Thorium sulfuric acid process

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 one acid digestion process, monazite sand is heated with 93% sulfuric acid at 210°C. The solution is diluted with water and filtered. Filtrate containing thorium and rare earths is treated with ammonia and pH is adjusted to 1.0. Thorium is precipitated as sulfate and phosphate along with a small fraction of rare earths. The precipitate is washed and dissolved in nitric acid. The solution is treated with sodium oxalate. Thorium and rare earths are precipitated from this nitric acid solution as oxalates. The oxalates are filtered, washed, and calcined to form oxides. The oxides are redissolved in nitric acid and the acid solution is extracted with aqueous tributyl phosphate. Thorium and cerium (IV) separate into the organic phase from which cerium (IV) is reduced to metalhc cerium and removed by filtration. Thorium then is recovered from solution. 

Monazite or Xenotime. The rare earth phosphate containing ores are attacked with either concentrated sulfuric acid or sodium hydroxide solution. The processing involves cracking the ore, removing the thorium, and separating the lanthanides. 

Loparite is decomposed in hot concentrated sulfuric acid and addition of ammonium sulfate. The rare-earths and thorium separate as double sulfates and are removed by filtration. The remaining solution of sulfates contains titanium, niobium and tantalum and is removed for separate processing. The double sulfates of rare-earths and thorium arc converted to carbonates followed by dissolution in add. Thorium is seperated by precipitation when the alkalinity of the solution is raised by the addition of sodium- or ammonium hydroxide. 

Monazite concentrate is processed either with sulfuric acid, like bastnasite, to produce a mixture of sulfates but the usual process is an alkaline treatment. The alkali process is preferred since it removes the phosphates more readily [9]. Whichever method is chosen the radioactive thorium must be completely removed. After benefication the monazite concentrate is finely ground and reacted with a hot concentrated sodium hydroxide at 140° to 150°C. Insoluble hydroxides of the rare-earths and thorium are formed while trisodium phosphate and excess sodium hydroxide remain in solution. The next step is hydrochloric acid attack on the solids portion. The thorium remains insoluble and a crude thorium hydroxide can be filtered off Trace contaminants that do carry through into solution, such as uranium and lead, as well as some thorium, are removed by coprecipitation with barium sulphate in a deactivation step. The cerium-containing product will be a rare-earth chloride differing only marginally in the proportions of the various rare- earths present, to the analogous rare-earth chloride produced from bastnasite. 

Sulfuric acid has been used to dissolve monazite in Europe, Australia, and the United States. The numerous processes used to separate thorium from the acid leach liquon are listed... 

Solvent extraction processes for recovering thorium from monazite sulfuric acid leach liquor are described in Sec. 8.6. 

Precipitation with oxalic acid. Figure 6.6 shows the principal steps in the process for separating the sulfuric acid solution of monazite into a thorium concentrate, a rare earth concentrate, and a uranium concentrate developed at the Ames, Iowa, Laboratory of the U.S. Atomic Energy Commission [Bl]. 

Attempts to separate thorium and uranium from sulfuric acid solution of monazite by solvent extraction with TBP were unsuccessful because distribution coefficients of uranium and thorium from monazite solutions were too low, as these elements are complexed by phosphate ion. Development of extractants with higher distribution coefficients for these metals has made solvent extraction a practical process for recovering uranium and thorium from monazite sulfate solutions and from sulfuric acid solutions of other thorium ores. This section describes processes tested on a pilot-plant scale by Oak Ridge National Laboratory [C5]. 

Because of the high acidity and high sulfate and phosphate content of sulfuric acid monazite leach solutions, distribution coefficients with primary and secondary amines are lower than in Table 6.18. In monazite sulfate solutions, thorium distribution coefficients with the primary amines of Table 6.18 are still greater than 500, however. The coefficient with di(tridecyl)amine is 4.6. These are still high enough for practical processes [C5]. 

Crouse and Brown [C5] describe pilot-plant studies on recovery of uranium and thorium from Canadian uraninite by sulfuric acid leaching followed by solvent extraction in a two-cycle amine extraction process using trioctylamine to extract uranium and di(tridecyl)amine to extract thorium. 

There are also thorium recovery processes based on extraction from sulfuric acid solutions, e.g., with primary, secondary, or tertiary amines or alkyl phosphorous acids such as bis-2-ethylhexyl phosphoric acid (HDEHP) or dibutylbutyl phosphonate (DBBP). Thorium is then stripped into a nitric add solution. The alkyl phosphorous acid processes are often employed when recovering thorium as a by-product in uranium production. 

Hydrometallurgical concentration processes. Because monazite is a relatively chemically inert mineral, only two hydrometallurgical dissolution processes can be efficiently used for recovering thorium and rare earths. Actually, the hydrometallurgical processing of monazite ore concentrate is carried out either by concentrated sulfuric acid or strong alkahne caustic hot digestion. 

On the treatment of monazite with sulfuric acid, thorium and many other elements are dissolved. The separation occurs with liquid-liquid extraction. The fluoride ThF is prepared from the thorium phase and reduced with magnesium. Fused salt electrolysis is also applied. The electrolyte is a mixture of equal parts of fused sodium and potassium chlorides, to which the double fluoride ThF -KF is added. The process temperature is 800°C. 

---