AMERICAN INSTITUTE OF MINING AND METALLURGICAL ENGINEERS Technical Publication No. a037 (CLASSH. INDUSTRIALMINERALS. NO. 147) DISCUSSION OF THIS PAPER IS INVITED. Discussion in writing (a co ies) may be sent to the Secretary. American Institute of Mining and.Metalluqgical Engineers. 29 West 39th gtreet. New York 18. N. Y. Unless spec~alarrangement is made, d~scuss~onof thls paper will close Dec. 31. 1946. Any d~scusslonoffered thereafter should preferably be in the form of a new paper. Technology and Uses of Monazite Sand

MONAZITEhas had a Cinderella-like sluiced away lighter materials and con- history. Although nearly go per cent pure centrated the heavy (D = 5.0 to 5.2) rare-earth compound (rare-earth phos- grains of monazite, together with ilmenite, phate) it was sought at first not for the zircon, garnet, and other dense minerals. rare earths but for the sake of a minor The Carolina deposits were in stream constituent-thorium. The thorium, essen- beds; most of the other deposits mentioned tial for the Welsbach gaslight mantle, was are on or near the seashore, where the present in only small quantities and the action of the waves has served to con- ~rincipal constituents, the rare earths, centrate the monazite. were largely discarded. Gradually, how- In most cases, today, the sands are ever, the rare earths have developed uses worked to obtain the ilmenite and zircon, of their own and today have left the as well as monazite. The minerals are thorium far behind in value. separated by means of sluices, dry tables, and magnetic separators, and each is finally obtained nearly pure. No crushing Monazite was first produced commer- or grinding is necessary. The concentrates cially about 1886, in North Carolina, are shipped in fiber bags. where it was collected in small sluices by farmers who found it in stream beds on their land. The deposits were later ex- As Table I illustrates, monazite is the ploited on a larger scale, but were mostly anhydrous orthophosphate of the cerium abandoned in a few years, when monazite from Brazil came upon the market in far TABLE ~.-TyPical Analysis of a Monazite Sand (India). larger quantities. Domestic production PER CENT ceased entirely in 1906. Thorium oxide...... 8.1 Ceriumoxide ...... 30.6 The Brazilian source is still active, Lanthanum oxide...... 15.7 Neodymium oxide...... 10.5 together with Indian, Dutch East Indies, Praseodymium oxide...... 2.9 Samarium oxide...... I.o and Australian deposits. Several other Europium, gadolinium, and terbium oxides.. .. 0.7 Yttrium oxide...... 0.4 localities in the United States have Dysprosium, holmium, erbium, thulium, ytter- yielded monazite, but none of these has bium. and lutecium oxides...... o. 1 Silicon oxide...... : ..... 2 .4 produced commercially as yet. Calcium, aluminum, and iron oxides...... I . o Uranium oxide...... 0.3 Monazite occurs originally in peg- Phosphorus pentqxide...... 26.2 'The values given for the rare earths follomng matite~ and gneisses, through which, samanum are approximate. being based upon analyses of concentrated fractions derived from monazite. unfortunately, it is scattered in highly dis- The composition given represents Travancore mona- persed form. All commercially useful zite carefully freed from other minerals. deposits consist of transported monazite group of rare-earth elements. A small sands liberated by erosion and concen- amount of yttrium and terbium group trated by the action of water, which has phosphate is also present.* All the rare- *Lindsay Light and Chemical Co., West Chicago, Illinois. Elements 57 to 62. inclusive, constitute Manuscript received at the office of the the cerium group. elements 63 to 65 inclusive Institute Sept. 4. 1945; revised Feb. 21, 1946. the terbium group, and elements 39 and 6671 Copyright, 1946, by the American Institute of Mining and Metallurgical Engineers. Inc. MINING TECHNOLOGY.July 1946. Printed in U. S. A. 2 TECHNOLOGY AND USES OF MONAZITE SAND

earth elements are of course isomorphous, Other uses of thorium are less well and react generally as one element. known. The oxide of thorium is used in Nonazite nearly always contains tho- radio tubes, as a constituent of special rium, which is chemically rather than optical glasses, as a high-temperature mechanically present, together usually refractory (M.P. ~ooo°F.), and as a with an approximately equivalent amount catalyst for petroleum cracking. of silica. The thorium content varies with the source of the ore. Rare-earth Salts The natural mixture of cerium and other DER~'ATNESOF MONAZITEAND THEIR rare earths as recovered from monazite USES has many large-tonnage uses in different Mesothorium forms. The mixed rare-earth oxide and the The radioactivity of monazite follows fluoride are used as a core material in from its content of thorium and its decom- searchlight and projector-arc carbons, position product, mesothorium, which where the brilliant spectrum of the rare- is an isotope of radium. Hocvever, the earth elements increases the visible light mineral also contains traces of uranium output. Rare-earth chloride is electrolyzed and hence radium. Mesothorium is similar in the fused condition to produce rare- in every way to radium and is useful for earth metal, which alone or alloyed with the same purposes. It decays at a much iron forms the well-known "flints" used faster rate, however, having a half-life in lighters. Rare-earth chloride and acetate of 6.7 years as compared with 1590 for are used as waterproofing agents and radium. Monazite actually contains a fungicides in textile manufacture. Rare- greater weight of radium than it does of earth oxalate is used medicinally as an mesothorium, but because of the greater agent to prevent nausea and seasickness. decay rate the activity is nearly all due to Cerium mesothorium. It is possible to recover radioactive material equivalent to one The high cerium content of monazite is gram of radium from about 300 tons of fortunate, as this element has unique and monazite. This value varies with the useful properties and may be readily thorium content of the ore. separated from the other rare earths. Because of its two valence states, cerium Thorium in some of its compounds is a valuable For many. years- monazite sands were oxygen carrier and catalyst. In this way priced according to their thorium content, it is used for decolorizing glass, through which was the sole useful constituent. its power of holding iron in the ferric state. The oldest and best known use for thorium Thus ordinary window and bottle glass, is the incandescent mantle, still used for as well as higher grades, may be produced street lighting in many cities. The well- with cheaper grades of glass sand. In known Aladdin kerosene lamps and Cole- paint and printing-ink driers, cerium man gasoline lanterns are widely used in naphthenate, though not widely used, rural districts. The armed forces have permits special effects to be obtained in used millions of mantles in the present some cases, since tremendous quantities of war. drier may be added without the usual ill effects. inclusive the yttrium group. These groups are not sharply characterized; the division is Various other catalytic reactions with based partly on the solubility of the double cerium are known. Cerium metal is used sulphates and partly on their distribution in certain minerals. in alloys, various salts are valuable labora- R. PHILIP HAMMOND 3 tory reagents, and the oxide (or hydrated silica, but consists entirely of the oxides of oxide) is used as a coloring agent in ceramic rare elements, with lanthanum oxide as glazes. the base. The glass, of very high density, A most important use is as a polishing has a high index of refraction and a rela- agent for lenses, prisms, and other sur- tively low dispersion, making possible faces. The development of a suitable lenses of improved color correction with form of cerium oxide for this use helped fewer elements and with curved surfaces break a critical bottleneck in optical of larger radius. equipment in the early days of the war. The tremendous demand for tank peri- Neodymium scopes, gun sights, binoculars, range Neodymium hds its principal use in finders, photographic lenses, and other the glass industry, where it serves to optical goods found the manufacturers neutralize the greenish tinge caused by of lcns-polishing equipment hopelessly iron. Whereas cerium decolorizes glass swamped. Rouge had been used for cen- chemically by oxidation of iron, neo- turies as the polishing agent, but in approxi- dymium is a physical decolorizer, since it mately three years has been almost imparts a complementary hue. Both are completely supplanted bv optical-grade often used in ordinary types of glass. cerium oxide. The outstanding advantage of In larger proportions, neodymium pro- cerium oxide, the use of which for optical duces an orchid tint of great beauty used polishing was unknown in this country in expensive stemware, and also in "Soft- before the war, is the tremendous speed Lite" spectacle lenses, where the ultra- with which a perfect polish is obtained. violet opacity of neodymium glass is Optical-grade cerium oxide polishes two utilized . For large-scale uses, neodymium and three times as fast as the best rouge, is often employed in its natural mixture and in addition has several other advan- with the other rare earths, except cerium. tages (such as cleanliness) greatly ap- The mixture, known commercially as preciated by lens polishers. With this new "didymium,"* contains about 33 per cent polish, the optical industry was enabled neodymium; the other rare earths present to double or triple its output with existing (mostly lanthanum) have little or no machinery. Today even spectacle lenses effect on glass in the amounts used. are polished with cerium. Large quantities of this mixture are also The peculiar combination of properties used in the manufacture of electronic required for lens polishing is not fully apparatus, particularly where a ceramic understood. The particle size is important, dielectric body is required having a zero but even more so is the ability to "break temperature coefficient. down" or disintegrate during the actual This summary of uses is abbreviated, polishing process. The ultimate particle and includes only the monazite derivatives size obtained is a fraction of a micron. available in tonnage quantities, but it Lanthanum serves to show the variety of fields into which the unique properties of the rare Lanthanum was of little use to anyone before the war, but this element has * Didymium. strictly speaking, consists only of the "twin elements" neodymium and played an important part in the defeat of praseodymium, but the term is used com- Germany as the major constituent of a mercially to denote the mixture with lan- thanum and samarium. The approximate new photographic lens de- composition is neodymium 33.5 per cent, veloped for use in aerial reconnaissance. lanthanum 50 per cent. praseodymium 9.5 per cent, samarium and other rare earths 7 per The glass used for this lens contains no cent. 4 TECHNOLOGY AND USES OF MONAZITE SAND

earths bring them. Many uses are still further. The separation of cerium from the restricted because of the mar; others are rest of the earths may be carried out in a being proposetl and tlevclo~~edcontinually. number of ways, all based upon the oxida- tion of cerium to its higher valence (te- travalent), in which form many of its salts are insoluble or hydrolyze, whereas the salts of the other rare earths, all Monazite may be opened in several trivalent, remain soluble. The cerium. ways, but the only one that has been once separated, is purified and marketed extensively used is that of roasting with as hydrate, chloride, nitrate, or oxide. strong sulphuric acid. The heating must Some of these processes, such as that for be carried out cautiously in a closed cast- optical cerium, require careful control to iron vessel, as the reaction is exothermic. produce exactly the desired properties in About 8 lb. of barium sulphate or other the finished product. suitable barium salt is added to each ton of The "didymium" from which the cerium monazite sand before roasting, the purpose has been removed is sold usually as the of which is to assist in isolating the radium oxide or the carbonate for use in glass or and mesothorium. ceramic manufacture. A portion of the The roasting is carried out in such a didymium, however, is separated still way that the thorium forms insoluble further. At this stage the elements present metaphosphate sulphate, and the rare are: lanthanum, neodymium, praseody- earths are left in soluble form, to be leached mium, samarium, and very small amounts out with water.* The barium, radium, and of other rare earths of atomic numbers mesothorium remain with the thorium as 63 to 71. All of these earths have almost insoluble sulphates. The thorium pre- identical properties as far as the ordinary cipitate is boiled with caustic soda and chemical reactions are concerned, and are then dissolved in sulphuric acid, where- isomorphous in all of their compounds. upon the soluble thorium sulphate may be Because of a slight variation in solubility separated from the mesothorium, barium, of certain salts, however, the elements and radium sulphates. These materials are may be separated by the method known further purified and the radioactive salts as fractional crystallization, which con- concentrated by fractional crystallization. sists of repeatedly crystallizing and re- The thorium undergoes a long series of combining a large number of fractions purifications, and is finally obtained, through dozens of cycles. Although tedious, chemically pure, in a variety of forms, the process may be carried out on any such as oxide, nitrate, carbonate, and desired scale, and in the past few years chloride. has been used to produce some hundreds The solution of the rare earths men- of tons of pure lanthanum salts. The other tioneti above is free from thorium and may elements may be isolated in any desired be converted to any desired form for sale degree of purity by suitable prolongation or further treatment. The rare-earth of the fractionation. mixture is usually soltl as hydroxides, chlorides, fluorides, or oxides. A large part of the mixed rare-earth material is not sold as such but is separated Monazite is a phosphate of the rare- earth metals. It occurs in stream beds or beach sands, where it has been concen- trated by wave action. Large deposits are R. PHILIP HAMMOND 5 found in Brazil, India, Australia, and the from monazite involves a long series of Dutch East Indies. Thorium, lanthanum, complex chemical and physical processes. cerium, and neodymium salts are produced REFERENCES in large tonnages from monazite, and find their way into such everyday products ~~,"~~,",$f~~y&?,"I!' s4~!~~y97~,,lr No. as spectacle lenses, window glass, cigarette (1895) and No. 25 (1916). U. S, Bur. Mines Tech. Paper IIO (19r 5). lighters, moving-picture projectors, and Soddy: The Chemistry of the Radio-Elements. medicines, in addition to more specialized Levy: The Rare-Earths. Friend: A Text-Book of Inorganic Chemistry, uses. The separation of these materials 4 and 5.