Patent GB-763278-A (GB)
Title: Preparation of alkali metal hyposulfites
IPC: C01B17/66
Filed: 1954-04-22
Abstract: <PICT:0763278/III/1> <PICT:0763278/III/2> Alkali metal, particularly sodium, hyposulphites are produced by reacting an alcoholic aqueous solution of an alkali metal bisulphite containing about 20 per cent of water miscible alcohol, with an alkali metal amalgam containing less than 0.1 per cent alkali metal while maintaining the pH at 5-7. In the preferred apparatus (Fig. 1) a reaction vessel 10 which may be provided with packing, is fed with alkali metal amalgam through an up-turned pipe 31 from a storage vessel 28.  Alkali metal bisulphite solution is passed in a cycle from the reaction vessel 10 through pipe 11 to a cooler 13, through a filter or centrifuge 15 and a pH flow cell 17 back to the reactor 10, the solution entering the top of the reactor and passing through line 23 to the bottom of the reactor. The recycled solution may also be supplied through line 21 to the top of the reactor.  The system is first freed from air by the admission of an inert gas such as carbon dioxide introduced through lines 52, 55, and maintained under a slight pressure of this gas at all times. Sulphur dioxide which reacts with sodium sulphite and/or sodium hydroxide formed is fed by line 48 to the surface of the liquid in the reactor to regenerate the necessary sodium bisulphate.  Spent amalgam falling through the liquid in the reactor is withdrawn through U-tube 40 to a storage vessel 44.  The circulating liquid containing sodium hyposulphite reaction product is cooled in cooler 13 to 10 DEG  or lower thereby depositing sodium hyposulphite dihydrate in the centrifuge or filter 15 from which it is recovered. Adhering reaction solution may be removed by an alcohol wash and the wash water returned to the reaction cycle.  The concentration of bisulphite is preferably 0.5 to 1 molar and the acidity of the circulating bisulphite solution is controlled by feeding an acidic substance to regenerate the bisulphite such as acetic acid but preferably sulphur dioxide.  The alcohol is preferably ethyl or methyl or a mixture of the two but butyl, propyl and isopropyl alcohols may be used and may be denatured, with a ketone for example. The dihydrate crystals obtained from the filter 15 may be delivered to the apparatus of Fig. 2 through shoot 66 to a vessel 67 in which the crystals are immersed in a 20 per cent sodium hydroxide solution maintained between 60 and 70 DEG  where the crystals are agitated and their water of crystallization removed by the sodium hydroxide.  A film of liquid hydrocarbon may cover the sodium hydroxide and the vessel 67 may be provided with an inert atmosphere.  The treated crystal slurry is then run into a centrifuge 80 to separate the hyposulphite crystals from the sodium hydroxide solution, again under an inert atmosphere and the solid particles are fed through a shoot 82 into a vessel 84 containing warm ethyl or methyl alcohol preferably at 60 to 70 DEG C. Alternative liquids are ketones such as acetone or methyl ethyl ketone.  The slurry in vessel 84 is agitated whereby the crystals are washed and residual water removed whereupon they are delivered to a centrifuge 94 to separate them from the alcohol. Alternatively the alcohol wash may be effected in the centrifuge 80.  Dehydration may also be effected by contacting the dihydrate crystals with a saturated solution of sodium chloride containing up to 50 per cent alcohol and an alkalizing agent such as sodium carbonate, bicarbonate or sulphite, ammonium hydroxide or borax.  Mercury contamination in the hyposulphite may be reduced by feeding the mercury as successive particles to the reaction zone and by the provision of a micro metallic filter through which the reaction solution is fed to the crystallizer whereby the mercury content may be reduced to 0.001 per cent.  For further reduction of the mercury content a carboxylic type cation exchanger may be placed in the feed line between the reaction chamber and the crystallizer. As an alternative arrangement the reaction solution may flow through the amalgam reaction zone, a pH flow cell, a heat exchanger which removes heat generated by the reaction and maintains the circulating liquor at below 35 DEG C., and a filter.  Liquid is bled off to a crystallizer assembly which is maintained at 10 DEG C. or lower and which includes a thickener from which the product is withdrawn and a recycle line for the solution back to the reaction zone.  Reference has been directed by the Comptroller to Specification 700,042.