Crystallization, fractional

In all crystallization phenomena, the crystals must first form and then grow. The formation of a new solid phase may occur on either an inert particle surface or in the solution itself, and is called nucleation. The increase in size of the nucleus by layer-upon-layer addition of solute is called growth. Both nucleation and crystal growth have supersaturation as a common driving force. If the solution remains (only) saturated, the crystals will not grow. If it becomes unsaturated, the crystal phase present will gradually disappear. 

Most early work on crystallization and nucleation was done with extremely pure solutions. There are numerous literature references in the high degree of supersaturation created and the energies associated with the formation of nuclei under such conditions this work is summarized in Chapter 2 of this volume. Within the last few years there has been a substantial rethinking of this aspect of crystallization. Most modem students of the subject agree that there are at least four identifiable sources of nucleation that must be considered in industrial crystallization  

In addition to these, one must consider intentional and accidental seeding as a surrogate form of nucleation, which is so often encountered industrially that it is convenient to consider it with other types of nucleation. 

Homogeneous and Heterogeneous Nucleation. Homogeneous and heterogeneous nucleation occur at very high levels of supersaturation, either in the solution, or in the case of heterogeneous nucleation, on other inert particles that are present in the form of crystals or as amorphous solid material. Most industrial crystallizers of the types in commercial use operate at levels of supersaturation far below those at which these types of seeding are expected, except under startup conditions. 

Attrition. Attrition causes small particles of crystalline material that have already been formed to be broken from identifiable crystals, and thereby, to be added to the body of solids that are present as individual or discrete particles. Their presence increases as the level of mechanical energy input in the system increases. They can be formed by contact of the crystals with a pump impeller or due to impact of the slurry on tube sheets, piping, or vessel walls. In some crystallizer designs, the attrition is sufficiently large so that if the crystallization rate is reduced to a very low value, the product size progressively decreases as particles separate from the existing solids and become new particles. Attrition can occur whether or not supersaturation is present. 

Fractional crystallization can be described as follows a part of a salt in solution is precipitated by a change in temperature or by evaporation of the saturated solution. If the solubility of the various compounds of the solution differs, the composition of the crystalline precipitate will not be the same as the composition of the original solution. The least soluble substance will be the first to crystallize. 

This method has been considered the best of the classical separation procedures for producing individual elements in high purity. The most suitable compounds are ammonium nitrates (for La, Pr, and Nd) and double magnesium nitrates (for Sm, Eu, Gd). Manganese nitrates have also been used for separation of lanthanides of the cerium group (La-Nd). Bromates and sulphates have been used in the separation of the yttrium group (being the heavy lanthanides or HREE) 

Erbium, thulium, lutetium and yttrium have been separated by application of a rare-earth hexa-antipyrine iodide salt. Other chemicals applied include a sodium rare earth EDTA salt for separating gadolinium, terbium, dysprosium, and yttrium. For these rare earths, a purity of 99% has been reached with this method. 

Fractional crystallization works best for the lanthanum end of the series, as there the differences in ionic radius are the largest. Fractional crystallization is very slow for the heavy rare earths and in the Sm(ni)-Gd(III)—region, because the differences in properties between the rare earths decrease as the atomic number increases (Gupta and Krishnamurthy 2005). 

Fractional crystallization is one of the oldest methods for the separation of rare earths and is now not used anymore for separation of rare earths (Habashi 2013). 

Fractional crystallization was the original method. It was an arduous task to make the separations in this way while searching for and finding the unknown elements. The fractionations were repeated hundreds or thousands of times to reach the goal of complete separation. 

The work often started by wetting the finely ground mineral powder with concentrated sulfuric acid. On heating to dull red heat, sulfates of the RE elements were formed and could be dissolved in ice water (the solubility increases with decreasing temperature). Oxalic acid was added to the aqueous solution and precipitated RE oxalates, which were transformed to oxides by calcination. This oxide mixture was the raw material in the separation work. 

One of the oldest methods used the difference in solubility of RE-potassium sulfates. The oxide mixture was dissolved in acid and potassium sulfate was added. Double salts of elements in the cerium group. La, Ce, Pr, Nd and Sm, have a low solubility and were precipitated first. The double sulfates of the elements Eu, Gd and Tb are a little more soluble but precipitated as a medium fraction. Elements of the yttrium group, Y, Dy, Ho, Er, Tm, Yb and Lu, remained in solution. 

Different, more specific, separation techniques were developed, each one having its special characteristics. It could be separation of other double salts - nitrates, oxalates - composed of a RE metal ion and another cation present in the solution, for example K+, Na+, NH or Mg. Double ammonium nitrate may be used for removal of lanthanum and the separation of neodymium from praseodymium. Members within the cerium group may be separated using double manganese nitrates, while elements in the yttrium group are separated using the differences in solubihty of their bromates. Auer von Welsbach did much work with RE-ammonium oxalate systems. Charles James in the USA developed an effective technique with RE-magne-sium double salts. 

Eractional crystallization is, as mentioned, time consuming and demands a great deal of work Eigure 17.15 describes the process in principle. 

If phenocryst compositions cannot explain trends in a rock series and a fractional and/racttonal crystallization model does not. appear to worlt, it is instructive to consider the crystallization possibility of simultaneous assimilation, of the country rock and fractional crystallization. This process, often abreviated to AFC, was first proposed by Bowen 

Bhatia, 1983). The increase in SiO reflects an increased mineralogical maturity, i.c. a greater quartz content and a smaller propartion of detrital grains. 

The identiji cation of former weathering conditions from sedimentary rocks 

A good measure of the degree of chemical weathering can be obtained from the chemical index of alteration (CIA Nesbitt and Young, 1982). 

Mixing in Banded gneisses from the Archaean Lewisian complex of northwest Scotland show metamorphic linear trends on major element variation diagrams. There are two possible 

In many cases, because the crystallizing minerals often have different densities from the magma, the crystals can separate from the magma, which is the scenario for fractional crystallization. 

The magma is in equilibrium with the instantaneous crystal, or, the last crystal of the formed solid, by the following relationship 

Note that the remaining magma is in equilibrium with instantaneous crystal, rather than the accumulated crystals that have separated from the magma. The concentration in the instantaneous crystal is related to the concentration in the accumulated fractionated crystals by 

Assuming constant D, the solution to Eq. (6.15) with initial magma condition Cn is 

This is the well-known Rayleigh fractionation law it is applicable to geological systems only when the bulk partition coefficients remain constant throughout fractional crystallization. 

The solution is evaporated to about 4.5 1. either by gentle boiling or by blowing a stream of air over the surface of the heated solution. The solution is allowed to cool about one-sixth of the mass should crystallize. The liquor is poured from the crystals into another dish and is again evaporated. To the crystals is added about 200 ml. of water and the dish heated until these crystals are dissolved. The resulting solution is poured into a 2-1. pyrex Florence flask and the dish rinsed into the same container this flask is labeled fraction 1. From the remaining rare earth solution are then removed further fractions in the same way, and each is transferred to a 2-1. flask and labeled in sequence 2, 3, etc. The final liquor, which will have a volume of about 200 ml., forms the fraction with the highest number. The series will now consist of fractions 1 to 7 aU should be nearly the same size except the last. 

While the series was set up by the process of removing relatively small crops of crystals from the liquor, the further fractionation of the material can best be carried on by removing small amounts of liquor from larger amounts of crystals. For the greatest efficiency all intermediate fractions (those other than the least soluble and the most soluble) should be nearly the same size, and the 

Commencing with the fraction next to the most soluble one (fraction 6 in this case) the liquor is poured into the most soluble fraction and the crystals allowed to drain as thoroughly as time allows. The liquor from fraction 5 is then poured onto the crystals in 6, that from fraction 4 onto the crystals in 5, etc., until the liquor from fraction 1 is poured onto the crystals in 2. Thus there has been accomplished a one-series fractionation, f Since fraction 1 is now dry, more solvent must be added to it. The volume of solvent must be somewhat smaller than the amount of solution that it is desired to pour from this fraction in the next fractionation. 

All fractions are now placed on gauze-covered tripods over individual M6ker burners (or better on a large 20- by 20-in. steel hot plate covered with thin sheet asbestos and heated with eight M6ker burners) and the crystals completely dissolved. Care must be taken during the dissolution of the crystals because of the fact that a hotter, more concentrated layer of solution is often formed at the bottom of the flask and this, if suddenly mixed into the upper layer, may cause much of the contents to be violently erupted 

As fractionation proceeds, the least soluble fraction (1) will decrease in volume, the intermediate fractions should remain constant, and the most soluble fraction wiU increase owing to the constant addition of liquors to it. The last fraction should be boiled down periodically or kept evaporated until it is of such volume that, when allowed to crystallize, it will have the desired volume of crystals and ratio of liquor to crystals. When this situation is realized, another flask is added to the series, the next higher number assigned to it, and the pouring of liquors into it commenced. 

A phase diagram showing complete miscibility in the solid state is known, for example, for the binary systems of phenanthrene and anthracene or naphthalene and /3-naphthol. 

---

KCl —NaCl —MgS04) and in many brines. Separated by fractional crystallization, soluble water and lower alcohols. Used in fertilizer production and to produce other potassium salts. 

Liquid-liquid and liquid-solid equilibria also find industrial applications in liquid-liquid extraction and fractional crystallization operations. 

This last interpretation makes P(0) the same as the fraction of a sample in the amorphous state. It is conventional to focus on the fraction crystallized 6 therefore the fraction amorphous is I - 6 and... 

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. 

Ciyst lliz tion. Low temperature fractional crystallization was the first and for many years the only commercial technique for separating PX from mixed xylenes. As shown in Table 2, PX has a much higher freezing point than the other xylene isomers. Thus, upon cooling, a pure soHd phase of PX crystallizes first. Eventually, upon further cooling, a temperature is reached where soHd crystals of another isomer also form. This is called the eutectic point. PX crystals usually form at about —4° C and the PX-MX eutectic is reached at about —68° C. In commercial practice, PX crystallization is carried out at a temperature just above the eutectic point. At all temperatures above the eutectic point, PX is stiU soluble in the remaining Cg aromatics Hquid solution,... 

The exact order of the production steps may vary widely in addition, some parts of the process may also vary. Metal formate removal may occur immediately after the reaction (62) following formaldehyde and water removal, or by separation from the mother Hquor of the first-stage crystallization (63). The metal formate may be recovered to hydroxide and/or formic acid by ion exchange or used as is for deicing or other commercial appHcations. Similarly, crystallization may include sophisticated techniques such as multistage fractional crystallization, which allows a wider choice of composition of the final product(s) (64,65). 

Fractional crystallization may be accompHshed on a batch, continuous, or semicontkiuous basis. Oil is chilled continuously while passkig through the unit and is then passed over a continuous belt filter which separates soHd fat from the Hquid oil. The process gives poorer separation compared to solvent fractionation because oils are viscous at crystallization temperatures and are entrained to a significant extent ki the soHd fraction. The Hquid fraction, however, is relatively free of saturated material. 

Wkiterization is a specialized appHcation of fractional crystallization that is utilized to remove saturates or waxes from Hquid oils. Salad oils, which do not cloud at refrigerator temperature, have been produced by winterizing lightly hydrogenated soybean ok. However, many producers now use refined, bleached, deodorized oks for this purpose (24). 

Solubility Properties. Fats and oils are characterized by virtually complete lack of miscibility with water. However, they are miscible in all proportions with many nonpolar organic solvents. Tme solubiHty depends on the thermal properties of the solute and solvent and the relative attractive forces between like and unlike molecules. Ideal solubiHties can be calculated from thermal properties. Most real solutions of fats and oils in organic solvents show positive deviation from ideaHty, particularly at higher concentrations. Determination of solubiHties of components of fat and oil mixtures is critical when designing separations of mixtures by fractional crystallization. 

The product salts were separated by fractional crystallization. However, the decline of natural saltpeter mining has virtually eliminated these processes as significant sources of KNO. ... 

Resources for Potash Fertilizers. Potassium is the seventh most abundant element in the earth s cmst. The raw materials from which postash fertilizer is derived are principally bedded marine evaporite deposits, but other sources include surface and subsurface brines. Both underground and solution mining are used to recover evaporite deposits, and fractional crystallization (qv) is used for the brines. The potassium salts of marine evaporite deposits occur in beds in intervals of haUte [14762-51-7] NaCl, which also contains bedded anhydrite [7778-18-9], CaSO, and clay or shale. The K O content of such deposits varies widely (see Potassium compounds). 

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. 

Fra.ctiona.1 Crystallization. Fractional crystallization, used until the early part of the twentieth century, is uneconomical for processing large quantities of lanthanides. Many recrystallization steps are required to recover high purity products. Several salts and double salts have been used ... 

RE(N0 )2 NH NO 4H20 for light lanthanide separation (La, Nd, Pr) 2RE(N02)3 3Mg(N03)2 24H20 for middle lanthanide separation (Sm, Eu, Gd). Bromates and ethylsulfates have been found useful. Fractional crystallization is particularly slow and tedious for the medium and heavy rare earths. 

A fourth ahoy separation technique is fractional crystallization. If shica is co-reduced with alumina, nearly pure shicon and an aluminum shicon eutectic can be obtained by fractional crystallization. Tin can be removed to low levels in aluminum by fractional crystallization and a carbothermic reduction process using tin to ahoy the aluminum produced, fohowed by fractional crystallization and sodium treatment to obtain pure aluminum, has been developed (25). This method looked very promising in the laboratory, but has not been tested on an industrial scale. 

Fractional crystallization processes are also used commercially to produce high purity metal from lower grade alurninum. These processes rely on the... 

Isomer separation beyond physical fractional crystallization has been accompHshed by derivatization using methyl formate to make /V-formyl derivatives and acetic anhydride to prepare the corresponding acetamides (1). Alkaline hydrolysis regenerates the analytically pure amine configurational isomers. 

Supercritical fluids can be used to induce phase separation. Addition of a light SCF to a polymer solvent solution was found to decrease the lower critical solution temperature for phase separation, in some cases by mote than 100°C (1,94). The potential to fractionate polyethylene (95) or accomplish a fractional crystallization (21), both induced by the addition of a supercritical antisolvent, has been proposed. In the latter technique, existence of a pressure eutectic ridge was described, similar to a temperature eutectic trough in a temperature-cooled crystallization. 

Solvent Extraction. The industrial separation of tantalum from niobium was carried out historicahy by the Marignac process of fractional crystallization of potassium heptafluorotantalate and potassium heptafluoroniobate (15,16) or the long-estabhshed Fansteel process (17), which involved the decomposition of the ore by a caustic fusion procedure. Processors have replaced these expensive processes by procedures based on solvent extraction. This technique was developed in the United States at Ames Laboratory and the U.S. Bureau of Mines (18). Figure 2 shows the flow sheet of an industrial instahation for the hydrometahurgical processing of tantalum—niobium raw materials. 

Chlorine and bromine add to benzene in the absence of oxygen and presence of light to yield hexachloro- [27154-44-5] and hexabromocyclohexane [30105-41-0] CgHgBr. Technical benzene hexachloride is produced by either batch or continuous methods at 15—25°C in glass reactors. Five stereoisomers are produced in the reaction and these are separated by fractional crystallization. The gamma isomer (BHC), which composes 12—14% of the reaction product, was formerly used as an insecticide. Benzene hexachloride [608-73-17, C HgCl, is converted into hexachlorobenzene [118-74-17, C Clg, upon reaction with ferric chloride in chlorobenzene solution. 

Boiling the solution speeds the conversion of intermediate hypobromites and bromites to bromate. The less soluble bromate can be separated from the hahde by fractional crystallization. A method that is often more economical is the oxidation of bromides into bromates by hypochlorites in aqueous solution. This can be done by passing chlorine into an alkaline bromide solution (75) ... 

Cocoa butter substitutes and equivalents differ greatly with respect to their method of manufacture, source of fats, and functionaHty they are produced by several physical and chemical processes (17,18). Cocoa butter substitutes are produced from lauric acid fats such as coconut, palm, and palm kernel oils by fractionation and hydrogenation from domestic fats such as soy, com, and cotton seed oils by selective hydrogenation or from palm kernel stearines by fractionation. Cocoa butter equivalents can be produced from palm kernel oil and other specialty fats such as shea and ilHpe by fractional crystallization from glycerol and selected fatty acids by direct chemical synthesis or from edible beef tallow by acetone crystallization. 

The ease of oxidation varies considerably with the nature and number of ring substituents thus, although simple alkyl derivatives of pyrazine, quinoxaline and phenazine are easily oxidized by peracetic acid generated in situ from hydrogen peroxide and acetic acid, some difficulties are encountered. With unsymmetrical substrates there is inevitably the selectivity problem. Thus, methylpyrazine on oxidation with peracetic acid yields mixtures of the 1-and 4-oxides (42) and (43) (59YZ1275). In favourable circumstances, such product mixtures may be separated by fractional crystallization. Simple alkyl derivatives of quinoxalines are... 

---