Heap Leaching: Analyzing Issues Preventing Effective Ore Recoveries

This report provides a comprehensive overview of the factors that affect the recovery processes during heap leaching mining operations. It highlights the critical need to consider the characteristics of the ore, including both mechanical and hydrological properties, percolation, heap construction, recovery processes, and the geographical location of the ore. The report recommends maintaining a maximum heaping height of about 3 meters and emphasizes that a combination of these factors impacts the sustainability of heap leaching activities, with construction being the most detrimental in terms of potential loss of life. Adequate testing of these properties is crucial for mining facilities to operate sustainably and recover minerals from the ore constituents in an economical and efficient manner.

Running head: HEAP LEACHING; ISSUES PREVENTING EFFECTIVE RECOVERIES 1
Heap Leaching; Issues Preventing Effective Recoveries
Student’s Name
Institutional Affiliation
Heap Leaching; Issues Preventing Effective Recoveries

HEAP LEACHING; ISSUES PREVENTING EFFECTIVE RECOVERIES 2
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).

HEAP LEACHING; ISSUES PREVENTING EFFECTIVE RECOVERIES 3
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
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

HEAP LEACHING; ISSUES PREVENTING EFFECTIVE RECOVERIES 4
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).
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

HEAP LEACHING; ISSUES PREVENTING EFFECTIVE RECOVERIES 5
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,

HEAP LEACHING; ISSUES PREVENTING EFFECTIVE RECOVERIES 6
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.

HEAP LEACHING; ISSUES PREVENTING EFFECTIVE RECOVERIES 7
References
Bouffard, S.C. and West-Sells, P.G. (2009). Hydrodynamic behavior of heap leach piles:
influence of testing scale and material properties, Hydrometallury, vol. 98. pp. 136-142.
Breitenbach, A.J., Thiel, R.S. (2005), A Tale of Two Conditions: Heap Leach Pad Versus
Landfill Liner Strengths, NAGS-GSI/GRI-19 Geosynthetics Conference, Las Vegas,
Nevada, USA, Dec.
Dixon, D.G. (2003). Heap leach modeling - The current state of the art. Hydrometallurgy2003,
Santiago, Chile.
Kelly, G. Ahlborn, G., Gunn, M., and Harvey, P. (2008). Laboratory and demonstration scale
optimization of the Quebrada Blanca heap leach bacterial regime using GeoLeachTM.
HydroProcess2008, Santiago, Chile.
Lampshire, D. and Braun, T. (2005). Heap leaching operations and practices at Cortez Gold
Mines. SME2005Annual Meeting, Salt Lake City, Utah.
Leiva, J., Rocha, M., Castro, S. Menacho, J., Troncoso, F., Arenas, A., and Dermergasso, C.
(2010). Bioleach of sulphide ores in mini-cribs: option to improve the sulphide leach
project in Minera Escondida. Hydro Process, 2010, Santiago, Chile.
Lewandowski, K.A., Kawatra, S.K., (2009), Binders for Heap Leaching Agglomeration,
Minerals & Metall. Process. Journal, SME, Littleton, Colorado, USA, Volume 26, No. 1.
Martin, V., Aubertin, M., Bussiere, B., and Chapuis, R. (2004). Evaluation of unsaturated flow in
mine waste rock. 57th Canadian Geotechnical Conference, Quebec, Canada.
McNab, B. (2006). Exploring HPGR Technology For Heap Leaching of Fresh Rock Gold Ores,
IIR Crushing & Grinding Conference, Townsville, Australia, March 29–30.

HEAP LEACHING; ISSUES PREVENTING EFFECTIVE RECOVERIES 8
Thiel, R.S., Smith, M.E., (2003), State Of The Practice Review of Heap Leach Pad Design
Issues, Proc. GRI-18, Las Vegas, Nevada, USA, vol. 22, pp. 555 - 568, Dec.
Zhou, J. and Yu, J-L. (2005). Influences affective the soil-water characteristic curve. Journal of
Zhejiang University, vol. 6A, no. 8. pp. 797-804.