Monazite production

A large portion of monazite production comes from mineral sand deposits. In the beneficiation of monazite from mineral sand deposits that contain garnet, ilmenite, shell and silicates, the physical concentration and combination of physical preconcentration-flotation is used. Several reagent schemes using flotation were developed throughout various studies [8-10] and some have been confirmed in continuous pilot plants. 

Both Australia and Malaysia have, so far, remained free of government controls on monazite production and export, unlike India and Brazil. Hence, all the monazite produced is exported, mainly to the U.S.A., France, and the UK. 

Commercial mining of rare-earth reserves began in the late 1800s. Monazite was the principal rare-earth source up until 1965. Thereafter bastnaesite production exceeded monazite production and as of 1992 bastnaesite, which is the world s principal source of rare earths, constituted 65% of world output of rare-earth minerals (see Table 5). In addition to the conventional ores, there are several other rare-earth resources having a low level of industrial production. 

Table 6.15 summarizes two sources of information on the annual rate of thorium production, by country. The first three columns give the production rate of monazite concentrates for the more recent years of 1976, 1977, and 1978 [El]. We have estimated total thorium production from a typical monazite thorium content of 6 weight percent (w/o). These columns do not include monazite production in the United States or Soviet Union, nor the small production of other thorium minerals. The last two columns give the U.S. Bureau of Mines figures [Ul] for total thorium production in 1973 and an estimate of total thorium production capacity in 1980, if demand were such as to support it. 

There are no significant amounts of monazite production or of heavy lanthanide production in the U.S. at present. Most U.S. and foreign needs for heavy lanthanides are met through placer operations in Malaysia, Thailand, Brazil, and India, where byproduct monazite and xenotime are recovered from the mining of titanium and tin. A heavy lanthanide concentrate (primarily Y) is produced in Canada from the processing stream of a uranium extraction mill. 

Bastnasite has been identified in various locations on several continents. The largest recognized deposit occurs mixed with monazite and iron ores in a complex mineralization at Baiyunebo in Inner MongoHa, China. The mineral is obtained as a by-product of the iron ore mining. The other commercially viable bastnasite source is the Mountain Pass, California deposit where the average Ln oxide content of the ore is ca 9%. This U.S. deposit is the only resource in the world that is minded solely for its content of cerium and other lanthanides. 

Thorium is widely but rather sparsely distributed and its only commercial sources are monazite sands (see p. 1229) and the mineral conglomerates of Ontario. The former are found in India, South Africa, Brazil, Australia and Malaysia, and in exceptional cases may contain up to 20% Th02 but more usually contain less than 10%. In the Canadian ores the thorium is present as uranothorite, a mixed Th,U silicate, which is accompanied by pitchblende. Even though present as only 0.4% Th02, the recovery of Th, as a co-product of the recovery of uranium, is viable. 

The separation of basic precipitates of hydrous Th02 from the lanthanides in monazite sands has been outlined in Fig. 30.1 (p. 1230). These precipitates may then be dissolved in nitric acid and the thorium extracted into tributyl phosphate, (Bu"0)3PO, diluted with kerosene. In the case of Canadian production, the uranium ores are leached with sulfuric acid and the anionic sulfato complex of U preferentially absorbed onto an anion exchange resin. The Th is separated from Fe, A1 and other metals in the liquor by solvent extraction. 

Primary Mined particularly for the molybdenum contained in the ores. In some instances, molybdenum could be the only valuable metal recovered from the ore. The Questa deposit in New Mexico is mined exclusively for molybdenum content. In other deposits molybdenum may be the main product recovered together with one or more products. In these deposits the molybdenum content alone would allow for a profitable operation. The ore at the Climax mine in Colorado is of this type. Currently, monazite, pyrite, tin, and tungsten are recovered from the ore none of these by-products exists singly nor together in sufficient quantity so that the ore could be mined profitably merely for the extraction of one or all of these by-products. 

Product Weight (%) % Monazite assay % Monazite recovery... 

The most abundant titanium sand deposits are black sands in streams and on beaches of volcanic regions. The principal black minerals are magnetite, titanoferous magnetite and black silicates, chiefly angite and homblend. It is quite difficult to produce an ilmenite suitable for pigment product from black sand, but other sand deposits that contain rutile, ilmenite and often monazite are found in Australia, USA, India and Africa. These deposits are either alluvial or marine in origin. 

Thorium is a radioactive metal that occurs naturally in several minerals and rocks usually associated with uranium. However, it is approximately three times more abundant in nature than uranium. On average, soil contains 6 to 10 ppm of thorium. Thorium is most commonly found in the rare-earth thorium-phosphate mineral, monazite, which contains 8% 10% thorium. Current production of thorium is, therefore, linked to the production of monazite, which varies between 5500 and 6500 tonnes per year, with approximately 300 to 600 tonnes of thorium recovered (NEA/IAEA, 2006a). 

Thorium is produced in smaller amounts than uranium, but if its production increases in the future the tailings problem will be very similar. The rare earth industry also produces comparable radioactive effluents because many minerals that contain rare earths (e.g., monazites) also include Th and U. 

The ores from which rare-earth elements are extracted are monazite, bastnasite, and oxides of yttrium and related fluorocarbonate minerals. These ores are found in South Africa, Australia, South America, India, and in the United States in Cahfomia, Florida, and the Carolinas. Several of the rare-earth elements are also produced as fission by-products during the decay of the radioactive elements uranium and plutonium. The elements of the lanthanide series that have an even atomic number are much more abundant than are those of the series that have an odd atomic number. 

It is found in ores such as monazite, gadohnite, and bastnasite. It was first separated into three elements in 1843 (yttria, erbia, and terbia). Erbium is also produced as a by-product of nuclear fission of uranium. 

Monazite occurs as anhedral to subhedral masses associated with phyllosilicate-rich domains in massive sulfides. Masses display simple concentric zoning, a primary core, and commonly show fracturing and healing involving later generations of monazite growth that are likely a product of metamorphism (Fig. 2a). 

Europeum generally is produced from two common rare earth minerals monazite, a rare earth-thorium orthophosphate, and bastnasite, a rare earth fluocarbonate. The ores are crushed and subjected to flotation. They are opened by sulfuric acid. Reaction with concentrated sulfuric acid at a temperature between 130 to 170°C converts thorium and the rare earths to their hydrous sulfates. The reaction is exothermic which raises the temperature to 250°C. The product sulfates are treated with cold water which dissolves the thorium and rare earth sulfates. The solution is then treated with sodium sulfate which precipitates rare earth elements by forming rare earth-sodium double salts. The precipitate is heated with sodium hydroxide to obtain rare earth hydrated oxides. Upon heating and drying, cerium hydrated oxide oxidizes to tetravalent ceric(lV) hydroxide. When the hydrated oxides are treated with hydrochloric acid or nitric acid, aU but Ce4+ salt dissolves in the acid. The insoluble Ce4+ salt is removed. 

Several other processes also are apphed for the commercial production of europium. In general, all processes are based upon the initial steps involving opening the mineral (bastnasite or monazite) with sulfuric acid or sodium hydroxide, often followed by roasting and solubihzation. In one such process after separation of cerium, the soluble rare earth chloride mixture in HCl solution is pH adjusted and treated with bis(2-ethylhexyl)phosphate to obtain europium sesquioxide, EuaOs. 

Holmium is obtained from monazite, bastnasite and other rare-earth minerals as a by-product during recovery of dysprosium, thulium and other rare-earth metals. The recovery steps in production of all lanthanide elements are very similar. These involve breaking up ores by treatment with hot concentrated sulfuric acid or by caustic fusion separation of rare-earths by ion-exchange processes conversion to halide salts and reduction of the hahde(s) to metal (See Dysprosium, Gadolinium and Erbium). 

Lutetium is produced commercially from monazite. The metal is recovered as a by-product during large-scale extraction of other heavy rare earths (See Cerium, Erbium, Holmium). The pure metal is obtained by reduction of lutetium chloride or lutetium fluoride by a alkali or alkaline earth metal at... 

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%. 

There are several processes for commercial thorium production from monazite sand. They are mostly modifications of the acid or caustic digestion process. Such processes involve converting monazite to salts of different anions by combination of various chemical treatments, recovery of the thorium salt by solvent extraction, fractional crystallization, or precipitation methods. Finally, metalhc thorium is prepared by chemical reduction or electrolysis. Two such industrial processes are outlined briefly below. 

Thorium dioxide is obtained as an intermediate in the production of thorium metal from monazite sand (See Thorium). 

Third example - Although ceric oxide really represents the active element in polishing media, most of the polishing plants are satisfied with about 50 % pure ceric oxide, which is available in the natural mixture of the light rare eai h elements as they are extracted either from bastnasite or monazite, in order to keep the cost of the product low. 

These materials are available through production of bastnasite at the Mountain Pass Mine in California, of monazite from Australia, India and Brazil, and of monazite as a by-product from the production of tin ores, rutile and various heavy mineral sands. 

From 1940 to 1965, the principal source of these rare earth products was the mineral monazite (Th, RE orthophosphate) which fortunately or unfortunately, depending on one s point of view, contains 4-6% thorium. Today, there is essentially no market for thorium in the U.S. The expense of separating out thorium-free rare-earth products from monazite is not only excessive, but bound tightly in governmental red tape because of the mild radioactivity of the thorium. This situation does not apply in France, Brazil, or India, whose governments are wisely stockpiling all extracted thorium for future atomic energy needs. 

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