Thermomechanical digestion process for enhancing the brightness of cellulose pulp using bleachants

An improved method for enhancing the brightness of a cellulose fiber pulp undergoing digestion in a thermomechanical process comprising the steps of:

• (a) providing a fiber source in a form suitable for input into a digester;
• (b) conveying the fiber source into a first chamber of a digester and contacting said fiber source with steam and a first quantity of a bleach solution at an autogenous pressure of 2 kg/cm to 15 kg/cm for a first period of time;
• (c) removing said fiber source from said first chamber, and rapidly decreasing the pressure on said fiber source at least 0.5 kg/cm to a first lower pressure;
• wherein water within said cellulose fiber source flashes to steam opening the fibers;
• and
• (d) inputing said fiber source at said first lower pressure into a second chamber of said digester for contact, with steam at said first lower pressure for a second period of time;
• the improvement comprising adding a second quantity of a bleach solution to said fiber source after removing said fiber source from said first chamber and prior to rapidly reducing the pressure on said fiber source to said first lower pressure, wherein part of the water of dilution of said second quantity of bleach solution converts to steam thereby concentrating said second quantity of bleach solution on said fiber source.