Hafnium Kroll process

It was originally separated from zirconium by repeated recrystallization of the double ammonium or potassium fluorides by von Hevesey and Jantzen. Metallic hafnium was first prepared by van Arkel and deBoer by passing the vapor of the tetraiodide over a heated tungsten filament. Almost all hafnium metal now produced is made by reducing the tetrachloride with magnesium or with sodium (Kroll Process). 

Table 3. Analysis of Kroll Process, Electrowon, and Refined Hafnium, ppm...

Refractory metals Zirconium Hafnium Titanium Kroll process, chlorination, and magnesium reduction Chlorine, chlorides, SiCli Wet scrubbers... 

Electrolysis. Electro winning of hafnium, zirconium, and titanium has been proposed as an alternative to the Kroll process. Electrolysis of an all chloride hafnium salt system is inefficient because of the stability of lower chlorides in these melts. The presence of fluoride salts in the melt increases the stability of Hf4+ in solution and results in much better current efficiencies. Hafnium is produced by this procedure in France (17). 

Refining. Kroll-process hafnium sponge and electrowon hafnium do not meet the performance requirements for the two principal uses of hafnium metal. Further purification is accomplished by the van Arkel-de Boer, ie, iodide bar, process (18) and by electron beam melting. 

Zirconium metal (mp 1855°C 15°C), like titanium, is hard and corrosion resistant, resembling stainless steel in appearance. It is made by the Kroll process (Section 17-A-l). Hafnium metal (mp 2222°C 30°C) is similar. Like titanium, these metals are fairly resistant to acids, and they are best dissolved in HF where the formation of anionic fluoro complexes is important in the stabilization of the solutions. Zirconium will burn in air at high temperatures, reacting more rapidly with nitrogen than with oxygen, to give a mixture of nitride, oxide, and oxide nitride (Zr2ON2). 

Derivation The ore is converted to a cyanonitride and is chlorinated to obtain zirconium tetrachloride. This is reduced with magnesium (Kroll process) in inert atmosphere. The metal can be prepared in a highly pure and ductile form by vapor-phase decomposition of the tetraiodide. Hafnium must be removed for uses in nuclear reactors... 

Chlorination of zircon has been the process mainly used in the United States because it produces ZrCU, which is used in the Kroll process for making zirconium metal (Sec. 8.3), and because ZrCU was the feed material for the first process developed for separating hafnium from zirconium, using thiocyanate extraction (Sec. 7.3). 

Zircon silicate is the most important source of hafnium. Ion-exchange and solvent-extraction techniques have supplanted fractional crystallization and distillation as the preferred methods of separating hafnium from zirconium. The metal itself is prepared by magnesium reduction of hafnium tetrachloride (the Kroll process), and by the thermal decomposition of tetraiodide (de Boer-van Arkel process). The annual world production of hafnium metal was about 40 tons at the end of the 1980s (Soloveichik... 

The most important commercial methods of production of metallic zirconium and hafnium are based on furnace reduction, either of MCI4 with magnesium or a Na-Mg mixture (the Kroll process), yielding a metal sponge, or of metal fluorides with Ca at 2000 °C to give an ingot. The... 

For hafnium electrowinning, electrolysis process is readily preferred to Kroll process Cezus developed the process in pure chloride media from HfCl4, reduced in Hf in a one single step [24]. However, the mechanism is controversial, other authors proposing two steps in pure chloride and a single step in chloride-fluoride melts [25]. 

The separation technique described and the Kroll process created the possibihty of manufacturing hafnium-free zirconium on an industrial scale for atomic energy... 

Most zirconium is used as an oxide in commercial applications. Only a few percent is converted to the metal and used in chemical process industries because of its excellent corrosion resistance, while a special grade of zirconium is used in the nuclear industry. There are no official statistics for the production and consumption of zirconium metal. The annual global production capacity is estimated approximately at 85001, and total production/consumption is about 7000 t/year. The main applications of zirconium metal are for the nuclear energy and chemical process industries. About 85% of zirconium metal, 5000-6000 t/year, is used in the nuclear industry. Commercial-quality zirconium still contains 1 -3% hafnium. This contaminant is unimportant except in nuclear applications. For nuclear reactor materials, the zirconium metal should have a very low hafnium content of less than 0.01 wt%. Most Zr metal is produced by the reduction of the zirconium (ZrCy chloride with magnesium metal in the Kroll process. 

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