Zirconium metal, or sponge, is used mainly in the nuclear industry with a requirement for minimum content of hafnium.
The chemical and physical similarity of zirconium and hafnium means that separation of these two metals is difficult.
The traditional process involved the direct chlorination of zircon flour in a fluidised bed containing carbon, the chemical reaction being:
The chlorination process produces tetra-chlorides of zirconium, hafnium and silicon from which the silicon is removed by sublimation, enabling the separation of the zirconium and hafnium tetra-chlorides by solvent extraction or distillation.
Zirconium tetra-chloride is then reduced to the metallic form with magnesium using the Kroll process:
The metallic zircon sponge is then crushed and milled to the required size. The schematic below is reproduced from the Areva website:
More recently, the use of fused zirconia as a feedstock material has become popular due to its lower uranium and thorium contents and lower silica content.
Zirconium sponge is produced in nuclear and non-nuclear grades (of which about 70% is nuclear grade), used for production of zirconium alloys for nuclear reactor components, such as cladding for fuel rods. A typical composition of nuclear-grade zirconium alloys is more than 95% by weight zirconium and less than 2% of tin, niobium, iron, chromium, nickel and other metals, which are added to improve mechanical properties and corrosion resistance.
Non-nuclear or industrial grade - with a minimum of 0.2% hafnium - is used for production of zirconium alloys used for applications such as chemical processing. ASTM allows 0.4-0.5% hafnium in zirconium alloys.
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