Two of the most environmentally problematic industries – mining and concrete – are moving closer to a tantalizing solution to use mining waste, or tailings, as a raw material in producing concrete.
Deadly tailings pond a concern worldwide
Tailings – the term for the waste produced from mineral processing – are a global problem, and a potentially deadly form of waste for communities around mining sites. Though there is no definitive count for the number of existing tailings ponds, recent estimates put it at approximately 8500. In Canada, mining operations produce some 950,000 tonnes of tailings alone.
Though the mining industry has recently made significant strides in managing tailings pond risks, with the establishment of the Global Industry Standard on Tailings Management in 2020, tailings storage facilities (TSFs) remain a danger to thousands of communities, with over 200 tailings dam failures occurring since 1917.
Tailings storage facilities are often in the form of ponds, as the water reduces the reactivity of components in the tailings sludge. However, storing tailings materials this way comes with a fair amount of peril, as earthquakes and severe weather can damage or destroy tailings storage facilities. These conditions have has led to flooding and environmental contamination.
There has been a movement towards different, less risk-laden methods of storing tailing, in particular a process called dry stacking. In this process, mine waste goes through a de-watering process, and is then packed and stacked.
Storing dry stacking is much more secure than holding tailings in a pond, though it comes at a high cost, can generate problematic dust, and does not fare well in wet weather.
Cement production is carbon-intensive
The production of cement, a component in concrete, emits an enormous amount of carbon; according to the National Resource Defense Council, while cement only accounts for 10-15% of the ingredients in concrete, it accounts for up to 90% of its total emissions. All told, cement accounts for 7% of global emissions.
Cement requires burning limestone at very high temperatures, which when using fossil fuels emits enormous amounts of carbon, into clinker. The clinker is then mixed with gypsum to generate cement.
Repurposing tailings to make cement?
Scientists have been working on the idea of reusing mine tailings to make cement, and lower cement’s carbon footprint in the process.
Authors Yipu Guo, Fulin Qu, and Wengui Li outlined the findings from dozens of papers seeking to connect tailings to concrete feed stock, finding that there could be pathways to using the mining waste to lower the carbon footprint of carbon, they wrote in npj Materials Sustainability earlier this month.
The literature clearly finds that tailings can be used for the raw clinker materials.
Moreover, scientists have found that iron tailings – depending on its chemical compositions and trace elements in the sample – can be used as a partial substitution of the clay used in cement. Because iron tailings have a lower heat threshold, which allows cement manufacturers to use less energy.
Additionally, iron tailings that contain a low amount of silicon dioxide can produce special cement, which are used for distinct applications.
Another benefit for the use of iron tailings is that they do not contain the volatile organic compounds that are usually found in other solvents used to make cement when other forms of pigmenting are used. This means that cement with iron tailings in pigment is more stable, and will not set off chemical or other reactions when the cement is used.
Materials extracted from tailings can also be used in the manufacture of lightweight cement aggregates. These aggregates make the cement tougher, as well as provide better fire resistance. This cement form also has thermal insulation and sound regulation.
There are many benefits that arise from using tailings materials as part of the cement-forming process.
Top of mind for cement producers will be the cost. Using tailings materials as 20% of the cement binder materials has achieved cost efficiency of 11% for cement, and 9.5% for lime mortar, the study found.
On the social front, communities living around tailing storage facilities are safer for the tailings materials to be used for other industrial processes. Fewer tailings in TSFs lessen the risk of failure, and also lessen the impact should one of these ponds fail.
Challenges that must be addressed before using tailings in cement
There are already cement companies that have used this process to lower costs and emissions. China’s Liaoning Shanshui Gongyuan Cement Co LTD is using iron tailings as part of their raw materials in their dry-process clinker production.
Australia’s University of Queensland has also teamed up with Australian miner Newcrest to build the HydroFloat flotation cells technology, which produces tailings sand. This sand reduces energy costs during milling, as well as increases the recovery rate of gold and copper in mine processing.
As enticing as it may be to use tailings in cement and lower cement’s problematically high amount of carbon emissions, there are several issues that need more research before more widescale use is possible.
Separation technologies of tailings materials needs significant improvement, the authors noted. There are several promising lines of inquiry currently underway, which involve either electrical pulse, microwave, or ultrasound methods.
There also needs to be more research on ensuring that tailings materials are more stable and less reactive to other materials. Studies have found that while tailings can be used in concrete, tailings materials are still more reactive than the non-tailings materials that concrete companies have been using.
Any attempt to find industrial uses for tailings is a good one – and even better if it can help clean up another industry long criticized for its carbon imprint.
