Recovery of rare earths from monazite

The second method of recovery of rare earths from monazite involves caustic soda digestion and is illustrated in Fig 1.4. 

Fig. 1.3. Sulphuric acid process for the recovery of rare earths from monazite [2].

The use of various techniques discussed so far is well illustrated by the processing steps adopted in the economic recovery of rare earths from monazite in India [25]. The first step in the processing of monazite is digestion with caustic soda and this is schematically shown in Fig. 1.11. The main reaction that takes place during digestion may be written as ... 

Source D. J. Crouse and K. B. Brown, Recovery of Thorium, Uranium, and Rare Earths from Monazite Sulfate Leach Liquors by the Amine Extraction (Amex) Process, Report ORNL-2720, July 16, 1959. 

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). 

Lanthanum is most commonly obtained from the two naturally occurring rate-earth minerals, monazite and bastnasite. Monazite is a rare earth-thorium phosphate that typically contains lanthanum between 15 to 25%. Bastnasite is a rare earth-fluocarbonate-type mineral in which lanthanum content may vary, usually between 8 to 38%. The recovery of the metal from either of its ores involves three major steps (i) extraction of all rare-earths combined together from the non-rare-earth components of the mineral, (ii) separation or isolation of lanthanum from other lanthanide elements present... 

Neodymium oxide is produced from the two principal rare earth minerals, monazite, and bastnasite. The oxide is obtained as an intermediate in the recovery of neodymium metal (See Neodymium). 

Yttrium oxide is produced as an intermediate in recovery of yttrium from xenotime and monazite (See Yttrium, Recovery). The oxide is produced after separation of rare earth sulfates obtained from digesting the mineral with sulfuric acid on a cation exchange bed, precipitating yttrium fraction as oxalate, and igniting the oxalate at 750°C. 

In this process, caustic soda (NaOH) is used. The phosphate part of the ore is recovered as trisodium phosphate. This product is marketable, which is already a major advantage over the acid process, and this process has been very favorable for commercial use. Fine-ground monazite is treated with a 60-70 % NaOH solution at 140-150 °C. The mixed rare earth thorium hydroxide cake resulting from this process is then treated for recovery of thorium and rare earths. A variety of methods is used to accomplish this (Gupta and Krishnamurthy 2005). An effective process for removing thorium completely and in a very pure state is solvent extraction with higher amines. This should preferably be carried out in a sulphate solution. However, leaching of the rare earths from the hydroxide cake with hydrochloric acid is also very effective. The rare earths are recovered from the leachate by solvents extraction. 

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. 

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