From October 15 - 17 in Mikkeli, we attended the workshop "Mine Water Management" guided by
Prof. Dr. habil. Christian Wolkersdorfer, the supervisor of Phuong, Daniela and Elham. We often call him just Chris. Chris is a world leader in mine water remediation and management, he has conducted and initiated several projects related to mine water and hydrogeology in Canada, Germany, Austria, Slovenia, Brazil, the United Kingdom, South Africa, Finland and Turkey. Originally from Germany, currently, Prof. Christian Wolkersdorfer holds two research chairs at Tshwane University of Technology in Pretoria, South Africa and as Finnish Distinguished Professor for Mine Water Management at Lappeenranta University of Technology in Mikkeli.
We were around 20 people, mostly from Finland or studying in Finnish universities, beside, we also had participants from UK, Germany, Kenya and Australia. The workshop introduced general mine water issues and treatment methods for contaminated mine water.
Four things that ruin a mine: War, diseases, inflation and listlessness. (Ex: wars in Ukraine, Yugoslavia; disease such as AIDS)
Sample mine rock with pyrite (fool's gold)
Olga and the rock
Walter Moers' "Mine Troll" [a funny character who often tells lie and eats only raspberry ;)]
Acidity - Alkalinity
Filtration methods
Membrane processes
Osmosis/Reverse Osmosis
Group photo on the last day
Some casual notes from the workshop:
Mine types:
- Deep mine, underground mine (gold, iron, graphite, baryte)
- Open cast mine, surface mine (iron, copper, uranium, gold, hard coal, soft coal)
- Quarries (granite, basalte, sand, limestone)
- Hydromining
More than 80% is open pit mining.
World's largest copper mine in Chile (2000 km of working length).
World's largest hard coal pit mine in UK.
World's deepest hard coal mine in Germany (Ruhr Area).
Many explosions of mines in China, because of no ventilation, methane is not collected.
Thousands of illegal mines in South Africa. People still use mercury for gold mining. Life expectancy of miners is only 40-50 years.
- It is poison!
- So what? If I don't eat, I'll die tomorrow.
Reminding us of the movie "Blood Diamond".
Pump room in mines is very noisy.
After mining ceases, the mine working areas are usually flooded. To predict or calculate mine flooding, it is necessary to understand the hydrogeological situation on-site. There are controlled flooding (with monitoring system, controlled raise of mine water table, active/passive flooding) for passive flooding, just turn off the pumps) and uncontrolled flooding (no geotechnical monitoring system, no chemical control, when mine budged is unclear and no risk for people or buildings, or during war times or crisis).
Chaos theory. More than 3 differentiate equations are needed to describe a chaotic system. (Weather forecast is not accurate over 2 weeks). Examples of chaotic systems: turbulent flow, car traffic.
Mine Water Geochemistry
Professor suggested not to use the term "heavy metal" but just metal instead, since there is no unique definition.
Pyrite: yellow
Ferrous, green
Ferric, orange red colour pH 2-3.
Once we have pyrite and water, immediately the weathering occurs, very fast process. Bacterial catalyse can increase the reaction speed upto million fold.
The highest pH measured in nature is 12.
The lowest pH measured is - 3.6 (minus).
Depending on the pH value, different metals coexist ('species'). pH value controls the release of contaminants ("master variable").
The weathering of minerals (except di-sulphides such as pyrite) produces alkalinity and, therefore, buffers the acid. Disulphides are abundant in nearly all rocks as trace minerals. Pyrite weathers more rapidly than silicates and therefore causes acid mine water. Small amounts of di-sulphide cause can cause severe problems due to different weathering kinetics of the minerals.
A neutral pH does not mean anything about the contamination of water (it just means there is the buffer).
(Coke, pH 3; blood, pH ~ 7.45)
Limestone pH 7.45 - 8.45: --> metal immobilized.
Types of mine water:
- acid mine drainage (pH < ~ 6)
- neutral mine drainage (pH > ~ 6)
- saline mine drainage (> ~ 1000 mg/L)
Factors affecting mobility and bioavailability:
- speciation: hydrolyses, complexation, solubility effects
- redox transformations
- sorption (adsorption/absorption), especially onto iron hydroxide mineral; silt, clay, wood, pore space.
Sources of contamination:
- Acidity: pyrite ("di-sulphide") weathering
- Metal ions: sulphide weathering
- Chemical reactants (ore processing)
- Organic substances (ex. timber impregnation)
- Tailings
- Waste rock stockpiles
- Ore stockpiles
- Heap leach material
- Pit walls
- Underground workings
- Processing wastes
Pathways of contamination:
- Alkalinity comes from calcite, aluminosilicate weathering
- Precipitation, sorption of metal ions
- Ochre precipitation
- Infiltration through mine waste
- Infiltration through soil/vadose zone
- Movement of mine waters
- Uptake by biota
- Runoff
- Groundwater, surface water
- Air
Targets of contamination: surface water, groundwater, sediment, air, soil.
Acidity is defined as 'base capacity', and alkalinity is 'acid capacity'.
Acidity of mine water is due to the mixing of infiltration waters that are in contact with pyrite and produce acidity; or in contact with carbonates or silicates and produce alkalinity. Acidic waters have pH values < 5.6; alkaline waters have pH values > 5.6 (boundary is due to the end point of carbon acid titration, use of buffer capacity). Acidic waters mobilize metal ions in a greater extend than alkaline ones. Neutralisation of acidity also demobilizes metal loads (attenuation of metal contamination: natural attenuation).
Microorganisims speed up chemical reactions, but they never enable reactions that are thermodynamical impossible.
INAP (The International Network for Acid Prevention) - Global Acid Rock Drainage Guide (GARD GUIDE)
Prediction
of mine flooding
: Black box modelling (regression).
Weathering kinetics:
- Calcite weathering is more than 1000 times faster than pyrite weathering. Pyrite weathering is 100-1000 times faster than weathering of silicates. Silicates are more abundant than sulphides, which are usually trace minerals, even in many ore deposits.
- The weathering rates have decisive effects on the development of the mine water or the tailings drainage water. At the beginning, the fast carbonate weathering dominates and the mine water is well buffered (alkalinity production). The pH decreases as soon as all the carbonates are weathered. At a later stage, after all of the pyrite has been weathered, the pH can increase again (buffer capacity of silicates).
Mine Water Treatment
To develop the most advantageous treatment strategy, the temporal, spatial and chemical development of mine flooding have to be understood. Based on that data, a conceptual model and a treatment option can be planned.
Nature usually tries to help itself.
Treatment technology categories:
- Neutralisation: lime based, sodium based, ammonia, biological sulphate reduction, constructed wetland.
- Metals removal: precipitation (hydroxides, carbonates, sulphates), constructed wetlands.
- Desalination: biological sulphate removal, membrane processes, ion exchange, constructed wetlands.
- Special treatment options: cyanide removal, radioactive compounds, Arsenic removal, electrocoagulation.
Active treatment methods are neutralization, ion exchange, reverse osmosis, nanofiltration, electrodialysis, solvent extraction, freeze separation, electrocoagulation, distillation.
Passive treatment system is a water treatment system that utilises naturally available energy sources (topographical gradient, microbial metabolic energy, photosynthesis and chemical energy) and requires regular but infrequent maintenance to operate successfully over its design life. Examples of passive treatment methods are aerobic/anaerobic constructed wetlands, anoxic limestone drains, SAPS - Successive Alkalinity Producing Systems (RAPS), reactive barriers, and vertical flow reactors, settlement lagoons.
Basically, every mine water can be treated to drinking water standards unless costs are of no consideration. Highly mineralised and aggressive water could be treated by the use of reverse osmosis, nanofiltration or distillation. All these methods consume a large amount of energy and, therefore, are extremely expensive.
In principle, passive treatment systems are low cost and labour system. However, highly mineralised mine water cannot be treated reliably down to a given standard. Some metal cannot be removed by passive systems (ex. high Zn amounts in neutral mine waters). To treat highly mineralised mine water (many metals, low pH, high water make) by using passive methods, a huge, and expensive area is needed.
To choose the appropriate treatment technology, the following points have to be considered:
- existing, available processes
- cost-benefit analyses of alternative methods (keep in mind: passive methods can be expensive in the first place, but maintenance is usually of lower cost whereas in active systems it is often vice-versa).
- possible changes of mine water quality (longevity of mine water pollution).
In many cases, the following approach proves to be useful:
- Treat the mine water actively until all the acidity has been depleted (as a rule: during the time of the first flush, sometimes up to 40 years).
- Thereafter, for the long term treatment of the mine water, install passive treatment systems (buffering of juvenile acidity).
Advantages of this step-wise approach:
- Design and construction of active and passive systems is done in different stages.
- The required water limits can be fulfilled at all time.
- Passive systems can be constructed before they are needed and, therefore, have enough time to mature before their first use.
