|Title of Invention||
A PROCESS FOR THE PREPARATION OF SILICON ALUMINIUM OXYNITRIDE (SiAlON) FROM FLY ASH IN A THERMAL PLASMA
|Abstract||A process for the preparation of silicon aluminium oxynitride (SiAlON) from fly ash in a thermal plasma which comprises a mixture of fly ash and carbon in an electrically conducting non-metallic crucible capable of forming the anode, employing mixture of nitrogen and argon as plasmagen gas in a plasma furnace at a temperature of atleast 1400°C for a time period ranging between 10-30 minutes and cooling the desired product in the furnace up to a temperature of 20-3 0°C and collecting it in the form of powder|
|Full Text||The invention relates to a process for the preparation of Silicon aluminium Oxynitride (SiAION) from fly ash in a thermal plasma.
SiAION is a thermal shock resistance refractory ceramic which exhibits many improved properties such as excellent resistance to thermal shock, high hardness strength at elevated temperature, good resistance to wear, creep and environmental corrosion. In contrast to silicon nitride (Si3N4), it can be directly sintered without the help of any sintering aid. The range of application of SiAION includes refractories and crucibles in metal industries, industrial components, gate valves for flow control of high temperature liquid metal, and ceramic cutting tools (nitride tools performance better than carbide tools).
The existing industrial processes of SiAION production involve simultaneous carbothermic reduction of alumino silicate and nitridation by nitrogen gas at 1400- 1450°C in electrically heated furnaces. The alumino silicates used for this purpose include kaolin, kyanite, mullite and sillimanite etc. Various carbothermic nitridation processes of SiAION preparation have been reported and discussed in literature by several workers out of which the following works are significant to mention:
K.H. Jack and W.I. Wilson, "Ceramics based on the SiAION and related system', Nature, Vo. 238, 1972, pp 28.
A. Hendry, "Thermodynamics of silicon nitride and oxynitride" in "Nitrogen ceramics' Ed. F.L. Riley, (International Publication, Reading, Mass.), 1977, pp 183.
I. Higgins and A. Hendry, 'Production of P' - silicon by carbothermal reduction of kaolinite', Tr. & J. Br. Cerm. Soc. Vol. 85,1986, pp 161.
A. Hendry, 'Structural ceramics - Processing, microstructure and properties, Ed. J.J. Bentzen, J.B. Blide - Serensen, N. Christiansen, A. Horsewell and B. Palph, Rise National Lab. Roskilde, Denmark, 1990.
Recently SiAION in P' phase have been prepared from a compacted mixture of nano oxide powders (alumina and silica) and carbon in pellet form by nitriding in a
constant flow of nitrogen at 1100-1400°C (Malgorzata Sopicka - Lizar, Mater. Ogniotrwale, Vol. 46 (3) 1994, pp 76 (Pol). LA. Rehman and M.I Saleh (Mater. Lett., Vol. 23, 1995, pp 157) have used rice husk with addition of alumina to prepare P -
SiAION by carbothermal reduction at 1430°C in nitrogen atmosphere. Of late, T. Ekstrom, Z. Shen, K.J.D. Mackenzie, I.W.M. Brown and G.V. White (J. Mater. Chem., Vol. 8, 1998, pp 997) have reported the synthesis of mixture of a and p SiAION and Y3Si3N11 by carboreduction and nitridation (CRN) of Y203 doped SiO2/clay or elemental Si/clay mixes at atmospheric pressure.
The production of SiAION powder from fly ash either in single phase or as a mixture of phases in thermal plasma has not been reported so far. The main object of this process is to prepare a mixture of SiAION phases from fly ash (generated from coal based thermal powder plants) by carbothermal reduction and nitridation in thermal plasma. It is also the object of this invention to produce the said oxynitride compound at lower cost by utilizing a polluting waste like fly ash with added thermal economy by taking advantage of the mullite phase present in the fly ash.
The composition of fly ash used in this process is as shown in Table -1.
Loss on Ignition (LOI) 0.1-1.0
Particles of+325BSS 3.5-7.0
It is evident that in the composition of fly ash SiO2 and A12O3 together can constitute up to as high as 94% and our x-ray diffraction (XRD) analysis shows that most of the SiO2 and A12O3 occur in mullite phase [2SiO2, 3(Al2O3)]. In conventional processes, SiAION is prepared from aluminosilicate clays or minerals by carbothermal
reduction and nitridation at 1400-1450°C where thermal energy is first utilized to synthesize mullite which then reacts with nitrogen to form SiAION. Since the mullite phase is already available in fly ash, the fly ash can be taken as a starting material in nitridation to produce SiAION. This would save a good amount of thermal energy.
In conventional (electrical, coal or oil fired) furnace, the reaction kinetics of nitridation is slow due to involvement of molecular nitrogen in the reaction. On the other hand, in a thermal plasma furnace nitrogen ions are continuously generated with high ion
temperature (>10,000°C). The reaction kinetics in plasma nitridation becomes very fast owing to such high temperature and by the participation of activated species including ions. Due to reduced processing time and higher throughput, the cost of production of SiAION by thermal plasma carbo-nitridation is greatly lowered, thereby it improves the process economics.
The main findings of the process are:
1. For the first time SiAlON has been produced by thermal plasma using fly ash as
2. The process makes use of low cost dc extended arc plasma formed by arcing
between two graphite electrodes and the heat generated by arc plasma brings
about the carbothermal reduction and nitridation to synthesize SiAION.
3. The process directly makes use of fly ash as charge without making
agglomeration or pelletisation, thus saving a considerable amount of energy and cost.
4. The process saves 1 1/2 hour and more time in synthesizing SiAION.
5. The process has the flexibility to produce SiAION of different oxygen and
nitrogen content by varying nitrogen percentage at the plasmagen (short form of plasma
generating) gas stage.
6. The process produces SiAION in mixed phases which is resistant to thermal
shock, creep, wear and environmental corrosion.
Accordingly the present invention provides a process for the preparation of silicon aluminium oxynitride (SiAlON) from fly ash in a thermal plasma which comprises characterized in that a mixture of fly ash and carbon in the weight ratio of fly ash to carbon ranges from 3.0: 1.5 to 3.0 : 2.5, in an electrically conducting non-metallic crucible capable of forming the anode, employing mixture of nitrogen and argon as plasmagen gas in a plasma furnace at a temperature of atleast 1400°C for a time period ranging between 10-30 minutes and cooling the desired product in the furnace up to a temperature of 20-30°C and collecting it in the form of powder.
In an embodiment of the present invention the carbon used is in the forms of graphite and coke to prepare the desired product.
In yet another embodiment the weight ratio of fly ash to carbon used is in the range of 3.0 : 1.5 to 3.0: 2.5.
In yet another embodiment the extended arc plasma used has an arc current in the range of 200-600 A and an arc voltage in the range of 30-60 V.
In yet another embodiment the ratio of nitrogen to argon in plasmagen gas used is in the range oO:l to 3:2.
In yet another embodiment the flow of plasmagen gas used is in the range of 1 to 2 litres per minute.
In still another embodiment the mole fraction x in SiAlON i.e. Si6_xAlxOxNg_x is varied by changing the ratio of nitrogen to argon in the plasmagen gas.
The invention is a process for preparation of SiAlON by thermal plasma which involves heating a mixture of fly ash and carbon in an electrically conducting nonmetallic crucible that forms the anode wherein the carbon cathode having provision for vertical movement and passing a mixture of nitrogen and argon gas at 3:1 and less volume ratio through a central axial hole, is made to contact the anode and then slowly withdrawn to form the arc plasma in non-transferred mode which subsequently changes to transferred mode of
operation soon after the charge becomes conducting at elevated temperature (>1000°C) and the carbothermal reduction and nitridation is carried out at high temperature (>
1400°C) generated by arc plasma and the reduction of oxides to oxynitride conversion is completed in a short time ( conditions are adopted to prepare the above SiAlON.
Arc voltage : 30-60V
Arc current : 200 - 600A
Plasmagen gas flow rate : 1-2 lit/min. at 3:1 and less volume ratio
(Nitrogen + Argon)
Plasma processing time : 10-30 min.
Characterisation of the above product by x-ray diffraction (XRD) identified the SiAlON to consist of a mixture of 9 , O' and 12H phases along with small amounts of mullite and silica occuring in the form of quartz in the tridymite phase. Fig.l shows the XRD pattern of the thermal plasma produced SiAlON recorded at room temperature
The following typical examples will illustrate how the process of the present investigation is carried out in actual practice and should not be construed to limit the scope of the investigation.
A graphite crucible was placed inside the plasma furnace with axis vertical and it was supported by a plurality of horizontal graphite rods which provided the structural support and the electrical connection to the crucible. The screw drive of the graphite rods making a forward and backward motion helped to load the crucible in position and remove the same whenever required. The crucible was connected to the positive terminal of power supply through the graphite rods, thus forming the anode of the arc plasma system. Another graphite electrode with a central axial hole was fixed over the crucible in a co-axial position. The electrode was mounted on a driving mechanism having provision to raise and lower the electrode for initiation and control of arc length which in turn control the stability of the plasma. The said electrode with central axial hole was connected to the negative terminal of power supply, thus making it the cathode of the
plasma system. The plasmagen gas was introduced through the said hole of the cathode and the arc was initiated across the electrodes to form plasma inside the crucible. Nitrogen and Argon at 3:1 ratio was used as the said plasmagen gas.
Initially, when the power was off, the cathode was brought in contact with the anode (crucible) and the annular space in the crucible was filled with charge material (200 g mixture of fly ash and carbon in the wt. ratio 3:2). The flow of the said plasmagen gas was maintained at 1 lit./min. Power was then switched on and the cathode was slowly pulled out forming an arc plasma. The arc was then stabilized at an electrode gap of 5-7
cm. A stable plasma with temperature > 8000°C with activated species including nitrogen ions caused carbothermic reduction and nitridation of SiO2 and A12O3 available
in the charged material. The time taken for completing the plasma processing was 10-15 minutes. Arc voltage and arc current in this operation were 30V and 350A respectively at full arcing condition. The product was cooled in the furnace up to room temperature and collected in the form of powder. Yield of the SiAlON powder product was in the range of 75-80% (by wt.).
Example - 2
In this case the charge taken was 1 kg with fly ash to carbon ratio 3:2 and repeated the procedure as discussed in Example-1 with the following change in parameters.
Arc voltage : 45 V
Arc current : 550 A
Plasmagen gas flow rate
(Nitrogen + Argon) : 2 lit/min at 3:1 (by volume)
Plasma processing time : 25-30 min.
The product was furnace cooled to room temperature and obtained in the form of powder with whitish gray colour. X-ray diffraction of the product in powder form identified the product to be made of a mixture of 9 , 0' and 12H SiAlON and small amounts of mullite and quartz (Fig.l). The yield of the product was found in the range of 72-76% (by wt.)
In figure 1 the corresponding peaks for 0, O, 12H, SiAlON and mulllite are represented by the following symbols D mullite (A16 Si2 On) O 12H SiAlON ( Si3 A15 O2N5) X O SiAlON(Si3Al,2O9NIC) O SiAlON (Al o.4 Si ,.6 N ,.6 O u) The main advantages are
1. Fly ash which is an air pollutant has been successfully used to prepare SiAlON, a
high valued structural ceramic, by thermal plasma process.
2. The process accepts both powdery as well as agglomerated fly ash for production
3. The process makes use of low cost extended arc thermal plasma formed by arcing
between graphite electrodes (non-transferred mode) and graphite electrode and charge
(transferred mode) while mixture of nitrogen and argon is continuously fed through the cathode.
4. The said process reduces synthesis time of SiAlON from two hours to half an
hour and less (at 0.2 - 1 kg scale charge) as compared to electrical operated conventional
furnace heating processes.
5. The process has the flexibility of varying the mole fraction x in SiAlON (Si6_x
AlxOxNg_x) by varying the nitrogen to argon ratio in the range 3:1 and less.
1. A process for the preparation of silicon aluminium oxynitride (SiAlON) from fly ash in a thermal
plasma which comprises characterized in that a mixture of fly ash and carbon in the weight ratio of
fly ash to carbon ranges from 3.0: 1.5 to 3.0 : 2.5, in an electrically conducting non-metallic
crucible capable of forming the anode, employing mixture of nitrogen and argon as plasmagen gas
in a plasma furnace at a temperature of atleast 1400°C for a time period ranging between 10-30
minutes and cooling the desired product in the furnace up to a temperature of 20-30°C and
collecting it in the form of powder.
2. A process as claimed in 1 wherein the carbon used is in the forms of graphite and coke.
3. A process as claimed in 1 to 2 wherein the extended arc plasma used has an arc current in the
range of 200-600 A and an arc voltage in the range of 30-60 V.
4. A process as claimed in claims 1-3, wherein the ratio of nitrogen to argon in plasmagen gas used
is in the range of 3:1 to 3:2.
5. A process as claimed in claims 1-4, wherein the flow of plasmagen gas used is in the range of 1
to 2 litres per minute.
6. A process for the preparation of silicon aluminium oxynitride (SiAlON) from fly ash in a
thermal plasma substantially as herein described with reference to the examples and drawing
accompanying this specification.
|Indian Patent Application Number||233/DEL/2001|
|PG Journal Number||13/2009|
|Date of Filing||28-Feb-2001|
|Name of Patentee||COUNCIL OF SCIENTIFIC AND INDUSTRIAL RESEARCH|
|Applicant Address||RAFI MARG, NEW DELHI- 110 001, INDIA|
|PCT International Classification Number||C04B 35/597|
|PCT International Application Number||N/A|
|PCT International Filing date|