Ozone (O₃) is a strong oxidant used in mining and metallurgy: pretreatment of refractory gold-bearing ores before cyanidation, heap leaching enhancement via oxygen supply, cyanide destruction in wastewater and tailings, and cleaning and passivation of metal surfaces during storage and processing.
Industrial trials in South Africa, Chile, Canada, and Southeast Asia have demonstrated 85–99% reduction of free and WAD cyanide when using on-site ozone generation. The technology complies with the International Cyanide Management Code (ICMC) and allows reuse of treated water in the process.
Advantages of ozone in mining and metallurgy
- Oxidation of sulfide minerals in refractory ores — release of gold and silver
- Heap leaching enhancement by increasing dissolved oxygen concentration
- Complete cyanide destruction to cyanate, then bicarbonate and nitrogen
- On-site ozone generation from air — no storage or transport of hazardous reagents
- Cleaning and passivation of metal surfaces during storage and processing
- Lower operating costs than hypochlorite at comparable effectiveness
Ozone pretreatment of refractory ores
Refractory gold ores contain gold locked in sulfide minerals (pyrite, arsenopyrite, pyrrhotite), which limits recovery by conventional cyanidation. Ozone pretreatment oxidizes sulfides and improves gold and silver extraction.
| Parameter | Without ozone | With ozone | Change |
|---|---|---|---|
| Gold recovery | 53% | 88% | +35% |
| Silver recovery | 26% | 78% | +52% |
| Leaching time | 40 h | 24 h | −40% |
| Ore type | Double refractory (Mexico) | Direct ozone pretreatment | — |
Cyanide destruction in wastewater
Ozone oxidizes free cyanide (CN⁻) to cyanate (CNO⁻), then with further oxidation and hydrolysis to bicarbonate and nitrogen. Typical reaction time 10–30 minutes. Industrial trials at gold operations in South Africa and Southeast Asia showed >99% reduction of free and WAD cyanide at 2–3 g O₃ per gram CN.
| Parameter | Before treatment | After treatment | O₃ dose |
|---|---|---|---|
| Free cyanide | 50–200 mg/L | < 0.5 mg/L | 2–3 g O₃/g CN |
| WAD cyanide | 100–400 mg/L | < 1 mg/L | 2–3 g O₃/g CN |
| Thiocyanate (SCN⁻) | — | requires 3–4 g O₃/g | Higher than for CN⁻ |
| Contact time | — | 10–30 min | — |
Ozone in the gold mining cycle
Crushing and grinding
Ore preparation for pretreatment or leaching
Ozone pretreatment (refractory ores)
Oxidation of sulfide minerals by ozone before cyanidation to improve gold and silver recovery
Leaching (heap or tank)
Cyanidation; in heap leaching, ozone or ozone ice can serve as an oxygen source to accelerate the process
Cyanide destruction in tailings
Ozonation of wastewater and slurry to destroy free and WAD cyanide to safe levels (ICMC)
Water reuse
Ozone-treated water can be returned to the process, reducing fresh water consumption
Economics and deployment
Capital costs for ozone systems are higher than for some chemical oxidants, but operating costs are often lower (e.g. vs. sodium hypochlorite). The technology does not require tailings stream changes, provides fast and irreversible reaction, and improves operator safety by eliminating handling of hazardous chemicals.
| Parameter | Ozone | Hypochlorite | Hydrogen peroxide |
|---|---|---|---|
| CN reduction (typical) | > 99% | > 99% | 90–98% |
| Hazardous residues | None | Chlorine, chlorates | Minimal |
| On-site generation | Yes (from air) | No | Partial |
| ICMC compliance | Yes | Yes | Yes |
Benefits of ozonation in mining and metallurgy
Higher recovery
Improved gold and silver recovery from refractory ores by 35–52% in studies
Cyanide safety
99% destruction of free and WAD cyanide, compliance with international ICMC standards
Eco-friendly
Ozone decomposes to oxygen, leaves no toxic residues in water or tailings
Metal cleaning
Cleaning and passivation of metal surfaces during storage and processing without aggressive chemistry
Sources
- Evaluation of ozonation technology for gold recovery and cyanide management — J. Southern African Inst. Mining and Metallurgy, 2017
- The Effective Use of Ozone for Cyanide Destruction in Mining and Mineral Processing — Academia.edu
- Ozone application in different industries: A review — ScienceDirect, 2022