Production of Chlorine and Sodium Hydroxide – Chlor-Alkali Electrolysis

Overview of the Chlor-Alkali Process

The chlor-alkali process is the central industrial method for producing three important basic chemicals from brine (usually sodium chloride solution):

• Chlorine gas: $\mathrm{Cl_2}$
• Sodium hydroxide solution: $\mathrm{NaOH}$ (caustic soda)
• Hydrogen gas: $\mathrm{H_2}$

The overall reaction using aqueous sodium chloride (brine) is:

$$
2\,\mathrm{NaCl(aq)} + 2\,\mathrm{H_2O(l)} \rightarrow 2\,\mathrm{NaOH(aq)} + \mathrm{Cl_2(g)} + \mathrm{H_2(g)}
$$

Electrolysis splits brine into these products by driving non-spontaneous redox reactions with electrical energy. Industrially, this is done in specialized electrolysis cells, optimized to keep products separate and to achieve high current efficiency and low energy consumption.

Chlor-alkali electrolysis is a key example of a large-scale electrochemical process and is one of the biggest single consumers of electrical energy in the chemical industry.

Electrochemical Basis

In chlor-alkali cells, brine acts as the electrolyte. The essential half-reactions (for sodium chloride in water) are:

Anode (oxidation, chlorine formation):
$$
2\,\mathrm{Cl^-} \rightarrow \mathrm{Cl_2} + 2\,\mathrm{e^-}
$$

Cathode (reduction, hydrogen and hydroxide formation):
$$
2\,\mathrm{H_2O} + 2\,\mathrm{e^-} \rightarrow \mathrm{H_2} + 2\,\mathrm{OH^-}
$$

The sodium ions $\mathrm{Na^+}$ do not get reduced to metal under these conditions; instead, they remain in solution and pair with $\mathrm{OH^-}$ to form sodium hydroxide solution:

$$
\mathrm{Na^+} + \mathrm{OH^-} \rightarrow \mathrm{NaOH(aq)}
$$

The central engineering challenge is to:

• Promote $\mathrm{Cl_2}$ formation at the anode rather than oxygen evolution.
• Produce $\mathrm{OH^-}$ and $\mathrm{H_2}$ at the cathode.
• Prevent chlorine from reacting back with sodium hydroxide or hydrogen.
• Minimize energy losses (overpotentials, ohmic resistance).

Different cell types solve these challenges in different ways.

Types of Chlor-Alkali Electrolysis Cells

Industrial chlor-alkali technology mainly uses three cell concepts:

• Mercury cells (historical, largely phased out)
• Diaphragm cells
• Membrane cells

All use brine as the starting material but differ in construction, environmental impact, and product purity.

Mercury Cell Process (Historical)

Principle

In the mercury cell (Castner–Kellner cell), the cathode is a flowing layer of liquid mercury. A concentrated sodium amalgam (a solution of sodium metal in mercury) forms at the cathode and is then reacted with water in a separate reactor (the decomposer) to produce sodium hydroxide and hydrogen.

Anode (in the cell):
$$
2\,\mathrm{Cl^-} \rightarrow \mathrm{Cl_2} + 2\,\mathrm{e^-}
$$

Cathode (in the cell):
$$
\mathrm{Na^+} + \mathrm{e^-} + \text{Hg} \rightarrow \text{Na(Hg)} \quad\text{(amalgam formation)}
$$

Decomposer (separate reactor with water):
$$
2\,\text{Na(Hg)} + 2\,\mathrm{H_2O} \rightarrow 2\,\mathrm{NaOH(aq)} + \mathrm{H_2} + 2\,\text{Hg}
$$

Chlorine is taken from the anode compartment, and sodium hydroxide solution is obtained with high purity and high concentration.

Advantages and Disadvantages

Advantages (technical):

• High purity NaOH (often ~50% by mass directly).
• Very low chloride content in the sodium hydroxide.
• Chlorine and hydrogen are well separated.

Disadvantages:

• Use of mercury, a highly toxic heavy metal.
• Risk of mercury emissions to air and water.
• High electrical energy consumption.

Due to environmental concerns and international agreements, mercury cells are being shut down worldwide and replaced by diaphragm or membrane cells.

Diaphragm Cell Process

Construction and Working Principle

Diaphragm cells use:

• An anode compartment (anolyte): where chlorine is formed.
• A cathode compartment (catholyte): where hydrogen and hydroxide ions form.
• A porous diaphragm (historically asbestos-based, now often polymer-based) between anode and cathode that:
• Allows brine and ions to pass.
• Reduces mixing of anolyte and catholyte.

The brine flows from the anode side through the diaphragm to the cathode side.

Typical half-reactions:

Anode:
$$
2\,\mathrm{Cl^-} \rightarrow \mathrm{Cl_2} + 2\,\mathrm{e^-}
$$

Cathode:
$$
2\,\mathrm{H_2O} + 2\,\mathrm{e^-} \rightarrow \mathrm{H_2} + 2\,\mathrm{OH^-}
$$

The flowing brine carries $\mathrm{Na^+}$ into the catholyte, where it joins $\mathrm{OH^-}$ to form NaOH.

Product Characteristics

• The catholyte leaving the cell contains:
• Sodium hydroxide at relatively low concentration (e.g. 10–12%).
• Significant amounts of unreacted NaCl.
• The NaOH solution typically must be:
• Separated from NaCl (e.g. evaporation and crystallization).
• Concentrated by further evaporation.

Chlorine leaves the anode zone as a gas mixed with water vapor and minor oxygen.

Advantages and Disadvantages

Advantages:

• No mercury.
• Established, robust technology.

Disadvantages:

• Historical use of asbestos diaphragms (health hazard); new materials mitigate this.
• Lower NaOH concentration; extra energy for concentration.
• Higher chloride contamination in NaOH compared with other processes.
• Lower current efficiency due to some mixing of anolyte and catholyte.

Membrane Cell Process

Construction and Working Principle

Membrane cells use an ion-selective membrane (typically a cation-exchange membrane) between anode and cathode. This membrane:

• Allows $\mathrm{Na^+}$ ions to pass from anolyte to catholyte.
• Blocks $\mathrm{Cl^-}$ and OH⁻ ions.
• Minimizes mixing of chlorine with sodium hydroxide.

The anode is in contact with purified brine; the cathode is in contact with deionized water or dilute NaOH.

Half-reactions:

Anode:
$$
2\,\mathrm{Cl^-} \rightarrow \mathrm{Cl_2} + 2\,\mathrm{e^-}
$$

Cathode:
$$
2\,\mathrm{H_2O} + 2\,\mathrm{e^-} \rightarrow \mathrm{H_2} + 2\,\mathrm{OH^-}
$$

Ion transport through the membrane:
$$
\mathrm{Na^+} \text{(anolyte)} \rightarrow \mathrm{Na^+} \text{(catholyte)}
$$

In the catholyte, $\mathrm{Na^+}$ and $\mathrm{OH^-}$ form relatively pure NaOH solution.

Process Features and Requirements

• The brine must be carefully purified before entering the anolyte (removal of $\mathrm{Ca^{2+}}$, $\mathrm{Mg^{2+}}$, heavy metals) to avoid fouling the membrane.
• The NaOH concentration typically reaches about 30–35% in the cell; further concentration may still be necessary.
• Cell voltage is lower than in mercury cells, so energy consumption is reduced.

Advantages and Disadvantages

Advantages:

• No mercury, no asbestos.
• High purity NaOH with low chloride content.
• Lower energy consumption per ton of product compared with mercury and often diaphragm cells.
• Good separation of chlorine from NaOH and hydrogen.

Disadvantages:

• Demands highly purified brine (additional treatment steps).
• Membrane materials are relatively expensive and have finite lifetimes.

Because of environmental and energy advantages, membrane cells are the current standard for new chlor-alkali plants and replacements.

Brine Preparation and Purification

For all chlor-alkali technologies, the quality of the brine feed is critical.

Brine Source and Dissolution

• Industrial brine commonly comes from:
• Rock salt dissolved in water.
• Natural brine deposits.
• Sea water (less common for direct chlor-alkali use because of impurities and low NaCl concentration).
• The NaCl concentration is adjusted to optimize conductivity and process efficiency, often around saturation conditions at the operating temperature.

Purification Steps

To protect electrodes, diaphragms, and especially membranes, impurities such as $\mathrm{Ca^{2+}}$, $\mathrm{Mg^{2+}}$, iron, and other heavy metals must be removed. Typical steps include:

• Precipitation of calcium and magnesium as carbonates or hydroxides (e.g. by adding $\mathrm{Na_2CO_3}$ and $\mathrm{NaOH}$).
• Filtration or sedimentation to remove solids.
• Polishing steps (ion exchange, clarification) for membrane-grade brine.

In diaphragm and membrane systems, brine concentration, pH, and flow are carefully controlled to maintain cell performance and product quality.

Handling and Utilization of Products

Chlorine ($\mathrm{Cl_2}$)

Chlorine leaves the anode compartment as a gas, usually saturated with water vapor and containing traces of oxygen and hydrogen. It is typically:

• Cooled and dried.
• Compressed or liquefied for storage and transport.
• Fed directly into downstream processes.

Major uses include:

• Production of organic chlorinated compounds (e.g. PVC monomers).
• Manufacture of inorganic chemicals (e.g. hydrochloric acid, chlorates).
• Water disinfection (usually after conversion to suitable forms).

Because chlorine is toxic and corrosive, its containment and safe handling are major design aspects in chlor-alkali plants.

Sodium Hydroxide ($\mathrm{NaOH}$)

NaOH is usually obtained as an aqueous solution and then:

• Concentrated by evaporation to commercial strengths (e.g. 30–50% by mass).
• Sometimes solidified as flakes, pellets, or prills.

Main applications include:

• Paper and pulp production.
• Soap and detergent manufacturing.
• pH adjustment and neutralization.
• Many synthesis and processing steps in the chemical industry.

NaOH from diaphragm cells requires additional purification and concentration compared with membrane and mercury cells.

Hydrogen ($\mathrm{H_2}$)

Hydrogen is a valuable by-product:

• Collected at the cathode side.
• Dried and compressed.

Uses include:

• Fuel for process heating or power generation (on-site use).
• Feedstock for ammonia synthesis and other hydrogenation reactions.
• Potential use in fuel cells or as an energy carrier.

Using the hydrogen efficiently improves the overall energy and economic balance of chlor-alkali plants.

Energy and Efficiency Considerations

The chlor-alkali process is highly energy-intensive because it drives non-spontaneous reactions:

• Electrical energy is converted into chemical energy stored in the products.
• The cell voltage must overcome:
• Thermodynamic decomposition potential.
• Overpotentials at electrodes.
• Ohmic losses (electrolyte, diaphragm/membrane, contacts).

Key performance metrics include:

• Current efficiency (faradaic efficiency):
• Fraction of total charge that actually produces the desired products.
• Losses can occur due to side reactions (e.g. oxygen evolution, chlorine reduction).
• Energy consumption:
• Typically expressed as kWh per ton of $\mathrm{Cl_2}$ (or NaOH).
• Lower in membrane cells than in mercury cells.

Engineering measures to increase efficiency:

• Optimized electrode materials and surface structures (reduce overpotentials).
• Short inter-electrode distance (reduce ohmic drop).
• Optimized brine concentration, temperature, and flow.
• Effective separation and cooling systems.

Because of the large global production scale, small improvements in energy efficiency can lead to substantial economic and environmental benefits.

Environmental and Safety Aspects

Chlor-alkali electrolysis is closely regulated due to:

Mercury and Asbestos Phasing-Out

• Mercury cell plants pose serious risks of mercury emission to air, water, and products.
• Many countries have mandated the shutdown or conversion of such plants.
• Traditional asbestos diaphragms pose health hazards; modern diaphragm technologies use alternative fibers or polymers.

Chlorine and Hydrogen Safety

• Chlorine is toxic, corrosive, and reacts with many substances.
• Hydrogen is flammable and forms explosive mixtures with air and chlorine.
• Plants must prevent:
• Mixing of chlorine with hydrogen and air.
• Leaks and accidental releases (containment, scrubbers, monitoring).

Safety measures include:

• Gas separation and monitoring.
• Vent gas treatment systems.
• Process control and emergency procedures.

Energy and Emissions

The environmental footprint of chlor-alkali production is strongly influenced by:

• The electrical energy source (fossil vs. renewable).
• Overall energy consumption per ton of product.

Transitioning to membrane cells and integrating renewable electricity can significantly reduce the process’s contribution to greenhouse gas emissions.

Industrial and Economic Significance

The chlor-alkali industry is foundational for many other chemical sectors:

• Chlorine and caustic soda are basic building blocks for polymers, pharmaceuticals, solvents, paper, and detergents.
• Production is often directly integrated with downstream plants (e.g. PVC, epichlorohydrin) to reduce chlorine transport and improve safety.
• The location of chlor-alkali plants is often chosen based on:
• Access to inexpensive electricity.
• Availability of salt resources.
• Proximity to major consumers of chlorine and NaOH.

Modern chlor-alkali electrolysis thus illustrates how electrochemical principles, process engineering, material science (membranes, electrode coatings), and environmental requirements are combined in large-scale industrial chemistry.