Using white liquor as the alkali source in oxygen delignification

Abstract

This paper considers the potential impact on oxygen delignification stage performance when unoxidized white liquor and partially oxidized white liquor containing thiosulfate, or (OWL[T]), are used as the alkali source. A literature review was conducted to estimate the oxygen consumption of the pulp, the raw black liquor carryover, and the sulfur compounds in the charged white liquor and in the postoxygen washer filtrate recirculated to the oxygen stage. These values were used to calculate the total oxygen consumption (kg O2/bone dry ton [BDT]) as a function of oxygen stage Δ kappa. The maximum oxygen charge was estimated using the generally accepted void volume (Xg) limit of 20%. A delignification model was developed using an empirical equation for the rate of change of lignin in the pulp as a function of operating parameters. The initial (OH-) concentration was adjusted to achieve 40% delignification at 0.70 MPa (100 psig) and 50% delignification at a pressure of 1.03 MPa (150 psig) for a 30 kappa pulp. The model predicted oxygen exhaustion and hence a reduction in achievable Δ kappa at operating pressures of less than 150 psig when OWL(T) was used as the alkali sources and the raw black liquor carryover was 30 kg/BDT