Spraying aqueous siliceous solutions

During the process, the aqueous silicate solution is introduced into the upper portion of the gas-fired spray dryer and passes through a spray nozzle or a disk atomizer (see Figure 22.5). The speed of the spray wheel may be about 11,000 rpm. The finely and evenly dispersed liquid comes into contact with upwardly directed hot air. Typical spray tower tanperatures are about 180°C [21] with inlet temperatures of about 260-300°C and outlet air temperatures of above lOO C. The resultant spray-dried droplets adopt the form of hollow microspheres. The silicate particles are collected at the spray dryer s bottom and are withdrawn by a screw conveyor. The amorphous sodium silicate may have a bulk density on the order of 250-500 g/L, an SiOjiNajO molar ratio of 2.04 1, and an ignition loss on the order of 19-20%. Its mean particle size can be on the order of 100-200 pm. The material may be subjected to further milling to modify the form and density of the powder [51,63]. 

J.7.2.5 Recipe from [2315,2316] Aqueous silicic acid was obtained from sodium silicate by ion exchange. AICI3 was dissolved in aqueous silicic acid, and the solution was sprayed into dilute ammonia. The product contains 72.5% of alumina by mass. 

The aqueous silicic acid solution that is useful in this invention contains silica of the proper particle size, that is, no greater than 5 nm, and provides a solution of sufficient stability to allow the formation of the slurry and subsequent spray drying. The silicic acid can be in the form of a monomer or in the form of low molecular weight polymeric units. For a review of the characteristics of silicic acid, see R. K. Her, The Chemistry of Silica, John Wiley and Sons, NY., 1979. Monomeric silicic acid Si(OH)4 has never been isolated. It is a very weak acid... 

The slurry which is spray dried is prepared by gradually adding catalyst, catalyst precursor or catalyst support particles to an aqueous silicic acid solution. The slurry is stirred until a uniform dispersion is obtained. The relative amounts of siUcic acid solution and particles are chosen so that the weight of the Si02 represents 3-15% of the total weight of the particles and the Si02. 

Engelhard s in-situ FCC catalyst technology is mainly based on growing zeolite within the kaolin-based particles as shown in Figure 3-9A. The aqueous solution of various kaolins is spray dried to form micR)spheres. The microspheres are hardened in a high-temperature l,3f)(TF/704°C) calcination process. The NaY zeolite is produced by digestion of the microspheres, which contain metakaolin, and mullite with caustic or sodium silicate. Simultaneously, an active matrix is formed with the microspheres. The crystallized microspheres are filtered and washed prior to ion exchange and any final treatment. 

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