Elec­tro­chem­i­cal Pro­cesses and In­dus­try – A Re­view

Dr. N C Datta, In­de­pen­dent Con­sul­tant

Chemical Industry Digest - - What’s In? - Dr N. C. Datta

There are chem­i­cal prod­ucts which can be best pro­duced only through the elec­tro­chem­i­cal route. Mak­ing the process en­ergy ef­fi­cient is a chal­lenge. Core de­vel­op­ments of the two most im­por­tant elec­tro­chem­i­cal in­dus­tries, vizchlor-al­kali man­u­fac­ture and alu­minium ex­trac­tion are de­scribed.

The elec­tro­chem­i­cal in­dus­tries have al­ways been a ma­jor chal­lenge in the roadmap for the con­trol of global warm­ing. World-wide in­tense ef­forts have been made to im­prove their en­ergy ef­fi­cien­cies and for abate­ment of the gen­er­a­tion of green­house gases. This ar­ti­cle de­scribes some of the core de­vel­op­ments in re­spect of the two most im­por­tant elec­tro­chem­i­cal in­dus­tries, viz. chlor-al­kali man­u­fac­ture and alu­minium ex­trac­tion.

Elec­tro­chem­i­cal pro­cesses are im­por­tant for the chem­i­cal in­dus­try. These pro­cesses are of two types: (1) Elec­trolytic, in which elec­tric­ity drives the chem­i­cal re­ac­tions in spe­cial re­ac­tors known as elec­trolytic cells, (2) Voltaic or Gal­vanic, in which the chem­i­cal re­ac­tions gen­er­ate elec­tric­ity in de­vices known as bat­ter­ies, fuel cells and sen­sors. Ex­am­ples of in­dus­trial elec­tro­chem­i­cal pro­cesses of the first type are many. Some of the bulk in­or­ganic and or­ganic chem­i­cals of prime im­por­tance such as caus­tic soda, chlo­rine, flu­o­rine, adiponi­trile, etc. and met­als such as alu­minium, mag­ne­sium, sodium, etc. are pro­duced only through elec­trol­y­sis. As of now, there are no other meth­ods to pro­duce these chem­i­cals and met­als.

Nev­er­the­less, the elec­tro­chem­i­cal in­dus­try is a ma­jor con­sumer of elec­tri­cal en­ergy, and thus, is re­spon­si­ble di­rectly and in­di­rectly for the re­lease of a very large vol­ume of green­house gases. Ad­di­tion­ally, the

1 elec­trolytic pro­cesses gen­er­ate some green­house gases them­selves by way of their process.

The elec­tro­chem­i­cal in­dus­tries have al­ways been a ma­jor chal­lenge in the roadmap for the con­trol of global warm­ing. World-wide in­tense ef­forts have been made to im­prove their en­ergy ef­fi­cien­cies and for abate­ment of the gen­er­a­tion of green­house gases. This ar­ti­cle de­scribes some of the core de­vel­op­ments in re­spect of the two most im­por­tant elec­tro­chem­i­cal in­dus­tries, viz. chlor-al­kali man­u­fac­ture and alu­minium ex­trac­tion.

The dif­fer­ence be­tween an elec­tro­chem­i­cal process and a con­ven­tional chem­i­cal process

Un­like a con­ven­tional chem­i­cal process, in which the rate of re­ac­tion and out­put are con­trolled by tem­per­a­ture, pres­sure, con­cen­tra­tion of re­ac­tants and cat­a­lysts, the rate and out­put in an elec­tro­chem­i­cal process are gov­erned by the mag­ni­tude of the elec­tri­cal cur­rent and the to­tal quan­tity of elec­tri­cal charges passed through the sys­tem, that is, by Fara­day’s laws of elec­trol­y­sis. Thus, the elec­tro­chem­i­cal equiv­a­lent (i.e. the equiv­a­lent weight of a sub­stance di­vided by 96485 coulombs) de­notes the up­per most limit to the out­put per unit charge ir­re­spec­tive of any fac­tors.

There­fore, it may ap­pear that the out­put could be in­creased by in­creas­ing the mag­ni­tude of the elec­tri­cal cur­rent, that is, by in­creas­ing the ap­plied voltage, but in re­al­ity this is not pos­si­ble be­cause a strong po­lar­i­sa­tion of the elec­trodes (that is, the for­ma­tion of a steep bar­rier of elec­tri­cal charges on the sur­face of the elec­trodes) sets in be­yond an op­ti­mum cur­rent, lead­ing to an in­creas­ing loss of elec­tri­cal en­ergy. Also, the other side re­ac­tions such as de­com­po­si­tion of wa­ter oc­cur

at higher voltage. Thus, the op­ti­mal de­sign of an elec­tro­chem­i­cal process plant is of­ten a trade off be­tween two op­pos­ing de­mands, viz. op­er­a­tion at rel­a­tively low cur­rents, which re­quires a large plant area with in­ci­den­tal ex­pen­di­tures vis-à-vis op­er­a­tion at rel­a­tively large cur­rents, which re­quires a rel­a­tively small plant area, and less main­te­nance and man­power, but some ad­di­tional cost due to equip­ment, mech­a­ni­sa­tion, en­ergy loss and more ero­sion of the elec­trode ma­te­ri­als at higher lev­els of re­ac­tion.

2

An­other ba­sic dif­fer­ence is that all elec­tro­chem­i­cal pro­cesses oc­cur only through elec­tron trans­fers, and these elec­tron trans­fer pro­cesses must oc­cur in two sep­a­rate zones of an­ode and cath­ode, as oth­er­wise the prod­ucts would re­act back. This is so be­cause the elec­trolytic re­ac­tions are, in gen­eral, ther­mo­dy­nam­i­cally up­hill pro­cesses and the re­verse re­ac­tions are ther­mo­dy­nam­i­cally more favourable. Thus, con­sid­er­able ef­forts are spent to sep­a­rate the an­odic and ca­thodic re­ac­tions, while keep­ing the ion and elec­tron trans­port pro­cesses in­tact. The prob­lem is more ag­gra­vated be­cause, by Ohm’s law, the dis­tance be­tween two elec­trodes must be as low as pos­si­ble and the area of the elec­trodes as large as pos­si­ble in or­der to min­imise the cell re­sis­tance and to max­imise the cur­rent pass­ing through the cell.

Some of the crit­i­cal ad­vances in elec­tro­chem­i­cal tech­nol­ogy in re­cent decades may there­fore be grouped into three cat­e­gories: (a) use of spe­cial elec­trodes with or with­out cat­alytic coat­ings to pro­mote / in­hibit spe­cific ionic pro­cesses, (b) use of mem­branes to se­lec­tively con­trol the ion trans­port pro­cesses and pro­vide sep­a­ra­tion be­tween ca­thodic and an­odic re­ac­tions, and (c) changes in the de­sign of elec­tro­chem­i­cal re­ac­tors to fa­cil­i­tate cer­tain ion trans­port pro­cesses. Some spe­cific in­stances of ad­vances are de­scribed in for two im­por­tant in­dus­tries.

Chlor-al­kali in­dus­try: Oxy­gen De­po­larised Elec­trode Tech­nol­ogy

Vol­ume-wise the largest elec­tro­chem­i­cal in­dus­try in the world is the chlor-al­kali in­dus­try pro­duc­ing chlo­rine, hy­dro­gen and caus­tic soda by the elec­trol­y­sis of brine. Both chlo­rine and caus­tic soda are in­dis­pens­able com­modi­ties that are re­quired in the man­u­fac­ture of about 55-60% of all types of spe­cialty chem­i­cals3. Most no­table is the use of chlo­rine and its com­pounds in pub­lic health, viz. wa­ter treat­ment and san­i­ta­tion.

World-wide, there are at present some 500 chlo­rinecaus­tic soda plants, with a pro­duc­tion ca­pac­ity of 58 mil­lion met­ric tons of chlo­rine and 62 mil­lion met­ric tons of caus­tic soda4, based on all three tech­nolo­gies – mer­cury cells, di­aphragm cells and mem­brane cells. In In­dia, there are 36 plants, with a pro­duc­tion ca­pac­ity of 3.2 mil­lion met­ric tons of caus­tic soda per an­num.

5 All over the world, the pro­duc­tion from mer­cury cells has dras­ti­cally re­duced for en­vi­ron­men­tal rea­sons, and the pro­duc­tion from mem­brane cells has equally in­creased.

The re­ac­tions which oc­cur dur­ing the elec­trol­y­sis are:

At an­ode: 2 Cl- j Cl + 2 e ; E0 = -1.36 V,

2

ΔG0 = + 262.4 kJ/mol of Cl ….. (1)

2

De­pend­ing on the ap­plied voltage, there may be two re­ac­tions at the cath­ode:

Ei­ther 2 H O + 2 e j H + 2 OH- E0 = - 0.83 V;

2 2

ΔG0 = + 160.2 kJ/mol of H ….. (2)

2

Or, Na+ + e j Na ; E0 = - 2.71 V, ΔG0 = + 261.5 kJ/mol ……… (3) Na then re­acts with wa­ter to pro­duce NaOH by re­ac­tion: 2 Na + 2 H O j 2 NaOH + H …….. (4) 2 2

The re­duc­tion of wa­ter (Re­ac­tion 2) into H and 2 OH- in­volves less en­ergy than the re­duc­tion of Na+ (Re­ac­tion 3). But if OH- ions and Cl are formed in the 2 cell with­out seg­re­ga­tion, then Cl would re­act with 2

NaOH form­ing NaOCl and NaClO . Ob­vi­ously the fo3 cus of the chlor-al­kali tech­nol­ogy, apart from get­ting more out­put at in­creas­ingly higher cur­rent den­sity, has been to pre­vent the mix­ing of Cl and Cl- with NaOH. 2

The mer­cury cell pro­vided for many years a clever ar­range­ment for the seg­re­ga­tion of Cl from NaOH.

2

In vogue ear­lier were the mer­cury cell and the di­aphragm cell, which have given way to the far su­pe­rior mem­brane cell tech­nol­ogy. Though mer­cury cell had cer­tain ad­van­tages, it is dis­banded pri­mar­ily be­cause

of pol­lu­tion con­cerns.

The ma­jor prob­lems in the di­aphragm cell are: (i) the di­aphragms al­low the pas­sage of all ions. Thus, Cl- ions mi­grate from an­ode re­gion to the cath­ode re­gion, con­tam­i­nat­ing NaOH, (ii) the con­cen­tra­tion of OH- ions in the cath­ode re­gion is to be re­stricted to 12%, as higher con­cen­tra­tion leads to the dif­fu­sion of OH- ions from the cath­ode re­gion back to the an­ode re­gion. Hence ad­di­tional en­ergy is re­quired to evap­o­rate the di­lute NaOH so­lu­tion to the de­sired con­cen­tra­tion of 50%. (iii) The cur­rent den­sity has to be main­tained usu­ally in the range of 0.15 – 0.20 A/cm2 be­cause of the high re­sis­tance in the cell. Hence a large elec­trode area is re­quired. (iv) The as­bestos di­aphragm has a rel­a­tively short life and needs re­place­ment ev­ery a few months.

The mem­brane cells work on the ba­sis of same prin­ci­ples as the di­aphragm cells (Fig 1), but do not have the same prob­lems. The sep­a­ra­tor is a semi-per­me­able poly­meric mem­brane, which al­lows the pas­sage of only Na+ ions, but does not al­low the per­me­ation of Cl- ions to­tally and OH- ions par­tially. The mem­brane cells are thus ca­pa­ble of pro­duc­ing NaOH con­cen­tra­tion up to 40%, free of Cl- con­tam­i­na­tion. Fur­ther, these do not have high re­sis­tance, and so can be op­er­ated at rea­son­ably higher cur­rent den­sity of 0.25 – 0.40 A/cm2. A ma­jor ad­van­tage is the “zero gap” tech­nol­ogy, that is, the two elec­trodes are al­most in con­tact with each other at the op­po­site sides of a mem­brane. This leads to a fur­ther re­duc­tion in the cell re­sis­tance and hence in en­ergy loss. Mod­ern mem­branes give a cur­rent ef­fi­ciency of 96-97% with a voltage drop of only 250 mV and en­ergy con­sump­tion is dras­ti­cally re­duced to about 2400 kWh/met­ric ton of chlo­rine. 6

Mem­brane Cell

The first mem­brane, based on cel­lu­lose ac­etate, was de­vel­oped by Loeb and Souri­ra­jan in 1962.7 Later on, its prop­er­ties like pore size, etc. were im­proved by adding var­i­ous ad­di­tives and con­di­tion­ing agents. But the real break­through in mem­brane tech­nol­ogy came with the de­vel­op­ment of NafionTM by Du Pont in 1960s. NafionTM are flu­oro-hy­dro­car­bon poly­mers with side chains con­tain­ing sul­phonic acids for easy ion ex­change. The molec­u­lar struc­ture of a typ­i­cal NafionTM poly­mer is shown in Fig.2.

The use of mem­branes has made a sea-change in the elec­tro­chem­i­cal tech­nol­ogy, and apart from chlo­rine cells, mem­branes are used in al­most all types of cells to sep­a­rate the elec­trode zones. Al­most all mem­branes are made from or­ganic poly­mers, but there are some in­or­ganic mem­branes, too, made from sil­i­cones, and even from steel, alu­mina and zir­co­nia, etc. for other elec­tro­chem­i­cal ap­pli­ca­tions.

6

Al­though, as de­scribed above, there has been some ma­jor sav­ing in en­ergy with mem­brane cells, there is a scope of con­sid­er­able im­prove­ment if oxy­gen re­duc­tion re­ac­tion (ORR, Re­ac­tion 5) is car­ried out in cath­ode in place of hy­dro­gen evo­lu­tion re­ac­tion (HER, Re­ac­tion 2):

O + 2H O + 4e j 4 OH-; E0 = + 0.401 V,

2 2

ΔG0 = - 154.8 kJ/mol of O …… (5)

2

The the­o­ret­i­cal cell voltage then drops to – 0.959 V at 25oC in place of - 4.07 V of mer­cury cell and - 2.19 V of di­aphragm/mem­brane cell, and the the­o­ret­i­cal en­ergy con­sump­tion per met­ric ton of chlo­rine there­fore re­duces to 725 kWh in place of 3077 kWh and 1656 kWh, re­spec­tively.

Next Gen­er­a­tion Elec­trodes

There­fore, the next ma­jor im­prove­ment in chlo­ral­kali in­dus­try is ex­pected to be the com­mer­cial im­ple­men­ta­tion of Oxy­gen De­po­larised Elec­trode (ODC) tech­nol­ogy be­cause of the re­duc­tion of O at cath­ode.

2

All ar­range­ments of the cell would be al­most the same as the mem­brane cell ex­cept that the cath­ode would be an elec­trode in Gas Dif­fu­sion Elec­trode (GDE) plat­form (Fig 3). It would be flooded with O to pre­vent the

2

H evo­lu­tion re­ac­tion (HER) and to fa­cil­i­tate the oxy

2 gen re­duc­tion re­ac­tion (ORR). The elec­tron trans­port pro­cesses are so fast in GDE that a cur­rent den­sity of ~ 1 A/cm2 may be re­alised with a po­ten­tial drop of less than about 100-200 mV. Gas dif­fu­sion elec­trodes (GDE) are al­ready es­tab­lished tech­nol­ogy in fuel cells, and in bat­ter­ies, and its use in chlor-al­kali cells is ex­pected to bring down the en­ergy con­sump­tion by about 30-35%.

8 There is huge lit­er­a­ture on both GDE and ODC. The GDE is ba­si­cally a three-phase (gas-liq­uid-gas) elec- trode sys­tem, of­fer­ing a very large sur­face area, and con­tain­ing cat­a­lysts to ac­cel­er­ate spe­cific ion/elec­tron ex­change re­ac­tions. In one type of prepa­ra­tion, the ac­tive cat­a­lyst ma­te­ri­als are dis­persed in aque­ous medium with a sur­fac­tant. The elec­trode sheet is made by floc­cu­la­tion of the par­ti­cles, fil­tra­tion, dry­ing and rolling. The sur­fac­tant is re­moved and the sheet is fur­ther hot-pressed. Some­times the ac­tive ma­te­ri­als are de­posited by chem­i­cal vapour de­po­si­tion or by elec­trophore­sis. The main struc­tural el­e­ment in GDE is car­bon black par­ti­cles (about 35%) bonded to PTFC layer. The layer has about 60% poros­ity, high hy­dropho­bic­ity, good elec­tri­cal con­duc­tiv­ity and very low pore di­am­e­ter of 30-60 nm, en­sur­ing the pore dif­fu­sion to be pre­dom­i­nantly of Knud­sen type. On the gas side the elec­trode has a metal­lic grid to act as cur­rent col­lec­tor as well as pro­vid­ing struc­tural re­in­force­ment. The elec­trolyte side con­tains ac­tive car­bon par­ti­cles, con­tain­ing spe­cific cat­a­lysts embed­ded in the car­bon ma­trix.

9

The ODC tech­nol­ogy is in a very ad­vanced stage, and its com­mer­cial im­ple­men­ta­tion by many re­puted tech­nol­ogy com­pa­nies is al­ready un­der progress.

Alu­minium Ex­trac­tion: In­ert An­ode Tech­nol­ogy

The alu­minium in­dus­try is the largest non-fer­rous metal in­dus­try in the world econ­omy, and alu­minium finds wide ap­pli­ca­tions in many ar­eas due to some of its very spe­cial prop­er­ties. But as on now, elec­trol­y­sis is the only eco­nomic means by which alu­minium can be sep­a­rated from oxy­gen. Alu­mina is an in­su­la­tor and also has a high melt­ing point (2072oC). But it forms a con­duct­ing so­lu­tion when dis­solved in molten cry­o­lite (Na AlF ) at 1060oC. This dis­cov­ery by Hall and

3 6

Héroult in 1886 has made the ex­trac­tion of alu­minium pos­si­ble.

The cell re­ac­tions are:

At an­ode: 3 O2- j 3/2 O + 6 e …… (6)

2

At cath­ode: 2 Al3+ + 6 e j 2 Al ………… (7)

Alu­mina, ex­tracted from baux­ite ore by Bayer’s process, is dis­solved up to 6% in a molten bath of cry­o­lite at 970oC con­tain­ing CaF and ex­cess AlF and is elec

2 3 trol­ysed be­tween car­bon-lined stain­less steel cath­odes and graphite blocks as an­odes. The use of graphite low­ers the the­o­ret­i­cal cell voltage from 2.21 V to 1.17 V due to the for­ma­tion of CO by the re­ac­tion of car­bon

2 with O . The ac­tual over­all re­ac­tion is:

2

2 Al O + 3 C = 4 Al + 3 CO ……… (8)

2 3 2

A typ­i­cal Hall – Héroult cell for alu­minium cell is shown schemat­i­cally in Fig.4.

Even though the the­o­ret­i­cal cell voltage is just 1.17

V, ac­tual cell voltage is be­tween 4.6-4.7 V be­cause of the over­all na­ture of op­er­a­tions and the need to over­come re­sis­tances of the cell com­po­nents. Thus, the

10 alu­minium ex­trac­tion con­sumes max­i­mum elec­tric­ity per unit quan­tity of any prod­uct among all in­dus­trial elec­tro­chem­i­cal pro­cesses. Specif­i­cally, at present 1316 MWh of elec­tri­cal en­ergy is re­quired to pro­duce 1 met­ric ton of elec­trolytic alu­minium, whereas the the­o­ret­i­cal en­ergy re­quire­ment is about 6.34 MWh/met­ric ton only. Apart from elec­tric­ity for elec­trol­y­sis, which con­sumes al­most 30% of the to­tal en­ergy, elec­tric­ity is re­quired to heat the cell bath, and to main­tain it at a tem­per­a­ture of 970oC. Be­sides, there is re­quire­ment of ther­mal en­ergy in the pu­rifi­ca­tion of baux­ite, its diges­tion in caus­tic soda, dry­ing and cal­ci­na­tions of the pre­cip­i­tated alu­minium hy­drox­ide. In fact, about 14% of to­tal en­ergy that is re­quired for alu­minium pro­duc­tion is con­sumed by these re­fin­ing pro­cesses.

10,11

Green­house gas emis­sion

Apart from the high cost of pro­duc­tion due to high en­ergy con­sump­tion, this in­di­rectly con­trib­utes to the ac­cu­mu­la­tion of CO in the at­mos­phere, as most

2 of the power gen­er­a­tion plants use fos­sil fuels as their source of en­ergy. Ad­di­tion­ally, the re­ac­tion of the car­bon an­ode with O pro­duces CO , and this is one of

2 2 the ma­jor prob­lems to­day as­so­ci­ated with alu­minium pro­duc­tion. It has been es­ti­mated that each met­ric ton of alu­minium con­sumes 330 kg of car­bon and de­posits cor­re­spond­ingly 1.22 met­ric ton of CO into the atmo

2 sphere. Se­condly, the flu­o­ride ions in the elec­trolyte

10 re­act with graphite at these tem­per­a­tures pro­duc­ing flu­o­ro­car­bons (CF and C F ), par­tic­u­larly when the

4 26 alu­mina con­tent in the cell falls be­yond a crit­i­cal limit (about 2%).

Ac­cord­ing to one es­ti­mate12, up to about 1985 about 1.3-1.6 kg of CF was emit­ted per met­ric

4 ton of alu­minium pro­duced. This quan­tity is now re­duced due to var­i­ous ad­vance­ments in tech­nol­ogy, but still the quan­ti­ties emit­ted are huge. Ac­cord­ing to the data of World-Alu­minium.org, in 2017

11

63.404 mil­lion met­ric tons of pri­mary alu­minium was pro­duced in the world, and this emit­ted on the av­er­age 0.64 kg of flu­o­rides per met­ric ton of Al, and per­flu­o­ro­car­bons 0.57 t CO e per met­ric ton of Al.

2

Need­less to say, these fluoro­car­bon gases are more dam­ag­ing than CO and to­day

2 the alu­minium in­dus­try con­trib­utes about 1% of all man-made green­house gases.

12 There­fore, a ma­jor part of the re­cent re­search ef­forts are di­rected to­wards the re­duc­tion of en­ergy con­sump­tion and abate­ment of emis­sion of green­house gases in alu­minium in­dus­try.

Tech­no­log­i­cal Ad­vances

The ba­sic fea­tures of the alu­minium ex­trac­tion tech­nol­ogy have im­proved con­sid­er­ably over the years, mod­i­fy­ing the types of the an­odes, the way the an­odes are in­tro­duced into the cell, the way alu­mina is fed, and the con­trol sys­tem au­tomat­ing the op­er­a­tions.

13 Apart from achiev­ing more op­er­a­tional ease, and bet­ter eco­nomics, most of these mod­i­fi­ca­tions have achieved higher en­ergy ef­fi­ciency and lower vol­umes of green­house gas emis­sions. It may be men­tioned that main­tain­ing the con­cen­tra­tion of alu­mina in the cry­o­lite bath at its op­ti­mum level is one of the most crit­i­cal op­er­a­tions. There has been a ma­jor break­through in 1980s with the in­tro­duc­tion of Point Feed Pre­bake (PFPB) process with com­puter con­trol of alu­mina con­cen­tra­tion pre­cisely. There has been con­sid­er­able im­prove­ment in the en­ergy con­sump­tion af­ter the in­tro­duc­tion of this tech­nol­ogy in most mod­ern alu­minium smelters.

13

Still, other ma­jor prob­lems are: (1) Main­tain­ing the

an­ode-to-cath­ode (ACD) dis­tance: ACD is the dis­tance be­tween the lower face of the graphite an­ode and the top sur­face of the molten alu­minium pad. By Ohm’s law, more the ACD, more is the re­quired voltage. Hence the ACD should be as low as pos­si­ble, but it has to be main­tained at an op­ti­mum dis­tance be­cause the mag­netic field due to high cur­rent pass­ing through the cell cre­ates a swirling mo­tion and de­for­ma­tion in the alu­minium pad. If the pad then touches the an­ode, there would be short cir­cuit. (2) Re­ac­tion of molten alu­minium with car­bon-lin­ing to form alu­minium car­bide: The steel pot is given car­bon-lin­ing so as to avoid its cor­ro­sion by molten alu­minium. But alu­minium car­bide for­ma­tion rup­tures the car­bon-lin­ing and ex­poses the steel. Hence a thin sheet of cry­o­lite is pro­vided be­tween alu­minium pad and car­bon-lin­ing, but this in­creases the cell re­sis­tance.

There­fore, sev­eral tech­nolo­gies are now un­der de­vel­op­ment / con­sid­er­a­tion to im­prove the en­ergy ef­fi­ciency of alu­minium smelt­ing and to re­duce the emis­sion of green­house gases:

(1) Use of in­ert elec­trodes in place of the graphite. (2) Use of flu­idised bed for cal­ci­na­tions of alu­minium

hy­drox­ide in place of ro­tary cal­cin­ers

(3) Cogeneration of elec­tric­ity to utilise fur­ther the

waste heat

(4) Op­ti­mi­sa­tion of the elec­trol­y­sis process such as (a) low­er­ing the cell bath tem­per­a­ture from the present 960oC to less by us­ing ad­di­tives like KF, (b) de­vel­op­ment of a drained cell tech­nol­ogy (or Wet­table Cath­ode Tech­nol­ogy), which uses ti­ta­nium boride (TiB ) lin­ing as the cath­ode so that the mag­net­i­cally

2 in­duced tur­bu­lence of molten alu­minium pad is avoided.

14

In­ert Elec­trodes

While the Wet­table Cath­ode tech­nol­ogy (which is yet to be com­mer­cially im­ple­mented, be­cause it needs a dif­fer­ent con­fig­u­ra­tion of the cell and im­proved heat bal­ance) would solve some rou­tine op­er­a­tional prob­lems and im­prove the en­ergy ef­fi­ciency to some ex­tent, the in­ert an­ode tech­nol­ogy, if suc­cess­ful, is con­sid­ered to be the most im­por­tant de­vel­op­ment since the in­ven­tion of the Hall - Héroult cell in 1886, as it would com­pletely elim­i­nate CO for­ma­tion from the an­ode.

15 2

Need­less to say, there is a ma­jor thrust in this area, and huge work is go­ing on. About 348 Patents have been reg­is­tered from 1945 to 1998 and many more in re­cent years.

15

The re­quire­ments of a good in­ert elec­trode for alu­minium ex­trac­tion are: (a) Must not re­act with or dis­solve in the cry­o­lite elec

trolyte

(b) Must be in­ert to oxy­gen or other cor­ro­sive gases at

the op­er­at­ing tem­per­a­ture

(c) Must be ther­mally sta­ble at about 1000oC and have

good me­chan­i­cal strength

(d) Must have an elec­tri­cal con­duc­tiv­ity greater than 120 ohm- cm- at the op­er­at­ing tem­per­a­ture of 950o

1 1

970oC

(e) Must not con­tam­i­nate molten alu­minium by form

ing al­loys or com­pounds

(f) Must be rel­a­tively in­ex­pen­sive in terms of the ma­te­rial cost and by achiev­ing a very high elec­tro-ac­tive sur­face area to weight ra­tio

(g) Retro­fit ca­pa­bil­ity of the in­ert an­ode assem­bly with the ex­ist­ing cell su­per­struc­tures (This is not re­quired for Green­field im­ple­men­ta­tions).

A num­ber of ma­te­ri­als such as metal ox­ides, mix­ture of metal ox­ides, cer­mets, met­als and their al­loys have been found to be suit­able as in­ert an­ode ma­te­ri­als. Cer­mets are com­pos­ites made of ce­ram­ics and met­als. There is huge lit­er­a­ture on the sub­ject. In­ter­ested reader may see, for ex­am­ple, the an­nual re­views of Pawlek.

16 Among many ma­te­ri­als, one cer­met con­tain­ing cop­per or sil­ver metal phase in nickel fer­rite (NiFe O ) with

2 4 dopants such as NiO and Fe O , has been found prom

2 3 ising. Of its many meth­ods of prepa­ra­tion and many com­po­si­tions, one is from Al­coa. The method is de

17 scribed briefly as an illustration. The com­po­nent ox­ides are mixed to­gether, pul­verised and then cal­cined at 1250oC for 12 h pro­duc­ing a mix­ture of NiFe O and

2 4 NiO phase. The mixed phase is milled to less than 10 mi­cron par­ti­cle size, made into a slurry with poly­meric bin­der (PVA) and wa­ter and spray-dried. The re­sul­tant pow­der is blended with very fine cop­per and sil­ver pow­ders (Cop­per : sil­ver = 70 : 30) to im­prove the elec-

tri­cal con­duc­tiv­ity and to en­hance the wear re­sis­tance. The mix­ture is com­pressed in an iso­static press un­der 200 MPa pres­sure into the an­ode shapes (Fig.5). The an­ode is then an­nealed in con­trolled ar­gon – oxy­gen mix­ture (O con­cen­tra­tion 17-350 ppm) at 1350o-1385oC

2 for2-4h. Atyp­i­cal com­po­si­tion of the a node by weight in terms of the con­stituents is: Ag 1.7%, Cu 15.3%, NiO 42%, Fe O 41%.

2 3

But the most im­por­tant road­block in the im­ple­men­ta­tion of the in­ert an­ode tech­nol­ogy in alu­minium pro­duc­tion is the im­bal­ance in the heat bal­ance. When car­bon an­odes are con­sumed by O , con­sid­er­able heat is

2 pro­duced dur­ing the process, which helps in main­tain­ing the bath tem­per­a­ture. In case of in­ert elec­trodes, ei­ther the heat is to be sup­plied ex­ter­nally and / or the heat loss from the bath is to be re­duced con­sid­er­ably. Thus, prima fa­cie, the use of in­ert an­ode would re­quire more in­put of en­ergy than what is re­quired at present. Ad­di­tion­ally, there are prob­lems of an­ode cor­ro­sion in the flu­o­ride elec­trolyte bath, and the huge cap­i­tal costs in­volved in retrofitting. Still there is a bright sil­ver lin­ing.

Fig.5 shows schemat­i­cally the di­a­gram of a typ­i­cal nickel fer­rite-based in­ter an­ode assem­bly. It has

18 been es­ti­mated that around 1120 in­ert an­odes would be re­quired for a 350,000 Amp cell con­tain­ing about 40 car­bon an­odes. The same au­thor18 has fur­ther es

18 ti­mated that for a 250,000 met­ric ton ca­pac­ity alu­minium smelter, the an­ode plant ca­pac­ity would be about 2,200 met­ric tons per an­num (that is, 124,000 an­odes per year), con­sid­er­ing the an­ode life as 3 years. The cap­i­tal cost of such a fa­cil­ity would be just about USD 10-15 mil­lion, com­pared with about USD 160 mil­lion for a con­ven­tional car­bon an­ode plant. Thus, the In­ert An­ode Tech­nol­ogy would not only elim­i­nate the emis­sion of CO from alu­minium smelters, but also would

2 make the op­er­a­tion and main­te­nance of such a plant prof­itable in the long run.

Con­sid­er­ing all as­pects, there­fore, the ex­pert group of the Euro­pean Com­mis­sion con­cludes that the fu­ture of alu­minium ex­trac­tion is in the In­ert An­ode and Wet­table Cath­ode Tech­nol­ogy, and ex­pects some com­mer­cial break­throughs in near fu­ture.

19

Ref­er­ences:

1. https://www.eia.gov/out­looks/ar­chive/ieo09/elec­tric­ity.ht

ml, Ac­cessed on Septem­ber 10, 2018

2. A. Sch­midt, “Cal­cu­la­tion of Prof­itabil­ity in Elec­trol­y­sis

In­stal­la­tions”, J. Elec­trochem. Soc., 118 (12), 2046 (1971).

3. G. G. Bot te ,“Elec­tro chem­i­cal man­u­fac­tur­ing in the chem­i­cal in­dus­try”, The Elec­tro­chem­i­cal So­ci­ety In­ter­face, 23 (3), p.49, 2014.

4. http:// www. world­chlo­rine. org/ wp- con­tent/ themes/ brick the­mewp/pdfs/ sus­tain­able fu­ture. pdf, Ac­cessed on Septem­ber 10, 2018. 5. www.ze­romer­cury.org/pho­cad­own­load/What­s_on_in_the_ re­gions/ In­dia/ Chlo­rine_ Eco­nomics_ of_ Con­ver­sion. pdf, Ac­cessed on Septem­ber 10, 2018.

6. J. A. How­ell( Ed .),“The Mem­brane Al­ter­na­tive: En­ergy Im­pli­ca­tions for In­dus­try – Watt Com­mit­tee Re­port No. 21”, CRC Press, 2014.

7. S. Loeb and S. Souri­ra­jan, “Sea Wa­ter Dem­iner­al­i­sa­tion by Means of an Os­motic Mem­brane”, Adv. Chem. Ser., 38, 117 (1962).

8. (a) Yo­hannes Kiros et al., “Low En­ergy Con­sump­tion in Chlor-Al­kali Cells Us­ing Oxy­gen Re­duc­tion Elec­trodes”, In­ter­na­tional J. Elec­trochem. Sci., 3, 444-451, 2008. (b) J. Kin­trup et al., “Gas Dif­fu­sion Elec­trodes for Ef­fi­cient Man­u­fac­tur­ing of Chlo­rine and Other Chem­i­cals”, The Elec­tro­chem­i­cal So­ci­ety In­ter­face, p.71, Sum­mer 2017.

9. For ex­am­ple, (a) N. Fu­ruya, “A New Method of Mak­ing a Gas Dif­fu­sion Elec­trode”, J. Solid State Elec­tro­chem­istry, 8 (1), 48-50, 2003; (b) US Patent 707, 4306, “Elec­tro­cat­alytic Com­po­si­tion for Oxy­gen-De­po­larised Cath­ode”, (De Nora), July 2006.

10. M. Obai­dat et al., “En­ergy and Ex­ergy Analy­ses of Dif­fer­ent Alu­minium Re­duc­tion Tech­nolo­gies”, Sus­tain­abil­ity, 2018, 10, 1216; doi:10.3390/su10041216.

11. http://www.world-alu­minium.org/sta­tis­tics, Ac­cessed on

Septem­ber 16, 2018.

12. Ralph E. We­ston, Jr .,“Pos­si­ble green­house ef­fects from tetraflu or om eth­ane and car­bon diox­ide emit­ted from alu­minium pro­duc­tion”, At­mo­spheric En­vi­ron­ment, 30 (16), 2901-10, 1996. 13. (a) Zheng Luo et al., “Prospec­tive Study of the World Alu­minium In­dus­try”, A Re­port by of the Joint Re­search cen­tre of The In­sti­tute for Prospec­tive Tech­no­log­i­cal Stud­ies, Euro­pean Com­mis­sion 2007 (http://ftp.jrc.es/ EUR­doc/JRC40221.pdf, Ac­cessed on Septem­ber 16, 2018). (b) H. Kvande, et al., “The Alu­minum Smelt­ing Process and In­no­va­tive Al­ter­na­tive Tech­nolo­gies”, JOEM, 56 (55), S23, 2014. DOI: 10.1097/JOM.0000000000000062

14. L. Box­all, et al., “TiB2 Cath­ode ma­te­rial: Ap­pli­ca­tion in

Con­ven­tional VSS Cell”, J. Light Ma­te­ri­als, 36, 35-39, 1984. 15. For ex­am­ple, US patent 655,1476, “Noble Metal Coated In­ert

An­ode for Alu­minium Pro­duc­tion”, (Scherba) April, 2003. 16. R. P. Paw lek ,“In­ert A nodes: An Up­date ”, in“Es­sen­tial Read­ings in Light Met­als” (Eds.) A. Tom­sett et al., 2016, Vol 4, Elec­trode Tech­nol­ogy for Alu­minium Pro­duc­tion, pp. 11261133.

17. US Patent 586,5980, “Elec­trol­y­sis with An In­ert Elec­trode Con­tain­ing a Fer­rite, Cop­per and Sil­ver”, (Siba P. Ray et al, Al­coa), Feb, 1999.

18. J. Keniry, “The eco­nomics of in­ert an­odes and wet­table cath

odes for alu­minium re­duc­tion cells”, JOM, 2001, May, p. 43. 19. A. Herbst, “Nav­i­gat­ing the Roadmap for Clean, Se­cure and Ef­fi­cient En­ergy In­no­va­tion : Is­sue Pa­per on Low-Car­bon Tran­si­tion of EU In­dus­try by 2050” (http://www.set-nav.eu/ sites/de­fault/files/com­mon_­files/de­liv­er­ables/wp5/Is­sue%20 Pa­per %20 on %20 low-car­bon %20 tran­si­tion %20 of %20 EU %20 in­dus­try %20 by %202050. pdf) Ac­cessed on Sept 16,2018.

Dr N C Datta, (phys­i­cal chemist, Ph.D.Chem­istry (1972) from IIT, Kharag­pur) is presently Con­sul­tant with Mod­i­con Pvt Ltd, Navi Mumbai. He is in the field of in­dus­trial catal­y­sis since 1972. He has worked in the Cat­a­lyst Divi­sion of Projects & De­vel­op­ment In­dia Ltd, Sin­dri; War­wick Man­u­fac­tur­ing Group, Univer­sity of War­wick, Coven­try, UK, and in the erst­while CATAD Divi­sion of In­dian Petro­chem­i­cals Cor­po­ra­tion Ltd (IPCL), Navi Mumbai. Be­sides catal­y­sis, Dr Datta’s other re­search in­ter­ests are in quan­tum chem­istry. He has sev­eral re­search papers, in­clud­ing a cou­ple of papers in quan­tum chem­istry, a book and a patent on wa­ter gas shift cat­a­lyst man­u­fac­ture to his credit.

Fig.1 Schematic Di­a­gram of a Mem­brane Cell 9

Fig. 3 Schematic Di­a­gram of a Mem­brane Cell with Oxy­gen De­po­larised Elec­trode

Fig. 4 Schematic Di­a­gram of a Typ­i­cal A typ­i­cal Hall – Héroult cell

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