The production of magnesium and its alloys by electrolysis has been practised commercially for more than a hundred years. Today, magnesium is mainly produced by electrolysis of fused compounds or by electrolysis of aqueous solutions, usually of chloride salts. In all cases, the electrolysis takes place between two graphite electrodes, the cathode being of larger surface area than the anode, allowing a greater number of electrons to enter the cathode. In the production of primary magnesium by electrolysis, the magnesium is deposited on the cathode and the anode reaction is evolution of chlorine.
In the process of primary magnesium production, a shaft furnace is used to provide the electricity and to heat the material. The furnace is lined with refractory bricks and the material to be processed is placed inside and heated to the melting point. The anode and cathode must then be fitted into the cell, which is made from graphite. The anodes are generally in the form of sheets of graphite, and the cathodes are formed from electrodes of graphite rod. When the furnace has reached its melting point, the molten material is circulated around the cell using an impeller. This agitation reduces the resistance to the current and helps the electrolysis process by keeping the ions in suspension and generating bubbles of gases at the anode.
The anodes are then passed in and out of the electrolyte at regular intervals. This allows the chlorine to escape from the anode and helps to control the current and thereby helps to sustain the reaction.
The graphite cathodes, on the other hand, are designed to act as a barrier, thus allowing only the magnesium ions to enter and deposit on the surface. The level of current can be regulated by varying the distance between the electrodes and the quality of the magnesium produced depends heavily on the precision of the cathode design. The currents used for electrolysis are usually high, and thus a large amount of heat is generated during the process.
Due to their superior conductivity and low cost, graphite electrodes are used in most magnesium industry plants. Graphite anodes are used because they are easy to manage, inexpensive, and show little reactivity. Their non-corrosive nature ensures long life of the electrodes and thus helps keep production costs lower.
In order to optimize the process, a balance must be struck between the rate of production and quality of the magnesium produced. It is also important to maintain a clean electrolyte in order to minimize the production of unwanted side products.
From an economic standpoint, the production of magnesium and its alloys via electrolysis using graphite electrodes is a very attractive and efficient process. The high quality of material produced further adds to its appeal. As such, it is no wonder that graphite electrodes remain a key component in magnesium production.
