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
West Bengal Pollution Control Board
18 May 2010
Chromium: Industrial Applications
Cr(VI)
Recycled
Cr(III)
Metallurgy
chromium
Ore dressings
Galvanizing
Recovered Cr
Tanning
hydroxide from
Electroplating
Pigments/dyes
tanning
Pigments /dyes
Fine chemicals
Fine chemicals
Chromite
Mining
How to avoid Cr(VI) formation?
Chromite
Processing
How to avoid Cr(VI) leaching?
How to overcome size
limitations in Cr recovery?
Downstream
Applications
How to respond to ecobans?
Tanning
What do we do with chromium
containing solid wastes?
Pigments
Can we replace/substitute
Cr in Tanning?
Can we replace/substitute
Cr in pigments?
Knowledge Gaps
Gap Areas in Green Chemistry for
Chromium Based Industries
Green Chemistry Initiative for
Chromite Mining Technologies
Chromite Ore
Milling
Cr(VI)
Fines
40-65 mgkg-1
Frictional Heat
• 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
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
0
Innovation of Commercial
Acceptance
Implemented at M/s. Tata Iron
& Steel Co., Jamshedpur
Chromium(VI) in Solid Residues from
Ore Processing
Chromite
FeCrO4
O2, 1300oC
Chromate
CrO42-
H+
Residue
Dichromate
Cr2O72-
 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
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
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?
Extractant
Na2O2
%Cr
Mobilized
Particle size
Reduction
(%)
0.2
mm
2.0
mm
0.2
mm
2.0
mm
96.2
95
90
85
Issue of Concern: Accessibility to occluded Cr
Options Available: Mechanical or Oxidative breakdown of particles
Leather: A Down Stream Chromium
Based Industry
Green chemistry foresight
Chromium in Tannery Wastewaters:
Needing Specific Solutions
Chrome Tanning
~3000 mg/L
Rechroming
~1000 mg/L
Dyeing &
Fatliquoring
~100 mg/L
Washings
~250 mg/L
Chromium in ETP
Sludge
~50000 mg/kg
Chromium in
leather wastes
~20000 mg/kg
Four Part Approach
 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
Chrome Tanning : Its Impact in India
 ~ 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
Possible Causes for Poor Uptake of
Chromium(III)
 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
Chromium Species: Varied Binding
Degree of Polymerization
Shrinkage Temperature (oC)
2 (dimer)
100
3 (trimer)
84
4 (tetramer)
68
Native collagen
60
5+
OH
Cr
Cr
4+
OH
Cr
Cr
OH
OH
HO
Cr
OH
Dimer, 1
Trimer, 2
4+
OH
Cr
Cr
OH
O
O
OH
Cr
Cr
OH
Tetramer, 3
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
Designing a High Uptake BCS Salt
 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
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 deacidification 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 ……
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




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%
Emission load (Kg/ton of raw
skins)
 Conventional method of chrome
160
140
120
100
80
60
40
20
0
TDS
Cl
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
Na2CO3/NaOH
Cr(OH)SO4
Neutralization
Tank
Cr(OH)3
Flow Tube
Primary Settler
Stable hydrostatic
Pressure
Improved aggregation
Better compaction
Instantaneous
pH 8.0-8.5
Increased friction
Improved interaction
Aggregation
Secondary
Settler
For trapping fine
particles
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 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
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 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
Closed Loop Tanning
Chromium Bearing Solid Wastes: Issues
 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)
Semi-Chrome Tanning: Unique Issues
of Concern to India
 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
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 adsorbent
 Predominant absence of COO- groups
 Presence of –CO, -CO-NH and NH2 groups for binding
vegetable tannins
 Selectivity!
Chromium-Collagen : Vegetable Tannin
Interactions
The Issue
The Solution
 Removal of vegetable tannin from a
 Conventional adsorption processes
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
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
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
Adsorbed
shavings
Cr(VI)
Reduction
Cr Shaving
Tannin/ dye free
Chrome liquor
Na2CO3
CrIII(OH)3
BCS
H2SO4
Advantage of the Three Pot Method
 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
Green Chemistry for
Chromium Based Industries: Some achievements with
industrial level success
Chromite
Mining
Chromite
Processing
How to avoid Cr(VI) formation?
How to avoid Cr(VI) leaching?
How to overcome size
limitations in Cr recovery?
Use myrobalan
Implemented at Jamshedpur
Mobilize & recover instead of
Immobilizing
Searching for industrial partner
Adopt semi-continuous methods
Implemented at Panruti
Downstream
Applications
How to respond to ecobans?
Adopt Cr-Fe tanning and
phenolic striking
Adopt three pot approach
Tanning
What do we do with chromium
containing solid wastes?
Pigments
Can we replace/substitute
Cr in Tanning?
Can we replace/substitute
Cr in pigments?
Substitute the chromium
Use rare earth pigments
Thank You