MINE LIFE CYCLE
Let's take a look at the Mining Life Cycle.
Ever wonder how mining operations work? A typical project goes through a series of different phases which we refer to as the mining life cycle, each presenting unique challenges and opportunities. Click on the categories below for a description of each phase.
In Michigan, all metallic mineral exploration and development activities on state and private property are regulated by the DEQ, including leasing and drilling. A metallic mineral lease within itself does not give a company permission to mine. If a deposit is found, separate applications and approvals will be required before mining activity can take place.
It can take years or even decades to discover a viable deposit. Geologists utilize aerial surveys, soil analysis, and drilling to determine if there is a sufficient mineral deposit to justify mining. Many factors control the economic viability of a deposit, but the most important are the grade, size, location, and demand. Exploration does mean a mine is going to be developed.
Detailed studies (e.g., feasibility study) are conducted to determine the capital requirements, community context, permitting requirements, critical environmental challenges, and other information vital in moving the project to the next step. If the outcomes yield positive results, additional research and planning will take place at a more granular level. As more information is gathered, companies gain a far greater understanding of mine plans, facility layouts, infrastructure, and environmental and social impact assessments.
All nonferrous metallic mines in Michigan are required to submit the necessary permit applications with the DEQ Office of Oil, Gas, and Minerals. The permits typically include mine, water discharge, and air. Government agencies thoroughly review each permit application and gather public input before issuing a permit. Each permit comes with conditions that must be met before, during, and after mining occurs. While these permits are a requirement for all mines, the exact details and specifications are unique for each project.
Upon receipt of permit approvals and capital investment, the project can prepare for development. Hundreds of workers will build the infrastructure required to support the operation. Examples include roads, water treatment plants, maintenance facilities, warehouses, contact water basins, and process plants. Construction of such sites is a massive undertaking and boost to the local economy. A typical project can take 15 to 20 years between discovery and construction. See what the Back Forty Mine will look post-construction here.
Mining involves extracting large amounts of rock from below the Earth’s surface. The goal is to separate the valuable minerals from the non-economical rock.
To do this, overlying rock is blasted, and the material is placed in a truck and hauled to the surface. The material that contains valuable minerals (e.g., ore) is sent to the process plant, while the non-economical material (e.g., waste rock) is taken to the temporary waste rock facility. The waste rock will be used to backfill the pit once mining ends.
In the process plant (e.g., mill) the zinc/copper/lead ore goes through a conventional process of crushing, grinding, and floatation to separate the minerals. The gold/silver ore needs a few more additional steps before the valuable minerals are captured.
When mining ceases, the site is reclaimed, and land is returned to a state compatible with adjacent properties. Examples of reclamation activities include removal of all buildings and infrastructure, capping and vegetating the tailings facility, backfilling the mine with waste rock. Postclosure monitoring lasts for 20 years following completion and approval of reclamation. The time may be shorter if the DEQ determines monitoring and maintenance activities to preserve the integrity of the area are complete. The DEQ requires an annual mining and reclamation report filed each year with the agency. The reports are available to the public.
TAILINGS MANAGEMENT FACILITY EXPLAINED
What are tailings and what is Back Forty’s management plan?
Tailings
We will process roughly 4,800 tonnes (5,300 tons) of ore per day onsite. We will use traditional crush-grind-float technology and equipment to separate zinc and gold from noneconomic rock. After removal of these valuable minerals, what remains is called tailings. Our tailings consist of particles of rock, water, and residual process chemicals (e.g., lime, copper sulfate, silica), and have the consistency of drywall mud. We will pump tailings to a double-lined tailings management facility (TMF). The TMF will cover a total footprint of 123 acres. During operation, the facility will be a maximum 118 ft tall, and after closure a maximum of 138 ft tall.
Tailings Management Facility
Water management is crucial to our design. Unlike a conventional TMF, our design is neither a pond nor does it store liquid tailings. The entire base of the TMF will be compacted and double-lined, consisting of a composite primary liner and a single secondary liner separated by a leak detection system. This lining extends under the perimeter wall and the surrounding berm. The coarse aggregate above the primary liner collects the water (e.g., residual tailings water, runoff, rain, snow) and gravity directs it to an exterior sump. We will also pump water that collects on top of the TMF to the contact water basins or the mill for reuse in the milling process. As a protective measure, if we experience an extreme storm event, an emergency spillway will channel water from the TMF into the open pit. Once mining ends, the TMF will be capped and revegetated to prevent any oxygen penetration or water percolation into the facility. Also, we will dewater tailings after capping. By doing so, the TMF will be near neutral pH and will not require perpetual care or treatment.
Performance Monitoring
Our monitoring program includes more than a dozen ongoing studies, including water balance, groundwater level and quality, perimeter wall settlement, and leak detection system analysis. Also, an engineering review by a qualified independent Geotechnical Engineer will take place on an annual basis. The performance of the facility will be reviewed closely during construction, operations, and post-closure to ensure that the design intent is being satisfied, to confirm design assumptions, and to identify any modifications that may be required.
The design mitigates known risks of traditional tailings facility construction. Similar and successful models include the Malartic Mine and the Musselwhite Mine in Canada, and the Neves Corvo Mine in Portugal. Before construction commences, EGLE will review and approve the plan.
Tailings Management Features
- Thickening tailings to 70:30 ratio of solids-to-water compared to traditional 30:70 ratio of solids-to-water.
- Reducing water in tailings recycles approximately 793M gallons of water back into the milling process on an annual basis.
- An emergency spillway channels water from the TMF into the open pit in case of an extreme storm event.
- Incorporates modern technologies and mitigates known risks of traditional TMF construction.
- Dewatering promotes tailings consolidation, increases tailings density, and extends strength characteristics of TMF.
- The entire base rests above a double-liner with a leak detection system.
- A competent and free draining perimeter wall roughly 108 ft wide made out of waste rock.