Page 121 - Radiochemistry and nuclear chemistry

Radionuclides in Nature 107

TABLE 5.3. Chemical bases for the Ames (amine) and Dapex (HDEHP) processes for uranium recovery from
sulfuric acid leach liquors. (From Musicas and Schulz.)

AMEX DAPEX
Extraction UO22+ + SO42- + 2(R3NH)2SO 4 ~=~ UO22+ 4- 2(HDEHP)2
(R3NH)4UO2(SO4) 3 UO2(HDEHP.DEHP) 2 + 2H +
R3N = Alamine 336 or
Adogen 364 (trialkylamines
with alkyl groups with
8 - 10 carbons)
Stripping Acidic stripping Alkaline stripping
(R3NH)4UO2(SO4) 3 + 4HX UO2(HDEHP.DEHP) 2 + 4Na2CO 3
4R3NHX + UO22+ + 3HSO 4- + H + UO2(CO3)34- + 4NaDEHP + H20 +
HX = HCi or HNO 3 CO 2 t + 4Na +
Neutral stripping (TBP must be added to avoid
(R3s~4uo2(s04) 3 + 0~I-I4)2s04 + the third phase formation
4 NH 3 ~ 4R3N + UO2(SO4)34- + owing to low organic phase
6NH4 + + SO42- solubility of NaDEHP.)
Alkaline stripping Acidic stripping
(R3NH)4UO2(SO4) 3 + 7Na2CO 3 UO2(HDEHP.DEHP) 2 + 2H +
4R3N + UO2(CO3)34- + 4HCO 3- + 2(HDEHP) 2 + UO22+
3SO42" + 14Na +

The chemistry of aqueous uranium is discussed in w together with the chemistry of
the other aetinides. Production of reactor fuel and reprocessing is described in Chapter 21.

5.5.4. Production wastes
The milling operation produces tailings consisting of fine-coarse particles in a water slurry
containing most of the radioactive uranium decay products, of which radium is the most
hazardous. As most tailings are not highly radioactive (e.g. from mining of low grade ores)
they are dumped on outside the plant; leach water from the dumps will then, combined with
mine water, enter local streams. In dry areas, dusts from the tailings may spread by winds.
This is the main picture and explains the high dose commitment values in Table 22.1.
However, many health authorities now require conditioning of the tailings to reduce harmful
effects to the environment: recycling of waste water, precipitation and removal of radium
from solution, and neutralization to precipitate heavy metals. The slurry is usually
transported to an impoundment basin where the solid particles settle out, and the effluent
is treated for removal of activities before discharge into a settling pond. Treatment ponds
may contain low permeability liners where appropriate to control seepage. Barium chloride
may be added to the tailings to precipitate dissolved 226Ra as Ra-Ba sulfate. Lime and
limestone may be added to the tailings to raise pH.
The final dry waste is stored either on surface or in shallow basins, though underground
storage also has been practiced (abandoned mines). In the former case, the tailings are
covered by up to 3 meters of earth fill to restrict erosion, and some water tight material to
protect it against rain. In some cases (e.g. Sweden) the land on top of mine waste and
tailings have been reclaimed for farming.