Vacuum salt

Where possible, solar salt is replacing vacuum salt because of rising energy costs. For example, in July the 81 x 10 (20,000 acres) of solar ponds... 

To manufacture the brine, a vacuum salt is used to which the producer needs to add a small amount of anti-caking agent which forms a ferrohexacyanide complex in the brine. Because of the acidic process conditions, Fe ions tend to migrate into the electrolyser membranes until encountering a sufficiently high pH and then precipitate [1]. This is an undesirable effect as it can cause void spaces within the membrane and thereby increase the voltage needed for the electrolysis. For this reason the ferrohexacyanide is depleted into Fe(OH)3 under well-defined conditions of temperature, residence time, free chlorine and pH in a process step prior to filtration [2]. 

A stream of a saturated brine suspension of crystals is continuously withdrawn from each evaporator-crystallizer, and the salt crystals separated on a continuous rotary filter with return of the brine to the evaporator. Much of the salt may be marketed in moist condition, or it may be passed through a drier moving countercurrently to heated air to give vacuum salt of typically 99.8-99.9% purity [21]. 

Biogenic amines are usually detected by LC with a pre- or postcolumn derivatization with o-phthalaldehyde in the presence of mer-captoethanol, and fluorimetric detection of derivatives. A sample derivatization also has to be done to perform GC/MS analysis of grape juice or wine. Amines are distilled from the alkalized sample and trapped in an acidified solution. After concentration under vacuum, salts of ethylamine, dimethylamine, ethylamine, diethyl-amine, n-propylamine, isobutylamine, a-amylamine, isoamylamine, pyrrolidine, and 2-phenethylamine are derivatized with trifluoroacetic (TEA) anhydride. Their derivatives are extracted with ethyl ether. GC/MS is performed using a capillary fused silica PEG column with an oven temperature programmed for 8 min at 70 °C, l°C/min to 160°C, isotherm for 90min (Daudt and Ough, 1980). 

Component Rock salt Washed solar salt Vacuum salt... 

Figure 7.13 shows PSDs for several salts typical of the various classes. Curve 1 is a Canadian potash. Curve 2 is an Italian vacuum salt. Curves 3 and 4 are two samples of the same Bahamian solar salt from two different final suppliers. Curve 5 is a typical dissolver-grade rock salt. All distributions depart from true lognormal by the presence of too much fine material. As shown by the uniformity coefficients, the solar salts have the widest PSDs and the vacuum salt the narrowest. Figure 7.14 shows the variation in PSD caused by screening. All three curves are for rock salt from Weeks Island (United States). These are well-screened fractions whose distributions are closer to lognormal. The uniformity coefficients are about 1.5. 

Small particles, with their high surface/volume ratio, pick up water more readily and are especially liable to caking. Vacuum salts and potash, as suggested by Fig. 7.13, are therefore the most sensitive. They are frequently treated with an anticaking additive before shipping. The one most widely used is sodium ferrocyanide, or yellow prussiate of soda (YPS). By the nature of the addition process, YPS also tends to concentrate at the surfaces of the particles. Therefore, when a batch of treated salt is dissolved, the first brine formed has a high concentration of YPS. Rain also selectively removes YPS from the salt, and this is one reason not to store such salts in the open. 

The salt itself is broadly classified as coarse or fine the dividing line is at a particle size of about 2 mm. Uncaked coarse salt flows fieely. It stands or flows on its angle of repose and can automatically be fed into a dissolver by gravity as the lower part of a bed dissolves. Fine salt, unless exceptionally dry, tends to cake and form arches in storage hoppers. It is often fed into a dissolver in batches and stored under a cover of saturated brine. Fine salts include vacuum salt, evaporator salt, fine crystal rock salt, and most KCl. 

The raw salt can be rock, solar or vacuum salt, where the latter has been purified by vacuum crystallization. Impurities as calcitun and magnesium etc. in the salt may harm the electrolysis operation by precipitating on the electrodes and result in high electrode potentials. Therefore purification of the salt is needed, and the quality of the salt set requirements on necessary purification steps. In Fig. 1 the incoming salt is first dissolved and then subject to ion exchange for removal of divalent cations as Ca " and Mg The evaporator illustrates re-crystallization of the... 

It is possible to economize the usual precipitation step for the first brine purification (see Fig. 1) and directly to use the irat exchanger for final purification of a vacuum salt solution. 

The example used below is a typical task definition and was realized in 1980-1982 in Croatia. A salt plant has to be designed for a production capacity of 9 t/h table salt. A part of that quantity (2.5 t/h) shall be recovered as granular salt with an average particle size d > 2 mm, whereas the rest (6.5 t/h) shall be produced as PDV (pure dried vacuum) salt with a mean particle size of around 0.4 mm. The purity of the vacuum salt had to be minimum 99.7% NaCl. 

Figure16.18 Quadruple-effiect evaporative crystallization plant with vacuum salt and granular salt production.

A plant of this type was erected for the treatment of 601 h concentrated liquor from a solar pond on the coast of the Mediterranean Sea. The plant is producing 2.5 th grain-size salt of >2 mm average crystal size and 6.51 h" of normal vacuum salt. Per hour it consumes lit of steam and evaporates 341 of water. By... 

The contaminants can be brought into the brine tystem by salt, by chemicals used in brine purification steps, by water for dissolving the salt, firom materials of tanks, pipework, and ceU components, or by the process itself [142]. The impurities in the salt depend upon the origin of the raw material. Rock salt, vacuum salt, sea salt, brine from well mining, or salt from waste incinerators serve as supplies of NaCl. The more varied the sources are, the more diverse the impurities. 

With the exceptions of high vacuum, salt baths, and chemically inert gases, such as izgon, all heat-treating atmospheres contain some hydrogen at tem-... 

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