Many mining and industrial processes generate wastewater that contains a variety of contaminants, such as metals and metalloids. These must be removed to ensure that the wastewater is suitable for reuse or safe discharge to the environment.
However, mining wastewater treatment processes have traditionally been difficult due to the large range of different contaminants present, requiring a number of complex steps.
In current processes, the mining industry generally adds lime to the wastewater to purify it. While often effective, the key issue with this method has been the volume of sludge that forms and the subsequent problems with dealing with this sludge – either to extract the contained water, which often requires additional treatment, or to find enough space for long-term disposal.
This complex practice could end soon, thanks to a new treatment solution that utilises hydrotalcites. Developed by CSIRO, the treatment overcomes the complexities of lime-based methods and offers a simpler and more water smart process.
Dr Grant Douglas, a CSIRO Senior Principal Research Scientist said the team found that hydrotalcites, which are layered minerals consisting of aluminium- and magnesium-rich layers, can simultaneously remove a variety of contaminants in wastewater in a single step.
“We realised that hydrotalcites begin to form when aluminium and magnesium are present at an ideal ratio and under conditions during neutralisation of acidic waters. As hydrotalcites form, the contaminants become trapped and are easily removed from the wastewater as a solid,” he said.
“Mining wastewater often contains substantial magnesium and aluminium concentrations. This means that we can create hydrotalcites utilising common contaminants that are already present in the wastewater, by simply adjusting their concentrations and adding alkaline compounds to rapidly increase the pH level.”
Initial applications have focussed on treating wastewater generated from the mining and extraction of uranium. A range of contaminants including uranium, rare earth elements, transition metals, metalloids and anions (negatively charged molecules such as arsenate) have been effectively removed from wastewater.
“This process purifies the wastewater from mines in a faster, more effective way that does not require large amounts of infrastructure or difficult chemistry to achieve it,” Dr Douglas said.
The advantages don’t stop there. The hydrotalcites themselves are easily removed using centrifugation, leaving behind a much cleaner sludge and a lot less of it. Initial results have shown the treatment produces around 80 to 90 per cent less sludge than that of lime-based treatments – so it does not present the same scale of handling and final disposal problems.
Dr Douglas adds that the hydrotalcite-treated water can be recycled back into the plant to lower the total cost of water used in the mining operations, ultimately helping to reduce water consumption as there will be less water drawn from the environment such as from the groundwater near to the mine. This would be particularly valuable for mining operations in arid regions with limited water supply such as in Australia and Chile.
“Around the world the minerals industry is keen to find more efficient ways to treat their wastewaters and reduce their environmental footprint. With the inherent technical advantages and added benefits of using hydrotalcites, there’s a high likelihood of the mining industry adopting this technology globally,” Dr Douglas said.
The steps to making this a reality have already been made, with commercialisation of the technology underway with Australian company Virtual Curtain Limited.
There is also the potential to use the treatment to reprocess and recover valuable commodities and produce ‘ore-grade’ material out of the contaminants. The material can be fed back into the recovery process to recover a greater proportion of the contained metals.
“This is a very real example of extracting ‘wealth from waste’, opening up the possibility of partially