The goal of drilling and blasting is more than just putting rock onto the ground for the processing plant. Many mine operators do not make efforts to optimize the drilling and blasting process until they run into problems, such as excessive oversize, which requires additional processing maneuvers such as breaking, or an abundance of fines, which can affect crusher wear parts and also creates waste.
“The goal with every shot should be to improve production and efficiency in the plant,” says Billy Zimmerman, senior blasting technician for Luck Stone. “It is a balancing act, through shot design, powder factor, and loading techniques — which are 90 percent of how your shot turns out — and fine tuning with timing, to create uniformity in your shot.”
The shot will affect the amount and size of material for the plant, and it should be optimized to achieve the best fragmentation size for loading and hauling, for crushing processes, and for helping to meet product specifications at the lowest cost, according to Andrew Heinemann, manager of geology and mine engineering at Benchmark Resources. “Generally, drilling and blasting has the highest effect on the cost to produce the final product,” he adds.
Matthew Boatman, president of General Drilling agrees. “Paying for professional drilling and blasting can reap benefits in the processing plant,” Boatman says. “Better drilling and blasting will result in a more efficient plant. This will improve the production and quality of your material in the plant, and it will lower your costs overall.”
Reduced overall mining and processing cost is influenced by the drill and blast design — specifically the drill-hole diameter, drill-hole spacing, type of blasting agent and initiation, subdrilling, stemming, burden width, and timing. These are the variables in a drill and blast design that can be controlled and adjusted to optimize fragmentation at the lowest cost, effectively lowering the overall cost of mining and processing.
Heinemann says an important parameter that often is linked to the distribution of explosive energy in the blast is the drill-hole diameter. It controls the distribution of energy in the blast and, therefore, affects fragmentation. “Large diameters are often associated with expansion of drilling patterns; however, large holes intersect fewer in-situ blocks of rock, resulting in more oversized material, especially in the case of jointed rock,” he says. Typically, the drill-hole diameter changes depending on the geology or the type of drill machine.
Similarly, changing the bench height (e.g., for a new loading machine) affects all dependent parameters or the fragmentation size of the blast muck pile. Modifying a drill-hole diameter, a bench height, or a product size tends to change all other relevant blast design parameters. Comparative calculations in every case allow the designer to determine the optimum cost parameters.
Drilling & Blasting: Optimizing for Production
For more than 18 years, Billy Zimmerman has been a senior Bblasting technician for Luck Stone, based in Richmond, Va., where he is responsible for blasting and also training of blasting personnel for the company at 16 different plants. Prior to joining the company, he also worked for blasting, explosives and engineering contractors as a blast designer, blaster, sales rep, and tech rep. Zimmerman began his career in 1978 working for the Virginia Division of Mined Land Reclamation.
Andrew Heinemann, manager of geology and mine engineering at Folsom, Calif.-based Benchmark Resources, is a California-registered geologist and mining engineer with more than 20 years of experience in mine planning; engineering; and plant management, operations, and maintenance. Heinemann earned a bachelor of science degree in geology from the California State University, Sacramento, in 1995 and a master of engineering degree in mining engineering from Missouri University of Science and Technology in 2010.
Matthew Boatman, president of General Drilling Inc., graduated from Purdue University in 1997 with a degree in geological engineering. Boatman has been active in the industry his whole life and has been a presenter at NSSGA’s drilling and blasting workshops and AGG 1 since 2004. He has previously served on the NSSGA Young Leaders Steering Committee and the NSSGA M&S Board of Directors. He has also presented on various safety topics for MSHA at both the local and national level.
Voices of Experience
“The cheapest cost to produce rock is at the face,” says Matthew Boatman, president of General Drilling. “It can also be the most expensive, if it produces oversize (materials) or excessive fines.” The value of a good drilling and blasting program is that, while the program may cost the producer a little more up front, the processing plant will be more efficient, which will increase production and improve product quality and overall cost in the end.
In addition to putting product on the ground that will sell, the goal of every operation is throughput, notes Boatman. Producers want quick drilling, an effective shot, and the material to be loaded into the primary and sent through the plant to the stockpile. This starts with a good blast design and proper drilling. In order to drill the prescribed pattern correctly, an experienced driller needs to know geology, and needs to be able to detail what was encountered du ring the drilling for purposes of the log, but also so that the blaster has a better idea of any geological anomalies within the shot location.
To optimize drilling rates, the drill operator must be a trained expert. The drill must be maintained at a very high level. If the operator is not able to get the hydraulic pressure correct or if the drill bit is worn, for example, the drill operator cannot achieve optimum drill rates.
“My value is in providing a good bore hole or blast hole in a timely manner by trained, qualified personnel,” Boatman says. “There is no one-size-fits-all with drilling. You have to have the right people and equipment for each unique situation.”
Production can be affected by numerous variables tied to drilling and blasting. But geology’s effect on the shot — and ultimately on material flow through the processing plant — is an element that cannot be altered. “Geology is always a factor with the shot,” says Billy Zimmerman, senior blasting technician for Luck Stone. “From the type of rock, to geological seams that can alter the shot — such as a mica seam that can affect the shape of the rock — you always have to take geology into consideration with the blast.”
Zimmerman explains that seams of quartz, or dikes of basalt and diabase will affect the shape of rock, creating more pointy, flat, and elongated stone. “There may not be much that can be done with some of these issues,” he says. “And typically, such geology may only be found in one section of a pit, so the answer will be to blend it with other material during production.”
Luck Stone handles its blasting in-house, with a professional plant blaster at each location. “We have an associate at each plant who is familiar with that pit and its unique geology. This helps with determining a blast pattern and powder factor combination that will give you a quality shot each time,” he says. Luck Stone also uses 2D face profiling systems, which help the blast designer to determine a perpendicular cross section of the face at each blasthole location and adjust the hole to the correct distance from the face. “We may decide to vary the angle of the blastholes or move the driller back from the wall — all with the goal of creating more consistent burden.”
Another geological factor affecting blasting is the presence of water. Zimmerman says the amount of water in each hole affects blast design. “If the holes are dewatered and a packaged explosive product is used to get above any remaining water, you lose 40 percent of the explosive power at the floor elevation,” he explains. “This is why many blasters will use a waterproof gassed emulsion in wet conditions.”
One challenge producers often face is the creation of fines with the shot. The amount of fines created by a shot is influenced by the explosive properties, priming methods, rock properties, blast geometry, and delay timing on the occurrence and extent of the fracture process. In order to reduce fines, the blaster must employ the proper explosive type, blast parameters, initiation sequence, and charge distribution. A high level of burden with the blast can actually cause crushing, produced by high gas pressures on the walls of adjacent radial cracks, resulting in more fines. Andrew Heinemann, manager of geology and mine engineering at Benchmark Resources, provides these tips to reduce fines with the shot:
Drill-hole spacing: If the drill-hole spacing is increased and the burden is kept the same, the fines generation decreases. An increase in burden and decrease in spacing lead to more fines.
Drill-hole diameter: If the powder factor is kept constant, increasing the drill-hole diameter typically increases fines.
Subdrilling: If subdrilling is increased too much, explosive energy has more time to fragment the rock and more fines are produced.
Stemming: Increased stemming length influences the fines generation in three ways: it provides explosive energy more time to work, which can increase fragmentation; it reduces the length of the explosive column, reducing fines; and it can result in more boulders.
Powder factor: A higher powder factor can increase the amount of fines. A powder factor that is too low results in oversized boulders.
Design for overall blast size and pattern: Having a length-to-width ratio of at least 3.0, if possible, is good practice because the blast must have space to move forward without sealing the back rows of the blast pattern. If a blast pattern row is not allowed to move forward, the explosion gases are trapped in the material for a longer time and unnecessary fines are generated. The bench should be wide and shallow, not narrow and deep.
Delay timing: To minimize fines, the rock in front of a row of the blast pattern must be loosened before the next row behind it starts to move to reduce confinement. The rock needs open space to move into. The amount of fines rises when shorter delay times and a greater number of boreholes are used.