Unoxidized and oxidized white liquor are often used as the alkali source for an oxygen delignification stage. The average amount required is dependent upon the NaOH makeup requirement of the mill. Some mills use 100% white liquor in the oxygen stage and add makeup caustic elsewhere. Other mills will vary the amount of white liquor based on liquor inventory. This paper considers the potential impact of using white liquor (unoxidized and oxidized) on the performance of an oxygen delignification stage. The amount of oxygen consumed by the white liquor charge to an oxygen stage that reduces the pulp kappa by 15 units is almost 6kgO2/BDt, if the white liquor is previously oxidized to thiosulphate. The oxygen consumption for delignification at 0.9kg/Akappa is 13.5kgO2/BDt. When 25kg/BDt of raw black liquor solids carryover is consider the total oxygen consumption would be 22.5kgO2/BDt. At an excess of 20% based on delignification oxygen demand, the total oxygen requirement is approximately 25kgO2/BDt. The amount of gas charged to any oxygen stage is limited to about 20 volume percent (Xg) to avoid channeling. Most mills source their oxygen from a Vacuum Swing Adsorption (VSA) oxygen plant which produces oxygen at 90% purity. Therefore at a pressure of 150psig and a consistency of 12%, the maximum oxygen charge is about 19.5kgO2/BDt. As a result, the available oxygen charge can be exhausted which would limit the kappa reduction. The high consumption rate will also reduce the oxygen "bubble" surface area (i.e., the "a" in Kla) which may cause the delignification rate to become mass transfer rate limited.
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