Patent GB-563051-A (GB)
Title: Process for the production of an alkali metal ferricyanide
IPC: C01C3/12
Filed: 1943-01-09
Abstract: 563,051. Alkali metal ferricyanides. MANCHESTER OXIDE CO., Ltd. (American Cyanamid Co.). Jan. 9, 1943, No. 444. [Class 1 (ii)] A water-soluble potassium salt, for example the chloride, sulphate, nitrate or carbonate, is added to an aqueous solution of sodium ferricyanide and dipotassium monosodium ferricyanide crystallized by cooling. The solid potassium salt is preferably added in molar ratio of 1-4 : 1 to a ferricyanide solution of at least 25 per cent concentration, the mixture heated to about 65‹ C., and then cooled to about 15‹ C. when the double salt precipitates from the solution substantially saturated with the potassium salt. It may be recrystallized once or twice from the mother liquor of a previous recrystallization. When crude sodium ferricyanide liquor is used as initial material, it may be treated with excess of caustic soda and the ferrichydroxide and other insolubles filtered off. Thus, a crude 27 per cent solution of pH 8.3 is treated at a temperature up to 65‹ C. with caustic soda until the pH is 9.5, and 0.75 per cent excess caustic soda added. It is cooled to 15‹ C. and filtered, potassium. chloride added in molar ratio of 3 : 1, the mixture heated to 65‹ C., cooled to 15‹ C., and centrifuged. The crystals are washed with recrystallization mother liquor and the wash liquor returned to the initial reaction stage. Water is added to the cycle, generally for dissolving crude crystals or, where a second recrystallization is effected, as solvent for the first recrystallization product. Where the quantity of this product gives a saturated solution with the added water, cooling to 15‹ C. will precipitate potassium ferricyanide instead of the double salt. Preferably, in operation, one pass is made using water as solvent producing potassium ferricyanide, and in the next several passes mother liquor is used. Sodium ferricyanide, made by supplying ferrocyanide, water, and chlorine to a reactor with stirrer, the pH being adjusted to 8-8.3 with caustic soda, is fed by line 11 to reactor 16 together with wash liquor from line 26. The mixture is circulated through pipes 17a, 17b, and heat exchanger 17, fed by steam line 17c, until the temperature is 60‹ C. Circulation is stopped while potassium chloride is added by line 14 and stirring effected. Further circulation raises the temperature to 74‹ C., steam being added by line 14a if necessary. The slurry is taken to a vacuum evaporator-crystallizer 20 and cooled to 15‹ C., and then led to tank 23 and centrifuge or filter press 24. Waste liquor is led by line 25 to tank 27 for reduction with SO2 and recovery of ferrocyanide. The crystals are washed with mother liquor, which is then fed to tank 30 for recycling, and pass to recrystallizer 32 where they are dissolved in recrystallization mother liquor from tank 34, and water. Steam is introduced to heat to 71‹ C. or circulation effected through heat exchanger 17 and lines 37, 37a. The solution is pumped through lines 41, 19 to crystallizer 20. After centrifuging, the crystals are washed with fresh water, dried in drier 42 and further processed.