DISPLACEMENT CHELATE TREATMENT OF PULP

摘要

Cellulose pulp, such as kraft pulp, having metals, is treated prior to bleaching or oxygen delignification so that it may be effectively bleached without chlorine, or with a minimum amount of chlorine. In a first stage, the pulp is washed with a first wash liquid, including an amount of chelating agent (such as EDTA, DTPA, or NTA) effective to ultimately remove the majority of the metals from the pulp, to produce pulp with a chelate. Chemicals for adjusting pH also may be added if necessary. In a second stage, after the first stage, the pulp is washed with a second wash liquid to displace the chelate from the pulp and to produce a washed pulp having the majority of metals removed from it. Subsequent washing stages can also be utilized. The pulp with removed metal is then subjected to oxygen delignifying and/or bleaching, preferably without the use of any chlorine (such as using a plurality of O, Z, P, A and/or E stages). A part of the spent wash liquid from the second stage may be recirculated to provide part of the wash liquid of the first stage, while part of the spent wash liquid from the second stage is sewered. The proportions of spent wash liquid that are sewered or recirculated may be controlled dependent upon the concentration of metals in the spent wash liquid. The washing preferably takes place at medium consistency.