Plastic and nonmetallic article shaping or treating: processes – Treatment of material by vibrating – jarring – or agitating... – By reciprocating or vibrating mold
Reexamination Certificate
2002-06-06
2004-08-17
Theisen, Mary Lynn (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Treatment of material by vibrating, jarring, or agitating...
By reciprocating or vibrating mold
C264S544000, C264S570000, C264S109000, C425S405200
Reexamination Certificate
active
06776941
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a cold isopressing method and mold for compacting a granular ceramic material for use in manufacturing ceramic tubes. More particularly, the present invention relates to such a method and mold in which the ceramic tubes are useful in forming ceramic membrane elements of the type exhibiting infinite hydrogen or oxygen selectivity. In other aspects, the present invention relates to a cold isopressing method and mold in which ceramic tubes are formed having enlarged end portions that are more amenable to sealing than prior art ceramic tubes.
BACKGROUND OF THE INVENTION
Ceramic tubes have many industrial uses, for instance, insulators, filters, etc. An important recent use involves the employment of such tubes in membrane modules to separate oxygen or hydrogen from a gas stream. The ceramic tubes used in membrane separation applications are manufactured from materials that exhibit infinite oxygen or hydrogen selectivity at high temperatures. In an oxygen-selective membrane, oxygen is ionized at one surface of the membrane to form oxygen ions. The oxygen ions travel through the membrane to the opposite surface thereof where the oxygen ions recombine to form elemental oxygen. In forming the elemental oxygen, electrons are given up from the ions. Depending upon the type of material used in fabricating the membrane, the electrons either flow through the membrane material itself or through separate conductive pathways to initially ionize the oxygen.
The traditional method fabricating ceramic tubes involves processes such as slip casting or extrusion. The use of extrusion to form solid-state membrane modules is disclosed in U.S. Pat. No. 5,599,383.
Another known method of fabricating ceramic tubes is isopressing. In cold isopressing, a tubular mold is utilized that is formed from a soft neoprene rubber. This mold, known as a bag, is used in conjunction with a mandrel that projects into the bag. The bag is filled with a ceramic material in granular form. The mold is subjected to a hydrostatic pressure within a vessel containing cold or warm water that is sufficient to compact the ceramic material into a green ceramic tube. After compaction, the hydrostatic pressure is relaxed and the green ceramic form is removed from the mold. In this regard, the mandrel is provided with a slight taper to permit separation of the green ceramic tube from the mandrel. The green ceramic tube can then be heated to burn out organic binder materials and the like and to sinter the ceramic.
The prior art method of isopressing has found application in the manufacture of short thick tubes. When longer tubes are attempted by this method, defects are found in the fired and sintered tubes. The reason for such defects is that it is impossible to introduce ceramic powder into the mold so that the powder is uniformly distributed. For instance, if there exists a slight wrinkle in the mold, the powder will tend to hang up on the wrinkle to produce a defect in the finished ceramic tube.
As will be discussed, the present invention in one aspect provides a cold isopressing method and mold that is particularly applicable to forming long thin, ceramic tubes that can be used in ceramic membrane applications. As will also be discussed, other aspects of the present invention are particularly useful in the fabrication of ceramic tubes having end configurations that are more amenable to sealing than prior art sealing methods.
SUMMARY OF THE INVENTION
The present invention provides a cold isopressing method for compacting a granular ceramic material. As used herein and in the claims, the term “granular ceramic material” means ceramic powder or a mixture comprising a ceramic powder, an organic binder and a plasticizing agent. In accordance with such method, the granular ceramic material is introduced into an isopressing mold having a cylindrical pressure bearing element and at least one mandrel located within the cylindrical pressure bearing element. After the cylindrical pressure bearing element is sealed, the cylindrical pressure bearing element is subjected to a hydrostatic pressure to compact the granular ceramic material. The hydrostatic pressure is relaxed after such compaction. The cylindrical pressure bearing element is sufficiently rigid so as to maintain its shape during the introducing of the granular ceramic material and is also sufficiently resilient in a radial direction thereof to deform and bear against the granular ceramic material upon the application of the hydrostatic pressure and to at least substantially return to its original shape upon the relaxation of the hydrostatic pressure.
The resiliency of the cylindrical pressure bearing element allows retraction of the cylindrical pressure bearing element from the granular ceramic material after compaction. In this regard, the use of the term, “at least substantially” with respect to the return of the cylindrical pressure bearing element to its original shape admits to the possibility that after relaxation of hydrostatic pressure, the cylindrical pressure bearing element might be slightly out of round. However, such cylindrical pressure bearing element should not be so out of round that the mold is unable to completely retract from the granular ceramic material.
As may be appreciated, the provision of a cylindrical pressure bearing element that will maintain its shape during filling allows for long thin tubes to be fabricated with the ceramic material having a uniform density along the length of the tube. Gross maldistributions of ceramic material that are caused by the wrinkling of the cylindrical pressure bearing element during filling are eliminated. Moreover, the resiliency of the cylindrical pressure bearing element ensures a clean separation of the mold from the compacted ceramic material to also prevent tube defects.
The at least one mandrel can be a single mandrel to produce a tube-like structure. Alternatively, multiple, parallel mandrels can be used to form a cylindrical structure having internal passageways. Both types of structures, the tube or the cylindrical structure having internal passageways would be useful in forming ceramic membrane elements.
The at least one mandrel can be connected to an enlarged base element that projects into one end of the cylindrical pressure bearing element, thereby to seal the cylindrical pressure bearing element at the one end. The granular ceramic material is introduced into the isopressing mold through the other end of the cylindrical pressure bearing element during filling. A removable end plug is positioned within the other end of the cylindrical pressure bearing element to seal such other end thereof. In this regard, the term “removable” as used herein and in the claims means that such end plug can be removed and is not permanently bonded attached to the cylindrical pressure bearing element.
One end of the cylindrical pressure bearing element can be provided with an enlarged, axial end bore positioned so that the compacted granular ceramic material has an enlarged end section. As will be discussed, such an enlarged end section can be used in a sealing arrangement at the connection of a finished ceramic tube to a tubesheet.
Preferably, the isopressing mold is vibrated while the granular ceramic material is introduced into the isopressing mold. The vibrations act on the cylindrical pressure bearing element member to help fill the mold and ensure that there is no hang up of the granular ceramic material within the mold. Such transmission of vibrations is possible due to the rigidity of a cylindrical pressure bearing element in accordance with the present invention as opposed to a soft rubber bag of the prior art. As may be appreciated, the use of such a vibration technique during filling is particularly important in the fabrication of long, thin tubes.
In case of a single mandrel, the cylindrical pressure bearing element can advantageously be sized such that the granular ceramic material prior to compaction occupies an annular space having a wall thickness no less
Apte Prasad S.
Callahan Shawn W.
Chen Jack C.
Schneider James S.
Praxair Technology Inc.
Rosenblum David M.
Theisen Mary Lynn
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