Caustic Soda
Recovery & Recycling

A great way to turn a problem into a benefit

Caustic soda recovery is manufacturing
your own chemical, instead of buying it

Multiple effect evaporation
The concentration of sodium hydroxide is increased by evaporation, simply said by boiling off water. To save energy, multiple-effect evaporation is applied. Steam is used to heat the first stage. Water evaporates out of the solution. These vapours of the first stage are used to heat the second stage, and so on for the subsequent stages.

More stages lead to less steam consumption. The more stages there are, the more often the heat is re-used and the lower is the steam consumption and therewith the running cost. Investment, however, obviously increases with more stages.

Re-use of heat
The driving force is a temperature and pressure drop over the stages of the evaporation, with the highest temperature and pressure in the first stage and the lowest temperature and pressure in the last stage. To use the vapours of one stage to heat the next stage, the boiling point in the next stage must obviously be lower. This is achieved, despite the higher concentration, by lower pressure in the next stage. The concentration increases over the stages, the temperature decreases An additional benefit of such a system is that the temperature is lowest at the highest concentration, in the last stage.

This is of particular interest to operate under non-corrosive conditions. The vapours of the last stage are not used for heating another stage, but are used to pre-heat the weak lye feed and are then condensed to liquid phase in a condenser by heating up cold water for process use. The heating energy entering the caustic soda recovery plant in form of steam is not lost, but used for clean hot process water production for the finishing mill, usable as wash water etc.


Vacuum
Vacuum is applied in case of higher concentrations to reduce the temperature of the highly concentrated caustic solution in the last stage below critical corrosion temperature. Vacuum is also required to increase the pressure gradient over a plant with more than 4 stages, to ensure sufficient driving force.

Create profits with a product that you don't need to sell !
You have the raw material : your mercerizing waste water

Purification of weak and strong lye
Cleaning of the weak lye feed is essential for a smooth operation of the recovery plant. Intervals between plant cleaning are increased therewith . For removal of lint, fluff and other solids, the weak lye is passed through a self-cleaning rotary filter. Alternatively, a pressure micro filter with finer mesh is used. By concentrating the caustic soda, also the impurities are concentrated, of course. Purification after recovery is very important, as the recovered caustic is recycled to the mercerising process and for other use.

There must be a constant purge of impurities from the caustic soda process and recovery cycle. Sedimentation is one possibility, however, for higher contamination, KASAG's unique LPS purification unit with hydrogen peroxide is superior.