Chloralkali process

With the chlor-alkali electrolysis the important basic chemicals chlorine, hydrogen and caustic soda are produced from sodium chloride. For this, rock salt, which is next to a few other salts, especially of sodium chloride (table salt ), activated with water from the salt domes and promoted as a concentrated brine in the electrolysis cells.

The chlor-alkali electrolysis is an endothermic reaction. The required energy of 454 kJ / mol is added in the form of electric current.

History

The three processes for the industrial production of chlorine and caustic soda, caustic potash were at the end of the 19th century and the diaphragm, the bell method and the mercury process.

The diaphragm cell process was developed by Breuer in 1885. Stroof and the brothers Lang built in Griesheim 1890, the first large-scale plant for the production of caustic soda and chlorine by means of Diaphragmaverfahrens. In 1895 the company Griesheim- Electron was founded. It created three large plants in Germany, in Griesheim, Rheinfelden and Bitterfeld as well as several works in other European countries (Spain, France, Russia). In France, a modified diaphragm process of Outhenin, Chalandres has been developed. However, the operation had to be discontinued soon.

Because besides soda and caustic potash Anyway chlorine was co-produced in the respective fraction, was quite a time constraint for the chemical industry to find market applications for the abundant chlorine. During the First World War some of the chlorine for the manufacture of war gases was used. Between 1950 and 1960, many toxic and poorly biodegradable organic chlorine compounds were prepared.

Basics

The salt is present in an aqueous solution ( " brine " ), which in addition to the Na - ions are present, the dissociation of water H3O ( oxonium cations) and OH - ( hydroxide anions ) - and Cl. One puts on the electrodes to a voltage, the ions are discharged preferentially requiring the lowest decomposition potential. With favorable choice of the electrode materials are the chloride and the oxonium ion. Remain the sodium and hydroxide ions, which form the hydroxide.

Technical Procedures

For the technical implementation, it is important that the resulting chlorine does not come into contact with hydroxide ion, as this one chloride / hypochlorite mixture would result.

In addition, care must be taken that the chlorine gas is not mixed with the hydrogen gas, as this chlorine-hydrogen gas would be produced.

These requirements are satisfied by three different methods.

Diaphragm process

In the diaphragm, the cathode is made ​​of iron or steel, the anode is made of ruthenium ( IV) oxide coated titanium. The cathode compartment is separated from the anode space by a current and cation -permeable porous barrier, so that the chlorine formed at the anode or the hydroxide ions is also formed with neither the hydrogen formed at the cathode, in contact. Chlorine would otherwise disproportionate OCl with the OH - ions and Cl -.

The redox couple H2/H3O has a higher potential than the Na / Na , also the overvoltage of hydrogen on iron is not very large, is therefore developed at the cathode and no sodium. By the discharge of the oxonium ions at the cathode in the cathode chamber solution is basic. The Cl - ions are discharged due to the overvoltage of the oxygen in the titanium on the anode.

Anode:

Cathode:

The diaphragm is usually made of asbestos. Today also diaphragms made ​​of polytetrafluoroethylene and inorganic additives are used. Because of this the diaphragm available hydroxide ions can not be completely separated from the anode compartment into the solution at an elevated concentration of hydroxide ions to a reaction of water and oxygen is possible. Therefore, only a sodium hydroxide solution are obtained up to a concentration of about 12-15%.

Advantages:

• Lower energy expenditure
• No environmental impact
• Hypochlorite ( bleach) or chlorates, such as fireworks can be formed.

Cons:

• By sodium chloride -contaminated caustic soda
• Health risks when the diaphragm is made of asbestos.

Amalgam process

The electrolysis of sodium chloride solution is the amalgam process between a graphite anode and the eponymous mercury cathode. At the anode, chlorine gas is separated. The sodium formed at the cathode immediately dissolves in the mercury as sodium amalgam. The amalgam is then treated with water, and form sodium hydroxide and hydrogen. The remaining mercury is returned to the process.

The deposition of sodium chloride and to the electrode based on the displacement of the deposition potential of the elements hydrogen and oxygen due to power surges.

Theoretically, the following electrode reactions could take place:

Anode reaction (oxidation ):

Cathodic reaction ( reduction):

Thus, hydrogen and oxygen would have to be deposited. However, by the choice of suitable electrode material ( graphite anode and mercury - cathode) and the appropriate concentration ratios it comes to separation of the sodium and chlorine.

The sodium reacts at the mercury cathode immediately to sodium amalgam:

For the production of caustic soda, sodium amalgam is reacted with water to the reaction in the amalgam decomposer. Decomposition reaction:

Overall reaction:

The advantages of this method lie mainly in the strict spatial separation of chlorine and hydrogen formation, so that the formation of a highly explosive chlorine-hydrogen gas mixture can be avoided.

The disadvantages are the high power consumption and the environmental hazard of mercury. Although the spread of mercury in the amalgam process is extremely low and was further reduced in recent years, mercury cell plants are increasingly being replaced by the membrane process (see below).

Another approach attempts to gain directly from the amalgam elemental sodium or other alkali metals such as potassium or lithium. This is done by means of electrochemical extraction of alkali metal from the amalgam eg ceramic ion conductors from beta -alumina type. This would have the advantage that the chemical and pharmaceutical industries would be reducing these important resources directly available and so the chlor-alkali electrolysis process could be run profitably.

Membrane processes

As with the diaphragm cell, the membrane method uses a titanium anode and an iron cathode. The key difference is that the diaphragm has been replaced by an approximately 0.1 mm thick, chlorine-resistant cation-exchange membrane of polytetrafluoroethylene (PTFE / Teflon), with negatively charged residues is SO3 (Nafion ). The anions such as OH - or Cl - can not happen, whereas reach the positively charged Na ions through. Due to the impermeability to Cl - ions results in a hardly contaminated by sodium chloride, 35% sodium hydroxide solution. The chemical processes at the electrodes are similar to those in the diaphragm process.

Anode ( positive pole ):

Cathode ( negative pole ):

This method, which is the newest of the illustrated here, is used today in about 2 /3 of a large scale operating at the site since this the end products Cl2, H2 and NaOH incurred almost the same purity as the amalgam process, but overall, a significantly lower energy input required is. Furthermore it can be completely dispensed with the use of the controversial environmental point of mercury.

Advantages:

• Environmentally friendly and not harmful, no contamination by asbestos or mercury
• Pure end products in the chlorine only small traces of oxygen
• Cost-saving

Problems:

• Membranes sensitive to alkaline earth metal ions, which can stick to this and hinder the passage of sodium ions, so the membrane has to be laboriously cleaned from time to time.
• Low stability and durability of the membrane

World production and shares in the process

The global production of caustic soda from rock salt is about 50 million t / a, this usually forcibly about 55 million t / y of chlorine and 1.56 million t / a hydrogen (stoichiometric and gas volume equal ) as by-products. The European chlorine production in 2010 was around 12.5 million tonnes, Germany this has a share of about 40%. The largest European producer with about 14 % share is Dow Chemical in Stade and Schkopau.

The share of manufacturing process in 1990: 39% amalgam, 45 % and 16 % diaphragm membrane processes; meanwhile, the proportion of the membrane process has grown to over 66 %.