Chemistry of inorganic compounds – Zeolite – Seed used
Patent
1993-01-05
1994-12-06
Bell, Mark L.
Chemistry of inorganic compounds
Zeolite
Seed used
423716, 423DIG21, C01B 3334
Patent
active
053708598
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
This invention relates to the growing of crystals of sodium type X zeolite, an alumino-silicate compound having molecular porosity. Sodium type X zeolite has the chemical formula Na.sub.2 O.Al.sub.2 O.sub.3. 2.5SiO.sub.2. It is usually referred to by the abbreviation NaX. When found as a naturally occurring mineral it is known as Faujasite.
NaX crystals are used in a variety of industrial and chemical processes, including use as a regenerative desiccant, that is to say a desiccant which may be used to absorb moisture from a gas until saturated, and may thereafter be heated to drive off the absorbed water to render the desiccant fit for re-use. The present invention was devised with the use of the crystals as the desiccant medium of a parallel plate dehumidifier in mind, and is described primarily in that context hereinafter, but it will be understood that crystals grown in accordance with the invention are not limited to that use.
BACKGROUND ART
Hitherto, NaX crystals have been produced commercially by a hydrogel process resulting in crystals ranging in particle size from 1 to 5 .mu.m. In applications requiring NaX particles of larger size it has been usual to agglomerate the hydrogel crystals into pellets held together by an inorganic adhesive.
DISCLOSURE OF THE INVENTION
In parallel plate dehumidifiers, the desiccant medium is applied as a layer to each side of the substrate "plates" (frequently a plastics film ribbon). In the case of a crystalline medium it is preferable for each crystal to be exposed to the gas to be dried. This calls for a single thickness layer of crystals, which means that agglomerated pellets are not suitable. It is also preferable for there to be a substantial amount of medium in each layer, which means that very small crystals are not suitable. These factors point to a need for commercial quantities of relatively large NaX crystals. The present invention aims to fulfil that need.
The invention consists in a method of growing crystals of sodium type X zeolite comprising the steps of preparing a super-saturated mother liquor, being an aqueous solution of sodium, aluminium and silicon wherein the ratio of the respective concentrations of aluminium to silicon is less than that corresponding to the molar proportions of those elements in sodium type X zeolite, maintaining the mother liquor at an elevated temperature, adding small seed crystals of sodium type X zeolite to the mother liquor, adding feed materials contributing further sodium, aluminium and silicon to the solution at a rate to maintain a degree, of super-saturation below that at which an amorphous gel of the zeolite would form at that temperature and provide for the elements in the growing crystals, and extracting larger crystals of sodium type X zeolite grown upon said seed crystals.
The method may be effected continuously using a continuous crystalliser with fractionation, or it may be effected as a single or multi-stage batch process.
BEST MODE OF CARRYING OUT THE INVENTION
By way of example, an embodiment of the above described invention is described in more detail hereinafter.
A mother liquor of approximate composition 0.94 Na.sub.2 O, 0.011 Al.sub.2 O.sub.3, and 0.200 SiO.sub.2 mol/kg is prepared and maintained at about 90.degree. to 100.degree. C. in a plastics container resistant to alkaline solutions.
The seed crystals are then added to the container following which the solution is continuously stirred.
Feed materials are added at a pre-determined rate to ensure the solution is super-saturated but to a degree less than 20% in respect of the alumina, so that a gel will not form. Those feed materials may comprise powdered silica, powdered alumina and sodium hydroxide.
One suitable silica powder which is presently commercially available is sold as 400 grit silica. It is high purity, while, crystalline silica, having a typical analysis of 99.0% SiO.sub.2, 0.03% Fe.sub.2 O.sub.3 and an ignition loss of 0.2%. The particle size may range from 3 to 30 .mu.m.
One suitable alumina powder which is p
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Bell Mark L.
Sample David
Unisearch Limited
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