It comes as no surprise to anyone in the industry that extracting coal leaves many by products at mine sites, currently classified as waste.
Our research team has found that in some places, coal waste may contain a 'higher than background' level of rare earth elements (REE) and sometimes also lithium – valuable for the energy transition.
The team is currently focusing on research supporting the mining industry to identify and recover critical minerals such as rare earth elements.
These can be found both in-situ, in existing mining operations, as well as in new mining areas.
We are looking at methods to characterise, extract and concentrate critical minerals as part of the wash plant process.
In addition, we are searching for productive uses of other coal waste.
Researchers Dr Jane Hodgkinson, Dr Nerrida Scott and Clint McNally answer some question on their work.
How are rare earths linked to coal waste in Australia?
Our current research focus is looking into waste products of mining to identify whether there is a presence of rare earth elements in tailings and waste streams.
We have also been looking for the original sources of these rare earths.
We hope to determine if the presence of rare earth elements can be predicted in new areas and whether they can be economically extracted in the future.
We also want to understand if they can be stored or quarantined in a concentrated stockpile within an operating mine for later extraction.
The team is looking at one mine in detail at this stage.
However, we hope to expand this research to investigate whether there are parallel findings within the same sedimentary layers throughout other areas of the coal basin.
How much coal waste isn't 'just waste', and is it worth going to all this effort?
We think we may need to look broader than just the coal itself in terms of a valuable resource.
There is potential for useful minerals to be found in the interburden layers of a coal deposit.
Some of these minerals may end up in the tailings, particularly if they are highly dense minerals.
Other minerals may end up in waste streams and not go through the processing plant at all.
If we can isolate the sources early in the mining process, we can potentially pre-concentrate the rare earths and critical minerals in a stockpile and avoid them ending up in the overburden or tailings altogether.
Ultimately it comes down to economics.
Do the dollars stack up to recover the minerals from the tailings and overburden or not? If we pre-concentrate, can we then economically recover them?
What concentrations of rare earth elements are we talking about?
We are aiming to identify where levels of rare earth elements are more than 2.5 times the naturally occurring elemental levels.
Where we see a spike, we know to look closer.
The ability to use those rare earths depends on the ease of extraction and the relative difficulty of separation.
At this stage, we cannot put a figure on how much is usable – it is too early in the research.
Every deposit has its own characteristics, and we can’t assume that the characteristics of one deposit, will apply across a whole coal basin.
How much is usable for other recycling?
From a mining perspective, if we can’t isolate the rare earth sources before mining has fully progressed, the task of recovery becomes bigger.
This is because we may have to get through a large amount of waste to find the minerals that we want to recover.
From a tailings perspective, this is an interesting question because, in the past, we may have viewed the material in a tailings dam as waste because it wasn’t the material (saleable coal) that we wanted to recover.
Looking for critical minerals in tailings opens a different perspective.
Instead of being a waste stream, it now becomes a partially concentrated resource, which has had the bulk of the coal removed.
We hope to find that the concentrations of critical minerals are high enough to enable efficient recovery of the new minerals that we want to extract from the tailings.
Even if we do identify higher concentrations of critical minerals in a tailings resource, it’s still not easy to recover the minerals that we want without reprocessing the entire contents of the tailings dam.
However, it may be viable to recover multiple high-value minerals, including magnetite and saleable fine coal.
With the global push for more stringent tailings standards, there is an increased focus on what else we can extract from our tailings, and how we can reduce the need for tailings storage long term.
Is this team the only team working on a project like this?
Part of our work has been funded by Australian Coal Industry's Research Program (ACARP), but we are certainly not the only ones looking into this.
In fact, a 2017 US Congress report flagged a pathway towards economic recovery of rare earth elements from coal and coal by-products!
A range of activities are taking place to find the best options for extracting rare earth elements, including research into physical separation techniques like roasting, leaching, bioleaching and biosorption.
Does Bowen Basin coal have a particularly high level of harvestable rare earth elements, compared to other major global coal resources?
We don't definitively know the answer to this question yet.
Which is why we are doing the research – to find out what's out there and identify where critical minerals reside in the existing mining resource.
We hope to expand our dataset by mapping credible rare earth element beds in various coal measures through the Bowen Basin.
This will provide some guidance on where more investigation should take place.
What spin-off potential uses are there for coal waste, with or without rare earth elements?
Aside from reclaiming coal mining areas for agriculture, which is already successful, coal wastes could potentially be used to recover other minerals, such as clay-based minerals, in the future.
Anecdotal research into making road base materials from coal waste has also been done.
Other construction materials may be possible depending on the waste content, so characterisation is critical.
Part of the challenge is seeing beyond our current mining needs to identify what we may need into the future – and that's a challenge we can all be a part of solving.
This article was originally submitted and published in the Bowen Basin Yearbook 2023 (page 94).