Metallurgy and Industrial Extraction of Calcium (Ca)

Natural Occurrence & Major Ores

Calcium (Ca) is the fifth most abundant element in the Earth’s crust, representing approximately 3.6% by mass. Due to its high reactivity as an alkaline earth metal, it is never found in its free, elemental state in nature. Instead, it occurs extensively in combined forms as various minerals.

Major Ores of Calcium:

• Limestone, Marble, Chalk: These are different forms of Calcium Carbonate ($\text{CaCO}_3$). Limestone is a sedimentary rock, marble is a metamorphic rock formed from limestone, and chalk is a soft, porous sedimentary rock composed of the shells of microscopic marine organisms.
• Gypsum: Hydrated Calcium Sulfate ($\text{CaSO}_4 \cdot 2\text{H}_2\text{O}$). Used in plaster of Paris and drywall.
• Fluorspar (Fluorite): Calcium Fluoride ($\text{CaF}_2$). A significant source of fluorine.
• Apatite: A group of phosphate minerals, predominantly Calcium Fluoro-phosphate ($\text{Ca}_5(\text{PO}_4)_3\text{F}$) or Calcium Chloro-phosphate ($\text{Ca}_5(\text{PO}_4)_3\text{Cl}$). A primary source of phosphorus.
• Dolomite: A double carbonate of Calcium and Magnesium ($\text{CaCO}_3 \cdot \text{MgCO}_3$).

Concentration of the Ore

For the industrial extraction of elemental calcium, the most common starting material is calcium carbonate, typically from limestone. Since limestone deposits are often relatively pure, extensive concentration steps like those required for sulfide ores are generally not necessary.

Primary Methods of Ore Concentration/Preparation:

• Crushing and Grinding: The quarried limestone is first subjected to crushing and grinding to reduce it to a suitable particle size for subsequent processing.
• Washing: In some cases, simple washing with water may be employed to remove adhering earthy impurities (gangue) if the ore is contaminated.
• Calcination: Although not strictly a concentration method, calcination is the crucial initial step in preparing a suitable compound for calcium extraction. Limestone ($\text{CaCO}_3$) is heated strongly (typically $900-1000^\circ\text{C}$) to decompose it into calcium oxide (quicklime) and carbon dioxide: $\text{CaCO}_3 (\text{s}) \xrightarrow{\text{Heat}} \text{CaO} (\text{s}) + \text{CO}_2 (\text{g})$ The calcium oxide is then converted to calcium chloride ($\text{CaCl}_2$) by reaction with hydrochloric acid: $\text{CaO} (\text{s}) + 2\text{HCl} (\text{aq}) \rightarrow \text{CaCl}_2 (\text{aq}) + \text{H}_2\text{O} (\text{l})$ The resulting $\text{CaCl}_2$ solution is then evaporated to obtain solid, anhydrous calcium chloride, which is the electrolyte for reduction.

Reduction to Crude Metal

Calcium is a highly electropositive metal, making chemical reduction methods (e.g., carbon reduction) impractical, as calcium carbide ($\text{CaC}_2$) would readily form, or calcium itself is too reactive. Therefore, industrial extraction of calcium exclusively relies on electrolytic reduction.

Electrolytic Reduction of Molten Calcium Chloride:

• Principle: Electrolysis of molten calcium chloride leads to the deposition of calcium metal at the cathode.
• Electrolytic Cell: An electrolytic cell is used, typically consisting of a graphite anode and an iron or copper cathode.
• Electrolyte: The electrolyte is molten anhydrous calcium chloride ($\text{CaCl}_2$). To lower the melting point of $\text{CaCl}_2$ (which is $772^\circ\text{C}$) and improve conductivity, calcium fluoride ($\text{CaF}_2$) is often added. This mixture typically melts at around $700^\circ\text{C}$.
• Reactions:
  • At Cathode (negative electrode, iron/copper): $\text{Ca}^{2+} + 2\text{e}^- \rightarrow \text{Ca}$ (Molten calcium is formed)
  • At Anode (positive electrode, graphite): $2\text{Cl}^- \rightarrow \text{Cl}_2 + 2\text{e}^-$ (Chlorine gas is evolved)
• Process Operation: The cathode is designed as a cooled iron rod that slowly descends into the molten electrolyte. As molten calcium is denser than the electrolyte, it collects around the cathode. The cathode is gradually raised so that the deposited calcium solidifies and forms a “calcium stick” or “calcium ingot,” which is continuously withdrawn. This method ensures that the molten calcium metal does not redissolve in the electrolyte or react with air.

Refining and Purification

The crude calcium obtained from the electrolytic process may contain small amounts of impurities such as magnesium, aluminum, and residual chlorides. For high-purity applications, further refining is necessary.

Common Refining Methods:

• Distillation: This is the most common method for purifying crude calcium. Calcium has a relatively low boiling point ($1484^\circ\text{C}$) compared to many common impurities like iron and certain other metals.
  • The crude calcium is heated under vacuum to a temperature above its boiling point.
  • Calcium vaporizes and is then condensed in a cooler part of the apparatus, leaving behind non-volatile impurities.
  • Volatile impurities (e.g., magnesium, if present) can be separated by fractional distillation due to differences in boiling points.
• Zone Refining: For extremely high-purity calcium required in specialized applications (e.g., semiconductors), zone refining can be employed.
  • A narrow molten zone is created in a bar of crude calcium and slowly moved along its length.
  • Impurities tend to remain in the molten phase and are carried to one end of the bar, while the purer calcium solidifies behind the moving zone.
  • Multiple passes result in very high purity metal. This method is generally more expensive and used for laboratory-scale purification or high-value materials.