Author: Peter D. Duryea
Conference: Tailings and Mine Waste 2019
Date: November 17-20, 2019

Proper design of a heap leach facility requires the development of a leach ore loading plan that can accommodate its ultimate capacity. The loading plan and associated capacity assessments will be more representative of actual field performance if they consider the variation in leach ore dry unit weight over time as the heap is loaded and the leach ore compresses under self-weight. That can be accomplished by careful evaluation of results from laboratory testing of representative samples of the leach ore.

Rigid wall permeability testing provides data about dry unit weight (and hence void ratio), saturated hydraulic conductivity, and vertical effective confining stress. These data can be fitted to relationships for compressibility (void ratio as a function of effective confining stress) and conductivity (saturated hydraulic conductivity as a function of void ratio) that were originally developed for self-weight consolidation of soft, fine-grained soils, e.g., tailings. The forms of those equations also typically fit laboratory test data from coarse-grained leach ore.

Once the compressibility and conductivity relationships are defined, incremental stress calculations with a spreadsheet allow for the development of: (1) profiles of dry unit weight and saturated hydraulic conductivity with depth in a unit area column of leach ore; and (2) the variation in depth-weighted average dry unit weight with average heap height. The average dry unit weight relationship may then be incorporated into heap loading plans and capacity assessments. Comparison of the planned leach application rate to the anticipated saturated hydraulic conductivity of the leach ore under the maximum heap height, assuming vertical percolation under unit hydraulic gradient, also provides a preliminary assessment of hydraulic performance.

Results for two case studies, including both a crushed and agglomerated copper heap and a run-of-mine gold heap, are presented. Actual results from mine production records compare well with predictions, although assessment of the run-of-mine gold heap required additional refinement to adjust for the effects of oversize material that could not be incorporated into the laboratory-scale testing.

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