Лако-красочные материалы — производство

The precipitates commonly encountered in the analytical labora­tory are practically insoluble and can be washed with unlimited quan­tities of liquid. Organic products, on the other hand, are usually more or less easily soluble in the solvent employed, and hence it is always neces­sary to use the smallest possible amount of wash liquid which will com­pletely remove the adhering mother liquor. For this reason, gravity filtra­tion is of little value. Instead, filtration methods must be used which pro­vide an appreciable pressure difference above and below the filter. This pressure difference is obtained by applying pressure above the filter, or suction below it.

The first method is preferred in plant operations, employing the filter press, in which pressure is supplied by compressed air (see Sec­tion L, page 348 ff.). The filter press is not applicable to operations on a laboratory scale.

On the other hand, the filter centrifuge, in which the pressure is supplied by centrifugal force, is well adapted for use with the quanti­ties usually encountered in industrial laboratory operations. The filter centrifuge consists of a metal or porcelain “basket,” closed at the bot­tom and open at the top, bearing sieve-like perforations around its periphery. This basket is mounted in such a way that it can be rotated at high speed within a surrounding shell serving to collect the liquid thrown out. For filtration purposes, the inside of the basket is lined with a suitable filter cloth. When the basket is rotated at a sufficiently high speed, the entrained liquid is expelled, and the precipitated ma­terial is obtained in a nearly dry condition. However, the filter centri­fuge is suitable only for the filtration of coarsely granular substances, since a fine precipitate either goes through the filter or packs into a dense mass through which the liquid cannot pass. Similar difficulties are encountered with substances which crystallize in plates. All in all, the number of industrial organic products which can profitably be centrifuged is not large. The most useful application of the centrifuge is in the separation of a mixture of isomers, one of which crystallizes from the liquid mixture on cooling (see, e. g., o — and p-nitrochloroben — zenes, page 90).

The filter centrifuge should not be confused with the sedimentation centrifuge widely used in biochemical laboratories. With a sedimentation centrifuge, fine pre­cipitates are not filtered, but are pressed against the walls of the container and thus separated from the liquid.

The other method of obtaining a pressure difference, namely, suc­tion applied below the filter, can be used in the plant only for well

crystallized, granular precipitates. Here, a fairly deep column of liquid must be used, and the filtration becomes exceedingly slow if the pre­cipitate is very fine. In laboratory operations, where the layer being fil­tered may not be more than a few centimeters deep, the suction filter is of more general application. Even here, however, failures are en­countered when soft, tarry, or gelatinous products are involved or when very finely crystalline materials are filtered. The latter first go through the filter and later stop it up. Difficulties are also encountered with plate-like crystalline materials which press down on the filter to form an impermeable mass. Some of these difficulties may be remedied by using a large filter so that the layer of liquid in it is quite shallow. It is also of assistance to maintain only a weak suction when the suspen­sion is poured into the filter and to stir the material constantly to keep the precipitate from settling.

For granular precipitates, filter cloth is used as the filter material, and for finer precipitates, filter paper. If the precipitate is very fine and has a tendency to go through the filter paper, two or three thicknesses of paper may be used, or, in some cases, hardened filter paper may be employed. It is recommended that both a filter cloth and a filter paper be used in the larger suction funnels to prevent tearing the paper and to facilitate the removal of the precipitate from the filter. The cloth and paper filters should be of exactly the same size as the bottom of the funnel. If they extend up the side wall of the funnel, wrinkles are formed which permit some the precipitate to escape into the filtrate. Wool felt may be used for filtering strongly acid liquids which attack paper and cotton. Still more suitable, and usable even with concentrated acids, are the sintered glass funnels recently made available commercially in a variety of pore sizes.

In order to achieve efficient washing of the precipitate with the least possible amount of liquid, it is important that the mother liquor held in the filter cake be removed as completely as possible before each addi­tion of wash liquid. After the main bulk of the filtrate has run through, the filter cake is stirred and pressed together with a spatula to pack down the individual particles and produce a cake without cracks. Then the cake is pressed down strongly with a pestle or inverted glass stopper until no more drops of filtrate are obtained. The vacuum is then inter­rupted and the wash liquid poured onto the filter cake, allowing the sponge-like cake to soak up the liquid. Suction is again applied and the cake is again pressed out. The operations of adding and sucking off small portions of the wash liquid are repeated until the impurities have been removed to the required extent. The success of the washing de-

pends on the choice of a filter funnel of the correct size. The filter cake should not fill the funnel but enough space should be left above it to hold the wash liquid. On the other hand, the use of a funnel which is too large gives a thin filter cake, and an excessive amount of wash liquid is required. Also, the formation of cracks and holes in the filter cake is hard to avoid.

Precipitates, which are so finely crystalline that they go through the suction filter or clog it, are filtered most satisfactorily through a large fluted filter. In this case, since the precipitate retains a large amount of liquid, the filter and its contents, after washing, are spread out evenly on a thick absorbent layer of cheap filter paper, for example. The precip­itate is separated from the filter without difficulty when enough liquid has been absorbed so that the residue has a pasty consistency, and the large amount of mother liquor still retained is removed in the screw press (Fig. 21). The precipitate is first wrapped in an ordinary filter cloth, then in a thick “press cloth” which can withstand the pressure of the press. Pressing is carried out slowly, with only slight pressure at first. The liquid must have time to make its way through the precipitate and the filter cloths, otherwise the hydrostatic pressure may become great enough to tear the cloth or force the precipitate through it. Only toward the end of the operation, when only a little liquid is present, is full pressure applied.

Filtration of amorphous, soft, flocculent, or gelatinous precipitates offers great difficulty. With these materials, the best results are often obtained by the use of a filter cloth, folded in the same manner that a filter paper is formed into a 60-degree cone, and placed in an ordinary glass funnel. The precipitate can be pressed out right in the filter cloth, carrying out the operation slowly and carefully.

Filtration of warm mixtures always proceeds more rapidly than the filtration of cold mixtures, because the viscosity is lower at higher tem­peratures. Hence, warm filtration is to be preferred if the stability of the material and the solubility relations are favorable.

In general, the filtering properties depend, in large measure, on the physical properties of the precipitate, and these in turn depend on the precipitation or crystallization conditions. It is often advantageous to carry out a precipitation or crystallization at elevated temperature and with stirring. Frequently, it is beneficial to add salts, or to maintain a certain degree of acidity or alkalinity. In other cases, separation of the precipitates takes place best in an exactly neutral medium. The condi­tions vary so greatly in individual instances that no general rules can be set up.