Hydrodynamic behavior of heap leach piles: Influence of testing scale and material properties

The dynamics of solution, soluble species, and gas in increasing scales of heap leaching apparatus were quantified with tracer tests to explain differences in their gold leaching behaviors. The tracer methods included tracer injection in the feed solution and monitoring soluble species during rinsing. Hydrodynamics of the heap systems were evaluated during (1) the scale-up from a 350 kg column to a 55 tonne crib to a 6300 tonne heap, and (2) the change in ore properties from run-of-mine to crushed to agglomerated ore in 55 tonne cribs. The tracer data was interpreted by the Levenspiel compartment flow model that considers a packed bed as a black box filled with ore, air, and moisture, the latter occurring as active flow, by-pass flow, plug flow volume, well-mixed volume, and stagnant/dead volume. This model was useful as a diagnostic tool but would not be a predictive tool. established that gold recovery could be correlated to the amount of stagnant solution in the system. Columns and cribs that achieved excellent gold recovery had as little as 0% to 18% of the solution retained in a stagnant form. The 6300 tonne heap had poor gold recovery and contained a significant stagnant volume (57% of the total solution retained in the heap). The proportion of stagnant solution was proportional to the proportion of fines in the ore and the lack of structural support (e.g. column walls) in the heap, the latter translating into a higher bed density. trast to the wide distribution of particle size in the run-of-mine and crushed ores, the agglomerated ore was more uniformly-sized, which led to a lower proportion of stagnant solution. The agglomerated ore had a lower bulk density of 1.7 t/m3 (vs 2 t/m3) and a lower proportion of stagnant solution 0% (vs 88%). Gold extraction from the agglomerated ore was aided by the air available in the pores, while good solution transport out of the packed bed was attributed to the absence of stagnant zones. s proved to be simple and effective to quantitatively diagnose the inner workings of heap leaching systems of various scales. A narrow particle size distribution, a low fines content, and a low bed bulk density were keys to achieving good solution, solute, and gas dynamics, hence high gold recovery.