Monazite phosphor

Accessory minerals commonly contain high concentrations of radioactive elements, and are a common target of radiogenic isotope measurements. Specific elements include uranium (zircon, apatite, titanite, monazite, xenotime, allanite) and thorium (monazite and allanite). Each accessory mineral is stabilized in a rock via a single element or suite of related elements, specifically phosphorous (apatite), REE (allanite, monazite, xenotime), zirconium (zircon), and titanium (titanite). Trace elements also occur in the major minerals (particularly phosphorous, zirconium, and titanium), so accessory minerals participate directly in major mineral reactions (Pyle and Spear, 1999, 2000, 2003 Ferry, 2000 Pyle et al, 2001 ... 

An alternative separation of thoria, based on the low solubility of lanthanides in a mixture of phosplioric and sulphuric acids, is to treat monazite at 225° with concentrated sulphuric acid and to leave most of the lanthanide sulphates behind by extracting the semi-solid mass with water. Tlie crude thorium sulphate is crystallised by concentrating the liquor, washed with cold concentrated sulphuric acid to remove phosphoric acid and redissolved in 25% sulphuric acid. The solution of the sulphate is boiled with ammonium carbonate to precipitate basic thorium carbonate. This is washed with a very little dilute nitric acid and calcined to thoria. 

Bo.M2(P04)3. The thorium, uranium, and neptunium phosphates of general formula Bo 5M2(P04)3 with B = Be, Mg, Ca, Sr, Ba are characterized by extensive polymorphism [5,53,60,61,62,63,64]. As a result of thermal treatment of Mg, Ca, and Sr neptunium phosphates with unknown structures in an Ar + 5% H2 atmosphere, these compounds were established [5,53] as substances crystallizing with the monazite structure type. The authors of [35] have obtained a series of neptunium phosphates with an analogous composition by interaction between solutions of salts of divalent metals (Mg, or Ca, or Sr) with neptunium oxide Np02 and phosphoric acid followed by thermal stage-by-stage treatment... 

Uranium was determined in phosphoric acid [156] and in ores [157,158] with the use of PAN. The PAR method was applied for determining uranium in natural waters [92,94], sewage [94], rocks and waters [159], geological samples and biological materials [22,36], monazite sands [160], tin [161], and phosphoric acid [96]. 

The iodate method1 is available in the presence of phosphoric acid, so may be applied directly to a solution of monazite in sulfuric acid. Add HNOa to the sulfuric acid solution, then KIO3 and HNO3. Repeat the precipitation and finally dissolve the thorium iodate in HC1, precipitate as Th(OH) t, and ignite. 

Thorium orthophosphate Th3(P04)4 is precipitated by phosphate ion from neutral or slightly acid solutions of thorium nitrate or sulfate. It is soluble in concentrated phosphoric or sulfuric acid, such as is present when monazite is dissolved in sulfuric add. 

Figure 14. The chain-like nature and interlocking mechanism of the monoclinic monazite structure illustrates the apical linking that forms the chains, shown in a stereo view. The large open circles represent phosphorous while the large solid circles represent the rare-earth ions. The small open circles represent oxygen (after Mullica et al. 1985a,b).

Figure 17. The variation of the RE-to-phosphate distances is shown as a function of the RE-ion radius for both the monazite- and xenotime-structure orthophosphate compounds. As shown in this figure, the shorter RE-to-phosphorous distances in the monoclinic structure compounds vary linearly with the trivalent RE ion radius with a slope that is close to 1. A similar variation is evident for the RE-P distances in the tetragonal xenotime-structure compounds—a trend that supports the comparison of the [001] polyhedron-P04 tetrahedron chain arrangement in the two structural types (after Ni et al. 1995).

Crystalline phosphates and phosphate glasses continue to receive attention as potential hosts for the immobilization and disposal of radionuclides, particularly actinides and waste streams with a high phosphorous content. The principal crystalline phases considered are apatite, silicates with the apatite-structure, and monazite. As has been discussed by Lutze and Ewing (1988a), there are a number of factors that have to be considered in selecting a nuclear waste form. The most important are ... 

Lanthanides in silicate rocks, commercial phosphoric acid, geological materials, refractory alloys, steel alloys, minerals, ores, monazite, and luminescent phosphors have been determined by HPLC. Determination of trace lanthanide impurities in nuclear grade uranium has been studied." An HPLC technique using the dynamic ion-exchange approach was also employed for the determination of Pm in urine. A typical application of lanthanide assay in nuclear industry is described next. 

An interesting feature of the anhydrous sulphate precipitation method is that the precipitation reagent (concentrated sulphuric acid) can be recycled and used for a subsequent monazite breakdown. The recycled sulphuric add is contaminated with a high proportion of phosphoric acid and within limits this can be tolerated. It has even been stated that sulphuric acid containing a proportion of phosphoric add allows more efficient breakdown than sulphuric acid alone, owing to the greater solubility of the thorium and rare earths in the mixed adds. 

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