The LPS process is excellently suitable for purification of caustic soda recovered
from weak lye (wash water, effluents) of the following mercerizing processes:

• mercerizing of desized and washed cloth
• mercerizing of scoured and washed cloth
• mercerizing after partially bleached cloth
• mercerizing of yarn dyed cloth

Purification in the LPS unit

Recovered caustic from mercerizing processes of yarn dyed goods can be purified effectively in the LPS unit, above all if it contains oxidizing dyestuffs.

The purified recovered caustic soda (recovered lye) can be employed in as wide a variety of applications as desired, e.g. the impregnation of gray cloth, alkaline treatment, mercerization, scouring and bleaching. The coloration of the recovered
caustic soda solution, varying from a nice yellowish to a coffee brown, has absolutely no influence, neither negative nor positive, on the effect and result of the mercerizing process and the quality of the fabric. Thorough tests under industrial process conditions by customers, plus the numerous users of caustic recovery plants around the world, have proven this. These natural colorants are washed off in the normal washing section of the mercerizer.

Undesired are impurities that increase the viscosity, for example, as this would not effect only the evaporation, but also the mercerizing effect. Accumulation of wax, paraffin, and the like is undesired as such could, at higher concentration levels, affect the wetting degree of the fabric mercerized. Negative are also remaining sizing agent washed out in the mercerizer, or substances that tend to foaming, and the like.

An undesired reaction of NaOH with CO₂ (carbon dioxide) leading to formation of Na₂CO₃ (soda ash) cannot be avoided if the strong lye is exposed to air, which is the case in most mercerizers. The carbon dioxide, even little in the air, reacts with the sodium hydroxide. The solubility of soda ash in strong caustic solution is much lower as compared to its solubility in water. Therefore, soda ash starts to precipitate as solid particles and to settle at the bottom of the precipitator or the settler of the LPS unit.

The process in the LPS unit
Purification is effected chemically and physically with the aid of hydrogen peroxide (H₂0₂). This chemical, well known from the bleaching process of cotton, is dissociated under alkaline conditions to form water (H₂0) and oxygen (0₂). Peroxide is added to the caustic soda feed and circulated over the loop reactor chamber during sufficient time and then pumped into the relaxation and flotation chamber.

The nascent oxygen ( "status nascendi") is created in form of extremely fine "bubbles", leading to a large specific surface and therewith to an excellent attack of contaminants. Besides, the oxygen is created directly on the surface of contaminants and therefore attached to very fine particles, as physically created air bubbles never could.

The so lightened particles and emulsified drops raise to the surface where they form a foam sludge layer. The relaxation chamber separates both the floating sludge layer and the settling sludge from the strong lye.

Cleaning effect
The separation of impurities such as size, fats, fiber remains and hemi cellulose is practically complete after treatment in the LPS unit. Pectin and dyestuffs leave a coloration of the lye, i.e. even very small concentrations of pectin or dyestuff obviously result in a coloration, which does, however, neither harm the mercerization process nor the quality of the textile product at all. KASAG has been gathering experience with a large number of differently contaminated mercerizing lye.

Vat dyes may become water soluble in the mercerizer, under alkaline conditions with size residues acting as reducing agent. In the LPS these vat dyes are oxidized again and made insoluble and removed. Other dyestuffs are also ecreased in the LPS.

Operating data

• caustic temperature 25-45 ^oC
• caustic concentration 35-40 ^oBé
• peroxide consumption
  (depending on contamination)
  3 - 7 ml H₂0₂ (50%) / liter lye
• rinsing water cons. (3 bar) 100 l/h
• air consumption (6 bar) 20 l/h
• power requirements 2-3 kW

Standard equipment of the LPS unit

The peroxide purification unit LPS comprises the following equipment

• feeding tank for recovered caustic soda, with level control
• feeding tank for peroxide, with metering pump and level control
• tank for purified caustic, with level control
• mixing and reacting unit with all necessary fittings
• relaxation chamber with automatic scum removal device
• 3 pumps, all valves, fittings and monitoring probes
• control panel
• support structure

The LPS is a completely pre-assembled and skid mounted unit in compact design,
suitable for container shipment ; no erection work is needed on site.

Available are models LPS-400 to LPS-2400 for various throughput of caustic soda solutions