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Waste Treatments - doe-std-1128-98_ch10228
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DOE Standard Guide of Good Practices for Occupational Radiological Protection In Plutonium Facilities
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Liquid-Solid Separation Techniques


DOE-STD-1128-98
8.4.3.2 Precipitation and Co-precipitation
Precipitation and co-precipitation are used to decrease the solubility of
some compounds. Precipitation involves making the contaminant into an
insoluble material by the adjustment of pH or the addition of a chemical.
For example, nickel may be rendered insoluble by the addition of sodium
dimethylglyoxime. Co-precipitation is similar but is used when the
contaminant is not present in sufficient quantity to form a filterable solid
but will incorporate into another precipitate as it forms or will adhere to the
surface of another precipitate. In some waste treatment processes, a stable
isotope of the radioactive contaminate is added to co-precipitate the
radioactive material that is not present in sufficient quantity to form a
precipitate on its own. Precipitation is always followed by some
liquid/solid separation technique.
8.4.3.3 Liquid-Solid Separation Techniques
Treatments such as flocculation and filtration are used to remove solid and
colloidal contaminates either directly from the waste stream or following a
precipitation or co-precipitation process. Centrifugation or settling are
sometimes used to remove gross quantities of solids preceding some
filtration processes. These processes separate the waste into a concentrated
and dilute waste stream, both of which will probably require further
treatment. The bulk liquid fraction may be subject to filtration before
recycling or disposal. The fraction with the high concentration of solids
may be subject to evaporation, or drum or filter-press filtration to remove
excess water, or it may be solidified as discussed below.
Where the contaminant is present as a colloid or extremely fine particulate,
co-precipitation or flocculation may be required before settling,
centrifugation, or filtration. Flocculation involves the addition of an
extremely small quantity of a long chain molecule that has the appropriate
electrostatic affinity for the contaminant present. The flocculent molecules
gather the contaminant into rather large particles that are amenable to
settling and filtration. The flocculent and dosage (addition ratio) are
usually selected by trial and error. Flocculents do not add appreciably to
the waste volume and usually do not add a contaminant that results in a
mixed waste. Residual flocculent may, however, foul ion exchange resins
or reverse osmosis membranes, so it is important that the quantity added be
closely controlled.
8.4.3.4 Ion Exchange
Ion exchange is one of the most useful waste treatment techniques.
Aqueous wastes that are free of oil and other organics and contain only
very minimal quantities of solids may be subject to ion exchange on cation
resin, anion resin, or specialty resins, either alone or in combination. If the
contaminant is present as a cation, such as sodium, ammonia, or calcium, a
cation resin can be used to replace the cation in solution. The cation from
the resin will go into the solution to replace the contaminant cation. If the
8-17


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