Urea precipitation

CHEMICAL SYNTHESIS OF MONAZITE, PRETULITE, AND XENOTIME Urea precipitation 

PRODUCTION OF SYNTHETIC MONAZITE FOR ACTINIDE WASTE STORAGE 

Rare-earth orthophosphates and the actinide phosphates Pu(P04) and PUP2O7 have been synthesized using a process developed by Bamberger (1982) and Bamberger et al. (1984) that consists of a metathesis reaction with boron phosphate B(P04). Bamberger first synthesized B(P04) by heating mixtures of H3(P04) and H3BO3 to 1000°C in air. 

Subsequently, the B(P04) was mixed with the rare-earth oxide and heated in a Pt boat at temperatures up to 1000°C. In the case of the formation of Ce(P04), the metathesis reaction proceeded according to the relation  

The evolution of oxygen in this reaction was reported to occur in the range of 550 to 600°C. A metathesis reaction using B(P04) was also shown to be effective in producing rare-earth orthophosphates starting with the trifluoride form of the rare earth, and this reaction proceeded according to the relation  

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The following are suitable anions for urea precipitations of some metals sulphate for gallium, tin, and titanium formate for iron, thorium, and bismuth succinate for aluminium and zirconium. 

However, rates of precipitation in soil systems may be quite different from those in solutions because precipitation is catalysed by adsorption of the reacting solutes onto soil surfaces the nature of the solid phases formed may be different and sorption may also alter the effects of inhibitors. There are very few data in the literature on these effects actually measured in soils. Figure 3.15 shows data of Huang (1990) for calcite precipitation in three soils incubated with urea. Precipitation was induced as the pH increased during urea hydrolysis ... 

Table 7.2 summarizes a range of homogeneous precipitation reactions. Details of all these reactions can be found in Ref. 2 (this book, in spite of its age, is required reading for anyone wishing to pursue this line more recent books may exist, but will probably not reduce its value). It should fu-st be stressed that the material in this final section relates to precipitates rather than to films. However, with some effort (in some cases only a little or none, as seen from the common film formation occurring in urea precipitations), it is reasonable to expect extension to form films of the same materials in at least some cases. 

The product structures available in this process are primarily determined by components in the paraffinic distillates used. The heterogeneity of virgin distillates has been illustrated by API sponsored work on distillate components 21). The preferred predominance of relatively straight-chain alkyl groups may be effected by combinations of selection of paraffin distillate source, and use of acid or solvent refining to effect enrichment of paraffinic components. With the advent of the urea precipitation technique of isolating the n-paraffins 26), particularly those in the decyl to hexadecyl range, it is now possible to produce products which are limited principally to the isomeric secondary phenyl alkanes a dodecyl benzene mixture prepared by this process from a 95+% n-dodecane would consist essentially of the 2-, 3-, 4-, 5-, and 6-phenyl-substituted dodec-anes. 

To produce Fe(oxide)/Si02 particles one could use a solution of Fe(II), in which case air has to be excluded to prevent its oxidation to Fe(III). Iron (II) starts to react markedly above pH = 4.8 (urea, 90°C). In this case, the reaction is not limited to Eqn. 9.14, but a bulk hydrosilicate is formed. Upon performing an injection experiment at 45°C, it is observed that the reaction with the support is less extensive at that temperature. As the slightly higher pH at the injection point brings about the formation of a less reactive iron species, attack of the support is less marked than in the urea case even at 90°C. The structures obtained in the three different experiments are indicated in Fig. 9.11. The different extents of hydrosilicate formation are reflected in the temperature-programmed reduction (TPR) experiments, as can be seen in Fig. 9.11. Previous air-drying partially oxidizes the Fe(II). As interaction with silica stabilizes Fe(II), the supported Fe samples show a separate reduction step to Fe(II), which is not displayed by bulk Fe oxide. The iron hydrosilicate obtained in urea precipitation at 90°C is fairly stable and is reduced only above 650°C. The Fe(II) precipitated at 45°C is more... 

Several routes exist for the synthesis of amides of halo acids. Cuatrecasas et al. (1969) used two different bromoacetylating reagents in the synthesis of water soluble inhibitors of staphylococcal nuclease. The N-hydroxysuccinimide ester of bromoacetate was one of the reagents used. It is particularly useful for the synthesis of radioactive derivatives since C-bromoacetic acid is commercially available. Synthesis of the hydroxysuccininimide ester is accomplished by dissolving 87 mg (630 /imoles) of bromoacetic acid and 86 mg of N-hydroxysuccinimide in 3 ml of dioxane. To this solution, 132 mg (700 /imoles) of dicyclohexylcarbodiimide is added. Urea precipitates immediately and after 1 hr, is removed. The solution of bromoacetyl N-hydroxysuccinimide ester is brought to 5 ml. It can then be used without any further purification. 

Preparation of nickel-modified ceramic filters by the urea precipitation method for tar removal from biomass gasification gas... 

Poly-unsaturated fatty acids (PUFA), especially n-3 fatty acids, are well known for their health benefits [26], Fish oil as starting material for the separation of PUFA is a cheap source, but its fractionation is difficult. Separation techniques based on urea precipitation or silver ion chelate formation are less favored because of waste... 

Irradiated ribonuclease is treated with H235S and the non-covalently bound 35S is removed by Sephadex chromatography. At this stage the specific activity of the ribonuclease was 0.089 /xc/mg., of which 0.043 /xc/mg. was caused by /2-labeling determined in a separate experiment. Reduction of this material with mercaptoethanol in 8M urea, precipitation, and carboxymethylation resulted in ribonuclease s having a specific activity... 

The procedure of forming inclusion conpounds requires several urea precipitation steps along with filtration, wadiing, and recovery of the fraction enriched in unsaturated fatty acids. These... 

An interesting complication of uremia is the development of so-called uremic pericarditis. In the early stage of pericarditis, urea precipitates at the surface of the pericardium. This precipitation results in the development of a fibrinous exudate, which covers both the parietal and visceral aspects of the pericardium, giving the typical appearance of the bread and butter pericardium. Despite this extensive fibrinous exudate, constrictive pericarditis is a rare complication of uremic pericarditis. 

Preparation of [ C]Bromoacetyl Hydroxysuccimide. In 3 ml of diox-ane, 87 mg (630 /unoles) of [l- C]bromoacetic acid (1.54 mCi/nmole) and 86 mg (750 /imoles) of iV -hydroxysuccinimide are dissolved. To this solution, 132 mg (700 moles) of dicyclohexylcarbodiimide are added. Urea precipitates immediately. After 1 hr the urea is removed and the solution is brought to 5 ml with dioxane. The compound is used without further purification. 

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