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Chemical formula: Na2CO3O

| |

Structural formula: Na -- O -- C -- O NACommon names: Sodium carbonate, calcined soda, disodiumcarbonate

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Trona: SodiumSesquicarbonate

2 3 3 2

2 3

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Na2CO3-NaHCO3 Solutions

& CO2

Equilibrium established between CO2 in air and

Na2CO3-NaHCO3 liquid

Na2CO3 + CO2 + H20 = 2NaHCO3

Want less than equilibrium value of NaHCO3 in solution

Two phase system with three components: Gibbs

Rule

[Na+]; Temperature

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The discovery of the chemistry of the ammonia-soda process can betraced back to the early 1800s. A few British and French plantsoperated in 1840-1860, but without success.

The ammonia-soda process is usually called the Solvay processbecause in 1865 Ernest Solvay started the first really successful plantat couillet in Belgium. In 1874, the first successful ammonia-soda plantwas erected in England.

The ammonia-soda process isthe dominant technology usedthroughout the world, hence this process is selected forproduction ofsoda ash.

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* SALT

* COAL

* LIME STONE

* WATER

* AMMONIA

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1) MOLECULAR WEIGHT = 106

2) MELTING POINT = 851 C

3) BOILING POINT - DECOMPOSES

4) SOLUBLE IN WATER

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Chemical reactions

(a) CaCO3 + 2NaCl Na2CO3 + CaCl2

(b) CaCO3 CaO + CO2

(c) C + O2 CO2

(d) CaO + H2O Ca(OH)2

(e) NH3 + H2O NH4OH

(f) NaCl + NH4OH NH4Cl + NaHCO3

(g) 2NaHCO3 Na2CO3 + CO2 + H2O

(h) 2NH4Cl + Ca(OH)2 2NH3 + CaCl2 + 2H2O

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1) AMMONIATION

2) CARBONATION

3) FILTERATION

4) CALCINATION

5) AMMONIA RECOVERY

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The SOLVAY process relative to the

production of soda ash could be summarizedby the theoretical global equation involving

the two main components: sodium chloride

and calcium carbonate.

2 NaCl + CaCO3 Na2CO3 + CaCl2

APPLIED PROCESS ANDTECHNIQUES

PROCESS

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In practice this direct way is not possible

and it needs the participation of other

substances and many different process

steps to get the final product: soda ash. First reactions occur in salt solution

(brine). First of all, ammonia is absorbed

and then, the ammoniated brine is reacted

with carbon dioxide to form successiveintermediate compounds: ammonium

carbonate , then ammonium bicarbonate .

Chemical reactions are as follows..

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NaCl + H2O + NH3 NaCl + NH4OH (1)

2 NH4OH + CO2 (NH4)2 CO3 + H2O (2)

(NH4)2CO3 + CO2 + H2O 2 NH4HCO3 (3)

2 NH4HCO3+ 2 NaCl 2 NaHCO3 + 2 NH4Cl (4)

Sodium bicarbonate crystals are separated

from the mother liquor by filtration, then

sodium bicarbonate is decomposed thermally

into sodium carbonate, water and carbon

dioxide .

2 NaHCO3 Na2CO3 + H2O + CO2 (5)

CO2 is recovered in the carbonation step (see

equations 2 and 3 above).

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Mother liquor is treated to recover ammonia.

The ammonium chloride filtrate (4) is reacted

with alkali, generally milk of lime (6), followed

by steam stripping to recover free gaseousammonia:

2 NH4Cl + Ca(OH)2 CaCl2 + 2 NH3 + 2 H2O (6)

NH3 is recycled to the absorption step (see

equation 1 above). Ammonia recovery cycle isshown in Figure 2.

Carbon dioxide and calcium hydroxide originate

from limestone calcination (7) followed by

calcium oxide hydration (8). CaCO3 CaO + CO2 (7)

CaO + H2O Ca(OH)2 (8)

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Brine (NaCl) has to be treated before the input

in the process to remove impurities : calcium

and magnesium. If not removed they would

react with alkali and carbon dioxide to

produce insoluble salts contributing to scale

formation inside equipment. Brine purificationreactions are described in the following

equations:

Ca2 + (CO3)2- CaCO3 (9)

Mg2 + 2 OH- Mg(OH)2 (10)

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Sodium carbonate formed (equation 5) iscalled "light soda ash" because its bulk

density is approximately 0.5 t/m3. A

subsequent operation called densification

enables this value to be doubled by

crystallisation into sodium monohydrate, by

adding water (equation 11) then followed by

drying (equation 12). Final product is "dense

soda".

Na2CO3 + H2O -------- > Na2CO3.H2O (11)

Na2CO3.H2O --------- > Na2CO3 + H2O (12)

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Brine PreparationSodium chloride solutions are occasionally available naturally butare more often obtained by solution mining of salt deposits to give raw, nearsaturated brine containing low concentrations of impurities such asmagnesium and calcium salts.Some brines contain significant quantities of sulfates. Brine purification isrequired to prevent scaling of processing equipment and contamination of theproduct. Brine is usually purified by a lime soda treatment where themagnesium is precipitated with Ca(OH)2 (milk of lime) and the calcium isprecipitated with soda ash. The brine, separated from precipitated impurities,is sent to the ammonia absorbers.

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Ammonia AbsorptionThe strong brine is saturated with ammonia gas in the absorption tower. The

ammonia, recycled from various process steps, contains water vapour and

carbon dioxide. Small amounts of ammonia are added to make up for losses.

During ammoniation, the brine requires cooling (approx 1650 MJ/t or 394

kcal/kg of product soda ash). The absorption operation is generally carried out

at atmospheric pressure. The brine descends through the main part of theabsorber counter current to the

Rising ammoniacal gases. The temperature of inlet brine is about 300C and

that of exit is about 360 to 420C.

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Precipitation of BicarbonateThe ammoniated brine from the absorber coolers is pumped to the top of onecolumn in a block of columns used to precipitate bicarbonate.

This column which has been fouled or partially plugged with sodium bicarbonate

after several days of crystallization is referred to as a cleaning column. Lime

kiln gas, compressed to about 414 kPa (60psi), enters the bottom of the

cleaning column and bubbles up through the solution to absorb most of the

carbon dioxide. The concentrationof carbon dioxide in the liquor is kept below the precipitation concentration.

Relatively little cooling is required. The scale is dissolved off the cooling

surfaces of the cleaning column by the fresh ammoniated brine, assisted by gas

agitation.

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Filtration of Bicarbonate

The slurry, collected from the crystallizing towers, is fed to

continuous vacuum filters or centrifuges which separate the crystals

from the filter liquor. The filter cake is carefully washed with fresh

water to control the residual chloride to meet customer specifications.The dewatering characteristics of the bicarbonate crystals are very

dependent on operating conditions in the crystallizing

columns. Air drawn through the vacuum filter (or the vent gas from the

centrifuge operation) is returned to the absorption section. The filter

cake, often called crude bicarbonate or ammonia soda, liquor and is

made up of sodium bicarbonate and small amounts (5 mol% on a dry

basis) of ammonia primarily in the form of ammonium

bicarbonate. The cake is then conveyed to the calcining operation.

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Recovery of Ammonia

The filter liquor contains unreacted sodium chloride and

substantially all the ammonia with which the brine was originally

saturated, present as fixed and free ammonia. The fixed

ammonia or ammonium chloride corresponds

stoichiometrically to the sodium bicarbonate that had been

precipitated. Free ammonia includes ammonium hydroxide,

bicarbonate, carbamate, and the several possible carbon

compounds of ammonia that decompose at moderate temperatures.

Before preheating, sulfide solution may be added for corrosion

protection. The sulfide is distilled for eventual absorption by the brine

in the absorber. The filter liquor is preheated by indirectcontact with the gases leaving the distiller.

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Lime Preparation

The most suitable limestone, hard and strong with lowconcentrations of impurities, is graded to reasonably uniform coarse size.

Although other fuels may be used, the limestone is usually mixed with about

7% metallurgical grade coke or anthracite and then burned in vertical shaft

kilns. Air is admitted continuously

into the bottom of the kiln an gas is sucked off the top. The fuel burns in a

zone a little below the middle of the kiln, and the stone burns to lime.Carbon dioxide is generated by decomposition of limestone and combustion

of carbon in the fuel. The kiln gases are

diluted with nitrogen from the air used to burn the fuel and usually stone

dust, ash particles, and gaseous impurities. The gas is partially cooled in the

kiln by the upper layers of stone, and further cooled and cleaned before

entering the compressors feedingthe carbonating columns.

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Calcining the Bicarbonate to

Soda AshTo prevent dilution of the decomposition gases, the crude filtered

bicarbonate is continuously calcined by indirect heating. Various

techniques are used to heat the material in which is recycled aftercompression to enrich the makeup kiln-gas feed to the carbonation

operation. The hot soda ash discharged from the calciner is cooled,

screened, and packaged or shipped in bulk. This product, called

light ash because of its low bulk density, is converted to dense ash

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By-ProductsCalcium Chloride

Relatively few synthetic soda ash plants recover calcium chloride,

and most of those that do utilize only a small part of the total

amount available in the distiller waste. To produce calcium

chloride, the distiller waste liquor is settled and thenevaporated in multiple effect evaporators. During concentration

most of the sodium chloride separates. The remaining solution is

further concentrated to the equivalent of CaCl2.2H2O. this solution

is cooled, forming flakes which are dried in a rotary dryer,giving a

product sold as 77-80% calcium chloride. A small amount isprocessed to the anhydrous state.

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Ammonium Chloride

Ammonium chloride is the principal salt present in the mother

liquor from the crude sodium bicarbonate filtration in the ammonia soda

process.

Small amounts have been produced in soda ash plants by carbonation of the

filter liquor, concentration, and crystallization of the ammonium chloride. Most

of the demands in the United States are low tonnage. The end uses areprimarily in dry cells and fluxing

agents.

However, ammonium chloride is also a good fertilizer for important crops in

rainy climates, particularly for rice.

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Carbonate-bicarbonate Ratio

http://www.cas.astate.edu/geochemistry/geochemistry/downloads/alk1.doc

Must have less than Equilibrium value of HCO3- to

absorb CO2

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FLOW SHEET

REPRESENTATION

Co2,n2 to brine

co2H2O

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co2

Purifiedbrine

solution

calciner

water

cooler

,

purifier

A

M

M

O

N

I

A

T

I

O

N

T

O

W

E

R

C

A

R

B

O

N

A

T

I

N

G

T

O

W

E

R

co2

S

O

D

A

A

S

h

F

R

E

E

N

H

3

S

T

I

L

L

C

O

M

B

I

N

E

D

N

H

3

S

T

I

L

l

H2O

H2O

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APPLICATIONS

water treatment;

as an additive in food and drinkseg

baking powder;

for blowing foams such as expanded

polystyrene; in pharmaceutical products as an antacid;

in personal care products such as

toothpaste.

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Advantages of Solvay process Can use low-grade brine

Less electric power

Less corrosion problems

No co-products to dispose of

Does not require ammonia plant investment

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Disadvantages of Solvay process

Higher salt consumption

Higher investment in ammonia recovery unit verses

crystallization units for ammonium chloride

Waste disposal of calcium chloride brine stream

More steam consumption

Higher capacity plant for economic break-even operation

With current fertilizer shortage, all of the ammonium chloride

will be used as a mixed chemical fertilizer ingredient, so co-

product disposal no problem

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PROJECT

DONE

BY

FIRST BATCH

STUDENTS

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