Fusion electrolysis

Scheme of fused-salt electrolysis for the production of aluminum

The fused salt electrolysis is an electrolysis process , no aqueous medium, but a hot in the molten salt serves as the electrolyte. It is used to manufacture or extract aluminum , all alkali metals and most alkaline earth metals , as well as some plastics. The process also provides fluorine and chlorine .

Since the so-called "base" metals potassium , sodium , magnesium and aluminum are below the water (H ₂ O) in the electrochemical series , they cannot be separated from aqueous solutions by electrolysis , as water is more likely to convert to H ₂ + 2 OH ^- would be reduced. To melt the salts you need very high temperatures: for the production of aluminum with cryolite you need 960 ° C, for sodium from sodium chloride approx. 620 ° C, for magnesium from MgCl ₂ and additives approx. 670 ° C. Even fluorine cannot be separated from aqueous solutions of fluorides by means of electrolysis, since oxygen is produced instead of fluorine .

To manufacture u. a. Electrolysis from a melt of their salts is therefore used for these substances , which is what gave the process its name. The fused -salt electrolysis in the Hall-Héroult process is described as an example .

Laboratory scale

On a laboratory scale, molten salts are sometimes carried out in a curved V-tube made of high-melting Pyrex, Jena glass or porcelain in an electric furnace. An iron rod can serve as the cathode material, and an iron or carbon rod ( Acheson graphite ) is often used as the anode . Carbon anodes can be attacked by the action of oxygen or fluorine in fused-salt electrolysis, with CO and CO ₂ or fluorine compounds being formed, with the electrode being gradually eroded.

production

In production, electrolysis cells are often used in which one electrode (usually cathode) is embedded in the ground, the other electrode (anode) is immersed in the molten salt parallel to the former. Concrete, stoneware, fireclay, porcelain or glass are suitable cell materials. Carbon or iron can serve as electrode materials.

Noteworthy things to note about fused-salt electrolysis

• Salt mixtures are often used instead of pure salts to reduce the often high melting temperature (formation of a eutectic ). Particularly popular salt additives are potassium chloride and calcium fluoride .
• The electrical resistance of the melt causes the melt to work like a resistance heater at the same time , this and other current losses maintain the melting temperatures and additionally provide heat for the endothermic reaction of the reduction. The stored energy ( enthalpy of reaction ) is released during the exothermic counter-reaction ( oxidation ) and applied and used in practice, for example, in thermite mixtures containing aluminum .
• The current density in fused flux electrolysis should be as high as possible. If the currents are too low, the Faraday conversion decreases sharply.
• If the distance between the electrodes is very small (distance less than 1 cm), the metal can migrate to the anode as a “metal mist” through diffusion or eddies and reduce the conversion. Metal mist can often be reduced by encapsulation (coating the cathode with Pyrex glass or porcelain).
• It only works with direct current and not with alternating current.

Reaction equations

Using the example of a sodium chloride melt:

The following reaction takes place on the positively charged anode:

${\displaystyle \mathrm {2\ Cl^{-}\longrightarrow 2\ e^{-}+Cl_{2}} }$

Two chloride ions are oxidized to form elemental chlorine.

The following reaction takes place at the negatively charged cathode:

${\displaystyle \mathrm {2\ Na^{+}+2\ e^{-}\longrightarrow 2\ Na} }$

Two sodium ions are reduced to elemental sodium.

The entire redox reaction looks like this:

${\displaystyle \mathrm {2\ NaCl\longrightarrow 2\ Na+Cl_{2}} }$

The chloride ion releases electrons and the sodium ion takes them up. In doing so, there is an electron transfer, which is reflected in the redox reaction .

Important melt flow electrolysis

metal	Starting salts	Temperature (° C)	Voltage (V)
sodium	Sodium chloride with CaCl 2 and BaCl 2	600	7th
potassium	Potassium chloride	360-380	4-7
magnesium	MgCl 2 , KCl, 1% CaF 2	670-730	12
Calcium	CaCl 2 , CaF 2	680-800	25-40
beryllium	5 BeF 2 , 2 BeO	600	55
Rare earths (Ce, La, Nd)	Corresponding chlorides, NaCl	800-900	8-15
aluminum	Cryolite, clay	935	5.5-7

Individual evidence

1. ↑ Volkmar M. Schmidt: Elektrochemische Verfahrenstechnik , ISBN 978-3-527-62362-4
2. ↑ Scriptum Elektrochemie (PDF; 1.3 MB) of the University of Siegen, page 184