3.5.2024

Recovering waste heat in copper refining.

Why high-temperature storage underpins future mining demands

In a paradoxical twist, the mining industry must ramp up its productivity to align with ambitious climate goals depending on facilitating low-carbon technologies on a large scale. It’s clear, however, that the current state of the mining industry, consuming up to 11% of the global energy and largely operating in water-stressed regions, is unsustainable (Hund et al., 2023). Here is where the mining industry reaches a crossroads and there is a need to forge an innovative path from a traditional one.

In a scenario where global warming is limited to a 2℃-temperature rise, solar PVs will consume 87% of the available aluminum. Meanwhile, wind and geothermal will account for most zinc and titanium. Further, solar PV and wind account for 74% of all copper demand, while battery storage accounts for most of the graphite and lithium demand. Graphite and lithium alone will require a 500% boost in production by 2050 (Northey et al., 2014).

As the demand for minerals steadily rises, productivity challenges will surely intensify, with the sector having the need to go clean itself. But where exactly will these challenges arise?

Based on data from a Polish (KGHM) case study (Kulczycka et al., 2016), the energy intensity of each process, averaged over the years 2003 to 2013, was distributed as seen in the chart to the right.

Green energy is an economic decision for the mining industry

While greenhouse gas (GHG) emissions are a global concern, energy-expensive operations must trim their expenditure and cut GHG emissions. Sourcing low-carbon energy to avoid upcoming carbon taxes is one way the mining industry can benefit financially. 

According to a recently published article by Forbes, Brent crude oil was trading at an all-time high of 93$ per barrel in November 2023 — a price hike that was 30% lower only 3 months prior (Duggan, Sept. 2023). The rapid rise incited fear that the cost per barrel would surpass the 100$ mark by 2024, while that has yet to be the case, the risk continues to loom. Meanwhile energy from renewable sources gets cheaper.

If it is not the price tag of each barrel, then ecological factors, such as declining ore grades and external factors, such as supply chain pressure to become carbon-neutral are factors for the mining industry to improve its energy efficiency.  

The Compelling Case of Copper

A collection of recent studies indicates the average grade of copper ore is steadily declining, and the energy demands and overall material output are surging (Calvo et al., 2016). In other words, the energy demand for copper production depends extensively on ore grade.

As ore grades decline, more material must be moved and processed to maintain current outputs. Specifically for copper mines, there's been a startling 25% drop in ore quality over a mere decade. During this period, energy consumption outpaced production. Estimates are a staggering 46% increase in energy use against a 30% rise in production. The copper mining industry will face critical efficiency challenges in the next decade as they race against rising global demand that could reach a 300% increase by 2050 (Northey et al., 2014). 

Declining copper ore grades, amongst others, are also an emerging topic for Nickel, Cobalt, and Platinum group metals (PGMs) (Magdalena et al., 2023). While scholars argue technological advances will offset the lack of high-ore-grade deposits (Ericsson et al., 2019) — those advances still need to happen.

Here and now, the mining sector has opportunities to reduce its energy costs and emissions effectively (Kulczycka et al., 2015). Especially when, for example, improvements are made to roasting or smelting processes conducted on-site at mining locations or in refinery plants.

Advances in high-temperature mining operations

Pyrometallurgy is one of the most common metal extraction methods today, but large quantities of heat are generally used throughout multiple stages of production. In this method,  a mineral is partially or completely converted from an elemental form to another chemical compound by dry, high-temperature physio-chemical changes. These processes include calcination, roasting, smelting and refining. With current setups, pyrometallurgy is considered economical for higher-grade ores (Whitworth et al., 2022). 

Overview of basic steps of mining and processing

Building on the case of copper production and its future scenario, most energy will be consumed during the mining phase, concentrate processing, the choice of smelting technology, the level of oxygen enrichment involved in the process, and both the generation and application of the process heat. 

Overview of heat sources and applications in copper refining.

Notably, inefficiencies exist with current setups for processing sulfide copper concentrate, which is the most widely available form. The drawbacks begin with the roasting stage, where a large quantity of heat is generated and then lost irreversibly. Then comes the smelting stage, which also requires a magnificent dose of thermal energy from heavy oil, coal or natural gas. 

Capturing and redistributing heat from flue gas in Copper pyrometallurgy

During copper production, multiple waste heat sources could be retrieved and redistributed from flue gas alone (Yu et al., 2011). These sources could be reused and redistributed if properly stored to match demand and lower energy expenditure. Many of them also can be covered by electrification which is beneficial with  Applications include but are not limited to:

  • Producing Steam: Using the high-temperature flue gas waste heat from the rotary anode furnace to produce steam in a waste heat boiler.
  • Preheating Air or Furnace Feed: Employing the lower-temperature flue gas, after heat exchange in the waste heat boiler, to preheat the intensifying combusted air or the furnace charge of the flash smelting furnace.
  • Producing Sulfuric Acid: The flue gas from the slag cleaning furnace, which is higher in temperature and contains sulfur dioxide, is mixed with the flue gas of the flash smelting furnace or converter to produce sulphuric acid.
  • Drying Furnace Feed: The lower-temperature flue gas from the hot blast stove and steam superheater stove is used to dry the furnace charge of the flash smelting furnace.
  • Generating Electricity: Utilizing the steam produced from the waste heat to power turbines and generate electricity.
  • Redistributing stored heat in remote mining locations

Remote mining locations, especially those in much cooler climates that aren't connected to the electric grid or natural gas lines, could particularly benefit from advanced or even portable heat storage and supply units.  Mines in remote areas usually depend on diesel generators. But even with the best technology, these diesel generators aren't very efficient. They only turn about 33% of the diesel fuel's energy into electricity. 

In particular, for remote mining operations, redistribution of wasted to heat to heat-demanding processes could help reduce costs (Baidyaby et al., 2019) by supplying heat for:

  • endothermic mineral processing systems
  • preheating of subfreezing mine intake air 
  • space heating for communal areas 
  • heating of sanitary water 

Recovering waste heat can make mining operations sustainable

Using clean heat from electrification and storage will assist the mining sector in sustainably meeting climate goals. This also can help offset the diminished quality and quantity of ore grades. Moreover, redistributing stored heat from flue gas to heat-demanding processes can further guarantee a more sustainable future for the mining industry. Storing heat is beneficial for the financial and environmental challenges of a demanding industrial and mineral market.

References

Baidya, D., De Brito, M. A. R., Sasmito, A. P., Scoble, M., & Ghoreishi-Madiseh, S. A. (2019). Recovering waste heat from diesel generator exhaust; an opportunity for combined heat and power generation in remote Canadian mines. Journal of Cleaner Production, 225, 785-805. https://doi.org/10.1016/j.jclepro.2019.03.340

Duggins, W, Why Is The Price Of Gasoline Rising? (2023), Forbes Advisor, Date of last access: January 01, 2023.

Ericsson, M., Drielsma, J., Humphreys, D. et al. Why current assessments of ‘future efforts’ are no basis for establishing policies on material use—a response to research on ore grades. Miner Econ 32, 111–121 (2019). https://doi.org/10.1007/s13563-019-00175-6

Hund, Kirsten; La Porta, Daniele; Fabregas , Thao P; Laing, Tim; Drexhage, John. 2023. Minerals for Climate Action: The Mineral Intensity of the Clean Energy Transition. © Washington, DC: World Bank. http://hdl.handle.net/10986/40002 License: CC BY-NC 3.0 IGO

Kulczycka, J., Lelek, Ł., Lewandowska, A., Wirth, H., & Bergesen, J. D. (2016). Environmental Impacts of Energy-Efficient Pyrometallurgical Copper Smelting Technologies: The Consequences of Technological Changes from 2010 to 2050. Journal of Industrial Ecology, 20(2), 304-316. https://doi.org/10.1111/jiec.12369

Magdalena, R., Valero, A., & Calvo, G. (2023). Limit of recovery: How future evolution of ore grades could influence energy consumption and prices for Nickel, Cobalt, and PGMs. Minerals Engineering, 200, 108150. https://doi.org/10.1016/j.mineng.2023.108150

Northey, S., Mohr, S., Mudd, G., Weng, Z., & Giurco, D. (2014). Modelling future copper ore grade decline based on a detailed assessment of copper resources and mining. Resources, Conservation and Recycling, 83, 190-201. https://doi.org/10.1016/j.resconrec.2013.10.005

Okosun, T., Nielson, S., Ray, S., Street, S., & Zhou, C. (2020). On the Impacts of Pre-Heated Natural Gas Injection in Blast Furnaces. Processes, 8(7), 771. https://doi.org/10.3390/pr8070771

Whitworth, A. J., Vaughan, J., Southam, G., Van der Ent, A., Nkrumah, P. N., Ma, X., & Parbhakar-Fox, A. (2022). Review on metal extraction technologies suitable for critical metal recovery from mining and processing wastes. Minerals Engineering, 182, 107537. https://doi.org/10.1016/j.mineng.2022.107537

Yu, H., Wang, L. Y., & Du, T. (2011). Waste heat recovery and reuse of flue gas in copper pyrometallurgy. Applied Mechanics and Materials, 71-78, 2239. doi:https://doi.org/10.4028/www.scientific.net/AMM.71-78.2239