Soda Ash(Sodium Carbonate) Process Flow Sheet
Basin & Brine purification
The basin is the part of plant where the raw brine is made. In basin, there are four stocks which
are used in the formation of brine. First of all, water from water reservoir is showered on piles of salt
(coming from khewra salt mine) and then this salt mixed water goes into sea sump stock by passing
through settler where mud is settled down. From sea sum tank, it is then showered on another pile of
salt using pump and stored in primary stock by passing through settler. And then, it is again showered
on slat piles and stored in dissolving cell by passing through settler and then goes to brine purification
section. The raw brine coming from basin should have a test of almost 105.6mls N/20mls.
Crude brine coming from basin is not
enough pure for use in the process as it
contains calcium and magnesium salts
and frequently potassium salts, sodium
sulphate and iron compounds. These
impurities are precipitated out of the
brine solution, so they do not scale the
equipment. In brine purification plant,
all the alkaline earth impurities are
removed by milk of lime and soda ash
In brine purification plant,
crude brine is dissolved in a reaction
vessel with milk of lime and then sent
to a time vessel so that complete mixing of mol and brine takes place. And then it is sent towards
primary settler for complete settling of red precipitates of magnesium hydroxide. And then it goes to
Ash mixing tank where brine mixes with ash and then goes to final settler for settling of red and white
precipitates of sodium and calcium carbonate respectively. The drain of reaction vessel, primary
settler and final settler goes to a decanter from where some brine is recovered, and the sludge goes to
calcium carbonate plant. (As see in fig.). After brine purification section, the brine should have a test
of almost 104.4 mls N/20mls
The main purpose of kiln is to produce
lime which is used to make milk of lime (MOL), and also
kiln produces CO2 which is used in the process. Inside
the kiln, CO2 is produced when coal combustion takes
place and also by decomposition of limestone. The top
product of kiln is almost 42% CO2 and bottom product
consist of unburnt limestone, lime and grit (impurities
like sand etc.).
In kiln process, first of all, limestone and coke are
stored in their respective two hopper through conveyer.
In the charging of feed into the kiln, there are two plate
feeder feed the coke in a weight tank and then wind loss
take it from weight tank and feed it into kiln or in some
old kilns, the feed is distributed in kiln by a Hannon flask
and air is fed by forced draft fan. The exiting kiln gas
goes to kiln gas scrubber where it is scrubbed with water
and then sent to plant. And at bottom of kiln, a rotary
lock is used to remove bottom products of kiln. Rotary
lock draws the products on buckets elevator which then takes the products to Lime
Bunker where it is stored and send to MOL Dissolver which is a revolving (a little bit inclined for
movement of materials) drum in hot water is showered to dissolve lime (CaO). There is also a sieve
inside MOL Dissolver by which grit and lime passes and goes to next MOL dissolver and back stone
is removes and sent back to limestone stock. And in the next MOL Dissolver
the grit is also removed and sent to dump yard. The remaining lime is then sent to MOL stock Tank
from which it goes to plant where it requires.
Absorber is a very important unit of plant. The objective of absorber is the absorption of ammonia
into brine as ammonia is act like an intermediate catalyst or an activator in the carbonization of
brine in carbonating tower. In absorber following reactions takes place:
NaCl.H2O + NH3 NaCl.NH4OH (Brine)
(Ammoniated Brine) Traces of CO2 causes following reaction:
2NH4OH + CO2 (NH4)2CO3 + H2
The combined product of above two reactions is known as Vat Liquor (Green Liquor).
The absorber has two main portions, the first and the upper
portion is usually called as under-vacuum absorber and also
tower washer (as it is using carbonating tower waste gases in
absorption). In this portion, purified brine is fed from top and
tower waste gases are fed from bottom and absorption takes
place. After exit from undervacuum absorber, the brine is sent
to plate heat exchanger to lower it temperature as absorption is
always favorable at high pressure and low temperature. Then,
the undervacuum absorber brine goes to Absorber Proper
which is the second portion of absorber tower. In this portion,
absorption takes place under vacuum. Here, the first three
compartments named as Abs. Vac. Washer are auto controlled
using showering of brine such that no gases can pass to Abs.
Vac. Engine. After these three compartments, the proper
absorber starts. From very bottom of Absorber Tower, the
ammonia coming from distillers and some makeup ammonia is
fed which by vacuum move upward. The liquor exiting the
proper absorber is called weak liquor and is sent to PHE for
lowering its temperature and then fed it back into Preliminary
Absorber. Preliminary Absorber consists of only two
compartment and the weak liquor is fed and at its exit we get strong liquor which is commonly
called as r
Vat Liquor. The Vat liquor is then sent to Vat liquor
Stock tank. Each absorber has a capacity of almost 350 tons vat liquor per day. And maintenance of
absorber tower mostly takes place after a year during plant partially shut down duration.
The function of mono-carbonating tower is to partially carbonate the ammoniated brine so that
NH3 absorbed in the brine is converted to ammonium
Following reaction takes place in mono-carbonating
2NH4OH + CO2 (NH4)2CO3 + H2O
This is an exothermic reaction and so the heat released,
and temperature of liquor increases from 38 to 50 °C.
The vat liquor from stock is pumped to the top
of mono-carbonating tower. The liquor is evenly
distributed into the tower which contains 2’’ mini
cascade polypropylene rings in random configuration.
The low-pressure blower suck kiln gas, containing 42%
CO2 deliver it into the bottom of the MCT. The unreacted
CO2, some NH3 (flashed off from liquor), and nitrogen leave MCT from top, and are scrubbed
of NH3 in the MCT- washer by purified brine and sent to tower washer
brine main while the other gases vent from top. The liquid product exiting the MCT from bottom is
then go to reammoniator in which part of distiller gases (mainly consisting NH3) are passed from
bottom. This to absorb more NH3 in the liquor exiting MCT so as to increase it NH3 concentration.
Following reaction takes place in reammoniator:
NH3 + H2O NH4(OH)
Here NH3 is absorbed in the liquor and unabsorbed gases leave the Reammoniator from
top and join the rest of the gases going to the absorber. The Liquor leaving the Reammoniator is
called Carbonated Vat Liquor (CVL), which then goes to the carbonating Towers.
There should be minimum or no crystallization due to formation of sodium bicarbonate as
monocarbonating tower washing maintenance is difficult and is usually done annually.
The carbonating tower is a very main unit as here the carbonization of CVL is completed by
further absorption of carbon dioxide to produce sodium bicarbonate, NaHCO3. In the carbonating
tower, the primary source of CO2 is kiln gas 42% CO2, and the secondary source is the strong gas
coming from calciner, 92-96% CO2
In the carbonating tower, CO2 is fed from bottom and middle by blower and the CVL is from
top of tower. The reaction inside the tower is exothermic, due to which, it is important to control the
temperature of liquor properly so that proper sodium bicarbonate crystal growth can occur,
maximum CO2 is absorbed and also ammonia retained in solution. Following reaction occur mainly
(NH4)2CO3 + CO2 + H2O 2NaHCO3 + 2NH4Cl
When the CVL is initially carbonated it has a high affinity for carbon dioxide and so the lower
strength kiln gas is used to control the temperature of liquor. However, as the reaction proceeds,
higher strength gas is required and a mixture of two sources of CO2 is used, making a CO2
concentration of 70%. The final temperature of liquor is important to make sure that the maximum
quantity of sodium bicarbonate has been precipitated. The temperature is maintained using an
intricate water circulation system with the help of shell and tube heat exchanger. The top, middle
and draw temperatures are 60, 40 and 25 °C.
The bottom product of tower is called magma which contain solid sodium bicarbonate in
suspension, which is then sent to the rotary filter. Due to the deposition of sodium bicarbonate,
ammonium carbonate and ammonium bicarbonate on the inside of towers a cleaning routine is
carried out every 72 hrs., during which ammoniated Vat Liquor is circulated in the towers to
dissolve the scale.
The magma from carbonating tower then goes to rotary filter. The purpose of rotary filter
is to separate the crystals of sodium bicarbonate from mother
liquor and also minimize moisture and chloride content in the
cake of sodium bicarbonate. The filter is a horizontally
revolving drum. The surface of drum is porous for suction
and blowing. A trough is fitted at 75% height of drum for
washing cake with wash water to remove the chloride
content as otherwise it will contaminate the final product.
There are press rulers on both sides of filter drum which
press the cake and water is removed from the cake. At the
bottom of drum, vacuum suction is used to suck cake on the
surface of drum leaving the mother liquor and then water is
showered for washing. And after that press rollers squeeze
out the water. After this, a fixed blade is used to scrape off the cake and then this cake is sent to
calciner. And the remaining liquor is then sent toward distillers where it is known as feeder liquor
and ammonia is recovered from it.
Here the calcination of sodium bicarbonate cake is done to turn it into light soda
The main objective of calciner is to make saleable grade soda
ash and to recover maximum strong gas (92-96% CO2). In the
calciner, the decomposition of sodium bicarbonate takes
place, by which CO2 and water is produced. As shown in
2NaHCO3 Na2CO3(s) + CO2(g) + H2O(g)
At calciner, the bicarb cake first goes to a double shaft
mixer with the help of star feeder. In the double shaft mixer, a
quantity of dry is also fed to reduce the mixer motor load and
this ash is called as return ash. After mixing, it goes to a
rotating drum in which it is heated with steam tubes. Here the
solid soda ash moves forward and exit from calciner at almost
200-210 C and its some amount is used as return ash and other
is sent to Asher house for packing or other purposes. The gases leave from the opposite side of ash
exit and move through a gas riser and go into a cyclone where particles of ash and bicarb are settle
down and send back to mixer while the gases are then go to wash box for washing and then used in
the plant as strong gas.
Ammonia Recovery Section
Ammonia recovery section consists of two prelimers, one distiller, a Cassie
Cooler, DBO vessel and a flash vessel. As ammonia is a very hazardous and expensive gas and
that’s why it is desired to recover maximum ammonia from the feeder liquor.
In the ammonia recovery section, feeder liquor coming from rotary filter goes to Cassie
Cooler where it is preheated to almost 75 C and then it goes to distiller heater section where
maximum free ammonia is removes. In this section, gases move upward from bottom and feeder
liquor moves downward in ceramic rings packing. And it is required to maintain a temperature of
almost 101C on gases entrance in heater section. The feeder liquor leaving the distiller heater section
is then fed to prelimers 1 where MOL is introduced and also IP steam is fed from bottom. In
prelimers following reaction takes place:
2NH4Cl + Heat(Steam) + Ca(OH)2 2NH3 + 2H2O + CaCl2
After overflow from prelimers-1 , feeder liquor goes to the prelimers-2 where only IP steam is
introduces from bottom. In prelimers, fixed ammonia is mainly removed and the recovered ammonia
both prelimers is sent to gases compartment. The feeder liquor exiting prelimers-2 is then fed to
distillation section of distiller. In the distillation section LP steam is used for heating purposes. After
distillation section the remaining liquid is sent to Flash vessel where by flashing some of the
ammonia is also recovered and then it is sent to DBO vessel from more ammonia and then it is send
to lime beds. The gases exiting from the top distiller column, are fed to Cassie cooler to cool down
and send to absorber.
Refined Sodium Bicarbonate Plant
Refined sodium bicarbonate, commonly known as baking soda, is widely used in
industry. The Chemical Formula is NaHCO3. Basic ingredients for the manufacture of refined
bicarb are soda ash and carbon dioxide
Na2CO3 + H2O + CO2 → 2NaHCO3
Here, Mother liquor dissolves LSA and is sent to the feed filter where it is filtered using a
sand filter. There are three sand filters and their combined capacity is 400m3/hr. CaCl2 is dosed in
it before sending it to the filter.
The feed is then sent to the saturator where hot kiln gas is pumped from the bottom by FD fan and
converts LSA to Sodium Bicarbonate. The gas is sucked from the top by suction fan.
The magma produced is sent to the magma tank and from there it is pumped to the centrifuge
decanters. The cake and mother liquor are separated using a filter cloth and the mother liquor is sent
back to the dissolver where LSA is dissolved.
The cake is removed by a knife and sent to the drying tube where the rest of the moisture is
removed. After that, it is sent to a cyclone where its impact is lowered, and smooth flow is
maintained. It then moves to the storage silos where RSB is packed in bags. About 250kg of
RSB is produced per hour.
Dense Ash Plant
The purpose of this plant is to convert light ash into dense ash
Na2CO3 + H2O → Na2CO3.H2O + Heat
(Monohydrate sodium carbonate)
Na2CO3.H2O + Heat → Na2CO3 + H2O
The first reaction takes place in Monohydrate mixer. Light ash is brought to the
mixer via conveyor but only 60% of it is fed into the mixer with soda liquor being sprayed on it. The
remainder of the 40% of light ash is fed into monohydrate mixer at a position further down the
mixer. Sodium carbonate Monohydrate crystals come out of the mixer and are fed into return ash
mixer where it is mixed with hot return ash. A vibratory screen then separates the ash and feeds it to
the fluidized bed. Hot air is used to fluidize the crystals and provide heat. The second reaction takes
place here where our desired product is separate out.
• Light Soda Ash.
• Dense Soda Ash.
• Sodium Bicarbonate.