Industrial Sodium carbonate (Na₂CO₃) Manufacturing Process, Solvay Process

Sodium carbonate production is a large scale industry in the world due to number of applications. Sodium carbonate (washing soda) is a white crystalline solid at room temperature. It exists as a decahydrate (Na₂CO₃.10H₂O) compound. Sodium carbonate is manufactured by Solvay process in industrial scale and have number of industrial and domestic uses.

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Definition of Washing soda, soda ash, baking soda

There are very similar names to identify some commonly used chemicals and those names can be confused with each other.

• Washing soda (Na₂CO₃)
• Baking soda (NaHCO₃)
• Soda ash (Na₂CO₃)

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Steps of Solvay Process of ammonia-soda process

Solvay process has five main steps on sodium carbonate manufacturing.

1. Ammonia absorber - saturation of brine solution with ammonia
2. Carbonation process in carbonation tower
3. Filtration
4. Calcination of sodium bicarbonate
5. Ammonia recovery tower

The Solvay process is an excellent example for a closed cycle chemical process where the by-products are efficiently reused in the manufacturing process.

Raw materials of sodium carbonate manufacturing

There are four main raw materials for Sodium carbonate manufacturing process and in this section we will see how those raw materials are supplied for this production.

Brine solution (NaCl) to provide Sodium

Brine solution is a concentrated Sodium chloride aqueous solution. Brine solution can be prepared by the sea water. For Sodium carbonate manufacturing, it is necessary to eliminate other cations and anions except Sodium and Chloride to enhance the quality of final product. Brine solution can be produced from dissolving Sodium chloride in water.

Limestone (CaCO₃) for CO₂ generation

Limestone is heated until it decomposes to Calcium oxide (CaO) and Carbon dioxide (CO₂) gas. It released large amount of heat as the result. Carbon dioxide is used to produce Ammonium carbonate which is an intermediate product.

Ammonia gas as a supportive raw material

Though ammonia is not consumed as a reactant at the end of the reactions, it helps to carry out the reaction during the reaction. Ammonia can be produced from the Haber Process in industrial scale. Because Ammonia is not consumed during the reaction, it is completely recycled back to the production process.

Water

Ammonia gas and Carbon dioxide gas are dissolved in an aqueous solution to prepare ammonium carbonate. Therefore, water is an essential raw material to manufacture Sodium carbonate.

Principles of Solvay process of sodium carbonate manufacturing

Following specific principles are used in Sodium carbonate manufacturing process to increase the yield and the reaction rate. In this section, we will look those aspects in detail.

• Common ion effect
• Counter current principle

Common ion effect

The main physicochemical principle is that when a saturated solution of sodium chloride (NaCl) is saturated with ammonia and carbon dioxide, then sodium bicarbonate gets precipitated. Sodium bicarbonate is generally soluble in water, but in a concentrated solution of NaCl, its solubility gets lowered as solubility product explains. Therefore, sodium bicarbonate gets precipitated. This is called the common ion effect. What this state is that the solubility of an electrolyte is lower in a medium where there is an excess of a common ion. In sodium bicarbonate manufacturing process, sodium cation is the common ion.

Counter current principle

It is required to increase the number of contact between reactants to increase the reaction rate. The counter current principle is used in sodium carbonate manufacturing process. Brine is saturated with ammonia gas and slowly passed through the Solvay tower from top to bottom. Carbon dioxide is sent from bottom to top. Inside the tower, there are a number of mushroom shaped perforated plates.

Steps of Sodium carbonate manufacturing process

Below figure shows outline of solvay process.

Ammonia absorber tower

Brine solution (NaCl, 30% solution) is pumped into the ammonia absorber tower. Ammonia and small amount of carbon dioxide from ammonia recycling tower is bubbled through the brine solution. Solution gets saturated with ammonia and form sodium bicarbonate. Calcium and Magnesium ions(in brine solution) are removed as carbonates in the filter.

Carbonation tower

Carbon dioxide is entered from the bottom of the tower and the Brine solution comes down from the bottom.
Both chemicals reacts with each other and form insoluble (due to common ion effect) sodium bicarbonate.

Solution coming from the carbonation tower contains ammonium chloride and small amount of ammonium bicarbonate. They are pumped back to the ammonia recovery tower.

Decomposition of Sodium bicarbonate

Sodium bicarbonate is not stable to heat and when exposed to the heat (higher temperature), Sodium carbonate, carbon dioxide gas and water form.

Ammonia recovering tower

Filtered ammonium chloride ,ammonium bicarbonate are pumped into ammonia recovering tower and mixed with calcium hydroxide. Then, mixture is heated. Ammonia gas, calcium chloride (CaCl₂), and water are given as products. This is an economical advantage because ammonia is recovered and returned back to the process. Ammonia is a relatively expensive material.

Lime kiln

Lime (CaCO₃) is heated to produce Carbon dioxide gas (CO₂) and Calcium oxide (CaO). Produced Calcium oxide is mixed with water to produce Calcium hydroxide (Ca(OH)₂).

HEAT + CaCO₃ → CaO + CO₂

CaO + H₂O → Ca(OH)₂

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By-products of solvay process

Ammonium chloride (NH₄Cl) is formed as a byproduct. It reacts with calcium hydroxide to regenerate ammonia gas for reusing the same procedure. To produce carbon dioxide gas, limestone (CaCO₃) is heated. Calcium oxide (CaO) is formed as a byproduct in this reaction. CaO is slaked with water to produce calcium hydroxide (Ca(OH)₂) which is used to regenerate ammonia from ammonium chloride.

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Environmental pollution due to solvay process

Ammonia is a very toxic gas to humans. If ammonia gas is leaked into water or air from plant, it pollutes the environment and may cause severe injuries ti humans. Therefore, there should be proper health and safety guidelines.

To produce carbon dioxide gas and heat, limestone is heated. This heat increases the temperature of the environment and around the plant. If limestone is taken from coral, it will cause to coastal erosion.

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Uses and applications of Sodium carbonate in the industry and domestic

Sodium carbonate is used in domestic purposes and industrial productions.

Industrial applications of Sodium carbonate

• for softening the hard water
• for washing purposes in the laboratory
• in textile industry
• in petroleum refining industry
• to produce soap, glass, paper, borax, caustic soda, etc.
• A mixture of Na₂CO₃ and K₂CO₃ is used as a fusion mixture.
• Na₂CO₃ is used as a reagent in the laboratory.
• For paint and dye stuffs.
• For metal refining

Use of Sodium carbonate in qualitative analysis in analytical laboratories

Sodium carbonate is used to identify cations in qualitative analysis. It precipitates insoluble carbonates of calcium, barium and strontium and many other metal cations.

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Technical Requirements to construct a solvay process plant

Building an industrial chemical plant should be feasible in both technically and financially. Ans also, that plant should be run for lone time duration to recover the investment. In this section, we will look what are the aspects you need to be think when building an industrial Sodium carbonate manufacturing plant.

Availability of Limestone

Limestone should be found easily in bulk quantities. Limestone is used in two ways in solvay process for heating purpose and obtaining carbon dioxide gas.

Availability of Brine water

If you can pump sea water to your plant, you can produce Brine solution within your factory premises. This Brine solution production will give you more by-products which have a chemical value in the market.

Logistic facilities

A good logistic facilities should be available to transport raw materials to factory and final products to destinations.

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Uses of Sodium bicarbonate and Calcium chloride

Sodium bicarbonate is produced as an intermediate product during the above process and Calcium chloride is generated as a by-product at the end of the production. Both these chemicals have a high demand in the chemical industry and here we will see them briefly.

Uses of sodium bicarbonate

1. to make baking powder. Baking powder contains sodium hydrogen carbonate and an acid like tartaric acid or citric acid.
   Baking powder should be added to cakes. If baking powder is not added, the obtained cake will be comparatively hard and small in size.
2. sodium bicarbonate is used in medicines as an antacid to remove the acidity of stomach.
3. to prepare aerated water(soda water)
4. sodium bicarbonate is used in fire extinguishers. Soda acid fire extinguishers contains a solution of sodium bicarbonate and sulfuric acid. These two substances can be brought in contact by pressing a knob or by inverting the extinguisher. Then carbon dioxide is released and surrounds the combustible substances and cut off the supply of air to put out the air.

Uses of calcium chloride (CaCl₂)

1. To prepare gypsum.
2. As a dehydrator

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Give Properties of sodium bicarbonate

1. Sparingly soluble in water
2. A white crystalline solid
3. When heating(373K), it decomposes into sodium carbonate, carbon dioxide and water.
4. Aqueous solution of sodium bicarbonate is alkaline in nature.

Sodium bicarbonate or baking soda, NaHCO₃

Sodium bicarbonate( baking soda, sodium hydrogen carbonate) is obtained as an intermediate product in solvay process for the manufacturing of sodium carbonate.

Why ammonia is used in solvay process?

The equilibrium reaction of carbon dioxide and water give H⁺ ions. Ammonia reacts with these H⁺ ions. Therefore equilibrium reaction of carbon dioxide and water shifts to the right according to the Le Chatelier principle.

This results more carbon dioxide dissolving in the medium. Therefore more NH₄⁺ ions are generated and it drive the reaction to produce more Na₂CO₃. Finally ammonia is recoverd again by CaCl₂ in the solvay process.

Why cannot manufacture potassium carbonate by solvay process?

Potassium carbonate (K₂CO₃) cannot be manufactured by solvay process because potassium bicarbonate (KHCO₃) fairly soluble in water. In sodium carbonate manufacturing, first sodium bicarbonate is precipitated, then Na₂CO₃ is produced. But due to higher solubility of KHCO₃, K₂CO₃ is unable to manufacture by solvay process.

Solvay process is very economical, why?

Your purpose of operating an industrial plant is earning profits as a business. You have to buy raw materials when you are doing a production. In solvay's process, brine solution (sea water) is used as raw material. For this, plant has to spend very low amount of money. Therefore their production cost is low.

Why do solvay plant situated near water mass?

Water is used as a raw material in two steps in solvay process. In ammonia absorber tower, brine solution (sea water) is fed to tower. Brine solution is a highly concentrated aqueous NaCl solution. Therefore, there is water in this stream.

To reproduce ammonia gas from ammonium chloride, calcium hydroxide is used. In the process calcium oxide is produced in the carbon dioxide generation section. Water is added to calcium oxide to produce calcium hydroxide.

Now, you can see water is a must for a solvay plant and plant should be located near a water source. Usually, these locations are situated close to the sea

How brine solution is taken in solvay process?

Brine solution is taken to the solvay process from sea.

At what temperature, sodium bicarbonate is precipitated in solvay process?

At a temperature about 15⁰C, sodium bicarbonate is precipitated and separated from the solution. Because this precipitation reaction is exothermic, the reaction tower is cooled by spraying water.

substances that are react to re generate ammonia gas in the solvay process and the chemical equation

Ammonium chloride is reacted with calcium hydroxide to regenerate ammonia.

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

Are there any exothermic reactions in solvay process?

Separating and precipitating NaHCO₃ is exothermic reaction. Therefore reaction tower is cooled by spraying water.

Economical features of the solvay process

Solvay process is a very economical because cost for raw materials is low.

Why there is a counter current principle in solvay process?

In counter current principle, reactants are collided with each other very well. Therefore it makes higher reaction rate to produce products.