Rare earth hydroxides

The oxalates obtained above, alternatively, are digested with sodium hydroxide converting the rare earth metals to hydroxides. Cerium forms a tetravalent hydroxide, Ce(OH)4, which is insoluble in dilute nitric acid. When dilute nitric acid is added to this rare earth hydroxide mixture, cerium(lV) hydroxide forms an insoluble basic nitrate, which is filtered out from the solution. Cerium also may be removed by several other procedures. One such method involves calcining rare earth hydroxides at 500°C in air. Cerium converts to tetravalent oxide, Ce02, while other lanthanides are oxidized to triva-lent oxides. The oxides are dissolved in moderately concentrated nitric acid. Ceric nitrate so formed and any remaining thorium nitrate present is now removed from the nitrate solution hy contact with tributyl pbospbate in a countercurrent. 

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. 

It is also possible, if the proper conditions are set, to dissolve selectively the rare earth hydroxides which are more basic than thorium hydroxide, see the right hand column of Figure 9. In such a case, the mixed hydroxide water slurry is brought to a pH of 3.4 by a slow and careful addition of hydrochloric acid. The undissolved thorium hydroxide is then separated from the solution by filtration. 

Ac, actinium, was initially identified in 1899 by Andr6-Louis Debierne, a French chemist, who separated it from pitchblende. He dissolved the mineral in acid, then added NH4OH, and found that a radioactive species was carried down with the rare earth hydroxides. He named the element actinium after the Greek aktinos which means ray. Because of its low abundance in U, the element is usually not obtained by isolation from U. It can be obtained in mlligram amounts by irradiation of Ra-226 in a nuclear reactor. The preparation of Ac metal involves reduction of AcFs by Li at high temperature. 

Table 18. Comparison of precipitation pH values, solubility product and solubility of the rare earth hydroxides from perchlorate [294], nitrate, sulphate and acetate [295] media... 

After filtering off the solid residue, the cold solution is treated with sodium sulphate. The precipitated double sulphate of the rare earths is washed with 0.3N sodium sulphate, and then boiled with excess (10%) sodium hydroxide to convert the double sulphate into a hydroxide. The rare earths hydroxides are filtered off and dried at 120° C to oxidize trivalent cerium into the tetravalent state. The dried material contains about half of the original thorium coprecipitated with rare earth double sulphates. 

Moeller et al. [294, 295] have thoroughly studied the pH values at which the rare earth hydroxides are precipitated from various salt solutions, and also the solubility and solubility product constants of the hydroxides. Their results are summarized in Table 18. 

Formates. — The simplest Tnowo-carboxylic acid is formic acid (HCOOH). Formate complexes have not been extensively investigated although Sakkar [353] has mentioned rare earth formates. The cerium group rare earths form spherulic formates which are hexagonal. This property is used to identify small amounts of these elements. These formates are prepared by dissolving freshly prepared rare earth hydroxides in formic acid. 

The bombarded oxides are dissolved In HHO3 from which the rare earth fluoride Is ppted. This Is dissolved and the hydroxide ppted. and dissolved In cone. HC1, which is sucked through a column of anion exchange resin (Dowex A-l). The rare earth hydroxide Is then prepared for counting sample. 

The subsequent sections are assigned by following the compound sequence of rare earth nanomaterials. Rare earth oxides are often regarded as the most important compoimd class while rare earth hydroxides are frequently used as their synthesis precursors. Therefore, we discuss the nanomaterials of ceria, R2O3 and other rare earth oxides along with the rare earth oxyhydroxides and hydroxides in Section 2. Rare earth oxysalts, such as phosphates, vanadates, and borates are... 

Since ceria exhibits a cubic fluorite crystal structure, the noncubic nanostructures, such as NRs, NWs, and nanoplates, are fabricated under experimental conditions that are suitable to break dovm the symmetry. One general way is to exploit an appropriate intermediate, such as Ce(OH)3 or Ce(OH)C03. The rare earth hydroxide crystalline NRs/NWs/ NTs are obtained in basic solutions imder hydrothermal treatment, which is discussed in Section 2.3. If certain oxidant is present in the hydrothermal treatment, the ceria NCs could be obtained in a one pot manner. In this way, rod-like, wire-like, or tube-like nanoceria could be synthesized. If the hydrothermal treatment is carried out under oxygen free... 

Rare earth hydroxide nanocrystals are commonly synthesized via the precipitation of to form gel-like R(OH)3 in basic aqueous solutions, which is quite straightforward with appropriate pH values of 6-8 for Y and La-Lu. Sc would precipitate even in an acidic solution. The crystallized rare earth hydroxide is then obtained after annealing or aging the gel-like precipitation with mother liquor. With elevated temperature, the dried rare earth hydroxides could be dehydrated into oxyhydroxide and oxide in steps. 

Rare earth hydroxides themselves find scarce applications, because of their instability in the presence of CO2. In addition, the presence of OH introduces deterioration effects for photoluminescence emissions. However, rare earth hydroxides can be easily converted into a number of other rare earth compounds through dry and solution chemical routes, therefore, they are often taken as intermediates for the s)mthesis of rare earth oxides, sulfides, and fluorides. 

The utilization of soft templates is helpful for the construction of rare earth hydroxide nanotubes. The s)mthesis of Y(OH)3 nanotubes could be assisted by PEG (Tang et al., 2003) or grafted with PMMA (Li et al., 2004 Mo et al., 2005). Hard templates like A AO are also studied for the fabrication of rare earth hydroxide nanowires (Bocchetta et al., 2007). 

Hu and coworkers developed a composite hydroxide method (CHM) to symthesize highly crystallized rare earth hydroxide and oxide nanocrystals (Hu et al., 2007). The eutectic mixture of alkaline hydroxides (NaOH KOH = 51.5 48.5, m.p. 165 °C) is used as solvent and the real synthetic process is done at approximately 200 °C in a sealed vessel. The highly crystallized La(OH)3 nanobelts are obtained through this method... 

There are also other ways to obtain rare earth hydroxide nanocrystals. The formation of rare earth hydroxide needles or nanotubes was observed as a corrosion product with LaNis in KOH (Maurel et al., 2000). Fang and Xu et al. reported that with medium pH values, that is, near the precipitation pH value of rare earth hydroxides, the hydration of medium rare earth (Tb, Dy, Y) oxides with a hydrothermal treatment will lead to rare earth hydroxide nanotubes (Fang et al., 2003a Xu et al., 2003 Figure 25). Lee and Byeon reported the hydration of LaOCl for the synthesis of La(OH)3 nanostructures (Lee and Byeon, 2006). 

Rare earth hydroxides are rarely obtained in nonaqueous systems. However, Djerdj et al. reported the synthesis of La(OH)s nanorods/nanofibers and manganese oxide nanoparticles through a nonaqueous sol-gel process involving the reaction of La(OiPr)3 (lanthanum f-propoxide) and KMn04 with organic solvents such as benzyl alcohol, 2-butanone, and their mixture (Djerdj et al., 2007). 

Properties and applications of rare earth hydroxide nanostructures... 

However, hydroxides proved to be applicable intermediate for preparing various nanocrystals of rare earth oxide, oxysulphide, oxyfluoride, and other rare earth compounds. In this route, the crystallized R(OH)s nanocrystals instead of gels were obtained and collected, later a next step is performed to convert the R(0H)3 nanocrystals into other compounds, without destroying the morphology. The obtained new nanocrystals may or may not have a certain crystal growth direction related to the precursor. A selection of typical works on the conversion of rare earth hydroxide nanostructures are listed in Table 1. 

TABLE 1 The conversion from rare earth hydroxide nanocrystals... 

Under general conditions, rare earth hydroxides RE(OH)3 H20 precipitate from a high pH solution as a gel. However, they are unstable during heating and usually lose water to become REO(OH) or RE2O3 when the temperature approaches or exceeds 200 °C. From lanthanum to lutetium, the dehydration temperature decreases with an increase in atomic number because of a decrease in the ionic ratio. 

Single crystals of rare earth hydroxides can be obtained by a hydrothermal method. At 190 20 °C and from 1.2 x 10 to 7 x 10 Pa, rare earth hydroxides can be grown from RE203-H20-Na0H systems after prolonged treatment. 

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... 

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