Denim Pollution – Solutions To Sulphur Dyeing Wastes

denim dyeing wastes

                        This is a guest post by Harry Mercer
The problem of treating wastewater from denim dye waste is significant in many developing countries. Either the costs of treating these wastes affects profit margins or the waste goes untreated and is discharged directly into rivers.

The most serious pollution problem results from the use of sulfur dyes in denim. Once the sulfur dye is applied the cotton is squeezed and washed. Typically, 50% or more of the sulfur dyes are washed off, producing water contamination that is difficult to remove from water.

Techniques have been developed to greatly reduce and even eliminate sulfur dyes as a source of textile pollution:

  • Unlike most types of cotton dyes, sulfur dyes can be recovered and re-used. The contaminated water from sulfur dyeing usually contains enough dye and chemicals to produce lighter shades without additions of dye or chemicals. For example, the wash-water from sulfur-black dyeing can be concentrated through evaporation, filtration or it can be re-used by adding enough dye and chemicals to produce the standard shade. There are methods for titrating sulfur dye with copper sulfate, reducing agents can be added to bring the ORP to the correct level and the alkali can be titrated with a 2-endpoint titration with HCl and formaldehyde. Using these techniques, the sulfur dye can be recycled which saves money and ends the pollution. It should be noted that sulfur dyes precipitated with an acrylic type flocculant cannot be re-used.
  • I have developed methods for sulfur dyeing that fix the dye completely, so that none is lost in washing, leaving the wash water completely clear. Depending on the quality of the dye,  which reducing agents are employed and the pH of dyeing, various inexpensive buffers can be employed that will fix 100% of the dye which eliminates the colored waste and also reduces the amount of dye needed by a sulfur black shade by 50% or more.These  methods were developed over a 20 year period and have employed them  in Brazil, Ecuador, Indonesia,  Phillipines and Thailand.
  • There are also cold-dyeing methods that also eliminate the dye waste 100%, saves energy and has colorfastness approaching that of a vat black.
  • Reducing agents are a major source of Chemical Oxygen Demand. Sodium dithionite sold commercially as sodium hydrosulfite has a COD of 0.22 kilograms per kilogram of 100% hydrosulfite. Since it is normally produced using zinc metal as a catalyst, there is also  some presence of heavy metal contamination. There are non-polluting substitutes that can be used with many Indigo-dyeing methods. Sodium sulfide-type reducers are commonly used, however they can be easily replaced for hot-dyeing methods with reducing sugars which includes sugar, dextrins (corn-sugar) and molasses. These have been offered by chemical companies for decades and basically break down under conditions of high temperature and high pH (11) into hydrogen and alcohol, which both escape into the air.
  • Sulfur blacks do not require chemical oxidation, in fact peroxide will reduce the colorfastness of sulfur blacks and acid-oxidation will weaken the yarn. – After applying sulfur black, it should be allowed to cool in air which takes the dye out of reduction and allows oxidation with atmospheric oxygen. The initial washing should be  cold in order to avoid washing off any color and also to allow further oxidize with  oxygen-rich cold water. The final wash should be warm, about 60 degrees, which will  remove alkalies and residual reducing agents. If noticeable color is removed, the dyeing procedure requires adjustment.

 

imageThis is a guest post by Harry Mercer. Mr. Mercer has 30 years experience in the denim business including 3 prominent U.S. denim companies.He is an expert colorist for measurement and color matching as well as textile testing.

  1. “however they can be easily replaced for hot-dyeing methods with reducing sugars which includes sugar, dextrins (corn-sugar) and molasses”… It is a theoretical talk. Have you ever tried with this method? The defects of this process will be 1) Low wet fastness, 2) Exhaustion of dye bath is very poor, and if you want zet-black then what percentage of dyes to be taken and most probably you have to repeat the dyeing process.
    Please excuse for this comment but sir it is different in commercial production than your laboratory trial for a small quantity trial.

  2. Hello Harry,

    I have been regularly reading mail & Quotes.

    On the specific subject, I want to know following:

    a. Importance of Redox Potential for Denim or Vat Dyeing.

    b. It can be acheived generally with Sodium Hydrosulphite.. Can you suggest any other ingrediants required to be added with hydrosulphite to increase and optimise REDOX value in the dye bath to get better shade of Indigo.

    c. Can any Suger/Dextros be added with hydrosulphite before dosing into dye bath?

    Please advise.

    Thanks, Niraj
    Best Regard,

    Niraj Sandesara
    —– Original Message

  3. Hi Amitabh:
    I take it that you are speaking of batch dyeing in package machines, jiggers etc. Dextrins have been used for many years in dyeing sulfurs and is especially easy in batch dyeing. Companies like Clariant have been selling dextrins like Reducer RDT for decades and they recommend it in order to reduce pollution that is caused by sulfur-containing reducers. Around pH 11 and at 85 degrees they produce reduction. I have been in some countries where dextrins are the only reducer used for sulfurs.
    Sulfur dyes have long been known for excellent wetfastness when the dyeing conditions are correct, which is not ususally the case in the last 20 years or so.
    I have presented many lectures around the world on dye chemistry and practice and if you are interested in dyeing theory, try to find a copy of “The Physical Chemistry of Dyeing” by Thomas Vickerstaff. It is a classic, but long out of print.
    I have used these procedures in production over many years,in the U.S. Brazil, Indonesia,Ecuador- but only a limited number of companies have taken advantage. Dyers are very difficult to change procedures. Actually, one of them, cold dyeing of sulfur black, I picked up from craft dyers in India who made these wonderfully dark and colorfast borders for saris. They understood more about sulfur dyeing than any dyeing manager I have seen. They did not waste dye.
    I learned the most valuable lessons about sulfur dyes as a dyeing superviser and researcher. I did conduct at least 1000 experiments in sulfur dyeing in the laboratory and proved that much of the guidande offered by dye companies is highly defective.

    Regards,

    Harry Mercer

  4. Hi Niraj:
    There are definitely some additives that will improve Indigo dyeing, but they vary depending on the type of Indigo dyeing. With powder Indigo, there are some additives that will improve shade consistancy and greatly reduce hydro use.There are some that will allow the use of much less Indigo for a dark shade. The pre-reduced Indigo is a bigger problem, but that depends on whether the supplemental hydro feed is as a liquid or powder.
    The reduction potential is critical for anthraquinoid vats, since different general structures require different reduction potentials for good dyeing. The reduction potential also determines the general colorfastness. With Indigo, low redction potential (-720/750) produces a darker, redder shade with lower amounts of Indigo, but it washes easily and the consistancy is not as good. At higher levels (-800/830) the shade is lighter with the same amount of Indigo, greener, but more colorfast and consistant.

    Regards,

    Harry Mercer

  5. Dear Harry,

    Thanks for your below feedback.

    I understand from your message that for Indigo Dyeing on Denim with Powder Indigo and to get better & consistent shade the preferred REDOX Range is between (-800/830).

    Using available Sodium Hydrosulphite we are getting REDOX in the range of (-710/750). What other additives can be added into Sodium Hydrosulphite to increase Redox upto -800

    Please Advise.

    Best Regards,

    Niraj Sandesara

  6. @Niraj Sandesara
    Hi Niraj:
    You can achieve Redox of around -950 with hydrosulfite. There must be a serious stability problem with your process. Send me the details of the machine type, number of boxes, speeds, abieny temperature, immersion time, % caustic(2-endpoint titration with formaldehyde and 0.1N HCl
    stock mix formula and mixing procedure and supplemental hydro feed. There is a buffer that can be added for extreme stability. The only source I know of currently is made in Indonesia by an Italian chemical company called Bozzetto. I think that the name is Isopon. E-mail me at Hme3065805@aol.com and I will do an analysis.

    Regards,

    Harry Mercer

  7. Hi Herry,

    Thanks for your message.

    Denim Manufacturers wanted to achieve max Redox with SHS to get better shade and stability. I do agree with you that SHS (hydrosulphite) is not the only one about stability but of course We can offer a solution to them as you advised.

    Is there any additive to be added in / alongwith SHS which can help them to enhance the performance of this product against standard SHS.

    Please suggest suitable additive to offer better solution to denim manufacturers.

  8. Hi Niraj:
    The fundamental problem is that hydrosulfite is unstable and breaks down into sodium bisulfite and sodium sulfite. A lot of hydrosulfite breaks down in the mixing tank because of exposure to air, excessive agitation or concentration. There are 2 types of hydro loss in the mixing tanks: aerobic, which is caused by exposure to air and anerobic which is a result of excessive concentrations, greater than 80 g/l. Additional hydro losses occur in the dye boxes because of air exposure at the surface, oxygen carried in by yarn and by high recirculation pressures. More boxes mean more hydro loss. Also the size of the box has an effect. Small Indigo boxes have a higher ratio of surface area to volume and as much as 50% of total hydro that arrives to the machine are lost in small boxes, but in the large boxes of most rope ranges the loss is as low as 15%. Those losses are in addition to losses in mixing, so 70-90% of the hydro is not made available for dyeing. The losses at the dye box surface is the reason that some machinery companies recommend using nitrogen to protect the surface of the boxes. Many years ago a company in Taiwan put ping-pong balls on the surface of the dye boxes. High Indigo foam on the boxes also reduces the hydro loss, but results in more crocking. In cases where powder feeders are used to deliver additional hydro, there are losses there as well since the hydro breaks down into something like meta-bisulfite. Therefore practices are important. The hydro is more stable with higher concentrations of sodium hydroxide.
    Sodium sulfide added at about 10 grams per liter to the mix will also stabilize. Sodium thiosulfite
    has been also used in the past. The best stabilizer is available from the Bozzetto company.

    Regards,

    Harry Mercer

  9. sir can i use sodium hrdrosulfite in sulfer dyeing and if yes than why we prefer sodium sulfide in sulfer dyeing

  10. Hi Alpha:
    There are at least 10 different ways to reduce sulfur dyes. The method depends on costs, quality, colorfastness requirements. Sodim hydrosulfite will reduce sulfur dyes, but it is unstable and can present dyeing porblems when used by itself, especially on continuous dyeing where the consistancy of shade is hard to control at normal dyeing temperatures. I have developed a method using a special buffer with hydrosulfite that allows perfect sulfur dyeing with all colors on Indigo machinery.
    That is one of my consulting secrets. It works best on loop machines and I have set up these procedures for 5 denim companies in the last 15 years.
    Dextrin-type reducers are used widely because of lower pollution than sulfides or hydrosulfite and I have used reducing sugars to dye Indigo. The dextrins do not result in the best sulfur dyeing, especially for colors other than black.
    Sulfides are used widwly in Asia because they are cheap. They have their best use for batch dyeing like package or jigs, but should not be used for continuous dyeing because they have low solubility which is important for continuous dyeing where the dye has only seconds to be applied as opposed to an hour or so in batch. The millivolt level of sulfide is only about -580 and continuous dyeing of sulfurs should be conducted at a minimum of -630, which requires other reducing agents.

    Regards,

    Harry Mercer
    Hme3065805@aol.com

    @Alpha

  11. Hi Mercer.

    You mentioned Reductor RDT is dextrin. Then is the Reductor poweder offered by them named as Reductor D Powder same as RDT? If not what is the difference.

    Also you mentioned that Dextrin is not suitable othet than black. Then which are the other eco friendly reducing agents available for Colours other than black for Sulphur dyes?

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