Summary

Deposit type

• Magmatic

Summary:

The NorthMet deposit is considered a magmatic Copper-Nickel±platinum group element (PGE) deposit.

The NorthMet deposit is a large tonnage, disseminated accumulation of sulfide in mafic rocks, with rare massive sulfides. Copper to nickel ratios generally range from 3:1 to 4:1. Primary mineralization is probably magmatic, though the possibility of structurally controlled re-mobilization of the mineralization (especially PGE) has not been excluded. The sulfur source is both local and magmatic. Extensive detailed logging has shown no definitive relation between specific rock type and the quantity or grade quality of sulfide mineralization in the Unit 1 mineralized zone or in other units, though local noritic to gabbronoritic rocks (related to footwall assimilation) tend to be of poorer PGE grade and higher in sulfur.

Mineralization
The metals of interest at NorthMet are copper, nickel, cobalt, platinum, palladium, silver, and gold. Minor amounts of rhodium and ruthenium are present though these are considered to have no economic significance. In general, except for cobalt and gold, the metals are positively correlated with copper mineralization. Cobalt is well correlated with nickel. Most of the metals are concentrated in, or associated with, four sulfide minerals: chalcopyrite, cubanite, pentlandite, and pyrrhotite, with platinum, palladium and gold also found as elements and in bismuthides, tellurides, and alloys.

Mineralization occurs in four broadly defined horizons or zones throughout the NorthMet property. Three of these horizons are within basal Unit 1, though they likely will not be discriminated in mining. The sulfide mineralization occurs as primarily as disseminated interstitial grains between a dominant silicate framework and are chalcopyrite > pyrrhotite > cubanite > pentlandite. The thickness of each of the three Unit 1 enriched horizons varies from 5 ft to more than 200 ft. Mineralization in Unit 1 occurs along the strike length of the NorthMet property and extends down dip from the surface to a depths 2,600 feet below surface. Mineralization in Unit 1 locally penetrates up into Unit 2 along strike and down dip of Unit 1. The copper-rich, sulfur-poor disseminated mineralization in the Magenta Zone crosscuts Units 4, 5 and 6 in the western part of the NorthMet. The Magenta Zone dips shallowly to the southeast and has a strike length of 8,700 feet, and average thickness of approximately 100 feet and occurs at depths starting at the surface to depths of 800 below surface. The mineralization within Unit 1, Unit 2, and the Magenta Zone accounts for over 90% of the mineralized material at NorthMet.

Mining Methods

• Truck & Shovel / Loader

Summary:

The NorthMet Project contains mineralization at or near the surface that is ideal for open pit mining methods.

The mine plan includes 225 million tons of ore at an overall strip ratio of 1.80:1. Mining is planned in three pits: The East Pit, the Central Pit, and the West Pit. As mining of the Central Pit commences, it will extend into the East Pit, thereby joining the pits. The combined pit will be referred to as the East Pit.

The method of material transport evaluated for this study is open pit mining using two 36.6-yd3 hydraulic front shovels as the main loading units with a 22.5-yd3 front end loader as a backup loading unit. The material will be loaded into 240-ton haul trucks and the ore will be hauled to the rail transfer hopper for rail haulage to the mill or ore surge pile (OSP) areas, and the waste rock to waste stockpiles or pit backfills.

During the first half of the operation, the more reactive waste rock mined will be placed in two temporary stockpiles (one west of the East Pit referred to as the Category 4 Stockpile, and one south of the East Pit referred to as the Category 2/3 Stockpile), and the least reactive waste rock will be placed in a permanent stockpile north of the West Pit (referred to as the Category 1 Stockpile). Once mining is completed in the East Pit, the more reactive waste rock mined will be placed directly in the East Pit as backfill. The more reactive waste rock in the Category 4 Stockpile (in the location of the future Central Pit) will then be relocated as backfill into the East Pit, thus clearing the area for mining of the Central Pit. The Category 2/3 Stockpile will be moved into the West Pit as backfill at the end of mining. Once mining is completed in the Central Pit, waste rock will be backfilled into that pit, also. By the end of the mine life, all of the more reactive waste rock will be placed as backfill in the pits. As the least reactive waste rock is mined, it will be placed in the permanent Category 1 Stockpile until it is completed then into the East and Central Pits as backfill. The three mine pits will flood with water after mining and backfilling are completed, which results in the more reactive waste rock being permanently disposed of sub-aqueously.

Pit design
The pits were designed into six phases with the East Pit mined in two phases, the Central Pit in one phase and the West Pit in three phases.

Haul roads were designed at a width of 122 ft, which provides a safe truck width (27’3” canopy width) to running surface width ratio of 1:3.5, including a 26.5-ft width for a bench on the edge of the road. Maximum grade of the haul roads is 10%.

Pit Design Criteria:
- Inter-ramp Angles with less than 400' between ramps - 51.4°;
- Inter-ramp Angles with greater than 400' between ramps - 49.1°;
- Face Angles - 65°;
- Catch Bench (< 400' between ramps) - 33.2 ft;
- Catch Bench (> 400' between ramps) - 33.2 ft plus an additional 27.2 ft to one of the catch benches;
- Catch Bench Vertical Spacing - 100 ft;
- Minimum Turning Radius - 200 ft;
- Ramp Widths - 122 ft;
- Ramp Grade - 10%.

Production schedule
The production schedule is driven by the nominal ore rate of 32,000 STPD equivalent to 11.6 million tons per annum (average of 362.5 days per year, or 99% availability) with a 20-year mill life. Mining is planned on a 7 day per week schedule, with two 12-hour shifts per day.

Comminution

Crushers and Mills

Summary:

PolyMet US plans to refurbish the plant and reuse the existing primary crusher and replace the downstream mill circuit with a new 40’ diameter x 22.5’ Effective Grinding Length (EGL) SAG mill and one new 24’ diameter x 37’ ball mill. 

Crushing and Material Handling
The Coarse Crushing building and equipment would be used for primary and secondary crushing of the plant ore feed. A new 60” primary crusher would be installed in the South Coarse Crushing facility. Only one primary crusher is required to achieve the plant throughput. The four existing 36” secondary gyratory crushers associated with the primary crushing system would require complete refurbishment. 

Primary and Secondary Crushing
ROM material is delivered to the two-stage crushing plant for size reduction, making it suitable for further liberation and beneficiation of the target economic metals. Two-stage crushing is used to achieve a final P80 crushed product size of 80% passing 4 in, which is then fed into the milling circuit for further liberation of the mineral. 

The crushing circuit consists of a primary crusher feed bin, a gyratory primary crusher, a primary crusher product surge bin, and four gyratory secondary crushers. 

Ore with a top-size of approximately 55 in is delivered by side-dumping rail cars to the primary crushing circuit. The rail cars dump their load directly into the gyratory crusher feed bin that in turn feeds the new 60" × 113" Traylor Type NT gyratory crusher on primary crushing duty. The P80 product, 80% passing 7 in from the primary crusher is discharged by chute arrangement to the Primary Crusher Product Surge Bin from where it is withdrawn via sliding gates into four parallel 36" × 72" Traylor gyratory secondary crushers. Each secondary crusher discharges 80% passing 4 in ore onto a dedicated variable speed apron feeder, which in turn feeds the Secondary Crusher Discharge Conveyor. 

The Crushed Ore Transfer Conveyor will receive material from the Secondary Crusher Discharge Conveyor and transports the crushed material to the Crushed Product Surge Bin. Material is withdrawn from the surge bin using an apron feeder, onto a conveyor which then discharges onto the tripper conveyor. The tripper belt conveyor transports the crushed ore to the Crushed Ore Storage Bin. 

Milling
The milling section consists of a SAG mill operating in open circuit and a ball mill operating in closed circuit with two clusters of classifying hydro cyclone clusters to give a product of 80% passing 120 µm. A pebble crushing circuit is incorporated to handle the SAG mill scats. 

Ore is transferred from the crushed ore storage bin to the SABC circuit, which consists of a SAG mill, ball mill and pebble crusher. The ball mill is fed by cyclone clusters. The overflow from the cyclones will discharge into a flotation feed tank that feeds the flotation circuit. 

Crushed ore is withdrawn from the crushed ore storage bin using 62 variable speed driven vibrating pan feeders. The pan feeders discharge through chute arrangements onto two reclaim conveyors. Between four and eight pan feeders per conveyor will operate at any one time. 

Both reclaim conveyors discharge onto the transfer conveyor which in turn delivers ore to the SAG mill feed conveyor. The SAG mill feed is measured and recorded using a weightometer installed on the SAG mill feed conveyor. The 40’ diameter × 22.5’ EGL SAG mill has a grate discharge and is fitted with a 28 MW motor. 

Process water is added to the SAG mill to achieve a slurry solids content of 75% by mass within the mill. Mill cooling water is provided by the mill cooling water pumps operating on a duty/standby configuration. The SAG mill discharge flows over a vibrating screen and the screen oversize is either conveyed to the pebble crushing circuit or to the scats bunker, via a diverter chute. 

The pebbles that are diverted to the pebble crusher feed conveyor are conveyed to the pebble crusher surge bin. A weightometer installed on the pebble crusher feed conveyor measures and records pebble crusher feed tonnage. A belt magnet removes ball scats prior to the pebble crusher and discharges the scats onto the scats removal conveyor. Pebbles are withdrawn from the pebble crusher surge bin using a variable speed driven pan feeder, fed through the crusher, and discharged onto the pebble crusher discharge conveyor. The crushed pebble transfer conveyor receives material from the crusher discharge conveyor and returns crushed pebbles to the SAG mill feed conveyor. 

Undersize from the SAG mill discharge screen discharges into the SAG mill discharge sump from where it is transferred to the cyclone cluster feed sump. Process water is added to both the SAG mill discharge sump and the cyclone feed sump at a controlled rate to achieve the required slurry solids content at the respective discharge points. 

Diluted slurry is pumped to the hydro cyclone clusters using hydro cyclone feed pumps. Overflow slurry from the cyclone clusters (33.2% solids by mass) gravitates to the flotation feed surge tank. Cyclone cluster underflow slurry (75% solids by mass) feeds the ball mill. 

The 24’ diameter × 37’ EGL ball mill has an overflow discharge and is fitted with a 14 MW motor and operates in closed circuit with the cyclone clusters. The discharge from the ball mill flows through a trommel screen and discharges into the cyclone cluster feed sump. Trommel screen oversize will be transferred by conveyor to the milling scats bunker. 

Bulk Cu-Ni Rougher concentrate Regrind
Bulk Cu-Ni Rougher concentrate is pumped to the Cu-Ni rougher regrind mill cyclone cluster. The cyclone underflow reports to the Bulk Cu-Ni rougher regrind screen. Screen oversize reports to a trash basket while the undersize gravitates to the mechanically agitated Cu-Ni rougher regrind mill feed tank as regrind mill feed. Cyclone overflow slurry is discharged into the Cu-Ni rougher regrind cyclone overflow sump.

Slurry from the Cu-Ni rougher regrind mill feed tank is pumped to the Cu-Ni rougher regrind mill. The feed is ground to give a product size of 80% passing 35 µm. Part of the regrind mill discharge is recycled back to the regrind mill feed tank while the balance flows to the Cu-Ni rougher regrind sump. A sample is taken from the rougher regrind discharge, using a Vezin sampler, which measures the grinding performance of the mill and ensures that the correct size distribution is sent to Bulk Cu-Ni cleaning.

Cu-Ni Separation Regrind
Concentrate slurry from the Bulk Cu-Ni cleaner flotation is pumped to the Cu-Ni separation regrind mill cyclone cluster. Cyclone underflow reports to the regrind mill feed tank as mill feed. Feed is ground to give a product size of 80% passing 15 µm. Part of the regrind mill discharge is recycled back to the regrind mill feed tank while the balance flows to the Cu-Ni separation cyclone overflow hopper. Process water is added to the cyclone feed tank to ensure the correct densities for cyclone separation.

Cyclone overflow is discharged into the regrind hopper. A sample is taken using a Vezin sampler prior to the regrind mill product being pumped to the concentrate aeration tank. This measures the grinding performance of the mill and ensures that the correct size distribution is sent to the Cu-Ni separation rougher flotation. Lime slurry is added to the regrind mill discharge tank for pH adjustment.

Po Rougher Concentrate Regrind
Po concentrate slurry from the Po rougher regrind cyclone feed tank is pumped to the Po regrind mill cyclone cluster. Cyclone underflow reports to the Po rougher regrind mill as mill feed. Part of the regrind mill discharge is recycled back to the regrind mill feed tank while the balance flows to the cyclone overflow hopper. Cyclone overflow is discharged into the Po regrind cyclone overflow hopper. The overflow slurry is pumped to the Po concentrate cleaning bank.

Processing

• Hydrometallurgical plant / circuit
• Hydrochloric acid (reagent)
• Sulfuric acid (reagent)
• Crush & Screen plant
• Autoclave
• Flotation
• Agitated tank (VAT) leaching
• Concentrate leach
• High Pressure Acid Leach (HPAL)
• Dewatering
• Filter press

Summary:

The NorthMet process plant will consist of an initial beneficiation plant in Phase I, and a hydrometallurgical plant in Phase II. The specific processing steps that will be involved in the hydrometallurgical plant include pressure treatment of concentrates and precipitation of gold and PGMs in separate processes. Additional facilities also include a hydrometallurgical residue facility.

Primary ground ore will be processed through a rougher flotation circuit to produce a bulk copper and nickel concentrate. The bulk concentrate will be reground and separated in cleaner flotation. The rougher tailings will be sent to the pyrrhotite flotation circuit so that PGM-rich iron sulfide can be captured as a pyrrhotite nickel concentrate.

Phase I: The Beneficiation Plant consisting of crushing, grinding, flotation, concentrate thickening and concentrate filtration. The Beneficiation Plant will produce and market concentrates containing copper, nickel, cobalt and precious metals.

Flotation
The overflow from the milling cyclone is pumped to the flotation feed tank. The flotation circuit consists of three separate flotation stages each with a regrind step:
- Bulk Cu-Ni circuit;
- Cu-Ni concentrate separation circuit;
- Pyrrhotite (Po) circuit.

Concentrate Thickening and Filtration
The three flotation concentrate products are dewatered via 2 stages, thickening followed by filtration. The recovered water from the dewatering stages is returned to the process water tanks for redistribution into the process plant.

The thickened concentrate is then filtered using a filter press to achieve a cake moisture of less than 12.1%.

Concentrate Storage
Front-end loaders transfer the selected filtered concentrate from the product stockpile onto the product transfer conveyors. The concentrate is then discharged into the rail cars via a bin and reversible shuttle conveyor. The transfer of concentrate to the rail cars is done separately so as not to contaminate the individual products.

Hydrometallurgical Processing
Phase II: In mine year 2, a hydrometallurgical plant is expected to be commissioned to process nickel sulfide and pyrrhotite concentrates, with processing starting in mine year 3. This concentrate stream will be processed through a single autoclave to recover high-grade copper concentrate, and recover the nickel-cobalt hydroxide and precious metals precipitates as by-products.

Hydrometallurgical processing will be used for downstream treatment and enrichment of metals into saleable products. The process involves high pressure and high temperature autoclave leaching in an oxygen environment, followed by solution purification steps to extract and isolate PGMs, precious metals, copper, nickel, and cobalt. All equipment used in the hydrometallurgical process will be located in the Hydrometallurgical Plant Building.

Once the hydrometallurgical plant becomes operational some of the concentrates produced in the beneficiation plant will be feedstock to the hydrometallurgical process.

Autoclave
The autoclave serves to oxidize sulfide minerals in the concentrates into soluble sulfates. Gold and PGMs, once liberated from encapsulating sulfides form soluble chloride complexes. Conversion of the metal sulfides into soluble metals species is achieved using under 440°F and 504 psi leaching conditions, in an acidic liquor and the presence of chloride ions in the autoclave slurry. The solid residue produced contains iron oxide, jarosite (iron sulfate) and any insoluble gangue (non-ore silicate and oxide minerals) from the two concentrate streams generated in the Beneficiation Plant.

Leach residue will be recycled (up to 230%) back to the mineral concentrate feed stream prior to introduction into the autoclave to maximize the extraction of Au/PGMs, thereby mitigating the requirement for a larger autoclave. Hydrochloric acid will also be added to maintain the proper chloride concentration in solution to enable leaching of the gold and PGMs. To ensure complete oxidation of all sulfide sulfur in the concentrate, and oxygen overpressure of 100 psi will be maintained in the autoclave. Leached slurry exiting the autoclave will be reduced to atmospheric pressure using a dedicated flash vessel, which allows the removal of excess heat through the release of steam from the slurry.

An autoclave gas scrubber will be provided to the flash vessel for initial scrubbing of the vapor streams to remove the majority of entrained process solids and liquor. Slurry discharging from the flash vessel is further reduced to 140°F using dedicated spiral heat exchangers. The cooled slurry is pumped to the leach residue thickener. The heat transferred in the heat exchangers will be used to pre-heat the feed solution for residual copper removal and mill process water. The contained solids will then be settled in a high-rate thickener, producing a thickened underflow containing 55% (w/w) solids. The underflow is split, with the majority of the slurry being recycled to the autoclave feed tanks. The remainder of the slurry reports to the leach residue filter, which separates the barren autoclave residue solids from the process liquor containing the solubilized metals. Residual entrained metals are recovered by washing the autoclave residue with filter wash water. The washed residue is filtered tails with process water and pumped to the hydrometallurgical residue facility (HRF). The HRF is being permitted for conventional tailing deposition.

The leach residue thickener overflow is then sent to other circuits to recover gold and PGMs by precipitation.

Gold and Platinum Group Metals Recovery
The leach residue thickener overflow is reacted with SO2 to reduce ferric ions in solution, followed by reaction with CuS to precipitate Au and PGMs in the second and third tanks. Complete reduction of ferric ions is subsequently achieved by the addition of CuS, recycled from the Residual Copper Sulfide Precipitation Thickener underflow. Secondly, CuS is also used to recover platinum, palladium, and gold from the autoclave leach liquor. This circuit produces a mixed Au/PGM sulfide with a large proportion of CuS and elemental sulfur. The discharge from the Au/PGM precipitation reactors is pumped to the Au/PGM thickener where CuS, enriched with Au/PGM metals, settles to produce thickened slurry suitable for filtration. The Au/PGM Thickener underflow is then pumped to the Au/PGM Filter which separates the Au/PGM precipitate solids from the process liquor which contain copper, nickel, and cobalt metal values. Residual entrained metal values are recovered by washing the Au/PGM precipitate with raw water and recycling to the Au/PGM thickener. The Au/PGM filter produces an Au/PGM Concentrate cake of 80% (w/w) solids.

Mixed Hydroxide Precipitation Recovery
The recovery of nickel and cobalt will be achieved by producing a mixed hydroxide precipitate for sale to a third-party refinery. The solution will be heated to 158ºF (70ºC) and reacted with 20% w/w Mg(OH)2 to precipitate out nickel and cobalt. The resulting discharge from the first stage of mixed hydroxide precipitation flows by gravity to the first mixed hydroxide precipitation thickener. With the aid of flocculant, the underflow of about 40% (w/w) solids containing the precipitated metals is achieved. The underflow will be pumped to a filter feed tank, which has a capacity to hold 12 hours’ worth of slurry to allow for filter maintenance. The slurry will then be pumped at a controlled rate into the hydroxide filter to produce a filter cake of about 75% (w/w) solids. The filter cake will be washed with raw water to remove entrained process solution. The final mixed hydroxide product has an approximate composition totaling 97% nickel, cobalt, and zinc hydroxides, with the remainder as magnesium hydroxide.

Pipelines and Water Supply

Summary:

The existing process water, raw water, spray water, fire water and gland water systems will require major re - engineering to suit the new process plant design. However, some of the major existing infrastructure including the Flotation Tailing Basin (FTB), fire water reservoir, reclaim water barge and pipeline, and Colby Lake supply system are still usable. New pipe racks will be required for the piping distribution systems within the Concentrator Building as well as all new buildings. Wherever practical, the piping distribution system will utilize the existing pipe tunnels to access these areas.

Raw Water
Raw water will be supplied to the plant from Colby Lake via a refurbished pipeline which PolyMet has acquired under its agreements with Cliffs Erie. The water appropriation permit that PolyMet has authorizes the withdrawal of the adequate quantities from Colby Lake for process make-up water. The existing 60-year-old pipeline that conveys raw water 5.6 miles will be lined in part or fully with a 34” diameter HDPE pipe. The process plant raw water distribution system will be modified to suit the new plant design. The reclaim water supply piping from the FTB will need to be routed to the newFlotation andConcentrate buildings.

Process Water
The five (5) existing 1,179 cy process water tanks will be installed for plant process water storage. The process water distribution system design will suit the proposed plant equipment layout. Piping from the process water tanks will be routed to the new Flotation and Concentrate handling buildings.

Gland Water
The gland seal water system would be fed by the raw water system, and will include a storage tank, pumps, filters, and recirculation piping. These services would be routed to the Concentrator Building and flotation areas.

Fire Water
The fire water system will be fed by the raw water reservoir and will include new pumps, recirculation piping, valves, hydrants, and hose reels.

The existing Plant Site fire water distribution system requires complete refurbishment. New fire water pumps, new piping in certain sections and new hydrants and hose reels are required. The distribution piping will also be extended into the new plant areas.

Potable Water Distribution
Bottled drinking water will be available at the mine and plant. Raw water will be treated to meet potable water standards for the plant use in safety showers.

Hydrometallurgical Plant Water
A separate hydrometallurgical plant process water stream is required due to the nature of the different process solutions involved in the hydrometallurgical versus the beneficiation processes. Hydrometallurgical process water will contain significant levels of chloride relative to the water in the milling and flotation circuits. The process water line would distribute reclaim water to various addition points throughout the hydrometallurgical plant from the hydrometallurgical residue facility. Make-up water could come from raw water when required.

All water that enters the Hydrometallurgical Plant will be recycled at each step of the process. The average annual water demand for the Hydrometallurgical Plant is estimated at 240 gpm but may vary from 114 to 406 gpm monthly as operating and climatological variations occur. To the extent possible, water used to transport residue to the tailing facility would be returned to the Hydrometallurgical Plant; however, losses may occur via evaporation and storage within the pores of the deposited residue. In addition, spilled solutions will be collected in sumps and returned to the appropriate process streams.

Commodity Production

Operational metrics

Personnel

Job Title	Name	Profile	Ref. Date
Consultant - Infrastructure	Alberto Bennett		Oct 31, 2022
Consultant - Recovery Methods	Laurie Tahija		Oct 31, 2022
Consultant - Recovery Methods & Costs	Nick Dempers		Oct 31, 2022
Director of Environment & Sustainability	Christie Kearney		Mar 5, 2025
Environmental & Permitting Manager	Kevin Pylka		Mar 5, 2025
President & General Manager	Tannice McCoy		Mar 5, 2025
Project Manager	Lucas Miller		Mar 5, 2025
Technical Manager	Jean-Charles Sztuke		Mar 5, 2025