Rare earth oxalates

Separation and Recovery of Rare-Earth Elements. Because rare-earth oxalates have low solubihty in acidic solutions, oxaUc acid is used for the separation and recovery of rare-earth elements (65). For the decomposition of rare-earth phosphate ores, such as mona ite and xenotime, a wet process using sulfuric acid has been widely employed. There is also a calcination process using alkaLine-earth compounds as a decomposition aid (66). In either process, rare-earth elements are recovered by the precipitation of oxalates, which are then converted to the corresponding oxides. 

A special technique was developed for rare-earth samples in which rapid hydration and carbonation occurred. The rare-earth oxalates were found to be more stable than the oxides and were used as sample material. In the rare-earth processing procedures that include an oxalate precipitate, the oxalate can be used as sample material. The advantages are that no diluent is required, weighing is eliminated, and recovery of the rare earths is simplified. 

There is an upper limit of about 23 microns in size. This may be due to the fact that the precipitation was accomplished at 90 C., or from the fact that rare earth oxalates tend to form very small particles during precipitation which then grow via Ostwald ripening and agglomeration to form larger ones. Nevertheless, it is clearly evident that when the oxalate is heated at elevated temperature ( 900 °C), the oxide produced retains the same PSD characteristics of the original precipitate. 

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 thermal decomposition of hydrated rare earth oxalates, M2(Ox)3 nSLsO, has attracted considerable attention [397—400]. Wendlandt and his coworkers [397, 39S] have used both thermogravimetric (TGA) and differential thermal analysis (DTA) for studying the thermal decomposition of the rare earth, Th4+ and U4+ oxalates. The DTA curve for Eu2(Ox)3 IOH2O consists of three endothermic peaks centered at about 160°, 200° and 285° C respectively. The thermogravimetric analysis [397] shows the presence of unstable intermediate hydrates on going from the decahydrated to the anhydrous oxalate. The thermal decomposition of Eu2(Ox)3 IOH2O can be summarized as... 

Glasner et al. [399], however, showed that Eu2(Ox)3 behaves somewhat differently from other rare earth oxalates, in being easily reduced to the divalent state. According to these investigators, the first step of the thermal decomposition of Eu2(Ox)3 at 320° C in a CO2 atmosphere involves the formation of Eu(Ox) with a weight loss of 28.6 per cent. In an oxidizing atmosphere, however, reoxidation takes place, and the final... 

Oxalates, determination of, in rare earth oxalates, 2 60 Oxalato salts, tri-, 1 35 Oxides, reduction of refractory metal, to metal powders with calcium, 6 47... 

Rare earths Oxalate to precipitate metal ions Precipitate titrated with KMn04 KMn04 titration [123]... 

E. Hansson, Thesis, On the Structures of Solid Rare Earth Oxalates, and Malonates, Lund, 1973. 

Oxalic acid is a precipitation agent for rare earth ions. The solubility of rare earth oxalates range from 10 to lO" mol in neutral solutions. The precipitate usually contains coordinated and/or lattice water molecules, RE2(C204)3 n H2O, where = 10 for lanthanum to erbium and yttrium while n = 6 for holmium, erbium, thulium, ytterbium to lutetium and scandium. 

An emulsion liquid membrane (ELM) system has been studied for the selective separation of metals. This system is a multiple phase emulsion, water-in-oil-in-water (W/O/W) emulsion. In this system, the metal ions in the external water are moved into the internal water phase, as shown in Fig. 3.4. The property of the ELM system is useful to prepare size-controlled aiKl morphology controlled fine particles such as metals, carbonates/ and oxalates.Rare earth oxalate particles have been prepared using this system, consisting of Span83 (sorbitan sesquioleate) as a surfactant and EHPNA (2-ethyl-hexylphospholic acid mono-2-ethylhexyl ester) as an extractant. In the case of cerium, well-defined and spherical oxalate particles, 20 - 60 nm in size, are obtained. The control of the particle size is feasible by the control of the feed rare earth metal concentration and the size of the internal droplets. Formation of ceria particles are attained by calcination of the oxalate particles at 1073 K, though it brings about some construction of the particles probably caused by carbon dioxide elimination. 

The decomposition products identified following reaction are not necessarily the primary compounds which result directly from the rate limiting step. Particularly reactive entities may rapidly rearrange before leaving the reaction interface and secondary processes may occur on the surfaces of the residual material which often possesses catalytic properties. The volatile products identified [144] from the decomposition of nickel formate were changed when these were rapidly removed from the site of reaction. The primary products of decomposition of thorium formate were identified [17] as formaldehyde and carbon dioxide, but secondary processes occurring on the residual thoria yielded several additional compounds. The oxide product similarly catalysed interactions between the primary products of decomposition of zinc acetate [145]. During the decomposition of rare earth oxalates, carbon monoxide disproportionates extensively to carbon dioxide and carbon [81,82]. 

If either thorium or zirconium is present in any amount, it is usually removed at this point by boiling the crude oxalates with (NH4)2C204, which dissolves all the zirconium and most of the thorium.2 Some rare earth oxalates are dissolved slightly,... 

Separation. — The separation of thorium from the rare earth metals with which it is still mixed may be accomplished by three methods (1) the carbonate separation depends on the fact that thorium carbonate is much more soluble in sodium carbonate than the carbonates of the rare earth metals (2) by the fractional crystallization of the mixed sulfates at 15°-20°, crystals of Th(S04)2 8 H20 are obtained at the insoluble end of the series (3) thorium oxalate forms a soluble double salt with ammonium oxalate, while the rare earth oxalates are almost insoluble in this reagent. Some other methods which have been suggested are fractionation of the chromates,4 of the hydrogen alkyl sulfates,5 of the acetates, by the use of sebacic add 6 and hydrogen peroxide. 

The solution of monazite in sulfuric acid containing about 50 to 60 g of thorium and rare earths per liter is diluted with about 4.5 volumes of water and brought to a pH of 1.5 by addition of NH4OH. Oxalate ion is added in the form of recycle sodium oxalate, plus sufficient oxalic acid in 10% aqueous solution to provide 110% of the oxalate ion needed to precipitate thorium and rare earth oxalates. The precipitate is filtered and washed with 1 % oxalic acid in... 

Because of the comparatively high cost of oxalic acid, economics requires recovery of oxalate ion. This is effected by digesting the thorium and rare-earth oxalates with a... 

Rare earth oxalate and oxide particles have been prepared using this system, and the particle size is controllable by changing the extractant (cation carrier). For example, some rare earth oxalate particles of submicrometer size were prepared in the ELM system containing 2-methyl-2-ethylheptanoic acid (VA-10) and bis(l,l,3,3-tetramethylbutyl)phosphinic acid (DTMBPA) as the extractant [58-61]. In the VA-10 system, ultrafine particles of 20-60 nm in diameter were obtained, while spherical oxalate particles of 0.2-0.6 pm in diameter were obtained in the DTMBPA system. Preparation of oxide particles are attained by calcination of the oxalate particles at 873-1073 K (Fig. 6-7), though it brings about some construction... 

This sludge contains 2-8 % rare earths. As large amounts of this sludge are produced, they are a valuable source of rare earths. The sludge is washed and leached with dilute nitric acid, to which calcium nitrate has been added. From the leach liquor, the rare earths are recovered by solvent extraction and finally precipitated as rare earth oxalates. The oxalates are calcined in a rotary furnace to yield mixed rare earth oxides of 89-94 % purity. 

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