Manufacturing of sodium carbonate by solvay process

Sodium carbonate (Na₂CO₃) is found naturally or is manufactured from natural salt i.e., sodium chloride (common salt). It has many uses but, one of the major notable applications is in making of glass. Soda ash is a key chemical for producing soap, paper making, baking soda production, and bleaching fabrics and paper.

Origin of soda ash term

The name "soda ash" is based on the principal historical method of obtaining alkali, which was by using water to extract it from ashes. The word "soda" (from the Middle Latin) originally referred to certain plants that grow in salt marshes; it was discovered that the ashes of these plants yielded the useful alkali "soda ash."

Why sodium carbonate called as washing soda?

CO₃^2- from dissolved Na₂CO₃ can precipitate Mg²⁺ and Ca²⁺ ions from hard water as the insoluble carbonates, preventing them from forming a precipitate with soap resulting in scum. For this reason, sodium carbonate is also known as washing soda.

Soda ash is used to produce the NaHSO₃ necessary for the sulfite method of separating lignin from cellulose.

Sodium carbonate removes grease from wool and neutralizes acidic solutions.

Na₂CO₃ is used to remove SO_2(g) from flue gases in power stations.

Na₂CO₃ was produced by two processes

• Leblanc process
• Ernest Solvay process

The Solvay Process (also known as the ammonia-soda process), developed in 1861, is the world's major industrial process for the production of sodium carbonate (NaCO₃), or soda ash.

Solvay process history

In 1861, after realizing the polluting impacts of the Leblanc Process, Belgian industrial chemist Ernest Solvay rediscovered and perfected Augustin Fresnel's reaction. This process recovered the ammonia in the reaction for re-use, therefore making it less detrimental to the environment.

In 1874, other companies had bought the right to use the Solvay process in their own plants. Now the sodium carbonate production market became a booming industry with several plants opening worldwide.

In 1890's, Solvay-based process plants produced the majority of the world's soda ash.

The materials needed in the Solvay process are nearly all readily available and inexpensive:

Solvay process feedstocks

Salt Brine – Salt brine as one of the feedstock provides salt and water and it can be easily sourced from both inland and the ocean.

Limestone

Ammonia

Manufacturing steps in solvay process

The following steps are involved in solvay process and in this process some amount of ammonia recycled back as feedstock to start new round of solvay process.

• Brine purification
• Ammoniation of brine
• Reactions in Solvay Tower
• Reactions in klin / Separation of solid sodium hydrocarbonate
• Formation of sodium carbonate
• Ammonia Recovery

Brine purification

Brine or brine solution is a solution of salt and water. It is mainly used as a preservative for vegetables, fish, fruit, and meat through a process known as brining. The high salt content in Brine prevents the growth of bacteria and thus helps to preserve the food for a long time without creating any difference in taste. Brine is widely used all over the world for various food preparations and is best suited for delicacies made from meat and fish.

Brine is both naturally found and artificially produced as well. Brine has many commercial applications. Brine is be prepared by dissolving rock salt post hydraulic fracturing of a well. The process of creating Brine is more than simple mixing. It is done by an electronically controlled and monitored process. Huge quantities of Brine are extracted by installing a self-contained hydraulic system from heavy salt concentrations areas.

Applications of brine

• Brine is an extremely useful substance used in domestic chores as well as commercial applications.
• Brine is used for the production of salts. The brine is generally evaporated and then processed to get the common salt or table salt (Nacl).
• Brine is widely used for food preservation.The brine can be mixed with a wide variety of herbs and spices for flavor.
• Brine is often used for marinating meat. It makes the flesh juicier, and tender. Apart for that, Brine kills the microorganisms that harbor in meat. The process of marinating the meat with brine, prior to cooking reduces the cooking time.
• Brine solution is an excellent prewetting agent and commercially used to treat the roads. The solution is non-corrosive, biodegradable, and environmentally friendly.
• Brine is used to transfer heat from one place to another place and is largely used for refrigeration.
• Brine has a lower freezing point than water and can be cooled to below zero Celsius. Hence it is effectively used as coolant other than plain water. Brine freezes at -21ºC (-6ºF). It is used for cooling steels and other metals.
• Brine is often used for Pickling.
• Brine is an important source of chlorides, sulphates of magnesium and potassium apart from natural salt. These salts are extracted through electrolysis.
• Brine solution cures Psoriasis, Osteoporosis, Arthritis, Gout and Herpes sores.
• Brine steam inhalation cures asthma, bronchitis, acute and chronic sinus and ear infections.
• Brushing teeth with concentrated Brine solution helps to protect tooth enamel.
• Brine poultice is used to sterilize open wounds.
• Brine baths improves circulatory system and control high temperature during viral fever.
• Brine acts an excellent detoxification solution and improves the metabolism. It maintains the body’s pH Factor and eliminates heavy metals.

The solvay process will commences with brine purification which involves

Brine solution is concentrated by evaporation to at least 30 per cent

Calcium, magnesium and iron are collected as precipitants in this reaction

Ca²⁺_(aq) + CO₃^2-_(aq) → CaCO_3(s)

Mg²⁺_(aq) + 2OH^-_(aq) → Mg(OH)_2(s)

Fe³⁺_(aq) + 3OH^-_(aq) → Fe(OH)_3(s)

Following the precipitation reaction brine solution is then filtered and passed through an ammonia tower to dissolve ammonia.

Through this process energy is released because of its exothermic nature, thereby, the ammonia tower will get cooled by this energy.

Ammoniation of brine

In the solvay process second step is the ammoniation of brine solution. Ammonia gas is absorbed in concentrated brine to give a solution containing both sodium chloride and ammonia.

Reactions in klin / Separation of solid sodium hydrocarbonate

Lime Kiln

A lime kiln is used to produce quicklime through the calcination of limestone. This reaction takes place at 900°C.

Types of kilns

• Shaft kilns
• Counter current shaft kilns
• Regenerative kilns
• Annular kilns
• Rotary kilns

Kilns are fed with a limestone/coke mixture (13:1 by mass).  The coke burns in a counter-current of pre-heated air

C_(s) + O_2(g) → CO_2(g)

The heat of combustion raises the temperature of the kiln and the limestone decomposes

CaCO_3(s) ↔ CaO_(s) + CO_2(g)

The gas, containing approximately 40 per cent carbon dioxide, is freed of lime dust and sent to the carbonating (Solvay) towers.  The residue, calcium oxide, is used in ammonia recovery.

Reactions in solvay tower

The Solvay Tower is tall and contains a set of mushroom-shaped baffles to slow down and break up the liquid flow so that the carbon dioxide can be efficiently absorbed by the solution.  Carbon dioxide, on dissolving, reacts with the dissolved ammonia to form ammonium hydrogen carbonate

NH_3(aq) +  H₂O_(l) + CO_2(g) → NaHCO_3(s)

The solution now contains ions Na⁺_(aq), Cl^-_(aq), NH4⁺_(aq) and HCO^3-_(aq).  Of the four substances which could be formed by different combinations of these ions, sodium hydrogencarbonate (NaHCO₃) is the least soluble. It precipitates as a solid in the lower part of the tower, which is cooled.

NaCl_(aq) + NH_3(aq) +H₂O_(I) +CO_2(g) → NaHCO_3(s) + NaCl _(aq)

Formation of sodium carbonate

Suspended sodium hydrogen carbonate is removed from the carbonating tower and heated at 300oC to produce sodium carbonate:

2NaHCO_3(s) → Na₂CO_3(s) + H₂O _(g) + CO_{2 (g)}

Ammonia Recovery

CaO is formed as a by-product of the thermal decomposition of limestone in the lime kiln.

This CaO enters a lime slaker to react with water to form calcium hydroxide:

CaO_(s) + H₂O _(l) → Ca(OH)_2(aq)

The calcium hydroxide produced here is reacted with the ammonium chloride separated out of the carbonating tower by filtration:

Ca (OH)_2(aq) + 2NH₄Cl_(aq) → CaCl_2(aq) + 2H₂O_(l) + 2NH_3(g)