Electrochemical Importance of Anode and Cathode Coating in a Membrane Cell Chlor Alkali Industry

8/3/2019 Electrochemical Importance of Anode and Cathode Coating in a Membrane Cell Chlor Alkali Industry

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Electrochemical importance of Anode and Cathode Coating in a Membrane Cell

Chlor Alkali Industry.

Document by:Bharadwaj

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Abstract

Membrane Cell operation for the production of Caustic and Chlorine is the latest environment and

energy friendly process for a Chlor-Alkali Plant. The membrane cell consists of Cathode (Nickel),

Anode (Titanium) and a florocarbonated membrane. By the continuous process and process

requirement in energy point of view in a normal running Chlor-Alkali Plant Anode and Cathode

coating is required in order to increase more lifetime and more productivity for a cell. This anode

and cathode coating not only increase the total productivity of a Chlor-Alkali Unit but also can

decrease the fixed cost for a unit by increasing process reliability. A study has been done to

understand the electrochemical importance of anode and cathode coating and the thrust zone for

more research in this area to increase the productivity as well as increasing the depreciation time

for the Cell.

Keywords: Anode Coating, Cathode Coating, Membrane, Cell, Caustic Soda, Chlorine.
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Introduction:

Chlor Alkali Industry is a special and separate zone of chemical Engineering process and design

which meets the need of tones of caustic for textile, pulp and paper, pharmaceutical industries

and chlorine for PVC, water purification, and bleaching industries. In industry, from the last

century caustic and chlorine is produced by Electrolysis of saturated Brine (320gpl of NaCl in

H2O) by diaphragm or mercury or Membrane cell operation.

But for operational and cost Excellence Membrane cell electrolysis process In Chlor-Alkali

Industry dominates over the other two processes from the last four to five decades. In Membrane

cell Electrolysis process, the membrane is set between a Cathode (Ni) and Anode (Ti) and the

electrolysis takes place in both Cathode and anode side by high current. Saturated Brine comes

in anode side and depleted into Na+ and Cl- ions. 26% caustic comes in Cathode side where

depletion of water into H+ and OH- ion takes place. Membrane is a very thin perfluroted substance

with carboxylic coating in Cathode side and sulphonic coating in anode side which only allows

Na+ ion in cathode side from anode side. NaOH forms in cathode side with H2 as a by product,

and Cl2 forms in the anode side with lean brine recycle after dechlorination process. Membrane

structure and selectivity is a separate zone of research and is not the scope of present study. The

Membrane cell dominates over other two electrochemical manufacturing process of Chlor-Alkali

Industry due to the following reasons.

The power requirement is very high in Chlor-Alkali industry. Applications of Membrane cell

technology decrease the power consumption over the other two processes. But reduction of

power is still a thrust zone of research because Chlor-Alkali Industry is still the largest consumer

of power. It is tried to reduce the voltage from the root i.e. one single cell voltage to reduce power

requirement in the Electrolysis process. The cathode and anode ageing is another problem and

replacement cost is also very high. Reminding both causes i.e. increasing cathode and anode life

and decreasing cell voltage; anode side and cathode side coating is generally done. In presents


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the need, factors affecting and methods for applying cathode and anode coating is highlight with

latest development in this zone. It is expected that the study will help to understand the anode

and cathode coating in details manner in Chlor-Alkali Industry and modification needed in this

zone. The objective of the study is given below:

1. To see the various causes of the failure of electrode coatings.

2. To discuss the effective coating process.

3. A one shop stop for the knowledge of anode coating.

Anode and Cathode Coating [1] [2]:

The key components of all electrochemical processes are the electrodes and, in some cases, the

separators. The last century has seen many developments in both for the electrode materials and

ion permeable membranes and allows successful implementation of many novel concepts. Where

optimum performance of electrode material is needed, electrodes made from noble metals or

noble metal oxides are the components of choice. The common features of the materials of

electrodes are

a) The use of the platinum group and there oxides as coatings on inert substances.

b) The minimization of platinum metal loading.

c) Minimization of interelectrode gaps and having high energy efficiency by fabrication of

membrane or the electrodes.

d) To increase electrode stability by the use of platinum group metal alloys that also helps to

minimize overpotentials.


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In late 50s particularly in the year of 1965 by Henri Beer, the Dimensionally Stable Anodes were

introduced that changes the pathway of electrochemical processing. A new approach to the

manufacture of coated electrodes and increased expectations of the performance of the

electrodes were introduced by the introduction of the DSA. The coated titatanium anodes are

found in rapid application as there has a unique combination of properties like a very low over

potential for chlorine evolution and a very long service life. These two applications have replaced

graphite which early used as anode in electrochemical applications. There are various series and

various groups of researches associated to increase the performance and life time of an

electrode by introduction of various coatings in Anode side or in Cathode side. The main focus

area has gone to the anode side as main problems appear in the anode side which is discussed

later. The improvement by the coatings of the electrodes was seen in extended lifetime, stable

low voltage operations and predictable inefficiency product splits. The Dimensionally stable

anode coatings are a unique combination of value metal, precious metal oxide catalysts,

chemically bound to a value metal substrate. For most commercial applications the value metal of

choice is titanium by which the anode side of the electrodes in membrane cell technology is

generally made. A valve metal is a metal that will form a protective oxide layer on exposed

metallic surfaces upon anodic operation in aqueous electrolytes. Use of the protective oxide layer

gives the valve metals a remarkable corrosion resistance in many operating environments but

also prevents the use of the valve metal independently as an anode without the addition of a

property formed catalytic layer.

Mainly all these dimensionally stable anodes are used for all the mercury diaphragm or

membrane cell technology in Chlor Alkali Applications. DSAs can be used in all types of

application whether high hypochlorite or chlorate content in anode side or oxygen evolving

applications. There are also protective layers or coatings for high current density applications in

Chlor alkali industries.


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In case of Cathode coatings, the over potential for the cathode reaction in an electrode for Chlor

alkali applications, is also critical to the process energy consumptions. Practically it is very difficult

to eliminate the significant over potential of the electrolysis of water which partially takes place in

cathode of an electrode. The elimination of the over potential of cathodic reaction in electrode in

Chlor alkali applications is partially achievable by using noble metal coatings over the nickel

cathode which is vastly used in Chlor alkali applications.

The Coating Materials and Types:

In cathodic coating, it is believed that platinum/ruthenium coatings are used. Nickel is used as

substrate, to ensure stability against corrosion and the coatings contain 3 to 3.5 gm/m2 of

platinum and 1-1.5 gm/m2 of ruthenium [5]. Immersion plating method is applied for cathode

coating. After appropriate surface preparation, the nickel substrate is simply dipped into the

solution of noble metal chlorides and left to stand the electrode at room temperature unless

required loading of noble metal is obtained. The Nickel metal is the reducing agent in this

reaction. Typically, the over potential of Hydrogen evolution is 100mV with a current density of

3kA/m2 in 35% NaOH at 363K [5] [6].

In anodic coating, there are 13 series of coatings available for both Chlorine and Oxygen evolving

anode applications. One series are ruthenium titanium oxide combinations. Each of the

formulations is suitable for use in all chlorine generating applications. Another series of anode

coating contain ruthenium, tin and titanium oxides and diaphragm anode applications. The latest

application is a series of anode coatings which contain ruthenium, iridium, and titanium oxides for

latest membrane cell applications in Chlor Alkali technology [6].

Factors affecting Anode and Cathode Coating [3] [4]:

There are three ways to interrupt the electrical circuit between the metal substrate and the

electrolyte in case of anode coating. The interface between the catalyst and the substrate can


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become non conductive preventing electron flow from the catalyst to the substrate. Secondly te

catalyst can be lost in service exposing the valve metal substrate, which oxides and cannot

accept electrons from solution without the presence of the catalyst. Finally a foreign material can

deposit on the surface of the catalyst effectively isolating the catalyst from the solution. Brine

impurities directly or indirectly impact on each of these failure mechanisms. The three ways

described is names as passivation, wear, and blinding deposits.

Substrate passivation is defined as the growth of a non conductive oxide barrier between the

catalyst and the valve metal. The passivation is observed as a case of brine impurities, low NaCl

concentration in brine, voltage increase, higher current density operations, Oxygen rate in anode

side, Sulphate presence in brine and the anode potential.

Passivation is the primary failure mode for use in oxygen evolving applications such as Electro

galvanizing [8]. Although this failure is not rare in Chlor Alkali applications especially in diaphragm

and membrane cell applications, the passivated anodes is observed in 15-20 years of operation.

In this life period, generally it is considered to an economic end of the service life of a coated

anode. A non destructive type Electrochemical Impedance Spectroscopy (EIS) technique can be

utilized to examine the anode and determine whether or not a passivation layer is present [3].

In case of wear, brine impurities can directly or indirectly impact. Typically a 10-15% loss of

catalyst is observed over the first six months of operation, followed by a gradual wear with time

throughout the useful life of the coating. Increasing wear in anode coating causes increasing in

voltage. Current will shift to remaining active areas, locally increasing current density and

observed operating voltage.

Fluoride ion in brine causes heavily to this wear problem of anode coating. Other brine impurities

that directly affect catalyst stability is sugar, EDTA and CN- ion [9].


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Blinding deposits are most frequently encountered problems associated with the coating erosion

in anode side. Both organic and inorganic deposits have been encountered over the years. In

general all the deposits are a result of electrochemical and electrophoretic processes. After

formation, they alter the current distributions within the cell and can result in locally high current

densities in non blinded regions. The first indication of this problem is usually rising cell potentials.

Inorganic deposits are normally visible on inspection.

Conclusions:

Electrochemical technology is the cleanest technology considered today. The Membrane cell

technology is the most environment friendly technology and energy efficient technology [7]. All this

technology depends on stable electrode materials and frequently only platinum group metal

materials fulfill this requirement. The objective of the electrode and coating catalyst design is

often therefore to place the noble metal in a stable matrix and to achieve the required

performance with a minimum noble metal loading. In today research is going on further reduction

of over potential of both sides of electrodes to reduce energy consumption and also to increase

its operating life and this is the challenge for the Chlor alkali industries in the 21st century.

ACKNOWLEDGEMENT:

The author wish to thankfully acknowledge the Mr. C S Babu, HOD, Chlor-Alkali Plant of RIL-

Dahej for providing sufficient infrastructure for the completion of this paper and also all the other

Plant personnel who have helped directly or indirectly to execute this work.


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References:

[1] Electrodes based on Noble Metals by G Kim Chandler, J David Genders and Derek Pletcher.

Platinum Metal Rev., 1997, 41, (2), 54-63.

[2] Comprehensive Treatise of electrochemistry Vol-2, eds, J. OM, Bockris, B E Conway E.

Yeager and R E white, by D L Caldwell, Plenum, Newyork, 1981, P 122.

[3] Modern Chlor Alkali Technology S Scaley by K L Hardee and R A Kus Vol 7 p 43-54 1998.

[4] Deactivation of thermally formed Ru/Ti Oxide electrodes by B V Tilak, V I Birss, J Wang, C P

Chen and S K Rangarajan, Journal of The Electrochemical Society, 148 (9) D112-D120 (2001)

[5] The Electrochemistry of Novel Materials, J Lipkowski and P N Ross editors, by S Trasatti, P

207, VCH Publications, New York (1994)

[6] The Effect of Brine Impurities on Dimentionally Stable Anodes by R C Carlson. 2005 Eltech

Chlorine seminar, Cleaveland, Ohio, Sept 13-15, 2005.

[7] Operating manual Of Chlor Alkali Plant of Dahej Manufacturing Division, RIL Vol 2.

[8] Modern Chlor Alkali Technology, Volume 4, N M Prout and J S Moorhouse eds. By D S

Cameron and P M Willis, Elsevier Applied Science, 1986.

[9] Encyclopedia Electrochemistry, Chlor-Alkali Processes: PP 1-3

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Electrochemical Importance of Anode and Cathode Coating in a Membrane Cell Chlor Alkali Industry