence increase further, the purity of the silica content. The purified ash was placed in an alumina crucible and heated in air in a muffle furnace to two selected temperatures of 500 and 700 oC at a mean heating rate of 300 oC /hr and maintained at this temperature for 1- 4 hours. The set up for this experiment is shown in Figure 4.4.
Separator Funnel
Teflon Coated Impeller
Teflon Beaker with Lid, Thermometer Insert and Sampling Port
Hotplate /Stirrer
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Figure 4.4 Set-up for roasting of acid leached RHA
4.5 Selection of Reducing Agent for RHA (SiO
2)
It was shown in Section 3.4 that the reactive metals Mg, Ca, Al and Ti are thermodynamically favourable to reduce SiO2 at comparatively lower temperature and forming a mixture of condensed phase products. However by considering factors such as cost and availability, ease of separating the by-product of reaction from the desired product (silicon), the effect of residual levels of these elements on the useful properties of the desired silicon product and above all safety and environmental concerns, magnesium was chosen as a suitable reducing agent over the others.
A look at the Si-O-Mg phase stability diagram in Figure 4.5 shows that other possible stable phases may be formed in addition to the main by-product of reaction, the MgO phase. This requires that the reduction reaction has to be controlled in order to minimise the formation of undesirable phases.
Leached RHA Sample
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Figure 4.5 Si-O-Mg phase diagram at 650 °C
4.6 Pelletizing of Reactants
The leached and roasted RHA having the highest silica content and lowest carbon content was selected for reduction experiments. The purified RHA and magnesium granules/turnings were wet blended in 4 wt% polyvinyl alcohol solutions and dried in argon atmosphere at a temperature of 105 oC for 10-15 minutes. The drying off of the wet alcohol improves the flowability of the reactants-mix into the compression die as well as improves the binding property of the PVA solution on the particulate reactants mix when compressed. The dried reactants-mix was poured into the compression die having a bore diameter of 16.3 mm in predetermined amounts. The
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interior surfaces of the compression die were lubricated with a solution of stearic acid in kerosene or alternatively benzene before introduction of the reactants-mix. The quick drying
interior surfaces of the compression die were lubricated with a solution of stearic acid in kerosene or alternatively benzene before introduction of the reactants-mix. The quick drying