Title of Invention

"A PROCESS FOR MAKING POROUS CERAMICS FOR PRESSURE FILTRATION"

Abstract

The invention relates to a process for making porous ceramics for pressure filtration. The main usage of porous ceramics are for separation of suspended particulate matter present in liquid and gaseous streams by passing the same through the porous ceramic tubes and plates for pressure filtration application. The porous ceramics has controlled pore size in the range of 1-15 urn. In the process of the present invention pore forming agents such as flour, saw dust, carbon, and charcoal are used in the range of 1-15 wt% which creates voids in the matrix after firing at temperature in the range of 1300°C - 1500°C. In the present invention it is necessary to control the ratio of plastic and non-plastic component in the range of 0.11 - 1.5 and to control the ratio of average grain size 0.04 - 0.53 to produce the aforesaid pore size in the range of 1-15 um according to specific requirement.

Full Text

The present invention relates to a process for making porous ceramics for pressure filtration. The main usage of porous ceramics are for separation of suspended particulate matter present in liquid and gaseous streams by passing the same through the porous ceramic tubes and plates for pressure filtration application. Another important use of such porous ceramic material are for preparation of ceramic membranes consisting of a coating of finer particles over the porous ceramic matrix to form a composite structure of graded pore size which are primarily used for micro, ultra and nano filtration application. Reference may be made to CSIR Patent No. 126508 wherein the raw materials used for the preparation of porous ceramic candles were coarser and angular grains of silica and a naturally occurring clay having much impurities causing wide pore size distribution, poor strength and formation of glassy matrix unsuitable for cleaning by strong alkalis and high pressure steam. The drawback of conventional porous ceramic candle filters are 1. Larger pore size varying in the range of 5-30 µm. 2. Owing to such broad pore size distribution, the efficiency of separation is much less and the chances of the pores clogging are very high due to penetration of the finer suspended particulates posing problems of frequent cleaning. 3. The suspended particles of relatively lower sizes passes through higher pore size of the porous candle when the filtration takes place under atmospheric pressure. To overcome this problem, cartridges of polymeric materials have been developed which has an integral skin of finer pore sizes as described in the reference Porter, M.C. , 1990, Handbook of Industrial Membrane Technology, Noyes Publication, Park Ridge, NJ. However, such materials also suffer from limitations of 1) poor chemical stability 2) low abrasiveness 3) low durability towards microbial attack. Ceramic materials have several key performance advantages over their polymeric counterpart due to which porous ceramics are gaining considerable importance and industrial applications. Reference may be made to the paper " The control of pore size in the manufacture of ceramic filters", R.A. Clark, M.F. Hall and J.W. Kirk, in the British Ceramic Proceeding, No. 43, December 1988, Institute of Ceramics, U.K. wherein the technique adopted for the manufacture of ceramic filters utilizes differently sized irregularly shaped non-plastic alumina particles following the theories of particle packing of different grain size range to control porosity and pore size in the filter. The drawbacks of this process are use of costly raw materials, difficulties in fabrication of the non-plastic component by slip casting, use of costly equipment like extruder and necessity of firing at high temperature above 1550 °C resulting in higher cost of filters. The main object of the present invention is to provide a process for making porous ceramics for pressure filtration which obviates above noted drawbacks. Another object of the present invention is to provide porous ceramics having controlled pore size in the range of 1 - 15 µm. Yet another object of the present invention is to provide a process for reducing the cost of production by using relatively cheaper raw materials, easier fabrication technique and lower firing temperature. In the process of the present invention pore forming agents such as flour, saw dust, carbon, and charcoal are used in the range of 1-15 wt% which creates voids in the matrix after firing at temperature in the range of 1300°C - 1500°C. In the present invention it is necessary to control the ratio of plastic and non-plastic component in the range of 0.11 -1.5 and to control the ratio of average grain size 0.04 - 0.53 to produce the aforesaid pore size in the range of 1-15 (am according to specific requirement. Accordingly, the present invention provides a process for making porous ceramics for pressure filtration having the steps of i) mixing 40 to 90 wt% non-plastic material powder such as metal oxides, metal hydroxides, metal silicates or mixture thereof 10 to 60 wt%, plastic material powder such as naturally occurring aluminosilicates containing impurities like soda, potash, calcium oxide and iron oxide less than 2.5 wt%, 0 to 15 wt% pore forming agents, 0 to 2% additives such as magnesia, titania, zinc oxide and deionised water, ii) adding deflocculants and binders in the concentration range of 0.5 to 2.0% w/v of water, optionally ageing to obtain a casting slip, iii) casting to shape, storing the shaped cast under humid condition for a period of 12 to 48 hours, iv) air drying for a period of 12 to 48 hours, oven drying at a temperature in the range of 40 to 150°C for a period of 12 to 24 hours, v) subjecting the dried cast to heating at the rate of 60 to 150°C at a temperature in the range 800 to 1000°C for 1 to 6 hours followed by heating at a temperature in the range of 1300 to 1500°C for 1 to 6 hours, cooling the resultant cast to room temperature at the rate of 60 to 150°C per hour to obtain the product. In an embodiment of the present invention the metal oxide, hydroxide, silicates used may be such as, alumina, boehmite, zirconia, yttria stabilized zirconia, mullite, calcined kyanite, sillimanite, diaspore, beach sand of rounded and sub-rounded grains in the size range of 1 t o 75 urn having a maximum amount of impurities like alkali oxide and iron oxide up to 0.5 wt%. In another embodiment of the present invention, the aluminosilicate materials used may be such as kaolinite, montmorillonite, illite containing impurities like soda, potash, calcium oxide, iron oxide less than 2.5 wt%. In still another embodiment of the present invention, the deflocculants and binders used may be such as polyvinyl alcohol, Darvan-7, sodium salt of carboxy methyl cellulose, sodium alginate, salts of p0olyacrylic acid, partially hydrolysed polyacrylic amide. In still another embodiment o the present invention, the ratio of average grain size of plastic and non-plastic components used may be in the range of 0.04 to 0.53 to produce the pore size in the range 1 to 15 µm. In yet another embodiment of the present invention the proportion of the plastic and non-plastic ingredients used may be in the ratio of 0.11 to 1.5 in such a way that there are at least 10 to 30 wt% of finer particles of less than 1.0 um size present in the mix. In still another embodiment of the present invention the pore forming agents used may be 0 15 wt % combustible materials with not more than 5 wt % ash content such as flour, saw dust, carbon, charcoal, polymeric materials of particle size below 20 um. In still another embodiment of the present invention the mixing may be effected in a plastic bottle or alumina lined pot / ball mill for a period of 4 to 24 hours to form a slurry of viscosity in the range of 100-1000 cP. The process steps of the present invention are described below: 1) Preparation of slurry in water of the non plastic components like oxide, hydroxides, silicates of metals, a combination of additives, plastic components like aluminosilicates, pore formers as combustible materials, binders and deflocculants. 2) Casting in the required shapes sucli as plates of 100 mm length or diameter and about 2 mm thick, tubes of about 1.5 to 4.0 mm wall thickness, 9 to 37 mm outer diameter and 300 to 1200 mm length by slip casting techniques. 3) Drying and firing at a temperature of 800 to 1000 ° C for 1 to 6 hours followed by heating at a temperature in the range 1300 to 1500 °C for 1 to 6 hours. The ability to produce porous ceramics with controlled pore size and porosity is less well documented. Simply underfiring a ceramic body will generate porosity of controlled pore size with low strength and permeability. Formation of closed pores or discontinuous pores following the burnout of organic fillers causes a problem. Sometimes bloating, pin-holes, surface waviness also occurs due to excessive emission of gasses through weaker zones during firing and also quick drainage of the slip, defective plaster moulds and such other reasons. Presence of fusible impurities causes excessive glass formation thereby disturbing the porous nature of the matrix. The critical features of the present invention is to utilize the theory of packing using narrow sized grains of non-plastic component and filling-up the inter-granular space of the particles without compromising the decrease in porosity and strength. The other parameters include homogenous mixing, control of impurity level and sintering temperature. The main novelty of the present invention is in providing porous ceramics shaped as required for pressure filtration having controlled pore sizes in the range of 1 to 15 µm. Another novelty is that the process is economical as it involves use of cheaper raw materials and firing at lower temperature. The above novel features have been achieved as a direct result of the inventive steps such as 1. Use of non-plastic component of rounded or sub-rounded grains, controlled grain size and lower purity. 2. Use of higher amounts of plastic component 3. Controlling the rheological properties of the slip made by mixing the components with water addition and adjusting the dose of deflocculants and binders 4. Firing the shape at lower temperature in the range of 1300° to 1500 °C. The following examples are given by way of illustration and therefore should not be construed to limit the scope of the present invention. EXAMPLE-1 About 60 gm of alumina powder of average particle size 9.6 u,m and 99.5 % purity was taken. 140 gm of washed china clay containing 1.2 wt% of iron oxide, soda, potash and calcium oxide impurities was added with the alumina. 30 gm of carbon powder containing 3.6 wt% ash was added to the aforesaid mix. The powder mix was charged in a plastic bottle with 400 gm alumina ball and 300 cc of deionised water. The materials were mixed for 6 hours and the slurry was unloaded in a flat container. 3 cc of 5% w/v Darvan-7 was added to the slurry to form a slip of viscosity 300 cP. The slip was poured in a two piece plaster of paris mould of about 16 mm internal diameter, the upper level of the slurry was maintained at a constant level by gradual addition of the slip time to time. The slip was allowed to remain within the mould for 2 minutes to maintain about 2 mm thickness of the cast after which the excess slurry was drained off. The cast body was allowed to remain within the mould for about 1 hour after which the two halves of the mould were separated out and the green cast was allowed to dry under humid condition for 2 days. The green tubes were slowly dried in air oven at 120 -130 °C for 24 hours. The average bulk density of green tubes was maintained in the range of 1.4 - 1.5 gm/cc. The green tubes were heated at the rate of 100 °C per hour maintained at 800 °C for 1 hour, then at 1500 °C for 4 hours and cooled to room temperature at the rate of 100 °C per hour. The apparent porosity of fired samples was found to be 44 % with average pore size of 4.15 \.im and fired MOR of about 30 MPa. The clean water permeability of the tubular samples was of the order of 12,000 LMH.bar. EXAMPLE- 2 About 80 gm of alumina powder (99.5 % purity) of average particle size 2.5 urn was mixed with 120 gm of processed and purified plastic clay containing less than 2.5 wt% total impurities (iron oxide, soda, potash and calcium oxide) and 30 grams of carbon powder containing 3.6 % ash. The powder mix was then charged in a plastic bottle with 400 gm alumina ball and 330 cc of deionised water. The materials were mixed for 3.5 hours and the slurry was unloaded in a flat container. 3 cc of 5% w/v Darvan-7 was added to the slurry to form a slip of viscosity 250 cP. The slip was poured in a two piece plaster of paris mould of about 16 mm internal diameter, the upper level of the slurry was maintained at a constant level by gradual addition of the slip time to time. The slip was allowed to remain within the mould for 1.5 minutes to maintain about 2 mm thickness of the cast after which the excess slurry was drained off. The cast body was allowed to remain within the mould for about 1 hour after which the two halves of the mould were separated out and the green cast was allowed to dry under humid condition for 18 hours. The green cast tubes slowly dried in air oven at 120 - 130 °C for 24 hours. The average bulk density of green tubes was maintained in the range of 1.35 - 1.4 gm/cc. The green tubes were heated at the rate of 100 °C per hour maintained at 800 °C for 1 hour, then at 1500 °C for 4 hours and cooled to room temperature at the rate of 100 °C per hour. The apparent porosity of fired samples was found to be 52 % with average pore size of 2.8 µm and fired MOR of about 26.48 MPa. The clean water permeability of the tubular samples was of the order of 6106 LMH bar. EXAMPLE -3 400 gm of alumina powder (91.5 % purity) of average particle size 37 um was mixed with 600 gm of processed and purified china clay containing 1.2 wt% iron oxide, soda, potash and calcium oxide as impurities. The powder mix was then charged in a plastic bottle with 1000 gm alumina ball and 1300 cc of deionised water. The materials were mixed for 3 hours and the slurry was unloaded in a flat container. 7 cc of 5% w/v Darvan-7 was added to the slurry to form a slip of viscosity 360 cP. The slip was poured in a two piece plaster of paris mould of about 16 mm internal diameter, the upper level of the slurry was maintained at a constant level by gradual addition of the slip time to time. The slip was allowed to remain within the mould for 1 minute to maintain about 2 mm thickness of the cast after which the excess slurry was drained off. The cast body was allowed to remain within the mould for about 1 hour after which the two halves of the mould were separated out and the green cast was allowed to dry under humid condition for 18 hours. The green cast tubes slowly dried in air oven at 120 - 130 °C for 24 hours. The average bulk density of green tubes was maintained in the range of 1.6 - 1.7 gm/cc. The green tubes were heated at the rate of 100 °C per hour maintained at 800 °C for 1 hour, then at 1500 °C for 4 hours and cooled to room temperature at the rate of 100 °C per hour. The apparent porosity of fired samples was found to be 55 % with average pore size 5 u,m. The clean water permeability of the tubular samples was of the order of 20,000 LMH.bar EXAMPLE 4 About 80 gm of alumina powder of average particle size 2.5 urn and 99.5 % purity was taken. 120 gm of washed bikaner clay containing 2 wt% of iron oxide, soda, potash and calcium oxide impurities was added with the alumina. 30 gm of carbon powder containing 3.4 wt% ash was added to the aforesaid mix. The powder mix was charged in a plastic bottle with 400 gm alumina ball and 300 cc of deionised water. The materials were mixed for 6 hours and the slurry was unloaded in a container. 4 cc of 5% w/v Darvan-7 was added to the slurry to form a slip of viscosity 230 cP. The slip was poured in a two piece plaster of paris mould of about 16 mm internal diameter, the upper level of the slurry was maintained at a constant level by gradual addition of the slip time to time. The slip was allowed to remain within the mould for 2.5 minutes to maintain about 2.5 mm thickness of the cast after which the excess slurry was drained off. The cast body was allowed to remain within the mould for about 1 hour after which the two halves of the mould were separated out and the green cast was allowed to dry under humid condition for 2 days. The green tubes were slowly dried in air oven at 120 -130 °C for 24 hours. The average bulk density of green tubes was maintained in the range of 1.4 - 1.5 gm/cc. The green tubes were heated at the rate of 100 °C per hour maintained at 800 °C for 1 hour, then at 1500 °C for 4 hours and cooled to room temperature at the rate of 100 °C per hour. The apparent porosity of fired samples was found to be 34 % with average pore si/e of 4.04 µm and fired MOR of about 28 MPa. The clean water permeability of the tubular samples was of the order of9160LMH.bar. EXAMPLES 900 gm of alumina powder (99.5 % purity) of average particle size 2.5 um was mixed with 100 gm of processed china clay containing less than 2.5 wt% total impurities (iron oxide, soda, potash and calcium oxide) and 100 grams of carbon powder containing 3.6 % ash. The powder mix was then charged in a plastic bottle with 1000 gm alumina ball and 1500 cc of deionised water. The materials were mixed for 5 hours and the slurry was unloaded in a flat container. 8 cc of 5% w/v Darvan-7 and 5 cc of 2 % w/v poly vinyl alcohol was added to the slurry to form a slip of viscosity 380 cP. The slip was poured in a two piece plaster of paris mould of about 16 mm internal diameter, the upper level of the slurry was maintained at a constant level by gradual addition of the slip time to time. The slip was allowed to remain within the mould for 2.5 minutes to maintain about 2 mm thickness of the cast after which the excess slurry was drained off. The cast body was allowed to remain within the mould for about 2 hour after which the two halves of the mould were separated out and the green cast was allowed to dry under humid condition for 24 hours. The green cast tubes slowly dried in air oven at 120 - 130 °C for 24 hours. The green tubes were heated at the rate of 100 °C per hour maintained at 800 °C for 1 hour, then at 1500 °C for 4 hours and cooled to room temperature at the rate of 100 °C per hour. The fired bulk density of the tubes was 2.9 gm/cc. The apparent porosity of fired samples was found to be 20 % and fired MOR of about 250 MPa. The clean water permeability of the tubular samples was of the order of 150 LMH.bar. EXAMPLE 6 48 Kg of alumina powder (99.5 % purity) of average particle size 37µrn was mixed with 72 Kg of processed and purified china clay containing 1.2 wt% iron oxide, soda, potash and calcium oxide as impurities. The powder mix was then charged in a alumina lined ball mill with charge : ball ratio of 3 : 1 and dry ground for 6 hours. 180 litres of deionised water was added to it and the slurry was wet ground for 17 hours.. The slurry was unloaded in a flat container. 1800 cc of 5% w/v Darvan-7 and 600 cc of 2% w/v polyvinyl alcohol were added to the slurry to form a castable slip of viscosity 360 cP. The slip was poured in two piece plaster of paris mould of about 36 mm internal diameter and 1200 mm in length, the upper level of the slurry in the mould was maintained at a constant level by gradual addition of the slip from time to time. The slip was allowed to remain within the mould for 3 minutes to maintain about 3 mm thickness of the cast after which the excess slurry was drained off. The cast body was allowed to remain within the mould for about 1.5 hour after which the two halves of the mould were separated and the green cast was allowed to dry under humid condition for 20 hours. The green cast tubes slowly dried in air oven at 120 -130 °C for 24 hours. The average bulk density of green tubes was maintained in the range of 1.6 - 1.7 gm/cc. The green tubes were fired in an oil fired furnace at 1400 °C with a total firing schedule of 14 hours and cooling schedule of 10 hours. The apparent porosity of fired samples were found to be 56 %, bulk density 1.32 -1.5 gm/cc and water absorption of 40 %. The clean water permeability of the tubular samples was of the order of 16 LPH at a pressure of 2 Kg/cm2 of cut piece of 200 mm length, 26.6 mm internal diameter and 32.8 mm outer diameter. The advantages of the present invention are i) Cheaper raw materials ii) Easy fabrication process requiring minimum investment iii) Feasibility of scaling-up upto 1000 - 1200 mm long tube of different porosity iv) Lower cost of production Claim: 1. A process for making porous ceramics for pressure filtration having the steps of i) mixing 40 to 90 wt% non-plastic material powder such as metal oxides, metal hydroxides, metal silicates or mixture thereof 10 to 60 wt%, plastic material powder such as naturally occurring aluminosilicates containing impurities like soda, potash, calcium oxide and iron oxide less than 2.5 wt%, 0 to 15 wt% pore forming agents, 0 to 2% additives such as magnesia, titania, zinc oxide and deionised water, ii) adding deflocculants and binders in the concentration range of 0.5 to 2.0% w/v of water, optionally ageing to obtain a casting slip, iii) casting to shape, storing the shaped cast under humid condition for a period of 12 to 48 hours, iv) air drying for a period of 12 to 48 hours, oven drying at a temperature in the range of 40 to 150°C for a period of 12 to 24 hours, v) subjecting the dried cast to heating at the rate of 60 to 150°C at a temperature in the range 800 to 1000°C for 1 to 6 hours followed by heating at a temperature in the range of 1300 to 1500°C for 1 to 6 hours, cooling the resultant cast to room temperature at the rate of 60 to 150°C per hour to obtain the product. 2. A process as claimed in claim 1 wherein the said metal oxide, hydroxide, silicates used are alumina, boehmite, zirconia, yttria stabilized zirconia, mullite, calcined kyanite, sillimanite, diaspore, beach sand of rounded and sub-rounded grains in the size range of 1 to 75 jam having a maximum amount of impurities like alkali oxide and iron oxide up to 0.5 wt%. 3. A process as claimed in claim 1 wherein the aluminosilicate materials used are kaolinite, montmorillonite, illite. 4. A process as claimed in claim 1 wherein the deflocculants and binders used are polyvinyl alcohol, Darvan-7, sodium salt of carboxy methyl cellulose, sodium alginate, salts of polyacrylic acid, partially hydrolysed polyacrylic amide. 5. A process as claimed in claim 1 wherein the ratio of average grain size of plastic and non-plastic material powder used are in the range of 0.04 to 0.53 to produce the pore size in the range 1 to 15 µm. 6. A process as claimed in claim 1 wherein the proportion of the plastic and non- plastic material powder used are in the ratio of 0.11 to 1.5 such that there are at least 10 to 30 wt% finer particles of less than 1.0 µrn size present in the mix. 7. A process as claimed in claim 1 wherein the pore forming agents used are 0 to 15 wt% combustible materials containing not more than 5 wt % ash of particle size below 20 (µm selected from flour, saw dust, carbon, charcoal, polymeric materials. 8. A process as claimed in claim 1 wherein the mixing is effected in a alumina lined ball mill for a period of 3 to 24 hours to form a slurry of viscosity in the range of 100-1000 cP. 9. A process for making porous ceramics for pressure filtration substantially as herein described with reference to the examples.

Documents:

351-del-2001-abstract.pdf

351-del-2001-claims.pdf

351-del-2001-correspondence-others.pdf

351-del-2001-correspondence-po.pdf

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

351-del-2001-form-1.pdf

351-del-2001-form-18.pdf

351-del-2001-form-2.pdf

351-del-2001-form-3.pdf

351-del-2001-petition-137.pdf

351-del-2001-petition-138.pdf