Challenges in predicting the reactivity of mine waste rocks based on kinetic testing: Humidity cell tests and reactive transport modeling

Humidity cell test represents one of the most popular geochemical characterization methods for predicting the reactivity and for estimating the leachate quality of mining waste materials. Yet, the interpretation of such laboratory test results can vary widely, and its validity and applicability can be impaired by poor characterization of the experimental details, sample representation, and the inability to correctly identify the key contributions of the relevant processes. In this study, we investigate the leaching behavior of mine waste rocks, collected from the Särkiniemi mine site in Finland, by means of humidity cell tests. The experiments were performed according to the ASTM standard D5744–18 and by utilizing two distinct experimental cells, packed with the same waste rock, with considerably different dimensions and shapes. The results show considerably higher chemical weathering rates of the waste rocks and mass loadings of different elements (approximately by two- to threefold) in the long/narrow shaped humidity cell setup compared to the short/broad cell. The observed leachate concentrations also suggest the possibility for potential influences from microscale chemical heterogeneity, despite the attempts to homogenize the waste rock samples by crushing and mixing prior to packing. The humidity cell test results were quantitatively interpreted with process-based multiphase and multicomponent reactive transport modeling, which allowed detailed examination of the complex interplay between chemical reactions and physical processes, helped distinguishing the dominant mechanisms, and facilitated the identification of the controlling factors leading to fundamental challenges associated with the analysis of such results. While the experimental results could be reproduced by fitting different conceptual models or by adjusting model parameters, the model suggests that such simulation outcomes cannot be fundamentally treated as predictive without the proper knowledge of the dynamics of water flow and solute/gaseous transport during these tests. •Humidity cell tests to study the weathering of waste rocks from Särkiniemi site•Distinct reactivity and elemental loading rates depending on cell shapes/dimensions•Influence of possible microscale heterogeneity on solute mass release rates•Process-based multiphase-multicomponent reactive transport modeling of the tests•Difficulties in mechanistic interpretation and prediction of the test results