Optimization of a Cover Design for Acid Rock Drainage Prevention in a Permafrost Environment

  • Author / Creator
    Smith, Hilary
  • The extraction of sulfide-bearing waste rock can contribute to the generation of acid rock drainage (ARD). ARD is the effluent generated due to the oxidation of sulfide minerals which can be toxic to the receiving environment. Waste rock material which contains potentially acid-generating (PAG) material is often covered with a soil cover to mitigate the production of ARD. When a soil cover system is implemented in a permafrost environment such as northern Canada, it may realize additional PAG management capabilities as a result of annual freezing temperatures.

    ARD is a known issue at the Diavik Diamond Mine (the Site). It is located on an island in Lac de Gras roughly 300 km northeast of Yellowknife, Northwest Territories. The Site is located on the boundary of continuous and discontinuous permafrost with an active layer ranging between 1.5 and 5 m in depth. The mean annual average temperature of the Site was determined to be -8.9 degrees Celsius.

    The waste material at this Site is separated according to sulfide content and therefore its acid-generating potential. The key material types included in this research include the Type I (T1) material which is classified as non acid generating (NAG), Type III (T3) material (PAG), and till material (NAG). The bulk of the material on Site is the PAG T3 material, which is stored in a large, 80 m tall waste rock storage area called the North Country Stock Pile (WRSA-NCRP).

    A proposed cover system for the WRSA-NCRA included 3.0 m of T1 waste rock overlying 1.5 m of a finer layer of till. The till material properties allow for the formation of a latent heat layer due to a low permeability and high moisture content. An adjustment to this design included the removal of fine fractions of the T1 material to form a higher permeability upper layer of the cover system with the intent to induce natural convective cooling. This material was denoted T1 (coarse), and the system was called an Air Convective Cover (ACC).

    Previous research included the development of a simulation model to analyze this effectiveness of this ACC design on a structure like the WRSA-NCRP in a program called COMSOL Multiphysics® (Comsol). The field conditions were approximated assuming heat transfer and fluid flow coupling. This simulation considered an annual surface temperature increase as a result of climate change. Results demonstrated the success of the soil cover system in maintaining the underlying PAG material frozen after 100 years.

    This thesis builds on these findings in order to optimize the soil cover design prior to moving towards test cell recommendations for the WRSA-NCRP. The optimization process was designed to minimize volume requirements while maintaining the material frozen for the same long-term timeline of 100 years. Similar to previous modelling, this research will also account for an annual surface temperature increase due to climate change.

    The key objectives in this research were to demonstrate a successful model benchmark analysis, conduct a sensitivity analysis, optimize soil cover thicknesses, and provide suggestions for future test cell constructions.

    A model benchmarking analysis was established and the rebuilt model in Comsol was deemed an acceptable equivalence to the original model simulation. A sensitivity analysis explored the parameter uncertainties of the till volumetric moisture content, the T1 (coarse) permeability, and the annual temperature increase as a result of climate change. Each parameter had a functional range that would produce accurate and expected results.

    The model optimization of the soil cover included different combinations of materials including T1 (coarse), T1 (all fraction), and till. The model optimization step provided options that account for material availability at the site. The accepted optimization combinations were further classified according to the definition of success for each simulation which provided four levels of added contingency. The optimized results from the lowest contingency buffer category were either 1.00 m T1 (Coarse) + 1.25 m Till or 2.00 m T1 (coarse) + 0.25 m Till. The highest contingency buffer resulted in an optimized prescription of 2.25 m T1 (coarse) + 1.25 m Till. Proposed field test plot prescriptions were generated according to these optimization results.

  • Subjects / Keywords
  • Graduation date
    Fall 2021
  • Type of Item
  • Degree
    Master of Science
  • DOI
  • License
    This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.