GB849816A - Production of chlorine dioxide - Google Patents

Abstract

<PICT:0849816/III/1> In the continuous or semi-continuous production of chlorine dioxide by the reduction of an alkali metal chlorate in aqueous sulphuric acid solution of a concentration of 1-5.5 molar, preferably 4.0-4.8 molar, the chlorate content of the solution being 0.01-0.5 molar, preferably 0.03-0.2 molar, the reaction is carried out in the presence of added chlorine, a hypochlorite, or a chloride so as to maintain a concentration of free or combined chlorine other than as chlorate or chlorine dioxide of 0.05-0.25 gm. atom/1. The flow of reactants and reaction products may be continuous or intermittent. The hypochlorite may be hypochlorous acid, or sodium, or potassium hypochlorite. The chloride may be hydrochloric acid, or sodium, or potassium chloride. The chlorine, hypochlorite, or chloride may be added as such, or as an aqueous solution. The reaction temperature may be 25-90 DEG C., preferably 25-35 DEG C. The reducing agent may be sulphur dioxide, methanol, sugar, or spent sulphite liquor. The ratio of sodium chloride to sodium chlorate charged may be 1-20 mole per cent, e.g. 10%. 5 1. of sulphur dioxide and 40 1. of air may be charged to 2.5 1. of the reactant solution per hour to produce chlorine dioxide of 125-180 mm. partial pressure. 18% sulphur dioxide, as produced by a sulphur burner, produces a chlorine dioxide partial pressure of ca. 260 mm. The partial pressure should not be above 300 mm. Product chlorine dioxide containing hydrogen chloride may be dissolved directly in water to produce an acid solution suitable for bleaching, or may be washed with water, constant-boiling hydrochloric acid, or sodium chlorate feed solution, and the hydrogen chloride absorbed distilled off and recycled. Alkali metal bisulphate or sesquisulphate may be crystallised from the product solution by cooling, which may be preceded by evaporation. The product solution or sodium bisulphate or sesquisulphate crystals obtained therefrom may be neutralised with caustic soda or soda ash as solid or solution, and sodium sulphate crystallised from the resulting solution by cooling, which may be preceded by evaporation. Examples specify particular proportions of reactants and conditions of reaction. As shown, sodium chlorate, sulphuric acid, and sodium chloride solution are passed into a water cooled reaction chamber 10 from above through lines 11, 13 and 47, respectively, while steam and a mixture of sulphur dioxide and air are passed in from below through lines 29 and 30, respectively. The gases may pass through a diffuser 33, and circulation of solution may be aided by a concentric circular tube 34. Product chlorine dioxide passes through line 35, while product solution is partly recycled via line 40, and partly bled off for the recovery of sodium sulphate via line 39.