Hazard
(Salts) Irritant to eyes and abraded skin
Chemical Properties
grey metal ingot
Physical properties
Neodymium is the third most abundant rare-earth element in the Earth’s crust (24 ppm). Itis reactive with moist air and tarnishes in dry air, forming a coating of Nd3O3, an oxide witha blue tinge that flakes away, leaving bare metal that then will continue to oxidize.
Its melting point is 1,021°C, its boiling point is 3,074°C, and its density is 7,01 g/cm3.
Isotopes
There are 47 isotopes of neodymium, seven of which are considered stable.Together the stable isotopes make up the total abundance in the Earth’s crust. Twoof these are radioactive but have such long half-lives that they are considered stablebecause they still exist on Earth. They are Nd-144 (half-life of 2.29×10+15 years) andNd-150 (half-life of 6.8×10+15years). All the other isotopes are synthetic and havehalf-lives ranging from 300 nanoseconds to 3.37 days.
Origin of Name
Derived from the two Greek words neos and didymos. When combined,
they mean “new twin.”
Occurrence
Although neodymium is the 28th most abundant element on Earth, it is third in abundanceof all the rare-earths. It is found in monazite, bastnasite, and allanite ores, where it isremoved by heating with sulfuric acid (H2SO4). Its main ore is monazite sand, which is amixture of Ce, La, Th, Nd, Y, and small amounts of other rare-earths. Some monazite sandsare composed of over 50% rare-earths by weight. Like most rare-earths, neodymium can beseparated from other rare-earths by the ion-exchange process.
Characteristics
As an element, neodymium is a soft silver-yellow metal. It is malleable and ductile. It canbe cut with a knife, machined, and formed into rods, sheets, powder, or ingots. Neodymiumcan form trivalent compounds (salts) that exhibit reddish or violet-like colors.
Neodymium reacts with water to form Nd(OHO)3 and hydrogen (H2), which can explodeif exposed to a flame or spark. It is shipped and stored in containers of mineral oil.
Production Methods
Neodymium is recovered mostly from mineral monazite and bastnasite, thetwo most abundant rare-earth minerals. Monazite is a rare earth-thorium phosphate usually containing between 9 to 20% neodymium. Bastnasite is a rare earth fluocarbonate ore containing 2 to 15% neodymium. Both ores are first cracked by heating with concentrated sulfuric acid or sodium hydroxide. The recovery process from monazite ore using sulfuric acid is described below:
Heating the ore with sulfuric acid converts neodymium to its water soluble sulfate. The product mixture is treated with excess water to separate neodymium as soluble sulfate from the water-insoluble sulfates of other metals, as well as from other residues. If monazite is the starting material, thorium is separated from neodymium and other soluble rare earth sulfates by treating the solution with sodium pyrophosphate. This precipitates thorium pyrophosphate. Alternatively, thorium may be selectively precipitated as thorium hydroxide by partially neutralizing the solution with caustic soda at pH 3 to 4. The solution then is treated with ammonium oxalate to precipitate rare earth metals as their insoluble oxalates. The rare earth oxalates obtained are decomposed to oxides by calcining in the presence of air. Composition of individual oxides in such rare earth oxide mixture may vary with the source of ore and may contain neodymium oxide, as much as 18%.
The oxalates obtained above, alternatively, are digested with sodium hydroxide converting the rare earth metals to hydroxides. Cerium forms a tetravalent hydroxide, Ce(OH)4, which is insoluble in dilute nitric acid. When dilute nitric acid is added to this rare earth hydroxide mixture, cerium(IV) hydroxide forms an insoluble basic nitrate, which is filtered out from the solution. Cerium also may be removed by several other procedures. One such method involves calcining rare earth hydroxides at 500°C in air. Cerium converts to tetravalent oxide, CeO2,while other lanthanides are oxidized to trivalent oxides. The oxides are dissolved in moderately concentrated nitric acid. Ceric nitrate so formed and any remaining thorium nitrate present is now removed from the nitrate solution by contact with tributyl phosphate in a countercurrent
After removing cerium (and thorium), the nitric acid solution of rare earths is treated with ammonium nitrate. Lanthanum forms the least soluble double salt with ammonium nitrate, which may be removed from the solution by repeated crystallization. Neodymium is recovered from this solution as the double magnesium nitrate by continued fractionation.
Three alternative methods may be mentioned here, which give high purity material and are less tedious than the one described above. These are (1) ion exchange, (2) metallothermic reduction, and (3) electrolysis
In the ion exchange process, the nitric acid solution of the rare earth oxides obtained above is passed through a sulfonated styrene-divinylbenzene copolymer or other cation exchange resin in the hydrogen form. The rare earths are selectively eluted by flowing down a chelating solution of ethylenediamine tetraacetic acid (EDTA), or citric acid, or nitrilotriacetate (NTA) through the loaded column. The most stable complexes are eluted first. Metal ions are selectively stripped out in successive stages.
In the metallothermal reduction, the mixture of rare earth oxides obtained above is first converted to their halide salts. This is done by heating the oxidesat 300 to 400°C with dry and purified hydrogen fluoride, or preferably, by allowing dry hydrogen fluoride to pass over rare earth oxides and ammonium fluoride at 300-400°C. If chloride salt is desired, the oxides must be heated with ammonium chloride. For example, neodymium oxide may be converted to its fluoride or chloride:
Nd2O3 + 6NH4F?HF → 2NdF3 + 6NH4F↑ + 3H2O↑
Nd2O3 + 6NH4Cl → 2NdCl3 + 6NH3↑ + 3H2O↑
Neodymium, along with lanthanum, cerium and praseodymium, has low melting points and high boiling points. The fluorides of these and other rare earth metals are placed under highly purified helium or argon atmosphere in a platinum, tantalum or tungsten crucible in a furnace. They are heated under this inert atmosphere or under vacuum at 1000 to 1500°C with an alkali or alkaline earth metal. The halides are reduced to their metals:
2NdF3 + Ca → 2Nd + 3CaF2
NdCl3 + 3Li → Nd + 3LiCl
The crucible is allowed to cool and is held at a temperature slightly above the melting point of neodymium for a sufficient time to allow separation of the metal.
In the electrolytic process, a fused mixture of anhydrous rare earth chlorides (obtained above) and sodium or potassium chloride is electrolyzed in an electrolytic cell at 800 to 900°C using graphite rods as the anode. The cell is constructed of iron, carbon or refractory linings. Molten metal settles to the bottom and is removed periodically.
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