Modern Mining for Essential Metals
PolyMet will employ modern mining and processing practices designed to protect worker safety, minimize environmental effects and achieve maximum productivity, to recover copper, nickel and precious metals from the NorthMet ore body.
The basic steps of how we will do this are outlined below.
Drilling and blasting
In the first step, powerful rotary drills will bore a pattern of approximately 45-feet-deep, 12-inch-diameter holes into the bedrock. The holes are then loaded with an explosive compound of ammonium nitrate and fuel oil (commonly referred to as ANFO). At carefully scheduled times, the explosives are discharged remotely, breaking thousands of tons of rock into more manageable-sized pieces. Blasting is expected to occur about every two to three days.
Through carefully orchestrated blasting and removal of the loose rock, 40-foot-high benches are formed within the pit on which ramps and roads are built. These rock benches allow for the work of deepening and expanding the mine to proceed so that all the ore can be accessed and removed safely and efficiently over the life of the mine.
Large excavator shovels with up to 30-cubic-yard-capacity and large front-end loaders will then load the ore into diesel-powered haul trucks, each having the capacity to carry 240 tons of material in a single load. The trucks will transport the ore out of the mine to a nearby facility known as a “rail transfer hopper” where it will be loaded onto 100-ton side dumping railcars. Sixteen-car trains pulled by new, energy-efficient locomotives will then transport the ore approximately six miles to the processing facility 20 times each day. As part of the acquisition of the Erie Plant facilities, we acquired 120 rail cars that we also will upgrade and modernize.
PolyMet plans to mine approximately 225 million tons of ore over a 20-year mine life.
Waste rock storage
Containment systems at each stockpile will ensure that all water that comes in contact with waste rock is treated to meet state and federal water quality standards.
Waste rock material that has no economic value and is not processed will be categorized, and stored in nearby rock stockpiles according to the amount of sulfur it contains.
Each stockpile will have an engineered containment system to collect rainwater that may come in contact with the rock and pump it to a water treatment plant. Containment systems such as ours are used widely and effectively at modern mining and mineral processing sites and landfills worldwide. They ensure that none of the water that comes in contact with waste rock leaves the facility without first being treated to meet state and federal water quality standards.
Some of the waste rock with the lowest sulfur content, called category 1 rock, will be placed in a permanent stockpile and surrounded by a containment system. Some of it may be used for building roads and berms in the mine, and some of it will be backfilled into the East Pit.
Once the East Pit has been mined out, waste rock with the highest sulfur content will be returned to the empty mine pit for underwater storage where the potential to weather and oxidize is greatly reduced. This higher-sulfide content rock represents about 6 percent of the total waste rock that will be mined. Extensive testing demonstrates that the other 94 percent of waste rock is not capable of generating acid.
Importantly, even before treatment, water on site will be in the pH-neutral range – about the same pH as milk. The Minnesota Department of Natural Resources has determined that the mine will not create acid rock drainage as a result of mine construction, operations and closure.
Crushing and grinding
PolyMet is reusing and reclaiming the former LTV Steel Mining site near Hoyt Lakes, and putting existing facilities, equipment, roads and other mining assets back to work.
PolyMet will rehabilitate, modernize and reuse much of the existing infrastructure of the former LTV Steel taconite processing site. The refurbished facilities will be used to process copper, nickel and precious-metals ore.
Ore being transferred to the processing facility by rail can be up to four feet in diameter. At the plant site, it will be offloaded into the Coarse Crusher Building, where a series of crushers reduce the ore to approximately 2.5 inches diameter. It is then fed by conveyor to the coarse ore bin located in the Fine Crusher Building.
Throughout this process, systems will be in place to control dust and other particulates in compliance with applicable federal and state air quality standards.
From the Fine Crusher Building, the ore will be conveyed to the Concentrator Building, a one-third-mile-long local industrial icon that processed taconite ore from the late 1950s to 2001. There, the ore will be further ground in a new milling facilities into particles 120 microns in size – about the diameter of a human hair.
Flotation is the final major stage of ore beneficiation for the NorthMet Project. In this stage of the process, copper and nickel are recovered from the pulverized ore particles and turned into a saleable concentrate.
In flotation, the pulverized ore particles are mixed and pumped with water into large tanks, commonly called flotation cells. The cell is mechanically agitated much like water in a washing machine, and air and alcohol is injected to create bubbles or froth. A collector agent also is added to the water to both strengthen the bubbles and attract and attach the target metal and sulfide to the bubbles. As the ore slurry circulates through the cell, the metallic particles separate from the ore and adhere to the bubbles. As the copper- or nickel-laden bubbles rise to the top, they are skimmed off and collected as concentrate.
The concentrate goes through one final step, filtering, to dewater it, creating a powdery form of concentrate containing copper and nickel. This is stored on site from where it can then be sold and shipped by rail to smelters and refineries for further processing by the buyer. The flotation stage is designed so that a copper-rich concentrate and a nickel-rich concentrate can be separately produced providing flexibility in marketing the products.
PolyMet will revise the existing tailings basin and add to the structure, applying similar design with enhanced engineering controls.
Tails or tailings, the leftover sand-like particles of rock that have been stripped of their economic metals in flotation, will be collected and pumped in a slurry to the tailings basin. The tailings basin already exists as a result of previous iron ore processing. The solid tailings drop out of the water – the solids are permanently stored there and the water is returned to the processing plant to be reused. Water from the tailings basin will be recycled over and over again in the processing circuit.
Tailings basin water, like other water on the site, will be in the neutral pH range. Many years of laboratory testing has demonstrated that these tailings will not generate acid.
The use of the existing tailings basin greatly reduces the environmental disturbance of the new mine. Before we put the existing tailings basin back into use, however, we will upgrade it with a rock buttress and new seepage capture systems to collect and control water and pump it back to the tailings basin. Any excess water will be discharged offsite after being treated to meet strict water quality standards. Tailings basins are highly engineered structures that are continuously maintained, monitored and inspected to ensure stability and acceptable environmental performance.
At closure, a permanent pond will be formed at the tailings basin. The pond bottom and exposed beach areas will be amended with bentonite to limit oxygen infiltration and water percolation into the tailings. Read more details in our Tailings Basin Stability Fact Sheet.
In a second phase of the project a new facility will be built to further refine nickel concentrate, increasing its value and providing better recovery of precious metals. Here’s how it works:
- Pressure oxidation. Nickel-rich concentrate from the flotation plant is put into a large sealed reactor called an autoclave. Oxygen is added to create a chemical reaction. Heat generated by the exothermic reaction and the high pressure extracts the nickel, gold, platinum group metals and some residual copper into a liquid solution. This solution then passes through a number of steps in which different metals are separated from the solution. This is a closed-loop process so water can be recycled and reused over and over again.
- Metal recovery. Precious metals are the first to be precipitated out of the solution, forming a filter cake. The copper is mixed with the copper concentrate to upgrade that product, and the nickel and cobalt are recovered as a hydroxide that can be sold to a metal refinery or could ultimately be further processed at site.
The solution, free of metals, impurities and waste, is recycled and returned to the beginning of the circuit to be reused.
Closure and Reclamation
Before the mine is even permitted and operational, mine closure is carefully planned. Having a closure and reclamation plan is a prerequisite to obtaining a permit to operate the mine. Its purpose is to ensure that the site used for mining and processing remains in compliance with applicable air and water regulations long after mining ceases. In reclamation, the property is restored to as close to a natural state as possible.
Download the DNR’s fact sheet on reclamation.
In some cases, reclamation activities will occur while mining is still underway, a process known as concurrent reclamation. For example, when the East Pit is mined out, waste rock that has been stored on a lined facility onsite will be backfilled into the pit. This will occur while mining continues at the West Pit. Also, a cover will be sequentially constructed on the permanent category 1 waste rock stockpile.
Mining is expected to be completed 20 years after operations begin. At closure, warehouses and other buildings will be removed, remaining temporary stockpiles backfilled into the pit, wetlands restored, various cover systems to minimize exposure and water seepage constructed, and vegetation planted in disturbed areas. Water will continue to be treated at the site for as long as necessary, and that which is discharged will meet Minnesota’s wild rice standards for sulfate.
By state law, the company must provide bankruptcy-proof financial assurance to the state to ensure that it has the appropriate level of funding to close and reclaim the site at any time in the event the company is unable to do so. This includes financial assurance to cover long term water treatment. This amount is reviewed annually and updated as appropriate.
The company will include the appropriate funds as a part of its operating budget to close the mine and monitor it post closure. Financial assurance is only used by regulatory agencies if the company is unable to meet its closure and reclamation obligations.