Zirconium electrolysis

ZIRCONIUM ELECTROLYSIS FROM A CHLORIDE-FLUORIDE MELT... 

Fig. 7.3. Zirconium Electrolysis cell (Shelton, S. M., Zirconium Production Methods, Metallurgy of Zirconium, McGraw-Hill. U.S.A.E.C. Copyright).

Zirconium trifluoride [13814-22-7], ZrP, was first prepared by the fluorination of ZrH2 using a mixture of H2 and anhydrous HP at 750°C (2). It can also be prepared by the electrolysis of Zr metal in KF—NaF melts (3). Zirconium trifluoride is stable at ambient temperatures but decomposes at 300°C. It is slightly soluble in hot water and readily soluble in inorganic acids. This compound is of academic interest rather than of any industrial importance. 

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

A number of attempts to produce tire refractory metals, such as titanium and zirconium, by molten chloride electrolysis have not met widr success with two exceptions. The electrolysis of caesium salts such as Cs2ZrCl6 and CsTaCle, and of the fluorides Na2ZrF6 and NaTaFg have produced satisfactoty products on the laboratory scale (Flengas and Pint, 1969) but other systems have produced merely metallic dusts aird dendritic deposits. These observations suggest tlrat, as in tire case of metal deposition from aqueous electrolytes, e.g. Ag from Ag(CN)/ instead of from AgNOj, tire formation of stable metal complexes in tire liquid electrolyte is the key to success. 

In the brine electrolysis system, silica is also contained in raw salt. Silica will precipitate on to membranes in the presence of calcium, strontium, aluminium and iodine resulting in the loss of current efficiency [8-10]. Silica can also be removed in a column filled with ion-exchange resin containing zirconium hydroxide, just like the iodide ion. 

As was mentioned previously, an effective system, RNDS , has been developed to remove particular impurities from brine used in membrane electrolysis procedures. The basic concept of RNDS is to bring the feed brine into contact with an ion-exchange resin containing zirconium hydroxide for the adsorptive removal of impurities. For the removal of the sulphate ion from brine, commercial plants utilising RNDS are already in service. For the elimination of iodide and silica, pilot-scale testing is being planned. 

The direct electrochemical synthesis of metal alkoxides by the anodic dissolution of metals into alcohols containing conducting electrolytes was initially demonstrated by Szilard in 1906 for the methoxides of copper and lead.19 More recently the method has received some attention particularly in the patent literature.29-25 The preparation of the ethoxides of silicon, titanium, germanium, zirconium and tantalum by electrolysis of ethanolic solutions of NH Cl has been patented, although the production of the ethoxides was found to cease after several hours.24,25... 

Hudgens and Cali 38) have described a determination of antimony at the parts per million level in zirconium oxide. Distillation and solvent extraction were used in the radiochemical purification and antimony was mounted, prior to counting, by electrolysis onto platinum foil. Atchison and Beamer (S) have determined arsenic, phosphorus, copper, sodium, strontium, calcium, and chromium in pure magnesium. 

Titanium diboride exhibits a high melting point, electronic conductivity, wetability by molten aluminum, and a resistance towards chemical attack of aluminum and molten fluorides. Due to these properties, TiB2 is considered to be the most promising material for inert cathodes in aluminum electrolysis. Also zirconium diboride belongs to the category of promising constructive materials due to its favorable properties. 

Zirconium(iii) and hafnium(iii) species have been generated in solution by the sodium amalgam reduction of ZrC in acetonitrile and by electrolysis of the same solution using a mercury cathode. ... 

Neutral zirconium sulfate, Zr(S04)2, is prepared by heating zirconia with an excess of concentrated sulfuric acid arid then expelling the excess acid. This salt dissolves slowly in water with the evolution of heat, presumably forming a tetrahydrate, which may be obtained in crystalline form. The normal sulfate is easily hydrolyzed, yielding a solution with a strong acid reaction. Such a solution yields no precipitate or one that forms slowly when oxalie acid or ammonium oxalate is added, while both these reagents produce an immediate precipitate when added to most zirconium salts. On electrolysis of a sulfate solution zirconium concen ... 

Electrolytical reduction of Tc-pertechnetate has been investigated (Benjamin 1969, 1970 Dworkin and Gutkowski 1971 Eckelman et al. 1971a Gil et al. 1976). When using zirconium or tin electrodes, anodic dissolution of metal ions produced in situ reduction of pertechnetate (Steigman et al. 1974). Electrolysis has been used as a reliable method for laboratory production of Tc pharmaceuticals. 

In electrocatalysis the use of metal-zirconium alloys in water electrolysis is a good example. It seems to be well established that the very high electrocatalytic activity of the HF-treated amorphous Ni and Cu electrodes is related to the presence of a rough, Raney-type surface structure. The beneficial effect of the amorphous structure is that it ensures the ready formation of a thick, stable, active surface layer. 

Processes for making ductile zirconium by electrolysis of KjZrFj dissolved in molten chlorides have been described by Steinberg et al. in the United States [S4, R2] and by Ogarev et al. [01] in the Soviet Union. An advantage over the Kroll process is that a coarsely crystalline product is obtained from which coproduced halides can be removed by leaching with acidified water without undue contamination of zirconium by oxygen. The washed crystals are then vacuum dried and consolidated by arc melting. 

Steinberg et al. [S4], of Horizons, Inc., developed a process for making ductile zirconium by electrolysis of K2ZrFj dissolved in fused sodium chloride. The essential features of the process were as follows ... 

Hydrophilic treatment of the surfaces of a cation exchange membrane is essential to decrease the voltage drop between electrodes during electrolysis. Such treatments involve coating with inorganic materials, e.g., metal oxides (zirconium... 

High temperature cells with solid electrolytes of zirconium dioxide have been investigated for the electrolysis of H2O vapor and as fuel cells for the generation of electric power. Examples include the combustion of synthesis gas (CO-H2 mixtures) in cells at temperatures of 1300 K. 

In contrast to titanium and zirconium, the preparation of thorium metal via reduction of the oxide with calcium (method II) acquires increased importance and rivals the reduction of the tetrachloride with sodium (method I). Melt electrolysis (method III) is another possibility. Neglecting the small oxide content (up to 1%), which in any case has never been determined precisely, the metal obtained by any of the three methods is already quite pure and contains only 0.1-0.2% of other impurities. The Th prepared by the refining process (method IV), is definitely oxygen-free and should in any case yield the purest product. 

Exotic metals such as titanium and zirconium are produced from their respective chlorides, which are formed by reacting chlorine with the oxides. Thus, metallic titanium is produced by reacting TiCLj with molten sodium or magnesium, while metallic zirconium is made by reaction of ZrCU with magnesium. Molten salt electrolysis of magnesium chloride generates magnesium metal and chlorine, and fused salt electrolysis of sodium chloride and calcium chloride mixtures produces metallic sodium. 

In recent years progress has been made in the manufacture of zirconium by electrolysis such that satisfactory nuclear grade metal can be produced. The economies of the process probably allow large scale production at a similar cost to the rival Kroll type reduction process. 

The rate of electrolysis is this near-commercial scale cell is 5-6 Ib/hr of zirconium at a current efficiency of about 60 per cent, and the overaU rate of production is 2-5 Ib/hr. Oxygen contents range from 0-06 to 0-09 per cent, nitrogen is about 0-003 per cent and carbon contents are about 0-02 to 0-06 per cent. Other impurities in typical electrolytic powder are as follows ... 

The stable double fluoride, K TiFe, dissolved in fused sodium chloride, has been employed by Messrs. Horizons Inc., by analogy with their zirconium process. Chlorine is evolved on electrolysis, leaving an electrolyte rich in fluorides which have too high an electrical resistance for satisfactory operation over a long period. Frequent replacement of the electrolyte is therefore necessary. 

---