Central Electrochemical Research InstituteCentral Electrochemical Research Institute

Environmental management in Environmental management in Indian Chlor-Alkali IndustriesIndian Chlor-Alkali Industries

- a presentation on CECRI’s Contribution- a presentation on CECRI’s Contribution

Dr.K.Subramanian, Senior Principal Scientist Chlor-Alkali Division, Central Electrochemical Research

Institute, KARAIKUDI – 630 006. TN.India

Chlor-Alkali ProcessesChlor-Alkali Processes

The chlorine and caustic soda are produced from the electrolysis of an aqueous solution of common salt or cooking salt

2NaCl + 2H2 O Cl2+ H2+ 2NaOH

The term Chlor-Alkali is derived from the two products produced: chlor from chlorine and alkali from sodium hydroxide, which is part of a group of chemicals called alkalis.

For one ton of Caustic soda produced, 0.88 ton of chlorine and 0.12 ton of Hydrogen are also produced

Production scenarioProduction scenario2003-2004

World’s production of caustic soda is about 700 lakh tonnes per annum

India’s Production amounts to 17 lakh tonnes

India’s Installed capacity is 23 lakh tonnes per annum

Conventional Technological Conventional Technological ProcessesProcesses

There are two conventional technologies for chlorine and caustic soda production:

Mercury cell process and Diaphragm Cell (asbestos) processes.

These processes lead to environmental pollution as they use hazardous materials (asbestos and mercury) as hardware and prone to emission of pollutants.

Diaphragm Cell ProcessDiaphragm Cell ProcessThere are two type of cells :1) Rectangular Hookers’ or Dow Cell and2) Dryden or Vorce cylindrical cell.

Asbestos was used as diaphragm separating anode and cathode

Asbestos is of good chemical stability and ion exchange properties and relatively inexpensive.

Asbestos in the form of fiber as diaphragm allows percolation of brine from anode side to cathode side

It is a homogenous mixture of Al2O3, SiO2, CaO, MgO and other acid insoluble salts mostly oxides. Asbestos is coated on to the cathode screen to a thickness of 3 to 4 mm by vacuum deposition. The permeability of the membrane is a very vital process parameter.

Brine enters the anode compartment and completely covers the anode and cathode fingers

Anolyte and Na+ ions flow through the diaphragm into the cathode compartment due to the difference in liquid level between the two compartments

Catholyte is 12 wt% NaOH and 16 – 18 wt % NaCl.

The cell liquor can be fed to a triple effect evaporator to get 50 wt% caustic (impurity 1 wt% NaCl).

 

Anode: Cl- ½ Cl2 + e ; Eo : 1.36 VCathode: H2O + e ½ H2 + OH- Eo : -0.83 V Overall reaction:H2O + Cl- Cl2 + H2+OH-

Eo : 2.19V

Hookers’ Rectangular Cell

Hookers’ Diaphragm Cell House(plant)

Mercury Cell ProcessMercury Cell Process The process involves two cells – an electrolyser

and a decomposer. In the electrolyser, flowing mercury is the

cathode and graphite or TSIA-(Titanium substrate insoluble anode) as anode.

Purified aqueous NaCl solution is electrolysed between anodes and flowing mercury cathode.

While chlorine is liberated at the anode, sodium ions are reduced at the Hg cathode forming Na-amalgam (0.20-0.25wt%).

The amalgam flows over to the decomposer where it reacts with water to form caustic (50wt%)solution, hydrogen gas and pure Hg. Hg is recycled to the electrolyser.

Caustic from this process contains only 50 ppm of NaCl.

 

Anode: Cl- ½ Cl2 + e ; Eo : 1.36 V

Cathode(Hg): Na+ + e Na(Hg)Eo : -1.77 VNa+ + Cl- Cl2 + Na(Hg) ;Eo : 3.13V

Decomposer reaction:H2O + Na(Hg) H2+NaOH- + Hg

Eo : 1.08V

A view down the middle aisle of 70,000-tonnes/year mercury process chlorine plant in The Netherlands. The circular vessels are amalgam decomposers

Graphite AnodesGraphite Anodes During operation, Graphite is consumed due to

electrochemical attack of O2, resulting from a side reaction.

2H2O_ 2H2 + O2

and chemical attack by HClO present in the electrolyte due to the dissolution of Cl2.

Cl2 + H2O HClO + HClC+ HClO CO + HCl

These reactions weaken intercrystalline bonds and graphite begins to spall off as result of mechanical erosion. Graphite consumption is of the order of 2 to 4 kg per tonne of caustic produced. Entire graphite anode set has to be replaced once in 10 to 12 months.

The graphite particles are plugged into the pores of asbestos diaphragm .This leads to higher resistance and it becomes ineffective

Periodic adjustment for maintaining the fixed (3-4mm) interelectrode gap between Hg and graphite/ replacement of graphite anodes increases operating cost and leads to substantial emissions and spillages of mercury to the environment in the case of mercury cells

Corrosion &Erosion of graphite results in the widening of the anode – cathode gap and cell voltage and power consumption increase.

Products are contaminated. Cl2 contained CO2 and NaOH has graphite particles.

ENVIRONMENTAL POLLUTION ENVIRONMENTAL POLLUTION FROM CONVENTIONAL CHOLR-FROM CONVENTIONAL CHOLR-

ALKALI MANUFACTURING ALKALI MANUFACTURING TECHNOLOGIESTECHNOLOGIES

Diaphragm Cell:

Asbestos is Carcinogenic.

Mercury Cell:

Mercury pollution

Environmental Pollution standardsEnvironmental Pollution standards( MINAS/CPACB standards for Chlor-Alkali Industries )( MINAS/CPACB standards for Chlor-Alkali Industries )

Parameter

MERCURY

Limit

Hg in liquid effluent < 0.01 mg/l

Hg from H2 gas holder stack 0.2 mg/Nm3

Hg in ambient air 0.03 mg/m3

ASBESTOS:

For Asbestos carrying waste water, slurry should be collected in containers which should be buried

To control asbestos in air, exhaust gases should be passed through bag filter limits 5 length and 3 width of fiber

Worldwide Norms of mercury Worldwide Norms of mercury DischargeDischarge

Country Norms of Hg discharge in all Effluent, gm/ton of

NaOH

Canada 2.50

France 10.00

India* 190.00 (* average)

Japan 0.63

Germany 3.00 ( practically now it is 1.3)

Sweden 3.25

USA 0.28

Pollution from Diaphragm CellPollution from Diaphragm Cell

In the case of diaphragm cells, when graphite anodes are used, graphite particles clog the diaphragm and impair the percolation of brine through it.

When the permeability of the diaphragm becomes very poor it has to be removed and re deposited.

As asbestos is carcinogenic, the frequency of its deposition is an environmental hazard

For one ton per day caustic production about 200-250 kgs of asbestos is required and it has to be replaced for every six months for Graphite anodes and for TSIA, it is about one year

With graphite anodes: graphite particles clog the diaphragm and impair the percolation of brine through it.

When the permeability of the diaphragm becomes very poor it has to be removed and re deposited. As asbestos is carcinogenic, the frequency of its deposition is an environmental hazard.

Handling of asbestos powder and its slurry in caustic solution causes cancer

Caustic will soften the skin of humans while handling asbestos slurry to make asbestos diaphragm over cathode and asbestos will get into touch with flush easily

Dry powder or asbestos in air goes to respiratory

system and causes cancer.

Pollution from Mercury CellsPollution from Mercury Cells In India, during 1976, 5.0 lakh tonnes of

caustic was produced, out of which 4.0 lakh tonnes were from mercury cells.

On an average 500 kg of mercury is required for the production of one tonne of caustic. O.25 kg of mercury is lost per tonne of caustic.

So in India, 1,00,000 kg of mercury was lost. This quantity is let out either to air or to water by way of industrial waste.

This quantity is needed every year for make up only provided, caustic production by this technology remains at the same level.

If production picks up, mercury requirement also will go up.

Ways of mercury emissions in mercury cell Plant

In chlor-alkali plant, the cells are running at 60 to 80oC. In such atmosphere, mercury, which acts as cathode can,

let out to air and water by the following outlets. 1) Cl2 gas, 2) H2 gas, 3) Caustic lye, 4) Clarifier and brine filter and caustic filter sludges, 5) Inlet and outlet end box flush water 6) Depleted brine which goes to the saturation tank. 7) Waste water from cleaning and building floors 8) Sulphuric acid from Cl2 drying system. 9) Handling losses 10) Cell room ventilation air. 

Spillages from the cell occur during opening of cell cover for anode adjustments / maintenance of cell

Frequency of opening of cell lead to larger volumes of spillages and emissions of mercury

The quantity of mercury that ultimately reaches the environment depends

on the anode material and its overall maintenance

on the operating efficiency of the plants and treatment processes installed.

Hazards of Hg pollution: Hg in water and air is a silent hazard. Hg , entering the environment through industrial

effluent discharges, enters human body either by inhalation of air containing Hg vapours or through food chain i.e., grass to cattle milk, thro, the soil to vegetables and thro, the tiny organisms in the river to fish.

Hg accumulated in human body affects the central nervous system thro’ brain damage, leads to convulsions and mental retardation and often death.

Hg vapour present in atmosphere is inhaled by human beings of which 75-80% is absorbed and accumulated in brain after diffusion across the membranes.

Subsequent Hg excretion from brain is very slow. Some metallic Hg oxidizes in blood and the Hg ions thus formed is bound to plasma proteins or haemoglobin

Environmental management (Pollution Environmental management (Pollution abatement)abatement)

Abatement of pollution from chlor-alkali industry can be done by

(i) treatment of effluents before discharging to air or waterways .

(ii) adopting technology which can lessen pollution.

(iii) adopting technology which does not involve pollutants at all

CECRI’s RoleCECRI’s Role

Development of Titanium Substrate Insoluble Anode (TSIA) – it replaced the Graphite both in diaphragm and mercury cells

Development of environment friendly Membrane Cell technology with innovative anode and cathode which completely avoids the pollutants (Hg & Asbestos)

Development of TSIADevelopment of TSIA

TSIA has a titanium base which is activated a catalytic coating consisting of a mixture of oxides of platinum group metals and oxides of valve metals. The coating is imparted by thermal decomposition.

The Technology has been licensed to three firms:1.Titanium Equipment and Anode Manufacturing Company Limited, Madras. (1976).(Production by 1978)2.Bharat Heavy Plates and Vessels Limited, Visakapatnam. (1981).3.Titanor Components Limited, Goa. (Formerly WIMCO, Bombay.) (1983). Now entire Chlor-Alkali production in India is from TSIA only

Advantages of TSIAAdvantages of TSIA

1.Dimensional stability. 2.Longer life. 3.Ability to function effectively at higher current densities. 4.Purer products. 5.Lesser cell interruptions and 6.10 –15 % power saving (350 Kwh / tonne of caustic) due

to a).Lower chlorine overvoltage . b).Lesser bubble effect resulting from higher free

surface for the escape of chlorine gas. c).Higher electrical conductivity. d).Lower and constant cell voltage and e).Operation at lower and constant inter electrode gap.

Life of TSIA The effective life of the electrocatalytic

coating ranges from 18 – 36 months depending upon the current density and the operating / cell working conditions in mercury cell and over 60 months in diaphragm cells

The structural life of Ti substrate is about 15 years

The life of graphite anode varies from 6 months to 10 months depends upon the CD and Cells

Frequency of anode adjustment and replacement is reduced: leading to reduction of Hg spillage & emission; Reduction in usage and dumping of asbestos

In addition to chlorine production, TSIA can be used for  1.Manufacture of chlorates, bromates and iodates. 2.Electrowinnig of metals. 3.Insitu generation of hypochlorite by the electrolysis of sea

water. 4.Cathodic protection by impressed current. 5.Electro - organic syntheses.

Environmental Protection & Power savingEnvironmental Protection & Power saving

POWER SAVED UPTO 31.03.2004 : 5300 MILLION Kwh COST OF POWER SAVED : Rs. 9500 MILLION

Hg pollution reduced by 60-70%: only 70-100 gms of Hg per ton of NaOH consumed in pollution process- lead to saving of 150-180 gms of Hg per ton of NaOH from reaching the environment

Total Hg saved by TSIA : 2108 tons (until 2002)

Asbestos consumption per ton of caustic is reduced from 900 gms to 400 gms saving about 500gm per ton

200-250 tons of asbestos getting polluted to indian earth per year until 1997.

As an environmental friendly and energy conserving alternate, Membrane cell technology for chlor–alkali production has been developed in India (CECRI) as well all over world.

In this technology, neither Asbestos nor Mercury is used.

The only imported component involved is CATION EXCHANGE MEMBRANE (CEM), which separates the anode compartment and the cathode compartment.

The anode compartment is made of Titanium and in it activated Titanium expanded mesh acts as anode.

The cathode compartment is made of Nickel and in it activated perforated Nickel sheet acts as cathode.

Membrane CellMembrane Cell

The first membrane cell plant in India was at M/s Chemfab Alkalis, Pondicherry in 1985 (Cl2 engineers, Japan).

CECRI has developed mono polar as well as bipolar membrane cell technologies with anodes and cathode indigenously developed at CECRI.

Dimensionally stable anodes and its coatings and catalytic cathodes are vital components of this technology. CECRI has developed ANODE and CATHODE (with innovative catalytic coatings) for these types of cells.

Catalytic anodes for membrane cells:

To fulfill the stringent conditions of membrane cell operation suitable anode electrocatalysts have been developed and licensed by CECRI.

The advantages of newly developed anode for use in membrane cells are as follows

1). Oxygen evolution is suppressed and product chlorine purity is high

2). High current efficiency of 94-95 3). Membrane damage due to improper

electrolyte circulation has been avoided and bubble effect has been eliminated

4). Service life of anode coating has been improved (>5 years)

Catalytic Cathodes for Membrane Cells: Catalytic cathodes consist of Nickel or Nickel-plated

SS activated by a coating of Nickel stabilised by a mixture of precious metals or their oxides.

Low hydrogen overvoltage of the order of 70 – 80 mV at 3 kA / m 2.

Energy saving due to these cathodes is 250 kWh per tonne of caustic.

The process for the manufacture of these cathodes was patented in India. (INDIAN PATENT: 179959 of 1990).

The Technology for activated cathodes were patented in USA.

(US PATENT: 5855751 of 1999)

Technological know how for the coating / recoating of membrane cell (1).Anodes and (2).Cathodes have been licensed to

M/S. GRASIM INDUSTRIES (CHEMICAL DIVISION) OF ADITYA BIRLA GROUP

They are using our technology for recoating the anodes and cathodes of their Membrane cells (Uhdenora). The performance of our electrodes is on par with the imported ones.

Uhdenora Cell with CECRI’s Anode and Cathode catalytic coatings

CECRI’s Anode Coating on UhDenora Anode PanCECRI’s Anode Coating on UhDenora Anode Pan

CECRI’s Cathode Coating on UhDenora Cathode PanCECRI’s Cathode Coating on UhDenora Cathode Pan