By Matt Wohl, President & CEO, The Doe Run Company

Across the globe, at shuttered smelters and active processing facilities alike, there are mountains of slag. Slag is what’s left behind after metal is extracted from ore — the byproduct of centuries of smelting. It piles up near mines, rivers, and rail lines. In many places, it has been there for decades. In some places, more than a century.

Most people think of slag as waste. They are not wrong, exactly. But they are not entirely right, either.

Locked inside that slag — in concentrations that are sometimes higher than in newly discovered primary ore deposits — are critical minerals the world urgently needs. Cobalt, germanium, indium, nickel, silver, tin, antimony, zinc, and other rare earth elements. The same materials driving the clean energy transition. The same materials embedded in defense systems, electronics, and electric vehicles. Academic and industry estimates suggest the unrecovered value of critical minerals in global metallurgical slag reaches into the hundreds of billions of dollars.

That is not a geological curiosity. That is one of the most significant untapped secondary mineral reserves on the planet.

Why Slag Has Been Overlooked

The reason slag has been largely ignored as a resource comes down to a simple technological constraint: until recently, we didn’t have an economically viable way to get the valuable minerals out.

Traditional metal extraction relies on pyrometallurgy — intense, high-heat smelting processes that require enormous energy inputs and generate significant greenhouse gas emissions. These methods are effective at extracting primary metals from ore, but they are poorly suited to recovering the complex mix of residual minerals left in slag. The heat destroys some minerals before they can be captured. Others require a level of chemical precision that high-temperature processing simply cannot provide.

Meanwhile, most hydrometallurgical processes — the water-based alternative to smelting — have historically been designed to target a single metal at a time. Running a complex, multi-mineral feedstock (unprocessed raw material) like slag through those systems is inefficient and expensive. The economics rarely pay off.

So, the slag sat. The minerals stayed locked inside. And the world continued mining new primary deposits to meet demand for materials that were already, in a very real sense, above ground.

What the Science Now Tells Us

Researchers have increasingly documented the scale of what is left behind. Metallurgical slags — particularly those from copper, tin, and zinc smelting — contain significant concentrations of materials that now appear on the U.S. government’s official critical minerals list.

Copper slag is rich in cobalt, zinc, and indium. Tin slag is a meaningful secondary source for niobium, tantalum, tungsten, and rare earth elements. Zinc slag contains gallium and germanium — materials essential to semiconductors and defense optics, and increasingly subject to export controls from countries that dominate their supply.

Researchers now describe legacy slag stockpiles as “technospheric deposits” — secondary reserves created not by geology but by human industrial activity. The concept is significant: it reframes slag from a liability to be managed into a resource to be recovered. Some of these deposits are more concentrated than the low-grade primary ores that mining companies are spending billions to develop.

The global steel slag market alone is valued at over $25 billion, and pressure to extract critical minerals from these secondary sources is accelerating as the clean energy transition drives up demand. The International Energy Agency and the U.S. Department of Energy have both identified secondary mineral recovery — including from slag — as a priority for reducing supply chain dependence on foreign primary sources.

Why Doe Run Is Positioned to Lead

At Doe Run, we have been thinking about slag for a long time. We know what it contains, because we have spent years developing a process specifically designed to handle it.

Our proprietary ACL HydroMet Process — more than a decade in development — uses liquid-based chemistry to recover a wide range of critical minerals from complex, multi-metal feedstocks. Unlike conventional hydromet systems that target a single element, our process is designed to work across a range of materials: ore concentrates, recycled batteries, mine tailings — and slag. We have tested it on materials from around the world and achieved recovery of copper, zinc, cobalt, nickel, lead, tin, antimony, and germanium from a single process.

Equally important, we operate with significantly lower emissions than traditional smelting. Slag piles at shuttered smelter sites around the world represent not just a mineral opportunity, but an environmental one. Material that would otherwise sit in a landfill can be reprocessed and put to productive use. That is the circular economy in practice — not as a marketing concept, but as an operational reality.

The U.S. Department of Defense recognized the potential of this approach when it awarded Doe Run $7 million in Defense Production Act funds in 2024 to build a demonstration plant at our Technology Center in Viburnum, Missouri. That investment reflects a growing appreciation in Washington for innovative tools and technologies that prioritize secondary recovery. It is a front-line strategy for securing the critical minerals America needs.

A Global Opportunity, A Missouri Solution

Our aim is to use our ACL Process not only with concentrates from our own Viburnum Trend mines, but with feedstocks from partners around the world: companies holding slag stockpiles that contain real value, with no current technology capable of unlocking it.

We already have partners in North America, Canada, and Mexico who hold feedstocks with minerals sitting untapped. The conversations are happening globally. And in each case, the same question arises: is there a process that can recover these minerals economically, at scale, with low environmental impact?

Our answer is yes. That is not a claim we make lightly. It is the result of more than a decade of rigorous research and testing on difficult and diverse material samples, and a DoD-backed demonstration program.

The Opportunity Ahead

The critical minerals conversation in the United States has rightly focused on mining and domestic processing. But there is a parallel conversation worth having: about the vast quantities of critical minerals that have already been mined, already been processed, and are sitting in slag piles — waiting.

Securing America’s critical mineral supply does not require us to start from scratch in every case. Sometimes it requires us to go back and finish the job — to recover what previous technology left behind, and to do it cleanly, efficiently, and here at home.

The world has been generating slag for centuries. The technology to unlock its value is finally here. And we believe Missouri is where that work will be done.

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Matt Wohl is President and CEO of The Doe Run Company, a privately held critical minerals and natural resources company based in St. Louis, Missouri. Doe Run has been mining, refining, and recycling minerals in Missouri for more than 160 years.