Two recent patents [М2, М3] by J. A. Megy describe a process in which zirconium metal is reduced from a salt and separated from hafnium in the same step, thus shortening the long series of steps in present processes for producing reactor-grade zirconium from natural zircon.

The Megy process has evolved from the finding of Petenev and Ivanovskii [PI ] that when a mixture of K3ZrF6 and K2HfF4 dissolved in molten alkali chlorides was reduced electro — lytically at a molten zinc cathode, the metal phase was enriched in zirconium relative to the
residual salt. Megy found that when a mixture of Na2ZrF6 and Na2HfF6 is reduced by aluminum dissolved in liquid zinc, a very high separation factor between hafnium and zirconium is obtained, with very little contamination of the zirconium by aluminum. Examples given in Megy’s patent [М2] indicate that the ratio of zirconium to hafnium in the metal phase may be as high as 328 times the ratio of these elements in the residual salt phase. This high separation factor, a = 328, makes possible production of reactor-grade zirconium containing less than 0.01 w/o hafnium from typical natural zirconium containing 2 w/o hafnium in two stages of salt-metal contact, such as shown in Fig. 7.9.

Material quantities in Fig. 7.9 are based on production of 1.000 mol zirconium at point 6. Feed to this process (point 1) consists of 1.058 mol Na2ZrF6 and 0.011 mol Na2HfF6, corresponding to 2 w/o hafnium in zirconium + hafnium. This feed is combined with 0.0558 mol Na2ZrF6 and Na2HfF6 of the same composition (point 5) recycled from a previous batch. In step A the salt feed is reacted in a graphite-lined container at 900° C with a metallic solution of 4 w/o aluminum in molten zinc containing 1.4078 mol aluminum. Reactions taking place are

3Na2 ZrF6 + 4Al(Zn) -*• 4(NaF)1<s A1F3 + 3Zr(Zn)

and 3Na2HfF6 + 4Al(Zn) -* 4(NaF)i. sAlF3 + 3Hf(Zn)

At the reaction temperature the products, (NaF)1.sAlF3 and the solution of zirconium and hafnium in zinc, are two immiscible liquids. The lighter salt phase, enriched in hafnium to 27 w/o, is drawn off at point 4, leaving a heavier metallic zinc solution of zirconium containing

0. 1126 w/o hafnium at point 3. These compositions are consistent with the separation factor of 328:

(27X100-0.1126) (?

(100-27X0.1126)

To reduce the hafnium content below the 0.01 w/o specified for reactor-grade zirconium, the solution of zirconium and hafnium in molten zinc at point 3 is contacted in step В with a liquid mixture of 0.1116 mol ZnF2 + and 0.1116 mol NaF. Reactions

Zr + 2ZnF2 + 2NaF -*• Na2ZrF6 + 2Zn

+Alternatively, an equivalent amount of natural sodium fluozirconate could be used, with only slight increase in hafnium content of product zirconium.

0.0 5 8 265 Na2Zr F6 0.011006 Na2Hf Fg 15.89 m/o Hf 26.99 w/o Hf I 4078 (NoR и A I Fj

0.0552 Na2Zr F6 0.0005759 Na2 Hf Fg 1.0 3 22 m/о Hf 2 0000 w/o Hf

Reactor — grade Zr in Zn

1.0000 Zr

0.0000318 Hf

0.003 18 m/o Hf

000622 w/o Hf

14.0570 Zn Solvent (Recycled!

Figure 7.9 Megy process for producing reactor-grade zirconium from natural sodium fluozircon — ate. Material quantities in moles. Basis, 1 mol zirconium product.———————————————————————————- salt;——- metal.

and Hf + 2ZnF2 + 2NaF -*■ Na2HfF6 + 2Zn

take place, with most of the hafnium and a small fraction of the zirconium reacting. The hafnium content of the residual zirconium metal at point 6 is 0.00622 w/o, thus meeting the

0. 01 w/o specification of reactor-grade zirconium.

The hafnium content of the salt at point 5 is at the feed level of 2 w/o, again satisfying the separation factor condition:

(2X100 — 0.00622) _

(100 — 2X0.00622) ~ 328 (7-5)

The salt at point 5 is recycled to step A of a later batch. The zinc solvent at point 6 is distilled from the zirconium product and recycled to a later batch at point 2.

The mixture of (NaF)i. sAlF3, Na2ZrF6, and Na2HfF6 is converted to more useful by-products by reduction with 4 w/o aluminum in zinc in step C. This produces a zirconium and hafnium-free fluoride salt by-product 7, (NaF)i.5AlF3, and a solution of 27 percent Hf, 73 percent Zr in zinc, 8. The (NaF)1.sAlF3 can be sold as a substitute for cryolite Na3AlF6, in electrolytic production of aluminum. The zinc can be distilled from the 27 percent Hf, 73 percent Zr and recycled to point 9, and the hafnium-zirconium alloy can be sold for metallurgical applications in which the high cross section of hafnium is not harmful.

Figure 7.10 shows how the Megy selective reduction process can be combined with K2SiF6
fusion to produce reactor-grade zirconium from zircon ore. The ore is fused with K2SiF6 in a graphite-lined arc furnace A at 1000°C to convert zirconium to K2ZrF6 and K2HfF6:

K2SiF6 + (Zr, ffi)Si04 ->■ K2 (Zr, Hf)F6 + 2Si02

Potassium is preferred to sodium because the potassium complex fluorides are more stable at this temperature. The K2(Zr, Hf)F6 is dissolved in water and filtered from insoluble Si02 at В and crystallized at C. To recover the relatively expensive potassium, the K2(Zr, Hf)F6 crystals are dissolved in heated NaCl brine and cooled to precipitate the less soluble Na2(Zr, Hf)F6 at D:

K2(Zr, Hf)F6 + 2NaCl -*■ Na2(Zr, Hf)F6 + 2KC1

The Na2(Zr, Hf)F6 is converted to zirconium metal by the Megy process described earlier. The KC1 is recycled and converted to K2SiF6 by countercurrent metathesis with purchased Na2SiF6 at E.

2KC1 + Na2SiF6 -*• K2SiF6 + 2NaCl

In this way zircon, Na2SiF6, and aluminum are converted to zirconium metal, hafnium-rich zirconium, and by-product (NaF)i.5AlF3 and Si02.