Extraction of Copper from Low Grade Ores & Scraps
Master the science of Leaching and Displacement using Scrap Iron.
While high-grade copper ores are extracted using traditional Pyrometallurgy (roasting, smelting, and auto-reduction in a Bessemer converter), these methods are too expensive and fuel-intensive for low-grade ores (ores with very small percentages of copper). For low-grade ores and recycled copper scraps, the industry turns to a more economical, liquid-based approach: Hydrometallurgy.
1. The Hydrometallurgical Approach
Hydrometallurgy involves two major steps to extract the metal from low-grade ores:
- Leaching (Dissolution): The solid ore is treated with a chemical solvent that selectively dissolves the copper, converting it into aqueous Copper(II) ions ($Cu^{2+}$), while leaving the rocky gangue behind.
- Displacement (Precipitation): A more reactive metal (or a reducing gas) is added to the solution to force the dissolved copper ions to turn back into solid copper metal.
2. Step 1: Leaching out the Copper
The low-grade ore is typically leached using an acid (like dilute Sulfuric Acid, $H_2SO_4$) or, increasingly, through Bio-leaching using specific strains of bacteria.
Acid & Bacterial Leaching
When the ore is sprayed with dilute acid in the presence of air, or digested by certain oxidizing bacteria, the insoluble copper minerals are oxidized and dissolved into the solution as vibrant blue Copper(II) ions.
This step separates the valuable copper from the massive amounts of worthless dirt and rock, creating a concentrated "pregnant leach solution" (PLS).
Figure 1: The two-stage Hydrometallurgical process for Copper extraction.
3. Step 2: Recovery (Displacement) using Scrap Iron
Once the copper is dissolved as $Cu^{2+}$ ions, it must be reduced back into solid, elemental copper ($Cu^0$). We need a reducing agent. According to the electrochemical series, Iron (Fe) is placed above Copper (Cu), meaning Iron is more reactive (more electropositive).
By tossing cheap Scrap Iron into the blue copper solution, a simple redox displacement reaction occurs. The iron dissolves, forcing the copper out of the solution to precipitate as a red-brown solid.
It is purely an economic choice. Iron is significantly cheaper than Copper. By sacrificing cheap iron scraps (like old cars or construction waste), the industry recovers highly valuable copper metal!
4. Alternative: Reduction by Hydrogen Gas
Instead of scrap iron, Hydrogen gas ($H_2$) can also be bubbled through the solution to reduce the copper ions. Hydrogen is also placed above Copper in the reactivity series (Hydrogen's standard reduction potential is $0.00 \text{ V}$, while Copper's is $+0.34 \text{ V}$).
An added benefit of using Hydrogen is that it regenerates acid ($H^+$ ions) as a byproduct, which can be recycled and used to leach the next batch of ore!
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