Thermal covers are widely used for freeze encapsulation of potentially reactive mine tailings and waste rock. The covers are designed to limit the depth of seasonal thaw to the cover system and in some cases promote cooling of the underlying waste to sustain perennially frozen conditions. Freezeback can reduce surface water infiltration, oxygen supply, biological activity, and availability of in-situ pore water; therefore, it can limit oxidation of sulfides within acid-generating material.
Site-specific factors important to thermal design of these covers include current and future climate and the physical, geochemical, and thermal characteristics of waste and cover material. Thermal performance of the cover design is routinely evaluated using numerical thermal models that consider these factors. In some cases, test piles are constructed to validate short-term model results prior to full-scale cover construction at closure.
This presentation will describe the design and dominant heat transfer processes for different thermal covers and will review cover performance for northern mines that have adopted the freeze strategy for closure. Three main thermal covers will be discussed: thermal rock-fill covers constructed of geochemically suitable, run-of-mine or run-of-quarry rock, thermal rock-fill covers with a high moisture latent heat layer, and air convection covers consisting of run-of-mine/run-of-quarry rock with a high air permeability.
Numerical thermal model results and field observations indicate that the thermal properties and the dominant mechanism for heat transfer control the required cover thickness for a site. Thermal rock-fill covers are typically dominated by thermal conduction with the greatest depth of seasonal thaw. The addition of a latent heat layer at the base of thermal rock-fill covers can be used to minimize thaw depth and cover thickness by increasing the material heat capacity and the latent heat of fusion. The last cover type, air convection cover, allows for the greatest heat loss due to convective movement of heat during the freezing season. During the thawing season, air convection covers experience reduced heat gain due to the low thermal conductivity material and settlement of relative cold air within the cover. Air convection covers are less sensitive to anticipated air temperature warming in the future.
Review of mining projects that have adopted the freeze strategy indicate thermal rock-fill covers are the most common design and are generally performing as expected under contemporary conditions. Nevertheless, alternative thermal designs should continue to be pursued to optimize long-term performance. Thermal covers are a viable approach for closure of some mine waste facilities in cold regions.
