Always Under Construction
by GD Jasuja  October 20, 2011 3:11 am
Caustic Soda Recovery
for Mercerizing Lye
Changing Mercerizing
Waste Water into Money
During the mercerizing process the diluted caustic soda (weak lye) is normally drained. However, several machinery manufacturers have found a way to recover this diluted caustic soda by evaporating water. They have been producing and supplying Caustic Soda Recovery System for mercerising lye to the textile industry. A large number of such Caustic Soda Recovery Systems are successfully working world-wide.
The advantages of caustic recovery are explained in the following example :
Example :
Fabrics for dry mercerizing: 50 000 m/day, width= 2.0 m, specific weight = 150….250 g/m² (average 200g/m² = 0.2 kg/m²). The mercerizing machine is considered to be operating for 20 hours a day.
Quantity of fabrics = 50,000 metres/day (20 hrs day) 
= 2500 metres/day = 2500 m (Length) x 2 m (Width) 
= 5000 sq. metres/day = 5000 x 0.2 (200g/m2 = 0.2kg/m2)
= 1000 kg/day
Hence, 1000 kgs of dry fabrics are processed every hour. Considering pick-up-values of 120% by weight after impregnation, 100% after stabilization and 80% after washing the flow rates can be seen below :
From the above example it is clear that the quantity of alkaline waste water amounts to 4800 kg/h at 8°Bé (5.2%) and its savings can be calculated quite simply as follows :
With the price of caustic soda equalling Rs 20 per kg and a production time of 20h/day, 25 days/month, wastage would amount to :
The savings of Rs. 300,00,000/year are just for caustic soda. Other savings are in the neutralization process, the waste water rates, waste water treatment and the generation of soft water for rinsing. These costs can also be reduced considerably by employing a suitable Caustic Soda Recovery System.
The pay-back time is less than one year!
Caustic Soda Recovery System separates the weak lye (wash liquor) into strong lye and vapour condensate (slightly alkaline soft water). The condensate can be used for pre-washing and the caustic soda can be reused in the mercerizing process.
The Caustic Soda Recovery System requires heating steam and cooling water. The cooling water is heated up to 60°C to 80°C. This hot water generation is a by-product, so some steam for the hot water generation can be saved somewhere else in the factory. The vapour condensate is slightly alkaline soft water with a temperature of approx. 80°C. It can be used for washing, e.g. in the mercerising or bleaching machine, or in other pretreatment. There is no direct contact between the heating steam and the lye, therefore the heating steam condensate can be reused as boiler feed water without cleaning.
Advantages :
Payback-time is less than one year!
No alkaline waste water from mercerising machine.
Generation of hot water from the waste energy.
Generation of soft water, the vapour condensate is slightly alkaline.
Recovery of surplus lye for wet-on-wet mercerizing.
No contamination of the heating steam condensate.
Environmental protection. Less chemicals for neutralisation are needed.
Operating method of the evaporation plant
The weak lye is reconcentrated by water evaporation. The Caustic Soda Recovery System is based on the natural circulation evaporation. The heating steam is condensed on the outside of the tubes and heats the lye inside. The lye boils up in the heating tubes, the mixture of lye and vapour flows into the a laterally arranged separator, where the vapour is separated from the circulating lye. The vapour is used as heating steam in the next stage. A partial vapour flow is used to pre-heat the weak lye. The separated lye flows back to the evaporator through a return pipe. A swirl droplet separator integrated into the separator prevents the alkaline liquid from being carried over into the vapour phase.
The evaporation plant is driven by the pressure gradient between the stages. The highest pressure is in the first stage. The last stage operates under a vacuum maintained by a steam jet vacuum ejector (v) with an after-condenser (ac), or by a liquid-ring-pump. In the first stage (1) live steam generates vapour which flows as heating steam into the second stage (2). The heating steam condensate from the first stage flows back to the boiler. The vapour from the second stage heats the third stage (3). The vapour from the last stage (here the 3rd stage) is condensed with cooling water in condenser (c). So cooling water becomes hot water by utilising waste heat from the last stage. 

The more stages a system has, the less heating steam is required. As textile units need large quantities of hot water, the number of stages of the Caustic Soda Recovery System should be as per the required amount of hot water. Sometimes a 3-stage evaporation plant is more economical than a 4-stage one. 
The above write up has been contributed by Shri Hardik Shah of Embee – a leading Ahmedabad based manufacturer and exporter of a very wide range of textile machineries and spare parts. Embee also manufactures an innovative Caustic Soda Recovery System based on principles similar to those outlined in the above write up. Embee exports to more than 40 countries. Embee's Caustic Recovery Plant offers an economical & efficient import substitution for similar imported systems. It's key features are : 
• Extremely Efficient & Cost Effective
• No Additional Chemical Cost
• Pay Back in 3 to 6 Months
• Easy to Run & Maintain
• All Contact Parts in Stainless Steel
• Heating Medium – Steam/Oil
• A Step Towards Zero Discharge
• Reduces Load On E.T.P.
• Full Capacity Utilization 2/3/4 Effect
• Plant from 2000 LPH to 10000 LPH 
• Eco – Friendly Plant 
HO : 426/A, GIDC, Odhav, Ahmedabad-382 415. India Ph : +91-79-2297 6411,22,33,55 Fax : +91-79-2297 6287
Web :




Mercerizing term is applied to a process, discovered in 1844 by John Mercer, a Lancashire calico printer, which consists in treating cotton (and to a limited extent other plant fibres) with strong caustic soda or certain other reagents, where-by morphological and chemical changes are brought about in the fibre . Thus, if a piece of bleached calico be immersed in caustic soda of 50° Tw. strength (sp. gr . 1.25), it rapidly changes in appearance, becoming stiff and translucent, but when taken out and well washed in running water it loses these properties and apparently reverts to its original condition . On closer examination, however, the fabric is found to have shrunk considerably both in length and breadth, so as to render the texture quite different in appearance to that of the original calico; it is also considerably stronger, and if dyed in the same bath along with some of the untreated fabric is found to have acquired a greatly increased affinity for colouring matters. 
By far the most important application of the mercerizing process is that by which a permanent lustre is imparted to cotton goods; this was discovered in 1889 by H.A. Lowe, who took out a patent for his process in that year, this being supplemented by a further patent in 1890. Since Lowe's invention did not receive sufficient encouragement, he allowed his patents to lapse and the process thus became public property. 
The production of a permanent lustre on cotton by mercerizing is in principle a very simple process, and may be effected in two ways. According to the first method, the cotton is treated in a stretched condition with strong caustic soda, and is then washed, while still stretched, in water. After the washing has been continued for a short time the tension relaxes, and it is then found that the cotton has acquired a permanent lustre or gloss similar in appearance to that of a spun silk though not so pronounced. According to the second method, which constitutes but a slight modification of the first, the cotton is immersed in caustic soda of the strength required for mercerizing, and is then taken out, stretched slightly beyond its original length, and then washed until the tension slackens.
The strength of the caustic soda employed in practice is generally between 55 and 60 Tw . The temperature of the caustic soda has a material influence on its action on the cotton fibre – very much stronger solutions being required to produce the same effect at elevated temperatures than at the.ordinary temperature, while, on the contrary, by lowering the temperature it is possible to obtain a good lustre with considerably weaker lyes. 
















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