The invention relates to a method for the production of ceramic hollow fibres, in particular hollow fibre membranes for microfiltration, ultrafiltration and gas separation.

Separation processes are of great economic importance for the concentration and separation of gases and liquids. Membrane technology is being used to an ever increasing extent in this field. The use of hollow fibre membranes in a separation or concentration process has a number of concomitant advantages: because the surface area/volume ratio is relatively high, the efficiency of the concentration or separation process will be high; the process is also easy to carry out and to adapt to the needs.

The abovementioned surface area/volume ratio is of vital importance to be able to compete with other types of membranes.

Hollow fibre membranes have already been available for years and are widely used. Commercially available hollow fibre membranes consist of a polymer material and consequently are sensitive to corrosive conditions and higher temperatures. In order to solve this problem attempts have been made to produce ceramic hollow fibre membranes, for example by condensing vapour on a carbon wire and then removing the wire. A sol-gel method for the production of ceramic thin membranes on a porous ceramic hollow fibre substrate has also been described in the "Journal of Membrane Science", 59 (1991). PP 81-99• However, these attempts have not led to a situation where ceramic fibres are commercially obtainable.

The aim of the invention is to provide a method indicated in the preamble, by means of which ceramic hollow membrane fibres for microfiltration, ultrafiltration and gas separation can be produced on an industrial scale.

According to the invention, this method is characterised in that a paste is made by filling a polymer binder system with a ceramic powder, said paste is processed by extruding through a spinneret to give hollow

fibres, the binder system is removed with the aid of thermal diffusion and the powder particles are sintered to each other.

Use of the method according to the invention can lead to fibre membranes having an external diameter of 0.5-10 mm, a wall thickness of between 30 and 00 μm and a surface area/volume ratio of at least 1000 m ² /m ³ .

For the ceramic powder, use is preferably made of silicon nitride (Si ₂ N/,) , but aluminium oxide, silicon carbide and other substances can also be used.

The binder system will be filled with ceramic powder to 40 to 60 % V/V.

It has been found that a binder system of the following composition leads to good results: 37 % m/m polyethylene vinyl acetate, 23 % m/m ethylenebisstearylamide, 15 % m/m dioctyl phthalate, 11 % m/m oleic acid, 12 % m/m polyglycol ester and 2 % m/m polyglycol ether.

The invention will now be explained in more detail with the aid of the diagrammatic figure. In the figure, an extruder 1 can be seen, which introduces the paste to be spun into the spinneret 3 by means of a spinning pump 2. A gas tank 4 feeds gas via a reducing valve 5 into a line which terminates centrally in the spinneret and, by means of a gas jet, ensures that the hollow fibre produced is held open and cooled. The hollow fibre is subsequently subjected to heat after-treatment in a burn- off furnace 6 and a sintering installation 7•

The paste which is spun to give a hollow fibre consists of a polymer binder system and a ceramic powder. The polymer system becomes plastic at a temperature of between 50 and 220 ^β C and serves as an aid during shaping (and sintering) . The percentage of ceramic powder is between 30 and 70 % V/V, preferably between 45 and 55 % V/V. The paste is mixed in a mixer and granulated after cooling. The granules are then introduced into the extruder 1, where they melt again and, via the spinning pump 2 and spinneret 3 are formed into a hollow fibre.

The polymer binder system is removed in the burn-off furnace 6, after which the residual shape is sintered in a sintering installation J .

The binder in the paste can have the following composition.

Component Make Type % m/m

Polyethylene vinyl acetate Esso Escorene Ultra UL 37

Ethylenebisstearylamide Hoechst 02020 23

Dioctyl phthalate Hoechst Wax C Micropowder PM 15

Oleic acid Merck 11

Polyglycol ester Hoechst 12

Polyglycol ether Hoechst Genagen C-100 2 Arkopal N-100

The ceramic powder in the paste preferably consists of silicon nitride (Si ₃ N _ή ), although aluminium oxide, silicon carbide, sialon and other ceramic powders can also be used. The gas which is blown into the interior of the spun fib^e is, for example, nitrogen in the case of silicon nitride powder and, for example, oxygen or air in the case of aluminium oxide powder. The type of gas is not a crucial factor for processing.

In the case of material based on silicon nitride, a certain amount of sinter aid, for example itrium oxide or aluminium oxide, can be added to the paste to control the porosity.

The temperature in the burn-off furnace is about 500 °C and that in the sintering furnace is about 1300 °C in the case of aluminium oxide powder and about 1650 °C in the case of silicon nitride powder. Sintering takes about 2 hours.

The hollow fibre membranes obtained are able to withstand a corrosive environment and a relatively high temperature. The external diameter is at most 10 mm and preferably less than 2 mm. The minimum dimension for the external diameter is in the region of 500 μm. The wall thickness is between 30 and 500 μm. The surface area/volume ratio is greater than 1000 m ² /m ³ , which is very high in comparison with existing ceramic tubular membranes. The porosity is 30 to 50 % .

Silicon nitride in particular has a high strength and density and, moreover, a very high temperature resistance and corrosion resistance.

Tne pore size can be adjusted between 0.1 and Oo μm ana tne aensi aim the pore size can be controlled with the aid of the sinter aid and with the aid of the sintering temperature. Obviously, various modifications and additions are possible within the framework of the invention. The ceramic powder chosen can also be hydroxyapatite, a ceramic powder that is biocompatible and that is used in artificial ossicles or as bone- replacement material. The particle size distribution of the powder can be important. A broad distribution leads to a higher degree of filling. The absolute particle size is important for the binder system removal process. The smaller the particles, the smaller will be the pores of the product formed and the more difficult it will be to remove the binder system.

By adding relatively large amounts of sinter additives and/or by using a higher sintering temperature and longer sintering time, it should be possible to dense-sinter the hollow fibres obtained by means of the invention. A possible use of such non-porous hollow fibres is in heat exchangers. An exchanger of this type is, for example, composed of a bundle of hollow fibres, a cold gas to be heated (for example a gas to be burned) Deing passed through the fibres and a hot off-gas being fed along the outside of the fibres, preferably in counter-current with the gas to be heated. Flanges of the same powder as that used to produce the fibres can be made on a bundle of hollow fibres by slurry casting of a ceramic powder suspension.