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Green Chemistry for Chromium Based Industries: A Case of Chromium In Leather Processing. Prof. Dr. Asit Baran Mandal Director Central Leather Research Institute Adyar, Chennai. Developing Environmental Compliance Assistance Centre for Tannery and Chrome Chemicals Manufacturing Sector

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Green chemistry for chromium based industries a case of chromium in leather processing l.jpg

Green Chemistry for Chromium Based Industries: A Case of Chromium In Leather Processing

Prof. Dr. AsitBaranMandal


Central Leather Research Institute

Adyar, Chennai

Developing Environmental Compliance Assistance Centre

for Tannery and Chrome Chemicals Manufacturing Sector

West Bengal Pollution Control Board

18 May 2010

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Gap Areas in Green Chemistry for Chromium Based Industries Chromium In Leather Processing



How to avoid Cr(VI) formation?



How to avoid Cr(VI) leaching?

How to overcome size

limitations in Cr recovery?



Knowledge Gaps

How to respond to ecobans?

What do we do with chromium

containing solid wastes?


Can we replace/substitute

Cr in Tanning?

Can we replace/substitute

Cr in pigments?


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Green Chemistry Initiative for Chromium In Leather ProcessingChromite Mining Technologies

  • HCrO4- + 3Fe2+ + 7H+ ----

    Cr3+ + 3Fe3+ + 4H2O

    • Practical difficulties for large volumes of treatment

    • Generates SO42- ions

  • A natural product containing gallic acid chosen as reductant and proton source

    • Myrobalan (Terminalia chebula) a hydrolysable tannin containing glucose and gallic acid used

    • Process does not produce sulfates


Chromite Ore



40-65 mgkg-1

Frictional Heat

Innovation of Commercial Acceptance

Implemented at M/s. Tata Iron & Steel Co., Jamshedpur

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Chromium(VI) in Solid Residues from Ore Processing Chromium In Leather Processing


O2, 1300oC

  • 1 ton chromite ore processing residue (COPR) generated per ton chromate produced

  • 15-20% of Cr in COPR is mobile

    • For e.g. hydrogarnet - (Ca3Al2(OH)12(CrO4)3), hydrocalumite - (Ca4Al2(OH)12(CrO4)6H2O) and ettringite - (Ca6Al2(OH)12(CrO4)3 26H2O)

  • Current day methodology is to immobilize these Cr(VI) forms as Cr(III), followed by land filling

    • Fe + CrO42- + 0.5 H2O + 2H+ → Fe(OH)3 + 0.5 Cr2O3

    • 6Fe2+ + 2CrO42- + 13 H2O → 6 Fe(OH)3 + Cr2O3 + 8H+

      Unsolved problem: Leaching of Chromium

      Insecurity with the Secure Landfill








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Green Chemistry for Chromium In Leather ProcessingChromite Ore Processing Industries

  • Currently employed process chemistries of chromite ore processing residues are based on immobilizing the 15-20% of mobile chromium

  • Our novelty is to mobilize totally and selectively the immobile chromium. This would take some doing by change of process chemistry

  • We have been able to select a process chemistry for mobilizing all chromium in COPR selectively

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Influence of Nature of Chromium In Leather ProcessingExtractant on Chromium Mobilization

Na2O2 + 2H2O  2NaOH +H2O2+ Heat

H2O2 H2O + ½O2

Cr2O3 + 4NaOH + 1.5O2 2Na2CrO4

Extractive strategies of chromium need to overcome diffusion problems. COPR sizes vary in mm to cm. How do we access chromium within this residue?

Issue of Concern: Accessibility to occluded Cr

Options Available: Mechanical or Oxidative breakdown of particles

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Leather: A Down Stream Chromium Based Industry Chromium In Leather Processing

Green chemistry foresight

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Chromium in Tannery Wastewaters: Needing Specific Solutions Chromium In Leather Processing

~3000 mg/L

Chrome Tanning

~1000 mg/L





tailor made


Dyeing & Fatliquoring

~100 mg/L

~250 mg/L


Chromium in ETP Sludge

~50000 mg/kg

Chromium in leather wastes

~20000 mg/kg

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Four Part Approach Chromium In Leather Processing

  • Recognize what is not absorbed and coax the unbound chromium to bind through structural modification

    • Incremental change approach

  • Recover the unbound chromium and reuse in another form

    • Near zero Cr discharge tanning methodology

  • Identify what is not used and avoid the formation of undesirables.

    • Near zero- (Cr) discharge tanning salt approach

  • When the rogue species persist, replace the element of chromium as a whole

    • Chromium free approach

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Chrome Tanning : Its Impact in India Chromium In Leather Processing

  • ~ 1 billion kgs of hides/skins processed annually

  • ~ 90% hides/skins receive chrome tanning

  • Chrome exhaustion levels vary in commercial units from 40 - 70% of Cr used

  • About 40,000 tons of BCS per annum consumed annually

  • About 4,000 tons of Cr wasted annually

  • Tanneries face legislative pressures

  • ~ 40 crore worth of chrome salts wasted annually

  • End of pipe treatment affords large amounts of chrome sludge (160,000 tons per annum)

  • Disposal of sludge is a serious problem

Secured Chrome Management: An Imminent Need

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Possible Causes for Poor Uptake of Chromium(III) Chromium In Leather Processing

  • High kinetic inertness of Cr(III) ions

  • Lower binding constants for the complexation with functional sites in collagen

  • Diffusion related difficulties

  • Lack of availability of Cr(III) binding sites in protein

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4+ Chromium In Leather Processing















Dimer, 1









Tetramer, 3


Trimer, 2

Chromium Species: Varied Binding

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A True Scientific Solution to the Problem of Poor Uptake Involves

  • Avoiding the formation of low affinity species

  • Modifying the chemical structure of tetramer

  • Converting low affinity into high affinity species

  • Developing a near zero waste material

  • Developing a near zero waste tanning method

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Designing a High Uptake BCS Salt Involves

  • Factors influencing the quality of BCS

    • Cr(VI)/acid ratio

    • Reaction temperature

    • Order of addition of reductant/acid

    • Rate of addition of reductant/acid

    • Basification pH

    • Ageing time

    • Mode of drying

  • Scavenging the precursors leading to the formation of tetramer in manufacture of BCS is the process logic.

  • A new modified BCS salt exhibiting >85% Cr exhaustion prepared and commercialized

Technology developed and transferred to

M/s. Golden Chemicals Ltd., Mumbai

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Chrome Tanning: Current Practices Involves

  • Currently employed chrome tanning methods require acidification (pickling) as a preconditioning process

  • This causes not only TDS emission but also requires a de-acidification step (Basification)

  • An ordinary pickle-free chrome tanning process employs conventional basic chromium sulfate salt at a higher pH

  • The unsuccessfulness of this process are the danger of swelling, poor penetration and low uptake of chromium

Calls for single step chrome tanning ……

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Pickle-less Chrome Tanning: Process Profile Involves

A new strategy has been designed to carry chrome tanning without pickling and basification steps with the help of formaldehyde free polymeric synthetic tanning agent

Know-how transferred to M/s BalmerLawrie and Co.

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Pickle–Basification Free Chrome Tanning Involves

  • Conventional method of chrome tanning leads to substantial TDS and chromium load

  • A pickle–basification free chrome tanning at pH 5.0 developed

  • A polymeric matrix based syntan has been prepared for pickle-less tanning

  • Chrome exhaustion is improved from 70 to 94%

  • The product enables reduction in TDS and chlorides by 85 and 99%

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Chrome Recovery and Reuse: InvolvesAn Easy and Practicable Solution

  • This involves the precipitation of all unbound chromium in the form of chromic hydroxide and redissolution for reuse under controlled conditions.

  • Batch processes are adopted based on the use of magnesium oxide as alkali

  • Adopted by all chrome tanning units in India

  • A continuous process developed to increase process capacity

  • Provides wealth from waste

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Semi-Continuous Chrome Recovery – Design Features Involves




Flow Tube

Primary Settler





Stable hydrostatic


Improved aggregation

Better compaction

For trapping fine



pH 8.0-8.5

Increased friction

Improved interaction


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Continuous Chrome Recovery: InvolvesIn Industrial Practice

  • Under steady state conditions, the fluid flow into and out of the settler are matched

  • The process is independent of nature of alkali and can be used to treat effluents containing oils and fats and low concentrations of chromium

Implemented at Tannex. Panruti (24 MLD)

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Chromium Recycling: Challenges Involves

  • Direct recycling of chromium into the pickling process leads to surface fixation and coarseness of the skin surface

  • This can be overcome through

    • Preacidifying the spent chrome liquor to pH 1.0 to convert cationics to anionics, thereby enabling direct recycling

Implemented in 4 Units under the Collaborative Program

with CSIRO Australia

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High Exhaust Chrome Tanning: InvolvesA New Methodology Based Logic

  • Employ a tailor made aluminium salt (Alutan) which resists hydrolysis and precipitation at tanning pH conditions

  • Alutan-BCS Combination Tanning

    • Employ easily available materials

    • Permits closed pickle-tan loop

    • Enables near zero waste tanning

    • Easy to adopt

    • Field experience gained and shared

    • Help in containing sulfate discharge

Implemented in 9 Units under the Leather Technology Mission, GoI

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Chromium Bearing Solid Wastes: Issues Involves

  • Generation: 5600 tons per annum

  • Cr concentration in dry waste: 2-3 g/kg

  • Moisture: 50%

  • Current disposal methods

    • Leather boards

    • Dumping low lying areas

    • Incineration

    • Hydrolysis to recovery gelatin

  • Issues of concern

    • Leaching of Cr into soil and groundwater

    • Air pollution

    • Risk of converting Cr(III) to Cr(VI)

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Semi-Chrome Tanning: Unique Issues of Concern to India Involves

  • Chrome tanning carried out after vegetable tanning

  • Process sequence employs a stripping process after vegetable tanning prior to chrome tanning

  • Wastewater after chrome tanning contains a mixture of polyphenols (6000 mg/L) and chromium (2500 mg/L)

  • Conventional precipitation of chromium as its hydroxide results in co-precipitation of quinones and other intermediates from the tannins

  • Oxidative treatment to remove tannins results in Cr(III)  Cr(VI) conversion

  • Current option is to dispose the chromium bearing wastewater to effluent treatment plants, where chromium precipitates along with tannins

Solution to the problem: Selective Removal of Tannins

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Chrome Shavings – As an Adsorbent Involves

  • Collagen contains both COO- and NH2+ groups

  • Chrome tanning process blocks the COO- groups significantly

  • Chrome shavings presents

    • A low/zero cost adsorbent

    • Predominant absence of COO- groups

    • Presence of –CO, -CO-NH and NH2 groups for binding vegetable tannins

      • Selectivity!

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Chromium-Collagen : Vegetable Tannin Interactions Involves

The Issue

The Solution

Removal of vegetable tannin from a waste water containing chromium and tannins increased with pH

Commonly used mimosa under the pH investigated is anionic and hence instantaneously complexes with Cr-Collagen (shaving)

However, at pH values above 5.0 chromium would precipitate as its hydroxide

To avoid formation of Cr-hydroxide and quinones adsorption studies were carried out at pH 3.5

Conventional adsorption processes adopt desorption as the subsequent step. Adsorbed tannins have no utility value

It has now been possible to use the adsorbed shavings as a reductant in the place of molasses in the manufacture of basic chromium sulfate for leather processing

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Three Pot Solutions to Leather Wastes & Waste Waters Involves

Cr-Veg effluent

Dye stream

  • In the first pot the chromium bearing shaving dusts have been used as adsorbents to selectively remove tannins from tannin-chromium mixed wastewaters or dyes from dye bearing streams

  • In the second pot the tannin/ dye adsorbed shaving is used as reductant and a source of electron, in the place of molasses, to generate basic chromium sulfate from Cr(VI) for use in tanning industry

  • In the third pot the tannin/dye free chrome liquor is precipitated, redissolved in sulfuric acid, to generate chromium(III) sulfate liquor for tanning




Tannin/ dye free

Chrome liquor

Cr Shaving







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Advantage of the Three Pot Method Involves

  • An effective and inexpensive method for treating phenolics/ dyes bearing waste waters

  • Simplicity in operation

  • No sludge formation

  • Aerobic oxidation methods require large space and higher levels of process control

  • Oxidation processes like Fenton’s method result in undesirable end products

  • Other catalytic methods are not economical in large scale applications

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Green Chemistry for InvolvesChromium Based Industries: Some achievements with industrial level success

Use myrobalan

Implemented at Jamshedpur



How to avoid Cr(VI) formation?

Mobilize & recover instead of


Searching for industrial partner



How to avoid Cr(VI) leaching?

Adopt semi-continuous methods

Implemented at Panruti

How to overcome size

limitations in Cr recovery?



How to respond to ecobans?

Adopt Cr-Fe tanning and

phenolic striking

What do we do with chromium

containing solid wastes?

Adopt three pot approach


Substitute the chromium

Can we replace/substitute

Cr in Tanning?

Use rare earth pigments

Can we replace/substitute

Cr in pigments?


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Thank You Involves