Precipitation from solution, filter media

This operation is the separation of the precipitate from the mother liquor, the object being to get the precipitate and the filtering medium quantitatively free from the solution. The media employed for filtration are (1) filter paper (2) porous fritted plates of resistance glass, e.g. Pyrex (sintered-glass filtering crucibles), of silica (Vitreosil filtering crucibles), or of porcelain (porcelain filtering crucibles) see Section 3.24. 

It is not necessary that the intermediate be separated from the reaction medium in the preparation of the end product. Instead, the reaction mixture, after cooling, is treated with 200 ml of water acidified with 42 ml 10% hydrochloric acid solution, and filtered. To the clear, light yellow filtrate is added dropwise a solution of 9.B g (0.07 mol) 5-nltro-2-furaldehyde in 100 ml ethyl alcohol. An orange solution of the hydrochloride results. The free base is precipitated asyellow plates by making the solution basic with saturated sodium carbonate solution. 14 g of the compound is filtered off by suction, washed with alcohol, and dried. The yield, MP 204°C to 205°C (dec.), is 53% of theoretical based on 3-(N-morpholinyl)-1,2-epoxy-propane. Recrystallization from 95% alcohol (75% recovery) raises the melting point to 206°C (dec.). 

The physical nature of the precipitate must be such that it can be readily separated from the solution by filtration, and can be washed free of soluble impurities. These conditions require that the particles are of such size that they do not pass through the filtering medium, and that the particle size is unaffected (or, at least, not diminished) by the washing process. 

Precipitation from aqueous solution is able to produce high purity materials of a wide chemical range and with very small particle sizes. Because of the formation of the filler as a suspension in an aqueous medium, precipitation methods lend themselves to the incorporation of wet coating procedures. The particles produced by precipitation are generally very small, and difficult to filter and often form a filter... 

Cross-flow filters behave in a way similar to that normally observed in crossflow filtration under ambient conditions increased shear-rates and reduced fluid-viscosity result in an increased filtrate number. Cross-microfiltration has been applied to the separation of precipitated salts as solids, giving particle-separation efficiencies typically exceeding 99.9%. Goemans et al. [30] studied sodium nitrate separation from supercritical water. Under the conditions of the study, sodium nitrate was present as the molten salt and was capable of crossing the filter. Separation efficiencies were obtained that varied with temperature, since the solubility decreases as the temperature increases, ranging between 40% and 85%, for 400 °C and 470°C, respectively. These workers explained the separation mechanism as a consequence of a distinct permeability of the filtering medium towards the supercritical solution, as opposed to the molten salt, based on their clearly distinct viscosities. 

There are some data that indicate sulfate ion aids in the formation of a more easily filtered plutonium peroxide precipitate. Ganivet (7) found, for instance, that peroxide precipitated from a nitric acid medium containing sulfate had better settling characteristics than precipitates from comparable solutions to which no sulfate was added. However, sulfate in the precipitate is undesirable, because of the corrosive effects it can have on processing equipment. Mainland, et al, (8) showed that by careful control of the precipitation parameters, it was possible to effectively precipitate peroxide in the absence of sulfate. 

Use Chemical (precipitant in the cyanide process, precipitant of iodine and lead salts from their solutions, catalyst, calcium carbide) decolorizing and filtering medium gas adsorbent component of black powder and other explosives fuel arc-light electrodes decolorizing and purifying oils solvent recovery deodorant. 

To Filter Precipitates. When filtration is employed to separate precipitated matter om the solution m which it Is suspended, the filtering medium should be such that the powder may bo easily reclaimed from it with as little loss os possible. Linen or smooth bibulous paper are the best for this purpose. A camel-hair pencil should be used, u needed, in preference to a knife, to remove... 

Based on the polarity difference between CO2 and the interior of the micelles, w/c microemulsions have found many applications as extraction media. Furthermore, by modifying pressure and temperature, solvent quality may be changed and it becomes, therefore, possible to exert a real control over the extraction process uptake of solutes inside micelles may be varied. This may be of use for separations/extractions involving bio-chemicals and proteins. In conventional solvents their separation from the reaction medium can be quite complicated, involving tedious processes such as fluid-fluid extraction, decantation, chromatography column, filtration, precipitation. Use of supercritical fluid technology with extraction in reverse micelles seems advantageous for proteins (e.g. 19, 76). This process was also used for the extraction of metals (77-79) and more recently of copper from a filter paper surface (1). 

The resistivity of the cake, however, very often increases with time. Under the pressure of filtration or the friction of the fluid that continues to pass through the cake, the filtered particles continue to compact and reduce the area available for flow. Section 7.S.4.2 also describes the empirical methods that are used to characterize the compressibility of the cake. It is significant that the particles produced by precipitation from the brine are often highly compressible. At the same time, they have a tendency to become lodged in the pores of the filter medium. Filtration of these particles can become very difficult. The usual solution to this problem is the use of a filter aid. The filter aid is applied to the surface of the filter medium before introducing the brine. This is the precoating operation. Filter aids are selected for their desirable characteristics, and the precoat material protects the pores from penetration by filtered solids while at the same time offering little resistance to the flow. Compression of the precipitates still occurs, and so filter aid ( admix ) is also added in small quantities to the brine to improve the characteristics of the cake. 

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