Metallurgy of Key Minerals - Thanatia: The Destiny of the Earth's Mineral Resources - page 240

by an alloy melting with FeB. The electrolytic reduction with Fe cathodes is an
alternative route followed by the alloy melting with FeB.
The main steps of the magnetisation process consist of milling, sintering and
annealing the alloy material. The milling process pulverises the material to ' 3
with a narrow size distribution. The ultimate objective should be a sizing as close
as possible to the internal domains of uniform magnetic polarisation. Next, the
powder is compressed ('200 MPa) and aligned in a magnetic field ('800 kA/m)
parallel to the magnetisation axes of the domains. The material is then sintered 31
to shape it into a solid form before undergoing a post-sinter annealing treatment at
625 o C to improve its coercitivity. The material is subsequently ready for grinding,
coating and finishing the magnet according to the market demand (Gupta and
Krishnamurthy, 2005).
The alloy compositions of the main magnets are SmCo 5 , Sm 2 Co 17 and
Nd 2 Fe 14 B. Their maximum energy products are '150 kJ/m 3 , '220 kJ/m 3
and '300 kJ/m 3 , respectively (Strnat, 1990). Various alloys based on sintered
Nd Fe B magnets have obtained an energy product higher than 400 kJ/m 3
at room temperature, an increased coercitivity and/or an improved resistance to
corrosion. Such alloys were prepared by simultaneously adding cobalt, dysprosium,
praseodymium amongst others, ensuring a preservation of the Nd 2 Fe 14 B crystal
structure. They are the strongest known permanent magnets and have found ap-
plication in high powered wind energy turbines (Kirchmayr, 1996).
Two representative routes for NdFeB alloys preparation and magnetisation
are shown in Fig. 8.16.

Fe, FeB

Nd

Powder
Compression
& Magnetization

Fluoride
reduction

Vacuum
melting

Alloy
melting

Nd 2 Fe 14 B
ingot

Nd 2 Fe 14 B
magnet

Sintering and
Annealing

Milling

NdF 3

OR BaF 2 , LiF

NdFe
Alloy

Electrolytic reduction
(Iron cathode)
(Carbon anode)

Vacuumpurification

NdF 3

Fig. 8.16 Representative routes for Nd-Fe-B alloy preparation and magnetisation

8.13.10 Energy and environmental issues ofREE

The overall environmental impact of rare earth production is among the highest
of all metals. This is because it involves beneficiation of very low mining grades,
containing radioactive elements such as Th and U, which is the case of monazite ores.

31 SmCo 5 is sintered at 600 o C, whereas the Nd 2 Fe 14 B is sintered in an Ar atmosphere at 1100 o C
to avoid oxidation.