Patent GB-626455-A (GB)
Title: Improvements in or relating to the manufacture of silica-alumina gels
IPC: B01J21/12
Filed: 1946-01-15
Abstract: A silica-alumina catalyst is prepared by reacting a dilute aqueous alkali metal silicate solution with an acid to form an aqueous slurry of gelatinous silica-containing alkali metal salt, ageing the slurry, precipitating hydrated alumina in the slurry and de-watering, washing, drying and calcining the silica-alumina mixture.  As shown in Fig. 1, a silica slurry is made in tank 1 by adding sulphuric acid to sodium silicate solution while continuously forcing the resulting slurry through a screen 14. The pH value of 4-6 of this slurry is raised by the addition of aquaammonia and the batch is transferred to tank 20 where it is aged under gentle agitation at a pH value between 5.5-8.1 and a solids content of 3-6 per cent of SiO2.  Coating of the aged silica may either be effected in tank 20 or in tank 22 by adding aluminium sulphate solution and, after thorough mixing, ammonia solution.  The batch is then passed through a feed tank 24 to rotary drum filters 25, 26, 27, filters 26 and 27 having feed tanks 28 and 29 which receive repulped slurry from a preceding filtration, in which the filter cakes are continuously <PICT:0626455/III/1> <PICT:0626455/III/2> wetted with acidified wash water during their passage over the rotating filters.  Each filter is provided with a wash-water trough on the pick-up side of the drum to collect surface wash-water and deliver if through lines 31, 32 and 33 to feed tanks 24, 28 and 29 respectively.  The cake from the last filter is dried and calcined in rotary kilns by countercurrent contact with hot combustion gases having a temperature of 1500 DEG  F. or below.  In another embodiment, Fig. 2, a water-glass solution is neutralised with a mineral acid in tank 41 and the contents discharged into a connection tank 43 and then into an ageing tank 44.  The aged slurry is then delivered either directly or through a tank 45 for the addition of an adhesive colloid, e.g. glue, gelatin or gluten, to a filter 48, and after washing is repulped at 49 and passed into one (53) of a series of tanks in parallel, where the alumina is precipitated; the resulting silica-alumina slurry is then aged in tank 55, filtered and washed at 56.  The impurities in the gel are further reduced by successive repulping, 57, 60, and filtering, 58, 62, and the final filter cake is transferred by a conveyer belt 63 to a calciner 64, the calcined material discharging into a grinder 66. The slurry fed to the filters preferably has a pH of 4 to 5.5, and the filter cake may be repulped, filtered and washed until its soda (Na2O) content is less than 0.05 per cent, though cakes containing a higher alkali content may be passed into the calciner where the majority of the alkali impurity is volatilised.  Modified SiO2-Al2O3 catalysts may be made containing small amounts of zirconium or titanium oxide.  Specification 523,252 is referred to.  The Specification as open to inspection under Sect. 91 comprises also the preparation of silica gels with or without oxides or hydrous oxides of metals of the third and fourth groups of the periodic system or their mixtures.  This subject-matter does not appear in the Specification as accepted.