THORIUM AS AN ENERGY SOURCE - Opportunities for Norway ...

4.1.1 Mining and Extraction
The Front End of the Thorium Fuel Cycle
Thorium is found in a number of minerals. Therefore, there are several basic process flow sheets
with modifications that can be considered for metal recovery [24]. The thorium is often associated
with other minerals and may be present as a by-product of another process. Examples of this are
the beach sands concentrates in which titanium and zirconium may be the metals of primary
interest, or the processing of uranium ores, such as those found in the Elliot Lake area of Canada,
where associated thorium can be recovered as a by-product. Hence there are several process
alternatives.
Physical and Magnetic Separation: Individual minerals - ilmenite, rutile, monazite, zirconium,
sillimanite and garnet - are separated by utilizing the differences in their physical properties, i.e.
specific gravity, magnetic susceptibility, electric conductivity and surface properties. The wet
concentrate is passed through rotary dryers at up to 150ºC, the dried feed then being
electrostatically or electromechanically treated. The electrically conducting ilmenite and rutile
constituents are separated first. The non-conducting monazite, being heavy and moderately
magnetic, is isolated by high intensity magnetic separators. The resultant concentrate contains
98 % monazite. Usually, feeds containing 1 - 2 % of heavy minerals found in combination with
monazite can be concentrated to 90 % heavy minerals with an overall recovery of 85 - 90 %.
Heavy Metal Chemical Extraction: The chemical properties of thorium and rare earth elements
associated with it are similar, so their separation is difficult and time consuming. Concentrates of
the rare earths must be chemically processed to separate them from the other components
forming the mineral and from impurities. Monazite, the chief commercial ore from which thorium
is extracted, is chemically inert and any chemical treatment for extracting thorium must initially
be very severe to achieve the complete dissolution necessary for the separation of the rare earth
elements, uranium and phosphates. The most common dissolution processes are: (1) acidic – using
highly concentrated sulphuric acid [25], and (2) alkaline – using highly concentrated sodium
hydroxide [22].
4.1.2 Refining
For nuclear applications thorium has to meet stringent requirements of purity, particularly
concerning neutron absorbing elements. This purity is obtained industrially by solvent extraction,
by ion exchange, or by direct chemical precipitation.
Solvent Extraction: This process makes use of the different solubilities of some heavy metal
compounds in organic solvents which are immiscible with water. Successive transfer operations
between the aqueous and organic phases lead to a relative concentration of one heavy metal in
the aqueous phase and another in the organic phase. In purifying thorium by this process, the
crude thorium produced is converted to a nitrate which exists in an aqueous phase. Contact
between the aqueous carrier and the organic solvent (tributyl phosphate (TBP) in kerosene or
hexane) transfers thorium to the solvent from which it is stripped by scrubbing with dilute nitric
acid. Nuclear-grade thorium nitrate is obtained with more than 99 % thorium purity.
Ion Exchange: This is an effective way of separating individual lanthanides in a pure state.
Direct Precipitation: Either foreign ions are added to precipitate insoluble salts, or variations in
solubility with pH of complex salts are exploited.
If there is uranium present with thorium and rare earth elements, a more sophisticated
extraction flow sheet, with two TBP concentrations and appropriate scrubbing and stripping
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