Ceramic hollow fiber membranes with improved mechanical properties

Abstract

A method for the manufacture of ceramic hollow fiber membranes in a spinning process by using a spinning mass, comprising the steps: providing a spinning mass formulation; providing a secondary phase; adding the secondary phase to the spinning mass formulation; manufacturing the ceramic hollow fiber membranes in a spinning process.

Claims

1. A method for the manufacture of ceramic hollow fiber membranes made of a spinning mass, comprising the steps: providing a solvent, a polymer and a ceramic powder; wherein the ceramic powder comprises: an oxide ceramic material selected from: aluminum oxide, ZrO.sub.2, TiO.sub.2 and mixtures thereof; or a non-oxide ceramic material selected from: Si.sub.3N.sub.4, AlN, WC, and mixtures thereof; providing a precipitating agent; manufacturing of a spinning mass made of the solvent, the polymer and the ceramic powder; adding a secondary phase to the spinning mass; injecting the spinning mass into a precipitation bath, solidifying the polymer upon contact with the precipitating agent; sintering the solidified polymer to form a green body; wherein the secondary phase is nanoscaled illite, the nanoscaled illite in the range of 1 to 200 nm.

2. The method according to claim 1, wherein in the step of adding a secondary phase, the secondary phase is added in an amount of 0.5 to 10% based on the total solid matter.

3. A ceramic fiber membrane produced by the method according to claim 1.

4. The method for the manufacture of ceramic hollow fiber membranes according to claim 1, wherein the nanoscaled illite comprises chemical major constituents: [SiO.sub.2: 47.0%3.5%; Al.sub.2O.sub.3: 21.0%2.0%; Fe.sub.2O.sub.3: 7.0%1.0%; CaO: 4.0%2.0%; K.sub.2O: 5.5%0.7%; MgO: 3.0%0.75%; Na.sub.2O: 0.15%0.05%; TiO.sub.2: 0.8%0.1%; P.sub.2O.sub.5: 0.3%0.08%; and TI (total impurity): 11.0%2.0%].

5. A ceramic hollow fiber membrane comprising: a ceramic powder, wherein the ceramic powder comprises: an oxide ceramic material selected from: aluminum oxide, ZrO.sub.2, TiO.sub.2 and mixtures thereof; or a non-oxide ceramic material selected from: Si.sub.3N.sub.4, AlN, WC, and mixtures thereof; and wherein the ceramic hollow fiber membrane is made with a secondary phase of nanoscaled illite, the nanoscaled illite in the range of 1 to 200 nm.

6. The ceramic hollow fiber membrane in claim 5 is an ultrafiltration, microfiltration or a gas separation membrane.

7. A water purification or waste water treatment membrane according to claim 5.

8. The ceramic hollow fiber membrane according to claim 5, wherein the nanoscaled illite comprises chemical major constituents: [SiO.sub.2: 47.0%3.5%; Al.sub.2O.sub.3: 21.0%2.0%; Fe.sub.2O.sub.3: 7.0%1.0%; CaO: 4.0%2.0%; K.sub.2O: 5.5%0.7%; MgO: 3.0%0.75%; Na.sub.2O: 0.15%0.05%; TiO.sub.2: 0.8%0.1%; P.sub.2O.sub.5: 0.3%0.08%; and TI (total impurity): 11.0%2.0%].

Description

DESCRIPTION OF THE INVENTION

(1) Hollow fiber membranes are manufactured in a spinning process by using a spinning mass, the spinning mass as a base formulation being made up of solvents, polymer and ceramic powder (aluminum oxide) and a dispersant for stabilizing the suspension. Usually, an increased strength of such hollow fiber membranes results in a lower porosity and, for this reason, also in a lower filtering performance.

(2) It has been discovered that the addition of a secondary phase to the base formulation of a ceramic hollow fiber for the manufacture of ceramic hollow fiber membranes on the basis of an oxide ceramic material, for example, aluminum oxide, ZrO.sub.2 or TiO.sub.2, or on the basis of a non-oxide ceramic material, for example, Si.sub.3N.sub.4, AlN or WC are counteracting this development. Preferably, the secondary phase is made up of a glass phase which is added in the amount of approximately 0.5-10% based on the total solid matter.

(3) Particularly suitable as a glass phase is nanoscaled (thus, in the range of 1 through 200 nm) illite, a naturally occurring three-layer sheet silicate, having the chemical major constituents 47.0%3.5% SiO.sub.2, 21.0%2.0% Al.sub.2O.sub.3, 7.0%1.0% Fe.sub.2O.sub.3, 4.0%2.0% CaO, 5.5%0.7% K.sub.2O, 3.0%0.75% MgO, 0.15%0.05% Na.sub.2O, 0.8%0.1% TiO.sub.2, 0.3%0.08% P.sub.2O.sub.5, 11.0%2.0% TI (total impurity). Alternatively to illite, synthetic glass powders or natural clay minerals may also be used.

(4) The conventional ceramic hollow fibers on the basis of aluminum oxide are mostly manufactured by a wet spinning method, the employed three-component system made of the solvent, the employed polymer and the precipitating agent having a miscibility gap, the reason of which the polymer dissolved in the spinning mass solidifies upon contact with the precipitating agent. By this process, the ceramic particles are embedded in the polymer matrix. After a drying step, the green compact manufactured that way still is made up of polymer and aluminum oxide. Subsequently, a sintering process occurs to bond the ceramic particles by the formation of sintering necks and to burn off the polymer, as a result of which a porous ceramic membrane is manufactured.

(5) The introduction of the secondary phase (glass phase) into the spinning mass according to the present invention, green compacts, which are made up of polymer, aluminum oxide and the glass phase, are successfully manufactured. The hollow fiber membranes subsequently manufactured by the sintering process have strength values which at approximately 13 N are twice as high as in conventionally manufactured hollow fiber membranes, the geometric fiber dimensions (outer diameter 0.8-5.0 mm, inner diameter 0.5-4.8 mm), the pore structure, the total porosity (40-60%) and the pure water permeability (10,000-30,000 l/m.sup.2/h/bar) of the glass-free systems being able to be maintained. Thus, for this reason, one succeeds in achieving substantially higher strength values (>30%) than have been possible, for example, by adding synthetically manufactured glass frits made up of ground glass powder.

(6) By substantially increasing the mechanical stability achievable by the present invention, the reject rates and, for this reason, the costs, which may arise from handling, further processing and transporting, can be significantly reduced.

(7) Such hollow fiber membranes according to the present invention are particularly suitable for microfiltration, nanofiltration, ultrafiltration and gas separation in the area of water purification (waste water treatment, drinking water purification, etc.). Further areas of application are filtrations in the food and beverage industry and in the chemical industry or the oil and gas industry.

(8) A reduction of the sintering temperature to 200 degrees Celsius during the manufacture is possible, the mechanical stability then being at a comparable level to the pure ceramic systems. In this manner, energy is conserved when manufacturing hollow fiber membranes in which the mechanical properties obtained without the addition of the glass phase are sufficient.

(9) If the glass phase in the structure of the hollow fiber membrane is, during the later operation in aggressive, for example, hot lyes at pH 14, partially or even completely removed, the basic mechanical strength is maintained at the level of glass-free systems, that is, the mechanical stability does not fall below values which are obtained without the addition of the glass phase.