Title of Invention

"A PROCESS FOR MANUFACTURE OF MICROPOROUS ALUMINA MEMBRANE SUBSTRATES"

Abstract

The present invention relates to a process for manufacturing microporous alumina membrane substrates with porosity in the range of 40-75% and pore size in the range of 0.2 µm to 1.0 µm comprising: reacting urea and formaldehyde to obtain methylol urea; dispensing the alumina powder in the range of 64 wt% to 84 wt% in methylol urea solution at a pH in the range of 2 -3, by ball milling to obtain a homogeneous alumina slurry; mixing the alumina slurry with additional urea so that final content of urea-formaldehyde in the slurry is in the range of 10 to 60% by weight of alumina, casting the slurry in moulds to obtain tubular or planar shaped gelled body, drying the gelled body; and heating the dried bodies followed by sintering at a temperature in the range of 1250 to 1450°C to obtain the microporous alumina membrane substrate.

Full Text

FIELD OF INVENTION This invention relates to a process for manufacture of microporous alumina substrate with high porosity and the substrate thereof. PRIOR ART Organic membranes used for industrial separation process has been increasingly replaced by inorganic membranes because of their high mechanical strength and chemical, biological and thermal inertness. Unlike polymer membranes inorganic membranes can be used under extreme conditions of pH, temperature, pressure, radiations and the like. 50% of the total market share in membrane separation technology is for microfiltration membranes with pore size in the range 0.1 to 10 urn.. Microporous inorganic membranes are used for separation of microbes from drinking water, concentration of fruit juices in food industry, filtration of beverages for their clarification and sterilization, water treatment and recovery of various chemicals from black liquor in paper industry and in biotechnology [Das et. al Brit. Ceram. Trans. 101 (5) 208-212 (2002)]. They also find application as substrate for nanofikration, ultra filtration and reverse osmosis membranes. Process such as powder pressing; slip casting, tape casting, extrusion and centrifugal casting followed by partial sintering of powders of appropriate particle size and size distribution are used for the preparation of microporus alumina membrane substrate in planar and tubular configuration [Das et. al j. Euro. Ceram. Soc, 19, 314-345 (1999); Lama et. al. Ceram. Int 19, 121-24 (1993); Darcovich et. al J, Am. Ceram. Soc, 82 (8) 2073-73 (1990); Nijmeijer, et. al Am. Ceram. Soc. Bull. 77 [4]95-98 (1998). Permeate flux of a membrane is directly proportional to its porosity [Clark et. al. Br. Ceram. Proc. 43, 77-91 (1998); Das et. al br. Ceram. Trans. 101(5) 208-212(2002)]. Literature shows porosity of membrane substrates prepared by the above methods was Gelcasting is a novel ceramic shape forming process for preparation of dense near-net-shape ceramic bodies [Omatete et. al. Am. Ceram.Soc., 70 (10)1641-1647 91991). This process based on gelation of concentrated ceramic powder suspensions containing an organic monomer in a mould by in situ polymerization of the monomer. Acrylic and urea formaldehyde monomers have been used for gelcasting process [Young et. al J. Am. Ceram. Soc. 74 (3) 612-18 (1991); Prabhakaran et. al. Ceram. (nt 26, 67-71 (2000)]. Recently, a freeform gelcasting process for fabrication of tubular microporous alumina membrane substrate with porosity - 42% based on urea formaldehyde gelation process has also been reported [Prabhakaran et. al. J. Am. Ceram. Soc., 85 (12) 3126-28 (2002). United State Patent 5,415,775 (May 16, 1995) describes a process for preparation of porous monolithic ceramic support with pore volume greater than 30% and pore diameter ranging from 1 to 20 µm from alumina particulates coated partially with TiO2 for composite inorganic micro/ultrafiltration devices. United States Patent 5,656,168 (August 12, 1997) describes a method of making porous substrate and depositing a sinterable layer of powder particle clusters on it for producing inorganic filter membranes. United States Patent 5,208,190 (May 4, 1993) describes a method for the preparation of microporus alumina ceramic membranes with mean pore size less than 100 Angstroms based on improved sol-gel technique. United State Patent 5, 895,572 (April 20, 1999) describes preparation of porous monolithic support for filtration membranes containing channels and deposition of microfiltration or ultrafiltration layer on the surface of the support. United State Patent 4,980,062 (December 25, 1990) describes a method for preparation of inorganic membrane on porous substrate from a colloidal suspension of inorganic particles and a polymer dissolved in a solvent by a phase separation process. United State Patent 5, 242,595 (September 7, 1993) describes a bacteria separation process by passing through a membrane prepared by coating two layers of alumina over porous alumina substrate. United State Patent No. 4,894,194 (January 16, 1991) describes a process for fabrication of complex ceramic shapes by gelation of non aqueous ceramic powder suspensions by in situ polymerization of organic monomers. United State Patent No. 5.028.362 (July 2, 1991) describes a process for fabrication of complex ceramic shapes by gelation of aqueous ceramic powder suspensions by in situ polymerization of water soluble acrylic monomers. NEED FOR THE PRESENT INVENTION The present processing techniques produce microporous alumina membrane substrate with porosity 40 to 50%. The pores are unstable at higher sintering temperature. This results in either poor strength if sintering is done at lower temperatures to get higher or lower porosity . In order to have strong and porosity if sintering is done at high temperatures to obtain substrates of high strength stable membranes of high permeate flux it is necessary to propose a process to manufacture membrane substrates having higher porosity even after sintering at higher temperatures. OBJECTS OF THE PRESENT INVENTION The primary object of the present invention is to propose a process for manufacture of microporous alumina substrates having high porosities. Another object of the present invention is to propose a process for manufacture of microporous substrates which have porosities in the range of 40% to 75% in tubular as well as planar configurations. Yet another object of the present invention is to propose a process to provide microporous substrates which can withstand extreme conditions of pH, temperature and pressure faced in any industrial separation process. Still another object of the present invention is to propose a process to provide microporous substrates having high mechanical strength and chemical, biological and thermal inertness. Further objects and advantages of the invention will more apparent from the ensuring description. It is to be understood that the features and concepts of the invention can be adopted to various embodiments by making changes, adaptions or modifications by those skilled in the art. Such variant embodiments are intended to be with in the scope of the present invention. STATEMENT OF INVENTION According to the present invention there is provided a process for manufacturing the microporous alumina membrane substrates with porosity comprising the following steps: a) reacting the urea and formaldehyde in the range of 1:2 to 1:6, in basic aqueous condition; b) dispensing the alumina powder in the methylol urea solution in acidic pH. in the range of 2 to 3, by ball milling to obtain a homogenous alumina slurry; c) mixing the alumina slurry with additional urea to obtain the urea to formaldehyde mole ratio t :2; d) casting the slurry in moulds to obtain tubular or planar shapes gelled body drying the gelled body under controlled humidity conditions in the range of 60 to 90% at a temperature of 25 to 40°C; e) heating the dried bodes by heating at a slow rate upto 600°C to burnout the binder from the dried bodies; f) sintering he debinderised bodies by heating at a rate of less than 10°C/min. and holding the temperature in the range of 1250 to 1450°C for a period of 1 to 4 hours to obtain the microporous alumina membrane substrate. According to the present invention there is further provided a microporous alumina membrane substrates obtained by using the process, wherein the porosity is in the range of 40 to 75% preferably 40 to 50% and the average pore size 0.2 µm to 1.0 µm. DETAILED DESCRIPTION OF THE INVENTION The present invention proposes a process for manufacture of microporous alumina substrate with porosities 40% to 75% and average pore size 0.2 µm in planar and tubular configurations. Porosity and pore size of the membrane is controlled by the amount of urea formaldehyde. Urea formaldehyde acts template for micropores. Porosity measurement using mercury porosimeter showed all pores are accessible (interconnected), suitable for microfiltration applications. They can also be used as substrate for ultrafiltration and reverse osmosis membranes. The process comprises of following steps: (i) Preparation of methylol urea by mixing urea and formaldehyde solution (35 to 37 w/v%) in basic condition preferably at pH in the range 8 to 9 and aging the solution for 24 hrs. Any base preferably sodium hydroxide is used for pH adjustment. The molar ratio of urea and formaldehyde used is in the range 1:2 to 1:6. (ii) Dispersion of alumina powder in the methylol urea solution in acidic pH preferably at pH in the range 2 to 3 by ball milling. Any mineral acid preferably nitric acid is used for pH adjustment. (iii) Mixing the alumina slurry with additional urea to make urea to formaldehyde mole ratio 1:2 and casting the slurry in moulds to fabricate tubular or planar shapes. (iv) Drying of the gelled body under humidity controlled conditions preferably at relative humidity in the range 60 to 90% and at temperature in the range 25% to 40% C. (v) Binder burnout from the dried bodies by heating at slow rate/up to 600°C at a rate of 60°C c/h preferably at 5° C/hrs. (vi) Sintering of the debinderised bodies by heating at a rate In basic medium urea react with formaldehyde to form methylol urea derivatives. Alumina slurry prepared in the methylolurea solution when supplemented with additional urea undergoes gelation in 15 to 30 minute due to condensation polymerization of urea and methylol urea. The gelled body has sufficient strength to be removed from the mould without deformation. The porosity and pore size of the membrane substrate is controlled by the amount of urea-formaldehyde used with respect to alumina powder and the amount of urea formaldehyde used is preferably in the range 10 to 60% by weight of alumina. According lo one preferred embodiment of the invention, the alumina used for the present process has an average particle size of from 0.1 5µn to 5µm. According to another embodiment, the gelation of alumina slurry is achieved by mixing urea, formaldehyde and methylol urea with alumina slurry. The moulds for fabricating the substrate are made of a metal. According to one embodiment the mould is made of any plastic material. According to another embodiment the mould is made of glass. According to one embodiment the mould is made of wax. According to a preferred embodiment the substrates obtained by this process have a thickness in the range of 1.5 mm to 4 mm. The following examples are provided to illustrate the invention in accordance with the principles of the invention, but are not construed as limiting the invention in any way except as indicated in the appended claim. Example I 100 ml formaldehyde solution was adjusted to 8.5 pH using sodium hydroxide solution 18.5 g urea was added to this solution and aged for 24 h. 398 g alumina powder was dispersed in the solution and adjusted the pH to 2.5 using dilute nitric acid. The slurry was ball milled for 6 hrs and then added 18.5 g urea. The slurry was cast in cylindrical open mould. The slurry after gelation was removed from the mould and dried at 75% relative humidity at 30°C. Dried body was heated in a furnace at a rate of 5° C/h up to 500°C and then 300° C/h up to I350°C with a hold of 2 h at 1350°C. Example II 100 ml formaldehyde solution was adjusted to 8.5 pH using sodium hydroxide solution 18.5 g urea was added to this solution and aged for 24 h. 133 g alumina powder was dispersed in the solution and adjusted the pH to 2.5 using dilute nitric acid. The slurry was ball milled for 6 hrs and then added 18.5 g urea. The slurry was cast in between two concentric pipes to prepare tubular shape. The slurry after gelation was removed from the mould and dried at 80% relative humidity at 30°C. Dried body was heated in a furnace at a rate of 5° C/h up to 600°C ad then 300°C/h up to 1450°C with a hold of 2 h at 1450°C. Example III 100 ml formaldehyde solution was adjusted to 8.5 pH using sodium hydroxide solution 18.5 g urea was added to this solution and aged for 24 h. 265 g alumina powder was dispersed in the solution and adjusted the pH to 2 using dilute nitric acid. The slurry was ball milled for 6 hrs and then added 18.5 g urea. The slurry was cast in cylindrical moulds. The slurry after gelation was removed from the mould and dried at 90% relative humidity at 40°C. Dried body was heated in a furnace at a rate of 5°C/h up to 600°C and then 300°C/h up to 1450°C with a hold of 2 h at 1450°C. Example III 100 ml formaldehyde solution was adjusted to 8.5 pH using sodium hydroxide solution 18.5 g urea was added to this solution and aged for 24 h. 265 g alumina powder was dispersed in the solution and adjusted the pH to 2 using dilute nitric acid. The slurry was ball milled for 6 hrs and then added 18.5 g urea. The slurry was cast in cylindrical moulds. The slurry after gelation was removed from the mould and dried at 90% relative humidity at 40°C. Dried body was heated in a furnace at a rate of 5°C/h up to 600°C and then 300°C/h up to 1450°C with a hold of 2 h at 1450°C. We Claim: 1. A process for manufacturing microporous alumina membrane substrates with porosity in the range of 40-75% and average pore size in the range of 0.2 µm to 1.0 µm comprising the following steps: (a) reacting urea and 35 to 37 w/v % of formaldehyde in the range of 1:2 to 1:6 in basic aqueous condition with pH in the range of 8 to 9, achieved by using any base preferably sodium hydroxide; (b) dispensing the alumina powder in the range of 64 wt% to 84 wt% in methylol urea solution at a pH in the range of 2 -3, by ball milling to obtain a homogeneous alumina slurry; (c) mixing the alumina slurry with additional urea to obtain urea to formaldehyde mole ratio 1:2; (d) casting the slurry in moulds to obtain tubular or planar shaped gelled body, drying the gelled body under controlled humidity conditions in the range of 60 to 90% at a temperature of 25 to 40°C; (e) heating the dried bodies by heating at a rate upto 60°C/hr, preferably 5°C/hr, upto 600°C to burnout the binder from the dried bodies; (f) sintering the debinderised bodies by heating at a rate of less than 10°C/min and holding the temperature in the range of 1250 to 1450°C for a period of 1 to 4 hours to obtain the microporous alumina membrane substrate; wherein the total amount of urea-formaldehyde in the slurry of step (c) is in the range of 10 to 60% by weight of alumina. 2. A process as claimed in claim 1, wherein in step (b) any mineral acid preferably nitric acid is added. 3. A process as claimed in claim 1 wherein the moulds to fabricate the substrate are made up of plastic, wax, glass or metals.

Documents:

2449-DEL-2004-Abstract-(21-05-2012).pdf

2449-del-2004-abstract.pdf

2449-DEL-2004-Claims-(21-05-2012).pdf

2449-DEL-2004-Claims-(24-10-2011).pdf

2449-del-2004-claims.pdf

2449-DEL-2004-Correspondence Others-(21-05-2012).pdf

2449-DEL-2004-Correspondence Others-(24-10-2011).pdf

2449-del-2004-correspondence-others.pdf

2449-DEL-2004-Description (Complete)-(21-05-2012).pdf

2449-del-2004-description (complete).pdf

2449-DEL-2004-Form-1-(21-05-2012).pdf

2449-del-2004-form-1.pdf

2449-del-2004-form-18.pdf

2449-DEL-2004-Form-2-(21-05-2012).pdf

2449-del-2004-form-2.pdf

2449-del-2004-form-3.pdf

2449-del-2004-form-5.pdf

2449-DEL-2004-GPA-(24-10-2011).pdf

2449-del-2004-gpa.pdf