Microwave treatment offers greener option for graphite processing

By Paul Rackstraw
Published: Thursday, 14 December 2017

Microwave irradiation has recently emerged as a promising method to process high-grade natural graphite from mineral ores in a way that is less environmentally damaging than many conventional beneficiation techniques, Industrial Minerals correspondent Rose Pengelly discovers.

Graphite resources around the world have been the focus of intensive exploration over the past seven years to help meet projected growth in demand for the carbon mineral from fast-expanding technologies such as lithium-ion batteries.

Projects under development range from greenfield sites in northern Canada to mothballed mines in South Australia, presenting a variety of ore types that require customized processing methods to produce the most lucrative high-grade materials.

The claims of environmental friendliness made by several of the end-markets that are driving the rise in battery-grade graphite consumption, such as electric vehicles and renewable energy storage, have attracted increased scrutiny on the supply chains of raw materials.

This has put graphite companies under pressure to demonstrate environmental sensitivity in the way they add value to products.

Currently, graphite ore is mostly beneficiated using flotation separation techniques, followed by acid leaching and caustic roasting.

According to Dr Saeed Chehreh Chelgani, adjunct professor at the University of Michigan in the United States, who has studied the effectiveness and environmental impact of different graphite processing methods, refining graphite through roasting and leaching is both costly and potentially polluting due to energy inefficiency and the production of chemical waste.

"There are different projects looking at ways to reduce the environmental problems of leaching and caustic roasting," Chelgani told Industrial Minerals.

"Microwave pre-treatment, as well as liquid-liquid flotation and bioleaching, are some examples of studies which have had successful results. But these trials are continuing, to meet the specific needs of industry," he said.

Proc1  
SEMs are used to analyses the effectiveness of different
graphite beneficiation methods.  
Penn State, via Flickr 

Benefits of microwave irradiation

Microwave irradiation has many advantages over roasting. It allows for rapid and selective heating, fast switch-on and -off, flexible modular design of treatment units and high energy efficiency. 

It is also relatively benign in environmental terms. Microwave pre-treatment makes the graphite more receptive to subsequent chemical treatment, meaning that smaller amounts of chemicals can be used to increase the mineral’s grade.

The capacity of microwaves to selectively heat graphite ore means that this technique is especially effective in targeting impurities.

"The organic component of graphite is a relatively weak absorber of microwave energy, whereas some minerals, such as pyrite, and water in their structures, readily heat within an applied electric field. Other minerals, such as quartz, appear transparent to the radiation," Chelgani explained.

"By applying various microwave energies, pyrite may be selectively heated and decomposed as pyrrhotite or iron sulfate," he added. "Furthermore, in response to the effects of microwave irradiation, the bonds of sulfur-carbon in organic sulfur compounds contained in the graphite ore are broken and the sulfur is released in gaseous form." 

Leaching experiments with hydrochloric acid (Cl) and nitric acid (HNO3) and microwave radiation showed increases in the grade of the graphite tested to 99.43% C from 95%.

Scanning electron microscope (SEM) and X-ray power diffraction (XRD) analysis of the resulting products indicated that the shape of the graphite flakes remained unchanged during this process. This is especially important for applications such as batteries, where large flake material is currently essential.

Microwave treatment of graphite dates back at least to around 1984, with many studies also performed on coal, but the technology has so far not been widely adopted in place of roasting. 

"Microwave irradiation was not considered a promising method of upgrading natural graphite until quite recently. I have not heard of its industrial application in graphite processing yet," Chelgani said. 

Chelgani is skeptical about claims from graphite producers that there is a looming shortage of large flake graphite, arguing that recycling and using artificial graphite can cover part of the anticipated future demand.

"For sure, natural graphite is performs better [than synthetic graphite] because it has been structured over a million years under geological conditions which created some specific properties in its structure," he said.

But ultimately, as Chelgani pointed out, commercial graphite users choose their raw material according to a cost-performance balance. Until new processing techniques can be shown to contribute positively to industry margins, incumbent methods will be difficult to dislodge.

MLA: Liberate to accumulate

Since size and grade are among the most important determinants of graphite’s sale value, high-grade ores with a large fraction of naturally occurring large flakes need to be processed in ways that do not crush these prized particles. 

Liberating these flakes within the ore is key to maximizing their recovery, studies have found. 

Mineral liberation analysis (MLA) by automated SEM-based image analysis can be used to determine which beneficiation techniques are the most effective in liberating large flakes.

MLA can also be used to gather useful data on impurities, grain size distribution and composition of mineral association of any given graphite sample, which cannot be obtained by other analytical tools that are currently available.

But MLA is a relatively expensive technique and the machines needed to perform the analysis are scarce, meaning that graphite samples must usually be shipped hundreds or even thousands of miles to research laboratories to be analyzed. 

While increasing use of MLA technology could potentially boost adoption of new methods of graphite processing, this will depend on commercial factors such as cost, perceived shortages of supply and the selling price of graphite products.