How do you purify copper from its ore?

Copper is purified from its ore through a multi-stage process involving heating the concentrated ore with silicon dioxide, calcium carbonate, and air or oxygen in furnaces. This process ultimately reduces copper(II) ions in the ore to copper metal.

Here's a breakdown of the process:

1. Concentration: The initial copper ore, which typically contains chalcopyrite (CuFeS₂), is concentrated. This often involves froth flotation, a technique that separates valuable minerals from waste rock (gangue).

2. Roasting: The concentrated ore is heated strongly in the presence of air. This process, called roasting, converts copper sulfides into copper oxides and removes volatile impurities like sulfur as sulfur dioxide (SO₂).

   • For example: 2CuFeS₂(s) + O₂(g) → Cu₂S(s) + 2FeS(s) + SO₂(g)

3. Smelting: The roasted ore is then mixed with silicon dioxide (SiO₂), calcium carbonate (CaCO₃), and heated to high temperatures in a furnace. The silicon dioxide acts as a flux, combining with iron oxide (formed during roasting) to form slag (iron silicate), which can be easily separated. The calcium carbonate helps to maintain a basic environment in the furnace, further promoting slag formation.

   • The copper is primarily present as copper(I) sulfide (Cu₂S) at this stage.

4. Conversion: The molten copper(I) sulfide (also called matte) is transferred to a converter. Here, air or oxygen is blown through the molten matte to oxidize the copper(I) sulfide to copper(I) oxide, which then reacts further with more copper(I) sulfide to produce blister copper (impure copper metal).

   • 2Cu₂S(l) + 3O₂(g) → 2Cu₂O(l) + 2SO₂(g)
   • 2Cu₂O(l) + Cu₂S(l) → 6Cu(l) + SO₂(g)

5. Refining: The "blister copper" (approximately 98-99% pure) is further refined to achieve higher purity. This is commonly done through electrolytic refining. In electrolytic refining, the impure copper is used as the anode in an electrolytic cell, and a thin sheet of pure copper serves as the cathode. The electrolyte is a copper sulfate (CuSO₄) solution. As electricity is passed through the cell, copper ions from the anode dissolve into the electrolyte and are deposited as pure copper onto the cathode. Impurities either dissolve in the electrolyte or fall to the bottom of the cell as anode sludge, which can be processed to recover valuable metals like gold and silver.

In summary, copper purification involves a series of chemical reactions and physical separations to transform copper ore into high-purity copper metal, typically suitable for electrical applications.