110 Crystal habit

Although crystals can be classified according to the seven general systems (Table 1.1), the relative sizes of the faces of a particular crystal can vary considerably. This variation is called a modification of habit. The crystals may grow more rapidly, or be stunted, in one direction thus an elongated growth of the prismatic habit gives a needle-shaped crystal (acicular habit) and a stunted growth gives a flat plate-like crystal (tabular, platy or flaky habit). Nearly all manufactured and...

TSK process

The Tsukishima Kikai countercurrent cooling crystallization process (frequently referred to as the TSK 4C process) is a development of Brodie technology with the scraped-surface chillers replaced by several cooling crystallizers connected in series. The flow sheet in Figure 8.21 shows three conventional agitated cooling crystallizers connected in series. Feed enters the first stage vessel and partially crystallizes. The first stage slurry is continuously pumped to a hydrocyclone to be...

Multistage vacuum crystallizer

The Standard Messo multistage vacuum crystallizer (Figure 8.52) provides a number of cooling stages in one vessel. The horizontal cylinder is divided into several compartments by vertical baffles that permit underflow of magma from one section to another but isolate the vapour spaces. Each vapour space is kept at its operating pressure by a thermocompressor, which discharges to a barometric condenser. Hot feedstock is sucked into the first compartment, which is operated at the highest pressure...

816 Industrial applications

It is not possible to attempt a comprehensive survey of the enormous range of industrial precipitation practice, but the examples below will serve to indicate something of its diversity. A few references are listed for each topic to serve as an introduction to relevant literature. Precipitated calcium carbonate is widely used industrially, with different applications usually demanding different physical and granulometric properties. Three different crystallographic forms of CaC03 can be...

H2oh2o

Phase diagrams for the system NaCl-Na2SO4-H2O (a) at 17.5 C (b) at 25 C regions S solution H hydrate Na2S04 10H20 S04 Na2S04 and CI NaCl. The solution above curve ABC is unsaturated. The lowest triangular region represents a solid mixture of Na2S04, Na2S04 10H20 and NaCl. Point B is the eutonic or drying-up point of the system. In Figure 4.23b, points A and D denote the solubilities of NaCl (26.6 mass per cent) and Na2S04 (21.6 per cent) in water at 250 C, point E the composition...

643 Structural compatibility

It is usually assumed that there is some form of affinity between an active impurity and the crystallizing species, and this can take a large variety of forms. For example, there may be some degree of structural compatibility between ionic groups in the modifying agent and the crystal, e.g. as in the case of calcium carbonate being modified by metaphosphates, or nitrilotriacetamide and nitrilotripropionamide for modifying NaCl and KCl, respectively (Sarig, Glasner and Epstein, 1975). There may...

91 Crystal size distribution CSD

One of the earliest investigations aimed at studying the size distribution of crystals in a continuous crystallizer was made by Montillon and Badger (1927) with Na2SO4 10H2O and MgSO4 7H2O. Shortly afterwards McCabe (l929) analysed the problem of crystal size distribution (CSD) and developed the AL law, making the following assumptions (a) all crystals have the same shape (b) they grow invariantly, i.e. the growth rate is independent of crystal size (c) supersaturation is constant throughout...

Example 91 Continuous cooling

A continuous cooling crystallizer is required to produce potassium sulphate crystals (density pc 2660 kg m-3, volume shape factor a 0.7) of 750 median size LM at the rate Pc 1000 kg h-1. On the basis of pilot-plant trials, it is expected that the crystallizer will operate with steady-state nucleation growth kinetics expressed (equation 9.39 with j 1 and i 2) as B 4 x 1018MtG2 m-3 s-1. Assuming MSMPR conditions and a magma density Mt 250 kg m-3, estimate the crystallizer volume and other...

Batch versus continuous crystallization

Continuous, steady-state operation is often regarded as the ideal procedure for many types of process plant equipment, but this is not always true for crystallization processes. Batch operation often offers considerable advantages, such as simplicity of equipment and minimization of encrustation on heat-exchanger surfaces. In many cases, only a batch crystallizer can produce the required crystal form, size distribution, or purity. On the other hand, the operating costs of a batch system can be...

93 Crystallizer specification 931 Crystallizer selection

The temperature-solubility relationship between the solute and solvent is of prime importance in the selection of a crystallizer. For solutions that yield appreciable quantities of crystals on cooling, the choice of equipment will generally lie between a simple cooling or a vacuum (flash cooling) crystallizer. For solutions that change little in composition with a reduction in temperature, an evaporating crystallizer would normally be used, although salting out might be considered in certain...

Rotary drum crystallizers

The rotary drum crystallizer (Figure 8.24) is another example of crystallization on a chilled surface. A horizontally-mounted cylinder, partially immersed in the melt, or supplied with feedstock in some other way, is supplied with coolant fluid entering and leaving the inside of the hollow drum through trunnions. As the drum rotates, a thin crystalline layer forms on the cold surface and this is removed with a scraper knife. Agitation of the melt, as near to the drum as possible, appears to...

Sparingly soluble salts

The solubility of sparingly soluble electrolytes in water, with the exception of the salts of weak acids or bases, may be determined from conductivity measurements on their saturated solutions. A variety of commercial instruments are now available for this purpose and experimental details may be found in handbooks of practical physical chemistry, e.g. Findlay, 1973 Matthews, 1985. If the equilibrium saturation concentration, c*, of the salt is expressed in molm-3, the molar conductivity, A,...

32

From equations 5.3 and 5.7 the radius of a spherical critical nucleus at a given supersaturation can be expressed as For the case of non-spherical nuclei, the geometrical factor 16 3 in equations 5.4 and 5.8-5.10a must be replaced by an appropriate value (e.g. 32 for a cube). Similar expressions to the above may be derived for homogeneous nucleation from the melt in terms of supercooling. The volume free energy AGV is given by where T* is the solid-liquid equilibrium temperature expressed in...

1

Where ai is the area of the ith face of a crystal bounded by n faces, and gi the surface free energy per unit area of the ith face. Therefore, if a crystal is allowed to grow in a supersaturated medium, it should develop into an 'equilibrium' shape, i.e. the development of the various faces should be in such a manner as to ensure that the whole crystal has a minimum total surface free energy for a given volume. Of course, a liquid droplet is very different from a crystalline particle in the...

Prilling

The name 'prilling' is given to a melt-spray crystallization process that results in the formation of solid spherical granules. It is employed widely in the manufacture of fertilizer chemicals such as ammonium nitrate, potassium nitrate, sodium nitrate and urea. In the ammonium nitrate prilling process (Shearon and Dunwoody, 1953) a very concentrated solution, containing about 5 water, is sprayed at 140 C into the top of a 30 m high, 6 m diameter tower in which the droplets fall...

73 Isolation of polymorphs

The ability of a single compound to crystallize in more than one crystallo-graphic form (polymorphism) is encountered in a wide range of industries including pharmaceuticals, dyestuffs, agrochemicals, photochemicals, and other specialty compounds, both organic and inorganic. Similarly, it may also be possible to crystallize several different solvates, e.g., hydrates, which although not strictly speaking polymorphs can also be included in the general considerations outlined below. As described...

Cooling disc crystallizer

The first cooling disc crystallizer to be developed, in the early 1930s, was the Werkspoor 'rapid' crystallizer which has widely used in the sugar industry for the processing of after-product massecuite. It was an open trough machine containing a horizontally mounted slow-speed agitator-cooler in the form of hollow discs through which cooling water was circulated. The discs had segmental openings to enable the crystal slurry to flow through the machine countercurrently to the cooling medium....

32 Solvent selection

Water is almost exclusively used as the solvent for the industrial crystallization of inorganic substances from solution. This fact is quite understandable because, apart from the relative ease with which a very large number of chemical compounds dissolve in it, water is readily available, cheap and innocuous. For these reasons water is used whenever possible even for the industrial crystallization of organic compounds, although for a variety of reasons other solvents may have to be used in...

34 Solubility correlations

In the majority of cases the solubility of a solute in a solvent increases with temperature, but there are a few well-known exceptions to this rule. Some typical solubilities for various salts in water are shown in Figure 3.1, where all concentrations are expressed as kg of anhydrous substance per 100 kg of water. In Figure 3.1a sodium chloride is a good example of a salt whose solubility increases only slightly with an increase in temperature, whereas sodium acetate shows a fairly rapid...

51 Primary nucleation 511 Homogeneous nucleation

Homogeneous Nucleation

Exactly how a stable crystal nucleus is formed within a homogeneous fluid is not known with any degree of certainty. To take a simple example, the condensation of a supersaturated vapour to the liquid phase is only possible after the appearance of microscopic droplets, called condensation nuclei, on the condensing surface. However, as the vapour pressure at the surface of these minute droplets is exceedingly high, they evaporate rapidly even though the surrounding vapour is supersaturated. New...

Sulzer MWB process

The Sulzer MWB process (Fischer, Jancic and Saxer, 1984) is a melt crystallizer that operates basically by crystallization on a cold surface, but with features which allow it to operate effectively as a multistage separation device. Consequently, it can be used to purify solid solution as well as eutectic systems. The effective multistage countercurrent scheme is illustrated for four-stage operation in Figure 8.23a. Stage 1, is fed with melt L2 and recycle liquor L1 - L, where L is the reject...

Wettedwall evaporative crystallizer

A somewhat unusual application of the wetted-wall column, frequently used in gas-liquid mass transfer operations, has been reported by Chandler (1959). A hot concentrated solution is fed into a horizontal pipe, and cold air is blown in concurrently at a velocity of about 30ms-1. The liquid stream spreads over the internal surface of the pipe and cools, mainly by evaporation (see Figure 8.47). The crystal slurry and air leave from the same end of the pipe. Only small Figure 8.47. Arrangement of...

30

Equations 9.41-9.44 are useful for basic design calculations (see section 9.3.2). For a constant magma density, equation 9.26 indicates that for different residence times r1 and r2 6apcn0i(GiTi)4 6apc n G )4 (9.45) which, from equation 9.18, may be written The effect of residence time on the median crystal size Lm (from equation 9.37) is Keeping the magma density constant and doubling the residence time (t2 2rL), the following effects might be noted 1. For a relative kinetic order i < 1, a...

Programmed controlled cooling

Crystal Vrowth Cooling Time Com

A typical batch cooling crystallization, for example, begins with a hot unsaturated feed liquor being charged to an agitated vessel equipped with a cooling jacket or coils. Supersaturation is created shortly after cooling is commenced, after which nucleation occurs, crystals grow and the supersaturation is depleted. The final product CSD is dependent on the supersaturation profile created over the batch time, so the cooling rate is of critical importance. Selective seeding can also be helpful...

18 Isomorphs and polymorphs

Two or more substances that crystallize in almost identical forms are said to be isomorphous (Greek 'of equal form'). This is not a contradiction of Hauy's law, because these crystals do show small, but quite definite, differences in their respective interfacial angles. Isomorphs are often chemically similar and can then be represented by similar chemical formulae this statement is one form of Mitscherlich's Law of Isomorphism, which is now recognized only as a broad generalization. One group...

Continuous belt crystallizer

The Sandvik continuous cooled belt crystallizer (Figure 8.25) may be considered as an alternative to the rotary cooled drum. The underside of the steel belt is sprayed with cooling water to provide a controlled temperature gradient along Figure 8.25. The Sandvik continuous belt crystallizer Figure 8.25. The Sandvik continuous belt crystallizer the belt. Melt is fed at one end and crystals are removed at the other, generally in the form of thin flakes. The unit can also produce small pastilles...

53 Metastable zone widths

The lack of success of the classical nucleation theories in explaining the behaviour of real systems has led a number of authors to suggest that most primary nucleation in industrial crystallizers is heterogeneous rather than homogeneous and that empirical relationships such as are the only ones that can be justified. J is the nucleation rate, kn the nucleation rate constant and Acmax the maximum allowable supersaturation (or metastable zone width). The exponent n, which is frequently referred...

Saltingout crystallization

Adductive Crystallization

A solute can be deposited from solution by the addition of another substance (a soluble solid, liquid or gas) which effectively reduces the original solute solubility. The process is often referred to as 'salting out', although it applies to electrolytes and non-electrolytes alike. A slow addition of the salting-out agent can change a fast precipitation of the solute into a more controlled crystallization process. For convenience, this topic is dealt with in more detail as one of the techniques...

48 Dynamic phase diagrams

One of the problems of trying to establish reliable phase equilibria in multi-component solid-liquid systems is that very long periods of contact between crystals and solution are often necessary before the equilibrium state is approached. In fact, some systems can appear to be unable to achieve a stable equilibrium, in which case a meaningful phase diagram cannot be constructed. Not only are reliable multicomponent phase equilibria difficult to measure in the laboratory, the measured data may...

No compound formed

This simplest case is illustrated in Figure 4.21 for the system KNO3-NaNO3-H2O at 500C. Neither salt forms a hydrate, nor do they combine chemically. Point A represents the solubility of KNO3 in water at the stated temperature (46.2g 100g of solution) and point C the solubility of NaNO3 (53.2g 100g). Curve AB indicates the composition of saturated ternary solutions that are in equilibrium with solid KNO3, curve BC those in equilibrium with solid NaNO3. The upper area enclosed by ABC represents...

628 Effect of impurities

The presence of impurities in a system can have a profound effect on the growth of a crystal. Some impurities can suppress growth entirely some may enhance growth, while others may exert a highly selective effect, acting only on certain crystallographic faces and thus modifying the crystal habit (see section 6.4). Some impurities can exert an influence at very low concentrations, less than 1 part per million, whereas others need to be present in fairly large amounts before having any effect....

3122 Measurement of supersaturation

If the concentration of a solution can be measured at a given temperature, and the corresponding equilibrium saturation concentration is known, then it is a simple matter to calculate the supersaturation (equations 3.67-3.69). Just as there are many methods of measuring concentration (section 3.9.2) so there are also many ways of measuring supersaturation, but not all of these are readily applicable to industrial crystallization practice. Solution concentration may be determined directly by...

618 Crystallization from melts

The rate of crystallization from a melt depends on the rate of heat transfer from the crystal face to the bulk of the liquid. As the process is generally accompanied by the liberation of heat of crystallization, the surface of the crystal will have a slightly higher temperature than the supercooled melt. These conditions are shown in Figure 6.14 where the melting point of the substance is denoted by T * and the temperature of the bulk of the supercooled melt by T. The overall degree of...

97 Downstream processes

An industrial crystallization process generally has to be followed by several separate operations in order to obtain the crystalline product in its final form. These downstream operations can include solid-liquid separation, washing, classification, drying and screening. Their requirements, or indeed the wish to avoid employing any one of them, can have an influence on the design of the crystallizer itself. The complete design of a crystallization plant, therefore, must take into consideration...

65 Polymorphs and phase transformations

It is not uncommon in crystallization processes for the first crystalline phase to make its appearance to be metastable, e.g. a polymorph or hydrate (Ostwald's rule of stages - section 5.7). Some metastable phases rapidly transform to a more stable phase while others can exhibit apparent stability for an exceptionally long time. Some transformations are reversible (enantiotropic) while others are irreversible (monotropic), as explained in sections 1.8 and 4.2.1. In some cases, the metastable...

612 Adsorption layer theories

The concept of a crystal growth mechanism based on the existence of an adsorbed layer of solute atoms or molecules on a crystal face was first suggested by Volmer 1939 . Many other workers have contributed to, and modified Yolmer's original postulation. The brief account of this subsequent development given below will serve merely to indicate the important features of layer growth and the role of crystal imperfections in the growth process. Volmer's theory, or as some prefer to call it, the...

Brodie purifier

The Brodie purifier Brodie, 1971 is a countercurrent melt purification system with the added feature of an imposed temperature gradient between the residue and product outlets. The unit thereby acts as a countercurrent multistage fractionator with partial melting and recrystallization occurring along its length. The essential features are shown in Figure 8.20. The heat exchangers, provided with slow-moving scraper-conveyors, are arranged into refining and recovery sections separated by the feed...

81 Precipitation

Precipitation is a widely used industrial process. It is also a very popular laboratory technique, especially in analytical chemistry, and the literature on this aspect of the subject is voluminous see Kolthoff et al., 1969 . Precipitation plays an important role not only in chemistry but also in metallurgy, geology, physiology, and other sciences. In the industrial field, the manufacture of photographic chemicals, pharmaceuticals, paints and pigments, polymers and plastics utilizes the...

Trough crystallizers

Swenson Walker Crystallizer Construction

The first truly continuous crystallizer to be introduced to the chemical industry, between 1905 and 1910, was the Wulff-Bock unit Figure 8.36 , frequently referred to as the crystallizing cradle or rocking crystallizer. It consists of a long shallow trough, about 1.2 m wide, rocked on supporting rollers. The solution to be crystallized is fed in at one end and the crystals are discharged at the other end, continuously. Transverse baffles may be fitted inside the trough to prevent longitudinal...

Oslo Krystal evaporating crystalllzer

The principles of the Oslo-Krystal process, already referred to above in connection with cooling crystallizers, can also be applied to evaporative crystallization. Three forms of the Oslo-Krystal evaporating crystallizer are shown in Figure 8.45. The construction of these crystallizers, commonly used in multiple-effect systems, is of the 'closed' form, i.e. the vaporizer is directly connected with the crystallizer body to form a sealed unit. There are two basic types, the waisted a and the...

Adiabatic evaporative cooling

An early report of adiabatic evaporative cooling Takegami, 1993 considered the purification of caprolactam melting point 690 C in the presence of water Figure 8.26. Apparatus for melt DCC crystallization, using air as a coolant. A Reservoir B Data acquisition system C Thermostatic baths and circulators D Particle analysis sensor E Dilution tank F Temperature recorder G Crystallizer H Flowmeter I Refrigerator J Temperature programming controller K Heat exchanger. After Kim and Mersmann, 1997...

Fines destruction

Even though significant improvements in product CSD can be made by employing controlled operation, the benefits are often diminished because fine crystals are created by secondary nucleation Mullin and Nyvlt, 1971 Jones and Mullin, 1974 . In an attempt to combat this problem, the effects of fines destruction FD , commonly employed to improve product CSD in continuous crystallizers section 9.1.2 was investigated by Jones, Chianese and Mullin 1984 . Runs were made with potassium sulphate in a 30...

473 Janecke diagrams

In order to simplify the interpretation of the phase equilibria in reciprocal salt pair systems, the water content may be excluded. The curves of the space model can then be projected on to the square base to give a two-dimensional graph, called a Janecke diagram as described in section 4.7.1. A typical projection is shown in Figure 4.31a the lettering is that used in Figure 4.30. The enclosed areas, which represent saturation surfaces, indicate solutions in equilibrium with one salt, the...

Factors Affecting Caking Of Crystals

Always remains lower than the partial pressure of the water vapour in the atmosphere. Calcium chloride, with a critical humidity of 32 per cent at 150C, is a well-known example of a deliquescent salt. The term 'efflorescence' refers to the loss of water of crystallization from a salt hydrate this occurs when the vapour pressure exerted by the hydrate exceeds the partial pressure of water vapour in the atmosphere. Sodium sulphate decahydrate, with a critical humidity of 93 per cent, is an...

14 Crystal systems

There are only 32 possible combinations of the above-mentioned elements of symmetry, including the asymmetric state no elements of symmetry , and these are called the 32 point groups or classes. All but one or two of these classes have been observed in crystalline bodies. For convenience these 32 classes are grouped into seven systems, which are known by the following names regular 5 possible classes , tetragonal 1 , orthorhombic 3 , monoclinic 3 , triclinic 2 , trigonal 5 and hexagonal 1 . The...

15 Miller indices

All the faces of a crystal can be described and numbered in terms of their axial intercepts. The axes referred to here are the crystallographic axes usually three which are chosen to fit the symmetry one or more of these axes may be axes of symmetry or parallel to them, but three convenient crystal edges can be used if desired. It is best if the three axes are mutually perpendicular, but this cannot always be arranged. On the other hand, crystals of the hexagonal system are often allotted four...

13 Crystal symmetry

Truncated Cube With Development Surfaces

Many of the geometric shapes that appear in the crystalline state are readily recognized as being to some degree symmetrical, and this fact can be used as a means of crystal classification. The three simple elements of symmetry which can be considered are 1. Symmetry about a point a centre of symmetry 2. Symmetry about a line an axis of symmetry 3. Symmetry about a plane a plane of symmetry It must be remembered, however, that while some crystals may possess a centre and several different axes...

113 Imperfections in crystals

Indeed, in many cases they are not required to be, since lattice imperfections and other defects can confer some important chemical and mechanical properties on crystalline materials. Surface defects can also greatly influence the process of crystal growth. There are three main types of lattice imperfection point zero-dimensional, line one-dimensional and surface two-dimensional . The common point defects are indicated in Figure 1.25. Vacancies are lattice sites...

12 Crystalline solids

Interfacial Angle

The true solid crystal comprises a rigid lattice of molecules, atoms or ions, the locations of which are characteristic of the substance. The regularity of the internal structure of this solid body results in the crystal having a characteristic shape smooth surfaces or faces develop as a crystal grows, and the planes of these faces are parallel to atomic planes in the lattice. Very rarely, however, do any two crystals of a given substance look identical in fact, any two given crystals often...