Softening Methods: Lime-Soda

Definition

The lime-soda process is a water softening method in which calculated quantities of lime $\text{Ca(OH)}_2$ and soda ash $\text{Na}_2\text{CO}_3$ are added to hard water to precipitate calcium salts as calcium carbonate and magnesium salts as magnesium hydroxide, thereby reducing hardness.

Main Content

1. Principle of Lime-Soda Softening

• The method works on the principle of chemical precipitation.
• Lime removes bicarbonates of calcium and magnesium and converts magnesium salts into magnesium hydroxide, while soda ash removes calcium salts by converting them into insoluble calcium carbonate.

In hard water, hardness is mainly due to the presence of bicarbonates, chlorides, sulphates, nitrates, and other salts of calcium and magnesium. The lime-soda process does not remove these salts by filtration alone; instead, it chemically transforms them into insoluble compounds. Lime reacts with bicarbonates and magnesium salts, while soda ash reacts with calcium salts that remain after lime treatment.

Examples of important reactions:

• Removal of temporary hardness:
• $\text{Ca(HCO}_3)_2 + \text{Ca(OH)}_2 \rightarrow 2\text{CaCO}_3 \downarrow + 2\text{H}_2\text{O}$

• $\text{Mg(HCO}_3)_2 + 2\text{Ca(OH)}_2 \rightarrow \text{Mg(OH)}_2 \downarrow + 2\text{CaCO}_3 \downarrow + 2\text{H}_2\text{O}$

• Removal of permanent hardness:

• $\text{CaSO}_4 + \text{Na}_2\text{CO}_3 \rightarrow \text{CaCO}_3 \downarrow + \text{Na}_2\text{SO}_4$
• $\text{MgCl}_2 + \text{Ca(OH)}_2 \rightarrow \text{Mg(OH)}_2 \downarrow + \text{CaCl}_2$, followed by
• $\text{CaCl}_2 + \text{Na}_2\text{CO}_3 \rightarrow \text{CaCO}_3 \downarrow + 2\text{NaCl}$

Thus, the basic principle is to convert dissolved hardness-causing ions into insoluble precipitates that can be separated from water.

2. Chemicals Used and Their Functions

• Lime $\text{Ca(OH)}_2$ is used to remove bicarbonates and magnesium salts.
• Soda ash $\text{Na}_2\text{CO}_3$ is used to remove dissolved calcium salts.
• Sometimes additional chemicals like coagulants are added to improve settling and clarification.

The efficiency of the process depends on using the correct quantity of chemicals. If insufficient lime or soda is added, some hardness remains in the treated water. If excess chemicals are added, they may increase alkalinity or leave residual chemicals in the softened water. Therefore, proper dosing is essential.

Functions in detail:

• Lime:
• Removes temporary hardness caused by calcium and magnesium bicarbonates.
• Converts magnesium compounds into insoluble magnesium hydroxide.

• Helps in reducing acidity and carbon dioxide in water.

• Soda ash:

• Removes non-carbonate hardness due to calcium salts such as sulphates, chlorides, and nitrates.
• Complements lime treatment by precipitating calcium ions remaining in water after lime addition.

Sometimes coagulants such as alum, sodium aluminate, or polyelectrolytes are added to help the fine precipitates clump together into larger flocs, making settling and filtration easier.

3. Types of Lime-Soda Process

• The process may be carried out as cold lime-soda process or hot lime-soda process.
• The hot process is generally more efficient than the cold process.
• Both are based on the same chemical principle but differ in temperature and operating conditions.

Cold lime-soda process

In this method, softening is carried out at room temperature. The reactions take place slowly, so the settling of precipitates may take more time. The water is usually treated in large tanks where lime and soda are mixed thoroughly with hard water. The formed precipitates are then removed by sedimentation and filtration.

Characteristics:

• Simple and economical
• Requires more time for reaction and settling
• Produces water with moderate residual hardness

Hot lime-soda process

In this method, water is heated to about 80–100°C before or during treatment. Higher temperature increases reaction rate, reduces solubility of calcium carbonate and magnesium hydroxide, and improves removal of gases like CO₂ and O₂.

Characteristics:

• Faster reactions
• Better softening efficiency
• Produces less residual hardness
• Also helps in removing dissolved gases and some microorganisms

The hot process is preferred for boiler feed water because it gives softer water and reduces corrosion and scale formation more effectively.

Working / Process

1. Calculation and addition of chemicals

The hardness of the water is first estimated by chemical analysis. Based on the type and amount of hardness present, the required quantities of lime and soda ash are calculated. These chemicals are then added to the hard water in proper proportion. If coagulants are needed, they are added at this stage as well.

2. Mixing, reaction, and precipitation

The water is stirred thoroughly so that the chemicals mix uniformly. Lime reacts with bicarbonates, magnesium salts, and dissolved gases, while soda ash reacts with calcium salts. Insoluble precipitates of calcium carbonate and magnesium hydroxide are formed. In the hot process, heating increases the speed and completeness of these reactions.

3. Sedimentation, filtration, and collection

The precipitated solids are allowed to settle in a clarifier or settling tank. The clear supernatant water is then filtered to remove remaining fine particles. The treated soft water is collected for use. In many installations, recarbonation may be used after treatment to remove excess lime and stabilize the water.

A typical sequence of operation is:

• Raw hard water enters the treatment unit.
• Measured lime and soda are added.
• The mixture is rapidly agitated for uniform reaction.
• Flocculated precipitates settle down.
• Water passes through filters to obtain softened water.

The overall process may also include:

• Preheating in the hot process
• Coagulant addition for better sludge formation
• Sludge removal from the bottom of clarifiers
• Final polishing by filtration

Advantages / Applications

• It is an economical method for treating large quantities of hard water.
• It removes both temporary and permanent hardness effectively when properly controlled.
• It reduces boiler scale formation, sludge deposition, fuel wastage, and maintenance problems.
• It is widely used for boiler feed water, industrial process water, and municipal water softening.
• The hot lime-soda process also helps remove dissolved gases such as CO₂ and O₂, lowering corrosion risk.
• It improves the suitability of water for laundry, textile, dyeing, paper, and chemical industries.
• The process can be combined with clarification, making it useful not only for softening but also for partial purification.
• It is a practical choice where water hardness is high and large-scale treatment is required.

Summary

• The lime-soda process softens hard water by chemically precipitating calcium and magnesium ions.
• Lime removes bicarbonates and magnesium compounds, while soda ash removes calcium salts.
• The method may be carried out in cold or hot form, with the hot process being more efficient.
• It is widely used in boiler water treatment because it reduces hardness and prevents scale formation.