Spherical particles of alumina, silica, silica-alumina, zeolite, and the like are extensively used, for example, as catalyst supports, drying agents, and adsorbents in the chemical industries, including petroleum chemistry and petroleum refining. When used as a catalyst, spherical particles having diameters of about from 0.5 to 5 mm are packed in a reactor called a fixed bed or moving bed. Because of their spherical shape, the particles readily flow, making charging and discharging of the catalyst easy. Further, since such a spherical catalyst is charged uniformly, no particular technique is required for the charging, and channelling or abnormal flow is less apt to occur during reaction, which contributes to safe operation. For use in the moving bed reactors which are of the type in which a catalyst bed gradually moves downward due to its own weight, an essential requirement of the catalyst particles is a spherical shape.
Such spherical catalysts are produced by a "dropping-into-oil" method as described, for example, in JP-B-26-4113, JP-B-54-163798, JP-B-38-17002 and JP-B-11-6771, or by a "rolling-and-moving" method as described, for example, in JP-B-52-14720 and JP-B-55-29930. (The term "JP-B" as used herein means an "examined Japanese patent publication".)
The dropping-into-oil method can produce spherical particles which are relatively uniform and nearly truly spherical. The rolling-and-moving method yields spherical particles which have a broad diameter distribution and are not so truly spherical, but this method is being extensively employed because of the low production costs involved. Spherical particles for use in moving beds are required not only to descend by their own weight, but to have such properties or performances as uniform size, a nearly true spherical shape, surface smoothness, high strength, high abrasion resistance and the like, because the catalyst is transferred in an air stream for regeneration. For this reason, the spherical particles for use as moving bed catalysts have been produced by the dropping-into-oil method.
In producing a spherical alumina catalyst by the conventional dropping-into-oil method, a reagent which decomposes at high temperatures to evolve ammonia, such as hexamethylenetetramine or urea, is used, and an alumina sol is caused to gel by alkalinity. It is, therefore, necessary, according to this method, that an alumina sol be dropped into a high-temperature oil having a temperature close to 100.degree. C. and that the resulting sol droplets be aged for as long as more than 20 hours. After being thus aged in the oil and then washed, the resultant particles are required to further undergo a post-treatment in which they are immersed in an aqueous solution of ammonium chloride or in ammonia water for several hours in order to increase their strength. If a spherical zeolite catalyst containing alumina as a binder is to be produced by this method, a long gelation time is required and this causes a problem in that the particles being produced are prone to coalescence, deformation, or breakage, leading to a low product yield. In addition, since the only technique for controlling the droplet diameters is to change the diameter of the dropping opening, it has been difficult to freely control droplet diameters and to cope with any viscosity change of the raw material due to a change in zeolite proportion. Furthermore, although spherical zeolite catalysts are required to contain a zeolite, the active ingredient, in a proportion as high as possible in order to have a high catalytic activity per unit weight, there is the problem that the larger the zeolite content, the lower the catalyst particle strength.