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






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

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Slide 1

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

Prof. Dr. AsitBaranMandal

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

Slide 3

Gap Areas in Green Chemistry for Chromium Based Industries

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

Knowledge Gaps

How to respond to ecobans?

What do we do with chromium

containing solid wastes?

Tanning

Can we replace/substitute

Cr in Tanning?

Can we replace/substitute

Cr in pigments?

Pigments

Slide 4

Green Chemistry Initiative forChromite 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

Cr(VI)

Chromite Ore

Milling

Fines

40-65 mgkg-1

Frictional Heat

Innovation of Commercial Acceptance

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

Slide 5

Chromium(VI) in Solid Residues from Ore Processing

H+

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

Chromite

FeCrO4

Chromate

CrO42-

Dichromate

Cr2O72-

Residue

Slide 6

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

Slide 7

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?

Issue of Concern: Accessibility to occluded Cr

Options Available: Mechanical or Oxidative breakdown of particles

Slide 8

Leather: A Down Stream Chromium Based Industry

Green chemistry foresight

Slide 9

Chromium in Tannery Wastewaters: Needing Specific Solutions

~3000 mg/L

Chrome Tanning

~1000 mg/L

Rechroming

Differing

Conditions

Needing

tailor made

Solutions

Dyeing & Fatliquoring

~100 mg/L

~250 mg/L

Washings

Chromium in ETP Sludge

~50000 mg/kg

Chromium in leather wastes

~20000 mg/kg

Slide 10

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

Slide 11

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

Slide 12

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

Slide 13

4+

4+

OH

OH

Cr

Cr

Cr

Cr

OH

OH

5+

O

O

OH

OH

Dimer, 1

Cr

Cr

Cr

Cr

OH

OH

OH

HO

Tetramer, 3

Cr

Trimer, 2

Chromium Species: Varied Binding

Slide 14

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

Slide 15

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

Slide 16

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 chromium

Calls for single step chrome tanning ……

Slide 17

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 BalmerLawrie and Co.

Slide 18

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%

Slide 19

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

Slide 20

Semi-Continuous Chrome Recovery – Design Features

Na2CO3/NaOH

Cr(OH)SO4

Cr(OH)3

Flow Tube

Primary Settler

Secondary

Settler

Neutralization

Tank

Stable hydrostatic

Pressure

Improved aggregation

Better compaction

For trapping fine

particles

Instantaneous

pH 8.0-8.5

Increased friction

Improved interaction

Aggregation

Slide 21

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)

Slide 22

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

Slide 23

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

Slide 24

Closed Loop Tanning

Slide 25

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)

Slide 26

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

Slide 27

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!

Slide 28

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

Slide 29

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

Tannin/ dye free

Chrome liquor

Cr Shaving

Na2CO3

CrIII(OH)3

Cr(VI)

Reduction

H2SO4

BCS

Slide 30

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

Slide 31

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

Use myrobalan

Implemented at Jamshedpur

Chromite

Mining

How to avoid Cr(VI) formation?

Mobilize & recover instead of

Immobilizing

Searching for industrial partner

Chromite

Processing

How to avoid Cr(VI) leaching?

Adopt semi-continuous methods

Implemented at Panruti

How to overcome size

limitations in Cr recovery?

Downstream

Applications

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

Tanning

Substitute the chromium

Can we replace/substitute

Cr in Tanning?

Use rare earth pigments

Can we replace/substitute

Cr in pigments?

Pigments

Slide 32

Thank You


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