GB1342580A - Oxygen bleaching process for lignocellulosic materials - Google Patents

Abstract

1342580 Oxygen bleaching process CANADIAN INDUSTRIES Ltd 18 Dec 1970 [19 Dec 1969] 60270/70 Heading D1P A process for bleaching lignocellulosic material comprises (i) treating the material in aqueous alkaline medium with oxygen gas or an oxygen-containing gas under supraatmospheric pressure, (ii) treating the oxygen-treated material in aqueous suspension with chlorine or chlorine dioxide or a mixture of chlorine and chlorine dioxide wherein the concentration of available chlorine is 0.11 to 0.30 x Kappa Number of the oxygen-treated material; and (iii) treating the chlorine and/or chlorine dioxide treated material in aqueous alkaline medium with oxygen gas or an oxygen containing gas under supraatmospheric pressure and optionally (iv) treating the oxygen-treated material of (iii) in aqueous suspension with chlorine dioxide. The treatment of the lignocellulosic material with oxygen may be carried out on pulped material in aqueous suspension at a consistency of 3 to 30% by weight. The pulp may be made alkaline by treatment with 1 to 5% by weight of the pulp of an alkali. Addition of a viscosity preserver such as magnesium carbonate, magnesium oxide or magnesium hydroxide can also be made at this stage. The alkaline pulp is then exposed in a pressure vessel to oxygen or an oxygen containing gas at a partial pressure of oxygen in the range 30 to 200 p.s.i. for 10 to 45 mins at a temperature of 100 to 200‹C. The pulp is preferably stirred continuously. After completion of the oxygen treatment the pulp is normally washed with water. The treatment with chlorine and/or chlorine dioxide is preferably carried out with the pulp in aqueous suspension at a consistency of 3 to 30% at 10 to 40‹C. The concentration of the chlorine and/or chlorine dioxide is such that the available chlorine is 0.11 to 0.30 multiplied by the Kappa Number of the oxygentreated material. The weight ratio of chlorine to chlorine dioxide in the chlorine/chlorine dioxide mixture can be 100:0 to 0:100, but is preferably 95:5 to 80:20. The treatment takes 10 to 60 mins and then the pulp is washed with water. In the second alkaline oxygen treatment, the pulp at 3 to 30% consistency is suitably treated with 1 to 3% alkali material and then subjected to treatment with oxygen or an oxygen-containing gas under a pressure of 30 to 200 p.s.i. preferably in the presence of the viscosity preserver. The treatment is for 10 to 30 minutes at 100 to 200‹C, preferably with stirring. The pulp is then washed with water. The pulp may then be treated with 0.1 to 1.0% chlorine dioxide at 60 to 90‹C, for 1 to 4 hours. The pulp is then washed with water. The expression "alkaline material" is defined as a compound selected from alkali metal hydroxides and carbonates and hydroxides of calcium, strontium and barium.