Title of Invention

A PROCESS FOR MAKING SILICON CARBIDE KILN FURNITURE

Abstract

A process for making silicon carbide kiln furniture which comprises mixing silicon carbide powder of different grit sizes (range of 60 grit to 1200 grit) with 3 to 10 wt % of additive cement containing 70 % Al2O3, 25% SiO2 and 5% CaO, adding stablising agent and a setting agent such as herein described, in an amount in the range of 0.1 to 1.0 wt % to the said mixture of silicon carbide and additive cement, adding not more than 1 . 5 wt % of water to the said mixture to form a slurry, wet casting the resultant mixture by conventional methods, followed by aging, curing, washing and drying the said wet cast, firing the resultant dried product under ambient atmosphere at a temperature in the range of 1350° to 1450°C to obtain silicon carbide kiln furniture.

Full Text

This invention relates to a process for making silicon carbide kiln furniture. The process of the present invention particularly relates to making silicon carbide kiln furniture by the addition of oxides derived from hydrogel. Kiln furniture is supporting structure like rack, saggar and the like used during firing of ceramic articles in a kiln. Though silicon carbide is an excellent material for high temperature application its use is limited due to fabrication difficulty associated with essential non-sinterability of the powder for its densification either alter the grain-boundary surface energy of the material or form a liquid at high temperature to assist densification. In both the cases oxidation of silicon carbide, which initiates around 800° C and increases rapidly with increasing temperature needs to be controlled for which in all the cases an inert atmosphere of firing is essential which leads to increase in the cost of materials produced. The present day methods of making silicon carbide kiln furniture consist of nitride bonding or silicon bonding between the grains of silicon carbide. The normal clay bonded kiln furniture contains less than 30% silicon carbide. The main drawbacks of the present day processes are listed below : 1. Completely new set up of plant and machinery required which includes a controlled atmosphere furnace which are not available with the present manufacturer of kiln furniture. 2. The process requires inert gas and nitrogen gas supply introducing new set of safety measures. 3. The processes are cost prohibitive that make high valued items, because at present these items are made in controlled atmosphere furnace and at much higher temperature (2000° C or above) which makes the manufacturing cost very high. 4. Clay bonded products are inferior and has limited applicability as the calcium silicate bonding out of clay has low thermal stability and poor mechanical properties. The object of the present invention is to provide a process for making silicon carbide kiln furniture which obviates the drawbacks of the hitherto known processes. Another object of the present invention is to provide a process using intermediate sol-gel formation. Yet another object of this invention relates to improvement on or relating to the production of silicon carbide kiln furniture of maximum working temperature 1550° C. Still another object of the present invention is to provide a process to make consolidated mass of silicon carbide as kiln furniture at much reduced cost and improved quality. Accordingly the present invention provides a process for making silicon carbide kiln furniture characterised by mixing silicon carbide powder of different grit sizes (range ot 60 grit to 1200 grit) with 3 to 10 wt % of additive cement containing 70 % Al203, 25% Si02 and 5% CaO, adding stablising agent and a setting agent in an amount in the range of 0.1 to 1.0 wt % to the said mixture of silicon carbide and additive cement, adding not more than 1 . 5 wt % of water to the said mixture to form a slurry, wet casting the resultant mixture by conventional methods, followed by aging, curing, washing and drying the said wet cast, firing the resultant dried product under ambient atmosphere at a temperature in the range of 1350° to 1450°C to obtain silicon carbide kiln furniture. For stablising the slurry agents such as aluminium nitrate, sodium carbonate, sodium silicate, poly vinyl alcohol may be used in an amount in the range of 0.1 to 1.0 wt % . For preventing the slurry from settling, setting agents such aluminum alginate, sodium alginate, molases may be used in a amount in the range of 0.1 to 1.0 wt % . The details of the process of the present invention are given below : a. Silicon carbide of three different sizes are mixed with specially prepared additive cement (3-10% by wt) containing 70% Al203 , 25% Si02 and 5% CaO in ball mill. b. The mixture is wet cast by usual methods by using not more than 1.5% water and aged for 24 hrs. c. The aged green product is cured for 7-10 days and washed thoroughly by flowing water. d. The cured green product is dried at below the boiling point of water, e. Dried product is fired under ambient atmosphere at 1350° to 1450°C. The process of the present invention can be used to produce consolidated silicon carbide of various green bodies (unfired articles) and sizes required for application as structural refractory materials. Good quality silicon carbide kiln furniture like rack, saggar etc used during, firing of ceramic articles in a kiln was made by adding alumina and silica which will be converted to gel from sol of aluraino silicate and will give go good green (unfired) strength. Alumino silicate gel thus produced will be of better purity and reactivity and eventually forms the bonding material of the silicon carbide grain after firing at much less temperature than those added in any other forms. The selection of appropriate grain size of silicon carbide powder from 60 grit to 1200 grit in appropriate proportion by weight is mixed with an additive the main constituent of which is alumina and silica. This additive acts as cementing material in the unfired articles made of silicon carbide powder and give good strength for handling. After firing of the silicon carbide articles this additive make the required alumina silicate at much lower temperature (1350-1450° C) which acts as binding material of the grains of silicon carbide and give the final fired strength of silicon carbide articles. The following example is given by way of illustration of the process of the present invention and should not be construed to limit the scope of the present invention. Example 1 Silicon carbide of 60 grit 350 gins 400 grit 350 gms and 1200 grit 200 gms are mixed with 50 gms of cement and 50 gms of aluminium nitrate. This mixture is thoroughly mixed in a ball mill for 4 hrs. Then the mixture was made into slurrry by adding 15 ml water containing 10 gms sodium alginate. The slurry was cast by ramming to stake green bodies (unfired article). Cast green bodies (unfired articles) were aged for 24 hrs. After ageing cast green bodies (unfired articles) were washed for three days under flowing water. After three days, those green bodies (unfired articles) were aged further under water for 4 days. Aged green bodies (unfired articles) were air dried at room temperature followed by drying at 110° C for 24 hrs. Dried green bodies (unfired articles) were fired at 1350° C in an electrically operated furnace for three hrs in an ambient atmosphere. The results obtained by the process of the present invention is given below by way of illustration : Table 1 Characteristics of raw material and fired products (Table Removed) Purity of SiC -98.8%, C-0.5%, SiO -0.15%, AI2O3-0.07%, Fe203 - 0.98% Table 2 Important features of the product (Table Removed) The main advantages of the process of the present invention are : 1. A good quality, high strength silicon carbide kiln furniture could be made by intermediate gel forming technique. 2. The best effect is obtained by careful selection of proportion of additives and fines in the composition. 3. The method developed is extremely flexible for green shaping of complicated green bodies (unfired articles) and large size . products. 4. The curcuring helps in developing green strength sufficiently strong for green transportation. 5. The method produces fired products that can be safely used upto 1550° C. 6. The entire process may be automised for continuous production. WE CLAIM: 1. A process for making silicon carbide kiln furniture characterised by mixing silicon carbide powder of different grit sizes (range of 60 grit to 1200 grit) with 3 to 10 wt % of additive cement containing 70 % Al2O3, 25% Si02 and 5% CaO, adding stablising agent and a setting agent such as herein described, in an amount in the range of 0.1 to 1.0 wt % to the said mixture of silicon carbide and additive cement, adding not more than 1 . 5 wt % of water to the said mixture to form a slurry, wet casting the resultant mixture by conventional methods, followed by aging, curing, washing and drying the said wet cast, firing the resultant dried product under ambient atmosphere at a temperature in the range of 1350° to 1450°C to obtain silicon carbide kiln furniture. 2. A process as claimed in claim 1 wherein stablising agents such as aluminium nitrate, sodium carbonate, sodium silicate, poly vinyl alcohol is used. 3. A process as claimed in claim 1 wherein setting agents such as aluminium alginate, sodium alginate, molases is used. 4. 4. A process as claimed in claim 1 wherein the aging is effected for atleast 24 hrs. 5. A process as claimed in claim 1 wherein the curing is effected for a period of 7 to 10 days. 6. A process as claimed in claim 1 wherein the washing is effected by flowing water. 7. A process as claimed in claim 1 wherein the drying is effected below the boiling point of water. 8. A process for making silicon carbide kiln furniture substantially as herein described with reference to the example

Documents:

1532-del-2003-abstract.pdf

1532-del-2003-claims.pdf

1532-del-2003-complete specification (granted).pdf

1532-del-2003-correspondence-others.pdf

1532-del-2003-correspondence-po.pdf

1532-del-2003-description (complete).pdf

1532-del-2003-form-1.pdf

1532-del-2003-form-19.pdf

1532-del-2003-form-2.pdf

1532-del-2003-form-3.pdf

1532-del-2003-petition-138.pdf