GB842745A - Production of potassium manganates - Google Patents

Abstract

Potassium manganate K3MnO4 is prepared by reacting a manganese oxide having a valency less than 5 with an aqueous potassium hydroxide melt at 170-350 DEG C. in the presence as oxidising agent of a free oxygen-containing gas and/or of a potassium manganate (including permanganate) with a valency greater than 5, the total amount of oxide added not in excess of that stoichiometrically equivalent to the potassium hydroxide and added continuously or intermittently throughout the reaction at a rate which is or in discrete amounts which are insufficient to cause the mixture to become too viscous for satisfactory operation. The product may be further oxidised simultaneously or subsequently in the same reactor or may be removed and oxidised in a separate reactor to K2MnO4, by means of free oxygen-containing gases. After the initial reaction at 170-310 DEG C., any further oxidation preferably takes place at 140-310 DEG C. For both reactions the aqueous melt is preferably a 65-90% potassium hydroxide melt which may be provided by the addition of a more dilute solution which evaporates under reaction conditions: a 10- or 20-fold excess of potassium hydroxide over that required for the reaction is preferably employed in the formation of K3MnO4. When the reactions are carried out in separate reactors it is preferred (a) when oxygen-containing gases are employed, to filter the product slurry from the second reactor and return the filtrate to the first reactor; filter cake washes comprising aqueous KOH are preferably returned to the second reactor and heat need only be supplied to the first reactor (b) where a manganate is employed in the first stage, to recycle part of the product slurry from the second reactor before filtering to the first reactor, to supply said manganate, and recycle the filtrate from the remainder of the slurry to the second reactor and to supply heat only to the second reactor, it is also preferred in the latter case to maintain a manganate: manganese dioxide ratio of at least 1.4:1. Initial materials to be used are pure or impure manganese oxides, hydrated or otherwise, such as manganese dioxide and calcined rhodochrosite (Mn3O4) and oxides such as those produced in permanganate reactions e.g. p 5MnO2.K2O.3-5H2O, which are combined with potassium oxide. As the free oxygen-containing gas, air, oxygen, air enriched with oxygen or a mixture of oxygen and an inert gas may be used, preferably in an amount at least four times that theoretically required. Sub- or super-atmospheric pressure may be used if desired. The gas may be passed through the melt or the melt may be very vigorously agitated while the gas is passed thereover so as to entrap bubbles of the gas. Continuous, semi-continuous or batch operation may be employed.