Title of Invention

A PROCESS FOR PREPARING CERAMIC CRUCIBLES

Abstract A process for preparing ceramic crucibles having thermal shock resistance and high slag penetration resistance useful for carbon and sulphur analysis of ferrous alloys and steel samples. The crucibles produced by the saiq process is highly cost effective compared to conventional crucibles for same application. The raw materials like sillimanite and quartz Used for the processing of the crucibles are cheap, readily and abundantly available. The peak temperature being 1300°C. The sintered crucibles when tested against N/ST standard steel calibration samples have shown excellent carbon and sulphur analysis values which are at par with the imported crucibles. The performance of the crucibles for carbon-sulphur analysis can be attributed to the formation of mullite phase coupled with controlled porosity in the crucibles.
Full Text

Field of the invention
The present invention relates to an improved process for preparing ceramic crucibles having high thermal shock resistance and high slag penetration resistance useful for carbon & Sulfur analysis of ferrous alloys and steel samples and the ceramic crucibles so prepared The crucible prepared by the process of the present invention is useful for carbon & Sulfur analysis of ferrous alloys and steel samples using conventional equipment.
The US Patent No. 3, 100, 155 dated August 6, 1963, discloses a process for the manufacture of ceramic crucible useful for making analytical combustion apparatus where different types of silica having various particle size distribution are used along with clay and bonding agents like lithium, sodium, potassium, magnesium, calcium, strontium and barium. The advantageous physical properties of this crucibles are dependent not only upon the amount of silica used in the composition, but also upon the degree of coarseness of the silica particles used in the composition. The composition is obtained by mixing coarse silica, clay, and a proper bonding agent in proportions. The minimum amount of coarse silica is suggested to be 30% and preferable amount 45% and maximum amount not more than 75% of the total mix. The proportion of fine silica preferably should not exceed 20%. The clay used acts as plasticizer to enable the article

to hold together while green. The bonding agent added in the ceramic mix are alkaline metal or alkaline earth. Two batches of ceramic mix were prepared in a muller mixer having same amount of clay and fine silica but different amount of coarse silica, feldspar and calcium carbonate. The mix were the subjected to granulation followed by pressing in a hydraulic press in the shape of crucibles. Pressed samples were then sintered and subjected to testing in an analytical combustion apparatus for carbon and sulfur analysis. The crucibles were found to withstand the thermal shock and resistance to molten metal penetration.
A relatively recent Romanian patent (RO 86,624 CIC04B35/14 dated April 30, 1985) discloses a process for making crucible for carbon and sulfur determination in steel samples where kaolin and silica are used as main constituents. The thermal shock resistant ceramic consists of 55-65% clay binder and balance filler containing silica > 95 and iron oxide - 1%. The mix was homogenized followed by aging for = 24 hrs. The mix were the subjected to granulation followed by pressing in a hydraulic press in the shape of crucibles. Pressed samples were then sintered and subjected to testing in an analytical combustion apparatus for carbon and sulfur analysis.
Both the above patented processes yield crucibles having silica as major constituent which is known as low thermal shock resistant material (except in the form of fused quartz) because of polymorphic transformation which takes place during heating from room temperature to high temperature. However, they are able to achieve sufficient thermal shock and slag penetration resistance by manipulating the size of silica particles and the use of proper amount of certain bonding agents and clay.

Further, the above Romanian patent discloses that more than 24 hr aging of the raw mix is required in their process. Hence, the above processes require close and careful control of the particle size of silica and the use of proprietary binder composition or long processing time.
Over and above, the crucibles, produced by the above method for the same application are costly.
The present process overcome the disadvantages faced in previous prior arts by judiciary selection of naturally occurring raw materials where a wide range of particle size distribution of the raw materials can be accommodated and prolonged ageing of the raw materials can be avoided.
The main objective of the present invention, accordingly, is to provide an improved process for the manufacture of ceramic crucibles suitable for the analysis of carbon and Sulfur content of ferrous alloys and steel employing conventional carbon and Sulfur analyzer.
Another objective of the present invention is to produce a ceramic crucible with high thermal shock resistance sufficient to prevent any major crack formation in the crucible during the analysis which can hamper the quality of analysis.
Yet another objective of the present invention is to produce a ceramic crucible with a high degree of slag penetration resistance sufficient to prevent any seepage of molten metal samples through the wall of the crucible during analysis .

Another objective of the present invention is to produce a ceramic crucible containing minimum amount of free silica to reduce polymorphic transformation of silica.
Still another objective of the present invention is to provide an improved process for the production of ceramic crucible useful for the above mentioned application where total processing time including sintering is less than 36 hrs.
Yet another objective of the present invention is to provide an improved process for the production of ceramic crucible useful for above said application where it is highly cost effective compared to conventional crucible for same application.
Another objective of the present invention is to provide a ceramic crucible having the above mentioned characteristics starting from cheap, readily and abundantly available raw materials.
Accordingly the present invention provides an improved process for the production of ceramic crucible suitable for application in analysis of carbon and Sulfur content of metal in a conventional carbon and Sulfur analyzer which comprises:
(i) mixing a mullite forming material and a source of silica
(ii) preparing a dough of the resulting mix in conventional kneading machine
employing a binder solution (iii) drying the resulting dough to maintain a moisture content of less than 15% (iv) granulating the semidried raw mix in a conventional granulator to a particle
size in the range of -16 to +200 mesh.

(v) drying the resulting granules to have a moisture content of less than 5%
(vi) moulding the granules in the form of crucibles
(vii) sintering the crucibles up to a temperature in the range of 1200-1300 °C degree C for a period in the range of 1- 2 hours at the heating rate in the range of 1-3 °C /min followed by cooling the resultant crucible.
According to another feature, the present invention also provides a crucible prepared by the process described above having improved characteristics.
Detailed description
The preparation of the crucibles of the present invention commences with selection of raw materials which are mullite forming material and a source of silica. The mullite forming material may be selected from andalucite, kyanite, sillimanite and the like. The source of silica may be selected from silica, beach sand, quartz, amorphous silica, microfine silica and the like. The Multite forming mterial and the source of silica materials may be mixed in the proportion of 50:50 to 100:0, preferably in the proportion of 75:2 and more preferably in the proportion of 80:20. The average particle sizes of mullite forming material and silica bearing material used may be in the range of 2 to 5 and 14 to 26 microns respectively. The total alumina and silica content, on loss free basis, of the mullite material used may be not less than 95% and the weight % of mullite forming material used in the raw mix may be within 60 to 80 %. The total alumina content, on loss free basis, of the silica bearing material used may be not less than 97% and the weight % of silica bearing material used in the raw mix may be within 20 to 40 %.

The mixing of the above said raw materials can be done in a conventional mixer like planetary mixer, ball mill, rod mill, Eirich mixer, etc. The mixing may be effected for a period of less than 4 hours, preferably for 0.75- 1hrs. The resulting mixture is then formed into dough using a polymeric binder solution in a conventional dough-making machine like muller mixer, sigma kneader, pug mill etc. The polymeric binder which can be used may be selected from dextrin, methyl cellulose, polyvinyl alcohol and the like The dough is then subjected to drying in a conventional drier to a moisture content of less than 15%. This semi-dried dough is granulated using conventional granulator to a definite size fraction which lies between the size range of -16 to +200 mesh. The granules thus obtained are again dried at a temperature around 120 °C in a conventional drier so as to have a moisture content of less than 5%. The dried granules are pressed in a conventional hydraulic press under a pressure between 3 to 8 tons to obtain the crucibles.
The crucibles so prepared may preferably have the dimensions: OD 26 mm, ID 20 mm and height 26 mm. The green density of the crucibles was maintained in the range of 2.0 to 3.2 g/cc. The green crucibles are finally dried by firing within a temperature in the range of 1150 °C - 1460 °C, for a soaking time of 0.5 - 2 hour maintaining a heating rate in between 1-3 °C /minute in a conventional
electrical/gas/oil fired static kiln. The sintered density and porosity of the resulting crucibles fall within the range of 30 to 40 %.
The Sintered crucibles are then inspected for any visual crack and a few Crucibles are selected randomly from each sintered batch and subjected to measurement of sintered density, and apparent porosity followed by carbon and Sulfur analysis of

standard sample using a conventional carbon Sulfur analyzer. Sintered pieces are then wrapped in aluminum foils to avoid any contamination during handling.
The following Examples are provided merely to illustrate the invention and therefore should not to be construed as limiting the scope of the invention. In the Examples, unless indicated otherwise, all temperatures are in degree centigrade and all compositions are in weight percent.
Example 1
As typical examples of the method for making the crucibles, seven batches of raw mix each of one kg were prepared by mixing of sillimanite and quartz powder in the proportion shown in the Table I.

Each of the raw mix was separately formed into dough with 200 ml, 5% polyvinyl alcohol solution in a sigma kneader. The dough was then dried in an electrically operated oven at 120 °C for 2 hrs followed by granulation in a granulator to obtain


granules of size fraction in between -40 to +100 mesh. The granules were then further dried in an oven at around 120 °C for a period of 2 hrs. Powder granules were then pressed in a tool steel mould at 4 tons pressure in the form of crucibles having dimensions of OD: 26 mm, ID: 20 mm and height: 26 mm. The pressed samples were then sintered at 1300 °C for a soaking period of one hour followed by furnace
cooling. Heating rate during sintering was maintained as 1 °C per minute up to 500 °C and then 2 °C per minute up to 1300 °C per minute.
The sintered crucibles were then visually examined for any type of surface defect. A few of the sintered crucibles were then selected randomly from the sintered batch and subjected to measurement of sintered density, and apparent porosity followed by carbon, Sulfur analysis in a conventional carbon - Sulfur analyzer using NIST



>From the analysis it is clear that none of the crucibles made from mullite forming material in the range of 60 to 80wt % have failed due to thermal shock in addition to that the analysis results for carbon and Sulfur were maintained within the tolerable range as mentioned in Table II in standard value column. With the increase of quartz percentage beyond 40%, cracks were observed on the surface of the sintered crucibles. These cracks were extended to a larger one during the analysis. In the case of crucibles containing more than 80% of mullite forming material, Sulfur percentage was found to exceed beyond the tolerable range and minor cracks were also observed during analysis.

Example 2
3.0 kg raw mix was prepared by mixing of 1.8 kg of sillimanite and 1.2 kg. of quartz in a ball mill for 3 hrs. The raw mix was then made into dough with 600 ml of 5% polyvinyl alcohol solution in a sigma kneader. The dough was then dried in an electrically operated oven at 120 °C for 2 hrs. The semidried dough was divided into three equal parts and granulated in the size range of -40 to +100 mesh, -80 to +150 mesh and - 100 to + 200 mesh, respectively using a granulator. The granules were then further dried at 120 °C for 0.5 hrs in an electrically operated oven to a moisture content of around 3.0% followed by pressing using a hydraulic press under pressure of 4.0 tons to form crucibles of dimensions mentioned in Example 1. The crucibles were then sintered in an electrically operated furnace at a temperature of 1300 °C for 1 hr. The sintered crucibles were then visually examined for the presence of any type of surface defect and a few of the sintered samples finally tested for carbon and Sulfur analysis using above-mentioned method. The test results are given in Table III.


>From the analysis as shown in the Table HI it is clear that crucibles made from granules in the size range of-40, +100 mesh is capable of withstanding high thermal shock and slag penetration encountered during analysis of carbon & Sulfur. The variation of porosity with granule size may be responsible for thermal shock resistance behavior of the crucible.
Example 3 3.0. kg raw mix was prepared by mixing of 1,8 kg of sillimanite and 1.2 kg. of quartz in a ball mill for 3 hrs. The raw mix was then made into dough with 600 ml of 5% polyvinyl alcohol solution in a sigma kneader. The dough was then dried in an electrically operated

oven at 120 °C for 2 hrs. The semidried dough was granulated using a granulator in the size range of -40 to +100 was. The granules were then further dried at 120 °C for 0.5 hrs in an electrically operated oven to a moisture content of around 3.0% followed by pressing using a hydraulic press under pressure of 4,0 tons to form crucibles of dimension mentioned in Example 1. The green crucibles were then divided into three equal parts and sintered in an electrically operated furnace to a temperature of 1250 °C, 1300 °C and 1350 °C each for 1 hr, respectively. The sintered crucibles were then visually examined for the presence of any type of surface defect and a few of the sintered samples finally tested for carbon and Sulfur analysis using above mentioned method. The test results are given in Table IV.


>From the analysis as shown in the table above it is clear that crucibles sintered at 1300 °C have the appropriate internal porosity which is mainly responsible of withstanding high thermal shock and slag penetration encountered during analysis of carbon & Sulfur. Advantages:
1. The process results in a High thermal shock resistance ceramic crucibles for analysis of ferrous alloys and steel samples from improved ceramic composition.
2. Crucibles of the present invention have resistance to penetration of molten metal slag or metal oxides.
3. The analysis conducted using the crucibles of the present invention does not result in inaccurate results
4. Since the process does not employ zirconium silicate, the crucibles are not contaminated thereby making the crucibles useful for the analysis of Sulfur.
5. The process does not require close and careful control of the particle sizes of raw materials.
6. The process does not also employ proprietary additive as binder or requires long processing time.
7. Since the ceramic crucibles produced by the process of the invention has high thermal shock resistance and high slag penetration resistance, they do not crack during analysis thereby providing accurate results.

8. Cost of crucibles having high thermal resistance manufactured by the process of the present invention is very low due to the employment of inexpensive and readily and abundantly available raw materials.
Disclaimer:
"No claim is made to the substance by which the crucible is made according to the present invention".



WE CLAIM:
1. An improved process for the manufacture of ceramic crucibles having high
thermal shock resistance and high slag penetration resistance useful for carbon & Sulfur analysis of ferrous alloys and steel samples employing a conventional carbon & Sulfur analyzer which comprises : (i) mixing a mullite forming material and a source of silica (ii) preparing a dough of the resulting mix in conventional kneading machine
employing a binder solution (iii) drying the resulting dough to maintain a moisture content of less than 15% (iv) granulating the semidried raw mix in a conventional granulator to obtain
granules in the range of -16 to +200 mesh, (v) drying the resulting granules to have a moisture content of less than 5% (vi) moulding the granules in the form of crucibles (vii) sintering the crucibles up to a temperature in the range of 1250 -1300 °C for
a period in the range of 1-2 hours at the heating rate in the range of 1-3 °C
/min followed by cooling the crucible.
2. An improved process as claimed in claim 1 wherein the mullite forming material such as mullite, sillimanite is used
3. An improved process as claimed in claims 1 & 2 wherein the silica bearing material such as silica, beach sand, quartz are used
4. An improved process as claimed in claims 1 to 3 wherein the average particle sizes of mullite forming material and silica bearing powder used are in the range of 2-5 micron and 14 - 26 micron respectively.

5. An improved process as claimed in claims 1 to 4 wherein the total alumina and silica content, on loss free basis, of the mullite forming material used is not less than 95%.
6. An improved process as claimed in claims 1 to 5 wherein the total silica content, on loss free basis, of the silica bearing powder used not less than 97%.
7. An improved process as claimed in claims 1 to 6 wherein the weight % of mullite forming material used in the raw mix composition is within 60-80%.
8. An improved process as claimed in claims 1 to 7 wherein the weight % of silica bearing powder in the raw mix composition is within 20 - 40%.
9. An improved process as claimed in claims 1-8 wherein the polymer binder such as dextrin, mehyl cellulose, polyvinyl alcohol is used for making the dough includes.
10. An improved process as claimed in claims 1-9 wherein the granulation size falls in the range of-40 to +100 mesh.

11. An improved process as claimed in claims 1-10 wherein the pressure employed for forming the crucibles is in the range between 3 - 8 tons.
12. An improved process as claimed in claims 1-11 wherein the green density of the crucibles formed fall within 2.5 to 3.2 g/cc.
13. An improved process as claimed in claims 1-12 wherein the sintering
temperature employed falls in the range of 1250 to 1300 °C.
14. A process as claimed in claims 1 to 13 wherein the heating rate employed during sintering falls within the range between 1-3 °C./min.
15. A process as claimed in claims 1 to 14 wherein the soaking period used for sintering fall within 1- 2 hrs.

16. A process as claimed in claims 1-15 wherein the sintered density and porosity of
the fired crucible falls within the range of 30 to 40%.
17. A process for the manufacture of ceramic crucibles having high thermal shock resistance and high slag penetration resistance useful for carbon & Sulfur analysis of ferrous alloys and steel samples conventional carbon & Sulfur analyzer substantially as herein described with reference to the Examples.
18. An improved ceramic crucible having high thermal shock resistance and high slag penetration resistance useful for carbon & Sulfur analysis of ferrous alloys and steel samples employing conventional analyzer carbon & Sulfur analyzer prepared by the process substantially as herein described with reference to the Examples


Documents:

806-mas-2000-abstract.pdf

806-mas-2000-claims duplicate.pdf

806-mas-2000-claims original.pdf

806-mas-2000-correspondence others.pdf

806-mas-2000-correspondence po.pdf

806-mas-2000-description complete duplicate.pdf

806-mas-2000-description complete original.pdf

806-mas-2000-form 1.pdf

806-mas-2000-form 19.pdf


Patent Number 207700
Indian Patent Application Number 806/MAS/2000
PG Journal Number 44/2007
Publication Date 02-Nov-2007
Grant Date 20-Jun-2007
Date of Filing 26-Sep-2000
Name of Patentee M/S. INTERNATIONAL ADVANCED RESEARCH CENTRE FOR POWDER METALLURGY AND NEW METERIALS (ARCI)
Applicant Address BALAPUR VILLAGE, RR DISTRICT, BALAPUR P.O., HYDERABAD-500 005.
Inventors:
# Inventor's Name Inventor's Address
1 Dr.SUBIR BHATTACHARJEE INTERNATIONAL ADVANCED RESEARCH CENTRE FOR POWDER METALLURGY AND NEW METERIALS (ARCI) BALAPUR VILLAGE, RR DISTRICT, BALAPUR P.O., HYDERABAD-500 005.
2 Dr. ROY JOHNSON INTERNATIONAL ADVANCED RESEARCH CENTRE FOR POWDER METALLURGY AND NEW METERIALS (ARCI) BALAPUR VILLAGE, RR DISTRICT, BALAPUR P.O., HYDERABAD-500 005.
3 BHASKAR PRASAD SAHA INTERNATIONAL ADVANCED RESEARCH CENTRE FOR POWDER METALLURGY AND NEW METERIALS (ARCI) BALAPUR VILLAGE, RR DISTRICT, BALAPUR P.O., HYDERABAD-500 005.
4 Dr.IBRAM GANESH INTERNATIONAL ADVANCED RESEARCH CENTRE FOR POWDER METALLURGY AND NEW METERIALS (ARCI) BALAPUR VILLAGE, RR DISTRICT, BALAPUR P.O., HYDERABAD-500 005.
5 Dr.YASHWANT RAMUCHANDRA MAHAJAN INTERNATIONAL ADVANCED RESEARCH CENTRE FOR POWDER METALLURGY AND NEW METERIALS (ARCI) BALAPUR VILLAGE, RR DISTRICT, BALAPUR P.O., HYDERABAD-500 005.
PCT International Classification Number F 27 B 14/10
PCT International Application Number N/A
PCT International Filing date
PCT Conventions:
# PCT Application Number Date of Convention Priority Country
1 NA