Thorium pyrophosphate precipitation

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%. 

Thorium orthophosphate, Th3(P04)4 4H2O, is precipitated by adding a solution of sodium phosphate to an acidic solution of a thorium salt. Thorium pyrophosphate. ThPiO 2FLO. precipitates when an acidic solution of thorium nitrate is treated with one of tetrasodium pyrophosphate. 

Moeller, T., Schweitzer, G. K., Chemistry of thorium. (Quantitative estimation of thorium by precipitation with radioactive pyrophosphate, Anal. Chem., 20, (1948), 1201-1204. Cited on pages 326, 327, 413. 

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). 

This dilution provides the proper pH for the complete precipitation of thorium pyrophosphate. 

Gadolinium is produced from both its ores, monazite and bastnasite. After the initial steps of crushing and beneficiation, rare earths in the form of oxides are attacked by sulfuric or hydrochloric acid. Insoluble rare earth oxides are converted into soluble sulfates or chlorides. When produced from monazite sand, the mixture of sand and sulfuric acid is initially heated at 150°C in cast iron vessels. Exothermic reaction sustains the temperature at about 200 to 250°C. The reaction mixture is cooled and treated with cold water to dissolve rare earth sulfates. The solution is then treated with sodium pyrophosphate to precipitate thorium. Cerium is removed next. Treatment with caustic soda solution fohowed by air drying converts the metal to cerium(lV) hydroxide. Treatment with hydrochloric or nitric acid sol-... 

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. 

From the sulfuric acid solution, thorium may also be obtained by precipitation with sodium fluosilicate, sodium hypo-phosphate,1 or sodium pyrophosphate.2 An ingenious method for removing the phosphorus has been proposed by Basker-ville3 and used on a large scale. It consists in heating in an electric furnace a mixture of monazite, coke, lime, and feldspar. The phosphorus is distilled out and the mass allowed to cool. When extracted with water, acetylene is evolved from the calcium carbide formed during the heating, and the remainder crumbles to a fine powder. This is dissolved in hydrochloric acid, and the cerium earths removed. 

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