Blasting method used in mining can be dangerous in mines closer to inhabited locations. Abhinay Hatewar explains the advantages of blast-free mining over the blasting method in mining.
The use of explosives is probably the most widely used means of fracturing the rock. This technique involves release of energy in a matter of milliseconds. If the energy release process is not adequately controlled, there can be a potential danger of environmental disturbance. This includes the effect of air blast, fly rock, changes in natural profile of ground, dust, fumes and ground vibrations.
The explosives' potential energy can be manifested as heat, rock fragmentation, fly rock air blast and ground vibrations. Depending upon the placement of explosive charge, the percentage distribution of the above charge may vary, except of heat emission, which will be constant in all situations.
Issues with blasting
Environmental issues: The rock blasting leads to a number of impacts on the environment. Open cast mining near the residential areas has become inevitable and therefore environmental impacts are required to be mitigated. Ground vibrations, noise and fly rock are the important environmental impacts as they may damage the properties.
Safety issues: Blasting near habitat, roads or establishment is out of the safety bond. Danger of fatality by fly rock is prominent. Other hazards are dust, poisonous fumes, ground vibrations, air blast (AOP), etc.
Selective mining: Loss of precious raw material mixed/sandwiched in ore body. Due to excess use of explosive, mixing of ore body with gangue can take place causing increase in time and work load for the excavation.
Explosive loss/safety: Handling the explosives is a dangerous and responsible job. Loss of explosives can be used for destructive forces. Also, if the rock is fractured, energy loss is obvious. The amount of explosives to be used can be controlled but destruction can be out of human limitations.
Secondary blasting/fly rock: When the rock is fractured or loosely compacted, there is formation of boulders. These boulders are blasted to break, in which, damage by fly rock is possible. Fly rocks are considered to be the most undesirable movement of rocks during the blasting activities. Damage by a fly rock cannot be refuted; the evidence is usually present and visible.
Disturbance of the natural ground profile: In an open cut mining operation by blasting, turmoil of the natural surrounding is often experienced with the end result of residual benches of bare rock. This can resemble a deep scar in the immediate environment.
Blast-free method as alternative
Considering the above-mentioned primary issues regarding blast mining, blast-free mining is an alternative, especially around inhabited locations. For blast-free mining, alternative to explosives, some machinery is used for progressive and sustainable development and for continuation in production.
Surface miner: This is a continuously operated mobile opencast machine. It cuts consolidated solids and semi-solid rocks without drilling and blasting, with cutting drum technology. The cut material is pre-crushed and make suitable for belt conveying, loading and transportation. Surface miner machines are the technology for extracting, crushing and loading material in one continuous process. When rock mass is softer, thinly bedded and flat bedded, this is widely considered as alternative to conventional drilling and blasting.
Dozer ripper: The ripper is the long claw-like device on the back of the bulldozer. The dozer ripper blade or the pusher blade can be raised or lowered through small angles horizontally by rams operated through hydraulic. The blades are used to push materials ripped, sand or soft weathered rocks. Rippers can come as single (single shank/giant ripper) or in groups of two or more (multi-shank rippers). Usually, a single shank is preferred for heavy ripping.
Vibro ripper: This technology uses the power of high frequency vibration and subsequent centrifugal force generated through it to extract stratified/fractured rocks from the bench. It works best on the surfaces with thin layers and significant cracks, which can be further expanded through ripper technology.
Hydraulic rock breakers: Large size hydraulic rock breakers (carrier size 40-100T class excavators) are used selectively for primary breaking applications. They use hydraulic percussion technology with multiple low energy impacts to extract the rock from the edges of the benches. A breaker is a percussion hammer fitted to an excavator for demolishing concrete structures or rocks.
Hydraulic rock splitter: The hydraulic rock splitter and concrete splitter/ breaking/bursting is driven by the high-pressed oil from hydraulic pump to work. The technology takes the advantage of hydraulic principle, which can generate tonnes of splitting force. Used for construction, tunnelling, etc,this is however limited by very low production.
Plasma blasting technology: This involves the production of a pulsed electrical discharge by inserting a blasting probe in a water-filled cavity drilled in a rock, which produces shocks or pressure waves in die water. These pulses then propagate into the rock, leading to fracture.
Foam injection: This method utilises a proprietary controlled foam injection (CFI) technique to fracture rock and concrete. The use of high-pressure foam as the fracturing medium has been shown to reduce significantly the air blast, fly rock and toxic fume problems associated with explosive-based techniques. The CFI method may be used in very close proximity to personnel, sensitive structures or equipment.
Houtec impact hammers: This is an ideal solution for extraction/primary breaking of hard rock without drilling/blasting. Very high amount of impact energy is being generated in this hammer which is enough for breaking rock in in-situ condition. The hammer is highly productive and most cost-effective solution in hard rock formations like black/blue basalt, granite, hard lime stone, iron ore, manganese, bauxite, etc. Microwaves: The use of microwaves is being experimented and tested. While it has shown positive results, the technology is in a very nascent stage and is yet to be evolved for practical or mass use.
Productivity & costing
As of now, conventional drilling and blasting is the most productive and cheaper. So use of any unconventional methods are generally out of compulsions, restrictions, laws, habitats, etc. The cost of unconventional methods also varies with the use of the type of rock, expected production, site limitations, use of machines, etc. When machinery/technologies are chosen wisely, the cost of blast-free mining remains marginally higher than the conventional methods.
Obviously there would be resistance to change. The sector also is generally unaware of the available technologies and matching machinery with respect to rock, site or production needs.
Environmental norms are getting stricter on drilling and blasting activities. Mines are being surrounded by habitats, establishments, roads, etc so that issues of safety and security of explosive have the potential for civil danger. Since mining and quarrying come under important economic activities, the need has been established for unconventional blasting methods with mechanical alternatives.
The author is Director, Houtec Engineering & Services Pvt Ltd.