Manufacture 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

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.

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.

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.

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.

(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 CO₂into 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.

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.

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 NH₃ which is recycled to saturation tank. CaCl₂ is a by-product.

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.