Mar 3, 2020

Screening hard granite at NCC Bornholm plant

Dumping a bucket of more than six cubic meters full of heavy pre-crushed granite every few minutes onto an apron feeder is a true test of a mobile scalping screen’s durability and capacity. But the Lokotrack® ST2.8™ mobile scalping screen keeps on working. At the NCC Bornholm plant in Denmark, it removes fines with reliable accuracy.

The mobile scalping screen is one of Metso’s bestsellers, and it’s part of a major modernization project at NCC’s Bornholm plant, where the previous stationary crusher was replaced by a mobile solution in 2016. The rock blasted in the new part of the quarry is now crushed with Jonsson’s jaw plant and transferred with a wheel loader for preliminary screening with the ST2.8.

The task of the Lokotrack® ST2.8™ is to remove fines with a diameter of 0–25 mm from the crushed aggregates. About one fifth of the feed material is fines. After screening, clean stones are transferred along a 400-meter field conveyor to secondary and fine crushing, where it is refined mainly as material for NCC’s asphalt production.

 

A bucket of more than six cubic meters full of pre-crushed material at a time is dumped onto the Lokotrack ST2.8 mobile scalping screen for processing.

Between 2,000 and 2,500 metric tons of screening per day

Bornholm is the only area in Denmark with a supply of hard and durable rock materials. The soil of the country’s other parts is primarily gravel, sand and sandstone. Thanks to this, the quarry next to the town of Rønne has been used since 1948.

The more than 120-meter-deep old quarry has now been closed, and rock material is being removed from the new 70–90 meter section. The production runs four days a week, from six in the morning to five in the afternoon, and during this period, 2,000–2,500 metric tons of crushed stone passes through the ST2.8 screen.

NCC Roads A/S’s quarry offers all the required product sizes for its customers, from boulders between 400 and 4,000 kilos for harbor breakwaters to crushed stone between 0 and 200 mm to be used as filler, and asphalt and concrete varieties of 11–16 mm and 2–8 mm.

Current annual production is approximately 300,000 metric tons, which doubled two years ago when the company produced crushed stone for the new 16-hectare harbor area. The end products are sometimes shipped to other parts of Denmark, as well as to Germany and Russia.

“ST2.8 is made for rock”

The new mobile scalping screen was supplied to NCC by Metso’s Danish dealer, Dansk Grusgrav Materiel ApS. The company’s Project Manager, Jørgen V. Nielsen, analyzes the suitability of the ST2.8 model for worksites like NCC as follows:

“ST2.8 has been designed to meet the requirements of demanding primary crushing, and it’s ideal for processing crushed rock material. It’s robust, and its steel-framed apron feeder is extremely resistant to hard impacts by the feed material,” Nielsen says.

“The screen effectively separates fines from the process before secondary and fine crushing and ensures steady and continuous production without unexpected downtime in facilities like the one in Bornholm.”

The ST2.8 mobile scalping screen separates fines from the pre-crushed material – about 60,000 metric tons per year. About one fifth of the feed material is this fine aggregate.

Fuel consumption is only a little more than 10 liters per hour

Although the screening site at the Bornholm quarry is demanding, this doesn’t translate into heavy fuel consumption. The approximate fuel consumption of the ST2.8 mobile screen is only 10.5 liters per hour.

The steel-framed apron feeder has proved very effective at handling crushed material. It’s much more durable than a conveyor made of rubber.

However, the screen is equipped with Metso’s rubber screen meshes for two key reasons. Steel meshes would be too loud for screening aggregates, as the closest neighbor lives only 60 meters from the quarry.

The second reason to use rubber is that it’s durable when handling crushed stones. The same meshes have been used for screening since January 2019, and the amount of materials passing through them has already exceeded 150,000 metric tons. There is still no need to replace them. The mesh size is 70 mm on the upper screen level, and 32 mm on the lower level.

Giant leap forward in effectiveness

Quarry and Production Manager Poul-Rikard Holberg Ebbesen has worked in crushing for more than 30 years. During his career, the quarry has relocated from the north of Bornholm to its current location close to the island’s largest town and airport.

Quarry and Production Manager Poul-Rikard Holberg Ebbesen demonstrates the size of a screened granite boulder.

“When I started my career, the quarry employed about 50 men. Today, the same production volume is handled by eight employees thanks to more effective equipment,” Holberg Ebbesen says.

“With the mobile primary crusher and scalping screen, we need less equipment to transfer aggregates. We only use a couple of wheel loaders and one excavator at the quarry. Thanks to the new conveyor, we no longer need any dump trucks.”

Lifetime for 40-50 years

The Bornholm quarry contains enough rock to crush for 40-50 years. Apart from asphalt and concrete, demand is growing for aggregates with a diameter of 4–64 mm, used for foundations and structures.

 

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