Prof. Dr. Asit Baran MandalDirector

Central Leather Research InstituteAdyar, Chennai

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

Developing Environmental Compliance Assistance Centre for Tannery and Chrome Chemicals Manufacturing Sector

West Bengal Pollution Control Board18 May 2010

Gap Areas in Green Chemistry for Chromium Based Industries

ChromiteMining

ChromiteProcessing

DownstreamApplications

How to avoid Cr(VI) formation?

How to avoid Cr(VI) leaching?

How to overcome size limitations in Cr recovery?

How to respond to ecobans?

What do we do with chromium containing solid wastes? K

now

led

ge G

ap

s

Can we replace/substitute Cr in Tanning?

Tanning

Pigments Can we replace/substitute Cr in pigments?

Green Chemistry Initiative forChromite Mining Technologies

• HCrO4- + 3Fe2+ + 7H+ ----

Cr3+ + 3Fe3+ + 4H2O– Practical difficulties for large

volumes of treatment– Generates SO4

2- 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 Milling Fines Cr(VI)

Frictional Heat

40-65 mgkg-1

Concentration of Myrobalan

(g/Kg Ore)

Residual Cr(VI)

(mg/Kg)

1.0’ 2.0’ 15’

7 2.5 1.1 0.09

15 1.0 0.03 0

65 0.05 0 0

125 0.02 0 0Innovation of Commercial Acceptance

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

Jamshedpur

Chromium(VI) in Solid Residues from Ore Processing

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 + CrO4

2- + 0.5 H2O + 2H+ → Fe(OH)3 + 0.5 Cr2O3 6Fe2+ + 2CrO4

2- + 13 H2O → 6 Fe(OH)3 + Cr2O3 + 8H+

Unsolved problem: Leaching of ChromiumInsecurity with the Secure Landfill

ChromiteFeCrO4

ChromateCrO4

2-

DichromateCr2O7

2-

O2, 1300oC H+

Residue

Green Chemistry for Chromite 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

Influence of Nature of Extractant on Chromium Mobilization

Issue of Concern: Accessibility to occluded CrOptions Available: Mechanical or Oxidative breakdown of particles

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

Na2O2 + 2H2O 2NaOH +H2O2+

Heat

H2O2 H2O + ½O2

Cr2O3 + 4NaOH + 1.5O2

2Na2CrO4

Extractant %Cr Mobilized

Particle size Reduction

(%)

0.2 mm

2.0 mm

0.2 mm

2.0 mm

Na2O2 96.2 95 90 85

Green chemistry foresight

Leather: A Down Stream Chromium Based Industry

Chromium in Tannery Wastewaters: Needing Specific Solutions

Chrome Tanning

Rechroming

Dyeing & Fatliquoring

Washings

Chromium in ETP Sludge

Chromium in leather wastes

~3000 mg/L

~1000 mg/L

~100 mg/L

~250 mg/L

~50000 mg/kg

~20000 mg/kg

Differing

Conditions

Needing

tailor m

ade

Solutions

Four Part Approach

Recognize what is not absorbed and coax the unbound chromium to bind through structural modificationIncremental change approach

Recover the unbound chromium and reuse in another formNear 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

Chrome Tanning : Its Impact in India

~ 1 billion kgs of hides/skins processed annually~ 90% hides/skins receive chrome tanningChrome exhaustion levels vary in commercial

units from 40 - 70% of Cr usedAbout 40,000 tons of BCS per annum consumed

annuallyAbout 4,000 tons of Cr wasted annuallyTanneries face legislative pressures~ 40 crore worth of chrome salts wasted

annuallyEnd 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

Possible Causes for Poor Uptake of Chromium(III)

High kinetic inertness of Cr(III) ionsLower binding constants for the complexation

with functional sites in collagenDiffusion related difficultiesLack of availability of Cr(III) binding sites in

protein

Chromium Species: Varied Binding

Degree of Polymerization Shrinkage Temperature (oC)

2 (dimer) 100

3 (trimer) 84

4 (tetramer) 68

Native collagen 60

Cr CrOH

OH

4+

Dimer, 1

Cr CrOH

OH

Cr

OHHO

5+

Trimer, 2

Cr CrOH

OH

Cr CrOH

OH

O O

4+

Tetramer, 3

A True Scientific Solution to the Problem of Poor Uptake Involves

Avoiding the formation of low affinity speciesModifying the chemical structure of tetramerConverting low affinity into high affinity speciesDeveloping a near zero waste materialDeveloping a near zero waste tanning method

Designing a High Uptake BCS Salt

Factors influencing the quality of BCSCr(VI)/acid ratioReaction

temperatureOrder of addition of

reductant/acidRate of addition of

reductant/acidBasification pHAgeing timeMode 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

Chrome Tanning: Current Practices

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 chromiumCalls for single step chrome tanning ……

Pickle-less Chrome Tanning: Process Profile

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 Balmer Lawrie and Co.

Pickle–Basification Free Chrome Tanning

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%

Chrome Recovery and Reuse:An 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

Semi-Continuous Chrome Recovery – Design Features

Neutralization

Tank

Primary Settler

SecondarySettler

Flow Tube

Na2CO3/NaOHCr(OH)SO4

Cr(OH)3

InstantaneouspH 8.0-8.5

Increased frictionImproved interaction

Aggregation

Stable hydrostaticPressure

Improved aggregationBetter compaction

For trapping fineparticles

Continuous Chrome Recovery:In 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)

Chromium Recycling: Challenges

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

This can be overcome throughPreacidifying 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

High Exhaust Chrome Tanning: A New Methodology Based Logic

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

Alutan-BCS Combination TanningEmploy easily available materialsPermits closed pickle-tan loopEnables near zero waste tanningEasy to adoptField experience gained and sharedHelp in containing sulfate discharge

Implemented in 9 Units under the Leather Technology Mission, GoI

Closed Loop Tanning

Chromium Bearing Solid Wastes: Issues

Generation: 5600 tons per annumCr concentration in dry waste: 2-3 g/kgMoisture: 50%Current disposal methods

Leather boardsDumping low lying areasIncinerationHydrolysis to recovery gelatin

Issues of concernLeaching of Cr into soil and groundwaterAir pollutionRisk of converting Cr(III) to Cr(VI)

Semi-Chrome Tanning: Unique Issues of Concern to IndiaChrome tanning carried out after vegetable

tanningProcess sequence employs a stripping process

after vegetable tanning prior to chrome tanningWastewater 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

Chrome Shavings – As an Adsorbent

Collagen contains both COO- and NH2+ groups

Chrome tanning process blocks the COO- groups significantly

Chrome shavings presents A low/zero cost adsorbentPredominant absence of COO- groupsPresence of –CO, -CO-NH and NH2 groups for

binding vegetable tanninsSelectivity!

Chromium-Collagen : Vegetable Tannin Interactions

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

Three Pot Solutions to Leather Wastes & Waste Waters

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

Cr-Veg effluentDye stream

Tannin/ dye freeChrome liquor

Tannin/dye Adsorbed shavings Na2CO3

BCSH2SO4

Cr Shaving

Cr(VI) Reduction

CrIII(OH)3

Advantage of the Three Pot Method

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

Simplicity in operationNo 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

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

ChromiteMining

ChromiteProcessing

DownstreamApplications

How to avoid Cr(VI) formation?

How to avoid Cr(VI) leaching?

How to overcome size limitations in Cr recovery?

How to respond to ecobans?

What do we do with chromium containing solid wastes?

Can we replace/substitute Cr in Tanning?

Tanning

Pigments Can we replace/substitute Cr in pigments?

Use myrobalanImplemented at Jamshedpur

Use myrobalanImplemented at Jamshedpur

Mobilize & recover instead of

ImmobilizingSearching for industrial

partner

Mobilize & recover instead of

ImmobilizingSearching for industrial

partnerAdopt semi-continuous methodsImplemented at Panruti

Adopt semi-continuous methodsImplemented at Panruti

Adopt Cr-Fe tanning and phenolic striking

Adopt Cr-Fe tanning and phenolic striking

Adopt three pot approachAdopt three pot approach

Substitute the chromiumSubstitute the chromium

Use rare earth pigmentsUse rare earth pigments

Thank You