The cell concept first developed by IG Farben Industrie in Germany in the 1930s is now in operation in the Russian Federation, the People's Republic of China, and the United States. Figure 1 illustrates the electrode configuration and the circulation of the electrolyte.
The brick-lined cell is divided into four to six compartments by semi-submerged refractory partition walls labeled semi walls (c). Three to five water- or air-cooled graphite anode plates (a) are installed and tightly sealed in the refractory cover of the cell. The semi walls on each side of the anodes separate the magnesium metal and the chlorine gas. Steel cathodes (b) are installed in the cathode compartments from above or through the sidewalls. Cells operated at 750-780 oC have a life span of ca. one year, limited by the deterioration of the semi walls. The metal is collected from each of several cathode compartments, and chlorine from each anode compartment. Extensive cathode chamber ventilation causes oxygen and water in the ventilation gases to react with metal and electrolyte to form sludge.
The current efficiency is typically 0.80-0.85, and the amperage is in the range 60-120 kA. The chlorine concentration may reach 90-95 wt% and is limited by air leaks through the semi walls. Anode consumption is 15-20 kg per ton of magnesium. Cell regularity (days in operation divided by total days available) is 95-98% or available lime, and the power consumption per ton of magnesium is 16-18 MW • h when operated on molten magnesium chloride.
Norsk Hydro Cell
This cell has been in operation since 1978 and consists of a sealed, brick-lined apparatus that is divided into two separate chambers for electrolysis and metal collection, respectively (Fig. 2). Densely packed, cooled graphite anode plates (b) are installed through the roof and double-acting steel plate cathodes (c) through the back wall. Chlorine (98 wt%) is collected from a central pipe in the anode compartment. Circulation of the electrolyte (h) is parallel to the electrodes bringing the magnesium metal to the collection chamber from where it is extracted by vacuum and transported to the foundry.
This cell operates on solid or liquid cell feed conveyed continuously or at intervals to the cell at 700-720 oC with a current load of 350-400 kA. Energy consumption is 12-14kW • h per kilogram of magnesium and cell lifetime exceeds five years.
The cell developed by VAMI, Saint-Petersburg, Russian Federation, is based on the same principles as the Norsk Hydro cell but operates at 150-180kA and 700-740 oC. Various electrode arrangements are reported. Current density varies between 1000 and 5000 A/m2, with a preferred optimum at 2000 A/m2. The low-carbon steel used for the cathodes is claimed to undergo transformation during operation from 0.2-0.3% carbon to 0.02% carbon, thus coarsening the ferritic grains 100-fold and improving current efficiency. When operated on dehydrated molten magnesium chloride, obtained after reduction of titanium (lV) chloride with magnesium, the current efficiency is ca. 0.80, with an electric energy consumption of 13.5 kW • h per kilogram of magnesium.
Magnesium cells developed by Alcan, Canada, for the electrolysis of anhydrous magnesium chloride have been operating since 1961 in Japan and the United States. They are used in titanium plants where magnesium is employed in the reduction of titanium (IV) chloride (Kroll process). The cells are divided by a curtain wall into a front compartment, where the metal accumulates on the heavier chloride bath, and an electrolysis compartment, where chlorine (≥ 97 wt%) is collected.
The two compartments have insulating covers through which water-cooled graphite anodes are installed. The cathodes are parallel to the anodes and are installed through the refractory back wall below the bath level. The operating temperature (660-680 oC) is controlled just above the melting point of the metal.
A new multipolar cell design has been in operation since 1982 in which electrodes are added between the anode and the cathode surfaces. The energy consumption of this cell, when operating on high-purity molten magnesium chloride from Kroll titanium production, is claimed to be 9.5-10 kW • h per kilogram of magnesium. Cell productivity has reached 1000 t/a. The cell operates at 100 kA; life span is up to two years.
The Ishizuka Research Institute, Japan, has developed a bipolar cell that has been used since 1983 for industrial magnesium production by Showa Titanium, Toyama (Fig. 3).
The outer, brick-lined, cylindrical, water- or air-cooled shell is divided in half by a refractory wall (k). Each half is subdivided with a refractory partition wall into a metal-collecting chamber 0 and an electrode chamber (i) where electrolysis is performed. The electrode chambers have three steel cathodes, one at each end and one in the center (b); a graphite anode (c) is located between each pair of cathodes. Five bipolar electrodes (d) are located between each set of anodes and cathodes. The bipolar electrodes are made of graphite with steel plates attached to the cathode side. The current load is connected to the cathodes and the graphite anodes, penetrating through the refractory-lined cover of the cell. The bipolar electrodes are submerged in the electrolyte. The electrolyte flow induced by gas production carries the metal into the collecting chamber through openings in the partition wall. The electrolyte returns to the electrode chamber in the bottom part of the cell. The cell operates at 670 oC and carries 50 kA, corresponding to 300 kA in a monopolar cell. Current density is 5600 A/m2, and the interpolar gap is 4 cm. At a current efficiency of 0.76, the power consumption is 11 kW • h per kilogram of magnesium, when operating on molten magnesium chloride from Kroll titanium production. The furnace cell's life is ca. three years.
The major advantage of the bipolar cells is reduction of heat losses. Special provisions are required to prevent current leaks from bypassing the bipolar electrode system. Ishizuka states a preferred electrolyte composition of 50% NaCl, 30% KCl, and 20% MgCl2. The cell, maintained at a pressure of 0-1.33 kPa, yields ≥ 96 wt% chlorine.
Electrolysis of hydrous cell feed (MgCl2 • 1.5 H2O) in the Dow cell (Fig. 4) requires graphite anodes, which can be lowered into the cells to compensate for consumed graphite. Densely packed cylindrical anodes (c) are led through openings in the refractory cell hood (d) and sealed to reduce air leaks. They are centered by insulating ceramic spacers.
The development of efficient anode seals to withstand severe corrosive environment and high temperature has been a challenge. Conical cathodes (b) are welded to the steel shell (a) holding the electrolyte. Magnesium metal is conducted by the electrolyte flow to a collection chamber (e) at the front of the cell. Water in the cell feed is partly flashed off, and partly reacts with chlorine (to form HCl) and with oxygen in the graphite (to form CO and CO2). Graphite consumption is probably 60-70 kg per ton of magnesium. The Dow cell is operated at 700 oC with an estimated current load of 180 kA and current efficiency of ca. 0.8.
In comparison with an anhydrous cell feed, the hydrous cell feed increases electric power consumption as well as the formation of sludge and dilute anode gases. The presence of hydrogen, chlorine, and carbon in the cell promotes formation of chlorinated hydrocarbons in off-gases, at high temperature.
Dow has patented an inert anode that is protected by a ceramic cover of doped and sintered Ti4+ and Ti3+ compounds, which could improve the performance of the cell.