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Microencapsulation to Prevent Acid Mine Drainage
Primary Issue Addressed: Source Control
Secondary Issue Addressed: Acid Mine Drainage
Project Site: Minnesota Department of Natural Resources
Collaborating Entities: MSE and Minnesota Department of Natural Resources
Cost Share: In-kind services provided by Minnesota Department of Natural Resources.
Project Description
Microencapsulation is the isolation of minerals by the precipitation of a chemical coating. For this project, unoxidized pyrite was coated to block oxygen transportation to the sulfide mineral surface and prevent the material from reacting with an oxidizing agent to produce ferric ions. The main project objective was to evaluate the potential success of field application of the microencapsulation process to preventing acid mine drainage. The second objective was to determine field application reagent requirements.
This project was a comparative laboratory study in which an unoxidized sulfidic rock material was tested and evaluated using modified humidity cells. Three application levels of two commercially available microencapsulation technologies: Klean Earth Environmental Company (KEECO) and Envirobond. The KEECO KB-SEA technology uses soluble silica to produce an insoluble ferric silicate precipitate that encapsulates solid media particles helping control future acid generation. The Envirobond EcoBond-ARD technology uses a soluble phosphate to form a ferric phosphate precipitate that prevents the leaching of metal contaminants by creating an impenetrable chemical bond.
Status
Humidity cells containing three application rates (high, medium, and low)
with duplicates for each along with control cells were tested and leached
weekly. The control reactors had acid drainage with a pH >6 after 1
week and a pH of 3.3 at 60 weeks. Over the first 60 weeks of testing,
the KB-SEA treatment was successful in preventing acid drainage; however,
it must be noted that initially very high pHs were generated in comparison
to the controls (see Figure 2).
The EcoBond treatment delayed the onset of acidification but was
not successful in preventing acid drainage. Project testing of
the EcoBond cells was discontinued at 60 weeks with one set of
the EcoBond-ARD duplicate cells being sent to the technology provider
for reapplication and continued leaching.
The humidity cell phase testing under the Mine Waste Technology Program is complete. A microscopic investigation of the test materials was conducted to evaluate the formation of the actual microencapsulation layers along with trying to determine any failure mechanisms of the microencapsulation layer. The project report will be completed in FY05.
Figure 2. Microencapsulation humidity cell testing.