Salt, known chemically as sodium chloride (\(\text{NaCl}\)), is an abundant compound that serves as the raw material for producing chlorine gas (\(\text{Cl}_2\)), a highly reactive chemical with extensive industrial uses. This transformation is not a natural occurrence but a forced chemical reaction powered by electricity, a process known as electrolysis. Electrolysis uses a direct electrical current to drive a non-spontaneous chemical reaction, breaking apart a stable compound. The industrial method for converting salt into chlorine is the Chlor-Alkali process, which also produces two other commercially useful co-products. This process requires a carefully controlled environment to manage corrosive materials and harness the electrical energy efficiently.
Preparing the Salt Solution
The first step in extracting chlorine from solid salt is to dissolve it in water to create a highly concentrated solution called brine. Sodium chloride is an ionic compound held together by the strong attraction between positively charged sodium ions (\(\text{Na}^+\)) and negatively charged chloride ions (\(\text{Cl}^-\)). When the salt dissolves, the polar water molecules surround and pull apart the \(\text{Na}^+\) and \(\text{Cl}^-\) ions from the crystal lattice in a process called ionic dissociation. This results in a solution where the sodium and chloride ions are freely moving, which is a prerequisite for the subsequent electrical separation. The dissociation is represented by the equation: \(\text{NaCl}(\text{s}) \rightarrow \text{Na}^+(\text{aq}) + \text{Cl}^-(\text{aq})\).
The Industrial Electrolysis Setup
To perform the separation on an industrial scale, an electrolytic cell is necessary to keep the products physically separated. Modern facilities primarily utilize membrane cells, which consist of two distinct compartments: the anode compartment and the cathode compartment, separated by an ion-exchange membrane. This membrane allows only positive ions, specifically sodium ions (\(\text{Na}^+\)), to pass through while blocking the movement of negatively charged ions like chloride (\(\text{Cl}^-\)) and hydroxide (\(\text{OH}^-\)).
The anode is the positive electrode, submerged in the incoming brine solution. The cathode is the negative electrode, located in the other compartment where water or a weak sodium hydroxide solution is introduced. When a direct current is applied, positively charged sodium ions are attracted to the negative cathode, while negatively charged chloride ions are drawn toward the positive anode. This controlled environment ensures that the chemical reactions can occur at each electrode without the resulting products instantly mixing.
Specific Chemical Reactions
The process involves two distinct electrochemical reactions that occur simultaneously at the separated electrodes, driven by the applied electrical current. At the positive anode, the chloride ions (\(\text{Cl}^-\)) are oxidized, losing an electron to the electrode. This transforms two chloride ions into a molecule of chlorine gas (\(\text{Cl}_2\)), which bubbles out of the solution and is collected. The reaction at the anode is represented as: \(2\text{Cl}^- \rightarrow \text{Cl}_2 + 2\text{e}^-\).
A reduction reaction takes place at the negative cathode. Here, water molecules (\(\text{H}_2\text{O}\)) are reduced by the electrons supplied by the current, splitting the water into hydrogen gas (\(\text{H}_2\)) and hydroxide ions (\(\text{OH}^-\)). The equation for the cathode reaction is \(2\text{H}_2\text{O} + 2\text{e}^- \rightarrow \text{H}_2 + 2\text{OH}^-\). The positively charged sodium ions (\(\text{Na}^+\)) that passed through the selective membrane combine with the newly formed hydroxide ions (\(\text{OH}^-\)) in the cathode compartment. The overall chemical transformation requires the continuous input of electrical energy to proceed.
Resulting Products and Commercial Uses
The Chlor-Alkali process yields three distinct products from the electrolysis of the salt solution: chlorine gas (\(\text{Cl}_2\)), sodium hydroxide (\(\text{NaOH}\)), and hydrogen gas (\(\text{H}_2\)). Chlorine, the primary target product, is a versatile chemical used in the manufacture of polyvinyl chloride (PVC) plastic and is a component in disinfectants and bleaches. It is also employed for cleaning approximately 98% of drinking water in the United States.
Sodium hydroxide, or caustic soda, is a valuable co-product utilized in several industries:
- The paper and pulp industry for processing wood fibers.
- The textile industry for fabric processing.
- The production of aluminum.
- The production of detergents.
The third product, hydrogen gas, is a fuel used in chemical synthesis, such as the production of ammonia, or captured to generate power within the plant. The simultaneous production of these three chemicals means that the industry must find uses for all of them to maintain an efficient operation.