Overview

One of the greatest environmental problems caused by the mining and refinement of strategic elements, in this case speaking primarily about rare earth elements, is the problem of waste management. This becomes a problem when mines and refineries that do not adhere to regulations regarding proper waste disposal. This can result in soil and water contamination by substances such as heavy metals and radioactive materials. This affects the ecosystem around the waste disposal site; and, if the contaminants get into the water table, it can affect areas beyond the site.

Description of Problem

Improper waste disposal is often the largest of mine and refinery pollution Waste is generally categorized into two different types: tailings and waste rock stockpiles.

Tailings have the most damage potential because they tend to be composed of smaller, finely milled particles, whereas waste rock stockpiles are made up of coarser particles, which are not absorbed into the water and the ground as easily. In addition to the small particles, tailings contain waste water and flotation chemicals. Tailings are typically placed in impoundment areas exposed to precipitation and water runoff, which can allow toxic substances to be washed out. Groundwater can be contaminated if the impoundment area is not leak-proof. Impoundment areas also run the risk of overflowing during periods of heavy rain, if the areas are not made large enough to contain great amounts of rain and runoff. The most serious risk, however, is that of a collapsed dam, which would allow the toxic tailings to flood the area. Although the exact composition of tailings is site-specific, they generally contain heavy metals, acids, fluorides, sulphides, and radioactive material. Waste rock stockpiles have many of the same problems, and are made out of the same materials, but in coarser mineral form. [2] The primary concern when determining how best to manage waste is preventing water pollution. A water body can be polluted in three different ways, which may occur in isolation or with each other: sedimentation, acid drainage, and metals deposition. [3]

Sedimentation:

Sedimentation is the process by which erosion of waste piles or runoff from impoundment areas adds layers of sediment to nearby bodies of water. This can alter the path and shape of streams, reduce the light available to aquatic plants, and smother small prey organisms, thus altering the food chain. More sediment layers can also make bodies of water shallower, which can increase flooding in wetter seasons [3].

Acid Drainage:

Acid drainage occurs when sulfuric acid is produced by the oxygenation (via exposure to air or water) of sulfide-bearing minerals (ex. pyrite) [3]. Acidic water facilitates further dissolution of sulfide minerals, thereby introducing more metals and acids into the environment. [1] This acid release can come from exposed waste, mine openings, and pit walls. This process continues until there are no more reactants, which, if there are large amounts of exposed rock, could continue for centuries. Acid drainage is harmful to aquatic life, particularly to fish, some of which cannot live below a pH of 5 or 6. [3]

Carbonate minerals, which are often the dominant minerals in rare earth element ores, are basic, and therefore act as a buffer for sulfides. However, too much carbonate dissolution is just as dangerous to aquatic ecosystems, because it can elevate the pH, and, like sulfides, can allow more contaminants to enter the water. [1]

Heavy Metals:

Rare earth element ores tend to contain high amounts of metals, similar to other types of hardrock. This means that the metal concerns of hardrock mining also apply to rare earth mining. In particular, the elements of concern at rare earth mines include, but are not limited to, aluminum, arsenic, barium, beryllium, cadmium, copper, lead, manganese, and zinc.lead, manganese, and zinc [1]. For more information about the harmful effects of metals, please refer to the environmental problems of refining rare earth elements.

Harmful Minerals:

Fluorine is another dangerous contaminant found in carbonate mineral dissolution. Fluorine is widely known to be harmful to the environment, as too much of it has been shown to cause a decline in plant growth as it accumulates, and can harm the bones of animals that eat plants with a high concentrations of fluorine. Some rare earth element deposits also include the asbestos mineral riebeckite, which is also known to be harmful to the respiratory systems of both humans and animals. [1]

Radionuclides:

Radionuclides are another contaminant associated with rare earth element ores. These are radioactive materials including thorium-232, uranium-238, and their non-stable decay products. These decay products are the most dangerous aspect of this contaminant, because the radiation emitted during radioactive decay can pose a risk of cancer among other things in humans and animals. [1]

Rare Earth Elements:

Rare earth elements also run the risk of contaminating the environment during production. This has also been discussed in the environmental risks of mining page. Rare earth elements are not well understood, but as they are considered metals, they are considered to be possibly harmful contaminants, since metals cannot be destroyed in the environment, but can only change their form. Because of this quality, many other heavy metals have been shown to be harmful, due to their tendency to accumulate in the bodies of plants and animals. [1]

Wastestreams:

Wastestreams are locations where it is likely that contaminants could be introduced into the environment. Contaminants may be released in pit mines or in waste rock piles, as well as at the mill. Contaminants are most likely to escape from waste rock piles, since they are less contained than contaminants in a pit. Careful monitoring is necessary, to ensure that waste products are not allowed into the environment. [1]

Case Studies

Bayan Obo Mine

The primary product of this mine is iron, with rare earth elements as secondary products. The tailings are taken to an area nearby and piled up; the mine impoundment is twelve kilometres long and has an area of eleven square kilometres. According to the Chinese Draft of Emission Standards of Pollutants from Rare Earths Industry, the amount of tailings totals to 150 million tons, including both the iron and rare earth concentration plants. Thorium (Th) is contained in the tailings and residues, and results from the Baotou Radiation & Environmental Management Institute in November 1998 show that the surrounding area is contaminated. The average Th content in the area is 0.0135% and the gamma-radiation dose rate of East, South, West and North ore bodies of Bayan Obo is 60.6-958.6 nGy/h, 54.5-546 nGy/h, 60.3-611.3 nGy/h and 49.7-599nGy/h, respectively, which is more radiation than would be there if the area was not contaminated. A normal range of background gamma radiation, for comparison, is between 50 and 200 nSv/h, or between 50 and 200 nGy/h [6] [5]. After testing plants for values of  232Th, 238U, 226Ra, and 40K, it was shown that the specific radioactivity in plant tests is a factor of 32 and in soil tests a factor of 1.7 higher than that of control specimens, which means that the plant was 32 times more radioactive than the control, and the soil was 1.7 times more radioactive. This provides even more evidence that plants and soil at Baotou region were contaminated by radioactive wastes. [2] This is corroborated by locals, who state that they observed their crops failing in the 1980s, several years after the impoundment/reservoir was built. [4] The Explanation of Compiling of Emission Standards of Pollutants also stated that Th-containing dust is emitted in a range of 61.8 t per year during the crushing process for the ores. Because of the elevated thorium levels in and around the mine, the Healthcare Research Centre performed a twenty-year follow-up study on health effects following long-term exposure to thorium dusts. This study concluded that the mortality rate of lung cancer has significantly increased for the workers in Baotou because of the thorium. There were also reports on groundwater pollution from the tailing pond which damaged the health of the area's inhabitants by contaminating the well water and the water and soil for crops and livestock. Approximately 200 tons of ThO2 in sludge are produced every year during the extraction phase from the 100 thousand tons of rare earth concentrates. Using the sulfuric acid-roasting method, which is a refinement method in which a sulfide and the solid ore are treated with very hot air in order to purify the metal in the ore, during the production of one ton of rare earth concentrate, between 9600-12000 m3 of waste gas containing fluoride, SO2, SO3 and dust may be emitted. Also, 75 m3 of acid-washing waste water and one ton of radioactive residues are generated per ton of rare earth concentrate. [2]

Molycorp Mountain Pass

Process wastewaters and tailings impoundments formed the bulk of the environmental contamination at the Molycorp Mountain Pass site. Before 1980, the facility used two different types of impoundments to dispose of waste; onsite percolation-type surface impoundments for wastewater, and conventional dam impoundments for tailings. Because of these sites, groundwater has been affected near the mine. The HCl in the wastewater was neutralized with sodium hydroxide (NaOH), so there has been an increase in total dissolved solids (TDS) as a result. Unlined impoundments have been reported to create TDS concentrations in the range of 10,000 mg/L, with background concentrations from 360 to 800 mg/L, including small concentrations of barium boron, strontium, and radiological constituents. Other constituents, such as metals, nutrients, and radiological constituents have potentially had a negative effect on groundwater as well. Two additional off-site evaporation ponds for containing wastewater were constructed in 1980 and 1987; while these two units were operating, there were several instances of failure in the pipelines that connected the new facilities with the Mountain Pass site, which resulted in surface soil contamination. Two wastewater spills were documented at the site during that time; the first in 1989, as reported by EPA, and the second in 1990. The first spill resulted in the surface discharge of 3.375 gallons of tailing and process wastewater from the failed pipeline. The second was larger, and resulted in the surface discharge of 45,000 gallons of process wastewater from the pipeline. Both spills were contained and deemed insignificant, since there happened to be low risk to human life and the environment at that time, but the Mountain Pass site stopped producing several years later.