Bull Mountain Coal Mining president and CEO John DeMichiei said he expected longwall mining to begin in December, which would ramp up over five years to produce 11 million tons of coal a year from the Mammoth coal seam.
The mine currently extracts coal using continuous miners, which leaves behind some coal as support pillars.
The Montana Department of Environmental Quality's Industrial and Energy Minerals Bureau said the mine expansion adds 2172 acres to the existing 4220 acres permitted, bringing the total permitted area to 6392 acres.
The initial mine plan called for longwall panels that were 800 feet wide and 10,000ft long but the revision enlarges the panels to 1250ft wide by 23,000ft long (more than four miles).
The DEQ has predicted subsidence will be an issue for the new longwall mine, which must be managed and repaired properly by the mining company.
