Sixty million years ago, the area where BNI now mines coal was a vast swamp. Plants died, fell into the water and decayed slowly. Eons later, the climate changed and rivers deposited material over the decaying vegetation. The weight of the overlying material pressed the vegetation into lignite seams. Geologists believe it took about 10 feet of decaying vegetation to form one foot of lignite.
Before mining begins, a rotary drill is used to define the lignite field, the depth and thickness of lignite seams, and the amount and type of material (or "overburden") overlying the lignite. This information is used by BNI engineers to determine where the best lignite deposits are located. These lignite deposits are then selected as potential places for mining to take place.
Topsoil & Subsoil Removal
After all necessary leases and permits have been acquired, lignite mining can begin. The first step in the mining process is the removal of the topsoil and subsoil. This soil, which contains nutrients needed for plant growth, is either stockpiled for future use or spread directly on previously mined areas. The process of removing and respreading topsoil and subsoil is typically done with scrapers. All areas of topsoil and subsoil removal must be approved by the North Dakota Public Service Commission before any overburden removal operations can take place.
Truck Shovel Overburden Removal
Overburden removal can begin after topsoil and subsoil have been removed. In areas of thick overburden, a hydraulic excavator and a fleet of end dump trucks are used to remove part of the overburden prior to dragline operations. The excavator and truck overburden removal step was introduced into BNI’s mining process in 2012.
Dragline Overburden Removal
The primary overburden removal tool at the Center Mine is the dragline. These machines can remove overburden up to 120’ thick and place it in piles 140’ high. The buckets of these draglines are larger than a typical school bus. They are powered with electricity produced from the same lignite they uncover.
After the lignite has been uncovered with the dragline, it must be broken into smaller pieces before it can be hauled to the power plant. This operation is done with bulldozers. The bulldozers at BNI are equipped with a large ripper shank built specifically for breaking coal into pieces small enough to be placed into BNI’s haul trucks.
The final step in the mining process is the removal of the lignite from the pit. This is done with a fleet of bottom dump trucks loaded with a large front end loader. Once loaded, the majority of the lignite is sent directly to the Milton R. Young Stations as fuel for electrical generation. A small amount of lignite is sent to the Center Coal Company where it is sold as a heating fuel. Once the coal is removed from the pit the mining process is complete and the reclamation process can begin.
Today, lignite coal-based electricity is a low-cost, efficient and increasingly clean energy source. BNI Coal brings over 70 years of mining experience to its process, partnering the skills and talents of its employees with state-of-the-art equipment to ensure safe, environmentally sound production.