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The advantages of ozone treatment applied to mining industries, especially to the gold mining industry.
Ozone can be used for successfully transforming toxic cyanide wastes into harmless substances. Hereby, ozone mitigates the adverse environmental effects associated with cyanide gold extraction processes.
The cyanide treatment with ozone is resulting in clean offsite effluents, permissible for indirect or even direct discharge from the gold-ore processing.
TES ozone treatment solutions provide state of the art oxidation processes for the mining industry.
Cyanide removal of wastewater/process water by oxidation.
Cyanide regeneration processes and pre-ozonization for process optimization and reduction of cyanide consumption.
Gold mining, depending on the exploitation methodology, can be a tedious, labor and cost intensive process. This is especially true for mines with low-grade deposits. Some ores contain as little as 0.001% gold. In such scenarios the chemical extraction processes (i.e. cyanide leaching) are one of the most efficient and economical extraction options. In general cyanide leaching (hydrometallurgical extraction) is now the dominant method used in the gold mining industry.
Annually ~2500 tons of gold are extracted from mines worldwide; 90% of which are extracted by cyanide leaching techniques.
The dominance of the cyanide leaching process is due to its efficiency compared to alternative processes that utilize complexing agents such as: chloride, bromide, thiourea and thiosulfate. All these alternatives form less stable complexes and thus require more aggressive conditions and oxidants to dissolve the gold. In contrast
the cyanide leaching exhibits much more efficient extraction rates (85% – 95%). In comparison the conventional complexing agents exhibit only extraction efficiencies between 40% – 50%. The cyanide process uses varying cyanide concentrations depending on the local mineralogy. Typically concentrations range from 300 to 500 mg/l.
Today there are commonly two predominant leaching techniques employed for gold extraction:
• VAT leaching
• Flooded heap leaching
VAT leaching utilizes reaction tanks. The excavated and ground up ore is collected and the cyanidation reaction is performed inside the vats. Further on the gold is separated from the ore resulting gold/cyanide mixture.
Flooded heap leaching process utilizes stockpiles of ground up ore. This method is more commonly in use than VAT leaching. The excavated ore is heaped onto, so called, leach pads.
Please note, despite its affinity binding to metals (i.e. gold, silver, etc.) cyanide is a highly toxic compound. Cyanide is normally stored and transported in solid form and is stable when dry (i.e. sodium cyanide). Many cyanide solids readily dissolve in water with a risk of even releasing actual toxic cyanide gas (HCN) when reacting with other compounds in the water matrix and depending on various parameters of the water body (i.e. pH, temperature, etc.). Dissolved cyanide solids are relatively stable in the environment until they are oxidized. Due to its toxicity the uncontrolled release to the environment must be prevented. This is very important as small amounts of cyanide can already harm or even kill a person (one teaspoon full of a 2% cyanide solution is already deadly).
Environmental research has reported that low concentrations as five parts per billion in surface waters can already inhibit fish reproduction.
The World Health Organization (WHO) has set threshold values for hydrogen cyanide (0.07 mg/l). However on worldwide perspective there is a large variety of national threshold limits for cyanide. These range from 0.1 mg/l in Argentina, South Africa and Germany, 1.5 mg/l in Mexico to limit up to 50 mg/l in Australia
Ozone Treatment of Gold Mining Discharges
As highlighted above, in order to protect wildlife, nature and people it is absolutely essential to treat cyanide containing effluents prior to discharge.
The oxidation and detoxification of cyanide containing waste waters can be achieved by introducing a variety of different treatment agents including chlorine gas, sodium hypochlorite.
The main disadvantages of the detoxification agents listed above are high operating costs, the associated bulk handling, and storage of highly toxic and dangerouschemicals on-site. This in itself poses an additional environmental risk alone that needs to be managed at high costs.
An alternative and also environmental safe oxidation agent is ozone, being one of nature’s most powerful oxidants. Ozone is easy to produce on-site and on-demand. Ozone can be easily applied to any industrial waters, waste waters and air treatment processes. During the treatment process ozone itself disintegrates and does not generate harmful byproducts. It just turns back to oxygen. Therefore ozone provides not only an environmentally friendly, but also a cost effective alternative to halogenated oxidants (i.e. chlorine, sodium hypochlorite), absorption techniques (i.e. activated carbon) or separation processes (i.e. reverse osmosis).
Chemistry of the Ozone Oxidation Process of Cyanide containing Waste Waters
The cyanide oxidation process can be described by two main chemical reaction pathways as highlighted below:
The first step is the oxidation of cyanide to cyanate (see reaction 1).
CNˉ + O3 → CNOˉ + O2 (1)
The second reaction (2) shows the reaction, where cyanate is hydrolyzed and oxidized in the presence of excess ozone to bicarbonate, nitrogen and oxygen:
2CNOˉ + 3O3 + H2O→ 2HCO3ˉ + N2 +3O2 (2)
The total reaction time for the entire oxidation process of cyanide ranges typically between 10 to 30 minutes.
Ozone Applications in Gold Extraction Process Optimization
Additionally, ozone can be used to enhance and improve the gold extraction yields by improving the exploiting refractory ores and cyanide regeneration.
The so-called “refractory” ores are naturally resistant to the cyanidation process due to strong cyanide consumers within the ore. Therefore the yield of the gold extraction
process can be impacted being less than 50% in some cases. In order to overcome this problem a preoxidation step is used to eliminate these cyanide consumers in the refractory ores.
At present, technical processes such as roasting, chemical oxidation, high- or low pressure oxidation or bacterial oxidation are used.
However, these techniques are disadvantageous due to very high capital costs, environmental problems and the need of highly trained personnel.
Ozone pretreatment of the ores can overcome these problems highlighted above by eliminating these cyanide consuming compounds. Pilot trials have demonstrated that the pre-oxidation of refractory ores is able to increase the gold extraction efficiency to over 85% from initially 53%.
Furthermore, research in this field has shown that the ozone pre-oxidation processes can increase gold recovery rates by more than 85%. Another advantage of ozone pre-treatment is the oxidation of sulphuric components within the ore. This in result prevents the formation of unwanted thiocyanates (SCNˉ) during the gold extraction procedure. In consequence the cyanide consumption is greatly reduced in the cyanidation process.
Ozone Oxidation for the Regeneration of Cyanide
During the cyanidation process the cyanide can react with several other scavenging constituents of the ore. This leads to increased cyanide consumption due to a fraction of the cyanide that is lost reacting with these scavenging compounds instead of reacting with the gold in the ore. This is resulting in a less efficient extraction process with negative economical impact.
One of these scavenging compounds is sulphur, being a strong cyanide consumer.
The sulfur content of the ores can be very high in certain regions. The reaction of sulfur and cyanide forms thiocyanate, following reaction (3).
S0 + CNˉ → SCNˉ (3)
Furthermore thiocyanate is also posing an environmental threat. Thiocyanates are also toxic and can harm aquatic species when released. Common waste water treatment systems in the mining industry might not be able to reduce thiocyanates sufficiently. Additional detoxification steps would be required for its removal. Ozone canovercome these negative impacts by oxidation of thiocyanate and the regeneration of cyanide. Research has shown that cyanide regeneration can reach up to 75% after short reaction time of approximately 20 minutes, see reaction (4).
SCNˉ +O3(g) + H2O → SO2 4 ˉ + HCN + H+ (4)
The ozone treatment and process optimization methods for the gold mining industry discussed here clearly demonstrate the advantages and effectiveness of ozone. Ozone is a very effective solution achieving detoxification of cyanide containing waste water and furthermore resulting in a significant reduction (up to 99%) of the general cyanide consumption in the gold mining/extraction process. Additionally ozone treatment offers further process benefits as highlighted below:
Key advantages of cyanide oxidation using ozone
- Ozone is produced on-site from oxygen feed gas or even ambient air. Many gold mines already have oxygen supply.
- Ozone is very reactive, so the reaction in the treatment process is very fast and does not require special conditions such as high temperatures or pressures.
- Extremely effective against most cyanides species (free and WAD complexed forms).
- Ozone does not form any undesirable by-products such as chlorinated organics or ammonia, which are generated by chlorine oxidation.
- Ozone is always generated on-site on a demand basis (on & off).
- The ozone treatment process does not require frequent purchases and delivery of large amounts of chemicals. Additionally there is no requirement for storage and handling of dangerous chemicals on-site (i.e. site safety protocols).
- Ozone can be used for increasing the gold extraction yields, when dealing with refractory ores and sulfur impurities.