Acid Mine Drainage

Acid mine drainage is one of mining’s most serious threats to water. A mine draining acid can devastate rivers, streams, and aquatic life for hundreds, and under the “right” conditions, thousands of years.

At metal mines, the target ore (like gold, silver, copper, etc) is often rich in sulfide minerals. When the mining process exposes the sulfides to water and air, together they react to form sulfuric acid. This acid can dissolve other harmful metals and metalloids (like arsenic) from the surrounding rock. Acid mine drainage can be released anywhere on the mine where sulfides are exposed to air and water — including waste rock piles, tailings, open pits, underground tunnels, and leach pads. Acid drainage is often marked by an orange-yellow substance (visible in the photo on this page) that occurs when the pH of acidic mine-influenced water raises above pH 3 (approaching more neutral conditions) so that the previously dissolved iron precipitates out.

Acid mine drainage can have severe impacts on fish, animals, and plants. Many impacted streams have a pH of 4 or lower — similar to battery acid. For example, acid and metals runoff from the Zortman Landusky mine in Montana has harmed biological life in a dozen streams in the Little Rocky Mountains. Acid mine drainage is especially harmful because it can occur indefinitely — long after mining has ended. A literature review on acid mine drainage concluded that “no hard rock surface mines exist today that can demonstrate that acid mine drainage can be stopped once it occurs on a large scale”. Many hard rock mines across the world may require water treatment for hundreds to thousands of years, or “in perpetuity” as a result of acid mine drainage or metals leaching. For example, over 40 hard rock mines in America will generate an estimated 17-27 billion gallons of polluted water every year, in perpetuity, and require costly water treatment.

Acid Mine Drainage in Finland

The Cu (Pb, Zn) mine of Orijärvi (1757–1956) was the first mining operation in Finland where flotation techniques (1911–1955) were used to enrich ore. Large quantities of tailings were produced. The impacts of past mining activities on the aquatic ecosystem of nearby Lake Orijärvi were studied using a combination of paleolimnological methods (analysis of sedimentary diatom frustules, chrysophyte cysts, metal concentrations, and radiometric datings). The acid mine drainage (AMD) – derived metal impact to the lake was found to be the strongest thus far recorded in Finland. Concentrations of Cu, Pb, and Zn in sediments are two to three orders of magnitude higher than background values. During the most severe loading phase, there were practically no algae in the lake. Achnanthes minutissima was the hardiest species able to tolerate increased metal contents. The metal load has changed the properties of sediments in such a way that chrysophyte cysts were impossible to identify because of coating and corrosion. Lake water still has elevated heavy metal concentrations, indicating that the impact from the tailings area continues to affect the lake. It has low productivity, and the planktic diatom community is still not developed (Salonen, 2006).

Acid Mine Drainage in Russia

The distribution of chemical elements (Zn, Cu, Fe, Pb, Cd, As, Sb, Be) in the water and bottom sediments of the Belovo swamp-settler was investigated in our integrated study of geochemistry, geophysics, and hydrobiology. This swamp collects drainage escaping from clinker heaps made up of waste of pyrometallurgical smelting of sphalerite concentrate. Water in the swamp has high TDS with extremely high contents of toxic elements. Bottom sediments in the swamp are the mixture of hydrogenic secondary Cu, Zn, Fe, and other elements minerals. High metal concentrations lead to drastic changes in biota: phytoplankton, zooplankton, and bacteria communities. The species richness and composition of plankton reflect the chemical composition of the water of swamp-settler and allow considering it as extreme habitat. About 90 % of zooplankton individuals have a genetic mutation expressed in morphological deformations. Infiltration of the high TDS swamp water into groundwaters was detected by vertical electric sounding (Bortnikova, 2010).

Acid Mine Drainage Treatment with Ferrate(VI)

The literature review reports a methodology of preventing the generation of acid drainage by applying ferrate (VI) to acid-generating materials prior to disposal in impoundments or piles. The solid potassium ferrate(VI) was successfully used to oxidize sulfide mine tailings. Oxidizing the pyritic material in mining waste could diminish the potential for acid generation and its related environmental risks and long-term costs at disposal sites. Preliminary results show that the oxidation of pyrite by ferrate has a half-life of about six hours. The stability of Fe(VI) in water solutions does not influence the reaction rate in a significant manner (Fernandes, 2008).

The advantage of the oxidation of the sulfide-rich tailings by ferrate(VI) in comparison with other remediation schemes is that it can be seen as a permanent solution, i.e., when the pyrite material is oxidized to appropriate levels, no long-term acid generation and leaching of metals from these materials will take place. This approach offers great advantages in relation to other treatment techniques that involve long-term maintenance because they do not serve as a definitive solution. The objective of the present project is to investigate the potential of a methodology to prevent the generation of acid drainage by applying ferrate (VI) to acid-generating tailings prior to their disposal in impoundments or piles. Oxidizing the pyritic tailings and diminishing the potential for an acid generation will reduce the long-term issues related to the disposal of this material and will reduce environmental risks at the disposal sites. The application of ferrate would take place in a slurry pipeline during the post-treatment of tailings prior to their disposal into heaps or dams.

The preliminary assessment of ferrate(VI) for the treatment of acid mine drainage, focused on precipitation of metals (i.e., iron [Fe] and manganese [Mn]) and subsequent removal. Two dosing approaches were studied to simulate the two commercially viable forms of Fe(VI) production: Fe(VI) only, and Fe(VI) with sodium hydroxide (NaOH). Subsequent metal speciation was assessed via filter fractionation. When only Fe(VI) was added, the pH remained <3.6, and the precipitation of Mn and Fe was <30 and <70%, respectively, at the highest, stoichiometrically excessive Fe(VI) dose. When NaOH and Fe(VI) were added simultaneously, precipitation of Mn was much more complete, at doses near the predicted oxidation stoichiometric requirement. The optimal dosage of Fe(VI) for Mn treatment was 25 mM. The formation of Mn(VII) was noted at Fe(VI) dosages above the stoichiometric requirement, which would be problematic in full-scale acid mine drainage treatment systems. Precipitation of Fe was >99% when only NaOH was added, indicating that oxidation by Fe(VI) did not play a significant role when added. The Fe(III) and Al(III) particles were relatively large, suggesting probable success in subsequent removal through sedimentation. The resultant Mn-oxide particles were relatively small, indicating that additional particle destabilization may be required to meet Mn effluent goals. Ferrate seems viable for the treatment of acid mine drainage, especially when sourced through onsite generation due to the coexistence of NaOH in the product stream (Goodwill, 2019).

The CBC OneDrop project is currently working on the applications of the unit with on-line sodium ferrate generation to treat acid mine drainage in Finland and Russia.