Nickel Poses a Processing Pitfall
U.S. Steel first discovered NorthMet in the late 1960s, initially targeting high-grade mineralization at depth before moving up dip into lower-grade material that outcropped. However, prior to the autocatalyst market and significant industrial demand for platinum group metals, the only metals of relevance were copper and nickel—and nickel contamination of copper concentrate made the economics of existing flotation technology unattractive. U.S. Steel, after conducting widely spaced drilling along the deposit’s 3-mile strike length, ultimately sold an automatically renewable 20-year lease for it to PolyMet in 1989.
In December 2006, Polymet closed an additional purchase from Cleveland Cliffs that included a railroad connection linking the mine development site and the Erie plant, plus a 120-railcar fleet, locomotive fueling and maintenance facilities, water rights and pipelines, administrative office facilities at the site, and approximately 6,000 acres to the east and west of and contiguous to PolyMet’s existing tailing facilities—adding extra tailings storage capacity should the company choose to expand its production in the future. These assets cost Polymet another $15 million and 2 million shares, giving Cleveland Cliffs ownership of about 7.7% of Polymet’s issued stock.
The Erie plant, built in the 1950s for about $350 million, is an enormous facility—the main concentrator building alone is one-quarter mile long—that was originally designed to process 100,000 t/d of tough taconite ore. It includes a railcar dumper building; dual primary, secondary, tertiary and quaternary crushing lines; a large number of rod and ball mills; and a large magnetic separation section that will be superseded by a new flotation circuit.
Joseph Scipioni, Polymet’s COO, is confident that the plant, which has been idle since 2001, can be restarted without major mechanical difficulty. But as Scipioni, who formerly was plant manager at U. S. Steel Corp.’s taconite plant in Keewatin, said last fall, “Since we only plan to run the plant at one-third of its capacity, we’ll have plenty of spare parts available no matter what.” An existing pellet plant on site is being demolished. Polymet is installing a new flotation circuit inside the existing plant building and also plans to construct a new hydrometallurgical plant. It will sell a bulk concentrate from the flotation section to fund construction and commissioning of the hydromet plant which, when completed, will produce copper cathode, nickel-cobalt-zinc hydroxides or purified hydroxides of each metal, a gold/PGM precipitate for toll smelting, and synthetic gypsum which will be a by-product of the pressure leaching technology employed by the plant.
The project’s DFS projected a mining rate of 81,000 t/d of rock over the life of mine, with 32,000 t/d of ore delivered to the mill via the rail link. Mining costs presented in the DFS are $1.14/t of rock mined and $3.13/t of ore, but Polymet executives feel that these early-estimate figures are probably on the high side. The project staff has been examining opportunities for mining plan optimization and, according to the company, has been able come up with improvements that streamline pit scheduling, lower unit mining costs, reduce the overall strip ratio and extend reserve life. Mine preparation, including prestripping, will be handled by a contractor; Polymet will take over for full-scale operations, employing two large shovels and seven or eight 240-ton-capacity haul trucks. The final operational mine plan will be completed in parallel with the final stages of the permitting process
Ore from the mine, after arriving at the plant by rail, will be unloaded into the coarse crusher dump pocket and subjected to four-stage crushing—using only about one-fourth of the facility’s total crushing capacity—down to a product sized at 80% minus 0.5 in. followed by rod and ball mill grinding to a coarse sandy texture of 100- 125 µm P80. Prior to commissioning of the hydrometallurgical plant, Polymet plans to sell concentrates, either as a bulk product or as separate Ni and Cu concentrates with precious metal values present in both products. During the time it is selling the bulk concentrate, it will additionally process the flotation product through a scavenging circuit after the cleaner stage, following by regrinding, to remove more gangue material and produce a higher-grade concentrate. One of the company’s current objectives, according to David Dreisinger, a Polymet director and metallurgical processing consultant, is to increase copper content in the concentrate to more than 20% with less than 0.3% Ni content. Once the plant is operational, the company will process all concentrate using the Platsol process, which employs chloride-assisted pressure leaching to allow extraction of precious metals concurrently with the copper, nickel and cobalt. Platsol processing requires a conventionally designed autoclave operating at a moderately high temperature of 225°C and employing 30-60 g/L of H2SO4, and 10 g/L NaCl. At full production, Polymet intends to produce about 72 million lb/y cathode copper, 15 million lb/y of nickel, 700,000 lb/y of cobalt and more than 100,000 oz/y of combined precious metals.