A maior fonte de terras raras estava escondida em nossos resíduos industriais

While governments race to secure new mineral deposits, a quieter opportunity is emerging in an unlikely place: vast mounds of long-forgotten waste.

For decades, heaps of industrial tailings were viewed purely as an environmental liability and an ongoing maintenance cost. Now, research indicates that some of this “rubbish” may actually be among the most strategic resources for the technology transition: a meaningful source of rare earths, metals that underpin mobile phones, electric vehicles, wind turbines and high‑precision military equipment.

Rare earths: from toxic rubble to a strategic “mine”

Despite the name, rare earths are not especially scarce in the Earth’s crust. The difficulty lies in how they are obtained: extraction is expensive, environmentally disruptive and concentrated in a small number of countries-creating geopolitical vulnerability. Under that pressure, scientists have begun reassessing a familiar material with fresh eyes: coal waste.

In the United States, coal tailings deposits in Pennsylvania alone may contain as much as 137,000 tonnes of rare earths with potential economic value. This material originates from coal processing prior to combustion in power stations and industrial facilities. What was once dismissed as worthless residue is increasingly being treated as a strategic mineral stockpile.

The same wastes that fill entire valleys and raise environmental alarm could become one of the most important “urban” sources of critical metals.

The barrier has always been technical. Rare earths are present, but they are effectively locked inside a complex mineral matrix-as though cemented into clay and silicate structures. Traditional acid leaching can release some of these metals, but typically with low recovery, high operating costs and large volumes of aggressive effluent.

How an alkaline bath and microwaves change the game for rare earths

Researchers at Northeastern University in the US have devised a process aimed directly at the mineral “padlock” that traps rare earths. Rather than simply applying acid to the tailings, the approach begins with an alkaline treatment using sodium hydroxide (NaOH), followed by rapid microwave heating.

This first stage alters the crystalline structure of the minerals. A key example is the conversion of kaolinite-a common clay in these wastes-into a phase known as hydrosodalite, which is more porous and more reactive.

By re‑engineering the minerals from the inside out, the method creates pathways so that the acid used later can reach hidden critical metals far more easily.

Tests on industrial samples found that this alkaline pre‑treatment-performed at around 180 °C using a 5 M NaOH solution under microwaves-followed by digestion with nitric acid, almost triples rare earth extraction yields compared with conventional routes.

What happens inside each grain of waste

When kaolinite dissolves or is converted into hydrosodalite, the solid becomes more porous. Internal surface area increases and channels and cavities develop. That makes it easier for acid to penetrate and release elements such as neodymium and cerium, which are vital for high‑performance permanent magnets used in electric motors and hard drives.

Spectroscopy and X‑ray diffraction measurements confirmed these mineralogical changes. Another significant finding is that a portion of the uranium present in the waste becomes soluble during the alkaline stage, which can help reduce radiological risks during the subsequent acid attack.

The results also indicate that rare earths are often associated with elements such as magnesium, calcium and iron. In practical terms, many of these metals share the same “mineral home”, reinforcing why targeted disruption of alumino‑silicate phases is central to releasing the full suite of valuable metals.

From lab bench to industrial plant: the real-world hurdles

The technical promise is substantial, but turning it into an operational production line is not straightforward. The economics and environmental balance must work: reagent consumption, the energy required for industrial‑scale microwave heating, and the handling of alkaline effluent all need to fit a competitive business model-ideally one integrated with other industrial supply chains.

Coal waste composition varies from one mine to another, and can even differ across layers within the same deposit. That variability forces careful tuning of parameters such as NaOH concentration, microwave time, temperature, the solid‑to‑liquid ratio and the number of treatment cycles.

• Required reagents: concentrated NaOH solution and nitric acid
  
• Energy: microwave‑based heating system at industrial scale
  
• Process control: continual adjustment to match the mineralogy of each batch of waste
  
• Effluent management: treatment and/or reuse of alkaline and acidic solutions
  
• Permitting: environmental compliance and monitoring of radionuclides such as uranium
  

Some of the highest‑recovery extraction scenarios-such as using a low liquid volume relative to solids or running multiple chemical attack cycles-can also produce large quantities of spent solutions that must be treated and, preferably, recycled.

Industrial success depends on fitting this route into a broader loop, where today’s reagent becomes tomorrow’s input-cutting both cost and environmental impact.

A further consideration for scale‑up is how the microwave step is powered. Pairing the process with low‑carbon electricity (for example, via on‑site renewables or grid contracts) could materially improve life‑cycle emissions and help meet tightening industrial decarbonisation requirements, without altering the chemistry of the method itself.

A new piece on the mineral security chessboard

Governments and companies are actively looking for ways to reduce reliance on a small number of global rare earth suppliers. Extracting these metals from existing wastes offers three clear benefits: it reduces pressure to open new mines, helps remediate degraded land affected by tailings, and strengthens supply security for strategic sectors-from renewable energy through to defence.

In practice, countries with a long history of coal mining or other resource‑intensive industries are sitting on an “archive” of waste that could become a critical asset. Large tailings impoundments, ash ponds and stockpiled materials can be reassessed specifically for their rare earth content.

Source	Advantages	Challenges
Conventional mines	High, concentrated volumes	Environmental impact; lengthy permitting
Coal waste	Existing infrastructure; dual benefit (clean-up plus extraction)	Variable composition; need for new technologies
Electronic waste	High metal content per tonne	Complex collection, sorting and dismantling

Concepts worth clarifying

The term rare earths refers to a group of 17 chemical elements, mostly lanthanides, including lanthanum, neodymium, praseodymium, dysprosium and terbium. They are considered critical for modern technology because they offer magnetic, optical and catalytic properties that are difficult to replace.

Meanwhile, urban mining describes the broader shift towards recovering valuable metals from industrial, electronic and municipal wastes rather than relying solely on natural ore bodies. The NaOH‑and‑microwave route fits squarely within this logic, adding a more mineralogically sophisticated tool to the reuse of tailings.

Future scenarios and the risks at stake

One plausible next step is the deployment of pilot plants in regions with large coal tailings deposits. Compact units could trial different combinations of temperature, reagent concentration and microwave duration, adjusting the process batch‑by‑batch to match local mineralogy.

The risks span both the environment and the market. If energy costs rise sharply, or if rare earth prices fall too far, projects may struggle to remain viable. Conversely, inadequate handling of alkaline and acidic liquors could create new liabilities-the very outcome this technology aims to avoid.

Beyond chemistry and economics, community confidence and regulatory scrutiny will heavily shape what is feasible. Transparent monitoring of effluent quality, radionuclide behaviour and site remediation outcomes can make the difference between a permitted recovery project and one that fails to earn a social licence to operate.

For those involved in energy planning, green economics or industrial policy, the strategic significance is growing. Tailings that currently do nothing but occupy land and worry local communities may, within a few years, be treated as critical reserves. The competition will not only be about who owns the richest mine, but also about who can design the most effective chemical process to extract value from what was previously discarded.