Product recovery caustic soda

Discussions of chlor-alkali technology and production usually focus on the production of caustic soda from NaCl. The use of KCl to produce caustic potash, an application that accounts for only a few percent of chlorine production, is often ignored. The authors have made a conscious attempt not to do this. Therefore, this section discusses sources and recovery of KCl. The comprehensive discussion of the potash industry and the processing of ore to cormnercial forms of KCl by Zandon, Schoeld, and McManus [28] is the basis for much of what follows. 

The crude liquid chlorobenzenes stream leaving the second reactor is washed with water and caustic soda solution to remove all dissolved hydrogen chloride. The product recovery system consists of two distillation columns in series. In the first column (the benzene column ) unreacted benzene is recovered as top product and recycled. In the second column (the chlorobenzene column ) the mono- and dichlorobenzenes are separated. The recovered benzene from the first column is mixed with the raw benzene feed and this combined stream is fed to a distillation column (the drying column ) where water is removed as overhead. The benzene stream from the bottom of the drying column is fed to the reaction system. 

Yet more research and development effort concentrates on the diaphragm cell caustic evaporator, finding ways to aid the evaporation of the 10-12% caustic soda in brine to make it into a saleable product. Work is directed into methods of removing the salt products and impurities and preventing corrosion of the equipment. Recovery of the salt from the evaporated caustic soda is an important part of a diaphragm cell plant as the recovered salt is used in the strengthening of the feed brine. 

Dialysis operates by the diffusion of selected solutes across a nonporous membrane from high to low concentration. An early industrial application of dialysis was caustic soda recovery from rayon manufacturing. It had been a viable process because inexpensive but alkali-resistant cellulose membranes were available that were capable of removing polymeric impurities from the caustic. Gradually however, dialysis is being replaced by dynamic membrane technology for caustic soda recovery because of the latter s much higher productivity. 

In the Kraft recovery process the green liquor, which is an aqueous solution of sodium carbonate, is heated with lime to produce white liquor or caustic soda, which is then returned to the pulp digestion operations. In the production of alumina, lime and soda are fed to bauxite digesters. The CaCO 3 produced during the course of the reaction is rebumed to lime and is recycled. The main difficulties associated with these processes include the extensive mechanical handling through the use of causticizers, settlers, and repulpers in order to produce caustic that is low in carbonate and the high fuel consumption needed to reconvert CaCO to lime. 

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. 

The heat recovery evaporator is a multistage, multi-effect evaporator which is different from conventional multiple effect evaporator or multistage flash evaporator. Asahi Chemical s heat recovery evaporator can concentrate the catholyte from 21% to about 40% without steam by utilizing heat generated during electrolysis. To obtain product caustic soda of 50% concentration, a small amount of steam is supplied to the finishing evaporator. 

Diffusion dialysis was initially applied in viscose rayon production to recover caustic soda using parchment paper as a membrane. Today the largest industrial utilization of diffusion dialysis is to recover acids or alkalis from waste acids and alkalis using anion or cation exchange membranes. Figure 6.28 shows the principle of diffusion dialysis for acid recovery from waste acid solution. The process and membrane performance are evaluated on the basis of the flux of acids or alkalis (dialysis coefficient) and the ratio of the flux of metal salt to that of acids or alkalis (separation coefficient). The total dialysis coefficient [molh-1 m 2 (moll-1)], Uo, is defined by... 

In this process, caustic soda (NaOH) is used. The phosphate part of the ore is recovered as trisodium phosphate. This product is marketable, which is already a major advantage over the acid process, and this process has been very favorable for commercial use. Fine-ground monazite is treated with a 60-70 % NaOH solution at 140-150 °C. The mixed rare earth thorium hydroxide cake resulting from this process is then treated for recovery of thorium and rare earths. A variety of methods is used to accomplish this (Gupta and Krishnamurthy 2005). An effective process for removing thorium completely and in a very pure state is solvent extraction with higher amines. This should preferably be carried out in a sulphate solution. However, leaching of the rare earths from the hydroxide cake with hydrochloric acid is also very effective. The rare earths are recovered from the leachate by solvents extraction. 

It is also possible simply to purge enough of this salt to maintain the sulfate specification in the product caustic soda. The purge is more efficient when the salt is removed through a centrifuge, as shown. Second, the liquor from the first effect is on its way to final product recovery. Better phase separation is desirable here, and so the slurry passes through a set of cyclones. The emphasis is on delivery of clear liquor, and the concentrated underflow joins the other slurries in the tank. 

A second Hydrina process involves recovery and recycle of caustic soda and a mixed solution of Na2S04 and H2SO4. This is similar to the two-compartment cell of Fig. 15.18, but it uses the hydrogen-depolarized anode instead of the conventional type. A third version uses a three-compartment cell with the hydrogen anode [106,107]. Saturated Na2S04 enters the central compartment, and water is added to the electrode compartments. The products are sulfuric acid and caustic soda of high pmity. 

In membrane electrol sis an electrolysis process is combined w ilh a membrane separation process. The classical example is the chlor-alkali process in which sodium chloride is converted into chlorine and caustic soda. Other examples are the electrolytic recovery of (hea 7) metals and the production of acid and base from the corresponding salts. 

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