Page 241 - Shale Shakers Drilling Fluid Systems

SOLIDS DEWATERINC 223
the cream formation is redispersed, it can cause
plugging of polymer feed pumps and resultant
dosage variations.
To minimize any makeup problems caused by
separation or creaming, the drum contents should
be thoroughly mixed before using. Although hori-
zontal drum rollers can be used for this purpose,
it is best to use a side-bung, angle-entry, air driven, of these floes is limited and usually not large
or electric drum mixer. A portable drum mixer can
be attached to the side bung for easy mixing and
redispersion of the emulsion polymer. Center-bung
mounted mixers can also be used but the effect
is not as great. A drum should be mixed in this
fashion for at least 15 minutes before using.
COAGULATION CONCEPTS
AND MECHANISMS
Colloid Stability
To understand the concept of the mechanisms
involved in coagulation reactions, it is important
to understand why particles in suspension remain
discrete and "stable" as a colloid.
Particles of a colloid remain separate and dis-
crete mainly due to electrostatic forces of repul-
sion that exist between the particles. In any stable
colloid, these electrostatic forces must be reduced
for particles to contact each other, thereby en-
abling coagulation to occur.
Charge neutralization. Theoretically, if the like
surface charge on particles could be reduced or
eliminated, the particles would be able to contact
each other and possibly bind together. By adding
counter-charged coagulant chemicals, the surface
charge on particles is neutralized. Charge neutral-
ization starts the coagulation process.
Zeta potential. The stability of a colloid depends
on the electrostatic repulsive forces between par-
ticles. The magnitude of these forces are variable
and can be measured. The charge sign and mag-
nitude is determined by following the path and
speed of particles through an electric field. The
"zeta potential" of the particles of a colloid allow
for the prediction of an effective coagulant charge
selection and dosage.
Interparticle bridging. Once the charge of col-
loidal particles has been neutralized, particle con-
tact will occur and point floes will form. The size
enough to provide for an optimal settling velocity.
The mechanism primarily responsible for further
floe growth is due to the joining, or "bridging,"
of micro floes by high-molecular weight, long-
chain molecules.
Van der Waals force of attraction. Van der
Waals force is a naturally occurring attractive force
between two bodies. Although not as powerful as
the electrostatic forces of repulsion between col-
loidal particles, it does enter into the coagulation
sequence once charge neutralization has occurred.
In addition to the above, other factors include:
• PH— Solution pH is an important influence on
particle charge, coagulant effectiveness, and
the formation of precipitates of dissolved
solids. The optimum pH range for cationic
polymers is 4.5 to 10.
• Temperature—As water temperature increases,
the viscosity decreases and the particles be-
come more active due to the decrease in the
shear conditions of the fluid.
• Mixing Energy—Refers to both the magnitude
of velocity gradients, as well as the duration
of mixing. The mixing energy affects the rate
of particle collisions and shear conditions.
• Particle Size—Affects the floe size and the
ultimate settling velocity of floes.
• Particle Concentration—Affects the opportunity
for particle collisions.
• Shear Forces—Probably the most limiting fac-
tor controlling floe growth.
• Particle Charge—Dictates the dosage and
charge of coagulant needed to attain charge
neutralization.