GB788933A - Process for the manufacture of alkali metal silicates - Google Patents

Abstract

<PICT:0788933/III/1> Hydrous alkali metal silicates are prepared by passing a suspension of silica in aqueous alkali metal hydroxide solution under 9-115 atmospheres pressure at 175-320 DEG C. through a reaction vessel, preferably a tube, the water content of the reaction mixture being greater than that desired in the final product and thereafter expanding the hot reaction product through an expansion valve to reduce the water content to the desired value. The initial suspension may have an alkali metal oxide to silica ratio from 1 : 0.66 to 1 : 1.5. The hot reaction product may be cooled before expansion through the valve but in general it is preferred to expand the hot reaction product directly. The reaction vessel may be provided with a stirrer in which case the alkali metal oxide to silica ratio may be increased to 1 : 2. Thus in the preparation of sodium metasilicate hydrate melts, the water content of the mixture may be 55-60 per cent, using a reaction tube without a stirrer and a temperature of 250-260 DEG C. The process is carried out in the apparatus shown in Fig. 1, where 1 indicates a mixing tank where the alkali metal hydroxide solution is mixed with finely ground silica. The suspension passes through a pump 4 to a reaction tube 5 which is provided with a rapidly rotating stirrer 6. The reaction tube is heated by oil passing through a pump 15 and an oil heater 16 and the reaction products flow out from the reaction tube through a cooling jacket 8 to an expansion/valve 13 to a collecting tank 14. In place of the stirred reaction vessel of Fig. 1, the reactor may take the form of a spiral tube immersed in an electrically heated oil bath. According to examples: (1) a suspension of quartz powder in 27.7 per cent strength sodium hydroxide solution at 80 DEG C. was forced into an alkali resistant steel tube under a pressure of 60-70 atmospheres, the quartz particles having a diameter between 70 and 150 microns. The reaction tube was heated by an oil bath to 260 DEG C. and a later part of the tube was cooled or heated to adjust the temperature to 220 DEG C. prior to expansion. At the end of the tube the reaction product was continuously expanded to atmospheric pressure by a throttle valve. (5) A suspension of quartz powder in 47.1 per cent strength sodium hydroxide at 80-90 DEG C. was passed into a cylindrical reaction tube, as shown in Fig. 1, at 60-70 atmospheres pressure. The quartz employed had a grain size below 70 microns. A rapidly rotating stirrer cage was provided in the reaction tube and the oil heating jacket was adjusted to about 70 DEG C. The reaction mixture was passed through the reactor tube with a dwell time of about six minutes. The reaction mixture then flowed through a tube which serves to adjust the temperature for expansion. If the expansion took place at 80 DEG C. the water content of the melt formed was 47 per cent corresponding to a 6 hydrate of sodium silicate whereas if the expansion took place at 200 DEG C. a sodium meta silicate hydrate melt with a water content of 38 per cent formed corresponding to the four hydrate of sodium metasilicate. Further examples relate to the production of sodium silicates having an Na2O : SiO2 ratio of 1 : 1.25 and a water content of 55 per cent, a ratio of 1 : 0.8 and a water content of 56 per cent, a ratio of 1 : 0.66 and a water content of 56 per cent, a ratio of 1 : 2 and a water content of 51 per cent. Further examples relate to the production of potassium silicates, for example potassium ortho silicate solution containing 63 per cent water.