Crystals caking

It may become necessary to work up solid products with the same solvent as used for crystallization. Cakes should not be allowed to dry out before washing, as air or inert gases result in evaporation of the solvent, and this may re.sult in the deposition of impurities, present in the dis.solved state, on the crystals. Such impurities when deposited are difficult to wash. In. some ca.ses washing of wet cake is done by replacing the solvent with another one, typically having a lower boiling point, so that in the final material loss of drying is within limits and the new solvent is more acceptable than the earlier one. 

For the stoichiometric liquid-liquid large-scale synthesis a flat steel pan (31x44 cm ) was charged with benzaldehyde (77k) (99.5% 848 g, 7.95 mol) and aniline (236s) (99.5% 744 g, 7.95 mol) (Scheme 51). The liquids were mixed at 18 °C. The temperature rose to a maximum of 32 °C and fell back to 24 °C when crystallization started with another increase in temperature to a maximum of 35 °C within 12 min when crystallization was virtually complete and water of reaction separated. Next day, the wet crystal cake was crunched with an ordinary household grain mill and dried in a vacuum at room temperature to give 1.438 kg (100%) of pure benzylidene-aniline (54s). 

Imperial Chemical Industries Ltd, BelgF 636956(1964) CA 62, 7580-81 (1965) [A Blasting Explosive was prepd by mixing for 15 mins 88 parts granular AN 12 ps of a 50% aq AN soln in a rotary blender equipped with paddles. The resulting mass was spread on sheets and dried at 45° for 14 hrs. The crystal cake was crushed, forced to pass a sieve (BS No 6) and then mixed with 9 parts of fuel oil for each lOOps of sieved material]... 

The cooled digest should be liquid, or liquid with a few small crystals. Caking before the addition of water indicates too little residual acid at the end of the digestion period and may result in low nitrogen values. 

The crude tartrate double salt is dissolved in 25 ml. of boiling water, and a solution of 5.4 g. of LiCl in 9 ml. of boiling water is added. The solution is filtered. The filtrate is heated to boiling and allowed to stand for 5 minutes. Thirty-five milliliters of absolute ethanol is added slowly with vigorous stirring, and the solution is allowed to cool by standing for 3 hours at room temperature. The crystal cake is broken up, and the mixture is cooled in ice. The precipitate is filtered, washed... 

A large excess of CINO3 is condensed at liquid nitrogen temperature onto the surface of 2-3 ml. of TiCl, frozen In a cold trap at high vacuum. The trap is connected to a surge vessel and the temperature is raised to —80°C. The reaction, which is accompanied by evolution of chlorine, is complete after a few hours. The volatile components (Clg and excess CINO3) are then distilled off in vacuum at room temperature. The Ti(N03)4 residue, in the form of a crystal cake, may be further purified by sublimation at 50°C. 

On the other hand, when the large amount of the mother liquid of di-L-phenylalanine sulfate recycled into the filtered fermentation broth was increased to increase the recovery rate of L-phenylalanine. When these thin plate form crystals appeared, the crystal cakes were hardly separated from the slurry by centrifugation. 

Di-L-phenylalanine sulfate monohydrate both in the needle form and in the thin plate form These two crystal cakes obtained from the industrial plant, separately, were washed with water and dried in a vacuum desiccatorwith sulfuric acid. 

This result indicates that colored substances and factors which were adsorbed onto activated carbon did not affect the change into thin plate form crystals. The figures in parentheses in Table 5 show the percentages of the mother liquid content included in crystal cakes. When Mc(-) is the mother liquid content in crystal cakes and Ms (ml/g) is... 

The frequency of the appearance of thin plate form crystals was dependent almost entirely on the mother liquid content in crystal cakes and was independent of the degree of removal of colored substances which were adsorbed onto the activated carbon. 

Thin plate form crystals belonged to the same crystal form as pillar form crystals and was obtained by the suppressed growth in the direction of the a-axis. This crystal form usually appeared under conditions of (a) pH less than 1 and (b) inclusion of the mother liquid at more than 40% in crystal cakes. 

Hakkinen et al. [51] investigated the filterability of sul-fathiazole crystals obtained by crystallization from one-solvent and two-solvent solutions. Their results showed that the choice of the mother-Uquor composition affected the obtained resistance of crystal cake. Lowering the cake resistance in a system usually decreases the moisture content of the cake that in turn facilitates the drying process. Organic compounds may also form amorphous solid particles that usually lead to low permeability of the solid bed due to the compression of the filter cake. 

In cases where crystal caking (section 9.6) is expected, a tight moisture content specification (surface moisture only) may be imposed on the final product. This will have a major influence on the choice of a dryer, which in turn will also have been influenced by the expected filter cake moisture content... 

Many physical characteristics, such as crystallization, caking, stickiness, collapse, molecular mobility, and difFusivity cannot be fully explained with the water activity concept corresponding relaxation times must also be taken into consideration. 

The crystal cake gradually increases in thickness with a subsequent increase in the pressure drop across the filter. When the pressure drop reaches a predetermined level, the cake is pulsed off the filter by backflushing (flow reversal) and collected in the bottom of the precipitate tank. 

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