The principal uses of ThF4 are as intermediate in the production of thorium metal or, potentially, as a compound in the fuel mixture of the molten-salt breeder reactor. For both applications anhydrous, oxide-free ThF4 is required.

Such ThF4 cannot be made by precipitation from aqueous solution, as the precipitate contains water that, during heating or evaporation, hydrolyzes some ThF4 to Th02 or ThOF2. Instead, ThF4 is produced by gas-phase hydrofluorination of Th02 with anhydrous HF:

Th02 + 4HF ThF4 + 2H20

This reaction is exothermic and proceeds rapidly at 566°C, but the equilibrium gas mixture contains some unreacted HF (Prob. 6.3). At lower temperatures nearly complete utilization of HF can be obtained, but the reaction is slow. The process was developed at Iowa State College

Table 6.21 Principal processes for producing metallic thorium

Electrolysis of fused salts Electrolysis of KThFs in NaCl Electrolysis of ThF4 in NaCl/KCl Electrolysis of ThC^ in NaCl/KCl Reduction with Reactive Metals Reduction of Th02 with Ca Reduction of ThCU with Mg Reduction of ThF4 with Ca Thermal Dissociation of Thl4

and used industrially for the U. S. AEC by the National Lead Company at Femald, Ohio. A summary of the process, described in detail by Cuthbert [C6], pp. 152-154, follows.

Equipment consists of four externally heated, screw-fed, horizontal reactors positioned vertically one above another. The reactors are made of 309 Nb stainless steel, and the screw of Inconel and Illium R. Solids flow through the four reactors in series. In the first reactor, counterflowing air at 650 to 675°C removes residual H20 and C02. Anhydrous HF vapor enters the fourth reactor4 and flows counter to the solids through the fourth reactor held at 566°C, the third at 370°C, and the second at 260°C. In this way, Th02 can be converted completely to ThF4, the highly exothermic reaction can be controlled and most of the HF can be reacted. However, the process was usually operated to produce 70 w/o aqueous hydrofluoric acid, which was sold as a by-product.

10.2 Production of ТЪСЦ

Several of the processes for producing thorium metal start with anhydrous ThCL,. As with ThF4, anhydrous ThCL, cannot be prepared from aqueous solution, but must be made by gas-phase chlorination. A process used in England [B3] involved chlorinating a mixture of Th02 and carbon at or above 600°C:

Th02 + 2C + 2C12 ->• ThCL, + 2CO

The chloride must be purified by distillation to free it from unreacted solids and from impurities in the carbon. This is difficult because of the hygroscopicity of ThCl4 and its high boiling point, 942°C. An alternative process [C6] reacts thorium oxalate with an excess of carbon tetrachloride and a small amount of chlorine as catalyst,

Th(C204)2 + ССЦ -* ThCl4 + 2CO + 3C02

batchwise in a vertical graphite reactor at 600°C. This uses more expensive materials but produces pure solid ThCl4 in a single step.