Contributed by Uno Sundelin, Manager, Mining & Materials Handling Hägglunds Products & Solutions – Bosch Rexroth
LKAB Malmberget in Sweden is an impressive iron ore mine in many ways. First, it is a high-tech facility that houses an entire underground community, including roads, mining operations, offices and canteens. Second, the mine has several ore bodies that are spread across the site. It is a full 8 km from one end of the mine to the other, which naturally places big demands on logistics and cost-effectiveness.
The fact that the mine is doing well, despite tough competition from open-cast mines around the world, is attributed to the high level of efficiency together with high-quality product, good environmental awareness, and highly skilled personnel.
One essential improvement in availability involved a 338-m long conveyor belt that was not living up to its task as the “main line” for carrying 80% of all the ore from the mine. LKAB Malmberget had been looking at alternatives for a couple of years. A new drive was installed during a scheduled shut-down recently, with Pär Sundqvist as project manager.
Sundqvist and his colleagues looked at everything from the latest electric drives without gearboxes to hydraulic drives, at other plants where these were in use. The only thing they knew for sure was that they did not want any gearboxes. This was partly for reasons of space, and partly to reduce the risk of breakdown.
“We already had some dual gearboxes and it was difficult to find spares for them. The old conveyor belt was like a patchwork quilt after all the hard starts and stops, and after a small fire. On average, we had one breakdown each year that led to an unscheduled shutdown of three to five days. This usually happened in the week after New Year, when it is coldest of all,” said Håkan Hansson, who is a field mechanic.
This loss of production was naturally unsustainable in the long term. A visit to LKAB in Kiruna gave very positive feedback on Hägglunds direct hydraulic drives from Bosch Rexroth.
The choice fell on two Hägglunds CBp motors with an installed power output of 2 x 630 kW and a maximum capacity of 3,000 tons per hour. The reason for choosing Hägglunds CBp motors was a requirement for compact design and maximum efficiency.
They have not regretted the decision. The biggest benefit of all, according to Sundqvist, is the built-in redundancy and high operational reliability.
Four drive units drive the belt, and a standby pump makes it possible to maintain operation even if one of the units stops. “We have not had a single incident, everything has just worked the way it should,” he says.“There was no need for any construction changes to install the new drive, so we completed everything during the May shutdown. Most of our other contact with Hägglunds specialists was for support with programming the control system.
“They have been very helpful in every way. Without the support we got from them we would never have achieved such an effective system. We visited their factory in Mellansel and they have visited us. We also had electrical planning meetings by Skype once a fortnight throughout the project. It has all worked perfectly,” Sundqvist said.
Automatic load sharing is another benefit, which leads to reduced vibration and wear in the drive chain compared to before, when there was a much higher load and demand on the single gearbox.
“The drive system works so well that you almost forget it is there. It just keeps on running. The only thing we have had to do is replace filters,” added Hansson.
The hydraulic drive is specially designed for dusty environments, and all the hydraulic components are isolated from the electric motor. The system is now easy to service, lightweight and easy to keep clean, according to Hansson.
“With the old 6,000-V motors and gearboxes it was not possible to wash off the ore dust with water, so we had to vacuum clean them regularly,” he said.
The soft start of the hydraulic drive is gentler on the mechanical components when you start up. However, the best feature of all, according to Hansson, is the creep drive. “It’s very useful when we need to inspect the belt for damage from rebar and other scrap that sometimes gets caught up from rock reinforcement. Now we can gradually advance the belt to the point of damage and check it,” he said.
The emergency stop function is also appreciated, as it enables the Hägglunds CBp motors to stop the belt in less than 1 m from full speed.
“The process was analyzed in detail and we carried out a thorough risk analysis. Naturally we want to be able to stop the belt as quickly as possible to prevent any injury to employees,” Sundqvist said.
In July 2018, the conveyor belt had an availability figure of 99.5% and ran for 23–24 hours per day. “The few stoppages we had were just caused by iron scrap along the belt. It is important that we carry out routine maintenance so that there are no problems in the future either. The big problems always arise after a breakdown, so we need to eliminate them completely,” Sundqvist concluded.