thorium processing

Acidic and alkaline digestion

Although monazite is very stable chemically, it is susceptible to attack by both strong mineral acids (e.g., sulfuric acid, H₂SO₄) and alkalies (e.g., sodium hydroxide, NaOH). In the acid treatment, finely ground monazite sand is digested at 155 to 230 °C (310 to 445 °F) with highly concentrated (93 percent) H₂SO₄. This converts both the phosphate and the metal content of the monazite to water-soluble species. The resulting solution is contacted with aqueous ammonia, first precipitating hydrated thorium phosphate as a gelatinous mass and then metathesizing the thorium phosphate to thorium hydroxide. Finally, the crude thorium hydroxide is dissolved in nitric acid to produce a thorium nitrate-containing feed solution suitable for final purification by solvent extraction (see below).

In alkaline digestion, finely ground monazite sand is carefully treated with a concentrated NaOH solution at 138 °C (280 °F) to produce a solid hydroxide product. Any one of several mineral acids is then used to dissolve this solid residue. For example, treatment with hydrochloric acid yields a solution of thorium and rare earth chlorides. Conventionally, thorium is partially separated from the rare earths by addition of NaOH to the acidic chloride solution. The crude thorium hydroxide precipitate is then dissolved in nitric acid for final purification by solvent extraction.

Solvent extraction

For the purification of thorium from residual rare earths and other contaminants present in nitric acid feed solutions, the crude thorium nitrate concentrate is usually contacted with a solution of tributyl phosphate diluted by a suitable hydrocarbon. The resulting organic extract, containing the thorium (and any uranium that may be present), is then contacted countercurrently with a small volume of nitric acid solution in order to remove contaminating rare earths and other metallic impurities to acceptable levels. Finally, the scrubbed tributyl phosphate solution is contacted with a dilute nitric acid solution; this removes, or strips, thorium from the organic solvent into the aqueous solution while retaining uranium (if present) in the organic phase. Thermal concentration of the purified thorium nitrate solution yields a product suitable for the fabrication of gas mantles (see below Chemical compounds). The nitrate can also be calcined to ThO₂, which is incorporated into ceramic fuel elements for nuclear reactors or is converted to thorium metal.

Reduction to the metal

Powdered ThO₂ can be fluorinated with gaseous hydrogen fluoride (HF), yielding thorium tetrafluoride (ThF₄). The metal is obtained by the Spedding process, in which powdered ThF₄ is mixed with finely divided calcium (Ca) and a zinc halide (either zinc chloride or zinc fluoride) and placed in a sealed, refractory-lined “bomb.” Upon heating to approximately 650 °C (1,200 °F), an exothermic reaction ensues that reduces the thorium and zinc to metal and produces a slag of calcium chloride or calcium fluoride:

After solidification, the zinc-thorium alloy product is heated above the boiling point of zinc (907 °C, or 1,665 °F) but below the melting temperature of thorium. This evaporates the zinc and leaves a highly purified thorium sponge, which is melted and cast into ingots.

Conversion to uranium-233

When bombarded by thermalized neutrons (usually released by the fission of uranium-235 in a nuclear reactor), thorium-232 is converted to thorium-233. This isotope decays to protactinium-233, which in turn decays to uranium-233:

The fissile properties of uranium-233 can be utilized immediately or after recovery from the irradiated reactor fuel.

Uranium-233 can be recovered and purified from neutron-irradiated thorium reactor fuels through the thorium extraction, or Thorex, process, which employs tributyl phosphate extraction chemistry. Irradiated fuel, containing either thorium metal or oxide, is dissolved in nitric acid containing a small amount of fluoride ion. Uranium-233 and thorium are coextracted into a tributyl phosphate solution, which is then contacted with an aluminum nitrate solution to remove traces of accompanying fission products. Dilute nitric acid is used to preferentially remove thorium from the scrubbed organic phase. Uranium-233 remaining in the tributyl phosphate solvent is stripped into acidified water; the resulting strip solution is passed through an ion-exchange resin bed in order to concentrate and purify the uranium-233.

Thorium nitrate

Aqueous solutions of highly purified thorium nitrate, Th(NO₃)₄, are produced when thorium ores are processed (see above Extraction and refining). The nitrate is extensively used in the commercial production of gas mantles. Such mantles are made by impregnating cotton or synthetic fibres with a 25 to 50 percent solution of Th(NO₃)₄ containing 0.5 to 1 percent each of thorium sulfate and cerous nitrate. The impregnated fibres are treated with aqueous ammonia, producing thorium hydroxide, Th(OH)₄, and this compound is calcined to produce ThO₂. The latter substance, when heated, emits brilliant white light. The added cerous nitrate improves spectral emission properties, while the small amounts of thorium sulfate yield mantles with improved mechanical properties.

Thorium dioxide

The only other thorium compound of any industrial significance is ThO₂, known as thoria. For nuclear applications, thoria is prepared by calcination of thoroughly purified Th(NO₃)₄. Thoria also finds some application as a refractory material in various high-temperature processes.