By Wynand Marx, CEO of BBE Group

While both underground and open-cast mining are common around the world, some open-cast mines transition to underground operations, hence the upper part of the ore body is exploited by open-cast method, and the deeper part is mined by underground method.

Open-cast mining is the obvious method for orebodies existing close to the surface such as vertical diamond or copper pipes continuing to greater depths or an orebody with a wide area of mineralisation exposed such as coal deposits.  In contrast, narrow-reef, or flat dipping orebodies typical for gold and platinum are located deep below the surface and have to start as underground mining operations. For orebodies extending to depth, mining starts as open-cast, if the intrusion is close enough to the surface, as it is more economical to mine this way.  The decision to transition from open-cast to underground mining depends on the depth and shape of the orebody and driving factors include declining ore grades and deeper ore deposits making it uneconomical to extend open-cast workings deeper.

One of the key mine design features to be considered when transitioning from open-cast to underground mining is the need for ventilation and/or refrigeration systems to ensure safe and feasible working conditions.

No ventilation required

Open-pit mines, which are exposed to the atmosphere, do not require any form of mechanical ventilation. The only real environmental factor, from a ventilation perspective, is potentially heat and the generation of dust. Open-cast operations can suffer from localised heat issues, caused by atmospheric conditions and vehicle-generated heat. Stagnant air could also be present as the lower levels of an open pit as it is not exposed to wind impacting on both dust and heat dilution. However, these factors are not severe enough to require the installation of a ventilation solution other than localised vehicle cabin air conditioning.

Conversely, underground mines must be ventilated through an intake and return system, using fans to extract and pull air through the underground workings. There must also be sufficient volume of airflow to the underground operations to dilute and remove dust, pollutants, and noxious gases and to regulate temperature. These gasses and pollutants are typically produced by diesel-powered equipment, blasting with explosives, mining activities as well as the orebody itself. Deep-level mining operations also typically require large refrigeration installations to create acceptable working conditions for miners.

One of the main challenges when transitioning from an open-cast to underground mining method is ensuring adequate ventilation during decline or shaft-sinking operations, as at this stage the mine will not have all the required airways in place. During this phase, it might be necessary to ventilate the shaft with long duct runs and auxiliary fans, until a downcast and upcast shaft decline/system is established, and a main fan is added to the system.

Capital expenditure

Underground ventilation and refrigeration installations require a significant amount of equipment to set up, requiring significant capital expenditure. In addition, the biggest component of the operating cost for mine ventilation is the electricity required to power the fans and refrigeration systems. This could account for up to one-third of a typical underground mine's entire electrical power cost.

Finally, as opposed to open-cast mining, underground mining is typically less energy efficient and have a larger carbon footprint than open-cast mines, due to the larger amount of electricity they consume to run the refrigeration and ventilation systems.  However, the correct ventilation system design will ensure minimal capital and maximum efficiency.  In addition, the correct quantity of airflow and a proper ventilation system ensure that mining has a minimal to no environmental impact from diesel exhaust gasses, blast fumes, dust, and other pollutants.