Introduction to Heap Leaching
Discuss about the Issues Preventing Effective Recoveries.
Heap leaching is a process of recovering metal and metallic components from crushed low-grade ores. It is a hydrometallurgical processing technique that was mainly used in the recovery of Au in the late 1960s but with time, it has become a well-established method of processing different commodities in the world. Fundamentally, the process accounts for about 20% of the global copper production and is considered for Nickel, zinc and uranium recovery. This paper aims at addressing the issues that hinder effective recoveries of the ore including ore characterization, mineralogy, construction of the heap, passivation, percolation and geographical location of the ore.
According to Breitenbach and Thiel (2005), “Heap leaching involves passing an appropriate chemical solution also known as lixiviant through crushed ore placed on an impermeable pad to enhance dissolution of the metals.” Upon dissolution of the metallic component, the ‘pregnant’ solution is extracted using either solvent extraction or electrowinning technologies. The mostly used lixiviant is either alkaline or acidic solution. Sulphuric acid is commonly used in copper extraction. Today, heap leaching has become an attractive processing technique as recovery can be conducted on-site without the need to take the ore to the processing facility. This has greatly reduced operating costs. However, the fact that the process is concerned with low-grade ore, the footprint of these operations are considered to occupy hundreds of hectares of land to allow for economic metal recovery. Also, the process is slow and poor heap permeability is likely to result in inefficient recovery operations. Characteristics of the primary rock are very important in determining the efficiency of the ore recovery processes and the hazardous nature of the spent heap leach piles. Some of which pose a contamination risk to water, air and the soil (Lewandowski, and Kawatra, 2009).
The characteristics of the ore and mineralogy of the base rock are of utmost importance when it comes to heap leaching. In conjunction with that, the hydraulic properties of the ore should be considered when designing the leach pad. Improper consideration of these properties will likely affect the performance of the process, the stability of the heap and consequently result in poor recovery. The mechanical properties of the ore affect the stability of the ore heap, the maximum possible slope of the heap, maximum ore height and the trafficability of the ore surface. As far as these factors are concerned, instability of the ore will lead to loss to the mining facilities as well as environmental contamination. The maximum possible slope affects the heap geometry and construction and the overall area needed for the recovery operation. The steeper the slope, the less the areas required and the converse is also true for gentle slopes. Friable, compressible and agglomerated ore materials are more sensitive to trafficking
Process of Heap Leaching
Construction of the heap requires that the maximum ore stacking height be maintained at 3 meters to prevent the collapse of the heap. Studies show that some metallic ores such as saprolitic, and agglomerated ores have the tendency to collapse while exposed to compression forces. This results in a reduction of both permeability and shear strength. Similar studies show that the maximum overall slope of any ore heap should be considered before design and ore recovery processes. The maximum possible slope usually offers a large setback and particularly if only a gentle slope can be achieved. If there is limited space to accommodate the ore, to avoid any future loss, additional space should be provided or the leaching operation terminated and the leached ore unloaded (Leiva, et al., 2010).
Hydraulic properties of the ore are of critical importance to heap leaching and the consequent recovery processes. The permeability of the ore determines the saturation characteristics of the material and affects the maximum solution the ore can accommodate. If more of the solvent is added to the ore beyond its saturation point, the solution will pool at the surface and as a result, affect the leaching environment. Additionally, such saturation conditions risk the stability of the heap and recovery operations. Percolation tests are the best in addressing the saturation point or the hydrological properties of an ore. Both percolation and drainage are gravity driven. In flat-bed leaching pads, and ponds can be a very viable solution when it comes to trapping the pregnant solution from the base. This optimizes the recovery time from the collected solution (McNab, 2006).
Recovery in heap reaching encompasses two processes; kinetics and solution flow. Without these two passive processes, no recovery can be achieved. Dixon (2003) notes that “Leaching kinetics describes the rate at which metals or other constituents are released from the ore.” There exist various theories to describe the rationale behind kinetics with one of them being the shrinking core model. According to this model, the reaction along the surface of the particles of the ore results in both a solid product and an aqueous solution that form on the surface of the same particle. As time goes by, further reactions take place reducing the size of the unreacted core. This results in more and more aqueous and solid particles being formed. Of late, this theory has undergone refinement so as to address the different observations being made from different experiments and experiences. To test for kinetics, column testing is used to measure the recovery rate from the ore, the PH, reduction-oxidation state, conductivity and the likely amount of biomass from the ore. How successful the kinetics of the ore is determined how resourceful the recovery process will be (Zhou, and Yu, 2005).
Issues Affecting Effective Recoveries
Kinetics is affected by the particle size of the ore and this is dependent on the constituents of the ore. It’s therefore worth noting that the factors affecting heap leaching and recovery operations are intertwined in one way or another. Column test should be used to offer more insight into the most appropriate size of the particles to facilitate kinetics and consequently recovery of minerals.
The second aspect or recovery is solution flow. This can be considered as the most critical property behind this mining procedure because without flow, recovery cannot occur. The major factors that affect solution flow are permeability and compression. The solvent containing the ore need to flow for extraction to happen, otherwise, other technologies need to be used as opposed to applying heap leaching (Kelly, Ahlborn, Gunn, and Harvey, 2008).
Geographical location has a great impact when it comes to mining. Some mines are located on the steep and rugged landscape while others are situated in gentle slope or generally flat landscape. Location determines the nature of the base rock and consequently the properties and the characteristics of the ore. Particularly, geographic location affects the type of soil in a given mine. Each of these landscapes poses their differing threats to the heap leaching procedures and recovery operations. One of the greatest threat to heap leaching is the amount of clay component in an ore. The structural properties of clay are based on the tetrahedral and octahedral components making the coordinated cations. These components occur as platy particles with very fine-grained aggregates. Upon mixing with water they offer some varying degree of plasticity and this has a big impact and cost when it comes to construction of the heaps and hence the mineral recovery (Martin, Aubertin, Bussiere, and Chapuis, 2004).
The amount of clay present in any mine depends on the nature of the parent rock and on the physiochemical environment. There are four major classifications of clay mineral namely; kaolinite, illite, vermiculite, and smectite. The clay mineral associated with copper ore is different from that of nickel or uranium ore. The most deleterious effect of clay is that, when the ore is piled dry, there is poor percolation because of the absorbent nature of clay soil. Similarly, this affects permeability and the overall success of mineral recovery from the ore (Bouffard and West-Sells 2009).
Conclusively, sustainability of the mining processes is a highly multi-faceted issue that requires input from many fields. Several considerations have to be looked at for successful recovery operations. The main aim of this paper was to review the different factors that affect the recovery processes during heap leaching mining operations. The paper highlights the need to overlook the characteristics of the ore, both mechanical and hydrological, percolation, construction, recovery processes and geographical location of the ore. The maximum heaping height of about 3 meters is recommended. A combination of these factors affects the sustainability of the heap leaching activity with construction being the most detrimental as far as loss of lives is concerned. Adequate testing of the above properties is very important because the aim of any mining facility is to sustainably operate and recover minerals from the constituents of the ore in the most economical and efficient manner.
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