The role of De Nora in the chlor-alkali process

The evolution of the chlor-alkali process

The chlor-alkali process is an industrial process for the electrolysis of sodium chloride or brine (NaCl) to produce chlorine gas and sodium hydroxide solution (caustic soda or NaOH). Originally, sodium hydroxide was produced by the reaction of slaked lime and soda ash. In 1892, the electrolysis of brine was discovered as a method of making both sodium hydroxide and chlorine. This method has been used since then and, in the 1960s, electrolysis became the predominant technique utilized to manufacture these two important chemicals.

How does the electrolysis of sodium chloride work?

Chlorine was first discovered in 1774 by the German chemist Scheele and was identified as an element in 1810 by an English scientist named Davy. Caustic soda, or sodium hydroxide, has been an important industrial chemical since 1853. In the process of electrolysis of sodium chloride, to both produce chlorine gas and a solution of sodium hydroxide, sodium chloride is supplied at the anode of an electrochemical cell where the chloride ions are oxidized, losing electrons to become chlorine gas (1):
2Cl− → Cl2 + 2e- (1).

At the cathode, water is reduced by the electrons provided by the cathode to hydrogen gas, releasing hydroxyl ions into the solution (2):
2Na+ + 2H2O + 2e- --> H2 + 2NaOH

The overall chemical reaction is displayed below (3):
2NaCl + 2H2O → Cl2 + H2 + 2NaOH (3).

Three electrolytic processes are available, each using different methods to keep the chlorine produced at the anode separated from the caustic soda produced at the cathode. In historical order, these cells include diaphragm cells, mercury cathode or "amalgam" cells, and membrane cells.

In the traditional electrolysis process, an asbestos-base diaphragm was employed to separate the anolyte from the catholyte. One disadvantage of the diaphragm cell is the low concentration of the caustic soda solution, which requires several concentration cycles to achieve the concentration needed for industrial use. Additionally, the produced caustic soda contains a minimum residue of sodium hydroxide, and it is therefore considered as a “lower grade” with respect to the caustic soda produced with the other two technologies. The cell is also quite polluting due to the asbestos fibers, which the diaphragm is made of; however, non-asbestos fibers to make diaphragm have been developed and are now commercially in use to address the asbestos concern.

For these reasons, mercury cathode cells began to compete with diaphragm cells. Mercury cells produce a much purer and concentrated caustic product that can be used without further treatment in most cases. However, mercury has severe ecological consequences, and for this reason the technology has been recently banned and very few plants using this technology are actually still in operation.

The development of Membrane Technology for the Chlor-Alkali industry

The membrane technology was developed to overcome the environmental challenges posed by both the diaphragm and mercury cells. In this case, a membrane, which is a cation exchange synthetic separator, is used to separate the anolyte from the catholyte while permitting the sodium ions to migrate from the anode to the cathode, thus forming the NaOH solution. Besides the sustainability benefit, the advantage of membrane cells is 1. the pure and relatively high concentration of the produced sodium hydroxide solution, and 2. lower electricity requirements than either diaphragm or mercury cells.

Today, membrane technology is the only technology adopted for new installations. Since the 1970s De Nora has been a pioneer, and played a pivotal role, in the development of the membrane technology- first by developing and commercializing its own electrolyser, and more recently by collaborating with thyssenkrupp Nucera in developing the most innovative membrane cell technologies actually on the market (BM Generation 6 and e-BiTAC v7). Concurrently, De Nora developed its proprietary anode and cathode coating formulations, which are now the reference for the industry and are able to sustain the high design current densities of modern membrane electrolyzers, providing:

superior performances in terms of voltage savings (specific energy consumption),
gas and caustic soda quality,
and longevity.

What has been the historical contribution of De Nora to the chlor-alkali process?

The production and manufacturing of chlorine (as a single element) and caustic soda is an industry with a long history. These substances are used throughout a myriad of different industries including, but not limited to construction, plastics and textiles, paper, and cleaning/disinfection. Our roots in the chlorine, caustic soda, and potash industry date back to the creation of the earliest commercial diaphragm cells in the 1920s, the first mercury cells in the 1940s, and the development of the very first membrane cell in the 1970s.

With more than 400,000 m2 of coated electrodes produced annually, De Nora, in addition to being a proven and qualified designer, a global manufacturer, and supplier of activated cathodes and anodes for the chlorine and caustic industry. Through its worldwide presence, with facilities in Europe, Asia, South America and North America, De Nora can also provide services in real-time wherever our customers are located.

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