Abstract:

One of the challenges in Panel Caving is the amount of dilution that might be expected from exhausted levels located above the current production levels. Generally, a panel caving mine is designed for a dilution entry point (DEP) to occur at 70-90% of the recovery, but if the waste is finer than the newly caved ore, a preferential influx of waste can enter earlier into the drawpoints, resulting in a low DEP. Another source of dilution can be expected when the draw rate exceeds the caving rate, allowing waste to rill along the solid boundary of the cave and enter into the drawpoint at an early stage.

Freeport-McMoRan Inc.'s Henderson Mine, located in Colorado, is a large underground panel caving mine that uses a mixing method that assumes that dilution can only flow vertically (Laubscher, 1994). In order to study other mechanisms of dilution such as fines migration and rilling, this paper presents a case study in which drawpoints located in different locations are back-analyzed using REBOP.

The results show that REBOP, by imposing a certain cave shape, is able to model and predict rilling. Although, back-analyzed drawpoints behaved differently depending on the location with respect to the cave front and their neighbours, no apparent relationship between the best-fit cave angle and the position of the drawpoint was found. This suggests that the evolution in cave shape and angle (and hence entry of waste via rilling) could be complex and hard to know ahead of time without three-dimensional predictions (e.g., via cave-scale modeling) that consider local rock mass strength and major structures.

Introduction

One of the challenges in Panel Caving is the amount of dilution that might be expected from waste material located above economic ore or from exhausted levels located above current production levels. Generally, a panel caving mine is designed for a dilution entry point (DEP) to occur at 70-90% of recovery, but if the waste is much finer than the newly caved ore, a preferential influx of fine waste can enter earlier into the drawpoints resulting in a lower DEP. Moreover, another mechanism of dilution can be expected when the draw rate exceeds the caving rate, allowing waste to rill along the solid boundary of the cave and enter the drawpoint at an early stage.

The case study described is taken from Freeport-McMoRan's Henderson Mine, an underground molybdenum mine that utilizes the panel caving method of extraction. An example of early DEP is illustrated in Figure 1 showing a typical grade-tonnage curve from Henderson Mine's 7700 production level. The darker curve represents the sampled grade profile and the dotted line represents the expected curve (using a standard mixing matrix).

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