Purifying Thorium Nitrate by Solvent Extraction
IN
NATURE, thorium is generally associated with the rare earth elements, some of which have extremely high thermal neutron-absorption cross sections. Because the chemistry of these materials is similar, their separation is difficult. As with uranium, however, thorium can be selectively extracted from an impure aqueous nitrate solution by organic materials such as tributyl phosphate. Using this mechanism, a liquidliquid extraction process was developed for purifying commercial, mantle-grade thorium nitrate. This work was done to provide basic process-design data to the Atomic Energy Commission for the Feed Materials Production Center thorium semiworks plant being built a t Fernald, Ohio, in late 1951 and early 1952. In three interconnected mixer-settler units, each consisting of 10 mixer-settler stages in series, counterflowing, immiscible aqueous and organic solutions were repeatedly contacted and separated. I n mixer-settler 3, thorium nitrate was selectively transferred from aqueous feed stream to the organic extractant stream, and in mixer-settler 2, minor traces of impurities were scrubbed from the resulting thorium-laden organic stream with aqueous nitric acid. I n mixer-settler 1, purified thorium was stripped from the organic stream into an aqueous stream for further processing. The organic extractant used was a nominal 30 volume % ' tributyl phosphate solution in a commercial hydrocarbon diluent, Solvesso-100 [Standard Oil Co.
Solvent-extraction pilot plant. mixer-settler units Course of thorium
(Ohio)]. Salting agent was nitric acid. The organic extractant was recovered, with low loss, purified by simple chemical treatment with soda ash, and re-used. Unfortunately, packed, spray, and pulse columns, then favored for countercurrent liquid-liquid extraction, were not readily adaptable to the stage-by-stage elucidation needed for the Fernald design. The mixer-settlers did permit stage-by-stage sample analysis during steady-state operation, which in turn led to a clearer picture of the nature of the extraction process. The mixer-settler units were of the centrifugal-pump impeller type, built from a design developed a t Knolls Atomic Power Laboratory (7, 2). With these units, the pump impeller and mixing chamber design was such that liquid-liquid interface Ievel control was automatic within each unit of 10 stages, and only the end stages needed monitoring. The total liquid holdup volume of each stage was only about 2 gallons. This permitted fairly large samples of uniform product to be made without requiring extremely long times or large quantities of solutions, to come to steady state operation. The thorium nitrate product, analyzed spectrographically a t Battelle and by neutron absorption techniques a t the AEC laboratory in New Brunswick, N. J., exceeded the AEC specifications for nuclear fuel purity for both physically and chemically undesirable contaminants. The product was not analyzed for uranium which was present, largely because methods were known for separating
The heart of the process i s three interconnected Aqueous streams
T,tank; P, pump; and R, rotameter
9
-
~
=
0
Organic streoms
1
,i' , J 7
i
From sttll
Aqueous
1
r i i Distilled
ond Solvesso-100
144
INDUSTRIAL AND ENGINEERING CHEMISTRY
$4
Average Flows and Composition
(Typical run) ~ l Compn. ~ ~GJL., Gal./&. Th "03 Aq. feed
Org. feed Scrub acid Strip water Org. ra5nate Aq. ra5nate Aq. product
1.608 16.95 0.68 9.0 16.35 2.01
10.00
435
... ... ...
0.7 0.4 67.5
96.4 39.1 95.1
...
3.1 97.7 106.4
Thorium Nitrate Analysis (Neutron absorption) Th, P.P.M.
Element B Cd co
Mo Ce
a
Feeda
14
