Minimization of padding liquor losses in denim dyeing

 

Technical description

 

Denim is mainly attributable to the attractiveness of its blue colour (mainly indigo and some indigo derivatives) and the finishing effects developed by various techniques. The blue colour characteristic of denim fabric is related to cotton warp yarns that are dyed by indigo, the most widely used vat dye.

There are 4 types of denim dyeing methods with Indigo which include the following:

  • Rope/Ball warp/Chain dyeing (Figure 1)
  • Slasher/Open warp/Sheet dyeing (Figure 2 and 3)
  • Loop dyeing
  • Beam dyeing

For dyeing denim yarns are passed through padding liquor in the dye boxes which can be seen in the following diagrams.

Figure 1: Typical scheme of a rope dyeing range. Image source: Meksi, N. (2015). Denim || Indigo dyeing technology for denim yarns.

Figure 2: Typical scheme of a slasher dyeing range. Image source: Meksi, N. (2015). Denim || Indigo dyeing technology for denim yarns.

Figure 3: Typical scheme of a slasher dyeing range. Image source: Meksi, N. (2015). Denim || Indigo dyeing technology for denim yarns.

In denim dyeing process, almost 100% of used padding liquor is wasted; in number which is around 12-24 m3 of liquor daily.

The main theme of the technology is to collect the used padding liquor in a tank and to reuse it at the padding baths.

Figure 4: Indigo dye bath. Image source: Meksi, N. (2015). Denim || Indigo dyeing technology for denim yarns.

The used padding liquor is collected in the storage tanks (Figure 5). The same is true for the padding liquor containing sulphur dyes where indigo dyeing is combined with dyeing with sulphur dyes (bottom or top dyeing). The tanks are connected to the padding baths through pipes and pumps. After the dyeing of a batch is finished, the remaining padding liquor is pumped to the reservoir tanks and stored for future use. For this purpose, the padding liquor is analyzed (Figure 5). Later the liquor is again slowly pumped to the relevant padding baths to be used with the virgin liquor.

Achieved environmental benefits:

  • Chemical saving
  • Water conservation
  • Groundwater protection
  • Reduction in hydraulic load of wastewater treatment plant

Operational data:

The feasibility of dye liquor reuse technology depends on the nature of the dye used and the shade required on the yarn.

However, if the shades required are much more varied and unpredictable, it would make reuse difficult. But, under the right conditions, dye liquor could be reused up to 10 times before the level of impurities limits further use (Rather, 2019).

The implementation of this technique will significantly reduce water, chemical consumption and wastewater generation. The water use can be reduced in the range of 12-24 m3/d (4380-8760 m3/year) and its value is USD 2000 ~ 3,000/yr. Moreover, the annual chemical savings achieved by this technology values USD 400,000 ~ 600,000. The potential of savings can vary from industry to industry.

Cross-media effects:

  • None identified.

Technical considerations relevant to applicability:

This measure is applicable to all denim fabric manufacturing industries where the yarns are dyed with indigo in padding methods.

This measure does not require any major intervention in the process, no major infrastructure development, and no technical human resource requirement.

The technique (storage tank, pumps, electrical motor, valves, electrical panel, pipelines) is simple easily available, and implementable.

The investment cost will be USD 6,660-18600 depending upon the amount of used liquor collection, storage, and conveyance.

Economics:

Capital cost = USD 18,600

Annual saving = USD 517,500

Annual O&M cost = USD 150,000

Simple payback period = < 1 month

An additional benefit will be the reduction in hydraulic load and treatment cost of wastewater treatment plant.

Driving force for implementation:

  • Financial benefits
  • Resource conservation
  • Sustainability of textile business

Reference industry:

Reference:

  • Based on the industry experience of Pioneer Denim Ltd., Shahjibazar, Bangladesh.
  • Rather, Luqman Jameel (2019). Water in Textiles and Fashion || Advances in the sustainable technologies for water conservation in textile industries, https://www.sciencedirect.com/science/article/pii/B9780081026335000105
  • Meksi, N. (2015). Denim || Indigo dyeing technology for denim yarns, doi:10.1016/b978-0-85709-843-6.00004-4


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