Modern mining for essential metals
PolyMet plans to mine important metals while meeting all state and federal environmental standards to protect our regions air and water quality.
We will apply well-established mining and processing practices all 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 50-feet-deep, 12-inch-diameter holes into the bedrock. The holes are then loaded with an explosive compound comprising 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, 45-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. New, energy-efficient locomotives will then pull 16- to 20-car trains loaded with ore approximately six miles to the processing facility. The rail cars, which will be upgraded and modernized, were used at the Erie Plant site formerly and purchased in our acquisition of the site.
PolyMet will mine approximately 32,000 tons of ore per day and, over the 20-year permitted mine life, expects to remove 225 million tons of ore and 308 million tons of waste rock – a total of 533 million tons of material.
Waste rock storage
Waste rock material that has no economic value and is not processed will be categorized, sorted 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 mine water treatment plant. Containment systems are used widely and effectively at mining and mineral processing sites and landfills the worldwide. They ensure that none of the water that comes in contact with waste rock leaves the facility without first being treated to meet 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.
Waste rock with the highest sulfur content will be temporarily placed on foundations, liners and containment systems. Then, starting in year 11 of operations, it will be returned to the empty mine pit for underwater storage where the potential to oxidize is greatly reduced, as it is in its natural state. This higher-sulfide content rock represents about 6 percent of the total waste rock that will be mined.
Importantly, even before treatment, water on site will be in the pH-neutral range – about the same pH as milk. The state Department of Natural Resources has determined that the mine will not create acid rock drainage as a result of this plan.
Crushing and grinding
PolyMet will rehabilitate, modernize and reuse much of the existing infrastructure of the former Erie Plant 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 await to reduce the ore to approximately 2.5 inches. It is then fed by conveyor to the coarse ore bin located in the Fine Crusher Building.
There, the ore is fed into another series of crushers that will reduce it to smaller than 0.315 inch, about the size of a sand particle. Throughout this process, systems will be in place to control dust and other particulates in compliance with applicable air quality standards.
From the Fine Crusher Building, the ore will be transported to the Concentrator Building, a one-third-mile-long local industrial icon built as part of the Erie Plant complex in the 1950s, where the ore will be further milled into particles 120 microns in size – about the diameter of a human hair.
Flotation is the final major stage of ore beneficiation for the PolyMet NorthMet Project. In this treatment stage, copper and nickel are separated 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 to the bubbles. As the ore slurry circulates through the cell, the metals and sulfur 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 about 28 percent copper. This is placed in dry storage on site from where it can then be sold and shipped by rail to 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.
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 slurry form to the tailings basin. The tailings basin already exists as a result of iron ore processing during the last half of the twentieth century. The solid tailings and the water are separated in the tailings basin. The solids are permanently stored there and the water is returned to the process 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, is 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 collection system to control leakage and pump it back into the tailings basin. Tailings basins are highly engineered structures that are continuously maintained, monitored and inspected to ensure stability.
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.
HydroMet process (phase two)
In a second phase of the project a new hydrometallurgical facility will be built to further refine nickel concentrate, increasing its value and providing for better recovery of precious metals. Here’s how it works:
1. Pressure oxidation. The nickel-rich concentrate from the flotation cells is put into a large sealed vessel called an autoclave. Oxygen is added to create a chemical reaction. Heat generated by the exothermic reaction and the high pressure drives 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 at different stages. This is a closed-loop process so water can be recycled and reused over and over again.
2. Metal recovery. Gold and PGMs are the first to be precipitated out of the solution, forming a filter cake comprising mixed gold and platinum group metal sulfide precipitate. For the remaining solution, which contains residual amounts of copper, a portion of copper concentrate from the dry concentrate storage area is mixed or re-pulped with the solution in a process known as cementation. All of the copper in the solution migrates and bonds to the copper concentrate, resulting in an enriched concentrate of about 33 percent copper that is then filtered and placed back into dry concentrate storage.
Limestone is then added to the leftover solution from cementation in a process known as solution neutralization. This creates synthetic gypsum that will be stored in a lined facility or potentially sold. Gypsum is the same white chalk-like material used to make wallboard. After neutralization, the solution is treated to remove iron and aluminum impurities.
With the gold, platinum group metals and copper extracted and impurities removed, the neutralized solution now contains only nickel and cobalt. Magnesium hydroxide is induced in this solution, creating a reaction that produces a high- grade nickel and cobalt precipitate that would have an approximate composition of 97 percent nickel and cobalt hydroxides.
The solution, now free of metals, impurities and waste, is recycled and returned to the beginning of the circuit to be reused over and over again.
Closure and Reclamation
Before the mine is even permitted and operational, mine closure is carefully planned. Having a closure and reclamation plan and financial assurance to accomplish that 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 the PolyMet project financial assurance and reclamation [PDF].
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.
Bankruptcy-proof funding, which is mandated by state law and over which the state has oversight, is established during permitting to ensure that funds are available to finance all closure and reclamation activities, including long-term water treatment, whether the mine operates for one day or 20 years.