Fig. 8.2-Flow diagram of ORNL water-treatment pilot plant. A, primary mixing tank; B, secondary mixing tank; C, flocculation, or coagulation, tank; D, settling tank; E, flow proportioning and sampling tank; F, overhead orifice feed tank for feeding 10 filter columns; and G, G-M tube connected to countrate meter and recorder. (Figure not to scale.)

nuclides in the mixtures varied considerably for the two tests. Radiochemical analyses are necessary to determine the removal of specific substances and should be performed in all instances.

8-1.4 Lime and Soda-ash Softening

Lime and soda-ash removals reported for a variety of radionuclides are summarized in Table 8.10. These data show that reasonable amounts of chemical will provide a 90% or better removal of soluble Ba 140-La 140, Sr89, Cd115, Sc46, Y91, and Zr95-Nb95 but that much larger quantities of chemical (up to 48 grains/gal) were ineffective for the removal of Cs137-Ba137m and W185. It will also be noted that lime alone was effective for the removal of Zr95-Nb95. Lime and soda-ash softening has also been tested for the removal of iodine, but the results have been negative."

Treatment with lime and soda ash finds its greatest use in the removal of potentially hazardous strontium. Many studies have been carried out to define the mechanism involved, namely, the coprecipitation of the strontium with the calcium. When stoichiometric amounts of chemical are used, a removal efficiency of approximately

with increased chemical dosages. 8, 18

Coprecipitation may be accomplished by (1) formation of mixe crystals in which foreign ions are incorporated homogeneously in th crystal lattice; (2) occlusion of impurities as imperfections scattere at random throughout the crystal; and (3) surface adsorption o foreign ions by the precipitate after it has formed. 20, 21

TABLE 8.11-REMOVAL OF STRONTIUM BY LIME AND SODA-ASI SOFTENING*

*From A. L. Downing et al., J. Inst. Water Engrs., 7: 555 (1953).
†As percentage of that equivalent to all the temporary calcium hardness.
*As percentage of that equivalent to the permanent calcium hardness.

Strontium is removed by mixed-crystal formation. Its removal is directly related to the removal of calcium. Therefore, for effective removal of strontium, it is imperative that the calcium hardness be reduced to a very low value. Such a requirement suggests a considerable excess of soda ash in the water during treatment and an initial reduction of calcium hardness to a low value. A system of repeated additions and precipitations of small quantities of calcium could then be used to reduce the radioactive strontium to a very low amount. This is the so-called "repeated-precipitation" process.

22

Although only 80 to 90% of the strontium can be removed by the initial softening reaction, a secondary process, which provides for the addition and elimination of small quantities of calcium in several steps, removes an equal percentage of activity in each stage. For instance, in a 10-stage plant, each step of which has a removal efficiency of 50%, 99.9% of the initial activity is removed. In 15 such stages, 99.996% of the initial activity is removed. These removals are ideal; in practice, however, where one treats highly variable chemical solutions, it may not be possible to obtain them.

In hot-lime and soda-ash softening at temperatures above 90°C, the addition of enough soda ash to produce a 50 mg/liter excess (as CaCO3) over that amount required to react with noncarbonate hardness and