Process for producing aqueous solution of calcium nitrite

摘要

An aqueous solution of calcium nitrite having high purity and high concentration is obtained by contacting a nitrogen oxides containing gas with an aqueous slurry of calcium hydroxide. It is necessary to use the aqueous slurry having high calcium hydroxide content such as 20 to 40 wt. %, whereby it is necessary to feed the gas having high nitrogen oxides concentration such as 5 to 10 vol. % to give high conversion to calcium nitrite. In order to prevent a formation of a by-product of calcium nitrate, a nitrogen oxides containing gas having 5 to 10 vol. % of a nitrogen oxides concentration and 1.2 to 1.5 of a molar ratio of NO/NO2 is fed as a first stage and then, the unabsorbed gas is oxidized and the resulting gas having 1 to 3 vol. % of a nitrogen oxides concentration and 1.2 to 1.5 of molar ratio of NO/NO2 is fed to the aqueous slurry having 3 to 10 wt. % of a calcium hydroxide obtained in the first stage so as to convert the residual calcium hydroxide to calcium nitrite without forming a by-product of calcium nitrate. The process is preferably carried out as continuous process having a first reaction zone and a second reaction zone.