Brine preparation

Dimeihylamine, C2H7N, (CH3)2NH. Colourless, inflammable liquid with an ammoniacal odour, mp -96" C, b.p. 7°C. Occurs naturally in herring brine. Prepared in the laboratory by treating nitrosodimetbyl-aniline with a hot solution of sodium hydroxide. Dimethylamine is largely used in the manufacture of other chemicals. These include the solvents dimethylacetamide and dimethyl-formamide, the rocket propellant unsym-metrical dimethylhydrazine, surface-active agents, herbicides, fungicides and rubber accelerators. 

Brine Preparation. Sodium chloride solutions are occasionally available naturally but they are more often obtained by solution mining of salt deposits. Raw, near-saturated brines containing low concentrations of impurities such as magnesium and calcium salts, are purified to prevent scaling of processing equipment and contamination of the product. Some brines also contain significant amounts of sulfates (see Chemicals FROMBRINe). Brine is usually purified by a lime—soda treatment where the magnesium is precipitated with milk of lime (Ca(OH)2) and the calcium precipitated with soda ash. After separation from the precipitated impurities, the brine is sent to the ammonia absorbers. 

Brine Preparation. Rock salt and solar salt (see Chemicals frombrine) can be used for preparing sodium chloride solution for electrolysis. These salts contain Ca, Mg, and other impurities that must be removed prior to electrolysis. Otherwise these impurities are deposited on electrodes and increase the energy requirements. The raw brine can be treated by addition of sodium carbonate and hydroxide to reduce calcium and magnesium levels to below 10 ppm. If further reduction in hardness is required, an ion-exchange resin can be used. A typical brine specification for the Huron chlorate ceU design is given in Table 6. 

Calculation of Brine Components. In order to satisfy U.S.D.A. regulations, care must be taken during brine preparation so that finished product protein meets or exceeds 17 percent. The following formula can be used to determine brine composition (Isolated soy protein, salt, dextrose, polyphosphate, etc.) (6). 

The electrolyzers producing sodium hydroxide are fed with saturated brine (300 to 310 NaCl per litre), which is prepared in most works by dissolving common salt. In mercury electrolyzers, the salt is dissolved iu the spent brine which as a rule is set free from the dissolved chlorine, and which contains after leaving the electrolyzer 260 to 280 g NaCl per litre. With the diaphragm electrolyzers, which yield a caustic solution containing 120 g NaOH and 200 g NaCl per litre, the so called return solution is added to the brine prepared by dissolving the salt. This return solution is obtained in a later stage of the process iu the course of evaporation of the caustic solutions. 

In case of rock salt, the most important impurities are (in wt%) water (<3), insoluble components (<2), calcium (0.2-0.3), magnesium (0.03-0.1), sulfate (<0.8), and potassium (<0.04). But there are not only Ca2+-, Mg2+-, K+-, and SO -ions in the brine prepared from the solid salt but also Ba2+- and Al3+-ions and certain amounts of heavy metal ions. And last but not the least impurities generated from the process itself (CIO-, CIO3-) or from materials of tanks, of pipe-work and of cell components have to be taken into consideration. And of course, the solving water can be a source of impurities, too [3, p. 84],... 

The final step in brine preparation is acidification. This is truly not a purification step and is not an essential part of the chlor-aUcali process, but it offers several improvements in cell performance. These are discussed in Section 7.5.6. The acid used is HCl, often produced by reacting together some of the chlorine and hydrogen produced in the plant. 

Many salt deposits contain sulfides and hydrocarbon inclusions. When brine rises from the mine to the surface, noxious or hazardous gases can evolve in the pipeline or at the surface as the pressure decreases. These must be separated and disposed of safely. When temperatures in the mine are higher than those at the surface, dissolved salt may drop from solution. Process design should recognize the possibility of precipitation and accumulation of solids. In this connection, the pressure at the bottom of the well also increases the solubility of other compounds. Frear and Johnston [43] and Brandani et al. [44] have shown that the solubility of calcium species increases when the partial pressure of CO2 increases. A final consideration is on rates of solution. The very long residence time in a brine well allows more of the slower-dissolving materials to enter the brine, which is generally of lower quality than brine prepared at the plant from the same salt. 

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