Thorium removal processes

Various thorium purification processes have involved the use of ion-exchange indirectly, e.g. to remove the uranium which is normally associ-... 

Purification or refining of thorium. Thorium produced previously is too impure to be used as nuclear fuel. In fact, impurities such as rare earths and uranium, owing to their elevated thermal neutron cross sections, are objectionable. Hence, the objective of the thorium refining process is to remove these impurities until concentrations below gg/kg (i.e., ppb wt.) are reached. Solvent extraction of an aqueous thorium nitrate solution with n-tributyl phosphate (TBP) in kerosene is a common procedure to perform the refining of thorium. At the end of the purification process, the thorium is recovered in the form of an aqueous solution of thorium nitrate or crystals of hydrated thorium nitrate. 

As is pointed out in Chapter 20, the easiest blanket to handle in the LMFR would be a 10 w/o thorium-bismuthide slurry in bismuth. Chemical processing of this blanket would be very similar to the core processes already described. The major problem consists in transferring the bred uranium and protactinium from the solid thorium bismuthide to the liquid bismuth phase, so that they can then be chemically processed. Two examples of proposed processes are shown in Fig. 22-11, which shows a process that can be used with the fused chloride salt FPS removal process, and in Fig. 24-19, which shows a flowsheet for a process to be used with the fluoride volatility process. 

Mona.Zlte, The commercial digestion process for m on a site uses caustic soda. The phosphate content of the ore is recovered as marketable trisodium phosphate and the rare earths as RE hydroxide (10). The usual industrial practice is to attack finely ground m on a site using a 50% sodium hydroxide solution at 150°C or a 70% sodium hydroxide solution at 180°C. The resultant mixed rare-earth and thorium hydroxide cake is dissolved in hydrochloric or nitric acid, then processed to remove thorium and other nonrare-earth elements, and processed to recover the individual rare earths (see... 

Coprecipitation is a partitioning process whereby toxic heavy metals precipitate from the aqueous phase even if the equilibrium solubility has not been exceeded. This process occurs when heavy metals are incorporated into the structure of silicon, aluminum, and iron oxides when these latter compounds precipitate out of solution. Iron hydroxide collects more toxic heavy metals (chromium, nickel, arsenic, selenium, cadmium, and thorium) during precipitation than aluminum hydroxide.38 Coprecipitation is considered to effectively remove trace amounts of lead and chromium from solution in injected wastes at New Johnsonville, Tennessee.39 Coprecipitation with carbonate minerals may be an important mechanism for dealing with cobalt, lead, zinc, and cadmium. 

Radium, thorium, and other radionuclides accumulate in uranium mill tailings. The potential environmental effects of these radionuclides has become of increasing concern to the public. In the future, it may be necessary to modify existing uranium recovery processes to accommodate removal of radium and perhaps other radioactive decay products of uranium. 

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. 

The vendor claims that the technology will treat uranium and possibly thorium. At this point, the technology has only been bench tested. The vendor hopes, however, that once the heap leaching technology is fully developed, it will be used to remove metals chemically (uranium in this case) from soil without damaging the soil. With some modification, the process can also be used to remove volatile organic compounds from soil by ex situ soil venting. 

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. 

There are several ways of treating the mixed hydroxide cake. The cake is either dissolved in HC1 or HNOs and thorium is removed. The original Rohden process involved a partial dissolution of the mixed hydroxide in HC1 at 70—80° C (pH 3.5—4). The crude thorium hydroxide was filtered off and the filtrate contained the rare earths practically free from thorium and phosphate. 

Some thorium is extracted along with calcium during the HOI treatment, and it is recovered by precipitation from the filtrate at pH 1.8 and returned to the process. The usual industrial practice is to attack the monazite with 60—70% NaOH at a temperature of 140—50° C for —4 hrs. Better results are obtained at 170° C under higher pressures [154]. After the reaction is complete the mixed rare earth-thorium hydroxides are filtered off while hot. The precipitates are carefully washed to remove... 

The uranium and thorium ore concentrates received by fuel fabrication plants still contain a variety of impurities, some of which may be quite effective neutron absorbers. Such impurities must be almost completely removed if they are not seriously to impair reactor performance. The thermal neutron capture cross sections of the more important contaminants, along with some typical maximum concentrations acceptable for fuel fabrication, are given in Table 9. The removal of these unwanted elements may be effected either by precipitation and fractional crystallization methods, or by solvent extraction. The former methods have been historically important but have now been superseded by solvent extraction with TBP. The thorium or uranium salts so produced are then of sufficient purity to be accepted for fuel preparation or uranium enrichment. Solvent extraction by TBP also forms the basis of the Purex process for separating uranium and plutonium, and the Thorex process for separating uranium and thorium, in irradiated fuels. These processes and the principles of solvent extraction are described in more detail in Section 65.2.4, but the chemistry of U022+ and Th4+ extraction by TBP is considered here. 

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. 

Uranium minerals may be obtained in solution, in a suitable condition for estimation, by the following process. The ore is dissolved in aqua regia, or, if necessary, fused with alkali bisulphate and extracted mth hot hydrochloric acid. After evaporation to drjmess, the residue is taken up with dilute hydrochloric acid, and the solution saturated with hydrogeir sulphide in order to remove any copper, lead, bismuth, arsenic, antimony, or any other metal yielding an insoluble sulphide. The filtrate is concentrated and treated with ammonium carbonate, which precipitates the carbonates of the alkaline earths, iron, and most of the rare earths. The filtrate is neutralised by hydrochloric acid, evaporated to dryness, and the residue ignited to drive off ammonium salts, and then redissolved in dilute acid. The remaining rare earths, and particularly thorium, are next precipitated by the addition of oxalic acid. The filtrate, which contains the uranium in the uranyl condition, may now be precipitated by any of the methods described above. 

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