Manufacture of sodium hydroxide

Manufacture of Sodium Hydroxide

1. Manufacture of Sodium Hydroxide
The major method for producing sodium hydroxide is electrolysis of concentrated brine
(sodium chloride solution)* produces chloride gas, hydrogen gas and sodium hydroxide.
2NaCl(aq) + 2H2O(l) -----> H2(g) + Cl2(g) + 2NaOH(aq)
At the anode (positive electrode), the chloride ions (Cl
-
) are oxidised to chloride gas
(Cl2(g)). At the cathode (negative electrode), the sodium ions (Na
+
) and water (H2O(l)) are
reduced to hydrogen gas (H2(g)) and sodium hydroxide (NaOH(aq)).
There are three types of electrolysis to produce sodium hydroxide from brine:
The mercury cell process (Castner-Kellner):









At the anode, titanium is the metal and chloride ions are oxidised to chloride gas:
2Cl
-
(aq) -----> Cl2(g) + 2e
-

At the cathode, mercury is flowing along the bottom of the cell. Sodium ions are reduced
to sodium metal:
Na
+
(aq) + e
-
-----> Na(s)
An amalgam is formed when sodium metal dissolves in the liquid mercury and it is
removed to the decomposer. The amalgam then reacts with water to form sodium
hydroxide, hydrogen gas and mercury:

2. 2Na/Hg(l) + 2H2O(l) -----> 2NaOH(aq) + H2(g) + 2Hg(l)

Sodium hydroxide and hydrogen gas are collected and the mercury is recycled through
the electrolytic cell. Mercury is toxic so care must be taken to prevent mercury loss.
Nelson diaphragm cell process







In the diaphragm cell, a permeable diaphragm of asbestos or metal with polymer
separates the anode area and the cathode area. Saturated brine enters into the anode
compartment and produces chlorine gas:
2Cl
-
(aq) -----> Cl2(g) + 2e
-

The saturated brine flows through the diaphragm and into the cathode compartment. At
the anode, carbon or titanium coated with Ru-Ti, the chlorine ions are oxidised. At the
cathode, steel mesh, hydrogen gas and hydroxide are produced:
2H2O(l) + 2e -----> H2(g) + 2OH
-
(aq)
The diaphragm allows the sodium ions to migrate across it and to combine with
hydroxide, forming sodium hydroxide:
2H2O(l) + 2Cl
-
(aq) + 2Na
+
(aq) -----> 2NaOH(aq) + H2(g) + Cl2(g)
The products are sodium chloride and sodium hydroxide. Solid sodium hydroxide can be
crystallised out. The sodium hydroxide must usually be concentrated to 50% and the salt
removed. The method that is used for this process is an evaporative process which uses
about three tones of steam per ton of sodium hydroxide. The salt separated from the
caustic brine which can be used to saturate diluted brine.
Membrane cell process

3. This process is most commonly used method for producing sodium hydroxide as it has
the lowest energy requirement producing very pure sodium hydroxide with no hazardous
waste. This is very similar to the diaphragm cell process, but with a Nafion membrane
separates the anode area and cathode area. The ion-exchange membrane selectively
allows sodium ions and water to flow to the cathode compartment but prevents products
from moving between the compartments. And the amount of steam needed for
concentration of the caustic is relatively small (less than one tone per ton of sodium
hydroxide). Saturated brine enters into the anode compartment and produces chlorine gas
(oxidised):
2Cl
-
(aq) -----> Cl2(g) + 2e
-

And it flows through the membrane to the cathode compartment. At the anode, titanium
is the metal. At the cathode the metal is nickel and water is reduced:
2H2O(l) + 2e -----> H2(g) + 2OH
-
(aq)
Sodium ions migrate across the membrane and combine with hydroxide to form sodium
hydroxide in the cathode compartment:
2H2O(l) + 2Cl
-
(aq) + 2Na
+
(aq) -----> 2NaOH(aq) + H2(g) + Cl2(g)
The product is concentrated sodium hydroxide.
Other methods:
Leblanc process
Sodium carbonate is produced and roasted to create carbon dioxide and sodium oxide.
The sodium oxide then absorbs water to create sodium hydroxide.
Metathesis reaction

4. The metathesis reaction between calcium hydroxide (lime) and sodium carbonate forms
sodium hydroxide:
Ca(OH)2 + Na2CO3 → CaCO3 + 2 NaOH