A new study showed a novel method to treat acid mine drainage. It could reduce environmental pollution and provide a source of critical rare elements.
The new method of treating acid mine drainage was developed by scientists at Pennsylvania State University (PSU), a public research university in the US. Their method could recover substantial amounts of critical rare earth elements, which could be utilized by several industries. On top of that, the method has been deemed cost-efficient and eco-friendly. These characteristics are highly needed in treating mine drains. They published their findings in the Chemical Engineering Journal.
What is Acid Mine Drainage?
Mining can provide numerous benefits to a community. First, it gives employment to various individuals, especially those skilled in manual labor. Second, mines can offer specific resources useful in trade and industry. And third, the resources and employment given by mines can improve a city's economy.
However, these benefits are not without a price. Mines are detrimental to the environment, particularly in pollution. Pollution emitted by mines can deliver adverse effects to nearby communities, including the one supposedly benefiting. Soil, water, and air pollution are some examples of the adverse effects in the mining industry. If unmitigated, communities will damage neighboring ecosystems and kill both animals and plants. Ecosystems loss can lead to forfeitures of natural sources, such as meat, herbs, and other raw materials.
According to the US Geological Survey, a scientific agency, mines create mine drainage or any surface water that drains from an active or abandoned mine. The drainage can either be contaminated or share a quality with natural waters. But oftentimes, mine drainage indicates pollution due to the high concentration of toxic, heavy metals. Generally, mine drainage is acidic than normal water and can harm living organisms.
Drains from mines tend to occur when water and rocks with sulfur-containing minerals chemically reacted. The runoff dissolves heavy metals like mercury, copper, and lead to surface water or groundwater. The runoff spikes the acidity of affected surface water that enhances the growth of specific bacteria, usually harmful to other organisms.
Countermeasures have been developed to reduce the acidity of mine drainage. But these efforts can be costly, time-consuming, and labor-intensive depending on the economic level of a country. Thus, mining operations in low-income regions may lack the necessary measures to reduce pollution from mine drainage. A common result if determined by authorities is the cancellation of the operation, which is not good for local employment.
Novel Acid Mine Drainage Treatment
At PSU, scientists developed a new way to treat mine drainage economically, environmentally-friendly, and resourcefully. Their method could reduce the environmental impact of mining operations without cutting the benefits. If approved for commercial use, it would further expand benefits from mines or even utilize leftovers from abandoned mining sites.
"Acid mine drainage has been a significant environmental concern for many decades. This research shows we can modify existing treatment processes in a way that not only addresses environmental concerns, but at the same time recovers valuable elements and actually decreases the cost of treatment," said Mohammad Rezaee, the corresponding author of the study and assistant professor of mining engineering at PSU.
Since mine drainage tends to have metals and other useful elements, processing them can yield resources that many industries use today. These elements can be utilized to manufacture fighter jets, smartphones, and other technologies. The US alone imports almost 100% of the 17 mineral groups of rare earth elements. While China alone supplies 80% of those groups to other countries. It means the US and other countries, except China, may have been neglecting these elements stored in mine drainage.
To enable resourceful actions, the team created a staged precipitation process to treat acid mine drainage or AMD. The process involves carbon dioxide mineralization to recover rare earth elements from mine drains. The process also allows a high recovery rate using a pH range. They cited pyrite rocks as an excellent source of rare elements. These rocks are unearthed during mining operations and produce iron sulfide when water and air interact.
After being oxidized, sulfuric acid is released that results in early AMD. The acid breaks down surrounding rocks that unleash heavy metals into the water, which produces AMD. Retention pods are common devices used to neutralize water acidity and reform dissolved metals into solids. Up to 70% of rare earth elements can be obtained from retention pods. The remaining percentage is released with treated water.
But if carbon dioxide was added into AMD treatment, the water acidity could be set to neutral or pH level 7. Scientists performed tests using carbon dioxide. Results showed that 90% of the aluminum was recovered at pH level 5. At pH level 7, 85% of rare elements were recovered. They explained that the gas induces chemical reactions to form solid minerals known as carbonites. The earth elements would bind with excess carbonites and precipitate out of the water, provided the pH levels were low.
Traditional methods could replicate the results of their study. By adding more chemicals, the pH level would go higher than 7. But doing so would increase the cost of the recovery. If the recovery cost was lowered, the market for rare earth elements could become more competitive.
According to Statista, a German portal for statistics, mining could create thousands of jobs worldwide. In the first quarter of 2017, the coal mining sector in the US produced 1,000 new jobs. Back in 2007, the sector created 77,500 jobs while 86,400 jobs in 2008. In 2016, 50,000 jobs were generated by the sector that was topped by 51,000 jobs in Q1 2017.
Coal power plants require a steady supply of coal, and coal mining ensures that. In 2018, lots of coal power plants were under construction in China. A total of 128,650 megawatts of coal power capacity were being constructed in the country. It was followed by India with 36,158 megawatt-capacity, Indonesia with 11,466 megawatt-capacity, Vietnam with 9,705 megawatt-capacity, and Japan with 8,742 megawatt-capacity.
The carbon dioxide mineralization is an emerging technology repurposing excess atmospheric carbon dioxide. The study is the first time to demonstrate the repurposing in mine drainage treatment.