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MEI Online: Hydrometallurgy: Latest News: March 22nd 2011

 
 

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:: Stirred In-Situ Leaching to Provide Vastly Improved Recoveries

CSIRO have developed a novel Subsurface Stirring concept that exploits the process of chaotic advection in porous media. Modelling studies and preliminary experimental investigations have shown that Subsurface Stirring allows in-situ leaching (ISL) lixiviant flow to be controlled to a far greater extent than can be achieved with current practice. It can potentially be put into effect in ISL well-fields and other leaching and groundwater remediation applications by transient switching of the pumping rate. This leads to a transient flow between inlet and outlet wells. When properly manipulated, such switching protocols create flows with fluid trajectories that produce rapid, complete mixing. Mixing can then be tuned to produce accelerated mass transfer and faster chemical reactions. These principles can also be used to produce confining flows for containment of lixiviant within a target zone.

Figure 1 shows how Subsurface Stirring can be used to achieve efficient mixing and hence contact of the lixiviant within the orebody.


Figure 1. Subsurface stirring is achieved by transient switching of injection (red) and extraction (blue) wells (inert wells coloured green). Periodic reorientation of the active well pair results in highly efficient mixing of the initially black/white fluid after a small number (n) of reorientations. Such mixing improves contact between the lixiviant and orebody, increasing yield and production.

The benefits of this method chiefly arise from the faster and more complete mixing within the fluid phase, and faster and more complete contact between fluid and solid phases. Computational modelling of a uranium ISL operation (Figure 2), predicts a nearly 100% increase in extraction rate, after the first bed volume, with Subsurface Stirring compared to conventional operation.

Figure 2. Uranium yield as a function of pumped fluid volume for conventional ISL and subsurface stirring. Both scenarios involved the same pumping energy consumption and well distribution.


 

As well as improving mixing, Subsurface Stirring can also produce encapsulating flows which help confine working fluids, as illustrated in Figure 3. Where a groundwater flow would usually sweep the black “fluid” downstream, Subsurface Stirring creates a so-called “kinematic barrier” which confines the black fluid separate from the background flow; improving recovery of pregnant liquors and reducing contamination of groundwater.

 


Figure 3. Periodic switching of the injection well (red, opposing extraction well not shown) produces an encapsulating flow which confines the black fluid to the cycle shown, even in the presence of a ground water flow.

 

The ability to produce confining flows using Subsurface Stirring represents an entirely new concept for control of ISL operations. The risks of unintended adverse environmental impacts will be lowered as a result. These improvements in the control of fluid flow are expected will also be applicable for site/groundwater remediation projects.

Research required to solve some key issues

A research project is proposed to confirm the findings of the experimental work undertaken to date. The project would be led by Dr Guy Metcalfe of CSIRO Materials Science and Engineering, with hydrometallurgical experts in CSIRO Process Science and Engineering providing support. If sufficient interest is received from industry, a full project proposal will be developed addressing the following core activities:

  • Experimental verification of the benefits of the concept, on synthetic and sponsor provided samples using an existing 1m diameter rig.
  • Computational scale up studies for sites nominated by sponsors (1 site/sponsor). Computational predictions based on the 1m rig experimental results. For one case study, operation under conventional ISL conditions at the site will also be modelled, to confirm the ability of the model to predict extraction characteristics and lixiviant consumption at larger scale.
  • Kinematic confinement experiments with minerals placed in crushed rock. These studies will include several different permeability distributions and accompanying predictive computations.
  • Preliminary evaluations of other potential applications. Scenarios will be simulated based on sponsor nomination, and prioritised for future experimental investigations.

 

 

   

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