Title of Invention

"AN IMPROVED PROCESS FOR THE PRODUCTION OF SILICON CARBIDE FROM RICE HUSK"

Abstract ^ An improved process for the preparation of SiC from rice husk, which comprises cleaning and drying of raw rice husk by conventional methods, subjecting the cleaned and dried rice husk to thermal plasma at a temperature in the range of 1200 to 2500oC for a period of 15 to 45minutes in an inert atmosphere, powdering the resultant product by conventional methods, removing excess free carbon in the powder so obtained by heating the powder at a temperature in the range of 700 to 900oC for a period of 1 to 3 hours in an oxidizing atmosphere, removing unreacted silica by hydro-fluoric (HF) acid treatment to obtain silicon carbide (SiC).
Full Text This invention relates to an improved process for the production of silicon carbide from rice husk. The present invention particularly relates to an improved process for the production of SiC from rice husk by thermal plasma process.
It has been estimated that in India 10 Mt of rice husk is produced annually as a byproduct of rice milling. Most of this product is considered as agricultural waste. A small quantity of the husk is burnt as fuel in the rice mill to per boil the rice. The burning of rice husk creates pollution problem as it produces fine silica ash which is toxic.
Rice husk contains 15 - 20% silica and a cellulosic component that can easily be converted to carbon upon pyrolysis. The intimate contact of amorphous silica with fine carbon makes the rice husk an attractive candidate for the preparation of silicon carbide, an industrially important advanced ceramic material. Thus, rice husk, a nuisance byproduct of rice milling can be utilised as a valuable raw material for the production of silicon carbide. The reference may be made to (i) J.G. Lee and I.B. Cutler, Amer, Ceram. Soc. Bull. 54 (1975) 195-198 (ii) F. Rodriguez-Reiniso and J. Narciso, Adv. Mater. 7 (1995) 209-211 (iii) R.V. Krishnarao and M.M. Godkhindi, Ceramics International 18 (1992) 35-42 (iv) Tanaka, et al., (1986) US patent 4591492. In all of the above processes, the rice husk is first cooked in the temperature range of 500-700 °C in an inert atmosphere to remove the volatiles. Subsequently the cooked rice husk is heat treated at a temperature 1400-1900 °C in inert atmosphere for several hours. The resultant product is then treated in air/oxygen in the temperature range of 700 to 900 °C to remove excess carbon. Finally the product is treated with HF to remove unreacted silica to produce silicon carbide. Silicon carbide is extensively used in metallurgical, abrasive and refractory industries. It is also used for heating elements in electric furnaces, in electronic devices and in applications where its resistance to radiation damage is advantageous.
The main drawbacks of the above processes are that the reaction takes several hours and usually a catalyst such as Fe, Ni, Si3N4 is used to enhance the reaction.
The main objective of the present invention is to provide an improved process for the production of SiC from rice husk by employing a thermal plasma process. Another objective of the present invention is to utilize rice husk, an agricultural waste product.
Accordingly the present invention provides an An improved process for the preparation of SiC from rice husk, which comprises cleaning and drying of raw rice husk by conventional methods, subjecting the cleaned and dried rice husk to thermal plasma at a temperature in the range of 1200 to 2500°C for a period of 15 to 45minutes in an inert atmosphere, powdering the resultant product by conventional methods, removing excess free carbon in the powder so obtained by heating the powder at a temperature in the range of 700 to 900°C for a period of 1 to 3 hours in an oxidizing atmosphere, removing unreacted silica by hydro-fluoric (HF) acid treatment to obtain silicon carbide (SiC).
In an embodiment of the present invention the thermal plasma used has provision for introducing plasmagen gas such as argon, nitrogen into the arc zone,
In another embodiment of the present invention the powdering is effected by conventional methods such as milling, grinding.
In yet another embodiment of the present invention the oxidizing atmosphere used is such as air, oxygen or mixture thereof.
The present invention relates to an improved process for the preparation of SiC from rice husk by thermal plasma method as detailed below.
Raw rice husk is cleaned by water to remove any mud or dirt sticking to the husk. It is then oven dried to remove moisture before charging into plasma.
An extended arc plasma reactor is sued to convert the said rice husk charge materials to SiC. The said thermal plasma is formed by striking an arc between a graphite crucible which forms the bottom electrode and a top graphite electrode having an axial hole to introduce the plasmagen gas such as initrogen, argon. An extended or expanded plasma arc is formed when the two electrodes are separated without extinguishing the plasma.
The oven dried rice husk is
heated in the plasma at a temperature in the range of 1200 to 2500°C for a period of 15 to 45 minutes. The temperature is dependant on the input power and size of the crucible used. Because of high energy content and high enthalpy of the thermal plasma, the heating is very rapid. Moreover, the ionic species generated in thermal plasma coupled with the high heat transfer in thermal plasma can be
instrumental in the formation of SiC in much faster rate compared to other known processes. The presence of nitrogen or argon gas also protects the SiC from oxidation at high temperature.
The plasma treated product is fragile and can be ground easily to fine powder by milling for few minutes.
The said powder is then heated at 700-900 °C in an oxidizing atmosphere of air/oxygen for few (1-3) hours to remove the excess free carbon by oxidation. Alternatively, the said carbon is removed by employing conventional froth flotation technique.
The carbon removed product is then treated in HF to remove the unreacted silica and the final product obtained is in the form of p-SiC.
The following typical examples are given to illustrate how the process of the present invention is carried out in actual practice and should not be construed to limit the scope of the invention. Ex ample-1
100 g of raw rice husk was cleaned in water and oven dried. The dried rice husk was then kept in a graphite crucible and heated in an extended arc thermal plasma with a current = 250 A; voltage = 40 V for about 20 minutes at a temperature above 1500 °C to form SiC. Argon was used as the plasmagen gas which also provides a protective atmosphere for the SiC. The resultant SiC product contained some free carbon and unreacted silica. The plasma treated product was then ground to fine powder in a ball mill. The fine powder was heated at 700 °C in air for 2 hr. to remove free carbon. It was then treated in hydro-fluoric (HF) acid to remove the unreacted silica. The final product was in the form of P-SiC powder. Example - 2
200 g of raw rice husk was cleaned in water and oven dried. The plasma treatment of the rice husk was then performed in a graphite crucible using extended arc plasma and argon as plasmagen gas with a current = 300 A : voltage = 50 V for about 30 minutes. The plasma treated powder was then ground and heated in 700 °C in presence of oxygen gas for 1 hr. followed by HF treatment to obtain P-SiC powder.
The main advantages of the improved process of the present invention are :
1. Rice husk contains silica in amorphous hydrated form which is in intimate contact
with the carbon of cellulose of the husk material. Thus unlike the conventional process no
grinding or intimate mixing of silica carbon is required. The large surface area of silica in
rice husk helps rapid formation of SiC.
2. The raw rice husk is directly converted to SiC in a very short time in the plasma
furnace as plasma synthesis provides (i) enhanced rate of reaction due to the presence of
ionic species (ii), high rate of heat transfer (iii) argon as plasmagen gas which also forms
a protective gas environment.
3. The process does not face any difficulty for scaling up.



We Claim:
1. An improved process for the preparation of SiC from rice husk, which comprises cleaning and
drying of raw rice husk by conventional methods, subjecting the cleaned and dried rice husk to
thermal plasma at a temperature in the range of 1200 to 2500°C for a period of 15 to 45minutes
in an inert atmosphere, powdering the resultant product by conventional methods, removing
excess free carbon in the powder so obtained by heating the powder at a temperature in the
range of 700 to 900°C for a period of 1 to 3 hours in an oxidizing atmosphere, removing
unreacted silica by hydro-fluoric (HF) acid treatment to obtain silicon carbide (SiC).
2. An improved process as claimed in claim 1 wherein the thermal plasma used has provision for
introducing plasmagen gas selected from argon, nitrogen into the arc zone.
3. An improved process as claimed in 1 wherein the powdering is effected by conventional
methods selected from milling, grinding.
4. An improved process as claimed in 1 wherein the oxidizing atmosphere used is selected from
air, oxygen or mixture thereof.
5. An improved process for the preparation of silicon carbide (SiC) from rice husk substantially as herein described with reference to the examples.

Documents:

1086-del-2001-abstract.pdf

1086-del-2001-claims.pdf

1086-del-2001-correspondence-others.pdf

1086-del-2001-correspondence-po.pdf

1086-del-2001-description (complete).pdf

1086-del-2001-form-1.pdf

1086-del-2001-form-18.pdf

1086-del-2001-form-2.pdf

1086-del-2001-form-3.pdf


Patent Number 242140
Indian Patent Application Number 1086/DEL/2001
PG Journal Number 34/2010
Publication Date 20-Aug-2010
Grant Date 16-Aug-2010
Date of Filing 30-Oct-2001
Name of Patentee COUNCIL OF SCIENTIFIC AND INDUSTRIAL RESEARCH
Applicant Address RAFI MARG, NEW DELHI-110001, INDIA.
Inventors:
# Inventor's Name Inventor's Address
1 SAROJ KUMAR SINGH REGIONAL RESEARCH LABORATORY, BHUBANESWAR-751013, INDIA.
2 BISHNU CHARNARABINDA MOHANTY REGIONAL RESEARCH LABORATORY, BHUBANESWAR-751013, INDIA.
3 SUKUMAR BASU I.I.T., KHARAGPUR-721302 INDIA.
PCT International Classification Number C04B 41/50
PCT International Application Number N/A
PCT International Filing date
PCT Conventions:
# PCT Application Number Date of Convention Priority Country
1 NA