Method and apparatus for bleaching high consistency pulp with gaseous bleaching reagents

A method and apparatus for bleaching high consistency pulps using gaseous bleaching reagents. The pulp is in contact with the gas in a riser tube. The gas contains a gaseous bleaching agent. The riser tube forms a metal plate contactor, which can delay the contact time between the slurry and the gas. The pulp stays in the contactor for a predetermined period of time to make the pulp. About 75% to 90% of the gaseous bleaching agent is consumed, and the high consistency pulp is delignified from the initial Kappa number to an intermediate Kappa number. The pulp and the contacted gas are then sent to a porous reaction bed, where The pulp reacts completely with the gaseous bleaching agent to make the pulp delignify from the intermediate kappa number to the end point kappa number.