Why is drilling and blasting used in underground mining? It is performed on confirmation of economic value in mineral deposits present in the rock masses that hold the deposits of interest, and the geotechnical nature of the parent rock. As a process it is a technique which follows a sequence of (1) setting up a blast pattern (2) drilling a blast hole into the rock surface (3) packing it with explosives (4) detonating it to achieve different rock fragments. Blasthole drilling is one of the most important operations from a cost and its influence on the performance of the crusher, mill and concentrator post blasting. Good drilling accuracy enables efficient controlled blasting operations. Different blasting techniques are used in underground mining. Smooth blasting is popular, and it comes in a variant of double smooth blasting as well. Drilling costs are generally the same for the two types of smooth wall blasting but the fractured area in the remaining rock is greater in smooth blasting as compared to double smooth blasting. This lowers reinforcement costs for double smooth blasting by 50%. Deviation of holes drilled for blasting purposes can affect production costs in certain mining operations. Therefore, it is key to give priority to invest in long hole drilling accuracy.
The initial form of blasting is known as primary blasting. The reduction of oversize boulders after primary blasting is known as secondary blasting. Drilling and blasting operations have a reliable measure of the effect of improvement in drilling precision based on the reduction in need to perform secondary blasting. This article will discuss drilling and blasting in underground mining operations focusing on smooth wall blasting operations, safety and environmental concerns and best industry practice in underground mining dust and gas control.
Drilling and blasting methods in underground mining – smooth wall blasting.
Drilling in underground mining is very diverse but used for two main purposes. Firstly for exploration which aims to obtain very detailed information about the formations below ground surface. Secondly for production to facilitate the labors of ore extraction. Smooth wall blasting is the most commonly used technique in underground mining and it was developed in Sweden. The smooth wall line is composed around the final excavation line underground where the holes are lightly loaded to reduce the amount of overbreak. The explosives are also decoupled from the sides of the borehole. The final feature of smooth wall blasting is that the smooth wall holes are initiated after the primary blast. Smooth wall blasting will allow blast damage to extend beyond the smooth wall line before they are initiated. The most damaged rock will be removed from the final excavation line and result in a smooth profile for the perimeter. The smooth wall technique also involves more perimeter drill holes when compared to conventional underground methods. Drilling costs have the greatest impact on the economics of blasting operation. Smooth wall blasting is known to have performance issues in weak rock formations. If the rock mass is too weak to support itself, the smooth wall blast will not eliminate the need for additional support. Drilling accuracy is a paramount concern with smooth wall blasting. Due to additional challenges of drilling and blasting in an underground heading, drill accuracy is critical to the success of a smooth wall blast. When the smooth wall holes cannot all be fired on a single delay, the relief is limited to the arch and partially down the rib because of the muckpile. Therefore, smooth blasting results will degrade further down the rib of the excavation line, which may cause safety concerns in larger entries.
Safety and environmental concerns of underground blasting and drilling
Drill and blast design is an important part of the mine planning and management, with major impact on safety and efficiency. Underground mines usually have designated primary times daily. Primary blasting is carried out in both development and stoping. Working safety in underground mining is related to blast operations, rock conditions and mining methods. The potential for roof problems, rock spalling, rock bursts, rock falls and brow damage is rife. It is a common problem that rock blasting in underground mines makes local underground water contaminated. The problem stems from two reasons. Firstly in multi-hole blasting, the explosives in one or more holes is not detonated. The undetonated explosive will partly go into the local underground water system and partly be transported to mineral processing plants together with the ores. When an upward hole is charged, for instance in a sublevel ring, the explosive sinks in boreholes and then falls to the floor during and after charging. This frequently happens in production charging, especially when emulsion is pumped into a wet hole. Guided by the need to increase ore recovery through reduction of resource loss in mining operations, safety in underground mining is non-negotiable and should not happen at the expense of mine workers. To increase profit in the mining process the worker safety and health should complement the economic gains from underground mining operations.
Managing risks – dust and gas control in underground mining
Every drill and blast operations generates dust. The amount of dust generated in these operations is a cause for great concern. Dust is a health hazard and more importantly dust kills. Exposure to dust from drilling should be separated out from blasting operations, as drilling will see more workers exposed to dust. Although a violent blast may produce more than the normal amount of dust, blasting is a relatively infrequent operation, and exposure can be managed by allowing an appropriate amount of time for the dust to settle, be suppressed or to be expelled before miners enter the blast area. Additionally, the total amount of dust produced in a day is less compared to the overall dust produced from other frequent operations such as loading, hauling, crushing and processing. Operationally, well-controlled blasts create little or no dust. Provision of dust palliatives in the drilling process is then best industry practice to eliminate dust when workers are exposed to it. GRT’s 12X technology is an innovative example of a product developed to manage drilling dust at the source.
The use of explosives in blasting poses danger of toxic gases. The most common are carbon monoxide and oxides of nitrogen. Blast fumes are quickly diluted to below toxic levels by the ventilation systems in underground mines. Therefore it is very important to use well calculated blasting agent mixtures, store blasting agents properly to avoid deterioration, use of non-water resistant products in wet blastholes and have enough confinement for efficient detonation.
Drill and blast operations – a key element of a successful operation
Drilling and blasting in underground mining is very important to achieve the intended exploration benefits. A proper plan must be set up which takes into account the rock formation properties as obtained from geological investigations. The drilling pattern chosen should suit the intended blasting operations to achieve minimal overburden at the primary blasting level. The need for secondary blasting to remove overburden should be minimal if the drilling patterns are set up properly. Finally, the economic value of drilling and blasting operations should not happen at the expense of worker safety and health. It is important to implement industry best practice to ensure dust and gas produced from drilling and blasting do not pose any health hazards to the mine workers. GRT takes an no compromise stance to saving lives and its sole mandate is to offer solutions at the apex of solving any dust related problem arising from drilling and blasting activities before it takes place.
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REFERENCES
Singh et al. 1993. Blasting in Ground Excavations and Mines. A.A. Balkema/Rotterdam.
Zhang. Z-X. 2016. Rock Fracture and Blasting. Theory and Applications. Elsevier.
