Brine Purification

Brine Purification is one of the key processes in the chlor-alkali production process. Only when the brine quality meets the required standards can the safe and stable operation of the electrolysis process be ensured.

1. Brine Preparation 
Brine preparation involves dissolving solid salt to produce saturated brine for use in the refining process.
Saturators can be divided into two main categories: above-ground type and underground type. In the early stages, above-ground saturators were commonly used. After 2010, underground saturators have become the preferred choice in most cases, except under special geological conditions. Through extensive long-term practical experience accumulated by chlor-alkali enterprises, the underground type has proven to be better suited to production, maintenance, and repair conditions.

2. Primary Brine Purification
2.1 Hazards of Impurities in Crude Brine
(1) Effects of Ca²⁺ and Mg²⁺
If the contents of Ca²⁺ and Mg²⁺ in the brine exceed the specified limits, the following three adverse conditions may occur during the electrolysis process:
First,Ca²⁺ and Mg²⁺ enter the cathode compartment and react with OH^- to form calcium hydroxide and magnesium hydroxide precipitates, which reduce the electrolyte concentration and decrease the current efficiency.
Second, Ca²⁺ and Mg²⁺ react with OH^- within the channels of the diaphragm/ion-exchange membrane in the electrolytic cell, forming precipitates that block the channels, resulting in increased cell voltage and reduced current efficiency.
Third, Ca²⁺ and Mg²⁺ react with OH^- that has migrated back into the anode compartment, forming precipitates that increase alkali loss and decrease current efficiency.
Therefore, it is essential that the contents of Ca²⁺ and Mg²⁺ in the brine fed into the electrolytic cell meet the required standards.
(2) Effects of SO₄^2-
SO₄^2- is the primary anion introduced into the brine through raw salt. Typically, a small amount of SO₄^2- in the brine does not affect electrolyzer performance. However, SO₄^2- tends to accumulate over time due to the recycling of depleted brine. When the SO₄^2- content becomes excessively high, the following issues may arise:
It promotes the discharge of OH⁻ at the anode to generate O₂, which increases the oxygen content in the chlorine gas and reduces the anodic current efficiency. It adversely affects the quality of the NaOH product.
Therefore, the SO₄^2- content in the brine should generally be maintained at no more than 5 g/L.
(3) Effects of NH₄⁺
When the pH on the anode side of the electrolytic cell is between 2 and 4, NCl₃ (nitrogen trichloride) will be generated. The reaction is as follows:

NH₃+3HOCI_-^pH<4.5_→NCI₃+3H₂O

NCl₃ has a significant risk of explosion and may explode in any area where it accumulates.
To mitigate the risks associated with NCl₃, the total ammonium content in the brine should be kept below 1 mg/L.
(4) Effects of other metal Ions
Other metal ions in the brine may either affect the performance of the diaphragm/ion-exchange membrane or impair the electrode performance, thereby impacting the cell voltage and current efficiency of the electrolyzer. These effects should also be given due attention.
It is required that the contents of Fe and Al in the brine be kept below 0.5×10^-6 (mass fraction), and the contents of Ba and Mn below 0.01×10^-6 (mass fraction).
2.2 Principles of Brine Purification
(1) Removal of Ca²⁺
Na₂CO₃ is added to the brine to react with Ca²⁺, forming CaCO₃ precipitate. To ensure sufficient removal of Ca²⁺, the amount of Na₂CO₃ added must exceed the theoretical stoichiometric requirement. The reaction is as follows:

Ca²⁺+CO₃^2-→CaCO₃↓

(2) Removal of Mg²⁺

NaOH is added to the brine to react with Mg²⁺, forming Mg(OH)₂ precipitate. To ensure sufficient removal of Mg²⁺, the amount of NaOH added must exceed the theoretical stoichiometric requirement. The reaction is as follows:

Mg²⁺+2OH⁻→Mg(OH)₂↓

(3) Removal of SO₄^2-
There are two types of process schemes for removing SO₄^2- from brine.
The first is the chemical refining agent method, such as adding BaCl₂ to the brine to react with SO₄^2- and form sparingly soluble BaSO₄ precipitate. This method offers high removal efficiency, but it introduces heavy metal barium, so the residual amount must be strictly controlled. In addition, barium chloride is a highly toxic chemical and requires careful handling. For this reason, our company has proposed a safer alternative solution.
The second approach employs physical separation technologies such as membrane filtration and freeze crystallization, which avoid the addition of chemical agents and reduce secondary pollution.
The selection of the appropriate process scheme should be based on the specific composition of the brine and the actual operating conditions.
(4) Removal of Ammonium
Sodium hypochlorite solution is added into the brine to generate NH₂Cl or NHCl₂. The reactions are as follows:

NH₃+Cl^_一pH>9_→NH₂Cl+HCl
NH₃+2Cl₂^_一pH>9_→NHCl₂+2HCl

NH₂Cl and NHCl₂ in the brine are removed by blowing with compressed air.
2.3 Brine Purification Process
(1) Traditional Process
Dorr clarifier + carbon sintered tube filter with α-cellulose.

(2) Membrane-Based Brine Treatment
Membrane separation is carried out at ambient temperature, with no phase change, no chemical reaction, good selectivity, and strong adaptability.
Early Membrane Process Flow

Current Membrane Process Flow

3. Secondary Brine purification
Ion exchange resin method is typically employed, utilizing chelating cation exchange resins to remove divalent metal ions such as calcium and magnesium.
This process is based on the selective adsorption of ions by chelating resins, ensuring extremely high purity of the brine entering the electrolytic cells. This protects the ion-exchange membranes from contamination and damage, extends their service life, and improves current efficiency. The impurities in the refined brine, particularly the total calcium and magnesium content, must be strictly controlled to below 20 ppb, to meet the stringent requirements of the modern chlor-alkali industry for high-quality brine and to ensure safe and stable production.
There are many types of chelating resins, with various series and grades available from different countries. Examples include:

3.1 Process Principles
3.1.1 Adsorption
Chelating resin is a type of ion exchange resin capable of adsorbing metal ions from solution to form chelate compounds, also known as inner complexes. These are formed by a central ion and a multidentate ligand, resulting in a complex with a ring structure, as shown in the figure below. Here, M represents the central ion (metal ion), and N and O are atoms that can donate electron pairs. The central ion forms two coordination bonds with them, creating a stable ring structure under suitable conditions.

3.1.2 Desorption
The formation and dissociation of complexes are two opposing yet interdependent processes. On the one hand, the central ion combines with the complexing agent through coordination bonds to form a complex, exhibiting a certain chemical attraction; on the other hand, due to the internal contradictory motion within the complex, some of them dissociate, exhibiting a certain chemical repulsion. Under certain external conditions (such as pH, temperature, and concentration), a relatively balanced state is achieved. Changing these conditions disrupts the equilibrium. The regeneration of chelating resins is based on this principle.
As shown in the following formula:

Due to the increase in acidity, the above equilibrium shifts to the right. 
In actual production, a 5% HCl solution is added to "wash" the resin. When the pH reaches 1, nearly all of the previously complexed metal ions are "eluted." At this point, the resin is in the so-called "H-form."
3.1.3 Regeneration
A 5% NaOH solution is added to the "H-form" resin from which the metal ions have been eluted, bringing the pH to 14. Under these conditions, the concentration of H⁺ in the solution is greatly reduced, causing the following equilibrium to shift to the right, and the resin returns to its state prior to adsorption.

3.2 Process Schemes
(1) Two-Column Scheme
This scheme employs two adsorption towers operating in series to ensure that the quality of the secondary brine meets the requirements of the electrolytic cells.
Advantages: Automatic operation, low investment;
Disadvantage: Operation stops during regeneration.
(2) Three Tower Scheme
This scheme employs three adsorption towers, with two towers operating in series while one tower is offline for regeneration. Through periodic switching of the three chelating resin towers, qualified refined brine can be produced continuously.
Advantages: Automatic operation, automatic regeneration, continuous operation;
Disadvantage: Higher investment in process equipment and control instrumentation compared to the two-column scheme.
It is generally recommended to adopt the second scheme.