Processing crystallisation

A clear distinction between crystallisation and precipitation is not always possible from a practical point of view [57] hence, it is more convenient to consider precipitation as a very fast crystallisation process. Crystallisation is a result of the combined effects of nucleation, nucleus growth and secondary processes inside the suspension such as agglomeration, ageing and recrystallisation. Depending on the reaction conditions, the above processes can occur together or sequentially during the crystallisation period. 

The result of any crystallisation process is a crystalline matter with a certain CSD, certain crystal habit and purity. These properties very often are quality requirements and well defined by market demands. Certainly, also the crystallisation process itself requires a minimum CSD, as the resulting suspension still has to be separated. This separation can be effected the more perfectly (purity) as more compact (crystal habit) and coarser (CSD) the crystals are. Quality and economy of the separation process crystallisation are therefore strongly dependent from crystal size, CSD and crystal habit. These properties also take influence to the storage ability, the dust-freeness and also the bulk density of the product. 

Students are familiar with the general process of recrystallisa-tion from their more elementary inorganic work. Friefly, it consists in first finding a solvent which will dissolve the crude material readily when hot, but only to a small extent when cold. The crude substance is then dissolved in a minimum of the boiling solvent, the solution filtered if necessary to remove any insoluble impurities, and then cooled, when the solute will crystallise out, leaving the greater part of the impurities in solution. The crop of crystals is then filtered off, and the process repeated until the crystals are pure, and all impurities remain in the mother-liquor. 

The separation of the solid phase does not occur readily with some liquid mixtures and supercooling is observed. Instead of an arrest in the cooling curve at /, the cooling continues along a continuation of c/ and then rises suddenly to meet the line f g which it subsequently follows (Fig. 1,13, 1, iii). The correct freezing point may be obtained by extrapolation of the two parts of the curve (as shown by the dotted line). To avoid supercooling, a few small crystals of the substance which should separate may be added (the process is called seeding ) these act as nuclei for crystallisation. 

The purification of solids by crystallisation is based upon differences in their solubility in a given solvent or mixture of solvents. In its simplest form, the crystallisation process consists of (i) dissolving the impure substance in some suitable solvent at or near the boiling point,... 

The high sodium ion concentration results in facile crystallisation of the sodium salt. This process of salting out with common salt may be used for recrystallisation, but sodium benzenesulphonate (and salts of other acids of comparable molecular weight) is so very soluble in water that the solution must be almost saturated with sodium chloride and consequently the product is likely to be contaminated with it. In such a case a pure product may be obtained by crystallisation from, or Soxhlet extraction with, absolute alcohol the sul-phonate is slightly soluble but the inorganic salts are almost insoluble. Very small amounts of sulphones are formed as by-products, but since these are insoluble in water, they separate when the reaction mixture is poured into water ... 

The Phillips process is a two-stage crystallisation process that uses a pulsed column in the second stage to purify the crystals (79,80). In the pulsed column, countercurrent contact of the high purity PX Hquid with cold crystals results in displacement of impurities. In the first stage, a rotary filter is used. In both stages, scraped surface chillers are used. This process was commercialized in 1957, but no plants in operation as of 1996 use this technology. 

Hydrothermal crystallisation processes occur widely in nature and are responsible for the formation of many crystalline minerals. The most widely used commercial appHcation of hydrothermal crystallization is for the production of synthetic quartz (see Silica, synthetic quartz crystals). Piezoelectric quartz crystals weighing up to several pounds can be produced for use in electronic equipment. Hydrothermal crystallization takes place in near- or supercritical water solutions (see Supercritical fluids). Near and above the critical point of water, the viscosity (300-1400 mPa s(=cP) at 374°C) decreases significantly, allowing for relatively rapid diffusion and growth processes to occur. 

Ak2o has been iastmmental ia developiag a new process for the stereospecific synthesis of 1,4-cyclohexane diisocyanate [7517-76-2] (21). This process, based on the conversion of poly(ethylene terephthalate) [25038-59-9] circumvents the elaborate fractional crystallisation procedures required for the existing -phenylenediamine [108-45-2] approaches. The synthesis starts with poly(ethylene terephthalate) (PET) (32) or phthaUc acid, which is converted to the dimethyl ester and hydrogenated to yield the cyclohexane-based diester (33). Subsequent reaction of the ester with ammonia provides the desired bisamide (34). The synthesis of the amide is the key... 

Re OPe . The final step in the chemical processing of rare earths depends on the intended use of the product. Rare-earth chlorides, usually electrolytically reduced to the metallic form for use in metallurgy, are obtained by crystallisation of aqueous chloride solutions. Rare-earth fluorides, used for electrolytic or metaHothermic reduction, are obtained by precipitation with hydrofluoric acid. Rare-earth oxides are obtained by firing hydroxides, carbonates or oxalates, first precipitated from the aqueous solution, at 900°C. 

The preferred route to higher purity naphthalene, either coal-tar or petroleum, is crystallisation. This process has demonstrated significant energy cost savings and yield improvements. There are several commercial processes available Sulser-MWB, Brodie type. Bets, and Recochem (37). 

NaOH solution is added dropwise to an aqueous suspension of this ester at 40—70°C over 1 h and the reaction mixture kept for 2 h to give 86.6% DHNA of 98.7% purity (74), which is then esterified with (CgH O) to obtain PDNA. The esterification process is dramatically improved by adding a small amount of inorganic or organic acid, preferably methanesulfonic acid, benzene sulfonic acid, or naphthalene sulfonic acid subsequent isolation and crystallisation gives a pure product (75). 

The stage of rapid crystallisation ends when crystallinity reaches approximately 0.5—0.7, ie, when HDPE is 50—70% crystallised. After that, the crystallisation rate falls drastically. The slower process that follows is known as secondary crystallisation (20), which can be described by the empirical equation CR = A - - S-log(t). Secondary crystallisation can be accelerated if HDPE is heated. 

Polymorphism. Many crystalline polyolefins, particularly polymers of a-olefins with linear alkyl groups, can exist in several polymorphic modifications. The type of polymorph depends on crystallisa tion conditions. Isotactic PB can exist in five crystal forms form I (twinned hexagonal), form II (tetragonal), form III (orthorhombic), form P (untwinned hexagonal), and form IP (37—39). The crystal stmctures and thermal parameters of the first three forms are given in Table 3. Form II is formed when a PB resin crystallises from the melt. Over time, it is spontaneously transformed into the thermodynamically stable form I at room temperature, the transition takes about one week to complete. Forms P, IP, and III of PB are rare they can be formed when the polymer crystallises from solution at low temperature or under pressure (38). Syndiotactic PB exists in two crystalline forms, I and II (35). Form I comes into shape during crystallisation from the melt (very slow process) and form II is produced by stretching form-1 crystalline specimens (35). 

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