GB772852A - Improvements in or relating to crystalline alkali metal silicates - Google Patents

Abstract

<PICT:0772852/III/1> Hydrated alkali metal silicates having a M2O:SiO2 ratio of 1.75:1 to 2.25:1 are prepared by mixing an alkali metal metasilicate liquor at 80-100 DEG C. with an alkali metal hydroxide solution of not more than 74 per cent strength at 70-90 DEG C. in amounts to give the desired M2O:SiO2 ratio and subjecting the resultant liquor to thermal crystallization. The preferred alkali metal hydroxide concentration is 70 to 74 per cent, NaOH, for example. The preferred products have an Na2O:SiO2 ratio of about 2:1 and contain 5 to 6 molecules of water. The invention is exemplified by the preparation of sodium silicate in which sodium metasilicate hydrate liquor is first dehydrated down to about 3.65 to 3.85 mols. of water and maintained at 80 to 100 DEG C. The aqueous caustic soda solution may be held at 70 to 90 DEG C. and the two raw materials may be combined in the ratio desired in the final product, it being preferred that the mixture is effected in small quantities which may be continuously delivered to a flaking drum. As shown in Fig. 1 of the drawings sodium hydroxide and sodium metasilicate are fed to a reaction vessel A provided with a stirrer C suitably rotated at 100 to 255 r.p.m., the mixture then flowing through line D to a steam-heated drum E mainained at 100 to 160 DEG C. The flake crystals are removed by blade G and are conditioned in a rotary conditioner J wherein the product is tumbled to cool and to permit the escape of water. According to an example, a solution of sodium silicate of formula Na2SiO3.3.75H2O at 87 DEG C. is mixed with 73 per cent NaOH at 90 DEG C. in a mixer and the mixture continuously withdrawn from the mixing zone to flaking drums maintained at 155 DEG C. The mixing time may be 26 seconds and the crystallization on the drums about 54 seconds; the final product analysis 2Na2O.SiO2.5.4H2O. Further Examples relate to the preparation of alkali metal silicate liquors having an Na2O.SiO2 ratio of 1.75:1, 2.25:1 and containing before passage to the flaking drum 5.7 and 7.1 mols. of water respectively. The invention is also applicable to potassium, lithium, calcium and rubidium silicates.