Method of bleaching pulp with an aqueous solution of chlorine dioxide and chlorine followed by a chlorine solution

A method of bleaching pulp using chlorine dioxide, which comprises:

• (a) continuously forming a gaseous mixture of chlorine dioxide, chlorine and steam by:
• (i) continuously feeding a sodium chlorate solution to an aqueous acid reaction medium present in a unilocular reaction zone,
• (ii) continuously feeding sulphuric acid to said aqueous reaction medium in an amount to maintain the total acid normality of the reaction medium in the range of about 2 to about 4.8 normal,
• (iii) continuously feeding sodium chloride, hydrochloric acid or a mixture of sodium chloride and hydrochloric acid to said aqueous reaction medium,
• (iv) continuously maintaining said reaction medium at a temperature of about 55.degree. to about 85.degree. C. while maintaining said reaction zone under a subatmospheric pressure of about 80 to about 300 mm Hg to cause the formation of chlorine dioxide and chlorine and the evaporation of water from the reaction medium,
• (v) continuously depositing anhydrous neutral sodium sulphate from the reaction medium in said reaction medium in said reaction zone once the reaction medium becomes saturated thereby after start up, and
• (vi) continuously removing the gaseous mixture of chlorine dioxide, chlorine and steam from the reaction zone;
• (b) continuously cooling said gaseous stream to a temperature of about 15.degree. to about 55.degree. C. to cause condensation of at least a substantial proportion of the steam therefrom to provide a chlorine dioxide- and chlorine-containing gas stream;
• (c) continuously contacting the latter gas stream with water having a temperature of about 3.degree. to about 10.degree. C. at a flow rate sufficient to form an aqueous solution of chlorine dioxide and chlorine containing about 8 to about 9 grams per liter of chlorine dioxide and about 1.5 to about 1.8 grams per liter of chlorine, and a gaseous chlorine stream;
• and
• (d) bleaching a cellulosic fibrous material pulp for about 10 to about 60 minutes at a temperature of about 35.degree. to about 70.degree. C. in aqueous suspension having a consistency of about 2 to about 16% by weight of pulp and containing no more dissolved organic material than about 2% by weight TOC on pulp at an overall equivalent chlorine concentration of about 2 to about 10% by weight of the pulp, by:
• (i) subjecting said suspension to a first bleaching step at an acid pH value using at least part of said aqueous solution of chlorine dioxide and chlorine formed in step (c), and
• (ii) without an intermediate washing step, subjecting the suspension to a second bleaching step at an acid pH using a chlorine solution and commencing about 5 seconds to about 10 minutes after commencement of said first bleaching step,
• the chlorine dioxide in said aqueous solution of chlorine dioxide and chlorine constituting about 20 to about 90% of the total available chlorine used in said first and second bleaching steps.