The Akselberg quarry in central Norway, about 400 km north of Trondheim, produces about 1.8 million tonnes of limestone carbonate annually, a product widely used in the European paper industry.
Since it was established in 1997, this 2.5 km long and 1.5 km wide pit has consistently improved its productivity in drilling and blasting. Today, however, the Brønnøy team is getting better results than ever due to professional planning and meticulous attention to every step in the production process.
As Raymond Langfjord, Production Manager at the site, explains: “Everything here starts with the drilling. If we get that wrong, it has consequences for every other phase in our operation – from blasting and loading through to crushing and, ultimately, the quality of our products.
“Our vision has always been to achieve optimal utilization of the limestone here and that requires optimizing the whole excavation process. That’s why we are very happy to be getting top results with our equipment and to see the effect of this on the total efficiency of our operation.”
Precision on the benches
The equipment on site consists of Atlas Copco SmartROC tophammer drill rigs complete with the Hole Navigation System (HNS), AutoPos function and ROC Manager, which creates drill plans and reports. HNS and AutoPos together are key functions that enable the operator to locate and collar the hole with accurate angles, first time every time.
There are three rigs at work in the quarry – a SmartRig ROC F9C, a SmartROC T40 and a SmartROC T45 – drilling on 15 m high benches. The holes are drilled at an inclination of 10°. Five rows are drilled with a burden of 2.8 m and a spacing of 3.5 m. After charging and firing, 20 000 to 80 000 tonnes of rock are removed with each blast and the quarry carries out an average of 10 blasts per month.
The rigs use T51 drill rods and 89 mm ballistic button bits from Atlas Copco Secoroc and have no trouble penetrating the overburden. On the contrary, the rock is relatively easy to drill with penetration rates of 1.8 to 2.5 m per minute. The bits only need to be reground every 250 m and, at best, one bit can last for up to 6 000 m.
But if the rock is easy to drill, Langfjord explains that it is more difficult to blast and that parallel holes are vital for success. “We have to make sure that all of the holes are drilled to the right depth and with the right inclination and absolutely parallel,” he says. “In addition, the hole bottoms have to be positioned in exactly the right spot. Any deviation, however small, will undermine the success of the blasted round. And as these rounds are spread out over such a large area, small errors can become big problems.”
The all-important last row
Fortunately, the drillers are able to meet these tough demands using the rigs’ HNS system together with the AutoPos function.
“The last row of holes, which is always at the rear of the bench, is the most important one,” continues Langfjord. “This is the master row and it has to be right. Otherwise it will negatively affect the rest.
“With HNS we can import the drill plan that has been created in ROC Manager, using a USB, and the system helps the operator to navigate the rig to the precise hole location. The rig’s AutoPos function then sets up the correct inclination on the feed, ensuring that the bit will hit the right spot on the ground at the touch of a button. The operator starts drilling and then simply switches to automatic mode and the rig drills the rest of the hole by itself.” In this way, the AutoPos function minimizes the risk of human error.
After the hole has been drilled to the correct depth, log files are stored and can be viewed as status reports in ROC Manager, enabling the actual hole to be compared with the plan. If there is any deviation, adjustments can be made to correct the error before continuing.
“In that case we can either adapt the charge for that hole or drill a new hole altogether, Langfjord points out. “It makes life very easy.”
Furthermore, HNS-equipped drill rigs can also be equipped with Atlas Copco’s optional Measure While Drilling (MWD) function, gathering information such as variations in rock type, hardness, the presence of fissures, cavities and so on, which is analyzed prior to blasting.
“Obviously, all of these devices are a great advantage,” Langfjord says. “They help us to achieve our prime goal which is to optimize the blasted rounds, and therefore create the conditions for making the operation as efficient as it can be.”
Increased drill pattern
Correctly blasted rounds produce the required fragmentation, which in the Brønnøy quarry, is maintained at an absolute maximum of 1.2 m x 1.6 m. Satisfactory fragmentation is important, both for optimizing the loading of the quarry’s wheel-bound loaders as well as for fast throughput at the crusher station.
With the correct dimensions of the rock, the crusher station runs evenly and smoothly, using the right amount of power required to crush the stone while consuming as little energy as possible to minimize costs.
An added advantage is that correctly blasted rounds increase safety and improve the environment for the 45 people at the site by reducing the amount of flyrock and dust.
“Obviously equipment like this costs extra money so you’ve got to get it out at the other end,” Langfjord maintains, “and that’s exactly what we’ve done as we have been able to increase the drill pattern from 8.6 to 9.8 square meters.”
Mats Birkestål, Product Manager at Atlas Copco’s Surface and Exploration Drilling Division, sums up the Brønnøy experience. “What’s important here is that this company has understood the value of each step in the process,” he says. “They plan the holes they are about to drill and follow up after each drilling operation in order to secure the best possible rock fragmentation. They know that this process is the key to total efficiency.
Epiroc operated under the trademark “Atlas Copco” prior to January 1, 2018.