NewsPolyMet Mining Corp. has released the results of the 2005 pilot plant testing of flotation and hydrometallurgical extraction of metals from NorthMet ore samples. The pilot plant program was completed at SGS Lakefield Research (Canada) under the supervision of Bateman Engineering (Australia). Bateman is responsible for completion of the definitive feasibility study on behalf of PolyMet.
Information contained in this news release was prepared by Dr. David Dreisinger, PolyMet's qualified person under National Instrument 43-101.
Testwork summary
The 2005 pilot plant program successfully confirmed the technical feasibility of the NorthMet flotation and hydrometallurgical treatment process. Copper, nickel, cobalt, gold, platinum and palladium were successfully floated from three ore composites and extracted via the autoclave process. The extracted precious metals were recovered into a precipitate for offtake/refinery treatment (after a further upgrading step). Copper was recovered as London Metal Exchange grade A copper cathode by conventional solvent extraction and electrowinning. Nickel and cobalt were recovered as either a combined mixed hydroxide precipitate or as separate purified nickel and cobalt hydroxide precipitate products for offtake/refinery treatment. The pilot plant provided engineering data for completion of the definitive feasibility study by Bateman. The pilot plant also provided environmental data for permitting of the NorthMet metallurgical facility.
The pilot plant testing comprised the following two main programs.
Ore preparation and flotation of concentrates
This consisted of preparation of three ore composites totalling approximately 40 metric tonnes with processing of the ore samples through a flotation recovery circuit to prepare concentrates for hydrometallurgical testing
Hydrometallurgical testing
This consisted of hydrometallurgical testing of the prepared concentrates to extract copper, nickel, cobalt, platinum, palladium and gold and recover metal and intermediate products. In the pilot plant program, platinum, palladium and gold were recovered as an upgraded precipitate. Copper was recovered as high-quality LME grade A copper cathode using solvent extraction and electrowinning. Two variations of the process for nickel and cobalt recovery were tested during the pilot plant. The first variation tested nickel recovery with cobalt as a mixed hydroxide precipitate. The second variation tested nickel recovery as a purified nickel hydroxide (cobalt free) with cobalt recovered in a second precipitate (as cobalt hydroxide). Selection of the preferred nickel recovery route will depend on the results of a cost benefit study and discussions with potential offtakers. Two byproducts were also produced in the pilot plant program. A high-quality gypsum precipitate was produced through neutralization of acidic solutions with limestone. A purified zinc sulphate solution was produced as part of the second nickel recovery process.
Ore preparation and flotation pilot plant results
The main objectives of this phase of the program were:
* to prepare three composites for testing from 40 tonnes of ore having varying sulphide mineralization;
* to study the breakage and grindability characteristics of the composites and generate design data for milling;
* to optimize flotation conditions in the laboratory for each composite including grind size, reagent conditions and flotation times;
* to process the three composites through a flotation pilot plant using conditions as set in the laboratory, confirming the established flowsheet and further optimizing conditions as required;
* to provide complete metallurgical balances and summaries describing the variability in performance between the three composites; and
* to provide detailed characterization of tailings and concentrate from the processing.
All objectives were met.
Three composites were produced from about 40 tonnes of ore. The tonnage distribution and contents, as averaged over the pilot plant campaigns, are given in the following table.
Composite
Sample weight Cu Ni S
(tonnes) (%) (%) (%)
Composite 1 9.5 0.31 0.095 0.73
Composite 2 19.2 0.35 0.10 0.77
Composite 3 9.6 0.40 0.11 0.87
Pt Pd Au
(g/t) (g/t) (g/t)
Composite 1 0.06 0.23 0.03
Composite 2 0.07 0.33 0.05
Composite 3 0.09 0.30 0.05
Comminution studies found the ore to behave consistently across the composites. The ore was found to have above-average breakage and abrasive characteristics. Ore hardness increased with finer grinding, with a Bond ball mill work index in the medium-hard range of 16 kilowatt hours per tonne at a P80 of approximately 100 microns.
Laboratory testing revealed that the composites all responded in a consistent manner to flotation. The general flotation flow sheet remained unchanged from past testing. SIPX, as applied in the past, was abandoned for PAX as the collector for maximized sulphide recovery. Copper sulphate was found to enhance sulphide flotation. Primary grind was set at 80 per cent passing 100 microns, consistent with past flotation programs. Dextrin was retained as a froth modifier and appeared to improve circuit stability in spite of showing no effect on flotation performance. Extended bulk scavenger residence times were found beneficial to sulphide recovery.
Pilot plant processing confirmed very little variability in flotation performance among the composites. About half of the ore was processed without copper sulphate addition and the remainder with copper sulphate addition. Average results for each general condition are provided in the table below.
S
Cu Ni (tot)
Conditions Product (%) (%) (%)
Feed 0.34 0.10 0.75
Concen-
No trate 11.9 2.55 20.0
CuSO4 Recovery,
(%) 94.3 69.3 72.4
Tailing 0.020 0.032 0.21
Feed 0.36 0.11 0.82
Concen-
CuSO4 trate 11.1 2.47 21.7
(+more PAX) recovery,
(%) 94.2 72.5 82.2
Tailing 0.022 0.030 0.15
S
equals Pt Pd Au
Condi- Product (%) (g/t) (g/t) (g/t)
tions
Feed 0.08 0.31 0.05
Concen-
No trate 2.02 8.61 1.11
CuSO4 Recovery, approx.
(%) 86 69.1 75.8 58.5
Tailing 0.09 0.03 0.08 0.02
Feed 0.08 0.28 0.05
Concen-
CuSO4 trate 1.81 7.61 1.01
(+ recovery,
more (%) approx.
PAX) 95 67.5 83.1 57.9
Tailing 0.04 0.03 0.05 0.02
Under the more aggressive flotation conditions including addition of copper sulphate, these ores were capable of recovering 94 per cent of the copper and 72 per cent of the nickel into a concentrate grading about 14 per cent Cu plus Ni. Platinum group element recoveries averaged 68 per cent for platinum, 83 per cent for palladium and 58 per cent for gold. Total sulphur content of the tails averaged 0.15 per cent total S and 0.04 per cent sulphide S equals.
Hydrometallurgical treatment of NorthMet flotation concentrate
The main objectives of this phase of the pilot plant program were to:
* demonstrate continuous operation of the hydrometallurgical extraction circuit for copper, nickel, cobalt, platinum, palladium and gold;
* demonstrate recovery of platinum, palladium and gold from leach solution by precipitation;
* demonstrate recovery of LME grade A copper cathode from the leach solution by solvent extraction and electrowinning;
* demonstrate recovery of nickel and cobalt in a co-product of mixed hydroxide precipitate;
* demonstrate separation of nickel from cobalt and minor impurities and production of a purified nickel hydroxide, a purified cobalt hydroxide and a zinc sulphate solution;
* collect important engineering data on each segment of the process plant and test a range of materials of construction for final selection in design; and
* collect all required environmental data.
All objectives were met.
Hydrometallurgical extraction
The concentrates from the flotation pilot plant were treated through the hydrometallurgical pilot plant. The concentrate compositions are listed below. Concentrates C2, C1-1, C1-2 and C3 were produced in the 2005 pilot plant. C2 represents the concentrate from composite 2 (see table above) with C1-1 and C1-2 from composite 1 and C3 from composite 3. A sample of concentrate from the year 2000 flotation pilot plant (C2000) was also treated in the current pilot plant for comparison to the new concentrate products and to supplement concentrate supply for the pilot plant. This concentrate (C2000) was preserved under nitrogen and frozen since 2000.
Concen- Quan- Assays
trate tity
sample (kg)
Ni Co Cu Fe S
(%) (%) (%) (%) (%)
C2 456 1.80 0.09 8.13 21.00 14.90
C1-1 45 2.46 0.1 10.50 25.50 20.15
C1-2 45 2.15 0.10 9.37 26.50 19.10
C3 177 2.46 0.12 10.90 29.80 23.15
C2000 139 3.04 0.14 15 32.7 27.2
Concen- Assays
trate
sample
S
equals SO4 Au Pt Pd
(%) (%) (g/t) (g/t) (g/t)
C2 14.40 0.60 0.91 1.45 5.97
C1-1 19.35 0.50 1.10 1.67 7.32
C1-2 19.05 0.50 0.87 1.36 6.03
C3 less
than
20.90 0.4 1.18 1.66 7.57
C2000 26.6 0.4 1.96 2.37 10.6
The autoclave extractions of the base metals were excellent throughout the pilot plant run. The extraction of copper ranged from 96 to 99 per cent with an overall average of 98 per cent. The extraction of nickel and cobalt ranged from 93 to 99 per cent for nickel (average of 97 per cent) and 97 to 99 per cent for cobalt (average of 98 per cent).
The autoclave extractions of gold, platinum and palladium were initially lower than expected due to the placement of corrosion coupons in the autoclave during the pilot plant. Corrosion coupons are small samples of alloys that are tested for compatibility to the solution chemistry in the autoclave as part of the engineering study. A number of the corrosion coupons corroded (and importantly, a number did not), likely leading to precious metal reprecipitation inside the autoclave during the pilot plant, and consequently, lower precious metal extractions. Gold is the element that is most sensitive to reprecipitation. Consequently, the autoclave extractions ranged from 37 to 77 per cent for gold, 83 to 94 per cent for platinum and 73 to 91 per cent for palladium. Importantly, during the last period of 51 hours of pilot plant operation, when the corrosion coupon effect was reduced (most of the corrosion having likely already occurred), the autoclave extractions were 76 per cent for gold, 94 per cent for platinum and 90 per cent for palladium.
Platinum group metal recovery from solution
Recovery of gold, platinum and palladium from the autoclave solution (after filtering and washing the solids) was accomplished by precipitation. Recoveries of gold, platinum and palladium into this precipitate concentrate were excellent, in excess of 99.5 per cent in each case. Base metal losses from solution into the PGM and gold residue were negligible.
Approximately four kilograms of precipitate were collected during the pilot plant analyzing 56 grams per tonne Au, 211 grams per tonne Pt and 907 grams per tonne Pd. Most of the precipitate mass was copper (35.7 per cent) and sulphur (49 per cent). Releaching of the precipitate to remove copper and sulphur resulted in an upgraded material analyzing approximately 1.6 per cent or 16,000 grams per tonne, contained gold, platinum and palladium. This high value material will be placed with an offtake/refinery partner for final separation into pure precious metal products.
Copper recovery from solution
Copper recovery from solution was performed by the conventional solvent extraction and electrowinning process. This process performed very well and produced excellent technical results.
A total of 68 kilograms of copper was extracted and electrolyzed during the pilot plant run. The copper metal cathode was analyzed and found to meet LME grade A copper purity specifications. This product can be sold directly to copper consumers.
Nickel and cobalt mixed hydroxide precipitate recovery
The first method of nickel and cobalt recovery tested involved treatment of a portion of the solution after recovery of copper, gold, platinum and palladium. The treatment culminates in precipitation of a mixed hydroxide precipitate using magnesia. This circuit performed very well and produced approximately 19 kilograms of products containing 31.5 to 40.6 per cent nickel and 1.67 to 2.17 per cent cobalt. The mixed hydroxide precipitate also carries the small amount of zinc in the NorthMet concentrate (4.27to 4.84 per cent zinc) and minor amounts of magnesium (0.62 to 1.04 per cent magnesium), sulphur (3.44 per cent) and other metals.
The mixed hydroxide material is high quality and may be placed with an offtake/refinery partner for final separation into pure nickel and cobalt and byproducts.
Purified nickel hydroxide precipitate recovery
A second method of nickel and cobalt recovery was tested as part of the pilot plant. In the second method, a purified nickel hydroxide was recovered using conventional solvent extraction removal of cobalt and zinc followed by precipitation of nickel hydroxide using magnesia. The reason for testing this second method was to allow for the option of producing a higher purity and higher value material for offtake.
This circuit performed very well and produced a total of 23 kilograms of precipitate containing 43.7 per cent nickel, 1.91 per cent magnesium, and 2.53 per cent sulphur.
The cobalt removed by solvent extraction was precipitated using magnesia to form a precipitate containing 35.1 per cent cobalt, 0.42 per cent zinc, 1.44 per cent copper, 1.23 per cent manganese, 0.016 per cent nickel and 9.11 per cent magnesium.
The zinc was recovered as a separate solution suitable for production of zinc sulphate solid crystals for offtake.
The purified nickel hydroxide route produced three products; a purified nickel hydroxide precipitate, a cobalt hydroxide precipitate and a zinc sulphate solution. The nickel and cobalt hydroxides were of high quality and may be placed with one or more offtake/refinery partners for final separation into pure nickel and cobalt products. The zinc sulphate solution was of high purity and may be used for manufacture of zinc sulphate by evaporative crystallization.
Engineering data
The demonstration pilot plant presented an opportunity to collect a wide range of engineering data by SGS Lakefield under the supervision of Bateman. Specialist vendors of solid-liquid separation equipment (filters and thickeners) were also invited to attend the pilot plant and perform tests.
The pilot plant engineering data will be used in the final design of the NorthMet metallurgical plant.
Environmental data
Barr Engineering of Minnesota, PolyMet's principal environmental review and permitting firm, was present throughout the pilot plant operation to collect environmental data for emission and waste characterization purposes. The pilot plant operation is essentially a scaled-down version of the full-scale plant under design and therefore represents an ideal opportunity to obtain key environmental data. Environmental data collected during piloting are being used by an independent contractor to prepare the mandated environmental impact statement and to define methods of mitigation and environmental management.