What to do about salt?

16 02 2011

Last week we talked about the use of salt in textile dyeing.  We always say the textile industry uses a LOT of three resources: water, chemicals and energy.  The use of salt (a chemical – benign, essential for life, but a chemical nevertheless) bumps up the other two considerably.   And though the salt itself is not expensive, using less salt delivers substantial benefits to the mill because the fabric requires less rinsing in hot water (and hence reductions in energy and water) as well as cost savings of up to 10% of the total process costs.[1] So we promised to look at options available to avoid salt.

To recap:

When fabrics made of cotton, linen, hemp or viscose are dyed,  they’re immersed in water which contains dyes which have been dissolved in the water.   These dye chemicals are usually reactive dyes which require  the addition of salt  to “push” the dyes out of solution and into the cloth.  The salt acts like a glue to hold the dye molecules in place.  But the percentage of dye that moves from the dye bath into the fiber, and permanently bonds with the fiber (called the fixation rate) is very low.  For conventional reactive dyes, the fixation rate is often less than 80%, resulting in waste of dyestuff, and also the need to remove that 20% from the fabric.[2] But this is incredibly difficult when the “unreacted” dyes are still “glued” onto the fabric by salt.  So vast amounts of water are required  to simply dilute the salt concentrations to a point where it no longer acts as glue.

There are a few things that mill owners can do:  simple process optimization can easily reduce salt concentrations in dyebaths by 10 to 15%.  Another simple method is to reduce liquor ratios (which is simply the ratio of water to fabric in a dyeing process).  It’s easy to see that using 10 gallons of 100 oz/gal of salt uses less salt than using 5 gallons of 100 oz/gal of salt.

There are also some “low salt” dyes that have appeared on the market.  These dyestuffs  require less “glue” to fix to the fibers.  Ciba Specialty Chemicals, a Swiss manufacturer of textile dyes (now part of BASF) produces a dyestuff which requires less salt.  As the company brochure puts it:  “ Textile companies using the new dyes are able toreduce their costs for salt by up to 2 percent of revenues, a significant drop in an industry withrazor-thin profit margins.”  However,  we’re told they’re not used because of uncompetetitive pricing.  (Remember, it’s all about the cost!).

Another alternative is to recycle the salt.  The effluent can be cleaned and the salt recovered through an energy intensive process to evaporate the water.  But the carbon footprint takes a beating.

We’re back to square one: to use less salt.

And that usually means we have to look to the dyeing machines.  There are low-liquor-ratio (LLR) jet dyeing mcahines that are based on the principle of accelerating water through a nozzle to transport fabrics through the machine.  They are designed to operate efficiently and at high quality with a very low ratio of water to material.  Although these types of machines have been used for over 40 years, recent technological advances have reduced water requirements so that liquor ratios of 8:1 and even 4:1 are possible, with average water consumption of less than 50 liters per kilogram of knit fabric.  Yet there is still salt infused effluent which must be treated.  And these new ultra low liquor ratio machines are very expensive.

What about using no salt at all?

There are two ways to dye fabrics without salt:  “continuous dyeing” and “cold pad batch dyeing”.  Continuous dyeing means that the dye is applied with alkali to activate the dye fixation; the fabric is then steamed for a few minutes to completely fix the dyestuff.  Cold pad batch dyeing applies the dyestuff with alkali and the fabric is simply left at room temperature for 24 hours to fix the dye.

Both of these methods don’t use salt, so the unfixed dye chemicals are easier to remove because there is no salt acting as the “glue” – and therefore less water is used.  And an additional benefit is having a lower salt content in the effluent.

So why don’t companies use this method?  Continuous dyeing requires investment in big, expensive machines that only make environmental sense if they can be filled with large orders – because they use lots of energy even during downtime.

Cold pad batch machines are relatively inexpensive to buy and run, they are highly productive and can be used for a wide range of fabrics.  Yet only 3% of knitted cotton fabric is dyed in Asia using cold pad batch machines.

Why on earth don’t these mills use cold pad batch dyeing?  I would love to hear from any mill owners who might let us know more about the economics of dyeing operations.

[1] “A Practical Guide For Responsible Sourcing”, The National Resources Defense Council (NRDC), February 2010.