Caustic Soda

Caustic Soda

Caustic soda is made from soda-ash, or from the "tank liquors" directly, by adding calcium hydroxide (milk of lime) to the solution : -

   Na2COa +Ca(OH)2 = CaCOa+2 NaOH.

When caustic soda is the ultimate product, it is generally customary to use this lime mud (CaCO3) instead of limestone, in the charge for the black-ash furnace, for the formation of caustic in the tank liquor is then of course not objectionable.
The tank liquor must not have a density of over 20° Tw. (1.10 sp. gr.), or it will attack the calcium carbonate formed, and cause a partial reversion of the reaction. Consequently it is diluted with the wash waters from the lime mud of a previous operation. The liquor is then heated to boiling, and run into large iron tanks, wher8 the "milk of lime" is added, and the mixture well stirred. Air or steam is usually blown into the liquor to assist in the mixing. The air, especially when aided by the addition of "Weldon mud", converts the sodium sulphide to sodium thiosulphate and sulphate:-

   2 Na2S +H20 +4 0 = 2 NaOH +Na2S202

The thiosulphate is afterwards destroyed by oxidizing it to the
sulphate.
The solution of caustic soda is allowed to settle and is drawn
into cast-iron kettles, which are heated by direct fire until the water is evaporated and the caustic soda remains as a fused mass. Some nitre is then added, or air is blown in to complete the oxidation of any thiosulphate to normal sulphate, which remains in the caustic, reducing its strength. To make very strong caustic, zinc oxide is often used to remove the sulphide from the tank liquors : -

  Na2S+ ZnO +H20 = 2NaOH + ZnS.

The precipitated zinc sulphide is settled out, before evaporating the caustic liquor. By calcining the zinc sulphide, the zinc is reconverted to oxide.
Sometimes the Yaryan system is used to evaporate the dilute caustic soda solution till it reaches a density of 60° Tw., at
which point the other salts, such as sodium carbonate, which are dissolved in the caustic liquor, begin to crystallize; the liquor is then transferred to the open pan and the evaporation continued, the salts being raked out as they separate.
   The fused caustic soda is run directly into the sheet-iron drums in which it is sold. These are sealed as soon as cold, to prevent the absorption of moisture by the caustic.
  Loewig's process for caustic soda depends on the formation of sodium ferrate (Na2Fe204), which is then decomposed with water. The soda liquors are mixed with ferric oxide, and the mass evaporated to dryness and calcined at a bright reel heat, usually in a revolving furnace. By the calcination, a reaction between the sodium carbonate and the iron oxide is brought about, carbon dioxide escaping and sodium ferrate remaining in the furnace. The mass is washed with cold water until all soluble matter is removed; then water at 900 C. is run over the sodium ferrate, by which it is decomposed, caustic soda formed, and iron oxide regenerated; the last is returned to the calcining process. '1'he ferric oxide used is a natural iron ore, very clean and free from silica or other impurities; that made by calcining a precipitated ferric hydroxide is not well adapted to the process, as it gives a product difficult to lixiviate. 1'he density of the resulting solution of caustic is about 620 Tw. (1.31 sp. gr.), and so less evaporation is necessary than in the lime process, where the density is only 150 or 200 Tw. Moeover, there are no other salts present, such as sulphate, thiosulphate, sulphide, or chloride, and the product is purer than that yielded by the lime process. But Loewig's process is not so well adapted to use with the Leblanc soda-ash, because the tank liquors must be evaporated to dryness before calcining the ferric oxide and sodium carbonate mixture, and the sodium carbonate must be quite pure. The process may be advantageously used with ammonia soda-ash, since this is obtained directly as a solid and no evaporation is necessary. Caustic soda of better quality can be made by Loewig's method, but it cannot be made so cheaply as by the use of lime with the tank liquor of the Leblanc process, especially in small works where the output is irregular and uncertain. For although there is no expense for lime, and less fuel is used for evaporation in the former method, yet an extensive and somewhat costly plant, designed to reduce labor to the minimum, is required, and considerable fuel is needed for the calcination.
  For the preparation of caustic soda by electrolysis of brine, see later on.