ALKALI MANUFACTURE - MANUFACTURE OF CAUSTIC SODA Caustic soda can be produced in two separate ways by the conversion of soda ash direct, or by subjecting brine to the action of the electric current (electrolysis) whereby caustic soda is produced simultaneously with chlorine. Two methods may be used for the direct conversion of soda ash, that of Lowig, a now obsolete process, and the lime-caustic process.
Inthis process a solution of soda ash is treated with lime, either in the form of milk of lime or by direct introduction of quicklime. The ensuing reaction causes a "double decomposition" to take place, which produces a solution of caustic soda and a precipitate of calcium carbonate.
(soda ash) (slaked lime) (caustic soda) (calcium carbonate) This reaction does not proceed to completion at once nor does the precipitate settle readily; so that a large part of the plant, af ter the reaction vessel, consists of time vessels and settling vats. After the liquor has passed through these, and the residual cal cium carbonate has been filtered out, it passes to a series of vacuum evaporators, working usually in two or three stages, where it is concentrated by steam heat to a degree that is limited by the economics of the method. The final stage of the concentration, which results in the complete elimination of all water, is accom plished in large hemispherical iron pots heated underneath by direct firing by coal or producer gas. The final product is molten caustic soda which is run or ladled off into thin iron drums where it solidifies (see fig. 7).
As the conversion of soda ash can never be taken to absolute completion whatever method be employed (the reaction being a "balanced" or "reversible" one), means must be found for the removal of unchanged ash and impurities. These "salts" separate out in the earlier stages of the concentration of the liquor. The precipitated calcium carbonate (chalk) may either be sold, after drying, for agricultural use, or may be calcined and returned to the first stage of the process.
Thismethod is entirely different and depends upon the effect produced when a current of electric ity is passed through brine. This is more particularly described under the heading CHLORINE IN INDUSTRY (Q.V.). The effect, in short, is to produce simultaneously a solution of caustic soda, chlorine gas, and hydrogen gas. The caustic solution is treated as already described; the chlorine is piped off to subsidiary plants; while the hydrogen may either be allowed to escape into the air or may be converted to ammonia or hydrochloric acid. This method is finding increased application and the caustic soda now made by it forms an appreciable proportion of the total: at the same time it is evident that the output is limited by the extent to which it may be found possible to dispose of the concomitant chlorine.
Animportant consumer of caustic soda is the artificial silk trade which uses it for digesting and purifying the cellulose which forms the basis of the industry. One-third or more of the total output of caustic soda is used for this purpose, no account being taken of the caustic soda made by soapmakers themselves. The largest consumer of caustic soda is the soap industry, in which the caustic is used to decompose the animal
and vegetable fats, with liberation of glycerine and production
of the sodium salt of the acid of
the fat; this sodium salt is in
fact soap of the "hard" variety.
"Soft" soaps contain potash in
place of soda. The textile
try uses caustic soda for a variety
of purposes, one of the more
miliar being the process of
cerization" by which cotton fibre
can be made to take on a gloss
somewhat resembling silk. Soap
and textiles account each for
about one-sixth of the total
sumption; and paper and oil
refining for about one-eighth
combined-. The remainder is split
up among a very large number of
minor uses. Fig. 8 gives a graphic representation of these proportions, which, as for soda ash, are necessarily only approximate.
