Zirconium extraction from zircon

De Wet A process proposed for extracting zirconium from zircon in the form of zirconium sulfate tetrahydrate. The sodium silicate byproduct should be free from radioactive elements. The key to this process is the formation of an insoluble basic zirconium sulfate as an intermediate. Invented by W. De Wet in South Africa. 

Ziiconium dioxide reacts with carbon to form the carbide ZrC. Zirconium dioxide is inert to the halogens, but when mixed with carbon reacts to form tetrahalides at hig temperatures. This is the basis of one process for extracting zirconium from zircon. 

Die foregoing processes for extracting zirconium from zircon, separating zirconium from hafnium, and reducing zirconium compounds to metal can be combined in a number of ways, some of which are shown in Fig. 7.15. The combination of processes used in the United States in 1978 is at the left of this figure. 

Most commercial thorium is extracted from monazite, which is not amenable to breakdown by dilute acids. Thorite, or thorium silicate, is an alternative thorium ore which occurs in fairly large quantities in some parts of the world, for example in the tailings from the tin beneficiation processes in Nigeria. The ore contains principally zircon (zirconium silicate), but this is of much smaller value than the 5 to 10 per cent thorite content. 

The other big problem was the hafnium content of natural zirconium. In the actual tubes the hafnium content must be lower than 100 ppm, because hafnium absorbs neutrons 550 times more strongly than zirconium. Zircon sand, the most important ore, has a hafnium content of 1.5-2.596 (relative to zirconium). Thus it was necessary to find a separation method with which hafnium-free zirconium could be manufactured. The solution of this problem was liquid-liquid extraction. From an impure zirconium solution in hydrochloric acid, iron is first removed by solvent extraction. Then ammonium thiocyanate is added to the chloride solution containing Zr -i- Hf. Hafnium is extracted into an organic phase of methyl isobutyl ketone. The pure zirconium solution in the water phase is worked up for zirconium. Hafnium is recovered from the ketone solution by scrubbing with dilute sulfuric acid. 

In France, Compagnie Europnene du Zirconium (CEZUS) now owned jointly by Pechiney, Eramatome, and Cogema, uses a separation (14) based on the extractive distillation of zirconium—hafnium tetrachlorides in a molten potassium chloride—aluminum trichloride solvent at atmospheric pressure at 350°C. Eor feed, the impure zirconium—hafnium tetrachlorides from the zircon chlorination are first purified by sublimation. The purified tetrachlorides are again sublimed to vapor feed the distillation column containing the solvent salt. Hafnium tetrachloride is recovered in an enriched overhead fraction which is accumulated and reprocessed to pure hafnium tetrachloride. 

In the drying of compound intermediates of refractory and reactive metals, particular attention is given to the environment and to the materials so that the compound does not pick up impurities during the process. A good example is the drying of zirconium hydroxide. After the solvent extraction separation from hafnium, which co-occurs with zirconium in the mineral zircon, the zirconium values are precipitated as zirconium hydroxide. The hydroxide is dried first at 250 °C for 12 h in air in stainless steel trays and then at 850 °C on the silicon carbide hearth of a muffle furnace. 

In the TBP process, which was developed in the USA283 and in England,286 zirconium(IV) hydroxide (produced, for example, by the hydrolysis of the material obtained from the caustic fusion of zircon sand) is dissolved in nitric acid to give a solution containing 30-100 g of zirconium (plus hafnium) per litre and 5-8 M free nitric acid. The zirconium is extracted into a 50-60% solution of TBP in a suitable hydrocarbon diluent, the loaded organic phase is washed in 5 M nitric... 

Separation of hafnium from zirconium. In a zirconium-hafnium separation process developed by Eldorado Nuclear, a mixture of sodium zir-conate and hafnate can be obtained by fusing zircon sand with NaOH and dissolving the product in water. Acidification with nitric acid then gives aqueous zirconyl (ZrO +, or hydrolyzed Zr + ) and hafnyl (HfO " ") nitrates. Extraction with TBP then gives an extract containing mainly [Hf0(N03)2(TBP) ], but most of the ZrO " " remains in the aqueous phase and can be recovered on evaporation as essentially Hf-free Zr0(N03)2(s). The effectiveness of this process can be ascribed to compounding of factors... 

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... 

One of the processes for extracting zirconium from the mineral zircon (ZrSi04) is to fuse the mineral with limestone or dolomite. The reaction product disintegrates on cooling into powdered calcium silicate and coarse crystals of calcium zirconate (equation 12.7). The zirconate is dissolved in acid and converted into zirconium salts or zirconium oxide, much of which are converted into the corrosion-resistant zirconium metal [12.38]. 

Zirconium is extracted mainly from the siUcate mineral, zircon (ZrSi04), which is found as a beach sand, usually in association with other minerals of commercial value, in Australia, India, Madagascar, Malaya, Ceylon, parts of Africa and the United States. The mineral is obtained in a relatively pure condition by ore dressing methods, except that it is usually associated with about 2 per cent of the isomorphous, and chemically very similar, hafnium silicate. The sihcate has been used directly, in admixture with charcoal, for chlorination to zirconium tetrachloride. The reaction can be represented as,... 

Semiindustrial tests were carried out on zirconium and hafnium extraction by TVEX-65% TBP from nitric silica-containing pulps with the following composition Zr 17.0-44.0 g/L, Hf 0.2-0.5 g/L, A10.2-0.37 g/L, Fe 1.7-4.0 g/L, Si 0.8-8.2 g/L, HNO3 260.0-420.0 g/L. The pulp was received as a result of HNO3 leaching of cake of zircon with soda. 

The metal zirconium is not as rare as might be expected. It is eighteenth in abundance in the earth s crust and more common than copper and zinc together. The most important mineral is zircon, zirconium silicate ZrSiO. Badddeyite, zirconium oxide ZrOj (Table 19.1) is also important There are, however, no zircon mines. Zircon is recovered as a by-product from the extraction of titanium from ores containing the minerals rutile and ilmenite. 

The pyrometallurgical methods were developed based on the differences between zirconium and hafnium in oxidation and reduction characteristics [11, 12] volatility [13-16] electrochemical properties [17-19] and molten metal-molten salt equilibrium [20, 21], The extractive distillation process, using carbochlori-nation of zircon [13], is in operation by CEZUS in France. Both chlorides are sublimated and run through a vertical distillation column containing molten aluminium chloride and potassium chloride. Both hafnium and zirconium tetrachloride chlorides dissolve, but hafnium tetrachloride has a higher vapour pressure and is therefore condensed from the top of the column in a hafnium-enriched mixture. The zirconium tetrachloride is partitioned to a liquid phase and recovered from a salt, typically containing less than 50 ppm hafnium. 

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