Method and apparatus for treating pulp with oxygen in a multi-stage bleaching sequence

Abstract

A wood pulp slurry is treated with oxygen in a mill with little change to the process or structure of the mill. No special pressure tanks are required. The consistency of the pulp need not be altered for the treatment step. It may be treated at the usual process consistency of the pulp; e.g., it may be treated at the usual consistency of the pulp leaving a washer or subsequent steam mixer without additional dewatering or additional dilution. The oxygen is added into a closed section of the system so that it cannot immediately vent to the atmosphere. Alkali should also be present when the oxygen is mixed with the slurry. The mixing should occur near to the point of oxygen addition. The mixing occurs in a relatively small mixer that intensively mixes the slurry and gas. The mixer has a mixing zone with a swept area of 10,000 to 1,000,000 square meters per metric ton of oven-dry pulp. A preferred range is 25,000 to 150,000 square meters per metric ton of oven-dry pulp and an optimum range of around 65,400 square meters per metric ton of oven-dry pulp. The pulp is treated several times during a sequence. Some sequences are O-X-O and O-O-X-O in which X may be chlorine, chlorine dioxide, a combination of chlorine and chlorine dioxide - mixture of chlorine and chlorine dioxide, C.sub.D, D.sub.c -, hypochlorite, peroxide and ozone. The sequence may be followed by a D stage. Other systems and specific mixer designs are also disclosed.

Dependencies are inferred from the claim text.