sought after of the two types and more valuable. light rare earth elements (La, Ce, Pr, Nd, Pm, Sm, Eu, Gd) are gener- ally more abundant and less valuable. Although yttrium is lighter than the light rare earth elements, it is included in the heavy rare earth group because of its chemical and physical associations with heavy rare earths in natural deposits.8
The
lighter REE are more strongly concentrated in the continen- tal crust than the heavier REE. In most RE deposits, the first four REE - La, Ce, Pr, and Nd - constitute 80% to 99% of the total.7
Rare earth elements are never found as free metals in rocks. They typically occur as mixtures of various REE-bearing minerals and require mineral separation from each other for commercial use. Bastnaesite, (Ce,La)(CO3)(OH,F); Xeno- time, YPO4; and Monazite, (Ce,La,Nd,Th)
PO4.SiO4 are the three most economically significant minerals of the more than 200 minerals known to contain essential or significant REE. Bastnaesite and monazite are sources of LREE and ac- count for 95% of REE currently used. Xenotime is a source of the HREE and yttrium.1
Monazite is also the principal ore of thorium, containing up to 30% thorium. Together with small amounts (up to about 1%) of uranium, thorium imparts radioactive properties to the monazite,1 cover.4
making the elements more expensive to re- Although thorium itself is only weakly radioactive, it
is accompanied by highly radioactive intermediate daughter products, particularly radium, that can accumulate during processing. Concern about radioactivity hazards has now largely eliminated monazite as a significant source of REE and focused attention on those few deposits where the REE occur in other, low-Th minerals, particularly bastnaesite.7 The cost of handling and disposing of radioactive material is a serious impediment to the economic extraction of the more radioactive REE-rich minerals, in particular monazite. In fact, imposition of tighter regulations on the use of radio- active minerals drove many sources of monazite out of the REE market during the 1980s.9
They are crucial to hybrid cars, wind turbines and many other green-tech innovations, but these elusive metals also have an environmental dark side. Many technologies rely on rare earths, but ironically, rare earth producers have a long history of harming the environment to extract the metals.10 Like many industries that process mineral ores, they end up with toxic byproducts known as “tailings,” which can be contaminated with radioactive uranium and thorium. Min- ing and refining rare earths makes an environmental mess, leading most countries to neglect their own reserves, even as demand soars.10
New regulatory rules are expected to raise global prices for the 17 REM. China, which supplies 97% of world rare earth oxides, has tightened its grip over the rare earth in- dustry by setting tough emission limits on miners produc- ing the lucrative metals. The emission caps on about 15
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pollutants will apply to all industry constituents including miners and smelters of rare earth alloys.11
The rare earth
production process is complex and expensive. The stages of production consist of mining, separating, refining, alloy- ing, and manufacturing rare earths into end-use items and components:6
1. Mining, where the ore is taken out of the ground from the mineral deposits.
2. Separating the ore into individual rare earth oxides. 3. Refining the rare earth oxides into metals with different purity levels; oxides can be dried, stored, and shipped for further processing into metals.
4. Forming the metals, which can be processed into rare earth alloys.
5. Manufacturing the alloys into devices and components.
Light rare earth elements, mainly including La, Ce, Pr and Nd are generally used for metallurgical applications, es- pecially as mischmetal, which is a rare earth alloy, as a mix of metals. It is a relatively impure alloy of Ce and La with other REE that is the direct result of refining rare earth mineral concentrates without separation of the individual elements. There is no exact formulation for mischmetal, but a common composition is approximately 50% cerium and 25% lanthanum with smaller amounts of other REE making up the balance. Cerium or La-rich mischmetal al- loys (Appendix 1), and metallic Ce or La are generally available for cast iron applications. With the creation of the first mischmetal from monazite ore, the REM industry was born, paving the way for isolation and purification of many rare earths.12
Despite the fact that these elements are used in the metal- lurgical industry (Appendix 1) and are generally more abun- dant and less valuable than the heavier HREE, they are still attractive for other applications (Appendix 2), so less LREE is available for use in cast iron graphite morphology control. And it is anticipated that this problem will be more acute. Consequently, the use of REE in ductile iron production must be carefully thought out in the future.
Rare Earth Element (REE) Applications in Ductile Iron
Ductile iron is a complex material, generally of high purity, requiring control of over 30 elements for an array of dif- ferent effects.21, 22
Generally, REE are employed in nodular
(spheroidal) graphite cast iron (ductile iron) to accomplish the following: (a) neutralize tramp elements like Ti, Pb, Bi, As etc; (b) assist in nodulizing or provide a supplementary effect to Mg to promote the graphite shape more spheroidal, i.e., to improve the roundness or spherical shape; (c) assist in nucleating graphite.
Rare earth elements are typically incorporated in Mg-FeSi alloys [0.10 - 3.5 wt-% REE range of available contents], the
International Journal of Metalcasting/Volume 8, Issue 2, 2014
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