Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Separator – retainer – spacer or materials for use therewith
Reexamination Certificate
2000-06-22
2003-03-04
Chaney, Carol (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Separator, retainer, spacer or materials for use therewith
C429S251000
Reexamination Certificate
active
06528214
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to industrial ceramics and more particularly but not exclusively to semi-permeable acid-resistant and alkali-resistant ceramic membranes for use in electrochemical processing of liquids or filtration of liquids and gases.
It is known, for example from the specification of WO 88/02742, to manufacture a ceramic membrane by impregnating a porous ceramic article with a suspension of submicron particles and by baking the impregnated article. A membrane manufactured by this method takes the form of a two-layered wall consisting of a macroporous layer and a thin (less than 0.1 mm) microporous layer, through which the actual filtration takes place. The thin microporous layer is formed by coalescence of the submicron particles in the suspension on the macroporous ceramic article during baking. The rest of the macroporous ceramic article acts mainly as a supporting structure.
One disadvantage of this method is that, after baking, mechanical stresses arise at the boundary between the micro- and macroporous layers owing to additional shrinkage of the microporous layer. Consequently, cracks appear in the microporous layer which sharply reduce the effectiveness of filtration. Furthermore, a significant number of the large pores in the macroporous layer are not filled during impregnation because small bubbles of air are tapped inside them.
However, the main disadvantage is the low strength of the membrane made by this method, which makes the membrane unsuitable for use in electrochemical processes using electrochemical cell such as those described in GB 2253860. This is especially the case when the electrochemical processes are intensified, for example when electrodes are brought closer together and the pressure difference across the membrane is increased.
FR 2587026 describes an alternative method for manufacturing a microporous ceramic membrane that involves moulding an article from a two-component mixture, containing 10-40% by mass of fine particles and 90-60% by mass of coarse particles, followed by baking the article. Membranes produced by this method are moulded by extrusion or by casting from thermoplastic compounds. This gives the ceramics membrane a high degree of homogeneity because the fine particles are evenly distributed among the large particles, while the pores take the form of a ramified network of submicron channels between the particles.
However, this method of manufacturing microporous ceramic membranes still does not make it possible to form a microporous ceramic membrane which simultaneously has high mechanical strength and low hydraulic resistance. The even distribution of the fine particles around the coarse particles leads to the particles being closely packed and to low porosity of the membrane even at the moulding stage (25%). Furthermore, during subsequent baking, when shrinkage of the article by 3-5% is vital in order to achieve a ceramic membrane of sufficient strength, there is a fall in porosity of up to 10-20%. The resulting low porosity of the membrane does not allow it to be used, for example in electrochemical cells in which electrochemical processes are intensified. Also, decreasing the thickness of the membrane in order to reduce the effect of the low porosity is impractical because it causes a reduction in structural strength with consequential breaking of the membrane by the hydraulic pressure of the liquid being processed.
EP 0 619 379 discloses a method of manufacturing a functionally gradient material by cast forming a slurry of non-metallic particles and metallic particles having about five times the specific gravity of the non-metallic particles in a porous mould. The porous mould is rotated using rollers and the cast product is sintered to form non-porous articles such as sealing caps for bulbs of metal vapour discharge lamps which are, by their very nature, impermeable. This method is not suitable for manufacturing semi-permeable ceramic membranes for use in electrochemical processing of liquids or filtration of liquids and gases.
EP 0 426 546 relates to a ceramic filter comprising a porous ceramic support having a thin film layer comprising particles of small diameter. It is concerned with the problem of excessively small film thicknesses causing the support to be partially uncovered or resulting in an excessively great difference in particle diameter between particles of the thin film and the particles of the support that tends cause peeling or cracking. The problem is solved by having a fine intermediate layer of particles with a particle size of not more than 500 Å and a thin film formed on the surface of the fine intermediate layer of particles with a particle size of not more than 300 Å. This provides a ceramic filter of good permeability and improved durability. Sols are used for the formation of the fine intermediate layer and film and are applied to the support after the addition of a thickening agent to each sol in order easily to control the thicknesses of the films to be and prevent cracks or peeling when the sol is formed into a gel or a coating is dried.
Taking the drawbacks of the prior art into consideration, Applicant has sought to solve the problem of reducing hydraulic resistance of a ceramic membrane while, at the same time, increasing the load capacity (mechanical strength) of the membrane.
Accordingly, the main object of the present invention is to provide a microporous ceramic membrane of which the hydraulic resistance is reduced without compromising its load capacity (mechanical strength).
SUMMARY OF THE INVENTION
To this end, the present invention resides in a method of manufacturing a semi-permeable ceramic membrane, comprising providing a mixture of at least two non-metallic mineral components comprising fine particles having an effective particle size of up to 1 &mgr;m and coarse particles having an effective particle size of 1 &mgr;m or above, the fine particles having a higher coefficient of thermal expansion than the coarse particles, suspending the mixture of fine and coarse particles in a liquid to form a slurry, applying the slurry to a porous mould so as to form a green intermediate product and baking or firing the green intermediate product to form a finished membrane having a density distribution of the fine particles that increases in one direction (in use) across the finished membrane and a density distribution of the coarse particles that decreases in the same direction across the finished membrane.
Expressed in another way, the density and porosity of the ceramic of the membrane are uniformly decreased across the membrane, during casting.
By means of the present invention, the hydraulic resistance of the wall of the ceramic membrane is reduced and, simultaneously, the load capacity (mechanical strength) of the membrane is increased. Moreover, the invention is particularly suitable for the production of membranes of tubular, for example cylindrical, shape having an internal and an external surface made using a tubular porous mould or form for use in electrochemical cells.
Applying a slurry to a porous mould made, for example of plaster of Paris (gypsum), is known as slip casting which is in widespread use as a ceramic process because of its industrial applicability and ability to form an infinite variety of shapes. The slurry or “slip” conventionally includes special additives to impart certain desirable characteristics. For example, sodium silicate may be added to keep the slip liquified with minimal water content so that, when the slip is stirred, it is thinned down sufficiently, i.e. has sufficient fluidity and low viscosity, to enable it to be poured and flow easily into the porous mould. The slip is delivered continuously into the porous mould to replenish the absorbed water and the membrane builds up as a soft, semi-rigid “green” intermediate solid product on the inner wall of the mould by absorption of some of the liquid from the slip into the mould.
The green intermediate product is then baked or fired. Firing is carried out by h
Djeiranishvili Nougzar
Peichev Viktor Georgievich
Pliner Sergei Yuryevich
Akin Gump Strauss Hauer & Feld L.L.P.
Chaney Carol
Sterilox Medical (Europe) Limited
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