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Dr Inam Ul Haq Jazbi s-Block ElementsManufacture of Soda Ash By Solvay-Ammonia-Soda Process
Introduction
Sodium Carbonate (Na2CO3) commonly called Soda Ash or Sal Soda and hydrated sodium carbonate
(Na2CO3.10H2O) generally called Washing Soda are commercially manufactured by Solvay’s Process or
Ammonia-Soda Process which was developed in 1861 by Belgian scientist, Ernest Solvay. Sodium
bicarbonate (NaHCO3) commonly known as Baking Soda or cooking soda is a valuable intermediate
product of Solvay’s process.
Merits of Advantages of Solvay’s Process
Solvay’s process has following advantages:
1. It gives soda of much higher purity (99.5).
2. It is a low cost process as raw materials are very cheap and less expensive.
3. Neither wastage of materials occurs nor harmful by-products forms.
4. No pollution problems as no gaseous pollutants are formed.
5. It is a highly efficient process as about 97% CO2 is used up in this process.
6. Sodium bicarbonate is obtained as a valuable intermediate.
7. Calcium chloride (CaCl2) is a useful by-product.
8. It is a cyclic, continuous and self-contained process as raw materials like NH3 and CO2 are
recovered and recycled.
Raw Materials and their Sources
1. Brine (20-31% Saturated solution of NaCl) [obtained from natural sources of common salt].
2. Ammonia [obtained by heating NH4Cl and quick lime/slaked lime].
3. Limestone; CaCO3 [for supplying CO2 and quick lime on heating in lime kiln]
Basic Principle
The process is based on the formation of ammonium bicarbonate by the reaction of NH3, CO2 and water
which on double decomposition with sodium chloride solution gives precipitate of sparingly soluble salt of
sodium bicarbonate which is ignited to give anhydrous sodium carbonate.
NH3 + H2O NH4OH(aq)
CO2 + H2O H2CO3(aq)
2NH4OH(aq) + H2CO3(aq) (NH4)2CO3(aq) + 2H2O
–
NH4 HCO3 (aq) +
+
NH4OH(aq) + H2CO3(aq) H2O
OR
NH3 + CO2 + H2O (NH4)2CO3(aq)
(NH4)2CO3(aq) + H2O + CO2 2NH4+HCO3–(aq)
OR
–
NH4 HCO3 (aq)
+
NH3 + CO2 + H2O
NH4+HCO3–(aq) + Na+Cl–(aq) NaHCO3 + NH4+Cl–(aq)
OR
NaCl + H2O + NH3 + CO2 NaHCO3 + NH4+Cl–(aq)
2NaHCO3(aq) Na2CO3(s) + CO2 + H2O
Different Steps of Process
1. Ammoniation of Brine in Ammoniating Tower (Saturation of Brine with Ammonia).
2. Carbonation of Ammoniated Brine in Carbonation Tower.
3. Filtration in rotary suction filter (vacuum filtration chamber).
4. Calcination and decomposition of Sodium Bicarbonate into soda ash and then its recrystallization
5. Ammonia Recovery.
Conceptual Inorganic Chemistry Text Book -1-
,Dr Inam Ul Haq Jazbi s-Block Elements
1. Ammoniation of Brine in Ammoniating Tower (Saturation of Brine with Ammonia)
In the ammoniating tower (ammonia absorber), a saturated solution of sodium chloride (about 28% by
mass) called brine containing some CO2 is thoroughly agitated with ammonia gas (coming from
Ammonia Recovery Tower or ammonia generator) until saturated ammoniated brine is obtained. [The
tower consists of mushroom-shaped baffles at short intervals which control the flow of brine ensuring
proper saturation with ammonia passing up the tower].
The ammonical brine so produced is allowed to stand for some time so that any impurities of calcium,
magnesium or iron salts present in brine are precipitated as carbonates and hydroxides settle down
which are filtered off. The impurities settle down and clear solution is passed through carbonating tower.
MgCl2(aq) + 2NH3 + H2O + CO2 MgCO3 + 2NH4Cl(aq)
OR
MgCl2(aq) + (NH4)2CO3(aq) MgCO3 + 2NH4Cl(aq)
MgCl2(aq) + 2NH3 + 2H2O Mg(OH)2 + 2NH4Cl(aq)
OR
MgCl2(aq) + 2NH4OH(aq) Mg(OH)2 + 2NH4Cl(aq)
2. Carbonation of Ammoniated Brine
(a) Production of CO2
For carbonation of ammoniated brine, CO2 is produced by heating lime stone to 1000°C in a furnace called
Lime Kiln.
CaCO3 CaO + CO2
(b) Saturation of Ammonical Brine with CO2
In Solvay or carbonation tower, carbonation of ammonical brine is carried out on the Counter-current
process. Ammonical brine is allowed to trickle down a carbonating (Solvay) tower (fitted with baffle-plates)
where it meets an upward current of carbon dioxide gas (coming from a lime kiln produced by heating lime
stone) introduced from the bottom of the tower at a pressure of 1-2 atmosphere. [The baffle-plates check
the flow of ammonical brine and break up the CO2 into small bubbles to ensure good conditions for the
reaction].
Here CO2 and NH3 reacts with water to give NH4+ and HCO3– ions which then react with Na+ and Cl– ions of
brine to precipitate less soluble sodium bicarbonate leaving ions of NH4+ and Cl– in solution. Since the
overall effect of these reactions is exothermic, the temperature of materials rises. This tends to increase
the solubility of NaHCO3 thereby inhibiting its precipitation. To counter this adverse effect, the lower part of
tower is cooled to 15°C.
NH3 + H2O + CO2 NH4+(aq) + HCO3– (aq)
(NH4+ + HCO3–) (aq) + (Na+ + Cl-) (aq) NaHCO3 + (NH4+ + Cl–)(aq)
3. Filtration in Rotary Suction Filter
The intermediate product soda bicarbonate is filtered in a vacuum filtration chamber where ammonium
salts are removed. (The thick milky suspension of NaHCO3 and NH4Cl from the base of carbonating tower
is them filtered (by means of rotary suction filter where two products are separated). The precipitated soda
bicarbonate left on filter cloth is scrapped off after washing with a spray of cold water and dried to free of
ammonium salts. The filtrate or mother liquor containing ammonium chloride is pumped to the ammonia
recovery tower).
4. Calcination of Sodium Bicarbonate followed by recrystallization
The precipitate of NaHCO3 is ignited (in specially constructed cylindrical vessels) to give anhydrous
Na2CO3 or soda ash. CO2 evolved is recycled to carbonating tower to use it again. Washing soda
(decahydrate Na2CO3.10H2O) is produced from soda ash by re-crystallization from its hot aqueous solution.
2NaHCO3 Na2CO3 + CO2 + H2O
Na2CO3+10H2O Na2CO3.10H2O
Conceptual Inorganic Chemistry Text Book -2-
,Dr Inam Ul Haq Jazbi s-Block Elements
5. Ammonia Recovery
In ammonia recovery tower, solution of NH4Cl is made to react with quick lime; CaO (obtained from lime
kiln) or with slaked lime (obtained by reacting CaO and steam) to give NH 3 which is recycled to saturation
tank. CaCl2 is a by-product.
CaO + H2O(g) Ca(OH)2
2NH4Cl + CaO CaCl2 + 2NH3 + H2O
2NH4Cl + Ca(OH)2 CaCl2 + 2NH3 + 2H2O
Summary
The manufacturing of washing soda from brine involves its ammoniation in ammoniating tower where
brine is saturated with ammonia and impurities are settled down followed by carbonation of ammoniated
brine in Solvay (carbonation) tower where brine reacts with ammonium bicarbonate to precipitate less
soluble baking soda which is thermally decomposed on calcination to yield soda ash which on
recrystallization from hot its hot aqueous solution gives washing soda.
Conceptual Inorganic Chemistry Text Book -3-
, Dr Inam Ul Haq Jazbi s-Block Elements
Extraction of Sodium from its Halite ore By Down’s Process
Type of Metallurgy
Sodium metal is extracted on large scale by the electrometallurgy though electrolysis of fused sodium
chloride in the presence of a flux fused calcium chloride in Down’s cell by Down’s Process.
Raw Materials
1. Fused sodium chloride as Electrolyte (42%)
2. Fused calcium chloride as Flux (58%)
Basic Principle
The metallurgy of sodium involves the reduction of sodium ions (Na+) of molten sodium chloride at the
cathode into molten sodium in Down’s Cell. Chlorine gas is also obtained as useful by-product.
Necessary Conditions
1. CaCl2 is used as a flux to lower the melting point of NaCl from 801°C to 600°C.
2. The electrolysis is carried out in absence of water (to avoid the formation of NaOH)
Construction of Down’s Cell
1. The cell consists of an outer steel tank lined with fire-bricks.
2. The cell is fitted with the central graphite anode, emerges from bottom of tank.
3. An inverted funnel shaped iron hood surrounds anode from above which leading to chlorine
container.
4. The cathode is rods of iron introduced through the side wall surrounding the anode.
5. Iron Gauze Diaphragm is suspended from iron hood separating cathode and anode thereby
preventing the mixing of electrolytic products (i.e. molten Na and Cl2 gas).
6. An inverted trough is placed over cathode to collect molten sodium.
Electrolytic cell Down’s cell Structural material Cylindrical steel tank with inner fire bricks
Cathode Iron rods Na+ and Ca2+ Na+ discharge as Na
Anode Graphite rod Cl‒ Cl‒ discharge as Cl2
Iron Gauze Diaphragm
Inverted trough
Conceptual Inorganic Chemistry Text Book -4-
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