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In order to study the characteristics of floc breakage in a
stirred vessel, kaolin flocs formed with various coagulation
and flocculation conditions are brokenup in a standard mixing
tank. Variations of floc size, floc apparent shape, cst value
and the residual turbidity of supernatant etc. under different
shearing action are measurement. Results indicate that floc
size measure- ment is more feasible than CST for floc's
breakage determination. Since the strength of kaolin-alum floc
is very weak, under ave- rage shear rate of 200 1/s its size
will decrease nearly to the same as the primary particle of
kaolin suspension. Addition of polymer flocculant to kaolin-
alum coagultaed system will increase floc size, floc sphericity
and its strength, thus even though shear rate increases to 300
1/s the floc after breakage is still two times the size of
kaolin primary particles. Photographic ob- servation shows that
the apparent structure of flocculated flocs is not varied
obviously under low shearing action, but after breaking under
higher turbulent intensity the structure of flocs become very
irregular. In this study, the Kolmogoroff's microscale of fluid
motion in mixing tank is the same order as floc size. By
analyzing previous theoritical studies andapplying the results
of floc density mea- surement, a relationship between maximum
floc size(d(f,max)) and rotational speed (N) of impeller is
found as d(f,max)∼N^{-(0.909∼1.158)} which agrees with the
results of floc breakage experi- ment. Therefore, both inertial
and viscous effects are significant for breaking the kaolin
flocs.
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