Following is a list of mining terms that will be included in TMM’s formal underground mine plan proposal, which the company expects to submit to state and federal regulatory agencies within the next few months for extensive and rigorous environmental review.


Backfill is material used to refill an excavated area of the mine. Approximately half of the tailings from the TMM Project will be stored as permanent cemented backfill in the underground mine.


Concentrates are the end product of the Twin Metals project. These concentrates are sold on a global market and contain the minerals that have been separated from rock in the mine.


When rock is removed from underground, the sizes of rocks can be small (like pebbles) or very large (like a car). Crushing of rock is necessary to send a consistently sized material to the processing site.

Environmental Impact Statement

An Environmental Impact Statement (EIS), under United States environmental law, is a document required by the National Environmental Policy Act (NEPA) for certain actions affecting the environment. Minnesota has a similar program, the Minnesota Environmental Policy Act (MEPA). Multiple state and federal agencies develop the EIS during a multiyear process that includes studies of project-specific engineering and site-specific data. The EIS process includes multiple mandated opportunities for public input.


Flotation is a physical separation using water, air, and a small amount of reagents to concentrate the desired metals.


Mining begins with the mineral-bearing ore (material with positive economic value) removed from the earth. In reaching the ore, developmental rock is also removed and used for barricading areas or to fill voids underground. Mining at the TMM project would occur below the ground surface from hundreds to several thousand feet. The project will process approximately 20,000 tons of mineralized ore per day.


Tailings are the leftover finely ground (milled) ore after the desired minerals have been physically separated and removed.

Dry Stack Tailings Storage

Dry stack tailings management is the most sustainable method used to store filtered tailings produced from the processing after the 4% of the ore that is copper, nickel, cobalt and other metals is recovered. Since the tailings are filtered and the majority of water is removed, a dry stack facility does not require a dam. The dry stack facility will be lined and filtered tailings are placed and compacted into an engineered mound, constructed in several stages. Learn more about Twin Metals’ plans for its dry stack tailings storage.

Waste Rock

Rock mined during operations below the targeted cut-off grade that would be managed underground and placed in mined out stopes for permanent storage.


The process of terminating and completing final steps in reclaiming any specific portion of a mining operation. Closure begins when, as set in the Permit to Mine, there would be no renewed use or activity by the permittee.


Mine reclamation is part of the lifecycle of a mine and is the process of converting an active mine into land that can be used for another natural or economic means. Planning for mine reclamation occurs during the design process, prior to permitting or mine operations. Closure and reclamation plans for the Twin Metals project will be determined later on through the regulatory process.