Purified thorium is usually produced in the form of an aqueous solution of thorium nitrate or crystals of hydrated thorium nitrate. The principal forms in which thorium is used in nuclear systems are the oxide Th02, the carbide ThC2, the fluoride ThF4, the chloride ThCl4, or the metal. Conversion to oxide, fluoride, chloride, and metal are discussed in this section; production of thorium carbide was discussed in Sec. 5.3.

1.14 Conversion of Thorium Nitrate to Th02

Three methods that have been used to convert thorium nitrate to Th02 are as follows:

1. Thermal denitration,

Th(N03)4-4H20 hea‘ > Th02 + 4HN03 + 2H20

2. Precipitation of thorium hydroxide from aqueous solution with NH3,

Th(N03)4 + 4NH3 + 4H20 -*• Th(OH)4 + 4NH4N03 followed by ignition of the hydroxide,

Th(0H)4 heat > Th02 + H20

3. Precipitation of thorium oxalate with oxalic acid,

Th(N03)4 + 2H2C204’2H20 -> Th(C204)2-2H20 + 4HN03 + 2H20

followed by ignition of the oxalate in air,

Th(C204)2 -2H20 + 02 heat—> Th02 + 4C02 + 2H20

Precipitation with NH3 has been used to prepare a colloidal sol of Th(0H)4 for formation into small spheres of Th(OH)4 gel, in the so-called sol-gel process, followed by ignition to small Th02 spheres [Zl] to be incorporated in fuel elements for an HTGR (Chap. 3, Sec. 7.3).

Precipitation with oxalic acid followed by ignition to oxide has the advantage of separating thorium from several impurities (uranium, iron, and titanium) that remain in nitric acid solution. Following is a brief description of the process developed at Iowa State College [W2], pp. 70-75, for the U. S. Atomic Energy Commission (ЛЕС) and used on a production scale by the National Lead Company at Femald, Ohio [C6], pp. 150-152. To an aqueous solution of Th(N03)4 containing about 200 g thorium/liter and 0.5 N in HN03 at 60°C is added about 105% of the oxalic acid, H2C204*2H20, needed to convert all thorium to the oxalate. The solution is stirred for about 5 min to complete precipitation. This results in an easily filtered crystalline precipitate of Th(C204)‘2H20. This is filtered on a vacuum filter and washed with about half the feed solution volume of distilled water at 35°C. The precipitate is dried in a twin-screw drier with a jacket temperature of 120°C and a screw temperature of 154°C to a water content of 10 w/o.

The dried oxalate is converted to oxide in an externally fired rotary kiln, with counterflow of air. The exit gas temperature is controlled at 820°C. This produces a reactive, free-flowing oxide containing less than 0.5% carbon and about 0.5% moisture.