Mine waters and effluents contain a wide array of contaminants, depending on the source of the water and the composition of surrounding earth and rocks.
A site’s mine water is analogous to a fingerprint, with factors such as ore type, groundwater composition, mineral refining and rainfall all contributing to the composition of the water.
Mine water at “The Big Hole”: Historical Diamond Mine in South Africa. Source: Wikimedia Commons
As such, no two mine waters are identical – and a tailored water treatment solution is required for each site.
While some mining effluents are highly contaminated with toxic metals and acidity, others can remain relatively clean and only require minimal treatment.
Professional water laboratories are used to analyse the contaminants within mine waters and effluents.
The following table shows a list of typical mine water contaminants:
Sulphates and acidity are common mine water contaminants, detected in waters that have contacted earth and rocks containing sulphide minerals.
These sulphide minerals dissolve into the water and form acid mine drainage (AMD); a particularly corrosive type of mine water causing issues around the world.
Acid Mine Drainage in the Río Tinto River in Spain. Source: Wikimedia Commons
With most mines located in areas where metal-grades are high (in the ores), it is inevitable that some of these metals dissolve into the water, particularly under acidic conditions (such as AMD).
As such, heavy metals (copper, iron, lead, zinc, etc.) and metalloids (elements containing both metal and non-metal properties) are commonly found in mine waters and effluents.
Sulphates, acidity and metals are removed in bulk from mine waters using a precipitation process call lime neutralisation.
Treatment processes are discussed further in Part 3 of this series.
Hardness (calcium, magnesium) and alkalinity (carbonates, bicarbonates) are typically detected in mine waters that have contacted earth and rocks containing limestone or dolomite.
These contaminants are relatively non-toxic, but form a tough scale within pipes, tanks and other process equipment.
These limestone and dolomite rocks cause hardness and alkalinity to dissolve in water. Source: Wikimedia Commons
The scale is typically a calcium carbonate compound which is a nuisance to operators and costly to remove.
Monovalent salts (sodium, potassium, chloride) are found at various concentrations in all waters and are generally non-toxic, unless the concentration is high.
Some ore refining processes use monovalent-based reagents – such as hydrochloric acid, caustic or soda ash - which contribute to elevated monovalent concentrations in mining effluents.
Nitrates in mine waters are typically due to residual ammonium nitrate – a key component of the explosives used in the mining industry.
The Australian National Health and Medical Research Council (NHMRC) states that concentrations up to 100 mg/L of nitrate in drinking water is safe for adults, with most mine waters falling below this guideline.
Despite this, many mines are still required to remove nitrate from their water prior to discharge, in order to match the quality of local creeks and rivers.
Suspended solids are undissolved macro-particles, often responsible for making mine water look dirty or murky.
Murky water due to suspended solids. Source: Wikimedia Commons
These solid elements are usually the easiest to remove because they remain undissolved and so can be settled, or filtered out.
Dirt, silt, chemical precipitations and bio-matter are common contributors to the suspended solids of mine waters and effluents.
Part 2 – Typical contaminants in mine water
Part 4 – Acid mine drainage