5.1 The chloralkali industry  (Page 71/2)

The chloralkali industry

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

The products of this industry have a number of important uses. Chlorine is used to purify water, and is used as a disinfectant. It is also used in the manufacture of many every-day items such as hypochlorous acid, which is used to kill bacteria in drinking water. Chlorine is also used in paper production, antiseptics, food, insecticides, paints, petroleum products, plastics (such as polyvinyl chloride or PVC), medicines, textiles, solvents, and many other consumer products. Many chemical products such as chloroform and carbon tetrachloride also contain chlorine.

Sodium hydroxide (also known as 'caustic soda') has a number of uses, which include making soap and other cleaning agents, purifying bauxite (the ore of aluminium), making paper and making rayon (artificial silk).

The industrial production of chlorine and sodium hydroxide

Chlorine and sodium hydroxide can be produced through a number of different reactions. However, one of the problems is that when chlorine and sodium hydroxide are produced together, the chlorine combines with the sodium hydroxide to form chlorate ( $Cl{O}^{-}$ ) and chloride ( $C{l}^{-}$ ) ions. This produces sodium chlorate, 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, and to produce chlorine and sodium hydroxide. All three methods involve electrolytic cells (chapter [link] ).

1. The Mercury Cell In the mercury-cell ( [link] ), brine passes through a chamber which has a carbon electrode (the anode) suspended from the top. Mercury flows along the floor of this chamber and acts as the cathode. When an electric current is applied to the circuit, chloride ions in the electrolyte are oxidised to form chlorine gas. $2{\mathrm{Cl}}_{\left(\mathrm{aq}\right)}^{-}\to {\mathrm{Cl}}_{2\left(\mathrm{g}\right)}+2{\mathrm{e}}^{-}$ At the cathode, sodium ions are reduced to sodium. $2{\mathrm{Na}}_{\left(\mathrm{aq}\right)}^{+}+2{\mathrm{e}}^{-}\to 2{\mathrm{Na}}_{\left(\mathrm{Hg}\right)}$ The sodium dissolves in the mercury, forming an amalgam of sodium and mercury. The amalgam is then poured into a separate vessel, where it decomposes into sodium and mercury. The sodium reacts with water in the vessel and produces sodium hydroxide (caustic soda) and hydrogen gas, while the mercury returns to the electrolytic cell to be used again. $2{\mathrm{Na}}_{\left(\mathrm{Hg}\right)}+2{\mathrm{H}}_2{\mathrm{O}}_{\left(\mathrm{l}\right)}\to 2{\mathrm{NaOH}}_{\left(\mathrm{aq}\right)}+{\mathrm{H}}_{2\left(\mathrm{g}\right)}$

   

   This method, however, 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, and although it is returned to the electrolytic cell, some always escapes with the brine that has been used. The mercury reacts with the brine to form mercury(II) chloride. In the past this 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 ( [link] ), a porous diaphragm divides the electrolytic cell, which contains brine, into an anode compartment and a cathode compartment. The brine is introduced into the anode compartment and flows through the diaphragm into the cathode compartment. When an electric current passes through the brine, the salt's chlorine ions and sodium ions move to the electrodes. Chlorine gas is produced at the anode. At the cathode, sodium ions react with water, forming caustic soda and hydrogen gas. Some salt remains in the solution with the caustic soda and can be removed at a later stage.

   

   This method uses less energy than the mercury cell, but the sodium hydroxide is not as easily concentrated and precipitated into a useful substance.

   Interesting 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. This was damaging to the environment, as large quantities of asbestos had to be disposed. Today, the asbestos is being replaced by other polymers which do not need to be replaced as often.
3. The Membrane Cell The membrane cell ( [link] ) is very similar to the diaphragm cell, and the same reactions occur. The main difference is that the two electrodes are separated by an ion-selective membrane, rather than by a diaphragm. The structure of the membrane is such that it allows cations to pass through it between compartments of the cell. It 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 positively charged sodium ions pass into the cathode compartment, which contains pure water.

   

   At the positively charged anode, $C{l}^{-}$ ions from the brine are oxidised to $C{l}_2$ gas. $2{\mathrm{Cl}}^{-}\to {\mathrm{Cl}}_{2\left(\mathrm{g}\right)}+2{\mathrm{e}}^{-}$ At the negatively charged cathode, hydrogen ions in the water are reduced to hydrogen gas. $2{\mathrm{H}}_{\left(\mathrm{aq}\right)}^{+}+2{\mathrm{e}}^{-}\to {\mathrm{H}}_{2\left(\mathrm{g}\right)}$ The Na ${}^{+}$ ions flow through the membrane to the cathode compartment and react with the remaining hydroxide ( $O{H}^{-}$ ) ions from the water to form sodium hydroxide (NaOH). The chloride ions cannot pass through, 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: $2\mathrm{NaCl}+2{\mathrm{H}}_2\mathrm{O}\to {\mathrm{Cl}}_2+{\mathrm{H}}_2+2\mathrm{NaOH}$ The advantage of using this method is that the sodium hydroxide that is produced is very pure because it is kept separate from the sodium chloride solution. The caustic soda therefore has very little salt contamination. The process also uses less electricity and is cheaper to operate.