Precipitation and Co-precipitation - doe-std-1128-98

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

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