GB713559A - Improvements in the manufacture of the thiosulphates and seleno-sulphates of the alkali metals, calcium and ammonium - Google Patents

Abstract

<PICT:0713559/III/1> A method of preparing the thiosulphates or selenosulphates of the alkali metals, ammonium and calcium, comprises digesting the corresponding sulphite with sulphur or selenium in the presence of a cationic surface-active agent. Preferred surface-active agents are alkyl pyridinium halides in which the alkyl group comprises an aliphatic chain of at least six carbon atoms, e.g. dodecyl pyridinium bromide, cetyl pyridinium bromide, decyl pyridinium bromide and octyl pyridinium bromide. The process permits operation below the melting point of sulphur and may be continuous. In an example, 50 parts by weight of crystalline sodium sulphate (Na2 SO37H2O), 7 parts by weight of finely divided sulphur and 10 parts by weight of water containing 1/100 parts of commercial cetyl pyridinium bromide "Fixanol C" (Fixanol is a Registered Trade Mark), are mixed and digested at 85 DEG -95 DEG C. for about 15 minutes, the solution is then filtered to remove the unused sulphur and crystallised to separate the sodium thiosulphate. Fig. 6 shows a flow diagram for a continuous process embodying the invention, sulphur, sulphite, and the cationic surface-active agent are fed from the vessels A, B and C, respectively, into the mixer D whence the mixture passes into a steam jacketed reactor E. The unused sulphur, filtered off at F is fed back to A while the filtered liquid is fed to a dehydrator or crystallising plant G. In the case of the preparation of sodium or ammonium thiosulphates, the filtered product is suitable for use as a photographic fixer (see Group XX).