Vacuum-Cooling Crystallisation

Ammonia, hydrochloric acid, and sodium perchlorate are mixed and the reaction mixture crystallised in a vacuum-cooled crystalliser. Ammonium perchlorate crystals are centrifuged, reslurried, recentrifuged, and then dried and blended for shipment. Mother Hquor is evaporated to precipitate sodium chloride and the depleted mother Hquor is recycled to the reactor. The AP product made by this method is 99% pure and meets the specifications for propeUant-grade ammonium perchlorate. The impurities are ammonium chloride, sodium perchlorate, ammonium chlorate, and water insolubles. 

The cmde oxide is pressure-leached in a steam-heated autoclave using water or circulating mother hquor. The arsenic trioxide dissolves, leaving behind a residue containing a high concentration of heavy metal impurities and sihca. The solution is vacuum-cooled and the crystallisation is controUed so that a coarse oxide is obtained which is removed by centrifuging. The mother hquor is recycled. The oxide (at least 99% purity) is dried and packaged in a closed system. 

The temperature-solubility relationship for solute and solvent is of prime importance in the selection of a crystalliser and, for solutions that yield appreciable amounts of crystals on cooling, either a simple cooling or a vacuum cooling unit is appropriate. An evaporating crystalliser would be used for solutions that change little in composition on cooling and... 

Solution crystallisation (adiabatic evaporation (vacuum cooling))... 

Converted to the semicarbazone (m 149 ) by adding 50g of semicarbazide hydrochloride and 44g of potassium acetate in 150mL of water to a soln of 85g of 6-ionone in EtOH. (More EtOH was added to redissolve any 6-ionone that ppted.) The semicarbazone crystallised on cooling in an ice-bath and was recrystallised from EtOH or 75% MeOH to constant m (148-149 ). The semicarbazone (5g) was shaken at room temperature for several days with 20mL of pet ether and 48mL of M H2SO4, then the ether layer was washed with water and dilute aqueous NaHCOs, dried and the solvent was evaporated. The B-ionone was distilled under vacuum. (The customary steam distillation of 6-ionone semicarbazone did not increase the purity.) [Young et al. J Am Chem Soc 66 855 1944]. 

Diphenylhydrazine [530-50-7] M 184.2, m 34.5 , 44 , b 120 /l.lmm, 220 /40-50mm, PKesi 9.1. Purify it via the hydrochloride [530-47-2], which has m 165-170°(dec) after crystallization from aqueous EtOH (h- a few drops of HCl) and recover the free base with aqueous NaOH, extract it into Et20, dry it (KOH), filter, evaporate and distil the residue under a vacuum. The distillate crystallises on cooling. The benzoyl derivative has m 194° (from Me2CO), and the 4-nitrophenyl hydrazone (with the aldehyde) has m 131-132°. [Koga Anselme J Org Chem 33 3963 1968, Beilstein 15 IV 55.]... 

It also crystallises from toluene/pet ether in the cold, from pet ether (b 40-60°) or benzene. Its solution in diethyl ether has been washed with aqueous 10% NaOH until colourless, then dried (Na2S04) and crystallised by cooling in powdered Dry-ice. It has also been purified by dissolving in benzene, washing with aqueous 5% NaOH, then dried with K2CO3 or MgS04, and distilled under reduced pressure and can be sublimed at high vacuum [Ebel Chem Ber 60 20867927]. [Beilstein 11 IV 375.]... 

Benzidine (4,4 -diaminobiphcnyl) [92-87-5] M 184.2, m 128-129°, pK 3.85, pK2 4.95. Its solution in benzene is decolorised by percolating through two 2-cm columns of activated alumina, then concentrated until benzidine crystallises on cooling. Reciystallise alternately from EtOH and benzene to constant absorption spectmm [Carlin et al. J Am Chem Soc 73 1002 7957]. It has also been crystallised from hot water (charcoal) and from diethyl ether. Dry it under vacuum in an Abderhalden pistol. Store it in the dark in a stoppered container. CARCINOGENIC. [Beilstein 13IV 364.]... 

Potassium chloride [7447-40-7] M 74.6, m 771°, d 4 1.98. Dissolve it in conductivity water, filter it, and saturate it with chlorine (generated from cone HCl and KMn04). Excess chlorine is boiled off, and the KCl is precipitated by HCl (generated by dropping cone HCl into cone H2SO4). The precipitate is washed with water, dissolved in conductivity water at 90-95 , and crystallised by cooling to about -5 . The crystals are drained at the centrifuge, dried in a vacuum desiccator at room temperature, then fused in a platinum dish under N2, cooled... 

A typical design feature is the way supersaturation is controlled and kept within the metastable range. The method is independently from the crystallisation process. Figure 9.6 explains this method based on the vacuum cooling crystallisation principle. This principle is also valid for the vacuum evaporation, surface cooling and reaction crystallisation. 

Vacuum-cooling crystallisation is the preferred cooling crystallisation method under continuous operation. Because cooling is generated by adiabatic expansion of the solvent no cooling surfaces can be incrusted. Vacuum-cooling becomes uneconomical only if cooling has to be effected at very lotv temperatures. 

The hydrochloride of the amine may be prepared precisely as that of the primary amine. For recrystallisation, boil a suspension of the powdered salt in petroleum (b.p. 60-80°), and then add acetone slowly in small drops until the boiling suspension just becomes clear allow the stirred solution to cool until crystallisation starts, and then chill in ice-water before collecting the colourless plates of the hydrochloride, which after drying in a vacuum desiccator have m.p. 132-134°. 

Dissolve 8 8 g. (9 0 ml.) of cyclohexanone in 50 ml. of glacial acetic acid, add 8 ml. of phenylhydrazine, and boil the solution under reflux for 5 minutes. Cool the solution, when the tetrahydrocarbazole will crystallise out. Filter at the pump, drain well, and recrystallise either from aqueous ethanol or (better) from aqueous acetic acid. The recrystallisation should be performed rapidly, for the tetrahydrocarbazole undergoes atmO" spheric oxidation in hot solutions after recrystallisation, the compound should be dried in a vacuum desiccator and not in an oven. Repeated recrystallisation should be avoided. The tetrahydrocarbazole, after thorough drying, is obtained as colourless crystals, m.p. 118° yield of recrystallised material, 11 g. 

Prepare acetophenonephenylhydrazone by warming a mixture of 20 g. of acetophenone (Section IV, 136) and 18 g. of phenylhydrazine on a water bath for 1 hour. Dissolve the hot mixture in 40 ml. of rectihed spirit, and shake or stir to induce crystallisation. Cool the mixture in ice, filter and wash with 12 ml. of rectified spirit. Dry in a vacuum desiccator over anhydrous calcium chloride for at least half an hour. The yield of phenylhydrazone, m.p. 105-106 , is 28 g. 

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