Zirconium possesses a very small neutron capture section, and that is why it is used in nuclear reactors. The mineral zircon nevertheless always contains hafnium, at a concentration of about 1% to 3% by weight. Unlike zirconium, hafnium absorbs neutrons strongly and will therefore greatly reduce neutron flux in a nuclear reactor. The use of zirconium in the nuclear field thus requires prior elimination of hafnium, and a content of less than 100 parts per million (ppm) is often recommended.
Hafnium and zirconium have properties that are very similar which makes separating them extremely difficult. Various techniques have been proposed and used. At present, only a few methods have been accepted and applied on an industrial scale. One such method is multiple crystallization of potassium and zirconium fluoride, another is liquid—liquid extraction using various organic solvents, e.g. TBP in an acid medium (nitric acid), n-octylamine in an acid medium (sulfuric acid), methyl isobutyl ketone (MIBK), and finally there is the method of chloride distillation (D. Sathiyamoorthy et al., High temperature materials and processes 1999, Vol. 18, No. 4, 213-226).
Conventionally, prior to performing extractive distillation and certain liquid—liquid extraction techniques, the mineral is subjected to carbochloration which produces the tetrachlorides ZrCl4 and HfCl4. In liquid—liquid extraction, the chlorides need to be put into aqueous solution and extraction leads to the formation of ZrO2 and HfO2, which then requires further carbochloration of the zirconium prior to moving on to the stage of recovering metallic Zr.
The method by extractive distillation uses a distillation column having a plurality of trays each supporting a layer of molten salts. The chlorides are introduced in the gaseous state. ZrCl4 is recovered in the solvent at the bottom of the column. HfCl4 is entrained with the gas to the top of the column. Various solvents have been proposed: sodium chlorozirconate and chlorohafnate in FR-A-1 537 218; phosphorus oxichloride in U.S. Pat. No. 1,582,860; pure zinc chloride; anhydrous tin chloride in U.S. Pat. No. 2,816,814; alkali metal chloride+aluminum or iron chloride in U.S. Pat. No. 2,928,722; sodium chloride in U.S. Pat. No. 3,671,186; alkaline chloroaluminate or chlorferrate in FR-A-2 250 707 (U.S. Pat. No. 4,021,531), FR-A-2 543 162; sodium and potassium chlorides U.S. Pat. No. 3,966,458; zinc chloride and lead chloride in U.S. Pat. No. 4,737,244; zinc chloride+calcium or magnesium chloride in U.S. Pat. No. 4,749,448; and lithium chloride+at least one chloride selected from those of sodium, potassium, magnesium, and calcium in U.S. Pat. No. 4,874,475.
Those various separation techniques present their advantages and their drawbacks. It is mentioned above that liquid—liquid extraction requires steps of putting chlorides into solution and a second carbochloration step. The solvent MIBK is volatile and highly explosive, which gives rise to problems with handling and reprocessing effluents, but nevertheless a significant fraction of worldwide Zr production is based on that technique. The solvent TBP turns out to provide lower performance and to be more expensive, which explains why it has been abandoned progressively. See A. B. V. da Silva, Jr. and P. A. Distin in CIM Bulletin 1998, Vol. 91, No. 1018, 221-224.
A drawback of extractive distillation results from the fact that the looked-for metal, i.e. Zr, is in the solvent which means that it must subsequently be recovered therefrom and thus that additional steps need to be performed, which gives rise to non-negligible costs. According to da Silva and Distin supra, the main drawbacks of that technique are its poor separation factor which requires a very large number of stages (about 90), and the highly corrosive nature of the solvents and the constraints associated with using vapor streams, which also has an impact on the materials that can be implemented. Still according to those authors, the cost of an installation operating with that method is therefore high, and that ought to constitute a brake on its use for new installations. Finally, that document concludes that the future lies more with liquid—liquid extraction using new aqueous solvents.