Microwaves for copper
An award winning copper extraction method using microwaves has been recognised for its contribution to the advancement of the field. Sam Kingman* looks at the method and its origins.
Demand for metals and minerals is increasing. Despite the mixed fortunes of industries over the last few years, it is clear that there is still a significant thirst for the Earth’s natural resources. One of the key areas of growing demand is in copper, but this also presents a major challenge as copper ore grade is falling quickly – so as demand goes up, so the amount of ore required to yield the metal volume increases exponentially, along with total energy consumption. In addition, many of the ores now being discovered are of a lower grade, require finer grinding and are also much harder, which again, means greater energy consumption.
This energy/grade dichotomy has led to significant research into energy reduction in comminution. Many important advances have been made in mill and equipment design and operation, along with process optimisation and modelling. However, it is unlikely that the significant step changes required in energy reduction and process efficiency can be delivered by such strategies alone, in order to provide this paradigm shift in the properties of the ore, or at the very least stopping the practice of grinding ore particles that contain no valuable minerals. These principles have led to significant developments in the use of ore sorting over the past decade, with such technologies now commonplace in flowsheets for the processing of certain commodities.
Copper processing – a hot topic
Given that fundamental breakage mechanisms used for size reduction in rocks have not changed in over 500 years, it is clear that such innovations are not easy to achieve. However, one technology that may offer opportunities for significant energy reductions in comminution is the use of microwave heating.
Microwave heating to process minerals is not a new concept – three reviews were published on the subject several years ago. At the time, these papers offered state-of-the-art reviews of applications of microwave heating in the minerals industry, including initial research and mineral heating studies, applications of microwave energy in thermally assisted comminution, studies in hydro and pyrometallurgy, and a brief review of microwave treatment of coal. The paper, Microwave energy for mineral treatment processes – a brief review, published in International Materials Review, specifically reviewed the basic concepts of microwave heating, heating system design, temperature measurement and control. These papers offer a basic introduction to microwave processing of minerals for any interested readers – Recent developments in microwave processing of minerals, published in International Materials Reviews, and Microwave treatment of minerals – a review, published in Miner.
Broadly, however, their conclusions were the same. Knowledge of the fundamental interaction of microwave energy with minerals was vital for the development of industrial-scale systems, significant research was required to develop concepts for industrial processing at throughputs appropriate to the mining industry, and microwave hardware developments were necessary to enable the technology to be used in the robust environments encountered in mineral processing plants.
In 2002, I realised the economics of microwave fracture could be revolutionised, if the degree of selective heating of the valuable mineral over the host rock could be increased. I established that the power densities in any mineral grain are directly related to its electrical conductivity, and the frequency and square of the electric field. With the conductivity of valuable minerals, such as copper sulphide, several orders of magnitude higher than that of the host rock, this meant the power density in the heated grain could be increased over the surrounding matrix through the use of high electric fields. Significant temperature gradients develop between the absorbing and non-absorbing phases if a very high power density is generated within a microwave-absorbing phase for a short residence time. This leads to large thermal stresses. I also identified that stress occurs at the grain boundaries, creating fracture, if the temperature rise is rapid enough.
This breakthrough represented the first acknowledgment that fracture in ores could be maximised if the power density in the heated phase and the heating rate were both high. Moreover, because the heating rate is high, the residence time in the microwave field needs to be low – less than one second – potentially allowing continuous high throughput with much lower energy inputs. These findings, supported by a patent application, were first published in 2004.
Experimental demonstration of the process brought additional critical insight, showing not only that the energy input and residence time could be reduced, as suggested, but that the process benefits could be further enhanced through the greater degree of thermal stress obtained. This phase of research was published in 2005.
Scaling technology and value
Due to the commercial potential, further industry funding was granted. Global mining and metals company Rio Tinto directly supported the research between 2002–2005, then again in 2009. And from 2005–2009, AMIRA International, an independent association of minerals companies, including Rio Tinto, BHP and Anglo American, provided backing.
Crucial to continued industrial engagement during 2005–2009 was the generation of a supply chain for the equipment. Even pilot demonstrations required the use of microwave energy at unprecedented scales.
In 2006, the University of Nottingham (UoN), UK, and I began working with RF-power technologies supplier e2v, which subsequently became Teledyne-e2v, to develop microwave hardware for the mining industry, and in 2008 this became a full strategic collaboration focused on harnessing the benefits of microwave heating in bulk material processing industries. It is this collaboration that has provided the underpinning research and microwave engines to some of the world’s largest microwave heating processes including technology for the mining industry.
In order to drive commercialisation, the value proposition for any technology must be understood and validated. In this context, the value proposition for microwave-enhanced fracturing as a pre-treatment was considered to relate to the following:
- Reducing the top-size of mill feed increasing mill capacity by 20–70%
- Increasing the grinding cut-point by 20–70 micrometres, saving 20–50% of the energy required to liberate the mineralisation within
- Increasing the mineral recovery by 1–2% points.
The combined impact of these attributes is to potentially increase the metal output of existing recovery plants by 20–70% or, in the case of greenfield installations, permit much smaller plants, hence reducing capital investment.
From 2010 until 2016, UoN and Teledyne-e2v worked together with other key partners, including Jenike and Johanson pty (USA), to develop the first high-throughput microwave processing system for treatment of ores, prove the potential value proposition and validate a series of complex computer simulations used for the design of large-scale processing systems.
This collaborative, multi-disciplinary research effort provided a route to commercialisation and demonstrated the technology in the largest throughput microwave processing plant ever built, proving for the first time that real commercially valuable processing benefits could be delivered at high material throughputs. This collaborative academic-industry team demonstrated consistently induced fracture around the mineral boundaries in ore processed continuously at a rate of 150t per hour (t/h), which, crucially, is scalable to 3,000t/h of treated material.
Microwave radiation interacts with different materials in different ways – it can be reflected, pass through a material or be absorbed. This pilot system was used to treat a number of copper ores and subsequent testing and flowsheet simulation gave spectacular results in terms of energy saving and throughput increases. It was demonstrated that the major benefit of the microwave pre-treatment, at microwave energy inputs of ~0.3kWh/t could change the liberation behaviour of the ore such that the same degree of recovery and grade could be produced following flotation, but at a grind size which was significantly increased – by 100-200 micrometres. The impact of these changes on plant operation was determined to be significant with a potential throughput increase up to 30% with a coupled energy reduction of up to 24%.
Breaking the habit
In 2018, the scale-up of the technology gained the Royal Academy of Engineering’s Colin Campbell Mitchell Award, presented to representatives of the project team. This prize is awarded to recognise the greatest contribution within the previous four-year period to the advancement of any field of engineering. And this technology does mark significant progress for mineral processing.
The global increase in population and rising wealth that is driving urbanisation, coupled with the growing scarcity of fossil fuels and growing carbon emissions means we cannot continue to simply grind more and more ore to meet future demand.
The continued scaling out of inefficient processes that exist today, primarily based on brute force to induce fracture, is not a viable and sustainable way to meet increased demand. We need a paradigm shift that enables increased recovery of metal from ore at a lower energy consumption allowing ores that are currently uneconomic to be processed with a strong value proposition.
For many years, microwaves were thought to have the potential to address these issues, but the concepts remained an academic curiosity for over 30 years. However, the recent work has shown microwave enhanced ore processing to be realistic and within touching distance for the mining industry.
* Professor Sam Kingman FIMMM, CEng, is a Pro-Vice-Chancellor at the University of Nottingham, UK. He was awarded a personal chair at Nottingham in 2006. He was previously the Director of the National Centre for Industrial Microwave Processing, which was one of the largest activities of its type in the world.