Associate Professor Dr Liza Forbes from the Julius Kruttschnitt Mineral Research Centre at The University of Queensland’s Sustainable Minerals Institute has taken on one of the big challenges in mineral processing – separating pyrite, or ‘fool’s gold’, from copper-bearing minerals to recover most of the base metal and reduce the amount lost to tailings.
Mineral processing is the means by which valuable metals are extracted from mined ore.
“Pyrite is the world’s most ubiquitous base metal sulphide mineral and is one of the most persistently problematic components of base metal processing operations, including copper, lead and zinc,” Dr Forbes says.
The processing of pyrite has major economic implications as it greatly affects base metal recoveries and concentrate quality, which then presents challenges for smelting operations.
“The earlier we can remove pyrite, the more we can mitigate its harmful effects on base metal recovery and the overall environmental impact of the process.”
In 2020, Dr Forbes received a $300,000 Advance Queensland Industry Research Fellowship to produce and test possible solutions.
What she has come up with to date has produced some good insight into how the problem might be solved, but there is still more work to be done.
She has been working with one of the world’s largest resource companies – Glencore – at their Mount Isa Copper Operation.
One of the major stages for recovering copper from mined ore is called mineral flotation.
In this process, minerals are mixed with water and placed in vessels called flotation cells. The mineral/water mixture is then treated with chemical reagents that modify the surface properties of certain minerals in a way that makes them attach to air bubbles. The mineral-laden bubbles then “float” to the surface of the cell, where they are collected for further processing.
The nature of pyrite is such that it readily attaches to the surface of air bubbles, which creates competition between pyrite and valuable copper minerals for available bubble surfaces.
This results in large amounts of copper being lost to tailings – the uneconomic materials left over after processing.
“If we get the pyrite out early, it becomes a valuable resource used to produce industrial sulphuric acid,” Dr Forbes said.
Sulphuric acid is used in the manufacture of fertilisers, pigments, dyes, drugs, explosives, and detergents as well in petroleum refining.
“Pyrite can also contain other valuable metals – specifically cobalt in the case of this study. Therefore, recovering pyrite early creates a possibility of recovering additional value from the ore.”
Cobalt is seen as one of the key commodities of the green economy. In 2021, cobalt-containing batteries accounted for three-quarters of the global electric vehicle market.
The other issue is that as high-grade ore deposits worldwide are being depleted, there is increasing demand to process low-grade ores, often rich in pyrite, to extract the valuable metals they contain.
According to the Australia Government’s Department of Industry, Science, Energy and Resources, the global demand for copper is expected to rise to over 30 per cent by 2030, driven by renewables, construction, energy and transport.
Dr Forbes says that initially the project team thought that they would be able to rely on a chemical reagent treatment pathway to improve the separation of pyrite and the valuable base metal.
“This was shown to be an untenable strategy for the Mount Isa ore, because of its inherent geochemistry.”
Like all good scientists, Dr Forbes didn’t let that defeat her. She and her team learned and moved on.
Their focus turned to the texture of pyrite.
The texture of a rock is the size, shape and arrangement of the grains or crystals in the rock. Pyrite can have a variety of textures depending on the conditions under which it was formed.
“The main benefit of the work was not the proposed treatment strategy, but rather the increased understanding of what makes pyrite behave the way it does – where pyrite is characterised as a function of its texture,” Dr Forbes said.
She says the next step is extending this study to other ore bodies, and other commodities.
“We are also working on creating automated pyrite texture identification and classification systems, that will allow plant personnel to better identify and predict problematic pyrite behaviour in their ores.”
Senior Glencore Zinc Project Metallurgist Roxanne O’Donnell said for her the most intriguing part of the work has been the identification of pyrite textures being linked to flotation performance.
“This is cutting edge research,” Ms O’Donnell said.
“The research the Julius Kruttschnitt Mineral Research Centre are doing will assist in providing a better understanding of the impacts of pyrite textures and how they can be identified.
“This will have impacts on mining operations worldwide and hopefully spur the creation of mineral texture identification tools which will assist in understanding the complicated mineralogy of the feeds that impact on flotation performance. Until you know what potential problems are in your feed, you can’t create treatment strategies to combat them.”
Senior Glencore Zinc Geometallurgist Catherine Curtis-Morar said she found it exciting that different types of pyrite have a different metallurgical response and how this can be linked back to the ore genesis.
“Deposit studies often use pyrite type and trace element geochemistry to fingerprint orebodies. The fact that similar variance can affect the metallurgical response is fascinating. This link between pyrite formation and metallurgy could be very useful for future geometallurgical domain modelling,” Ms Curtis-Morar said.
Geometallurgy domain modelling is a system used by resource companies to determine the costs and profits of a mining project.