Mining wastewater and mine-water aeration must be selected around water quality, treatment objective and oxygen-transfer requirements. These streams may originate from underground dewatering, open-pit drainage, mineral processing, tailings facilities, waste-rock runoff, equipment washing or domestic services at the mine site.
Some mine waters contain acidity, dissolved iron, manganese, aluminium, sulfate and suspended mineral solids. Others may contain ammonia, nitrate, residual flotation reagents, hydrocarbons or biodegradable organic matter. No single aeration process is suitable for all of these contaminants.
Roots blowers are used where a mine-water treatment system requires a continuous supply of low-pressure air for oxidation, biological treatment, carbon-dioxide stripping or basin mixing. Their positive-displacement characteristic provides predictable airflow against changing water depth and diffuser resistance.
Aeration must be treated as one unit operation within a complete treatment process. It does not independently neutralize acidity, remove dissolved salts or separate precipitated metals from water.
Sources of Mining Wastewater
Water requiring treatment can originate from:
- Underground mine dewatering
- Open-pit sumps
- Acid mine drainage
- Acid rock drainage
- Waste-rock facilities
- Tailings storage areas
- Heap-leach drainage
- Mineral-processing plants
- Coal preparation plants
- Equipment-washing stations
- Contaminated stormwater
- Explosives residues
- Mine camps and sanitary facilities
- Water recovered from filter presses
- Reclaim-water circuits
The treatment objective may be environmental discharge, reuse in mineral processing, dust suppression, wash-water production or protection of downstream equipment.
Before an aeration system is selected, the water should be characterized for:
- pH
- Acidity and alkalinity
- Dissolved oxygen
- Temperature
- Ferrous and ferric iron
- Manganese
- Aluminium
- Sulfate
- Suspended solids
- Conductivity and salinity
- Chemical oxygen demand
- Biochemical oxygen demand
- Ammonia and nitrate
- Dissolved carbon dioxide
- Sulfide
- Residual reagents
- Hydrocarbons
- Potentially volatile contaminants
- Acid Mine Drainage Treatment
Acid mine drainage develops when sulfide minerals are exposed to oxygen and water. The resulting reactions can generate acidity and dissolve metals from the surrounding material.
A conventional active-treatment process may include:
- Flow equalization
- Chemical neutralization
- Aeration or chemical oxidation
- Metal precipitation
- Flocculation
- Sedimentation or clarification
- Sludge dewatering
- Final polishing
Aeration is commonly used to oxidize dissolved ferrous iron to ferric iron. After the pH has been adjusted to an appropriate range, ferric iron hydrolyses and forms insoluble iron hydroxide solids that can be removed by clarification or filtration.
A roots blower can supply the oxidation air through submerged diffusers. It does not perform the neutralization or solid–liquid separation steps.
Ferrous-Iron Oxidation
The simplified oxidation reaction is:
Iron Oxidation Reaction
- Fe2+ = ferrous iron
- Fe3+ = ferric iron
- O2 = dissolved oxygen
- H+ = hydrogen ion
The theoretical oxygen requirement is approximately:
Theoretical Oxygen Mass Required for Iron Oxidation
- O2,required,Fe = theoretical oxygen mass required
- mFe2+ = mass of ferrous iron to be oxidized
The factor 0.143 represents the stoichiometric oxygen requirement per unit mass of ferrous iron. The actual process oxygen requirement is higher because oxygen-transfer efficiency is below 100% and other oxidizable constituents may consume oxygen.
Ferrous-iron oxidation by dissolved oxygen is strongly influenced by pH. The reaction is generally much slower in strongly acidic water. Neutralization and oxidation order must therefore be based on water chemistry, chemical consumption and sludge characteristics.
Iron Precipitation
After oxidation, ferric iron can hydrolyse:
Ferric Hydroxide Precipitation Reaction
- Fe(OH)3(s) = precipitated ferric hydroxide
This reaction releases acidity. Sufficient neutralizing chemical must therefore be supplied not only to raise the initial pH but also to neutralize acidity generated during metal hydrolysis.
The blower supplies oxygen for the oxidation step; it does not eliminate the requirement for alkalinity.
Manganese Oxidation
Dissolved manganese is often more difficult to remove than iron. Its oxidation by air can require a higher pH, longer reaction time or catalytic surfaces.
A simplified reaction is:
Manganese Oxidation Reaction
- O2,required,Mn = theoretical oxygen mass
- mMn2+ = mass of dissolved manganese being oxidized
Actual manganese-removal performance depends on pH, temperature, oxidation-reduction potential, reaction time and the mineral form of the precipitate. Installing a larger blower does not overcome unfavourable chemistry.
Chemical oxidants or catalytic media may be required where air oxidation is too slow.
Carbon-Dioxide Stripping
Mine water can contain dissolved carbon dioxide, which contributes to acidity and chemical demand. Aeration transfers carbon dioxide from the liquid into the gas phase.
Removing dissolved carbon dioxide before adding alkaline chemicals can reduce neutralization-chemical consumption in suitable waters.
The carbon-dioxide mass-transfer rate can be represented as:
Carbon-Dioxide Transfer Rate
- NCO2 = carbon-dioxide transfer rate, mass/time
- KLa,CO2 = volumetric carbon-dioxide mass-transfer coefficient, 1/time
- V = aeration-basin volume
- CCO2 = actual dissolved carbon-dioxide concentration
- CCO2,eq = equilibrium concentration corresponding to the gas phase
The efficiency of stripping depends on bubble size, gas-to-liquid ratio, basin geometry, mixing, temperature and water chemistry.
Aeration may increase the pH of carbon-dioxide-rich water without adding chemicals, but the resulting pH change must not be assumed sufficient for metal precipitation.
Biological Treatment of Mining Wastewater
Some mine-water streams contain biodegradable contaminants, ammonia or nitrogen compounds. Aerobic biological treatment may be used where the wastewater composition supports microbial activity.
roots blowers can supply diffused air to:
- Activated-sludge reactors
- Sequencing batch reactors
- Aerated lagoons
- Moving-bed biofilm reactors
- Membrane bioreactors
- Nitrification systems
- Biological polishing stages
Mining wastewater can be difficult for biological treatment because it may have:
- Low biodegradable-carbon content
- High salinity
- Toxic metals
- Extreme pH
- Variable temperature
- Residual flotation chemicals
- Sudden hydraulic loading
- Nutrient imbalance
A conventional municipal-wastewater design should not be transferred directly to a mine without treatability testing.
Organic-Matter Oxidation
For a simplified carbonaceous process:
Actual Oxygen Requirement (Carbonaceous)
- AORC = actual oxygen requirement for carbonaceous treatment
- Qwater = wastewater flow
- S0 = influent biodegradable-substrate concentration
- Se = effluent biodegradable-substrate concentration
- FC = process factor accounting for biomass synthesis and solids handling
Unit conversion must be included when flow and concentration are not expressed in directly compatible units.
Nitrification
The theoretical oxygen requirement for ammonia oxidation is approximately:
Theoretical Oxygen Demand (Nitrification)
- O2,required,N = theoretical oxygen demand
- mNH4-N = mass of ammonia nitrogen oxidized
Nitrification also consumes alkalinity. Mine water with limited alkalinity may require chemical addition to maintain a suitable biological pH.
Aeration cannot remove nitrate under aerobic conditions. Nitrate removal generally requires an anoxic denitrification stage with an appropriate electron donor.
Glossary
Acid mine drainage: Acidic water formed when sulfide minerals oxidize and generate acidity.
Carbon-dioxide stripping: Removal of dissolved CO2 by contacting water with a gas stream.
Dissolved-oxygen setpoint: Target oxygen concentration maintained in the mine-water treatment basin.
Ferric hydroxide: Solid iron precipitate formed when ferric iron hydrolyzes in water.
Ferrous iron: Dissolved iron in the +2 oxidation state.
Iron oxidation: Reaction converting ferrous iron to ferric iron, often using oxygen.
Oxygen demand: Rate of oxygen supply required by the treatment reaction.
Oxygen-transfer rate: Mass of oxygen transferred from supplied gas into the liquid per unit time.