Rare earths, cerium separation from

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. 

Separation based on valency change.—The easy oxidation of Ce3 to Ce4+ permits its isolation from other rare earths. The separation of cerium is usually performed by selective leaching with acids, or by complete dissolution [129, 130] followed by hydrolysis. The solvent extraction behaviour of Ce(N03)4 has been extensively studied. Among the various extractants, alcohols, ethers, organic and inorganic acids, ketones etc., TBP proved to be most advantageous in large scale operations [131,132]. 

Note that aqueous flotation (with appropriate detergent eompounds) is used to separate the rare-earth-rich particles from the siliceous materials. After drying, the ore is calcinated (fired in air to form the oxides) and then treated with concentrated HCl. A cerium-rich concentrate is left after the other rare earths have dissolved. We have not shown how the cerium concentrate is processed since the main object is to obtain the more expensive oxides as a final product. The rare earth chloride solution is then adjusted to pH = 1.0, purified with activated carbon to remove some of the heavy metals and then extracted with DEHPA. The first extraction separates the "heavy" and "light" fractions of the rare earths. The resulting extracted solutions are then subjected to further extraction processes to produce the individual products, including La, Pr and Nd. 

The separation of light rare earths (cerium group) sulfates obtained from bastnasite by primary amine with or without an aqueous phase chelating reagent was studied by Bauer et al. (1968). Kerosene was used as the diluent. The pH of the lanthanide-amine system affects the molecular species present in both the aqueous and organic phases. The extraction of rare earths at high acid concentrations is inhibited due to the formation of stable amine bisulfate... 

Although rare-earth ions are mosdy trivalent, lanthanides can exist in the divalent or tetravalent state when the electronic configuration is close to the stable empty, half-fUed, or completely fiUed sheUs. Thus samarium, europium, thuUum, and ytterbium can exist as divalent cations in certain environments. On the other hand, tetravalent cerium, praseodymium, and terbium are found, even as oxides where trivalent and tetravalent states often coexist. The stabili2ation of the different valence states for particular rare earths is sometimes used for separation from the other trivalent lanthanides. The chemicals properties of the di- and tetravalent ions are significantly different. 

Separation Processes. The product of ore digestion contains the rare earths in the same ratio as that in which they were originally present in the ore, with few exceptions, because of the similarity in chemical properties. The various processes for separating individual rare earth from naturally occurring rare-earth mixtures essentially utilize small differences in acidity resulting from the decrease in ionic radius from lanthanum to lutetium. The acidity differences influence the solubiUties of salts, the hydrolysis of cations, and the formation of complex species so as to allow separation by fractional crystallization, fractional precipitation, ion exchange, and solvent extraction. In addition, the existence of tetravalent and divalent species for cerium and europium, respectively, is useful because the chemical behavior of these ions is markedly different from that of the trivalent species. 

The rare earth (RE) ions most commonly used for applications as phosphors, lasers, and amplifiers are the so-called lanthanide ions. Lanthanide ions are formed by ionization of a nnmber of atoms located in periodic table after lanthanum from the cerium atom (atomic number 58), which has an onter electronic configuration 5s 5p 5d 4f 6s, to the ytterbium atom (atomic number 70), with an outer electronic configuration 5s 5p 4f " 6s. These atoms are nsnally incorporated in crystals as divalent or trivalent cations. In trivalent ions 5d, 6s, and some 4f electrons are removed and so (RE) + ions deal with transitions between electronic energy sublevels of the 4f" electroiuc configuration. Divalent lanthanide ions contain one more f electron (for instance, the Eu + ion has the same electronic configuration as the Gd + ion, the next element in the periodic table) but, at variance with trivalent ions, they tand use to show f d interconfigurational optical transitions. This aspect leads to quite different spectroscopic properties between divalent and trivalent ions, and so we will discuss them separately. 

Lanthanum - the atomic number is 57 and the chemical symbol is La. The name derives from the Greek lanthanein for to be hidden or to escape notice because it hid in cerium ore and was difficult to separate from that rare earth mineral. It was discovered by the Swedish surgeon and chemist Carl-Gustav Mosander in 1839. In 1842, Mosander separated his lanthanium sample into two oxides for one of these he retained the name lanthanum and for the other he gave the name didymium (or twin). 

Cerium was the first rare-earth element discovered, and its discovery came in 1803 by Jons Jakob Berzelius in Vienna. Johann Gadohn (1760—1852) also studied some minerals that were different from others known at that time. Because they were different from the common earth elements but were all very similar to each other, he named them rare-earth elements. However, he was unable to separate or identify them. In the 1800s only two rare-earths were known. At that time, they were known as yttria and ceria. Carl Gustav Mosander (1797—1858) and several other scientists attempted to separate the impurities in these two elements. In 1839 Mosander treated cerium nitrate with dilute nitric acid, which yielded a new rare-earth oxide he called lanthanum. Mosander is credited with its discovery. This caused a change in the periodic table because the separation produced two new elements. Mosander s method for separating rare-earths from a common mineral or from each other led other chemists to use... 

Cerium is separated from other rare-earth elements by an ion-exchange process in which it reacts with fluoride. This compound is then reduced with calcium metal (3Ca +2CeF — 2Ce + 3CaFj). Cerium can also be produced by the electrolysis of molten cerium salts. The metal ion collects at the cathode, and the chlorine or fluorine gases of the salt compound at the anode. 

This paper identifies alumina, rare earths, platinum, and magnesia as important SOx capture materials. Alumina is either incorporated directly into the matrix of a cracking catalyst or added as a separate particle. Cerium is shown to promote the capture of SO2 on high alumina cracking catalyst, alumina, and magnesia. Other rare earths are ranked by their effectiveness. The promotional effect of platinum is shown between 1200 and 1400 F for SO2 capture on alumina. Silica, from free silica or silica-alumina in the matrix of cracking catalyst, acts as a poison by migrating to the additive. 

Cerium(III) nitrate is used for the separation of cerium from other rare-earth elements. It also is used as a catalyst in hydrolysis of phosphoric acid esters. 

Acid soluble rare earth salt solution after the removal of cerium may be subjected to ion exchange, fractional crystalhzation or solvent extraction processes to separate individual rare earths. Europium is obtained commercially from rare earths mixture by the McCoy process. Solution containing Eu3+ is treated with Zn in the presence of barium and sulfate ions. The triva-lent europium is reduced to divalent state whereby it coprecipitates as europium sulfate, EuS04 with isomorphous barium sulfate, BaS04. Mixed europium(ll) barium sulfate is treated with nitric acid or hydrogen peroxide to oxidize Eu(ll) to Eu(lll) salt which is soluble. This separates Eu3+ from barium. The process is repeated several times to concentrate and upgrade europium content to about 50% of the total rare earth oxides in the mixture. Treatment with concentrated hydrochloric acid precipitates europium(ll) chloride dihydrate, EuCb 2H2O with a yield over 99%. 

Praesodymium may be recovered from its minerals monazite and bastana-site. The didymia extract of rare earth minerals is a mixture of praesodymia and neodymia, primarily oxides of praesodymium and neodymium. Several methods are known for isolation of rare earths. These are applicable to all rare earths including praesodymium. They include solvent extractions, ion-exchange, and fractional crystallization. While the first two methods form easy and rapid separation of rare earth metals, fractional crystaUization is more tedious. Extractions and separations of rare earths have been discussed in detail earlier (see Neodymium and Cerium). 

The discovery of samarium is credited to Boisbaudran, who in 1879 separated its oxide, samaria from Mosander s didymia, the mixture of rare earth oxides from which cerium and lanthanum were isolated earher. Demarcay in 1901 first identified samaria to be a mixture of samarium and europium oxides. The element got its name from its mineral, samarskite. The mineral, in turn, was named in honor of the Russian mine official Col. Samarki. 

After separation from other rare earths, ytterbium is usually obtained as its oxide, Yb203. If separated as oxalate, oxalate is converted into oxide by high temperature. Ytterbium oxide is reduced to metallic ytterbium by heating with lanthanum metal in high vacuum. The metal is purified by sublimation and collected over a condenser plate. Aluminum, zirconium, and cerium also are effective reducing agents and may be used instead of lanthanum. 

Cerous iodates and the iodates of the other rare earths form crystalline salts sparingly soluble in water, but readily soluble in cone, nitric acid, and in this respect differ from the ceric, zirconium, and thorium iodates, which are almost insoluble in nitric acid when an excess of a soluble iodate is present. It may also be noted that cerium alone of all the rare earth elements is oxidized to a higher valence by potassium bromate in nitric acid soln. The iodates of the rare earths are precipitated by adding an alkali iodate to the rare earth salts, and the fact that the rare earth iodates are soluble in nitric acid, and the solubility increases as the electro-positive character of the element increases, while thorium iodate is insoluble in nitric acid, allows the method to be used for the separation of these elements. Trihydrated erbium iodate, Er(I03)3.3H20, and trihydrated yttrium iodate, Yt(I03)3.3H20,... 

Double sulphate precipitation is one of the most common methods used in industry for the separation of cerium group from yttrium group rare earths. Various other precipitants such as chromates, double chromates, ferrocyanides, phosphates etc. have been tried. 

Cement, laboratory, 1 189 Cerite, extraction of, 2 44 Cerium, phosphor containing strontium sulfide and, 3 23 separation of, from rare earth mixtures, 2 43, 47, 48 test for, 2 50 Cerium amalgam, 1 15 Cerium-group earths, separation of, from yttrium earths by doublesulfate method, 2 44, 46 Cerium (III) magnesium nitrate, 2Ce(N03)s-3Mg(N03)2-24H,0, separation of praseodymium from lanthanum by, 2 57 Cerium(III) nitrate, 2 51 Cerium (IV) nitrate, basic, 2 49 Cesium, cesium azide for preparation of, 1 79... 

Magnesium cerium(III) nitrate, 3Mg(N03)2-2Ce(N03)3-24H20, separation of praseodymium from lanthanum by, 2 57 Magnesium chloride, anhydrous, 1 29 5 154n. 6 9 Magnesium cyclopentadienide, 6 11 Magnesium rare earth nitrates,... 

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