WHITE LIQUOR OXIDATION

The concept of White Liquor Oxidation is a natural development following the
commercialization of oxygen delignification. The use of white liquor as an alkali source
in oxygen bleaching dictates that the sodium sulfide be oxidized in an external vessel.
This is necessary since the oxygen atmosphere in the delignification reactor will oxidize
the Na2S, liberating heat and thereby making the temperature control in the reactor
difficult. The well-defined technology of weak black liquor oxidation may be directly
applied to white liquor oxidation.

The A. H. Lundberg, Inc. System accomplishes the oxidation of sodium sulfide in white
liquor with atmospheric oxygen in the presence of a suitable catalyst. The most
convenient catalytic material in an integrated kraft mill is weak black liquor. Additions of
weak black liquor up to 10% of the volume of the white liquor being oxidized have proven
to allow bleaching to proceed without interference, 5% addition being the norm.

The Oxidation Reactor is a sparged and baffled tank. The white liquor enters the tank
and passes over the air spargers. As the flow proceeds through the baffled path to the
outlet, the liquor will be subjected to a series of air sparged zones. A stilling area
permits separation of entrained air prior to removal by the process transfer pump.

During oxidation, the air approaches saturation at the discharge temperature. The heat
removed from the liquor as latent heat of vaporization is usually greater than the heat
generated by oxidation of sodium sulfide. This deficiency in heat can be partially returned
to the system by direct steam addition or the use of heating coils.

In any system where a large volume of air passes through liquor, it is expected that liquor
entrainment in the gases may occur. Since little foaming should occur, the removal of the
entrainment can be achieved by one of two methods, (1) chevron type separator, or (2)
cyclone separator. Either unit is installed in the vent gas stream just prior to the exhaust
stack. The emissions from the oxidizer are essentially saturated air, depleted somewhat
in oxygen content.

The primary product of the oxidation will be sodium thiosulfate. The product will be
dependent upon maintaining the liquor above 160 degrees F. in order to minimize the production
of polysulfide. The oxidation efficiency which is indicated by the oxidation products may
be affected by the rate of reversion. This is dependent upon the elemental sulfur
formation and polysulfide formation. Polysulfide, in the absence of oxygen, will revert to
sulfide. Likewise, elemental sulfur will react with the thiosulfate to form sodium sulfide.
160 degrees F. Reversion may be expected to be about 5% of the oxidized sulfide. The amount
of sodium sulfate that is formed as oxidation products will be small.

White Liquor Oxidation Flow Sheet
SW-1031-B