Rare earth oxalates mixtures

Heating the ore with sulfuric acid converts neodymium to its water soluble sulfate. The product mixture is treated with excess water to separate neodymium as soluble sulfate from the water-insoluble sulfates of other metals, as well as from other residues. If monazite is the starting material, thorium is separated from neodymium and other soluble rare earth sulfates by treating the solution with sodium pyrophosphate. This precipitates thorium pyrophosphate. Alternatively, thorium may be selectively precipitated as thorium hydroxide by partially neutralizing the solution with caustic soda at pH 3 to 4. The solution then is treated with ammonium oxalate to precipitate rare earth metals as their insoluble oxalates. The rare earth oxalates obtained are decomposed to oxides by calcining in the presence of air. Composition of individual oxides in such rare earth oxide mixture may vary with the source of ore and may contain neodymium oxide, as much as 18%. 

The oxalates obtained above, alternatively, are digested with sodium hydroxide converting the rare earth metals to hydroxides. Cerium forms a tetravalent hydroxide, Ce(OH)4, which is insoluble in dilute nitric acid. When dilute nitric acid is added to this rare earth hydroxide mixture, cerium(lV) hydroxide forms an insoluble basic nitrate, which is filtered out from the solution. Cerium also may be removed by several other procedures. One such method involves calcining rare earth hydroxides at 500°C in air. Cerium converts to tetravalent oxide, Ce02, while other lanthanides are oxidized to triva-lent oxides. The oxides are dissolved in moderately concentrated nitric acid. Ceric nitrate so formed and any remaining thorium nitrate present is now removed from the nitrate solution hy contact with tributyl pbospbate in a countercurrent. 

Equally precise and considerably more accurate atomic weight values may be obtained if the potassium permanganate solution is standardized against the oxalate of that rare earth element which is the principal constituent of the mixture. The explanation of the phenomenon may be that the precipitated rare earth oxalates are, in part. 

The anhydrous alkali double carbonates of the rare earths have been synthesized from mixtures of M2CO3 (M = Li, Na, K) and rare earth oxalate hydrate under carbon dioxide pressure of 200-300 MPa and at temperatures of 350-500°C (fig. 26). The sodium and potassium compounds can also be synthesized by dehydration of MR(C03)2 H20 under the same experimental conditions. At lower pressures (20 MPa) litliium forms an oxycarbonate, LiROC03 (Kalz and Seidel, 1980). The compounds have been characterized from powder samples by IR and X-ray investigations and by thermal decomposition studies. 

Insoluble silica residues are removed by filtration. The solution now contains beryllium, iron, yttrium, and the rare earths. The solution is treated with oxalic acid to precipitate yttrium and the rare earths. The precipitate is calcined at 800°C to form rare earth oxides. The oxide mixture is dissolved in an acid from which yttrium and the rare earths are separated by the ion-exchange as above. Caustic fusion may be carried out instead of acid digestion to open the ore. Under this condition sihca converts to sodium sihcate and is leached with water. The insoluble residue containing rare earths and yttrium is dissolved in an acid. The acid solution is fed to an ion exchange system for separating thuhum from other rare earths. 

Various processes separate rare earths from other metal salts. These processes also separate rare earths into specific subgroups. The methods are based on fractional precipitation, selective extraction by nonaqueous solvents, or selective ion exchange. Separation of individual rare earths is the most important step in recovery. Separation may be achieved by ion exchange and solvent extraction techniques. Also, ytterbium may be separated from a mixture of heavy rare earths by reduction with sodium amalgam. In this method, a buffered acidic solution of trivalent heavy rare earths is treated with molten sodium mercury alloy. Ybs+ is reduced and dissolved in the molten alloy. The alloy is treated with hydrochloric acid, after which ytterbium is extracted into the solution. The metal is precipitated as oxalate from solution. 

Weaver [44], in a series of studies, found that mandelic acid is specially selective for rare earths. The / value for a Sm—Nd mixture in oxalate precipitation is 1.4, and the value for mandelate is 3.8. A large P value of 14 was obtained for La—Nd mandelate. However, the rapidity and completeness of precipitation is dependent on the pH, temperature and concentration of both the rare earths and mandelic acid. 

On the basis of this charge difference transition metals can be eluted with PDCA, while lanthanides are retained at the beginning of the column. After the transition metals have been completely eluted, one switches in a second step (as described above) to the mixture of oxalic and diglycolic acid which elutes the rare-earth elements. It is possible to analyze transition metals and lanthanides in the same run by optimizing this technique, as shown in Fig. 3-163. 

Er203 and Gd203 (Er203 doped concentration 5%) with purity of 99.999% were used as starting materials. The starting materials were dissolved into the nitric acid solution. Then, a suitable amount of oxalic acid was dissolved into distilled water. The rare earth solution was added into the oxalic acid solution at suitable speed. The pH value of the mixture was adjusted to 3.5 4.0 using... 

The eluate fractions are brought to a boil and the rare earths precipitated with oxalic acid. The mixtures are allowed to stand for 20 minutes at 80°C and filtered hot, and the solids are calcined to the oxides. The first fractions, which may contain minute quantities of Zn, are reprecipitated. More than 70% of the nitrilotriacetic acid used in the process can be recovered from the eluate by precipitation with HCl. 

Moeller (1973) reminds us that homogeneous generation of anions, such as carbonate (from urea) and oxalate (from dimethyl oxalate), is more efficient than adding a soluble carbonate or oxalate, and that fractional precipitation by such means in the presence of selective complexing agents could have applications in resolving rare earth mixtures. 

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