Metallurgy and Industrial Extraction of Magnesium (Mg)

Natural Occurrence & Major Ores

Magnesium is the eighth most abundant element in the Earth’s crust and the third most abundant dissolved element in seawater. It always occurs in combined state due to its high reactivity.

• Seawater and Brines: Large reservoir of dissolved Magnesium Chloride (MgCl₂).
• Magnesite: MgCO₃ (Magnesium Carbonate).
• Dolomite: MgCO₃·CaCO₃ (Mixed Carbonate of Magnesium and Calcium).
• Carnallite: KCl·MgCl₂·6H₂O (Hydrated Double Salt of Potassium Chloride and Magnesium Chloride).
• Asbestos: Silicate minerals like Chrysotile Mg₃Si₂O₅(OH)₄.

Concentration of the Ore

The primary industrial sources of magnesium are seawater, brines, and dolomite. The concentration process varies depending on the source.

From Seawater (Dow Process Principle)

1. Precipitation of Magnesium Hydroxide: Seawater (containing dissolved MgCl₂) is treated with milk of lime (Ca(OH)₂), which is prepared by heating limestone (CaCO₃ → CaO + CO₂) and slaking (CaO + H₂O → Ca(OH)₂). MgCl₂(aq) + Ca(OH)₂(aq) → Mg(OH)₂(s) + CaCl₂(aq) Magnesium hydroxide is highly insoluble and precipitates out.
2. Conversion to Magnesium Chloride: The precipitated Mg(OH)₂ is filtered, washed, and then reacted with hydrochloric acid (HCl). Mg(OH)₂(s) + 2HCl(aq) → MgCl₂(aq) + 2H₂O(l)
3. Dehydration: The resulting MgCl₂ solution is evaporated and carefully dehydrated to obtain anhydrous MgCl₂. Complete dehydration is crucial to prevent hydrolysis: MgCl₂·6H₂O(s) → MgCl₂(s) + 6H₂O(g) (Requires heating in an atmosphere of HCl gas to suppress hydrolysis: MgCl₂ + 2H₂O → MgO + 2HCl).

From Dolomite

1. Calcination: Dolomite (MgCO₃·CaCO₃) is calcined at high temperatures to form a mixture of magnesium oxide and calcium oxide. MgCO₃·CaCO₃(s) → MgO(s) + CaO(s) + 2CO₂(g)
2. Conversion to Chloride: The mixed oxides can then be converted to MgCl₂ using HCl as described above, or MgO can be used in the Pidgeon process (thermal reduction).

Reduction to Crude Metal

The industrial reduction of magnesium is primarily achieved by electrolysis. Thermal reduction methods are also used, but less frequently for primary production.

Electrolytic Reduction (Dow Process)

This is the main method for producing magnesium.

1. Electrolyte: Anhydrous MgCl₂ is fused with NaCl and CaCl₂ (or KCl). These salts serve to:
   • Lower the melting point of the electrolyte mixture (from 714°C for pure MgCl₂ to ~450°C).
   • Increase the electrical conductivity.
   • Decrease the density of the electrolyte, allowing the molten magnesium to float and be easily collected.
2. Cell Design: An electrolytic cell typically consists of a steel tank lined with refractory material. Graphite rods act as anodes, and steel plates serve as cathodes.
3. Electrolysis: Direct current is passed through the molten electrolyte at approximately 700°C.
   • At Cathode (Steel): Magnesium ions gain electrons and are reduced to molten magnesium metal. Mg²⁺(l) + 2e⁻ → Mg(l)
   • At Anode (Graphite): Chloride ions lose electrons and are oxidized to chlorine gas. 2Cl⁻(l) → Cl₂(g) + 2e⁻
4. Collection: Molten magnesium, being lighter than the electrolyte, floats to the top and is siphoned off periodically. Chlorine gas is collected as a valuable byproduct and can be recycled to produce HCl.

Thermal Reduction (Pidgeon Process)

This process involves reducing MgO (obtained from calcined dolomite or magnesite) with ferrosilicon (FeSi).

1. Reactants: Calcined dolomite (MgO·CaO) is mixed with ferrosilicon (an alloy of iron and silicon).
2. Reaction: The mixture is heated in a vacuum furnace at high temperatures (1100-1200°C). Silicon reduces MgO, and CaO acts as a scavenger for silica, shifting the equilibrium to the right. 2(MgO·CaO)(s) + SiFe(s) → 2Mg(g) + Ca₂SiO₄(s) + Fe(s)
3. Distillation: Magnesium vaporizes at these temperatures and is condensed in a cooler part of the furnace to produce crude magnesium. This process is energy-intensive and generally used for smaller-scale production or specific applications.

Refining and Purification

The crude magnesium obtained from electrolysis or thermal reduction typically has a purity of 99-99.8%. Further purification is done to achieve higher purity.

1. Distillation: This is the most common method for obtaining high-purity magnesium.
   • Crude magnesium is heated in a vacuum furnace to its boiling point (1091°C).
   • Magnesium vaporizes, leaving behind less volatile impurities like iron, silicon, and dissolved chlorides.
   • The pure magnesium vapor is then condensed in a cooler section of the furnace to yield magnesium metal with purity up to 99.99%.
2. Flux Refining: Molten crude magnesium can be treated with specific fluxes (mixtures of chlorides and fluorides, e.g., MgCl₂-BaCl₂-CaF₂). These fluxes help in removing non-metallic inclusions and some metallic impurities by forming dross, which separates from the molten magnesium. This is often an initial step before further refining.
3. Recycling: A significant portion of magnesium metal is obtained through the recycling of scrap, which involves melting and purification processes similar to flux refining and distillation.