Sodium hydroxide caustic soda evaporation

In this cell, not all of the dissolved sodium chloride is electrolyzed. Consequently, the solution that seeps through the perforated cathode contains sodium hydroxide together with some unchanged sodium chloride. The solution is concentrated by evaporation, whereupon most of the less soluble sodium chloride crystallizes and the very soluble sodium hydroxide remains in solution. This concentrated solution of sodium hydroxide (caustic soda) may be sold as such, or the remainder of the water may be driven off by heating to form solid sodium hydroxide. If a purer product is desired, the solid is dissolved in alcohol, which does not dissolve the remaining traces of sodium chloride. Pure sodium hydroxide is then secured by filtration, followed by evaporation of the alcohol. 

Demand for Caustic Soda Types. Approximately 99% of the sodium hydroxide produced in 1987 was 50% caustic solution (5). Higher concentrations require additional evaporation and therefore increased prices relative to the sodium oxide values. To obtain maximum value, users have learned to adapt manufacturing processes to the 50% caustic soda. 

In the decomposer, deionized water reacts with the amalgam, which becomes the anode to a short-circuited cathode. The caustic soda produced is stored or evaporated, if higher concentration is required. The hydrogen gas is cooled by refrigeration to remove water vapor and traces of mercury. Some of these techniques are employed in different facilities to maximize the production of chlorine, minimize the consumption of NaCl, and also to prevent the buildup of impurities such as sulfate in the brine.26 The production of pure chlorine gas and pure 50% sodium hydroxide with no need for further concentration of the dilute solution is the advantage that the mercury cell possesses over other cells. However, the cell consumes more energy and requires a very pure brine solution with least metal contaminants and above all requires more concern about mercury releases into the environment.4... 

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. 

Cone. soln. of sodium hypochlorite with up to 42 per cent, of available chlorine have been made under the trade name chloros, by passing chlorine into a soln. of caustic soda of such a strength that the sodium chloride which is formed separates out. The temp, is kept below 27°. The crystals of sodium chloride are removed, and more chlorine is introduced, but the sodium hydroxide is always kept in excess or the soln. will be unstable. A. J. Balard prepared potassium, sodium, and lithium hypochlorites by neutralizing a well-cooled soln. of the base with the acid. E. Soubeiian evaporated in vacuo the liquid obtained by treating a soln. of calcium hypochlorite with sodium carbonate, and obtained, before the liquid had all evaporated, crystals of sodium chloride and of sodium hypochlorite. P. Mayer and R. Schindler obtained solid potassium hypochlorite mixed with potassium hydrocarbonate by the action of chlorine—developed from 10 parts of sodium chloride—on a soln. of 24 parts of potassium hydrocarbonate and one of water. 

From 20 to 30 c.c. of the alcoholic distillate, rendered distinctly acid with dilute sulphuric acid, are evaporated on the water-bath to expel the alcohol, but should not be taken to dryness. The residue is made alkaline with caustic soda and distilled the first 2 3 c.c. of the distillate (which has a marked disagreeable odour in presence of pyridine) are acidified with a few drops of concentrated hydrochloric acid and treated with 1% aqueous gold chloride solution a pale yellow crystalline precipitate is formed if pyridine is present. If the precipitate is washed and dried and heated with a few drops of sodium hydroxide solution in a small test-tube, the characteristic odour of pyridine is observed. 

Another possibility to use PET recyclate for food applications is the URRC process (United Resource Recovery Corporation). In this method the cleaned PET flakes are covered with concentrated caustic soda. After evaporation of the water in a rotary kiln the sodium hydroxide etches the surface of the PET at a temperature of more than 200°C. The resulting sodium terephthalate is removed by washing. The remaining flakes are used for the production of bottle preforms. Since 2000 several plants in Switzerland and Germany have been established [1],... 

Historically, a classic example of an evaporation process is the production of table salt. Maple syrup has traditionally been produced by evaporation of sap. Concentration of black liquor from pulp and paper processing constitutes a large-volume present application. Evaporators are also employed in such disparate uses as desalination of seawater, nuclear fuel reprocessing, radioactive waste treatment,preparation of boiler feed waters, and production of sodium hydroxide. They are used to concentrate stillage waste in fermentation processes, waste brines, inorganic salts in fertilizer production, and rinse liquids used in metal finishing, as well as in the production of sugar, vitamin C, caustic soda, dyes, and juice concentrates, and for solvent recovery in pharmaceutical processes. 

What became known as the Leblanc process was actually several interrelated processes. Salt was first reacted with sulphuric acid in a cast-iron pan, then in a reverberator furnace (in which heat was apphed from a flame blown from a separate chamber, not in direct contact with the salt), to produce saltcake (sodium sulphate), with hydrochloric acid released as a waste gas. Saltcake was used to make sodium carbonate, or roasted with limestone (calcium carbonate) and coal or coke to produce black ash. This mixture of sodium carbonate, calcium sulphide, sodium sulphide, hme, salt, carbon, and ash could be treated further with hot water to produce impure sodium carbonate in solution, evaporated into soda crystals (washing soda), or heated to yield anhydrous sodium carbonate. The latter, in turn, could be reacted with lime to made caustic soda (sodium hydroxide), the strongest commercial alkali then available. 

A diaphragm is porous and cannot discriminate between species. All will diffuse through its pores where there is a concentration difference. For this reason the caustic soda produced in a diaphragm cell is always contaminated with chloride ion and the catholyte leaving the cell cannot contain more than 10% sodium hydroxide since otherwise hydroxide ion diffusion to the anode becomes significant and oxygen as well as chlorine is evolved. Thus prior to sale, the sodium hydroxide produced in a diaphragm cell must be concentrated by evaporation to a 50% solution. 

The limitation of the caustic soda concentration to 10% means that an additional step must be introduced into the process. To increase the concentration to the 50% solution normally traded, water must be evaporated and this requires additional energy and plant. The evaporation stage does, however, reduce the problem from chloride contamination since on cooling much of the sodium chloride crystallizes out from the 50% sodium hydroxide solution. Even so, the chloride level remains about 1% and this is not an acceptable level for all the applications of sodium hydroxide. 

Nearly all applications of caustic soda require separation of the chloride from the hydroxide. In sodium-brine electrolysis, fortunately, the phase equilibrium allows a rather effective separation by evaporation of the liquor. If water is removed until the concentration of NaOH approaches 50%, nearly all of the NaCl falls out of solution. After cooling, the residual concentration is about 1.0-1.1%. This removal of salt causes the concentration of NaOH to increase. The solution produced by evaporation therefore can contain somewhat less than 50% NaOH. This is discussed in some detail in Section 9.S.3.3. Dissolved salt is not acceptable in some uses of NaOH, and so there has always been a split market. Part has been reserved to a purified version of the diaphragm-cell product and to mercury-cell, and now membrane-cell, NaOH. 

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