It’s an inconvenient fact that most of Earth’s largesse of useful minerals is locked up in, under, and around a lot of rock. Our little world condensed out of the remnants of stars whose death throes cooked up almost every element in the periodic table, and in the intervening billions of years, those elements have sorted themselves out into deposits that range from the easily accessed, lying-about-on-the-ground types to those buried deep in the crust, or worse yet, those that are distributed so sparsely within a mineral matrix that it takes harvesting megatonnes of material to find just a few kilos of the stuff.

Whatever the substance of our desires, and no matter how it is associated with the rocks and minerals below our feet, almost every mining and refining effort starts with wresting vast quantities of rock from the Earth’s crust. And the easiest, cheapest, and fastest way to do that most often involves blasting. In a very real way, explosives make the world work, for without them, the minerals we need to do almost anything would be prohibitively expensive to produce, if it were possible at all. And understanding the chemistry, physics, and engineering behind blasting operations is key to understanding almost everything about Mining and Refining.

First, We Drill

For almost all of the time that we’ve been mining minerals, making big rocks into smaller rocks has been the work of strong backs and arms supplemented by the mechanical advantage of tools like picks, pry bars, and shovels. The historical record shows that early miners tried to reduce this effort with clever applications of low-energy physics, such as jamming wooden plugs into holes in the rocks and soaking them with liquid to swell the wood and exert enough force to fracture the rock, or by heating the rock with bonfires and then flooding with cold water to create thermal stress fractures. These methods, while effective, only traded effort for time, and only worked for certain types of rock.

Mining productivity got a much-needed boost in 1627 with the first recorded use of gunpowder for blasting at a gold mine in what is now Slovakia. Boreholes were stuffed with powder that was ignited by a fuse made from a powder-filled reed. The result was a pile of rubble that would have taken weeks to produce by hand, and while the speed with which the explosion achieved that result was probably much welcomed by the miners, in reality, it only shifted their efforts to drilling the boreholes, which generally took a five-man crew using sledgehammers and striker bars to pound deep holes into the rock. Replacing that manual effort with mechanical drilling was the next big advance, but it would have to wait until the Industrial Revolution harnessed the power of steam to run drills capable of boring deep holes in rock quickly and with much smaller crews.

The basic principles of rock drilling developed in the 19th century, such as rapidly spinning a hardened steel bit while exerting tremendous down-pressure and high-impulse percussion, remain applicable today, although with advancements like synthetic diamond tooling and better methods of power transmission. Modern drills for open-cast mining fall into two broad categories: overburden drills, which typically drill straight down or at a slight angle to vertical and can drill large-diameter holes over 100 meters deep, and quarry drills, which are smaller and more maneuverable rigs that can drill at any angle, even horizontally. Most drill rigs are track-driven for greater mobility over rubble-strewn surfaces, and are equipped with soundproofed, air-conditioned cabs with safety cages to protect the operator. Automation is a big part of modern rigs, with automatic leveling systems, tool changers that can select the proper bit for the rock type, and fully automated drill chain handling, including addition of drill rod to push the bit deeper into the rock. Many drill rigs even have semi-autonomous operation, where a single operator can control a fleet of rigs from a single remote control console.

Proper Prior Planning

While the use of explosives seems brutally chaotic and indiscriminate, it’s really the exact opposite. Each of the so-called “shots” in a blasting operation is a carefully controlled, highly engineered event designed to move material in a specific direction with the desired degree of fracturing, all while ensuring the safety of the miners and the facility.

To accomplish this, a blasting plan is put together by a mining engineer. The blasting plan takes into account the mechanical characteristics of the rock, the location and direction of any pre-existing fractures or faults, and proximity to any structures or hazards. Engineers also need to account for the equipment used for mucking, which is the process of removing blasted material for further processing. For instance, a wheeled loader operating on the same level, or bench, that the blasting took place on needs a different size and shape of rubble pile than an excavator or dragline operating from the bench above. The capabilities of the rock crushing machinery that’s going to be used to process the rubble also have to be accounted for in the blasting plan.

Most blasting plans define a matrix of drill holes with very specific spacing, generally with long rows and short columns. The drill plan specifies the diameter of each hole along with its depth, which usually goes a little beyond the distance to the next bench down. The mining engineer also specifies a stem height for the hole, which leaves room on top of the explosives to backfill the hole with drill tailings or gravel.

Prills and Oil

Once the drill holes are complete and inspected, charging the holes with explosives can begin. The type of blasting agents to be used is determined by the blasting plan, but in most cases, the agent of choice is ANFO, or ammonium nitrate and fuel oil. The ammonium nitrate, which contains 60% oxygen by weight, serves as an oxidizer for the combustion of the long-chain alkanes in the fuel oil. The ideal mix is 94% ammonium nitrate to 6% fuel oil.

How the ANFO is added to the hole depends on conditions. For holes where groundwater is not a problem, ammonium nitrate in the form of small porous beads or prills, is poured down the hole and lightly tamped to remove any voids or air spaces before the correct amount of fuel oil is added. For wet conditions, an ammonium nitrate emulsion will be used instead. This is just a solution of ammonium nitrate in water with emulsifiers added to allow the fuel oil to mix with the oxidizer.

ANFO is classified as a tertiary explosive, meaning it is insensitive to shock and requires a booster to detonate. The booster charge is generally a secondary explosive such as PETN, or pentaerythritol tetranitrate, a powerful explosive that’s chemically similar to nitroglycerine but is much more stable. PETN comes in a number of forms, with cardboard cylinders like oversized fireworks or a PETN-laced gel stuffed into a plastic tube that looks like a sausage being the most common.

Being a secondary explosive, the booster charge needs a fairly strong shock to detonate. This shock is provided by a blasting cap or detonator, which is a small, multi-stage pyrotechnic device. These are generally in the form of a small brass or copper tube filled with a layer of primary explosive such as lead azide or fulminate of mercury, along with a small amount of secondary explosive such as PETN. The primary charge is in physical contact with an initiator of some sort, either a bridge wire in the case of electrically initiated detonators, or more commonly, a shock tube. Shock tubes are thin-walled plastic tubing with a layer of reactive explosive powder on the inner wall. The explosive powder is engineered to detonate down the tube at around 2,000 m/s, carrying a shock wave into the detonator at a known rate, which makes propagation delays easy to calculate.

Timing is critical to the blasting plan. If the explosives in each hole were to all detonate at the same time, there wouldn’t be anywhere for the displaced material to go. To prevent that, mining engineers build delays into the blasting plan so that some charges, typically the ones closest to the free face of the bench, go off a fraction of a second before the charges behind them, freeing up space for the displaced material to move into. Delays are either built into the initiator as a layer of pyrotechnic material that burns at a known rate between the initiator and the primary charge, or by using surface delays, which are devices with fixed delays that connect the initiator down the hole to the rest of the charges that will make up the shot. Lately, electronic detonators have been introduced, which have microcontrollers built in. These detonators are addressable and can have a specific delay programmed in the field, making it easier to program the delays needed for the entire shot. Electronic detonators also require a specific code to be transmitted to detonate, which reduces the chance of injury or misuse that lost or stolen electrical blasting caps present. This was enough of a problem that a series of public service films on the dangers of playing with blasting caps appeared regularly from the 1950s through the 1970s.

“Fire in the Hole!”

When all the holes are charged and properly stemmed, the blasting crew makes the final connections on the surface. Connections can be made with wires for electrical and electronic detonators, or with shock tubes for non-electric detonators. Sometimes, detonating cord is used to make the surface connections between holes. Det cord is similar to shock tube but generally looks like woven nylon cord. It also detonates at a much faster rate (6,500 m/s) than shock tube thanks to being filled with PETN or a similar high-velocity explosive.

Once the final connections to the blasting controller are made and tested, the area is secured with all personnel and equipment removed. A series of increasingly urgent warnings are sounded on sirens or horns as the blast approaches, to alert personnel to the danger. The blaster initiates the shot at the controller, which sends the signal down trunklines and into any surface delays before being transmitted to the detonators via their downlines. The relatively weak shock wave from the detonator propagates into the booster charge, which imparts enough energy into the ANFO to start detonation of the main charge.

The ANFO rapidly decomposes into a mixture of hot gases, including carbon dioxide, nitrogen, and water vapor. The shock wave pulverizes the rock surrounding the borehole and rapidly propagates into the surrounding rock, exerting tremendous compressive force. The shock wave continues to propagate until it meets a natural crack or the interface between rock and air at the free face of the shot. These impedance discontinuities reflect the compressive wave and turn it into a tensile wave, and since rock is generally much weaker in tension than compression, this is where the real destruction begins.

The reflected tensile forces break the rock along natural or newly formed cracks, creating voids that are filled with the rapidly expanding gases from the burning ANFO. The gases force these cracks apart, providing the heave needed to move rock fragments into the voids created by the initial shock wave. The shot progresses at the set delay intervals between holes, with the initial shock from new explosions creating more fractures deeper into the rock face and more expanding gas to move the fragments into the space created by earlier explosions. Depending on how many holes are in the shot and how long the delays are, the entire thing can be over in just a few seconds, or it could go on for quite some time, as it does in this world-record blast at a coal mine in Queensland in 2019, which used 3,899 boreholes packed with 2,194 tonnes of ANFO to move 4.7 million cubic meters of material in just 16 seconds.

There’s still much for the blasting crew to do once the shot is done. As the dust settles, safety crews use monitoring equipment to ensure any hazardous blasting gases have dispersed before sending in crews to look for any misfires. Misfires can result in a reshoot, where crews hook up a fresh initiator and try to detonate the booster charge again. If the charge won’t fire, it can be carefully extracted from the rubble pile with non-sparking tools and soaked in water to inactivate it.