Cost, rare earth oxides

The relatively high cost and lack of domestic supply of noble metals has spurred considerable efforts toward the development of nonnoble metal catalysts for automobile exhaust control. A very large number of base metal oxides and mixtures of oxides have been considered, especially the transition metals, such as copper, chromium, nickel, manganese, cobalt vanadium, and iron. Particularly prominent are the copper chromites, which are mixtures of the oxides of copper and chromium, with various promoters added. These materials are active in the oxidation of CO and hydrocarbons, as well as in the reduction of NO in the presence of CO (55-59). Rare earth oxides, such as lanthanum cobaltate and lanthanum lead manganite with Perovskite structure, have been investigated for CO oxidation, but have not been tested and shown to be sufficiently active under realistic and demanding conditions (60-63). Hopcalities are out-... 

TABLE I Rare Earth Oxides Abundance and Cost ... 

Noble metal catalysts are highly active for the oxidation of carbon monoxide and therefore widely used in the control of automobile emissions. Numerous recent studies on noble metal-based three-way catalysts have revealed characteristics of good thermal stability and poison resistance(l). Incorporation of rare earth oxides as an additive in automotive catalysts has improved the dispersion and stability of precious metals present in the catalyst as active components(2). Monolith-supported noble-metal catalysts have also been developed(3). However, the disadvantages of noble metal catalysts such as relative scarcity, high cost and requirement of strict air/fuel ratio in three-way function have prompted attention to be focused on the development of non-noble metal alternatives. 

Non-noble metal catalysts A and B were developed in view of the rich resources of transition metal oxide and rare earth oxides as well as low cost in China. The catalysts were characterized by high effectiveness for the conversion of CO and HC, high crush strength and high stability to prevent poisoning by S02 and Pb. The catalytic converter can also be used in place of exhaust muffler while the consumption of fuel does not increase. 

In addition, the hydrogen plasma-metal reaction (HPMR) method was applied to synthesize rare earth oxide nanoparticles [13], which has been used to prepare better metallic ultrafine particles industrially at low cost. Pure Sm and Nd oxide nanoparticles were prepared through the HPMR method followed by oxidation treatment. The first step is to fabricate hydride nanoparticles the second is to oxidize them at room or elevated temperatures. Cubic neodymium and samarium oxides were synthesized with their average sizes of 20 and 40 nm, respectively. 

Higher in activity, but also more costly, are catalysts that contain precious metals such as rhodium, ruthenium, platinum, palladium and rhenium or mixtures thereof [107], while alumina or magnesia [214] and rare earth oxides such as ceria and zirconia or mixtures thereof serve as the carrier material. Rare earth metals have an oxygen storage capability, they interact with the precious metal and generate active sites for hydrocarbon activation [164]. 

Preparation of rare earth standards as metals is not feasible due to the cost and complexity of preparing accurate and homogeneous samples doped at trace levels. However, rare earth oxides present a favorable chemical system for quantification. Three different methods of preparing oxide standards were reported by Griffith et al. (1971) which will be reviewed briefly. 

Since the R-Co alloys have become technically significant, many efforts have been made to lower the cost of the R-Co alloys which are the starting materials for the magnet production. In the last three years new methods have been developed, which avoid the preparation of pure R metals which are later melted with Co and other transition metals, but start with rare earth oxides (R20a) and Co oxide. These processes are known under the names reduction-diffusion process and co-reduction process . 

The cost of 99.9% erbium metal is about 21/g. Erbium is finding nuclear and metallurgical uses. Added to vanadium, for example, erbium lowers the hardness and improves workability. Most of the rare-earth oxides have sharp absorption bands in the visible, ultraviolet, and near infrared. This property, associated with the electronic structure, gives beautiful pastel colors to many of the rare-earth salts. Erbium oxide gives a pink color and has been used as a colorant in glasses and porcelain enamel glazes. 

Benefits of the kind outlined above would suggest that the best resistance to sulphidation is achieved by oxide scales containing rare-earth elements (mainly Ce and Y). This being the case, one of the most cost-effective routes for enhancing resistance to sulphidation is to incorporate the rare-earth oxides by pre-oxidation of the substrates upon which rare-earth oxides have superficially been applied. The sequential development of this approach is illustrated in fig. 13, where the most effective sulphidation resistance is achieved on Incoloy 800H which has been pre-oxidized in air at 850"C and coated with Ce02 (Fransen et al. 1985). 

In summary, the determination of lanthanides in high-purity rare-earth oxides depends on the ultimate use of rare-earth oxides in commercial devices. Derivative spectrophotometry and EDXRF (which are less costly) can be reliably used for monitoring lanthanides in 90% to 3 N purity rare-earth oxides. The determination of traces of lanthanides in high-purity rare-earth oxides (4N to 6N) invariably requires costlier ICP-MS or GD-MS techniques for liquid and solid samples. 

Rare earths have been used either as neutron absorbers (Eu, Dy) or as burnable poisons (Gd, Er) either as oxides or titanates (see Section 15.3.4.1). Their main drawback (excepted for Dy) is a very high specific activity after neutron irradiation, leading to complicated management after use. Moreover, rare earths are presentiy in high demand for magnets, catalysis, technical ceramics, or electronic applications, etc. [20] with the demand increasing [21]. The main reserves are located in the USA, Australia, China, and Russia. Due to lower processing costs, rare earths are now mainly extracted and refined in China [22] (Fig. 15.2). Extraction and separation... 

The cerium concentrate derived from bastnasite is an excellent polish base, and the oxide derived direcdy from the natural ratio rare-earth chloride, as long as the cerium oxide content is near or above 50 wt %, provides an adequate glass poHsh. The polishing activity of the latter is better than the Ce02 Ln0 ratio suggests. Materials prepared prior to any Ln purification steps are sources for the lowest cost poHshes available used to treat TV face plates, mirrors, and the like. For precision optical polishing the higher purity materials are preferred. 

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