Potential sources and types of emissions associated with dyeing processes

most of the emissions are emissions to water.

Due to the low vapour pressure of the substances in the dye bath, emissions to air are generally not significant

and can be regarded more as problems related to the workplace atmosphere (fugitive emissions from dosing/dispensing chemicals and dyeing processes in "open" machines).

A few exceptions are the carrier dyeing, thermosol process, pigment dyeing

In pigment dyeing the substrate is not washed after pigment application and therefore the pollutants are quantitatively released to air during drying

Emissions from carriers are to air and water.

Environmental issues related to the substances employed Water-polluting emissions may originate from

  the dyes themselves (e.g. toxicity, metals, colour)

  auxiliaries contained in the dye formulation

   basic chemicals (e.g. alkali, salts, reducing and oxidising agents) and

 auxiliaries used in dyeing processes

 contaminants present on the fibre when it enters the process sequence (residues of pesticides on wool are encountered in loose fibre and yarn dyeing and the same occurs with spin finishes present on synthetic fibres).

 Spent dye baths (discontinuous dyeing),

 residual dye liquors and water from washing operations

% of non-fixed dye that may be discharged in the effluent for principle classes of dyes

Dyes are non-biodegradzable

Some azo dyes may degrade under anaerobic (Absence of oxygen) conditions

Water insoluble dyes (disperse, vat, sulphur, pigment) removed from waste water by

• Coagulation

• Adsorption on biomass/activated sludge

• Water soluble unfixed dyes difficult to remove

• Responsible for colour effluent

Impact of colour in the effluent

• Coloured effluent aesthetically not accepted

• Interupt photosynthesis affecting aquatic life

• Presence of heavy metals give aqueous toxicity

• Liberation of free chlorine may lead to AOX problem

AOX emissions Vat, disperse and reactive dyes may contain halogen in their molecule

Vat Dyes

• High fixation

• Insoluble in water

Removed from effluent to high degree by

• coagulation/settling, adsorption on activated sludge

Reactive dyes •Water soluble

•Low fixation

•Non-biodegradable

Difficult to remove by

• Coagulation/precipitation

• Adsorption on activated sludge

• Usualy removed by destructive techniques such as advance oxidation using Fenton reagent

AOX problem is not serious because

• Halogen usually converted into salt NaCl by reaction with alkali

•Bound halogen if present is not available in the free form

Heavy metal emission

Presence of metals in dyes

• Used as catalyst during dye manufacture

• bound metal in the form of metal complex dyes

Dyes containing Bound metal

• copper and nickel in phthalocyanine groups,

•copper in blue copper-azo-complex reactive dyes

• chromium in metal-complex dyes used for wool silk and polyamide.

•The total amount of metallised dye used is decreasing

• Use of metal catalyst is more harmful than bound metal

toxicity of dyes • Aquatic toxicity

• Human toxicity

• 60-70% of dyes used today are azo dyes

• Under reductive conditions produce amine

•Some amines are found to be carcinogenic

• Dyes producing carcinogenic amines are banned

• Still some of these dyes are available from small scale dye manufacturers

List of carcinogenic amines

Mechanism of Human carcinogenicity• Dye leaching in contact in contact with skin due to perspiration• Entry of dye into body metabolism through skin pores• Breaking of dye molucule• Bioacumulationof amine

Auxiliaries contained in dye formulations

•different additives are present in the dye formulations.

•these substances are not absorbed/ fixed by the fibres,

•they are completely discharged in the waste water.

•additives are not toxic to aquatic life,

• general poorly biodegradable

•not readily bioeliminable.

•This applies in particular to the dispersants present in the formulations of vat, disperse and sulphur dyes.

Other not readily eliminable additives

•anti-foam agents.

Dispersants

•Present water-insoluble dyes and alao used in dyeing

• consist mainly of

•naphthalene sulphonate-formaldehyde

•condensation products and lignin sulphonates,

• sulphomethylation products derived from the condensation of phenols with formaldehyde and sodium sulphite

difference between liquid and powder formulations of dyes

•liquid form contain only one third of the amount of dispersing agent normally contained in powder dyes

reason

• powder dyes: the very small particles generated during grinding must be protected during the subsequent drying process and this is possible only by adding high proportions of dispersing agents.

Proportion of additives for powder and liquid dyes

Basic chemicals and auxiliaries used in the dyeing process

Sulphur containing reducing agents

Waste water from sulphur dyeing contains sulphides from

•reducing agents.

• dye formulation

•sulphide ends up in the waste water.

•Sulphides are toxic to aquatic organisms

•contribute to increasing COD load.

• sulphide anions are converted into hydrogen sulphide under acidic conditions,

• giving rise to problems of odour and corrosivity

Sodium hydrosulphite

•commonly used not only in sulphur and vat dyeing

•reductive after-cleaning agent in PES dyeing.

• Sodium hydrosulphite is less critical than sodium sulphide.

•during the dyeing hydrosulphite is converted into

• sulphite (toxic to fish and bacteria)

• further oxidised into sulphate. In lhe waste water treatment plant.

•Sulphate, cause corrosion of concrete pipes or

•may be reduced under anaerobic conditions into hydrogen sulphide.

Oxidising agents

Dichromate •Banned in Europe as an oxidising agent when dyeing with vat and sulphur dyes,

•Also widely used for the fixation of chrome dyes in wool dyeing.

•Chromium III exhibits low acute toxicity,

•chromium VI is acutely toxic and is carcinogenic towards animals.

•During the dyeing processes with chrome dyes, Cr VI 'is reduced to Cr III

• emissions of Cr VI may still occur due to inappropriate handling of dichromate during dye preparation

•care must be taken as dichromate is carcinogenic

• may cause health problems for workers handling.

bromate, iodate chlorite, hypochlorite

•oxidising agents in vat and sulphur dyeing

• hypochlorite as stripping agent for faulty dyeing

• may produce AOX emissions.

• only hypochlorite and

•elemental-chlorine-containing compounds (e.g. sodium chlorite containing Cl2 as activator for formation of chlorine dioxide gas)

•likely to give rise to hazardous AOX

Salt

Salts of various types are used in dyeing processes

• promote level dyeing

• promote dye erhaustion.

•large amounts of salt are used in cotton batch dyeing processes with reactive dyes.

•Efforts are being made by dye manufacturers to solve this problem

Amount of salt used for exhaust dyeing of cotton with reactive dyes

• salt as raw material,

• neutralization of commonly used acids and alkalis produces salts as a by-products.

• Salts are not removed in conventional waste water treatment systems

• ultimately discharged in the receiving water.

• mammalian and aquatic toxicity of the salt is very low,

• in arid or semi-arid regions their large-scale use can produce concentrations above the toxic limit

• increase the salinity of the groundwater.

• Countries have set emission limits at 2000 ppm or below.

Carriers

• widely employed in the past, for PET dyeing

Presently the use reduced

• ecological and health problems.

• still an issue in dyeing of polyester/wool blend

• Carriers may also be added to the dyes by manufacturers.

•In this case textile finishers will have little knowledge of the loads discharged

•Carriers include a wide group of organic compounds,

• many of them steam volatile,

•poorlv biodegradable

• toxic to humans and aquatic life.

•However, active substances usually have high affinity for the fibre (hydrophobic types),

•75 - 90 % are absorbed by the textile

• only the emulsifiers and the hydrophilic-type carriers such as phenols and benzoate derivatives are found in the waste water.

•The carriers that remain on the fibre after dyeing and washing, are partially volatilised during drying and fixing operations

• give rise to air emissions.

•Traces can still he found on the finished product,

• potential problem for the consumer.

Other auxiliaries of environment interest Other substances that may be used for dyeing may give rise to water pollution

• Fatty amine ethoxylates (levelling agent)

alkylphenol ethoxylates APEO (levelling agent)

quaternary ammonium compounds (retarders for cationic dyes)

polyvinylpyrrolidone (levelling agent for vat, sulphur and direct dyes)

cyanamide-ammonia salt condensation products (auxiliaries for fastness improvement)

Acrylic acid-maleic acid copolymers (dispersing agent)

Metal chelating agents

Ethylene diamine tetra acetate (EDTA)

diethylenetriaminepentaacetate (DTPA)

ethylenediaminetetra(methylenephosphonic acid) (EDTMP)

•diethylenetriaminepenta(methylenphosphonic acid) (DTPMP)

These auxiliaries are

•water-soluble

•hard-to-biodegrade

• pass untransformed or only partially degraded, through waste water treatment systems.

•In addition, some of them are toxic (e.g. quaternary amines)

• can give rise to metabolites which may affect reproduction in the aquatic environment (APEO).

Environmental issues related to the process

Batch dyeing processes

higher water and energy consumption than continuous processes.

factors

•higher liquor ratios involved in batch dyeing

• higher liquor ratios mean not only higher water and energy uses, also

•higher consumption of chemicals and auxiliaries dosed based on the volume of the bath.

all machine manufacturers offer

•machines with reduced liquor ratios.

• "low liquor ratio'‘ machines

•Liquor ratio from 1:5-1:8 for cotton and

•1:3-1:4 for PES.

• ultra-low liquor ratio"

•machines that can be operated at liquor ratios as low as the minimum volume required to completely wet out the substrate and avoid cavitations of the pumps.

•This term applies only to machines for dyeing fabric in rope form.

shade matching

•responsible for higher water and energy consumption,

•especially when dyeing is carried out without the benefit of laboratory instruments.

•bulk of the dyestuff is added to obtain desired shade

• followed by a number of matching operations

•small quantities of dye are applied to achieve the final shade.

•Shades which are difficult to match may require repeated shade additions with cooling and reheating between each addition

Continuous and semi-continuous dyeing processes

•consume less water

•higher dyestuff concentration in the dye liquor.

•In batch dyeing the dye concentration varies from O.I to I g/l,

• in continuous processes this value range from 10 to 100 g/l.

• residual padding liquor in the troughs, pumps and pipes must be discarded when a new colour is started.

•The discharge of this concentrated effluent can result in a higher pollution load compared with discontinuous dyeing,

•especially when small lots of material are processed.

•Modern continuous machines use

•small pipes, pumps, small pad-bath troughs

•reduce the amount of concentrated liquor to be discharged.

•it is also possible to minimise the discard of leftovers using automated dosing systems,

• meter the dye solution, ingredients and deliver the exact amount needed

Washing

•In hoth continuous and batch dyeing processes, final washing and rinsing operations are water- intensive steps

•Washing and rinsing operations actually consume greater quantities of water than dyeing itself

PRINTING

Emission Sources

• printing paste residues

• waste water from wash-off and cleaning operation

• volatile organic compounds from drying and fixing processes

Printing paste residues

Two main causes

• incorrect measurements

• common practice of preparing excess paste to prevent a shortfall.

•at each colour change, printing equipment and containers (dippers, mixers. homogenizers, drums, screens, stirrers, squeegees, etc.) have to be cleaned up.

•Print pastes adhere to every implement due to their high viscosity and it is common practice to use dry capture systems to remove them before rinsing with water.

•In this way these residues can at least be disposed of in segregated form, thus minimising water contamination.

•Another significant, but often forgotten source of printing paste residues is the preparation of sample patterns.

• Digital printing offers a solution to these problems

•Another significant, but often forgotten source of printing paste residues is the preparation of sample patterns.

• Digital printing offers a solution to these problems

Waste water from wash-off and cleaning operations

•Waste water in printing processes is generated from

•final washing of the fabric after fixation,

•cleaning of application systems in the printing machines,

•cleaning of colour kitchen equipment and cleaning of belts.

•Waste water from cleaning-up operations accounts for a large share of the total pollution load.

• in the case of pigment printing,

• considerable amounts of waste water arise from cleaning operations,

•pigments are completely fixed on the fibre without need for washing-off.

Pollutants in waste water from printing processes

Pollutants in waste water from printing processes

Volatile organic compounds from drying and fixing

• Kerosene or MTO in case of emulsion thickener

• aliphatic hydrocarbons (C10-C20) from binder

      Monomers such as acrylates, vinyl acetates, styrene, acrylonitrile, methylol acrylamide, butadiene,

•• methanol and formaldehyde from fixing agents (Fixer)

  • other alcohols, esters, polyglycols from emulsifiers

• ammonia from

• urea decomposition and from Synthetic thickeners

  • N-methylpyrrolidone from emulsifiers

•• phosphoric acid esters

• phenycyclohexene from thickeners and binders.

Finishing

Anti-felt finishing of wool Processes

Oxidation

Use of

•Sodium hypochlorite

•Di-isocyanurate

•Formation of AOX

•Resin treatment (Hercosett Process)

•Oxidation followed by resin treatment (Polyamide epichlorohydrin resin)

•Formation of AOX both from oxidant and resin

•Fish toxicity due to cationinc nature of resin

Environmental issues associated with continuous Finishing processes

Pad - Dry – Cure Sequence

• No washing after cure (Exception Flame retardant finishes based on organo phosphorous)

emissions of water pollution restricted to the system losses and to the water used to clean all the equipment.

  potential system losses at the end of each batch are:

• the residual liquor in the pad trough

• the residual liquor in the pipes

• the leftovers in the batch storage container from which the finishing formulation is fed to the pad trough.

System losses

•Normally 1-5%, based on the total amount of liquor consumed:

• in some cases, with small finishers, losses up to 35 or even 50 % may be observed.

•This depends on the application system e.g.

•size of pad trough) and t

•The size of the lots to be finished

•Solution

•Application of finishes by

•Spraying

•Foam

•In this case system losses are

•Less in terms of volume

•Concentrted in terms of active substances

•Residues of concentrated liquors are re-used, if the finishing auxiliaries are stable or

•disposed of separately as waste destined to incineration.

•The range of pollutants that can be found in the waste water varies widely depending on the type of finish applied.

•In particular, the release of the following, substances in the environment gives rise to significant concerns:

• ethylene urea and melamine derivatives in their "not cross-linked form" (cross-linking agents in easy-care finishes)

• organo-phosphorous and polybrominated organic compounds (flame retardant agents)

• polysiloxanes and derivatives (softening agents)

• alkyl phosphates and alkyletherphosphates (antistatic agents)

• fluorochemical repellents.  

Air pollutionIn the drying and curing operation air emissions are produced due to

•the volatility of the active substances themselves as well as that of their constituents (e.g. monomers, oligomers, impurities and decomposition by-products).

•The emission loads depend on the drying or curing temperature, the quantity of volatile substances in the Finishing liquor

•The range of pollutants is very wide and depends on the active substances present in the formulation and on the drying, curing conditions.

• 

Environmental issues associated with discontinuous processes

The application of functional finishes in long liquor by means of batch processes is used mainly in varn finishing and in the wool carpet yam industry in particular.

For the resulting water emissions, the efficiency exhaustion of the active substance from the liquor to the fibre, influences the emission loads.

The efficiency depends on the liquor ratio and on many parameters such as pH, temperature and the type of emulsion (micro- or macro-emulsion).

 

•Maximising the efficiency is particularly important when biocides are applied in mothproofing finishing.

• mothproofing agents are not water-soluble they are applied from emulsions.

•The degree of emulsification and the pH are critical in the application of mothproofing agents

• the efficiency of the process is higher when the active substance is applied from micro-emulsions and at acidic pH).

• the finishing agents are dosed based on the weight of the fibre and not on the amount of bath (in g/litre).