Blast Hole Drilling
Blast hole drilling techniques are used in quarrying, mining and tunneling to derive economic value from rock masses through:
- Creating a blast pattern
- Blast hole drilling into the rock surface
- Charging the blast hole with explosives
- Detonating the charges in a planned sequence to achieve different rock fragments.
The bulk of blasting in mines involves charging explosives into holes drilled in rock. The mouth of the drill hole is called the collar and the end is the toe. Drilling is carried out by hand, hammer and chisel which tend to be tedious and slow processes in hard-rock mining. The limitations to hand-drilled holes does not exceed 400 mm in length. In modern day operations, mechanical rockdrills are predominantly used. Blasting in a mine generally occurs in two phases, primary blasting and secondary blasting. Primary blasting involves breaking down of in-situ rock in development and stoping as well as by drilling holes in rocks. Secondary blasting is performed to get rid of overburden. Drill and blast design is a key component of mine planning and management. Its major impacts are on safety and efficiency. Calculation of charge density (kg of explosives per cubic meter or tonne of rock) for different applications, rock types and explosives is very theoretical. In practice, operation experience is beneficial for success in designing drilling and charging patterns for different conditions.
What is blast hole drilling?
Blast hole drilling is a technique used in mining whereby a hole is drilled into the surface of the rock, for the purposes of packing it with explosives and detonation. The intention is to induce cracks in the inner geology of the surrounding rock, in order to facilitate further drilling and associated mining activity. The initial hole into which the explosives are packed is known as the “blast hole”. Blast hole drilling is a primary surface drilling procedure used in mining operations.
Where is blast hole drilling used? Traditionally it is used wherever the mining company wants to explore the mineral composition or potential mineral yield of the area of their mining interests. Blast holes are a fundamental step in the exploratory mining process and can be used in both surface and underground mining operations. Why is blast hole drilling used? It is carried out in order to break up rock and hard minerals to make it easier for mining crews to get to the resources being mined.
What steps are taken in blast hole drilling process:
- Survey the location
- Researching the rock types and formations
- plan up the drilling patterns
- drilling the blast holes
- breaking up the rock - blasting
- clean up process
What techniques are used for rock blasting?
There are different techniques and methods used in rock blasting. The following are the most common:
- Open mine blasting
- Underground blasting
- Controlled blasting and
- Secondary blasting
Open pit mining involves mining minerals from an open pit or any similar shallow mining operation. Open mine blasting plays a pivotal role in determining the needs of all other mining activities to come. From loading and unloading, to transport plans, the initial blasting will affect the cycle of activities to come.
Underground mining, blasting is the primary method for rock excavation. Research is done on the effect of the vibration of the explosion, to the likely impact of the blast on the existing mining structures, before blasting takes place. Given that underground blasting is controlled cave-in, safety is a great concern hence calculations and potential outcomes are checked and rechecked. On achieving the desired blasting, debris is cleared out and the resultant tunnel is reinforced.
Controlled blasting is employed when one aims to reduce the amount of overbreak or to control the extent of the resultant ground vibrations. Controlled blasting techniques include pre-splitting, trim or cushion blasting, smooth, perimeter or contour blasting and muffle blasting.
Secondary blasting is blasting done for the second time in order to reduce the previously blasted portion of rock to a manageable size for excavators and crushers. The two methods available include plaster or mud-cap blasting and pop-shooting.
Risks and Hazards in Drilling and Blasting
There are risks and hazards involved in drilling and blasting operations. Drilling hazards include rotating and moving parts such as drill rods, drill strings and rod handlers. The direct risk factors include exposure to rotating or moving parts which have contributing risk factors such as inadequate or lack of safeguarding and severity of the risk can lead to amputation and scalping.
Compressed air is by far the most dangerous of all fluids used in the drilling industry. To put this into perspective, unlike oil or water, a given volume of air can be reduced by compression, which can be likened to a compressed spring. The uncontrolled release of stored energy can be lethal. An example is debris from blown hoses can travel at speeds such that there is no time to react.
Hazardous substances and dangerous goods such as diesel, petrol, acids, oils, and grease can lead to spills and leaks which present a fire hazard. Exposure to hydrocarbons may result in dermatitis and other illnesses. Contact with acids can cause chemical burns. Spilt or leaking oil and fuel can present a slip hazard to workers. Spilt hydrocarbons can also attract dirt and dust from the drilling operations, which makes detecting and monitoring of leaks difficult.
Manual tasks undertaken during drilling operations include physical work such as lifting, lowering, pushing, and pulling. Repetitive actions like hammering, sustained postures and concurrent exposure to vibration can lead musculoskeletal disorders. Drilling operations generate substantial amounts of dust. Dust results in long-term health effects such as lung disease, systemic toxicity, and irritation of the respiratory tract.
Hazards in the drilling and blasting process includes the presence of some ground structures such as hard rock boulders and voids, heavy seepage of ground water into the blast-holes, blast-holes that have high temperature within them and deviation of blast-holes.
Commonly used explosive ANFO has the AN component that dissolves in water leading to a feeble blast on no blast occurring at all. Hot blast holes create temperature zones which can lead to explosive deflagration or detonation immediately or after some time.
Hazards of the blasting process include misfires, ground vibrations, air blast, fly rocks, air pollution and environmental changes. Misfires occur when a charged blast-hole does not explode or when a part of explosive column in a blast-hole has not exploded along with other blast-holes. A neglected misfire can suddenly explode which presents risks and hazards to the environment and can lead to fatalities.
On blasting an explosive in a blast-hole, the chemical reaction evolves a huge quantity of energy. The ground vibrations radiate out from the blast-hole, the intensity continues to reduce and at long distance it becomes too low to be perceived. At nearby distances the ground vibrations have sufficiently high energy to shatter many structures firmly embedded into ground.
Air blast, often called air overpressure because the rapid movement of rock mass, and the ejection of gases under extremely high pressure. These air blasts can lead hearing loss in humans. Buildup of excessive pressure within the blast-hole exerts a propelling force on the pieces of rock and makes them fly like a projectile. Direction of fly rock is very unpredictable.
Respirable dust as a hazard
Dust from blasting activities contains inhalable and respirable crystalline silica. Silicosis because of silica exposure kills people all over the world. GRT offers innovatory chemical dust suppression products that eliminate dust at its source. As dust suppression global leaders, GRT introduced two new products on the market for drill and blast dust control. Be sure to read more about GRT: 12X for blast and drill dust control and GRT DC Binder for blast pattern dust control which are cutting edge chemical technologies fulfilling the mandate of eliminating dust generated from drilling and blasting operations in Australia
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