PURPOSE: To provide an improved ozone-bleaching process for paper pulp, particularly medium consistency paper pulp, and apparatus therefor.

CONSTITUTION: The ozone in oxygen and the medium consistency paper pulp are fed to a fluidizing mixer; the intimate uniform mixture produced is then passed upwardly in a reaction vessel until bout 99% of the ozone has been consumed, and then separation of gas from the pulp is initiated by moving the pulp mixture into a generally horizontal tube leading to a gas zone in a large diameter retention vessel. A pressurized gas pad is maintained at and above the introduction of the pulp into the retention vessel, while the level of pulp in the retention vessel is maintained below the point of introduction. The pulp may be withdrawn from the retention vessel with a fluidizing degassing pump, and the pressurized gas removed from the top of the vessel may be fed to an apparatus using oxygen gas under pressure.