(96a) A Tale of Two Mines: Insights to the Similarities and Differences of Surface and Underground Critical Mineral Mines Life Cycle Analyses

Decarbonization technologies are potential solutions to reducing greenhouse gas emissions and mitigating climate change. However, the decarbonization transition will undoubtedly be mineral-intensive with critical minerals like cobalt, lithium, and nickel serving as the backbone of many decarbonization technologies. Mining is the primary acquisition method for critical minerals but wields many environmental and social effects – both positive (i.e. job opportunities and boosting the local economy¹) and negative (i.e. land degradation², reduced water quality³, and community displacement²). Life cycle analysis (LCA) is a tool that can assess effects and sustainability. Newer interest in critical minerals has sparked LCA research on critical minerals acquisition, but there are limited studies. Most of studies only investigated mine operations – such as extraction and chemical processing – and often excluded closure and reclamation. To the best of our knowledge, there are no studies that consider the whole life cycle of the mine – encompassing the first ground break to reclamation and mine closure.

Additionally, the location of most studies mirrored top critical mineral-producing countries, like Australia, Chile, and China. Few studies investigated U.S. mines. Moreau et al. (2021) compared manual versus automated operations for six different underground copper mines, two of were in the U.S. They found automation reduced global warming potential, acidification potential, and human toxicity potential up to 20%. Twin Metals Minnesota (TMM), a mining company in Minnesota, completed a prospective LCA of their proposed underground copper-nickel mine. Their LCA, like many others, included basic operations like mining, beneficiation, and electricity burdens but excluded land clearing, water treatment, and reclamation. Notably, they assumed electricity will come from wind farms in Iowa. They calculated the global warming potential (GWP) per kilogram of copper and nickel in concentrate. TMM found that explosive emulsions and quicklime were the largest GWP contributors.

Methods

We conduct LCAs on the whole mine life cycle of two proposed copper-nickel mines in Minnesota – which would mark the first non-ferrous mines in the state. One mine, PolyMet, is surface and the other, Tamarack, is underground. We employ two function units: one ton of mineral product equivalent and one year of mining operation. We define the system boundary as cradle-to-grave to encompass the entire life cycle of the mine, from first ground break to mine reclamation and closure. We use foreground system processing and site information from legal documents as an appropriate, open data source. We use Greenhouse Gases, Regulated Emissions, and Energy Use in Technologies (GREET) Model5, and literature for background system inventory data. We include uncertainty and scenario analyses to improve the robustness and validity of our LCAs. We calculate energy consumption, water use, and greenhouse gas emissions.

Results

The results will illuminate interesting dynamics between surface and underground mining LCAs and provide insights into the associated environmental costs. For example, PolyMet may require more land clearing and result in a larger carbon stock loss than Tamarack. Conversely, Tamarack may require more fuel emissions to dig and haul rock and ore out miles beneath the surface. This work will showcase how two different mine types with different life expectancies and operations can be compared using LCA. Their geographic proximities – a mere 90 miles apart – are an advantage to truly understanding the burdens, similarities, and differences of surface and underground mining. We will implement the same background data for many processes. For example, electricity from the Minnesota grid mix will source both projects. Reducing data variances limits additional uncertainties and allows dynamics to shine.

Implications

These results will help guide decision-making on what type of mine to implement for a given ore deposit to minimize environmental costs and will expand the limited LCA research of critical mines in the United States. These results can guide the development of a critical mineral LCA framework to standardize comparison between mines regardless of type, mineral product, and geographic location. These results expand, which can help guide policy and decision-making. Also, the two mines are close to Indigenous reservations so future work could encompass including Indigenous perspectives into critical mineral LCAs to broaden the definition of sustainability.