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The "rock-to-metal ratio" indicates how much ore and waste rock must be mined, moved and processed to produce a refined unit of a mineral commodity. Data on the quantities of ore mined and waste rock removed during mining and processing was not previously known on a global level and is crucial to understanding the future supply of these minerals and the potential environmental impacts from mining them.

The new rock-to-metal ratio is explained in a USGS and Apple study recently published in Environmental Science & Technology. The authors used the most current data available to determine the rock-to-metal ratio of 25 of the most commonly used mineral commodities. The ratio they developed considers various mining factors, such as ore grades and recovery yields, to estimate the amount of ore and waste rock produced at more than 1,900 mining operations worldwide.

"The rock-to-metal ratio and the underlying data have many potential uses," said Nedal Nassar, chief of the Minerals Intelligence Research Section at the USGS National Minerals Information Center and lead author of the study. "For instance, manufacturing companies could use our data as one of several deciding factors on where to source minerals and or which materials they use in their products."

While the rock-to-metal ratio can provide some clues to the environmental impacts of various mining operations around the world, Nassar is quick to add that "this is only one piece of a larger puzzle, and the rock-to-metal ratio needs to be used in conjunction with other data to make informed decisions about mineral sourcing or material choice.”

The study shows there is a wide range in rock-to-metal ratios among different mineral commodities. For example, iron ore – one of the most heavily mined commodities – has a rock-to-metal ratio of 9-to-1. This means for every nine metric tons of waste rock and ore moved and processed, one metric ton of iron is produced. This was one of the lowest ratios found in the study. Gold, on the other hand, was found to have the highest ratio at about 3,000,000-to-1, which means for every three metric tons of ore and waste rock moved and processed, only one gram of gold is produced.

"These iron and gold ratios are the global averages for these particular commodities," said Nassar. "We found wide variability among the ratios for each mineral commodity based on the particular mining operation that it was extracted from."

This variability in the ratios for a single mineral commodity is one of the key pieces of data Apple was interested in when they initially approached the USGS to collaborate on this study. The rock-to-metal ratio can also provide companies an additional way to quantify the benefits of recycling by showing how much waste removal and ore mining could be avoided by recycling these materials.

“As Apple continues to pioneer new innovations in recycling, the rock-to-metal ratio will help advance scientific understanding of the value of using more recycled materials and accelerate progress toward a circular economy,” said Sarah Chandler, Apple’s senior director of Environment and Supply Chain Innovation.

A key finding of the study revealed that the worldwide total of ore and waste rock moved in 2018 for the 25 mineral commodities was about 37.6 billion metric tons. This enormous number can be difficult to comprehend, but it's roughly the equivalent of almost 7,000 Great Pyramids of Giza being moved each year.

The 25 minerals analyzed in the study are used by virtually every manufacturing industry in the global economy and include: aluminum, chromium, cobalt, copper, gallium, gold, iridium, iron, lithium, magnesium, molybdenum, nickel, palladium, platinum, rhodium, ruthenium, silicon, silver, tantalum, tin, titanium, tungsten, vanadium, zinc, and zirconium.

The full study can be read here: https://pubs.acs.org/doi/10.1021/acs.est.1c07875