Abstract

Mining sector is one of the oldest and fastest growing industries in Canada. This sector generates several hundred million tonnes of wastes per year and is also one of the biggest contributors to greenhouse gas (GHG) emissions in Canada. One critical challenge associated with mining wastes is their potential to generate acid mine drainage (AMD). This acidic drainage commonly leaches out environmentally toxic levels of sulfate and heavy metals. Among various methods for AMD remediation, neutralization with lime is the universal approach that has being widely applied in AMD treatment plants. The statistical data revealed that almost all Canadian plants utilize lime during AMD remediation. At the same time, lime treatment of AMD dramatically increases the concentration of dissolved calcium (Ca) in water (up to 10-fold), which is known as a potential source for CO₂ sequestration and storage.

Carbon dioxide (CO₂) sequestration via mineralization is an emerging and promising technology for stable CO₂ storage; however, its integration into mining practises has not been well investigated. In addition, the relatively high cost of the process is the major obstacle towards its scale-up and commercialization. The objective of this study was to investigate the potential application of neutralized AMD as a resource for CO₂ mineralization/sequestration and to address the gaps associated with this approach. In order to offset the cost of the mineralization, the potential recovery of valuable minerals, iron and aluminium in this case, from an actual Canadian AMD stream was also studied. Moreover, this work included a techno-economic analysis and CO₂ footprint of the proposed single-stage and the multistage CO₂ mineralization plants.

In this work, the physicochemical processes taking place in both single-stage and multistage processes were investigated, along with CO₂ sequestration capacity of a Canadian AMD stream. The staged precipitation and recovery of iron and aluminium using an alkaline agent and oxidant prior to carbonation was optimized in order to maximize their recovery and minimize the impurities in each step. CO₂ (in the form of gaseous and solid carbonate/bicarbonate) was then injected into AMD and formed Ca/Mg carbonates during neutralization. In each step, the impact of pH, temperature, pressure, residence time and the dosage of the oxidant, alkaline agent and CO₂ were optimized, if applicable.

The techno-economic results revealed that the high cost of the process can be improved by employing a multi-stage carbonation process. Another major strength of the multistage mineral carbonation compared to the single-stage route is its high carbonation efficiency at relatively low temperature and pressure as well as the potential production of high purity and sellable carbonates and non-carbonates products. Based on these results, it was concluded that technical and economic potentials exist for integrating mineral carbonation technology into an AMD neutralization plant. The high cost of the process was mainly related to the high cost of sodium hydroxide solution utilized in carbonation contactor. Application of industrial alkaline wastes such as ultramafic mine tailings can potentially considerably reduce the costs of the mineral carbonation.