Rare earth salts cerium solutions

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. 

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%. 

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. 

In their use as catalyst components, rare earth Y zeolites are freauently prepared by ion exchange with commercial rare earth salt solutions. Such commercial salts are mixtures of different rare earths, in which the major components are lanthanum, cerium, praseodymium and neodymium (5). These rare earth elements therefore play a major role in determining the physico-chemical characteristics and stability of Y zeolites that are used in many commercial catalysts. 

The hydrothermal method composes of three types of processes hydrothermal synthesis, hydrothermal oxidation, and hydrothermal crystallization. Hydrothermal synthesis is usually used to synthesize oxides from their component salts, oxides or hydroxides. Pressures, temperatures, and mineralizer concentrations control the size and morphology of the particles. Forced hydrolysis of solutions of a rare earth salt is effective to obtain uniform and fine particles. For example, cerium oxide fine particles were prepared from tetravalent cerium salt solution (CeS04-4H20, (NH4)4Ce(S04)4 2H20, and (NH4)2Ce(N03)6) in low concentrations by low temperature aging in a sealed vessel (see Fig. 6-4) [38-41]. The metal ions are solvated by water molecules which can be deprotonated to give hydroxide or oxide particles. This method is very sensitive to the concentration, temperature, and pH value of the solution. 

The rare-earth salts of trifluoromethanesulfonic acid (triflic acid) are popular reagents for lanthanide-mediated organic reactions. Especially scandium(III) triflate, Sc(CF3S03)3, and ytterbium(lll) triflate, Yb(CF3S03)3, are often used as mild Lewis acids for reactions in water (Kobayashi et al., 2002). It is therefore surprising that only very few studies of cerium(IV)-mediated reactions describe the use of cerium(IV) triflate, Ce(CF3 803)4 or Ce(OTf)4, as a reagent. This salt was first reported by Kreh et al. (1987), who prepared a solution of cerium(lV) in aqueous triflic acid by electrochemical oxidation of a cerium(III) triflate solution. They illustrated the use of this reagent for oxidation of alkylaromatic and polycyclic aromatic compounds. Imamoto et al. (1990) prepared cerium(IV) triflate by reaction... 

The first paper published by Hinton et al. (1984) describes the use of cerium chloride salts as aqueous inhibitors of corrosion for 7075-T6 aliuninum alloy in a O.IM solution of aerated sodimn chloride. Three years later, the same author published a study which stated that additions of small concentrations of rare earth salts (1000 ppm of LaCl3, YCI3, PrCl, NdCl, or CeCy, to a 0.1M NaCl solution induce a decrease in the corrosion rate of AA7075 (Amott et al., 1987). The best degree of inhibition was achieved with Ce ions (Fig. 3.1) when added as chloride compounds (Hinton et al., 1986), because of the formation of a compact film of cerium oxides and hydroxides (Amott et al., 1985). 

On the other hand, the inhibitory efficiency is also dependent on the cation of the rare earth salt. The level of inhibition of different cations of the rare earths on the altrminum alloys 7075 and 8090 is clearly seen in the stuface appearance. Plate I (see color section between pages 174 and 175) shows the level of inhibition after 96 hottrs of immersion in a solution of 3.56% NaCl with 1000ppm of cerium chloride or lanthanrrm chloride in the medium. Both alloys do not show any attack in the Ce containing solution, whereas pitting is observed on the stuface in the presence of La catiotts. 

After removing cerium (and thorium), the nitric acid solution of rare earths is treated with ammonium nitrate. Lanthanum forms the least soluble double salt with ammonium nitrate, which may be removed from tbe solution by repeated crystallization. Neodymium is recovered from this solution as the double magnesium nitrate by continued fractionation. 

Cerium Thioarsenates.—The addition of sodium orthothio-arsenate to an aqueous solution of a cerous salt produces a pale yellow precipitate of cerous orthothioarsenate.1 With sodium hydrogen ortho-thioarsenate the precipitate approximates in composition to cerous pyro-thioarsenate. Ceric salts also give pale yellow precipitates, probably ceric orthothioarsenate. Thioarsenates of other rare earth metals have not been described. 

The organic amine extractants are the most commonly used anion exchangers. Secondary amines have been used to recover uranium from leach liquors (GlO) secondary and tertiary amines to recover molybdenum from uranium mill circuits (L13) a primary amine, diethylenetriamine-penta-acetic acid (DTPA) to extract cerium group lanthanides (B6) tri-,V-butylamine-3-methyl-2-butanonc to separate yttrium and rare earth nitrates (G13) tricaprylyl amine (Alamine 336) and methyltrioctyl-ammonium salt (Aliquat 336) to recover vanadium from acidic solutions (A3) and Aliquat 336 to extract vanadium from slightly acidic or alkaline leach liquor (S36). 

The process of rare earth recovery is based on rare-earth double-salt precipitation. However, yttrium and the heavy rare earths go with thorium. The rare earths are recoverable from the thorium fraction during the solvent extraction step used for the purification of uranium and thorium. Solvent extraction with TBP (tribulyl phosphate ), from an aqueous 8 N nitric acid solution of thorium and mixed rare earths, enables the recovery of thorium, uranium, cerium and cerium free rare earths (Gupta and Krishnamurthy 2005). Other significant processes involve precipitation of thorium pyrophosphate, or precipitation as basic salts from the leach fiquor. After that comes recovery of the rare earths from solution as double sulphates, fluorides, or hydroxides, and also selective solubilisation of thorium itself in the ore treatment stage. The sulphuric acid route does yield impure products, but it is not used anymore (Gupta and Krishnamurthy 2005). 

The cerium may now be removed from this solution by the bromate method (synthesis 14). Then the resulting solution is treated as before with excess oxalic acid or sodium suKate, the precipitates are again converted to hydrous oxides, and the latter are again dissolved in nitric acid. The rare earths from monazite are generally converted to double magnesium nitrates, 3Mg(N03)2-2(R.E.)-(N03)3 24H20 (synthesis 15), for preliminary fractionation. The rare earths from xenotime, after complete removal of cerium, may be converted to bromates (synthesis 17), and fractional crystallization of these salts may be commenced. 

Of great interest is the use of intermetallic compounds of platinum with rare-earth metals such as cerium and praseodymium for anodic methanol oxidation, known from the work of Lux and Cairns (2006). This combination is attractive inasmuch as it involves two metals that differ strongly in their own electrode potentials Pt with = -1-1.2 V and Pr with = —2.3 V(SHE), and thus in their electronic structure. However, for the same reason, traditional methods of preparing joint disperse deposits of these metals by chemical or electrochemical reduction in a solution of the corresponding salts fail in such a situation. Lux and Cairns developed a new technology for preparing disperse powders of such compounds by thermal decomposition of complex cyanide salts of these metals. The catalyst obtained had some activity in ethanol oxidation (although somewhat... 

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