BLACK LIQUOR OXIDATION
The oxidation of kraft mill black liquor prior to direct contact evaporation is recognized
throughout the industry as essential to the reduction of odor from conventional recovery
furnace. Two types of oxidation systems are now commonly used: weak liquor systems
that operate on 15 to 20% solids liquor, and strong liquor systems that operate on 40 to
50% solids liquor.
Weak liquor systems were first developed in Scandinavia in the late thirties, and quickly
won widespread acceptance for their sulfur savings and reduced evaporator operating
problems. However, as their application was extended to mills with relatively high soap
content liquors (primarily in the southern United States), they were quickly seen to be
impractical without equipment capable of containing the large volume of foam produced.
While mechanical foam breaking equipment was being developed, strong liquor oxidation
was introduced to the Southern mills. These systems offered many of the benefits of the
earlier weak liquor systems, but since they operated on the liquor after most of the soap
had been removed by skimming, they did not encounter severe foam-control problems.
The strong liquor system soon became the standard for all mills with high soap content
liquors, and for those mills were weak liquor oxidation caused other problems (such as
the evaporator fouling problems associated with Eucalyptus Rostrata).
A. H. Lundberg Inc. has designed and installed black liquor oxidation (BLOX) systems for
many years, and have systems operating throughout the world. In recognition of the
more stringent pollution requirements of recent years, two-stage BLOX systems of
extremely high efficiency have been developed and offered for oxidizing both foaming and
non-foaming liquors from weak to strong. Each system is especially designed to suit the
liquor type, flow rate, and TRS (Totally Reduced Sulfur) loading of the particular mill.
The purpose of oxidation is to remove residual Na2S in black liquor by oxidizing it to the
stable sodium thiosulfate prior to a direct contact evaporator. As this reaction proceeds
and a sufficient amount of thiosulfate has been formed, secondary oxidation processes
commence, forming among others, sodium polysulfide. The amount of polysulfide formed,
or the ratio of thiosulfate-polysulfide, depends on the original sodium sulfide
concentration, reactor temperature, pH, and retention. The formation of polysulfide
proceeds faster once a large amount of thiosulfate is available. The latter reaction is
reversible and the reconversion from polysulfide is accelerated when the oxidized liquor
is stored after oxidation. At prolonged storage, and particularly at lower temperatures,
some of the reconverted sulfide appears as elemental sulfur. From the foregoing, it may
therefore be concluded that two steps are essential for continuous efficient operation:
1. Reduce oxidized liquor inventory to a practical minimum.
2. Maintain a product liquor temperature as high as possible.
It is now possible to produce oxidized liquor at below 0.1 gm/I Na2S content continuously,
which will normally result in a stack emission level of less than 5 ppm H2S, provided the
recovery boiler is not excessively overloaded and its operation is closely controlled. In
order to achieve such low levels efficiently, two-stage oxidation systems are required.
The first stage of oxidation can be either on weak black liquor (15 to 20% solids) or on
strong black liquor (45 to 50% solids). The second stage of oxidation should be
immediately before the direct contact evaporator in order to minimize reconversion effects
and to insure minimum TRS emissions. The first stage of oxidation normally is utilized
to oxidize 80 to 90% of the TRS in the liquor. The second stage is called upon to
complete the oxidation. The design of the oxidation for each stage is dependent upon
TRS loading, liquor flow rates, and wood species pulped.
Black liquor oxidation will cause the retention of sulfur in the system which was previously
lost. This retention of sulfur can result in a drastic increase in liquor sulfidity. In order to
control sulfidity at an acceptable level, many mills have to replace salt cake as make-up with
caustic soda or sodium carbonate.
Strong Liquor Oxidation
When strong liquor oxidation is used as the first stage, foam generation is generally
reduced. However, it is recommended that due to higher emission levels in the
evaporator, additional pollution abatement equipment be installed for treatment of
evaporator condensates together with digester condensate, as well as collection of
evaporator noncondensibles together with digester vent gases.
The A. H. Lundberg, Inc. Strong Liquor Oxidation System has been designed to overcome
the problems frequently encountered in earlier strong liquor systems where sparger
plugging is commonplace. To overcome the disadvantage of air sparger pipe with holes
to two inches in diameter, A. H. Lundberg Inc. has designed a small hole, multiple stage,
self-cleaning sparger assembly. This sparger, which permits continuous trouble-free
operation, releases small bubbles forming the maximum air-liquor interface for mass
transfer. Air for strong liquor oxidation is supplied by a turbo-compressor with a delivery
pressure of 8 to 9 psig.
Second Stage Oxidation
To insure that an oxidation plant operates on a continuous basis to provide minimum
emission levels, a second stage of oxidation is essential. This second stage oxidation
should occur with the oxidized liquor passing as quickly as possible to the direct contact
evaporator of the recovery furnace. The second stage of oxidation, if not provided with
an initial BLOX plant, can be later added as a "touch-up" stage when emission
The principles of design for the second stage of oxidation are similar to those of a first
stage strong liquor system. The design capacity of this stage is dependent upon the two-
stage system design or upon the efficiencies of a previously installed first stage.
A flow diagram for a two-stage strong BLOX system is included.
As the absolute amount of TRS following the second stage of operation is very low,
virtually no polysulfide is formed. The liquor transferred from the secondary stage,
directly to the direct contact evaporator, is virtually completely oxidized. The resulting
stack emissions of TRS are at a level of 5 ppm or less under furnace design operating
When black liquor is mixed with spent NSSC liquors it is expected that tall oil recovery
will be reduced. The solubility of soap is expected to be the same, but the liquor volume
is now considerably larger. If tall oil production is of major importance, consideration
should be given to oxidizing only weak kraft liquor in the first stage, mixing the liquors
prior to evaporation and reoxidizing the total strong liquor. Experiments have shown that
mixed kraft and NSSC liquor as strong liquor can be oxidized quite readily. In fact, during
mixing and storage of these two types of liquors, some of the Na2S is reconverted,
depending on the liquor volume ratio and the resulting pH. It must be noted that the mixed
kraft-NSSC liquors are more corrosive and additional equipment protection should
Two-Stage Strong BLOX Flowsheet
Black Liquor Oxidation Flow Sheet
Black Liquor Oxidation Plant Inquiry
BLACK LIQUOR OXIDATION PLANT INQUIRY
(For Kraft Pulp Mills)
Wood Species Pulp__________________________
If hardwood and softwoods, are they
_______pulped separately? _____ mixed during pulping?
If pulped separately, are black liquors
_____ held separate? _______mixed, before, after evaporation?
Strong Liquor: Flow ________
Solids, % _________
Desired Oxidation Level as ________%
or ________________ Na2S in outlet
Special Concern: Soap content or yield_____________________________
Limits of Supply:_______________________________________________________