Lecture Slides by Prof. Imitaz Ali at Ghulam Ishaq Khan Institute for Engineering Sciences and Technology (GIKI).

2. Sodium carbonate

3. Introduction
• Sodium carbonate (Na2CO3) also known as washing soda or
soda ash, is a sodium salt of carbonic acid.
• Commonly occurs as a crystalline heptahydrate, which
readily effloresces to form a white powder, the
monohydrate.
• Sodium carbonate is domestically well known as a water
softener.
• It can be extracted from the ashes of many plants.
• Commercially it is produced from salt and limestone in a
process known as the Solvay process.
• Soda ash is the most important high tonnage, low cost,
reasonably pure, soluble alkali available to the industries as
well to the laboratory.

4. Uses
• fluxing agent in glass manufacture
• alkali in many soap and detergent applications.
• flue gas desulfurization
• sulfite paper pulp process
• green liquor recovery section of the Kraft pulping
process
• production of sodium hydroxide by the lime soda
process
• production of baking powder
• as a dry-powder fire extinguisher

5. Manufacture
• Leblanc process
• Solvay’s process
• Dual process (modified Solvay‘s process)
• Electrolytic process

6. Leblanc process
• Raw materials
– Common salt -- sea water, salt lake and sub soil
water
– Sulfuric acid -- contact process
– Lime stone -- after removing clay, slit and sand
from mineral calcite or aragonite
– Coke

9. Leblanc process

10. Leblanc process
• Common salt is first mixed with the conc. H2SO4
in equivalent quantities and heated in a cast iron
salt cake furnace by flue gases from adjacent coal
of fire.
NaCl + H2SO4  NaHSO4 + HCl
• HCl is passed to tower packed with coke and is
absorbed through a spray of water.
• The paste of NaHSO4 is taken out and heated to a
high temperature on the hearth of a furnace
along with some more common salt.
NaHSO4 + NaCl  Na2SO4 + HCl

11. Leblanc process
• The salt cake is broken or pulverized, mixed with coke
and limestone and charged into black ash rotary
furnace consisting of refractory lined steel shells.
• The mass is heated by hot combustion gases entering
at one end and leaving at the others.
• The molten porous gray mass thus formed known as
black ash is separated from the calcium sludge and
then crushed and leached with water in absence of air
in a series of iron tank.
Na2SO4 + 2C  Na2S + 2CO2
Na2S + CaCO3  Na2CO3 + CaS

12. Leblanc process
• The extract containing Na2CO3 , NaOH and other
impurities is sprayed in counter current to the flow of
hot gases from the black ash furnace in a tower.
• The sodium carbonate thus obtained is concentrated in
open pans and then cooled to get sodium carbonate.
• The product is calcined to get soda ash which is re-
crystallized to Na2CO3 .10H2O.
• The sludge containing mostly CaS is left behind as alkali
waste.
• The liquor remaining after removal of first batch of
soda ash crystals is purified and then causticized with
lime to produce caustic soda.

13. Leblanc process
Recovery of sulfur from alkali waste
• Alkali waste is charged into cylindrical iron vessels
arranged in series and CO2 delivered from lime kilns is
passed through it, the H2S gas thus obtained is then
conduced together with a regulated amount of air in a
Claus kiln containing iron oxide as catalyst.
• The exothermic reaction proceeds without further
external heat.
CaS + H2O + CO2  CaCO3 + H2S
2H2S + O2  2H2O + 2S
• Recovered sulfur is used in the manufacture of sulfuric
acid.

16. • Poor economics and excessive pollution
caused by the hydrochloric acid and calcium
sulfide by-product led to the eventual demise
of the Leblanc process.

17. Solvay's ammonia soda process
• Raw materials
– Salt
– Limestone
– Coke
– Ammonia

18. Solvay's ammonia soda process
CaCO3 + 2NaCl  Na2CO3 + CaCl2

20. Solvay's ammonia soda process
Preparation and purification of brine
• Crude sodium chloride brine is first purified to prevent
scaling of downstream process equipment and to prevent
contamination of the final product.
• Magnesium ions are precipitated with milk of line, Ca(OH)2,
and the calcium ions are precipitated with soda ash.
Ammoniation of brine:
• NH3(aq) + H2O(l)  NH4OH(aq) + 34900 kJ
Most of the ammonia is recycled from downstream steps,
although some make-up is required.

21. Solvay's ammonia soda process
Precipitation of Bicarbonate
• The ammoniated brine is then sent to the
carbonating columns where sodium bicarbonate
is precipitated by contacting the brine with
carbon dioxide
2NH4OH + CO2 (NH4)2CO3 + H2O + heat
(NH4)2CO3 + CO2 + H2O  2NH4HCO3
NH4HCO3 + NaCl  NH4Cl + NaHCO3
ammonium chloride is a marketable fertilizer product

22. Solvay's ammonia soda process
• NaHCO3 is less soluble and precipitates on the
internals of the carbonating column.
• At the end of the make cycle, the slurry is drained
and the solid NaHCO3 is filtered. However,
considerable amounts of NaHCO3 remain in the
column after the slurry is drained.
• A series of five or more columns with appropriate
piping interconnections are used for continuous
operation.
• The carbonation is favored by higher pressures
and low temperature.

23. Solvay's ammonia soda process
Filtration of bicarbonate
• The slurry from the carbonating columns is fed to continuous
vacuum filters or centrifuges where NaHCO3 crystals are recovered.
• The filter cake is carefully washed to control residual chloride while
maintaining acceptable yield.
• Yield losses on washing are on the order of 10%.
Calcining the Bicarbonate to Soda Ash
• The filtrate is then calcined at 175–225◦C to produce sodium
carbonate, carbon dioxide and water vapor:
• 2NaHCO3  Na2CO3 + CO2 + H2O
• CO2 is recovered, compressed and recycled back to the carbonating
columns as needed.

24. Solvay's ammonia soda process
Recovery of Ammonia
• The traditional Solvay process recovers
ammonia by reacting the ammonium chloride
in the filtrate liquor with milk of lime
• 2NH4Cl + Ca(OH)2  2NH3 + CaCl2 + 2H2O

25. Solvay's ammonia soda process
Production of milk of lime
• The milk of lime and much of the carbon dioxide needed in
the Solvay process are produced from limestone. The
reaction is carried out in a kiln at 950–1100◦C.
• CaCO3  CaO + CO2
• C(s) + O2 (g)  CO2 (g)
• Usually metallurgical grade coke is mixed with the
limestone as a fuel.
• CO2 is recovered from the exhaust by filtration to remove
entrained dust, compressed and sent to the carbonization
columns. The lime is cooled and slaked with water.
• CaO(s) + H2O (l)  Ca(OH)2

26. Leblanc Process
Solvay’s Process

28. Major engineering problem
• The carbonation tower should be constructed to
permit the downward travel of growing NaHCO3
crystals.
• This is done by having each unit simulate a very
large single bubble cap with down sloping floors.
• The absorption is carried out in towers filled with
liquid. Hence CO2 must be compressed.
• Due to the compression the partial pressure and
solubility of CO2 increased at the end of
carbonating cycle.

29. Major engineering problem
• NaHCO3 formed in the making tower is drawn off as a
suspension, it is necessary to ensure that the
precipitated NaHCO3 is easily filterable and efficiently
washable.
• It is carried out by regulating the temperature and
concentration in the making tower.
• During the precipitation cycle, the temperature
gradient is maintained at 20°C at the both ends and
45°C in the middle and fine crystals of NaHCO3 are
allowed to grow.
• The temperature is increased from 20°C to 45-55°C by
heat of reaction and reduced by using coils.

30. Major engineering problem
Development of suitable calcining equipment
• Moist NaHCO3 will cake on sides of kiln, preventing effective
heat transfer through shell.
• Kiln must be equipped with heavy scraper chain inside and
wet filter cake must be mixed with dry product to avoid
caking.
• These problems can be avoided by using fluidized bed
calciner.
Filtration unit
• Filtration should be carried out by using vacuum on the
drum filter. As it helps in drying NaHCO3 and in recovering
NH3

31. Major engineering problem
Ammonia recovery
• NH3 recovery costs 4-5 times more than that of Na2CO3
so losses must be kept low.
• By proper choice of equipment design and
maintenance, losses can be as low as 0.2% of recycle
load.
Waste disposal
• Large quantities of CaCl2-NaCl liquor is generated
during the process.
• Either used elsewhere or disposed as waste.

32. Advantage of Solvay process
• Less electric power
• Less corrosion problem
• Use of low grade brine
• Not a problem of disposal of co-product
• Does not require ammonia plant

33. Disadvantage of Solvay process
• Higher salt consumption
• Waste disposal of CaCl2-brine stream
• Higher investment in ammonia recovery units
than crystallization unit of NH4Cl
• Higher capacity plant set up require for
economic break even operation

34. • This process combines
Na2CO3 production with
NH4Cl production.
• The importance of the
process in Japan is due
to the high cost of
imported rock salt and
use of the ammonium
chloride as a fertilizer.
Solution A: 4.1 mol/L NH4Cl + 1.05 mol/L NaCl; =1.108 g/cm3; t =20 ◦C
Solution B: 3.45 mol/L NH4Cl + 1.1 mol/L NaCl;  =1.110 g/cm3; t =5◦C
Solution C: 1.86mol/L NH4Cl + 3.73 mol/L NaCl;  =1.187 g/cm3; t =10 ◦C
Dual Process

35. Dual Process
• NH3 is absorbed by the NaHCO3 mother liquor,
and solid NaCl is added.
• On cooling, NH4Cl separates, is recovered in
centrifuges, and is then dried in rotary dryers
with air at 150 ◦C.
• The mother liquor is recycled to the
carbonation towers where sodium NaHCO3 is
precipitated.

38. Dual Process
Difference compared to Solvay’s process
• In the dual process, NH3 is not recovered; hence no
NH3 recovery tower (distillation equipment)t is
required.
• Also, lime kilns are not required if other sources of
CO2 are available.
• As the mother liquor is recycled, special attention
must be paid to the water balance of the system.
• The amount of water introduced into the system (e.g.,
for washing NaHCO3 and NH4Cl) must be controlled
continuously to maintain the correct quantity and
composition.

39. Major engineering problem
Salt purification
• Solid salt which is used to obtain better crystallization yields of NH4Cl
cannot be purified compared to brine feeds in Solvay process.
• Only purification method is mechanical washing and dewatering.
Corrosion
• NH4Cl solution is quite corrosive to equipment involved in crystallization
and solids recovery.
• So, corrosion resistant material or rubber-lined units are preferred.
Refrigeration Cost
• Actual refrigeration cost is variable but to maintain the temperature
around 0°C, the electric requirements are still double than Solvay's
operation.

40. Electrolytic process
• Will be covered in the production of caustic
soda (NaOH)