Monazite separation processes

A commercial digestion process is currently in use for the extraction of REE, including yttrium from monazite. The process is based on the application of caustic soda, and one of the products is REE hydroxide. The rare earths are leached from bastnaesite with hydrochloric acid (or sulfuric acid), followed by calcination at >600°C they are then treated with 16 M nitric acid (Kirk-Othmer 1999). Yttrium is produced as pure silver metal, both on the laboratory and industrial scale, by molten salt electrolysis and metallothermic reduction of the fluoride, oxide, or chloride with calcium following an enrichment process, after separation by fractionated crystallization, ion exchange... 

The sulfates are probably the most frequently studied group of inorganic complex compounds formed by the rare earths. Ever since the discovery of the rare earths, the sulfates have also played an important role in separation processes. Even today, the low solubilities of the sodium double sulfates are exploited for the precipitation of the rare earths from monazite and xenotime in the acid process (Hedrick, 1985). 

The electrostatic separation method is the exclusive choice in some specific situations, for example in the cases of rutile and ilmenite deposits. These deposits generally contain minerals of similar specific gravities and similar surface properties so that processes such as flotation are unsuitable for concentration. The major application of electrostatic separation is in the processing of beach sands and alluvial deposits containing titanium minerals. Almost all the beach sand plants in the world use electrostatic separation to separate rutile and ilmenite from zircon and monazite. In this context the flowsheet given later (see Figure 2.35 A) may be referred to. Electrostatic separation is also used with regard to a number of other minerals. Some reported commercial separations include those of cassiterite from scheelite, wolframite from quartz, cassiterite from columbite, feldspar from quartz and mica, and diamond from heavy associated minerals. Electrostatic separation is also used in industrial waste recovery. 

Lanthanide, as a pure metal, is difficult to separate from its ores, and it is often mixed with other elements of the series. It is mosdy obtained through an ion-exchange process from the sands of the mineral monazite, which can contain as much as 25% lanthanum as well as the oxides of several other elements of the series. The metal is malleable and ductile and can be formed into many shapes. Lanthanum is considered the most basic (alkaline) of the rare-earth elements. 

Although neodymium is the 28th most abundant element on Earth, it is third in abundance of all the rare-earths. It is found in monazite, bastnasite, and allanite ores, where it is removed by heating with sulfuric acid (H SO ). Its main ore is monazite sand, which is a mixture of Ce, La, Th, Nd, Y, and small amounts of other rare-earths. Some monazite sands are composed of over 50% rare-earths by weight. Like most rare-earths, neodymium can be separated from other rare-earths by the ion-exchange process. 

Of all the 17 rare-earths in the lanthanide series, terbium is number 14 in abundance. Terbium can be separated from the minerals xenotime (YPO ) and euxenite, a mixmre of the following (Y, Ca, Er, La, Ce, Y, Th)(Nb, Ta, Ti O ). It is obtained in commercial amount from monazite sand by the ion-exchange process. Monazite may contain as much as 50% rare-earth elements, and about 0.03% of this is terbium. 

Lutetium is the 60th most abundant element on Earth, and it ranks 15th in the abundance of the rare-earths. It is one of the rarest of the lanthanide series. It is found in monazite sand (India, Australia, Brazil, South Africa, and Florida), which contains small amounts of all the rare-earths. Lutetium is found in the concentration of about 0.0001% in monazite. It is difficult to separate it from other rare-earths by the ion-exchange process. In the pure metallic form, lutetium is difficult to prepare, which makes is very expensive. 

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). 

The monazite sand is heated with sulfuric acid at about 120 to 170°C. An exothermic reaction ensues raising the temperature to above 200°C. Samarium and other rare earths are converted to their water-soluble sulfates. The residue is extracted with water and the solution is treated with sodium pyrophosphate to precipitate thorium. After removing thorium, the solution is treated with sodium sulfate to precipitate rare earths as their double sulfates, that is, rare earth sulfates-sodium sulfate. The double sulfates are heated with sodium hydroxide to convert them into rare earth hydroxides. The hydroxides are treated with hydrochloric or nitric acid to solubihze all rare earths except cerium. The insoluble cerium(IV) hydroxide is filtered. Lanthanum and other rare earths are then separated by fractional crystallization after converting them to double salts with ammonium or magnesium nitrate. The samarium—europium fraction is converted to acetates and reduced with sodium amalgam to low valence states. The reduced metals are extracted with dilute acid. As mentioned above, this fractional crystallization process is very tedious, time-consuming, and currently rare earths are separated by relatively easier methods based on ion exchange and solvent extraction. 

Terbium is recovered from the minerals, monazite, xenotime, and euxenite. The recovery processes are quite similar to those of other lanthanide elements (See individual lanthanide elements). The metal is separated from other rare... 

In one acid digestion process, monazite sand is heated with 93% sulfuric acid at 210°C. The solution is diluted with water and filtered. Filtrate containing thorium and rare earths is treated with ammonia and pH is adjusted to 1.0. Thorium is precipitated as sulfate and phosphate along with a small fraction of rare earths. The precipitate is washed and dissolved in nitric acid. The solution is treated with sodium oxalate. Thorium and rare earths are precipitated from this nitric acid solution as oxalates. The oxalates are filtered, washed, and calcined to form oxides. The oxides are redissolved in nitric acid and the acid solution is extracted with aqueous tributyl phosphate. Thorium and cerium (IV) separate into the organic phase from which cerium (IV) is reduced to metalhc cerium and removed by filtration. Thorium then is recovered from solution. 

Monazite or Xenotime. The rare earth phosphate containing ores are attacked with either concentrated sulfuric acid or sodium hydroxide solution. The processing involves cracking the ore, removing the thorium, and separating the lanthanides. 

The crude monazite sands are first concentrated by the general mechanical and physical treatments of mineral dressing. Brazilian monazite sands are processed mainly by the electromagnetic separation technique. The Indian monazite industry is essentially based on the recovery of rutile and ilmenite. Ilmenite has a magnetic susceptibility very close to that of monazite and thus direct electromagnetic separation cannot be applied to the Indian Travancore monazite sands. 

There are a number of minerals in which thorium is found. Thus a number of basic process flow sheets exist for the recovery of thorium from ores (10). The extraction of monazite from sands is accomplished via the digestion of sand using hot base, which converts the oxide to the hydroxide form. The hydroxide is then dissolved in hydrochloric acid and the pH adjusted to between 5 and 6, affording the separation of thorium from the less acidic lanthanides. Thorium hydroxide is dissolved in nitric acid and extracted using methyl isobutyl ketone or tributyl phosphate in kerosene to yield Th(N03)4,... 

Mineral Beneficiation Electrostatic methods are widely used in the processing of ores with mineral concentrates. Generally, electrostatic separation is used as a part of an overall flow sheet comprising various combinations of physical separation procedures. It is particularly well established in the processing of heavy-mineral beach sands from which are recovered ilmenite, rutile, zircon, monazite, silicates, and quartz. High-grade specular hematite concentrates have been recovered at rates of 1000 tons/h in Labrador. Applications also include processing tin ores to separate cassiterite from columbite and ilmenite. Refer to Fig. 19-61 . 

A fraction of Ce, La. Nd and Pr derived from bastnasite or monazite is a typical feedstock in the recovery process of cerium on a commercial scale. Separation of the rare-earth elements may be achieved by splitting the mixed rare-earth elements into a cerium/lanthanum and didymium (Nd/Pr) fraction first. The cerium/lanthanum fraction may be used as a further feedstock in a second extraction stage and will yield high pure cerium and lanthanum solution respectively. Cerium can then be precipitated as. for example, an oxalate or a carbonate which may be used as precursor for cerium derivatives. 

Zircon is usually accompanied by quartz, ilmenite and rutile, and in lesser amounts by garnets, kyanite and monazite, The treatment processes utilize the differences in density ilmenite and rutile are extracted by magnetic and electrostatic separation respectively. These processes provide technical-grade zircon purer zircon is produced by acid leaching. The product usually contains particles mainly 75 — 150 im. 

Monazite is usually a minor constituent of deposits of other minerals, all of which must be separated and processed for a profitable venture. As an example, the mineral constituents of beach sands in Travancore, India, which are dredged for their zirconium, titanium, thorium, and rare-earth content, are as follows ... 

Sulfuric acid has been used to dissolve monazite in Europe, Australia, and the United States. The numerous processes used to separate thorium from the acid leach liquon are listed... 

Figure 6.6 Iowa process for separating thorium, rare earths, and uranium from monazite sulfate solutions.

Precipitation with oxalic acid. Figure 6.6 shows the principal steps in the process for separating the sulfuric acid solution of monazite into a thorium concentrate, a rare earth concentrate, and a uranium concentrate developed at the Ames, Iowa, Laboratory of the U.S. Atomic Energy Commission [Bl]. 

Attempts to separate thorium and uranium from sulfuric acid solution of monazite by solvent extraction with TBP were unsuccessful because distribution coefficients of uranium and thorium from monazite solutions were too low, as these elements are complexed by phosphate ion. Development of extractants with higher distribution coefficients for these metals has made solvent extraction a practical process for recovering uranium and thorium from monazite sulfate solutions and from sulfuric acid solutions of other thorium ores. This section describes processes tested on a pilot-plant scale by Oak Ridge National Laboratory [C5]. 

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