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Isolation of the Rare Earths Form Monazite
Monazite (R.E + Th) P04 -i-* (R.E)2(S04)8 + Th(S04)2 + H,P04 (5)
After filtering off the solid residue, the cold solution is treated with sodium sulphate. The precipitated double sulphate of the rare earths is washed with 0.3N sodium sulphate, and then boiled with excess (10%) sodium hydroxide to convert the double sulphate into a hydroxide. The rare earths hydroxides are filtered off and dried at 120° C to oxidize trivalent cerium into the tetravalent state. The dried material contains about half of the original thorium coprecipitated with rare earth double sulphates.
The filtrate from the double sulphate precipitation contains the other half of the thorium and small amounts of the heavy rare earth double sulphates. This solution is heated to—90°, and oxalic acid is added. The precipitates contain phosphato-oxalates of thorium and rare earths (heavy).
There are various modifications of the process just described. Direct extraction of thorium from monazite after conversion to the sulphate has been considered. Recovery of thorium by selective precipitation as oxalate [145, 146], sulphate  or fluoride  has been attempted.
Alkaline breakdown of monazite. —The flowsheet for the process is given in Fig. 5. For this process it is necessary to remove calcium from the monazite by boiling the mineral with nitric or hydrochloric acid. It has long been known that monazite can be attacked by alkali. Rohden and Peltier  have shown the practical applicability of the alkali process for monazite decomposition. The optimum conditions necessary for alkaline decomposition of monazite have been extensively studied by Kaplan and his coworkers [150—153].
Some thorium is extracted along with calcium during the HC1 treatment, and it is recovered by precipitation from the filtrate at pH — 1.8 and returned to the process. The usual industrial practice is to attack the monazite with 60—70% NaOH at a temperature of 140—50° C for ~4 hrs. Better results are obtained at 170° C under higher pressures . After the reaction is complete the mixed rare earth-thorium hydroxides are filtered off while hot. The precipitates are carefully washed to remove
(R.E + Th)P04-^i R.E(OH)3 + Th(OH)4 + NasP04 (6)
all phosphate. Air oxidation of cerium during the early stage of the process is particularly undesirable and should be avoided.
The trisodium phosphate is recovered from the filtrate by concentration and crystallization. The mother liquor from this crystallization process is concentrated, and about 50% of the sodium hydroxide is recycled.
22 Methods of Separation of Individual Rare Earth Elements
Acid extraction of calcium
70% NaOH 4 hrs. at 150° Filter
Residue Rare Earths Thorium Hydroxide
HC1 leaching ~80°, pH 3-5—4
Residue Rare Earth
Thorium Chlorides Soln.
Fig. 5. A schematic representation of the alkaline breakdown of monazite
There are several ways of treating the mixed hydroxide cake. The cake is either dissolved in HC1 or HNOa and thorium is removed. The original Rohden process involved a partial dissolution of the mixed hydroxide in HC1 at 70—80° C (pH 3.5—4). The crude thorium hydroxide was filtered off and the filtrate contained the rare earths practically free from thorium and phosphate.
The possibility of dissolving the mixed hydroxide in HNO3 and obtaining direct extraction of thorium (and uranium) from the nitrate solution has been studied [155,156], but does not seem to be too promising, possibly due to the partial oxidation of tripositive cerium to the tetrapositive state. Kkaitzer,  was able to separate thorium from the mixed hydroxide cake by extracting the cake with sodium carbonate buffer at pH 9.5—10. Thorium was found to form a soluble carbonate complex and a recovery of better than 99% of thorium was claimed after only four extractions.
Sulphuric acid vs. caustic soda breakdown. — Although a greater variety of monazite sands can be processed by sulphuric acid treatment than by the
Tsolation of the Rare Earths Form Monazite
caustic soda method, higher yields and cleaner separations are the main advantages of the latter process. Moreover, the byproduct, trisodium phosphate, can be marketed as a technical grade chemical. However, the sulphuric acid method has the advantage over the caustic soda process of being slightly cheaper to operate. In the caustic soda breakdown of monazite, concentration of the rare earths is achieved at a relatively early stage of the process.
Preparation and Properties of Europium
Early in 1827 when Mosander  first prepared rare earth metals by the reduction of their chlorides with potassium in a hydrogen atmosphere, the pattern for modem research was actually set. During 1935 Urbain, Weiss, Trombe , Klemm and Bommer  made significant contributions to this field.
Various types of reducing agents were tried for the preparation of rare earth metals. Keller et al.  used calcium as a reductant in the metallothermic preparation of cerium. An excess of calcium was used to provide the heat neccessary to fuse the reaction products. The reaction vessels were sealed steel bombs having a dolomite lining. Cerium was recovered by vacuum melting the product which removed calcium. By using this technique Spedding et al.  prepared a few other metals, La, Pr and Nd, but the results were not very satisfactory when the technique was extented to the preparation of Gd and Y. In the case of Sm the process completely failed, giving no free metal. The reaction involved in the reduction process is given by the equation