Ore is extracted from LKAB's underground mines by means of sub-level caving. The method is based on creating cavities in the ore body through drilling and blasting. When the ore is removed, waste rock collapses down and automatically fills the cavity.
Sub-level caving is based on creating cavities in the ore body and allowing the ore to fall down to underlying levels by means of gravity, from where it is transported onwards by train or loader. When the ore is removed, waste rock collapses down and automatically fills the cavity. This causes clogging, and the more ore that is mined, the more the ground above sinks. Sub-level caving is an effective mining method for steep ore bodies, allowing maximum extraction from the ore body with a high degree of safety.
The mining in the underground mines takes place in a 'drill-and-blast cycle', i.e. a process that includes activities ranging from development to hoisting during the course of a day.
Underground mining takes place in several stages:
In order to mine iron ore underground, we have to build transport routes in the rock, i.e. tunnels known as drifts. Development consequently entails making preparations for mining. A development drift is blasted directly through the ore body and, if necessary, the walls and roof are reinforced with bolts, mesh and sprayed concrete.
When the development drifts are ready, long holes measuring up to 55 metres in length are drilled upwards through the ore body in a fan-shaped pattern, known as a fan cut. A single drift can include 15-40 fan cuts, with each fan cut containing eight drill holes. Drilling is performed with a remotely controlled drilling unit, which is operated and monitored by operators from a control room. When production drilling is ready throughout the drift, a special explosive with a creamy texture is pumped into the holes by robot. Blasting is performed every night.
When blasting is complete and the blast fumes have been ventilated away, the iron ore is removed from the drifts by underground loaders, which can carry 17-30 tonnes of ore per bucketful. The ore is tipped into vertical shafts, known as ore passes. The ore falls with the aid of gravity, and is then collected in rock bins just above the main level.
The ore is transported from the rock bins at the ore pass to huge crushers. In Kiruna, transport takes place using driverless trains to the main level, 1,365 metres below ground. In Malmberget, where mining is performed in several ore bodies at the same time, the ore is transported to the crushers in large trucks.
The trains or trucks empty the ore into large rock bins for onward delivery to the crushers. The crushers break the ore into pieces measuring around 10 centimetres, which are then transported on long conveyor belts to the skip hoists.
The ore is loaded automatically into the skips, which hoist it to the surface. Each skip can lift up to 40 tonnes of ore at a time at a speed of 17 metres per second. In the Kiruna mine, hoisting and reloading take place via different distribution levels. In Malmberget, the ore is transported on a long conveyor belt to a hoisting facility.