1. The Down's cell is an electrolytic cell that operates at 800°C to produce sodium metal through the electrolysis of molten sodium chloride. 2. It consists of a graphite anode and iron cathode separated by an iron screen inside a lined steel chamber. Molten sodium chloride mixed with calcium chloride and barium chloride is used as the electrolyte. 3. During electrolysis, chloride ions are oxidized at the anode to produce chlorine gas while sodium ions are reduced at the cathode to produce molten sodium metal. Some calcium is also produced but separates from the sodium due to differences in density.

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An Electrolytic Cell
Operating Temperature = 800oC
Voltage Of Cell = 3.5 to 4.5 V
Cell current = ≤ 45
Cell efficiency = 80 – 90 %
Lifetime of cell = 1100 – 1200 days
Anode = Graphite
Cathode = Iron

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Down’s cell consists of:
• Rectangular steel chamber lined inside with firebricks.
• Anode at the centre in the shape of large cylindrical block
made of graphite.
• Cathode in the form of large circular ring of Iron around the anode.
• Both anode and cathode are separated by an iron screen with gauze
to prevent the flow of ions.
• Dome-Shaped area above anode for the removal of chlorine gas.
• Sodium metal outlet connected above the anode for the extraction
of Na metal.

5.  In Down’s cell Sodium chloride is used
in molten state.
 The melting point of NaCl is 8010C.
 At this elevated temperature efficiency
of cell is reduced due to the formation
of metallic fog(emulsion of Na in
molten NaCl), Which is impossible to
separate.
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 To lower the melting point of NaCl we add CaCl2 & small amount
of BaCl2 as an impurity in a measured ratio.
 Ratio can be vary according to the desired conditions.
 The most general one is 1 mole of NaCl in 3 mole of CaCl2.
 Melting point of this mixture is red about 6000C.

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• Both the electrodes are connected to a D.C. source of about 7 to 8 V.
• Cathode is at low potential turns -ve due to the excess of electrons.
• On the other hand anode is at high potential turns +ve due to the
deficiency of electrons.
• When we add molten electrolyte into the chamber the ions Na+ and
Cl- migrates to their respective electrode due to the electrostatic
forces of attraction.
Na+ Na+
Na+
Na+
Cl- Cl-
Cl-
Cl-

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When chloride ions approaches to anode, they oxidize by losing
one electron by each individual atom and turns into chlorine gas.
2Cl-
(aq) Cl2 (g) + 2e-
When Sodium ions approaches to cathode, they reduce to sodium
metal by gaining one electron.
Na+
(aq) + e- Na(s)

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2Cl-
(aq) Cl2 (g) + 2e-
( Na+
(aq) + e- Na(s) ) × 2
2 Na+
(aq) + 2Cl-
(aq) 2 Na(s) +
Cl2 (g)
• Chlorine gas produced in the reaction is evaporated through
dome-shaped cavity above the anode
• Sodium metal produced is added into the container attached
at above the cathode.

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• Since the decomposition potentials of both the salts NaCl & CaCl2
are so close (3 to 4 V) that we are unable to maintain the potential
difference of electrodes to decompose only NaCl.
• We also have to apply some extra voltage ( 7 to 8) to overcome the
potential due to cell resistance as well as the for the evolution
of chlorine gas.
• Due to this high potential difference Ca2+ also reduced to Ca and
released into Na container.

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• Sodium and calcium do not react with each other .
• Density of sodium is 0.971 gcm-3.
• Density of Calcium is 1.54 gcm-3.
• Sodium moves up due to the low density than calcium.
Na
Ca

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Both the metals are separated by means of different separation
techniques:
• Generally Na is cooled to temperature
upto 110oC.
• At this temperature solid calcium
make a separate layer with Na.
• Through which we can separate easily
both the metals.

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• Even after this separation, sodium contain minute amount
of calcium ( 200 to 400 ppm)
• But quantity is too small to render the properties of sodium.
• We can use this sodium industrially for the manufacturing of
many things.

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In Some fields:
we need 100% pure sodium to use in:
 Manufacturing of electronics
 Nuclear field

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 We use mixture of salts ( NaCl, CaCl2 and BaCl2 ) to lower
the melting point of electrolyte.
 If we mix 58 wt% CaCl2 with 42 wt% NaCl in electrolyte,
then cell will work at 585oC.
 If we mix 66.8 wt% CaCl2 with 33.2 wt% NaCl in electrolyte,
then cell will work at 505oC.
 By increasing the concentration of CaCl2, sometimes a layer
of Na-Ca is formed on cathode. Which often resists charges
to make connect with the surface of cathode.
 Sometimes increased concentration of CaCl2 can cause
short circuiting.

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 Some times current efficiency is reduced by the cell resistance
 Due to this energy loss heat is produced
 This heat leads to the corrosion of graphite anode.
 Also reduce the efficiency of cell.
 This extra heat is removed by cell walls.
 Current efficiency is also reduced by the diaphragm, due to the
restriction of flow of charges between the electrodes.

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 Sometimes we need pure Na metal without any
impurities for some applications:
 To produce pure sodium metal from the mixture of
sodium and Calcium, an electrolyte NaCl / SrCl2 has
been developed.
 It lowers the concentration of calcium metal in sodium
metal from 300 ppm to 50 ppm.

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We can also use two kinds of mixtures of salts:
Ternary electrolyte:
NaCl , SrCl2 & BaCl2
Quaternary electrolyte:
NaCl , SrCl2 , BaCl2 & NaF

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Pros and cons of these electrolytes:
 By using these electrolytes we can get pure sodium metal
 Cell works at the same temperature (600o C )
 Have same current efficiency 86% to 88%.
 But by using strontium salts, electrolyte attacks the
fire brick lining of cell.

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Anode can be:
 Electrically conducting carbon ( Graphite )
 Metals ( Platinum, palladium or nickel )
 Oxides of these metals
The anode should not be corroded by
the reaction of Chlorine formed at anode:

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Cathode can be:
 Electrically conducting carbon ( graphite )
 Steel, Iron
 Metals of Group 8 elements
Diaphragm used to separate anode from cathode
non-reacting porous material usually made of
glass, any non-reacting fabric and asbestos.

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In the later research on Down’s cell research in modified Down’s cell
 In this electrolytic cell cathode and anode are used in the form
of horizontal plates.
 Electrolyte is a mixture of ( NaCl, co-electrolyte and
chlorine donating compound)
 works at between temperature range ( 20 to 200oC )
 Can be operate like batch or continuous process.
 Cell should be gas tight, so that no moisture enter the chamber.

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 NaCl can be mixed with co-electrolyte before starting the
electrolysis.
 Mixer of electrolyte should be moisture free.
 Because the water in the electrolyte can react with chlorine
to make hydrochloric acid.
 Product formed by this cell is Na and co-electrolyte.
 Na can be purified by fractional crystallization as well as
decantation, filtration.
 For the further purification we perform solvent extraction
technique.

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Batch process:
To operate this cell at less than 150oC,
we should remove the solid layer of sodium formed at cathode.
For this purpose process should be queued.
Continuous process:
To make the process continuous,
temperature should be kept higher than the melting point of
sodium metal (97.5o C). Normally for continuous process cell
should be operate at 100o C.

25. References 25
 Keppler, Stephen John; Messing, Thomas A. Proulx, Kevin Bernard;
Jain, Davendra Kumar (2001-05-18).
"Molten salt electrolysis of alkali metals, U.S. Patent 6669836".
Retrieved 2010-07-17.
 Stephen Ernest Jacobson, Dennie Turin Mah (2004-09-07).
“Low temperature alkali metal electrolysis. U.S. Patent 6787019B2”
 Aurbach, Doron. Nonaqueous electrochemistry. CRC Press, 1999.
pp. 531-535
 www.researchgate.net
 www.citycollegiate.com

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