Mischmetal

Mischmetal, from the German for "mixed metal," is an alloy of rare-earth elements, also known as cerium mischmetal or rare-earth mischmetal. Its typical composition is about 55% cerium, 25% lanthanum, and 15–18% neodymium, with traces of other rare earths making up 95% lanthanides and 5% iron. It is most familiar as the pyrophoric material in ferrocerium "flints" used in lighters and torches. Because a pure rare-earth alloy would be too soft to produce good sparks, it is hardened by blending with iron oxide and magnesium oxide to create ferrocerium. In chemical formulas, it is often abbreviated as Mm, for example in MmNi5.

The alloy was invented by Carl Auer von Welsbach, who discovered neodymium and praseodymium, co-discovered lutetium, and invented the gas mantle and the rare-earth industry. After extracting thorium from monazite sand, he was left with many lanthanides that had no commercial use, and he sought applications for them. Mischmetal was among his first discoveries.

Historically, mischmetal was prepared from monazite, an anhydrous phosphate of light lanthanides and thorium. The ore was cracked at high temperature with concentrated sulfuric acid or sodium hydroxide. Thorium was removed by exploiting its weaker basicity compared to trivalent lanthanides; its daughter radium was precipitated by entrainment in barium sulfate. The remaining lanthanides were converted to their chlorides, forming "rare-earth chloride" (hexahydrate), also called "lanthanide chloride," a major commodity chemical. This hexahydrate was dehydrated by careful heating, preferably with ammonium chloride or in a hydrogen chloride atmosphere, to yield anhydrous chloride. Electrolysis of the molten anhydrous chloride (mixed with other anhydrous halides to improve melt behavior) produced molten mischmetal, which was cast into ingots. Any samarium in the ore was not reduced to metal but accumulated in the molten halide, from which it could later be profitably isolated. Monazite-derived mischmetal typically contained about 48% cerium, 25% lanthanum, 17% neodymium, and 5% praseodymium, with the balance other lanthanides. When bastnäsite began being processed for rare earths around 1965, it was also converted to rare-earth chloride and then to mischmetal, but this version was higher in lanthanum and lower in neodymium.

As of 2007, high demand for neodymium made it profitable to remove all heavier lanthanides and neodymium (and sometimes all praseodymium) from the natural mixture for separate sale, leaving only La-Ce-Pr or La-Ce in the most economical forms of mischmetal. Because the light lanthanides are so similar in metallurgical properties, any application suitable for the original composition works equally well with these truncated mixtures. The traditional "rare-earth chloride" was also used as a commodity chemical by companies that did not process ores directly, to extract individual rare earths. As of 2007, mischmetal typically cost less than $10 per kilogram, and the underlying rare-earth chloride mixtures less than $5/kg.

Mischmetal is used in preparing virtually all rare-earth elements, because these elements are nearly identical in most chemical processes and ordinary extraction methods cannot distinguish them. Highly specialized processes, such as those developed by Carl Auer von Welsbach, exploit subtle solubility differences to separate mischmetal into its constituent elements, each step producing only an incremental change in composition. These processes later informed Marie Curie in her search for new elements.

In the Galfan galvanising process for steel wire, traces of a cerium and lanthanum mischmetal are sometimes added to the zinc and 5–10% aluminium coating.