Elsevier

Hydrometallurgy

Volume 133, February 2013, Pages 1-6
Hydrometallurgy

Visualization of flow behavior during bioleaching of waste rock dumps under saturated and unsaturated conditions

https://doi.org/10.1016/j.hydromet.2012.11.009Get rights and content

Abstract

A simulation framework is outlined for the analysis of the saturated and unsaturated flow behaviors within a waste rock dump. The traditional inherent “black-box” approach for treating waste rock dumps has resulted in generalizations that sometimes prove inaccurate. The hydrodynamics play an important role in terms of leaching reaction kinetics, heat and solute transport. The coexistence of saturated and unsaturated flows within the dump makes the flow behavior complicated. In this study, X-ray CT (SOMATOM Sensation 16) was used to obtain the real pore structure of the packed waste rocks. The meso-scale saturated flow process was visualized through solving the governing equation by the COMSOL Multiphysics software. The velocity field depends strongly on the distribution of channels, pore connectivity and curvature. The macro-scale unsaturated velocity field within the dump was obtained. The information drawn from this model can be used to improve the understanding of the hydrodynamics within waste rock dumps.

Highlights

► X-ray CT was used to obtain the real pore structure of the paced waste rocks. ► The meso-scale saturated flow process between the particles was visualized. ► The macro-scale unsaturated velocity field within the dump was obtained.

Introduction

Waste rock dump is a major environmental concern, common to most hard rock mines that contain sulfidic minerals such as pyrite or chalcopyrite. The joint action of oxygen and water on these reactive minerals causes a complex sequence of oxidation–reduction reactions that can produce an acidic leachate (Poisson et al., 2009). Water and circulation into the pile initiate and sustain the acid mine drainage process (Lefebvre et al., 2001). The reactions occurred inside the dump can be summarized as follows:CuFe2 + O2 + 4H+  Cu2 + + Fe2 + + 2S0 + 2H2O2FeS2 + 7O2 + 2H2O  2Fe2 + + 4SO42  + 4H+2S0 + 3O2 + 2H2O  4H+ + 2SO42 .

Those reactions induce the release of protons and set off a sequence of other processes that result in the release of cations and formation of sulfate, sulfuric acid, and a low pH environment. The presence of micro-organisms (e.g. Acidithiobacillus ferrooxidans) can further speed up the processes (Brierley and Brierley, 2001). Water infiltrating the dump carries these pollutants into the surrounding environment. There is the potential of migration of dissolved metals away from the mine wastedisposal area, causing the degradation of groundwater and surface water resources, threatening flora, fauna, and aquatic life.

With increasing use of dump leaching technology, it is clear that deep insight into the effects of the driving parameters including chemical, physical, microbial factors, and their interactions, is the key to successful implementation and improvement of the technology (Pradhan et al., 2008). The study of the thermodynamics, kinetics and mechanisms of the chemical reactions in dump leaching (with and without microbial mediation) has provided much insight into the effects of limiting reactions (Petersen and Dixon, 2002). This has led to various strategies for optimizing the reactions. However, dump leaching technology still faces serious problems related to the hydrology, resulting in relatively low rates and extent of extraction and necessitating long retention time to achieve a high degree of mineral extraction.

During the leaching process, dumps are irrigated in order to transport leaching reagents to reaction sites within the dump, to transport the products of reaction out of the dump, and to maintain dump temperatures within the limits of activity of microbes. Two distinct phenomena are of interest in the study of dump leaching: fluid flow and physicochemical reactions (Cariaga et al., 2005). These two phenomena can be studied separately if the extent of leaching does not influence the flow pattern (Cariaga et al., 2007). The liquid phase hydrodynamics in dump leaching, however, in spite of their economic importance as a metal recovery and cleanup method, has been the object of only a very few systematic studies and has not yet been completely elucidated.

Phenomenological mathematical models for dump leaching, taking into account fluid flow throughout the porous bed and the reactions that take place in the solid particles, can be used for the design and optimization of these processes (Bouffard and West-Sells, 2009). Understanding flow through waste rock dumps is important to enhance the performance of dump leaching with respect to design and operating considerations. Column leaching of zinc sulfide with various column heights and various irrigation rates found that the reaction kinetics are proportional to the irrigation rate divided by heap height (Lizama et al., 2005). FLUENT has been used to study the flow behavior through random packing of non-overlapping spheres in a cylindrical geometry (Jafari et al., 2008). Analyzing the best heap conditions from the economic standpoint reveals that leaching time and heap height are the variables, which had the greatest effects when determining the most optimal circuits (Padilla et al., 2008). Numerical modeling of flow and basic transport in waste rock dumps could provide visualization of probable flow patterns and transport pathways which provide the basis for possible remedies. The volume of fluid method was used to simulate the fluid motion in the heap leaching system (Mousavi et al., 2006). An unsteady and two-dimensional model is developed based on the mass conservation equations of liquid phase in the bed and in the particles (Sheikhzadeh et al., 2005). Fundamental-based modeling tools have been applied for industrial process design and process optimization of existing plants (Menacho et al., 2007). HeapSim modeling tool has been put forward for the HydroZinc™ heap bioleach process (Petersen and Dixon, 2007). The interaction of mesophiles and moderate thermophiles in a 3-phase computational fluid dynamics model for heap bioleaching of chalcocite is investigated (Leahy et al., 2007). The analytical models, based on the Bernoulli equation, are presented to describe heap leaching (Mellado et al., 2009). The objective of this paper is to realize the visualization of saturated and unsaturated flows within the waste rock dump.

Section snippets

Flow patterns inside the waste rock dump

The movement of solution through the waste rock pile has important impact on efficiency of bioleaching operations, because the solution transport lixiviates into and metal ions out of the dump. Maximizing solution contact and minimizing preferential flow leading to significant bypassing of ore by the leach solution are crucial for enhancing leaching efficiency and ore recovery. But the flow behavior in the rock dump is very complex due to a wide range of particle size, complex configuration of

The leaching column and X-ray CT technology

The flows in porous dump depend on the geometric properties of the packed particle bed. It is difficult to obtain the in-situ inner structure of the rock dump. The column leaching test involves passing leaching solutions through a stationary ore sample to model the dissolution process. It is not so much to duplicate the results that can be expected from a commercial dump leaching operation but to collect kinetic information on the ore being evaluated so that scale-up equations can be validated.

The simulation tool and boundary conditions

COMSOL Multiphysics is a perfect simulator for problems of variable density flow in porous media. Script-driven by a programming language that extends the Matlab language, it provides all stages of modeling – CAD design, meshing, solution, and visualization – via control-panel operation. The Earth Science module within this simulation tool is used for both heat and water simulations. The porous media is defined by the volume fractions of solid particles and water. Once the DXF file derived from

Meso-scale flow between ore particles

The investigation of flow phenomena at the meso-scale level could be an important step in the optimization of dump leaching process. By combining the X-ray Computed Tomography with robust modeling tools, it is possible to show the pore scale fluid flow modeled by the Navier–Stokes equation.

It is assumed that saturated liquid flow exists within a certain part of the region consisting of compacted small particles. Fig. 5, the plot of velocity field, shows the solution predicted with a

Macro-scale variably saturated liquid flow within dump

Dumps under leach are subject to an application of a solution of reactants and occasional rain events. The solution flow velocity field is largely related to the irrigation rate. Increased irrigation increases the flow velocity and water content in the dump, and reduces the air-filled porosity. An optimum irrigation rate exists to provide sufficient both reagents and oxygen for the leaching reaction. Other factors that can influence flow conditions include decrepitation of the substrate,

Conclusion

The flow inside the waste rock dump is divided into saturated and unsaturated, and the saturation ratio depends on many factors, such as dump structure, irrigation rate and so on. Navier–Stokes equation and Richards' equation are selected to describe the flow behavior in intra-particle and in the bulk dump, respectively. Using the X-ray CT technology, the inner structure of the packed rocks could be visualized. The distribution of flow velocity between the particles is simulated by importing

Acknowledgments

The authors would like to acknowledge the financial support for this work provided by the National Natural Science Foundation of China (50934002 and 51104011), the Program for Changjiang Scholars and Innovative Research Team in Universities (IRT0950), and the project supported by State Key Laboratory of Comprehensive Utilization of Low-Grade Refractory Gold Ores.

References (28)

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This paper was originally presented at the International Biohydrometallurgy Symposium (IBS), Changsha, China, 18–22 September 2011.

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