Rare earth fluoride precipitation

Re OPe . The final step in the chemical processing of rare earths depends on the intended use of the product. Rare-earth chlorides, usually electrolytically reduced to the metallic form for use in metallurgy, are obtained by crystallisation of aqueous chloride solutions. Rare-earth fluorides, used for electrolytic or metaHothermic reduction, are obtained by precipitation with hydrofluoric acid. Rare-earth oxides are obtained by firing hydroxides, carbonates or oxalates, first precipitated from the aqueous solution, at 900°C. 

This precipitate of 94, which was viewed under the microscope and which was also visible to the naked eye, did not differ visibly from the rare-earth fluorides. 

The reason for the ultramicrochemical test was to establish whether the bismuth phosphate would carry the plutonium at the concentrations that would exist at the Hanford extraction plant. This test was necessary because it did not seem logical that tripositive bismuth should be so efficient in carrying tetrapositive plutonium. In subsequent months there was much skepticism on this point and the ultramicrochemists were forced to make repeated tests to prove this point. Thompson soon showed that Pu(Vl) was not carried by bismuth phosphate, thus establishing that an oxidation-reduction cycle would be feasible. All the various parts of the bismuth-phosphate oxidation-reduction procedure, bulk reduction via cross-over to a rare earth fluoride oxidation-reduction step and final isolation by precipitation of plutonium (IV) peroxide were tested at the Hanford concentrations of... 

All the early work on plutonium was done with unweighable amounts on a tracer scale. When it became apparent that large amounts would be needed for the atomic bomb, it was necessary to have a more detailed knowledge of the chemical properties of this element. Intensive bombardment of hundreds of pounds of uranium was therefore begun in the cyclotrons at Berkeley and at Washington University in St. Louis. Sepa-ration of plutonium from neptunium was based on the fact that neptunium is oxidized by bromate while plutonium is not, and that reduced fluorides of the two metals are carried down by precipitation of rare earth fluorides, while the fluorides of the oxidized states of the two elements are not. Therefore a separation results by repeated bromate oxidations and precipitations with rare earth fluorides. 

The classic method for the isolation of the rare earth group which is used for both qualitative and quantitative determination involves three methods. In the first method, rare earths are precipitated as fluorides in acidic medium. The elements precipitated include Mg, Cu, Fe, rare earths Th, Ca and Sr. The second method consists of precipitation as hydroxides resulting in the removal of alkaline-earth elements like calcium from the mineral. In the third method rare earths are precipitated as oxalates from moderately acidic solutions and the elements Ca, Zn, Pb, Cu, Cd, and Ag may be coprecipitated. In early times the above methods were repeated several times to isolate, the rare earth group in a relatively pure form. 

Another process, developed at Molycorp, consists of the treatment of bastnaesite with HCl, yielding rare earth chlorides. These are subsequently treated with NaOH to convert them onto RE-hydroxides. After separation, are dissolved again in HCl, yielding rare earth chlorides. In the next step, the rare earth fluoride in the solid residue is converted to rare earth hydroxide using NaOH. Next follows neutralisation and purification. This involves hydrochloric acid, yielding a solution with a pH of about 3. By addition of hydroxide, iron precipitates as iron hydroxide. Sulphuric acid is used to precipitate lead sulphate. Then barium chloride is added to precipitate excess sulphate and to act as a carrier for the removal of any thorium daughter products present in the ore. At this pH, thorium hydroxide is insoluble and can be removed. Filtration finally leads to a clear solution of rare earth chlorides. This solution is then either concentrated by evaporation or by evaporation made into a solid form (Gupta and Krishnamurthy 2005). 

Extraction of columbate-tantalates, titanocolumbates, and titanosilicates may also be initiated by treatment of the mineral with hydrofluoric acid. The procedure has the advantage that columbium, tantalum, uranium(VI), scandium, titanium, zirconium, and hafnium are dissolved, while silica is volatilized as silicon tetrafluoride and the rare earth elements, together with thorium and uranium(IV), remain as slightly soluble fluorides. The residue is then heated with concentrated sulfuric acid to remove hydrogen fluoride and to oxidize uranium (IV), the thorium is separated by precipitation of the phosphate (synthesis 12), and the rare earths are precipitated as oxalates. 

There are numerous possible methods for reprocessing molten-salt fuels. The behavior of the rare-earth fluorides indicates that. some decontamitia-tion of molten-fluoride fuels may be obtained by substitution of CeF.i or LaFs, in a sidestream circuit, for rare earths of higher cross section. It seems likely that Pul bj can be recovered with the rare-earth fluorides and subsequently separated from them after oxidation to Pul T. Further, it appears that both selective precipitation of various fission-product elements and active constituents as oxides, and selective chemisorption of these materials on solid oxide beds are capable of development into valuable separation procedures. Only preliminary studies of these and other possible processes have been made. 

Americium may be separated from other elements, particularly from the lanthanides or other actinide elements, by techniques involving oxidation, ion exchange and solvent extraction. One oxidation method involves precipitation of the metal in its trivalent state as oxalate (controlled precipitation). Alternatively, it may be separated by precipitating out lanthanide elements as fluorosilicates leaving americium in the solution. Americium may also he oxidized from trivalent to pentavalent state by hypochlorite in potassium carbonate solution. The product potassium americium (V) carbonate precipitates out. Curium and rare earth metals remain in the solution. An alternative approach is to oxidize Am3+ to Am022+ in dilute acid using peroxydisulfate. Am02 is soluble in fluoride solution, while trivalent curium and lanthanides are insoluble. 

Assay of beryllium metal and beryllium compounds is usually accomplished by titration. The sample is dissolved in sulfuric acid. Solution pH is adjusted to 8.5 using sodium hydroxide. The beryllium hydroxide precipitate is redissolved by addition of excess sodium fluoride. Liberated hydroxide is titrated with sulfuric acid. The beryllium content of the sample is calculated from the titration volume. Standards containing known beryllium concentrations must be analyzed along with the samples, as complexation of beryllium by fluoride is not quantitative. Titration rate and hold times are critical therefore use of an automatic titrator is recommended. Other fluoride-complexing elements such as aluminum, silicon, zirconium, hafnium, uranium, thorium, and rare earth elements must be absent, or must be corrected for if present in small amounts. Copper—beryllium and nickel—beryllium alloys can be analyzed by titration if the beryllium is first separated from copper, nickel, and cobalt by ammonium hydroxide precipitation (15,16). 

Rare-earth element hydroxides, M(OH)3, precipitate from nitrate solution at pH values above 6.3-7.8 and reveal no amphoteric properties. Like thorium, the rare-earth elements yield acid-insoluble fluorides and oxalates, and soluble EDTA-, tartrate-, and citrate-complexes. 

Rare-earth elements may be isolated by precipitation as their oxalates, fluorides, or hydroxides. When the oxalates are precipitated from a weakly acidic medium (pH 1-4), Ca is used as a collector [14,15],... 

The separation of Th as the sparingly soluble thorium fluoride is equally selective [4]. The solubility of ThF4 is lower than that of the oxalate. Usually, the sample solution in hydrofluoric acid is evaporated to a small volume and diluted with water to precipitate Th, U(IV), and rare-earth metals. La, Ce, or Ca are used as collectors for traces of Th. Since the fluoride precipitate is difficult to filter off, centrifugation is advisable. 

The finishing stages for strontium separation are shown in Fig. 4. The main separation of the rare earths from the alkaline earths is made by ammonia gas precipitation of the rare earths as hydroxides in a carbonate-free medium. The alkaline earths pass into the filtrate and are removed in the next step as the carbonates. Since the separation of rare earth hydroxides and the only moderately soluble alkaline earth hydroxides is not clean, a re-precipitation step is required. The alkaline carbonates are then passed to packaging, either as the dried carbonates, or are first converted to sulfates, oxides, or fluorides for subsequent packaging in multiple-walled, weld-sealed, containers for storage. The... 

The typical purification method for rare earths is coprecipitation with ferric hydroxide, dissolution in dilute acid, precipitation as fluoride in strong mineral acid solution, dissolution in strong nitric acid with boric acid to complex fluoride, and precipitation for counting as the oxalate in dilute acid solution (Stevenson and Nervik 1961). Because Pm has no stable isotope, another rare earth (such as lanthanum) is added as carrier. The " Pm precipitate can be counted with a proportional counter, or can be dissolved and measured with an LS counter because of the low beta-particle energy. If small amounts of the other rare earth radionuclides are detected by gamma-ray spectrometric analysis, the beta-particle count rate of Pm can be calculated by difference. 

The trivalent actinides such as " Am follow the same precipitation reactions as the trivalent rare earth radionuclides, notably with insoluble hydroxides, fluorides, and oxalates. Numerous solvent extraction and ion-exchange separations from other trivalent radionuclides are reported. Americium radionuclides can be... 

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). 

Some authors have used precipitation techniques to concentrate the lanthanides. The most commonly used species are oxalates and fluoride. Rare-earth oxalates (R2(C204)3) have solubility products ranging from 10 to 10 M. Isolation of lanthanide cations as oxalate precipitates is often followed by ignition to the oxide, then acid dissolution of R2O3. This procedure can be expected to provide samples suitable for almost any type of detection/quantitation method. The solubility products of the fluorides (RF3) are found in the range of 10 to 10 NT. Whether precipitation techniques can be applied is partly determined by the concentration of rare-earth ions in the sample, and whether a carrier precipitation is acceptable for those samples in which the lanthanide concentration is too low. The detection method most directly impacts the viability of carrier precipitation techniques. 

The term upconversion describes an effect [1] related to the emission of anti-Stokes fluorescence in the visible spectral range following excitation of certain (doped) luminophores in the near infrared (NIR). It mainly occurs with rare-earth doped solids, but also with doped transition-metal systems and combinations of both [2, 3], and relies on the sequential absorption of two or more NIR photons by the dopants. Following its discovery [1] it has been extensively studied for bulk materials both theoretically and in context with uses in solid-state lasers, infrared quantum counters, lighting or displays, and physical sensors, for example [4, 5]. Substantial efforts also have been made to prepare nanoscale materials that show more efficient upconversion emission. Meanwhile, numerous protocols are available for making nanoparticles, nanorods, nanoplates, and nanotubes. These include thermal decomposition, co-precipitation, solvothermal synthesis, combustion, and sol-gel processes [6], synthesis in liquid-solid-solutions [7, 8], and ionothermal synthesis [9]. Nanocrystal materials include oxides of zirconium and titanium, the fluorides, oxides, phosphates, oxysulfates, and oxyfluoiides of the trivalent lanthanides (Ln ), and similar compounds that may additionally contain alkaline earth ions. Wang and Liu [6] have recently reviewed the theory of upconversion and the common materials and methods used. 

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. 

---