Caustic soda evaporation

Copper-alloy corrosion behavior depends on the alloying elements added. Alloying copper with zinc increases corrosion rates in caustic solutions whereas nickel additions decrease corrosion rates. Silicon bronzes containing between 95% and 98% copper have corrosion rates as low as 2 mil/y (0.051 mm/y) at 140°F (60°C) in 30% caustic solutions. Figure 8.2 shows the corrosion rate in a 50% caustic soda evaporator as a function of nickel content. As is obvious, the corrosion rate falls to even lower values as nickel concentration increases. Caustic solutions attack zinc brasses at rates of 2 to 20 mil/y (0.051 to 0.51 mm/y). 

Figure 8.2 Corrosion rates of copper and copper-nickel alloys in a 50% caustic soda evaporator (Courtesy ofF. L. LaQue, Corrosion 10 391 1954.)...

Six caustic soda evaporators were anodically protected against stress corrosion in the aluminum industry in Germany in 1965 [27]. Each evaporator had an internal surface area of 2400 m. The transformer-rectifier had a capacity of 300 AJ 5 V and was operated intermittently for many years. Automatic switching on of the protection current only took place in case of need when the drop in potential reached... 

The external heating boxes for the caustic soda evaporator with forced circulation must be anodically protected separately. Ring-shaped impressed current electrodes of carbon steel are mounted and insulated on supporting brackets (see Fig, 21-12). 

The high purity of the caustic soda obtained by the membrane process eliminates the need for a caustic soda evaporator in cases where it is to be supplied to customers such as pulp mills which utilize a dilute caustic soda. This is in marked contrast to the diaphragm process which inevitably requires evaporation to separate sodium chloride. For the general trade, in which caustic soda at 50% concentration is required, a conventional multiple effect evaporator is generally utilized to concentrate the catholyte. Caustic soda from the membrane process contains a very slight amount of sodium chloride which does not cause corrosion of the evaporator materials or precipitation of sodium chloride, and thus allows easier and more stable evaporator operation than in the diaphragm process. 

The brine used in the mercury cell and membrane cell processes is normally saturated with solid salt although there are some installations that use solution-mined brine on a once-through basis. The brine supply for diaphragm cells is always used on a once-through basis, although the salt recovered from caustic soda evaporators may be recycled into the brine supply. 

In the United States and Europe, rock salt is most commonly used. The most important impurities are shown in Table 3. The concentrations of these impurities depend on the method of production and on the different grades crude rock salt, prepared rock salt, and evaporated salt. Solar salt is used in Japan and many other parts of the world, the most important sources being Australia, Mexico, China, Chile, India, and Pakistan. The salt produced by solar evaporation is usually much less pure than rock salt. In a few cases the salt may be obtained from other processes, such as caustic soda evaporation in the diaphragm process. 

Caustic Soda Evaporation. A flow diagram for a typical triple-effect caustic soda evaporator is shown in Figure 44. The evaporator is of the backward-feed design and concentrates 10 -11.3 wt % NaOH cell liquor to 50 wt % NaOH. Liquor flows from... 

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. 

Lime Soda. Process. Lime (CaO) reacts with a dilute (10—14%), hot (100°C) soda ash solution in a series of agitated tanks producing caustic and calcium carbonate. Although dilute alkaH solutions increase the conversion, the reaction does not go to completion and, in practice, only about 90% of the stoichiometric amount of lime is added. In this manner the lime is all converted to calcium carbonate and about 10% of the feed alkaH remains. The resulting slurry is sent to a clarifier where the calcium carbonate is removed, then washed to recover the residual alkaH. The clean calcium carbonate is then calcined to lime and recycled while the dilute caustic—soda ash solution is sent to evaporators and concentrated. The concentration process forces precipitation of the residual sodium carbonate from the caustic solution the ash is then removed by centrifugation and recycled. Caustic soda made by this process is comparable to the current electrolytic diaphragm ceU product. 

Manufacture, Shipment, and Analysis. In the United States, sodium and potassium thiocyanates are made by adding caustic soda or potash to ammonium thiocyanate, followed by evaporation of the ammonia and water. The products are sold either as 50—55 wt % aqueous solutions, in the case of sodium thiocyanate, or as the crystalline soHds with one grade containing 5 wt % water and a higher assay grade containing a maximum of 2 wt % water. In Europe, the thiocyanates may be made by direct sulfurization of the corresponding cyanide. The acute LD q (rat, oral) of sodium thiocyanate is 764 mg/kg, accompanied by convulsions and respiratory failure LD q (mouse, oral) is 362 mg/kg. The lowest pubhshed toxic dose for potassium thiocyanate is 80—428 mg/kg, with hallucinations, convulsions, or muscular weakness. The acute LD q (rat, oral) for potassium thiocyanate is 854 mg/kg, with convulsions and respiratory failure. 

Evaporators have performed successfully in a number of industrial applications. Typical materials that are processed in evaporators include Caustic Soda, Caustic Potash, Sodium Carbonate, Sodium Dichromate, Sodium Nitrate, Ammonium Nitrate, Phosphoric Acid Superacid, Potash, Urea, Glue, Glycerine,... 

Methyl cinnamate (16 parts) is dissolved in methyl alcohol (20 parts) and treated with bromine (20 parts). The mixture solidifies in the cold. It is shaken with a solution of caustic soda (12 parts) in water (24 parts), the temperature being kept down to 40°. After two hours the mixture is neutralised with dilute sulphuric acid, and an oily layer separates. This is mixed with water (to 250 parts) and sodium carbonate (5-5 parts) added, and the aldehyde distilled in a current of steam, and extracted with ether, and the ether evaporated. The yield is about 75 per cent, of the theoretical. 

Methyl-heptenone also forms a bromine derivative which is well suited for the identification of the ketone. This body, which has the formula CgHjjBrgO. OH, melts at 98° to 99°, and is obtained as follows Three grams of methyl-heptenone are mixed with a solution containing 3 grams of caustic soda, 12 grams of bromine, and 100 c c. of water. After a time an oily substance is deposited, which is extracted with ether. The solvent is evaporated, and the residue, redissolved in ether, is treated with animal charcoal and filtered. On slow evaporation the product is obtained in well-defined crystals. 

The dihydrochloride is converted into monohydrochloride by dissolving 26.36 g (0.066 mol) of dihydrochloride into 158 cm of water, adding drop by drop a suitable amount (0.066 mol) of caustic soda (1 N), evaporating the aqueous solution to dryness, drying by means of benzene, filtering the formed sodium chloride (3.8 g) and crystallizing the cooled obtained benzene solution. 22.6 g (95%) of monohydrochloride are obtained, MP 120° to 121 t. 

After twice washing with 100 ml of diethyl ether, the aqueous phase is made alkaline with 50% caustic soda solution. The liberated base Is twice extracted with 150 ml of diethyl ether. After the ether has been evaporated, the residue Is distilled under reduced pressure and has a boiling point of 184°C/0.1 mm, np70 = i. 5539. 77 g of the pure base in the form of a viscous liquid is thus obtained. The hydrochloride, which is prepared in conventional manner, has a melting point of 128°C. 

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