Purification of melts

Strictly speaking, solubility is also a function of pressme, but the effect is generally negligible in the systems normally encountered in crystallization from solution. In the purification of melt systems, however, pressure manipulation can be utilized for separating organic isomers (section 7.3.2). 

The gases present in the atmosphere over molten salts can react both with their principal components and with impurities. In reactions of the first kind, which include, in particular, hydrolysis, the gas (water) behaves as a Lux base since its action results in the increase of O concentration. In reactions of the second type, which are usually employed for the purification of melts, the gas has acidic properties. 

The effectiveness of the purification of melts based on lithium salts by hydrogen halides is much lower. Water dissolves in the KCl-LiCl melt in appreciable amounts and is firmly retained at temperatures up to 400 C. When dry HCl is passed for 1 h, removal of H2O is incomplete. 

The use of silicon halides for purification of melts used to grow single ciystals does not lead to formation of additional impurities, because the processes are carried out in quartz (i.e., Si02) containers. 

Its charge transfer complexes with aromatic hydrocarbons have characteristic melting points and may be used for the identification and purification of the hydrocarbons. 

Zone refining can be appHed to the purification of almost every type of substance that can be melted and solidified, eg, elements, organic compounds, and inorganic compounds. Because the soHd—Hquid phase equiHbria are not favorable for all impurities, zone refining often is combined with other techniques to achieve ultrahigh purity. 

The infrared spectmm of caprolactam has been given (3). Melting point data for the caprolactam—water system, as shown in Eigute 1, ate indicative of successful purification of caprolactam by crystallization from aqueous solution such purification is very effective for separating and rejecting polar impurities. 

In a modem carbon disulfide plant, all operations are continuous and under automatic control. On-stream times in excess of 90% are obtainable. The process is in three steps melting and purification of sulfur production and purification of carbon disulfide and recovery of sulfur from by-product hydrogen sulfide. A typical process appears in the flow diagram of Figure 1 (50). 

Purification of a chemical species by solidification from a liquid mixture can be termed either solution crystallization or ciystallization from the melt. The distinction between these two operations is somewhat subtle. The term melt crystallization has been defined as the separation of components of a binaiy mixture without addition of solvent, but this definition is somewhat restrictive. In solution crystallization a diluent solvent is added to the mixture the solution is then directly or indirec tly cooled, and/or solvent is evaporated to effect ciystallization. The solid phase is formed and maintained somewhat below its pure-component freezing-point temperature. In melt ciystallization no diluent solvent is added to the reaction mixture, and the solid phase is formed by cooling of the melt. Product is frequently maintained near or above its pure-component freezing point in the refining sec tion of the apparatus. 

High or ultrahigh product purity is obtained with many of the melt-purification processes. Table 22-1 compares the product quality and product form that are produced from several of these operations. Zone refining can produce very pure material when operated in a batch mode however, other melt ciystallization techniques also provide high purity and become attractive if continuous high-capacity processing is desired. Comparison of the features of melt crystalhza-tion and distillation are shown on Table 22-2. 

Figure 22-8 shows the features of a horizontal center-fed column [Brodie, Au.st. Mech. Chem. Eng. Tran.s., 37 (May 1979)] which has been commercialized for continuous purification of naphthalene and p-dichlorobenzene. Liquid feed enters the column between the hot purifying section and the cold freezing or recovery zone. Ciystals are formed internally by indirect cooling of the melt through the walls of the refining and recovery zones. Residue liquid that has been depleted or product exits from the coldest section of the column. A spiral conveyor controls the transport of solids through the unit. 

The dominant mechanism of purification for column ciystallization of sohd-solution systems is reciystallization. The rate of mass transfer resulting from reciystallization is related to the concentrations of the solid phase and free hquid which are in intimate contac t. A model based on height-of-transfer-unit (HTU) concepts representing the composition profQe in the purification sec tion for the high-melting component of a binaiy solid-solution system has been reported by Powers et al. (in Zief and Wilcox, op. cit., p. 363) for total-reflux operation. Typical data for the purification of a solid-solution system, azobenzene-stilbene, are shown in Fig. 22-10. The column ciystallizer was operated... 

Performance information for the purification of p-xylene indicates that nearly 100 percent of the ciystals in the feed stream are removed as produc t. This suggests that the liquid which is refluxed from the melting section is effectively refrozen oy the countercurrent stream of subcooled crystals. A high-meltingproduct of 99.0 to 99.8 weight percent p-xylene has been obtained from a 65 weight percent p-xyfene feed. The major impurity was m-xylene. Figure 22-12 illustrates the column-cross-section-area-capacity relationship for various product purities. 

Column crystalhzers of the end-fed type can be used for purification of many eutectic-type systems and for aqueous as well as organic systems (McKay loc. cit.). Column ciystaUizers have been used for xylene isomer separation, but recently other separation technologies including more efficient melt ciystaUization equipment have tended to supplant the Phillips style ciystaUizer. 

One of the most widely applicable and most commonly used methods of purification of liquids or low melting solids (especially of organic chemicals) is fractional distillation at atmospheric, or some lower, pressure. Almost without exception, this method can be assumed to be suitable for all organic liquids and most of the low-melting organic solids. For this reason it has been possible in Chapter 4 to omit many procedures for purification of organic chemicals when only a simple fractional distillation is involved - the suitability of such a procedure is implied from the boiling point. 

For materials with very low melting points it is sometimes convenient to use dilute solutions in acetone, methanol, pentane, diethyl ether or CHCI3-CCI4. The solutions are cooled to -78° in a dry-ice/acetone bath, to give a slurry which is filtered off through a precooled Buchner funnel. Experimental details, as applied to the purification of nitromethane, are given by Parrett and Sun [J Chem Educ 54 448 7977]. 

Amino acids have high melting or decomposition points and are best examined for purity by paper or thin layer chromatography. The spots are developed with ninhydrin. Customary methods for the purification of small quantities of amino acids obtained from natural sources (i.e. l-5g) are ion-exchange chromatography (see Chapter 1). For general treatment of amino acids see Greenstein and Winitz [The Amino Acids, Vols 1-3, J.Wiley Sons, New York 1961] and individual amino acids in Chapters 4 and 6. 

In addition to the above, purification of /V-methylacetamide can be achieved by fractional freezing, including zone melting, repeated many times, or by chemical treatment with vacuum distn under reduced pressures. For details of zone melting techniques, see Knecht in Recommended Methods for Purification of Solvents and Tests for Impurities, Coetzee Ed. Pergamon Press 1982. 

The esierUial oil detived from ihc hoa-rds is filled with crystaJa which have heeo ideotificd as c dtir camphor after purification they melt at 6G to 67 C., and have a specific rotation of + 10T2 ... 

Modem refining technology uses tantalum and niobium fluoride compounds, and includes fluorination of raw material, separation and purification of tantalum and niobium by liquid-liquid extraction from such fluoride solutions. Preparation of additional products and by-products is also related to the treatment of fluoride solutions oxide production is based on the hydrolysis of tantalum and niobium fluorides into hydroxides production of potassium fluorotantalate (K - salt) requires the precipitation of fine crystals and finishing avoiding hydrolysis. Tantalum metal production is related to the chemistry of fluoride melts and is performed by sodium reduction of fluoride melts. Thus, the refining technology of tantalum and niobium involves work with tantalum and niobium fluoride compounds in solid, dissolved and molten states. 

This gave a viscous sludge, and some ammonia was evolved. The temp was raised steadily to 59-61° over a period of 8 minutes, and held there for 23 minutes, while stirring the mixt continuously. After the mixt was cooled to 6°, a white ppt of methylnitroguanidine (MeNGu) was filtered off, washed with 30ml of cold w and dried. The crude product weighed about lQg (84.7% yield) and melted at 151—54°. Purification of the product was achieved by crystn, first from hot w (3ml per g) and then from 95% ethanol (8ml per g)... 

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