Caking of crystals

for example, the case of a water-soluble substance. If the partial pressure of water vapour in the atmosphere is greater than the vapour pressure that would be exerted by a saturated aqueous solution of the pure substance at that temperature, water will be absorbed by the crystals. If, later, the atmospheric moisture content falls to give a partial pressure below the vapour pressure of the saturated solution, the crystals will dry out and bind together. Small fluctuations in atmospheric temperature and humidity, sufficient to bring about these changes, can occur several times in one day. 

Substance Formula Stable phase at 15 °C Percentage relative humidity 

Commercial crystalline salts frequently exhibit hygroscopy at atmospheric humidities lower than those given for the pure salts in Table 93. Usually impurities present in the product cause the trouble. For example, traces of calcium chloride in sodium chloride would render the crystals damp at very low atmospheric humidities. Removal of the hygroscopic impurity would be the answer, but this is not always economical. Coating of the crystals with a fine inert dust will often prevent the mass becoming damp table salt, for instanee, can be coated with magnesium carbonate or calcium aluminium silicate. 

Shape and uniformity of the particles also affect the behaviour of the product under storage. If the crystals are packed in bags and stacked on top of one another, the pressure in the bags near the bottom of the pile tends to force the crystals into closer contact with non-xmiform crystals this compaction may be quite severe and, in extreme cases, many of the crystals may be crushed. If the solubility of the salt in water increases with an increase in pressure, traces of solution may be formed under the high local pressure at the points of contact. The solution will then tend to flow into the voids, where the pressure is lower, and crystallize. Storage of crystalline materials xmder pressure should always be avoided if possible. 

Controlled crystallization, coupled with some form of classifying action in the crystallizer, helps to produce crystals of uniform size. The production of granular crystals, however, may demand the careful control of other conditions of crystallization to modify the crystal habit the rate of cooling, the degree of supersaturation and the pH of the crystallizing solution can exert considerable influence. The deliberate addition of traces of impurity, in the form of active ions or surface-active agents, may also help to produce the right type of crystal. 

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At this point, you may have a cake of crystals in your Buchner funnel. The easiest way to handle this is to carefully lift the cake of crystals out of the funnel along with the filter paper, plop the whole thing onto a larger piece of filter paper, and let the whole thing dry overnight. If you are pressed for time, scrape the damp filter cake from the filter paper, but... 

In the later stages of the addition, a cake of crystals forms preventing adequate stirring. This difficulty is overcome by temporarily interrupting the addition and swirling the flask manually—stirring then continues normally. 

Stir well with a thermometer and regulate the addition of nitric acid so that the temperature does not rise above 5°. Remove the beaker from the ice-salt mixture, cover with a watch glass, and allow to stand for 30 minutes. Pour into a mixture of 10 ml water and 10 g of ice. Return the diluted material to the 250-ml beaker in which the reaction took place. Stir well, cool, and filter. Wash with water. Transfer the cake of crystals into a 250-ml beaker and suspend in 30 ml of water. Add gradually 5 g of sodium carbonate, with stirring. The reaction of the suspension should be distinctly alkaline. Heat almost to boiling, then cool to 40-50° and filter, washing with water twice. The cake of crystals is ready for hydrolysis. 

It was found that even if the crystals were compacted and the liquid fraction was considerably reduced, the paths of liquid removal were maintained as pinholes through the inside of the cake of crystals, as long as the liquid remained impure. These pinholes vanish at the end of separation. 

Figure 9.28. Effect of particle shape on the caking of crystals a) large uniform granular crystals good) b) non-uniform granular crystals poor) (c) large uniform elongated crystals poor) d) non-uniform elongated crystals very bad)...

The soiution is aliowed to cool and the crystals of the P2P-bisulfite addition compound are then separated by vacuum filtration, washed with a little clean dH20 then washed with a couple hundred mLs of ether, DCM or benzene. The filter cake of MD-P2P-bisulfate is processed by scraping the crystals into a flask and then 300mL of either 20% sodium carbonate solution or 10% HCi soiution are added (HCI works best). The soiution is stirred for another 30 minutes during which time the MD-P2P-bisulfite complex will be busted up and the P2P will return to its happy oil form. The P2P is then taken up with ether, dried and removed of the solvent to give pure MD-P2P. Whaddya think of that ... 

In a flask the chemist mixes 50g piperonal into 200mL glacial acetic acid, then adds 45mL nitroethane and 17g ammonium acetate. The solution is then refluxed 4 hours and takes on the color of yellow to yellow-orange. After 4 hours and cooling, yellowish crystals of p-nitropropene will spontaneously form. If not, the solution can be diluted with 50ml of dHjO and chilled in an ice bath for an hour to form the crystals with some slushy glacial acetic acid and water intermixed. The mass of crystals is broken up and plopped into a Buchner funnel to be vacuum filtered. The filter cake is washed with a little extra acetic acid or water. All of the filtrate is saved. 

Most of the final product producing recipes in this book will provide for the chemist to take up the final free base product in DCM. Usually the freebase oil in the DCM is dark. Used to be that Someone-Who-ls-Not-Strike (SWINS) would have to distill the freebase to get clear yellow oil before crystallizing because when SWINS used ether or ethanol as a crystallization solvent, the colored crap would contaminate the final product. But not with DCM. Even with the grungiest (well...not too grungy) freebase, the crystals that come out are pure snow. The DCM so strongly solvates the contaminants that none remain in the mass of crystalled final product. The filter cake is sooooo clean even in the darkest solvent ... 

The way the chemist knows that she has methylamine and not ammonium chloride is that she compares the look of the two types of crystals. Ammonium chloride crystals that come from this reaction are white, tiny and fuzzy. The methylamine hydrochloride crystals are longer, more crystalline in nature and are a lot more sparkly. The chemist leaves the methylamine crystals in the Buchner funnel of the vacuum filtration apparatus and returns the filtrate to the distillation set up so it can be reduced one last time to afford a second crop. The combined methylamine hydrochloride filter cake is washed with a little chloroform, scraped into a beaker of hot ethanol and chilled. The methylamine hydrochloride that recrystallizes in the cold ethanol is vacuum filtered to afford clean, happy product (yield=50%). 

Dicalcium hexakiscyanoferrate [13821 -08 ] Ca2[Fe(CN)g], is formed as yellow crystals by reaction of Hquid or gaseous HCN with iron(II) chloride in water containing Ca(OH)2 or CaCO and having pH > 8. It is used to prevent caking of other substance and serves as a useful starting material in the preparation of other [Fe(CN)g] salts. Examples of mixed salts include calcium dicesium hexakiscyanoferrate [15415-35-7] CaCs2[Fe(CN)g], and calcium dipotassium hexakiscyanoferrate [20219-00-5] CaK2[Fe(CN)g]. 

Sepa.ra.tlon, Sodium carbonate (soda ash) is recovered from a brine by first contacting the brine with carbon dioxide to form sodium bicarbonate. Sodium bicarbonate has a lower solubiUty than sodium carbonate, and it can be readily crystallized. The primary function of crystallization in this process is separation a high percentage of sodium bicarbonate is soHdified in a form that makes subsequent separation of the crystals from the mother hquor economical. With the available pressure drop across filters that separate Hquid and soHd, the capacity of the process is determined by the rate at which hquor flows through the filter cake. That rate is set by the crystal size distribution produced in the crystallizer. 

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