Recovering Rare Earth Metals Using Simple Chemistry

Introduction

Coal extraction has been a crude and inefficient process, creating a large amount of wastes that are detrimental to the natural environment. In 2018, approximately 8.7 million people were killed from increasing carbon emissions. Despite this, coal remains the world’s most dependable energy source with powerful energy consumers such as the US and China creating 640 million and 3.7 billion metric tons of coal, respectively.

One of these forms of coal pollution is acid mine drainage — one of the worst forms of pollution in the coal industry. The effects are devastating for wildlife, as acid ends up seeping into waterways, creating an inhabitable environment for wildlife.

However, not all hope is lost for coal. Despite its dirty extraction and usage processes, an imminent solution is at hand: extraction of rare earth metals.

What Even Is Acid Mine Drainage? How Is It Dangerous?

Acid Mine Drainage (AMD) is a form of environmental pollution. It is caused when a subsurface mine gets flooded with water, typically a coal mine. Since many different minerals are found with veins of coal, they are incorporated into the AMD. Pyrite is one of the most common minerals found with coal. The pyrite, which contains sulfur, reacts with the water and other gasses inside the mine, creating sulfuric acid that dissolves metals. Since sulfuric acid is soluble in water, the water inside the mine will drain the sulfuric acid into nearby watersheds, as subsurface mines absorb lots of groundwater.

AMD is dangerous because it contaminates water with heavy metals such as iron, rendering it toxic and undrinkable. AMD, with a pH of around 4, is also extremely corrosive just like battery acid. It is also very unpredictable and the undesirable reaction could occur indefinitely (average is around 100 - 2000+ years). The transfer of water also makes AMD much more dangerous, because the solution can seep into groundwater and contaminate not only water sources, but soils too, making it dangerous for plants, animals, and humans. In 2013, a report on AMD concluded that subsurface mining pollutes 17-27 billion gallons of water per year, in perpetuity.

What Are Rare Earth Metals? What Are They Used For?

Rare earth metals are a series of 17 metals known for their silvery white appearance and sporadicity around the world. Aside from scandium and yttrium, these metals belong in the lanthanide group on the periodic table. These metals are well-known for their excellent magnetic capabilities, their electron configuration, and their versatility in modern technology. 

The United States has sought after these metals because of their use in technology applications to maintain their technological edge over its competitors, but the only mass producer of these metals today is China, a major competitor of the United States, who has only one mine for these metals. Because of this, many researchers have attempted to find an alternative to extract these rare earth metals.

The Ultimate Solution to Extracting Rare Earth Elements: Extremely Simple and Eco-Friendly

Rare earth metals are among the most common minerals found with coal deposits within the Earth’s lithosphere, the solid upper part of the Earth. In the late 19th Century and early 20th Century, the United States relied primarily on coal for fueling but disregarded the rare earth metals that were found with the coal, as at the time they thought the rare earth metals were nothing more than waste. However, as the United States started using oil and natural gas as a replacement for coal for a variety of reasons, the coal mines were gradually shut down. The result was AMD carrying rare earth metals and iron precipitates into nearby watersheds. Since the late 2010s, the demand for rare earth metals skyrocketed, and researchers have attempted to use acid mine drainage as a way of getting these significant minerals. 

The recent proposal of extracting rare earth metals from acid mine drainage is relatively simple: the acid water is treated and raised from an acidic pH (around 2-5) to around a pH of 7 (neutral pH) using weak bases and carbon dioxide gas. The metals, now insoluble in the resulting solution, come out as a precipitate, or sludge. An average of 90% aluminum is recovered at a pH of 5, and 85% of rare earth metals are recovered at a pH of 7. The sludge is then recovered and the rare earth metals are extracted, ready for use.

Extracting Rare Earths from AMD: Better for the Environment, Better for Everyone?

The benefits of extracting rare earth metals from acid mine drainage is that the metals have already been dissolved away by the acid, rendering the need for mechanically extracting the metals from the surface obsolete. Aside from this, the metals are also very pure, removing the need and cost for processing the ores. The United States also has a large amount of acid mine drainage sites in former coal and heavy metal mines, making this method very practical for extracting tons of rare earth metals in a long period of time, as AMD happens in perpetuity. And because of the consequences of AMD; a huge concern for wildlife and climate change, it is beneficial for the effort against global warming and climate change.

In recent years, as the demand for rare earth metals increased, researchers also found ways to use the metals to power the sustainable technology of the future, further increasing the value of the metals. Despite not being used on the commercial scale, researchers from around the world have received the attention of governments and corporations. In the near future, these corporations will be able to acquire these well-sought after metals on a commercial scale, which will hopefully commence an excellent start to the relatively new yet practical proposal. 

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