1. Mg Extraction - Dow Process

What is the name of the industrial process used for the production of metallic Magnesium?

The Dow Process

What principal raw materials are required for the industrial production of Magnesium?

1. Sea water (or bittern) as the source of Mg2+ ions.\n2. Limestone (CaCO3) or Dolomite (CaCO3.MgCO3).\n3. Concentrated Hydrochloric acid (HCl).

What are the four main sequential steps involved in the Dow process for producing magnesium?

Step 1: Calcination / Production of CaO\nStep 2: Precipitation of Mg(OH)2\nStep 3: Neutralization and dehydration to form anhydrous MgCl2\nStep 4: Electrolysis of molten MgCl2

Write the balanced chemical equation with state symbols for the calcination of Limestone inside the kiln.

CaCO3(s) -> CaO(s) + CO2(g) (under heating)

Write the balanced chemical equation with state symbols for the calcination of Dolomite inside the kiln.

CaCO3.MgCO3(s) -> CaO(s) + MgO(s) + 2CO2(g) (under heating)

Why must CO2(g) be continuously removed from the calcination kiln during the Dow process?

The thermal decomposition reaction is reversible. Continuous removal of CO2(g) shifts the equilibrium to the right according to Le Chatelier's principle, ensuring maximum yield of CaO and preventing contamination with unreacted carbonate.

Write the balanced chemical equation for the slaking step where the kiln product is prepared for precipitation.

CaO(s) + H2O(l) -> Ca(OH)2(s/aq)

Write the balanced ionic equation for the selective precipitation of magnesium from sea water/bittern.

Mg2+(aq) + 2OH-(aq) -> Mg(OH)2(s)

Why does Magnesium Hydroxide precipitate instead of Calcium Hydroxide when Ca(OH)2 is added to sea water?

The solubility product (Ksp) of Mg(OH)2 is significantly lower than the solubility product (Ksp) of Ca(OH)2. Therefore, Mg(OH)2 reaches its saturation limit and precipitates at a much lower OH- concentration.

Write the balanced chemical equation for the neutralization of the magnesium precipitate with concentrated hydrochloric acid.

Mg(OH)2(s) + 2HCl(aq) -> MgCl2(aq) + 2H2O(l)

Write the chemical equation for the dehydration step of magnesium chloride before it undergoes electrolysis.

MgCl2.6H2O(s) -> MgCl2(s) + 6H2O(g) (under controlled heating)

What are the specific anode and cathode materials used in the electrolysis stage (Dow cell)?

Anode: Graphite (Carbon)\nCathode: Steel (Iron)

What specific temperature range is maintained inside the Dow cell during electrolysis?

700–800 °C

What is the exact composition of the electrolyte used in the Dow cell?

Molten anhydrous MgCl2 (often mixed with NaCl/CaCl2 to lower the melting point and increase electrical conductivity).

Write the balanced half-reaction that occurs at the graphite anode during the electrolysis of molten MgCl2.

2Cl-(l) -> Cl2(g) + 2e-

Write the balanced half-reaction that occurs at the steel cathode during the electrolysis of molten MgCl2.

Mg2+(l) + 2e- -> Mg(l)

What physical factor facilitates the easy separation of liquid magnesium from the liquid electrolyte mixture in the cell?

Density difference: Molten metallic Magnesium has a lower density (1.58 g cm^-3) than the molten MgCl2 electrolyte mixture (1.68 g cm^-3), causing the pure liquid Mg to float to the surface where it can be continuously skimmed off.

Which industrial by-product of the Dow process can be used to regenerate a raw material, and what is that raw material?

Chlorine gas (Cl2) produced at the anode during electrolysis is collected and can be reacted with hydrogen/methane to regenerate Hydrochloric acid (HCl).

List two major engineering/industrial applications of metallic Magnesium.

1. To manufacture strong, lightweight structural alloys for aircraft, spacecraft, and automotive components.\n2. Used as a reducing agent in metallurgy and for preparing Grignard reagents in organic synthesis.

During which steps of the overall Dow process is Carbon Dioxide (CO2) gas released into the atmosphere?

1. Thermal calcination of limestone or dolomite in the kiln.\n2. Combustion of fossil fuels used to generate high temperatures for the kiln and electrolysis cell.

What primary global environmental problem is caused by the release of CO2 gas from the Dow process?

Enhanced greenhouse effect leading to global warming and global climate change.