Solar salt fractional crystallization

In most commercial processes, the compound is either derived from the sea water or from the natural brines, both of which are rich sources of magnesium chloride. In the sea water process, the water is treated with lime or calcined dolomite (dolime), CaO MgO or caustic soda to precipitate magnesium hydroxide. The latter is then neutralized with hydrochloric acid. Excess calcium is separated by treatment with sulfuric acid to yield insoluble calcium sulfate. When produced from underground brine, brine is first filtered to remove insoluble materials. The filtrate is then partially evaporated by solar radiation to enhance the concentration of MgCb. Sodium chloride and other salts in the brine concentrate are removed by fractional crystallization. 

Magnesium chloride can be also recovered from its mineral carnallite by similar processes involving concentration of the liquor by solar evaporation followed by separation of other salts by fractional crystallization. 

Sulfate of potash (K2S04), unlike the earlier-discussed potash salts, does not occur as natural deposits. It can be recovered by fractional crystallization from such natural brines as those of the Great Salt Lake in Utah and Searles Lake in California. Here separation and recovery are achieved by solar evaporation in shallow ponds. These processes can be utilized only where a suitable brine source is available, and where solar evaporation rates are high. 

Solar evaporation, from primary and secondary ponds of 100 and 30 km in extent, the initial stage for potassium chloride recovery from the Dead Sea brines [26]. The smaller number of constituent ions present in these waters significantly simplifies salts recovery, and the fact that they contain nearly twice the relative potassium chloride concentration of seawater also improves profitability. Developed from a process, which was first operated in 1931, evaporation in the first pond reduces the volume of the brine to about one-half of the initial volume and brings down much of the sodium chloride together with a small amount of calcium sulfate (Fig. 6.5). The concentrated brines are then transferred to the secondary pond where evaporation of a further 20% of the water causes carnallite (KCl MgCli 6H2O) and some further sodium chloride to crystallize out. With care, a 95% potassium chloride product on a scale of some 910,000 tonne/year is obtained either by countercurrent extraction of the carnallite with brines, or by hot extraction of potassium chloride from the sylvinite matrix followed by fractional crystallization for its eventual recovery [16]. 

Some potassium chemicals are currently separated from sea water by fractional crystallization of the bittern from solar salt production. Several years ago the Dutch operated a process which utilized dipicrylamine to precipitate potassium from sea water, but this reagent is rather costly. We are developing a... 

A flowchart of the production of potassium sulfete from the Great Salt Lake, U.S.A., is shown in Rgure 5.31. Water from the lake is pumped and distributed to shallow ponds covering an area of 19,200 acres. Solar evaporation causes the brine to reach saturation with sodium chloride, and the salt settles on the pond floor. Further evaporation of the brine in subsequent ponds results in the precipitation of kainite, sylvite, and camaF lite, which are harvested from the ponds for further processing. Fractional crystallization is used to separate the final potassium sulfate product from the other minerals 1110]. 

Rock salt is obtained from mines and it is typically 95% sodium chloride. Alternatively, water is pumped into salt deposits to form brine underground. This brine is then removed through a brine well. After purification, the water is removed to make mechanically evaporated salt that is 99.99% sodium chloride. Controlled evaporation and fractional crystallization of seawater yields solar salt that is 99.7% sodium chloride. ... 

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