Do Tall Columns Truly Represent Industrial Heaps? A Critical Review of Nickel Heap Leach Testwork

Tall column tests are widely used as an intermediate step between laboratory-scale experiments and industrial heap leaching, aiming to improve the reliability of scale-up predictions by capturing hydro-mechanical and geochemical processes under more representative conditions. However, their predictive value remains inherently limited. This review critically evaluates the extent to which tall columns (2–6 m) reproduce key mechanisms governing industrial heap performance, including progressive compaction, unsaturated flow, preferential pathways, and coupled transport–reaction phenomena. Evidence shows that while tall columns can partially capture vertical chemical gradients, permeability evolution, and delayed reagent consumption, they still fail to represent large-scale heterogeneity, long flow paths, and structural evolution typical of industrial heaps. As a result, the observed extraction kinetics reflect system-dependent effective rates rather than intrinsic reaction kinetics, and direct extrapolation to recovery, hydraulic stability, or long-term performance is unreliable. The analysis also identifies systematic limitations in experimental design, including insufficient column diameter, limited instrumentation, and short test durations, which further constrain data interpretation. A scale-aware framework is proposed that integrates mineralogical characterization, staged testing (from bottle roll to pilot), and coupled hydro-geochemical modeling to improve decision-making and reduce scale-up risk. Tall column tests are therefore best interpreted as diagnostic tools for mechanistic understanding and trend identification, rather than standalone predictors of industrial heap leaching performance.