Salting out crystallization

The absorption and/or reduction is usually accomplished by causing the chlorine to react with sodium hydroxide solution or, in certain situations, with another alkaline compound. Absorption and/or reduction can take place in an appropriately designed tank or in a scrubber. The sodium hydroxide concentration should be less than 20 percent to prevent precipitation of sodium chloride crystals (salting-out) and excessive heat of reaction. 

Two cocrystallization processes employ dibasic crystals as intermediates. The PPG process (199—202) is discussed under commercial processes. The PPC process (203) forms dibasic crystals from lime and recovered filtrates. The dibasic crystals are separated from thek mother liquor by decantation, slurried in caustic solution and chlorinated to produce a cocrystalline slurry of Ca(OCl)2 and NaCl. The slurry is sent to a flotation cell where the larger salt crystals settle out and the smaller hypochlorite crystals float to the top with the aid of ak and flotation agent. The hypochlorite slurry is centrifuged the cake going to a dryer and the centrate to the flotation cell. The salt-rich bottoms from the flotation cell are centrifuged and washed with dibasic mother Hquor. The centrates are recycled to the precipitation step. 

Salting-out crystalli tion operates through the addition of a nonsolvent to the magma ia a crystallizer. The selection of the nonsolvent is based on the effect of the solvent on solubiHty, cost, properties that affect handling, iateraction with product requirements, and ease of recovery. The effect of a dding a nonsolvent can be quite complex as it iacreases the volume required for a given residence time and may produce a highly nonideal mixture of solvent, nonsolvent, and solute from which the solvent is difficult to separate. 

Whether a vessel is called an evaporator or a crystallizer depends primarily on the criteria used in arriving at its sizing. In an evaporator of the salting-out type, sizing is done on the basis of vapor release. In a ciystaUizer, sizing is normally done on the basis of the volume required for crystallization or for special features required to obtain the proper produc t size. In external appearance, the vessels could be identical. Evaporators are discussed in Sec. 11. 

Application Systems where high capacity near-design rates to be maintained in continuous service. Handles suspended crystal and small solid materials, as well as polymer forming materials. Holes become plugged in salting-out systems where trays run hot and dry (as underside of bottom tray). Good in vacuum or low-pressure-drop design. 

Similarly, concepts of solvation must be employed in the measurement of equilibrium quantities to explain some anomalies, primarily the salting-out effect. Addition of an electrolyte to an aqueous solution of a non-electrolyte results in transfer of part of the water to the hydration sheath of the ion, decreasing the amount of free solvent, and the solubility of the nonelectrolyte decreases. This effect depends, however, on the electrolyte selected. In addition, the activity coefficient values (obtained, for example, by measuring the freezing point) can indicate the magnitude of hydration numbers. Exchange of the open structure of pure water for the more compact structure of the hydration sheath is the cause of lower compressibility of the electrolyte solution compared to pure water and of lower apparent volumes of the ions in solution in comparison with their effective volumes in the crystals. Again, this method yields the overall hydration number. 

Alkali ions (salts) influence the formation of the precursor gel for most of the synthetic zeolites (3,34,39,40). Na+ ions were shown to enhance in various ways the nucleation process (structure-directing role) (40-42), the subsequent precipitation and crystallization of the zeolite (salting-out effect) (JO and the final size and morphology of the crystallites (34,43). Informations on the various roles played by the inorganic (alkali) cations in synthesis of ZSM-5, such as reported in some recent publications (7,8,10,14,17,29,30,44,45) remain fragmentary, sometines contradictory and essentially qualitative. 

Role of alkali and NH cations in the crystallization of ZSM-5 Introduced in an aqueous (alumino) silicate gel (sol), the bare alkali cations will behave in various ways firstly, they will interact with water dipoles and increase the (super) saturation of the sol. Secondly, once hydrated, they will interact with the aluminosilicate anions with, as a result, the precipitation of the so formed gel (salting-out effect). Thirdly, if sufficiently small, they also can order the structural subunits precursors to nucleation species of various zeolites (template function-fulfilled by hydrated Na+ in the case of ZSM-5 (11,48)). ... 

For salt-out, add solute which reduces effective solubility of component to be crystallized (salt having common ion is usual choice) salting-out compound should be cheap, as maybe difficult to recover. 

Methylene Blue [61-73-4] M 319.9, 454 94,000 (EtOH), 44481,000 (HjO). Crystd from O.IM HCl (16ml7g), the crystals were separated by centrifugation, washed with chilled EtOH and ethyl ether and dried under vacuum. Crystd from 50% aqueous EtOH, washed with absolute EtOH, and dried at 50-55 for 24h. Also crystd from benzene-MeOH (3 1). Salted out with NaCl from a commercial cone aqueous soln, then crystd from water, dried at 100° in an oven for 8-1 Oh. 

The salting-out may produce an interface that often has a high pigment concentration and occasionally even crystals that begin to precipitate. It is important to collect the interface and wash it with salt solution or an appropriate solvent to allow the pigments to combine with... 

Crystallographers grow crystals of proteins by slow, controlled precipitation from aqueous solution under conditions that do not denature the protein. A number of substances cause proteins to precipitate. Ionic compounds (salts) precipitate proteins by a process called "salting out." Organic solvents also cause precipitation, but they often interact with hydrophobic... 

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