Oxygen Delignification and Post Oxygen Washing - process info

All modern kraft pulp mills producing bleached pulp continue the delignification after cooking in an oxygen delignification stage.

Like cooking, oxygen delignification occurs under alkaline conditions and the dissolved wood substance and spent delignification chemicals can therefore be circulated through brown stock washing, screening, digester washing and be recovered together with the spent liquor from cooking in the recovery cycle of the pulp mill.

In an oxygen stage the pulp is primarily delignified, but also bleached, becoming brighter, why it is s also called oxygen bleaching.

Oxygen delignification requires pressure and rather high temperature (About 100 °C) and is therefore carried out in pressurized reactors. Depending on the raw material and the aimed degree of delignification it is carried out in one or two reactors in series. The reactors give the time required for chemical reactions. It is important to wash the pulp well after the oxygen stage to minimize carry-over to the bleach plant and hence to decrease the consumption of chemicals in the bleach plant and the effluent load from the bleach plant.

Typically a two reactor oxygen delignification includes a pulp feed pump, a mixer for bleaching chemicals and steam, a 1st reactor, an intermediate mixer and a pump, a 2nd reactor, a blow tube or tank and finally washers equalling two or possibly three washing stages. It is very common to use two wash presses in series or one or two parallel 2-stage DD washers. If one or two 2-stage DD washers are needed depends on the production level. In the flow sheet DD washers are shown.The oxygen delignified and washed pulp is usually stored at medium consistency before entering the bleach plant, but not shown in this flow sheet.