Rare Earth Oxalate Compounds

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. 

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]. 

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. 

The rare earths may Ik defined as a group of trivalent metals, forming basic oxides, wi<,h oxalates insoluble in dilute mincml acids. Their fluorides are also difficultly soluble, hence they may be separated, in general, from other elements by adding oxalic or hydrofluoric acid to their solution, to which some mineral acid has previously ls m added. Doubtless the most striking fact which characterizes these elements is the remarkable similarity in both the physical and chemical projmrties of their compounds, Their main differences are in the solubilities of their salts and the basicity of their oxides, which varies l e-tween that of the alkaline earths and that of aluminium. 

Chemical precipitation is a popular method for synthesizing solid materials from solution in which a liquid-phase reaction is utilized to prepare insoluble compounds. The precipitates are composed of crystalline or amorphous fine particles. Usually, rare earth oxides are prepared by calcinations of the hydroxide or oxalate gel precipitated from a reaction of an aqueous or alcohol solution of inorganic salt (nitrate, chloride, sulfate, and ammonium nitrate, etc.) with an alkali solution (NaOH, NH4OH, and (NH2)2 H20) or an oxalic acid solution [15-21]. However, it is very difficult to obtain ultrafine particles because of growth and sintering of the particles during the calcinations. 

Komissarova, L.N., V.M. Shatski and G.Ya. Pushikina, L.G. Shchorbakova, L.G. Mam-surova and G.E. Sukhanova, 1984, Rare Earth Compounds (Carbonates, Oxalates, Nitrates, Titanates) (Nauka, Moscow) p. 234. In Russian. 

The precipitation with oxalic acid separated the radioactive impurities from the rare earths. The hydrogen fluoride gas was absorbed by water then reacted with alkaline substances for the recovery of fluoride compound. 

Gypsum and/or anhydrite were separated in the insoluble residue. The formation of either calcium sulfate compounds is independent on leaching factors. The rare earth elements were transferred to the leach liquor with efficiency reached 97.8% by leaching the ore sample with 9M sulfuric acid at 100 C for 2 houi s at the acid/ore ratios 10.0/1.0. The precipitation of rare earth from sulfate leach liquor by oxalic acid is preferred at lower acid/ore ratio. 

The catalyst are prepared from aqueous solution of rare earth nitrates, lithium carbonate or lithium hydroxide. The solids are obtained by evaporation to dryness at 110 C-120"C of the solution or suspension in which the rare earth is preciplted as oxalate by oxalic acid (pH - 2) or as hydroxide by ammonia (pH - 9). After heat-treatment at 750 C for 24h we obtain the definite compounds as shown in figure 1. 

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