Process for gas phase delignifying wood pulp using chlorine dioxide at subatmospheric pressure

摘要

A process for gas phase delignifying wood pulp comprising: contacting wood pulp in the first stage of pulp bleaching with chlorine dioxide-containing gas to delignify said wood pulp; wherein (a) said wood pulp has a consistency of at least about 15 percent; (b) said chlorine dioxide-containing gas contains less than about 3% by weight chlorine gas, based on the weight of chlorine dioxide in said gas; and (c) said contacting is conducted at a subatmospheric pressure from about 15 to about 750 mm Hg. This process reduces the formation of chloroform and significantly reduces the amounts of toxic by-products such as dioxin and other organic chlorides. These undesirable by-products, collectively identified as absorbable organic halides (AOX), require effluents from pulp bleaching processes to undergo expensive treatment methods to eliminate AOX's prior to discharges to sewers. The effluent discharge from this process generally has less than 2.0 kg adsorbed organic halides (AOX) per ton of wood pulp. The pulp produced by this process has a brightness and viscosity that meet or exceed the requirements of industrial papermaking.