Industrial electrolytic cells differ in electrode configuration, flow pattern of the electrolyte, and collection system for reaction products. Steel cathodes and graphite anodes are arranged vertically or at an angle. Electrodes may be monopolar, bipolar, or a combination of both, with interpolar gaps ranging from 3 to 12 cm and current densities from 2000 to 8000 A/m2.
Electrolyte, filled with chlorine gas adjacent to the anodes, has a lower density than the bulk electrolyte, this causes circulation. The circulating electrolyte carries magnesium globules to the collecting area. Small bubbles of chlorine may follow the flow and escape into the ventilation system.
Under operating conditions (10% MgCl2 and 1000 K) the energy of formation of magnesium is 520.9 kJ/mol, and the enthalpy 597.3 kJ/ mol. Consequently, the reversible decomposition voltage is 2.70 V. The thermoneutral voltage (ΔH/2 F) is 3.095 V. According to Faraday's law, 4.534 x 10-4 kg of magnesium is formed per ampere per hour.
Under thermoneutral conditions energy consumption is 6.8 kW • h/kg. In industrial cells, however, power consumption may range from 10 to 20 kW • h/kg; 40-65% of the energy input generates heat as a result of ohmic resistance of the electrolyte and the recombination of magnesium and chlorine. Heat generation is necessary to compensate for heat losses from the electrodes and the cell surface. Current efficiencies normally range from 0.75 to 0.95. The main cause for reduction of current efficiency is the recombination of dissolved chlorine and magnesium in the electrolyte.
The composition and primary characteristics of electrolytes used in the electrolysis are listed in table 1; they normally contain 8-20 wt% magnesium chloride. Other chlorides (BaCl2 or LiCl) may be added in order to influence the density; lithium chloride also increases the conductivity of the melt. The density of the lithium-potassium chloride electrolyte is lower than that of liquid magnesium, allowing magnesium to accumulate at the bottom of the cell. The solubilities of magnesium and chlorine in commercial electrolytes at 730 oC are 0.005 wt% and 6.27 x 107 mol/cm3, respectively. Solubilities increase proportionally to temperature increase.
Excess electrolyte components brought to the cell with the cell feed are removed by pumping them into molds or by closed, vacuum-operated containers mounted on trucks. At 750 oC the standard decomposition voltages of the electrolyte components are: MgCl2 2.51 V, NaCl 3.22 V, KCl 3.27 V, LiCl 3.30 V, CaCl2 3.33 V and BaCl2 3.40 V. Sodium is deposited when magnesium chloride contents are below ca. 5%; this lowers current efficiency and causes a temperature rise due to the recombination of sodium and chlorine.
Impurities in the cell feed disturb cell performance by passivating the cathode surface and hindering the coalescence of metal globules or their free rise to the surface. The upper permissible limits for common electrolyte impurities, in parts per million, are:
Small additions of calcium fluoride, sodium fluoride, amorphous carbon, and alkaline-earth metals to the electrolyte counteract these undesired effects.
Magnesium chloride and magnesium react with oxygen in air or with oxygen-containing compounds to form magnesium oxide, which has a low solubility in the melt. The oxide can form a layer on the cathode surface, resulting in higher electrical resistance. Magnesium oxide with entrapped electrolyte and magnesium globules form a layer of sludge at the bottom of the cell which must be removed at intervals. The amount of sludge is significantly lower in sealed, as opposed to open cell, designs.
Theoretically, 2.918 t of chlorine is produced per ton of magnesium; commercial cells operating with dehydrated magnesium chloride may yield 2.7-2.8 t of chlorine with a concentration of ≥ 95 wt%. The chlorine gas, from cells operated on dehydrated cell feed, contains small amounts of air, dust from the electrolyte components, and minor amounts of chlorinated hydrocarbons requiring efficient systems for collection and destruction. Electrolytic magnesium contains typically ≥ 99.8 wt% magnesium.