Recovery of alkali from Mercerizing
During the mercerisation process, cotton yarn or fabric (mainly woven fabric but also knit fabric) is treated in a solution of concentrated caustic soda (270 - 300 g NaOH/l, or also 170 - 350 g NaOH/kg textile substrate) for about 40 - 50 seconds. The textile substrate is then rinsed in order to remove caustic soda. This rinsing water is called weak lye (40 - 50 g NaOH/l) and can be concentrated by evaporation for recycling. The principle is shown in the figure below.
After removal of lint, fluff and other particles (using self-cleaning rotary filters or pressure micro-filtration), the weak lye is first concentrated, for instance in a three-stage evaporation process. In many cases, purification of the lye is required after evaporation. The purification technique depends on the degree of lye contamination and can be simple sedimentation or oxidation/flotation with injection of hydrogen peroxide.
Main achieved environmental benefits
The alkaline load of waste water is reduced drastically and acid required for waste water neutralisation is minimised.
The concentration of weak lye is usually 5 - 8 °Bè (30 - 55 g NaOH/l) and is increased to 25 - 40 °Bè (225 - 485 g NaOH/l), depending on the mercerising process applied. When mercerisation is carried out on the greige dry textile substrate (raw mercerisation) it is possible to achieve a concentration of caustic soda not higher than 25 - 28 °Bè, whereas a concentration of 40 °Bè can be obtained in non-raw mercerisation. In raw mercerisation, the concentration of impurities is significantly higher, as is viscosity, which makes it difficult to reach higher concentrations (circulation in evaporators is disturbed).
The higher the number of stages for evaporation, the more often the heat is re-used, the lower the steam consumption and, therefore, the running cost. Investment, however, obviously increases with the number of stages.
Evaporation requires approximately 0.3 kg steam /kg water evaporated in a 4-stage evaporation plant. This corresponds to 1.0 kg steam/kg of recovered NaOH at 28 °Bé or 1.85 kg steam / kg of recovered NaOH at 40 °Bé.
Representation of the caustic soda recovery process by evaporation followed by lye purification
The technique is applicable to both existing and new installations.
Due to the action of active oxygen generated by the decomposition of hydrogen peroxide it is possible to recover and decontaminate colored alkali for re-use (hydrogen peroxide is already used in the water stream when applying the oxidative route).
Investment costs mainly depend on plant size and purification technique and typically vary from 200000 to 800000 euros. Payback time depends on plant size and operating time per day. Usually, if mercerisation is practised full-time, the payback period is less than one year. In companies where non-recovered caustic soda lye has to be neutralised with acid, payback time is less than six months. Thus, from the economic point of view, caustic soda recovery may be very attractive.