Lecture NoteEnviron Annauniv

8/3/2019 Lecture NoteEnviron Annauniv

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ENVIRONMENTAL MANAGEMENT IN INDIAN CHLOR-ALKALI INDUSTRIES

Dr.K.Subramanian, Scientist, Chlor-Alkali Division,Central Electrochemical Research Institute, Karaikudi-630006

Chlor-Alkali process is one of the chemical industry's leadingworkhorses and consists in the production of chlorine, sodiumhydroxide (caustic soda) and hydrogen as a by-product.

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

2NaCl + 2H 2 O Cl2 + H 2+ 2NaOH This industry is the second largest consumer of electricity next to

Aluminum.

The term Chlor-Alkali is derived from the two products produced:chlor from chlorine and alkali from sodium hydroxide, which is part of agroup of chemicals called alkalis. For one ton of Caustic sodaproduced, 0.88 ton of chlorine and 0.12 ton of Hydrogen are alsoproduced.

The caustic soda and chlorine industry is one of the largestchemical industry in the world as well as in India.

Chlorine and Caustic soda have wide spread usage. Chlorine is mainly used for the production of PVC, Inorganics,

pharmaceuticals(health sectors), automobiles, fire safety cloths,bleaching, water disinfection and purification.

Caustic soda is used for the production of organics, soaps, paper,bleach and mineral oils.

Hydrogen is used as a fuel, in fuel cells , for making hydrochloricacid and in hydrogenation of fatty acids to make refined foodproducts


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At present there are three processes available. Until 1980, there were onlytwo conventional production processes for Chlor-Alkali Production 1) Diaphragm celland 2) Mercury (amalgam) cell processes. Environmental and energy constraints leadto the development of modern membrane cell Process .

The worlds production of caustic soda comes around 700 lakh tonnes during2003-2004 and in India, it is about 17 lakh tonnes (23 lakh tonnes-installed capacity).Indian chloralkali industry is 60 years old. Electrolytic caustic production started inIndia in 1940. First cell was a Diaphragm cell at installed at Mettur Chemicals, Mettur,Tamil Nadu in 1940. First mercury cell plant was also at the same place in 1951. Firstdemonstration membrane cell plant was also set up in the same factory. All first in theIndustry occurred in Tamilnadu only.

The two conventional technologies for chlorine and caustic sodaproduction: Mercury cell process and Diaphragm (asbestos) processeslead to environmental pollution as they use hazardous materials(asbestos and mercury) as hardware and prone to emission of pollutants. The pollutant Hg comes from Hg cell process, the asbestosfrom diaphragm cell process. In this lecture the role played by CECRI incontaining the environmental hazards from this industry is discussed.Though the residual chlorine from liquefaction affects the environment to a great extent,contribution made in the electrochemistry angle only is discussed.

Let us see the each process in detail and how far it adds up thepollutant to the environment. Chlorine is produced by electrolysis whenan electric current is passed through a solution of brine (common saltdissolved in water). Co-products are caustic soda (sodium hydroxide)and hydrogen. All three are highly reactive, and technology has beendeveloped to separate them and keep them separate

2.1.DIAPHRAGM CELLS

Cathode separates the cell into two compartments onecontaining the anode and the other containing the cathode. Diaphragmis deposited on the cathode screen. Brine enters the anodecompartment and completely covers the anode and cathode fingers.
http://www.eurochlor.org/graphsect/cp12.htmhttp://www.eurochlor.org/graphsect/cp14.htmhttp://www.eurochlor.org/graphsect/cp12.htmhttp://www.eurochlor.org/graphsect/cp14.htm


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Chlorine leaves the cell through an outlet in the cell hood. Anolyte andsodium ions flow to through the diaphragm into the cathodecompartment because of the difference in liquid level between the twocompartments. Catholyte is 12 wt% NaOH and 16 18 wt % NaCl.

Hydrogen is produced at the cathode and leaves through an outlet onthe cathode. Cell liquor flows out through a level control pipe on thecathode chamber. The cell liquor can be fed to a triple effectevaporator operating on backward feed mode to get 50 wt% causticwhich contains 1 wt% NaCl. The diaphragm with all cell hardware isshown in Fig.2.1 and with specific details of asbestos with cathode andanode is shown in cross sectional view in Fig.2.3.

Hookers Cell


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Hookers Diaphragm Cell Plant

ENVIRONMENTAL HAZARDS OF ASBESTOS:Most vital component of the Diaphragm cell is the diaphragm, whichallows percolation of the brine from the anode side to the cathode side.Widely used Diaphragm material has been asbestos in the form of fiber. Asbestos was chosen because of its good chemical stability andits ion exchange properties. Asbestos is relatively inexpensive. It is ahomogenous mixture of Al 2O3 , SiO 2, CaO, MgO and other acid insolublesalts mostly oxides. Asbestos is coated on to the cathode surface to athickness of 3 to 4 mm by vacuum deposition. The permeability of themembrane is a very vital process parameter.

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 becomesvery poor it has to be removed and re deposited. As asbestos iscarcinogenic, the frequency of its deposition is an environmentalhazard.

Hazards of Hg pollution:


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Hg in water and air is a silent hazard. Hg , entering the environment throughindustrial effluent discharges, enters human body either by inhalation of air containingHg vapours or through food chain i.e., grass to cattle milk, thro, the soil to vegetables andthro, the tiny organisms in the river to fish. Hg accumulated in human body affects thecentral nevous system thro brain damage, leads to convulsions and mental retardation

and often death. [ Hg vapour present in atmosphere is inhaled by human beings of which75-80% is absorbed and accumulated in brain after diffusion across the membranes.Subsequent Hg exeretion from brain is very slow. Some metallic Hg oxidizes in bloodand the Hg ions thus formed is bound to plasma proteins or haemoglobin.

Hg Pollution:There are 25 Hg cell plants in our country having a production capacity of

9.63LTA. A conservative estimate of 0.25 kg of Hg loss per tonne of NaOH makes theenvironment richer by 200T of Hg. Every year around 80-85% of our annual import of about 250T is consumed by this industry. A major fraction of this Hg consumed (apartfrom carry our with products) is released into the environment through cell room

ventilation, brine sludge and waste streams

ASBESTOS IS CARCINOGENIC:Handling of asbestos powder and its slurry in caustic solution cause cancer.

Caustic will soften the skin of humans while handling asbestos slurry to make asbestosdiaphragm over cathode for diaphragm cell and asbestos will get into touch with flusheasily and dry powder or asbestos in air goes to respiratory system and causes cancer.

Mercury Cell Process :The electrolytic cell has titanium anodes located above a mercury cathode, which

flows along the bottom of the cell. Under the action of a direct current on brine, chlorineis released at the anode and sodium dissolves in the mercury cathode to give an amalgam.The sodium amalgam passes out of the electrolytic cell into a separate reactor, away fromthe chlorine. Here, it reacts with water to give hydrogen and 50% caustic soda . Thisregenerates the mercury, which is then returned to the electrolytic cell. Salt is added tothe brine leaving the cell and the brine is recirculated. The mercury process producesextremely pure, high quality caustic soda, suitable for use in textile applications.The Principle of mercury cell process is shown below


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Electrochemical Reactions:Primary Cell:ANODE: Cl - Cl 2 + e ; Cathode: Na + + e + Hg Hg-Na(0.25wt%)Secondary Cell (Denuder or decomposer):Hg-Na(0.25%) + H2O NaOH (50wt%) + H 2 + Hg


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


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Additional industrial sources of mercury emissions include :Iron and steel sectors Cement industryMunicipal and hospital waste incineratorsCoal-burning for power generation and industrial uses

Gold mining and refiningThermometersDental amalgam.Due to the complexity of identifying anthropogenic sources and possible emission rates,the OECD considers estimates of global emissions are extremely difficult to make.Global anthro-pogenic emissions of mercury to air were estimated at 3,560 tonnes in1983, and to water and soil at 4,600-8,300 tonnes, respectively (including atmosphericfall-out, but excluding disposal of mine tailings, smelter slags and waste).

Natural sourcesMost natural mercury deposits are at fairly shallow depths (a few metres to about 700metres). Mercury is extracted by underground and opencast mining of cinnabar ore.

Heating the ore, followed by condensation of the vapour produces liquid mercury. Globalmercury production has fallen steadily from 6,500 tonnes in 1986 to 3,260 tonnes in1996. Some 1,350 tonnes of mercury were mined in the EU in 1996, according to thereport prepared for EU Commission DG III. In addition to primary extraction, there issignificant secondary production of mercury, including recycling, recovery and industrialreprocessing. In 1982, the OECD estimated it to be up to 40% of primary production.OECD estimates of natural emissions of mercury to air, water and land range from 2,500-15,000 tonnes/year. These originate from soil, vegetation, forest fires, water surfaces andgeological sources:Degassing from geological mineral depositsEmissions from volcanic activities

Photoreduction of divalent mercury in natural watersBiological formation of elemental mercury (or possibly dimethylmercury) frommethylmercuryVolatilisation from soilSeismic activities such as earthquakesGeothermal sources - including the oceanic crust - related to submarine volcanoes.

Substantial improvements have been made, with mercury emissions reduced byover 95% from 1997 to 1999. This compares with estimated global total man-made andnatural emissions of 20,000 tonnes per year

DurationYears Production byHg Hgconsumed Production byDiaphragmcells

Asbestos consumed

1975-80 100000 x 51981-85 90000 x 51986-90 700001991-95 400001996-97 0


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1997-981998-991999-002000-012001-02

2002-03

2.3. GRAPHITE ANODESCarbon was the first anode material used for the

electrolytic production of caustic soda and chlorine. While usingcarbon, the products were contaminated with carbon particles. Thisdrawback was eliminated with the discovery of artificial graphite at thebeginning of the century. Though graphite possessed better physicaland chemical properties than carbon, it also has its owndisadvantages.

1.During operation, Graphite is consumed due to electrochemicalattack of O 2 , resulting from a side reaction.

2H 2O _ 2H 2 + O 2and chemical attack by HClO present in the electrolyte due to thedissolution of Cl 2.

Cl2 + H 2O HClO + HClC+ HClO CO + HCl


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

2.Gradual oxidation of graphite to CO and CO 2 results in the wideningof the anode cathode gap, thereby increasing the cell voltage andhence power consumption for the production of caustic.3. The graphite particles are plugged into the pores of asbestosdiaphragm and rendering the diaphragm offering higher resistance andineffective4.Periodic adjustment for maintaining the fixed (3-4mm)interelectrode gap between Hg and graphite/ replacement of graphiteanodes increases operating cost and leads to substantial emissionsand spillages of mercury to the environment in the case of mercurycells.4.Products are contaminated. Cl 2 contained CO 2 and NaOH has graphiteparticles.

4. POLLUTION ABATEMENT AND ROLE OF CECRI FOR THAT:

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

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

(ii) adopting technology which can lessen pollution.

(iii) adopting technology which does not involve pollutants.

Among the above, CECRI has developed

(i) an alternate anode which keeps its dimensions constant therebyfrequent cell openings are not required, which lessen pollution to aconsiderable extentand(ii) developed components for a technology which does not involvepollutants.


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4.1. DEVELOPMENT OF TITANIUM SUBSTRATE INSOLUBLEANODE - (TSIA)

The disadvantages of the usage of graphite anodes in theproduction of Caustic soda and chlorine have been discussed earlier.Further Graphite, itself is being produced by an electrolytic processconsuming 9000 Kwh per tonne. Further the cost of electrical energywas also increasing at a steady and constant rate. All these

necessitated a concerted search, international in scope to find analternate anode material, which could keep constant, dimensions withtime.

Central electrochemical research institute, Karaikudi hasdeveloped a novel type of dimensionally stable anodes known as

Titanium substrate insoluble anode (TSIA) by depositing a combinationof mixed oxides of platinum group metals such as Ruthenium, Iridium,Platinum etc with oxides of valve metals such as Titanium, Tantalum ,Zirconium etc over a corrosion resistant substrate like Titanium,

Tantalum ,Zirconium or their alloys. In simple terms, TSIA has atitanium base which is activated a catalytic coating consisting of amixture of oxides of platinum group metals and oxides of valve metals.

The coating is imparted by thermal decomposition. The process waspatented in 1972. Industrial TSIA and Graphite anodes are shown inFig.4.1.


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The advantages of TSIA are numerous. Though the initial cost of such anodes is four times higher than that of conventional graphiteanodes, it has been established that they are highly economical in the

long run due the following advantages:

1.Dimensional stability.2.Longer life.3.Ability to function effectively at higher current densities.4.Purer products.5.Lesser cell interruptions and6.10 15 % power saving (350 Kwh / tonne of caustic) due to

a).Lower chlorine overvoltage .

b).Lesser bubble effect resulting from higher free surface for theescape of chlorine gas.

c).Higher electrical conductivity.d).Lower and constant cell voltage ande).Operation at lower and constant inter electrode gap.

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.


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All raw materials for the production of TSIA are at presentavailable indigenously. The effective life of the electrocatalytic coatingranges from 18 36 months depending upon the current density andthe operating / cell working conditions in mercury cell and over 60

months in diaphragm cells. The TSIA structures can be recoated withthe electrocatalyst as and when the coating wears out and reinstalled. The life cycle of the structure varies with the operating and cellconditions : but usually last for four to five recoatings.

The cost of the raw materials especially titanium and preciousmetal compounds are showing an upward trend always. So the cost of

TSIA per m 2 geometrical area mainly depends on the design of structure, current density of the cell, type of cell and prevailingelectrocatalyst cost. Approximately it varies from Rs 30000 70000per m 2deponding on the current density and type of cell.

To prove the technical viability of the invention field trials wereconducted in various commercial chlor - alkali plants in the country.

The first field trial was conducted in a diaphragm cell at DCM chemicalworks, New Delhi in 1972. Favourable results in this plant encouragedother units to follow suit. Field trials were then successfully conductedin low current density mercury cells and high current density mercurycells. By 1976, Indian chloralkali industry was convinced of thetechnical feasibility, commercial viability and the energy conservationaspects of TSIA developed by CECRI..

The process know - how was released for commercial productionto three industrial establishments namely :

1.Titanium Equipment and Anode Manufacturing CompanyLimited, Madras. (1976).

2.Bharat Heavy Plates and Vessels Limited, Visakapatnam.(1981).

3.Titanor Components Limited, Goa. (Formerly WIMCO,Bombay.) (1983).

The first commercial caustic soda unit with the TSIA supplied bythe first licensee was commissioned in 1978. The plant recorded apower saving of 12 %. Orders started pouring in from other units aswell. Hence the process was released to two more parties. By 1982,the entire caustic production was based on TSIA.


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TOTAL POWER SAVED UPTO 31.03.2004 : 5500 MILLIONKwh

COST OF POWER SAVED : RS 9500 MILLION

When graphite anodes are used, frequent opening of the cellsare needed for the removal of graphite pieces fallen down, for theadjustment of inter electrode gap, when graphite anodes wears andalso for the regular cleaning of the cell. In all these cases, mercuryvapours escape from the mercury cells into the atmosphere. When

TSIA is used in the cells, as the electrodes maintain their dimensionsconstant through out their life time, Periodical opening of the cell arenot required. This itself has been estimated to reduce mercury

emissions by over 50%.When TSIA is used in the place of graphite anodes in the case of Diaphragm cells,the permeability of the diaphragm is preserved for longer time. The life of the diaphragmhas been found to be increased by over 40%.

Advantages of Graphite in Diaphragm cell: For every ton of NaOH produced, about 800 gm of asbestos

is consumed or let to the environment. But with TSIAnodes life of diaphragm is about 12 months,

i.e hazard to environment is halved For one ton per day caustic plant with graphite anode, 200

kg of asbestos is required for every 6 months, afterwards,it should be renewed. (hence for 180 ton of causticproduced using graphite anode, about 200 kg of asbestosis lost or buried to environment

The graphite fitted as well as TSIA fitted diaphragm cells are shown here:


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4.2. ENVIRONMENTAL FRIENDLY MEMBRANE CELLS

The two conventional technologies for the chlor alkaliproduction are not environmentally acceptable as the Diaphragm cell

technology involve carcinogenic asbestos as separator and theMercury amalgam process use the hazardous pollutant mercury ascathode. Further these processes consume 3200 kWh electricalenergy per tonne of caustic produced.

As an environmental friendly and energy conserving alternate,Membrane cell technology for chloralkali production has beendeveloped in India (CECRI) as well all over world. In this technology,neither Asbestos nor Mercury is used. The only imported componentinvolved is CATION EXCHANGE MEMBRANE (CEM), which separatesthe anode compartment and the cathode compartment. The anodecompartment is made of Titanium and in it activated Titaniumexpanded mesh acts as anode. The cathode compartment is made of Nickel and in it activated perforated Nickel sheet acts as cathode. Ultrapure saturated brine is fed to the anode compartment and depletedbrine is discharged. 30% Caustic is fed to the cathode compartment.Chlorine gas is evolved at the anode and Hydrogen gas and Hydroxideions are generated at the cathode by the reduction of water molecules.CEM permits only Sodium ions to pass through to the cathodecompartment where it combines with hydroxide ions to form sodiumhydroxide. 33% caustic flows out of the cathode compartment. As CEMprevents the migration of chloride ions to the cathode compartment,the caustic produced is highly pure. The Principle of membrane cell isshown in Fig.4.2


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The first membrane cell plant in India was established at M/s

Chemfab Alkalis, Pondicherry in 1985. Depending on the manner inwhich electrical connections are made between the cell units,electrolysers are classified as mono polar or bipolar. In the mono polarcells all anode and all cathode elements are arranged in parallel,forming an electrolyser having high amperage and low voltage. In thebipolar cells the cathode of a cell is connected to the anode of the


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subsequent cell so that cells are connected is series forming anelectrolyser with low amperage and high voltage.

4.2.1. CECRI has developed mopolar as well as bipolarmembrane cell technologies with anodes and cathode indigenously

developed at CECRI.4.2.2.Dimensionally stable anodes and its coatings and catalyticcathodes are vital components of this technology. CECRI hasdeveloped ANODE and CATHODE (with innovative catalytic coatings)for these types of cells .The Developmental process in CECRI is shown here :


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Further in Membrane cells the higher % conversion per pass (33%) is allowed, while in the conventional technologies, 5 conversionsper pass are 15 % only. Volume of electrolyte required in Membranecells is 10 m 3 per tonne of caustic while in the conventionaltechnologies it is four times this value. This also helps indirectly toreduce the volume of effluent from the process considerably.4.2.1. Catalytic anodes for membrane cells:

To fulfill the stringent conditions of membrane cell operationsuitable anode electrocatalysts have been developed. The advantagesof newly developed anode for use in membrane cells are as follows1). Oxygen evolution is suppressed and product chlorine purity is high

2). High current efficiency of 94-953). Membrane damage due to improper electrolyte circulation has beenavoided and bubble effect has been eliminated4). Service life of anode coating has been improved (5 years)5). The performance of the newly developed TSIA for use in membranecells was evaluated in 6000-Ampere prototype membrane cell.

4.2.2. Catalytic Cathodes for Membrane Cells:Conventional cathodes namely MS, SS or Nickel used in the

electrolysis of aqueous alkali metal halide solutions exhibit very high


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hydrogen over voltage of the order of 350 mV at a current density of 300 mA per square cm.

In order to save energy, we have developed Catalytic cathodes.

Catalytic cathodes consist of Nickel or Nickel-plated SS activated by acoating of Nickel stabilised by a mixture of precious metals or theiroxides. The coating is imparted by the Thermal Decomposition

Technique developed by the group already for the production of titanium Substrate Insoluble Anodes. The hydrogen over voltage of these cathodes is only 70 mV at a current density of 300 mA persquare cm. The life of the active coating has been evaluated in 30-wt%NaOH solutions at a current density of 3 kA per square meter at 80 0Cto be over Three Years. Energy saving due to these cathodes is 200kWh per tonne of caustic. The process for the manufacture of thesecathodes was patented in India. (INDIAN PATENT: 179959 of 1990).

Further improvements have been made and the present activatedcathode is superior the previous one in terms of performance namelyimproved coating life and enhanced tolerance towards metallicimpurities present in the electrolyte. The activated cathodes werepatented in USA.( US PATENT: 5855751 of 1999 )Advantages of the Catalytic Cathodes for membrane cells:

Low hydrogen overvoltage of the order of 70 80 mV at 3 kA / m2 .

Energy saving of the order of 250 kWh per tonne of caustic. Enhanced tolerance towards metallic impurities in the

electrolyte. Easier method of processing of the electrodes.

Recently Technological know how for the coating / recoating of

membrane cell (1).Anodes and (2).Cathodes have been licensed toM/S. GRASIM INDUSTRIES ( CHEMICAL DIVISION) OF ADITYABIRLA GROUP. They are using our technology for recoating theanodes and cathodes of their Membrane cells. The performance of ourelectrodes is on par with the imported ones.

5. SUMMARY:


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In Indian as well as in the world scenario, the environmentalmanagement of chlor-alkali industries has been done to a great extentby the invention and adoption of metal anodes (TSIA) in the place of graphite anodes and replacement of conventional mercury and

diaphragm cell processes by the modern membrane Cell technology. The scientific innovations in this field saved not only the earth but alsothe energy to a great extent.

. MERCURY CELLSMercury cells contain no diaphragm. Here separation of anode

and cathode products is achieved by the use of an Hg cathode. Themercury (sodium amalgam) cell (Fig.2.2) process actually involves twocells an electrolyser and a decomposer. In the electrolyser, flowingmercury is the cathode and graphite or TSIA-(Titanium substrate

insoluble anode) as anode. Purified aqueous NaCl solution iselectrolysed between anodes and flowing mercury cathodes. Whilechlorine is liberated at the anode, sodium ions are reduced at the Hgcathode forming sodium amalgam(0.20-0.25 wt%).The amalgam flowsover to the decomposer where it reacts with water to form causticsolution, hydrogen gas and pure Hg. Hg is recycled to the electrolyser.Concentration of NaOH produced in the process can be maintained at


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any level between 40 and 50 wt% by regulating the water fed to thedecomposer. Caustic from this process contains only 50 ppm of NaCl.

For every tonne of caustic produced, 0.25 kg of Hg is lost bymeans of spillage and evaporation. But Hg pollution is not as alarming

as in developed countries, as the plants are well scattered.

The electrolyser consists of a long shallow inclined (5 10 mmper m) trough provided with a cover containing hole at regularintervals for holding the anodes. Cell cover and the sides of the celltrough are rubber lined. Bottom of the trough is rubber lined in thecase of low current density cells and are bare in the case of highcurrent density cells. In the case of rubber-lined bottom, bare MS discsare provided at regular intervals for current connection. Denuder ordecomposer may be vertical or horizontal design and made of cast ironfilled with treated graphite blocks. If the denuder is vertical, it ispositioned at the outlet end of the electrolyser. If it is horizontal, it iskept beneath the electrolyser.

Anode: Cl - Cl 2 + e ; E o : 1.36 VCathode(Hg): Na + + e Na(Hg) E o : -1.77 VNa + + Cl- Cl2 + Na(Hg) ; E o : 3.13 VDecomposer reaction:H2O + Na(Hg) H2+NaOH - + Hg E o : -1.08V

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