GROWING SODIUM TYPE X ZEOLITE CRYSTALS

Abstract: Sodium type X zeolite crystals of a particle size of 50 mu m or more are grown from seed crystals at a rate of about 5 mu m per day in a mother liquor having an approximate composition 0.94 Na2O, 0.011 Al2O3, and 0.200 SiO2 mol/kg by adding a fine silica powder, a somewhat coarser alumina powder, and sodium hydroxide from time to time. The addition rates are such as to maintain the mother liquor at a super-saturation level of less than 20% with respect to the alumina to avoid the formation of a gel. Throughout the growing period the mother liquor is stirred and maintained at a temperature of about 95 DEG C.

Description: GROWING SODIUM TYPE X ZEOLITE CRYSTALS 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 2 O.AI 2 0 3 .2.5Si0 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 μ 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 in respective concentrations substantially 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 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 2 0, 0.011 Al 2 0 3 , and 0.200 Si0 2 mol/kg is prepared and maintained at about 90 to 100°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 a high purity, white, crystalline silica, having a typical analysis of 99.0% Si0 2 , 0.03% Fe 2 0 3 and an ignition loss of 0.2%. The particle size may range from 3 to 30 μ m. One suitable alumina powder which is presently commercially available is sold as smelter grade calcined alumina. It is a high melting point aluminium oxide in the alpha phase form, having a typical analysis as follows Al 2 0 3 98.7 - 99.3% Si0 2 0.02 - 0.04% The sodium hydroxide may be any commercially available substantially pure product. It should be noted that the alumina is coarser than the silica. This substantially compensates for their differing solubilities in the alkaline mother liquor, to the extent that they may be added thereto simultaneously without substantially altering their relative concentrations in the solution. It should also be noted that the addition of the silica and alumina in the form of solids having low solubility enables them to be added as discrete doses at timed intervals without producing sudden changes in the composition of the solution. This greatly facilitates the control of the degree of super-saturation and the rate of growth of the zeolite crystals which is preferably not more than 5 μ m per day. However in other embodiments, possibly presenting more control problems insofar as the feed rates are concerned, the make up materials may be in other forms and may comprise any other appropriate chemicals, for example the sodium and the silicon may be fed in as water glass, that is a viscous liquid sodium silicate. Typically the preferred feed materials mentioned above (powdered silica, powdered alumina and sodium hydroxide) may be added two to four times per day over a period of say five to ten days to produce NaX crystals of about 50 to 100 μ m. particle size. The optimum size for use as the desiccant of a parallel plate dehumidifier is substantially 50 μ m. The quantity of feed material added in each instance is preferably increased as time progresses to compensate for the greater rate of take up as the growing crystals increase in surface area. At the end of the batch operation's growing period the NaX crystals may be harvested by filtering them from the solution in conventional manner. They may then be washed and dried to make them suitable for application as a desiccant to the supporting substrate of a parallel plate gas dehumidifier. Alternatively, and preferably, the first growing period may be reduced and the crystals harvested when they have attained a particle size of about 20 μ m. Those newly harvested crystals may then be divided into parcels and each parcel used as the seed crystals in respect of subsequent batch operations. This two stage process permits more efficient use of reaction vessels smaller than a vessel able to accommodate the full growth of all of the original seed crystals.

Claims: CLAIMS 1. 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 in respective concentrations substantially 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 extracting larger crystals of sodium type X zeolite grown upon said seed crystals. 2. A method according to claim 1 wherein the feed material contributing the aluminium is powdered calcined alumina. 3. A method according to claim 1 wherein the feed material contributing the silicon is powdered silica. 4. A method according to claim 1 wherein the feed material contributing the aluminium is powdered calcined alumina, wherein the feed material contributing the silicon is powdered silica and wherein the alumina powder is coarser than the silica powder, so that the rates of dissolution of the powders are more nearly equal than would otherwise be the case. 5. A method according to claim 4 wherein the elevated temperature is within the range of from 90°C to 100°C. 6. A method according to any one of the preceding claims wherein the feed materials are added at a rate producing a crystal growth rate of substantially 5 μ m per day. 7. A method according to claim 6, being a batch process, wherein the growing period is substantially five days to produce crystals having a particle size of substantially 50 μ m.

Inventors: MACLAINE-CROSS IAN LACHLAN, MACLAINE-CROSS, IAN, LACHLAN
Applicants: UNISEARCH LTD, UNISEARCH LIMITED
Application year: 1991
Main classification: C01B33
IPC codes: B01D53, C01B39