Mine waters are often contaminated with toxic metals and acids, as discussed previously in this series: Part 1 - Mine Water Sources, and Part 2 - Contaminants.
Thankfully, all mine waters and effluents are treatable with the wide range of water treatment technologies available today. However selecting the correct combination of technologies – and using them in correct sequence – is critical when designing a water treatment system.
Mistakes in technology selection are common, and can make a water treatment plant inefficient and difficult to operate; often causing the following issues:
Fouling or scaling of equipment
Reduction in treated water quality
Low treated water recoveries
Increased brine volumes
Increased operating costs
Extensive sludge production
Treating contaminated pit water at a copper mine in Queensland. Source: Wikimedia Commons
Lime precipitation is a cornerstone technology of mine water treatment – due to its ability to treat a wide range of contaminants typically associated with acid mine drainage: acidity, sulphate and metals.
The process uses limestone (calcium carbonate) or hydrated lime (calcium hydroxide) reagents to neutralise the water’s pH and precipitate 70–80 per cent of contaminants into a solid sludge.
The water/sludge mixture is then passed through a clarifier that extracts the solid materials as a low-volume waste, which can be combined with tailings or disposed in landfill.
Lime precipitation is an efficient and robust mine water treatment technology. However, treated water quality is limited by contaminant solubility (highly soluble contaminants are unable to precipitate).
Lime precipitation plant at the historical Mount Morgan Copper Mine in Queensland. Source: State of Queensland
Thus lime precipitation often leaves 20-30 per cent of contaminants in the water (mostly hardness, sulphate, sodium and chloride) and requires additional treatment technologies, such as ion exchange and reverse osmosis, to remove the remaining contaminants.
Ion exchange comprises a set of vessels containing chemically activated resins, which adsorb specific elements from process streams and effluents. Many different elements can be targeted, depending on the type of resins used.
Ion exchange is useful in many areas of mining due to its ability to extract valuable elements – such as gold, silver, cobalt and uranium.
In mine water treatment, ion exchange allows manipulation of water chemistry by removing specific contaminants – such as hardness, metals and sulphate – that typically cause equipment scaling downstream.
The highly targeted treatment of ion exchange can be contrasted with reverse osmosis (RO), which instead provides a blanket removal of nearly all elements.
Reverse osmosis (RO) membranes being extracted from their vessels. Source: Wikimedia Commons
Reverse osmosis is a useful water treatment technology that uses semi-permeable membranes as molecular barriers, removing contaminants based on molecule size and charge.
RO produces very high-quality treated water (TDS < 100 ppm) and concentrates the contaminants into a brine stream, which either requires further treatment or disposal.
Many industries (including the mining industry) successfully use RO, however the membranes are prone to scaling and thus become particularly effective once scaling elements (hardness, metals, sulphates) are removed.
Below is an example of a mine water treatment flowsheet, which highlights the position of each technology and the contaminants they typically treat:
Mine water treatment technologies are combined to produce high quality treated water.
The flowsheet shows contaminants are removed at each step, with the final treated water suitable for recycling at the mine, or saleable to local farmers.
The contaminants are transferred into sludges and brines, which require handling and treatment in their own right.
Evaporation is one such process for handling brines. Evaporation technologies are high-cost and usually only applied once a stream's volume is reduced.
There are two main types of evaporation processes:
Heated Evaporation/Crystallisation – uses electricity, steam or gas to raise the brine's temperature to boiling, and steam is removed. The contaminants do not evaporate and instead precipitate into solids once solubility limits are reached.
Passive Evaporation Ponds – these are large, purpose-built shallow ponds that use the sun and atmosphere to evaporate water. They require a large amount of land and are particularly effective in regions with low rainfall and hot temperatures.
Salt evaporation ponds in California, USA. Source: Wikimedia Commons
Applying the correct combination of water treatment technologies allows for efficient and effective mine water treatment; and should be a major focus for all mines with water issues.
Part 3 – Mine water treatment technologies
Part 4 – Acid mine drainage