A comprehensive review is presented of the extractive metallurgy of rare earths. The topics covered are: world rare earth resources and production; ore processing and separation of individual rare earths; reduction, refining, and ultrapurification of rare earth elements; methods for rare earth materials analysis; and a selection of the numerous rare earth applications. World rare earth reserves are abundant and would last for well beyond the next century. However, all of the 16 naturally occurring rare earth elements are not equally distributed in the ore minerals. This, compounded with the problems specific to the isolation and recovery of each of the rare earths, sets the stage for an unequal rare earth availability. The close chemical similarity of rare earths looses its importance when divergent physical properties determine the processes for rare earth element reduction and refining. The rare earth metals, alloys, and compounds have been as pure as could be determined. Finally, the commercial...

Extractive Metallurgy of Rare Earths

Determination of the crystal structure of ${\mathrm{Nd}}_{2}$${\mathrm{Fe}}_{14}$B, a new ternary phase, is reported. It has recently been demonstrated that permanent magnets having large coercivities and energy products can be formed from this phase, underscoring its potential technological importance. We relate the crystal structure and intrinsic magnetic properties by considering analogies with previously known rare-earth---transition-metal materials.

Relationships between crystal structure and magnetic properties in Nd 2 Fe 14 B

The temperature dependence of the saturation magnetization and the magnetocrystalline anisotropy field have been measured on single‐crystal samples of the R2Fe14B compounds for R=Y, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, and Tm from 4.2 K to the magnetic ordering temperatures. A spin reorientation transition of the Nd2Fe14B type has been found in Ho2Fe14B at 57.6 K in zero field. Another type of spin reorientation caused by anisotropy compensation between the Fe and the R sublattices exists in Er2Fe14B and Tm2Fe14B. The temperature dependence of the angle of the easy direction of magnetization from the c axis has been measured for R=Nd, Ho, Er, and Tm. The relation between the magnetocrystalline anisotropy and the sublattice magnetization is investigated by employing a simplified two‐sublattice molecular field model.

Magnetization and magnetic anisotropy of R2Fe14B measured on single crystals

Structural and metallographic studies were carried out on the Nd-Fe-B alloy system as well as the Nd-Fe-B tetragonal compound on which record high energy magnets have been developed using a powder metallurgical technique. The study on the new magnet has also been extended to other R-Fe-B componds containing various rare earths (R) and to R-Fe-Co-B alloys. The results are as follows; (1) The sintered Nd-Fe-B magnet is composed of mainly three phases, the Nd 2 Fe 14 B matrix phase plus Nd-rich phase and B-rich phase ∼ Nd 2 Fe 7 B 6 ) as minor phases. (2) Nd 2 Fe 14 B has the space group of P4 2 /mnm. The crystal structure of this phase can be described as a layer structure with alternate stacking sequence of a Nd-rich layer and a sheet formed only by Fe atoms. The sheet of Fe atoms has a structure similar to the σ-phase found in Fe-Cr and Fe-Mo systems. (3) The Nd-rich phase containing more than 95 at.% Nd, 3∼5 at.% Fe and a trace of B has fcc structure with a=0.52 nm. This phase is formed around grain boundaries of the matrix phase. Nd 2 Fe 7 B 6 phase has an one-dimentional incommensurate structure with a=a o and c\simeq8 C o , based on a tetragonal structure with a o =0.716 nm and c o =0.391 nm. (4) In the as sintered Nd 15 Fe 77 B 8 alloy periodic strain contrasts are observed along grain boundaries, which disappear after annealing at 870K. This may be related to the enhancement of the intrinsic coercivity of the sintered magnet by post sintering heat treatment. (5) Stable R 2 Fe 14 B phases are formed by various rare earths except La. Of all the R 2 Fe 14 B compounds, Nd 2 Fe 14 B has the maximum saturation magnetization as high as 1.57 T. Dy and Tb form R 2 Fe 14 B phases with the highest anisotropies. Small additions of these elements greatly enhance the coercive force of the Nd 2 Fe 14 B base magnet. (6) Partial replacement of Fe by Co raises the Curie temperature of the Nd 2 Fe 14 B compound, which improves the temperature coefficient of the remanence of the magnet. But the intrinsic coercive force is decreased by the Co addition.

Permanent magnet materials based on the rare earth-iron-boron tetragonal compounds

Permanent magnets: Plugging the gap

A new compound composed of Nd, Fe, and a small quantity of B (about 1 wt. %) has been found, which has a tetragonal structure with lattice constants a=0.880 nm and c=1.221 nm. This phase, which has the approximate composition, 12 at. % Nd, 6 at. % B and balance Fe, possesses remarkable magnetic properties. From the approach to saturation an anisotroy constant of about 3.5 MJ/m3 can be calculated, while saturation magnetization amounts to 1.35 T. The magnetization versus temperature curve shows a Curie temperature of 585 K, which is much higher than those of the Fe and light rare earth binary compounds. Based on the new compound, sintered permanent magnets have been developed which have a record high energy product. Permanent magnet properties and physical properties of a typical specimen which has the composition Nd15B8Fe77 are as follows: Br =1.23 T, HcB =880 kA/m, HcI =960 kA/m, (BH)max =290 kJ/m3, temperature coefficient of Br =−1260 ppm/K, density=7.4 Mg/m3, specific resistivity=1.4 μΩm, Vickers hardn...

Norio Togawa

Papers

New material for permanent magnets on a base of Nd and Fe (invited)