19
Hafnium [Hf, 72]

19.1 INTRODUCTION
Hafnium (Hf), a metal of the group 4 in the periodic table of elements, has
crystallo graphic and chemical properties very analogous to those of Zr, a metal of the
same group. Its terrestrial abundance ranges between 3 and 5 mg/kg, and similar contents are in igneous rocks. A bit higher amounts, at about 6 mg/kg, may also be present
in some sandstones and argillaceous sediments. Its lowest contents, <0.4 mg/kg, is in
calcareous rocks. Hard coal contains Hf within the range of 0.5–1 mg/kg.
The oxidation state of Hf is +4. It is highly associated with Zr, and occurs mainly
in the following Zr minerals: hafnon (called also zircon), (Zr,Hf)SiO 4 and baddeyite,
(Zr,Hf,Ti,Fe,Th)O 2 . It may also be absorbed by other minerals, mainly biotite and
pyroxene.
World primary Hf production statistics are not available. However, some earlier
estimation was for about 60 t/yr (Kabata-Pendias 2011). Hafnium is produced as a
by-product during Zr processing. Resources of Hf in the United States are estimated
to be about 130,000 t.
Hafnium is used mainly in filaments and electrodes, and in nuclear industries,
especially in power plants. Hf and Zr are used interchangeably in certain superalloys, but in others, only Hf may be used. The alloys with Nb, Ti, and W have a special
application in nuclear processes, mainly for reactor-control rods and coatings of cutting tools. Also, alloys of Hf with Pd and Pt are relatively common. Some amounts
of Hf are applied in photographic flashes. Biometallic materials may contain Hf, as
it has a good biocompatibility and osteoconductivity (Szilagyi 2004).
19.2 SOILS
The worldwide mean Hf content of soils is estimated at 6.4 mg/kg, within the range
of 2–20 mg/kg, in minerals soils. Its abundance in organic soils is reported to be
much more lower (Table 19.1). Hafnium content in soils is reported to range/average
in various countries as follows (in mg/kg): Bulgaria, 1.8–18.7; Canada, 1.8–10;
Russia, 20.8; Sweden, 7.6; the United Kingdom, 5 (Kabata-Pendias 2011).
Mean Hf content in surface soils, with high organic matter content, is 0.07 mg/kg,
and is a bit lower than mean value (0.1 mg/kg) for all other organic soils in Norway
(Nygard et al. 2012).
Increased Hf levels in coarse fractions of desert dusts resulted from its association
with detrital fraction (Castillo et al. 2008).
137