Mining in focus: Time to go with the flow

There are many benefits to using pumpable emulsion explosives in underground mines, write Gerrie Bezuidenhout and Dr Nicolaas Steenkamp.

Burnstone gold mine 070
The advent of emulsion explosives eliminates the need to store explosives in the underground working environment.
Image credit: Leon Louw

Pumpable emulsion explosives have been available for more than two decades. Recent debates in the underground mining industry have focused on the efficiency and safety of this relatively new technology. The conservative school of thought holds that conventional blasting methods are more productive and efficient, while a more modern approach considers that emulsion is much safer than traditional blasting. 

Conventional explosives, in the form of factory-packaged final products, are delivered to the mine and stored in an explosives bunker on surface, before being dispatched underground and distributed to the faces for blasting. In contrast, the components of pumpable explosives are mixed together by the delivery system and then pumped directly into the blasthole.

Benefits of emulsion

The benefits of pumpable explosives compared to conventional explosives are many, but the primary consideration is that of safety, and not having to be treated with the same stringent regulations as conventional explosives. The use of UN Classified Class 1 explosives is much more arduous, time consuming, and regulatory binding than the use of UN Classified Class 5.1 blasting agents. The pumpable emulsion system is a Classification 5.1 oxidiser until it is sensitised during the charging process at the face, and therefore only becomes an explosive within the blasthole. As a result, the legislation applicable to their storage and transportation is also significantly less stringent, easing the burden of explosive control on the underground operation. This in turn requires less labour to ensure explosives are handled in accordance with regulations for magazine masters and transport personnel.

Other advantages of using pumpable emulsion explosives include improved productivity of the charging crew, and potentially reducing the overall cost for blasting and drilling operations. The increased blast performance of such products is also notable. However, despite its obvious benefits, the uptake by underground mines in South Africa has been slow, to say the least. Many mines have not yet adopted this technology.

Emulsion composition

Emulsion explosives were first patented by Americans Richard Egly and Albert Neckar in 1961, and were introduced into South African underground tabular mining operations in cartridge form in the early 1980s. Since then, the technology has advanced to ensure that it is safe to be pumped through a pumping system and delivered directly into the blasting hole.

Burnstone gold mine 043
Drilling according to the blasting pattern at the now defunct Burnstone underground gold mine in 2012.
Image credit: Leon Louw

Emulsion explosives are composed of two immiscible liquids with anaqueous oxidiser phase and a fuel oil phase making up the explosive. During the manufacturing process, the aqueous ammonium nitrate phase of the emulsion is divided repeatedly through a blending process, forming microscopic droplets of oxidiser suspended within the oil matrix. The microscopic size ensures an even distribution of oxidiser droplets within the matrix, which creates the intimacy of the oxidiser and fuel within the emulsion. The intimacy achieved through this process is higher than in ammonium nitrate fuel oil (ANFO) and slurry explosives. This results in emulsion being insoluble in water, making it ideal for use in wet mining operations, where it displaces water within wet blastholes due to the high initial density. This has been a huge benefit especially in bottom holes or holes sloping downwards, which are normally prone to water accumulation.

Improved fragmentation

Due to the intimacy between the constituents of the explosives, the emission of noxious gases is less than that of packaged explosives, while it ensures a comparatively high velocity of detonation (VOD) of pumpable emulsions. The high VOD of pumpable emulsions deliver a greater percentage of energy to the rock through shock (wave) energy as opposed to gas (expansive) energy, increasing the fragmentation of ore and reducing over-break and damage to surrounding rock strata.

Within this statement lies the age-old drive to ensure stability of surrounding rock and reduction of dilution of ore. This would be hugely beneficial to narrow reef mining and only within the last two years have significant strides been made to supply a product for this application by respective explosive suppliers.

The first applications of pumpable explosives were in opencast operations and in trackless mechanised mines that have large excavations. This is because a utility vehicle (usually fitted with an emulsion cassette) is required to transport the emulsion down the shaft and to effectively charge the development ends that are accessible. These units use pumps to pump the emulsion from the containers to the blastholes and also ensure the mix with the gassing agent, resulting in the emulsion turning into an explosive.

Face drilling of blast holes
Face drilling of blastholes in an underground gold mine.
Image credit: Dr Nicolaas Steenkamp

In-stope charging at conventional mines has been challenging due to safety concerns, as well as the remoteness and inaccessibility of large equipment that can perform the pumping work. Currently there are several explosives manufacturers that have developed solutions to perform charging of blastholes in narrow reef mining operations. This usually entails a portable charging unit that can be handled easily and carried up the travelling way into the raise. Being less than 15kg in total weight, the portable charging unit is a compact and lightweight piece of equipment that can be carried by one person, with an easy-to-handle pump and a closed emulsion system for the supply of pumpable emulsions to the working face. This means a single person can charge the face as opposed to an entire team that is required for charging a face with conventional explosives. The emulsion bags, weighing on average 20kg, are carried separately, but the introduction of emulsion bag filling stations underground, near the blasting face, will reduce the overall effort.

Pipelines for emulsion

Other operations pump the explosives via a pipeline into the stoping area. These systems consist of a vertical pipeline running from a surface location to a central underground storage facility. It is then dispatched to the filling stations in the stoping area via a flexible piping system.

The associated benefits of using pumpable explosives are mostly cost-effectiveness. This includes reduction in costs associated with transporting, packing, and storing bulk blasting agents, and reduced labour costs, as stated previously. With the correct blasthole layout, the number of holes drilled can be reduced and consequently, less drill steel is consumed. The emulsions can also be stored for extended periods with no reduction in quality because of crystal growth or product degradation.

With new technology, the product can be re-pumped several times (this differs from supplier to supplier and huge strides have been made to ensure that pumpable explosives are not desensitised by pumping actions). Some suppliers minimise product losses by installing radio frequency identification (RFID) to track the location of pumping units. The introduction of the underground filling stations will reduce the number of explosive transport vehicles used underground and could potentially increase flexibility in shaft access mines, as these emulsions can now be transported along with other materials.

Potential drawbacks

The negative aspects of emulsion are that although it can be stored longer than conventional explosives, it does have a finite shelf life and cannot be stored indefinitely, such as stored for use at an operation that is under care and maintenance. Eventually, the effectiveness of emulsion explosives is reduced through the growth of ammonium nitrate crystals. The emulsion will then have an improper detonation and minimum rock fragmentation will occur. In addition, ground vibration and air overpressure can increase up to five times that of a normal blast. Crystallised emulsions will produce large amounts of noxious gases, nitrous oxides, and carbon monoxide.

Automating the systems

Mechanisation and the introduction of robotics are considered as the way forward to increase productivity and mine safety. Underground equipment manufacturers are investigating opportunities to produce ultra-low profile mechanised equipment to reduce stoping width to below a metre in height. This advancement is essential to maintain the competitiveness of South African narrow reef operations. Emulsion explosives will be essential in these areas to ensure strict hangwall control and effective blasting in constrained areas.

Narrow Reef Mining
Blasting is an extremely risky undertaking in narrow reef mines, of which there are many in South Africa.
Image credit: Dr Nicolaas Steenkamp

The development of cruiser-based systems in trackless mines is also underway. These systems will be highly modernised and computerised. The development of these systems will see a low-energy pumping unit requiring less than 20% of the power of conventional systems, with an increase in pumping efficiency. The units would be adaptable to the power source used by different mines and could be operated with multiple energy sources, including compressed air, water, or electricity. The automated system aims to assist with optimal loading functionality, monitoring, and record flow rate, pump pressure, operating temperature, mass per hole, and the total mass charged per face. The system would ideally also generate automatic blast reports to allow for maximum operational control and ongoing blast optimisation.

The use of underground filling stations has also gone one step further with suppliers, in conjunction with mines, coming up with the idea of pumping emulsion explosives down a vertical borehole to an underground silo that is close to the current workings. This ensures that no strain is placed on the mine’s shaft or decline for transporting explosives. This has a huge saving on transport, and the payback for drilling the vertical borehole would thus be achieved within a year or two — if not sooner. The vertical borehole can also be moved once workings move further from the shaft, which is advantageous to ensure minimal transport of explosives are required and that emulsion explosives are always close to the stoping areas.

The benefits of pumpable explosives can only be fully realised by integrating mining engineering and geological considerations, as each hardrock environment poses unique challenges. Though these ‘semi-new’ technologies, like pumpable emulsions, are readily available, it is not widely adopted by operating mines. The question then must be asked: is the resistance to change too deep-seated on mines? This is an intimidating question, which requires an in-depth analysis from each mine to ensure that both appropriate and technologically advanced products are adopted.


Click below to read the October 2017 issue of Mining Mirror

MM Oct2017 160

 

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