M Saleem Chohan
Chemical Engineering NEDUET, Pakistan
Chemical & Plant Report
Part I: Manufacturing Process & Plants of denim
1.1 Evolution of Denim
1.2 Peculiarity of Denim
1.3 Defining Denim
1.4 Manufacturing Of Denim
2. Warping (Ball Warping)
3. Dyeing (Indigo Rope Dyeing)
3.1 Process : Chemical Reduction & Oxidation
3.2 Some Facts And Figures
3.3 Dyestuff Vs. Pigment
3.4 Listing Defect in Vat Dyeing
3.5 Process Control of Rope Dyeing
4. Slashing (Sizing)
5.1 Structure of Denim Fabrics
6. After-Finishing (Or Finishing)
PART II: CHEMICALS FOR DENIM
7. 1 Various Chemicals Used In Denim Industry
7.1.1 Pre-treatment Chemicals
7.1.2 Dyeing Chemicals
7.1.3 Sizing Agents
7.1.4 Printing & Coating Chemicals
7.1.5 Finishing Chemicals
7.2 REACH system & Substances of very high concern (SVHC)
7.2.1 hazards of dyestuffs
7.2.2 hazards of non-dyestuffs
7.2.3 hazards of chemicals used in finishing Process
7.2.4 Other hazards
7.2.5 Fire and explosion hazards
7.3 Critical Consumable Item List in Denim Industry
7.4 Effluent treatment in denim industry
Evolution of Denim
The term “Denim” comes from the city of Nimes in France where “serge de Nimes” was
made.The term “Jeans” came from the cotton workpants worn by sailors inItaly, who were
known as “Genes”
Peculiarity of Denim
Denim is made from Indigo dye, a vat dye, which is attached to cotton fabric in loosly held
form in layers .When washed, it fades differentially, which gives its characteristic washed
Denim is a rugged cotton twill textile in which the weft passes under two (twi -
double) or more warp fibers. This produces the familiar diagonal ribbing identifiable
on the reverse of the fiber, which distinguishes denim from cotton duck. It is a 3/1 warp-
faced twill fabric made from a yarn dyed warp and an undyed weft yarn. Traditionally, the
warp yarn is indigo dyed
Manufacturing Of Denim
The warp yarn (length-wise) used in denim fabrics is uniquely prepared for denim
manufacturing compared to conventional woven fabrics. The yarn goes through
numerous processing steps before it is placed on the weaving machine. Unlike the warp
yarn, most filling yarn (width-wise) is put onto yarn packages and delivered directly to the
weaving machine where it is inserted into the fabric without any further preparation in the
same manner as conventional woven fabrics.
The following flow chart reveals the necessary steps in the manufacture of denim
fabrics, beginning with the production of the warp to yarns used. The chart forms an outline
for most of the topics that will be covered in this report .
WARPING (BALL WARPING)
Warping is the process of transferring multiple yarns from individual yarn packages onto a
single package assembly
In ball warping ,250 to 400 yarn ends are pulled from the creel. The yarns then pass
through a comb-like device (sometimes called a hack or reed), which keeps each
warp yarn separate and parallel to its neighboring ends. At intervals of every 1000
or 2000 yards (or meters), a lease string is placed across the sheet of warp yarns to aid
yarn separation for the re-beaming operation, which will occur later. The yarns then go
through a funnel-shaped device called a trumpet or condenser, which collapses and
condenses the sheet of yarn into rope form. This device is located at the base of the
warper head and traverses back and forth, guiding the newly formed rope of yarn onto a
log. The rope must be wound at a constant tension to keep the yarns from tangling.
DYEING (Indigo Rope Dyeing)
“Dyeing” is the homogenous colouration of textile substrates using dyes, which are substantive
colourants for the substrate, and other necessary chemical auxiliaries, such as surfactants, to
improve wetting, dispersing, levelling or other desirable dyeing properties.
Most denim is yarn-dyed fabric with the warp yarns dyed with indigo dye and the filling
yarnleft undyed. There are a number of modifications or alternatives in the dyeing process
that are routinely used to change the overall look or performance of the fabric. With the
advent of denim garment washing techniques, the consistencies of the indigo dyeing
process and its modifications have become crucially important in determining the quality
and performance of indigo denim products.
Indigo is unique as a major textile dye, because it has a very low affinity for the cotton fiber.
Because of the low substantivity of the indigo, the ball warp dyeing process ring dyes
cotton. Unlike almost all other commercially successful dyestuffs, the indigo dye
concentrates in the outer layers of the cotton yarn and fiber during the dyeing
process. This produces an intense ring of color around a white core in the cotton yarn
and the cotton fiber thus the name ring dyeing. When using most other dyes, if the ring-
dyeing effect occurs, it would be considered a dyeing defect.
Indigo dye in its normal form is a vibrant blue, it is insoluble in water, and it will not dye
cotton fiber. In order to dye cotton, the indigo must be converted to a water-soluble“leuco”
form and then applied to the cotton. This process is known as chemical reduction.
Reducing agents such as sodium hydrosulfite with sodium hydroxide chemically convert
the indigo dye to its soluble form. This also temporarily converts the dye from its blue color
to a very pale greenish yellow color. The leuco form of indigo is readily absorbed by the
outer layers of the cotton yarn.
Once in the fiber/yarn, the indigo is made insoluble by oxidizing the yarn by passing the
yarn through the air (skying). In fact, the dye will start to oxidize immediately when
exposed to the air. The oxygen in air converts the dye back to its original blue and
insoluble form. Thus the dye becomes trapped inside the outer layers of the cotton
yarn. This results in a small amount of dye being deposited on the surface resulting in only
light blue dyed yarn.
DYE BOXES & MULTIPLE DIPS
In order to obtain deep blue indigo dyed yarns, the color must be built in layers. The dye is
layered by using passes of the rope of yarn into the soluble dye and then exposing it to the
air for oxidation. This multiple passing of yarn into dye is called dips. Normally, this
process is repeated from three to twelve times to build up a deep indigo blue color. The
number of dips is limited to the number of dye boxes on the dye range. If the concentration
of indigo dye in the dye boxes is doubled, this will result in slightly darker denim. This acts
as a multiplier when labeling the denim. A double concentration of dye in nine dye
boxes makes it an 18-dip denim. Tripling the concentration makes it a 27-dip denim.
When even darker shades are desired, a sulfur black or blue dye can be applied to
the yarn before indigo dyeing. This is known as a sulfur bottom. If the sulfur dye is
applied after the yarn has been indigo dyed, it is known as a sulfur top. After either rinsing
following indigo dyeing or rinsing following sulfur topping, the yarn ropes pass through
squeeze rolls to mechanically extract water. The yarns are then dried and coiled into large
tubs. The typical type of drying apparatus is a multiple stack of drying cans.
DYEING- SOME FACTS AND FIGURES
Many Problems of Fastness take place because pretreatment is not done properly.
These have very poor fastnes but these are easy to apply. They dont react chemically with
the fiber. But they have affinity for the substrate.
These react chemically with fibers. They have very good fastness as compared to the direct
They have the best fastness.In vat there is no reaction. The molecule size of vat dyes is
large. It is made smaller by reducing and then again assembled to its original size by
oxidising. Once they are inside the fiber they cannot come out.
These are different from vat dyes.They are also reduced before applying and contain dull
dark shades like black, brown.
Nepthol Base- Azoic or Ice Colors
Rapid dyes come in this class .The reaction is a bit complicated. The color is made inside
the fiber.The color is formed when Napthol is coupled with base.Very bright colors and
shades can be obtained. Fastness is very good, except rubbing fastness.
As a rule of Thumb, Yellows are low priced
Blues are high priced.
Reds are somewhere in between
For all dyestuff.
Reactive dyes are about twice costlier than direct dyes.
Vat dyes are about 5-10 times costlier than direct dyes.
Consumption of Dyestuff on Yarn
In Light Shades: It is about 1% of the weight of the fabric
In Heavy Shade: It is about 3% of the weight of the fabric.
Requirement for Dyeing to Penetrate
c. Material to Liquor ratio
Dyestuff Vs. Pigment
The main difference between dyestuff and pigment is that dyestuff is soluble in the media of
application whereas pigment is not soluble in the media of application.
Thus Vat Dyes, Disperse Dyes and Sulpur are actually pigments as they are not soluble in
the media of application.
In case of pigment printing the pigment is sandwitched between the binder and the fabric,
the binder is colorless therefore whatever the color of pigment is, it is shown.
The colorfastness of pigment is dependent upon three factors:
- Quality of binder
- Quality of pigment
- Quality of curing.
Listing Defect in Vat Dyeing
Listing Defect in Vat Dyeing and the Precautions to Prevent the Defect
Listing is the variation in color of centre of the fabric with the selvedge. Some of the
reasons for this defect are:
1. Improper batching.
2. Non Uniformity in the selvedge
3. Redyeing of the fabric
4. Foam on the two sides of the jigger.
5. Slippage of the fabric from the roller during dyeing.
6. Shortcomings in the machine such as malfunctioning of guide roll, expander roller or
7. Improper filteration of the colors, improper circlation of the liquor during dyeing.
Difference in temperature of liquor in the centre and at the ends.
8. Mixing of colors which are not properly compatible.
9. Improper singeing
The remedies are:
1. There should not be any mechanical fault in the machine.
2. Take proper care during dyeing, like filter the color solution before using, ensure that all
the controlling instrument ( temperature, time) work properly. Circulate the liquor
continuously during dyeing.
3. If some shade is not coming out proper, dye with a slighly heavy shade without taking
out the earlier color.
4. Join the fabrics of the same width while making a lot.
Process Control of Rope Dyeing for denim
1.Concentration of Hydrosulphite
It is measured by vatometer. It should be from 1.5 gpl to 2.5gpl , or by redox potential of dye
bath which should be from -730 mV to -860 mV.
2. Caustic Soda or pH value
Should be from 11.5-12.5
Effect of pH
At pH of 10.5 to 11.5, there will be formation of more monophenolate ions, which lead to
higher color yield, as strike rate of the dye to the yarn bundle is very high, and wash down
activities will be very good.
At pH higher than this, dye penetration will be less and wash down characteristics are also
1. Alkalanity in Dye Bath Liquor
2. Hydro in Dye bath Liquor
Addition of chemicals
1. Red Tinge: reduce addition of NaOH, increase slightly Na2S2O3
2. Darkish Red: increase Hydro
3. Light Greenish: decrease Hydro
4. Dark Green: Increase Caustic
3. Dye concentration in Dye bath
it is measured by spectrophotometer. It should be in g/l
High Indigo Concentration --> Shade is greener and lighter
Low Indigo Concentration --> Shade is dull and Red.
High pH or Caustic Concentration --> Redder and lighter
Low pH or caustic concentration --> greener and darker
COLORFASTNESS PROBLEMS IN VAT DYEING-
How to improve colorfastness in Vat Dyeing
1. Dont frequently add colors during dyeing
2. Ensure proper reduction clearance
3. Ensure proper oxidation
4. Choose right colors and chemicals
5. Control Temperarue. Maintaining a consistent pressure of steam within the cylinder
can accurately control the temperature of the surface of each cylinder.
6. Use soft water
7. Add Dispersing or levelling agent as per need
8. Dont redye finished cloths
9. Ensure proper washing after dyeing.
10. Treat the material with small amount of Ammonia and Sodium Hydrosulphite
11. Treat the cloth with large amount of reducing agent and alkali.
12. Care must be taken not to attempt to dry the rope of yarn too quickly, which causes the
dye to migrate to the surface of the rope. Additionally, if the surface of the drying
can is too hot, the yarn can be overstressed producing an undesirable glazed
appearance that reduces absorbency in later processing
The main purpose for sizing warp yarns is to encapsulate the yarn with a protective coating.
This protective coating reduces yarn abrasion that takes place during the weaving
operation and reduces yarn hairiness preventing adjacent yarns from entangling with
one another at the weaving machine. Also, this protective coating keeps the indigo dye
from rubbing off during the weaving process.
Figure shows a schematic diagram of a SLASHER. In slashing, section beams are
combined to create a loom beam and at the same time apply the appropriate size to the
warp yarns.The section marked (1) is the let-off station where one or more section beams
are combined and fed through the rest of the range. The warp yarns are let-off as a flat
sheet and then pass through a size applicator (2) consisting of a trough containing the size
formulation and squeeze rolls. The yarns pass over heated cans (3) to dry. Located at the
exit end of the slasher is an arrangement of bust bars which separate each warp end from
The weaving process involves interlacing two directions of thread -- the warp and the weft -
to make fabric or cloth. The associated machine is called the loom, and it uses a jet of air or
water to insert the weft. The loom ensures pattern diversity and faultless fabrics by a
flexible and gentle material handling process. Fabrics can be in one plain color with or
without a simple pattern, or they can have decorative designs; they are generated from the
loom with one of three basic weaves -- plain weave, satin weave or twill.
Structure of Denim Fabrics
Denim fabrics are woven by interlacing two sets of yarns (Figure 15) perpendicular
to one another in fabric form. Yarns in the machine direction are called warp yarns or warp
ends, and these are interlaced with filling yarns or picks. The sequence or order of
interlacing the two sets of yarns can be varied to produce many different weave
designs. The finished fabric construction is determined by the number of warp and filling
yarns per square inch or centimeter.
AFTER-FINISHING (OR FINISHING)
“After-finishing” refers to the treatment of textile substrates to impart desirable properties
(other than colouration), such as wash-and-wear finish, anti-shrink finish, water-repelling
finish and flame-retarding finish. It is often simply called “finishing” and thus may
cause confusion with its another meaning that carries a broader implication.
Singeing is the process of removing the hairs of fabrics or fibers. At the end of this
process, fabrics is smoother than before and the fabrics wet ability is increased.
VARIOUS CHEMICALS USED IN DENIM INDUSTRY
Pre-treatment is very important in textile processing. Major steps involved are desizing,
scouring and bleaching. such as wetting agents, lubricants, antifoam agents, dispersing
and sequestering agents, fixing and leveling agents, hydrogen peroxide stabilizers,
scouring agents and many others.
To remove the different types of sizes (Starch, waxes etc.) from the fabric/yarn
Wetting and Penetrating Agent
To reduce the surface tension of water and thus increase the absorbency of the water to
Sequestering Agent & Protective Colloid
To reduce the hardness of water and thus make ideal conditions for washing
To remove the oil , fats etc from the fabric.
Silicone & Non Silicone Defoamers
To reduce the foam created during the process of treatment of fabric
Non-Silicate Peroxide stabilizer
To stabilize the peroxide of hydrogen peroxide in the pretreatment process.
This is used to clear the residual peroxide from the fabric
Polyester Weight Reducing Catalyst
To reduce the weight of the polyester fabric during the caustic addition process
Anti-back Staining Agent
To prevent the staining of the pockets during the denim fabric treatment.
To reduce the hardness of water and thus make ideal condition for processing
Silicone & Non Silicone Defoamers
These are used to reduce the foam created during the process
These are used to maintain the pH of dye bath throughout the dyeing process
Polyester Dyeing Carriers
To facilitate easy absorption and penetration of dyes by the polyester fabric
Dispersing Agent & Oligomer Removing Agent
To maintain the dispersion of dyes in the dyeing process and help to remove oligomers
To get even dyeing and even colour depth effect
To reduce the friction between fabric to fabric and fabric to machine and to reduce the
creation of creases in the fabric
Washing off Agent
To remove the unfixed dyes from the fabric
To fix the dyes on to the fabric
Soda Ash Substitute
To substitute soda ash in the dyeing process of cotton.
Cationising Agent for Pigment Dyeing
To provide required cationic charge to the fabric in the pigment dyeing process
Natural sizing agents are based on natural substances and their derivatives:
• Starch and starch derivatives; native starch, degradation starch and chemical modified
• Cellulosic derivatives; carboxymethlycellulose (CMC), methylcellulose and
• Protein-based starches; glue, gelatin, albumen
Synthetic Sizing Agents
• Modified polyesters
• Polyvinyl alcohols (PVA)
• Styrol/maleic acid copolymers
Dispersing, Penetrating, Swelling, Levelling & Defoaming Agent
This is used to provide depth and even level printing effect, being added in the printing
These are used to provide depth and fixation of dyes to the polyester printing fabric in
loopager machine during the disperse printing process
Binders Acrylic, Self Thickening for Gold & Flock
These are used to bind the pigment or dyes onto the fabric
Fixers in Pigment Printing
These are used to provide fastness to the print
These are used to provide viscosity to the printing paste to facilitate required printing effect
These are used to provide printing effect onto the fabric
Washing Off Agent
This is needed to remove the unfixed dye from the printed fabric
This is used to provide stiff finish effect to the fabric
These are used to provide soft finish effect to the fabric
This is used to provide silky and soft finish effect to the fabric
Wax Finishing Agent
This is used to provide waxy finish to the fabric
Anti Static Agent
This is used to reduce the static power of the fabric
Water Repellant & Soil Resisting Agent
This is used to provide water repellency and dust repellency to the fabric
Polyurethane Finishing Agent
This is used to provide bouncy feel to the fabric
Crease Recovery Agent
This is used to reduce the crease and provide wrinkle free effect to the fabric
This is used to remove the luster from the viscose fabric
Anti-pilling & Anti-Slip Agents
These are used to remove the hairing and pilling problems from the fabric and also provide
Enzymatic Bio-polishing Agent
This is used to remove the surface protruding fibers from the knit substrates and denim and
thus improvise the surface feel.
Anti-pilling & Anti-Slip Agents
These are used to remove the hairing and pilling problems from the fabric and also provide
Enzymatic Bio-polishing Agent
This is used to remove the surface protruding fibers from the knit substrates and denim and
thus improvise the surface feel
THE CHEMICAL HAZARDS
REACH is a new European Community Regulation on chemicals and their safe use. It
deals with the following
CH: Restriction of Chemical Substances
REACH entered into force on 1st June 2007. The basic aim of this Regulation is to ensure
a high level of protection of human health and the environment, through a better and earlier
identification of the intrinsic properties of chemical substances. It also includes the
promotion of alternative methods for assessment of hazards of substances. This
Regulation is based on the principle that it is for manufacturers, importers and downstream
users to ensure that they manufacture, place on the market or use such substances that do
not adversely affect human health or the environment.
Manufacturers and importers will be required to gather information on the properties of their
chemical substances, which will allow their safe handling, and to register the information in
a central database run by the European Chemicals Agency (ECHA) in Helsinki. The
Agency will act as the central point in the REACH system: it will manage the databases
necessary to operate the system, co-ordinate the in-depth evaluation of suspicious
chemicals and run a public database in which consumers and professionals can find
Substances of very high concern (SVHC)
Under REACH regulation, SVHC are the hazardous substances which may be inadvertently
introduced in various consumer products due to the complexities of both supply chain &
manufacturing process. To secure their sustainable competitive advantage, EU-based companies
have started requesting their suppliers to justify the presence of SVHC in their products.
REACH considers substances to be of very high concern if they are:
• Carcinogenic, mutagenic or toxic for reproduction (CMR), categories 1 & 2 according to Directive
• Persistent, liable to bioaccumulate & toxic (PBT), or very persistent & very liable to bioaccumulate
• Others such as endocrine disruptors, PBTs & vPvBs for which there is scientific evidence of
probable serious effects to human health or the environment.
The hazards of dyestuffs:
Many textile dyes have complex chemical structures, and there is a large range of such
products in commercial use. Their impacts on health (such as toxicity and
carcinogenicity) and safety thus vary considerably. Dyes that have not been associated
with any adverse health effects in light of current medical knowledge should also be
handled with care as they may pose health hazards that have not yet been known. It
should be prudent to minimize exposure to dyes of all types. It is also essential to read
the material safety data sheet (MSDS) supplied with each product.
The following dyes are known to be hazardous to health:
(a) Certain reactive dyes are recognized as respiratory sensitizers. Inhaling such dyes
can cause occupational asthma. Once a person is sensitized, re-exposure to even very
small amounts of the same dye may result in allergic symptoms, such as wheezing,
chest tightness and breathlessness.
(b) Certain reactive, vat and disperse dyes are recognized as skin
(c) Certain azo dyes are known to be carcinogenic.
What are Azo Dyes. Are All Azo Dyes Banned ?
No. There are three components of any dyestuff: Chromophore- the color producing
component, Auxochrome- The Supportive component that helps the dyestuff to go from
dyebath to the fiber and Binary Site: The site where reaction takes place.
The Chromophore groups of the dyes can be :
a. AZO ( -N=N-) 70% of all dyes available are azo based
b. Anthroquinone- Present in vat dyes
In 1994, In Germany, dyestuff was divided on the basis of max. working place
concentration into three classes:
Class I- They defined 22 banned amines which are present in these dyes. thus
Benzindine - Material of direct dyes fall in this category eg. Congo Red. Out of these 22:
4: Amines are Carcinogenic
16: Harmful to animals- may be harmful to human being
2: Sensitizers- Allergic- Cause itching
Now if an Azoic dyestuff after catalytic cleavage it releases any of the banned amines then
it is banned.
Remember in Vats - there is no azo- all vats are safe.
The hazards of non-dyestuffs:
The denim finishing also uses many hazardous chemicals other than those for dyeing. It
is not possible for this Guidance Notes to elaborate all these chemicals individually
but they can be broadly grouped into oxidising agents, reducing agents, corrosives,
irritants and flammables.
Oxidizing agents used in denim finishing are mainly bleaching agents such as hydrogen
peroxide, sodium hypochlorite and sodium dichlorocyanurate. Violent reactions may
be caused by uncontrollable decomposition of such chemicals. For example, if the
chemical reaction during peroxide bleaching is not strictly controlled, a rapid evolution of
oxygen can be caused by decomposition of hydrogen peroxide. The volume of
oxygen thus produced may exceed the capacity of the relief devices causing the
vessel to fail catastrophically. Mixing oxidizing agents with incompatible substances
such as reducing agents will generate a lot of heat and toxic gases, or lead to explosion.
Moreover, precautions must be taken to prevent occurrence of the following
hazardous reactions between:
(a) sodium hypochlorite and acid;
(b) sodium peroxide and sodium hydrosulphite.
Reducing agents like sodium hydrosulphite are used in reducing bleach bath, in
stripping (i.e. removal of a dye from a substrate) and in applying vat dyes. Other
reducing agents are thiourea dioxide, sodium formaldehyde sulphoxylate (trade name
“Formosul” or “Rongalite”), sodium bisulphite and sodium metabisulphite. Sodium
bisulphite, when contaminated with acid, gives rise to hazardous fumes of sulphur
dioxide. Sodium hydrosulphite, when contaminated with water, generates considerable
amount of heat and may even ignite spontaneously.
The hazards of chemicals used in finishing Process :
Many chemicals used in textile finishing pose health and physical-chemical hazards.
As some processes are operated at elevated temperatures, fire and explosion may
In finishing, strong acids and alkalis, such as sulphuric acid and caustic soda, are
commonly used. These are very corrosive chemicals that can cause serious burns and
react dangerously with other chemicals.
The dyeing and after-finishing processes involve many chemicals that are irritating to
eyes, skin and respiratory tract. These include formaldehyde-based resins, ammonia,
dilute acetic acid, aqueous ammonia, some shrink-resist chemicals and optical whiteners,
soda ash, various bleaches and acrylate (for preparation of screens).
When flat or rotary screens are made or repaired at the printing workshops, solvent
vapour hazards can be caused by the use of volatile adhesives and varnishes. If
epoxy adhesive is used to secure rotary screens to their end rings there is a skin contact
risk until the adhesive is fully cured.
Screen inks may contain such solvents as aliphatic and aromatic hydrocarbons (e.g.
white spirit, trimethylbenzenes), ketones (e.g. cyclohexanone), alcohols (e.g. diacetone
alcohol) and certain glycol ethers and their esters (e.g. 1-methoxy-2-propanol and 2-
butoxyethyl acetate). There has been particular concern about the health effects of some
glycol ethers and their acetate derivatives.
The use of pressure equipment for high temperature operation can be found in dyeing by
high temperature dyeing machines – the substrates requiring dyeing at high temperature
C are usually poly(ethylene terephthalate) (PET) or its blends. The high
temperature dyeing machines include package dyeing machine for yarns (mostly cheese
or cones), jet dyeing machine for knitted goods and high pressure jigger for woven
Explosion of pressure equipment may result in serious accidents.
These equipment are under the control of the Boilers and Pressure Vessels Ordinance
and its subsidiary legislation. Here are some examples of the risks:
(a) Pressure vessels or components such as opening covers may fail under pressure, due
to mechanical failure or faulty closing.
(b) Vessels may fail during bleaching operations using hydrogen peroxide as a result
of decomposition of the peroxide leading to the rapid evolution of oxygen beyond the
capacity of the relief devices.
(c) Operators may be engulfed with steam, boiling liquid, or hot water, when:
(i) temperature of dyeing liquor is at or above 100o
C and a lid or sample chamber is
opened, allowing liquor to flash violently into steam and erupt from the vessel;
(ii) doors are opened and hot liquor allowed to spill out, e.g. from horizontal vessels not
completely drained or vertical vessels filled above cover-flange level;
(iii) one of a bank of machines is inadvertently pressurised from an adjacent
machine, via a common blow-down or drain line.
Fire and explosion hazards :
Fire and explosion hazards may arise from the use of flammable substances, or
oxidizing agents which can intensify a fire by supplying more oxygen. Flammable
substances (e.g. petroleum spirit) are often used for preparation of emulsion thickening in
pigment printing. This incurs significant fire hazards to the workplace, particularly when the
printed articles are subsequently baked at high temperature. Sodium hydrosulphite, a
widely used reducing agent, may spontaneously ignite when wet. Some aqueous
organic acids, such as acetic acid, are flammable at high concentrations. The solvent
base of resin coatings or adhesives, which is intended to be easily vaporized during the
drying process, is usually flammable, e.g. white spirit. In pigment printing, the thickening
commonly used is oil-in-water emulsion, in which over 65% of the constituents is
flammable solvent (e.g. white spirit). Subsequent evaporation in oven can give rise to
significant fire and explosion risks.
Critical Consumable Item List in Denim Industry
1. Vat Indigo dye
2. NaoH--> flake, lye
4. Dispersing Agent
5. Wetting Agent
6. Potessium persulphate
7. Thin boiling Starch
9. Mutton Tallow- or equivalent
10. Acrylic Polymer
11. Hessian Cloth- a. 45"width, 10 oz./linear yard, b. 45"width, 14 oz. per linear yard
12. High Molecular high density polyethylene: 65" guage x 44 "width, 65" guage x 50" width
EFFLUENT TREATMENT IN DENIM INDUSTRY
-Dark Blue Indigo color
-High Dissolved Solids ( Decomposed products of hydro)
-High Chemical Oxygen Demand (COD)
-Chlorides and Sulphates of Suspended matter
Characteristics of Effluent
Appearance: Dark Blue
Suspended Solids: 250ppm
Dissolved Solids: 3500-5000 ppm
BOD ( 5 days, 20 deg C): 160-350 ppm
COD: 570-1100 ppm
Alkalinity (pH): 400 ppm
,,,,,,,,,,,,,,,,,(MO): 1700 ppm
Total Hardness: 220 ppm
Chlorides: 2 10-480 ppm
Sulphates ( SO4): 1200 ppm
Calcium: 15 ppm
Magnesium: 45 ppm
Ammonical Nitrogen: 2.5 ppm
Color (pt.Co): 250
Effluent is characterized by “high strength low volume”, as most of the most
contaminated (“high strength”) effluent come from comparatively small quantity (
“low volume”) of wash waters used for rinse after yarn dyeing with indigo. Major
contaminant is Indigo.
EFFLUENT TREATMENT SCHEME
Equalization tank in two compartments. Retention time of at least 7-8 hours.
2. Flash Mixing
Equalisation clarifloculator Unit ( Alum/Poly flash mixer ( to adjust pH)Tank Aluminium
Chloride) for coagulation/segmentation
( it is a physico chemical process with 35-40% COD removal, 25-30% BOD and 70-80%
color removal, also >95% color removal is possible if PAC overflow rate at CFUand
polymer dosage increased) < 20 m^3/day
The sludge withdrawal should not be too less or too large ( can take place in lamella unit
Effluent aeration process ( timeafter CFU > 18 hours) ( New recent aerators use injectors
which produce very fine bubble resulting in a large air/water interface. Waste water is used
as pressure water fro the operations of injection. Water +air stream are subjected at the
bottom of the tower to prevent any possible longsedimentation. Gas bubble rise to full
height of the tower resident time. Good utilization of oxygen upto 80% is possible.
clarifierEffluent from aeration activated sludge recalculated from( resident time 3-4
hours) centrifugal decanter thickened sludgeclarifier to aeration tank filtrate is then
discharged to another tank.
Clarified deficient in dissolved oxygen (DO)( for bioassay parameters)Effluent DO make
uptank ( 2 hours resident time) – the output is expected to meet the criteria.
Process for filtration of particles >5 n meters, from feed water made to flow at low pressure
through membrane having pore size of 4-5 nm.
Useful for elimination of high molecular weight organic compounds. By using this ( the
original indigo concentration in rinse water is 0.05%) fully usable 5% dispersion of indigo
dye is obtained.
There are two types of membranes available. 1. Organic 2. Mineral – resistant to pH 0-14,
resistant to mechanical and thermal conditions and are unaffected by solvents.
Burning of waste
Major threat to possible health
Destruction of resources
8. Sludge Disposal
85% of the waste is biodegradable. Can be used for compost. Lime sludge has
agricultural value as it is free from pathogenic microorganisms
Bugs convert dyes into colorless substances
Microorganisms filament fungus isolated from soil) can( Geotrichum Candidum
decompose 18 different kinds of dyes in to colorless substances. Preferred pH for them is
4-7 at a temperature of 20-30 deg C. Can destroy dye in two days ( at a concentration of 12
g/lit). They can eat indigo also.
Process Control for Effluent
Usually 10% of the applied indigo is washed off in rinses. Indigo fixation of yarn could be
-Slightly lower pH- can reduce indigo consumption for a given visual depth of shade
-Use of pre reduced indigo and indigo dyeing under nitrogen blanket. Can cut hydro
-Use of prereduced sulpher dye and maintain reduction potential with hydrol ( glucose +
other oligomeric reducing agent) instead of sod. Sulphide.
Effluent volume can be reduced through water conservation
-Washing in counter current type
-Decrease size of wash tanks
-Use Na2CO3 (Sod. Bicarbonate) in first rinse tank
-Use Co2 for neutralization of alkali
-Use as many nips as possible during washing to squeeze out alkali to maximum (
squeezed liquor should not drop back into bath)
-Relying more on spray rather than immersion into the bath