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.


What can be considered the “good” chemicals in textile processing?

9 02 2011

We’re often asked if ALL the chemicals used in textile processing are harmful.  And the answer is (surprisingly maybe)  no!   Many chemicals are used, many benign, but as with everything these days there are caveats.

Let’s look at the chemical that is used  most often in the textile industry:  salt.  That’s right.  Common table salt.  Safe, natural salt is used in textile dyeing.

Salt shaker painting by Jeff Hayes

The way the dyestuff bonds to the fibers is very important – and the most permanent, wash fast dyes are the most tightly attached to the fiber molecules (called reactive dyes).  Here’s how salt comes into the picture:

To dye a fabric made of a cellulosic fiber (i.e., cotton, hemp, linen) or its close cousin (viscose),  the fabric is put into water, where its surface gets covered in negative ionic charges.  The reactive dyes used most often to dye cellulosic fabrics also develops a negative charge, so the fibers actually repel the dye – like two magnets repelling each other.   If we try to dye a cellulosic fabric without using  salt, the dye molecules just roll off the surface of the fibers and the fabric does not show much color change.

But when salt is added to the water, the solution splits into positive sodium ions (Na+) and negative chlorine ions (Cl-).  The  positive Na+  ions then dive into the surface of the fabric to neutralize the negative charge.

The dye molecules are then attracted to the fiber by weak Van der Waals forces and as the dyes get close to the fiber molecules, the salt acts like a glue to hold the dyes in place.  If we add alkali, the dyestuff will permanently grab hold of the fiber and become a part of the fiber molecule rather than remaining as an independent chemical  entity.

The color fastness of reactive dyes is so good that  it’s no wonder that they have become so widely used.  And natural salt has been crucial to their success.

We sprinkle salt on our foods – indeed salt is essential for life itself.  But (there is always a “but”) the “dose makes the poison”  – and the textile industry uses a LOT of salt!

The concentrations to suppress those negative ions can be as high as 100 gm per liter.  In the worst cases, 1 kg of salt is used to apply reactive dye to 1 kg of fabric.  Think of the billions of yards of fabric that’s produced each year:   In Europe alone, 1 million tons of salt is discharged into our waterways each year.[1] In areas where salt is discharged into the ecosystem, it takes a long, long time for affected areas to recover, especially in areas of sparse rainfall – such as Tirupur, India.

Tirupur is one of the world’s centers for clothing production , home of 765 dyeing and bleaching industries.  These dyehouses  had been dumping untreated effluent into the Noyyal River for years, rendering the water unsuitable or irrigation – or drinking.   In 2005, the government shut down 571 dyehouses  because of the effluent being discharged into the Noyyal.  The mill owners said they simply couldn’t afford to put pollution measures into place.   The industry is too important to India to keep the mills closed for long, so the government banned the discharge of salt and asked for an advance from the mills before allowing them to re-open.     But … on February 4, 2011, the Madras high court ordered 700 dye plants to be shut down because of the damage the effluent was doing to the local environment.  Sigh.  (Read more about Tirupur here.)

Unfortunately, the salt in textile effluent is not made harmless by treatment plants and can pass straight through  to our rivers even if treated.  This salt filled effluent can wreak havoc with living organisms.

There are some new “low salt” dyes that require only half the amount of “glue”, but these dyes are not widely used because they’re expensive – and manufacturers are following our lead in demanding ever cheaper fabrics.

Recycling the salt is possible, and this has been used by many of the dyers in Tirupur, and elsewhere, who operate zero discharge facilities.  The effluent is cleaned and then the salt is recovered using an energy intensive process to evaporate the water and leave the solid, re-useable salt.

This sounds like a good idea – it reduces the pollution levels – but the carbon footprint goes through the roof, so salt recovery isn’t necessarily the best option.  In fact, in some areas of the world where water is plentiful and the salt can be diluted in the rivers adequately, it may be better to simply discharge salt than to recover it.

But the best option is to avoid salt altogether.

Next week we’ll look at how to do that.

[1] Dyeing for a change: Current Conventions and New Futures in the Textile Color Industry (2006, July) www.betterthinking.co.uk