Ciystals from solution

Ciystallization from solution is an important separation and purification process in a wide variety of industries. These range from basic materials such as sucrose, sodium chloride and fertilizer chemicals to pharmaceuticals, catalysts and specialty chemicals. The major purpose of crystallization processes is the production of a pure product. In practice however, a number of additional product specifications are often made. They may include such properties as the ciystd size distribution (or average size), bulk density, filterability, slurry viscosity, and dry solids flow properties. These properties depend on the crystal size distribution and crystal shape. The goal of crystallization research therefore, is to develop theories and techniques to allow control of purity, size distribution and shape of crystals. 

DOUBLE SALT. A hydrated compound resulting from crystallization ol a mixture of ions in aqueous solution. Common examples arc the alums, made by ciystallizing from solution either potassium or ammonium sulfate and aluminum sulfate Rochelle salt (potassium sodium tartrate), made from a water solution of potassium acid tartrate treated with sodium carbonate and Mohr s salt (fertous ammonium sulfate), crystallized Iruin mixed solutions of ferrous sulfate and ammonium sulfate. 

This reaction is effected by heating a suspension of beta-naphthol in water vitli equivalent quantities of formaldehyde and sodium bisulfite. The sulfo acid is reported to ciystallize from solution on cooling after acidifica tion with acetic acid. The formation of this sulfonate of a naphthol analogue of saligenin is of particular interest, since monomethylolnaphthols haw never been isolated. 

CiystaUization may be carried out from a vapor, from a melt, or from a solution. Most of the industrial applications of the operation involve crystalhzation from solutions. Nevertheless, crystal sohdifica-tion of metals is basically a crystalhzation process, and much theoiy has been developed in relation to metal ciystallization. This topic is so specialized, however, that it is outside the scope of this subsection, which is hmited to ciystaUization from solution. 

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. 

Hydroxy-N-Methylisomorphinan. 24 mg of the 3-hydroxy-delta-6-dehydro-N-methyl-morphan is hydrogenated over 5 mg of Adams catalyst in 15 cc of dcohol. H2 uptake is complete after 35 min. The solution is filtered, concentrated, and the residue is ciystallized from benzene to yield 24 mg of colorless fine prisms. 

The precipitated precursor can be dissolved and re-ciystallized from fluorine-free solutions. This provides excellent conditions for deep purification of the material and reduction of problematic impurities such as titanium, fluorine, etc. Peroxometalates decompose at relatively low temperatures forming tantalum or niobium oxides containing small amount of absorbed water. The absorbed water separation is achieved by further thermal treatment - drying and calcination - of the product ... 

Test for the Presence of Blood. Gunning has discovered in acetate of zinc a rea ut that precipitates the slightest traces of tee coloring matter of blood from solutions, even where the liquids are so dilate as to be colorless. Blood washed from the hands in a pail of water can readily be detected in this way. The flocculent precipitate thrown down by acetate of zinc must be washed by decantation, and Anally collected on a watch-glass, and allowed to dry, when the microscope will readily reveal ciystals if any blood bo present. (See Ab. 6415.)... 

Selid salution. Mixed ciystals formed when isomotphous substances crystallize together out of a solution. Supersaturation. The departure from solution saturation usually caused by cooling of the traxture and/ or by evaporating solvent,... 

Simultaneously the ciystallization of C-S-H from the solution occurs. This mechanism belongs to the theory of ciystallization through solution . 

Regarding the ciystallization devices, crystallization from solution and ciystalhza-tion from melts can be distinguished. The process principles of crystallization from melts can again be divided into two groups ... 

The simplest polymer ciystal conformation is formed from linear poly(methylene), - [CHj-] . While more ordered crystals can be made by evaporation of or precipitation from solution, the commonly encountered polymeric solid is formed from the melt and it is crystallization in these materials that will be considered here. 

Benzyllithium ciystallizes from hexaneAolucr.c solution in the presence of l,4-diazabicyclo[2.2.2]oc-tane (DABCO) in infinite polymeric chains. Insoection of the in vidual monomeric units of this structure reveals a unique interaction of the lithium atoms in an q -manner with the benzylic carbanion. This bonding is based upon the three relatively short Li—C contacts as indicated in structure (53). The two protons on the benzylic carbon center were located crystallographically one of these lies in the plane of the aromatic ring and the other is significantly out of this plane. A similar q -Li—CCC interaction is observed in the diethyl ether solvate of triphenylmethyllithium (54). This latter structure is depicted as (55). 

Crystallization kinetics. Expressions that describe crystal growth and nucleation rates from solution. CSD. Ciystal size distribution. 

Nyvit, Industrial Crystallization from Solutions, Buttetwoiths, London, 1971. Nyvh discusses the theoretical foundatioas of ciyslallizalioa and presents ciystallizer design methods. 

A Figure 13.10 Sodium acetate readiiy forms supersaturated solutions in water, (a) When a seed crystal of NaC2H302 is added, excess NaC2H302 ciystallizes from the solution, as shown in (b) and (c). 

Crystal Formation There are obviously two steps involved in the preparation of ciystal matter from a solution. The ciystals must first Form and then grow. The formation of a new sohd phase either on an inert particle in the solution or in the solution itself is called nucle-ation. The increase in size of this nucleus with a layer-by-layer addition of solute is called growth. Both nucleation and ciystal growth have supersaturation as a common driving force. Unless a solution is supersaturated, ciystals can neither form nor grow. Supersaturation refers to the quantity of solute present in solution compared with the quantity which would be present if the solution were kept for a veiy long period of time with solid phase in contac t with the solution. The latter value is the equilibrium solubility at the temperature and pressure under consideration. The supersaturation coefficient can be expressed... 

This type of equipment can also be used for apphcations in which the only heat removed is that required for adiabatic cooling of the incoming feed solution. When this is done and the fines-destriiction feature is to be employed, a stream of hqiiid must be withdrawn from the settling zone of the ciystaUizer and the fine ciystals must be separated or destroyed by some means other than heat addition—for example, either dUiition or thickening and plwsical separation. 

In some crystalhzation apphcations it is desirable to increase the solids content of the shiny within the body above the natural consis-tencw, which is that developed by equilibrium cooling of the incoming feed solution to the final temperature. This can be done by withdrawing a stream of mother liquor from the baffle zone, thereby thickening the shiny within the growing zone of the crystallizer. This mother liquor is also available for removal of fine ciystals for size control of the product. 

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... 

CiystaUization is the preferred method of forming many final prod-uc ts because veiy high purification is possible. High purity antibiotic ciystals can be produced from colored, rather impure solutions if the filter cake is uniform and amenable to good washingto remove the mother hquor. When a sterile pharmaceutical produc t is desired, ciystals are formed from liquid streams that have been sterihzed by filtration. 

Ammonium nitrate is made at Fisons Ltd and stored in solution in two heated tanks to prevent ciystalization. One tank holds 3,100 tonnes and the other 6,200 tonnes of 92% aqueous ammonium nitrate solution. Trains laden with oil refinery products from the Mobil and Shell refineries pass the factory on a near embankment. A derailment could spill and ignite hydrocarbons from a rail tank car to explode an ammonium nitrate storage tank. Suggestions were made to mitigate or prevent such a domino effect... 

Reaction.—Make a solution of 4 grams stannous chloride in TO c.c. cone, hydrochloric acid, add 2 grams aminoazobenzene, and boil for a few minutes. On cooling ciystals of the hydrochlorides of aniline and yi-phenylenediamine separate out. The liquid is filtered and washed with a little cone, hydrochloric acid to remove the tin salts. If the precipitate is dissolved in water and made alkaline with caustic soda, a mixture of liquid aniline and solid/-phenylenediamine is precipitated, from which the former may be removed by filtering, washing, and draining on a porous plate. 

In systems where the liquid phase interaction between the solute and solvent is close to ideal, then Eq. 2 can be used successfully on it s own to fit and extrapolate solubility data with respect to temperature. The technique is valuable in an industrial setting, where time pressures are always present. Solubility data points are often available without any additional effort, from initial work on the process chemistiy. The relative volume of solvent that is required to dissolve a solute at the highest process temperature in the ciystallization is often known, together with the low temperature solubility by analysis of the filtrates. If these data points fit reasonably well to the ideal solubility equation then it can be used to extrapolate the data and predict the available crystallization yield and productivity. This quickly identifies if the process will be acceptable for long term manufacture, and if further solvent selection is necessary. 

---