Zinc Electrolytic Refining from Sulfide Ore
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The fourth most consumed metal produced by electrowinning
Overview
Zinc is the fourth most consumed metal globally, primarily produced by the roast-leach-electrowin (RLE) process from zinc sulfide (sphalerite) ore. The ore is roasted to zinc oxide, leached in sulfuric acid, and the purified zinc sulfate solution is electrolyzed to deposit high-purity zinc on aluminum cathodes. Over 85% of the world's zinc is produced by this hydrometallurgical route. Zinc's primary use is galvanizing steel (60%), followed by zinc alloys, brass, and zinc compounds.
Chemical Process
Zinc sulfide concentrate is roasted in a fluidized bed at 900-950 degrees C, converting ZnS to ZnO and producing SO₂ for sulfuric acid. The calcine is leached in spent electrolyte (H₂SO₄) at 60-80 degrees C. Iron is precipitated as jarosite or goethite. The solution is purified by cementation with zinc dust to remove Cu, Cd, Co, and Ni. Purified ZnSO₄ solution is electrolyzed at 3.3-3.5 V to deposit zinc on aluminum cathodes.
ZnO + H₂SO₄ → ZnSO₄ + H₂O (leaching)
Cathode: Zn²⁺ + 2e⁻ → Zn(s) (zinc deposition)
Anode: H₂O → ½O₂ + 2H⁺ + 2e⁻ (oxygen evolution, regenerates H₂SO₄)
Raw Materials
-
Zinc sulfide concentrate (ZnS, sphalerite) — Flotation of zinc ore (Zinc source (50-60% Zn))
-
Sulfuric acid (H₂SO₄) — Contact process using SO₂ from roasting (Leaching acid (recycled as spent electrolyte))
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Aluminum cathodes — Aluminum smelting (Zinc deposition substrate (peeled after 24-48 hours))
End Products
-
Special high grade (SHG) zinc cathode — Galvanizing, die casting, brass, zinc oxide (99.995% Zn (4N5), LME deliverable grade)
Environmental Impact
Zinc roasting produces SO₂ which is captured as sulfuric acid. Iron precipitation generates large volumes of jarosite or goethite residue containing entrapped impurities. Cadmium recovered during purification is a toxic heavy metal requiring careful management. However, zinc galvanizing extends steel life by 50-100 years, reducing overall metal consumption and environmental impact.
Safety Considerations
- ⚠ SO₂ gas from roasting is toxic — scrubbing and acid plant required
- ⚠ Sulfuric acid is corrosive throughout the process
- ⚠ Cadmium dust from cementation purification is a carcinogen
- ⚠ Electrolysis produces acid mist containing H₂SO₄ and zinc
Recent Innovations
Direct leaching of zinc sulfide concentrates using oxygen pressure leaching eliminates roasting.
Solvent extraction for zinc recovery from complex ores improves selectivity.
Zinc-air batteries for grid energy storage could create new demand.
Hot-dip galvanizing with zinc-aluminum-magnesium (ZAM) alloys provides 3-5x longer corrosion protection.
Production Scale
14000000
tons/year
$45 billion
market value
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