Fractional crystallization and precipitation

Fractional crystallization and precipitation are classical methods of separation of rare earth metal ions. Complex forming agents may be used to give better separations than simple or double salts. Some of the complexing agents used in fractionation separation are given below. 

So far we have considered only the newer countercurrent methods for fractionating peptides. These will probably play a predominant role in the future, though the classical methods of fractional crystallization and precipitation should not be forgotten. They are still the most effective methods of fractionating proteins and probably larger polypeptides, such as the oxidation products of insulin (Sanger, 1949a). 

The separation of RE metals is one of the most difficult problems within inorganic chemistry. The chemical properties of the elements are so exceptionally similar. When the separation work started with fractional crystallization and precipitation chemists used the small differences in solubility between the RE salts. To some extent the differing basicities of the oxides were also used. As soon as advanced separation techniques, ion exchange and Hquid-Hquid extraction, were developed the situation changed drastically. 

Although the complete inseparability of isotopes by chemical means has been frequently asserted, the evidence on which this assertion is based has always seemed insufficient. The methods used have been fractional crystallization and precipitation, but these processes have seldom been carried out more than ten times in a particular case, and frequently six or seven crystallizations have been thought a sufficiently thorough test of inseparability. A search of the literature revealed only one investigation, that of Radiothorium and Thorium by McCoy and Ross, where as many as one htmdred repetitions of a given process had been made. 

The early history of the rare earth elements is primarily dominated by the attempts to separate and purify the individual elements using the classical techniques of fractional crystallization and precipitation (Vickery, 1953). These procedures generally involved aqueous solutions which contained the hydrated ion and in this sense could be considered as the earliest examples of studies of the complexing properties of the rare earths. From a practical standpoint, however, the existence of the complexed ions was only incidental and was probably not even considered by the early workers. Early reference works of the 1920 s which summarize the extant information discuss only some double salts and adducts of the rare earths and do not consider them in terms of coordination compounds (Moeller, 1967). The first chelates prepared were probably the acetylacetonates used by Urbain (1896) in a separation procedure. 

A mixture of l,cw-3-dibromo-/ram-l,/ran.v-3-dinitro-dM-2,rran,s-4-diphenylcyclobutane (4 7,6 g, 17 mmol) and pyridine (1.6 g, 20 mmol) in benzene at 20 nC within a few seconds became yellow and deposited a white precipitate. After 10 min, the mixture was filtered and concentrated to dryness in a stream of air. Fractional crystallization and chromatography gave fine yellow needles of 5 yield 4.47 g (73%) mp 129-131 C. 

Under the pressure change option, the user can specify a lower and upper pressure range and an incremental pressure interval (AP) at which equilibrium is calculated at a fixed temperature (e.g., 1 to 1001 bars with AP = 100 bars would result in equilibrium calculations at 1, 101,. ..and 1001 bars). Under equilibrium crystallization , solid phases that have precipitated are allowed to dissolve and repreciptate as different solids when environmental drivers such as temperature, pressure, or evaporation change. Under fractional crystallization , a precipitated solid phase is not allowed to subsequently dissolve and reprecipitate when environmental drivers change this option allows for hypothetical removal (layering) of precipitates in basins when environmental drivers change. 

Classical methods of separation [7] are (1) fractional crystallization, (2) precipitation and (3) thermal reactions. Fractional crystallization is an effective method for lanthanides at the lower end of the series, which differ in cation radius to a large extent. The separation of lanthanum as a double nitrate, La(N03)3-2NH4N03-4H20, from praseodymium and other trivalent lanthanide with prior removal of cerium as Ce4+ is quite a rapid process and is of commercial significance. Other examples are separation of yttrium earths as bromates, RE(Br03>9H20 and use of simple nitrates, sulfates and double sulfate and alkali metal rare earth ethylenediamine tetraacetate complex salts in fractional crystallization separation. 

Crystallization and precipitation Fractionates based on melting point In situ removal of long-chain and hydroxy FFA and polyol monoesters... 

The separation of yttrium from the lanthanides is performed by selective oxidation, reduction, fractionated crystallization, or precipitation, ion-exchange and liquid-liquid extraction. Methods for determination include arc spectrography, flame photometry and atomic absorption spectrometry with the nitrous oxide acetylene flame. The latter method improved the detection limits of yttrium in the air, rocks and other components of the natural environment (Deuber and Heim 1991 Welz and Sperling 1999).Other analytical methods useful for sensitive monitoring of trace amounts of yttrium are X-ray emission spectroscopy, mass spectrometry and neutron activation analysis (NAA) the latter method utilizes the large thermal neutron cross-section of yttrium. For high-sensitivity analysis of yttrium, inductively coupled plasma atomic emission spectroscopy (ICP-AES) is especially recommended for solid samples, and inductively coupled plasma mass spectroscopy (ICP-MS) for liquid samples (Reiman and Caritat 1998). 

Analysis of complex mixtures often requires separation and isolation of components, or classes of components. Examples in noninstrumental analysis include extraction, precipitation, and distillation. These procedures partition components between two phases based on differences in the components physical properties. In liquid-liquid extraction components are distributed between two immiscible liquids based on their similarity in polarity to the two liquids (i.e., like dissolves like ). In precipitation, the separation between solid and liquid phases depends on relative solubility in the liquid phase. In distillation the partition between the mixture liquid phase and its vapor (prior to recondensation of the separated vapor) is primarily governed by the relative vapor pressures of the components at different temperatures (i.e., differences in boiling points). When the relevant physical properties of the two components are very similar, their distribution between the phases at equilibrium will result in shght enrichment of each in one of the phases, rather than complete separation. To attain nearly complete separation the partition process must be repeated multiple times, and the partially separated fractions recombined and repartitioned multiple times in a carefully organized fashion. This is achieved in the laborious batch processes of countercurrent liquid—liquid extraction, fractional crystallization, and fractional distillation. The latter appears to operate continuously, as the vapors from a single equilibration chamber are drawn off and recondensed, but the equilibration in each of the chambers or plates of a fractional distillation tower represents a discrete equihbration at a characteristic temperature. 

We wanted to identify beyond any doubt the chemical properties of the parent members of the radioactive series which were separated with the barium and which have been designated as radium isotopes. We have carried out fractional crystallizations and fractional precipitations, a method which is well-known for concentrating (or diluting) radium in barium salt solutions ... 

Scientists relied too much on the spectroscopic method as soon as a new line was observed in the spectrum, they announced the discovery of a new element. The spectral analysis of that time was relatively young and it was not always possible to establish when the new line really was due to a new element and when it belonged to an impurity of some known element. This was, perhaps, the main cause of false discoveries of REEs. Another was that separation methods were few only fractional crystallization and fractional precipitation. The first method was based on different... 

None of the solid forms of sodium hypochlorite have sufficient stability for commercial use. However, hydrates of sodium hypochlorite can be separated from sodium chloride by fractional crystallization and then dissolved to make sodium hypochlorite solutions with low salt. Salt precipitates as a solution of 50% sodium hydroxide is chlorinated at 25-30 C. A solution with 32% of sodium hypochlorite and -6% sodium chloride is separated from the salt. This solution is cooled to 10-22°C to crystallize NaOCl 5H2O. The crystals are then dissolved to make a solution of 13% sodium hypochlorite and 0.1-2% sodium chloride. ... 

Isomers of /-Bu,OH f-Bu,OH-Bio were separated by fractional crystallization and column chromatography. A solution of isomeric mixture of i-Bu,OH /-Bu, OH-Bio (2g) dissolved in chloroform (lOmL) and hexane (25 mL) was kept in a refrigerator overnight to induce formation of solid. The solid formed was separated by filtration and contained a mixture of isomers with intensity of 8 2 (isomer-1 isomer-2). Hexane (15mL) was added to the filtrate and the mixture kept at —30°C for 24 h to induce further precipitation, which contained an isomeric ratio of about 2 8 (isomer-l isomer-2). 

The two methods most widely used, prior to the development of ion-exchange elution, as means of separating rare earth mixtures were fractional crystallization and fractional precipitation. Such practices have been summarized by numerous experts on rare earth separations, e.g., Spencer (1919), Yost et al. (1947), Kremers (1953), Vickery (1953), Healy et al. (1961), Bouisseres et al. (1959), Bril (1964), Moeller (1963), and Moeller (1973). 

This technique is similar in concept to fractional crystallization and has generally been effected by addition of hydroxide ion, or by the generation of this ion in solution. Hydrolysis occurs less extensively with La than with the other lanthanons and, due to the greater sdlubility product of La(OH)3 compared to those of the smaller tervalent rare earth cation hydroxides (ca. 10 for La(OH)3 compared to 10" for Lu(OH)3), it is feasible to precipitate the bulk of the other rare earths in the presence of ammonium ion and leave La, for the most part, behind. 

For preparative purposes batch fractionation is often employed. Although fractional crystallization may be included in a list of batch fractionation methods, we shall consider only those methods based on the phase separation of polymer solutions fractional precipitation and coacervate extraction. The general principles for these methods were presented in the last section. In this section we shall develop these ideas more fully with the objective of obtaining a more narrow distribution of molecular weights from a polydisperse system. Note that the final product of fractionation still contains a distribution of chain lengths however, the ratio M /M is smaller than for the unfractionated sample. 

Fluorozirconate Crystallization. Repeated dissolution and fractional crystallization of potassium hexafluorozirconate was the method first used to separate hafnium and zirconium (15), potassium fluorohafnate solubility being higher. This process is used in the Prinieprovsky Chemical Plant in Dnieprodzerzhinsk, Ukraine, to produce hafnium-free zirconium. Hafnium-enriched (about 6%) zirconium hydrous oxide is precipitated from the first-stage mother Hquors, and redissolved in acid to feed ion-exchange columns to obtain pure hafnium (10). 

Separation Processes. The product of ore digestion contains the rare earths in the same ratio as that in which they were originally present in the ore, with few exceptions, because of the similarity in chemical properties. The various processes for separating individual rare earth from naturally occurring rare-earth mixtures essentially utilize small differences in acidity resulting from the decrease in ionic radius from lanthanum to lutetium. The acidity differences influence the solubiUties of salts, the hydrolysis of cations, and the formation of complex species so as to allow separation by fractional crystallization, fractional precipitation, ion exchange, and solvent extraction. In addition, the existence of tetravalent and divalent species for cerium and europium, respectively, is useful because the chemical behavior of these ions is markedly different from that of the trivalent species. 

Another method of fractional crystallization, in which advantage is taken of different ciystallization rates, is sometimes used. Thus, a solution saturated with borax and potassium chloride will, in the absence of borax seed ciystals, precipitate only potassium chloride on rapid coohng. The borax remains behind as a supersaturated solution, and the potassium chloride crystals can be removed before the slower borax crystalhzation starts. 

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