Page 118 - Radiochemistry and nuclear chemistry

104 Radiochemistry and Nuclear Chemistry

Because thorium often occurs with other valuable metals (in addition to the lanthanides)
such as niobium, uranium and zirconium, it can be produced as a byproduct.
The following procedure is used for producing thorium from monazite sand. The sand is
digested with hot concentrated alkali which converts the oxide to hydroxide. The filtered
hydroxide is dissolved in hydrochloric acid and the pH adjusted between 5 and 6, which
precipitates the thorium hydroxide but not the main fraction of lanthanide dements. The
thorium hydroxide is dissolved in nitric acid and selectively extracted with methyl isobutyl
ketone or tributyl phosphate in kerosene. This gives a rather pure organic solution of
Th(NO3) 4. The thorium is stripped from the organic phase by washing with alkali solution.

5.4.3. Uses

Thorium metal is used as electrode material in gas discharge lamps, and as getter for
absorption of rest gases in high vacuum technique. ThO 2 (melting point 3300~ is highly
refractory and used for high temperature furnace linings. Thorium salts are of little practical
use. Because Th 4+ is a stable tetravalent ion with properties very similar to the tetravalent
actinides, Th 4+ is often used as an analog for the An(IV) ions; most common to use are
228Th (1.91 y) or 23~ (7.54 x 104 y) and 234Th (tt,~ 24.5 d), which can be isolated from
old 232Th or 238U, see Fig. 1.2. Thorium may become important to the nuclear energy
industry as a fuel in high temperature gas-cooled reactors and may be used in the future in
thorium-breeAer reactors (Ch. 20).

5.5. Uranium

5.5.1. Isotopes

Natural uranium consists of 3 isotopes, 234U, 235U and 238U, members of the natural
decay series discussed in w Uranium is an important raw material for nuclear energy
production (see Ch. 19 and 21).
The specific radioactivity of natural uranium makes it a weak radiological hazard (see Ch.
18). It is also chemically toxic and precautions should be taken against inhaling uranium
dust for which the threshold limit is 0.20 mg/m 3 air (about the same as for lead).

5.5.2. Occurrence, resources and production capacity

Uranium appears in a large number of minerals (at least 60 are known). The earth's crust
contains 3 - 4 ppm U, which makes it about as abundant as arsenic or boron. Uranium is
found at this relative concentration in the large granitic rock bodies formed by slow cooling
of the magma about 1.7 - 2.5 cons ago (1 con = 109 y = 1 billion years). It is also found
in younger rocks at higher concentrations ('ore bodies').
Geochemists now begin to understand how these ore bodies were formed. They are
usually located downstream from mountain ranges. As the atmosphere ~me oxidizing
about 1.7 cons ago, rain penetrated into rock fractures and pores, bringing the uranium to