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DOE-STD-1128-98
Guide of Good Practices for Occupational Radiological Protection in Plutonium Facilities
example, an acidic solution containing iron may show a copious precipitate of
ferric hydroxide upon the addition of a base.
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.
8-17


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