Process of tanneries and pollution caused by leather tanneries

1. Unit 3
Various finishing process in tanneries
The leather produced by tannage should be prepared to give the final shape ,which is
called the finishing stage.
Drying
The leather is subjected to drying, which can achieved by air, machine or vaccum
drying.
Bleaching
Titannium dioxide, sodium polyphosphate, wet blue they are directed to whitening
and bleaching of semi chrome finished leather.
Dyeing
Dyeing can be done in conventional drum fitted with pegs and feed pipe in a hollow
axie from which hot water dye cid can be added.
Fat liquoring
This process of oiling the fibre structure of hide and skin.
Calendering
It is a finishing process used to smooth ,coat meterial. Calender roller is at high
temperature and pressure.
Pollution caused by leather tanneries
The tanning industeries have been identified as one of the main causes of
environmental pollution.
Treatment of tannery effluents primary, secondary, tertiary treatment
Primary is a removal hair and small suspended partical. secondary is a coaculation,
filtation, BOD,COD. Tertiary is a discharge is fed uniformly on to an area of land
with a gentle down ward slope.

2. Finishing.
 After drying, vegetable tanned leather is subjected to mechanical
operations (setting and rolling) and given a final polish.
 The finishing process for chrome leather includes a series of mechanical
operations and, normally, the application of a covering layer to the leather
surface.
 Staking is a mechanical beating operation used to make the leather soft.
To improve the final appearance, the grain side of the leather is buffed
using a sanding drum.
 This process generates a tremendous amount of dust.
 A final surface finish is applied, which may contain solvents, plasticizers,
binders and pigments.
 These solutions are applied by pads, flow coating or spraying. Some
tanneries employ hand labour to apply the finish using pads, but this is
usually carried out by machines.
 In flow coating, the solution is pumped into a reservoir above the conveyor
carrying the leather and flows down onto it. In most cases, painted or
sprayed leathers are not dried in ovens, but on trays on shelves. This
practice provides a wide evaporating surface and contributes to air
pollution.

3. Drying mechanism
 In some products having a relatively high initial moisture content, an initial linear
reduction of the average product moisture content as a function of time may be observed
for a limited time, often known as a "constant drying rate period".
 Usually, in this period, it is surface moisture outside individual particles that is being
removed. The drying rate during this period is mostly dependent on the rate of heat
transfer to the material being dried.
 Therefore, the maximum achievable drying rate is considered to be heat-transfer limited.
If drying is continued, the slope of the curve, the drying rate, becomes less steep (falling
rate period) and eventually tends to nearly horizontal at very long times.
 The product moisture content is then constant at the "equilibrium moisture content
where it is, in practice, in equilibrium with the dehydrating medium. In the falling-rate
period, water migration from the product interior to the surface is mostly by molecular
diffusion, i,e. the water flux is proportional to the moisture content gradient.
 This means that water moves from zones with higher moisture content to zones with
lower values, a phenomenon explained by the second law of thermodynamics. If water
removal is considerable, the products usually undergo shrinkage and deformation, except
in a well-designed freeze-drying process.
 The drying rate in the falling-rate period is controlled by the rate of removal of moisture
or solvent from the interior of the solid being dried and is referred to as being "mass-
transfer limited".
 This is widely noticed in hygroscopic products such as fruits and vegetables, where
drying occurs in the falling rate period with the constant drying rate period

4. Bleaching
 oiling
 brushing
 padding
 impregnation
 buffing
 spraying
 roller coatingFor some leathers a surface coating is applied.
Tanners refer to this as finishing
 curtain coating
 polishing
 plating
 embossing
 ironing
 combing (hair-on)
 glazing

5. FATLIQUORING
 . The process of introducing oil into a skin following tannage but
before the leather is dried.
 In fatliquoring, which is usually applied to light leathers, the oil is
introduced into the leather in such a manner that the individual fibers
of the skin are uniformly coated.
 Re-tanning, colouring and fat-liquoring. After tanning, most leathers
except sole leathers undergo colouring (dyeing).
 Generally, colouring is performed in a batch mode; and re-tanning,
colouring and fat liquoring operations are all performed in sequence in
the same drum with intermediate steps of washing and drying.
 Three major types of dyes are used: acid, basic and direct. Blends of
dyes are used in order to obtain the exact shade desired, so the
composition is not always known except by the supplier.
 The purpose of fat-liquoring is to lubricate leather to give it strength
and flexibility.
 Oils, natural fats, their transformation products, mineral oils and
several synthetic fats are used.

6. The dyer's objectives
 To eliminate the random element which occurs in dyeing, by applying an
understanding of the mechanism of dye fixation, in order to advance:
 - dyehouse process control: actual process parameters and equipment
status congruent with parameters specified in the 'Recipe'.
 - predictable and consistent results: consistent process conditions produce
consistent product quality test results.
 - accurate color matching - within agreed tolerances.
 - specific Fastness Properties achieved: Fastness to Light, Washing,
Perspiration, Migration in PVC, Heat & Damp,
 - economic process, with minimised waste, avoiding re-dyeing, correction
dyeings and multiple shading additions.
 - economic process, with minimised waste, avoiding re-dyeing, correction
dyeings and multiple shading additions. The dyer's objectives
 To eliminate the random element which occurs in dyeing, by applying an
understanding of the mechanism of dye fixation, in order to advance:
 - dyehouse process control: actual process parameters and equipment
status congruent with parameters specified in the 'Recipe'.
 - predictable and consistent results: consistent process conditions produce
consistent product quality test results.

7. Pollution casued by leather tanneries
 Based on new data and requests from interested users, the revised
second edition of the paper Pollutants in tannery effluents was
prepared drawing on technical inputs by J. Buljan, I. Král, M. Bosnić, R.
Daniels.
 This training material is primarily intended to meet the needs of
tanners and people of different profiles associated with environmental
protection in the leather industry in developing countries.
 The environment is under increasing pressures from solid and liquid
wastes as by-products from leather manufacture and tannery effluent
create significant pollution unless there has been a form of treatment
before discharge.
 The industry has gained a negative image in the society with respect to
its pollution potential and therefore the leather processing activity is
facing a serious challenge.

8. Leather tanneires
 The paper presents the main sources of pollution and typical
pollution loads generated by tanning processes adopted by the
tanneries in developing countries, volume(s) of wastewater
discharged, the corresponding concentrations of main pollutants
as well as the the table of widely prevailing discharge standards.
 In this edition the main pollutants' parameters are elaborated in
great detail, together with descriptions of their negative
environmental impact.
 Air pollution and toxicity aspects are expanded and a concise
chapter on Substances of Very High Concern, SVHC (
carcinogenic, mutagenic, bioaccumulative, persistent etc.)
introduced. One can also find photos of equipment used for
laboratory analysis.
 For the country-wise overview of discharge standards
(admittedly somewhat obsolete) please refer to Part II of the first
edition.

9. Primary treatment
 The removal of hair and small suspended particals can be accomplised
by a mechanical screening system.
 Large quantities of solid materials require primary filteries brush filters
or drum filters these also avoid blockage of the system.
 The further sedimentation of tannery effluents are carried out in tanks
by coaagulation.the traditional method is lead all process drains into
one common mixing tank where the materials of tannery interact
mutually.
 For the example acid of the pickle neutralizers alklinity of the lime thr
alkalinity of the lime precipitate chrome salt vegetable tans dyestult.
 Soluble proteins interacts to form a sludge which can be later removed
by settling
 This process forms a wet sludge which is difficult and unpleasent to
handle.
 Special drains and tanks are installed to handle more contaminated
efflent the remainder being wash waters etc which are relatively pure
enter the discharge system by passing those of high conatamination.

10. Secondary treatment
 The effluent from sedimentation tank after primary treatment is send into
large lagoons for giving secondary treatment. This treatment is performed
by the following process.
 Chemical treatment for coagulation
 Filtration to remove coagulated materials
 Biological Treatments to reduce BOD, COD, and colour
 Second filtration
 i) chemical treatment
 Sulphuric acid alum
the effluent is treated with sulphuric acid alum soltion and aerated by
blowing atmospheric air.
 Ferric chloride
The tannery effluent is treated with ferric chloride. With improves the clarity
and removes objectionable sulphide from the effluents.
 Flue-gas –lime
In this process flue gas is blown and lime is addes two or three times the
carbon di oxide of flue gas reacts with lime to precipitate calcium
carbonate.