GB2250508A - Cellular inorganic material - Google Patents

Abstract

The material is obtained by (a) forming hollow microspheres of an aluminosilicate with a SiO2:Al2O3 molar ratio of at least 0.75:1, each microsphere having a shell defining an internal cavity. (b) subjecting (a) to a mechanical action to cause adherence to bodies comprising microspheres of diameter from 0.2mm to 4.0mm; (c) calcining the bodies at 1300 DEG C to 1800 DEG C for at least one hour; (d) treating with concentrated aqueous alkali metal hydroxide at at least 50 DEG C whereby silica formed during calcination is removed to leave ceramic crystals which define between them interconnecting pores; (e) washing bodies formed in step (d) until free of silicate and alkali metal ion; and (f) dewatering and drying. Product can be used as a biological support for enzymes and whole living cells.

Description

CELLULAR INORGANIC MATERIAL This invention relates to a rigid cellular inorganic material suitable for supporting and immobilising biological catalysts such as enzymes and whole living cells.

Our British Patent Application No. 8822079.3 describes a porous cellular material comprising a plurality of cavities each defined by a substantially spherical wall formed of a rigid intermeshing matrix of ceramic needles said wall being pierced by at least one aperture to provide access to the cavity, and the or each aperture having a diameter such that the ratio of the diameter of the aperture to the diameter of the cavity into which the aperture opens is in the range of from 0.1:1 to 1:1.

The predominant part of the description filed with the above application concerns the preparation of individual, hollow, apertured particles each having a substantially spherical wall formed of a rigid intermeshing matrix of ceramic needles. The diameter of these particles generally lies within the range of from about 20 microns to about 100 microns. It has been found that, while the size of the internal cavity and of the apertures of these particles makes them ideal for the purpose of capturing and immobilising whole biological cells, if the particles are used to pack a column their relatively small size is a disadvantage because the particles pack so closely together that the volume of free space surrounding them is limited and the resistance of flow of a liquid through the packing of the column is correspondingly high.

A more advantageous material for use as a packing for a column through which a flowing liquid is passed while at the same time supporting and immobilising catalyst would comprise particles having cavities and apertures of generally the same size as those of the material described in Example 9 of the British Patent Application No. 8822079.3 but having overall particle diameters in the range from about 0.2mm to about 4.0mums In the description filed with Application No.

8822079.3 it is suggested that microspheres formed by spray drying an aqueous suspension prepared under certain special conditions may be fused together to form pellets or bodies of various shapes and sizes, each body comprising a plurality of individual microspheres. Such bodies may be formed by subjecting microspheres formed by spray drying the specially prepared suspension to light pressing in a mould of appropriate dimensions, for example a tablet press, and calcining the resultant pressed body under the conditions specified in the description of the application.

We have now discovered that for most advantageous results certain precautions must be observed in carrying out the tablettingprocess and that other suitable processes exist for forming bodies comprising a plurality of spray dried microspheres.

According to the present invention, there is provided a method of preparing a rigid, cellular, inorganic material comprising the following steps: (a) forming hollow microspheres of an aluminosilicate material having an SiO2:A1203 molar ratio of at least 0.75:1, each microsphere having a shell defining an internal cavity.

(b) subjecting the hollow microspheres formed in step (a) to a mechanical action which causes the microspheres to adhere or agglomerate together to form bodies which each comprise a plurality of microspheres and which have a diameter in the range of from 0.2mm to 4.0mum; (c) calcining the bodies formed in step (b) at a temperature in the range of from 13000C to 18000C for at least one hour; (d) treating the calcined bodies formed in step (c) with a concentrated aqueous solution of an alkali metal hydroxide at a temperature of at least 500C whereby the silica formed during calcination is removed to leave ceramic crystals which define between them interconnecting pores; (e) washing the alkali metal hydroxide treated bodies formed in step (d) until the washing medium is substantially free of silicate and alkali metal ion; and (f) dewatering and drying the washed product of step (e).

In step (a) the hollow microspheres are preferably formed by spray drying an aqueous suspension of the aluminosilicate material, the suspension containing from 20 to 60% by weight of solid aluminosilicate material and up to 40% by weight, based on the weight of dry aluminosilicate material, of a viscosifying agent.The aluminosilicate material may be, for example, kyanite, sillimanite or andalusite, all of which can be represented by the chemical formula Al203.SiO2; dumortierite which can-be represented by the chemical formula 8Al203.B203.6SiO2.H2O; a clay mineral of the kandite group, which can be represented by the chemical formula Al203.2SiO2.2H2O; or pyrophillite which can be represented by the chemical formula Al203.4SiO2.H2O. Other possible aluminosilicate starting materials include topaz, pinite, illite and clay minerals of the smectite class.

It is also possible to use as the aluminosilicate starting material a mixture of an alumina-rich material and a silica-rich material or a mixture of substantially pure alumina and silica provided that the molar ratio Sio2:A1203 is at least 0.75:1 and preferably at least 1:1 in each case. Especially suitable are clay minerals of the kandite class, namely kaolinite, dickite, nacrite and halloysite and, in particular, kaolin clay which consists predominantly of kaolinite.Also very suitable are materials formed by calcining certain clay minerals of the kandite class at a temperature and for a time such as just to drive off substantially all of the chemically bound water to give a material which can be represented by the chemical formula Al203.2SiO2. Still.further examples of aluminosilicate starting materials include magnesium aluminum silicates such as talc. In this case, after the material has been subjected to steps (a) to (f) above, the product will consist of a cellular material with a porous network of crystals of cordierite of chemical formula 2MgO.2Al203.5SiO2.

The viscosifying agent may be a water dispersible synthetic polymer such as poly(vinyl alcohol) or poly(vinyl acetate), or a water dispersible natural polymer which may be, for example, carbohydrate-based such as dextran, starch, agar, guar gum, hydroxyethyl cellulose or sodium carboxymethyl cellulose, or protein-based, for example casein or gelatin.

The hollow microspheres formed in step (a) will generally have sizes substantially all of which are in the range from 10 microns to 250 microns and microspheres having sizes in the range from 50 to 100 microns are especially suitable.

The hollow microspheres formed in step (a) each preferably have a cavity which has an overall diameter in the range of from 20 microns to 120 microns. The wall thickness is preferably in the range of from 2 microns to 20 microns, more preferably from 5 microns to 10 microns.

Preferably, the shell of each microsphere formed in step (a) is pierced by one, two or three apertures, most preferably by a single aperture, and the ratio of the diameter of the aperture to the overall diameter of the microsphere is preferably in the range from 0.1:1 to 0.75:1. The volume of the internal cavity of each microsphere is preferably from 50% to 90% of the volume of the microsphere.

When prepared by spray drying, the hollow spheres, before mechanical treatment and calcination, are usually produced with openings of the desired size communicating with the internal cavities.

Alternatively, the spray dried particles may be produced with a wall which is thinned at certain regions which regions, after calcination, are held together by silica. On leaching, their thinned regions will break to yield openings into the internal cavity.

In step (b) the mechanical action may take the form of light pressing in the mould of a tablet press.

The smallest mould which is at present commercially available for a tablet press has a diameter of about 1.6mm. For the purposes of this invention it is desirable to use the smallest moulds which are available, in particular those having a diameter in the range from 1.6 to 2.0mm. In general the tablet formed in step (b) will contract during the calcining step (c), so that, for example, a tablet formed in a press of diameter 1.6mm in step (b) will be found to have a diameter of 1.2mm at the end of step (c). The thickness of the pressed tablet will be in the range from 1.4 to 2.0mm and most preferably about 1.5mm.If the diameter of the tablet is significantly greater than 2.0mm or the thickness is significantly greater than 1.5mm it becomes difficult to obtain a uniformly etched body in step (d) because the aqueous solution of alkali metal hydroxide does not easily penetrate to the centre of the body. On the other hand, if the thickness of the tablet is significantly less than 104mm the tablet becomes difficult to handle The pressure applied in the tablet press should be just sufficient to form a tablet or body which is able to withstand a crushing force in the range of from 50 to 200 g.wt. without breaking. If the tablet is crushed by a force of less than about 50 g.wt. the tablet will, in general,-be too weak to withstand further handling in steps (c) to (f).If the pressure applied in the tablet press is much greater, so that a force of say 250 g.wt. is required to crush the tablet, then it will usually be found that the hollow microspheres have been collapsed and the cellular nature of the final product will be lost. Preferably the tablet will withstand a crushing force in the range from 70 to 120 g.wt. without breaking, and most preferably about 90 g.wt.

The crushing force required to break the tablet is measured by means of an analytical beam balance to one arm of which is mounted a hammer head which co-operates with an anvil on which a formed tablet may be placed.

A small beaker is placed on the pan on the side of the balance on which the hammer is mounted and weights are placed on the opposite pan to counterbalance the beaker and the hammer. Water is then allowed to flow into the beaker until the tablet is just crushed between the hammer and the anvil. The volume of water contained in the beaker at this point gives a measure of the crushing strength required to break the tablet.

As an alternative to the tabletting process, granules having a diameter in the range of from 0.2 to 2.0mm may be formed by rolling the hollow microspheres formed in step (a) with a binder in a suitable granulator, for example a pan granulator. The quantity of binder required is generally from 1% to 30% by weight based on the weight of dry microspheres. Most advantageously the size of the granules should be in the range from 0.5 to 1.0mum.

The binder may be for example a natural waterdispersed polymer such as starch, casein, gelatine; a water dispersible synthetic resin such as poly(vinyl acetate) or a latex of an elastomeric material, for example a synthetic rubber such as styrene-butadiene rubber or an acrylic copolymer.

When a latex of styrene-butadiene rubber containing 50% by weight of elastomer solids is used as the binder it has been found advantageous to use from 400 to 500 ml. of latex per kilogram of dry microspheres which corresponds approximately to 20-25% by weight of elastomer solids based on the weight of the dry microspheres.

The bodies formed by granulation in this way are generally sufficiently strong to withstand the handling required to convey them to the apparatus in which step (c) is performed. During step (c) the binder is generally removed by combustion and the microspheres fuse together to form the required rigid, cellular material.

The bodies formed in step (b) may be calcined in step (c) by the process known as soak calcination in, for example, a tunnel kiln, in which the material is exposed to a temperature in the range of from 13000C to 18000C for at least 1 hour. Preferably, the material is heat-treated at a temperature greater than 13500C but not greater than 16000C for a time in the range of from 5 hours to 24 hours. After heat-treatment by soak calcination the mixture of ceramic crystals and silica is comminuted and subjected to one or more particle size separations by sieving, air classification and/or centrifugal or gravitational sedimentation.

If, in step (d), the silica in the heat-treated mixture of ceramic crystals and silica is to be readily soluble in the concentrated aqueous solution of an alkali metal hydroxide then the aluminosilicate starting material should preferably contain from 1.5% to 2.5% by weight of M20 where M is sodium and/or potassium. If the starting material contains less than this amount of M20, the M20 content may be increased by adding an appropriate quantity of an M20-containing mineral, such as feldspar.

In step (d) the alkali metal hydroxide is most conveniently sodium hydroxide and the molarity of the alkali solution is preferably at least 3M.

Advantageously, the reaction between the hollow microspheres of step (a) and the alkali metal hydroxide solution is performed at a temperature between 800C and the boiling point of the alkali metal hydroxide solution.

The purpose of step (d) is to dissolve substantially completely from the mixture of ceramic material and silica, the silica component which is generally present in a glassy form. After the silica has been removed by dissolution each body consists predominantly of ceramic needles joined together in the form of a three-dimensional lattice which has a high proportion of interconnecting voidage, the passages of which are relatively wide in relation to the width of the ceramic needles.

After leaching with the alkali metal hydroxide solutions, the calcined cellular body is washed and dried as described in steps (e) and (f) above.

In step (e) the alkali-treated particulate product of step (d) is preferably washed first with an alkaline solution weaker than that used in step (d) and then repeatedly with water until the washing medium is substantially free of silicate and alkali metal ions.

The alkaline solution used in this washing procedure preferably has a molarity of 1M or less.

The matrix of ceramic needles left after leaching preferably consist of mullite. Since the melting point of mullite is about 18100C the particular product can be subjected to high temperatures for long periods without fusing or sintering, and it has been found that the material has good resistance to thermal shock, being able to withstand repeated quenching from 12000C to room temperature with no deleterious effect on its structure. The cellular porous material formed from mullite is also able to withstand prolonged exposure to strong acids of pH 1 and strong alkalis of pH14. Of the known comparable materials silica is soluble in strongly alkaline solutions and alumina and clays are attacked by strong acids.

Preferably, the ceramic crystals, normally mullite, are needle-shaped and form an intermeshing matrix. The needles have a-width generally in the range of from 0.1 microns to about 0.5 microns. The pores defined between the needles'preferably are in the range of from 0.1 to 1.0 microns i.e. of a width to permit penetration of nutrients and material essential to the immobilised cell for growth. The crystals of mullite are preferably substantially uniform in size, preferably having a length in the range of from 1 micrometre to 5 micrometres, and the interconnecting pores preferably have a width no greater than 2 micrometres and a width preferably no smaller than 0.1 micrometre, more preferably no smaller than 0.5 micrometre.

The specific surface area of the particulate porous material of the present invention, as determined by the B.E.T. nitrogen adsorption method, is less than lOm2 per gram, and usually is less than 7m2 per gram.

EXAMPLE 1 A kaolin clay having a particle size distribution such that 80% by weight consisted of particles having an equivalent spherical diameter smaller than 2 microns and 0.1% by weight consisted of particles having an equivalent spherical diameter larger than 10 microns was mixed with water to form a suspension containing 40% by weight of dry kaolin, there being mixed with the suspension as a viscosifier 9% by weight, based on the weight of dry kaolin, of sodium carboxymethylcellulose.

This mixture was sprayed by means of an atomizer at a rate of aproximately 1 litre per minute to a labatory spray dryer operating at an inlet temperature of 3800C and an outlet temperature of 1750C. The dried product, which was in the form of hollow spheres of substantially uniform overall diameter of about 50 microns, was compressed in a tablet press to form tablets of diameter 4mm and thickness 2mm. The pressure used to form the tablet was such that the crushing strength, as measured by the technique hereinbefore described, was in the range from 90 to 100g.wt.

The tablets were calcined in a tunnel kiln under conditions such that they were exposed to a temperature of 15000C for eight hours. The calcined tablets were then boiled with 3M sodium hydroxide solution for one hour, the tablets being supported meanwhile on perforated trays which were rotated in sodium hydroxide solution in order to avoid breakage of the tablets.

The sodium hydroxide solution dissolved the silica phase leaving a rigid intermeshing matrix of mullite needles. The liquor was removed by filtration and the tablets were washed quickly in hot water. The calcined tablets were then treated for a second time with 3M sodium hydroxide solution for one hour, filtered and washed for ten minutes in boiling water. The steps of washing the tablets in hot water and removing the washing water by filtration were then repeated until the washing water was found to be substantially free of sodium and silicate ions. The washed tablets were finally dried in an oven at 600C.The dry product consisted of tablets each of which comprised a rigid, intermeshing matrix of mullite needles of length in the range of from 1 to 5 micrometres and of width in the range from 0.1 to 0.5 micrometre, the needles defining between them interconnected pores of width in the range of from 0.5 to 1.0 micrometre. The specific surface area of the tablets as determined by the B.E.T.

nitrogen adsorption method was 4.0m2 per gram. Each tablet consisted of clusters of hollow microspheres each of which had a large internal cavity. The hollow.

microspheres defining the outer layer of the tablet could be seen by electron microscopy to have walls pierced by at least one aperture of diameter approximately in the size range of from 5 to about 30 micrometers.

EXAMPLE 2 A further batch of spray dried kaolin clay, having the form of.hollow spheres of substantially uniform overall diameter of about 50 microns, was prepared, using the same starting material and the same method as were described in Example 1.

The spray dried kaolin was rotated in a pan granulator while there was sprayed on to it from a trigger spray device a latex of poly(vinyl acetate), which latex contained 50% by weight of polymer solids.

The amount of latex used was 400 ml of latex per kilogram of dry spray dried kaolin which corresponded to 20% by weight of polymer solids based on the weight of microspheres. When'the full amount of latex had been sprayed on to the spray dried kaolin it was found that the microspheres had agglomerated to form granules. The granules thus formed were passed through a sieve having a nominal aperture size of 1.6mm and then screened through a sieve having a nominal aperture size of 0.5mm. The product retained on the second sieve was collected and comprised granules having diameters in the range of from 0.5 to 1.6mm.

The granules were calcined in a tunnel kiln, treated with 3M sodium hydroxide solution, washed, dewatered and dried in the same manner as was described for the tablets in Example 1.

The dry granules each comprised a rigid, intermeshing matrix of mullite needles of length in the range from 1 to 5 micrometres and of width in the range from 0.1 to 0.5 micrometre, the needles defining between them interconnected pores of width in the range of from 0.5 to 1.0 micrometre.. The specific surface area of the granules as determined by the B.E.T.

nitrogen adsorption method was 4.7m2 per gram. Each tablet consisted of clusters of hollow microspheres each of which had a large internal cavity. The hollow microspheres defining the outer layer of the tablet could be seen by electron microscopy to have walls pierced by at least one aperture of diameter approximately in the size range of from 5 to about 30 micrometers.

Claims (18)

1. A method of preparing a rigid, cellular, inorganic material comprising the following steps: (a) forming hollow microspheres of an aluminosilicate material having an SiO2:Al203 molar ratio of at least 0.75:1, each microsphere having a shell defining an internal cavity.

(b) subjecting the hollow microspheres formed in step (a) to a mechanical action which causes the microspheres to adhere together to form bodies which each comprise a plurality of microspheres and which have a diameter in the range of from 0.2mm to 4.0mm; (c) calcining the bodies formed in step (b) at a temperature in the range of from 13000C to 18000C for at least one hour; (d) treating the calcined bodies formed in step (c) with a concentrated aqueous solution of an alkali metal hydroxide at a temperature of at least 500C whereby the silica formed during calcination is removed to leave ceramic crystals which define between them interconnecting pores; (e) washing the alkali metal hydroxide treated bodies formed in step (d) until the washing medium is substantially free of silicate and alkali metal ion; and (f) dewatering and drying the washed product of step (e).

2. A method according to claim 1, wherein the hollow microspheres formed in step (a) are formed by spray drying an aqueous suspension of the aluminosilicate material.

3. A method according to claim 2, wherein the suspension to be spray dried contains from 20 to 60% by weight of solid aluminosilicate material.

4. A method according to claim 3, wherein the suspension further comprises up to 40% by weight, based on the weight of aluminosilicate material, of a viscosifying agent.

5. A method according to claim 4, wherein the viscosifying agent is a water dispersible natural or synthetic polymer.

60 A method according to any one of the preceding claims, wherein the hollow microspheres formed in step (a) have sizes all of which are in the range of from about 10 microns to 250 microns in diameter.

7. A method according to-any one of claims 1 to 5, wherein the hollow microspheres formed in step (a) have sizes substantially all of which are in the range of from 50 to 100 microns in diameter.

8. A method according to any one of the preceding claims, wherein, in step (b), the mechanical action comprises light pressing of the hollow microspheres formed in step (a) in a mould.

9. A method according to claim 8, wherein the mould is a tablet press.

10. A method according to claim 8 or 9, wherein the pressure applied to the mould is sufficient to form a tablet which is able to withstand a crushing force in the range of from 50 to 200g.wt. without breaking.

11. A method according to claim 8 or 9, wherein the pressure applied to the mould is just sufficient to form a tablet which is able to withstand a crushing force in the range of from 70 to 120g.wt. without breaking.

12. A method according to claim 8 or 9, wherein the pressure applied to the mould is just sufficient to form a body which is able to withstand a crushing force of about'90g.wt. without breaking.

13. A method according to any one of claims 1 to 7, wherein the mechanical action comprises rolling the hollow microspheres formed in step (a) with a binder in a granulator to form granules comprising an agglomerate of the hollow microspheres.

14. A method according to claim 13, wherein the quantity of binder employed is in the range of from 1% to 30% by weight, based on the weight of dry microspheres.

15. A method according to claim 13 or 14, wherein the size of the granules formed is in the range of from 0.2 to 2.0mm.

16. A method according to any one of claims 13 to 15, wherein the binder is a natural water-dispersed polymer,' a water-dispersible synthetic resin or a latex of an elastomeric material.

17. A rigid cellular material whenever produced by a method as claimed in any one of the preceding claims.

18. A biological support comprising a rigid cellular material as claimed in claim 17.