Hall–Héroult process

The Hall -Heroult process is a process for the preparation of pure aluminum from alumina. The obtained from the Bayer process alumina starting material is dissolved in cryolite and then represented the pure aluminum by igneous electrolysis.

History

1886 Charles Martin Hall and Paul have Heroult this method invented about the same time independently. This method is - still used in industry - with some improvements.

Method

Before the step of the actual electrolysis, the aluminum oxide ( melting point 2045 ° C), cryolite ( Na3 [ AlF6 ] ) were mixed, in order to lower the melting temperature. The resulting existing 80 to 90 percent of cryolite mixture has a melting temperature of about 950 ° C. only Thus, the required working temperature is significantly reduced, which in the first place, the fused-salt electrolysis ..

The working temperatures are maintained, because due to the electrical resistance of the melt them simultaneously as a resistance heater works and delivers the supplies heat for melting. In the process continuously heat is lost:

• By the cooling action endothermic reaction of reduction ( reaction enthalpy ),
• By aspiration of the liquid hot aluminum ( runoff after the siphon principle ),
• Because the reaction gases deduct hot exhaust gas.

The reduction of alumina occurs in the fused salt electrolysis ( briefly also melt electrolysis). The electrolytic cell comprises a steel shell, which is lined with carbon material (graphite / charcoal ). In this tub is the liquid electrolyte ( cryolite with an excess of AlF3 ). Dip into the electrolyte from above the anode ( calcined petroleum coke from graphite blocks) which are connected to the positive pole of a voltage source. The cathode pan on the other hand is connected to the negative pole.

With a voltage of 4 to 5 volts and a current of up to 330,000 amperes ( a current density of 8,000 amperes / square meter) is split, the aluminum oxide ( Al2O3). The high current strength is required to appreciable amounts of aluminum to produce a large scale, because according to the Faraday's law the amount by weight of a substance electrolytically formed is proportional to the flowed electric charge and finally requires the heating in accordance with the Ohm's law, such high currents.

The melt contained in the positively charged aluminum ion Al3 migrate to the cathode ( negative pole). There they take up electrons and are reduced to aluminum atoms. The negative oxygen ions ( O2 - nascent ) migrate to the anode (positive pole). There they give off their excess electrons and are oxidized to oxygen molecules. These oxygen molecules react with the carbon of the graphite anode to carbon monoxide and carbon dioxide, which then escape as gas.

The resulting molten aluminum has a greater density than the molten cryolite - alumina mixture, and therefore accumulates on the bottom of the cathode pan. From there it is drawn off with a suction tube. The resulting pure aluminum still contains about 0.1 to 1 percent contaminants. These are iron, silicon and titanium substantially.

In this process, undesirable side reactions can occur which constitute major environmental part. For example, be no significant amounts of hydrogen fluoride, carbon monoxide, and the greenhouse effect enhancing fluoroalkanes, in particular tetrafluoromethane.