Page 238 - Shale Shakers Drilling Fluid Systems

220 SHALE SHAKERS AND DRILLING FLUID SYSTEMS

the first step in water and waste water clarifica- The mechanism of flocculation by high molecu-
tion. This process requires rapid mixing for thor- lar weight polymers is specific to this type of
ough integration of the chemical and water. chemical. Functionally, the polyelectrolyte assumes
an elongated shape in the solution because of the
electrostatic repulsion between charged groups,
which are adjacent to one another along the poly-
FLOCCULATION mer chain. Generally, nonionic polymers form a
random coil due to the lack of charged groups to
The process of agglomerating small, neutralized provide an uncoiling force. The extremely long
floe particles to form large, settleable floes is called length and charge density of the synthetic poly-
"flocculation." A flocculating agent gathers to- mers are important characteristics in the polyelec-
gether small floe particles by means of bridging trolyte's behavior. It is generally accepted that a
and enmeshment to form large floe particles that "bridging" mechanism accounts for the flocculation
readily settle. Bridging results from the long poly- behavior of these compounds. In this process, the
mer chains of the flocculant that reach out and polymer molecules attach themselves to the sur-
gather small floe particles. Enmeshment results face of suspended particles at one or more sites
from the netting effected on the cross-linking poly- with part of the long chain extending into the bulk
mer structure. This traps and retains the small floe of the solution. The free end of the molecule is
particles and allows the floe size to grow. then able, upon contact, to be adsorbed into other
The same inorganic chemical salts used as co- suspended particles, thus forming a "bridge," or
agulants have also been used as flocculants. link, between turbidity particles. This dynamic
The major drawback to inorganic flocculants is process is one in which adsorption and desorption
that they are extremely ineffective because they rapidly take place. It continues until a condition
lack the necessary molecular weight. Polymer floc- is established in which almost all of the polymer
culants, on the other hand, provide excellent water segments become attached, by one or more sites,
clarity at relatively lower dosages, without volu- to two or more turbidity particles. The reactions
minous sludge. Effective flocculation requires slow, are fast, with approximately 85% of the polymer
gentle mixing to allow the floe particles to build being adsorbed within about 10 seconds, and an
and grow. This is one reason why polymer floccu- additional 10% becoming attached after a more
lants must be diluted prior to application. Dilution prolonged contact. For this reason, extra effort is
facilitates proper mixing of the polymer and wa- taken to significantly dilute polymer solutions to
ter, which allows the polymer thins to reach out allow them to be well distributed into the floccu-
and contact the floe particles. lating medium. The progressive linking of more
The major benefit obtained with polymer floccu- and more particles results in an ever-increasing
lants is the large increase in floe size, resulting in sized floe, which is limited only by its ability to
more rapid sedimentation. It must be remembered withstand the hydraulic shear gradient imposed by
that these products function primarily as high the existing turbulence. Theoretical and experi-
molecular weight flocculants by providing more mental evidence has shown that the turbidity par-
efficient and effective bridging mechanisms. This ticle is partially occupied by the adsorbed polymer.
should be contrasted to cationic polyelectrolytes of Some studies have shown that the maximum floc-
lower molecular weight that function as primary culation rate is obtained when the turbidity par-
coagulants (neutralization). The flocculants do not ticles have one-half of their surfaces covered with
normally function effectively as primary coagulants polymer segments. If too many adsorption sites
except in cases where the charge is of a low or- are occupied by polymer segments, bridging is re-
der magnitude. The dosage of polymer flocculants stricted, and if all sites are occupied, bridging is
for water clarification is normally very low, rang- impossible. Both conditions lead to a dispersed
ing from 0.05 to 2 ppm (for highly turbid systems). rather than flocculated state. Conversely, if too few
In practice, flocculants have a considerable ad- sites are occupied by polymers, the possibility of
vantage over activated silica due to the more pre- bridging between adjacent particles is reduced,
dictable control over solution preparation and and the bonds may be so weak that the floe is
feeding, as well as lower space requirements for unable to withstand shear.
storage. Difficulties in handling high molecular
weight flocculants, are primarily due to the lack
of understanding in the area of dissolving the EMULSION POLYMERS
pulverized polymer product. Overdosage, particu-
larly when the polymers are being used as a filter Emulsion polymers represent the latest technol-
aid, can cause plugging and should be avoided. ogy in the development of water and waste water