Abstract
Liquid is a crucial medium to contain soluble oxygen, valuable metal ions, and bacteria in unsaturated heap leaching. Liquid retention behavior is the first critical issue to be considered to efficiently extract low-grade minerals or wastes. In this study, the residual liquid holdup of an unsaturated packed bed was quantitatively discussed by liquid holdup (θ), residual liquid holdup (θresidual), relative liquid holdup (θ′), and relative porosity (n*) using the designed measuring device. The detailed liquid holdup and the hysteresis behavior under stepwise irrigation are indicated and discussed herein. The results show that relative porosity of the packed bed was negatively related to particle size, and intra-particle porosity was more developed in the −4.0 + 2.0 mm packed bed. The higher liquid retention of the unsaturated packed bed could be obtained by using stepwise irrigation (incrementally improved from 0.001 to 0.1 mm/s) instead of uniform irrigation (0.1 mm/s). It could be explained in that some of the immobile liquid could not flow out of the unsaturated packed bed, and this historical irrigation could have accelerated formation of flow paths. The θ was sensitive to superficial flow rate (or irrigation rate) in that it obviously increased if a higher superficial flow rate (u) was introduced, however, the θresidual was commonly affected by n* and θ′. Moreover, the liquid hysteresis easily performed under stepwise irrigation condition, where θ and θresidual were larger at u of the decreasing flow rate stage (DFRS) instead of u of the increasing flow rate stage (IFRS). These findings effectively quantify the liquid retention and the hysteresis behavior of ore heap, and the stepwise irrigation provides potential possibility to adjust liquid retention conditions.
Highlights
As an environmentally friendly, lower cost, and efficient recycling method of valuable metal resources, the solution mining method has been widely utilized in the recycling of lower grade minerals, mine tailings, and wastes [1,2,3]
In the unsaturated ore packed beds, McBride et al (2015) explored the potential effect of heap porosity on liquid retention and preferential flow paths, and the results reveal that flexible irrigation could disturb and affect initial flow paths [30]
It well known that liquid spreading in unsaturated ore heaps is basically divided into two existential statuses [27,37]: (1) preferential flow mainly exists in the macro-porous flow channels, which is driven by gravity forces and rapidly appears if the liquid breaks through the dry ore heap; (2) diffusion flow mainly exists in micro-porous flow channels, which is driven by capillary forces and slowly appears if the solute transfer and the liquid bridge are well developed
Summary
Lower cost, and efficient recycling method of valuable metal resources, the solution mining method has been widely utilized in the recycling of lower grade minerals, mine tailings, and wastes [1,2,3]. It is commonly recognized that valuable metal ions continuously transfer between the reaction interface and the leaching solution, eventually obtaining pregnant leaching solution (PLS) and metal products [4]. In this leaching procedure, the leachate is regarded as the key medium of bacteria, soluble oxygen, and metallic ions [5,6,7]. The leachate could mainly include mobile and immobile liquids, and both simultaneously exist in this unsaturated ore heap. To obtain a desirable leaching efficiency, the first step should be to deeply understand the liquid holdup and the hysteresis behavior in the unsaturated ore heap
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