Chlorine production

Chlorine gas can be produced by extracting from natural materials, including the electrolysis of a sodium chloride solution (brine) and other ways. Chlorine can be manufactured by the electrolysis of a sodium chloride solution (brine), which is known as the Chloralkali process. The production of chlorine results in the co-products caustic soda (sodium hydroxide, NaOH) and hydrogen gas (H2). These two products, as well as chlorine itself, are highly reactive. Chlorine can also be produced by the electrolysis of a solution of potassium chloride, in which case the co-products are hydrogen and caustic potash (potassium hydroxide). There are three industrial methods for the extraction of chlorine by electrolysis of chloride solutions, all proceeding according to the following equations: Cathode: 2 H+ (aq) + 2 e− → H2 (g) Anode: 2 Cl− (aq) → Cl2 (g) + 2 e− Overall process: 2 NaCl (or KCl) + 2 H2O → Cl2 + H2 + 2 NaOH (or KOH) Mercury cell electrolysis, also known as the Castner–Kellner process, was the first method used at the end of the nineteenth century to produce chlorine on an industrial scale. The "rocking" cells used have been improved over the years. Today, in the "primary cell", titanium anodes clad with platinum or conductive metal oxides (formerly graphite anodes) are placed in a sodium (or potassium) chloride solution flowing over a liquid mercury cathode. When a potential difference is applied and current flows, chlorine is released at the titanium anode and sodium (or potassium) dissolves in the mercury cathode forming an amalgam. This flows continuously into a separate reactor ("denuder" or "secondary cell"), where it is usually converted back to mercury by reaction with water, producing hydrogen and sodium (or potassium) hydroxide at a commercially useful concentration (50% by weight). The mercury is then recycled to the primary cell by a pump situated at the bottom. The mercury process is the least energy-efficient of the three main technologies (mercury, diaphragm and membrane) and there are also concerns about mercury emissions.