Study on the leaching behavior of cemented paste backfill containing arsenic trioxide roaster waste

After decades of mining at Giant Mine in Yellowknife, Northwest Territories, a significant amount of arsenic trioxide roaster waste (ATRW), containing around 60% arsenic, was stored underground, posing serious health hazards. This study explored solidification and stabilization of ATRW through its incorporation into cemented paste backfill (CPB). It evaluated the stability of arsenic-bearing compounds and the mechanisms of arsenic trioxide stabilization within CPB. Based on the results of unconfined compressive strength (UCS) tests on CPB samples, certain samples were selected for monolithic tank leaching tests (TLT) and a range of microstructural analyses, including thermogravimetry, X-ray absorption spectroscopy, Fourier-transform infrared spectroscopy, and computed tomography. These tests aimed to examine the leaching behavior of arsenic and the microstructure of the selected CPB samples, and to investigate the relationship between their strength, leaching behavior, and pore characteristics. Findings indicated significant arsenic release from CPB surfaces. Up to 41% of arsenic was leached from the CPB samples, and the leaching of arsenic, calcium, and sulfate showed no signs of stabilizing, suggesting potential long-term leaching risks. The pH levels ranged between 9.2 and 10.0, with the dissolution of ATRW contributing to a decrease in pH. This lower pH inhibited the formation of portlandite, while calcium-silicate-hydrate (C-S-H) gels were identified as the primary hydration product, albeit in limited quantities. Thermogravimetric analysis showed weight losses primarily due to calcite decomposition, while X-ray absorption spectroscopy revealed arsenic was present predominantly in its + 3 oxidation state, with no significant As-Ca bonding observed in high-strength samples. CT scans highlighted a relationship between higher strength and larger pore volumes. The study concluded that while ATRW incorporation in CPB can provide mechanical stability, modifications are necessary to reduce arsenic solubility and mitigate environmental risks.