Method for ozone bleaching of cellulosic pulp at low consistency

A method of bleaching an aqueous cellulosic pulp at low consistency comprising:

• (i) exposing an aqueous cellulosic pulp at low consistency to a bleaching with at least one of chlorine and chlorine dioxide to effect partial bleaching of the pulp,
• (ii) flowing aqueous partially bleached cellulosic pulp from step (i) along a flow line effective to provide a high ratio of flow path of aqueous pulp to length of effective travel of the aqueous pulp,
• (iii) introducing ozone under pressure into said aqueous pulp in a total amount effective to further bleach the pulp, said ozone being injected into said flow path through a plurality of spaced apart injection ports,
• the ozone injected at each port being a fraction of said total amount,
• the fraction of ozone injected at each port and the spacing of the ports being such that a low partial pressure of ozone is established in said flow path, effective for chemical oxidation of lignin in the pulp by the ozone with low chemical attack of cellulose in the pulp by the ozone, and with the amount of ozone injected at the spaced apart injection ports along the flow path being a controlled amount such that the aqueous cellulosic pulp arriving at each immediate downstream port has substantially consumed all of the ozone entrained during the flow of the aqueous cellulosic pulp from each immediate upstream injection port.