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Abstract

Flow in heaps and dumps is essentially a two-phase flow phenomenon, though for many applications it could be simplified as unsaturated flow. Unlike saturated flow (typical of groundwater flow) where permeability is independent of other hydraulic parameters, unsaturated flow permeability depends on the degree of saturation and/or on capillary pressure. Several recent field studies suggest that flows in heaps and dumps tend to concentrate in preferred pathways, bypassing much of the ore. Different preferential flow phenomena are triggered, promoted, and influenced by different heap structures, by pretreatment, by the composition of the leach solution, and by application rates and schedules. The structure of heaps and dumps is determined and affected by each and every stage of their construction - from blasting to crushing to conveying and stacking of the material. History of heap construction and operation combined with monitoring, mapping, and certain field tests can reveal patterns of flow and transport in a given heap, and can shed light on the physical mechanisms behind it. Such insight can provide a first cut on heap leaching enhancement by either eliminating or bypassing physical barriers to flow and leaching. Based on the understanding of the heap structure, advanced flow and transport models can be constructed and further enhance the insight. This insight is essential for understanding cause and effect, and can provide a basis for decision making. Further, based on the physically-based models, an intelligent control agent such as MRDS1 can learn and build a self-learning representation (or a model) of the complex heap leaching system, followed by optimization and planning of heap leaching and heap rinsing.

Introduction

Flow in heaps and dumps is essentially two-phase flow of liquid and air in porous media, though for isothermal applications it could be simplified as unsaturated flow by neglecting flow of air. Unlike saturated flow (typical of groundwater flow) where permeability is independent of other hydraulic parameters, unsaturated flow permeability depends on the degree of saturation and/or on capillary pressure. Therefore, one cannot characterize a heap or a dump by a single permeability value, but rather define relationships between permeability, saturation, and capillary pressure throughout the heap. Unsaturated flow in porous media is a complex phenomenon that has been studied extensively for the last 60 years in the areas of soil science and hydrology. Two- and three- phase flows are typical of oil reservoirs, and have been studied extensively in the field of petroleum engineering. This suggests that we can use the knowledge, experience, and physical intuition accumulated in hydrological sciences and petroleum engineering to simulate, optimize, and improve current heap leaching operations today. Concurrently, additional planning (discussed later n the text) based on field and laboratory experiments combined with theoretical, conceptual, and numerical models is needed to improve ore leaching.

One of the consequences of two-phase/unsaturated flow is preferential flow. In a recent investigation of the Aitik waste rock heaps (of copper-bearing mineralized rocks) in Sweden, Eriksson and Destouni (1997) concluded that the only possible explanation for low copper concentrations in the drained solution was flow heterogeneity in the form of preferential flow. Several other field investigations around the world have suggested that bypassing of ore by leach solution is a common phenomenon. Preferential flow phenomena are common in natural soils, as can bee seen in Figure 1.

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President and CEO, Shlomo Orr, PhD, Peng, has over 33 years of extensive consulting, research, and project management experience in the field of Hydrology and Water Resources. He earned a PhD in Hydrology and Water Resources with a minor in Soil and Water Sciences, and BSc and MSC in Civil Engineering. Dr. Orr's background includes modeling, planning, and controlling complex subsurface flow and transport phenomena. He has a broad background in conceptual and computational-numerical modeling of fluid flow and solute transport in saturated and unsaturated porous and fractured formations, including stochastic models that account for uncertainties and provide the basis for risk assessment.

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