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The Chloralkali Industry

The chloralkali industry

The chlorine-alkali (chloralkali) industry is an important part of the chemical industry, which produces chlorine and sodium hydroxide through the electrolysis of the raw material brine. Brine is a saturated solution of sodium chloride (\(\text{NaCl}\)) that is obtained from natural salt deposits.

Definition: Brine

A saturated aqueous solution of sodium chloride.

Fact:

Remember that electrolytic cells are used to transform reactants into products by turning electric current into chemical potential energy.

The products of the chloralkali industry have a number of important uses:

Chlorine is used:

  • to purify water

  • as a disinfectant

  • in the production of:

    • hypochlorous acid (used to kill bacteria in drinking water)

    • paper, food

    • antiseptics, insecticides, medicines, textiles, laboratory chemicals

    • paints, petroleum products, solvents, plastics (such as polyvinyl chloride)

Sodium hydroxide (also known as ‘caustic soda’) is used to:

  • make soap and other cleaning agents

  • purify bauxite (the ore of aluminium)

  • make paper

  • make rayon (artificial silk)

One of the problems of producing chlorine and sodium hydroxide is that when they are produced together the chlorine combines with the sodium hydroxide to form chlorate (\(\text{ClO}^{-}\)) and chloride (\(\text{Cl}^{-}\)) ions. This leads to the production of sodium chlorate, \(\text{NaClO}\), a component of household bleach.

To overcome this problem the chlorine and sodium hydroxide must be separated from each other so that they don’t react. There are three industrial processes that have been designed to overcome this problem. All three methods involve electrolytic cells.

  1. The Mercury Cell

    In the mercury-cell (see figure below):

    • The anode is a carbon electrode suspended from the top of a chamber.

    • The cathode is liquid mercury that flows along the floor of this chamber.

    • The electrolyte is brine (\(\text{NaCl}\) solution) that is passed through the chamber.

    • When an electric current is applied to the circuit, chloride ions in the electrolyte are \(\color{blue}{\text{oxidised at the anode}}\) to form chlorine gas.

      \(\color{blue}{\text{2Cl}^{-}\text{(aq)} \to \text{Cl}_{2}\text{(g) + 2e}^{-}}\)

    • At the same time sodium ions are \(\color{red}{\text{reduced at the anode}}\) to solid sodium. The solid sodium dissolves in the mercury making a sodium/mercury amalgam.

      \(\color{red}{\text{Na}^{+}\text{(aq) + Hg(l) + e}^{-} \to \text{Na(Hg)}}\)

    • The amalgam is poured into a separate vessel, where it decomposes into sodium and mercury.

    • The sodium reacts with water in the vessel and produces sodium hydroxide and hydrogen gas, while the mercury returns to the electrolytic cell to be used again.

      \(\color{red}{\text{2Na(Hg) + 2H}_{2}\text{O(l)} \to \text{2NaOH(aq) + H}_{2}\text{(g) + Hg(l)}}\)

    The Chloralkali Industry

    The mercury cell.

    Fact:

    In a mercury cell the sodium dissolves in the liquid mercury to form a liquid amalgam of the two metals. This separates the \(\text{Cl}^{-}\) and \(\text{Na}^{+}\) ions. 

    This method only produces a fraction of the chlorine and sodium hydroxide that is used by industry as it has certain disadvantages:

    • mercury is expensive and toxic

    • some mercury always escapes with the brine that has been used

    • mercury reacts with the brine to form mercury(II) chloride

    • the mercury cell requires a lot of electricity

    • although the chlorine gas produced is very pure, mercury has to be removed from the sodium hydroxide and hydrogen gas mixture.

    In the past the effluent was released into lakes and rivers, causing mercury to accumulate in fish and other animals feeding on the fish. Today, the brine is treated before it is discharged so that the environmental impact is lower.

  2. The Diaphragm Cell

    In the diaphragm-cell (see figure below):

    • a porous diaphragm divides the electrolytic cell into an anode compartment and a cathode compartment

    • brine is introduced into the anode compartment and flows through the diaphragm into the cathode compartment

    • an electric current is passed through the brine causing the salt’s chlorine ions and sodium ions to move to the electrodes

    • Chlorine gas is produced at the \(\color{blue}{\text{anode}}\)

      \(\color{blue}{\text{2Cl}^{-}\text{(aq) + 2e}^{-} \to \text{Cl}_{2}\text{(g)}}\)

    • At the \(\color{red}{\text{cathode}}\), sodium ions react with water forming caustic soda (\(\text{NaOH}\)) and hydrogen gas.

      \(\color{red}{\text{2Na}^{+}\text{(aq) + 2H}_{2}\text{O(l) + e}^{-} \to \text{2NaOH(aq) + H}_{2}\text{(g)}}\)

    • Some \(\text{NaCl}\) salt remains in the solution with the caustic soda and can be removed at a later stage.

    The Chloralkali Industry

    The diaphragm cell.

    The advantages of the diaphragm cell are:

    • uses less energy than the mercury cell

    • does not contain toxic mercury

    It also has disadvantages however:

    • the sodium hydroxide is much less concentrated and not as pure

    • the chlorine gas often contains oxygen gas as well

    • the process is less cost-effective as the sodium hydroxide solution needs to be concentrated and purified before it can be used

    Fact:

    To separate the chlorine from the sodium hydroxide, the two half-cells were traditionally separated by a porous asbestos diaphragm, which needed to be replaced every two months.

    The Chloralkali Industry

    This was damaging to the environment, as large quantities of asbestos had to be disposed. Asbestos is toxic to humans, and causes cancer and lung problems. Today, the asbestos is being replaced by other polymers, which do not need to be replaced as often, and are not toxic.

  3. The Membrane Cell

    The membrane cell (see figure below) is very similar to the diaphragm cell, and the same reactions occur. The main differences are:

    • the two electrodes are separated by an ion-selective membrane, rather than by a diaphragm

    • the membrane structure allows cations to pass through it between compartments of the cell but does not allow anions to pass through (this has nothing to do with the size of the pores, but rather with the charge on the ions)

    • brine is pumped into the anode compartment, and only the \(\color{red}{\text{positively charged}}\) \(\color{red}{\text{sodium ions}}\) pass into the cathode compartment, which contains pure water

    The Chloralkali Industry

    The membrane cell.

    • At the \(\color{blue}{\textbf{positively charged anode}}\), \(\text{Cl}^{-}\) ions from the brine are oxidised to \(\text{Cl}_{2}\) gas.

      \(\color{blue}{\text{2Cl}^{-}\text{(aq)} \to \text{Cl}_{2}\text{(g) + 2e}^{-}}\)

    • At the \(\color{red}{\textbf{negatively charged cathode}}\), hydrogen ions in the water are reduced to hydrogen gas.

      \(\color{red}{\text{2H}_{2}\text{O(l) + 2e}^{-} \to \text{H}_{2}\text{(g) + 2OH}^{-}}\)

    • The \(\text{Na}^{+}\) ions flow through the membrane to the cathode compartment and react with the remaining hydroxide (\(\text{OH}^{-}\)) ions from the water to form sodium hydroxide (\(\text{NaOH}\)).

      \(\color{red}{\text{Na}^{+}\text{(aq) + OH}^{-}\text{(aq)} \to \text{NaOH(aq)}}\)

    • The chloride ions cannot pass through the membrane, so the chlorine does not come into contact with the sodium hydroxide in the cathode compartment. The sodium hydroxide is removed from the cell. The overall equation is as follows:

      \(2\text{NaCl}(\text{aq}) + 2\text{H}_{2}\text{O}(\text{l})\) \(\to\) \(\text{Cl}_{2}(\text{g}) + \text{H}_{2}(\text{g}) + 2\text{NaOH}(\text{g})\)

    The advantages of using this method are:

    • the sodium hydroxide that is produced is very pure because it is kept separate from the sodium chloride solution

    • the sodium hydroxide has a relatively high concentration

    • this process uses the least electricity of all three cells

    • the cell is cheaper to operate than the other two cells

    • the cell does not contain toxic mercury or asbestos

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