Rayon-grade caustic

Fig. 15.3 The balance factoring the price of rayon-grade caustic soda production from membrane technology.

Another aspect that may be taken into account is that of membrane electrolysers having a lower power consumption (Fig. 15.4). Not only does the new technology save power but it also requires less steam to evaporate the cell caustic product to 50%. Additionally, salt removal equipment required in diaphragm plants uses power. This benefit can also be turned around so that for the same power consumed by a diaphragm cell room extra volumes of rayon-grade caustic soda can be produced from the membrane electrolysers. 

There have been many attempts to utilize the approximately 1.8 V generated by the electrochemical reactions of the decomposer. Elowever, it has not been found possible to do this and to maintain high concentrations of sodium hydroxide and low residual sodium in the stripped mercury. The sodium hydroxide product obtained from the decomposer of a mercury cell is very pure, containing 0.001% or less sodium chloride. This product is referred to as rayon grade caustic because the high purity and low sodium chloride content makes it particularly suitable for rayon manufacture. This is achieved without the special purification steps required for the diaphragm cell product. 

Caustic from the mercury-cell process is called Rayon-grade caustic. Rayon-grade caustic and membrane-grade caustic are referred to as low salt caustic soda, which is a premium product. 

The sodium hydroxide solution from the mercury process is the purest of the three the amounts of NaCl and NaClOs are especially low. However, the quality of caustic from the membrane process is almost as good. A main drawback of the diaphragm process is the high concentration of NaCl and NaClOs in the caustic solution. This sodium hydroxide solution cannot be used for some processes. A chloride-free grade, commonly referred to as rayon-grade caustic, is required for 20-30% of the demand in industrialized countries. Even the use of purification processes (see p. 73) does not reduce the NaCl content below 0.03 wt%. In addition to the NaCl and NaClOs, the levels of Si, Ca, Mg, and sulfate impurities are higher than for the mercury and membrane processes. 

Three forms of caustic soda are produced to meet customer needs purified diaphragm caustic (50% Rayon grade), 73% caustic, and anhydrous caustic. Regular 50% caustic from the diaphragm cell process is suitable for most appHcations and accounts for about 85% of the NaOH consumed in the United States. However, it caimot be used in operations such as the manufacture of rayon, the synthesis of alkyl aryl sulfonates, or the production of anhydrous caustic because of the presence of salt, sodium chlorate, and heavy metals. Membrane and mercury cell caustic, on the other hand, is of superior quaUty and... 

FIFRA amendments, 18 537, 538 50% rayon grade of caustic soda, 22 838... 

Now that there is a choice of caustic soda grades as increased quantities of rayon grade become available it is evident that fresh water pulp mills are seeing a difference in their process. The price differential will enable the diaphragm caustic soda to be sold, but as in any free market, market forces may wish to move to rayon grade as it becomes more widely available. 

This premium price is now seen as independent of the cyclical market pricing of caustic soda. So how long could you expect the differential to last The current view is that the more rayon grade, and the less diaphragm grade available, then the more the... 

High Mono Stearyl Acid Phosphate Caustic Soda 50% Rayon Grade Light Density Soda Ash... 

Crude 10% sodium hydroxide containing sodium chloride is purified in a similar manner to the product of the causticization process. The water is evaporated in nickel or nickel-clad steel (to reduce corrosion) multiple-effect evaporators to about 50% sodium hydroxide concentration. At this concentration, sodium chloride is only about 1% soluble (2%, on a dry basis) in the more concentrated caustic so that the bulk of it crystallizes out and is filtered off. This quite pure sodium chloride is recycled to the cells. Lor many purposes, such as for pulp and paper production, this purity of 50% sodium hydroxide is quite acceptable. If higher purities are required, sodium hydroxide may be separated from residual water and salt by chilling to the double hydrate crystals NaOH 2HiO, m.p. about 6°C, or as NaOH 3.5HiO, with a m.p. of about 3°C, or by counter-current extraction [9]. The sodium hydroxide obtained after these steps contains 2-3 ppm sodium chloride, equivalent to the purity of the mercury cell product ( rayon grade ) [10]. Concentrations of 73% and 100% sodium hydroxide (see details, Section 7.5) are also marketed. 

Mercury cell caustic soda liquor is a high-grade product, low in chloride and chlorate impurities and, because of its traditional associations, known as Rayon grade. It is produced directly by amalgam decomposition at the strength at which it is sold. This usually contains 46.8% NaOH (i.e. 100Tw caustic liquor). 

Liquid caustic soda, 50% and 73% solutions, is available in the following regular grades standard, low-iron, rayon and mercury-cell. The rayon and mercury-cell grades contain lower percentages of sodium chloride than do the standard and low-iron grades. 

The high-performance rayons overcome this disadvantage. The HWM fiber has cotton like mechanical properties and a caustic resistance that allows mercerization. It is compatible in blends with all grades of cotton where it adds strength, improved luster and appearance, and a softer hand. In blends with nylon, polyester, acrylics, and triacetate, it has good strength retention after resination, and the blended fabrics have superior wash-and-wear performance and resistance to pilling. 

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