PRODUCTION OF HIGH PURITY SILICON FROM RICE HUSK FOR USE IN S O U R CELLS Dr. B.K. Dhindaw Assistant Professor Material Science Centre I.I.T. Kharagpur W.B., India.
Mr. Rajvir Singh Research Engineer Rice Process Engg. Centre I.I.T. Kharagpur W.B., India.
silicon for solar cells converting the solar energy to electrical energy directly.
ABSTRACT White ash obtained by firing rice husk seems to be a potential source to obtain cheap solar grade silicon. The paper describes the structure and crystallization of silica obtained by firing rice husk at different temperatures. The results indi cate that ash contains considerable amount of amorphous and active silica mixed with active car bon which is possible to reduce at lower tempera tures compare to quartz and graphite. The paper also deals with the possibility and methods of obtaining pure silicon by making silicon tetrachloride as the intermediate compound and direct reduction of white ash by carbon or pyrolysed rice husk to silicon and its subsequent purification.
At present ferro-silicon as the starting material is processed and purified into polycrystalline silicon from which semiconductor devices are made. The iodide process, the trichlorosilane process and the silane process are the three major methods of chemical purification. In these processes, ferro-silicon is converted to some halide form such as S1I4, S1HCI3 and S1H4 since halides can be purified by chemical methods. Formation and purification of these halides make the process very expensive. An approach through metallurgical grade silicon may prove less expensive for produc tion of solar grade silicon.
STRUCTURE AND CRYSTALLIZATION OF SILICA
Rice husk is an unique waste product of agricul ture. The, unique nature of rice husk concerns their high ash and silica (SÌO2) content. No other agricultural residue even approaches the amount of silica found in rice husk which contains about 20 per cent silica in very finely dispersed form and constitutes a unique source of high grade silica. Amounts of deleterious contaminants such as boron, arsenic and tin are generally very low compared to other cheap sources such as sand, bentonite and diatomaceous earth.
As a first step towards extraction and subsequent purification of silicon from rice husk, an attempt has been made to study the nature of silica in ash by different techniques, viz., differential ther mal analysis, thermogravematric analysis, X-ray analysis and Scanning Electron Microscope so that the advantages of using white ash as a starting material can be fully appreciated. Differential Thermal Analysis (DTA)
Silicon in rice husk - as in the remainder of rice plant occurs as hydrated amorphous form of silica. Silicon is taken up and transported in the plant as mono-silicic acid. It moves to outer surfaces where it becomes concentrated by evaporation and polymerised to form cellulose silica membrane. For the production of nonmetallic inorganic material, the high specific surface in rice husk has an accelerating effect in chemical reaction between silica and carbon (Cutler, 1972).
DTA is a convenient method of making mineralogical analysis. The method involves measuring and graph ing the difference in temperature between the sam ple being examined and an inert substance (alumina) as the two are heated rapidly. The temperature of the sample during a test is influenced by the reaction taking place in this unknown sample, loss of carbon dioxide leads to the absorption of heat while crystallization of amorphous material or the burning of organic matter results in evolution of heat.
The core of the electronics equipment are the semi conductor devices - diodes, transistors, rectifiers and integrated circuits. The active materials in these devices are the elemental semiconductors ger manium and silicon. Although germanium dominated the field initially, silicon with its remarkable blend of chemical and physical properties has now opened up exciting possibilities. Apart from elec tronics equipments there would be a demand for
The ash samples were prepared by burning rice husk in an electric furnace at temperatures varying from 300-1400°C. The ash sample and the inert material alumina were placed in separate cells side by side inside a furnace and were heated rapidly. The DTA aparatus was connected to a X-Y recorder which plotted the difference in temperature of the ash and alumina as the ordinate against the tempera ture of the cells as the abscissa.
Thermogravimetric Analysis A small q u a n t i t y (0.5 gm) of f i n e l y ground r i c e husk was taken in a small platinum container and was hung from the bottom of the pan of a chemical balance placed in an e l e c t r i c a l l y heated furnace. The temperature of the furnace was raised(200°C/hr) upto 800°C and the weight of the husk was recorded at 3 minutés i n t e r v a l . The corresponding temperat u r e of the furnace was measured with the help of Ptg7RHi3-Pt thermocouple in conjunction with a potentiometer. The weight of the husk corresponding to d i f f e r e n t temperatures i s p l o t t e d against the temperature of the furnace. The a n a l y s i s was also repeated for white ash. X-ray Diffraction X-ray d i f f r a c t i o n s t u d i e s have been c a r r i e d out for a l l the samples of white ash f i r e d at d i f f e r e n t temperatures varying from (300-1400°C) t o study amorphous and c r y s t a l l i n e nature of s i l i c a . The s i l i c o n obtained by the reduction of white ash (S1O2) with Mg, and C was a l s o examined by X-ray diffraction. Results and Discussion D i f f e r e n t i a l thermal a n a l y s i s of r i c e husk as given in F i g . l i n d i c a t e s some endothermic and exothermic peaks in curve (b) where as t h e curve .(a) for alumina i s f l a t . Peak I occurring at about 100°C and representing endothermic r e a c t i o n may be a t t r i b u t e d t o the removal of moisture from the husk. Peak I I which i s also an endothermic one and occurring at about 200°C i s probably due to the chemical decomposition of the organic compounds present in r i c e husk. Above 400° C the curve slowly r i s e s i n d i c a t i n g a slow exothermic r e a c t i o n and t h i s i s believed to be due t o the gradual change i n the s t r u c t u r e of s i l i c a . Possibly r e s u l t i n g from c r y s t a l l i z a t i o n of amorphous s i l i c a or from a chang of a disordered t o an ordered s t r u c t u r e . The white ash samples prepared at temperatures from 300°C t o 700°C in e l e c t r i c furnace did not show any exothermic peak i n DTA (curve c ) , which suggests the presence of s i l i c a in amorphous s t a t e or lack of aforesaid transformation. On the other hand, a l l the white ash samples prepared at higher temperatures ranging between 750°C and 1300°C show an endothermic peak with temperature i n the range of 100-110°C for the evaporation of water and an exothermic peak in the range of 410-430°C (curved). Within t h i s temperature range some c r y s t a l l i z a t i o n of amorphous s i l i c a occurs which gives r i s e to exothermic r e a c t i o n r e s u l t i n g in evolution of heat in the system. Peak I I I i n d i c a t e s the s i n t e r i n g of s i l i c a p a r t i c l e s at about 8$0°C Fig. 2 represents the r e s u l t s of thermogravimetric a n a l y s i s of r i c e husk and white ash. The curves i n d i c a t e the burning of carbon and r e l e a s e of volat i l e matters from husk when i t i s subjected t o temperature upto 800°C. I t may be observed t h a t the l o s s in weight of the husk takes place in two d i s t r i c t s t e p s , one occurr-777-
ing between 100 and 110°G, the other between 230° and 250°C. I t may be suggested t h a t the f i r s t one p r e s e n t s the removal of moisture and second one i s due t o the decomposition and subsequent removal of the organic compounds. I t continues slowly upto 500°C, which i n d i c a t e s the burning of fine carbon i n s i d e the s i l i c a p o r e s . About 20 per cent of ash remains unburnt, which i s mostly s i l i c a . DTA of white ash a l s o confirms the change i n s t r u c t u r e of s i l i c a in white ash, which i s i n d i c a ted by a big exothermic peak i n DTA curve at 420°C. The white ash which was prepared a t 750°C in e l e c t r i c furnace was subjected to thermogravimetric a n a l y s i s . The r e s u l t s in F i g . 2 (curve d) i n d i c a t e t h a t a t about 420°C there was no appreciable change in weight of white ash threby i n d i c a t i n g the absence of any chemical r e a c t i o n . I t i s , t h e r e f o r e , concluded t h a t the peak i n DTA a t about 420°C i s only due to the change i n i n t r i n s i c s t r u c t u r e of s i l i c a of white ash. X-ray analysis of r i c e husk s i l i c a prepared at d i f f e r e n t temperature in e l e c t r i c furnace has been i l l u s t r a t e d in F i g . 3 · I t i n d i c a t e s t h a t s i l i c a in r i c e husk ash e x i s t s as amorphous in nature upto 700°C. C r y s t a l l i z a t i o n occurs above 750°C as revealed by the peak at 22° 2THETHA. This peak remains and increases in i n t e n s i t y at temperatures above 750°C· The humps in X-ray p l o t s show t h a t the amorphous s i l i c a e x i s t s even at very high temperatures. Scanning E l e c t r o n Micrographs The main requirement of a specimen for examination by scanning e l e c t r o n microscopy i s t h a t i t be of a s i z e small enough to f i t i n t o the SEM chamber. The c i r c u l a r brass p a l l e t s were made of size 4 cm d i a . and 9 cm l e n g t h . The ash samples were spread i n the powder form on the surface of brass p a l l e t s on which the aquadaq was pasted which adheres the s i l i c a p a r t i c l e s s t r o n g l y on the surface. The inductive carbon cootings were made on a l l the samples and scanning e l e c t r o n micrographs were obtained which are shown in F i g . 4 to 6. In the p r e s e n t study the s t r u c t u r e and c r y s t a l l i zation of s i l i c a in ash prepared a t d i f f e r e n t temperatures (500-1200°C) has been examined by Scanning Electron Microscope. At lower temperatures t h e r e i s no s i n t e r i n g of s i l i c a p a r t i c l e s and even m i e r o - c r y s t a l l i n e s t r u c t u r e i s not v i s i b l e . Fig*4 i n d i c a t e s t h a t the whole s i l i c a mass i s in amorphous s t a t e and the whole mass i s v i s i b l e as homogenous. A t y p i c a l Scanning E l e c t r o n Micrograph (Fig.3) shows the s i n t e r e d s i l i c a p a r t i c l e s and m i c r o - c r y s t a l l i n e s t r u c t u r e of s i l i c a in ash prepared a t 900°C. The SEM (Fig.6) shows the outer c r u s t s of the burnt husk (1200°C) and e v i d e n t l y c o n s i s t s e n t i r e l y of s i l i c a . The carbon and other organic matter completely burnt out and r e t a i n e d 20% s i l i c a . Therefore Scanning Electron Microscopy has revealed t h a t f i r i n g at higher temperatures the s i n t e r i n g and some m i c r o - c r y s t a l l i n e s t r u c t u r e develop at
MANUFACTURE OF SILICON TETRACHLQRIDE FROM RICE HUSK
above 800°C Below 700°C the whole silica is in amorphous state. The sintering can be accelerated by increasing heating rate and temperature and change in structure morphology was also observed at higher temperatures. This study is useful in studying the kinetics of reduction of silica to silicon. The study shows that the silica in rice husk is high grade and very active and the reac tions are possible at lower temperatues thus sav ing energy.
The cheapest raw materials for the manufacture of silicon tetrachloride by conventional means are sand, chlorine, and coke. All methods using the above raw materials require temperatures in excess of 1300°C. Yery fine dispersal of the coke and sand by grinding is also necessary. The potential advan tages of using rice husk as the raw material for making SiCl^ are lower cost, dispersion much finer than possible by mechanical means, and the resul tant lower temperature of chlorination (Basu, et al. 1972).
REDUCTION OF WHITE ASH BY MAGNESIUM »ALUMINIUM AND CARBON TO SILICON The alkali and alkaline earth metals and alumi nium reduce the silica more easily than carbon, since the affinity of these for oxygen is greater. The reactions once started, proceed to completion, accompanied by evolution of heat. Mixture of mag nesium and white ash was heated to redness in one spot in an electric muffle furnace with a small flame, the reaction spreads throughout the entire mass which soon became incandescent.
Si0 2 + 2 Mg
The conversion of rice husk silica to silicon tetra chloride occurs when pyrolysed husk were chlorina ted at 1000°C under the proper flow conditions and reactor geometry. The two possible overall reactions between carbon, silica and chlorine are : SiCl^ + C0 2
Si0 2 + C + 2C1 2 Ä Hn ° a 1300°K
800°C - ^ Si + 2 Mg
-42.54 Kcal/mol -56.53 Kcal/mol
The aluminium reduction of white was also tried successfully. The kinetics of the reactions are under investigation. The results were examined by X-ray diffraction which is shown in Fig.7.
Si0 2 + 2C + 2C1 2 = [i
Acid Mashing of Silicon In this method the silicon is washed with the mixture of acid that reacts with the impurities and inclusions but not with silicon. The main impurities in technical silicon are silicates, suicides of calcium, magnesium and aluminium etc. these lie at the boundaries between grains. Finely ground silicon (99.8$) obtained by the reduction of white ash was washed in sequence with hydrofluoride acid mixed with sulphuric acid and hydro-chloric acid to yield a material of 99.9999Î pure containing very little impurities. Silicon was also obtained by the melting the ash in electric arc furnace and was purified by acid leaching and up graded upto solar grade silicon. The pyrolysis experiment was carried out in a steel pot through which nitrogen could be passed. The rice husk was heated at 900°C in the absence of air for 2 hour. The pyrolysed rice husk was washed by mixed Hcl and H 2 S0^. The process was repeated 4 to 5 times to remove out the impurities. The pyrolysed rice husk which contains about 46.4/6 ash and 53·4ί carbon, was reduced to silicon by melting it in electric arc furnace. The silicon metal thus obtained was pulverised into very fine size and was treated with mixed HCl and HF. The purity of silicon powder could be increased to 99.999$ after repeating this process 9 times. This high purity silicon would be melted and cast into a bar in an atm. of purified argon for fur ther purification.
SiCl^ + 2C0
Zinc Reduction of Silicon Tetrachloride to Silicon The s i l i c o n t e t r a c h l o r i d e obtained from r i c e husk was reduced t o s i l i c o n by z i n c . F i g . 8 shows a system for t h i s purpose. The r e a c t i o n chamber was a tube coupled t o two evaporaters containing, r e s p e c t i v e l y , zinc and s i l i c o n t e t r a c h l o r i d e , and t o head for condensing the zinc chloride and any unreacted a g e n t s . The p a r t s were made of fused s i l i c a , which were joined by fused s e a l s . The temperatures were used as follows : 950-1000°C ( r e a c t i o n chamber), also (zinc evaporator and i t s connecting t u b e s ) , 30-60°C ( s i l i c o n t e t r a chloride evaporator) and ö50-1000°C (connecting tube from t h a t e v a p o r a t o r ) . The s i l i c o n t e t r a c h l o r i d e was fed by means of c a r r i e r gas n i t r o g e n . The n i t r o g e n gas reduces the d e p o s i t i o n of zinc in the connecting t u b e s . The Reaction i s SiCl^ + Zn = Si + 2ZnCl2 Zinc and silicon tetrachloride vapours were fed into the reaction chamber which was heated to 1000°C Silicon is formed as needless in the reactor. It is further crushed digested with HF and subjected to further washing treatments before drying and consolidating.
CONCLUSIONS The silica in rice husk ash exists as amorphous and very active. The intimate mixture of fine carbon and amorphous silica has an advantage to reduce it at lower temperatures compare to quartz and graphite. For production of silicon the high specific surface in rice husk has an accelerating effect in chemical reaction between silica and carbon. Absence of arsenic and boron in particular is the most important factor for this ash as the source material for the production of solar grade silicon.
J b ) WHITE ASH
J.G. Lee and I . B . C u t l e r , Formation of S i l i c o n Carbide from Rice H u l l s , U.S. Patent 3754976.
P. Bartha and E.A. Huppertz, S t r u c t u r e and c r y s t a l l i z a t i o n of s i l i c a in Rice H u l l s . Proceedings I n t e r n a t i o n a l conference on Rice By-product U t i l i z a t i o n , Valeneia, Spain. Sept.30 - O c t . 2 , 1974.
RICE HUSK FIRED AT
P.K. Basu, C.J. King and S. Lynn "Manufacture of S i l i c o n Tetra Chloride from Rice Hulls. AiChE J o u r n a l , Vol.19, No.3, May, 1973(
Thermogravimetric analysis of rice husk and white ash
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(C) RICE HUSK FIRED AT 700°C
(a) (a) ALUMINA 32 200
400 600 TEMPERATURE °C
BRAGG ANGLE,2Θ (degrees)
Differential thermal analysis plot
X-Ray Diffraction pattern of rice husk ash fired at (a) 500°» (b) 700°. (c) 750°. (d) 850° and (e) 1000°C
4 SEM of White Ash Obtained at 500°C
5 SEM of White Ash Obtained at 900°C
6 SEM of White Ash Obtained at I200OC
BRAGG ANGLE t 2 · (degrtes)
X-Ray Diffraction pattern tor Mg - Reduction of White Ash at (a) 800°, (b) 850° and (c) 900°C
RE ACT r ON CHAMBER (QUARTZ TUBE)
HEAO FOR COLLECT WG UN RE ACTE 0 MATERIALS
PRODUCTION OF SILICON
BY Zn REDUCTfON