Recent numbers from the International Copper Study Group (ICSG 2017) show that the global demand and production of copper has more than tripled in the last 50 years. This has been in response to increasing populations and economic growth. Demand is expected to increase even further in response to the shift towards electric cars and renewable energy sources, which are heavily reliant on copper. However, the ICSG (2017) also identified several constraints on copper supply, which includes declining ore grades, especially as near surface resources are exhausted and mining is pushed deeper.
Forecasts indicate that there will be an increase in copper production from underground operations over the next decades as available resources trend deeper. Already the industry is seeing the transition of several large open pits to underground mass mining operations to access deeper resources, with mass mining methods such as block and panel caving being favoured due to the tonnages and economics achievable when dealing with lower grade ore.
These methods involve developing an extraction level beneath the ore, followed by undercutting to initiate caving. As the broken rock is mined from different drawpoints on the extraction level, the cave propagates upwards through the orebody providing a constant feed of broken ore to be mined. This has seen the use of cave mining directed towards weak, massive, and steeply-dipping orebodies, for example copper-gold porphyry and diamond kimberlite deposits. As noted above, it is expected that block and panel caving methods will be increasingly used in the copper-gold industry. Their implementation holds significant potential to extend the life of existing open-pit operations by providing an economic means to access ore located at depths that are beyond the practical limits of open-pit mining. Examples of cave mines proposed for British Columbia alone include the Kwanika Mine, Red Chris underground, Kemess Underground project, and the Iron Cap and Mitchell cave mines of the Kerr-Sulphurets and Mitchell (KSM) project (Moose Mountain Technical Services, 2017; Imperial Metals, 2017; Golder Associates, 2017; Tetra Tech, 2016). New Gold operates an existing caving operation at their New Afton mine.
With the need to mine deeper, comes several key challenges that expose limitations in existing mining practices and engineering design tools. Current techniques are based on poorer quality rock masses and lower stress environments, coupled with energy-demanding material handling processes. Moss (2014) argues that step change is required for the industry to transition to cave mining at the scale required to achieve similar tonnages to open pit mining, while also managing the project risk and increased energy demands resulting from mining deeper. These include: i) more limited geological data due to greater drilling depths and therefore greater geological uncertainty, ii) encountering higher stresses and greater risk of ground control resulting in increasing safety hazards and costly interruptions to production, and iii) increased energy demands for material handling and processing. Compounding these project risks is that approximately 70 percent of capital costs are consumed before any revenue is generated (Oancea, 2013).
Thus, caving has a very different risk profile than open pit mining and other underground methods with one of the critical areas being Run of Mill (ROM) grades that are delivered to the mill. Though caving can match pits in terms of operating cost per ton, it suffers from limited grade selectivity with ROM grades trending toward the orebody average due to consequences of mixing in cave columns.
Research and subsequent operational trials have demonstrated that bulk sorting can manage this lack of selectivity by “pre‐concentrating” the ROM ore, providing the mill with a more consistent and higher grade feed. Their implementation holds significant potential to extend the life of existing open‐pit operations by providing an economic means to access ore located at depths beyond the practical limits of open‐pit mining.
Building on momentum from recent industry-sponsored caving research programs, the caving team at the University of British Columbia established the International Caving Research Network (ICaRN) that targets a series of research projects aimed at optimizing the value of cave mining operations.