Praseodymium
Occurrence and production

Praseodymium is not particularly rare, despite it being in the rare-earth metals, making up 9.2 mg/kg of the Earth's crust. Praseodymium's classification as a rare-earth metal comes from its rarity relative to "common earths" such as lime and magnesia, the few known minerals containing it for which extraction is commercially viable, as well as the length and complexity of extraction. Although not particularly rare, praseodymium is never found as a dominant rare earth in praseodymium-bearing minerals. It is always preceded by cerium and lanthanum and usually also by neodymium.

The Pr ion is similar in size to the early lanthanides of the cerium group (those from lanthanum up to samarium and europium) that immediately follow in the periodic table, and hence it tends to occur along with them in phosphate, silicate and carbonate minerals, such as monazite (M PO) and bastnäsite (M CO F), where M refers to all the rare-earth metals except scandium and the radioactive promethium (mostly Ce, La, and Y, with somewhat less Nd and Pr). Bastnäsite is usually lacking in thorium and the heavy lanthanides, and the purification of the light lanthanides from it is less involved. The ore, after being crushed and ground, is first treated with hot concentrated sulfuric acid, evolving carbon dioxide, hydrogen fluoride, and silicon tetrafluoride. The product is then dried and leached with water, leaving the early lanthanide ions, including lanthanum, in solution.

The procedure for monazite, which usually contains all the rare earth, as well as thorium, is more involved. Monazite, because of its magnetic properties, can be separated by repeated electromagnetic separation. After separation, it is treated with hot concentrated sulfuric acid to produce water-soluble sulfates of rare earth. The acidic filtrates are partially neutralized with sodium hydroxide to pH 3–4, during which thorium precipitates as hydroxide and is removed. The solution is treated with ammonium oxalate to convert rare earth to their insoluble oxalates, the oxalates are converted to oxides by annealing, and the oxides are dissolved in nitric acid. This last step excludes one of the main components, cerium, whose oxide is insoluble in HNO. Care must be taken when handling some of the residues as they contain Ra, the daughter of Th, which is a strong gamma emitter.

Praseodymium may then be separated from the other lanthanides via ion-exchange chromatography, or by using a solvent such as tributyl phosphate where the solubility of Ln increases as the atomic number increases. If ion-exchange chromatography is used, the mixture of lanthanides is loaded into one column of cation-exchange resin and Cu or Zn or Fe is loaded into the other. An aqueous solution of a complexing agent, known as the eluant (usually triammonium edtate), is passed through the columns, and Ln is displaced from the first column and redeposited in a compact band at the top of the column before being re-displaced by NH. The Gibbs free energy of formation for Ln(edta·H) complexes increases along with the lanthanides by about one quarter from Ce to Lu, so that the Ln cations descend the development column in a band and are fractionated repeatedly, eluting from heaviest to lightest. They are then precipitated as their insoluble oxalates, burned to form the oxides, and then reduced to metals.
language:response_language[Dutch]