The hot-wire process was developed by Van Arkel and de Boer [V2], who used it to produce the first pure, massive specimens of many refractory metals, notably titanium, zirconium, hafnium, and thorium. An interesting account of early uses of this process is given in

Figure 7.14 Hot-wire reactor for zirconium produc­tion.

Van Arkel’s book Reine Metalle [VI]. This method was used by the Foote Mineral Company and the Westinghouse Electric Corporation to produce the first zirconium pure enough for nuclear reactors.

The apparatus used by Foote is shown in Fig. 7.14. It consists of an Inconel tube through which are led insulated tungsten leads capable of carrying a heavy electric current. Inside the tube, the leads are connected to a thin tungsten wire. The tube is charged with crude zirconium and evacuated, and a few grams of iodine are distilled into it. The tube is heated to a temperature at which iodine reacts with the zirconium and at which the iodide produced has a vapor pressure of several torn. The tungsten wire is heated electrically to a temperature high enough to dissociate the iodide, but below that at which the metal melts or has a substantial vapor pressure. Tetraiodide, formed from the crude metal, diffuses through the iodine vapor and deposits pure metal on the tungsten wire. As the latter increases in cross section, the electric current through it is increased to keep it above the dissociation temperature of the iodide. The run is concluded when the tungsten leads are carrying the maximum possible current.

In this way rods, or “crystal bars,” of compact, ductile zirconium or hafnium have been prepared. The usual crystal bar is 0.25 to 0.4 in in diameter in lengths up to 2 ft, but Westinghouse and Battelle Memorial Institute have produced zirconium bars as large as 1.7 in in diameter and 50 ft overall length [HI].

The hot-wire process eliminates oxygen, nitrogen, and carbon, the impurities most difficult to keep out of zirconium in other processes, but other metals that form volatile iodides are not removed completely. The main disadvantage of the process is its low capacity, the rate of production being limited by the rate of diffusion of iodide vapor to the small wire. Temperatures used for producing metals of the IVA group by the hot-wire process are listed in Table 7.13.

Eighty-five percent of the zirconium used in the first land-based prototype of a submarine reactor was made by the hot-wire process. In 1952, the hot-wire process for zirconium was superseded by the lower-cost Kroll process. However, the hot-wire process is still used to produce hafnium for control rods in U. S. naval reactors.