-realworld of indwtrial c hemiftry
edited by
W C FERNELIUS Kent State Unlverstty Kent, OH 44242 HAROLD WITTCOFF Chem Systems, Inc 303 South Bmadway Tarrytown, NY 10591
.
Technology of the Rare Earths Howard E. KremerS 3 High Lake Avenue, West Chicago, IL 60185
Commercial production and use of the rare earths date from 1891, when Carl Auer von Welsbach invented a practical incandescent gas mantle for light production. The gas mantle, based on thorium oxide with 1% cerium oxide. required the processing of monazite ore and the production of pure cerium nitrate. The early 1900's saw the production of lighter flints based on mixed rare earth metal, initially for ianitina mantle lamvs, and the use of mixed rare earth fluoride h the cores b f a r c light carbons. Developments in commercial rare earth technology over the next seven decades, and particularly in the last 30 years, have resulted in a wide range of uses for rare earths. Worldwide production and consumption of rare earth compounds and metals are on the order of 35,000 to 40,000 short tons of contained rare earth oxide per year. About half of this is produced and consumed in the United States. Rare earths impact on all of us daily through their applications in petroleum cracking catalysts, metallurgy, ceramics, glass polishina, o ~ t i c aglass. l and electronics. ~ h e l f r a r r&th e dements (Sc, Y and the lanthanides, La rhruuah 1.~1'account for one-fifthof the 83 narurallv occurring eiements, and collectively rank as the 22nd most abundant "element." The rare earths are strongly electropositive and form stable compounds with the strongly electronegative elements. The chlorides, bromides, iodides, nitrates, sulfates, and many other salts are water soluble; the fluorides, hydroxides, phosphates, and carbonates are among the least soluble of these compounds, and the oxides are refractory and stable. Tervalence is normal. However. Ce. Pr. and T b also show tetravalence, and Eu, Sm, and ~b ard divalent under nonoxidizing conditions. The ionic radii of the rare earths are quite large, comparable to those of Na+ and Ca2+,and decrease in size in going from La t o Lu (the lanthanide contraction), resulting in gradual changes in some properties of their compounds. The most important feature of the rare earths is their chemical similarky. Treatment of Rare Earth Ores
Approxlmate Composition of Benellclated Commercial Rare Earth Ores Percent
Oxide
Bastnssite
Monazite
Xenotime
-60 22
-60 0.5
-z
Total rare earth oxides
boa
60-70 32
\
@asis total rare earth oxides
' = lOO%/O'
)
0.1 o,3
0.1
4 0.1 2 0.2 1 0.1 0.4
trace 0.2
trace
of monazite and xenotime, and hastnasite, a rare earth fluorocarbonate, REFC03. Monazite and xenotime are recovered from alluvial placer deposits in the United States, Australia, Brazil, India, and Malaysia as a coproduct of ilmenite, rutile, and zircon sand recovery. Ilmenite and rutile are sources of titanium dioxide, and rutile is used in refractories and as a source of zirconium. Bastnasite is mined lareelv in California and provides most of the rare earths now usLd ~ommercially. xenotime ~roductionis relativelv small. since its occurrence in nature is minor compared to monazite. Monazite and xenotime contain a greater portion of the heavier rare
Commercial rare earth production is derived from two types of ore: rare earth-thorium orthophosphates in the form
'
Although considered to be a rare earth by the International Union of Pure and Aoolied . , Chernistrv.,. scandium does not occur in nature in significant concentrations with the other rare earths, and it is not considered a part of the industrial processing of rare earths. Yttrium. although not a lanthanide, occurs with the lanthanides, and its sirnilarity to the lanthanides classes it as a rare earth.
Howard E. Krsmersisa retired director of marketing of Kerr-McGee Chemical Corporation. His experience with rare earths goes back morethan 40 years, and he was at one time director of research for the formerLindsay Chemical Company. He wasdeeply involved in promoting the commercial availability of both Inexpensive rare earth mixtures and then highly purified rare earn materials when they fimt became commercially availablethrough ion exchange technology.
Volume 62 Number 8 August 1985
665
earths and yttrium than does bastnasite. Processors of rare earth ores elect to use either monazite or bastnasite depending on their access to long-term availability of ore and on demands for thorium and the heavier rare earths and yttrium. The treatment of monazite and xenotime differs from the treatment of bastnasite, a t least until the rare earths have been isolated and freed from imourities such as thorium. phosphate, radioactive materials,Hnd common metals. Historically, monazite was digested with sulfuric acid to convert the phosphates to water-soluble sulfates. However, acid treatment is no loneer used because of difficulties in adequately separating thorium and phosphate from the rare earths and problems in the management of acid wastes. Treatment of the ore with caustic soda, the preferred method, permits recovery of the phosphate content of the ore, does not present corrosion problems, and makes for better separation of thorium and phosphate. In the caustic process, ground ore containing about 60% rare earth oxide is heated with 65% NaOH a t 140 "C for several hours to convert the phosphates to rare earth and thorium hydroxides and trisodium phosphate. The reaction mixture is treated with hot water to dissolve the trisodium phosphate, leaving the rare earths and thorium as insoluble hvdroxides which are removed hv filtration. Cooline the fiitrate crystallizes trisodium phosphate, leaving a mother liquor containina- excess caustic soda which is recvcled to the ore treating process. The washed thorium-rare earth hydroxides are selectively extracted with a mineral acid, usually HC1 or HNOa a t pH 3.5-4. This dissolves the rare earths, leaving insoluble thorium hydroxide, unreacted ore and gangue, and some rare earth phosphate. The rare earth solution is processed into commercid rare earth compounds or is subjected to separation into other rare earth mixtures or into purified rare earth materials. The thorium residue is reserved for future thorium recoverv. or i t mav he treated with nitric acid to eenerate a thorium nitrate solution for purification by liquib-liquid extraction. Radiation management is necessary in dealing with thorium-rontainina ores. 'Monazite contains thorium and uranium in the raGo of about 0.1 and 0.01 parts per part of rare earths, respectively. Radiation protection is concerned largely with controlling g8Ra ("mesothorium I," a radium isotope), ii6Ra (radium), and their daughters i$n Pthoron." a radon i s o t o ~ eand ) %E2Rn(radon) and t h e ~ asr soclated gamma and alp'ha radldiions. Thenresence of thorium in rare earth maceriala is undesirable, and thorium and uranium radioactive daughter products in the ore must he removed and suitably of. . disposed In the initial processing of ores containing thorium, a barium compound, generally barium sulfate, is added to the ore to act as a carrier for radium and mesothorium. Eventually, the harium-radium sulfate is recovered with the insoluble eaneue and unreacted ore. ~ i principal e commercial bastnasite deposite occurs a t Mountain Pass, California, associated with barite (BaSOa), celestite (SrSOa), and calcite (CaC03). The open pit ore averaging 7-10% RE oxide is crushed and ground, and subjected to flotation t o recover a hastnasite-calcite concentrate containing ahout 60% RE oxide. Some of the 60% concentrate is soid for metallurgical use. Treatment of the 60% concentrate with dilute HC1 dissolves most of the calcite, yielding an insoluble 70%RE oxide concentrate, which is sold for some applications or is further processed into more refined rare ea;
