Impurities fractional crystallization

The chief impurity is l,5-di- -tolyl-3-methyl-1,4-pent-azadiene (m.p. 148°). This can be removed by fractional crystallization, but it is easier to sublime the triazene from the reaction mixture. 

Separation of niobium from tantalum and impurity metals is the most important step in its extraction from the ore. It may be achieved by several methods that include solvent extraction, ion exchange, fractional crystallization, fractional sublimation, and other techniques. Solvent extraction is apphed mostly in several large-scale commercial processes. Although the classical fractional crystalhzation method forms effective separation, it is a tedious... 

Liquid-liquid extraction is a relatively simple and efficient process compared to Marignac s older fractional crystallization method. Ore concentrate in its finely-ground form is heated with hydrofluoric acid to dissolve the oxides of tantalum and niobium and separate them from associated impurities. 

Tantalum metal is prepared from potassium fluotantalate or tantalum pen-toxide produced from the ore concentrate by solvent extraction or fractional crystallization as described. The metal is produced industrially by Balkes electrolysis process. Fused potassium fluotantalate is electrolyzed at 900°C in a cast iron pot. While the latter serves as a cathode, a graphite rod is used as the anode. A small amount of tantalum oxide is added to the melt. The unreduced potassium fluotantalate is separated from the tantalum metal produced by leaching with water. Impurities are removed from the metal by acid wash. 

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 The former methods have been historically important but have now been superseded by solvent extraction with TBP.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. 

C. Aromatic Hydrocarbons. The methods listed for saturated hydrocarbons may be used with benzene. Thiophene and similar sulfur-containing impurities are removed by sulfuric acid washes. Very-high-purity benzene may be prepared by fractional crystallization from ethanol followed by distillation. 

So-called superlattices of 5mn alkylthiolate protected silver particles having truncated octahedral shapes as well as thiol-stabilized 5 6 mn gold particles can be obtained from solution. Fractional crystallization is a very usual method to separate chemical compounds from other compounds and impurities. Mixtures of thiolate stabilized gold nanoparticles between 1.5 and 3.5 nm could successfully be fractioned into real monodisperse species containing 140, 225, 314, and 459 atoms. 2D assemblies have also become available of these fcc-structured nanoparticles. The decisive criterion to successhilly fraction and crystallize metal nanoparticles is to protect them perfectly by strongly bound ligand molecules in order to avoid coalescence. 

A closely related and highly important method of purification is fractional crystallization from a solution. The impure substance is dissolved in a suitable solvent, the solution is filtered, and the filtrate is then allowed to evaporate until some of the substance has crystallized. These crystal are usually purer than the original material (Fig. 2-11), Thus nearly pure... 

---