A LANTHANIDE LANTHOLOGY (.pdf) - Davidson Physics

MISCHMETALWhen a lanthanide mineral is processed – without separation into the individual lanthanides -to yield a metal, the product, termed Mischmetal, will still contain the Ln's in the "natural-ratio" asthey occur in the ore body. The close similarity in Ln content between bastnasite and monazitemeans that mischmetals from those two sources will also be similar. In addition it often usual toremove, before metal production, the small amount of heavy lanthanides, of high value, from theprecursor. Consequently mischmetal contains predominantly the first four lanthanides, the lightLn's.MischmetalTypical CompositionLa ≈ 25 %Ce ≈ 50 %Pr ≈ 6 %Nd ≈ 15 %heavies,etc. ≈ 1 %Fe ≈ 3 %It is prepared by electrolysis at ≈850 °C of molten rare earth chloride, as anhydrous as ispractical, preferably under an inert atmosphere, see metals. The few percent of iron is present inorder to lower the melting point towards the eutectic point and to make the casting of shapes easier.The material does not have a definite melting point but melts over a range near 800 °C, close to thatof Ce. It is a strong reductant, comparable to magnesium, and will react with hot water plus alsoforming surface oxides on exposure to air.The start of the lanthanide metals industry was the production of lighter flints (a 1903 patent),based on a mischmetal- iron (≈65 % : ≈35 %) alloy, Auer metal. Due to the high content ofpyrophoric cerium this alloy, when struck to create small fragments, produces glowing particles,sparks, and is used to ignite flammable gases.The major use of mischmetal is as an additive for steel treatment[1], an application that is nolonger as dominant for Ln usage in most countries as it once was. The prime purpose of mischmetaladdition during steel alloy production is to tie up sulfur impurities, through the high affinity of Ln'sfor O and S, as stable lanthanide oxysulfides. A trend towards "cleaner" starting materials andmore efficient use of competitive additives has reduced mischmetal consumption. In China though,with its abundant resources, mischmetal accounts for the major portion of internal lanthanidedemand.Mischmetal-based alloys are promising electrode materials in nickel-metal-hydriderechargeable batteries that offer high capacity and long-life[2].[1] The Rare Earth Metals in Steel, L.A. Luyckx, in Industrial Applications of Rare EarthElements ed.K.A.Gschneidner, Am.Chem.Soc. Symposium 164, publ. 1981, p. 43[2] Rare-Earth-Based Hydrogen Storage Alloys for Rechargeable Nickel-Metal-HydrideBatteries,T.Sakai et al., J. Alloys Comp., 1993, 192, 1557