Thorium is mainly extracted from the monazite ore via a multistage process. The first stage is the process of digestion that involves dissolving monazite sands in concentrated sulfuric acid (93-98%) at 120-150 °C for several hours [11]. As an alternative, alkaline digestion process can also be followed. In the digestion pro­cess, thorium, uranium, and rare earth metals pass into solution to form sulfates

in phosphoric acid. Following the digestion process, the resulting solution is diluted to pH of 1 using ammonium hydroxide, and all the thorium from solution gets precipitated out of the solution along with some rare earths. But subsequently increasing the pH to ~2.5, the rest of the rare earth metals and uranium also get precipitated. The precipitate residue is then collected and treated with nitric acid (solvent extraction process) and thorium compound is separated.

There are several methods to obtain thorium from thorium compounds. Metallic thorium can be obtained by reduction in a sealed container or bomb-reacting thorium tetrachloride (ThCl4) or tetrafluoride (ThF4) with calcium, sodium, or mag­nesium. Because thorium has a high melting point, zinc is often added to create a low-melting eutectic from which Zn is later distilled off under vacuum to obtain the so-called “bomb-reduced” thorium. A relevant chemical reaction is shown below:

ThF4 + Zn + 2Ca! Th-Zn + 2CaF2 (7.12)

Highly pure thorium (>99.9%) can be obtained by using an iodide treatment (DeBoer process).

The morphology of the bomb reduced thorium is sponge-like and iodide thorium is loosely packed crystals of highly pure thorium. That is why they need to be consoli­dated through ingot or powder metallurgy process. In ingot metallurgy, two types of methods are generally employed: induction melting/casting under vacuum and arc melting/casting. If thorium is low in oxygen, silicon, nitrogen, and aluminum impurities, it can be fabricated by various deformation processing techniques like extrusion, hot and cold rolling, hot forging, and swaging. However, wire drawing presents challenges since thorium has great tendency to stick with the drawing dies. In the powder metallurgy, thorium can be fabricated into cold compacts (with 95% of theoretical density) from powders produced by hydride method. Then, hot pressing under vacuum at 650 °C at a nominal pressure can produce almost full density metal. The machining of thorium has been found to be easier, especially with greater tool feed rate and low spindle speeds.