BLEACH PLANT (FIRST STAGE) MIXER


Successful chemical mixing with pulp slurry occurs when an optically or chemically
induced signal is sensed and related to proper addition of chemical for CEK number
control at the end of the reaction period. Historically, three types of mixers have been
used: 1) the large mechanical mixers with 2 to 3 minutes retention, 2) the inline static
mixer with limited retention of 5 to 9 seconds, and more recently 3) the hi-shear mixer
with essentially no retention time in the mixing zone. The extended retention time of the
large mechanical mixer creates control problems due to the large feedback lag. The
inline static mixers and high shear mixers with plug flow provide neither adequate
retention for good CEK prediction nor any backmixing for smoothing short term variations
n incoming pulp K number, flow or consistency.

A. H. Lundberg, Inc. offers a chemical retention mixer with the desired 20 to 30 second
retention time for good back flow characteristics. The control signal, derived from the
pulp leaving the mixer, is measured on pulp which has had sufficient reaction time to
permit CEK number prediction; yet the feedback time such that an instrument control loop
can function properly. Chemical addition and "runaway" leading to over/under chemical
addition low viscosity/shieve excesses are eliminated.

The mixer can be used for the addition of chlorine, chlorine dioxide, or a combination of
the two. The first chemical is added to the pulp as it enters the mixer. The second
chemical is introduced at the proper predetermined position in the interior of the mixer.

The retention mixer is an agitated FRP lined carbon steel vessel. The agitation is
supplied by four (or more) turbine blades mounted on a vertical shaft driven by a low
horsepower motor. Typically, pulp at 3% consistency enters the bottom of the mixer into
a zone of intense, rapid mixing in which chemical is intimately mixed with the fiber and
initial reactions occur. In the case of chlorination, sufficient chlorine reacts to permit
further solution of dispersed gas bubbles. The pulp then passes through a "choke" ring
upward into the retention-backmixing zone prior to discharge. In the retention-backmixing
zone, sufficient agitation occurs to insure uniform distribution of chemicals through out the
pulp. Additional ports are provided at each agitation zone for installations desiring to add
more chemical. The high shear addition zone can be relocated to a higher position in the
mixer if required by chemical addition sequence. Each mixer is custom designed for the
application desired by the customer.

Increased pumping horsepower is not required for the retention mixer installation since
there is no pressure drop through the unit. The mixer is constructed of carbon steel with
FRP lining and coverings. The agitator shaft is provided with Hastalloy C stub-ends for
stuffing box and steady bearing. The vertical shaft is suspended by an internal,
removable water flushed bearing inserted at the bottom of the mixer vessel.