GB1594588A

GB1594588A - Drying of gel materials

Info

Publication number: GB1594588A
Application number: GB44475/76A
Authority: GB
Original assignee: UK Atomic Energy Authority
Current assignee: UK Atomic Energy Authority
Priority date: 1976-10-26
Filing date: 1976-10-26
Publication date: 1981-07-30
Also published as: JPS5354353A; FR2369524B1; FR2369524A1; JPS618910B2; IT1093019B; DE2747783C2; DE2747783A1

Description

(54) IMPROVEMENTS IN OR RELATING TO THE DRYING OF GEL MATERIALS (71) We, UNITED KINGDOM ATOMIC ENERGY AUTHORITY, London, a British Authority, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- The present invention relates to the drying of gel materials and particularly, but not exclusively, to the drying of gel particles.

According to one aspect of the present invention there is provided a process for the drying of a gel material comprising mixing together steam and gas in controlled amounts to form a mixture, feeding the mixture to a gel material to be dried thereby to contact the gel material with steam and gas in controlled amounts and thereby dry the gel material.

In one embodiment of the invention the gel material comprises gel particles, for example particles of a hydrous inorganic oxide gel or particles of a gel precipitated gel.

Thus the invention finds an application in the production of ceramic particles (e.g. for nuclear fuels as disclosed in our British Patent Specification No. 1313750) wherein gel particles in the form of microspheres are first formed and then subsequently dried and heated to produce a ceramic particle (microsphere) product.

Ceramic microspheres, particularly for nuclear applications, often have to meet rigorous requirements with regard to integrity, internal structure, density and size and it has been found that the nature of the drying of gel microspheres prior to conversion to ceramic particles as hereinbefore mentioned can influence the properties of the final product ceramic microspheres particularly with regard to the likelihood of meeting rigorous specifications.

Organic solvents may be used for the drying of gel microspheres but suffer from the disadvantages of flammability, contamination, and radiolytic and/or thermal breakdown. Drying can also be effected by using air or gas, but this method of drying suffers from the disadvantages that very careful control of humidity, pressure and temperature is required in order to avoid cracked or broken ceramic microsphere product.

The use of steam in drying is also known but suffers from the disadvantage that generally the drying conditions are difficult to control in order to avoid product of poor integrity (e.g. cracked or broken product) or a product of inconsistent quality (both in respect of inter and intra "batch" quality).

We have found that the use of steam in conjunction with a gas, as a diluent, may be used in the controlled drying of a gel .material and can give rise to a product having improved properties.

Thus it is one object of the present invention to avoid or substantially overcome at least some of the difficulties of prior art processes.

Gel (materials (e.g. gel particles) which may be treated in accordance with the present invention may be, for example, gel microspheres produced by the so-called gelprecipitation process (reference may be made to our British patents 1175834, 1231385, 1253807, 1313750 and 1363532 regarding gel precipitation processes).

Alternatively, the gel particles may be, for example, microspheres produced by other gelation processes such as those involving sol-gel techniques or internal gelation.

Thus particles which may be treated in accordance with the present invention can comprise inorganic compounds in association with an organic gelling agent (sometimes also called in gel-precipitation technology a "gelating agent" or a "protective agent") or may comprise only an inorganic compound or compounds (e.g.

alumina, zirconia and ceria). The gel particles may contain mixtures of oxides (e.g. oxides of uranium and thorium). The gel particles may optionally contain carbon.

In carrying out the process of the present invention to dry gel microspheres we prefer that the said microspheres are first conditioned by contact with an atmosphere of wet steam (optionally containing a diluent gas) prior to being dried by being contacted with an atmosphere containing steam and a diluent gas therefor. This has been found to prevent or reduce substantially the microspheres taking on a dimpled, mis-shapen appearance.

Conditioning of gel materials is further discussed hereinafter. This contacting with wet steam acts as an "in situ" hot wash and can be replaced by a hot water wash (e.g.

preferably with water having a temperature of 60 to 1000C).

If the temperature of the gel material is less than that of the atmosphere of steam and gas, condensation will generally occur on the gel material upon initially contacting it with the atmosphere. It will be appreciated that condensation will not tend to occur once the temperature of the gel material has risen sufficiently. Where the nature of the gel material is such that it can be pre-heated without detrimental effects, initial condensation can be substantially avoided by pre-heating.

It will be appreciated that where the gel microspheres are produced by a gel process which gives a water dispersible gel (e.g.

dehydration of a sol) the use of wet steam or a hot water wash is not desirable.

From the point of view of obtaining a particle product with the best integrity it is in general very preferable to exclude oxygen and oxygen-containing gases from, or minimise the amount thereof in, the atmosphere used in the drying step when treating certain gel materials at high temperatures (e.g. 100"C) (e.g. gel precipitated gels of uranium, thorium or aluminium containing a gelling agent which is prone to oxidation). Thus, the gas chosen for the atmosphere should be such that it does not lead to undesirable chemical reactions (e.g. oxidation) at the temperature used during the drying.

Examples of suitable diluent gases which are suitable, inter alia, for the drying of gel precipitated gels are nitrogen and "inert" gases (e.g. argon and helium), and mixtures thereof. By "oxygen-containing" we mean containing oxygen as such (e.g. air) and we do not include in this term gases containing chemically combined oxygen. Thus, carbon dioxide may be used although some surfaces etching may occur under certain processing conditions with some gel materials.

Other gases may be used as diluent gases (e.g. hydrogen and methane) providing undesirable chemical reactions do not occur during drying.

In one embodiment we prefer to prepare the atmosphere containing steam and gas in controlled amounts by introducing the gas into wet steam and subsequently heating the mixture formed thereby to produce an atmosphere containing dry steam and gas.

By way of example, steam to gas ratios in the range 25:1 to 125:1 have been used to dry gel materials in accordance with the present invention.

The process of the invention may be conveniently carried out by passing a mixture of steam and diluent gas at a temperature in the range 373A23 K, preferably at 378-3880K, through a bed of gel material to be dried for a period determined by the amount of gel material in the bed.

We have found that with gel precipitated microspheres produced using certain organic gelling agents (e.g. polyacrylamide) some inorganic species (e.g. uranium or tungsten containing ions) alone do not give a sufficiently rigid gel structure to permit entirely satisfactory drying by the process of the invention after contact with wet steam.

Thus, microspheres containing respectively uranium-containing ions and tungsten-containing ions swelled in size on contact with wet steam and were not entirely satisfactorily dried subsequently with a steam/diluent gas atmosphere. It is believed that the valency of certain inorganic species (e.g. members of Group VIa) does not permit production of sufficient rigidity in the gel with the result that the gel structure expands on contacting wet steam.

However, if other ions are also present in the gel (e.g. plutonium) or thorium in the case of uranium) satisfactory drying may be effected.

Also, it is believed that whilst the certain inorganic species hereinbefore mentioned do not produce gels of sufficient rigidity when polyacrylamide is used as the organic gelling agent, these species may produce gels which are susceptible to satisfactory drying by the process of the invention if these gels are produced using other organic gelling agents.

The treatment with wet steam before drying in an atmosphere of steam and gas in accordance with the foregoing aspect of the present invention permits an open pore structure to be maintained in the gel, which in the case of gel precipitated gels aids crack-free debonding and sintering. It is to be understood that "debonding" means the removal of organic materials from the gel.

This is normally achieved by controlled oxidation at elevated temperature.

Whilst reference may be made to our British patents hereinbefore mentioned for details of gel precipitation processes, a brief description is included in this specification.

Thus, gel precipitated microspheres may be produced by contacting droplets of a gel precipitation feed solution, containing inorganic species (e.g. uranium and/or thorium salts) and an organic gelling agent (e.g. a polyacrylamide such as Superfloc (Registered Trade Mark) 16 or Magnafloc (Registered Trade Mark) 900), with a precipitating agent (e.g.

ammonia/ammonium hydroxide).

Gel particles comprising gel precipitated microspheres in the size range 20O3000 y diameter have been satisfactorily dried using the process of the present invention, but the invention is not, of course, limited to the drying of particles of any particular size.

(It is to be understood that gel precipitated gel microspheres of 200--3000 ,*4 diameter give ceramic product microspheres in the range 701000 y after drying and heating).

According to another aspect the present invention provides a gel material dried by a process in accordance with the invention.

As hereinbefore disclosed the gel material, optionally, may be conditioned by wet steam or hot water prior to drying in accordance with the present invention.

It will be appreciated that the contacting .with steam or hot water is continued for a sufficient time to cause the desired degree of conditioning.

The wet steam may optionally contain a diluent gas (e.g. nitrogen, argon, helium or carbon dioxide). The wet steam may optionally be at a pressure of greater than one atmosphere.

Wet steam is defined as steam which contains drops or particles of liquid water above the saturated vapour pressure at the prevailing temperature. Dry steam is steam which contains no liquid water.

The contacting with hot water may be effected by boiling the gel material in water.

The nature of the conditioning is complex and not fully understood, however conditioning does increase the stability of the gel material with respect to maintaining its integrity and consistency of quality during subsequent drying. Conditioning also assists in the maintenance of an open pore structure in the gel material (e.g. in a gel precipitated gel). It has been observed that dried gel microspheres produced from gel microspheres which have been conditioned prior to drying, have a larger diameter than dried gel microspheres produced from essentially similar gel microspheres not conditioned prior to drying.

The degree of conditioning can be determined by experimentation for a particular gel material.

If hot water is used for conditioning the gel material we prefer that it has a temperature in the range 60"C to 1000C.

The upper limit of the temperature which can be used will normally be limited by the decomposition temperature of the gel material.

A gel material dried in accordance with the present invention (optionally conditioned prior to drying) may be heated to give a ceramic product.

The invention finds application in the production of ceramic particles (e.g. for nuclear fuels as disclosed in our British Patent Specification No. 1313750) wherein gel particles in the form of microspheres are first formed, and subsequently conditioned, dried and heated to produce a ceramic particle (microsphere) product. Thus, in one embodiment the present invention provides a process comprising conditioning particles of gel material, drying the said particles and heating the dried particles to give a ceramic particle product wherein to cause conditioning of the particles prior to drying the particles of gel material are contacted with wet steam, or hot water for a sufficient time to cause the desired degree of conditioning.

The particles of gel material may contain nuclear fuel materials (e.g. uranium containing ions or thorium ions or plutonium ions).

The conditioning and drying in accordance with the present invention finds one particular application in the treatment of large diameter gel particles where water diffusion paths are relatively long and diffusion of water is slow. (Examples of large diameter gel particles are nuclear fuel particles having a final diameter after drying and heating of 500--1000 4m) It will be appreciated that where reference is made to a gel material containing ions of an element (e.g. uranium containing ions) the "ions" may generally be in combination with other elements (e.g.

as a compound or in association with the gelling agent in the case of a gel precipitated gel).

The invention will now be particularly described and illustrated, by way of example only, as follows (Examples 1, 2, 9 and 11 are examples showing the use of steam and an oxygen-containing diluent gas in the drying of gel precipitated gels and Examples 3 to 8 and 10 and 12 show the use of steam and a non-oxygen-containing diluent gas in the drying of gel precipitated gels): Example 1 In this Example gel precipitated micro spheres, containing uranium and thorium (70:30 U/Th ratio), prepared in accordance with known gel precipitation procedures were dried by use of an atmosphere containing steam and air.

The gel precipitated microspheres were prepared by contacting droplets of a gel precipitation feed solution, said solution containing uranium and thorium salts, a polyacrylamide organic gelling agent (Superfloc (Registered Trade Mark) 16) and an organic modifying agent (formamide) (as disclosed in our British Patent No. 1363532), with a precipitating agent comprising ammonium hydroxide. The resulting gel microspheres were washed with water and subsequently dried by use of a mixture of steam and air.

Thus wet steam at 373 K was passed through a stainless stell spiral tube placed inside a tube furnace. Air was used as a diluent gas and was introduced into the wet steam prior to entering the tube furnace.

The resulting atmosphere of steam plus diluent gas was heated to between 390 and 400 K and subsequently passed, at approximately atmospheric pressure, into a stainless steel vessel containing a bed of wet gel precipitated microspheres prepared as above described. The atmosphere of steam and diluent gas was passed up through the bed at a temperature of approximately 380 K and water was condensed at the outlet of the vessel.

Flow meters and needle valves were used to control the amount of diluent gas added to the steam and the temperatures and flowrates were monitored.

A batch of 10 g of wet gel spheres was treated for 1 hour with wet steam (373 K) and subsequently for 1 hour at 380 K with an atmosphere comprising dry steam (steam flow rate 25 litre min-') to which air had been added (as hereinbefore described) at a rate of 1 litre per minute.

The microsphere product produced in this way was substantially crack-free but when de-bonded to remove carbonaceous products and sintered to produce ceramic microspheres the resulting product was found to be cracked and broken.

Example 2 The general procedure of Example I was followed using a 10 g bed of wet gel microspheres. However the conditions were varied such that wet steam was contacted with the gel spheres for 30 minutes and subsequently the spheres were treated at 380 K with dry steam (steam rate 25 litres min-') to which air had been added at 0.2 litres per minute for 1.5 hours.

The microsphere product after the drying stage was observed to be substantially crack-free but after the de-bonding stage some cracks had appeared. Also the final sintering step in treating the microspheres resulted in the production of broken microsphere products. The final density (Hg immersion) was 86.5% theoretical.

Example 3 The general procedure of Example 1 was carried out with a 10 gram bed of wet gel microspheres. However the conditions were varied such that the microspheres were treated with wet steam at 373 K for 30 minutes and subsequently at 380 K for 1.5 hours with dry steam (steam rate 30 litres min-') to which argon gas had been added at 0.5 litres per minute.

After the drying stage the gel microspheres were found to be substantially crack-free with no aggregated lumps of microspheres.

After the de-bonding and sintering stages the ceramic microsphere product was found to be substantially free of cracked or broken spheres. The density (Hg immersion) was found to be 96.5% of theoretical.

Example 4 The general procedure of Example 1 was followed with 80 grams of wet gel microspheres.

The conditions were varied such that the microspheres were treated for 1 hour with wet steam at 373 K and subsequently at 380 K for 5 hours with an atmosphere of dry steam (steam rate 30 litres min-') to which argon had been added at 0.5 litres per minute.

The product was of good quality in that it was substantially crack-free and of substantially spherical shape after the drying stage, the de-bonding stage and sintering stage. The final density (Hg immersion) was 97% of the theoretical value.

Example 5 The general procedure of Example I was carried out with a batch of 80 grams of wet gel microspheres.

The conditions were such that the microspheres were first contacted for 2 hours at 373 K with wet steam. Subsequently the microspheres were treated at 380 K for 5 hours with an atmosphere containing dry steam (steam rate 30 litres min-1) to which carbon dioxide had been added at a rate of 0.5 litres per minute.

The dried gel microsphere product was found to be substantially crack-free.

However after de-bonding and sintering the microspheres were found to have a rough (etched) surface.

The density (Hg immersion) was 94.3% of theoretical value.

Example 6 The general procedure of Example 1 was repeated with 200 grams of wet gel microspheres.

In this example the conditions were such that the microspheres were contacted for 2 hours with wet steam at 373 K and subsequently for 10 hours at 380 K with an atmosphere containing dry steam (steam rate 25 litres min-') to which carbon dioxide had been added at a rate of 0.4 litres per minute.

As in Example 5 the microsphere product after this drying stage was found to be substantially crack-free but after debonding and sintering was found to have a rough (etched) surface. The density (Hg immersion) was 93.8% of the theoretical value.

Example 7 The general procedure of Example 1 was followed with a bed of 200 grams- of wet gel microspheres.

In this example wet steam was contacted for 2 hours with the microspheres and subsequently the microspheres were treated at 380 K for 10 hours with dry steam (steam rate 30 litres min-') to which argon had been added at the rate of 0.5 litres per minute.

After the drying, de-bonding and sintering stages the microspheres were found to be substantially free of cracks and had no rough surface as was noticed in the cases of Examples 5-and 6 where carbon dioxide was used as a diluent for the dry steam.

The density (Hg immersion) of the sintered spheres was 98.8% of theoretical value.

Example 8 The general procedure of Example 1 was followed with 200 grams of wet gel microspheres.

The conditions were selected such that the gel microspheres were contacted with wet steam at 373 K for 2 hours followed by treatment for 10 hours at 380 K with dry steam (steam rate 30 litres min-') to which nitrogen was added at a rate of 0.5 litres per minute.

The microspheres were found to besubstantially crack-free after the drying, debonding and sintering stages. The density (Hg immersion) was found to be 99% of the theoretical value.

Example 9 The procedure of Example 1 was followed once again with 200 grams of wet gel microspheres and the conditions were selected such that the microspheres -were first contacted with wet steam for 2 hours and then treated at 380 K for 10 hours with steam (steam rate 30 litres min-') to which air had been added at 0.5 litres per minute.

The microspheres after the drying stage were found tobe of good appearance in that no cracks were observed. However after the de-bonding stage cracks had begun to show in the product and after the final sintering stage many cracked and broken micro spheres were observed.

Example 10 140 ml of aluminium chlorohydrate solution (50.312 g Al,Odml) were mixed with 200 mls of a 5% solution of Wisprofloc P (a cationic water soluble starch-ex Allied Colloids) and added dropwise to 0.880 NH4OH.

The microspheres thereby formed by gel precipitation were washed and dried. using an atmosphere. of steam and argon gas at 105"C (steam flow rate 35 I min-', argon flow rate 0.35 min-').

The microspheres dried satisfactorily to give a dried gel product showing no cracks nor aggregation.

Example 11 The procedure of Example 10 was repeated with the exception that the gel microspheres were dried using an atmosphere of steam and air at 1050C (steam flow rate 35 I min-l, air flow rate 0.35 1 in').

The microspheres dried but every one showed cracks or had broken portions.

Example 12 In this Example gel precipitated microspheres (produced in a manner similar to that disclosed in Example 1 using polyacrylamide as the organic gelling agent) containing uranium and thorium (70:30 U/Th ratio) were dried in !accordance with the present invention under a stepwise temperature programme rather than under isothermal conditions as in the previous examples. The drying atmosphere was steam and argon gas (ratio 100:1), steam flow rate 30 1. min-', and the temperature was raised in steps from 950C to 1250C in 5 deg C steps, the increases in temperature being made every 15 minutes.

After 4 hours the product has a similar appearance to gel microspheres of similar composition dried under isothermal conditiions.

WHAT WE CLAIM IS: 1. A process for the drying of a gel material, comprising mixing together steam and gas in controlled amounts to form a mixture, feeding the mixture to a gel material to be dried thereby to contact the gel material with an atmosphere containing steam and gas in controlled amounts and thereby dry the gel material.

2. A process as claimed in Claim 1, wherein the atmosphere containing steam and gas in controlled amounts is prepared by introducing the gas into wet steam and

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **.

In this example the conditions were such that the microspheres were contacted for 2 hours with wet steam at 373 K and subsequently for 10 hours at 380 K with an atmosphere containing dry steam (steam rate 25 litres min-') to which carbon dioxide had been added at a rate of 0.4 litres per minute.

As in Example 5 the microsphere product after this drying stage was found to be substantially crack-free but after debonding and sintering was found to have a rough (etched) surface. The density (Hg immersion) was 93.8% of the theoretical value.

Example 7 The general procedure of Example 1 was followed with a bed of 200 grams- of wet gel microspheres.

In this example wet steam was contacted for 2 hours with the microspheres and subsequently the microspheres were treated at 380 K for 10 hours with dry steam (steam rate 30 litres min-') to which argon had been added at the rate of 0.5 litres per minute.

After the drying, de-bonding and sintering stages the microspheres were found to be substantially free of cracks and had no rough surface as was noticed in the cases of Examples 5-and 6 where carbon dioxide was used as a diluent for the dry steam.

The density (Hg immersion) of the sintered spheres was 98.8% of theoretical value.

Example 8 The general procedure of Example 1 was followed with 200 grams of wet gel microspheres.

The conditions were selected such that the gel microspheres were contacted with wet steam at 373 K for 2 hours followed by treatment for 10 hours at 380 K with dry steam (steam rate 30 litres min-') to which nitrogen was added at a rate of 0.5 litres per minute.

The microspheres were found to besubstantially crack-free after the drying, debonding and sintering stages. The density (Hg immersion) was found to be 99% of the theoretical value.

Example 9 The procedure of Example 1 was followed once again with 200 grams of wet gel microspheres and the conditions were selected such that the microspheres -were first contacted with wet steam for 2 hours and then treated at 380 K for 10 hours with steam (steam rate 30 litres min-') to which air had been added at 0.5 litres per minute.

The microspheres after the drying stage were found tobe of good appearance in that no cracks were observed. However after the de-bonding stage cracks had begun to show in the product and after the final sintering stage many cracked and broken micro spheres were observed.

Example 10

140 ml of aluminium chlorohydrate solution (50.312 g Al,Odml) were mixed with 200 mls of a 5% solution of Wisprofloc P (a cationic water soluble starch-ex Allied Colloids) and added dropwise to 0.880 NH4OH.

The microspheres thereby formed by gel precipitation were washed and dried. using an atmosphere. of steam and argon gas at 105"C (steam flow rate 35 I min-', argon flow rate 0.35 min-').

The microspheres dried satisfactorily to give a dried gel product showing no cracks nor aggregation.

Example 11 The procedure of Example 10 was repeated with the exception that the gel microspheres were dried using an atmosphere of steam and air at 1050C (steam flow rate 35 I min-l, air flow rate 0.35 1 in').

The microspheres dried but every one showed cracks or had broken portions.

Example 12 In this Example gel precipitated microspheres (produced in a manner similar to that disclosed in Example 1 using polyacrylamide as the organic gelling agent) containing uranium and thorium (70:30 U/Th ratio) were dried in !accordance with the present invention under a stepwise temperature programme rather than under isothermal conditions as in the previous examples. The drying atmosphere was steam and argon gas (ratio 100:1), steam flow rate 30 1. min-', and the temperature was raised in steps from 950C to 1250C in 5 deg C steps, the increases in temperature being made every 15 minutes.

After 4 hours the product has a similar appearance to gel microspheres of similar composition dried under isothermal conditiions.

WHAT WE CLAIM IS: 1. A process for the drying of a gel material, comprising mixing together steam and gas in controlled amounts to form a mixture, feeding the mixture to a gel material to be dried thereby to contact the gel material with an atmosphere containing steam and gas in controlled amounts and thereby dry the gel material.
2. A process as claimed in Claim 1, wherein the atmosphere containing steam and gas in controlled amounts is prepared by introducing the gas into wet steam and

subsequently heating the mixture formed whereby to produce an atmosphere containing dry steam and gas.
3. A process as claimed in Claims I or 2 wherein a mixture of steam and gas at a temperature in the range 373423"K is passed -through a bed of the gel material to be dried.
4. A process as claimed in Claim 3 wherein the mixture of steam and gas is at a temperature in the range 378-3880K.
5. A process as claimed in any preceding claim, wherein the gas in the atmosphere is nitrogen, argon, helium or carbon dioxide.
6. A process as claimed in any one of the preceding claims, wherein the ratio of steam to gas in the atmosphere is in the range 25:1 to 125:1.
7. A process as claimed in any one of the preceding claims wherein prior to the drying by contacting with an atmosphere containing steam and gas in controlled amounts, the gel material is conditioned by contacting with wet steam or hot water.
8. A process as claimed in Claim 7, wherein the wet steam contains a diluent gas.
9. A process as claimed in Claim 8, wherein the diluent gas is nitrogen, argon, helium, or carbon dioxide.
10. A process as claimed in Claim 7, wherein the wet steam is at a pressure of greater than one atmosphere.
11. A process as claimed in Claim 7, wherein the gel material is boiled in water.
12. A process as claimed in Claim 7, wherein the hot water has a temperature in the range 600C to 100"C.
13. A process as claimed in any one of the preceding claims wherein the gel material has been produced by a gel precipitation process, or by a sol-gel technique, or by internal gelation.
14. A process as claimed in any one of the preceding claims, wherein the gel material comprises gel particles.
15. A process as claimed in Claim 14, wherein the gel particles have a diameter in the range 203000 4.
16. A process as claimed in any one of the preceding claims, wherein the gel material contains uranium-containing ions or tungsten-containing ions, thorium ions or plutonium ions, or alumina, zirconia or ceria.
17. A process as claimed in any one of claims 1 to 15 wherein the gel particles contain a nuclear fuel material.
18. A process as claimed in any one of the preceding claims wherein the gel material is heated subsequently to drying to produce a ceramic product.
19. A process as claimed in Claim 18, wherein the ceramic product comprises ceramic particles.
20. A process as claimed in Claim 19, wherein the ceramic particles are nuclear fuel particles.
21. A process for the drying of gel material substantially as hereinbefore described with reference to any one of Examples 3 to 8, and 10 and 12.
22. A material whenever dried by a process as claimed in any one of Claims 1 to 21.
23. A ceramic product whenever prepared by a process as claimed in any one of Claims 18 to 20.
24. A dried material substantially as hereinbefore described with reference to any one of Examples 3 to 8, or 10 or 12.
25. A ceramic product substantially as hereinbefore described with reference to any one of Examples 3 to 8, 10, or 12.