Patent GB-1430095-A (GB)
Title: Method for making high proosity glass articles
IPC: C03C3/089
Filed: 1973-03-01
Abstract: 1430095 Process for producing porous glass PPG INDUSTRIES Inc 1 March 1973 [1 Dec 1972] 22436/75 Divided out of 1430094 Heading C1M A method for making a porous glass article having a high thermal stability comprises: (a) heating a phase-separable borosilicate glass having the following composition expressed as per cents by weight: 30-50% SiO 2 , 40-55% B 2 O 3 , 5-15%' of a fluxing agent selected from alkali metal oxides, alkaline earth metal oxides, PbO, CdO, SnO and ZnO, and 0-4% Al 2 O 3  to a temperature sufficient to separate the glass into two phases, one of which is a borate-rich phase and is soluble in water or an aqueous solution; (b) cooling said phase-separated borosilicate glass; (c) a first leaching of the cooled phase separated glass with water or an aqueous solution, other than the acid used in step (d) below, to remove a sufficient quantity of the borate-rich phase to form a porous glass, and (d) a second leaching of the porous glass with an acid to remove metal oxide fluxing agent and borate not removed in the first leaching, thereby forming a low metal oxide borate containing microporous glass structure. The glass article obtained has a composition, in per cent by weight, of at least 96% SiO 2 , less than 4% B 2 O 3 , less than 0À05% of fluxing agent, and less than 0À4% Al 2 O 3 . It is believed that the acid leaching step (c) forms additional pores in the glass article smaller than those formed by the first leaching step (b), by extruding portions of the soluble phase not removed by the first leaching step. Phase separation conducted for 1-8 hours at 420-500 C. results in a porous glass article having pore size diameters of 50-200 , and phase separation conducted at 580-650 C. for 8-24 hours in an article having pore size diameters of 200-1000 . The process is particularly applicable to glass particles. The first leaching is preferably conducted with an aqueous solution of a metal borate. The volume of water to glass in this leaching may be 2-8 volumes to 1 volume. The water solution containing the soluble phase is then separated from the particles. Subsequently the wet glass particles are transferred to an acid bath for a second leaching. Useful acids are mineral acids and oxalic acid. In general the acid leach is carried out at 80-100 C. for at least ¢ hour, e.g. 2-4 hours, the volume ratio acid to porous glass being 1-8 : 1. When the preceding phase separation has not been complete acids having a normality of 2-3 are used whilst if it is complete acids of 0À1-0À3 N are satisfactory. The porous glass particles may be used as catalyst supports. Catalysts may be impregnated into the porous particles by soaking the particles in solution or by vapour depositing the catalyst in the pores. Suitable catalysts incorporated therein are Pt, Pd, TiO 2 , V 2 O 5 , FeO, CoO, NiO 2 , CuO, Al 2 O 3 , ZrO 2  and SnO 2 . A deposited metal oxide may be subsequently reduced to the metal by passing a stream of hydrogen gas over the support. A specific example refers to treating beads with SnCl 4 .5H 2 O, Al(NO 3 ) 3 .9H 2 O, CrCl 3 .6H 2 O and ZrOCl 2 .8H 2 O solutions and subsequent calcination to form the oxides.