Magnesium (Mg) is a silvery-white element and the lightest structural metal available. This low density, combined with its favorable mechanical properties when alloyed, makes it highly sought after in modern industry. The metal is highly reactive, meaning it is found only in compounds in nature. Transforming these naturally occurring compounds, found in the Earth and oceans, into pure metallic form requires a complex industrial process.
Identifying Magnesium’s Natural Sources
Magnesium is an abundant element. The primary terrestrial sources are magnesium-rich minerals like magnesite and dolomite, which are mined from the Earth’s crust. Magnesite is composed of magnesium carbonate (\(\text{MgCO}_3\)), while dolomite is a calcium magnesium carbonate.
The oceans represent an enormous reservoir of magnesium, dissolved in seawater as magnesium chloride (\(\text{MgCl}_2\)) and magnesium sulfate (\(\text{MgSO}_4\)). Although the concentration is low, the sheer volume makes it a viable source. High-concentration underground salt solutions, known as brines, are another significant natural source, containing dissolved chloride salts.
Initial Extraction and Compound Preparation
Extracting magnesium requires pre-processing steps to create a purified feedstock. For mineral sources like magnesite and dolomite, the first step is calcination. This involves heating the carbonate ore to high temperatures, typically between \(950^\circ\text{C}\) and \(1200^\circ\text{C}\). This heat drives off carbon dioxide, converting the raw ore into chemically active magnesium oxide (\(\text{MgO}\)), often called magnesia.
When utilizing seawater or brines, a different chemical route begins with precipitation. Calcium hydroxide, derived from calcined limestone, is introduced to the liquid source to selectively precipitate the magnesium ions. This reaction creates solid, insoluble magnesium hydroxide (\(\text{Mg(OH)}_2\)), which can then be filtered out.
The resulting magnesium oxide or hydroxide must be prepared for the final reduction by converting it into anhydrous magnesium chloride (\(\text{MgCl}_2\)). This conversion is necessary because the presence of water during the final high-temperature metal production would lead to the formation of undesirable magnesium oxide. The preparation involves mixing the oxide or hydroxide with a carbon source and chlorine gas in a process called carbo-chlorination. This mixture is heated to yield the essential anhydrous magnesium chloride feedstock.
Transforming Compounds into Magnesium Metal
The prepared magnesium compounds are converted into pure metal through two distinct industrial routes: electrolysis and thermal reduction.
Electrolysis
The electrolytic process is historically the most widespread method, relying on electrical current to separate the elements in a compound. This process utilizes the anhydrous magnesium chloride feedstock, which is melted in an electrolytic cell. A mixture of molten salts, such as sodium chloride and potassium chloride, is added to lower the melting point. An intense direct electrical current is passed through this molten salt bath, causing the magnesium chloride to dissociate. At the cathode, magnesium ions gain electrons to form liquid magnesium metal, while chlorine gas is released at the anode. The liquid metal floats to the surface where it is collected. This method yields a high-purity product and recovers valuable chlorine gas, but it requires a substantial supply of electricity.
Thermal Reduction
The alternative method is thermal reduction, most famously represented by the Pidgeon process. This process uses magnesium oxide, typically sourced from calcined dolomite, as its primary magnesium source. The magnesia material is mixed with ferrosilicon, an alloy of iron and silicon, which acts as the reducing agent. This mixture is compacted into briquettes and heated inside sealed, evacuated retorts to extremely high temperatures, often exceeding \(1200^\text{C}\). Under these high-temperature and vacuum conditions, the silicon reacts with the magnesium oxide to produce magnesium vapor. The vapor is then condensed in a cooler section of the retort into solid, crystalline rings, often referred to as “crowns.” This method is less efficient and more labor-intensive than electrolysis, but it is the dominant production method in regions with inexpensive heat sources.
Key Uses of Manufactured Magnesium
The primary demand for manufactured magnesium metal stems from its ability to form lightweight alloys that are approximately one-third less dense than aluminum. When alloyed with metals like aluminum, zinc, and manganese, magnesium retains high strength while significantly reducing weight. This characteristic makes magnesium alloys indispensable in the transportation industry, particularly for aircraft components and automotive parts such as steering wheels, seat frames, and engine casings.
Magnesium metal is also employed in metallurgical processes as a strong reducing agent. It is used to remove sulfur from molten iron and steel in a process called desulfurization, improving the quality of the final metal. Furthermore, magnesium oxide serves as a material for refractory bricks in furnaces and kilns due to its exceptional resistance to heat.